Designing with plastics offers many benefits, one of which is the ability to embed the color into the polymer itself, eliminating the production bottlenecks created by finishing lines. But the decisions to be made for coloring plastics are multi-faceted, involving aesthetics, color stability, processing, cost, and the effect of pigments on mechanical properties. All of these factors must be carefully weighed to make the optimal selection of color and coloring method, and with new special effect resins being introduced on a regular basis, the right choice becomes a moving target.

In choosing a color, designers are often faced with a dizzying number of pigments, dyes, and special additives, and an almost infinite number of material combinations from which to choose. These “recipes” of colors can give the colored-plastic product a unique and attractive look while offering a whole range of functionality.

In addition to the broad array of stock colors from which to choose, designers also have the option of working with a color supplier to create a custom color that no one else has. The selection of a particular color for a product can be a short-lived flirtation, used for one model year, but not the next. Or it can be an enduring commitment, where the color serves as a brand identifier, as with John Deere green or DeWalt yellow. Either way, designers must be assured of getting exactly what they want, and not just “close enough.”

The human eye can discriminate between almost 10 million colors, which means getting the color right is imperative. Getting it right means that the color on the finished part not only matches the color specified, but the color continues to match from batch-to-batch and part-to-part. Appliances assembled from multiple components made at multiple sites, and even multiple countries, must have color-matched parts. Slightly skewed color matches negatively affect the aesthetics of a product and create the impression of poor quality in the mind of the consumer.  Beyond aesthetics, the pigments often must be functional, and must be compatible with the chosen resin. Of course, the choice must be cost effective as well.

Enlarge this picture Special effects pigments differ in cost, compatibility to polymers, and durability. Here, six popular special effects categories are examined. Source: The Sabreen Group

Decisions also need to be made as to the method of how to add colors to plastics. Colors can come in many forms, as powders, pellets and even liquids.

Many raw pigment and dye materials come as a powder, and are usually converted by the color or resin supplier into pellet form for easier handling and improved color consistency. The powders can be used on their own, however, but color consistency can be an issue. Often a color specialist is needed to get the appropriate ratio of powdered pigments and dyes to plastic resin in order to achieve a good color match and ensure color uniformity from part to part. Before molding, powders must first be dispersed throughout the resin in a mixing operation. Many molding machines do not have the capability and special mixing equipment is required. On the positive side, of all of the options available, buying pigments in powder form offers low inventory costs, fast color changes, and the ability for a company to create custom colors on their own.

Most often, however, molders use materials that have been converted into pellets or in some cases into liquid colors because of their ease of use and superior color consistency. Masterbatches and precolored resins are two options for designers to consider.

Of these, masterbatches, which are also called color concentrates, are considered to be the most often used. Comparatively, they are more economical, and can be delivered in small volumes. They offer very consistent color uniformity and can be used with a wide range of natural resins. Masterbatches typically come in pellet form, but are available as liquid colors. Each pellet contains all of the pigments, dyes, and additives needed for a specific job. They are then “let down” into natural resins in precise ratios. Let down ratios are normally from 3 percent to 5. If 100 lbs of resin is needed for a run, then 3 lbs of masterbatch is added. This typically requires metering equipment and the ability to heat and mix the pellets into the resin. During the mixing process, Scott Sabreen, president of The Sabreen Group, a Plano, Texas-based secondary plastics manufacturer,  says that shear forces are created and these can have an effect on the colorants being mixed and may cause a slight color aberration. He adds that results are typically good and within tolerance.

Bayer MaterialScience released four new special effects pigments to its LEDA line of mold-in colors. LEDA is part of the company’s Fantasia line of colorful products.

Liquid colors sometimes have somewhat less opacity or brilliance, Sabreen says, but they are easy to use. They require initial mixing to get good dispersion, but will usually keep properly dispersed over time. To ensure dispersion, some molding operations agitate the liquid prior to molding, especially with special effects pigments that have greater densities.

An easier, but more expensive method, is to use precolored resins. In this case, the colorants are added to the resin by the supplier and turned into pellet form. The pellets will contain the resins, pigments, dyes, and additives all within the resin pellet. Resin pellets are more expensive, have slightly longer lead times and are not as desirable for short runs because if using different colors, the machine needs to be shut down and completely cleaned prior to introducing a new color. However, they are easier to use, deliver better color consistency, and do not require mixing or metering equipment. Whatever method of dispersion is chosen, a proper mix is essential. Colorants can be made from organic or inorganic pigments, and dyes. Typically, color formulas contain four colorants, says Sabreen. Two of which are white and black to control value (lightness and darkness) and chroma (color intensity). Two pigments and dyes establish hue. The pigments themselves can be made from different particle sizes and orientations, which can change a product’s appearance and functionality. The color mixtures can be “tuned” through the use of particle sizes to produce the needed luster and opacity.

In general, smaller particles provide opacity while larger particles provide luster, says Sabreen. Small flakes, about 5 microns in size, can give pearlescent pigments a satiny appearance with good opacity. Larger flakes of around 25 microns can give a more lustrous effect. By blending the particle sizes, the appropriate mix of luster and opacity can be achieved. To add luster to the base color, transparent dyes can be added.

Adjusting the size of particles in pigments can also affect light scattering, which in turn can affect a color’s shading, he says. Finer particles shift red pigments toward blue, while bigger, coarser particles move it toward yellower shades.

The Pensonic blender is made from several different polymers and comes in a range of colors. Clariant Masterbatches supplied the company with a series of “combibatches.”

Colorants can also be used to reduce secondary operations. Metallic pigments, for example, due to their conductive properties, can provide the additional benefit of EMI shielding, eliminating the need for applying conductive materials in a post fabrication operation.

Metallic pigments  with a larger particle size and a lower loading (concentration) can reduce the visibility of flow and weld lines compared to smaller-sized particles, and resins with a lower viscosity can cause the least particle orientation resulting in fewer flow lines, says John Skabardonis, polycarbonates marketing manager, North America, for Pittsburgh-based Bayer MaterialScience LLC.

The thermal conductivity properties of metallic pigments also allows them to help with thermal management issues, which may impact part design.

While pigments and additives can add color and functionality to a plastic part, they must be used appropriately for the material. For instance, of the newer special effects pigments to hit the market in recent years, not all of them  can be used with all plastics; some fluorescent dyes cannot be used in plastics such as polyethylene and polypropylene, but they will work in transparent plastics such as acrylics, polycarbonate, and polyester. Some grades of titanium dioxide white pigment can promote the breakdown of polymer chains in polycarbonate, reducing its impact strength.

Clariant Masterbatches chip cube is part of the company's ColorForward program that helps designers identify coming color trends.

Special effects pigments and dyes have been known to lower a plastics’ impact resistance. According to Skabardonis, special effects additives  can act like stress concentrators in plastic. Seen under a microscope, the additives appear as hard particles with sharp edges. As the plastic part is stressed, the stress goes down to these sharp edges and gets concentrated at single points where the polymer experiences a multiplied stress effect. He says that the colored plastic parts are simply not going to have the same mechanical properties as the virgin polymer material and designers need to be aware of this reduction in properties.

In these cases, Skabardonis says designers need to compensate for this durability loss. They can specify stronger base materials, increase the thickness of parts at certain areas, and add structural design elements such as ribs. He suggests that special effects be used on accent pieces, whether they are appliqués or parts surrounding the main component. If it is an existing part, he says that designers should thoroughly test the part to make sure that it holds up to the types of loads that it will experience.

For the most part, all of these problems can be overcome, he says, if the OEM and supplier work together on the project. Whether purchasing “off-the-shelf” colors or custom-blending them, this relationship is pivotal toward making products to specification. Color matching is one key to consistent product quality.

Matching colors can be tricky because of a number of factors, according to Thierry Chevrier, Director, Performance Chemicals-Coatings, Plastics and Specialties, for BASF in North America, Florham Park, N.J. Chevrier says that color matching for traditional absorption colors is different than color matching for effect pigments and achieving color matches on parts made of different materials can also be a challenge. Often, little tolerance is acceptable and color matching tolerances for typical organic and inorganic colorants can be quite tight, Chevrier says. The deviation of two colors is noted as delta-e, and many applications require a delta-e of less than 1.0. Typically, a delta E of less than 1.5 is considered a match. When effect pigments are employed, the delta E should be less than 3.0 on five different detection angles.

Clariant Masterbatches is using clarifying additives from Milliken Chemical to produce clear colors and sparkling reflectivity.

To help ensure consistency, resin and pigment suppliers have developed large databases that house tens of thousands of color combinations from which a designer can search. While these can help a designer color match correctly, until recently there wasn’t a central location for designers to go and see what and how certain resins, pigments, dyes, and additives can work together. That place is now MatchMyColor, says Gareth Morgans, business manager for matchmycolor.com. Ciba Specialty Chemicals, the Switzerland-based manufacturer of pigments and resins, developed the site.

Traditionally, Morgans says that problems with color consistency often occur during product development when inaccurate information is disseminated. While millions of colors can be discerned by the human eye, words generally cannot describe those colors. At least not well enough to use in a production setting. Matchmycolor brings a common dialogue to the product design equation, Morgans says. It features a massive database that includes detailed information on available materials and how they can be used most effectively. It links color and additive options from multiple sources including DuPont, Engelhard, and the sites’ parent company, CIBA.

Morgans says that the designer only needs to know the application parameters to use the system. By entering information such as whether the product needs to be flame retardant, UV protected, EMI shielded, or FDA approved, the number of resin and colorant choices can be whittled down to a more reasonable number.

Whether using matchmycolor.com or some other method of color matching, product designers need to think about color at the front end of the project and discuss their wants and needs with the material supplier, says Stephen Duckworth, global head of the consumer goods market segment for Switzerland-based Clariant Masterbatches. Clariant Masterbatches sells masterbatches that can be formulated for specific and demanding applications.

The Suggar washing machine features a transparent plastic lid made of polypropylene clarified with additives from Milliken Chemical.

For instance, Pensonic Holdings, a Malaysian manufacturer of consumer electronics, developed a new kitchen blender that featured ABS for the main housing, nylon for the control knob, and polypropylene for the lid. Colors needed to be produced and matched despite being made from different material types. Working on a tight deadline, Clariant needed to resolve the issues surrounding the coloring of the three polymers, and, in less than two weeks, deliver masterbatches to a molding facility in Malaysia for production trials.

In another case, a European maker of electrical components was launching a new product line in a range of colors and materials. Products incorporated multiple polymers including high-impact polystyrene, ABS, polypropylene and nylon, which were sourced and processed in different parts of the world. In addition to color matching multiple polymers, the product’s needed to meet UL and IEC specifications for flame retardance and provide protection against heat and UV light regardless of where the product was manufactured. Clariant’s solution was a series of “combibatches.” These are custom masterbatches that contain color and as many as five additives, all formulated to function when combined with different natural resins wherever they happen to be processed.

In some applications, functional properties are not as important. In others, color isn’t even that important. Clariant Masterbatches is using a clarifying agent produced by Milliken Chemical, Spartanburg, S.C., on some of its special effects products to produce what the company calls sparkling clear colors and dazzling reflectivity on polypropylene.

Clear and colored transparent plastic for household appliances have been a long-term trend, but designers sometimes had to weigh clarity against cost and durability issues. A new clarifying agent from Milliken, Spartanburg, S.C., supposedly will solve that dilemma. In at least one application, it allowed a manufacturer to replace a higher-cost plastic with lower cost polypropylene for use on its clothes washing machines, says Brian Burkhart, Global Market Manager, Polypropylene Clarifiers.

The material, Milliken’s Millad® NX8000, is added to a polypropylene resin stream prior to molding, to make the plastic transparent. In addition, the polypropylene can be custom tinted for a more upscale, sophisticated look, says Lee Rieth, Global Market Manager, PE Nucleation & Light Absorbing Additives. He adds that custom tinting with Milliken’s ClearTint® colorants can give customers the look that they require, while keeping the polypropylene optically clear. Clarity is just one of its benefits, Burkhart says. The additive also acts as a nucleating agent that very quickly crystallizes the polymer chains for faster processing and better dimensional stability, says Burkhart.

Choosing colors and effects from the multitude of options can be daunting for designers, but web site’s such as Ciba’s matchmycolor.com and Xymara.com can help

Transparency and impact resistance were two properties that were needed by Suggar, a Brazilian appliance maker that produces 2,500 clothes washing machines per day. The company had created a machine with a transparent lid in three of its washer models using styrene acrylonitrile (SAN). The see-in washing machines became almost immediately popular and sales increased, but durability issues began to creep up and the company began to look for alternatives. While the clarity and cost of SAN were good, its impact resistance was not quite good enough. Suggar then looked at polycarbonate (PC), which is a naturally transparent material that offers high strength, toughness, heat resistance, and dimensional and color stability, but its price point was too high for the application.

Polypropylene (PP) was then considered. PP is a very versatile material, durable and inexpensive, but there were some concerns about its clarity, says Burkhart. With Milliken’s additive, those concerns were put to rest. With the additive, Suggar was able to replace the SAN material with the less expensive and more durable clarified PP, reducing haze by 50 percent compared to standard clarified polypropylene, thus preserving the see-through clarity desired by consumers. Impact resistance was improved through the use of PP. In Izod pendulum tests, impact resistance was improved by more than five times as compared to SAN.

In this case, by choosing a material and an additive, the O EM was able to manufacture an appliance that was more durable and less costly to produce. As color palettes continue to grow, and more and more functionality is required from the plastics and the materials that color them, matching the right color to the right material for the right application will become ever more paramount.

For more information, email:
Bayer MaterialScience: john.skabardonis@bayerbms.com
CIBA: gareth.morgans@ciba.com
Clariant: steve.duckworthhu@clariant.com
Milliken: brian.burkhart@milliken.com
Sabic:  yvette.bogaert@sabic-ip.com