Intermolecular hydrogen bonding in novolak resins In the solid state (homoassoclatlon) and with other hydrogenaccepting substances (heteroassociation) has been investigated by using differential scanning calorimetry to observe the changes In the glass-transition temperatures. It has been demonstrated that solid novolaks are self-associated aggregates interacting through the polar hydroxyl groups of neighboring molecules. Heteroassociations with nitrogen containing substances make stronger hydrogen bonds at temperatures above the glass-transition temperature of novolak resin. Such a hydrogen bonding in the case of hexamethylenetetramine leads to the complete proton transfer and cleavage of the carbon-nitrogen covalent bond and can be considered a driving force for the curing reaction. An appropriate mechanism for the novolak curing reaction with hexamethylenetetramine is proposed.
SynopsisThe process of resole-type phenol-formaldehyde resins was studied by differential thermal analysis and infrared spectroscopy. It was shown that in neutral media the first reactions that occur are those between free phenol present in the resin and monosubstituted methylol phenol with free reactive positions on the benzene ring. The formation of methylene linkages is followed immediately by the condensation of methylol groups to give dibenzyl ether linkages. These are subsequently destroyed at about 210°C. It is believed that the entire curing process is governed by a free-radical mechanism. It is also shown that oxidation of the resin occurs slowly a t room temperature and humidity.
A Novolac p‐fluorophenol‐formaldehyde (NFF) resin was prepared by condensation of p‐fluorophenol with formaldehyde. DSC showed the glass transition effect or coinciding endothermal peak depending upon the thermal history of samples. It is supposed that the peaks are caused by breaking of the intermolecular bondings in the resin during the glass transition. The bondings are formed in the resin during the storage at room temperature. Thermally stimulated depolarization current (TSDC) measurements were carried out in the temperature range of 290 to 350 K, with the samples having an average number molecular weight M̄n of 375 and 434. TSDC curves mainly showed the dipolar relaxation α peaks. The influence of poling temperatures, the influence of M̄n, the activation energy Ea, and the physical ageing of the samples were investigated. Physical ageing was determined as the reciprocal polarizability Rp vs. time of ageing. Samples. with higher M̄n showed a higher glass transition temperature, a lower Ea, and a higher increase in Rp than the sample with lower M̄n. The increase in M̄n increased the rigidness of NFF samples. The effects are attributed to the strong hydrogen bonding. The comparison with analogous results in novolac phenol‐formaldehyde resin without fluorine is given.
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