Epoxy resin systems for the application in new additive manfacturing process of fiber-reinforced polymers

Carbon fiber reinforced polymers (CFRP) with thermosetting matrices are increasingly applied in transportation due to their lightweight potential. Wet Fiber Placement (WFP), a process currently developed in the interdisciplinary junior research group “TopComposite“ at Leibniz-Institut für Verbundwerkstoffe (IVW) represents a novel additive manufacturing process for the production of CFRP. In this process, the roving (fiber bundle) is impregnated with a low viscous epoxy resin system at room temperature and conveyed via a special arrangement of driven rollers (exploitation of belt friction effects). Afterwards, the impregnated roving is gradually placed layer upon layer into a mold. In this manner, both straight and curved specimens can be placed. Finally, the thermoset can be cured upon heating in a pressing process. Process technologies, design methods and materials are simultaneously developed within the research group in order to achieve the best possible results.

One of the biggest challenges with regard to the processed materials is the development of an epoxy resin system which is able to fulfil the quite specific processing requirements. On the one hand, the resin formulation should show a low viscosity during the impregnation and generally a long pot life at room temperature. On the other hand, the impregnated rovings should be sufficiently stabilized in order to avoid subsequent displacements in the pressing process. The final element should display good thermal and mechanical properties which comply with typical requirements in the field of mobility and transport.

To meet the process requirements, the resin system should be pre-cured at low temperature in the first curing step, so that there’ll be a minimal resp. no relevant change in viscosity in the pressing process in second curing step. This means, the material should already be sufficiently solid after the first curing step and completely cured or post-cured in the pressing process in the second curing step without undesirable flow and deformation processes in the fibre composite material. In the view of above‑described aspects, an approach commonly referred to as ´dual-curing´ was chosen. Dual-curing is a combination of two compatible and well-controlled polymerisation processes which take place simultaneously or sequentially [1]. In the case of sequential dual-curing, a stable but high deformable intermediate material is formed after the first polymerisation step which is further transformed into the final material after the second polymerisation step. For this purpose, both polymerisation processes can be initiated, for instance, by different stimuli such as UV light or heat [1, 2]. In this project, resin systems  are primarily developed in which both polymerisation reactions take place at different temperatures [2]. The DSC diagram of one currently investigated resin formulations based on a combination of two curing agents in which both polymeristion steps are separated, is shown in Figure 1. The ´dual-cured´ resin systems are off-stoichiometric [1, 2]. Therefore, the stability of the intermediate material as well as the properties of the final material (e. g. the glass transition temperature) are strongly dependent on the degree of conversion [1]. A series of DSC measurements has been already carried out in different chosen stoichiometric and off-stoichiometric ratios in order to control the separation of both peaks and to make an initial prediction about the glass transition temperature. The next key step is to investigate both polymerisation processes with common methods (such as DSC, FTIR, DMA) and to verify the stability of the intermediate material (experiments in an oven). On the one hand, the first resin formulation is intended to create a new ´dual-cured´ resin system which can help to better understand the correlations already described in the literature. On the other hand, the potential implementation of this approach to WFP should be verified. Thus, the pot life of the first resin system, which is mainly conditioned by the first curing agent, should be long enough for manufacturing a test plate for classical tensile testing (300x300x2 mm³). In the case of successful experiments, the main goal will be to prolong the pot life of the resin formulation (e. g. by replacing the first hardener) which can make the production of more and/or bigger elements possible. In the future, a ´dual-cured´ resin system with two latent curing agents (able to cure at different temperatures) will be developed . This will enable an continuous  manufacturing of elements which is an important prerequisite for the potential application of such resin systems in industry.

The project "TopComposite – Topology-optimized and resource-efficient composites for mobility and transport" is funded by the Federal Ministry of Education and Research (funding reference 03XP0259).

Projektbeirat:

• ACE Advanced Composite Engineering GmbH
• AVK – Industrievereinigung Verstärkte Kunststoffe e.V.
• BMW AG
• Centre for Advanced Composite Materials (CACM), The University of Auckland, New Zealand
• Westlake Epoxy GmbH
• Leibniz-Institut für Verbundwerkstoffe GmbH

Contact:

Anna Dlugaj, M. Sc.
Junior Research Group TopComposite
Phone: +49 631 2017-292
Email: anna.dlugaj@ivw.uni-kl.de