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bioplasticsMAGAZINE_0905

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bioplasticsMAGAZINE_0905

Basics Basics of

Basics Basics of Starch-Based Materials Starch is a reserve of energy for plants and is widely available in cereals, tubers and beans all over the planet. The present annual production of starch worldwide is about 44 million tonnes and comes mainly from corn, where worldwide production is about 700 million tonnes, as well as from wheat, tapioca, potatoes etc.. Today the main uses of starch available annually from corn and other crops, produced in excess of current market needs in the United States and Europe, are in the pharmaceutical and paper industries. Starch is totally biodegradable in a wide variety of environments and can permit the development of totally biodegradable products for specific market demands. Biodegradation or incineration of starch products recycles atmospheric CO 2 sequestered by starch-producing plants and does not increase potential global warming. All of these reasons aroused a renewed interest in starch-based plastics over the last 20 years. Starch graft copolymers, starch plastic composites, starch itself, and starch derivatives have been proposed as plastic materials. Starch consists of two major components: amylose (Fig. 1), a mostly linear a-D-(1,4)-glucan; and amylopectine (Fig. 2), an a-D-(1,4) glucan that has a-D-(1,6) linkages at the branch point. The linear amylose molecules of starch have a molecular weight of 0.2–2 million, while the branched amylopectine molecules have molecular weights as high as 100–400 million. In nature starch is found as crystalline beads of about 15–100 mm in diameter, in three crystalline design modifications: A (cereal), B (tuber), and C (smooth pea and various beans), all characterised by double helices - almost perfect left-handed, six-fold structures, as elucidated by X- ray-diffraction studies. Starch as a filler Crystalline starch beads can be used as a natural filler in traditional plastics [1]; they have been used particularly in polyolefines. When blended with starch beads, polyethylene films biodeteriorate on exposure to a soil environment. The microbial consumption of the starch component, in fact, leads to increased porosity, void formation, and loss of integrity of the plastic matrix. Generally, starch is added at fairly low concentrations (6–15%); the overall disintegration of these materials is obtained, however, by transition metal compounds, soluble in the thermoplastic matrix, used as pro-oxidant additives to catalyse the photo and thermooxidative processes [2]. Starch-filled polyethylenes containing pro-oxidants have been used in the past in agricultural mulch film, in bags, and in six-pack yoke packaging. According to St. Lawrence Starch Technology, regular cornstarch is treated with a silane coupling agent to make it compatible with hydrophobic polymers, and dried to less than 1% of water content. It is then mixed with the other additives such as an unsaturated fat or fatty-acid autoxidant to form a masterbatch that is added to a commodity polymer. The polymer can then be processed by convenient methods, including film blowing, injection molding, and blow molding. The non compliance of these materials with the international standards of biodegradability in different environments and the increasing concern for micropollution that can be enhanced by their fragmentability, together with the potential negative impact on recyclability of traditional plastics, and their limited performances with time, have not permitted serious consideration of this technology as a real industrial and environmental option. Thermoplastic starch There are two different conditions for loss of crystallinity of starch: at high water volume fractions (>0.9) described as gelatinization; and at low water volume, fractions (

Article contributed by Catia Bastioli, CEO, Novamont S.p.A., Novara, Italy Fig. 3: Droplet-like structure of thermoplastic starch / EVOH blend above. It can show other forms of crystallinity, different from the native ones, induced by the interaction of the amylose component with specific molecules. These types of crystallinity are characterised by single helical structures and are known as V complexes [7]. Moreover thermoplastic starch is characterised by a melt viscosity comparable with that of traditional polymers [8]. This aspect makes possible the transformation of destructurised starch in finished products through the use of traditional manufacturing technologies for plastics. Thermoplastic starch alone can be processed as a traditional plastic; its sensitivity to humidity, however, makes it unsuitable for most applications. Thermoplastic starch composites Starch can be destructurised in combination with different synthetic polymers to satisfy a broad spectrum of market needs. Thermoplastic starch composites can reach starch contents higher than 50%. EAA (ethylene-acrylic acid copolymer) / thermoplastic starch composites EAA/thermoplastic starch composites have been studied since 1977 [9]. The addition of ammonium hydroxide to EAA makes it compatible with starch. The sensitivity to environmental changes and mainly the susceptibility to tear propagation precluded their use in most of the packaging applications; moreover, EAA is not at all biodegradable. Starch / vinyl alcohol copolymers Starch/vinyl alcohol copolymer systems, depending on the processing conditions, starch type, and copolymer composition, can generate a wide variety of morphologies and properties. Different microstructures were observed: from a droplet-like (Fig. 3, 4) to a layered (Fig. 5) one [10], as a function of different hydrophilicity of the synthetic copolymer. Furthermore, for this type of composite, materials containing starch with an amylose/amylopectine weight ratio of >20/80 do not dissolve even under stirring in boiling water. Under these conditions a microdispersion, constituted by microsphere aggregates, is produced, whose individual particle diameter is

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