Ask A Scientist© Environmental Science Archive

Author:     ssample
Text:       I am a junior high school science teacher and I have a question 
about recycling of plastics. I know there are seven plastic categories, but I 
do not know anything about the process involved to recycle plastics. Do you 
reheat them up until melting point is reached or do you have to add chemistry 
before they are reusable liquid? In other words, can a junior high school 
teacher actually take a #2 milk container and reform it in school or develop 
an activity whereby my kids can do this?

Response #: 1 of 1
Author:     Eric Dallman
Text:       About 86% of the plastic in use today falls into the category of 
thermosetting plastic. This total consists of about 55% polyethylene and 
polypropylene, 20% polystyrene, and 11% PVC. Your #2 milk jug, high density 
polyethylene, is a thermosetting plastic. Unfortunately, thermosetting 
plastics have cross linked polymers and cannot be remelted. If heated until it 
melts, the plastic will decompose. This is a problem from two points of view. 
First, if you melt thermosetting plastic and then attempt to reset it, you 
will have a non-cohesive blob of goo to contend with. Second, if heated to the 
point that the cross links of the polymer degrade, some actual degradation of 
the plastic and (if present) plasticizer release will occur. PVC, for example, 
contains a polyethalate ester to keep the final product from being hard and 
brittle. Polyethalate ester vapors are not pleasant compounds that I would 
like to introduce to a junior high classroom. Also, the point at which plastic 
melts and that which it decomposes (burns) are very close. Melting plastic 
would result in a release of a host of unpleasant compounds (as anyone who 
every burned a polystyrene foam cup will attest to). Re-softened and remolded under 
very controlled conditions with appropriate environmental controls. It 
probably is not a good idea to attempt this in a classroom setting. I am 
however involved in a sort of a community awareness type presentation for 
local school districts. Right now we (USDOE) are only speaking at grade 
schools (topics include waste reduction, recycling, pollution prevention, 
composting, etc.) I would be very much interested in hearing from you on any 
ideas you have, and perhaps we can come up with some type of recycling 
experiment that does not involve releasing hazardous compound.