Painting with rubber

While exploring my creativity, trying to prove I can reinvent myself and find a lost connection with a free-spirited self, I just couldn’t kick the chemistry habit. I was coming up with all sorts of questions like ‘What is this pigment made of?’ ‘Does the state of California actually have something against ultramarine blue? Is it going to follow the faith of cinnabar vermilion and lead white?’ And inevitably, ‘How does paint dry?’

Oil paints have been used for centuries, which explains why so much is known about how they work. I refer you to this 36-page review covering in detail how oil paint dries. Reading about the curing of oil paints led to another mystery; ‘But how do acrylic paints cure?’

First, let’s cover some terms:

Monomer is a small molecule which can be used to build a bigger one like links in a chain.

Polymer is the chain: a big molecule formed from many little ones.

Acrylic refers to a molecule derived from acrylic acid. It contains an alkene double bond and a carboxylic acid. These are important for two reasons: the alkene is used to connect the monomers amongst them, the carboxylic acid is useful because it allows to easily add extra features to our molecules.

Emulsion is a mixture of two liquids which cannot form a solution; instead, one is dispersed as small droplets throughout the other. An emulsion of acrylic monomers in water is used to prepare acrylic polymers – ready suspended in water – by a process called emulsion-polymerisation. Britannica have a very good explanation here.

Glass phase transition is the point at which an amorphous solid (i.e. its molecules are disordered) such as a polymer transitions between the glass and rubber phases. In the glass phase, below the glass phase transition temperature (Tg), the long polymer molecules can’t change shape: the solid is a glass, rigid and brittle. In the rubber phase, above the glass phase transition temperature (Tg), the long polymer molecules can change shape: the solid is rubbery, elastic. Here is a good summary.

Crosslinking is forming chemical bonds across from one molecule to another, linking them together.

Covalent bond is the strongest kind of chemical bond.

Think of rubber: if it’s too cold it becomes stiff and it’s not stretchy anymore, because the molecules can’t change shape and move past each other. Bring it back to the rubber state by warming up and you can stretch and bend it. But only to a certain extent. If you put too much force, it will eventually tear. The molecules come apart and the material with it. That is why rubber gets vulcanised. Vulcanisation is a form of crosslinking by which actual bonds are made between the rubber molecules. Some of the stretchiness is lost, but the rubber is much harder to break apart. The molecules can’t move as freely, but they are much harder to pull apart.

Introduction to polymers over (and me refreshing my memory), let’s discuss acrylic paint and how it dries (I get curious about the strangest things). The compulsory Google search yielded some pretty good and consistent results: acrylic paint drying is a physical process. Some resources here and here. Let’s compare oil paints and acrylic paints for perspective. In oil paints monomer molecules crosslink (bond) under the influence of air, light and additives to form a paint film held together by covalent bonds. Acrylic paints don’t contain monomers. They contain polymers held apart in emulsion form. As water and other solvents evaporate after the paint is applied, the polymer molecules in the emulsion move from their droplet form to come together and form a solid. Sounds simple.

However, I didn’t find this explanation enough. If there is no reaction taking place and no bonds are formed between the polymer molecules, how can the film formed be so strong?

I strayed outside of my comfort zone and into the fearsome field of polymer chemistry. Lucky for me, I found this research article: Acrylic Paints: An Atomistic View of Polymer Structure and Effects of Environmental Pollutants, published by Aysenur Iscen, Nancy C. Forero-Martinez and Omar Valsson, headed by Kurt Kremer. They were looking from the same angle I wanted to approach this: from the atom scale. Theirs is a thorough computational investigation, but let’s see what the main learnings are.

  • Acrylic paints are mostly composed from pigment (colour, <10%), binder (the acrylic polymer, ~30%) and carrier (water, ~40%). When the paint dries, the polymer content goes up to ~60-70% because of water loss
  • The binders used are P(MMA-co-EA) and P(MMA-co-nBA). MMA, EA and nBA are the monomers. The P stands for polymer, and the parentheses tell us what monomers go into making the polymer
Image reproduced from The Journal of Physical Chemistry B 2021 125 (38), 10854-10865
DOI: 10.1021/acs.jpcb.1c05188 under a CC-BY-4.0 licence
  • The glass transition temperature (Tg) for the polymers used in acrylic paints is around room temperature. This means the paint has a good compromise of qualities under normal usage conditions: it is flexible enough so it won’t crack easily, but is not too sticky and will hold its shape
  • The polymer molecules are pretty much stuck in one place at room temperature. They can wriggle but can’t move easily to another location
  • nBA is used more than EA nowadays because it makes the paints soft and rubbery, so they are easier to work with. nBA lowers the glass transition temperature and makes the polymer molecules more mobile. This is essentially because the bigger nBA is more demanding in terms of space, preventing polymer molecules coming together and staying together
  • Using the bigger nBA has another effect: it stretches out the polymers. Each polymer chain has many nBA fragments, and they can’t bump into each other. The polymer has to stretch out to give them space. This stretching actually makes the material weaker and leaves gaps in which water or other molecules can travel

Let’s summarise all this. There are two microscopic properties we care about: how easily individual polymer molecules change shape and how easily polymer molecules move past each other. There are two macroscopic properties we care about: how flexible our material is (i.e. is it more like rubber or more like glass?) and how tough it is (i.e. how much force can I put on it before it breaks?). Keep in mind ‘breaking’ means different things. Glass will shatter by breaking cleanly into sharp pieces, rubber will tear into shredded pieces.

How easily polymer molecules change shape determines how flexible the material is. How easily polymer molecules move past each other determines strength.

So far we covered in quite a lot of detail how changing the polymer changes its flexibility (and that of the material it comprises). But what about its strength, i.e. how well it holds together? This has to do with entanglement. Steven Abbott has a great explanation on his website. In summary, long polymer molecules, like the ones used in commercial products, become tangled. And as you’ve probably noticed with any tangled mess (rope, cooked spaghetti, cables, Christmas lights) simply grabbing on to it and pulling will not bring the components apart. The more strands you have and the longer they are, the harder it is to untangle the mess.

Engineering the polymers and paint composition to make the paint work is tricky business. One composition change can impact more than one property. It can improve one feature, but be detrimental to another (flexibility vs strength). Balancing everything is key to optimising properties.

To wrap up, how strong is artist’s acrylic paint anyway? I realised I didn’t really have a feeling for this. I started by believing it’s really strong with the way it creates a film on the painting surface. After writing this article I wonder, is it not more like rubber?

I came up with a little test. I used my white acrylic paint and made a little circle. The thickness was between 4 and 6 mm. I then left it to dry for 3 weeks (but didn’t watch) and removed it from the support. I was left with a little rubbery creation that is really stretchy and strong.

Check out the video below.

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