By Andreas Ferencz, Head of PU
Composites Resins, Henkel.
Henkel has given more details of the physical and processing properties of the polyurethane matrix resin it has developed for resin transfer moulding (RTM).
Henkel believes that this process will speed the production of composite components in the automotive industry with resins that can cure in one minute.
Composite components have become an increasingly important factor in the multi-material designs of modern cars, because they offer lower weight for equivalent performance compared with metal components.
Typically, fibre-reinforced composites in the automotive sector are glass- or carbon-fibre-reinforced. Particularly for mass-produced automobiles, resin injection processes such as epoxy RTM now predominate.
Henkel developed and launched its Loctite Max 2 resins in 2012 (see Urethanes Technology International October/November 2012 p 7). The product is a combination of Henkel’s composite matrix resins expertise and fundamental research on adhesives and surface modification. Henkel says its resin system is a complete and coherent system for the manufacture and integration of composite components in the automobile.
Compared with the epoxy resins normally used for RTM, Loctite Max 2 offers a very attractive combination of good mechanical properties and high damage tolerance. In processing terms, short cycle times of five minutes are possible.
One special feature of the polyurethane matrix resin is its exceptionally high stress intensity factor, a measure of toughness. The polar interactions and hydrogen bonding between polyurethane chains in the reacted matrix, in addition to chemical cross-linking, help to enhance this property. The third aspect of toughness is the good compatibility between polyurethane systems and glass fibres which are used for composites.
The resin’s excellent toughness also has a positive effect on the fatigue behaviour of parts under load. Correctly formulated polyurethane systems have high tolerance to stress peaks. Because of this intrinsic toughness it is hard for cracks to form and this helps to prolong part life.
Additionally, PU resins have high damage tolerance. This is shown, for example, in compression after impact experiments. In such experiments comparing glass-reinforced polyurethane and glass-reinforced epoxy systems, at large impact energies, the remaining compressive strength, as well as the high fatigue resistance of polyurethane systems, are significantly higher than comparative standard epoxies.
In automobiles, which are constantly subjected to dynamic loading under driving conditions, materials with a high fatigue tolerance are essential in order to be able to exploit light weighting potential to its fullest extent.
But material properties are not the only important factors. In the mass-production of automobiles, speed is paramount, and RTM is a process that enables short cycle times to be applied. Typically, Loctite Max 2 resins allow a cycle time of 5 minutes or less.
For example, in a carbon-fibre reinforced centre console it was possible to produce a component with four layers of 300 g/m2 carbon fibre reinforcement in a polyurethane matrix. The mould temperature was 120oC, 280g resin was injected in 4 seconds and demoulding took place after 1 minute.
Graph 2 shows the viscosity behaviour of Loctite Max 2. It’s extremely low viscosity, even at low temperatures, permits very fast resin injection without distorting the fibres while also achieving good wetting even with high fibre volumes. Typically, viscosity at moulding is between 10 and 200m Pa.s (10-200cP) and glass fibre reinforcement would be 60% of part weight.
Furthermore, the polyurethane chemistry makes it possible to control the curing reaction reliably, either by adjusting the temperature or adding an accelerator. The generally low-heat generation during curing allows fast curing even of thick parts with many layers of fibres and reduces the risk of local overheating and resulting defects. In all cases the resin components are degassed before the reaction starts and the reaction conditions control the amount of foaming that takes place within the resin transfer process.
KraussMaffei has worked with Henkel to show that using the new resin system on high-pressure RTM equipment can significantly reduce manufacturing cycle times. Loctite Max 2 cures much faster than the comparable epoxy resins that are mainly used in the RTM process today. This was exemplified by achieving a cure time of just one minute with a real-life three-dimensional component. The machine settings are listed in Table 2.
The de-gassed resin is injected into the preheated mould under vacuum and removed after one minute. The fibre volume was approximately 50%, with no fibre distortion being detected regardless of the laminate structure. Milling to the final shape was performed directly after cooling of the components.
Machine technology adapted for polyurethane RTM
Even though high-pressure polyurethane processing and high-pressure RTM are already state-of-the-art, the machine technology does require some adaptation for the polyurethane RTM process. Building on Henkel’s processing expertise, KraussMaffei’s engineers further optimised the proven mixing and dispensing stations and the mixing heads in order to improve the high-precision dispensing technology and thus the controllability in high temperature processes. The results can be seen on the part itself. Excellent mixing of the resin components under high pressure ensures homogeneous product properties without any air pockets. As a result, suitable and proven systems are now available for mass-production.
However, no application solution for the automotive industry is ever complete if it cannot be integrated into an overall package. Henkel therefore offers individually formulated adhesives to ensure reliable integration of the different component materials in modern multi-material designs.
New opportunities for volume production
The polyurethane-based matrix resin technology Loctite Max 2 offers a very attractive properties profile tuned to the requirements of the automotive industry. Good handling capability in the RTM process combined with low injection viscosity and controllable cure speed permits short cycle times (< 5 min) in composite component manufacture. The excellent intrinsic toughness of the resin results in higher resistance to dynamic loading and greater fatigue tolerance.
Combining Loctite Max 2 with glass or carbon fibres opens up new opportunities for cost-efficient mass production of lightweight components in the automotive industry. The first applications are already in the commercialization phase.
Takeaways
Andreas Ferencz:
Dr. Andreas Ferencz graduated in chemistry from the Max Planck Institute for Polymer Research in 1994 when he joined corporate research of Henkel. He held various positions in Research and Development and now leads the development of PU resins for composites.
