ABS is made by polymerizing styrene and acrylonitrile in the presence of polybutadiene rubber, so that part of the styrene-acrylonitrile copolymer (SAN) grafts onto the rubber particles and the rest forms the rigid matrix around them. The three monomers come from petrochemical streams, and the polymerized resin is compounded and cut into pellets before it ships.
This article covers how ABS resin is made; forming parts from the pellets by injection molding, extrusion or thermoforming is a separate subject.
Step 1: Producing the Monomers
Each of the three monomers is made from petrochemical feedstocks, by one dominant route per monomer:
- Acrylonitrile: made by ammoxidation, the Sohio process, in which propylene, ammonia and air react in the vapor phase over a catalyst.
- Butadiene: recovered from the C4 stream that a steam cracker produces as a co-product of ethylene, then concentrated by extractive distillation or solvent extraction.
- Styrene: made by dehydrogenating ethylbenzene, which is itself made by alkylating benzene with ethylene.
In finished ABS, the rubber content ranges from 5 to 30 % and acrylonitrile is about 25 % by weight, with styrene making up the balance.
Step 2: Polymerizing the ABS
Styrene and acrylonitrile are polymerized in the presence of polybutadiene rubber: some of the SAN chains graft onto the rubber, and the rest form a rigid SAN matrix in which the rubber sits as fine particles. The graft holds the two phases together.
Plants run this step by one of two main routes, emulsion and mass, and the emulsion route dominates. Hybrid/emulsion technology, in which the graft rubber is made in emulsion and then blended with separately made SAN, accounts for about 97 % of global ABS production, according to a 2023 paper whose authors work for a licensor of the competing mass process.
A third route, suspension, starts like the mass process and finishes the reaction in water with suspending agents.
Emulsion ABS has smaller rubber particles and a glossy finish and is suited to injection molding, but the soap and water it is made in leave emulsifier in the product, and its base color varies, often with a light yellow hue. Mass ABS has a consistent light base color, but its larger rubber particles give a matte finish, and it is better suited to extrusion.
Heat-resistant and transparent grades are made at this step by changing the monomer recipe, adding a further monomer such as alpha-methylstyrene or methyl methacrylate.
Emulsion Polymerization
Emulsion ABS is built in water, starting from butadiene monomer and ending as a dry graft-rubber powder:
- Rubber latex: butadiene is polymerized in a water emulsion into a polybutadiene latex, and the latex particles are grown to the size the grade needs.
- Grafting: styrene and acrylonitrile are fed into the latex and polymerize onto the rubber particles.
- Coagulation: a salt or acid solution is added to break the latex, and the graft precipitates out of the water.
- Dewatering and drying: the precipitate is centrifuged and dried into a graft-rubber powder, ready to be compounded with SAN in Step 3.
Mass Polymerization
Mass ABS, also called bulk ABS, is made by dissolving a preformed polybutadiene rubber in styrene and acrylonitrile and polymerizing the solution continuously, with no water. Commercial mass processes often carry some solvent, such as ethylbenzene, so the route also goes by the name solution polymerization.
As the monomers polymerize, the growing SAN becomes the continuous phase and the rubber breaks up into particles, a change called phase inversion.
The reaction is stopped short of full conversion. The polymer solution passes through vacuum devolatilizers that strip out unreacted monomer and solvent for recycling, and the devolatilized melt goes straight on to pelletizing.
Step 3: Compounding and Pelletizing
ABS is turned into pellets by melt extrusion, and the path depends on the route. Emulsion graft powder is melt-blended in a twin-screw extruder with separately made SAN and with additives such as lubricants, stabilizers and pigments, and the extruded strands are cut into pellets. Mass ABS needs no separate blending step: additives are dosed into the devolatilized melt on its way to the pelletizer.
For emulsion ABS, the ratio of graft powder to SAN sets the finished resin’s rubber content, so one rubber-rich graft can be diluted into ABS grades of several impact levels.
Before shipment, each lot is tested against the producer’s sales specification. For one general-purpose emulsion ABS, the contractual specification lists four properties: melt volume rate, Vicat softening temperature, notched Charpy impact and yellowness index, the values a certificate of analysis reports for each lot.