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Scattering Sunshine

There is no investment you can make which will pay you so well as the effort to scatter sunshine and good cheer through your establishment.

– Orison Swett Marden (1850-1924)

770_4665645Scattering sunshine metaphorically is a great idea. The stress of the workplace places great burdens on people and spreading a little sunshine is a simple way to create a more positive environment. But scattering sunshine also has a far less romantic side to it.

Clouds and shaving cream are white, but they do not contain any white pigment. Driving through dense fog can be treacherous, but it’s just water! Bathroom mirrors also fog, especially if you take a really hot shower on a cold winter day. And titanium dioxide is a clear crystal, yet we call it a white pigment. Scattering of light explains all.

One can discuss scattering by pointing to detailed physics formulae, but that method produces more fog than clarity. I prefer the much simpler method that assumes that the brainy physicists have worked out all the details and that they can be trusted. If that works for you (and it certainly works for me), read on.

Scattering always occurs as light (and not just visible light; infrared, UV, X-rays…they all scatter) encounters a particle that has a different refractive index compared to the medium through which the light is traveling. (No need to fret over “refractive index.” Just know that air has a value of 1.000 and water has a value of 1.333. Frankly, just remember that they are different. How different is not that important for your long-term memory. Any difference between two materials creates scattering.) When light traveling through air strikes a water droplet, the light rays bend a little (refracts). You wouldn’t think that this is a big deal, and it’s not if the droplet is huge—like a raindrop. But as the water droplet size gets smaller and smaller, there are more and more droplets for the light to strike. At a certain droplet size (we’re not going to get into that detail either; it just complicates the discussion), so much scattering takes place that the object containing all those droplets—a cloud, a mound of shaving cream—appears white.

Fog is created by the same effect, but you might be wondering why fog normally occurs at night. Great question! You already know that water droplets must be involved, and you know that those droplets must be small. And from your own experience you already know that fog is not a usual occurrence, and normally occurs during the evening hours. So what gives? Fog formation requires a “just right” set of conditions. The air must contain enough water vapor (humidity) and the temperature has to be low enough for the vapor to condense. But not too much; that would produce dew, not fog. And you cannot have any breeze because that would knock the just-right droplets together and they become too large to effectively scatter light. As the sun rises in the morning and the fog lifts, that’s simply the effect of rising temperature. As the temperature exceeds the dew point temperature, the droplets return to their vapor forms (i.e., no more droplets).

White paint is no different. In this case the medium is some kind of coating resin (refractive index around 1.4-1.5). The white pigment is a titanium dioxide crystal with a refractive index of 2.6, and it’s just-right particle size is 0.2 microns. (The physicists figured this all out, and we’ve already agreed earlier to take their word on it. Remember: refractive index differences cause scattering.) Light enters the paint film, encounters a large number of very small titanium dioxide particles, each of which creates a little scattering as light passes through the particles. (Remember that these titanium dioxide particles are clear.). But a lot of little things combine to produce an important set of paint film properties: A white film that is opaque.

A low-gloss clear coating is low in gloss because of the addition of a flattening agent. These are usually mineral-like materials and they usually have about the same refractive index as the resinous binder. When light strikes them, no scattering takes place and, therefore, the film has no color. No refractive index difference = No scattering.

Colored pigments also scatter light, but they also absorb some of the wavelengths of light, which produce their color. We tend to think of a pigment as something that adds color, but—as mentioned earlier—the titanium dioxide crystal is clear, so is titanium dioxide a pigment? Of course it is. In my opinion, life stays much simpler if we stick with how we perceive an object.

Scattering is a phenomenon that explains many of our observations. It helps to know that it might be at play in one of your own projects. So always give scattering some thought. You might even find yourself deconstructing a song verse:

I’ve looked at clouds from both sides now, [scattering sunshine metaphorically and physically]

From up and down [clouds are up; fog is ground cloud], and still somehow [just-right conditions are paramount]

It’s clouds illusions I recall, [they really aren’t white, but nothing more than a bunch of tiny water particles scattering light]

I really don’t know clouds at all. [After this blog, hopefully you do know clouds, shaving cream, and white paint.]

To dive deeper into the discussion on the latest news and trends in the industry, be sure to share these posts via Facebook, Twitter, and other social media using the buttons below and join the NCCA LinkedIn group here. The NCCA LinkedIn group provides a forum for discussing issues pertinent to the process, industry participants, users and potential users of pre-painted metal.

– David Cocuzzi, NCCA Technical Director

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