Title: BLACK PEARLESCENT PIGMENT FOR POWDER COATING AND PLASTICS Authors: Jonathan Doll*, Michael Willis, Calvin Richardson *- Corresponding author: jonathan.doll@sunchemical.com, Sun Chemical Corporation, 5020 Spring Grove Ave., Cincinnati OH 45232. Jonathan Doll is the Leader - Effects Pigments Research at Sun Chemical Performance Pigments. To reach Jonathan and learn more about this pigment, call 513.681.5950 ext. 4364 or email him at jonathan.doll@sunchemical.com. Other contributing authors to this article are Michael Willis, Manager - Advanced Applications Plastics Lab, and Calvin Richardson, Technician, Advanced Applications Plastics Lab at Sun Chemical. Abstract: The paradoxical nature of black pearlescent pigments is due to the contradictory facts that 1) the color black is characterized by the absence of light and 2) the iridescence of pearlescent pigments is due to the structure-induced interference of light. Due to this apparent contradiction, the current black pearl technology is limited to pigments that either have a jet, matte-black appearance with no iridescence, or to iridescent black pearls that have blue, violet or green undertones and lack sufficient jetness. None of these pigments show the sparkle that pearlescent pigments are traditionally known for. A new black pearlescent pigment has been developed that has exceptional luster and sparkle and is sufficiently jet. The new pigments can be incorporated into a range of plastics with no reduction in effect. The properties of this pigment with respect to plastics will be described. Background Information Pigments are insoluble materials that give objects distinctive colors. There are two main pathways by which most pigments impart color to an object: 1) through absorption of light; and 2) through physical interaction of light with the structural matrix of the pigments. 1 Absorption is the traditional pathway by which most organic and inorganic pigments interact with light. When color arises from absorption, certain wavelengths of light will be absorbed by the pigment. 1 These wavelengths will no longer be visible. The color of the pigment is determined by the wavelengths of light that are not absorbed, but are instead reflected or transmitted. For example, a pigment that is described as red will reflect the red wavelengths of light, but absorb the blue and green wavelengths. The color observed in absorption pigments is essentially angle
independent, and the same color can be perceived at any viewing angle under equal lighting conditions. The second pathway, also known as structural color, display color by constructively or destructively interfering light as a result of periodic structures within the pigment. In the world of effect pigments, the most well-known pigments that display color via this pathway are known as pearlescent pigments. The color perceived after light interacts with a pearlescent pigment is determined by a combination of the pigment s physical structure and the absorbance of its components. Unlike absorption pigments, the perceived color of pearlescent pigments has a strong dependence on the viewing angle due to their structure. This angle-dependent color is described as iridescence or luster and is a characteristic of pearlescent pigments. 1 Structurally, pearlescent pigments are similar. They are typically comprised of a flat, plateletshaped substrate that is coated with one or more layers of a metal oxide. Typical metal oxides used to make pearlescent pigments include titanium dioxide (TiO2), iron oxide and silicon dioxide, among others. By varying the order, thickness, and number of layers, on a pearlescent pigment, the color of pearlescent pigments can be changed, even though the ingredients are the same. For titanium dioxide-coated pearlescent pigments in the bulk powder form, the viewing angles of the pigment are averaged over all directions, and thus appear white. When added to a medium and given a net average orientation, the reflected iridescent color of the pearlescent becomes apparent when viewed over a black background. Over white, the pearlescent pigment may display a complimentary color that is comprised of a combination of the reflected and transmitted light. 2 Traditional pearlescent pigments can be grouped into ten basic colors: silver/white, interference gold, red, violet, blue, green, metallic gold, bronze, copper and red russet. Substrates (including mica, synthetic mica, glass, alumina etc.) coated with TiO2 produce the first six, while iron oxide and/or iron oxide-tio2 coatings produce the remaining 4 colors. Because iron oxide is a colored oxide, pearlescent pigments coated with it will have an absorption component in addition or iridescence. Besides color, pearlescent pigments can also display a range of effects that are dictated by the shape of the substrate. Small particle size substrates (<15 micron) tend to give a more uniform appearance and finish that is termed satin. Large particle size substrates (> 250 micron) tend to impart an appearance of graininess and/or sparkle, depending on the smoothness and uniformity of the substrates. Expanding the pearlescent pigment color gamut beyond the basic ten allows formulators to produce unique design concepts. Perhaps the most sought after pearlescent color is black. Black is one of the most popular colors in consumer products because it imparts a sense of quality and refinement. Despite this inherent demand, there are currently no truly black pearlescent pigments available on the market. Most currently marketed black pearlescent pigments tend to be a deep shade of blue or violet or they lack the characteristic pearlescent luster. This is because producing a black pearlescent effect presents many challenges due to the nature of the color black. Black is defined physically as the absence or minimization of light reflection at all visible wavelengths. It is the ultimate result of subtractive color blending and is characterized by absorption across all wavelengths. In the perfect black body, no light is reflected or emitted. Pearlescent pigments, on the other hand, produce color through interference. When interference
pigments are blended, they are subject to additive color mixing, resulting in a whitening of the overall appearance. This represents a fundamental paradox about black pearlescent pigments: How do you make a pigment that absorbs all light but also shows interference? Despite these difficulties there are black pearls available in the market. However, in many cases, these pigments are not truly black. Most of them can be described as gun-metal gray (too high lightness or L* value) or black olive or deep blue (too high chroma or C* value). True blacks are achromatic and have low L* and C* values. On the other side, there are some black pearlescent pigments that have a high degree of jetness, however, these pearls lack the sparkle effect that is typical of traditional pearlescent pigments. Objective of Work In this article, we highlight a new development in the realm of black pearlescent pigments. We present a pigment that has exceptional luster, sparkle and jetness when compared to traditional commercially available black pearlescent pigments. These new pigments can be incorporated into many different applications, including plastics with little reduction in the perceived effect. Additionally, they can be used to impart a unique sparkle effect plastic parts and film while remaining robust enough to handle high temperatures. The application of these pigments in coatings and plastic applications will be described and compared to other black pearlescent pigments. Experimental procedure Materials For the pigment evaluations, the following commercial pigments were used: Sample A new black pearlescent pigment; Samples B and C standard black pearlescent pigments. Coating Test Method - Displays In order to measure their coloristic properties in a smooth, flat, and uniform surface, Samples 1-4 were loaded into a clear solvent-borne acrylic paint at 10% loading w/w. The samples were mixed well and drawn down on a piece of black and white Leneta paper using a #3 Byrd applicator. The samples were allowed to dry overnight. Coating Test Method Color The color of the displays was measured via two different instruments: Datacolor SF600 plus spectrophotometer using a diffuse reflection sphere and an X-Rite MA-98 multiangle spectrophotometer using D65 illumination. From these data the jetness, Mc, and the flop index, F. I., of the displays was calculated using Equation (1) and (2) 1..... (1) 100 log log log (2)
Where L15*, L45* L110* are the brightness values measured at 15 o, 45 o, and 110 o from the specular reflection angle (at 45 o incident light). Xn, Yn and Zn are the tristimulus values of the incident light source and X, Y and Z are the tristimulus values of the reflected light, and are defined by: 1 116 / 16 (3) 500 (4) 200 (5) Equations 3-5 define the CIELAB system in terms of the standard color values X,Y, and Z and well as the standard color values for the illuminating light source, Xn, Yn, and Zn, and a standard observer. 1 Incorporation into Polypropylene To a mixture of 363.3 g polypropylene 6301 and 1.85 g Zn Stearate, 0.5% (w/w) black pearlescent pigment was added and mixed until homogeneous. The mixture was charged to a Battenfeld Injection Molder, equipped with an injection die to make 2 x 3 chips in both flat and step configuration. The temperature of the injection molder was set at 232 o C. After approximately four chips were made, the fifth chip was collected to measure the color and transparency. The process was repeated to make a sample containing 1.0% (w/w) black pearlescent pigment. The color and transparency of the chips were measured on a Datacolor SF600 plus spectrophotometer using a diffuse reflection sphere using D65 light. The color measurements were made using the flat chips, while the transparency measurements used the step chips. The transparency was measured over both white and black background at three different thicknesses of 1.5 mm, 2.3 mm, 3.1 mm. Thermal Stability Test To a mixture of 60.0 g linear, low density polyethylene (LDPE) LDPE 722 and 5.0 g zinc stearate, 7.1% (w/w) black pearlescent pigment was added to give a total weight of 72. The mixture was mixed completely to make a premix concentrate. The premix concentrate was combined and homogenized with and 305.0 g LDPE 722 and 625.0 LDPE 710-20 to make a 0.5% w/w pigment mixture. The 0.5% pigment mixture was loaded into a Boy 30M injection molder at 163 o C. After one minute, five black LDPE chips were ejected. The sixth chip was kept for color stability measurements. The plastic flow was stopped and a second chip was collected in the same way after five minutes. This procedure was repeated at a gradual ramp of temperature up to 300 o C at 13-14 o C increments. The color was measured by Datacolor SF600 plus spectrophotometer using a diffuse reflection sphere using D65 illumination. A change in de of >1.0 units was considered to be significant.
Discussion of Data and Results Black Pearl has a Unique Appearance To demonstrate the uniqueness of the new black pearlescent pigment, Figure 1 shows a picture of a masstone comparison of a drawdown of Samples A-C at 10% loading w/w. The pictures were all taken in D65 light. Of all of the black pearls on display in Figure 1, only Sample A has the ideal combination of high jetness, luster and sparkle. The remaining pigments either do not have appreciable luster (Sample B), or they are more blue (Sample C) when compared to Sample A. None of the comparative pearls display the characteristic, attractive sparkle of Sample A Figure 1: Pictures of the masstone in a liquid coating for Samples A-D The flop index (F.I.) and jetness (Mc) of Samples A-D were measured in order to quantitatively understand their color. The displays from Figure 1 were used to measure these values. The flop index can be used as a measure of the overall luster and travel seen by the pearl, because it takes into account the brightness at three different viewing angles. A higher flop index will correspond to a more lustrous appearance when the sample is viewed by an observer. The jetness is a measure of how black the pigment appears. Larger Mc values mean that the pigment is more black or jet. Table 1 shows that Sample A, the new black pearlescent pigment, has a good balance of jetness and flop, Sample B has good jetness but poor color flop, and Sample C has the worst jetness, but high flop. It would appear that Sample A offers that best balance of properties. In addition, the masstone pictures in Figure 1 show that Sample A has a sparkle effect, which is not observed in Samples B and C. The combination of the sparkle effect, flop and jetness creates an effect unlike any of the other pearlescent pigment in this study, demonstrating the uniqueness of the pigment. Table 1: Table showing the Flop index and Jetness of the Samples A-C when put into a coating. Sample F.I. Mc Sample A 7.4 361.9 Sample B 5.4 375.1 Sample C 16.6 348.6
Tinting polypropylene A key application for Sample A is to use as a coloring and tinting agent in plastics. To look at the color, a number of masstone chips in polypropylene were made for Samples A-C and the color was measured at two different loadings, 0.5% and 1.0%. The flop index and jetness were measured for each sample and shown in Table2 Table 2: Table showing the Flop index and jetness of the Samples A-C when put into polypropylene at 0.5% and 1.0% Sample Loading F.I. Mc Sample A 0.5% 9.9 374.3 1.0% 11.6 375.4 Sample B 0.5% 6.4 388.78 1.0% 7.3 389.4 Sample C 0.5% 32.0 354.6 1.0% 36.2 351.0 The black pearlescent pigments show the same trends in polypropylene as they did in the coating when comparing A to B and C and actually appear to have larger value of both FI and Mc. This is likely due to the increased thickness of the chip when compared to the coatings, leading to less contribution to the optics from the substrate. The change in the FI and MC with pigment loading is minimal, suggesting that 0.5% is a good loading from a coloristic perspective. Not reflected in these numbers is the presence of sparkle in the samples. Indeed, Sample A is the only material in this study that has a high degree of sparkle. Again, Sample A performs well in terms of finding a good value for flop while maintaining a high degree of jetness. In addition, Sample A has a well-defined, polychromatic sparkle that is not extant in either Sample B or C. Sample A has the most optimized coloristics and effect of the three samples studied. To see if 0.5% is the optimum loading, an opacity test was performed using step chips at 0.5% pigment loading in polypropylene with gradations of 0.8 mm. The color was then measured at each of the gradations over a white and a black background using Datacolor SF600 plus spectrophotometer using a diffuse reflection sphere using D65 light. To calculate the opacity, the ratio of the L* over white to the L* over black was taken. Generally an opacity of 1.0 is required for the sample to be opaque / (6) The opacity results are plotted in Figure 2 for Samples A-C. At this loading, the three pigments have similar opacity at 2.3 mm and 3.1 mm thickness. The main divergence in this result is that
Sample C has lower opacity at 1.5 mm thickness. From above, Sample A has an optimized appearance. Moreover, Sample A is just as opaque as Sample B in this test. Figure 2: Opacity measurements for Samples A-C for samples at 0.5% at three different chip thicknesses. From these data, we conclude that the hiding power of Sample A and B are similar and that they can be used within similar ranges, while the hiding for Sample C is slightly inferior at low thicknesses and loading. Temperature stability in LDPE Because most black pearlescent pigments contain black iron oxide (Fe3O4) there is the possibility that there may be some color instability due to the thermally induced oxidation of Fe3O4 to Fe2O3. This would cause the color of the pigment to shift to red or have more reddish undertones. The new black pearlescent pigment (Pearl A) was tested for thermal stability against existing black pearlescent pigments (Pearl B and C). For thermal stability an overall color change or de should be 1.0 after a heating cycle at a specified temperature and time. Materials that have a de > 1.0 are considered not thermally stable past the conditions that caused the color shift. The de is determined by the Pythagorean relationship between the change in lightness, (L*), redto-green contribution (a*), and yellow-to-blue contribution (b*) in CIELAB color space: 1 (7) Figure 3a and b charts the de after holding the pigments in an LDPE matrix at 0.5% for one and five minutes at the specified temperature. This figure shows that Sample A and Sample B perform similarly across the entire studied temperature range and can be thought of as thermally stable by this test. Sample C shows a large asymptotic color shift at c.a. 200 o C for a one minute
a) b) Figure 3: Plot of the de versus holding temperature for a hold time of a) 1 minute, and b) 5 minutes for Samples A-C. holding time. This color shift occurs more gradually for Sample C at a five minute holding time, which suggests that time and temperature both play a role in this pigment s stability. The color data for Sample C suggests that the pigment gets lighter and more yellow with temperature and time. Conclusion In conclusion, a new black pearlescent effect has been developed for both liquid and powder coatings that shows exceptional sparkle and jetness. In comparison to other black pearlescent pigments, the reported pigment shows the right balance of luster and jetness, as well as a unique sparkle. It is just as opaque and thermally stable as current industry standards. Recommendations It is recommended that Pearl A is used in application where an exceptional pearlescent luster and sparkle are needed in conjunction with good opacity and thermal stability. References 1) Klein, G.A. Industrial Color Physics; Springer: New York, 2010. 2) Pfaff, G. and Weitzel, J., Pearlescent Pigments/Flakes, in Coloring of Plastics: Fundamentals; Charvat, R. A., Ed., John Wiley & Sons, Inc., Hoboken, NJ 2003