Rubber ozone resistance tests

Laboratory ozone resistance tests involve the exposure of stretched test pieces, usually strips or dumbbells, to a specified combination of ozone concentration and temperature. The standard concentrations permitted in ISO 1431/1 (BS903, Part A43) are 25, 50. 100. and 200 pphm ozone. For most purposes, SOpphm is considered suitable and is now found in a wide range of material and product specifications. The lowest concentration is intended for applications used under low-severity conditions, whereas 100 and 200 pphm are used to ensure that there is sufficient protection for badly polluted areas or to ensure the use of an inherently resistant rubber type. 

Annulus Test An ozone resistance test for rubbers that involves a flat-ring specimen mounted as a band over a rack, stretched 0 to 100%, and subjected to ozone attack in the test chamber. The specimen is evaluated by comparing to a calibrated template to determine the minimum elongation at which cracking occurred. 

Table 4.111 displays examples of ageing and chemical property ranges for some TPEs. These comparisons are very schematic and cannot be used for designing. Ozone resistance is a specific property of rubbers used in numerous specifications. The rating system is the same as previously 5 is always attributed to the most attractive materials. These general indications should be verified by consultation with the producer of the selected grades and by tests under operating conditions. 

DOPDA h as been used as an addidve to rubber.composidons at the time of manufacture for the purpose of providing ozone resistance to elastometers. Mixtures of DOPDA with solvents such as acetone (usually in 50/50 ratio) are flammable and toxic, causing skin irritation. The material covered by US Military Specification MIL-D-50000A(MR), July 1966 is intended for use as an externally applied (brush or dip) solution to rubber items, particularly tires. This chemical functions as an antiozonant, preventing cracking of stressed, vulcanized rubber items in outdoor storage Requirements and tests covered by the above Spec are as follows ... 

The areas where a test piece is attached to clamps and cut edges are preferential sites for cracking. It is generally good practice to coat clamped areas with an ozone resistant paint (which does not affect the rubber in any way) but cut edges are best left. For most purposes a Hypalon-based paint is satisfactory. Clamps, even when made of material such as aluminium, should be soaked in ozone prior to use. Any pattern or flaws on the test piece surface will also tend to act as stress raisers and show preferential cracking. 

Stress rupture tests on test pieces are very important under conditions where, in addition to the stress, the atmosphere is chosen to accelerate failure. The best known t> pe of test is a test of the so-called environmental. stress cracking of plastics, where the aggressis e atmosphere is a chemical that causes cracking when the material is in a strained state. These tests are usually considered as a form of chemical resistance test and are cosered in Chapter 14. Ozone cracking of rubber, also an environmental resi.stance test, is another example. 

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 continuous flexing. ASTM test method D 1149 covers static testing and D 3395 covers dynamic testing in a controlled ozone atmospheres. 

In addition to the weathering tests for elastomers described in Section 2.2, testing ozone resistance is significant for rubber products. Here, parts molded from rubber are stored statically (tensile strain) or dynamically (alternating strain) in an ozone chamber at a specified strain, temperature, humidity, and ozone concentration. Subsequently, their ozone resistance is evaluated based on the resulting crack formation. 

The static ozone chamber test permits adjustment of ozone concentration (0.5 to 2.0 ppm) and temperature (20 to 50°C) as well as relative humidity (10 to 90%) besides numerous strain levels for the rubber specimens the typical loading time ranges from 2 to 96 hours. The corresponding materials and product standards provide recommendations for the use of certain testing conditions the atmospheric conditions in the ozone chamber should, however, be tuned as closely as possible to the crack resistance of the particular elastic material short-term loading at a low ozone concentration and temperature in the case of crack prone materials, a high ozone concentration and temperature over several days for relatively ozone resistant rubbers. 

Less than 1 pphm (parts per hundred million) of ozone in the atmosphere can severely attack non-resistant rubbers. However, there is far less, if any, problem with plastics and tests are rarely made. If required, the methods standardised for rubbers in ISO 1431 [24] could be used, which involve exposure in a special cabinet with controlled levels of ozone. 

ISO 1431-1 2004 Rubber, vulcanized or thermoplastic - Resistance to ozone cracking -Part I Static and dynamic strain testing... 

DuPont was looking for a synthetic rubber (SR). Carothers assigned Arnold Collins to carry out this research. Collin s initial task was to produce pure divinylacetylene. While performing the distillation of an acetylene reaction, in 1930, he obtained a small amount of an unknown liquid, which he set aside in a test tube. After several days the liquid turned to a solid. The solid bounced and eventually was shown to be a SR polychloroprene, whose properties were similar to those of vulcanized rubber but was superior in its resistance to ozone, ordinary oxidation, and most organic liquids. It was sold under its generic name neoprene and the trade name Duprene. ... 

The second class includes tests for ageing, oxygen and ozone attack, adhesion, resistance to wear and tear, fatigue, etc. This class also includes tests under simulated service conditions under laboratory environments and immersion in fluids. These two classes of tests are conducted on vulcanized rubber. 

Tests for elastomer resistance to air have to be considerably accelerated in the laboratory by exposure to pure oxygen (pressure test, 21 bar, 70°C) or in the ozone test chamber (standard pressure, 50-200 ppm ozone, 25°C, s. Section 2.2.8). While oxygen influence is rated as a percentage loss of properties, comparable to heat aging tests in air, ozone influence is evaluated according to crack formation in the rubber transverse to the tensile or bending direction (evaluation levels 0 to 3) [229]. 

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