Thickness losses in plywood panels during hotpressing are well known but recent trends toward gluing at higher veneer moisture content (MC) and press temperature result in losses much greater than previously expected. This study of 5/8-inch plywood made from conventionally dried and platen-dried veneer examined the effect of veneer MC , press temperature , press pressure (single and multiple-step cycles), and wetting of hot panels on percent thickness loss. The greatest thickness losses were with veneer at 8 percent MC, press temperature , and steady pressure . These losses can be minimized by reducing pressure at press closure or during the press cycle. Panels wetted immediately after hot-pressing recovered 1percent of thickness, regardless of veneer and pressing conditions.
Hot-pressing veneers to cure plywood adhesives causes densification of the wood with a resulting loss in wood volume. Because of the dramatic increase in log costs during the past 10 years, the cost of veneer has increased from less than 50 percent to more than 60 percent of the cost of production of construction plywood. This increase has stimulated major efforts for veneer recovery in plywood plants. The easy gains from what were large losses of veneer have been made. Therefore, attention is now focused on the sources of smaller losses. Reducing loss from compression in the hot-press is a logical next step. For example, 1percent reduction with a 1 percent decrease in the required thickness of dry veneer would save more than $100,000 per year in an average Douglas-fir plywood plant. Losses from compression and the variables influencing them have been reported in several studies. Redfern and Fawthrop (6) showed that thickness of Douglas-fir plywood decreased 10 percent with an increase from 125 to 200 psi pressure and from 1 percent to 9 percent moisture content (MC).MacDonald (3,4)showed further detrimental effects of long press time, high temperature, and high pressure. He designed equipment that would minimize compression by continuously reducing pressure during the press cycle. Currier (1,2)found that losses from compression at 175 psi varied from 5 to 10 percent, depending on the density of the species, but that the losses could be nearly halved by reducing pressure in steps during the press cycle. He also showed that compression lessens as panels adsorb moisture from the air. However, few plywood plants have implemented press cycling or panel humidification, presumably because of other greater opportunities for saving wood, not because of mechanical limitations. During the 20 years since these earlier studies, construction plywood plants have changed. To consume less energy during veneer drying, to minimize veneer shrinkage, and to improve adhesion, many mills dry veneer to an average 5 to 8 percent MC rather than the former zero to 2 percent MC. But the more moist veneer is more easily compressed. Most plants now prepress to consolidate panels before hot-pressing — a step that may increase total loss in thickness. Hot-press temperatures today are 25°F to 50°F higher than those of the 1960s (increasing compression) but phenolic adhesives are faster curing,requiring only three-fourths the previous press time (reducing compression). Some mills have had to increase hot-press pressure from 175to 200 psi in order to obtain intimate contact between the rough veneer surfaces obtained by peeling small, coarsegrained, second-growth logs. To compensate for these losses, a few mills now spray water on panels shortly after hot-pressing. Because this humidification step was not studied earlier, we do not know if gains in thickness from wetting and humidification are in addition to, or in lieu of, gains from reduced pressure. Thus, it is difficult to use earlier data to estimate accurately the potential savings from humidification and from pressure reduction during hot-pressing. The three objectives of this study were, therefore, to determine the magnitude of losses from compression occurring during typical construction plywood manufacture, to determine the extent to which thickness can be retained by reducing hot-press pressure or be recovered by wetting panels after pressing, and to determine the location of compression in the plywood panels.
Materials and methods Five-ply panels (5/8 in. by 2 ft. by 2 ft.) were made from commercially peeled and dried veneer (1/8 in. thick, CD grade) of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), southern yellow pine (Pinus spp.), and hem-fir, which is a commercial mixture of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and true firs (Abies spp.). Some green Douglas-fir veneers were platen-dried in the laboratory to 5 percent MC at 425°F with a contact pressure of 35 psi. All veneers were conditioned before gluing either to 5.8 ± 0.5 percent MC at 90°F and 30 percent relative humidity (RH) or to 1.0 ± 0.5 percent MC by storing them in a convection oven at 145°F for 24 hours. Four replicate panels were glued for each combination of hot-press variables. Phenolic adhesive (Monsanto Mix PF 3098 with wheat and alder bark flours) was applied to the veneers (65 to 67 lb./1,000 ft.2 of double glueline). Panels were assembled immediately and allowed to wait 5minutes before a 5-minute prepressing (ambient temperature at 150 psi) and 10 minutes before hot-pressing, (total closed assembly time 20 min). The temperature, pressure, and length of time of hotpressing are given in Tables 1, 2, and 3. Immediately after hot-pressing, each panel was cut in half. One half was placed while hot in an insulated box for 24 hours to simulate hot-stacking, then stickered for conditioning to about 10 percent MC (70°F, 60%RH). The other half was dipped in water (70°F) for 20 seconds, then placed in a separate hot-stack for 24 hours before being conditioned to 10 percent MC. The water soak added an average 5 percent moisture to the panels. After conditioning for 60 days or more, each half panel was cut and tested according to product standard PS-1-74 (5)to produce five plywood shear specimens that were vacuum-pressuresoaked and sheared while wet. Wood failure was estimated from the dried specimens. Panels were measured for thickness in different ways, depending on when the measurement was made. The thickness of uncured panels going into and out~ of the prepress was measured at 1 psi pressure by a dial gauge (sensitivity = 0.0001 in.) calibrated to record the distance between the prepress plates. The thickness of panels being cured in the hot-press was measured and recorded continuously by a pair of linear variable differential transducers (sensitivity >0.0001 in.) mounted on opposite corners of the hot-press and calibrated to the average distance between the plates. The thickness of cured panels was measured at midline along the length of each half panel at zero, 2, and 60 minutes and at 1, 7, and 60 days with a (Trienco) noncontact thickness gauge, a laser-based instrument with repeatability better than 0.001 inch. All thickness gauges were standardized to read the correct values from both metal and wood plates whose thicknesses were uniform and known within 0.0005 inch. Microscopic examinations of gluelines and end surfaces of veneers of various panels were made with a (Zeiss) stereomicroscope, an incident (Leitz) fluorescence microscope, and a (AMR 1000A) scanning electron microscope. Specimens were sawn from each panel and the surface to be examined was cut cleanly with a razor blade. No other wetting, softening, or sectioning was done, except that some surfaces and sections were prepared for photomicrography. Results and discussion This study clearly demonstrates the need to consider carefully the potential for losses from compression when plywood-manufacturing variables are changed. Product standard PS-1-74 requires panel thickness to be within 1/32 inch of the stated nominal — in this instance, 0.625 inch ± 0.031 inch. When made from veneer at 6 percent MC, the average thickness of panels entering the hot-press was 0.640 inch ± 0.005 inch, unacceptably thin if thickness losses exceeded 7 percent. We measured losses as low as 4percent and ashigh as 11 percent. The excessive losses could be avoided by considering not only the gluing variables but the advantages of panel wetting and humidification. Anatomical observations show clearly why large compression losses should be avoided. Gluing variables affecting thickness Prepressing significantly affected panel thickness, as measured in this study. On the average, panels were thinner by 3.1 ± 0.8 percent (95% C.I.) when they entered the hot-press than when they entered the cold-press (0.660 in. versus 0.640 in,) but we doubt that the true effect of prepressing was that large. By definition, the thickness of a panel going into the prepress was the distance between the prepress plates at 1psi pressure on the incoming panel. However, if the veneer thickness was not uniform, or if the veneer was wavy, daylight could be seen between portions of the panel and the press plates at 1 psi. The average dry veneer thickness was 0.126 inch ± 0.002 inch at 6 percent MC, five plies of which would be 0.630 inch. It is unlikely that four gluelines would add 0.030 inch to the panel. Thus we believe that the high values for average thickness of panels entering the prepress were biased. Because almost one-third of the panels lost only 1/2 to 1 percent during prepressing and most others lost more than 3percent, we suspect that 1/2 to 1percent is the real compression loss during prepressing. All panels leaving the prepress were flat so a similar bias did not exist in subsequent measurements at the hot-press. For that reason, the thickness of panels entering the hot-press was used for comparing all future measurements and any effect of prepressing was not included in subsequent measurements and results. Press temperature and MC had major effects on compression (Tables 1 and 2). Increasing press temperature from 270°F to 330°F reduced panel thickness by 2 to 3 percent at 200 psi. Increasing MC from 1 to 6 percent reduced it by 3 to 5 percent. Panels made from veneer at 1 percent MC averaged 0.630 ± 0.006 inch as they entered the hot-press, 1.4 percent thinner than those made from veneer at 6 percent MC – but this difference was only one-third to one-half the added loss in thickness from hot-pressing when veneer MC increased from 1 to 6 percent. Thus, the recent industry trend to higher press temperatures and higher veneer MC (Fig. 1) could mean loss to compression of as much as 11 percent if pressure is not reduced. Because wood is plasticized both by heat and water, less pressure should be needed to consolidate the glueline at higher temperatures and MC. The larger losses in thickness at higher temperature and MC could easily offset economic gain from shorter press time and veneer drying time if pressure is not reduced. Pressure during the press cycle had an effect similar to that of temperature and veneer MC (Table 2, Fig. 2). Panel thickness decreased almost instantly by 5 to 10 percent, depending on the closing pressure. During the pressure cycle, further loss in thickness occurred almost linearly with time. Reducing the closing pressure from 200 psi to 150 psi at the higher temperatures and MC reduced loss from 11 percent to only 7 percent (Fig. 2). To effectively minimize compression, closing pressure should be low and pressure should be reduced as quickly as possible during the cycle (Fig. 1). Reducing pressure during the first half of the press cycle was less effective than reducing pressure duting the first quarter of the cycle. The most effectivepressure cycle was a four-stepreduction that kept loss to 6 percent or less, even at the highest temperatures and pressures. With either one or four steps in pressure reduction, we were able to reduce pressure to 100 psi with no measurable loss in bond quality. However, reduction to 75 psi during the press cycle caused the percentage of wood failure to drop below the 85 percent average required by PS-1-74 (5). The minimum allowable pressure depends on the roughness of the veneer. Rough veneer will increase loss in thickness because greater pressure is required to maintain intimate contact at the glueline during hot-pressing. The effect of species on hot-press compression was not analyzed in detail. Douglas-fir was compared to southern yellow pine only to determine whether species of similar density had similar losses from compression with the variables we examined (Table 3). Lower density woods (e.g., the hem-fir mixture) compressed more than Douglas-fir under similar conditions. Platen-dried Douglas-fir veneer was compared to that dried by convection. An earlier study showed that platen drying caused 8 percent shrinkage in thickness rather than the normal 4 percent (7). Though more thickness loss during drying might result in less compression during hot-pressing, the platen-dried veneer did not differ significantly (at the 95%level) in hot-press compression from Douglas-fir veneer dried conventionally (Table 3). Wetting and conditioning of panels After hot-pressing, the plywood panels recovered somewhat from compression. Instantaneous increases in thickness ranged from 2 to 5 percent. Thereafter, small but consistent decreases in thickness occurred during the first days if panels were kept dry. Presumably, this decrease resulted from continued about 10 percent. This increase in thickness is apdrying of the hot plywood. Currier (1) found a similar proximately what would be expected with a 5 percent decrease in thickness during the day after hot-pressing. increase in MC. After 1 day the dry plywood slowly gained 1/2 to 1 Even more thickness was recovered when the percent in thickness as it conditioned to an EMC of plywood panels were wetted immediately after hot pressing. Wetted panels were 1/2 to 1-1/2 percent thicker than dry panels after only 2 minutes. This increase was retained even after wetted panels were conditioned to essentially the same MC as the dry panels (MCdry = 9.8 ± 0.3%; MCwetted = 10.6 ± 0.3%). Apparently the thickness recovery after wetting was from springback and not simply from swelling due to increased MC. Of special significance is the fact that the approximately 1 percent thickness recovery from wetting panels occurred whether compression was 4 percent or 11 percent. Thus the benefit from wetting can be gained in addition to the benefit from pressure reduction .Wetting the panels caused no significant change in wood failure. Location of compression The loss in thickness of plywood panels compressed in the hot-press may be a result of uniform fiber shrinkage throughout the panel and of fiber compression at the panel surface, in earlywood zones of growth rings, or in adhesive-soaked wood at the glueline. Anatomical changes in fibers showing uniformly distributed shrinkage or compression are not easily seen and were not measured but zones of highly compressed fibers were easily identified. A zone of highly compressed earlywood fibers was a normal feature at gluelines of the plywood panels, regardless of the amount of compression. Figure 3 shows a typical compression of these thin-walled fibers that are adhesive-saturated and bonded in place. Such compression is essential if the two irregular surfaces are to conform to one another. However, when thick-walled, latewood fibers were adjacent to the glueline, little or no compression occurred even when they were heated and plasticized by the adhesive. When panel compression was less than 5 percent, the glueline was the major zone of compression. When panel compression was greater than 7 percent, additional bands of thin-walled fibers were compressed in earlywood zones of growth rings. The compressed fibers may have been near the glueline, as in Figure 4, or well removed from it, as in Figure 5. Such compression failures would occur only when the compressive strength of those fibers has been exceeded and the wood has been weakened from overloading. Although expected, no consistent zone of compressed fibers was found at the plywood surfaces adjacent to the hot-press plates, even in panels with large thickness losses from compression.
Conclusions 1. The trend to higher MCs and press temperatures for manufacturing veneer can result in losses from 34 compression from 4 percent to as much as 11 percent during hot-pressing of Douglas-fir plywood. 2. Reducing pressure at press closure, during the press cycle, or at both times, compensates in large part for the greater thickness losses at higher temperatures and MC. 3. Douglas-fir plywood wetted immediately after hot-pressing recovered 1 percent of thickness, regardless of the amount of compression. 4. As losses from compression increase above 5 to 7 percent, zones of compression failure occur not only in adhesive-saturated fibers compressed in the glueline but in thin-walled fibers scattered throughout the veneer.
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