Elastomer service temperature

Plasticizers (qv) are usually present at lower concentrations compared to general-purpose mbber-based compounds, because of their volatility at typical ACM service temperatures and/or their partial extractabiflty by the aggressive fluids where acryflc elastomers are employed. Other additives are therefore required to improve processibiflty. These processiag aids act as lubricating agents and enhance the release characteristics of the acryflc compound and/or reduce compound viscosity. 

In thermoplastic polyurethanes, polyesters, and polyamides, the crystalline end segments, together with the polar center segments, impart good oil resistance and high upper service temperatures. The hard component in most hard polymer/elastomer combinations is crystalline and imparts resistance to solvents and oils, as well as providing the products with relatively high upper service temperatures. 

Adhesives, Coatings, and Sealants. Eor these appHcations, styrenic block copolymers must be compounded with resins and oils (Table 10) to obtain the desired properties (56—58). Materials compatible with the elastomer segments soften the final product and give tack, whereas materials compatible with the polystyrene segments impart hardness. The latter are usually styrenic resins with relatively high softening points. Materials with low softening points are to be avoided, as are aromatic oils, since they plasticize the polystyrene domains and reduce the upper service temperature of the final products. 

Polydithiazoles Polyoxadiazoles Polyamidines Pyrolyzed polyacrylonitrile Polyvinyl isocyanate ladder polymer Polyamide-imide Polysulfone Decompose at 525°C (977°F) soluble in concentrated sulfuric acid. Decompose at 450-500°C (842-932°F) can be made into fiber or film. Stable to oxidation up to 500°C (932°F) can make flexible elastomer. Stable above 900°C (1625°F) fiber resists abrasion with low tenacity. Soluble polymer that decomposes at 385°C (725°F) prepolymer melts above 405° C (76l.°F). Service temperatures up to 288° C (550°F) amenable to fabrication. Thermoplastic use temperature —102°C (—152°F) to greater than 150° C (302°F) acid and base resistant. 

Yu J.M., Dubios P., and Jerome R., Synthesis and properties of polypsobomylmethacrylate (IBMA)-b-butadiene (BD)-b-IBMA] copolymers New thermoplastic elastomers of a large service temperature range. Macromolecules, 29, 7316, 1996. 

TPEs from thermoplastics-mbber blends are materials having the characteristics of thermoplastics at processing temperature and that of elastomers at service temperature. This unique combination of properties of vulcanized mbber and the easy processability of thermoplastics bridges the gap between conventional elastomers and thermoplastics. Cross-linking of the mbber phase by dynamic vulcanization improves the properties of the TPE. The key factor that controls the properties of TPE is the blend morphology. It is essential that in a continuous plastic phase, the mbber phase should be dispersed uniformly, and the finer the dispersed phase the better are the properties. A number of TPEs from dynamically vulcanized mbber-plastic blends have been developed by Bhowmick and coworkers [98-102]. 

Before briefly discussing each type it is necessary to consider the performance of thermoplastic elastomers, and the problem of defining service temperature limits for them. The structural features that convey the ability to be processed as a thermoplastic are also a limiting factor in their use. Since it is the pseudocrosslinks that allow these materials to develop elastomeric behaviour, any factor which interferes with the integrity of the pseudocrosslinks will weaken the material, and allow excessive creep or stress relaxation to occur under the sustained application of stress and strain. Temperature is obviously one such factor. 

The description of the physical properties of fluoroelastomers is necessarily less precise than that of fluoroplastics because of the major effect of adding curatives and fillers to achieve useful cross-linked materials of a given hardness and specific mechanical properties Generally, two parameters are varied increasing cross-link density increases modulus and decreases elongation, and raising filler levels increases hardness and decreases solvent swell because of the decreased volume fraction of the elastomer In addition to these two major vanables, the major determinants of vulcanizate behavior are the chemical and thermal stabilities of its cross-links The selection of elastomer, of course, places limits on the overall resistance to fluids and chemicals and on its service temperature range... 

Some of the conditions used in rubber test methods may need modifying for application to thermoplastic elastomers because of their intrinsic thermoplastic nature. If the temperatures generally used in ageing and compression set tests on thermosetting rubbers were applied to thermoplastic materials they could appear to perform extremely badly. Whether this was significant would depend on the service temperature. Data sheets need to be checked as those for thermoplastic elastomers may have used much lower temperatures that would be found for conventional rubbers, and it is only too easy to get a misleading impression of performance. 

A large variety of elastomers and plastics are currently available for seals in valves. At present, there is no single material suitably resilient to all pressures, temperatures and chemicals. Therefore, each resilient seat application should be selected after considering the specific fluid and service conditions. Where certain materials may be excellent with respect to chemical resistance, they may not be suitable for the intended service temperatures, and vice versa. 

Perfluoroelastomers, such as Kalrez (copolymer of TEE and PMVE), can maintain their thermal stability to temperatures as high as 300°C (572°F) or even higher, with a maximum continuous service temperature of 315°C (599°F). Moreover, instead of hardening, the elastomer becomes more elastic with aging.13... 

Fluorocarbon elastomers, such as copolymers of VDF and HFP, typically have a maximum continuous service temperature of 215°C (419°F). Some metal oxides may cause dehydrofluorination at a temperature of 150°C (302°F) or even lower.16 Copolymers of VDF and CTFE (e.g., Kel-F ) have a maximum long-term service temperature of 200°C (392°F). Fluorocarbon elastomers based on copolymers of VDF/HPFP (hydropentafluoropropylene) and on terpolymers of VDF/HPFP/TFE have lower thermal stability than copolymers of VDF/HFP because they have a lower fluorine content than the latter.17 A detailed study of thermal stability of fluoroelastomers was performed by Cox et al.18... 

Polysulfide resins combine with epoxy resins to provide adhesives and sealants with excellent flexibility and chemical resistance. These adhesives bond well to many different substrates. Tensile shear strength and elevated-temperature properties are low. However, resistance to peel forces and low temperatures is very good. Epoxy polysulfides have good adhesive properties down to -100°C, and they stay flexible to -65°C. The maximum service temperature is about 50 to 85°C depending on the epoxy concentration in the formulation. Temperature resistance increases with the epoxy content of the system. Resistance to solvents, oil and grease, and exterior weathering and aging is superior to that of most thermoplastic elastomers. 

Several types of diisocyanates (aromatic, aliphatic, cyclo aliphatic) and many different glycol-chain extenders (open-chain aliphatic, cyclo aliphatic, aromatic aliphatic) can be used to produce TPU-elastomer hard segments. In the more conventional and practical formulations only a single diisocyanate component is used to make a TPU, so the diisocyanate is common to both the hard and soft segments. The polymer chemist makes his diisocyanate and glycol-chain-extender component selections based on such considerations as desired TPU mechanical properties, upper service temperature, environmental resistance, solubility characteristics, and economics. 

PNB has a glass transition temperature of 35C (95F). This plastic can easily be plasticized with large amounts of oil, subsequently vulcanized into an elastomer with a low service temperature of -65C (-85F). The damping properties of the elastomer can be adjusted to meet different performance requirements. 

This compounded cured elastomer or rubber99 shares with all the other methyl silicone products the common characteristic of exceptional thermal stability. The material does not melt when heated in air at 300° C., which is far above the decomposition temperature of natural rubber or of any of the synthetic organic elastomers. Service over long periods of time at 150° C. does not destroy its elasticity. 

FZ elastomer offers a broad service temperature range, namely, from -65°C to 175°C (-85°F to 347°F) [110], excellent flex fatigue resistance, damping properties, and resistance to chemicals and fluids. 

The thermal stability of fluorocarbon elastomers also depends on their molecular structure. Fully fluorinated copolymers, such as copolymer of TFE and PMVE (Kal-rez), are thermally stable up to temperatures exceeding 300°C (572°E). Moreover, with heat aging this perfluoroelastomer becomes more elastic rather than embrittled. Eluorocarbon elastomers containing hydrogen in their structures (e.g., Viton, Dyneon, and DAI-EL EKM) exhibit a considerably lower thermal stability than the perfluori-nated elastomer. Eor example, the long-term maximum service temperature for FKM... 

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