ABS plastics

ABS plastics are a family of opaque thermoplastic resins formed by copolymerizing acry-lonitrile, butadiene, and styrene (ABS) monomers. ABS plastics are primarily notable for especially high impact strengths coupled with high rigidity or modulus. Consisting of particles of a rubberlike toughener suspended in a continuous phase of styreneacrylonitrile (SAN) copolymer, ABS resins are hard, rigid, and tough, even at low temperatures. Various grades of these amorphous, medium-priced thermoplastics are available offering different levels of impact strength, heat resistance, flame retar-dance, and platability.

Most natural ABS resins are translucent to opaque, but they are also produced in transparent grades, and they can be pigmented to almost any color. Grades are available for injection molding, extrusion, blow molding, foam molding, and thermoforming. Molding and extrusion grades provide surface finishes ranging from satin to high gloss. Some ABS grades are designed specifically for electroplating. Their molecular structure is such that the plating process is rapid, easily controlled, and economical.

Compounding of some ABS grades with other resins produces special properties. For example, ABS is alloyed with polycarbonate to provide a better balance of heat resistance and impact properties at an intermediate cost. Deflection temperature is improved by the polycarbonate, molding ease by the ABS. Other ABS resins are used to modify rigid polyvinyl chloride (PVC) for use in pipe, sheeting, and molded parts. Reinforced grades containing glass fibers, to 40%, are also available.

Related to ABS is SAN, a copolymer of styrene and acrylonitrile (no butadiene) that is hard, rigid, transparent, and characterized by excellent chemical resistance, dimensional stability, and ease of processing. SAN resins are usually processed by injection molding, but extrusion, injection-blow molding, and compression molding are also used. They can also be thermoformed, provided that no post-mold trimming is necessary.

The triangle in Figure A.1 illustrates the properties and characteristics that each constituent acrylonitrile, butadiene, and styrene contributes to the final product. Polymerization of these materials produces the ABS ter-polymer, a two-phase system consisting of a continuous matrix of styrene-acrylonitrile copolymer and a dispersed phase of butadiene rubber particles. Properties are varied principally by adjusting the proportions in which the materials are combined and by altering the molecular weight of the SAN.

Properties

The unique combinations of excellent impact strength with high mechanical strength and rigidity plus good long-term, load-carrying ability or creep resistance are characteristic of the ABS plastics family. In addition, all types of ABS plastics exhibit outstanding dimensional stability, good chemical and heat resistance, surface hardness, and light weight (low specific gravity), Table A.1.

These materials yield plastically at high stresses, so ultimate elongation is seldom significant in design; a part usually can be bent beyond its elastic limit without breaking, although it does stress-whiten. Although not generally considered flexible, ABS parts have enough spring to accommodate snap-fit assembly requirements.

The individual commercially available ABS polymers span a wide range of mechanical properties. Most suppliers differentiate types on the basis of impact strength and fabrication method (extrusion or molding). Some compounds feature one particularly exceptional property, such as high heat deflection temperature, abrasion resistance, or dimensional stability.

FIGURE A.1 Properties and characteristics of acrylonitrile, butadiene, and styrene.

TABLE A.1

Properties of ABS and SAN

ASTM			Standard ABS Grades
or UL Test	Property	High Impact	Super high Impact	Medium Impact
D792	Specific gravity	1.01-1.05	1.02-1.05	1.04-1.06
D792	Specific volume (in.3/lb)	27	27	28
D638	Tensile strength (psi)	6,000	5,000-6,300	6,000-7,500
D638	Elongation (%)	5-20	5-70	5-25
D638	Tensile modulus (103 psi)	3.3	2.0-3.4	3.6-3.8
D790	Flexural strength (psi)	10,500	6,000-11,500	11,500
D790	Flexural modulus (103 psi)	3.4	2.0-3.5	3.6^1.0
D256	Impact strength, Izod (ft-lb/in. of notch)	6.5	7.0-8.0	4.0-5.5
D785	Hardness, Rockwell R	103	69-105	107
D696	Coefficient of thermal	5.3	5.6	4.6
	expansion (10~3) in./in.-°F
D648	Deflection temperature1‘ (°F)
	At 264 psi	188	192	184
	At 66 psi	203	208	201
UL94	Flammability rating	HB	HB	HB
D149	Dielectric strength (V/mil)
	Short time, 1/8-in. thk	400	350-500	350-500
D495	Arc resistance (s)	89	50-85	50-85
a Density has a marked effect.
b Unannealed.
c 0.060-	in.-thick samples.

ASTM = American Society for Testing and Materials; UL = Underwriters’ Laboratories

	Special-Purpose ABS Grades
	Flame
High Heat	Retardant	Clear	Expandable	Plating	SAN Grades
Physical
1.04-1.06	1.19-1.22	1.05	0.55-0.85	1.05-1.07	1.07-1.08
28	—	26	—	26	26
Mechanical
6,000-7,500	5,500-10,000	5,800-6,300	3,000^1,000	5,500-6,600	9,000-12,000
3-20	5-25	25-75	—	—	l-A
3.0-4.0	3.2-3.7	3.0-3.3	1.0-2.5	3-3.8	4.5-5.6
10,000-13,000	9,000-12,250	10,500	3,000-8,000	8,700-11,500	14,000-17,000
3.1-4.0	3.0-3.4	3.4-3.9	1.4-2.8	3.0-3.8	5.5
2.3-6.0	4.0-13.0	2.5-4.0	—	5.0-7.0	0.35-0.50
111	90-117	100-105	60-70a	103-109	M85
Thermal
3.9-5.1	3.7^1.6	4.6	4.9	4.7-5.3	3.0
220-240	180-220	168	160	189	210
230-245	198-238	180-185	185	214	—
HB	V-0 to V-lc	HB	HB-V-0	HB	HB
Electrical
350-500	400+	400	_	_
50-85	20-60	120-130	—	—	—

Impact properties of ABS are exceptionally good at room temperature and, with special grades, at temperatures as low as -40°C. Because of its plastic yield at high strain rates, impact failure of ABS is ductile rather than brittle. Also, the skin effect, which in other thermoplastics accounts for a lower impact resistance in thick sections than in thin ones, is not pronounced in ABS materials. A long-term tensile design stress of 6.8 to 10.3 MPa (at 22.8°C) is recommended for most grades.

General-purpose ABS grades are adequate for some outdoor applications, but prolonged exposure to sunlight causes color change and reduces surface gloss, impact strength, and ductility. Less affected are tensile strength, flexural strength, hardness, and elastic modulus. Pigmenting the resins black, laminating with opaque acrylic sheet, and applying certain coating systems provide weathering resistance. For maximum color and gloss retention, a compatible coating of opaque, weather-resistant poly-urethane can be used on molded parts. For weatherable sheet applications, ABS resins can be coextruded with a compatible weather-resistant polymer on the outside surface.

ABS resins are stable in warm environments and can be decorated with durable coatings that require baking at temperatures to 71 °C for 30-60 min. Heat-resistant grades can be used for short periods at temperatures to 110°C in light load applications. Low moisture absorption contributes to the dimensional stability of molded ABS parts.

Molded ABS parts are almost completely unaffected by water, salts, most inorganic acids, food acids, and alkalies, but much depends on time, temperature, and especially stress level. Food and Drug Administration (FDA) acceptance depends to some extent on the pigmentation system used. The resins are soluble in esters and ketones, and they soften or swell in some chlorinated hydrocarbons, aromatics, and aldehydes.

Properties of SAN resins are controlled primarily through acrylonitrile content and by adjusting the molecular weight of the copolymer. Increasing both improves physical properties, at a slight penalty in processing ease. Properties of the resins can also be enhanced by controlling orientation during molding. Tensile and impact strength, barrier properties, and solvent resistance are improved by this control.

Special grades of SAN are available with improved ultraviolet (UV) stability, vapor-barrier characteristics, and weatherability. The barrier resins — designed for the blown-bottle market — are also tougher and have greater solvent resistance than the standard grades.

Fabrication and Forms

ABS plastics are readily formed by the various methods of fabricating thermoplastic materials extrusion, injection molding, blow molding, calendering, and vacuum forming. Molded products may be machined, riveted, punched, sheared, cemented, laminated, embossed, or painted. Although the ABS plastics process easily and exhibit excellent moldability, they are generally more difficult flowing than the modified styrenes and higher processing temperatures are used. The surface appearance of molded articles is excellent and buffing may not be necessary.

Moldings

The need for impact resistance and high mechanical properties in injection-molded parts has created a large use for ABS materials. Advances in resin technology coupled with improved machinery and molding techniques have opened the door to ABS resins. Large complex shapes can be readily molded in ABS today.

Pipe

The ABS plastics as a whole are popular for extrusion and they offer a great deal for this type of forming. The outstanding contribution is their ability to be formed easily and to hold dimension and shape. In addition, very good extrusion rates are obtainable. Because ABS materials are processed at stock temperatures of 400 to 500°F, it is generally necessary to preheat and dry the material prior to extrusion.

The largest single ABS end product is plastic pipe, where the advantages of high long-term mechanical strength, toughness, wide service temperature range, chemical resistance, and ease of joining by solvent welding are used.

Sheet

ABS sheet is manufactured by calendaring or extrusion and molded articles are subsequently vacuum-formed. The hot strength of the ABS materials coupled with the ability to be drawn excessively without forming thin spots or losing embossing have made them popular with fabricators. The excellent mechanical strengths, formability, and chemical resistance, particularly to fluorocarbons, are largely responsible for the rapid increase in the use of ABS.

Applications

Molded ABS products are used in both protective and decorative applications. Examples include safety helmets, camper tops, automotive instrument panels and other interior components, pipe fittings, home-security devices and housings for small appliances, communications equipment, and business machines. Chrome-plated ABS has replaced die-cast metals in plumbing hardware and automobile grilles, wheel covers, and mirror housings.

Typical products vacuum-formed from extruded ABS sheet are refrigerator liners, luggage shells, tote trays, mower shrouds, boat hulls, and large components for recreational vehicles. Extruded shapes include weather seals, glass beading, refrigerator breaker strips, conduit, and pipe for drainwaste-vent (DWV) systems. Pipe and fittings comprise one of the largest single application areas for ABS.

Typical applications for molded SAN copolymers include instrument lenses, vacuum-cleaner and humidifier parts, medical syringes, battery cases, refrigerator compartments, food-mixer bowls, computer reels, chair shells, and dishwasher-safe house ware products.