Hot-press times

Fig. 7. Schematic representation of the temperature increase in wood particleboard core as a function of hot-pressing time.

Figure 2. Relationship between hot-press time and wet-bond adhesion strength for lignin-resol resin adhesives with and without alkylresorcinol. Legend Q without alkylresorcinol 0 with 10 parts alkylresorcinol. Note Numerical values in parentheses are percentages of wood failure phenola-tion 200°C, 60 min, without catalyst hot-press 120°C, 6 min.

The fourth research category mentioned in the NFPA review relates to enhancing the performance of established natural adhesives for wood bonding to provide durability equivalent to that of the synthetics. This is especially desirable where natural adhesives presently offer significant performance advantages over synthetics in terms of fast hot-press times, short-cycle cold cure, or improved gap-filling properties. Specific examples would include ... 

From liquified solutions of untreated wood, almost the same products have been prepared as those from chemically modified wood [7,8,23,26]. For example, resol-type phenol resin adhesives prepared from five parts of wood chips and two parts of phenol, liquified at 250°C without catalyst, did not require severe adhesion conditions and were comparable to the corresponding commercial adhesives in their gluability. Acceptable waterproof adhesion was attained from the adhesives after gluing wood veneers at 120-130°C with a hot-pressing time of 0.5 min to 1 mm-thick plywood [27]. 

A MUF resin, at parity of all other conditions, yields a lower pH drop after addition of the hardener than a UF resin [46]. This lower drop of the pH due to the buffer capacity of the triazine ring of melamine, however, also eauses a deerease of the hardening rate of the resin and, therefore, a lengthening of its gel time [1], henee a lengthening of the hot press time is necessary. This is also seen in the shifts of the exothermie differential scanning calorimetry (DSC) peak of hardening whieh are observed in thermal experiments [47]. 

In order to increase the capacity of a production line, especially by shortening the panel hot press times, adhesive resins with a reactivity as high as possible should be used. This includes two parameters a short gel time and a rapid and instantaneous bond strength development, even at a low degree of chemical curing. 

A. Rezaie. W.G. Fahrenholtz, and G.E. Hilmas, Effect of hot pressing time and temperature on the microstructure and mechanical properties of ZrB2-SiC, V. Mater Sci, 42 2Ti5-21-AA (2007)... 

Figure 1. Dependence of Bond Quality on Hot Press Time and Assembly Time for A), Commerical P-F...

The homogeneous southern pine particleboards with a density of 50 pounds per cubic foot were pressed to 5/8 inch stops using a closing pressure of 500 psi with a platen temperature of 360 F for the prescribed press time of 5, 6, or 7 minutes. A resin solids of 6%, based on oven dry weight chips, was used for all boards. No wax or other material was added. Immediately upon removal from the hot press, boards were stored in an insulated, but unheated, oven for an overnight period of simulated hot stacking. Two duplicate boards were made for each hot press time and resin combination. 

Figure 2. Dependence of Homogeneous Particleboard Internal Bond on Hot Press Time for Various Resins.

Dependence of Flake Board Thickness (Immediately after Release from Hot Press) on Hot Press Time for Different Resins and Wood Species. 

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