There are a significant number of biodegradable polymers (biopolymers) that come from both natural and synthetic sources. Utilizing agricultural products in plastic applications is a promising way to reduce surplus farm products and develop non-food applications.
Thermoplastic starch (TPS) or plasticized starch (PLS) undergoes processing like synthetic plastics through extrusion and injection units. Unfortunately, TPS is hydrophilic, and reducing the hydrophilic character of the starch chains is sometimes necessary. Starch has some limitations in developing starch-based products due to its poor mechanical properties and high moisture sensitivity. Starch, an environmentally friendly and common biopolymer, comes from various sources, including corn, wheat, rice, and potatoes. Wheat and rice granular starch tend to be the least expensive. However, pure starch is brittle and rapidly degrades when exposed to water.
To overcome this disadvantage, there have been several attempts to combine starch with synthetic polymers. If starch were used as an additive in blends with polymers, it should increase their biodegradability and reduce the cost of synthesis. However, mixtures of starch and polymers have resulted in materials with poor physical qualities since starch does not mix well with polymers. Some researchers have used high-amylose starch to improve the mixtures, but it is relatively expensive and negates the cost benefits of these mixtures. Gelatinized starch obtained from processing raw starch has also been used to improve these mixtures. However, both gelatinized starch and high-amylose starch require a plasticizer (e.g., glycerol), increasing the cost and water absorption of polymer/starch mixtures. The addition of glycerol (plasticizer) not only damages the mechanical properties of starch-filled PE but also has a negative effect on the mechanical properties of starch-filled PE after storage since glycerol is not compatible with PE.
Starch and hydrophobic compounds like biodegradable polyester don’t mix well and tend to produce blends with poor interfacial properties and separated phases. To improve this, one strategy is to combine starch with a moisture-resistant polymer that has good mechanical properties while maintaining the overall biodegradability of the product. This is necessary to prevent moisture sensitivity and critical aging, which have led to the need to combine TPS with other biopolymers to preserve the biodegradability of the final blends.
Polymer association can take the form of blends or multilayer products. Multilayers can be obtained through coating or coextrusion processes. Starch-based multilayers can also be made by compression molding of plasticized starch and polyesters. These efforts have led to commercialized starch blends like Mater-Bi from Novamont (Italy) or Bioplast from Biotec (Germany), which can be produced by blending starch with non-biodegradable polymers (polyolefins) or biodegradable polyesters (e.g., PCL) for use in packaging, disposable cutlery, gardening, leisure, hygiene, and more.
Most research focuses on blending PLS with biodegradable polyesters like PCL, PEA, PHBV, PHBO, PBSA, PBAT, PLA, or PHEE. These commercially available polyesters have interesting and reproducible properties like more hydrophobic characters, lower water permeabilities, and improved mechanical properties relative to PLS. The preparation of the blends is the main factor affecting their properties and behavior during processing. The solid-state properties of the blends depend on the nature of the polyester phase, with different mechanical properties that can be adjusted depending on the type of polyester used. PEA has the highest surface tension due to its highly polar component, while PLA has the lowest. Low compatibility between the polymers can lead to special behavior and properties, like preferential migration of the polyester toward the mold surface during injection molding, resulting in a polyester-rich skin and a starchy core after cooling.
PLS has been extensively used in combination with other polymers, resulting in numerous patents on the subject. This stratified pseudo-multilayer structure offers good water resistance compared to PLS alone, thanks to the protection provided by the polyester surface. The blend’s water sensitivity decreases significantly for polyester contents below 10%. The blends’ biodegradabilities are altered, with degradation occurring from the starchy core towards the skin.
Wang et al. proposed a provisional patent application for biodegradable materials made from starch-grafted polymers, cross-referencing related applications. Common plastics such as PE, PP, PS, PB, SEBS, PVF, PVC, and PET are typically non-biodegradable and synthesized from petroleum products. Various starch types, including high-amylose starch, processed starch like gelatinized starch and starch ethers and esters, may be used as alternatives to replace plastic polymers. The innovation only requires granular starch and provides a blend with physical characteristics very similar to those of the pure polymer. Additionally, any type of starch can be used, including wheat and rice starch, greatly enhancing the invention’s cost efficiency.
Using granular starch eliminates the need for an additional processing step to prepare plasticized or gelatinized starch. The starch can come from different sources, such as corn, rice, and potato, in either modified or unmodified form. It is crucial to note that using starch to replace PE significantly reduces production costs by 15%. The patent involves mixing starch with a primary polymer and a compatibilizer that has grafted compounds attached to it. These grafting compounds bond covalently to the hydroxyl groups located on granular starch, making it susceptible to binding. The resulting mixture has substantially the same physical properties as the pure polymer since the starch is chemically grafted to the compatibilizer, which physically interacts with the polymer. Up to 30% of the mixture may be granular starch.
In addition, starch greatly enhances the mixture’s biodegradability. Polymers with grafting compounds already attached to them are commercially available and only slightly more expensive than unaltered polymers. Granule size may vary significantly without significantly affecting the invention’s physical properties. Although granules of approximately 25 micrometers in diameter are preferred, large variations in granule diameter have only a minimal effect on the physical properties of the end product. The invention’s ability to utilize a variety of starches is highly advantageous since different starches are more plentiful in different parts of the world. Wheat, corn, rice, and potatoes are just a few of the many suitable starches.
The method described in U.S. Patent No. 6,218,532 involves the synthesis of materials from amylose starch derivatives, which are chemically modified to form starch ethers or esters. The resulting polymers have degrees of substitution ranging from 35% to 95% and are cross-linked to produce permanent entanglements. The polymers are then mechanically stretched to produce a biodegradable and mechanically superior material. The resulting sheets, films, fibers, threads, or other articles are swollen in thermodynamically acceptable solvents or solvent mixtures, deformed in uniaxial or biaxial extensions, and stretched from 1% to 500%. The solvents are then removed, resulting in homogeneous, highly-ordered materials with improved mechanical properties due to an increase in van der Waals bonding between the molecules.
In contrast, when starch is mixed with a hydrophilic biodegradable polymer, such as polyethylene oxide, no compatibilizer is needed for covalent attachment to granular starch. To convert native starch or starch derivatives into thermoplastic starch, at least one hydrophilic biodegradable polymer is added as a plasticizer or swelling agent, which may be an aliphatic polyester, copolyester with aliphatic and aromatic blocks, polyester amide, polyester urethane, polyethylene oxide polymer, polyglycol, or mixtures of these. The water content is reduced to <1% by weight based on the weight of the mixture during the homogenization process.
Another method involves grafting polyester onto starch, creating a polyester-grafted starch/polymer alloy that imparts flexibility and toughness to moldings without the need for plasticizers. The alloy comprises a blend of a polyester-grafted starch and an independent polyester, which are uniformly mixed together. The patent also discloses a method of preparing the alloy and thermoplastic resin compositions containing the alloy.
U.S. Patent No. 5 569 692 describes a starch-based composition for the production of articles of biodegradable plastic materials. The starch is heated with a destructuring agent in order to destructure the starch. It is then mixed with a polymer, preferably polyvinyl alcohol or ethylene vinyl alcohol. This patent does not mention covalent grafting of granular starch onto polyethylene.
In order to prepare a starch-based composition usable for the production of articles of biodegradable plastics material, starch is mixed in a heated extruder with a plasticizer with high boiling temperature. A destructuring agent is added for a time sufficient to cause the starch to be destructured. The process temperature should be below the boiling point of the plasticizer and between 120 – 170 ° C.
The composition thus obtained is particularly suitable for the formulation of mixtures with polymers of relatively high melting points, because it can be processed at temperatures higher than 120 ° C and is suitable for extrusion at low pressure. In particular, compositions thus obtained and mixed with polyvinyl alcohol and/or ethylene vinyl alcohol are suitable for the formation of films by blowing, since they have the desired characteristics of mechanical strength and resistance to tearing and perforation, or for the formation of articles finished by injection molding, thermoforming, or mold blowing.
The most popular method for processing of starch/polymer blends is injection molding for rigid biodegradable products, because of high production rates and accurate product size. During flow, a polymeric material is simultaneously subjected to mechanical and thermal influences, and depending on the morphology, introduces orientation under residual stress. Shrinkage in injection-molded products affects the physical properties and dimensional stabilities of the finished products.
Biodegradable PLS-based multilayer films are useful for packaging or coatings. Multilayer structures present some advantages over blends. Moisture sensitivity is not fully addressed in a blend because of the presence of starchy material close to the surface. Better resistance to moisture in starch-based products can thus be achieved by use of multilayers, allowing the preparation of sandwich-type structures with PLS as the central layer and a hydrophobic component as the surface outer layers.
These blends can, for instance, be obtained by different processes – that is, coextrusion, casting, and hot-melt techniques – to protect starch-based materials with waxy layers. Stratified materials can also be obtained by a multistep process based on compression molding. Instances of coating have also been mentioned in the literature. Coating has been achieved by spraying or painting solutions made from biodegradable polyester onto the starch-based material.
Coextrusion seems the best option, because it offers the advantages of being a one-step, continuous, and versatile process. Multilayer coextrusion has been widely used in the past decades to combine the properties of two or more polymers in one single multilayered structure. However, some problems inherent to the multiphasic nature of the flow are likely to occur during coextrusion operations; these include non-uniform layer distribution, encapsulation, and interfacial instabilities, which are critical because they directly affect the quality and functionality of the multilayer products.
Layer encapsulation is essentially the surrounding of the more viscous polymer by the less viscous one. Different stratified structures have been processed by coextrusion and studied: with PCL, PBSA, PEA, PLA, PBAT, or with PHBV. Some research into PLS/PEA/PLS systems has shown that the key parameters are the skin-layer viscosity and thickness, the global extrusion rate, and the die geometry after determination of the stable and unstable flow conditions. The occurrence of instabilities is strongly related to the shear stress at the interface.