SOLSPERSE™ Hyperdispersants: More efficient ways of dispersing pigment in water-based

coatings

Andrew Shooter, Tom Annable, Stuart Richards

Lubrizol

Abstract

With increasing regulatory and environmental requirements, there is a drive towards developing water

based VOC free pigmented coating formulations with the same aesthetic properties as solvent based

systems. The pigment dispersant in addition, to providing fast wetting and desirable colouristic effects

will also impact the overall durability of the coating.

Novel anchor groups have enabled us to design improved dispersants which can be formulated into

coatings with superior jetness and colour strength. These dispersants can reduce the particle size in

much less time than conventional dispersants using less energy during the milling process while

providing the required stability and compatibility in the coating formulation. In addition, through

efficiency in design couple with judicious selection of stabilising chains, the dispersant can be optimised

to minimise its impact on water sensitivity and corrosion resistance.

After proving that these principles deliver superior dispersants on organic pigments and carbon black,

our recent focus has been to deliver the same benefits on high end inorganic pigments for industrial

applications such as transparent iron oxides. In this paper we will discuss these design principles and

show how this has translated into superior performance in waterborne industrial coatings

Introduction

To achieve desirable aesthetic properties in a coating a pigment should be well dispersed to a particle

size appropriate for the application. In some applications, high transparency is critical (smaller particles)

where as in other coating opacity and hiding are more important (larger particles). Pigment dispersants

are essential for stabilising pigment particles, preventing re-agglomeration which would otherwise lead

to instability in both the millbase or final coating in both the wet and dry state.

A well designed dispersant is critical, as in addition to achieving excellent tinctorial properties such as

such as gloss, haze and colour strength, there should be no negative impact on the final coating

properties such as water or corrosion resistance. Polymeric pigment dispersants are designed to have

a segmented structure with a pigment loving anchoring segment and a solvent loving stabilisation chain.

In aqueous dispersions, a hydrophobic anchor is advantageous as this will be more attracted to the

pigment surface. In aqueous systems stabilisation can be achieved by having either water soluble steric

chains or surface charge to enable electrostatic repulsion between the pigment particles. Dispersion in

a water borne system can be complicated by the dispersion media, as let down environments may be

of a different polarity and contain other thickeners, binders, co-solvents or surfactants. Changes in

electrolyte concentration or pH can also impact dispersion stability.

The amount of dispersant required is based on the pigment surface area. As a rule of thumb, 2 mg/m2

of polymeric Hyperdispersant is a good starting point. The theoretical amount of dispersant used in

formulations can be referred to as the agent on weight of pigment (AOWP), and the %AOWP can be

calculated by dividing the pigment surface area by 5. The optimum dosage is then determined by

preparing a ladder series based around this calculated value. There will always be an optimum AOWP

for a dispersant, using dispersants below the optimum AOWP provides too little surface coverage (low

dosage) and can lead to bridging flocculation whereas too much dispersant (high dosage) can lead to

depletion flocculation both of which will cause undesirable increases in viscosity

Figure 1.0 Optimising the dosage of a dispersant and the relationship to flocculation

A well designed pigment dispersant will have a well-defined structure with good control of molecular

weight and an optimum balance of stabilising segments to anchor segments. When this is achieved the

actual amount of dispersant required can be less than the theoretical requirement. This is advantageous

as less hydrophilic material is introduced to the coating formulation, which may eventually impact

coating durability by attracting water from the surface environment.

There are many ways to classify pigment dispersants. In this paper we have classified dispersants into

two main types Single Anchor (SA) dispersants which tend to be low molecular weight with only one

anchor group and Multiple Anchor (MA) dispersants which tend to be higher molecular weight with

several different anchor groups. MA dispersants benefit from being able to disperse many different

pigments as they will have an affinity for different surfaces.

Figure 2.0 Classification of dispersants

Dispersants anchor to the pigment surface using a combination of Van der Waals, ionic and hydrogen

bonding interactions. A good MA dispersant will have a distribution of functionalities that can achieve

all these functions.

The disadvantage of multiple anchor dispersants may be longer milling times due to the slower wetting

associated with the presence of higher molecular weight species. Lower molecular weight single anchor

dispersants which are faster diffusing can offer superior performance on selected pigments due to faster

wetting and stabilisation. Often a disadvantage of a single anchor dispersant is reduced milling stability

of the dispersion. We have developed novel Single Anchor technology that enhances the interaction

with the pigment surface and can provide equivalent stability to an MA dispersant. Figure 3.0 highlights

the ability of Lubrizol’s novel Single Anchor dispersant to reduce the particle size of a high surface area

carbon black pigment (25% Pigment loading,70% AOWP ) after 6, 60 and 240 minutes.

Figure 3.0 Dispersion of high surface area carbon black pigment with SA dispersant

0

50

100

150

200

0 100 200 300

Part

icle

szie

D50/

nm

Time / Minutes

Commercial Control, milling energy 0.5 KW/Kg

Novel Single Anchor, milling energy 0.15 KW/hr

The milling energy required for the novel SA dispersant is considerably less than the competitive control.

The novel SA dispersant reduces the pigment particle size to less than 60nm whereas the competitive

control does not achieve a particle size to less than 100 nm after 240 minutes. The resulting mill base

when let down into an automotive acrylic resin, produced a coating with exceptionally high jetness

(Mc>300). This novel dispersant is excellent for carbon black and organic pigments but it’s anchoring

group will not interact well with inorganic pigments.

The dispersion of carbon black, organic and inorganic pigments can only be achieved using a multi

anchor (MA) dispersant. It is challenging optimising the different functionalities in the dispersant

molecule for each surface. The chart below in Figure 4.0 shows how this is achieved by changing the

polarity of the anchor group. Mill base viscosity is a good indicator of dispersion performance, when this

is low the pigment can be considered well dispersed. Any flocculation will result in gelation or

sedimentation and the viscosity of the mill base will increase. The novel SA polymer as anticipated

performs very well on an industrial grade Carbon Black 7 but high viscosity occurs when dispersing a

Pigment Yellow 42. Dispersants MA1, MA2 and MA3 are experimental candidates which have different

polarity anchor groups. As the dispersion performance is improved on the Pigment Yellow 42 the

performance on Carbon Black 7 diminishes. The novel MA dispersant, which has chemically has a

different back bone is very effective across the pigment range.

Figure 4.0 Developing an effective MA dispersant and impact on millbase viscosity

Our novel multiple anchor (MA) dispersants are designed so they can be used at a dosage of around

15% to 20% less than current commercial materials. As the dispersants are more efficient we observe

less negative impact on final film properties such as water or corrosion resistance. In a carbon black

formulation at 25% pigment loading, the optimum AOWP using the Multiple Anchor (MA) dispersant is

reduced from 60% to 50%. Even at this lower dosage when the millbase was let down into an industrial

white base the tinctorial properties were better than the competitive control (Figure 4.0a). In an industrial

Pigment Yellow 42 formulation, the optimum AOWP using the MA dispersant is reduced from 4% to

0

1

2

3

4

5

6

Novel SA MA1 EXP MA2 EXP MA3 EXP Novel MA

Mill

bas

e vi

sco

sity

Pa.

S

Dispersant

Pigment Blue 15:1

Pigment Yellow 42

Pigment Black 7

3%. This results in an equivalent gloss and colour strength in an industrial white base to the competitive

control (Figure 4.0b).

Figure 4.0a Carbon black dispersion gloss and colour strength when let down into an industrial white

base

Figure 4.0b Pigment Yellow 42 gloss and colour strength when let down into an industrial white base

0

20

40

60

80

100

120

60°Gloss 20° Gloss Colour strength%

Glo

ss o

r colu

or

str

ength

valu

e

Competitive control - 60% AOWP

MA Dispersant - 50% AOWP

0

20

40

60

80

100

120

60° Gloss 20° Gloss Colour strength %

Glo

ss o

r colo

ur

str

ength

Competitive control - 4% AOWP

MA Dispersant - 3% AOWP

Dispersion of Transparent Iron Oxides

Transparent iron oxides used in wood finish or automotive coatings are much smaller than standard

iron oxides which make them agglomerate very strongly. They have a needle like shape and are more

challenging to disperse. The SEM images in Figure 5.0 highlight the similarities between a commercially

available red and yellow transparent iron oxide, most grades have needle like particles of size 20-100

nm.

Figure 5.0 SEM Images of transparent iron oxides

Yellow iron oxide Red iron oxide

We have determined the optimum conditions for milling these pigments using our novel MA dispersants.

Initial work focused on a low energy milling test using a paint shaker with 3mm glass beads. In Figure

6.0 the pigment loading was varied in water maintaining the ratio of dispersant to pigment constant at

20% AOWP. At 30% pigment loading, despite the viscosity being very low after milling, the particle size

was not reduced. We believe at this pigment concentration there was not enough shear in the milling

process. Despite the relatively short milling time (1hr) we observed good particle size reduction at 35

and 40% pigment loading. However at pigment concentrations above 45% we observed an increase in

viscosity and particle size.

Figure 6.0 Millbase viscosity and particle size at different concentrations of transparent iron oxide

At 40% pigment loading we investigated lowering the dosage of MA dispersant but the reduction in

particle size was not as efficient. Increasing the dosage to 25% improves the particle size reduction and

overall stability as shown in Figure 7.0.

Figure 7.0 Transparent iron oxide millbase viscosity and particle size at different loadings of dispersant

0

20

40

60

80

100

120

140

160

180

200

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

30 35 40 45

Pa

rtic

le s

ize

Za

ve

/nm

Mill

ba

se

vis

cosity /

Pa

.S

Pigment Loading %

Viscosity Pa.S Zave/nm

0

20

40

60

80

100

120

140

160

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

15 20 25

Part

icle

siz

e Z

ave/n

m

Mill

base v

iscosity

/Pa.S

Dispersant loading %AOWP

Viscosity Zave/nm

In order to improve transparency of the coating, milling on a paint shaker had to be increased to 8 hrs.

The Contrast Ratio Opacity (OP) was used to compare the effectiveness of the dispersion formulation

once the mill base was let down into an automotive acrylic clear coat. Contrast Ratio Opacity (OP) can

be determined after the coating has been applied to black and white card. Contrast Ratio Opacity (OP)

is the ratio of the Y value over the black card to the Y value over the white card. Y values are determined

using a spectrophotometer. As anticipated a high energy mill was more efficient at reducing the particle

size of the inorganic pigment and low opacity was achieved in 2.5hrs as shown in Figure 8.0

Figure 8.0 Opacity of yellow iron oxide dispersions in an automotive clear coat vs milling time

Yellow iron oxide was also dispersed using a high energy mill (40% pigment loading with 25% AOWP)

and we observed a milling time of 1.5 hrs to reduce the particle size to under 90 nm. Reducing the

particle size is important, but for exceptional transparency it is essential to break down all the large

aggregated particles. Examining the actual particle size distribution helps to identify dispersants that

are particularly effective at dispersing the transparent pigments. In Figure 9.0 the particle size average

was similar at 10, 60 and 150 minutes but the optimised dispersant MA2 was more effective than MA1

at breaking down agglomerated pigment particles of 1990-6440 nm size.

Figure 9.0 Particle size distributions of pigment dispersion during the milling process

6.9

4.9 5.0

0

1

2

3

4

5

6

7

8

Shaker mill 1hr Shaker mill 8 hrs High energy mill 2.5 hrs

Contr

ast

Ratio O

pacity

Dispersion conditions

We have observed a linear relationship between particle size and viscosity as shown in Figure 10.0

Figure 10.0 Relationship between particle size and opacity for yellow iron oxide dispersion

Using a well-designed dispersant for transparent iron oxides we have demonstrated that a contrast ratio

opacity of less than 4.0 can be achieved which is better than all the competitive dispersants that we

have evaluated.

3

3,5

4

4,5

5

5,5

6

60 80 100 120

Opacity (

OP

)

Particle size / nm

10

,1

18

,17

32

,67

58

,77

10

5,7

19

0,1

34

2

61

5,1

11

06

19

90

35

80

64

39

PARTICLE SIZE / NM

60 minutes

10

,1

18

,17

32

,67

58

,77

10

5,7

19

0,1

34

2

61

5,1

11

06

19

90

35

80

64

39

150 minutes

10

,1

18

,17

32

,67

58

,77

10

5,7

19

0,1

34

2

61

5,1

11

06

19

90

35

80

64

39

INTE

NSI

TY

10 minutes

MA1 MA2

Dispersion of Titanium Dioxide

Titanium dioxide pigments present a different challenge for our dispersion technology, the key attributes

for this pigment type is high opacity and whiteness. TiO2 pigment properties are determined by the base

pigment and finishing process. We have developed a well-designed MA dispersant for titanium dioxide

and have demonstrate the effectiveness on several titanium dioxide grades at a pigment concentration

of 75% with 2% AOWP. In our screen tests the pigments were dispersed in water with a small quantity

of humectant and defoamer. The MA dispersant provided a very stable millbase with respect to viscosity

and was more stable than the four competitive controls as shown in Figure 11.0.

Figure 11.0 Dispersion of titanium dioxide with MA dispersant vs competitive controls

The resulting titanium dioxide millbase was let down into an acrylic resin (25% TiO2 in coating). The MA

dispersant produced the highest Berger whiteness at equivalent opacity to the competitive controls Over

all the gloss of the coating was improved, with the 20o gloss significantly better than competitive controls.

We have discovered that producing a stable low mill base viscosity translates to improved coating

properties. The spider diagrams in Figure 12.0 highlight the benefits of our MA dispersant on two

different titanium dioxide pigments. Grade 1 is manufactured by the chloride process with Al and Si

treatment, whereas Grade 2 is manufactured by the sulphate process with Al and Zr treatment.

Viscosity, storage stability, particle size stability, drawdown quality (gloss and bits), Berger whiteness

and opacity have all been rated out of 10. Note that the MA dispersant shows improvements in coating

quality on both cases over the four competitor samples.

Figure 12.0 Performance of MA dispersant on titanium dioxide pigments

0 weeks 40°C2 weeks 40°C

4 weeks 40°C

0

0,2

0,4

0,6

0,8

1

Mill

bas

e vi

sco

sity

Pa.

S

0 weeks 40°C 2 weeks 40°C 4 weeks 40°C

Grade 1 Grade 2

The MA dispersant in addition to providing fast wetting and desirable colouristic effects must not

negatively impact the overall durability of the coating. For this reason a further study was conducted to

assess the corrosion performance of our novel MA dispersant. Titanium dioxide pigment was dispersed

at 55% with 2.6% AOWP using a high-speed disperser until a fineness <20µm was achieved. The

resulting millbase was let down into either a Waterborne 1K acrylic paint or Waterborne 2K epoxy paint

and coated on to cold rolled steel. All coatings were air dried for seven days then subjected to salt spray

or salt immersion in an 5% NaCl solution at 35oC. In figure 13.0 shows the panels with the acrylate

system at the end of this test. Note in this system there is 0.5% Dispersant in the total formulation.

Figure 13.0 Corrosion testing of titanium dioxide dispersion in acrylic coating

In the 1 K acrylate system, it is obvious that the Competitor 2 sample is failing, this was to be expected

as the type of dispersant was a sodium polyacrylate type and very hydrophilic. The novel MA dispersant

showed improvement over Competitor 1 which had more blisters. The same result was observed in a

2K epoxy system as shown in Figure 13.0

Figure 13.0 Corrosion testing of titanium dioxide dispersion in 2K Epoxy coating

Conclusion

The design of pigment dispersants is very important for the dispersion of pigments. Single anchor (SA)

dispersants work very effectively on specific pigments whereas multi anchor (MA) dispersants have

better compatibility with many pigment types and can help disperse carbon black, organic and inorganic

pigments. We have highlighted the performance of MA type dispersants on inorganic pigments and

challenges in designing these molecules. We have been able to synthesise MA dispersants that work

well not only on both opaque or transparent iron oxides but also titanium dioxide. As our MA dispersants

have been designed to minimise hydrophilicity in the molecule we see benefits in the final coatings with

respect to corrosion resistance. We have also demonstrated that we can use our MA dispersants at a

lower dosage than competitive controls.

Disclaimer

The information contained herein is believed to be reliable, but no representations, guarantees or

warranties of any kind are made as to its accuracy, suitability for particular applications or the results to

be obtained. The information is based on laboratory work with small scale equipment and does not

necessarily indicate end product performance. Because of the variations in methods, conditions and

equipment used commercially in processing these materials, no warranties or guarantees are made as

to the suitability of the products for the applications disclosed. Full-scale testing and end product

performance are the responsibility of the user. Lubrizol Advanced Materials, Inc. shall not be liable for

and the customer assumes all risk and liability of any use or handling of any material beyond Lubrizol

Advanced Materials, Inc.’s direct control. The SELLER MAKES NO WARRANTIES , EXPRESS OR

IMPLIED , INCLUDING , BUT NOT LIMITED TO , THE IMPLIED WARRANTIES OF

MERCHANTABILITY AND FITNESS FOR A PARTICULAR

PURPOSE. Nothing contained herein is to be considered as permission, recommendation, nor as an

inducement to practice any patented invention without permission of the patent owner.