Infrared Analysis of Toluene-2,4-diisocyanate and Toluene- 2,6-d iisocyanate Mixtures S. S.
LORD, Jr.
Jackson laboratory,
E. 1. du font de Nemours & Co., Wilmington, Del.
b Two infrared spectrophotometric methods have been developed for analyzing mixtures of toluene-2,4-diisocyanate and toluene-2,6-diisocyanate. One procedure is applicable to mixtures containing 5 to 9570 of the 2,4-isomerl and the other to mixtures containing more than 95% of the 2,4isomer. Both methods are based on a quantitative intensity measurement of bands arising from C-H deformation vibrations of the aromatic ring ( 1 2.35 and 12.8 microns). The precision at 95% confidence limits for the mean of duplicate determinations is *0.8701 absolute, for the former method and *0.0870, absolute, for the latter. The 2,5-isomer will interfere slightly with the 2,4-isomer determination, but the concentration of toluene-2,5-diisocyanate in these mixtures is normally sufficiently low to prevent serious error.
T
are rapid$ increasing in importance as cheniical intermediates, because of their use in polyurethane foams and elastomers. As the relative proportions of the 2,4and 2,6- isomers present in toluenediisocyanate mixtures are known to affect the physical properties of these foams and elastomers, a suitable method for analyzing theee mixtures is extremely important. The conventional chemical determination of isocyanates by ieaction with excess piimarg amine followed by an acidimetric titration of the unreacted amine (3-5) is suitable for determining total isocyanate, but fails to distinguish between the different isomers. Cryoscopic methods are sensitive to isomeric differences, but they are nonspecific, and the presence of a third component generally leads t o serious errors. An infrared spectrophotometric method has been developed which may be used to analyze mixtures of toluene-2,bdiisocyanate and toluene-2,6-diisocyanate reliably. Two procedures are described: one for analyzing mixtures n-here the 2,4-isonier content is between 5 and 957& and the other for mixtures where the 2,44somer content is greater than 95%.
Sodium chloride sealed liquid absorption cells, 0.1 and 0.2 mm. Cyclohexane, 99% grade,Shell Chemical Co. Store over sodium or silica gel. Diisocyanitte Standards. The toluene-2,4-diisooganate, toluene-2,6-diisocyanate, an,d toluene-2,5-diisocyanate used in this study were prepared by phosgenating the corresponding pure amines and vacuum distilling the product. These isocyanates may discolor if allowed to rimiain in contact with air, and are very sensitive to moisture. It is recommended that they be stored and handled under dry nitrogen. ANALYTICAL PROCEDURE
930 a
10
"
OLUEKELUISOCY~44NATES
12.0 12.5 13.0 13.5 WAVE LENGTH ( MICRONS 1
Figure 1. Infrared absorption spectra of toluene-2,4diisocyanate and toluene-2,6diisocyanate
- _ _Cyclohexane, 0.208-mrn. cell - Taluene-2,4-diisocyan a t e,
0 . 0 2 0 3 gram per ml. in cyclohexane, 0.208-mm. cell. Toluene-2,6-diisocya n a t e, 0.0200 gram per ml. in cyclohexane, 0.208-mm. cell
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APPARATUS A N D REAGENTS
Perkin-Elmer Rlodel 21 Infrared Spectrophotometer. Although the procedures described herein were developed using t.he Perkin-Elmer instrument, any single- or double-beam instrument having a comparable resolution may be used. The following instrumental settings and conditions were used: Prism Scanning speed Gain Response Resolution
NaCl 0.5 micron/niinute 6 1 slit schedule 3
Slit schedule 3 corresponds to a mechanical slit width of approximately 0.26 mm. a t the analytical wave lengths (12.35 and 12.8 microns).
Procedure I. Mixtures Containing 5 to 9Syoof Toluene-2,4-diisocganate. CALIBRATION,Weigh t o the nearest milligram 2.0-gram samples of toluene2,4-diisocyanate and toluene-2,6-diisocyanate into separate, dry 50-nil. glassstoppered volumetric flasks and dilute to volume with dry cyclohexane. From each of these solutions prepare a series of dilutions in dry cyclohexane such that the final solute concentrations are 100, 80, 60, 40, 20, 10, and 5% of the original. Using a 0.2-mm. sodium chloride sealed liquid absorption cell, superimpose the spectra of each dilution series from 11.5 to 13.5 microns upon the spectrum of pure cyclohexane. bfeasure the absorbance of the toluene-2,4-diisocyanate calibration samples a t the 12.35-micron peak and the toluene-2,6-diisocyanate calibration samples a t the 12.80-micron peak using the cyclohexme blank as lo. For each calibration series plot the absorbance against the absorptivity, which is calculated as follows:
a=----d b X c
a = absorptivity, ml./gram-mm. A = absorbance b = cell thickness, mm. c = concentralion, grams/ml.
PROCEDURE. Weigh to the nearest milligram a 2.0-gram sample of the mixture to be analyzed into a dry, 50ml. glass-stoppered volumetric flask and dilute to volume with dry cyclohexane. Fill the 0.2-mm. cell with a portion of the solution, record the spectrum from 11.5 to 13.5 microns, and superimpose that of pure cyclohexane in the same cell without changing VOL. 29, NO. 4, APRIL 1957
497
the balance controls. If the absorbance of any band is greater than 1.0, dilute the sample solution further with dry cyclohexane and rerun its spectrum. Measure the absorbance of the toluene-2,4-diisocyanate band a t 12.35 microns and the toluene-2,6-diisocyanate band a t 12.80 microns, using the cyclohexane blank as Io (Figure 1). Correct the measured absorbances for the slight mutual absorption band overlap as follows:
- 0.043 - 4 1 2 . 3 5 ~ A Z ,= ~ A i z . 3 ~- 0.024 A i 2 . 8 0 ~ A2,s =
Aiz.80~
coyected isomer Az,l = cofrected isomer A1~.80p= measured microns All.asfi = measured microns A2,6
=
(2)
Table 1. Accuracy of Toluene-2,4-diisocyanateand Toluene-2,6-diisocyanate Determinations
Prccedure I Found %
Known yo
2,4-isomer 12.75 30.10 55.78 74.99 84.76 93.46
2,4isomir (av. of duplicates) 13.5 31.0
55.0 76.4
84.1 93.5
=
