HIGH VOLUME STRUCTURAL COMPOSITE PART PRODUCTION: PAINTABLE PARTS RIGHT OUT OF THE MOLD THROUGH SURFACE RESIN TRANSFER MOLDING PROCESS

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1 HIGH VOLUME STRUCTURAL COMPOSITE PART PRODUCTION: PAINTABLE PARTS RIGHT OUT OF THE MOLD THROUGH SURFACE RESIN TRANSFER MOLDING PROCESS Dr. Olaf Lammerschop, Dr. Andreas Ferencz, Dr. Stefan Kreiling Henkel AG&Co. KGaA, Adhesive Technologies, Heidelberg/Duesseldorf, Germany First published at the 2014 SPE ACCE, Novi, Mich., Sept. 9-11, 2014, Abstract Driven by fuel efficiency targets the automotive industry increasingly considers fiberreinforced plastics as lightweight material option in modern car concepts. In future, broader usage of composites will be realized by fast manufacturing processes that enable mass production of composite parts. In addition, painting and assembly of composites via adhesive bonding are important steps along the process chain where further efficiencies can be realized by selection of polyurethane matrix resin technology. Henkel s novel polyurethane matrix resin enables high-speed RTM processes. Besides the already proven fast infusion and cure speed with polyurethane matrix resins, a major challenge has so far been the manufacturing time and process to achieve high quality automotive exterior parts. In cooperation with a major equipment supplier and several other composite expert companies, a new in-mold coating RTM process has been developed to facilitate manufacturing of high quality surfaces. The process utilizes a new generation of polyurethane matrix resin comprising an internal mold release agent. Due to the excellent compatibilities of chemistries for matrix resin, paint and mold release, a far quicker process has been demonstrated compared to current labour- and costintensive surface treatment and painting procedures. The presentation outlines key processing and performance characteristics of the new polyurethane matrix resin as well as the surface resin transfer molding process. Introduction Composite materials based on carbon fiber continuously gained importance for aerospace applications since the 1950 s. Formulations based on multifunctional epoxy resins enabled high crosslink densities of the networks and thus high mechanical stiffness and high service temperatures of the respective composite parts. For the automotive industry, various opportunities for weight savings via substitution of metals by composite materials have been identified, e.g. for BIW, interior, exterior and chassis structures. The performance and processing requirements for automotive applications are different compared to aerospace. Structural composite parts based on glass and carbon fibers are being manufactured e.g. via sheet molding compound (SMC) or via fast resin transfer molding processes (RTM) in a single press-form operation. 1 Early approaches have adopted epoxy matrix resin chemistries. Lower service temperature requirements allow usage of standard bisphenol A type epoxy matrix resin systems.

2 Today novel polyurethane matrix resins are gaining increasing interest especially in the automotive industry due to an attractive combination of mechanical and durability characteristics. In addition, fast processing does enable cycle time advantages to realize composite parts mass production as it is required for larger series car models. Polyurethane Matrix Resin Materials The polyurethane thermosetting matrix systems described in the following sections have been commercialized by Henkel Corporation under the trademark LOCTITE MAX 2 TM and LOCTITE MAX 3 TM. These are two-component polyurethane matrix resins which can be accelerated by addition of a cure accelerator depending on the cure speeds required in the respective process. Laminates were made from non-crimp fabric (glass, carbon) commercially available e.g. from Zoltek, Saertex or SGL Carbon. Processing and reactivity Low matrix resin viscosities allow fast injection of the resin into the respective molds and at the same time good fiber wetting and fabric penetration even at high fiber volume contents. 2 The rheological behavior of the thermosetting resin has been evaluated in dependency of the temperature in order to define a processing window for injection at minimum resin viscosity. Isothermic viscosity curves were recorded to determine the temperature effect on cure kinetics. Figure 1: Rheological behavior of polyurethane matrix resin as a function of temperature (a) and isothermic cure kinetics (b). Injection of the resin can be done in a temperature window between 30 C and 80 C with low injection viscosities of mpa s. Depending on the mold design and fiber volume content injection processes can be accomplished within e.g. < 10 seconds. The cure speed can be drastically increased by increasing mold temperatures. Even without addition of accelerator a rapid cure of the resin can be achieved in the temperature range of C.

3 However, cure times until demolding are strongly depending on the accelerator concentration. The resin system can be tailored specifically to the required design and setup. Typical reaction times may be varied between a range from as low as 30 seconds and to as high as 10 minutes. Internal or external mold release agents assure a fast release out of the mold. Post-cure of the composite parts might increase thermal resistance of the resulting laminates further depending on the initial cure time requirements. Neat polyurethane matrix resin properties Table I shows selected properties of the neat polyurethane thermosetting resin after a complete in-mold cycle Table I. Properties: Neat polyurethane resin cured Neat Resin Property Polyurethane matrix resin Tensile modulus [MPa] 3100 Tensile strength [MPa] 95 Elongation [%] 5-10 Glass transition temperature [ C] * Critical stress intensity factor K 1c [MPa*m 0.5 ] 1.3 * depending on post cure cycle applied Polyurethane matrix resins provide a high modulus in combination with an elongation of break of 5-10 %. The critical stress intensity factor is a measure for the neat resin toughness performance. In this content, polyurethane chemistry induces a unique intrinsic toughness to the cured resin due to its specific polymer backbone structure. A combination of soft polymer segments with strong H-bridging of the urethane moieties is responsible for this advantageous characteristic. No further addition of cost-intensive and viscosity-increasing toughening agents is necessary as compared to epoxy chemistry. The intrinsic toughness of the resulting composite is especially important when the application demands high damage tolerances and durabilities of the part. Different toughening mechanisms have been described in the literature (crack-pinning mechanism, microcracking mechanism, crack-path deflection mechanism, massive shearbanding mechanism). 3 In the case of polyurethane, plastic deformation of the localized shear yielding mechanism is believed to be the major contributor to toughness. The excellent toughness properties of polyurethanes transfer also into a superior fatigue behavior which positively affects durability of the composite in the application. 4,5 Especially for chassis applications, composite matrices offering high fatigue resistance are being selected in order to withstand the cyclic loads occurring during driving operations.

4 Figure 2: Investigation of fatigue resistance under cyclic loading (neat resin): Comparison of epoxy versus polyurethane matrix resins Surface Resin Transfer Molding Process Process The newly developed surface resin transfer molding process enables cost-effective manufacturing of paintable fiber-reinforced visible components for vehicle manufacturing for series applications. Paintable components are created by flow-coating the component surface produced by surface resin transfer molding with a polyurethane layer directly in the cavity, without the need for any additional intermediate steps. That saves on timeconsuming rework, which can amount to up to 60 percent of the costs per component. 6 However, the Surface-RTM process consists of more than adding an additional layer of base paint. Several key elements have to fit to make the process efficient. Most important of these necessary supplements is the modification in the resin formulation, which provides the right balance between release properties for easy demolding and adhesive force for reliably attaching the paint. This can be achieved by adjusting the chemical properties of an internal release agents. The internal release agent is an additive to the RTM resin, which minimizes the adhesive forces to the RTM mold surface to provide an easy release of the cured composite part. Hence, a reduction in these interfacial forces, typically also lead to a lower adhesion of the composite surface to the subsequently applied coating or adhesive. Therefore, the goal was to obtain an easy part release from a mold surface but keep sufficient adhesion to an organic coating or adhesive. Developing a solution, it is important that the solubility of the internal mold release agent at the given temperature is adjusted in a way that the release agent forms a thin layer upon cure on top of the matrix resin. This is done by design of molecular structure mainly with respect to polarity, diffusibility and intermolecular interaction. The development and optimization of the internal release agents had to be done in several steps: a) Screening test b) Process test c) System test

5 Taking into account the large number of possible combinations of formulations and varying process conditions, it was crucial to establish a valid screening test. The test set up used for the screening, matrix resin is cured in a defined geometric shape on a specific surface. The measurement itself was done by shearing of the cured chunk parallel to the test surface. The recorded load quantifies the adhesion to the surface and indicates the efficiency of the internal mold release agent under the given conditions. This setup is used to determine structure-property relationships and the effect of influencing factors on the release properties like e.g. molecular structure of the release agent, cure temperature & time and different surfaces Figure 3: Evaluation of surface roughness (metal) on adhesion strength of the cured resin In Figure 3 one can see four different measurement series performed with one release agent on surfaces of different roughness. For this measurement the cure-release operation was repeated without cleaning the surface in between the measurements to get information on an absolute level of release force as well as the development over time. Due to the experimental conditions, changes occur faster than in a real production situation and are therefore very suitable for the screening test.

6 Figure 4: Evaluation of surface roughness on adhesion strength Figure 4 displays the mean of such a repeated measurement for different release agent formulations on one kind of substrate without change of the cure conditions. This setup allows for rapid screening of a wide range of formulations due to the large difference in absolute values observed and the additional indication of possible long term evolution by recording systematic changes of release forces in a test series. After selection of a small range of release agent candidates, a process test is necessary to distinguish the finer differences between them. Due to the significant influence of mixing and cure time on the real efficiency of the release agents, actual optimization has to be performed under real process conditions. Nevertheless the screening results can reliably indicate the property trends necessary for speedy adaption into a working system. Besides giving valuable information on the release properties, the screening test also can indicate trends in the adhesive properties. The surface on which the resin is cured can be modified or even exchanged to simulate metals, or organic matter like external release agents or even paints. Naturally, the setup for a release agent differs from a setup for a paint or adhesive. Nevertheless, the principle is the same: screening to get a selection of candidate with process testing as follow up. In a final step, the resin in combination with the internal mold release agent is tested in the Surface RTM process mentioned above. Here, the resin is cured in a first step followed by an injection of paint after opening the mold slightly to allow the paint to overmold. This way it is possible to obtain a prepainted composite part right out of the mold. In the process the adhesion to the metal surface has to be as low as possible to guarantee a full release over the part surface and at the same time provide good adhesion to the paint providing a durable coating. This was achieved and process conditions (Table II) as well as a picture (picture 1) of a demo part is shown below. Table II. Materials and injection conditions for composite part produced by S-RTM process

7 Resin LOCTITE MAX 3 TM PU Surface Coating Layup Mold temperature 80 C Rühl puroclear (200 mm) 6 layers Zoltek Chomarat CF fabric 300 g/m2 Injection time Resin injection rate Fiber Content Demold time 14,6 sec 82 g/sec (total weight 1.200g) 50% (no fiber displacement) 5.5 min Picture 1: Prepainted part manufactured by Surface RTM process Conclusion Polyurethane-based matrix resin technology offers a very attractive performance tailored to the requirements especially of the automotive industry. Convenient processability within RTM processes in combination with high reactivity enable shortest cycle times (<1 min) for composite parts manufacturing. The excellent intrinsic toughness of the resin leads to highly impact and fatigue durable composite materials. The resin is compatible to various types of commercial carbon and glass fibers. Especially the combination of polyurethane resin with glass fiber offers unique opportunities for cost-efficient mass production of automotive composite parts andfirst commercialized application is in the market.

8 In addition to the regular RTM process, it was demonstrated that a so called Surface RTM process is feasible with polyurethane resin and paint to obtain prepainted parts right out of the mold. This reduces significantly the rework for painted exterior composite parts resulting in a cost saving of up to 60%. Besides the introduction of the polyurethane matrix resin, Henkel Corporation is offering innovative composite bonding solutions in order to support the full process chain from composite manufacturing towards composite part integration in modern car design concepts. Acknowledgements The authors wish to thank Prof. Volker Altstaedt and Manuel Kempf, University of Bayreuth, Department of Polymer Engineering, Germany for support regarding the characterization of the polyurethane matrix resin and the determination of mechanical and processing parameters. Activities were supported by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (Project FKZ D). Furthmore, the authors wish to thank the consortium of the Surface RTM process in particular Diffenbacher, KraussMaffei, Rühl and Zoltek for their contributions in the process development. References 1. K. Potter, Resin transfer moulding, Chapman & Hall, London, (1997) 2. J. Summerscales, A model for the effect of fibre clustering on the flow rate in resin transfer moulding Composites Manufacturing 4(1), 27 31, (1993) 3. Y. Huang et al. Toughened Plastics I edited by C.K. Riew, and A.J. Kinloch, Advances in Chemistry Series, 233, Amer. Chem. Soc., Washington DC (1993) 4. W.J. Cantwell, J. Morton, The impact resistance of composite materials a review Composites 22(5), , (1991) 5. M. Kempf, S. Schwaegele, A. Ferencz, V. Altstaedt, Effect of impact damage on the compression fatigue performance of glass and carbon fibre reinforced composites 18th International Conference on Composite Materials, Jeju, Korea, (2011) 6. KraussMaffei,