Tire crumb polyethylene composite with matrix ligaments
28 August 2007
Tires, along with the other thermoset materials, are difficult to recycle as they cannot be reshaped once they have been cross-linked. However, they can be ground into a crumb that can be combined with another material, such as polyurethane, to make a new products, such as playground surfacing. Such materials are however quite expensive. This work examined the feasibility of combining tyre crumb with a second waste stream product, post consumer milk container polyethylene.
The addition of rubber to thermoplastics is common and has given rise to classes of materials, such as thermoplastic elastomers (TPE), and toughened engineering polymers, such as high impact polystyrene (HIPS). However, post consumer tires are not a suitable source of rubber for these materials. Tires are in fact a mixture of many component materials, with different rubbers such as SBR and EPDM being used for different parts of the tire. Steel and polyester are also used for the belts and cords. While the belts and cords can be removed during grinding, the final product is a mixture of the various rubbers that constitute the tire. Additionally, the economics of tire reduction generally set the lower limit of crumb size to around 100 µm which is 100 fold the diameter of the rubber particles used in toughened engineering polymers.
In this work tire crumb polyethylene composites were made by reaction injection moulding. The tyre crumb rubber and polyethylene are incompatible and compatibilisation was carried during the moulding step (i.e., on screw) using brominated dimethyl-phenolic resin and dicumyl peroxide. Figure 1 shows the fracture surface of an impact specimen broken at room temperature. Ligaments can be seen joining the tire crumb to the polyethylene matrix indicating that compatabilisation has been successful and that good bonding between the tyre crumb and the polyethylene matrix has been achieved.
Figure 1: Fracture surface through tire crumb polyethylene composite. A piece of tyre crumb can be seen on the left. While the fracture has accused separation of the rubber from the matrix ligaments joining the two components remain indicating that successful bonding has been achieved.