2017 Asia-Pacific Engineering and Technology Conference (APETC 2017) ISBN: 978-1-60595-443-1

A Study on Preparation of Hygroscopicity Porous Polyacrylate Resin by Adding Porogenic Agent Hui Zhong, Hong-Jun Huang, Hong-Jing Wan and Xiao-mei Wang

ABSTRACT

Acrylic acid as the main raw material, N, N-methylene bisacrylamide as crosslinking agent, potassium persulfate as initiator, N, N'- dimethyl formamide (DMF), dimethyl sulfoxide and isopropanol as porogenic agents, porous sodium polyacrylate resin was synthesized by solution polymerization. The pore structure parameters were characterized by scanning electron microscope and mercury injection test, which was indicated that the pore size distribution was uniform, the pore was the relatively typical cylindrical hole. At the same time, it was also compared the moisture absorption property with silica gel and pure sodium polyacrylate resin, indicating that absorption performance of the porous resin was relatively excellent. Moreover, the pore size distribution of the porous resin was wider and the moisture absorption was best when the dimethyl sulfoxide was used as pore agent, which might contribute to the drying filed.

INTRODUCTION Sodium polyacrylate is a kind of super absorbent polymer resin, which is also used to the research of hygroscopic material owing to its strong adsorption capacity to water vapor molecules. However, the actual moisture absorption performance has not reached the level of the theory [1], so the current research is focused on how to improve the adsorption capacity, Harada N[2] mixed the sodium polyacrylate resin with hygroscopic inorganic salt, the moisture absorption capacity of the mixture increased. But more scholars believe that the hygroscopicity of polymer absorbent material depends more on its chemical and physical structures. Diamant Y[3] pointed out that the absorption process was mainly water diffusion process, and it was mainly affected by two factors, namely was that the micropore and the interaction between the resin and water molecules. At present now, scholars of domestic and overseas have done many related researches, mainly including the

Hui Zhong1, Hong-jun Huang, Hong-jing Wan and Xiao-mei Wang Department of Physics and Chemistry, Ordnance Engineering College, Hebei, Shijiazhuang 050003; China [email protected]

1066 research of the synthetic preparation process and the exploration of the microstructure of resin, hoping for introducing the porous structure to form the capillary channel, which might attribute to the diffusion and transportation of water molecular. At the same time, the specific surface area is increased to maximize exposure the hydrophilic groups on the surface of the resin, improving the hygroscopicity [4]. The methods for preparing porous materials include phase separation, porogenic agent method, foaming method and freeze drying technology. Shu Xiulin[5] prepared the porous γ-polyglutamic acid / chitosan by the method of graft polymerization and freeze drying technique, which had good water absorption, water retention and biodegradability; Zhang Chunxiao[6] synthesized porous sodium polyacrylate / urea resin with foaming method, which improved the moisture absorption capacity; Zhang Mushi[7] studied the surface morphology and the yield ratio of the porous phenolic resin microspheres by aqueous suspension polymerization with the mixed porogenic agents of toluene, dioctyl phthalate and octadecanol; Hongmei Joe[8] adopted the dispersion polymerization to prepare the monodisperse porous polymer microspheres with adding the toluene porogenic agent, and studied the influence of porogenic agent on the pore structure of the resin, which was applied to high performance liquid chromatography. But in actual, it is not easy to achieve control of the pore structure by foaming method due to its low polymerization temperature, and the common porogenic agents like toluene, n-hexane and some linear polymers is inappropriate in this system because of its excellent hydrophilic properties, meanwhile rinsing method is not a good means with regard to removing the porogenic agent in the late period on account of the tacky surface after absorbing water. For that reasons, the main experiment conditions were as follows: N, N '- dimethyl formadide(DMF), dimethyl sulfoxide and isopropanol were the porogenic agents, the porogenic agent was removed by heating evaporation, SEM and mercury porosimeter were used to analyze the micro structure, and adsorption isotherm was used to characterize the moisture absorption performance.

EXPERIMENT

Experimental Reagents and Instruments Acrylic acid , sodium hydroxide, N, N-methylene bisacrylamide(DMF), potassium sulfate, N, N'- dimethyl formadide, dimethyl sulfoxide, isopropanol(All the reagents are AR); SU-8010 scanning electron microscopy, Autopore IV 9510 mercury porosimeter, FD-1-50 vacuum freeze dryer, FW80 miniature high-speed universal grinder, DGG-101-2 electric heating blower, FC204 electronic balance. The Preparation of Porous Sodium Polyacrylate Resin The porous sodium polyacrylate was prepared by combination solution polymerization and porogenic agent method. And the procedures were as follows: A certain concentration of NaOH solution was prepared and then dropped slowly into acrylic acid in an ice water bath, obtaining a certain neutralization degree of acrylic acid sodium solution. DMF, potassium persulfate and porogenic agents were added into the mixed solution to act as the pre-polymerization solution. Then it took 5h to react at 70℃, later on, the elastic rubbery polymer was formed. Next, the polymer

1067 was shredded into small pieces to dry 24h under 120℃, crushing and sifting to set aside. Structure analysis and performance testing Morphology Analysis Scanning electron microscopy images were obtained from SU-8010 type field emission scanning electron microscopy. The dried resin samples were placed on the test specimens with conductive adhesive tape, and the surface morphology of the samples was observed after the spray treatment. Pore Structure Analysis The analysis of pore was test by the Autopore IV 9510 mercury porosimeter from the test center of Tsinghua University.

Moisture Absorption Test The process of moisture absorption was as follows: A certain amount of distilled water was added into the dryer at room temperature, at the same time the samples were placed on the surface of dishes, which were put into the dryer to test the moisture absorption, weighing regularly and making record. The moisture content at a certain time was calculate according to the formula 1-1: m −m Q = 2 1 (1-1) m1

Where Q is the moisture absorption for a time, g/g; m1 is quality of drying sample, g; m2 is the quality of the sample after moisture absorption, g.

RESULTS AND DISCUSSION SEM Analysis The structure of the resin would be affected greatly because of the molecular structure, molecular weight, polarity and other parameters of the porogen agent, therefore, it is necessary to select the corresponding porogenic agent according to the structure of resin matrix [9]. Due to the strong water absorption and the tacky surface of sodium polyacrylate, the traditional washing method to remove the porogen agent is inappropriate. Based on the above mentioned reasons, in this experiment, the porogenic agents are DMF, dimethyl sulfoxide and isopropanol, all whose common characteristics are good compatibility with the monomer to form a homogeneous and the high boiling point compared to the polymerization temperature. And the porous sodium polyacrylate resin was prepared by heating evaporation in this experiment. The pore forming process mainly is that the porogenic agent volatilizes to form pore in the drying stage of 120℃.

1068

Figure 1. Surface morphology of polyacrylate sodium resins (a. pure sodium polyacrylate resin, b. DMF, c. dimethyl sulfoxide d. isopropanol).

1069 Figure 1 is the surface morphology of porous polyacrylate sodium resins. In figure a, it was see that the surface of the pure polyacrylate sodium resin was plain, no pore structure. Figures b, c, d were the surface morphology of the resins with porogenic agents. From these figures, it can see that the pore was typical cylindrical hole whose diameter was about 10μm. Among them, the pore size of DMF resin was more uniform, the dimethyl sulfoxide resin existed both macropores and micropores, while the isopropanol resin had a comparatively small amount pores, mainly because of the low boiling point of isopropanol and volatilizing easily, the isopropanol volatilized partly in polymerization process, making the content of porogenic agent reduce later, so hole number reduced correspondingly. DMF and dimethyl sulfoxide had relatively high boiling point, they consumed very little during the polymerization process, and evaporation process was relatively slow when dried, obtaining relatively uniform pore size. The results and analysis of pore structure with mercury intrusion method Mercury is non wetting to most of the solid, so it’s needed to use the external force to overcome the surface tension of mercury and be pressed into the sample pore. The pressure of mercury and the pore size meet the formula: cosθ p = −4γ (2-1) d Where p is injection pressure (psia), θ is contact angle between mercury and the hole wall, γ is surface tension of mercury, d is pore size(nm). Based on the formula, it can be seen that mercury pressure and pore size are inversely proportional when the contact angle and surface tension is certain, the smaller pore size, the greater the external force, so it can be obtained the pore diameter and the pore volume recording mercury content under different pressures[10].

Figure 2. Mercury injection curve of sodium polyacrylate resin. Figure 2 is the mercury injection curve of porous sodium polyacrylate resin by mercury porosimetry. In the figure 2, curve a, b, c represented the mercury injection curves of the resins with DMF, dimethyl sulfoxide and isopropanol respectively. From Figure 2, the trend of three kinds of curves was roughly the same, mercury injection curve increased rapidly in pressure 0.4-1.2psia, corresponding 100 μm

1070 pore size. It was inferred that the emergence of 100μm pore size should be the clearance of resin particles by combination the images of scanning electron microscope figure. When the pressure increased, the increase of the amount of mercury decreased because of mercury entering micropore. In curve a, when the mercury injection pressure achieved at 100psia, mercury rate increased significantly, corresponding to the pore size within 1μm, and the total mercury content of curve a was greater than the curve b and c, indicating that the porosity of DMF resin was slightly higher than others. In curve b, when the mercury pressure was 30000psia, mercury content significantly increased, corresponding to the pore size of 5 nm, indicating that dimethyl sulfoxide of resin had micropore.

Moisture Absorption Performance

Figure 3. Moisture absorption curve of absorbent materials. 1. dimethyl sulfoxide 2. DMF 3. Isopropanol 4. pure sodium polyacrylate resin 5. silica gel

Figure 3 is the moisture absorption curve. It can be seen from the figure that the variation trend of three kinds of porogenic agents and the pure sodium polyacrylate resin was roughly similar. The absorption rate was fast at early stage and then got slowly at 150h, when the absorption time was long enough, the rate began increasing rapidly, indicating that the moisture absorption of the resins had not achieve equilibrium. While silica gel arrived at equilibrium state at 50h. At the same time, moisture absorption performance of three kinds of porogenic agents was significantly better than that of pure sodium polyacrylate resin, showing that porous resin with porogenic agents had better moisture absorption. The absorption properties of dimethyl sulfoxide was most prominent among them, it can reached at 2g/g, compared other porogenic agents resins remaining at 1.7g/g, while the silica gel was only 0.4g/g. The reason was that it may attribute to widely distributed pore size in the resin with dimethyl sulfoxide porogenic agent. The existence of the micropore increased specific surface area, the capillary condensation was more obvious, which improved the adsorption performance; at the same time, dimethyl

1071 sulfoxide possessed strong hygroscopicity, so the residual dimethyl sulfoxide contributed to the moisture absorption.

CONCLUSIONS Porous polyacrylate sodium resin was prepared by adding porogenic agents, DMF, dimethyl sulfoxide and isopropanol. The surface morphology was observed by scanning electron microscope, and the pore size distribution was tested by mercury analyzer. The results showed that the pore was cylindrical and the pore size distribution was about 10 μm. By comprehensive comparison of pore size and moisture absorption, pore size distribution of the resin with dimethyl sulfoxide was wider, and the moisture content was the highest about 2g/g, significantly exceeding the moisture content of silica gel. The result might contribute to expand the application field of the desiccant. However, it was founded that porous resin had not yet reached the maximum moisture content, meanwhile, the pore-forming mechanism and the relationship between pore structure and wettability were still relatively vague. So it also need to be find a better porogenic agent to prepare mesoporous and microporous structure, realizing the aim of controlling of gradient porous structure of sodium polyacrylate resin, laying a foundation for build a model between micro structure control and moisture absorption.

REFERENCES [1] Liu Jian. Study on moisture absorption function of acrylic resin [D]. Jilin University, 2009. [2] Harada N, Okamoto K, Kobayashi H, et al. Production of hydroscopic resin: JP,5105704[P]. 1993-04-27. [3] Diamant Y G M, Broutman L J. The effect of network structure on moisture absorption of epoxy resins[J]. Journal of Applied Polymer Science, 1981, 26(9):3015-3025. [4] Lei Guangcai. A study on the formation ,control and the pore-forming mechanism of acrylic acid superabsorbent resin porous microspheres structure [D]. Xiamen University, 2008. [5] Shu Xiu Lin, Shi Qingshan, Lin Xiaoping, et al. Study on preparation, characterization and properties of γ-chitosan porous scaffold materials [J]. Natural products research and development, 2013, 25 (4):514-518. [6] Zhang Chunxiao, Zhang Wanxi, Pan Zhenyuan, et al. Preparation and research of moisture absorption properties of sodium polyacrylate / urea porous materials [J]. Polymer materials science and engineering, 2009 (7):144-147. [7] Zhang Mushi. Effect of pore forming agent on the preparation of porous phenolic resin microspheres by suspension polymerization in aqueous solution [J]. Journal of Henan Institute of Science and Technology: Natural Science Edition, 2013, 41:68-72. [8] Qiao Hongmei. Monodisperse porous polymer microspheres and its application in reversed-phase chromatography[J] Fine chemicals, 2015, 32 (7): 721-725. [9] Yang Feng. Study on synthesis and application of super high crosslinked adsorption resin [D]. Nanjing University, 2011. [10] Huang Wenqiang. Adsorption and Separation Materials [M]. Chemical Industry Press, 2005.

1072