Controlling Color Change in Adhesives and Coatings

The column in this space last month discussed color in adhesives and coatings with emphasis on avoiding color during the manufacture of the products. Now we will consider how converters and users can control the color and color development of any adhesive or coating product by addition of specific additives and careful attention to the conditions of use.

The most common conditions causing color change are exposure to heat, light, and the environment. Avoiding or minimizing such exposure is the best way to ensure an adhesive or coating will maintain the same initial color over an extended time. High temperatures, UV light, and materials such as oxygen and ozone in the atmosphere will cause many adhesives and coatings to discolor. First they will turn yellow followed by tan. Under severe exposure conditions, they even can change to a brown or black color. Acrylic materials generally are resistant to color change.

What are some common ways adhesives and coatings find exposure to these unfavorable conditions? The first is the drying oven that many products pass through to remove solvents or water during the coating step. Excessive temperature in the oven or a long period of time in the oven can cause an adhesive or coating to discolor.

The color change might be small, depending on the specific temperature and time, but the material nevertheless will start to discolor. Once that happens, any exposure to heat or another cause of discoloration will result in the product reaching a darker color considerably faster. In use, adhesives and coatings placed in hot environments that include being near an oven, in direct sunlight, etc., often will discolor.

Because UV light is such a common cause of discoloration, adhesives and coatings should not find use in environments where they will receive such exposure. An excellent example of such use is a decal or similar product applied to a glass window of a store or automobile. Adhesives and coatings that find their way into outdoor uses run the risk of discoloration from oxygen and ozone. Before using any material outdoors or in direct sunlight, make sure the product undergoes testing to determine the effect of UV light, oxygen, and ozone on it. Without such testing, a user may experience a surprise some time later when the materials darken considerably.

Fortunately, compounds do exist that can negate or retard the changes in color described above. These include light stabilizers, antioxidants, and similar materials. Generally, they work by reacting with the color-changing cause such as UV light to prevent it from reaching the adhesive or coating. They are very effective and often are useful in small quantities. They are costly, but they can prevent color change.

When using stabilizers or similar materials in a formulation, the user first must make certain they dissolve in the solvent or water present or are incorporated uniformly in a 100% solids product. They must dissolve or have proper dispersion in the adhesive or coating to do their job.

Some experimental work also is necessary to determine the proper amount to add. Because they are expensive, users should not add any more than the amount necessary to protect the product from the conditions of use it will experience. Any amount above this simply is wasteful and uneconomical. Testing at various levels under the actual use conditions will allow one to determine the proper levels of additives.

The important point to remember from this column and the previous one is that converters and end-users should be aware of the importance of color in their application. They need to know if initial color is important and if color stability is important. Often, it may not be critical. In those applications where it does have significance, use the hints given here and in last month's column.


David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at mailto: dbentley@unm.edu.

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