Page 279 - Carrahers_Polymer_Chemistry,_Eighth_Edition

242 Carraher’s Polymer Chemistry

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Typically, NBR has a wide-operational temperature range of about –40 C to 120 C, making it
useful for extreme automotive applications as well as cooling units. Its good resistance to oils and
other chemicals allows its use around ketones, hydrocarbon liquids, esters, and aldehydes. In the
lab, many of the gloves are made of nitrile rubber. These gloves are also used in home and industrial
cleaning and medically as examination and disposable gloves. Nitrile gloves have greater puncture
resistance compared to “rubber gloves.” NBRs ability to withstand extreme temperatures and resis-
tance to oils encourages their automotive uses as hoses, seals, belts, oil seals, and grommets. NBR

is also used as adhesives, expanded foams, floor mats, and surface treatment of paper, synthetic
leather, and footwear.
NBR is often synthesized using a radical initiating agent. Thus, emulsifier, acrylonitrile, and

butadiene, catalysts, and radical initiators are added to the reaction vessel. The vessel is heated
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to 30 C–40 C for the formation of so-called hot NBR aiding in the polymerization process and
promoting branch formation. Reaction continues for about 5–12 h to about 70% conversions when
a terminating agent such as dimethyldithiocarbamate or diethylhydroxylamine is added. The pro-
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duction of cold NBR is similar except the polymerization temperature is in the range of 5 C–15 C.
Lower-temperature NBR contains less branching generally producing a somewhat stiffer product.
Unreacted monomer is removed and reused in a subsequent reaction. The latex is fi ltered, removing
unwanted solids, and then sent to blending tanks where an antioxidant is added. The resulting latex
is coagulated by addition of aluminum sulfate, calcium chloride, or other coagulating compound.

The latex is dried giving a flaky crumb like product that is then used to produce the desired product.
Because of the variation in reactants and conditions of polymerization, there exists a variation in
product structure.

7.11 ACRYLONITRILE–BUTADIENE–STYRENE TERPOLYMERS

The terpolymer formed from reaction of acrylonitrile, butadiene, and styrene is referred to by the

first letters of the three monomers, ABS. It is made by polymerizing acrylonitrile and styrene in
the presence of polybutadiene. The proportions of reactants vary widely but are generally in the
range of 15%–35% acrylonitrile, 5%–30% butadiene, and 40%–60% styrene. The resulting product
contains long butadiene chains captured by shorter chains of poly(acrylonitrile-co-styrene). The
polar nitrile groups attract one another binding the mixture together, resulting in a material that is
stronger than simply polystyrene. The “plastic” styrene contributes a shiny product with surface that
is largely impervious. The “rubbery” polybutadiene contributes resilience and a wide range of oper-
ating temperatures (OT) between –25°C and 60°C. This combination results in what is referred to as
“rubber toughening” where the rubbery polybutadiene is dispersed within a plastic or a more rigid
styrene matrix bound together by the acrylonitrile units. Typically, impacts are transferred from the
more rigid styrene-rich portions to the rubbery butadiene-rich regions that are able to help absorb
the impact through segmental chain movement. Impact resistance can be increased by increasing
the amount of polybutadiene to a limit. Aging is also dependant on the ABS composition and is
generally dependent on the amount of butadiene since the unsaturation is typically responsible for
limiting the use time because of the increased cross-linking, and consequently increased brittleness,
with time. Thus, it is customary to add an antioxidant to curtail aging.
The properties of the end product are dependant on the processing conditions. For instance, pro-
cessing the ABS at higher temperatures increases the gloss and heat resistance of the material while
lower processing temperatures result in increased impact resistance and strength.
The electrical properties of ABS are relatively independent over a wide range of applied frequen-
cies, making it a good material where varied electrical frequencies are present.

ABS is used to make rigid-molded products such as piping and fittings, fuel tanks, automotive
body parts, toys, wheel covers, enclosures, and where good shock absorbance is needed such as golf
club heads and protective head gear. In fact, our ever-present Lego building blocks are made from
ABS plastic.

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