Compatibility of plastics depends on whether or not the use of a secondary material within a component or sub-assembly will cause impurities that will disrupt the recycling of the primary material (see the chart below). Preferably all parts within a component/ sub-assembly are made of the same polymer, but if this is not possible, make sure the polymers are compatible.

Source: Maier, C. Design Guides for Plastics; Econology Ltd., 2009

 

Plastic incompatibility can cause problems in separation of plastic waste streams if two plastic types have properties that are indistinguishable during the separation. For example, float-density separation is a technique used to separate plastic waste streams by density. Two plastic types that have the same density cannot be separated by such a technique. For example, PLA cannot be separated from PET using float-density separation (CE Delft, 2021).  In addition, in certain combinations, small concentrations of one plastic can ruin the properties of another waste stream. For example, PET becomes brittle and yellow when it is blended with a small concentration of PVC (Dorigato, 2021).

On the next page we will discuss a number of plastic-specific design guidelines for recycling.