A Three Step Process for Automotive Adhesive Selection

Posted on 2/2/2017 10:32:42 AM By Marc Benevento

Adhesive manufacturers often claim that cars and trucks are not designed with adhesive choices in mind, and that adhesive selection can seem like an afterthought in many cases.  There is an element of truth to this – after all, adhesives are an enabling technology rather than a primary material of construction.  Automotive design doesn’t begin with fastener selection or development of weld schedules, either.

Although adhesive selection methods include formal and informal processes and may vary across organizations, some common themes are generally addressed during the adhesive selection process for automotive applications.  These processes examine three areas:

1)      Substrates to be joined

2)      Service demands of the application

3)      Process parameters and constraints

Adhesive selection normally begins with identification of the substrates to be joined.  In light vehicle production, the most common substrate is steel, which can be readily spot welded to itself, and previous blogs have discussed how structural adhesives are used in concert with spot welding.  As vehicle manufacturers strive to reduce weight, the introduction of aluminum, composites, and other lightweight materials is creating opportunities for adhesives and joining techniques other than resistance spot welding.  Acrylic, epoxy, and urethane chemistries, which are the most common structural adhesives, can generally be used to bond nearly every combination of these materials. Therefore, it is not the materials themselves, but the performance requirements and process constraints that truly dictate structural adhesive selection.

Once the substrates have been identified, an evaluation of the service requirements of the joint is required to take the next step in proper selection of an adhesive.  This includes an assessment of the required strength and elasticity expected over the service life of the vehicle.  The amount and type of loading can vary greatly depending upon the part and joint design, even for similar applications.  Other considerations, such as environmental and service life expectations are fairly uniform, but not identical, across the light vehicle and heavy truck industries. A thorough evaluation of these factors will further narrow the acceptable adhesives choices, but may not identify a solution. 

It is likely that multiple adhesive chemistries could deliver satisfactory performance given the substrate choices and service requirements.  It is often the case that process constraints dictate the final choice of structural adhesive.  These constraints include cycle time, available curing mechanisms, plant layout, and other factors.  Light vehicle manufacturers require short cycle times, process simplicity, and strive to minimize the footprint process equipment.  As a result, they desire to use one-component adhesives in their plants as often as possible, which minimize storage space, and can be dispensed with simple pumping systems. Adhesives applied in the weld shop can be cured using the heat of  the e-coat and paint processes, while adhesives applied in the assembly area must be able to cure at ambient temperatures.

In OEM weld shops, bonds are typically metal-to metal, and adhesives are used to supplement or replace spot welding or mechanical fasteners.  The adhesive is cured using the heat of the e-coat oven, which is roughly 180 C for 25 – 30 minutes.  This allows for quick curing, but limits adhesive choices significantly.  Epoxies are the primary chemistry used in this environment, as they are best suited to survive these temperatures without degrading.  In some cases, sub-assemblies are provided to the OEM weld shop from another OEM plant or a supplier.  Because these parts are shipped between facilities before they are painted, two-component, reactive adhesives are sometimes employed, because these adhesives can develop handling strength before being cured with heat, which can improve dimensional stability of the final assembly.  Of course, the adhesive selected for these applications must be capable of surviving the paint process, so epoxy chemistries are most commonly selected.

On the OEM assembly floor, the primary structural bonding application is for installation of front and rear glass.  In unibody construction, bonded glass is a structural element that contributes significantly to the vehicle’s torsional rigidity. Therefore, a strong, yet flexible bond is required.  Because one-component adhesives are desired for their ease of use, and adhesives that cure at ambient temperatures are needed in the assembly area, one-component urethanes are most commonly used in this application. These adhesives cure using moisture that is present in ambient air, and offer a good combination of strength and elasticity.   A ceramic frit is normally applied to the glass, which shields the polyurethane adhesive from ultraviolet light that would degrade it and provides a uniform bonding surface.

The applications discussed above are just a few of many in the automotive industry, but are good examples of common structural adhesive selection processes. Adhesive selection for automotive applications can be complex, and the final decision is often driven by consideration of factors other than the substrates being joined. However, a three step progression of substrate, service conditions, and process constraints is simple yet effective, model of most selection processes.