tailieunhanh - Handbook of Plastics Technologies Part 15

Tham khảo tài liệu 'handbook of plastics technologies part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | PLASTICS RECYCLING AND BIODEGRADABLE PLASTICS PLASTICS RECYCLING AND BIODEGRADABLE PLASTICS dation were equally unclear. This brought the initial generation of biodegradable plastics which most often were mixtures of starch with low-density polyethylene. As time went on information increased about both the behavior and composition of solid waste in landfills. It became clear that the majority of material in landfills was in fact biodegradable consisting of paper food waste and yard waste see Fig. . Therefore if biodegradation was the solution to landfill problems there should be no problem However studies of landfill behavior showed clearly that when landfills are kept dry as is required by regulations designed to help prevent groundwater contamination degradation is a very slow process taking decades or more to be complete. Hence simply making the plastics in a landfill biodegradable was not a real solution when conditions prevent rapid microbial destruction of landfilled materials. Furthermore calling a plastic biodegradable on the basis of loss of tensile strength or even complete loss of structural integrity is questionable at best. The starch polyethylene bags may disappear from view when the starch fraction is metabolized by microorganisms but this is no guarantee that there has been any substantial change in the polyethylene fraction of the bag. The current view is generally that complete biodegradation means complete destruction of the molecular structure a return to nature of the carbon content of the polymer rather than conversion of a plastic item to unidentifiable plastic powder. Implicit in labeling a polymer biodegradable is the assumption that this molecular destruction will take place in some reasonable time frame. Even the most recalcitrant plastic will probably biodegrade eventually but eventually may mean centuries. Obviously such slow biodegradation is of no practical value in the short term. In recent years there has been some tendency