Structural adhesives appear where high load-bearing capability is needed, e.g. in the face-to-core bond of honeycombs. They are characterized by
having high strength and modulus; good toughness and peel strength are important factors for structural adhesive bonds.
Non-structural adhesives (glues, bonding agents) are found particularly in, for example, solar-cell assembly, optical-component bonding and screw locking. Adhesives are most useful in the bonding of dissimilar materials which are difficult (or impossible) to assemble by other means: e.g. glass and ceramics. Some ensure good thermal contact and low stress
concentration at the joint, but such assemblies are difficult to take apart after fabrication. Electrically conductive adhesives find a use as grounding points for conductive surfaces.
Coatings and varnishes appear as electrical insulating layers, corrosion protection and mechanical protection mainly in electronic circuitry. Finished layers can be thin (e.g. varnishes) or rather thick (e.g. conformal coatings).
Adhesives: in current use are epoxies, phenolics, "modified" epoxies,
acrylates, polyurethanes, silicones, polyimides and cyano-acrylates. Their consistency is quite variable: liquid, paste, powder, supported or unsupported films. Some have to be kept cold until used; others are prepared by mixing two or more components just before application. They can contain fillers or be clear and transparent. Adhesives are in general quite complex (and proprietary) formulations, and appear on the market under many trade names; it is frequently difficult to determine their
basic chemical nature from the manufacturer's data. Anything can be bonded with adhesives, but no adhesive exists that can effectively bond everything.
-Coatings and varnishes: Current polymer bases are alkyd, epoxy, polyester, polyimide, polyurethane, silicone, polyesterimide and polybenzimide. Coatings appear as one- or two-component systems, frequently containing solvents (thinners) to give the necessary low viscosity. Some are crystal clear, while some contain organic dye (mainly for
quality control in the application). There are also products containing fillers. From the mechanical point of view, all grades are found from quite rigid to elastic products. As in the case of adhesives,
coatings are frequently proprietary mixtures, the composition of which is difficult to trace.
-Some adhesive coatings have quite different properties above or below their Tg. This shall be carefully considered during selection for a particular application. |
