Other elastomers

This field has been reviewed by Parkinson [426]. Common hydrocarbon elastomers (with the exception of butyl rubber), nitrile and acrylic rubbers were found to be as resistant to radiation as silicones, fluorocarbon and polyurethane rubbers. 

In this chapter brief consideration has been given to the major tonnage mbbers. Derivatives of natural mbber such as ebonite are discussed in Chapter 30 and thermoplastic rubbers are reviewed in Chapter 31. Other important speciality mbbers (with their ASTM designations) include  

Ethylene-vinyl acetate mbbers (EAM)—this chapter Ethylene-acrylate mbbers (AEM)—this chapter Fluorombbers (FKM, CFM, FFKM, FZ, AFMU etc.)—Chapter 13 Silicone and fluorosilicone mbbers (MQ, VMQ, PMQ, PVMQ and FVMQ)— Chapter 29 

Epichlorhydrin rubbers (CO, ECO and AECO)—Chapter 19 Propylene oxide mbber (GPO)—Chapter 19 

310 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers 

Chlorinated (CM) and chlorosulphonated polyethylenes (CSM and ACSM)— this chapter 

Polynorbomene (PNR), polyoctenamer and hydrogenated nitrile rubber (HNBR)—this chapter 

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Nacconate 100 A lachrymatory liquid b.p. 25l°C. Manufactured from phosgene and 2,4-diaminotoJuene. Used for preparing polyurethane foams and other elastomers by reaction with polyhydroxy compounds. Produces skin irritation and causes allergic eczema and bronchial asthma. 

Polyacrylate elastomers find limited use in hydrauhc systems and gasket apphcations because of their superior heat resistance compared to the nitrile mbbers (219,220). Ethylene—acrylate copolymers were introduced in 1975. The apphcations include transmission seals, vibration dampers, dust boots, and steering and suspension seals. Further details and performance comparisons with other elastomers are given in reference 221 (see also Elastomers, SYNTHETIC-ACRYLIC ELASTOTffiRS). 

Latex mbber foams are generally prepared in slab or molded forms in the density range 64—128 kg/m (4—8 lbs/fT). Synthetic SBR latexes have replaced natural mbber latexes as the largest volume raw material for latex foam mbber. Other elastomers used in significant quantities are polychloroprene, nitrile mbbers, and synthetic i j -polyisoprene (115). 

One method (116) of producing cellular polymers from a variety of latexes uses primarily latexes of carboxylated styrene—butadiene copolymers, although other elastomers such as acryUc elastomers, nitrile mbber, and vinyl polymers can be employed. 

Rubbers. Plasticizers have been used in mbber processing and formulations for many years (8), although phthaHc and adipic esters have found Htde use since cheaper alternatives, eg, heavy petroleum oils, coal tars, and other predominandy hydrocarbon products, are available for many types of mbber. Esters, eg, DOA, DOP, and DOS, can be used with latex mbber to produce large reductions in T. It has been noted (9) that the more polar elastomers such as nitrile mbber and chloroprene are insufficiendy compatible with hydrocarbons and requite a more specialized type of plasticizer, eg, a phthalate or adipate ester. Approximately 50% of nitrile mbber used in Western Europe is plasticized at 10—15 phr (a total of 5000—6000 t/yr), and 25% of chloroprene at ca 10 phr (ca 2000 t/yr) is plasticized. Usage in other elastomers is very low although may increase due to toxicological concerns over polynuclear aromatic compounds (9). 

It is estimated that 27,000 t/yr of CSM have been commercially used in the United States. However, due to environmental problems in the manufacturing process, it has been necessary to develop a process that is much mote expensive. As a result many companies using CSM ate trying to replace the CSM with CPE or other elastomers. The result is a decline in the usage of this polymer. Chlorosulfonated polyethylene is sold under the trade name Hypalon (DuPont—Dow Company). 

Fluoroelastomers excel compared to aU other elastomers in heat, chemical, flame, weathering, fuel, and o2one resistance. In addition oU, oxygen, and water resistance ate very good. The fluoroelastomers, however, ate attacked by amines and some highly polar solvents. The abrasion resistance and low temperature properties ate adequate for most appHcations. 

Ozonc-rcsjstant elastomers which have no unsaturation are an exceUent choice when their physical properties suit the appHcation, for example, polyacrylates, polysulfides, siHcones, polyesters, and chlorosulfonated polyethylene (38). Such polymers are also used where high ozone concentrations are encountered. Elastomers with pendant, but not backbone, unsaturation are likewise ozone-resistant. Elastomers of this type are the ethylene—propylene—diene (EPDM) mbbers, which possess a weathering resistance that is not dependent on environmentally sensitive stabilizers. Other elastomers, such as butyl mbber (HR) with low double-bond content, are fairly resistant to ozone. As unsaturation increases, ozone resistance decreases. Chloroprene mbber (CR) is also quite ozone-resistant. 

The bulk viscosity control parameter for CSM, as with other elastomers, is molecular weight M and molecular-weight distribution (MWD). Mooney viscosity for CSM is determined by selection of the polyethylene precursor. 

Peroxide curing systems are generally the same for CSM as for other elastomers but large amounts of acid acceptor must be present to complete the cure. A small amount of a polyfunctional alcohol, ie, pentaerythritol (PER) in the compound significantly reduces the amount of base required by acting as a solubiHzer. TriaHyl cyanurate [101-37-17 is an additional cure promoter and leads to higher cross-link density. 

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 ...

Mixing. Ethylene—acrylic elastomers are processed in the same manner as other elastomers. An internal mixer is used for large-scale production and a mbber mill for smaller scales. In either case, it is important to keep the compound as cool as possible and to avoid overmixing. Ethylene—acryflc elastomers require no breakdown period prior to addition of ingredients. Mixing cycles for a one-pass mix are short, typically 2.5—3.5 min. When compounds are mixed on a mbber mill, care should be taken to add the processing aids as soon as possible, after the polymer has been banded on the mill. Normal mill mixing procedures are followed otherwise. 

Nitiile rubber presents no unusual hazards when processed and handled in areas with good housekeeping and ventilation. Like other elastomers, nitrile... 

The development of the mechanical seal advanced in parallel with elastomer technology. Mechanical seals using o-rings and other elastomer forms, benefited with improved shelf-life, maximum and minimum temperature limits, better chemical resistance, and higher pressure ratings. 

Misalignment - O-rings can compensate for some misalignment in the seal chamber face and bore better than any other elastomer configuration. 

Because the polybutadiene component is liable to oxidation, ABS materials are embrittled on prolonged exposure to sunlight. By replacing polybutadiene rubber with other elastomers that contain no main chain double bonds it has been possible to produce blends generally similar to ABS but with improved weathering resistance. Three particular types that have achieved commercial status are ... 

NBR is a polar polymer and shows superior compatibility with resins as compared to other elastomers. The major ingredients, in addition to the elastomer, for the NBR adhesives are given below. 

Although pressure-sensitive and hot-melt adhesives can be manufactured, they are not common because lower-cost formulations using other elastomers can be prepared. 

The properties of the solvent-bome CR adhesives depend on the molecular weight, degree of branching and rate of crystallization of the polymer. The ability of polychloroprene adhesives to crystallize is unique as compared to other elastomers. The higher the crystallization rate, the faster the adhesive strength development. 

Ceramic materials have found acceptance as cyclone liners for the apex orifice as well as other areas which exhibit severe abrasion. These include areas such as the lower cone liner and vortex finder. Nihard has also proven to be an acceptable wear material, especially for vortex finders and other areas which require strength as well as abrasion resistance. Other elastomer materials such as neoprene and nitrite are also utilized when hydrocarbons are present or when the temperature exceeds 60 °C. Urethane has found acceptance, especially in areas where the solids are relatively fine,... 

Thermoplastic polyurethane (TPU) is a type of synthetic polymer that has properties between the characteristics of plastics and rubber. It belongs to the thermoplastic elastomer group. The typical procedure of vulcanization in rubber processing generally is not needed for TPU instead, the processing procedure for normal plastics is used. With a similar hardness to other elastomers, TPU has better elasticity, resistance to oil, and resistance to impact at low temperatures. TPU is a rapidly developing polymeric material. 

Excepting polyurethanes which are designed solely for chemical resistance, most abrasion-resistant polyurethanes do not have the chemical resistant capabilities of the other elastomers. Chemical-resistant polyurethanes have elevated temperature limitations, but have proved to have an adequate resistance range especially when contamination by oils and certain solvents have to be handled. 

As previously stated, polyurethanes do not have the degree of chemical resistance enjoyed by the other elastomers. Specially designed chemical resistant polyurethanes are suitable for use in dilute non-oxidative acids and salts, but are not normally suitable for alkalis. They show good resistance to oils and solvents. Maximum temperature of use is usually about 80°C, but this temperature is very dependent on the chemical environment. 

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