Devulcanised rubber compound

A devulcanisation process that has some similarities to the DeLink process has been published as a US patent by Tang (US 6590042). In this process, a specially designed twin-screw extruder is used, with the waste rubber in a crumb form, and a reclaiming agent that includes the following rubber compounds ... 

One estimate of the depth to which these devulcanisation reactions penetrate into the snrface of the rubber particles is <1 pm [104]. The process can also be regarded, therefore, as a technique for surface modification (other types of surface modification and activation are reviewed in Chapter 6, Section 6.6), which, by desulfurisation, activates the surface of the rubber particles and improves their interfacial bonding within the matrix of the rubber compounds into which they are incorporated, hence enhancing the quality of the subsequent products. 

Devulcanising rubber can result in products that have bad odours, and this has been addressed in this system by the use of a novel deodorisation process that involves the injection of high-pressure water into the extruder. Odour-producing compounds are trapped in the high-pressure water vapour and removed via vents. 

Once a rubber has been devulcanised to the desired level, with the Mw retained at the target level, it has to have cure characteristics (e.g., scorch time) that ensure it is processible. This is particularly true if the devulcanised rubber is to be used with little, or no, modification, but also important if it is to be blended into another rubber compound, although the amount that is to be incorporated into the blend obviously has an impact on the significance of this property. 

Some of the earliest cited practical, third-party trials of DeLink were reported in Plastics and Rubber Weekly [6]. The article covered the official launch for DeLink at the International Seminar and Workshop on Devulcanisation Using DeLink R Process that was held at the TARRC laboratories (then MRPRA). Papers that were presented generally endorsed the claims made by STI-K that the rubber devulcanised by DeLink can be used in manufacture or blended with fresh rubber compound to obtain the desired levels of technological performance. Rubber that had been devulcanised with DeLink had been used successfully to ... 

A more recent article that addressed the use of the DeLink process was published in Rubber Journal Asia [13]. It featured the Gujarat Reclaim and Rubber Products Company, which produces reclaimed rubber from both synthetic and NR waste, as well as a light-coloured reclaim from latex scrap using the Rubplast process. The company uses Green Rubber s patented devulcanisation agent, DeLink, at a ratio of two parts DeLink to 100 parts crumbed waste rubber, to produce a product that can be incorporated back into new rubber compounds. The article claims that this is very advantageous to the rubber industry as it enables them to re-use the 5-15% of waste, which they typically throw away. 

These accelerators (ratio MBTS 0.65, CBS 0.48 and DPG 0.15) had also been used to vulcanise the original SBR rubber compound (along with 2 phr sulfur, 5 phr zinc oxide and 1 phr stearic acid) that was being devulcanised in these experiments ... 

The properties of the fine, devulcanised rubber crumb that results from the process varies according to the types of additives used and their levels, and the crumb has been shown to be used effectively to replace virgin rubber in new rubber compounds. For example, experimental trials have been undertaken with rubber crumb obtained using this technology from car tyre treads incorporated into SBR compounds. The results that were obtained on the resulting vulcanisates were compared to those produced using conventional rubber of the same... 

The investigations and studies presented in this section are concerned with the blending of waste rubber powder or crumb, which has not been through any devulcanisation processes of the type described in Chapter 4, into rubber compounds to produce new products. The rubber crumb in question may or may not have been surface-activated by one of the processes described in Chapter 6, Section 6.6. The properties of the resulting blends will depend upon whether this activation has taken place or not, as well as upon some of the variables already listed at the start of Section 7.2, namely the origin and type of rubber crumb, the proportion of rubber crumb in the blend, and... 

Devulcanised rubber can then be sold in the form of masterbatches for blending into new rubber compounds or, occasionally, other polymers such as thermoplastics. 

Once the reaction vessel is charged with waste rubber, solvent and chemical agents, it is sealed and then heated, often at a relatively high temperature (e.g., 180 °C for 1 h), to achieve the devulcanisation. High levels of devulcanisation (e.g., up to 100%) have been claimed for gum stock rubbers, but lower levels are usually obtained with compounded rubbers, e.g., tyre rubber. 

A lot of relatively recent work, using specially prepared sulfur-cured NR and polyisoprene compounds, has been carried out by a group at Kyoto University led by K. Kojima [10]. In addition to studying the fundamental factors surrounding the use of supercritical CO2 as a reaction medium with polyisoprene-type rubbers, they have used supercritical CO2 in conjunction with a number of devulcanisation agents, for example ... 

The use of supercritical CO2 to facilitate the devulcanisation of rubber with chemical agents has been investigated by a number of workers. As mentioned already, this compound can assist the process in a number of ways, for example by acting as a swelling agent to open up the rubber matrix and so act as a carrier for the chemicals, aiding their penetration and, hence, their interaction with the crosslinks. 

The scale-up goal was achieved and the DevulCOi devulcanisation technology was successfully transferred to a number of sulfur-cured rubbers, including nitrile rubber, EPDM rubber and NR. In addition, the results of the evaluation of the performance of the DevulCOi rubber in tyre retread compounds were encouraging. A more extensive description of these two projects and the results of the manufacturing trials that were imdertaken during them, is provided in Chapter 5, Section 5.2.2.I. 

Because of the possibility of over-heating, microwave devulcanisation can sometimes cause problems with diene rubber-based compounds, due to their limited thermal stability. So, although Section 4.7.2 shows that successful work with these materials has been carried out, workers have often tended to concentrate on rubbers that have greater heat resistance, such as butyl rubber and EPDM rubber. The work reported below also shows that these studies have often been successful, with, in the case of butyl rubber, the DR showing a tendency to revulcanise without the need for recompounding. 

Other work that has been carried out with microbial devulcanisation includes use of the microorganism Pyrococcus furiosus to carry out devulcanisation under anaerobic conditions. This type of bacteria reduces the sulfur in the crosslinks to hydrogen sulfide [108]. Another type of bacteria, Sulfolobus acidocaldarius, can be used to selectively cleave carbon-sulfur bonds to generate sulfate compounds that stay in the rubber and so the sulfur remains available for any revulcanisation treatments [109]. 

Antonio [16] has demonstrated how the final properties of a revulcanisate can vary according to a combination of the type of scrap rubber that is used as the starting material and the devulcanisation conditions that are used to devulcanise it. The study demonstrated this by using four NR compounds with tensile strengths varying from... 

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