Study On The Structures And Properties Of Polypropylene Foams Via Supercritical CO₂ Extrusion Foaming

Polypropylene(PP)foam is the fastest-growing environmental materials among general polymer foams due to its advantages of lightweight,heat resistance,recyclability and easy-degradation.Nevertheless,there are still many problems and challenges waiting for PP foams,such as the intrinsic flammability of PP materials and the lack of knowledge for senior cellular structures.In recent years,though the solo flame-retardant treatment or foaming behavior of PP has already got notice,there are still severely scarce reports about the flame-retardant PP foams.Because it is difficult to balance the relationship between lightweight,flame-retardant and mechanical properties of the materials.In addition,the cellular structures formed by the long-term evolution and optimization in the biological body endow the nature organism with special functions,which provides novel ideas for the design and development of advanced functional PP foams.However,the application of biomimetic idea in the fields of polymer foams is still in its infancy.In this dissertation,the biomimetic concepts were introduced for the structural design and preparation of novel PP foams with different functionalities,and the environmental-friendly supercritical carbon dioxide(scCO₂)foaming method together with other interdisciplinary technologies were utilized to construct diverse structures inside the PP foams,such as the dense structure,uniform closed/open porous structure,hollow hierarchical porous structure,surface micro-nano structure and flower-like porous structure.Moreover,the formation mechanisms of the corresponding structures were also studied.The main research contents are as follows:(1)The fabrication of PP/intumescent flame retardant(IFR)composites with more uniform dispersion was demonstrated through the scCO₂ foaming technique followed by defoaming owing to the advantages of scCO₂ plasticization and biaxial bubble stretching.The results show that the scCO₂ can plasticize the melt and diffuse into the IFR agglomerates,and then effectively break the IFR agglomerates under the help of high-speed rotating screw.Moreover,the biaxial stretching force formed by bubble growing can further improve the dispersion of IFR particles in PP matrix.The increasing of the foam expansion ratio could lead to the better IFR dispersion of the final defoamed solid PP/IFR composites due to stronger bubble stretching force.Moreover,the improved IFR dispersion could result in the greatly enhanced flame retardancy and mechanical properties in comparison with those of uneven dispersed ones.(2)Inspired by natural anisotropic cellular structures,a novel structure was designed and constructed inside the PP foams to coordinate the relationship among lightweight,flame-retardant and mechanical properties of the PP foams.Besides,lightweight,flexible,ultrastrong anisotropic PP foams with superior flame retardancy were fabricated through the draw-assisted scCO₂continuous extrusion-foaming technique.There are special structures inside the PP25S3,such as the anisotropic cellular structure,oriented molecular chains and uniform dispersion of IFR particles on the PP cell wall.Above unique structures endowed the PP25S3 materials with excellent performance,such as lightweight(density,0.08 g/cm³),superior flame retardant(self-extinguish within 2 s after 60 s of combustion)and ultrastrong(lift up ten thousand times heavier objects than itself)properties.(3)Inspired by a variety of special structures in nature,the biomimetic PP tubular foams were design and massively fabricated by scCO₂foaming technique.Besides,the inner diameter of the as-prepared biomimetic PP tubular foams can be easily adjusted from extreme low(⁰.4 mm)to very high(²0.0 mm)size through a fixed 2.0 mm circle extruder die.At the same time,the as-prepared biomimetic PP tubular foam has the following characteristics:(a)hollow architectural like the straw,which is in favor of high throughout during gas/liquid flow;(b)hierarchical structure like the honeycomb,which is help to increase space utilization;(c)rough inner surface like the lotus,which can endow the foams super-oleophilic properties.In addition,the formation mechanism of the hollow hierarchical structure was proposed and the corresponding schematic diagram was drawn up.More interestingly,thanks to the above biomimetic structure,the as-prepared PP foam strips exhibited both superior absorption and filtration ability during the oil-water separation,in which the deionized water could be filtered quickly through the foam strips while the oil was absorbed in seconds.Besides,the relationship between pore size and hydrophobicity of polymer foams was theoretically analyzed by the Young-Laplace equation.(4)Inspired by the strong adhesion of the mussel,the dopamine was used to modify the surface of PP materials with diverse structures,such as the dense structures,uniform open/closed porous structures,and hollow hierarchical porous structures,for studying the self-polymerization and assembly behavior of the dopamine.For example,compared to the PP samples without dopamine modification,the surface energy of the samples with hollow hierarchical porous structures increased by 47.71 mJ/m² while the dense samples only increased by 14.11 mJ/m² after modification,indicating that dopamine has a poorer coating effect on smooth surface rather than the rough porous surface.Moreover,the relationship between both physical(such as pore structure,type and size)and surface chemical properties of the porous media,and wettability were studied.Besides,fundamental theoretical was also conducted to reveal the mechanism of the penetration or surface repellency toward a drop.Based on the above research,corresponding super-wetting materials were developed and prepared,such as super-hydrophobic and super-amphiphilic.