Effect of Resin Type on Properties of Steam-press-cured Flakeboards

Six potentially important wood adhesives for gluing southern pine and white oak flakeboards were evaluated for their perfonnances in steam-injection pressing and conventional platen pressing. Of the six resins tested, polyisocyanate resin perfonned well in both steam injection and conventional platen pressings. Phenol-fonnaldehyde (PF) and melamine urea-fonnaldehyde (MUF) resins perfonned poorly in steam-injection pressing. However, adding a small amount of polyisocyanate improved perfonnance considerably. Bending strengths were consistently higher with conventional platen pressing than with steam-injection pressing. The lower density face in steam-injection pressing would result in lower bending properties. In PF andMUF resin systems, steam-injection pressing resulted in low bond strength and high dimensional stability, whereas conventional platen pressing resulted in high bond strength and low dimensional stability. The pairing of low strength with high stability suggested that bonding strength could not be the cause of improvement in dimensional stability. Most probably, steam pressing reduced internal stresses and stabilized the flakes, which in turn resulted in an improvement in dimensional stability. The improvement in dimensional stability may also be related to less strength loss of the board in steam-injection pressing when exposed to high relative humidity treatment. The objective of this study was to compare the properties of steam-injection pressed flakeboards made from either southern pine or white oak, using six different resin adhesives, with control boards pressed in a conventional fashion. EXPERIMENTAL PROCEDURES The six resin combinations chosen for the study were: phenol-formaldehyde (PF), melamine urea-formaldehyde (MUF), phenol-formaldehyde/melamine urea-formaldehyde (pF/MUF), polymeric isocyanate (PIC), pol yi socyanate/pheno I-formal dehyde (pIClPF), and polyisocyanate/urea-formaldchyde (PIC/UF). The resin contents, based on ovendry weight of wood furnish, were: PF = 4.5 percent; MUF = 4.5 percent; PIC = 2.5 percent; PF/MUF = blended as in a three-layer board with 1/3 PF-blended flakes at each face and 1/3 MUFblended flakes at the core, the resin content was 4.5 percent (resin solids based on ovendried wood) in all three layers; PIC/PF = blend as in situ polymerization in which PIC (0.625%) was applied before PF (3.375%); and PIC/UF = blend as in situ polymerization in which PIC (0.833%) was applied before UF (3%). Steam-press curing is a method of hot-pressing wood composite panels by injecting steam directly into the board during the pressing process. The injected steam raises the internal mat temperature,which in turn rapidly cures the resin binder in the composite panels. It was shown, for instance, l-inch-thick, 45pcf phenolic-bonded particleboard was fully cured in I minute at a steam pressure of 300 psi (430°F). This time was substantially shorter than the 15 minutes needed with a conventional platen pressing method (16). The economic potential of this process has recently stimulated attention on steam-injection pressing by a number of researchers (1-7,9-11,13-22). Most earlier processes used superheated steam (l,ll,lS). More recently, saturated steam was used in the developments of self-sealed (9) and unsealed (3) steaminjection pressing processes. These new techniques provide better control oftemperature and board moisture in the manufacturing process and were chosen for this study. The authors are, respectively, Principal Wood Scientist. USDA Forest Serv., Southern Forest Expt. Sta., Pineville, LA 71360; Research Wood Technologist, USDA Forest Serv., Forest Prod. Lab., Madison, WI 53705-2398; Composite Development Scientist, Forintek Canada Corp., Ottawa, ON Canada; and Professor, School of Forestry, Auburn Univ., Auburn, AL 36849-5418. This paper was received for publication in August 1993. C> Forest Products Society 1994. Forest Prod. J. 45(1):57-62. FOREST PRODUCTS JOURNAL VOL. 45, No.1 57