There are two major types of resin used in the production of composites — thermosetting and thermoplastic. Thermosetting resins are currently the most common, but as composite use expands, thermoplastic resins are increasingly being explored.
Thermosetting resins are hardened via curing, using heat, to form heavily cross-linked polymers with insoluble or infusible rigid bonds that will not melt on exposure to heat. On the other hand, thermoplastics are branches or chains of monomers that soften when heated and solidify once cooled — a reversible process with no chemical bonding. Put simply, you can re-melt and re-form a thermoplastic, but not a thermoset.
Why thermosets rule
Thermosetting resins, such as epoxies or polyesters, are popular for composite production because their low viscosity helps achieve good penetration into the fibre network. This allows for the use of more fibres and increases the strength of the final composite material.
The process for a thermosetting resin begins, in the pultrusion process, with the fibres being immersed in resin, which are then pulled into a die where heat is applied. This starts the curing reaction that converts the low molecular weight liquid resin into solid three-dimensional network structure, locking the fibres into this newly formed network.
Because most curing reactions are exothermic, once the reaction has begun it will readily propagate, making thermoset production easily scalable. Once set, the three-dimensional structure locks the fibre in place and gives the composite its strength and rigidity.
The rise of the thermoplastic
Thermoplastics and thermoplastic composites have been around for some time, especially for short fibre applications. But new attention is being drawn towards thermoplastics, due to the rising need for additional light weighting without the loss of structural stability, especially in the automotive industry.
One specific example is the potential to use thermoplastic composite to reduce the weight of the interior components of a car door. In fact, a major Japanese car manufacturer recently began redesigning its interior door components with thermoplastic composites. It is believed that this material shift could reduce the weight of the doors by almost half.
The success of thermoplastics in the composite industry will depend on businesses developing products and processes that work. Exel Composites, the world’s largest producer of thermoset pultrusion and pull winding composites, is already developing its thermoplastic offerings.
It’s not just cars that will benefit from the adoption of more thermoplastic resins, especially since new commercial aircraft often comprise of more than 50 per cent composite parts.
There are many reasons why thermoplastic composites could easily become strongholds of the transport market. Components made from the material can be welded, reducing the need for adhesives, as well as over moulded to produce advanced geometries with superior mechanical properties compared to other materials.
The universal advantage of thermoplastic resins is that they can be softened and reformed endlessly without major loss of physical properties. Once a thermoplastic product reaches the end of its life cycle, it can be melted and reformed for a new application, decreasing material waste. Other advantages can be found from the physical properties of the materials themselves, as well as potential new applications where thermosets have not been suitable.
There is still much research to complete before thermoplastic pultrusions become mainstream, especially because most production methods cater for thermosetting resins, and will have to be adapted.
Thermoplastic resins are showing great potential to produce strong, lightweight composites that are easily recyclable. While it’s not time to abandon tried and tested thermosets, it is wise to keep an eye on developments in thermoplastics, particularly if sustainability is a priority.