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Acrylic adhesive is a binding agent made from the polymerization of acrylic acid. Variations of the chemical composition produce types of acrylic adhesive for different purposes that balance the performance characteristics of shear, peel, and tack. Other distinctions include the viscosity of the formula, the setting or curing time, and the required temperature for setting. Acrylic adhesive is sold in liquid, paste, and tape form. The chemical properties can be designed for permanent installations or to allow for removal by application of a substance to break the chemical bond.
Polymers are the compounds created by the linking of quantities of simple molecules, or monomers. The cross linking of the polymers improves adhesion, or shear, i.e., the measure of the material's ability to stick to itself. The chemical design of acrylic adhesive is a balance between internal cohesion, or shear, and the tack and peel, which are external measures. Peel, or adhesion, is the measure of the bond between the adhesive and the materials it is binding. Adhesion usually increases for a period of time after application while the adhesive is curing. Tack measures the degree to which the adhesive bonds immediately to the other material.
Engineering of acrylic compounds can produce a wide range of adhesives suitable for many purposes. The viscosity of the material is often linked to the shear rating, with firmer adhesives having higher shear. Many acrylics require a 24-hour curing period to reach full adhesion, but softer variations tend to have better tack and peel offset by lower shear. Some types are designed for heat curing. There tends to be an inverse relationship between the curing temperature and curing time required; a higher curing temperature requires a shorter cure time and vice versa.
Acrylic adhesive is often used as a two-part application, with an initiator applied to one of the materials to be bonded and a bonding agent on the other. The two parts are clamped or held together until the curing is completed. An alternative is a form where the two compounds are mixed prior to application. Acrylic adhesive is also sold in tape form as a roll of pressure sensitive material.
Permanent installations require adhesive able to withstand temperature fluctuations, humidity, UV, and resist solvents. Acrylic adhesive exhibits these qualities and is often the preferred option for outdoor uses. For installations that may require removal of the adhesive after an interval, removable adhesives are designed with lower tack and peel; this allows the bond to be broken without damage to the materials that have been joined.