Degradation behavior of PPC/PBS as urea coating in soil

【Objectives】Biodegradable resin has been used as coating materials in manufacture of controlled / slowreleased fertilizers in order to prevent the persistent polyolefin resin from potential damage to soil. Different coating materials have different degradation behavior in soils,which influence the final application effects of coated fertilizers. Therefore,the study of the degradation characteristics of the coating materials will provide directly support for the correct application of coated fertilizers and the safety of soil ecological environment. 【Methods】Two kinds of biodegradable polymers,poly( propylene carbonate)( PPC) and poly( butylenes succinate)( PBS)( mass ratio,PPC ∶ PBS = 7 ∶ 5) were used as coating materials for urea in a burial soil experiment. Infrared spectroscopy( FTIR),scanning electron microscopy( SEM),and thermo-gravimetric analyses( TGA) were used toinvestigate the degradation characteristics of the PPC / PBS film. The degradation behavior of the coating material in soil including the weight loss,structure change,thermal properties and the surface morphology of polymer films was simulated. 【Results 】1) The FTIR analysis shows that the absorption strength of- OH around 3480 cm- 1increases firstly and then decreases,and the carbonyl absorption peaks are strengthen at 1849- 1543 cm- 1significantly. The asymmetry stretching vibration absorption peaks of the cumulative double bonds,- C = C = C and- C = C = O,appear at 2363 and 2331 cm- 1,respectively,indicating that the molecular chain of the coating materials is broken up firstly and then oxidized,forming new compounds. The random cleavage of the ester bonds are initiated in the biodegradable resin structure. 2) The thermo-gravimetric analysis shows that the thermal decomposition temperature of the degraded membrane moves to higher temperature compared to the initial,which demonstrates that the PPC resin in the membrane is degraded firstly in the soil and then the PBS section with larger proportion plays more inhibitory effect to reduce the further decomposition of PPC. 3) The SEM analysis shows that the PPC / PBS film is found suitable to the rugged surface of urea and combined tightly on the surface of urea. The initial surface is smooth with uniform density and no holes on it. As the buried time elongated,the membrane surface becomes rough and a few of micropores of 3-5 μm appears. With the degree of degradation increasing,the membrane structure becomes loose,and the micropores gradually grow up in the internal surface and then extend into the surrounding structure. The pore size increases to about 20 μm and the thickness of the film is thinned to 40-50 μm. However,the PPC / PBS coating still keeps the original frame,which shows that the degradation process of the membrane material needs a certain time. 4) The tested degradable films have good biological degradabilities with a low initial degradation rate. As the buried time extension,the sample biodegradation degrees increase gradually,about 5% of weight loss is found within five months,after that,the weight loss rate is significantly accelerated and reaches to 78% in October,decomposed completely within 12 months. 【Conclusions 】 The degradation behavior of PPC / PBS as urea coating material in soil is initiated on the coating surface caused by the cleavage of chemical structure of film, then followed by biodegradation of the ester bonds at random. The degradation rates of the coating materials are slow within the first five months,then increased dramatically within the following five months,and biodegraded completely in 12 months.