Air aging resistance

Sulfur cures do not always use elemental sulfur. Sometimes with special EV (efficient vulcanization) cure systems no elemental sulfur is used at all. Instead a sulfur donor chemical is used. The most common sulfur donor is dithiodimorpholine (DTDM), which donates two sulfur atoms from the center of its molecule to participate in the sulfur vulcanization process. These EV cures are more expensive than conventional sulfur cures based on elemental sulfur. This is because sulfur donors such as DTDM are more expensive per pound than sulfur itself. Elowever, the EV cure will usually impart better air aging resistance than that of a conventional sulfur cure system using a significant concentration of elemental sulfur. 

Another example is the use of halobutyl in neoprene compounds as an economy measure. Table 27 illustrates the influence of bromobutyl content on compound cost, tensile strength, oil resistance, compression set, and specific gravity. Air ageing resistance is unchanged. 

The carboxylated types (XNBR) contain one, or more, acrylic type of acid as a terpolymer, the resultant chain being similar to nitrile except for the presence of carboxyl groups which occur about every 100 to 200 carbon atoms. This modification gives the polymer vastly improved abrasion resistance, higher hardness, higher tensile and tear strength, better low temperature brittleness, and better retention of physical properties after hot-oil and air ageing when compared to ordinary nitrile rubber. 

The sidewall compound is compounded to protect the ply and must possess resistance to weathering, ozone, abrasion, and tearing while providing excellent flex fatigue resistance. The innerliner compound must provide good air-permeation resistance and resist moist/hot air aging. The inside of a hot... 

Accelerated aging or heat resistance tests Heat resistance and accelerated air aging test... 

A group of new, fully miscible, polymer blends consisting of various styrene-maleic anhydride terpolymers blended with styrene-acrylonitrile copolymer and rubber-modified versions of these materials have been prepared and investigated. In particular the effects of chemical composition of the components on heat resistance and the miscibility behavior of the blends have been elucidated. Toughness and response to elevated temperature air aging are also examined. Appropriate combinations of the components may be melt blended to provide an enhanced balance of heat resistance, chemical resistance, and toughness. 

In addition to the excellent oil resistance and heat aging resistance, the ECH homopolymer has particularly good air retention properties, by a factor of two better than butyl rubber. 

Elastomeric covered yam uses spandex as the core wrapped with various yams and fibers such as nylon, polyester, rayon, cotton, acryUc, and so forth, for end products. Based on chemical research, it draws better properties than mbber yam in terms of intensity, stress, and age-resistance. Elastomeric covered yams are divided into conventional covering yam and air-jet covered yam. Elastomeric covered yam gives fabric elasticity and a natural sense of touch. 

Uses of Chlorinated Polyethylene Vulcanizates. CM is used in applications where aging resistance in hot air, oils, or chemicals is required and where good ozone, weathering, and flame resistance are required. Many such applications are in the wire and cable industry. 

Another extrusion-grade polyamide blend is a reactively compatibihzed PA6/polyolefin (PP or LDPE) alloy with some additional olefinic elastomer present for lowering the modulus (Jacquemet et al. 2000). Such low-modulus PA blends (Orgalloy LT series, Arkema) have been qualified for use in automotive under-the-hood air-intake systems such as clean air tubes and ducts. Low density, high chemical resistance to oil and greases, heat-age resistance, fatigue, and vibration resistance have been the main criteria for the selection of this blend in automotive air-intake duct applications. 

Geolast (polypropylene plus nitrile rubber) has a higher resistance than Santoprene (polypropylene plus EPDM) to oils (such as IRM 903) and fiiels, plus good hot-oil/hot-air aging. Geolast applications include molded fuel filler gasket (Cadillac Seville), carburetor components, hydraulic lines, and engine parts such as moimts and tank liners. 

EPDM is a terpolymer of ethylene, propylene, and a small amount of an unsaturated diene as a third monomer to provide a cure site. Unlike the elastomers previously discussed, the unsaturation in EPDM is not in the main chain, but it is pendent to the chain. Peroxide-based cure systems afford better aging resistance and low compression set. A comparison of a sulfur-based cure to two different peroxides in EPDM is shown in Table 21 (4). Initial properties for these three compounds are reasonably close. However, after air aging, the advantages of peroxide curing are apparent. Most dramatic is the improved compression set... 

Heat resistance a rubber compound s ability to undergo exposure to some specified level of elevated temperature, and still retain a high level of its original properties also known as heat aging or air aging. 

A second test which is even more discriminating is the so-called "contamination test, in which the rubber is soaked in the fluid for several hours at room temperature and is then subjected to hot air ageing at high temperature. This test simulates the environment of many engine components (e.g. seals, gaskets, hoses, etc.). The main rubbers commonly used in such applications are nitrile rubbers which are not appreciably swelled by hydrocarbon oils and ethylene-propylene (EP) and their ter-polymers (EPDM) which are resistant to the phosphate ester fluids (e.g. Skydrol) used in aircraft hydraulic systems. The following discussion is concerned with the performance of antioxidants under these veiry aggressive conditions. 

Carboxylic nitrile combines excellent oil resistance with abrasion resistance. It is available in nominal hardness of 80 Shore A. It exhibits a high tensile modulus, low elongation, improved hot tear and tensile resistance, and better resistance to hot oil and air aging than most nitriles. 

A major dry natural rubber type is ribbed smoked sheet. For this the coagulum is passed between pairs of even-speed steel rollers that squeeze the water out. The final set of rollers has channels cut in them so that the sheet emerging from them has a ribbed surface. The ribbed surface facilitates drying. This rubber is dried in smokehouses. The ribbed sheets are hung over poles mounted on trolleys. Rubber tree wood fires produce the smoke, which dries the rubber and gives the rubber some age resistance from its components such as cresols. Drying takes 48 to 96 hours, with entrance temperatures at about 40 °C and exit temperatures at about 60 °C. Some ribbed sheet rubber is dried in hot air out of contact with smoke. This produces a lighter colored rubber, which commands a premium and is referred to as air-dried sheets. 

The physical properties of the formula shown in Table 3 are listed in Table 4. The resultant resistance to hot-air ageing is shown in Fig. 1. 

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