Why is it good to “rough up” a surface to be glued? Wouldn’t a smooth surface give the best adhesion?
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There’s two types of glue. There’s filling glue, and non-filling glue.
A feeling glue bonds strongly to itself and will fill voids between the materials. An extreme example of a filling glue that is not particularly adhesive is the silver amalgam fillings that dentists use. When the amalgam hardens. It’s just a lump but it doesn’t stick to anything which is why it doesn’t glue your jaw shut. But since it’s a lump, if the dentist makes the whole bigger as you go deeper, he can fill that wedge with the filling glue and when it hardens the filling stays in place because it literally can’t fit back out of the hole it was stuck in through.
A non-filling glue, such as super glue, does not stick well to itself, but it can make an excellent connection to certain kinds of materials. So as the original Wonder glue, the fact that you could glue two pieces of steel together was pretty darn amazing. You want the two surfaces to be as smooth and evenly mating as possible because your ideal is to get basically a one molecule layer thick connection that is connected to both smooth surfaces. That doesn’t actually happen but it’s the ideal.
Now in the practical world, most glues are a mixture of the two. That is, they do some degree of filling and they do some degree of directed adhesion without filling.
There’s an obvious benefit to a filling glue being applied to a roughed up surface because there’s more surface area for it to stick to and it will conform to and become a solid piece of that things being joined.
There’s also the chemistry of the joining material. For instance, wood glue is very good at sticking to cellulose but there are materials. It doesn’t stick too well at all.
White glue will stick paper together quite well, but you can peel it right off your hands .
And so on .
So the directly adhesive property and the filling property and the chemical compatibility between the glue itself and the surface it’s going to be sticking to. I’ll have to be taken into account to make a good solid bond .
Glues that effectively don’t fill hardly at all are very rare, not counting super glue which is super common, but it’s only one kind of glue basically.
For the most part, roughing just increases the surface area which is something you want when your glue will conform to that surface area.
Most glue is really good at sticking to itself.
There are a whole bunch of other interesting things about glue such as how fast it cures and, how fast it might degrade under certain circumstances. How resilient it can be. In other circumstances where different glues would be crap.
It’s actually a fascinating material science.
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