Starch, a major component of corn, is a linear polysaccaride made up of repeating glucose groups with glycosidic linkages in the 1-4 carbon positions with chain lengths of 500 to 2,000 glucose units. There are two major polymer molecules in starch—amylose and amylopectin. Amylose, which has alpha linkage, is both flexible and digestible.
As the starch content of a polymer increases, it becomes increasingly biodegradable and leaves fewer intractable residues. Biodegradation of starch-based polymers is the result of enzymatic attack on glycosidic linkages between the sugar groups, which leads to decreases in chain length and the “splitting out” of lower molecular-weight sugar units (monosaccharides, disaccharides, and oligosaccharides) that can be readily used in biochemical pathways.
Biodegradable starch-based polymers are being investigated for potential use in several biomedical applications. For example, new processing techniques and reinforcement with various fillers have led to the development of materials with mechanical properties comparable to those of bone (Reis et al., 1997a). These polymers may be suitable for bone-replacement implants (Reis and Cunha, 1995), bone cements (Reis et al., 1997b), drug-delivery systems (Malafaya et al., 2001), and tissue-engineering scaffolds (Gomes et al., 2001).
When biodegradable thermoplastic-starch plastics, which have a starch (amylose) content of more than 70 percent, are combined with specific plasticizing solvents, they produce thermoplastic materials with good performance properties and inherent biodegradability. The hydrophilic nature of high-starch content plastics, which readily disintegrate on contact with water, can be overcome by blending and chemically modifying the material by acetylation, esterification, or etherification. Starch-based polymers are often plasticized, destructured, and/or blended with other high-performance polymers (e.g., aliphatic polyesters and polyvinyl alcohols) to provide useful mechanical properties for different packaging applications.
Blends of biodegradable, synthetic, aliphatic polyesters and starch are often used to produce high-quality sheeting and films for packaging. Approximately 50 percent of synthetic polyester (which costs approximately $4.00/kg) could be replaced with natural polymers, such as starch (approximately $1.50/kg). In addition, polyesters can be modified by incorporating different functional groups (e.g., hydroxy, amine, carbonyl, etc.) that are capable of reacting with natural starch polymers.