written by: John Moehring
• edited by: Lamar Stonecypher
• updated: 12/27/2009
What do gecko feet and screws have in common? They both utilize a form of adhesion, one of the first fasteners used in the construction trade. But of course, there's more to the story...
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Two Plus Two Equals One
Fastening two construction elements together is a challenge as old as construction itself. For example stone walls were fabricated by expert artisans using friction and geometry as the only means of keeping the structure intact. Nails, nuts, and screws utilize friction adhesion as well. In this sense, friction can be considered the first construction adhesive. Even though ancient examples of these fastening methods still exist, faster and more versatile bonding systems are constantly desired.
Especially in the last one hundred years with the advent of synthetically manufactured construction materials, like plastics and composites, along with innovative applications of traditional metal, wood, masonry, and textile materials. The result is a plethora of current adhesive bonding materials along with development of new and usually improved adhesives from which to choose for construction. In fact the largest markets (as defined by quantities and revenues) for all adhesives are generated by civil engineering and construction applications.
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Bonding For Life vs. Just Hanging Around
Adhesives used in construction today are classified by function into two broad categories, structural and non-structural. Structural adhesives are used to bond materials which must withstand loads or stresses acting to compromise structural integrity. Plywood laminate glues and epoxy used to put mounting bolts into concrete are typical examples of these bonding systems. Non-structural adhesives keep decorative or protective materials in place such as tiles, laminates, floor coverings, millwork, and other elements that are not subject to critical stress. Clearly to prevent disaster the distinction between these two categories must be considered prior to selecting an adhesive for engineering applications.
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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.
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Example By Lizard, Before There Was Insurance
Function is not the only performance factor when selecting construction adhesives. Cost, application methods, and extremes of environment must also be considered. New approaches and formulations are always on the research laboratory horizon. Nature also provides some some interesting case studies in adhesion, such as gecko lizard feet, sticky insect traps, and seed burrs. All of these may continue to yield some unusual construction adaptations. Analysis of seed burrs gave rise to the familiar Velcro tape fasteners. Sticky spider webs are some of the strongest filaments in existence. Geckos control extremely fine tissue and molecular moisture on their foot pads to adhere to many dry surfaces, even upside down. Who knows, retaining walls of the future may be held together with technology derived from cockleburs, lizard feet, and a spider web or two. Or maybe it's just that time of year…