TECHLINE3
FOR THE CONSTRUCTION INDUSTRY
ABSTRACT 01 | INTRODUCTION 01 | Cellulose Ethers 02 | Redispersible Polymer POWDER 07 |
INTERACTION OF CELLULOSE ETHER AND REDISPERSIBLE POLYMER POWDER 08 | Summary 11 |
REFERENCES 12 | Authors 12
Interaction of Cellulose Ethers and Redispersible
Polymer Powders in Cement Based Tile Adhesives
Abstract
Cellulose ether products and redispersible polymer powder products are commonly used as additives to improve the per­
formance properties of cement based tile adhesives. Cellulose ethers provide water retention for open time, tile correction
time, and proper hydration development of the cement paste. Cellulose ethers also provide rheological properties for en­­
hanced application characteristics. Redispersible polymer powders improve the adhesion and flexibility of the tile adhesive.
Formulators of cement based tile adhesives seek to optimize the formulation cost and performance by selecting the right
combination of cellulose ethers and redispersible polymer powders. Therefore, it is important to understand the types of
synergies and interactions that might occur between these additives in the tile adhesive formulation. A designed experiment
was carried out to screen the impact of cellulose ether and redispersible polymer powder on the performance properties of
a tile adhesive formulation. The test formulation parameters included variation of the cellulose ether chemical substitution
type, cellulose ether degree of substitution (DS/MS), redispersible polymer powder type, and the redispersible polymer
powder content. Definite synergies and interactions were observed with variations in the combinations of cellulose
ether / RDP and RDP concentration; however these observations were dependent on the property examined
INTRODUCTION
Redispersible polymer powders (RDP) and cellulose ethers
(CE) are used as additives in many construction applications.
Cellulose ethers like methyl hydroxypropyl cellulose (HPMC)
or methyl hydroxyethyl cellulose (HEMC) help to control fresh
mortar properties like workability, sliding resistance, open
time and water retention. They have also a retarding effect on
the hydration kinetics of cement which reduces the bonding
strength of the mortar. High substitution levels of the CE
minimize this effect. Redispersible polymer powders are
additional binders to the cement in dry-mix mortar formulations
which enhance the strength and the flexibility of the tile
adhesive. The increasing acceptance of norms and regulations
has contributed to improved quality of tile adhesives, which
require higher levels of additives. As both CE and RDP constitute
a significant cost factor in CBTA formulations, a detailed
knowledge of their interaction will help to identify the right
product combinations to optimize dosage and performance.
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1.5% viscosity [Pa.s]
Before investigating potential interactions of CE and RDP it is
important to understand the individual roles of these additives.
Many of the performance tests have been run according to
European Norm EN 12004. However, our experience shows
that the trend of test results with structural parameters of the
additives correlates well with ANSI standards. The qualitative
results of these studies are therefore also valid for North
American conditions.
CELLuLoSE EthErS
Cellulose ethers (HPMC and HEMC) are available in different
molecular weights and substitution levels. The following data
have been generated based on a range of various commercially
available cellulose ethers. Their structural properties were
analyzed and they were all tested according to EN 12004 in
the following standard tile adhesive formulation:
Portland Cement CEM I 42.5
40.00 pbw
Quartz Sand (0.1 – 0.3 mm)
60.00 pbw
Cellulose Ether
0.35 pbw
Starch Ether
0.08 pbw
rDP
0.50 pbw
Water
24.00 pbw
weight average molecular weight
Figure 1: Plot of CE viscosity in Pa�s
(1.5% aqueous solution at a share rate of 2.0 s-1)
against weight-average molecular weight
The aqueous viscosity of the cellulose ether has an important
influence on the rheology of the tile adhesive formulation. The
rheology of mortars can be reliably measured with a Brookfield
RVT rheometer equipped with a T-spindle and Helipath.
In order to understand the impact of additives on CBTA
formulations, both the raw mortar properties as well as the
final adhesive strength properties have to be considered. They
are largely interrelated. Some of the raw mortar properties
can even be correlated with the rheology of the aqueous
solutions of CE.
The predominant factor controlling the viscosity of CE
solutions in water is the molecular weight of the linear
polymer. The weight-average molecular weight of the cellulose
ethers Mw has been determined by gel permeation
chromatography. Plotting the molecular weight against the
viscosity of a 1.5% aqueous solution gives a linear relationship
(see figure 1).
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BF visco 5.0 rpm
Table 1: Standard tile Adhesive Formulation
1.5% viscosity
Figure 2: Plot of mortar consistency (as Brookfield viscosity
at 5.0 rpm) against aqueous viscosity of the
cellulose ether
Control of the substitution levels of both substituents in
HEMC and HPMC products allows for adjustment of the
surface activity of cellulose ethers. The higher the surface
activity of the cellulose ether, the more air will be entrapped
during the preparation of the cementitious tile adhesive
reducing its fresh mortar density.
BS after 20 min. OT [N/mm2]
mortar density (kg/l)
In order to achieve the same rheology and water retention,
high viscous cellulose ethers can be used at a lower addition
level; thus saving formulation cost. The viscosity of the
cellulose does not only affect the raw mortar properties of
the adhesive; it can also have a strong impact on the final
strength proper ties. Reducing the viscosity of the CE has a
positive effect on the open time of the tile adhesive as a
function of bond strength.
surface tension 1% [dyn/cm]
1.5% viscosity
Figure 5: Fresh mortar density as a function of CE
surface tension
Figure 3: bonding strength after 20min open time as a
function of CE viscosity
BS after 20 min. OT [N/mm2]
BS after water aging
The reason for this is enhanced wetting capabilities of
cellulose ethers with lower molecular weight at the same
dosage level. Larger tile surface coverage of the adhesive
results in higher bonding strength.
Low mortar density is beneficial for the workability of the
adhesive; thus, requiring less force to spread the mortar.
Mortar density has also a strong impact on the strength
properties; particularly after the wet aging step.
The reason for this relationship will be explained later.
mortar density (kg/l)
Mw
Figure 4: Surface coverage (wetting) as a function of CE
weight-average molecular weight
Figure 6: Plot of bonding strength after wet aging against fresh
mortar density
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Cellulose ether with a weak surface activity will contribute to
higher mortar density and improve bond strength. This trend,
however, is in conflict with the need for easy workability,
which requires low fresh mortar densities. Low fresh mortar
density contributes to lower bond strength; particularly after
the open time test. In order to find out if the lower mortar
densities also result in higher evaporation rates (drying out of
the mortar at a faster rate), we have compared the water loss
with time of four different CBTA formulations.
Figure 8: Schematic cross section of an applied tile
adhesive ridge
Water Content [%]
After a period of time, the tile is placed in the mortar bed
and a weight is applied. The initial contact of the tile to the
adhesive is at the top edge of the ridges. The skin prevents
the adhesive from wetting the tile. After the weight is applied,
the wet adhesive from the center of the ridges squeezes out
at the sides, and wets the tile between the initial ridges.
Open Time [min]
Figure 7: Water loss of tile adhesive formulations with time
Within the accuracy of the experiment, the water loss is
independent from the fresh mortar density. The water loss
seems to be more pronounced during the first five minutes
after the adhesive has been applied. It slows down during
the next ten minutes, and starts to become more pronounced
again after 15 minutes. This is in line with a mechanism
proposed by Jenni et al. . The explanation for this behavior
could be that water evaporates quickly on the surface of the
CBTA ridges. A thin CE film forms after 5 minutes, preventing
further evaporation. More water migrates from the center of
the ridges, partially dissolves the initial film, and eventually
evaporates. The water flux also transports additional CE to
the surface of the tile adhesive where it enriches with time,
forming a more solid skin. This skin will not dissolve anymore;
thus preventing the adhesive from wetting the tile.
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Figure 9: Photograph of tile breaking pattern after open time
The picture above illustrates the breaking pattern after
the tile is pulled off after 28 days. There is cohesive failure
(breakage within the adhesive) only in the space between
the ridges. The mechanism of skin formation is still subject
to internal research.
The degree of substitution (DS) of methyl groups has a strong
influence on the kinetics of cement setting.
Temperature [deg C]
It was confirmed that sucrose acts via nucleation poisoning/
surface adsorption. The same effect was demonstrated on the
polymer as well. H.J. Weyer et al. have monitored the cement
hydration with the help of synchrotron radiation in-situ in the
presence of cellulose ethers. They found out that the level of
methylation has a significant effect on the hydrolysis of the
C2S/C3S phases. With increasing methyl-substitution, the
anhydroglucose unit of the cellulose ether becomes more
hydrophobic, adsorbs less on the clinker phase surface, and
consequently has less retardation effect (see figure 11).
time [hours]
A strong retardation effect on the cement setting enables the
water to evaporate and dry out the adhesive before the cement
can completely hydrate; thus, we observe lower strength
properties of the adhesive (figure 12) with low methyl DS.
Figure 10 shows the temperature development of cement
setting in the presence of various types of cellulose ethers.
The three curves with temperature peaks at 7 hours (see
arrow) represent samples without cellulose ether. Plotting
the delay of the cement setting against the degree of methyl
substitution DS reveals a linear relationship.
bonding strength [N/mm2]
Figure 10: Adiabatic temperature development of cement
setting in the presence of various cellulose ethers
delay time [hours]
Methyl DS
Figure 12: CBtA bonding strength as a function of
methyl degree of substitution
Methyl DS
The cement setting will resume once the adhesive contacts
water. The bonding strength after the water aging step is at
a higher level, and almost independent from the methyl DS,
as can be seen in figure 13. This effect is more pronounced
in formulations containing CE with low methyl DS.
Figure 11: Cement setting delay time plotted against
methyl substitution level
It has been shown that sucrose and other glucose derivatives
accelerate ettringite formation, but are more effective at retarding
C3S (tri-calcium silicate) phase hydration.
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bond strength [N/mm2]
1% RDP
Methyl DS
Figure 13: CBtA bonding strength after various aging
steps as a function of methyl substitution level
The bond strength after open time is usually weaker and
follows a different mechanism as described earlier. Upon heat
aging at 70ºC, the adhesive is subject to thermal stress at the
tile/adhesive interface. This reduces the bond strength significantly.
The addition of RDP helps to make the adhesive more flexible,
and improves the bond strength. Depending on the dosage
level of the polymer powder, the cohesive strength is improved
because the interface between tile and adhesive gets stronger.
Adding more RDP to the tile adhesive formulation causes the
adhesive to break within the cement matrix and not at the
interface as illustrated in figure 14. The breaking pattern of the
tile after the heat aging test is shown at 1%, 3% and 5%
addition level of a redispersible polymer powder.
3% RDP
5% RDP
Figure 14: Breaking pattern of tiles after the heat aging
test with various amounts of rDP
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Liquid polymer dispersions can be spray-dried and used as
a powder component in dry-mix mortar formulations. When
mixed with water, the powder redisperses and forms films
in the cement matrix, which further enhance the strength
properties of cementitious mortars. Vinylacetate/Ethylene
copolymers are the preferred polymers in the market for
producing redispersible polymer powders. We have compared
two RDP products in a CBTA formulation at two different
dosage levels.
Composition
tg [ºC]
MFFt [ºC]
Polymer 1
Vinylacetate/Ethylene
6
0
Polymer 2
Vinylacetate/Ethylene
17
3
share bond strength [psi]
rEDISPErSIBLE PoLYMEr PoWDEr
Table 2: CBtA Formulation
Polymer 2 has a higher glass transition temperature compared
to Polymer 1. The adhesives have been tested according to
some of the shear bond specifications of ANSI A118.4 and
A118.11 in the following formulations:
Component
unit
Formulation 1
Formulation 2
Cement
pbw
40.0
40.0
Sand
pbw
56.7
54.7
rDP
pbw
2.0
4.0
hPMC
pbw
0.3
0.3
Water
pbw
24.0
24.0
Table 3: ANSI A118.4 / A118.11 Formulations
Figure 15: ANSI A118.4 – 5.0 Shear Strength to Impervious
Ceramic Mosaic tile (vitreous tile)
The shear bond strength of two impervious ceramic mosaic
tiles (low absorption vitreous tiles) after various aging periods
is illustrated in figure 15. The shear bond strength after 7 days
is the case of 2% RDP dosage level, specific to this evaluation,
higher compared to the 28 days. The reason for this is still
unknown to us. When comparing the shear bond strength
after 28 days for the five formulations, it becomes obvious
that that the use of RDP in the formulation improves the
strength properties of the adhesive. The harder polymer
(Polymer 2) provides higher strength.
After the test specimens have been immersed in water for
7 days, the shear bond strengths decrease to the level
observed for the mortar without polymer powder. In
formulations with 4% RDP the drop in strength is even more
pronounced. The water immersion step seems to completely
reduce the contribution of the RDP to the mortar strength.
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share bond strength [psi]
The base formulation was:
Quartz Sand (0.1 – 0.3 mm)
60.00 pbw
Portland Cement CEM I 42.5
40.00 pbw
Cellulose Ether
0.35 pbw
Water
26.00 pbw
Three independent variables:
Variable
Parameters
CE Chemistry
rDP dosage
rDP grade
Figure 16: Shear bond 4”x 4” Quarry/Wood
When quarry tile / plywood composites are tested for shear
bond strength, the impact of the polymer becomes more
pronounced, as illustrated in figure 16. Adding 2% of polymer
1 to the formulation doubles the strength of the adhesive.
With 4% of polymer 2 the strength is almost four times the
value of the adhesive without polymer powder. An increase
in bond strength is also observed between formulations
containing polymer 1 and polymer 2; most likely due to the
higher glass transition temperature of polymer 2.
INtErACtIoN oF CELLuLoSE EthEr
AND rEDISPErSIBLE PoLYMEr PoWDEr
Formulators of cement based tile adhesive try to optimize
the cost/performance ratio by selecting the right combination
of cellulose ethers and redispersible polymer powders.
The understanding of synergistic effects or incompatibilities
between these additives is critical. We have used an
experimental design to screen the impact of CE and RDP in
a CBTA application and to identify potential interactions.
hEMC
hPMC
1%
3%
5%
rDP1
rDP2
rDP3
Both cellulose ethers have an aqueous viscosity of
35,000 mPa*s (2% aqueous solution, at 20 rpm Brookfield
RVT, spindle 4, 23ºC)
Variable
CE Chemistry
Parameters
hEMC
hPMC
DS Meo
1.6
1.8
MS hPo/hEo
0.27
0.15
The RDP grades represent:
rDP1
Vinylacetate/ethylene copolymer
with neutral rheology
rDP2
Vinylacetate/ethylene/VeoVa* terpolymer with
neutral rheology
rDP3
Vinylacetate/VeoVa* copolymer
with thickening effect
Fresh mortar density is an important parameter affecting the
strength properties of cement based tile adhesives. Both the
type of CE and the type and dosage of the RDP play an important role for the density of the mortar. Figure 17 illustrates the
possible combinations and their effect on mortar density.
* VeoVa = vinyl ester of versatic acid
(a synthetic, highly branched-C10 tertiary carboxylic acid)
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1
2
3
4
Figure 18: Fresh mortar density, consistency and sliding
resistance as a function of CE type and rDP type and amount
at 3% rDP dosage
Figure 17: Combinations of CE and rDP and
their effect on fresh mortar density
Figure 17 shows a matrix of some of the possible combinations
of CE and RDP in the adhesive and their effect on mortar
density. When switching from the HEMC to the HPMC (case
1 to case 2) in the presence of RDP 1, the density slightly
increases due to the lower surface activity of the HPMC.
RDP 2, however, has a strong interfacial activity as well, and
decreases the density due to more effective air entrainment
(case 3). The strong surface activity of RDP 2 masks the effect
of the cellulose ether. In the three first cases, the amount of
RDP has a negative effect on the mortar density. However,
when using RDP 3 this trend is reversed. RDP 3 has a
defoaming and thickening effect. This property needs to
be considered when it is used in combination with strongly
modified cellulose ethers.
The main effects of redispersible polymer powder in tile
adhesive formulations are to increase bond strength and
flexibility. The right combination of CE and RDP is critical in
optimizing the cost/performance ratio. The following test
results (figure 19) were obtained in accordance with
EN 12004 test methods.
1
2
3
Fresh mortar density and mortar consistency typically show
the same trend if no additional thickeners are present. The
increase in mortar density and consistency has also a positive
effect on the tile sliding resistance as figure 18 illustrates.
High consistency of the adhesive mortar typically results in
less sliding of the tile on a vertical substrate.
4
Figure 19: Bond strength as a function of combinations of CE
and rDP at 3% rDP dosage
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Using HEMC in combination with RDP 1 gives less bond
strength compared to the HPMC (case 1 and case 2). The
reason for higher strength is the higher methyl DS of the
HPMC, which has less impact on the cement retardation
(see figure 12: CBTA bonding strength as a function of methyl
degree of substitution). When using RDP 2, the bond strength
slightly improves (case 3). This polymer powder also
compensates for the decreased bond strength caused by
the retarded cement setting of the low substituted HEMC.
The adhesive strength is independent from the selected
cellulose in case 3 and case 4 (RDP 3).
The measured bond strength is composed of two noted
contributors: cement matrix and polymer film. The right choice
of the cellulose ether allows for optimization of the cement
matrix strength. If the polymer film is strong enough, it can
compensate for poor CE performance (RDP 2 and RDP3).
Standard CBTA products contain lower levels of RDP; just
enough to pass the standards for the bond strength after heat
aging. The cellulose ether effect on the cement dominates the
bond strength of the adhesive (see figure 20).
Figure 21: Bonding strength after 20 min open time
Increasing the amount of RDP improves the bond strength
after open time in all cases. RDP 2 is the most effective
product in high performance tile adhesives (RDP dosage > 3%)
for improving the bond strength after 20 minutes of open time
as illustrated in figure 21.
Standard RDP products based on VA/E co-polymers do
not help to improve bond strength after water immersion
(wet aging). The polymer film swells and looses all strength
properties in the wet state. The measured strength is only
based on the contribution of the cement matrix. The presence
of a hydrophobic monomer like VeoVa in the polymer backbone
has a positive effect on the hydrolytic stability of such CBTA
products (figure 22).
Figure 20: Bonding strength at low rDP dosage
At an RDP addition level of 1.0% (or lower) the cement
component of the overall strength property prevails, which
makes the highly substituted HPMC the CE of choice in
combination with RDP 1.
Bond strength after open time is an important performance
criterion for CBTA products. Within the scope of the described
experimental design, the bond strength after open time can
only be controlled by the type and dosage of the polymer
powder.
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Figure 22: Bonding strength after water immersion
using different types of rDP
SuMMArY
The difference in composition of RDP 1, 2 and 3 were listed
earlier. Their content of VeoVa co-monomer is 0%, 20% and
40%, respectively. To resist wet aging the content of this
hydrophobic co-monomer is critical. The impact of the CE
is insignificant.
When using RDP 1, the bond strength after water immersion
even slightly decreases with increasing addition levels.
Using RDP 2 in place of RDP 1 reverses this trend, enhancing
the hydrolytic stability of the CBTA formulation. RDP 3
(VA/VeoVa copolymer 60:40) provides further enhancement
of the hydrolytic stability of CBTA’s.
To pass the heat aging test, the composition of the RDP
is of minor importance. The main contributor to the bond
strength of the adhesive is addition level of RDP (figure 23).
Figure 23: Bonding strength after heat aging at 70ºC
The HPMC provides slightly higher bond strength compared to
the HEMC.
Cellulose ethers and redispersible polymer powders are
key performance additives in cement based tile adhesives.
They both determine fresh mortar properties as well as the
final strength of the mortar. We were able to demonstrate
that the detailed understanding of the individual roles of
these components can be successfully used to optimize the
performance of CBTA formulations.
The impact of the cellulose ether on the performance of tile
adhesives is based on their rheological characteristics and
on the retardation of the cement setting kinetics. Many of
the effects caused by their molecular structure are in conflict
with each other. The thickening effect of cellulose ethers
is strongly related to their molecular weight. High viscous
cellulose ethers have a high water demand, but they also
reduce sliding resistance and limit open time. The surface
activity of CE is determined by the substitution pattern.
Highly surface-active grades incorporate more air during the
mixing process of the CBTA; thus, reducing the fresh mortar
density. This effect is beneficial to the workability of the
mortar. However, the strength properties of the adhesive are
negatively affected. The cement retardation can be linearly
correlated with the methyl degree of substitution of the CE.
CE with higher methyl DS will retard the cement setting to a
lesser extent, resulting in higher bond strength.
The use of redispersible polymer powders in CBTA
formulations, enhance the strength of the adhesive mortar.
VA/E copolymer based RDP’s have a different effect on the
bond strength depending on their hardness, as indicative of
the Tg. A higher glass transition temperature of the polymer
results in superior strength properties. VA/E based polymer
powders do not help to improve bond strength per the water
immersion test. The presence of a hydrophobic co-monomer
like VeoVa is required to improve the hydrolytic stability of the
tile adhesive.
Experimental design is a very useful tool to screen the impact
of additives in complex formulations. Interactions of CE’s and
RDP’s in CBTA formulations were observed, which can be
applied synergistically to improve the cost/performance ratio.
In standard tile adhesives with low RDP levels, the mortar
strength properties are largely influenced by the cement
retardation effect of the CE. Products with high methyl DS
will give superior strength properties. High performance tile
adhesive contain higher levels of RDP (3%– 5%) to meet the
requirements for extended bond and flexural strength. High
performing RDP products can compensate the negative effect
of CE on the cement setting.
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AuthorS
Robert Baumann, Ken Tarantul, Cindy Powell, Andreas Wichmann, Christa Tepper, Marga Perello, Mark van
Heeringen, Wolfgang Koch, Phil Griggs, Michael Radler / The Dow Chemical Company
REFERENCES
1.A . Jenni, L. Holzer, R. Zurbriggen, M. Herwegh
“Influence of polymers on microstructure and adhesive strength of
cementitious tile adhesive mortars” Cement and Concrete Research 35
(2005) 35– 50
2.Bishop, Maximilienne; Barron, Andrew R.
“Cement Hydration Inhibition with Sucrose, Tartaric Acid, and
Lignosulfonate: Analytical and Spectroscopic Study”; Industrial
& Engineering Chemistry Research (2006), 45(21), 7042-7049
3.H.J. Weyer, I. Müller, B. Schmitt, D. Bosbach, A. Putnis, “Time-resolved
monitoring of cement hydration: influence of cellulose ethers on reaction
kinetics“; I. Nucl. Instr. and Meth. in Phys. Res. B 238 (2005) 102-106
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