Improved outlook for a biodegradable plastic.
Agarwal, S and C Speyerer. 2010. Degradable blends of semi-crystalline and amorphous branched poly(caprolactone): Effect of microstructure on blend properties. Polymer 51(5):1024-1032.
A new way of concocting a promising “green” plastic called polycaprolactone (PCL) makes it clearer and more biodegradable – critical features for alternatives to PVC plastic or other conventional packaging materials.
PCL was transformed into a more transparent plastic when two different varieties of the same starting material were combined in the laboratory. The blends broke down faster when buried in a compost. The results show that the new blends improve traits – transparency and degradability – necessary to develop PCL into a viable plastic product.
PCL degrades easily and thus has been studied for decades as an alternative plastic for use in agriculture, medicine, pharmacy, biomedical and as an environmentally friendly material for packaging. Because it has some disadvantages – for example it cannot form a transparent film – it must be blended with other plastics in industrial applications.
PVC, like many other plastics, is not biodegradable, and therefore, it persists in the environment. PVC is rigid unless other chemicals are added to the formulation. Phthalates are among the most commonly used additives to make PVC flexible. Human health concerns have been raised about exposures when these chemicals migrate out of the plastic, especially effects on the male reproductive system.
Ironically, PVC is often chosen for blending with PCL because the two polymers can be mixed very easily. This takes away from the environmental benefits of PCL, since in the blended plastic, after the PCL degrades, the PVC persists just as it normally would on its own.
The new PCL plastic reported in this study does not use PVC. It can be fine-tuned so that the transparency increases from 8 percent to 45 percent and the plastic films break down much more quickly than ordinary PCL. The blends were less flexible and stretchy, but the researchers did not discuss whether the impact this would have on a potential packaging material.
The technique that led to the new plastic was a method of changing the structure of the PCL chain. Ordinary PCL and the new PCL contain the same repeating units, but the new PCL is not perfectly linear. It has branches, forcing the chains to take on a different overall shape. There are many different ways to make branched-chain PCLs, so more research could increase the number of options for manufacturers who want to use environmentally friendly plastics.