Page 226 - Handbook of Plastics Technologies

ELASTOMERS

4.18 CHAPTER 4

usually stretched at low strains and mounted on wooden supporting racks for outdoor ex-
posure. Three procedures are specified by ASTM D 518:
• Procedure A—Straight strips elongated 20 percent.
• Procedure B—Bent loops, where the elongation varies from 0 to 25 percent at different
points on the loop.
• Procedure C—Tapered samples elongated to strains of 10 to 20 percent.
The specimens in their supporting racks are usually exposed facing south for maximum
sunlight at an angle of 45° with the horizon. Samples may also be buried in the ground to
determine resistance to moisture, soil chemicals, insects, bacteria, and oxidation effects in
the absence of direct sunlight.
Simulated weathering tests are carried out by using such testing devices as the
Weather-o-meter (ASTM D 750) and Fade-o-meter. In the Weather-o-meter, test speci-
mens are continuously or intermittently exposed to water spray and artificial light pro-
duced by a carbon arc. This light is of wavelengths similar to those of natural sunlight but
with increased intensity in the ultraviolet range. In the Fade-o-meter, specimens are ex-
posed to constant controlled humidity (instead of water spray) along with carbon-arc arti-
ficial light. Specimens may be exposed unstrained or under a slight elongation
As in the other accelerated aging tests, tensile properties are measured after aging and
compared with original properties. In addition, visual observations are made for cracking,
crazing, or color changes.
Deterioration by the Action of Ozone. Ozone, although found in relatively small
amounts in air, is a highly reactive gas. Its action on many types of rubbers can cause prod-
ucts to crack and fail prematurely unless they are protected by antiozonants or made from
ozone-resistant elastomers.
To test for resistance to ozone attack, samples are stretched to 20 or 40 percent strain
on a test rack or bent in a loop to produce a surface strain. The specimens are then placed
in a chamber equipped with an ozone generator. Usual controlled test concentrations are
about 0.25, 0.5, 1, or 2 ppm (parts ozone per million parts of air by volume). The test tem-
perature is usually 40°C. The test specimens are inspected at various time intervals until
initial cracking occurs.
In dynamic testing of ozone resistance, a fabric-backed vulcanized rubber specimen is
continuously flexed in the ozone chamber over a roller. The fabric backing is in the form
of a belt. Any protective chemical films (e.g., certain waxes and antiozonants) that might
build up on the surface of the specimen in static testing are quickly broken by the continu-
ous flexing. ASTM test method D 1149 covers static testing and D 3395 covers dynamic
testing in a controlled ozone atmospheres.
Low-Temperature Properties. As elastomer compositions are gradually cooled, they
gradually stiffen until the region of the glass transition, T , is reached where a very sharp
g
increase in stiffness occurs as a function of decreasing temperatures. This temperature is
near the brittle temperature or stiffness temperature discussed below. At temperatures be-
low the transition temperatures, the elastomer composition becomes brittle and can crack
or break on sudden impact or bending. The temperature at which this occurs becomes
higher as the impact or bending becomes more rapid.
In the test for brittleness temperature (ASTM D 746), a small rectangular strip is
clamped in a holder at one end and cooled to a predetermined temperature in a chilled liq-
uid bath. The specimen is then given a sudden sharp blow by a solenoid-actuated striking
arm. If the specimen does not fracture, the temperature of the liquid bath is lowered, and
the process is repeated until the specimen breaks when struck.
A torsional stiffness test measures the modulus of rigidity of an elastomer composition
over a broad temperature range. There are two commonly used tests: ASTM D 1043,

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