Ambient ground rubber

If the starting material is a composite one, such as tyres, it has been found that ambient grinding processes tend not to remove as much fabric and metal from the final crumb product as cryogenic processes. For example, an ambient ground rubber has been found to have a fibre content of 0.5% and a steel content of 0.1%, whereas that produced by a cryogenic process had levels below the detection limit for these two components [10, 11]. 

FIGURE 38.8 Ambient ground acrylonitrile butadiene rubber (NBR) waste powder. (Reprinted from Anandhan, S., De, P.P., Bhowmick, A.K., Bandyopadhyay, S., and De, S.K., J. Appl. Polym. Sci., 90, 2348, 2003. With permission from Wiley InterScience.)... 

Figure 1. Ground rubber tire partides. (left) cryogenically ground, (right) ambiently ground.

In order to be used as a filler in polymer composites, tires are first ground into a fine powder on the order of 100-400 im. This is accomplished typically through either cryogenic or ambient grinding. General reviews of the size-reduction process have been published. A typical process generally involves tire splitters to cut the tire initially, followed by a two-roll grooved-rubber mill or hammer... 

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). 

Another example of applying characterisation tests in this way concerns a group in Brazil [2], who started with scraps generated during the extrusion of SBR and EPDM profiles, and ground them under ambient conditions. The rubber crumb materials that were obtained had irregular-shaped particles with a high surface area and... 

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... 

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