The importance of emulsion polymerization in the coatings industry has increased in the last decades due to environmental awareness and the replacement of solvent-based paints by water-based systems.1,2 Acrylic resins, which have an important commercial application in the paint industry, are prepared through the emulsion polymerization of acrylic or methacrylic acids and their corresponding esters as well as vinyl monomers, such as styrene.
High-solid-content latexes are of growing interest since these allow for increasing reactor output and developing paint formulations with higher solid content, as well as reducing transportation and storage costs.3 Several studies have highlighted the importance of controlling particle size distribution and the number of particles in order to develop high-solid latexes.3,4 It is very well known that surfactants, due to their important role in nucleation, growth and stability of latex particles,5-7 impact particle size distribution and number of final latexes.
Alkylphenol ethoxylates (APEs) are one of the most important classes of surfactants used in emulsion polymerization due to their cost-effectiveness and performance.8 However, concerns about APE ecotoxicity have led to an effort to replace them with more environmentally friendly alternatives.9
In this article, the properties of a new APE-free surfactant system, containing an optimized ratio of anionic and non-ionic surfactants designed for emulsion polymerization of conventional and high-solid latexes, are explored. Also, the effect of the new APE-free surfactant system on nucleation, growth, particle size distribution and number of the high-solid styrene-acrylic latexes obtained through unseeded and seeded processes is presented. Finally, the properties of coatings containing the aforementioned latexes are examined in order to prove the effectiveness of the new APE-free surfactant latexes.
Industrial-grade inhibited monomers from Unigel and Vetta Química, and potassium persulfate (K2S2O8 99% purity grade from Merck) were used in the polymerizations. Industrial-grade monoethanolamine (MEA) was used as a neutralizer of latexes obtained. All materials used in paint preparations were used as received.
General Procedure for Emulsion Polymerizations
Polymerizations were performed in a 3 L three-neck cylindrical glass reactor equipped with mechanical stirrer, thermometer and reflux condenser. The temperature of the water in the cooling jacket was adjusted by a controlled temperature bath. All polymerizations were carried out at 82 ± 2 °C. The monomeric compositions and formulations used in all polymerizations are described in Tables 1 and 2, respectively. The finished latexes were filtered through a 200-mesh screen to collect the filtered solids. Coagulum adhering to the agitator, thermometer and reactor was also collected. The solid content of the latex was determined by a gravimetric method (ASTM D1489-9).
The surface tension of surfactant solutions was determined using a Data Physics OCA-15 analyzer.
Viscosity and pH
The pH of latexes and paints was determined in an UB-10 pH meter from Denver Instrument. Latex viscosity was measured at 25 °C and 100 rpm in an LVT viscometer from Brookfield. Paint viscosity was measured at 25 °C in a Brookfield KU-2 viscometer.
Latex average particle size was determined in a Malvern Zetasizer NS. Average particle size of slurries used in paint formulations was determined using a Mastersizer 2000 G analyzer.
The density of latexes and paints was determined with a Mettler DE-40 analyzer.
Wet Scrub Test
Films were applied at a wet thickness of 175 mm to polyvinyl chloride substrates by using a BYK film applicator. Films were dried for 7 days at 25 ± 2 °C and 60 ± 5% RH. The wet scrub test was performed in a BYK Gardner Abrasion Tester AG-8100 according to the ABNT NBR 14940 Standard for paints with PVC 30% and 55 %, respectively. The data shown is an average of three experiments for each sample.
Previous works from McKenna et al.3 and Schneider et al.4 have shown that the control of the number of particles along the polymerization, as well as the particle size distribution, is essential for developing latexes with high solid content. Monomodal latexes with solid content up to 60 wt% and bimodal latexes with solid content up to 72 wt% were obtained controlling particle size distribution and number. In the present work, the APE-free surfactant concentration and process were optimized in order to control particle size distribution and number, which resulted in latexes with solid content up to 63 wt%. Also, APE-free latexes containing different surfactant concentrations were polymerized through unseeded and seeded processes. These two processes were investigated for understanding which process promotes a better control of number of particles avoiding the nucleation of new particles along polymerizations.
The main properties of the APE-free surfactants investigated are presented in Table 3. As mentioned previously, the new APE-free surfactant system has an optimized ratio of anionic and non-ionic surfactants designed for emulsion polymerization of latexes containing conventional to high solid content. The APE-free anionic surfactant was only evaluated in the polymerizations of seeds used in the seeded process due to its effectiveness for generating stable seeds.
In the first part of this work, semi-continuous unseeded polymerizations were carried out using different concentrations of the APE-free surfactant blend in order to show the effect of the APE-free surfactant system on particle nucleation, growth and number. The main characteristics of latexes obtained from polymerizations carried out using 1 and 2 wt% of APE-free surfactant during the polymerizations are presented in Table 4.
Figure 1 shows the number of particles as a function of solid content during the unseeded polymerizations. According to Figure 1, the polymerization containing 2 wt% of APE-free surfactant system, P24, generated a higher number of particles in comparison to the one containing 1 wt% of surfactant, P41. In both polymerizations, there is an increase of the number of particles along the polymerization, even using a minimal surfactant concentration of 1 wt%. Polymerizations carried out using surfactant concentration lower than 1 wt% destabilized.
In the seeded process, seed particles were previously generated through semi-continuous emulsion polymerizations using only APE-free anionic surfactant. The concentration of APE-free anionic surfactant used in the polymerizations of seeds was about 3 wt%. The main physico-chemical characteristics of seeds are shown in Table 5.
Polymerizations containing 2 x 1017 seed particles, APE-free surfactant system concentration of about 1.0 wt% during the polymerization and 1.0 wt% of post-stabilization, produced stable latexes with solid content higher than 60 wt%. The main characteristics of the latexes obtained are shown in Table 6.
According to Table 6 and Figure 2, there was an increase of number of particles along the polymerizations in the seeded process and, due to this, the latexes obtained have particle size of about 200 nm instead of the predicted particle size of 220 nm considering that the number of particles was kept constant at 2 x 1017.
However, based on Figure 3, this increase in the number of particles observed for the seeded process is lower than the one observed for the unseeded process, suggesting that the seeded process allows for a better control of the number of particles along the polymerization. Due to this, the use of the seeded process for generating high-solid latexes will be further explored considering the effect of seed size and number.
The latexes containing solid content of about 50 wt% and solid content higher than 55 wt% were evaluated in paint formulations having a PVC of 55%. The wet scrub resistance of these paints is presented in Figure 4. According to Figure 4, both paints have similar wet scrub resistance.
Additional investigation of the effect of high-solid latexes obtained through the seeded process on film formation and properties of coatings containing PVC ranging from 30-80% will be explored.
The knowledge of the role of APE-free surfactant system on the number of particles through nucleation and growth stages was shown to be important for developing latexes with high solid contents. Unseeded and seeded emulsion polymerization processes using very low concentration of an APE-free surfactant system along the polymerization (1 wt%) and with post-stabilization using additional surfactants, allow formulators to generate stable latexes with solid content higher than 55%. Paints formulated with the experimental high-solid latexes have wet scrub resistance similar to the ones formulated with conventional latexes having solid content of about 50 wt%.
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6Tadros; T.F. Colloids in Paints; Wiley-VCH Verlag GmbH & Co. KGaA, 2010. p 254.
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This paper was presented at the 2015 Waterborne Symposium in New Orleans, LA.
By Juliane Pereira Santos, Silmar Balsamo Barrios, Pedro Henrique Invencione, Cíntia Fávaro and Nadia Andrade Armelin, Oxiteno Brazil; Servaas Engels, Sinergia; and Kip Sharp, Oxiteno USA