Styrene, from thermal degradation polystyrene

C, and styrene production was also more by 11 wt% in the case of waste expanded polystyrene. Oil yield increased from 24 to 98 wt% with increase in reaction temperature from 350 to 480 °C, while styrene selectivity, which was about 76 wt% up to 450 °C, decreased sharply to 49 wt% at 480 °C. This decrease in styrene selectivity takes place with increase in styrene dimer formation from 4 to 10 wt% and production of other chemicals. Also the production of toluene, ethylbenzene and methylstyrene decreased with the same rise in pyrolysis temperature. Thermal degradation of PS was reported to have started with random initiation to form polymer radicals, the main products being styrene and its corresponding dimers and trimers. These results were comparable with studies reported elsewhere on oil yield of 99 wt% with 60 wt% styrene monomer and 25 wt% for other aromatics. Another work has reported on recovery of 58 wt% styrene from thermal degradation of PS at 350 °C after a time of 60 min [a.379]. Furthermore, oil yields of 82 wt% with 70 wt% styrene selectivity and 77 wt% styrene recovery at 580 °C have been reported [a.380]. 

Polystyrene - Thermal degradation is the simplest of the current techniques used to recover feedstock chemicals from styrene-based polymers and has therefore been studied extensively. Investigations of the product distributions from thermal degradation of polystyrene have mainly focused on liquid products. It has been observed that the yield and the composition of liquid products vary strongly with temperature and the reactor configuration,... 

Carniti et al. examined the product distributions evolved when polystyrene was reacted in batch mode at temperatures of 623 and 673 K up to 120 min in the presence of HY and REY zeolites. The catalyst concentration was 10 wt% for all runs. The product selectivities for these two catalysts were similar and markedly different from thermal degradation. The yields of benzene and ethylbenzene increased, and the yield of styrene decreased to zero with the addition of the catalyst. Selected product yields from the more comprehensive table provided by Carniti et al are summarized in Table 5. Values for a silica-alumina catalyst with a Si02/Al203 ratio of 6.5 are also included for comparison. 

All of tl products l,3 triphenylbenzene, 1,3-diphenylpropane, 1,3,5-triphenylpen-tane, ethylbenzMie, methylben2 ne, and styrene are volatile products of thermal degradation of polystyrene This kind of intramolecular cyclization is not likely to be important in thermal degradation of poly(methyl methacrylate) and poly(a-methylstyrene), because the reactive site is linked to two bulky substituents which restrict the rotation of the terminal carbon atom containing the unpaired electron and thus prevent it from coming into proper orientation to attack the other carbon atoms in the chain. 

Product distribution, variable or stepped pyrolysis and kinetic studies have all been used to expose mechanistic events during polymer degradation. For example, the thermal degradation of polystyrene has been elucidated using the block copolymer poly(styrene-b-styrene-dg). Hybrid and homo monomers (styrene) and dimers (2,4-diphenylbut-l-ene) were detected but without hybrid trimers (2,4,6-triphenylhex-l-ene) (Figure 10). The amount of hybrid dimer far exceeds that which might arise from adjacent residues, while similar proportions of dimers were obtained when the two homopolymers were pyrolyzed together. Data indicate intermolecular reaction rather than the previously proposed 1,3-transfer. 

The thermal degradation pathway of pure PS involves chain scission followed by depolymerization. The resulting products are styrene monomer, dimer, and trimer through an intrachain reaction. However, the presence of clay causes changes in the degradation pathway of polystyrene. In the thermal decomposition of PS nanocomposites, the presence of products from interchain reactions is significant, because the radicals have more opportunity for transfer [20] (Figure 3.3). 

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