Longread

Watch/Let Paint dry in different functional layers

Watching paint dry, a proverb that we use if something is boring and takes a long time. However the processes during the drying of paint are essential in many applications. Many surfaces need to be painted or coated to protect it from the environment and to provide a color or a gloss/shine. To provide these different qualities most often different layers of paint/coatings are needed. Take for example the coating on a car as shown in figure 1 at the right. Many layers are needed with  their own property to properly to provide the total layer with the correct functionalities.

At this moment each layer of paint need to be added separately. You can image that this takes much time and energy. Therefore, we like to develop a paint that can separate into multiple layers during evaporation.

We can do this by mixing the different paint beforehand. Paints are colloidal suspensions of different particles. The most prominent particles are latex particles (sort of plastic spheres). They provide attachment to the substrate once the paint is dried. By adding another type of particle for example silica particles (kind of very small glass spheres), we could create a paint consisting out of a more soft attachment layer, due to the latex particles and the more hard layer due to the silica particles.

If we make sure the size between these particles is different we can use a specific process called stratification. Stratification occurs during the evaporation of the paint. Both, small silica particles and the large latex particles will accumulate at the evaporation interface, since they are too big to diffuse away from the descending evaporation interface. This gives a concentration gradient in the evaporating paint mixture. The smaller silica particles like to move away from this high concentration, this creates a diffusiophoretic force. This will draw also the large particles along. Since their size is larger than that of small particles they will experience this force more than the small particles. Therefore the large particles are pushed away from the evaporation front, while the small particles will more and more accumulate over there. In this way a paint layer can be formed with more silica particles at the top and more latex particles near to the substrate. (see figure 2)

Schematic of Evaporating latex/silica mixture. The concentration gradient occurs due to the evaporating water at the top. This creates the concentration gradient and therefore the diffusiophoretic force. Finally ,leading to a dry sample with a higher concentration of silica at the top.
Schematic of Evaporating latex/silica mixture. The concentration gradient occurs due to the evaporating water at the top. This creates the concentration gradient and therefore the diffusiophoretic force. Finally ,leading to a dry sample with a higher concentration of silica at the top.

In this research we like to measure and tune this stratification, with different particles and see the effect of for example different particles sizes, evaporation speed, temperature, humidity etc.

  • Use Raman confocal spectroscopy to measure stratification and see how the different parameters influence the stratification. (see Figure 3)
Schematic of Raman measurements in a dried silica latex mixture. The silica and the latex gives different Raman signals. Using the ratio between these signals we can determine the relative amount of both at different depths inside the film.
Schematic of Raman measurements in a dried silica latex mixture. The silica and the latex gives different Raman signals. Using the ratio between these signals we can determine the relative amount of both at different depths inside the film.
    • Use fluorescently labeled particles to measure the stratification during evaporation with confocal/multiphoton spectroscopy.

    Stratification in a drying particle suspension inside a capillary. The green fluorescence comes from small particles ~ 400 nm and the red particles from bigger particles ~ 1000 nm. Clearly two different colors thus two different layers of particles can been seen.
    Stratification in a drying particle suspension inside a capillary. The green fluorescence comes from small particles ~ 400 nm and the red particles from bigger particles ~ 1000 nm. Clearly two different colors thus two different layers of particles can been seen.
    • Use simulations on mixtures of spherical particles and rod like particles. Do we still get stratification? Do we see other interesting effects? (see Figure 5)
    Stratification also occurs in binary colloidal mixtures of Rods and spherical particles. Interestingly depending on the evaporation rate we can tune the ordering of the rods inside the sample. Here the rods are color coded according to their orientation. This can lead to promising new optical effects of paints.
    Stratification also occurs in binary colloidal mixtures of Rods and spherical particles. Interestingly depending on the evaporation rate we can tune the ordering of the rods inside the sample. Here the rods are color coded according to their orientation. This can lead to promising new optical effects of paints.

    Please contact me if you are interested via ellard.hooiveld@wur.nl