Mixed Plastics has been the subject of industry-wide debate due to the various interpretations of what the waste stream consists of. Much of the discussion has centered on whether or not plastic bottles and commercial waste should be included in any classification of mixed plastics.
In order for the issue of mixed plastics to be properly addressed, it is necessary to establish a comprehensive definition for the term to stop potential confusion and inconsistent specifications for mixed plastic waste.
As part of its work to drive a world class recycling industry and help individiuals, businesses and local authorities to reduce waste and recycle more, WRAP (Worldwide Responsible Accredited Production) uses the following definition when referring to mixed plastics:
"Mixed plastic is a term that covers all non-bottle plastic packaging sourced from the domestic waste stream, and it includes rigid and flexible plastic items of various polymer types and colours that are typically found in the household waste bin. http://www.wrap.org.uk/content/definition-mixed-plastics (Source: http://www.wrap.org.uk/content/definition-mixed-plastics)
This definition of mixed plastics include the ordinary, everyday packaging items people routinely throw away such as plastic trays, films, pots and tubs. This definition does not encompass plastic bottles or other plastic items which are not used in packaging such as plant pots or toys.
Over one million tonnes of mixed plastics packaging from households are disposed of in the UK each year and the drive to develop sustainable waste management options for mixed plastics packaging is gaining momentum.
The move towards greater recycling levels is being fuelled by a number of factors including the rising cost of waste disposal alongside increasing public demand for more efficient and environmentally sound ways of diverting waste from landfill
We use plastic every day of our lives, at work and at home, and much of it has a very short life cycle. The question of what we do with end of life plastic is of growing importance due to environmental concerns and in some parts the developed world, the escalating costs of disposal (often the result of deliberate government policy). So, given these commercial and other pressures why is all plastic not recycled?
The two most significant stumbling blocks are material variability and the costs associated with identifying and separating waste plastics into recognizable grade ranges. The more knowledge you have about the material you intend to recycle the easier it is to put into valuable second use applications. Plastics are highly engineered products manufactured in sophisticated and highly controlled manufacturing operations to meet clear specifications which correlate to a well defined set of physical properties. These properties are chosen to fit specific end-use applications and set the manufacturing specifications.
For example, polyethylene is produced using numerous different technologies in a wide range of densities and molecular weights. The resulting materials are designed to suit thousands of specific applications, ranging from assorted films, blown/rotationally/injection molded bottles, sealing caps, oil tanks, specialised gas & potable water pipe grades through to Ultra High Molecular Weight Polyethylene (UHMWPE) often used in biomedical applications. This already diverse supply chain is further complicated via blending, the use of additives (colour, UV protection etc.) and fillers.
As a result, single plastics with known properties and additives are the easiest to reuse, assuming they can be collected and reprocessed cost effectively. However, in many cases it is not cost effective to collect waste plastics due to low volumes and material diversity. Technically it is possible to separate most mixed plastics into recognizable streams, but commercially this is currently only economically feasible for higher value plastics (e.g. PET or the HDPE used in bottles). Effective recycling therefore is dependent upon knowing the property range of the materials you separate, having an efficient collection infrastructure that can consistently gather the desired products and a method of reprocessing the waste plastic that produces a product suitable for re-use.
There are two extremes; at one end of the scale is the factory which reprocesses its own off-spec products, and at the other end of the scale is material such as the plastic blend found in domestic black bag waste which can contain large numbers of different polymers which are often contaminated. Variability and the investment cost of separation equipment are key reasons why only 25% of domestic plastic waste and 40% of bottles are recycled.
Kevin Ross of Impact Solutions, believes if this technology can be commercialised it could significantly increase the volume of available recycled plastic. Most currently available recovered plastic streams will almost certainly be a mixture of different polymers, different grades, and different colours. This is especially true of plastics in domestic waste streams.
Currently the market for lower value mixed plastic is limited with much being exported to low labour cost economies where manual separation of higher value fractions is feasible, and where residues can be used in other applications such as fuels. There are however applications being developed to utilise lower grade mixed plastics some examples being:
Powder Impression Moulding (PIM): A process where low value and even highly diverse plastic blends can potentially be used to create a range of moulded items. This process is now being successfully used in the production of hoarding panels by the UK’s 2K Manufacturing.
Encapsulation: Using the recycled plastic as a “former” which is then encapsulated by a thin layer of higher value plastic. This enables the product to have the appearance and much of the performance of the higher value encapsulated plastic.
Fibre Plastic Composites (FPC): Have experienced significant growth since the early 1990’s in the US and have been successfully developed for niche applications in Europe. Composites can contain a wide range of both plastics fibres from high value PET with carbon nano-tubes to low value wood and other waste cellulosic fibres combined with mixed polyolefins.
After this study, I was pondering, what does the future hold for mixed pastic recycling? Well, there are many technologies exist which facilitate the separation of mixed waste plastic into fractions of various purities and values, but the economic argument for using these on the majority of low value mixed plastics is still very far from proven. However, it is Impact’s view that current developments in large scale, more efficient processes for handling un-segregated waste will change both these economics and step-change the availability of consistent plastic waste. Surely the key challenge for us all is to view ALL plastic waste as a valuable resource and to develop techniques and processes that convert these materials into ever higher value products.