Page 222 - Handbook of Plastics Technologies

ELASTOMERS

4.14 CHAPTER 4

ber surface and reading the scale after a designated period of time. The scale is calibrated
in arbitrary units from a value of 0 to 100. There are two scales used in rubber and elas-
tomer technology. The A scale durometer is used for most soft rubber products, whereas
the D scale durometer is used for stiff rubber compounds and plastic-like materials. The
chart of Fig. 4.10 shows the ranges of hardness with respect to rubbers and plastics.
At the upper end of the hardness scales (i.e., near values of 100), the hardness value is
of little significance because, at these levels, the indentor moves only slightly. However, at
the lower and intermittent values (i.e., 15 to 85), the hardness test is valuable as a quick
and easy method to characterize the stiffness of a rubber compound. In fact, the hardness
value correlates fairly well with some types of modulus measurements.
The hardness of a rubber band is about 40 on the A scale. That of a tire tread compound
might be from about 55 to 65 on the A scale, whereas a shoe sole might have a hardness of
about 80 A or 30 D.
Stress-Strain (Tensile) Properties. Ultimate tensile strength (UTS), MPa (or psi); ul-
timate elongation (UE), percent; and stress at 100, 200, and 300 percent, MPa (or psi)
strain are measured during the stretching of a dumbbell-shaped specimen (ASTM D 412).
For rubber testing, the strain rate is generally 2000 percent/minute. They are normally
measured at room temperature (23°C). These properties can also be measured at much
higher or lower temperatures, which greatly affects them. Generally, engineering tensile
strength (rather than true strength) is recorded. Engineering stress is the force to stretch is
divided by the original cross-sectional area. For elastomers, true stress at break (force di-
vided by cross-sectional area at break) is approximated by multiplying the engineering
tensile strength by the extension ratio, λ , at break (λ = 1 + [UE%/100]).
B
B
Tensile properties are important and are measured on various compositions during
compound development, because they are generally part of the material specifications.
Measurements of these properties are also used to, in part, control manufacturing pro-
cesses. Also, resistances to various types of deterioration (i.e., by the action of heat, oil,
weather, and so on) are estimated on the basis of changes in mechanical properties dur-
ing exposure. For example, during aging in dry heat (in air), UE generally decreases,
tensile strength can decrease or increase, and the moduli M100, M200, or M300 fre-
quently are increased. Hardness also typically changes as a result of exposure to stress-
ful environments.
Tearing Resistance. Tearing resistance of rubber is measured as force per specimen
thickness. There are several types of tear specimens, with limited correlation between test
results obtained with the various types of specimens. Some are cut (nicked) to provide a
starting point or initiation for tearing, whereas others are not. A frequently used sample is

FIGURE 4.10 Hardness ranges for elastomers and plastics.

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