IMPLICATIONS OF AGGREGATION AND MASS FRACTAL NATURE OF AGGREGATES ON THE PROPERTIES OF ORGANIC PIGMENTS AND POLYMER COMPOSITES By N. Agashe *, G. Beaucage *, D. Kohls *, S. Sukumaran *, G. Skillas †, G. Long ‡, J. Ilavsky #, P. Jemian §, L. Clapp &, R. Schwartz & * Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45220, USA † Inst. f. Verfahrenstechnik, ETH Zentrum ML F24, CH–8092 Zurich, Switzerland. ‡ Ceramics Division NIST, Gaithersburg, MD 20899, USA. # University of Maryland, College Park, MD 20742, USA. § University of Illinois at Urbana–Champaign, IL 61801, USA. & Colors Group, Sun Chemical Corp., Cincinnati, OH 45232, USA. What are Pigments ?? Most Common types of Pigments are, Inorganic Pigments Organic Pigments Other types include Metallic and Pearlescent. The smallest size of an aggregate necessary for scattering is given by Bragg’s Law, The optimum size of the aggregate can be estimated by integrating the Guinier Law, Why Organic Pigments ? Most of the research in the pigment industry is concentrated on inorganic pigments like titania. Until now all work on organic pigments has examined only the surface fractal nature of the organic pigment particles. We make the first attempt to study the aggregation of organic pigments, the mass fractal nature of these aggregates and their relationship to the optical properties. The typical size of primary particles of organic pigments is 0.05 to 0.1  m. The optimal size necessary for good scattering is about 0.5  m. Fractal Structures – have Fractional Dimension Surface Fractal Object, (d s ) Irregular Surface Mass Fractal Object, (d f ) Irregular Structure Common Organic Pigments are, Azo Pigments: Monoazo (-NH-) or Diazo (-N=N-) – Quinacridones, Naphthol Reds, Diarylides, Rhodamines, and Naphthoic Acid. Phthalocyanines: (Naphthol) & (-CN) – Metal and Non-metal Perylenes Carbazoles Triphenyl Methane Anthraquinone and Indigoid Pigments Denotation: C.I. PR 122 (Pigment Red 122) Regimes of Aggregation Diffusion Limited Aggregation, d f < 2 (d f ~ 1.8) Reaction Limited Aggregation, d f > 2 (d f ~ 2.5) Intermediate Regime Transport Limited Reaction Limited Aggregation of Organic Pigments Forces behind aggregation of organic pigments are weak electrostatic forces like van Der Waal’s forces, static charge, chemical polarity and surface tension. Processing also dictates the nature of the aggregates. Color is produced in pigments by scattering. Any ScatteringBest Scattering For visible light, ~ 0.5  m The typical particle size for organic pigments is 0.05 to 0.1  m. Aggregation is necessary for good scattering. Scattering in Organic Pigments Mie Scattering Dilute Systems, Large Particles, Higher Index Difference Rayleigh Ganz Approximation All Systems, All Particles Particles, Lower Index Difference For X-Rays, the comparable contrast difference is very small between the pigment and the polymer. Small and Ultra Small Angle X-Ray Scattering (SAXS/USAXS) Range of q for SAXS is 0.01 to 0.1, while USAXS can go down to q value of B3B3 B2B2 B1B1 Log (I) Log (q) (q in Å -1 ) d f - 4 G 2, Rg 2 G 1, Rg 1 d f is the mass fractal dimension For fractal objects, Unified Function for Mass Fractal Model Unified Function, used to fit the scattering data, is based on six parameters, Guinier Prefactors: G 1 and G 2 Radius of Gyration: Rg 1 and Rg 2 Power law Prefactor: B 1 Fractal Dimension: d f The diameter of a sphere having similar Rg as the aggregate can be used to estimate the size of the aggregate. Results The results are a survey of some of the behavior seen when organic pigments are milled into polymers. This is the first attempt to characterize the aggregation according to the process by which the aggregates are formed. The mass fractal behavior of these aggregates is studied. The primary particle of each organic pigment is examined to see if it is made up of a single crystal or multi crystals. Diazo Pigment Red 170 ( ) TEM – D pp = 0.2  m LS – D agg = 0.4  m Powder – Non Mass Fractal D pp = 0.2  m 20% in PMMA – Mass Fractal, d f = 2.5 (RLA) D agg = 2.35  m Diazo Pigment Red 170 ( ) Higher Luminosity TEM – D pp = 0.15  m LS – D agg = 0.35  m Powder – Non Mass Fractal D pp =  m 20% in PMMA – Mass Fractal, d f = 2.67 (RLA) D agg = 2.01  m PMMA, used earlier, is a non-crystalline polymer. PP is a semi-crystalline polymer. The addition of pigments has an effect on the crystallinity of PP. Monoazo Pigment Red 122 TEM – D pp = 0.1  m (length) LS – D agg = 0.2  m Powder – Non Mass Fractal D pp = 0.13  m 1% in PP – Mass Fractal, d f = 1.91 (DLA) D pp = 0.13  m 5% in PP – Mass Fractal, d f = 1.5 (DLA) D pp = 0.13  m C. I. PR 122 Monoazo Pigment Violet 19 TEM – D pp = 0.05  m LS – D agg = 0.4  m Powder – Mass Fractal, d f = 2.32 (RLA) D pp = 0.03  m, D agg = 1.2  m 1% in PP – Mass Fractal, d f = 1.55 (DLA) D pp = 0.16  m, D agg = 1.0  m 5% in PP – Mass Fractal, d f = 1 (DLA) D pp = 0.26  m C. I. PV 19 Disazo Pigment Yellow 13 TEM – D pp = 0.1  m LS – D agg = 0.5  m Powder – Mass Fractal, d f = 2.34 (RLA) D pp =  m, D agg = 0.32  m 5% in PP – Mass Fractal, d f = 2.7 (RLA) D pp =  m, D agg = 0.49  m C. I. PY 13 Conclusions Organic pigments field has ignored the importance of aggregates to optical properties. Mass fractal aggregates were observed for all the pigments when milled into polymers. The size of a crystal is too small to scatter visible light. Aggregation is critical to have good optical properties, and this issue has been dealt with for the first time. There is an incredible range of behavior in terms of aggregation, based on the polarity of the compound and the particle size. Some contradictions in the behavior can be seen. There is a potential to control and design the aggregate size and structure of organic pigments if we had a bit more understanding. Acknowledgements The UNICAT facility at the Advanced Photon Source (APS) is supported by the Univ of Illinois at Urbana-Champaign, Materials Research Laboratory (U.S. DOE, the State of Illinois-IBHE-HECA, and the NSF), the Oak Ridge National Laboratory (U.S. DOE under contract with UT-Battelle LLC), the National Institute of Standards and Technology (U.S. Department of Commerce) and UOP LLC. The APS is supported by the U.S. DOE, Basic Energy Sciences, Office of Science under contract No. W ENG-38.