Thermal degradation of high density polyethylene and polystyrene wastes and their 50:50 mixture have been investigated over used FCC and Ga/used FCC catalysts and different heating rates. The pyrolysis experiments were carried out in a semi batch stirred reactor and TGA instrument. The results show that pyrolysis of polystyrene leads to the production of aromatics, although the role of un-zipping mechanism in the presence of zeolite based catalysts was significantly reduced, but the produced dimer, trimer and tetramers were also decreased and products were often in the gasoline range. In pyrolysis of the 50:50 mixture, aromatics were the main product, which significantly increased with Ga/used FCC. Wax was the main product of thermal degradation of polyethylene, while used FCC and Ga/used FCC catalysts produced liquid in the gasoline range however Ga/used FCC increased the aromatics significantly. Heating power as function of heating rate decreased the cyclic products and the produced coke in the catalytic pyrolysis of polyethylene obviously while heating power had no obvious effect on the polystyrene pyrolysis. By using TGA instrument, the results showed that the DTG curve of polyethylene and polystyrene mixture at low and moderate heating rates was different from the high rate.

Novelty Statement
The results of this paper show that the heating rate has a significant effect on the degradation of polymers. At low heating rates, the type of polymer structure and chemical band is very effective in degradation, while its effect decreases with increasing heating rate. On the other hand, the heating rate and the type of catalyst especially Ga/used FCC, have significant effect on the pyrolysis products that can be helpful in the economization of pyrolysis.

Keywords
HDPE; Polystyrene; Polymeric Waste; Used FCC; Ga/used FCC; Heating Rate; Pyrolysis.