Professor Carmen Torres-Sanchez dives into her work in the Multifunctional Materials Manufacturing Lab at Loughborough University, exploring how reinforcing polymeric foams with short fibres can achieve lightweight materials with greater mechanical properties.
Loughborough University
Lightweighting
Composites that use foams in their core and are reinforced with carbon fibre to give them mechanical integrity are a successful approach to lightweighting. Now more than ever, the automotive industry is hungry for solutions to ease mass in our cars and vans, as the emission targets become stringent. A larger mass requires more fuel to move it, and more fuel creates fumes that are expelled into the atmosphere.
However, the problem is not unique to internal combustion engines. Electric vehicles carry a heavy load: their battery. Added to the actual mass of the chassis itself and the passengers, we encounter another heavy mass that needs “fuel” to be moved around. In this example, we want to lower the mass to maximise battery duration and reduce power consumption.
Adding carbon fibre reinforcement to polymeric foamed matrices gives synergistic properties to the composite. The carbon fibre load is typically 30 or 40% vol., which is a blow to the lightweighting approach since carbon fibre itself is quite heavy. It’s widely recognised that increasing the content of fibre reinforcement and the degree of fibre alignment in short-fibre composites leads to an increase in mechanical properties.
However, no studies report on the lower threshold for reinforcement content to enhance mechanical properties. Most research uses comparatively large percentage volume or percentage weight loading of fibre reinforcement, which over-designs the composite and leads to it being heavier than required.
We wanted to study whether increasing mechanical properties with a lower carbon fibre than the typical %vol. content is possible. Although counter-intuitive at first, we aligned the fibres in a desired direction to maximise load transfer in the direction where stiffness is required. Despite the fibres being less, they’re more effective, improving the overall performance. How low could we get? 10%, 5%, 1%. We set out to explore the minute addition of fibre (circa 0.5% vol.)
Would such a minuscule addition make a difference?
Manufacturing these composites requires the selection of the cellular polymeric core and its reinforcement. We wanted to keep this work relatable to the current industrial practice, so we chose polyurethane as the polymer and carbon fibre as the reinforcement.
Polyurethane (PU) composites are widely used due to their strength, low density and low cost as well as production scalability. The PU raw material comes in two separate liquid chemicals that, once combined in equal amounts, rapidly expand to 200% of the original volume and cure hard, taking the shape of the mould where it’s placed. For us, there was an opportunity to shape and further increase the strength of these parts by forming a composite of PU and short carbon fibres that are added at the liquid and expanding stages.
However, if the fibres are left to mix with the expanding foam in the mould, their final location will be random when the foam is fully cured. While the resulting composite will be lighter, the random reinforcements will not deliver the enhanced desired mechanical properties. This is why we had to find a way of “guiding” short fibres to the locations where they are needed when the composite is deployed in service.
After much thinking and early prototyping, we decided to use weak magnetic fields to guide the short fibres to the desired zone in the composite bulk. This incurred the alignment that computational mechanics had indicated the localised reinforcements would be most beneficial. In this case, with a structural application in mind, the desired location for the short fibres was around the perimeter of the beam.
However, carbon fibre doesn’t have natural magnetic properties, meaning it wasn’t attracted by our magnets. But what if we coat the carbon fibres with a magnetic ‘skin’ that makes them active in a magnetic field?
In the pursuit of this functionalisation of the reinforcement, we created a novel way of coating the carbon fibres using magnetite nanoparticles attached via a facile electrochemistry route – quick and suitable for mass production. Once coated, these magnetised fibres could be aligned within the reacting polyurethane foam using a relatively weak source magnetic field created with magnets. The magnets aided the positioning of the magnetised fibres and their orientation, improving mechanical properties (Figure 1).
We tested the % vol. content of short, magnetised fibres as low as 0.1, 0.2 and 0.4% to achieve the minimal loading of fibre into the composite. The beams were manufactured using the insitu magnetisation of carbon fibres. These were then guided, oriented and positioned in the zones of the reacting polymeric core where reinforcement was most needed. The reinforced composites were tested in tension and compression.
The results
The results demonstrate that the performance of the reinforced composites improved over the blanks significantly (Figure 2). In addition to the contribution of the intently located reinforcement, the nanoparticles that coated the fibres improved the interfacial adhesion between the matrix and the fibres. This means more force was required to pull the fibres out of the cellular core when these were coated. The reinforcement with short carbon fibre and the increase in interfacial matrix-fibre properties led to improved mechanical properties of the lightweight beams.
This offers a novel method to align reinforcing fibres within a polymeric matrix using magnetic fields. The composites’ mechanical properties can be tailored to specific applications and load requirements. Since the addition of extra mass is minimal, these results are promising for the manufacturers of structural beams and composites.
In summary
We used a hybrid manufacturing process that comprised:
(1) insitu functionalisation of carbon fibres with magnetite nanoparticles created and deposited via electrodeposition that gave them magnetic properties as well as a rougher surface, chemically compatible with the matrix. Due to that newly acquired property, the short fibres could be guided, oriented and positioned in a mould where a magnetic field has been deployed.
(2) Informed by structural mechanical simulation, the composite reinforcement was required on the beam’s outer perimeter.
(3) Using a weak magnetic field, applied simultaneously to the polyurethane foam reacting in the mould, the short functionalised fibres were positioned in those zones of the reacting polyurethane foam core where reinforcement was most needed.
(4) The composite beams’ mechanical properties were improved with content loading as low as <0.4%vol, making this approach low-cost for manufacturing lightweight composites.
(5) The presence of well positioned and oriented short fibres, as well as the coating on the fibres that improved interfacial adhesion with the matrix, contributed to the composites’ improved performance.