Abstract
The fouling of AISI 316L stainless steel during themanufacture of polymeric methylene diphenyl diisocyanate (pMDI) has been investigated. Studies have been carried out using a laboratory-based rig that simulates the process chemistry of the production plant. A variety of solution concentrations and treatment times have been employed to represent different stages in the production process. Following exposure, steel coupons have been removed and studied by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The thickness of the fouling layer, determined by XPS, is found to vary inversely with exposure time and solution concentration. This is a result of the solubility of the different pMDI derivatives that have been formed at different stages, and a reaction scheme is developed to explain these inverse relationships. ToF-SIMS indicates the formation of metal chlorides as a result of the initial treatment of the steel in the reaction vessel with hydrogen chloride. Fragment ions characteristic of reacted and unreacted pMDI (at m/z = 106 and 132 au, respectively) were used as an indicator of the degree of reacted isocyanate groups within the fouling layer and show a decrease with increasing exposure time, as a result of the formation of intermediates such as amines, ureas, carbodiimides, and uretonimines. The ToF-SIMS data was also processed by principal component analysis (PCA). This generally reinforced the conclusions reached by XPS and ToF-SIMS but, in addition, gave confidence in the repeatability of the analyses with the repeat data (of four analyses) clustering very tightly in the PCA score plots.
Highlights
Polyurethanes are a versatile group of polymeric materials with applications as elastomers, coatings, adhesives, and foams.[1,2] Polyurethanes are produced through a polyaddition reaction between polyisocyanates and polyols
These deposits are darkest on 316L-04, which shows a highly fouled region across the wash region with solid deposits adhered to the surface. The coverage of these solid deposits appeared more extensive prior to rinsing with monochlorobenzene (MCB) and is assumed to be polymeric MDI (pMDI)-based material adhered to the surface
The chemical characteristics of 316L steel samples exposed to conditions related to those found in the pMDI manufacture have been fully characterized
Summary
Polyurethanes are a versatile group of polymeric materials with applications as elastomers, coatings, adhesives, and foams.[1,2] Polyurethanes are produced through a polyaddition reaction between polyisocyanates and polyols. The MDI production process is described in detail elsewhere;[2,3] the main steps involve the polycondensation of aniline with formaldehyde and subsequent phosgenation. Crude MDI is obtained through a series of heat treatment steps used to remove excess reactants and solvents. Pure MDI and polymeric MDI (pMDI) are obtained from crude MDI through a distillation step. The heat exchangers used in the heat treatment and distillation steps are commonly made from 316L or Duplex stainless steels, chosen for their corrosion resistance properties. Fouling and degradation of the metal surface can still occur, reducing the efficiency of the heat exchanger, requiring periodic cleaning and the disposal of contaminated water and other residues to restore the metal surface
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