7.1 Introduction

The two biggest problems associated with plastics recycling of post consumer products are:

1. The degradation of the plastics as a result of both processing and service life, and

2. The difficulties associated with getting acceptable material properties when processing mixed plastics [1].

To slow down the degradation of plastics, stabilisation additives are employed. Other additives such as fillers or modifiers can be incorporated to improve the properties of the recyclates. These modifiers can also be of benefit in mixed plastic fractions. First the issues associated with the degradation of materials will be discussed.

7.2 The Degradation of Plastics

Due to their chemical make-up, most plastics are susceptible to damage from the influence of the environment due to the action of light, heat and weathering. This ageing process is accompanied by a drop in both the visual (commonly seen in a yellowing of white carrier bags) and mechanical properties of the plastic, and it affects both virgin and recyclate materials alike. Plastics are also susceptible to damage from the shear force that can arise during standard manufacturing processes and granulation. This action is accelerated by the high temperatures involved. The result is chemical degradation due to molecular damage in the form of chain cleavage, crosslinking or the formation of a series of double bonds.

Waste plastics, therefore, will have degraded to some degree following their original usage. The level of this degradation depends on the polymer type, on thermal and shear stress history and on...