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Identifying plastics

Product analysis: the Tribopen

The market
The Tribopen was originally developed to sort and recycle automotive waste. Now it is also used to:

In the future, the Tribopen could be used for a range of other applications where it is necessary to discriminate between different types of plastics. The biggest potential market is domestic and industrial waste management and recycling.

Technical specification
There are three lights at the top of the Tribopen. The red and green lights indicate which group of plastics the product belongs to. The yellow light shows when the battery is being charged. The pen can operate within a wide temperature range (0°C to 40°C), but the permitted humidity range is narrower (35 to 55% RH) because the samples tested need to be dry.

The pen is 155mm long and has a diameter of 28mm. It is powered by a NiCd rechargeable battery.

Product function
The Tribopen is a hand-held device that measures the tribo-electric (static) charge generated when its sensor head is rubbed against the surface of a plastic sample. The sensor head can be changed according to the materials being sorted.

Different plastics have unique molecular structures, so their surfaces generate different static charges. Within a few seconds of the pen's head touching plastic, a red or green LED indicates whether the plastic is in one of two polymer groups. The process can then be repeated using different heads or pens to test for different polymers, until the Tribopen makes a definite identification.

The Tribopen can identify a range of materials, including nylon (PA), acrylic (PMMA), ABS, metal, polypropylene (PP), PVC and polyethylene (PE).

Wolfson Electrostatics has developed a conveyor-belt system with a number of pens in series. Plastic products (e.g. bottles) are loaded into a hopper and brush past each of the pens in turn. As the pens identify the plastics, the products either stay on the belt or are pushed off. At the end, the plastics that are most useful for recycling have been identified and sorted, and the remainder can be burnt for energy. This conveyor-belt system is still at the laboratory stage, but its commercial applications are obvious.

The Tribopen case is made from anodised aluminium. Its heads are made from metal, PVC or acrylic.

The Tribopen is made in batches, according to demand.

The pen operates at 12V. It is non-toxic and doesn't give off any harmful emissions.

The pen is comfortable to use and can be carried in the pocket. Its heads can be changed easily. No special training is needed before using the pen, although a new user will need to practise.

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Practical Action - Technology challenging povertyEuropean Commission - Department for International Development

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