超临界CO2发泡制备硬质聚氨酯微孔材料过程中聚合反应与气泡成核和生长的协调机制

Rigid PU microcellular foams are promising advanced thermal-insulation lightweight materials. The fine cell size smaller than 10 microns will lead to obviously much lower thermal conductivity of 0.01W/m.K. Supercritical carbon dioxide (CO2) as blowing agent was an emerging technology to manufacture microcellular thermoplastic polymer foams which have drawn much attention in academic research and industrial application in past two decades. The preparation of PU microcellular foams with high expansion ratio in supercritical CO2 is a new challenging field, since foaming occurs during the generation of cross-linking network structural macromolecules simultaneously, a coupling process between complex chemical changes and physical changes actually. The interaction between supercritical CO2 and PU multicomponent reactants, PU prepolymer with different molecular weight and different cross-linking degree will be studied by the multi-scale molecular simulation and a series of high pressure experiments, and the PU formulas will be evaluated whether they are suitable to prepare PU microcellular foams or not. Compared PU polymerization (also called curing reaction) at atmospheric pressure, the reaction characteristics of PU polymerization in supercritical CO2 will be explored, and the influence of both solvent effect as well as heat effect of supercritical CO2’s will be analyzed. The reaction kinetics and rheo-kinetics of PU polymerization in supercritical CO2 will be built. The effect of curing rate, reaction degree, PU’s cross-linking network structure and rheological property on bubble nucleation and bubble growth will be investigated. The foamable range should be determined, and the polymerization progress should be controlled to have a good match with foaming stages of CO2 saturation, bubble nucleation, bubble growth and fixing in turn, both higher cell nucleation density and limited growing of bubble size are expected. The changes including polymer’s network structure and physical properties caused by curing reaction in supercritical CO2 will be taken into consideration in the equations of bubble nucleation and bubble growth. This work involves reaction and transfer phenomena under unconventional conditions. The results will provide theoretical basis and fundamental data for microcellular rigid PU foams prepared by the coupled process of reaction and foaming in supercritical CO2. Therefore, this work has high theoretical value and potential application prospect.

超临界CO2发泡制备具有优异绝热性能的高发泡倍率硬质聚氨酯微孔材料具有挑战性，发泡在反应生成交联网络结构大分子的同时进行，是复杂的化学变化与物理变化耦合过程。拟通过多尺度分子模拟和实验研究超临界CO2与聚氨酯反应物和具有不同分子量不同交联程度聚合物的相互作用，评估聚氨酯配方是否适合于微孔泡沫的制备。研究超临界CO2的溶剂效应和热效应对聚氨酯聚合（即固化反应）的影响，建立超临界CO2环境中聚氨酯固化反应动力学和流变动力学。将认识反应速率、反应程度、聚合物网络结构和流变特性等对气泡成核和生长的影响规律，确定可发泡区间，控制聚合反应进程使之能与CO2饱和、气泡成核和气泡生长与固定等各阶段协调匹配，增加气泡成核数量、控制泡孔长大；并能将反应导致的聚合物网络结构和物性等动态变化正确体现在气泡成核和生长模型中。研究结果将为超临界CO2环境反应与发泡耦合制备聚氨酯微孔材料提供理论依据和基础数据。

超临界CO2发泡制备硬质聚氨酯微孔材料是复杂的化学变化与物理变化耦合过程。本项目采用分子模拟和实验结合的方法研究了聚氨酯体系与CO2的相互作用，考察了多元醇性质、聚氨酯齐聚物分子量和交联程度对CO2溶解度和扩散系数的定量影响，CO2在多元醇中的溶解度随压力的变化规律可以利用Henry-Langmuir双模模型进行较好关联，优选出了适合CO2发泡的多元醇。利用高压DSC和原位红外研究了超临界CO2下聚氨酯体系固化反应特征，高压CO2的存在对PU固化过程有着明显的促进作用，建立了等温/非等温聚氨酯固化反应动力学模型，不同压力氛围中的聚氨酯固化反应符合自催化Setak- Berggren模型。利用高压流变、高压视窗釜分别测定聚氨酯预聚物/多元醇的粘度与表面张力的变化，发现多元醇/CO2体系的界面张力与两相密度差之间的关系可以用Macleod方程较好的关联；CO2的存在对于聚氨酯反应粘度的急剧上升过程具有延缓作用，主要由于CO2对体系的传热、吸热、稀释及塑化效应等造成。采用高压搅拌釜在线扭矩测定方法考察了超临界CO2环境中聚氨酯聚合反应过程中粘度随时间的变化，结合系列发泡可行性实验，建立了确定可发泡粘度窗口的方法，尽管可发泡时间窗口随温度、压力等变化，但对于特定发泡体系其可发泡粘度窗口皆保持恒定。优化设计了 “两步升压饱和法”发泡过程，优化表面活性剂、压力及其泄压速率，控制反应进程匹配CO2饱和、气泡成核和气泡生长与固定等发泡各阶段，实现了泡孔形貌调控，成功制备了发泡倍率达到10、泡孔尺寸小于10 μm、泡孔密度超过108个/cm3的硬质聚氨酯微孔泡沫。利用经典成核理论和细胞模型，结合固化反应动力学和物系性质随固化程度的变化趋势，模拟分析CO2直接发泡聚氨酯过程中影响气泡成核和生长过程的关键因素。研究结果为超临界CO2环境反应与发泡耦合制备聚氨酯微孔材料提供理论依据、基础数据和发泡过程优化策略。