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EC number: 701-029-8
CAS number: -
The results of the IR-spectroscopic analyses are shown in Table 1. The
NCO content of 4,4’-MDI, dissolved in ‘dry’ DMSO (0.03-0.04% water),
dropped below 40% initial value within 15 min. After 2 h, the NCO
absorptions began to disappear completely. In the course of the
hydrolysis reaction, the 500 mg 4,4’-MDI (2 mM) originally used would in
theory consume 36 mg of water (2 mM) to form an unstable carbamic acid
which decomposes into carbon dioxide and a highly reactive intermediate,
the (4-aminophenyl)-(4-isocyanateophenyl)methane. A solution containing
0.04% of water (2.22 mM), as is the case here, could therefore
stoichiometrically convert all the MDI into this intermediate, which, in
turn, can react with any of the remaining isocyanate groups to form a
number of monomeric, oligomeric and polymeric ureas. These ureas may
have NCO and / or NH2 ends. All these reactions steps are strongly
accelerated by DMSO.
OCN-R-NCO + H20 -> [OCN-R-NH-COOH] -> OCN-R-NH2 + CO2
OCN-R-NH2 .............+ ........OCN-R-NCO......
-> Oligoureas -> -> Polymericureas
The above reaction sequence demonstrates that the NCO groups of the
initial diisocyanate may disappear or relocate in a new molecule. The
IR-spectrum, however, does not provide information on the location of
the NCO groups.
Compared with the findings in ‘dry’ DMSO, solutions of MDI in commercial
EGDE can be considered relatively stable. Even after 4 h, more than 98%
of the NCO groups of 4,4’-MDI still exist. Increasing the water content
to 0.23% (12.78 mM), which means by a factor of more than 10, did not
influence the stability of the solution tremendously, although there was
sufficient water available to convert all NCO groups to amines and / or
polymeric ureas. Isomers of monomeric MDI as well as polymeric MDI,
dissolved in EGDE, behaved in a similar manner to 4,4’-MDI. Increasing
the water content in a solution that contained isomers of monomeric MDI
had no pronounced influence on its stability either (Table 2). It can
therefore be concluded that solutions of MDI in EGDE can be stored for a
few hours before use.
The stability of solutions of 4,4’-MDI in DMSO and in EGDE with varying
amounts of water was additionally analyzed by HPLC. The advantage of
this method is that the concentrations of MDI and of the possible
degradation products can be monitored and quantified, if suitable
reference substances are available.
Signals that relate to the reaction product of 4,4’-MDI and
dibutylamine, indicating the presence of 4,4’-MDI, as well as to
4,4’-MDA, indicating the presence of one of the possible degradation
products of 4,4’-MDI, can be identified in the chromatogram. Their
reference substances are readily available. This is not the case with
different ureas of MDI, which are not easily accessible. As the location
of their signals has already been described in the literature, they were
identified by analogy.
Table 3 shows the influence of the two solvents as well as the effect of
their water content on the stability of solutions of 4,4’MDI. Within 30
min 2.13 mM (532 mg) of MDI, dissolved in relatively dry DMSO (0.04%,
2.2 mM of water), were almost completely degraded to a number of
reaction products such as ureas, carbon dioxide, and as a minor
fraction, 4,4’-diphenylmethanediamine (4,4’-MDA). After 45 min no more
MDI could be detected. 4,4’-MDA, with a final concentration of 3% in
DMSO, could not be found at all if EGDE was the solvent. This indicates,
that the mode of degradation of 4,4’-MDI in EGDE is different to that in
DMSO. 4,4’-MDI (2.12 mM (531 mg)) dissolved in 100 ml of EGDE, which is
a concentration comparable to that in DMSO, and a nearly 3-fold
increased water content of 6.11 mM (0.11%), was relatively stable. Of
the original 4,4’-MDI, 95.3% was detectable after 30 min and 87.6% after
4 h. The influence of increased amounts of water on the stability of
4,4’-MDI was monitored in a supplementary experiment. In nearly
equimolar solutions (4.03 mM 4,4’-MDI and 3.89 mM water) 99.1% of the
MDI was still present after a period of 4 h. Raising the water content
to 26.11 mM, which brings the MDI : water ratio to approximately 1:6.5,
led to a solution still containing 78.9% of the MDI after 4 h.
These findings can only be explained by the fact that the degradation of
MDI is considerably accelerated in the presence of DMSO and may be
complete in less than an hour. On the other hand, the presence of EGDE
does not influence the stability of solutions of MDI tremendously. Even
after 4 h and in an excess of water, most of the 4,4’-MDI is still
Table 1. Shelf-life of solutions of 4,4'-MDI in DMSO:
IR-spectroscopic determination of the relative NCO content as a function
Weight of 4,4'–MDI in 100 ml solvent
Water content of solvent
Table 2. Shelf-life of solutions of MDI in EGDE: influence of the
water content of EGDE and IR-spectroscopic determination of the relative
NCO content as a function of time.
Type of MDI / solvent
Monomeric MDI isomers
Weight of MDI in 100 ml of solvent
Water content of EGDE
Table 3. Shelf-life of solutions of 4,4'-MDI in DMSO and in EGDE:
HPLC determination of residual free MDI and one of its reaction products
as a function of time.
Weight of 4,4'-MDI in 100 ml of solvent
(a) ND, not detectable, e.g. <0.05 mg/100 ml solvent
All MDI isomers and forms are highly unstable in dimethylsulhpoxide solvent, water content of the DMSO increasing breakdown. The corresponding diamine is identified as one of the breakdown products. MDI is more stable in ethyleneglycoldimethylether as solvent.
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