Dielectric cure monitoring of composite panels during hot pressing : a fundamental understanding Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qn59q786f

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  • Dielectric quantification of material properties is a technology well established in many industries. The application of this concept to the forest products industry to measure adhesive cure, however, has been belated in part due to a lack of proven technology directed at industrial processes and products. It is of great interest to manufacturers to minimize production costs and maximize output. This means being able to identify the minimum time required to cure composite panel products during hot pressing. In the hot-pressing process, material is currently pressed based on a conservative schedule that is actually longer than necessary. The schedule provides what temperatures and pressures are to be used throughout the press cycle to ensure resin cure (cross-linking). In the case of urea-formaldehyde, the resin is subject to strength loss if heat remains applied too long. The objective of the schedule is to cure the resin to an acceptable level and remove the product before degradation can occur. It is difficult to exactly predict the optimum point given all the variables to consider such as panel thickness; moisture and time; press pressure and temperature; and particle geometry, etc. This is where dielectric monitoring can help. Since the critical variable is degree of resin cure, it is logical to design a monitoring system that measures and utilizes it in a feedback control system. As the resin is curing, the molecules and ions become interlaced in a lattice structure during polymerization, reducing rotational and migrational mobility. Rotational and migrational mobility can be quantified dielectrically by applying an oscillating electric field to the material. Included in any monitored dielectric quantity is the effect due to moisture and wood. Studies were conducted to determine their contribution to the readings. Particleboard panels were manufactured in a laboratory environment and monitored with a dielectric system developed by the author. Three different adhesives were used urea - formaldehyde (U F), phenol formaldehyde (PF), and polymeric diphenolmethane diisocyanate (isocyanate, MDI). Dielectric response curves were obtained for each of the resins, and internal bond strength (IB) measurements were taken throughout the curing process of the boards. IB data were charted with dielectric data to show characteristics in the dielectric response curves that could be used to indicate cure status. The dielectric response curves show very encouraging peaks, valleys, and inflection points that seem to correspond to respective cure data. These characteristics could be incorporated into a full-scale system and used in an industrial setting to control and optimize press operations.
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