Structural adhesives for the joining of similar and dissimilar substrates

• Adhesives based on epoxy, polyurethane, methacrylate & phenolic
• Lower weight, improved vehicle durability
• Application: automotive (incl. application examples)

In an age of substrate change, where manufacturers are looking to use different materials that will have a significant impact on weight reduction, adhesives play an important role. Structural adhesives can be used to join a variety of similar and dissimilar substrates and can help to reduce weight, improve vehicle durability and reduce manufacturing costs. By contrast to mechanical fixings, their use can help reduce fatigue and failure commonly found around spot welds and fasteners. Based on epoxy, polyurethane, methacrylate and phenolic materials,Huntsman’s adhesives have specific characteristics that help improve manufacturing processes, secure long-term performance and the safety of assemblies.

How can adhesives help?

In the assembly of parts, structural adhesives can be used across a wide range of areas instead of rivets and other fastening elements to have a cumulative effect on weight reduction. Without holes, rivets or fastening elements that can weaken structures, adhesives facilitate the strength of materials and provide better aesthetics on finished parts. Adhesive bonds that have smooth joint surfaces also provide auto makers with greater design flexibility. In providing a continuous bond which is less susceptible to fatigue cracks, a bonded structure is often described as a safer structure. This type of bond facilitates more uniform stress distribution within a leak proof solution that is less prone to corrosion and able to provide a longer service life under load.

Joining of dissimilar materials

Adhesives can also join dissimilar materials together, including aluminium, magnesium, composites and carbon fibre, and compensate for variants in the coefficients of thermal expansion in different materials, which contract and expand at different rates. Whilst producing a bond that is both strong and flexible, this key feature also assists in lowering ongoing maintenance costs.

High-performance epoxies

For structural and semi-structural assemblies, such as transmission shafts or door modules, epoxy adhesives provide high strength, high stiffness, high temperature resistance, very high fatigue and thermal shock resistance on metal and thermoset composites.

For example, used by an Italian manufacturer to bond two parts of a carbon fibre spoiler together, Araldite 2015 of Huntsman is an example of an epoxy system selected for its flexible properties. The characteristics of this material proved essential for helping the spoiler withstand mechanical stresses and vibrations when the car reached speeds up to 200mph. Araldite 2015’s high temperature resistance also acted as a key enabler for autoclave curing the finished part at 90°C for an hour. The same material is also used for bonding couplings onto the GRE transmission shaft of a French-made 4×4 vehicle, where it met important criteria with its high strength capabilities to successfully withstand torsion loadings.

Another epoxy adhesive that offers high strength in combination with easy processing is Araldite 2022. This epoxy has been used by a German manufacturer to provide adhesion on the steel and ABS components that make up the interior parts of a mini-van’s tailgate. Helping to reduce the overall weight of the unit, this epoxy’s short curing time at room temperature allowed the parts to be handled after just one hour, significantly improving the assembly process.

Versatile methacrylates

Methacrylate adhesives are suitable for similar and dissimilar materials and are most commonly selected for the time saving advantages created by their speed of cure characteristics. For a Polish vehicle maker, for instance, these fast curing properties proved particularly beneficial in allowing the PA6 and steel parts of a tailgate plug to be securely bonded and rapidly cured, making it the adhesive of choice for high speed production repeatability.

As well as offering a special balance of high tensile, shear and peel strengths with the maximum resistance to shock, stress and impact across a wide temperature range, methacrylates are extremely versatile in being tolerant to mix-ratio variations and remain strong and durable under severe environmental conditions.

Flexible polyurethanes

Similar to epoxy and methacrylate adhesives, polyurethane adhesives are suitable for multi-material assemblies. By contrast, due to their flexible properties, polyurethanes also work well on tough-to-bond engineering thermoplastics, ridged plastics and composites. They are typically applied in the production of headlights, brake lights, reflector housings and often used to bond bumpers onto vehicles. Araldite 2029-1 for example, a cold curing polyurethane adhesive of Huntsman, has been used on the dashboards of high-end sports cars and has primary applications where bonding of Carbon Fiber Reinforced Polymer to ABS is required.

Friction resistant phenolics

Phenolic adhesives are best used on metals and friction materials and are mostly used on brake shoes, brake pads, friction lining materials for clutches and dip-coating applications. Araldite 64-1 is a phenolic adhesive that has been used by a Bulgarian car maker to bond anti-friction composite liner onto the stainless steel plate of a brake pad. Fulfilling the manufacturer’s objective to provide better resistance to brake fluid, use of this adhesive also resulted in the successful application of lighter weight materials alongside reduced costs.

Picture: Araldite (R) adhesives cover a wide range of automotive applications, using the latest epoxy, polyurethane, methacrylate and phenolic technologies (Source: Huntsman)