Cure Systems for Polychloroprene

Cure systems for nitrile rubber are somewhat analogous to those of NR, SBR or BR except that magnesium carbonate (MC) treated sulfur is usually used to aid sulphur dispersion into the polymer (a. 12). Common accelerator systems include thiazole, thiuram, thiazole/thiuram, or sulfonamide/ thiuram types. Examples of these systems are shown in Table 18. 

As operating requirements for nitrile rubber become more stringent, improved ageing and set properties become important. In order to address those requirements, formulations with sulfur donor systems (as a partial or total replacement of rhombic sulfur) have been proposed. These are summarised in Tables 19 and 20. The advantages of these systems are improved set and ageing resistance while adequate processing safety and fast cures are maintained. 

Chlorobutadiene or chloroprene rubbers (CR), also called neoprene rubbers, are usually vulcanised by the action of metal oxides. The crosslinking agent is usually zinc oxide in combination with magnesium oxide (a. 16). CR can be vulcanised in the presence of zinc oxide alone, but magnesium oxide is necessary to confer scorch resistance. The reaction may involve the allylic chlorine atom, which is the result of the small amount of 1,2 polymerisation (a.l7). 

Recipe Medium acrylonitrile NBR 100 N-550 40 N-770 40 plasticiser di(2-ethylhexyl)phthalate (DOP) 15 zinc oxide 5 stearic acid 1 TMQ 1 6PPD 2  

Most accelerators used in the sulfur vulcanisation of other high diene rubbers are not applicable to the metal oxide vulcanisation of CR. An exception is the use of a so-called mixed curing system for CR, in which metal oxide and accelerated sulfur vulcanisation are combined. Along with the metal oxides, TMTD, DOTG and sulfur are used. This is a good method to obtain high resilience and dimensional stability. 

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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... 

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. 

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. 

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). 

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. 

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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