Donata Favretto CNR Area di Ricerca, Corso Stati Uniti 4, I Padova, Italy. Adriano Marcelli McWorther, Sant Albano Stura, Cuneo, Italy.

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1 D. Favretto, P. Traldi and A. Marcelli, Eur. Mass Spectrom., (1998) 371 Analysis of Butylated Amino Resins by HPLC/ESI and APCI D. Favretto, P. Traldi and A. Marcelli, Eur. Mass Spectrom., (1998) High performance liquid chromatography/ electrospray ionisation and atmospheric pressure chemical ionisation mass spectrometry for the analysis of butylated amino resins Donata Favretto CNR Area di Ricerca, Corso Stati Uniti, I-3100 Padova, Italy. Pietro Traldi CNR, Centro di Studi sulla Stabilita e Reattivita dei Composti di Coordinazione, Via Marzolo 1, I-3100 Padova, Italy. Adriano Marcelli McWorther, Sant Albano Stura, Cuneo, Italy. Etherified amino resins are used in conjunction with alkyds, polyesters, epoxy or acrylic resins in coating formulation. They are formed from the reaction between formaldehyde and compounds containing NH groups, for example, amide, amine or imide groups. Control of this reaction is important in order to obtain the appropriate mixture targeted for the intended use and, as a consequence, the development of analytical procedures able to describe the different species present in the final resin mixtures is necessary. The use of high-performance liquid chromatography, coupled with electrospray ionisation or atmospheric pressure chemical ionisation mass spectrometry, proved to be efficient in the characterisation of n-butylated melamine-formaldehyde resins. The same analytical approach was also applied to describe the first steps in the synthesis of i-butylated urea-formaldehyde resins. Keywords: Melamine resins, HPLC/MS, ESI, APCI, ion trap. Introduction Etherified amino resins are widely used, normally in combination with other polymeric species, in surface coating systems. Since 190, the most commonly used amino resins are those produced from urea, melamine and, to a lesser extent, benzoguanamine. 1 More particularly, the principal types of amino resins used as cross-linking agents in industrial stoving paints and lacquers are butylated ureaformaldehyde resins and butylated melamine-formaldehyde resins; more recently, methylated melamine-formaldehyde resins have gained prominence. The first step in the preparation of the etherified amino resins is hydroxymethylation, i.e. the reaction between the amino groups of urea or melamine and formaldehyde to form hydroxymethyl groups: IM Publications 1998, ISSN

2 37 Analysis of Butylated Amino Resins by HPLC/ESI and APCI Once formed, the hydroxymethyl groups can react to give a condensed structure by loss of water and/or formaldehyde and, by this means, stepwise polycondensation can occur with a gradual increase in molecular weight. RNHCH OH + RNHCH OH RNHCH NHR + CH O + H O RNHCH OCH NHR +HO The second synthetic step is the etherification of the produced hydroxymethyl compounds. Alkylation can, in fact, restrict the amount of condensation by removing hydroxymethyl groups and thus achieving a product suitable for use as a surface coating. The degree of etherification can be controlled by temperature, reaction time and ph of the reaction as well as by the molar ratio of the alcohol used. Normally, low ph and excess alcohol favours alkylation over the competing polymerisation reaction. The most widely used alcohols for etherification are n-butanol, i-butanol and methanol and the solubility of the etherified resin products in either water or organic solvents then varies according to the alcohol used. Thus, methylated amino resins are soluble in water and methanol whereas butylated amino resins are soluble in less polar organic solvents. + H R NHCH OH + R OH R NHCH OR +H O where R =CH, C H, i C H 3 n 9 9 Etherified amino resins are used in conjunction with alkyds, polyesters, epoxy or acrylic resins in coating formulations such as stoving paint or acid curing systems. They are described as polymers and react according to the general acid catalysis. Urea resins are known for better adhesion properties than melamine resins, which show, in turn, better chemical resistance and exterior durability. The etherified amino resins available on the market differ in the degree of alkylation and molecular weight distribution. The control of the reaction and, therefore, of the degree of hydroxymethylation and etherification is important in order to obtain a mixture targeted for the specific intended use. The development of analytical procedures able to describe, and possibly quantify, the different species present in the final resin mixtures is consequently necessary. Both electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI) were recently successfully applied to the high-performance liquid chromatography/mass spectrometry (HPLC/MS) analysis of methylated amino resins, which are easily amenable to HPLC procedures due to their relatively low molecular weight and high solubility in aqueous solvent systems. Butylated amino resins, however, present a more selective solubility (for example, they are not water-soluble) and higher molecular weights. Their analytical profile is checked, during their industrial synthesis, using gelpermeation chromatography (GPC) with refractive index (RI) as a detection system, but HPLC analysis is used, when possible, in order to achieve a better detailed description of the mixtures produced. In particular, n-butylated formaldehyde-melamine resins, even though less soluble, less polar and with higher molecular weights than their methylated analogues, can be subjected to HPLC procedures. i- butylated ureic amino resins show a molecular weight distribution, as determined by RI index measurement, from 00 to 0,000. The feasibility of using reversed-phase HPLC procedures for such a wide distribution is, therefore, practically negligible. However, with the description of the composition of the lowest molecular weight, most soluble fractions of such resins could be of interest. In particular, the lack in the ureic structure of chromophores, which absorb at appropriate wavelengths, hampers UV detection so that HPLC/MS can offer an interesting analytical approach for the characterisation of ureic amino resins with a low degree of polymerisation although it remains ineffective for the analysis of cross-linked systems with a high degree of polymerisation. Pursuing our interest in the application of the newest ionisation techniques to the knowledge of the composition of synthetic polymers, we thought it would be of interest to undertake an HPLC/MS study of commercial n-butylated melamine resins and i-butylated ureic resins. Experimental Samples Sample a is an n-butylated melamine-formaldehyde resin with a medium degree of alkylation, a low to medium content of hydroxymethyl groups and a medium content of imino functionality. This resin is a very efficient crosslinking agent for backbone polymers containing hydroxyl and amide functional groups, such as epoxy, alkyd/polyester or acrylic resins. These resins are specially designed for medium solid systems, providing a fast cure response and low formaldehyde release during cure. The main physicochemical characteristics are as follows: Non volatile content (ISO 31, DIN 31) 70% Dynamic viscosity at 3 C 900 mpa White spirit tolerance at 3 C, % 1000 Molecular weight by GPC (Mw) 1700 Combined molar ratio, detected by 13 C NMR, is melamine/formaldehyde/n-butanol 1::.. Sample b is the most soluble fraction of a commercial isobutylated urea-formaldehyde resin, obtained by preparative GPC, with molecular weight distribution from 10 to 000 Da as determined by GPC using a RI detector. The isobutylated urea-formaldehyde resins are known as crosslinking agents with a medium degree of alkylation, medium hydroxymethyl content and low imino functionality and this resin is especially designed for use in a fast-curing wood coating system. Its main physiochemical characteristics are reported in the following table:

3 D. Favretto, P. Traldi and A. Marcelli, Eur. Mass Spectrom., (1998) 373 Non volatile content (ISO 31, DIN 31) % Viscosity at 3 C 900 mpa White spirit tolerance at 3 C, % 10 Molecular weight by GPC (Mw) 000 Instrumentation HPLC/MS measurements were performed on a Finnigan (San Jose, CA, USA) LCQ ion trap mass spectrometer equipped with either an electrospray (ES) or an atmospheric pressure chemical ionisation (APCI) interface and connected to a SpectraSeries P000 (Thermo Separation, San Jose, CA, USA) HPLC pump. The electrospray system employed a kv spray voltage, with nitrogen as sheath and auxiliary gas set at flow rates of 0 and 0 (arbitrary units, au) respectively, and a heated capillary temperature of 00 C. The APCI system employed a source temperature of 0 C, a discharge current of ma and nitrogen as sheath and auxiliary gas with flow rates of 0 and 10 au, respectively. The mass spectrometer was operated in positive-ion mode with a scan range from m/z 10 to 000 at a scan rate of 0. scan s 1. The HPLC conditions were as follows: (i) For sample a, a Prodigy ODS(3) column (Phenomenex, Torrance, CA, USA) (. 10 mm, µm) was used with a linear gradient from 80% H O and 0% methanol to 100% methanol in 0 min, then isocratic for 0 min. The mobile phase flow rate was 0.8 ml min 1. Sample loading was 0 ml of a 0.% solution in n-butanol. (ii) For sample b, a Prodigy ODS(3) column (. 10 mm, µm) was linked to a shorter Prodigy ODS(3) column (. 0 mm, µm) and used with a linear gradient from 80% H O (containing 0.1% acetic acid) and 0% methanol (containing 0.1% acetic acid) to 100% methanol (also containing 0.1% acetic acid) in 0 min, then isocratic for 0 min. The mobile phase flow rate was 0.8 ml min 1. Sample loading was 0 ml of a 0.% solution in i-butanol. In HPLC/ESI-MS experiments, the flow was split in order to send 0. ml min 1 to the ESI interface and 0. ml min 1 to a SpectraSystem UV1000 (Thermoquest, San Jose, CA, USA) UV detector operated at a wavelength of 3 nm. Results and discussion The HPLC/UV and HPLC/ESI-MS profiles of sample a, the n-butylated melamine resin, are reported in Figure 1. As may be seen, the two detection systems yield analogous results in terms of the qualitative profile but different relative abundances of the various components. As was previously observed for methylated melamine resins, ESI-MS detection seems to accentuate higher molecular weight components that are only at the noise level in the UV trace. Under these conditions, MNa + adduct ions represent the base peak of the spectrum for all the components of the chromatogram, while a scarcely abundant MH + ion can be detected only in the case of the early eluting components of the mixture. For the higher molecular weight components, cluster ions of composition [M + Na] + are as abundant as the MNa + species. Figure 1. HPLC/UV and HPLC/ESI-MS chromatograms obtained for sample a.

4 37 Analysis of Butylated Amino Resins by HPLC/ESI and APCI Figure. ESI mass spectrum of the components with retention time.7 min in sample a. This drawback often complicates the analysis of the ESI mass spectra when two or more components co-elute. As an example, the ESI mass spectrum of the component with a retention time of.7 min in sample a, which by UV detection seemed to be a pure compound, is reported in Figure. In the m/z range , MNa + ions are detected for each component (A D) of the chromatographic peak, while in the higher m/z region of the spectrum, ionic species corresponding to [M + Na] + are present showing combinations of different molecular species. In order to avoid the formation of such cluster adducts, we tried to perform the same HPLC procedure by adding 0.1% acetic acid to the aqueous mobile phase to enhance the production of protonated molecules, 3 but no change in the abundance, either of the MNa + or of the multimolecular clusters, was observed, thus proving that cluster formation is very easy for these compound types under positive-ion ESI conditions. We were therefore persuaded to perform HPLC/APCI experiments on the same sample; in this case, mass spectra with readily detectable MH + species were obtained for the s, even though a high degree of fragmentation is often observed. The fragmentation is particularly extensive for s and oligomers, for which no ionic species MH + could be detected, thus hampering the use of APCI for a complete description of this resin. In Figure 3, the ESI and APCI spectra of the component of sample a eluting at 1.8 min are reported. As can be seen, the APCI mass spectrum shows sequential losses of 30 mass units from the ion at m/z 7 (MH + ). This behaviour is particularly useful in assigning the chemical structure of unknown components, because it is typical of melamine compounds containing one or more hydroxymethyl functionalities. The same fragmentation data can be obtained, as previously stated, by performing multi-stage tandem mass spectrometry (MS n ) experiments on ESIproduced MNa + cations. However, the molecular weight of the components, as inferred from their ESI mass spectra, was sufficient in most cases to identify the chemical structure in terms of or multimer units, as well as in terms of n- butylated or free hydroxymethyl functionalities, while MS n experiments were reserved for the more puzzling cases. As Figure 3. ESI and APCI mass spectra of the component with retention time 1.8 min in sample a.

5 D. Favretto, P. Traldi and A. Marcelli, Eur. Mass Spectrom., (1998) 37 Table 1. n-butylated melamine-formaldehyde resin. Retention time MW Structure No. of functionality Total F FBu FF Retention time MW Structure No. of functionality Total F FBu FF s trimers trimers s or 3 for 1 for or 3 for 3 for tetramer tetramer A 8 B 98 C 78 D 78 E s or 3 for trimers tetramers tetramers for 1 for 3 for

6 37 Analysis of Butylated Amino Resins by HPLC/ESI and APCI Table. i-butylated urea-formaldehyde resin. Retention time MW Structure no. of functionalities total F FBu FFBu Retention time MW Structure no. of functionalities total F FBu FFBu trimer trimer tetramers trimer trimer for trimer trimers trimer s 3 s tetramer.39 8 tetramers 1 for trimer trimer 7. 3 trimer n > n > n > n > trimer trimer 3 an example, tandem mass spectrometry (MS/MS) was performed to check the presence of a single hydroxymethyl functionalityfor component eluting at 3.7 min in sample a [by observing a loss of 30 mass units in the collision-induced decomposition (CID) mass spectrum of its MNa + ions], as well as to investigate the form of the s that are linked together in the structures of s and trimers. The data obtained for resin a are reported in Table 1. In this Table, we report the number of the different functionalities present in each molecule, i.e. when possible, the total number of hydroxymethyl groups (F) and the number of butylated hydroxymethyl groups (FBu). Furthermore, when propagation of the formaldehyde group occurs as a side reaction, CH OCH OH groups are formed and can undergo etherification; in Tables 1 and we refer to them as an FF functionality. It must be remarked that, as previously observed, the ESI mass spectrum of a component containing ach OCH OC H 9 substituent cannot be distinguished from that containing one free CH OH and one CH OC H 9 group, unless MS n experiments are performed. Positional isomers are also presented with the same molecular weight but no further measurements were performed to better identify their structure because information provided by ESI, i.e. the number of hydroxymethyl and/or butyl functionalities, was deemed sufficient to describe the trend of the synthetic reaction and composition of the final resin mixture. However, in order to ascertain the kind of sub-units present in the polymeric structures eluting at high retention times (with poor chromatographic separation), MS n experiments remained essential. From the chromatographic point of view, the elution order reflects the polarity of the molecules (and their molecular weight); s with a low number of FBu functionalities elute earlier than the more extensively alkylated ones. However, with an increase of alkylation and/or oligomerisation the elution order is no longer easily

7 D. Favretto, P. Traldi and A. Marcelli, Eur. Mass Spectrom., (1998) 377 predictable, so that s and trimers often elute before s with a high degree of alkylation. From the quantitative point of view, the main components of the mixture in the UV trace seem to be those at 1.3, 1.8 and 1.8 min with molecular weights of 1, and 7 Da, respectively (see Table 1). They all contain three butyl groups (as targeted by the synthetic approach used) and have a number of free hydroxymethyl groups which varies from 1 to 3. At higher retention times, the ESI mass spectra often demonstrate the co-elution of a number of components. When the mass shifts of the main peaks in the ESI spectrum correspond to 30 mass units (a CH O functionality) the coeluting species are assigned to oligomers with the same number of butylated hydroxymethyl residues but with a different number of total hydroxymethyl groups (a different degree of formylation ). For example, HPLC/MS/MS experimental data were essential in order to differentiate the set of oligomers eluting at.71 min from the set of oligomers present at.30 min with the same molecular weight distribution. In fact, the MS/MS spectrum of the sodiated ions at m/z 803 at.71 min provides evidence for the ic nature of this component; thus, among its collisionally-induced fragment ions, a sodiated ion at m/z 0 corresponding to cleavage between the two s is detected which, in turn, fragments by sequential losses of three CH OC H 8 functionalities. For the ureic amino resin, the possibility of polymerisation and cross-linking reactions is much greater than for melamine 1 and high degrees of polymerisation are obtained for etherified ureic resins. This necessarily results in higher molecular weights. However, we nevertheless decided to examine only the most soluble fraction of an i-butylated urea-formaldehyde resin, whose molecular weight ranged, as observed by GPC, from 00 to 000 Da. The chromatogram with, of course, only ESI detection is reported in Figure. As may be seen, five major peaks are present with comparable abundance. The data obtained from the ESI spectra are reported in Table. Again, we report the number of F and B functionalities. For components with retention times that are less than min, their signal-tonoise level in the total-ion chromatogram of Figure is low (this often occurs with ESI detection). However, selectedion chromatograms of the molecular ions enhance the signalto-noise ratio and allow an unequivocal assignment to the retention times of all the components listed in Table. It can be noted that, due to the fast and facile condensation of any small urea s during the synthesis of amino ureic resins, simple s are practically absent from the final mixture, whereas oligomers are abundant and this is the most relevant difference with respect to sample a for which a wide variety of s was observed. In particular, all the polar components with retention times < 3 min show a medium or high degree of hydroxymethylation (F functionality) and a low degree of alkylation (FBu functionality). However, the five most intense peaks correspond to oligomers that exhibit a medium degree of FBu functionality. The most abundant are trimers (at 3., 3.8 and.3 min) with three or four FBu groups, while the tetramers (at.91 min) show a lower degree of alkylation ( FBu per molecule, corresponding to 1 FBu per ). Finally, the s eluting at.9 min show the highest degree of alkylation in terms of FBu groups per unit. MS/MS experiments were also useful in the case of sample b. For example, they allowed us to establish that the Figure. HPLC/ESI-MS chromatogram obtained for sample b.

8 378 Analysis of Butylated Amino Resins by HPLC/ESI and APCI eluting at.80 min, with molecular weight, shows propagation of the hydroxymethyl group (FFBu), as well as allowing a determination of the number of FBu functionalities present for each unit of the various oligomers. These data are essential in order to understand whether the synthetic approach (in particular the molar ratio of alcohol to amino groups of the urea-formaldehyde resin) has lead to the intended F/FBu ratio. In conclusion, HPLC/ESI-MS has proved to be efficiently applicable to the analysis of the production of n- butylated melamine-formaldehyde resins. It can also be applied to follow the first steps of the synthesis of i-butylated urea-formaldehyde resins, although requiring other methodologies (possibly GPC/ESI-MS) for their complete characterisation. References 1. R.McD. Barrett, in Surface Coatings,Vol. 1, 3rd Edition. Chapman and Hall, London, pp. (1993).. A. Marcelli, D. Favretto and P. Traldi, Rapid Commun. Mass Spectrom. 11, 131 (1997). 3. T.T. Chang, Anal. Chem., 37 (199).. M.W.F. Nielen and H.J.F.M. van de Ven, Rapid Commun. Mass Spectrom. 10, 7 (199). Received: 10 July 1998 Revised: 1 December 1998 Accepted: December 1998 Web Publication: 17 March 1999