Crumb manufacture

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. 

Toxic pollutants (kg/Mg) Emulsion crumb rubber manufacturing ... 

Some recycling alternatives use whole tires, thus requiring no extensive processing other alternatives require that tires be split or punched to make products and still other alternatives involve tires that are finely ground enabling the manufacture of crumb rubber products. Some applications for each alternative are listed below ... 

Various rubber products can be manufactured using rubber from scrap tires to replace some or all of the virgin rubber or other material in the product. Tires may be either split, punched, or stamped to yield shapes suitable for fabrication, or the tires may be processed to crumb size to make new products, usually by mixing with other materials. 

Rubber Reclaim. For the traditional rubber "reclaim," crumb rubber is mixed with water, oil, and chemicals and heated under pressure, thus rupturing the carbon-sulfur bonds that cross-link the molecular matrix. The resulting partially devulcanized rubber may be formed into slabs or bales and shipped to manufacturers who process and vulcanize it for use as an alternative to virgin rubber to use in tires or to make mats and other rubber products. 

Poly(vinyl chloride) films are produced in two main forms—unplasticized and plasticized—and over the years different machines have been manufactured to handle the two types. When calendering unplasticized PVC there is a tendency for small particles, usually referred to as crumbs , to fall away from the edges of the film and from the feed nip. Such crumbs then could fall on to the finished film, where they would stick and form defects. To avoid this, producers of unplasticized film usually prefer an L configuration in which the product travels up the stack and surface contamination of this kind is prevented. With plasticized PVC the problem of crumbs does not occur to any great extent and, as it is an advantage to have good access to the part of the calender where the finished film is made, an inverted L configuration is the most popular. 

Both of the solid forms are supplied by the manufacturer either as a slab or as a uniform, free-flowing crumb and may contain a suitable food-grade antioxidant. The crumb form, in addition, may contain a suitable food-grade partitioning agent. 

The hood and other inaccessible parts of fryer are equipped with spray nozzles as part of the cleaning-in-place (CIP) system installed by the fryer manufacturers. The CIP systems are common with equipment designed for operation in the plants processing meat, poultry, or fish where daily sanitation of the frying system is mandatory. However, the fryer pan, the piping, and the external oil heater must be washed with caustic solution or sanitizing chemical to remove the residual oil and crumbs left after the fryer is emptied. 

In converting ESBR latex to the dry mbber form, coagulating chemicals, such as sodium chloride and sulfuric acid, are used to break the latex emulsion. This solution eventually ends up as plant effluent. The polymer crumb must also be washed with water to remove excess acid and salts, which can affect the cure properties and ash content of the polymer. The requirements for large amounts of good-quality fresh water and the handling of the resultant effluent are of utmost importance in the manufacture of ESBR and directly impact on the plant operating costs. 

Styrene-butadiene copolymers are extremely important to the rubber industry. They are particularly important in tire manufacture. Styrene-butadiene polymer is produced by emulsion polymerization and solution polymerization. Most of the volume is by emulsion polymerization. This affords the opportunity to prepare polymer nanocomposites by several avenues. One can blend an aqueous dispersion of the nanoparticles with the styrene-butadiene latex before flocculation to produce the rubber crumb, disperse an organically treated nanoparticle in the styrene-butadiene solution polymer before the solvent is stripped from the polymer, disperse the organically treated nanoparticles into the monomers, or prepare the rubber nanocomposite in the traditional compounding approach. One finds all of these approaches in the literature. One also finds functional modifications of the styrene-butadiene polymer in the literature designed to improve the efficiency of the dispersion and interaction of the nanoparticles with the polymer. 

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. 

Tyre rubber is developed specifically for tyre manufacturing. It is prepared by coagulating NR latex containing oil with wet crumb rubber in the following proportion 30% latex rubber, 30% unsmoked sheet, 30% field coagulum and 10% plasticizer (oil). 

Pasta products are manufactured continuously by a vacuum extruder, which consists of a mixing trough and press segments. The vacuum is used to retard oxidative degradation of carotenoids. The semolina and added water (30%) and, when necessary, egg or egg powder are mixed in a mixing trough to form a crumb dough (diameter 1-3... 

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. 

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