Polychloroprene heat resistance

Interpenetrating networks have been made by co-curing polychloroprene with copolymers of 1-chloro-1,3-butadiene [627-22-5]. The 1-chloro-1,3-butadiene serves as a cure site monomer, providing a cure site similar to that already in polychloroprene. The butadiene copolymer with 1-chloro-1,3-butadiene (44) and an octyl acrylate copolymer (45) improved the low temperature brittieness of polychloroprene. The acrylate also improved oil resistance and heat resistance. 

If tertiary chlorine atoms are indeed critical to heat resistance, then reactions that consume them should improve polymer stabiUty. This is indeed the case. Post-reaction of polychloroprene with dodecyl mercaptan (111), use of higher levels of ethylene thiourea for curing (112), and inclusion of reactive thiols such as mercaptobenzimidazole in cure systems (113) all improve heat resistance. This latter technique is especially effective in improving the heat resistance of mercaptan modified polychloroprene. 

Two propylene oxide elastomers have been commercialized, PO—AGE and ECH—PO—AGE. These polymers show excellent low temperature flexibihty and low gas permeabihty. After compounding, PO—AGE copolymer is highly resiUent, and shows excellent flex life and flexibiUty at extremely low temperatures (ca —65°C). It is slightly better than natural mbber in these characteristics. Resistance to oil, fuels, and solvents is moderate to poor. Wear resistance is also poor. Unlike natural mbber, PO—AGE is ozone resistant and resistant to aging at high temperatures. The properties of compounded ECH—PO—AGE he somewhere between those of ECH—EO copolymer and PO—AGE copolymer (22). As the ECH content of the terpolymer increases, fuel resistance increases while low temperature flexibihty decreases. Heat resistance is similar to ECH—EO fuel resistance is similar to polychloroprene. The uncured mbber is soluble in aromatic solvents and ketones. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

New Neoprene M- and XD grades. These polychloroprenes were developed in the 1990s and combine low temperature flexibility, improved heat resistance and dynamic properties. 

Resistance to weathering. Zinc oxide and magnesium oxide stabilize poly-chloroprene against dehydrochlorination. Further, zinc oxide helps vulcanize the rubber, and magnesium oxide reacts with /-butyl phenolic resin to produce a resinate which improves heat resistance of solvent-borne polychloroprene adhesives. 

Formulation of a solvent-borne CR. A typical formulation of a solvent-borne CR adhesive may include the following components (fillers are not commonly added and curing agents are added to improve heat resistance) (1) polychloroprene elastomer (2) metal oxides (3) resins (4) antioxidants (5) solvents (6) fillers (7) curing agents (8) other modifiers. 

Little comment can be made on the uses of this material except that it is used in some mouldings where the advantages of heat resistance, low temperature performance and oil resistance, roughly equivalent to that of polychloroprene, can be utilised. It has been investigated for use in engine mounts and transmission belting. 

Neoprene is the generic name for polychloroprene rubber. It has been produced commercially since 1931 and had rapid and wide acceptance because it is much superior to natural rubber for heat and oil resistance. Heat resistance is far better than NR, BR or SBR. but less than EPDM. When heated in the absence of air, neoprene withstands degradation better than other elastomers which are normally considered more heat resistant, and retains its properties fifteen times longer than in the presence of air. Compression set at higher temperature is better than natural rubber and 100°C is typically the test temperature rather than 70°C. Abrasion resistance is not as good as natural rubber but generally better than most heat resistant and oil resistant rubbers. This is also true for tear strength and flex resistance. 

Chloroprene rubber (Neoprene—trade name of DuPont) was one of the earliest synthetic rubbers, first commercialized in 1932. It has a wide range of useful properties but has not become a true general purpose synthetic rubber, probably because of its cost. It does possess properties superior to those of a number of general purpose polymers, such as oil, ozone, and heat resistance but for these properties other specialized polymers excel. Polychloroprene thus is positioned between the general purpose elastomers and the specialty rubbers. 

Contact adhesives can also be processed in the form of aqueous dispersions, whereby the longer time required for evaporation of the water must be taken into account. These contact adhesives are also more sensitive to moisture. Typical examples of contact adhesives are solutions of polychloroprene (CR) (e.g., Baypren firom Bayer AG, Neoprene from DuPont). The solvents are xylene, benzine, cyclohexanone, ethyl acetate, or methylene chloride. CR is characterized by a pro-notmced crystallization behavior with a crystallite melting temperature of 53 C. The crystallinity is controlled in accordance with the specific formula in each case (adhesive contains Zn and Mg oxides that result in partial crosslinkage of chlorine to carbon with double bonds). The adhesive strength and heat resistance increase with the degree of crystallinity, UV stability and flexibility are very high. 

Curing agents Curing agents are generally added to CR adhesive formulations to increase heat resistance. Thiocarbanihde and polyisocyanates can be used as curing agents. The reaction of an isocyanate with polychloroprene, which leads to improved heat resistance property, has not been fiiUy explained. 

The development of two new polychloroprene latices, designated Dispercoll C LS-2324 and Dispercoll C LS-2373, which are suitable for the manufacture of waterborne contact adhesives, is reported. Examples are presented of formulations in which blends of these polychloroprene latices are used to optimise initial green strength, contact bond life and heat resistance. 

Polychloroprenes give vulcanizates which are broadly similar to those of natural rubber in physical strength and elasticity. (See Table 18.1.) However, the polychloroprenes show much better heat resistance in that these physical properties are reasonably well maintained up to about 150°C in air. The heat resistance and compression set resistance of the G type polymers are inferior to those of the W type polymers due to the presence of labile polysulphide bonds. (On the other hand, for the same reason, the G types are easier to process.) As might be expected from the highly regular structure of polychloroprene, normal grades readily crystallize and become stiff when cooled below -10°C. 

Isocyanates can be added to solvent-bome polychloroprene adhesive solutions as two-part adhesive systems. The reaction of an isocyanate with polychloroprene that leads to improved heat resistance property has not been fully explained as there are no active hydrogen atoms in the polychloroprene to allow reaction with isocyanate group. This two-part adhesive system is less effective with mbber substrates containing high styrene content and for butadiene-styrene block (thermoplastic rubber) copolymers. To improve the specific adhesion to those materials, addition of a poly-alpha-methylstyrene resin to solvent-bome polychloroprene adhesives is quite effective (Tanno and Shibuya 1967). 

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