Ozone cracking

Ozone concentrations Ozone control Ozone cracking Ozone depletion... 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

Typically, ozone cracking initiates at sites of high stress (flaws) on the mbber surface. Thus, in general, mbber articles should be designed to rninirnize potential sites of high elongation such as raised lettering. Similarly, the use of clean molds helps to reduce the incidence of surface flaws. 

Ozone cracking is a physicochemical phenomenon. Ozone attack on olefinic double bonds causes chain scission and the formation of decomposition products. The first step in the reaction is the formation of a relatively unstable primary ozonide, which cleaves to an aldehyde or ketone and a carbonyl. Subsequent recombination of the aldehyde and the carbonyl groups produces a second ozonide [58]. Cross-linking products may also be formed, especially with rubbers containing disubstituted carbon-carbon double bonds (e.g. butyl rubber, styrene-butadiene rubber), due to the attack of the carbonyl groups (produced by cleavage of primary ozonides) on the rubber carbon-carbon double bonds. 

Table 11.14 gives critical stress, critical strain, and critical stored energy of NR-EPDM blends for initiation of ozone cracking. All the properties show an increase on addition of TOR, especially the critical stored energy. Stored energy is a strong indicator of ozone resistance and shows an increase by about 80% on addition of 20 parts of TOR to the compound. This was confirmed by SEM pictures of surface ozone cracks and the results of dynamic mechanical moduli and tan d measurement. 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

ISO 1431, Parts 1-3, Rubber, vulcanized or thermoplastic - Resistance to ozone cracking, 1989-2000. 

Standard method of test for accelerated ozone cracking of vulcanized rubber [Test method D-1149-64 (reapproved 1970)], pp. 554-560. In 1972 Annual Book of ASTM Standards. Part 28. Rubber Carbon Black Gaskets. Philadelphia American Society for Testing and Materials, 1972. 

Haagen-Smit, A. J., M. F. Brunelle, and J. W. Haagen-Smit, Ozone Cracking in the Los Angeles Area, Rubber Chem. Technol., 32, 1134-1142(1959). 

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

Ozone cracking - The surface cracks, checks or crazing caused by exposure to an atmosphere containing ozone. 

Temperature does affect the rate of ozone cracking but it cannot be said simply that higher temperatures accelerate the effect. The blooming characteristics of different waxes can make an increase in temperature increase or decrease ozone resistance. Above about 70°C all ozone is destroyed. In the current major standards, 40°C is specified, just a small degree of acceleration above ambient and practically the lowest level which can be controlled without cooling. 

ISO 1431 Part 1, 2004. Resistance to ozone cracking - Static and dynamic strain testing. 

ASTM D1149,1999. Surface ozone cracking in a chamber. 

Vulcanized rubber was ozonized as follows. Natural rubber was compounded with the ingredients shown in Table III and cured at 141 °C. for 13 minutes. The vulcanizates were cut off from the sheets with JIS No. 1 dumbbell cutter to obtain the specimens for the ozone crack test. The test pieces were exposed to an oxygen atmosphere containing 0.01% ozone under an elongation of 50%, and a time, t0, required for the initial crack formation was measured. 

Under some circumstances it is possible to observe very low 5 values for polymer solids, namely when the energy to fracture chemical bonds is provided by chemical reaction. An excellent example of this is the ozone cracking of unsaturated hydrocarbon elastomers where crack propagation occurs at threshold values of as low as 0.05 Jm Under such low stresses the toss function of Eq. (5) is effectively... 

Antioxidants such as ketone-amine condensation products or phenols are used to prevent natural ageing and atmospheric ozone cracking. 

The above units are randomly arranged in emulsion copolymers, but in solution the polymers may occur in uniform blocks of varying length. These block copolymers are resilient and rubber-like at room temperature, but are thermoplastic at higher temperatures. SBRs are similar to natural rubber, in that they are susceptible to atmospheric oxidation and ozone cracking when stretched in air. They are swollen and weakened by hydrocarbons and halogenated hydrocarbons. However, their resistance to abrasion and ageing is superior to natural rubber. 

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