Series production of composite leaf springs with PU matrix resin

Lightweight construction technologies are steadily gaining ground in the automotive industry as the weight savings they achieve in modern cars help to significantly reduce fuel consumption and carbon emissions. Working with Benteler-SGL, Henkel has developed a process for resin transfer molding (RTM) of glass fibre-reinforced leaf springs that uses a polyurethane matrix resin. Compared to conventional leaf springs made of steel, these composite leaf springs are up to 65 % lighter.

With Loctite MAX 2, Henkel is offering a polyurethane-based composite matrix resin that cures significantly faster than the epoxy products usually employed for the RTM process. Due to its low viscosity, the polyurethane resin penetrates and impregnates the fibre material more easily and less harshly, thus enabling very short injection times to be applied. One special feature of Loctite MAX 2 is its exceptionally high stress intensity factor, which is a measure of toughness. This toughness also has a positive effect on the fatigue behaviour under load. Leaf springs in cars are constantly subjected to dynamic loading under driving conditions, so the use of flexible materials with a high fatigue tolerance considerably prolongs the life of such components.

Resin injection processes such as RTM predominate in the manufacture of automotive composites for mass-produced automobiles because they make it possible to control the curing reaction more reliably, either by adjusting the temperature or adding an accelerator. Furthermore, the risk of local overheating and resulting shrinkage is reduced, as the polyurethane resin Loctite MAX 2 generates less heat overall during curing than epoxy resins do. As a result, even thick components with many fibre layers cure fast. The RTM process is especially attractive for volume production of cars as fast curing of the matrix resin permits short cycle times to be achieved.

The product range of Benteler-SGL, a joint venture of Benteler Automobiltechnik and the SGL Group, includes body shell components such as side blades, doors and visible carbon components. The composite components, which are designed specifically for mass-production consist mainly of carbon and glass fibres in various textile forms such as stitched or woven fabrics. Customers include almost all the big names in the automotive industry, primarily in the premium class.

“While developing these composite leaf springs, the competencies of Benteler-SGL and Henkel complemented each other perfectly,” said Frank Deutschländer, Global Market Manager Automotive at Henkel AG & Co. KGaA. “With our matrix resin technology Loctite MAX 2, we were able to establish a new process at Benteler-SGL that is tailored to the automotive industry and gives this fibre composite component an attractive properties profile.”

Given the more stringent CO₂ exhaust standards, lightweight construction is becoming ever more important for the automotive industry. The composite components from Benteler-SGL help to reduce fuel consumption while offering a high level of safety. “Thanks to our intensive collaboration with Henkel, we have succeeded in producing a resin transfer moulded glass fibre-reinforced leaf spring for the automotive industry that not only combines low weight with high strength but is also mass production capable. It thus also satisfies the highest expectations in terms of cost-efficiency and process reliability,” said Frank Fetscher, Head of Sales and Marketing at Benteler-SGL Automotive Composites.