Investigating the Curing of Amino Resins with TGA-MS and TGA-FTIR

Introduction 

The monomers used for preparing amino resins (aminoplasts) are urea (for UF resins) or melamine (for MF resins) and formaldehyde. The latter undergoes an addition reaction with amino groups with the formation of N-methylol groups. In principle, with a primary amine, two methylol groups can be formed.

With melamine (Fig. 1), the addition reaction of formaldehyde to the primary amine group (Fig. 2) is faster than to the secondary amine group; there is nevertheless the possibility of six-fold methylolation. Urea however only undergoes two-fold methylolation. Amino resins are available commercially in precondensed form, usually in aqueous solution, with molar masses of 500 to 1500 g/mol. During curing, the amino resin undergoes cross-linking as a result of condensation reactions (Fig. 3).

The curing reaction is acid-catalyzed; p-toluenesulfonic acid (PTS) is normally used. The methylol groups react with amino or other methylol groups and form either methylene bridges according to reaction (1), or ether bridges according to reaction (2). The methylol groups of the MF resin are present in partially methylated form. Correspondingly, the cross-linking reaction results in the liberation of not only water but also methanol. The conversion of ether bridges to methylene bridges with the elimination of formaldehyde according to reaction (3) does not occur until higher temperatures. The cross-linking density of the cured aminoplast is determined by the number of methylol groups on an amine.

The aim of the TGA-MS and TGA-FTIR measurements was to gain an insight into the reactions taking place by analyzing the volatile substances evolved. The detection of methanol, for example, should enable one to decide whether the MF prepolymer is involved in the crosslinking reaction or whether it serves only as an external plasticizer. It should also be possible to obtain information on the reaction kinetics. The evolved gases were to be identified by coupling the thermobalance (TGA) with mass spectrometry (MS) and Fourier transform infrared spectroscopy (FTIR).

Experimental Details 

An open aluminum crucible of 100 µl volume was used for the measurements. The TGA temperature program ran dynamically from 25 °C to 250 °C at a heating rate of 10 K/min; nitrogen was used as inert gas with a flow rate of 70 ml/min. The gaseous components evolved were detected by coupling the METTLER TOLEDO TGA/SDTA851e thermobalance either to a mass spectrometer (Balzers Thermostar©) or a Fourier transform infrared spectrometer (Nicolet Nexus). 

Conclusions

The objectives defined were to a large degree achieved through the use of TGA coupled with MS and FTIR spectrometers:

• The analysis of volatile gases up to 220 °C showed that during the curing reaction only water was eliminated in agreement with equations (1) and (2) in Figure 3. During the degradation of the aminoplast (220 °C to 250 °C) methanol, ammonia, formaldehyde and carbon dioxide were liberated
  
• The quantity of water eliminated was determined quantitatively and was found to be 12.2 weight%.
  
• Methanol was not detected until above 220 °C. This indicates that the ether methylol groups do not take part in the curing reaction. It does not exclude the participation of melamine in the reaction because this was only partially but not completely etherified.
  
• The FTIR measurements confirmed that methanol, carbon dioxide and ammonia as well as smaller amounts of formaldehyde were liberated.

^{Investigating theCuring of Amino Resins with TGA-MS and TGA-FTIR | Thermal Analysis Application No. UC 145 | Application published in METTLER TOLEDO Thermal Analysis UserCom 14}