Thermal aging and degradation of epoxy coatings at oxidised interfaces

人们普遍认为，镀层与金属的附着力是影响防腐镀层使用性能的关键因素。界面破坏很少“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））也可以检测到原始聚合物涂层的化学残留物特征。因此，人们普遍认为涂层失效是以一种内聚的方式发生的，并通过靠近金属-聚合物界面的结构独特、弱的聚合物“界面”区域传播。这种相间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粘合剂的情况下，其形成和结构也存在争议。对于环氧树脂，人们提出了多种形成机制。这包括聚合物在刚性基体附近的受限运动，树脂组分在金属表面的选择性吸附，金属诱导的固化或氧化催化引起的局部化学梯度，以及热梯度或有机金属物质在聚合物中的形成和扩散导致的交联密度变化。这种相间结构的模糊性源于历史上缺乏能够直接分析埋藏区域的技术。对于热塑性聚合物，超薄膜可以作为界面的模型近似来研究。然而，对于热固性聚合物，这种方法有严重的局限性，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，或分层后留在金属基板上的有机材料。迄今为止，所选择的振动光谱学技术一直受到光学衍射极限约为2-3米和灵敏度差的限制。替代技术，如分析透射电子显微镜（a - tem），可用于在高（nm）空间分辨率下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，但横向分辨率不超过100米左右。在曼彻斯特大学，我们率先使用了原子力显微镜-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结合在一起，用于检查金属-聚合物间相区域。最近，我们使用AFM-IR来检测70℃热老化后拉脱附着力测试后残留的聚合物残留物。初步工作发现，在1658 cm2处有一个强带，这表明环氧树脂正在发生界面氧化。我们建议检验在金属衬底和颜料相邻的相间区域高度局部聚合氧化影响界面附着力和整体腐蚀性能的假设。我们将主要使用AFM-IR方法，辅以纳米tg和电化学测定界面阻抗的辅助分析方法。我们将探讨一些变量，包括：树脂化学计量，高于或低于Tg的固化，在高温下的后固化和“过固化”作为促进聚合物氧化应激的装置。特别令人感兴趣的是金属界面也可能开始明显氧化的温度。在这里，我们可以通过使用预氧化铁底物以稳定的方式模拟这一过程。

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.