Preparation And Evaluation Of Polyurethane Based Controlled-release Material For Coated Urea

With the world's population continuously rising,demand for crop has kept increasing dramatically,but the area of cultivated land is declining,Thus increasing more fertilizers will inevitably be used to increase crop production.Therefore,the pollution of the fertilizer to the environment has attracted more and more attention.Traditional fertilizers have low utilization rates,slow/controlled release fertilizer is an effective method to improve the utilization rate of fertilizers.Most of the controlled release fertilizers on the market use petroleum-based coating materials,such as polyolefins and polyurethanes.But the controlled-release effect is not ideal for the controlled-release period is short.Therefore,this dissertation studied a new type of polyurethane-based controlled-release material for coated-urea,which can effectively improve the controlled-release period of slow/controlled release fertilizers,reducing the amount of slow/controlled release fertilizers.It has important practical significance for promoting green development of agriculture.In this dissertation,polyurethane coating and polyurethane coated urea were prepared base on polyoxypropylene triol BHD-540 and castor oil polyol as soft segment,and polymerized diphenylmethane diisocyanate(5305)as hard segment.Composite coating and composite coated urea were prepared base on the polyurethane and modified SiO2 filler.The details are as follows:1.Polyurethane coating and polyurethane coated urea was prepared by using castor oil,BHD-540,and 5305 as raw materials.The effects of castor oil content on polyurethane coating properties and polyurethane coated urea were studied.It was found that with the increase of castor oil content,the degree of hydrogenation of the coating decreased,and the degree of swelling increased.The tensile strength and hardness of the coating decreased,and the elongation at break increased.The water resistance of the coating increased,the water contact angle increased,and the water absorption rate decreased.The addition of BHD-540 can reduced the viscosity of the mixture composing of polyol and the 5305.TG and DTG graphs showed that the coating with mixed soft segments had good thermal stability.When the mass ratio of castor oil to BHD-540 was 1:4,using low viscosity liquid paraffin300#,adding no catalyst and intermittently feeding the raw materials,the release period of coated fertilizer with 4 wt%coating can reached 65 days,and that of coated fertilizer with 9 wt%coating can reached 120 days.2.Silica(SiO2)was modified using the silane coupling agent KH550 to prepare the modified SiO2/polyurethane composite coating and polyurethane composite coated urea.The results showed that compared with the viscosity of the mixture without modified SiO2,that with modified SiO2 increased.Composite coating with modified SiO2 showed an increased water resistance and tensile strength,while a decreased elongation at break.When the modified SiO2 content in the coating film layer is 2 wt%,the prepared coated urea with 4 wt%coating had a long controlled-release period ranging from 65 days to 96 days.3.The composite coating and the shell with 2 wt%modified SiO2 were chosen to hydrolyze in alkaline and acid solution,and deionized water.The coating carbamate index decreased and the water contact angle decreased,but the mass retention rate was higher than 99.26%.After hydrolysis in acid solution,the outer surface of the shell turned yellow and cracks appeared on the inner surface.After hydrolysis in alkaline solution,the outer surface of the shell turned black and cracks and holes appeared on the inner surface.No obvious changes were observed when the shell hydrolyzed in deionized water.