Antiozonants, rubber ozone resistance

Vulcanox 3100 is a powerful, persistent antioxidant and anti-flexcracklng agent for most diene rubbers (NR, IR, SBR, NBR), imparting moderate antiozonant properties. Ozone resistance can be improved by the addition of microcrystalline waxes. 

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 ... 

Antiozonents protect the rubber surface by the formation of a protection layer, the ozonides on the surface of rubber by reaction of the antiozonents with ozone. Certain polymers also provide good ozone protection. The use of 10-20 parts of EPDM, a low diene rubber, in natural rubber compound significantly increases ozone resistance. 

The heat and ozone resistant [126] EPR was made by incorporating acrylic rubber, dicumyl peroxide, triaUyl cyanurate, ZnO and carbon-black into the matrix. Triallyl cyanurate increases the crosslinking efficiency probably due to an addition reaction between polymeric and aUyl radicals and leads to stable chemical crosslinks. Thus ozone because there is no unsaturation cannot initiate a degradation reaction. Digteva et al. [127] prepared sealants for use at high temperature by adding aromatic diaminodisulfide, MgO, ZnO and carbon black in EPR. The aromatic diaminodisulfide is an antiozonant and functions both as an antioxidant and a... 

Deterioration by the Action of Ozone. Ozone, although found in relatively smaU amounts in air, is a highly reactive gas. Its action on many types of rubbers can cause products to crack and fail prematurely unless they are protected by antiozonants or made from ozone-resistant elastomers. 

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. 

Most compounders use a combination of physical and chemical antiozonants and achieve excellent protection in this way. For more severe ozone-resistance problems, there are, of course, a number of specialty elastomers that are saturated and therefore completely ozone-resistant ethylene/propylene rubber, chlorinated and chlorosulfonated polyethylene, ethylene/vinyl acetate, ethylene/acrylic esters, butyl rubber, SEES, plasticized PVC, butyl acrylate copolymers, polyepichlorohydrin and copolymers, polyetherester block copolymer, polyurethane, and silicone. 

Ethylene propylene diene terpolymers (EPDM) can be used to improve the ozone resistance of bromobutyl/natural rubber binary polymer blends, eliminating the need for chemical antiozonants. Addition of 10 phr of EPDM (with a high ethylidene norbornene, ENB, content of9%) to a 50/50 bromobutyl rubber/natural rubber blend results in a compound vdth good static and dynamic ozone resistance. EPDM with a 5.7% ENB level is another suggested grade of polymer. 

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). 

The influence of polymers and additives on the service life of non-tread tyre components is discussed on the basis of results obtained from studies of inner liner, belt and sidewall compounds. The effects of the bromine content of bromobutyl rubbers on the performance of inner hners, and of sulphenamide accelerators on the adhesion of NR belt compounds to brass coated steel cords were investigated. The mechanical properties and ageing and ozone resistance of black sidewall compounds consisting of NR blends with neodymium catalysed polybutadiene protected with different antioxidants and antiozonants were also evaluated. 5 refs. 

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. 

EPM and EPDM have no unsaturation in their backbone and thus are quite weathering and ozone resistant. Antiozonants are not required. EPDM can even be used in blends to improve the ozone resistance of sensitive rubbers [697],... 

In terms of weathering resistance and ozone resistance, NBR is comparable with natural rubber and SBR. The effect of antiozonants is small [697]. 

In contrast to nitrile rubber, fully hydrogenated NBR is not attacked by ozone. Even under extreme conditions (e.g., 2 ppm ozone, 40 °C, 50% rel. humidity, 60% strain, 168 h) no cracks appear. Even under cyclic load these rubbers exhibit excellent ozone crack resistance. Elastomer parts based on HNBR are highly ozone-resistant without the addition of antiozonants. Partially-hydrogenated grades behave differently, depending on the vulcanization system. Ozone resistance quickly decreases with increasing double bond content. However, peroxide vulcanized rubbers behave more favorably than sulfur vulcanized rubbers [697]. 

Halobutyl rubbers with almost completely saturated backbones usually do not require extra antioxidants or antiozonants the rubber producer adds traces of stabilizers. For severe service circumstances, particularly in blends with unsaturated rubbers, the addition of some antidegradant may be necessary. In the selection of any antidegradant care should be exercized, particularly regarding amine derivatives that may interfere in the crosslinking process, but with proper technology vulcanizates with enhanced stability may be obtained [103]. Ozone resistance and weathering behavior of halobutyl rubber vulcanizates are also dependent on the curing system, on the nature and amount of fillers and plasticizers. 

The requirements for static ozone resistance vs. dynamic ozone resistance are very different, so that choiee of antiozonant depends greatly on the expected service of the rubber produet. Static protection is provided by petroleum waxes, usually paraffin and/or microcrystalline waxes. The waxes work by blooming to the rubber surface to form a physical barrier to ozone attack. The choice of wax or wax blend is based upon migration temperature where mobility and solubility of the wax in the rubber are balanced so that sufficient bloom occurs for optimum protection. Because the wax film is inextensible, it will rupture under deformation and expose the elastomer. Waxes protect only under static conditions. 

Deterioration by Ozone - Ozone is a highly reactive gas which can cause rubber products to crack and fail prematurely imless they are protected by antiozonants or made of an ozone-resistant elastomer. Ozone is present in the atmosphere, and is produced constantly by arcing in high voltage electrical devices. 

The ozone resistance of unprotected NBR is comparable with that of other diene rubbers. Unlike that of natural rubber and styrene-butadiene rubber, however, it cannot be greatly improved by including paraffinic hydrocarbons and antiozonants in the compound. In cases where increased thermoplasticity and reduced low-temperature flexibility are acceptable this difficulty is often overcome by blending PVC with the NBR (see Section 5.3.1). 

Provides antiozonant and antioxidant properties with high temperature, fatigue and flex resistance to rubber compounds. Used in pneumatic tire components, solid tires, belts, hoses, cables, automotive mounts, bushings and general mechanical products that are exposed to continuous and intermittent dynamic operating conditions that require protection from ozonation. 

Involvement of the Ozonized Rubber Moieties in Antiozonant Mechanism. The rubber chain relinking theory (30) is consistent in part with the self-healing film formation theory (37) a reaction between an antiozonant or some of its transformation products and ozonized elastomer is considered. Either scission of ozonized rubber is prevented in this way or severed parts of the rubber chain are recombined (i.e., relinked). A "self-healing" film resistant to ozonation is formed on the rubber surface. Such a film formed by the contribution of nonvolatile and flexible fragments of the rubber matrix should be more persistent than any film suggested in the protective film theory. 

Zinc dialkyl dithiocarbamates reduce the rate of crack growth, but their scorch resistances are prohibitively low. A few other classes of compounds have been reported to react rapidly with ozone, such as phosphines and stibines. These materials also exhibit antiozonant activity when swollen into rubber vulcanizates after the cure. However, they cannot be used practically for two reasons ... 

Photo-oxidation proceeds relatively slowly. That is why elastomers made from chlo-roprene rubber exhibit considerably better resistance to ozone attack and weather influences than those made from NR, SBR, and NBR. Moreover, chloroprene rubbers respond well to anti-aging and antiozonant agents so that they can be recommended for use under difficult climatic conditions, e.g., in the tropics. By extrapolating mechanical property changes after several years of weathering in ozone-rich atmosphere, the current technology allows the prediction that chloroprene rubber products will exhibit sufficient strain for most static applications even after 50 years in service [697]. 

Blends of petroleum waxes are commonly used as antiozonants in the rubber industry. Just as 6PPD gives excellent protection against ozone attack of the surface of the rubber under dynamic conditions, waxes will bloom to the surface of the rubber to impart excellent resistance to ozone attack under static conditions. The wax actually exudes (or blooms) to the surface of the rubber article to create a physical protective barrier to protect the rubber surface against ozone attack, which can create cracking of the rubber. 

A variety of materials are commercially available for giving a rubber compound certain desired properties needed for functional performance in actual use. Antioxidants are commonly added for resistance against attack by environmental oxygen (Oj), whereas antiozonants are employed for resistance from attack by ozone (O3). 

Vulkanox 4010 NA/LG antiozonant excels in antiflexcracking properties and is used in tires and mechanical goods subjected to dynamic 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. 

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