- February 01, 2003, David J. Bentley Jr., RBS Technologies
Most people have color preferences. My favorite color is black. Some individuals have very strong feelings about their favorite color. A granddaughter absolutely adores pink. A grandson totally despises pink. In the case of adhesives and coatings, the absence of color usually is preferable. People who work with adhesives and coatings generally do not want any color in their products. Color in an adhesive or coating can interfere with the use of the product in its final application.
A dark color in an adhesive or coating can detract from the esthetic qualities of a product and make it unsuitable for a particular application. Likewise, an adhesive or coating that initially is free of color in its intended use, but develops color over time, will be unsatisfactory.
The initial color of a material and its resistance to change depend on its chemical composition. Among the common ingredients for making adhesives and coatings, acrylic materials generally have the least amount of initial color and provide the most resistance to color change exhibited as yellowing or darkening. Products formulated using most rubbers, common tackifiers, phenolic resins, PUs, and similar ingredients often will have an initial yellow or brown color and can darken considerably in their area of use depending on the conditions of heat, light, etc., to which they receive exposure.
Coating weight of an adhesive or coating plays an important role in the resulting color of the material. The liquid product that is coated onto a substrate can have some degree of color as supplied in a drum. In a drum or in a glass jar, it may appear yellow or even light brown. When used at very low coating weights of perhaps 1 mil or less, the color may not be objectionable. Use of more material obviously would increase the color until some point occurred when the coating thickness made the color unacceptable. With a very clear product based on an acrylate, this problem would not happen — regardless of how thick the coating weight became.
For some applications, initial color and color development may not be important. A paper label with a PSA (pressure-sensitive adhesive) on the backside can have some color, provided the paper is sufficiently opaque to hide the color. The color of an adhesive or coating on the backside of aluminum foil or a metallized film also will not cause any problem. In similar fashion, a heat-seal adhesive formulated from an EVA resin tackified with various materials applied to the backside of a 100% printed substrate usually does not cause any color concerns.
Contrast this with a PSA on a clear film intended for use as a protective covering for a surface. In this case, the user wants to preserve all the characteristics of the surface such as color, clarity, gloss, etc. Any deterioration of color due to the use of the p-s product would be totally unacceptable, because it would make the product look bad.
Sometimes the color development in an adhesive is desirable. An example would be a label that protected a material whose exposure to heat or light would make the material unsatisfactory. Looking at the label that had yellowed or darkened with such exposure would indicate the product was no longer acceptable for its intended use.
A second example would involve a critical balance where the color change in the adhesive or coating coincides with another color to achieve the desired color. This is very difficult and often relies on conditions over which the user does not have complete control.
Converters using adhesives and coatings can control the color and color development of a product by the addition of specific additives and careful attention to the conditions of use. This will be the subject of next 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 firstname.lastname@example.org.