Department of Chemistry and Biochemistry, 103 Chemistry and Biochemistry Building, Montana State University , Bozeman, Montana 59717, United States.
ACS Appl Mater Interfaces. 2017 Jun 28;9(25):21422-21437. doi: 10.1021/acsami.7b03919. Epub 2017 Jun 16.
Material response models for phenolic-based thermal protection systems (TPSs) for atmospheric entry are limited by the lack of knowledge of the nonequilibrium processes that may govern the decomposition pathways of phenolic resin at heating rates up to tens of degrees Celsius per second. We have investigated the pyrolysis of phenolic impregnated carbon ablator (PICA) by measuring the molar yields of the volatile decomposition products as a function of temperature at four nominal heating rates of 3.1, 6.1, 12.7, and 25 °C s, over the temperature range of 100-1200 °C. A mass spectrometer was used to probe the 14 significant gaseous products directly as PICA samples were heated resistively in vacuum. Four products, H, CH, HO, and CO, overwhelmingly dominated the molar yields. However, in terms of mass yield, phenol and its methylated derivatives, cresol and dimethyl phenol, were significant. The temperature-dependent molar yields of the observed products exhibited a marked dependence on heating rate. The heating-rate-dependent behavior of the molar yields has been attributed to two main competing decomposition processes that occur as the temperature passes from roughly 300 to 500 °C: (1) cross-linking reactions that produce ether functional groups and carbon-carbon bonds and eliminate HO and (2) breakdown of the polymer backbone through scission of methylene bridges and liberation of phenol and its methylated derivatives. The latter process competes more effectively with the former as the heating rate increases. The relative rates of these processes appear to have a significant effect on the molar yields of volatile products from subsequent decomposition processes as the temperature is increased further. Thus, the heating rate strongly affects the pathways taken during the pyrolysis of the phenolic resin in PICA. The new data may be used to test nonequilibrium models that are designed to simulate the response of TPS materials during atmospheric entry of spacecraft.
用于大气进入的基于酚醛的热保护系统 (TPS) 的材料响应模型受到缺乏知识的限制，这些知识可能支配酚醛树脂在加热速率高达每秒几十摄氏度时的分解途径。我们通过测量在四个标称加热速率 3.1、6.1、12.7 和 25°C s 下作为温度函数的挥发性分解产物的摩尔产率，研究了酚醛浸渍碳烧蚀体 (PICA) 的热解。在 100-1200°C 的温度范围内，使用质谱仪直接探测 14 种重要的气态产物，因为 PICA 样品在真空中被电阻加热。四种产物，H、CH、HO 和 CO，绝对主导了摩尔产率。然而，就质量产率而言，苯酚及其甲基衍生物、甲酚和二甲基苯酚是显著的。观察到的产物的温度依赖性摩尔产率表现出对加热速率的显著依赖性。摩尔产率的加热速率依赖性行为归因于两个主要的竞争分解过程，这些过程在温度从大约 300 到 500°C 时发生：(1) 通过交联反应产生醚官能团和碳-碳键并消除 HO 和 (2) 通过亚甲基桥的断裂和苯酚及其甲基衍生物的释放来破坏聚合物主链。随着加热速率的增加，后一个过程与前一个过程竞争更加有效。这些过程的相对速率似乎对进一步升高温度时后续分解过程中挥发性产物的摩尔产率有显著影响。因此，加热速率强烈影响 PICA 中酚醛树脂热解过程中所采取的途径。新数据可用于测试旨在模拟航天器大气进入期间 TPS 材料响应的非平衡模型。
