Study on modified VAc-VeoVa10 latex prepared by soap-free emulsion polymerization

Abstract

Modified latex is prepared via the soap-free emulsion polymerization of ethyl acetate (VAc) and vinyl versatate (VeoVa10). DFHMA is used as modified monomers. The composite surfactants of SR-10 and ANPEO-10 are used as the composite emulsifiers. The polymerization is initiated by potassium sulfate (KPS). The structure of latex film was characterized by Fourier transform infrared spectroscopy (FTIR). The latex films were tested by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and contact angle (CA) determinator. The particle size of the latex emulsion particle size is determined by the Zetatrac dynamic light scatter. Results show that the optimum condition of preparing the modified latex can be obtained, i.e. the amount of emulsifier is 4.0%; the mass ratio of SR-10 to ANPEO-10 is 1:1; the amount of initiator is 0.5%; the mass ratio ofVAc to VeoVa10 is 1:1; the amount of DFHMA is 5.0%. The appearance of the latex is blue and translucent with a small particle size. The conversion rate is more than 98%. Compared with the conventional latex, the hydrophobicity and heat resistance of the film have been improved. Moreover, the latex has good chemical stability and mechanical stability.

Keywords

Reactive emulsifier soap free emulsion polymerization modification latex

Introduction

Tertiary vinyl carbonate has a similar reactivity rate with vinyl acetate, which makes the chemical reaction of emulsion copolymerization between tertiary vinyl carbonate and vinyl acetate easier.^1–5 Since tertiary vinyl carbonate can improve the water resistance of vinyl acetate emulsions, there are some reports on tertiary vinegar copolymer latex, which is prepared with the efficient and environmentally friendly emulsifiers and new functional monomers.^6–11 Soap-free emulsion polymerization can be used to obtain emulsion particles with clean surface and uniform size. The way of bonding is combined with the polymer chain. This strong bonding makes the emulsifier molecules not migrate and desorb when the polymer is stored and used.^12–18 As a reactive emulsifier, SR-10 not only has environmental protection properties but also has the excellent emulsification ability, which do not reduce various physical properties (water resistance, adhesiveness, weather resistance and so on) imparted to the polymer film.

Fluorine-modified polymer latex refers to the addition of fluorine-containing monomers in the emulsion polymerization stage to participate in the emulsion polymerization together with the main reaction monomer, thereby introducing fluorine into the macromolecular chain of the polymer latex, which can protect other molecules and structures on the main carbon chain from the interference of external factors.^19–24 Shi et al reported that chlorotrifluoroethylene, vinyl acetate and vinyl tertiary carbonate were used as co-monomers, potassium persulfate was used as initiator, ammonium perfluorooctanoate was used as emulsifier and the deionized water was used as the reaction medium to prepare a series of stable fluoropolymer latexes with a solid content of about 33% and a latex particle size of 151–205 nm.²⁵ However, there are few reports on the modified VAc-VeoVa10 latex prepared by soap-free emulsion polymerization. In this work, the modified VAc-VeoVa10 latex prepared by soap-free emulsion polymerization, which dodecafluoroheptyl methacrylate (DFMA) is used as a modified monomer. The emphasis was put in the present work on factors which have on the properties of the resultant latex and the soap free emulsion polymerization. The specific synthetic pathway is shown in Scheme 1.

Scheme 1.

Synthetic pathway of modified latex.

Experimental

Materials

VAc and Veova10, which were chemically pure, were obtained from Aladdin reagents and Guangdong Wengjiang Reagent Co. Ltd (China), respectively. DFMA was purchased from Aladdin reagent. SR-10 and ANPEO-10, which were the industrial grade, were from Shenzhen Xiangrun New Materials Co. Ltd and Shanghai Macin Biochemical Technology Co. Ltd (China), respectively. KPS, which was chemically pure, was supplied by Shanghai United Company (China). The deionized water was distilled in our laboratory.

Preparation of modified latex

The semi-continuous seed emulsion polymerization was used to prepare the modified latex. The monomers used in the experiment included VAc, VeoVa10 and DFMA. The first step was to add 40 mL of the deionized water and 4% of the emulsifier, which was composed of 0.6 g of SR-10 and 0.6 g of ANPEO-10 in a 250 mL four-neck bottle. The bottle, which was equipped with a reflux condenser, a mechanical mixer and a constant pressure dripping funnel was heated in the water bath. The stirring speed was maintained at 200–250 r/min and the temperature increased to 80°C. The second step was to dissolve 0.15 g of KPS in 30.00 g of the deionized water to produce the initiator solution. 1.5 g of DFMA, 14.25 g of VAc and 14.25 g of VeoVa10 were used as mixed monomers. When the temperature reached 80°C, part of the initiator solution and the mixed monomers was added to the four-neck flask via two different the constant pressure dripping funnels within 15 min and then the temperature was kept constant for another 15 min under the condition of the moderate agitation. The third step was to drip the rest of the initiator solution and the mixed monomers via two different the constant pressure dripping funnels within more than 3 h. After the completion, the temperature is increased to 90°C and the reaction was maintained for another 45–60 min to further improve the monomer conversion rate. Finally, the mixture in the reactor was naturally cooled to about 40°C and the gel was separated by filtration with 100 mesh gauze. Thus, the modified latex was obtained. The typical recipe of preparing the modified latex was given in Table 1.

Table 1.

Recipe of preparing modified latex.

Ingredient	Weight/g	Ingredient	Weight/g
VAc	14.25	SR-10	0.6
VeoVa10	14.25	ANPEO-1	0.6
DFMA	1.5	DI-water	40
KPS	0.15

Characterizations

The molecular structure of the polymer was characterized by Fourier transform infrared spectroscopy (FTIR, Thermo-Nicolet-infrared AVATAR370,USA). The thermal stability of the polymer was performed from 40 to 60°C with a heating rate of 10°C/min under a nitrogen atmosphere with a thermogravimetric analyzer (TGA, Q50, and USA). The glass transition temperature of the polymer was carried out by differential scanning calorimetry (DSC, Q100, and USA). The contact angle (CA) of latex film was detected by the solid drop method on the data physical contact angle instrument (oca-20, Germany). The average particle size of the latex was tested by NanoBrook Omni (Brookhaven Instruments Corporation, USA). The water absorption was calculated according to formula (1):

X (%) = \frac{M_{0} - M_{1}}{M_{1}} \times 100 %

(1)

where X was the water absorption; M₀ and M₁ were the weight of wet film and dry film, respectively. The conversion rate was measured by gravimetric analysis, and the formula was calculated according to formula (2)

Y (%) = \frac{(\frac{W_{2} - W_{0}}{W_{1} - W_{0}}) - A}{B} \times 100 %

(2)

where Y was the conversion rate; W₂ was the weight of bottle and dry latex; W₁ was the weight of latex and bottle; W₀ was the weight of weighing bottle; A was the weight percentage of chemicals excluding monomers; B was the weight percent of the total monomer. The solids deposited on the walls of the four-necked flask and agitator and the latex remaining after filtration were collected. The coagulation rate was calculated according to formula (3)

Z (%) = \frac{M_{2}}{M_{3}} \times 100 %

(3)

where Z was the gel percentage; M₂ was the weight of coagulum was collected; M₃ was the weight of the total monomer in the recipe. The mechanical stability of the dispersion was investigated by centrifugation at 4000 rpm for 30 min via the centrifugation. 2 mL of an aqueous solution of CaCl₂ (0.5 wt %) was added to the test tube to 8 mL of latex. The stability of Ca²⁺ was measured by observing at room temperature for more than 48 h.

Results and discussion

FTIR and DSC of latex film

FTIR of the latex film is shown in Figure 1. 2963.5 cm⁻¹and 2875.9 cm⁻¹ are the characteristic stretching peaks of C-H (CH₃, CH₂). 1737.4 cm⁻¹ is the stretching vibration of C=O. The absorption peak at 1463.8 cm⁻¹ indicates that it is a characteristic peak of CH₃-CO-O-. The characteristic peak at 1370.6 cm⁻¹ is the C-C bending vibration peak. 1128.1 cm⁻¹ is the symmetrical absorption peak of C-O-C. The characteristic peak at 1020.3 cm⁻¹ is C-O vibration peak and the characteristic peak at 944.9 cm⁻¹ is the absorption peak of the C-F absorbing peak. Furthermore, there is no stretching vibration in the range of C=C of 1500-1700 cm⁻¹, which indicates that all the monomers are involved in the emulsion polymerization.

Figure 1.

FTIR of latex film.

Tg is the minimum temperature that the molecular chain segment in the polymer can move. Through the morphological change trend of polymer at different temperatures, the state of polymer within different temperature range is obtained, and the heat analysis curve is drawn based on the measured data. This temperature is the critical temperature of polymer from glass state to high-elastic state, and it is also the minimum temperature for free movement of non-crystal polymer molecular segments. The DSC curves of the resultant latex film were shown in Figure 2. From Figure 2, it can be found that the Tg of the polymer latex film is 8.79°C, which is different from those of VAc homopolymer (28–40°C) and VeoVa10 homopolymer (−3°C).²⁶ In addition, only one Tg is detected, which means that all monomers are introduced into the polymerization and the latex is the random copolymer.

Figure 2.

DSC of latex film.

Effect of emulsifier on emulsion polymerization and latex

In this study, the emulsification system for preparing latex is based on ANPEO-10 and SR-10. The influence of the emulsified system on emulsion polymerization and latex is studied. From Figure 3, it can be found that the amount of composite emulsifier has an effect on the conversion percentage and coagulation percentage. The latex is translucent and blue, which indicates that the composite emulsifier has a good emulsified capacity. With the increased amount of the emulsifier, the monomer conversion rate is increased first and then decreased. This may be caused by the fact that the increased amount of the emulsifier content will form more micelles, which provide more reaction centers. Thus, the conversion rate of mixed monomers continues to increase. When the amount of emulsifier is more than 4%, the coagulation percentage increased first and then decreased slightly but the monomer conversion rate is decreased. When the amount of emulsifier reaches a certain level, the emulsifier will cover the latex particles, which results in low monomer conversion and more coagulation. Thus, the optimum amount of emulsifier is 4%. Furthermore, influences of different mass ratios of emulsifiers on the conversion rate and coagulation percentage is presented in Table 2. Table 2 shows that the monomer conversion rate is the maximum and the gel rate is correspondingly lower when the mass ratio of SR-10 to ANPEO-10 is 1:1. The average particle size and particle size distribution of the latex are given in Table 3. The results indicate that the particle size range of the emulsion is between 57.6 nm and 80.12 nm. As the amount of emulsifier is increased, the average particle size of the latex is decreased. This is mainly caused by the fact that the reaction site of emulsion polymerization is micelles. When the amount of emulsifier is increased, more micelles are formed, which is equivalent to increasing the reaction site. The number of monomers, which takes part in the reaction in the system, is certain. Thus, the number of monomers that can be obtained for each micelle is reduced. Hence, the length of the polymer chain becomes shorter and the particle size of the polymer becomes smaller. The PDI value indicates that the particle size distribution is correspondingly uniform.

Figure 3.

Effect of amount of emulsifier on conversion percentage and coagulation percentage.

Table 2.

Influence of mass ratio of emulsifier on emulsion polymerization.

m (SR-10):m (ANPEO-10)	3:1	2:1	1:1	1:2	1:3
Conversion percentage (%)	97.96	95.78	98	96.3	96.82
Coagulation percentage (%)	1.5	1.2	0.6	1.1	0.9

Table 3.

Particle size of latex.

Amount of emulsifier/%	Particle size/nm	PDI	Appearance of latex	Mechanical stability	Ca²⁺ stability
2	80.12	0.099	Blue translucent	√	√
4	79.96	0.120	Blue translucent	√	√
6	71.52	0.098	Blue translucent	√	√
7	67.81	0.140	Blue translucent	√	√
8	57.6	0.153	Blue translucent	√	√

√ stands for stable latex.

Influence of amount of initiator on conversion percentage and coagulation percentage

The influence of initiator amount on conversion rate and coagulation percentage is given in Figure 4. In the early stage of polymerization, the amount of free radicals is few, the polymerization reaction is slow and the monomer conversion is low. With the increased amount of KPS, the conversion rate rises and the gel rate declines. When the amount of KPS is more than 0.5%, the conversion rate is gradually decreased and the coagulation rate is gently increased. The reasonable explanation is that when the initiator concentration is small, the number of free radicals entering the colloidal particles is relatively reduced. The monomer polymerization rate is gentle. The reaction is incomplete and the coagulation is large. With the continuous addition of the initiator, the number of active free radicals dissolved in water rises. The probability of entering the micelles for initiation improves, thus causing the conversion rate to improve. Too many initiators will lead to too many active centers and release more reaction heat in the system. The heat of reaction is not easy to eliminate, which makes the polymerization system be difficult to control and the stability of the emulsion decreases. At the same time, a small number of monomers outside the micelles will also initiate polymerization, which will easily collide with each other to produce coagulations and the conversion rate will also decrease. Furthermore, KPS is also an electrolyte in water solution. With the increase in electrolyte concentration, the thickness of double layer of emulsion particles decreases due to the salt effect of electrolyte, and the cohesion rate of the system increases. Therefore, the optimal amount of initiator in this study is 0.5%.

Figure 4.

Effect of amount of initiator on conversion percentage and coagulation percentage.

Determination of mass ratios of monomer

Usually, in the copolymer latex, hard monomers have a high applicable temperature for polymer dispersion, which can increase the hardness, water resistance and transparency of the latex film. The soft chain has a long side chain alkyl, which brings the polymer dispersing body lower temperature. Thus, the latex has good adhesion and liquidity. In the study, it can be found that when the amount of VAC is too large, the film of the resultant latex is very hard and even full of cracks. However, when the amount of VeoVa10 is too large, the film of the resultant is sticky. In the VAc-VeoVa10 copolymer, VAc belongs to the hard monomer because of its higher glass transition temperature. Besides, VeoVa10 belongs to the soft monomer because of its lower glass transition temperature. The hardness of the film can be improved because of its high glass transition temperature of VAc-VeoVa10 copolymer when moderate amount of VAc is used. However, the film tension is very strong because of because of its higher glass transition temperature of VAc-VeoVa10 copolymer when excessive amount of VAc is used, which causes that it is easier for the resultant film to be fragile and even full of cracks. In addition, influence of different mass ratios of monomers on the emulsion polymerization is shown in Table 4. Table 4 indicates that the conversion rate is the highest and the coagulation rate is the lowest when the mass ratio of monomers is 1:1. This may be caused by the fact that the spatial blocking effect generated by the side chain makes the molecular chain rotate difficultly. The molecular chain is so hard that the probability of colliding with other molecules is very low. Thus, the mass ratio of monomers is determined to be 1:1.

Table 4.

Effect of different mass ratios of monomers on emulsion polymerization.

m (VAc): m (VeoVa10)	3:1	2:1	1:1	1:2	1:3
Conversion rate/%	94.3	96.52	98	97.2	96.5
Coagulation rate/%	1.4	1.2	0.6	1.3	1.6

Contact angle of film

The contact angle of the liquid on the solid surface is an important parameter to evaluate the surface of hydrophobicity. The larger the contact angle is and the greater the hydrophobicity of the film is. The effect of the amount of DFMA on the contact angle of film is presented in Figure 5. Figure 5 shows that the contact angle is increased with the increased amount of DFMA when it is less than 5%. With the addition of DFMA, the contact angle of the latex film is increased, which is attributed to the introduction of fluoride monomer, the fluorinated groups of acrylate preferentially migrate and accumulate on the surface during the course of film forming. Thus, the surface energy of film is decreased, which leads to the increase of the hydrophobicity.²⁷ However, the contact angle is decreased with the increased amount of DFMA when it is more than 5%. This may be explained by the following fact. The stability of the latex will be destroyed by the excessive DFMA, which leads to less fluorine in the polymer. Fewer fluorinated groups of acrylate preferentially migrate and accumulate on the surface during the course of film forming, thus causing the contact angle to be decreased.

Figure 5.

Influence of amount of functional monomer on contact angle of film.

Thermal properties of latex film

The comparison of the thermal properties between the conventional latex and modified latex is given in the Figure 6. The thermal decomposition of the modified polymer latex film begins at 302.25 C, which is higher than that of the latex film without fluorine (294.81 C), and the heat resistance is improved. It demonstrates that when the fluorinated monomer is polymerized to the macromolecular chain, the thermal decomposition temperature of the latex film is improved owing to the existence of the C-F bond with high bond energy.

Figure 6.

TGA of latex film (A: conventional latex without fluorine; B: modified latex).

Conclusions

The modified latex is synthesized by soap free emulsion polymerization, which VAc and VeoVa10 are used as the main monomers and DFMA is used as modified monomers. The composite surfactants of SR-10 and ANPEO-10 are used as the composite emulsifiers and KPS was used as the initiator. FTIR confirms that DFMA has been successfully introduced into the modified latex. The optimum synthetic conditions are as follows: the amount of emulsifier is 4.0%; the mass ratio of SR-10 to ANPEO-10 is 1:1; the amount of initiator is 0.5%; the mass ratio of VAc to VeoVa10 is 1:1; the amount of DFMA is 5.0%. The appearance of the latex is blue and translucent with a small particle size. The conversion rate is more than 98%. Compared with the conventional latex, the hydrophobicity and heat resistance of the film have been improved. Moreover, the latex has good chemical stability and mechanical stability.

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) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Lijun Chen

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