Of Paints and Varnishes: Watching Paint Dry can be Fun

by Ignacio Martín-Fabiani*

Department of Physics, University of Surrey, GU2 7 XH Guildford, United Kingdom.

*I am part of the Soft Matter Group, where we are particularly interested in all sorts of applications of soft materials. The project I am working on aims to develop new paints and coatings which are more environmentally friendly than the ones in the market. So I could say I am living proof that you can get a salary for watching paint dry!

Have you ever wondered what is paint made of? And why a milky liquid like a varnish becomes transparent when it dries? Well, I will try to provide answers to these questions in this post. The main component of any paint or varnish is a dispersion of small particles in a liquid. How small? Really small. And how small is that?

The basic component of paint is a dispersion of very small plastic particles in water: A latex. Image by tookapic,  via Pixabay. CCO 1.0 Public Domain.

Just take one of your hairs and imagine that it is cut into a thousand pieces. Each of those pieces would have the size of the particles floating in your paint. That means you cannot see this particles with you bare eye, you need a microscope to visualize them. And it turns out they are polymer particles! So under the microscope you would see a world of plastic spaghetti within them. What about the liquid surrounding the particles? That depends on the manufacturer – if it is a water-based paint, it will be more enviromentally friendly. Why? Because when it dries, only water vapour is released to the atmosphere. However, a solvent-based paint contains organic solvents such as benzene or toluene. You will recognize this type of paint by the strong odor of something that has been fresh painted. These solvents are released when the paint dries and are hazardous to both the environment and human health. This is why there is a trend to reduce the amount of solvent-based paints in the market. So for this article we will stick to water-based paints, that is: A dispersion of small polymer particles in water. There is another name for that combination, which might sound familiar to you: latex.

Latex is natural, extracted from the rubber tree. However, most latexes used in paints are synthetic. Image by Konstantino Dafalias, via Freeimages.com . Freeimages license.

What comes to mind when you think of latex? Right, gloves and condoms. In fact, these things are made by dipping moulds with the desired shape in latex dispersions and letting it dry. But of course paints are not only made of latex – they have plenty of other additives in their formulation. There are wetting agents, which allow paints to spread more easility onto surfaces. Without them a paint would not spread homogeneously, forming droplets instead of a continous layer - like rain does on a window. Also pigments are necessary when a coloured paint is needed. Depending on the color different pigments can be used, mainly metal oxides: Titanium oxide (white), iron oxide (red, orange, yellow, brown) and chromium oxide (green). Black colour is obtained by adding carbon particles. In fact, anytime you leave your food in the oven for longer than necessary, those black spots you see are just carbon. Because you burned away all the other elements! That is called the carbon black colour. And why do we see paints white, red or black? Why the different colours? Well, here is a bit of Physics. Sunlight is composed of lights of all the different colours: red, orange, yellow, green, blue and violet. The combination of all these together is called white light. When an object is iluminated, it absorbes some or all of the sunlight colours and reflects others. The reflected colours are the ones that we perceive. So for example a tomato (a ripened one!) absorbs all colours but red, and that is why it looks red, because it is reflecting that particular colour into our eyes. For example, coming back to pigments, white paint looks white because the titania dioxide inside it is reflecting all the light components, thus generating white light. 

Different paint colours are obtained by adding pigments to latex: They reflect the colour that we see with our eyes. Image by Mark Tullos, via FreeImages.com. Freeimages license.

But what about clear varnishes? They look milky when they are in the can, and even after application. So why do they become transparent upon drying? In fact, its two main components (water and tiny polymer particles) are both transparent. But when you put them together they look milky! To explain this, we have to introduce a new concept: the refractive index of a material. Light travels through air, water or other mediums at different velocities, depending on their optical properties. Its refractive index is defined as the ratio of the velocity of light in the air (more precisely, in vacuum, that is, in space) to that in the material. When light rays cross the boundary between two media with different refractive index, in addition to a change in velocity they also experience a direction change. This effect is known as refraction, and in the process some light can be reflected.  This reflected light does not have a preferential color because the arrangement of particles is completely random and that is why it looks white to us. When the varnish dries, water evaporates and polymer particles pack together. We still have two refractive indexes: one for the polymer particles and another one for the air that is in the small gaps between them. But polymer particles in paints and varnishes have their glass transition temperature well below room temperature. That means that they can deform and entangle their chains with the adjacent polymer particles, thus eliminating air voids and creating a continous polymer layer. Therefore, at the end of the process we end up with only one refractive index (that of the polymer particles) and a trasparent layer of varnish on our surface!

As you can see, behind simple objects such as paints and varnishes there is a lot of science involved: chemistry, physics and materials science. So next time you are painting something, and it takes ages to dry, try recalling the process that is taking place. That will make the wait less painful. I guess.

References

Keddie, J. L. And Routh, A. F. 2010. Fundamentals of Latex Film formation. Springer-Verlag Berlin Heidelberg.