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Surface Finishing Tutorial

#269 Ultraviolet-Resistant Paints

Category : Special paints
December 9, 2016

Ultraviolet rays degrade coatings of painted products and fade their colors.
The phenomena of coating degradation include the followings: [1] Loss of gloss or image definition; [2] Cracking, checking, and loss of perfection; [3] Chalking; [4] Flaking and loss of adhesion; [5] Yellow discoloration; [6] Fade-out of pigment colors; [7] Biological degradation. All of these degradation events are closely related to ultraviolet rays.
It has been discovered that ultraviolet rays contribute significantly to the degradation of the coating ingredients, such as acrylic polymer, polystyrene copolymer, aromatic polyester, and aromatic polyurethane.

Traditional methods of protecting polymers from ultraviolet rays are the use of UV-absorbing pigments or opaque pigments. Pigments like carbon black or titanium dioxide absorb ultraviolet rays and prevent degradation of coating films. Such pigments that act as the UV stability enhancers may be referred to as UV shielding agents.

However, using such pigments may result in undesirable colors, loss of transparency, poor ability to protect the surface and various disadvantages in product design. As an extreme example, color selection is no longer available if you use carbon black.

Because clear coating is becoming more and more important in automobile painting, using opaque pigments that act as UV shielding agents is not an option. The shielding agents only protect the polymers directly underneath. Thus, they cannot protect high-gloss coatings.
Since UV-shielding pigments do not solve all the problems, more UV shielding agents have been developed to this day, such as molecular UV absorbent, light-stable antioxidant, hindered phenol, and hindered amine light stabilizer, for example.

Just like the moisturizer with sunscreen popular among women nowadays, the ultraviolet absorber has improved the performance of protection clear coating. This effect is achieved by combining light stabilizer and ultraviolet absorber.
In this double-block protection, only about one percent of ultraviolet absorber converts unfavorable solar radiation of short wavelength into thermal energy. Hindered amine light stabilizer will act as free radical scavengers to prevent coatings from being degraded. This mechanism has made the base coat plus clear coat application possible in automobile painting systems, which resolved most of the troubles caused by ultraviolet rays.

Chinese white (zinc oxide) absorbs and dissipates ultraviolet rays. Optical transmittance of Chinese white changes drastically at the wavelength range of 380 to 400 nanometers. It transmits visible lights but does not transmit ultraviolet rays. Zinc oxide also absorbs infrared radiation at 1,000 nanometers or less and starts transmitting it at the higher range.
The reflectance is lower for ultraviolet rays, but it increases in the blue range. It becomes the highest at 480 nanometers and slightly decreases in the infrared territory.

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