Clear Coat & UV Protection

 

A clear coat system consists of one or more primer layers, a flat colour layer and a glossy, clear top layer.  The primer is a corrosion inhibitor and a bonding agent for the bare metal and the colour layer.  It prevents corrosion and provides a stable substrate for the colour and clear coats.  The colour layer is applied to the primer and is typically very thin.  Its only purpose is to provide colour.  The clear coat is two to three times the thickness of the colour layer, adding to the appearance of paint depth and offering additional protection. 

Automotive clear coats are generally based on urethanes, acrylics, melamine’s, silanes, etc. which are hydrophobic and repel water and other contaminants initially. However as the paint surface becomes oxidized water beading changes to sheeting and environmental contaminants will bond to the surface. Polishing paint will remove the oxidized layer and makes the paint surface hydrophobic again for a short period of time.

Specific gravity [: the specific gravity of water is 1.0 if an object or liquid has a specific gravity greater than one, it will sink. If the specific gravity of an object or a liquid is less than one, it will float]

Density (or specific gravity); different materials usually have different densities, so density is an important concept as less dense fluids float on more dense fluids if they do not mix, causing it to rise to the top (we have Archimedes to thank for this discovery)

OEM paint specifications usually require the addition of ultraviolet absorber technology in their clear coat systems for protection against photo degeneration (sun fading). UV absorbers have a greater specific gravity than clear coat resin (1.15 g/ml for instance) which is heavier than water (0.80 g/ml) and much higher than the solvents used so there is reason to believe that it migrates throughout the clear coat. On the other hand fumed silica (CeramiClear) is low (less than 1.0 g/ml) and therefore migrates to the top of clear coat as the solvents evaporate and the paint cross-links and hardens.

Modern Isocyanate resins (clear coat) finishes are so good today that they lull people into thinking that vehicle paint has protection and shine when in reality there is not really much there, the clear coat that has a thickness of ~25.4 µ (micron). As a point of reference a sheet of copy paper is 89 µ. 

 A micron (µ) is a metric unit that equals one millionth of a meter, or 1/1000 of a millimetre. A micron is much smaller than a Mil. A human hair is about 2 Mil (50 µ) thick and individual bacteria are 0.1 mil (2.5µ). There are 25.4 millimeters in an inch and a micron is 1/1000 of a millimetre.

 Using the micron (metric) measurement system gives you a much better idea of paint thickness as the numbers used are so much smaller. Most detailers are or should be aware of how thin clear coat paint is and it puzzles me why people over-polish paint causing it to fail prematurely

 Radiation Damage

 Ultra violet (UV) radiation is unrelenting, and can never be totally eliminated. It affects human skin, the paint and polycarbonate headlights of motor vehicles and aircraft as well as the gel coats of boats, and trailers. This process is called oxidation. It also affects paint, plastic, rubber, and vinyl, just to name a few.

Paint turns dull, then chalky. Rubber seals and bumpers turn white, dry out, and become hard as a rock. Vinyl trim will also turn white and eventually crack. Hard plastic (headlights, air dams, side mirrors, trim) react similarly to paint.

Many natural and synthetic materials are attacked by ultra-violet radiation and products made using these materials may crack or disintegrate. This problem is known as ultra violet photo degradation, and is a common problem in products exposed to sunlight.

 Infrared (IR) rays create heat in much the same way as rubbing your hands together does. The energy contained in the infrared rays causes the molecules of the substance it hits to vibrate back and forth. However, the energy contained in ultraviolet rays is higher, so instead of just causing the molecules to shake, it actually can knock electrons away from the atoms, or causes molecules to split. This results in a change in the chemical structure of the molecule

IR dries out the binder system causing structural failure; it will dry the resin in paint; leading to oxidation. A paint surface will often show cracking as the resin binder dries out the paint draws up on itself forming ‘crow’s feet’. It will also dry out the oils and plasticizers in vinyl and other materials and may lead to structural damage (this is especially relevant to open top convertibles)

 Ultra violet (UV) radiation exposure leads to gloss and colour instability (photo degradation or photo oxidation and surface fading stains. But before UV light can cause harm, it must first be absorbed. If it is not turned into heat or transferred to a nearby stabilizer molecule called a quencher, it breaks weak chemical bonds. This is the beginning of UV damage.
 

Some materials absorb UV radiation more readily than other materials. Materials that readily absorb (UV) radiation are quickly damaged...rubber, vinyl, gel coat fibreglass, and many other plastics.
 

When radiation is absorbed, it starts to break (cleave) weak chemical bonds, which leads to photochemical degradation (bleaching, (fading), discoloration, chalking, brittleness and cracking) all indications of UV deterioration. The bond cleavages resulting from UV absorption cause the formation of “radicals.” Each free radical can trigger a chain of reactions (in the presence of air), leading to more bond cleavages and destruction. These oxidising chain reactions require no further UV exposure, just the presence of air

Photo degradation [: decomposition of a compound by radiant energy] a common reaction is oxidation.

Today’s water-based products have a number of obvious environmental benefits, but some are more susceptible to photo degradation (fading) over time, a significant drawback. In addition, because of ozone depletion, higher levels of solar ultra violet (UV) radiation now reach the surface of the earth. This further contributes to the rate of fading.

 Polymers use in auto paint, plastics and vinyl, where they are routinely exposed to sunlight; the UV radiation adversely the mechanical properties of these materials, often causing structural failure, which limits their useful life.

 Photo-oxidation of polymers

 Sometimes incorrectly described as oxidation; is the degradation of a polymer surface in the presence of ozone. This is a consequence of ultra violet (UV) radiation, which instigates a chemical change that reduces the polymer's molecular weight. As a consequence of this change the material becomes more brittle, with a reduction in its tensile, impact and elongation strength. Discoloration and loss of surface smoothness accompany photo-oxidation. Infra-red (heat) radiation or high surface temperature significantly increases the effect of photo-oxidation by drying.

 

Ultra violet Blockers and absorbers

There are a couple things that should be realized; auto paint or chemical companies don't state the amount of ultra violet protection is in their clear coat, nor do they tell you what the specific ultra violet stabilizing chemical is in most cases. Most automotive ultra violet (UV) absorbers are very durable and exhibit a 5 year half-life, meaning that every five years the concentration is reduced by 50% percent.

UV stabilizers are a group of chemical agents with the ability to counteract or neutralize the harmful effects of UV and infrared (IR) light. Competitive absorbers provide protection by converting UV light to heat so it can dissipate harmlessly All UV stabilizers are consumed as they do their job. In a way, they serve as sacrificial molecules, taking the abuse from the UV light instead of the material they are protecting.

Ultraviolet Light Absorbers (UVAs) are molecules used in organic materials (polymers, paints, etc.) to absorb UV light in order to reduce the UV degradation (photo-oxidation) of a material. A number of different UVAs exist with different absorption properties. UVAs can disappear over time, so monitoring of UVA levels in weathered materials is necessary.

Ingredients which absorb UVA/UVB rays, such as Avobenzone, Benzotriazole and Octyl Methoxycinnamate, are known as absorbers. UV absorbers do not add to the hardness and are heavier than clear coat resin. They are contrasted with physical "blockers" of UV radiation such as titanium dioxide and zinc oxide.

 Ultra violet light (UV) radiation and environmental contaminant exposure leads to photo degradation of materials and surface staining. But before UV light can cause harm, it must first be absorbed. If it is not turned into heat or transferred to a nearby stabilizer molecule called a quencher, When UV light is absorbed, it starts to break (cleave) weak chemical bonds, which leads to bleaching (fading), discoloration, chalking, brittleness and cracking, all indications of UV deterioration.

The bond cleavages resulting from UV absorption cause the formation of "radicals." Each free radical can trigger a chain of reactions leading to more bond cleavages and destruction. These oxidising chain reactions require no further UV exposure, just the presence of oxygen. This is the beginning of UV damage. Some materials absorb UV light more readily than other materials.

Be cognizant that infrared radiation (heat) can also affect the structural integrity of materials by drying out and therefore removing their elasticity and causing structural damage (cracking).

 UV Protection

Ultra violet stabilizer - providing protection from ultra violet radiation (UV) is very important to avoid photo synthesis (colour fading / yellowing) ultra violet heat (IR) infrared radiation can also affect the structural integrity of materials by drying out and therefore removing their elasticity and causing cracks.

Be cognizant that Carnauba wax or polymers do not contain a natural ultra violet radiation protection, they both oxidize as a way of providing a sacrificial protection against ultra violet radiation, neither of these elements contains UV stabilizers (protection) they must be added. This renewable barrier is probably less than 0.1 µ (4 Mils) thick.

There is no such thing as a permanent UV stabilizer, it a matter of physics, not chemistry. This chemical must be added to a car care product, it is not inherent in any wax, sealant or coating. Ultra violet protection is a sacrificial and therefore a renewable protection; this is due to the UV protection layer being degraded by exposure to the elements (sun, sand, road or sea salt, and etc.) so it is imperative that you renew it and it needs to be re-applied on a regular 45 to 60 day basis (dependent upon location climatic condition)

Protecting the Protection
To ensure the UV blockers can absorb and reject the radiation heat without drying itself out it is encapsulated with Silicon dioxide. Among the inorganic pigments, TiO2white is, by far, the most important one.

The photo catalytic activity of TiO2leads to a rapid degradation of the organic binder matrix, so the surface of the TiO2particles has to be covered by an inorganic coating to prevent the matrix from being photo oxidized. This is done by applying layers of Silicon dioxide, also known as silica (SiO2) and Aluminium oxide, better known as Alumina (Al2O3)

 Summary

The main premise of this article is inform detailers that there is a very limited amount of clear coat available and to keep them from unnecessarily removing their clear coat. New polymer technologies are now available to add clear coat to the existing layer without the need for an expensive re-paint and at a fraction of the cost. Depending on the resin technology, these clear coats can be more resistant to scratch and marring, more resistant to chemical etching, and more resistant to oxidation, therefore minimizing the need for polishing. Even if you have to polish it off at some point, you can always add a new layer without removing the thin layer of factory clear and avoiding the possibility of premature failure of the paint system and the need to repaint.

Bibliography

1.             UV Protection and Coatings for Plastics in the Automobile Industry - Paint & Coatings Industry (PCI)
 
2.             “Protecting UV-absorbing Clear Coats from Sunburn," Polymers Paint Colour Journal, February 2000

3.             Evolution of Automotive Paint and Protectant Technology by Gene Praschan and David Ghodussi

References

1. Distribution of Stabilizers in Multi-Layer-Coatings and Plastic Coatings - 3rd International Coatings for Plastic Symposium, Troy, MI, June 2000.
2. Potential Reasons for Yellowing of Coatings over Plastic Substrates -  4th International Coatings for Plastic Symposium, Troy, MI, June 2000

 

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