Waste rubber crumb

Often, the initial attractiveness of using waste rubber crumb (particularly tyre crumb) in products such as concrete, asphalt and wood lies in the importance of finding a use for the material itself. However, in addition to the environmental benefits mentioned above, improvements in important performance parameters can also result from its use (e.g., improvements in wear, durability, impact performance, insulation and so on), and another major benefit is that it takes the place of important raw materials (e.g., ceramics, wood, aggregate and so on) in these large tonnage products. 

The expression added value is often encountered in initiatives and activities that seek to re-use waste rubber in new products. An illustration of how this can be put into practice was provided in a presentation to the RubberCon 2014 (Manchester, UK) by Kind [22]. The presentation concerned the use of tyre crumb as a major ingredient in retread compound, and some up-to-date and informative price comparisons were provided that showed the potential cost benefits of using waste rubber crumb to manufacture new products. The figures provided are shown in Table 3.3. 

Figure 4.9 Schematic diagram of the PRL manufacturing process for the production of new products from waste rubber crumb. Reproduced with permission from Polymer Recyclers Ltd, Dordon, Tamworth, UK. 2014, Polymer Recyclers Ltd...

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. 

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. 

A considerable amount of work has been carried out to establish if it is possible to produce good-quality rubber/thermoplastic blends by incorporating waste rubber crumb, whether surface-activated or not, into thermoplastics matrices. This work has often been carried out using commodity semi-crystalline thermoplastics, such as low-density polyethylene (LDPE), high-density polyethylene (HOPE) and polypropylene (PP), with the aim to produce final products that are either modified (e.g., improvements in impact strength) or have properties that are usually associated with thermoplastic rubbers. 

Guzman and co-workers [27] investigated whether it is possible to use waste tyre crumb as a replacement for zinc oxide as an activator in the sulfur vulcanisation of natural rubber (NR). They used the unsaturated organic compound squalene as a model compound for NR in their work, and followed the course of the vulcanisation reaction using the analytical technique high-performance liquid chromatography. The results confirmed that waste rubber crumb was an alternative to zinc oxide as an activator in the curing of NR compounds by sulfur-based cure systems. 

Waste rubber crumb from a number of diene rubbers such as SBR, butadiene rubber (BR) and polyisoprene has been compounded with cheap mineral fillers and additives, such as monoethanolamine, hy Ushmarin and co-workers [32] at Chuvash State University and used to manufacture general rubber goods such as car mats and sleeping policemen for railway lines. They found that the use of a two stage mixing process in an internal mixer produced materials with the best physical and mechanical properties. 

Jevtic and co-workers [51] looked into the possibility of using waste rubber crumb to replace some of the natural river aggregate that is... 

For some applications, such as when it is necessary to dissolve or solvate rubbers, or rubber compounds to form doughs and solutions it is desirable to use a machine in this category. Various types of machines can be used to produce the required form. Grinders are also used to reduce scrap vulcanisate to form a rubber crumb for purposes of working this waste product back into cheap compounds, or prior to reclaiming rubber by chemical processes. 

The PAT process has been unable to stabilize organic materials such as rubber crumbs or wood chips because these substances have a memory effect and rebound to their original size after compression. The process requires low moisture content in the waste stream because waste loading is sensitive to moisture content. For hazardous and mixed wastes, influent particle size must be less than 0.25 inches in order to achieve proper brick formation and adequate stabilization. 

Another application of the injection system consists of recycling old and waste rubber. The reactive mixing of crumb rubber with a melted polymer represents an innovative way of recycling these waste rubbers [6]. Some thermoplastic elastomer may contain up to 60% recycled rubber derived from used vehicle tires, and thus through the reactive injection system the rubber scrap is revulcanized [7]. 

Keeping environmental and economic factors in mind, it was envisaged in a review [18] that the disposal problem could be solved by incorporating ground waste rubber into virgin polymer matrices. At the end of 2001, the Asphalt Rubber Technology service has reported to be planning to use crumb rubber in several applications in the construction of its new research facility [19]. 

Reactive mixing of crumb rubber with polypropylene by melting represents an innovative way of recycling old and waste rubber. A machine technology can easily be applied through injection molding. Results are examined for quasi-static and dynamic tests applied to the final materials. Moreover, the properties of this elastomer alloy remains constant after several recycling processes [59]. 

In addition to the use of organic chemicals, rubbers can be devulcanized by means of inorganic compounds. Discarded tires and tire factory waste were devulcanized by desulfurization of suspended rubber vulcanizate crumb (10 to 30 mesh) in a solvent such as toluene, naphtha, benzene, cyclohexane, etc., in presence of sodium [44]. The alkali metal cleaves mono-, di-, and polysul-fidic crosslinks of the swollen and suspended vulcanized rubber crumb at around 300°C in absence of oxygen. However, this process may not be economical because the process involves swelling of the vulcanized rubber crumb in an organic solvent where the metallic sodium in molten condition should reach the sulfidic crosslink sites in the rubber crumb. Also, the solvent may cause pollution and be hazardous. A technology was also proposed to reclaim... 

The compatibility between the phases of the blends is even more needed when these phases are recovered from previously manufactured products. T6th et al. [165] made an approach in improving the adhesion between the compounds and thus the mechanical properties of the resulting materials in waste polyethylene/crumb rubber blends by using compatibiUzing additives such as olefin-maleic-anhydride copolymers the preparation method consisted of two-roll mixing at temperatures of 145-180°C followed by compression molding at 180°C. 

Strenuous efforts are being made to market various waste materials as fillers for plastics, with varying degrees of success. They include fly ash from power stations, newsprint and various agrowaste products. In the past five years Asian organisations have promoted empty fruit bunch fibres, oil palm mesocarb fibres, rice husk ash and various nutshells. Scrap rubber crumb from used tyres has also been evaluated by Composite Particles, Inc. for use in mouldings, foams and pipes. 

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. 

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. 

A popular reaction vessel to carry out this type of work is an autoclave. Because of its low cost and good health and safety credentials, supercritical (or liquid) CO2 is often used, although its nonpolar nature means that it has its limitations as a solvent, particularly for certain chemical agents. In common with the majority of the other processes that are reviewed in this section, the waste rubber is usually in the form of crumb or powder to present a large surface area to the solvent/reactants in the autoclave. 

A devulcanisation process that has some similarities to the DeLink process has been published as a US patent by Tang (US 6590042). In this process, a specially designed twin-screw extruder is used, with the waste rubber in a crumb form, and a reclaiming agent that includes the following rubber compounds ... 

In this process, which has been in existence as a concept for at least 30 years and is often referred to as a microbial process, chemolithiotrophic bacteria in aqueous suspensions in the presence of oxygen can be nsed to selectively attack the sulfur crosslinks in the snrface of waste rnbber crumb particles. The process can also be regarded as a type of biodegradation process, the by-prodncts of which can inclnde elemental sulfur, sulfates, sulfides and snlfnric acid. Specific examples of the bacteria that have been used to devulcanise rubber in this way inclnde species of Thiobacillus, e.g., T. thiooxidans. 

A group of workers drawn from the Universities of Kocaeli and Hacettepe [123] have carried out a comparative study of the results that were obtained from waste butyl rubber crumb that had been devulcanised using conventional methods and end-of-life butyl rubber inner tubes that had been irradiated using a dose of 120 kGy of... 

A company called Revultec have developed a devulcanisation system called the Revultec system [128], which uses a relatively inexpensive solvent, such as hexane, in its supercritical state to devulcanise waste tyre crumb rubber. At the end of the process, up to 98% of the solvent can be recovered for re-use, and it is claimed that the DR is of a sufficiently high quality to make new rubber products, including tyres. 

A more recent article that addressed the use of the DeLink process was published in Rubber Journal Asia [13]. It featured the Gujarat Reclaim and Rubber Products Company, which produces reclaimed rubber from both synthetic and NR waste, as well as a light-coloured reclaim from latex scrap using the Rubplast process. The company uses Green Rubber s patented devulcanisation agent, DeLink, at a ratio of two parts DeLink to 100 parts crumbed waste rubber, to produce a product that can be incorporated back into new rubber compounds. The article claims that this is very advantageous to the rubber industry as it enables them to re-use the 5-15% of waste, which they typically throw away. 

Irrespective of whether the source of waste rubber is whole tyres, tread buffings or GRG, the crumbing operation is sometimes carried out under cryogenic conditions (i.e., it is cooled to below around... 

Other examples of the types of tests that are mentioned above have been covered in Chapter 5, Section 5.1.1, where they are used to characterise rubber crumb and other waste rubber products to assist in devulcanisation work. 

To assist in the marketing of tyre-derived rubber products, a categorisation system has been established as part of the UK Waste Resources Action Programme (WRAP) Quality Protocol (Section 6.4.1.5), which will help avoid possible confusion in the use of terms such as rubber crumb and rubber powder . The categories and definitions that are in the protocol are shown in Table 6.1. 

Preparation of Waste Rubber Products prior to Grinding or Crumbing... 

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