Nanotech adhesives: the new frontier
The European adhesives and sealants market accounts for 35% of the global adhesives market. Here Peter Crossen, VP of the Maintenance and Partsmaster Innovation platform of global water, energy and maintenance solutions provider NCH Europe, explores the recent developments in super structural adhesives that are paving the way for better joining methods. The global adhesives market is worth over €13.4bn and growing at four per cent per year.
Much of this demand is being driven by the changing use of lightweight materials and smarter processes in the manufacturing, construction and automotive sectors.
In September 2006, scientists writing in the Journal of Archaeological Science, reported findings from Italy, which showed the earliest use of glue-type adhesives by early humans in the Middle Pleistocene era around 200,000 years ago. They found stone flakes partially covered with birch-bark tar, thought to be the oldest discovered human use of tar-hafted stones.
Adhesives have come a long way since then. From tree sap used by cavemen in South Africa and animal glues used by Egyptian pharaohs, to plant-based and synthetic glues developed over the last 70 years, adhesive technology has become stronger and more versatile, offering a cheap and quick method of joining a variety of materials including metal, wood and plastics.
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. Here the surface of the substrate being joined will need to handle a variety of stresses and strains when in use.
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 existing adhesives are perceived to not offer the required bond strength. The cheap, relatively reliable and widely available nature of these traditional methods has served manufacturers well for many years.
However, they are not perfect. While welding creates a permanent join, it can only be used to join metals and requires specialist equipment as well as a skilled hand 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.
The holes created by riveting and fastening also expose the material to ingress from the elements. This can remove the galvanic coating on many metals like steel, leading to corrosion, which then needs further coating or encapsulation to protect. This method can also add significant weight.
This is why NCH Europe has formulated Super Structural Technology. This consists of a range of structural acrylic adhesives that use nano particles to increase the bond area and subsequent bond strength.
A structural adhesive consists of two parts; a base and activator. When the base and activator are mixed together, it creates a compound that the user can 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 normal structural adhesives. These adhesives simply sit on the surfaces of the peaks, providing limited surface-area contact.
What we've done with our proprietary Super Structural Technology is introduce 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.
With the increased bond strength achieved through the use of Super Structural Technology, engineers can now start to consider adhesives for applications where they would have previously relied on more traditional joining methods.
As manufacturing, construction and automotive environments become more flexible and adaptable; the old methods of joining are fast becoming obsolete. Take the automotive sector, for example. As cars, buses and caravans move to electric-motor propulsion, automotive manufacturers will look to reduce weight by using lightweight exotic plastic materials.
These exotic or low energy plastics such as polypropylene (PP) and high density polyethylene (HDPE) have not been served well by existing structural adhesives and, until now, they have been near impossible to bond, forcing manufacturers to use mechanical fasteners. However, these advances in structural adhesive technology mean that design engineers now have the ability to bond these exotic plastic materials.
NCH Europe has combined the unique Super Structural Technology with the latest low energy plastic adhesive formulations to create Mega Cryl LE, which produces a high-strength bond with these difficult materials. Tests have shown that the bond is so strong that the substrate fails before the bond breaks.
The list of materials suitable for use with these next-generation adhesives is a long one: from aluminium, stainless steel, carbon steel and low energy plastics such as PP and HDPE, to brass, copper, glass reinforced plastic (GRP), glass, nylon, ceramic, ABS and more.
Considering the flexibility and strength offered by Super Structural adhesives over traditional adhesive and mechanical joining methods, it's easy to see why many people have predicted rapid growth in the adhesives market. Adhesives have certainly come a long way in 200,000 years.