Polychloroprene cure systems

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. 

Curing Systems. Polychloroprene can be cured with many combiaations of metallic oxides, organic accelerators, and retarders (114). The G family of polymers, containing residual thiuram disulfide, can be cured with metallic oxides alone, although certain properties, for example compression set, can be enhanced by addition of an organic accelerator. The W, T, and xanthate modified families require addition of an organic accelerator, often ia combination with a cure retarder, for practical cures. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

Thermoplastic or thermosetting While many contact bond applications require no curing process because an extra strength requirement is not present, in certain formulations polychloroprene will provide ambient cure for improved properties, and can be cured by several different mechanisms for high performance properties. Ambient cure systems are typically one component, while high performance formulations are often two-part systems, or one-component systems cured in elevated temperature conditions. 

Several rubbers may be crosslinked using divalent metal oxides, usually zinc oxide. There are a limited number of polymers that utilise this method, which is used with halogenated polymers such as polychloroprene [8], chloro- and bromobutyl, and chlorosulfonated polyethylene and carboxylated nitrile rubbers. The system may utilise the metal oxide alone or in combination with the organic accelerators used with sulfur-curing systems. In the case of halogenated polymers, magnesium oxide may be added to act as an acid scavenger. 

Generally, the remaining 10% of ruhher compounds have cure systems based mostly on peroxide curatives. However, a small number of compounds based on halogen-ated elastomers (such as polychloroprene) have cure systems based on metal oxides. Also, resin cures are used in special cases to cure some compounds such as curing bladders for tires. 

The saturated backbone of CPE imparts outstanding ozone-, oxidative-, and heat-resistance to a compound s performance [4]. The inherent nature of the polymer backbone allows compounds of CPE to be formulated that meet stringent high heat requirements, for example, up to 150°C for certain automotive applications and 105°C for various wire and cable applications using a peroxide cure system [5]. CPE typically provides better heat-aging resistance than polymers containing backbone unsaturation, for example, natural mbber and polychloroprene (CR) (Figure 8.2). 

Compounds of either type of colour must contain a non-discolouring type antioxidant (particularly Antioxidant 2246), which will inhibit discolouration in sunlight, and must avoid the use of aromatic oils in favour of organic ester plasticisers. These compounds are normally made from mercaptan modified types of polychloroprene which do not darken during cure, provided the curing system is free from sulphur or lead oxide. 

Brass or zinc coated steel may be bonded directly to polychloroprene compounds provided that a sulphur (1 5 phr) based curing system is used. Higher and more consistent bond strengths may be obtained by adding 5 phr of the cobalt complex Manobond C to the compound. 

Rubber Chemistry and Technology 55,No.4,Sept./Oct. 1982,p.949-60 DEVELOPMENT OF SYNERGISTIC CURING SYSTEMS FOR POLYCHLOROPRENE... 

Of the dihydric phenols, catechol exhibited a significant synergistic accelerating effect on joint use with TMTD in curing systems for polychloroprene. Ageing... 

Most Rubber-based adhesives may be cured by a sulphur-based vulcanizing system (see Rubber-based adhesives compounding), however, as mentioned in Polychloroprene rubber adhesives applications and properties, CR adhesives are cross-linked by various reactions involving the labile chlorine atoms in the repeat unit. This is reflected in the additives used, as discussed below. ... 

Examples of the use of blocked diisocyanates for rubber-fabric adhesion are as follows vulcanized polychloroprene and SBR can be adhered strongly to nylon and polyester fibre fabric by means of aqueous adhesive systems (Table 8.4). This combination is spread or roller coated on to the fabric which is then allowed to dry. Bonds to sheet rubber stock can be made immediately after the treated fabric is dried or at any time thereafter. When the sheet rubber is applied it should be held under moderate pressure to provide intimate contact with the treated fabric and to prevent lifting if any gases are emitted during cure. Press cures of 20-40 min at 140°C are sufficient to cure the adhesive and most elastomer compositions being adhered. If a latex film is applied to the treated fabric, the assembly can be cured in a hot-air oven at 120"C. A chemical bond results between fabric and the diphenylmethane-/7,/ -diisocyanate generated on the thermal cleavage of the blocked diisocyanate. 

With the sulphur-modified polymers cure may be brought about by zinc oxide and magnesium oxide in combination either alone or together with an accelerator such as ethylene thiourea. In the case of the homopolymers it has been common practice to support the zinc oxide/magnesium oxide/ethylene thiourea system with a further component. This component consists of a sulphide or a blend of sulphides of the type more commonly used as accelerators for the diene hydrocarbon polymers. These include mercaptobenzothiazole disulphide (MBTS), diorthotolyl guanidine (DOTG) and tetramethyl thiuram monosulphide (TMTM). In the polychloroprene homopolymers these materials appear to act as retarders of cure at processing temperatures but are accelerators at vulcanization temperatures. Their mechanism does not appear to have been fully elucidated. 

The halogenated butyl rubbers may be cross-linked via either allylic hydrogen or the halogen group. As a broad generalization it may thus be considered that the materials may not only be cured by the methods used with butyl rubber but also those employed with polychloroprenes. The main systems to consider are ... 

Polychloroprene is commercially produced by emulsion polymerization and exists as a one- and two-component system. The one-component polychloroprene sets to touch in 4-6 hours and cures slowly (3-7 daysj.t The two-component system is a solvent-free, non-shrinking, slump resistant sealant which sets quickly, but cures slowly Polychloroprene sealants have poor color retention (usually ate pigmented with black pigments), and high shrinkage. 

Curatives, The function of curatives is to cross-link the polymer chains into a network the most common ones are the sulfur type for unsaturated rubber and peroxides for saturated polymers. Chemicals called accelerators may be added to control the cure rate in the sulfur system these materials generally are complex organic chemicals containing sulfur and nitrogen atoms. Stearic acid and zinc oxide usually are added to activate these accelerators. Metal oxides are used to cure halogenated polymers such as polychloroprene or chlorosulfonated polyethylene. 

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