Tuesday, June 24, 2014

Why are polymers more complex to recycle?

In principle, recycling polymers is no more difficult than recycling metals or glass. Heat them up until they melt and then reshape them into whatever form you desire. But in reality, it's actually not that simple. There are 3 major reasons for this:
  1. Part of the complexity arises from consumers having access to so many different polymers. You've got the Big 6 for starters, polyethylene (both high- and low-density - HDPE and LDPE respectively), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and polyethylene terephthalate (PET). And then there is nylon, acrylonitrile-butadiene-styrene (ABS), polycarbonate, maybe some polylactic acid (PLA) and others in just about every household. All these plastics need to be kept separate. Besides differences in melting temperatures that can sometimes be quite large, thermodynamics prevents these materials from being compatible with it other (although there are a few exceptions). You cannot melt PET and PP together and get a product of any value.

    Compare this with recycling metals. While metals need to be kept separate as well, consumers have access to large amounts of steel and aluminum and that's about it. And the steel and aluminum can easily be sorted with electromagnets rather than by hand. The same is true with glass and paper. There just isn't much sorting that can be done or needs to be done.
  2. Other issues arise from crosslinked polymers. Crosslinked materials, whether rubbers or not, cannot be recycled because they do not melt. This doesn't mean that nothing can be done with them, as there are a number of options to degrade them to other materials that can then be processed, but that isn't really recycling. And so we have massive piles of used tires around the countryside.
  3. Lastly, the range of physical shapes that polymers can be formed into adds to the complexity. Unless the plastic is already in small pieces, the plastic needs to be resized prior to being reprocessed. While all plastics can be resized, the initial form plays a huge role in deciding what equipment needs to be used. Sometimes grinders are used, sometimes choppers are used, sometimes shredders are use. This isn't really an issue that consumer is directly involved with, but it does explain why plastics films are so seldom recycled. They can be recycled quite easily, but they also can jam equipment that isn't designed to handle films. That leads to many municipalities not accepting them for recycling and then the perception that they can't be recycled.
So the complexity in recycling polymers arises from the large number of polymers available and all the uses we put them to. If we lived in a world that only had polyethylene and polystyrene available, recycling polymers would involve nothing more than separating the cloudy plastic from the clear plastic.

Previous Years
June 24, 2011 - Art - from Ocean Beach Junk

June 24, 2010 - Environmental Stress Cracking

1 comment:

Ken Glick said...

Guess I'll have to stop putting plastic film in our company's recycling bin.