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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,

3

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.

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Acknowledgements

• Dr Gregory Beaucage.

• Dr George Skillas.

• Sun Chemical Corporation.

• Advanced Photon Source, ANL.

• Research Group and the Department.

• My friends and my family.