Journal of Chemical Engineering and Chemistry
Revista de Engenharia Química e Química - REQ2
ISSN: 2446-9416
Vol. 01 N. 02 (2015) 065–079
doi: 10.18540/2446941601022015065
AMPHOTERIC POLYMERS TO IMPROVE PAPER
DRY STRENGTH
D. J. SILVA1, S. W. PARK2, M. A. HUBBE3 and O. J. ROJAS3,4
Universidade Federal de Viçosa, Chemistry Department
1
Polytechnic School University of São Paulo, Chemical Engineering Department
3
North Carolina State University, Forest Biomaterials Science and Engineering
Department
4
Aalto University, Forest Products Technology Department
E-mail: deusanilde@ufv.br
2
SUMMARY: The objective of the present work is to evaluate the application of an
amphoteric polymer, with acidic and basic groups on the polymer chain, as a paper
dry strength additive. The polymer studied here is random terpolymer of high
electrostatic charge density, and high molecular weight. The results of our work
showed that the balance between the charge densities of the surface and the
polymer structures is an important factor to be considered when using this polymer.
The highest paper strength value, measured by tensile index, was found at the
polyampholyte’s isoelectric point (ca. pH of 7.3) when the charge of the fiber
surface was negative and the polymer structure charge was symmetric. This
observation agrees with our dynamic light scattering results which, demonstrated
that at the isoelectric point there was a maximum in association among
polyampholyte molecules, leading to a maximum in size of molecular aggregates.
When adsorbed on an electrically charged surface, the maximum amount of
adsorbed polymer, measured by the shift in resonance frequency in a quartz crystal
microgravimetric balance, was observed for the same isoelectric point. Better
results for paper dry strength were found when the fiber surface and the polymer
structures were oppositely charged or at the isoelectric point of the polymer. Less
effective addition strategies were found in the case when the fiber surfaces and the
polymer structures had same sign of charge.
KEYWORDS: Nanotechnology; Polyampholytes; Paper strength; Surface charge
density; Electrostatic interaction; Colloidal chemistry.
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
1. INTRODUCTION
The application of wet end agents in papermaking is a common practice to
meet the specific demands of the process and the paper product. Mechanical
strength is important in printing grades for use in modern high-speed operations.
Among those chemical additives, the polyacrylamides and starches are the most
often applied for the purpose of increasing the dry strength of the paper. Other
additives studied to increase the dry strength of papers are the polyampholytes
(Carr et al., 1977; Song, 2006; Hubbe et al., 2007; Silva et al. 2009a).
Polyampholytes are polymers that have positive and negative charges in the
same molecular chain and have different properties when compared with
monocharged polyelectrolytes (Bohidar, 2002). This author notes that a particular
feature that these molecules is their antipolyelectrolyte behavior. When in solution,
the solubility increases and the molecule expands with increasing ionic strength at
the isoelectric point. Another peculiarity is that these macromolecules assume
different states of aggregation under different pH conditions. At the pH of the
isoelectric point, when the charge density is symmetric, polymer precipitation also
occurs, the solubility is low (Patrickios, 1999), and the aggregation process
increases.
According to the literature, the first work applying polyampholyte as a
paper dry strength additive was published in 1977 by Carr et al. (1977).
Polyampholytes based on starch were prepared by the xanthation of the cationic
corn starch derivatives, which had tertiary amine groups, [-CH2CH2N (C2H5)2] or
ammonium quaternary [-CH2CHOHCH2N+(CH3)3]. Xanthate anionic groups were
introduced in aminated cationic starch. The authors showed that the addition of
starch-based polyampholyte in the wet end was effective to increase the dry and the
wet strength, and the strength values were higher than those obtained by starch
polyelectrolytes only, either cationic or anionic (Carr et al., 1977).
More recent study concerning the use of polyampholytes as a dry strength
agents for paper was undertaken by Hubbe et al. (2007). These authors observed an
increasing in the breaking length when this kind of polymer was added to the fibers
suspension. More favorable results for this application were found when the pH
condition was close to the isoelectric point of the polymer.
The charge densities of the polymer and the solid surface are considered as
parameters that control the adsorption behavior. The adsorption of additives on the
cellulosic surface occurs in the solid-liquid interface mainly through electrostatic
interactions. The charge development in aqueous medium is due to the presence of
functional groups. In papermaking, where cellulosic material is used, the
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
carboxylic groups are mainly responsible for the development of charge on the
fiber surfaces (Fardim, 2005; Zhang, 2006). In the case of polymers, besides
carboxylic groups, other groups such as quaternary ammonium groups also develop
charge. An important aspect in an aqueous medium which influences the
development of electrical charge, both surface and additives, is the concentration of
hydrogen protons in the solution, which determines the pH. It is expected that at
lower values of pH, the carboxylic groups, for example, are found more in the
protonated form than in ionized form. On the other hand, the degree of ionization
of those groups at higher pH values is higher, i.e., more deprotonated. At this pH
range, the net charge of the cellulosic material is negative, as reported in the
literature (Radtchenko, 2005). This author measured the charge densities of silica
and cellulose surfaces by using atomic force microscopy technique. The results
were as follows: silica -0.40 and -2.0 mC.m-2, and for cellulose -0.21 and -0.80
mC.m-2, for pH 4.0 and 9.5, respectively.
Adsorption behavior is also dependent on the concentration of electrolytes
in a dispersion medium (Fleer, 1993). This author attributed adsorption, at low salt
concentrations, to ion exchange mechanism that occur in the electrical double layer
due to the net gain of entropy by the release of counter-ions for the liquid phase,
favoring electrostatic interactions. For intermediate salt concentrations, beside the
electrostatic interactions, nonionic interactions may also occur. In this case, the
increasing in salt concentration reduces the repulsion between the polymer chains
increasing surface adsorption. On the other hand, for high concentrations of salt,
the electrostatic interaction between the polyelectrolyte and the surface charged is
reduced by electrostatic screening (Fleer, 1993).
The polyampholyte behavior in solution, associated with its adsorbed layer
viscoelasticity, has been studied to explain the performance of these polymers as
dry strength agents (Song, 2008; Silva et al., 2008; Silva et al., 2009a; Song et al.,
2010). In these studies, by using quartz crystal microbalance technique, it was
observed that the energy dissipation of adsorbed layers was related to their
hydration degree and, therefore, the amount of water entrapped within the
aggregates formed at different pH levels.
The objective of the present work is to evaluate the application of an
amphoteric polymer as a paper dry strength additive, measured by paper tensile
index, for different net charge densities of the substrate and polymer.
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
2. EXPERIMENTAL
2.1. Materials
A milli-Q unit was used as a source of ultra-pure water in our experiments.
Sodium hydroxide and hydrogen chloride, both at 0.1N aqueous concentrations,
were used to adjust the pH. Sodium chloride was used as a supporting salt to adjust
the ionic strength of the buffer solutions. All inorganic chemicals used in this work
were of analytical grade.
Polyampholyte: The amphoteric polymer (PAmp) used was prepared by
random, free-radical polymerization. The cationic monomer was N-[3-(N′,N′dimethylamino)propyl] acrylamide (DMAPAA), a tertiary amine. The anionic
monomer was methylene butanedioic acid, also known as itaconic acid (IA).
Sufficient neutral acrylamide monomer was added to achieve each selected molar
composition (Figure 1). The molecular weight of the polyampholyte synthesized
was 2.93 x 106 Da with cationic to anionic group ratio of 5:4, as measured by
NMR. Aqueous solution (1g.L-1) viscosity was 2,400 mPa.s (25 °C) and the iep
was 7.5 (Wang, 2006).
Figure 1 - Polyampholyte molecular composition prepared by random, freeradical polymerization.
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
Cellulosic fibers: Pre-treated Eucalyptus bleached kraft pulp from Brazil was used
in this study. The pre-treatment consisted in removing the fine fraction by washing with
plenty running water through a 100-mesh screen. After this step, the pH and salt (NaCl)
concentration were adjusted to 6.5 and 1 mM, respectively. A sample was collected to
carry out fiber analysis with a Fiber Quality Analyzer LDA02 (see Table 1). The
suspension was dewatered by centrifugation and the wet pre-treated pulp was
conditioned in a polyethylene bag under refrigeration. The moisture was determined in
triplicate.
Table 1 - Fiber Quality Analysis.
Parameter
Mean
length
(mm)
Mean
width
(µm)
Fines
(%)
Vessel
(#)
Vessel
length
(mm)
Vessel
width
(µm)
Arithmetic
average
0.719
16.43
0.689
6.5
0.533
133.48
Standard
deviation
0.006
0.096
0.059
2.082
0.032
9.194
Note: The number of fibers counted was 3000. The fine fraction was classified as such if the fibrous
elements had an effective dimension between 0.05 and 0.2 mm.
Model surface: Negatively charged silica substrates supplied by Q-Sense, Sweden
were used in quartz crystal microgravimetry (QCM-D) experiments. The substrate
consisted of a 50 nm silicone dioxide outer layer (active side) coated on both sides of
layers consisting of Ti (10 nm), Au (100 nm) and Cr (5 nm) and quartz. Before use the
silica substrates were cleaned according to the following procedure: they were
immersed in a 2% w/w Hellmanex solution (HELLMA Worldwide) and placed in an
ultrasonic cleaner for 20 min followed by rinsing with milli-Q water. In order to
activate the silanol groups (Duval, 2002), the substrates were immersed in a 10% w/w
NaOH solution for 3 min followed by rinsing with milli-Q water and drying with
nitrogen gas and final conditioning in a desiccator. Before use, the substrates were
exposed to UV-ozone light (28 mW.cm-2 at 254 nm) for 15 min.
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
2.2. Methods
Polyampholyte adsorption on fiber surfaces, handsheet preparation and
paper strength: Handsheets were prepared following TAPPI method T205. The
fibers were re-suspended as 0.5 % slurry. The required pH was adjusted at a fixed
salt concentration of 10 mM and PAmp dosage of 0.3% on fiber. Fresh 1 g.L-1
PAmp solutions (168 mg.L-1 active product) were mixed under constant stirring at
the selected dosage and pH. The volume of each solution added was determined to
adjust the slurry consistency. The equilibration time was 30 min. The pH was
continuously controlled. In order to maintain the same pH and salt concentration
during the sheet formation, a calibration curve with distillated water was carried
out in order to ensure the same set of conditions during formation of each
handsheet. Tensile strength was measured following the TAPPI method T404.
Polyampholyte solution behavior: Solution properties with dissolved
polymer were determined for each pH at a fixed salt concentration of 10 mM.
Turbidity measurements for polyampholyte solutions at different pH’s were carried
out with a DRT-15CE turbidimeter from HF Scientific, Inc.
The theoretical maximum charge density of the PAmp used in this study
was 20 mol% for the cationic groups and 16 mol% for the anionic groups (Wang,
2007). As the ionizable groups on this kind of polymer are in equilibrium with OHions and H3O+ ions, we expect that the net charge in the bulk solution will change
with pH. Charge density measurements were done at 10 mM of salt. A 10 ml
aliquot of 168 mg.L-1 PAmp solution was titrated with 0.0025N polyvinyl sulfate
potassium salt (PVSK) to a neutral streaming current endpoint, as determined by a
Mütek PCD-03pH device from BTG. Three replicate runs were carried out for each
PAmp solution.
The hydrodynamic diameter of the polymer in solution at different pH
levels was obtained by using a dynamic scattering system, the Beckman Coulter N4
Plus. This system employs a 10-mW Helium-Neon laser (λ=632.8 nm) as a light
source. The measurements of the Autocorrelation Function (ACF) were performed
for each pH solution, and a set concentration of 336 mg.L-1 was used in order to
ensure diffusion particle range per sec of recommended by the manufacturer, 5.0 x
104 to 1.0 x 106. The results from the ACF analysis were related to diffusion
coefficient, and therefore the particle sizes were calculated by using StokesEinstein equation (Eq. 1). Each sample was investigated at three different scattering
angles, 32.6, 62.3, and 90 deg. Three replicates were performed, and the results
with smaller standard deviation are reported here. All experiments were performed
at room temperature.
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
D
K BT
3d
( 1)
Where, D = diffusion coefficient, K B = Boltzmann constant (1.38x10-16
erg.°K-1), T = temperature (°K); = diluent viscosity (poise), and d = equivalent
spherical diameter (nm).
Polyampholyte adsorption behavior on model surface: Quartz Crystal
Microgravimetry with Energy Dissipation (QCM-D) was used to study
polyampholyte adsorption phenomena. The instrument consisted of an E4 (Q-Sense,
Sweden) coupled with a syringe pump for flow-through operation. The changes in
resonance frequency of a silica quartz crystal electrode upon polyampholyte
adsorption were monitored with time, enabling us to study the dynamics of the
adsorption process. The energy dissipation, related to the damping of the sensor
oscillation, was used to study the viscoelastic properties of the adsorbed layer. This
information was relevant to understand the relationship between the structure of the
adsorbed layer and the hydration behavior of the macromolecules at the interface.
The changes in QCM frequencies were assumed to be proportional to the
adsorbed mass, as is the case for adsorbed rigid layers, as reported by Sauerbrey
(Sauerbrey, 1959). The fundamental frequency fo (oscillating frequency without
adsorbed mass) was 4.95 MHz, and the sensitivity constant ( C ) used in the
Sauerbrey equation (Eq. 2) to calculate the added or adsorbed mass Δm was 17.7
ng.Hz-1.cm-2:
m
C.f
n
(2)
Where n is the overtone number (n = 1, 3, 5, 7). More information about
this technique can be found in (Silva et al., 2008; Silva et al., 2009b; Song et al.,
2010).
The thickness and the specific adsorbed PAmp layer were calculated
considering the density of 1255 kg.m-3 (Silva et al., 2009b).
The QCM-D frequency and dissipation data were monitored with time
before and after polymer injection. Rinsing with buffer solution at the end of the
adsorption experiments was typically performed to determine the amount that was
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
irreversibly adsorbed. During the measurements, the QCM liquid chamber was
temperature-stabilized to 25 °C and buffer solution was injected at a flow rate of
130µL.min-1. All experiments with the QCM were repeated at least two times.
Although all the measurements were recorded at five harmonic frequencies in
continuous mode (130µL.min-1 flow rate), the third overtone (15 MHz) was
primarily used in the evaluation of the data presented here.
3. RESULTS AND DISCUSSION
3.1. Polymer solution behavior
In case of the highly charged (asymmetric) polyampholyte studied here, the
pH responsiveness in solution was very significant. The PAmp turbidity and net
charge density showed different values in the range of pH studied (Figure 2 a 3). At
extreme pH’s the PAmp seems to be fully solubilized, forming single or small
aggregates that have little tendency to scatter light. In this case, no turbidity and
small structure sizes are expected (Figure 2, left), and we have charge densities
close to a single charged polyelectrolyte (Figure 2, right). On the other hand, the
low values of turbidity associated with intermediate pH (pH 6 to 8.7) could be due
to either (a) low light scattering efficiency when the polymers are all gathered
together into a few big agglomerates, and (b) the likelihood that such agglomerates
would quickly precipitate out of solution and not remain in the beam of the
turbidimeter. In this case the charge of the polymer tends to symmetry. Some
studies in the literature also found changes in turbidity of polyampholyte solutions
with the changing in pH (Mahltig, 1999; Patrickios, 1999; Mahltig, 2000; Sezaki,
2006).
The change of the polymer charge balance with the pH affected the size of
the aggregates as we can see at Figure 3, on the left. Smaller structures could be
found at pH far from the iep, conditions under which the polymer presents
asymmetrical positive charge (acidic pH – Figure 2, right) or negative (alkaline pH,
Figure 2, right) and its behavior is closer to that of a monocharged polymer. Larger
structures could be found at pH close to iep (pH 7.3). However, there was wide
variation in the results for polydispersity of the structures along the pH range
studied (Figure 3, right). According to the results, wider size distribution can be
observed at extreme pH’s where that intra and inter molecular polymer chain
interaction would take place. Moreover, the tendency to lower values of
polydispersity was found for intermediate values of pH close to the iep.
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
Figure 2 - Effect of pH on the turbidity and charge density of the polymer in
solution.
Figure 3 - Effect of pH on the hydrodinamic diameter and polydispersity
index of the polymer in solution.
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3.2. Effect of the polymer addition on the paper tensile strength
In Figure 4 the results of handsheet paper tensile index were plotted against
PAmp charge density. This graph considers all of our results at a fixed pH (4.3) for
different ionic strength (5, 10 and 100 mM), and with fixed ionic strength (10 mM)
for different pH (4.3, 5.35, 6.35 and 8.48). We are also considering that the
cellulosic material developed negative charge within a wide pH range, 4 up to 9.5,
in agreement to what is reported in the literature (Radtchenko, 2005).
We verified that larger tensile index results were found when the PAmp and
the cellulosic materials showed opposite charges (right side region of the graph).
On the other hand, worse results were found were fibers and the polymer showed
the same charge (left side region of the graph). In addition, the tensile index
appeared to increase when the cellulose material was negative and the charge of the
polymer tended to be symmetric.
Figure 4 - Effect of PAmp charge density on the paper tensile index.
This tend can be better verified when the results of paper tensile strength
are plotted against the polymer aggregate diameter (Figure 5). In this graph, the
curve was built with the results where the condition or charge density is opposite
for substrate, fibers, and PAmp, and the polymer charge symmetric. On the hand,
the tensile index average result where the fibers and the PAmp have the same
charge density is out of the curve (full triangle). It looks like an out-lier. The
behaviour verified in Figure 5 showed that, besides the size the polymer aggregates,
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
the substrate and the polymer charges, dictated by the pH of the suspension, is an
important factor that has to take in an account. Even though in this case (filled
triangle) the diameter is higher than the diameter results of the points where the
substrate and the polymer exhibit opposite charge, the paper tensile strength was
lower.
Figure 5 - Effect of PAmp structure diameter on the paper tensile index.
The Figure 6 shows the results of paper tensile strength as a function of the
thickness and the specific adsorbed mass. These results where obtained from the
adsorption Quartz Crystal Microbalance with Energy Dissipation analyses after
rinsing step.
These results are in agreement with the last results. Higher thickness and
specific adsorbed mass were found for the condition where the substrate was
negative and the polymer structures showed charge symmetry. On the other hand,
even though the thickness and the adsorbed mass are high for the condition with
substrate and polymer with the same charge, the paper tensile strength results were
lower than the other points.
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
Figure 6 - Effect of thickness and the specific adsorbed mass on the paper
tensile index.
4. CONCLUSIONS
According to the results obtained in this study, the following conclusions can
be drawn: (a) the balance in charge densities of the surfaces and the polymer
structures is an important factor to be considered when using the amphoteric
polymer studied here; (b) the optimal paper strength value, measured with the
tensile index, was found to be at the polyampholyte’s isoelectric point (ca. pH of
7.3) when the polymer structure charge was symmetric. This is in agreement with
dynamic light scattering results, which demonstrated that at the isoelectric point
occurred a maximum in association among polyampholyte molecules, leading to a
maximum in size of molecular aggregates, and possible more hydrated structures;
(c) favorable results for paper dry strength was found when the fiber surface and
the polymer structures were oppositely charge or at the isoelectric point of the
polymer. Less effective addition strategies were found in the case when the fiber
surfaces and the polymer structures had same sign of net charge.
Journal of Chemical Engineering and Chemistry - Vol. 01 N. 02 (2015) 065–079
5. ACKNOWLEDGEMENTS
The financial support of CAPES, Brazilian Government Organ, is
acknowledged. Harima Chemical Co. is also thanked for providing the
polyampholyte sample. The authors also are thankful for Dr Wilson Hirota for his
help in the DLS analyses, and Antônio Santos Filho and Patrícia Kaji Yasumura
from IPT for their help in paper handsheet preparation and paper mechanical
measurements.
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POLÍMERO ANFÓTERO PARA MEJORAR LA
RESISTENCIA DEL PAPEL
RESUMEN: El objetivo de este trabajo fue la evaluación de un polímero anfótero,
conteniendo grupos ácidos y básicos en su cadena, como aditivo para la resistencia
en seco del papel. El polímero en cuestión fue un terpolímero aleatorio de alta
densidad de carga y alto peso molecular. Se determinó que el balance entre la
densidad de carga del substrato y de las estructuras poliméricas es un factor crítico
a considerar en el uso de estos polímeros. El máximo valor de resistencia del papel,
cuantificado mediante el índice de tensión, se consiguió a valores de pH
equivalentes al punto isoeléctrico del polímero, iep (pH equivalente a 7.3); en esta
condición la carga de la superficie de las fibras es negativa y la distribución de
carga en el polímero es simétrica. Esta observación coincide con resultados de
dispersión de luz láser de soluciones del polímero que mostró un máximo en la
asociación polimérica y por tanto un máximo tamaño molecular en el iep. Al
adsorberse sobre superficies con carga electrostáticas la máxima cantidad de
polímero adsorbida, cuantificada mediante técnicas de microgravimetría de balanza
de cristal de cuarzo, también ocurrió en el iep. Los mejores valores de resistencia
en seco del papel se consiguieron en condiciones donde las carga electrostáticas del
polímero y de las fibras eran de signo opuesto o en el iep del polímero. Otras
estrategias de adición menos efectivas se consiguieron cuando la carga del
polímero y de las fibras eran del mismo signo.
PALABRAS CLAVE: Nanotecnología; polianfóteros; Resistencia del papel;
Densidad de carga superficial; Interacción electrostática; Química coloidal.