Within the field of composite materials, we find a wide range of processing techniques based on the use of reinforcing materials mainly focused on textile structures and matrices of polymeric materials. Depending on the thermoplastic or thermosetting nature of the matrix used, the most effective techniques are chosen.
Thermoforming has been for a long time the most traditional technique, used for processing continuous-fibre thermoplastic composites, but thanks to the development of new materials and formats in recent years, many more techniques are emerging.
Thermoplastic acrylic resins
One of these new materials are thermoplastic acrylic resins. Acrylic resins are generally characterized by its transparency, bright surface, resistance to different weather conditions and high hardness. Due to their characteristics, these materials are used in a large number of applications, such as cars, household appliances or in construction.
The novelty in this family of materials lies in its adaptation to the field of thermoplastic composites.
On the one hand, it has been achieved that they are acrylic resins with thermoplastic nature. These resins act like a thermoplastic regarding thermoforming, reprocessing, recycling and welding possibility, but with mechanical properties similar to the thermosetting resins commonly used in the sector.
On the other hand, they have been adapted so they can be used when producing composites with techniques already available, such as manual lamination, infusion, RTM or pultrusion.
Adapting the designed processes for composites with thermosetting resins to those with thermoplastic materials is a major challenge. The main problem lies in the high viscosity and the low Flow Rate of thermoplastic polymers, making the impregnation of the reinforcement and the consolidation of the composite materials more difficult. For this reason, they are a great progress for the use of thermoplastics in the field of composites, thus increasing the number of possible applications.
The most used continuous-fibre thermoplastic prepregs are currently commingleds, towpregs and pre-consolidated tapes (PCT).
Commingleds are a type of pre-impregnated thermoplastic that consist in the combination of reinforcement fibres with thermoplastic fibres, thus forming a mixed roving of reinforcement and matrix.
Towpregs are thermoplastic prepregs that consist in the addition of particles of a thermoplastic polymer to reinforcement fibres.
Finally, PCTs are a type of thermoplastic prepreg that consist of a fibre roving, which has been impregnated by melted polymer and which has been given the shape of a flat tape.
This type of materials can be used in processes such as pultrusion, filament winding or 3D printing by making small changes in the equipment's configuration.
Consumption and market trends
The continuous-fibre composite materials’ sector is clearly dominated by thermosetting composites, but thermoplastic composites have also found their market niche.
Within the sector of carbon fibre-reinforced composites with polymer matrix (CFRP), thermoplastic matrices are 26.3 % of the turnover (around 3.48 billion dollars) and have progressively increased over the last years.
On the other hand, we find glass fibre-reinforced composites (GRP), 95 % of the total volume in the composites market. Within GRPs, continuous-fibre thermoplastic composites represent 13 % of the production volume, thus placing it around 145 kilotons per year in Europe.
The increase of this type of materials is mainly due to their use in a large number of projects in the automotive industry and to their advantages regarding processing, cycle times, reprocessability and recyclability.
In the Plastics Technology Centre (AIMPLAS), different R&D projects are being developed at European and national level in the field of continuous-fibre thermoplastic composites. An example of this is the project RECOTRANS, within the framework of the programme Shift2Rail. The aim is to cover one of the current demands of the market, which is the achievement of high-performance materials with more flexibility regarding the processing possibilities (reprocessing), as well as a greater environmental responsibility (recyclability and repairability).