Analysis of Diisocyanate Monomers in Chemical Products Containing Polyurethanes by High Pressure Liquid Chromatography

S. C. Rastogi

Ministry of Environment, National Environmental Research Institute, M~rkh6j Bygade 26 H, DK-2860 S6borg, Denmark

K e y w o r d s

High-pressure liquid chromatography Chemical products MDI monomer TD I monomer Polyurethane

Summary

High pressure liquid chromatographic methods for the determination of diphenylmethane-4,4'-diisocyanate (MDI) and toluene diisocyanate (TDI) in chemical pro- ducts are described. The MDI- and TDI monomers were determined as their urea derivative formed by the re- action with 9-(methyl aminomethyl)-anthracene. Using these methods MDI- and TDI monomer concentratior, s have been determined in 55 chemical products: sealing waxes, insulating- and adhesive foam, hardener, primer, adhesives and surface coatings. The recovery of both MDI and TDI monomers from various types of chemical product was found to be 92-97 %, and the relative stan- dard deviations of the methods was < 5% for all types of products.

adhesives. Thus, during the use of chemical products con- taining polyurethane/prepolymer diisocyanate, a great number of people besides those involved in the production of polyurethanes may be exposed to MDI/TDI monomers. For the safe use of such products, a number of countries have adopted a directive according to which chemical pro- ducts containing polyurethane/prepolymer diisocyanate must be classified and labelled harmful/toxic depending upon the concentration of free diisocyanate monomer. Thus, for the purpose of control and supervision, it may be necessary to determine the free diisocyanate content in chemical products containing polyurethane/prepolymer diisocyanate.

A number of methods for the determination of diiso- cyanate monomers in work-room air [7 -10 ] and in com- mercial polyurethanes [11-16 ] have been published. How- ever, there is no method available for the quantitation of diisocyanate monomers in chemical products containing polyurethanes/prepolymer diisocyanate. In the present communication liquid chromatographic methods for the determination of MDI- and TDI monomers in finished chemical products have been described. The methods have been applied to the determination of MDI/TDI monomers in various types of product: adhesives, insulating foam, sealing waxes, surface coatings etc.

I n t r oduc t i on

Polymers of the polyurethane family are produced by con- densation reaction of di- and multifunctional isocyanate with a polyol containing several hydroxy end groups [1]. Thus, the polyurethane and low- molecular-weight diiso- cyanate prepolymers may contain residues of unpolymeris- ed diisocyanate monomers. The diisocyanate, i.e. diphenyl- methane-4,4'-diisocyanate (MDI) and toluene diisocyanate (TDI), are extensively used in the production of poly- urethanes, and they have been shown to be potentially harmful to human health [2 -6 ] .

A number of chemical products containing polyurethane/ prepolymer diisocyanate, in the form of surface coatings, paints, lacquers, adhesives, insulating foam, sealing waxes, inks etc. are used throughout industry as well as in private homes. Furthermore, monomer MDI in concentrations of 10 to 100% is frequently used as a hardener for many

Exper imen ta l

Principle

Diisocyanates are reacted with 9-(methyl aminomethyl)- anthracene (MAMA) to form urea derivatives. The amount of diisocyanate is calculated from the UV-absorption of its urea derivative at 254 nrn after separation by high pressure liquid chromatography (HPLC) [7, 8]. Hexamethylene diisocyanate (HDI)-urea derivative is used as internal stan- dard (i.s.). MDI is determined as the 4,4'-isomer, and TDI is determined as a mixture of the 2,4- and 2,6-isomers.

Chemicals and Reagents

MAMA (99 %) was purchased from Aldrich (FRG), triethyl- amine from Fluka (Switzerland) and HPLC-grade acetoni- trile from Rathburn (Scotland). HDI, MDI (4,4'-isomer),

Chromatographia Vot. 28, No. 1/2, July 1989 Originals 15

0009-5893/89/7 0015-04 ~ 03.00/0 �9 1989 Friedr. Viewe9 & Sohn Verlagsgese.llschafl mbH

TDI (mixture of 80 % 2,4-isomer and 20 % 2,6-isomer), and other chemicals of analytical grade were obtained from E. Merck (FRG). Traces of water were removed from the dichloromethane by adsorption on sodium sulphate.

The standard urea derivatives of HDI, MDI and TDI were prepared by the reaction of these with MAMA under de- fined conditions [8]. The urea derivatives were stored in dark bottles in a refrigerator.

Samples

Concentrations of MDI/TDI monomers were determined in 55 chemical products containing polyurethanes/prepolymer diisocyanate (Table I). These products were obtained from Danish producers or importers. Twenty of the investigated products, i.e. foam products, were in aerosol cans.

Sample Preparation

MAMA reagent 1% (V/V) in dichloromethane was freshly prepared every day. The reaction of the sample with MAMA reagent was initiated as soon as possible after weigh- ing the sample for analysis.

For the MDI analysis, a sample containing approximately 100 mg MDI (maximum 3 g sample) was weighed accurately and dissolved in 100 ml dichloromethane. In a 50 ml volu- metric flask, 1 ml of the sample solution together with 3ml of MAMA reagent and 6mr dichloromethane was stirred under a nitrogen atmosphere, for 1.5h at room temperature (22~ To this, 2 ml of the i.s. (O,8g HDI-urea derivative/liter dimethylformamide (DMF)) was added, and the volume was made up to 50 ml with DMF. 2 ml of this solution were diluted to 10 ml with the HPLC-mobile phase (see MDI-HPLC). Any persistent precipitate was removed by fi ltration or by centrifugation.

For the TDI and subsequently MDI analysis, a sample con- taining approximately 10mg TDI (maximum 3g sample) was weighed accurately and dissolved in 100ml dichloro- methane. To 10 ml of the sample solution, 3 ml of MAMA reagent was added and the sample preparation procedure was followed as for the MDI. The reaction product was diluted with the TDI-HPLC mobile phase (see conditions for TDI-HPLC).

HPLC

HPLC of MDI- and TDI-urea was carried out under the following conditions:

Table I. Polyurethane products analysed for free MDI/TDI monomer content,

Products No. of samples

Sealing waxes 12 InsuLating- and adhesive foam 20 Adhesives 9 Hardener for adhesives and sealing waxes 6 Primer 6 Surface coating 2

MDI

Precolumn:

HP LC-column:

Mobile phase:

Flow:

Injection volume:

Detector:

M D I standard :

2cm x 0,46cm packed with C1~ Corasil, 37-50/~.

25cm x 0,46cm ODS Hypersi RP18, 5P (from Bischoff, FRG)

Acetonitri le/4% aqueous triethyI amine adjusted to pH 3.0 with ortho phosphoric acid (75/25).

1,5 ml/min, pressure 1500 psi.

20pl.

UV at 254 nm.

14mg MDI-urea derivative was dis solved in 8ml i.s. (0,8g HDI-urez derivative/liter DMF), and the mix ture was diluted to 50 ml with DMF 0,5 ml of this solution was diluted tr 10ml with the mobile phase. Thf diluted standards were stable for on~ week at 4 ~

TDI and subsequently

HP LC-conditions:

HP LC-column:

Mobile phase:

TDi Standard:

TDI+MDI Standard:

IVlDI

as for MDI but with the followin! changes:

25 cm x0,46 cm ODS Hypersil RP18, 5/ l + 25 cm x 0,46 cm Spherisorb $5 ODS, 5/~.

Acetonitrile/2 % aqueous triethyI. amine adjusted to pH 3.0 with ortho phosphoric acid (80/20).

20mg TDI-urea derivative was dis solved in 50 ml DMF. To 1 ml of thi! solution, 0,8ml i.s. solution (se~ above) was added and the mixturf was diluted to 10ml with DMF. 1 rnl of this was diluted to 10 ml with the mobile phase.

20 mg TDI-urea derivative and 70 rn! MDI-urea derivative were dissolved ir 50 ml DMF. Thereafter procedure a~ for the TDI.

Reagent blanks were prepared and analysed as the samples At least two determinations of MDI/TDI in each sampl~ were carried out. The ratio of the peak heights of the MDI, TDl-urea derivatives to the peak height of i.s. were used f01 the calculation of MDI/TDI concentrations in the samples.

HPLC of MDI-urea derivative (0-66/Jg MDI/ml) and TDI urea derivative (0--80pg TDI/ml) containing HDI-urea deri vative (6,4Mg HDI/ml) were performed to prepare the cali bration curves. The reproducibility of the method was cal culated by performing 6 -8 analyses of 4 different sample (adhesive, sealing wax, primer and insulating foam) fe MDI, and a surface coating material for TDI. Analyses 0'. these samples were also performed after spiking them witt MDI/TDI to calculate the recovery of these substances bt the present methods of analysis.

16 Chromatographia Vol. 28, No. 1/2, July 1989 Originari

Resul ts and C o n c l u s i o n s

The identification of MDI and TDI in the sample was done on the basis of retention times (RT) of their urea deriva- tives after separation by HPLC. The HDI-urea derivative and the 2,6-TDI urea derivative had the same RT when they were analysed by the MDI-HPLC method. Therefore, the HDl-urea derivative could not be use as an i.s. for the ana- lysis of samples containing TDI monomer. The samples containing TDI monomer and eventually MDI as well as HDI monomer were analysed by the TDI-HPLC method. Although the speed of the analysis was reduced by approxi- mately 40% compared to that by the MDI-HPLC method (Figures 1-3) the separation of the three urea derivatives was perfect by the TDI-HPLC method. Furthermore, no interference was observed for the analysis of MDI/TDI when polyurethane-containing products were analysed by the TDI-HPLC method. Thus, the method of choice for the routine analysis of MDI wi l l obviously be the MDI-HPLC method, unless it is observed that some other substance present in a chemical product interferes with the analysis of MDI.

It was found that the detection limits for the MDI- and TDI monomers by the present methods of analysis were 50 ppb and 5 ppb respectively, The calibration curves for MDI and TDI were found to be linear over the concentration ranges

,,8,

cP

m ~ M N

o, ra too

m l l l | ~ l i l l ~ l l i I I I

R T

M I N

Figure 1 Analysis of a foam adhesive by the MDI-HPLC method. RT 6, 96: HDI-urea derivative used as internal standard, RT 10,73: MDI-urea derivative.

,2

V*

S I o I I ~ I I I ! o ! I I I | a | I I I = I o

= - ~ - ~ M I N . ~ r

R T Figure 2

Separation of standard urea derivatives of 2,4-TDI (RT 11,88), 2,6- TDI (RT 12,86), HDI (RT 14,86), and MDI (RT 18,14) by TDI- HPLC method.

N

~, =. ' =a**

. . . . ~ . . . . $ . . . . ~ . . . . $ , , , - - - M I N

R T

Figure 3 Analysis of a primer by TDI-HPLC method. RT 11,88: 2,4-TDI-urea derivative, RT 12,86: 2,6-TDi-urea derivative and RT 18,12: MDI- urea derivative.

Chromatographia Vol. 28, No. 1/2, July 1989 Originals 17

investigated in the present study. The recovery of MDI and TDI from various types of chemical products was found to be 92-97 %. The relative standard deviation of the analysis was found to be < 5 % in all cases.

Sampling of the foam products from aerosol cans was achieved, wi thout any problem, by directly transferring the sample from the aerosol can into the reaction flask. The dif- ference between the weights of aerosol can before and after the sampling was considered as the weight of the sample analysed.

Of the 55 products analysed, 5 products (2 surface coating, 2 primer, 1 sealing wax) were found to contain 0 ,006- 1,12 % (w/w) TDI monomer. 44 of the investigated pro- ducts contained 0 ,02-100% MDI-monomer. One of the primers contained both MDI- and TDI monomer and an other primer contained all three monomers i.e. HDI-, MDI- and TDI monomer. The concentrations of these diiso- cyanate monomers in the last mentioned sample were deter- mined using appropriate calibration curves prepared with- out the use of any i.s.

The methods reported in the present communication may be used by industry for the control of free diisocyanate mo- nomer content in polyurethane products. They may also be used for the control and supervision of polyurethane pro- ducts by national authorities. According to a E.E.C. direc- tive (83/265/E.E.C.) chemical products must be classified and labelled "harmfu l " when they contain i> 2% MDI mo- nomer or 0 ,5 -2 % TDI monomer. However, manufacturers of the products containing diisocyanate are not obliged to declare the concentration of free monomer in the products. In the present study it was found that among the MDI con- taining products < 10 % MDI was present in 31% of the products, 10-20% MDI was present in 29% of the pro- ducts, 20 -30% MDI was present in 24% of the products, and the remaining products contained 30 -100% MDI monomer. As the hazard potential of a product may de- pend upon the concentration of MDI present in it, it may

be argued that the declaration of the concentrations of di. siocyanate monomer in a chemical product may be of value to all consumers, but especially, to those in small and medium scale industry.

Acknowledgements

Technical assistance was provided by Mette R. Nielsen and Kirsten Marijnissen.

References

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Received: May 5, 1989 Accepted: May 10, 1989 B

18 Chromatographia Vol. 28, No. 1/2, July 1989 Originals