Reprocessing of Thermoplastic Recyclates

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Mechanical recycling is the most common method of recycling. Here plastics are physically ground back to a suitable size (regrind) and reprocessed. The end use can be the original one or something different.

In the plastics industry it has long been common practice to reprocess waste material arising from normal production. This in-house recycling, known as primary recycling, makes economic sense as it reduces both production waste and utilisation of raw materials. For example, with injection moulding, regrind from start-up waste and production waste such as reject parts, can be fed directly back into the production machine.

For reclaiming used material or recyclate outside of this scenario, the situation is slightly different and greater effort may be required on the part of the reprocessor. This type of mechanical recycling is termed secondary recycling (see Table 5.1).

Table 5.1 Types of mechanical recycling

Primary Recycling

'In-house' reprocessing of production waste

Secondary Recycling

Mechanical recycling of single or mixed plastic materials from external sources

The material from external sources may be received in a variety of forms such as bales, mouldings or large lumps. It will probably need to be reduced in size, cleaned, separated and possibly recompounded and regranulated before it can be reprocessed in production. Often little is known about the history of the material to be recycled, for example:

  • How many times it has been reprocessed previously?
  • How much thermal degradation it has already undergone?
  • Is it in-house or external waste?
  • Is it a single material or a combination of plastics (commingled)?
  • Is it contaminated or uncontaminated?
  • What it was used for previously?
  • Has it been damaged due to outdoor exposure, weathering, etc?

The characteristics of plastics can change depending on the exposure to thermal, mechanical (shear), oxidative and photochemical degradation processes. The characteristics of the recyclate may be quite different from those of the original virgin plastic.

Ideally, to produce high quality products, high quality materials are required. For this, consideration must be given to a number of factors. It must be determined as to whether the material is pure or commingled and whether it is contaminated, for example, with metal or wood. For ease of feeding into the processing machines be they injection moulding, extrusion or blow moulding, the size and shape of the regrind (i.e., the bulk density) must be suitable. If the material is hygroscopic (water absorbing), for example polyamide, it may require pre-drying. Finally, should the recyclate be reprocessed on its own, mixed with other virgin material or modified with additives?

Within a closed loop cycle, it is easy to recycle materials and this is the reason that primary recycling is so commonplace. The key is having the knowledge of, and confidence in, the materials that are being used.

One example of a closed loop cycle in action is seen in the automotive industry. Since 1991 Volkswagen have recycled scrap bumpers made of a modified grade of polypropylene (PP). Their supplier reclaims the material, which is then mixed with virgin and returned to the bumper production process. The properties of the bumpers produced are as good as those made using virgin material alone. In tests it was found that no significant difference in characteristic properties occur until the material has been melted and extruded eight times [1].

It is easy to see the benefit of recycling in these closed loop environments. One of the biggest factors against the use of recyclate is the concern on the part of users that recyclate will reduce the quality of their product or damage their processing machinery. For this reason in the UK a number of groups such as Waste and Resources Action Program [2] (WRAP) and Consortium for Automotive Recycling [3] (CARE), have championed a recyclate materials standardisation programme. It is hoped that by providing a clear set of recyclate material properties, designers will be better positioned to specify, and have confidence in, recyclate materials.

The effects on material properties of mechanical recycling can be explored by repeated cycling of material through processing machines (see Section 3.4). Material from each cycling loop can be assessed. For example, in the production of the Volkswagen bumpers mentioned earlier, tests found that properties changed significantly, only after eight cycles of reprocessing. Experiments of this kind have shown that short-term properties do not vary too greatly if the material does not contain glass fibres. Glass fibre is a very common reinforcement used in plastics. However, this material tends to become damaged when reprocessed. The mechanical strength of the plastic is dependent on the length of the fibres used and the act of processing reduces this residual length.

However, a word of caution should be applied here. Note that these are short-term properties only. The long-term effects of repeated processing on plastic properties are still under investigation. Whether these materials, when mixed with virgin, will undergo accelerated degradation is still the subject of current research. It does seem clear however, that incorrect processing parameters (too high temperatures) cause far more damage to the plastics than repeated processing at suitable temperatures.

One important criteria for high quality processing is the homogeneity of the material.

When recyclates are mixtures of different viscosity and colour, it is important that they are mixed adequately together to form one coherent material. Special screws are available for processing equipment. These homogenising screws improve both the product quality and the reproducibility.

Homogeneity is a word that you will encounter throughout this book. It is a most important concept, as much of the processing work done on a material is carried out to get it to a consistent state. This has a number of benefits:

  • Highly automated machinery can be used in production with the knowledge that it will not need to be adjusted greatly.
  • The properties and quality of the production component are known and will not vary drastically.
  • Quality procedures are less time consuming and faults are easier to identify and trace.
  • The customer will get a high quality product of consistent quality.

In fact, achieving homogeneity with recyclates, especially mixed materials, is difficult and sometimes impossible. The technology of reprocessing recycled plastics still has a way to develop. This means that quality control for recyclates is every bit as necessary as for virgin material.

For example, imagine you have a recycled material that was a 40% glass filled polypropylene with a data sheet of properties provided from the supplier of virgin material.

How will those properties have changed?

Is that datasheet a true reflection of the material you have in front of you?

Hopefully, having read Chapters 1-4, you will now be in a position to answer this question or, perhaps more specifically, to know which questions to ask!

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