| A good choice to reduce environment pollution is that food packaging films could be prepared with biodegradable biomass polymers instead of petroleum-derived plastics.Moreover,plastics packaging films always contained unhealthful residues including styrene,diester phthalate and heavy metal ions,so biopolymer packaging films are better for the safety of packaged food and for human health.The properties of packaging materials about proteins,polysaccharides and polyesters always have their own limitations which lead to corresponding defects for the films.Among the many ways to improve the film performances,preparation of multilayer films made by different materials is a promising research field.This paper aims to establish polylactic acid(PLA)based bilayer(multilayer)film systems with good mechanical and barrier properties,and to study the antimicrobial and antioxidant effect of active bilayer(multilayer)films embedded thymol and the controlled release and directed diffusion of thymol.Firstly,PLA film plasticized with different amount of polyethylene glycol(PEG2000)was prepared,and then the mechanical and barrier properties of plasticized PLA films with different PEG amount and different storage time were studied by tensile experiment and permeability/transmittance tests of oxygen,water and light to analyze the correlation between the properties and the conditions of different PEG additive amount and storage time.The reasons that films had corresponding properties were revealed by the microstructure of PLA under the conditions of different PEG additive amount and different storage time.Ultimately,PLA film with good tensile properties and moisture barrier property was prepared successfully.Secondly,new-type PLA-chitosan emulsion double film,PLA-zein/chitosan Pickering emulsion double film and PLA-gliadin bilayer/trilayer films were prepared respectively to establish PLA based double(multilayer)film systems.Experiments about tensile properties,water vapor permeability(WVP),oxygen permeability(OP)and light transmittance were carried out to study the mechanical and barrier properties of films;Scanning electron microscopy(SEM)and confocal laser scanning microscopy(CLSM)were used to reveal the film microstructure;Fourier transform infrared spectroscopy(FTIR)and differential scanning calorimetry(DSC)were used to analyze the interfacial chemical structure between two layers and the thermodynamic behavior of films,respectively.The results showed that film layers about zein/chitosan particles stabilized chitosan emulsion layer,zein/chitosan Pickering emulsion layer and gliadin layer were uniform and compact,and they had good oxygen barrier ability,too.Chitosan emulsion film had nice ultraviolet barrier ability,while zein/chitosan Pickering emulsion film and gliadin film had superior barrier ability,their ultraviolet transmittance was even as low as 0.02%.The combination between layer and layer of double films was firm which could be proved by continuous horizontal tensile curve,and could also be verified by SEM,FTIR and DSC.PLA layer endowed double(multilayer)films with good mechanical and moisture barrier properties,and emulsion layer and gliadin layer endowed double(multilayer)films with good oxygen and UV barrier ability.Thus,double(multilayer)films that complemented each layer's advantages were prepared.Last,thymol was added into the oil phase of zein/chitosan Pickering emulsion layer and gliadin layer of double(multilayer)films to prepare active films with antimicrobial and antioxidant ability.The results showed that double(multilayer)films embedded thymol had obvious antibacterial and antioxidant properties.In addition,different layer of double(multilayer)films had different antibacterial effect because PLA layer blocked the diffusion of thymol,so double films had the characteristic of directed diffusion of thymol.The release of thymol which was embedded in Pickering emulsion layer and gliadin layer achieved the goal of controlled release and the diffusion kinetics of thymol conformed to Fick's second law.In conclusion,establishing PLA based bilayer(multilayer)film systems with good mechanical and barrier properties is one effective approach to improve the properties of biopolymer films. |
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