This website requires certain cookies to work and uses other cookies to help you have the best
experience. By visiting this website, certain cookies have already been set, which you may delete and
block. By closing this message or continuing to use our site, you agree to the use of cookies. Visit our
updated privacy and cookie policy to learn more.
Redispersible Powders for Sealing Slurries
Mixing two-component flexible cementitious sealing slurries onJune 1, 2004
site immediately before use may soon become a thing of the past.
A redispersible polymer powder has been developed that enables
the formulation of one-component products. Polymer powders of this type can also be used
successfully, in other ‘elastic' applications, such as highly flexible tile adhesives
The effects of polymer dispersions on the physical and chemical properties of flexible
cementitious sealing slurries are well known. 1The resulting products have been used for many
years as surface-protection systems.2Second-generation sealing slurries of this type are
characterized by their high polymer fraction with the polymer-cement (p/c) ratio being typically
0.8 or more. The polymer, which ensures that the membrane remains flexible down to low
temperatures, must have a very low glass-transition temperature (Tg) (typically in the region -5
to -60°C). The technical properties of this type of building material are described elsewhere. 1-2

Figure 1. The action of spray additives in the spray drying of aqueous polymer dispersions
Powdered Polymeric Starting Material
Until recently, all commercially available low-temperature flexible sealing slurries were twocomponent systems. This meant that, immediately prior to use, the user had to mix a powdered
component with a liquid component on-site. Both logistic considerations (reduction in freight
volume, no need to protect against freezing during storage, simplified disposal of containers)
and the improved handling properties (ready-made powdered product that can be mixed to the
required consistency by simply adding water, thus making preparation more reproducible and
less prone to error), led to considerable effort in developing a suitable single-component
cementitious slurry product. The main difficulty in this development work was the availability
of a powdered polymer that exhibited adequate low-temperature flexibility and good processing
properties, especially with regard to the drying of the slurry coat after application.
The aim of this article is twofold: to explain the difficulties in producing a redispersible powder
with adequate low-temperature flexibility, and to present a new solution to this problem,
including a description of the associated physical and chemical characteristics. The use of
powdered products of this kind in other "elastic" applications, such as flexible tile adhesives,
will also be discussed.
Figure 2. Fluorescence microscopy image of an acrylate dispersion powder recorded with a
confocal laser-scanning microscope
Redispersible Powders from Drying Aqueous Polymer
Dispersions
Redispersible powders3-7are usually made by carefully drying aqueous polymeric suspensions in
a spray drier.8Typically, drying temperatures are 120-200°C at the drier entrance and 60-80°C
at the drier exit (the cooling being due to evaporation of the water of dispersion). At such high
drying temperatures there is a risk that a soft polymer (e.g., one with a Tgwell below 0°C) will
undergo irreversible particle-particle coalescence and film formation during the drying process.
Initial deterioration of this type often results in formation of undesirable large size
agglomerates and poor redispersion properties when water is added on the job site. However,
since it is only the redispersible fraction of a dispersion powder that modifies the properties of
the building material (a fact especially evident whenever the dispersion powder alone is
responsible for binding9), poor redispersibility can only be compensated for by the undesirable
step of increasing the amount of powder used.
Spray Additives Improve Redispersion of the Dry
Powder
Generally, premature film formation of the polymer particles can be prevented by including
hard, water-soluble additives (spray additives) to the dispersed binding agent prior to the
drying step (see Figure 1).
The spray additive distributes itself between the continuous aqueous phase and the surface of
the dispersed particles. Since the spray dries within seconds, the distribution of particles at that
time becomes frozen in, with the spray additive acting as an inter-particle spacer. Irreversible
coalescence of the polymer particles is thus prevented. If the dried powder is subsequently
mixed with water again, even low-level shear is sufficient to cause the spray additive to dissolve
completely, releasing the essentially unchanged primary dispersion particles. The polymer
powder is thus re-dispersible again.
The validity of this model has been established with microscopic measurements. Fluorescence
microscopy using a confocal laser-scanning microscope (CLSM) is a non-destructive optical
technique that can generate high-resolution images of very thin, well-defined sections of a
prepared specimen. The method is, therefore, especially suited to imaging the interior of
particles in the dispersion powder.
Figure 2 shows a CLSM image of a BASF product after embedding in silicone oil. In addition, a
powder sample was ground in a mortar at -120°C and the fragments examined in a scanning
electron microscope.
Figure 3. Tensile strain test on redispersion films — dependence of extension on glass transition
temperature and spray additive
Preparation of Dispersion Polymer/Spray Additive
Mixtures
Four model polymer dispersions were prepared using a standard semi-batch polymerization
process. The dispersions differed in their styrene-acrylate ratio and therefore had
correspondingly different Tgs. All other experimental parameters were kept constant to assist
the comparison of results.
The spray additives used included an aryl sulfonate-based low-molecular-weight condensation
product and a polyvinyl alcohol (PVA) with a degree of saponification of approx. 88% and a
weight-average molecular mass of approx. 26,000 g/mol.
Spray additive solutions were stirred into the dispersions and mixtures were then adjusted to a
solids content of 30% using added deionized water. Spray drying was performed in a laboratory
apparatus with maximum 2 kg material/hr throughput capacity, using a binary nozzle atomizer
under standard conditions.
A qualitative assessment was made of the redispersibility of the polymer powders based on the
amount of sediment that formed during a 72-hour redispersion period with a solid fraction of
30% w/w.
Figure 4. Tensile stress testing of redispersion films — dependence of maximum tensile strength
on glass transition temperature and spray additive
Tensile Strain Test Results
To compare the flexibility of the polymer powders, air-dried films were tested using a tensile
strain experiment. All powders were redispersed in water in the ratio 30% w/w and left to form
a film at room temperature. The free polymer films achieved their final hardness within two
weeks at the most. Film samples were pulled in a commercial tensometer at temperatures of 25
to -5°C, until fracture.
At a temperature of 25°C, the elongation at break of these redispersion films -and thus their
elasticity - was essentially independent of the Tg. Effects first become observable at lower
temperatures. At -5°C, redispersible powders with a Tg above approx. 10°C are essentially
inflexible. Soft powders with a Tg under about 5°C are still elastic at this temperature, the extent
of elasticity being determined by the spray additive (see Figure 3).
The fact that the redispersion powder with a Tg of -33°C at a test temperature of -5°C is not
more flexible than that with a Tg of -16°C is presumably a result of a small amount of prior film
formation during the spray drying process.
In contrast, the maximum tensile strength (stress) is solely determined by the difference
between the glass transition temperature of the polymer and the measurement temperature,
and it falls monotonically as a function of decreasing Tg (see Figure 4). The type of spray
additive used has no significant effect on the tear strength.
Figure 5. Temperature dependence of the tensile moduli of polymer films — the influence of the
spray additive and spray drying (Tg = –35°C, from DSC –33°C)
Spray Additive Influences the Temperature
Dependence of the Tensile Modulus
The qualitative property of a polymer film known as ‘flexibility' is associated with the material's
tensile strength and its elongation at break. However, this behavior is best interpreted in terms
of the temperature dependence of the film's tensile modulus. The tensile modulus can be
interpreted as the initial gradient of the stress-strain (load-extension) curve from a simple
tensile loading test, and comprises an elastic part (storage modulus E') and a viscous part (loss
modulus E").
To measure these quantities, the initial dispersions and the polymer/spray additive mixtures
were left to form a film at room temperature both before and after spray drying. The
temperature dependence of the polymer film's storage and loss moduli were then determined in
a dynamic mechanical analyzer.11
The influence of the aryl-sulfonate-based spray additive is demonstrated in Figures 5-6.
Figure 6. Temperature dependence of the tensile moduli of polymer films — the influence of the
spray additive and spray drying (Tg = +13°C, from DSC +14°C)
The low-molecular-weight (hard) aromatic spray additive is fully miscible with the styreneacrylate copolymer; as a result, the tensile moduli of the polymer film at temperatures above the
polymer Tgis increased by several orders of magnitude. Eventually, an essentially temperatureindependent region (plateau) is achieved due to the physical crosslinking of the dispersion
polymer particles by the spray additive. This phase structure is also retained after spray drying;
the temperature dependence of E' and E" are practically identical before and after spray drying.
The redispersion film, therefore, demonstrates the desired rubber-like elasticity that is
responsible for the flexibility and strength observed in the elongation at break experiment.
Very different behavior is observed when the partially saponified polyvinyl alcohol spray
additive is used. The much greater molecular mass of the polyvinyl alcohol means that this
additive is effectively immiscible with most copolymers, including the styrene-acrylate
copolymers used in the present study. The two polymer phases, therefore, remain separated in
the mixture; as a result, a second glass transition - that of the polyvinyl alcohol - is observed at
around 60-70°C. In this temperature range, the tensile moduli decrease with increasing
temperature (see Figure 6).
The absence of physical crosslinking means that there is only minimal cohesion within the
redispersion film when subjected to thermal or tensile stress. The low level restoring forces are
also manifest in the low flexibility observed in the tensile strain test (see Figure 3).
These experimental observations are of direct relevance to product applications characterized
by a high polymer fraction and a desired high degree of low-temperature flexibility. Examples of
such applications are flexible sealing slurries and flexible tile adhesives, both of which are
discussed in greater detail in the following two sections.
Tensile Testing of Single-Component Sealing Slurries
Each of the six redispersible powders described was used to manufacture a single-component
(‘one-pack') sealing slurry using the simplest possible formulation (see Table 3).
Figure 7. Tensile testing of low-temperature flexible single-component (‘one-pack’) sealing
slurries — dependence of the tear strength on the glass-transition temperature and the spray
additive
The polymer-cement ratio (p/c) was 0.8, with total polymer content of approximately 17% in all
dry mixtures. Mixing water level was adjusted to produce a slurry with a consistency suitable for
application with a roller.
Once mixed for use, each sealing slurry was spread wet-on-wet in two to three coats onto an
inert polyethylene sheet (application rate: 3 kg/m2), left to dry for 24 hours and then peeled off
as a film. The films were then stored for a further 14 days in a standard reference atmosphere.
The tensile breaking strength and the elongation at break of each slurry film was then measured
at room temperature, 10°C and -5°C (see Figures 7-8).
Figure 8. Tensile testing of low-temperature flexible single-component (‘one-pack’) sealing
slurries — dependence of the elongation at break on the glass-transition temperature and the
spray additive
The room temperature tests demonstrate the decreasing tensile tear strength and the increasing
elongation at break of the sealing slurries as the Tgdecreases. The reason why the polyvinyl
alcohol/acrylate powders exhibit lower tensile strength and reduced elongation at break
compared to the aryl sulfonate containing samples is that the former has a lower cohesion due
to the incompatibility of the polyvinyl alcohol and the acrylate copolymer.
As the investigations of the elongation at break at 10°C and, in particular, at -5°C show, a
sealing slurry will only exhibit sufficient flexibility if the Tg of the polymeric component is far
enough below the measurement temperature. However, the price paid for this effect is the
significant reduction in the inherent strength of such a product (see the results for the softest
polymer in Figure 7). Even at the lower measurement temperatures, the polyvinyl alcoholcontaining acrylate powders are again characterized by their lower tensile strengths and lower
extensibility compared to the aryl sulfonate samples.
From the results discussed in this section, the sealing slurry with the most balanced set of
properties in terms of inherent strength and low-temperature flexibility is the aryl sulfonate
redispersible powder with a glass transition temperature of -16°C.
Bond Strengths of Flexible One-Component Tile
Adhesives
Two aryl-sulfonate-modified acrylate powders with Tgs of +14°C and -16°C (A1 and C1,
respectively) were used in the preparation of powdered flexible tile adhesives. Testing of the
corresponding polyvinyl-alcohol-modified powders was not possible because the bond strengths
of the corresponding tile adhesive formulations were far too low.
As with the sealing slurries, the simplest possible recipe was used for the investigations of the
polymer-modified cementitious tile adhesives (see Table 4).
To understand the effect of polymer content in the formulation, the polymer-cement ratio of the
dry mixture was varied between 0.04 and 0.31, corresponding to a polymer fraction of between
approx. 1.5 and 12% w/w in the tile adhesive. For comparison purposes, a common,
commercially available EVA-based polymer powder recommended for this purpose was also
tested (Tg = -5°C; contains polyvinyl alcohol; EVA = ethylene vinylacetate).
The amount of mixing water required was that which gave the best workability coupled with
sufficient stability of the tile on the test substrate. As shown in Figure 9, the amount of water
required was found to decrease with increasing p/c with the acrylate powder systems but was
approximately constant with the EVA-based powder. As expected, the differences between the
two product groups are marginal at very low polymer fractions.
Tile adhesive bond strengths were determined using the old DIN 18 156 standard (tile pulloff
test) on samples with p/c ratios of 0.04 and 0.23 (corresponding to a polymer fraction of 1.5%
w/w and 10% w/w, respectively). Bond strengths were measured under four conditions: after 28
days dry storage at room temperature ("standard reference atmosphere"); after 28 days in the
standard reference atmosphere followed by additional high-temperature storage at 70°C; after 7
days in the standard reference atmosphere and then 21 days wet storage; and after 7 days in the
standard reference atmosphere followed by freeze-thaw cycling.
The failure pattern observed during these tests was always either cohesive failure or concrete
substrate breakout and never adhesive failure. The measured bond strengths, therefore,
represent minimum values and reflect the inherent strength of the tile adhesive. Their
magnitude may, however, also be a result of the reduced amount of water required by these
modified materials.
Figure 9. The amount of water required for ‘one-pack’ flexible tile adhesives
Bond Strength and Freeze-Thaw Cycling
The bond strength results shown in Figure 10 demonstrate that the differences between the
three test powders are slight at low polymer fractions. At the higher polymer level tested (p/c =
0.23), in spite of the lower Tgand high inherent flexibility, C1 yielded bond strengths that were
indeed somewhat less than the higher Tgpolymer A1 but, importantly, were comparable to the
harder commercial EVA-based powder - with its much weaker ability to impart flexibility.
As the experiments on the sealing slurries showed (see Figure 8), the addition of a sufficient
quantity of soft acrylate powder C1 results in a dramatic increase in the flexibility of the
cementitious compound material. The lower inherent strength of the polymer is the reason for
the reduced bond strength compared with the harder polymer A1 (see Figure 10). This effect is
compensated to some extent by the reduced amount of water required by the acrylatecontaining formulations (see Figure 9). It is noteworthy that, compared with the EVA-based
product, the two acrylate-containing powders offer significant advantages when subjected to the
particularly critical freeze-thaw cycling test.
The choice of the right acrylate powder for flexible and highly flexible tile adhesives depends
upon the desired material properties. If a high inherent strength is of primary importance for
an application, then the harder powder A1 is certainly preferable. If the user attaches greater
weight to a product with good (low-temperature) flexibility and adequate inherent strength
(e.g., when tiling on a substrate that may be subject to movement), a softer powder such as C1 is
the material of choice.
Figure 10. Bond strengths of flexible ‘one-pack’ tile adhesives on concrete*
* Normal storage: 28d standard reference atmosphere; Wet storage: 7d standard reference
atmosphere + 21d wet storage; High-temp. storage: 28d standard reference atmosphere + 14d
70°C + 1d standard reference atmosphere; Freeze-thaw cycling: 7d standard reference
atmosphere + 21d wet storage + 25 “–15°C/Water” cycles)
Summary
The present work provides insights to the challenges associated with manufacturing soft
redispersible polymer powders (i.e., ones that are flexible at low temperatures). By using a new
spray drying additive technology, the elastic properties of a soft acrylate dispersion can be
retained in the resulting redispersible acrylate powder. Powder products of this type can be
used successfully in ‘elastic' applications such as flexible cementitious sealing slurries and
(highly) flexible tile adhesives. In the latter, high bond strengths are combined with excellent
resistance to freeze-thaw stress and exceptionally good flexibility, even at low temperatures.
For more information on redispersible polymer powders, contact Luke Egan, eganl@basf.com
.
References
1 Angel, M.; Denu, H.-J. Dichtungsschlämmen für den Betonoberflächenschutz [Sealing slurries
for protecting concrete surfaces].Farbe&Lack103 (1997), No. 8, p. 92.
2 Volkwein, A..; Petri, R.; Springenschmid, R. Oberflächenschutz von Beton mit flexiblen
Dichtungsschlämmen - Teil 1 [Concrete surface protection using flexible sealing slurries - Part
1]. Betonwerk und Fertigteiltechnik 54 (1988), No. 8, p. 30. b) Volkwein, A..; Petri, R.;
Springenschmid, R. Oberflächenschutz von Beton mit flexiblen Dichtungsschlämmen - Teil 2
[Concrete surface protection using flexible sealing slurries - Part 2]. Betonwerk und
Fertigteiltechnik 54 (1988), No. 9, p. 72.
3 Schulze, J. Redispersionspulver im Zement [Redispersible powders in cement]. TIZFachberichte 109 (1985) No. 9, p. 698.
4 Rietz, U.; Dispersionspulver, Herstellung und Verwendung [Redispersible powders:
manufacture and use]. Chem. Tech. makromol. Stoffe (FH-Texte, 12. Koll. FH Aachen) 53
(1987), p. 85.
5 Walters, D.G. VAE Redispersible-Powder Hydraulic-Cement Admixtures. Concrete Int. 14
(1992) No. 4, p. 30.
6 Tsai, M.C.; Burch, M.J.; Lavelle, J.A. Acrylic Dry Polymer Modified High Solids Cementicious
Coatings. Proc. Water-Borne Higher-Solids Powder Coat. Symp. (1993), p. 322.
7 Denu, H.-J.; Pakusch, J. Ein neues Dispersionspulver mit integrierter Bindemittel- und
Verlaufsfunktion [A new redispersible powder with integrated binding and leveling
properties]. European Coatings Show (Nürnberg 1999) Bauchemiekongreß.
8 Armbruster, W. Die Zerstäubungstrocknung [Spray drying]. Chem. App. Verfahrenstechnik
93 (1969) No. 12, p. 469.
9 Dörr, H.; Lohmann, W. Ohne Wasser geht's auch - optimales Formulieren und Produzieren
von Dispersions-Pulverfarben. [Doing without water - optimized formulations and the
production of dispersible powder pigments]. Farbe&Lack 101 (1995), No. 8, p. 708.
10 Richardson, M.J. Comprehensive Polymer Science Vol. I. Hrsg. C. Booth, C. Price, Pergamon
Press Oxford (1989), p. 867.
11 Zosel. A. Lack- und Polymerfilme - viskoelastische Qualitätsmerkmale [Paint and polymer
films - viscoelastic quality characteristics). Hrsg. U. Zorll, C.R. Vincentz Verlag Hannover
(1996), p. 21.
12 Mächtle, W. Coupling Particle Size Distribution Technique. Angew. Makromol. Chem. 162
(1988), p. 35.
The latest news and
information
Content focused on adhesive &
sealant manufacturing, formulations
and finished products
SUBSCRIBE TODAY
Copyright ©2021. All Rights Reserved BNP Media.
Design, CMS, Hosting & Web Development :: ePublishing