Preparation and Characterization of colloidal pure polyacrylate Read more...

a b s t r a c t

A series of acrylic polymer colloids were prepared via semi-continuous seeded emulsion polymerization of BA and MMA in water phase when OP-10 and AIBI is used to be emulsifier and initiator, respectively. FTIR spectrum identifies the formation of copolymers of P (MMA-co-BA). DSC confirms that the colloid is a kind of random copolymer and the consistency among the chain segment is fairly good. The emulsion polymerization conditions of preparing acrylic polymer colloid are optimized. Results show that the conversion rate is high and the coagulum is low and the particle size of the acrylic polymer colloids is small when the amount of AIBI is 0.75 g. The polymerization temperature is 70 ◦C, which is lower than the one that the emulsion polymerization is initiated with the persulfate. 

 Introdution-

Acrylic polymer colloids play an important role in many industrial processes such as the production of synthetic rubber, surface coatings, adhesives, as additives in paper, in textiles and many other products. At present, emulsion polymerization of butyl acrylate (BA) and methyl methacrylate (MMA) are one of the most common methods for preparing acrylic polymer colloids. Emulsion polymerization involves dispersion of a relatively water-insoluble monomer (e.g. BA and MMA) in water at the reaction temperature with the aid of surfactants, followed by addition of the persulfate initiator solution [1]. However, the oxygen free radicals, which are induced by the decomposition of persulfate initiator, have strong hydrogen abstraction ability. Furthermore, pH value of the reaction system is decreased because of the ionization reaction of persulfate initiator during the course of emulsion polymerization. These have a negative influence on the stability of the acrylic polymer colloids [2]. A simple, promising way to overcome such a drawback can be realized by using water-soluble azo initiator, which is prepared via introducing the hydrophilic groups based on the oil-soluble azo initiator. 

 The water-soluble azo initiator has high initiation rate. No induced decomposition reaction takes place in the course of initiating the vinyl monomers. Furthermore, the residual water-soluble azo initiator has no effect on the stability of polymer colloids ∗ Corresponding author. Tel.: +86 18968048502. E-mail address: این آدرس ایمیل توسط spambots حفاظت می شود. برای دیدن شما نیاز به جاوا اسکریپت دارید (L. Chen). [3]. There are some reports on using the water-soluble azo initiator to prepare polymer colloids. Yang et al. [4] reported a facile, green, and tunable method to functionalize carbon nanotubes with water-soluble azo initiators by one-step free radical addition. Luna-Xavier et al. [5] synthesized and characterized silica/poly (methyl methacrylate) nanocomposite latex particles through emulsion polymerization using a cationic azo initiator. Gu et al. [2] studied the inverse emulsion polymerization of sodium acrylate via water-soluble azo initiators. Zhang et al. [3] reported the homopolymerization of dimethyldiallylammonium chloride by using water-soluble azo initiators. However, the reported researches are focused on the homopolymer. To our knowledge, there is no other example, in the open literature, of using the water-soluble azo initiator to prepare the copolymer such as acrylic polymer colloids. In the present paper, using the water-soluble azo initiator such as 2,2-azobis[2-(2-imidazolin-2- yl)propane] dihydrochloride (AIBI, Scheme 1), we would like to report the convenient method to synthesize an acrylic polymer colloid. The emphasis is put in the present work on the influence of the water-soluble azo initiator on the emulsion polymerization and properties of the acrylic polymer colloids.

2. Experimental 

2.1. Materials

BA and MMA were obtained from Shanghai Chemical Reagents Supply Procurement of Five Chemical Plants (China) and were distilled under reduced pressure prior to polymerization.

AIBI was a gift from Tangshan Chenhong Industrial Co. Ltd. Potassium persulfate (KPS) was obtained from the Second Chemical Reagent Factory in Yixin (China). Sodium dodecyl benzene sulfonate (SDBS) was supplied by Shanghai Yingpeng Chemical Reagent Co., Ltd. (China). Polyethylene glycol mono-nonyl phenyl ether (OP-10) was obtained from Shanghai Minchen Chemical Co., Ltd. (China). The water used in this experiment was distilled followed by deionization. 

2.2. Emulsion polymerization 

 The emulsion polymerization was carried out in a 250 ml fourneck flask equipped with reflux condenser, mechanical stirrer, dropping funnels and heated with the water bath. A typical procedure for the emulsion polymerization is presented in Table 1. First, OP-10 was introduced into the four-neck flask and charged with 100.00 g of de-ionized water. The flask was then placed in water bath at the constant temperature of 70 ◦C with a stirring rate of 200 rpm. After complete dissolution of the emulsifier in de-ionized water, the mixture of 1.40 g of MMA and 1.60 g of BA and AIBI aqueous solution (0.15 g of AIBI was dissolved in 6.00 g de-ionized water) were dropped into the reactor within 15 min simultaneously. And the reaction maintained for another 15 min. Thus, the polyacrylate seed latex particles were obtained. Then the mixture of the residual monomer of 12.60 g MMA and 14.40 g BA in the dropping funnel was fed into the reactor containing seed latex under starved-feed addition. Simultaneously, AIBI aqueous solution (0.50 g of AIBI was dissolved in 24.00 g de-ionized water) in the dropping funnel was fed into the reactor with an appropriate dropping rate to initiate the polymerization of mixed monomers at the constant temperature of 70 ◦C. When the mixed monomers and AIBI solution were fed completely, the temperature was kept at 70 ◦C for another 30 min. The latex was then cooled to about 40 ◦C, and NH4OH (25 wt.%) was added to adjust the pH to about 8.0. Finally, the mixture in the flask was cooled and filtered. Thus, acrylic polymer colloid was obtained.

 2.3. Characterizations

 Fourier transform infrared (FTIR) spectrometric analyzer (Thermo Nicolet AVATAR370, USA) was used to analyze the chemical structures of the latex films. The particle size of the latexes was determined by Zetatrac dynamic light scattering detector (Microtrac Limited Corporation, USA) at 25 ◦C. The power and the wavelength of the diode laser used in the dynamic light scattering measures were 3 mW and 780 nm, respectively. The coagulum rate was measured by collecting the solid deposited on the reactor walls and stirrer, and by the residual of filtered latex.

It is expressed as the weight of coagulum per total weight of monomer added.

Conversion rate was determined by the mass difference of a sample taken before and after evaporation of the liquid phase. The sample was dried completely, and the residual polymer was weighed. Conversion rate of the monomer was calculated according to the following equation [6]: X (%) = ((((W2 − W0)/(W1 − W0)) − A)/B) × 100%; where X is the conversion ratio; W0 is the weight of the weighing bottle; W1 is the weight of the latex and bottle; W2 is the weight of the dried latex and bottle; A is the weight percent of the total non-volatile ingredients in the recipe; and B is the weight percent of the total monomer in the recipe. The film of latex is obtained from coating the latex on the clean glass and drying at 80 ◦C under constant pressure.

 3. Results and discussion

3.1. FTIR of the film

Fig. 1 is FTIR of the film. In Fig. 1, 2957 cm−1 and 2873 cm−1 were the characteristic stretching peaks of C–H (CH3, CH2); 1731 cm−1 was stretching vibration peak of C O; 1455 cm−1 was distortion vibration of –COO–; 1163 cm−1 was the stretching vibration absorption peak of C–O–C; 957 cm−1 was the stretching vibration of C–C. 843 cm−1 was the stretching vibration absorption peak of C O in the acrylic group. The stretching vibration of C C disappeared within the range of 1500 cm−1 and 1700 cm−1. FTIR spectrum shows that BA and MMA are copolymerized to form the acrylic polymer colloid which is initiated with water-soluble AIBI.

3.2. Glass transition temperature

DSC is often used to determine Tg of the polymer emulsion. Tg can be obtained directly from the inflection temperature of DSC curve. Fig. 2 is DSC curve of the acrylic polymer colloid. The measured value of Tg is −4.47 ◦C. Tg of the acrylic polymer colloids is different from ones of homopolymer of BA and MMA, which also directly confirms that the acrylic polymer colloid has been prepared successfully. Besides, in Fig. 2, it can be seen that the latex has only one Tg, which shows that the latex is a kind of random copolymer and the consistency among the chain segment is fairly good.

3.3. Influence of amount of AIBI on coagulum and conversion rate

Influences of amount of AIBI on coagulum and conversion rate are shown in Fig. 3. In Fig. 3, it is found that the conversion rate of monomers is gradually increased with the increase of the amount of AIBI. The probability that the latex particles obtain free radicals is fewer when the amount of AIBI is small.

Thus, the number of effective latex particles, which takes part in the polymerization reaction, is fewer, and the polymerization rate is fewer. The ultimate conversion rate is lower after the specific reaction time. When the amount of AIBI is increased, the probability that the particles of the polymer obtain free radicals is increased. Thus the reaction rate is quickened, and the ultimate conversion rate is raised. Fig. 3 also shows that the coagulum rate is decreased firstly, then is increased with the increase of the amount of AIBI. AIBI is a kind of cationic electrolyte. Part of AIBI will act as the electrolyte if the amount of AIBI is excessively large.

 

The concentration of the electrolyte in the system is increased. However, the increase of concentration of cation has a negative effect on the stability of the emulsion polymerization causing the coagulum rate to be increased. Therefore, AIBI is continuously dripped into the reaction system and the dripping rate is strictly controlled to keep the polymerization rate constant besides that the overall amount of AIBI is strictly controlled. In this experiment, the amount of AIBI is 0.75 g.

3.4. Effect of amount of AIBI on particle size of acrylic polymer colloid

Influence of the amount of AIBI on the particle size of acrylic polymer colloid is given in Fig. 4. Fig. 4 shows that the particle size of colloid is decreased with the increase of the amount of AIBI when it is less than 0.75 g. However, the particle size of the colloid is increased with the increase of the amount of AIBI when it is more than 0.75 g. This phenomenon can be explained by the following facts. The concentration of AIBI is increased with the increase of AIBI and the concentration of the free radical in the water phase is also increased accordingly, thus causing the particle size of the colloid to decrease when the amount of AIBI is less than 0.75 g. However, the excessive AIBI accelerates the nucleation rate in the system, and the number of the droplet is increased when the amount of AIBI is more than 0.75 g. It is easy to collide and aggregate among the droplets. In addition, the polymerization rate is increased by the excessively high concentration of AIBI. The liberated heat of polymerization accelerates Brownian motion among the latex particles, and the probability of collision among the latex particles is increased, thus causing the particle size of colloid to increase.

3.5. Effect of mass ratio of OP-10 to SDBS

In this study, the mass ratio of OP-10 to SDBS is varied when the amount of emulsifiers is constant. Effect of the mass ratio of OP-10 to SDBS on the coagulum rate is given in Fig. 5. Fig. 5 shows that coagulum rate is decreased i.e. the stability of the emulsion polymerization is increased with the decrease of the amount of SDBS. This may be explained by the following facts. AIBI belongs to the cationic initiator, which can lead to charge neutrality with the anionic emulsifier SDBS. Thus, the actual amount of emulsifier is decreased. Only some part of the surface of the emulsion particles is covered with the molecules of emulsifiers. In this case, particles of the emulsion are easy to coalesce and the coagulum rate is increased. In addition, the formed stable colloid is demulsified to coagulate because of the charge neutrality. In our study, the stability of the emulsion polymerization is very high when no SDBS is added in the system.

3.6. Polymerization temperature

Influences of polymerization temperature on conversion and coagulum rate are shown in Fig. 6. Fig. 6 indicates that the conversion rate is increased but the stability of the emulsion polymerization is decreased with the increase of the polymerization temperature. In the experiment, although the stability of the emulsion polymerization is fairly good, the reaction velocity is very slow and the conversion rate is very low when the polymerization temperature is 55 ◦C. Fig. 6 also shows that the variation of the conversion rate is not obvious but the coagulum rate is increased greatly when the polymerization temperature is more than 70 ◦C. The excessive coagulum is not beneficial to the emulsion polymerization. This is mainly caused by the fact that the reaction velocity is quickened and the reaction heat cannot be transferred in time, thus aggravating the polymerization reaction when the reaction temperature is excessively high. In the study, the conversion rate is very high and the coagulum rate is very low when the reaction temperature is 70 ◦C. This polymerization temperature is lower than the one that the emulsion polymerization is initiated with the persulfate [1,7,8], which is helpful to save energy.

4. Conclusions

A series of acrylic polymer colloids is prepared via semicontinuous seeded emulsion polymerization of BA and MMA in water phase when OP-10 and AIBI is used to be emulsifier and initiator, respectively. The conversion rate is high and the coagulum rate is low when the amount of AIBI is 0.75 g. The particle size of the prepared acrylic polymer colloid is small. The polymerization temperature is 70 ◦C, which is lower than the one that the emulsion polymerization is initiated with the persulfate. The structure of the acrylic polymer colloid is confirmed with FTIR spectrum. DSC confirms that the colloid is a kind of random copolymer and the consistency among the chain segment is fairly good.

Acknowledgements

This work has been supported by the Science and Technology Department of Zhejiang Province under Grant No. 2010C31040. In addition, the financial support of Zhejiang Provincial Natural Science Foundation of China (No. Y4100152) and Zhejiang University of Technology Natural Science Foundation (No. 20100202) is gratefully acknowledged. The authors are also very grateful to Tangshan Chenhong Industrial Co. Ltd. for offering AIBI.