Study On Improving Water Vapor Barrier Property And Hydrophobic Modification Of Starch Films

In recent years,"white pollution and microplastic migration"caused by the extensive use of non-degradable plastic packaging products has become a worldwide environmental problem that threatens human health and social development.The development of degradable materials has become a research hotspot in the field of packaging today.Thanks to its biodegradability,renewability,biocompatibility and food-safety characteristics,starch has become one of the most promising biodegradable materials which has shown great potential in the field of food packaging.However,a large number of hydroxyl groups on the starch molecular chain make starch materials extremely hydrophilic.Thus,the mechanical properties of starch materials in high-humidity or water contact conditions will sharply decrease,which limits their applications.To expand the applications of environmental-friendly starch-based materials,typical sheets（commercially available laver and montmorillonite）were selected to improve the water vapor barrier property of starch films.A high-barrier composite starch film with"micro-nano barrier"and"maze"effects was prepared by adding those sheets.Then,a hydrophobic layer of epoxy soybean oil acrylate（AESO）was bridged on the hydrophilic starch substrate through a coupling agent,which successfully enhanced the surface hydrophobicity of the starch film.Finally,superhydrophobicity was achieved on the surface of the starch film for the first time by constructing a special surface morphology（bionic lotus leaf）.The primary contents and results are as follows:（1）Based on the tortuous path theory and the molecular interaction between protein and starch,a commercially available food-grade seaweed with a unique lamellar structure and rich in protein was selected as the enhancer and gas barrier in starch-based edible film.The relationship among the processing,microstructure,and properties was elucidated by investigating the viscosity,micromorphology,interfacial compatibility,mechanical properties,and barrier properties of the composite systems.In the cold mechanical blending method,it was found that the barrier properties of the composite films were improved due to the laver sheet barrier effect.Meanwhile,the laver fiber can enhance its mechanical properties by dispersing the stress exerted on the matrix of the composite film.While in the hot method,a tighter structure can further improve the barrier ability and mechanical properties of the composite system.The denser composites were obtained thanks to the heat-enhanced molecular interaction between starch and laver protein.At the same time,the laver cell wall was softened by the thermal treatment,which improves the compatibility with the starch matrix.Compared to the pure starch film,the water vapor transmission rate of the starch-protein-laver fiber composite film（hot method）added with 30%（w/w）laver decreased by 29.5%,and the contact angle value increased by 25.4%.Since both the enhancer and the matrix are of food origin,the film material has the potential to be safe for food packaging and as edible films.（2）Based on the magnetization driving force of the low-magnetic field,the magnetized filler montmorillonite（MMT@Fe₃O₄）was oriented in the starch matrix,and a new starch-based composite material with high barrier properties was developed.Modern analytical methods such as SEM,X-ray diffraction,and small-angle X-ray scattering were used to study the relationship between different loading concentrations（0～5%）of MMT@Fe₃O₄,magnetization field loading,the micromorphology,aggregated structure,mechanical properties,and barrier properties of composite starch films.The results showed that the Fe₃O₄grafting processing expanded the interlayer spacing of MMT,which facilitated stronger intermolecular interactions between starch chains and MMT.The magnetic field-oriented layered filler MMT@Fe₃O₄ worked as the gas diffusion wall with longer path,thereby effectively enhancing the water vapor barrier properties of the layered filler composite starch film（about 56.7%for sample H5.0）.（3）The interaction of coupling agent APTES on the interface between hydrophilic starch and lipophilic epoxidized soybean oil was studied based on the bidirectional cross-linking properties of coupling agent molecules.The permeation,hydrolysis,adsorption and deposition of APTES in the surface layer of starch film was studied.The relationship among the bidirectional bridging effect between the hydrophobic AESO molecular layer and the starch molecular layer,and its macroscopic water sensitivity of starch film was established.Firstly,the coupling agent molecule 3-aminopropyltriethoxysilane（APTES）is used as a"molecular bridge"to enhance the compatibility between hydrophilic and hydrophobic interfaces.APTES penetrated into the upper layer and self-assembled on the surface of starch film.Secondly,the AESO hydrophobic layer was cross-linked to the silanized hydrophilic starch substrate.A stable and effective waterproof starch film was successfully obtained.The APTES assembly rule on starch surface was determined by EDS and FTIR.It was found that APTES binds to starch molecules through hydrogen bonding interactions,and 1.6%APTES can be uniformly deposited on the surface of starch molecules,while when the concentration exceeds 1.6%,APTES will self-aggregate on the starch surface.Subsequently,APTES linked the AESO coating to the starch film via a Michael addition reaction,which improved the overall water resistance of the starch film.Adhesion and cross-cut tests showed that the interfacial bond strength increased by more than 4 times after treatment with 1.6%APTES.The starch films treated with APTES and AESO coatings remained intact after soaking in water for more than 2 h.After double coating treatment,the water vapor transmission rate of starch film decreased by 60.4%),The contact angle increased by 18.4%.（4）Inspired by lotus leaf,a superhydrophobic starch-based film（Contact angle>150°and Sliding angle<10°）was prepared for the first time by using poly（dimethylsiloxane）（PDMS）and ball-milled montmorillonite（MMT）coating.The effects of PDMS assistance and ball milling time on the MMT exfoliation during wet ball milling were investigated.The correlations relationship among the PDMS/MMT ratio,ball milling time,surface roughness of the coating,and surface wettability of the coated film were established.It was found that PDMS-assisted wet ball milling has an effective exfoliation effect on MMT.Besides,PDMS can not only act as both a hydrophobic modifier of MMT,but also as a binder between MMT and starch matrix.The ball milling time and the PDMS/MMT ratio are crucial factors of forming superhydrophobic surface.Without assistance of PDMS and enough milling time,micro-nano roughness cannot form;while too high PDMS/MMT ratio could submerge the micro-nano surface morphology.Only the samples 3h/3-7 and 6h/3-7 achieved the superhydrophobicity standard.Biodegradable superhydrophobic starch-based films have great application potential since they showed excellent anti-adhesion properties to various liquids.