Study On Preparation And Modification Of Degradable Polylactic Acid/Polyvinyl Alcohol Food Packaging Materials

Currently,developing degradable packaging materials substituted for petrol derived packaging materials has became the main focus in the packaging fild for achieving the sustainable society.Most of all,the packaging materials derived from Polylactic acid shows the advantages of excellent mechanical processing dynamics,good hydrophilicity and biocompatibility,endowing it a wide range of industrial applications and prospects in the field of biomedical and food packaging materials processing.Nonetheless,the essential imperfection of poor hydrophilicity and mechanical dynamic properties hinder its application prospect.Accordingly,the researches on the modification of PLA to overcome its inherent defects has become a hot research topic.With regard to the PLA,the polyvinyl alcohol（PVA）not only has the advantages of low cost and excellent biocompatibility,but also outstanding hydrophilicity properties.Furthermore,the high hydroxyl group concentration favors it a certain degree of compatibility with polyester by forming hydrogen bonding,effectively adjusting the hydrophilicity and mechanical strength of PLA.Based on the above refered properties,nano-silica particles were used to modify PLA by the in-situ copolymerization-blending methodto optimize the mechanical properties and heat resistance.The research results are as follows:（1）The influence of the reaction conditions on the synthesizing PLA from lactic acid monomer by the melt polycondensation method were explored,and the optimal reaction condition was established.The results showed that the optimum reaction conditions for the preparation of PLA by melt polycondensation were as follows:stannous chloride 0.5 wt.%,p-toluenesulfonic acid 0.25 wt.%,reaction temperature 170℃,reaction pressure 0.9Kpa,reaction time 12 hours.The viscosity average molecular weight and yield of PLA were the best.（2）A polyvinyl alcohol（PVA）/polylactic acid（PLA）composite bio-food packaging film was synthesized by physical blending in presence of polyethylene glycol（PEG）as a solubilizer.A series of characterization methods such as infrared spectroscopy（IR）,thermogravimetry（TG）and scanning electron microscopy（SEM）were used to systematically study the amount of solubilizer polyethylene glycol and the PLA/PVA blending ratio to absorb moisture and swell the composite film,degradation performance and thermal stability.The results show that:the PVA can significantly improve the compatibility of the PVA/PLA phase in the mixed membrane and the binding force between the phase interfaces with less than 1.0 wt.%PEG doped.However,excessive PEG caused the swelling rate and moisture absorption rate of the composite membrane decreased adversely.In addition,the introduction of PEG can significantly optimize the degradation performance of polylactic acid composites.When the content of polylactic acid in the mixed film was 20.0 wt.%,The degradation rate increased to 43%after 144 h.The thermal degradation temperature of the blended system was delayed from 287.9℃to 294.5℃.The permeability of the blended system has also been improved,with water vapor permeability（WVP）improved by about 14%,oxygen permeability（OP）improved by about 29%,and nitrogen permeability（NP）improved by about 28%.（3）Polylactic acid（PLA）/silica（SiO₂）composite membrane was synthesized by in-situ copolymerization-blending method with KH-550 as a coupling agent.A series of characterization methods such as infrared spectroscopy（IR）,differential scanning calorimetry（DSC）,x-ray diffraction（XRD）,thermogravimetry（TG）and scanning electron microscopy（SEM）were used to systematically explore the effect of loading and agglomeration of coupling agent on PLA/SiO₂ film formation Experimental mechanism.The results ensured that hydrogen bonds N-H???O and O-H???O formed in the PLA/SiO₂ composite.In addition,a double melting peak appears,and the cold crystallization temperature drops by as much as16℃with 0.3 wt.%SiO₂ content and the 0.1 wt.%KH-550 content is.On the other hand,increased coupling agent loading favor improved thermal stability of PLA/SiO₂ composite membrane,of which the thermal degradation temperature increased by nearly 20℃.