Page 225 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.17

Accelerated Aging in Hot Air. A variety of laboratory accelerated hot-air aging tests
have been developed to predict the service life and aging characteristics of elastomers. For
hot-air oven aging, specimens are exposed at specified elevated temperatures. Their physi-
cal properties are determined (ASTM D 573) as a function of aging time. Point changes in
hardness and percentage changes in tensile strength and elongation at break are reported.
Hot-Air Test Tube Aging. In this method (ASTM D 865), specimens are heated in
air but confined within individual test tubes. This prevents cross-contamination of com-
pounds due to transfer of volatile materials (e.g., antioxidants, curatives, plasticizers, deg-
radation products, and so on) from one sample to another. Thus, this test is free of some of
the complications that can occur when numerous compounds are aged in the same enclo-
sure. As before, hardness and tensile mechanical properties are measured before and after
aging.
Deterioration by Pure Oxygen under Pressure (“Oxygen Bomb” Test). Pure oxy-
gen under pressure is used to accelerate oxidative degradation (ASTM D 572). Test
specimens are placed in a strong metal vessel under an atmosphere of pure oxygen gas
pressure of up to 2070 kPa (300 psi) at a temperatures of 70 or 80°C. After the aging pe-
riod, hardness and tensile properties are measured and compared with original proper-
ties.
Fluid Resistance (Chemical Resistance). Fluid resistance is the extent to which a
rubber product retains its original physical characteristics and ability to function after ex-
posure to oil, water, organic fluids, or any other liquid encountered in its use. Fluid resis-
tance tests do not necessarily correlate with service performance, because service
conditions are not easily defined. However, they are useful for screening compounds, be-
cause they give comparative assessments of expected performance.
A specimen’s weight or volume and mechanical properties are measured before and af-
ter exposure to selected fluids for a specified time at a specified temperature. The effect of
the fluid on the specimen is judged on the basis of the change in properties during the test.
Volume, weight, hardness, tensile strength, and elongation at break of specimens are often
measured. Changes in tear strength, compression set, and low-temperature properties are
also frequently evaluated. ASTM D 471 describes the tests and lists the composition of
various ASTM-designated fluids that have been given special ASTM designations.
Oil Resistance. This is a special case of fluid resistance. Elastomers may be swollen
and weakened by oil. Standard petroleum-based ASTM test oils are used.
General-purpose rubbers (e.g., natural rubber, SBR) placed in oil absorb the fluid
slowly until the oil is all imbibed or the rubber has disintegrated. Oil-resistant specialty
elastomers can absorb some oil, especially at elevated temperatures, but only a limited
(and sometimes negligible) amount. In most end uses requiring oil-resistant elastomers,
some swelling or volume increase is acceptable. In some cases, some swelling is even de-
sirable, e.g., to tighten up a seal. If the oil extracts a plasticizer of extender oil from a com-
position, a seal could even fail due to shrinkage.
Resistance to Outdoor Exposure. Elastomer compositions progressively degrade
with respect to physical properties upon prolonged exposure to the elements (air, sunlight,
rain, and so forth) because of chemical changes in the elastomer molecule. These changes
are largely caused by the action of ozone and oxygen in the atmosphere, ultraviolet light,
and in some cases water (e.g., when the polymer or other components, such as plasticizers,
are subject to hydrolysis). Typical effects are surface hardening, crazing and cracking,
gradual changes in tensile strength, and elongation.
To determine the actual effects of normal weathering, specimens are exposed outdoors
in different locations and different climates. Changes in color, cracking, crazing, chalking,
and stress-strain properties are recorded at various time intervals. (Mildew might also be
observed.) Typical check periods are 1, 2, 5, 10, and 20 years exposure. Test specimens are

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