There are a large number of adhesive types for various applications. They may be classified in a variety of ways depending on their chemistries (e.g. epoxies, polyurethanes, polyimides), their form (e.g. paste, liquid, film, pellets, tape), their type (e.g. hot melt, reactive hot melt, thermosetting, pressure sensitive, contact, etc.), or their load carrying capability (structural, semi-structural, or non-structural).
Structural adhesives refer to relatively strong adhesives that are normally used well below their glass transition temperature, an important property for polymeric materials, above which polymers are rubbery and below which they are glassy. Common examples of structural adhesives include epoxies, cyanoacrylates, and certain urethanes and acrylic adhesives. Such adhesives can carry significant stresses, and lend themselves to structural applications.
For many engineering applications, semi-structural (applications where failure would be less critical) and non-structural (applications of facades, etc. for aesthetic purposes) are also of significant interest to the design engineer, and provide cost-effective means required for assembly of finished products. These include contact adhesives where a solution or emulsion containing an elastomeric adhesive is coated onto both adherends, the solvent is allowed to evaporate, and then the two adherends are brought into contact. Examples include rubber cement and adhesives used to bond laminates to countertops.
Pressure sensitive adhesives are very low modulus elastomers which deform easily under small pressures, permitting them to wet surfaces. When the substrate and adhesive are brought into intimate contact, van der Waals forces are sufficient to maintain the contact and can provide relatively durable bonds for lightly loaded applications. Pressure sensitive adhesives are normally purchased as tapes or labels for non-structural applications, although can also come as double-sided foam tapes which can be used in semi-structural applications. As the name implies, hot melts become liquid when heated, wetting the surfaces and then cooling into a solid polymer. These materials are increasing used in a wide array of engineering applications using more sophisticated versions of the glue guns widely used by consumers. Anaerobic adhesives cure within narrow spaces deprived of oxygen; such materials have been widely used in mechanical engineering applications to lock bolts or bearings in place. Cure in other adhesives may be induced by exposure to ultraviolet light or electron beams, or may be catalyzed by certain materials such as water which are ubiquitous on many surfaces.
Adhesives of various chemistries are available in many different forms as well. For structural applications, adhesives are available as pastes, liquids, films, and supported films. The latter are supported on loose knit or mat scrim cloth to improve the handling properties and also to offer some measure of thickness control. Many of these adhesives produce little or no out-gassing when cured, significantly reducing the likelihood of voids within the adhesive. It is important that these adhesives be kept dry, as absorbed moisture can create significant void problems. Thermosetting structural adhesives are normally available in two-part forms that are mixed through carefully controlled stoichiometry into a product that cures within the desired time window. One-part forms are also available in which the resin and hardener (cross-linking agent) are already mixed together. These one-part forms must be kept at sufficiently low temperature that the reaction does not occur prematurely, sometimes utilizing latent cross-linking agents that are not active at low temperatures. One-part thermosetting adhesives often have limited shelf life, and often must be stored at low temperatures, but do offer very high performance capabilities. Pot life refers to the time after a two-part adhesive is mixed during which it is workable and will still make a satisfactory bond. Materials with too short of a pot life will harden too fast, and do not give the workers sufficient time to assemble the product. An excessively long pot life may delay the cure time and slow the assembly process.
Adhesives may be applied in a variety of ways depending on the form it comes in. Adhesives may be spread on a surface manually, or may be dispensed using a variety of sophisticated nozzles and robotic equipment that is currently available. Maintaining adherend cleanliness, providing proper jigs and fixturing during cure, and providing adequate cure conditions may all be important considerations for certain types of adhesives.
The glass transition temperature (Tg) is one of the most important properties of any polymer, and refers to the temperature vicinity in which the amorphous portion of the polymer transitions from a hard, glassy material to soft, rubbery material. Although specific temperatures are often quoted for the glass transition temperature, it is important to remember that this transition temperature is a rate dependent process. For thermosetting structural adhesives, the glass transition temperature should normally be 50°C higher than the expected service temperature. Unless there are significant exotherms associated with the cure process, the glass transition temperature of an adhesive seldom exceeds the cure temperature. High performance structural bonds often require an elevated temperature cure to provide a sufficiently high Tg in a reasonable cure time. One concern with such conditions, however, are the residual stresses which may develop with an assembled joint is cooled from the cure temperature to the service conditions.
For example, silly putty at room temperature will readily flow when pulled slowly, will bounce like a rubber ball when dropped on the floor, or can shatter in a brittle fashion when struck with a hammer.
The glass transition temperature of epoxies and other adhesives can be significantly reduced by moisture absorption, a factor which should be considered when designing for humid applications.