How are 3M glass bubbles used to achieve high levels of Total Solar Reflectance (TSR) in coatings?

How does the light from the sun warm the Earth?

Light waves from the sun travel through the vacuum of space and reach the Earth in just over 8 minutes. Once they reach the Earth they encounter the particles that make up the protective gas atmosphere. Some of the light is reflected by the very outer particles of our atmosphere, but around 70% still makes it through. As the light waves meet particles, they are either reflected back the way they came or absorbed by the particle at which point, some of the energy is transferred to the particle. For the purposes of this article, we can think of this energy as heating the particle. If you are interested in the chemistry behind photons and the different energy states an electron can sit in, check out our blog article about fluorescence which goes into more detail on the effect of light on particles.

The total solar emission spectrum in Figure 1 shows the irradiance, a measure of light energy, of different wavelengths emitted by the sun and the percentage split between different types of light. UV light is invisible to us but can cause damage to our cells such as sunburn, hyperpigmentation or eventually skin cancer, visible light describes the wavelengths that our eyes see and IR radiation is responsible for the heating effect. Therefore, the more a material reflects IR radiation rather than absorbing it, the less energy will be transferred to the particle to heat it. The measure by which a material's ability to reflect light is compared is called the total solar reflectivity (TSR) and measures the percentage of light reflected by the material.

Figure 1: Solar irradiance spectrum showing the contributions from UV, Visible and IR radiation.

How to increase the TSR of coatings?

Increasing the total solar reflectivity of a material allows it to reflect light and therefore heat energy away. For buildings, this keeps them cooler and reduces the need for energy-intensive cooling systems which in turn saves energy and money. Using white pigments and fillers is a traditional method of doing this, as the pigment doesn't absorb any visible light wavelengths in contrast to black pigments which absorb every visible light wavelength. This is why white is often referred to as the absence of colour, while black represents all the visible light colours. The other method of increasing TSR is by the shape of particles.

3M glass bubbles for increased TSR

Glass Bubbles from 3M are spherical in shape and also white in colour. The perfectly spherical nature of the bubbles allows them to scatter light in all directions away from the material, particularly at the wavelengths associated with IR radiation. 

A study was done with waterborne coatings to test this. A traditional coating with calcium carbonate as a white filler at 22.5 % volume was used as the reference. The calcium carbonate was then replaced on an equal volume basis with different grades of glass bubbles, also at 22.5 % volume. For a more detailed breakdown of the coating, please see Table 1 in the appendix.

A Perkin-Elmer spectrophotometer was used to analyse the TSR of the coatings at 400 microns. All 3M glass bubbles outperformed the calcium carbonate reference filler, however, it was the smaller, high-strength grades that gave the highest reflectance (Figure 2). The smaller grades were found to be more effective due to the higher surface area and greater packing potential which means more lightwaves can be reflected. Due to the strength of these grades, these coatings are also able to be applied using an airless sprayer without breaking the bubbles.

Figure 2: The total solar reflectance of the waterborne coatings formulated using CaCO3 as a reference and different grades of glass bubbles. For detail of this formulation, see Table 1 in the appendix.

3M glass bubbles for cooling buildings

While the TSR improvements are important, it is the thermal diffusivity properties through the coating that matter. To understand the correlation between TSR and thermal diffusivity, an experiment was set up (Figure 3) where each coating was painted onto an aluminium panel and exposed to IR radiation from an IR lamp. The thermocouple on the other side of the coated panel was measured over time to investigate the thermal barrier presented by the coating.

Figure 3: An experimental lab set up to test each coating on its ability to reflect IR light and act as a thermal barrier between the light source and a measured thermocouple.

A good correlation is found between total solar reflectance and the level to which heat transfer is reduced through the coating. S28HS and S32HS both perform very well in this experiment - with a reduction of 10°C when compared to the coating containing only calcium carbonate (Figure 4).

Figure 4: Temperature readings from the experimental setup using different grades of glass bubbles within a waterborne coating to achieve higher TSR and reduced temperatures.

Other benefits to 3M glass bubbles in exterior coatings

Glass bubbles can also impart other benefits to coatings...

• Light-weighting - Hollow in nature with a vacuum centre, Glass Bubbles have a very low density which improves handling, processability, and application properties. Reduced transport weight can also lower the CO2 footprint of a product.
• Strength - Through both a spherical design and soda lime borosilicate glass composition, a high isocratic crush strength of up to 27,000 psi is achieved providing good processability and durability.
• Improved flow characteristics - Microspheres can move over each other easily which helps achieve coatings with lower viscosity and better flow for improved handling and product application.
• Reduced binder demand - A sphere offers the lowest surface area to volume ratio of any shape. As such, when Glass Bubbles are used, less binder is required to wet out the surface than with other fillers.
• Higher volume loading - Spheres are able to pack more efficiently than most other shapes. This, combined with the reduced need for binders, allows paints to have high solid content and lower shrinkage.

Summary

The small, spherical shape of glass bubbles in coatings formulations allows them to effectively reflect light rays from the sun with a TSR of over 94 % for certain grades. By doing so, they prevent the material below the coating from absorbing the heat energy and raising in temperature. Therefore, adding glass bubbles to an exterior roof or wall coating can keep a building cooler than the traditional filler of calcium carbonate (up to 10 °C cooler). Glass bubbles also have other benefits for handling, transporting and formulating making it an all-around improved filler option. For more information, get in contact with us and one of our technical team would be happy to talk you through switching to glass bubbles. 

Appendix

Formulation 1 - Shows the breakdown of the components from the experiment listed above with calcium carbonate. 

Formulation 2 - Shows the same formulation but with the calcium carbonate replaced with glass bubbles (different grades hence the different densities).

Lawrence Industries can also supply you with the following components of the formulation: dispersing agents, acrylic binders, HEUR thickeners and the polysiloxane defoamer.