Study On The Degradation Behavior Of Polylactic Acid Catalyzed By Lewis Acid

The environmental pollution caused by the non-degradability of petroley-based polymer materials is becoming more and more serious.With the general enhancement of people's environmental awareness,the use of biodegradable polymer to replace petroley-based polymer materials is considered as an important method to alleviate the environmental pollution.Polylactic acid?PLA?has become a research hotspot due to its good biocompatibility and completely degradable properties in recent years.However,with the acceleration of social rhythm,in the fields of fast moving consumer goods?FMCG?,agricultural mulch and disposable medical machinery plastics,the contradiction between high scrap rate and low degradation efficiency of PLA products has become more and more acute,making the problem of garbage accumulation impossible to solve,which caused its solid waste to be recycled into the ecosystem for too long,limiting its application in this field.Therefore,it is necessary to accelerate the degradation rate of PLA products in some specific fields.Lewis acid with good catalytic activity is often used as a catalyst in the synthesis of PLA.Its coordination function can shield the hydrogen bonds between the molecular chains of PLA,so that the PLA molecular force is weakened.The influence of Lewis acid remaining in polymer system on the thermal stability of modified material has been reported for a long time,but its degradation behavior of polymers,especially PLA materials,has been rarely studied.In this paper,Lewis acid modified PLA material was prepared by melt blending method,the degradation behavior of material was studied in different environments.The degradation effects and comprehensive properties of the materials were analyzed by a variety of detection methods.Scanning electron microscope?SEM?was used to observe the change of sample morphology;combined with Fourier transform infrared?FTIR?spectroscopy,X-ray photoelectron spectrometer?XPS?and nuclear magnetic resonance spectroscopy?NMR?to analyze the mechanism of action.The main research content includes the following parts:1.Anhydrous zinc chloride?Zn Cl₂?,calcium chloride?Ca Cl₂?,lithium chloride?Li Cl?,ferric chloride?Fe Cl₃?and other metal chlorides and Lewis acids of different iron salts such as ferric sulfate?Fe₂?SO₄?₃?and ferric phosphate?Fe PO₄?were used to melt modify PLA in a torque rheometer to explore the changes in degradation behavior and comprehensive properties of modified materials and the mechanism of action of Lewis acids.The results show that Fe Cl₃ has the best degradation effect on PLA among several salts of Lewis acids;Fe Cl₃/PLA materials were prepared by solution method and melt blending method.The better degradation performance of modified PLA was due to the presence of Fe Cl₃ rather than the thermal effect in melting blending.As the amount of Fe Cl₃ added increases,the molecular weight of modified sample becomes smaller and the molecular weight distribution becomes wider.Fe Cl₃ has obvious regulation effect on the crystallization behavior of PLA,and its crystallinity changes with the content from 43.7%to 1.17%;In terms of degradation effect,the modified sample containing 2.95 phr Fe Cl₃ has the best degradation effect,and the degradation rate is more than 10 times higher than that of pure PLA.The degradation process follows the hydrolysis degradation mechanism,but the difference is that Fe³⁺bonds with the carbonyl oxygen in PLA and the adjacent carbon,forming stable Fe-O and Fe-C chemical bonds,which can weaken the ester bonds of PLA more than other types of complexation.2.In order to reduce the adverse effect of excessive degradation caused by adding Fe Cl₃ in the processing process,triphenyl phosphite?TPPi?modified Fe Cl₃/PLA materials were prepared by melting and blending of TPPi into Fe Cl₃/PLA modified samples,which were endowed with certain comprehensive mechanical properties.The experimental results show that the modified material has the best performance when added in the ratio of TPPi:Fe Cl₃=3:1.The material has certain comprehensive mechanical properties while maintaining good degradation performance,the tensile strength and bending strength reaches 43.78 MPa and 99.04MPa,respectively.The addition of TPPi significantly improves the thermal stability of Fe Cl₃/PLA materials,and also has a regulatory effect on the molecular weight and crystallization behavior.The chain extension effect of TPPi on PLA inhibits the excessive degradation of PLA by Fe Cl₃ to maintain certain degradation effect and mechanical property.3.Biodegradable polyadipic acid/Poly?butyleneadipate-co-terephthalate??PBAT?was introduced to blend with PLA to give the modified material a certain toughness,and the degradation behavior and comprehensive performance of PLA/PBAT were regulated by changing the content of Fe Cl₃.When the amount of Fe Cl₃ was 0.3 phr,the best comprehensive performance of modified materials was obtained.The addition of Fe Cl₃ not only accelerated the degradation of PLA/PBAT modified materials,but also promoted their compatibility.The degradation of the modified material by Fe Cl₃ first acts on the PBAT chain segment,while the residual PBAT in PLA system can increase the molecular chain movement ability and toughen the effect,but when the PBAT chain segment is completely degraded,the Fe Cl₃ will degrade the PLA,and the comprehensive performance of the material will still decline significantly in the later stage.The degradation of modified materials by Fe Cl₃includes complex reaction and bonding reaction.The first reaction is the complex reaction of the ester bond near the benzene ring,and the second is the bonding reaction to the fatty segment.