Cracks can be formed in many differentelastomers byozone attack, and the characteristic form of attack of vulnerable rubbers is known asozone cracking. The problem was formerly very common, especially intires, but is now rarely seen in those products owing to preventive measures.
However, it does occur in many other safety-critical items such asfuel lines and rubberseals, such asgaskets andO-rings, where ozone attack is considered unlikely. Only a trace amount of the gas is needed to initiate cracking, and so these items can also succumb to the problem.
Tiny traces of ozone in the air will attackdouble bonds in rubber chains, withnatural rubber,polybutadiene,styrene-butadiene rubber andnitrile rubber being most sensitive to degradation.[1] Everyrepeat unit in the first three materials has adouble bond, so every unit can be degraded by ozone.Nitrile rubber is acopolymer ofbutadiene andacrylonitrile units, but the proportion of acrylonitrile is usually lower than butadiene, so attack occurs.Butyl rubber is more resistant but still has a small number of double bonds in its chains, so attack is possible. Exposed surfaces are attacked first, the density of cracks varying with ozone gas concentration. The higher the concentration, the greater the number of cracks formed.
Ozone-resistant elastomers includeEPDM,fluoroelastomers likeViton andpolychloroprene rubbers likeNeoprene. Attack is less likely because double bonds form a very small proportion of the chains, and with the latter, the chlorination reduces the electron density in the double bonds, therefore lowering their propensity to react with ozone.Silicone rubber,Hypalon andpolyurethanes are also ozone-resistant.
Ozone cracks form in products under tension, but the critical strain is very small. The cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are very dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow. Seals are also susceptible to attack, such asdiaphragm seals in air lines. Such seals are often critical for the operation ofpneumatic controls, and if a crack penetrates the seal, all functions of the system can be lost.Nitrile rubber seals are commonly used in pneumatic systems because of its oil resistance. However, ifozone gas is present, cracking will occur in the seals unless preventative measures are taken.
Ozone attack will occur at the most sensitive zones in a seal, especially sharp corners where the strain is greatest when the seal is flexing in use. The corners representstress concentrations, so the tension is at a maximum when the diaphragm of the seal is bent under air pressure.
The seal shown at left failed from traces of ozone at circa 1ppm, and once cracking had started, it continued as long as the gas was present. This particular failure led to loss of production on asemi-conductor fabrication line. The problem was solved by adding effective filters in the air line and by modifying the design to eliminate the very sharp corners. An ozone-resistant elastomer such asViton was also considered as a replacement for theNitrile rubber. The pictures were taken usingESEM for maximum resolution.
The reaction occurring between double bonds and ozone is known asozonolysis when one molecule of the gas reacts with the double bond:
The immediate result is formation of anozonide, which then decomposes rapidly so that the double bond is cleaved. This is the critical step in chain breakage when polymers are attacked. The strength of polymers depends on the chainmolecular weight ordegree of polymerization, the higher the chain length, the greater the mechanical strength (such astensile strength). By cleaving the chain, the molecular weight drops rapidly and there comes a point when it has little strength whatsoever, and a crack forms. Further attack occurs in the freshly exposed crack surfaces and the crack grows steadily until it completes a circuit and the product separates or fails. In the case of a seal or a tube, failure occurs when the wall of the device is penetrated.
The carbonyl end groups which are formed are usuallyaldehydes orketones, which can oxidise further tocarboxylic acids. The net result is a high concentration of elemental oxygen on the crack surfaces, which can be detected usingenergy-dispersive X-ray spectroscopy in the environmental SEM, orESEM. The spectrum at left shows the high oxygen peak compared with a constantsulfur peak. The spectrum at right shows the unaffected elastomer surface spectrum, with a relatively low oxygen peak compared with the sulfur peak.
The problem can be prevented by addingantiozonants to the rubber beforevulcanization. Ozone cracks were commonly seen in automobiletire sidewalls, but are now seen rarely thanks to the use of these additives. A common and low costantiozonant is a wax which bleeds to the surface and forms a protective layer, but other specialist chemicals are also widely used.
On the other hand, the problem does recur in unprotected products such as rubber tubing and seals, where ozone attack is thought to be impossible. Unfortunately, traces of ozone can turn up in the most unexpected situations. Using ozone-resistant rubbers is another way of inhibiting cracking.EPDM rubber andbutyl rubber are ozone resistant, for example.
For high value equipment where loss of function can cause serious problems, low cost seals may be replaced at frequent intervals so as to preclude failure.
Ozone gas is produced duringelectric discharge bysparking orcorona discharge for example.Static electricity can build up within machines likecompressors with moving parts constructed from insulating materials. If those compressors feed pressurised air into a closed pneumatic system, then all seals in the system may be at risk from ozone cracking.
Ozone is also produced by the action ofsunlight onvolatile organic compounds or VOCs, such as gasoline vapour present in the air of towns and cities, in a problem known asphotochemical smog. The ozone formed can drift many miles before it is destroyed by further reactions.
