A novel polymerization method based on pressure drop for monomodal high solid content low viscosity latexes of poly(ethylene-co-vinyl acetate)

Abstract

A novel polymerization route to synthesize monomodal high solid content latexes of poly(ethylene-co-vinyl acetate) with low viscosity based on pressure drop was designed. This route consists of a semi-continuous polymerization step (step I) and a pressure drop step (step II). An investigation of the effect of pressure drop on the polymerization showed that pressure drop led to a secondary reaction. Secondary nucleation of a large number of particles followed by limited flocculation was caused by the pressure drop, which further led to an optimization of the particle size distribution (PSD) of latex. The effect of pressure drop on formation and growth of new particles was investigated and a possible mechanism was inferred. Latex with solid content of 70.1 wt% and a viscosity of 640 mPa s at a constant shear rate of 20 s⁻¹ was obtained through this method.

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Introduction

High solid content (HSC) latexes with low viscosity are widely used in adhesives, coatings, paints, cigarettes, and other applications in industrial fields and in daily life.^1–3 HSC latexes exhibit many advantages, including rapid drying times, high space/time yield of reactor, great transport efficiency, low production and storage costs.⁴ Thus, these latexes attracted considerable research attention.

In conventional latexes, the macroscopic viscosity always remains at an acceptably low value when the solid volume content is below 60%. However, once solid content is beyond the critical point the macroscopic viscosity will increase rapidly with the increase in solid content. With the development of latex research, many papers^5–8 and patents^9,10 that focus on HSC latexes have been published in the past decades. HSC latexes can be prepared mainly through conventional emulsion polymerization, self-emulsion polymerization, miniemulsion polymerization, concentrated emulsion polymerization and agglomerating method.^5,11 The only purpose of all the abovementioned methods is to optimize the particle size distribution (PSD) because a polymer latex with optimized PSD including bimodal PSD, trimodal PSD, and monomodal PSD with large particle size and broad distribution is generally accepted to present lower apparent viscosity. Mariz et al.¹² investigated a process based on an iterative strategy to determine the optional PSD and obtained a bimodal latex with solid content of 70 wt% through seeded semi-continuous emulsion polymerization. A trimodal latex was obtained by Chu et al.¹³ who prepared the latex by mixing monomodal latexes with different diameters and obtained a latex with a solid content of more than 72 wt% and a viscosity of 1077 mPa s.

Compared with monomodal latex, latex with bimodal or trimodal distribution has attracted significant attention in recent years mainly because of its high theoretical maximum packing factor.⁵ However, the preparation of these types of latexes are time-consuming and usually require several steps, additional seeds, or excess surfactants during the reaction process, thereby making them disadvantageous in industries and economy.

In this manuscript, a novel route based on pressure drop was developed to prepare monomodal HSC latexes with low viscosity. The effect of pressure drop on the formation and growth of particles was investigated. The latex was polymerized with ethylene and vinyl acetate.

Experimental

Materials

Ethylene (99.95% purity) and vinyl acetate (VAc, 99.0% purity) supplied by Guangxi Guangwei Chemical Limited Liability Company were used as monomers. VAc was further distilled at 40 °C under reduced pressure to remove the hydroquinone inhibitor and subsequently stored at 5 °C in the dark. PVA-205 (polyvinyl alcohol with 86.5–89 mol% hydrolysis degree and a polymerization degree of approximately 500, Kuraray) and PEG-400 (polyethylene glycol with a molecular weight of approximately 400, Aladdin) were used as protective colloids. Sodium hexametaphosphate (SHMP, Aladdin) was used as dispersant. Tween 20 [polyoxyethylene (20) sorbitan monolaurate, Macklin], Tween 40 (polyoxyethylene sorbitan monopalmitate, Macklin), and sodium dodecyl sulfate (SDS, Macklin) were used as surfactants. Ascorbic acid (AsAc, Aladdin), hydrogen peroxide (H₂O₂, Aladdin), n-dodecyl mercaptan (DDM, Aladdin) were used as reductant, oxidant, chain transfer agent, respectively. Ferrous sulfate (FeSO₄, Aladdin) was used as catalyst of the redox reaction to improve the system. All the chemicals mentioned above, except VAc were used as received. Double-deionized (DDI) water was used throughout the work.

Apparatus

Polymerizations were carried out in a 2 L stainless steel autoclave equipped with a stainless anchor agitator at the bottom, a cooling coil, a heat exchanger, a nitrogen/ethylene inlet, a sample device, three feed inlet tubes, and three constant flow pumps used for continuous feeds of oxidant, reductant, and monomer, respectively. The apparatus employed in the experiment are shown schematically in Fig. 1.

Fig. 1 Experimental apparatus for the emulsion copolymerization of ethylene and VAc.

Preparation of aqueous phase

The preparation of aqueous phase comprises two steps, and the recipe is presented in Table 1. In the first step, an aqueous solution was obtained. The aqueous solution was prepared by dissolving the nonionic surfactant (Tween 20 and Tween 40), the anionic surfactant (SDS), the dispersant (SHMP), the chain transfer agent (DDM), and the protective colloids (PVA-205 and PEG-400) in the DDI water and stirred with a magnetic stirrer at 30 °C. In the second step, the aqueous phase was obtained by adding the FeSO₄ aqueous solution and the reductant (AsAc) to the aqueous solution obtained in the first step and dissolved under nitrogen protection.

Table 1 Recipe for the preparation of the aqueous phase

Ingredients	Quantity (g)
a 0.1 wt% aqueous solution.
PVA-205	10.0
PEG-400	10.0
SHMP	6.25
DDM	9.40
Tween 20	4.00
Tween 40	1.25
SDS	1.00
AsAc	1.31
FeSO4^a	5.00
DDI water	185.0

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Latex polymerization

Step I. The initial VAc monomer and the obtained aqueous phase were introduced into the reactor at room temperature. The initial charge was purged once with nitrogen and then purged twice with ethylene under 1.0 MPa to remove dissolved oxygen. Pressurized the autoclave to the reaction pressure of 3.0 MPa, the stirring speed was set to 500 rpm, and the heater voltage of the heating jacket was set at 80 V. Immediately the reactor temperature was brought to 55 °C, fed the oxidant solution into the reactor at a given rate. Subsequently, the addition of neat VAc monomer was start at a given rate after 5 min of the oxidant solution feed. The reaction temperature began to increase after the addition of the oxidant solution, and the reaction temperature was limited below 85 °C with the heat exchanger. After a given feed duration, the additions of oxidant solution and the VAc neat monomer were stopped. The reaction system temperature was maintained at the final value for 20 min and then cooled down to 60 °C. Table 2 shows the formulation of the polymerizations used in step I. In this step, runs R1–R3 were carried out under the same conditions, runs R2, R4 and R5 were carried out with different feeding durations, and run R6 was carried out at a higher concentration of redox couple.

Table 2 Formulation for semi-continuous polymerization in step I

	R1	R2	R3	R4	R5	R6
Initial charge
Aqueous phase (g)	233.21	233.21	233.21	233.21	233.21	233.21
VAc (g)	370	370	370	370	370	370
Ethylene (MPa)	3.0	3.0	3.0	3.0	3.0	3.0
Feeds
Feed duration (min)	120	120	120	240	360	120
Stream 1
VAc (g)	250	250	250	250	250	250
Stream 2
H2O2 (g)	0.51	0.51	0.51	0.51	0.51	1.53
DDI water (g)	110	110	110	110	110	110

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Step II. After the reaction mixture was cooled down to 60 °C, the pressure in the autoclave was reduced to atmospheric pressure within 15 min. The oxidant solution (when used) and the reductant solution (when used) were fed into the reactor at 1.0 mL min⁻¹ at the same time. The addition of both oxidant solution and reductant solution lasted for 15 min. Step II lasted for 30 min. The formulation for the polymerizations in step II is shown in Table 3.

Table 3 Formulation for semi-continuous polymerization in step II

	R1	R2	R3	R4	R5	R6
a The code used: (+) reduce the pressure to atmospheric pressure, (−) remain constant.
Pressure drop^a	+	+	−	+	+	−
Stream 3
H2O2 (g)	0.03	0	0.03	0.03	0.03	0.09
DDI water (g)	15	15	15	15	15	15
Stream 4
AsAc (g)	0.09	0	0.09	0.09	0.09	0.27
DDI water (g)	15	15	15	15	15	15

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Characterization

Solid content and conversion were determined gravimetrically. During polymerization, relatively small samples were withdrawn from the reactor at various intervals, and the polymerization was stopped by adding 0.4 wt% aqueous hydroquinone solution.¹⁴ It is worth pointing out the conversion refers to VAc that was used as an indicator for the rate of polymerization because the ethylene conversion is hard to determine experimentally due to the vaporization of the unconverted ethylene monomer upon sampled.¹⁵ The conversion of VAc monomer was calculated through the following expression.
where X(t) is the vinyl acetate conversion at time t, W_R(t) is the total weight of the reactant in the reactor at time t, W(t)_VAc is the weight percent of vinyl acetate in copolymer at time t, S(t) is the solid content at time t, C is the solid fraction that do not contributed by polymer, W_VAc(t) is the total weight of vinyl acetate has been introduced into reactor by the time t. The ethylene percent in copolymers was determined by FTIR and ¹H NMR spectroscopy technique.

The z-average diameter (D_z) of latex particles were measured with dynamic light scattering using a Zetasizer Nano ZS device (Malvern Instruments, UK). The latex samples withdrawn from the reactor were diluted in the DDI water followed by ultrasonic dispersion. Each sample was tested five times. The particle number (N_p) was obtained by the following expression:^16,17

where M₀ is the monomer/water ratio (grams per liter of water), X is the total conversion, ρ_p (approximately 1.055 g cm⁻³) is the density of the copolymer and D_v is the volume-average diameter. It is worth to point out that the D_z determined by the dynamic light scattering device was used instead of D_v to calculate the N_p. Nevertheless, the values of N_p show valuable trends.

Transmission electron microscope (TEM) was used to characterize the PSD. Latex samples were diluted and stained with 0.5 wt% aqueous solution of phosphotungstic acid and analyzed with a Tecnai™ G² F30 S-TWIN device (FEI Electron Microscopes, USA). Viscosity (η) was measured with a rotary viscosimeter (ShangYi, China) at a constant temperature of 25 °C and a constant shear rate of 20 s⁻¹.

Results and discussion

The entire polymerization process of HSC latex is shown in Fig. 2. Non-isothermal polymerizations were performed in this work to decrease energy consumption. The approach consists of two steps. In step I, an initiator system (H₂O₂/AsAc, AsAc was used as initial charge) was employed, and latex with a broad PSD was obtained through the semi-continuous polymerization process. In step II, the reactor pressure was reduced at the beginning of the reaction, and lower concentration aqueous solutions of H₂O₂ and AsAc were employed.

Fig. 2 Scheme for preparing HSC latex through the method based on pressure drop.

Fig. 3 presents the evolution of the VAc monomer conversion for polymerization runs R1–R3 in step I. As shown, good reproducibility of the experiment was achieved in step I. The polymerization rate was slow at the beginning of the reaction probably due to the small amount of initiator that was used and the low rate of radical generation due to low temperature (55 °C). Later, the polymerization rate began to accelerate with the increase in reaction temperature. The polymerization rate began to decrease gradually at approximate 90 min. Conversion of bout 0.83 was encountered at the end of the semi-continuous feeding period.

Fig. 3 Conversion versus time profiles for semi-continuous polymerization runs R1, R2, and R3 in step I (run R1, R2, and R3 were performed under the same condition).

Fig. 4 shows the evolution of the VAc monomer conversion for polymerization runs R1–R3 in step II. As shown in the figure, the polymerization rate increases in the following order: R2 > R1 > R3. In the case of R3, the simple addition of redox couple could not increase the overall conversion. In the case of R1, the pressure drop at the beginning of step II led to a significant increase in polymerization rate and a higher overall conversion.

Fig. 4 Conversion versus time profiles for polymerization runs R1, R2, and R3 in step II (run R1, feeding DDI water during pressure drop, run R2, feeding redox couple during pressure drop, and run R3, feeding redox couple without pressure drop).

In the case of R2, rapid increase in polymerization rate was observed immediately after the pressure dropped and the addition of redox couple, furthermore, almost complete conversion was obtained. Fig. 4 suggests that pressure drop can cause a secondary reaction and improve the polymerization rate.

Fig. 5 illustrates the conversion evolution for the semi-continuous polymerization with different feeding durations in step I. The polymerization rate decreased with an increase in feeding duration. In other words, the polymerization rate was increased with the increase in oxidant aqueous solution feeding rate (R2: 0.92 mL min⁻¹, R4: 0.64 mL min⁻¹, R5: 0.3 mL min⁻¹) because the total amount of oxidant aqueous solution remained constant. This phenomenon can be explained based on the rate of radical generation. All the reaction curves did not reach complete conversions in the figure, and the low and limited conversion obtained in the case of three processes could not be attributed to the low concentration of initiator or the diminution of radical generation along the polymerization. To check those hypotheses, reaction R6 was carry out using thrice the initiator concentration (run R2), and the step II was performed at 70 °C. The result was shown in Fig. 6. It can be seen that a faster polymerization rate was observed but complete conversion was not achieved in step I, moreover, the continuous feeding of initiator at a higher reaction temperature in step II didn't improve the polymerization rate and conversion. The limited conversion presumably due to the following reasons. Firstly, the oxygen centered hydroxyl radicals given by H₂O₂ are hydrophilic and, hence, they must propagating with the monomer present in the aqueous phase to a critical degree that hydrophobic enough instead of entering into the polymer particles directly, moreover, once the monomer droplet disappeared (interval III, according to the classical Smith–Ewart theory) the consumption of residual monomer present in the aqueous phase gradually will slow the propagation rate;^18,19 secondly, the viscous surfactant layer formed by the nonionic surfactant used in the reaction can reduce the oligomer radical entry rate;^20,21 thirdly, the large particles (large than 0.2 μm) formed in the latex caused the limited conversion according to the shell-growth mechanism.^22–24 After the disappearance of monomer droplets, the monomer consumed by the reaction in the shell would be supplied by the diffusion of internal monomers. However, the decreased internal monomer concentration, the increased internal viscosity of particle as well as the large size of particle resisted the diffusion of monomer to the reaction loci. Consequently, the further polymerization of monomer was inhibited.

Fig. 5 Effect of the feeding duration on the conversion versus time for the semi-continuous polymerization in step I (R2: 120 min, R4: 240 min, R5: 360 min).

Fig. 6 Conversion evolution for the entire polymerization process for run R6.

Fig. 7 presents the evolution of particle number as a function of conversion for different feeding durations in step I. As shown, a shorter feeding duration (or a higher oxidant aqueous solution feed rate) can create more particles, which gives a further explanation for the polymerization rate decreased with an increased in feed duration. The number of particles decreased during the conversion of 0.1 to 0.3 and then increased slightly after 0.4 because of homogeneous nucleation along the semi-continuous process due to the high water solubility of VAc monomer.

Fig. 7 Effect of the feeding duration on the evolution of the number of particles along the semi-continuous polymerization process in the step I.

Fig. 8 presents the kinetics of the polymerizations carried out in step II, and secondary reactions were clearly observed. Reactions in this step were started at 60 °C in case of a large quantity loss of VAc monomer and system instability caused by violent secondary reaction. It can be seen that the polymerization rates increased rapidly after the pressure dropped at the beginning of step II. Then, almost complete conversions were finally achieved. Fig. 9 shows that a large number of new particles were nucleated at the beginning of the process. The nucleation of new particles could provoke a redistribution of the surfactant molecules in the reactor, which caused a redistribution in the coverage of the particles and further led to a limited flocculation in the system, thereby reducing the increased surface area and further attaining a new equilibrium.⁷ In addition, the limited flocculation could improve the kinetics of polymerization compared with that not happen according to the literature²⁵ by Boutti et al.

Fig. 8 Effect of the pressure drop on the conversion versus time for polymerization in step II.

Fig. 9 Effect of the pressure drop on the evolution of the number of particles along the process in the step II.

As generally accepted, nucleation of a large number of new particles requires sufficient amount of monomers.^7,12,26,27 However, the total conversion of VAc monomer reached more than 0.8 at the end of step I, i.e., the monomer droplets completely disappeared and all the unreacted monomer existed in the polymer particles.²⁸ As a result, nucleation of a large number of new particles in the emulsion polymerization system became impossible since the polymerization reaction could take place only in monomer-swollen micelles. As clearly observed that the secondary nucleation of larges number of new particles was attributed to the pressure drop at the beginning of step II. We present a possible explanation for the phenomenon and changes caused by pressure drop, which deserves deeper investigation. The pressure drop of the reactor broke the pressure balance between the internal and external of polymer particles, thereby causing intense diffusion of unreacted VAc monomer and the dissolved ethylene monomer from the interior of the polymer particles due to the saturated vapor pressure and low solubility respectively, further, the diffused monomer supported the secondary nucleation of new particles. This process can be described by Fig. 10.

Fig. 10 Concept of the secondary nucleation caused by pressure drop.

The main characteristics of the final latexes at the end of steps I and II are shown in Table 4. As shown, the PSD of latex can be sensitive to the feed duration of semi-continuous polymerization process from the solid content and viscosity obtained in step I. In addition, it can be inferred that the feed duration of 4 h (R4) could lead to a better PSD. Compared with the result obtained in steps I and II, it is obviously observed that the solid content obtained at the end of each process of step I was improved significantly after the process of step II except in run R3. However, the viscosity decreased instead of rapidly increasing with the increase in solid content as predicted, thereby suggesting that the pressure drop could optimize the PSD of latex by causing a secondary nucleation of a large number of particles followed by limited flocculation. Additionally, the secondary nucleation could not led to a bimodal latex due to the broad PSD obtained by the semi-continuous polymerization in step I and the effect of the limited flocculation, instead, a broader monomodal PSD was obtained. Generally, the latexes obtained by this method exhibit the D_z of larger than 1 μm. It should be noted that the effect of pressure drop on the ethylene content in the copolymer is not investigated in this paper because this paper mainly focus on the investigation of the effect of pressure drop on the formation and growth of particles.

Table 4 Main characteristics of the final latexes from runs R2, R4, and R5

Run	Step I			Step II
	SC (wt%)	η (mPa s)	Dz (nm)	SC (wt%)	η (mPa s)	Dz (nm)
R1	55.9	4210	1022	62.3	2430	1145
R2	55.8	4160	1010	68.2	1120	1200
R3	55.9	4190	987	57.6	6900	1004
R4	61.0	1870	1120	70.1	640	1280
R5	58.3	2630	1069	68.6	880	1265

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The volume PSDs obtained at the end of steps I and II for run R4 are shown in Fig. 11. The PSDs were measured with dynamic light scattering using the Zetasizer Nano ZS device. It is clearly observed that a monomodal latex with broad PSD was obtained at the end of step I, furthermore, PSD was get broader after the step II.

Fig. 11 Volume PSDs of the latexes at the end of steps I and II for R4.

Fig. 12 shows the TEM micrographs of latex particles formed for run R4 in step II at three different instants of the polymerization process. Sample (a) was withdrawn before the beginning of step II, sample (b) was withdrawn at 10 min after the beginning of the step II, and samples (c) was withdrawn at the end of step II. Broad PSD is shown in micrograph (a). As shown in the micrograph (b), secondary nucleation was clearly observed. Micrograph (c) shows that broader PSD was obtained compared with the observation in micrograph (a). Micrograph (d) was taken from the same sample with (c). Interestingly, partial hollow can be seen clearly in micrograph (d), which is apparent in the bright field inside the particle. This interesting phenomenon was likely attributed to the shell-growth mechanism of large particles and the intense diffusion of unreacted monomer, which may supported the ideas that the secondary nucleation was due to the pressure drop. However, the phenomenon was not observed for all the particles and was most likely related to the amount of unreacted monomer in each particle. Moreover, it seems that the approach based on pressure drop provides a potential preparation method for hollow latex.

Fig. 12 TEM micrographs for three samples of latex for run R4 in step II at (a) 0 min, (b) 10 min, (c) and (d) 30 min.

Conclusions

A route that consists of two steps was designed to prepare HSC latex with low viscosity. The mechanism and effect of pressure drop on the formation and growth of particles in the copolymerization of ethylene and VAc were investigated.

A semi-continuous non-isothermal preparation method was introduced in step I, in which the first nucleation and growth of particles occurred. Limited conversion of the VAc monomer was obtained at the end of this step. The PSD is sensitive to the feed duration, and the feed duration of 4 h could lead to a better PSD. The pressure drop approach was introduced in step II. The secondary nucleation of a large number of new particles occurred immediately after the pressure drop. It may be speculated that the secondary nucleation caused by pressure drop was a result of the imbalance between the internal and external pressure of the latex particles and the intense diffusion of unreacted monomer, which deserves deeper investigation. The nucleated particles could cause a limited flocculation. Moreover, the limited flocculation improves the polymerization, which plays a important role in achieving complete conversion of the VAc monomer. The PSD of latex was got optimized through the approach of pressure drop, HSC latex with low viscosity was obtained. The evolution of polymer particles throughout the entire polymerization process can be described by Fig. 13.

Fig. 13 A schematic representation of the evolution of polymer particles throughout the entire polymerization process.

Hollow particles were observed due to shell-growth mechanism and the intense diffusion of the unreacted monomer inside the particles caused by the pressure drop, which support a idea that hollow latex could be prepared through this method.

Acknowledgements

This work is supported by the Scientific and Technological project of Guangxi Autonomous Region (14122005-34, 1598007-24), the Scientific Research innovative project of Guangxi Autonomous Region (2014CXJHA08).

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