The Role of Thickeners in Optimizing Coatings Formulation Clemens Auschra #) , Iván García Romero #) , Immanuel Willerich #) , Robert Reichardt #) , Cindy Muenzenberg #) , Elena Martinez #) , Hunter He* ) #) BASF SE, Ludwigshafen, Germany *) BASF Company Ltd., Shanghai, China Abstract: Waterborne interior decorative paints consist of many components that interact with each other to provide a proper viscosity for storage and application. Suitable rheology is achieved through the thickener selection, which depends on the formulation, and the application method (brush or roller). In this work we report our fundamental investigations on the interactions between rheology modifiers and polymer binders and correlate these to the application performance in paint systems. Selecting the right thickeners enables to achieve optimized coating performance in regards to thixotropy, sagging, stain resistance, color acceptance and paint stability. The results from these studies help for optimal paint formulation, as well as for the design of new rheology modifiers for next generation paints based on new binder systems. Introduction In waterborne coatings, rheology is a key property for the efficient handling and application of paints. Proper control of the rheology under different shear regimes is important for the overall performance of a paint system (Figure 1). For example, at very high shear rates, paint viscosity should stay in a defined range to allow easy brush and roller application, but avoid spatter. Figure 1: Paint rheology in relation to its impact on handling and application properties in different shear regimes. Lower chart: accessible shear rate measurement range of common rheology tests in paint industry. At low shear rates, viscosity build must be high enough to prevent sagging, but still low enough to allow good levelling. The sag-levelling balance of the drying paint is critical for the film properties and appearance of the final coating. Modern waterborne coatings are complex formulations constituting many components, which together form a multiphase fluid (Figure 2). The main components, like the polymer latex and the pigments and fillers, are dispersed particles in the aqueous carrier. Together with formulation additives, like film forming aids, wetting agents, dispersants and rheology modifiers, the resulting overall paint rheology is the sum of several contributions in addition to the chosen rheology modifier system.
Transcript
The Role of Thickeners in Optimizing Coatings Formulation
Clemens Auschra#)
, Iván García Romero#)
, Immanuel Willerich#)
, Robert Reichardt#)
, Cindy
Muenzenberg#)
, Elena Martinez#)
, Hunter He*)
#) BASF SE, Ludwigshafen, Germany
*) BASF Company Ltd., Shanghai, China
Abstract:
Waterborne interior decorative paints consist of many components that interact with each
other to provide a proper viscosity for storage and application. Suitable rheology is achieved
through the thickener selection, which depends on the formulation, and the application
method (brush or roller). In this work we report our fundamental investigations on the
interactions between rheology modifiers and polymer binders and correlate these to the
application performance in paint systems. Selecting the right thickeners enables to achieve
optimized coating performance in regards to thixotropy, sagging, stain resistance, color
acceptance and paint stability. The results from these studies help for optimal paint
formulation, as well as for the design of new rheology modifiers for next generation paints
based on new binder systems. Introduction
In waterborne coatings, rheology is a key property for the efficient handling and application of
paints. Proper control of the rheology under different shear regimes is important for the
overall performance of a paint system (Figure 1). For example, at very high shear rates, paint
viscosity should stay in a defined range to allow easy brush and roller application, but avoid
spatter.
Figure 1: Paint rheology in relation to its impact on handling and application properties in different
shear regimes. Lower chart: accessible shear rate measurement range of common
rheology tests in paint industry.
At low shear rates, viscosity build must be high enough to prevent sagging, but still low
enough to allow good levelling. The sag-levelling balance of the drying paint is critical for the
film properties and appearance of the final coating. Modern waterborne coatings are complex formulations constituting many components, which
together form a multiphase fluid (Figure 2). The main components, like the polymer latex and
the pigments and fillers, are dispersed particles in the aqueous carrier. Together with
formulation additives, like film forming aids, wetting agents, dispersants and rheology
modifiers, the resulting overall paint rheology is the sum of several contributions in addition to
the chosen rheology modifier system.
Figure 2: Typical composition of a high PVC (pigment volume concentration) interior architectural paint.
Paint formulators experience shows that the rheology modifier has to be carefully chosen for
a given paint in order to achieve a stable rheology and fulfilling all the needs of handling,
application and final coating properties. In this contribution, we report our fundamental
investigations on the interactions of the three main synthetic polymeric rheology modifier
classes with polymer latex binders. Rheology measurements of binary model systems are
supplemented with selected analytical techniques to study the interactions and better
understand the mode of action of the rheology modifiers. The model studies are correlated to
the observed rheology and performance characteristics in paints. These results help to select
and design new rheology modifiers with improved efficiency and “optimum fit” for new
generation polymer binders for VOC-free paints.
Theory of suspensions - Rheology of pure polymer latex
The rheology of simple suspensions can be adequately described by hard sphere models.
The well-known equation (1) describes the viscosity of a dilute suspension of uniform and
non-interacting hard spheres [1].
(1)
( η = dynamic viscosity, Φ = volume fraction of spheres, ηr = “relative viscosity = “thickening efficiency
effect” of the dispersed phase)
In the case of higher concentrated suspensions with interacting particles, empirical equations
including the higher term Φ2 can be applied [2]:
(2)
(K = constant)
All such equations show that, in real suspensions at high particle concentrations, viscosity
shows strong dependency on the volume fraction Φ of particles [2]. For polymer colloids, hard sphere models can be applied to fit experimental rheological data
by using an effective volume fraction Φeff. Depending on the surface characteristics of the
latex, its rheological effective volume fraction can be increased by solvated, adsorbed or
grafted polymer chains or other formulation components [3,4]. Table 1 shows the polymer dispersion binders which were used in this study. We selected
four well-known binders from different regions, which are frequently used for interior paints.
These small particle sized latex binders were used as commercial grades without further
treatment, in order to serve as realistic models to study the interactions with different
rheology modifiers.
Binder Region Chemistry SolidsParticle Size
(DLS)MFFT Remark
Acronal ECO 338 ap SA-1 Asia styrene / acrylate 50% 158 nm ~16°Cexcellent water resistance
AC-3 NAFTA all acrylic 50% 126 nm ~10 °Csuitable for zero VOC paints
excellent cleanability
AC-4 Europe all acrylic 50% 198 nm ~2°Csuitable for low VOC paints
broad formulation latitude
Table 1: Benchmark polymer dispersion binders used in this study. Particle size is determined by
dynamic light scattering (DLS), MFFT = minimum film forming temperature.
In a first step, we characterized the rheology of these latex binders in aqueous phase under
normalized conditions of same active content of 40% wt and same pH of 8.5. Figure 3
compares the flow curves of the pure latex binders. The styrene-acrylate latex SA-2 shows
visible higher viscosity over the whole shear range and much higher pseudoplasticity, i.e. the
base thickening of this latex is higher. Looking at the characteristics of the four binders, we
see no obvious correlation of the latex rheology to particle size or the bulk monomer
chemical composition.
Figure 3: a) Rheology of pure binders at normalized conditions: 40% wt active polymer, pH = 8.5
b) Pseudoplasticity index and comparison to selected characteristics of the binders. The binder SA-2 is an example of a latex with enhanced thickening due to the surface
chemistry. Using the model of equivalent spheres, the latex particles SA-2 show higher
hydrodynamic effective volume compared to the other polymers, i.e. the effective volume
fraction Φeff in the dispersion is enhanced.
Binary system: latex and rheology modifier
For our study on the interaction of rheology modifiers with polymer dispersion binders, we
selected the three main classes of synthetic polymeric rheology modifiers which are used in
high quality waterborne coatings. Figure 4 shows the generic structures of the three types of
rheology modifiers.
Figure 4: Different classes of synthetic rheology modifiers:
Table 2 shows the characteristics of the three rheology modifiers which we used in this study.
These are well-known benchmark rheology modifiers from each class, which are used
globally in various types of architectural coatings primarily for low shear and mid shear
thickening. As in the case of the latex binders, commercial grade samples were used without
further treatment. For simplicity, we use the generic designations “HASE”, “HEUR” and
“ASE” to name the three different rheology modifiers throughout this paper.
Rheology
ModifierChemistry Product form pH Solids
Viscosity
(mPas)
HASE
associative anionic polyacrylate
(hydrophobe modified alkali swellable
emulsion copolymer)
aqueous
emulsion~3.5 35% ~5
HEUR
associative nonionic polyurethane
(hydrophobe modified polyethyleneoxide
urethane copolymer)
aqueous
solution~7 30% ~2700
ASEanionic polyacrylate
(alkali swellable emulsion copolymer)
aqueous
emulsion~3.5 30% ~40
Table 2: Benchmark low shear rheology modifiers used in this study.
Next to the pure binders, we studied the rheology of the different binders combined with the
different types of rheology modifiers. For this binary system we used the normalized
conditions of 40% wt active latex and 0.28% wt active rheology modifier polymer (relative to
latex solids) at pH of 8.5. The chart in Figure 5 shows the low shear thickening effect of the different rheology modifiers
with the four different latex binders. Not unexpectedly, we see different low shear response
of the binary systems depending on the type of latex and depending on the type of rheology
modifiers.
Figure 5: Binary system latex + rheology modifier: low shear thickening efficiency at shear rate 0.1 s-1
normalized conditions: 40% wt active latex polymer, 0.28% wt active rheology modifier
(versus binder solids), pH = 8.5
The HASE thickener which contains a “very strong” hydrophobe shows, with all binders, the
strongest low shear thickening. The HEUR rheology modifier contains a “somewhat weaker”
hydrophobe and therefore is more “selective” towards the associative interaction with
different latex surfaces. We will further investigate this picture in the following paragraphs. The selected three rheology modifiers are primarily designed for low shear and mid shear
thickening. However, all these rheology modifiers will also show an effect at higher shear
rates. In Figure 6, we give an overview on all binary systems by plotting the low shear
viscosities at 0.1 s-1 versus the high shear viscosities at 1000 s-1. The comparison to the data
of the pure binders shows that HASE brings strongest thickening effect with all binders on
both ends: low shear and high shear. ASE shows a more pseudoplastic trend (“north-west”),
compared to HEUR which is more balanced with a more Newtonian trend (“south-east”).
Again we see that the response of HEUR is weaker on latex AC-4 compared to the other
latex binders. To further investigate the interactions and the observed “selective” thickener response, we
recall the thickening mechanism of associative thickeners in Figure 7, with the example of
HEUR. The thickening effect is primarily caused by hydrophobe interactions, forming micelle
structures and by adsorption to the latex surface. This dynamic association is influenced by
surfactants which can interfere with, both the micelle formation as well as by competing for
the adsorption to the latex surface. This picture explains that the thickening efficiency
critically depends on the surface characteristics of the latex and on the hydrophobe structure
(“hydrophobe strength”).
Figure 6: Binary system latex + rheology modifier: low shear viscosity data at 0.1s-1 plotted versus
high shear data at 1000 s-1; included for reference: data of pure binders
normalized conditions: 40% wt active latex polymer, 0.28% wt active rheology modifier
(versus binder solids), pH = 8.5
Micelle
Surfactants
Latex particle
Mixed Micelle
Figure 7: Schematic picture of the associative thickening mechanism of HEUR in latex based paint
formulations The interaction of the HEUR associative thickener with the different latex polymers was
studied by measuring the electrophoretic mobility in dilute solution by laser light scattering.
Details of the method have been given elsewhere [4]. Figure 8 shows the changes of the
electrophoretic mobility of the different latex particles upon the addition of increasing
amounts of the associative HEUR.
Figure 8: Electrophoretic mobility of the different latex particles measured by laser light scattering in
dilute solution by varying the amount of added HEUR; latex concentration: 0.01% wt; the
concentration of HEUR is given in % wt active polymer relative to latex solids.
For all four types of latex particles, Figure 8 reveals a visible decrease of the electrophoretic
mobility by increasing the concentration of the associative HEUR. This gives direct evidence
of the interaction of the HEUR with the latex particles. With increasing HEUR concentration,
more polymers adsorb via the hydrophobic groups and thereby reduce mobility. Most
interestingly, the latex AC-4 shows the lowest decrease in mobility compared to all the other
latex polymers. This indicates that the surface chemistry of the latex AC-4 is “less capable”
for the adsorption of the hydrophobes of HEUR. This reduced adsorption strength is the
reason for the observed lower thickening response of the HEUR versus latex AC-4 (compare
to Figure 9).
Figure 9: Binary systems: latex + HEUR: high shear viscosity data at 0.1s-1
plotted versus low shear
data at 1000 s-1
; extract from Figure 6. Despite the fact that the measurements on electrophoretic mobility must be conducted in
very dilute solution, the results are a direct measure of the inherent capability of associative
interaction between a latex and a rheology modifier.
Testing of rheology modifiers in paints
The results from the model experiments in the binary latex-thickener systems were extended
by testing in fully formulated paints. Paints from different regions, based on the four selected
polymer dispersion binders, were used for application testing. Figure 10 gives an overview
on the selected paints. In Table 3, more details are given on the paints A and B from Asia.
Figure 10: Overview on the type of white paints used for testing of the rheology modifiers
Rheology Modifier, HEUR 0.25% Rheology Modifier, ASE 0.13%
Total 100.00% Total 100.00%
Paint A (PVC = 69%)based on styrene acrylate binder SA-1
Paint B (PVC = 80%)based on styrene acrylate binder SA-2
Table 3: Details of the formulations of paint A and paint B To compare the results from the binary systems to the fully formulated paints, in a first step
the different rheology modifiers were added to the base paints at same active polymer
content of 0.175% wt relative to the wet paint. In Figure 11, we compare the low shear
thickening efficiency of the three rheology modifiers in the different paints.
Figure 11: Low shear thickening efficiency at 0.1 s-1
of the rheology modifiers in different base paints;
all paints with same active content of rheology modifier of 0.175% wt relative wet paint. Again, we see that the HASE rheology modifier shows visibly higher thickening efficiency in
all paints, except paint D which is based on AC-4. Paint D, in general, shows a rather weak
response to all the three different types of rheology modifiers. In Figure 12, we look at the
overall thickening response of the four paints on the low shear as well as on the high shear
side and compare to the starting point base paints. Starting already from very different
rheology in the base paints, the “2-D”-chart of Figure 12 demonstrates that paint A, paint B
and paint C show the strongest response with HASE and good response with HEUR and
ASE. Again, Paint D shows, in general, a rather low response on thickening with almost no
differentiation between the different types of rheology modifiers. These thickening results in
the paints qualitatively correlate with the observed behavior of the rheology modifiers in the
binary systems.
paint B
paint A
paint D
paint C
Figure 12: Thickening response of four different base paints by the addition of the same amount of
rheology modifier: 0.175% wt active polymer relative to total wet paint; the shaded areas
are included as a guide for the eye to show the spread within the four paint groups. In the next step, the paints A and paint B were formulated with varying amounts x of the
different rheology modifiers, such that the resulting paint achieved a KU value of 100. This
corresponds to the practical scenario of the formulation of wall paints, for which mid-shear
viscosity is typically adjusted to KU values of about 100. Table 4 shows the amount of active
polymer of rheology modifiers used in these KU-adjusted formulations.
Rheology
Modifier
Paint A
(based on SA-1)
Paint B
(based on SA-2)
HASE 0.175 0.250
HEUR 0.175 0.310
ASE 0.150 0.450
% wt active polymer in
final paint
Table 4: Amounts of active rheology modifier used in the formulations of paint A and paint B to adjust
KU to 100. Table 4 shows that the exact amounts of rheology modifiers which are needed for the
adjustment of mid-shear viscosity to KU value of 100 are different in the two paint systems.
However, if we look to the flow curves of these formulated paints in Figure 13, we see the
same qualitative picture for the three different rheology modifiers. In both paints, the HASE
results in very pseudoplastic rheology, followed by ASE, also with strong contribution on the
low shear side. In both paints HEUR provides are more balanced, more Newtonian rheology,
which can be expected to result in better flow and levelling performance.
Figure 13: Flow curves of paint A and paint B which were adjusted with different rheology modifiers to
KU = 100.
Both paints were further tested concerning their different application properties. For the
example of paint B, Figure 14 shows results of testing the sagging and flow and levelling
behavior. As expected from the relative high low shear viscosities, all paints of formula B
show very good sag resistance. However, only the HEUR based paint also provides good
levelling performance, due its more balanced rheology.
Figure 14: Paint B: testing of sagging and levelling behavior; paints were adjusted with the different
rheology modifiers to KU = 100.
The classic challenge of finding the best compromise between sagging and levelling is based
on the conflicting rheology requirement of high versus low viscosity in the low shear region.
In addition, the dynamic rheological behavior of the paint is important, which cannot be
deducted from simple flow curves as shown in Figure 13. To characterize the viscoelastic
behavior of paints in the low shear region, more sophisticated rheology testing is required,
like dynamic shear jump / viscosity recovery experiments or dynamic mechanical analysis. In the example of paint B, in Figure 15, we show the dynamic mechanical analysis of the
paints with the different rheology modifiers.
Figure 15: Paint B: dynamic mechanical analysis of paints; strain sweep at constant frequency of 10 rad/s; paints were adjusted with the different rheology modifiers to KU = 100; the crossover points of G’ and G” are highlighted for the different paints.
The crossover point between the elastic modulus G’ and the loss modulus G’’ can be taken
as a measure how easy the paint begins flowing and levelling upon the application of a small
mechanical force. In line with the good results from the levelling tests, the paint with HEUR
shows the crossover point already at low strain level, but still with a high level of the elastic
modulus G’, which is important for sag resistance. This explains the good balance between
levelling and sag offered by the paint with HEUR. This in contrast to the situation with the
paints including ASE or HASE, for which the crossover point only happens at much higher
strain level.
New developments of nonionic associative thickeners
Aside from the basic wish to further improve thickening efficiency and formulation robustness,
new developments for associative thickeners also aim to offer optimum performance for the
next generation of VOC reduced or VOC-free paints. The corresponding new types of
polymer binders often will require well selected and adopted associative rheology modifiers
to offer optimum paint application performance. In the last part of our paper, we present one example of a new development for HEUR
associative thickeners. Figure 16 shows the basic concept of branched and hyperbranched
thickeners.
Figure 16: Design concept of new branched and hyperbranched nonionic thickeners.
The branched polymer architecture and the option to combine with new types of branched
hydrophobes offers routes to increase the basic thickening efficiency as well as to adopt the
associative hydrophobe groups to specific types of latex surfaces. In Figure 17, we show an
example of the thickening efficiency of the new nonionic rheology modifiers with branched
hydrophobe groups.
Figure 17: Thickening efficiency of new nonionic rheology modifiers with branched hydrophobes
compared to analog polymers with linear hydrophobes; tested in binary system with a latex
similar to AC-4; 0.25% active thickeners relative to total binary formulation.
The improved thickening efficiency, as seen in the binary system with a selected binder, also
translates to efficiency advantages in fully formulated paints. Figure 18 shows performance
results of the new rheology modifiers on the example of a high PVC paint. The data shows
that, compared to a commercial benchmark HEUR 2, the new rheology modifiers achieve the
target Brookfield viscosity at significant reduced treat level of about 35% lower active
polymer content. Important to note is that this efficiency gain is not compromised by reduced
performance concerning sagging or levelling, as can be seen from the data in Figure 18.
Figure 18: Testing of new rheology modifiers in a high PVC paint similar to Paint A Summary and Outlook
The interaction of synthetic rheology modifiers of the three classes of HASE, HEUR and ASE
with selected benchmark polymer dispersion binders was fundamentally studied by rheology
and by supplementary scientific methods. The interaction of the associative thickeners, as
measured by electrophoretic mobility, could be well correlated to the rheology in the binary
systems with different types of latex particles. The results from the fundamental studies are qualitatively in good correlation to the rheology
observed in different paint systems. For the optimization of paint properties like the critical
balance between sagging and levelling, the characterization of paints by dynamic mechanical
analysis proved to be useful. The interaction between rheology modifiers and the latex particles of the binder is one of the
key aspects to understand how the interactions between the formulation components
influence the paint rheology and how this impacts critical performance characteristics of the
paint. Ongoing and future investigations will include further key components like dispersants
and surfactants to complement the basic understanding about the key interactions in a paint. The gained understanding of the interactions between rheology modifiers and latex binders
forms a valuable basis for the optimization of paint formulations as well as the development
of new rheology modifiers in a system approach, which includes next generation polymer
binders. An example is given with new nonionic rheology modifiers based on branched and
hyperbranched structures, which offer improved base thickening efficiency and which can
also be tailored towards specific types of binders. References
[1] A. Einstein, Annalen der Physik, 1906, 19(2), 289
[2] S. Mueller, Proceedings of the Royal Society A 2010, 466, 1201-1228
[3] D. Quemada et al, Advances in Colloid and Interface Sci. 2001, 98, 51-85
[4] P. Schurtenberger, I. Willerich et al, BASF, Langmuir 2013, 29, 10346-10359
