Let’s begin with some definitions: thermoplastics are a polymer material that becomes moldable at a certain elevated temperature and solidifies upon cooling. In the context of composites, thermoplastic tapes are narrow reinforced strips used for a structural purpose made from a mouldable polymer and a reinforcing fibre.
Thermoplastic tapes are a family of composite materials – they comprise two distinct components: a thermoplastic polymer (often referred to as the matrix) and a fibre reinforcement. See our explainer on Composites for the non-engineer for a great introduction to composite materials. Typically, the reinforcing fibre is carbon or glass but can also be aramid, natural fibres (such as flax, jute or hemp) or a combination of different fibre types. The fibre is pre-impregnated with the thermoplastic polymer to produce the fibre reinforced thermoplastic tape.
Thermoplastic tapes are often referred to as unidirectional (UD) since the reinforcing fibre is in a single direction and continuous. This allows for a wide range of processing techniques which will be discussed further in this article.
Thermoplastic tapes offer different advantages and disadvantages to their thermoset counterparts due to the difference in the matrix properties. One of the key advantages of thermoplastics is their ease of processability by melting, which is in contrast to the chemical processing requirement of thermosets. This not only makes the tape production process simpler, but also the subsequent processing steps to turn the tape into a formed part. Additionally, the nature of the polymer means that thermoplastic tapes can be stored almost indefinitely, compared to the limited shelf-life of pre-impregnated thermoset tapes. This reduces the expense of large freezers for storage of the materials.
Thermoplastic matrices such as PEEK, PEKK can offer similar mechanical properties to epoxies (thermoset matrix) but with higher toughness. In addition to this, many thermoplastics are corrosion resistant making these materials ideal for applications in aggressive environments.
The ability to remelt and reprocess thermoplastics makes recycling them much easier than thermosets. This is becoming an ever-increasing driver for markets such as automotive where such large volumes are involved. The ability to remelt thermoplastics also offers part bonding techniques, such as welding, that cannot be applied to thermosets. This allows multiple processes to be applied in sequence so thermoplastic tapes can be overmoulded/inserted into other parts to provide localised reinforcement.
A drawback of manufacturing thermoplastic tapes is the need for high processing temperatures, especially in the case of advanced engineering polymers such as PEEK and PEKK. These materials require processing temperatures of up to 400°C, which create additional design challenges to the processing equipment.
Additionally, these high-grade engineering polymers are more expensive than the equivalent epoxy thermosets. This is in part due to the lower volume of demand, and is expected to change as demand increases for thermoplastic materials.
For decades much of the production of reinforcing fibres, particularly carbon fibre, has focused on the thermoset market. This had led to sizing (polymer coating on fibres to aid processing and interfacing with the matrix) options developed specifically for thermosets. As a result, the options available for thermoplastic tape processing are somewhat limited. Manufacturers of fibre are certainly working on developing fibre products targeted at thermoplastic matrices, but the options are still limited.
The combination of thermoplastic polymers and continuous unidirectional composite materials offer some interesting benefits to OEM and other users of composite materials.
The combination of low void content tapes and a high level of automation in tape laying processes can allow for low void content structures without the need for autoclave.
Automated fibre placement (AFP) and automated tape laying (ATL) are in-situ consolidation processes that place slit tapes onto a tool using a combination of a heat source in the form of a laser or lamp and pressure from an end effector. For more information on these processes see our explainer, What is Automated Fibre Placement?
At present these methods often require some form of further consolidation (e.g. autoclave) to remove the porosity of the parts. An autoclave is often required because of the high viscosity of thermoplastic polymers, and in some part to account for the inconsistency of the tape materials. The temperature and pressure of an autoclave allow the polymer to flow between the fibres and produce a much more consistent part. If an autoclave is used, 3 to 4 hours is sufficient, significantly less time than a typical thermoset autoclave cure cycle.
The cycle times of thermoplastic tapes are generally much lower than other composite materials. This is for a number of reasons including the ability to do multiple operations, pre-forming methods, and avoiding long cure times. The ability to reform parts allow much wider flexibility in part manufacturing, where one preform could be made into multiple different parts further down a production line depending on mechanical requirements.
Typically thermoplastic tapes have been produced using PEEK and PEKK by a solvent or water-based powder application process. More recently melt-extrusion processes have been developed, removing the need for a solvent or water-based powder, increasing the number of materials that can be used. These methods generally produce thermoplastic tapes with fibre volume fractions in the 40 – 60% range. A high volume fraction would suit applications where mechanical performance is the priority, while lower volume fractions (higher polymer content) are preferred for higher speed and lower pressure processes.
The most common method currently used to manufacture parts from thermoplastic tapes is stamp forming. In this process tapes are cut to a predefined shape, stacked, heated and pre-consolidated. The preform is then inserted into a metal tool to fully consolidate under pressure. The other significant methods for processing thermoplastic tapes are the automated methods such as AFP and ATL discussed in the section above.
The application of tapes requires the manufactured materials to be slit to the desired width for the subsequent manufacturing process. Because of the toughness and springy nature of thermoplastic polymers, slitting can be challenging, requiring alternative winding patterns and methods. However, this rigidity of slit tapes when processing is a potential benefit for automated processes such as AFP and ATL. It is these automated processes that potentially offer the most opportunity for thermoplastic tapes. It is highly likely that automation will continue to grow, and with it so will the uptake and application of thermoplastic tapes.
The real advantages presented by thermoplastic tapes have drawn significant investment in the research and development of the technologies and processes, often coming from the material manufacturers themselves. Whilst there are difficulties presented by these materials, they are manageable and with innovation, in automation and scale-up of infrastructure, we are only going to see the demand for thermoplastic tapes increase.
Coventive Composites has significant experience in the development and use of fibre-reinforced tapes. We can manufacture pilot-scale quantities of a variety of tapes for initial evaluation, and can provide technical support to assist with application development.
Please feel free to contact us to discuss your requirements.
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