Introduction to Yellowing and Color Change of Oligomers
Yellowing or high color is very undesirable in several oligomer and coating applications. The yellowing of a material causes the distortion of the intended color on a part or coating, for example, the color of a furniture or wood coating yellowing over time, causing an unappealing aesthetic to wood color. Oligomers and formulated materials have varying ranges of color or yellowness that can depend on long-term/short-term aging, cure method, or materials used. The increase of color or yellowing of a material occurs by several mechanisms: inhibitor choice, polyol degradation, aromatic groups in the urethane, heat exposure, and photoinitiator.
Color change is an “irreversible” sign of aging. However, there is a phenomenon called photobleaching that occurs in select materials over time, where color is developed by curing and the color reduces over time. Yellowing can occur at varying stages of a material being formulated or cured. Certain ingredients to an oligomer or formulated material may cause yellowing or color. There are three basic types of yellowing; yellowing caused by curing the product, yellowing caused be the exposure to sunlight, and yellowing caused by heat. It is essential to take a color sample prior to curing the product or exposing it to any aging conditions, such as heat or light exposure, to determine the color behavior of the material.
|Inhibitor||As inhibitors oxidize, they can contribute to color|
|Polyol Degradation||Chain scission|
|Aromatics in Urethane||Chain scission, usually with light|
|Photoinitiator||Color issues from longer wavelength PI, effects caused by concentration of PI (ex. TPO creates color then photobleaches)|
|Oxidation||If amines and other nitrogen containing ingredients are added to the product, yellowing can be an issue, as oxides of nitrogen can be formed. These oxides are yellow in color.|
Measurement of Yellowness
There are several ways to test color, but color change or yellowing of liquid oligomers is most commonly measured on the Hazen or Gardner scales. For cured coatings or films generally a photo spectrophotometer is used.
The Hazen scale, also known as the Platinum Cobalt (Pt-Co) scale was initially created by the American Public Health Association to determine the quality of water and measures the yellowness index, or APHA\Pt-Co color of a liquid as it compares to a standard. This scale typically measures very low levels of color, or yellowness, in liquids and ranges from 0-500 units described by the ppm concentration of platinum cobalt to water (a zero being distilled water and 500 being light yellow).
A much coarser scale, known as the Gardner scale, measures higher concentrations of color in drying oils, varnishes, fatty acids, polymers, and resins. The Gardner scale ranges from 1-18, 1 being the lightest (light yellow), 18 being the darkest (deep red/brown).
For color of a cured film or coating it is more typical to measure it with a color spectrophotometer. This can read the visible light reflected or transmitted through the product. Most commonly the color is reported as a change in overall color (dE), change in the b axis (blue-yellow) of CIELab calculation (db) or change in yellowness (dYe).
There are several oligomer and coatings applications where low color or non-yellowing is essential. Nail coatings require low yellowing characteristics as to not distort the nail color. The yellowing of nail coatings generally happens during the curing process, but in many cases will be reduced or disappear after a short time. If the yellowing is too severe this will be an issue even if it disappears in a few hours. It may be possible to compensate for the yellowness with blue dye.
Yellowing Over Time
Other applications that are concerned about color are exterior coatings; automotive coatings, wood coatings, inks for exterior applications, and applications that will see sunlight through windows. In this case sunlight and heat can both contribute to the color change. It is important to use raw materials that will withstand the exposures to both. The protection can be enhanced by using UV absorbers and hindered amines. These are not a substitute for stable raw materials but can enhance and prolong resistance to color change.
Another part of this degradation by sunlight and heat, is that gloss can also be lost in the process. This is a concern especially in exterior coatings with a high gloss. The loss of gloss indicates a breakdown of the surface film allowing a route for the elements to enter and further deteriorate the coating surface.