Mixed blessing

A technique that can process mixed plastics and create new recyclable products may help companies cut the high cost of sorting waste.

A technique that can process mixed plastics and create new recyclable products may help companies cut the high cost of sorting waste.

London-based Environmental Recycling Technologies (ERT), with Bovis Lend Lease and Tesco, is using its powder impression moulding (PIM) process to develop products such as site hoarding systems (building site fencing) and reusable plastic containers for use in retail logistics.

The work is part of a £1.2m project partly funded by the Technology Strategy Board.

‘We are looking at returnable transit packaging and other products that could potentially make use of the supermarket waste and start building a closed-loop model,’ said Paul Gallen, a project leader at ERT.

‘There are not many processes that will allow the use of mixed plastics in the moulding. It is an open mould technology where we take the mixed plastic to the mould and melt it in situ, rather than, say, injection moulding where you would melt the material and then move it quickly to the mould tool. If you had mixed plastics in there it would not process well.’

In the PIM process, an external heat is applied to the mould set, in which skin layers of mixed-plastic granules (or single plastic, depending on product specifications) are laid to form the exterior skin of the product. After that a core of powdered mixed-plastic, which also contains a small amount of chemical blowing agent, is put into the mould.

‘We then close the mould set together, bring up the temperature and melt the plastics within the skin and the core, decompose the chemical blowing agent and what we get is a foamed-core, solid plastic product,’ said Gallen.

He said this process would result in a product with a core of much lower density than typical thermoplastics, which can weigh anything from 900kg/m3, yet still have a solid outer layer.

ERT’s technology can process a wide range of plastics, from the most common family of packaging polymers — olefinic materials — to recycled polystyrene materials. Olefinic polymers include polypropylenes (such as shampoo bottles) and polyethylene, (such as cling film, plastic bags and milk bottles).

‘Initially it started off with polyethylene but we found that we can use a wide variety of different thermoplastics to different effects. We’ve also worked with PET, which is what coke bottles are made from, and engineering thermoplastics such as nylons,’ said Gallen.

‘We can adapt the process depending on the product specification and the availability of the materials.’

So if a product needs to be lightweight and cheap, a suitable ‘raw’ material for ERT would be something like recycled polyethylene.

In certain circumstances, the company might blend other materials with the polymer waste streams in the PIM process or just prior to it, to achieve the right material properties. ‘We also have the opportunity of introducing other non-polymer materials into the core, such as fibre or mineral reinforcements. For a shower tray, a specialist building product, we have actually put aluminium reinforcement within the core,’ said Gallen.

Although this might pose a problem in recycling the product, Gallen said the non-polymer materials could easily be removed in the PIM process: ‘You can partially melt it and pull it out, but it really does depend on the application.’

ERT’s technology allows the skin and the core of the product to be made from different materials so if a specific recycled polymer is required for the product surface (for example, if it were to come into contact with food) it is possible to use mixed plastic for the core. This also has cost benefits.

‘With post-consumer plastics, most recycling will start out either by collecting it in such a way that you are able to sort it at source, or it gets sorted at some point in the material improvement chain,’ said Gallen.

‘You then have all these costs in terms of segregating, sorting and improving and trying to get it back to something like a virgin grade of the material.

‘Through our process, which allows stuff like paper, glue and inks to be left in, instead of spending a lot of money on trying to improve it, we say there is a use for it in the core, where we will not have to spend too much money on improving the material.’

Material engineers at Brunel University are helping by analysing the available sources of plastic waste, classifying them and defining the suitability of the waste for use in ERT’s process.

Other industry partners include waste management companies St Regis Paper Company and Philip Tyler Polymers and technology consulting group Pera.