Zargarnezhad Hossein, Asselin Edouard, Wong Dennis, Lam C N Catherine
Department of Materials Engineering, The University of British Columbia, 309-6350 Stores Road, Vancouver, BC V6T 1Z4, Canada.
Shawcor Ltd., 25 Bethridge Road, Toronto, ON M9W 1M7, Canada.
Polymers (Basel). 2021 May 9;13(9):1517. doi: 10.3390/polym13091517.
The barrier performance of organic coatings is a direct function of mass transport and long-term stability of the polymeric structure. A predictive assessment of the protective coating cannot be conducted a priori of degradation effects on transport. Epoxy-based powder coatings are an attractive class of coatings for pipelines and other structures because application processing times are low and residual stresses between polymer layers are reduced. However, water ingress into the polymeric network of these coatings is of particular interest due to associated competitive sorption and plasticization effects. This review examines common analytical techniques for identifying parameters involved in transport in wet environments and underscores the gaps in the literature for the evaluation of the long-term performance of such coating systems. Studies have shown that the extent of polymer hydration has a major impact on gas and ion permeability/selectivity. Thus, transport analyses based only on micropore filling (i.e., adsorption) by water molecules are inadequate. Combinatorial entropy of the glassy epoxy and water vapor mixture not only affects the mechanism of membrane plasticization, but also changes the sorption kinetics of gas permeation and causes a partial gas immobility in the system. However, diffusivity, defined as the product of a kinetic mobility parameter and a concentration-dependent thermodynamic parameter, can eventually become favorable for gas transport at elevated temperatures, meaning that increasing gas pressure can decrease selectivity of the membrane for gas permeation. On the other hand, reverse osmosis membranes have shown that salt permeation is sensitive to, among other variables, water content in the polymer and a fundamental attribute in ionic diffusion is the effective size of hydrated ions. In addition, external electron sources-e.g., cathodic protection potentials for pipeline structures-can alter the kinetics of this transport as the tendency of ions to dissociate increases due to electrostatic forces. Focusing primarily on epoxy-based powder coatings, this review demonstrates that service parameters such as humidity, temperature, and concentration of aggressive species can dynamically develop different transport mechanisms, each at the expense of others. Although multilayered coating systems decrease moisture ingress and the consequences of environmental exposure, this survey shows that demands for extreme operating conditions can pose new challenges for coating materials and sparse data on transport properties would limit analysis of the remaining life of the system. This knowledge gap impedes the prediction of the likelihood of coating and, consequently, infrastructure failures.
有机涂层的阻隔性能是质量传输和聚合物结构长期稳定性的直接函数。在未事先考虑降解对传输的影响时，无法对防护涂层进行预测性评估。环氧基粉末涂料对于管道和其他结构而言是一类有吸引力的涂料，因为其应用加工时间短，且聚合物层之间的残余应力减小。然而，由于相关的竞争吸附和增塑效应，水进入这些涂料的聚合物网络特别值得关注。本综述研究了用于识别潮湿环境中传输所涉及参数的常见分析技术，并强调了在评估此类涂层系统长期性能的文献中存在的差距。研究表明，聚合物水合程度对气体和离子的渗透性/选择性有重大影响。因此，仅基于水分子微孔填充（即吸附）的传输分析是不够的。玻璃态环氧树脂和水蒸气混合物的组合熵不仅影响膜增塑的机制，还会改变气体渗透的吸附动力学，并导致系统中部分气体无法移动。然而，扩散系数定义为动力学迁移率参数与浓度相关的热力学参数的乘积，在高温下最终可能有利于气体传输，这意味着增加气体压力会降低膜对气体渗透的选择性。另一方面，反渗透膜表明，盐渗透对聚合物中的含水量等变量敏感，离子扩散的一个基本属性是水合离子的有效尺寸。此外，外部电子源，例如管道结构的阴极保护电位，会改变这种传输的动力学，因为由于静电力，离子解离的趋势增加。本综述主要关注环氧基粉末涂料，表明湿度、温度和侵蚀性物质浓度等使用参数会动态地产生不同的传输机制，每种机制都会以其他机制为代价。尽管多层涂层系统减少了水分进入和环境暴露的后果，但本次调查表明，极端操作条件的要求可能给涂层材料带来新挑战，而关于传输特性的稀疏数据会限制对系统剩余寿命的分析。这一知识差距阻碍了对涂层失效可能性的预测，进而阻碍了对基础设施失效可能性的预测。
