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Coloration of Polypropylene with InterferencePigment and the Effect of Wetting Agents On Its Optical Properties

R.N. Jagtap,† C.K. Nere, and K.G. Patel—University of Mumbai*

Presented at the Symposium on Color and Appearance Instrumentation (SCAI), cosponsored by the Inter-Society Color Council (ISCC) andthe Federation of Societies for Coatings Technology (FSCT), April 15-16, 2003 in Chicago, IL.

* Paints and Plastics Division, Institute of Chemical Technology (formerly UDCT), Matunga, Mumbai – 400 019, India.† Author to whom all correspondence should be addressed. Email: jagtap_68@yahoo.com.

Coloration of polypropylene with interference pigment with and without dispersing agents, namely propylene glycol and poly(ethylene glycol) were studied. Effect of pigment concentrations and dispersing agents on various color properties of themolded specimens, such as reflectance versus wavelength characteristics, angular dependency, K/S data, and flop index wereevaluated using the CIE system with goniospectrophotometer. Surface image analysis was also conducted. Based on theabove parameters, design equations were developed. The study revealed that the effective concentration of the interferencepigment was 0.5% and better color characteristics were found when poly (ethylene glycol) was used as a dispersing agent.

Keywords: Coloration of polypropylene, interference pigments, optical characterization, pigments, appearance, colormeasurement, color, color matching, process modeling, simulation, pigmentation, pigment optics, plastics

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Color technology has brought enormous changes inour day–to–day life and interference pigments havebrought revolutionary changes in the optical proper-

ties of plastics, paints, printing inks, textiles, cosmetics, etc.1

Interference pigments have a multilayer structure, con-sisting of thin alternating layers having high and low re-fractive indexes, which partially reflect and partially trans-mit light, and thus cause the color to change with theangle of viewing.2-4 An extensive review of interferencepigments has been reported in the literature.5

Since the colors imparted to the polymers by interfer-ence pigments are angle dependent, the compounding ofthese pigments is very critical from the perspective of dis-persion and orientation of the particles. There is much lit-erature on coloration of polymers with ordinary pig-ments,6-11 but scarce material is available on the use ofinterference pigments for coloration of polymers.

In the present work, inclusion of interference pigmentsin the chemically inert polypropylene (PP) was studiedand the effects of propylene glycol (PG) and poly (ethyl-

Figure 1—Reflectance vs. wavelength curves for VPP system at 20º. Figure 2—Reflectance vs. wavelength curves for VPP system at 45º.

ene glycol) (PEG) as wetting and dispersing agents on itsoptical characteristics were evaluated. These optical datawere used in developing the design equations.

EXPERIMENTAL

Materials

Polypropylene, R120MK, (MFR 12 gm/10 min) was ob-tained from Reliance Industries Ltd., India. Blue interfer-ence pigment, Iriodin225, having a particle size of 40–60μm, was obtained from Emerck (India) Ltd., India. PG andPEG were procured from S. D. Fine Chemicals, India. Allthe raw materials were used as received.

Coloration

The compounding was done in a co–rotating twin-screw batch mixer, Haake Rheomix 600, Germany. Thecompounding was carried out at 190°C and at a rotorblade speed of 40 rpm. The compounding was continueduntil the torque became constant. Precaution was taken toavoid damaging the crystal shape and size by adding thepigment towards the end. After compounding, the lumps

obtained were compression molded into a 2-mm thicksheet at 190°C and at a pressure of 150–170 kgf/cm2 usingSterling Machine (India). In these compounds, the con-centration of the pigment was varied from 0.1 to 2.0 wt%and the concentration of wetting agents was kept con-stant at 0.2 wt%.

CharacterizationThe flat molded specimens were analyzed using a

GretagMacbeth Goniospectrophotometer—Auto Eye641 (U.S.) for optical characterization. Normal spec-trophotometers with a single angle of illumination orviewing were unable to measure reflectance characteris-tics of interference finishes. These finishes requiremeasurement at different angles of view and, therefore,the analysis was done using a multiangle spectropho-tometer, which is also referred to as a goniospectropho-tometer. The instrument has four angles of viewingnamely 20º, 45º, 75º, and 110º which are in respect tospecular angle and are called aspecular angles. The in-strument is based on the reverse viewing system or re-verse optics; i.e., there are four light sources (one foreach angle) and a detector system. Because of the re-verse optics, the measurement time is much less (150

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Figure 3—Reflectance vs. wavelength curves for VPP system at 75º.

Figure 4—Reflectance vs. wavelength curves for VPP system at 110º.

Figure 5—Reflectance vs. wavelength curves for PP + PEG system at 20º.

Figure 6—Reflectance vs. wavelength curves for PP + PEG system at 45º.

milliseconds). The instrument is equipped with apulsed xenon light source and a photodiode array witha monochromator as observer, as well as internal andexternal temperature sensors. The compounded sam-ples were backed by a dark and light Lenete Card and

the instrument provided the average reading over lightand dark in the reflectance mode. A Leica MLB (U.S.)compounded microscope was used for surface imageanalysis. The average of seven readings was taken andhad a ± 5% standard deviation.

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Figure 7—Reflectance vs. wavelength curves for PP + PEG system at 75º.

Figure 8—Reflectance vs. wavelength curves for PP + PEG system at 110º.

Figure 9—Reflectance vs. wavelength curves for PP + PG system at 20º.

Figure 10—Reflectance vs. wavelength curves for PP + PG system at 45º.

Figure 11—Reflectance vs. wavelength curves for PP + PG system at 75º.

Figure 12—Reflectance vs. wavelength curves for PP + PG system at 110º.

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RESULTS AND DISCUSSION

Reflectance versus Wavelength Characteristics

WITHOUT DISPERSING AGENT (VPP): Figures 1-4 show re-flectance versus wavelength curves at 20º, 45º, 75º, and

Figure 13—Angular dependency curves for VPP system.

Figure 14—Angular dependency curves for PP + PEG system.

Figure 15—Angular dependency curves for PP + PG system.

Figure 16—Color variation properties at 20°.

Figure 17—Color variation properties at 45°.

Figure 18—Color variation properties at 75°.

110º, respectively. Maximum reflection was observed at20º. Reflection decreased with an increase in the angle ofviewing, according to Bragg’s Law and Snell’s Law. Thefigures also show absorption of red color, as minimum re-flection was observed in 580 to 640 nm wavelength re-gions and, hence, the object appeared greenish-blue.

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Virgin polypropylene showed minimum reflectance dueto its translucent nature. Opacity and reflectance in-creased as the concentration of pigment increased.Chroma or dullness can be predicted by the difference inthe maximum and the minimum reflectance values. Itwas observed that the sample dullness increased with anincrease in the angle of viewing.

WITH PEG AS DISPERSING AGENT (PP + PEG): Figures 5-8show that when PEG was added to pigmented PP, re-flectance increased with an increase in the pigment con-centration, but decreased with an increase in the angle ofviewing. The dullness increased with the angle of viewing.

WITH PG AS DISPERSING AGENT (PP +PG): Figures 9-12show the effect of PG on the reflectance of pigmented PP.Reflectance decreased with an increase in the angle ofviewing and increased with an increase in the pigmentconcentration. Dullness increased with an increase in theangle of viewing.

On comparison, it was found that reflectance and dull-ness increased in the order of PP+PEG < PP+PG < VPP at thesame angle of viewing and same pigment concentration.

Angular Dependency

The L*-value indicates whiteness and darkness of thesample. Figures 13-15 show that there was not much vari-ation in the whiteness with the change in the angle ofviewing at fixed level of pigment concentration, but thewhiteness increased with an increase in the pigment con-centration for the same angle of viewing. VPP exhibitedthe highest L*-value at the same angle of viewing andsame pigment concentration. The L*-value of PP + PEG

Figure 19—Color variation properties at 110°.

Figure 20—K/S vs. concentration curves at 20°.

Figure 21—K/S vs. concentration curves at 45°.

Figure 22—K/S vs. concentration curves at 75°.

Figure 23—K/S vs. concentration curves at 110°.

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Table 1—Flop Index Values for Systems With and WithoutDispersing Agents

Sr. No. Concentration VPP System PEG System PG System

1 0.1 25 15 172 0.2 29 15 203 0.3 24 18 214 0.4 18 12 145 0.5 17 18 186 1.0 18 15 217 1.5 13 14 188 2.0 14 15 17

Figure 24—Surface image of VPP system.

Figure 25—Surface image of PP + PEG system.

Figure 26—Surface image of PP + PG system.

Figure 27—Design equation for VPP system.

system was slightly higher than that of the PP + PG systemat higher pigment concentrations. At lower pigment con-centrations, the difference was larger, showing that PEGwas better than PG as a dispersing agent at lower pigmentconcentrations.

The a*-value provides redness and greenness, while theb*-value is the measure of yellowness and blueness of thesample.

WITHOUT DISPERSING AGENT (VPP): Figures 16-19 showthat the a*-value increased with an increase in the angleof viewing. The a*-value decreased with the pigmentconcentration, i.e., the tone became greener. This trend

was observed up to 1% concentration; but at higher con-centrations, it became lighter as saturation was achieved.

Figures 16-19 show that the b*-value increased with anincrease in the angle of viewing as well as with an increasein pigment concentration, i.e., the tone became yellowishfrom bluish. Thus, color changed from greenish-blue toreddish-yellow. Color changes were more predominant at20° and 45° than at 75° and 110°.

WITH PG AS DISPERSING AGENT (PP +PG): Figures 16-19show that the a*-value had the same tendency as that ofthe a*-value of the system without dispersing agent.

Figures 16-19 show that the b*-value had the same ten-dency as that of the b*-value of the system without dispers-ing agent. The b*-values were higher as compared to thatof the system without dispersing agent because of betterdispersion of pigment particles in the polymer matrix.Thus, color shifted from greenish-blue to reddish-blue andeven to reddish-yellow at higher angles of viewing.

WITH PEG AS DISPERSING AGENT (PP + PEG): Figures 16-19 show that the a*-value had the same tendency as thatof the a*-value of the system without dispersing agent.

Figures 16-19 show that the b*-value had the same ten-dency as that of the b*-value of the system without dispers-ing agent. Thus, color changed from greenish-blue to red-

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Figure 29—Design equation for PP + PG system. Figure 28—Design equation for PP + PEG system.

dish-yellow and was more yellowish as compared to the PP+ PG system because the refractive index of PEG, whichwas 1.45, was higher than that of PG, which was 1.42.

K/S versus ConcentrationThe K/S value is the ratio of absorption coefficient to

scattering coefficient. Figures 20-23 show that K/S versusconcentration curves show linear behavior up to 0.5% pig-ment concentration, except for the PP + PEG system,where linear behavior was seen up to 1% pigment concen-tration, indicating that PEG acted as a better dispersingagent than PG at lower pigment concentrations. At higherpigment concentrations, i.e., more than 0.5%, curvesshowed nonlinear behavior because of saturation of thepigment, except for PP + PEG. As the pigment concentra-tion increased, scattering increased. Thus K/S versus con-centration showed nonlinear behavior at higher pigmentconcentrations, which is consistent with the Kubelka-Munk theory. Yellowness was seen more in the PP + PEGsystem than in the PP + PG system and blueness was seenmore in the VPP system than in others.

Flop IndexFlop index is a measurement of the change in the re-

flectance of metallic colors as it is rotated through therange of viewing angles. A flop index of zero indicates asolid color while a very high flop metallic or pearlescent-based coat or clearcoat may have a flop index of 15–17.Flop index is calculated as

Flop Index = [2.69 (L150º–L110º)1.11]/ (L45º)

0.86.

where L is the grayness scale.

Flop index helps one to identify the worthiness of aninterference pigment. For better interference pigment,the flop index should be between 15 and 17. Table 1shows the values of flop index. Nonlinearity was ob-served when flop index values were compared with eithera*, b*, or K/S values at pigment concentrations higherthan 0.5%. At lower pigment concentrations, i.e., up to0.5 wt% for the PP + PEG system and 0.3–0.5 wt% for thePP + PG and VPP systems, the flop index values were wellwithin the desirable range, i.e., 15–17, indicating that the

effective concentration of interference pigment was0.5%.

Surface Image Analysis

Figures 24-26 are surface images of VPP, PP + PEG, andPP + PG systems, respectively. In the case of the PP + PEGsystem, the well dispersed pigments in the PP matrix ex-hibit a stronger color emergence than the PP + PG andVPP systems, thus indicating PEG is a better dispersingagent than PG.

Design Equations

Figures 27-29 show the predicted reflectance for all thesystems. The design equations for the reflectance character-istics were formulated assuming linear behavior for all sys-tems. From the design equations, pigment concentrationscan simply be determined without studying the K/S evalu-ation. Mean sum of square R2 ~ 1 and precisely R2 ~ 0.98.

Rt1 = 25.2428 C + 0.1622 θ – 4.3299 a* – 1.6062 b* – 26.1973 (1)Rt2 = 16.8843 C – 0.04866 θ – 0.4412 a* – 1.7677 b* + 11.5525 (2)Rt3 = 18.6426 C + 0.1130 θ – 0.4654 a* – 2.1462 b* - 2.1464 (3)

where,Rt1 is the predicted reflectance of the VPP system; Rt2 isthe predicted reflectance of the pigmented PP + PEG system; Rt3 is the predicted reflectance of the pig-mented PP + PG system; C is the concentration of pig-ment; and θ is the angle of viewing.

CONCLUSIONSThe addition of wetting agents gave better color char-

acteristics. At low concentrations, PEG was better than PG,but at higher concentrations, the reverse was true. Flop in-dex, K/S data, and surface image analyses also revealed thesame. As the concentration and angle of viewing in-creased, the color shifted from greenish-blue to reddish-yellow and the transition of color towards yellow wasgreater for the PP + PEG system. By knowing the a*, b*,and reflectance values, one could easily determine the pig-ment concentration from the matrix by making use of de-sign equations.

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ACKNOWLEDGMENTWe would like to thank M/S Emerk (India) for gener-

ously providing samples of blue interference pigment,Iriodin225.

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