The ABCs of ABS

July 11, 2002
Tweaking mix ratios of acrylonitrile/butadiene/styrene in ABS resins lets designers boost a part’s heat stability, chemical resistance, toughness, stiffness, and impact resistance.

By Kevin Dunay
Product Specialist
Bayer Corp.
Plastics Div.
Pittsburgh, Pa.

Edited by Jean M. Hoffman

The Kiwi Too travel trailer from Jayco Inc., Middlebury, Ind., sports a durable ground-effects package consisting of a front cap assembly that covers the well for holding LP bottles, wheel-well skirting, and the rear bumper. The parts are coextruded sheets of Lustran ABS resin capped with Centrex 825 weatherable polymer.

The front panels and sinks for the QPA-160 and QPA-310 ice dispensers from Manitowoc Ice Inc., Manitowoc, Wis., come from Lustran 456 resin. The opaque, matte-gloss ABS resin has a tensile yield stress of 1,900 lb/in.2, a flexural yield stress of 4,100 lb/in.2, and a flexural modulus of 173,000 lb/in.2

The aftermarket golf-cart top from Primex Plastics Corp., Oakwood, Ga., and Piper Plastics Inc., Orlando, is made from Centrex 833 acrylonitrile-styreneacrylic (ASA) and Lustran 752 ABS. The ABS gives the hood its sturdy structure while the high-gloss ASA resists fading, cracking, and yellowing from the sun.

Acrylonitrile butadiene styrene (ABS) use has grown over the last 50 years as more and more molders and OEMs take advantage of the resin's processing and property versatility. Applications run the gamut from automotive interiors and exteriors, boat hulls and appliances to food-storage products, garden tools, electrical/electronic components, and industrial equipment.

In general, ABS has high impact strength and rigidity, stands up well to chemicals and weather, and may easily be processed with good as-molded surfaces. Properties of specific grades of ABS resin are determined by several factors including acrylonitrile-butadiene-styrene mix ratio, manufacturing method, rubber particle size, and polymer chain length.

In setting the mix ratio, designers increase the amount of acrylonitrile for better heat stability and chemical resistance. Likewise, tweaking the amount of butadiene improves toughness and impact resistance. And adjusting the styrene ratio changes stiffness and processibility.

ABS is manufactured in one of three ways. Emulsion polymerization is the most common. ABS made in this way produces highgloss parts with good impact strength and heat resistance. It also makes a good coextrusion substrate. The second processing method, continuous-mass polymerization, produces less-expensive resins with lower gloss and better natural color. The third method, suspension polymerization, produces grades with low-gloss, greater toughness, and better chemical resistance.

The size of the rubber particle also plays a role in the resin's final properties. For example, larger particles improve toughness but lower strength and gloss. Conversely, smaller particles produce high-gloss parts with better surface finishes. Additionally, small particle size boosts strength and stiffness but sacrifices toughness. Longer polymer chains enhance strength and toughness but worsen flow characteristics, which makes processing trickier.

Such versatility in ABS formulation makes the resin suitable for a wide range of products including those exposed to weather or significant abuse, or both. In addition, ABS has a broad processing window and easily flows so it holds tight tolerances for both small and large parts. ABS can be thermoformed and injection or blow molded. It's also extruded or coextruded as profiles and sheets that can later be thermoformed. Common secondary processing steps include machining, bonding and fastening, and plating.

ABS is considered a midrange polymer in both cost and performance. ABS grades are typically classified according to gloss and impact strength. Additional ranking comes from its ability to withstand weather, heat, chemicals, and abuse. Processibility is yet another common metric.

High-gloss, general-purpose ABS for injection molding can be either medium or highimpact grade. Medium-impact grades serve in small appliances, toys, and food-contact applications. The high-impact variety finds use in housings for kitchen appliances, tools, and vacuums, as well as in lawn and garden gear.

Extrusion ABS grades range from low-tohigh gloss and from medium-to-high impact strength. Typical applications include vehicle instrument panels, luggage shells, carrying cases, and wall panels. Extrusion grades can also serve as substrates for coextruded parts that require good weather resistance, such as motorcycle fairings and boat hulls. One recent application replaced RV fiberglass body panels with ABS.

Specialty medical ABS grades offer medium impact and high gloss. These grades meet the requirements of FDA-Modified ISO 10993, Part I, and are prime candidate materials for intravenous (IV) systems, diagnostic test kits, and surgical instruments.

For automotive applications, high-heat (HH) ABS grades deliver good heat resistance, toughness, and flow properties, while low-gloss grades are more rigid with better impact strength. The HH grades are used for doors, sail panels, and consoles. The lowgloss grades build scuff plates, map pockets, and interior quarter trim panels.

ABS also can be blended with other plastics to produce alloys that combine the best of both resins.

ABS/polycarbonate (PC) alloys extend the properties of ABS and are more cost efficient than PC alone. These resins exhibit high rigidity and hardness, good impact strength at high and low temperatures, and dimensional stability. Other attributes include creep resistance and low moisture absorption. Parts made with this material are easily colored and textured.

Flame-retardant grades are used extensively in business machines and electrical/electronic equipment. General-purpose grades serve consumer or industrial/mechanical, automotive, appliance, sports, and leisure markets.

ABS/nylon blends process nearly the same as ABS though finished parts have a nylon-rich surface. This gives better abrasion and chemical resistance as well as higher impact strength. Typical applications include housings, shrouds, and handles for lawn and garden equipment, power tools, appliances, and sporting goods. It's also well suited for automotive components such as fasteners and interior functional parts, housing, and shrouds.

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