Thermal Devulcanisation Processes

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 types of processes that are reviewed in this section are those that have an elevated temperature as a common denominator and, by the use of this, they target the thermally labile nature of the sulfur-sulfur bonds in the crosslinks. There are quite a few examples where chemical agents have also been used in the process to assist in the devulcanisation of the rubber. In these cases, the chemical agents that have been chosen have often been similar to those that are used in the mechanical-chemical processes (Section 4.5) and often their assimilation into the rubber matrix has been assisted by solvents that have a high affinity for both the rubber and the chemical agents. Other processes of this type have employed solvents (e.g., supercritical carbon dioxide (CO2), supercritical water, alcohols and so on) on their own, without any other chemicals, and sometimes they have reacted with the crosslinks and/or the polymer chains. 

In common with the thermal processes (Section 4.3), these processes and mechanical processes that use chemical agents (Section 4.4) often use supercritical fluids (e.g., supercritical CO2) as a process aid. A supercritical fluid is chosen that is compatible with the rubber that is being devulcanised so that it swells the rubber within the process equipment and so facilitates devulcanisation by ensuring a high degree of fill and, hence, shearing efficiency. It is, however, harder to maintain the fluid in a supercritical state in these types of processes, due to leakage and loss of pressure, than in sealed vessels (e.g., autoclaves), as are often used in thermal processes. 

The use of CO2 in the supercritical, or liquid state, to facilitate the devulcanisation of rubber has already been mentioned in Sections 4.3 and 4.4. However, because it was used on its own as a non-reactive process aid in those thermal and mechanical processes, it was not regarded as acting as a chemical devulcanisation agent, and so those systems were not included in this section. CO2 has also been used in these types of processes by some workers to swell... 

Rubber can also be ground at ambient temperatures by the use of any one of a number of extrusion-based processes. These can be twin-screw extruders that have been fitted with grinding elements and can be operated at conditions of high shear. Other extrusion processes can deliberately heat up the rubber, so that it is subjected to both high shear and high temperatures. These types of processes can be regarded as having a dual role, as they can also start to devulcanise the rubber, particularly those that are sulfur-cured, as the sulfur-sulfur crosslinks are both more thermally labile than main-chain carbon-carbon bonds and are less flexible, and so will preferentially break under conditions of shear (Chapter 4, Section 4.2). The occasions when these types of... 

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