Adhesives

Adhesives have been shown to have been used throughout the last 6000 years. In about 1700 the first commercial glue factory was started to manufacture animal glue from hides. From about the 1750's, glues based on the following materials: fish, animal bones, starch, rubber, and a protein isolated from milk (casein) were patented. In the early 1900's plastic polymer glues were produced - allowing for the creation of specialist glues that improve various properties - such as flexibility, toughness, curing or setting time, temperature and chemical (and water) resistance.

These modern glues are known as synthetic adhesives. Some synthetic adhesives, such as the epoxy resins, are strong enough to be used in construction in place of welding or riveting. Adhesive tapes have a coating of pressure-sensitive adhesive.

The following hyperlinks provide great resources for teachers and students.

The following adhesives websites have very useful information, along with links to other adhesives & sealants sites.

The Adhesives Mart site, linked below, advises website users that they are "The Adhesives Matchmaker for Industry". Answer a few questions and specific adhesive recommendations will be provided along with detailed technical data for each adhesive selected. AdhesivesMart claims to have "the largest searchable industrial adhesives database on the web, with many thousands of adhesives from over 50 suppliers".

The "About Woodworking" link, below, has information on various woodworking adhesives and glues, and also links to an article on understanding woodworking glues.

Adhesion: theory

Theories or Mechanisms of Adhesion should consider: 1) Surface science 2) Polymer Characteristics 3) Interactions between polymers and surfaces [2].

There is no unifying theory that describes all adhesive bonds. There are, arguably, four mechanisms or theories of adhesion. They are: 1. Mechanical Interlocking 2. Diffusion Theory 3. Adsorption and Surface Reaction 4. Electrostatic [2].

The bonding of two materials using an adhesive is the net effect of the mechanical (physical) and chemical forces involved. When discussing the theory of adhesion, it is not possible to separate these forces from one another [1].

Mechanical interlocking forces: one aspect of adhesion is the mechanical fastening of the adhesive in the surfaces that are joined - even surfaces that look quite smooth have pores and uneven regions.

Physical or electrostatic forces: is where one charged particle exerts a force over another charged particle. The transfer of electrons to or from the particle is what creates the charge.

Chemical forces: refers to intermolecular and chemical bonding forces. There are three types.

a) Chemical adhesion is explained by adsorption - which is adhesion of the molecules of liquids, gases, and dissolved substances to the surfaces of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium. This theory is the most important mechanism in achieving adhesion between two surfaces.

For more information on these forces search for information on "van der Waals forces". Research indicates that Geckos cling to surfaces using these forces [3]. The other commonly mentioned force that forms where the adhesive and surface to be joined (adherend) meet and is believed to impact on the intrinsic adhesion strength is known as "acid-base interactions" [1]. To obtain good adsorption with commercially available adhesives, sufficient adhesive must be applied to achieve satisfactory "wetting" - meaning that enough adhesive must be applied to the surface to spread spontaneously when the join is formed.

Hairs give gecko feet their incredibly strong, dry, reversible adhesion. © PNAS [3]

b) Chemical interactions can be explained by the chemisorption theory. Chemisorption is to take up and chemically bind onto the surface of another substance. Chemical bonds are strong and make a significant contribution to the intrinsic adhesion in some cases.

c) Diffusion theory. This theory only applies to the bonding of polymers. Most common polymers are made up of large chains of carbon atoms. It is believed that the molecular chains interpenetrate at the interface where the adhesive and surfaces being bonded meet [1]. In simple terms, the two surfaces become interlocked at a molecular level, and therefore, become one.

If exploring diffusion theory further, three key terms that are worth to investigating are: entanglement coupling; cooperativity; and reptation [1].

Also, if more detailed information is required, see the two hyperlinks below (Citations).

Next >> Sealants

Glossary

adhesive: a substance that unites or bonds.

absorption: the process of taking up or sucking up (as a sponge absorbs water, or charcoal absorbs gas).

adsorption: the adhesion in an extremely thin layer of molecules to the surfaces of solid bodies or liquids with which they are in contact.

sealant: a substance used to seal a surface to prevent passage of a liquid or gas.

seta: (Biology) [plural: setae] a stiff hair, bristle or part on an organism.

Citations

[1]. SpecialChem S.A. (2002). Adhesives & Sealants: Adhesion theory - Basics of adhesion. [WWW Document] URL: www.specialchem4adhesives.com/techcenter/adhesionguide/index.aspx?id=theory (visited October 2002).

[2]. Gardner, Dr D.J. [Adhesion Research Group, University of Maine] (2002, March). Theories or Mechanisms of Adhesion. [Portable document format] URL: www.umaine.edu/adhesion/gardner/540lecture/adhesion%20lecture9.pdf (visited October 2002).

[3]. Autumn, Sitti et al. (August, 2002). Proceedings of the National Academy of Sciences, published online. "Evidence for van der Waals adhesion in gecko setae". [WWW Document] URL: www.pnas.org/cgi/content/abstract/99/19/12252 (visited October 2002).

Note: This page is under construction, and will be added to as time permits.

Copyright D. L. Christiansen [Last updated Nov 2002] Images: respective copyright owners noted/cited.