Commonwealth of Australia 2000

6.2.2SAN, ABS and ABS/PC alloy resin pellets

SAN, ABS and ABS/PC alloy resin pellets are compounded from the respective polymers by blending and extrusion. The compounding and extrusion of SAN and ABS resin pellets at Huntsman Chemical is typical of this process. The SAN polymer beads are fed into a blender from a weigh hopper or added manually via a chute. The ABS polymer pellets/powder and PC powder are charged manually. To meet product specifications, a great deal of attention is paid to the weighing and addition of pigments and additives prior to manual charging by gravity into the blender. Following blending, the compound is fed into the heated barrel of an extruder and extruded continuously through uniform die heads. SAN and ABS polymers do not decompose at or near extrusion processing temperatures (230ºC) under normal processing conditions (Radian Corporation, 1986). Volatile vapours from additives and residual acrylonitrile and styrene are removed from the die heads and extruder barrels by a vacuum system. The two vapour streams are combined and discharged from a vent. The hot resin strands are cooled through a water bath and pulled up through cutters. The resulting pellets are sifted to eliminate those of non-uniform size for disposal and sent to a hopper for bulk packaging into containers or bags. Dust is removed from the bead storage, weighing and blending and packaging areas by extraction and collected on a filter.

The further processing of resin pellets to plastic articles is described below. Some ABS resin pellets are exported to New Zealand for moulding.

6.2.3Polymer emulsions

Polymer emulsions are submicroscopic, spherical polymer particles of high molecular weight finely dispersed in water. They are made by dissolving an emulsifier in water until micelles are formed. As acrylonitrile and other hydrophobic monomers are mixed in, they migrate to the interior of the micelles where polymerisation occurs upon contact with the initiator. Emulsion polymerisation is always performed with water soluble free radical initiators, such as peroxides or persulfates.

In the manufacture of polymer emulsions, acrylonitrile may be co-polymerised with a number of other monomers, including n-butyl acrylate, 2-ethylhexyl acrylate, vinyl acetate, methyl methacrylate, styrene, ethyl acrylate, 1,3-butadiene, itaconic acid, acrylic acid, and N-methylolacrylamide. The concentration of acrylonitrile to other monomers ranges from 2-20% depending on the final product. In most cases, the specifications require that the concentration of residual acrylonitrile in the final product is <10 ppm, however, some require a level of <0.1 ppm (BASF, 1998). Each batch is tested for conformity with these requirements.

Acrylonitrile is delivered to the manufacturing sites in a dedicated road tanker. A discharge line is hooked up between the road tanker and a discharge pump for transfer of acrylonitrile into a sealed storage tank. The chemical is pumped directly to the reactor when required for production and the polymerisation process takes place under sealed conditions.

In the reactor, acrylonitrile is mixed with other monomers, aqueous emulsifier solution and other chemicals such as initiators, buffers and defoamers. Quantities in the reaction mixture are according to the specifications required in the final product. Some companies premix the emulsifier, water and other additives in a tank before being added to the reactor.

Polymerisation occurs in a stirred-tank enclosed reactor at temperatures between 20° and 130°C and depending on the components with or without pressure (up to 15 bar). Reactor cooling and further evaporative cooling if necessary reduces heat produced during the polymerisation process. The reaction is stopped at conversion levels of 80 to 99%. Steam stripping then removes vapours containing residual monomers from the reactor. The vapours from the steam are vented to an external caustic scrubber or an incinerator. During cooling, specific reagents may be added to scavenge any unreacted monomers in the emulsion. Following completion of the reaction, the polymer emulsion may pass through a coarse filter and is then transferred to a conditioning vessel, where final product adjustments are made to stabilise the mixture by adding preservatives, defoamers, plasticisers, thickeners and pH adjusters. If necessary, the product may be passed through a fine filter before being pumped into bulk product storage tanks or direct to packaging. The product is transported to the customer in road tanker, 1 t tanks (Schutz or collapsible Palecon containers), 200 kg drums or 20 L pails.