Salt-resistance Superabsorbent Polymer’s Synthesis And Application In Slow-release Fertilizer

Superabsorbent(SAP)as known as new polymer materials that can absorb hundreds or even thousands of times water of its own quality,has been widely used in hygienic,medical,agriculture,water saving,wastewater treatment and other fields.However,the decrease in repeating water absorbency(RWA)and lower salt-tolerance retarded the widespread utilization of SAP.In this paper,carboxymethyl cellulose(CMC)was used as raw material with Epichlorohydrin(ECH)as crosslinking agent in NaOH solution to prepare SAP.The effects of CMC’s molecular weight,CMC concentration and NaOH concentration on the water absorption rate of the synthesized products were discussed.Ft-IR,TG,13C-NMR and low temperature DSC were used to characterize the microstructure and study the main influence factor of WA and mechanism of cations’ influence SAP’s absorption.At last,the waterabsorbing and water-retaining slow release fertilizer was prepared with stearic acid and SAP as first and second coating layer respectively and gypsum urea as fertilizer core.The main results and conclusions are as follows:1.The synthesis product of carboxymethyl cellulose with low molecular weight has higher water absorbency(WA)and better performance of RWA with 5%CMC,3%NaOH and 5%ECH.Using the same molecular weight CMC,the WA of the synthetic product decreases with the increase of CMC concentration and the decrease of NaOH concentration.2.The first WA decreased with the increase of cations’ valence under the same concentration of salt solution.After repeating water absorption for 7 times,the WA of SAP CCMC53 decreased from 969g/g to 200g/g in deionized water,from 68.8g/g to 39.8g/g in 0.1mol/L NaCl,from 30.7g/g to 19.8g/g in 0.1mol/L CaCl2,from 296g/g to 112g/g in saturated Ca(OH)2 solution,Thus,CCMC53 showed good salt-resistance and repeated water absorption performance.3.It’s discovered that in the process of absorbing and swelling,segments with lower crosslinking density would break the linkage with the rest of SAP and dissolve in solution,causing the loss of carboxyl group.When the solution contains Ca2+ and A13+,Ca2+ would combine with carboxyl by bidentate chelating while Al3+ would combine with carboxyl by bidentate bridging.And these binding would retain the carboxyl,but after combining with multivalent cations,SAP’ s hydrophilicity would decline.3.Gypsum urea was prepared by solution crystallization with urea and gypsum to improve the slow release performance of urea.Stearic acid and SAP were used as the coating layer to prepare a slow-release nitrogen fertilizer with nitrogen content of 18.81%and WA of 23.40g/g under normal pressure.Total nitrogen release in water for 28 days was 85.27%.With gypsum urea and potassium dihydrogen phosphate as the core of fertilizer,stearic acid and SAP as the double-layer coating,a slow-release compound fertilizer with a nitrogen content of 14.89%,a phosphorus content of 3.85%,a potassium content of 4.73%and WA of 23.92g/g under normal pressure was prepared.Total nutrient release in water for 28 days is Nitrogen 83.12%,Phosphorus 75.82%and potassium 90.1%.