Why don't we recycle more waste plastics? It's an economically, as well as environmentally, important question. In the UK only 7% of the 2.3 million tonnes of waste plastics generated each year are recycled into new products. However, thanks to new legislation this figure is set to increase rapidly over the next few years. The European Commission has set recycling targets for packaging of at least 15% of each material used including plastics - that must be met by 2001. Similar targets could easily be incorporated into draft automotive and waste electronic and electrical equipment directives.
These draft directives on waste management are fueling the debate about how best to recycle plastics. The National Physical Laboratory (NPL) is looking at a number of technical issues that must be resolved if recycled plastics are to be used with any degree of confidence in new or demanding applications. In particular, the strengths and weaknesses of recycled plastics need to be much better understood.
Cost and Complexity of Recycling
The cost and complexity of recycling very much depends on the material. Manufacturers have recycled the scrap plastics produced during processing for many years. This material comprises sprues and runners, poorly coloured mouldings and short shots. It is ‘clean’ material that is known to the processor and - perhaps more importantly - if it were not reused the wastage would adversely affect company profit margins. However, there are occasions on which processors cannot reuse this material. Customer demands and technical issues mean that safety critical items or mouldings that need to be, say, optically perfect are not made from recycled plastic.
When Recycled Plastics are Unsuitable
These issues are becoming more important as companies introduce complex co-injection processing routes. In this type of processing, mouldings are manufactured in a single shot from a number of different plastics or from the same plastic but in different colours. This technology is already used to make keyboard keys, in which black plastic is co-injected to form the letter all the way through the key. Rear lights ‘clusters’ for cars are also made from red, amber and clear polycarbonate. Manufacturers can also use these techniques to produce complex assemblies such as vehicle parcel shelves, which may now be produced complete with a carpet finish in a single step moulding. Recycling process scrap from these complex moulding operations is much more difficult than for simple mouldings. Multi-coloured plastic rear light clusters that fail quality checks obviously cannot be granulated and re-moulded to form new rear light clusters because of colour contamination.
Recycling of Post Consumer Scrap Plastics
Such problems are only the tip of the iceberg. Scrap plastics from processing represent only a small fraction of the plastic waste generated. Post consumer scrap, which makes up by far the highest proportion of plastic waste, often has an unknown service history and may be contaminated. It is the most expensive scrap plastic to recycle and is usually perceived as being the least useful. The price of this material is high as it includes transport and labour costs incurred during collection and sorting. Additional costs are incurred if the material has to be cleaned before it can be reprocessed.
Low Tech Applications for Recycled Plastics
Much of the recycled post consumer plastics scrap is used to form ‘low’ technology goods, applications in which the plastic typically is being used as a replacement for wood - for example, in the manufacture of pallets and street furniture. In these applications, mixed plastic waste from the municipal waste stream is used as a feedstock for compression moulding. High levels of metal foil, paper or adhesive contaminants can be tolerated in these woodlike substitutes. In one instance, used milk cartons lined with plastic are being granulated and successfully compression moulded to form a board-like composite that looks and behaves like chipboard.
Recycled Plastics in Clothing
Not all post-consumer scrap has to be used in such low-tech applications, though. It is also possible to produce mouldings with more demanding specifications. Outdoor jackets made from waste plastic have been on the market for several years. The fleece used to make these jackets is made of fibres spun from recycled PET soft drinks bottles. Only clear bottles that have been sorted and cleaned to remove potential contaminants such as plastics or paper are used.
Collection of Recycled Materials
This is a good example of how companies source a particular product for recycling the plastic. Using this approach gives some perceived consistency in the properties and processability of the feedstock. However, companies using single sources of feedstock need a process for collecting and sorting the material. In the case of vehicle batteries or white goods, this could readily be achieved through a ‘take back’ scheme or by using the local council's collection procedures. Obtaining recyclate from single sources requires a well developed infrastructure and tends to be the exception rather than the rule.
Effect of Contaminants
Post consumer waste often contains a wide range of contaminants, from chemical residues to fragments of metal and paint. Most users of recyclate firmly believe that plastics such as these need to be extensively cleaned before it they can be reused. This belief comes from valid reasons, in some cases - no one wishes to accelerate screw wear by processing plastics that contain shards of steel, or to block nozzles with globules of solid immiscible plastic. However, contrary to popular opinion, contaminants can have very little effect on many properties of plastics. Admittedly this sweeping statement is based on a limited amount of scientific research, but the results obtained so far look promising.
NPL has recently shown that doping high density polyethylene with copper turnings has very little influence on its mechanical properties such as Young's modulus, strain to failure and impact behaviour. These results are encouraging, but they were obtained using a specific grade of polyethylene. Different or mixed grades may give different answers.
CARE (Consortium for Automotive Recycling) has obtained similar results for mixed grades of polypropylene. It has shown that the performance of components made solely from polypropylene obtained from scrap bumpers was similar to components made from virgin material. This is quite an achievement, especially as the bumpers were granulated in the condition in which they were received from the breakers yard. Paint flakes, strands of copper wire and road grime do not seem to affect significantly the performance of the material.
Reasons for Cleaning Recyclates
It is important to be very careful when using recyclates from post consumer plastics waste as they may contain food or chemical residues. In some cases these materials retain odours that can be undesirable. Who would buy a car with air ducting made from waste fish crates? On a more serious note, materials such as oil can pose processing problems. Oil that permeates into the walls of containers made from polyethylene is released on reprocessing as noxious fumes. Removing these containers from the waste stream before the material is granulated eliminates this problem however, this process is relatively expensive as it is currently done by hand.