Applications of structural adhesives have grown as more manufacturers learn of their superiority to conventional joining techniques. Today, adhesives bond materials across every industry, ranging from automotive to aerospace, electronics, electrical, medical, sports, and construction. They can provide a multitude of functions, such as building up laminates, assembling structures, sealing against corrosion, damping vibrations, packaging electronics, and many more.
Most structural adhesives today are based on polymers, long chainlike molecules in which the links are smaller molecules called “mers,” and the backbones are carbon atoms. These polymers become adhesives when combined with various additives that improve their ability to adhere to different surfaces. Today’s polymer adhesives are complex blends of polymer resins and additives that are carefully designed for each application. Although polymer adhesives may be classified in many ways, the most basic classifications are those of thermosets and thermoplastics, each of which contains thousands of formulations.
Thermosets are polymer resins that are cured, set, and hardened into a permanent shape. Adjacent chains are not strongly bonded until they are crosslinked through vulcanization, a heating process that produces covalent bonds between chains. This is an irreversible reaction in which the long chains of mers link together across the chains. Once cross-linked, a thermosetting plastic cannot be remelted. Most structural adhesives are thermosets, and they have the capability of fastening structural materials for long periods of time, even when the adhesive joint is under load.
Epoxies are the most widely used thermoset adhesives, because they have low shrinkage during curing, excellent adhesion and chemical resistance, and high-temperature capability. Properties can be tailored by blending with other polymers or by selecting various additives. This enables epoxies to bond different materials, and to improve characteristics such as resistance to fracture.
In the automotive industry, reliability and durability are the most important characteristics, and epoxy adhesives meet these criteria better than almost all others. They can be customized to bond to almost any surface, they provide very high strength, and they function in a wide range of temperatures. In fact, epoxies usually form a bond stronger than the materials they join together.
As a result, epoxy adhesives have replaced spot welding and fasteners in many steps of the auto manufacturing process, reducing costs and weight while improving performance. Epoxies bond metal components, attach mirrors, join plastic parts, and assemble a variety of other materials. Epoxies are used for load-carrying structures and in parts requiring perfect joints. They provide higher strength than mechanical fasteners such as rivets, while enhancing appearance.
Polyurethane adhesives are unique thermosets that provide strong bonding, elasticity, toughness, impact resistance, and durability. Automakers typically use them for vehicle interiors and to bond windshields. They can be used to seal fuel tanks, where they maintain the bond even during prolonged periods of high temperatures. Polyurethane adhesives are more rubbery than epoxies, and are used to damp vibration, seal out water, prevent corrosion, and insulate electrical devices.
Elastomers (elastic polymers) are soft and deformable, and elastomer adhesives often function as seals. Elastomers consist of polymer chains with coiled shapes that stretch when pulled, and return to their original shapes when released. Adjacent elastomer chains are not strongly bonded until they are crosslinked through vulcanization, and are therefore considered thermoset elastomers. With the proper formulation they can provide resiliency at both high and low temperatures, as well as resistance to creep and retention of sealing force during repeated temperature and pressure cycling.
Thermoplastics are polymers that soften when heated and harden when cooled. However, unlike thermosets, no cross-linking takes place, and the process is totally reversible. In other words, hardened thermoplastics can be remelted and hardened over and over again. Thermoplastics can deform under load and they exhibit strong resistance to fracture.
Thermoplastic adhesives generally have low resistance to heat and creep, and are therefore normally used under moderate service conditions. In general, thermoplastic adhesives have low to medium shear strength and are susceptible to creep at high loading. They have good resistance to oils but poor resistance to water.
However, properties of thermoplastic adhesives have been enhanced over the past 30 years as the industry has developed additives that improve their toughness and chemical resistance. One example is cyanoacrylates, better known as superglue. Other thermoplastics include silicone resins (which have backbones of silicon rather than carbon) and polyamides. Thermoplastic adhesives bond metals, glass, ceramics, rubber, and many other materials.