Because of environmental problems with huge piles of plastic garbage, biodegradable plastics are still widely studied. Personally,  I thought this was a relatively easy problem to solve because of today’s advancements in chemistry and technology. Many papers on thermoplastic starch, including those I used as references, are published no more than 4 years ago, suggesting that the problem of finding substitutes to petroleum based plastics is still an open and active area.

One of the most widely studied biodegradable plastic is thermoplastic starch (TPS) which is basically made of starch, glycerol and water. Different starch sources produce thermoplastics of varying physical properties because of differing contents of two types of polymers: amylose and amylopectin. Both polymers are composed of chains of sugar units the difference being that amylose has a “strand like” structure while amylopectin is branched.  We expect that amylose is the freer to move than amylopectin based on structure so that it contributes to the “flexibility” of the material. While on the other hand, amylopectin makes up the “rigidity”.

Monmorillonite (MMT) nanoclay has been found to improve the mechanical properties of thermoplastic starch. Incorporation of this type of clay into TPS is has been the subject of some studies [1,2]. In these studies the strength are increased but the elongation is limited and could be due to the fact that the polymer chains are restricted in motion upon adding this type of clay. Our research made use of locally found

MMT for use in our TPS. It has thesame basic MMT clay microstructure but has a uniquely different elemental composition.  Our data from TPS-MMT mechanical tests showed the same improvement in strength but a sudden decline was observed when too much clay was added (up to about 9% by weight).

But how does MMT clay improved the properties of TPS? The answer lies in the microstructure of MMT. MMT clay structure can be visualized as plates sandwiching ions with positive charges like Na, Ca and Al.  These ions can be replaced by positive charged surfactants to render it "polymer compatible".

Each of these plates has a thickness of approximately 1 nm and width of about 200 nm. Spectral measurements show that hydrogen bonding is promoted in the presence of MMT. The strengthening mechanism may be likened to reinforcement by incorporating pieces of high strength material like that of steel to concrete. Here, the pieces are very small and involves chemical bonding—this makes the material remarkably stronger than conventional composites. Furthermore, our results also suggest that adding MMT makes TPS easier to process by making the material softer upon heating.

[1] Renet. al. J Polym Environ  (2009) 17:203–207 DOI 10.1007/s10924-009-0139-6

[2] Dai et. al. J Polym Environ (2009) 17:225–232DOI 10.1007/s10924-009-0142-y