Elastomer grades

Manufacturer Trade Type name elastomer Grade designation Oil content (phr) Mooney viscosity d + 8 ) at 127°C Ethylene content Diene type Diene content Molecular weight distribution classification... 

Tacticity (stereoregularity) % stereo-regular propene in elastomer grades Vanadium or metallocene catalysis 0 (2-5, 8)... 

Cold grades, that is, one-component room temperature vulcanizable elastomer grades (RTVs) dominate in the case of liquid silicone rubbers. In these cases, the rubbers are branched poly(dimethyl siloxanes) with silanol end groups that can be cross-linked with tetrabutyl titanate or methyl triacetoxy silane. The cross-linking starts on contact with the humidity in the air, whereby in the case of methyl triacetoxy silane, for example, acetic acid is liberated and the methyl trihydroxysilane produced reacts with the silanol groups of the polymer ... 

At strains over 300% the stress occurs mostiy in the amorphous regions up to the point where the sample breaks. AH of the grades exhibit permanent set, and the curves of grades with a Shore Hardness of 55 and higher exhibit a yield point. This means that parts have to be designed for low strains to stay within the area of elastic recovery. Special grades of elastomer are available to provide hydrolysis resistance (194), improved heat aging (195), and improved uv-stabihty (196). 

The more important grades of thermoplastic natural mbber, which fall into the olefinic class of thermoplastic elastomers, are prepared with the natural mbber phase partially cross-linked during blending, a process known as dynamic vulcanization. The hardness of the soft blends is controlled by the natural mbber content, and typical properties of those of 50—90 hardness (Shore A) are shown in Table 7. 

Grades of polyisobutylene, butyl mbber, halogenated butyl mbber, and partially cross-linked isobutylene—isopiene—divinylbenzene terpolymer have been developed to meet specific processing and property needs. Recently, two new polyisobutylene-based elastomers have been developed. One is now available commercially as Exxon SB Butyl Polymers (32) and the other is under market development as Exxon bromo XP-50. 

Fig. 3. Heat and oil resistance of CSM compared to other elastomers by ASTM D2000. A—K iadicate grades of CSM. The other ASTM designations are as follows AM, acryhc elastomers CR, chloroprene mbber EPDM, ethylene—propjiene—diene mbber FKM, fluorocarbon elastomers FQ, fluorosiUcones ...

Commercial Forms. Eour different base polymers of VAMAC ethylene—acryhc elastomer are commercially available (Table 1). Until 1990, existing grades of ethylene—acryhc elastomers were based on a single-gum polymer. VAMAC G, defined as a terpolymer of 55% methyl acrylate, ethylene, and a cure-site monomer (5). In 1991, a higher methyl acrylate terpolymer, VAMAC LS, was introduced. The composition of this polymer was specifically chosen because it significantly increases the oil resistance of the polymer while minimizing losses in low temperature fiexibihty (6). 

Compression Set Resistant. One property of fluorocarbon elastomers that makes them uniquely valuable to the sealing industry is their extreme resistance to compression set. Figure 4 plots compression set vs time for compounds prepared especially for compression set resistance (O-ring grades). 

Extruded Articles. In extmded article compounding, the most important parameters are scorch safety and flow characteristics (53). The bisphenol cure system again offers the best scorch resistance of the available fluorocarbon elastomer cure systems. Good flow characteristics can be achieved through proper selection of gum viscosities. Also, the addition of process aids to the formulation can enhance the flow characteristics. Typical formulations for extmsion grade fluorocarbon elastomers are given iu Table 7. 

Table 7. Fluorocarbon Elastomer Extrusion Grade Compound...

Extrusion. Extmsion techniques are used in the preparation of tubing, hose, O-ring cord, preforms and shaped gaskets. Typical extmsion conditions are 70 to 85°C for the barrel temperature and 95 to 110°C for the head temperature. The extmded forms are normally cured in a steam autoclave at 150 to 165°C. Some special grades of peroxide curable fluorocarbon elastomers can be hot air vulcanized. 

Certain grades and formulations of the fluorocarbon elastomers are qualified under the code of Eederal Regulations, 21, Eood and Dmgs, Part 177.2600 for use as mbber articles whose intended appHcations requite repeated or continuous contact with food. Elastomer suppHers will provide assistance in formulating for specified uses. 

Among the polyurethane, polyester, and polyamide thermoplastic elastomers, those with polyether-based elastomer segments have better hydrolytic stabihty and low temperature flexibiUty, whereas polyester-based analogues are tougher and have the best oil resistance (43). Polycaprolactones and aUphatic polycarbonates, two special types of polyesters, are used to produce premium-grade polyurethanes (12). 

---