Chemical devulcanisation agent

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 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. 

Due to their underlying chemical properties and atomic structures, there is also a difference in the chemical reactivity of the different types of bonds that are present in the rubber matrix and so if chemical agents (sometimes referred to devulcanising agents in the literature) are used that specifically target the sulfur-sulfur and sulfur-carbon bonds, usually aided by other agencies such as heat, this difference in reactivity can be exploited to achieve devulcanisation. 

In addition to the work referred to above, workers at Twente University [55] have also carried out an investigation on the effectiveness of three different devulcanisation agents on the devulcanisation of two different types of EPDM rubber. The three chemical agents were ... 

A group of Chinese workers [58] have carried out work to devulcanise a NR compound using snpercritical CO2 in conjunction with the chemical devnlcanising agent diphenyl disulfide. During the study they investigated the effects of a number of parameters, such as CO2 density, temperatnre, pressnre and reaction time. They also used two analytical techniqnes, GPC and NMR, to study the products of the various devulcanisation experiments. 

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. 

Once the reaction vessel is charged with waste rubber, solvent and chemical agents, it is sealed and then heated, often at a relatively high temperature (e.g., 180 °C for 1 h), to achieve the devulcanisation. High levels of devulcanisation (e.g., up to 100%) have been claimed for gum stock rubbers, but lower levels are usually obtained with compounded rubbers, e.g., tyre rubber. 

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 processes that are included in this section are those that employ both shear forces and chemical agents to devulcanise the rubber. In a number of cases, heat is also deliberately applied to the rubber by the use of a heated extruder barrel or mill. Even in those cases where a process may be described as operating at ambient , they... 

A number of chemicals have been used to devulcanise rubber and a very good review of the different types that are used to devulcanise and reclaim rubber has been published recently by Myhre and coworkers [36]. In this review they divided the types of organic chemical agents that could be used into the following categories ... 

The use of supercritical CO2 to facilitate the devulcanisation of rubber with chemical agents has been investigated by a number of workers. As mentioned already, this compound can assist the process in a number of ways, for example by acting as a swelling agent to open up the rubber matrix and so act as a carrier for the chemicals, aiding their penetration and, hence, their interaction with the crosslinks. 

Once devulcanised in the Banbury using a 3.5 min (0.5 min initially, then 3 min with the mix of chemical agents) programme, the devulcanised samples were mixed with 0.4 phr cyclohexyl benzothiazole (CBS), 5 phr zinc oxide and 1 phr stearic acid and revulcanised at 160 C. The time that was used for each material was obtained from the T90 (i.e., 90% cure state) time value recorded in a rheometer trace. 

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