Why Does Glue Dry & Act As an Adhesive?
Glues have existed in many forms for thousands of years, from ancient uses of beeswax and tar to the modern epoxy and superglue. They use a combination of adhesion (whereby the glue attaches itself to the objects it is spread on) and cohesion (where the glue holds itself together) to fix items together with varying degrees of success.
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The Need for a Solvent
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Glue needs to dry before it can act as an adhesive because the material with adhesive qualities is suspended in a solvent. This not only allows the glue to be applied to the surfaces it will be holding together, but also allows the glue to be removed from its original container. Without a solvent, the glue would need to be heated (or liquified by some other means) prior to application.
Mechanical Bonding
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While in a solution, the dissolved adhesive is able to enter the spaces between the surface molecules of the objects being glued. This results in part of the glue being inside the surface and part outside when the glue dries and the solvent ceases to take effect. As a result, the molecules are attached to one another like keys on a key ring, producing a strong adhesive effect.
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Covalent Bonding
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Covalent bonds are formed when two atoms come into close enough contact that one or more electrons can be shared between them. The free movement of adhesive molecules dissolved in a solvent allows for numerous covalent bonds to form between the adhesive and the items to be glued. These bonds are very strong but not all chemicals react with one another; hence not all glues are suitable for all purposes.
Van der Waal's Force
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Water molecules cling to one another because of van der Waal's force. The van der Waal's force is the minute attraction felt between oppositely charged atoms and molecules. Because electrons are not always shared equally in covalent bonding, different ends of a molecule can become charged. These charged molecules will then be weakly bound to molecules with the opposite charge, which is why water seems to cling to your skin. With enough molecules polarized in this fashion, a strong adhesive is formed.
The Combined Effect
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The exact nature of glue's adhesive properties is not fully understood, but each of the chemical processes plays some part in each type of glue. The exact proportion of the adhesive effect that is due to each process depends on the chemicals in the glue and therefore varies, but it is clear from the relative strengths of each bonding method that a combination of methods plays its part in the adhesive process.
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References
- Photo Credit bottle of glue on black marble image by phizics from Fotolia.com water splash - bottle and water in a moment image by Stasys Eidiejus from Fotolia.com
