The Science of Glue

In a bit of a departure from talking models, Mark N. discusses the science behind keeping your troops together, in the literal sense.

Science time

I’ll preface this article by saying, I’m not usually one to brag, but at the time of writing it’s approximately a week after my final exams to achieve a BSc in Chemistry.
A large part of this course was looking at ‘polymers, plastics and carbohydrates’. Something that kept coming up during the studying of polymers was how a few of the most famous ones were ones we, as hobbyists, use almost every day (usually cursing a little as we get them stuck to our fingers and not to the fiddly little machine gun we were trying to stick down).

So, if you’ll indulge me, I’d like to talk about little bottles of adhesive that turns a sprue of uninspired plastic into anything from a tiny Stuart to a lumbering King Tiger.

The White Stuff – PVA

We’re not talking milk here, we’re talking wood glue/PVA glue/Elmer’s glue, or whatever you happen to call it in your country. PVA/PVAc (poly-vinyl-alcohol or poly-vinyl-acetate for a slightly different blend) glue is the one most of us probably use the least. It’s useful for sticking flock, scatter basing and the like to bases of our models, but utterly useless in keeping your resin-metal or plastic tank together. PVA glue tends to be mixed with a little bit of water or oil, which you can see for yourself if you leave a bottle of the stuff sitting long enough, the two phases will separate and need a right good mixing to get back together.
PVA glue works by solidifying on exposure to air, and usually heat. This makes it useful for quick applications on substances such as paper and wood, with the latter usually backed up by more permanent features such as nails or screws. Sadly, PVA is almost useless when applied to plastic or resin/metal models. I’ve found the best use for PVA is gluing a model to a nail-head, strip of wood or the like for painting, and then thanks to the less tactile nature, removed easily. Some smart, and highly skilled people are even able to use PVA glue for masking when painting camouflage.

 

The good old PVA, like you used to use in school

Toxicity rating: Non-toxic (but please don’t eat any)
Removal advice: Peel off hands [best feeling ever – ED] and then wash thoroughly with water.

The science bit: A bracketed off monomer of the vinyl-alcohol group.

Something for the plastic: Polystyrene Cement (poly-cement)

Plastic glue is something of a new invention; given that plastics themselves are relatively new, and even within plastics themselves there’s been a bit of an evolution. Old plastic used to be hard, brittle and difficult to mould. Anyone over a certain age will remember a substance known as ‘Bakelite’, this plastic was used universally from telephone casings to jewellery. Modern plastic, such as that used by Battlefront, Games Workshop, and other model makers is much improved over the older stuff. This new plastic has a lower melting point, is more malleable, but less hard-wearing than the older plastics of the past.

A sprue of modern plastic

So, what is plastic glue, and why is it so effective in bonding our little tanks and men together? Plastic glue works by reacting with your plastic and the air through an exothermic (giving out heat), reaction. You can feel this for yourself if you accidentally spill a little on your skin, there will be a warm sensation as the glue reacts with the air and heats up. This is slightly different to two-part epoxy cement, which I, myself, have not had experience with.
In essence, plastic glue works by melting the plastic slightly to form a tactile liquid joining point between two parts. You’ll see this for yourself is you ever accidentally get some on a nice smooth part of the model you wish to keep clean, you grab your cloth to rub it off and… there’s a frustrating melting happening. So, be careful when using plastic glue.

Plastic glue, not much else to say about it.

Toxicity rating: Very toxic, avoid contact with eyes, skin and mouth, avoid breathing fumes
Removal advice: Wash immediately with warm soapy water

 

The mother of all glues: Super/Crazy Glue

We all know super glue (called crazy glue by some very strange people across the pond).  This highly volatile glue will stick almost everything to everything else. We’ve all had experiences where the glue will bind more closely to your fingers, table, almost everything except the metal/resin model you are desperately trying to stick together – I’m looking at you tiny little machine-guns on the older tanks.
We’ve also all heard the urban legend of super glue, and the ‘fact’ that it was invented as an instant bandage to be used for wounds during battle. Well, this is half-right. Initially, the trials proved mildly successful, but it was found that super glue had too high a toxicity and can cause permanent skin damage if applied to an open wound. However, the sticky nature of the substance gave it a new lease of life as the wonderful(?) adhesive it is today.

So, what is super glue? Super glue is a substance called: Cyanoacrylate (The cyano part coming from the Carbon-Nitrogen group at the bottom of the diagram below). This substance is a thick liquid in it’s natural form, but upon contact with water, or the moisture in the air, it polymerises and forms a crystalline substance which is very difficult to get unstuck from whatever it happens to be sitting on.

This is Super Glue at the most basic level

Naturally, this is why the glue likes to stick to your fingers more than the clean piece of resin or metal you happen to be trying to glue. Your skin, despite being dry as a bone, has natural oils which contain trace amounts of water, which this lovely substance latches onto and sticks around like an absolute git. Ghetto modellers like myself use small amounts of water, or if you’re being disgusting: spit, to help the glue set quickly on your model. As mentioned above though, the glue sets in a crystalline form and this form can lead to the bonds between parts being rather brittle (ever dropped a resin model from about 5′?). So, whilst super glue will bond everything to everything, it does tend to be rather fragile unless you get a really good bond going.

Toxicity rating: Extremely toxic, if inhaled or swallowed seek medical help immediately
Removal advice: Super glue is easily removed with either an alcohol based liquid such as methylated spirits, or acetone. Be aware though that these can also harm your skin and should be washed off immediately after use

That’s all for now. I hope this was mildly interesting to some of you out there, and gives a better understanding towards the science of glue and why things just work better on some materials.

Category: Flames of WarRambling

6 comments

  1. Really interesting article, and really well written. Congratulations on your degree! (Also, Crazy Glue is a brand name we have over here, I’ve never heard it called that, but I live in the south, so maybe them northern folks call it that.)

  2. Margarine can also be used to break the resin bonds formed by Cyano. Especially useful if its skin to skin contact. Found this in a medical article from a London Hospital after a toddler had bitten into a tube of it!
    Not saying why I felt the need to look that up a few months ago while gluing some models together 😀

  3. Yeah, Crazy Glue was the first brand of cyanoacrylate glue that many people were exposed to. For me it was in the upper Midwest in the mid-late 70s.

  4. In my humid climate I use both a dot of pva and a dot of supaglue for the bond so its not so brittle over time………works good.

  5. Hi,

    does anyone know what the solvent is in polyfiller? I was watering down some ready- made from a tube in a jam jar lid that had dried acrylic paint. the solvent dissolved the paint and smelt like acetone or methanol/ ethanol. Will the solvent damage hard plastic? thanks

    1. Hi Dennis, a quick glance at the COSHH instructions for Polyfilla reveals that an active, and harmful ingredient in it is a polymer form of Acetate, and it’s quite possible that the polymer monomised into Acetone. I don’t believe that acetone has much of an effect on hard plastic so long as it’s not dunked into a vat of it. But in case of harm, keep the exposure to a minimum and wipe away any major droplets that may form.

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Article by: Mark Nisbet