Preparation And Characterization Of Polylactic Acid Based Porous Membranes For The Application Of Slow/Controlled Release Fertilizer

Chemical fertilizers have changed the traditional cultivation methods and greatly improved the productivity of crops since their invention.With the development of chemical fertilizers,numerous environmental problems,such as the massive loss of nutrients and the resulting environmental pollution,have impaired the sustainable development of agriculture.For this reason,the invention of slow/controlled release fertilizers can greatly ease the problems of nutrient loss and environmental pollution.However,the widespread use of petrochemical materials as coating materials in the field of slow/controlled release fertilizers has led to the disadvantages of high price and narrow application range of slow/controlled release fertilizers.Incomplete degradation of petrochemical materials also poses a risk of"secondary pollution"to the environment.As a natural polymer material,Polylactic acid?PLA?,as a natural polymer material,has the advantages of high strength,low cost,complete degradation and good biocompatibility.The purpose of this thesis is to develop low-cost,fully degradable coating materials for slow/controlled release fertilizers.Polylactic acid?PLA?is used as the membrane substrate,cellulose acetate?CA?or modified chitosan?CS?-O,O-lauroyl chitosan?OCS?is used as an additive,while PEG or nano-Si O₂is used as a porogen.PLA-based porous slow/controlled release membranes can be prepared with the method of immersion precipitation?NIPs?.The research content of this thesis is shown as follows.The section of Introduction first introduces the research background,development history,overseas and domestic research status for slow/controlled release fertilizers.It also introduces structure,properties and applications for polylactic acid?PLA?,cellulose acetate?CA?and chitosan?CS?.In addition,it also describes the research significance and main contents of this thesis.In the second chapter,PLA/CA porous membranes and the relative coated fertilizers are prepared and characterized.First,cellulose acetate?CA?with different contents is added into PLA solution.Then,the porogen,such polyethylene glycol?PEG?and nano-Si O₂,is added into the PLA/CA mixture.Last,membrane samples are prepared with the method of NIPs to investigate the effects of cellulose acetate content,porogen content and types on the performance of PLA/CA porous membranes.After that,several representative membrane solutions are selected to prepare coated slow/controlled release fertilizers,and releasing performance of the resulting fertilizers are calculated.Results show that the PLA/CA porous membranes prepared by the NIPs method have a sponge-like pore structure.After adding different amounts of cellulose acetate,the number of pores on the membranes'surface decreases,while the pore size has no obvious effect.The hydrophilicity,permeability and flexibility of the obtained PLA/CA porous membranes are significantly improved,but the mechanical strength is relatively reduced.As the mass fraction of nano-Si O₂and PEG increases,the resulting PLA/CA membranes have further improved in hydrophilicity,permeability and flexibility compared to membranes without any porogen,and the mechanical strength also continues to decrease.The release performance of coated fertilizers here conforms to the regular pattern of mathematical models reported in the references.Also,the releasing performance of fertilizers coated with membranes of PLA,cellulose acetate and porogen is more excellent than that of fertilizers coated with membranes of only PLA and cellulose acetate.The third chapter prepares and characterizes PLA/O,O-lauroyl chitosan?OCS?porous membranes..First,O,O-lauroyl chitosan is prepared with dodecanoyl chloride and chitosan as raw materials and methanesulfonic acid as a reaction medium.Fourier infrared spectroscopy and nuclear magnetic resonance hydrogen spectroscopy are used to characterize whether O,O-lauroyl chitosan has been successfully synthesized.Then,with O,O-lauroyl chitosan as additive,porogen polyethylene glycol or nano-Si O₂as porogen,PLA/OCS porous membranes are prepared by the NIPs method.It investigates the effect of O,O-lauroyl chitosan content and porogen type on the performance of PLA/OCS porous membranes.The result shows that the addition of O,O-lauroyl chitosan can cause spherical protrusions on the surface of PLA/OCS/PEG membranes.As the mass fraction of O,O-lauroyl chitosan increases,the area of spherulite protrusions expands and agglomerates into package-like protrusions.The distribution distance of the internal membrane pores is extended and the pore depth is gradually increased.The surfaces of PLA/OCS/Nano Si O₂membranes have micro-pores blocked by agglomerated nano-Si O₂.With the increment of O,O-lauroyl chitosan'mass fraction,the pore diameter doesn't change significantly,and the internal pore diameter of the membranes keeps shrinking.In terms of membranes'releasing performance,PLA/OCS/PEG membranes show a downward trend as the mass fraction of O,O-lauroyl chitosan increases,but there is no obvious pattern for PLA/OCS/Nano Si O₂membranes.The summary section is the final chapter.The results in the second and third chapters show that PLA/CA porous membranes with polyethylene glycol or nano-Si O₂can effectively improve the shortcomings of the polylactic acid,such as the long degradation cycle itself and the poor flexibility.The coated slow/controlled release fertilizers prepared by coating the urea with the above membranes still have regular release behavior.Only one drawback of these membranes is that the strength of the polylactic acid material itself is reduced accordingly.However,the PLA/OCS porous membranes can't effectively improve the defects of the polylactic acid material due to the slow phase inversion process,which still needs to be explored in the future.