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A Study of Low Temperature Rubber Curing Accelerators
Khirud B. Chakraborty, Boyd Grover, Max Liu and Ranvir S. Virdi
Robinson Brothers Limited, Phoenix Street, West Bromwich, UK. B70 0AH
Abstract
Current production processes in the rubber industry lean towards greater productivity and faster throughput times using shorter cure cycles at higher curing temperature. This process is however energy intensive and increasing concerns over volatility and upward shifts in the energy market, as well as environmental impact, has seen a growing interest in developing accelerator cure systems which can operate at lower temperatures to mitigate these concerns, without adversely affecting existing production rates, cure times or physical properties of the final vulcanisate.
The use of elemental sulphur in combination with chemical accelerators remains the preferred method of choice for cross-linking most rubbers. Conventional accelerators such as dithiocarbamates, thiurams, thiazoles and sulphenamides when used alone, or in combination, are highly effective and efficient curatives in most dry rubber applications. However, ultra-accelerators such as dithiocarbamates and thiurams, although fast curing, suffer from greater reversion at higher temperatures resulting in vulcanisates with deteriorated physical properties. Sulphenamides and thiazoles on the other hand can withstand high temperature processing but are not effective at lower temperatures. In addition, health and safety and environmental concerns are growing with the use of conventional accelerators, for example, the occurrence of toxic fumes containing carcinogenic N-nitrosamines at work places in the rubber industry is well documented.
This paper aims to show that by using safer accelerators, the processing temperature of vulcanisation can be lowered by at least 30ºC in natural rubber. Energy consumption is therefore reduced affording cost savings and environmental issues addressed whilst maintaining the performance integrity of the final vulcanisate.
Our scientific approach considers the reaction pathway of the vulcanisation process in selecting suitable accelerators for lower temperature cure systems. These accelerators proceed via a lower activation energy barrier compared to conventional accelerators. We also consider other parameters which affect accelerator selection including structural variation and chemical functionality. This paper describes the selection procedure for lower temperature rubber curing accelerators and their performance in natural rubber at curing temperatures from 160ºC to 110ºC.