Resistance to ozone attack

The process yields a random, completely soluble polymer that shows no evidence of crystallinity of the polyethylene type down to —60°C. The polymer backbone is fully saturated, making it highly resistant to ozone attack even in the absence of antiozonant additives. The fluid resistance and low temperature properties of ethylene—acryUc elastomers are largely a function of the methyl acrylate to ethylene ratio. At higher methyl acrylate levels, the increased polarity augments resistance to hydrocarbon oils. However, the decreased chain mobiUty associated with this change results in less fiexibihty at low temperatures. 

Nitrile rubbers have high resistance to oils and organic solvents, and, although their mechanical properties are not as good as those of natural rubber, they are subject to much less deterioration in the presence of oils and solvents. They are not resistant to ozone attack. 

Good resistance to weathering, resistant to ozone Attack... 

EPDM tends to show good resistance to ozone attack, oxidation resistance, and moisture resistance. It is therefore used in applications that require good weather resistance and heat stability. Roofing materials, outer covers of high-voltage electric cables, and selected automotive hoses use EPDM. 

Hypalon is one of the most weather-resistance elastomers available. Oxidation takes place at a very slow rate. Sunlight and ultraviolet light have little, if any, adverse effecfs on its physical properties. It is also inherently resistant to ozone attack without the need for fhe addition of special antioxidants or antiozonants to the formulation. 

Urethane rubbers exhibit excellent resistance to ozone attack and have good resistance to weathering. However, extended exposure to ultraviolet light... 

Weather Resistance Many rubbers are affected adversely by outdoor exposure, particularly by the traces of ozone, which are always present in the atmosphere and which cause rapid cracking of lightly stressed rubber articles. Nitrile rubber is no exception and thus it is not suitable for use in exposed conditions. However, the incorporation of a proportion of PVC (Polyvinyl chloride) into the nitrile compound results in improved resistance until, with a blend of 70/30 nitrile/PVC, a material with almost complete resistance to ozone attack is produced. The presence of PVC produces stiffer compounds, with lower resiliance and inferior low temperature properties, but the oil and solvent resistance is enhanced. 

To test for resistance to ozone attack, samples are stretched to 20 or 40 percent strain on a test rack or bent in a loop to produce a surface strain. The specimens are then placed in a chamber equipped with an ozone generator. Usual controlled test concentrations are about 0.25, 0.5, 1, or 2 ppm (parts ozone per milhon parts of air by volume). The test temperature is usually 40°C. The test specimens are inspected at various time intervals until initial cracking occurs. 

Cadmium sulfide KR 138S Epoxy Resistance to ozone attack in photosensitive plates [1] (Table 34)... 

Whatever the cause of the problem, the judge was not convinced that the failure was caused by a manufacturing defect and the claimant lost her case. Disclosure of the manufacturer s complaints records did show that there were many complaints of premature failure, and the company had researched and introduced a synthetic rubber condom based on polyurethan into the market. This elastomer was much more resistant to ozone attack, but was ultimately withdrawn owing to the greater expense of the product. 

The urethane mbbers exhibit excellent resistance to ozone attack and have good resistance to weathering. However, extended exposure to ultraviolet light will reduce their physical properties and wiU cause the mbber to darken. Table 4.17 lists the compatibility of urethane with selected corrodents. 

Ethylene acrylic mbber is manufactured by M/s Dupont USA under the trade name of Vamac, and is about half ethylene and half methylacrylate. A small amount of cure site monomer in the molecule provides the ability to cross-link chemically. This rubber is the combination of two major chemicals which give its unique balance of properties. For instance, the backbone structure of the polymer molecule is saturated, and so it is inherently resistant to ozone attack. The acrylic segment provides oil resistance, and the ethylene segment yields low temperature performance. The added feature of this mbber is that there is no halogen present to become corroded. There is slightly more tendency to swell than a homopolymer, such as polyacrylate or acrylonitrile mbber, but it is approximately equal to silicone, chloroprene and Hypolan (chlorosulfonated polyethylene) mbbers. 

Rubber surfaces contains many flaws, where cracks can be initiated via ozone attack. Increasing stress will increase the number of flaws, which leads to a larger number of cracks. The depth of the cracks is inversely related to their number, and so, low stresses that produce long deep cracks are more damaging to rubber than high stresses. Rubbers which do not contain olefinic double bonds, such as silicone and fluorocarbon rubbers, are completely resistant to ozone attack under any conditions of concentration, stress or temperature. 

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]. 

Complete saturation of the polymer backbone as shown above imparts much better resistance to ozone attack, extended service temperature range, and improved hot air resistance, compared to compounds based on conventional NBR. In other words, compounds based on HNBR are much more resistant to heat than those based on regular NBR elastomers. HNBR compounds have better thermo-oxidative aging resistance, qualifying them for automotive under the hood applications. 

CSM compounds possess excellent resistance to ozone attack, oxidation, and weathering, even for nonblack products. Also, these CSM compounds usually possess some oil, water, and chemical resistance. Therefore, CSM compounds are used in environmental applications (especially pond linings), insulation for cables, spark plug boots, coated fabrics, hose covers, single-ply roofing, rafts, and folding kayaks. 

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. 

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