Pyrolysis enables successful composite recycling

The plastic polymer components commonly used include thermoset polyester, phenolic resin, epoxy resin, polypropylene and vinylester resin. It is this mix that makes composites difficult to recycle.

During pyrolysis, the polymer breaks down to produce an oil/wax, a gas and a char product, leaving a solid friable residue. Its advantage is that potentially all of the by-products from the process can be used.

A two-year research project at the University of Leeds has recovered glass fibres from composite plastic waste and produced an oil/wax product suitable for use as a liquid fuel or as a chemical feedstock to produce new plastics.

The pyrolysis gas has sufficient energy content to provide the energy requirements of the pyrolysis process plant.

The gas composition is dependent on the original plastic used in the composite. For oxygen-containing plastics such as polyester and phenolic resins, the main gases are carbon dioxide and carbon monoxide.

For other plastics, higher concentrations of hydrogen and hydrocarbons such as methane and ethane are produced.

The researchers decided to concentrate on glass fibre reinforced styrene-polyester co-polymer composite as a typical high usage material.

Pyrolysis produced 40% conversion of the composite plastic waste to oil that had fuel properties similar to those of petroleum-derived gas oil. In addition, the oil contained 25wt% of styrene that was used in the production of the styrene-polyester copolymer.

The plastic was thermally degraded to a wax, which was composed of more than 95wt% of phthalic anhydride – used in the production of the composite material.

The use of the oil and wax as a chemical feedstock in the production of new plastic materials has much greater value than their use as a substitute for petroleum derived fuels.

Glass fibre is a major constituent of composite plastic wastes and also has a significant value. The problem for glass fibre recovery is to use process conditions that do not degrade the fibre strength and the key parameter is the temperature.

Above 800°C, the fibres become brittle and quickly lose strength. Pyrolysis uses low temperatures of less than 500°C – this retains the strength and flexibility of the virgin glass fibre.

Separation of the glass fibre from the filler was achieved using a drum carder machine, which gently separates the fibres from the friable char and filler matrix.

Test sample results showed that up to 25wt% of recycled fibre could successfully be incorporated into new composite, whilst still meeting manufacturers specifications.

Overall the application of pyrolysis as a process route for a difficult waste stream in the form of composite plastic waste appears to show potential.

University of Leeds

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