Yellowing in the finishing.

Yellowing is a common phenomenon in the wood finishing industry. Yellowing is caused by color changes of the clear coating on the finishing film layer. The color of the finishing film layer (coating)  will be changing after passing a certain time period. The yellowing  (color turn to yellow) is the most often color changing happened in the NC finishing and some other material. Some type of finishing materials such as: nitrocellulose, varnish and polyurethane will turn to yellow after certain time. The yellowing will become stronger when the coating is thicker.
The yellowing can interfere the appearance and color of the finishing for white and light color finish. In the finishing with dark colors, the yellowing may not be so disrupt the final color, since it is invincible by the dark color in the finishing. But at the finishing with light color or white color, then a slightly color changes at the coating film will obviously visible and disrupt to the finishing color.

There are some tips to anticipate the yellowing.

• Use the proper clear coating.

Some types of clear coating will turn yellow after a period of time. The yellowing is the natural characters of some clear coating such as: nitrocellulose, oil varnish and regular polyurethane clear coating. While the film layers produced by the acid curing clear coating will tend to be slightly yellow red or orange that can be mean as yellowing so it is not recommended for white and light color finish. To make the finishing which does not allow the yellowing, those clear coatings should not be used. For the white or light color we should use the non-yellowing clear coating type. For the lacquer finish, the butyrate or acrylic clear coating is highly recommended. The acrylic or butyrate  has better resistant to the UV light and avoid the yellowing problem. The clear film layer wont be yellow, or take very long time to yellowing. These materials are the standard clear coating used for the white and light color in the lacquer finish. Meanwhile for the polyurethane finish, the non-yellowing polyurethane type clear coating has also been widely available. It is specially formulated polyurethane clear coating that has high resistant to the UV light.

The clear coating in the other types such as: acrylic water based coating and UV coating, so they can be used well in white color finish  The water based and UV coating film layer tends to be slightly whitish and it will not interfere the performance finishing in the white color. The character of each clear coating type has to be considered when we develop finishing process to make color. Some samples of the finish with the variety systems can be made to be compared with each other when we develop new finishing system.

• Be careful in the uses of glaze

Glaze is a material that uses oil solvent. The oil is that is trapped in the clear coating will make a little yellow color in the finishing result. The rest the oil in the finish layer will make yellowish in color when the finishing is dry. This case could interfere to the finishing color produced. Therefore, the application of glaze in the white color need to be done carefully, make sure that the glaze is completely dry when it coated. The oil solvent in the glaze can give slight yellow color result and mess up the finishing color.

The yellowing test on the finishing layer.
 
A yellowing test for the finishing layer some time is necessary to ensure that a certain finishing process is quite safe for a particular color. The yellowing test is a standard test in the furniture finishing industry. The furniture manufacturers who make the furniture products in a big volume need a certainty result of their finishing processes. The yellowing test is the standard test for the white colors for indoor or outdoor use, to see the deviation color after certain time.
There is some test method that can be done, one of which is a test method that refers to the ASTM standard: ANDG ASTM-WDL-W030
This test is done on a finished color panel. A finished color panel is partially wrapped with an aluminum foil and then the panel is irradiated with the ultra-violet light with certain intensity in a specific period time. Then the aluminum foil is wrapped off, and the color is viewed to find the deviation occurs. The color deviation is measured and used as the yellowing number of the finishing on the color panel.