The reaction in water of UF resins with isocyanates at short curing times: A ¹³C NMR investigation

Abstract

CP MAS13 C NMR spectra of hardened resins have shown that urethane bridges derived from the reaction of the isocyanate group with the hydroxymethyl group of urea do form even at fast curing times comparable to what was used in the wood panels industry, in lower proportions than what was shown earlier. Polyureas and biurets obtained from the reaction of isocyanate with water are the predominant crosslinking reactions of pMDI alone and in UF/pMDI resin systems under fast curing conditions. Residual, unreacted isocyanate groups in the hardened network are consistently observed. Their proportion markedly decreases when the original proportion of urea-formaldehyde (UF) resin is high and that of pMDI is low. Under these fast curing conditions, the UF resin appears to self-condense through an unusually high proportion of methylene ether links rather than methylene bridges alone. A marked proportion of residual, unreacted hydroxymethyl groups is also noticeable, initially, in the UF self-condensation network. Direct NMR tests on thin hardboard bonded under fast pressing conditions with different proportions of UF/pMDI confirmed that crosslinking due to polyureas and biurets formation are predominant in the crosslinking of pMDI when alone and in UF/pMDI resin systems. They confirmed that residual, unreacted isocyanate groups are present in the finished panel. Their proportion is higher when the proportion of pMDI in the system is high. The presence or absence of urethanes could not be confirmed directly on the panels as the relevant peaks are masked by the wood carbohydrates signals of wood cellulose and hemicelluloses.

Bio-Renewable Sources for Synthesis of Eco-Friendly Polyurethane Adhesives—Review

“…mole ratios to prepare adhesives. These adhesives were used to investigate adhesive properties and bond performance in wood application [124] [125] [126]. Reaction of Synthesis of MHSBO and polyurethanes is shown in Figure 6.…”

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

“…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.…”