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ANALYSES OF DENSELY CROSSLINKED PHENOLIC SYSTEMS USING LOW FIELD NMR

Abstract
A uniform dispersion of reactants is necessary to achieve a complete reaction involving multi-components, especially for the crosslinking of rigid high-performance materials. In these reactions, miscibility is crucial for curing efficiency. This miscibility is typically enhanced by adding a third component, a plasticizer. For the reaction of the highly crystalline crosslinking agent hexamethylenetetramine (HMTA) with a strongly hydrogen-bonded phenol formaldehyde resin, furfural has been traditionally used as the plasticizer. However, the reason for its effectiveness is not clear. In this doctoral thesis work, miscibility and crosslinking efficiency of plasticizers in phenolic curing reactions are studied by thermal analysis and spectroscopic methods to elucidate the role of furfural. By combining information from NMR, infrared spectroscopy and differential scanning calorimetry, we show that the presence of furfural increases segmental mobility, disrupts the hydrogen-bonded matrix and frees the hydroxyl units, which further increases HMTA solubility. The higher solubility and segmental mobility increases the extent of crosslinking in the phenolic system. Extent of crosslinking is determined from the spin lattice relaxation time T1 measured by low field NMR (LFNMR). We demonstrate the effectiveness of this method for phenolic systems, where others methods like infrared spectroscopy and differential scanning calorimetry are inappropriate. For validation, we have also correlated extent of crosslinking with T1 using epoxy. The utilization of LFNMR in this work demonstrates its value for characterizing crosslinking of rigid thermosets. Two alternative plasticizers to furfural are substituted in these phenolic curing reactions because of their environmental friendliness: the non-reactive methyl benzoate and the reactive methyl anthranilate. Their effect on the extent of crosslinking is evaluated and compared with the traditional reactive plasticizers furfuryl alcohol and furfural.
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