Abstract
"Polyurethanes are amongst the most versatile polymeric materials covering a broad range of
applications such as elastomers, adhesives, and coatings. Of the polyisocyanates used in the
production of polyurethanes, methylene diphenyl diisocyanate (MDI) is the most widely employed.
The final stage of MDI production is the separation process, where a series of heat exchangers are
used to remove excess reactants and solvents. Many heat exchangers are made from 316L stainless
steel (316L) or Duplex stainless steel (DSS), however, fouling and degradation of the metal surface can
occur. Foulant layers decrease the thermal efficiency of the heat exchanger and ultimately, production
process efficiency.
Tantalum, commonly employed for its high corrosion resistance and thermal conductivity, is examined
for its potential to reduce fouling. X-ray photoelectron spectroscopy (XPS) and time-of-flight
secondary ion mass spectrometry (ToF-SIMS) were employed to investigate the interface chemistry
between polymeric MDI (pMDI) and tantalum. Specific interfacial interactions, previously unknown to
the scientific community, have been identified.
The fouling behaviour of 316L, DSS and tantalum was examined in a laboratory scale experiment
devised to replicate the initial fouling conditions found in the separation process. Small differences in
the proposed initial deposition mechanisms of 316L and DSS were identified.
The variables established to replicate initial fouling conditions were modified to reflect later
separation stages. The small observed differences identified between 316L and DSS in initial fouling
conditions were exaggerated in the more severe late conditions, resulting in large differences in
antifouling performance. The fouling process was found to coincide with the corrosion of the metal
substrate and urea formation was associated with the initial deposition process. Tantalum showed improved antifouling performance over 316L and DSS in all experiments conducted in this work and
initial investigations show promise for tantalum as a potential improved antifouling material for MDI
production."