POLYMERIZATION REACTION MONITORING FOR PSA PRODUCTION
USING AN ATR-FTIR PROBE
Renata Jovanović, Doctoral student, Department of Chemical Engineering, University of Ottawa,
Ottawa, Canada, ([email protected])
Marc A. Dubé, Associate Professor, Department of Chemical Engineering, University of Ottawa,
Ottawa, Canada, ([email protected])
Abstract
The utilization of Attenuated Total Reflectance – Fourier Transform Infrared (ATR-FTIR) spectroscopy
for the characterization of pressure sensitive products is well established. Some recent advances in
sensor technology have opened new application areas of ATR-FTIR spectrometry. One of them is
monitoring of chemical reactions, which are of interest for pressure sensitive adhesive production. In
this work, ATR-FTIR spectrometry was used for monitoring polymerizations of acrylic monomers.
Initially, the ATR-FTIR probe was used for off-line monitoring of solution polymerizations. The
obtained results were used to establish real-time in-situ monitoring strategies for more complex,
heterogeneous systems such as emulsion or miniemulsion polymerizations. In all cases, the performance
of ATR-FTIR spectroscopy was compared to traditional monitoring techniques (i.e. gravimetry, 1HNMR spectroscopy) and good agreement between the techniques was obtained. However, some
limitations of this technique were also uncovered. Current results offer a basis for the development of
ATR-FTIR spectroscopy as not only a monitoring tool, but also as a tool for process control.
Introduction
The manufacture of pressure sensitive adhesives includes synthesis and formulation. Two common
synthesis methods for compounds with built-in PSA properties are polymerization in solution and
emulsion (1). When producing compounds with built-in PSA properties, the variation in the chemical
and physical characteristics of the reaction components and/or the disturbance of process parameters can
seriously affect the desired structure-property relationships of the final PSA. Modeling and/or
monitoring of polymerization reactions are the means to ensure desired final product properties despite
these disturbances. Models can be used effectively in these situations, but they are usually required to be
robust and have an extensive parameter database. Reaction monitoring systems combined with feedback
control strategies can be a solution to real-time process control.
Among several reaction monitoring techniques, in-line monitoring is preferred (2) because it overcomes
the major disadvantages of off- and on-line monitoring techniques. This includes considerable time lags,
in one case, and the necessity of sampling loops, which are prone to polymer build-up or sample
disturbance in the other. Different techniques are suitable for polymerization reaction monitoring (e.g.
gas chromatography, calorimetry, ultrasound, spectroscopic techniques (3,4)). Among the spectroscopic
techniques, midrange infrared (MID IR) offers kinetic and structural information about reaction
components without the application of complex chemometrics methods for analysis or expensive
modifications to the reactor configuration. Recent advantages in sensing and signal transfer technologies
have enabled the utilization of Attenuated Total Reflectance – Fourier Transform Infrared (ATR-FTIR)
spectroscopy for remote, real-time reaction monitoring of chemical reactions. Curing, solution and
emulsion polymerizations have been monitored using a range of homemade and commercial monitoring
systems based on ATR-FTIR spectroscopy (3,4).
In this study, a commercially available ATR-FTIR reaction monitoring system was used. It operates in
the MID IR region (4000-650 cm-1) and consists of electronic and optical modules. A purged path for
the IR beam from source to detector and back was ensured using a set of mirrors and conduits ending
with a remote sampling device. The sampling device consists of a stainless steel body and a sixreflection bi-layer diamond-composite ATR element. The external processor is used for data acquisition
and manipulation. Details of the probe are discussed elsewhere (5). The basis for monitoring the reaction
is the determination of characteristic absorbances assigned to monomer consumption or polymer buildup. These can be used to calculate polymer composition and overall conversion.
In the work presented here, an ATR-FTIR probe was used to monitor solution and batch emulsion
polymerization reactions of different monomers suitable for PSA production. The use of the ATR-FTIR
probe started with off-line monitoring of homogeneous systems such as solution homo- and copolymerizations (5) and continued with the more demanding real-time monitoring of heterogeneous
systems such as emulsion or miniemulsion polymerizations (6). The data obtained using ATR-FTIR
spectroscopy were compared to those obtained using traditional off-line monitoring techniques such as
gravimetry, 1H-NMR spectroscopy and gas chromatography. The results presented here were obtained
for butyl acrylate and vinyl acetate homo- and co-polymerizations. Successful ATR-FTIR monitoring
was also performed with other monomers of interest for PSA production such as styrene and methyl
methacrylate (7).
Experimental Methods
Solution Polymerizations. Six homo- and co-polymerizations of butyl acrylate (BA) and vinyl acetate
(VAc) in toluene were performed. The concentration of toluene was varied from 50 to 80 wt.%. Ndodecyl mercaptan was used as a chain transfer agent and 2,2’- azobisisobutyronitrile (AIBN) as an
initiator. Standard procedures were followed for reagent purification (5). All reactions were performed
in glass ampoules at 60oC. For each measurement, the reaction in two ampoules was simultaneously
quenched and the contents of one ampoule were analyzed by gravimetry and 1H-NMR (Proton Nuclear
Magnetic Resonance) Spectroscopy. The contents of the second ampoule were used for off-line
determination of conversion and copolymer composition using the ATR-FTIR probe. The probe was
inserted into a vial containing the reaction mixture. For all samples, air was collected as a background
spectrum and automatically subtracted from the sample spectra. All spectra were collected at 64 scans
and a resolution of 8 cm-1.
Emulsion Polymerizations. Eight BA/VAc emulsion copolymerizations were performed in two
automated reactors of 1.2 and 5L. Two polymerization temperatures were used (60 and 80oC) and solid
contents were 30 wt.% and 50 wt.%. For two runs, sodium dodecyl sulfate was used as stabilizer while
poly(vinyl alcohol) was used in all other runs. Ammonium persulfate and NaHCO3 were used as initiator
and buffer, respectively. N-dodecyl mercaptan was used as a chain transfer agent in all reactions.
Standard procedures were followed for monomer purification (5). In this case, the ATR-FTIR probe was
used in-line for real-time reaction monitoring. The background spectrum was collected at the appropriate
temperature and stirring rate prior to the addition of monomers and initiator. Collection of the reaction
spectra was started simultaneously with the injection of initiator. The best signal to noise ratio for the
spectra collection was determined to be 256 scans at resolution of 4 cm-1. To ensure continuous
monitoring, the spectra were collected every 2 min.
Miniemulsion Polymerizations. Several miniemulsion homo- and co-polymerizations of BA and VAc
were also monitored using ATR-FTIR spectroscopy. These reactions were performed in a 1.2L,
automated reactor at 60oC. The solids content was 30%. The same number of scans and resolution was
used as for the emulsion polymerizations. The difference was in the collection of background spectra.
Due to the nature of the miniemulsion polymerization, the reaction components were sonicated prior to
charging to the reactor. Thus, the background spectrum in this case included monomer(s). The reaction
spectra were also collected beginning with the injection of initiator.
Results and Discussion
Solution Polymerization Monitoring. In this part of the project, the main objective was the identification
of characteristic absorbances and their utilization for copolymer composition and overall conversion
monitoring. Given this objective and the nature of the experimental runs, off-line measurements were a
good initial start for the development of a measurement method based on ATR-FTIR spectroscopy. The
results obtained were compared to gravimetric measurements.
The basis for the application of ATR-FTIR spectroscopy in monomer conversion determination is
Beer’s law. The absorbance of the component in the reaction mixture is directly proportional to its
concentration. The absorbance can be measured as peak height, peak height ratio, peak area or peak area
ratio. Thus, a simple expression for conversion is obtained:
X (mole fr .) =
Absorbancet =0
Absorbancet =0 − Absorbancet =t
(1)
where X is conversion and t is time.
The first step was the homopolymerization of BA and VAc in toluene, in order to identify the
characteristic absorbance peaks for each monomer that exhibited a change with reaction time. In Figure
1, a typical spectra of BA homopolymerization in toluene is shown. For the purpose of clarity, only a
fingerprint region of the spectra is shown. The absorbances at several wavenumbers (e.g. 810 cm-1, 984
cm-1, 1409 cm-1, 1640cm-1) showed potentially useful time trends for reaction monitoring. In all cases,
the absorbances decreased with time, indicating a decrease in monomer concentration due to its
consumption during the polymerization. Similar trends were found at 876 cm-1, 949 cm-1, 1293 cm-1, and
1648 cm-1 when spectra of the VAc homopolymerization in toluene were investigated.
After peak identification, the next step was the determination of the best mathematical expression for the
absorbance. Theoretically, regardless of the peak selection and the expression for absorbance, the results
should be similar. However, in this case, the best fit between gravimetry and ATR-FTIR data was
obtained when the absorbances at 810cm-1 for BA and at 1293 cm-1 for VAc (Figure 2) were used. A
peak height to two-point baseline was used in both cases. Thus, Equation 1 becomes:
X (mole fr .) =
Peak Height t =0
Peak Height t =0 − Peak Height t =t
(2)
In Figures 3 and 4, typical results for BA and VAc homopolymerizations in toluene are shown,
respectively. Similar results were obtained when different solvent concentrations were used. In all cases,
ATR-FTIR spectroscopy showed good agreement with the standard gravimetric method. The estimated
error for ATR-FTIR spectroscopic data was 2.5 wt.% (7).
After the determination of conversion vs. time data of the solution homopolymerizations, ATR-FTIR
spectroscopy was used for off-line monitoring of BA/VAc solution copolymerizations. In addition to
low toluene absorbance, an additional challenge was to find the characteristic absorbances for each of
the two monomers that would not overlap, in order to use them for the determination of individual
monomer conversions. In Figure 5, the results are shown for a BA/VAc solution copolymerization in
toluene.
In the copolymerization case, the following expression was used to calculate overall conversion using
ATR-FTIR spectroscopic data:
X (wt. fr.) =
m2
m1
x 2 (mol fr.)
x1 (mol fr.) +
m1 + m2
m1 + m2
(3)
where mi/(mi+mj) is the weight fraction of monomer i in the reaction mixture and xi is the individual
monomer conversion determined from Equation (2). The data obtained using ATR-FTIR spectroscopy
were compared to standard 1H-NMR spectroscopy and gravimetric data. As in the homopolymerization
cases, good agreement between the different techniques was obtained.
Emulsion Polymerization Monitoring. After the successful completion of off-line monitoring of
homogenous reaction systems, the ATR-FTIR probe was used for the more complex, in-line monitoring
of the heterogeneous emulsion polymerization system. Real-time monitoring of such a system is
interesting not only because there are several phases present, but also because of the
compartmentalization of monomer(s) among them, especially for highly water soluble monomers such
as VAc. Typical reaction spectra collected in-line are shown in Figure 6. Real-time peak profiles i.e.
normalized absorbances vs. time (Figure 7) were used as input data in Equation 2 to determine the
individual monomer conversions. Once the individual monomer conversions were obtained, Equation 3
was used to determine overall conversion.
Several different copolymer compositions were investigated. In addition, two different stabilizers were
used: one was an electrostatic stabilizer, sodium dodecyl sulfate (SDS) and the other was a steric
stabilizer, poly(vinyl alcohol) (PVOH). Due to the presence of grafting reactions when PVOH was used,
the obtained polymers were not soluble in the range of organic solvents acceptable for 1H-NMR
analysis. Thus, gas chromatography was employed as a standard technique for off-line monitoring of
copolymer composition. A typical result using SDS as stabilizer is shown in Figure 8. Once again, ATRFTIR spectroscopy showed good agreement compared to standard techniques. In this case, the
heterogeneity of the system did not affect the results obtained using the probe. This was possibly due to
several reasons. First, good mixing of the reaction mixture was obtained. Second, the probe was
positioned just 2mm above the impeller element ensuring that a representative sample was in contact
with the probe. In addition, each spectrum was recorded from an average of 256 scans. Similar results
were observed when PVOH was used for the same initial BA/VAc composition but at a higher solids
content (50%). A typical conversion vs. time curve under such conditions is shown in Figure 9.
In emulsion polymerization monitoring, the ATR-FTIR probe has shown great potential. While off-line
monitoring of copolymer composition using 1H-NMR spectroscopy was not possible due to the
insolubility of the obtained polymers, in situ measurements using ATR-FTIR spectroscopy were
successful. In addition, the probe was used to successfully monitor an induction period when it occurred
(Figure 8). This enables the operator to estimate early during the reaction the amount of initiator to be
added in the reactor for corrective purposes. Furthermore, in the case of a catastrophic coagulation, the
spectra obtained showed the disturbance along the time axis much earlier than the separation of phases
was observed in the samples taken from the reactor (Figure 10). Further investigation is needed in order
to obtain a more quantitative representation of these phenomena but, nonetheless, this information can
save time and financial resources that are required for reactor cleaning.
On the other hand, the probe was found to be extremely sensitive to temperature variations (5). The
temperature control had to be within ± 0.2oC. Large deviations from the set point followed by a long
stabilization period negatively affected the results while small deviations and a short stabilization period
showed disagreement. After the set temperature was established the ATR-FTIR data showed good
agreement with the standard techniques. In addition, at high concentrations of VAc in the presence of
PVOH, deviations beyond experimental error were observed. Further investigation is needed to resolve
this issue. After successfully monitoring several systems over a range of concentrations and solids
contents (5-7), the speculation is that the presence of PVOH or its subtraction in the background might
have caused this discrepancy. At lower VAc concentrations this method was appropriate, while at higher
VAc concentrations, when more grafting of VAc occurs, the subtraction of PVOH as a background
might not be appropriate.
Miniemulsion Polymerization Monitoring. Monitoring of miniemulsion polymerizations of BA/VAc and
other monomers is currently in progress in our lab. Due to the fact that the preparation of miniemulsions
includes homogenization of the reaction mixture prior to charging it to the reactor, the collected
background contains monomer in addition to other components. In this case, the identification of
characteristic peaks is more complex and possibly requires the application of chemometrics methods
such as principal component analysis for data extraction.
Conclusions
For the production of PSA with the desired built-in performance characteristics real-time process
monitoring and control are essential process tools especially for systems where compositional drift can
occur or where variation in physical and chemical properties of reactants is expected. Even though there
are different techniques for reaction monitoring, ATR-FTIR spectroscopy has advantages over other
monitoring techniques. It has a potential for real-time monitoring of kinetic and structural changes of the
reaction components with no requirements for sampling loops or complex changes to reactor design.
In this study, it has been shown that an ATR-FTIR probe can be successfully used for off-line and inline monitoring of homogeneous and heterogeneous polymerization systems, that are commonly
employed in PSA synthesis. Used in the study for off-line monitoring of BA/VAc solution
polymerizations, it has accurately monitored conversion and copolymer composition. When used offline, no disadvantages were observed. The time lag was minimal, no calibration was required and the
technique was accurate compared to standard techniques. When similar principles were applied for inline monitoring of BA/VAc emulsion polymerizations, in most cases the probe was able to correctly
monitor copolymer composition and conversion compared to traditional techniques. When used in realtime, the probe was also able to follow the occurrence of an induction period or to offer an early
indication of catastrophic coagulation. On the other hand, strict temperature control was found to be a
major requirement for the successful application of ATR-FTIR spectroscopy. The extent and the
duration of the disturbance were major factors affecting the performance of the probe. In addition, some
problems were experienced at high concentrations of water-soluble monomer combined with PVOH
presence and high solids content. At this point, one speculation is that the methods developed for other
feed compositions might not be applicable when grafting reactions are likely. The performance
evaluation of the probe for real-time monitoring of miniemulsion polymerizations is still in progress.
References
1. Benedek, I. Heymans, L.J. Pressure-Sensitive Adhesives Technology, Marcel Dekker, Inc. New York,
1997
2. Dallin, P. Proc. Control Qual. 1997, 9(4), 167-172
3. Hergeth, W.D. In Polymeric Dispersions: Principles and Applications. Asua, J.M. Eds.; Kluwer
Academic Publishers: Dordrecht, 1997; 267-288
4. Kammona, O.; Chatzi, E.G.; Kiparissides, C. J. Macromol. Sci.—Rev. Macromol. Chem. Phys. 1999,
C39(1), 57-134
5. Jovanović, R.; Dubé. M.A. J. Appl. Polym. Sci. 2001, 82(12), 2958-2977
6. Jovanović, R.; Dubé. M.A. Polym. React. Eng. J. (in press)
7. Hua, H.; Dubé. M.A. J. Polym. Sci.: Polym. Chem. 2001, 39, 1860-1876
Acknowledgements
The authors wish to acknowledge the National Engineering and Science Research Council of
Canada and the Canada Foundation for Innovation for the financial support of this project.
Figure 1. Typical ATR-FTIR Spectra of BA Solution Polymerization in Toluene
(Published with permission from Ref. 5)
BA
VAc
Figure 2. Decrease in the Absorbance for BA and VAc Characteristic Peaks with Time Indicates
Monomer Consumption during the Reaction (Published with permission from Ref. 5)
Conversion (mol %)
100
80
60
40
20
gravimetry
ATR-FTIR
0
0
200
400
600
800
Time (min)
Figure 3 BA Solution Homopolymerization Monitored using Gravimetry and ATR-FTIR Spectroscopy
(Published with permission from Ref. 5)
100
Conversion (mol %)
80
60
40
20
gravimetry
ATR-FTIR
0
0
200
400
600
Time (min)
800
1000
Figure 4. VAc Solution Polymerization Monitored using Gravimetry and ATR-FTIR Spectroscopy
(Published with permission from Ref. 5)
100
Conversion (mol %)
80
60
40
20
1H NMR data - closed symbols
ATR-FTIR data - open symbols
overall conversion
BA conversion
VAc conversion
0
0
200
400
600
800
1000
Time (min)
Figure 5. BA/VAc Solution Polymerization in Toluene (Published with permission from Ref. 5)
Figure 6. Typical Real-Time Reaction Spectra of BA/VAc Emulsion Copolymerization
Obtained using an ATR-FTIR Probe (Published with permission from Ref. 6)
Normalized absorbance
1
BA
VAc
0.8
0.6
0.4
0.2
0
0
100
200
300
400
Time (min)
Figure 7. Real-Time Peak Profiles (Published with permission from Ref. 6)
Conversion (wt. fraction)
1
0.8
0.6
0.4
gravimetric data and 1H-NMR data - closed symbols
GC data - closed symbols with bars
ATR-FTIR data - open symbols
0.2
overall conversion
BA conversion
VAc conversion
0
0
100
200
300
400
500
600
Time (min)
Figure 8. BA/VAc Emulsion Copolymerization (SDS Stabilizer) Monitored using an ATR-FTIR
Probe and Standard Techniques (Published with permission from Ref. 6)
Conversion (wt. fraction)
1
0.8
0.6
0.4
0.2
overall conversion
BA conversion
VAc conversion
GC & gravimetric data - closed symbols
ATR-FTIR data - open symbols
0
0
100
200
Time (min)
300
400
Figure 9. Conversion vs. Time for a BA/VAc Emulsion Copolymerization (PVOH Stabilizer) using InLine and Off-Line Monitoring Techniques (Published with permission from Ref. 6)
Figure 10. Early Indication of Catastrophic Coagulation (Published with permission from Ref. 6)