Isomer Ratio of TDI by Infrared Spectroscopy

Infrared spectroscopy permits the determination of the isomer ratio by suitable IR absorption measurements on several commercially obtainable toluene diisocyanates. It is based on the quantitative measurement of absorption bands due to out-of-plane C—H deformation vibrations of the 

LITERATURE CORRELATIONS OF GROUPS SPECIFIC TO THE IR CONSTITUTION OF A POLYURETHANE 

A 012-mm liquid cell is preferred for this analysis however, if this cell size is not available, the same results may be produced using a different known path-length cell and adjusting the sample weight as follows  

It is necessary to prepare a calibration graph before results can be obtained by the following procedure known weight concentrations of the isomers from pure 2,4- and 2,6-TDI in the analytical range desired are prepared. The absorbance for each isomer is plotted versus concentration. The x-axis intercept (% 2,4- or % 2,6-isomer) and the -axis intercept (absorbance) that corresponds to the x-axis intercept are determined. The concentration (cone.) of each isomer is given by absorbance at 12 35 or 12 80 fim minus absorbance that corresponds to % isomer intercept on the x-axis divided by the units of absorbance per unit of isomer (slope) plus the % isomer intercept on the x-axis. The TDI isomer absorption is read or calculated from the cyclohexane baseline at 12 35 and 12 80 jum. 

Note an alternative teehnique to normalize the absorbanee is to divide the absorbanee of the sample by the eell thiekness (in mm)  

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