A life cycle analysis of novel lightweight composite processes: Reducing the environmental footprint of automotive structures

In this study, three novel thermoplastic impregnation processes were analyzed towards automotive applications. The first process is thermoplastic compression resin transfer molding in which a glass fiber mat is impregnated in through thickness by a thermoplastic polymer. The second process is a melt-thermoplastic Resin Transfer Molding (RTM) process in which the glass fibers are impregnated in plane with the help of a spacer. The third process, stamp forming of hybrid bicomponent fibers, coats the fibers individually during the glass fiber production. The coated fibers are used to produce a fabric, which is then further processed by stamp forming. These three processes were compared in a life cycle analysis (LCA) against conventional resin compression resin transfer molding with either glass or carbon fibers and metal processes with either steel or aluminum that can be new, partly or fully recycled using the case study of the production, life and disposal of a car bonnet. The presented LCA includes the main phases of the process: extraction and preparation of the raw materials, production and preparation of the mold, process, and energy losses. To include the life of the analyzed bonnet, the amount of diesel that is used to drive the weight of the bonnet for 300 ' 000 km is calculated. In this LCA, the disposal of the bonnet is integrated by analyzing the used energy for the recycling and the incineration. The results show the potential of the developed thermoplastic impregnation processes producing automobile parts, as the used energy producing a thermoplastic bonnet is in the same range as the steel production.

Phase separation at the fiber-matrix interface in composites based on a thermoplastic/polyester blend

Xavier Bulliard, Jan-Anders Månson, Véronique Michaud

A study of the microstructure developing at the surface of glass fibers in a poly(vinyl acetate) (PVAc)/polyester blend is presented. Three different experimental methods are used: a technique based on the Wilhelmy method to measure the wettability of the fibers before curing, and both optical microscopy and atomic force microscopy in the pulsed-force mode to characterize potential phases splitting at the fibermatrix interface after curing. It was found that, depending on the curing conditions and the concentration in PVAc, the surface treatment of the fiber could have a significant influence on the microstructure. For a concentration in PVAc lower than 5 wt% and a curing temperature of 80°C, extreme cases, such as the development of layers of one of the phases at the surface or the formation of lenses of one phase, were observed. In other cases, in particular for elevated temperatures and higher concentrations in PVAc, the fibers did not exert a significant influence on the morphology. It was also found that in such a reactive system, surface tension considerations alone are insufficient to explain the configuration of the phases at the surface of the fibers.

2005

Wettability of Glass Fibers in Reactive Thermoplastic/Polyester Blends: Comparison between Compatible and Incompatible Thermoplastics

Xavier Bulliard, Jan-Anders Månson, Véronique Michaud

The evolution of morphology of reactive thermoplastic/unsaturated polyester blends at the surface of glass fibers is investigated during curing. The study focuses on two different types of thermoplastics, incompatible and compatible respectively with the polyester resin. Poly(dimethylsiloxane) and poly(methyl methacrylate) are chosen as incompatible thermoplastics, and poly(vinyl acetate) as a compatible thermoplastic. In the presence of incompatible thermoplastics, the blends form an emulsion during the entire course of curing. In that case, a correlation exists between the surface tensions of the components of the blend measured before curing and the final morphology at the surface of the fibers. For a compatible thermoplastic, on the other hand, a reactioninduced phase separation occurs during curing. In that case, the morphology at the surface of the fiber after phase separation cannot be fully determined by the surface tensions of the components.

2005

Strategies for faster impregnation in melt thermoplastic resin transfer molding process

Damiano Salvatori

Continuous fiber thermoplastic composites are potential substitutes of steel in structural automotive components, since they combine fiber strength and rigidity with ductility, potential recyclability and low density of the thermoplastic (TP) matrix. Resin transfer molding (RTM) is a process for composite production, involving direct impregnation of a reinforcing fabric in a closed rigid mold with a liquid resin. The duration of the impregnation step is directly proportional to the resin viscosity and inversely proportional to the fabric in-plane permeability. As a result, the high viscosity of melt thermoplastic resins (mTP) generally precludes their use in RTM, in particular to reach production volumes in line with the automotive sector. However, recent development of high-fluidity (HF) TPs, with medium range viscosity (10-50 Pa s) opens new possibilities for mTP-RTM. The goal of this thesis is to explore strategies to enhance the impregnation kinetics in mTP-RTM with HFTP by further increasing the fabric in-plane permeability. The effect of modified fabric architectures was investigated through experiments allowing visualization of flow propagation and permeability measurement using model fluids on a modified non-crimp fabric. A saturated permeability of ca 10-9 m2 for 46 vol% fibers was measured. The mesoscopic pore-space of the compacted fabrics was imaged with X-ray Tomography, and analyzed to propose permeability predictions based on the large channels geometry, which agreed well with the experimental results. Capillary effects were assessed over a broad range of capillary numbers (from 4.10-5 to 0.4) by direct comparison of unsaturated and saturated permeability. Permeability in these dual-scale fabrics is governed by viscous flow in the meso-channels. As a result, provided that the capillary number exceeds a threshold value, their permeability can be rather accurately measured by carrying out unsaturated measurements, neglecting micro-flow and capillary effects. Further improvement up to 10-8 m2 for 46 vol% fibers was achieved by introducing a flow-enhancing structure, made by polymer 3D-printing in the core of a woven fabric stack. The resulting flow was markedly dual-scale, going first very quickly in the core and later through the fabric. Optimized impregnation time was obtained by pulling vacuum prior to injection, letting the fluid flow within the core to the outlet, which is then closed, and finally continue the injection through thickness to complete saturation. A proof of concept with epoxy resin showed that a poly(latic acid) spacer led to an increased bending stiffness of 20%, but strong reinforcement gradients. Using a sacrificial spacer made of polycaprolactone, which melted during resin cure (80°C) allowed full fabric relaxation and a more homogeneous fibers distribution. Finally, these findings were applied to the development of a mTP-RTM mold to produce plates with HF polyamide 6 matrix. An 11 cm-long plate with 46 vol% glass fibers was impregnated in 7 min (1 min in the core, 6 min through the fabric) using high temperature spacers compared to more than 45 min without, also with improved bending stiffness by 26%. This concept is conceivably scalable to larger parts, since the bottleneck of the whole process is transferred to the saturation step. A preliminary cost analysis was carried out to evaluate the scalability of this technique as compared to alternative compression TP-RTM or a baseline epoxy RTM process.

EPFL2018

Strategies for faster impregnation in melt thermoplastic resin transfer molding process

Damiano Salvatori

EPFL2018