Curing Elastomers

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. 

Latex gloves, balloons, condoms, and other elastomeric products are produced by dipping metal molds into a latex that can consist of either natural or synthetic rubber. The mold is pretreated with a chemical that causes the latex to coagulate on the mold. Then the latex-covered mold passes through an oven where the elastomer cures (crosslinks). A modem factory can produce on the order of a million latex gloves or balloons a day. 

These are silicones in liquid or paste form that can be transformed into cross-linked elastomers at or near room temperature. There are two basic types, one component and two component. The one-component room temperature vulcanizing (RTV) elastomers cure upon exposure to air. A typical material of this type is synthesized from a dihydroxy-terminated siloxane oligomer and methyltriacetoxysilane (Scheme 2). The resulting tetraacetoxy compound will flow as a paste and so can be molded into shape. Hydrolysis of the acetoxy groups takes place over a period of hours. Condensation of the resulting silanols gives a highly cross-linked end product. 

In this study, the properties and processing of elastomers cured with HER were compared with those cured with HEQ. Studies were made using a variety of prepolymers and catalysts. 

Plasma clean dishes (2 min, high intensity) and immediately add 500 pL of the well mixed, bubble-free silicone elastomer/curing reagent mixture on the plasma-cleaned side of a glass coverslip. 

Carefully clean the silicon wafer with canned air. Then add about 4 mL of the bubble-free silicone elastomer/curing reagent mixture on the wafer. Plasma clean round cover glasses at high intensity for 2 min. Then place them with the plasma-cleaned surface facing the silicone elastomer/ curing reagent mixture on the wafer. Press them down to the silicon wafer and make sure to get rid of aU the bubbles. 

Uses Modifier for elastomers, adhesives, fibers reactive diluent for epoxies reactive intermediate for coatings sizing/finishing agent for fiberglass silane intermediate in elec, coatings stabilizer of chlorinated compds., vinyl resins, rubber defoamer comonomer for polyepichlorohydrin rubber, unsat. polyester resins, and crosslinked PU elastomers curing system additive in bisphenol A-epichlorohydrin epoxies for food-contact articles... 

Elastomers cured with sulfur frequently have improved mechanical properties when the curing conditions are chosen to give cross links that... 

Fig. 33. Proton NMR images of unfilled polyisobutilene-based elastomers, cured with 1,6-hexamethylenediamine, after equilibrium swelling in CCI4. The degree of cross-linking increases from samples 1 to 4. The regions of interest indicate the locations used to determine the local relaxation decay times. Reproduced from Ref 181, with permission from American Chemical Society...

Dry RubbGr. Because of its enhanced crystallizability, guayule rubber can exhibit superior failure properties in unfilled rubber compositions. As a double network, an elastomer cured a second time while in a deformed state guayule rubber exhibits substantially better fatigue resistance than deproteinized Hevea rubber (118). When compounded with carbon black, guayule rubber and Hevea rubber behave similarly (119). In tread and wire skim stocks, the compoimding and performance behavior of guayule rubber was comparable to that of Hevea rubber (Tables 7, 8) (120). 

The measurement of the degree of conversion by enthalpy measurement applies to elastomer cure also, since the vulcanization reaction is highly exothermic. Unlike for thermosets, Tg is not a measure of degree of cure for elastomers. Sircar discusses this and numerous other aspects of the characterization of elastomeric materials using thermal analysis in chapter 5 of Reference 5. 

Fig. 6. Tensile failure envelope for Adiprene polyurethane block copolymer elastomer cured at 25°C given by a double logarithmic plot of 298 a j/T versus...

Cure RT 50 secs (Al), 15 - 20 secs (Plastic), 7 -10 secs (Elastomer). Cure RT 95secs (Al). 15-25 secs. (Plastic), 15-20 secs (Elastomer) ... 

Both (A) and (C) curves indicate a final exothermic reaction initiated in the vicinity of 285 °C, whereas the elastomer cured by Diak No3 shows some reaction as low as 200 °C. In contrast, the untreated Viton A-HV starts to decompose from 430 °C. Hence, vulcanization results in a decrease of the thermal stability of fluorinated elastomer. 

Other mechanical properties and resistance to bases and to acids are studied for a poly(TFE-alt-P) elastomer cured with a peroxide (o, Q -bis-(t-butylperoxy)-p-diisopropylbenzene), and a poly(VDF-ter-HFP-ter-TFE) ter-polymer cured with a diamine (NyN dicinnamylidene-l,6-hexanediamine) (Table 24) [39]. 

X-ray microanalysis and BEI provide information regarding the elemental composition of dispersed phase particles and also of the contaminants responsible for failure. These techniques are useful for the study of multiphase polymers, if one of the phases contains an element not contained by the other. For example. Price et al. [409] used SEM/EDS to study the interface between two polymers, one of which was PVC, to measure the local composition during interdiffusion. Failure of elastomer compounds used in tank track pads, natural rubber, SBR, EPDM, and 50/50 SBR/ BR was studied as a function of elastomer cure and degradation in service [410]. Andrade et al. [411] used BEI of osmium tetroxide stained PS/ PB to provide images that showed the dispersed and stained PB as a result of atomic number... 

The proper choice of elastomer, cure system and filler will ensure that the desired properties are obtained from a rubber product. Antidegradants are used to prevent these properties from changing during service. The primary degradant effects are oxidation and ozone attack. 

K. Hosoya, T. Nakagawa, and S. Yagishita, A New Polyacryhc Elastomer Cure System for Automatic Transmission Ruid Hose Apphcation, SAE Meeting, Detroit, Febmary 29-March 4, 1988. 

Elastomers cured with two crosslinking systems such as sulphur and the polymerisation products of p-benzoquinone are shown to have much improved overall mechanical properties. Non-Gaussian behaviour of quinone polymer crosslinked elastomers viewed as bimodal networks was studied. The study focused on the effect of ageing time on the reduced stress values of the networks in relation to the elongation of the samples. The study is also extended to cover the possible effect of these bound antioxidants on the onset of the vulcanisation process and the hardness values of the elastomeric networks. 15 refs. 

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