Advance in structural adhesives promises breakthrough for lightweight vehicle manufacturing
Material scientists at diversified technology company 3M have achieved a breakthrough in structural adhesives for vehicle body-in-white applications. A unique combination of material properties will allow manufacturers to store bonded sub-structures for up to two months before curing them at much lower temperatures than before. The company believes that 3MTM Structural Adhesive SA9820 (B/A), a two-part epoxy adhesive, could help carmakers use aluminium and carbon composite structures in a wider range of vehicles as a means to reduce weight and hence CO2 emissions.
SA9820 enables Tier One suppliers to build sub-assemblies that can be stored for more than 60 days prior to the final curing stage, giving production much more flexibility. When applied on oiled surfaces, conventional two-component structural adhesives can have a shelf-life of just three or four days. Single-part adhesives pose even greater challenges: uncured, they lack joint strength and absorb moisture during storage that can affect paint processes.
3M says that SA9820’s prolonged stability prior to curing and its ability to create structures as tough as those made with single component adhesives is attracting strong interest from the industry. Dr Bernard Sikkel, Adhesives Senior Specialist for 3M’s Automotive Division says: “Low-volume manufacturers and companies producing higher volume premium vehicles are interested in the new adhesive’s benefits. There is clearly an unsatisfied demand for SA9820’s unique combination of properties.”
The ability to cure SA9820 at temperatures as low as 115°C supports manufacturers’ efforts to reduce energy consumption in their plants by reducing the temperature at which they bake their painted cars. “This issue is challenging the entire industry,” says Sikkel. “Even in normal oven bake cycles, not all the car’s structural components may be able to reach the critical temperatures required by conventional adhesives. This produces under-performance, or leads to over-design, with manufacturers adding extra mechanical fasteners to support joints.”
SA9820’s combination of prolonged room temperature stability and activation during a low-temperature bake will help manufacturers build lightweight structures from aluminium and a variety of materials, says 3M. As the adhesive cures well below the glass transition temperature at which the resin degrades in many types of composites, it is ideal for bonding carbon fibre components to other materials. The epoxy’s excellent gap-filling properties also ensure that surfaces marry better and joints have greater integrity.
The innovative adhesive’s structural performance is also superior, says 3M. The company claims that SA9820 exceeds the joint stiffness, strength and crash performance of the best existing two-part adhesives. This is important to manufacturers who cannot compromise vehicle refinement or crash performance as they increase the use of lighter materials.
Sikkel also emphasises structural bonding’s superiority over welding or mechanical fastening when it comes to stress distribution in joints, stiffness and the ability to join dissimilar materials readily. “The end-user can feel the difference,” says Sikkel. “A good structural adhesive contributes to a stiffer, quieter, safer car.”
The adhesive’s developers focused on application quality. SA9820 can be dispensed easily by robots for high-volume applications or using hand-held applicators for smaller volumes. Regardless of application method, it retains excellent gap-filling properties and is non-sag and non-slump.
As a two-part adhesive, SA9820 is produced at the point of application by mixing two components. The adhesive turns bright orange when the mixture is correct, providing a straightforward visual check of mix-quality.