Properties and characterization of acrylic latex prepared with novel emulsifiers

Abstract

The polyacrylate latex has been successfully prepared by semicontinuous seeded emulsion polymerization with methyl methacrylate (MMA), butyl acrylate (BA), and acrylic acid (AA), which were initiated with potassium persulfate and emulsified with the novel green mixed surfactants of alkyl polyglycoside (APG1214) and disodium laureth sulfosuccinate (MES). The particle size of the latex was measured by Zetatrac dynamic light scattering detector. The structure of the latex was tested by Fourier-transform infrared spectroscopy. The film of latex was tested by differential scanning calorimetry and thermogravimetric analysis. Factors, which had an influence on the properties of the latex, were studied in detail. The optimum conditions for preparing the polyacrylate latex were as follows: the amount of emulsifiers was 7.0%, the mass ratio of APG1214 to MES was 3:1, the amount of the initiator was 0.7%, the mass ratio of MMA to BA was 1:1, and the amount of AA was 2.0%. In this case, the conversion of the mixed monomers was high and the mechanical and ionic stability of the latex was good.

Keywords

Polyacrylate latex green surfactants characterization

Introduction

Acrylate latex has lots of excellent performance, such as weather resistance and aging resistance. In the process of emulsion polymerization, surfactant is an important component to stabilize the latex. Usually, conventional surfactant is adsorbed on the surface of latex through physical function.^1,2 Generally, the combination of anionic and nonionic surfactant has a positive effect on the stability of the latex and the following mixed surfactants are used widely in the emulsion polymerization such as SDS/OP-10, SDBA/OP-10, and LAS/OP-10.^3

-6 However, with the increasing awareness of environmental protection in different countries, polyoxyethylene nonylphenyl ether emulsifiers such as OP-10 were seriously forbidden in industrial production area for their properties of biological accumulation and durability in the environment.^7

-10

Alkyl polyglycoside (APG1214) is a nonionic surfactant synthesized by fatty alcohol and glucose. In recent years, APG1214 has been widely used in many fields due to its low toxicity, excellent biodegradability, and other remarkable properties.^11
-13 In addition, disodium laureth sulfosuccinate (MES) has lower skin irritability than other anionic surfactants, and it also has easy-cleaning property, excellent hard water resistance, limpness, biodegradability, and so on. But the study of emulsion polymerization emulsified with the mixed surfactants of APG1214 and MES is rarely reported in the open literatures. In this work, the mixed surfactants APG1214 and MES were first used as the emulsifier to prepare the polyacrylate latex via the semicontinuous seeded emulsion polymerization. Butyl acrylate (BA) and methyl methacrylate (MMA) were used as the main monomers which were initiated with potassium persulfate (KPS). In addition, acrylic acid (AA) was introduced as the functional monomer, which could improve the adhesive force between the film and the substrate. The technological condition and preparing recipe are optimized and the structure of the latex is characterized and the properties of the latex are tested.

Experimental

Materials

MMA and BA, which are analytically pure, are purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (China) and are distilled under reduced pressure prior to polymerization. AA, which is chemically pure, is obtained from Shanghai Chemical Reagents Supply Procurement of Five Chemical Plants (China). MES, which is the industrial grade, is bought from Guangzhou Nanjia Chemical Technology Co., Ltd. (China). APG, which is industrial grade, is provided by Shandong Usolf Chemical Technology Co., Ltd. (China). KPS, which is chemically pure, is obtained from Shanghai United Initiators Co., Ltd. (China). The water used in the experiment is deionized.

Preparation of polyacrylate latex

The polyacrylate latex was prepared with the semicontinuous seeded emulsion polymerization. First, the mixed emulsifiers of 3.15 g of APG, 1.75 g of MES, and 37.20 g of deionized water were simultaneously added into a 250-ml four-necked flask equipped with a mechanical stirrer, a reflux condenser, and two dropping funnels. The stirrer was stirred at 200 r min⁻¹ to homogenize the mixture under 80°C in the water bath. Then, 10 wt% of initiator solution consisting of 0.21 g of KPS and 30.00 g of deionized water and 10 wt% mixed monomers consisting of 14.7 g of MMA, 14.7 g of BA, and 0.6 g of AA were added into the reactor dropwise under stirring within 15 min. The seeded latex was obtained when the reaction was kept for another 15 min. Second, the rest of the initiator solution and mixed monomers were added subsequently by two separate dropping funnels within 3.0 h. Then, the temperature was raised to 90°C and maintained for another 40 min to increase monomer conversion after the feed was completed. Finally, the latex was cooled and filtered. Thus, the polyacrylate latex was obtained.

Characterizations

The differential scanning calorimetry (DSC Q100, TA Instruments Corporation, USA) was applied to determine the glass transition temperature (T _g) of the film of the latex. The chemical structure of the latex films was analyzed by a Fourier-transform infrared (FTIR) spectrometric analyzer (Thermo Nicolet infrared AVATAR370, Waltham, Massachusetts, USA). The particle size of the latex was determined by the laser particle size analyzer (Malvern Zetasizer Nano S90, UK) at 25°C. The mechanical stability of the latex was tested by the centrifugal machine with the rotational speed of 3000 r min⁻¹ for 30 min. The calcium ion stability of the latex was tested with 16 ml of the latex and 4 ml of the calcium chloride whose concentration was 5 wt%.

Results and discussion

FTIR of film

The FTIR of the latex film is shown in Figure 1. The peaks at 2956 and 2874 cm⁻¹ are the characteristic stretching of C–H (CH₃, CH₂). The peak at 1726 cm⁻¹ is the stretching vibration of C=O. The peak at 1449 cm⁻¹ is the bending vibration of –CH₂–. The peak at 1385 cm⁻¹ is the flexural vibration of C–H in CH₃.¹⁴ The peak at 1236 cm⁻¹ is the absorption of C–O–C in the esters. The peak at 1144 cm⁻¹ is the vibration absorption of C–H in CH₃. The peak at 1065 cm⁻¹ is the vibration absorption of C–O.¹⁵ The peaks at 989 and 842 cm⁻¹ are the characteristic absorption of C–H in BA.¹⁴ There are no absorption peak in the wave number of 1600–1680 cm⁻¹ and no characteristic absorption peak of =C–H is detected in 3000–3100 cm⁻¹, which indicates that the C=C does not exist in the copolymer and all the monomers have participated in the reaction and the latex is synthesized successfully.¹⁶

Figure 1.

FTIR spectra of the film.

Glass transition temperature

T _g is the polymer conversion temperature from a glassy state to elastomeric state. The polymer appears hard and brittle when the T _g is high but it is more elastic and flexile when the T _g is low. DSC curve of the film of the latex is presented in Figure 2. It shows that the T _g of the film is 5.61°C, which is different from those of homopolymers of BA (−54°C) and MMA (100°C). This also directly confirms that the polyacrylate polymer latex has been prepared successfully. In addition, the latex has just only one T _g, which shows that the latex is a kind of random copolymer.^17,18

Figure 2.

DSC of the film.

Influence of amount of emulsifier on properties of latex

According to the mechanism of emulsion polymerization, the number of colloid particles can be calculated with the following equation⁸

N_{P} = X {(\frac{ρ}{μ})}^{\frac{2}{5}} {(a_{s} [S])}^{\frac{3}{5}}

where X is the constant, ρ is the generation rate of free radicals, μ is the volume growth rate of colloid particles, a_s is an area per emulsifier molecule on the surface of colloid particle, and S is the total concentration of emulsifier. The influence of the amount of the emulsifier on the properties of the latex is given in Table 1. Table 1 indicates that the particle size is decreased with increased amount of emulsifier, which can be explained by the fact that the number of colloid particle is increased with the increased amount of emulsifier and more micelles are generated. Thus, the particle size becomes smaller relatively.¹⁹ In addition, in Table 1, it can be seen that the monomer conversion rate is comparatively higher and the coagulum rate is lower when the amount of emulsifier is 7% through the trend of coagulum rate and conversion rate. At the same time, the deposition after high-speed rotation is lowest. Thus, the amount of emulsifier is 7% in this study.

Table 1.

Influence of amount of emulsifier on properties of latex.

Amount of emulsifier (%)	4	5	6	7	8	9
Coagulum rate (%)	0.012	0.015	0.015	0.008	0.017	0.006
Conversion rate (%)	96.47	92.57	98.19	97.15	93.33	96.25
Particle size (nm)	125.8	119.7	111.2	105.6	100.5	98.98
Calcium ion stability	×	√	√	√	√	√
Mechanical stability	▴	▴	▴	•	▴	▴

√: good calcium ion stability of the latex; ×: poor calcium ion stability of the latex; •: the emulsion has a little deposition after high-speed rotation; ▴: the emulsion has more deposition after high-speed rotation.

Effect of mass ratio of emulsifier on properties of latex

In this work, the mixed surfactants of APG1214 and MES are used as emulsifiers. APG1214 is a nonionic emulsifier, which has good chemical stability and can bring copolymer latex good ionic stability because of the electrostatic repulsion. However, the latex mechanical stability is poor when the nonionic emulsifier is adsorbed on the surface of latex. MES, which is an anionic emulsifier, has high efficiency of emulsification and produces a lot of micelles. Furthermore, it brings the negative charge to the outer layer of the latex by which the gathering of ion can be avoided. This is beneficial to the mechanical stability of the latex.²⁰ Both the absorption of emulsifier molecule onto the surface of latex and the stability of ion and sedimentation are improved by the mixed surfactants of APG1214 and MES.¹⁹ The coagulum percentage is lowest and conversion percentage is highest as shown in Table 2 when the mass ratio of APG1214 to MES is 3:1.

Table 2.

Effect of mass ratio of emulsifier on properties of latex.

Mass ratio of emulsifier	3:1	2:1	1:1	1:2	1:3
Coagulum rate (%)	0.006	0.008	0.009	0.007	0.007
Conversion rate (%)	94.77	91.30	93.87	89.49	93.04
Calcium ion stability	√	√	√	√	√
Mechanical stability	▴	▴	•	▴	▴

√: good calcium ion stability of the latex; •: the emulsion has a little deposition after high-speed rotation; ▴: the emulsion has more deposition after high-speed rotation.

Influence of amount of functional monomer on properties of latex

The influence of the amount of functional monomer on the properties of latex is shown in Table 3. Table 3 shows that the calcium ion stability of all the latexes is good and the latex has a little deposition after high-speed rotation when the amount of AA is ranged between 2% and 4%. When the amount of AA is 2%, the monomer conversion rate is highest and the coagulum generates lowly. In combination with the above observations and considering excessive amount of AA, which contains hydrophilic group –COOH and decreases the water resistance of latex,²⁰ the amount of AA is 2%.

Table 3.

Influence of amount of functional monomer on properties of latex.

Amount of AA (%)	2	4	6	8	10
Mechanical stability	•	•	▴	▴	▴
Calcium ion stability	√	√	√	√	√
Coagulum rate (%)	0.005	0.006	0.004	0.008	0.039
Conversion rate (%)	94.71	87.84	89.43	90.75	91.20

AA: acrylic acid; •: the emulsion has a little deposition after high-speed rotation; ▴: the emulsion has more deposition after high-speed rotation; √: good calcium ion stability of the latex.

Conclusions

The polyacrylate latex was prepared successfully by semicontinuous seeded emulsion polymerization of BA, MMA, and AA, which was emulsified with mixed surfactants of APG1214 and MES and initiated with KPS. The optimum conditions for preparing the polyacrylate latex were as follows: the amount of emulsifiers was 7.0%, the mass ratio of APG1214 to MES was 3:1, the amount of the initiator was 0.7%, the mass ratio of MMA to BA was 1:1, and the amount of AA was 2.0%. In this case, the conversion of the mixed monomers was high and the mechanical and ionic stability of the latex was good.

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.

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