adhesive bonding

adhesive bonding

Adhesive bonding

The process of using an adhesive to manufacture an assembly. The adhesive-bonded assembly is known as an adhesive joint, and the materials to which the adhesive adheres are known as the adherends.

Adhesive joints are designed by first knowing the loads that are to be supported by the joint. Adherends and adhesives are chosen according to the needs of the application, that is, the stiffness, toughness (fracture resistance), and elongation. Mechanical engineering principles are applied to ensure that the joint can support the necessary load. A properly designed adhesive joint will provide for adherend failure rather than adhesive failure unless the joint is designed to be reworked or reused. Usually, the design is subjected to a test protocol before going into production.

Adhesive joints are made by means of surface attachment; thus the condition of the adherend surface must be taken into account. This is particularly important when the adhesive joint is to be exposed to adverse environmental conditions such as temperature and humidity. In general, the purpose of a surface preparation is to remove weak boundary layers (such as oils and greases), increase the adherend surface energy, and provide a surface with enough mechanical roughness to “key” the adhesive into the surface of the adherend. In some cases, a primer is applied to the adherend before applying the adhesive.

For a proper adhesive joint, the adhesive must “wet” the adherend; that is, the adhesive must come into intimate contact with the adherend. As a guideline, the adhesive must have a liquid surface energy less than the critical wetting tension of the adherend. If the adherend's surface has been properly prepared or primed, this is usually achievable. Alternatively, the correct adhesive can be chosen such that this condition of intimate contact is achievable.

Joint assembly is an important consideration in adhesive bonding. In many cases, the adhesive has a “set time”; that is, the adhesive has little, or no strength until some solidification takes place. During the solidification process, the adherends must be kept in place.

Pressure-sensitive adhesives usually require no processing to solidify, as they are already viscoelastic solids; that is, they display both liquidlike and solidlike character. Adhesives such as rubber-based adhesives and contact bond adhesives require the evaporation of solvent or water to solidify. Other adhesives undergo a chemical reaction to solidify. For example, two-part epoxy adhesives must be properly mixed in order to effect the solidification or “cure” of the adhesive. Some adhesives require the application of heat to cure the adhesive. Other adhesives are cured by the action of light or some other actinic source of energy. Hot-melt adhesives are heated to and applied in the liquid state, and solidify upon cooling.

Adhesives are used in a wide range of applications, including electronics, automotive, aircraft, furniture construction, and plywood manufacture, to name some. Adhesives are also used in many noncritical applications such as paper binding, carton sealing (hot-melt adhesives), and envelope sealing. In medicine, adhesives are used as tissue sealants during surgeries and transdermal drug delivery systems. See Adhesive