Crystallization And Foaming Behavior Of Polylactic Acid With Supercritical CO₂

As a common form of biodegradable material, poly (lactic acid) has many superiorperformance like environmentally friendly, biological compatibility and security.With the factthat it can be widely used in biological tissue engineering, packaging materials and otherfields, it has become a research hotspot in the field of polymer science in recent years.As akind of typical semi-crystalline polymer with a relatively slow crystallization rate, thecrystallization behavior of PLA has significant impact on its forming process and productperformance.In addition, the foaming process of PLA would inevitably involve thecrystallization process, and therefore the crystallization behavior and foaming processof PLAis closely related to and has important impact on each other.This paper aims to study theinfluence of process condition on the crystallization properties and foaming properties ofpolylactic acid respectively.And on the basis of this, linking crystallization properties withfoaming performance will also help us explore the interplay between the twoprocesses.Therefore,this study has huge theoretical and practical meaning.Based on the crystallization and melting theory of polylactic acid, and also the theory ofsupercritical CO2foaming, these experiments were conducted in a high pressure kettle(selfdesignation of Cao’s group).The influence of different temperature and CO2saturationpressure on the crystallization and foaming properties of different PLA samples werestudied,as well as the relationship between crystallization behavior and foaming process.Theexperiment selected three different PLA materiasl:1) PLA4032D;2) Modified PLA with highcrystallization rate;3) Blend materialof Modified PLA and PLA4032D, and according to theweight percentage of the modified material in the system, called PLA20，PLA30and PLA40.The results showed that when the isothermal crystallization process were conducted, fordifferent PLA materials, the existence of CO2would decrease the crystallinity of PLA and alsoreduce its melting point.The XRD results showed that it was related to the appearance of anewα’crystalline phase under the environment of CO2.For PLA4032D pure material, theincrease of crystallization temperature and saturation pressure could make its crystallinity andmelting point increase.As for modifiedPLA and three kinds of blending PLA,because of theappearance of thenewα’crystalline phase under certain temperature and saturation pressure, itscrystallinity and melting point showed discontinuous change with crystallization temperatureand saturation pressure increasing.On one hand,the foaming process of PLA would also have a significant impact on itscrystallization property.First,the difference between the foaming temperature and saturation pressure would make the changes in the crystallinity and melting point of poly (lactic acid)(PLA).Second, the pressure relief stage (especially when higher foaming temperature andsaturation pressure were set) of the foaming process can induce polylactic acid produce manymore loose stack-α’crystal.Therefore,with the increase of foaming temperature and saturationpressure,melting point and crystallinity also present discontinuous change. On the other hand,the crystallization behavior of poly PLA can also affect the foaming properties.For purePLA4032D and modified PLA material, low crystallinity is advantageous to the foamingprocess.For the three kinds of blending PLA, the change tendency of the crystallinity and thechange trend of bubble density has a good consistency, within a certain range, the increase ofcrystallinity is helpful for the decrease of the bubble hole diameter and the increase of thebubble density, achieve the requirement of microcellular foaming.