Hand Tools Archive

Universal adhesive considerations

Bill Tindall, E.Tn.
Adhesives fundamentals are quite simple:

1.The adhesive must wet the surface being bonded. Wet means spread like water on a perfectly clean piece of glass and not like water on a waxed car. In this regard water is an excellent adhesive for fastening glass together except.....

2. If the adhesive wets the surface this adhesive property can't get any better. Wetting reveals how strong the adhesive sticks to the surface being bonded. . However, if the mated surfaces are not molecularly close together, or the mated surfaces are subject to a force that will slide them apart, then the mechanical strength of the cured adhesive comes into play. Water fails as an adhesive for glass because the water itself is weak and the glass surfaces are easily slid apart. if the are separated by a thick water layer they are easily pulled apart. In each case the strength of the water is determines how well the glass surfaces are stuck together. But, if the glass surfaces have only a few molecules of water between then they can't be pulled apart ("stiction" when sharpening on a smooth stone). In this case wetting alone results in the force holding the surfaces together.

So, what makes an adhesive strong? Adhesive molecules don't stick to one another very strongly. However, the bonds holding these molecules together are boggling strong.

Picture a very long red rope and green rope. Rope doesn't stick to itself strongly so if the ropes are laying side by side they are easily separated. (molecule sticking to adjacent molecule analogy). Now take these 100' long ropes and tangle them together. Grab a red rope in one hand and green in the other. Pull. At first there is a bit of give and then the separation stops. To separate the ropes by force any further the tangled rope loops will need to be broken (analogy is breaking the strong bonds holding the adhesive molecules together.)

A strong adhesive will be one where the adhesive molecules are long and tangled like rope, and the longer the better. Epoxy curing is turning small molecules into big ones by chemical reaction of the curing resin. As cure proceeds it gets ever harder for the molecules to wiggle into the right position to hook together ideally.

Slower curing epoxy, all else being equal, allows more time for the molecules to wiggle into optimum position to make big, and hence, more tangled, stronger cured resin.

"Urethane" and "epoxy" describes the chemistry for hooking together (bonding together with chemical bonds) the small molecules into bigger more tangled, hence stronger molecules. These names provide no clue what chemicals are being hooked together. The names are near useless for predicting properties of the hooked together polymer.

Longer setting epoxy is probably stronger.

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