Structural adhesives, also known as acrylic adhesives, have been used for many years in industrial manufacturing, automotive and transport applications. Here, engineers need to consider the type of material or component being bonded, its size and weight, the strength of the bond and cure times.

Many applications require the joining of dissimilar materials, for which structural adhesives are ideal — whether it is a new roof facia on a building, an engine cover on a mechanical digger or the polycarbonate passenger car in an elevator.

To appreciate the stresses placed on a bond, it is important to understand that for an adhesive to bond it needs to effectively wet and penetrate the surface being bonded and produce a chemical reaction that causes cross-linking. Once penetrated, the surface forms small solid blocks, which lock into the surface, creating a secure bond. Think of it like the hook and loop principle used on Velcro fasteners.

The forces acting on a bond can cause various stresses such as tensile, shear and peel stress. This can weaken the join and ultimately cause it to fail under prolonged loading and in dynamic loads where moving parts can exert forces from all angles. This is why traditional joining methods including welding, riveting and fastening are still often the preferred choice in manufacturing environments for applications where adhesives are perceived to not offer the required bond strength. The cheap, relatively reliable and widely available nature of these methods has served manufacturers well for many years.

However, they are not always perfect. While welding creates a permanent join, it can only be used to join metals and requires specialist equipment and skill to perform the join. Once joined, further finishing is needed to remove cosmetic blemishes and weld marks. Similarly, riveting and fastening — using screws and bolts — creates holes in the material, which can create weaknesses and structural stress points.

Nano particles

NCH Europe's Super Structural Technology consists of structural acrylic adhesives that use nano particles to increase the bond area and subsequent bond strength. A structural adhesive consists of two parts; base and activator. When mixed, they create a compound that can be spread onto the surface of the material being joined.

While surfaces like glass, stainless steel and glossy plastic appear smooth to the naked eye, in reality the tiny peaks and troughs that make up the substrate of the material at a microscopic level can cause problems for some structural adhesives. These adhesives simply sit on the surfaces of the peaks, providing limited surface-area contact.

NCH Europe's Super Structural Technology introduces nano particles into the compound, which fills the gaps in the microscopic substrate, increasing the surface area and strength of the bond.

In industry-standard ASTM D1002 tests, NCH Europe's lead Super Structural Technology solution, Mega Cryl SST, has been shown to increase bond strength by over 40% compared to standard structural adhesives. Engineers can now consider adhesives for applications where previously they would have relied on more traditional joining methods.

Low energy plastics

As cars, buses and caravans move to electric-motor propulsion, automotive manufacturers look to reduce weight by using lightweight exotic plastic materials. These exotic or low energy plastics such as polypropylene and high density polyethylene have been difficult to bond, forcing manufacturers to use mechanical fasteners. Advances in structural adhesive technology mean engineers can now bond these materials.

NCH Europe has combined its Super Structural Technology with low energy plastic adhesive formulations to create Mega Cryl LE, which produces a high-strength bond with these materials. Tests have shown the bond to be so strong that the substrate fails before the bond. 

Materials suitable for use with these adhesives range from aluminium, stainless steel, carbon steel and low energy plastics such as PP and HDPE, to brass, copper, glass reinforced plastic, glass, nylon, ceramic, ABS and more.