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Influence of concentration of redox couple on freeze-thaw stability of styrene-acrylic copolymer latex.

Abstract Styrene-acrylic copolymer latexes were synthesized by emulsion polymerization process using sodium formaldehyde sulphoxylate (SFS) and tertbutyl hydroperoxide (TBHP) as redox initiators. Approximately 100% conversion was achieved successfully at 50[degrees]C. The redox polymerization was carried out at varied dosages of redox concentration; the lowest dosage of 0.3% was sufficient to achieve ~100% conversion. The molecular weight of the polymer was decreased with an increase in redox concentration as expected. The properties such as viscosity, specific conductivity, particle size, molecular weight, and [T.sub.g] (by differential scanning calorimetric and dynamic mechanical analysis) of the redox polymers are reported. The other important property of the latexes is their freeze-thaw (F-T) stability. Even though ~100% conversion was achieved at 0.3% redox initiator couple, it failed in F-T stability test. Latexes prepared above 0.5% redox initiator couple successfully pass the five F-T cycles. The effect of sulfate from SFS, and tertiary butyl from TBHP species on the stability of the latexes is discussed. The F-T stability of the emulsions is very crucial in the coatings industry, and this article brings some insight into the effect of concentration of redox species on the stability of emulsions. Moreover, low-temperature processes are always appreciated in terms of safety and energy savings.

Keywords Emulsion polymerization, Redox initiation, Styrene-acrylic latex, Freeze-thaw stability of latexes

Introduction

Emulsion polymerization is a very well accepted technique for the production of waterborne polymer latex in the paint and coatings industry. In the emulsion polymerization process, surfactant-stabilized oil in water system is polymerized using a suitable initiator. In general, two types of initiation are being used in the industrial world. The first type is thermal initiation where free radicals are generated by pyrolytic cleavage of covalent bond, whereas the second type is redox initiation where free radicals are generated by redox reaction between an oxidant and a reductant. Thermal initiation requires higher temperature (above 70[degrees]C) whereas redox initiation can be done at a low range of temperature (0-50[degrees]C). Redox polymerization was first reported by Bacon, Baxendale, and Morgan in 1946. (1-3) Subsequently, much literature is available on redox-initiated polymerization. (4-12) Generally, in the thermal initiation process also, industries use redox initiators as a chaser catalyst to polymerize the residual amount of monomers at the end of the polymerization process. Thermal initiation depends on half-life of initiator which in turn very much depends on temperatures. At low temperature, the radical generation rates of thermal initiators are so poor that they lead to incomplete conversion. On the other hand, depending on the redox couple, polymerization can be carried out in a wide range of temperatures. Comparatively lower activation energy of redox polymerization provides the freedom to choose polymerization temperatures ranging from 0 to 50[degrees]C. (13) Since initiation of a process in the industrial scale is a very critical task, it always requires an induction period at the very beginning of the process. The commercial grade monomers contain inhibitors which in turn increase the induction period. Redox initiators offer short induction time (8) compared to thermal initiators, hence, initiation occurs at a faster rate. However, temperature is a very crucial factor for emulsion polymerization. The polymer microstructure, gel content, swelling etc., largely depend on the reaction temperature. (14-16) Redox polymerization results in an emulsion with very high molecular weight and broader molecular weight distributions. (8,17) Generally, redox polymerization is carried out in an [N.sub.2] atmosphere (18) in the presence of some catalysts like FeS[O.sub.4], [H.sub.2]S[O.sub.4], or ethylenediaminetetraacetic acid (EDTA). [N.sub.2] is used to remove dissolved [O.sub.2] of water which acts as inhibitor for the redox system, whereas catalysts are added for faster initiation. Nitrogen purging has a great influence on the polymerization kinetics. The emulsion polymerization can be carried out at 50[degrees]C in the absence of [N.sub.2] whereas the reaction carried out at 40[degrees]C required [N.sub.2] purging as shown by Nicolas et al. (15) Surprisingly, the use of catalysts like FeS[O.sub.4], [H.sub.2]S[O.sub.4], or EDTA did not result in any significant improvement in the per hour conversion.

Apart from the challenges in polymerization processing, storage and transportation of the emulsions are of great concern since they may encounter extreme temperature conditions such as freezing and overheating. So, it is a necessity of the emulsions to withstand such extreme temperature changes. Hence, the emulsions need to pass certain tests such as an accelerated stability test for shelf life and a freeze-thaw (F-T) stability test to withstand the extreme temperature variations. However, there are few reports on the parameters affecting the F-T stability of emulsions prepared with thermal initiation. (19) There is still an ambiguity on the effect of [T.sub.g] and molecular weight on the F-T stability of the emulsions. (19,20) There are articles which have discussed the stability/F-T stability of food grade emulsions. (21-23) David et al., have reported a mechanism of radical entry in redoxinitiated emulsion polymerization and its impact on the emulsion stability. (24) The electrospray mass spectroscopy (ES/MS) spectrum evidenced the ionic species of the redox reactions and the oligomers where the later species primarily stabilizes the latex particles. To the best of our knowledge, there are no articles focused on F-T stability of high solid (50%) emulsions synthesized with redox initiation and at low temperature. Please note that the term emulsion is also used throughout the article in place of latex which carries the same meaning.

This article presents the redox polymerization of styrene-acrylic copolymer system at ~50[degrees]C. Tert-butyl hydroperoxide (TBHP, oxidant), sodium formaldehyde sulphoxylate (SFS, reductant) are used as redox couple to initiate the polymerization process. The chemical structure of potassium persulfate (PPS), SFS, and TBHP, and the probable radicals/ionic species generated during the redox reaction are given in Fig. 1. The effect of concentration of redox couple on the properties of the emulsions is reported. The important highlight of this article is the effect of concentration of redox initiator on the F-T stability of the latexes.

[FIGURE 1 OMITTED]

Experimental

Materials

Commercial grade monomers such as styrene (ST) from J P Dyechem Pvt. Ltd., Ahmedabad, India, butyl acrylate (BA) from BASF Petrochemicals CDN BHD, Selangor, Malaysia, and methacrylic acid (MAA) from Evonik Roem GmbH, Germany were procured and used as received without further purification. A commercial grade anionic surfactant (sodium salt of alkyl ether sulfate, with C12-14) was procured from Cognis Deutschland GmbH & Co KG, Germany and used as emulsifying agent. A commercial grade non-ionic surfactant (alcohol ethoxylate) was procured from Croda Chemicals. Commercial names of the surfactants are not used to avoid the product promotion that may arise out of this work. Sodium bicarbonate (SBC) was procured from V S Chemical Corporation, Mumbai, India. Tert-butyl hydroperoxide (TBHP, oxidant) and sodium formaldehyde sulphoxylate (SFS, reductant) were received from Roma Organics, India and Demosha Chemicals Pvt. Ltd. India, respectively. Potassium persulfate (PPS) was procured from Gujarat Persalts Pvt. Ltd., India. The polymerization process was carried out using deionized water.

Emulsion polymerizations

Approximately 50% solid styrene-acrylic copolymer latex was synthesized by a semi-continuous, seeded emulsion polymerization process. The reaction was carried out in a 2 1 glass reactor equipped with agitator and reflex condenser. The reaction temperature was set at 50[degrees]C by means of a thermostatic water bath. After attaining the set temperature, redox initiator couple, buffer, and known amount of seed were added to the reactor. The reaction temperature decreased due to the seed addition. Once the reaction temperature attained its set temperature, the pre-emulsion (PE) was fed to the reactor over a period of 4 h and subsequently, the heating and reflux was continued for another 1 h to polymerize any unreacted monomers left. A typical recipe of the redox polymerization of styrene-acrylic system is shown in Table 1. As a standard emulsion sample, similar polymerization was also carried out at 80[degrees]C with PPS as thermal initiator.

In the polymerization process, a couple of approaches were tried for the addition of redox initiators into the reactor. These approaches include (a) initiators directly charged into the reactor, (b) distributing the initiator between reactor and PE, and (c) addition of initiator couple to the reactor at regular time intervals. Out of all these approaches, addition at regular time intervals (approach c) resulted in better per hour and final conversion whereas, the first two approaches resulted in poor conversion. Hence, the addition of initiator at regular time intervals was chosen. Table 2 summarizes the addition of the redox couple.

Characterization

Gel Permeation Chromatography (GPC): The molecular weight of the polymers was determined by GPC (Varian-Prostar, Model-410) equipped with an auto sampler (100 [micro]L), refractive index (RI) detector, and mixed bed (60 cm) column of poly divinyl benzene. The emulsion polymer was dissolved in tetrahydrofuran (THF) and the measurement was run for 40 min. at the flow rate of 1 ml/min maintaining the oven temperature at 40[degrees]C.

Viscosity: Viscosity was measured by Brookfield viscometer (DV II + pro LV) at 25[degrees]C.

Latex particle size: Latex particle size distribution was measured by dynamic light scattering technique (Nano ZS from Malvern Instruments Ltd.).

Non-volatile materials: 1 g of latex sample was weighed in a steel lid and dried for 1 h at 120[degrees]C. The final non-volatile material (% NVM) was estimated by the following equation:

% NVM = [W.sub.d]/[W.sub.i] x 100

where [W.sub.i] is the initial weight of the sample and [W.sub.d] is the weight of the dried sample. The conversion of monomer into polymer was calculated by the following equation:

% Conversion = [NVM.sub.determined]/[NVM.sub.calcuiated] x 100

Minimum film formation temperature (MFFT): The MFFT was measured by ASTM D2354 method using MFFT Bar 60.

Differential scanning calorimetry (DSC): The glass transition temperature ([T.sub.g]) was measured by DSC, model Q 10, TA instrument. The samples of polymer films were prepared by drying the emulsion at ~30[degrees]C for 7 days. The heating rate of 3 degree/min and temperature range of -20[degrees]C to + 100[degrees]C were used to get the thermogram.

Dynamic Mechanical Analysis (DMA): The [T.sub.g] was also determined by DMA, model Q800, Multi-Frequency --Strain, TA Instruments. The samples of polymer films were prepared by drying the emulsion at ~30[degrees]C for 7 days. The heating rate of 3 degree/min and temperature range of -30[degrees]C to +150[degrees]C were used to get the thermogram.

[FIGURE 2 OMITTED]

Result and discussion

Styrene-acrylic copolymers were prepared successfully using SFS-TBHP as redox initiators. The reaction was carried out at 50[degrees]C. A stable, coagulum-free emulsion of ~50% solid content was obtained. The reactions were repeated at various amounts (0.3, 0.4, 0.5, and 0.6%) of redox initiators to understand their effects on the properties of the emulsions. The percent NVM, which gives information on the monomer conversion, was determined at every hour during the reaction. Figure 2a shows the % NVM determined at every hour during the polymerization process for all the initiator concentrations used in the study.

[FIGURE 3 OMITTED]

The percent NVM increased with time, and the difference between the experimental and theoretical NVM was decreased, and finally they overlap after complete addition of the pre-emulsions (4th hour). The data points in the figure are average of minimum 3 experiments and the deviation for these data points is approximately [+ or -] 2%. Figure 2 clearly shows that the complete conversion could be achieved at 0.3% initiator concentration (SFS:TBF1P, 0.15:0.15). Growth of particle size as a function of reaction time for 0.5% redox couple is shown in Fig. 2b. The growth of particle size is quite similar to the growth of particle size of thermal-initiated polymerization.

The initiator systems strongly influence the molecular weight. Difference in molecular weight and molecular weight distribution was observed for redox and thermal-initiated reactions due to the difference in reaction temperature and different radicals. (17) Figure 3 shows the variation in number averaged, weight averaged, and z-averaged molecular weights at different initiator concentrations. Molecular weight of the emulsion polymer prepared with thermal initiator (PPS) at 80[degrees]C (Std.) is also given for comparison.

The molecular weight decreased with an increase in the concentration of redox pairs as expected, since a large number of active centers were generated due to high concentration of redox couple. It is worth to noting that, in case of thermal initiation (Std.), the total amount of initiator used was ~0.25%. In redox polymerization, at the similar amount of redox pairs (0.3%), approximately 3 times higher molecular weight was obtained. A decrease in molecular weight was observed with an increase in redox concentration which is in good agreement with the literature where, the molecular weight of poly butyl methacrylate decreased with increase in the redox couple ascorbic acid and hydrogen peroxide. (17) Interesting to note that, in our study, the molecular weight obtained with thermal initiators is close to the molecular weight obtained at 0.5% of redox couples.

[FIGURE 4 OMITTED]

Some of the properties of emulsions are tabulated in Table 3. It is clearly evident that the emulsions prepared with redox couples at all four concentrations are comparable with the emulsions prepared with thermal initiator (Std.) except for the F-T stability test.

The emulsion prepared with thermal initiator passed the F-T stability test whereas the emulsions prepared with redox couple SFS-TBHP showed variations in viscosity pickup. Figure 4 demonstrates the variation in viscosity of the emulsions prepared with redox couple. The viscosity of the samples was measured before keeping them in the freezer at -5[degrees]C (CO in the x-axis of Fig. 4), and all the samples show similar viscosity of ~80 g. Emulsion samples at -5[degrees]C for 8 h (freezing) and the same samples after freezing placed at room temperature (25[degrees]C) for 8 h (thawing) complete 1 F-T cycle. From Fig. 4, cycle I (Cl), the emulsion sample prepared with 0.3% redox couple did not show any thawing in the first cycle itself even at extended thawing and hence was considered as a failure in F-T stability. Hence, data are not shown for 0.3% sample and the rest of the samples recovered their original viscosity during thawing. It is to be noted that the sample with 0.4% redox couple shows a pickup in viscosity with an increase in F-T cycles, whereas the other two samples remain similar. Thus, it can be concluded that the samples below 0.4% fail in the F-T stability test, which is one of the important tests for coating products.

[FIGURE 5 OMITTED]

The parameters such as molecular weight, [T.sub.g], particle size significantly affect the F-T stability of oil in water emulsions. (17) The effect of [T.sub.g] on F-T stability is not clear; a few report no correlation between the latex [T.sub.g] and F-T stability, (20,25) while reports from King (26) and Naidus (27) showed a correlation. The latexes with higher [T.sub.g] require very little or no stabilizing groups for F-T stability. As shown in Fig. 5, our findings with the DSC thermograms are similar to the later report, where the latex with 0.3% redox couple had lower [T.sub.g] (26[degrees]C) compared to the standard and the rest of the samples (~31[degrees]C). The latex sample with 0.3% initiator fails in the F-T stability test in the first cycle itself. However, the significance of the difference of ~5[degrees]C on the F-T stability is not very clear.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The [T.sub.g] was also determined by dynamic mechanical analysis (DMA). The DMA thermograms are shown in Fig. 6. There was no significant difference in [T.sub.g] among all the samples. However, a clear difference in the shape of the curve for sample of 0.3% was seen. The curve was broad with two shoulders, such curves are generally observed for the samples with very high crosslinking or very high molecular weight with high poly dispersity index (PDI). In our case, the broadness is probably due to the very high molecular weight (see Fig. 3) and high PDI (4.2) than the other samples which have PDI less than 3.9.

Brian et al. have reviewed the influencing factors on the F-T stability of food-based emulsions. (23) During the F-T test, the freezing process involves a number of different physicochemical processes such as phase separation of latex particles and water, crystallization of ice, crystallization of latex particles, etc. A partial coalescence was observed if the oil phase (latex particles) crystallizes before the water phase which leads to instability of the emulsions. In the other case, where the water phase crystallizes before the oil phase, the concentration of latex particles increases in the non-frozen region which eventually brings the latex particles closer to the other latex particles. Such systems lead to flocculation and coalescence and finally phase separated. The previous studies have also shown that during freezing, the ionic strength of the nonfrozen water increases due to the crystallization of ice, which will directly affect the electrostatic interactions between the latex particles and thereby impose instability in emulsions. (28)

The specific conductivity of the emulsions was measured to understand the ionic nature of the emulsions. The specific conductivity increased with an increase in the concentration of the redox couple as shown in Fig. 7. The specific conductivity increased with an increase in concentration of SFS and TBHP until 0.5% (0.25:0.25), and beyond this point, the slope decreased which is clearly reflected on the F-T stability of the latexes. The redox couple, SFS and TBHS, produces a variety of ionic species. (24) The ES/MS data evidenced the possible species generated during the redox reaction of SFS and TBHP, which include the reactive species, S[O.sup.2-], S[O.sup.3-], and HOC[H.sub.2], and the unreactive species like S[O.sub.2], S[O.sup.4-], and HCHO. We made an attempt to understand the effect of the amount of sulfate species on the stability of latexes. Table 4 summarizes the theoretical amount of sulfate ions generated during the redox reaction of SFS-TBHP.

In the case of thermal initiation, 0.21% PPS (7.77 x [10.sup.-4] mol) generates 1.55 x [10.sup.-3] mol of sulfate ions, whereas emulsion with 0.25% SFS (Table 4, B3) generates nearly same amount of sulfate ions. Hence, the stability against F-T was comparable. The failure of samples B1 and B2 in the F-T stability might be due to the lack of sulfate species which, in turn, stabilizes the latexes against F-T. The ionic species produced at the saturation point 0.5% initiator couple is sufficient enough to stabilize the latex particles against the F-T stability test.

Conclusions

Co-polymerization of styrene-acrylic system was carried out with the redox couple SFS-TBHP as initiator at 50[degrees]C. The processing, kettle/stirrer hygiene, and the conversion, were comparable with the standard sample (co-polymerization with thermal initiator PPS at 80[degrees]C). Almost 100% conversion was achieved with 0.3% of SFS-TBHP (1:1) system. The effect of concentration of redox initiator on the properties of the emulsions was examined. A noticeable effect on molecular weight and F-T stability was observed. Though the 0.3% redox couple results in 100% conversion, it failed in the F-T stability test. The emulsions prepared with 0.5% and above passed the F-T stability test. Based on the experimental results, it is concluded that concentration of redox couple (various ionic species generated from the reactions of SFS and TBHP) strongly influences the F-T stability of the redox co-polymerized styreneacrylic system. The F-T stability of the emulsions is a very crucial property in the coatings industry, and this article brings some insight into the effect of concentration of redox species on the stability of emulsions. Moreover, low-temperature processes are always appreciated in terms of safety and energy savings. Redox polymerization at low temperatures (<50[degrees]C) is an approach towards designing a greener product in terms of energy savings.

DOI: 10.1007/s11998-015-9727-7

C. R. Haramagatti ([mail]), S. Sikdar, S. Bhattacharya

Research & Technology Centre, Asian Paints Limited, Plot No. C3-B/1, TTC MIDC, Pawane, Thane--Belapur Road. Navi Mumbai, Maharashtra 400703, India

e-mail: crharamagatti@gmail.com; chandrashekara. haramagatti@asianpaints.com

Acknowledgments Authors would like to thank Dr. Swapan K Ghosh and Dr. B. P. Mallik for their kind support. Our thanks are also due to Mrs. Sonali Bivalkar, Analytical lab (Asian Paints Limited) who did the characterizations in time.

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Table 1: A typical recipe of the styrene-acrylic
copolymer

                              %

Reactor charge
  Deionized water             16.9
  Anionic surfactant          0.6
  Sodium bicarbonate          0.15
Pre-emulsion
  Deionized water             22.1
  Anionic surfactant          1.5
  Non-ionic surfactant        1
  Butyl acrylate (BA)         20.2
  Styrene (ST)                26.5
  Meth acrylic acid (MAA)     1.5
Additives
  Defoamer                    0.2
  Neutralizer                 1.0
  Deionized water             8.35

The quantity of the redox couple added is adjusted with the
deionized water in the reactor charge

Table 2: An approach of addition of redox couple
SFS:TBHP (1:1) to the reactor

Batch   Added                                       Total
no.     in RC   1st h   2nd h   3rd h   4th h   initiator (%)

B1      0.06    0.06    0.06    0.06    0.06         0.3
B2      0.08    0.08    0.08    0.08    0.08         0.4
B3      0.10    0.10    0.10    0.10    0.10         0.5
B4      0.12    0.12    0.12    0.12    0.12         0.6

RC reactor charge

Table 3: Some of the properties of the emulsions prepared at varied
amount of redox couple, and the properties of the emulsion prepared
with thermal initiator (Std.) are also given for comparison

Properties                      Std                 B1

Particle size (nm)       145 ([+ or -] 36)   140 ([+ or -] 36)
Viscosity (g)            85                  86
MFFT ([degrees]C)        39.5                38.3
AS                       Pass                Pass
F-T                      Pass                Fail
[T.sub.g] ([degrees]C)   31                  26
  (by DSC)

Properties                      B2                  B3

Particle size (nm)       160 ([+ or -] 40)   170 ([+ or -] 37)
Viscosity (g)            84                  82
MFFT ([degrees]C)        37.5                35.6
AS                       Pass                Pass
F-T                      High viscosity      Pass
[T.sub.g] ([degrees]C)   32                  30
  (by DSC)

Properties                      B4

Particle size (nm)       160 ([+ or -] 34)
Viscosity (g)            85
MFFT ([degrees]C)        36.6
AS                       Pass
F-T                      Pass
[T.sub.g] ([degrees]C)   32
  (by DSC)

Numbers in the parentheses are standard deviations

MFFT minimum film formation temperature, AS accelerated stability
test (at 50[degrees]C), F-T freeze-thaw stability test (freezing
at -5[degrees]C for 8 h and thawing at room temperature for 8 h),
[T.sub.g] (DSC) [T.sub.g] determined by DSC thermograms

Table 4: Summary of the theoretically calculated total sulfate ions
generated during the redox reaction of SFS-TBHP

                                                      Concentration of
Concentration   Concentration     Concentration        sulfate ion in
of initiator       of SFS         of SFS in mole            mole

B1                  0.15        0.97 x [10.sup.-3]   0.97 x [10.sup.-3]
B2                  0.20        1.30 x [10.sup.-3]   1.30 x [10.sup.-3]
B3                  0.25        1.62 x [10.sup.-3]   1.62 x [10.sup.-3]
B4                  0.30        1.95 x [10.sup.-3]   1.95 x [10.sup.-3]
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Author:Haramagatti, Chandrashekara R.; Sikdar, Subhadip; Bhattacharya, Shruti
Publication:Journal of Coatings Technology and Research
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Date:Jan 1, 2016
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