¹³C NMR investigation of the reaction in water of UF resins with blocked emulsifiable isocyanates

Abstract

A solid state 13 C NMR study of hardened networks obtained by the reaction of blocked and nonblocked isocyanates (pMDI) with urea-formaldehyde (UF) resins in water showed different results according to the temperature of the reaction. At high temperature, in water, both a nonblocked or an emulsifiable, blocked isocyanate, appear to crosslink with UF resins through the formation both of traditional methylene bridges connecting urea to urea and of urethane bridges. The latter have been confirmed by 13 C NMR to form in water by reaction of the isocyanate -NACAO group with the hydroxymethyl groups of the UF resin. At ambient temperature, UF/pMDI resins where the pMDI is a emulsifiable blocked isocyanate, do not appear to form urethanes to any great extent but rather to crosslink through the usual UF resin urea to urea methylene bridges. Even in this case, when urethane bridges appear to be absent, evidence of crosslinking in water through reaction of the isocyanate with the -NH 2 and -NHamide of the UF resin has not been observed.

Reaction Mechanism of Hydroxymethylated Resorcinol Adhesion Promoter in Polyurethane Adhesives for Wood Bonding

“…1 shows the reactions which appear to occur between the still active isocyanate (-N=C=O) groups of the polyurethane and the groups of the low condensation HMR. Unreacted -N=C=O groups are noticeable in both CP-MAS 13 C-NMR spectra between 123 and 126 ppm [21][22][23]. Both in the HMR/isocyanate reaction in Fig.…”

Gluten Protein Adhesives for Wood Panels

“…However, their presence could be masked by the dominant peaks belonging to the protein in this same region of the spectrum. Some condensation has, however, occurred as can be noticed by the increase in the 69.5-70 ppm band corresponding to the appearance of methylene ether bridges (-CH 2 OCH 2 -) [23,24] and is even more evident in the spectrum of GC (Fig. 6) as the 57.5-58.5 peak, although appearing quite clearly, is less pronounced than for GA (Fig.…”

“…The features of this are the dominant peaks at 117, 126.8 and 134 ppm [23,24]. These are all due to the pMDI used, but the most important of these is the 126-127 ppm peak that corresponds to the unreacted isocyanate group (-N=C=O) [23,24]. The 134 ppm peak belongs to the aromatic carbon of pMDI reacted with the secondary amide group of the ureas obtained by the reaction of pMDI with water.…”

“…5) the most evident features differentiating the two are the appearance of a definite peak at 58.5-59 ppm which is due to the formation of the methylol group as a consequence of the reaction of formaldehyde with the protein [1,[22][23][24]. As the reaction was carried out under alkaline conditions there is no evidence of methylene groups in the 35 to 45 ppm range [23,24], as it would be expected. However, their presence could be masked by the dominant peaks belonging to the protein in this same region of the spectrum.…”

“…The 134 ppm peak belongs to the aromatic carbon of pMDI reacted with the secondary amide group of the ureas obtained by the reaction of pMDI with water. The peak at 117 ppm indicates the presence of the relatively unstable carbamic acid [23,24], its presence indicating the reaction between isocyanate and methylolated protein is diffusion-controlled, hence limited, possibly due to the too high viscosity of the system. Lower viscosity would favour cross-linking.…”