Vulcanization elastomers

Initially, vulcanization was accomplished by heating elemental sulfur at a concentration of 8 parts per 100 parts of rubber (phr) for 5 h at 140°C. The addition of zinc oxide reduced the time to 3 h. Accelerator in concentrations as low as 0.5 phr have since reduced time to 1-3 min. As a result, elastomer vulcanization by sulfur without accelerator is no longer of commercial significance. An exception is the use of about 30 or more phr of sulfur, with httle or no accelerator, to produce molded products of hard mbber called ebonite. 

M.C. Kirkham, Current Status of Elastomer Vulcanization, in Progress of Rubber Technology, Vol. 41, Plastics and Rubber Institute, London, 1978. 

Although the vulcanization mechanisms are well established for other elastomers, vulcanization of CR is a complicated process and has not been, until now, well understood. To our knowledge, there are only a few studies that discuss the vulcanization chemistry of CR [87]. The vulcanization mechanism for CR proposed in these pioneering studies was considered to be a planar three-component reaction mechanism in which the structural and special peculiarities were fully taken into consideration. Here, we propose an explanation of the chemical coupling mechanism between the CR matrix and electron-modified PTFE powder based on the similar vulcanization mechanism of CR. Such a mechanism can also be applied to vulcanization of CR in the presence of a ethylenethiourea compound. 

Rubber - An elastomer, generally implying natural rubber, but used loosely to mean any elastomer, vulcanized and unvulcanized. By definition, a material that is capable of recovering from large deformations quickly and forcibly. 

Figure 4.2 Structure of silicones (a) silicone fluid (Dow Coming 360 Medical Fluid) (b) silicone foam elastomer (c) silicone elastomer (vulcanized Silastic 382 Medicalgrade Elastomer) and (d) silicone elastomer (vulcanized Silastic Medical Adhesive Type A)...

Elastomers Vulcanization accelerator Notural and synthetic rubber 411-415... 

I elastomer vulcanization thermoplastics cross-linking I curing of coatings I grafting... 

The technology of controlled crosslinking during reactive extrusion has been surveyed by Brown (1992) and there have been many reviews of the technology of elastomer vulcanization (Brydson, 1988, Morrison and Porter, 1989). 

Crosslinking Perfluorophenoxy-Containing Elastomers. Vulcanization of the perfluoroelastomer containing a phenoxy group may be carried out several ways. One method uses diamines such as p-phenyl-enediamine, tetraethylenepentamine, or hexamethylenediamine carbamate (Du Pont DIAK No. 1) (3). A typical recipe is shown in Figure 8. 

Here again is a formulation compromise between the brittle, high-temperature characteristics of the phenolic resin and the rubbery, low-temperature performance of the elastomer. Note also that the other ingredients are the type of material combination that is normally associated with nitrile elastomer vulcanization. 

Some properties of sodium and zinc vulcanizates are compared with those of the acid precursor in Table 5.16. High tensile strength is a characteristic of ionic vulcanizates. As a comparison, a standard polybutadiene elastomer vulcanized with sulfur gives a strength of 1.9-5.8 MPa (275-841 Ibf/in. ), whereas an equivalent copolymer containing 1.5 mole% methacrylic acid, and vulcanized with... 

Synonyms BIIR Brominated butyl rubber Brominated isobutylene/isoprene copolymer Bromobutyl rubber 1,3-Butadiene, 2-methyl-, polymer with 2-methyl-1-propene, brominated Butyl rubber, brominated Isobutylene, isoprene polymer, brominated Definition Elastomer vulcanized by sulfur systems vulcanizate offers low gas permeability, good weather/ozone resist., better chem./heat resist, than butyl rubber... 

Definition Elastomer vulcanized by peroxide crosslinking vulcanizates have good chem./weather/flame resist. 25-40% chlorine content... 

Definition Elastomer vulcanized by sulfur or peroxide produces very soft, but strong, rubber goods... 

Eight types of dmometers are described in the standard, including A, B, C, D, DO, O, OO, and M." The dirrometers are used for determination of indentation hardness of thermoplastic elastomers, vulcanized rabber, and elastomeric and cellular materials. Specific... 

In contrast, thermoplastic elastomers vulcanize by a physical cross-linking, that is, by formation of hard domains in a soft matrix. Here, hard and soft refer to glass transition temperatures relative to application temperatures. The properties of these thermoplastic elastomers follow directly from their structures. All thermoplastic elastomers (TPEs, plastomers) are copolymers with long sequences of hard and soft blocks. They can be block polymers, segment polymers, or graft polymers. 

Thermoplastic elastomers (TPEs) exhibit properties characteristic of chemically cross-linked elastomers (vulcanized rubbers) at room temperature but, at elevated temperatures, behave as thermoplastics (thermo from the Greek word 0ep x6i , meaning hot). Consequently, they can soften and flow and, thus, can be processed with high speed, efficiency, and economy on conventional thermoplastic equipment. Furthermore, unlike vulcanized mbber, TPEs can be reused. 

Process of converting of raw rubber compounds into lightly cross-linked network elastomer. Vulcanization of diene rubbers involves compounding it with sulfur or sulfur compounds and then heating at about 140 °C for sometimes several hours. The process can be sped up by addition of catalyst, viz., ZnO. 

In the raw state elastomers tend to be soft and sticky when hot, and hard and brittle when cold. Compounding increases the utility of rubber and synthetic elastomers. Vulcanization extends the temperature range within which they are flexible and elastic. In addition to vulcanizing agents, ingredients are added to make elastomers stronger, tougher, or harder, to make them age better, to color... 

It seems appropriate to close this discussion of textile-to-elastomer vulcanization bonding with the same type of schematic portrayal as was shown earlier for elastomer-to-metal adhesion (Figure 23). Not all of these phenomena or processes can be operative for any single bonding system, but each would appear to be a possibility in one or more of the material approaches that have been cited. 

Fig. 5.18 Variation of the stress in an elastomer (vulcanized natural rubber), as a function of temperature, held at a fixed extension of350% (data of Meyer and Ferri reported by Treloar). Treloar).

Elastomers are polymers with rubberlike properties. The word elastic refers to the ability of a material to return to its original dimensions when unloaded, and the term mer refers to the polymeric molecular make up in the word elastomer. Vulcanized rubber materials typically have more than 200% elongation in a tensile test and are capable of returning rapidly and forcibly to their original dimensions when load is removed. This elastic response is due to the three-dimensional cross-linked network molecular structure they have. An elastomer, on the other hand, typically has elongation rates of 100% or more and a significant amount of resilience. Resilience is represented by the area under the elastic portion of the stress-strain curve, and therefore, refers to a material s ability to undergo elastic deformations. 

Vulcanization is a very complex reaction and involves activators for the breakage of die sulphur ring (Sg) and accelerators for the formation of sulphur intermediates, which facilitate sulphur-to-double bond crosslinking. Elastomer vulcanization by sulphur wifliout any accelerators takes several hours and is of no commercial importance. By using accelerators in the sulphur curing system, die optimum curing time can be decreased to as little as 2-5 min. 

---