As the auto industry adopts an increasingly wide mix of materials in its pursuit of constantly tougher weight-saving and service-longevity targets, joining mixed materials using precisely-compatible adhesives is a crucial aspect of the formula. Considering the expanding role of adhesives in the mixed-materials matrix, a new warning to design engineers comes from Peter Swanson, Managing Director of adhesives specialist Intertronics, of the need to fully appreciate through-life performance.

“Adhesives are commonly specified during product development, often based on functional requirements,” he said. “But when production is upscaled, the chosen adhesive can be less than optimal; application and curing may be difficult, unreliable, challenging to automate and demand too much time. Intended production quantities for a product should always influence adhesive choice.” 

Stressing that there is no such thing as a perfect adhesive, specification should not be left to the production team, he added: “A holistic consideration during the design stage would ensure later efficiencies and savings—essential considerations include functionality, process ability and commercial factors.

“In an ideal world, all surfaces would be bonded by one perfect adhesive,” Swanson continued. “But a single adhesive that can cure on demand, operate at any temperature, resist all solvents, that sticks to everything and is completely and instantly workable—is an impossibility! However, via the acceptance of compromise, there are certain things design engineers should consider to get as close as possible to the perfect adhesive.” 

Choosing substrates and adhesives wisely is a significant part of achieving successful bonding. Swanson said, “Normally, an adherent force can only be established when the molecules in the substrate are really close to the molecules in the adhesive, of the order of 5x10^-7 mm. For this to occur, wetting of the adhesive to the substrate must be achieved. If the adhesive behaves like water on a greasy plate and forms little balls rather than a homogenous film, wetting (therefore adhesion), will not occur. Achieving that necessitates the adhesive having a lower surface energy than the substrate.” 

Stainless steel, for example—used in a variety of roles throughout the auto industry—has a considerably higher surface energy than most adhesives, which usually makes it readily bondable. But at the opposite end of the materials’ spectrum, polyolefin plastics such as polypropylene are attractive to the industry for things like vehicle bumpers; but they have inherently low surface energy, which makes them quite difficult to bond.

UV light through substrates  
Use of UV light that can pass through substrates can provide rapid, highly-efficient light-curing adhesive solutions (an adhesive that takes a long time to cure would probably be totally unsuitable for high-volume vehicle production), with many process advantages, including no requirement for mixing. This can equate to overall parts-cost savings. 

But it isn’t just the initial bonding efficiency per se that is vital. The effects of thermal expansion—caused by anything from ambient weather conditions to hot exhaust temperatures and including torsional stresses and vibration—subject vehicle materials and associated adhesives to extraordinarily hostile conditions. If the CTE (coefficient of thermal expansion) is different for each substrate, the adhesive may need to be tough or flexible to withstand the different amounts of thermal expansion. The CTE for stainless steel, for example, is 17 ppm/^o C and 6 ppm/^o C for glass. Depending on the size of the assembly and the temperate excursions, there may be significant stress imposed on the bond line. The adhesive must cope with this.  

Adhesive types must also meet many other criteria, including application to vertical, angled, or horizontal surfaces. Engineers should consider using viscous adhesives for vertical applications and runnier solutions for gap filling, Swanson added. But when some normally viscous adhesives are shaken, agitated or stressed in any way, they can become runny; the property is called thixotropy.

“Design engineers must identify the most appropriate adhesive for an application with multiple factors to consider,” said Swanson. “The designer may aim for the very best performance, while the production manager wants an adhesive that is simple to use and problem-free.” 

Swanson’s "do and don’t" list for helping design engineers make decisions regarding adhesives includes avoiding specifying more strength or heat resistance than is needed, as this may exclude lower-cost alternatives that have simpler production capabilities. “Ideally, the substrate fails before the adhesive, so a low-strength substrate does not need a high-strength adhesive.” 

Swanson’s Intertronics also is a specialist in plasma treatment to alter the surface properties of substrates. If the surface energy of a substrate is too low for successful wetting, plasma can be applied to change the chemical groups on its surface. This increases wettability and creates bonding “anchors” for the adhesive. It also can clean surfaces and remove dust particles. Auto-sector examples of plasma-treated adhesive applications are LED headlight and taillight units that must be watertight, as keeping moisture out is essential for LED longevity. To ensure this, use of plasma treatment can provide uniformity of the adherent force and increase the strength of adhesion.