Crystallization Of Polycarbonate In Supercritical CO₂ And Its Effect On Foaming

Polycarbonate(PC),is a widely used thermoplastic engineering plastics,has excellent impact resistance,heat resistance,low dielectric constant,high transparency and other advantages,but easy to stress cracking.Through the foaming process,the introduction of microporous structure in the PC can effectively prevent crack growth,enhance mechanical properties,and has the properties of lightweight,low cost,sound insulation and insulation,reduce the dielectric constant and so on.How to obtain high rate PC foaming material with high bubble density and small size is the most important research hotspot and direction.Firstly,the dissolution and diffusion characteristics of supercritical CO₂ in PC were systematically studied in this paper.On this basis,a non-isothermal growth model of PC/CO₂ foaming was established.Combined with the results of intermittent foaming experiment,the reliability of the model was verified.The effects of process parameters and material properties on bubble growth were analyzed by numerical simulation.In terms of process parameters,increasing the pressure drop rate and cooling rate is beneficial to increase the cell nucleation density and reduce the cell size.While the increase of viscosity and the decrease of diffusion coefficient caused by temperature decrease can significantly reduce the bubble growth rate,which is conducive to the reduction of bubble size.In terms of physical parameters,the melt viscosity and surface tension of the polymer hinder the bubble growth,while the diffusion coefficient and Henry's constant promote the bubble growth.Secondly,organic nano-clay was added to promote CO₂ induced crystallization of PC,and micro-grains were used as physical crosslinking points to improve viscoelasticity of PC melt,and could also be used as bubble nucleation points to promote nucleation of bubble pores,improve nucleation density and reduce pore size.The effects of the addition amount of nano-clay,saturation temperature and saturation time on the viscoelasticity,crystallization and pore structure of PC melt were systematically studied by means of differential scanning calorimeter,rotary rheometer and electron scanning microscope combined with intermittent foaming experiments.The results showed that the addition of nano-clay reduced the melt viscosity of PC,increased the activity of molecular chains,and significantly increased the crystallization rate of CO₂-induced PC.At 170? and 15MPa CO₂,the crystallization rate of PC can be induced after saturation of 0.25%sample for only 10min.The crystallization rate of PC increases with the increase of the content of nano-clay.The crystallinity increases rapidly with the prolongation of saturation time.Under the same conditions,the crystallinity of the sample with 3%added amount reached 18.85%after saturation for 15min.However,with the increase of saturation temperature,the crystallization rate decreased,the crystallinity increased first(24.02%at 180?)and then decreased,and the double melt peak in the DSC curve of the foamed sample increased and fused,indicating that the crystal structure was gradually improved and the proportion of primary crystals increased.Crystallization has a significant effect on the foaming process,and the uniformly dispersed crystals can induce a large number of bubbles to nucleate.At 185? and 15MPa,compared with PC,the crystallinity of the sample with 3%addition increased to 11.39%with time,the cellular density increased by 8.39 times,the cellular diameter decreased by 80%,but the foaming rate was only 2.27.Under the same conditions,when the crystallinity of the sample was only 0.33%,the cellular density increased by 1.92 times,the cellular diameter decreased by 35%,and the foaming rate was 4.78.Thirdly,the CO₂-friendly PMMA was blended to improve the solubility of CO₂ in PC blends and enhance the CO₂-induced crystallization of PC.Optimize the blending process and add catalyst to improve the compatibility between PMMA and PC.The effects of the addition amount of PMMA,the saturation temperature and saturation time of CO₂ on the cellular structure and crystallization properties of PC foamed samples were systematically studied,and the quantitative relationship between the crystallization and the cellular structure was established.The results show that the crystallization rate of the blends is lower and easier to be controlled than that of adding nano-clay,and it also decreases with the increase of temperature.At 170 MPa and 15MPa,the crystallinity increased with the prolongation of time,and the foaming times firstly increased and then decreased.The higher the PMMA content,the greater the maximum foaming rate.When the crystallinity is less than 2%,with the increase of crystallinity,the bubble density increases by 2.06 times and the bubble diameter decreases by 30.81%.However,when the crystallinity is greater than 4%,the dissolved CO₂ will be discharged,which greatly limits the nucleation and growth of bubbles and reduces the density of bubble pores.When the temperature is lower than 185?,the bubble nucleation is mainly affected by the relative ratio of crystallinity to viscosity as the temperature increases.With the decrease of the crystal proportion,the bubble nucleation is mainly affected by the relative ratio of crystallinity to viscosity.The cell density increased from 1.25 × 108cells/cm³ to 1.83 × 108cells/cm³.However,when the temperature was higher than 185?,the decrease of CO₂ solubility played a dominant role in the nucleation of bubbles,and the bubble density decreased but still had 1.43 × 108cells/cm³.Finally,the modified PC optimized the process conditions and found that when the content of nano-clay was 0.25%or PMMA content was 10%,the foaming effect was the best at 15MPa and 180-185?,and the preparation of high rate PC foaming materials with high bubble density,small size and uniform distribution could be achieved by adjusting the crystallinity by extending the saturation time.