| Microcellular foaming injection molding is an advanced manufacturing technology to prepare microcellular plastic parts with supercritical N2 or CO2 as physical blowing agent,which is one of the most promising lightweight and high-performance microcellular injection molding process.As one of the most widely used thermoplastics,polypropylene(PP)is widely applied in the fields of automobile manufacturing,rail transportation,etc.Research on microcellular foaming injection molding of PP and its composite materials is important for promoting the development of injection molding technology and the transformation and upgrading of related industries in China,upgrading the level of lightweighting of important areas of the national economy such as automobile manufacturing,rail transportation,communication electronics,electrical equipment and so on.It is of great theoretical and engineering significance to promote China’s sustainable development strategy of "Carbon Peak and Carbon Neutral".Due to the low strength and poor foaming properties of PP,it is difficult to obtain fine,dense and homogeneous cell structures,and the addition of certain fillers is usually necessary to improve the rheological behavior,crystallization behavior and foaming properties of PP.However,the crystallization behavior of PP under supercritical N2 environment,the cohesive behavior of the crystallization and cell nucleation and growth of PP melt/filler/supercritical N2 multiphase system,the influence of enhanced filler on the rheological and crystallization behavior of PP as well as its effect on the performance of composite microcellular plastics parts and other scientific issues are not yet mastered.This leads to the lack of theory and process data to support the design and optimization of the microcellular foaming injection molding process of PP and its composites and the regulation of PP and its composite plastic parts performance,resulting in the poor performance of microcellular plastic parts,which restricts the development of the microcellular foaming injection molding and the engineering applications of the PP microcellular plastic parts.In this paper,the crystallization behavior of PP and its composites under supercritical N2 was systematically investigated by using two kinds of graphite nano sheets(GN)and carbon fibers(CF)with different aspect ratios as reinforcing fillers,the mechanism of PP crystallization behavior induced by GN and CF was elucidated,and the secondary nucleation model of PP and its composites under supercritical N2 was established.The incorporation of GN and CF realized the regulation of PP composites melt rheological behavior,crystallization behavior and foaming performance,and successfully prepared lightweight and high-performance PP composites microcellular plastic parts.The study elucidated the influence of material components,foaming process,different mold opening distance,etc.,on the cellular structure and performance of microcellular plastic parts.The main research work is as follows.To clarify the mechanism of PP crystallization behavior under supercritical N2,the in situ high-pressure multi-optical test system developed by the group was employed to investigate the grain evolution law of PP in supercritical N2 It was found that increasing the N2 pressure had a significant inhibitory effect on PP grains when the N2 pressure was lower,and further increasing the N2 pressure had a significant refining effect on PP grains when the N2 pressure was higher than 13.79 MPa,and the regulation of PP grains could be realized by adjusting the supercritical N2 pressure.Within the research scope of this paper.it was not found that high-pressure N2 induced PP to produce other crystal types,but thickened the z-axis of PP grains.In addition,high-pressure N2 exhibited an inhibitory and then promotional effect on the growth rate of PP grains.Based on the above study,a secondary nucleation model of PP/N2 system was established considering the effect of extra free energy caused by high-pressure N2,and the influence mechanism of high-pressure N2 on the growth rate of PP spherulites was investigated from the energy point of view.The calculation results of the secondary nucleation model are in good agreement with the results of PP isothermal crystallization experiments.which can accurately predict the trend of PP grain growth rate under high-pressure N2.The influence mechanisms of GN and supercritical N2 on the crystal morphology and crystallization behavior of PP were investigated.Based on the secondary nucleation model of the PP/N2 system,the effect of the interfacial free energy induced by GN on nucleation is further considered,and the secondary nucleation model of the PP/GN/N2 three-phase system is established.The intrinsic influence of high-pressure N2 and GN content on the growth rate of spherical crystals is analyzed,and it is found that the increase of translational free energy caused by desorbed N2 in the homogeneous system under high-pressure N2 is the fundamental reason for the increase of the secondary nucleation rate.The calculation results of the secondary nucleation model of the three-phase system are in good agreement with the results of isothermal crystallization experiments,which can accurately predict the trend of the growth rate of PP/GN composite grains under high-pressure N2.The addition of GN produced more irregular lamellar crystals and made the stacking of lamellar crystals more disordered,which further indicated the heterogeneous nucleation effect of GN.To systematically study the influence rules of GN on the foaming behavior and microcellular plastic properties of PP composites,experiments on the microcellular foaming process of PP/GN composites were carried out to reveal the influence rules of different GN contents and different microcellular foaming process on the microcellular plastic properties of PP/GN composites.It was discovered that the addition of GN not only directly improved the melt strength and viscoelasticity of the material.but also had the dual effects of refining the grains and accelerating the crystallization process of the material.which significantly improved the crystallization behavior and foaming properties of the PP.After the incorporation of GN.the cell structure of PP/GN composites was significantly refined,but the cell density decreased continuously and the cell size increased with mold opening distance.The regulation of the cellular structure of PP/GN composite microcellular plastic parts was achieved by changing the GN content and the mold opening distance.Finally,the PP/GN composite microcellular parts with small,uniform and dense cells and excellent mechanical properties were successfully prepared by MOFIM technology.To solve the problems of slightly poor GN reinforcement and performance of PP/GN microcellular parts.CF was adopted to reinforce PP.A method was proposed to prepare CF reinforced polymer films to study the fiber-induced interfacial crystallization behavior,and the mechanism of CF-induced transcrystallization formation under supercritical N2 as well as the periodic growth mechanism of the crystal front and the multi-isothermal temperature crystallization behavior under supercritical N2 were investigated.CF still has excellent heterogeneous nucleation effect under supercritical N2,which induces the formation of transcrystallization at the PP/CF interface,and the grain refinement becomes more and more significant with the increase of CF content.The surface tension effect of CF was visualized for the first time during the in situ multiple isothermal crystallization process.This process induces the transformation of spherical crystals into dendritic structures,and the degree of dendritization becomes more obvious with the increase of the number of isothermal crystallizations.Based on the secondary nucleation model of the three-phase system.the formation mechanism of CF-induced transcrystallization crystals and the periodic growth mechanism of the crystal front are elucidated under supercritical N2.The higher the CF content.the higher the interfacial free energy to be overcome during nucleation,which results in the decreasing grains growth rate.In this paper,the fiber-induced PP crystallization behavior under supercritical gas conditions is investigated for the first time,which provides theoretical support for the preparation of fiber reinforced composite microcellular plastic parts with excellent properties.The effects of CF on the rheological and crystallization behavior of copolymer polypropylene(CPP)and homopolymer polypropylene(HPP)were elucidated.CF significantly improved the crystallization and rheological behavior of the two types of PP,and the enhancement of the melt strength and viscoelasticity provided the necessary conditions for the formation of dense and stable cellular structures.To investigate the intrinsic influence mechanism of different microcellular foaming processes on fiber orientation distribution,PP/CF composite microcellular plastic parts with 20%weight reduction were prepared by using regular foam injection moulding(RFIM)and MOFIM technologies.Based on the micro comuted tomography(Micro-CT)data,the internal fibers of PP/CF microcellular parts were reconstructed,and the fiber orientation distribution of the microcellular parts prepared by MOFIM was more random,which formed a more perfect 3D fiber network structure.The inherent influence mechanism of different microcellular foaming processes on the microcellular structure was investigated,and the packing pressure process in MOFIM reduces the coarse macrocellularity of the primary foaming,and forms a finer and denser cellular structure.The improvement of the cellular structure is conducive to the enhancement of the mechanical and impact properties,and the intrinsic mechanism of the augment of the electromagnetic shielding performance of the MOFIM microcellular plastic parts is analyzed based on the fiber orientation distribution.To obtain more lightweight,high strength PP/CF composite microcellular parts with excellent electromagnetic shielding properties,the influence of mold opening distance on the cellular structure and properties of PP/CF microcellular molded parts was investigated based on the MOFIM process.The regulation of the internal cellular structure of PP/CF composite microcellular parts was realized by adjusting mold opening distance.With the increase of mold opening distance,the cell size gradually increases,the cell density gradually decreases,and the tensile and bending properties also decline to different degrees,but the specific mechanical properties of the microcellular parts do not reduce significantly after eliminating the effect of the density.In addition,HPP/CF microcellular parts can still maintain a high level of electromagnetic shielding at a higher mold opening distance. |
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