Next generation polyurethane

Additives
Dr Stefan Mößmer, BYK Additives & Instruments discusses the development of
DISPERBYK ® -2155, which provides a high molecular weight for stabilisation and solids content
Next generation polyurethane
Growing environmental awareness is leading to
changes in coating formulations and one way to fulfil
stricter requirements, eg, in terms of VOC regulations,
is to change to higher solids or solvent-free systems.
Polyurethane-based dispersants have historically
been used in many coating systems to deflocculate pigments;
resulting in lower mill base viscosities and improved
colouristic properties such as gloss, colour strength or
transparency. Especially in high quality applications such as
automotive coatings, polyurethane-based high molecular
weight wetting and dispersing additives have set the
industry standards for pigment stabilisation since the late
‘80s; especially for organic pigments and fine particle size
carbon blacks.
Since high solid systems were not common and the dispersants
were not designed for that purpose, these high
molecular weight polyurethane additives often suffer, due
to their chemical nature, from poor flowability; requiring
dilution with solvents to below 50% solids for handling.
On the other hand, this low solids content limits their use
in modern high solid or solvent-free systems.
High solids and solvent free systems generated the
demand for new wetting and dispersing additives combining
both high molecular weight to give outstanding stabilisation
and high solids content. These apparently contradictory
requirements were combined in the development
of DISPERBYK ® -2155, a 100%, high molecular weight
polyurethane of the next generation that exhibits exceptional
flow due to its innovative spherical structure.
Wetting and dispersing additives are structured in such
a way that, on one side, they contain compatibility-promoting
groups (represented by the blue lines in figure 1), ie
groups that make for optimum compatibility with the binding
agent or the solvent of the paint/coating, while on the
Fig1. Comparison between polyurethane-based dispersants with conventional and spherical structure
22 PPCJ – September 2008 www.coatingsgroup.com
other side, they contain stabilising groups (represented by
the red dots) which effectively adsorb on to the particle surface.
Both structures, shown in figure 1, fulfil the requirement
profile for a wetting and dispersing additive. In the
case of the conventional molecular structure, compatibilising
units and many pigment anchoring groups are aligned
along the polymeric backbone, whereas in the case of the
spherical structure these groups are ordered ‘star-like’.
These structural differences show a strong influence on
polymer viscosity. A conventional polymeric polyurethane
dispersant with an assumed molecular weight of around
20,000g/mol at a hypothetic 100% solids content does not
exhibit flow. Therefore, polyurethanes of conventional
structure are dissolved in solvents to achieve manageable
viscosities. In contrast, DISPERBYK ® -2155, a
polyurethane-based dispersant of the next generation displays
at the same molecular weight, due to its star-like or
spherical structure, a remarkably low product viscosity at
100% solids. This property makes it an ideal product that is
solvent-free and yet a flowable liquid, allowing for easy handling
of the additive and enables its use in ultra high solid
systems and 100 % solids UV coatings.
PIGMENT STABILISATION IN HIGH
SOLID SYSTEMS
The performance of DISPERBYK ® -2155 in terms of pigment
stabilisation was compared with a conventional
structured polyurethane dispersant, long regarded as the
industry standard for automotive and high quality industrial
coatings, supplied at 30% solids.
For a low VOC high solid system, pigment concentrates
were prepared with a polyester grinding resin to assist in
the wetting and stabilisation of pigments and helps minimising
potential ‘shock’ effects during paint manufacturing.
An acrylic/NCO 2K system was selected as let down.
The mill base viscosities for three different organic pigments,
stabilised either with DISPERBYK ® -2155 or with
the conventional state-of-the-art polyurethane dispersant,
are shown in figure 2.
Low mill base viscosities are preferable due to better handling
and the possibility of higher pigment loadings in the
paste. Based on equal solids content, the mill base viscosities
with these three organic pigments are in all cases far
lower with the DISPERBYK ® -2155 than with the industry
standard; thus enabling higher pigment loading in the concentrate
with the new polymeric dispersing agent.
Additionally, the mill base viscosities developed with
Author:
Dr Stefan Mößmer, Product Group Manager, Wetting and Dispersing Additives, BYK-Chemie GmbH, Abelstraße 45, 46483 Wesel, Germany
Tel : +49 (0) 281 670-617 ; Fax : +49 (0) 281 65735 ; M +49 (0) 160 8198958; Stefan.Moessmer@altana.com; www.byk.com

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Fig 2. Mill base viscosities for PY 151 (25% dispersant s.o.p.), PR 254 (30% dispersant s.o.p.) and PG 36 (25%
dispersant s.o.p.) (s.o.p. = solid on pigment)
Fig 3. Transparencies of drawdowns for
different pigments
DISPERBYK ® -2155 show Newtonian flow behaviour,
whereas the mill base viscosities obtained with the state-ofthe-art
polyurethane dispersant exhibit strong thixotropic
flow behaviour.
After tinting the let down system with the pigment concentrates,
the transparency of the draw downs after cure
was judged visually, ranging from ‘1’, excellent transparency
to ‘5’, very poor transparency. This is summarised
in figure 3 for six different pigments; an organic
yellow pigment (PY 151), two organic red pigments (PR
178 and PR 254), a blue and a green phthalocyanine pigment
(PB 15:2 and PG 36) and a High Colour Channel
Carbon Black.
‘Control’ means that the pigments were ground only with
grinding resin without any dispersant. In this case, these
organic pigments and carbon black were only marginally dispersed;
therefore the transparency of the final paint after
cure was very poor. Aside, these cured films show pigment
flocculates on the paint surface. Where a dispersant was
used with the polyester grinding resin, the pigments were
effectively ground, resulting in highly transparent films after
cure with no visible flocculates on the surface. The next generation
polyurethane-based dispersant, DISPERBYK ® -2155,
matches in terms of transparency, the excellent perform-
Fig 4. Transparency in a 2-pack Acrylic let down system, left: 25% s.o.p. industry standard additive; right: 25%
s.o.p. DISPERBYK ® -2155
24 PPCJ – September 2008 www.coatingsgroup.com
ance of the industry standard for all pigments tested. High
transparency means effective deflocculation and stabilisation
of the pigments. Due to the greater amount of pigment
anchoring groups, DISPERBYK ® -2155 forms a more
durable adsorption layer on the surface of the organic pigments
and the carbon black and is able to optimise the
desired colouristic properties. For the phthalocyanine
green pigment, the performance with the DISPERBYK ® -
2155 is superior to the state-of-the-art dispersant. This can
be seen on figure 4, where photographs were taken
through the transparent drawdowns.
The stabilisation properties of DISPERBYK ® -2155 for
inorganic pigments were evaluated by making pigment
concentrates with titanium dioxide (PW 6), bismuth vanadate
(PY 184) and an iron oxide red pigment (PR 101) in
a polyester grinding resin. The dosage was 2% solid on
TiO2, 10% s.o.p. (solid on pigment) for both the inorganic
yellow and inorganic red pigments. These pigment
pastes were mixed with a high solids 2K acrylic/NCO let
down system. Stabilised and deflocculated pigments
always give low haze and high gloss values. After making
draw downs on polyester foil, gloss values at 20° geometry
were measured, again in comparison to the industry
standard. For all inorganic pigments tested, the same high
gloss values as for the state-of-the-art additive were
achieved, proving the ability of this novel dispersant to
also stabilise inorganic pigments very effectively. This is
shown in Table 1.
Table 1. Gloss values in a 2 pack Acrylic/NCO let down system
Pigment Industry standard DISPERBYK ® -2155
Gloss [20°]
PW 6 84 86
PY 184 92 91
PR 101 88 88
RESULTS IN WHITE REDUCTIONS
Of great importance for a dispersant is its performance in
terms of rub out stability. Sometimes, if stable coloured
pigment concentrates are mixed with white paints, where
none or the inappropriate wetting and dispersing additive
is used to stabilise the TiO2, the system will flocculate.
Additionally, if different wetting and dispersing additives
are used to stabilise the titanium dioxide and the organic
pigments in the pigment pastes, compatibility issues may
occur when mixing the coloured pastes with the titanium
dioxide paste in the base paint. To minimise possible compatibility
problems, ideally the same dispersant should be
used to deflocculate both the white pigment as well as the
coloured pigments. For this study, another high solid system
based on a 2K polyurethane was chosen as the let
down. Three different white pigment pastes were prepared;
in the first, no wetting and dispersing additive was
used, in the second, the titanium dioxide was stabilised
with the industry standard additive and in the third,
DISPERBYK ® -2155 was used. The amount of dispersants
on the TiO2 was in all cases 2% s.o.p. Then, the different
coloured pigment concentrates, produced with
DISPERBYK ® -2155, were mixed into the clear paint
together with the white paste stabilised with DISPERBYK ® -
2155. The same procedure was performed with the con-

Additives
trol (no additive) and with the industry standard. The wellknown
rub out test gives valuable information on how well
the pigment dispersions are stabilised in the final paint.
The colour differences (∆E) of the rub-outs performed
with the different white-reductions were determined; the
colour differences between the rubbed area and the nonrubbed
area of the paint are shown in figure 5.
In high quality applications, ∆E values below 0.5 are
acceptable. In the case of the control, it is not surprising
that the non-stabilised pigments result in dramatically high
∆E values, indicating total flocculation of the system. The
agglomerated pigment particles can actually be seen on
the surface of the polyester foil. In the case of the stabilised
pigments, here again the excellent performance of
the low solids polyurethane dispersant is matched by the
new 100% solids polyurethane dispersant DISPERBYK ® -
2155 with its novel spherical structure. The coloured pigment
concentrates and the white pigment concentrate are
stabilised in such a way, that after mixing them together in
the clear paint, excellent rub-out values are obtained; ∆E
values below 0,5 for all pigments used, indicating no
flooding or floating.
Besides providing excellent stabilisation properties, a
high performance dispersant is asked not to degrade the
physical properties, such as hardness or adhesion of the
final coatings, nor to negatively affect the curing of the
coating in any way. Especially in 2K systems, some dispersants
can reduce the pot-life in a very significant manner.
With the next generation polyurethane dispersant
DISPERBYK ® Fig. 5: ∆E values in white reductions
-2155, no acceleration of curing was
observed in 2K systems. Similarly, there is no yellowing
with this new wetting and dispersing additive in baking
enamels. To evaluate resistance to overbake yellowing, a
white pigmented alkyd/melamine paint was prepared and
applied on to a metal panel. This panel was then put into
an oven with different temperature zones (120°C, 140°C,
160°C and 180°C) and baked for 30min. As controls, the
industry standard dispersant was used, as well as another
dispersant (dispersant 2), that is not designed for TiO2 Fig 6. Yellow index Y in an alkyd/melamine baking system
26 PPCJ – September 2008 www.coatingsgroup.com
stabilisation in baking enamels. The influence of the dispersant
on the yellowing is evidenced by measuring the
yellow index Y; shown in figure 6.
The higher the Y value, the better the stability against yellowing.
Independent of the baking temperature,
DISPERBYK ® -2155 shows no yellowing tendency, whereas
the dispersant 2 is highly sensitive to yellowing at increasing
bake temperatures. The same behaviour holds true for
a white pigmented acrylic/melamine baking enamel.
In terms of stabilisation properties and influence on
the final paint characteristics, the new DISPERBYK ® -2155
shows the same excellent results as the industry standard
but has the outstanding advantage, thanks to its 100%
solids content, to be used in modern high solid systems,
like 2K or high solid alkyd systems, as well as 100%
solids UV coatings.
DISPERBYK ® -2155 is not only an excellent additive for
high solid systems but also due to its wide compatibility,
is perfectly suitable for many other types of let-down systems,
covering the whole polarity range from lower polarity
long oil alkyd systems to highly polar 2K polyurethane
systems. Stable and highly filled pigment concentrates
can be prepared with aldehyde, acrylic or short oil alkyd
grinding resins, leading to excellent results in terms of
high gloss, low haze, high transparency and excellent rub
out stability, in tinting many common let-down systems
used in general industrial or wood and furniture coatings,
such as alkyds, baking enamels (acrylic/melamine,
alkyd/melamine, polyester/melamine), 2K
polyurethanes, thermoplastic acrylics, Nitrocellulose lacquers
or acid curing systems.
SUMMARY
Due to its innovative sphere-like structure, DISPERBYK ® -
2155 combines high molecular weight with high solids
content, therefore opening up new horizons in the wetting
and dispersing of pigments in high solid as well as in conventional
coating systems. It displays outstanding deflocculation
and stabilisation properties, effectively reducing
mill base viscosity, is storage-stable and shows wide compatibility
with many different types of resin systems.
The next generation polyurethane wetting and dispersing
additive DISPERBYK ® -2155 is designed to be used in
pigment concentrates for automotive coatings, general
industrial coatings, wood and furniture coatings, floor
coatings (based on polyurethane and epoxy),
polyurethane foams and thanks to its 100% solids content
also for solvent-free UV systems. In addition, the additive
has been successfully used in co-grinding systems.
Acknowledgements:
The author would like to thank Andrea Eßer and Dr Wolfgang
Pritschins for the synthetic design of this innovative dispersant, Ralf
Koch for performing the application tests as well as Frank Kother
and Manfred Knospe for valuable discussions.
The author:
Dr Stefan Mößmer studied chemistry at the University of Ulm,
Germany, where he completed his Ph.D thesis in 1999 in the field of
macromolecular chemistry (Title of the thesis: Size Control of
Metallic Nanoparticles and Surface Patterning by Self-Assembly of
Diblock Copolymers). Since 2000 he has been working at BYK, initially
in the R&D Department and, since 2006, as the person overseeing
the product group of wetting and dispersing additives.