Colorants have become an increasingly popular plastics additive over the years. Color improves product appearance and can help distinguish your brand in a competitive marketplace. Color-coded parts facilitate selection, assembly and operations. Simply put, color adds value.
There are two types of colorants: dyes and pigments. Pigments do not dissolve in plastics and must be thoroughly dispersed to achieve a uniform color. Dyes are soluble in plastics, which is an important consideration when coloring transparent plastics. They are usually more expensive than pigments and tend to be less thermally stable.
Industry regulations can affect the choice and cost of colorants. One new set of rules that’s having a significant impact is the Restriction on Hazardous Substances (RoHS) standard, which took effect in the European Union last July. Added into the mix are existing testing standards and regulations for food contact from the U.S. Food and Drug Administration. Expect to pay more for newer colorants free of heavy metals, especially those formulated for high processing temperatures.
Precolored compounds and color concentrates are two primary methods used to add colorants to plastics. Consider the reason the colorant is being used to decide which is right for you. “When I start working with a customer, one of the first things I ask is if the color will be used as an identifier or for aesthetics,“ comments Dave Dickerson, color specialist for Solvay Advanced Polymers in Alpharetta, GA. “For aesthetics you need lot-to-lot color matching, and that’s going to cost more.”
Color concentrates are pelletized plastic compounds that have a high percentage of colorant. The concentrate is blended in appropriate amounts with a neutral (uncolored) base resin in order to achieve the desired color. Concentrates can significantly lower materials inventory costs when a variety of colors are used with the same base resin. Typically, it is more difficult to achieve a precise color match with a concentrate. If color is a critical requirement, consider specifying a precolored compound from your material supplier.
Precolored compounds are made by adding the colorant to the resin at the same time other additives, such as stabilizers, fillers and impact modifiers, are added. They simplify molding (no mixing or blending) and provide very consistent color. On the down side, resin for each color must be purchased and stored. A premium is applied to colors other than black and white.
Look Beyond Data Sheets
Be aware that colorants can adversely affect properties of the plastic. The more you add, the bigger the effect (see graph). Consider testing molded plaques of materials as compounded because data sheets may contain none of the final performance information or may not accurately reflect the specific mix of materials you plan to use.
“We publish data sheets that are based on natural (uncolored) or black compounds,” comments Greg Warkoski, process technology manager for Solvay Advanced Polymers. “If you are banking on a certain number for impact, modulus, or tensile strength, and then you decide you need a perfect color match, you need to talk to your materials supplier to understand how the colorants you’re using might affect those properties.”
Color is measured with a spectrophotometer over a range of wavelengths by determining transmitted light for transparent plastics and reflected light for opaque. Colorists use the curve generated as a reference point when matching a color. Specifications are typically listed as a delta E value (delta E of 0.5 for example), which is the difference between the sample color and the reference color.
Keep in mind that actual color can differ from perceived color. “Color is relative to surface appearance, which is why the color of a textured surface can look very different from the color of a smooth surface even though both were made from the same precolored resin,” notes Ted Moore, a research engineer at Solvay Advanced Polymers. Another factor to consider is processing conditions. It’s important to stay within the parameters recommended by the resin supplier to avoid color shifts caused by thermal degradation.