Research on the Microcellular Foaming Process and Properties of Polypropylene Foams
Author:Hou Jun Ji
Supervisor:zhao guo qun
Foamed plastics have advantages of light weight,thermal and sound insulation,cushion,and good absorption performance,and thus are widely used in various fields of the national economy.Under the background of advocating green manufacturing,microcellular foaming process has become a research hotspot in the processing of foamed plastics.Polypropylene(PP)has a high heat distortion temperature,good hydrophobicity and oleophylicity,excellent mechanical properties,and good chemical stability,so the PP foams show a good application prospect in the fields of thermal insulation,oil spill clean-up,and automobile lightening.However,there are still some issues in the preparation of PP foams.Firstly,owing to the high crystallinity,rapid crystallization rate,and low melt strength of linear PP,the high-expansion foam with closed cells is very difficult to fabricate.In addition,the effect of crystal evolution under the action of CO2 on the foaming behavior of PP is not clear.Secondly,the relation between expansion ratio and cell opening of the foam is mutual opposed,so this brings challenges to the preparation of high-expansion open-cell foam used in the oil spill clean-up filed.Finally,because the foaming occurs in a confined space,high temperature environment,and complicated flow field,the foamed part molded by conventional microcellular injection molding often encounters several problems including limited weight reduction,inferior surface quality,and poor mechanical properties,which seriously restrict its application in the automobile lightening filed.Therefore,this paper used the environmentally friendly supercritical fluid as blowing agent,and adopted the heating batch foaming process in which foaming is triggered in a semi-molten state,cooling batch foaming process in which foaming is triggered in a non-crystalline state,and mold-opening microcellular injection molding and gas-assisted microcellular injection molding by increasing expansion space to prepare the high-expansion closed-cell PP foam,high-expansion open-cell PP foam,and lightweight microcellular injection molded PP part,respectively.The thermal insulation and compression properties of the closed-cell foam,the oil absorption performance of the open-cell foam,and the surface quality and mechanical properties of the injection molded part were studied.The main work and conclusions are shown as follows:(1)An in-situ observation apparatus was designed,and the crystal melting behavior of PP during the CO2 adsorption process was in-situ studied.Under the guidance of the in-situ observation results,the foaming experiments were carried out,and the relation between crystallization and foaming was intuitively revealed.The thermal insulation and compressive property of the prepared high-expansion PP foams were also studied.It was found that there must be a certain number of crystals in the polymer matrix to ensure PP can be foamed.When the crystals were nearly melted,the foam with a highest expansion ratio could be prepared.Under the condition of 15 MPa and 154℃,a high-expansion closed-cell foam with an expansion ratio of 45-fold using pure linear PP was prepared for the first time.It had a density as low as 0.0202 g/cm3,a cell size of 49 μm,and a thermal conductivity as low as 37.2 mW·m-1 k-1.(2)A cooling batch foaming process was developed to prepare high-expansion open-cell PP foam.To reveal the relation between crystallization and foaming of PP,and the influence mechanism of crystallization on cell opening in the designed foaming process,the effect of CO2 on the non-isothermal crystallization of PP during cooling process was studied using an in-situ observation apparatus.The influences of foaming temperature and pressure on the expansion ratio and cellular morphology of the foams were analyzed.The hydrophobicity,oil absorption capacity,oil absorption kinetics,and reusability of the high-expansion open-cell foam were also studied.The results showed that when the foaming was triggered in a non-crystalline state of the melt,the high-expansion foam could be prepared.The following crystallization of PP after triggering foaming could promote cell opening in the cell growth process.Under the condition of 20 MPa and 135℃,the prepared open-cell foam using pure linear PP had an expansion ratio as high as 23.9-fold.In addition,it had a water contact angle up to 151.5°,an adsorption capacity of between 17.0-48.9 g/g for different oils,and a good reusability.(3)The PP/talc composites were prepared,and the effects of talc on the crystallization and rheological behavior of PP were studied.The cell disappearing process in the packing stage was studied using a visual mold,and the effect of packing time on the cell disappearance in the mold-opening microcellular injection molding were investigated.The effects of packing time,mold-opening distance,and talc content on the cellular morphology of the injected parts were also studied.Finally,the surface quality of parts molded by mold-opening microcellular injection molding and conventional microcellular injection molding were compared.It was found that the long time packing process with high pressure in the mold-opening microcellular injection molding and heterogeneous nucleation of talc could significantly improve the foaming behavior of PP.When the talc content was 10 wt%,the mold-opening distance was 6 mm,and the packing time was 12 s,the weight reduction of the molded part was 66.7,the average cell size was 56.5 μm,and the cell density was 1.54×107 cells/cm3.In addition,the packing pressure used in the mold-opening microcellular injection molding could partly eliminate the cells on the part surface,and thus the surface quality of the part was improved.(4)A novel gas-assisted microcellular injection molding was proposed by introducing gas-assisted injection molding into microcellular injection molding.The experiment system of this novel process and the related mold were constructed and designed.The weight reduction,cellular morphology,surface quality,and mechanical properties of the PP parts molded by the conventional microcellular injection molding and gas-assisted microcellular injection molding were compared.The results demonstrated that the new process increased the maximum weight reduction of the foamed part from 30%to 45%.The packing effect of the high-pressure gas could effectively decouple the melt filling and foaming process.After second foaming,the cells of the foamed part were very uniform,and the cell density was one order of magnitude higher than that of the sample molded by the conventional microcellular injection molding.Moreover,the molded part had a compact solid skin layer.The long packing time of the high-pressure assisted gas could also improve the surface quality of the part.Compared with the part molded by the conventional microcellular injection molding,the surface roughness of the part molded by gas-assisted microcellular injection molding was decreased by 2.63 times,and the surface gloss was increased by 2.87 times.The tensile,flexural,and impact properties of the part molded by gas-assisted microcellular injection molding were improved because of the inner uniform and fine cells,and external compact skin layer.(5)The crystallization behaviors of the sample molded by conventional injection molding and gas-assisted injection molding were studied.A three-dimensional numerical simulation model for filling and cooling process of gas-assisted injection molding was established to reveal the effect of high-pressure assisted gas on the melt crystallization.The cell evolution in conventional microcellular injection molding and gas-assisted microcellular foaming injection molding under different packing times was studied.The improvement mechanism of the foaming behavior of PP in gas-assisted microcellular injection molding was revealed from the point of view of crystallization.It was found that the penetration of the high-pressure assisted gas produced a strong shear on the polymer melt,and thus promoted nucleation during melt crystallization process.Meanwhile,the gas penetration caused a thin residual wall thickness of the sample,so the melt cooling rate was increased,thus leading to an inadequate growth of crystals and decreasing the crystallinity of PP.In gas-assisted microcellular injection molding,the polymer melt was cooled by the packing of the high-pressure gas,so the melt strength was enhanced when the foaming was triggered.Moreover,a large number of nuclei generated in the melt could provide heterogeneous nucleating sites for foaming.Therefore,the foaming behavior of PP was significantly improved.