Blends of Synthetic Polymers and Starch

The blends of starch and a synthetic polymer (usually polyethylene) are products of commercial importance. Two families of blends are obtained those using dehydrated starch pellets and those using gelatinised or thermoplastic starch. In both cases, the mixture with the synthetic polymer is done by extrusion. Further processing by moulding or blowing is still possible, depending on the kind of starch used. 

Dehydrated starch acts as a filler and diminishes the mechanics properties of the polymer. It is not possible to add more than 20%. Normally, in practice, the rate oscillates between 6 and 10%. In order to get a better compatibility between starch and the polymer, starch might be rendered hydrophobic by chemical treatment, as described above for cellulose. 

There are some examples at a commercial level of starch-containing blends  

These kinds of products are normally used in mulch films, bags and packing. Other systems have been investigated. The combination starch/ polyester has been claimed to be fully biodegradable (Tokiwa et al., 1993). Others are partially biodegradable, like starch/polyethylene/poly-e-caprolactone blends (Corp, 2001) and their derivatives or combinations of starch and modified polyesters. 

A particular mention goes to Mater-Bi, produced by Novamont, who have revolutionised starch-based biomaterials for two decades. The commercial success of this biodegradable and biocompostable plastic relies on two main factors the scale economy that allows the reduction of costs, and the diversity of formulations to develop different end products (plastic bags, tableware, toys, etc.). More than 210 references in Chemical Abstracts are available on this (registered) keyword, and the number of patents related to different formulations and developments is also impressive. Mater-Bi can be essentially described as a blend of starch with a small amount of other biodegradable polymers and additives. The actual compositions are still known only by a very few people. 

---