Degradation Behavior And Mechanism Of Poly (Lactic Acid) Based Composites

Sustainable development oriented by environment,energy,and economy is a considerable challenge for the current society.The application and development of petroleum-based plastics have caused serious environmental pollution and energy consumption.It is urgent and an inevitable trend for the sustainable development to replace petroleum-based plastics with renewable and biodegradable polymer materials.Poly(lactic acid)(PLA),as one of the most promising environmental and biodegradable materials,has been widely developed and applied in various fields.As considered from environment protection,PLA can be naturally degraded and transformed into carbon dioxide and water,which is beneficial to relieve the pressure of environmental pollution.In the view of energy consumption,the monomers for PLA can be derived from renewable biomass resources,which can effectively reduce the consumption of fossil energy.From an economic standpoint,solid waste of PLA materials can be biodegraded by landfill and/or composting,which can reduce the cost of waste management.However,the biodegradable rate of PLA is very slow,especially in natural soil.Therefore,some methods,such as blending with easily biodegradable biomass resources,are used to increase the biodegradation rate of PLA-based materials.Starch is one kind of renewable biomass resources with extensive resources and low price.Blending starch with PLA can increase the biodegradation rate of PLA-based composites,as well as reduces the production cost.The ester groups in the PLA molecular chains are susceptible to environmental conditions,such as humidity,temperature and ultraviolet,which can caused a breakdown of the ester groups.PLA-based materials will loss some performances,such as mechanical strength,transparency,toughness,and so on,leading to a reduce of their service life.The purpose of this thesis was focused on the degradation behavior of PLA-based composites,and disclosing the degradation mechanisms under different degradation conditions.These can provide theoretical and practical guidance for the processing,application,recycling and disposal of PLA-based materials.This thesis mainly studied influences of four different types of degradation on the performances of PLA-based composites.First,natural soil burial degradation of starch/PLA composites was investigated,which disclosed a fact that starch had an accelerated effect on the soil burial degradation of PLA matrix.The homogeneity of soil burial degradation for PLA-based materials were characterized by using IR imaging and XPS mapping techniques.Artificial weathering of PLA-based composites was carried out to analyze the different effects that were caused by starch and wood flour.Mathmatical models were established to describe the relationship between molecular weight/mechanical strength and weathering time,which could provide a theoretical basis for evaluating the environmental durability of materials.Thermal degradation kinetics in nitrogen and oxygen atmosphere were analyzed,and the thermal degradation mechanism was explored,which should give more important informations for the thermal processing and recycling of PLA materials.Hydrolytic degradation behavior and mechanism of starch/PLA composites were systematically studied.Mathmatical functions of the relationship between the mechanical strength of composites and hydrolysis time were established,which should provide more theoretical knowledge for the application of PLA materials in related fields and predicting their service lives.The specific research contents are as follows:(1)Degradation behavior and mechanism of starch/PLA composites were investigated in real soil environment by using various characterization techniques.The results indicated that the biodegradation rate of starch/PLA composites was higher than that of pure PLA.Starch played a key role in the composites that can be used as carbon source for the growth of microorganisms on the surface of composites.Weight loss and component analysis showed that the weight loss rate of PLA in the composites was higher than that of pure PLA,confirming that the biodegradation rate of PLA could be accelerated by starch.With the prolonged soil burial degradation,the glass transition temperature of starch/PLA composites exhibited an obviously decrease while it had a slight variation for PLA.The thermal stability of starch/PLA composites shifted towards to that of pure PLA when they were subjected to soil burial for the same time.The mechanical strengths of starch/PLA composites decreased abruptly,while the molecular weight of PLA did not show obvious decrease.This indicated the limited degradation at molecular level when the composites were subjected to short-term natural soil burial degradation.Modification of chemical structure characterized by XPS and IR indicated that the biodegradation of PLA occurred at the ester groups in PLA chains,and the breakdown of starch molecular took place through the cleavage of C-O linkages between glucose rings.The distribution uniformity of carboxyl group intensity and carbon atomic percent decreased with the soil burial time,reflecting that the biodegradation of starch/PLA composites was heterogenous.Moreover,the decrease in the uniformity of carbon atomic percent distribution was higher in the bulk of composites,indicating the bulk erosion mechanism of the soil burial degradation of starch/PLA composites.(2)The durability of sustainable PLA-based composites was studied by using artificial weathering method,and impact of two biomass materials(starch and wood flour)on the UV survivability was compared.During the artificial weathering,the molecular weights of all the samples were significantly decreased,which were caused by simultaneous degradation reactions of photolysis and hydrolysis.The average number of chain scissions followed the following order:PLA>starch/PLA>wood flour/PLA.Three different mathemathical functions were used to fit the relationship between molecular weight and artificial weathering time.The three linear fitting results showed that the rate constant of chain scissions followed the order:PLA>starch/PLA>wood flour/PLA.Thermal stability of the materials decreased with the artificial weathering time.PLA showed the largest decrease magnitude in the thermal stability,followed by starch/PLA composites,while wood flour/PLA composites only showed a slight decrease.As the artificial weathering time increased,the crystallinity of the materials first increased,followed by a decrease.However,the variation of crystallinity for wood flour/PLA composites was lower than the crystallinity change of starch/PLA composites.After artificial weathering,tensile strength of PLA monotonously decreased,while starch/PLA and wood flour/PLA composites exhibited a steady decrease in their tensile strength.All of the changes in the materials' properties,including molecular weight,chain scission number and rate,thermal stability,crystallinity,and tensile strength,demonstrated that starch and wood flour had a stabilizing effect on the weathering of PL A,and WF/PLA had a better weathering durability than starch/PLA composites.Finally,mathematical models were established to evaluate the relationship between the tensile strength of material and artificial weathering time.The result showed that the mathematical models for different materials were first-order expotential decay models.(3)Thermal degradation behavior and kinetics of starch/PLA composites in nitrogen and oxygen atmospheres were investigated by usingTG and TG-FTIR techniques.Meanwhile,effect of annealing temperature on the thermal degradation kinetic of starch/PLA composites was also studied.Thermal stability analysis indicated that oxygen could promote the thermal degradation of starch/PLA composites,which resulted in a reduction in the thermal stability of composites.At higher degradation temperature,the residual chars and some gaseous products were further cracked and turned into carbon dioxide under the oxygen condition.Annealing promoted the conformation rearrangement of PLA chains,which was found to be beneficial to weakn or even elimate the physical relaxation phenomenon of starch/PLA composites.In addition,the higher the annealing temperature was,the more obvious the weaken effect.Annealing could promote the crystallization of PLA at annealing temperatures that were higher than the Tg of PLA.The crystallinity of starch/PLA composites was increased with the annealing temperatures.During the annealing process,the crystal structure of PLA was translated from unstable ?'-crystal to stable a-crystal through molecular chain rearrangement.Thermal degradation kinetic under different conditions were analyzed by using the iso-conversional Flynn-Wall-Ozawa(FWO)model.The results showed that the apparent activation energy of thermal degradation for starch/PLA composites was lower in the oxygen than that in the nitrogen,and the activation energy did not show obvious dependence on the annealing temperature.As the thermal decompose conversion degree increased,the activation energy of starch/PLA composites gradually increased,indicated that the thermal degradation of the composites was a complex process,which may include at least two different pyrolysis mechanisms.The thermal decomposition process was a competitive process between the intramolecular transesterification and cis-elimination reactions.At higher conversion degree,this competitive process transformed into cis-elimination reaction.Evolution of gaseous products of starch/PLA composites under nitrogen and oxygen atmospheres was analyzed.The results showed that the main gaseous products of the composites in different atmospheres did not show any difference.The main gaseous products of the composites mainly contained lactide,cyclic oligomers,aldehydes,CO2,CO,H2O and CH4.However,the yield of CO2 for the thermo-oxidative degradation was higher than that of the thermal degradation in nitrogen.This was due to the fact that the generated residual chars,bio-oil,and part of the gaseous products can be further oxidized and transformed into CO2 at higher temperatures.(4)Hydrolytic degradation behavior of starch/PLA composites at different immersion temperatures(30?,40?,and 50?)was comprehensively studied.Water uptake of starch/PLA composites increased with the immersion time,and the water uptake increase faster at higher immersion temperature.Contrary,the water uptake of PLA showed a Fickian diffusion behavior at 30?,while it exhibited a Fickian diffusion behavior at early hydrolytic stage and a deviation from the Fickian diffusion behavior for the longer hydrolysis at 40? and 50?.The molecular weight of PLA in the materials showed immersion temperature-and time-dependence.The higher the immersion temperature was,the faster the molecular weight decreased.When the materials were hydrolyzed at 30?,the molecular weight of PLA did not show obvious decrease.Whilst immersing at 50?,the molecular weight of PLA showed an obvious dependence on the immersion time,it decreased dramatically at the initial period,and then tended to remain unchanged.The pH values of the water medium for both materials decreased slightly at 30?and 40?,while it decreased dramatically at 50?.This was due to the release of acidic products produced by hydrolysis of PLA.Combining the water uptake,molecular weight changes,and reduce of pH values together,the materials underwent more severe hydrolysis at higher immersion temperatures,and the hydrolysis rate was faster.The crystallinity of materials did not show obvious increase,and the materials still had amorphous structure at 30?.At 40?,the crystallinity of starch/PLA composites increased after a longer immersion time.When the materials were immersed at 50?,the crystallinity of materials first increased significantly,then the crystal phase was hydrolyzed and the crystallinity decreased slightly.With the increase of immersion time,thermal stability of the materials decreased gradually,as well as the mechanical strengths of the materials.Mathmatical models of the relationship between the tensile strength and immersion time for the materials at different temperatures were established by polynomial fitting.Comparing all the modifications of the properties for both materials that was hydrolyzed at 50?,we can conclude that the modifications for starch/PLA composites were lower than the modifications for PLA.This conclusion indicated that the hydrolytic degradation rate of starch/PLA composites was slower than that of the pure PLA.Meanwhile,the reduce of molecular and pH values reduction of hydrolytic mdium for starch/PLA composites were lower than that of pure PLA during the hydrolysis process.These results suggested that the starch may slightly slow down the hydrolysis rate of PLA.However,the final molecular weight and pH values were almost same for the composites and pure PLA,indicated that starch did not influence the hydrolysis degree of PLA.The hydrolysis behavior of materials revealed the hydrolysis mechanism.Chain cleavage caused by hydrolysis took place at the ester bond in the PLA structure.Acidic products genetrated from the hydrolysis could further catalyze the hydrolysis reaction,which was called autocatalysis hydrolysis effect.The hydrolysis of materials first took place at the amorphous phase,while the dense packed crystal phase was hydrolyzed after a longer immersion time.