Rubber stress-ozone cracking

When diene rubbers are exposed to ozone under stressed conditions cracks develop which are perpendicular to the direction of stress. Whilst ozone must react with unstressed rubber no cracking occurs in such circumstances nor when such rubber is subsequently stressed after removal of the ozone environment. For many years such rubbers were protected by waxes which bloomed on to the surface of the rubber to form an impermeable film. This was satisfactory for static applications but where the rubber was operating under dynamic conditions the wax layer became broken and hence less effective. 

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. 

The Gough Joule effect, shown as an increase in modulus with an increase in temperature and the retraction of stressed rubber on heating An ability of some elastomers to undergo strain-induced crystallization The susceptibility of un.saturated rubbers to ozone attack and subsequent cracking in the stretched state... 

Typically, ozone cracking initiates at sites of high stress (flaws) on the rubber surface. Thus in general the rubber article should be designed to minimize potential sites of high elongation such as raised lettering. Similarly, clean molds should be used to reduce the incidence of surface flaws. 

Increase in the critical stress of the rubber article. The critical stress is the applied static stress at which ozone cracking appears on the surface of a rubber article exposed to ozone. 

Vulkanox 4010 NA/LG antiozoneuit excels in antiflexcracking properties and is used in tires and mechanical goods subjected to dyneunic stress, e.g. conveyor belts, hoses, spring components and elastic couplings. In static applications and in cables and seals, its main function is resistance to ozone cracking, which can be further improved by the simultaneous use of an ozone protective wax. Use should be restricted to dark colored rubber articles where staining and discoloration are of no concern. 

A review is presented of the literature on the protection of rubber against ozone. Particular attention is paid to the historical background, ozone formation, chemistry of the ozone-rubber reaction, physical requirements for ozone cracking, physical methods of ozone protection, chemical antiozonants, chemical antioxonants for polychloroprene, mechanism of action of chemical antiozonants, chemistry of the reaction of ozone and p-phenylenediamine, free-radical mechanism, and critical stress and antiozonants. 88 refs. USA... 

Natural rubber is composed of polymerized isoprene units. When rubber is under tension, ozone attacks the carbon-carbon double bond, breaking the bond. The broken bond leaves adjacent C = C bonds under additional stress, eventually breaking and placing shll more stress on surrounding C = C bonds. This "domino" effect can be discerned from the structural formulas in Fig. 9-4. The number of cracks and the depth of the cracks in rubber under tension are related to ambient concentrations of ozone. 

The effect of ozone is complicated in so far as its effect is largely at or near the surface and is of greatest consequence in lightly stressed rubbers. Cracks are formed with an axis perpendicular to the applied stress and the number of cracks increases with the extent of stress. The greatest effect occurs when there are only a few cracks which grow in size without the interference of neighbouring cracks and this may lead to catastrophic failure. Under static conditions of service the use of hydrocarbon waxes which bloom to the surface because of their crystalline nature give some protection but where dynamic conditions are encountered the saturated hydrocarbon waxes are usually used in conjunction with an antiozonant. To date the most effective of these are secondary alkyl-aryl-p-phenylenediamines such as /V-isopropyl-jV-phenyl-p-phenylenediamine (IPPD). 

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 ozone concentration in the atmosphere is only a few pphm. In certain chemical plants as in electrolytic mercury cell houses in the chloralkali industry, the ozone concentration is higher. Although the atmospheric ozone level is low, it reacts with rubber double bonds rapidly and causes cracking of rubber products. Especially when rubber is under stress (stretching and bending as in the case of flexible cell covers), the crack development is faster. Neoprene products resist thousands of parts per hundred million of ozone for hours without surface cracking. This nature of neoprene is quite suitable for cell house application in chlor-alkali industries. Natural rubber will crack within minutes when subjected to ozone concentration of only 50 pphm. 

In cell houses, cracks can be visually seen on the upper neoprene surface of the flexible covers which are subjected to stress at a temperature of 80°C, during eighteen to twenty four months of operation, necessitating mandatory replacement of the covers. Ozone reacts with double bonds so rapidly that it has no chance to diffuse into the rubber and therefore all action is at the surface. Thus it implies surface protective agents are most useful against ozone attack. For example, waxes that bloom to the surface of rubber to form an inert film are used effectively for static protection. 

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. 

Typical examples of stress corrosion are crack formation in strained rubber vulcanizates under the influence of ozone, hair cracks in PE under stress in the presence of a surface active agent (see also 7.4.2), crack formation in PC, when exposed to e.g. CCI4, within a few seconds after the application of a small stress. 

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