Page 223 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.15

an unnicked 90° angle specimen (ASTM D 624 Die C Tear). In still another test, the test
piece is a razor-nicked crescent-shaped specimen (ASTM D 624 Die A Tear). In another
test, the trouser-tear specimen (so called because it resembles a pair of men’s trousers) is
used.
Tear-test specimens are generally pulled on a tensile tester at a cross-head rate of
8.5 mm/sec (20 in/min). The maximum force required to initiate or propagate tear is re-
corded as force per unit thickness. Tear strength measurements can indicate gross differ-
ences in performance in some applications. They are also useful in production control.
Abrasion Resistance. Abrasion is the wearing of material from a rubber surface due
to the action of an abrasive surface in contact with and moving with respect to the rubber
surface. It is measured under a specified load, at a specified speed and type of abrasive sur-
face. Laboratory tests may not predict service life, because the many and complex factors
affecting abrasion vary greatly from application to application. However, laboratory tests
are useful for quality control of rubber products intended for rough service.
The National Bureau of Standards (NBS) Abrader is a frequently used abrasion tester.
Test samples are pressed under a specified load against a rotating drum covered with abra-
sive paper. The number of revolutions of the drum required to wear away a specified thick-
ness of the test specimen is recorded and compared to the number of revolutions required
to wear away the same thickness of a standard reference material (ASTM D 1630).
For the Pico Abrader test, a pair of tungsten carbide knives of specified geometry and
controlled sharpness are rubbed over the surface of a pellet-shaped sample in a rotary fash-
ion under controlled conditions (load, speed, and time). The volume loss of the test speci-
men is measured and compared to that of a reference compound (ASTM D 2228).
In the Taber Abrader (or Abraser) test, weight loss or thickness loss is measured. The
specimen is placed in contact with an abrasive wheel under a load of 500 or 1000 g
(ASTM D 1044). This test is generally performed on clear plastics; however, the apparatus
has also been used with rubber.
Flex and Fatigue Resistance. Flex resistance refers to the ability of a rubber part to
withstand numerous flexing cycles without failure. Two common flex resistance tests are
the DeMattia Flex Test (ASTM D 813) and the Ross Flex Test (ASTM D 1052). The De-
Mattia flex tester alternately pushes the test specimen ends together, bending it in the mid-
dle, then pulls it back to straighten it. The Ross flex tester repeatedly bends and then
straightens the specimen over a metal rod. In both tests, a small cut or nick of prescribed
size and shape may be made in the center of the test specimen. Data generated from these
tests include flex life, which is the number of test cycles a specimen can withstand before
it reaches a specified state of failure (i.e., length of grown nick or cut), and crack growth
rate, which is the rate at which the cut propagates itself as the sample is flexed. These tests
are particularly useful in evaluating compositions that are intended for use in products that
undergo repeated flexing or bending.
Another fatigue-testing device is the (Monsanto) Fatigue to Failure Tester. This instru-
ment measures the ultimate fatigue life as cycles to failure. Tensile-like samples are
stretched at 100 cycles per minute at a preselected extension ratio. Samples can be strained
over a range of 10 to 120 percent. The fatigue performance of compounds can be mea-
sured (number of cycles to failure, i.e., rupture) and compared either at constant extension
ratio (strain) or at constant strain energies (work input).
Resilience. Resilience is the stored energy that is rapidly (essentially instantaneously)
returned by a vulcanized rubber object when it is suddenly released from a state of strain
or deformation. Generally, the strain is also applied very rapidly. Resilience is what causes
a rubber ball to bounce. It can be expressed as 100 minus the percentage of energy loss
upon recovery from deformation. Either high resilience or low resilience (damping) may
be required in a given application, depending on the application.

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