Coloration of polypropylene with interference pigment and the effect of wetting agents on its optical properties.

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 the molded specimens, such as reflectance versus wavelength characteristics, angular dependency, K/S data, and flop index were evaluated using the CIE system with goniospectrophotometer. Surface image analysis was also conducted. Based on the above parameters, design equations were developed. The study revealed that the effective concentration of the interference pigment 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, color measurement, color, color matching, process modeling, simulation, pigmentation, pigment optics, plastics

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Color technology has brought enormous changes in our day-to-day life and interference pigments have brought revolutionary changes in the optical properties of plastics, paints, printing inks, textiles, cosmetics, etc. (1)

Interference pigments have a multilayer structure, consisting of thin alternating layers having high and low refractive indexes, which partially reflect and partially transmit light, and thus cause the color to change with the angle of viewing. (2-4) An extensive review of interference pigments has been reported in the literature. (5)

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Since the colors imparted to the polymers by interference pigments are angle dependent, the compounding of these pigments is very critical from the perspective of dispersion and orientation of the particles. There is much literature on coloration of polymers with ordinary pigments, (6-11) but scarce material is available on the use of interference pigments for coloration of polymers.

In the present work, inclusion of interference pigments in the chemically inert polypropylene (PP) was studied and the effects of propylene glycol (PG) and poly (ethylene glycol) (PEG) as wetting and dispersing agents on its optical characteristics were evaluated. These optical data were used in developing the design equations.

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EXPERIMENTAL

Materials

Polypropylene, R120MK, (MFR 12 gm/10 min) was obtained from Reliance Industries Ltd., India. Blue interference pigment, Iriodin 225, having a particle size of 40-60 [micro]m, was obtained from Emerck (India) Ltd., India. PG and PEG were procured from S. D. Fine Chemicals, India. All the raw materials were used as received.

Coloration

The compounding was done in a co-rotating twinscrew batch mixer, Haake Rheomix 600, Germany. The compounding was carried out at 190[degrees]C and at a rotor blade speed of 40 rpm. The compounding was continued until the torque became constant. Precaution was taken to avoid damaging the crystal shape and size by adding the pigment towards the end. After compounding, the lumps obtained were compression molded into a 2-mm thick sheet at 190[degrees]C and at a pressure of 150-170 kgf/[cm.sup.2] using Sterling Machine (India). In these compounds, the concentration of the pigment was varied from 0.1 to 2.0 wt% and the concentration of wetting agents was kept constant at 0.2 wt%.

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Characterization

The flat molded specimens were analyzed using a GretagMacbeth Goniospectrophotometer--Auto Eye 641 (U.S.) for optical characterization. Normal spectrophotometers with a single angle of illumination or viewing were unable to measure reflectance characteristics of interference finishes. These finishes require measurement at different angles of view and, therefore, the analysis was done using a multiangle spectrophotometer, which is also referred to as a goniospectrophotometer. The instrument has four angles of viewing namely 20[degrees], 45[degrees], 75[degrees], and 110[degrees] which are in respect to specular angle and are called aspecular angles. The instrument is based on the reverse viewing system or reverse optics; i.e., there are four light sources (one for each angle) and a detector system. Because of the reverse optics, the measurement time is much less (150 milliseconds). The instrument is equipped with a pulsed xenon light source and a photodiode array with a monochromator as observer, as well as internal and external temperature sensors. The compounded samples were backed by a dark and light Lenete Card and the instrument provided the average reading over light and dark in the reflectance mode. A Leica MLB (U.S.) compounded microscope was used for surface image analysis. The average of seven readings was taken and had a [+ or -] 5% standard deviation.

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RESULTS AND DISCUSSION

Reflectance versus Wavelength Characteristics

WITHOUT DISPERSING AGENT (VPP): Figure 1-4 show reflectance versus wavelength curves at 20[degrees], 45[degrees], 75[degrees], and 110[degrees], respectively. Maximum reflection was observed at 20[degrees]. Reflection decreased with an increase in the angle of viewing, according to Bragg's Law and Snell's Law. The figures also show absorption of red color, as minimum reflection was observed in 580 to 640 nm wavelength regions and, hence, the object appeared greenish-blue. Virgin polypropylene showed minimum reflectance due to its translucent nature. Opacity and reflectance increased as the concentration of pigment increased. Chroma or dullness can be predicted by the difference in the maximum and the minimum reflectance values. It was observed that the sample dullness increased with an increase in the angle of viewing.

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

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

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

Angular Dependency

The L*-value indicates whiteness and darkness of the sample. Figures 13-15 show that there was not much variation in the whiteness with the change in the angle of viewing at fixed level of pigment concentration, but the whiteness increased with an increase in the pigment concentration for the same angle of viewing. VPP exhibited the highest L*-value at the same angle of viewing and same pigment concentration. The L*-value of PP + PEG system was slightly higher than that of the PP + PG system at higher pigment concentrations. At lower pigment concentrations, the difference was larger, showing that PEG was better than PG as a dispersing agent at lower pigment concentrations.

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

WITHOUT DISPERSING AGENT (VPP): Figures 16-19 show that the a*-value increased with an increase in the angle of viewing. The a*-value decreased with the pigment concentration, i.e., the tone became greener. This trend was observed up to 1% concentration; but at higher concentrations, it became lighter as saturation was achieved.

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Figures 16-19 show that the b*-value increased with an increase in the angle of viewing as well as with an increase in pigment concentration, i.e., the tone became yellowish from bluish. Thus, color changed from greenish-blue to reddish-yellow. Color changes were more predominant at 20[degrees] and 45[degrees] than at 75[degrees] and 110[degrees].

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

Figures 16-19 show that the b*-value had the same tendency as that of the b*-value of the system without dispersing agent. The b*-values were higher as compared to that of the system without dispersing agent because of better dispersion of pigment particles in the polymer matrix. Thus, color shifted from greenish-blue to reddish-blue and even 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 that of the a*-value of the system without dispersing agent.

Figures 16-19 show that the b*-value had the same tendency as that of the b*-value of the system without dispersing agent. Thus, color changed from greenish-blue to reddish-yellow and was more yellowish as compared to the PP + PG system because the refractive index of PEG, which was 1.45, was higher than that of PG, which was 1.42.

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K/S versus Concentration

The K/S value is the ratio of absorption coefficient to scattering coefficient. Figures 20-23 show that K/S versus concentration curves show linear behavior up to 0.5% pigment concentration, except for the PP + PEG system, where linear behavior was seen up to 1% pigment concentration, indicating that PEG acted as a better dispersing agent than PG at lower pigment concentrations. At higher pigment concentrations, i.e., more than 0.5%, curves showed nonlinear behavior because of saturation of the pigment, except for PP + PEG. As the pigment concentration increased, scattering increased. Thus K/S versus concentration showed nonlinear behavior at higher pigment concentrations, which is consistent with the Kubelka-Munk theory. Yellowness was seen more in the PP + PEG system than in the PP + PG system and blueness was seen more in the VPP system than in others.

Flop Index

Flop index is a measurement of the change in the reflectance of metallic colors as it is rotated through the range of viewing angles. A flop index of zero indicates a solid 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 ([L.sub.150[degrees]]-[L.sub.110[degrees]])[.sup.1.11]]/([L.sub.45[degrees]])[.sup.0.86].

where L is the grayness scale.

Flop index helps one to identify the worthiness of an interference pigment. For better interference pigment, the flop index should be between 15 and 17. Table 1 shows the values of flop index. Nonlinearity was observed when flop index values were compared with either a*, b*, or K/S values at pigment concentrations higher than 0.5%. At lower pigment concentrations, i.e., up to 0.5 wt% for the PP + PEG system and 0.3-0.5 wt% for the PP + PG and VPP systems, the flop index values were well within the desirable range, i.e., 15-17, indicating that the effective concentration of interference pigment was 0.5%.

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Surface Image Analysis

Figures 24-26 are surface images of VPP, PP + PEG, and PP + PG systems, respectively. In the case of the PP + PEG system, the well dispersed pigments in the PP matrix exhibit a stronger color emergence than the PP + PG and VPP systems, thus indicating PEG is a better dispersing agent than PG.

Design Equations

Figures 27-29 show the predicted reflectance for all the systems. The design equations for the reflectance characteristics were formulated assuming linear behavior for all systems. From the design equations, pigment concentrations can simply be determined without studying the K/S evaluation. Mean sum of square [R.sup.2] [approximately] 1 and precisely [R.sup.2] [approximately] 0.98.

[R.sub.t1] = 25.2428 C + 0.1622 [theta] - 4.3299 a* - 1.6062 b* - 26.1973 (1)

[R.sub.t2] = 16.8843 C - 0.04866 [theta] - 0.4412 a* - 1.7677 b* + 11.5525 (2)

[R.sub.t3] = 18.6426 C + 0.1130 [theta] - 0.4654 a* - 2.1462 b* - 2.1464 (3)

where, [R.sub.t1] is the predicted reflectance of the VPP system; [R.sub.t2] is the predicted reflectance of the pigmented PP + PEG system; [R.sub.t3] is the predicted reflectance of the pigmented PP + PG system; C is the concentration of pigment; and [theta] is the angle of viewing.

CONCLUSIONS

The addition of wetting agents gave better color characteristics. At low concentrations, PEG was better than PG, but at higher concentrations, the reverse was true. Flop index, K/S data, and surface image analyses also revealed the same. As the concentration and angle of viewing increased, the color shifted from greenish-blue to reddish-yellow and the transition of color towards yellow was greater for the PP + PEG system. By knowing the a*, b*, and reflectance values, one could easily determine the pigment concentration from the matrix by making use of design equations.

ACKNOWLEDGMENT

We would like to thank M/S Emerk (India) for generously providing samples of blue interference pigment, Iriodin225.

References

(1) Elvers, B., Hawkins, S., and Schulz, G. (Eds.), Ullman's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A-20, 348, 1992.

(2) Hawe-Grant, M. (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 19, 1, 1996.

(3) Bikales, N.M. (Ed.), "Nacteous Pigments," Encyclopedia of Polymer Science and Technology, Vol. 10, 193, 1970.

(4) Zarrel, U. (Ed.), Special Effect Pigments, European Coating Literature, Vincentz, 1, 1998.

(5) Rane, R.H., Nere, C.K. and Jagtap, R.N., "Iridescent Pigments for Plastic Application," Popular Plastics & Packaging, Vol. 52, No. 2, 57 (2002).

(6) Margolis, J.M. (Ed.), Decorating Plastics, Hanser Gardner Publications, NY, 111, 1987.

(7) Teige, W., "Colorants for Mass Coloration of Polyester," Colourage, Vol. 40, No. 11, 59 (1993).

(8) Taylor, J.R. and Foster, H., "The Pigmentation of Acrylic Resins," J. Soc. Dyers and Col., Vol. 85, No. 12, 579 (1969).

(9) Freireich, S., Gertner, D. and Zilkha, A., "Coloration of Polyacrylonitrile and Polymethacrylonitrile by Organotin Compounds," J. Polym. Sci., Part A, Vol. 10, No. 10, 3109 (1972).

(10) Hay, J.N., "Coloration in Vinyl Polymers," Makromol. Chem., Vol. 67, 31 (1963).

(11) Bond, J.R., "The Coloration of Automotive Finishes," Rev. Prog. Coloration, Vol. 12, 17 (1982).

R.N. Jagtap, ([dagger]) 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) and the 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.

([dagger]) Author to whom all correspondence should be addressed. Email: [e-mail].

Table 1 -- Flop Index Values for Systems With and Without Dispersing
Agents

Sr. No.  Concentration      VPP System     PEG System     PG System

1        0.1                25             15             17
2        0.2                29             15             20
3        0.3                24             18             21
4        0.4                18             12             14
5        0.5                17             18             18
6        1.0                18             15             21
7        1.5                13             14             18
8        2.0                14             15             17