Hydrophilic Coatings Enabled by Wetting Agents and Phenolic Resins
Luuk Moone successfully defended his PhD thesis at the Department of Chemical Engineering and Chemistry on May 7th.
Phenolic resins (PRs) are widely used as adhesives for wood, in laminates, and as binders for fibrous materials. While the field of PRs is quite mature, the area of water-loving (hydrophilic) PRs and PR coatings has not been explored much. Such resins are for instance used in stone wool substrates, which enable a soilless method to grow fruits and vegetables. Desired water characteristics of hydrophilic stone wool are attained by addition of wetting agents (WAs) to the resin formulation. Knowledge of how processing conditions of PR coatings, the interaction between WAs and the PR, and the behaviour of the PRs and WAs when exposed to water (vapour) affects surface characteristics of PR coatings provides valuable insights. These insights can be crucial for the optimisation and selection of the appropriate WAs for applications where phenolic resin coatings with hydrophilic surfaces are required. This thesis aims to cultivate more understanding of surface properties of hydrophilic phenolic resin.
First, the influence of resin composition and curing conditions on surface characteristics such as morphology, chemistry and wettability was studied. This was done by using different resin compositions (Phenol-Formaldehyde; PF, and Phenol-Urea-Formaldehyde; PUF) and using various hardening conditionss. Curing at an industrially common temperature of 200 掳C yields surfaces with varying degrees of oxidation, differences in linkages between phenolic and urea species, and a lower overall nitrogen content for the urea-containing coatings compared to coatings cured at 150 掳C. In turn, this results in stronger fluctuations in water-wettability for coatings hardened at elevated temperatures compared to those cured at lower temperatures.
Hydrophilic PR coatings were prepared by addition of wetting agents to the formulation. The interaction between these WAs and a PUF and PF resin was then studied by successive advancing contact angle measurements. This way, not only the WA persistence in the surface layers but also the response of the PR coating to the presence of water could be studied. Successive wetting of the PR coating surfaces was seen to yield a plasticising effect, evidenced by a partially reversible reduced water contact angle value with increasing exposure time to water. This water plasticisation was observed even when anionic WAs were present in the coatings. A fundamental difference in the compatibility between WAs and PUF and PF was observed, as anionic WAs demixed in PF upon curing.
WAs are efficient when they are well-distributed throughout the PR coatings, resulting in an improved hydrophilicity of the coating surface. The distribution of WAs depends on the balance between miscibility with the PR resin and forming polymer network upon curing, as well as the surface segregation. The research of compatibility between PR and WA was therefore extended by systematically changing the type and composition of WAs. Overall, anionic WAs were found to be most suited to use to enable hydrophilic PF and PUF coatings.
The life- and service-time of hydrophilic PUF coatings was studied by monitoring the hygrothermal stability of WAs and PUF(-WA) coatings. Additionally, the role of the glass substrate in ageing phenomena of the PUF(-WA) coatings was taken into account. Samples were exposed to accelerated ageing conditions of 75% RH at 70 掳C for a prolonged time. WAs were found to be remarkably stable, but PUF(-WA) coatings became more hydrophobic over time, partly due to post-curing reactions. The coating substrate also played an important role in the ageing process. Only coatings applied on soda lime glass showed inhibited ageing as gauged from limited changes in hydrophilicity. This was attributed to WAs remaining more mobile at the coating-air interface due the leaching of mobile charges from the glass substrate. Addition of an organic sodium salt to the PUF(-WA) resin inhibits ageing even for inert substrates. These results can provide a tool to improve the stability of hydrophilic PRs.