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. 

Specimens were exposed to an ozone concentration of 50+5 parts per hundred million at 4O+2°0 and relative humidity of 55+5%. During the ozone resistance tests, specimens were under a strain of 20+2%. 

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. 

Architectural coatings, 18 55-56 economic aspects of, 18 73-74 Architectural fabrics, 13 394 Architectural paints, 18 72 Archives, preservation of, 11 414 Arch Raschig process flow sheet, 13 578 Arc melting techniques, 25 522-523 ARCO process, 23 342 Arc-resistance furnace, 12 304 Arc resistance testing, 19 587 Arctic polar stratospheric clouds, effect on ozone depletion, 17 789-790 Arc vaporization, 24 738 Arc welding, copper wrought alloys,... 

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. 

Because antiozonants and waxes, which to be effective must form a surface bloom, are used to enhance ozone resistance it is usual to condition test pieces in the strained state before exposure. The usual conditioning period is between 48 and 96h and the test pieces should be kept in the dark and in an ozone-free atmosphere. For this treatment to be effective, the test piece surface must not of course be touched in the course of subsequent handling. Where specifications wish to specifically exclude compounds which rely on an adequate wax film for protection, the conditioning period is dispensed with. Hill and Jowett47 in a criticism of ozone test methods strongly make the point that the conditioning process should be relevant to service conditions if a discriminating evaluation of waxes is to be made. 

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. 

Various tests and analytical methods are used for the characterisation and evaluation of the properties of vegetable oil-based polymer composites. Mechanical tests for properties such as tensile, flexural, compressive, impact, hardness and wear are carried out by a universal testing machine (UTM), and by equipment for testing impact, hardness, abrasion loss, and so on. Weather and chemical resistance tests are performed in UV/ozone, an artificial environmental chamber and in different chemical media. Water uptake and biodegradability tests are carried out by standard ASTM methods. Biodegradability and biocompatibility may be studied by the same procedure as described in Chapter 2. However, in practice only a few such studies have been performed for vegetable oil-based composites. 

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. 

Of auxiliary agents, linseed oil, nnsatnrated vegetable oils, rapeseed oil, and hydrocarbon waxes enhance the efficiency of an antiozonant in a dynamic ozone test as they bloom to the snrface of the pol5mier. Ester plasticizers snch as dioctyl sebacate impair ozone resistance presnmably becanse of enhanced solnbility of polar esters in the polar polychloroprene network polymer. Finally, the mechanism for ozone attack differs appreciably from oxygen attack at allylic atoms. Thus, an antioxidant is generally nsed in combination with an antiozonant for polychloroprene. 

Dynamic and static moduli Kinetic energy Creep and set Tensile properties Mechanical stability Abrasion resistance Elastic modulus Peel tests Ozone resistance... 

As one illustration of quality control lapses causing failure, nitrile/PVC hoses analysed by the author failed from ozone cracking. Although suitably formulated, there had been a processing error and because the quality assurance (QA) testing for ozone resistance had not been carried out this went undetected. For this case, there was in theory a control procedure which is one stage better than no provisions for control, although the result was the same. 

The ozone resistance of hoses and hose lines is tested at 40°C for 72 h according to ISO 7326 [452] (rel. humidity 55 10%, ozone concentration 50 5 ppm, strain 20%). No cracks must be visible under twofold magnification. 

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. 

Tests for weather and ozone resistance depend on visual observation of specimens (e.g., for the appearance or growth of checks or cracks), and are therefore somewhat inaccurate and qualitative. The differences are typically so gross, nevertheless, that they can be demonstrated dramatically. A given ozone containing atmosphere can cause visible attack on one vulcanizate in a few minutes and no attack on another in a week. 

Note Optimization = 2 factor, 2 and 3-level full factorial design—12 compounds and experiments. Responses (properties) to be measured Static ozone resistance dynamic ozone resistance flex-fatigue resistance extrusion processing high temperature, pressure pulse simulated application test of hose sample. 

Antiozonants may also be included in CO, ECO, and GECO compounds for improved resistance to ozone. The selection of the proper antiozonant is important as some will actually reduce ozone resistance, as may be seen in Table 7.13. NBC and MBI are the only effective ones tested, although methyl niclate would also be good. 

Fuel, heat, and ozone resistance, adhesion test... 

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