Compound composite challenges
Sheet moulding compound (SMC) composites offer potential advantages to automotive manufacturers, but their use has been limited due to concerns surrounding the reproducibility of high quality parts. A EUREKA SURFAS project made up of French and Swiss researchers sought to address these problems, and their results have been unveiled.
Produced from a mixture of glass fibres and a paste containing a thermoset resin (such as unsaturated polyester), a thermoplastic resin (polyvinyl acetate or polyethylene), styrene and calcium carbonate fillers, SMCs are lightweight, have high-corrosion resistance, and are suitable for medium-to-high rate processing. If used in vehicle body panels, they could reduce the car’s weight and, in turn, decrease carbon emissions.
But while SMCs are used increasingly in bumper beams and panels, they have yet to be fully accepted as a suitable replacement for steel and aluminium. Cracks and blisters often appear in the finished components, making them unacceptable for body panels. The rework rates for SMC parts are, on average, eight per cent, while the reject rates of total production are three per cent, rising to 50% for complex horizontal shapes.
‘Surface quality per se is not the issue, as there are many [original equipment manufacturers] that already use SMCs on production vehicles,’ says Simon Buckingham, a Technical Specialist at Jaguar and Land Rover in the UK. ‘The challenge is more about delivering a true “A” class, defect-free surface first time, every time.’
Veronique Michaud, Project Leader from the Polymer and Composite Technology Laboratory, Switzerland, explains, ‘We needed to better understand the parameters that affect surface quality, so that reproducibility can be achieved.’
Through research carried out by the SURFAS team, it was found that air becomes trapped in the paste during mixing, which can lead to component blistering.
The sizing of the glass fibres also appears to affect the rigidity of the fibre bundle and its surface energy, which impacts the air pocket or void content in the SMC. The researchers noted that an optimised glass fibre should have a low expansion rate during impregnation of the paste to improve wetting, but a high expansion rate during moulding to increase shear in the paste and evacuate potential voids remaining in the bundles.
Scientists also investigated low-profile additives (LPAs) such as polyvinyl acetate. These are combined with the paste to decrease roughness, but can contribute to gelation of the SMC in the early stages of curing, increasing the number of voids.
‘The biggest achievement of SURFAS from an LPA point of view was the identification of spinodal decomposition (a reaction-induced instability of the unsaturated polyester/LPA/styrene mixture that causes a phase separation) as the mechanism for how LPAs work,’ says Marcel Montagne, formerly of SURFAS industrial partner Dow Automotive, based in Switzerland.
This understanding differed from the previous nucleation and growth theory, which suggested that the LPA acted as a barrier around the unsaturated polyester, making it difficult for styrene to diffuse into the reacting site. This belief limited the role that LPAs could play in composite forming.
While Buckingham applauds the research into SMCs, he notes that complications still exist with further adoption into the automotive industry, specifically regarding the EU End of Life Vehicle Directive on recycled material content. Sheet moulding compounds may be difficult to recycle. ‘Some challenges remain regarding infrastructure and the quantity of recyclate material available, allied to the market demand and cost for such material.
‘Once in production, it is important to consider repair. Is it possible to fix minor damage [as with metals] or is it necessary to replace the whole assembly? If these issues can be addressed, then it may be possible to step around any first-time-through paint shop issues,’ he adds.
‘We have made steps towards understanding some issues and improving some materials, [but] a lot remains to be done,’ says Michaud. While the SURFAS project ended some time ago, Michaud hopes that the automotive industry will continue to develop the information gleaned.