Characterisation of complex MDI oligomers using supercritical fluid chromatography and mass spectrometry

Abstract

A new analytical approach for the separation and detection of oligomeric methylene diphenyl diisocyanate (MDI) sample components has been developed using packed column ultra-high performance supercritical fluid chromatography - mass spectrometry (UHPSFC-MS). The polymeric MDI bulk properties are well known, although the intricate differences in physical properties of MDI, such as colour and viscosity, are affected by the amounts of minor HMW components. No analytical methods have previously been developed to separate isocyanate oligomers from side-reaction products formed. One of the challenges is that the oligomeric isocyanates are very complex, containing mixtures of additional components with isocyanate functional groups. Capillary SFC (cSFC), using GC-like columns, was investigated in the late 1980s for isocyanate characterisation, although proved problematic for non-volatile high molecular weight (HMW) component detection. MDI oligomers were separated and detected up to four aromatic rings, some minor components were observed, although not identified. This research project was focused into the use of packed column SFC (pSFC) to develop and optimise a method to characterise oligomeric MDI components. Isocyanate groups are very reactive towards nucleophiles, therefore non-conventional aprotic, polar organic modifiers, such as acetonitrile, were used in the absence of mobile phase additives. Chromatographic separation methods for MDI components were developed using fully porous and core-shell particle columns. The resulting lower system backpressure core-shell particle column application allows columns to be connected in series, obtaining high peak resolution. The current UHPSFC-MS methods enable baseline separation and quantitation of isocyanate oligomers that lead to further total amine equivalent content (NCO value) calculations, important for MDI characterisation. MDI is industrially produced through methylene diphenylamine (MDA) phosgenation processes that lead to impurities. Minor components formed, such as carbodiimides, ureas, uretonimines, and biurets, are the possible suspects of MDI colouring. This becomes an issue when high quality materials are required for polyurethane supply. Polyurethanes are made by the reaction of isocyanates with alcohols or amines to form crosslinked synthetic polymer matrices. MDI is a precursor of two-component polyurethane materials related to a variety of applications, such as foams, adhesives and coatings. MDI was analysed using atmospheric pressure ionisation (API) techniques. Electrospray ionisation (ESI) was used for methanol-treated samples; the resulting polar urethane groups were formed that are ionisable by ESI. Only ions that correspond to isocyanate urethanes were observed, lacking minor component information. Atmospheric pressure photo ionisation (APPI) was used for non-polar, aromatic MDI compounds. Dopant-assisted APPI, with toluene as a dopant, was used and proved to be a successful method for minor component ionisation, as well as isocyanate oligomers.

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Identifiers

ORCID for Graham Langley: orcid.org/0000-0002-8323-7235

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