It's All About Sticking With It
Construction adhesives can be further defined by the materials, or substrates, to be bonded. To start a bond the process of adhesion must occur by chemically, mechanically, and/or physically adhering material to the substrates. After successful adhesion is accomplished the important process of cohesion between materials must begin. Cohesion involves the thickening or setting of the adhesive to withstand the stresses expected in the application. For structural bonding this means it must permanently meet or exceed the engineering properties of the substrates without losing adhesion. For non-structural applications it is usually desirable that cohesion is reversible, or defeated with reasonable effort, in order to facilitate replacement of the one or both substrates.
For adhesion to occur the substrates must be compatible with the adhesive. Developing the most broadly compatible yet cost effective formulation to initiate adhesion while maintaining cohesion is a subject of constant research, but as yet there is not a one stop solution for all construction materials. A typical categorization of adhesives based on substrates would be:
Smooth surface contact requiring minimal adhesive thickness: includes hard, finished, and impervious substrates such as dense concrete, plastics, metals, hardwood, glasses, etc. Chemical, heat, or radiation reactive components adhere to the substrate without need for solvent evaporation or absorption. Chemical and/or physical changes then occur to build cohesion. Examples include epoxies, thiokols, thermosets, RF and UV activated, and some humidity activated adhesives such as certain silicones and polyurethanes.
Rough surface contact requiring greater adhesive thickness: includes permeable, absorptive, and unfinished substrates such as softwood, bricks, lightweight concrete, joints, and seams. Solvent based solutions, emulsions, dispersions, and other similar adhesives are used which have a low viscosity carrier to promote adhesion. The carrier then reacts, evaporates, or is absorbed by the substrate to build cohesion. Examples include acrylics, mastics, pressure sensitive formulations, cements, mortars, PVC resins, some silicones, and certain foaming polyurethanes.