Mechanical devulcanisation

Yagneswaran and co-workers [25] incorporated mechanically devulcanised waste tyre powder into a vinyl ester resin matrix at various levels of addition and characterised the resulting materials using physical tests, DSC and TGA. As the level of rubber powder increased the composites were found to have reduced heat stability and flexural strength, but increased flexural modulus. 

Mechanical with chemicals Shear/chemical devulcanisation... 

Because of the dominance of sulfur-cured rubbers in the market, which is principally due to their use in manufacturing the single most important rubber product (i.e., tyres), the majority of the work that has been done on devulcanisation has concentrated on breaking the sulfur-sulfur crosslinks in these type of rubbers, and this is reflected in the information that is presented in Sections 4.3 to 4.9. Before the different devulcanisation technologies are described in detail, it is useful to consider the chemistry that is associated with these type of crosslinks, as it is this that has played such an important role in determining the fundamental choices that have been made with regard to the mechanisms that have been employed and the development work that has been carried out. 

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 ... 

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. 

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. 

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. 

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... 

The exact mechanism by which the ultrasonic treatment causes devulcanisation is still under study. One theory is that acoustic cavitation occurs within the rubber and it is the collapse of these cavities that causes devulcanisation to occur. However, workers have also postulated that the collapse of these cavities was not the primary method of devulcanisation and that degradation of the network around the cavities should also be considered. The theories surrounding the mechanism by which devulcanisation is achieved by the use of ultrasound, and other important characteristics of the technique, are covered by a comprehensive review of ultrasonic devulcanisation written by Isayev and Ghose [62]. 

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. 

In an earlier study, the group at Beijing used a mercapto-containing yeast to devulcanise ground NR [115], and then characterised a number of the properties of the devulcanised NR, including its mechanical properties, swelling behaviour in solvents, crosslink density, and the amount of sulfur present on the surface of the particles. 

In the case of the devulcanised rubber, properties such as crosslink density, extractable fraction and gel fraction were obtained. For the two different series of vulcanisates, cure characteristics, and morphological and mechanical properties were among those that were investigated. 

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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