Impact of curing time on ageing and degradation of phenol-urea-formaldehyde binder

Phenol-urea-formaldehyde (PUF) resin is one of the most important thermosetting polymers. It is widely used in many industrial and construction applications as an organic coating and adhesive. For example, in production of mineral wool for insulation, PUF is used together with the coupling agent (3-aminopropylsilane, APS) and serves as a binder for attaching mineral fibers to each other and to create the necessary mechanical integrity and shape of the final product. However, during ageing under high humidity (95%) and temperature (70 °C), hydrolysis can degrade PUF, decreasing product quality. A better understanding of the chemical processes caused by hydrolysis would promote development of more stable PUF binders. We investigated the composition and stability changes during ageing of cured PUF powder binder and mineral wool fibers where PUF binds the fibers together. We aged the samples in hot water (80 °C) or in a controlled climate chamber (70 °C; 91% RH) and analyzed them using X-ray photoelectron spectroscopy (XPS), element analysis and thermogravimetric (TG) analysis. We investigated the composition of species released from PUF during hydrolysis by electrospray ionization (ESI) of the aqueous solutions. The results show that the extent of PUF curing and the presence of APS as the coupling agent have an important impact on its stability. XPS revealed that poorly cured PUF contains a high fraction of NHCH2OCH2NH bonds which are easily hydrolyzed, while longer curing results mostly in more stable methylene bridges, NHCH2NH. We also observed evidence for urea NHCO bond decomposition by ESI analysis. Mineral wool fiber ageing studies showed that PUF rearranges on the fiber surface and detaches from it, together with the APS coupling agent. This improved understanding of the effects of ageing provides clues for designing a more robust binder, leading to increased quality and stability of mineral wool insulation.