Replacing the glass fibres in sheet moulding compound (SMC) automotive parts with natural wood or plant cellulosic fibres is the focus of research at the College of Forest Resources, Mississippi State University, USA.

The team claims this work will reduce the cost of vehicle components, as well as improve their environmental credentials by employing a lighter weight, non-petroleum based, biodegradable and organic component. A fall in weight will improve the vehicle’s fuel efficiency and in turn decrease carbon emissions.

Sheet moulding compounds are composites that are conventionally produced from a mixture of glass fibres, thermoplastic and thermoset polymeric resins, and styrene and calcium carbonate fillers. They can be used as lighter weight alternatives to steel and aluminium in automotive bumpers and body panels.

Using natural fibres could advance SMCs, says Assistant Professor Sheldon Shi at the University. He explains, ‘The density of the natural fibre is in a range of 0.6-1.5g/cm3, while the fibre glass is above 2.6g/cm3. The cost of natural fibre is in a range of US$0.33-0.88/kg, while the cost of E-glass is about US$2.5/kg’.

Replacing the 30% glass fibre content with natural materials is involving extensive research into a number of areas, such as a chemical retting process to produce high quality fibres and a biodegradable soy-based adhesive to further improve interfacial compatibility between the hydrophilic fibres and hydrophobic polymers. The materials under investigation are wood pulp fibres from Southern pine and agricultural stems from kenaf.

For the wood, Shi says the challenge is to obtain longer length fibres from the pulping process, as shorter fibres inhibit strength, and hence ability to reinforce composites. This is compounded by the fact that pulping introduces additional micropores in the cell wall structure – these micropores could cause defects such as interfacial failure or air pockets during SMC composite manufacture.

Impregnating these micropores with sodium carbonate and calcium chloride nanoparticles in a pressurised reactor is said to work as a void filler. ‘We have optimised the [impregnation] reaction temperature, time and molar ratio. The [amount of ] nanoparticle loading should depend on the fibre condition, around 15-25%,’ explains Shi.

The final SMC part is then produced by laminating, rather than conventional mixing, of the impregnated fibres into a mat or sheet. The formulated resin and soy adhesive is applied to the mat, followed by the usual compression moulding of the raw materials into a component.

Shi describes, ‘Nanoparticles deposited on the fibre surface will serve as nucleation sites to initiate crystal formation of the [molten] polymer matrix around the fibre’, improving compatibility. The fibre itself could not stimulate nucleation due to ‘unbalanced free energy’ between the two materials. He adds, ‘The inorganic nanoparticles modify the natural fibre surface to be more hydrophobic [in line] with the polymer’.

Preliminary tests have revealed that natural fibre SMC parts can compete with existing composites in terms of density and flexural strength. Tests into water absorption, weathering, impact resistance and damping are ongoing.

Dr Brett Suddell, Senior Materials Scientist at environmental consultancy ADAS UK Ltd, Preston Wynne, UK, believes the work has potential. He says, ‘This is a very sensible approach to addressing a number of the inherent limitations that come with using natural fibres. Natural fibres by definition suffer from variable properties that can be attributed to the growth cycle [and processing]. The one main issue, as in all cases, will be transferring the lab-based trials and scaling these up into full scale manufacture’.

He adds, ‘Natural fibres have before been confined to predominantly interior car components due to their poor impact resistance. Work is ongoing to create a range of products that have structural strength and improved impact properties but this is in combination with synthetic fibres such as carbon for example. Wood can be a stronger material than bast fibres such as kenaf, but they are shorter. Kenaf is also attractive in that is grows so quickly when compared to wood’.

Lifecycle analysis is also an area where investigation is ongoing, therefore issues remain surrounding the environmental credentials, for example, if the researchers can separate the biodegradable content from the synthetic polymer for disposal at end-of-life.

Shi says, ‘Incorporating natural fibre into the SMC increases the biodegradability of the material, and it should reduce the landfill cost for the material. It would be nice to separate, however, I would anticipate a high cost, since a chemical process might have to be used’. The team is exploring the use of biodegradable polymer matrices and claims the nanoparticles do not impact on the fibres’ biodegradability.