Color is so much more than meets the eye. It has emotional elements that impact product positioning and consumer preferences. It is also a chemical variable that affects not only appearance but also the functionality, processing, and cost of plastic products. Processors need a solid understanding of how color is created in plastic and an appreciation for how valuable the assistance of a knowledgeable color supplier can be when color and plastics collide.
Basic color chemistry is well established. Colorists have a choice of using dyes, which are soluble and disperse in the polymer on a molecular level (like sugar in hot water), or pigments, which are insoluble solids that must be dispersed in the polymer matrix. Dyes make it easier to achieve bright, clear, transparent colors, and are the best solution for coloring transparent resins. Pigments, being solids, are better for deep, saturated, opaque or transparent colors. In recent years, there has been a dramatic move toward use of organic dyes or pigments and away from inorganic colorants that rely on regulated metals like cadmium or lead chromates.
Inorganic pigments are finely ground complexes of metals. They have outstanding lightfastness, but have poor color strength, high density, and lower intensity and chromaticity than organics. Common inorganic pigments include oxides of metals like iron, titanium, or cobalt; inorganic salt complexes, such as ultramarine blue; and sulfides or sulfates of zinc, barium, and cerium.
Organic pigments, on the other hand, are finely ground particles of reactor-synthesized organic molecules. Depending on their base chemistry and functional groups, they can be widely variable in their light and temperature stability. The different classes of organic pigments are too numerous to mention here, but they are available in a wide variety of colors. Many offer high levels of brightness and tint strength. Particle sizes vary, with smaller particles being more transparent and having higher tint strength, and larger ones being more opaque with lower tint strength.
RESIN TYPE MATTERS
What kind of color you use in a given application will be determined by the basic structure of the polymer. Even similar polymers, produced by different companies or in different regions of the world, can vary in base color so that they respond to colorants differently. A particular color that works well in a polycarbonate produced in North America, for instance, may yield a totally different result if it is used in material sourced in Europe or Asia.
The molecular structure of the polymer determines its processing characteristics, which in turn will influence color choices. A crystalline resin like PP has a white semi-opaque appearance due to the mixture of ordered and disordered domains, which bend light differently. Amorphous (non-crystalline) resins are almost all disordered so that light passes straight through. This is why they are transparent. Naturally, extremely transparent colors are only possible in amorphous resins.
Crystalline resins have a very sharp melting point and require significant energy (heat) input before they undergo a phase change and melt to a low viscosity. For this reason, colors used in crystalline polymers may need to be more heat stable than those used in other materials. This usually suggests the use of inorganic pigments that are more stable at high-temperatures. It also explains why colors may change when regrind is blended with virgin resin. The color in the regrind, which has a longer heat history, degrades.
Amorphous resins, on the other hand, have a glass-transition temperature above which they start to soften until their melt viscosity is low enough to flow. Amorphous resins have significantly more free volume than crystalline resins, so they are more receptive to dyes, which are able to stay in solution without surface blooming or tool plate-out.
The selection and ultimate performance of colorants also depends on whether a plastic resin is a homopolymer (involving a single polymeric chemistry) or a copolymer, which involve two types of chemistry that respond differently. A homopolymer may be either crystalline or amorphous, but appropriate colorants will still be homogeneously distributed though out the resin.
Copolymers, on the other hand can be either random (having uniform phases) or block copolymers, which segregate into aggregates of the two chemistries. Impact-modified materials like ABS, HIPS and acrylic, for instance, contain cross-linked rubber particles. Colorants cannot get into the rubber phase and the rubber changes how light moves through the toughened resin, making it appear very white. Rubber alloys like super-tough nylon involve very different chemistries between the nylon and rubber, so they can be very difficult to color, with limited options.
The solubility of a resin (the way in which hydrogen atoms are bonded and the polarity of the material) will affect how well dyes will stay in the polymer. Dyes that are soluble in PS or ABS will slowly migrate to the surface of nylon. There are also pigments that will become soluble in the wrong resin. Consequently, if a dye is to be used to color plastic, it is very important that it be compatible with the resin.
Still another factor to consider is the refractive index of the resin, which defines the angle that light is bent as it passes through a polymer. Aliphatic resins (PE, PP, and EVA) have low refractive indices, while aromatic resins (PS, PC, PSU, PET) have high refractive indices. When resins of different refractive indices are mixed together, to improve impact strength or chemical resistance, for instance, light does not have a single path to follow, so it appears to scatter, making the material look opaque white. Again it is important to understand factors like these that limit the colorability of a resin.
The reactivity of a pigment can dramatically degrade the stability of the polymer. TiO2 will reduce the thermal stability of polyesters and polycarbonates, while iron salts will do the same for PVC. Improper TiO2 selection reduces the UV stability of polyethylene. Likewise, the reactivity of a polymer end group can alter the chemistry of certain colorants and lead to a color change. Amides, for instance, can affect chlorine- or bromine-containing pigments, while anhydrides can adversely affect metal salts of vat dyes.
In all cases, the functionality of the resin should be considered first and then color can be worked around the application.
IMPACT ON PROPERTIES, PROCESSING
Colorants can affect physical properties of plastic components. Color particles can function as a void in the polymer matrix, creating a tear point or causing poor interfacial adhesion. In fact, inorganic colorants can actually notch fiberglass reinforcements. All of these conditions can limit tensile, elongation, and impact properties in many applications. The best pigments for outdoor weatherability, for instance, actually are likely to reduce these properties the most. Proper color choice and formulation, however, can limit this reduction to no more than a 10%.
Living-hinge properties, which require a combination of toughness and orientation, are seriously affected by colorants. Hard voids can reduce the flex life of a hinge. If the masterbatch supplier knows upfront that toughness is going to be an issue, carrier resins can be formulated with additional tougheners to compensate for the loss. In many cases, it may even be possible to improve impact resistance.
Certain combinations of pigments, dyes, and resins can result in a phenomenon called phototendering, in which products lose strength and flexibility from exposure to sunlight. Some of the more challenging combinations included uncoated TiO2 or iron salts in polyolefins and standard coated grades of TiO2 in polycarbonate.
The thermal stability of “touchy” resins can also be compromised by trace metals often found in metallized dyes, lake pigments, and non-synthetic inorganic pigments. Of particular concern are ultramarine blue in acetal, manganese violet in ABS, and iron oxide in PVC. In order to achieve the most robust formulations, the best approach may be to formulate for light and thermal stability first and then, if necessary, compromise on color.
Rheological effects of colorants are often investigated only after there is a problem, but at that point formulation and tooling changes can be very expensive. Therefore it is important to consider the effect certain color-related materials can have on the melt viscosity of a resin. For instance, the high structure of carbon black and calcium carbonate will cause them to act like fillers and increase melt viscosity. Solvent dyes can reduce viscosity, as can some carriers used in liquid colors. And any colors or additives that cause degradation reactions in the polymer, as discussed above in the context of reactivity, can also reduce viscosity.
Processing temperature is another consideration. When running a material that requires high temperatures or high shear, it is best to avoid colorants that lack stability at elevated temperatures—inexpensive salt-based pigments, for instance. The color may degrade, sometimes causing light or dark streaks in the part, or an adverse reaction with the polymer may reduce important mechanical properties in the end product.
A related issue is dwell time. Even if the processing temperatures do not seem too high for the colorant, extended exposure—due to oversized barrels with too much capacity, for instance—may have the same degrading effect. If scrap regrind is returned to the process, the additional heat history may cause the color to degrade. As a general rule, the less expensive organic pigments tend to be the least stable, so a lowest cost formulation may not be the most robust, and any raw-material savings will be wasted in lower yields.
Many types of pigments can affect rates of shrinkage and warpage. General-purpose phthalocyanine green or blue, for instance, has a crystalline structure that can increase nucleation in semi-crystalline resins like PP and cause unexpected shrinkage. Computerized flow-simulation programs can predict the effect so that formulations can be adjusted before you notice that the green parts and the white parts are not the same size.
Pigments can affect the way that a resin will reheat, hold heat, or transfer heat. For instance, carbon black absorbs heat quickly and transfers it well, while ceramic (mixed-metal oxide) pigments tend to hold heat for a long time and special grades reflect heat. Aluminum pigments transfer heat extremely well. All of these effects can impact cycle time, part dimensions, and secondary processes like welding. If a job is trialed and quoted based on cycle times achieved with natural (uncolored) resin, it may become unprofitable due to changes in thermal conductivity caused by colorants.
THINK ABOUT COLOR EARLY
Costs balloon when color decisions are made late in the product development process. Maximum value for plastics products cannot be realized unless color and additive decisions are integrated into the design process early, before tooling is cut and certainly before a molding job is quoted. Attempting to cut costs by simply ordering a less expensive colorant is asking for trouble. On the other hand, a brief conversation with a knowledgeable colorant supplier can go a long way toward avoiding problems. The supplier will be able lead you to formulations that are both robust and cost-effective.
To help bring color into the product development process as early as possible, processors and their customers would be well advised to take advantage of color consultation services offered by larger color suppliers. Laboratory analysis, flow simulation, and performance testing can identify failure risks so they can be avoided before costly commitments of resources are made.
About the Author
Justin Christie is Colorworks laboratory manager at Clariant Masterbatches, Holden, Mass. He was previously technical manager for Clariant at Milford, Del., in custom compounds and fiber concentrates. He has held QC, manufacturing, and research positions at several plastics and color compounding operations. (800) 782-7333 • clariant.masterbatches.com