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Chitosan: Emergence as potent candidate for green adhesive market
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Chitosan: Emergence as potent candidate for green adhesive market Anil Kumar Patel ∗ Clermont Université, Université Blaise Pascal, Institut Pascal UMR CNRS 6602, Polytech’Clermont- Ferrand, 24 Avenue des Landais, BP 20206, Aubière cedex 63174, France
a r t i c l e
i n f o
Article history: Received 27 November 2014 Received in revised form 12 January 2015 Accepted 21 January 2015 Available online xxx Keywords: Chitosan Adhesive Absorption Glucose Glycerol Proteins
a b s t r a c t Chitosan has gained significant attention during last decades as a potent natural adhesive. Its lower concentrations (<10% w/v) offer competitive strength as synthetic adhesives which would reduce economic constrains of adhesive production. There is increasing commercial interest on chitosan as it possesses biodegradability, biocompatibility, non-toxicity and anti-microbial properties which are of high interest for industries and consumers. Moreover, it has reactive amino side groups, which offer possibilities of chemical modification, increased ionic interactions and graft-reactions etc. Degree of deacetylation (DD) and molecular weight (Mw) is important in bonding mechanism. Most of the synthetic adhesives are comprised of petrochemicals that leave toxic residues such as formaldehyde and volatile organic compounds (VOCs), which are injurious to health and environment. Therefore, development of cost-effective, environmental and health-friendly green adhesives, based on renewable resources is main interest of adhesive industries these days. Rising oil prices are another driving force in research for development of bioadhesives as substitute of synthetic adhesives. This review is focused on current developments of chitosan adhesives for structural and general bonding applications during last decades as well as its current market potential worldwide. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The majority of commercial adhesives such as poly (vinyl acetate), epoxy, phenol-formaldehyde and polyurethane are based on non-renewable and depleting petrochemical resources. Furthermore, numerous adhesives consist of residual toxic chemicals and volatile organic compounds (VOCs). These are costly, harmful for the health of living beings as well as are pollutants of environments. Environmental Protection Agencies from various countries aims to diminish the use of these materials and encourage the development of adhesives from bioresources. Compared to synthetic adhesives, biopolymer based adhesives are expected to offer reduced price, a lesser degree of price volatility and a more favourable environmental footprint. Currently none of the bioadhesives offer very good strength as synthetic adhesive without containing VOCs. Hence research in this area is in great scope today. The effective bioadhesives are still under investigation and their bonding properties are actually an industrial challenge and an important research area [1].
∗ Correspondence address: DBT IOC, Centre for Advanced Bio Energy Research, Indian Oil Corporation Ltd; R and D Centre, Sector 13, Faridabad 121007, Haryana, India. Tel.: +91 888 2060134/129 2294 345. E-mail address: [email protected]
Carbohydrates were well explored as food additives and as potent adhesive candidate for many decades viz. gums, polysaccharides, oligomers and monomers. For carrying adhesive properties, the fundamental structural feature of polysaccharides mainly comprises of high molecular weights and polar-functional groups [2,3]. Polymer based adhesives must have many physical, chemical and mechanical properties to promote adhesion. The most explored polysaccharides for adhesive development have been the starch, pullulan, levan, dextrin, Gum arabic, chitosan etc. [4–6]. Among various polysaccharides the most attractive polysaccharide for adhesive development appeared to be chitosan. It is a polymer of -(1,4)-linked 2-acetamido-2-deoxy-d-glucopyranose (N-acetyl glucosamine) and 2-amino-2-deoxy-d-glucopyranose (glucosamine). Chitosan is a heteropolymer obtained by alkaline deacetylation of chitin at C-2 position (Fig. 1). Chitin is the second most abundant natural polymer in nature after cellulose. It is the main structural element of a large number of invertebrates such as crustaceans (exoskeleton) insects (cuticles) and the cell walls of fungi [7–11]. In contrast, chitosan only occurs naturally in some Mucoraceae fungi [12]. Production of chitosan is economical and environment-friendly [13]. Chitin is highly hydrophobic and is insoluble in water and majority of organic solvents whereas chitosan is water soluble due to amino groups present in it. The amino group has a pKa value of ∼6.5, which leads to a protonation
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Fig. 1. Chemical structure of chitosan showing -(1,4)-glycosidic linkage between (a) 2-acetamido-2-deoxy-d-glucopyranose (GlcNAc) and (b) 2-amino-2-deoxy-dglucopyranose (GlcN) units.
in acidic to neutral solution with a charge density dependent on pH and the % deacetylation value. Chitosan is an expensive polysaccharide approx. 10 USD/kg, but it is used in aqueous solutions less than 10% (w/v), bringing the cost down to less than 1 USD/L of solution, which is acceptable. Chitosan solutions are prepared in 1–2% (v/v) of acetic acid in water which forms viscous solution. These solutions are converted to solid adhesive bonds by the loss of water and solvent or by a chemical setting mechanism. Loss of water takes place either by evaporation or by absorption from porous adherend materials. Additives or plasticizers (citrate, glycerol, glucose) are often used to improve properties of the final adhesive film such as early strength development, moisture resistance and ultimate bond strength [5,6,14]. This review focuses on exploration of different molecular interactions between the components of chitosan adhesives and adherends for obtaining maximum bonding strength. The first part reviews the background information relative to adhesion theories, ideal properties of adhesives and the different tools available to evaluate adhesive performance. The second part focuses on exploration of chitosan adhesives on various bonding applications based on their properties. The last part highlights the bioadhesive advances, bottleneck of chitosan research and current market potential. Bonding strength discussed in this review is actually shear strength of the adhesive formulation. 2. Background information of adhesion Adhesion is a process of attachment between surface of two substances that can be similar or dissimilar in structures. Adhesion requires energy that may come from chemical or physical linkages. These linkages can be reversible when reasonably more energy is applied. The bond strength of an adhesive to an adherend is the sum of various mechanical, physical and chemical forces that overlap and influence one another. The strength of adhesion depends on many factors. There are different adhesion theories proposed viz. Mechanical adhesion: adhesion occurs by the penetration of adhesive in irregular surface, pores or micro-cavities [15]. Adsorption adhesion: adhesion occurs due to inter-atomic or inter-molecular forces between adhesive and adherend molecules, based on associated energy (in kJ/mol) covalent (150–950), ionic (400–800), H-bond (40) and van der Walls (2–15) forces are respectably considered to be strong in above order [16]. Electrostatic adhesion: adhesion occurs due to electrostatic forces at interface having difference in electrical charges, this adhesion is applicable between non-compatible materials such as polymeric and metallic substances [16]. Diffusion adhesion: adhesion particularly effective with polymer chains where one end of the molecule diffuses into
the other material (atoms diffuse from one particle to another). This adhesion can be applied in mutually miscible and compatible polymer [17]. This theory is relevant when temperature would be lower than the ‘Glass transition temperature’ (Tg) [18]. Tg is defined as transition of polymer material between solid and viscous phases [19]. Wetting phenomenon was introduced then thermodynamic theory of adhesion was proposed, in which inter-atomic and intermolecular force was the measurement of contact angle between adhesive and adherends at thermodynamic equilibrium [20]. There are several influencing factors determined which governs the degree of adhesion. Temperature was most governing factor in diffusion adhesion, likewise, pH in electrostatic adhesion, wettability and roughness are important factors in mechanical and diffusion adhesions [21] and void (air bubble) formation in mechanical adhesions [22]. To characterize the adhesion, different types of mechanical tests were used in order to evaluate the strength of adhesives. These test methods were based on loaddependent stresses on adhesive joints viz. tension, compression, cleavage, shear and peel stresses [21]. Likewise, different types of adhesive joints pattern have been discussed for carrying shear loads in which joints were employed for an overlapping adherend’s arrangement. Shear loaded joints are the most popular because most of the bonded configurations induce shear type of failure in the bonded joints [23]. Double-lap joints configuration (Fig 2) has been largely used because it offers low peeling effect and more reliable strength measurement. Moreover, types of failure in the adhesive (cohesive failure) or interface of adherends and adhesive (adhesive failure) or in the adherend (structural failure) must be characterized, which has described in our previous article in details [21]. 3. Salient features of chitosan for adhesion Three properties must be present in a polymer to become an ideal adhesive, Surface tension: Lesser or equal on adherend’s surface to achieve better molecular interaction, viscosity: more viscous to avoid fluidity and penetration ability: higher upon adherend’s surface. Hence the lower the surface tension of an adhesive solution, the easier it will form an adequate wet film over adherend’s surface. If the surface tension of the adhesive is greater than the surface energy of the substrate, it will not spread out and form a film. Metals possess higher surface tension than polymers [24,25]. Surface tension of viscoelastic thermal compressed wood ranged between 28.6 and 35.5 mN/m and 0.5% (w/v) chitosan shows 64 mN/m surface tension initially and decreased with time [26]. Chitosan surface tension decreases with increasing chitosan concentration. A 2% (w/v) chitosan in 1% (v/v) acetate
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Fig. 2. Schematic of specimen preparation: Double lap configuration (offer low peeling effect and accurate bonding assessment). In this arrangement shear stress peaks appear near the free edges of the specimen in the adhesive layer which can provoke the failure of the joint.
solution exhibited 38.59 mN/m surface tension at dispersive end and 1.10 mN/m at polar end [25]. It explains that acid-base Lewis interaction dominates, this property of chitosan favours bonding with material having low surface energy. However, low surface tension and high dispersive part reveals that it can easily spread out upon all type of adherend materials. The chitosan carries polar and H-bonding functional groups, viz. hydroxyls and amines. They show superior adhesion on low and high surface energy wood and metals adherends. The OH, NH3 + , CH2 OH, NHCOCH3 , and NH2 groups also serve as possible sites for chemical modification and cross-linking to improve adhesive properties. These groups also support non-covalent, inter- and intra-chain interactions and promote adhesion. Chitosan is the only cationic polysaccharide due to its positive charges (NH3 + ) at acidic pH (pH < 7). These charges increase retention at the site of application [27]. At acidic pH, positively charged chitosan in wet condition (swelled) interacts more strongly with negative charged surface via electrostatic forces, H-bonds and van der Waal’s forces between glucosamine and hydrated surface of adherend [28]. Among these interactions, H-bonds are most important for bonding strength because it would be numerous which can further be improved with blending of additive. Moreover, other parameters influencing the desirable characteristics of chitosan are its Mw and DD. The Mw of most of the commercially produced chitosan is between 3800 and 20,000 Da. Its adhesive properties can, however, be weakened when both the DD and the Mw are low [29,30]. The DD directly affects the number of protonatable amine groups, it basically determines the polymer properties such as its solubility, hydrophobicity and the ability to interact electrostatically with polyanions [31,32]. Chitosan exhibits higher viscosity which is measure of its resistance against spreading. Its viscosity increases with increasing chitosan concentration but decreases with increasing temperature [33,34]. Chitosan viscosity has also been attributed with their varying molecular weight, it varies from 3.2 to 1080 Pa.s respectively for chitosan Mw 35 and 350 kDa, and such a large range of viscosity is an advantage of this polymer to be used in adhesion applications [35]. It shows shearthinning behavior in rheological studies due to disorientation and
disentanglement of macromolecular chain under influence of shear rate [33]. Thermal property of chitosan does not take part directly into adhesion mechanism but improved thermal property increases the scope of its applications. It has been characterized based on their decomposition temperature and Tg. Decomposition of chitosan film occurs in two stages in thermo-gravimetric analysis [36]. Thermal degradation of chitosan occurs at 250 ◦ C [14,26]. Reported values of Tg varied significantly from 103 to 150 ◦ C due to differences in concentrations, molecular weights, degree of deacetylation and crystallinity [19,37,38].
3.1. Mechanical test of chitosan based composites For an ideal adhesive, higher tensile strength (TS) is desirable. TS denote material’s resistance ability against tensile force. TS of chitosan specimens varied significantly from 7 to 150 MPa and from 4.1 to 117% respectively depending on their Mw and the solvent used for solubilisation [39]. The highest TS (150 MPa) was obtained with 92 kDa from 2% (w/v) chitosan solution in 2% (v/v) acetic acid, whereas the highest elongation (inversely proportional to hardness) at break (117%) was obtained with 37 kDa 2% (w/v) chitosan in 2% (w/v) citric acid solution. These results explained that the ratio of H-bonds between hydroxyl and amino groups are accountable for these properties in chitosan films [26]. The TS of rectangular specimens (40 × 8 mm) of glucose or cellobiose cross-linked LMW chitosan and pure chitosan was determined. The maximum average TS 64.8 MPa was offered by 20% (w/v) glucose cross-linked chitosan which was substantially better than that of the pure chitosan specimens. Improvement in TS with glucose and cellobiose blending was respectively 45 and 30% (w/v) [40]. Chitosan has high cohesive strength, it shows shear modulus value approx. 0.76 GPa [4], During contact in wet condition, Hbonds increased by structural changes from relaxed to extended molecule of 2-fold chitosan helix due to the confinement of the film. After drying, it possesses high quality mechanical properties such as traction and shear resistance and also resistance against water and temperature effect [28]. Chitosan as adhesive material finds major applications in two areas based on its application,
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biomedical adhesive: It was widely described especially for drug delivery and tissue bonding in medical segment but is not focus of current review and natural adhesive, which is widely explored for bonding application on surface of metal, wood, glass, paper etc. 4. Chitosan in bonding applications The interactions between adherends and chitosan are strong at acidic pH at which the charge density of chitosan is high [13]. Different types of formulation have been adopted in various studies. Pure 2–4% (w/v) chitosan solution in acetic acid was the common practice. Moreover, the mechanical strength of chitosan has been improved by blending of plasticizer also. Plasticizers are natural additives blend in chitosan to increase its resistance against applied mechanical constraints. They are widely used to control brittleness of the bond line and to regulate the speed of drying of polysaccharide adhesives. The main plasticizers described in previous studies were polyethylene glycol (PEG) [41,42], glycols, glycerol [5,6] sorbitol, saccharose [43] citrate [5,6] etc. These plasticizers act as hygroscopic agents to decrease the drying time of the adhesive. Apart from this, cross-linking of other polysaccharides with chitosan was also evaluated for bonding application. 4.1. Metal bonding In context of chitosan adhesive application on metal adherends, various eco-friendly polyanionic polysaccharides, acids and plasticizers in single or multiple formulations were attempted using double-lap shear configuration in previous studies [5,6,21]. The adhesive glues the area of two adherend surface is known as lap area. The adopted lap area was 50 × 20 = 1000 mm2 . Pure chitosan was glued on treated aluminum surfaces such as chemically (NaOH and CH3 OH), mechanically (bench pressed) metal file scratched (big deeper groove), sand paper (thin swallow groove) and untreated. Among these treatments chemically treated aluminum adherend was found best based on surface morphology and bonding strength. From various chitosan-based formulations, the maximum shear strength of 40.8 MPa, obtained from 7% (w/v) chitosan (in 2% v/v acetic acid) and 1% (v/v) glycerol (plasticizer) cross-linked polymer on chemically treated Al adherend which was the most significant finding of this study. This bonding strength was equivalent to the bonding strength offered by synthetic adhesive used in industry [5]. Moreover, evaluation of the effect of chitosan concentration (4–9% w/v in 1–2% v/v acetic acid), 6% (w/v) chitosan concentration exhibited best bonding strength (31.4 MPa) whereas further chitosan concentrations were limited with moisture content. Chitosan concentration more than 6% (w/v) in 2% (v/v) acetic acid solutions increased the contact angle of the film which did not support good film formation on metal surface. Increasing chitosan concentration improved the shear strength with increasing moisture content in this study [5], likewise, increase in Mw of chitosan also resulted in stronger adhesion [44]. Both the factors were increased the attachment points due to increase in -NH2 groups which was resulted into better bonding strength. 4.1.1. Cross-linking of chitosan with other polymer for metal bonding Formulation of alginate (ALG) and carrageenan (CRG) in chitosan was tested. These polysaccharides were tested at different concentration ratios with 4% (w/v) chitosan (4:3, 4:4, and 4:5). It was hypothesized that ionic interactions between these acidic polysaccharides and cationic chitosan can increase the bonding strength. But the pKa of alginate lies between 3.38 and 3.65, whereas the pKa of chitosan is ∼6.3 [45,46]. It was resulted low bonding strength
that of the chitosan solution alone due to low degree of crosslinking, it did not homogenize completely and hence did not lead to good adhesion. Likewise, carrageenan was also tested for jellifying properties with chitosan at low pH. The anionic charges of carrageenan are due to sulfate substituents. It has shown higher solubility than that of alginate exhibited at pH 4.5. However, the CS: CRG mix exhibited a poor solubility due to rapid reticulation between them [5]. 4.2. Wood bonding The potential of chitosan as wood adhesive was evaluated using double-lap shear tests. Various lap lengths (20–40 mm) were evaluated and 20 mm lap length was found the best (optimum) compared to the others tested lap lengths in bonding strength assessment. Cohesive mode of failure at lap area was usually set as desire objective for correct assessment of the bonding strength but in this study cohesive strength of chitosan was better than the structural strength of the soft-wood adherend (Pinus pinaster) hence the objective of the assessment therefore structural or cohesive failure at lap area was set to achieve this objective. Three formulations were tried: CS 4%, CS 6%, and CS 6%: gly 1%: cit 5 mmol/L which showed dry bond strengths 4.2, 6.1, and 6.0 MPa respectively with 100% structural wood failure at lap using a 20 mm lap length. The best formulation based on bonding strength obtained was 6% (w/v) of chitosan, 1% (v/v) of glycerol and 5 mmol/L of trisodium citrate dihydrate [6]. The bonding strength of chitosan (Mw 35–350 kDa) and glucose (10–70% w/v) was evaluated on 3-ply plywood (300 × 300 × 1.6 mm) veneer sheets. While curing, it was hot pressed at 130 ◦ C at 1 MPa pressure for 5 min. The bond strength more than 1.7 MPa was obtained with 10% (w/v) glucose addition in low-molecular-weight (LMW) chitosan solution. This formulation also showed 1.1 MPa wet bond strength which was substantial. In contrast, the glucose addition in the high molecular-weight (HMW) chitosan did not exhibit good dry and wet bonding strength. Maillard reaction in above formulation formed brownish melanoidins which occurred between COOH of glucose and NH2 of chitosan that was improved adhesive properties of glucose cross-linked LMW chitosan [14]. 4.2.1. Cross-linking of chitosan with other polymer for wood bonding The structure of chitosan is unique which provides viscoelastic properties and specific interaction with biological substrates. Such type of interactions was not found in other modified polymers. Chitosan under certain circumstances (acidic pH) is a cationic polymer containing NH3 + groups. Thus the chitosan forms polyelectrolyte complex with anionic polymers. Moreover, NH2 groups of chitosan can react covalently with CHO or COOH groups of other polymers. For example, chitosan can crosslink with oxidized starch more as compared to native starch [47]. The adhesion of chitosan to the other polymers is the sum of different types of interactions such as H-bonds, electronic and hydrophobic interactions. Cross-linking of chitosan was attempted with other polysaccharides for bonding application in wood specimens. Primarily, blending of 2–16 g/m2 Konjac-glucomannan (KGM) and 2–16 g/m2 chitosan were tested in veneer plywood preparation (3 ply: 300 × 300 × 1.6 mm). The H-bonding was supported cross-linking between these polysaccharide molecules. The best strength of 2.13 MPa was offered by 8:8 and 8:10 g/m2 CS: KGM blends. Specimens were fabricated at 0.98 MPa by cold-pressing and then hot-pressed at 130 ◦ C. The dry-bond strength increased with increasing chitosan amount in blend. The dry-bond strengths offered by these blends under these conditions exceeded the
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strength offered by CS and KGM alone and also with ureaformaldehyde (UF) resin adhesive and casein glue [14]. The chitosan was cross-linked with modified-lignin (ML) using laccase (copper-containing oxidase enzymes) and bonding strength of the cross-linked CS: ML polymer was tested in wood specimens. The bonding strength obtained from CS: ML was higher than that of the CS alone and similar to lignin blended soy protein and polyethylenimine [48]. Likewise, phenolic compound (PhC) was also cross-linked with chitosan in the presence of laccase and formed CS: PhC (0.48: 10 mM: 60 U) adhesive formulation. The adhesion mechanism of this preparation was similar to the mussel adhesive proteins. The bonding assessment in wood veneer was resulted 1.7 MPa bonding strength. This result did not exhibit any correlation between number of the OH group and the phenolic compound, and also with viscosities and adhesive strength [49]. The chitosan potential was evaluated as adhesive or binder of plant particles for fabricating insulating panels. Sunflower stem particles (6.3 mm size) were used for making insulating panels with ratio chitosan/sunflower of 4.3 and density 150 kg/m3 . Maximum stress at break of this panel was obtained around 2 MPa [50]. Wooden materials comprise very small porosities, but the roughness is more apparent. chitosan solution having low surface tension and approved viscosities which facilitate it to penetrate these asperities [6]. The penetration of the chitosan was tested in wood tissue using microtome technique for deep insight of bonding mechanism. Impregnation degree of rhodamine-labeled 4% (w/v) chitosan in pinewood matrix was superficial. It did not penetrate the cell wall therefore, was not able to reach the cell lumens. It only interacted with the outer surface of the plant cell. It could penetrate the ray cells of the wood matrix up to some extent. The molecular size of the chitosan in this study was approx. 303 kDa with 75% DD [6]. Formulation with high-molecular weight chitosan can elasticate into cell wall, enter into intercellular capillary spaces and expend thus being trapped and unable to enter across the cell wall [51]. Chitosan with low-molecular weight can penetrate wood cell wall and entered into cell lumen [52].
4.3. Glass bonding Chitosan bonding has also been tested on glass slides (lap area 25 × 25 mm). Chitosan alone did not exhibit bonding property on glass material. When chitosan was cross-linked with 3,4 dihydroxyphenylalanine (DOPA) in presence of tyrosinase (50 mg) to form CS: DOPA polymer which exhibited adhesion on glass material. Shear strength of glass slides bonded with CS: DOPA adhesive offered 0.4 MPa which was marginally low compare to the strengths obtained in metal and wood adherends. In the same study chitosan with glutaraldehyde (cross-linker) blend showed 0.3 MPa shear strength under the same conditions [53]. Apparent and peak shear strength obtained in various types of adherends using chitosanbased green adhesives have been summarised in Table 1.
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5. Water resistance property Water resistance property of chitosan-based adhesive formulations has not been found promising. For its improvement, several molecules were explored for cross-linking chitosan based on their structure and types of interactions for bonding application viz. DOPA, glucose, glycerol, citrate, glutaraldehyde, phenolic compounds etc. [5,6,49,53,55,56]. Note that the chemical structures of these compounds had little effect on shear strength and viscosity. However, the cross-linked polymer obtained had not offered significant water resistance property. DOPA-containing proteins were found to be responsible for water resistance property of adhesive synthesized in water by marine animals. Such property was due to cross-linking of intermediate DOPA-derived quinones and primary amines of proteins. Structural application of the bio-based adhesives cannot be realized until these formulations would show water-resistance property. Different blending approaches were adopted to increase such property. In a study, increased glycerol concentration in CS: Gly: Cit (1: 1: 9) formulation led to reduced water absorption upto 44% against 96% weight of pure chitosan. After soaking, the modified film swelled by 11% which was lower than that of the 42% swelling range of pure chitosan [56]. Supporting to above result, formulation CS 6%: Gly 5%: Cit 5 mmol/L gave maximum bond strength of 0.55 MPa as compare to chitosan and chitosan with glycerol formulations in water soaking test. This formulation was protected with oil coat which led to the best wet bond strength of 1.6 MPa. It could retrieve 27% of its dry bond strength [6]. However these approaches are not enough to compete water resistant synthetic adhesives in the market. Water-soluble 2% (w/v) chitosan acetate over the wood particles was converted to water-insoluble chitin by high heat application at 140 ◦ C for 90 min. This chitin was acted as layer of water barrier over the wood particles and reduced significantly the thickness swelling of chitosan-treated panels [57]. Similar effect was observed on wools where anti-shrinking effect of chitosan was recorded [58]. The water resistance property of CS: KGM blend was determined with water immersion test. The bond strength of the blend was increased above 1.2 MPa with increasing amount of the chitosan [14]. Paper industries mainly focus in the improvement of drystrength product properties, utilization of cost effective adhesive formulations and also to improve their environmental foot-print image of their products in the market. In this context, chitosan found to be a good candidate. The potential of chitosan has been evaluated in development of improved bagasse paper, during this process hot water pre-extraction step positively improves surface arrangement and orientation of chemical groups in a fashion that a hydrophobic groups increased in the top of the surface. This arrangement has resulted more water resistance property, however dry-strength agent acts as a protecting film or glaze on the surfaces of bagasse paper sheets [59].
6. Biological properties 4.4. Other applications of chitosan Apart from the bonding applications on surface of metal, wood and glass adherend, the use of chitosan biopolymer has emerged as new generation epoxy: chitosan cement slurry. The newly formed cement slurry was able to conserve its crystalline property after long-term contact with HCl aqueous solution whereas standard cement exhibited drastic decrease in crystalline pattern after reaction with HCl. Improved property of modified epoxy: chitosan cement slurry offered its increased application as environmentalfriendly acidizing polymer in oil wells [54].
The desirable biological features of chitosan adhesives are biodegradability, biocompatibility, non-toxicity and anti-microbial properties which are of industrial interest. Chitosan and its derivatives offer extra advantage among polysaccharides which increase its selection in adhesive applications. It exhibited antibacterial properties due to their polycationic nature at lower pH ranging pKa 6.3–6.5 [60,61]. One hand, it offers significant mechanical strength and another hand the advantage of wood protection [52]. Chitosan impregnated wood material exhibits enhanced life because chitosan absorption into wood tissues increases its protection against
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Table 1 Apparent shear strength obtained in types of adherends using chitosan based green adhesives. Specimen failure
Reference
2.13 (0.39) 2.13 (0.42) 1.7 (0.25)
20 50 35
[14] [14] [40]
1.7
–
[49]
124 × 18 × 4 A: 20 × 18 124 × 18 × 4 A: 20 × 18 124 × 18 × 4 A: 20 × 18
4.2 (0.31)
100
[6]
6.1(0.22)
100
[6]
6.0 (0.20)
100
[6]
40.8 (0.51)a
[5]
31.4 (0.15)a
[5]
Adhesive formulation
Test standard
Specimen type
Specimen dimension (mm)
(I) Wood adherend CS CS: KGM CS:Glc
JIS K6851 JIS K6851 JIS K6251
(3ply) Shorea sp. (3ply) Shorea sp. (3ply) Shorea sp.
300 × 300 × 1.6 300 × 300 × 1.6 300 × 300 × 1.6 A: 40 × 8 170 × 25 × 1 A: 25 × 25
CS: PhC: laccase (0.48%: 10 mM: 60U) CS 4%
ASTM-D-906 (1961)
(3ply) Maple veneer
ASTM D 5573–99
Pinus pinaster
CS 6%
ASTM D 5573–99
Pinus pinaster
CS:Gly:Cit (6%:1%:5 mM)
ASTM D 5573–99
Pinus pinaster
(II) Metal adherend CS: Gly (7%: 1%)
–
CS (6%)
–
Aluminum alloy 2014 Aluminum alloy 2014 Aluminum alloy 2014 Aluminum alloy 2014 Aluminum alloy 2014
150 × 20 × 2 A: 50 × 20 150 × 20 × 2 A: 50 × 20 150 × 20 × 2 A: 50 × 20 150 × 20 × 2 A: 50 × 20 150 × 20 × 2 A: 50 × 20
glass slide
A: 25 × 25
CS (4%)
–
CS: Cit (6%: 3 mM)
–
CS: Glut (4%: 0.010%)
–
(III) Glass adherend CS: DOPA: tyrosinase
–
Apparent shear strength (MPa)
a
28.7(1.1)
[5]
28.4(2.1)a
[5]
21.2(3.9)a
[5]
0.41
–
[53]
CS: chitosan; Gly: glycerol; Cit: citrate; Glc: glucose; DOPA: dihydroxyphenylalanine; KGM: Konjac-glucomannan; PhC: phenolic compound; Glut; glutaraldehyde. a peak shear strength
deteriorating microorganisms. It is noticeable that chitosan is not only fills cell lumens and cavities but also impregnates cell walls establishing a tight seal against invading microorganisms. Thus, wood protection is based on chemical properties of adhesive as well as physical protection of cell walls. Chitosan is naturally-occurring compound showing potential in agriculture for controlling plant diseases. The molecule exhibit toxicity and inhibit fungal growth and development. It was reported to be active against viruses, bacteria and other pests [62]. Oil coating and painting approaches are useful to reduce moisture intake in adhesive which ultimately lead to reduce the incidence of microbial attack in bonded line. Biodegradability is another advantage of this polymer material. After completion of life cycle, each product undergoes biodegradation in the nature and converted into simplest elemental form. In this context, chitosan is a biodegradable polymer, does not pollute environment as synthetic adhesive which is desirable. Biocompatibility defines degree of polymer acceptability by the living system. Chitosan offers such property and finds applications largely in biomedical area. 7. Bottleneck and market potential Chitosan offer competitive dry bonding strength with synthetic adhesive whereas wet bonding strength is still lower which reduces its wide spectrum applications. The need for robust bioadhesives in wide niche area is expected which can completely replace the utilization of toxic petrochemical based commercial synthetic adhesives. Water resistance and thermal properties are two important areas to be fixed at this stage of chitosan adhesive development to increase its scope for wide spectrum commercial exploitation by various industries. Thermal degradation of chitosan occurs at 250 ◦ C whereas Tg has reported up to 150 ◦ C. Polyamide blending has been recently attempted to improve its thermal property marginally [63]. More research is needed to improve these properties by blending other ionic compounds which may offer wider
application range and niche area in the market. Likewise, water resistance property has been partially improved yet by approaching temperature ramping at high temperature during curing process. Moreover, coating practices which are required to be improved further by seeking various formulations for removing hydrophilic property of polymer and encourage hydrophobic nature. Crosslinking of chitosan with other additives such as polysaccharide, proteins and chemicals must be explored for removal of non-linked NH2 groups which were left after bonding. These free groups are largely accountable for hydrophilic interactions of the polymer. Apart from this, low cost stable coating material should be identified which can protect adhesive material from reaction with water, weathering and moisture. Another aspect of high interest is the cost of adhesive for bonding application, although biopolymer based adhesive formulation offers lower price than that of the commercially established synthetic adhesives. Based on the application quantity, study showed solid based spread rate (16 g/m2 ) of chitosan was lower than that of the UF-resin adhesive (74 g/m2 ), casein glue (67 g/m2 ) and soyabean glue (50 g/m2 ) [14]. It can be economized further by improving its mechanical strengths by blending of additives or plasticizer. If the blending practices offer improved bioadhesive properties that will drive its application at higher application segment and help to reduce prize volatility. This can also help to replace important and inflated synthetic adhesives in the market. Adhesives from bioresources offer huge market potential worldwide. Green adhesives and sealants markets are projected to reach nearly $1.24 billion in 2017, according to a new study done by IntertechPira in association with ASC and FEICA [64]. 8. Conclusion Development of potential natural adhesives is in high demand from consumers worldwide due to health and environmental problems emerged after utilization of synthetic adhesives. Chitosan has
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been emerged as potential candidate of natural adhesive based of its properties. It is unique cationic heteropolymer among polysaccharides which offer more ionic interactions upon negatively charged wood, paper and metal adherends surface due to presence of NH2 groups. However, higher degree of deacetylation and higher molecular weight support for improved interaction and dense polymeric network formation. Additive blending and cross-linking of plasticizer in chitosan solution increased its bonding strength but thermal property was marginally improved. Reverse acetylation of chitosan into chitin from temperature ramping improved water resistance property partially. More research is needed to improve its water resistance and thermal properties to increase its scope worldwide for more commercial exploitation.
Acknowledgements The author thanks University of Auvergne for post doctoral fellowship and Dr. Reeta R. Singhania for help in this study.
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Chitosan: Emergence as potent candidate for green adhesive market Anil Kumar Patel ∗ Clermont Université, Université Blaise Pascal, Institut Pascal UMR CNRS 6602, Polytech’Clermont- Ferrand, 24 Avenue des Landais, BP 20206, Aubière cedex 63174, France
a r t i c l e
i n f o
Article history: Received 27 November 2014 Received in revised form 12 January 2015 Accepted 21 January 2015 Available online xxx Keywords: Chitosan Adhesive Absorption Glucose Glycerol Proteins
a b s t r a c t Chitosan has gained significant attention during last decades as a potent natural adhesive. Its lower concentrations (<10% w/v) offer competitive strength as synthetic adhesives which would reduce economic constrains of adhesive production. There is increasing commercial interest on chitosan as it possesses biodegradability, biocompatibility, non-toxicity and anti-microbial properties which are of high interest for industries and consumers. Moreover, it has reactive amino side groups, which offer possibilities of chemical modification, increased ionic interactions and graft-reactions etc. Degree of deacetylation (DD) and molecular weight (Mw) is important in bonding mechanism. Most of the synthetic adhesives are comprised of petrochemicals that leave toxic residues such as formaldehyde and volatile organic compounds (VOCs), which are injurious to health and environment. Therefore, development of cost-effective, environmental and health-friendly green adhesives, based on renewable resources is main interest of adhesive industries these days. Rising oil prices are another driving force in research for development of bioadhesives as substitute of synthetic adhesives. This review is focused on current developments of chitosan adhesives for structural and general bonding applications during last decades as well as its current market potential worldwide. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The majority of commercial adhesives such as poly (vinyl acetate), epoxy, phenol-formaldehyde and polyurethane are based on non-renewable and depleting petrochemical resources. Furthermore, numerous adhesives consist of residual toxic chemicals and volatile organic compounds (VOCs). These are costly, harmful for the health of living beings as well as are pollutants of environments. Environmental Protection Agencies from various countries aims to diminish the use of these materials and encourage the development of adhesives from bioresources. Compared to synthetic adhesives, biopolymer based adhesives are expected to offer reduced price, a lesser degree of price volatility and a more favourable environmental footprint. Currently none of the bioadhesives offer very good strength as synthetic adhesive without containing VOCs. Hence research in this area is in great scope today. The effective bioadhesives are still under investigation and their bonding properties are actually an industrial challenge and an important research area [1].
∗ Correspondence address: DBT IOC, Centre for Advanced Bio Energy Research, Indian Oil Corporation Ltd; R and D Centre, Sector 13, Faridabad 121007, Haryana, India. Tel.: +91 888 2060134/129 2294 345. E-mail address: [email protected]
Carbohydrates were well explored as food additives and as potent adhesive candidate for many decades viz. gums, polysaccharides, oligomers and monomers. For carrying adhesive properties, the fundamental structural feature of polysaccharides mainly comprises of high molecular weights and polar-functional groups [2,3]. Polymer based adhesives must have many physical, chemical and mechanical properties to promote adhesion. The most explored polysaccharides for adhesive development have been the starch, pullulan, levan, dextrin, Gum arabic, chitosan etc. [4–6]. Among various polysaccharides the most attractive polysaccharide for adhesive development appeared to be chitosan. It is a polymer of -(1,4)-linked 2-acetamido-2-deoxy-d-glucopyranose (N-acetyl glucosamine) and 2-amino-2-deoxy-d-glucopyranose (glucosamine). Chitosan is a heteropolymer obtained by alkaline deacetylation of chitin at C-2 position (Fig. 1). Chitin is the second most abundant natural polymer in nature after cellulose. It is the main structural element of a large number of invertebrates such as crustaceans (exoskeleton) insects (cuticles) and the cell walls of fungi [7–11]. In contrast, chitosan only occurs naturally in some Mucoraceae fungi [12]. Production of chitosan is economical and environment-friendly [13]. Chitin is highly hydrophobic and is insoluble in water and majority of organic solvents whereas chitosan is water soluble due to amino groups present in it. The amino group has a pKa value of ∼6.5, which leads to a protonation
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Fig. 1. Chemical structure of chitosan showing -(1,4)-glycosidic linkage between (a) 2-acetamido-2-deoxy-d-glucopyranose (GlcNAc) and (b) 2-amino-2-deoxy-dglucopyranose (GlcN) units.
in acidic to neutral solution with a charge density dependent on pH and the % deacetylation value. Chitosan is an expensive polysaccharide approx. 10 USD/kg, but it is used in aqueous solutions less than 10% (w/v), bringing the cost down to less than 1 USD/L of solution, which is acceptable. Chitosan solutions are prepared in 1–2% (v/v) of acetic acid in water which forms viscous solution. These solutions are converted to solid adhesive bonds by the loss of water and solvent or by a chemical setting mechanism. Loss of water takes place either by evaporation or by absorption from porous adherend materials. Additives or plasticizers (citrate, glycerol, glucose) are often used to improve properties of the final adhesive film such as early strength development, moisture resistance and ultimate bond strength [5,6,14]. This review focuses on exploration of different molecular interactions between the components of chitosan adhesives and adherends for obtaining maximum bonding strength. The first part reviews the background information relative to adhesion theories, ideal properties of adhesives and the different tools available to evaluate adhesive performance. The second part focuses on exploration of chitosan adhesives on various bonding applications based on their properties. The last part highlights the bioadhesive advances, bottleneck of chitosan research and current market potential. Bonding strength discussed in this review is actually shear strength of the adhesive formulation. 2. Background information of adhesion Adhesion is a process of attachment between surface of two substances that can be similar or dissimilar in structures. Adhesion requires energy that may come from chemical or physical linkages. These linkages can be reversible when reasonably more energy is applied. The bond strength of an adhesive to an adherend is the sum of various mechanical, physical and chemical forces that overlap and influence one another. The strength of adhesion depends on many factors. There are different adhesion theories proposed viz. Mechanical adhesion: adhesion occurs by the penetration of adhesive in irregular surface, pores or micro-cavities [15]. Adsorption adhesion: adhesion occurs due to inter-atomic or inter-molecular forces between adhesive and adherend molecules, based on associated energy (in kJ/mol) covalent (150–950), ionic (400–800), H-bond (40) and van der Walls (2–15) forces are respectably considered to be strong in above order [16]. Electrostatic adhesion: adhesion occurs due to electrostatic forces at interface having difference in electrical charges, this adhesion is applicable between non-compatible materials such as polymeric and metallic substances [16]. Diffusion adhesion: adhesion particularly effective with polymer chains where one end of the molecule diffuses into
the other material (atoms diffuse from one particle to another). This adhesion can be applied in mutually miscible and compatible polymer [17]. This theory is relevant when temperature would be lower than the ‘Glass transition temperature’ (Tg) [18]. Tg is defined as transition of polymer material between solid and viscous phases [19]. Wetting phenomenon was introduced then thermodynamic theory of adhesion was proposed, in which inter-atomic and intermolecular force was the measurement of contact angle between adhesive and adherends at thermodynamic equilibrium [20]. There are several influencing factors determined which governs the degree of adhesion. Temperature was most governing factor in diffusion adhesion, likewise, pH in electrostatic adhesion, wettability and roughness are important factors in mechanical and diffusion adhesions [21] and void (air bubble) formation in mechanical adhesions [22]. To characterize the adhesion, different types of mechanical tests were used in order to evaluate the strength of adhesives. These test methods were based on loaddependent stresses on adhesive joints viz. tension, compression, cleavage, shear and peel stresses [21]. Likewise, different types of adhesive joints pattern have been discussed for carrying shear loads in which joints were employed for an overlapping adherend’s arrangement. Shear loaded joints are the most popular because most of the bonded configurations induce shear type of failure in the bonded joints [23]. Double-lap joints configuration (Fig 2) has been largely used because it offers low peeling effect and more reliable strength measurement. Moreover, types of failure in the adhesive (cohesive failure) or interface of adherends and adhesive (adhesive failure) or in the adherend (structural failure) must be characterized, which has described in our previous article in details [21]. 3. Salient features of chitosan for adhesion Three properties must be present in a polymer to become an ideal adhesive, Surface tension: Lesser or equal on adherend’s surface to achieve better molecular interaction, viscosity: more viscous to avoid fluidity and penetration ability: higher upon adherend’s surface. Hence the lower the surface tension of an adhesive solution, the easier it will form an adequate wet film over adherend’s surface. If the surface tension of the adhesive is greater than the surface energy of the substrate, it will not spread out and form a film. Metals possess higher surface tension than polymers [24,25]. Surface tension of viscoelastic thermal compressed wood ranged between 28.6 and 35.5 mN/m and 0.5% (w/v) chitosan shows 64 mN/m surface tension initially and decreased with time [26]. Chitosan surface tension decreases with increasing chitosan concentration. A 2% (w/v) chitosan in 1% (v/v) acetate
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Fig. 2. Schematic of specimen preparation: Double lap configuration (offer low peeling effect and accurate bonding assessment). In this arrangement shear stress peaks appear near the free edges of the specimen in the adhesive layer which can provoke the failure of the joint.
solution exhibited 38.59 mN/m surface tension at dispersive end and 1.10 mN/m at polar end [25]. It explains that acid-base Lewis interaction dominates, this property of chitosan favours bonding with material having low surface energy. However, low surface tension and high dispersive part reveals that it can easily spread out upon all type of adherend materials. The chitosan carries polar and H-bonding functional groups, viz. hydroxyls and amines. They show superior adhesion on low and high surface energy wood and metals adherends. The OH, NH3 + , CH2 OH, NHCOCH3 , and NH2 groups also serve as possible sites for chemical modification and cross-linking to improve adhesive properties. These groups also support non-covalent, inter- and intra-chain interactions and promote adhesion. Chitosan is the only cationic polysaccharide due to its positive charges (NH3 + ) at acidic pH (pH < 7). These charges increase retention at the site of application [27]. At acidic pH, positively charged chitosan in wet condition (swelled) interacts more strongly with negative charged surface via electrostatic forces, H-bonds and van der Waal’s forces between glucosamine and hydrated surface of adherend [28]. Among these interactions, H-bonds are most important for bonding strength because it would be numerous which can further be improved with blending of additive. Moreover, other parameters influencing the desirable characteristics of chitosan are its Mw and DD. The Mw of most of the commercially produced chitosan is between 3800 and 20,000 Da. Its adhesive properties can, however, be weakened when both the DD and the Mw are low [29,30]. The DD directly affects the number of protonatable amine groups, it basically determines the polymer properties such as its solubility, hydrophobicity and the ability to interact electrostatically with polyanions [31,32]. Chitosan exhibits higher viscosity which is measure of its resistance against spreading. Its viscosity increases with increasing chitosan concentration but decreases with increasing temperature [33,34]. Chitosan viscosity has also been attributed with their varying molecular weight, it varies from 3.2 to 1080 Pa.s respectively for chitosan Mw 35 and 350 kDa, and such a large range of viscosity is an advantage of this polymer to be used in adhesion applications [35]. It shows shearthinning behavior in rheological studies due to disorientation and
disentanglement of macromolecular chain under influence of shear rate [33]. Thermal property of chitosan does not take part directly into adhesion mechanism but improved thermal property increases the scope of its applications. It has been characterized based on their decomposition temperature and Tg. Decomposition of chitosan film occurs in two stages in thermo-gravimetric analysis [36]. Thermal degradation of chitosan occurs at 250 ◦ C [14,26]. Reported values of Tg varied significantly from 103 to 150 ◦ C due to differences in concentrations, molecular weights, degree of deacetylation and crystallinity [19,37,38].
3.1. Mechanical test of chitosan based composites For an ideal adhesive, higher tensile strength (TS) is desirable. TS denote material’s resistance ability against tensile force. TS of chitosan specimens varied significantly from 7 to 150 MPa and from 4.1 to 117% respectively depending on their Mw and the solvent used for solubilisation [39]. The highest TS (150 MPa) was obtained with 92 kDa from 2% (w/v) chitosan solution in 2% (v/v) acetic acid, whereas the highest elongation (inversely proportional to hardness) at break (117%) was obtained with 37 kDa 2% (w/v) chitosan in 2% (w/v) citric acid solution. These results explained that the ratio of H-bonds between hydroxyl and amino groups are accountable for these properties in chitosan films [26]. The TS of rectangular specimens (40 × 8 mm) of glucose or cellobiose cross-linked LMW chitosan and pure chitosan was determined. The maximum average TS 64.8 MPa was offered by 20% (w/v) glucose cross-linked chitosan which was substantially better than that of the pure chitosan specimens. Improvement in TS with glucose and cellobiose blending was respectively 45 and 30% (w/v) [40]. Chitosan has high cohesive strength, it shows shear modulus value approx. 0.76 GPa [4], During contact in wet condition, Hbonds increased by structural changes from relaxed to extended molecule of 2-fold chitosan helix due to the confinement of the film. After drying, it possesses high quality mechanical properties such as traction and shear resistance and also resistance against water and temperature effect [28]. Chitosan as adhesive material finds major applications in two areas based on its application,
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biomedical adhesive: It was widely described especially for drug delivery and tissue bonding in medical segment but is not focus of current review and natural adhesive, which is widely explored for bonding application on surface of metal, wood, glass, paper etc. 4. Chitosan in bonding applications The interactions between adherends and chitosan are strong at acidic pH at which the charge density of chitosan is high [13]. Different types of formulation have been adopted in various studies. Pure 2–4% (w/v) chitosan solution in acetic acid was the common practice. Moreover, the mechanical strength of chitosan has been improved by blending of plasticizer also. Plasticizers are natural additives blend in chitosan to increase its resistance against applied mechanical constraints. They are widely used to control brittleness of the bond line and to regulate the speed of drying of polysaccharide adhesives. The main plasticizers described in previous studies were polyethylene glycol (PEG) [41,42], glycols, glycerol [5,6] sorbitol, saccharose [43] citrate [5,6] etc. These plasticizers act as hygroscopic agents to decrease the drying time of the adhesive. Apart from this, cross-linking of other polysaccharides with chitosan was also evaluated for bonding application. 4.1. Metal bonding In context of chitosan adhesive application on metal adherends, various eco-friendly polyanionic polysaccharides, acids and plasticizers in single or multiple formulations were attempted using double-lap shear configuration in previous studies [5,6,21]. The adhesive glues the area of two adherend surface is known as lap area. The adopted lap area was 50 × 20 = 1000 mm2 . Pure chitosan was glued on treated aluminum surfaces such as chemically (NaOH and CH3 OH), mechanically (bench pressed) metal file scratched (big deeper groove), sand paper (thin swallow groove) and untreated. Among these treatments chemically treated aluminum adherend was found best based on surface morphology and bonding strength. From various chitosan-based formulations, the maximum shear strength of 40.8 MPa, obtained from 7% (w/v) chitosan (in 2% v/v acetic acid) and 1% (v/v) glycerol (plasticizer) cross-linked polymer on chemically treated Al adherend which was the most significant finding of this study. This bonding strength was equivalent to the bonding strength offered by synthetic adhesive used in industry [5]. Moreover, evaluation of the effect of chitosan concentration (4–9% w/v in 1–2% v/v acetic acid), 6% (w/v) chitosan concentration exhibited best bonding strength (31.4 MPa) whereas further chitosan concentrations were limited with moisture content. Chitosan concentration more than 6% (w/v) in 2% (v/v) acetic acid solutions increased the contact angle of the film which did not support good film formation on metal surface. Increasing chitosan concentration improved the shear strength with increasing moisture content in this study [5], likewise, increase in Mw of chitosan also resulted in stronger adhesion [44]. Both the factors were increased the attachment points due to increase in -NH2 groups which was resulted into better bonding strength. 4.1.1. Cross-linking of chitosan with other polymer for metal bonding Formulation of alginate (ALG) and carrageenan (CRG) in chitosan was tested. These polysaccharides were tested at different concentration ratios with 4% (w/v) chitosan (4:3, 4:4, and 4:5). It was hypothesized that ionic interactions between these acidic polysaccharides and cationic chitosan can increase the bonding strength. But the pKa of alginate lies between 3.38 and 3.65, whereas the pKa of chitosan is ∼6.3 [45,46]. It was resulted low bonding strength
that of the chitosan solution alone due to low degree of crosslinking, it did not homogenize completely and hence did not lead to good adhesion. Likewise, carrageenan was also tested for jellifying properties with chitosan at low pH. The anionic charges of carrageenan are due to sulfate substituents. It has shown higher solubility than that of alginate exhibited at pH 4.5. However, the CS: CRG mix exhibited a poor solubility due to rapid reticulation between them [5]. 4.2. Wood bonding The potential of chitosan as wood adhesive was evaluated using double-lap shear tests. Various lap lengths (20–40 mm) were evaluated and 20 mm lap length was found the best (optimum) compared to the others tested lap lengths in bonding strength assessment. Cohesive mode of failure at lap area was usually set as desire objective for correct assessment of the bonding strength but in this study cohesive strength of chitosan was better than the structural strength of the soft-wood adherend (Pinus pinaster) hence the objective of the assessment therefore structural or cohesive failure at lap area was set to achieve this objective. Three formulations were tried: CS 4%, CS 6%, and CS 6%: gly 1%: cit 5 mmol/L which showed dry bond strengths 4.2, 6.1, and 6.0 MPa respectively with 100% structural wood failure at lap using a 20 mm lap length. The best formulation based on bonding strength obtained was 6% (w/v) of chitosan, 1% (v/v) of glycerol and 5 mmol/L of trisodium citrate dihydrate [6]. The bonding strength of chitosan (Mw 35–350 kDa) and glucose (10–70% w/v) was evaluated on 3-ply plywood (300 × 300 × 1.6 mm) veneer sheets. While curing, it was hot pressed at 130 ◦ C at 1 MPa pressure for 5 min. The bond strength more than 1.7 MPa was obtained with 10% (w/v) glucose addition in low-molecular-weight (LMW) chitosan solution. This formulation also showed 1.1 MPa wet bond strength which was substantial. In contrast, the glucose addition in the high molecular-weight (HMW) chitosan did not exhibit good dry and wet bonding strength. Maillard reaction in above formulation formed brownish melanoidins which occurred between COOH of glucose and NH2 of chitosan that was improved adhesive properties of glucose cross-linked LMW chitosan [14]. 4.2.1. Cross-linking of chitosan with other polymer for wood bonding The structure of chitosan is unique which provides viscoelastic properties and specific interaction with biological substrates. Such type of interactions was not found in other modified polymers. Chitosan under certain circumstances (acidic pH) is a cationic polymer containing NH3 + groups. Thus the chitosan forms polyelectrolyte complex with anionic polymers. Moreover, NH2 groups of chitosan can react covalently with CHO or COOH groups of other polymers. For example, chitosan can crosslink with oxidized starch more as compared to native starch [47]. The adhesion of chitosan to the other polymers is the sum of different types of interactions such as H-bonds, electronic and hydrophobic interactions. Cross-linking of chitosan was attempted with other polysaccharides for bonding application in wood specimens. Primarily, blending of 2–16 g/m2 Konjac-glucomannan (KGM) and 2–16 g/m2 chitosan were tested in veneer plywood preparation (3 ply: 300 × 300 × 1.6 mm). The H-bonding was supported cross-linking between these polysaccharide molecules. The best strength of 2.13 MPa was offered by 8:8 and 8:10 g/m2 CS: KGM blends. Specimens were fabricated at 0.98 MPa by cold-pressing and then hot-pressed at 130 ◦ C. The dry-bond strength increased with increasing chitosan amount in blend. The dry-bond strengths offered by these blends under these conditions exceeded the
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strength offered by CS and KGM alone and also with ureaformaldehyde (UF) resin adhesive and casein glue [14]. The chitosan was cross-linked with modified-lignin (ML) using laccase (copper-containing oxidase enzymes) and bonding strength of the cross-linked CS: ML polymer was tested in wood specimens. The bonding strength obtained from CS: ML was higher than that of the CS alone and similar to lignin blended soy protein and polyethylenimine [48]. Likewise, phenolic compound (PhC) was also cross-linked with chitosan in the presence of laccase and formed CS: PhC (0.48: 10 mM: 60 U) adhesive formulation. The adhesion mechanism of this preparation was similar to the mussel adhesive proteins. The bonding assessment in wood veneer was resulted 1.7 MPa bonding strength. This result did not exhibit any correlation between number of the OH group and the phenolic compound, and also with viscosities and adhesive strength [49]. The chitosan potential was evaluated as adhesive or binder of plant particles for fabricating insulating panels. Sunflower stem particles (6.3 mm size) were used for making insulating panels with ratio chitosan/sunflower of 4.3 and density 150 kg/m3 . Maximum stress at break of this panel was obtained around 2 MPa [50]. Wooden materials comprise very small porosities, but the roughness is more apparent. chitosan solution having low surface tension and approved viscosities which facilitate it to penetrate these asperities [6]. The penetration of the chitosan was tested in wood tissue using microtome technique for deep insight of bonding mechanism. Impregnation degree of rhodamine-labeled 4% (w/v) chitosan in pinewood matrix was superficial. It did not penetrate the cell wall therefore, was not able to reach the cell lumens. It only interacted with the outer surface of the plant cell. It could penetrate the ray cells of the wood matrix up to some extent. The molecular size of the chitosan in this study was approx. 303 kDa with 75% DD [6]. Formulation with high-molecular weight chitosan can elasticate into cell wall, enter into intercellular capillary spaces and expend thus being trapped and unable to enter across the cell wall [51]. Chitosan with low-molecular weight can penetrate wood cell wall and entered into cell lumen [52].
4.3. Glass bonding Chitosan bonding has also been tested on glass slides (lap area 25 × 25 mm). Chitosan alone did not exhibit bonding property on glass material. When chitosan was cross-linked with 3,4 dihydroxyphenylalanine (DOPA) in presence of tyrosinase (50 mg) to form CS: DOPA polymer which exhibited adhesion on glass material. Shear strength of glass slides bonded with CS: DOPA adhesive offered 0.4 MPa which was marginally low compare to the strengths obtained in metal and wood adherends. In the same study chitosan with glutaraldehyde (cross-linker) blend showed 0.3 MPa shear strength under the same conditions [53]. Apparent and peak shear strength obtained in various types of adherends using chitosanbased green adhesives have been summarised in Table 1.
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5. Water resistance property Water resistance property of chitosan-based adhesive formulations has not been found promising. For its improvement, several molecules were explored for cross-linking chitosan based on their structure and types of interactions for bonding application viz. DOPA, glucose, glycerol, citrate, glutaraldehyde, phenolic compounds etc. [5,6,49,53,55,56]. Note that the chemical structures of these compounds had little effect on shear strength and viscosity. However, the cross-linked polymer obtained had not offered significant water resistance property. DOPA-containing proteins were found to be responsible for water resistance property of adhesive synthesized in water by marine animals. Such property was due to cross-linking of intermediate DOPA-derived quinones and primary amines of proteins. Structural application of the bio-based adhesives cannot be realized until these formulations would show water-resistance property. Different blending approaches were adopted to increase such property. In a study, increased glycerol concentration in CS: Gly: Cit (1: 1: 9) formulation led to reduced water absorption upto 44% against 96% weight of pure chitosan. After soaking, the modified film swelled by 11% which was lower than that of the 42% swelling range of pure chitosan [56]. Supporting to above result, formulation CS 6%: Gly 5%: Cit 5 mmol/L gave maximum bond strength of 0.55 MPa as compare to chitosan and chitosan with glycerol formulations in water soaking test. This formulation was protected with oil coat which led to the best wet bond strength of 1.6 MPa. It could retrieve 27% of its dry bond strength [6]. However these approaches are not enough to compete water resistant synthetic adhesives in the market. Water-soluble 2% (w/v) chitosan acetate over the wood particles was converted to water-insoluble chitin by high heat application at 140 ◦ C for 90 min. This chitin was acted as layer of water barrier over the wood particles and reduced significantly the thickness swelling of chitosan-treated panels [57]. Similar effect was observed on wools where anti-shrinking effect of chitosan was recorded [58]. The water resistance property of CS: KGM blend was determined with water immersion test. The bond strength of the blend was increased above 1.2 MPa with increasing amount of the chitosan [14]. Paper industries mainly focus in the improvement of drystrength product properties, utilization of cost effective adhesive formulations and also to improve their environmental foot-print image of their products in the market. In this context, chitosan found to be a good candidate. The potential of chitosan has been evaluated in development of improved bagasse paper, during this process hot water pre-extraction step positively improves surface arrangement and orientation of chemical groups in a fashion that a hydrophobic groups increased in the top of the surface. This arrangement has resulted more water resistance property, however dry-strength agent acts as a protecting film or glaze on the surfaces of bagasse paper sheets [59].
6. Biological properties 4.4. Other applications of chitosan Apart from the bonding applications on surface of metal, wood and glass adherend, the use of chitosan biopolymer has emerged as new generation epoxy: chitosan cement slurry. The newly formed cement slurry was able to conserve its crystalline property after long-term contact with HCl aqueous solution whereas standard cement exhibited drastic decrease in crystalline pattern after reaction with HCl. Improved property of modified epoxy: chitosan cement slurry offered its increased application as environmentalfriendly acidizing polymer in oil wells [54].
The desirable biological features of chitosan adhesives are biodegradability, biocompatibility, non-toxicity and anti-microbial properties which are of industrial interest. Chitosan and its derivatives offer extra advantage among polysaccharides which increase its selection in adhesive applications. It exhibited antibacterial properties due to their polycationic nature at lower pH ranging pKa 6.3–6.5 [60,61]. One hand, it offers significant mechanical strength and another hand the advantage of wood protection [52]. Chitosan impregnated wood material exhibits enhanced life because chitosan absorption into wood tissues increases its protection against
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Table 1 Apparent shear strength obtained in types of adherends using chitosan based green adhesives. Specimen failure
Reference
2.13 (0.39) 2.13 (0.42) 1.7 (0.25)
20 50 35
[14] [14] [40]
1.7
–
[49]
124 × 18 × 4 A: 20 × 18 124 × 18 × 4 A: 20 × 18 124 × 18 × 4 A: 20 × 18
4.2 (0.31)
100
[6]
6.1(0.22)
100
[6]
6.0 (0.20)
100
[6]
40.8 (0.51)a
[5]
31.4 (0.15)a
[5]
Adhesive formulation
Test standard
Specimen type
Specimen dimension (mm)
(I) Wood adherend CS CS: KGM CS:Glc
JIS K6851 JIS K6851 JIS K6251
(3ply) Shorea sp. (3ply) Shorea sp. (3ply) Shorea sp.
300 × 300 × 1.6 300 × 300 × 1.6 300 × 300 × 1.6 A: 40 × 8 170 × 25 × 1 A: 25 × 25
CS: PhC: laccase (0.48%: 10 mM: 60U) CS 4%
ASTM-D-906 (1961)
(3ply) Maple veneer
ASTM D 5573–99
Pinus pinaster
CS 6%
ASTM D 5573–99
Pinus pinaster
CS:Gly:Cit (6%:1%:5 mM)
ASTM D 5573–99
Pinus pinaster
(II) Metal adherend CS: Gly (7%: 1%)
–
CS (6%)
–
Aluminum alloy 2014 Aluminum alloy 2014 Aluminum alloy 2014 Aluminum alloy 2014 Aluminum alloy 2014
150 × 20 × 2 A: 50 × 20 150 × 20 × 2 A: 50 × 20 150 × 20 × 2 A: 50 × 20 150 × 20 × 2 A: 50 × 20 150 × 20 × 2 A: 50 × 20
glass slide
A: 25 × 25
CS (4%)
–
CS: Cit (6%: 3 mM)
–
CS: Glut (4%: 0.010%)
–
(III) Glass adherend CS: DOPA: tyrosinase
–
Apparent shear strength (MPa)
a
28.7(1.1)
[5]
28.4(2.1)a
[5]
21.2(3.9)a
[5]
0.41
–
[53]
CS: chitosan; Gly: glycerol; Cit: citrate; Glc: glucose; DOPA: dihydroxyphenylalanine; KGM: Konjac-glucomannan; PhC: phenolic compound; Glut; glutaraldehyde. a peak shear strength
deteriorating microorganisms. It is noticeable that chitosan is not only fills cell lumens and cavities but also impregnates cell walls establishing a tight seal against invading microorganisms. Thus, wood protection is based on chemical properties of adhesive as well as physical protection of cell walls. Chitosan is naturally-occurring compound showing potential in agriculture for controlling plant diseases. The molecule exhibit toxicity and inhibit fungal growth and development. It was reported to be active against viruses, bacteria and other pests [62]. Oil coating and painting approaches are useful to reduce moisture intake in adhesive which ultimately lead to reduce the incidence of microbial attack in bonded line. Biodegradability is another advantage of this polymer material. After completion of life cycle, each product undergoes biodegradation in the nature and converted into simplest elemental form. In this context, chitosan is a biodegradable polymer, does not pollute environment as synthetic adhesive which is desirable. Biocompatibility defines degree of polymer acceptability by the living system. Chitosan offers such property and finds applications largely in biomedical area. 7. Bottleneck and market potential Chitosan offer competitive dry bonding strength with synthetic adhesive whereas wet bonding strength is still lower which reduces its wide spectrum applications. The need for robust bioadhesives in wide niche area is expected which can completely replace the utilization of toxic petrochemical based commercial synthetic adhesives. Water resistance and thermal properties are two important areas to be fixed at this stage of chitosan adhesive development to increase its scope for wide spectrum commercial exploitation by various industries. Thermal degradation of chitosan occurs at 250 ◦ C whereas Tg has reported up to 150 ◦ C. Polyamide blending has been recently attempted to improve its thermal property marginally [63]. More research is needed to improve these properties by blending other ionic compounds which may offer wider
application range and niche area in the market. Likewise, water resistance property has been partially improved yet by approaching temperature ramping at high temperature during curing process. Moreover, coating practices which are required to be improved further by seeking various formulations for removing hydrophilic property of polymer and encourage hydrophobic nature. Crosslinking of chitosan with other additives such as polysaccharide, proteins and chemicals must be explored for removal of non-linked NH2 groups which were left after bonding. These free groups are largely accountable for hydrophilic interactions of the polymer. Apart from this, low cost stable coating material should be identified which can protect adhesive material from reaction with water, weathering and moisture. Another aspect of high interest is the cost of adhesive for bonding application, although biopolymer based adhesive formulation offers lower price than that of the commercially established synthetic adhesives. Based on the application quantity, study showed solid based spread rate (16 g/m2 ) of chitosan was lower than that of the UF-resin adhesive (74 g/m2 ), casein glue (67 g/m2 ) and soyabean glue (50 g/m2 ) [14]. It can be economized further by improving its mechanical strengths by blending of additives or plasticizer. If the blending practices offer improved bioadhesive properties that will drive its application at higher application segment and help to reduce prize volatility. This can also help to replace important and inflated synthetic adhesives in the market. Adhesives from bioresources offer huge market potential worldwide. Green adhesives and sealants markets are projected to reach nearly $1.24 billion in 2017, according to a new study done by IntertechPira in association with ASC and FEICA [64]. 8. Conclusion Development of potential natural adhesives is in high demand from consumers worldwide due to health and environmental problems emerged after utilization of synthetic adhesives. Chitosan has
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been emerged as potential candidate of natural adhesive based of its properties. It is unique cationic heteropolymer among polysaccharides which offer more ionic interactions upon negatively charged wood, paper and metal adherends surface due to presence of NH2 groups. However, higher degree of deacetylation and higher molecular weight support for improved interaction and dense polymeric network formation. Additive blending and cross-linking of plasticizer in chitosan solution increased its bonding strength but thermal property was marginally improved. Reverse acetylation of chitosan into chitin from temperature ramping improved water resistance property partially. More research is needed to improve its water resistance and thermal properties to increase its scope worldwide for more commercial exploitation.
Acknowledgements The author thanks University of Auvergne for post doctoral fellowship and Dr. Reeta R. Singhania for help in this study.
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