HTC-15 - Abstract

Abstract Information :

This research is focused on the application of UHPSFC-MS for the analysis of methylene diphenyl diisocyanate (MDI) samples. Diisocyanates are di-functional pre-polymer materials used in polyurethane chemistry for a variety of applications, such as foams, adhesives and coatings. Industrially polyurethanes are made by the reaction of alcohols and isocyanates into crosslinked synthetic polymer matrices. These oligomeric isocyanates are very complex, containing mixtures of additional components with isocyanate functional groups. Other components are carbodiimides, ureas, uretonimines, biurets, dimers and trimers.

The polymeric MDI bulk properties are well known but the complete composition needs to be understood in much finer detail, hence the analysis of such samples is required to quantify the relative abundances of each component series.

Isocyanates are extremely reactive and susceptible to nucleophilic attack, hence they must be handled in the absence of protic solvents, and therefore the use of RP-HPLC is not suitable for MDI analysis. GC-MS has been used for the analysis of volatile components within these products, although its use is limited to short oligomers.

Historically, capillary supercritical fluid chromatography (cSFC), using GC-like columns was used for isocyanate characterisation.1 The results provided information regarding sample composition, although confirmation of peak identities required analysis by SFC coupled to mass spectrometry. Previous assays in polymeric MDI analysis used mass spectrometry techniques, such as MALDI ToF-MS and field desorption (FD).2 These also show the challenges of observed isocyanate reactivity as most information was presented by already derivatised ('methanol-killed') prepolymer isocyanates.

This project utilised positive ion atmospheric photoionisation mass spectrometry (APPI-MS) coupled with UHPSFC. Positive ion APPI is the ionisation technique of choice for ionisation of the non-polar, aromatic compounds, more specifically, dopant assisted APPI is applied.

Baseline separation of the individual components in the isocyanate series was achieved using a Waters UPC2™ with an Acquity UPC2™ columns. Positive ion APPI methods were optimised over the range of m/z 200-1000 using Waters SQD. Acetonitrile was used as a modifier with the gradient of 2-40 % over 3 minutes at a flow rate of 1.5 mL/min. Toluene was introduced at 0.45 mL/min as a make-up solvent and dopant.

The peak areas of integrated RICCs reflects the ratio of the individual components expected to be present within the oligomeric MDI. The relative component abundances should allow the prediction of expected physical, chemical properties and inter batch comparison.