Thermoplastic Starch-Based Blends: Processing, Structural, and Final Properties Flávia Debiagi, Léa Rita P.F. Mello and Suzana Mali State University of Londrina, Londrina, PR, Brazil
6.1 INTRODUCTION In recent decades, the plastic industry and the academic community have been looking for new raw materials to replace the petrochemical polymers, which are produced from nonrenewable resources. Bio-based polymers have been shown to be a viable alternative to replace these fossil sources and have environmental advantages, such as decreasing carbon dioxide emissions (Imre & Pukánszky, 2013). Moreover, the accumulation of conventional plastic materials can contribute to environmental pollution. Most traditional plastics are inert to microbial attack, and the development of biodegradable packaging derived from renewable natural resources has gained increasing interest (Nafchi, Moradpour, Saeidi, & Alias, 2013). Target markets for biodegradable polymers include packaging materials (trash bags, loose-fill foam, food containers, film wrapping, and laminated paper), hygiene products (diaper back sheets and cotton swabs), consumer goods (fast-food tableware, containers, egg cartons, and toys), and agricultural tools (mulch films and planters). However, the commercialization of these materials is hampered by competition with commodity plastics that are inexpensive and familiar to the customers (Gross & Kalra, 2002). Starches from different sources have been the object of intensive academic and industrial study for several reasons, including their renewable source, biodegradability, low cost and wide availability. Starch has received growing attention since the 1970s, but over the last two decades, several authors have reported the use of this polysaccharide as a main component to produce biodegradable packaging. This biopolymer is a promising material for use as a Starch-Based Materials in Food Packaging. DOI: http://dx.doi.org/10.1016/B978-0-12-809439-6.00006-6 © 2017 Elsevier Inc. All rights reserved.
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viable alternative to the traditional nonbiodegradable polymers, especially in short lifetime applications and when recycling is difficult and/or not economical (Kaseem, Hamad, & Deri, 2012). In its granular form, starch is mostly composed of linear amylose and highly branched amylopectin. Native starches have a semicrystalline character with granular morphology and an overall crystallinity of approximately 20%–45% (Nafchi et al., 2013). Native starch cannot be processed as a thermoplastic material due to the strong intermolecular and intramolecular hydrogen bonds, which means that its thermal processing requires plasticizers, such as water, glycerol, glycol, sorbitol, or urea (Ning, Jiugao, Xiaofei, & Ying, 2007a). Starch granules are desestructurized under the action of heat and shear in the presence of water and plasticizers. As a result, a continuous phase gives rise in the form of a viscous melt that can be processed using the traditional plastic processing techniques, such as injection molding, blow molding, and extrusion. This type of starch material is called thermoplastic starch (TPS) (Aichholzer & Fritz, 1998; Kaseem et al., 2012). Starch materials have interesting characteristics and several applications. However, TPSs are susceptible to moisture when they are stored in high relative humidity (RH) and have poor mechanical properties and processability (Martins & Santana, 2016; Wang, Yang, & Wang, 2003). These are limiting factors for the use of starch-based materials. In materials science, blending can be done to improve the unsatisfactory physical properties of the existing polymers. The modification of polymers by blending is a mature technology that was developed in the 1970s (Imre & Pukánzky, 2013). When TPS is melt-mixed with any other thermoplastic, the mixture can be considered a polymer blend (Nafchi et al., 2013). Blending TPS with other polymers represents an important route to overcoming the aforementioned limitations of TPS (Martins & Santana, 2016). Polymer blending is often a convenient industrial process, and according to Avella, Martuscelli, and Raimo (2000), some economy-related reasons can sustain the blending procedures, such as improving polymer performance by dilution with a low cost material. Blending polymers is easier and cheaper than synthesizing new ones. The main drawback of the blends is linked to the nonmiscibility of polymers, which is due to the differences in their chemical structures (Schwach & Averóus, 2004). In a miscible blend of two or more polymers, a homogeneous material is formed after a mixing process, resulting in a single-phase system with improved properties. In contrast, a compatible blend is a heterogeneous system in which the components show good adhesion among them. In immiscible blends, it is necessary to use compatibilization methods because of the poor adhesion between the polymers (Avella et al., 2000). Immiscible polymers may be described as compatible when their blends do not exhibit gross signals of polymer segregation and when there is good adhesion between the constituents (Verhoogt, Ramsay, & Favis, 1994).
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The compatibility of polymer pairs is often modified by physical (compatibilizers and block copolymers) or chemical (e.g., reactive processing) treatments. Compatibilizers can be used to improve the interfacial adhesion between the polymers, resulting in smaller dispersed phase and producing blends with improved mechanical properties (Imre & Pukánzky, 2013; Martins & Santana, 2016). Thus, the challenges for researchers and the packaging industry in terms of producing starch-based blends with commercial utility are: (1) overcoming miscibility problems at high starch contents, (2) avoiding mechanical property deterioration at high starch contents, even in compatibilized blends, and (3) reducing costs, especially for biodegradable starch-polyester blends at low starch contents (<30 wt.%) (Kalambur & Rizvi, 2006). This chapter discusses the advantages, disadvantages, processing methods, and properties of different blends obtained by mixing synthetic or biodegradable polymers with starch. Besides their mechanical and morphological properties, as well as, the use of compatibilizers to improve the blend properties were analyzed.
6.2 BLENDS OF THERMOPLASTIC STARCH AND SYNTHETIC POLYMERS Petroleum-based polymers are the main constituents of conventional packaging and have been widely used because of their high specific strength, durability, and low cost. Furthermore, the mechanical properties and thermal stability of synthetic polymers are better than those of naturally occurring polymers (Debiagi, Kobayash, Nakazato, & Panagio, 2014; Mali et al., 2010; Sionkowska, 2011). According to Peres, Pires, and Oréfice (2016), conventional synthetic polymers are widely used due to their easy processing, low cost and versatility. However, there are serious environmental problems associated with use of nonbiodegradable polymers due to their very slow degradation. The recycling process is an eco-friendly alternative; however, large amounts of postconsumer packaging cannot be recycled, and in some cases, the process is expensive. According to Dave, Rao, and Desai (1997), the biodegradation process can improve environmental waste management. In this sense, biopolymers and petroleum-based polymer blends are an alternative that can reduce the waste management problem associated with synthetic plastics. These materials exhibit higher biodegradation rates, which results in a reduced volume of plastic waste and in the conservation of petrochemical resources (Salmah & Azieyanti, 2010). Blending starch with synthetic polymers can accelerate the decomposition of conventional packaging materials (Cerclé, Sarazin, & Favis, 2013). Griffin (1977) was the first to employ granular starch as filler in poly(ethylene) materials, and its concentration was as low as 6–15 wt.%. Blends of TPS with traditional polymers were prepared in efforts to obtain new materials with low cost and high biodegradability (Kaseem et al., 2012).
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According to Mohanty, Misra, and Hinrichsen (2000), the most important commodity plastics are poly(ethylene) (PE), poly(propylene), poly(styrene), and poly(vinyl chloride), which are processed into a variety of forms, such as films, flexible bags, and rigid containers. PE is one of the most extensively produced nondegradable polymers, and various types of PEs are extensively used in many fields; therefore, PE is the synthetic polymer that has been studied most frequently in blends with starch in attempts to enhance its biodegradability. Among the various types of PEs, the most studied in blends with starch is LDPE (low-density PE), followed by LLDPE (linear low-density PE) and HDPE (high-density PE). The main influence that starch has in blends with synthetic polymers is related to its higher biodegradation. When starch is associated with synthetic polymers, it can low the cost and increase the degradability of the blends (St-Pierre, Favis, Ramsay, Ramsayt, & Verhoogt, 1997). There have been many efforts to make PE easily degradable, such as using starch as a natural filler in PE. When blends are exposed to a soil environment; the starch component is consumed by microorganisms, which leads to increased porosity, voids formation, and the loss of integrity of the plastic matrix. Thus, this matrix is broken down into smaller particles (Kiatkamjornwong, Thakeow, & Sonsuk, 2001). LDPE has gained attention in film packaging applications. This synthetic polymer has a long branched chain, which provides good processability and high melt strength. However, more research is required to use LDPE-starch blends as biodegradable packaging because of the reduction of the mechanical properties of PE when starch content increases (Hemati & Garmabi, 2011; Sabetzadeh, Bagheri, & Masoomi, 2015). In general, the mechanical properties decrease with starch addition because of the poor compatibility between starch and PE. The starch granules are highly hydrophilic due to the hydroxyl groups located at their surfaces, whereas LDPE is mostly hydrophobic. In many studies, when 20% starch was added, the highly deformable matrix of the synthetic polymer was transformed into a fragile material (St-Pierre et al., 1997; Shujun, Jiugao, & Jinglin, 2005). According to Ramkumar et al. (1996), the presence of starch can also increases the water absorption of the PE matrix. The proportion of starch has a strong influence on the water vapor diffusion coefficient, and the variations in these coefficients between different starch blends indicate dissimilar properties. Blends of starch and synthetic polymers can be processed by extrusion, blown, and injection molding (Ning et al., 2007a). According to Fishman, Coffin, Konstance, and Onwulata (2000), extrusion is often employed as the production method because this process is faster and less energy is required to remove the water. Moreover, the starch can be gelatinized in situ during the extrusion process. These factors are ideal because they reduce the production costs. Many authors have employed the extrusion process to produce starch-synthetic polymer blends (Bhattacharya, 1998; Mani & Bhattacharya, 1998; Nguyen, Vu, Grillet, Ha Thuc, & Ha Thuc, 2016; Ramkumar et al., 1996; St-Pierre et al.,
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1997; Rodríguez-González, Ramsay & Favis, 2003; Rosa, Guedes, & Carvalho, 2007). St-Pierre et al. (1997) studied the behavior of TPS and PE blends that were produced by continuous process in a corotating twin-screw extruder fed by a single-screw extruder. The use of the single-screw as a side feeder allowed the gelatinization of the starch before feeding it into the twin-screw at a controlled temperature and pressure. The TPS content varied from 0 to 36 wt.% of the blend, and the blends presented the morphology of an immiscible system formed by two distinct phases. However, the final material maintained very high elongation properties at break, though the blend did not contain an interfacial modifier. They also reported that the experimental data for elongation in this study suggested some level of adhesion of the hydrophilic starch to the hydrophobic PE. Bhattacharya (1998) stressed that the adhesion of phases is directly related to the mechanical properties of the polymer blends and a strong interfacial adhesion results in a good mechanical performance. Psomiadou, Arvanitoyannis, Biliaderis, Ogawa, and Kawasaki (1997) produced blends of LDPE and wheat or soluble starch by extrusion followed by a hot-press process to produce films. LDPE:starch proportions ranging from 100:0 to 60:40 were used. With higher starch contents, the mechanical and barrier properties of these materials exhibited worse performance, but they were more biodegradable. Arvanitoyanis, Biliaderis, Ogawa, and Kawasaki (1998) produced films based on blends of LDPE and rice or potato starch by extrusion followed by a hot-press process. They observed that high starch contents (>30 wt.%) had an adverse effect on the mechanical properties of the blends. The gas permeability and water vapor transmission rate increased proportionally to the starch content, and the biodegradability rate of the blends was enhanced when the starch content exceeded 10 wt.%. Mani and Bhattacharya (1998) analyzed blends of cornstarch with different contents of amylopectin and amylose with synthetic polymers, which were melt-blended in a corotating twin-screw extruder. They observed that the water absorption decreased when the amylose content increased in the blends, which was associated with the increased degradation of amylopectin in the starch blends during the thermal processing, thus making the materials more water sensitive. Rodriguez-Gonzalez, Ramsay, and Favis (2003) studied the production of blends with 55 and 45 wt.% LDPE and TPS, respectively. The materials were prepared using a one-step process, and had ductility and moduli similar to those of virgin PE, even at very high loadings of TPS without the addition of any interfacial modifier. They also reported that it is possible to achieve blends in which the TPS morphology can be controlled, yielding a wide range of morphologies. According to Pedroso and Rosa (2005), there is a significant interest in the production of blends of recycled LDPE with cornstarch. This new material can substitute the virgin synthetic matrix by postconsumer materials, obtaining
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end-products that would be biodegradable and inexpensive. They compared the properties of LDPE-TPS blends prepared with a corotating twin-screw extruder to virgin and recycled LDPE with TPS contents of 30, 40, and 50 wt.%. The blends had lower tensile strength and elongation at break than did pure LDPE. Therefore, the type of LDPE did not affect the mechanical performance of final materials. Regardless of whether virgin or recycled LDPE was used, the authors observed that the blends were immiscible and that the interfacial interaction was weak. Rosa et al. (2007) reported that the incorporation of TPS (30, 40, and 50 wt.%) within a PE matrix resulted in the separation of phases. Tena-Salcido, Rodríguez-González, Méndez-Hernández, and Contreras-Esquivel (2008) produced blends by mixing TPS and LDPE by one-step extrusion process, with TPS contents ranging from 32 to 64 wt.%. Biodegradation studies indicated that the bacterial attack resulted in higher losses of weight as the TPS content increased. An increment of TPS content resulted in differences in the blends morphology. At 32 wt.%, materials displayed a discrete morphology consisting of a large population of small round LDPE domains dispersed around larger deformed TPS ones. Whereas at 64 wt.%, TPS rich-phases were larger than 1 mm, which allowed the morphological interconnection among them at macroscopic level. Euaphantasate, Prachayawasin, Uasopon, and Methacanon (2008) obtained LLDPE-TPS blends by extrusion, containing starch ranging from 10 to 40 wt.%. Their properties were greatly affected by the starch content. Besides, the mechanical behavior was influenced by the water content and decreased with the starch content increment. The blends evidenced a two-phase morphology with the presence of micro-voids, affecting material properties. Thipmanee and Sane (2012) studied the incorporation of zeolite at low contents (1–5 wt.%) on PE-TPS blends (70 and 30 wt.%, respectively), observing an improvement in the miscibility and mechanical properties of the blends. Zeolite was considered both as a promising physical compatibilizer and as a reinforcing filler, improving the miscibility between hydrophilic and hydrophobic polymers and their final properties. Nguyen et al. (2016) developed LLDPE and TPS films blends, reaching a degree of biodegradation of 63%. These films were degraded into CO2, H2O, methane, and biomass after 5 months in compost soil. These authors concluded that LLDPE/TPS blends were decomposed more quickly than pure LLDPE because of the starch consumed by the soil microorganisms provides a fracture in LLDPE chains. According to Kiatkamjornwong et al. (2001), when starch was exposed to a soil environment, it was consumed by microorganisms. Thus, an increase in porosity with a loss of integrity of the plastic matrix was achieved and broken down into smaller pieces. Peres et al. (2016) reported that introducing natural and biodegradable polymers into synthetic polymers reduces the levels of recyclability compared to the original polymers. This issue would have a negative impact on both environmental and industrial point of view. These authors studied the reprocessing
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of TPS-LDPE blends (50 and 50 wt.%, respectively) simulated by up to 10 extrusion cycles and no alteration of the mechanical, rheological, nor dynamic mechanical properties was observed. Currently, the main challenge for biodegradable plastic researchers is to achieve the compatibility between synthetic polymers and TPS phases. Over the last several years, researchers have reported that it is necessary to use a compatibilizer to increase the interfacial adhesion and to enhance the mechanical properties of blends (Mortazavi, Ghasemi, & Oromiehie, 2013). Among the usually added compatibilizers, it can be mentioned citric acid (Ning et al., 2007a; Ning, Jiugao, Chang, & Xiaofei, 2007b) and maleic anhydride (Bikiaris et al., 1998; Gupta & Sharma, 2010; Mortazavi et al., 2013; Sabetzadeh, Bagheri, & Masoomi, 2012; Sabetzadeh et al., 2015).
6.3 BLENDS OF THERMOPLASTIC STARCH AND BIODEGRADABLE POLYMERS The most common biodegradable materials are blends of TPS and aliphatic/ aromatic polyesters, such as poly(lactic acid) (PLA), poly(butylene adipate terephthalate) (PBAT), poly(hydroxybutyrate) (PHB), and poly(vinyl) alcohol (PVA). PLA and PHB have attracted attention because they are obtained from renewable resources. Among these biodegradable polymers, PVA was the first to be studied in blends with starch. These biopolymers, pure or in blends, have been used to manufacture packaging materials due to their physical and chemical properties are similar to those of the conventional plastics, as observed in Table 6.1. Several authors have proposed to blend these polymers with TPS to reduce cost and to obtain new materials with specific properties. It can be observed that the biodegradable polymers present some comparable properties to synthetic polymers (LDPE, LLDPE, or HDPE). However, TPS, PHB, and PLA (Table 6.1) are brittle, having much lower values of elongation than synthetic ones, attributed to the their chemical structure. This mechanical behavior can prejudice their thermal processability.
6.3.1 Blends of TPS and PVA PVA is a particularly well-suited synthetic polymer for the formulation of blends with natural polymers because it is highly polar and water soluble. Besides, depending on its polar grade, it can be soluble in some organic solvents (Chiellini, Cinelli, Imam, & Mao, 2001). PVA is considered a truly biodegradable synthetic polymer; however, its biodegradation rate is lower than that of poly(hydroxyalcanoates) (PHAs) or PLA. Blending PVA with starch could be a solution to improve the biodegradation rate and to lower the overall cost (Tang & Alavi, 2011). PVA is synthesized by the partial or complete hydrolysis of polyvinyl acetate to remove acetate groups. Its physical properties depend on various factors,
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TABLE 6.1 Properties of Some Synthetic and Biodegradable Polymers Used in Blends with Thermoplastic Starch Polymer
Melting Point (°C)
Tensile Strength (MPa)
TPS (from Author)
PLA (Nature Works 2002D)
Adapted from Müller, Yamashita, Grossmann, and Mali (2012).
such as the production processes used to make the polyvinyl acetate precursor and convert it to PVA, the degree of hydrolysis, the molecular weight, and the water content. Thus, the properties of PVA are a function of its degree of hydrolysis (DH) and of polymerization (DP). Considering the DH, PVA can be classified as partially (86.0%–89.0%), intermediately (90.0%–97.0%), fully (98.0%–98.8%), or highly (98.9%–99.3%) hydrolyzed. Whereas, regarding the DP, PVA can be classified as very low (3–4 cP; DP = 150–300), low (5–6 cP; DP = 350–650), medium (22–30 cP; DP = 1000–1500), or high (45–72 cP; DP = 1600–2200) viscosity (Mansur, Sadahira, Souza, & Mansur, 2008; Sekisui Specialty Chemicals America, 2016). According to Tang and Alavi (2011), an increase in DH results in: (i) lower water solubility, (ii) higher energy stabilization promoted by intra- and interchain hydrogen bonds of the polymer, and (iii) increased adhesion to hydrophilic surfaces, viscosity, and tensile strength. Partially hydrolyzed PVA contains residual acetate groups, which reduce the overall degree of crystallinity. Generally, their formulations have lower melting points, easier processability, lower strength, and water dissolution temperatures than those fully hydrolyzed polymers. TPS-PVA applications are limited by their lack of water resistance and poor mechanical properties (Xiong, Tang, Tang, & Zou, 2008). TPS and PVA have excellent compatibility, and these polymers can be blended in various ratios,
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resulting in materials with improved tensile strength, elongation, and processability compared to pure TPS. However, the blends have a negative effect on the rate of starch degradation, and increasing the amount of PVA will decrease this rate (Mao, Imam, Gordon, Cinelli, & Chiellini, 2000; Vroman & Tighzert, 2009). Westhoff, Kwolek, and Otey (1979) produced starch-PVA films by casting, evaluating the effect of several polyols. They observed a loss of plasticizer effectiveness in high starch content blends. Liu, Feng, and Yi (1999) obtained wheat starch-PVA-glycerol blends by injection molding and reported that starch and PVA were partially compatible based on the morphology of the blends. Tudorachi, Cascaval, Rusu, and Pruteanu (2000) developed blends film by casting based on cornstarch and PVA (DH = 88%). They studied the biodegradation behavior of the blends and the variation in mechanical and thermal properties during this process. The biodegradation of polymeric materials based on PVA, starch and plasticizers (glycerol or urea) in the presence of microorganisms occurs with an important decrease in their physical-mechanical characteristics as well as significant weight losses. The highest values of weight loss were obtained for samples with a high starch content. Microorganisms consumed firstly the starch and then the PVA amorphous phase. Jayasekara, Harding, Bowater, Christie, and Lonergan (2004) reported that blend films containing 20 wt.% PVA (DH = 85%), 60 wt.% starch, and 20 wt.% glycerol were homogeneous and flexible and had an intermediate surface roughness. In these blends, the presence of hydroxyl groups tends to form strong hydrogen bonding among the molecules, resulting in a synergistic stability and a better system integrity (He, Zhu, & Inoue, 2004). Sin, Rahman, Rahmat, and Khan (2010) stressed that when PVA is introduced into a blending system containing starch, the hydroxyl groups of PVA interact freely with those of starch, and this interaction is strong enough to increase the energy stability. Sin, Rahman, Rahmat, and Mokhtar (2011) also claimed that these blends exhibit enhanced thermal stability. Increasing the percentage of starch content in PVA matrix retains the mechanical properties, supporting the evidence of the formation of aforementioned hydrogen bonds (Siddaramaiah, Raj, & Somashekar, 2009). Faria, Vercelheze, and Mali (2012) produced cast films based on a blend of starch and PVA (DH = 86.5%–89.5%). They observed that the introduction of PVA into the starch matrix led to films with lower water vapor permeability, as well as, high tensile strength and elongation. Besides, the surface of films with a TPS:PVA ratio of 95:5 was more homogeneous than the corresponding to pure starch ones, observed by SEM (Fig. 6.1). This is indicative that PVA and starch can interact, leading to films with better properties than those based on pure starch, even at lower PVA contents. According to Chai, Chow, and Chen (2012), the common properties of TPS and PVA make them an excellent pair for blending, and the water solubility of PVA makes it easy to mix evenly with the starch. These authors studied
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FIGURE 6.1 SEM micrographs of films surfaces based on: (A) pure TPS and (B) 95:5 TPS:PVA.
blends employing PVAs with different molecular weights (75,000–120,000) and several compositions of cross-linked starch to understand the effects of the molecular weight of PVA on the biodegradable characteristics of the blends. They observed that PVA with higher molecular weights displayed high biodegradability, which increased with the starch content. The tensile strength was augmented and the elongation diminished with the molecular weight of PVA, which was related to the notable toughness of the PVA chains as result of high molecular weights. Sreekumar, Al-Harthi, and De (2012) reported the effect of glycerol content on properties of TPS-PVA (1:1) blends. They obtained films by casting varying glycerol content from 1 to 5 mL for each 100 mL of filmogenic solution. At high glycerol contents the crystallinity decreases and it was observed a lowering of tensile properties and energy at break. In this blend, maximum of 3 mL of glycerol could be used without causing sharp drop in the ductility, mechanical, and dynamic mechanical properties. Zanela et al. (2015a) produced biodegradable sheets by extrusion with different proportions of cassava starch, PVA (DH = 88%) and glycerol. The opacity of the materials ranged from 31% to 56% and increased with starch concentration. In other work, Zanela et al. (2015b) processed these blends by flat extrusion, obtaining biodegradable sheets. They observed that PVA was the main component that improved the sheets properties; meanwhile glycerol presented the opposite effect. By observing the morphology of the blends, good compatibility between their components was found because the resulting structures were continuous and cohesive and did not exhibit phase separation (Fig. 6.2). Moreover, PVA was able to improve the mechanical and barrier properties of starch-based sheets. Several works reported the use of starch-PVA blends to produce starch-based loose-fillers or foam plates as a replacement for expanded polystyrene (Chiellini, Cinelli, Ilieva, Imam, & Lawton, 2009; Cinelli, Chiellini, Lawton, & Imam, 2006; Debiagi, Mali, Grossmann, & Yamashita, 2011; Debiagi & Mali, 2012). These authors stressed that the addition of PVA improved the mechanical properties of starch foams, possibly due to the interactions between these biopolymers.
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FIGURE 6.2 SEM micrographs of blend films surfaces of: (A) 63:7:30 and (B) 49:21:30 starch:PVA:glycerol. From Zanela et al. (2015b).
6.3.2 Blends of TPS and PLA PLA is a thermoplastic aliphatic polyester with a hydrophobic character. According to Mohanty et al. (2000), PLA is synthesized by the condensation polymerization of D- or L-lactic acid or ring opening polymerization of the lactide, which may be derived from the fermentation of sugar feedstock at competitive prices. PLA can be molded into bottles, containers, and other products by injection molding or blown and can be extruded into films and sheets (Soroudi & Jakubowicz, 2013). PLA possesses some advantages, such as good biocompatibility and processability, high strength and modulus. However, PLA is very brittle under tension, develops serious physical aging during application, and has a higher cost compared with other biodegradable polymers (Jun, 2000; Lu, Xiao, & Lu, 2009; Li et al., 2016; Yang, Tang, Xiong, & Zhu, 2015). The blends of starch and PLA have many advantages, such as low cost, biodegradability, low-density, high toughness, thermal resistance, and renewability (Li et al., 2016). Several studies have investigated the production and characterization of these blends using several processes and proposing different applications (Acioli-Moura & Sun, 2008; Arboleda, Montilla, Villada, Varona, & Huang, 2015; Jacobsen & Fritz, 1996; Ke & Sun, 2003; Ke, Sun, & Seib, 2003; Li et al., 2016; Li, Wang, Zhuang, Hu, & Chu, 2011; Li & Huneault, 2011; Mano, Koniarova, & Reis, 2003; Mihai, Huneault, Favis, & Li, 2007; Muller, Pires, & Yamashita, 2012; Park et al., 1999; Park, Im, Kim, & Kim; 2000; Shirai et al., 2013; Wang, Sun, & Seib, 2002). Jacobsen and Fritz (1996) reported that PLA is a very brittle material and blending PLA with native starch may be unrealistic because the dispersed starch granules increase this characteristic. To avoid this drawback, these authors employed low molecular weight poly(ethylene glycol) (PEG) in the PLA matrix to enhance crystallization and lower the glass transition temperature significantly under possible usage temperatures. PLA and starch blends combine the good mechanical properties of PLA and the low cost of starch; however, the improvement of the mechanical behavior
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is dependent on the adhesion between the polymers. Park et al. (1999) reported that PLA and starch interact by hydrogen bonding but the adhesion between them is poor. Park et al (2000) claimed that in starch-based blend systems, the interfacial affinity between both polymers is one of the most important factor, which affects the blends mechanical properties. The crystalline structure of starch must be destroyed in the native granules before they can be mixed with other polymers in order to improve the compatibility between them. The properties of starch-PLA blends are affected by several variables, including the starch ratio and moisture content, blend heat treatment, plasticizers, and coupling agents. PLA dominates the mechanical properties of the blends, and the microstructure of this biopolymer, which is formed during thermal processing, has a significant effect on the ultimate products quality (Ke & Sun, 2003). Ken et al. (2003) studied the morphology, mechanical properties, and water absorption of four corn starches with different amylose levels (0%, 28%, 50%, and 70%) that were blended with PLA using twin-screw extrusion. They reported that the PLA phase was continuous at low starch contents (<40 wt.%) but became discontinuous as the starch content increased further than 50 wt.%. They also reported that the adhesion between starch and PLA was poor based on the space gaps observed on the film surfaces by SEM. Blends of starch and PLA can be affected by physical aging, which is a natural phenomenon that affects the amorphous phase in glassy polymers (Cai, Dave, Gross, & McCarthy, 1996; Acioli-Moura & Sun, 2008). According to Pan, Liang, Zhu, Dong, and Inoue (2008), physical aging occurs above the glass transition temperature (Tg) in the glassy state, where the chain mobility decreases and the molecules are unable to reach an equilibrium packing density and conformational structure with respect to a given temperature. The process of the relaxation toward to the equilibrium state is commonly referred to as physical aging, and this phenomenon has been studied because it is important for determining the long-term performance of a material system. The effects of aging can be observed by a shrinkage of a specific volume, a decrease in specific enthalpy, entropy, and in molecular mobility. These effects are associated with the motion of large segments of the polymer chains, affecting their mechanical and barrier properties. Besides, RH plays an important role in materials physical aging. Neat PLA and PLA-starch blends had a significant reduction in tensile strength when they were stored at a RH fluctuating between 30% and 90% (Acioli-Moura & Sun, 2008). Pan et al. (2008) studied PLA/starch blends and reported that the fracture strain in the tensile test decreased from more than 300% to ∼6% during physical aging, which was accompanied by the fracture mechanism varying from ductile to brittle. Li et al. (2011) investigated blends of PLA and starch found in acorn kernels, which are an important starch source at wildlife forestry and are the seed for oak tree regeneration. They observed poor adhesion between acorn powder and PLA matrix, but they also reported that the blends did not show any difficulties
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in the extrusion, injection molding, and hot-compression processes, indicating the possibility of industrial production of these materials by conventional manufacturing methods. Müller et al. (2012) also reported that both extrusion and thermopressing can be successfully used in the obtaining of starch-PLA blends. Several plasticizers have been studied to improve the properties of TPS, but glycerol and sorbitol are the most studied compounds. Li and Huneault (2011) reported the use of glycerol and sorbitol as TPS plasticizers in TPS-PLA blends, and they observed that the sorbitol-plasticized TPS phase can be more finely dispersed and uniformly distributed in the PLA matrix, even in absence of compatibilizers. The sorbitol-plasticized TPS-PLA blends exhibited much higher tensile strength and modulus than those plasticized with glycerol. Shirai et al. (2013) found that glycerol is not a good plasticizer for PLA, although it is proper for starch. Müller et al. (2012) reported the immiscibility of the TPS-PLA blends and the presence of two phases. They studied the incorporation of 10, 20, and 30 wt.% PLA into the starch matrix and observed that there was an increase in both tensile strength and modulus for blends with 20 and 30 wt.% PLA. The lowest water vapor permeability was observed for the blend with 30 wt.% PLA. Several authors have reported poor interfacial adhesion between starch and PLA when these polymers are directly blended, which resulted in brittle materials. The lack of affinity between the TPS and PLA was a severe limitation and emphasized the need for a compatibilization strategy (Martin & Averóus, 2001; Wang et al., 2002; Ken et al., 2003; Muller, Pires & Yamashita 2012, Shirai et al., 2013, Yang et al., 2015). Thus, several efforts have been made to improve the compatibility between these polymers to enhance the final blend performance (Schwach, Six, & Averóus, 2008). Some compounds that are used as compatibilizers, plasticizers, flexibilizers, and hydrophobic agents. They can improve the compatibility between PLA and starch granules, plasticize starch granules to enhance their dispersion into PLA matrix, and improve the toughness of the PLA matrix or the hydrophobicity of the blends (Li et al., 2016). The use of compatibilizers is discussed in Section 6.4 of this chapter.
6.3.3 Blends of TPS and PHB Other polymers that have been studied include the PHAs family of biodegradable polyesters. These polymers, which are produced by fermentation, occur as intracellular inclusions within the cytoplasm of many prokaryotic organisms. In particular, poly-3-hydroxybutyrate (PHB) and poly-(3-hydroxybutyrate-cohydroxyvalerate) [P(HB-co-HV)] are the most promising PHAs (Kaseem et al., 2012). Similarly, PLA, PHB, and derivatives are expensive polymers, and their blends with starch are economically advantageous. PHB is a semicrystalline thermoplastic polyester with a high melting point (173–180 °C) that is insoluble in water, soluble in chloroform, biocompatible,
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and biodegradable. With mechanical and physical properties similar to those of polypropylene, the main drawbacks of PHB and P(HB-co-HV) are their highly brittle structures and thermal instability (D’Amico, Iglesias Montes, Manfredi, & Cyras, 2016; Ten, Jiang, Zhang, & Wolcott, 2015). Ramsay, Langlade, Carreau, and Ramsay (1993) produced biodegradable films from wheat starch and P(HB-co-HV) blends by casting using chloroform as solvent. They observed that increasing the starch content from 0% to 50% decreased tensile strength and flexibility, but they also reported that the blend obtained with a proportion of 50:50 of starch:P(HB-co-HV) exhibited useful thermoplastic properties. Godbole, Gote, Latkar, and Chakrabarti (2003) observed that blending starch with PHB in a ratio of 30:70 could be advantageous for cost reduction with improved properties compared to virgin PHB. Lai, Don, and Huang (2006) prepared and characterized TPS-PHB blends using three types of starch, including potato, corn, and soluble potato starch, with different ratios of glycerol. In all cases considered in their study, mechanical properties of TPS blended with PHB conferred higher performance than those of pure TPS. Thiré, Ribeiro, and Andrade (2006) produced cornstarch and PHB blends with starch contents that ranged from 0% to 50%. PHB was melt processed (165 °C, 20 min) and mixed with TPS plasticized with glycerol. The mixtures were hot-pressed (160 °C, 30 min) to obtain sheets. The incorporation of starch did not affect the thermal stability of PHB; however, higher starch contents led to poorer mechanical properties. These results may be explained by the lack of interfacial adhesion between starch and PHB and by the heterogeneous dispersion of starch granules within the PHB-rich matrix. Parulekar and Mohanty (2007) reported that both TPS- and PHA-based materials demonstrate aging behavior, which results in a loss of properties as a function of storage time. TPS is a hydrophilic material that has high moisture uptake when it is stored in high RH, resulting in a decrease in mechanical properties. Another reason for TPS aging is the leaching of the plasticizer from the system. For PHA materials, this aging is a result of the progressive reduction of the amorphous content in the semicrystalline polymer and the development of secondary crystallization during storage. The small crystallites produced during the secondary crystallization reduce the mobility of the chain segments, thereby increasing the stiffness and embrittling the material. However, these authors observed that TPS-PHB blends did not exhibit discernable aging behavior during 30 days of storage at 30°C and 50% RH. Reis et al. (2008) prepared blends of P(HB-co-HV) with maize starch by casting, and they assayed starch contents ranging from 0 to 50 wt.%. These authors observed that no intermolecular interactions occurred between the polymers, which suggests that P(HB-co-HV) and starch are immiscible. The blends showed a lack of interfacial adhesion between starch and P(HB-co-HV) and a heterogeneous dispersion of starch granules over the P(HB-co-HV) rich
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matrix. Zhang and Thomas (2010) reported that intermolecular hydrogen bonding occurred between PHB and maize starch with different amylose and amylopectin contents. Lai, Sun, and Don (2015) reported that starch and PHB are incompatible and have brittle behavior and that the blend of these polymers does not form intact films. They also reported that it is necessary to use a compatibilizer to improve the TPS-PHB blend properties.
6.3.4 Blends of TPS and PBAT Poly(butylene adipate-co-terephthalate) (PBAT) is a biodegradable aliphaticaromatic copolyester that has been shown to be an interesting polymeric partner in starch-based blends (Raquez et al., 2008). However, several authors have reported that PBAT and starch are thermodynamically immiscible, which leads to materials having poor adhesion characteristics as well as deficient physical and mechanical properties (Garcia et al., 2011; Nabar, Raquez, Dubois, & Narayan, 2005; Ren, Fu, Ren, & Yuan, 2009). Averóus and Fringant (2001) studied blends of TPS with different biodegradable polyesters, including PBAT. They observed that PBAT and TPS blends presented better interphase compatibility than other polyester blends, such as polycaprolactone (PCL) or polybutylene succinate-adipate (PBSA). They also demonstrated that the blends had better mechanical properties and a more hydrophobic character than the pure starch materials. Ren et al. (2009) obtained binary and ternary blends of TPS, PLA, and PBAT using a one-step extrusion process. The concentration of TPS in both binary and ternary blends was fixed at 50 wt.%, with the rest being PLA and/ or PBAT. They produced blends both with and without a compatibilizer and observed that the addition of a small amount of compatibilizer greatly increased the final mechanical properties. Mechanical performance of the blends exhibited a dramatic improvement in elongation at break with increasing the PBAT content. The water uptake of the compatibilized blends was lower, and the time required to reach equilibrium water uptake was longer than those of noncompatibilized blends. Bilck, Grossmann, and Yamashita (2010) developed black and white biodegradable films by blown-extrusion from cassava starch and PBAT blends to use as mulching film in strawberry production. TPS was produced by mixing cassava starch with glycerol (75:25), and this mixture was extruded, pelletized, and blended with PBAT (30 TPS:70 PBAT) to the white films. For the black ones, 2% pigment was added to the blend. They observed that PBAT films provided efficient mulching for strawberry production, similar to PE film performance; but their mechanical properties were altered after 8 weeks on the ground, and the weight mulching decreased, due to its possible biodegradation, cross-linking and photodegradation.
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Brandelero, Yamashita, and Grossmann (2010) produced TPS-PBAT film blends employing a surfactant (Tween 80) to improve the interfacial adhesion between the polymers. Starch-PBAT films were processed in the amounts of 50, 65, and 80 g starch/100 g PBAT and 2 g of Tween 80 per 100 g starch/PBAT. However, the addition of the surfactant did not result in the desired effect since resulting films showed less structural integrity and lower tensile strength than did control films (without surfactant). The addition of soybean oil to TPSPBAT blend compatibilized with Tween 80 improved the mechanical properties of the films and resulted in a more homogeneous microstructure (Brandelero, Yamashita, & Grossmann, 2012). The method of producing blends of starch-PBAT affected their mechanical, barrier, and microstructural properties. Brandelero, Yamashita, and Grossmann (2011) processed blends using two different methods. In the first one, starch granules were mixed with glycerol and extruded to produce pellets of TPS, which then were extruded with PBAT and doubly pelletized to improve blends homogeneity. The PBAT-TPS pellets were blown-extruded to produce films. In the second method, the blend was obtained by mixing granular starch, glycerol, and PBAT. These blends were extruded twice, and the films were produced by blown-extrusion. The first method resulted in films with better mechanical properties when the concentration of PBAT was 50 wt.%. With an increased concentration of starch (>50 wt.%), blends can be prepared from the second method (granular starch) in a single extrusion without a loss of mechanical properties, which can result in lower production costs. Several authors reported the use of compatibilizers to improve the mechanical and microstructural properties of TPS-PBAT blends, such as maleic anhydride (Mohanty & Nayak, 2010; Nabar et al., 2005; Raquez et al., 2008,), citric acid (Garcia et al., 2011, Olivato, Grossmann, Yamashita, Eiras, & Pessan, 2011; Olivato, Grossmann, Bilck, & Yamashita, 2012), and tartaric acid (Olivato, Müller, Carvalho, Yamashita, & Grossmann, 2014). Therefore, among the synthetic and biodegradable polymers that were blended with starch, PVA is the most compatible with starch. TPS-PVA blends present improved mechanical properties, evidencing interactions between the chains of both polymers. Several authors have also reported that blending starch with other biodegradable polymers with high cost, such as PLA, PBAT, or PHB, or with other synthetic ones can be accompanied by disadvantageous changes in other characteristics, which are related to the poor interfacial adhesion between starch and these polymers. Most of them are hydrophobic (or more hydrophobic than starch) and are thermodynamically immiscible with hydrophilic starch; thus, simple mixing results in phase incompatibility and poor mechanical properties. According to Imre and Pukánszky (2013), if a polymer is not miscible with starch, blending can result in inferior mechanical properties compared to both components, and compatibilization is often needed to result in smaller dispersed domains, which can lead to improved mechanical properties.
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6.4 EFFECT OF COMPATIBILIZERS ON THE PROPERTIES OF BLENDS BASED ON TPS Polymer blends are either miscible or immiscible, depending on the thermodynamic characteristics of the constituents. Miscible blends are homogeneous at molecular level, and there is an association with the negative value of the free energy of the mixture. Immiscible blends are those that have more than one phase and are characterized by a positive value of free energy, resulting from the high molecular weight of the polymers and the unfavorable enthalpic interaction between the constituent components (Utracki, 1999, Paul & Newman, 1978). Most polymer blends are immiscible; they are thermodynamically unstable and should be stabilized to prevent coalescence during melting. This process of stabilizing polymer blends is commonly called compatibilization. This is a process of modification of the interfacial properties and may be analyzed under three aspects: (i) reduction of the interfacial tension between the dispersed phase and the matrix; (ii) morphology stabilization against high stress and deformation applied in the material during the processing; and (iii) improvement of the adhesion between the phases in the solid state. Therefore, compatibilization is an essential process that converts a mixture of polymers into a material with the desired set of performance characteristics (Utracki, 2002). Compatibilization methods can be separated into two categories: (i) physical compatibilization utilizing a block copolymer and (ii) chemical compatibilization with an in situ reactive compatibilizer. Incompatible polymer blends exhibit coarse morphology and poor mechanical properties. Therefore, compatibilization techniques have been widely used to improve the poor properties of immiscible polymer blends (Jeon & Kim, 1998; Utracki, 2002). Chemical compatibilization has been widely researched (Moad, 2011). The morphology of incompatible polymer blends with a compatibilizer is fine because the copolymers formed by the reaction between the functional groups that exist in the components of the blend can reduce the interfacial tension between the dispersed phase and the polymeric matrix (Jeon & Kim, 1998). The chemical structures of biopolymers often contain reactive groups that exhibit excellent possibility for the compatibilization. The structure and properties of the blends are controlled by the proper selection of agents, blend composition, and processing conditions. Modification involves the formation of groups on one component that are able to react with the second component during blending, and the phases can be coupled chemically (Imre & Pukánszky, 2013). In addition to the interfacial tension, several authors have reported that the morphological properties of the blends are a key parameter controlling their properties and that the morphology can be affected by the rheological properties of the components, processing conditions, and blends composition (Steinmann, Gronski, & Friedrich, 2001; Wu, 1987; Serpe, Jarrin, & Dawans, 1990). In most cases, the major component of the blend forms the continuous phase, whereas
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the minor component is the dispersed phase (Shujun et al., 2005). A wide range of different morphologies can be obtained by melt mixing, such as spherical, dispersed, lamellar, fibrillar, and even cocontinuous (Steinmann et al., 2001; Rodriguez-Gonzalez et al., 2003). Cocontinuous microstructures are distinguished by the mutual interpenetration of the phases (Galloway & Macosko, 2004). This morphology is particularly interesting because both components can contribute fully to the properties of the blend, resulting in a synergistic improvement of the final properties. In blends, depending on the polymer concentrations, phase inversion can occur, a process in which two phases switch their functions (He, Bu, & Zeng, 1997; Steinmann et al., 2001; Willemse, de Boer, van Damand, & Cotsis, 1998; Willemse, de Boer, van Damand, & Cotsis, 1999). This occurs when both polymers are present in approximately equal amounts or at high concentrations of the minor phase (Schwach & Averóus, 2004). The cocontinuous structure depends on the relative viscosities, the interfacial tension, and the mixing conditions. This type of morphology often occurs near to the phase inversion point (Steinmann et al., 2001; Willemse et al., 1998; Willemse et al., 1999). Several authors have described the production and characterization of PE-TPS blends, but most of these works were performed using large amounts of PE (matrix) and lower TPS contents (dispersed phase). In blends with higher TPS contents, the morphology of the blends changes significantly, and phase inversion occurs. Above the phase inversion concentration, TPS, as a dispersed phase, turns into the matrix, which results in poor mechanical properties and high sensitivity to moisture (Mortazavi et al., 2013). Copolymerization, grafting, transesterification, and the use of reactive coupling agents have all been successfully utilized to achieve blends with improved properties. Grafting of a polymer is an important method to obtain polymers with functional groups. This starch modification with anhydrides results in the formation of free acid groups, and the obtained reactive polymers are frequently used for the compatibilization of starch-based blends that contain a large number of hydroxyl groups (Coltelli, Toncelli, Ciardelli, & Bronco, 2011; Imre & Pukánszky, 2013). Graft modification of polyolefins with maleic anhydride (MA) to improve their hydrophilicity dates back to the 1969s (Rzayec, 2011), and much later it was employed to graft biodegradable polymers (Vaidya & Bhattacharya, 1995; Vaidya, Bhattacharya, & Zhang, 1995). MA and its isostructural analogs (maleic, fumaric, citraconic, and itaconic acids and their amide, imide, ester, and nitril derivatives) as polyfunctional monomers are being widely used in the synthesis of reactive macromolecules to prepare high performance materials. These monomers are also successively utilized for the graft modification of various thermoplastic polymers (polyolefins, polystyrene, and polyamides, among others), biodegradable polymers, polysaccharides, natural, and synthetic rubber, and biopolymers, among others (Rzayev, 2011).
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MA is a strong hydrophilic monomer, which is grafted onto polymers, giving a dense distribution of carbonyl or free carboxylic groups. These reactive groups can also serve as sites for further macromolecular reactions of copolymers and grafted polymers, especially for compatibilization of immiscible polymers and preparation of various reactive blends with high performance and controlled morphology and mechanical properties (Rzayev, 2011). Maleated polymers can be prepared in the presence of organic peroxide initiators and at temperatures around 200°C. Several authors proposed a grafting mechanism based on a macroradical formation from reaction between the initiator and polymer chain. Then, it is carried out the grafting of MA to polymer macrochain with formation of -MA• in the side chain, and transformation of this radical to succinic anhydride units via H-transfer. However, MA oligomerization occurs more frequently, leading to end chains of poly(MA) (Russell, 1995, Mani, Bhattacharya, & Tang, 1999, Rzayev, 2011). The reactive compatibilization of starch blends with MA has been extensively studied (Ramkumar, Bhattacharya, & Vaidya, 1997; Sailaja & Chanda, 2001; Shujun et al., 2005; Wang, Sun, & Seib, 2001, 2003; Yu, Dean, Yuan, Chen, & Zhang, 2007; Zhang & Sun, 2004). MA is one of the foremost approaches to improve compatibility in starch blends and it has been used to modify many different biodegradable and nonbiodegradable polymers, both hydrophobic and hydrophilic, to produce functional polymers (Shujun et al., 2005). Grafting with MA may be performed by different methods, such as in solution, in suspension, and in the melt and solid states. The most widespread method is the melt state process often called reactive extrusion method (Mani et al., 1999; Moad, 2011; Raquez et al., 2008). This is a viable process in an industrial application due to its high conversion efficiencies, an absence of byproducts (or the ability to simply and efficiently remove or recycle excess reagents and byproducts) and a rapid production rate. Moreover, extrusion processing can provide a continuous reactor environment for a combination of thermomechanical and chemical treatment (Moad, 2011). The processing parameters (temperature, time, initiator, and monomer concentrations) must be optimized to avoid side reactions, such as cross-linking, polymer degradation, oligomerization of monomer, and β-scission of C-C bonds in the main-chain (Kalambur & Rizvi, 2006, Rzayev, 2011). In the plastic industry, MA-grafted polymers are generally used as compatibilizers between ungrafted and other polar polymers. The use of these functional compatibilizers was found to improve the modulus, strength, and elongation by forming a continuous phase in the blend. Thus, these derivatives can act as compatibilizers between nonfunctional polymers and starch (Kalambur & Rizvi, 2006). The nonbiodegradable synthetic polymer that is most frequently grafted with MA is PE because of its economic relevance, resulting in polyethylene-g-maleic anhydride (PE-g-MA). This functionalization lowers the hydrophobicity of PE,
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thereby imparting polarity, which makes it more compatible with hydrophilic compounds, such as starch (Shujun et al., 2005). The interfacial tension is largely controlled by the polarities of the two phases (PE and starch) (Wu, 1987). Thus, the addition of grafted copolymer reduces the interfacial tension between these phases and promotes the adhesion between them. Besides, the presence of this compatibilizer increases the surface area and stabilizes the morphology of the dispersed one. PE-g-MA was considered to be the most effective compatibilizer between starch and PE. The efficiency of PE-g-MA as a compatibilizer in the case of TPS-PE blends has been attributed to the esterification reaction between the maleic anhydride groups of PE-g-MA and the hydroxyl groups of starch, as well as, the good interaction of its nonpolar chain with the PE matrix (Ramkumar et al., 1997; Yoo et al., 2002). However, this derivative presents some disadvantages, such as its high price and difficult manufacturing process (Shujun et al., 2005). Another advantage that can be attributed to the use of PE-g-MA in blends of starch with PE is that these blends have better flow behavior during extrusion process, which arises from two factors: (i) the arrangement of molecules was more orderly and (ii) the molecular weight reduction of starch and PE by MA presence (Wang, Yu, & Yu, 2005). Chandra and Rustgi (1997) grafted MA onto LLDPE (LLDPE-g-MA) and then blended with cornstarch using different concentrations (between 10 and 60 wt.%). They observed that the tensile strength and modulus increased and the percentage elongation decreased with starch content, suggesting good interfacial adhesion between LLDPE-g-MA and starch. All of the blends had a high percentage of degradation when they were exposed to a soil environment. Bikiaris and Panayiotou (1998) studied the effect of incorporating LDPEg-MA as a compatibilizer of blends based on LDPE and native cornstarch, emphasizing that the biodegradation of compatibilized blends was only slightly lower than that of uncompatibilized blends. Sailaja and Chanda (2001) used cassava starch (20 and 40 wt.%) in both glycerol-plasticized (TPS) and unplasticized blends with HDPE using HDPE-g-maleic anhydride as compatibilizer. In general, blends of HDPE and TPS with added compatibilizer had better mechanical properties than unplasticized starch blends. The addition of 25 wt.% compatibilizer improved the dispersion of starch in HDPE matrix. The compatibilizer promoted better adhesion between starch and HDPE because starch contains a large number of hydroxyl groups. On the other hand, the formation of ester groups by reaction with the anhydride groups of the compatibilizer promotes the better dispersion of starch in the HDPE matrix. Matzinos, Bikiaris, Kokkou, and Panayiotou (2001) produced LDPE-TPS with different TPS content (10, 20, 30, 40, and 50 wt.%) using PE-g-MA as compatibilizer by extrusion, injection molding, and film-blowing. TPS containing 20 wt.% glycerol could act as a reinforcing agent of LDPE matrix in injection molded materials but not in films. LDPE-TPS injection-molded products containing up to 50 wt.% TPS can be easily prepared, but in the blown films
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TPS presence appears to cause processing problems, which was attributed to the higher water content. Liu, Wang, and Sun (2003) studied the effects of LDPE-g-MA on thermal properties, morphology, and tensile properties of blends based on LDPE (75 wt.%) and cornstarch (25 wt.%). The interfacial properties between starch and LDPE were improved after PE-g-MA addition, and the tensile strength and elongation at break of the blends was greater than that of uncompatibilized LDPE-starch blends. Wang et al. (2005) confirmed the effectiveness of MA as a compatibilizer of TPS-LLDPE blends. They produced TPS-LLDPE blends with five different levels of TPS (50, 60, 70, 80, and 90 wt.%). MA and dicumyl peroxide were used as monomer and initiator at 1 and 0.1 wt.%. They observed that the blends containing MA showed higher tensile strength, elongation at break, and thermal stability than those of blends without MA. They also stressed that LLDPE-g-MA was used as compatibilizer based on: (i) the ester-forming ability of anhydride groups with hydroxyl groups on starch, (ii) the hydrogen bond-forming ability between carboxyl groups of hydrolyzed MA and hydroxyl groups on starch, and (iii) the substantial compatibility between grafted LLDPE and LLDPE phase. Shujun et al. (2005) produced TPS-LLDPE blends by one-step reactive extrusion in a single-screw extruder, using high TPS levels in (50–90 wt.%). They observed that MA could graft onto the LLDPE chain using the same operational conditions as the preparation of the blends, which had better mechanical properties and higher thermal stability than did the blends without MA. Additionally, the morphology of the blends with MA was improved due to the enhanced compatibility between TPS and LLDPE. Majid, Ismail, and Taib (2009) analyzed the effects of LDPE-g-MA on the properties of LDPE/TPS blends. TPS was blended with sago starch using 35 wt.% glycerol and different TPS loadings, both with and without the addition of PE-g-MA. The mechanical properties of LDPE/TPS/PE-g-MA blends were greater than those of LDPE/TPS blends, particularly at higher TPS concentrations. The interfacial properties between LDPE and TPS were improved after PE-g-MA addition, confirmed by FTIR. Gupta, Kumar, and Sharma (2010) produced extruded blown films from blends of LDPE grafted with MA, LDPE, and TPS (30 wt.% glycerol). They employed LDPE and LPDE grafted with MA (1:1), in 0, 2.5, 5, 7.5, 10, 12.5, and 15 wt.% ratios of potato starch. They reported that mixing TPS up to 15 wt.% did not affect the basic properties of LDPE, which are required for packaging applications. Besides, thermal stability and melting behavior of the blown film remained undisturbed. Gupta and Sharma (2010) observed that these blends resulted in a uniform continuous phase and that the MA played a key role in reducing the interfacial energy and promoting the adhesion between potato starch and LDPE-g-MA/LDPE (1:1). TPS and LDPE containing LDPE-g-MA were studied by Mortazavi et al. (2013). TPS was prepared with 36 wt.% glycerol, and TPS:LDPE proportions
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ranging from 75:20 to 20:75 and 5 wt.% of LDPE-g-MA. The mixtures were blended at 130°C for 5 min with and 80 rpm rotor speed. Then, samples were formed into the desired shapes by compression molding. To obtain higher efficiency LDPE-TPS blends, TPS content must not exceed the phase inversion concentration, which occurred beyond 75 wt.%. At low TPS concentration, a droplet-matrix morphology was formed, but near the phase inversion point, continuous one was observed. LDPE-LLDPE-TPS blend films containing high TPS content in a twinscrew extruder followed by a blowing process were developed by Sabetzadeh et al. (2015). They observed good dispersion of TPS in PE matrices and confirmed the compatibility between polymers using PE-g-MA as compatibilizer. The tensile strength and elongation decreased when TPS increased from 5 to 20 wt.%. However, the required mechanical properties for packaging applications were attained when 15 wt.% starch was added, and 12% increase in water uptake was achieved. As discussed above, MA has been grafted onto synthetic polymers (PE, LDPE, and HDPE) to produce functional polymers with very good results in terms of improving the compatibility of these polymers with starch. This process could also be used in biodegradable polyesters to enhance their compatibility with TPS in polymeric blends (Kalambur & Rizvi, 2006). Maleated TPS (MTPS) was produced employing an one-step and continuous plasticization (with glycerol—20 wt.% starch basis) and maleation reaction of starch by reactive extrusion. When 2.5 wt.% MA was used, reaction of glycerol with starch backbone occurred during the maleation process. Increasing MA content, it was observed the occurrence of hydrolysis and glucosidation promoted by MA moieties grafted onto the starch backbone, which reduced the melt-viscosity and the relative molecular weight of MTPS. It was observed a complete disruption for the granular structure of native starch in the reactive extrusion. It was a clear evidence of grafting and that also the reaction occurred preferentially at the C-6 hydroxyl, but the efficiency was not precisely determined (Raquez et al., 2008). Carlson, Nie, Narayan, and Dubois (1999) reported that a free-radical-initiated grafting of MA onto a PLA backbone was performed by reactive extrusion. The process resulted in a little degradation of PLA. The maleation of PLA was efficient in promoting strong interfacial adhesion with the corn native starch obtained by melt blending. Zhang and Sun (2004) produced blends with PLA (55 wt.%) and wheat starch (45 wt.%) compatibilized by MA using a lab-scale coextruder. An initiator (2,5-bis(tert-butylperoxy)-2,5 dimethylhexane—L101) was used to improve the compatibility among PLA, starch, and MA. The interfacial adhesion between PLA and starch was significantly improved by MA presence, as observed by the mechanical properties, which increased markedly compared to the virgin composites of PLA-starch. The PLA-starch blends compatibilized by 1 wt.% MA and initiated by 10 wt.% L101 (MA basis) resulted in the highest tensile strength and elongation.
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Wang, Yu, and Ma (2007) obtained TPS–PLA compatibilized blends in a single-screw extruder by one-step reactive extrusion. They used MA as compatibilizer in the presence of dicumyl peroxide as initiator. Homogeneous blends could be achieved as observed their morphology, and the tensile strength of compatibilized blends was higher than that of the original ones. The blend became more thermally stable as shown by thermogravimetric analysis results. They stressed that the better dispersion and compatibility between PLA and TPS results from the formation of branched and cross-linked macromolecules caused by reaction of the carboxyl groups of PLA-graft-MA with the hydroxyl groups of starch. One-step reactive extrusion was a promising way to prepare starch-based materials. Li and Huneault (2007) studied the properties and interfacial modification of PLA and glycerol-plasticized TPS. This modification was achieved by freeradical grafting of MA onto PLA and then by reacting the modified PLA with the starch. TPS was employing in concentrations ranging from 27 to 60 wt.%. A twin-screw extrusion was used to gelatinize the starch, devolatilize the water to obtain a water-free TPS and then to blend with PLA matrix. Elongation at break of compatibilized blends was in the 100%–200% range compared to 5%–20% for uncompatibilized ones and for the pure PLA. This improvement was related to enhanced interfacial adhesion between TPS and PLA, which resulted in a more homogeneous blend and smaller TPS domains. In order to improve compatibility and interfacial adhesion between PLA and starch, Orozco, Brostow, Chonkaew, and López (2009) produced functionalized PLA with MA (PLA-g-MA) using dicumyl peroxide as initiator of the grafting process. The blends had TPS:PLA-g-MA ratios ranging from 15:85 to 60:40, and a uncompatibilized sample was produced with a TPS:PLA ratio of 35:65. Uncompatibilized blend shows two phases, PLA matrix and starch granules, and there are holes and cavities at the interface between both phases. Meanwhile all compatibilized blends show better interaction between PLA and starch. Chemical interactions resulted from a transesterification reaction among starch hydroxyl groups, grafted acid hydroxyl groups or anhydride groups on PLA. Clasen, Müller, and Pires (2015) reported that TPS–PLA blends obtained using a torque rheometer showed a reduction in the average molar weight. These blends, with free or grafted MA, showed higher strain than the blends without MA, suggesting that free MA can act as a plasticizer, and grafted MA as a compatibilizer. Detyothin, Selke, Narayan, Rubino, and Auras (2015) reported that although several works had investigated the use of grafted MA for reactive compatibilization of starch with biodegradable polymers, the effect of molecular weight of the functionalized derivatives on the blends properties has scarcely been investigated. They obtained PLA functionalized with MA, using 2,5-bis(tertbutylperoxy)22,5-dimethylhexane (Luperox 101 or L101) as initiator in a twin-screw extruder. PLA-g-MA with the highest grafting degree had the lowest molecular weights and intrinsic viscosity. Besides, this grafted polymer
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presented a broad molecular weight distribution, due to the dominant side reaction during melt free radical grafting, polymer degradation, or chain scission. Mani et al. (1999) also reported a significant chain scission of PLA during grafting due to its thermal instability. Poly(butylene adipate terephthalate) (PBAT) grafted with MA was extensively studied to improve compatibility of PBAT-TPS blends. Adhesion between phases in the mixtures strongly influences their ultimate properties. Thus, the blend obtaining with adequate physicomechanical properties results from the ability to control interfacial tension, generating a small-dispersed phase and strong adhesion. In this sense, stress transfer between the component phases is improved (Barlow & Paul, 1984; Nabar et al., 2005; Mani et al., 1999). Nabar et al. (2005) reported the grafting of MA onto PBAT by reactive extrusion employing 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane as free-radical initiator. MA concentration varied between 1.0% and 5.0 wt.% at 0.5 wt.% peroxide content. The amount of MA-grafted on PBAT backbone ranged from 0.194 to 0.691 wt.%, which was enough to successfully compatibilize the polyester and starch blends. The grafting of MA appeared to cause the chain scission of the polyester backbone as indicated by the decrease in molecular weight and intrinsic viscosity of the grafted PBAT, as the concentration of the free-radical initiator was increased. Stagner, Dias Alves, and Narayan (2012) studied the effect of different content (0, 10, 20, 30, 40, and 50 wt.%) of MTPS in MTPS-PBAT films produced by extrusion. Increasing MTPS content decreased the films tensile strength, and the permeability to oxygen and carbon dioxide decreased. However, films water vapor permeability was increased with MTPS. They also observed in films morphology that PBAT was the continuous phase and MTPS was the dispersed one. As the amount of MTPS increased, the dispersed phase became finer and more regularly spaced. Several studies have shown promising results with the use of poly(carboxylic acids), such as citric, malic and tartaric acids, as compatibilizer agents, like an alternative of MA. These organic acids are inexpensive, naturally present in fruits and vegetables, or they can be synthesized for microorganisms (Sailaja & Seetharamu, 2008). The use of these acids has been a good possibility to replace synthetic agents, once they have a compatible chemical structure with biodegradable polymers. Besides, they have the added advantage of health safety when the objective is food packaging, being nontoxic and nonvolatile. They act promoting esterification (grafting) and transesterification reactions (cross-linking) between polymers, improving the compatibility between the starch chains (hydrophilic) and polyolefins or biodegradable polyesters (hydrophobic). These agents also can exert a plasticizing effect and acid hydrolysis of starch chains (Martins & Santana, 2016; Olivato et al., 2012; 2013; Reddy & Yang, 2010). Citric acid (CA) is a tricarboxylic acid, and according to several authors, its structure is the reason why CA can be used for different functions: plasticizer, cross-linking, and hydrolytic agent, and compatibilizer (Da Róz, Zambon,
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Curvelo, & Carvalho, 2011). Reddy and Yang (2010) used CA as an additive in cornstarch films to promote cross-linking. They observed that CA cross-linked starch films showed about 150% higher tensile strength than non–cross-linked ones. These authors also stressed that CA is preferable for starch cross-linking since low levels (5 wt.% or less) are required. Moreover, it can derived from fermentation and could therefore be considered as a green chemical, and it has price advantages over a few other compounds commonly used. Garcia et al. (2011) reported that reactive extrusion was efficient for compatibilization of starch-PBAT blends using CA as a compatibilizing agent. This effect was mainly observed by decrease in water vapor permeability of the blends with CA addition, which was related to the introduction of ester groups in the starch chains. Thus, a more homogeneous and hydrophobic structures were obtained due to the increase in the compatibilization among the components promoted by CA. These authors also observed that this acid was responsible for other different functions in the blends: plasticizer (when in excess), cross-linking and hydrolytic agent. SEM micrographs of the films fracture containing 0.5 wt.% CA, 7.5 wt.% glycerol, and 92 wt.% starch/PBAT (60/40) show starch granules with altered conformation, with donuts shaped. This is an evidence of cross-linking between CA and starch inside the granule. As the concentration of CA increased from 0.5 to 2.5 wt.%, the amount of this kind of granules morphology decreased, which was an indicative of the starch fragmentation, due to the specific action of CA. Garcia et al. (2014) also reported the use of sodium hypophosphite as catalyst in the production of cassava starch/PBAT blown films with CA as esterifying agent. Films showed a more homogeneous structure and the combined effect of CA and sodium hypophosphite slightly increased their thermal stability. Several authors emphasized that CA assumed different functions on starch films depending on its concentration. Low concentrations (<5%) promote insufficient cross-linking, while higher concentrations, the excess cross-links limit starch chains mobility, decreasing elongation. When CA is used in excess, the residual free acid can act as plasticizer, improving the dispersion of polymers. However, excess plasticizer (glycerol + residual cCA) can increase the adhesiveness of the material and affect its processability (Garcia et al., 2011; Garcia et al., 2014; Olivato et al., 2012; Reddy & Yang, 2010). Olivato et al. (2011) employed reactive extrusion to obtain TPS-PBAT blown films, using MA and CA, alone or combined, as compatibilizers. MA, CA or MA-CA 1:1 mixture (2 wt.%) was added to starch-PBAT (55:45) blends after their dispersion in glycerol. More rigid films were produced when CA was used, while MA addition resulted in poor tensile strength and elongation, which was related to the difficulty in blend homogenization during the one-step extrusion. Water vapor permeability was improved by use of both compatibilizers. CA and MA were able to promote esterification and transesterification reactions, but blends containing CA also showed a better phase compatibilization. Olivato et al. (2012 and 2013) reported that CA, malic acid (MAc) and tartaric acid (TA) can be use as compatibilizers in TPS-PBAT (55:45) blown
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films. The compatibilizing effect of MAc was less efficient than CA and TA. The inclusion of organic acids contributed to starch hydrolysis, facilitating the destructuring and disruption of the granules. Thus, the material viscosity decreased, improving the processing properties, and producing a more homogeneous matrix. In addition, they generated cross-links that interconnect the polymeric chains, leading more resistant and less permeable films.
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