Page 285 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.77

hexamethylenediamine with MgO, CaO red lead, or ZnO/dibasic lead phosphite as acid
acceptors, (2) bisphenol A/organophosphonium salt with MgO/calcium hydroxide as acid
acceptor, and (3) organic peroxides for certain grades, again with acid acceptors.
FKM elastomers are available in a range of different viscosities.
Properties of Fluoroelastomers. FKM elastomers can have reasonably low glass
transition temperatures, in the range of –18 to –40°C. The tensile properties of fluoroelas-
tomers vulcanizates are fairly good but can decrease considerably with increasing temper-
atures.
FKM vulcanizates have excellent heat resistance, giving continuous service for 1000
hr at 220°C. Useful service is even possible at 250°C. These elastomers are also highly
resistant to weathering and ozone attack. FKM vulcanizates are resistant to swelling in
hot oils and aliphatic compounds. They also are resistant to aromatics, chlorinated hydro-
carbons, and motor fuels. In addition, they are very resistant to most mineral acids. The
gas-permeability resistance of FKM vulcanizates even exceeds that of butyl rubber vul-
canizates.
Uses of Fluoroelastomers. FKM elastomers are expensive, but their demand is high
because of their unusual stability in very severe environments. They are used in specialty
products, e.g., shaft seals of internal combustion engines, and components in aircraft and
rockets. Products include seals, gaskets, liners, hoses, protective fabric coatings, dia-
phragms, roll covers, and cable jacketing.

4.5.4 Fillers, Plasticizers, and Other Compounding Ingredients
In addition to polymers, vulcanization-system ingredients, and antidegradants, other com-
pounding ingredients include fillers, pigments, plasticizers, reinforcing resins, processing
aids, flame retarders, and others.
4.5.4.1 Fillers. Fillers for elastomers are not generally used just to fill space and
cheapen the compositions. They are very important to modify the properties of rubber
compositions in very positive ways. This is especially true for the so-called reinforcing
fillers. Their presence in the compound can improve the strength- and durability-related
properties of vulcanizates and can strongly enhance processing characteristics. The choice
and amount of filler can have a profound effect on vulcanizate properties. These effects de-
pend on several factors: level of use (concentration), primary particle size, surface area
(inverse function of primary particle size), and structure (shape factor, e.g., spherical,
chain or rod-like, plate-like, and so forth).
One can produce soft natural rubber vulcanizates or chloroprene rubber vulcanizates
with good strength-related properties because of their tendency to crystallize during defor-
mation. This is not true for most of the synthetic elastomers (e.g., SBR, BR, IIR, EPDM,
and others), which do not significantly crystallize due to strain. It is only with the use of
reinforcing fillers that serviceable vulcanizates can be made from such elastomers. Rein-
forcing fillers even improve many properties of the crystallizing rubbers.
In addition to increasing the stiffness, hardness, and modulus of a vulcanizate, the pres-
ence of a reinforcing filler increases properties such as tensile strength, abrasion resis-
tance, and tear resistance. (Other properties, such as rebound or resilience, can be
reduced.)
The major types of fillers (in the approximate descending order of their amounts used
in the market) are carbon black, silica, kaolin clay, and calcium carbonate. These probably
account for 95 percent of the filler used in rubber vulcanizates.
Reinforcement. There is little agreement on the mechanism for reinforcement of
elastomers by fillers in elastomer vulcanizates. Some investigators believe that chemical

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