Unravelling the Phases of Melamine Formaldehyde Resin Cure by Infrared Spectroscopy (FTIR) and Multivariate Curve Resolution (MCR)

Abstract

Here, we study resin cure and network formation of solid melamine formaldehyde pre-polymer over a large temperature range via dynamic temperature curing profiles. Real-time infrared spectroscopy is used to analyze the chemical changes during network formation and network hardening. By applying chemometrics (multivariate curve resolution, MCR), the essential chemical functionalities that constitute the network at a given stage of curing are mathematically extracted and tracked over time. The three spectral components identified by MCR were methylol-rich, ether linkages-rich and methylene linkages-rich resin entities. Based on dynamic changes of their characteristic spectral patterns in dependence of temperature, curing is divided into five phases: (I) stationary phase with free methylols as main chemical feature, (II) formation of flexible network cross-linked by ether linkages, (III) formation of rigid, ether-cross-linked network, (IV) further hardening via transformation of methylols and ethers into methylene-cross-linkages, and (V) network consolidation via transformation of ether into methylene bridges. The presented spectroscopic/chemometric approach can be used as methodological basis for the functionality design of MF-based surface films at the stage of laminate pressing, i.e., for tailoring the technological property profile of cured MF films using a causal understanding of the underlying chemistry based on molecular markers and spectroscopic fingerprints.

Prediction of Residual Curing Capacity of Melamine-Formaldehyde Resins at an Early Stage of Synthesis by In-Line FTIR Spectroscopy

“…As shown earlier [ 19 ], condensation of the resin does not progress much during impregnation and drying (second process step, Figure 2 b) under the process conditions typically applied in the industry [ 19 ]. It was also shown earlier [ 20 , 21 , 22 ] that the curing conditions applied in process step 3 ( Figure 2 c) have a significant effect on the final properties of cured MF resin films. Laminate properties can be tuned by applying appropriate hardening schemes to one and the same liquid resin [ 20 , 21 , 22 ].…”

“…It was also shown earlier [ 20 , 21 , 22 ] that the curing conditions applied in process step 3 ( Figure 2 c) have a significant effect on the final properties of cured MF resin films. Laminate properties can be tuned by applying appropriate hardening schemes to one and the same liquid resin [ 20 , 21 , 22 ]. However, this is only possible within certain limits and both the processing behavior and the performance in solid state of a resin are mainly determined by the applied synthesis conditions and, in particular, by the melamine-to-formaldehyde ratio [ 23 ].…”

“…In the spectral region from 1750 to 750 cm −1 , melamine shows the characteristic absorbance bands found in melamine formaldehyde resins ( Figure 4 b). The following interpretation of infrared bands is based on peak assignments given in [ 20 , 21 , 28 , 29 , 30 ]. The absorbance peak at 815 cm −1 is assigned to the triazine ring.…”

“…This confirms earlier findings [ 23 ] and can be explained by the fact that resins with lower M:F ratios need to form less bridges during the synthesis to reach the same water tolerance as resins with high M:F ratio. MF resins with high formaldehyde content (M:F 1:4) show high content of methylol moieties (potential cross-linker), but due to the already rather advanced cross-linking state the rigidity of the network suppresses further bridge formation [ 21 ].…”

Cure Kinetics Modeling of a High Glass Transition Temperature Epoxy Molding Compound (EMC) Based on Inline Dielectric Analysis

“…These drawbacks have given rise to alternative in situ cure monitoring methods based on Raman spectroscopy [27], IR-spectroscopy [26,[28][29][30], ultrasonic monitoring [31] or dielectric analysis (DEA) [9,25,27,29,32]. DEA especially holds great potential for industrial application as a process control tool to inline monitor EMC cross-linking in molding tools.…”