Starch-based Plastics See Also Chapter

Granular starch and copolymers of ethylene and acrylic esters and alkyl(meth)acr-ylates or vinylacetate are produced as master batches for the production of mulch films, geotextiles and molded articles.146 The polar copolymers act as compatibilizers by lowering the interfacial energy between starch and the polyolefin and eliminate the need to coat granules. Processing via a vented twin-screw extruder also eliminates the need for anhydrous starch. 

The drive to use starch at higher addition levels requires it to contribute to the expected strength properties. For this to happen, the starch must be disrupted or destructured so that it can form a continuous phase in an extruded matrix. This can be done by extrusion of starch under low moisture conditions, which effects granular fragmentation, melting of hydrogen-bonded crystallites and partial depolymerization. Thermoplastic blends of up to 50% starch and poly(ethylene-co-acrylic acid) (EAA) were produced in the presence of aqueous base, which solubilized EAA and increased its compatibility with starch and urea, which aids in starch gelatinization.147,148 

Another proposed process employed injection molding in which starch and limited amounts of plasticizing water are heated under pressure to temperatures above the Tg and Tm to transform the native starch into a homogenous, destructured, thermoplastic melt. The process melt is then cooled to below the Tg of the system before pressure release to maintain the moisture content. Additives include natural and synthetic polymers, plasticizers and lubricants.136-139 159 160 The technology has been used to prepare pharmaceutical capsules and shaped objects, such as disposable cutlery, straws and pens. 

Strengths of films made from a starch-poly(vinyl alcohol) blend containing glycerol and poly(ethylene-co-acrylic acid) have been examined.166 High-amylose starches produced films with the most consistent properties. In a process for extruded blown film, blending of high-amylose starch with starches with more typical amylose contents and plasticizers or gelatinization aids improved properties.167 Extruded 

Adherent starch particles were prepared by partially swelling pregelatinized starch in 30% 2-propanol and/or solutions of inorganic salts. A pest control agent was stirred into the gelled mass, which was dried, broken up and ground. The material 

---