Study On The Mixing Mechanism And Properties Of Poly(Lactic Acid)-based Materials Processed By Tri-Screw Extruder Under The Effect Of Vibration Field

Over the last few decades,the petroleum-based polymer materials have been increasingly applied.This greatly aggravates the risk of petroleum shortage,as well as the environmental pollution problems.Poly lactic acid(PLA)has the advantages of high tensile strength,easy processing,and most importantly,good bio-compatibility and biodegradability.Therefore,it has been considered as one of the most promising bio-material to substitute the petroleum-based polymer.Since the introduction of "plastic restriction" policy,this biodegradable polymer materials attracted more attention.However,pure PLA is a brittle material,with low ductility and weak impact strength,which limits its application scope.To improve its toughness,the ductile materials,nanomaterials,etc.are often added.Nevertheless,the performance of the modified PLA is significantly relying on the mixing effects.How to achieve better mixing effects and improve the comprehensive performance of PLA-based materials in the processing of PLA through the innovation of processing technology and equipment not only has a significant impact on the widespread application of bio-based PLA,but also reduces the the extensive use of petroleum-based polymers has significant practical significance in the waste of petroleum resources and environmental pollution.In this context,our research group designed an innovative balanced tri-screw dynamic extruder,which has important scientific significance for the preparation of PLA-based materials and the development of mixing theory of vibration force field processing.The self-developed balanced three-screw dynamic extruder adopts a "balanced" structure with symmetrical feed at both ends and the materials acting on the spirals to cancel each other.The vibration force field is introduced into the polymer material mixing process and successfully prepared PLA-based blending/composite materials.Based on the theoretical analysis,the physical and mathematical models of plasticizing transportation system during the mixed transportation process of the balanced tri-screw dynamic extruder were established.Through numerical solution,the relationship between the vibration force field parameters and the circumferential and axial speed of the screw and the change of its deformation rate is obtained,which proved that the axial vibration applied to the intermediate screw can cause the periodic change of the composite flow field of shear-stretch between the screws.The kneading block of the tri-screw extruder was numerically simulated by Polyflow.The influence of the vibration force field on the velocity field,mixing index,shear rate field and particle trajectory was studied.It was found that the axial vibration causes the axial kneading clearance change periodically between the main kneading block and auxiliary kneading block,the material has axial forward velocity,as well as leakage flow phenomenon.The results show that the mixing index(?)changes periodically in the whole simulation period,and there are regions greater than 0.5,which verifies the existence of shear-tensile composite flow field.Based on theoretical analysis,experimental research methods are adopted,PLA/EPDM,PLA/PBS/EGMA and PLA/OMMT were prepared by balanced tri-screw dynamic extruder.Compared with the steady-state processing force field,the experimental results showed that the particle size of dispersed EPDM particles in the PLA/EPDM blends decreased and the distribution was uniform,In PLA/PBS/EGMA blends,the ratio of EGMA epoxy group to PLA molecules and hydroxyl groups in PBS molecules increased,resulting in the increase of PLA/ PBS-g-EGMA copolymers distributed on the interface between PLA and PBS.In the PLA/OMMT composites,the lamellar OMMT spacing in PLA matrix increased,and the degree of agglomeration decreased significantly.Based on the experimental results,the influence law and dispersion mechanism of the vibration force field on the mixing process were deeply analyzed,and the influence law of the vibration force field amplitude and vibration frequency on the properties of the PLA-based material prepared by the balanced three-screw dynamic extruder was revealed,Established the relationship between processstructure-performance,which provided a theoretical basis for the preparation of highperformance PLA-based materials.In this paper,vibration force field is introduced into tri-screw extruder,which breaks through the mixing mechanism of single and double screw extruders driven by shear deformation.The axial vibration causes melt to undergo certain tensile deformation in the process of conveying.The complex shear-tensile flow field with periodic transformation between screws plays a role in strengthening mixing and promoting dispersion of materials plasticization.Therefore,the balanced tri-screw dynamic extruder will surely become an important equipment for polymer mixing,kneading and processing,and play an important role in the development of polymer composite materials processing.