Turning Archive 2004

Subject:
Some thoughts on adhesive chemistry (long)

JL work
>What makes an adhesive stick?
The answer is - it depends on the adhesive.
Polyurethanes
One-part (i.e. Gorilla Glue)- You have a somewhat long urethane molecule that is has an end isocyanate group dissolved in a suitable solvent. Water or a suitable hydroxyl group comes along and "activates" it. Ideally you get a nice, long bulky chain (the growing chain often cross-links with another chain giving it bulk). This can intermesh with protruding wood fibers (aren't you glad you sponged it with a little water to raise the grain?). If you are very lucky, it chemically binds with the molecules making up the wood fibers.
Two-part - You have a polyol component which can consist of long chain poly alchols. This is usually polyester based or acrylic based. You have an isocyante component which can consist of TDI (toluene di-isocyanate), MDI or more complex isocyanates. The isocyanate component reacts with the polyol component creating a nice long bulky chain. It can intermesh with the wood fibers and hopefully react with some of the molecules making up the wood fibers.

PVA adhesives
These are typically "solutions" of polyvinylacetate (PVA) in water. PVA is very water soluble. The PVA also likes the cellulose in the wood fibers. As the water from the glue evaporates, the PVA is attracted to the wood fibers and intermeshes with them. Again, long bulky chains of molecules are best.

Casein based adhesives
Again, these are solutions of a material (casein, a protein) in water. Similar to PVA in how it acts. Long bulky chains of molecules are attracted to the wood fibers and intermesh with them as the water dries.

Hide glues
Again, typically solutions of a material (collagen and similar proteins) in water. Hide glues hold on to water a little better, which is why they have a longer working life. Long bulky chains of molecules are attracted to the wood fibers and intermesh with them.

As you can sort of tell, it is the long bulky chains intermeshing with the wood fibers that sort of does the trick of gluing the wood together.

Ethanol might act to soften or solvate some of the shorter chain polyurethane molecules and thereby weaken the bonds between the pieces/parts.

I haven't gotten into acrylates and acrylate chemistry as this post is probably too long already.

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Some thoughts on adhesive chemistry (long)
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