Page 276 - Handbook of Plastics Technologies

ELASTOMERS

4.68 CHAPTER 4

Properties of Nitrile Rubbers. Used with reinforcing fillers, NBR vulcanizates of
excellent mechanical properties are obtained. With proper compounding, a wide range of
hardness grades are possible, with good resistance to compression set. Elastic properties
of unplasticized NBRs are somewhat less favorable than those of NR or SBR. The use of
ester-type plasticizers and non- or semireinforcing fillers can nevertheless give rise to
compositions of good elasticity. The presence of reinforcing fillers can give abrasion resis-
tance that is considerably better than that of comparable NR or SBR vulcanizates, and X-
NBR vulcanizates have extremely good abrasion resistance. The heat resistance of NBR
vulcanizates is generally better than for vulcanizates of NR of SBR. With reduced
amounts of oxygen, as in an oil environment, the heat aging resistance is even more im-
pressive. The weather and ozone resistance of NBR vulcanizates is similar to that of NR
vulcanizates, but antiozonants are somewhat less effective with NBR. Low-temperature
flexibility improves with decreasing amounts of nitrile content but improves with increas-
ing concentrations of ester-type plasticizers, e.g., adipate esters.
Because nitrile rubbers are polar elastomers, their vulcanizates are very resistant to
swelling in hot oil, gasoline, grease, and other nonpolar substances. The resistance to
swelling in nonpolar oils, solvents, and so on, is improved greatly with increasing nitrile
content. Here we see the need for compromises: increasing nitrile content improves oil re-
sistance but reduces elasticity and set resistance and most severely reduces low-tempera-
ture flexibility. Plasticizers are very necessary to surmount this problem.
NBR vulcanizates are less permeable to gasses than are NR and SBR, with permeabil-
ity decreasing with increasing nitrile content. Permeability by gasoline vapors and the like,
however, is very high.
Nitrile rubber vulcanizates have a considerably higher electrical conductivity than do
those of nonpolar elastomers and are, thus, not generally used in parts that require low
electrical conductivities.
Completely saturated HNBR vulcanizates have the excellent resistance to hot air and hot
oils, and they have high resistance to oxidative and ozone attack. They have good resistance
to sulfur-containing oils, sulfur and nitrogen-containing oil additives, and industrial chemi-
cals in general. The fully saturated H-NBRs have very high tensile strength, good low-tem-
perature flexibility, and good abrasion resistance. Unfortunately, H-NBRs are costly.
Uses of Nitrile Rubbers. NBR vulcanizates are used where, in addition to good me-
chanical properties, good resistance to swelling in oils and resistance to abrasion are re-
quired. Typical uses are in seals, O-rings, packings for crank shafts and valves,
membranes, bellows for coupling, hose, high-pressure hose, and others. NBRs are also
used in oil-rig applications, roll coverings, conveyor belts, linings, containers, work boots,
shoe soling, and so on. NBRs are also used in products for the food industry. Liquid grades
of NBR, whose molecules contain reactive groups, are used to make liquid-cast elasto-
meric parts and used as impact modifiers for epoxy resins. NBR gloves are prepared by the
latex dipping process.

4.5.3.9 Chlorinated Polyethylene (CM). Commercial grades of CM are produced by
random chlorination of high-density polyethylene in aqueous suspension. Uniform chlori-
nation of the polymer requires elevated temperatures for the reaction. The general chemi-
cal structure of CM can be represented as follows:

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