The Sun Also Destroys by William Rice

Everyone has used a sun blocker or sun protector on their skin to help protect themselves from the damaging effects of solar radiation. Specifically, UVA and UVB radiation are the primary elements that can cause temporary sunburn and long-term tissue damage, even to the extent of cancerous melanoma.

The pigmented paint we all have on our cars, boats, buildings and homes is much the same as the skin on our bodies when it comes to the need to be shielded from the harmful effect of the sun's rays. The pigmented paint must have three things to keep it shielded from the elements. First and foremost, it needs a binder as its backbone. Hopefully, this binder will be comprised of a resin or resin system that is capable of long-term exposure to the sun without any appreciable deleterious effect. It is important to understand that it is the resin or resins in the binder system of a paint that is the primary determinate of durability.

The next thing is the type of pigment that is used - whether it is an architectural or an automotive grade pigment. Automotive grade pigments cost more but have increased durability over the architectural. A subset of this type of pigment is whether it is organic or inorganic, such as those that are based on chrome or lead. Since lead pigments have been out of favor for years (primarily because of the potential risk to children of ingesting lead-based paints), they are used mostly in specialty applications. While organic pigments are brighter, have a broader spectrum of colors, and are more environmentally acceptable, they do not have the same durability or color hiding strength as inorganics.

Which brings us to the third element of the paint formulation - materials to absorb, scatter, or reflect the sunlight. The primary one of these is titanium dioxide.  In the case of the organic pigments, TiO2 also has the essential characteristics of adding hiding power to what otherwise is a transparent pigment making it seem to be a solid color.Much the same way you put zinc oxide on your nose to protect it from the sun, TiO2 performs similarly to provide protection from UVA and UVB rays.

All the above are required ingredients of a good-pigmented paint.

However, in the last two decades, there has been increased research into additives that have the potential for improving the ability of paints to retain their color. These additives are either organic or inorganic. First, let's take a look at what the organic additives can do.

The first class of organic additive is ultraviolet light absorbers or UVAS. These convert some of the UV radiation into harmless levels of heat energy, which can be dissipated throughout the coating. (For those interested, this is called "ketoenol tautomerism." Look it up if you are so inclined.)

Essentially, the concentration of UVAS necessary to completely protect the surface of a coating is so high as to be impracticable. So, for additional protection, we turn to the second class of additives which is called hindered amine light stabilizers or simply HALS. You have probably heard the term free radical applied in the context of your body's health as something negative. And you've heard of using vitamins A, C, E and others as anti-oxidants or free radical inhibitors to try to control these free radicals. Well, paint has to deal with them too.

UVAS minimize the number of these radicals while HALS control, not eliminate, but control, the harmful effects of them. When combined with HALS, UVAS have high light stabilizing efficiency and the potential to increase the durability of pigmented coatings. Remember, that neither UVAS or HALS in and of themselves can provide protection to a coating that lacks a quality binder supporting a quality pigment. Their job is simply to add to the durability of a first class formulation.

An important note to realize is that these organic additives to increase protection to prevent sunlight degradation of the paint is that their effect is only temporary. In the first year at most, they have dissipated and left the coating. That brings us to the inorganic additives that graft onto the polymer backbone in the paint. These recently developed additives are nano-particles of zinc oxide and titanium oxide. Their effects are continuous and the benefits more long term.

The word “nano” comes from Greek nanos, meaning “dwarf”. It is also an SI (International System of Units) prefix denoting a factor of 10-9, or 1-Billionth of a meter (a nanometer, abbreviated nm). To give you a point of relevance, a average human hair is about 80,000 nm in diameter, which is 20,000 nm shy of 1 millimeter (mm). Seems pretty small, doesn't it? So, small it can't be seen by the human eye. Which makes inorganic nano-particles of TiO2 and zinc oxide ideal for clear coatings.

In paint technology, nanotechnology refers to the study, creation and application of molecular materials, in particular nano-coatings, with a particle size that is typically less than 100 nanometers. Nano-particle UV inhibitors are perfectly clear, yet they absorb UV light energy. These pigments of titanium dioxide and zinc oxide absorb the UV energy like a pillow absorbs a punch. Zinc oxide was and is used as a sunscreen on people's noses when they go out in the sun.

This is a rather simplistic explanation of what it takes to make a good-pigmented paint. A clear topcoat has only the binder and optional organic UVA's and HALS plus inorganic nano-particles of zinc oxide and titanium dioxide to give it the capacity to protect the underlying pigmented coating or base coat from damage by the elements.

In the next article section (Accelerated Weathering Devices), I will explain how you test each constituent part of the paint to determine whether one resin, pigment, or additive is better than another and how that effects the choices one makes in formulating a product for exterior uses.