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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.
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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, 2
R
sin22.0~RThe optimum size of the aggregate can be
estimated by integrating the Guinier Law,
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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.
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Fractal Structures – have Fractional Dimension
• Surface Fractal Object, (ds)
Irregular Surface
• Mass Fractal Object, (df)
Irregular Structure
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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)
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Regimes of Aggregation
• Diffusion Limited Aggregation, df < 2 (df ~ 1.8)
• Reaction Limited Aggregation, df > 2 (df ~ 2.5)
• Intermediate Regime
Transport
Limited
Reaction
Limited
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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 Scattering
2R
Best Scattering
sin22.0~R
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.
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Scattering in Organic Pigments
Mie ScatteringDilute Systems, Large Particles,
Higher Index Difference
Rayleigh Ganz ApproximationAll Systems, All Particles Particles,
Lower Index Difference
For X-Rays, the comparable contrast difference is very small between the pigment and the polymer.
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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 0.0001
B3
B2
B1
Log
(I)
Log (q) (q in Å-1)
- 4
- df
- 4
G2,
Rg 2
G1,
Rg 1
df is the mass fractal dimensionFor fractal objects,
fdRM ~
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Unified Function for Mass Fractal Model
Unified Function, used to fit the scattering data, is based on six parameters,
• Guinier Prefactors: G1 and G2
• Radius of Gyration: Rg1 and Rg2
• Power law Prefactor: B1
• Fractal Dimension: df
The diameter of a sphere having similar Rg as the aggregate can be used to estimate the size of the aggregate.
RgD 6.2~
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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.
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Diazo Pigment Red 170 (235-0170)
TEM – Dpp = 0.2 m
LS – Dagg = 0.4 m
Powder – Non Mass Fractal
Dpp = 0.2 m
20% in PMMA – Mass Fractal, df = 2.5 (RLA)
Dagg = 2.35 m
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Diazo Pigment Red 170 (235-1170) Higher Luminosity
TEM – Dpp = 0.15 m
LS – Dagg = 0.35 m
Powder – Non Mass Fractal
Dpp = 0.156 m
20% in PMMA – Mass Fractal, df = 2.67 (RLA)
Dagg = 2.01 m
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0.001
0.01
0.1
1
10
100
Inte
nsi
ty (
a.u
.)
0.0012 3 4 5 6 7 8 9
0.012 3 4 5 6 7 8 9
0.12 3 4 5 6 7 8 9
1
q (Å)-1
Plain PP
1% PY14 in PP
1% PV19 in PP
5% PR122 in PP
1% PR122 in PP Peak for PP (q ~ 0.027 Å)
Shifted Peak for PP (q ~ 0.048 Å)
• 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.
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108
106
104
102
100
10-2
10-4
10-6
Inte
nsi
ty (
a.u
.)
0.0001 0.001 0.01 0.1 1
q (Å)-1
-4
-4
-1.91
-4
-1.5
0.13 µm
0.13 µm
0.13 µm12 µm
12 µm
Raw Data PR122 Powder Unified Fit PR122 Powder Raw Data PR122 (1% in PP) Unified Fit PR122 (1% in PP) Raw Data PR122 (5% in PP) Unified Fit PR122 (5% in PP)
Monoazo Pigment Red 122
TEM – Dpp = 0.1 m (length)
LS – Dagg = 0.2 m
Powder – Non Mass Fractal
Dpp = 0.13 m
1% in PP – Mass Fractal, df = 1.91 (DLA)
Dpp = 0.13 m
5% in PP – Mass Fractal, df = 1.5 (DLA)
Dpp = 0.13 mC. I. PR 122
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1010
108
106
104
102
100
10-2
10-4
Inte
nsi
ty (
cm)-1
10-5
10-4
10-3
10-2
10-1
100
q (Å)-1
-4
-4
-2.32
-1.55
-1
-4
-4
0.03 µm
1.2 µm
0.16 µm
1.0 µm
0.26 µm
Raw Data PV19 Powder Unified Fit PV19 Powder Raw Data PV 19 (1% in PP) BGS Corr Data PV19 (1% in PP) Unified Fit PV19 (1% in PP) Raw Data PV 19 (5% in PP) Unified Fit PV19 (5% in PP)
Monoazo Pigment Violet 19
TEM – Dpp = 0.05 m
LS – Dagg = 0.4 m
Powder – Mass Fractal, df = 2.32 (RLA)
Dpp = 0.03 m, Dagg = 1.2 m
1% in PP – Mass Fractal, df = 1.55 (DLA)
Dpp = 0.16 m, Dagg = 1.0 m
5% in PP – Mass Fractal, df = 1 (DLA)
Dpp = 0.26 mC. I. PV 19
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1010
108
106
104
102
100
10-2
10-4
Inte
nsi
ty (
cm)-1
10-5
10-4
10-3
10-2
10-1
100
q (Å)-1
-4
-4
-4
-2.34
-2.7
0.035 µm
0.32 µm
0.43 µm
0.052 µm
Raw Data PY13 Powder Unified Fit PY13 Powder Raw Data PY13 (5% in PP) Unified Fit PY13 (5% in PP)
Disazo Pigment Yellow 13
TEM – Dpp = 0.1 m
LS – Dagg = 0.5 m
Powder – Mass Fractal, df = 2.34 (RLA)
Dpp = 0.035 m, Dagg = 0.32 m
5% in PP – Mass Fractal, df = 2.7 (RLA)
Dpp = 0.052 m, Dagg = 0.49 m
C. I. PY 13
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1010
108
106
104
102
100
10-2
10-4
10-6
Inte
nsi
ty (
cm)-1
10-5
10-4
10-3
10-2
10-1
100
q (Å)-1
-4
-4-2.77
-2.63
0.125 µm
0.52 µm
0.08 µm
0.43 µm
Raw Data PY14 Powder Unified Fit PY14 Powder Raw Data PY14 (5% in PP) Unified Fit PY14 (5% in PP)
Disazo Pigment Yellow 14
TEM – Dpp = 0.1 m
LS – Dagg = 0.5 m
Powder – Mass Fractal, df = 2.63 (RLA)
Dpp = 0.125 m, Dagg = 0.52 m
5% in PP – Mass Fractal, df = 2.77 (RLA)
Dpp = 0.08 m, Dagg = 0.43 m
C. I. PY 14
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108
106
104
102
100
10-2
10-4
Inte
nsi
ty (
cm)-1
0.0001 0.001 0.01 0.1 1
q (Å)-1
-4
-4
-4-1.62
-1.4
0.14 µm
0.14 µm
0.12 µm
4.5µm
1.1µm
Raw Data PY83 Powder Unified Fit PY83 Powder Raw Data PY83 (1% in PP) BGS Data PY83 (1% in PP) Unified Fit PY83 (1% in PP) Raw Data PY83 (5% in PP) Unified Fit PY83 (5% in PP)
Pigment Yellow 83
TEM – Dpp = 0.1 m (length)
LS – Dagg = 1.2 m
Powder – Non Mass Fractal
Dpp = 0.14 m
1% in PP – Mass Fractal, df = 1.4 (DLA)
Dpp = 0.14 m, Dagg = 1.1 m
5% in PP – Mass Fractal, df = 1.62 (DLA)
Dpp = 0.12 m, Dagg = 4.5 mC. I. PY 83
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Phthalocyanine Pigment Green 7
TEM – Dpp = 0.05 m
LS – Dagg = 0.1 m
Powder – Non Mass Fractal
Dpp = 0.018 m
50% in PE – Mass Fractal, df = 1.4 (DLA)
Dpp = 0.036 m, Dagg = 0.767 m
X=Cl
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Effect of Pigments on the lamellar thickness of PP
0.001
0.01
0.1
1
10
100
Inte
nsi
ty (
a.u
.)
0.0012 3 4 5 6 7 8 9
0.012 3 4 5 6 7 8 9
0.12 3 4 5 6 7 8 9
1
q (Å)-1
Plain PP
1% PY14 in PP
1% PV19 in PP
5% PR122 in PP
1% PR122 in PP Peak for PP (q ~ 0.027 Å)
Shifted Peak for PP (q ~ 0.048 Å)
The Long Period decreased from 233Å to 131Å on addition of pigments
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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.
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Future Work
• Carry out mass fractal analysis using different pigments from different classes and different polymers.
• Study of processing effects on the nature of aggregates.
• Study of effect of additives on the behavior of aggregates.
• Mass fractal analysis of digital electron micrographs of organic pigment powders and polymer samples.
• Simulations of the process of formation of aggregates starting from primary particles.
• Simulate growth processes for different systems like asymmetric particles, polydispersity and roughness of surfaces.