The effect of maleic acid diesters type on the stability of vinyl acetate emulsion polymers

Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 233–241

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

The effect of maleic acid diesters type on the stability of vinyl acetate emulsion polymers Hale Berber Yamak a,∗ , Yasemin Tamer b , Hüseyin Yıldırım b a b

Department of Metallurgical and Materials Engineering, Yıldız Technical University, 34220 Istanbu, Turkey Department of Polymer Engineering, Yalova University, 77100 Yalova, Turkey

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Maleic acid diesters as comonomer contribute to the latex stability of vinyl acetate. • Type and amount of these diesters play an important role on the latex properties. • Dibutyl maleate allows obtaining more homogeneous copolymer latex particles. • It significantly reduces the average particle size above a certain amount.

a r t i c l e

i n f o

Article history: Received 14 January 2016 Received in revised form 5 March 2016 Accepted 7 March 2016 Available online 9 March 2016 Keywords: Semi-continuous emulsion polymerization Comonomer ratio Vinyl acetate Maleate monomers Latex stability

a b s t r a c t Vinyl acetate (VAc) was copolymerized with two different maleic acid diesters, dibutyl maleate (DBM) and dioctyl maleate (DOM) at different comonomer ratios by semi-continuous emulsion polymerization method. The effects of maleic acid diesters type and amount on the latex properties of VAc emulsion polymers were investigated. The particle size, viscosity, zeta potential, surface tension and conductivity of the latexes were determined by Dynamic Light Scattering (DLS), Brookfield viscometer, zeta potential analyzer, Du Nouy tensiometer and electrical conductometer, respectively. The latex freeze-thaw stability and ionic strength resistance were also analyzed. The experimental results have shown that the variety in comonomer type and amount created significant differences on the latex properties of VAc emulsion polymers. With the use of DBM or DOM in a certain amount, the particle size and particle size distribution of VAc emulsion polymers were decreased while their latex stability were increased. DBM was also more effective in improving the latex stability than DOM. Consequently, it was clearly observed that these two comonomers enhanced the overall latex properties of VAc homopolymer depending on their type and amount. © 2016 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author at: Yıldız Technical University, Department of Metallurgical and Materials Engineering, 34220 Istanbul, Turkey. E-mail addresses: [email protected], [email protected] (H. Berber Yamak). http://dx.doi.org/10.1016/j.colsurfa.2016.03.014 0927-7757/© 2016 Elsevier B.V. All rights reserved.

Emulsion polymerization is an unique process employed for radical chain polymerizations and it is widespread in the chemical industry in order to produce a great variety of homopolymers and copolymers [1]. The homopolymer of vinyl acetate (PVAc) is

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successfully produced by different emulsion polymerization techniques including batch, continuous and semi-continuous on large scales in industry. Therefore, it is used in a wide range of applications such as adhesives, coatings, paints, sealants, binders, wood processing, construction, textile, paper, inks, leather, biomedical and other fields [2–4]. As a result of being water-based, its availability at low cost, its high durability and the fact that it can be manufactured easily, these polymers eliminate the drawbacks of organic based emulsion polymers that are toxic, flammable and expensive. Furthermore, these emulsions provide advantages in many applications since they can maintain their fluidity even high solids ratio and they can form continuous films when dried [5,6]. On the other hand, PVAc has some disadvantages such as poor water resistance, poor hydrolytic stability, higher film forming temperature, low heat resistance and higher glass transition temperature (Tg ∼33 ◦ C) [7,8]. The properties of PVAc latexes can be changed in order to enhance their performance by using internal and external plasticizers [6,9]. The presence of the external plasticizer in the emulsion polymer causes the phase separation between the plasticizer and polymer matrix against organic solvents, and this case results in the loss of its effectiveness against aging or atmospheric conditions. Conversely, the internal plasticization of PVAc is a more satisfactory approach that can be achieved by copolymerizing of VAc with various functional monomers such as vinyl chloride [10], vinyl propionate [11], vinyl versatate [12], butyl acrylate [13], 2-ethyl hexyl acrylate [14], acrylonitrile [15], ethylene [16], various metahcrylates [17,18], maleates [19] and fumarates [20]. These comonomers allow the latex particles to be produced with different molecular and morphological properties depending on their types and composition. The maleic acid diesters such as dibutyl maleate (DBM) and dioctyl maleate (DOM) are preferred in plastification of PVAc [9]. In addition to their plastification effect, the maleic acid diesters provide better stability to the copolymer latexes by creating a strong interaction with latex particles, and also enhance the certain properties of VAc polymer such as water, alkali and solvent resistance, durability, hardness and flexibility depending on their concentration [6,9,21]. The emulsion copolymers of VAc-maleic acid diesters are considerably affected by the large differences in the monomer properties such as water solubilities (23–25 g L−1 for VAc, 0.35 g L−1 for DBM and no solubility for DOM at 20 ◦ C), reactivity ratios (for VAc/DBM system rVAc :0.171 and rDBM :0.046; for VAc/DOM system rVAc :0.195 and rDOM :0.945) and molecular weights (MWDBM : 228.3 g mol−1 and MWDOM : 340.5 g mol−1 ) [22,23]. DBM and DOM show a tendency to copolymerize with vinyl acetate instead of homopolymerization. This results in covalently incorporation of these diesters to the polymer chain which provides the hydrophilic hydrophobic balance control of the particles [9]. Due to this feature, some derivatives of maleic acid diesters have been used as a polymerizable surfactant in emulsion polymerizations [24]. The maleic acid diesters provide unique properties to the emulsion polymers. However, there are a few studies in the literature on the use of them in emulsion polymerization applications. The earliest studies about the VAc-maleic acid diester copolymers belong to Donescu et al. [25–27]. They studied the semi-continuous emulsion copolymerization of VAc with dibutyl maleate which was used both as a coemulsifier and a comonomer. They observed that DBM acted as a chain transfer agent in this type emulsion copolymerization of VAc/DBM [25]. In their following study, Donescu et al. copolymerized VAc with DBM using a semi-continuous process on different copolymer compositions. The retarding effect of DBM on the VAc polymerization was determined and they observed a simultaneous decrease in particle size with an increase in the amount of DBM monomer [26]. Also, they used 2-ethylhexyl maleate as a comonomer in the VAc semi-continuous emulsion polymerization by continuous addition of monomer mixture. In the presence

of DOM, the conversion and latex properties were found to be significantly affected depending on the changing polymerization conditions [27]. Schork and Wu synthesized the VAc/DOM latexes by both macroemulsion and miniemulsion copolymerizations and examined the effect of DOM on physical and kinetic properties such as the monomer droplet and particle size, copolymer composition, conversion and molecular weight. They also compared VAc/DOM system with the other monomer systems; VAc/butyl acrylate and VAc/N-methylol acrylamide. It was seen that water-insoluble DOM significantly affected the reaction rate, monomer transport and nucleation mechanism. Hence, a rise in DOM concentration in the system resulted in a decrease in the polymerization rate and molecular weight and an increase in coagulation and viscosity. A significant deviation of the composition of the VAc/DOM copolymer from the ideal case was also detected [22]. In our previous work, we studied the effect of DBM and DOM concentration on the film properties of the VAc emulsion polymers. The copolymer films having surfaces in various hydrophobic characters were obtained depending on the comonomer type and ratio. The plasticizing effects of these maleic acid diesters on the PVAc were confirmed by the minimum film forming temperature (MFFT) and glass transition temperature (Tg) results. Also the copolymer structure, molecular weights, water contact angle and water resistance of VAc based copolymer films were investigated in detail [21]. In the present study, unlike our previous work, we completely focused on the latex properties of VAc emulsion polymers and in particular the effect of maleic acid diester type and amount on the latex stability. For this purpose, the semi-continuous emulsion copolymerizations of VAc were carried out by using DBM and DOM as comonomers, and also nonionic emulsifiers and a reactive surfactant were used. Feeding was performed with two separate paths including pre-emulsion and initiator solution. The effect of type and ratio of maleic acid diester comonomers on the VAc latex properties such as particle size, viscosity, zeta potential, surface tension, conductivity, freeze-thaw stability and ionic strength resistance were examined as all the other components and reaction conditions were kept constant. According to the obtained results of the analysis of these properties, the stability of copolymer latexes was discussed in detail.

2. Experimental 2.1. Materials Monomers, vinyl acetate (VAc) from Elsan Fiber Co., Turkey, and maleic acid diesters (DBM and DOM) from Plastifay Co., Turkey were used. Initiator, potassium persulfate (K2 S2 O8 ) and pH adjuster, sodium bicarbonate (NaHCO3 ) were purchased from Merck Co. 30 mol ethoxylated nonyl phenol (NP30) and 10 mol ethoxylated nonyl phenol (NP10) from Turk-Henkel Co. were used as emulsifiers. Reactive co-surfactant, oligomeric N-methylol acrylamide (o-NMA; 48 wt% in water) was prepared as reported [28,29]. All chemicals were used without further purification. Deionized water was used throughout the experiments.

2.2. Synthesis of copolymer latexes Two sets of emulsion copolymerization experiments were carried out by semi-continuous process; in set 1, DOM was used as comonomer with the main monomer, VAc and in set 2, DBM was used instead of DOM. In both sets, the weight ratios of the monomers, VAc/DBM and VAc/DOM, were 60/40, 80/20, 90/10, 95/5 and 100/0.

H. Berber Yamak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 233–241 Table 1 Recipe for Semi-continuous Emulsion Copolymerization of VAc (for 100 g latex).

Table 2 Synthesized VAc Copolymer Latexes.

Ingredients

Initial charge (g)

Feed Ia (g)

Feed IIb (g)

Symbol

Monomer mixture Initiator (K2 S2 O8 ) Emulsifier (NP10 + NP40) Reactive surf. (o-NMA) NaHCO3 H2 O

3.9680 0.1300 0.8125 0.8500 0.1100 24.7500

35.7120 – 2.4375 0.8500 – 13.7500

– 0.1300 – – – 16.5000

PVAc DBM5 DBM10 DBM20 DBM40 DOM5 DOM10 DOM20 DOM40

a b

Feeding rate; 1 mL/min. Feeding rate; 0.14 mL/min.

a

To prepare pre-emulsion, nonionic emulsifiers (75 wt% of total emulsifiers) and co-surfactant (50 wt% of total co-surfactant) were dissolved in deionized water at room temperature (RT). Monomer mixture (90 wt% of total monomers) was added to the emulsifier solution and the pre-emulsion was stirred for 30 min at constant rate. The rest of the monomer mixture and the other remaining ingredients were added to the reactor and the temperature was raised to 70 ◦ C. The polymerization was started by adding the first part of the aqueous initiator solution (50 wt% of total K2 S2 O8 ). The reaction was carried out under constant stirring of 120 rpm at 70 ◦ C for 30 min, then heated to 75 ◦ C and kept for 30 min at this temperature. At the end of this period, the temperature was raised to 80 ◦ C and two streams consisting of the pre-emulsion and the initiator solution (rest of 50 wt% K2 S2 O8 ) were fed as dropwise into the reactor under constant stirring of 220 rpm for 5 h. The flow rates of these streams were kept constant during the feeding. Then, the polymerization system was heated to 85 ◦ C and maintained for a further 1 h to increase the monomer conversion. At the end of the completion period, the synthesized latex was allowed to cool down to RT. The recipe and polymerization conditions for different monomer systems are shown in Table 1.

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VAca 100 95 90 80 60 95 90 80 60

DBMa

DOMa

– 5 10 20 40 – – – –

– – – – – 5 10 20 40

Weight percent.

7 h at RT under atmospheric conditions and then repeated for 5 cycles. The stability or instability was observed visually. The ionic strength resistance of copolymer latexes was determined with five different salts having mono-, di- and tri-valent metal cations and anions (NaCl, Na2 SO4 , Na3 PO4 , CaCl2 , AlCl3 ) at various concentrations (0.1 M, 0.5 M and 1 M). 2 mL of electrolyte solution was added to test tube containing the same amount of latex. The latex stability was observed visually after addition and 1 week and 3 months later. 3. Results and discussion The semi-continuous emulsion polymerizations of VAc copolymer latexes were carried out by using two different types of maleic acid diesters, DBM and DOM, and the synthesized VAc copolymer latexes are listed in Table 2. The total solid contents were 45% (w/w). At the end of the polymerizations, the final conversions were measured gravimetrically and they were found to be 95 ± 1%. 3.1. Particle size and particle size distribution

2.3. Characterizations The particle size and particle size distribution (polydispersity index, PDI) of the latexes were determined by Dynamic Light Scattering (DLS) using a 90 Plus/BI MAS Particle Size Analyzer (Brookhaven Instrument Corporation). The latex samples were diluted to 0.4 wt% of the solid content with deionized water before testing to prevent multiple scattering. DLS measurements were performed using 15 mW solid-state laser at 660 nm and the scattering angle was fixed at 90◦ . The measurement time was 10 min (average of 5 measurements of 2 min) at 25 ◦ C. The viscosity measurements of latexes were carried out using a Brookfield LVDV-II+ Programmable Viscometer with spindle LV66 at a rotational speed of 10 rpm at 23 ± 1 ◦ C. The zeta (␰) potential of latex particles was determined at 25 ◦ C by using ZetaPlus Zeta Potential Analyzer (Brookhaven Instrument Corporation). 0.1 g latex was diluted to 25 g with aqueous 10−2 mol L−1 KCl solution, and then each sample was sonicated at RT for 1 min. The zeta potential was calculated using the Smoluchowski equation. Each data used in the evaluation was obtained by taking the average of 10 measurements. The surface tension of latexes was measured by the Du Nouy ring method using a platinum ring attached to digital tensiometer (KSV Instruments Ltd., model Sigma 701). Measurements were performed in triplicate under atmospheric pressure at RT. The electrical conductivity measurements were carried out at under constant temperature conditions (26.0 ± 0.1 ◦ C) by using a conductometer (Eutech/Oakton- PC 510) operating in the range of 1 ␮S–199.9 mS. The freeze–thaw stability of latexes was examined on the basis of ASTM standard test method (D2243-95). Plastic tubes containing 10 mL latex was stored at −18 ◦ C for 17 h and then was thawed for

For the emulsion polymerization processes, particle size and particle size distribution are important parameters influencing the resulting latex properties and, its processing and product performance. The size and distribution of the particles formed during the polymerization and in the final latex are primarily affected by the applied emulsion polymerization method, solid content, monomer characteristics, type and amount of emulsifier and initiator, aqueous phase composition, and other polymerization conditions [30–33]. In this study, all polymerization parameters except the comonomer type and ratio were kept constant, and we particularly investigated the effect of type and ratio of maleic acid diesters on the particle and other colloidal properties of the final VAc latexes. The number average particle sizes of PVAc, VAc/DBM and VAc/DOM latexes are given in Fig. 1. The particle size of the PVAc was found to be about 164 nm. The values of the latex particle size were varied by using different amounts of maleic acid diesters. It was clearly observed from this figure that the particle size values increased for low amounts of maleic acid diesters while they decreased above a certain amount. This variation in average particle size can be explained by the major differences in the monomer properties of VAc, DBM and DOM such as water solubility, reactivity ratio and mass transfer resistance. These features of the monomers directly influence the polymerization kinetic, particle nucleation and growth mechanism, the distribution of the monomers between aqueous phase, monomer droplets and monomer swollen polymer particles, and also the behavior of emulsifier in aqueous phase, which play significant role in size, distribution, stability of the particles during polymerization and in the final latex, and also on particle composition [23,25–28,30–32]. The evaluation of the particle size and distribution results as a function of monomer type and amount is mainly carried out by the nucleation mechanism, which is

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Fig. 1. Particle Size of VAc/DBM and VAc/DOM Copolymer Latexes.

excessively affected by the monomer characteristics, in particular the degree of water solubility. In a conventional emulsion polymerization, there are two main particle nucleation mechanisms; micellar nucleation and homogenous nucleation. Micellar nucleation takes place when the particle nuclei are formed by capturing free radicals into the large monomer swollen micelles [34–37]. In the homogenous mechanism, nucleation takes place in the aqueous phase; monomer molecules dissolved in aqueous phase are initiated by the free radicals and followed by the formation of oligomeric radicals. When the propogating oligomer chains reach critical lengths that are no longer soluble in aqueous phase, they precipitate to form precursor particles. These precursor particles can either directly grow or coagulate with each other or with the formed large latex particles to form stable particles [38–41]. The coagulation process of the colloidally unstable precursor particles nucleated by either micellar or homogeneous nucleation to form stable polymer particles is also called as coagulative nucleation mechanism [34,39,42–44]. The nucleation stage of VAc having higher water solubility predominantly occurs by homogeneous mechanism [23,45]. In the VAc emulsion polymerizations, the number of small-sized particles nucleated by homogeneous mechanism, is expected to increase depending on increasing VAc amount. The small-sized particles having sufficient stability can grow directly. On the other hand, the emulsifier adsorption on the particle surface cannot be sufficient to ensure the particle stability if the number of these particles with hydrophilic surfaces rise. The decreased stability results in coagulation of the small-sized particles, and so an increase occurs in the size of the particles of PVAc [5,25]. By the addition of small amounts of hydrophobic maleic acid diesters (DBM5, DOM5, DOM10) to the monomer mixture, the particle sizes of the final latexes further increased. This indicated that the nucleation mechanism and the particle stability were primarily affected by the addition of hydrophobic comonomer. In all polymerizations, the total emulsifier concentration was above the critical micelle concentration (CMC) and in this case the nucleation stage took place with the co-existence of micellar nucleation and homogeneous nucleation. The total monomer concentration in the aqueous phase and the number of small-sized particles obtained by homogeneous nucleation decreased compared with the homopolymerization of VAc [5,46] and the monomer-rich primary polymer particles were obtained. These primary small-sized particles nucleated by either micellar or homogeneous nucleations are present with the micelles, monomer droplets and polymer particles already

formed during nucleation stage. Although the emulsifier concentration is above CMC, the emulsifier system, which is also used to form micelles and stabilize the monomer droplets and the formed polymer particles, is insufficient in providing the stability of the primary small-sized particles especially nucleated with high rate of homogeneous nucleation due to the presence of water soluble monomers like VAc in high ratios [27,42]. At this stage, these unstable primary particles coagulate to minimize the surface between polymeric particle and water, reduce the surface free energy and maintain the particle stability. Thus, a greater number of particles are lost due to this limited coagulation (coagulative nucleation) and the particle size increases [43,47]. The final particle number of the latex is a result of this coagulation and the particle nucleation [25,48]. If the amount of emulsifier covering the polymer particles is too low, excessive agglomeration is observed [25,42,43]. For the DBM5, DOM5 and DOM10, an excessive agglomeration was not detected, but creaming was observed. The increasing particle size for these copolymers can be also explained by this creaming that causes an increase in mass transfer resistance of the monomers [23]. The increasing monomer transfer resistance decreases the homogeneity of the copolymer particles, and so the emulsifier adsorption on the particle surface decreases. Therefore, the nucleated particles tend to coagulate in order to protect their stability and this tendency increases the average particle size [38,39,43]. Through these limited coagulations, the colloidal stability is maintained and the stable particles of the final latex are obtained, even when the protective system is insufficient in providing stability during polymerization [27,49]. On the other hand, the average particle size decreases as the weight ratio of the maleic acid diesters rises above a certain amount (above 5 wt% of DBM and 10 wt% of DOM). This decrease is a result of micellar nucleation that predominantly takes place when the emulsifier concentration is above CMC and the less water-soluble or water-insoluble monomers exist in the emulsion polymerization medium, instead of homogenous mechanism [42]. The number of the smaller particles formed by the micellar nucleation increases compared with the polymerizations including a high percentage of water-soluble monomer. Moreover, the emulsifier adsorption on the surface of these particle increases due to the increasing hydrophobic character of the particles with the maleic acid diesters, the better particle stability achieves and so the nucleated particles are protected against coagulation. As a result of the predominant micellar nucleation and the successful emulsifier adsorption on the nucleated particles during polymerization, the smaller and more stable particles of the final latex are obtained [5,44]. While comparing the VAc/DBM and VAc/DOM latexes according to their average particle size results, it was found that the copolymer latexes containing DBM (except DBM5) had smaller particles. For instance, the smallest particles were obtained for DBM40 as 80.1 nm and 125.7 nm for DOM40. This variety is caused by the differences in homogeneity of the copolymer particles. Although the application of semi-continuous emulsion polymerization under monomer-starved conditions leads to control of copolymer composition, so the large differences in water solubility and mass transfer resistance of the monomers directly affect the sharing of monomers during polymerization and consequently the composition of copolymer particles [48]. Due to the excessive water insolubility of DOM, the difference between the mass transfer resistance of VAc and DOM arise, and this causes a shift in the homogeneity of the produced VAc/DOM latex particles [23]. The inhomogeneity of the resulting copolymer particles decreases the emulsifier adsorption on the particle surface. Owing to the more hydrophobic character of DOM than DBM, the emulsifier adsorption on the VAc/DOM particle surface is less effective due to the higher shift in the homogeneity of the VAc/DOM particles. When we focused on the behavior of DBM in the semi-continuous monomer-

H. Berber Yamak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 233–241

Fig. 2. Particle size distribution of VAc/DBM Copolymer Latexes.

Fig. 3. Particle size distribution of VAc/DOM Copolymer Latexes.

starved emulsion polymerization system of VAc, it was observed that the growing stage was not limited by the mass transfer of the DBM, because the water solubility of DBM was enhanced by VAc [12]. With applied feeding type, this behavior of DBM allows producing homogeneous copolymer particles. The homogeneous distribution of the DBM on the particle surface enables a more effective emulsifier adsorption, and this can explain the obtained smaller average particle size of the VAc/DBM systems than VAc/DOM. In addition, the glass transition temperature, Tg values of VAc/DBM and VAc/DOM copolymers, analyzed in our previous study, supported the homogeneous structure of the copolymers. The single transitions was observed in DSC thermograms for all the copolymers and the Tg of PVAc, which was found as 17 ◦ C, decreased linearly with increasing contents of DBM and DOM in the copolymer. As expected, DOM was more effective than DBM for lowering the Tg of PVAc. For example, Tg values decreased up to 2.8 ◦ C and −11.8 ◦ C for DBM and DOM, respevtively [5,21,50]. Figs. 2 and 3 show the polydispersity curves of PVAc, VAc/DBM and VAc/DOM copolymer latexes. The monomodal particle size distributions were obtained and all the values of the polydispersity

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indexes (PDI) were less than 0.100 and the values were found to change 0.040–0.095 and 0.048–0.092 for VAc/DBM and VAc/DOM, respectively. These results indicated that the latex particles were homogeneous and had narrow particle size distribution. It was clearly seen from the polydispersity curves and the PDI values that the particle size distributions of VAc based final latexes prepared in same polymerization conditions were significantly related to the type and ratio of maleic acid diester comonomers. For PVAc, a partly broad particle size distribution was obtained and the PDI value was 0.088. As expected, this result supported the presence of the secondary particles generated by the aqueous phase polymerization of VAc, including homogeneous-coagulative nuclation, as well as the particles obtained from micellar nucleation [5,23,25,38,45,51]. When the evaluation of particle size distribution was made for the VAc copolymer latexes having small amount of maleic acid diesters (especially for DBM5 and DOM10), PDI values were found to be higher and their polydispersity curves were rather broad, and also these latexes had larger particle sizes. It can be possible to say that these particle size and particle size distribution results arised as a result of coagulative nucleation of the small-sized particles generated by either homogeneous nucleation or micellar nucleation. In other words, the positive skewness of the particle size distribution implies the coagulative nucleation [34,42]. However, the copolymer latexes containing maleic acid diesters (both DBM and DOM) above a certain amount had narrower particle size distribution than those of PVAc and the copolymer latexes containing small amounts of DBM and DOM. This gradually narrowing in particle size distribution occurred by increasing DBM and DOM amount in monomer mixture is a result of predominant micellar nucleation as explained above. The low PDI values for these copolymers meant that the particle stability was provided more effectively. When the particle size distribution results were compared according to the comonomer type, the copolymer latexes containing DBM (except DBM5) were found to have narrower particle size distribution than the VAc/DOM latexes. This result indicated that DBM increased the copolymer stability more effectively in a wide range of comonomer ratio. Consequently, the investigation of the particle size and particle size distribution of the copolymer latexes consisting of two different maleic acid diesters with different ratios showed that the nucleation mechanism and the particle stability is affected by the nature of monomers, especially hydrophilicity. Comparing vinyl acetate with DOM and DBM, the most obvious difference in physical property is monomer solubility in water. In VAc copolymerizations with the maleic acid diesters, the number of particles generated by homogeneous nucleation and the extent of coagulative nucleation raised with increasing the ratio of VAc having high water solubility in the monomer mixture [10–13]. In addition, it was detected that the extent of coagulative nucleation depending DBM and DOM, especially of their small amounts. These two comonomers have different hydrophilicity, DOM is water-insoluble while DBM has low water solubility. This difference also affects the mass transfer resistance of comonomers, the particle homogeneity of copolymers and the adsorption capability of an emulsifier molecule on the copolymer particle surface [12,13]. All of these determines the particle stability both during the copolymerization and in the final latex. It can be said that the low water solubility of DBM contributes to the homogeneous nucleation while the high mass transfer resistance property of DOM causes a shift in the particle homogeneity that decreases the emulsifier adsorption on the particle surface. For the low concentrations of the comonomers, these effects increased the extent of coagulative nucleation. Conversely, the extent of micellar nucleation and the adsorption of the emulsifier molecule on particle surface increased above a certain amount of DBM and DOM, which lead to improve the particle stability and reduce the potential of

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Fig. 4. Viscosity of VAc/DBM and VAc/DOM Copolymer Latexes.

particle coagulation. These results are in agreement with literature [33,38,49,52].

3.2. Viscosity The latexes with a very wide viscosity range (250–2800 cP) were obtained depending on the maleic acid diester type and amount in the copolymerization of VAc (Fig. 4). The viscosity is highly influenced by the particle size, the particle size distribution and the inter-particle interactions (such as Van der Waals attraction forces, electrostatic repulsion forces, entropic repulsion forces) of the latexes. The forces between the particles become important when the distance between particles decreases and the viscosity increases due to the repulsive interactions between double layers surrounding the neighboring particles. This effect becomes important for the latexes having low ionic strength, low shear rate and certain solid content (45–55 wt%) [53]. The viscosity of PVAc was found to be 237 cP as a result of large particle size and broad particle size distribution. The addition of 5–10 wt% DBM and 5–20 wt% DOM to the monomer mixture significantly increased the viscosity of the PVAc latex. The high viscosity values of these copolymer latexes could be explained by creaming and broad particle size distribution. Especially for DBM5 and DOM5-10, the effect of creaming was more predominant on the viscosity. By creaming, the distance between particles is reduced and due to that the inter-particle interaction increases which leads to an increment in viscosity. When the amount of maleic acid diesters was slightly increased in the monomer mixture (DBM10, DOM20), it was seen that the inter-particle interactions took place because of the small particle size and broad particle size distribution as well as creaming. This could explain the high viscosity values of these latexes. On the other hand, a marked reduction in viscosity of the copolymer latexes occurred when the maleic acid diesters ratio in monomer mixture was above a certain value, 20 wt% for DBM and 40 wt% for DOM. The amount of maleic acid diesters located on the particle surface and so the charge density formed by the hydrolysis of ester groups rose with the increasing maleic acid diester ratio. This increment in the charge density holds the particles apart from each other, increases the distance between particles and reduces the interactions among particles. Consequently, the viscosity decreases despite the small particle size and narrow particle size distribution of these latexes.

Fig. 5. Zeta potential of VAc/DBM and VAc/DOM Copolymer Latexes. The error bars indicate the standard deviation for 5 runs of measurements.

3.3. Zeta potential Zeta potential can be defined as the difference in potential between the medium in which the particles are dispersed and the stationary layer of aqueous phase surrounding these particles, so the zeta potential is closely related to the total electrical charge on the particle surface [54]. Furthermore, the magnitude of the zeta potential gives an indication of the potential stability of the colloidal system [53]. The electrostatic repulsion between the particles that are highly charged (positive or negative) makes the latex more stable. This means that stabilization arises from the mutual repulsion between the electrical double layers of particles [55]. However, the lowest zeta potential values cannot prevent the particles coming together due to the attractive forces that exceed the repulsion force between particles which leads to observation of flocculation. Consequently, the stability of the latexes increases with the resistance increment of the particles against coming together in the colloidal system when the zeta potential value increases. As shown in Fig. 5, the obtained zeta potential values were negative for VAc homopolymer and all copolymer latexes as expected since they were negatively charged. This negative charge was due to the presence of anionic moieties derived from the decomposition of the initiator (sulphate groups) and hydrolysis of the ester groups of the monomers (carboxylate groups). It was found that PVAc had the lowest zeta potential value (−7 mV), and then the zeta potential values slightly rose to −18 mV with the increase in the amount of maleic acid diesters. This shows the zeta potential of copolymer latexes would be affected by the amount of maleic acid diesters adsorbed at the oil/water interface. With the increasing ratios of both DBM and DOM, the amount of dispersed charge on the particle surface and the thickness of electrical double layer surrounding the particles were increased. This formation on the particle surface increased the zeta potential value and improved the stability of the latexes. Briefly, we can say that the presence of maleic acid diesters has a positive effect on the colloidal stability of the VAc copolymer latexes. Also, the maleic acid diesters type plays an important role in the zeta potential behavior of the VAc based copolymer latexes. The higher zeta potential values were detected for the copolymer latexes containing DBM than those of DOM. As discussed in detail in section 3.1, the low mass transfer resistance of DBM comparing with DOM resulted in an increase in the amount of DBM located on the particle surface and particle homogeneity. This fact reinforced

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Fig. 6. Surface tension of VAc/DBM and VAc/DOM Copolymer Latexes. Fig. 7. Conductivity of VAc/DBM and VAc/DOM Copolymer Latexes.

that the amount of carboxylate groups formed by the hydrolysis of maleic acid diesters located on the particle surface which more predominant in case of DBM. Accordingly, it is possible to say that this effect would increase the amount of dispersed charge and the thickness of electrical double layer surrounding the particles, and so the VAc copolymer latexes having higher zeta potentials and higher stability were obtained with DBM according to DOM. 3.4. Surface tension The surface tension measurements were done by ring method and the results were given in Fig. 6. The surface tension values were between 33.10–34.32 mN m−1 depending on the maleic acid diester type and ratio in the copolymerization of VAc. The lowest value was found for PVAc and the surface tension took various values with the addition of maleic acid diesters to the monomer mixture. For the low maleic acid diesters ratios, 5–10 wt% of DBM and 5–20 wt% of DOM, the higher surface tension values were observed while the values started to decrease again above a certain value, 20–40 wt% of DBM and 40 wt% of DOM. The surface tension is directly related to the amount of free emulsifier in water phase and its value increases when the amount of free emulsifier in the water phase reduces. In other words, more emulsifier adsorption to the particle surface causes an increase in the surface tension. For instance, the obtained lowest surface tension value for PVAc shows that the amount of free emulsifier in the aqueous phase is more predominant compared to the adsorbed emulsifier on the particle surface. The emulsifiers do not prefer to be adsorbed on the particle surface because of the more hydrophilic character of vinyl acetate. On the other hand, two different behaviors were observed with the presence of hydrophobic maleic acid diester comonomers depending on their amounts. For low ratios of DBM and DOM, the surface tension got higher values. As expected, the emulsifier molecule prefers to be adsorbed on the particle surface instead of being in aqueous phase due to the increasing hydrophobic character of the copolymer particles comparing with PVAc. Despite the increase in hydrophobic character of the particle surface above a certain ratio of DBM and DOM, the more negatively charged particles were created by hydrolysis of these groups and they do not allow the effective adsorption of emulsifier on the particle surface. Moreover, the free emulsifier concentration in the aqueous phase increases and the surface tension decreases again. When we compared the effect of maleic acid diester type on the surface tension, the higher values were generally observed for DBM containing copolymer latexes. This indicated that the emulsifiers

preferred to be adsorbed on the more homogeneous VAc/DBM particle surfaces than VAc/DOM particles, although DOM has a more hydrophobic character. Considering the obtained surface tension results, it can be clearly said that the emulsifier adsorption on the particle surface does not significantly affect the stability of the copolymer latexes, and the homogenous presence of a certain amount of maleic acid diester on the particle surface is more effective to provide the stability. 3.5. Conductivity The electrical conductivity change in a colloidal system is related with the amount of free water or number of ions dissolved in the aqueous phase. When the conductivity and surface tension results were evaluated, it was seen that the conductivity behavior of the copolymer latexes was completely different from the surface tension results related with the free water content. Accordingly, it can be said that the number of ions dissolved in the aqueous phase determines the electrical conductivity behavior of these latexes rather than free water content. So, the ion type and amount of ions placed to the particle surface plays a significant role in conductivity [56,57]. The VAc-maleic acid diester copolymer latexes provide ions from the strong and weak acid groups of persulphate initiator and monomers [28]. As seen in Fig. 7, the highest electrical conductivity value belongs to PVAc as 2.20 mS cm−1 . This highest value can be explained by the excess amount of the initiator strong acid groups passing through the aqueous phase, because the more hydrophilic character of PVAc latex particle surfaces limits the adsorption of initiator groups on the surface. So, the ionic strength and the conductivity value of PVAc latex increased. By the addition of hydrophobic maleic acid diesters (5–10 wt% for DBM and 5–20 wt% for DOM) into the monomer mixture, a decrease in conductivity values was observed. The more hydrophobic surfaces of these latexes compared with PVAc increased the adsorption of hydrophilic initiator on the particle surface, the initiator groups passing through the aqueous phase was inhibited, and so the ionic strength of the aqueous phase decreased. On the other hand, the conductivity increased again above a certain value of maleic acid diesters as a result of excess amount of the weak acid groups in the aqueous phase. When an evaluation was made according to the type of maleic acid diesters, it was found that VAc/DOM latexes had lower conductivity values than those of VAc/DBM latexes. In addition, the increasing amount of maleic acid diesters made a large

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Table 3 The Ionic Strength Resistance of VAc Copolymer Latexes. Copolymer

Time (week)

The type of salt NaCl concentration (M)

CaCl2 concentration (M)

AlCl3 concentration (M)

Na2 SO4 concentration (M)

Na3 PO4 concentration (M)

0.1

0.5

1.0

0.1

0.5

1.0

0.1

0.5

1.0

0.1

0.5

1.0

0.1

0.5

1.0

PVAc

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

DBM5

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

DBM10

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

DBM20

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

DBM40

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

DOM5

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

DOM10

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

DOM20

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

DOM40

0 1 12

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

+ + +

+ + +

+ + +

– – –

– – –

+ + +

– – –

– – –

+: stable. –: precipitation and/or phase separation.

difference between conductivities of these latexes due to the more hydrophobic character of DOM. Finally, it was concluded that the electrical conductivity values of the latexes can be determined by strong acid groups of the initiator passing through the aqueous phase depending on the hydrophilicity of the particle surface and the amount of charged maleic acid diesters that hydrolyzed in the aqueous phase, in comparison with the amount of free water which depends on the quantity of emulsifier adsorbed on the particle surface. 3.6. Freeze–thaw stability The freeze-thaw stability test was performed to all VAc copolymer latexes as five cycles [58]. The latexes showed resistance to these freeze-thaw cycles independently from the maleic acid diester type and amount. The obtained results indicated that all the latexes protected their stability and any flocculation or coagulation was observed in these conditions. 3.7. Ionic strength resistance The ionic strength resistance of the VAc/maleic acid diester copolymer latexes was detected against the different concentrations of electrolyte solutions. Collapse and/or phase separation were observed for some electrolyte solutions while adding and/or after 1 and 12 weeks. As shown in Table 3, all VAc/DBM and VAc/DOM latexes showed the same behavior against the electrolyte solutions that had different ionic strength. They were resistant against all concentrations of NaCl and AlCl3 , but cannot protect their

stability against the increasing concentrations of CaCl2 , Na2 SO4 , Na3 PO4 and precipitation or phase separation occurred. 4. Conclusions The latex properties of the VAc based emulsion copolymers synthesized by dioctyl maleate and dibutyl maleate comonomers were investigated. The main factors affecting the properties of these latexes were type and percentage of the maleic acid diesters in the monomer mixture. When all the obtained results were evaluated, it was seen that the large differences in water solubility and mass transfer resistance of the monomers played an important role on the nucleation mechanism and particle homogeneity of the resulting latexes. The variety in the particle homogeneity was determining factor on the latex properties such as particle size, viscosity, zeta potential, surface tension and conductivity. The copolymer latexes synthesized with DBM above a certain value had significantly smaller average particle size and narrower particle size distribution than PVAc and VAc/DOM latexes. The viscosity of the latexes was found to generally increase when both types of the maleic acid diester existed in the structure of the latex particles, but the values were varied with the ratio of the diesters. The PVAc latex had the minimum zeta potential value and the values increased in magnitude when the amount of DBM and DOM increased. The conductivity and surface tension values showed the variation depending on the maleic acid diesters type and ratio. Consequently, these obtained analysis results supported that the maleic acid diesters improved the latex stability of the VAc when they were used above a certain amount, and also DBM were gen-

H. Berber Yamak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 233–241

erally more effective in increasing the latex stability according to DOM.

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