Plastic Recycling - Challenges

Challenges

When compared to other materials like glass and metal, plastic polymers require greater processing (heat treating, thermal depolymerization and monomer recycling) to be recycled. Due to the high molecular weight of their large polymer chains, plastics have a low entropy of mixing. A macro molecule interacts with its environment along its entire length, so total energy involved in mixing it is large compared to that of an organic molecule with a similar structure (see enthalpy). Heating alone is not enough to dissolve such a large molecule, so plastics must often be of nearly identical composition to mix efficiently.

When different types of plastics are melted together, they tend to phase-separate, like oil and water, and set in these layers. The phase boundaries cause structural weakness in the resulting material, meaning that polymer blends are useful in only limited applications.

Another barrier to recycling is the widespread use of dyes, fillers, and other additives in plastics. The polymer is generally too viscous to economically remove fillers, and would be damaged by many of the processes that could cheaply remove the added dyes. Additives are less widely used in beverage containers and plastic bags, allowing them to be recycled more often. Yet another barrier to removing large quantities of plastic from the waste stream and landfills is the fact that many common but small plastic items lack the universal triangle recycling symbol and accompanying number. A perfect example is the billions of plastic utensils commonly distributed at fast food restaurants or sold for use at picnics.

The use of biodegradable plastics is increasing. If some of these get mixed in with the other plastics for recycling, the reclaimed plastic is not recyclable because of the variance in properties and melt temperatures.