聚酯超临界挤出发泡系统中复杂通道内的热质传递过程数值模拟与调控

Processing of foamed poly(ethylene terephthalate) (PET) through extrusion foaming process by using supercritical carbon dioxide (ScCO2) is a green foaming technology. The interaction between supercritical fluid and polymer and the interphase transfer behavior in the process, which are insufficient due to the harsh environment, play an important role in determining the cell nucleation and growth and therefore, the properties of the foams. In this project, numerical simulation of complex flow field the hybrid finite element method is to be applied to study the interphase transfer behavior in the extrusion foaming process of PET with ScCO2. The dissolubility, plasticization and diffusion behaviors of ScCO2 on m-PET melt film are to be fully investigated with Magnetic Suspension Balances(MSB) and Differential Scanning Calorimetry (DSC), and the transfer process model for interface coupled with melt bulk under supercritical conditions is to be established. The constitutive equation that applied to the ScCO2/PET complex system is also to be obtained by study of rheological properties of the mixed fluids under saturated and oversaturated status. Then the complex flow field and the dispersion- distribution of ScCO2/PET complex system in mixing and conveying sections of extruder is to be simulated by coupling with governing equations and constitutive equation. And the flow field, crucially influencing the heat mass transfer process, should be optimized and controlled to meet the demands of rapid formation of ScCO2/PET homogeneous solution. In die head area, the phase separation behavior of ScCO2/PET, which are quite critical for the bubble nucleation and growth process, is also to be simulated by coupling with governing equations, constitutive equation, classic bubble nucleation model and cell model. The bubble nucleation and growth of foaming process could be controlled by optimizing the distributions of temperature, pressure and velocity, which can be changed by the structure parameters and process conditions of die head., which are quite critical for the bubble nucleation and growth process, will be also simulated coupling with governing equations, constitutive equation, classic bubble nucleation model and cell model. Based on the polymer rheology and fundamental theory of transportation process, this project involves the establishment of constitutive equation and transportation model, simulation and monitoring of the extrusion foaming process, and the interphase transfer behavior with supercritical fluid. The result will be of great theoretical and practical importance for PET extrusion foaming process with ScCO2, and will provide theoretical guidance for other supercritical fluid assisted polymer processing.

超临界挤出发泡过程中聚合物/超临界流体复杂系统的流变行为、相际传递行为以及流场分布等均会影响聚合物泡孔结构与分布，从而影响泡沫材料性能。本项目针对高熔体强度PET挤出发泡过程，研究超临界CO2/PET复杂系统流变行为基础上，构建超临界CO2在PET熔体中的溶解-扩散模型，并采用新的有限元技术对挤出发泡过程中复杂通道内PET/CO2流体流动、热质传递以及气泡均相成核-生长过程实现数值模拟，研究超临界环境下复杂流场分布规律及相际混合-分散传递行为，获得挤出发泡过程气液混合-分散、热质传递及气泡成核-生长的调控手段，以实现挤出发泡制品泡孔结构和分布可调控。研究从聚合物流变学和传递过程基本理论出发，建立非常规条件下聚合物本构方程和传递过程理论模型，以数值模拟方法研究复杂相际传递过程机理与调控手段，研究结果对形成超临界PET挤出发泡新技术有重要理论和实际意义，可为其他聚合物超临界加工提供理论基础。

聚对苯二甲酸乙二醇酯（PET）是一种性能优异的热塑性芳香族聚酯，轻量化是应用领域对PET材料提出的新要求，而发泡是实现PET轻量化的主要手段。超临界挤出发泡过程中聚合物/超临界流体复杂系统的流变行为、相际传递行为以及流场分布等均会影响聚合物泡孔结构与分布，从而影响泡沫材料性能。.本项目针对高熔体强度PET挤出发泡过程，系统研究超临界CO2/PET 体系热行为、流变行为基础上，着重测定了CO2在PET熔体中的表观溶解度，并进行了溶胀度校正，构建了超临界CO2在PET熔体中的溶解-扩散模型。在250~280ºC，1~6MPa下CO2的溶解行为符合亨利定律，4~6 MPa下溶解度约10-2（g CO2/g PET）量级。表面张力测试结果表明，PET熔体在CO2环境中的表面张力随着温度和压力的增加而减小。.针对螺杆挤出发泡过程，构建了单/双螺杆物理模型--同轴圆筒模型，采用网格重叠技术，进行有限元划分，对不同元件结构及操作条件下螺杆挤出过程中的速度场、压力场分布进行了数值模拟。结果表明：小导程螺纹元件有利于流体多次分流和翻复，螺纹元件增大导程后，流动的返混加剧，径向混合不均匀；增大错列角有助于混合；使用大导程对称元件和六棱柱元件等非捏合元件，物料与机筒的热交换差，不利。以开孔模头为例，进行了挤出发泡过程模头段的模拟计算，表明压降诱导气泡成核主要发生在模头毛细管入口处和毛细管中，孔径1mm时的压降速率明显高于其它孔径。协调螺杆、模头等关键结构以及相应发泡窗口及工艺，设计构建了连续挤出发泡实验装置，研究了螺杆、模头温度等对挤出发泡过程的影响，成功制备了泡孔直径150-450μm之间，孔密度在1*105-9*105 cells/cm3的高发泡倍率低密度PET泡沫材料。.本项目研究从聚合物流变学和传递过程基本理论出发，建立了超临界环境下传递过程相关理论模型，以数值模拟方法研究了复杂相际传递过程及其调控，研究结果对形成超临界PET挤出发泡新技术有重要意义，也为其他聚合物超临界加工提供了理论基础。