Sulfur Free Curing Systems

Some special vulcanising agents can cure diene rubbers such as NR, SBR and BR. These are now discussed. 

Composition Vistalon 7500,100 N-550 100 Flexon 815, 45 TMQ 1 zinc salt of 4- and 5-methyl-2-mercapto-benzimidazole (ZMMBl) 3 coagent ethylene dimethacrylate (EDMA) 0.5, Perkadox 14-40, 7.  

Diene rubbers can be vulcanised by the action of phenolic compounds like phenol-formaldehyde resin (5-10 phr). Resin cured NR offers good set properties and low hysteresis (a.31). 

Resin curing of SBR and BR imparts excellent cut growth and abrasion resistance. Resin cured nitrile rubber shows high fatigue life and high relaxation, while resin-cured butyl rubber shows outstanding ozone and age resistance (409). 

Another vulcanising agent for diene rubbers is m-phenylenebismaleimide. A catalytic free-radical source such as dicumyl peroxide or benzothiazyldisulfide (MBTS) is commonly used to initiate the reaction (a.36). Phenolic curatives, benzoquinonedioxime, and m-phenylenebismaleimide are particularly useful where thermal stability is required. 

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An Evaluation of Sulfur Free Cure Systems in Low Durometer Roll Compounds, Zeon Chemicals LLC Internal Report TN94-104. 

In contrast, the EV cure systems employ much lower levels of free sulfur (0.1—1.0 phr) or they use sulfur donors such as TMTD or DTDM combkied with higher accelerator levels. The short mono- and disulfide cross-links that form often do not exhibit the excellent physical properties of the conventional systems but they do retain thek properties much better after aging. 

Cold-Box Process. In the cold-box process which takes place at room temperature, a gas catalyst is passed through the sand to promote curing. The catalysts are triethylamine or meth-ylethylamine for phenolic isocyanate binders, sulfur dioxide for vinyl-unsaturated urethane binders, and methyl formate for an alkali metal salt of a phenolic resole binder. Yii -ortho resins have been developed for the phenolic component by using water-free systems and salts of divalent metals as catalysts. A recent patent describes improvements in the two-package phe-... 

When a rubber is subject to vulcanisation, with sulfur or any other curing agent, it is converted into a thermoset system that is lightly crosslinked, with a molecular weight (Mw) between each crosslink of approximately 10,000 Da. This leaves the majority of the rubber chain free and, as a consequence, the glass transition temperature of the material is below ambient temperature. This gives rubber its... 

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