Thermoplastic composites are presented as an alternative for the industry compared to thermoset composites. The latter use as matrix a thermosetting resin that acquires rigidity when the crosslinking of its polymeric chains occurs.
As a result of this reaction, the composites cannot be reprocessed again, making their recyclability difficult. Therefore, the alternative is to replace the thermosetting matrix with easily recyclable thermoplastic polymers, since they can be remelted during several processing cycles.
Thermoplastic composites are formed by a matrix of thermoplastic polymer reinforced by long fibers that can be made of carbon, glass, basalt, polymer or any other type of material, including natural fibers or polymeric fibers. In addition, additives that provide the composite with other functionalities such as fire resistance, electrical conductivity, antibacterial, among others, can be included.
Its main benefits are:
- Under weight.
- High mechanical resistance without losing ductility: The incorporation of long fibers allows to obtain high rigidity, without compromising impact resistance, as occurs with reinforcement with short fibers.
- Adaptable to different manufacturing processes: injection, compression or automatic fiber deposition.
- Short manufacturing cycle times, allowing high production volumes.
- Combination with other materials, composing lower cost hybrid materials according to the requirements of each piece: over molding, compression and welding are examples of processes that can be used to form hybrid solutions.
- Circular economy: the advantage of thermoplastic composites over traditional thermosets is that they are easily recyclable. In addition, a biodegradable polymer can be used as a matrix and even as a reinforcing fiber, having a totally sustainable solution.
Other highlights
The development of thermoplastic composites began in the aeronautical sector, using technical polymers with high mechanical properties and high resistance to temperature, such as PEEK (Polyetheretherketone), PEI (Polyetherimide), PI (Polyimide), PPS (Phenylene Polysulfide) and PAEK (Polyaryletherketone), reinforced with carbon fibers.
In the automotive sector, the most widely used polymers are PP (Polypropylene) and PA (Polyamides), almost always reinforced with fiberglass, being a cheaper composite and greatly exceeding the properties of traditional short-fiber thermoplastic composites .
A cheaper alternative is the use of recycled polymers. Its reinforcement with long fiber allows to increase its properties, which can be a very suitable strategy for the recovery of plastic waste.
Other sectors in which thermoplastic composites can be of great interest are rail, naval, space and defense.
The production process of long fiber pellets and tapes is based on the pultrusion of thermosetting composites in which the reinforcing fibers are guided to the resin impregnation bath and subsequently they are passed through a guide mold or gauge that will give the shape of the profile.
In the case of thermoplastic composites, the process is somewhat more complicated since the impregnation bath is fed with an extruder ensuring that the polymer is melted and the fiber is correctly impregnated.
The selection of the matrix polymer is crucial, its rheological properties being a highly influential factor in the correct impregnation of the fiber.
In case the fiber is not well impregnated, the interface between the matrix and the reinforcement will not be appropriate. Gaps will be generated between both phases that will be the beginning of cracks and will enhance the mechanical failure of the material.
The chemical compatibility between fiber and matrix is another factor to take into account. The ensimage of the fiber and the incorporation of compatibilizing additives in the matrix will help to improve this compatibility and to equalize polarity and surface tension of both materials.
On the other hand, the incorporation of additives in the matrix must be carried out ensuring the correct mixing thereof, obtaining a matrix with isotropic properties.
Incorrect mixing will result in agglomerates that will act as stress concentrators and will invalidate the stress behavior simulations performed in finite element calculation (FEM) software.
To ensure the correct mixture, twin screw co-rotating extruders are used. Spindle configuration and processing conditions can be optimized through simulation.
Participation of Aimplas in the sector
Aimplas, Instituto Tecnológico del Plástico, has Ludovic software, which allows reducing the number of experimental tests and optimizing the quality of the compound with fewer experimental tests, as well as a pilot plant scale long fiber impregnation line with which samples can be obtained with 1-3 kg of material.
This pultrusion line is coupled to a twin screw co-rotating extruder, with which unidirectional “tapes” are obtained for fiber deposition (AFP and ATL) and long fiber pellets for injection or compression molding.
The matrix can be modified to provide these materials with new functionalities, such as fire resistance, electrical or thermal conductivity, electromagnetic shielding, among others.
The line has been designed and optimized thanks to the FIBRALLARGA project, which is financed by the Conselleria d’Economia Sostenible, Sectors Productius, Comerç i Treball de la Generalitat Valenciana through grants from IVACE with co-financing from FEDER funds from the EU, within the FEDER Operational Program of the Valencian Community 2014-2020.
These grants are aimed at technological centers of the Valencian Community for the development of non-economic R&D projects carried out in cooperation with companies for the 2019 financial year.
