However, LSE polymers are more difficult to bond. It’s imperative to select an adhesive or tape that will bond well and also maintain high-temperature performance and other necessary mechanical characteristics. “In particular for automotive applications, you get a lot of thermoplastic olefin and polypropylene usage,” says Uebbing. “We have acrylic-based products that will bond to them and don’t require surface preparation – flame treating or corona treating in order to get a structural strength bond back.”
A third issue for mixed-material applications is bondline read-through. “If one of the substrates is destined to be a Class A panel – say an exterior body panel on an automobile or truck – we also need to solve for the fact the adhesive, if not selected properly, will telescope through the exterior panel and read out, causing a surface defect,” says Barker. Ashland and other suppliers have studied the phenomenon and ways to mitigate it.
If you’re bonding composites to metal, such as CFRP to aluminum, then galvanic corrosion is also a concern. “You need to make sure you have separation there either with a fiberglass layer, spacer beads or anodization,” says Brekke.
Suppliers offer solutions to tackle multiple concerns simultaneously. For example, 3M created a family of urethanes with a unique formulation and adhesion promoters that bond well to both metals and composites. “They are flexible in comparison to MMAs (methyl methacrylates) or acrylics, so they have some real benefits in reduced read-through and low exotherm,” says Brekke.
Some companies are tackling joining issues by combining technologies. For instance, adhesive company DELO and assembly specialist BÖLLHOFF are using stud welding as the basis for their combination bolt and base product called ONSERT®. Fixing elements such as thread inserts or clips are bonded with special light-curing adhesives.
Creating Solutions Through Collaboration
Perhaps the best way to head off any problems and ensure a joining method works is to run laboratory tests. “I don’t think we will ever have a roadmap that encompasses every variable that can go into a composite and whatever else it might be bonded to,” says Barker. “At the end of the day, you need laboratory data to support your objectives.”
Using the customer’s substrates, Ashland cures candidate adhesives under the exact conditions expected in production. The company then runs adhesion tests – such as lap shear versus temperature, peel and impact – on the material sample and after environmental conditioning exposure to salt spray, high humidity, water soak and other conditions. In addition, using the load generated from the mismatched CTE and expected temperature excursions, the company runs fatigue tests to demonstrate long-term performance.