Wheeling out microcrystalline cellulose
Partially replacing silica with renewable microcrystalline cellulose (MCC) could make tyre production and use more energy efficient, claim researchers at Oregon State University, USA.
In their study, MCC was used as the third reinforcing filler (alongside silica and carbon black) in styrene butadiene rubber and poly-butadiene rubber composites, replacing 12% of silica.
This approach is said to consume less energy in dispersing the lighter MCC in the matrix than the same amount of silica, particularly as the MCC particles are less likely to aggregate. Reduced aggregation also decreases the build up of heat in breaking the clusters down and the Mooney viscosity, enhancing production.
Laboratory tests have also revealed that MCC does not negatively impact upon the mechanical properties of un-aged and aged rubber composites. The tensile strengths and modulus at 100%, 200% and 300% elongation were higher than for those materials without MCC. The lower Young’s modulus of MCC compared to silica, however, prevents complete replacement of the latter.
Furthermore, the delta tan (traction) of the MCC composites is said to be comparable to
conventional materials at low temperatures (23ºC), ensuring high rolling resistance between the tyres and the road during rainy seasons. While the decrease in delta tan at higher temperatures using MCC will reduce rolling resistance during the summer, and thus improve fuel efficiency and increase vehicle mileage.
Research into renewable fillers for tyres is not new, but the team at Oregon believes it may be the first to explore MCC in this application.
‘Carbon black and silica both come from non-renewable sources. Their production requires huge amounts of energy and they both have high density, which has a negative impact on the fuel efficiency of automobiles,’ explains Kaichang Li, Professor of Wood Science and Engineering at the University.
Microcrystalline cellulose is produced by acid hydrolysis of cellulose from plant fibres to remove the amorphous regions of the substance.
The researchers say the new rubber composite could be incorporated into conventional tyre manufacturing with no modification, however, collaboration with tyre manufacturers is ongoing to demonstrate this. A ‘slight decrease’ in tear strength and hot tear strength will require
investigation of the impact on the tyre’s durability.
Dr Jane Clark, a specialist in rubber research at Loughborough University, UK, says, ‘The work [at Oregon] has potential but these are only basic measurements. The existing fillers that are used in tyres work extremely well. You have to work hard to get the same properties. Their process might be more efficient but if the tyre wears out quickly that’s not good enough.
‘With biopolymer renewable fillers, unless the particle size is reduced and interaction with the elastomer is increased, it won’t be possible to substitute more than a token amount of conventional filler.
She adds, ‘At the moment, renewable fillers are mixed using conventional methods, however a better way to do the blending [may] be in the aqueous phase – mixing rubber latex and a solution or dispersion of the biofiller. It can solve the particle size problem (the particles dissolve or disperse very finely). An added benefit is that it is a low energy process. However, only a few products are made using latex (like rubber gloves) and so the tyre and other rubber engineering industries are not set up for handling [it]. It is certainly a future possibility, providing that the product performance can be proved’.