Vehicle manufacturers have set some ambitious targets to significantly reduce carbon emissions to 95g CO₂/km on the sale of all new cars by 2020. Laurent Pourcheron, Marketing Manager for Adhesives at Huntsman Advanced Materials looks at how structural adhesives can help the automotive industry to successfully integrate the latest lightweight materials and comply with future environmental legislation.

Carbon emissions limits are part of the EU's drive to switch Europe to a low-carbon economy and slow the impact of climate change. In Europe, legislation approved by the European Parliament stipulates that cars must abide by a carbon pollution limit of 130g/km by next year, and no more than 95g/km on all new vehicles sold by 2020.

Improving the components that generate power and reducing the rolling resistance of tires offer potential to boost energy efficiency and make progress on carbon emission reductions. But the most significant target by far is weight reduction.

A commonly accepted guideline states that for every 100kg in vehicle weight reduction, fuel consumption falls by 0.251l/km, delivering a reduction in carbon emissions of approximately 7g CO₂/km.

How Can Adhesives Help?

Manufacturers and suppliers now recognize that structural adhesives can help across the spectrum. In the assembly of parts for example, they can be used across a wide range of areas instead of rivets and other fastening elements to have a cumulative effect on weight reduction.

Adhesives can be seen as a potentially disruptive technology that will allow the sector to successfully embrace multi-material automotive lightweighting in the application of new weight-saving materials including aluminium, magnesium, composites and carbon fibre.

Lightweight, Heavy Impact

Without holes, rivets or fastening elements that can weaken structures, adhesives facilitate the strength of materials and provide improved 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. Adhesives can also join dissimilar materials together 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. Adhesives also help manufacturers overcome traditional production challenges by adopting faster, cheaper processes and simplified assembly procedures.

Huntsman's Role

As a strategic partner, Huntsman has tailored its products and services to meet the evolving needs of the global automotive market for many years. The company's products help overcome a variety of challenges such as low emissions, fast curing cycles with high initial green strength, durability under strength, impact resistance and multi-substrate joining. Based on state-of-the-art epoxy, polyurethane, methacrylate and phenolic technologies, Huntsman's adhesives have specific characteristics that help improve manufacturing processes, secure long-term performance and the safety of assemblies, and all importantly, facilitate the use of the range of materials that enable car lightweighting.

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.

Used by an Italian manufacturer to bond two parts of a carbon fibre spoiler together, Araldite 2015 is an example of an epoxy system selected for its tough and flexible properties.

These characteristics proved essential for helping the spoiler withstand mechanical stresses and vibrations when the car reached speeds up to 200mph. The material's high temperature resistance also acted as a key enabler for autoclave curing the finished part at 90°C for an hour. The same system has also proven its worth in bonding couplings onto the GRE transmission shaft of a French-made 4x4 vehicle, where it met important criteria with its high strength capabilities to successfully withstand torsion loadings.

Demonstrating the value that epoxy adhesives offer in combining high strength with easy processing, Araldite 2022 is another good example. This epoxy has been used by a German manufacturer to provide excellent 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 enhancing and simplifying 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, 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, has been successfully 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 antifriction 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.

Conclusion

Structural adhesives can be used to join a variety of similar and dissimilar substrates, helping to improve vehicle durability, reduce weight and manufacturing costs. By contrast to mechanical fixings, their use can help reduce fatigue and failure commonly found around spot welds and fasteners.

In an age of substrate change, where manufacturers are looking to use different materials that will have a significant impact on weight reduction, adhesives certainly have a big part to play. They are likely to be used in combination with mechanical fastening techniques for the foreseeable future, but it's true to say that the use of nuts and bolts will decrease as time goes on and targets for the low carbon-economy escalate.