Starch has been of interest for polymer applications for almost two centuries. Its use in biodegradable plastics has been motivated in part by concerns over the negative impact of conventional plastics on the environment. Starch has advantages such as being a low-cost, annually renewable resource that is biodegradable. However, its use in plastics is challenged by its hydrophilic nature and branched structure, which can result in changes in physical and mechanical properties and reduce its ability to form entanglements. Several approaches have been used to incorporate starch into plastics, including chemical modification, granular starch composites, starch graft copolymers, thermoplastic starch, and expanded starch foams.
Starch-based plastics were initially modified chemically, but they were brittle at low humidity. To improve properties, esters of starch or isolated amylose were prepared. Amylose mixed esters had good tensile strength and elongation. High-amylose starch esters were prepared with butyrate, valerate, and hexanoate radicals, and blends with unmodified starch were claimed. Starch esters with higher levels of substitution or longer chain lengths biodegraded faster. Water-repellent starch esters and biodegradable polyesters were blended to create materials with good stability and properties comparable to general purpose polystyrene.
Granular Starch Composites
Fillers can be used to change the properties of plastics, but they can also decrease other properties. When starch is added to a polymer without bonding, it can cause a decline in tensile and impact properties due to the high energy at the surface of the starch granules. In the 1960s and 1970s, researchers investigated the use of granular starch in various plastics, including low-density polyethylene, polyurethanes, and poly(vinyl chloride), to impart paper-like characteristics to films. In the 1970s, researchers tried using starch in biodegradable materials by mixing it with synthetic resin, which reduced the strength and stretchiness of the material, but made it more susceptible to microbial attack. Later, researchers used various approaches to improve the properties of starch-filled polyolefin materials, such as using copolymers of ethylene with methyl acrylate, ethyl acrylate, or butyl acrylate, coextrusion with oxidative prodegradants, and reactive extrusion. These approaches helped to maintain good properties and adhesion between the starch granules and the plastic matrix.
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Starch in Rubber
Scientists have been exploring ways to use starch xanthate (SX) to reinforce rubber compounds, as starch granules are typically larger than the required 2 μm for reinforcement. Research has shown that SX-reinforced rubber compounds improve the mechanical properties of carboxylic-modified and styrene-butadiene rubber, but not natural rubber. SX-reinforced rubber also accommodates high amounts of pigments and can be used to develop formulations suitable for various applications. Starch xanthide-reinforced rubber compounds can swell more than regular reinforced compounds, but this effect is reversible when the sample is dried. Cationic starch products have also been used to reinforce carboxylic elastomers, resulting in compounds with improved physical properties and reduced solubility.
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Starch Graft Copolymers
Starch graft copolymers can be formed through various methods, such as combining starch with chloroformylated poly(ethylene oxide) or anionically polymerized polystyrene. They can also be synthesized by initiating free radicals on the starch molecule in the presence of unsaturated monomers. Starch graft copolymers have been used as flocculants, dispersing agents, paper additives, and superabsorbents. Different methods of free radical initiation and monomers can produce a wide range of products. The properties of the copolymers depend on factors such as the monomers used, the type of starch, and the graft content. Various studies have been conducted on the effects of copolymer content on tensile strength and elongation at break, as well as the effects of moisture content and temperature during extrusion. Reactive extrusion of starch has also been studied as an alternative to batch polymerization methods.
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Thermoplastic Starch Blends
Describing the limitations of granular starch composites and the benefits of converting them into thermoplastic materials, which can be blended with synthetic hydrophilic polymers. Poly(vinyl alcohol) was one of the first polymers blended with starch, and glycerol was used as a plasticizer. Sorbitol and glycol glycoside were found to reduce the embrittlement tendency caused by glycerol. Ethylene – acrylic acid copolymer (EAA) was added to improve elongation, but too much EAA made the films brittle. Strong alkali can be used in starch-EAA formulations to create water-stable films that are more transparent and permeable. The interaction between starch and EAA has been studied extensively, and a helical inclusion complex was formed between the two. Destructurized starch can be blended with hydrophilic and hydrophobic polymers to create commercialized thermoplastic compositions. Mater-Bi products developed by Novamont are blends of at least 60% starch and biodegradable synthetic polymers that have properties similar to low- and high-density PE and can be used to make water impervious films.
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Starch can be used as an extender in polyurethane foams, with different effects depending on the type of foam. Starch-based foams have been developed as an eco-friendly alternative to polystyrene in packaging and serving applications. Expanded products from compositions including starches with amylose contents up to 35% have been claimed. Starch foam trays can be made by mixing starch, water, fibers, and additives and then heating the mold to generate steam that causes foaming. Microcellular foams can be made using starch by preparing an alcogel and then displacing the water with ethanol to make an aerogel.
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