Fatigue Crack Growth of Recycled Rubber in Natural Rubber/Butadiene Rubber Blends (2015-2019)
Researcher: Dayang Habibah, Abang Ismawi Hassim
The increasing use of rubber in many applications results in a growing volume of rubber waste. The disposal of waste automotive tyres in particular is an increasing environmental problem worldwide due to the ever-increasing numbers of vehicles on the road. Various methods of handling waste tyres have been reviewed as alternative solutions to the tyre recycling issues. Inclusion of rubber powder made from End of Life Vulcanised Rubber tyres into new rubber products will be of significant environmental and economic benefit, provided it does not considerably affect the quality of the product. To date, most rubber or non-rubber companies are able to produce ground rubber from tyre waste, but the main issue is relatively low quantities being used in the manufacturing of new high performance products or re-used in new tyres.
The method of production of recycled rubber powder (RRP), such as cryogenic or ambient temperature milling of the End of Life Vulcanised Rubber Tyres, affects the morphological structure and size of the particles. The ambient process produces a coarse structure, whereas the cryogenic process results in smooth textures. Smaller particles than 400 μm for ambient ground RRP, are currently only produced in small batches, so not readily commercially available, with the only source being cryogenic ground production. Thus, the emergence of smaller sized RRP produced using cryogenic methods in micronised form certainly appear to have great potential for many applications.
The overall aim of this research is to investigate the mechanical and crack growth behaviour with incorporation of RRP in Natural and Butadiene rubber blends. In this research, two types of RRP, smaller size particles (~ 74 μm) produced by the cryogenic method and common commercial particles sizes (~ 400 μm) from ambient ground production are incorporated in rubber compound as recycled filler.
For quasi-static tensile tests, the tensile failure of RRP filled materials is dependent on the filler size and type of rubber matrix. The relationship of the interface morphology with tear strength and abrasion resistance were also discussed. Quasi-static tests are relatively useful for comparing the performance of fillers in rubber matrices, but those tests do not reflect the performance under dynamic applications for real world products. Wear loss in tyres or cracking in rubber mountings is usually associated with crack growth due to repeated cyclic stress. Therefore, crack growth resistance is of great importance in determining the strength and durability of rubber products. How does these particle size of RRP influence the crack growth rates of the rubber compound?
The effect of an artificial sharp cut on the fatigue crack growth (FCG) rate in carbon black filled NR/BR containing CRP400 or MRP074 was investigated in detail. The fracture mechanics approach was used in the crack growth measurement which relates crack growth rate (dc/dN) and strain energy release rate (T, also called tearing energy). Single-edge notched tensile (SENT) specimens were used and crack growth rates were monitored during cyclic deformation with different tearing energies. The dynamic behaviour of RRP in filled rubber compounds was studied in terms of absolute and relative hysteresis loss.
Results indicated a correlation between absolute/relative hysteresis loss and fatigue crack growth rate under specific dynamic strain amplitudes. Differences in relative hysteresis loss showed that additional energy dissipation, due to multiple new crack surfaces at the crack tip, contributes to the FCG of the RRP compounds. At higher tearing energy, beside other factors affecting the FCG performance of the RRP compounds, both higher absolute and relative hysteresis loss are slightly detrimental to the crack growth rates. Microscopic and macroscopic fracture surfaces after FCG test corresponding to the crack path deviation have also been studied.