How Coloring Plastics Affects Secondary Processes
by Scott R. Sabreen
The Sabreen Group
Figure 1. Solubility of Pigment vs. Dye
Coloring plastics is integral to most products both aesthetically and functionally. Improperly formulated color compounds can cause harm or damage to secondary processes including bonding and printing. Many colorants can be used only in a limited selection of polymers. For example, dyes have low solubility in polyethylene (PE) and polypropylene and migrate out. Polyamides (nylon) react with most organic pigments, and many pigments and dyes have insufficient heat stability to withstand the high processing temperature of polycarbonate.
So, how is secondary processing affected? For example, products of the same nominal color will appear radically different if all appearance components are not taken into consideration. Consider a molded nylon tool housing that is decorated using pad printing. The colorants typically used in pad printing inks are organic pigments, whereas nylon colors are usually formulated with inorganic pigments and dyes. If the decoration is intended to match the nylon’s base color, "metamerism" may occur. The colors will appear to match under the stores fluorescent lighting, but will not when viewed at home under an incandescent bulb.
Plastics can be colored in many different ways. Coloring of plastics uses either pigments or dyes. Both methods are substantially different and produce specific results. Pigments are organic or inorganic solid particles that are insoluble in polymers. Conversely, dyes are soluble in the media in which they are incorporated. Pigments are always incorporated by simple physical mixing with the medium. Organic pigments provide strong translucent or transparent color, and have smaller average particle size and lower thermal stability than inorganics. Organic pigment compounds are based on carbon chains and carbon rings and include quinacridones (red) and phthalocyanines. Interestingly, carbon black is often classified with inorganic pigments, but it is actually organic in nature. Inorganic pigments include titanium dioxide (white), carbon black and metal oxides. Inorganic pigments provide opaque color and possess high thermal stability. However, they do not typically have as bright a color as organic pigments. The average particle size of inorganic pigments is much larger than organic pigments. The optimum particle size needed to achieve maximum light scattering, resulting in opacity, is between 400 and 800nm (wavelength). The particle sizes of inorganic pigments are much closer to this optimum than those of organic pigments, which tend to be lower. This is the main reason why most organic pigments are considered transparent and most inorganic pigments opaque. With their larger surface area, organic pigments provide much higher color strength. However, for similar reasons, their dispersibility is usually poorer.
Dyes are organic liquids that are soluble (dissolve) in the plastic in which they are dispersed, losing their crystal or particulate structure. (Reference Figure 1 - Solubility of Pigment vs. Dye.) Dyes are generally used when bright, clear, transparent colors are needed because they have little effect on light transmission through the colored plastic. There are no visible particles and the transparency of the medium is unchanged. The selection of color, pigments or dyes used in a given application will be determined by the basic structure of the polymer. Resin type is critical. Even similar polymers produced by different companies, or even in different geographic locations, can vary in base color in which they respond to colorants differently.
Subtractive color mixing is used for formulating plastic colorants, paints and printing. Desktop digital inkjet printers also make use of the same Yellow, Cyan and Magenta color set. When mixed together, Black is produced. Each color in subtractive color differentially absorbs some wavelengths of light and allows others to scatter and reflect back to the observer. The color that you see is the light that is not absorbed and is scattered. In contrast, Additive color mixing is used in lighting, computer monitors and television. The three primaries are Red, Blue and Green. When combined, they produce White light.
Pigments and dyes are just part of the total color formulation. Almost without exception, color formulations will include processing additives to aid pigment dispersion. The most cost effective dispersion aids are metal soaps, such as zinc and calcium stearate. Ethylene bisstearamide (EBS) wax also is commonly used. Such additives can cause adhesion problems with secondary processing. This problem is more likely to occur with PE and PP. These are very non-polar plastics, and during melt processing (e.g., injection molding) the more polar dispersion aids tend to migrate to the surface. When this happens, even pretreatments such as corona discharge fail to assure good adhesion of paint and printing. Fortunately, there are many less polar, higher molecular weight dispersants that can be substituted for post bonding or decorating processes. Oxidized PE wax is an example of a higher molecular weight dispersant that can be used in a wide range of polymers. Other additive packages that can lead to migration and cause problems with secondary processing are plasticizers, flame retardants, internal mold release agents and slip agents. These hydrocarbon based materials cannot be removed by conventional oxidative pretreatments.
Color is an integral part of the plastic material. During the design phase, consider the following questions:
- Which polymer(s) is being used?
- How are the polymers being processed? (Note: injection molding subjects the polymers to much higher shear rates than extrusion processing and requires more heat stable colorants.)
- At what temperature is the plastic being processed and what is the residence time at this temperature?
- What functional additives are required (stabilizers, plasticizers, flame retardants, slip agents, etc.)?
- What chemicals or solvents will the part encounter in its use, and what are the chemical/solvent resistance requirements?
"Color" needs to be dealt using a total systems approach. If you do not view color as part of the total material system, you risk problems at the secondary processing stage or, worse yet, in the field.