Measuring adhesive cure by dielectric analysis Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/76537538r

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  • Present methods of determining proper glueline cure are inadequate in either production or laboratory situations because they are time consuming and "after the fact". Monitoring the glueline while the wood composite is still in the press would provide many advantages over existing methods. Such a system would provide greater control over the degree of cure, helping eliminate variable quality in the glue bond. Dielectric spectroscopy has been proposed as a method capable of continuously monitoring adhesive cure and a specific machine, the Audrey II dielectrometer, has been advertised as being capable of detecting cure of resin systems common to the forest products industry. The objectives of this research were to determine if the Audrey II dielectrometer could measure cure of a highly caustic phenolic plywood adhesive and a resorcinol-formaldehyde laminating adhesive in Douglas-fir gluelines under conditions analogous to mill production. The dielectric spectrometer used in this study produced frequencies of 100-1000 Hz and measured voltage, capacitance, phase angle, and dissipation of the glued assembly placed between its electrodes. Capacitance was found to be the property that was most sensitive to changes in the wood-adhesive composite during the cure process. The dielectric spectrometer was able to accurately and consistently measure the capacitance of oven dried wood. Dielectric measurements were very sensitive to the presence of water, and the increase in capacitance with an increase in moisture content was reproducible. Problems were encountered when using the dielectric spectrometer to monitor cure of a resin in wood composites. The lack of repeatability in capacitance measurements led to the conclusion that the Audrey II dielectric spectrometer could not measure cure of a highly caustic phenolic adhesive or a resorcinolformaldehyde laminating adhesive in Douglas-fir gluelines. Components of a phenol-formaldehyde resin were examined to determine what caused the lack of reproducibility. The capacitance was reproducible with either water or aqueous caustic on the veneers. However, the presence of either phenol or formalin on the veneers resulted in a total lack of repeatability in capacitance. Both phenol and formalin are excellent swelling agents for wood and as wood swells, its microstructure is opened and plasticized. This would increase the capacitance. Other factors that would influence variation in swelling and capacitance would include depth of lathe checks, wood damage, porosity, density, and natural anatomical variation that influences the rate of penetration of chemicals into wood. Even considering the swelling effects and natural variation of wood, the variability in capacitance of the wood-adhesive composite is not readily understood.
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