Anal. Chem. 1994,66, 3261-3213
Characterization of (Methoxymethy1)melamine Resins by Liquid Chromatography/Mass Spectrometry Ted T. Chang Cyfec Research Division, Cyfec Industries, 1937 West Main Street, Stamford, Connecticut 06904
(Methoxymethy1)melamine resins (melamine resins) are complex mixtures of melamines with different degrees of hydroxymethylation and methoxymethylation. In spite of the extensive industrial applications of melamine resins, characterization of these resins has been a difficult task because of their complex compositions. Following the development of an efficient HPLC separation procedure, LC/MS was utilized for identification of separated components. Structures were assigned from molecular weight and fragmentationinformation. Twenty monomericcompoundsand 13dimeric compoundswere identified. (Methoxymethy1)melamine resins, commonly known as melamine resins, are important synthetic thermosetting resin products. They are widely used in various industries as binder components and cross-linkers.’ Melamine resins are synthesized by reacting the primary amino groups of melamine with formaldehyde to form hydroxymethyl groups and converting these hydroxymethyls to methoxymethyl groups by condensation with methanol. The degrees of hydroxymethylation and methoxymethylation are varied according to the intended end use applications. Melamine resins are mixtures of components with different degrees of substitutions. The complex nature of melamine resins has been confirmed by high-performance liquid chromatography (HPLC)2+3and mass s p e c t r ~ m e t r y . Since ~~~ separation and identification of these mixtures is not an easy task, the structures of melamine resins are expressed in a general composition to indicate the mole ratios of formaldehyde and methyl groups to melamine. These ratios are determined by NMR analysis. Two research groups have recently attempted to identify individual components in melamine resins. Edbon, Hunt, and O’Rourke used preparative HPLC to collect major components for 13C NMR analysk6 Longordo, Papazian, and Chang compared the profile of HPLC with the profile of fast atom bombardment (FAB) for major Both approaches attained only partial peak a~signments.~ success. Recently liquid chromatography/mass spectrometry (LC/ MS) has proven to be an efficient technique for complex mixture analysis.* Using LC/MS, we have successfully (1) Blank, W. J. J. Coat. Technol.1979,51,61-70. (2) Kawai, S.; Nagano, H.; Maji, T.J. Chromatogr. 1989, 479, 467-470. (3) Oguri, H. Shikizai Kyokaishi 1982, 55, 812-17. (4) Saito, J.; Toda, S.;Tanaka, S.Netsu Kokasei Jushi 1980, 1, 18-25. (5) Simonsick, W. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1989,43,257-74. (6) Ebdon, J.; Hunt, B.; O’Rourke, W. Br. Polym. J . 1987, 19, 197-203. (7) Longordo, E.; Papazian, L.; Chang, T. J . Liq. Chromatogr. 1991.14.204363. (8) Nissen, W.; van der Gretf, J. Liquid Chromatography-Mass Spectrometry; Marcel Dekker: New York, 1992.
0003-2700/94/0366-3267$04.50/0 0 1994 American Chemical Soclety
identified most components in melamine resins without resorting to laborious and error prone fraction collection. Combining the molecular weight and fragmentation information, structures of 20 monomeric compounds and of 13dimeric compounds were identified. This study characterizes melamine resins by identifying major components and their composition profiles. This is a significant improvement over the traditional method of the past 20 years, which is based upon the bulk ratio of three major constituents. The traditional method is also a more time consuming process, because these three constituents are analyzed by different analytical procedures. Another advantage of the new technique is that its component specificity has provided us with a powerful tool for several physicochemical investigations currently in progress, such as reaction mechanism, equilibrium study, and product evaluation.
EXPERIMENTAL SECTION Instrumentation. The LC/MS system employed in this study consisted of a Finnigan 700 TSQ quadrupole mass spectrometer, a Finnigan TSP-2 LC/MS interface unit, and a Hewlett-Packard 1090 HPLC system equipped with a diodearray detector. The mass spectra were acquired from m / z 100 to m / z 1400 at a scanning rate of 2 s/scan. UV absorbance at 235 nm from the HPLC detector was incorporated into the data system of the mass spectrometer. The HPLC conditionsbasically followedthose of Longordo, Papazian, and Chang7 with minor modifications to accommodate LC/MS operation. Two Waters Nova-Pak C18 columns (3.9 X 150 mm, 60A, 4 pm) were connected in tandem for the separation. Methanol and HzO were the mobile phases. Both mobile phases contained 0.05 M ammonium acetate. Analysis was carried out with a linear gradient system. The starting methanol concentration was 25%. The final methanol concentration was 100% at 50 min. Flow rate was 0.75 mL/min. Sample size was 10 pL of a 0.2% solution. Column temperature was ambient. The dimer fraction was not preseparated. The gradient system was slightly modified for dimer analysis. The starting methanol concentration was 60%. The final methanol concentration was 90% at 40 min. The column was washed with 100%methanol for 10min before reuse. A larger sample size, 20 pL of a 0.2%solution, was used. Materials. All (methoxymethy1)melamineresins (melamine resins) are products of Cytec Industries. Although these products are generally considered safe, they should be handled with the usual safety precautions. The 0.2% sample solutions were prepared by dissolving 20 mg of sample in a IO-mL Analytical Chemism, Vol. 66,No. 19, October 1, 1994 3267
volumetric flask with a 20/80 water/methanol solution. The sample solutions were filtered prior to HPLC analysis.
RESULTS AN0 DISCUSSION Terminology. The structures of melamine resins are traditionally expressed by a “general formula” MF,Me,, where M represents the melamine ring, F represents the formaldehyde units on the three primary amino groups, and Me represents the end-capping methyl group. In this general formula, x and y need not be equal, but y cannot be larger than x . Most commercial products use this general formula to indicate the degrees of hydroxymethylation, x, and of methyl end-capping, Y. In this study, we introduce “structure formulas” for additional structural information beyond the general formula. The structure formula is expressed as M(FMe),F,H,. Since each methylation process always occupies an F group, FMe is treated as one unit in this expression. The unreacted hydrogen atom of the amino group is expressed as H functionality. The sum of x, y , and z is always 6 for monomeric compounds, since there are six reaction sites at the three primary amino groups of melamine. Similarly, the sum of x, y, and z is always 10 for dimeric compounds. Figure 1 shows four representative structures of melamine resins to illustrate the terminology used in this study. Structure I (peak 18), M(FMe)6, is a fully methoxymethylated compound. Structure I1 (peak lo), M(FMe)3(FFMe)FH, has three FMe (CH2OCH3) functionalities, one FFMe (CH1OCH20CH3) functionality, one F (CHIOH) functionality, and one H functionality. Structures I11 and IV are both fully methoxymethylated dimeric compounds. Structure I11 (peak 31), [M-Me-M](FMe)lo, is a methylene linkage compound, and structure I V (peak 28), [M-FMe-M](FMe)lo is an ether linkage compound. Several expressions are frequently used in this text: both hydroxymethylation and formylation refer to the F functionality, methylation refers to the end-capping of F functionality, and methoxymethylation refers to the FMe functionality. HPLC Chromatograms of Melamine Resins. Melamine resins are mixtures of melamine derivatives with various degrees of methylolation and methylation of the terminal hydroxy groups. The degrees of substitution of commercial products are varied according to their applications. Figure 2 shows the HPLC chromatograms of four typical commercial amino resins. Resin A is a highly substituted melamine resin, and resin D is the least substituted melamine resin. Dimeric and trimeric fractions usually present in melamine resins are not included in this figure. Since melamine has six substitution sites, it is apparent that most melamine resins are highly complex mixtures. This figure also indicates that the polarity of a melamine resin decreases with increase of substitution, leading to longer retention time. LC/MS of Melamine Resins. A sample containing equal weight amounts of resins B, C, and D was prepared for LC/ MS analysis. Figure 3 compares the total ion current (TIC) chromatogram from the mass spectrometer and the HPLC chromatogram obtained with UV detector. The close similarity of the two chromatograms suggests that all monomeric components have comparable mass spectrometry ionization efficiencies and UV absorbencies at 235 nm. 3260
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Structure I M(F’Me), CHzOCHs
CH30CH2/N\
CH20CH3 I
CH20CHzOCH3
Structure I1 M(FMe), (FFMe)F
Structure IV [M-FMe-MI ( m e ) Flgure 1. Structures of typical melamine reslns.
Mass Spectra of Melamine Resin. Thermospray ionization produces stable MH+ ions for melamine resin components. This is in sharp contrast to FAB ionization, where the spectra consist mostly of fragment ions with barely detectable MH+ ions. As shown in Figure 4, under thermospray ionization conditions, some compounds produce mainly MH+ ions, while other compounds also produce fragment ions. In general, the fully methylated compounds produce few fragment ions, and partially methylated compounds produce intense fragment ions. There are three major fragmentation pathways: loss of CH30H (-32 Da), loss of C2H40 (-44 Da), and loss of HCHO (-30 Da). Loss of CH3OH, which is prevalent in FAB, is not a favored pathway for monomers but becomes more significant for dimers. Loss of C2H40 is insignificant for higher molecular weight compounds but becomes more significant for lower molecular weight compounds, especially for the partially substituted compounds. It appears that in a partially substituted amino group, the presence of an H functionality acts as a contributing factor for this fragmentation. The loss of HCHO is observed for all compounds containing F functionality (CH20H). Figure 4 shows the mass spectra
100
50
Resin A
100
Resin C 50
ir
min.
AL
(T-
0 Figure 2. HPLC chromatograms of amino resins, UV = 235 nm. 8
of four typical compounds with different numbers of F functionalities. Peak 6 does not contain a F functionality, because the numbers of F and Me are equal. The other three compounds contain F functionalities because the numbers of Fare larger than the numbers of Me. These three compounds are peak 6 , M(FMe)4H2; peak 7, M(FMe)4FH; peak 9, M(FMe)4F2; and, peak 10, M(FMe)s(FFMe)FH. The F-containing compounds will continue to fragment until all F functionalities are depleted. Peak 10 has the same general formula as peak 9 but loses only one formaldehyde, suggesting
1
40
25:'OO
3 3 :'20
that the extra F group is incorporated into one of the FMe functionalities as an FFMe functionality. Interpretation of this fragmentation scheme provides useful structural information without resorting to an MS/MS study. The vulnerability of the F functionality in thermospray ionization is comparable to the deformylation process of the same compound in the presence of an acid ~ata1yst.l.~The structure of the MH+ ion produced by thermospray ionization (9)Wilson, R.FATIPEC-Kongr., 20rh 1990, 155-158.
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100
18
14
7
19
50
17
h
2o
J
min.
8:20
1
$0
25
:.oo
33: 0
Figure 3. TIC and UV chromatograms of monomer fraction.
is similar to the structure of the acid-catalyzed intermediate. When the protonated molecule has both F (CHIOH) and FMe (CH20CH3) functionalities, deformylation appears to be the favored process. H‘
I -N--CH?OH I
CHPOCH~
-
H’
I
-NH
I
+ HCHO
CH20CH3
Assignment of Structure Formula. Structure formulas are assigned with the following considerations (detailed discussions are given separately): (1) the general formula; (2) the number of F functionalities; (3) the number of FMe functionalities; (4) the number of H functionalities; ( 5 ) the number of FFMe functionalities. (1) Determination of the General Formula. The molecular weight of each separated component is obtained from the observed MH+ ion. Assignment of the general formula is a straightforward task. The moleculat mass of melamine is 126 daltons. Addition of one formaldehyde increases the molecular mass by 30 Da. Each methylation increases the molecular mass by 14 Da. In all cases, only one logical general formula can be assigned to the molecular weight of each component, excluding unusual reactions. (2) Confirmation of the Structure Type. In general, the number of Me functionalities equals the number of FMe 3270
Analytical Chemktry, Vol. 66,No. 19, October I , 1994
functionalities. Exceptions are compounds with FFMe functionalities. As was discussed previously, all M( FMe),F, compounds will continuously deformylate to produce a fragment ion with the M(FMe), basic structure (the H functionality is not expressed in this paragraph for ease of discussion). Therefore, by tracing the mass numbers of various M(FMe), basic structures in the chromatogram, it is possible to confirm the structure type of the separated compounds. Figure 5 shows the mass chromatograms of key ions representing five basic structures: mlz 259 (MF3Me3), m/z 303 (MF4Me4), m / z 341 (MFsMes), and m / z 391 (MFsMea). It is evident that peaks 1-5 have M(FMe)sF, structures, peaks 6-9 have M(FMe)4Fy structures, peaks 13 and 14 have M(FMe)5Fy structures, and peak 18 has the M(FMe)6 structure. Peaks 8 and 9 have a similar structure formula. Peak 8 probably is an asymmetric isomer of peak 9, having both F functionalities at the same amino group. Similarly, peak 4 probably is an asymmetric isomer of peak 5 . (3) Determination of the Number of F Functionalities. The number of F functionalities is determined by the number of deformylations from the M H ion to the basic structure. As shown previously in Figure 4, this number is easily obtained from the sequential loss of 30 Da. (4)Determinationof the Number of H Functionalities. The three primary amino groups of melamine offer six reaction sites. If the sum of F functionalities and FMe functionalities is less than six, then the remainder is the number of H
303
Peak 6
303
333
333
F
I
In/ 2 250 Flguro 4. Mass spectra of melamine reslns contalnlng F groups.
functionalities. This number is not shown in the general structure but is expressed in the structure formula. Many early eluting compounds belong to this category. (5) Determination of FFMe Functidties. In some highly formylated compounds, the total F number in the general structure often exceeds the sum of F numbers in the F functionalities and the FMe functionalities. This excess F number is assigned as the number of FFM functionalities.
1.
,
I
300
The number of FM functionalities is accordingly reduced. Under excess formaldehyde, an F functionality can propagate to an FF functionality (CHzOCH20H). Methylation of this functionality results in formation of a FFMe functionality. Using these evaluation steps, 20 major peaks in Figure 3 are identified. Table 1 lists their molecular weights, general formulas, numbers of F, FMe, and H functionalities, and proposed structure formulas. Ana!vticalChemistry, Vol. 66,No. 19, October 1, 1994
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1 1 3
50-
loo-
M(FMe)3 Fy
m/z:303 6 17 9
M(FMe)4 Fy
50-
Ah
m/z:347
50-
M(FMe)cj Fy
d
100-
50-
e
Tabh 1. Structure of Monomeric Compounds In Melamlne Rerlnr Functionality structure peak MW F FMe FFMe H formula
1 2 3 4 5 6 7 8 9 10 11 12 13 14
15 16 17 18 19 20
258 288 318 348 348 302 332 362 362 362 392 392 346 376 376 406 406 390 420 450
1 2 3 3 1 2 2 1 2 2 1 1 1
3 3 3 3 3 4 4 4 4 3 3 3 5 5 4 4 4 6 5 4
1 1 1 1 1 1 1 2
3 M(FMehH3 2 M(FMe)sFHz 1 M(FMe),F2H M(FMe)Ps M(FMW3 2 M(FMehH2 1 M(FMe)4FH M(FMe14Fz M(FMe)& 1 M(FMe)l(FFMe)FH M(FMe)s(FFMe)Fz M( FMe)3(FFMe)Fz 1 M( FMe)sH M(FMe)sF 1 M(FMe),(FFMe)H M(FMe),(FFMe)F M(FMe)d(FFMe)F M(FMe)6 M(FMe)s(FFMe) M(FMe)4(FFMe)2
Characterizationof the Dimer Fraction. Melamine resins are known to contain a considerable amount of oligomers, especially dimers. The compositions of oligomer fractions are more complex than that of the monomer fraction. Figure 6 shows the TIC chromatogram and UV chromatogram of 3272 AnalyttCalChemistry, Vol. 66,No. 19, October 1, 1994
Tabio 2. Sttrwture ol Dlnmic Compoundr In Molmho R d n A peak MW general formula structure formula
21 22 23 24 25 26 27 28 29 30 31 32 33
720 690 616 646 690 660 690 734 720 660 704 734 734
MzFllMelo M~FIOMC~O MzFsMw M2FgMeg MzFloMelo MzFgMeio MzF~oMelo M2F1lMe1I M ~ F mel I lo M2FgMe10 M2FloMell M ~ FlMel1 I M~FIIM~II
[M-FMe-M](FMe)gF [M-FMe-M](FMe)9H [M-Me-MI (FMe)sHz [M-Me-MI (FMe)aFH [M-FMe-M](FMe)gH [M-Me-M](FMe)gH [M-Me-M](FMe)gF [M-FMe-MI (FMe) 10 [M-Me-MI (FMe)s( FFMe)F [ M-Me-MI (FMe)gH [M-Me-M](FMe)lo [M-Me-MI (FMe)g(FFMe) [M-Me-M](FMe)g(FFMe)
the dimer fraction of Resin A. Resin A contains a major peak and three minor peaks in its monomeric fraction (see Figure 2). However, its dimer fraction produces over a dozen components. The general formulas and tentatively identified structure formulas of 13 compounds are listed in Table 2. The linkage between two melamine groups is a subject of interest, at least academically. There are two major possible types of linkages: the methylene linkage, [M-CHz-MI or [M-Me-MI, and the ether linkage, [M-CHzOCHz-M] or [M-FMe-MI. The major compound, peak 3 1, is a fully methoxymethylated compound with only one possible struc-
19 10-
31 TIC
5-
30
,
20
25
26 27
A
33
w
min.
1o:'oo
13 :'20
Flgure 8. TIC and UV chromatograms of dimer fraction.
ture, a methylene linkage structure. This result suggests that the methylene linkage is the predominant linkage. The presence of the less favored ether linkage is also confirmed. Adding an extra formaldehyde to peak 31 produced MW = 734 compounds. There are three possible sites to place this additional formaldehyde: an ether linkage compound and two methylene linkage compounds, one at the methylene-linked amino group and the other at the unlinked amino group. As shown in Table 2, we also observed three compounds with MW = 734. From the study of the monomer fraction, we found that an FFMe functionality causes a compound to elute later (compare peaks 18, 19, and 20 of Table 1). Accordingly, peaks 32 and 33, which elute later than peak 3 1, are assigned as methylene linkage compounds. The FFMe position of the larger peak 33 probably is at one of the eight unlinked amino groups, and the FFMe position
of the smaller peak 32 probably is at one of the two linked amino groups. The remaining peak 28, which elutes earlier than peak 3 1, is assigned as an ether linkage compound. The assumption that the ether linkage is more polar than the methylene linkage is also applied to structure assignment of other peaks. Lacking sufficient model compounds, this assumption remains hypothetical. Considering the fact that sample A produces a complex dimeric profile from a simple monomeric profile, it would be a difficult task to characterize its trimeric profile. Similarly, characterization of the dimeric fraction of lesser derivatized melamine resins was not attempted. Received for review May 11, 1994. Accepted June 21, 1994. Abstract published in Advance ACS Abstracts. August 1, 1994.
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