A new concept for dispersing agents in aqueous coatings

JoÈrg Schmitz*, Harald Frommelius, Ulrich Pegelow, Heinz-GuÈnther Schulte, Rainer HoÈfer

Henkel KGaA, Henkelstr. 67, 40191 DuÈsseldorf, Germany

Received 15 July 1998; accepted 20 February 1999

Abstract

After a short overview about the development of synthetic dispersing agents for solvent-borne coatings up to the actual state of technique,

analogous concepts of polymeric additives for water-borne coatings are introduced. The performance of these polyfunctional dispersants will

be critically reviewed and described in comparison to dispersing agents based on new concepts. With these new concepts using mono- and

difunctional oleochemical agents or a combination of them it is often possible to reach better performance than with polyfunctional polymers.

During our study different physicochemical methods (Zeta potential, adsorption, desorption) and paint application tests including numerous

binders and pigments were done. These investigations lead to the discussion of new models for the dispersing of pigments in water-borne

coatings and to more knowledge about the design of a dispersant to reach high gloss, optimal rub-out and high color-strength. Furthermore

the formulation of solvent- and resin-free water-borne pigment pastes is possible using dispersing agents selected by the new concepts.

q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Dispersing agents; Pigment-stabilisation; Oleochemical block co-polymers; Monofunctional dispersants; Aololitive combinations

1. Introduction

In the colorful world of paints and coatings pigments are

an important component in the formulation. These pigments

are made to have an optimum particle size yields maximum

hiding power and color strength. During transportation and

storage, however, the so called primary particles build up to

form agglomerates and aggregates. In the process of paint

production it is important to break down these structures

back into ®ne particles and to stabilize this state [1±3].

Only this provides good color strength [4,5], good color

stability [6±8] and also prevents pigment settling [9,10].

First step in the dispersion process is the wetting of the

particles by the liquid phase, which consists of mainly

solvent and binder, followed by mechanical destruction of

the agglomerates. Stabilizing this stage and preventing reag-

glomeration is the designated task of the dispersing agent

[11].

There is a signi®cant difference in this process between

solvent-borne and water-borne systems. The wetting step in

solvent-borne systems is generally quite easy because of the

low surface tension of ordinary solvents and resins. Special

additives to improve wetting are not required.

² Xylene: 30 mN/m

² Melamine resins: 58 mN/m

² Butylacetate: 25 mN/m

² Oil-free polyesters: 47 mN/m

² White spirit: 24 mN/m

² PMMA: 41 mN/m

² Soya-bean long oil alkyd: 37 mN/m

Due to the extremely high surface tension of water,

special additives are required to lower the surface tension

and enable suf®cient pigment wetting [12]

² Aqueous dispersions: 35±45 mN/m

² Water: 72.7 mN/m

In addition, stabilization of pigments in aqueous systems

functions by a different mechanism. While in solvent-borne

systems a sterical barrier prevents reagglomeration

[2,13,14] water-borne systems require the ionic mechanism

of electrostatic repulsion [2,16]. In architectural latex

paints, by far the largest area of water-based paints, anionic

dispersing agents like polyacrylic acid [15] are widely used.

2. Fundamentals of dispersing agents in solvent-bornesystems

Already in 1957 Henkel introduced their ®rst synthetic

dispersing agent under the name of Texaphorw [17]. Mono-

functional dispersing agents like Texaphor provide good

stabilization of inorganic pigments and ef®ciently prevent

Progress in Organic Coatings 35 (1999) 191±196

0300-9440/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.

PII: S0300-9440(99)00028-4

* Corresponding author.

pigment sedimentation. With the introduction of high grade

organic pigments as well as extremely durable but poorly

wetting polyacrylic binders, the need for the development of

new dispersants arose. Those products are oligomeric to

polymeric and have a plurality of anchoring groups

[2,3,18±21] (Fig. 1).

The introduction of two pack polyurethane systems in the

automotive re®nish and even OEM markets boosted the

demand for polymeric dispersants. Various structures of

anchoring groups and steric chains are discussed in the

literature.

3. Structures of dispersing agents for aqueous systems

The area of aqueous coatings is dominated by architec-

tural latex coatings where inorganic pigments and ®llers are

stabilized by polyacrylic acid (PAS) [15] or maleic acid

derivatives. Organic pigments in colorants were stabilized

mainly with alkyl phenol ethoxylates [22,23]. With the

appearance of new resins for aqueous industrial coatings

in recent years better additives which allow for higher

gloss, better color stability and better water resistance had

to be developed. Furthermore, paint formulators are moving

away from nonyl phenol ethoxylates because of ecotoxico-

logical discussions connected to them [24].

For the development of a new class of dispersants for

aqueous coatings, concepts used in solvent-based coatings

were initially transfered to the water-borne ®eld. Several

additive suppliers introduced individual polyfunctional

products [25±27] with the general structure as follows

(Fig. 2):

² anchoring groups with good adhesion to pigments;

² ionic groups for electrostatic stabilization;

² bulky hydrophilic groups for water-phase steric stabili-

zation.

This fairly new generation of additives has been available

to the market for several years. Even though this concept

looks convincing, market success seems to be quite limited.

The fairly high price of the products also hurts market pene-

tration.

These were the reasons why Henkel started searching for

alternative concepts based on oleochemical raw materials.

An interesting structure was found in the latex polymeriza-

tion surfactant department. This material was developed as a

substitute for Nonylphenolethoxylates [28] and can be

described as monofunctional Oleo-Alkyleneoxide Block

co-polymer (Oleo ABC) (Fig. 3). Variations of this structure

lead to longer or shorter chain, ionic or alkyleneoxide-free

monofunctional types. In addition, similarly constructed di-

and trimer structures of the ABC-compounds are available.

In cases where electrostatic stabilization is required,

oligofunctional phosphonates provide a reasonable alterna-

tive to polyacrylic acid and maleic acid copolymers with

excellent properties (Fig. 4).

Based on these building blocks the new concept combines

selected mono- or oligofunctional molecules to a universal

mixture at a reasonable cost (Fig. 5).

In the practical work discussed later a proof of this

concept is examined. Aside from the principal work of

pigment dispersing some structure/performance relations

of oleo ABCs will be reported. The following work was

carried out:

² Physicochemical measurements (adsorption and deso-

rbtion studies, Zeta-potential),

² Coating lab tests in aqueous coatings.

² Tests in pigment pastes.

J. Schmitz et al. / Progress in Organic Coatings 35 (1999) 191±196192

Fig. 1. Dispersants for solvent-based coatings.

Fig. 2. Polyfunctional dispersants for aqueous coatings.

4. Physicochemical measurements

4.1. Zeta-potential

Zeta-potential measurements were carried out with the

Laser Zee Meler 501 (Pen Kern).

As expected, non-ionic oleo-ABCs do not signi®cantly

in¯uence the Zeta-potential. The polyfunctional polymeric

additives with a limited number of ionic groups moderately

reduce the Zeta potential (to higher negative numbers), e.g.

in case of titanium dioxide (Kronos 2310) from 2 30 to 245 mV. Other ionic oleo-ABCs as well as polyacrylic acid

derivatives and the organophosphonates create even higher

negative values, up to 2 60 mV on TiO2. This ®gure means

that electrostatic stabilization is the dominant and effective

stabilization mechanism.

4.2. Quantities of absorbed dispersant on Titanium dioxide

Results:

² excellent absorption of mono-oleo-ABCs with low HLB

(approx. 2 mg/m2);

² no adsorption of mono-oleo-ABCs with medium and

high HLB;

² increased adsorption of dimer and trimer oleo-ABCs

compared to monomeric ones of equal HLB;

² good adsorption of ionic mono-oleo-ABCs (0.8 mg/m2);

² good adsorption of polyfunctional (polymeric) disper-

sants (0.8 mg/m2) little adsorption of polyacrylic acid

and oligophosphonates (0.3 mg/m2).

As expected hydrophobic organic pigments such as Chro-

mophthalred 2B (Ciba) and Heliogenblue L 6900 (BASF)

gave signi®cantly different results:

² excellent adsorption of mono-oleo-ABCs with low and

medium HLB (1.5±2 mg/m2);,

² good adsorption of ionic and non-ionic mono-oleo-ABCs

with high HLB-value (approx. 1 mg/m2);

² good adsorption of dimer and trimer oleo-ABCs and

polyfunctional (polymeric) types (0.5±0.8 mg/m2);

² no adsorption of polyacrylic acid and oligophosphonates.

4.3. Comprehensive desorption studies

A certain amount of the particular dispersant was ®rst

adsorbed on the pigment surface and the loaded pigment

separated from the solution. This was mixed with pure

water and the amount of desorbed pigment determined

according to DIN 38409.

² Monofunctional types desorb with 5% less than difunc-

tional (15%) and polyfunctional types (11%).

² In aqueous systems it seems that the amount of adsorbed

material on the pigment surface is much more deter-

mined by the HLB than by the number of anchoring

groups.

² Polyfunctional types do not show an advantage

compared to monofunctional types per se.

5. Results in aqueous coatings

A mixture of TiO2 and Phthaloblue (20:1) was used as

reference. Water-based alkyds and polyesters, polyurethane

and polyacrylate dispersions as well as alkyd and polyester

emulsions were studied. Some resins were air drying, others

stoving or two-pack-systems.

Two general ®ndings of these studies were of particular

interest:

(1) The binders had a strong in¯uence on the pigment

dispersing process. We could distinguish between: (a)

good wetting resins (water-soluble alkyds, polyesters

and polyurethane dispersions) and (b) non-wetting resins

J. Schmitz et al. / Progress in Organic Coatings 35 (1999) 191±196 193

Fig. 3. New Oleo-Alkyleneoxide-Block-Copolymer dispersants.

Fig. 4. Ionic dispersants for aqueous coatings.

(emulsion polymers and emulsi®er stabilized polyesters,

alkyds and epoxies).

Conclusion 1: The resin has a strong in¯uence on the

selection of the correct dispersant.

(2) The second general ®nding was that gloss, color

strength and rub-out were not simultaneously determined

by a speci®c chemical structure. Furthermore, certain

chemical structures will give high gloss, others high

color strength and others excellent rub-out.

Conclusion 2: The combination of different chemical

structures enable optimal gloss, color strength and rub out.

6. Correlation of physicochemical measurements withpractical results

The statistical results out of tests with 22 different resins

gave clear indications regarding suitable dispersants and

dispersant combinations. They correlate very well with the

physicochemical studies. Also, some structure/performance

correlations were found. The results were also useful in

®nding new effective dispersant combinations.

6.1. Color strength

To achieve an excellent color strength in a paint with a

pigment mixture of an inorganic pigment like TiO2 and a

hydrophobic organic pigment like Heliogenblue the disper-

sant has to have a good adsorption on the organic pigment

and good stabilizing properties. These requirements,

however, are contradictory. A hydrophobic additive (low

HLB) shows strong adsorption. The short hydrophilic

chain on the other hand can only provide limited steric

stabilization. The compromise here is equivalent to the

well-known nonylphenol ethoxylate £ 10EO [20] in the

range of medium HLB-values.

The ionic and oligomeric types have a small adsorption

potential and showed poor results. Polyacrylic acid and

oligophosphonates do not adsorb on organic pigments and

are not useful here. With regard to the nature of the resin we

did not ®nd any signi®cant in¯uence on color strength.

A mono-ole-ABC with medium HLB gives good color

strength also in dispersant mixtures even if the other compo-

nents by themselves do not give good results with regard to

color strength.

6.2. Rub-out

For a good rub-out, wetting and stabilizing of the organic

and the inorganic pigment are important. Contrary to color

strength, the nature of the resin has signi®cant impact on

rub-out.

Resins with pigment-wetting properties often provide

stabilization of the inorganic pigment, and, in some cases

even the organic pigment. Best results are achieved with an

additive which wets and stabilizes the organic pigment

(similar to color strength).

In formulations where the resin is unable to wet and

stabilize the pigments this has to be done entirely by the

dispersant. Here additives are required which will adsorb on

inorganic as well as organic pigments and provide both

excellent steric and electrostatic stabilization. Ionic Mono-

Oleo-ABCs, polyfunctional additives and additive-mixtures

are the only dispersants capable of this.

Oleo ABCs with low HLB-values do absorb on both

kinds of pigments but their steric stabilization is poor

because of their short hydrophilic chain.

J. Schmitz et al. / Progress in Organic Coatings 35 (1999) 191±196194

Fig. 5. The new concept of additiv-mixtures for aqueous coatings.

Table 1

Results in pigments pastes

Organic pastes Inorganic pastes

Mono-Oleo-ABC (low HLB) 11 11

Mono-Oleo-ABC (medium HLB) Hydropalat 1080 111 0

Mono-Oleo-ABC (high HLB) 11 0

Ionic Mono-Oleo-ABC 1 1

Tri-Oleo-ABC (medium HLB) 1 11

Oligophosphonate 0 1

Mono-Oleo-ABC 1 Oligophosphonate Hydropalat 1080 1

Hydropalat 3204

11 111

J. Schmitz et al. / Progress in Organic Coatings 35 (1999) 191±196 195

Tab

le2

Dis

per

san

tre

com

men

dat

ion

sfo

raq

ueo

us

coat

ing

s

Ad

dit

ive

Ty

pe

Sta

bil

izat

ion

Posi

tive

in¯

uen

ceon:

Posi

tive

in:

Ele

ctro

-

stas

tic

Ste

ric

Org

an.

pig

men

ts

Anorg

pig

men

ts

Glo

ssW

ater

resi

stan

ce

Colo

r-

acce

pta

nce

Poorl

y

wet

ting

resi

ns

Good

wet

ting

resi

ns

2K

-PU

R

syst

ems

Dis

po

nil

O5

Mo

no

-Ole

o-A

BC

0(1

)1

11

11

11

12K

-Epoxy-S

yst

ems

Hy

dro

pal

at5

35

NO

lig

oca

rbo

xy

late

11

01

01

10

21

1W

ater

-solu

ble

resi

ns

Hy

dro

pal

atz

75

9O

lig

oph

osp

ho

nsaÈ

ure

11

02

11

02

01

01

TiO

2-s

lurr

ys

and

pas

tes

Hy

dro

pal

at8

93

Mo

no

-Ole

o-A

mid

0(1

)1

11

11

01

11

Hig

hglo

ssco

atin

gs

Hy

dro

pal

at1

08

0M

ono

-ole

o-A

BC

01

11

21

11

11

11

1O

rgan

icpig

men

tpas

tes

Hy

dro

pal

at3

03

7O

lig

o-o

leo

-AB

C0

11

21

11

11

11

Colo

r-ac

cepta

nce

Hy

dro

pal

at3

06

5M

ono

-ole

o-A

BC

01

11

21

01

11

11

1O

rgan

icpig

men

tpas

tes

Hy

dro

pal

at3

20

4A

min

esa

lto

f7

59

11

02

11

02

01

02

TiO

2sl

urr

ys

and

pas

tes

Hy

dro

pal

at3

21

6A

dd

itiv

em

ixtu

re1

11

11

11

10

11

11

11

Univ

ersa

lin

aqueo

us

coat

ings

Hy

dro

pal

at3

27

5P

oly

-Ole

o-A

BC

11

11

10

11

11

1P

igm

ent

pas

tes

Additive mixtures perform very well with regard to rub-

out as long as they contain one component with suf®cient

stabilization for inorganic and organic pigments or one

component for inorganic and another component for organic

pigments.

6.3. Gloss

High gloss values are determined by two factors. First, all

pigments must be optimally stabilized (see also rub-out).

Secondly the (absorbed) additive must be very compatible

with the resin.

This was the case in most of the studied systems with

Mono-Oleo-ABCs with low HLB-values. Reduced gloss is

caused by additives with higher HLB-values. Ionic types

also lower the gloss depending on their content of ionic

moieties.

Mono-Oleo-ABCs with low HLB-values in additive

mixtures generally have a positive in¯uence on gloss.

7. Results in pigment pastes

Due to economical advantages, pigment pastes have

become more important in recent years [29]. Besides the

well established use of pastes in on-site mixing systems in

automotive re®nish or architectural tint shops, more and

more paint companies use ready made pastes for the manu-

facture of various kinds of industrial, automotive or wood

paints. The need for solvent-and binder-free pastes based on

Nonylphenol-free dispersants has become more important

[24].

Some of the above mentioned dispersants are very suita-

ble for use in pigment pastes. Tables 1 and 2 give an over-

view of our results.

8. Conclusions

Physicochemical and technological results of this work

proof that a direct transfer of knowledge from the classical

solvent borne systems is only possible to a certain extend.

The selection of a suitable dispersing agent is not only

in¯uenced by the type of pigment but often to a larger extent

by the type of binder.

Furthermore the properties high gloss, excellent color

strength and color stability are often linked to individual

agents while in solvent-borne systems often one additive

ful®ls all requirements. Water-based coatings, therefore,

need several different additives or sophisticated combina-

tions of them.

A combination of additives with a compatible agent with

low HLB (for high gloss), an (often ionic) agent with excel-

lent adsorption on inorganic surfaces (good rub-out), and

another one with strong adsorption on organic pigments

(good color strength/rub-out) creates an optimal additive.

Such an additive like Hydropalat 3216, provides universal

performance in a variety of different resins with numerous

pigments.

To formulate solvent- and binder-free pigment concen-

trates with inorganic and organic pigments, the polyfunc-

tional additive Hydropalat 3275 does an excellent job.

However, the monofunctional Hydropalat 1080 for organic

pigments and a combination of Hydropalat 1080 with

Hydropalat 3204 for inorganic pigments respectively often

provides a reasonable alternative.

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