Requirements for Modern Car Body Adhesives
Adhesion as a joining process for implementing lightweight construction concepts is often discussed at present. For the bonding of composite structures (e.g. steel/aluminium or steel/CFRP), special adhesive systems have been developed, which do not cause the bonding to fail even when the individual substrates have different coefficients of thermal expansion. Such adhesive systems can make a significant contribution to lightweight construction. Sika sells these adhesives under the brand name SikaPower MBX.
When discussing new innovative lightweight construction concepts and suitable joining processes for these, pure steel/steel or aluminium/aluminium bonding almost becomes overshadowed, although most structural adhesive applications still involve such conventional adhesives. Steel, followed by aluminium, is the most important material for building automotive structures worldwide. This requires regular rethinking of the properties of conventional structural adhesives.
In the past, structural adhesives were often highly viscous. These adhesive systems rapidly cool down on the cold sheet steel after hot application and, like hot melt adhesives, restore the high initial viscosity, which results in very high wash-out resistance. Fully heatable application systems are required to process these adhesives. A new adhesive technology now combines low viscosity with excellent wash-out resistance. This makes it possible to pump and convey the adhesives at lower temperatures, meaning that procurement and maintenance costs for the systems, as well as energy costs during operation, can be saved without having to forego their usual properties. The lower viscosity also improves compression and hence leads to a more stable joining process.
Due to their lower viscosity, these adhesive systems can also be used at lower application pressures. With nozzle diameters of 0.8 – 1.4 mm often used in shell construction, 30 to 40 % lower pressures can be achieved with low-viscosity systems depending on the discharge rate. An application temperature of at least 45°C at the nozzle is achieved gradually over the feed zones in order to create uniform application conditions throughout the year. When the outlet temperature at the nozzle is increased to 55°C and a nozzle diameter smaller than 0.8 mm is used, thin-jet application with a component distance of up to 15 mm is possible. The materials can be applied using both the E-Swirl and Air-Swirl processes.