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Chemical and physical modifications of starch for renewable polymeric materials
Journal Pre-proof Chemical and physical modifications of starch for renewable polymeric materials Ewomazino Ojogbo, Emmanuel O. Ogunsona, Tizazu H. Mekonnen PII:
S2589-2347(19)30056-9
DOI:
https://doi.org/10.1016/j.mtsust.2019.100028
Reference:
MTSUST 100028
To appear in:
Materials Today Sustainability
Received Date: 7 May 2019 Revised Date:
25 October 2019
Accepted Date: 7 November 2019
Please cite this article as: E. Ojogbo, E.O. Ogunsona, T.H. Mekonnen, Chemical and physical modifications of starch for renewable polymeric materials, Materials Today Sustainability, https:// doi.org/10.1016/j.mtsust.2019.100028. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
Chemical and physical modifications of starch for renewable polymeric materials Ewomazino Ojogbo, Emmanuel O. Ogunsona, Tizazu H. Mekonnen* Department of Chemical Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, Canada
* To who correspondence should be addressed: [email protected] Abstract Biodegradable polymers obtained from renewable natural resources have received increasing attention due to their potential as alternatives to traditional petroleum-based plastics. Among the various sources, polysaccharides stand out as a highly convenient feedstock because they are readily available, renewable, inexpensive and provide great stereochemical diversity. Starch, a renewable polysaccharide polymer, has attracted a substantial research and commercial interest as a feedstock due to its renewability, biodegradability, low cost, and abundance of –OH chemistry leaving it open to endless modification possibilities and melt processability in the presence of plasticizers. However, its hydrophilicity, thermal and mechanical properties limitations, rapid degradability, and strong intra and intermolecular hydrogen bonding of the polymer chains hinder its melt processability and limit its widespread commercial application as a renewable biopolymer. Therefore, modification is necessary to mitigate these limitations and bring about other desirable properties. This article critically reviews the recent progress achieved in the modification of starch for industrial biopolymer material applications where starch is used as one or the major constituents. Keywords: Starch bioplastics, Biopolymers, Thermoplastic starch, Starch modifications, starch processing, Starch biodegradation
1
1. Introduction Polymers offer an unparalleled combination of low cost and toxicity, processability ease, excellent thermal stability and balance of physical properties. Consequently, they are used in a variety of advanced and commodity applications. The simplistic design and low cost of these products bring about a short service life; typically, in single-use applications after which they become waste. As a result, a significant amount of municipal and industrial wastes consists of commodity
polymers
such
as
polyethylene,
polyethylene
terephthalate,
polystyrene,
polypropylene, etc., which are utilized on a massive scale in packaging and containment applications1. The short service life of these inexpensive polymers is in sharp contrast with their remarkable resistance to biodegradation. Moreover, concerns over depleting fossil resources coupled with unstable supply, energy requirements for processing, price, and limitation in geographical distribution are further concerns for typical fossil-derived polymers. As a result, there is significant research interest in the utilization of natural feedstock (e.g. starch, cellulose, hemicellulose, lignin, protein, lipids, etc.) for polymeric applications as these are susceptible to microbial attack for biodegradation and safe disposal. In general, renewable polymers provide environmental sustainability, unique performance, and economic benefits2–4. However, due to limitations in the intrinsic properties of these renewable polymers, they cannot be used directly in combination with polymeric materials for targeted applications. For instance, due to the strong inter- and intramolecular hydrogen bonding in starch, protein and cellulose molecules, they exhibit melting temperatures beyond their thermal degradation temperatures. Consequently, such polymers cannot be directly melt-processed. The high moisture sensitivity of these biopolymers is another challenge that needs to be mitigated before they can be applied in useful applications. This manuscript focuses on reviewing the 2
recent progress achieved in starch modification pertaining to industrial thermoplastics applications. 1.2. Bio-sourced polymers for sustainable material development Biopolymers can be extracted from various sources; for instance, cellulose, hemicellulose, lignin, and polyphenols can be sourced from plants. Animal-sourced biopolymers include chitin from crustaceans, collagen and keratin proteins from animals, proteins and cellulose whiskers from mollusks, and other bacterial polymers5. The most extensively utilized biopolymers for non-food industrial applications are starch and cellulose because of the cost and availability. Contrarily, research and development efforts into the utilization of bio-sourced monomers as building blocks of polymers, or as a carbon sources to produce sustainable polymers via a fermentation process has resulted in important scientific and commercial success. For instance, lactic acid produced via the bacterial fermentation of starch is a monomer that can be polymerized into poly(lactic acid)2. Similarly, succinic acid produced via fermentation is used as a monomer or co-polymer of a range of polyesters and polyamides6. The polymerization or co-polymerization of lipid-derived fatty acids for a range of polymer applications constitutes an example of the utilization of bio-resourced polymer7,8. Among the naturally occurring biopolymers, cellulose is the most abundant structural polysaccharide polymer9. It is a straight-chain polymer that is comprised of glucose monomers linked by β-1,4 glycosidic bonds. The major applications of cellulose are in the production of paper and fibers for textiles. Cellulose also finds use in medicine, production of films10 and fillers for consumables. The hydroxyl functional group in cellulose can be partially or wholly esterified or nitrated to produce derivatives with desirable properties for other applications11,12.
3
More recently, the extraction of the crystalline fraction of cellulose via acid hydrolysis for material applications has received extensive attention. Hence, developing more efficient ways of producing them at a lower cost. Hemicellulose is a branched polysaccharide polymer found in plant cell walls. It includes xyloglucans, xylans, mannans, glucomannans, and glucans13. Hemicellulose biopolymer is used as hydrogels, cosmetics, and drug carriers14,15. However, its extensive use is still limited due to its hydrophilicity and rather poor mechanical performance16. Proteins are also an attractive feedstock for industrial and functional material applications. This is because of their capability to form a cohesive and continuous matrix and an impressive range of functionality spanning from catalytic activity to selective binding and mechanical strength, thereby making them attractive biomaterials17,18,19. Moreover, the rich functional groups (e.g. –NH2, -OH, -COOH, -SH) of proteins associated with the amino acid residues, provides a lot of opportunities for modification and successively makes them appealing biomaterials20,21. Starch, a polysaccharide that can be synthesized by plants and found mainly in fruits, roots tubers, legumes, and cereals is typically found in the range of 25-90 %13. It is a semi-crystalline polymer having about 1,000-2,000,00022 glucose monomers linked by α-1,4 glycosidic bonds. There are three reactive hydroxyl groups on the glucose unit of the starch chain, with a primary and two secondary hydroxyl groups, which can act as anchor points for modification chemistries. With several reviews already published on starch with focuses on food applications23,24, advanced function applications25, and surface modifications26,27, this literature review focuses on capturing the progress on the physicochemical properties and chemical modification of starch for use as feedstock in petro- and bio-plastics material applications. Prior to date, to the best of our knowledge, only one review written ten years ago has been published on completely 4
biodegradable starch-based polymer materials28, relating to the application of starch in polymeric materials. Researchers involved in renewable and sustainable polymeric materials with starch as a focus can significantly benefit from this review as an update to advances and knowledge in this area, and act as a stepping stone for investigations into niche areas of research which need exploitations. 1.3. Starch as a renewable and sustainable feedstock material Unlike other polysaccharide polymers that are harvested or extracted from through the destruction of the plants, starch can be harvested in most cases without destroying the plant. Plants produce starch granules through the polymerization of glucose via photosynthesis of carbon dioxide. Starch is largely consumed as food or used in food preparation across the world, and finds use in diverse industrial applications29. Besides being consumed as food, other major uses are as components in paper binders and adhesives, feedstock for fermentation, textiles, chemical production, and other industrial products30. The low cost, abundance and wide geographical distribution and ease of growth have sparked research interest in starch as a biopolymer. Some of its chemical, physical and functional properties attributes such as gelatinization, water retention properties, ease of functional property optimization, ease of dissolution in solvents, and temperature-induced pasting make it intriguing and aresearch focus31. Furthermore, starch, like other natural fibers and biomass can be processed via traditional polymer processing techniques such as extrusion and injection molding in the presence of plasticizers32–39. The retrogradation, low thermal stability, hydrophilicity, and brittleness of starch has restricted its widespread use in and for industrial polymer applications that require tailored properties like mechanical integrity. Hence, modification of the hydroxyl
5
functional groups on its surface is required to alleviate the aforesaid limitations to achieve the required properties for its use in industrial material applications. Native starch granule is an unprocessed and/or unmodified starch granule that is made up of a linear glucose chain linked by α-1,4 glucosidase bond and a branched glucose chain with branching at α-1,6 positions called amylose and amylopectin, respectively13. The amyloseamylopectin ratio has been shown to affect the chemical properties of the starch and hence, it’s functionality as a biopolymer22,31. Starch containing various amylose-amylopectin ratios from various botanical sources are shown in Table 1. Table 1: Amylose-amylopectin ratios and crystallinities of starch obtained from various plant Source Rice Potato Cassava Waxy cassava Wheat Corn Sorghum sources.
Amylose (%) 20-30 23-31 16-25 0 30 28 24-27
Amylopectin (%) 80-70 77-69 84-75 100 70 72 76-73
Crystallinity (%) 3840 23-5340 31-5942 36-3940 43-4840 22-2845
Ref. 5 41 43 43 5 44 46
The physical and chemical properties of starch are both determined by its composition and structure. Several factors such as the plant source, geographic growth location, soil type and condition, and growing climate conditions as well as its amylose/amylopectin ratio determine the overall structure of the starch41. Starch contains three crystal structures; A, B and C, and are functions of its origin. It has small granules which vary in shape depending on the source13. The granule comprises of glucose monomer units linked by α-1,4 glycosidic bonds, forming amylopectin and amylose polymer units. Amylose is a linear polymer with α-1,4 glycosidic 6
bonds which links the glucose monomers with an approximate molecular weight of 1 x 106 g/mol. This represents the amorphous structure within the starch granule. Conversely, amylopectin, which accounts for the crystalline structure of starch, is a higher molecular weight polymer with an average molecular weight of approximately 1 x 108 g/mol47 while linked by short α-1,4 glycosidic bonds with high branching at the α-1,6 positions13. Branching of amylopectin polymer creates a double helix of approximately 5 nm in length which aligns in the crystalline region within the starch granule48 as shown in Figure 1. Positive birefringence as determined through x-ray diffraction (XRD) of the macroscopic view of starch under illuminated light showed by a Maltese cross, indicating an arrangement of the macromolecular units represented by a helix in the starch morphology22, and disappeared upon disruption of the starch granule. The amorphous and crystalline lamellae in the starch granule caused by the interchanging arrangement of the molecular unit is responsible for the semicrystallinity of starch. The crystallinity is reported to range from 20 to 45 %48.
7
Figure 1. Acid hydrolysis of native starch showing the breakdown of constituents: amorphous and crystalline growth rings Multi-scale structure of starch49. A non-carbohydrate component found in starch is phosphorus, which exists as a phosphate monoester and phospholipids. Depending on which macro-polymer the phosphorus within the starch molecule bonds with, the lucidity, gel strength and solubility of the starch varies13. Starch also contains 0.1-0.7 % protein by weight and lipids (up to 1.5 % by weight) that are present as free fatty acids and lysophospolipids50. Other minerals such as calcium, magnesium, potassium, and sodium also exist in granular starch40. 1.4. Physical properties of starch 1.4.1. Solubility in organic solvents
8
Starch is hydrophilic in nature with limits its solubility in cold water. When in contact with cold water, starch disperses well but settles with time if left undisturbed. However, when cold waterdispersed starch is heated, it solubilizes to form a gel-like paste. The chemical process responsible for solubility and gel-like paste formation of heated water-dispersed starch is the formation of hydrogen bonds between water molecules and the hydroxyl groups on the separated amylose-amylopectin molecules. Although starch is soluble in water when heated, water is not always a suitable solvent for starch modification for industrial applications. This is because water may partake in some reactions. Thus, the study of the solubility of starch in organic solvents is desirable to elucidate an appropriate inert solvent as a reaction media. Starch is soluble in some polar solvents like dimethyl sulfoxide (DMSO), dimethylformamide (DMF), pyridine, 90/10 v/v mixed solvent of DMSO/water, DMSO with lithium chloride (LiCl), DMF with LiCl and N,N-dimethyl acetamide (DMAc)51. Generally, the more polar the solvent, the higher the solubility of starch will be in that solvent. DMSO appears to be a promising solvent media for starch modification owing to its high polarity, its stability in neutral and alkaline pH and its stability at a fairly high temperature. It is worth noting that the choice of solvent depends on the desired application. 1.5.2. Gelatinization and retrogradation of starch Many starch modification processes involve the granular disruption of starch known as gelatinization, mainly to access the –OH functional groups. Gelatinization, in general, is an irreversible order disruption of the granular structure of starch molecule leading to a loss in bifrigerence properties52. This occurs when starch is heated between 60-70 °C53 in excess water, leading to maximum granular swelling and bursting of the granule. It occurs in two stages;
9
firstly, amylose-amylopectin separation resulting from the absorption of water and swelling of the granule leading to a loss in semi-crystallinity13 of starch. This separation occurs when the intermolecular hydrogen bonds are broken to loosen the double helices54. It usually begins in the amorphous region because of the ease of water percolation that results in the weakening of the hydrogen bonds. Secondly, separation and loss of amylose leaching from granule into the solution. The amount of water affects gelatinization; in a low water-starch ratio, granular swelling is incomplete, leading to a partial loss of crystallinity called melting55. Contrarily, the crystallites will be separated because of excessive swelling with no crystallites to be disordered in the case of excessive water. Additionally, the ratio of amylose and amylopectin of the starch granule affects the gelatinization temperature and the quality of the paste. For instance, high amylose starch with amylose to amylopectin ratio of 70:30 gelatinizes at 160 – 170 °C 56. Another method to achieve the gelatinization of starch is through the application of high pressures. While separation of amylose-amylopectin molecules also occurs with high pressure, granule swelling is minimized and solution leaching of amylose is reduced. Like thermal gelatinization, the amount of water and treatment time affect high-pressure gelatinization55. A study by Baks et al
55
revealed that at constant temperature in different starch samples,
gelatinization was faster at higher pressures (above 400 MPa). As gelatinization and granular disordering occur, starch granules lose birefringence, which is a characteristic of gelatinized starch. When gelatinized starch is cooled, the segregated amylose-amylopectin molecules realign themselves to a crystalline structure in a process known as retrogradation. Retrogradation is usually accompanied by expulsion of water, an increase in viscosity and gel formation. Furthermore, when retrogradation occurs, amylose links up with multiple glucose units, forming 10
a double helix and the short chains of amylopectin crystallize simultaneously. As well, components present in the starch granule affect retrogradation. While the presence of protein has been shown to slow down retrogradation during refrigeration, temperatures below 0 oC accelerate it13. In summary, retrogradation of starch can also be influenced by the botanical origin, storage length and conditions, and the amount of water54. The resulting product of retrograded starch is the formation of a gel. In native starch with a high amylopectin ratio, the gel formed is typically soft. Contrarily, starch containing a high amylose ratio forms a flexible and strong gel that exhibits resistance to deformation5. Since the soft amylopectin gels display low molecular strength, their desire for industrial use is rather limited13. Hence, for most industrial applications, starch with high amylose content is preferred. Figure 2 summarizes the steps involved in the gelatinization and retrogradation process of starch.
11
Figure 2. Processes that occur during gelatinization and retrogradation. (a) undisrupted starch granule; (b) absorption of water, swelling of granule, molecular segregation and loss of amylose to solution; (c) realignment of amylose molecules due to cooling (d) recrystallization of amylopectin molecules during storage. Adapted and modified from Liu et al 48. Elsevier copyright © 2009. 1.5.3 Melt processability of starch To process polymers in a typical thermoplastic processing operation, the polymer is heated above its melting point to enable a viscous flow. Nevertheless, when starch is heated, it begins to degrade before any observation of melting. This is because its melting temperature is above the degradation temperature, stemming from the strong inter- and intramolecular hydrogen bonds in the starch polymer chains. Plasticizers are customarily added to starch to reduce the number of sites available for inter- and intra-molecular hydrogen bonds and as a result improve melt processability48. There are several plasticizers employed for use with starch including water, polyols, sorbitol, urea, formamide, and citric acid with water being the most common plasticizer57. Starch in its native form contains 9-10 % w/w of water. However, water from an external source is required to plasticize starch. The higher the water concentration in starch, the lower the melting temperature. Unfortunately, starch plasticized with water has undesirable mechanical properties and suffers from retrogradation. Hence, it cannot be used as a lone starch plasticizer. Plasticized starch, also referred to as thermoplastic starch (TPS), is prepared by melt processing of starch at an elevated temperature and shearing in the presence of an added plasticizer57. Gelatinous starch is referred to as thermoplastic starch if it is stable and retrogradation does not occur. Retrogradation can be prevented by the addition of other polymers or the use of substituted starch that restrict recrystallization by interrupting the hydrogen bonds in starch.
12
Glycerol, polyols, sugar alcohols, ionic and non-ionic surfactants amongst others are common plasticizers employed for TPS. However, care should be taken when using glycerol as it has a high affinity for bonding with water molecules, thereby reducing its efficiency as a plasticizer to interrupt the hydrogen bonds in starch. This anti-plasticization phenomenon results in increased phase transition temperatures and brittleness of the starch. Also, most common plasticizers are known to leach out of the polymer with age as they do not chemically bond with the starch molecules. 1.5.4. Rheological and thermal properties of starch Dry starch molecules is frozen at room temperature, making them immobile. As the temperature is increased, starch molecules become mobile and move past one another resulting to its glass transition. When starch is heated, it undergoes several phase transitions including granule swelling, gelatinization, decomposition, melting and crystallization. Methods employed to study starch phase transitions can be divided into two major groups: (1) Transitions studied in the absence of shear, including, wide-angle X-ray diffraction scattering (WAXS), small-angle X-ray diffraction scattering (SAXS), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and hot-stage microscopy; and (2) transitions studied while shearing including rheometry48.
13
Figure 3. (a) A viscosity profile of native tapioca starch as a function of thermal treatment and time. The onset of viscosity between 339 and 347 K (66 to 74 °C) is marked by the grey zone, and (b) In situ WAXS data measurement for native tapioca at a heating rate of 2 K/min. Adapted from Huang et al.58 IUCrJ copyright © 2014. Shearing aids the mechanical segregation of the starch molecules, thereby enhancing the loss of the crystalline structure. The paste obtained after gelatinization contains viscous leached granules, swollen granules and the rest of the starch components. The viscosity of the starch gel is impacted by temperature change, amylose/amylopectin ratio, starch components, and shear rate41. Structural and viscosity changes during gelatinization and gelation of starch granule were studied in detail by Huang et al58. The viscosity and crystallinity changes during the gelatinization of starch are presented in Figure 3 (a and b). Figure 3a shows that the viscosity exhibits a sharp increase in the melting range of 66 to 74 °C during the heating of starch, indicating the gelatinization of starch. Subsequent cooling of the gelatinized starch caused an additional significant viscosity increase suggesting the occurrence of gelation. The WAXS result in Figure 3b exhibits the decaying of peaks with an increase in temperature, indicating starch crystal melting.
14
According to a study carried out on rice starch by Simi et al59, a fixed concentration of starch was heated from 50 to 95 °C and the temperature kept constant at 95 °C for 2 mins. The recorded viscosity profile exhibited a high peak viscosity, which is the maximum viscosity at which the starch granules hold water before they break. When granules disruption followed by amylose leaching begins, the breakdown viscosity occurs due to amylose loss to the solution. On the contrary, when starch granules begin to realign themselves during cooling and retrogradation, it is accompanied by an increase in viscosity; this is the setback viscosity. A schematic representation of the melting of starch is presented in Figure 4.
Figure 4. Schematic representation of the melting process of a semicrystalline starch lamellae. (a) stacked lamellae of well packed crystalline nanoclusters of amylopectin (orange) and amylose (black) chains, (b) the melting of defect-rich regions leads to partial loss of integrity of the lamellae, into layers alternatingly rich and poor in tilted and partially disordered nanoclusters (shaded background in torquoise), and (c) in melted starch, the molten amylopectin nanoclusters (bright orange), are loosely associated with relatively untangled linear amylose chains (black). Adapted and redrawn from Huang et al58. IUCrJ copyright © 2014. 2. Modification of starch Though, native starch is hydrophilic in nature, it is insoluble in water at room temperature and suffers from retrogradation. Its very low thermal stability means it cannot be melt processed without thermal degradation occurring, resulting in poor mechanical integrity. These deficiencies have constrained it as a filler or second phase in polymer blending and use for polymer applications that require thermal stability and mechanical strength. This is especially critical in the plastics manufacturing industry which relies heavily on melt processing in most cases.
15
Likewise, the limited cold water solubility of starch further restricts its potential application as an additive in the huge oil industry in applications such as drilling fluid additives and enhanced oil recovery (EOR). Therefore, modification of starch in most cases is required and desirable not only to alleviate these challenges but also to bring about other functional properties. Several improvements to the properties of starch have been achieved after modification include amphiphilicity, paste clarity,
thermal
stability,
hydrophobicity,
freeze-thaw stability,
retrogradation resistances and mechanical strength amongst others59–64. These starch modification processes used in achieving these improvements have been well detailed and reported in the literature, and can be subdivided into physical65 66, chemical67–70, enzymatic71–73, biotechnological5 or the combinations. 2.1. Physical and thermal modifications Physical modification is an appealing process, as it does not involve the use of chemicals, which could contaminate the starch especially when the modified starch is to be utilized for food contact or in the biomedical industry. Moreover, such modification processes are relatively inexpensive compared to other techniques. Starch modified by physical means can be classified into two broad categories: (i) Physical modification with the destruction of granular structure (pre-gelatinization processes) and (ii) Physical modification without the destruction of the granular structure (hydrothermal treatment)74. Physical modifications could involve changes in the granular structure, thereby modifying its gelatinization behavior, swelling, and solubility behavior. These processes are commonly implemented when modifying starch for food applications. Alcazar-Alay and Meireles75 conducted a detailed review of such physical modifications of starch.
16
2.1.1. Pre-gelatinized starch Cooking of starch till the conclusion of the gelatinization process and drying thereafter to a powder, is referred to as pre-gelatinized starch (PGS)33 . The process of PGS produces starch with improved swelling capacity that solubilizes in cold water without the application of heat31. Unfortunately, there is an accompanying loss of granular structure, granule break-down and, loss of birefringence, and crystalline properties with this physical treatment. The solubility in cold water and pasting ability allows for the extension of PGS application in the food industry such as thickeners for soups and as an adhesive in many products. Chemical properties of starch treated by this method are dependent on the starch botanical source, methods and conditions of the cooking and drying process34 . The techniques for drying are drum drying30 , spraying drying35 , and conventional extrusion cooking30 . Drum drying is the most accepted technique applied industrially. Pre- gelatinization of starch using this drying technique could occur in one or two stages. In the one stage technique, the starch slurry fed into the drum is gelatinized and dried simultaneously, while, the two-stage technique, involves cooking the starch slurry until it gelatinizes before it is drum dried31 . The PGS from this drying process has various textures and porosities with an effect on the functional properties and, product rheological properties. These properties are dependent on the time spent in the dryer, hence, the residence time should carefully be selected30 . Research results carried out by Majzoobi et al on PGS with twin drum drier showed PGS with no crystalline structure, intrinsic viscosity significantly lower than that of native starches indicative of degradation of starch molecules due to high temperature in drums, higher viscosities at temperatures below the gelatinization temperatures of native starches and, higher water adsorption and solubility than the native counterpart. Spray drying on the other hand commonly used in the food industry for 17
encapsulation involves instant water removal from starch slurry by atomizing it in a hot gas current35. This technique prepares PGS without the loss of granular integrity30. Finally, conventional extrusion is a high temperature, pressure and shear technique that compresses the starch with an accompanied change in granular structure from semi-crystalline to a plastic material with high viscosity30. 2.1.2. Hydrothermal modification Hydrothermal treatment is a technique that modifies starch while keeping the granular structure intact. This treatment requires the starch molecules to be kept in the mobile state above the glass transition temperature (Tg) and below the gelatinization temperature (Tgel)33 . Techniques for hydrothermal modification are annealing (ANN) and hydrothermal treatment (HMT). Both techniques require temperature and moisture level for the duration of the treatment to achieve modification30 31. While HMT requires low levels of moisture, typically <35 % w/w, annealing is carried out in excess water content of >65 % w/w31. 2.1.2.1. Annealing (ANN) Annealing is the rearrangement of starch molecules in the presence of water between 40 and 76 %w/w, and heat between the glass transition and gelatinization onset temperature32 36 4. Annealing could lead to partial gelatinization; however, starch can only be classified as annealed when the onset of gelatinization temperature exceeds that of the native counterpart37. Therefore, this process causes an upward shift in the endotherms of starches38 characterized by elevated gelatinization temperatures and shortening of the time from onset to the conclusion of gelatinization. This increase in temperature (up to 14 oC) is because of the rearrangement of the
18
granular structure, leading to a reduction of the amorphous polymers and an increase in crystallinity38. Figure 3 shows the DSC thermograms of annealed starch versus native starch.
Figure 5. DSC thermograms of commercial wheat starches: (a) native; (b) after annealing in excess water (45 oC, 100 days) (Adapted and modified from Tester et al37 ) Parameters for annealing to occur are moisture content, temperature and time; the extent of treatment changes as these conditions are varied39. Other characteristics of annealed starch are a decrease in solubility, increase in enzyme hydrolysis, increase in crystallinity and decrease in retrogradation39. 2.1.2.2. Heat moisture treatment (HMT) This treatment method is similar to annealing except it is carried out in low moisture content of <35 % w/w, usually for a period between 15 minutes to 16 hours33 and at 80-140 oC30 . This treatment affects the physical, chemical and morphological properties of starch depending on their botanical source. The properties affected include crystallinity and granular interactions as
19
shown from the XRD patterns, gelatinization, paste properties, amylose leaching, and recrystallization and enzyme hydrolysis31. Starch treated with this method finds industrial applications in the food industry, especially in baby foods. Baking properties along with freezethaw stability has also been shown to improve in potato starch treated by this method30. Although, thermal treatments improve the physicochemical properties of starch, the shortfall of these processes is a loss in nutrients, vitamins, and taste in food starch due to temperature exposure. As mitigation strategy, non-thermal treatments can be used to reduce the impact of microbes while keeping nutrients, flavors, and vitamins intact31 40. The desired property of treated starch determines the technique of treatment applied. The methods include ultrasound effect, osmotic pressure treatment41 , microwave treatment, and high hydrostatic pressure (HHP) treatment. A study carried out by Chirdchan et al41on the effect of osmotic pressure on starch showed results similar to those of HMT treated starch except a broadening of the gelatinization enthalpy. 2.2. Chemical modifications Chemical modification of starch is an important modification route that involves the blocking or introduction of functional groups to impart desirable physical and chemical properties while maintaining chain integrity, thereby extending its application. Because of the intrinsic advantage of an abundance of hydroxyl groups on the structure of starch, different chemical modifications have
been
studied,
including
etherification76–78,
oxidation67,79,80,
acetylation56,62,81,82,
esterification83–86, polymer grafting87–89, crosslinking90,91, silyation92–94, hydrolysis,95–97 etc. A combination of physical and chemical modifications can also be conducted. The physicochemical properties that can be modified include gelatinization, retrogradation, viscosity
20
and pasting properties, thermal stability, solubility, hydrophilicity, and compositions. The extent of modification depends on the crystallinity of the starch, amylose to amylopectin ratio, reaction conditions, and molecular distribution of starch74. Some of the common chemical modification of starch as applied to the material applications are reviewed in the following sections. 2.2.1 Etherification of starch Starch ethers namely, cationic starch, anionic starch, amphoteric starch, and non-ionic starch are produced when the hydroxyl group of starch molecules reacts with groups that add charges to the resulting starch69. Cationic modified starch is based on the substitution of the hydroxyl functional group of starch via etherification with positive charges. Chemical compounds such as tertiary or quaternary ammonium, imino, amino, butyl, sulphuric and phosphate groups (cationization agent) using organic or inorganic bases are commonly used for such cationic modifications75,98. The use of monomers like 2,3-epoxypropyltrimethylammonium chloride (ETA) or 3-chloro-2hydroxypropyltrimethyl ammonium chloride(CTA) in a dry or wet process has been getting significant attention in recent years69. In a typical dry process, a cationic agent is sprayed on the dry starch during an extrusion process. A scheme of the cationic modification of starch using ETA is shown in Figure 6. In a semi-dry process, a cationic agent is sprayed on the starch prior to thermal treatment, while in the wet process, the homogenous reaction is carried out in dimethyl sulphoxide or heterogeneous reaction in an alkaline suspension (e.g. under dissolution in aq. NaOH)74. In general, homogenous etherification is more efficient as compared to heterogeneous methods. This is because of the impeded diffusion of the cationizing reagent in the heterogeneous reaction as compared to the homogeneous process in which the starch granules are solubilized, which results in enhanced diffusion of the cationizing agent.
21
Low degree of substitution (DS) cationic starch (DS up to 0.2) are commercially used as additives in paper, textile or in the cosmetic industry. More recently, the application of cationic starch has expanded by obtaining high DS derivatives. Heinze et al76 prepared cationic starch of DS 1.5 using 2,3, epoxypropyltrimethyl-ammonium chloride in a two-step homogenous and heterogeneous reaction. The modified starch obtained was soluble in cold water and exhibited promising properties such as flocculation agents for wastewater treatment and antibacterial behavior. It can be expected that these materials can be used as EOR additives as well. Zhang et al99 synthesized a high DS (0.49) cationic hydrolyzed starch using 2,3, epoxypropyltrimethylammonium chloride. The derivative, when employed in salt-free dyeing of reactive dyes, showed high dye fixation without affecting rub and wash fastness. Compared to conventional dyeing, this showed higher efficiency and reduced pollution from salt. This modified starch can be applied as a substitute for salt in reactive dyeing.
22
Figure 6. Reaction scheme for the synthesis of 2-hydroxy-3-(trimethylammonium) propyl cationic starch (HTPS) under different media and conditions. Adapted from Heinze et al.76. Wiley copyright © 2014. Although processes with an aqueous solution of starch have been shown to produce cationic starch with a high degree of substitution, other side reactions may occur as a result of the presence of water, thereby, hindering the etherification reaction74. Etherification modification usually alters the morphology and granular structure of starch accompanied by a loss of
23
crystallinity. Furthermore, it improves water uptake, pasting, and thermal properties of modified starch. The extent of improvement depends on the structure of starch and reaction conditions such as cationization agent, temperature, catalyst efficiency, water content, and reaction time5. Overall, starch modified as such find applications in wastewater treatment, paper making, oilfield drilling, cosmetic products, and textile industry because they provide excellent properties that can be geared towards these applications and are relatively cheap and biodegradable99. Cationic starch derivatives with a high degree of substitution (between 1 and 1.5) are soluble in cold water. Such derivatives can be used as additives in Enhanced Oil Recovery (EOR) technologies78 . Anionic starch is prepared by modifying starch with anionic groups such as carboxymethyl and sulphonic,
thereby
improving
its
physicochemical
properties
like
solubility,
hydrophobicity/hydrophobicity, and flocculation behavior69,78 . Oxidation of starch to carboxylic acids, the formation of inorganic and organic polycarboxylic acids, and an etherification of starch with halocarboxylic acids, acylation of starch with cyclic anhydrides of dicarboxylic acids (e.g. succinic, maleic, phthalic) are other starch anionic modifications reported in the literature77. However, carboxymethyl starch (CMS) is perhaps the most studied anionic starch since CMS has a range of appealing properties, and the modification process is relatively simple. Commercial CMS production is carried out by the reaction of starch with monochloroacetic acid or its sodium salt after activation of the starch with aqueous NaOH in an aqueous organic solvent (e.g. ethanol). A scheme for carboxymethyl starch modification is presented in Figure 7. Due to the low cost, solubility, and water absorption properties of CMS, it has found applications in pharmaceuticals (e.g. disintegration aid for tablets, laxatives), food industry (emulsification
24
stabilizing agent, colloid and suspension stabilizing agent),
textile, paper making, water
treatment and laundry detergent, superabsorbent additive etc100.
Figure 7. Scheme for carboxymethyl starch modification. Adapted from Yanli et al101. Elservier copyright © 2009 Amphoteric starch is the product of the simultaneous synthesis of native starch with both cationic and anionic groups. Common anionic groups used are carboxyl, sulfate, phosphate, sulfonate, and phosphonate groups. Non-ionic starches are used in medical and surgical applications with the main non-ionic starch being alkyl ester. Common examples are hydroxypropyl starch and hydroxyethyl starch69. Etherification of starch can be useful in plastic applications as well. For instance, Wokadala et al102 studied and reported butyl-etherification (C4) of starch to induce enhanced compatibility of the etherified starch and polylactic acid (PLA) polymer. The results of the study revealed that the butyl-etherification brought about hydrophobicity of the starch, thereby improving the compatibility with PLA. 2.2.2 Cross-linked starches Cross-linked starch is formed when the intermolecular hydrogen bond is reinforced with chemical bonds that form bridges by interconnecting the polymer chains. This happens when modifying reagents esterify or etherify the hydroxyl functional group in starch. The result of this modification is a polymer with strong chemical bonds due to the formation of three-dimensional 25
network. Hence, cross-linked starch has high thermal and shear resistance with improved viscosity and consistency103. Crosslinked starch finds application in industries requiring high shear and thermal resistance. Zhou et al104 investigated citric acid crosslinking of starch for the production of nanocrystals. The results of this study showed that the crosslinking caused a reduction in the starch crystal size, hydrophobicity, and improved dispersibility in non-polar solvents. Starch nanocrystals produced as such can find application in cosmetics, emulsion stabilizers, rheology modifier, drug delivery vehicles, and reinforcing agents of polymers for nanocomposites development. EcoSynthetix Inc, a renewable chemical company based in Canada, developed a commercial starch nanoparticle called Eco-Sphere by crosslinking starch with glyoxal in a continuous reactive extrusion process. In an effort to understand the mechanism of starch nanoparticle formation using glyoxal crosslinking, Song et al
105
conducted a detailed investigation of the
reactive extrusion crosslinking process. The results of the study indicated that wet extrusion of starch produces starch particles of about 300 nm in size. However, with the addition of glyoxal crosslinkers (2 %), the starch nanoparticle size reduced to 160 nm. Such nanoparticles could have applications as a filler in polymers and other functional applications. Sodium trimetaphosphate (STMP), sodium tripolyphosphate (STPP), epichlorohydrin (ECH), dicarboxylic acids, anhydrides, dialdehydes and phosphoryl chloride (POCl3) are other common cross-linking reagents of starch75. In most cases, the crosslinking agent has multiple moieties, which can react with the hydroxyl group in starch, leading to cross-linking between hydroxyl functional groups on the same or on different molecules. The final product of cross-linking which can be classified into one of three types depends on the reagent used. The stages of starch crosslinking with POCl3 as shown in Figure 8 and are: a) Starch esterification with 26
orthophosphoric acid, potassium or STPP to produce monostarch phosphate, b) Reaction with POCl3 or STMP to produce distarch phosphate or, c) combined treatment of a and b to produce phosphate distarch phosphate. This study revealed that the crosslinked starch has enhanced stability, hardness and adhesiveness of the gels formed from it, and a reduced swelling power, solubility, pasting temperature, heat of gelatinization, and retrogradation 106.
Figure 8. Crosslinking of starch with phosphorus oxychloride. Adapted and modified from Miyazaki et al. Elservier copyright © 2006 68. 2.2.3 Esterification of starch Starch esterification is a vital process to alter the structure, and physicochemical properties of starch, and thereby making it suitable for industrial and functional material applications. Starch esters can be obtained when it is modified with a wide range of derivatives of organic acids (e.g. anhydrides, chlorides) or inorganic acids (e.g. phosphate, sufhate). Inorganic starch esters (e.g. starch phosphate) exhibits higher viscosity and transparency, making it suitable as adhesives, thickener, stabilizer, and drug bulking agent69. A high degree of substttution of the –OH functional group via esterification is usually desired to directly utilize starch in thermoplastic
27
polymer applications. Esterification of starch for material applications can be conducted to enhance its processability, promote hydrophobicity, reduce crystallinity, improve flexibility, etc.
Figure 9. Summary of starch esterification. Adapted from Haroon et al70. RSC copyright © 2016. Figure 9 presents a schematic summary of various starch esterification pathways. Haroon et al70 has also included a compilation of the various esterifying agents including citric acid, acid chloride, acetic acid, acid anhydride, together with the process
(Figure 9). Common
esterification of starch that are pertinent to material applications are reviewed in the next sections. 2.2.4. Fatty acid esterification of starch
28
Starch modification with fatty acids (FA) has received substantial attention in recent times because of the hydrophobicity of FA and the ability to incorporate desired properties when grafted on starch. Generally, hydrophobically modified starches are amphiphilic, which means their back-bone structure is hydrophilic with hydrophobic side groups107. Therefore, the solubility of fatty acid starch esters depends on the extent of substitution of –OH groups, type of FA and starch, and the modification process (e.g. solvent media used, reaction temperature, etc). FA modification has been shown to alter the crystallinity of starch as observed by x-ray diffraction; however, at low DS, the crystalline change is insignificant
107,108
. As the DS
increases, the level of disorganization of the granular structure of starch also increases proportionally as observed by Chi et al62 . Short-chain fatty acids (SCFA), medium-chain fatty acids (MCFA), and long-chain fatty acids (LCFA) are used for the esterification of starches. While longer fatty acid chain FAs induces improved hydrophobicity to the starch, steric hindrance associated with the chain length of the LCFA usually limits the level of modification61. Lopez-Rubio et al108 investigated the effect of FA esterification on the granular structure of high amylose corn starch (HAMS) and low amylose corn starch (LAMS) using acetic, propionic and butyric acid at low, medium and high DS. As shown in the SEM images of Figure 10, esterification of starch at low and medium DS had no effect on the size and shape of starch. On the contrary, at the highest DS, granular distortion and roughness occurred resulting in aggregation as seen in Figures 10f and 10h. A comparative structural evolution study between native starch and the corresponding starch-laurate esters were monitored by FTIR and NMR spectroscopy and results are shown in Figure 10 (I and J). These spectroscopy displayed an
29
almost complete substitution of the –OH functional group of starch with laurate, resulting in a complete disruption of the crystalline structure of starch with no re-crystallization behavior.
Figure 10. SEM images of HAMS: base starch (a), acetylated DS-0.32 (b), propionylated DS = 0.39 (c) and butyrylated DS = 0.4. Magnified images (x2000) of HAMS base (e), acetylated HAMS DS 0.32 (f), LAMS base (g) and acetylated LAMS DS = 0.29 (h). Adapted from Amparo Lopez-Rubio et al108. Elsevier copyright © 2009. (I) FTIR spectra of (a) starch, (b) Starch ester DS 0.5, (c) Starch ester DS 1.2, (d) Starch ester DS 2.1, and (e) Starch ester DS 2.9. (J) 1H-NMR spectra of (a) native starch in DMSO, (b) 0.5 DS ester in pyridine, (c) 1.2 DS ester in pyridine, (d) 2.1 DS ester in CDCl3, and (e) 2.9 DS ester in CDCl3. Adapted from Ojogbo et al.110 Wiley copyright © 2018.
30
Simi et al109 investigated the solubility of starch grafted with oleic and stearic acid in various organic solvents. The results of this study demonstrated that the FA esterified starches were soluble in warm DMSO and partially soluble in DMF and chloroform. Although the values of DS of the modified starches were not quantified, it was stated that the DS was low. The solubility of these modified starches were not investigated at high DS, which makes these results inconclusive. Rajan et al71 obtained similar results from the enzymatic modification of starch using palmitic acid by liquid state and microwave esterification. Results showed that the esterification increased the thermal degradation temperatures, reduced swelling power, and reduced enzyme digestibility when compared to native starch. The changes observed in this study are desirable for material applications. Octanoyl, lauroyl and palmitoyl chloride fatty acids as starch modifiers were investigated by Namazi et al107. These starches exhibited various levels of hydrophobicity depending on the degree of substitution. Another study56 highlighted that the successful esterification of starch with fatty acid-starch can be carried out using medium-chain fatty acids (containing between 610 carbon chains) in alkali media as others outside this range could be hydrolyzed. However, Namazi et al107have successfully esterified longer carbon chain FAs (C12) by optimizing the reaction conditions such as alkali concentration, reaction time and temperature. Winkler et al61 compared starch synthesized by fatty acid vinyl ester in DMSO and lauroyl chloride in pyridine and showed the importance of reaction parameters (Starch/FA ratio, temperature, catalyst type and mole ratio, reaction time and solvent choice) on the DS of the FA-starch ester. Ojogbo et al110 esterified starch to varying degrees of substitution and elucidated the effect of the extent of modification on the structure and properties of starch. These investigations agreed that the level of hydrophobicity of the FA modified starch proportionally increases with the increase in the
31
level of starch –OH substitution. Table 2 summarizes starch-fatty acid esterification modifications reported in the literature.
32
Table 2. Common fatty acid modifications and conditions Fatty Acid C2-C4 acid
Solvent Water108
Catalyst -
Temperature /Time -/30 mins108
DS 0.16 – 0.29108
-
Butyric acid
NaOH56
-
Room temp/1 h56
0.3 – 356
-
Octanoyl chloride
NaOH56
-
Room temp/1 h56
0.5 - 356
-
80 oC/ 30 mins86
>0.3 - 386 -
Water/DMAC86 Pyridine86 Pyridine84 Lauroyl Chloride
-
Water/DMAC86 Pyridine86
115 oC/3 h84 80 oC/ 30 mins86
1.884
-
>0.3 - 386 -
Pyridine61
-
110 oC/1 h61
0.5-2.861
-
Pyridine
Pyridine
110 oC/1 h
0.45-2.9
-
Results No change in granular size and shape at DS less than 0.2108. Deformation and increased roughness of some granule at DS higher than 0.2108 Water soluble at low DS and insoluble at DS > 1.556
High DS > 1.5 soluble in most organic solvent. Low DS insoluble in toluene and chloroform at all temperatures56 Soluble at low DS in most organic solvents with partial solubility in chloroform, dichloromethane and hot toluene86 Loss in crystallinity, Tm of 174 , decreased Tg, hydrophobic84 Soluble at low DS in most organic solvents with partial solubility in chloroform, dichloromethane, and hot toluene, Greater thermal stability than unmodified starch with the onset of decomposition of 300 compared to 200 of modified starch 86 Partially insoluble in organic solvent61
Soluble in most organic solvents at low DS and chloroform at high DS110 Decreased onset degradation temperature due to chain swelling and exposure of hydroxyl groups available to partake in degradation mechanism. Increase in inset degradation at DS >2110 33
Vinyl ester (C6-16) Vinyl ester (C18) Oleic acid
DMSO
Carbonate
110 /2 h
0.3 – 2.5
-
DMSO
Carbonate
110 /2 h
0.3 – 2.5
-
DMSO109
K2S2O8109
100 oC/8 h109
-
-
-
Palmitol chloride Stearic acid
Water/DMAC86
-
80 oC/30 mins86
0.5 - 386
-
Water/DMAC86
-
80 oC/30 mins86
0.5 - 386
-
DMSO109
K2S2O8109
100 oC/8 h109
-
-
Highly soluble in organic solvents, hydrophobic, can be cast into films61 Highly soluble in organic solvents, hydrophobic, too brittle61 Reduction in swelling power from 4.11 % in native starch to 2.03 % in modified starch Reduced degradation start temperature of 180 in modified starch compared to 275OC in native starch Reduced gelatinization temperature of 84 oC indicating better processability than native starch with temperature of 100 oC Destroyed granular structure characterized by amorphous particles109 Soluble at low DS in most organic solvents with partial solubility in chloroform, dichloromethane and hot toluene86 High DS, soluble in organic solvents with Poor solubility at low DS86 Reduction in swelling power from 4.11 % in native starch to 2.9 % in modified starch Reduced degradation start temperature compared to native starch109
34
2.2.5. Acetylated starches Acetylation is the esterification of starch by introducing acetate groups to replace the hydroxyl functional groups in starch. This is achieved by the use of acetylating reagents such as acetic anhydride, acetic acid, or vinyl acetate in the presence of an alkaline catalyst like sodium hydroxide, potassium hydroxide, sodium carbonate amongst others74 . The general mechanism of acetylation reaction with starch occurs by the elimination of the hydroxyl group from starch and the addition of acetate groups on the starch molecule as presented by Figure 11. The application of acetylated starch depends on the degree of substitution (DS) of the hydroxyl functional groups on the native starch. Acetylated starch with low DS (0.1-0.2) finds applications in films, bindings, adhesives, thickeners, and stabilizers62. On the other hand, high DS acetylated starches find more extensive material applications because of properties such as hydrophobicity, melt processability, solubility in certain solvents such as acetone and chloroform, and thermoplasticity62.
35
Figure 11. Acetylation of starch showing the addition-elimination reaction mechanism. Adapted from Oliveira et al111. Wiley copyright © 2018 Singh et al82 studied the effect of acetylation on corn and potato starches in sodium hydroxide. They showed that that acetylation decreased thermal transition temperatures, increased swelling power, improved light transmittance with negligible retrogradation properties of cooked starch paste. The authors reported that the observed properties were greater in potato starch compared to corn starch, which was attributed to the higher crystallinity in corn starch.. Hence, it was safely concluded that the change in functional properties of acetylated starch is dependent on the morphology of the native starch (associated with the botanical origin) as well as other factors including concentration of reactants, reaction time, and pH. 2.2.6. Other modifications Oxidation, enzyme hydrolysis, and acid hydrolysis are other common chemical modification of starch to alter its structure and properties. Acid hydrolysis forms soluble starch in water at temperatures below Tg using common acidic solutions like hydrochloric and sulphuric acid75 . The hydrolysis typically occurs in two stages to break and shorten the polymer chains. While the initial stage that is associated with the preferential breakdown of the amorphous region of the starch granule is fast, the final stage for the hydrolysis of the crystalline segment happens at a slower rate95 . The mechanism of acid hydrolysis of starch is depicted in Figure 12. The reaction begins by an electrophilic attack of the hydroxonium ion (H3O+) on the oxygen atom as shown in Figure 12A. This is followed by a transfer of electrons from one of the carbon-oxygen bonds to the oxygen atom as seen in Figure 12B, resulting in an uneven, high energy carbocation intermediate (Figure 12C), which is a Lewis acid that reacts with water to produce a Lewis base (Figure 12D), thereby reproducing the hydroxyl group (Figure 12E)112 . The extent of hydrolysis
36
in the first stage depends on the granular size, surface pores, amount of lipid content in amylose chains and amylose content. Whereas the second stage is influenced by branch distribution, crystallite degree of packing and amylopectin content. Hydrolyzed starches find use in food, textile, and pharmaceutical industries. The hydrolyzed starch also be modified further for functional material applications.
Figure 12. Acid hydrolysis of starch69. Adapted from Chen et al. RSC © copyright 2015 Oxidation is the introduction of an oxidizing reagent in the starch molecule under controlled temperature and pH to break down the polymer. Although hypochlorite causes environmental concerns like water and air pollution due to chlorine persistence113, it is the most commonly used starch oxidizing reagent because it oxidizes the molecule without any residue as by product80. Other common oxidizing agents are potassium permanganate, ozone, hydrogen peroxide and persulphate79. When starch is oxidized, the hydroxyl groups on the starch molecules varies depending on the amount of carbonyl and carboxyl groups present69. The Figure 13 shows the oxidation of starch with sodium hypochlorite to produce carbonyl and carboxyl groups. These modified starches are less viscous, more stable, form better films and have lower molecular size than their native counterpart67.
37
Figure 13. Scheme for Sodium hypochlorite oxidized starch67 2.2.7. Starch surface modification Modifications that alter only the surface properties of starch while maintaining the crystal structure of the molecule are classified as surface modifications. The –OH substitution or DS in most surface modification of starch processes is usually low. Surface grafting, cationic and anionic modifications, plasma treatment and other chemical modifications that only involve alteration of the surface –OH groups can be categorized under surface modification. Some benefits obtained from surface modification include decreased or increased hydrophilicity, improved paper printability, decreased surface roughness, reduced particle agglomeration, improved adhesion amongst others114–123. Table 3 summarizes common techniques employed for surface modification of starch. Plasma modification of starch is among the most extensively studied processes of surface modification of starch. It is typically conducted by exposing starch to partially ionized gas. The major advantage of this method is that it modifies only the surface of starch. Thirumdas et al124 modified starch using cold plasma treatment. The modified material exhibited a decrease in
38
molecular weight, viscosity, and gelatinization temperatures. Additionally, this process increased surface energy and hydrophilicity. The reaction mechanism which depends on feed gas used and treatment time is shown in Figure 14. Table 3. Surface modification techniques Technique Grafting
Agent Succinic anhydride Phenyl isocyanate Plasma Helium Hexamethyldisiloxane Plasma Argon Cold plasma Oxygen, nitrogen, ammonia
Effect Altered morphology, retained crystallinity, increased hydrophobicity No change in surface hydrophobicity Increased paste clarity Increased wettability and surface energy. Decreased molecular weight, viscosity and gelatinization temperatures
Ref. 117,125
114
126 124
39
Figure 14. Mechanism of cold plasma (a) cross-linking of chains (b) depolymerization of branched chains (c) etching of granule. Adapted from Thirumdas et al124. Springer copyright © 2009. 3. Graft modification of starch Graft modification can be employed to modify the surface, structural and functional property of starches. Grafting is a modification process that introduces other monomers or polymers on the starch backbone chain linked by covalent bonds, resulting in modified co-polymer with combined functional properties of the starch and polymers used. Furthermore, the resulting properties of the copolymer may be influenced by the nature and degree of substitution of the side chain. Starch grafting is carried out by two major routes as displayed in Figure 15. The first
40
route, “grafting from”, reduces stearic hindrance by direct polymerization of the monomer from the main chain and, the second route, “grafting to”, directly grafts a functionalized polymer with functional groups reactive with starch on the starch backbone127,128.
Figure 15. Routes in graft co-polymerization127. Graft co-polymerisation is conducted by the free radical initiation of granular or pre-gelatinized starch with the side chains in the presence of an initiator. Techniques of free radical initiation are chemical initiation using peroxides, redox systems, and transition metals with high valence, and initiation by irradiation129. Since the side chains grafted on the starch backbone are bonded covalently, one advantage of this process is that the grafted species does not leach out. On the other hand, low grafting efficiency and high homopolymer percentage are disadvantages especially of grafting by chemical technique128.
41
3.2.1 Grafting co-polymerization by free radical initiation and atom transfer radical polymerization Grafting polymerization by free radical initiation is carried out in three steps: initiation, propagation, and termination. During the initiation step, hydrogen is transferred to the initiator, producing radicals on starch, the starch radical produced is highly reactive, and hence, it bonds easily to the monomer, resulting in starch macro radical. The starch macro radicals as such can react with other monomers nearby, which become free radical donors, and again contribute free radicals, thereby propagating grafting. Propagation is terminated by a combination of disproponation of two reactive chains127. Since grafting efficiency of 100 % cannot be obtained in many cases, the unreacted homopolymer (starch and monomer/polymer) can be removed by extraction (e.g., soxhlet extraction). The general synthesis of grafting a vinyl monomer using ceric ammonium nitrate on starch is represented in Figure 16.
42
Figure 16. The mechanism for grafting vinyl monomer on starch using Ce4+ as initiator. Adapted from Haroon et al70. RSC copyright © 2016.
43
Generally, the concentration of initiator, concentration of monomer, temperature, and time affect the grafting yield. Apopei et al130 grafted potato starch with acrylonitrile using two initiators cerium sulfhate Ce(SO4)2 and ceric ammonium nitrate (CAN) and found the grafting percentage using Ce(SO4)2 to be three times higher than the CAN counterpart. Lutfor et al131 grafted sago starch with acrylonitrile (AN) using ceric ion as the initiator in the presence of sulphuric acid and reported that the grafting percentage, efficiency, yield, and rate were dependent on the concentration of CAN, AN, starch, sulphuric acid, reaction temperature and time. The reaction scheme is shown in Figure 17. The optimum value of their grafting reactions is summarized in Table 4. Avval et al87 prepared superabsorbent starches by grafting polyacrylamide (PAA) and polyhydroxyethylacrylate (PHEA) by using radical polymerization and atom transfer radical polymerization (ATRP) for drug use according to the reaction scheme shown in Figure 17. Copolymers by the ATRP method had longer drug release than that of free radical polymerization. This is attributed to the benefits of ATRP, which usually creates uniform polymers chains that result in a regular release as opposed to the radical polymerization that results in erratic release. Zou et al132 graft copolymerized and characterized pea starch and acrylamide for waste-water treatment using ceric ammonium nitrate initiator and obtained a maximum graft yield of 38.3 %. The resulting copolymer from their study showed enhanced thermal stability as indicated by an increase in the degradation temperature. Qu et al133 grafted poly(vinyl acetate) onto starch using KMnO4-H2SO4 redox system and obtained a 38.3 % graft yield at an optimum concentration of redox system at 40
with 3 hours of reaction time. The
concentration of initiator, concentration of monomer, temperature and time were varied and arrived at optimum values of these parameters, grafting was affected negatively beyond the
44
optimum. The resulting copolymer showed increased swelling power and reduced solubility at different temperatures and different grafting yields133. Initiation:
Propagation:
Termination:
Figure 17. Mechanism for grafting AN on starch using CAN initiator131. In other studies, novel starch-g-poly(benzyl methacrylate) copolymer was prepared by grafting benzyl methacrylate monomer onto starch backbone using potassium persulphate as an initiator. The resultant copolymer was hydrophobic, resistant to basic and acidic conditions, that exhibited thermal stability and higher swelling in non-polar solvents 134.
45
Table 4. Grafting initiators and optimum conditions for the graft modification of starch Starch source Corn
Initiator /Catalyst Potassium persulphate (KPS)/ tetramethylene diamine (TMEDA)
Monomer /Polymer Methacrylic acid (MAA
Pea
Ceric ammonium nitrate (CAN)
Acryl amide (AM)
Potato Sago
CAN/(CeSO4)2 CAN
Acrylonitrile Acrylonitrile
Optimum conditions -
Potato
Potassium persulphate
Potato
Potassium persulphate
Citronellyl Methacrylate Benzy methacrylate
-
10 mmol/L KPS 8 mmol/L TMEDA 50% monomer concentration 10 liquor ratio, 60mins reaction time 60 oC temperature CAN: 0.02 mol/L Temp: 65 oC Starch-AM ratio: 0.5 CAN: 9.61e-3 AN: 0.653 AGU: 0.152 H2SO4: 0.187 mol L-1 Temp: 50 Time: 90 mins Temp: 80 Time: 2 h Temp: 80 Time: 2 h
Graft %
Ref.
38.3
128
38.3
132
218.38 54.46
130
-
88
53
88
131
46
3.2.2 Graft copolymerization mechanism by irradiation Irradiation grafting technique is extensively used for modifying polymers because of its low cost, relative ease of the process, and absence of waste. Literature has reported grafting of starch using ultraviolet (UV)135, microwave power136–138, electron beam139,140, and gamma-ray140,141. For instance, Sheikh et al
141
synthesized starch grafted with polystyrene using gamma rays and
investigated how varying the starch-styrene ratio and amount of irradiation doses affected the grafting percentage. They arrived at a graft percentage of 252.9 % at optimum conditions of starch to styrene weight ratio (one to three ratio) and applied gamma-ray dose of 10 kGy. The radiation grafting mechanism is shown in the scheme represented in Figure 18.
Figure 18. Radiation grafting mechanism of styrene on starch using gamma rays. Adapted from Sheikh et al141. Elsevier copyright © 2013.
47
Jian-Kun et al138 grafted acrylic acid (AA) on pre-gelatinized corn starch using microwave irradiation. They studied and reported the impact of initiator content and radiation time on the grafting ratios. Results showed a grafting ratio of 19.57 % at an optimal grafting time and initiator content of 3.5 mins and 4.55 %, respectively. The resulting copolymer was less brittle compared to native starch. Salimi et al136 investigated the effects of power radiation (W), monomer ratio, and NaOH catalyst concentration on the grafting efficiency of a “green” approach for graft polymerization of L-lactic acid onto the starch backbone. They obtained a maximum grafting at 450 W microwave power, 1:5 monomer ratio, and 0.4 M NaOH. 3.3. Reactive extrusion process for the modification of starch Reactive extrusion enables the modification of starch in a continuous process. It combines heat and mass transport operations in a single process while simultaneously carrying out chemical reactions. In most studies, starch has been modified using a batch reactor (BR) or a continuous stirred tank reactor (CSTR). However, wet processes are generally more expensive as compared to reactive extrusion modification process as it involves heating of extra reaction media (e.g. water or solvent). Downstream processes for the recovery of the product and handling of generated waste also contribute to the cost of using reactors for starch modification. Thus, the reactive extrusion process has received significant attention for a range of polymer modifications, including renewable polymers such as starch142. In a typical modification process, starch is first gelatinized to form a viscous paste, which is often difficult to manage using conventional reactors. To mitigate such mixing hurdles associated with viscosity, modifications are conducted using salts to limit gelatinization and, by extension, to eliminate problems associated with viscosity and mixing. However, this requires
48
the product to be purified after modification, thereby, incurring additional costs and time. Another way to manage viscosity and mixing problems is to substitute the reactor. Table 5 presents a comparison between conventional reactors and extruders for starch modification. Table 5. Comparison between conventional reactors and extruders. Adapted from Graeme Moad 142
-
Conventional Reactor Can handle 30-50 % weight starch Long residence time (2-24 h) Relatively low operating temperature Challenges in mixing in gelatinized starch
-
Extruder Can handle 60-80 % weight starch Accelerated residence time (2-5 mins) High operating temperature (between 70-140 °C) Efficient mixing
Modification of starch using extruders has been studied extensively in the literature142–149. Some of the reported processes include grafting of monomers and polymers onto starch, grafting of polymers from starch, crosslinking starch with epichlorohydrin, and degradation of starch142. Details of these processes are not reported in this review; however, they can be found in literature143–145. Table 6 presents some of the processes, parameters studied, and modification effects reported in the literature.
49
Table 6. Chemical modification of starch using extruder Extruder Type Twin-screw Twin-screw
Twin-screw
Reaction type/ Reagent Esterification/Maleic anhydride Crosslinking/sodium trimetaphosphate, sodium tripolyphosphate Oxidation/Polystyrene
Micro extruder Twin-screw
Grafting/ Polycarbonate
Twin extruder Twin extruder
Hydroxypropylation
Cationic /Polyurethane
Grafting/ Polyacrylamide
Reaction parameters 70-155 °C, 125 rpm screw speed
Effect
Ref. 143
Increased grafting efficiency
144
40-50 wt.% starch, 150 rpm screw Retained granular structure speed, 1 min. reaction time, no mixing elements or conveying elements Increased molecular weight, thermal 165-195 °C resistance, hydrophobicity Improved interfacial adhesion 230 °C, 2 mins, 3-6 wt.% glycerol, 200 rpm Improved mechanical properties, 5-20 wt% starch, 90 °C hydrophobicity, and thermal properties No impact on the extrusion process 250-350 rpm, 150 °C 90 °C, 100 rpm, 3 mins
145
146
147
148
149
Controlled grafting
50
4. Industrial biopolymers based on modified starch Native starch needs to be modified to improve its physicochemical and functional properties, thereby enabling its use for a wide variety of biopolymer and other functional material applications. As discussed earlier in this review, starches can be modified chemically by introducing other functional groups to the highly reactive functional groups of starch. Some of the modifications result in an altered material compared to native starch (e.g. hydrophobic, amorphous, and melt-processible) that can be used for industrial bioplastic applications61,150–152. In other cases, the modified starch exhibits good surface properties to be used as a co-blend component of other renewable polymers. Modified starch-based biopolymers find industrial use in the food industry for food packaging, in agricultural industry as mulch films and controlled release materials for fertilizers, as a carrier for biocides153–155 and other active agents, as well as in the biomedical industry for drug delivery28,156. 4.1. Modified starch films and sheets Starch films and sheets commonly used for packaging and agricultural mulch are a substitute for those made from synthetic polymers, thereby, managing the pollution problem resulting from the use of non-degradable polymers. Furthermore, the wide variety of starches along with their reactive nature allow physical and chemical derivatization, enable the production of films and sheets tailored to specific end-use. The common drawback, with the use of unmodified starch for plastic films and sheets, is their poor mechanical properties (brittle and inflexible). To overcome this, external plasticizers are incorporated during the thermoplastic process. Plasticizers are generally low molecular weight polymer additives used to enhance processability, and flexibility of polymers by reducing the glass transition temperature and other second-order transition
51
temperatures63,157. However, most thermoplastic starch produced by using external plasticizers (e.g. glycerol) suffers from poor moisture tolerance behavior, too rapid degradation, and aging because of leaching out of the plasticizers. An alternative to the use of plasticizers chemically modified starch can be utilized to make films and sheets. Several researchers have reported the use of modified starch to obtain films and sheets. Zhou et al90 studied the surface crosslinking of corn starch sheets through ultra-violet irradiation using sodium benzoate as a photosensitizer. The results of this study from contact angle measurements and moisture absorption evaluation revealed the improvement in water resistance. The film-forming capacity of acetylated starch was studied by Lopez et al
63
with various
concentrations of glycerol plasticizer to improve their mechanical properties. They showed that a 5 % w/w concentration of acetylated starch and 1.5 w/w glycerol is required to reduce the water vapor permeability and flexibility by 87 % and 34 %, respectively. Fang et al158 modified native potato and high amylose (HA) potato starches using lauroyl chloride at various levels. The results of the study displayed that all modifications improved the hydrophobicity of starch irrespective of the DS. The modified starch with DS of 1.5-3 can be extruded and processed like traditional synthetic polymers to produce (a) strong films with limited extensibility from the native potato starch, or (b) a low strength with good elongation from the high amylose modified starch; (c) a 50:50 high amylose and native potato starch mixture with balanced properties. Lafargue et al159 prepared and characterized films from hydropropylated pea starch/Kcarrageenan mixture to study the mechanical and calorimetric properties of the starch films. They reported a dramatic increase in the rheology of the mixture, however, the modification had no effect on film properties. Films that were based on silicon dioxide and poly(vinyl acetate) (PVA) modified starch were studied by Xiong et al160. Characteristics of the films produced as such
52
exhibited an increase in the tensile strength, breaking elongation and transmittance by 79.4 %, 18 %, and 15 %, respectively accompanied by a decrease in crystallinity from 41.2 to 32.9 % and water absorption by 70 %. This was due to the formation of intermolecular hydrogen bonds the starch-PVA composite, and strong chemical bonds formed in the nano SiO2/starch/PVA blend, thereby forming a network structure, which increased the water resistance and mechanical properties. Table 7 presents a summary of the characteristics of starch films reported in recent researches with important highlights.
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Table 7: Modified starch-based films and their characteristics Starch Source Corn
Modification Acetylated
Potato (native and HA)
Acylation with lauroyl chloride
Pea
Acid hydrolyzed hydroxypropylated / k-carrageenan Citric acid modified/ carboxymethyl cellulose (CMC)
Corn
Corn
Wheat
Nano silicon dioxide and PVA
Lipid (raspseed oil)
Plasticizer Glycerol -
-
Glycerol
-
-
-
Glycerol
-
Results 87 % reduction in water vapor permeability 34 % increase in flexibility Improved processability of starch Improved water resistance Strong film, poor extensibility with modified native starch Low strength, good elongation films with HA modified starch Stronger films, ease of processing and improved tensile strength with a 50:50 combination of native and HA starches Increased rheology of mixture No effect on film properties Increase in ultimate tensile strength by 59 % without any decrease in strain. Reduced water vapor permeability with increased CMC content. Improved water resistance of starch films. Improved optical properties with increased CMC content Improved mechanical, transmittance and resistance properties by 79.4 %, 18 % and 15 %, respectively. Reduced water vapor by 70 %. Decrease in crystallinity Increased resistance to water vapor and oxygen permeability. Improved surface hydrophobicity. No much change in barrier efficiency. Decreased film transparency
Ref. 63
158
159
161
160
162
54
4.2. Modified starch-polymer blend systems Synthetic polymers, although non-biodegradable, possess desirable thermoplastic properties compared to starch. Blending biodegradable and non-biodegradable polymers and making plastics from them can lead to partially degradable plastics. Starch is also blended with other biopolymers of desired properties to improve the properties of the overall polymer blend and biodegradability. One drawback, however, is the inhomogeneity and incompatibility between hydrophilic starch granules and hydrophobic polymers in blends, requiring the reduction of the interfacial tension to improve the properties of blends. This is typically achieved by modifying native starch before blending or the addition of compatibilizers which improve interfacial adhesion between polymers. Modifying the starch prior to blending creates a physically rough surface on the starch granule, allowing for better adhesion and anchoring of the blend163. More importantly, targeted modifications can reduce the surface energy of starch making it more compatible with non-polar polymers. The mechanical, thermal and morphological properties of blends of low-density polyethylene (LDPE)/starch and LDPE starch phthalate (Stath) was studied by Thakore et al163. The Surface morphology of LDPE/stath blend showed a well-distributed and firmly rooted stath particles in the blend while the LDPE/starch blend showed loose starch granules. This has resulted in better tensile properties of the LDPE/Stath blends as compared to the LDPE/starch blend due to the improvement of compatibility. Researchers have been interested in starch blended with polylactic acid (PLA) because PLA is biodegradable and sourced from plants, hence it is renewable. One group investigated a blend of PLA with glycerol plasticized starch (TPS)81. Morphology study of PLA/TPS blend exhibited
55
rough particle dispersion of component blends with sizes ranging from 5 to 30 µm. The study further carried out interfacial modification by grafting maleic anhydride (MA) on PLA and made a blend with TPS. Such modification caused fine particle size and homogenous blend, which resulted in improved ductility of the modified blend. The observed elongation was between 100200 % for the modified blend, as compared to only 5-20 % elongation for the non-modified blend systems81. Xiong et al164 carried out a similar blending study of PLA with starch with or without tung oil anhydride plasticizer. The PLA-starch blends showed a void between the PLA matrix and starch granules indicating poor compatibility. Contrarily, the plasticized PLA starch blend showed fewer voids indicative of improved compatibility. While the addition of starch to PLA made it more brittle exhibiting a poor elongation (6 %) and low impact strength, the addition of 5 wt % tung oil anhydride plasticizer to the blend significantly enhanced the elasticity (17 %) and impact strength as compared to the baseline PLA. The use of propionic anhydride-modified starch as a co-blend component of polyurethane was studied by Santayanon et al165, and a comparison of properties of the modified starch blend with native starch blended polyurethanes was reported. An improved interfacial adhesion was noted by using the modified blend from an SEM micrograph. The modified starch samples were remarkably well dispersed and embedded in the polyurethane matrix as opposed to the native starch the displayed poor interfacial adhesion in the polyurethane matrices. A three-month biodegradability study carried out in soil in these blend systems revealed increased weight loss with an increase in the native starch content of the blend. The modified starch-based blend, on the other hand, showed little or no weight loss (3 %) for the duration of the study. This indicates that the moisture resistance and compatibility of the blend were improved at the expense of the rate of biodegradation.
56
Berruezo et al166 characterized polystyrene (PS)/TPS blends for packaging applications and reported a 48 and 62 % decrease in the young modulus and tensile strength accompanied by a 62 % increase in elongation at break for a 50:50 TPS/PS blend compared to neat PS.
The
biodegradability test revealed an increase in the weight loss of the blend with increasing starch concentration. Table 8 summarizes starch - polymer blends systems reported in recent literature.
57
Table 8. Characteristics of starch polymer blends for industrial polymer applications. Modification Type Esterified starch
Polymer LDPE
TPS
Maleic anhydride grafted PLA
-
Propionic anhydride-modified starch Tung anhydridemodified starch TPS
Polyurethane
-
PLA PS
TPS
Thermoplastic chitosan (TPC)
TPS
Poly Propylene (PP)
TPS
PVA/Chitosan
Starch
Ethylene and vinyl alcohol copolymer, Cellulose acetate, Poly- ε-caprolactone Anhydride functionalized i) Poly- ε-caprolactone
TPS
-
-
Results Improved mechanical, thermal and morphological properties. No significant difference in melting temperature. Greater biodegradation of modified starch blends compared with normal starch blends Improved interfacial adhesion. Elongation at break of 100 to 200 % range compared to 5-20 % for starch and pure PLA Improved interfacial adhesion. Slower biodegradation rate than starch. No significant effect in tensile toughness Improved tensile and impact properties. A downward shift in thermal properties Decreased young modulus and tensile strength, increased elongation at break Increased biodegradation rate Decrease in tensile strength and stiffness Good thermal stability Improved extensibility Temperature-sensitive flow behavior compared to PP. Decreased stress and strain at break with increased TPS Small patches of individual components on microscopic scale. No new functionality created. Increased hydrophobicity Improved water resistance by 15 %. Reduced degradation rate. Improved mechanical properties. Comparable tensile strength of blend with polyesters at up to 70 % starch content.
Ref. 163
167
165
164
168
169
170
118
171
172
58
Maleated TPS (MTPS)
ii)Polybutylene succinate iii) butanediol-adipateterephthalate copolymer Plasticized PLA
-
50 % reduction in elongation at 10% starch content in the blend.
-
Increased tensile strength and modulus with increased starch content, blending temperature and time. Decreased toughness and elongation values. Superior mechanical properties of MTPS/PLA blends compared to TPS/PLA blends
173
Increase in young modulus Decreased yield stress, tensile strength, and elongation at maximum strength. Mechanical properties of blends not suitable for disposable packaging Voids and poor interfacial tension in uncompatibilized blends, hence poor mechanical properties. PP/TPS/C14 showed the highest impact energy
174
-
TPS
Isotactic polypropylene (iPP)
-
TPS
PP, C14-C18 Compatibilizers
-
175
59
4.3. Biodegradation of modified starch-based polymers Biodegradation is the disintegration of polymer chains by the activities of microorganisms to yield carbon dioxide and water in aerobic conditions, and methane and carbon dioxide in anaerobic conditions. Native starch by itself is completely biodegradable. Completely biodegradable composites of starch can be obtained from blending starch with other biopolymers such as cellulose or blending with other degradable polymers like aliphatic polyesters and PVA28.
Although recent environmental concerns have led to an increased interest in
biodegradable resources for commercial products, the stability of the product is important as well; hence, the rate of biodegradation should be controlled176. The general mechanism of starch degradation by enzymatic hydrolysis is the diffusion of enzymes and binding of the same to the starch substrate, followed by bond cleavage42. Therefore, biodegradation is dependent on the chemical structure of the polymer, moisture content, surface area, pH, porosity, crystallinity, presence and type of microorganisms in the environment, oxygen content, wettability, etc., of the material177. Therefore, altering the chemical structure of starch by modification or blending it with other materials is expected to change its biodegradability. Research has reported the effect of modification on starch scission. Olivera et al178 blended PS with starch and evaluated the biodegradability in soil. Their results revealed that the rate of biodegradation of the blend was a function of the starch content on the blend and incubation time.. This is expected as the starch is the only degradable component in the blend system. Thakore et al163 esterified starch with phthalate and blended with LDPE and compared the biodegradability with native starch/LDPE blend. From soil burial tests experiments results, they reported reduced biodegradation rate with esterification of native starch, revealed by a slower degradation of modified starch/LDPE blend compared to the unmodified starch/LDPE 60
blend. The effect of propionic anhydride as an esterifying agent on biodegradation of starch was studied by Santayanon
165
. The results from soil burial test showed a slower degradation of
esterified starch than unmodified starch, which was attributed to the reduced hydrophilicity in modified starch leading to slower hydrolysis of the chemical bonds. From all reviewed literature, it is reported that chemical modification of starch deters the rate of degradation, which can be correlated to the degree of substitution of the functional groups on the starch molecule. Rivard et al studied the anaerobic degradation of acetylated starches with DS ranging between 0.3 to 2.4 for 98 days incubation period. The authors reported a drastic reduction in degradation levels for samples with DS levels between 1.2 and 1.7. Above DS of 1.7, the acetylated starches were not biodegradable179. The resistance of acetylated starches to degradation could result from poor wetting, and therefore limited microbe/ polymer surface contact resulting from the increased hydrophobicity of acetylated starches, or steric hindrance from the attached functional groups which hinders biocidal activity179. 5. Outlooks, prospects, and conclusions The environmental concerns from landfilling, leaching of chemicals into the soil and water table caused by non-degradable polymers used in commercial and industrial applications have intensified the interest for the development of biodegradable polymers. Starch is proposed as a front runner potential candidate due to its many attractive properties and benefits it has to offer. The drawbacks of high degradation rate, water sensitivity, and poor mechanical properties can be minimized by modification. Physical modification combines temperature, moisture, shear and irradiation to improve its water solubility and reduce granular size. Chemical modification introduces functional groups to the reactive hydroxyl group in starch without altering the
61
morphology. Blending starch with other polymers has been shown to synergize the properties of these polymers to obtain desirable materials suited to specific end-use. Through the tuning of starch, modification degrees and levels, the physicochemical properties of the final product can likewise be varied. For instance, single-use food packages and plastic bags can be designed to have a high degradation rate while other materials with expected longer expected life cycles can be designed for a low degradation rate. With a focus on sustainability, renewability and bioresources utilization, starch is deemed as one of the leading and most desirable materials for the design and development of polymeric materials in functional and commodity applications. Likewise, the modification of starch has found applications beyond the bioplastics industry, as the applications have extended to use in the medical industries as sutures, pharmaceutical industries as a drug carrier, scaffolds for tissue engineering and even the metallurgical industries for porous media manufacturing.
62
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Conflicts of interest statement
There is no conflicts of interest to disclose.
S2589-2347(19)30056-9
DOI:
https://doi.org/10.1016/j.mtsust.2019.100028
Reference:
MTSUST 100028
To appear in:
Materials Today Sustainability
Received Date: 7 May 2019 Revised Date:
25 October 2019
Accepted Date: 7 November 2019
Please cite this article as: E. Ojogbo, E.O. Ogunsona, T.H. Mekonnen, Chemical and physical modifications of starch for renewable polymeric materials, Materials Today Sustainability, https:// doi.org/10.1016/j.mtsust.2019.100028. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
Chemical and physical modifications of starch for renewable polymeric materials Ewomazino Ojogbo, Emmanuel O. Ogunsona, Tizazu H. Mekonnen* Department of Chemical Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, Canada
* To who correspondence should be addressed: [email protected] Abstract Biodegradable polymers obtained from renewable natural resources have received increasing attention due to their potential as alternatives to traditional petroleum-based plastics. Among the various sources, polysaccharides stand out as a highly convenient feedstock because they are readily available, renewable, inexpensive and provide great stereochemical diversity. Starch, a renewable polysaccharide polymer, has attracted a substantial research and commercial interest as a feedstock due to its renewability, biodegradability, low cost, and abundance of –OH chemistry leaving it open to endless modification possibilities and melt processability in the presence of plasticizers. However, its hydrophilicity, thermal and mechanical properties limitations, rapid degradability, and strong intra and intermolecular hydrogen bonding of the polymer chains hinder its melt processability and limit its widespread commercial application as a renewable biopolymer. Therefore, modification is necessary to mitigate these limitations and bring about other desirable properties. This article critically reviews the recent progress achieved in the modification of starch for industrial biopolymer material applications where starch is used as one or the major constituents. Keywords: Starch bioplastics, Biopolymers, Thermoplastic starch, Starch modifications, starch processing, Starch biodegradation
1
1. Introduction Polymers offer an unparalleled combination of low cost and toxicity, processability ease, excellent thermal stability and balance of physical properties. Consequently, they are used in a variety of advanced and commodity applications. The simplistic design and low cost of these products bring about a short service life; typically, in single-use applications after which they become waste. As a result, a significant amount of municipal and industrial wastes consists of commodity
polymers
such
as
polyethylene,
polyethylene
terephthalate,
polystyrene,
polypropylene, etc., which are utilized on a massive scale in packaging and containment applications1. The short service life of these inexpensive polymers is in sharp contrast with their remarkable resistance to biodegradation. Moreover, concerns over depleting fossil resources coupled with unstable supply, energy requirements for processing, price, and limitation in geographical distribution are further concerns for typical fossil-derived polymers. As a result, there is significant research interest in the utilization of natural feedstock (e.g. starch, cellulose, hemicellulose, lignin, protein, lipids, etc.) for polymeric applications as these are susceptible to microbial attack for biodegradation and safe disposal. In general, renewable polymers provide environmental sustainability, unique performance, and economic benefits2–4. However, due to limitations in the intrinsic properties of these renewable polymers, they cannot be used directly in combination with polymeric materials for targeted applications. For instance, due to the strong inter- and intramolecular hydrogen bonding in starch, protein and cellulose molecules, they exhibit melting temperatures beyond their thermal degradation temperatures. Consequently, such polymers cannot be directly melt-processed. The high moisture sensitivity of these biopolymers is another challenge that needs to be mitigated before they can be applied in useful applications. This manuscript focuses on reviewing the 2
recent progress achieved in starch modification pertaining to industrial thermoplastics applications. 1.2. Bio-sourced polymers for sustainable material development Biopolymers can be extracted from various sources; for instance, cellulose, hemicellulose, lignin, and polyphenols can be sourced from plants. Animal-sourced biopolymers include chitin from crustaceans, collagen and keratin proteins from animals, proteins and cellulose whiskers from mollusks, and other bacterial polymers5. The most extensively utilized biopolymers for non-food industrial applications are starch and cellulose because of the cost and availability. Contrarily, research and development efforts into the utilization of bio-sourced monomers as building blocks of polymers, or as a carbon sources to produce sustainable polymers via a fermentation process has resulted in important scientific and commercial success. For instance, lactic acid produced via the bacterial fermentation of starch is a monomer that can be polymerized into poly(lactic acid)2. Similarly, succinic acid produced via fermentation is used as a monomer or co-polymer of a range of polyesters and polyamides6. The polymerization or co-polymerization of lipid-derived fatty acids for a range of polymer applications constitutes an example of the utilization of bio-resourced polymer7,8. Among the naturally occurring biopolymers, cellulose is the most abundant structural polysaccharide polymer9. It is a straight-chain polymer that is comprised of glucose monomers linked by β-1,4 glycosidic bonds. The major applications of cellulose are in the production of paper and fibers for textiles. Cellulose also finds use in medicine, production of films10 and fillers for consumables. The hydroxyl functional group in cellulose can be partially or wholly esterified or nitrated to produce derivatives with desirable properties for other applications11,12.
3
More recently, the extraction of the crystalline fraction of cellulose via acid hydrolysis for material applications has received extensive attention. Hence, developing more efficient ways of producing them at a lower cost. Hemicellulose is a branched polysaccharide polymer found in plant cell walls. It includes xyloglucans, xylans, mannans, glucomannans, and glucans13. Hemicellulose biopolymer is used as hydrogels, cosmetics, and drug carriers14,15. However, its extensive use is still limited due to its hydrophilicity and rather poor mechanical performance16. Proteins are also an attractive feedstock for industrial and functional material applications. This is because of their capability to form a cohesive and continuous matrix and an impressive range of functionality spanning from catalytic activity to selective binding and mechanical strength, thereby making them attractive biomaterials17,18,19. Moreover, the rich functional groups (e.g. –NH2, -OH, -COOH, -SH) of proteins associated with the amino acid residues, provides a lot of opportunities for modification and successively makes them appealing biomaterials20,21. Starch, a polysaccharide that can be synthesized by plants and found mainly in fruits, roots tubers, legumes, and cereals is typically found in the range of 25-90 %13. It is a semi-crystalline polymer having about 1,000-2,000,00022 glucose monomers linked by α-1,4 glycosidic bonds. There are three reactive hydroxyl groups on the glucose unit of the starch chain, with a primary and two secondary hydroxyl groups, which can act as anchor points for modification chemistries. With several reviews already published on starch with focuses on food applications23,24, advanced function applications25, and surface modifications26,27, this literature review focuses on capturing the progress on the physicochemical properties and chemical modification of starch for use as feedstock in petro- and bio-plastics material applications. Prior to date, to the best of our knowledge, only one review written ten years ago has been published on completely 4
biodegradable starch-based polymer materials28, relating to the application of starch in polymeric materials. Researchers involved in renewable and sustainable polymeric materials with starch as a focus can significantly benefit from this review as an update to advances and knowledge in this area, and act as a stepping stone for investigations into niche areas of research which need exploitations. 1.3. Starch as a renewable and sustainable feedstock material Unlike other polysaccharide polymers that are harvested or extracted from through the destruction of the plants, starch can be harvested in most cases without destroying the plant. Plants produce starch granules through the polymerization of glucose via photosynthesis of carbon dioxide. Starch is largely consumed as food or used in food preparation across the world, and finds use in diverse industrial applications29. Besides being consumed as food, other major uses are as components in paper binders and adhesives, feedstock for fermentation, textiles, chemical production, and other industrial products30. The low cost, abundance and wide geographical distribution and ease of growth have sparked research interest in starch as a biopolymer. Some of its chemical, physical and functional properties attributes such as gelatinization, water retention properties, ease of functional property optimization, ease of dissolution in solvents, and temperature-induced pasting make it intriguing and aresearch focus31. Furthermore, starch, like other natural fibers and biomass can be processed via traditional polymer processing techniques such as extrusion and injection molding in the presence of plasticizers32–39. The retrogradation, low thermal stability, hydrophilicity, and brittleness of starch has restricted its widespread use in and for industrial polymer applications that require tailored properties like mechanical integrity. Hence, modification of the hydroxyl
5
functional groups on its surface is required to alleviate the aforesaid limitations to achieve the required properties for its use in industrial material applications. Native starch granule is an unprocessed and/or unmodified starch granule that is made up of a linear glucose chain linked by α-1,4 glucosidase bond and a branched glucose chain with branching at α-1,6 positions called amylose and amylopectin, respectively13. The amyloseamylopectin ratio has been shown to affect the chemical properties of the starch and hence, it’s functionality as a biopolymer22,31. Starch containing various amylose-amylopectin ratios from various botanical sources are shown in Table 1. Table 1: Amylose-amylopectin ratios and crystallinities of starch obtained from various plant Source Rice Potato Cassava Waxy cassava Wheat Corn Sorghum sources.
Amylose (%) 20-30 23-31 16-25 0 30 28 24-27
Amylopectin (%) 80-70 77-69 84-75 100 70 72 76-73
Crystallinity (%) 3840 23-5340 31-5942 36-3940 43-4840 22-2845
Ref. 5 41 43 43 5 44 46
The physical and chemical properties of starch are both determined by its composition and structure. Several factors such as the plant source, geographic growth location, soil type and condition, and growing climate conditions as well as its amylose/amylopectin ratio determine the overall structure of the starch41. Starch contains three crystal structures; A, B and C, and are functions of its origin. It has small granules which vary in shape depending on the source13. The granule comprises of glucose monomer units linked by α-1,4 glycosidic bonds, forming amylopectin and amylose polymer units. Amylose is a linear polymer with α-1,4 glycosidic 6
bonds which links the glucose monomers with an approximate molecular weight of 1 x 106 g/mol. This represents the amorphous structure within the starch granule. Conversely, amylopectin, which accounts for the crystalline structure of starch, is a higher molecular weight polymer with an average molecular weight of approximately 1 x 108 g/mol47 while linked by short α-1,4 glycosidic bonds with high branching at the α-1,6 positions13. Branching of amylopectin polymer creates a double helix of approximately 5 nm in length which aligns in the crystalline region within the starch granule48 as shown in Figure 1. Positive birefringence as determined through x-ray diffraction (XRD) of the macroscopic view of starch under illuminated light showed by a Maltese cross, indicating an arrangement of the macromolecular units represented by a helix in the starch morphology22, and disappeared upon disruption of the starch granule. The amorphous and crystalline lamellae in the starch granule caused by the interchanging arrangement of the molecular unit is responsible for the semicrystallinity of starch. The crystallinity is reported to range from 20 to 45 %48.
7
Figure 1. Acid hydrolysis of native starch showing the breakdown of constituents: amorphous and crystalline growth rings Multi-scale structure of starch49. A non-carbohydrate component found in starch is phosphorus, which exists as a phosphate monoester and phospholipids. Depending on which macro-polymer the phosphorus within the starch molecule bonds with, the lucidity, gel strength and solubility of the starch varies13. Starch also contains 0.1-0.7 % protein by weight and lipids (up to 1.5 % by weight) that are present as free fatty acids and lysophospolipids50. Other minerals such as calcium, magnesium, potassium, and sodium also exist in granular starch40. 1.4. Physical properties of starch 1.4.1. Solubility in organic solvents
8
Starch is hydrophilic in nature with limits its solubility in cold water. When in contact with cold water, starch disperses well but settles with time if left undisturbed. However, when cold waterdispersed starch is heated, it solubilizes to form a gel-like paste. The chemical process responsible for solubility and gel-like paste formation of heated water-dispersed starch is the formation of hydrogen bonds between water molecules and the hydroxyl groups on the separated amylose-amylopectin molecules. Although starch is soluble in water when heated, water is not always a suitable solvent for starch modification for industrial applications. This is because water may partake in some reactions. Thus, the study of the solubility of starch in organic solvents is desirable to elucidate an appropriate inert solvent as a reaction media. Starch is soluble in some polar solvents like dimethyl sulfoxide (DMSO), dimethylformamide (DMF), pyridine, 90/10 v/v mixed solvent of DMSO/water, DMSO with lithium chloride (LiCl), DMF with LiCl and N,N-dimethyl acetamide (DMAc)51. Generally, the more polar the solvent, the higher the solubility of starch will be in that solvent. DMSO appears to be a promising solvent media for starch modification owing to its high polarity, its stability in neutral and alkaline pH and its stability at a fairly high temperature. It is worth noting that the choice of solvent depends on the desired application. 1.5.2. Gelatinization and retrogradation of starch Many starch modification processes involve the granular disruption of starch known as gelatinization, mainly to access the –OH functional groups. Gelatinization, in general, is an irreversible order disruption of the granular structure of starch molecule leading to a loss in bifrigerence properties52. This occurs when starch is heated between 60-70 °C53 in excess water, leading to maximum granular swelling and bursting of the granule. It occurs in two stages;
9
firstly, amylose-amylopectin separation resulting from the absorption of water and swelling of the granule leading to a loss in semi-crystallinity13 of starch. This separation occurs when the intermolecular hydrogen bonds are broken to loosen the double helices54. It usually begins in the amorphous region because of the ease of water percolation that results in the weakening of the hydrogen bonds. Secondly, separation and loss of amylose leaching from granule into the solution. The amount of water affects gelatinization; in a low water-starch ratio, granular swelling is incomplete, leading to a partial loss of crystallinity called melting55. Contrarily, the crystallites will be separated because of excessive swelling with no crystallites to be disordered in the case of excessive water. Additionally, the ratio of amylose and amylopectin of the starch granule affects the gelatinization temperature and the quality of the paste. For instance, high amylose starch with amylose to amylopectin ratio of 70:30 gelatinizes at 160 – 170 °C 56. Another method to achieve the gelatinization of starch is through the application of high pressures. While separation of amylose-amylopectin molecules also occurs with high pressure, granule swelling is minimized and solution leaching of amylose is reduced. Like thermal gelatinization, the amount of water and treatment time affect high-pressure gelatinization55. A study by Baks et al
55
revealed that at constant temperature in different starch samples,
gelatinization was faster at higher pressures (above 400 MPa). As gelatinization and granular disordering occur, starch granules lose birefringence, which is a characteristic of gelatinized starch. When gelatinized starch is cooled, the segregated amylose-amylopectin molecules realign themselves to a crystalline structure in a process known as retrogradation. Retrogradation is usually accompanied by expulsion of water, an increase in viscosity and gel formation. Furthermore, when retrogradation occurs, amylose links up with multiple glucose units, forming 10
a double helix and the short chains of amylopectin crystallize simultaneously. As well, components present in the starch granule affect retrogradation. While the presence of protein has been shown to slow down retrogradation during refrigeration, temperatures below 0 oC accelerate it13. In summary, retrogradation of starch can also be influenced by the botanical origin, storage length and conditions, and the amount of water54. The resulting product of retrograded starch is the formation of a gel. In native starch with a high amylopectin ratio, the gel formed is typically soft. Contrarily, starch containing a high amylose ratio forms a flexible and strong gel that exhibits resistance to deformation5. Since the soft amylopectin gels display low molecular strength, their desire for industrial use is rather limited13. Hence, for most industrial applications, starch with high amylose content is preferred. Figure 2 summarizes the steps involved in the gelatinization and retrogradation process of starch.
11
Figure 2. Processes that occur during gelatinization and retrogradation. (a) undisrupted starch granule; (b) absorption of water, swelling of granule, molecular segregation and loss of amylose to solution; (c) realignment of amylose molecules due to cooling (d) recrystallization of amylopectin molecules during storage. Adapted and modified from Liu et al 48. Elsevier copyright © 2009. 1.5.3 Melt processability of starch To process polymers in a typical thermoplastic processing operation, the polymer is heated above its melting point to enable a viscous flow. Nevertheless, when starch is heated, it begins to degrade before any observation of melting. This is because its melting temperature is above the degradation temperature, stemming from the strong inter- and intramolecular hydrogen bonds in the starch polymer chains. Plasticizers are customarily added to starch to reduce the number of sites available for inter- and intra-molecular hydrogen bonds and as a result improve melt processability48. There are several plasticizers employed for use with starch including water, polyols, sorbitol, urea, formamide, and citric acid with water being the most common plasticizer57. Starch in its native form contains 9-10 % w/w of water. However, water from an external source is required to plasticize starch. The higher the water concentration in starch, the lower the melting temperature. Unfortunately, starch plasticized with water has undesirable mechanical properties and suffers from retrogradation. Hence, it cannot be used as a lone starch plasticizer. Plasticized starch, also referred to as thermoplastic starch (TPS), is prepared by melt processing of starch at an elevated temperature and shearing in the presence of an added plasticizer57. Gelatinous starch is referred to as thermoplastic starch if it is stable and retrogradation does not occur. Retrogradation can be prevented by the addition of other polymers or the use of substituted starch that restrict recrystallization by interrupting the hydrogen bonds in starch.
12
Glycerol, polyols, sugar alcohols, ionic and non-ionic surfactants amongst others are common plasticizers employed for TPS. However, care should be taken when using glycerol as it has a high affinity for bonding with water molecules, thereby reducing its efficiency as a plasticizer to interrupt the hydrogen bonds in starch. This anti-plasticization phenomenon results in increased phase transition temperatures and brittleness of the starch. Also, most common plasticizers are known to leach out of the polymer with age as they do not chemically bond with the starch molecules. 1.5.4. Rheological and thermal properties of starch Dry starch molecules is frozen at room temperature, making them immobile. As the temperature is increased, starch molecules become mobile and move past one another resulting to its glass transition. When starch is heated, it undergoes several phase transitions including granule swelling, gelatinization, decomposition, melting and crystallization. Methods employed to study starch phase transitions can be divided into two major groups: (1) Transitions studied in the absence of shear, including, wide-angle X-ray diffraction scattering (WAXS), small-angle X-ray diffraction scattering (SAXS), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and hot-stage microscopy; and (2) transitions studied while shearing including rheometry48.
13
Figure 3. (a) A viscosity profile of native tapioca starch as a function of thermal treatment and time. The onset of viscosity between 339 and 347 K (66 to 74 °C) is marked by the grey zone, and (b) In situ WAXS data measurement for native tapioca at a heating rate of 2 K/min. Adapted from Huang et al.58 IUCrJ copyright © 2014. Shearing aids the mechanical segregation of the starch molecules, thereby enhancing the loss of the crystalline structure. The paste obtained after gelatinization contains viscous leached granules, swollen granules and the rest of the starch components. The viscosity of the starch gel is impacted by temperature change, amylose/amylopectin ratio, starch components, and shear rate41. Structural and viscosity changes during gelatinization and gelation of starch granule were studied in detail by Huang et al58. The viscosity and crystallinity changes during the gelatinization of starch are presented in Figure 3 (a and b). Figure 3a shows that the viscosity exhibits a sharp increase in the melting range of 66 to 74 °C during the heating of starch, indicating the gelatinization of starch. Subsequent cooling of the gelatinized starch caused an additional significant viscosity increase suggesting the occurrence of gelation. The WAXS result in Figure 3b exhibits the decaying of peaks with an increase in temperature, indicating starch crystal melting.
14
According to a study carried out on rice starch by Simi et al59, a fixed concentration of starch was heated from 50 to 95 °C and the temperature kept constant at 95 °C for 2 mins. The recorded viscosity profile exhibited a high peak viscosity, which is the maximum viscosity at which the starch granules hold water before they break. When granules disruption followed by amylose leaching begins, the breakdown viscosity occurs due to amylose loss to the solution. On the contrary, when starch granules begin to realign themselves during cooling and retrogradation, it is accompanied by an increase in viscosity; this is the setback viscosity. A schematic representation of the melting of starch is presented in Figure 4.
Figure 4. Schematic representation of the melting process of a semicrystalline starch lamellae. (a) stacked lamellae of well packed crystalline nanoclusters of amylopectin (orange) and amylose (black) chains, (b) the melting of defect-rich regions leads to partial loss of integrity of the lamellae, into layers alternatingly rich and poor in tilted and partially disordered nanoclusters (shaded background in torquoise), and (c) in melted starch, the molten amylopectin nanoclusters (bright orange), are loosely associated with relatively untangled linear amylose chains (black). Adapted and redrawn from Huang et al58. IUCrJ copyright © 2014. 2. Modification of starch Though, native starch is hydrophilic in nature, it is insoluble in water at room temperature and suffers from retrogradation. Its very low thermal stability means it cannot be melt processed without thermal degradation occurring, resulting in poor mechanical integrity. These deficiencies have constrained it as a filler or second phase in polymer blending and use for polymer applications that require thermal stability and mechanical strength. This is especially critical in the plastics manufacturing industry which relies heavily on melt processing in most cases.
15
Likewise, the limited cold water solubility of starch further restricts its potential application as an additive in the huge oil industry in applications such as drilling fluid additives and enhanced oil recovery (EOR). Therefore, modification of starch in most cases is required and desirable not only to alleviate these challenges but also to bring about other functional properties. Several improvements to the properties of starch have been achieved after modification include amphiphilicity, paste clarity,
thermal
stability,
hydrophobicity,
freeze-thaw stability,
retrogradation resistances and mechanical strength amongst others59–64. These starch modification processes used in achieving these improvements have been well detailed and reported in the literature, and can be subdivided into physical65 66, chemical67–70, enzymatic71–73, biotechnological5 or the combinations. 2.1. Physical and thermal modifications Physical modification is an appealing process, as it does not involve the use of chemicals, which could contaminate the starch especially when the modified starch is to be utilized for food contact or in the biomedical industry. Moreover, such modification processes are relatively inexpensive compared to other techniques. Starch modified by physical means can be classified into two broad categories: (i) Physical modification with the destruction of granular structure (pre-gelatinization processes) and (ii) Physical modification without the destruction of the granular structure (hydrothermal treatment)74. Physical modifications could involve changes in the granular structure, thereby modifying its gelatinization behavior, swelling, and solubility behavior. These processes are commonly implemented when modifying starch for food applications. Alcazar-Alay and Meireles75 conducted a detailed review of such physical modifications of starch.
16
2.1.1. Pre-gelatinized starch Cooking of starch till the conclusion of the gelatinization process and drying thereafter to a powder, is referred to as pre-gelatinized starch (PGS)33 . The process of PGS produces starch with improved swelling capacity that solubilizes in cold water without the application of heat31. Unfortunately, there is an accompanying loss of granular structure, granule break-down and, loss of birefringence, and crystalline properties with this physical treatment. The solubility in cold water and pasting ability allows for the extension of PGS application in the food industry such as thickeners for soups and as an adhesive in many products. Chemical properties of starch treated by this method are dependent on the starch botanical source, methods and conditions of the cooking and drying process34 . The techniques for drying are drum drying30 , spraying drying35 , and conventional extrusion cooking30 . Drum drying is the most accepted technique applied industrially. Pre- gelatinization of starch using this drying technique could occur in one or two stages. In the one stage technique, the starch slurry fed into the drum is gelatinized and dried simultaneously, while, the two-stage technique, involves cooking the starch slurry until it gelatinizes before it is drum dried31 . The PGS from this drying process has various textures and porosities with an effect on the functional properties and, product rheological properties. These properties are dependent on the time spent in the dryer, hence, the residence time should carefully be selected30 . Research results carried out by Majzoobi et al on PGS with twin drum drier showed PGS with no crystalline structure, intrinsic viscosity significantly lower than that of native starches indicative of degradation of starch molecules due to high temperature in drums, higher viscosities at temperatures below the gelatinization temperatures of native starches and, higher water adsorption and solubility than the native counterpart. Spray drying on the other hand commonly used in the food industry for 17
encapsulation involves instant water removal from starch slurry by atomizing it in a hot gas current35. This technique prepares PGS without the loss of granular integrity30. Finally, conventional extrusion is a high temperature, pressure and shear technique that compresses the starch with an accompanied change in granular structure from semi-crystalline to a plastic material with high viscosity30. 2.1.2. Hydrothermal modification Hydrothermal treatment is a technique that modifies starch while keeping the granular structure intact. This treatment requires the starch molecules to be kept in the mobile state above the glass transition temperature (Tg) and below the gelatinization temperature (Tgel)33 . Techniques for hydrothermal modification are annealing (ANN) and hydrothermal treatment (HMT). Both techniques require temperature and moisture level for the duration of the treatment to achieve modification30 31. While HMT requires low levels of moisture, typically <35 % w/w, annealing is carried out in excess water content of >65 % w/w31. 2.1.2.1. Annealing (ANN) Annealing is the rearrangement of starch molecules in the presence of water between 40 and 76 %w/w, and heat between the glass transition and gelatinization onset temperature32 36 4. Annealing could lead to partial gelatinization; however, starch can only be classified as annealed when the onset of gelatinization temperature exceeds that of the native counterpart37. Therefore, this process causes an upward shift in the endotherms of starches38 characterized by elevated gelatinization temperatures and shortening of the time from onset to the conclusion of gelatinization. This increase in temperature (up to 14 oC) is because of the rearrangement of the
18
granular structure, leading to a reduction of the amorphous polymers and an increase in crystallinity38. Figure 3 shows the DSC thermograms of annealed starch versus native starch.
Figure 5. DSC thermograms of commercial wheat starches: (a) native; (b) after annealing in excess water (45 oC, 100 days) (Adapted and modified from Tester et al37 ) Parameters for annealing to occur are moisture content, temperature and time; the extent of treatment changes as these conditions are varied39. Other characteristics of annealed starch are a decrease in solubility, increase in enzyme hydrolysis, increase in crystallinity and decrease in retrogradation39. 2.1.2.2. Heat moisture treatment (HMT) This treatment method is similar to annealing except it is carried out in low moisture content of <35 % w/w, usually for a period between 15 minutes to 16 hours33 and at 80-140 oC30 . This treatment affects the physical, chemical and morphological properties of starch depending on their botanical source. The properties affected include crystallinity and granular interactions as
19
shown from the XRD patterns, gelatinization, paste properties, amylose leaching, and recrystallization and enzyme hydrolysis31. Starch treated with this method finds industrial applications in the food industry, especially in baby foods. Baking properties along with freezethaw stability has also been shown to improve in potato starch treated by this method30. Although, thermal treatments improve the physicochemical properties of starch, the shortfall of these processes is a loss in nutrients, vitamins, and taste in food starch due to temperature exposure. As mitigation strategy, non-thermal treatments can be used to reduce the impact of microbes while keeping nutrients, flavors, and vitamins intact31 40. The desired property of treated starch determines the technique of treatment applied. The methods include ultrasound effect, osmotic pressure treatment41 , microwave treatment, and high hydrostatic pressure (HHP) treatment. A study carried out by Chirdchan et al41on the effect of osmotic pressure on starch showed results similar to those of HMT treated starch except a broadening of the gelatinization enthalpy. 2.2. Chemical modifications Chemical modification of starch is an important modification route that involves the blocking or introduction of functional groups to impart desirable physical and chemical properties while maintaining chain integrity, thereby extending its application. Because of the intrinsic advantage of an abundance of hydroxyl groups on the structure of starch, different chemical modifications have
been
studied,
including
etherification76–78,
oxidation67,79,80,
acetylation56,62,81,82,
esterification83–86, polymer grafting87–89, crosslinking90,91, silyation92–94, hydrolysis,95–97 etc. A combination of physical and chemical modifications can also be conducted. The physicochemical properties that can be modified include gelatinization, retrogradation, viscosity
20
and pasting properties, thermal stability, solubility, hydrophilicity, and compositions. The extent of modification depends on the crystallinity of the starch, amylose to amylopectin ratio, reaction conditions, and molecular distribution of starch74. Some of the common chemical modification of starch as applied to the material applications are reviewed in the following sections. 2.2.1 Etherification of starch Starch ethers namely, cationic starch, anionic starch, amphoteric starch, and non-ionic starch are produced when the hydroxyl group of starch molecules reacts with groups that add charges to the resulting starch69. Cationic modified starch is based on the substitution of the hydroxyl functional group of starch via etherification with positive charges. Chemical compounds such as tertiary or quaternary ammonium, imino, amino, butyl, sulphuric and phosphate groups (cationization agent) using organic or inorganic bases are commonly used for such cationic modifications75,98. The use of monomers like 2,3-epoxypropyltrimethylammonium chloride (ETA) or 3-chloro-2hydroxypropyltrimethyl ammonium chloride(CTA) in a dry or wet process has been getting significant attention in recent years69. In a typical dry process, a cationic agent is sprayed on the dry starch during an extrusion process. A scheme of the cationic modification of starch using ETA is shown in Figure 6. In a semi-dry process, a cationic agent is sprayed on the starch prior to thermal treatment, while in the wet process, the homogenous reaction is carried out in dimethyl sulphoxide or heterogeneous reaction in an alkaline suspension (e.g. under dissolution in aq. NaOH)74. In general, homogenous etherification is more efficient as compared to heterogeneous methods. This is because of the impeded diffusion of the cationizing reagent in the heterogeneous reaction as compared to the homogeneous process in which the starch granules are solubilized, which results in enhanced diffusion of the cationizing agent.
21
Low degree of substitution (DS) cationic starch (DS up to 0.2) are commercially used as additives in paper, textile or in the cosmetic industry. More recently, the application of cationic starch has expanded by obtaining high DS derivatives. Heinze et al76 prepared cationic starch of DS 1.5 using 2,3, epoxypropyltrimethyl-ammonium chloride in a two-step homogenous and heterogeneous reaction. The modified starch obtained was soluble in cold water and exhibited promising properties such as flocculation agents for wastewater treatment and antibacterial behavior. It can be expected that these materials can be used as EOR additives as well. Zhang et al99 synthesized a high DS (0.49) cationic hydrolyzed starch using 2,3, epoxypropyltrimethylammonium chloride. The derivative, when employed in salt-free dyeing of reactive dyes, showed high dye fixation without affecting rub and wash fastness. Compared to conventional dyeing, this showed higher efficiency and reduced pollution from salt. This modified starch can be applied as a substitute for salt in reactive dyeing.
22
Figure 6. Reaction scheme for the synthesis of 2-hydroxy-3-(trimethylammonium) propyl cationic starch (HTPS) under different media and conditions. Adapted from Heinze et al.76. Wiley copyright © 2014. Although processes with an aqueous solution of starch have been shown to produce cationic starch with a high degree of substitution, other side reactions may occur as a result of the presence of water, thereby, hindering the etherification reaction74. Etherification modification usually alters the morphology and granular structure of starch accompanied by a loss of
23
crystallinity. Furthermore, it improves water uptake, pasting, and thermal properties of modified starch. The extent of improvement depends on the structure of starch and reaction conditions such as cationization agent, temperature, catalyst efficiency, water content, and reaction time5. Overall, starch modified as such find applications in wastewater treatment, paper making, oilfield drilling, cosmetic products, and textile industry because they provide excellent properties that can be geared towards these applications and are relatively cheap and biodegradable99. Cationic starch derivatives with a high degree of substitution (between 1 and 1.5) are soluble in cold water. Such derivatives can be used as additives in Enhanced Oil Recovery (EOR) technologies78 . Anionic starch is prepared by modifying starch with anionic groups such as carboxymethyl and sulphonic,
thereby
improving
its
physicochemical
properties
like
solubility,
hydrophobicity/hydrophobicity, and flocculation behavior69,78 . Oxidation of starch to carboxylic acids, the formation of inorganic and organic polycarboxylic acids, and an etherification of starch with halocarboxylic acids, acylation of starch with cyclic anhydrides of dicarboxylic acids (e.g. succinic, maleic, phthalic) are other starch anionic modifications reported in the literature77. However, carboxymethyl starch (CMS) is perhaps the most studied anionic starch since CMS has a range of appealing properties, and the modification process is relatively simple. Commercial CMS production is carried out by the reaction of starch with monochloroacetic acid or its sodium salt after activation of the starch with aqueous NaOH in an aqueous organic solvent (e.g. ethanol). A scheme for carboxymethyl starch modification is presented in Figure 7. Due to the low cost, solubility, and water absorption properties of CMS, it has found applications in pharmaceuticals (e.g. disintegration aid for tablets, laxatives), food industry (emulsification
24
stabilizing agent, colloid and suspension stabilizing agent),
textile, paper making, water
treatment and laundry detergent, superabsorbent additive etc100.
Figure 7. Scheme for carboxymethyl starch modification. Adapted from Yanli et al101. Elservier copyright © 2009 Amphoteric starch is the product of the simultaneous synthesis of native starch with both cationic and anionic groups. Common anionic groups used are carboxyl, sulfate, phosphate, sulfonate, and phosphonate groups. Non-ionic starches are used in medical and surgical applications with the main non-ionic starch being alkyl ester. Common examples are hydroxypropyl starch and hydroxyethyl starch69. Etherification of starch can be useful in plastic applications as well. For instance, Wokadala et al102 studied and reported butyl-etherification (C4) of starch to induce enhanced compatibility of the etherified starch and polylactic acid (PLA) polymer. The results of the study revealed that the butyl-etherification brought about hydrophobicity of the starch, thereby improving the compatibility with PLA. 2.2.2 Cross-linked starches Cross-linked starch is formed when the intermolecular hydrogen bond is reinforced with chemical bonds that form bridges by interconnecting the polymer chains. This happens when modifying reagents esterify or etherify the hydroxyl functional group in starch. The result of this modification is a polymer with strong chemical bonds due to the formation of three-dimensional 25
network. Hence, cross-linked starch has high thermal and shear resistance with improved viscosity and consistency103. Crosslinked starch finds application in industries requiring high shear and thermal resistance. Zhou et al104 investigated citric acid crosslinking of starch for the production of nanocrystals. The results of this study showed that the crosslinking caused a reduction in the starch crystal size, hydrophobicity, and improved dispersibility in non-polar solvents. Starch nanocrystals produced as such can find application in cosmetics, emulsion stabilizers, rheology modifier, drug delivery vehicles, and reinforcing agents of polymers for nanocomposites development. EcoSynthetix Inc, a renewable chemical company based in Canada, developed a commercial starch nanoparticle called Eco-Sphere by crosslinking starch with glyoxal in a continuous reactive extrusion process. In an effort to understand the mechanism of starch nanoparticle formation using glyoxal crosslinking, Song et al
105
conducted a detailed investigation of the
reactive extrusion crosslinking process. The results of the study indicated that wet extrusion of starch produces starch particles of about 300 nm in size. However, with the addition of glyoxal crosslinkers (2 %), the starch nanoparticle size reduced to 160 nm. Such nanoparticles could have applications as a filler in polymers and other functional applications. Sodium trimetaphosphate (STMP), sodium tripolyphosphate (STPP), epichlorohydrin (ECH), dicarboxylic acids, anhydrides, dialdehydes and phosphoryl chloride (POCl3) are other common cross-linking reagents of starch75. In most cases, the crosslinking agent has multiple moieties, which can react with the hydroxyl group in starch, leading to cross-linking between hydroxyl functional groups on the same or on different molecules. The final product of cross-linking which can be classified into one of three types depends on the reagent used. The stages of starch crosslinking with POCl3 as shown in Figure 8 and are: a) Starch esterification with 26
orthophosphoric acid, potassium or STPP to produce monostarch phosphate, b) Reaction with POCl3 or STMP to produce distarch phosphate or, c) combined treatment of a and b to produce phosphate distarch phosphate. This study revealed that the crosslinked starch has enhanced stability, hardness and adhesiveness of the gels formed from it, and a reduced swelling power, solubility, pasting temperature, heat of gelatinization, and retrogradation 106.
Figure 8. Crosslinking of starch with phosphorus oxychloride. Adapted and modified from Miyazaki et al. Elservier copyright © 2006 68. 2.2.3 Esterification of starch Starch esterification is a vital process to alter the structure, and physicochemical properties of starch, and thereby making it suitable for industrial and functional material applications. Starch esters can be obtained when it is modified with a wide range of derivatives of organic acids (e.g. anhydrides, chlorides) or inorganic acids (e.g. phosphate, sufhate). Inorganic starch esters (e.g. starch phosphate) exhibits higher viscosity and transparency, making it suitable as adhesives, thickener, stabilizer, and drug bulking agent69. A high degree of substttution of the –OH functional group via esterification is usually desired to directly utilize starch in thermoplastic
27
polymer applications. Esterification of starch for material applications can be conducted to enhance its processability, promote hydrophobicity, reduce crystallinity, improve flexibility, etc.
Figure 9. Summary of starch esterification. Adapted from Haroon et al70. RSC copyright © 2016. Figure 9 presents a schematic summary of various starch esterification pathways. Haroon et al70 has also included a compilation of the various esterifying agents including citric acid, acid chloride, acetic acid, acid anhydride, together with the process
(Figure 9). Common
esterification of starch that are pertinent to material applications are reviewed in the next sections. 2.2.4. Fatty acid esterification of starch
28
Starch modification with fatty acids (FA) has received substantial attention in recent times because of the hydrophobicity of FA and the ability to incorporate desired properties when grafted on starch. Generally, hydrophobically modified starches are amphiphilic, which means their back-bone structure is hydrophilic with hydrophobic side groups107. Therefore, the solubility of fatty acid starch esters depends on the extent of substitution of –OH groups, type of FA and starch, and the modification process (e.g. solvent media used, reaction temperature, etc). FA modification has been shown to alter the crystallinity of starch as observed by x-ray diffraction; however, at low DS, the crystalline change is insignificant
107,108
. As the DS
increases, the level of disorganization of the granular structure of starch also increases proportionally as observed by Chi et al62 . Short-chain fatty acids (SCFA), medium-chain fatty acids (MCFA), and long-chain fatty acids (LCFA) are used for the esterification of starches. While longer fatty acid chain FAs induces improved hydrophobicity to the starch, steric hindrance associated with the chain length of the LCFA usually limits the level of modification61. Lopez-Rubio et al108 investigated the effect of FA esterification on the granular structure of high amylose corn starch (HAMS) and low amylose corn starch (LAMS) using acetic, propionic and butyric acid at low, medium and high DS. As shown in the SEM images of Figure 10, esterification of starch at low and medium DS had no effect on the size and shape of starch. On the contrary, at the highest DS, granular distortion and roughness occurred resulting in aggregation as seen in Figures 10f and 10h. A comparative structural evolution study between native starch and the corresponding starch-laurate esters were monitored by FTIR and NMR spectroscopy and results are shown in Figure 10 (I and J). These spectroscopy displayed an
29
almost complete substitution of the –OH functional group of starch with laurate, resulting in a complete disruption of the crystalline structure of starch with no re-crystallization behavior.
Figure 10. SEM images of HAMS: base starch (a), acetylated DS-0.32 (b), propionylated DS = 0.39 (c) and butyrylated DS = 0.4. Magnified images (x2000) of HAMS base (e), acetylated HAMS DS 0.32 (f), LAMS base (g) and acetylated LAMS DS = 0.29 (h). Adapted from Amparo Lopez-Rubio et al108. Elsevier copyright © 2009. (I) FTIR spectra of (a) starch, (b) Starch ester DS 0.5, (c) Starch ester DS 1.2, (d) Starch ester DS 2.1, and (e) Starch ester DS 2.9. (J) 1H-NMR spectra of (a) native starch in DMSO, (b) 0.5 DS ester in pyridine, (c) 1.2 DS ester in pyridine, (d) 2.1 DS ester in CDCl3, and (e) 2.9 DS ester in CDCl3. Adapted from Ojogbo et al.110 Wiley copyright © 2018.
30
Simi et al109 investigated the solubility of starch grafted with oleic and stearic acid in various organic solvents. The results of this study demonstrated that the FA esterified starches were soluble in warm DMSO and partially soluble in DMF and chloroform. Although the values of DS of the modified starches were not quantified, it was stated that the DS was low. The solubility of these modified starches were not investigated at high DS, which makes these results inconclusive. Rajan et al71 obtained similar results from the enzymatic modification of starch using palmitic acid by liquid state and microwave esterification. Results showed that the esterification increased the thermal degradation temperatures, reduced swelling power, and reduced enzyme digestibility when compared to native starch. The changes observed in this study are desirable for material applications. Octanoyl, lauroyl and palmitoyl chloride fatty acids as starch modifiers were investigated by Namazi et al107. These starches exhibited various levels of hydrophobicity depending on the degree of substitution. Another study56 highlighted that the successful esterification of starch with fatty acid-starch can be carried out using medium-chain fatty acids (containing between 610 carbon chains) in alkali media as others outside this range could be hydrolyzed. However, Namazi et al107have successfully esterified longer carbon chain FAs (C12) by optimizing the reaction conditions such as alkali concentration, reaction time and temperature. Winkler et al61 compared starch synthesized by fatty acid vinyl ester in DMSO and lauroyl chloride in pyridine and showed the importance of reaction parameters (Starch/FA ratio, temperature, catalyst type and mole ratio, reaction time and solvent choice) on the DS of the FA-starch ester. Ojogbo et al110 esterified starch to varying degrees of substitution and elucidated the effect of the extent of modification on the structure and properties of starch. These investigations agreed that the level of hydrophobicity of the FA modified starch proportionally increases with the increase in the
31
level of starch –OH substitution. Table 2 summarizes starch-fatty acid esterification modifications reported in the literature.
32
Table 2. Common fatty acid modifications and conditions Fatty Acid C2-C4 acid
Solvent Water108
Catalyst -
Temperature /Time -/30 mins108
DS 0.16 – 0.29108
-
Butyric acid
NaOH56
-
Room temp/1 h56
0.3 – 356
-
Octanoyl chloride
NaOH56
-
Room temp/1 h56
0.5 - 356
-
80 oC/ 30 mins86
>0.3 - 386 -
Water/DMAC86 Pyridine86 Pyridine84 Lauroyl Chloride
-
Water/DMAC86 Pyridine86
115 oC/3 h84 80 oC/ 30 mins86
1.884
-
>0.3 - 386 -
Pyridine61
-
110 oC/1 h61
0.5-2.861
-
Pyridine
Pyridine
110 oC/1 h
0.45-2.9
-
Results No change in granular size and shape at DS less than 0.2108. Deformation and increased roughness of some granule at DS higher than 0.2108 Water soluble at low DS and insoluble at DS > 1.556
High DS > 1.5 soluble in most organic solvent. Low DS insoluble in toluene and chloroform at all temperatures56 Soluble at low DS in most organic solvents with partial solubility in chloroform, dichloromethane and hot toluene86 Loss in crystallinity, Tm of 174 , decreased Tg, hydrophobic84 Soluble at low DS in most organic solvents with partial solubility in chloroform, dichloromethane, and hot toluene, Greater thermal stability than unmodified starch with the onset of decomposition of 300 compared to 200 of modified starch 86 Partially insoluble in organic solvent61
Soluble in most organic solvents at low DS and chloroform at high DS110 Decreased onset degradation temperature due to chain swelling and exposure of hydroxyl groups available to partake in degradation mechanism. Increase in inset degradation at DS >2110 33
Vinyl ester (C6-16) Vinyl ester (C18) Oleic acid
DMSO
Carbonate
110 /2 h
0.3 – 2.5
-
DMSO
Carbonate
110 /2 h
0.3 – 2.5
-
DMSO109
K2S2O8109
100 oC/8 h109
-
-
-
Palmitol chloride Stearic acid
Water/DMAC86
-
80 oC/30 mins86
0.5 - 386
-
Water/DMAC86
-
80 oC/30 mins86
0.5 - 386
-
DMSO109
K2S2O8109
100 oC/8 h109
-
-
Highly soluble in organic solvents, hydrophobic, can be cast into films61 Highly soluble in organic solvents, hydrophobic, too brittle61 Reduction in swelling power from 4.11 % in native starch to 2.03 % in modified starch Reduced degradation start temperature of 180 in modified starch compared to 275OC in native starch Reduced gelatinization temperature of 84 oC indicating better processability than native starch with temperature of 100 oC Destroyed granular structure characterized by amorphous particles109 Soluble at low DS in most organic solvents with partial solubility in chloroform, dichloromethane and hot toluene86 High DS, soluble in organic solvents with Poor solubility at low DS86 Reduction in swelling power from 4.11 % in native starch to 2.9 % in modified starch Reduced degradation start temperature compared to native starch109
34
2.2.5. Acetylated starches Acetylation is the esterification of starch by introducing acetate groups to replace the hydroxyl functional groups in starch. This is achieved by the use of acetylating reagents such as acetic anhydride, acetic acid, or vinyl acetate in the presence of an alkaline catalyst like sodium hydroxide, potassium hydroxide, sodium carbonate amongst others74 . The general mechanism of acetylation reaction with starch occurs by the elimination of the hydroxyl group from starch and the addition of acetate groups on the starch molecule as presented by Figure 11. The application of acetylated starch depends on the degree of substitution (DS) of the hydroxyl functional groups on the native starch. Acetylated starch with low DS (0.1-0.2) finds applications in films, bindings, adhesives, thickeners, and stabilizers62. On the other hand, high DS acetylated starches find more extensive material applications because of properties such as hydrophobicity, melt processability, solubility in certain solvents such as acetone and chloroform, and thermoplasticity62.
35
Figure 11. Acetylation of starch showing the addition-elimination reaction mechanism. Adapted from Oliveira et al111. Wiley copyright © 2018 Singh et al82 studied the effect of acetylation on corn and potato starches in sodium hydroxide. They showed that that acetylation decreased thermal transition temperatures, increased swelling power, improved light transmittance with negligible retrogradation properties of cooked starch paste. The authors reported that the observed properties were greater in potato starch compared to corn starch, which was attributed to the higher crystallinity in corn starch.. Hence, it was safely concluded that the change in functional properties of acetylated starch is dependent on the morphology of the native starch (associated with the botanical origin) as well as other factors including concentration of reactants, reaction time, and pH. 2.2.6. Other modifications Oxidation, enzyme hydrolysis, and acid hydrolysis are other common chemical modification of starch to alter its structure and properties. Acid hydrolysis forms soluble starch in water at temperatures below Tg using common acidic solutions like hydrochloric and sulphuric acid75 . The hydrolysis typically occurs in two stages to break and shorten the polymer chains. While the initial stage that is associated with the preferential breakdown of the amorphous region of the starch granule is fast, the final stage for the hydrolysis of the crystalline segment happens at a slower rate95 . The mechanism of acid hydrolysis of starch is depicted in Figure 12. The reaction begins by an electrophilic attack of the hydroxonium ion (H3O+) on the oxygen atom as shown in Figure 12A. This is followed by a transfer of electrons from one of the carbon-oxygen bonds to the oxygen atom as seen in Figure 12B, resulting in an uneven, high energy carbocation intermediate (Figure 12C), which is a Lewis acid that reacts with water to produce a Lewis base (Figure 12D), thereby reproducing the hydroxyl group (Figure 12E)112 . The extent of hydrolysis
36
in the first stage depends on the granular size, surface pores, amount of lipid content in amylose chains and amylose content. Whereas the second stage is influenced by branch distribution, crystallite degree of packing and amylopectin content. Hydrolyzed starches find use in food, textile, and pharmaceutical industries. The hydrolyzed starch also be modified further for functional material applications.
Figure 12. Acid hydrolysis of starch69. Adapted from Chen et al. RSC © copyright 2015 Oxidation is the introduction of an oxidizing reagent in the starch molecule under controlled temperature and pH to break down the polymer. Although hypochlorite causes environmental concerns like water and air pollution due to chlorine persistence113, it is the most commonly used starch oxidizing reagent because it oxidizes the molecule without any residue as by product80. Other common oxidizing agents are potassium permanganate, ozone, hydrogen peroxide and persulphate79. When starch is oxidized, the hydroxyl groups on the starch molecules varies depending on the amount of carbonyl and carboxyl groups present69. The Figure 13 shows the oxidation of starch with sodium hypochlorite to produce carbonyl and carboxyl groups. These modified starches are less viscous, more stable, form better films and have lower molecular size than their native counterpart67.
37
Figure 13. Scheme for Sodium hypochlorite oxidized starch67 2.2.7. Starch surface modification Modifications that alter only the surface properties of starch while maintaining the crystal structure of the molecule are classified as surface modifications. The –OH substitution or DS in most surface modification of starch processes is usually low. Surface grafting, cationic and anionic modifications, plasma treatment and other chemical modifications that only involve alteration of the surface –OH groups can be categorized under surface modification. Some benefits obtained from surface modification include decreased or increased hydrophilicity, improved paper printability, decreased surface roughness, reduced particle agglomeration, improved adhesion amongst others114–123. Table 3 summarizes common techniques employed for surface modification of starch. Plasma modification of starch is among the most extensively studied processes of surface modification of starch. It is typically conducted by exposing starch to partially ionized gas. The major advantage of this method is that it modifies only the surface of starch. Thirumdas et al124 modified starch using cold plasma treatment. The modified material exhibited a decrease in
38
molecular weight, viscosity, and gelatinization temperatures. Additionally, this process increased surface energy and hydrophilicity. The reaction mechanism which depends on feed gas used and treatment time is shown in Figure 14. Table 3. Surface modification techniques Technique Grafting
Agent Succinic anhydride Phenyl isocyanate Plasma Helium Hexamethyldisiloxane Plasma Argon Cold plasma Oxygen, nitrogen, ammonia
Effect Altered morphology, retained crystallinity, increased hydrophobicity No change in surface hydrophobicity Increased paste clarity Increased wettability and surface energy. Decreased molecular weight, viscosity and gelatinization temperatures
Ref. 117,125
114
126 124
39
Figure 14. Mechanism of cold plasma (a) cross-linking of chains (b) depolymerization of branched chains (c) etching of granule. Adapted from Thirumdas et al124. Springer copyright © 2009. 3. Graft modification of starch Graft modification can be employed to modify the surface, structural and functional property of starches. Grafting is a modification process that introduces other monomers or polymers on the starch backbone chain linked by covalent bonds, resulting in modified co-polymer with combined functional properties of the starch and polymers used. Furthermore, the resulting properties of the copolymer may be influenced by the nature and degree of substitution of the side chain. Starch grafting is carried out by two major routes as displayed in Figure 15. The first
40
route, “grafting from”, reduces stearic hindrance by direct polymerization of the monomer from the main chain and, the second route, “grafting to”, directly grafts a functionalized polymer with functional groups reactive with starch on the starch backbone127,128.
Figure 15. Routes in graft co-polymerization127. Graft co-polymerisation is conducted by the free radical initiation of granular or pre-gelatinized starch with the side chains in the presence of an initiator. Techniques of free radical initiation are chemical initiation using peroxides, redox systems, and transition metals with high valence, and initiation by irradiation129. Since the side chains grafted on the starch backbone are bonded covalently, one advantage of this process is that the grafted species does not leach out. On the other hand, low grafting efficiency and high homopolymer percentage are disadvantages especially of grafting by chemical technique128.
41
3.2.1 Grafting co-polymerization by free radical initiation and atom transfer radical polymerization Grafting polymerization by free radical initiation is carried out in three steps: initiation, propagation, and termination. During the initiation step, hydrogen is transferred to the initiator, producing radicals on starch, the starch radical produced is highly reactive, and hence, it bonds easily to the monomer, resulting in starch macro radical. The starch macro radicals as such can react with other monomers nearby, which become free radical donors, and again contribute free radicals, thereby propagating grafting. Propagation is terminated by a combination of disproponation of two reactive chains127. Since grafting efficiency of 100 % cannot be obtained in many cases, the unreacted homopolymer (starch and monomer/polymer) can be removed by extraction (e.g., soxhlet extraction). The general synthesis of grafting a vinyl monomer using ceric ammonium nitrate on starch is represented in Figure 16.
42
Figure 16. The mechanism for grafting vinyl monomer on starch using Ce4+ as initiator. Adapted from Haroon et al70. RSC copyright © 2016.
43
Generally, the concentration of initiator, concentration of monomer, temperature, and time affect the grafting yield. Apopei et al130 grafted potato starch with acrylonitrile using two initiators cerium sulfhate Ce(SO4)2 and ceric ammonium nitrate (CAN) and found the grafting percentage using Ce(SO4)2 to be three times higher than the CAN counterpart. Lutfor et al131 grafted sago starch with acrylonitrile (AN) using ceric ion as the initiator in the presence of sulphuric acid and reported that the grafting percentage, efficiency, yield, and rate were dependent on the concentration of CAN, AN, starch, sulphuric acid, reaction temperature and time. The reaction scheme is shown in Figure 17. The optimum value of their grafting reactions is summarized in Table 4. Avval et al87 prepared superabsorbent starches by grafting polyacrylamide (PAA) and polyhydroxyethylacrylate (PHEA) by using radical polymerization and atom transfer radical polymerization (ATRP) for drug use according to the reaction scheme shown in Figure 17. Copolymers by the ATRP method had longer drug release than that of free radical polymerization. This is attributed to the benefits of ATRP, which usually creates uniform polymers chains that result in a regular release as opposed to the radical polymerization that results in erratic release. Zou et al132 graft copolymerized and characterized pea starch and acrylamide for waste-water treatment using ceric ammonium nitrate initiator and obtained a maximum graft yield of 38.3 %. The resulting copolymer from their study showed enhanced thermal stability as indicated by an increase in the degradation temperature. Qu et al133 grafted poly(vinyl acetate) onto starch using KMnO4-H2SO4 redox system and obtained a 38.3 % graft yield at an optimum concentration of redox system at 40
with 3 hours of reaction time. The
concentration of initiator, concentration of monomer, temperature and time were varied and arrived at optimum values of these parameters, grafting was affected negatively beyond the
44
optimum. The resulting copolymer showed increased swelling power and reduced solubility at different temperatures and different grafting yields133. Initiation:
Propagation:
Termination:
Figure 17. Mechanism for grafting AN on starch using CAN initiator131. In other studies, novel starch-g-poly(benzyl methacrylate) copolymer was prepared by grafting benzyl methacrylate monomer onto starch backbone using potassium persulphate as an initiator. The resultant copolymer was hydrophobic, resistant to basic and acidic conditions, that exhibited thermal stability and higher swelling in non-polar solvents 134.
45
Table 4. Grafting initiators and optimum conditions for the graft modification of starch Starch source Corn
Initiator /Catalyst Potassium persulphate (KPS)/ tetramethylene diamine (TMEDA)
Monomer /Polymer Methacrylic acid (MAA
Pea
Ceric ammonium nitrate (CAN)
Acryl amide (AM)
Potato Sago
CAN/(CeSO4)2 CAN
Acrylonitrile Acrylonitrile
Optimum conditions -
Potato
Potassium persulphate
Potato
Potassium persulphate
Citronellyl Methacrylate Benzy methacrylate
-
10 mmol/L KPS 8 mmol/L TMEDA 50% monomer concentration 10 liquor ratio, 60mins reaction time 60 oC temperature CAN: 0.02 mol/L Temp: 65 oC Starch-AM ratio: 0.5 CAN: 9.61e-3 AN: 0.653 AGU: 0.152 H2SO4: 0.187 mol L-1 Temp: 50 Time: 90 mins Temp: 80 Time: 2 h Temp: 80 Time: 2 h
Graft %
Ref.
38.3
128
38.3
132
218.38 54.46
130
-
88
53
88
131
46
3.2.2 Graft copolymerization mechanism by irradiation Irradiation grafting technique is extensively used for modifying polymers because of its low cost, relative ease of the process, and absence of waste. Literature has reported grafting of starch using ultraviolet (UV)135, microwave power136–138, electron beam139,140, and gamma-ray140,141. For instance, Sheikh et al
141
synthesized starch grafted with polystyrene using gamma rays and
investigated how varying the starch-styrene ratio and amount of irradiation doses affected the grafting percentage. They arrived at a graft percentage of 252.9 % at optimum conditions of starch to styrene weight ratio (one to three ratio) and applied gamma-ray dose of 10 kGy. The radiation grafting mechanism is shown in the scheme represented in Figure 18.
Figure 18. Radiation grafting mechanism of styrene on starch using gamma rays. Adapted from Sheikh et al141. Elsevier copyright © 2013.
47
Jian-Kun et al138 grafted acrylic acid (AA) on pre-gelatinized corn starch using microwave irradiation. They studied and reported the impact of initiator content and radiation time on the grafting ratios. Results showed a grafting ratio of 19.57 % at an optimal grafting time and initiator content of 3.5 mins and 4.55 %, respectively. The resulting copolymer was less brittle compared to native starch. Salimi et al136 investigated the effects of power radiation (W), monomer ratio, and NaOH catalyst concentration on the grafting efficiency of a “green” approach for graft polymerization of L-lactic acid onto the starch backbone. They obtained a maximum grafting at 450 W microwave power, 1:5 monomer ratio, and 0.4 M NaOH. 3.3. Reactive extrusion process for the modification of starch Reactive extrusion enables the modification of starch in a continuous process. It combines heat and mass transport operations in a single process while simultaneously carrying out chemical reactions. In most studies, starch has been modified using a batch reactor (BR) or a continuous stirred tank reactor (CSTR). However, wet processes are generally more expensive as compared to reactive extrusion modification process as it involves heating of extra reaction media (e.g. water or solvent). Downstream processes for the recovery of the product and handling of generated waste also contribute to the cost of using reactors for starch modification. Thus, the reactive extrusion process has received significant attention for a range of polymer modifications, including renewable polymers such as starch142. In a typical modification process, starch is first gelatinized to form a viscous paste, which is often difficult to manage using conventional reactors. To mitigate such mixing hurdles associated with viscosity, modifications are conducted using salts to limit gelatinization and, by extension, to eliminate problems associated with viscosity and mixing. However, this requires
48
the product to be purified after modification, thereby, incurring additional costs and time. Another way to manage viscosity and mixing problems is to substitute the reactor. Table 5 presents a comparison between conventional reactors and extruders for starch modification. Table 5. Comparison between conventional reactors and extruders. Adapted from Graeme Moad 142
-
Conventional Reactor Can handle 30-50 % weight starch Long residence time (2-24 h) Relatively low operating temperature Challenges in mixing in gelatinized starch
-
Extruder Can handle 60-80 % weight starch Accelerated residence time (2-5 mins) High operating temperature (between 70-140 °C) Efficient mixing
Modification of starch using extruders has been studied extensively in the literature142–149. Some of the reported processes include grafting of monomers and polymers onto starch, grafting of polymers from starch, crosslinking starch with epichlorohydrin, and degradation of starch142. Details of these processes are not reported in this review; however, they can be found in literature143–145. Table 6 presents some of the processes, parameters studied, and modification effects reported in the literature.
49
Table 6. Chemical modification of starch using extruder Extruder Type Twin-screw Twin-screw
Twin-screw
Reaction type/ Reagent Esterification/Maleic anhydride Crosslinking/sodium trimetaphosphate, sodium tripolyphosphate Oxidation/Polystyrene
Micro extruder Twin-screw
Grafting/ Polycarbonate
Twin extruder Twin extruder
Hydroxypropylation
Cationic /Polyurethane
Grafting/ Polyacrylamide
Reaction parameters 70-155 °C, 125 rpm screw speed
Effect
Ref. 143
Increased grafting efficiency
144
40-50 wt.% starch, 150 rpm screw Retained granular structure speed, 1 min. reaction time, no mixing elements or conveying elements Increased molecular weight, thermal 165-195 °C resistance, hydrophobicity Improved interfacial adhesion 230 °C, 2 mins, 3-6 wt.% glycerol, 200 rpm Improved mechanical properties, 5-20 wt% starch, 90 °C hydrophobicity, and thermal properties No impact on the extrusion process 250-350 rpm, 150 °C 90 °C, 100 rpm, 3 mins
145
146
147
148
149
Controlled grafting
50
4. Industrial biopolymers based on modified starch Native starch needs to be modified to improve its physicochemical and functional properties, thereby enabling its use for a wide variety of biopolymer and other functional material applications. As discussed earlier in this review, starches can be modified chemically by introducing other functional groups to the highly reactive functional groups of starch. Some of the modifications result in an altered material compared to native starch (e.g. hydrophobic, amorphous, and melt-processible) that can be used for industrial bioplastic applications61,150–152. In other cases, the modified starch exhibits good surface properties to be used as a co-blend component of other renewable polymers. Modified starch-based biopolymers find industrial use in the food industry for food packaging, in agricultural industry as mulch films and controlled release materials for fertilizers, as a carrier for biocides153–155 and other active agents, as well as in the biomedical industry for drug delivery28,156. 4.1. Modified starch films and sheets Starch films and sheets commonly used for packaging and agricultural mulch are a substitute for those made from synthetic polymers, thereby, managing the pollution problem resulting from the use of non-degradable polymers. Furthermore, the wide variety of starches along with their reactive nature allow physical and chemical derivatization, enable the production of films and sheets tailored to specific end-use. The common drawback, with the use of unmodified starch for plastic films and sheets, is their poor mechanical properties (brittle and inflexible). To overcome this, external plasticizers are incorporated during the thermoplastic process. Plasticizers are generally low molecular weight polymer additives used to enhance processability, and flexibility of polymers by reducing the glass transition temperature and other second-order transition
51
temperatures63,157. However, most thermoplastic starch produced by using external plasticizers (e.g. glycerol) suffers from poor moisture tolerance behavior, too rapid degradation, and aging because of leaching out of the plasticizers. An alternative to the use of plasticizers chemically modified starch can be utilized to make films and sheets. Several researchers have reported the use of modified starch to obtain films and sheets. Zhou et al90 studied the surface crosslinking of corn starch sheets through ultra-violet irradiation using sodium benzoate as a photosensitizer. The results of this study from contact angle measurements and moisture absorption evaluation revealed the improvement in water resistance. The film-forming capacity of acetylated starch was studied by Lopez et al
63
with various
concentrations of glycerol plasticizer to improve their mechanical properties. They showed that a 5 % w/w concentration of acetylated starch and 1.5 w/w glycerol is required to reduce the water vapor permeability and flexibility by 87 % and 34 %, respectively. Fang et al158 modified native potato and high amylose (HA) potato starches using lauroyl chloride at various levels. The results of the study displayed that all modifications improved the hydrophobicity of starch irrespective of the DS. The modified starch with DS of 1.5-3 can be extruded and processed like traditional synthetic polymers to produce (a) strong films with limited extensibility from the native potato starch, or (b) a low strength with good elongation from the high amylose modified starch; (c) a 50:50 high amylose and native potato starch mixture with balanced properties. Lafargue et al159 prepared and characterized films from hydropropylated pea starch/Kcarrageenan mixture to study the mechanical and calorimetric properties of the starch films. They reported a dramatic increase in the rheology of the mixture, however, the modification had no effect on film properties. Films that were based on silicon dioxide and poly(vinyl acetate) (PVA) modified starch were studied by Xiong et al160. Characteristics of the films produced as such
52
exhibited an increase in the tensile strength, breaking elongation and transmittance by 79.4 %, 18 %, and 15 %, respectively accompanied by a decrease in crystallinity from 41.2 to 32.9 % and water absorption by 70 %. This was due to the formation of intermolecular hydrogen bonds the starch-PVA composite, and strong chemical bonds formed in the nano SiO2/starch/PVA blend, thereby forming a network structure, which increased the water resistance and mechanical properties. Table 7 presents a summary of the characteristics of starch films reported in recent researches with important highlights.
53
Table 7: Modified starch-based films and their characteristics Starch Source Corn
Modification Acetylated
Potato (native and HA)
Acylation with lauroyl chloride
Pea
Acid hydrolyzed hydroxypropylated / k-carrageenan Citric acid modified/ carboxymethyl cellulose (CMC)
Corn
Corn
Wheat
Nano silicon dioxide and PVA
Lipid (raspseed oil)
Plasticizer Glycerol -
-
Glycerol
-
-
-
Glycerol
-
Results 87 % reduction in water vapor permeability 34 % increase in flexibility Improved processability of starch Improved water resistance Strong film, poor extensibility with modified native starch Low strength, good elongation films with HA modified starch Stronger films, ease of processing and improved tensile strength with a 50:50 combination of native and HA starches Increased rheology of mixture No effect on film properties Increase in ultimate tensile strength by 59 % without any decrease in strain. Reduced water vapor permeability with increased CMC content. Improved water resistance of starch films. Improved optical properties with increased CMC content Improved mechanical, transmittance and resistance properties by 79.4 %, 18 % and 15 %, respectively. Reduced water vapor by 70 %. Decrease in crystallinity Increased resistance to water vapor and oxygen permeability. Improved surface hydrophobicity. No much change in barrier efficiency. Decreased film transparency
Ref. 63
158
159
161
160
162
54
4.2. Modified starch-polymer blend systems Synthetic polymers, although non-biodegradable, possess desirable thermoplastic properties compared to starch. Blending biodegradable and non-biodegradable polymers and making plastics from them can lead to partially degradable plastics. Starch is also blended with other biopolymers of desired properties to improve the properties of the overall polymer blend and biodegradability. One drawback, however, is the inhomogeneity and incompatibility between hydrophilic starch granules and hydrophobic polymers in blends, requiring the reduction of the interfacial tension to improve the properties of blends. This is typically achieved by modifying native starch before blending or the addition of compatibilizers which improve interfacial adhesion between polymers. Modifying the starch prior to blending creates a physically rough surface on the starch granule, allowing for better adhesion and anchoring of the blend163. More importantly, targeted modifications can reduce the surface energy of starch making it more compatible with non-polar polymers. The mechanical, thermal and morphological properties of blends of low-density polyethylene (LDPE)/starch and LDPE starch phthalate (Stath) was studied by Thakore et al163. The Surface morphology of LDPE/stath blend showed a well-distributed and firmly rooted stath particles in the blend while the LDPE/starch blend showed loose starch granules. This has resulted in better tensile properties of the LDPE/Stath blends as compared to the LDPE/starch blend due to the improvement of compatibility. Researchers have been interested in starch blended with polylactic acid (PLA) because PLA is biodegradable and sourced from plants, hence it is renewable. One group investigated a blend of PLA with glycerol plasticized starch (TPS)81. Morphology study of PLA/TPS blend exhibited
55
rough particle dispersion of component blends with sizes ranging from 5 to 30 µm. The study further carried out interfacial modification by grafting maleic anhydride (MA) on PLA and made a blend with TPS. Such modification caused fine particle size and homogenous blend, which resulted in improved ductility of the modified blend. The observed elongation was between 100200 % for the modified blend, as compared to only 5-20 % elongation for the non-modified blend systems81. Xiong et al164 carried out a similar blending study of PLA with starch with or without tung oil anhydride plasticizer. The PLA-starch blends showed a void between the PLA matrix and starch granules indicating poor compatibility. Contrarily, the plasticized PLA starch blend showed fewer voids indicative of improved compatibility. While the addition of starch to PLA made it more brittle exhibiting a poor elongation (6 %) and low impact strength, the addition of 5 wt % tung oil anhydride plasticizer to the blend significantly enhanced the elasticity (17 %) and impact strength as compared to the baseline PLA. The use of propionic anhydride-modified starch as a co-blend component of polyurethane was studied by Santayanon et al165, and a comparison of properties of the modified starch blend with native starch blended polyurethanes was reported. An improved interfacial adhesion was noted by using the modified blend from an SEM micrograph. The modified starch samples were remarkably well dispersed and embedded in the polyurethane matrix as opposed to the native starch the displayed poor interfacial adhesion in the polyurethane matrices. A three-month biodegradability study carried out in soil in these blend systems revealed increased weight loss with an increase in the native starch content of the blend. The modified starch-based blend, on the other hand, showed little or no weight loss (3 %) for the duration of the study. This indicates that the moisture resistance and compatibility of the blend were improved at the expense of the rate of biodegradation.
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Berruezo et al166 characterized polystyrene (PS)/TPS blends for packaging applications and reported a 48 and 62 % decrease in the young modulus and tensile strength accompanied by a 62 % increase in elongation at break for a 50:50 TPS/PS blend compared to neat PS.
The
biodegradability test revealed an increase in the weight loss of the blend with increasing starch concentration. Table 8 summarizes starch - polymer blends systems reported in recent literature.
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Table 8. Characteristics of starch polymer blends for industrial polymer applications. Modification Type Esterified starch
Polymer LDPE
TPS
Maleic anhydride grafted PLA
-
Propionic anhydride-modified starch Tung anhydridemodified starch TPS
Polyurethane
-
PLA PS
TPS
Thermoplastic chitosan (TPC)
TPS
Poly Propylene (PP)
TPS
PVA/Chitosan
Starch
Ethylene and vinyl alcohol copolymer, Cellulose acetate, Poly- ε-caprolactone Anhydride functionalized i) Poly- ε-caprolactone
TPS
-
-
Results Improved mechanical, thermal and morphological properties. No significant difference in melting temperature. Greater biodegradation of modified starch blends compared with normal starch blends Improved interfacial adhesion. Elongation at break of 100 to 200 % range compared to 5-20 % for starch and pure PLA Improved interfacial adhesion. Slower biodegradation rate than starch. No significant effect in tensile toughness Improved tensile and impact properties. A downward shift in thermal properties Decreased young modulus and tensile strength, increased elongation at break Increased biodegradation rate Decrease in tensile strength and stiffness Good thermal stability Improved extensibility Temperature-sensitive flow behavior compared to PP. Decreased stress and strain at break with increased TPS Small patches of individual components on microscopic scale. No new functionality created. Increased hydrophobicity Improved water resistance by 15 %. Reduced degradation rate. Improved mechanical properties. Comparable tensile strength of blend with polyesters at up to 70 % starch content.
Ref. 163
167
165
164
168
169
170
118
171
172
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Maleated TPS (MTPS)
ii)Polybutylene succinate iii) butanediol-adipateterephthalate copolymer Plasticized PLA
-
50 % reduction in elongation at 10% starch content in the blend.
-
Increased tensile strength and modulus with increased starch content, blending temperature and time. Decreased toughness and elongation values. Superior mechanical properties of MTPS/PLA blends compared to TPS/PLA blends
173
Increase in young modulus Decreased yield stress, tensile strength, and elongation at maximum strength. Mechanical properties of blends not suitable for disposable packaging Voids and poor interfacial tension in uncompatibilized blends, hence poor mechanical properties. PP/TPS/C14 showed the highest impact energy
174
-
TPS
Isotactic polypropylene (iPP)
-
TPS
PP, C14-C18 Compatibilizers
-
175
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4.3. Biodegradation of modified starch-based polymers Biodegradation is the disintegration of polymer chains by the activities of microorganisms to yield carbon dioxide and water in aerobic conditions, and methane and carbon dioxide in anaerobic conditions. Native starch by itself is completely biodegradable. Completely biodegradable composites of starch can be obtained from blending starch with other biopolymers such as cellulose or blending with other degradable polymers like aliphatic polyesters and PVA28.
Although recent environmental concerns have led to an increased interest in
biodegradable resources for commercial products, the stability of the product is important as well; hence, the rate of biodegradation should be controlled176. The general mechanism of starch degradation by enzymatic hydrolysis is the diffusion of enzymes and binding of the same to the starch substrate, followed by bond cleavage42. Therefore, biodegradation is dependent on the chemical structure of the polymer, moisture content, surface area, pH, porosity, crystallinity, presence and type of microorganisms in the environment, oxygen content, wettability, etc., of the material177. Therefore, altering the chemical structure of starch by modification or blending it with other materials is expected to change its biodegradability. Research has reported the effect of modification on starch scission. Olivera et al178 blended PS with starch and evaluated the biodegradability in soil. Their results revealed that the rate of biodegradation of the blend was a function of the starch content on the blend and incubation time.. This is expected as the starch is the only degradable component in the blend system. Thakore et al163 esterified starch with phthalate and blended with LDPE and compared the biodegradability with native starch/LDPE blend. From soil burial tests experiments results, they reported reduced biodegradation rate with esterification of native starch, revealed by a slower degradation of modified starch/LDPE blend compared to the unmodified starch/LDPE 60
blend. The effect of propionic anhydride as an esterifying agent on biodegradation of starch was studied by Santayanon
165
. The results from soil burial test showed a slower degradation of
esterified starch than unmodified starch, which was attributed to the reduced hydrophilicity in modified starch leading to slower hydrolysis of the chemical bonds. From all reviewed literature, it is reported that chemical modification of starch deters the rate of degradation, which can be correlated to the degree of substitution of the functional groups on the starch molecule. Rivard et al studied the anaerobic degradation of acetylated starches with DS ranging between 0.3 to 2.4 for 98 days incubation period. The authors reported a drastic reduction in degradation levels for samples with DS levels between 1.2 and 1.7. Above DS of 1.7, the acetylated starches were not biodegradable179. The resistance of acetylated starches to degradation could result from poor wetting, and therefore limited microbe/ polymer surface contact resulting from the increased hydrophobicity of acetylated starches, or steric hindrance from the attached functional groups which hinders biocidal activity179. 5. Outlooks, prospects, and conclusions The environmental concerns from landfilling, leaching of chemicals into the soil and water table caused by non-degradable polymers used in commercial and industrial applications have intensified the interest for the development of biodegradable polymers. Starch is proposed as a front runner potential candidate due to its many attractive properties and benefits it has to offer. The drawbacks of high degradation rate, water sensitivity, and poor mechanical properties can be minimized by modification. Physical modification combines temperature, moisture, shear and irradiation to improve its water solubility and reduce granular size. Chemical modification introduces functional groups to the reactive hydroxyl group in starch without altering the
61
morphology. Blending starch with other polymers has been shown to synergize the properties of these polymers to obtain desirable materials suited to specific end-use. Through the tuning of starch, modification degrees and levels, the physicochemical properties of the final product can likewise be varied. For instance, single-use food packages and plastic bags can be designed to have a high degradation rate while other materials with expected longer expected life cycles can be designed for a low degradation rate. With a focus on sustainability, renewability and bioresources utilization, starch is deemed as one of the leading and most desirable materials for the design and development of polymeric materials in functional and commodity applications. Likewise, the modification of starch has found applications beyond the bioplastics industry, as the applications have extended to use in the medical industries as sutures, pharmaceutical industries as a drug carrier, scaffolds for tissue engineering and even the metallurgical industries for porous media manufacturing.
62
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Conflicts of interest statement
There is no conflicts of interest to disclose.