Pigments TiO2

7/28/2019 Pigments TiO2

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Mike DiebolDuPont de Nemours, Inc

[email protected]


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Important Properties of Small Particles

Particle size and shape Surface Area

Surface Energy

Friction and Bulk Flow ac ng an ens

y

Surface Charge

Absorption Scattering


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Surface area

Depends on particle size and shape. Increases linearl with article size decrease.

Typical values as a function of size:

rea m g

1 mm 0.003

mcrons .1 micron 310 nm 300

*Assumes density of 2 g/cc


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Origin of Surface Energy


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Magnitude of Surface Energy

epen s on two t ngs:

Amount of surface Chemical state of the surface atoms


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Consequences of Surface Energy

Particles are attracted to other surfaces. Particlesstick to ethe

Particles stick to bin walls, etc.

r1 r2

an er aa s orce equa on:

21

rrWF

L

21 rrLW = Constant


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Friction and Bulk Flow

Friction is a surface phenomenon: proportional to amount of contact between two surfaces

(i. e. surface area).

proportional to strength of contact between two. . .

Friction is a very significant force for small

Because of friction, the flow of particles (movingarticles ast one another is difficult.


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Friction between Small Particles

are very light

ne part c e can o up1,000,000 others!

Wall Pigment Bed


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Particle Packing

Depends on particle shape Round articles ack themostdensel

acicular (rod-like) particles pack least densely

Depends on particle size

arge par c es pac e mos ensey Small particles pack least densely


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Effect of Particle Shape on Packing


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Eff t f P rti l iz nP kinMaterial Particle Size True Density Bulk Density Void Fraction

SiO2 Sand 1,000 2.6 g/cc 1.6 g/cc 38%

TiO2 0.25 4.1 g/cc 0.8 g/cc 80%

fumed SiO2 0.015 2.2 g/cc 0.05 g/cc 98%

100%

tio

n

60%

Void

Fra

0%

20%

Packing

0.01 0.10 1 10 100 1000

Particle Size (microns)


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Static Electricity

Excess electrical charge on a particle will accumulate onthe particle surface.

c arge s too g or part c es are too c ose, t e sur acewill discharge (~10,000 volts/cm).

independent of particle size.

Therefore, the amount of excess charge in a pound of material.

Electrostatic forces from static charge become relatively moreimportant for smaller particles.


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in Slurries

+++ --

OHOH O--

+ H2

O

Acid (Low pH) Base (High pH)


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Surface Potential

60.0

(mv)

Basic SolidAcidic Solid

pH20.0

. TiO2 ZnO

6.0-20.0

.

2.0 4.0 8.0 10.0 12.0

-60.0

-40.0


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Some Solids are Both Acidic and Basic

O--O O

Si Si

O

Si Si

OO

O OH2+O

OO

O O O OH2+

O O O

Si Si

O O

HSi Si

O

H

--


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Some Solids are Both Acidic and Basic

Negative Faces


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Kaolinite

House of Cards


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Im ortant Pro erties ofPigments and Fillers

Particle size and shape Tinting properties

Oil Absorption

ur ace emstry

DispersionDensity / Bulking Value

Hidin Power O acit

Surface Charge

Influenceonfilmdurabilit

Manufacture


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Film PropertiesGloss, DOI

Degree of

Dispersion

Optimal forLight Absorption

Optimal forLight Scattering

0.1 1.00.5 5.00.2 2.0

Diameter (microns)

0.01 0.050.02


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Effect of Particle Shape on Paint Film Properties

Shape affects packing characteristics. Rod shaped particles improve film strength (like reinforcing

ars .

Rod shaped particles can hurt gloss (if the protrude from the

Platy particles overlap and make penetration by water, lightmore difficult (c. f., roof shingles).

Round particles have lesser effects on film strength,permeability, etc.


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Particle Porosity

Diatomaceous Earthan empty sphere

CPVC = 0.18

Fumed Silica

very open structureCPVC = 0.23


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Oil Absorption (OA) Value

Grams linseed oil required to form a paste from100 ram i ment.

Oil first coats surface and fills voids; excess

.

Depends on:

ur ace area ar c e sze e Packing - presence of voids / porosity (like CPVC)

Wetta ty


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Oil Absor tion vs. Surface AreaTiO2

Grade Surface Area Oil Absorption(m2/ ) lbs/100 lbs

R-700 9.6 12

R-706 11.4 14R-900 13.5 16

R-960 14.2 19

R-902 15.2 17R-931 42.1 37


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Effect of White Pigment on Tint

strong scattering

short path length

weak scattering

long path length


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Effect of Pigment Flocculation on Tin

If the white pigment flocculates: Color darkensbecause athlen thof li ht

increases.

Color lightens because flocculated particles do

not absorb li ht as efficientl as dis ersed

particles.


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Surface Chemistry

Hydrous oxide surfaces tend to beh dro hilic.

Pure oxide surfaces are marginally

.

Organic surfaces (carbon black, organic

.


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Surface Modification - Theory

pac y an o or - u proper

Refractive index

Absorption coefficient

Particle size

dispersibility, dispersion stability, bulk

an ng, p otocatayt c act v ty, rate oincorporation into polymer resin:

ALL ARE SURFACE PROPERTIES


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1. Surface Deposition

TiO2 Slurry Treated TiO2Soluble Oxide

Precursor

Change in

Conditions

NaAlO2 Al2O3nH2OH+

Na2SiO3 SiO2nH2OH+


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Surface Deposition


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Modifying Pigment Surfaces

Mild ox enation creates -OH and -O H

.

moieties on the surface of organic pigments.

COOH

OH

CO

- +

HOOC

COOH

COOHOH

OH


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Modifying Pigment Surfaces

Two forms:

.

Small, high functionality molecules

e. g., TMP, AMP

Grafted alkanes e. g.,

Si-RXX

Decrease surface energy

enera y ncrease spers y


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Dispersion of Pigment

Particles have a natural attraction to one another(Van der Waals attraction).

To keep them separated, must force them to repel

one another (electrostatic charge). Repulsion must be large enough to overcome

attraction:

Particle size Charge need for stability0.25 micron > 20 mv (or < -20 mv)

-.


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Dispersion Process

Wetting

[Van der Waals Attractions]

Stabilization

[Surface charge]


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Formation of Surface Charge

+++ --

OHOH O--

+ H2O



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Surface Potential

60.0

(mv) rea o stability

20.0

. TiO2 ZnO

Zone of instability

6.0-20.0

.

2.0 4.0 8.0 10.0 12.0

-60.0

-40.0 rea of stability

pH


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Formation of Surface Charge-H+

OHOH O-

+ H2O


dispersant

Odispersant


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Influence of Pigment on Film Durability

Major problem is UV component of sunlight Energetic enough to break chemical bonds

Organic binder reacts with oxygen and water to

eventually give CO2 + more H2O Inorganic pigments tend to be inert (high

thermodynamic stability)

Organic pigments can fade easily TiO2 has complex role on film durability


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TiO2 and Paint Durability

TiO2 is an EXCELLENT UV light absorber One article 0.25microns absorbs>99%of

UV from sunlight.

binder in a paint

,2

the UV light energy into chemical energy in

.


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Binder Protection by TiO2

PigmentedBinder


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Radical Reactions

TiO2 + UV light e- + hole+

e- + hole+ TiO2 + heat

- -hole+ + OH- OH

2 2O2

- + OH + (-CH2-) intermediatesO2

- + OH + intermediates CO2 + H2O

UV light + O2 + (-CH2-) intermediates 2 2


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Improving TiO2 Durability

Surface of TiO2 is reactive on exposure toUV li ht.

Solution - encapsulate surface with inert

TiO2


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Titanium Dioxide PigmentsTitanium Dioxide Pigments

Type I Type II Type II I Type IVProduct NameTiO2 min.%

LW94

R-90092

R-900, R-90180

R-96080

Chalking

Surface treatment

free

none

mediumresistant

Al2O3

mediumresistant

SiO2 +Al2O3

mediumresistant

SiO2 +Al2O3

Chalkin : loose i ment articles form on the surface from the

More complete encapsulation to protect TiO2 from UV free radical reaction

erosion of the binder as a result of photodegradation.


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Conclusions

Pigment and filler particles are PARTICLES.

do, but the relative importance of these laws

-surface forces; we are dominated by volume

.

Many particle properties translate directly to.

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Pigments TiO2