Dual-Curable Epoxy Resin Systems for Wet Fiber Placement and Thickness-Adaptive Compression Molding

Wet Fiber Placement (WFP), a process currently developed in the interdisciplinary junior research group “TopComposite“ at Leibniz-Institut für Verbundwerkstoffe (IVW), is a novel additive manufacturing technology for carbon fiber-reinforced polymers (CFRP) with thermosetting matrices. On the one hand, WFP combines elements of well-established manufacturing processes such as winding or pultrusion, but overcomes their geometric limitations of the final products by programmability advantages of 3D printers [1, 2]. On the other hand, WFP offers the advantage of cost-effective system technology opposed to automated fiber/tape placement processes [2].

WFP in-situ impregnates roving (fiber bundle) with thermoset, transports the impregnated roving through a special arrangement of driven rollers called “Euler-Eytelwein Unit”, places it and cuts it layer by layer on a tooling mold. The generated prepreg exhibits different thicknesses by introducing two additional layers in the middle on its top and is then cured upon heating in elastomeric/metallic mold in the press to realize  thickness-adaptive compression molding. This tooling mold is shown in figure 1. The differences in the thickness of the final components are measured after pressing in order to validate the effectiveness of this process [1, 2]. Apart from the manufacturing and pressing technology, the thermoset also has to adapt to the whole process in terms of viscosity.

The viscosity of the thermoset should be sufficiently low and ideally constant at room temperature during the impregnation and processing at WFP to ensure good wettability of the molds and to avoid air entrapment, non-uniform resin distribution and weakened structures; the requirements correspond to those during winding [3]. The subsequent adjustable decrease in viscosity during the pressing process via thickness-adaptive compression molding is optimal to test different scenarios for maintaining the thicknesses in final components.

The most promising approach to adjust individual viscosity profiles of a resin in different processing scenarios or to produce components with complex shapes is the so-called sequential dual curing. This methodology is a combination of two compatible and well-controlled polymerization processes which take place one after the other. The viscosity increases during the first curing stage to form an intermediate material. The decrease in viscosity of the intermediate material can be tailored during the activation of the second curing stage by a proper choice of a relative contribution of both polymerization processes, i.e. the deformable shape of an intermediate material can be fixed or deformed in a controlled manner again by spontaneous decrease in viscosity [4]. In this processing scenario, the prepreg generated by WTP is placed in the oven at low temperature to complete the first curing stage. Afterwards, the prepreg is further heated up at higher temperature during pressing to activate the second curing step and cause a desired (smaller or larger) decrease in viscosity. The thickest part in the middle of the composite was still about 18 ‑ 20% thicker than the base laminate after pressing, opposed to the required 30 % thicker part before pressing [1, 2]. Further research on this topic is ongoing.

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).

References:

[1] May, D., Eckrich, M., Dlugaj, A. M. et al., Wet fiber placement for additive manufactoring with thermoset resins. In Additive Manufactoring of Polymer-Based Composite Materials. Materials, Processes and Properties, 2024, DOI: https://doi.org/10.1016/B978-0-443-15917-6.00009-8, pp. 281 – 302.

[2] Arrabiyeh, P. A., Dlugaj, A. M., Eckrich, M. et al., Applied Composite Materials2024, 31, pp. 1237 – 1258.

[3] Shibley, A. M., Filament Winding. In Handbook of Composites; Lubin, G. (eds); Springer, Boston, MA, 1982, DOI: doi.org/10.1007/978-1-4615-7139-1_16, pp. 449 – 477.

[4] A. Belmonte, X. Fernández-Francos, Á. Serra et al., Materials and Design 2017, 113, DOI: https://doi.org/10.1016/j.matdes.2016.10.009, pp. 116 – 127.