What's happening to polymer chains when...

When heating (or radiating) basically what happens is that polymer chains scisson (are cut) due to energy provided by the heat. In melt processing the original molecular weight of the raw material always significantly decreases. This leads to poorer mechanical properties. Dissolving the polymer to a decent solvent shouldn't necesseraly decrease the molecular weight.
Anyway, that's the reason why thermoplastic polymers cannot be recycled in such a way that they could be reprocessed several times. On the other hand plastics are mostly manufactured from the by-products of the petrochemical industry. Common plastics such as polyethylene and polypropylene are excellent and clean fuel when burned to gain energy. Best way to recycle polymers is to burn them in suitable facilities.
-Petrus

Thanks for your response. Very interesting!
So, if solvents could be used in the recycling process, the loss of molecular weight would not be a problem. I would imagine that emissions from using solvents would be a big problem.
I agree with the burning. I can't believe how we dump trash in landfills where it does not degrade. I wonder if the potential energy in the plastics make it worth sorting, hauling and incinerating in an equipped power producing facility? What other polymers are / are not desirable for incineration? I'm pretty sure PVC is not due to the chlorine in it. -S

Thanks for your response. Very interesting!
So, if solvents could be used in the recycling process, the loss of molecular weight would not be a problem. I would imagine that emissions from using solvents would be a big problem.
I agree with the burning. I can't believe how we dump trash in landfills where it does not degrade. I wonder if the potential energy in the plastics make it worth sorting, hauling and incinerating in an equipped power producing facility? What other polymers are / are not desirable for incineration? I'm pretty sure PVC is not due to the chlorine in it. -S

While all plastics are somewhat sensitive to heat. Some plastics are more sensitive to than others. PVC is very sensitive and must be heavily stabilized in order to be processed. Polyethylene is relatively insensative and requires only a around 0.1% of antioxidants for stable processing. That said, even PVC can and is recycled. The reason that more is not is simple economics. Even at today's "high" prices, in most cases the cost of virigin plastic (i.e. plastics that have not been processed) is less than cost of collecting, sorting and cleaning post consumer recycle plastics. Most plastics that are recycled are typically recycled in house - the scrap and trim form extrusion or molding processes are ground up and blended back into the process at levels between 10-20%. Or this scrap or trim is sold to a broker who then grinds and sells the material as a wide spec product.
This is way most governments have to institute a minimal level of recycled content into plastics parts. It's just not economical. That said, there are some companies that have made it economical, e.g. Trex. In order to do so, they typically deal with a limited set of selected recycle suppliers who supply them realtively clean source of one particlar plastic. In Trex's case they take recycled polyethylene bags and bottles and use them as the polymer for their plastic/wood composite lumber. However, Trex is more the exception than the rule.
Incinerating plastics for fuel is a very good idea. However, most muncipalities are reluctant to pursue the concept due to sniping from environmentalists.
Larry Effler

Thank you Larry for bringing us detailed knowlidge of plastics recycling. In my opinion enviromentalists sniping about burning plastics for fuel are way out of track. I consider myself an enviromentalist of some sort. I also know that clean and safe burning facilities for municipal waste can and have been constructed. By regulations that oppose these facilities we are just biting our own legs off. By burning waste we would gain energy and reduce the amount of environmental problems created by the landfills that keep growing and growing...