Blends of Thermoplastic Starch and Biodegradable Polymers

Biodegradable materials are commonly blends of TPS and aliphatic/aromatic polyesters, such as PLA, PBAT, PHB, and PVA. PLA and PHB are renewable resources and PVA was the first to be studied in blends with starch. These biopolymers have physical and chemical properties similar to those of conventional plastics and have been used to make packaging materials. Blending these polymers with TPS can reduce cost and obtain new materials with specific properties. However, TPS, PHB, and PLA are brittle and have lower elongation values than synthetic polymers, which may affect their thermal processability.

Blends of TPS and PVA

PVA is a good synthetic polymer to mix with natural polymers because it dissolves easily in water and some organic solvents. It’s biodegradable, but not as fast as other biodegradable polymers like PHAs or PLA. Mixing PVA with starch can help it break down faster and be cheaper.

PVA is made by removing acetate groups from polyvinyl acetate. The properties of PVA depend on how much hydrolysis and polymerization it undergoes. Hydrolysis refers to the amount of acetate groups removed, and PVA can be partially, intermediately, fully, or highly hydrolyzed. Polymerization refers to the length of the PVA chains, and PVA can have very low, low, medium, or high viscosity.

Increasing the degree of hydrolysis (DH) of PVA results in lower water solubility, higher energy stabilization due to hydrogen bonding, increased adhesion to hydrophilic surfaces, and increased viscosity and tensile strength. Partially hydrolyzed PVA has residual acetate groups, which reduce overall crystallinity and make their formulations easier to process, but with lower melting points, lower strength, and lower water dissolution temperatures than fully hydrolyzed polymers.

Blending TPS and PVA can result in materials with improved tensile strength, elongation, and processability compared to pure TPS, but the blends have a negative effect on the rate of starch degradation. Increasing the amount of PVA in the blend will further decrease the rate of degradation.

Westhoff et al. (1979) produced starch-PVA films and found that high starch content blends resulted in a loss of plasticizer effectiveness, while Liu et al. (1999) obtained wheat starch-PVA-glycerol blends and observed that starch and PVA were partially compatible based on the morphology of the blends.

Tudorachi et al. (2000) made films by mixing cornstarch and PVA (DH = 88%) and studied how microorganisms affected their properties. The films degraded as the microorganisms consumed the starch and the PVA, causing weight loss and changes in mechanical and thermal properties. Films with higher starch content degraded more quickly.

Jayasekara et al. (2004) created homogeneous and flexible blend films with 20wt.% PVA (DH = 85%), 60wt.% starch, and 20wt.% glycerol. The hydroxyl groups in PVA interacted with those in starch, forming strong hydrogen bonds that resulted in increased stability and integrity. He et al. (2004) observed that the blends had an intermediate surface roughness.

Sin et al. (2010) noted that when PVA was mixed with starch, the hydroxyl groups in PVA interacted strongly with those in starch, increasing the energy stability. Sin et al. (2011) also reported that these blends had enhanced thermal stability.

Increasing the starch content in PVA matrix maintains mechanical properties due to the formation of hydrogen bonds (Siddaramaiah, Raj, & Somashekar, 2009). Faria, Vercelheze, and Mali (2012) found that adding PVA to the starch matrix created films with better properties than pure starch films, with lower water vapor permeability, high tensile strength, and elongation. Chai, Chow, and Chen (2012) discovered that TPS and PVA are an excellent pair for blending, with high molecular weight PVA leading to high biodegradability and increased starch content resulting in better properties.

TPS and PVA make an excellent blend, with PVA’s water solubility making it easy to mix with starch (Chai, Chow, & Chen, 2012). Chai, Chow, and Chen studied blends with different molecular weight PVAs and cross-linked starch compositions to understand the effects on biodegradability. Higher molecular weight PVA displayed high biodegradability, and the tensile strength increased while the elongation decreased with increasing molecular weight. Sreekumar, Al-Harthi, and De (2012) observed that adding too much glycerol to TPS-PVA blends decreased crystallinity, tensile properties, and energy at break.

Adding more starch content to PVA matrix maintains mechanical properties by creating hydrogen bonds (Siddaramaiah, Raj, & Somashekar, 2009). Faria, Vercelheze, and Mali (2012) found that adding PVA to the starch matrix improved properties, resulting in films with lower water vapor permeability, high tensile strength, and elongation. Chai, Chow, and Chen (2012) discovered that TPS and PVA are an excellent pair for blending, with higher molecular weight PVA displaying high biodegradability and better properties with increased starch content. Sreekumar, Al-Harthi, and De (2012) found that adding too much glycerol to TPS-PVA blends decreased crystallinity, tensile properties, and energy at break.

Zanela et al. (2015a) made biodegradable sheets by extrusion using cassava starch, PVA (DH=88%), and glycerol with different proportions. The opacity of the sheets increased with starch concentration. In another study, Zanela et al. (2015b) processed these blends using flat extrusion, and observed that PVA improved the properties of the sheets while glycerol had the opposite effect. The blends had good compatibility between their components and PVA improved their mechanical and barrier properties.

Several studies have used starch-PVA blends to create foam plates and loose-fillers as an alternative to expanded polystyrene. These studies found that adding PVA improved the mechanical properties of the starch foams, possibly because of interactions between the biopolymers.

Blends of TPS and PLA

PLA is a hydrophobic thermoplastic aliphatic polyester that can be molded into products such as bottles, containers, films, and sheets. It has good biocompatibility and processability, but is brittle under tension, experiences physical aging during application, and is more expensive than other biodegradable polymers.

Blends of starch and PLA are advantageous due to their low cost, biodegradability, low-density, high toughness, thermal resistance, and renewability. Many studies have investigated the production and characterization of these blends, proposing different applications.

Researchers have synthesized PLA by the condensation polymerization of D- or L-lactic acid or ring opening polymerization of lactide. The fermentation of sugar feedstock at competitive prices can provide lactide. The addition of PLA to starch has resulted in lower cost and higher toughness compared to pure PLA. Many studies have investigated the production and characterization of these blends, proposing different applications.

PLA is brittle, and blending it with native starch may not be practical as the dispersed starch granules can worsen this characteristic. Jacobsen and Fritz (1996) solved this issue by adding low molecular weight PEG to the PLA matrix to improve its properties.

Blending PLA with starch combines their good properties, but the improvement of the mechanical behavior depends on their adhesion. Park et al. (1999, 2000) reported that hydrogen bonding and interfacial affinity between the polymers are essential factors that affect the mechanical properties of the blends. The crystalline structure of starch must be disrupted before mixing with other polymers to improve compatibility.

Several variables affect the properties of starch-PLA blends, such as starch ratio, moisture content, heat treatment, plasticizers, and coupling agents. PLA dominates the blends’ mechanical properties, and their microstructure, formed during thermal processing, affects the product quality (Ke & Sun, 2003).

Ken et al. (2003) found that blending PLA with corn starch resulted in a continuous PLA phase at low starch contents, but the adhesion between starch and PLA was poor, causing gaps in the film surfaces.

Blends of starch and PLA can be affected by physical aging, which is a natural process that affects the amorphous phase in glassy polymers when the chain mobility decreases. Physical aging is important for determining the long-term performance of a material system as it can lead to shrinkage, decreased specific enthalpy, entropy, molecular mobility, and affect mechanical and barrier properties. Relative humidity (RH) also plays a significant role in physical aging of neat PLA and PLA-starch blends.

The mechanical properties and water absorption of blends of four corn starches with different amylose levels and PLA were studied by Ken et al. (2003). They reported that PLA phase was continuous at low starch contents but became discontinuous as the starch content increased beyond 50wt.%.

Pan et al. (2008) found that the fracture strain in PLA/starch blends decreased significantly during physical aging, which caused the fracture mechanism to change from ductile to brittle.

Li et al. (2011) investigated PLA/starch blends made from acorn kernels and found poor adhesion between acorn powder and PLA matrix. However, they were still able to produce these blends using conventional manufacturing methods like extrusion and injection molding. Müller et al. (2012) also reported success with extrusion and thermopressing in creating starch-PLA blends.

Glycerol and sorbitol are common plasticizers used to improve the properties of thermoplastic starch (TPS). Li and Huneault (2011) found that the sorbitol-plasticized TPS phase can be more uniformly distributed in the PLA matrix, leading to higher tensile strength and modulus than glycerol-plasticized TPS-PLA blends. Shirai et al. (2013) noted that glycerol is not a good plasticizer for PLA but is suitable for starch.

Müller et al. (2012) found that adding PLA to starch led to two phases and increased tensile strength and modulus. The blend with 30wt.% PLA had the lowest water vapor permeability.

Many studies have reported poor adhesion between starch and PLA, resulting in brittle materials. This requires a compatibilization strategy, which has been extensively studied to improve blend performance.

Compounds like compatibilizers, plasticizers, flexibilizers, and hydrophobic agents can enhance the compatibility between PLA and starch, improve the dispersion of starch granules in PLA matrix, and enhance the toughness of PLA or the hydrophobicity of blends.

Blends of TPS and PHB

PHAs such as PHB and P(HB-co-HV) are biodegradable polymers that are produced by fermentation and are promising for use in blends with starch due to their economic advantages (Kaseem et al., 2012).

PHB and P(HB-co-HV) are semicrystalline thermoplastic polyesters that have properties similar to those of polypropylene, but are highly brittle and thermally unstable. (D’Amico et al., 2016; Ten et al., 2015).

Blends of wheat starch and P(HB-co-HV) produced useful thermoplastic properties when a proportion of 50:50 was used, while blending starch with PHB in a ratio of 30:70 could be advantageous for cost reduction with improved properties compared to virgin PHB (Ramsay et al., 1993; Godbole et al., 2003).

Lai et al. (2006) studied TPS-PHB blends using potato, corn, and soluble potato starch with varying amounts of glycerol. In all cases, the TPS-PHB blends had better mechanical properties than pure TPS.

Thiré et al. (2006) created cornstarch and PHB blends with different starch contents and found that higher starch contents led to poorer mechanical properties due to poor interfacial adhesion and heterogeneous dispersion. However, the incorporation of starch did not affect the thermal stability of PHB.

Parulekar and Mohanty (2007) noted that TPS and PHA materials exhibit aging behavior, but TPS-PHB blends did not show any aging behavior during a 30-day storage period at 30°C and 50% RH. The authors explained that TPS aging is due to moisture uptake and plasticizer leaching, while PHA aging is due to secondary crystallization, which stiffens and embrittles the material.

Reis et al. (2008) made blends of P(HB-co-HV) with maize starch and tested different amounts of starch, ranging from 0% to 50%. The authors found that there were no interactions between the two polymers, indicating that they are not soluble in each other. The blends showed poor adhesion between the two materials and uneven distribution of the starch granules in the P(HB-co-HV) matrix. In contrast, Zhang and Thomas (2010) discovered that PHB and maize starch with different amylose and amylopectin contents can form intermolecular hydrogen bonds.

According to Lai, Sun, and Don (2015), starch and PHB do not work well together and result in brittle behavior. The blend of these polymers does not make complete films. To improve the properties of TPS-PHB blends, a compatibilizer is necessary.

Blends of TPS and PBAT

PBAT and starch have poor adhesion and physical and mechanical properties when blended together, as they are thermodynamically immiscible (Raquez et al., 2008; Garcia et al., 2011; Nabar et al., 2005; Ren et al., 2009).

PBAT and TPS blends have better interphase compatibility than other polyester blends, with better mechanical properties and a more hydrophobic character than pure starch materials (Averóus and Fringant, 2001).

The addition of a small amount of compatibilizer to TPS, PLA, and PBAT blends greatly improves the final mechanical properties. The elongation at break increases with increasing PBAT content, and the water uptake of the compatibilized blends is lower, with a longer time required to reach equilibrium water uptake than non-compatibilized blends (Ren et al., 2009).

Bilck, Grossmann, and Yamashita (2010) made biodegradable black and white films from cassava starch and PBAT blends for strawberry production. PBAT films functioned well as mulching films similar to PE film, but their mechanical properties deteriorated over time and the mulching weight decreased due to possible biodegradation, cross-linking, and photodegradation.

Brandelero, Yamashita, and Grossmann (2010) used a surfactant to improve the adhesion between TPS and PBAT in film blends, but it resulted in less structural integrity and lower tensile strength. The addition of soybean oil improved the mechanical properties of the films and made the microstructure more uniform.

Brandelero, Yamashita, and Grossmann (2011) used two methods to make starch-PBAT blends for films. The first method resulted in better mechanical properties when the PBAT concentration was 50wt.%. For starch concentrations higher than 50wt.%, the second method using granular starch can be used for single extrusion production without sacrificing mechanical properties, which can save production costs.

Several authors have used compatibilizers to improve TPS-PBAT blend properties. These include maleic anhydride, citric acid, and tartaric acid, as reported by various studies (Mohanty & Nayak, 2010; Nabar et al., 2005; Raquez et al., 2008; Garcia et al., 2011; Olivato, Grossmann, Yamashita, Eiras, & Pessan, 2011; Olivato, Grossmann, Bilck, & Yamashita, 2012; Olivato, Müller, Carvalho, Yamashita, & Grossmann, 2014).

PVA is the most compatible synthetic and biodegradable polymer to blend with starch. Blending starch with other expensive biodegradable polymers or synthetic ones can lead to poor mechanical properties due to the hydrophobic nature of most polymers, which is not compatible with starch. This phase incompatibility can be improved with compatibilization, resulting in smaller dispersed domains and improved mechanical properties.

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