New manufacturing technology for thermoplastic liquid impregnation of fiber reinforced polymers at IVW

Continuous fiber-reinforced thermoplastics (TP-FRP) offer a number of advantages compared to continuous fiber-reinforced thermosets (DP-FRP): For example, they can be melted, enabling forming and welding processes as well as material recycling. TP-FRP are usually manufactured by thermoforming of flat plates, so-called organo sheets. Thermoforming is suitable for large series production but requires comparatively large investments in equipment and semi-finished products, while the production of DP-FRP in liquid impregnation processes, such as vacuum infusion and resin transfer molding (RTM), is relatively inexpensive and flexible but only suitable for low to medium series.

An alternative approach to the production of TP-FRP is to use adapted variants of thermoset liquid impregnation processes in order to combine the processing advantages of liquid impregnation with the material advantages of thermoplastics. For this purpose, impregnation is not carried out with fully-polymerized thermoplastic melts (due to their high viscosity) but with the low-viscosity precursor materials of the polymers, which then polymerize in the mold (in-situ polymerization).

However, there are a number of things to consider when moving from thermoset to thermoplastic processes. There are numerous potentially interesting systems but most are still in the development phase. Currently dominating is the use of ε-caprolactam, which polymerizes to polyamide 6. With this system, very fast cycle times and good part qualities can be achieved but processing is challenging: the precursor materials are very sensitive to oxygen and moisture, the material is corrosive and it must be melted at about 70-90  °C before processing, but then viscosity is in the same order of magnitude as it is for water.

In order to help with the industrialization of in-situ polymerization and support companies in its introduction, Leibniz-Institut für Verbundwerkstoffe is currently creating the necessary manufacturing technology and expertise as part of an EFDF project (European Regional Development Fund). Core of the project is the in-house development of an injection machine in order to meet the special requirements of the technology and to have full control on all aspects of the machine. The machine is primarily designed for the processing of ε-caprolactam to polyamide 6 but can also be adapted to other systems thanks to its modular design. During development, particular attention was paid to ensuring that the plant is not only suitable for research purposes but also for small to medium-sized prototypes and pilot series of industrial customers due to its 10 kg monomer reservoir and an injection mass flow of 100-1,000 g/min. A special feature is the integration of a syringe pump in addition to the two conventional injection lines, which allows the direct addition of liquid additives in small, precisely defined doses.

The injection system is complemented by an oil-heated stainless steel injection mold for laminates with a size of 400 mm x 600 mm. Three injection ports/vents and four modular inserts for sensors (e.g. for temperature, pressure or dielectric properties) allow flexible use and precise monitoring of the injection and the polymerization process.

The system development is accompanied by laboratory tests in which, among other things, the compatibility of different sealing materials with different monomers, the correct dosage of activator and catalyst, and the influence of moisture and oxygen on the polymerization reaction are investigated.