Study On Degradation Of Molten Polylactic Acid Using In-line Raman Spectroscopy

Polylactic acid(PLA)is one of the most successful biodegradable materials in industrial conversion.Its synthesis does not depend on petroleum-based raw materials.It is a typical environment-friendly material and has great potential for development.Although PLA has good properties such as reproducibility,biodegradability,biocompatibility,high modulus,and high strength,PLA is prone to degradation during processing,resulting in decreased product performance.Therefore,it is necessary to conduct in-depth research on the PLA degradation process in the processing process,in order to optimize the processing technology and equipment.The current research method mainly uses offline testing methods to conduct research,which has the disadvantages of destructiveness,hysteresis,long time consumption and high cost,and it is difficult to dynamically track the degradation process of the polymer during processing in real time.This article applies in-line Raman spectroscopy measurement technology to the realtime monitoring of PLA degradation process,explores the degradation behavior and degradation kinetics of PLA during processing,guides the optimization of PLA processing parameters,controls the degradation of PLA during processing,and Improving the quality of its products is of great significance.In this paper,combined with stoichiometry and degradation kinetics,the self-developed Raman spectroscopy monitoring device has been mainly used for research in two aspects.One is to study the static thermal degradation process of PLA at different temperatures and atmospheres.Through characteristic peaks and two-dimensional correlation spectrum analysis,the thermal degradation behaviors at different temperatures and atmospheres are compared.Combined with the degradation kinetics,the in-line Raman spectroscopy calculation The kinetic constant of the PLA thermal degradation reaction is obtained,and the activation energy of the PLA degradation reaction in different atmospheres is calculated by the Arrhenius equation.The second is to study the thermomechanical degradation process of PLA in the extruder at different speeds.The thermomechanical degradation behavior of PLA and the influence of speed on it are analyzed in depth.Combined with the degradation kinetics,the thermal and mechanical shear effects of PLA degradation during processing are decoupled.The main research result are as follows:(1)Regardless of whether it is thermal degradation or thermomechanical degradation,the Raman spectra collected in-line are sufficiently sensitive to the above processes,proving that in-line Raman spectroscopy measurement technology can be used for real-time monitoring of polymer degradation processes.(2)The thermal degradation behavior and thermal degradation kinetics of PLA were studied by in-line Raman spectroscopy.It was found that the thermal degradation behavior of PLA in nitrogen and oxygen is very similar.The two-dimensional correlation spectroscopy analysis showed that the increase of temperature and oxygen concentration can promote the breakage of PLA molecular chain,so that the content of its ester group,methyl group and carbonyl group decreased;revealed the sensitivity of ester group,methyl group and carbonyl group under different external disturbances,the sensitivity to temperature is ranked as ester group> methyl> carbonyl group,and the sensitivity to oxygen concentration is ranked as ester group> carbonyl group> methyl group.At the same time,the increase in temperature can promote the vibration of the carbonyl group,and the presence of oxygen makes the activity of the carbon group and methyl group enhanced.Combined with the degradation kinetics,it is shown that the increase of temperature and oxygen concentration can increase the thermal degradation rate of PLA and shorten the reaction half-life.In addition,comparing the reaction activity energy in different atmospheres,it is found that the activation energy of the degradation reaction is the lowest in oxygen,and the activation energy is the highest in nitrogen,so PLA degradation reaction is more likely to occur in oxygen.(3)Research on PLA thermomechanical degradation behavior and thermomechanical degradation kinetics through in-line Raman spectroscopy.It was found that the thermomechanical degradation behavior of PLA was very similar to its thermal degradation behavior,but a new carbonyl stretching vibration peak was found in the thermomechanical degradation of PLA,which indicated that molecular chain regularity decreased due to the strong shearing.Two-dimensional correlation spectroscopy analysis showed that with the increase of rotation speed,the content of PLA ester,methyl and carbonyl groups all decreased,and the increase of rotation speed can promote the thermomechanical degradation of PLA;revealing that under the external disturbance of rotation speed,the group pair The sensitivity of rotation speed is ranked as carbonyl> ester> methyl.In addition,the increase in rotation speed also reduced the regularity of PLA.Combined with the degradation kinetics,it is shown that the increase in rotational speed can accelerate the thermomechanical degradation rate of PLA and shorten its half-life.In addition,according to the kinetic equation,the mechanical degradation of thermomechanical degradation is separated,and it can be seen that as the speed increases,the mechanical degradation rate of PLA also increases,and the half-life shortens.