Preparation And Physical Foaming Behavior Of Polypropylene Composites

Polypropylene?PP?is an important engineering plastic,its performance and PP composites have attracted more and more attention owing to their excellent processability.The performance of PP composites not only depends on the properties of the reinforcement and the polymer matrix,but also on the processing technology to a large extent.Compared with solid polymers and conventional polymeric foams,microcellular polymers usually exhibit reduced weight,high mechanical strength,improved fatigue life as well as excellent thermal stability.Therefore,microcellular foam shave attracted extensive attention for itspotential applications in various areas,such as aerospace,biomedical,microelectronics,auto parts,building environmental protection etc.Thus microcellular polymers are claimed as“novel materials for the21st century”.In recent years,microcellular polymers and corresponding processing technology,mechanism of cell nucleation and growth,as well as relationships between microstructure and macroscopic properties have become hot research topics in the field of polymer materials and processing.Fabrication of PP composites and microcellular foam process were combined in this research.Based on PP composites with various reinforcement systems,supercritical CO₂ was used separately in solid batch foaming and microcellular injection molding process as physical blowing agents.The microcellular foaming behavior and properties of PP composites with different reinforcement systems under static and dynamic microcellular foaming conditions were investigated.The effects of PP composite composition,processing conditions and microstructure on microcellular foaming behavior and cell structure were studied by DSC,rheology,SEM and other characterizations test equipment.Main contents can be listed as below:1.The viscoelastic behavior,crystallization properties,tensile properties and intermittent foaming behavior of PP/PS blends were investigated.The addition of PS increased the viscosity and storage modulus of PP,accelerated the viscoelastic response,inhibited crystal nucleation and growth.Meanwhile,the crystallinity of the blend was decreased with the increase of PS content.The blend modulus increases,elongation at break and yield strength decrease.During the solid batch foaming process,PS significantly increased the foaming density and expansion ratio of PP as a heterogeneous nucleating agent and reduced the cell size.Due to the significant difference in solubility of supercritical CO₂ in PP and PS,the PP/PS blend porous material exhibits a distinct bimodal cell structure.2.Based on multi-wall carbon nanotube,the viscoelastic behavior,crystallization properties and tensile properties of PP/CNT composites were investigated.The dispersed CNT in the PP matrix interact to form a three-dimensional ordered structure,and the viscoelastic response of the composite is accelerated.The presence of CNT promotes nucleation of crystals,but crystal growth is inhibited and crystallinity is lowered due to an increase in viscosity of the complex and interaction between a large number of crystal nuclei.The CNT were dispersed in the PP matrix in agglomerated form,indicating that the conventional melt blending method cannot effectively distribute the CNT uniformly in the PP matrix,but the CNT agglomerate are not large in size.Compared the batch foaming behavior of PP/CNT with PP/PS/CNT,the CNT as heterogeneous nucleating agent in the PP/CNT composite greatly increases the cell density and reduces the cell size.The presence of CNT creates a heterogeneous melt structure inside the PP melt.As the cells of the high-density composite grow continuously,the cell walls become thinner and thinner,and the peeling at the interface between PP and CNT form an open-cell structure.After adding PP at the same time as PS,the novel three-phase composite system exhibits a novel micro-nano composite structure with both bimodal cells and high openings.3.Based on the single-wall carbon nanotube and research of solid-state batch foaming of PP composites,the continuous microcellular injection molding process of PP/CNT was studied.The thermal,mechanical and rheological properties of PP/CNT composites were analyzed.Compared with the high opening ratio micro-nano composite porous structure obtained by the intermittent foaming process,the microcellular injection foamedmolded sample had larger internal cell size,poor cell uniformity,and the cell wall was thicker and completely closed.The heterogeneous nucleation effects of the same filling system under the micropore foaming process vary greatly.Comparing the mechanical properties of the samples before and after foaming,the Young's modulus of the unexpanded PP composite samples with the same CNT content was larger than that of the microporous foamed samples,and the Young's modulus of the solid and microporous samples of the PP/CNT composites with different CNT contents.It changed in a nonlinear form but has a small range of variation.The elongation at break of the solid PP/CNT is about 120%,but the elongation at break of the PP/CNT composite after microcellular injection is above550%.4.Based on the single-wall carbon nanotube and microcellular injection molding processed PP/CNT composites,glass microspheres?GB?treated with silane coupling agent were added to prepare CNT/GB reinforced PP composites with various components.The addition of GB enhances the interface between the PP matrix and CNT and GB,and the viscosity of the composite system increases.In addition,the PP/CNT/GB composite with the same CNT content increased by an average of 5°C compared to the PP/CNT composite after the addition of the quantitative GB.The average cell size of the three-phase PP composite microporous injection sample is35?m,which is 60%lower than that of the PP/CNT micropore injection.The cell density is increased by an order of magnitude,indicating that the composite filling system can produce more in the micropore foaming process.Many heterogeneous nucleation points.The tensile stress-strain curve of PP/CNT/GB samples is consistent with the trend of PP/CNT samples.The Young's modulus of PP/CNT/GB solids and microporous samples after adding GB is also nonlinear,but the range of variation smaller.The solid samples of PP/CNT/GB composites are larger than the elongation at break of microporous samples,and the elongation at break of solid PP/CNT/GB is about 150%,compared with PP/CNT composites with the same CNT content.The elongation at break of the solid increases by about 30%.