Ozone-resistant rubbers

The cost of neoprene is very high when compared to NR, BR, or SBR and even other heat resistant and ozone resistant rubbers such as EPDM. So when EPDM can be used for such conditions for ozone and heat and not flame resistance, it should be the preferred rubber in place of CR. 

For most sealing applications, attack by ozone is only likely during storage, and this can be reduced by packing the seals properly. With exterior seals, ozone attack is possible, and ozone-resistant rubbers should be selected. If this is not possible, anti-ozonants should be added to the compound. 

Ozone-resistant rubbers have also been prepared using j3-pinene instead of a conventional diene (Kennedy, 1963 1968). 

The ozone resistance of ENR-25 is comparable with that of NR and it shows a similar response to waxes and chemical antiozonants. Uncompounded ENR-50 has superior ozone resistance, but it does not respond as well to antiozonants, and the ozone resistance of protected ENR-50 vulcanizates is inferior to that of NR and ENR-25 at the same level of protection. The ozone resistance of both ENR-25 and ENR-50 may be improved by blending with ozone resistant rubber (e.g. a 50 20 blend of ENR-50 and EPDM, without antiozonant and strained to 20%, was crack-free after 14 days at 40 C and 50pphm ozone). 

A review is made of the literature describing the surface discolouration of black tyre sidewalls caused by exposure to ozone and formulation studies undertaken to overcome this problem. Methods examined include the use of nonstaining antiozonants and blending inherently ozone resistant rubbers such as EPDM, halobutyl rubbers and brominated isobutylene-paramethylstyrene copolymers with NR and/or polybutadiene. 67 refs. 

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. 

It is a very lightweight rubber and has very good weathering and electrical properties, excellent adhesion, and excellent ozone resistance. 

Nitrile mbber finds broad application in industry because of its excellent resistance to oil and chemicals, its good flexibility at low temperatures, high abrasion and heat resistance (up to 120°C), and good mechanical properties. Nitrile mbber consists of butadiene—acrylonitrile copolymers with an acrylonitrile content ranging from 15 to 45% (see Elastomers, SYNTHETIC, NITRILE RUBBER). In addition to the traditional applications of nitrile mbber for hoses, gaskets, seals, and oil well equipment, new applications have emerged with the development of nitrile mbber blends with poly(vinyl chloride) (PVC). These blends combine the chemical resistance and low temperature flexibility characteristics of nitrile mbber with the stability and ozone resistance of PVC. This has greatly expanded the use of nitrile mbber in outdoor applications for hoses, belts, and cable jackets, where ozone resistance is necessary. 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

The level of unsaturation is much lower, giving rubbers of much better heat, oxygen and ozone resistance. 

At one time it was widely expected that EPDM rubbers would become of significance in the tyre sector. For a variety of reasons but primarily because of the high cost of the diene monomers this expectation has not been realised. On the other hand these rubbers have become widely accepted as moderately heat-resisting materials with good weathering, oxygen and ozone resistance and they find extensive use in cars, domestic and industrial equipment. 

More recently, in 1975, Du Pont introduced a terpolymer (Vamac) based on ethylene, methyl acrylate and a third monomer of undisclosed composition which contained a carboxylic acid group to provide a cure site for use with peroxides or amines. Both types of rubber exhibit good heat, oxygen and ozone resistance. 

Vulcanisation can be effected by diamines, polyamines and lead compounds such as lead oxides and basic lead phosphite. The homopolymer vulcanisate is similar to butyl rubber in such characteristics as low air permeability, low resilience, excellent ozone resistance, good heat resistance and good weathering resistance. In addition the polyepichlorohydrins have good flame resistance. The copolymers have more resilience and lower brittle points but air impermeability and oil resistance are not so good. The inclusion of allyl glycidyl ether in the polymerisation recipe produces a sulphur-curable elastomer primarily of interest because of its better resistance to sour gas than conventional epichlorhydrin rubbers. 

Rubbers differ in their resistance to ozone. All the highly unsaturated rubbers (natural rubber, styrene-butadiene rubber, butyl rubber, nitrile rubber) are readily cracked while the deactivated double carbon-carbon bonds rubber (such as polychloroprene rubber) shows moderate ozone resistance. 

Ozone attack of rubbers can be prevented in three ways (1) coating the surface (2) adding a chemical antioxidant (3) relieving internal stresses by adding ozone-resistant polymers. 

D 2770 Ozone-Resist Ethylene-Propylene Rubber Integral Insulation Jacket for Wire Cable. 

D 2802 Ozone-Resistant Ethylene-Propylene Rubber Insulation for Wire Cable. 

Natural rubber Solid Good physical properties and resistance to cutting and abrasion. Low heat and ozone resistance. Gaskets. 

Blends of carboxylated nitrile rubber (XNBR) with EPDM are likely to provide an attractive combination of properties including oil resistance, heat and ozone resistance, high tensile strength, modulus, and hardness. However, the polar curing ingredients often diffuse from the nonpolar to polar component, thereby producing cure rate mismatch and inferior properties. Three different measures have been used to overcome the cure rate mismatch [29] ... 

The most prevalent approach to achieve long-lasting and nonstaining ozone protection of rubber compounds is to use an inherently ozone-resistant, saturated backbone polymer in blends with a diene rubber. The ozone-resistant polymer must be used in sufficient concentration (minimum 25 phr) and must also be sufficiently dispersed to form domains that effectively block the continuous propagation of an ozone-initiated crack through the diene rubber phase within the compound. Elastomers such as ethylene-propylene-diene terpolymers, halogenated butyl mbbers, or brominated isobutylene-co-para-methylstyrene elastomers have been proposed in combination with NR and/or butadiene rubber. 

Butyl rubber (a copolymer of isobutylene and 1-3 mole per cent isoprene) and its halogenated derivatives have unsaturation in the carbon-carbon backbone and consequently do not have as good aging properties as EPDM. There are also reports (9-12) that ozone-resistant butyl rubber with a high degree of unsaturation can be prepared by copolymerization of isobutylene with either cyclopentadiene or 9-pinene. 

Since the main chain of both EPM and EPDM rubbers is saturated, both co- and terpolymers exhibit excellent stability to oxygen, UV light, and are ozone resistant. 

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. 

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