Waste polystyrene used for gas separation

CHEMICAL engineers in Taiwan have found that waste polystyrene can be used to produce effective membranes for gas separations.

Growing quantities of plastic waste are causing disposal concerns, and many scientists are seeking new uses for it. The researchers, led by Ming-Yen Wey at National Chung Hsing University, say that their new membranes are economically feasible and environmentally friendly. The membranes could be used for such processes as carbon capture, oxygen enrichment and hydrogen purification.

The researchers looked at three different types of polystyrene waste – oriented polystyrene, expandable polystyrene and high-impact polystyrene. The first two are made from what is known as general purpose polystyrene (GPPS) which is made solely from styrene monomer, while high-impact polystyrene (HIPS) also includes butadiene to increase its strength. Samples of waste polystyrene were obtained from local markets, which were then crushed, cleaned using ultrasonication with water and ethanol, and dried overnight at 75?C.

Ming-Yen and the team used a process called solution casting to make the membranes. This involves dissolving the polystyrene in a toluene solvent, and then casting the solutions onto a glass slide. These are dried out overnight at 75?C to evaporate the solvent, washed in deionised water and dried again.

In various properties tests, the team determined that the decomposition temperatures of the membranes ranged between 400-440?C, and that the presence of inorganic components, for example colourants, does not affect them. Comparisons with membranes made from virgin materials showed very little difference. The membranes’ gas permeability was tested using a lab-made instrument. The researchers placed samples of each membrane in the instrument, degassed on each side using a vacuum. They then tested the permeability at a variety of pressures.

Too high a feeding pressure resulted in a decrease in permeability for all membranes due to membrane saturation. Expandable polystyrene was the least effective for separation, due to instabilities in the membrane structure. The researchers found that HIPS membranes were slightly more stable than the membranes made from oriented polystyrene and expandable polystyrene. The HIPS membranes showed better separation ability than either of the GPPS membranes. The researchers say these improved properties are as a result of the butadiene molecules.

“This novel process for reusing plastic waste is applicable for polystyrene as a material to fabricate polymeric gas-separation membranes to aid in reducing the consumption of raw materials and the pollution caused by waste,” conclude the researchers.

Chemical Engineering Research and Design DOI: 10.1016/j.cherd.2016.03.033