SynopsisThe chemical compositions and molecular size distributions of a series of soluble resole phenolics was studied by high resolution 13C NMR, IR, GPC, and viscosity techniques. The curing reactions were then followed by IR and solid state 13C NMR techniques. The ultimate degree of cure increases with curing time and temperature. The molecular weight of the precursor resole can be increased with increasing formaldehyde/phenol mole ratio and increased condensation reaction time and temperature. The degree of cure achieved under given conditions is directly proportional to the molecular weight of the precursor resole. The condensation catalyst has a great effect on resole composition and molecular size. It also influences preferential methylolation at the para position to the phenolic OH group in the following increasing order Ba(OH)2, NaOH, Na,CO,. The curing pH affects the degree of cure as well as the type of linkages formed. Methylene bridges are almost exclusive at high or very low pH's, while dibenzyl ether bridges predominate at neutral pH. High resolution 13C NMR spectroscopy is the most powerful tool to study soluble resoles. Infrared spectroscopy supplies qualitative results. Solid state 13C NMR is useful to study polymers during the curing process but there are inherent limitations and potential errors in this method.
SynopsisThe degree of cure of various resole phenolic resins was assessed from their resistance to acetone extraction. Cured woodflour composites of resoles were also characterized by dynamic mechanical tests in linear viscoelastic behaviour regions. Acetone extraction is not a sensitive discriminator between highly cross-linked resins. In addition, residual basic catalyst promotes aldol condensation reactions of acetone to produce materials that are absorbed in the phenolic resin so as to provide negative weight loss data on extraction. Despite its inaccuracies, however, this solvent resistance test is still useful as a go/no-go specification technique. Dynamic mechanical tests were performed on molded resole/woodllour composites. This is a useful method for characterizing degree of cure and residual reactivity of the phenolic resin. Degree of cure increases with curing time, temperature and resin pH, with pH being the most important variable. Resins cured at pH's near neutral are linked by ether linkages rather than the more stable methylene bridges. The final degree of cure of woodflour composites depends heavily on the curing conditions (pH, time, temperature). It does not appear to be crucial how the initial resoles were made (i.e., catalyst, phenol/formaldehyde mole ratio or duration of the condensation reaction time) so long as the resoles have relatively high molecular weight. Curing should preferably be at temperatures below 180°C to minimize degradation of the polymer.
SynopsisResole phenolic resins are widely used for gluing wood structures. Solid state I3C-NMR and wide angle X-ray scattering experiments show that a typical resole adhesive disrupts some of the crystallinity of the cellulose. Several explanations are offered for this observation. It is possible that this effect is connected with the adhesive action of the phenolic polymer. If this is so, then measurements of the type described here could be used to understand the adhesion better and to develop better wood glues.
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