Characterisation of network structure in recycled rubber
Produced by existing size reduction technologies, rubber crumb is the most valuable type of recycled material. However, only a limited amount of untreated crumb can be used in new tyres due to significant property loss. The de-vulcanisation process offers the potential for providing high quality recycled material that can be used as a substitute for virgin rubber. Rubber materials produced during industrial trials that formed the development stages of the novel DeVulCO”2 de-vulcanisation technology, were used in this work. A practical way to characterise de-vulcanised and re-vulcanised waste tyre materials and blends with virgin compound was established. This work showed that simple to obtain and understand parameters, acquired from tests that are commonly used in the rubber industry, such as Mooney viscosity, Mooney relaxation and curing can be successfully used to indicate the efficiency of the de-vulcanisation process and hence, to assist the research and development stages of the de-vulcanisation process and to assure quality control. Good correlation was evident between these parameters and the network characterisation parameter (Vr) obtained from the swelling measurements that are known to be a reliable guide but are more time consuming to perform. In this work, the Mooney relaxation test was successfully extended to characterise some de-vulcanised compounds (automotive weather strip and clothing tape). In addition, the swollen-state 1H-NMR technique was successfully applied to characterise de-vulcanised and subsequently re-vulcanised “closed-loop” tyre materials. A new tool to measure the efficiency of the de-vulcanisation process has been devised, based on a comparison of the peak areas in the olefinic region of rubber extracts analysed by the liquid 1H-NMR technique. This technique is capable of characterising different components of a rubber blend (e.g. NR, BR) and is suitable for carbon black filled compounds. The results of this work have confirmed the potential of the DeVulCO”2 technology and have indicated that up to 20% of de-vulcanisate can be blended with virgin tyre compound without significant deterioration of the key performance properties.
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