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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Depends entirely on what you’re gluing. As people have pointed out, textured surface = more surface area within the same area. Which is normally good.
But if you’re gluing something like a broken piece of a statue back together, and both sides are already perfectly smooth (polystone has this problem a lot), you don’t want to rough up the surfaces necessarily, because they already fit together more or less perfectly. Doing any damage to the surface could cause the repair to be off-kilter or not hold as well, at least not in the position you want.
Rough up for joining two formerly unrelated surfaces.
Do not rough up if you’re trying to repair a clean break. At least with any substance I’m used to working with, which admittedly is not all of them by any means.
Have you heard the reason they say why the brain has so many wrinkles? It’s because the wrinkles give more surface area, which means there can be more “brain” in the same space. A similar thing is true for glues. A rough surface gives more area for the glue to stick to. All those little bumps and crevices provide that extra surface area.
When you scuff it up, you’re increasing the surface area. Therefore you have more space with which to bond the surfaces together. That’s really all there is to it. Pretty simple.
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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