RockTron filler offers greener green tyres

By David Shaw, ERJ Staff

London -- RockTron fillers are derived from pulverised fuel ash (PFA). In comparison with manufactured raw materials, they are cheap - in the range of hundreds of euro per tonne, rather than thousands.

PFA is composed of small spheres of alumino silicate glass and magnetite and other materials. Some are solid, others are hollow.

RockTron has found a way of separating out these different components and turning them into useful materials for a wide variety of industries. It has received multi-millions in funding and has a pilot plant in the UK and has recently completed a much larger plant, capable of processing 800 000 tonnes/year of the ash, which is currently producing material for a range of industries.

The surprising thing is that adding small percentages of the alumino silicate glass spheres to rubber tread compounds appears to improve both wet grip and rolling resistance at a reduced cost.

Furthermore, using the same filler in EPDM extrusions appears to offer significant cost savings as well as reduced density.

Godfrey Short, director of RockTron Advanced Products Business Development said he has funded initial tests of these compounds at Artis, the former Avon Rubber research unit. The independent researchers reported that substituting a small percentage of silica with the RockTron material resulted in “significantly increased tan-delta at 0°C in conjunction with slightly reduced tan-delta at 70°C.” In addition, it indicated that the performance is enhanced with lower levels of organosilane.

On the tyre side, Artis also noted that abrasion performance is not so good while hardness and the M300 modulus also suffered. 

Short said he has approached two tyre companies with this initial research and they have been very keen to take the research forward. 

On the extrusions side, Short said his emphasis has been on the plastics business, so he has not made contact with any rubber extrusion company. 

Pulverised Fuel Ash

Pfa is produced whenever a coal-fired power station generates electricity. The coal is burned and results in flue gases. These are hot enough to melt glass and magnetite, but in the emissions protection systems, many of the gases are cooled down to the point where glasses and other materials freeze out of the emissions. These materials tend to freeze into small spherical particles, up to 100 µm in size. This is the source material for RockTron's process. 

Pfa comprises 82 percent solid alumino silicate spheres; 10 percent carbon; 7 percent spherical magnetite and 1 percent hollow cenospheres made from alumino silicate glass. These hollow glass spheres are a bit bigger than the solid ones, ranging up to around 300 µm.

RockTron removes the carbon, which is returned to the power plant for burning. It separates out the magnetite with magnets and then removes the hollow glass spheres from the remaining solid glass spheres. These solid spheres are graded into size classifications and sold. 

Short said the company is targeting the rubber industry with a D50 size of 7 µm. The colour of the product is a dull grey, so it is not suited to bright white applications, but fits in well in formulations containing carbon black.

Historically, said Short, some other companies have manufactured glass spheres in this size range, which have been used in the plastics and rubber industry as passive fillers. These spheres disperse well, and offer lower density than some other fillers.

Short said RockTron's products sell for prices around a few hundreds of euro/tonne, as opposed to thousands for the manufactured glass products.

Glass spheres in rubber