Controlled polymers for pigment dispersants

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Controlled polymers for pigment dispersants
Clemens Auschra, Ernst Eckstein, Ralf Knischka, Frank
Pirrung, Peter Harbers.
Techniques based on controlling free radical polymerization
have recently been commercialized for pigment dispersions.
Such systems based on acrylic block copolymers show
good dispersant properties and could help to meet the
challenges of future coatings.
Polymeric pigment dispersants are an indispensable class of
additives to realize high quality pigment dispersions for use
in modern paints [1-3]. A uniform and stable dispersion state
of the pigments is necessary to achieve good colouristic
properties and high gloss in the final coating. In addition,
well-selected pigment dispersants can contribute to
increased production economics by allowing higher pigment
loads and faster grinding. A prominent example of advanced
production concepts is the use of resin minimal or resin free
pigment concentrates.
New developments in pigment dispersants are geared
towards more efficient products which offer superior
rheological performance and which can be applied broader
in different coating systems. The polymer architecture and
the type of pigment affinic anchoring groups are key
parameters to tailor dispersant performance [4,5]. Good
control on polymer architecture requires appropriate
advanced polymerization techniques. Only very recently has
the novel pioneering technique of controlled free radical
polymerization (CFRP) been commercialized in the field of
pigment dispersants [6].
In this contribution acrylic block copolymer type "controlled
pigment dispersants" are presented based on controlled free
radical polymerization technology. Examples are given of
how the polymer design capabilities of CFRP can be
translated into improved solutions in pigment dispersion.
Novel polymerization regulators for CFRP
Amongst the different methodologies for controlled free
radical polymerization which have been studied in academia
and industrial research, the nitroxyl-mediated controlled free
radical polymerization has proven very useful for the
synthesis of defined block copolymers [7, 8]. The chemical
mechanism of nitroxyl-mediated controlled free radical
polymerization is based upon the reversible capping of a
growing polymer chain radical by a stable nitroxyl radical
(Figure 1). During polymerization, only a very small
concentration of active polymer chains is present in
equilibrium with "dormant" polymer chains. This reduces
unwanted side reactions and, if certain kinetic conditions are
fulfilled, a "controlled" polymerization results. This leads to
well-defined polymers with narrow molecular weight
distribution [9]. The "living" character of the controlled
polymerization also enables the synthesis of block
copolymers by sequential addition of different monomers.
New classes of sterically hindered alkoxyamine compounds,
like special open chain NOR [10], piperazinone-type NOR
[11], piperidine-type NOR [12] and 7-ring heterocyclic NOR
[13] have been developed. These NOR polymerization
regulators represent a versatile and robust toolbox for the
synthesis of functional polyacrylates with controlled
structure. Some representative examples of NOR
compounds, which have been shown to be especially useful
for the synthesis of polyacrylate dispersants are shown
(Figure 2).
The reaction conditions for the use of such NOR regulators
are compliant with the basic requirements of industrial
polymer production, e.g. there are no stringent requirements
on the purity of the raw materials and the process does not
require special precautions for the handling of very reactive
or toxic compounds.
A representative example for the controlled polymerization
of butylacryate using the regulator NOR 3 is shown (Figure
3). The GPC-analysis of the polymer formed at different
polymerization times displays all the elements of a
well-controlled polymerization process: The molecular
weight Mn continuously increases with monomer conversion
and the molecular weight distribution stays narrow without
broadening towards the low molecular weight side.
Synthesis of controlled block copolymer dispersants
Controlled polymerization using the specific NOR
compounds shown in figure 2 enables the synthesis of
acrylic block copolymers by sequential polymerization of
different monomers or monomer compositions. A series of
different AB-type dispersants containing an aminic
anchoring block was synthesized by using the regulators in
Figure 2; Table 1. All samples are very comparable in
molecular weight in the range of 10000 to 12000. Effects of
the variation of the molecular weight of similar AB-type block
copolymers have been described elsewhere [14]. The steric
stabilizer block of the block copolymers tested was always
selected from monomers of medium polarity. The block
copolymers BC-3 to BC-5 differed primarily by the type and
amount of pigment anchoring groups in the B-block.
Different degrees of cationic charges, as well as
modification with special acidic anchoring groups were used.
Application testing of controlled dispersants
The block copolymer dispersants were evaluated on
different pigments in comparison to with convetional
benchmark dispersants R-1 to R-5. In one set of
experiments resin minimal pigment concentrates (RMPC)
were prepared and let down into different industrial coating
systems including white reductions (Table 2).
Carbon black pigments have high specific surface area
Carbon black pigments, especially the type HCC are difficult
to disperse, because most grades have extreme high
specific surface area. High concentrations of active
dispersant of typically 30 - 70% relative pigment are needed
to achieve acceptable stabilization and reduction of millbase
viscosities.
Compared to the references R1, R2 and R5, the controlled
dispersant BC-1 combined excellent millbase rheology and
a fully stable behavoir in the paint. At use levels of 50% or
higher, very low viscosity with almost ideal Newtonian flow
profile is achieved (Figure 4). The masstone pour outs also
showed very high gloss and perfect transparency,
demonstrating complete flocculation free behaviour in the
paint.
Performance of controlled dispersants in RMPC
Resin minimal pigment concentrates (RMPC) is a concept
by which pigment concentrates with low resin content can
be used in different coating systems. The controlled
dispersant BC-2 is a product that is optimized for broad
applicability in RMPC. Pigment concentrates according to
Table 2 were compared in rheology and concerning the
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quality of colouration in typical industrial coating systems.
Out of the group tested pigments, the controlled dispersand
BC-2 provided superior milbase rheology with most
pigments as exemplified with the iron oxide red (Figure 5).
Only in the case of the tested blue pigment concentrate, the
block copolymer BC-2 could not provide the same strong
viscosity reduction, but nevertheless it gives excellent
uniform colouration in different coating systems, what is an
even more important criterion for this application.
The blue pigment concentrate containing the controlled
dispersant BC-2 provided very uniform colouration in the
different coating systems under conditions of low shear
incorporation (Figure 6a, b). The colour positions are much
closer together than in the case of the reference dispersants
R-4 and R-5.
The dispersant BC-2 also provided similar advantages
concerning uniform colouration with other pigments such as
the iron oxide red. These results demonstrate that the
controlled dispersant BC-2 offers a very efficient solution for
resin minimal pigment concentrates with broad applicability
to different coating systems.
Controlled dispersants for specific organic pigments
For very demanding applications like those with transparent
organic pigments, it can be beneficial to optimize the
dispersant structure for a specific pigment. In particular,
pigments that have not been surface treated with polar
derivatives can cause severe dispersion problems because
they do not allow good interaction with common anchoring
groups. In this section an example is given for the
optimization of AB-type controlled pigment dispersants for
the non-surface treated pigment PR 254-I, which belongs to
the diketo-pyrrolo-pyrrole (DPP) class and has a very high
specific surface area of BET = 94 m2/g.
Typical commercial dispersants and also BC-1 did not
provide significant lowering of the millbase viscosity, even at
very high addition levels. Also the paints derived from these
dispersions showed poor transparency and flocculation.
These dispersants did not perform well because they do not
contain the right anchoring groups. Only the block
copolymer BC-3, which was modified with special aromatic
acidic anchoring groups, showed somewhat improved
rheology and good transparency.
Starting from BC-3 the anchoring block was further
optimized with special chemical groups which have
improved adsorption capability on PR 254-I. The block
copolymers BC-4 and BC-5 have the same structure as
BC-3 except that the B-Block contains different anchoring
groups and different degrees of cationic charges. BC-4 and
BC-5 gave very strong viscosity reduction even at much
higher pigment load of 18% wt (Figure 7).
The paints that were derived from the concentrates using
BC-4 and BC-5 showed excellent transparency and very
high gloss. This proves that the dispersion of the nano-sized
pigment particles was very stable. Such an improved
dispersion of transparent pigments like PR 254-I allows for
more brilliant colouration, for example in automotive metallic
base coats.
Conclusions and Outlook
Special designed NOR-polymerization regulators allow for
the synthesis of acrylic copolymers with very well defined
structures. Based on the NOR-technology, novel AB-type
block copolymer dispersants have been developed for
different solvent-based paint applications. Key parameters
for the optimization of dispersant performance are the
selection of the steric stabilizer chain and the type of
anchoring groups.
Novel "controlled" pigment dispersants have been
developed. These include BC-1, which offer excellent
performance on difficult pigments like carbon black, and
BC-2, which is an ideal dispersant for multipurpose resin
minimal pigment concentrates for industrial coatings. By
optimizing the pigment anchoring chemistry, AB-type
dispersants can be tailor-made for specific pigments. This
has been demonstrated on the example of a transparent
DPP pigment.
In the area of acrylic chemistry, NOR-technology gives for
the first time access to advanced polymer architectures
under viable conditions of industrial polymer production.
Controlled pigment dispersants are the first example of the
commercialization of functional polymers made by controlled
free radical polymerization. The versatility of acrylic
chemistry combined with the polymer design possibilities of
NOR-technology forms a powerful technical platform to
realize novel functional materials. Development work
continues to expand the product offering on controlled
polymer pigment dispersants and will be extended to other
types of coating additives. It can be expected that new
materials based on controlled polymerization will make a
significant contribution to meet the challenges of future
coating technologies.
Acknowledgements
The authors would like to thank all their colleagues at Ciba
SC and EFKA Additives for their contributions and
cooperation, especially: Peter Nesvadba and Andreas
Muehlebach for their pioneering research work on
polymerization regulators and controlled polymers; Almut
Staniek and Werner Steiner for assistance in polymer
synthesis; Tissa Rebmann, Matthias Graber and Piet van
der Steeg for assistance in paint applications; and Martin
Philipoom for application expertise and discussions.
Literature
[1] J. Bielemann in : 'Lackadditive', Ed. J. Bieleman,
Wiley-VCH Verlag GmbH, Weinheim, 1998, p.67
[2] J. D. Schofield, in L.J. Calbo (Ed.), 'Handbook of Coating
Additives', Vol.2, Marcel Decker, New York, 1992, pp.
71-104
[3] F.O.H. Pirrung, P.H. Quednau, C. Auschra, Chimia, 56,
(2002), 170
[4] H.L. Jakubauskas, J. Coat. Techn. 58 (736), (1986), 71
[5] H.J.W. van den Haak, J. Coat. Techn., 69 (873), (1997),
137
[6] P. Harbers, product presentation EFKA-4300 and
EFKA-4330, European Coatings Show, Nuernberg, April
2003
[7] K. Matyaszewski, J. Xia, Chem. Rev., 101, (2001), 2921
[8] D.H. Solomon, G. Waverly, E. Rizzardo, P. Cacioli, US 4
581 429, 1986
[9] H. Fischer, J. Polym. Sci.: Part A: Polym. Chem., 37,
(1999), 1885
[10] M.O. Zink, A. Kramer, P. Nesvadba, Macromolecules,
33, (2000), 8106
[11] P. Nesvadba, A. Kramer, M.O. Zink, GB 2 342 649,
2000
[12] A. Kramer, P. Nesvadba, GB 2 335 190, 2000
[13] P. Nesvadba, A. Kramer, M.O. Zink, US 6 479 608,
2002
[14] C. Auschra, E. Eckstein, A. Mühlebach, M.O. Zink, F.
Rime, Prog. Org. Coat., 45, (2002), 83
Results at a glance
- Special designed NOR-polymerization regulators allow
acrylic copolymers with very well defined structures to be
made.
- Novel AB-type block copolymer dispersants have been
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developed for different solvent-based paint applications. Key
parameters for the optimization of dispersant performance
were the selection of the steric stabilizer chain and the type
of anchoring groups.
- The novel "controlled" pigment dispersant BC-1 offers
excellent performance on difficult pigments like carbon
black.
- The novel "controlled" pigment dispersant BC-2 is an ideal
dispersant for multipurpose resin minimal pigment
concentrates for industrial coatings.
- By optimizing the pigment anchoring chemistry, AB-type
dispersants can be tailor-made for specific pigments.
The authors:
> Clemens Auschra received his Ph.D in polymer chemistry
from University of Mainz in 1992 and afterwards worked for
Röhm GmbH and RohMax GmbH. After joining Ciba
Specialty Chemicals in 1998, he has been engaged in the
development of coating additives, now responsible as R&D
manager for Polymer Specialties.
> Ernst Eckstein received the BS degree (chemical
engineering) from the Basle Institute of Technology in 1987.
He has been working in R&D for Dynamit Nobel and later in
technical service and R&D for Ciba Specialty Chemicals.
Since 1998 he has been responsible for the application
laboratory for Polymer Specialties.
> Ralf Knischka received his PhD in Macromolecular
Chemistry from the University of Freiburg in 2000. After
working for mnemoScience GmbH, he joined Ciba Specialty
Chemicals in 2001. He has been engaged in R&D of
Polymer Specialties as coating additives.
> Frank Pirrung received his Ph.D in organic chemistry from
the University of Amsterdam in 1995. He joined EFKA
Additives as development chemist in the field of polymers.
Since 2000, he has been Head of R&D at EFKA, being
responsible for the R&D programs for coating, graphic arts
and plastic additives.
> Peter Harbers worked since 1988 for Chemie Gro, later
known as Exachem and Beer Lakfabrieken, responsible for
developing automotive and car refinish paints. In 2000 he
joined EFKA Additives as an area manager in technical
service, now responsible as Head Application Development.
Comité of scientists awarded Clemens Auschra and his
co-authors with the "Fatipec 2004 Prize for Excellence".
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Figure 1: Mechanism of nitroxyl-mediated controlled free radical polymerization.
Figure 2: New NOR-polymerization regulators for acrylic monomers.
Figure 3: GPC-analysis of the controlled polymerization of n-butylacrylate in bulk with
NOR 3.
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Figure 4: Millbase viscosities with carbon black.
Figure 5: Viscosities of red pigment concentrates.
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Figure 6a: Colour measurement on white reductions of blue pigment concentrates;
shaded area as guide for the eye to compare the spread of data.
Figure 6b: Colour measurement on white reductions of blue pigment concentrates;
shaded area as guide for the eye to compare the spread of data.
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Figure 7: Millbase viscosity of pigment concentrates with 18% pigment load of PR 254-I
.
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Table 1: Overview on the dispersants used in this study; *) estimate.
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Table 2: Characteristic data of resin minimal pigment concentrates; "Laropal A81" is
used as grinding resin.
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