Microwave devulcanisation

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. 

In addition to the two reviews that are mentioned at the beginning of this section, a number of other relatively recent reviews of the processes and methods that have been developed and evaluated for the devulcanisation of waste rubber are available. For example, Majumdar published an overview in 2009 in the Chemical Weekly journal [4], which covered the challenges that face workers in this field and covered the main types of systems that have been developed (i.e., chemical, microwave, ultrasonic and so on). This article also covered the production and use of rubber crumb from waste rubber. In another article [5], Majumdar reviews the three main sources of reclaimed rubber that are available in the marketplace (rubber crumb, rubber powder and chemically digested reclaimed sheet) and describes their properties and uses. 

This process can be regarded as a form of thermal devulcanisation, and the use of microwave energy to devulcanise rubber by causing molecular motion within it, resulting in heat generation, has been assessed by a number of workers. With regard to how the process is carried out in practice, the review published by Myhre and MacKillop [1] stated that a number of the processes that have been developed refer to a Goodyear Tire and Rubber Company patent (US 4104205) that was published in 1978. 

The use of microwaves in conjunction with a chemical devulcanisation agent is a variation of this technology that assists the process to be more targeted towards the removal of crosslinks. In this variation of the technology, the microwaves are used to generate the heat to enable the devulcanisation reaction to proceed. 

Vega and co-workers [89, 90] have used squalene as a model compound for studies to assess the ability of microwave heating rather than conventional heating to control the devulcanisation of... 

An article by Landini and co-workers [91] describes how small samples (250 g) of butyl rubber have been devulcanised by the use of a high-frequency (2.45 GHz) microwave. In this preliminary batch assessment, various powers (e.g., 1,000 and 3,000 W) and heating times (e.g., 9-25 min) were used, and the test results obtained indicated that devulcanisation had occurred and that upon subsequent heating some samples were foimd to revulcanise without the addition of any crosslinking agents. 

Workers in Brazil [98] have recovered scraps of industrial SBR waste and then, after preparing crumb from them by an ambient grinding process, employed microwaves to devulcanise the rubber. Once devulcanised, the vulcanisation behaviour of the rubber was determined by oscillatory disk rheometry and samples vulcanised with and without a post-cure. The samples were then tested so that their mechanical and crosslink densities could be compared. 

A lot of work has also been carried out in Brazil when it comes to using this technique for the devulcanisation of EPDM [99]. Waste EPDM from the automotive sector was exposed to microwave radiation for between 2 and 5 min and the DR produced characterised by a DSC and TGA. The degree of devulcanisation was assessed using gel content measurements. Workers from the same Brazilian university [100] have also devulcanised EPDM rubber by microwaves and then blended it with low-density polyethylene (LDPE) in the presence of a peroxide to improve the interfacial interaction between the two phases. The presence of the devulcanised EPDM in the LDPE matrix resulted in a reduction in the deformation and traction strength, but a significant increase in the elastic modulus values and impact strength. DSC data obtained on the mixture showed that the... 

At Pisa University, work has also been carried out on EPDM by Bani and co-workers [103]. Carbon-black-filled EPDM crumb samples were devulcanised by exposing them to microwaves so that their temperature increased to over 300 "C. This process, in a short period of time, produced a rubber that had a relatively low crosslink density and could produce high-quality blends with virgin EPDM due to good interfacial adhesion. Once vulcanised, these blends were found to have superior physical properties to those produced using untreated EPDM crumb. 

Workers at the Chinese university of Yangzhou [35] have used microwaves to modify the surface of waste rubber crumb by devulcanising it and then blending it with NR in various proportions. These mixtures were then vulcanised and the mechanical properties, compression set, swelling behaviour and crosslink density investigated. The results obtained were compared with those of blends that had been prepared using crumb that had not been treated with microwaves. 

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