Research On Microcellular Foaming Properties Of Graphene-enhanced Polylactic Acid

Excessive use of petroleum-based polymers has brought about series of energy and ecological problems.Therefore,how to use bio-based and biodegradable polymers to replace petroleum-based polymers has become a heated issue in the world today.As a typical bio-based and biodegradable material,polylactic acia(PLA)has been widely used in daily life due to its superb biocompatibility,excellent mechanical property,good processing ability and so on.However,PLA has shortcomings such as low initial impact,poor heat resistance,and poor thermal stability,which seriously hinder its further application.What is worse,PLA itself has low melt strength and slow crystallization rate,making it difficult to perform foaming processing.So far,it is still challenging to prepare PLA foams with low density and good cell structure.To this end,this paper used supercritical CO₂ foaming technology to discuss the foaming behavior of neat PLA and its composite system.Graphene nanoplatelets(GNP)and polybutylene adipate-terephthalate(PBAT)were added to PLA matrix to improve its performance and foaming ability.On the one hand,the experimental technology and parameters were determined by studying the foaming behavior of neat PLA.On the other hand,PLA/GNP and PLA/PBAT/GNP composite materials were prepared by melt extrusion and injection molding to realize the modification of PLA.And then,analyze the solid and foamed samples.The main research conclusions are as follows:1.By changing the process parameters,a series of PLA foams with different cell structures were prepared by sc-CO₂ foaming technology.Through the analysis of foam morphology and cell structure,how the process parameters such as foaming temperature and saturation pressure influenced the foaming performance of PLA were discussed.Under the conditions of 120?and 20.7 MPa,a PLA foam with a foaming ratio of 39.2 folds and an open-cell structure was prepared.What's more,experiments showed that PLA foam has good hydrophobicity and lipophilicity,and can selectively absorb oil from water.For various organic solvents and oils,the adsorption capacity of PLA foam is as high as 4.6-15.1 g/g.Furthermore,during 20 adsorption-desorption cycles,the foam's adsorption capacity for organic solvents does not significantly attenuate.2.PLA/GNP composites with different GNP content were prepared by melt blending and injection molding.The microscopic morphologies,mechanical properties,rheological behaviors,and foaming properties of the composites were investigated.The results show that GNP has a good dispersion in the PLA matrix.Compared with neat PLA,the tensile strength and stiffness of the composite with a GNP content of 3 wt%are improved by nearly 20%.In addition,with the gradual increase of GNP content,the movement of PLA chains is restricted,which significantly enhances the viscoelasticity of the composite melt and is beneficial for PLA foaming ability.Due to the heterogeneous nucleation of GNP,the composite foam has a large foaming ratio of more than 30 folds.The compressive strength and modulus of the composite foams are effectively improved as well.3.In order to overcome the shortcoming of PLA foam's brittleness,PLA/PBAT/GNP composites with a PBAT content of 20 wt%was prepared by melt blending and injection molding.And the effects of the addition of PBAT and GNP on the mechanical,thermal properties and rheological behavior of composites were discussed.It has been proved that the addition of PBAT and GNP increased the melt viscoelasticity of the composites.When the GNP content is low,the cell morphology of the composite foam after foaming with sc-CO₂ is more uniform,and the maximum foaming ratio can reach 53.0 folds.However,when the GNP content exceeding 3 wt%,the composite's melt viscoelasticity would be too high to produce foams.In addition,the composite foam has a shape memory property due to the addition of highly flexible PBAT.Whereas,when the GNP content is less than 3 wt%,the composite foam can be rebound significantly when it is compressed.