Thermal aging and degradation of epoxy coatings at oxidised interfaces

人们普遍认为，涂层与金属的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，即使在没有可见薄膜的情况下，X射线光电子能谱 (XPS) 等表面分析方法也可以检测原始聚合物涂层的化学残留特征。因此，人们普遍认为涂层失效以内聚方式发生，并通过位于金属-聚合物界面附近的结构独特且较弱的聚合物“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，但即使在广泛研究的粘合剂（例如环氧胺热固性材料）的情况下，其形成和结构也存在争议。对于环氧树脂，已经提出了多种形成机制。这些包括刚性基材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产生的局部化学梯度，以及由于热梯度或有机金属物质的形成和扩散到聚合物中而导致的交联密度的变化。相间结构的这种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近似模型进行检查。然而，对于热固性聚合物，这种方法具有严重的局限性，这是由于空气-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材上的有机材料。迄今为止，所选择的技术——振动光谱法，一直受到约 2-3 m 的光学衍射极限和较差的灵敏度的限制。分析透射电子显微镜 (A-TEM) 等替代技术可用于以高 (nm) 空间分辨率确定界面区域的元素组成，而 XPS 提供真正的原子表面灵敏度，但横向分辨率不超过 100 m 左右。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，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起来，用于检查金属-聚合物界面区域。最近，我们使用 AFM-IR 来检查 70℃ 热老化后的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 cm1 处存在强谱带，这意味着环氧树脂正在发生界面氧化。我们建议检验以下假设：与金属基材和颜料相邻的相间区域中的高度局部聚合物氧化会影响界面附着力和整体腐蚀性能。我们将主要使用 AFM-IR 方法，并辅以补充分析方法，包括纳米 Tg 和界面阻抗的电化学测定。我们将探索许多变量，包括：树脂化学计量、高于或低于 Tg 的固化、后固化和高温下的“过度固化”，作为促进聚合物内氧化应激的装置。特别令人感兴趣的是金属界面也可能开始显着氧化的温度。在这里，我们可以使用预氧化铁基质以稳定的方式模拟该过程。

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.