Mechanical Devulcanisation Processes

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. 

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. 

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. 

In order to develop processes and techniques capable of devulcanising sulfur-cured rubbers, polymer chemists have often referred to a number of fundamental studies that have been carried out in the past to characterise the nature and properties of sulfur bonds. These original studies, carried out by workers such as Tobolsky [6], and Murakami and Ono [7] among others, highlighted mechanisms and routes (e.g., exploitation of the relative weakness of the sulfur-sulfur crosslink bonds) that had the potential to selectively cleave the sulfur crosslinks without damaging the rubber molecules themselves. The research work that has resulted showcases the existence of three principal mechanisms that can be utilised in the devulcanisation of sulfur-cured rubbers. These three mechanisms are associated with differences that exist in the three fundamental properties of the different chemical bonds that are present in the sulfur crosslinks and the rubber molecules, namely ... 

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... 

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. 

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