Ageing phenomena in hydrophilic phenolic resin coatings

• Phenolic resin coatings post-cure upon exposure to accelerated ageing conditions. • Wetting agents in phenolic resin coatings are chemically stable but physically age. • Ageing of hydrophilic phenolic resins was inhibited by addition of an organic salt. • Sufficient water and mobile charges are needed to limit physical ageing of wetting agents. Phenolic resins (PRs) have been of industrial interest for over 100 years due to their excellent thermal and mechanical properties after curing. To adjust their surface hydrophilicity, wetting agents (WAs) are commonly added. The long-term stability of surface properties governs the durability and service life of hydrophilic PR coatings. This research focusses on the effects of physical and chemical ageing on the surface wettability of hydrophilic phenol-urea-formaldehyde (PUF) coatings. Ageing experiments at 70 °C and 75 % relative humidity revealed post-cure reactions at the surface, seen from both a change in surface chemistry and water contact angle values. Coatings containing anionic WAs showed a moderate increase in average water contact angle value upon ageing, attributed to chemical and physical changes in the PUF surface layers (e.g., post-curing of the polymer matrix and reduction of the amount of WA at the coating−air interface) rather than WA degradation. The presence of mobile charges in the coatings, for instance by leaching from ion-rich glassy substrates, leads to an inhibition of a decrease in hydrophilicity of the PR coatings. Addition of sodium methanesulphonate to the resin formulation reduces ageing even of coatings on glass substrates which are not prone to leaching of such mobile charges. These findings provide for extending the life and service time of hydrophilic phenolic resin coatings.