Devulcanisation process

One of the major problems facing any organisation that is trying to develop a novel devulcanisation process, or run an existing one... 

A brief description of each of the principal devulcanisation processes shown in Table 4.1 is provided in Sections 4.3 to 4.8, along with a summary of their strengths and weaknesses. Also, for each type of process, a review of the literature has been carried out to provide a selection of the organisations, researchers and companies that have carried out investigations and studies in that particular field. 

It can be seen from Sections 4.2.1 to 4.2.4 that four generic categories of difference are available to research scientists that can be used as the basis for a devulcanisation process. Some processes mainly use just one mechanism, hnt it is often the case that in order to develop an optimised system that has the potential to be used commercially, more than one process is nsed, and often all three of the mechanisms that are described above are employed. Processes of this type are included in Section 4.5 and employ shear and a degree of heat, using either a two-roll mill or an extrnder, and chemical agents. 

By the generation of high shear forces within the rubber, these processes mainly work by exploiting the difference in the elasticity of sulfur-sulfur bonds in the crosslinks compared to the carbon-carbon bonds in the main-chain polymer molecules. However, as the Watson Brown HSM process has demonstrated (Section 4.4.2), this type of devulcanisation process does not have to be confined to sulfur-cured rubbers. Irrespective of the type of cure system that has been used (i.e., sulfur-type, peroxide, metal oxide and so on), there is often some difference in the intrinsic chemical properties, particularly the degree of flexibility, associated with the chemical bonds present... 

Balasubramanian [16] has described a devulcanisation process that uses a counter-rotating twin-screw extruder to devulcanise GTR. The DR was then blended with virgin NR in various proportions and the blends revulcanised using a sulfur cure system. The Mooney viscosity, cure characteristics and mechanical properties of the resulting vulcanisates were characterised and a four-parameter rheometric equation, based on the standard logistical model for the curing behaviour of extrusion processed blends, was derived and validated for the different levels of virgin NR. 

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 ... 

As is usually the case, the need for analysis and characterisation does not stop once a devulcanisation process has been fully developed and commercialised, as it is important to carry out quality control checks at regular intervals as an integral part of any quality assurance and good manufacturing practice system. Carrying out such tests provides the purchasers of the devulcanised product with reassurance that the system is not only inherently capable of producing a high-quality product, but that it is also able to do this consistently over an extended period of time. 

A large number of the results that have been obtained in this way, or recorded on samples to demonstrate the effectiveness of a particular process for commercial reasons (e.g., marketing), have been left embedded in the various sections that make up Chapter 4 if it was thought appropriate to do so in the context of describing a particular devulcanisation process. They were also left embedded if the amount... 

One of the main reasons for placing this type of information in a separate section within the book was to keep Chapter 4 reasonably balanced and to not overload certain sections within that chapter with information, particularly Section 4.5. It was also thought that it was more appropriate to place this information within a characterisation chapter rather than a chapter that was focused on describing different devulcanisation processes. So, the information in this section is intended to complement the information provided in Chapter 4, and inclusion in this section does not imply any special merit to a particular process or study, but is often a consequence of the amount of information that is available in the public domain. 

This section has been divided up, for convenience, into studies that have reported on the results obtained from a single process and comparative studies that have compared the products that have been generated by two or more different devulcanisation processes. 

Once the rubber has been granulated, the next step is usually to sieve it into products that have well-defined particle sizes and particle size distributions. This operation confers the dual benefit of enabling a producer to deliver consistent high-quality material to a customer who has developed and optimised a process (e.g., a devulcanisation process), and enabling the provision of different grades of material, helping to ensure that the potential number of end-uses is not restricted. 

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... 

Although the technical argument can now be regarded as essentially won for recycling options that involve either devulcanisation or the use of ruhher crumb, the principal factor that has often restricted commercial exploitation of both is the cost of the replacement product relative to the existing product. As has been demonstrated with rubber crumb, the use of it in, say, a road surface, often results in a product with better properties, but the cost has to be justified. With the devulcanisation of rubber for return into new rubber products, often the best that can be aimed for technically is equivalence with the existing virgin product, and the additional cost of the devulcanisation process can be a problem. 

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