Synthesis and characterization of fluoro-silicone polyacrylate latex emulsified with novel green surfactants

Abstract

The novel fluoro-silicone polyacrylate latex was successfully synthesized in the presence of a novel green mixed emulsifier of β-cyclodextrin (β-CD) and sodium lauroyl glutamate (SLG) by semi-continuous seeded emulsion polymerization, which was initiated with potassium persulphate. Methyl methacrylate (MMA) and butyl acrylate (BA) were used as the main monomers, while hexafluorobutyl methacrylate (HFMA) and vinyltriethoxysilane (VTES) were used as the functional monomers. The resultant latex and its film were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry and contact angle determinator, respectively. The optimum conditions of preparing the fluoro-silicone polyacrylate latex are as follows: the amount of compound emulsifiers are 7.0%; β-CD: SLG = 1:2; the amount of initiator is 0.7%; the appropriate proportion of main monomer of MMA: BA = 1:1; and the amounts of HFMA and VTES are 6.0% and 4.0%, respectively. In this case, the resultant latex has high conversion rate, low gel rate and good stability. Results showed that both thermal stability and hydrophobic property of the latex are improved with the incorporation of a small amount of HFMA and VTES.

Keywords

Fluoro-silicone polyacrylate latex green surfactants synthesis characterization

Introduction

The increasing pollution problems caused by our living style and modern industry drive researcher’s attentions to develop eco-friendly materials.¹ The polyacrylate latex has been widely used in various areas, such as coatings, adhesives and painting inks, owing to the good performance of weather resistance, film-forming property, adhesion, mechanical properties and its environmental friendliness.^2,3 However, their drawbacks such as poor water resistance and thermal stability limit its further applications. Therefore, it is essential to improve the water resistance and thermal stability of the polyacrylate. One approach to overcome the aforementioned drawbacks is to introduce functional groups in polymers during synthesis such as fluorine and silicon groups. Fluoropolymers have been widely studied because of their unique properties such as superior surface hydrophobicity, thermal and good chemical.^4

-7 Consequently, fluoropolymers have been found wide application in modern industry ranging from biomaterials, building, anticorrosion applications and surface coatings for textile and paper.^8
-10 However, the application of fluoroacrylate copolymers is limited mainly due to the high cost of fluorinated monomers. Silicone acrylate emulsion has also been widely used as adhesives and coatings because of its various advantages, such as good film-forming property, high adhesive strength, resistance to high and low temperature, chemicals and anti-contamination.¹¹ Silicone polymers display an unusual combination of physical and chemical properties compared with homologous carbon-based polymers, which is mainly due to the unusual physicochemical properties of the siloxane (–Si–O–) bond.^12

-16 Therefore, acrylate polymers containing fluorine and silicon are of considerable scientific interest and widely applied in coating and adhesives due to their extraordinary compound properties.¹⁷

Emulsion polymerization is by far one of the most important techniques for producing acrylate latex from an industrial point of view. In the process of emulsion polymerization, monomer is polymerized in an aqueous medium containing surfactant micelles. Surfactants play a significant role in the process. They are very important for the nucleation of the latex particles, emulsification of monomer droplets and stabilization of polymer particles during polymerization.¹⁸ Therefore, it is essential to select the emulsifier with good performance and environmental protection. The use of conventional surfactants such as alkylphenol ethoxylates (OP-10), but it is forbidden to use because of its toxicity and bad degradation. β-Cyclodextrin (β-CD), due to its unique molecular structure, consisting of seven d-glucopyranose units and possessing a hydrophobic internal cavity and a hydrophilic external rim, which can selectively form inclusion complexes with hydrophobic molecules.¹⁹ Recently, β-CD shows strong bonding behaviour with anionic surfactants, widely used in food, chemical industry, agriculture and cosmetics. Sodium lauroyl glutamate (SLG) is also a new environmental protection anionic surfactant used in acrylate emulsion synthesis. At present, application of mixed emulsifier system of β-CD and SLG in preparing the novel fluoro-silicone polyacrylate latex has not been reported in the literature. In this work, a new green emulsion system of β-CD and SLG surfactant is used to replace the traditional surfactants. Influences of amount of hexafluorobutyl methacrylate (HFMA) and vinyltriethoxysilane (VTES) on the properties of resultant latex and its film are investigated in detail. The synthetic pathway is given in Figure 1.

Figure 1.

Synthetic pathway of preparing novel fluoro-silicone polyacrylate latex.

Experimental

Materials

Methyl methacrylate (MMA) and butyl acrylate (BA), which were the chemically pure grade, were purchased from Shanghai Chemical Reagents Supply Procurement of Five Chemical Plants (China) and were distilled under reduced pressure prior to polymerization. HFMA, which was the industrial grade, was obtained from Harbin Xeogia Fluorine-Silicon Material Co Ltd (China). VTES, which was the industrial grade, was supplied by Huarong Chemical Co Ltd (Qufu, China). β-CD, which was the chemically pure grade, was purchased from Shanghai Aladdin Biochemical Technology Co Ltd (China). SLG, which was the chemically pure grade, was supplied by Guangzhou Bafeorii Chemical Co Ltd (China). The water used in this experiment was distilled followed by deionization.

Preparation of fluoro-silicone polyacrylate latex

The fluoro-silicone polyacrylate latex was prepared with semi-continuous emulsion polymerization, which was carried out in a 250-mL four-necked flask equipped with reflux condenser, mechanical stirrer and dropping funnels and heated with the water bath. First, the mixed emulsifier of 0.70 g of β-CD and 1.40 g of SLG was charged into the reactor which contained 40.00 g of deionized water and was heated with water bath to the temperature of 80°C under the condition of moderate agitation at 200 r min⁻¹. Then, 10 wt% of the initiator solution composed of 0.21 g of potassium persulphate (KPS) and 30.00 g of deionized water and 10 wt% of mixed monomers composed of 13.50 g of MMA, 13.50 g of BA, 0.18 g of HFMA and 0.12 g of VTES were added into the reactor dropwise under stirring within 15 min, respectively. The seeded latex was obtained when the reaction was maintained for another 15 min. Then, the rest of the mixed monomers and initiator solution was charged into the reactor within 3 h simultaneously by two separate dropping funnels, respectively. After the feed was completed, the temperature was raised to 90°C and maintained for another 45 min to increase monomer conversion. The latex was then cooled to about 40°C. Finally, the mixture in the reactor was cooled and filtered. Thus, the fluoro-silicone polyacrylate latex was obtained. The recipe for the preparation of the fluoro-silicone polyacrylate latex was presented in Table 1.

Table 1.

Recipe of preparing a fluoro-silicone polyacrylate latex.

Ingredients	Amount (g)	Ingredients	Amount (g)
MMA	13.50	β-CD	0.70
BA	13.50	SLG	1.40
HFMA	1.80	KPS	0.21
VTES	1.20	Water	70.00

MMA: methyl methacrylate; BA: butyl acrylate; HFMA: hexafluorobutyl methacrylate; VTES: vinyltriethoxysilane; β-CD: β-cyclodextrin; SLG: sodium lauroyl glutamate; KPS: potassium persulphate.

Characterizations

Fourier transform infrared (FTIR) spectroscopy was recorded between 4000 and 400 cm⁻¹ with an FTIR spectrometer (Thermo Nicolet Infrared AVATAR370, Waltham, MA, USA). Glass transition temperature (T _g) of the copolymers was tested via differential scanning calorimetry (DSC; DSC Q100, TA Instruments Corporation USA) under the nitrogen atmosphere at a heating rate of 10°C min⁻¹ under nitrogen (N₂) atmosphere and the temperature scale was from −60°C to 60°C. Thermogravimetric analysis was utilized to characterize the thermal stability of copolymers. The temperature range was from 40°C to 500°C at a heating rate of 10°C min⁻¹ with N₂ protection. The fluoro-silicone polyacrylate latex particle size and its distribution were determined by the dynamic light scattering detector (Malvern Zetasizer Nano S90, UK) at room temperature. Water contact angles (WCAs) were tested to study the hydrophobicity of the copolymer films. The measurements were performed on DataPhysics CA meter (OCA-20, Germany) by the sessile drop method at room temperature. Water absorption of film was determined according to HG/T3344-1985 and calculated via the equation: water absorption (wt%) = $\frac{w_{1} - w_{0}}{w_{0}} \times 100 %$ , where w ₁ is the weight of a certain amount of latex film and w ₀ is the weight of the latex film after putting it in water for 48 h. Conversion rate was measured by gravimetric analysis.²⁰ Approximately 2–3 g of acrylate emulsion was weighted in a Petri dish and four drops of inhibitor of 5% hydroquinone were added and dried to a constant weight in a dry oven at 80°C. Conversion rate was calculated by the following formulas: Solid content (wt%) = $\frac{w_{2} - w_{0}}{w_{1} - w_{0}} \times 100 % = A$ and conversion (wt%) = $\frac{w_{3} \times A - w_{4}}{w_{5}} \times 100 %$ , where w ₀ was the weight of the Petri dish, w ₁ and w ₂ were the weight of the copolymer latex before and after drying to a constant weight, w ₃ was the total weight of all the materials put in the four-necked flask in polymerization, w ₄ was the weight of materials that cannot volatilize when they were dried and w ₅ was the total weight of monomers. Gel rate (wt%) = W ₁/W ₀ $\times$ 100%, where W ₁ was the weight of the coagulum that was measured by collecting the solid deposited on the stirrer, reactor walls and by residual of filtered latex; W ₀ was the weight of the total monomer in the recipe. The mechanical stability of the copolymer latex was determined by a centrifugal method (4000 r min⁻¹, 30 min).

Results and discussion

FTIR and DSC film analysis of films

Figure 2 is the FTIR spectrum of the film of the fluoro-silicone polyacrylate latex. The strong peaks in the 2955 and 2863 cm⁻¹ are corresponded to C–H (CH₃, CH₂). The characteristic absorptions at 1726 cm⁻¹ are attributed to the stretching vibration of the C=O; 1449 and 1385 cm⁻¹ were the distortion vibration of –CH₂ and C–H in –CH₃, respectively. The absorption bands corresponding to the stretching vibration of C–F at 1236 cm⁻¹ can be detected; 1143 cm⁻¹ was the stretching vibration peak of C–H. The peaks at 1063 and 961 cm⁻¹ are due to absorptions of Si–O–C and Si–O–Si, respectively; 841 cm⁻¹ is the stretching vibration absorption peak of C–O in the acrylic group and the absorption peak at 754 cm⁻¹ is assigned to the stretching vibration of C–F. It can be seen that the disappearance of the characteristic absorption of C=C indicates that all acrylate monomers have been copolymerized. This means that the fluorine and silicone groups have been successfully introduced into the copolymer through emulsion polymerization. The DSC curves of the samples are shown in Figure 3. The sample exhibits only one T _g at 4.58°C. This also confirms that the emulsion has been prepared successfully and the latex is a kind of random copolymer.

Figure 2.

FTIR of latex film. FTIR: Fourier transform infrared.

Figure 3.

DSC analysis of latex film. DSC: differential scanning calorimetry.

Particle size of latex

Influence of varied amount of emulsifiers on average particle size and its distribution of latex is given in Figures 4 and 5, respectively. It indicates that the average particle size of the latex is decreased with the increased amount of the emulsifiers. This phenomenon can be explained by the mechanism of micelle nucleation. There are more chances to form more micelles when the amount of the mixed emulsifiers is increased, thus causing the average particle size of the latexes to be decreased.

Figure 4.

Influence of varied amount of emulsifier on average particle size.

Figure 5.

Influence of varied amount of emulsifier on average particle size and its distribution.

Influence of amount of emulsifier on properties of latex

The influence of the varied amount of the emulsifier on the properties of the emulsion is presented in Table 2. It can be seen that it has the highest conversion percentage, smallest coagulation percentage, better mechanical stability and the appearance of latex is translucent with blue light when the amount of emulsifier is 7%. In addition, it is also found that the varied amount of the emulsifier has no obvious effect on the coagulation percentage. This phenomenon may be attributed to the fact that the increase of emulsifier leads to forming more micelles, which provides more reaction centre, thus causing the increase of the conversion percentage. However, the latex particles will be completely covered when the amount of emulsifier is excessive. It is difficult for the free radicals to enter inside of latex particles. Thus, the conversion percentage is decreased.

Table 2.

Influence of amount of emulsifier on properties of emulsion.

Amount of emulsifier (%)	Conversion percentage (%)	Coagulation percentage (%)	Appearance of latex	Mechanical stability
4.0	98.3357	1.50	Milky with no blue light	A little sediment
5.0	98.5962	1.33	Milky with weak blue light	Passed
6.0	99.0501	1.03	Translucent with blue light	Passed
7.0	99.4598	0.63	Translucent with blue light	Passed
8.0	99.2820	0.83	Translucent with blue light	Passed
9.0	98.7531	0.90	Translucent with blue light	Passed

Influence of amount of initiator on conversion percentage and coagulation percentage

The influence of the amount of the initiator on the conversion and coagulation was shown in Figure 6. It can be seen that the conversion percentage is highest when the amount of initiator is 0.7%. On the contrary, the coagulation percentage was decreased with the increased amount of the initiator when it is less than 0.7%. The phenomena can be explained by the fact that the increased amount of initiator will lead to more free radicals and there are more chance for free radical to diffuse to the micelles, which results in an increase in the conversion percentage and a decrease in coagulation percentage. However, the excessive amount of the initiator will lead to the obvious increase of the polymerization reaction rate. It is easy to collide with each other for the particle of the emulsion. Thus, polymerization stability of the emulsion is also decreased. Moreover, the massive reaction heat, which is generated in the polymerization, cannot be removed in time, which is more likely to produce gel. These will result in the decrease of conversion percentage and the increase of coagulation percentage. The amount of initiator was 0.7% in the study in view of the monomer conversion and the stability of polymerization system.

Figure 6.

Influence of initiator on conversion percentage (a) and coagulum percentage (b).

Influence of different mass ratios of emulsifier and monomer on properties of latex

The influence of different mass ratios of emulsifier on properties of latex is shown in Figure 7. It can be found that the conversion percentage is the highest and the coagulation percentage is the lowest when the mass ratio of β-CD to SLG is 1:2. It has better synergistic effect when the mass ratio of β-CD to SLG is 1:2. From Table 3, it can be found that the conversion percentage is the highest and the coagulation percentage is the lowest when the mass ratio of MMA to BA is 1:1 and all the conversion percentages are more than 98%. The results also indicate that the varied mass ratio of the monomer has a slight effect on the conversion percentage and coagulation percentage. The influence of varied amount of mass ratio of monomers on the properties of the film is shown in Figure 8, which indicates that the film is moderately hard when the mass ratio of MMA to BA is 1:1. MMA is a hard monomer and BA is a soft monomer. When the amount of MMA is excessive, the film will be hard. On the contrary, the film will be soft. Therefore, the selection of the appropriate monomer mass ratio has a great influence on the film-forming property and performance of the latex. Based on the further analysis of the theoretical value of T _g, it can be found that the data tested by DSC (see Table 4) are consistent with those calculated via Fox equation.

Figure 7.

Different mass ratio of emulsifier on conversion percentage (a) and coagulum percentage (b).

Figure 8.

Appearance of film for varied mass ratio from MMA to BA. (a) 2:1. (b) 1:5. (c) 1:1. (d) 1:1.5. (e) 1: 2. MMA: methyl methacrylate; BA: butyl acrylate.

Table 3.

Influence of mass ratio of monomer on properties of emulsion.

Mass ratio from MMA to BA	2:1	1.5:1	1:1	1:1.5	1:2
Conversion percentage (%)	98.84	99.10	99.51	99.42	99.06
Coagulation percentage (%)	0.62	0.68	0.46	0.58	0.61
Appearance of film	Brittle	Hard	Moderate	Soft	Sticky
Appearance of emulsion	•	•	•	•	•
Mechanical stability	√	√	√	√	√

MMA: methyl methacrylate; BA: butyl acrylate; •: appearance of the emulsion is translucent with blue light; √: emulsion is stability.

Table 4.

Effect of mass ratio of monomer on T _g.

Mass ratio from MMA to BA	T _g (°C)
1:0	105.00
0:1	−54.00
2:1	30.66
1.5:1	18.51
1:1	2.73
1:15	−11.73
1:2	−20.50

T _g: glass transition temperature; MMA: methyl methacrylate; BA: butyl acrylate.

TG analysis

TG analysis results of the polyacrylate films are shown in Figure 9. It is taken as a criterion for evaluating the thermal stability of the latex films.²¹ It can be seen from Figure 9 that decomposing temperature of polyacrylate latex containing silicon and fluorine is 348.35°C, which is 11.12°C higher than that of the latex film without fluorine and silicon. It is evident that thermal stability of the copolymer is enhanced by the introduction of a small amount of HFMA and VTES. The fluoro-silicone chain containing C–F bound and Si–O bond has high bond energy, which shields and protects the non-fluorinated segment beneath the fluorinated segment sufficiently. Both the construction of a cross-linked silica network, which is produced from the hydrolysis and condensation of Si (OR)₃ groups and the temperature insensitiveness of the silica itself contributed to the improved thermal stability of latex films.

Figure 9.

TGA of latex film. (a) Acrylate copolymer and (b) acrylate copolymer modified by fluorine and silicon monomer. TGA: thermogravimetric analysis.

WCA of film

WCA is tested to study the hydrophobicity of the copolymer films. The influence of amount of HFMA and VTES on the CAs of film is shown in Figure 10. The result indicates that the WCA of the film is obviously improved when the fluorine and silicon monomers are incorporated by copolymerization. These phenomena can be explained by the fact that the fluorine atom in the molecule of the latex tends to locate on the film surface during the film formation to minimize the interfacial energy, which can cover the hydrophilic group in the molecule of latex.²² On the other hand, formation of silica network can prevent water from entering the films, which also cause the increase of the initial WCA.

Figure 10.

Influence of amount of HFMA and VTES on WCA. HFMA: hexafluorobutyl methacrylate; VTES: vinyltriethoxysilane; WCA: water contact angle.

Water absorption of film

The influence of the content of HFMA and VTES on the water resistance of the film is shown in Figure 11. The results show that the water resistance of the film is improved when the moderate amount of HFMA and VTES is used. This may be caused by the fact that the branched chain of the fluorine group in copolymer will arrange outwards spontaneously and form hydrophobic layer after the film of the latex is formed.²³ On the other hand, hydrolysis and condensation of Si (OR)₃ groups in VETS can construct a cross-linked network will also help to prevent water molecules penetrating further. In combination with the price of fluorine and silicon and the effect of hydrophobic property, the appropriate amounts of fluorine and silicon monomers were 6.0% and 4.0%, respectively.

Figure 11.

Influence of amount of HFMA and VTES on water absorption rate of film. HFMA: hexafluorobutyl methacrylate; VTES: vinyltriethoxysilane.

Conclusions

The novel fluoro-silicone polyacrylate latex is successfully synthesized in the presence of a novel green mixed emulsifier of β-CD and SLG by semi-continuous seeded emulsion polymerization, which is initiated with KPS. The optimum recipe of preparing the latex is as follows: the amount of compound emulsifiers are 7.0%, β-CD: SLG = 1:2; the amount of initiator is 0.7%; the appropriate proportion of main monomer of MMA: BA = 1:1; and the amounts of HFMA and VTES are 6.0% and 4%, respectively. The thermal stability and the water resistance of latex films are improved significantly because of the introduction of fluorine and silicon into the polymer.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Zhejiang Provincial Natural Science Foundation of China (no. Y4100152).

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