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Stability in organic solvents and identity of relevant degradation products

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Reference
Endpoint:
stability in organic solvents and identity of relevant degradation products
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Well reported non-standard study good published details
Qualifier:
no guideline followed
Principles of method if other than guideline:
Time course of isocyanate monitored by either IR spectroscopy of NCO or HPLC.
GLP compliance:
not specified
Test substance stable:
no
Transformation products:
yes
No.:
#1

IR-spectroscopy

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

.....Diisocyanate........................Carbamicacid......................Intermediate

OCN-R-NH2 .............+ ........OCN-R-NCO...... -> OCN-R-NH-CO-NH-R-NCO

....Intermediate.........................Diisocyanate.............................Urea

...........................................................................-> -> 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.

HPLC analysis

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 available.

Table 1. Shelf-life of solutions of 4,4'-MDI in DMSO: IR-spectroscopic determination of the relative NCO content as a function of time

 .

Solvent

DMSO

DMSO

Weight of 4,4'–MDI in 100 ml solvent

50 mg

500 mg

mM 4,4'–MDI

0.20

2.00

Water content of solvent

0.03%

0.04%

mM water

1.67

2.22

Start

100%

100%

15 min

37.3%

38.6%

30 min

15.6%

19.8%

45 min

8.0%

10.1%

1 h

3.2%

5.1%

4 h

1.2%

0.0%

 

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

4,4'-MDI

Monomeric MDI isomers

Polymeric MDI

EGDE

EGDE

EGDE

EGDE

EGDE

EGDE

EGDE

Weight of MDI in 100 ml of solvent

100 mg

500 mg

500 mg

500 mg

500 mg

100 mg

500 mg

mM MDI

0.40

2.00

2.00

2.00

2.00

0.40

2.00

Water content of EGDE

0.02%

0.02%

0.23%

0.04%

0.27%

0.02%

0.02%

mM water

1.11

1.11

12.78

2.22

15.00

1.11

1.11

Start

100%

100%

100%

100%

100%

100%

100%

15 min

99.6%

99.7%

99.6%

99.3%

99.4%

100%

99.6%

30 min

99.3%

99.6%

99.5%

99.2%

99.0%

99.8%

100%

45 min

99.3%

99.3%

99.6%

99.2%

98.9%

99.6%

99.7%

1 h

99.1%

99.3%

99.2%

99.0%

98.7%

100%

99.7%

4 h

98.1%

98.5%

97.3%

98.5%

95.9%

100%

99.7%

 

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.

 .

Solvent

DMSO

EGDE

EGDE

EGDE

Weight of 4,4'-MDI in 100 ml of solvent

532 mg

531 mg

1007 mg

1007 mg

mM 4,4'-MDI

2.13

2.12

4.03

4.03

Water content of EGDE

0.04%

0.11%

0.07%

0.47%

mM water

2.22

6.11

3.89

26.11

 .

Analyzed products

MDI

MDA

MDI

MDA

MDI

MDI

Start

86.5%

0.2%

100%

ND(a)

100%

100%

15 min

22.1%

8.6%

98.0%

ND(a)

-

-

30 min

1.0%

4.5%

95.3%

ND(a)

100%

96.6%

45 min

ND(a)

3.4%

95.3%

ND(a)

-

-

1 h

ND(a)

3.0%

92.3%

ND(a)

99.1%

93.3%

4 h

ND(a)

3.0%

87.6%

ND(a)

99.1%

78.9%

(a) ND, not detectable, e.g. <0.05 mg/100 ml solvent

Description of key information

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 .

Additional information