Adhesives are substances used for bonding materials together, while sealants form a seal between materials in order to prevent the passage of a gas or liquid. In practice, the distinction between adhesives and sealants is not clear, since many sealants also act as adhesives and vice versa.
Until the 20th century, the only adhesives available were derived from natural sources, such as bones and plant saps. With the development of synthetic polymers, however, an enormous diversity of new adhesives became available. Today, adhesives range from the gum on postage stamps to the epoxy resins used in aircraft and bridge building.
Adhesives work in several ways; adsorption, in which adhesion takes place through intermolecular forces between the surfaces of the adhesive and the adherend; interdiffusion, which is the dissolving of the adherend surface by a liquid adhesive, causing the two surfaces to become mixed together; mechanical interlocking, which occurs as a result of the adhesive flowing into small pores in the surface of the adherend; and a combination of adsorption and surface reactions, in which chemical reactions occur between the adhesive and the surface of the adherend. In practice, more than one of these processes is likely to be involved in any particular adhesive reaction.
An adhesive must normally be applied to both surfaces that are to be joined together, because no matter how smooth they seem, they will be full of irregularities at a molecular level and must be evened out if the process is to be successful.
Design of joints
A normal adhesive joint has five parts, which may be considered five links in a chain-the joint being as strong as the weakest link. They are the inherent strength of one material, the strength of the bond of the adhesive to it, the inherent strength of the adhesive itself, the strength of its bond to the other material, and the strength of the other material.
With most types of adhesives, the strength of the bond between adhesive and adherend is stronger than the cohesion of the adhesive itself. For this reason, it is important to keep the adhesive film as thin as possible to prevent the joint from failing. The joining surfaces of the adherends must therefore fit together exactly. They must also have a large enough area and the right shape so as not to overstress the joint. Adhesive joints resist shearing (sideways) forces and tension as well but do not stand up to peeling forces, where there is tension at one edge of the joint that can cause a split to form and spread.
More and more joints in manufactured goods of all types are being made with adhesives instead of more traditional methods, such as bolting or welding. These joints even include metal-to-metal joints: for example, aluminum chair leg assemblies and certain parts of aircraft, where honeycomb structures of light alloy are bonded between two aluminum panels. In joints of this type, the adhesive cannot normally be used as direct replacement for the earlier fastening method; the joint has to be redesigned from scratch. Sometimes adhesive is used as a supplement to mechanical fastening: parts are spot-welded together and the space between the welds filled with adhesive to steady the parts against vibration.
It is important to choose the right adhesive for a job. For example, joints between flexible materials must be made with a flexible adhesive. In industry, other factors are also important, for example, setting time. It is no use cutting manufacturing costs by using adhesive joints instead of more expensive fastenings if the whole industrial process is held up while everyone waits for the adhesive to set.
