Why is plastic so hard to recycle?

by Jeffrey R. Seay, PhD, PE 

             In principle, plastic is recyclable. It may certainly appear that the plastic waste crisis is simply one of management – if we did a better job of collecting plastic and recycling it we could keep it out of the ecosystem entirely – right? Unfortunately, it is isn’t that simple. The recycle number printed on plastic products is only part of the story.  Plastic is of course a generic term referring to a multitude of products. There are about 70 types of plastic, and over 80,000 grades. There are a variety of reasons as to why we have so many types and grades of plastic. One of course has to do with intellectual property. The desire to maintain control over a new product has led to the development of many plastics with similar properties. However, the most prominent reason is due to the ability to modify properties precisely to meet very specific needs. Chemical additives can be used to tweak the properties to suit an exact purpose – and these additives complicate the picture dramatically!

In plastic materials used in most products the basic polymer is incorporated into a formulary (plastic compound) with different ‘additives’, which are chemical compounds added to improve the performance, functionality and ageing properties of the polymer. The most commonly used additives in different types of polymeric packaging materials are: plasticizers, flame retardants, antioxidants, acid scavengers, light and heat stabilizers, lubricants, pigments, antistatic agents, slip compounds and thermal stabilizers. Each of them plays a distinct role in delivering and/or enhancing the (final) functional properties of a plastic product.

For instance, catalyst deactivators neutralize any remaining catalyst residues, nucleators increase resin clarity and reduce processing time, and pigments provide a variety of colors. Anti-static agents permit the discharge of static electricity from the film or part, and the addition of flame retardants allows the use of PP in electronics, construction, and transportation applications. Anti-block and slip agents are commonly used in films, to prevent the latter from sticking together, or to metal surfaces. These additives can be mainly divided into 4 categories: Functional additives; Colorants; Fillers; and Reinforcements. All of these additives make post-consumer plastic very difficult and expensive to recycle.