Devulcanisation of rubber

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 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. 

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. 

This section will review the different technologies that are currently available for the devulcanisation of waste rubber from waste tyres, or from products in the general rubber goods (GRG) sector. Devulcanisation, although important, is only one way of recycling waste rubber and the extensive range of other technologies that have been developed for this purpose are covered in later sections of this book e.g., crumb manufacture and the use of crumb in the production of a variety of different products are reviewed in Chapters 6 and 7. 

Table 4.2 Organisations associated with the devulcanisation of waste rubber ...

In addition to the two reviews that are mentioned at the beginning of this section, a number of other relatively recent reviews of the processes and methods that have been developed and evaluated for the devulcanisation of waste rubber are available. For example, Majumdar published an overview in 2009 in the Chemical Weekly journal [4], which covered the challenges that face workers in this field and covered the main types of systems that have been developed (i.e., chemical, microwave, ultrasonic and so on). This article also covered the production and use of rubber crumb from waste rubber. In another article [5], Majumdar reviews the three main sources of reclaimed rubber that are available in the marketplace (rubber crumb, rubber powder and chemically digested reclaimed sheet) and describes their properties and uses. 

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 ... 

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. 

A research group in Canada [34] have used supercritical CO2 in a twin-screw extruder to devulcanise waste rubber from the automotive sector. They have used the process on both waste tyre rubber and EPDM-based products, such as door seals, and claim to have achieved good results with both types of material. The process uses that fact that as the CO2 swells the rubber in the high shear environment within the twin-screw extruder, the sulfur-sulfur crosslink bonds break preferentially compared to the carbon-carbon main-chain bonds (Section 4.2). 

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 team of workers in India [52] have described the use of tetramethyl thiuram disulfide (TMTD) in the presence of Spindle oil for the devulcanisation of ground tyre rubber at near ambient temperatures. Small-scale (-100 g batches) devulcanisation experiments were carried out on a two-roll mill and the products produced characterised using a variety of analytical techniques (e.g., dielectric analysis and TGA). The amounts of TMTD and Spindle oil were 2.75 and 10%, respectively. The DR was mixed with virgin, uncompounded NR and a cure system, with the DR content varying from 0 to 60%. Vulcanisates were produced, which were characterised by both physical (e.g., tensile strength and swelling) and analytical (e.g., SEM and DMA) tests. The tensile strengths that resulted were found to vary from 7.7 MPa (60% DR) to 14 MPa (0% DR). 

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 ... 

The technique has been found by workers such as Isayev to be suitable for use with a great variety of rubbery networks (Section 4.6.2), and in the presence of rigid filler particles. Whereas the ultrasound reactor and power settings to produce the desired devulcanisation performance vary significantly between the types of rubbers, the general nature of the network destruction and degradation are thought to be relatively similar. Post-process NMR studies on the different rubber systems have shown structural variations. 

Because of the interest in the recycling of waste tyre rubber, the University of Akron team has carried out a large number of studies on the application of ultrasound to the devulcanisation of SBR. These studies have evaluated the effect that variables such as crosslink type [69], styrene content [70], and crosslink density and molecular mobility [71] have on the results that are obtained. 

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. 

A lot of work has also been carried out in Brazil when it comes to using this technique for the devulcanisation of EPDM [99]. Waste EPDM from the automotive sector was exposed to microwave radiation for between 2 and 5 min and the DR produced characterised by a DSC and TGA. The degree of devulcanisation was assessed using gel content measurements. Workers from the same Brazilian university [100] have also devulcanised EPDM rubber by microwaves and then blended it with low-density polyethylene (LDPE) in the presence of a peroxide to improve the interfacial interaction between the two phases. The presence of the devulcanised EPDM in the LDPE matrix resulted in a reduction in the deformation and traction strength, but a significant increase in the elastic modulus values and impact strength. DSC data obtained on the mixture showed that the... 

The types of processing and physical property tests that are required to assess the important characteristics referred to above are also continually cited in the later sections of this book dealing with the use of waste rubber crumb in rubber products, thermoplastics, and thermosets, for the same reasons (Chapter 7). Many other specific property tests (e.g., acoustic) are also referred to in this book but, unfortunately, there is insufficient space here to cover them. A reasonably detailed section on the characterisation of rubber crumb is provided in Chapter 6, Section 6.4, because understanding the nature of this material is as important as understanding the properties of devulcanised rubber when it comes to its re-use applications. 

It is important to stipulate therefore that this section covers a different type of recycled product than Chapter 7, which reviews the work that has been carried out to produce and characterise products from blends of rubber crumb with virgin rubber, thermoplastics and other materials (e.g., asphalt). In Chapter 7, the rubber crumb that is used is either unmodified, or has been through a surface activation process, but it has not been devulcanised. 

A paper was published by Ishiaku and co-workers [11], who investigated the optimum concentration of DeLink to use in the devulcanisation of a sulfur-cured, NR-hased powder that had originated from waste rubber balls and artificial eggs. A solubility test was also developed to assess the degree of crosslink destruction that had taken place. The results obtained showed that the optimum... 

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... 

A solid-state mechanochemical milling process has been developed that combines both the devulcanisation of tyre waste and its mixing into an HDPE matrix [23]. The resulting product was also dynamically vulcanised within the mill, resulting in a thermoplastic elastomer (TPE) that SEM analysis showed to have good interfacial compatibility between the rubber and thermoplastic phases. 

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. 

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... 

One objective of this book is to describe the techniques used to devulcanise waste rubber so it (fin he reprocessed into high specification products. The production of rubber crumb from waste rubber products and its use to manufacture a wide range of products are also covered. 

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