Sustainable use of cassava (Manihot esculenta) roots as raw material for biocomposites development

Industrial Crops and Products 65 (2015) 79–89

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Sustainable use of cassava (Manihot esculenta) roots as raw material for biocomposites development Florencia Versino a , Olivia V. López a,b , M. Alejandra García a,∗ a Centro de Investigación y Desarrollo en Criotecnología de Alimentos, CIDCA (UNLP–CONICET), Facultad de Ciencias Exactas, UNLP, 47 y 116, 1900 La Plata, Argentina b Planta Piloto de Ingeniería Química, PLAPIQUI (UNS–CONICET), Departamento de Ingeniería Química, UNS, Camino La Carrindanga km. 7, 8000 Bahía Blanca, Argentina

a r t i c l e

i n f o

Article history: Received 1 August 2014 Received in revised form 14 November 2014 Accepted 30 November 2014 Available online 8 December 2014 Keywords: Natural fillers Cassava root peel and bagasse TPS composite processing Microstructure characterization Cassava TPS reinforcement Compression moulding

a b s t r a c t This work is focused on the use of cassava roots peel and bagasse as natural fillers of TPS materials based on cassava starch. A deep insight into biocomposites microstructure was performed in order to support mechanical and barrier properties of the final materials. Cassava byproducts chemical composition and particle size distribution helped to explain TPS SEM morphology, mechanical and barrier properties modifications. Processing conditions favored starch-filler interactions leading to lower mixing energy requirements. The matrix-filler compatibility was demonstrated by FTIR and thermal analysis of TPS composites. Filler addition increased UV-barrier capacity and opacity of TPS materials, though water vapor permeability was maintained. Both byproducts reinforce TPS matrices even though low filler concentrations were used. Bagasse addition (1.5%) increased 260% elastic modulus and 128% maximum tensile stress of TPS composites, being the most efficient reinforcing agent due to its high residual starch content and lower proportion of smaller particles. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Cassava bagasse is a byproduct of cassava-processing industry, which is a fibrous material that contains about 30–50% starch on dry weight basis. By cassava bagasse fermentation non-food products can be obtained, such as pullulan (Sugumaran et al., 2014) or n-butanol (Lu et al., 2012) and food-related products such as nutritionally improved animal feed (Sriherwanto et al., 2009) or aroma compounds (Christen et al., 2000). The use of fiber obtained from cassava bagasse has been studied as natural filler intended to reinforce biomaterials, modifying their hydrophobicity and final properties (Pasquini et al., 2010; Teixeira et al., 2012, 2009). Fiber derived from cassava bagasse is also attractive as food additive; however, to the best of our knowledge, there are no reports about the use of cassava peel as filler of polymeric matrices. Natural cellulosic fibers are playing an important role in a number of applications due to their inherent eco-friendly advantages since the last few decades. They are being explored as green reinforcement alternatives to traditional synthetic fibers for diverse applications. Composites including natural cellulosic fibers offer a

∗ Corresponding author. Tel.: +54 221 4254853; fax: +54 221 4249487. E-mail address: [email protected] (M.A. García). http://dx.doi.org/10.1016/j.indcrop.2014.11.054 0926-6690/© 2014 Elsevier B.V. All rights reserved.

number of advantages over conventional materials such as considerable toughness, flexibility, easy processing, recyclability, and eco-friendliness. In the light of its wide availability and renewable character, thermoplastic materials obtained by the plasticization of starch are among the most promising alternative to develop biomaterials (Averous and Boquillon, 2004; Shirai et al., 2013; Yu et al., 2006). Understanding of different processes involved on the transformation of starch into a thermoplastic polymer has considerably advanced in the last years (Da Róz et al., 2006; Liu et al., 2009; Mano et al., 2003; Xie et al., 2013). Nowadays, thermoplastic starch (TPS) is an interesting alternative for synthetic polymers in applications that do not require long-term durability nor elevated mechanical performance. Due to the multiphase transitions of starch, the microstructure and mechanical properties of starch-based materials strongly depend on the processing techniques and conditions (Liu et al., 2009, 2013). For example, compression molding of starch led to sintered, relatively brittle materials while, by injection molding of native starch amorphous materials with enhanced mechanical properties are obtained (Cagiao et al., 2004). In order to fulfill their potential utilization as alternative to synthetic thermoplastics, mechanical properties of TPS must be enhanced. Reinforcement of thermoplastic matrix with natural lignocellulosic fibers is a good

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option to increase their mechanical performance and to preserve the environmental-friendly character of the final material. Thus, Azwa et al. (2013) and Shalwan and Yousif (2013) have extensively reviewed the use of fibers as reinforcement materials in biodegradable matrices. However, the use of industrial byproducts as reinforcement and/or support is still under study. Within this context, cellulose fibers isolated from wood pulp (Da Róz et al., 2006), sugarcane bagasse (Chiellini et al., 2001), jute (Khondker ˜ et al., 2012), and wheat bran (Famá et al., 2006), cotton (Luduena et al., 2009) have been incorporated into hydrocolloid matrices. Moreover, Slavutsky and Bertuzzi (2014) reported that addition of cellulose nanocrystals obtained from sugarcane bagasse helped improve the water barrier properties of starch films. Nevertheless, there are some disadvantages associated with the use of natural fibers as reinforcement of polymer composites, such as their incompatibility with hydrophobic polymer matrices, tendency to form aggregates during processing, poor moisture resistance, inferior fire resistance, limited processing temperatures, lower durability, variation in quality and price, and difficulty in using established manufacturing process (Azwa et al., 2013; Dittenber and GangaRao, 2012). Chemical treatment of fibers has been identified as an alternative to reduce their susceptibility to moisture and improve the filler-matrix interfacial interactions and thermal stability of the composites (Azwa et al., 2013). Thus, in order to evaluate the capacity of natural fibers as fillers of polymeric materials, it is necessary to gain a deep understanding of their inherent characteristics and composition. This work is focused on the use of cassava roots peel and bagasse as natural fillers of TPS materials based on cassava starch. Composition of these byproducts, as well as, their compatibility with TPS matrix was investigated. A deep insight into biocomposites microstructure was performed in order to support mechanical and barrier properties of the final materials. 2. Materials and methods 2.1. Materials Manihot esculenta (cassava) roots were provided by INTA Montecarlo Experimental Station (Misiones, Argentina; 26◦ 33 40.15 latitude south and 54◦ 40 20.06 longitude west). Starch was extracted from the roots following the procedure described by López et al. (2010) roots peel (P) and bagasse (B), obtained from the same extraction process, were dried at 50 ◦ C to constant weight, ground and sieved (through a 500 ␮m mesh sieve) to be used as filler of thermoplastic cassava starch matrices. 2.1.1. Characterization of cassava byproducts: peel and bagasse 2.1.1.1. Scanning electron microscopy (SEM). Microstructure and morphology of byproducts were studied by SEM, using a JEOL JSM 6360 microscope (Japan). Samples, mounted on bronze stubs using a double-sided tape and metalized with gold layer (40–50 nm), were analyzed under high vacuum mode. 2.1.1.2. Chemical composition. Dry matter, ash, lipid fraction, crude protein, total dietary fiber, and lignin content of cassava byproducts was determined following standard methods, as thoroughly described in a previous work (Versino and García, 2014). 2.1.1.3. Solvent retention capacity (SRC). Cassava bagasse and peel solvent retention capacity was analyzed following the AACC method 56–11.02 (Rosell et al., 2009). Solvents used were deionized water, sucrose (50% w/w), sodium carbonate (5% w/w), and lactic acid (5% w/w) solutions. SRC was expressed as percentage (%).

2.1.1.4. Particle size distribution. Filler particle size distribution was studied as described in a previous work using laboratory test sieves (Versino and García, 2014). 2.2. Composites processing Mixtures of cassava starch, glycerol (30% w/w), distilled water (45% w/w), and residues (0.5 and 1.5% w/w) were prepared. Components concentration was expressed in g per 100 g of starch (dry basis). Formulations were named TPS, TPS-P0.5, TPS-P1.5, TPS-B0.5, and TPS-B1.5, where P is referred to peel and B to bagasse, numbers indicate filler concentration. Byproducts were premixed with starch to achieve good dispersion within the matrix. Then, glycerol and distilled water were added, and samples were mixed and conditioned at 25 ◦ C during 24 h. Mixtures were processed in a Brabender Plastograph (Brabender, Duisburg, Germany) at 140 ◦ C and 50 rpm for 15 min. Curves corresponding to torque (Nm) versus processing were recorded. Thermal degradation of developed biocomposites was carried out in a thermogravimetric balance TA Instrument Discovery Series (New Castle, USA). Samples, previously conditioned at 25 ◦ C and 60% relative humidity (RH), were heated from 30 to 700 ◦ C at 10 ◦ C/min, employing an air flux (40 mL/min). Curves of loss weight as function of temperature were recorded and the maximum decomposition temperature of each component was obtained from first derivative curves. 2.3. Films preparation Films were obtained by thermo-compression using an hydraulic press, following the processing conditions reported by Castillo et al. (2013). Mixtures were previously conditioned at 25 ◦ C and 60% relative humidity (RH), and films were prepared at 140 ◦ C and 150 kg cm−2 during 6 min. Before characterization, films were conditioned at 25 ◦ C and 60% RH. Film thickness was measured at least in ten different locations using a digital coating thickness gage Check Line DCN-900 (New York, USA). 2.4. Microstructural characterization 2.4.1. Scanning electronic microscopy (SEM) Homogeneity and appearance of films were examined by SEM. This study was performed in a JEOL JSM-35CF electron microscope (Japan), with a secondary electron detector. Films were cryofractured by immersion in liquid nitrogen, mounted on bronze stubs and coated with a gold layer (∼30 Å), using an argon plasma sputter coater (PELCO 91000). Samples were observed using a 10 kV accelerating voltage, under high vacuum mode. 2.4.2. Fourier transform infrared spectroscopy (FTIR) Spectra were obtained using a Thermo Nicolet Nexus spectrophotometer (Milwaukee, USA). Samples were prepared by mixing thermoplastic starch mixtures as fine powder with KBr (Sigma–Aldrich, 99%) at 3% w/w. Mixture was pressed and a transparent sample was obtained. Spectra were achieved from 100 accumulated scans at 4 cm−1 resolution in the range 4000–400 cm−1 . 2.5. Thermal properties Modulated differential scanning calorimetry (MDSC) assays were performed in differential scanning calorimeter Q100 (TA Instruments, New Castle, USA). Approximately, 10 mg of film, previously conditioned at 25 ◦ C and 60% RH, was weighted in hermetic pans in order to avoid water loss. An empty hermetic pan was used as reference. Samples were heated from 0 to 200 ◦ C at 10 ◦ C/min,

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under nitrogen atmosphere. From these thermograms, the following parameters were obtained: onset (To ) and melting (Tm ) temperatures and enthalpy associated to this thermal transition (Hm ). Temperatures and intensities of relaxation phenomena for conditioned TPS films were determined by DMTA. Measurements were carried out in a dynamic-mechanical thermal equipment Q800 (TA Instruments, New Castle, USA) with a liquid nitrogen cooling system, using a clamp tension. Multi-frequency sweeps at fixed amplitude from −100 to 100 ◦ C at 5 ◦ C/min were carried out. Storage (E ) and loss (E ) moduli and tan ı curves as a function of temperature were recorded and analyzed using the software Universal Analysis 2000. Temperatures of the relaxation processes associated to glass transition temperatures (Tg ) were determined through the inflexion point of the storage modulus E curve, as well as, the maximum peak in both the loss modulus E and tan ı curves.

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isothermal conditions. Assays were performed at 25 ◦ C from 0.1 N up to specimen breakage using a constant displacement velocity of 0.3 N/min and a preload force ranged from 0.1 to 0.5 N. Stress–strain curves were recorded and analyzed using the software Universal Analysis 2000. Maximum tensile strength (MPa), strain at break (%), and elastic modulus (MPa), which corresponds to the slope of the initial linear portion of the stress–strain curve, were determined. Reported results correspond to the mean value of each mechanical property. 2.9. Statistical analysis A completely randomized experimental design was used to characterize the composite films. Analysis of variance (ANOVA) was used to compare mean differences of samples properties. Besides, comparison of mean values was performed by Fisher’s least significant difference test conducted at a significance level p = 0.05.

2.6. Optical properties 2.6.1. Opacity and UV barrier capacity Absorbance spectra (200–800 nm) were recorded using a U1900 spectrophotometer (HITACHI, Japan). Films were cut into rectangles (3 × 1 cm) and placed on the internal side of a quartz spectrophotometer cell. Film opacity (AU × nm) was defined as the area under the recorded curve between 400 and 800 nm, determined by an integration procedure according to Piermaria et al. (2011) and the standard test method for haze and luminous transmittance of transparent plastics recommendations (American Society for Testing and Materials, 2000). Likewise, UV-barrier capacity of films was estimated as the area under curve between 200 and 400 nm, and expressed in AU × nm. 2.6.2. Color measurements Films color measurements were performed using a Minolta colorimeter CR 400 Series (Konica Minolta Sensing Inc., Osaka, Japan) calibrated with a standard (Y = 93.2, x = 0.3133, and y = 0.3192). Color parameters L, a*, and b* were recorded according to the CIELab scale, in at least ten randomly selected positions for each film sample. Color assays were performed by placing the film samples over the standard. Hue angle [h◦ = tan−1 (b*/a*)] and chroma [C* = (a × 2 + b × 2)1/2] were calculated; while color differences (E) were determined according to Rivero et al. (2009). 2.7. Water vapor permeability Water vapor permeability (WVP) was determined according to ASTM F 1249-89 standard method using a PERMATRAN-W® Model 3/33 (Mocon Inc., USA) as described in a previous work (López et al., 2015). Results were expressed in g/m s Pa. 2.8. Mechanical properties Mechanical behavior of TPS films was evaluated by performing quasi-static assays in uniaxial condition using a dynamicmechanical analyzer Q800 (TA Instruments, New Castle, USA) with liquid nitrogen as cooling system and a tension clamp. At least ten specimens (60 × 30 mm) were tested for each film formulation at

3. Results and discussion 3.1. Characterization of cassava byproducts Processing of cassava tubers yields mainly two byproducts: peel and bagasse. Peel could represent 5–15% of the roots (Aro et al., 2010) and are obtained after the tubers have been sanitized and peeled manually or mechanically. Depending on the cassava variety analyzed, they may contain high amounts of cyanogenic glycosides and have higher protein content than other tuber parts. In our case, the variety used is a sweet one, which does not contain detectable levels of cyanogenic glycosides (Dini et al., 2014). Cassava bagasse is a solid fibrous residue (around 17% of the tuber) that remains after the flour or starch has been extracted (Aro et al., 2010). Quality and appearance of these byproducts vary with plant age, time after harvest, as well as, industrial equipment and method used (Leonel et al., 2003). Cassava byproducts are rapidly fermentable, so in the present work they were dried at 50 ◦ C up to constant weight in order to stabilize them. Chemical composition of cassava residues is shown in Table 1. While cassava peel is majorly composed by 55% fiber (dietary fiber) and 24% lignin; bagasse consists of approximately 82.6% carbohydrates, mainly starch, as has been well-described in a previous work (Versino and García, 2014). Likewise, cassava peel is characterized by their high protein, ash and lipid content. Obtained results were within the range of those reported for cassava dry peel and cassava pomace used for animal feed (Ahamefule et al., 2006; Aro and Aletor, 2012; Kosoom et al., 2009). SEM morphological characterization of the cassava bagasse is shown in Fig. 1A, where the starch granules surrounded by the remaining cell walls of the parenchyma tissue can be observed. Identified starch granules exhibited both round and polygonal shapes, as was described in a previous work (Versino and García, 2014). Morphology of cassava peel is similar to the bagasse one, although a larger contribution of fibrous components can be observed (Fig. 1B), as could be expected considering their chemical composition (Table 1). Cassava byproducts presented a heterogeneous particle size distribution, peel showed a higher contribution of larger particles

Table 1 Chemical composition of natural fillers derived from cassava roots. Natural filler

TDFa (%)

Lipids (%)

Bagasse Peel

4.18 ± 2.76 55.14 ± 0.82

0.17 ± 0.015 0.90 ± 0.187

Value ± standard deviation. a TDF stands for total dietary fiber.

Moisture (%) 10.56 ± 1.58 9.67 ± 0.03

Protein (%)

Ash (%)

1.25 ± 0.11 7.05 ± 1.09

0.86 ± 0.039 10.70 ± 0.12

Klason lignin (%) 1.62 ± 0.579 23.59 ± 1.452

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(mainly 300 ␮m), while in bagasse predominate particles smaller than 53 ␮m (Fig. 1A and B). Solvent retention capacity is generally used to evaluate flours, although in the present work it was proposed in order to study the cassava byproducts ability to interact with different test-solvents, complementing their physicochemical characterization. In general, cassava peel exhibited higher retention capacity than bagasse for all the solvents tested (Fig. 1C), being these results in agreement with their chemical composition (Table 1). In the case of peel, sodium carbonate retention capacity, which has been related to the damaged starch content (Gaines, 2000) was similar to water retention capacity, which has been correlated with total dietary fiber content (Duyvejonck et al., 2012). Cassava peel retained more sucrose solution than other solvent, suggesting a higher proportion of com-

ponents related to soluble dietary fiber (Dini et al., 2013). With regard to lactic acid retention capacity, the higher values of peel than bagasse could be explained considering its higher protein content (Table 1). 3.2. TPS composite processing Processing of starch based materials is more difficult than the corresponding to synthetic polymers. Starch processing involves multiple chemical and physical reactions, such as water diffusion, granule expansion, gelatinization, decomposition, melting, and crystallization (Liu et al., 2009). When starch is processed at high shear stress and temperature in presence of plasticizers, it behaves like common thermoplastic synthetic polymers.

Fig. 1. Characteristics of cassava byproducts used as natural filler: scanning electron microscopies (SEM) and particle size distribution of (a) bagasse and (b) peel (magnification is indicated in the micrographs); and (c) solvent retention capacity of cassava byproducts.

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Table 2 Characteristics parameters of plasticization process of biocomposites based on thermoplastic starch (TPS) with 0.5 and 1.5% w/w of cassava roots peels (P) and bagasse (B). Biocomposites formulation

TPS

TPS-P0.5 TPS-P1.5 TPS-B0.5 TPS-B1.5

Plasticization energy (Nm min) 165.95 131.15 Maximum torque (Nm) 67.49 64.1 Steady torque (Nm) 57.76 55.91

Fig. 2. Torque curves as a function of processing time obtained for TPS blends and those containing cassava byproducts: (a) peel and (b) bagasse.

During plasticization, some of carbohydrate polymer molecules are released from the granular structure (Córdoba et al., 2008). Thus, thermoplastic starch melt presents highly plasticized amorphous regions and some remaining granular regions, and the amount of each phase in a given blend depends on processing time and conditions. Fig. 2 shows the curves of torque as function of mixing time for plasticization and melting process of the developed biocomposites. According to Corradini et al. (2007), torque curves corresponding to polymers plasticization rise to a maximum in the initial processing stages and then decrease, reaching a plateau with an almost constant torque value. Particularly, in the case of assayed starch blends, curves presented a broad and intense plasticization peak, torque increased to a maximum value and then decreased to a stable value in time. At the beginning of processing, torque increases rapidly due to the weakening of strong hydrogen bond interaction between starch molecules and swelling of granules (Qiao et al., 2011). Then, starch microcrystals melt under the action of mechanical shear force and heat, decreasing torque values. Finally, starch melt is homogenized and torque tends to equilibrium. Mo´scicki et al. (2012) reported similar plasticization curves for melting processing of potato starch, even though higher plasticizer content and slightly different operating conditions were employed. Regarding residues incorporation, it could be observed that the presence of cassava bagasse and peel did not affect plasticization profile of thermoplastic cassava starch. Table 2 shows

103.73 59.05 51.28

125.98 59.70 54.52

103.99 53.46 50.08

the characteristics parameters associated to starch plasticization and melting process, derived from torque curves. Plasticization energy was calculated as the integral under the torque-time curves, from the time when the torque starts to increase until the time where the maximum torque is reached (Córdoba et al., 2008). This region of the mixing process is associated to the plasticizer diffusion inside the polymer molecules. Bagasse and peel addition to TPS formulations led to a reduction in plasticization energy, evidencing an enhancement in the thermodynamic favorability of the diffusion process of plasticizers in the starch granules when cassava residues were present (Córdoba et al., 2008). Besides, biocomposites presented lower maximum torque and time to complete the plasticization stage than TPS. These results indicated that residues addition improved the ability to process starch under the used operating conditions (Qiao et al., 2011). Finally, bagasse and peel incorporation led to a reduction in the steady torque reached at the end of processing and it could be associated to a decrease of TPS melt viscosity (Córdoba et al., 2008). Thermal degradation and stability of starch based materials during processing are important issues from both scientific and industrial perspectives (Liu et al., 2013). Thermo gravimetric analysis (TGA) has been the most widely used technique to study thermal stability and decomposition of starch from different sources (Aggarwal and Dollimore, 1996; Guinesi et al., 2006; Soares et al., 2005; Soliman et al., 1997). The knowledge of degradation and mode of decomposition under the influence of heat is highly recommended in the processing optimization (Mano et al., 2003). Dehydration and decomposition have generally been considered as the main two processes associated with the degradation mechanisms of starch (Liu et al., 2009, 2013). Although polymer degradations are commonly studied under inert atmospheres, degradation in an oxygen or air environment is equally important. Thus, thermal oxidative degradation of TPS biocomposites was studied in order to provide relevant information on how these materials behave under more realistic atmospheric conditions (Acar et al., 2008; Peterson et al., 2001). Fig. 3 shows weight loss as a function of heating temperature, as well as, first derivative curves corresponding to the developed composites. Along with temperature increase, several solid-state reactions and phase transitions take place, such as melting, evaporation and sublimation, as well as, chemical condensation, decomposition, and finally carbonization at very high temperatures (Pielichowski and Njuguna, 2005). As it can be observed, all curves presented a similar behavior, showing the occurrence of four weight loss steps, indicating that cassava byproducts addition did not modify TGA curves of TPS composites (Fig. 3). Each stage corresponds to a peak in first derivative curve which represents a separate event in a particular temperature range. Weight loss corresponding to water desorption, which generally starts just above room temperature (∼70 ◦ C), was not observed in any of the processed biocomposites. This result could be an indicative of the small quantity of water absorbed in the material structures. Similar results were reported by Mano et al. (2003) for thermoplastic starch/synthetic polymer blends. Degradation process observed around 150 ◦ C could be attributed to the loss of glycerol, as it has been previously reported by other authors (Mano

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Fig. 3. Weight loss as a function of heating temperature and first derivative curves of: (a) TPS; (b) TPS-B0.5; (c) TPS-B1.5; (d) TPS-P0.5; and (e) TPS-P1.5.

et al., 2003; Vega et al., 1996). The most important weight loss, which occurs at 318 ◦ C, was associated to the degradation of starch constituent molecules. During this process, ether bonds and unsaturated structures are formed via thermal condensation between hydroxyl groups of starch chains, which eliminates water and other small molecules, and by dehydration of hydroxyl groups in the glucose ring (Ruseckaite and Jimenez, 2003). The shoulder in the starch degradation peak may be due to the different degradation rate of amylose and amylopectin. The linear structure of amylose results in more exposed hydroxyl groups, while the branched structure of amylopectin hinders the access to these functional groups. In regards to the last weight loss stage, which appeared around 490 ◦ C, it was attributed to the reactions between carbonaceous residues with oxygen, forming simple gases such as CO, CO2 , and H2 O. This additional decomposition step named “glowing combustion” happens when thermal degradation occurs in oxygen atmosphere (Liu et al., 2013; Rudnik et al., 2005). 3.3. Composites microstructural characterization FTIR spectroscopy was used for investigating changes in TPS structure on a short-range molecular level and to identify the potential interactions between starch, glycerol, and cassava byproducts. Yin et al. (1999) stressed that when two or more substances are mixed, physical blends versus chemical interactions are reflected by changes in characteristic spectral bands.

Fig. 4 shows the FTIR spectra corresponding to cassava peel and bagasse, TPS matrix, and biocomposites containing the fillers. Regarding cassava byproducts, characteristics bands were detected and assigned. The main differences between the spectra of both byproducts are the presence of the peak at 1518 cm−1 , typical signal of lignin, as well as, the width and position of the band around 3400 cm−1 . These results can be related to differences in chemical composition, as has been previously described, mainly to protein and lignin contents in cassava peel (Table 1). Cassava TPS composites presented bands corresponding to the distinctive functional groups of starch and glycerol. The broad peak observed around 3400 cm−1 , is associated to the stretching of the OH groups. Bands identified at 2929 and 2881 cm−1 are attributed to C H stretching. The peak at 1642 cm−1 was assigned to the bending mode of the absorbed water. The peaks around 1420 cm−1 are assigned to O H bonding. Peaks at 1155 and 1040 cm−1 were attributed to the C O bond stretching of the C O H group in cassava starch and the C O bond stretching of the C O C group in the anhydroglucose ring, respectively (Kaewtatip and Thongmee, 2013; Prachayawarakorn et al., 2013). Additionally, the peak detected at 1456 cm−1 was attributed to glycerol (Zhang and Han, 2006). TPS composite spectrum presented the signals corresponding mainly to TPS matrix, regardless the cassava byproduct incorporated. Besides, filler bands intensities in TPS composite were relatively weak due to their low concentration in the formulations.

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were homogeneous, since no individual starch granules could be identified (Fig. 5A), which indicated that, during starch mixtures processing, the starch granules were completely disintegrated. Particle filler dispersion was studied by SEM technique, being the particles near the surface coated by plasticized starch matrix (Fig. 5B and C). This observation is indicative of a good adhesion of the natural fillers to the matrix. This result is in accordance with those observed by FTIR. Ma et al. (2005) also noticed the same effect in TPS films containing micro winceyette fiber and attributed it to the strong interaction between the fiber and the matrix. The presence of both bagasse and peel seems to increase slightly the roughness of the surface, leading to a less homogeneous one. This effect was more evident in the formulations containing peel as filler, despite the low concentrations used (Fig. 5B), probably due to the higher content of large particle size (Fig. 1B). A similar trend was reported by Bodirlau et al. (2013) while working on starchbased biocomposites containing chemical modified microparticles, keratin fibers, spruce cellulose, or boost lignin. 3.4. Thermal properties of composite TPS films

Fig. 4. FTIR spectra of TPS blends containing cassava byproducts as natural fillers: (a) peel and (b) bagasse.

The corresponding band located at 3421 cm−1 shifted to 3408 and 3412 cm−1 for TPS containing 0.5% bagasse and peel, respectively. In biocomposites containing 1.5% cassava bagasse, the mentioned peak shifted to 3396 cm−1 , the higher the filler content the higher the shift, while in the case of cassava peel it shifted to 3418 cm−1 . These changes could be attributed to the interactions by hydrogen bonding between hydroxyl groups of cassava starch and specific groups of lignin, cellulose, and hemicellulose of byproducts. Bodirlau et al. (2013) stressed that these hydrogen bonding interactions possibly occurs between hydroxyl, carbonyl groups in starch and carbonyl, hydroxyl, ether groups in lignin and hydroxyl groups in cellulosic components. Thus, FTIR results revealed changes in TPS matrix due to cassava byproducts addition attributed mainly to chemical interaction between starch and filler component molecules by hydrogen bonding formation, which is favored during blends processing. Likewise, considering that cassava peel contained 7% proteins (Table 1), the presence of NH2 groups could contribute to enhance the hydrogen bonding interactions of blends, which allow explaining the observed shifts. The characteristic peak of lignin at 1518 cm−1 , identified in cassava peel spectrum, could not be detected in composites containing this byproduct, probably due to its low concentration (Fig. 4). Once TPS cassava blends were submitted to thermocompression, films were obtained. Cassava TPS surface films

Thermal properties of the material (glass-transition temperatures and melting properties) were studied by DMTA and DSC, respectively. DMTA assays of cassava TPS and composites exhibit two clear transitions: the first one, at lower temperatures (Tg1 ), is attributed to a rich glycerol phase; whereas, the second one, usually above room temperature (Tg2 ), is considered the TPS proper glass transition (Fig. 6A). An increase in this second transition temperature was observed for TPS reinforced with cassava byproducts, resulting this shift higher for TPS with peel as filler. Similar behaviors have been reported by other authors working on corn TPS reinforced with natural fibers (Gironès et al., 2012; Muller et al., 2014). Higher transition temperatures could be indicative of strong hydrogen interactions between the reinforcing material and the starch matrix, which is in agreement with the shifts observed by FTIR in the band located around 3400 cm−1 . Moreover, a decrease in tan ı peak intensity was observed in both reinforced TPS films that, according to Gironès et al. (2012), indicates that the filler presence promotes the inhibition of relaxation process, thus leading to a more rigid system. Correspondingly, melting transition of TPS films appeared as a marked single endothermic peak Fig. 6B. Reinforced TPS showed less cooperative melting process than cassava TPS, probably because of the major number of interactions involved in matrices with more components. Furthermore, TPS with 1.5% cassava peel presented de higher melting temperature, which is in agreement with the higher Tg2 as was previously described. 3.5. Optical properties General appearance of all TPS films studied was homogeneous and transparent, with a brownish tone. The effect of filler addition on plasticized films color parameters is shown in Fig. 7. On one hand, luminosity parameter (L*) decreased significantly (p < 0.05) with peel content while chroma (C*) and color difference (E) parameter increased. The change in hue angle (h◦ ), however, decreased slightly with this filler. Bodirlau et al. (2013) stressed that filler particle-sizes larger than the visible wavelength would obstruct light, minimizing the transparency of starch-based biocomposite films, being the greater effect due to lignin presence. Consequently, the larger the particle size the lesser the light transmission. In our case, cassava peel with larger particle sizes than bagasse led to TPS composites with lower luminosity values and higher opacity ones at the same filler concentration (Fig. 7A). On the other hand, composite TPS containing bagasse presented a moderate effect, being color parameters similar to those of TPS. These

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Fig. 6. (a) Glass transition temperatures (Tg1 and Tg2 ) and (b) DSC thermograms of TPS films with cassava bagasse and peel as fillers.

Fig. 5. Films surface scanning electron microscopies of (a) TPS, (b) TPS-P0.5 and (c) TPS-B0.5. Magnification is indicated in the micrographs. Fig. 7. (a) Optical properties and (b) UV–vis spectra of TPS films containing cassava byproducts as natural fillers.

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Fig. 8. (a) DMA stress–strain curves and (b) mechanical properties of TPS films containing cassava byproducts as natural fillers.

results correlate with the chemical composition of the filler, which as was previously described, exhibits a major proportion of carbohydrates, mainly starch. In accordance with the color increment of the TPS reinforced samples; there is an increase in the absorbance across the UV–vis spectrum portion of TPS films when cassava byproducts are added to the starch matrix (Fig. 7B). Mbey et al. (2012) reported a similar trend for cassava starch- kaolinite composite films. The study of the UV-light absorption capacity of the biodegradable films is especially relevant to determine their possible applications for packaging. Fig. 7B shows the obtained UV–vis spectra for the studied plasticized films. In order to estimate the UV-barrier capacity of reinforced films, the area under the curve in the UV region (200–400 nm) was calculated. As it can be expected, this parameter increased with filler content, although peel had a more important blocking effect than bagasse. The UV-barrier capacity enhancement of TPS films reinforced with cassava peel could be mainly attributed to the phenolic components of lignin (Buranov and Mazza, 2008). Likewise, opacity was calculated from the spectra visible range (400–800 nm), showing a similar, although less pronounced, trend (Fig. 7A). 3.6. Mechanical and barrier properties Incorporation of reinforcement agents to starch based materials aims to enhance their final mechanical properties. Fig. 8 shows tensile test curves, obtained by quasi-static assays, corresponding to cassava TPS films, and biocomposites containing both byproducts. Bagasse and peel addition did not modify the ductile stress–strain behavior of TPS films (López et al., 2015). Table insert in Fig. 8 shows mechanical properties obtained from stress–strain curves. As it could be observed, incorporation of both cassava byproducts as reinforcement agents of TPS matrix led to stronger films, even though low filler concentrations were used. This result was evidenced by the increment in maximum tensile strength ( m ) and elastic modulus (E) values of TPS films due to byproducts addition (Fig. 8B). Even though biocomposites with 0.5% w/w bagasse exhib-

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ited higher values of  m and E than TPS films, these increments were only significant (p < 0.05) in the case of E. However, the presence of 1.5% w/w of bagasse led to an important increase (p < 0.05) in both mechanical properties. In this sense,  m of TPS-1.5B composites resulted 128% higher than the corresponding value of TPS matrix. On the other hand, E of starch films increased 2.6 times with incorporation of 1.5% w/w bagasse. Fibrous nature of bagasse as well as remaining starch content could be responsible for the reinforcement capability of this organic filler. These results are in agreement with those reported by several authors related to starch based films filled with fibers from different origins (Bodirlau et al., 2013; Gáspár et al., 2005; Gironès et al., 2012; Lu et al., 2006; Müller et al., 2009; Vallejos et al., 2011). This significant increase in TPS films rigidity could be attributed mainly to the intrinsic adhesion between filler and matrix due to the similar chemical structures of starch and the bagasse fibers (Ma et al., 2005; Müller et al., 2009). Specifically, Vallejos et al. (2011) also stressed that the addition of bagasse fibers improved significantly  m and stiffness of materials based on cassava and corn TPS. On the other hand, TPS films containing 1.5% w/w bagasse exhibited higher  m and E than those with the same amount of peel, evidencing the higher efficiency of bagasse as filler reinforcement (Fig. 8). This result could be mainly attributed to two contributions. First, it may be associated to the higher content of residual starch in bagasse than in peel, which contributes to the development of a denser matrix structure. On the other hand, the difference in fiber content in both byproducts could be the other responsible factor of their differential reinforcement ability. Particularly, the presence of 1.5% w/w of cassava peel in TPS matrix led to a decrease in  m and E respect to biocomposites containing 0.5% w/w. This behavior could be related to the higher content of larger particles, which led to a less homogenous material (Fig. 5B), and could promote structural defects. With respect to elongation at break (B ), addition of 0.5% w/w bagasse did not affect significantly (p > 0.05) this mechanical property of TPS films (Fig. 8). However, incorporation of the highest assayed concentration of this byproduct increased around 42% TPS B values. This result could be ascribed to the orientation and dispersion of the bagasse on TPS matrix, attributed to the efficient melt processing and thermo-compression, improved the material ability to deform up to fracture. The fact that TPS films ductility increases with bagasse presence is a positive aspect due to this mechanical property is relevant for industrial processing and applications. Even though peel addition at 1.5% w/w led to an increment in B values of starch films, this effect was not significant (p > 0.05) (Fig. 8). Since one of the main functions of a packaging film is often to impede moisture transfer between the product and the surrounding atmosphere, WVP should be as low as possible. Because of the hydrophilic character of starch, TPS based materials have poor water vapor barrier properties and an alternative to overcome this limitation is to incorporate different fillers. Thus, several authors have reported the reduction of WVP of materials based on starch by the addition of different natural fillers, especially fibers from diverse origins (Gilfillan et al., 2012; Prachayawarakorn et al., 2013). Fig. 9 shows the effect of bagasse and peel presence on WVP of cassava TPS films. Obtained value for cassava TPS was similar to those reported for materials based on starch from different botanical origins. Thereby, López et al. (2015) informed that corn TPS films, developed by melt mixing and thermo-compression, presented a WVP of 1.36 × 10−10 g/s m Pa. Phan et al. (2005) stressed that films based on normal rice had a WVP of 1.67 × 10−10 g/s m Pa. On the other hand, cassava and corn starch based films plasticized with 1.5% glycerol, obtained by casting method, exhibited WVP values of 1.4 and 1.2 × 10−10 g/s m Pa, respectively (López and García, 2012). Besides, Han (2014) reported a value of 1.12 × 10−10 g/s m Pa for cassava starch films. Contrary to the expected, no significant influence (p > 0.05) was observed in WVP of films with the incorporation

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F. Versino et al. / Industrial Crops and Products 65 (2015) 79–89 1.8

WVP x 1010 (g/s m Pa)

1.6

a

a

a

Acknowledgements a

a

The financial support provided by ANPCyT (Project PICT 20111213) of Argentina is gratefully acknowledged. Authors wish to thank EEA Montecarlo (INTA, Misiones) for cassava roots provision.

1.4 1.2

1

References

0.8 0.6 0.4

0.2 0

TPS

TPS-B0.5

TPS-B1.5

TPS-P0.5

TPS-P1.5

Fig. 9. Water vapor permeability (WVP) of TPS films containing cassava byproducts as natural fillers.

of cassava byproducts. However, when cassava starch films containing 0.75, 1.5, and 3% cassava bagasse were developed by casting technique, an increase in WVP was observed (Versino and García, 2014). In this work an increase of 54.13% was reported by addition of 1.5% bagasse fiber. This result reveals a clear effect of processing method on final properties of the developed materials, especially on water vapor barrier property. This behavior could be attributed to two offset factors. First, it is well-known that filler presence increases the tortuosity of water molecules pathway, decreasing WVP values. On the other hand, the processing method could promote the generation of voids and structural defects in the matrix which facilitates the water molecules transport, increasing WVP. The compensation of these two factors help to explain the obtained results.

4. Conclusions Cassava byproducts were obtained and their chemical composition and morphology was characterized. Peel contained higher amounts of fiber and protein than bagasse, being starch the main component of the later. Chemical composition determined the solvent retention capacity of cassava byproducts. Moreover, peel presented a higher proportion of larger particles, which affected TPS SEM morphology, mechanical, and barrier properties. Under the processing conditions studied starch-filler interactions were favored, thus leading to lower mixing energy requirements, which were evidenced by lower maximum torque values. The matrix-filler compatibility expected, due to their similar chemical nature, was demonstrated by FTIR and TGA analysis. Likewise, thermal properties of TPS composites are in agreement with band shifts observed by FTIR. Observed increments in Tg2 and Tm are attributed to a more heterogeneous matrix composition as well as the interaction among the components. Filler addition increased UV-barrier capacity and opacity of TPS materials, especially those containing peel; this effect depended on the filler concentration. On the contrary, TPS water vapor barrier properties were not affected by cassava peel or bagasse inclusion. In spite of the low concentration used both byproducts reinforce TPS matrices, being bagasse the most efficient filler due to its high residual starch content and lower proportion of smaller particles. An integral approach of cassava roots use was proposed: biocomposites from cassava TPS and the remaining components of starch extraction (peel and bagasse) as reinforcement agents were developed. Nevertheless, further studies considering higher filler concentrations, processing conditions and chemical modifications are necessary in order to optimize final properties for customized applications of these biocomposites.

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