Summary of the GPPS process

High impact poiystyrene (HiPS) process Process description 

Generally, the process is very similar to the GPPS process. The main difference is the addition of the rubber component. Polybutadiene rubber received in the form of 35 kg bales is ground into small chips. These rubber chips are added to the dissolving tank by gravity or pneumatic conveyor. There, with strong agitation, the chips are dissolved in styrene to obtain a rubber solution that can contain up to 15 % of rubber. 

The antioxidant is usually also added in the dissolving tank. In addition, other chemicals can be added there such as white oil, peroxides, recycled styrene, ethylbenzene or chain transfer agents. The dissolved mixture is then fed continuously to the reactor train where bulk polymerisation occurs. Chemicals not added in the dissolving tank are added into the feed stream or directly into the reactors. 

The reactor train usually includes continuous stirred tank reactors (CSTR) and/or plug flow reactors (PFR). The styrene itself acts as the solvent of the reaction. Moreover, up to 10 % of ethylbenzene is added to ensure a better reaction control. The reactors temperatures are between 110 and 180 °C. The pressure is up to 1 MPa in a PFR, whereas reactions in CSTR are carried out under atmospheric or sub-atmospheric pressure. At the end of the reactor train, the styrene monomer conversion reaches 60 - 90 % solid. 

The process flow then goes through a devolatilisation section where it faces one or two flashes (one or two devolatilisation vessels) to separate the pol5mier from the unreacted monomers. The devolatilisers are operated at high temperature (220 - 260 °C) and under high vacuum ( 40 mbar). 

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A tabulated summary of the GPPS process is shown in Table 4.5. 

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