Starch-based plastics

In previous editions of this textbook, no mention was made of starch polymers because they were not easily processed by extrusion until recently, as the melting point of starch is above its decomposition temperature. Starches were used exten- 

If starch is modified by partially fermenting it, or esterifying or etherification, the resulting material can be extruded by adding plasticizers. Starch-based plastics are readily biodegraded. In most cases, the polymers are also water-soluble. Several suppliers now offer thermoplastic starch. Major markets include soluble films for industrial packaging, films for bags and sacks, and loose fill. 

In addition to the ability to dispose of the used material by dissolving it, an interesting advantage of starch-based loose fill cushioning materials is they do not tend to build up electrostatic charges, so they do not cling the way packaging peanuts made of expanded polystyrene tend to do. 

Thermoplastic starch is also blended with other polymers to improve its properties for particular applications. For example, a bag for collection of household food waste for composting that readily dissolved when it got wet would not function very well In applications such as this, the resins used for blending are also biodegradable, so that they do not interfere with the composting operation. In other cases, starch is blended with nonbiodegradable resins such as polyolefins. 

Other effective plasticizers for starch for imparting melt processibility include a variety of low molecular weight compounds, such as glycerol and diethylene glycol, and also polymers such as poly(ethylene-co-vinyl alcohol) [55]. Furthermore, starch plasticized in that manner can be melt blended with minor amounts of hydrophobic thermoplastics, such as polyethylene and poly( methyl methacrylate), to obtain biodisintegratable molded articles with good mechanical properties. 

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Gaspar M., Benko Zs. Dogossy G. Reczey K., Czigany T. Reducing water absorption in compostable starch-based plastics. Polymer Degradation and Stability 90 563-569... 

Historically, chemical modification of starch was the first widely-investigated method for producing starch-based plastics. Attempts at forming cast films from starch or amy-lose showed that the properties were highly dependent on relative humidity the films became brittle at low humidities.4 Figure 19.2 shows data for films of various amylose/ amylopectin ratios conditioned at 50% relative humidity and 23°C. Efforts to overcome... 

These discoveries provided the technology base for commercialization efforts of starch-based plastics by the Novon division of the Warner-Lambert Company.157,158 The thermoplastic compositions were based on blends of destructurized starch and various hydrophilic polymers, such as ethylene-vinyl alcohol copolymers (EVOH). Hydrophobic polymers and additives, such as plasticizers and lubricants, could also... 

Several companies have developed starch-based plastics. By using carefully selected starch feedstocks, and water as a plasticizer, they produce thermoplastics from nearly 100% starch, or from blends of starch with other biodegradable components. Many of these materials are water-soluble in addition to being biodegradable. The major target application has been as a replacement for polystyrene foam, including both molded cushions and loosefill. 

Warner-Lambert was the first major producer of starch-based plastics, which it sold under the Novon name, beginning in about 1990. It discontinued manufacture in 1993 and sold the operation to EcoStar International, which was shortly thereafter acquired by Churchill Technology, which then declared bankruptcy. Other suppliers of biodegradable starch-based plastics include StarchTech Inc. of Minnesota, FP International, Biotec in Germany, Mellita, and Novamont, among others. 

The starch-based plastics do not degrade totally, since only the starch constituent is consumed by microbial activity, and the plastic residues can be harmful to the soil and to birds and insects. 

All starch-based plastics on the market contain polyols such as sorbitol, glycerol or mixtures thereof ensuring flexibility and mechanical properties stability of the finished product. 

The largest share in terms of volume corresponds to the starch-based plastic market. The main applications are compost bags, shopping bags, loose-fill packaging and mulch films. 

Generally speaking, bio-based plastics include starch-based plastics, protein (soybean protein) based plastics, and cellulose-blended plastics. They can also be blended with conventional plastics such as polyethylene (PE), polypropylene (PP), and poly(vinyl alcohol). However, such bio-based plastics are only partially biodegradable. The residual petroleum-based plastics remain as broken pieces, creating additional pollution. In addition, these plastics have intrinsic thermal and mechanical weaknesses, and they are now discouraged for applications. 

Shogren R.L., Fanta G.F., Doane W.M., Development of starch based plastics - A reexamination of selected polymer systems in historical perspective, Starch/Starke, 45, 1993, 276-280. 

Lorcks J., Properties and applications of compostable starch-based plastic materials, Polym. Degrad. Stab., 59, 1998, 245-249. 

Starch-Based Plastics. As can be seen in Table 8.7, a variety of starch-based plastics have been produced by several companies. Starch-based plastics are often water-soluble as well as biodegradable. Some contain almost entirely starch others contain blends of starch with other biodegradable components. 

Freedonia predicts demand for starch-based plastics will increase an average of 11.6 percent per year through 2008, reaching a total of 83 imlhon pounds, compared to 48 million in 2003. °... 

This led some environmental groups, such as the Environmental Defense Fund (EDF) and the Environmental Action Foundation (EAF), to call for a consumer boycott of degradable plastics. In contrast to these materials, starch-based plastics have been developed which are truly biodegradable. Some contain nearly 100% starch, and others are blends of starch with other biodegradable components. Most use carefully controlled amounts of water as a plasticizer to convert the starch into a thermoplastic, along with careftilly controlled temperature and pressure. 

Research on starch-based plastics has taken place in many coim-tries around the world. The Australian government funded a 1995 research project on development of starch-based plastics from corn and wheat, using water and glycerine as a plasticizer. In Japan, the Biodegradable Plastics Society was formed in 1989, with 48 member companies located mainly in Japan. By 1990, the membership had expanded to 69 companies, and included a significant number of non-Japanese members. In 1992, the U.S. Bio/Environmentally Degradable Polymer Society was formed, and had over 200 members by 1998.2 2... 

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