Temperature and pressing

The penetration behavior of resins into the wood surface also is influenced by various parameters, like wood species, amount of glue spread, press temperature and pressure and hardening time. The temperature of the wood surface and of the glue line and hence the viscosity of the resin (which itself also depends on the already reached degree of hardening) influence the penetration behavior of the resin [79]. 

Influence parameters are especially the press temperature and the moisture content in the mat. Additional parameters are the wood density, the porosity, the swelling and shrinking behavior of the wood, the structure at the surface and the wetting behavior. During the press cycle several mechanisms are in operation ... 

The goal of all fabricators is to minimize the cycle time and maintain part quality. The process parameters involved are winding speed, mold and press temperatures and polymer formulation. 

The mold and sample were placed in the press also preheated to 185°C and the cure cycle started immediately. Press temperature and pressure were microprocessor-controlled for consistent cure cycles. The cure cycle consisted of an initial 30 minute hold at 185°C with 1300 psi of pressure followed by 45 minutes at 221°C with 850 psi pressure and finally 15 minutes at 260°C with 300 psi pressure. The mold was removed from the press immediately following the completion of the final step and the sample removed from the mold while still hot. 

Future wood composites will likely require the gluing of low-quality wood with higher moisture content. Faster curing rates (compared to phenol-form-aldehyde resins) will also likely be desirable in new adhesives. Employment of faster curing adhesives to shorten press time, lower press temperature, and glue... 

Fig. 24. Flexural strength of hot-pressed specimens containing two phases, a -SiAION plus /S —Si3N4. The numbers indicating volume percent of the a -SiAION are given on the line. The hot-pressing temperatures and times are given in the figure [36].

High temperatures and high relative humidity can result in odor problems in a room containing particleboard manufactured with UF resins [25]. The release of formaldehyde from UF particleboard is caused by two factors. It can be due to free formaldehyde present in the board that has not reacted, and it can be due to formaldehyde formed by hydrolysis of the aminoplastic bond as a result of temperature and relative humidity [2,25]. While the first type of release lasts only a short time after manufacture of the particleboard, the second type of release can continue throughout the entire working life of the board. A considerable number of variables influence the emission of formaldehyde from a UF-bonded particleboard. The main ones are the molar ratio of urea to formaldehyde (which influences both types of release), the press temperature, and in service, the ambient temperature and relative humidity. 

Pizzi A, Cameron FA 1981 Decrease of pressing temperature and adhesive content by metallic ion catalysis in tannin-bonded particleboard. Holz Roh-Werkst 39 463-467... 

FIGURE 2.18 Hie effects of hot-pressing temperature and time on the polarization curves of MEAs prepared at (a) 500 psi and (b) 1500 psi. [72], (Fot color version of this figure, the reader is referred to the online version of this book.)... 

Sintering additives such as metal (Ni, Nb, Mo), silicides (MoSij, TaSi, HfSi ), carbide (SiC, B C), nitride (AIN, SijN ) and oxides (ZrO, rare earth oxides) helps in bringing down the hot pressing temperature and results in near theoretical densities. The selection of sinter additive is very important as these additives also affect the mechanical, thermal, and oxidation properties of UHTCs. 

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