Page 275 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.67

Accelerated sulfur vulcanization systems can be employed, but generally as compo-
nents of mixed vulcanization systems containing both metal oxide and accelerated sul-
fur.
Properties of Chloroprene Rubber Vulcanizates. Unfilled (nonreinforced) gum
vulcanizates are much stronger for CR than for other synthetic rubbers (except for syn-
thetic polyisoprene) because of the tendency for strain-induced crystallization (similar to
the case of isoprene rubbers). Similarly, resistance to tearing is very good for CR vulcani-
zates.
Because of the chlorine content, CRs are more flame resistant than other elastomers.
However, when chlorinated polymers burn, they liberate great amount s of corrosive, toxic
vapors. They are not used in areas where there is a strong need for safety.
The stiffening of CR vulcanizates with cooling can be due to crystallization. Fillers
have little effect on this, but ester-type plasticizers can greatly reduce the crystallization
temperature.
Because of the increased polarity of CR over other (i.e., hydrocarbon) rubbers, CR vul-
canizates are sufficiently oil resistant for many uses. Also, CR vulcanizates are fairly resis-
tant to chemicals such as concentrated alkalis, dilute acids, and aqueous salt solutions.
CR is superior to NR or SBR as a barrier against gas permeation, but it does not exhibit
as low permeability as do butyl rubbers, epichlorohydrin rubbers, or nitrile rubbers.
Uses of CR. CR vulcanizates are used in many rubber products that are flame resis-
tant, resistant to fats and oils, and resistant to weathering and ozone (when compounded
with proper antidegradants). Products include moldings, extrusions, seals, hoses, rolls,
belts, shoe soles, bearings, rubberized fabrics, linings, and cable jackets. However, in
some areas, the use of CRs has been reduced by competition form nitrile rubbers (for bet-
ter oil resistance) and EPDM compositions because of price.
In contrast to the low crystallinity required for rubber products, crystallization is a
great benefit for adhesives. In solution, crystallization doesn’t occur but, after drying, the
adhesive film hardens rapidly due to crystallization.
4.5.3.8 Nitrile Rubber (NBR). Nitrile rubbers are copolymers of butadiene and acry-
lonitrile produced by emulsion polymerization. There are “hot” and “cold” polymerized
types. The hot-polymerized types generally have the higher green strengths but are some-
what more difficult to process. The acrylonitrile monomer repeat units impart resistance to
oil swelling. There are grades containing 18 to 50 percent acrylonitrile-derived backbone
units. Glass transition temperatures and oil resistance increase with the nitrile content, the
glass transition temperatures ranging from –38 to –2°C. An example of a structure for ni-
trile rubber can be given as follows:

Relatively small amounts of acrylic acid can be used in the monomer polymerization
mix to give carboxylated nitrile rubber (X-NBR), whose polymer chains have carboxylic-
acid side groups.
NBR can be partially or even completely hydrogenated (to eliminate carbon-carbon
double bonds) in nonaqueous solution by using suitable catalysts (e.g., cobalt, rhodium,
ruthenium, iridium, or palladium complexes) to give hydrogenated nitrile rubbers (H-
NBRs). Completely saturated H-NBR grades are cross-linked with peroxides.

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