Study On Pressure-induced-flow Processing Strengthened And Toughened Crystalline Polymers And Their Microcellular Foaming

Owing to the multi-scale oriented structures that are formed under pressure-induced-flow processing(PIF) and result in the strengthening and toughening of polymer materials, PIF processing provides a new thinking for polymer processing and manufacturing. However, the effect of PIF processing on the properties of semi-crystalline polymers and their blends needs to be further studied. The high pressure in PIF processing prevents its potential applications in many fields and more discussion and experimental study need to be conducted to solve this problem.In this work, the effect of PIF processing on properties of linear PP, MWCNT coated PP nanocomposites and PP/PA6 blends in which PA6 acts as dispersed domain was investigated. It was found that PIF processing has a same effect of strengthening and toughening of PP and its composites and blends. Because carbon nanotube particles are interconnected, PIF processed PP/MWCNT-Coated remains electrical conductive. The results showed that a highly oriented structure was observed under SEM and the crystals also oriented along the flow direction under high pressure. For PP/PA6 blends, both the crystals in PP phase and PA6 phase had certain deformation under high pressure. Similarly, due to the change of microstructure under high pressure, PIF processing could strengthen and toughen engineering plastic PA6 and its blends at the same time.A mechanism model of “brick-mud” structure was proposed and established to explain the phenomenon of the strengthening and toughening of PIF processing. For linear PP or PA6, the oriented structure under high pressure included the crystalline phase, amorphous phase and the two phases inside spherulites. The oriented structure is called “brick-mud” structure, in which, the crystal is the “brick” and the amorphous phase is the “mud”. On macroscale, the crystalline phase as the “brick” and the amorphous phase as the “mud” are deformed and oriented along the flow direction; on microscale, inside the spherulites, the neighboring lamellaes as the “brick” and the amorphous phase between the lamellaes as the “mud” are also deformed and oriented along the flow direction. For PP/MWCNT-Coated nanocomposites, another “brick-mud” structure was formed, in which the outside MWCNT layer acts as the “brick” and the PP acts as the “mud”. It is the multi-scale oriented “brick-mud” structure formed under high pressure that results in the strengthening and toughening of polymers. Similarly, due to the change of microstructure under high pressure, PIF processing could strengthen and toughen engineering plastic PA6 and its blends at the same time.To lower the high pressure in PIF processing, firstly, an ultrasound vibration assisted PIF processing(UAPIF) was investigated. It was found that through the optimization of the ultrasound parameters the pressure could be lowered to 20.7MPa. In addition, the mechanical property of UAPIF processed PP was close to that of PIF PP. A higher temperature in PIF processing also could lower the pressure. It was found that when the temperature was increased up to 150℃ the pressure needed to deform the solid polymer decreased to 34.5MPa. In addition, the results also showed that the holding time in PIF processing could be much shortened to 10 s.Through combining PIF processing and supercritical CO2 foaming technology the effect of PIF processing on linear PP was investigated. Compared to linear PP, the foamability of PIF processed PP was significantly improved; the foamability of PIF processed PP/MWCNTCoated was also much improved, which had a morphology of “layer by layer” bimodal structure. Not only did PIF processed PP and PP/MWCNT foams have low densities, but also they had a higher compressive strength. Through foaming the warping problem of PIF processed samples at high temperature was also solved by releasing the residual pressure. The foamability of PIF processed linear PP depends on the property of PP. PP with higher viscosity, that is, lower MFI, has better foamability under PIF processing. But PIF processing is necessary for the improvement of foamability of linear PP. A foaming process with short saturation time and low foaming pressure is preferred by researchers and industry. The results showed that there is a critical foaming pressure of 13.8MPa existed and above which foams with low density could be obtained. It was also found that the saturation time could be shortened from 2 h to 10 min to obtain foams with similar densities. The effect of dispersed phase PA6 on the foamability of linear PP was studied. Due to the low melt strength of PA6 and low solubility of CO2 in PA6, PA6 had little effect on the foamability of linear PP. However, PIF processed PP/PA6 had a much better foamability and the cell density increased obviously and a higher expansion ratio. It was found that the foamability of PA6 was not improved under PIF processing at foaming temperature of 155℃. Meanwhile, the foamability of PA6 showed small improvements with blending linear PP; However, Combing blending with PP and PIF processing the foamability was much improved with much higher cell density and low density.