Cryogenic ground rubber

The fourth trend is spurred by environmental sustainability concerns and the need for increased recyclability and reuse of polyolefin blends. In this regard, there is increasing replacement of PVC by polyolefin-polyolefin blends. There is also an increase in recyclability of EPDM mbber vulcanizates since EPDM is the fastest growing elastomer among synthetic rubber and the most used of nontire rubbers. Also, cryogenically ground rubber tires are being used as fillers for polyolefin blends such as LLDPE/HDPE. 

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

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]. 

WRAP have also funded a project that looked into the use of cryogenically ground 0.5-1.5 mm rubber particles from waste car tyres [79] to help create a new artificial football pitch at Woodley Sports Football Club in the UK (Figure 7.7). The rubber particles were used in the FieldTurf third-generation artificial sports pitch product, which is supplied worldwide by the American company... 

Figure 7.7 The third-generation artificial football pitch at Woodley Sports Football Club, near Stockport, UK, contains cryogenically ground car tyre rubber. Reproduced with permission from the WRAP Quality Protocol, Waste and Resources Action Programme, Banbury, UK. WRAP (see note with Figure 7.1)...

Rubber tyres are by far the most visible of rubber products. Identification is trivial and collection is well organized. Recycling and disposal, however, are less evident. A major route for tyres is their use as a supplemental fuel in cement kilns. Major compounds in tyres are styrene-butadiene rubber (SBR), synthetic and natural polyisoprene rubber, steel cord, carbon black, zinc oxide, sulphur and vulcanization-controlling chemicals. Tyres can be retreaded, which is economic for large sizes (truck tyres), or ground to crumb or powder (cryogenic grinding). Such materials have some limited market potential as an additive in asphalt, and in surfaces for tennis courts or athletics. 

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... 

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