Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 12939−12952
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Kinetic Description of Ascorbic Acid Decomposition in Redox Initiator Systems for Polymerization Processes Baldur Schroeter,† Sven Bettermann,† Henning Semken,† Timo Melchin,‡ Hans-Peter Weitzel,‡ and Werner Pauer*,† †
Institute for Technical and Macromolecular Chemistry, University of Hamburg, Bundesstraße 45, 20146 Hamburg, Germany Wacker Chemie AG, Johannes-Hess-Strasse 24, 84489 Burghausen, Germany
‡
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S Supporting Information *
ABSTRACT: The temperature dependent redox decomposition rate constants of ascorbic acid (AsAc) in aqueous solutions were quantified in the presence of ammonium iron(III) sulfate dodecahydrate (Fe-cat) and tert-butylhydroperoxide (TBHP). Decomposition rates were determined in a temperature range from 35−75 °C with respect to the component ratios. First-order kinetics were observed up to specific conversions (∼20−80%) for AsAc decomposition. The application of a flexible Weibull model provided a good and comparable description (R2 ≥ 0.992) of decomposition processes up to high AsAc conversions (96%) at all studied reaction conditions. The activation energy was determined as a function of AsAc and Fe-cat ratios (38−79 kJ mol−1). Statistical modeling facilitated the prediction of rate constants. Thus, it was possible to adjust the reaction rate of emulsion copolymerizations of vinyl acetate and Versa10 in a broad temperature range (10−70 °C) by variation of the Fe-cat content.
solvent, pH value, presence of oxygen, and additives.16 Therefore, the design of tailor-made redox initiator systems for various temperatures, monomer systems, and processes requires precise knowledge about the decomposition kinetics of the systems. Redox initiator systems containing ascorbic acid (AsAc), tert-butylhydroperoxide (TBHP), and ammonium iron(III) sulfate dodecahydrate (Fe-cat) are established for the emulsion copolymerization of vinyl acetate (VAc) and neodecanoic acid vinyl ester.17−19 Despite the practical relevance, the reaction mechanism of these systems has not been clarified yet. Following the principle reactions of the iron catalyzed Fenton reaction between H2O2 and Fe(II) as well as the interactions of AsAc with Fe(III),20,21 a formal reaction scheme for the reactions between AsAc, TBHP, and Fe-cat can be accordingly proposed (Figure 1). The direct quantification of the radical generation rate based on in-line detection of radical species during the reaction is difficult to implement under process conditions due to the short lifetime of the resulting radical species and their numerous subsequent reactions. However, conclusions about the radical generation rate may be drawn based on the analysis of a single component decomposition kinetics. In the given case, AsAc decomposition was chosen as indicator for the determination of the radical generation rate.
1. INTRODUCTION Emulsion polymerization is one of the most important processes for the production of waterborne latex polymers in industry.1−4 It is typically carried out in a temperature range from 75 °C to 90 °C in order to achieve high space-time yields. Thermal initiators are commonly used in this temperature range providing an adequate radical generation rate.5−9 The homolytic cleavage of covalent bonds of most practical thermal initiators requires a bond dissociation energy in the range of 125−160 kJ mol−1. Compounds with values above or below this range provide either too slow or too rapid radical generation rates at the polymerization temperatures.10 It is desirable to expand the reaction temperatures to lower values to enhance the range of accessible polymer microstructures, which depend on polymerization processing temperatures.11−15 This minimizes the possibility of side chain reactions, provides access to high molecular weight polymers, and offers also the possibility to influence latex particle sizes.7 Redox initiator systems provide a high radical flux at milder conditions than thermal initiators due to their lower activation energies (40−80 kJ mol−1).10 Additional advantages of these systems can be found in their low induction periods as well as high flexibilities of the radical generation rates.3,7 However, this flexibility comes along with the downside of a high sensitivity regarding the reaction conditions. In comparison to thermal initiators, whose radical generation rate is mainly controlled by temperature, the radical generation of redox initiators depends on additional effects, e.g., the component ratios of the initiators as well as the © 2019 American Chemical Society
Received: Revised: Accepted: Published: 12939
February 4, 2019 June 17, 2019 June 27, 2019 June 27, 2019 DOI: 10.1021/acs.iecr.9b00710 Ind. Eng. Chem. Res. 2019, 58, 12939−12952
Article
Industrial & Engineering Chemistry Research
The Weibull model approach showed an interesting potential for the kinetic description of enzymatic, microbial, and chemical degradation kinetics.49 Its original application was to statistically analyze the ultimate strength of materials.50 A flexible Weibull distribution approach proved to be appropriate for the description of nonlinear aerobic AsAc decomposition processes in orange juice by Manso et al.51 Up to now, AsAc kinetics has been described by the Weibull model in several studies, including comparisons with linear model evaluations.52−55 Nevertheless, the use of a Weibull distribution model has not been reported yet to determine AsAc decomposition kinetics in redox initiator systems. The aim of this this work was to quantify the reaction rates of AsAc decomposition in the redox initiator system consisting of AsAc/TBHP/Fe-cat in a broad temperature range, which covers the majority of important settings in emulsion polymerization. In order to identify the most appropriate kinetic model, the suitability of different linear models and the nonlinear Weibull approach were compared. In addition, changes of the reaction order during the decomposition processes were identified as a function of the reaction conditions. Kinetic data were provided from concentration profiles of AsAc determined by UV−vis spectroscopy. The main focus was the adjustment of the reaction rate utilizing the temperature as well as variations of the AsAc and Fe-cat component ratios. The applicability of AsAc decomposition kinetic models was finally proved by emulsion polymerization processes.
Figure 1. Formal reaction scheme for the reactions of the redox initiator system AsAc/TBHP/Fe-cat. AsAc-deg. represents the variety of AsAc decomposition intermediates and final products.
Challenges for the comparable description of AsAc kinetics at different reaction conditions are the high complexity of AsAc decomposition itself. Further formation of numerous AsAc decomposition and intermediate products, which may be also reactive,22 occurs. Furthermore, the complex reaction mechanism between the three initiator components and the solvent constitutes challenging aspects as well. As a consequence, a kinetic description of AsAc decomposition has been described by many different kinetic models, e.g., zero-order,23,24 first-order,25−30 or second-order31,32 kinetics. Furthermore, Sakai et al. discussed a mixed model assuming a consecutive reaction, which consists of zero-order followed by first-order kinetics.33 While the given examples focus on AsAc decomposition in food and pharmaceutical products, reactions of AsAc with Fe ions are also relevant in a redox initiator context. These result in AsAc-Fe complex formation of different stoichiometry and stability, which depend, inter alia, on the presence of oxidizing agents, pH value, AsAc to Fe ion ratio, and the valence state of Fe.34−39 Martell and Khan showed that AsAc decomposition rates have a linear dependency with the Fe ion concentration (pH range of 1.5−3.9) assuming that neutral and monoionic species of AsAc react independently with Fe ions.34 In addition, the ambivalent nature of AsAc reactivity has to be considered, especially in the presence of redox initiator components. AsAc works as an antioxidant and radical scavenger, on the one hand, but is also widely used as a reducing agent with pro-oxidative properties on catalytic metals, on the other.20,40,41 The crossover effects of AsAc as pro- or antioxidant are considered to be variable. In general, the effect as antioxidant dominates at low AsAc levels in comparison to metal catalysts.20 The complexity of redox initiator reactions may explain the fact that numerous reports about the application of redox initiators for polymerization processes in the presence of monomers have been published in the form of articles and patents.2,18,42−48 However, no systematic study about the performance and kinetics of the pure redox components in the absence of additives has been reported yet. From a practical point of view, rate constants of AsAc decomposition are most essential, specifically to adjust the systems for polymerization processes. The AsAc decomposition time has to outlast the mean residence time of the reactor to provide a sufficient radical flux during the entire process. Here, a flexible kinetic model is needed to comparably quantify AsAc decomposition rates at various reaction conditions, which describes AsAc decomposition up to high conversions, adapts to changes in the reaction order, and is applicable to all desired reaction conditions.
2. MATERIALS AND METHODS 2.1. Materials and Experimentation. Preparation of Redox Initiator Solutions. A 490−499.00 g quantity of demineralized and bidistilled water was placed in a 500 mL round-bottom flask and stirred (250 rpm) at room temperature with a cylindrical PTFE-coated magnetic stirring rod (38 mm · 5 mm). Dissolved oxygen was replaced with argon, and a residual O2-content of ≤0.1 mg·L−1 was ensured by measurements with an oxygen sensor (Mettler Toledo Inpro 6850i). AsAc and TBHP (70% solution), obtained from SigmaAldrich, were dissolved, and the pH value was measured (laboratory pH meter 765, KNICK). The Fe-cat solution was added after 2 min of stirring. Two mL of the redox initiator solution was transferred under argon atmosphere after 30 s of additional stirring into an argon filled cell (Hellma Analytics, 117-QS), which contained a PTFE-coated magnetic stirring rod (4 mm · 1 mm). The cell was immediately sealed and placed in a heated cell holder into the optical path of the UV− vis spectrometer (Cary 50 UV−vis, Varian). Preparation of Fe-cat Solutions. A 100 g quantity of demineralized and bidistilled water was placed in a 100 mL round-bottom flask and stirred (250 rpm) at room temperature with a cylindrical PTFE-coated magnetic stirring rod (20 mm · 4 mm). Dissolved oxygen was replaced with argon, and an O2content ≤0.1 mg L−1 was ensured by measurements with an oxygen sensor. Ammonium iron(III) sulfate dodecahydrate (Fe-cat), obtained from Merck, was added, and the solution was stirred for an additional 30 s. A 1 g−10 g quantity of the Fe-cat solution was added immediately under argon atmosphere to the redox initiator solution after this period. Ascorbic Acid Decomposition Experiments. AsAc concentrations of the redox initiator solutions were determined by UV−vis spectroscopic measurements (λ = 190−500 nm). The measurement interval was adapted to the reaction rate in the 12940
DOI: 10.1021/acs.iecr.9b00710 Ind. Eng. Chem. Res. 2019, 58, 12939−12952
Article
Industrial & Engineering Chemistry Research
Figure 2. Calibration of AsAc UV−vis spectra (dilution series of AsAc in water at room temperature).
Figure 3. Calibration of AsAc UV−vis spectra on pH-value (left, double determination) and dependence of pH-value on AsAc concentration in aqueous solutions of AsAc at room temperature (right, single determination).
2.3. Variation of Component Ratios and Temperatures. The temperature was varied in the range from 25−75 °C by intervals of 10 °C. All molar ratios were related to the component TBHP, which was kept constant at an initial concentration of 0.44 mmol L−1 in all experiments. Molar ratios of the Fe-cat were varied in the range from 0.003−0.03 at equimolar ratios of AsAc and TBHP (Table 1). Molar ratios of AsAc were varied in the range from 0.3−1.0 at a fixed molar ratio of 0.003 Fe-cat (Table 2)
range from 0.15−5.00 min. The solution was stirred (400 rpm) during the measurement process. The temperature was adjusted beforehand to ±0.2 °C accuracy by temperature measurements of 2 mL of water in the same reaction room (thermostat: R22, Lauda). 2.2. Calibration of Ascorbic Acid UV−Vis Spectra. The AsAc signal in UV−vis spectra has an absorption maximum λmax depending on the pH value varying from 242.5 nm (pH = 2.5) to 259.5 nm (pH = 5).56 Two calibration curves were used to determine AsAc concentration as well as changes of pH-values during the reaction. The calibration of the absorbance (abs) at λmax to the AsAc concentration c was carried out as 3-fold determination by measuring UV−vis spectra of aqueous AsAc reference solutions in the concentration range from c = 0.02−0.25 mmol L−1 (Figure 2(l)). A strictly linear correlation (R2 = 0.999) was found for the intensity of λmax and AsAc concentration (Figure 2(r)). The bathochromic shift of λ max toward lower AsAc concentrations corresponds to a change of pH value from 3.73 to 4.26. The pH calibration of λmax was carried out in the pH range from 2.85−4.49 at 25 °C and a constant AsAc concentration of 0.22 mmol L−1 (pH = 3.76). Target pH-values < 3.76 were adjusted by addition of formic acid (97% obtained from SigmaAldrich) and pH-values > 3.76 by addition of aqueous sodium hydroxide solution (0.1 mol L−1, sodium hydroxide obtained from Sigma-Aldrich). The correlation between the wavelength of λmax and the pH-value can be described by a polynomial function with high correlation (R2 = 0.999) (Figure 3(l)).
Table 1. Recipes for Variation of Fe-cat Content at Equimolar Ratios of AsAc and TBHP
a
H2O [g]
AsAc [mg]
TBHP(70%) [μL]
Fe-cat solutiona [g]
molar ratio of AsAc/ TBHP/Fe-cat [-]
500.0 498.0 497.0 496.0 491.0
38.6 38.6 38.6 38.6 38.6
30 30 30 30 30
1.00 3.00 4.00 5.00 10.0
1.00:1.00:0.003 1.00:1.00:0.009 1.00:1.00:0.012 1.00:1.00:0.015 1.00:1.00:0.030
c Fe-cat in each solution = 6.5 · 10−3 mol L−1.
Emulsion Polymerization. 2.4. Materials and Experimentation. Industrial grade VAc/Versa10 (molar ratio 9:1) monomers (Wacker Chemie AG) were used for emulsion copolymerization processes in batch reactors. Details about the setup can be found in the Supporting Information. Poly(vinyl alcohol) (PVOH; Celvol 4-88, 8 wt % regarding monomer mass) was used as emulsifier. All polymerizations were carried out with an initial amount of 1 wt % AsAc regarding monomers and equimolar ratios of AsAc and TBHP. Two reaction series 12941
DOI: 10.1021/acs.iecr.9b00710 Ind. Eng. Chem. Res. 2019, 58, 12939−12952
Article
Industrial & Engineering Chemistry Research
300 rpm during the following processes. The emulsions were heated/cooled to the desired temperatures, dissolved oxygen was replaced with argon, and an argon atmosphere was contained above the solutions during the processes. AsAc was added, and an acidic environment was verified (pH = 3.9). Polymerizations were initiated by adding Fe-cat dissolved in a total volume of 1−2 mL of demineralized water and TBHP as one shot to the emulsions. The total conversion of the monomers was determined by gas chromatography (Agilent Technologies 7820A; column: WCOT fused silica 50 m, 320 μm; mobile phase: hydrogen; detector: FID, injector temperature: 200 °C, detector temperature: 250 °C, sample volume: 0.4 μL). For this, 500 mg of the emulsion sample was dissolved in 5 mL of N,N-dimethylformamide (Merck), and 50 μL of toluene was added as internal standard. Samples for gas chromatography were quenched directly after sampling by addition of a 7 wt % aqueous solution of hydroquinone (Merck). In-line detection of conversion was carried out via Raman spectroscopy (RAMAN RXN1, Kaiser Optical Systems, probe IO-1/4S-NIR, Kaiser Optical Systems), by evaluation of the mixed C=C-signal at ν = 1646 cm−1, which shows a signal intensity proportional to monomer concentration.57 Temperature development in the dispersions during the reaction was monitored in-line via a Pt-100 temperature sensor. Particle sizes of the final latexes were obtained from dynamic light scattering (DLS, Malvern Zetasizer Nano ZS, temperature: 25 °C, scattering angle: 173°) by measuring samples dissolved in demineralized water.
Table 2. Recipes for Variation of AsAc Content at a Fixed Molar Ratio of Fe-cat H2O [g]
AsAc [mg]
TBHP(70%) [μL]
Fe-cat solutiona [g]
molar ratio of AsAc/ TBHP/Fe-cat [-]
500 500 500 500
38.6 29 19.3 11.6
30 30 30 30
1.00 1.00 1.00 1.00
1.00:1.00:0.003 0.75:1.00:0.003 0.50:1.00:0.003 0.30:1.00:0.003
c Fe-cat in each solution = 6.5 · 10−3 mol L−1.
a
under variation of Fe-cat content were performed. Series one was conducted at 70 °C initial temperature under active counter cooling (Table 3). Series two was conducted at 10 °C initial temperature and constant jacket temperature (Table 4). Table 3. Recipes for Emulsion Copolymerization at 70 °C Initial Temperature, Variation of Fe-cat Content H2O [g]
VAc [g]
Versa10 [g]
PVA [g]
Fe-cat [mg]
TBHP [g]
AsAc [g]
molar ratio Fe-cata [-]
328 328 328 328
66.9 66.9 66.9 66.9
17.1 17.1 17.1 17.1
6.72 6.72 6.72 6.72
0.72 0.96 1.45 2.41
0.46 0.46 0.46 0.46
0.88 0.88 0.88 0.88
3.0 4.0 6.0 1.0
· · · ·
10−4 10−4 10−4 10−3
a
Molar ratio regarding AsAc and TBHP.
Table 4. Recipes for Emulsion Copolymerization at 10 °C Initial Temperature, Variation of Fe-cat Contenta H2O [g]
VAc [g]
Versa10 [g]
PVA [g]
Fe-cat [mg]
TBHP [g]
AsAc [g]
molar ratio Fe-cata [-]
195 195 195 195
39.8 39.8 39.8 39.8
10.2 10.2 10.2 10.2
4 4 4 4
8.54 12.8 21.4 42.8
0.27 0.27 0.27 0.27
0.52 0.52 0.52 0.52
7.2 9.0 1.5 3.0
· · · ·
3. RESULTS AND DISCUSSION 3.1. Modeling of AsAc Decomposition. AsAc decomposition was modeled using the linear zero-order (eq 1) and first-order (eq 2) relations
10−3 10−3 10−2 10−2
ca = c0 − (k·t )
(1)
ca = c0·e−k·t
(2)
a
Molar ratio regarding AsAc and TBHP.
Polymerization processes were carried out in two steps. First, an emulsion consisting of monomers, PVOH, and water was prepared, directly followed by initiation with the redox initiator. The emulsion containing monomers, PVOH, and demineralized water was prepared as follows: PVOH and monomers were stirred in demineralized water at 500 rpm for 30 min. The prepared solutions were continuously stirred at
with ca and c0 = concentrations of AsAc at time t and zero, t = reaction time, and k = zero-order and first-order rate constants. The AsAc concentration was also determined using the Weibull model (eq 3) t
ca = c0·e−( α )
β
(3)
Figure 4. Kinetic evaluation of AsAc decomposition rates (left) and residuals of the individual fits (right) for a molar ratio of AsAc/TBHP/Fe-cat 0.5:1.0:0.003 and T = 65 °C. A first-order model (black line) was applied to the first 30% of the data set (in this case: t = 4.5 min, conversion = 36%), and a Weibull distribution model (red dashed line) was applied to the complete data set (in this case: t = 15 min, conversion 96%) as well as a zero-order model (red dotted line). 12942
DOI: 10.1021/acs.iecr.9b00710 Ind. Eng. Chem. Res. 2019, 58, 12939−12952
Article
Industrial & Engineering Chemistry Research
Figure 5. Averaged correlation coefficient values for the different kinetic models as functions of temperature (top left) including related standard deviations σ(R2) (top right) and as a function of the initial AsAc ratio regarding TBHP (bottom left) including related standard deviations (bottom right). X̅ = number of individual experiments used for calculation of mean average and standard deviation.
Figure 6. Dependence of the molar AsAc ratio and temperature on the decomposition rates (left) and dependence of the molar ratio of AsAc on β (right, temperature independent). α−1-values at different temperatures are based on Arrhenius relationships.
with α = Weibull scaling parameter, and β = shape parameter. The Weibull model (eq 3) includes an additional degree of freedom due to the shape parameter β and is therefore more flexible with respect to deviations from ideal first-order or zeroorder behavior in comparison to linear kinetic models. The Weibull model coincides with a first-order model in the case β = 1. The inverse of the scale constant α corresponds to the rate constant k. An exemplary evaluation of a decomposition experiment illustrates the comparative approach (Figure 4). The first 30% of the AsAc decomposition curve can be described by a firstorder model (first-order30%) (Figure 4(l)). The application of a Weibull model fit enabled the completion of the data set up to a high AsAc conversion of 96% (Figure 4(l)). Both models
correlated highly (R2first‑order,30% = 0.992, R2Weibull = 0.995) within the given range, while first-order evaluation of the complete data set (first-ordercomplete) resulted in a poor correlation (R2 = 0.926) as also illustrated by the residuals of the extrapolated first-order30% fit. Absolute values of Weibull model residuals were in the range from 4.3 · 10−3−8.7 · 10−3 over the complete data set, which is equivalent to 1−11% in comparison to values of the first-order model in the range up to 30% reaction time and
