Cure acceleration of phenol-formaldehyde (PF) adhesives for three-layer medium-density fiberboard (MDF)

The cure accelerations by the addition of three carbonates (propylene carbonate, sodium carbonate, and potassium carbonate) into PF resol resin have been investigated to characterize thermal behavior and chemical structure of cure-accelerated PF resins in order to improve slower cure rate of PF resin and apply the cure-accelerated PF resins for the manufacture of three-layer MDF panels. The result showed that using the cure accelerated PF resin with 3% propylene carbonate, quality three-layer MDF panels can be manufactured with 6% PF resin content under 5 minutes hot pressing time. For the synthesis of a base PF resin for the cure acceleration, a series MDF panels bonded with PF resol resins of various weight average molecular weights (Mw) showed that an optimum Mw and viscosity of PF resin for making MDF panels was approximately 1100 and 300 mPa.s, respectively. Isothermal thermal characterization of the cure-accelerated PF resins with DSC showed that PC-accelerated PF resin follows predominately autocatalytic kinetics while the other two carbonates (sodium and potassium carbonates) show nth-order kinetics with a partial autocatalytic effect. Liquid-phase 13C-NMR spectra indicated that the cure-acceleration of PC in PF resin seems to be involved in increasing reactivity of phenol rings, while the addition of both NC and KC into PF resin apparently increased the quantity of ortho-ortho methylene linkages. Solid-state 13C CP/MAS NMR results showed that the cure-accelerated PF resins that have smaller proton spin-lattice rotating frame relaxation times (T 1rhoH) are less rigid than that of the control PF resin. A response surface methodology used for the optimization of hot-pressing process for three-layer MDF panels bonded with PF resole resin has determined an optimum condition based on the IB strength: face-layer moisture content: 13.4 %; closing time: 3.8 minutes; platen position: 111% of the target board thickness. The application of cure-accelerated PF resins by the addition of PC was successful not only to reduce overall PF resin content, but also to reduce total hot-pressing time for the manufacture of three-layer MDF panels.