Elastomers ageing

AGE-Gontaining Elastomers. The manufacturing process for ECH—AGE, ECH—EO—AGE, ECH—PO—AGE, and PO—AGE is similar to that described for the ECH and ECH—EO elastomers. Solution polymerization is carried out in aromatic solvents. Slurry systems have been reported for PO—AGE (39,40). When monomer reactivity ratios are compared, AGE (and PO) are approximately 1.5 times more reactive than ECH. Since ECH is slightly less reactive than PO and AGE and considerably less reactive than EO, background monomer concentration must be controlled in ECH—AGE, ECH—EO—AGE, and ECH—PO—AGE synthesis in order to obtain a uniform product of the desired monomer composition. This is not necessary for the PO—AGE elastomer, as a copolymer of the same composition as the monomer charge is produced. AGE content of all these polymers is fairly low, less than 10%. Methods of molecular weight control, antioxidant addition, and product work-up are similar to those used for the ECH polymers described. 

Three methods that were used to measure the chemical changes associated with oxidative degradation of polymeric materials are presented. The first method is based on the nuclear activation of lsO in an elastomer that was thermally aged in an, 802 atmosphere. Second, the alcohol groups in a thermally aged elastomer were derivatized with trifluoroacetic anhydride and their concentration measured via 19F NMR spectroscopy. Finally, a respirometer was used to directly measure the oxidative rates of a polyurethane foam as a function of aging temperature. The measurement of the oxidation rates enabled acceleration factors for oxidative degradation of these materials to be calculated. 

The tensile properties measured on the unaged and heat-aged elastomer materials made from HER and butanediol are given in Table 8.16. 

For 120 °C heat-aged samples, 85-90% retention of G values for HER compared to 63-89% for butanediol were seen. The same kind of trend is seen with 135 °C heat-aged elastomer storage modulns values. Thus the HER extended elastomer has better retention of storage modulus at higher temperatures than the butanediol elastomer. 

Oxidative aging, elastomers n. Breaking down of an elastomer through the action of oxygen on the polymer itself or on other ingredients of the compound. The process may be signaled by change of color, visible deterioration of the part surface, or lowered performance in service. 

TMO-AGE Elastomers. The sulfur vulcanizate of the 96-4 TMO-AGE copolymer had high tensile, modulus, and tear strength at 23 C. at a good elongation level (Table V). The hot tensile properties at 100 C. are somewhat lower but still good. Crystallinity and/or crystallization on stretching could enhance tensile properties at 23°C. apparently this is not the case, since the hot tensile results, which could not involve crystallinity... 

An approach to altering the TMO-AGE copolymer conposition would be to make a terpolymer containing PO as described previously. Vulcanizate data are given on the PO-TMO-AGE elastomers in Table VIII. The properties are fair but indicate that the vulcanizates are overcured. The AGE level was evidently too high for the cure system used. The results do confirm the preparation of the desired terpolymer and indicate that this approach is feasible. 

Other Oxetane Elastomers. The 1M)-butadiene monoxide (BMO) copolymer elastomer (Run 6, Table II) containing 6.5% BMO gave inferior properties (Table VIII) to the TMO-AGE elastomer with 4.4% AGE described in the previous section. Actually the properties were even slightly inferior to a 98-2 TMO-AGE copolymer vul-canizate (Table VIII). Thus AGE appears about three times more effective (on a weight basis) in conferring sulfur vulcanizability than BMO. On a molar basis, the difference is even greater, i.e., AGE is five times more effective. This same difference between BMO and AGE was previously observed on comparing PO-BMO elastomer with PO-AGE elastomer. 

Aging Elastomer/post-elastic stiffiiess, Minor increase ... 

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Nickel dialkyldithiocarbamates stabili2e vulcani2ates of epichlorhydrinethylene oxide against heat aging (178). Nickel dibutyldithiocarbamate [56377-13-0] is used as an oxidation inhibitor in synthetic elastomers. Nickel chelates of substituted acetylacetonates are flame retardants for epoxy resins (179). Nickel dicycloalkyldithiophosphinates have been proposed as flame-retardant additives for polystyrene (180—182) (see Flame retardants Heat stabilizers). 

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). 

Chloroprene Elastomers. Polychloroprene is a polymer of 2-chloro-l,3-butadiene. The elastomer is largely composed of the trans isomer. There are two basic polymer types the W-type and the G-type. G-types are made by using a sulfur-modified process W-types use no sulfur modification. As a result, G-types possess excellent processing and dynamic properties, and tend to be used in V-belts. However, they have poorer aging properties than W-types. The W-types tend to be used in appHcations requiring better aging, such as roUs and mechanical goods (see Elastomers, SYNTHETIC-POLYCm.OROPRENE). 

SiHcone elastomers possess outstanding resistance to heat aging. The Si—O—Si backbone imparts resistance to oxygen, o2one, uv, and to some polar fluids. However, the strength of these elastomers is usually just adequate. They have low abrasion resistance and tear strength (see Silicon compounds, silicones). 

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

EPDM is a terpolymer of ethylene, propylene, and a small amount (<10%) of an unsaturated diene 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 cure gives superior aging resistance and low compression set. 

I ew Rubber-Modified Styrene Copolymers. Rubber modification of styrene copolymers other than HIPS and ABS has been useful for specialty purposes. Transparency has been achieved with the use of methyl methacrylate as a comonomer styrene—methyl methacrylate copolymers have been successfully modified with mbber. Improved weatherability is achieved by modifying SAN copolymers with saturated, aging-resistant elastomers (88). 

Ethylene—Propylene (Diene) Rubber. The age-resistant elastomers are based on polymer chains having a very low unsaturation, sufficient for sulfur vulcanization but low enough to reduce oxidative degradation. EPDM can be depicted by the following chain stmcture ... 

Polysulfides. The polysulftde elastomer, best known under the trade name Thiokol, represents the earliest commercially developed synthetic mbber, developed ia 1930 by J. C. Patrick as a highly solvent and age-resistant elastomer (15). It is stiH considered the most solvent-resistant mbber, but its poor mechanical properties provide a serious disadvantage (see Polymers containing sulfur). 

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