Abstract
Methylene diphenyl diisocyanate (MDI) is the most widely employed diisocyanate for the production of polyurethanes (PUs). This family of polymers is used for many applications including adhesives and coatings where an important characteristic is good adhesion. Considering how widely PUs are employed, the importance of MDI production in the world becomes evident. The phosgenation step, in the MDI production process, ultimately leads to fouling of the climbing lm evaporators (CFEs) employed, resulting in the loss of their thermal e- ciency. This work is concerned with the interactions between MDI (and related compounds) with metal substrates such as steels, which are employed in the CFEs as well as substrates for adhesion. For this study, surface analysis techniques, in particular X-ray photoelectron spectroscopy (XPS) and time of ight secondary ion mass spectrometry (ToF-SIMS), have been employed. Both substrates (316L and duplex steels) and adsorbates (MDI, polymeric MDI, methylene diphenyl amine and amine hydrochloride) have been characterised. The interaction between phenyl ring electrons (present in MDI and related compounds) and metal have been studied by observing the XPS - shake-up satellite at high spectral resolution. The interface between MDI and related compounds was then investigated. After the investigation of the model samples, plant facsimile samples were produced and analysed. Finally, actual plant samples were characterised. Proof was found of interaction between the phenyl ring electrons and the silicon substrate, as well as the formation of covalent bonds at the interface between MDI and steel as a result of the reaction between isocyanate and metal oxides and hydroxides, present on the surface of the steel. The facsimile samples showed the same types of interactions observed in the model samples and it was also found that the corrosion of the metal strongly influences the adhesion and fouling mechanisms. The samples from the plant showed similarities with the model and facsimile samples, proving that they provide a good basis for understanding the real world scenario.