Mechanical recycling refers to operations that aim to recover plastics waste via mechanical processes (grinding, washing, separating, drying, re-granulating and compounding), thus producing recyclates that can be converted into new plastics products, often substituting virgin plastics.
For mechanical recycling only thermoplastic materials are of interest, i.e. polymeric materials that may be re-melted and re-processed into products via techniques such as injection moulding or extrusion. Thermosets cannot be reprocessed in this way but may be chemically recycled back to feedstock or used as a carrier (e.g. cement kilns).
Thermoplastics represent a variety of multiple polymers with different physical and mechanical properties. A major hurdle for mechanical recycling is that these different polymers are generally non-miscible or compatible with each other. This means that a mixture of different polymers can have inferior mechanical properties which make the recyclates unsuitable for many applications. Consequently, the mechanical recycling of plastics waste is generally only feasible for homogeneous, single polymer streams or for defined mixtures of polymers that can be effectively separated into the individual polymers.
Most mechanical recyclers obtain their input material from collecting and sorting organisations. The market value of recyclates and the costs of the recycling process determine the value of the input material, rather than the actual costs of collecting and sorting which in general should be lower.
Since recyclates aim to partly substitute virgin polymers in existing applications, their market value is directly linked to virgin prices. Converters, however, are only willing to pay a lower price than the corresponding virgin resin price because of the assumption that the quality of recyclates is lower than that of virgin materials. It appears that the marketing advantage of using eco-friendly recyclates is not yet strong enough to overcome the abovementioned price gap.
Examples of mechanical recycling of post-consumer plastics waste:
Collection and grinding of sorted, clean PP crates and blending of the regrind with virgin polymer to mould new crates;
Collection of PE-LD films used in agriculture and industrial packaging, pre-washing, grinding, washing, separating, drying and melt-filtration/re-granulation and processing into refuse bags;
Collection and sorting of PET bottles used for drinks packaging, grinding, washing, separat-ing, drying and processing into polyester fibres, sheets or containers.