Rubber vulcanization accelerators

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Accelerators for rubber vulcanization

Introduction

There are many accelerators available for the vulcanization of rubber. That is because there is a wide range of rubber articles on the market with a wide variety of properties. For instance in a car tire alone there can be already up to eight different rubber compounds, each with specific properties. For instance the tread in a typical passenger car tire consists of a mixture of SBR (styrene-butadiene rubber) and BR (butadiene rubber). This rubber should have high abrasion resistance and high grip on both dry and wet roads. The side wall of the tire should have a high flexibility, that means that it should resist many flexings during the running of the tire, without cracking. It consists normally of a mixture of natural rubber and butadiene rubber. Inside the tire there is a rubber compound with as major function the adhesion between rubber and the steel cord of the belt. It typically consists of natural rubber with a very high sulfur level (up to 8 phr), to get a relatively stiff rubber, with sulfur promoting the adhesion with the steel cord. The basis of the tire is formed by the carcass, normally a mixture of NR(natural rubber), SBR and BR. It should have a very good adhesion to the polyester cord, used as reinforcement. And the inner side of the tire is formed by the inner liner, normally consisting of halogenated butyl rubber (IIR) For all these compounds with their different properties different accelerators and mixtures of accelerators have to be used to obtain the required properties. A vulcanization accelerator is typically used in combination with sulfur as the cross-linker, and with zinc oxide and stearic acid as activators. Other additives can be added too, but for the cross-linking reaction the abovementioned ones are the most important. The various types of rubber used in the various tire compounds all have different vulcanization characteristics, like speed of cure (cure is the crosslinking reaction) and extent of cure (the number of cross-links). A typical passenger car tire is vulcanized for 10 minutes at 170 degrees C. This means that all the different compounds have to be cured to their optimum state of cure in this same 10 minutes. This is the reason why a lot of different accelerators or mixtures thereof are used in the same tire.

Classification of accelerators

Primary accelerators

There are two major classes of vulcanization accelerators, primary accelerators and secondary accelerators or ultra accelerators. Of the primary accelerators the major group used in tire manufacture is formed by sulfenamides.These are produced by an oxidative coupling reaction of mercaptobenzthiazole (MBT) with a primary amine like cyclohexylamine or tertiary butyl amine. Secondary amines like di-cyclohexyl-amine can be used also but result in much slower accelerators. Such a slow accelerator is rquired in the steel cord adhesion compound mentioned above, because for optimal adhesion a slow cure is required. Another important group of primary accelerators is formed by the thiazoles. The two main products are mercaptobenzthiazole (MBT) and mercaptobenzthiazole disulfide (MBTS), a product formed by oxidative coupling of two MBT molecules). The thiazoles are used for the vulcanization of thick articles, and as basic accelerator in EPDM compounds (ethylene-propylene-diene rubers), in combination with mixtures of ultra-accelerators.

Secondary accelerators

Of the secondary or ultra-accelerators the main categories are the thiurams and the dithiocarbamates. In vulcanozation of tire compounds they are used as small addition to sulfenamides to boost the speed and state of cure. They have a very vulcanization speed and therefore, next to boosters in tire compounds they are used as main accelerator in EPDM compounds and in latex compounds. EPDM compounds have much less cure sites than natural rubber or SBR, and therefore need a rapid vulcanization system to have sufficient cure speed. Latex is cured at relatively low temperature (100- 120 degrees C)and therefore need an inherently rapid accelerator. The major thiurams used are TMTD (tetramethylthiuramdisulfide) and TETD( tetraethylthiuramdisulfide), They are produced by the reaction between dimethyl amine ar diethylamine and carbondisulfide. The major dithiocarbamates are the zinc salts ZDEC (zinc diethyldithiocarbamate and ZDBC (zinc dibutyldithiocarbamate).

References

Natural Rubber Science and Technology, Editor: A.D. Roberts, Oxford University Press, Oxford 1988