A better understanding of how pearlescent pigments work in plastics can help compounders and processors get the effects they are looking for and sidestep common pitfalls, such as pigment separation and flow and weld lines.
Today's pearl pigments have the thermal and chemical stability to survive polymer processing and resist weathering. They do not bleed or bloom. But they are somewhat more complex to use than other colorants. Compounders, molders, and extruders who lack a clear grasp of how these pigments function can run into problems in formulating, mixing, and subsequent processing. This article offers tips that can help you avoid difficulties with pearlescents and thus get the effects they are designed for.
Given the many pearl pigment grades now available, and the many transparent absorption colorants that can be deposited on or blended with them, users can choose from an almost unlimited array of color combinations and special effects. These range from iridescence, luminescence, and luster to the ability to change color as viewing and illumination angles change.
The most widely used pearl pigments consist of mica platelets coated with titanium dioxide or iron oxide that give white and colored effects. The crystalline pearlescent layer is formed by calcining mica coated with a metal oxide at about 1350 F. The heat creates an inert pigment that is insoluble in resins, has a stable color, and withstands the thermal stress of plastics processing.
Color in these pigments develops through interference between light rays reflecting at specular angles from the top and bottom surfaces of the metal-oxide layer. Depending on the coating thickness, these colors can be gold, red, blue, or green. The pigments lose color intensity as viewing angle shifts to non-specular angles. In addition, a second color is transmitted through the pigment platelet that is complementary to the reflected one.
The relative contributions of the transmitted and reflected colors can depend on how the plastic is viewed. When a transparent plastic containing pearl pigment is placed against a white surface, the weaker transmission color is reflected back through the film and supplements the stronger reflection color. Against a dark background, the transmission color is absorbed and only the pigment's reflection color is seen.
Combining absorption and interference colors can create a variety of single- or dual-color effects. The presence of an absorption pigment--either coated onto the interference pigment or used in conjunction with it--produces intense specular reflection colors that give way to the color of the absorption pigment at non-specular angles. There is also a transmission color that combines the effects of the two pigments.
Light reflected from pearl platelets as they lie essentially parallel to each other at different levels in the plastic creates a sense of depth and luster. The best luster, brightness, and color intensity occurs with platelets that are 10 to 40 microns long. Smaller platelets impart a smooth, silky luster, and larger ones confer sparkle and glitter.
Bismuth oxychloride crystals provide a white pearlescence that ranges from smooth, subdued, "frosted" effects to a brighter, livelier appearance. These pigments' pearlescent effect derives from the crystals themselves, not a coating.
Mica-based pearlescents can be used in nearly all thermoplastics and most processes. Their effects are most intense in transparent resins like PS, PP, PE, PVC, acrylic, styrene block copolymers, and silicone.
It is also possible to attain pearlescence and luster in polymers having little or no transparency--such as nylon 6, ABS, and HIPS--but you may need higher pigment loadings. Pearlescents combined with dark absorption pigments in opaque polymers can yield a strong reflection color and produce pearlescent effects of great richness and depth.
Modest amounts of transparent absorption pigments--for example, 0.01 to 0.50% of phthalocyanine blue or green or quinacridone red--can give interesting effects when used with pearlescents. But opaque, inorganic, high-coverage pigments should be limited to very small amounts in order to retain any pearlescence. For products such as white HDPE shampoo bottles, some compounders add up to 5% TiO2 to pearlescent color concentrates in order to enhance opacity.
Highly filled plastics are not good candidates for pearlescents because opaque fillers scatter light, eliminating the pearlescent effect. Most users limit fillers to less than 1% in systems containing pearl pigments.
Pearlescents can also be incorporated in many thermosets, including unsaturated polyester, acrylic, urethane, and epoxy. Bismuth oxychloride pearl pigments are often used in cast polyester buttons for brilliance and luster. Mica-based pearls are added to cast polyester cultured marble and onyx components for countertops, floors, furniture, and fireplace elements. Pearls can also be added to cast acrylic sheet and alloyed or blended materials such as rubber-toughened polycarbonate, nylon/ABS, and PC/ABS.
Pearlescent pigment powders can be predispersed in most resins by drum tumbling or mixing in twin-shell, ribbon, or high-speed blenders. They are usually incorporated in powdered polymers such as PVC and PE by simple blending. Injection molders who dryblend often follow a three-step process. First, they blend powdered resin and mineral oil for 10 minutes. Then, all colorants--except pearl pigments--and a dispersing aid are added and blended for 10 more minutes. Pearlescents are then added and blended for another 20 minutes.
When pearl pigments are used with pelletized polymers, the disparity in size between the pellets and the powdered pigment may cause separation after blending. Compounders often use shorter blending times with pellets than with powders because the pellets can fracture the pearlescent particles. They also find that blending works best if pellets are made slightly tacky by preblending with mineral oil before adding pigment. Many compounders avoid preblending and potential separation problems altogether by feeding pearl pigments into the molten polymer through a downstream feeder.
Dispersion aids help reduce viscosity to improve mixing. These aids include low-molecular-weight polyethylene waxes and calcium or magnesium stearate. Molders often add 1% LMW-PE wax to polyolefins and mix for 20-30 min. With PS they typically use only about 0.1% of a dispersion aid.
Pearlescent color concentrates commonly contain 25% pearl pigment, though amounts can be as high as 50%. The different pigments are added in a specific order to the mix in order to optimize color development. Addition usually begins with the harder-to-disperse organic pigments, followed by inorganic pigments, and lastly the pearlescents.
Pearlescents need gentle handling during mixing. They should not be ground or subjected to extended cycles or heavy shear because these can strip off the metal oxide and break the platelets. Banbury-type or continuous mixers are most often used to create pearl concentrates, although two-roll mills, calenders, vertical intensive mixers, and double planetary mixers are also suitable as long as you avoid excessive shear.
In liquid colorants, mica-based pearls may settle and pack. It is important to stir pearlescent liquid colors thoroughly before use.
Intensity of the pearlescent effect depends on how well the platelets align parallel to the surface of the plastic part. In injection molding, orientation can be disturbed by drag when platelets pass through gates, causing dark flow lines. Such flow lines are less noticeable than those that form with opaque, metallic pigments, however. Pigment "disorientation" also can occur at weld lines. Molders use a number of strategies to help prevent flow and weld lines:
Raise resin and mold temperatures as high as you can without harming part appearance. This lowers melt viscosity to allow better platelet orientation and reduces cooling, which causes weld lines. Molders often run molds 75-100° F above normal with pearlescents. They also use higher injection speed and pressure.
Use single-gate molds with wedge-shaped fan or tab gates. Or use a smaller gate that will generate more shear heating to keep the melt hot enough to minimize flow lines.
Place the gate in a thicker section of the part as far as possible from flow obstructions. In radially symmetric parts, such as caps, the gate is placed at the center of the part so that resin flows concentrically.
Minimize the length of sprue and runner to keep the melt hot.
Keep part thickness as uniform as possible.
Use overflow vents to remove flow lines.
Use concentrates rather than powdered pigment for easier handling and better dispersion.
Dies and mandrels must be clean and free of defects. Scratches, burrs, and burnt deposits can tip platelets and create surface marks .
Screen packs should be properly sized. As a rule of thumb, mesh sizes of 40 to 60 give good pigment dispersion. Backpressure can become excessive if screens are too small or too many are used, especially with large sized pearls.
Combining pearlescents with other pigments is as much art as science, given the complexity of how their colors interact. The interference colors formed by pearlescent pigments add when mixed so that red and blue form magenta, blue and green form blue-green, and red and green form yellow. White results when the three primary colors are mixed in the right proportion. By contrast, absorption colors created by other pigments subtract when mixed so that yellow and blue form green, red and yellow form orange, and red and blue form violet. Black results when the three primaries are mixed in the right proportion. Note that the primary and secondary colors are different for additive and subtractive colors.
In additive mixing, complementary colors mix to form white. In subtractive mixing, they mix to form gray or black.