Electron radiation for natural fibre-reinforced composites
Researchers in Germany have shown that electron radiation can improve interfacial bonding in natural fibre-reinforced composites. The process may be faster, cheaper and more environmentally friendly than existing methods.
The research has been conducted at Hochschule Bremen – University of Applied Sciences, alongside radiation specialist BGS Beta-Gamma-Service in Bruschel. The scientists have measured the fibre-matrix interface adhesion between a polypropylene matrix and bundles of flax or hemp, or single fibres of cotton and ramie, using the single fibre fragmentation test on composite samples of about 0.22-0.28mm in thickness.
The study has found that although irradiation reduces polypropylene’s tensile strength, it increases fibre adhesion to the matrix by up to 50% compared with current coupling agents, even at intensities as low as 5kGy. Higher intensities lead to better adhesion.
Professor Jörg Müssig, one of the authors at Bremen, says, ‘The improvement in adhesion is greater than the reduced tensile strength. The process is also quicker to use than existing coupling methods and, because it uses no extra chemicals, would be cheaper and more sustainable.
‘The levels of irradiation are typically of the same order as used in food processing,’ he adds. ‘It is a commercialised industrial process on a conveyor belt system, and is also a standard technique in the plastics industry.’
Dr John Summerscales, Associate Professor of Composites Engineering at the University of Plymouth, UK, sees potential in this work. ‘I like the idea of using the fibres in their “raw” state and subjecting the composite to electron-beam processing. The alternative approaches to modify the interface involve potentially hazardous chemicals and process waste’.
The study does not, however, identify an ideal radiation intensity for [balancing] tensile strength and adhesion. Dr Summerscales says, ‘Limitations are the low number of samples (especially just two for flax), [the need to] scale up to realistic fibre volume fractions (rather than single fibre), and a need for scanning electron microscopic images to confirm where the failure is occurring (at the interface or in one of the bulk components).
‘[Also] are the electron beams attenuated quickly and hence of less use in thick composites?’ As a result, he says, ‘I suspect commercial application is about five years away.’
Dr Brendon Weager, Technical Manager at UK-based consultancy NetComposites, says, ‘Fibre-matrix adhesion is one of the key issues to overcome if natural fibres are to achieve significant breakthroughs in the composites market. The results in this paper suggest that irradiation can improve the interfacial adhesion of a range of natural fibres to polypropylene beyond that of a particular maleic anhydride modifier, which is encouraging.
‘The next steps might be to conduct larger-scale tests on composite laminates and to assess the long-term durability of the interfaces in different environments according to temperature, humidity and so on.’
Prof Müssig concedes that research is still in its infancy. Tests will commence on injection-moulded samples with and without coupling agents and irradiation. He also hints that the process could be applied in the automotive industry in the future.