Vulcanization steps

Labile Chlorine Containing Monomers. Chlorine is introduced in the acryhc elastomer chain by analogy to polychloroprene (19). The monomers are characterized by the simultaneous presence of a double bond available for polymerization with acrylates and a chlorine atom ready to react easily during the vulcanization step. The general formula is as follows where R is a group that might enhance the reactivity of the double bond and/or of the vicinal chlorine atom. 

The shaping and vulcanization steps are combined in a number of processes such as transfer or injection molding or may be separated as in the extrusion and subsequent vulcanization sequence. An outline of these steps is given in Figure 18.4. 

Another cure system consideration is the compound scorch behavior. Prior to vulcanization, rubber is plastic-like and can be processed into desired shapes such as tires, hoses, belts, or other articles. The time available to accomplish this processing depends laigely on the cure system and is referred to as the scorch time. If a compound cures prematurely during the processing step, it usually becomes useless scrap. Therefore, a key requirement of the vulcanization step is to minimize premature vulcanization or scorch (Fig. 4). 

As with all thermoplastic elastomers, the copolyesterethers can be processed as thermoplastics. They are linear polymers and contain no chemical cross-links, thus the vulcanization step needed for thermosetting elastomers is eliminated and scrap elastomer can be re-used in the same process as virgin material (176—180). 

It should be noted that all elastomers, prior to the vulcanization step, are compounded (mixed) with a reinforcing agent such as carhon hlack or silica (in addition to the crosslinking agents) in industrial applications. The reinforcing agents increase the strength and elastomeric properties of the final crosslinked product (often referred to as a vulcanizate). 

As for pure elastomers, the vulcanization step provides sulfur bridges between elastomeric chains and connects them into an infinite network. Vulcanization is supposed to be mainly unaffected by the presence of the reinforcing fillers and transforms the roughly plastic green mixes into viscoelastic vulcanizates. 

For silica mixes, the high temperature of the vulcanization step allows the reaction of the polysulfidic moiety of the coupling agent with elastomer, ensuring the chemical covalent bonding of polymer to silica surface. 

Powder and liquid rubbers simplify the working in of additives, but these rubbers still require a laborious chemical vulcanization procedure. Irreversible cross-link points are produced by chemical cross-linking the cross-linking can only be eliminated by destroying the primary polymer molecules. The waste produced in the vulcanization step can therefore not be reworked as rubber. 

Carboxylic nitrile liquid and solid elastomers are used to prepare elastomer-modified epoxy liquid and solid resins when it is desirable to have the modifier in the epoxy portion of the system. This is effected through alkyl-hydroxy esterification reactions. This is covered in the literature for uncatalyzed liquid resins (9), for tert-amine catalyzed systems (10), for tert-phosphine catalyzed systems (11), for low molecular weight solid resins advanced from the liquid state (12) and for liquid and solid resins where an additional rubber vulcanization step is carried out in addition to the alkylhydroxy ester adducting step (13). Such adduct preparations offer formulation stability with a wide range of hardener types amines, anhydrides, catalytic, Lewis acids/bases. 

The production route is exactly the same as in the one-step process, with the exception of introducing the pre-vulcanization step immediately after the thermoforming of NR. In this step, temperature and time were set up respectively to 140 °C for 5 min. 

Step 2, reaction with polymer, occurs in the vulcanization step (with heat, after curatives have been added in a lower temperature stage addition). Figure 9 shows the two-step coupling mechanism in the general case. 

Especially in tire building (and some other applications as well) a tackifier must be used for the rapid, facile assembly of the components of the rubber article prior to the vulcanization step. The production of a rubber article with decreased bulk density requires the use of a blowing agent that will generate gases at vulcanization temperature. Flame retardants are commonly added to a rubber compound to retard burning under service conditions. 

The most viable recyclable alternatives to traditional rubber materials are thermoplastic elastomers (Section 15.19). Being thermoplastic in nature, they are not chemically crosslinked and, thus, are easily reshaped. Furthermore, production energy requirements are lower than for the thermoset rubbers because a vulcanization step is not required in their manufacture. 

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