A method that uses magnetic fields to align tiny structural elements within a polymer matrix has been developed by scientists in Switzerland. The technique allows a polymer composite to be reinforced in three dimensions - something that is difficult to achieve using conventional reinforcement techniques.

'Nature is very clever at producing materials with reinforcement in 3D,' says André Studart at the Swiss Federal Institute of Technology (ETH) in Zurich, who led the research. 'For example in seashells there are two layers, an inner one consisting of tiny platelets of calcium carbonate that are organised parallel to the shell's surface; on top is an outer layer of calcium carbonate rods that are aligned perpendicular to the surface. This provides high hardness and wear resistance on the outer surface, as well as an ability to arrest cracks that reach the inner layer.' 

It has proved tricky to achieve similar 3D alignment of structures in polymer composites.   Standard 2D reinforcement using fibres or mesh is strong in one plane but relatively weak out-of-plane, says Studart. 

The first step of the team's approach was to attach a small quantity of iron oxide nanoparticles to the surface of micrometre-sized aluminium platelets - magnetising the platelets. Rods of calcium sulfate hemihydrate were also magnetised this way.

The platelets or rods were then suspended in a solution of precursors of a range of polymers, including polyurethane, epoxy resins and acrylate. When a low magnetic field was applied, the structures aligned in a specific orientation, becoming locked in place once polymerisation was initiated.

To obtain 3D reinforcement, the structures can be aligned in one direction in one layer of polymer matrix, and in a second plane in a subsequent layer. Or two types of platelet can be included in the same polymer precursor - one with a denser iron oxide coating than the other. 'If we apply a high field initially, all the platelets will align in one direction. If we then apply a lower field in a different direction, the more densely coated platelets alone will respond,' Studart says.   

In tests, the composite materials were shown to have  improved strength and  wear resistance compared to the plain polymers. 

Additionally, the extremely low magnetic fields needed to align the particles - much lower than that emitted by a credit card strip - should mean the method can be scaled up easily and inexpensively, Studart says. 

Emile Greenhalgh, a composites expert at Imperial College London in the UK, describes the approach as 'interesting' and worth pursuing but says that 'particulate reinforcement is not the most efficient way of stiffening/strengthening a material'. Overall, says Greenhalgh, 'this could be a process which could be utilised to develop [specialised] materials, but not to replace high performance structural composite materials.'

Simon Hadlington

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