Elastomers vulcanization agents sulfur cures

Nonsulfur Vulcanizing Agents. Many high performance specialty elastomers do not contain diene moieties in their molecular structure and therefore cannot be sulfur-cured. These elastomers require cross-linking agents capable of reacting with the specific functional group(s) contained by the specific elastomer. Some common nonsulfur curatives include peroxides, difunctional resins, and metal oxides. 

The blends of EPDM terpolymers and isotactic PP with curing agents, such as peroxide, phenol resins, and sulfur, are termed as thermoplastic vulcanized elastomer (TPV) since the rubber domains are vulcanized. Polyolefin copolymers, such as random copolymer of propylene with ethylene, copolymers of other olefins, elastomeric PP, and elastomeric PE, are developed with recent advances of... 

Activators. Activators are chemicals that increase the rate of vulcanization hy reacting first with the accelerators to form rubber-soluhle complexes. These complexes then react with the sulfur to form sulfurating agents (eqs. 14,15). The most common activators are combinations of zinc oxide and stearic acid. Other fatty acids used include lauric, and oleic, acids. Soluble zinc salts of fatty acid such as zinc 2-ethylhexanoate are also used, and these rubber-soluble activators are effective in natural rubber to produce low set, low creep compoimds used in load-bearing applications. Weak amines and amino alcohols have also been used as activators in combination with the metal oxides. Natural rubber usually contains sufficient levels of naturally occurring fatty acids to solubihze the zinc salt. However, if these fatty acids are first extracted by acetone, the resultant clean natural rubber exhibits a much lower state of cime. Therefore, to ensme consistent cure rate, fatty acids are usually added. Synthetic rubbers, especially the solution polymerized elastomers, do not contain fatty acids and require their addition to the cure system. 

Peroxides are probably the most common materials used instead of sulfur for rubber vulcanization, because of their ability to cross-link a variety of unsaturated and saturated elastomers and also their ability to produce thermally stable carbon-carbon cross-links. There are also some disadvantages of peroxide cross-linking. The most important are the poor mechanical properties of vulcanizates and low crosslink efficiency [1,2]. The use of co-agents coupled with peroxides to cure elastomers has been a common practice... 

True properties are determined by the polymer and compound formulation and, for synthetic rubbers, additionally by type of curing agents such as sulfur or peroxides and the state of vulcanization. Typical properties for four polymers often used in automotive applications [acetals, nylons, polycarbonates, and unsaturated polyester (vinyl esters)] are shown in Tables 10.1 through 10.4. Typical property data is readily available from resin producers and compounders. Property data for elastomers is found in the chapter on elastomers in this handbook. 

---