Study On Continuous Extrusion Foaming Of Blending Modified Polypropylene/supercritical CO₂ And Underlying Mechanism Analysis

Microcellular plastics with the merits of reduced material usage, reduced cost, reduced environment pollution and excellent physical and mechanical properties, play an important part in many fields.Polypropylene(PP), as a cheap material with superior mechanical properties, is widely used. With the good degradability, retrievability, environment protection, heat and chemical resistance, Polypropylene foams are taken as the substitute for Polystyrene(PS) and Polyethylene(PE) foams in industrial applications. If let PP microcellular foamed, then its application will be further widened. But common PP is linear, crystalline polymer, the viscosity of which will decrease dramatically when heated above its melting temperature, making its melt strength very low, so the foamability of common PP is vey poor. It is extremely difficult to obtain foamed PP plastics with satisfying cell structures.A modified cell nucleation mechanism with the presence of additives is presented and analyzed in this thesis, in which the definition of"negative pressure"is introduced. The effect of addition of Nano-clay on cell nucleation and cell growth is analyzed from the view of cell nucleation. Meanwhile a mechanism for effect of PDMS on foaming of PP is analyzed.This research, through the blending modification with other materials, studied the effect of various blending formulation on improvement of PP's foamability in continuous extrusion foaming process, using supercritical CO₂ as the blowing agent. PDMS has high CO₂ solubility, low surface tension and CO₂ penetration ability, so if PDMS is introduced into foaming process of PP, it will probably be beneficial for foaming of PP. This thesis, by blending PDMS with three kinds of PP with different polymerization patterns, investigated the effect of PDMS on improvement of PP's foamability systematically in continuous extrusion foaming process for the first time. The results showed the addition of PDMS could entrap more gas after the extrudate came out of die, which will induce a dramatic decrease in pressure. When the"gas-rich field"around the PP matrix is depleting, as the concentration of CO₂ in PDMS phase is much higher than PP, the great difference in gas concentration between PDMS and PP will cause a portion of CO₂ in PDMS phase diffuse into PP phase, which could later be used for growth of nucleated cells. As the size of bubbles must be larger than critical nucleated cells for survival, so the presence of PDMS phase will prolong the life span of cell growth to a great degree by supplying the nucleated cells with CO2 timely, which will avoid the cell shrinkage effectively. So the foamability of PP is improved with the larger expansion ratio and higher cell-population density. Experimental results showed foamability of R-PP is worst of the three kinds of PP while foamability of B-PP is the best. After blending modification, foamed PP material with cell diameter of 50-100μm, cell-population density as high as 2.27×10⁷cells/cm³ and expansion ratio almost as high as 22 folds was obtained using high-melt-strength H-PP. When supercritical CO2 contents were 3% and 5%, the addition of PDMS and PP-g-MAH increased cell-population density of H-PP obviously. This implies on the one hand, the addition of PP-g-MAH and PDMS acted as the nucleating agent; on the other hand, heterogeneous cell nucleation was promoted in the interfaces between PP phase and PDMS phase, PP phase and PP-g-MAH phase, and the interface between PP-g-MAH phase and PDMS phase, which showed heterogeneous nucleation is the main factor determining the cell-population density. When CO2 content was high (7%), a superhigh thermodynamic instability will be induced, which made homogeneous nucleation as the main mechanism determining the cell-population density. So for the high CO2 content case, the addition of PDMS and PP-g-MAH didn't show an obvious influence on cell-population density of foamed samples. The addition of PDMS and PP-g-MAH improved foamability of R-PP and B-PP to some extent, but the improvement was not as obvious as for the H-PP case. After modification, foamed PP material with cell diameter of 50-100μm, cell-population density as high as 4×10⁷cells/cm³ and expansion ratio as high as 13 folds was obtained using B-PP as 3% CO2 was used; cell diameter of 100μm or so, cell-population density of 2.3×10⁷cells/cm³ and expansion ratio as high as 24 folds was obtained using B-PP as CO2 content was increased to 5%.The effect of addition of Nano-clay on foamability of two kinds of PP with different polymerization patterns in continuous extrusion foaming process was also studied in this thesis,using supercritical CO2 as the blowing agent. Experimental results showed the addition of Nano-clay and PP-g-MAH decreased melt strength of PP to some extent. As PP-g-MAH with very high melt flow rate was used as the coupling agent for PP and Nano-clay, so the decrease of melt strength might be caused by the presence of PP-g-MAH. In spite of the decrease of melt strength for PP/Nano-clay/PP-g-MAH blends system, the introduction of Nano-clay improved the foamability of PP, especially for LH-PP. The possible explanation might be as follows: on the one hand, nano-clay provided more nucleating sites as a nucleating agent, which made more gas used for cell nucleation and growth, as a result of which gas usage efficiency was improved; on the other hand, the addition of PP-g-MAH made the interface between PP and PP-g-MAH as the potential nucleating sites, which is beneficial for increase of cell-population density of PP foamed samples.In the last part of the thesis, the synergistic effect for addition of PS and Nano-clay in improvement of PP's foamability was investigated in a continuous extrusion foaming process for the first time, with supercritical CO₂ as the blowing agent. Experimental results showed blending with PS increased crystallization temperature of PP. As PP and PS are typical incompatible polymers, the interface between them may provide a better place for cell nucleation as a result of decrease of cell nucleation free energy barrier, which resulted in improvement of PP's foamability. And the addition of Nano-clay further improved the foamabiltiy of PP/PS blends system. When low content of Nano-clay(0.5%) was used, foamed samples with cell diameter of 50-100μm, cell-population as high as 6.08×10⁷cells /cm³ and expansion ratio higher than 14 fold were obtained. As higher content(3% and 5%) Nano-clay was loaded, viscosity and melt strength of PP/PS blends system were improved. When 5% Nano-clay was added to PP/PS blend, fine-cellular foamed samples with the average cell size between 10-30μm, cell-population density as high as 2.16×10⁸cells/cm³ were obtained while the maximum expansion ratio and cell-population density of pure PP foams used for the study was only 6 folds and 2.5×10⁶cells/cm³ respectively.