Diene elastomers Polychloroprene

Diene Types The diene elastomers are based on polymers prepared from butadiene, isoprene, their derivatives and copolymers. The oldest elastomer, natural rubber (polyisoprene), is in this class (see Section 9.2). Polybutadiene, polychloroprene, styrene-butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR) are also in this class. 

The Swiss company, WW Fischer, offers PTFE (Teflon PTFE or Hostaflon), PBT (Celanex, Crastin, Ultradur or Valox) or PEEK (Victrex) insulator material options in its 405 series of cylindrical connectors according to the requirements of working temperature and other criteria. PEEK is an expensive polymer which tends to be employed when other materials fail to meet the specification requirements of the application. Other Fischer connector types use polyamide-imide (Torlon) or POM (Celcon, Delrin or Hostaform). Elastomeric seals used by Fischer in conjunction with their connectors are made from acrylonitrile-butadiene rubber (NBR N BUNA) or to MIL-P-25732, fluoroelastomer (FPM VITON), polychloroprene elastomer (CR Neoprene), ethylene-propylene diene elastomer (EPDM) and styrene-ethylene-butadiene-styrene thermoplastic elastomer (TPE-S or TPE-O) where each compound is followed by its trade name. Fischer s Swiss competitor, Lemo, manufactures a similar range of connectors including the Redel types which have a plastic body. 

The most widely used elastomers are natural rubber [17], synthetic polyisoprene and butadiene rubbers, styrene-butadiene copolymers, ethylene-propylene rubber (specifically EPDM), butyl and halobutyl elastomers, polyurethanes, polysiloxanes, polychloroprenes, nitrile rubber, polyacrylic rubbers, fluorocarbon elastomers, and thermoplastic elastomers [18-20]. The examples which have unsaturation present in the repeat units (such as, the diene elastomers) have the advantage of easy cross-linkability, but the disadvantage of increased vulnerability to attack by reactants, such as oxygen and ozone. 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

Besides butadiene, another important monomer for the synthetic elastomer industry is chloroprene, which is polymerized to the chemically resistant polychloroprene. It is made by chlorination of butadiene follow by dehydrochlorination. As with most conjugated dienes, addition occurs either 1,2 or 1,4 because the intermediate allyl carbocation is delocalized. The 1,4-isomer can be isomerized to the 1,2-isomer by heating with cuprous chloride. 

Many of the synthetic elastomers now made are still polymerized by a free radical mechanism. Polychloroprene, polybutadiene, polyisoprene, and styrene-butadiene copolymer are made this way. Initiation by peroxides is common. Many propagation steps create high molecular weight products. Review the mechanism of free radical polymerization of dienes given in Chapter 14, Section 2.2. 

Elastomers, synthetic-polychloroprene Elastomers, synthetic-ethylene-propylene-diene rubber). Tires, hoses, bdts, molded and extmded goods, and asphalt products consume ca 80% of the redaimed mbber manufactured. Typical properties of reclaimed mbbers are shown in Table 5. 

Elastomers, synthetic -acrylic elastomers [ELASTOMERS, SYNTHETIC - ACRYLIC ELASTOMERS] (Vol 8) -butyl rubber [ELASTOMERS, SYNTHETIC - BUTYL RUBBER] (Vol 8) -chlorosulfonated polyethylene [ELASTOMERS, SYNTHETIC - CHLOROSULFONATED POLYETHYLENE] (Vol 8) -ethylene-acrylic elastomers [ELASTOMERS, SYNTHETIC - ETHYLENE-ACRYLIC ELASTOMERS] (Vol 8) -ethylene-propylene-diene rubber [ELASTOMERS,SYNTHETTC - ETHYLENE-PROPYLENE-DIENE RUBBER] (Vol 8) -fluorocarbon elastomers [ELASTOMERS, SYNTHETIC - FLUOROCARBON ELASTOMERS] (Vol 8) -nitrile rubber [ELASTOMERS, SYNTHETIC - NITRILE RUBBER] (Vol 8) -phosphazenes [ELASTOMERS, SYNTHETIC - PHOSPHAZENES] (Vol 8) -polybutadiene [ELASTOMERS, SYNTHETIC - POLYBUTADIENE] (Vol 8) -polychloroprene [ELASTOMERS, SYNTHETIC - POLYCHLOROPRENE] (Vol 8) -polyethers (ELASTOMERS, SYNTHETIC - POLYETHERS] (Vol 8) -polyisoprene [ELASTOMERSSYNTHETTC - POLYISOPRENE] (Vol 9) -survey [ELASTOMERS, SYNTHETIC - SURVEY] (Vol 8)... 

Elastomers include natural rubber (polyisoprene), synthetic polyisoprene, styrene-butadiene rubbers, butyl rubber (isobutylene-isoprene), polybutadiene, ethylene-propylene-diene (EPDM), neoprene (polychloroprene), acrylonitrile-butadiene rubbers, polysulfide rubbers, polyurethane rubbers, crosslinked polyethylene rubber and polynorbomene rubbers. Typically in elastomer mixing the elastomer is mixed with other additives such as carbon black, fillers, oils/plasticizers and accelerators/antioxidants. 

Commercial samples of l,4-c/5 -polybutadiene (SKD, E-BR) polybutadiene (Diene 35 NFA, BR) l,4-c/5 -polyisoprene (Carom IR 2200, E-IR), and polychloroprene (Denka M 40, PCh) were used in the experiments (Table 10.1). The 1,4-irara-polyisoprene samples were supplied by Prof A. A. Popov, Institute of Chemical Physics, Russian Academy of Sciences. All rubbers were purified by three-fold precipitation from CCl solutions in excess of methanol. The aforementioned elastomer structures were confirmed by means of H-NMR spectroscopy. Ozone was prepared by passing oxygen flow through a 4-9 kV electric discharge. 

Although all of these attempts had a noble purpose indeed, the means used could hardly be considered a contribution to science, as the transformation of the simple molecules of a diene into the colloidal substance known as rubber was then far beyond the comprehension of chemical science. As a matter of fact, the commercial production of synthetic rubber was already well established, at least in Germany and Russia, before Staudinger laid the basis for his macromolecular hypothesis during the 1920s (Staudinger, 1920). Even such relatively modern synthetic elastomers as polychloroprene and the poly(alkylene sulfides) were... 

Though natural rubber, SBR, and BR represent the largest consumption of elastomers, several additional polymers merit a brief discussion because of their economic significance—nitriles, polychloroprene, butyl, and ethylene-propylene-diene monomer (EPDM) elastomers (Datta, 2004 The Synthetic Rubber Manual, 1999). 

It is proposed that this is due to attack of carbonyl oxides, in their biradical form, on the rubber double bonds. Typical diene rubbers (polyisoprene and polybutadiene) have rate constants several orders of magnitude greater than polymers having a saturated backbone (polyolefins). Other unsaturated elastomers having high reaction rates with ozone include styrene-butadiene (SBR) and acrylonitrile-butadiene (NBR) rubbers. As an example, Polychloroprene (CR) is less reactive than other diene rubbers, and it is therefore inherently more resistant to attack by ozone. 

Examples of vulcanizable elastomers include natural rubber (NR), styrene butadiene rubber (SBR), butadiene rubber (BR), ethylene-propylene-diene monomer-rubber (EPDM), butyl rubber (HR), polychloroprene or neoprene (CR), epichlorohydrin rubber (ECO), polyacrylate rubber (ACM), millable polyurethane rubber, silicone rubber, and flu-oroelastomers. Examples of thermoplastic elastomers include thermoplastic polyurethane elastomers, styrenic thermoplastic elastomers, polyolefin-based thermoplastic elastomers, thermoplastic polyether-ester (copolyester) elastomers, and thermoplastic elastomers based on polyamides. 

Both natural and synthetic rubbers are used as elastomeric linings. The most commonly used synthetic elastomers are NBR (acrylonitrile-butadiene), Hypalon (chlorosulfonated polyethylene), EPDM (ethylene-propylene-diene monomer), EPT (ethylene-propylene-diene terpolymer), SBR (styrene-butadiene), and neoprene (polychloroprene). A maximum use temperature of nS F/SOX is typical. 

Synthetic rubbers are produced as commodities. Polybutadiene, polybutylene, polychloroprene and polyepichlorohydrin are examples of elastomeric homopolymers. Copolymeric rubbers comprise poly-(butadiene-co-styrene), poly(butadiene-co-acryloni-trile), poly(ethylene-co-propylene-co-diene), and poly-(epichlorohydrin-co-ethylene oxide). The unsaturated group in the comonomer provides reactive sites for the crosslinking reactions. Copolymers combine resilience with resistance to chemical attack, or resilience in a larger temperature range, and thermoplastic-like properties. There are several studies in the literature describing the preparation of blends and composites of elastomers and conductive polymers. A description of some significant examples is given in this section. 

Polychloroprene, nitrile, natural rubber (polyisoprene), styrene butadiene rubber (SBR) and butyl are amongst the types of rubber that can be readily bonded with cyanoacrylates. Ethylene propylene diene monomer (EPDM) and fluroelastomers (Viton, registered trade mark of DuPont) can also be bonded, although only with specific grades of cyanoacrylate. Silicone rubber and thermoplastic rubber (Santoprene, registered trade mark of Advanced Elastomer Systems) can be bonded with the aid of a primer. Typical applications and techniques for bonding different grades of rubber are discussed in Section 10.11. 

When X H, the resulting pol)mier is polybutadiene when XsCHj, it is polyisoprene and when X = C1, it is polychloroprene. The double bond may be cis or trans and would thus give the cis or trans forms of these polymers. It is the 1,4 addition form that predominantly goes into the formation of commercial dienes which are all elastomers. 

Tg measurements have been performed on many other polymers and copolymers including phenol bark resins [71], PS [72-74], p-nitrobenzene substituted polymethacrylates [75], PC [76], polyimines [77], polyurethanes (PU) [78], Novolac resins [71], polyisoprene, polybutadiene, polychloroprene, nitrile rubber, ethylene-propylene-diene terpolymer and butyl rubber [79], bisphenol-A epoxy diacrylate-trimethylolpropane triacrylate [80], mono and dipolyphosphazenes [81], polyethylene glycol-polylactic acid entrapment polymers [82], polyether nitrile copolymers [83], polyacrylate-polyoxyethylene grafts [84], Novolak type thermosets [71], polyester carbonates [85], polyethylene naphthalene, 2,6, dicarboxylate [86], PET-polyethylene 2,6-naphthalone carboxylate blends [87], a-phenyl substituted aromatic-aliphatic polyamides [88], sodium acrylate-methyl methacrylate multiblock copolymers [89], telechelic sulfonate polyester ionomers [90], aromatic polyamides [91], polyimides [91], 4,4"-bis(4-oxyphenoxy)benzophenone diglycidyl ether - 3,4 epoxycyclohexyl methyl 3,4 epoxy cyclohexane carboxylate blends [92], PET [93], polyhydroxybutyrate [94], polyetherimides [95], macrocyclic aromatic disulfide oligomers [96], acrylics [97], PU urea elastomers [97], glass reinforced epoxy resin composites [98], PVOH [99], polymethyl methacrylate-N-phenyl maleimide, styrene copolymers [100], chiral... 

Sircar and co-workers [8] compared experimental and data from the literature for the Tg of some common elastomers determined by different thermal analysis techniques, including DSC, TMTA, DMTA, dielectric analysis and thermally stimulated current methods. Elastomers examined include natural rubber, styrene-butadiene rubber, polyisoprene, polybutadiene, polychloroprene, nitrile rubber, ethylene-propylene diene terpolymer and butyl rubber. Tg values obtained by DSC, TMA and DMTA were compared. Experimental variables and sample details, which should be included along with Tg data were described, and the use of Tg as an indication of low temperature properties was discussed. 

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