Production, chemistry and degradation of starch-based polymers

Abstract

As reflected in the number of publications, research on starch and starch-based plastics is widespread and continuing to grow. The characteristics of starch plastics are highly influenced by the amylase/amylopectin ratio, humidity, type and content of plasticizer, processing method and final crystallinity. In this respect, it is very important to monitor processing and storage conditions because these will also influence the properties of starch-based plastics. Thermoplastic starch is usually based on starch with amylose content greater than 70% and is use of gelatinised starch. The addition of specific plasticisers produces thermoplastic materials with good performance properties and inherent biodegradability. Starch is typically plasticised, destructured, and/or blended with other materials to obtain products with useful mechanical properties. Importantly, thermoplastic starch can be processed on existing plastic procesing equipment. High starch content plastics are highly hydrophilic and readily disintegrate on contact with water. The use of starch-polymer blends provides a route to materials with adjustable degradation rates and the chemical modification of starch to form new materials or for the purpose of compatibilization in blends is facilitated by the availability of free hydroxyl groups in starch, which can undergo a number of reactions such as acetylation, esterification and etherification. The fine phase structure is an important parameter in terms of obtaining films with useful properties and is determined by the interface and interphase, the weight ratio of the components and features of the processing method such as shear, residence time and temperature. Biodegradation of starch and starch-based polymers has been studied by means of laboratory tests using specific extracellular enzymes and also by means of simulation tests, mainly in soil burial and compost environments. Even if simulation tests do not entirely reflect the real biodegradation conditions in a natural environment, these assays have proved suitable for assaying the extent of polymer degradation in different environments whilst under controlled conditions. In the case of starch granules, enzymatic hydrolysis has been widely studied. The rate and extent of enzymatic degradation has proved to be dependent on a number of factors, among which the botanical origin plays the most important role by defining the granule size and shape, the amylose/amylopectin ratio and the starch morphology. In blends, starch generally enhances the degradation rate acting as the initial point of biological attack; and starch concentration determines the extent of degradation of the blend. Although many research groups have been interested in studying the degradation of starch-based polymers they synthesize, the literature shows a wide diversity of assays and conditions used to do so (e.g., time of exposure to degrading environment, temperature, humidity, methods of measurement), sometimes making it difficult to compare results for the degradation of starch-based blends.