synthesis of cationic styrene-acrylic acid ester copolymer ......mixture of styrene, butyl acrylate,...

Journal of Bioresources and Bioproducts. 2016, 1(1): 36-41 Peer-Reviewed

www.Bioresources-Bioproducts.com 36

Synthesis of cationic styrene-acrylic acid ester copolymer emulsion for paper sizing

Xinlei Wua, Youjia Cuia, Yi Jinga*, and Joseph Mosselerb

a) Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing, 210037, Chinab) Department of Chemical Engineering & Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick,

Canada, E3B 5A3*Corresponding author: [email protected]

ABSTRACT

Surface sizing is an effective way to increase paper’s water-resistance and printability. The purpose of this study was to study synthesis process to develop an efficient cationic styrene-acrylic acid ester emulsion (SAE) for the surface sizing of paper. Dimethylaminoethyl methacrylate methyl chloride (DMC) was used as the cationic monomer, and cationic starch or native starch was used as the emulsion stabilizer to copolymerize with styrene and butyl acrylate. The results indicated that the SAE synthesized with cationic starch and DMC had a high cationic charge density and a high DMC conversion rate. Paper sized with the cationic SAE had higher surface strength and lower Cobb value than the paper sized with other surface sizing agents such as, anionic SAE, and cationic or oxidized starch. Scanning electron micrographs revealed that the paper sized with a combination of oxidized starch and cationic SAE had smoother surface morphology when compared to the paper sized with oxidized starch alone, or with oxidized starch and anionic SAE.

Keywords: Surface sizing agent; Synthesis; Polymer; Styrene; Butyl acrylate; Cationic modification; Paper

1. INTRODUCTION

Surface sizing is one of the most efficient approaches toimprove paper qualities and printability, as the chemicals added in the surface sizing process are completely retained on the paper, and nothing is in the process. Styrene-maleic anhydride (SMA), styrene-acrylic acid (SAA), alkyl ketene dimmer (AKD), styrene acrylic acid ester emulsion (SAE), and polyurethane (PU) are often used as size-press additives in combination with starch solutions. SAE has an advantage over other sizing agents. The film-forming properties of SAE are better than SMA, the cost of SAE is lower than PU, and SAE sizing is superior to SAA. Furthermore, it can readily form a water-resistant film without need for additional curing after drying. By contrast, alkylketene dimer (AKD) may require over 24 hours to achieve its full hydrophobic effect.1,2 In recent years, the styrene-acrylic acid ester copolymer (SAE) surface sizing agent has been widely used in the papermaking industry. Not only can it improve the water resistance of paper, but it can also increase the surface strength of paper.3, 4 However, SAE cannot bond effectively with fibers due to its anionic charge.5 Aluminum sulfate (papermaker’s alum) is often used to enhance the bond between the fiber and the SAE. Papermaker’s alum can assist in orienting SAE’s hydrophobic groups onto the surface of paper. To improve the bonding ability of SAE with fibers, SAE needs to be modified. At present, there are three methods to modify SAE to increase its bonding with

fibers.6 The first method is to improve the traditional emulsion synthesis. New methods of synthesis, such as seed emulsion polymerization, soap-free emulsion polymerization, and micro-polymerization, have been applied in the process of SAE synthesis. The second is by adding functional components during the synthetic process. The third is to change the anionic groups on the surface of SAE into cationic groups.7,8,9 Cationic starch and quaternary ammonium salts are often used as cationic modifiers. Due to quaternary ammonium salts maintain a cationic charge over a wide pH range and can be used in acid, alkaline or neutral sizing, they are widely adopted as cationic monomers. The goal of the present research is to synthesize a cationic SAE which is more efficient in sizing paper. Dimethylaminoethyl methacrylate methyl chloride (DMC) was used as the cationic monomer, and cationic starch or native starch was used as the emulsion stabilizer to copolymerize with styrene and butyl acrylate. The present work focuses on the effects of different surface sizing agents on paper properties. The morphologies of the paper sized with different sizing agents were observed by SEM.

2. EXPERIMENTAL

2.1 Materials Native starch, oxidized starch, and cationic starch were provided by Goldeastpaper Co. Ltd. (Zhenjiang, China). Styrene, acrylic acid, butyl acrylate, alkylphenol

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polyoxyethylene (OP-10), H2O2, FeSO4, NaHCO3, and DMC were obtained from Chemical Reagent Co. Ltd. (Nanjing, China). 2.2 Methods In a four-neck flask equipped with a thermometer and a mixer, native starch or cationic starch (5 g) was dispersed in distilled water (45 g). Then, the solution was slowly heated to 90°C at a stirring rate of 600 rpm and α-amylase (0.1 mL) was added and mixed at 90 °C for 10 min. Afterwards the temperature was lowered to 45 °C. In a separate container, a mixture of styrene, butyl acrylate, and acrylic acid (5:14:1) was mixed with different dosages of DMC. Then, the mixture in the container and alkylphenol polyoxyethylene (5 g) were added into the four-neck flask. The total amount of monomers (starch, styrene, butyl acrylate, and acrylic acid) was 250 g. The mixture in the four-neck flask was stirred at a rate of 1500 rpm for 30 minutes until a stable pre-emulsion was produced. 80% of the pre-emulsion was separated into a separate flask, leaving 20% in the four-neck flask. The initiator (FeSO4) was added to the four-neck flask, the temperature was raised to 80 oC and the solution was stirred at 600 rpm. When the emulsion color changed from white to light blue, the remaining 80% of the pre-emulsion and the initiator (H2O2) were slowly added in the four-neck flask over the course of 2 h, keeping the stir rate at 600 rpm and a constant temperature of 80 oC. Then, the mixture was cooled to 35 oC, and the pH was adjusted to 7.8, resulting in a blue, milky emulsion. The formed emulsion was kept in a cool dark place. 2.2.1 Characterization The apparent charge density of the prepared emulsion was measured with a Mütek Particle Charge Detector-03 (PCD-03; Mütek Analytic GmbH, Germany). The conversion rate of DMC was calculated using the following equation,

𝑑𝑑 = 𝑉𝑉𝑉𝑉𝑚𝑚

(1)

%10011

×=WM

VCη (2)

where d (mmol/g) is the colloidal particles’ apparent charge density, C (mol/L) is the molar concentration of the standard volumetric reagents, V (mL) is the volume of the standard volumetric reagent consumption, m (g) is mass of the polymer, M1 (mol/g) is the molar mass of DMC, and W1 (g) is the mass of DMC. The apparent viscosity (η) was measured with a NDJ-79 rotational viscometer (Shangyi; Shanghai, China) at 30°C with a #3 spindle and 60 rpm. The glass transition temperature (Tg) was measured using a differential scanning calorimeter (DSC). The molecular structures of the products

were determined by infrared spectroscopy (FT-IR). The surface morphology of the paper sized with the cationic sizing agent was observed using a scanning electron microscope (SEM). 3. RESULTS AND DISCUSSION 3.1 Effect of Emulsion Stabilizers and DMC on the Sizing Agent´s Properties DMC was used as the cationic monomer. Native starch or cationic starch was used as the emulsion stabilizer. The colloidal particle surface charge density is an important factor affecting emulsion stability and performance of the products.10 According to the Derjaguin-Landau-Verwey- Overbeek (DLVO) theory, the higher the absolute value of the surface charge density of the colloidal particles, the greater the mutual repulsion between particles becomes. The dosage of DMC affected not only the surface charge density of particles, but also the hydrophilic properties of the copolymer products. Table 1 shows the effects of different materials and dosages on the polymer properties. It can be seen that the viscosity of the product decreased as the amount of DMC increased, as DMC could build a bridge between the styrene and the butyl acrylate, which reduced the surface tension of the reaction system. Furthermore, with increasing DMC dosage, the charge density of the product gradually changed from anionic to cationic. For example, without DMC addition, the charge density of the product was -1.6665 mmoL/g; it changed to -0.6795 mmoL/g when 1% DMC was added; and to +0.0486 mmol/g with 4% DMC addition. The theoretical DMC conversion rate was calculated from the cationic charge density of the system. It can be found from Table 1 that when DMC was used in conjunction with native starch, the DMC conversion rate was very low. When the dosage of DMC was less than 4%, the theoretical conversion rate was difficult to calculate. It is evident that part of the DMC applied was necessary to neutralize the anionic charge of native starch. Table 1 also illustrates that as the dosage of DMC increases, the viscosity of the product decreases. When the dosage of DMC was 0%, the viscosity of the product was 98.0 mPa·s. When the dosage of DMC was 5%, the viscosity of the product was 28.1 mPa·s. The viscosity of the latter decreased by 71.3%. The reason was that the cationic starch itself had cationic groups and that the DMC could increase the cationic charge density of the product. With the charge density of the system becoming cationic, the repulsive forces among particles increased. Therefore, the viscosity of the product decreased.

When native starch or cationic starch was added, the product exhibited a weak negative electrical charge and charge density. This effect was attributed to ionization of acrylic acid, which produced anions and counteracted the




positive charge of the cationic starch. Because DMC was present as a quaternary ammonium salt, it provided a substantial amount of positive charge, depending on its dosage, which resulted in the cationic charge density increasing. In addition, as the dosage of DMC increased, its theoretical conversion rate was gradually reduced. Because DMC was the cationic monomer, the repulsion of the positive charge and its large steric effect causes the reactivity of polymerization to be less than for other monomers. When DMC was used in conjunction with cationic starch, to a certain percentage, the cationic effect was better than when either DMC or cationic starch were used alone. With the increase in the dosage of DMC, the cationic density of the product gradually increased. When the DMC dosage was 5% of the total dosage of monomers, the cationic charge density increased to 0.2060 mmol/g.

Table 1. Effect of emulsion stabilizers and DMC on the sizing agent’s properties (NS=natural starch; CS=cationic starch)

Raw Material proportion

Viscosity (mPa·s)

Charge Density

(mmoL·g-1)

Conversion Rate DMC (%)

NS 39.6 -1.6665 /

NS:DMC=99:1 37.3 -0.6795 /

NS:DMC=98:2 35.1 -0.3278 /

NS:DMC=97:3 31.8 -0.1103 /

NS:DMC=96:4 28.5 +0.0486 37.8

NS:DMC=95:5 28.0 +0.0559 35.4

CS 98.0 -0.0808 /

CS:DMC=99:1 87.2 +0.0752 67.2

CS:DMC=98:2 48.7 +0.1256 64.9

CS :DMC=97:3 33.8 +0.1546 58.1

CS :DMC=96:4 30.5 +0.1776 56.6

CS :DMC=95:5 28.1 +0.2060 49.0 3.2 Effect of Surface Sizing on Paper Properties The sizing results with the various sizing agent compositions are shown in Figure 1. It was found that aluminum sulfate had an impact on the sizing effect of the anionic SAE. Cobb sizing values decreased from 49.3 g/m2 to 39.2 g/m2 when aluminum sulfate was used with anionic

SAE. This was due to the multivalent cationic charge of the aluminum ion could cause the anionic SAE to absorb onto the anionic fiber surface. In general, the Cobb size value of paper gradually decreased as the dosage of DMC increased in the SAE. For example, the Cobb value decreased to the minimum of 31.4 g/m2 when the DMC dosage was 3% in the SAE. This was most likely a result of the presence of absorption sites on the surface of fibers. Anionic trash decreased with increasing aluminum addition, so cationic SAE was easily absorbed onto the anionic surface of fibers. However, if the DMC was in excess and exceeded the maximum absorption of fibers, it was lost in the white water and the paper’s Cobb sizing value increased. The reason for this observation is that the DMC itself is a hydrophilic monomer. The paper’s Cobb values with 100% cationic starch or oxidized starch was lower than that without a sizing agent. This can be attributed to the formation of a partially impermeable starch film coating.

Fig. 1 Effect of surface sizing on the paper’s Cobb size value

Table 2. Experimental Program for Sizing No. Composition of sizing agent

1 Anionic SAE (without aluminum sulfate)

2 Anionic SAE (with aluminum sulfate)

3 Cationic SAE (99% cationic starch+1% DMC)





8 100% cationic starch

9 100% oxidized starch

10 Without sizing agent

01020304050607080

1 2 3 4 5 6 7 8 9 10

Cob

b （

g/m

2）

Sample No.




Fig. 2 Effect of surface sizing on the surface strength of paper

The surface strength of paper depends on the bonding between the film of the surface agent and the fibers on the surface of paper. The density of the formed film is another factor affecting the surface strength of paper.11 Figure 2 shows that the surface strength of paper was increased by the cationic SAE compared to the anionic SAE. It was also found from Figure 2 that the cationic SAE synthesized by adding 3% DMC had a high surface strength, 400 cm/s. However, the surface strength of paper with the same dosage of anionic SAE was only 343.2 cm/s. Comparing sample No. 1 to sample No. 2, it is clear that aluminum sulfate was a key factor affecting the surface strength of paper. Without adding aluminum sulfate, the surface strength of paper was 265.3 cm/s. The main reason for this is that aluminum sulfate had a positive charge after hydrolysis, thus increasing the bond between anionic SAE and the paper surface. When compared to the anionic SAE, the surface strength of paper with cationic SAE was significantly higher because of the cationic SAE’s high cationic density. From Figure 2, it is clear that the surface strength of the paper sized with cationic starch or oxidized starch was lower than that with SAE, but higher than the blank sample. This was due to the weak bond between starch and fiber.

3.3 SEM Analyses of the Surface sized Paper The surface morphologies of the base paper sized by different sizing agents were observed by SEM. The sizing agents consisted of three different components: 100% oxidized starch, 95% oxidized starch with 5% anionic SAE, and 95% oxidized starch with 5% cationic SAE. Figure 3 shows the surface morphology of base paper. The surface morphology of paper sized by oxidized starch alone can be observed in Figure 4. Figures 5 and 6 display the surface morphology of paper sized with 95% oxidized plus 5%

anionic SAE, and 95% oxidized starch plus 5% cationic SAE, respectively.

Fig. 3 SEM image of the base paper

050

100150200250300350400450

1 2 3 4 5 6 7 8 9 10

surfa

ce st

reng

th(c

m/s

)

Sample No.




Fig. 4 SEM image of the papers sized with the oxidized starch alone

Fig. 5 SEM image of the paper sized with the oxidized starch and anionic SAE

Fig. 6 SEM image of the paper sized with the oxidized starch and cationic SAE Comparing Figure 3 with Figure 4, it can be observed that the surface of the paper was smooth and that there were many dents. A discontinuous film layer formed on the surface of paper, which could improve the surface strength and water resistance. However, the film layer was discontinuous because oxidation starch has a high viscosity and weak film-forming character. Figure 5 shows that the film on the paper surface was greatly improved and that most of the gaps between the fibers were filled. This indicated that the anionic SAE contained hydrophobic groups and had good fluidity. Aluminum salt could cause the hydrophobic membrane to spread onto the surface of

paper. It can be seen from Figure 6 that there was a layer of dense hydrophobic film on the surface of the paper. Compared with the anionic SAE, the cationic SAE had better film-forming performance. This observation may be attributed to the cationic sites of the SAE copolymer forming ionic bonds with the anionic fibers. Therefore, the paper’s water resistance, strength and bursting strength were greatly improved with the cationic SAE 3.4 Chemical Analyses of the Cationic SAE The chemical structure of the cationic SAE was characterized by FT-IR. The position of absorption bands confirmed the formation of cationic SAE. Figure 7 illustrates the IR absorption spectra of a representative cationic SAE. The peak at 3370 cm-1 can be attributed to the O-H stretching vibration of starch. The peak at 3027 cm-1 is the –CH3 characteristic absorption of the quaternary ammonium cations. Non-symmetric vibration of the methylene group was observed around 2927 cm-1. The peak at 1731 cm-1 was assigned to the C=O stretching vibration of the butyl acrylate. The aromatic ring vibration absorption of styrene is shown at 1602, 1494 and 1453 cm-1. The peak at 958 cm-1 is the -CH2-N + (CH3)3 characteristic absorption peak of the DMC. The peaks at 759 and 700 cm-1 show C-H flexural vibration of the mono-substituted benzene ring. The appearance of these adsorption peaks demonstrates the successful copolymerization of the primary monomers.

Fig. 7 FT-IR spectrum of the cationic SAE Figure 8 shows the chemical reactions in the synthesis of the cationic SAE. Styrene is a hard monomer and can copolymerize with acrylates or other vinyl monomers. It can increase the bond strength, water resistance, and transparency of copolymers. Butyl acrylate is a soft monomer. It can give rise to a low glass transition temperature (Tg) copolymer; also, it can improve the softness of the copolymer film. Acrylic acid is a modified monomer that can promote polymerization, speed up the




aggregation, and improve the stability of copolymers. The OP-10 is a non-ionic emulsifier.1 The emulsion’s solid content was 27.7%, the viscosity was 33.8 mPa·s, the cationic charge density was 0.1546 mmoL/g, and the glass transition temperature was 65.8°C.

Fig. 8 Chemical reactions in the synthesis of the cationic styrene–acrylic acid ester (SAE) 4. CONCLUSIONS A cationic styrene acrylic acid ester copolymer emulsion (SAE) was successfully fabricated and used for the surface sizing of paper. The results indicated that the cationic SAE synthesized with cationic starch and DMC had a high cationic charge density and a high DMC conversion rate. The dosage of cationic monomer had a significant influence on the paper sizing performance. The appropriate dosage of the DMC cationic monomer was 3% of the SAE polymer. Paper sized the the cationic SAE had higher surface strength and lower Cobb values than paper sized using other surface sizing agents, such as anionic SAE, cationic starch, or oxidized starch. The Cobb value of paper sized by cationic SAE was 31.4 g/m2, and the surface strength was 400 cm/s. Scanning electron micrographs illustrated that the paper sized with oxidized starch and cationic SAE had the best surface morphology when compared to papers sized by either oxidized starch alone or oxidized starch with anionic SAE. ACKNOWLEDGMENTS Financial support for this work was provided by the

National Natural Science Foundation of China (No.31370583) and Brand Specialty Construction First Stage Projects of Jiangsu Universities 2015. The article was partly presented at the 4th International Conference on Pulping, Papermaking and Biotechnology on Nov. 7-9, 2012 at Nanjing China. The organizer agreed for the contents to be submitted to Journal of Bioresources and Bioproducts.

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synthesis of cationic styrene-acrylic acid ester copolymer ......mixture of styrene, butyl acrylate,...

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