Structural adhesives: Plus ça change ….

Back in 2000, in just my second year as an independent consultant, then publisher/editor-in-chief Judy Hazen asked me to join the writing staff of High-Performance Composites and sister publication Composites Technology, the two magazines that, in 2015, were merged to become this magazine, CompositesWorld. My first feature article appeared in the July/August issue of High Performance Composites. Titled “The new glue: Stronger, tougher, more versatile,” it was what we called a “roundup.” I contacted major suppliers in the structural adhesives market, plus a few end-users, and compiled the trends and a description of the latest products.

Back in those days, every magazine issue contained inserts called Reader Service Cards (nicknamed “bingo cards”) with numbers corresponding to specific products or topics of interest. Readers would circle numbers on the card corresponding to a numbered list of suppliers at the end of each article, mail in the card, and these “leads” would be passed along to suppliers. At the time, my article generated more leads than any previously published article in either magazine, a clear indicator that structural adhesives were of high interest and, perhaps, poised for major breakthroughs. Not to mention that I had a strong future as a writer in the field of composites ….

It’s clear that joining, and the subset of adhesive bonding, is an important topic in the composites industry. The growth in structural composites in aircraft, wind turbines and automobiles certainly suggests a market fueled by advances in how to “put things together.” However, when I reviewed that article from 2000 and subsequent roundups from 2004 and 2007 penned by other staff members, and then looked around at the industry today, I was reminded of the famous French phrase, “plus ça change, plus c’est la même chose,” which translates roughly to “the more things change, the more they stay the same.”

Processing innovations have led to wider adoption of advanced composites in many industries and with it, the need to assemble multiple structural elements in complex structures. But the adoption of structural adhesive technologies for those assemblies has trailed by comparison. It’s not that new products haven’t been introduced — it’s just that these have not been able to overcome the industry tendency to prefer alternate solutions to adhesive bonding for critical structures or final assemblies.

There are exceptions: The marine and wind turbine industries have used paste adhesives for structural bonding since at least 2000, with a high degree of confidence. With dimensional tolerances much less stringent than aerospace or automotive, these adhesives often must, and successfully do, fill gaps as wide as 50 mm. These are not static applications, either — boats, especially large sailboats, see significant loads on the water, and wind turbine blades are expected to endure fatigue loads over 20 years, at finished part costs of under $15/kg.

Where things tend to stay the same, but ought not to, is the automotive and aerospace realms. Although the automotive industry relies on bonding at the component level, opportunities exist in the assembly plants, particularly for attaching composite components to metallic structures. Many molded parts are adhesively bonded together — for example, SMC inner and outer panels rely on urethane adhesives, and the numerous molded components of the BMW i3 and i8 are joined with epoxy. It’s logical that panels that have a need to be removed, such as hoods or fenders, are attached with fasteners, but fixed structures made of composites, like roofs, floor pans and pickup boxes incorporate metallic inserts or are bonded to metal fittings, then are mechanically fastened to the vehicle during assembly. As the industry continues to evolve into multi-material vehicles, there is significant upside to developing robust adhesive approaches.

In the aerospace industry, film adhesives are the staple for making honeycomb sandwich panels used for secondary structures, and will continue to be a large consumer of such materials. But aerocomposites manufacturers prefer co-cured structures, despite the increased complexity in bagging and fixtures, to bonding less complex shapes. Further, work in infusion or RTM seeks to make entire assemblies in one shot, using very complicated tooling to achieve this, and at some higher manufacturing risk. And the industry is most averse to using adhesives to bond critical structures, preferring to drill thousands of holes in composites (knowing full well such holes reduce performance), using bolts and rivets in combination with adhesives to build aircraft — not trusting the glue to do the job alone. I understand the theory behind such “chicken rivets,” but the practice seems archaic. If reducing the costs of building aerospace composites is a priority, figuring out how to do this without fasteners merits some increased attention.