Twenty years ago, aerospace-grade structural adhesives were manufactured by a small group of companies whose customers qualified a relatively limited number of adhesives to reduce the cost of extensive testing for qualification. Consequently, the adhesives were formulated to suit a wide variety of applications. In structural bonds, adhesives were primarily the "backup" system - a supporting act for the real stars of the show, mechanical fasteners.
Times have changed
As composites have proliferated in aerospace applications, structural adhesives have taken a place at center stage as primary bonding materials, reducing or eliminating mechanical fasteners and the labor, weight and strength-reducing holes they require. Further, adhesives are asked to perform many functions. They attach metals to composites, cores to prepregs, and they secondarily bond precured composite laminates to each other or to core or metal. But their uses go beyond simply bonding. They also separate dissimilar resins in cocured assemblies, provide smooth, pinhole-free surfaces to minimize finishing, and sometimes carry an electrical charge by means of embedded metal meshes.
From a material science viewpoint, therefore, today's structural adhesives are asked to do a lot. Adhesives have two distinct phases: They must flow with very low viscosity to thoroughly coat the substrate or adherend surfaces (the terms are interchangeable), and then harden into a cohesively strong solid for in-service use. The flow period must be long enough and at viscosity sufficiently low for excellent wetout of the substrate surface. However, if the flow period is too long, then the adhesive will run beyond the edges and the bondline will be adhesive-starved. The trick is to have a brief period of high flow, then the adhesive must gel to hold its position.
Sometimes, adhesives also must tolerate severe in-service conditions, including temperature fluctuations and exposure to vibration, moisture and/or chemicals, such as aircraft fuels and hydraulic fluids. In some applications they must have a high flame resistance and, if they do burn, they must meet strict smoke and toxicity limits. Most importantly, the adhesives must do the job for which they were formulated. The adhesive bondline often must be stronger than the substrates themselves. They must be stiff (glassy) in most structural applications, or flexible (rubbery) in others. They must not shear, fracture or peel apart under in-service loading. And they must do all this while being relatively easy to manufacture, ship, store and use.
Formulators of adhesives have benefited from improved manufacturing technologies: a wider variety of high-quality chemical raw materials, better mixing equipment, and much greater control and batch-to-batch consistency. Much of the variability has been taken out of the manufacturing process, and the resulting greater precision has enabled formulation of adhesives to narrower specifications. Whether the application is a limited-production space vehicle, a moderate-production business jet or a high-production commercial application, increased technological sophistication has enabled the formulator to develop specialized adhesives to meet specific customer specifications.