Thermal aging and degradation of epoxy coatings at oxidised interfaces

氧化界面处环氧涂层的热老化和降解

• 批准号: 2481469
• 金额: --
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2020
• 资助国家: 英国
• 起止时间: 2020 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2481469
• 关键词: Thermal; aging; degradation; epoxy; coatings

It is generally understood that coating-metal adhesion is a critical controlling factor in the service performance of corrosion protective coatings. Interfacial failure rarely occurs "cleanly" and even in the absence of a visible film, surface analytical methods such as x-ray photoelectron spectroscopy (XPS) can detect chemical remnants characteristic of the original polymer coating. Thus, it is generally accepted that coating failures occur in a cohesive manner and propagate through a structurally distinct, and weak, polymeric 'interphase' region located close to the metal-polymer interface. The nature and extent of this interphase region has been the subject of much research, and yet even in the case of widely studied adhesives such as epoxy-amine thermosets, its formation and structure are disputed. For epoxies, a variety of formation mechanisms have been proposed. These include restricted polymer motion in the vicinity of a rigid substrate, the selective adsorption of resin components onto the metal surface, local chemical gradients as a result of metal-induced catalysis of the cure or oxidation, and variations in cross-linking density as a result of thermal gradients or the formation and diffusion of organometallic species into the polymer. This ambiguity around interphase structures stems from a historical dearth of techniques capable of analysing the buried region directly. For thermoplastic polymers, ultrathin films can be examined as a model approximations of the interphase. However, for thermoset polymers this approach has serious limitations, which stem from the increased influence air-polymer interactions on the structure of thin films. An alternative approach is to examine resin-metal cross-sections, or the organic material remaining on the metal substrates after delamination, directly. The technique of choice, vibrational spectroscopy, has heretofore been restricted by the optical diffraction limit of around 2-3 m and by poor sensitivity. Alternative techniques such as analytical transmission electron microscopy (A-TEM) can be used to determine the elemental composition of interphase regions at high (nm) spatial resolution while XPS delivers true atomic surface sensitivity but with a lateral resolution no better than around 100 m. At the University of Manchester we have pioneered the use of the atomic force microscopy - infra-red (AFM-IR) technique, which couples the lateral resolution of scanning probe microscopy with the chemical sensitivity of infrared spectroscopy, in the examination of metal-polymer interphase regions. Recently we have use AFM-IR to examine polymeric residues remaining after pull-off adhesion tests after thermal aging at 70 C. Preliminary work finds a strong band at 1658 cm1 implying interfacial oxidation of the epoxy is occurring.We propose to examine the hypothesis that highly local polymeric oxidation in the interphase region adjacent to metallic substrates and pigments influences interfacial adhesion and overall corrosion performance. We will use predominantly the AFM-IR method supported by complementary analytical approaches including nano-Tg and electrochemical determination of interfacial impedance. We will explore a number of variables including: resin stoichiometry, curing above or below Tg, post-cure and "overcure" at elevated temperature as a device to promote oxidative stress within the polymer. Of particular interest is the temperatures where the metallic interface might also begin to oxidise significantly. Here we can model this process in a stable fashion by using pre-oxidised iron substrates.

通常认为，涂层-金属附着力是腐蚀防护涂层使用性能的关键控制因素。界面失效很少“干净”地发生，即使在没有可见膜的情况下，表面分析方法如X射线光电子能谱（XPS）也可以检测原始聚合物涂层的化学残留物特征。因此，通常认为涂层失效以内聚的方式发生，并通过位于金属-聚合物界面附近的结构上不同且弱的聚合物“界面”区域传播。该界面区域的性质和范围一直是许多研究的主题，然而即使在广泛研究的粘合剂如环氧-胺热固性材料的情况下，其形成和结构也是有争议的。对于环氧树脂，已经提出了各种形成机制。这些包括在刚性基底附近的受限聚合物运动、树脂组分在金属表面上的选择性吸附、由于金属诱导的固化或氧化催化而导致的局部化学梯度、以及由于热梯度或有机金属物质形成和扩散到聚合物中而导致的交联密度的变化。这种关于界面结构的模糊性源于历史上缺乏能够直接分析掩埋区的技术。对于热塑性聚合物，可将热塑性薄膜作为界面相的模型近似物来研究。然而，对于热固性聚合物，这种方法具有严重的局限性，这源于空气-聚合物相互作用对薄膜结构的影响增加。另一种方法是直接检查树脂-金属横截面或分层后残留在金属基材上的有机材料。迄今为止，所选择的技术，振动光谱学，受到约2-3 m的光学衍射极限和差的灵敏度的限制。替代技术如分析透射电子显微镜（A-TEM）可用于以高（nm）空间分辨率确定相间区域的元素组成，而XPS提供真正的原子表面灵敏度，但横向分辨率不优于约100 μ m。在曼彻斯特大学，我们率先使用原子力显微镜-红外（AFM-IR）技术，该技术将扫描探针显微镜的横向分辨率与红外光谱的化学灵敏度相结合，用于金属-聚合物界面区域的检查。最近，我们使用AFM-IR来检测在70 ℃下热老化后，在拉脱粘合试验后残留的聚合物残留物。初步工作发现，在1658 cm-1的强带，这意味着界面氧化的环氧树脂是happening.We建议检查的假设，即高度局部聚合物氧化的界面区域附近的金属基板和颜料的影响界面附着力和整体的腐蚀性能。我们将主要使用AFM-IR方法，并辅以包括纳米Tg和界面阻抗电化学测定在内的补充分析方法。我们将探讨一些变量，包括：树脂化学计量，固化高于或低于Tg，后固化和“过度固化”在高温下作为一种设备，以促进聚合物内的氧化应力。特别感兴趣的是金属界面也可能开始显著氧化的温度。在这里，我们可以通过使用预氧化的铁基质以稳定的方式模拟这个过程。