What Is ABS Plastic

ABS plastic is acrylonitrile butadiene styrene, an amorphous thermoplastic in which a rigid styrene–acrylonitrile copolymer is toughened with polybutadiene rubber. It is chosen for its combination of stiffness, impact strength and surface finish.

It is a high-volume engineering plastic, a class that sits above commodity resins such as polyethylene, polypropylene, polystyrene and PVC, at an intermediate cost level.

The health questions about ABS concern its residual monomers and the fumes of severe overheating: processed correctly and with ventilation, it has shown no adverse effects on processing staff. For food contact in the US, 21 CFR 181.32 allows ABS with under 30% acrylonitrile only as films, coatings, food-handling piping and repeated-use articles, and only within limits on the acrylonitrile monomer it releases.

How ABS Plastic Is Made

ABS is made from three monomers, acrylonitrile, butadiene and styrene, by polymerising styrene and acrylonitrile onto and around polybutadiene rubber. Each monomer contributes part of its properties:

Styrene: stiffness, easy processing and lower cost. It is typically at least half of the resin.
Acrylonitrile: heat and chemical resistance.
Butadiene: rubber-like ductility and impact resistance.

Producers vary the shares of acrylonitrile and butadiene from grade to grade to tune the balance of properties. The registry formula, (C₈H₈·C₄H₆·C₃H₃N)ₓ under CAS RN 9003-56-9, names the three monomers without fixing their ratio.

ABS is made commercially by two main routes, emulsion and mass. In the emulsion route, butadiene is polymerised in water into a rubber latex, styrene and acrylonitrile are grafted onto the latex, and the grafted rubber is blended with styrene–acrylonitrile copolymer. It is usually a batch process and suits glossy grades.

In the mass route, the rubber is dissolved in styrene, acrylonitrile is added, and the solution polymerises continuously; the unreacted monomers are then stripped off under vacuum and the polymer is pelletised.

Properties of ABS Plastic

ABS is tough and hard to break in normal use, but a notch or cold sharply cuts its impact strength.

ABS is amorphous, so it has no melting point. It softens through its glass transition at about 105 °C and is processed as a melt at 220 to 260 °C; the “melting points” quoted for ABS are one figure or the other.

ABS is stiff, scratch-resistant and dimensionally stable, and its glossy surface takes colour, paint and electroplating. It is also a good electrical insulator, and a general-purpose grade has a tensile stress at yield of 45 MPa under ISO 527.

The table gives typical values for a general-purpose grade, GP-22, and a high-impact grade, HI-10, from their producer’s ISO datasheets.

Property	Test method	GP-22	HI-10
Density (kg/m³)	ISO 1183	1040	1030
Charpy notched impact, 23 °C (kJ/m²)	ISO 179/1eA	22	35
Charpy notched impact, −30 °C (kJ/m²)	ISO 179/1eA	8	13
Heat deflection temperature, 1.8 MPa (°C)	ISO 75	94	93
Vicat softening temperature, B/50 (°C)	ISO 306	96	90
Linear mould shrinkage (%)	ISO 294-4	0.4–0.7	0.4–0.7

These are typical values for uncoloured material, not specification limits.

The clearest difference from rigid PVC is fire behaviour: PVC, with an oxygen index of about 47, stops burning once the flame is removed, while ABS, at about 19, keeps burning in air.

Disadvantages of ABS Plastic

ABS is vulnerable to sunlight, flame, solvents and heat, and a part has to be designed or specified around each:

Sunlight: UV light embrittles the surface. Outdoor parts need a UV-stabilised grade or a weather-resistant styrenic such as ASA.
Flammability: standard grades rate only HB in the IEC 60695-11-10 flame test, so parts that need a V rating call for a flame-retardant grade.
Solvents: ketones, esters and chlorinated solvents attack ABS. Coating and printing solvents have to be chosen so they do not stress-crack the part.
Heat: a standard grade deflects at 94 °C under a 1.8 MPa load, so hotter parts need a heat-resistant grade or a PC/ABS blend.

Clean ABS scrap, such as moulding regrind, recycles well by remelting. Used parts may have lost properties with age or be mixed with polyolefins, polystyrene or polyamides, so recycled ABS goes into parts that do not need high impact strength or clean colours.

Grades of ABS Plastic

ABS grades are told apart by melt flow, notched impact strength, Vicat softening temperature and tensile modulus, the four properties ISO 19062-1 classifies ABS by, and producers sell them in families built for one process or one property:

General purpose: easy-flow injection-moulding grades with balanced impact and heat resistance, used for a wide range of parts and especially housings; GP-22 is one.
High flow: grades for thin walls and long flow paths, such as GP-35.
High impact: the toughest grades, at lower flow, which are extruded as well as injection moulded; HI-10 is one.
Heat resistant: grades for parts where standard ABS reaches its heat limit.
Flame retardant: classed per thickness under IEC 60695-11-10.
Electroplating: grades made to take a plated metal finish.
Extrusion and thermoforming: low-flow grades for sheet, profiles, edge-bands and pipe.
Transparent: MABS, which adds methyl methacrylate to the three monomers.

ABS is also sold blended with polycarbonate (PC/ABS) for higher heat resistance, and ABS sheet is co-extruded under a PMMA or ASA cap layer where the surface must be glossier or weather-resistant.

Most producers make either standard ABS, sold uncoloured for the converter to colour, typically with masterbatch, or pre-coloured specialty ABS.

A melt-flow figure identifies an ABS grade only together with its test condition. ISO datasheets give the melt volume rate of ABS at 220 °C under 10 kg (ISO 1133), where GP-22 reads 19 cm³/10 min; the same grade’s ASTM D1238 datasheet also lists it at 1.5 g/10 min at 200 °C under 5 kg and 4.8 cm³/10 min at 230 °C under 3.8 kg. Before setting a certificate of analysis against a datasheet, or one supplier’s offer against another’s, check that the temperature, load and unit match.