Public service announcement – super glue

I’m fixing and converting some second hand models tonight and wanted to toss this out there, since it’s on my mind.

When you use super glue, use as little as you possibly can.

I tried to do some research to grab a second source on why this is, but it seems that modellers aren’t the target market for most cyanoacrylate (CA) glues. There are lots of other applications, but they don’t seem to care how much you use!

My first guess at why you use as little as possible was that because it has a very poor shearing strength and because it’s very brittle, the more you use, the more weak link you’re adding to your model. (in a perfect world, your model would just be entirely plastic/metal/resin, with no glue, which would be much stronger of a join!) The problem with this idea is that it really shouldn’t matter how much you add, since adding any is weakening.

I came across an article that finally gave me a decent answer.

The cyanoacrylate glue hardens very quickly when trapped between two surfaces. The reaction is caused by the condensed water vapour on the surfaces (namely the hydroxyl ions in water). The water comes from the surrounding air, so obviously the air humidity is a factor that may affect bonding capabilities, or cause them to differ from application to application.

The curing reaction starts at the surface of the bonded material and develops towards the centre of the bond. Because of this, thick seams or large blobs of glue may harden less satisfactorily than surface-to-surface bonds with good fit. In a thick blob of glue, a polymerisation reaction may stop before it reaches the centre of the blob. A rule of thumb is that seams thicker than 0,25 mm should be avoided. Thick seams will also take longer time to cure.

The other thing I’m thinking about (not a chemist), is that CA joins two things because of a chemical reaction between molecules, in particular the moisture on another object. But the stuff in the middle crystallizes, and rather than creating a strong bond between molecules creates a lattice of CA molecules that just isn’t as strong as if you have moisture:CA:moisture molecules all touching.

Regardless of the science behind it, we all know that thick applications are just more brittle and break easier. Don’t do it, you’ll thank yourself when your model stays in one piece in your case when you’re traveling by plane! (baggage handlers…*shudder*)