Synthesis Of Wasted Branch-based Superabsorbent Polymer And Its Urea Release Behavior

Superabsorbent polymer(SAP) is a kind of new functional material with three-dimensionalnetwork, which is able to absorb water as much as hundreds or thousands times of its ownweight, and possesses excellent water retaining capacity. SAP has been widely used inagriculture and horticulture fields. At present, masses of agro-industrial residues are removedby burning or discard as waste. Polysaccharide, which could be used as skeleton material withhydrophilic molecular grafted, is very rich in wasted plant branches. SAP based on wastedbranches would serve as a carrier, combining with fertilizer or nutrient needed by the growth ofplants, to prepare the material with the ability of water absorbing&retention and urea slow&controlled release. It is of great practical significance for improving agricultural economicbenefit and reusing disused resources.In this work, the synthetic conditions of SAP were optimized. The properties such as waterabsorption&retention, biodegradability, etc. were studied. The urea load capacity and releasebehavior of the sample obtained via immersing SAP in the urea solution were researched. Thecompound of SAP, sodium alginate and urea was cross-linked by calcium chloride to acquireanother sample, then its urea loading capacity and release behavior were studied as well.1. Synthesis of SAP based on mulberry branches and its water absorption&retention anddegradationThe SAP was synthesized by initiation with the hot free radical and polymerization inaqueous solution. The synthetic conditions were optimized as follow: the mass ratio of AA+AMand PMB was6g/g, AA/AM=4:1,60%neutralization degree of AA, the mass ratio of APS andPMB was0.05g/g, and the mass ratio of MBA and PMB was0.02g/g. The prepared samplepresented a maximum water absorbency of570g/g in deionized water. When the temperaturewas5°C,25°C and45°C, the water absorbency in deionized water was482g/g,533g/g and542g/g, respectively. In the pH range of4~10, the water absorbency was high, the maximum waterabsorbency reached to570g/g. The sample with0.8-1mm particle size achieved swellingequilibrium in20min, however, the sample with less0.1mm particle size could complete thisprocess only in less than2min. The sample reserved more than50%of the absorbed water at45°C for12h. When0.5g sample was added into100g soil, it could keep more than10%moisture content for the soil after the11th day. The degradation experiment indicated that the weight loss of the sample could come to above50%via immersing in the soil extracting solutionfor30days and33%via burring in natural soil. In view of these facts, the sample had excellentwater absorption&retention capacity and biodegradability.2. Preparation of urea-adsorbed sample and its urea load/release behaviorThe sample was attained via immersing SAP in the urea solution directly. The urea loadingpercentage increased along with the increase of the urea solution concentration. The release ofurea showed a typical first-order release behavior which was an initially high release ratefollowed by a rapid decline. The urea release rate of the sample with initially0.8-1mm particlesize was similar to others with initially smaller size, As time went on, the release rate of theformer slowed down. The higher the temperature was, the faster the urea was released. In theacidic environment, the initial urea release rate slowed down, while in basic environment, theurea release period was extended. When the soil water content kept for30%, the urea release ofthe sample could sustained more than10days, in which the SAP played a function of ureaslow-release.3. Preparation of urea-wrapped sample and its urea load/release behaviorTo fully take into account the properties of water absorption and urea loading, the syntheticconditions of the SAP/Ca-Alg/urea composite were optimized as follow: the dropper diameterwas2mm, the mass ratio of SAP and Alg was1:1, and the CaCl2concentration was0.5%. Withthe increasing of the urea concentration, both the sample diameter and urea loading rateincreased. Initially the urea release rate of the sample was fast. Then it slowed down. At thelater period of the urea release, urea release content happened to fluctuation in release medium.To compare with the release rate of the urea-adsorbed sample, the released urea of theurea-wrapped sample was slower within5days in the soil.