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Emulsion copolymerization of vinyl acetate-ethylene in high pressure reactor-characterization by inline FTIR spectroscopy
Progress in Organic Coatings 76 (2013) 1798–1804
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Emulsion copolymerization of vinyl acetate-ethylene in high pressure reactor-characterization by inline FTIR spectroscopy Ida Poljanˇsek a,b,∗ , Ema Fabjan a , Klemen Burja a , Dolores Kukanja a,c a b c
Centre of Excellence for Polymer Materials and Technologies, Tehnoloˇski park 24, SI-1000 Ljubljana, Slovenia Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia Mitol d.d., Partizanska cesta, Seˇzana, Slovenia
a r t i c l e
i n f o
Article history: Available online 24 June 2013 Keywords: Emulsion polymerization Vinyl acetate-ethylene Pressure Glass transition temperature Differential scanning calorimetry (DSC) Inline FTIR spectroscopy
a b s t r a c t The objective of this research was to investigate the effect of temperature, pressure, initiator concentration and agitation rate, in ethylene-vinyl acetate emulsion copolymerization, on copolymer composition. The inline React-IR ATR system was used to monitor the reaction as well as to determine residual free vinyl acetate. Pressure, temperature and agitation rate have great influence on mass transfer of ethylene monomer to the reaction sites. The vinyl acetate was introduced in semi-batch mode as well as ethylene since the copolymerization was carried out under a constant pressure of ethylene. The higher temperature results in lower content of ethylene incorporated in copolymer. Increase of pressure has a direct effect on the ethylene content in the copolymers through increasing solubilization of ethylene monomer which in turn increases ethylene content in the copolymers. Copolymers of up to 15 wt.% of ethylene content have been synthesized at an ethylene pressure of 30 bar and a temperature of 75 ◦ C. Analytical methods, such as differential scanning calorimetry, nuclear magnetic resonance, thermogravimetric analysis, and infrared spectroscopy were used for characterization of copolymers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The free radical polymerization reaction of vinyl acetate in emulsion is commercially conducted in both batch and continuous reactors at atmosphere pressure. In order to improve the properties of poly(vinyl acetate) (PVA) a second monomer is polymerized in conjunction with the vinyl acetate [1,2]. The vinyl acetate ethylene (VAE) copolymer is a product based on the copolymerization of vinyl acetate and ethylene in which the vinyl acetate content can range between 60–95%, and the ethylene content ranges between 5–40% of the total formulation. VAEs offer considerable performance advantages over PVA homopolymers due to the ability to alter the glass transition temperature (Tg ◦ C) through the incorporation of the ethylene monomer. Adding of ethylene (for example from 10 to 25%) will reduce Tg and generally improve peel strength. The presence of ethylene in effect reduces the glass transition temperature (Tg ) of polyvinyl acetate from +42◦ to −30 ◦ C depending on its concentration [3]. VAE emulsions can often be looked upon as modification
∗ Corresponding author at: Centre of Excellence for Polymer Materials and Technologies, Tehnoloˇski park 24, SI-1000 Ljubljana, Slovenia. Tel.: +386 1 320 36 50; fax: +386 1 25 72 297. E-mail addresses: [email protected] (I. Poljanˇsek), [email protected] (D. Kukanja). 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.05.019
of polyvinyl acetate with ethylene as an internal plasticizer. In contrast to the emulsion polymerization of vinyl acetate, the syntheses of vinyl acetate ethylene copolymers utilize high-pressure reactions ranging from 20 to 60 bar or even more. Generally, VAE copolymer emulsions can be used without additives or modifiers and ideally combine low environmental impact with effective performance in paints and coatings. VAE emulsions may be used as coatings and also as adhesives for different applications. Coating gives clear and flexible film which gives protection from direct sun light and it is resistant to yellowing colour on ageing effect. The primary factor affecting the properties of this material is the percentage of ethylene. Preparation of stable dispersion of ethylene/vinyl acetate copolymer polymerization in a carbon dioxide continuous phase with different stabilizers was investigated [4]. Gruber et al. [5] studied effects of different colloidal stabilizer on vinyl acetate-ethylene copolymer emulsion and films. Not much literature exists on the topic of the synthesis and the kinetic study for the preparation of vinyl acetate–ethylene copolymer dispersions. The process of free radical copolymerization of vinyl acetate and ethylene at high pressure is presented in the patent publications. The course of the copolymerization reactions is influenced by numerous parameters, so the preliminary experiments were done in which the parameters with the greatest impact was determined [6,7]. Scott et al. [8–12] described ethylene-vinyl acetate semi-batch emulsion copolymerization and the influence of
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redox initiator system, the effect of n-hexane as a co-solvent, effect of temperature, vinyl acetate feed rate, emulsifier type and pressure on the structure of the copolymer. There were some attempts to synthesize vinyl acetate-ethylene copolymer emulsion with a continuous process [13]. Guo et al. reported the copolymerization of ethylene and vinyl acetate via emulsion and miniemulsion polymerizations [14]. Thermal properties of vinyl acetate-ethylene copolymers were studied by differential scanning calorimetry and thermogravimetric analysis [15,16] and for structure characterization of copolymers nuclear magnetic resonance and infrared spectroscopy were used [16–18]. Our goal was to synthesize the copolymer in batch reactor with the highest portion of ethylene at lowest temperature as possible in order to decrease the percentage of branches [20] and to proceed the process at reduced pressure due to the safety reasons and the associated costs. In order to better understand the process during the course of copolymerization reaction and to accumulate reliable data we used an ATR-FTIR spectrometry technique (ReactIR 4000) with light conduit and diamond-composite sensor to perform in-line monitoring of emulsion copolymerization of vinyl acetate-ethylene in high pressure reactor. ATR-FTIR spectrometry technique was found to be ideal for determining residual free vinyl acetate, individual vinyl acetate and ethylene conversions and copolymer composition changes as a function of time when the copolymerization reaction was carried out. The kinetics data obtained through the ReactIR 4000 in-line reaction analysis system agreed well with those determined by the traditional GC method. ReactIR technology replaced time consuming (and inaccurate) off-line methodology. We examined the effect of agitation and mixing, temperature, pressure as well as initiator concentration on the copolymer composition and composition drift.
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Table 1 Reaction conditions of vinyl acetate-ethylene copolymers synthesis, agitation rates, dosing times, ethylene pressure, weight of initiator and sodium bicarbonate, and temperature of reaction synthesis.
2R 3R 4R 5R 7R 8R 10R 11R 12R 13R 14R 15R 16R 17R 18R 19R 20Ra 21Ra 22R 25R 26R 28R 30Ra 34R 35R 37R 38R 39Rb a b
T (◦ C) PEthylene (bar) mAPS (g) mNaHCO3 (g) tVAc dosing (min)
Agitation rates (rpm)
70 70 70 75 70 70 75 75 70 70 75 75 70 75 70 75 80 80 80 80 70 75 75 75 75 65 65 75
200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 250 300 200 250 200
20 20 20 20 30 30 30 30 30 30 30 30 20 20 20 20 20 30 20 30 30 30 30 30 30 30 30 /
0.5 0.5 0.5 0.5 0.5 0.75 0.75 1.0 0.75 1.0 0.75 0.5 1.0 1.0 0.75 0.75 0.5 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75
0.5 0.5 0.5 0.5 0.5 0.75 0.75 1.0 0.75 1.0 0.75 0.5 1.0 1.0 0.75 0.75 0.5 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75
90 180 250 180 180 180 180 180 240 240 240 180 180 180 180 180 180 180 180 180 240 180 180 180 180 180 180 180
Longer time of reaction after dosing at reaction temperature. Pure polyvinyl acetate dispersion (the reference batch).
2. Experimental 2.1. Materials Vinyl acetate (99.5%) was purchased from Lyondell Chemie Nederland. Ethylene (99%) was supplied by Istrabenz, Koper. Poly(vinyl alcohol) solution was prepared from MOWIOL 26–88 (3.5 wt.%, (4%) = 26 mPas, Mw = 160,000, degree of hydrolysis 88 mol%) and MOWIOL 4-88 (7 wt.%, (4%) = 4 mPas, Mw = 31,000, degree of hydrolysis 88 mol%) with deionised water (90 wt.%). Polyether siloxane copolymer (Tego Foamex 1488) was purchased from Evonik Tego Chemie GmbH. The initiator ammonium persulphate (APS, ≥98.0%), dimethylsulfoxide (DMSO-d6 + 0.03% TMS, 99.80%D) and sodium bicarbonate (NaHCO3 , ≥99.5%) were supplied by Sigma Aldrich, Steinheim, Germany. All chemicals were used as obtained without further purification.
2.2. Synthesis of co-ethylene-vinyl acetate dispersions The reaction configuration is important variable which influences the rate of polymerization, copolymer composition, molecular weight and limiting end conversion. Polymerisations were carried out on RC1-reaction calorimeter equipment (Mettler Toledo) in 1.8 L HP60 high pressure stainless steel semibatch reaction vessel, a stainless steel-anchor stirrer, a digital thermometer, a calorimeter and FTIR K6 conduit 16 mm Dicomp probe. Ethylene consumption during the polymerization process was measured by gas uptake unit and the pressure in the vessel was controlled by external automatic gas valve. Prominent Micro delta Optodrive pump was used for dosing of vinyl acetate. The temperature, dosing and pressure during the processes were well controlled by Mettler Toledo iControle software.
A typical procedure was as follows: 0.5–1.0 g of APS, NaHCO3 and 0.5 g of 24% emulsion of a polyether siloxane copolymer were diluted in 200 g of deionised water at room temperature and 200 g of 10% water solution of PVOH were charged into a reaction vessel. While stirring 25 g VAc was added at room temperature. Vessel was closed and flushed with ethylene thoroughly to remove air completely and to saturate emulsion with ethylene. Pressure was increased to 5 bar less than the end pressure of the experiment, which were 20 or 30 bar. Temperature of the reactor was increased to 65–80 ◦ C in 20 min. Pressure was kept constant at 20 or 30 bar during the whole process at selected temperatures. Right after the temperature and pressure was reached VAc was added dropwise into the reactor. 225 g of VAc was charged at a constant flow rate during 90–240 min. Concentration of VAc was measured with online FTIR to pursue the conversion of VAc. After the dosing of VAc finished the reaction mixture was kept at chosen temperature for 140 min before cooling. Overpressure of ethylene was released after cooling to room temperature. The experiments were monitored by the inline FTIR–ATR spectroscopy, using the Mettler Toledo ReactIR- iC IR 4.2 instrument coupled with DiComp diamond probe. IR spectra were collected every 3 min, in the wave number range between 4000 and 650 cm−1 at resolution 8 cm−1 . The first spectra were collected before the reaction mixture was heated to the chosen temperature. At the end of the synthesis the traditional GC method for determining free VAc was carried out. The solid content of all synthesized copolymer dispersions was about 45%. Several semibatch copolymerizations of vinyl acetate and ethylene were conducted at 70, 75, 80, and 85 ◦ C. All the experiments’ initiator dosing, dosing times and ethylene pressure are gathered in Table 1.
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2.3. Characterization methods 2.3.1. 1 H nuclear magnetic resonance The 1 H NMR spectra were measured using Unity Inova 300 Varian NMR spectrometer operating at 300 MHz. Samples for NMR measurements were first diluted (1:3) with deionised water, dispersed in a round bottom flask and frozen by liquid nitrogen. Frozen samples were then transferred into a freeze dry system (VirTis SP Scientific sentry 2.0) overnight, to remove water. The pressure within the freeze dryer was set at 0.026 mbar and the temperature of the condenser was set at −105 ◦ C. DMSO-d6 was used as a solvent and 1 H NMR signals were referred to tetramethylsilane (TMS) as an internal standard. During the sample preparation for 1 H NMR measurements no gel formation was observed, VAE samples were completely dissolved in the DMSO-d6. All the spectra were obtained at 25 ◦ C. The conditions for the 1 H NMR was as follows: a 90◦ pulse angle, a 5-s delay between the pulses, an acquisition time of 5 s, and up to 32 repetitions. VNMRJ rev. 1.1D software was applied for the peak integration. Assignation of individual signals was made on the basis of the available literature [16,17,19] on co-ethylene-vinyl acetate dispersions. The vinyl acetate-to-ethylene monomer ratio was calculated according to the procedure described by Guo at al. [14] and Gupta et al. [16]. 2.3.2. Dynamic scanning calorimetry For the determination of glass transition temperature Tg of the copolymers the differential scanning calorimetry was used. Approximately 8 mg of the sample was placed in an aluminium pan, sealed and analyzed. The measurements were performed on a METTLER TOLEDO 821e instrument with intracooler, using the STAR software. In and Zn standards were used for the temperature calibration and for the determination of the instrument time constant. For the heat flow calibration, the In standard was used. The experiments were performed under nitrogen atmosphere (60 mL min−1 ) in two scans in the temperature range of −80 to 150 ◦ C and at the heating rate of 10 ◦ C min−1 . An empty pan served as a reference. 3. Results and discussion 3.1. Semibatch VAE emulsion copolymerization ATR-FTIR spectrometry was utilized to monitor the reaction of vinyl acetate-ethylene copolymer dispersions at 65, 70, 75, and 80 ◦ C. In-line data acquisition was performed by immersing the transmission probe directly into the pressure reactor. Assignment of the characteristic peaks of vinyl acetate and ethylene was done using the previously collected spectra of both reactants in aqueous solution. The spectra of vinyl acetate and ethylene solutions were collected at the beginning of the measurements (not presented). The assignment of characteristic peaks was done on the basis of relevant literature and of our previous experimental work [20,21]. These signals were helpful in identifying the components of the copolymerization reaction of vinyl acetate and ethylene. We have to emphasize that the literature assigned absorption bands to the various chemical groups did vary, as might be expected [20]. The peak at 1138 cm−1 which corresponded to the vinyl acetate O C C stretch vibration (observable in Fig. 3) diminished with the reaction time, because of the copolymer formation. The normalized vinyl acetate-ethylene copolymer spectra at the wavelengths in the range of interest for the copolymer synthesis are shown in Fig. 1. The difference in spectra of homo polyvinyl acetate and copolymer polyvinyl acetate ethylene is minimal; we can observe the peak at 2800 cm−1 which correspond to the C H stretch vibration of ethylene. As the reaction proceeds this peak increased with reaction time. Another difference in spectra of homo and copolymer can be
Fig. 1. The IR spectra in the entire wave number range of interest for the coethylene-vinyl acetate dispersions synthesis at 70 ◦ C.
observed at 760 cm−1 but in our case this peak is overlapped with peaks of water. In Fig. 2 the concentration profiles for vinyl acetate, ethylene and reaction product during copolymer synthesis at 70 ◦ C are presented. The concentration of ethylene during the copolymerization reaction decreases, while the concentration of vinyl acetate increases due to the addition of it. When the addition of vinyl acetate is completed, the concentration of vinyl acetate dropped rapidly as a result of the polymerization reaction. We also observed that longer dosing times of vinyl acetate led to an increase of ethylene weight percent in the copolymers. In Fig. 3A the effect of reaction temperature versus ethylene incorporation in the copolymer is presented for VAE37, VAE8, VAE28 and VAE21 reactions which proceed at reaction temperature of 65, 70, 75 and 80 ◦ C, respectively. At the highest temperature the lowest content of incorporated ethylene in copolymer structure is observed due to the lower solubility of ethylene monomer in this copolymerization system. At 65 ◦ C, the proportion of incorporated ethylene in the copolymer was 12.92% [15,16]. We believe that lower ethylene percent in copolymer structure at 65 ◦ C is attributed to the slower polymerization reaction [22]. In our system, the temperature of 65 ◦ C was too low for the desired rate of polymerization and for decomposition of initiator.
3.2. Effect of agitation The effect of agitation was investigated through VAE28, VAE34 and VAE35 reactions with agitation rates of 200, 250 and 300 rpm, respectively (Fig. 3B). Higher agitation rates, for instance 300 rpm, increases the mixing turbulence of the liquid phase and thus increases the mass transfer, but also increases the solubility of ethylene due to better contact between gas and liquid phases. Here we must emphasize that the higher mixing speed does not increase the solubility of ethylene, but reduces the mass transfer limitations, which is reflected as an increase in solubility [23]. Thus, the presence of ethylene monomer in the reaction sites is increased with the higher agitation rate. Low agitation rate, for instance 200 rpm, caused a decrease in ethylene solubilization and its participation in the reaction sites due to lower solubility or due to poorer contact between the latex and the gas phase. Since vinyl acetate and ethylene are in competition in participating in polymerization reaction, there is variation in the ethylene content of polymers and the higher ethylene percentage in the samples is attributed to the lower vinyl acetate conversion. We have to consider that mass transfer limitations increases due to the gradual increase in the viscosity of the
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Fig. 2. The concentration profiles in weight percent for vinyl acetate, ethylene, and reaction product during copolymer synthesis at 70 ◦ C.
B 16,0
15,5
A
Ethylene incorporated
Ethylene incorporated
15,0 15,5
Ethylene incorporated (%)
Ethylene incorporated (%)
14,5 14,0 13,5 13,0 12,5 12,0
15,0
14,5
14,0
13,5
11,5 11,0
13,0 64
66
68
70
72
74
76
78
80
82
200
220
C
260
280
300
28
30
Agitation rate (min )
14
Ethylene incorporated
D
15
Ethylene incorporated at 70 °C Ethylene incorporated at 75 °C Ethylene incorporated at 80 °C
Ethylene incorporated (%)
14
Ethylene incorporated (%)
240
-1
Temperature (°C)
12
10
13
12
11
10
9
8
8 0,4
0,6
0,8
Concentration of initiator (%)
1,0
20
22
24
26
P (bar)
Fig. 3. (A) Effect of reaction temperature on ethylene percent in copolymers; (synthesis at 30 bar; monomer addition rate 1.25 g/min; 0.75% of APS; 200 rpm); (B) effect of agitation rate on ethylene percent in copolymers, (synthesis at 75 ◦ C, 30 bar; monomer addition rate 1.25 g/min; 0.75% of APS); (C) the influence of initiator amount on the co-ethylene-vinyl acetate composition (synthesis at 70 ◦ C, 20 bar; monomer addition rate 1.25 g/min; 200 rpm) and (D) Effect of pressure on ethylene percent in copolymers at different temperatures (synthesis; 0.75% of APS; 200 rpm).
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latex with the progression of the polymerization [24]. The higher mass transfer barrier can be overcome in the higher agitation rates. When the data of ethylene content and vinyl acetate conversion are considered simultaneously, it can be concluded that these experimental values increase at higher agitation rate at the constant temperature and initiator concentration. At lower agitation rates mass transfer of vinyl acetate and ethylene to the reaction sites is limited. The overall conversion of vinyl acetate and ethylene is satisfactory at agitation rates 200 rpm. Higher agitation rate is a high energy consuming process and a lot of foaming occurred during the polymerization. In order to optimize the procedure, copolymerization reaction were carried out at different temperatures, pressures and at different initiator concentrations. 3.3. Effect of initiator concentration To study the influence of initiator concentration a set of three reactions with initiator concentrations of 0.5, 0.75, and 1 wt.% of ammonium persulphate was investigated designated as VAE3, VAE18, and VAE16, respectively. At constant agitation rate of 200 rpm, we used three levels of initiator concentration and found that successive increase of the initiator concentration increased the conversion simultaneously. Fig. 3C shows the influence of initiator concentration on the proportion of incorporated ethylene. It can be seen that ethylene weight percent in the copolymers of VAE16, prepared with 1 wt.% of ammonium persulphate is the highest in comparison with those obtained in the other two reactions. When the initiator concentration was increased from 0.5 to 1.0 wt.%, ethylene weight percent in the copolymer showed a very high increment. In VAE16 reaction (Fig. 3C) with 1.0 wt.% of initiator concentration, higher nucleation and thus, higher number of particles were formed in the initial stage of the reaction. Therefore, the extent of ethylene mass transfer and its presence in the reaction sites have increased so that ethylene weight percent in VAE16 (1.0 wt.%) copolymers showed the highest value compared to VAE18 (0.75 wt.%) and VAE3 (0.5 wt.%). We assume that at the highest initiator concentration the greater numbers of particles were formed in the initial stage of the reaction, thus the higher polymerization rate of ethylene and vinyl acetate proceeds. Higher polymerization rate of vinyl acetate inhibits ethylene polymerization and reduces its conversion and vice versa. Scot et al. have noticed a similar trend in the installation of an ethylene copolymer as a function of initiator concentration in 1.8 L reactor [12,13]. 3.4. Effect of pressure One of the most important effects of pressure is the increasing ethylene solubility at the reaction sites. By increasing pressure from 20 to 30 bar ethylene weight percent in copolymer shows significant increase. Considerable increase in ethylene content of the copolymers is observed by increasing pressure from 20 and 30 bar at all introduced temperatures (Fig. 3D). Copolymer, which was synthesized at 20 bar and 75 ◦ C, has the proportion of incorporated ethylene 9.59 mol %, while the copolymer synthesized at 30 bar and 75 ◦ C, has the proportion of incorporated ethylene 14.4 mol%. This is due to increase in ethylene solubility. When the pressure increases, ethylene solubilization and thus, its presence in the reaction sites increase as well [25]. Therefore, ethylene comes to react with vinyl acetate and reduces its homopolymerization. Consequently vinyl acetate conversion decreases. Similar finding was reported by Scot et al. [9,11,12] which performed reactions in 1.8 L reactor. These results are in good agreement with those where the influence of the initiator and stirring on the conversion of ethylene and VAc were studied. We noticed the reciprocal influence of VAc conversion and ethylene.
50 Tg as a function of the ethylene content using the Flory-Fox equation Tg as a function of the ethylene content from actual consumption
40
30
Tg (°C)
1802
20
10
0
-10 0
5
10
15
20
25
Ethylene incorporated (%) Fig. 4. Tg values as a function of ethylene incorporation for the vinyl acetateethylene copolymer of different compositions synthesized as indicated in the Table 1.
3.5. Glass transition temperature In order to investigate the effect of ethylene on copolymer properties, glass transition temperatures, Tg of the copolymers were determined. Since the aim of this copolymerization is to reduce the Tg of poly(vinyl acetate) through internal plastification of vinyl acetate homopolymer, this measurement proved to be very useful. Nevertheless, Tg is affected by polymer molecular weight, chain structure, branching, crystallinity, polarity, etc. In our study some of these parameters control the Tg of the synthesized copolymers. As a matter of fact all copolymers show significant decrease in Tg regardless of different conversion, temperature, and initiator concentration (Fig. 4). This is due to the incorporation of ethylene in the samples. Note that the Tg of poly(vinyl acetate) is about 43 ◦ C [20]. However, the differences in Tg arise from different molecular weights and structure, for instance, branching in the copolymers. Fig. 4 shows the Tg values of the synthesized vinyl acetate-ethylene copolymer of different compositions as a function of the ethylene content. Ethylene content was determined regarding the consumption during the polymerization process by gas uptake unit. Fig. 4 presents also the results, which were calculated using Flory–Fox predictions equation. The ethylene content of the synthesized VAE copolymer determined by gas uptake unit follow the predictions of the Flory–Fox equation (Fig. 4, Eqs. (1)–(3)). The ethylene content in VAE copolymer was calculated using the following equations (Flory–Fox equation): 1 ω(ethylene) ω(VAc) = + Tg Tg (ethylene) Tg (VAc)
(1)
1 − ω(ethylene) ω(ethylene) 1 + = Tg Tg (ethylene) Tg (VAc)
(2)
ω(ethylene) =
Tg (ethylene)[Tg (VAc) − Tg ] Tg [Tg (VAc) − Tg (ethylene)]
=
(Tg (VAc)/Tg ) − 1 (Tg (VAc)/Tg (ethylene)) − 1
(3)
Higher is the content of incorporated ethylene lower is Tg of vinyl acetate-ethylene copolymer. In Fig. 5 1 H NMR spectra for the vinyl acetate-ethylene copolymer of different compositions are presented. In Fig. 5, the peak at 4.78 ppm corresponds to the CH proton in VAc segment, the broad peak from 1.59 to 1.86 ppm represents
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Fig. 5.
1
H NMR spectra for the vinyl acetate-ethylene copolymer of different compositions as indicated in the legend.
the CH2 in VAc segment, and the signal of the CH3 protons of VAc appears at 1.95 ppm. Additional ethylene protons signals at 1.21 and 1.45 ppm were observed. The peak at 0.82 ppm corresponds to the CH3 group derived from H-abstraction by ethylene CH2 radical and represents methyl group from polyethylene branches [14]. Ethylene proton signals at 1.45 ppm overlap with the signals of methylene and methyl protons of polyvinyl acetate. In order to compare the data with the ethylene content obtained by prediction Flory–Fox equation and results determined by consumption of ethylene during the synthesis the weight fraction of ethylene was calculated from 1 H NMR spectra. From NMR spectra the molar ratio of ethylene to vinyl acetate was calculated using the Eq. (4) [14,16] and transformed to the weight fraction following the Eqs. (5) and (6): I0.74−2.1 ppm − 5I4.78 ppm n (ethylene) = 4I4.78 ppm n (VAc)
(4)
where I represents the integral of a peak in the 1 H NMR spectra; X(ethylene) =
n (ethylene) n (ethylene) + n (VAc)
(5)
where X represents the molar fraction; ω(ethylene) =
1803
X(ethylene) × Methylene X(ethylene) × Methylene + X(VAc) × MVAc
(6)
where ω represents the weight fraction and Methylene and MVAc represent molar mass of ethylene and vinyl acetate unit, respectively. In Table 2 the comparison of ethylene content in vinyl acetateethylene copolymers determined from the actual consumption of ethylene during the polymerization, by prediction using the Flory–Fox equation (from DSC) and by 1 H NMR measurements are presented. Results obtained from the actual consumption of ethylene during the polymerization agreed well the predictions of the Flory–Fox equation. Results obtained from 1 H NMR spectra also
Table 2 Ethylene content in Vinyl acetate-ethylene copolymers determined by DSC, 1 H NMR measurements and from the actual consumption of ethylene. Sample
3R 12R VEA reference
Ethylene weight ratio in VAE copolymer Prediction Flory–Fox equation (from DSC)
1
0.085 0.149 0.223
0.083 0.163 0.231
H NMR
Actual consumption 0.095 0.149 Ref.
agreed well with the results obtained from the actual consumption as well as the predictions of the Flory–Fox equation. We believe that minor deviations in the results can be attributed to the various methods of measurements and due to the overlapping of proton signals of ethylene with the signals of polyvinyl acetate. 4. Conclusions VAEs are a better cost-performing polymer offering improved adhesive and binding properties with more user friendly compounding characteristics. The use of vinyl acetate ethylene (VAE) emulsion compositions in adhesives has grown rapidly over the last two decades with the development of a broad spectrum of end users. Generally, VAE copolymer emulsions are used without additives or modifiers. The primary factor affecting the properties of these adhesives is the percentage of ethylene. During the synthesis of VAE lattice the appropriate combination of different parameters (pressure, temperature, initiator and agitation rate) let to the VAE copolymer with desired percent of ethylene incorporated in the structure. At higher reaction temperature, ethylene solubility decreased which caused a decrease of ethylene weight percent in the copolymer. Longer dosing times of vinyl acetate led to an increase of
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ethylene weight percent in the copolymers. By increasing the agitation rate, ethylene solubilization and its presence in the reaction sites (increase in mass transfer) increased which caused growth of ethylene weight percent in the copolymers. Decreasing agitation rate was led to the decrease in mass transfer and presence of ethylene and vinyl acetate monomers in the reaction sites which in turn lowered vinyl acetate conversion. Furthermore, a decreasing trend in the ethylene percent was observed. At constant agitation rate of 200 rpm, we used three levels of initiator concentration and found that successive increase of the initiator concentration increase the conversion simultaneously. The optimum initiator concentration was 1.0 wt.% of ammonium persulphate on the total amount of vinyl acetate. Considerable increase in ethylene content of the copolymers is observed by increasing pressure from 20 bar to 30 bar. Higher pressure has a direct effect on the copolymerization system, because of the superior solubility of ethylene, leading to the formation of a copolymer with higher amount of ethylene in the final structure of the copolymer. At the same time the vinyl acetate conversion decreased. Copolymer of the highest ethylene content 15.59 wt.% (35R) have been synthesized at an ethylene pressure of 30 bar and at the temperature of 75 ◦ C with dosing time of 180 min, agitation rate of 300 rpm and initiator concentration of 0.75%. Acknowledgements The authors wish to gratefully acknowledge Mitol, Seˇzana for their generous financial support to the presented work and for providing all chemicals used in this study and for permission to publish this paper. This work was supported by the Ministry of Education, Science, Culture and Sport of the Republic of Slovenia Grant number 3211-10-000057 (Centre of Excellence for Polymer Materials and Technologies), whose contribution is fully recognized. References [1] K. Geddes, Polyvinyl and ethylene-vinyl acetates, in: A. Pizzi, K.L. Mittal (Eds.), Handbook of Adhesive Technology, Marcel Dekker, New York, 1994. [2] T.M. Goulding, Polyvinyl acetate wood adhesives, in: A. Pizzi (Ed.), Wood Adhesives: Chemistry and Technology, Marcel Dekker, New York, 1983. [3] P.A. Lovell, M.S. El-Aasser, Emulsion polymerization and emulsion polymers, John Wiley, New York, 1997, pp. 18, Chs 16. [4] D.A. Canelas, D.E. Betts, J.M. DeSimone, M.Z. Yates, K.P. Johnston, Poly(vinyl acetate) and poly(vinyl acetate-co-ethylene) latexes via dispersion polymerizations in carbon dioxide, Macromolecules 31 (1998) 6794–6805. [5] B.A. Gruber, M.S. Vratsanos, C.D. Smith, Effect of colloidal stabiliser on vinyl acetate-ethylene copolymer emulsion and films, Macromol. Symp. 155 (2000) 163–169.
[6] G.E.J. Reynolds, Vinyl acetate-ethylene copolymer dispersions, Polym. Int. 1 (1969) 233–238. [7] H. Salehi Mobarakeh, M.R. Rostami Darankola, Effect of different parameters in emulsion copolymerization of vinylacetate-ethylene in 7.5 liter high pressure reactor, Iran. Polym. J. 14 (2005) 579–587. [8] P.J. Scot, A. Penlidis, G.L. Rempel, A.D. Lawrence, Ethylene-vinyl acetate semi-batch emulsion copolymerization: experimental design and preliminary screening experiments, J. Polym. Sci., Part A: Polym. Chem. 31 (1993) 403–426. [9] P.J. Scot, A. Penlidis, G.L. Rempel, A.D. Lawrence, Ethylene-vinyl acetate semibatch emulsion copolymerization: Use of factorial experiments for improved process understanding, J. Polym. Sci. A: Polym. Chem. 31 (1993) 2205–2230. [10] P.J. Scot, A. Penlidis, G.L. Rempel, Reactor design consideration for gas–liquid emulsion polymerization: the ethylene-vinyl acetate example, Chem. Eng. Sci. 49 (1994) 1573–1583. [11] P.J. Scot, A. Penlidis, G.L. Rempel, A.D. Lawrence, Ethylene-vinyl acetate semibatch emulsion copolymerization: Use of factorial experiments for process optimization, J. Polym. Sci. A: Polym. Chem. 32 (1994) 539–555. [12] P.J. Scot, A. Penlidis, G.L. Rempel, Ethylene-vinyl acetate emulsion copolymerization: monomer partitioning and preliminary modeling, Polym. React. Eng. 3 (1995) 93–130. [13] F.P. Petrocelli, C.F. Cordeiro, Continuous process for the production of vinyl acetate-ethylene emulsion copolymers, Macromol. Symp. 155 (2000) 39–51. [14] J. Guo, K.Y. Choi, F.J. Schork, Miniemulsion copolymerization of ethylene and vinyl acetate, Macromol. React. Eng. 3 (2009) 412–418. [15] N. Gospodinova, T. Zlatkov, L. Terlenezyan, Relationship between microstructure and phase and relaxation transitions in ethylene-(vinyl acetate) copolymers prepared by emulsion copolymerization, Polymer 39 (1998) 2583–2588. [16] N. Gupta, R.K. Srivastava, V. Choudhary, I.K. Varma, S. Patnaik, Thermal characterization of vinyl acetate-ethylene copolymers, J. Thermal Anal. Calorim. 58 (1999) 509–515. [17] J. Rottstegge, K. Landfester, M. Wilhelm, H.W. Spiess, C. Heldmann, Different types of water in the film formation process of latex dispersions as detected by solid-state nuclear magnetic resonance spectroscopy, Colloid Polym. Sci. 278 (2000) 236–244. [18] J. Wu, J.P. Tomba, J.K. Oh, M.A. Winnik, R. Farwaha, J. Rademacher, Synthesis of dye-labeled poly(vinyl acetate-co-ethylene) (EVA) latex and polymer diffusion in their latex films, J. Polm. Sci. A: Polym. Chem. 43 (2005) 5581–5596. [19] D. Britton, F. Heatley, P.A. Lovell, Chain transfer to polymer in free-radical bulk and emulsion polymerization of vinyl acetate studied by NMR spectroscopy, Macromolecules 31 (1998) 2828–2837. [20] I. Poljanˇsek, E. Fabjan, K. Burja, D. Kukanja, Emulsion copolymerization of vinyl acetate-ethylene in high pressure reactor – influence of different parameters, in: Slovenski kemijski dnevi 2011, Portoroˇz, 2011, str. 1–8. [21] I. Aizpurua, M.J. Barandiaran, Comparison between conventional emulsion and miniemulsion polymerization of vinyl acetate in a continuous stirred tank reactor, Polymer 40 (1999) 4105–4115. [22] L.-G. Tang, Z.-X. Weng, Z.-R. Pan, Kinetic studies on emulsion copolymerization of vinyl acetate and acrylics in the batch process, Eur. Polym. J. 32 (1996) 1139–1143. [23] P.G.T. Fogg, W. Gerrard, Solubility of Gases in Liquids, John Wiley, New York, 1991, pp. 7, Chs 3. [24] H. Salehi Mobarakeh, M.R. Rostami, B. Ismailipour, Synthesis of vinyl acetateethylene copolymer through batch emulsion polymerization. II: Effect of buffer and emulsifier concentration, Iran. J. Polym. Sci. Technol. 16 (2003) 231–239. [25] H. Salehi Mobarakeh, M.R. Rostami, B. Ismailipour, Synthesis of vinyl acetateethylene copolymer through batch emulsion polymerization. I: Effect of pressure, Iran. J. Polym. Sci. Tech. 16 (2003) 165–173.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Emulsion copolymerization of vinyl acetate-ethylene in high pressure reactor-characterization by inline FTIR spectroscopy Ida Poljanˇsek a,b,∗ , Ema Fabjan a , Klemen Burja a , Dolores Kukanja a,c a b c
Centre of Excellence for Polymer Materials and Technologies, Tehnoloˇski park 24, SI-1000 Ljubljana, Slovenia Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia Mitol d.d., Partizanska cesta, Seˇzana, Slovenia
a r t i c l e
i n f o
Article history: Available online 24 June 2013 Keywords: Emulsion polymerization Vinyl acetate-ethylene Pressure Glass transition temperature Differential scanning calorimetry (DSC) Inline FTIR spectroscopy
a b s t r a c t The objective of this research was to investigate the effect of temperature, pressure, initiator concentration and agitation rate, in ethylene-vinyl acetate emulsion copolymerization, on copolymer composition. The inline React-IR ATR system was used to monitor the reaction as well as to determine residual free vinyl acetate. Pressure, temperature and agitation rate have great influence on mass transfer of ethylene monomer to the reaction sites. The vinyl acetate was introduced in semi-batch mode as well as ethylene since the copolymerization was carried out under a constant pressure of ethylene. The higher temperature results in lower content of ethylene incorporated in copolymer. Increase of pressure has a direct effect on the ethylene content in the copolymers through increasing solubilization of ethylene monomer which in turn increases ethylene content in the copolymers. Copolymers of up to 15 wt.% of ethylene content have been synthesized at an ethylene pressure of 30 bar and a temperature of 75 ◦ C. Analytical methods, such as differential scanning calorimetry, nuclear magnetic resonance, thermogravimetric analysis, and infrared spectroscopy were used for characterization of copolymers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The free radical polymerization reaction of vinyl acetate in emulsion is commercially conducted in both batch and continuous reactors at atmosphere pressure. In order to improve the properties of poly(vinyl acetate) (PVA) a second monomer is polymerized in conjunction with the vinyl acetate [1,2]. The vinyl acetate ethylene (VAE) copolymer is a product based on the copolymerization of vinyl acetate and ethylene in which the vinyl acetate content can range between 60–95%, and the ethylene content ranges between 5–40% of the total formulation. VAEs offer considerable performance advantages over PVA homopolymers due to the ability to alter the glass transition temperature (Tg ◦ C) through the incorporation of the ethylene monomer. Adding of ethylene (for example from 10 to 25%) will reduce Tg and generally improve peel strength. The presence of ethylene in effect reduces the glass transition temperature (Tg ) of polyvinyl acetate from +42◦ to −30 ◦ C depending on its concentration [3]. VAE emulsions can often be looked upon as modification
∗ Corresponding author at: Centre of Excellence for Polymer Materials and Technologies, Tehnoloˇski park 24, SI-1000 Ljubljana, Slovenia. Tel.: +386 1 320 36 50; fax: +386 1 25 72 297. E-mail addresses: [email protected] (I. Poljanˇsek), [email protected] (D. Kukanja). 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.05.019
of polyvinyl acetate with ethylene as an internal plasticizer. In contrast to the emulsion polymerization of vinyl acetate, the syntheses of vinyl acetate ethylene copolymers utilize high-pressure reactions ranging from 20 to 60 bar or even more. Generally, VAE copolymer emulsions can be used without additives or modifiers and ideally combine low environmental impact with effective performance in paints and coatings. VAE emulsions may be used as coatings and also as adhesives for different applications. Coating gives clear and flexible film which gives protection from direct sun light and it is resistant to yellowing colour on ageing effect. The primary factor affecting the properties of this material is the percentage of ethylene. Preparation of stable dispersion of ethylene/vinyl acetate copolymer polymerization in a carbon dioxide continuous phase with different stabilizers was investigated [4]. Gruber et al. [5] studied effects of different colloidal stabilizer on vinyl acetate-ethylene copolymer emulsion and films. Not much literature exists on the topic of the synthesis and the kinetic study for the preparation of vinyl acetate–ethylene copolymer dispersions. The process of free radical copolymerization of vinyl acetate and ethylene at high pressure is presented in the patent publications. The course of the copolymerization reactions is influenced by numerous parameters, so the preliminary experiments were done in which the parameters with the greatest impact was determined [6,7]. Scott et al. [8–12] described ethylene-vinyl acetate semi-batch emulsion copolymerization and the influence of
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redox initiator system, the effect of n-hexane as a co-solvent, effect of temperature, vinyl acetate feed rate, emulsifier type and pressure on the structure of the copolymer. There were some attempts to synthesize vinyl acetate-ethylene copolymer emulsion with a continuous process [13]. Guo et al. reported the copolymerization of ethylene and vinyl acetate via emulsion and miniemulsion polymerizations [14]. Thermal properties of vinyl acetate-ethylene copolymers were studied by differential scanning calorimetry and thermogravimetric analysis [15,16] and for structure characterization of copolymers nuclear magnetic resonance and infrared spectroscopy were used [16–18]. Our goal was to synthesize the copolymer in batch reactor with the highest portion of ethylene at lowest temperature as possible in order to decrease the percentage of branches [20] and to proceed the process at reduced pressure due to the safety reasons and the associated costs. In order to better understand the process during the course of copolymerization reaction and to accumulate reliable data we used an ATR-FTIR spectrometry technique (ReactIR 4000) with light conduit and diamond-composite sensor to perform in-line monitoring of emulsion copolymerization of vinyl acetate-ethylene in high pressure reactor. ATR-FTIR spectrometry technique was found to be ideal for determining residual free vinyl acetate, individual vinyl acetate and ethylene conversions and copolymer composition changes as a function of time when the copolymerization reaction was carried out. The kinetics data obtained through the ReactIR 4000 in-line reaction analysis system agreed well with those determined by the traditional GC method. ReactIR technology replaced time consuming (and inaccurate) off-line methodology. We examined the effect of agitation and mixing, temperature, pressure as well as initiator concentration on the copolymer composition and composition drift.
1799
Table 1 Reaction conditions of vinyl acetate-ethylene copolymers synthesis, agitation rates, dosing times, ethylene pressure, weight of initiator and sodium bicarbonate, and temperature of reaction synthesis.
2R 3R 4R 5R 7R 8R 10R 11R 12R 13R 14R 15R 16R 17R 18R 19R 20Ra 21Ra 22R 25R 26R 28R 30Ra 34R 35R 37R 38R 39Rb a b
T (◦ C) PEthylene (bar) mAPS (g) mNaHCO3 (g) tVAc dosing (min)
Agitation rates (rpm)
70 70 70 75 70 70 75 75 70 70 75 75 70 75 70 75 80 80 80 80 70 75 75 75 75 65 65 75
200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 250 300 200 250 200
20 20 20 20 30 30 30 30 30 30 30 30 20 20 20 20 20 30 20 30 30 30 30 30 30 30 30 /
0.5 0.5 0.5 0.5 0.5 0.75 0.75 1.0 0.75 1.0 0.75 0.5 1.0 1.0 0.75 0.75 0.5 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75
0.5 0.5 0.5 0.5 0.5 0.75 0.75 1.0 0.75 1.0 0.75 0.5 1.0 1.0 0.75 0.75 0.5 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75
90 180 250 180 180 180 180 180 240 240 240 180 180 180 180 180 180 180 180 180 240 180 180 180 180 180 180 180
Longer time of reaction after dosing at reaction temperature. Pure polyvinyl acetate dispersion (the reference batch).
2. Experimental 2.1. Materials Vinyl acetate (99.5%) was purchased from Lyondell Chemie Nederland. Ethylene (99%) was supplied by Istrabenz, Koper. Poly(vinyl alcohol) solution was prepared from MOWIOL 26–88 (3.5 wt.%, (4%) = 26 mPas, Mw = 160,000, degree of hydrolysis 88 mol%) and MOWIOL 4-88 (7 wt.%, (4%) = 4 mPas, Mw = 31,000, degree of hydrolysis 88 mol%) with deionised water (90 wt.%). Polyether siloxane copolymer (Tego Foamex 1488) was purchased from Evonik Tego Chemie GmbH. The initiator ammonium persulphate (APS, ≥98.0%), dimethylsulfoxide (DMSO-d6 + 0.03% TMS, 99.80%D) and sodium bicarbonate (NaHCO3 , ≥99.5%) were supplied by Sigma Aldrich, Steinheim, Germany. All chemicals were used as obtained without further purification.
2.2. Synthesis of co-ethylene-vinyl acetate dispersions The reaction configuration is important variable which influences the rate of polymerization, copolymer composition, molecular weight and limiting end conversion. Polymerisations were carried out on RC1-reaction calorimeter equipment (Mettler Toledo) in 1.8 L HP60 high pressure stainless steel semibatch reaction vessel, a stainless steel-anchor stirrer, a digital thermometer, a calorimeter and FTIR K6 conduit 16 mm Dicomp probe. Ethylene consumption during the polymerization process was measured by gas uptake unit and the pressure in the vessel was controlled by external automatic gas valve. Prominent Micro delta Optodrive pump was used for dosing of vinyl acetate. The temperature, dosing and pressure during the processes were well controlled by Mettler Toledo iControle software.
A typical procedure was as follows: 0.5–1.0 g of APS, NaHCO3 and 0.5 g of 24% emulsion of a polyether siloxane copolymer were diluted in 200 g of deionised water at room temperature and 200 g of 10% water solution of PVOH were charged into a reaction vessel. While stirring 25 g VAc was added at room temperature. Vessel was closed and flushed with ethylene thoroughly to remove air completely and to saturate emulsion with ethylene. Pressure was increased to 5 bar less than the end pressure of the experiment, which were 20 or 30 bar. Temperature of the reactor was increased to 65–80 ◦ C in 20 min. Pressure was kept constant at 20 or 30 bar during the whole process at selected temperatures. Right after the temperature and pressure was reached VAc was added dropwise into the reactor. 225 g of VAc was charged at a constant flow rate during 90–240 min. Concentration of VAc was measured with online FTIR to pursue the conversion of VAc. After the dosing of VAc finished the reaction mixture was kept at chosen temperature for 140 min before cooling. Overpressure of ethylene was released after cooling to room temperature. The experiments were monitored by the inline FTIR–ATR spectroscopy, using the Mettler Toledo ReactIR- iC IR 4.2 instrument coupled with DiComp diamond probe. IR spectra were collected every 3 min, in the wave number range between 4000 and 650 cm−1 at resolution 8 cm−1 . The first spectra were collected before the reaction mixture was heated to the chosen temperature. At the end of the synthesis the traditional GC method for determining free VAc was carried out. The solid content of all synthesized copolymer dispersions was about 45%. Several semibatch copolymerizations of vinyl acetate and ethylene were conducted at 70, 75, 80, and 85 ◦ C. All the experiments’ initiator dosing, dosing times and ethylene pressure are gathered in Table 1.
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2.3. Characterization methods 2.3.1. 1 H nuclear magnetic resonance The 1 H NMR spectra were measured using Unity Inova 300 Varian NMR spectrometer operating at 300 MHz. Samples for NMR measurements were first diluted (1:3) with deionised water, dispersed in a round bottom flask and frozen by liquid nitrogen. Frozen samples were then transferred into a freeze dry system (VirTis SP Scientific sentry 2.0) overnight, to remove water. The pressure within the freeze dryer was set at 0.026 mbar and the temperature of the condenser was set at −105 ◦ C. DMSO-d6 was used as a solvent and 1 H NMR signals were referred to tetramethylsilane (TMS) as an internal standard. During the sample preparation for 1 H NMR measurements no gel formation was observed, VAE samples were completely dissolved in the DMSO-d6. All the spectra were obtained at 25 ◦ C. The conditions for the 1 H NMR was as follows: a 90◦ pulse angle, a 5-s delay between the pulses, an acquisition time of 5 s, and up to 32 repetitions. VNMRJ rev. 1.1D software was applied for the peak integration. Assignation of individual signals was made on the basis of the available literature [16,17,19] on co-ethylene-vinyl acetate dispersions. The vinyl acetate-to-ethylene monomer ratio was calculated according to the procedure described by Guo at al. [14] and Gupta et al. [16]. 2.3.2. Dynamic scanning calorimetry For the determination of glass transition temperature Tg of the copolymers the differential scanning calorimetry was used. Approximately 8 mg of the sample was placed in an aluminium pan, sealed and analyzed. The measurements were performed on a METTLER TOLEDO 821e instrument with intracooler, using the STAR software. In and Zn standards were used for the temperature calibration and for the determination of the instrument time constant. For the heat flow calibration, the In standard was used. The experiments were performed under nitrogen atmosphere (60 mL min−1 ) in two scans in the temperature range of −80 to 150 ◦ C and at the heating rate of 10 ◦ C min−1 . An empty pan served as a reference. 3. Results and discussion 3.1. Semibatch VAE emulsion copolymerization ATR-FTIR spectrometry was utilized to monitor the reaction of vinyl acetate-ethylene copolymer dispersions at 65, 70, 75, and 80 ◦ C. In-line data acquisition was performed by immersing the transmission probe directly into the pressure reactor. Assignment of the characteristic peaks of vinyl acetate and ethylene was done using the previously collected spectra of both reactants in aqueous solution. The spectra of vinyl acetate and ethylene solutions were collected at the beginning of the measurements (not presented). The assignment of characteristic peaks was done on the basis of relevant literature and of our previous experimental work [20,21]. These signals were helpful in identifying the components of the copolymerization reaction of vinyl acetate and ethylene. We have to emphasize that the literature assigned absorption bands to the various chemical groups did vary, as might be expected [20]. The peak at 1138 cm−1 which corresponded to the vinyl acetate O C C stretch vibration (observable in Fig. 3) diminished with the reaction time, because of the copolymer formation. The normalized vinyl acetate-ethylene copolymer spectra at the wavelengths in the range of interest for the copolymer synthesis are shown in Fig. 1. The difference in spectra of homo polyvinyl acetate and copolymer polyvinyl acetate ethylene is minimal; we can observe the peak at 2800 cm−1 which correspond to the C H stretch vibration of ethylene. As the reaction proceeds this peak increased with reaction time. Another difference in spectra of homo and copolymer can be
Fig. 1. The IR spectra in the entire wave number range of interest for the coethylene-vinyl acetate dispersions synthesis at 70 ◦ C.
observed at 760 cm−1 but in our case this peak is overlapped with peaks of water. In Fig. 2 the concentration profiles for vinyl acetate, ethylene and reaction product during copolymer synthesis at 70 ◦ C are presented. The concentration of ethylene during the copolymerization reaction decreases, while the concentration of vinyl acetate increases due to the addition of it. When the addition of vinyl acetate is completed, the concentration of vinyl acetate dropped rapidly as a result of the polymerization reaction. We also observed that longer dosing times of vinyl acetate led to an increase of ethylene weight percent in the copolymers. In Fig. 3A the effect of reaction temperature versus ethylene incorporation in the copolymer is presented for VAE37, VAE8, VAE28 and VAE21 reactions which proceed at reaction temperature of 65, 70, 75 and 80 ◦ C, respectively. At the highest temperature the lowest content of incorporated ethylene in copolymer structure is observed due to the lower solubility of ethylene monomer in this copolymerization system. At 65 ◦ C, the proportion of incorporated ethylene in the copolymer was 12.92% [15,16]. We believe that lower ethylene percent in copolymer structure at 65 ◦ C is attributed to the slower polymerization reaction [22]. In our system, the temperature of 65 ◦ C was too low for the desired rate of polymerization and for decomposition of initiator.
3.2. Effect of agitation The effect of agitation was investigated through VAE28, VAE34 and VAE35 reactions with agitation rates of 200, 250 and 300 rpm, respectively (Fig. 3B). Higher agitation rates, for instance 300 rpm, increases the mixing turbulence of the liquid phase and thus increases the mass transfer, but also increases the solubility of ethylene due to better contact between gas and liquid phases. Here we must emphasize that the higher mixing speed does not increase the solubility of ethylene, but reduces the mass transfer limitations, which is reflected as an increase in solubility [23]. Thus, the presence of ethylene monomer in the reaction sites is increased with the higher agitation rate. Low agitation rate, for instance 200 rpm, caused a decrease in ethylene solubilization and its participation in the reaction sites due to lower solubility or due to poorer contact between the latex and the gas phase. Since vinyl acetate and ethylene are in competition in participating in polymerization reaction, there is variation in the ethylene content of polymers and the higher ethylene percentage in the samples is attributed to the lower vinyl acetate conversion. We have to consider that mass transfer limitations increases due to the gradual increase in the viscosity of the
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1801
Fig. 2. The concentration profiles in weight percent for vinyl acetate, ethylene, and reaction product during copolymer synthesis at 70 ◦ C.
B 16,0
15,5
A
Ethylene incorporated
Ethylene incorporated
15,0 15,5
Ethylene incorporated (%)
Ethylene incorporated (%)
14,5 14,0 13,5 13,0 12,5 12,0
15,0
14,5
14,0
13,5
11,5 11,0
13,0 64
66
68
70
72
74
76
78
80
82
200
220
C
260
280
300
28
30
Agitation rate (min )
14
Ethylene incorporated
D
15
Ethylene incorporated at 70 °C Ethylene incorporated at 75 °C Ethylene incorporated at 80 °C
Ethylene incorporated (%)
14
Ethylene incorporated (%)
240
-1
Temperature (°C)
12
10
13
12
11
10
9
8
8 0,4
0,6
0,8
Concentration of initiator (%)
1,0
20
22
24
26
P (bar)
Fig. 3. (A) Effect of reaction temperature on ethylene percent in copolymers; (synthesis at 30 bar; monomer addition rate 1.25 g/min; 0.75% of APS; 200 rpm); (B) effect of agitation rate on ethylene percent in copolymers, (synthesis at 75 ◦ C, 30 bar; monomer addition rate 1.25 g/min; 0.75% of APS); (C) the influence of initiator amount on the co-ethylene-vinyl acetate composition (synthesis at 70 ◦ C, 20 bar; monomer addition rate 1.25 g/min; 200 rpm) and (D) Effect of pressure on ethylene percent in copolymers at different temperatures (synthesis; 0.75% of APS; 200 rpm).
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latex with the progression of the polymerization [24]. The higher mass transfer barrier can be overcome in the higher agitation rates. When the data of ethylene content and vinyl acetate conversion are considered simultaneously, it can be concluded that these experimental values increase at higher agitation rate at the constant temperature and initiator concentration. At lower agitation rates mass transfer of vinyl acetate and ethylene to the reaction sites is limited. The overall conversion of vinyl acetate and ethylene is satisfactory at agitation rates 200 rpm. Higher agitation rate is a high energy consuming process and a lot of foaming occurred during the polymerization. In order to optimize the procedure, copolymerization reaction were carried out at different temperatures, pressures and at different initiator concentrations. 3.3. Effect of initiator concentration To study the influence of initiator concentration a set of three reactions with initiator concentrations of 0.5, 0.75, and 1 wt.% of ammonium persulphate was investigated designated as VAE3, VAE18, and VAE16, respectively. At constant agitation rate of 200 rpm, we used three levels of initiator concentration and found that successive increase of the initiator concentration increased the conversion simultaneously. Fig. 3C shows the influence of initiator concentration on the proportion of incorporated ethylene. It can be seen that ethylene weight percent in the copolymers of VAE16, prepared with 1 wt.% of ammonium persulphate is the highest in comparison with those obtained in the other two reactions. When the initiator concentration was increased from 0.5 to 1.0 wt.%, ethylene weight percent in the copolymer showed a very high increment. In VAE16 reaction (Fig. 3C) with 1.0 wt.% of initiator concentration, higher nucleation and thus, higher number of particles were formed in the initial stage of the reaction. Therefore, the extent of ethylene mass transfer and its presence in the reaction sites have increased so that ethylene weight percent in VAE16 (1.0 wt.%) copolymers showed the highest value compared to VAE18 (0.75 wt.%) and VAE3 (0.5 wt.%). We assume that at the highest initiator concentration the greater numbers of particles were formed in the initial stage of the reaction, thus the higher polymerization rate of ethylene and vinyl acetate proceeds. Higher polymerization rate of vinyl acetate inhibits ethylene polymerization and reduces its conversion and vice versa. Scot et al. have noticed a similar trend in the installation of an ethylene copolymer as a function of initiator concentration in 1.8 L reactor [12,13]. 3.4. Effect of pressure One of the most important effects of pressure is the increasing ethylene solubility at the reaction sites. By increasing pressure from 20 to 30 bar ethylene weight percent in copolymer shows significant increase. Considerable increase in ethylene content of the copolymers is observed by increasing pressure from 20 and 30 bar at all introduced temperatures (Fig. 3D). Copolymer, which was synthesized at 20 bar and 75 ◦ C, has the proportion of incorporated ethylene 9.59 mol %, while the copolymer synthesized at 30 bar and 75 ◦ C, has the proportion of incorporated ethylene 14.4 mol%. This is due to increase in ethylene solubility. When the pressure increases, ethylene solubilization and thus, its presence in the reaction sites increase as well [25]. Therefore, ethylene comes to react with vinyl acetate and reduces its homopolymerization. Consequently vinyl acetate conversion decreases. Similar finding was reported by Scot et al. [9,11,12] which performed reactions in 1.8 L reactor. These results are in good agreement with those where the influence of the initiator and stirring on the conversion of ethylene and VAc were studied. We noticed the reciprocal influence of VAc conversion and ethylene.
50 Tg as a function of the ethylene content using the Flory-Fox equation Tg as a function of the ethylene content from actual consumption
40
30
Tg (°C)
1802
20
10
0
-10 0
5
10
15
20
25
Ethylene incorporated (%) Fig. 4. Tg values as a function of ethylene incorporation for the vinyl acetateethylene copolymer of different compositions synthesized as indicated in the Table 1.
3.5. Glass transition temperature In order to investigate the effect of ethylene on copolymer properties, glass transition temperatures, Tg of the copolymers were determined. Since the aim of this copolymerization is to reduce the Tg of poly(vinyl acetate) through internal plastification of vinyl acetate homopolymer, this measurement proved to be very useful. Nevertheless, Tg is affected by polymer molecular weight, chain structure, branching, crystallinity, polarity, etc. In our study some of these parameters control the Tg of the synthesized copolymers. As a matter of fact all copolymers show significant decrease in Tg regardless of different conversion, temperature, and initiator concentration (Fig. 4). This is due to the incorporation of ethylene in the samples. Note that the Tg of poly(vinyl acetate) is about 43 ◦ C [20]. However, the differences in Tg arise from different molecular weights and structure, for instance, branching in the copolymers. Fig. 4 shows the Tg values of the synthesized vinyl acetate-ethylene copolymer of different compositions as a function of the ethylene content. Ethylene content was determined regarding the consumption during the polymerization process by gas uptake unit. Fig. 4 presents also the results, which were calculated using Flory–Fox predictions equation. The ethylene content of the synthesized VAE copolymer determined by gas uptake unit follow the predictions of the Flory–Fox equation (Fig. 4, Eqs. (1)–(3)). The ethylene content in VAE copolymer was calculated using the following equations (Flory–Fox equation): 1 ω(ethylene) ω(VAc) = + Tg Tg (ethylene) Tg (VAc)
(1)
1 − ω(ethylene) ω(ethylene) 1 + = Tg Tg (ethylene) Tg (VAc)
(2)
ω(ethylene) =
Tg (ethylene)[Tg (VAc) − Tg ] Tg [Tg (VAc) − Tg (ethylene)]
=
(Tg (VAc)/Tg ) − 1 (Tg (VAc)/Tg (ethylene)) − 1
(3)
Higher is the content of incorporated ethylene lower is Tg of vinyl acetate-ethylene copolymer. In Fig. 5 1 H NMR spectra for the vinyl acetate-ethylene copolymer of different compositions are presented. In Fig. 5, the peak at 4.78 ppm corresponds to the CH proton in VAc segment, the broad peak from 1.59 to 1.86 ppm represents
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Fig. 5.
1
H NMR spectra for the vinyl acetate-ethylene copolymer of different compositions as indicated in the legend.
the CH2 in VAc segment, and the signal of the CH3 protons of VAc appears at 1.95 ppm. Additional ethylene protons signals at 1.21 and 1.45 ppm were observed. The peak at 0.82 ppm corresponds to the CH3 group derived from H-abstraction by ethylene CH2 radical and represents methyl group from polyethylene branches [14]. Ethylene proton signals at 1.45 ppm overlap with the signals of methylene and methyl protons of polyvinyl acetate. In order to compare the data with the ethylene content obtained by prediction Flory–Fox equation and results determined by consumption of ethylene during the synthesis the weight fraction of ethylene was calculated from 1 H NMR spectra. From NMR spectra the molar ratio of ethylene to vinyl acetate was calculated using the Eq. (4) [14,16] and transformed to the weight fraction following the Eqs. (5) and (6): I0.74−2.1 ppm − 5I4.78 ppm n (ethylene) = 4I4.78 ppm n (VAc)
(4)
where I represents the integral of a peak in the 1 H NMR spectra; X(ethylene) =
n (ethylene) n (ethylene) + n (VAc)
(5)
where X represents the molar fraction; ω(ethylene) =
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X(ethylene) × Methylene X(ethylene) × Methylene + X(VAc) × MVAc
(6)
where ω represents the weight fraction and Methylene and MVAc represent molar mass of ethylene and vinyl acetate unit, respectively. In Table 2 the comparison of ethylene content in vinyl acetateethylene copolymers determined from the actual consumption of ethylene during the polymerization, by prediction using the Flory–Fox equation (from DSC) and by 1 H NMR measurements are presented. Results obtained from the actual consumption of ethylene during the polymerization agreed well the predictions of the Flory–Fox equation. Results obtained from 1 H NMR spectra also
Table 2 Ethylene content in Vinyl acetate-ethylene copolymers determined by DSC, 1 H NMR measurements and from the actual consumption of ethylene. Sample
3R 12R VEA reference
Ethylene weight ratio in VAE copolymer Prediction Flory–Fox equation (from DSC)
1
0.085 0.149 0.223
0.083 0.163 0.231
H NMR
Actual consumption 0.095 0.149 Ref.
agreed well with the results obtained from the actual consumption as well as the predictions of the Flory–Fox equation. We believe that minor deviations in the results can be attributed to the various methods of measurements and due to the overlapping of proton signals of ethylene with the signals of polyvinyl acetate. 4. Conclusions VAEs are a better cost-performing polymer offering improved adhesive and binding properties with more user friendly compounding characteristics. The use of vinyl acetate ethylene (VAE) emulsion compositions in adhesives has grown rapidly over the last two decades with the development of a broad spectrum of end users. Generally, VAE copolymer emulsions are used without additives or modifiers. The primary factor affecting the properties of these adhesives is the percentage of ethylene. During the synthesis of VAE lattice the appropriate combination of different parameters (pressure, temperature, initiator and agitation rate) let to the VAE copolymer with desired percent of ethylene incorporated in the structure. At higher reaction temperature, ethylene solubility decreased which caused a decrease of ethylene weight percent in the copolymer. Longer dosing times of vinyl acetate led to an increase of
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ethylene weight percent in the copolymers. By increasing the agitation rate, ethylene solubilization and its presence in the reaction sites (increase in mass transfer) increased which caused growth of ethylene weight percent in the copolymers. Decreasing agitation rate was led to the decrease in mass transfer and presence of ethylene and vinyl acetate monomers in the reaction sites which in turn lowered vinyl acetate conversion. Furthermore, a decreasing trend in the ethylene percent was observed. At constant agitation rate of 200 rpm, we used three levels of initiator concentration and found that successive increase of the initiator concentration increase the conversion simultaneously. The optimum initiator concentration was 1.0 wt.% of ammonium persulphate on the total amount of vinyl acetate. Considerable increase in ethylene content of the copolymers is observed by increasing pressure from 20 bar to 30 bar. Higher pressure has a direct effect on the copolymerization system, because of the superior solubility of ethylene, leading to the formation of a copolymer with higher amount of ethylene in the final structure of the copolymer. At the same time the vinyl acetate conversion decreased. Copolymer of the highest ethylene content 15.59 wt.% (35R) have been synthesized at an ethylene pressure of 30 bar and at the temperature of 75 ◦ C with dosing time of 180 min, agitation rate of 300 rpm and initiator concentration of 0.75%. Acknowledgements The authors wish to gratefully acknowledge Mitol, Seˇzana for their generous financial support to the presented work and for providing all chemicals used in this study and for permission to publish this paper. This work was supported by the Ministry of Education, Science, Culture and Sport of the Republic of Slovenia Grant number 3211-10-000057 (Centre of Excellence for Polymer Materials and Technologies), whose contribution is fully recognized. References [1] K. Geddes, Polyvinyl and ethylene-vinyl acetates, in: A. Pizzi, K.L. Mittal (Eds.), Handbook of Adhesive Technology, Marcel Dekker, New York, 1994. [2] T.M. Goulding, Polyvinyl acetate wood adhesives, in: A. Pizzi (Ed.), Wood Adhesives: Chemistry and Technology, Marcel Dekker, New York, 1983. [3] P.A. Lovell, M.S. El-Aasser, Emulsion polymerization and emulsion polymers, John Wiley, New York, 1997, pp. 18, Chs 16. [4] D.A. Canelas, D.E. Betts, J.M. DeSimone, M.Z. Yates, K.P. Johnston, Poly(vinyl acetate) and poly(vinyl acetate-co-ethylene) latexes via dispersion polymerizations in carbon dioxide, Macromolecules 31 (1998) 6794–6805. [5] B.A. Gruber, M.S. Vratsanos, C.D. Smith, Effect of colloidal stabiliser on vinyl acetate-ethylene copolymer emulsion and films, Macromol. Symp. 155 (2000) 163–169.
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