Preparation Of Superabsorbent Polymers By Dispersion Polymerization Of Acrylic Acid In Saline Solution

Superabsorbent polymer(SAP)is a functional polymer material which has strong water absorption and excellent water retention.SAP has been widely used in agriculture,forestry,horticulture,health care,construction materials,oil exploitation and so on.In this paper,the poly(acrylic acid)superabsorbent particles were prepared by dispersion polymerization in saline solution.Firstly,using a soluble poly(sodium acrylate)(PAA-Na)instead of the primary polymers in the dispersion polymerization,the salting-out effect of different inorganic salts on the soluble poly(sodium acrylate)and three macromolecular dispersants,i.e.polyacrylamide(PAM),polyvinyl pyrrolidone(PVP)and carboxymethyl cellulose sodium(CMC-Na),were investigated.The results showed that NaCl aqueous solution and CMC-Na were appropriate as disperse medium and di'persant respectively for the dispersion polymerization of acrylic acid in salt solution.Secondly,the poly(acrylic acid)(PAA)superabsorbent particles were prepared by dispersion polymerization.The effects of reaction conditions on the particle size and water absorption were investigated.The results showed that the particle size of PAA absorbent were in the range of 200?500 ?m.The maximum water absorption reached 410 g/g and 72 g/g in distilled water and 0.9%NaCl solution,respectively.In addition,the swelling could reach equilibrium within 3min.The water-absorbing process of PAA followed the pseudo first-order kinetic model in distilled water or 0.9 wt%NaCl solution.The diffusion of distilled water in the PAA particles belonged to relaxation diffusion and the diffusion behavior was under the control of relaxation rate of polymer chains;however,the diffusion of 0.9 wt%NaCl solution in the PAA particles belonged to non-Fickian diffusion,and the diffusion behavior was controlled by the diffusion rate of water molecules and the relaxation rate of polymer chains.The PAA particles had a superior reusability and water retention capacity.The water-lossing process accorded with the zero-order kinetic model.Then,the dispersion polymerization kinetics of acrylic acid in saline solution was studied.The content of acrylic acid in the sample was measured by bromize-titration,then the conversion of acrylic acid and polymerization reaction rate of the dispersion polymerization were decided.The results showed that the conversion versus time curve was "S" type;the polymerization reaction was moderate in the early and late reaction stages,while it became faster in the middle stage.The polymerization rate increased with increasing the initiator concentration[I],the initial monomer concentration[M]and the reaction temperature,but decreased with the dispersant concentration[S].The kinetics relation of the dispersion polymerization was Rp?[I]1.96[M]2.45[S]-0 69,and the apparent activation energy was 52.0 kJ/mol.Finally,in order to improve the salt-tolerance of poly(acrylic acid)super absorbent polymer,the non-ionic monomer acrylamide(AM)was introduced into the dispersion polymerization.The copolymerization of acrylic acid and acrylamide was confirmed by infrared spectroscopy and thermal gravimetric analysis,and the copolymer(PAA-co-AM)particles were obtained.The results of water absorption of PAA-co-AM particles showed that the water absorption in 0.9 wt%NaCl solution was increased but the water absorption in distilled water was gradually reduced with increasing the amount of acrylamide.It indicated that the salt-tolerance of SAP was improved.