Green Preparation And Characterization Of PHBV Microcellular Foaming Material

With the increasing awareness of environmental protection and the urgent need for sustainable development,it is of great significance to develop biodegradable microcellular foam materials.Among them,poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV)not only has excellent mechanical properties,oxygen barrier and water vapor resistance,but also can be completely degraded by bacteria and fungi in the ecosystem after being discarded.,so it is expected to become a superstar in the biodegradable plastic group.But during the foaming process,the crystallization and melting behaviors of PHBV increase the difficulty in preparing the microcellular foaming material.In order to achieve pollution-free preparation process,we used supercritical carbon dioxide(scCO₂)as physical foaming agent.Through determining reasonable batch foaming strategy and controlling process parameters,PHBV microcellular material were successfully foamed.The crystallization behavior and foaming mechanism during the foaming process were also investigated.Furthermore,the toughness of PHBV was effectively improved by regulating the cell size of PHBV and its toughening mechanism was revealed.PHBV samples were successfully foamed using supercritical carbon dioxide as physical foaming agent by 2-step batch foaming process.PHBV sheets were first saturated at 175°C to completely melt the crystalline regions.This step aimed at facilitating dissolution of scCO₂ into PHBV matrix.Subsequently,the sample was cooled down to the foaming temperate in the range of145-165°C to improve melt strength and prevent large amounts of CO₂ from escaping.To investigate crystallization behavior during foaming process and explore the corresponding foaming mechanism,differential scanning calorimetry(DSC),wide angle X-ray diffraction(WAXD)and small angle X-ray scattering(SAXS)characterizations were carried out.Stretching behavior during the cell growth stage may increase the crystal nucleation rate,and the generated crystal nucleus accelerates the crystallization rate as well as thickens PHBV crystal lamellae.Further,scCO₂-assisted annealing process was adopted to toughen PHBV by tuning the pore size.PHBV samples were successfully toughened by adjusting different saturation temperature(60 to 100°C)and saturation pressure(10 to 20 MPa).Compared with pure PHBV,the PHBV elongation at break increased by nearly 30 times,the tensile toughness was improved by about 26 times and the impact strength increased by nearly 2 times.The toughening mechanism was studied by differential scanning calorimetry(DSC),wide angle X-ray diffractometer(WAXD)and dynamic mechanical analyzer(DMA).It was found that the plasticization of supercritical carbon dioxide and the annealing process not only reduced the chain density of the amorphous phase in the matrix and increased the chain density of the crystal phase,promoted the growth of nanopores in the amorphous phase,but also changed the crystallization behavior of PHBV.This allows the material to effectively counteract the applied tensile stress when undergoing stretching,resulting in significant toughening.