U.S.D.A. FOREST SERVICE RESEARCH PA PER FPL 208 1973 FACTORS AFFECTING PARTICLEBOARD PRESSING TIME: INTERACTION WITH CATALYST SYSTEMS U.S. Department of Agriculture/Forest Service Forest Products Laboratory - Madison, Wis.
ABSTRACT In laboratory manufacture of particleboards, use of catalyst, hight press temperature, fast closing, and nonuniform mat moisture content produced adequate 1/2-inch-thick, urea-bonded panels in 1 minute and phenolic-bonded panels in 2 minutes presstime. The use of catalyst allows the reduction of presstime by approximately 1 minute for urea-formaldehyde resin systems, but no reduction was obtained with catalyst phenol-formaldehyde systems.
FACTORS AFFECTING PARTICLEBOARD PRESSING TIME: INTERACTION WITH CATALYST SYSTEMS By W. F. Lehmann, Technologist R. L. Geimer, Technologist and F. V. Hefty, Technician Forest Products La bora tory 1 Forest Service United States Department of Agriculture ---- INTRODUCTION This study is the third of a series to determine the individual effects of many variables on minimum pressing time of particleboards, Part I evaluated the effects of pertinent variables using uncatalyzed resin binders at uniform mat moistures while part II studied the same variables with nonuniform mat moisture contents. The results of parts I and II are contained in a previous report. 2 Part III, dealing with the interaction of catalyzed systems with factors affecting pressing time, is discussed in this report. The intent of this study was to investigate several types, levels, and combinations of catalysts used in gluing both interior- and exterior-type particleboards and to determine their effects, in combination with other variables, on reducing presstime. RESEARCH PLAN Catalyst Types and Levels Initial investigations in this study were directed toward finding optimum levels of several catalyst mixtures using the revised (1968) standard Forest Products Laboratory (FPL) board. These boards are made with Douglas-fir 0.015- by 1-inch flakes with 6 percent resin at 40 pounds per cubic foot (p.c.f.). Additional details are described in appendix A and, except where otherwise noted, were followed in manufacture of all boards. Minimum presstimes were determined on a series of boards made with each combination of resin, catalyst, and mixture ratio. Presstimes were shortened on replicate boards of each type until a "blow" occurred or the board emerged from the press with uncured corners. Presstime reductions were as follows: At 1-minute intervals to 3 minutes' presstime; 1/2-minute intervals between 3 and 1 minute presstimes; and 114-minute intervals below 1 minute presstime. 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Heebink, B. G., Lehmann, W. F., and Hefty, F. V. An Exploratory Study of Factors Affecting Particleboard Pressing Time. USDA Forest Serv. Res. Pap. FPL 180. Forest Prod. Lab., Madison, Wis. 1972.
The catalysts evaluated included: Urea-resin.--Catalyst A (20 pct. ammonium chloride, 2.5 pct. ammonium hydroxide, 1 pct. triethylamine, and 76.5 pct. water). Added at 1, 2, and 4 percent based on 1iquid:liquid weights. Catalyst B (16.8 pct. ammonium chloride, 14.8 pct. hexamethylenetetramine, and 68.4 pct. water). Added at 1, 2, and 4 percent based on 1iquid:liquid weights. Standard phenolic resin.--catalyst C (methylethyl ketone hydroperoxide). Added at 1 and 2 percent levels based on 1iquid:liquid weights. Catalyst D (5 pct. potassium chromate and 95 pct. water). based on liquid: liquid weights. Added at 1 and 2 percent levels Catalyst E (a commercial phenol-resorcinol resin containing its own catalyst). 4 and 8 percent based on 1iquid:liquid weights. Added at Fast-curing phenolic resin.--a phenolic resin having a shorter gel time than the "standard" resin was further catalyzed with 8 percent (1iquid:liquid basis) of catalyst E described above. Thickness, Density, and Species After analyzing the data, the most promising catalyst combinations were used to determine their effect on presstime in combination with selected variables of thickness, density, and species. Thicknesses of 3/4 and 1 inch and densities of 45 and 50 p.c.f. with Douglas-fir flakes were evaluated using: (1) Urea resin with 2 percent catalyst B. (2) Urea resin with 1 percent catalyst B in the face material and 3 percent catalyst B in the core material. (3) Standard phenolic resin with 4 percent of catalyst E. Minimum presstimes were determined at standard conditions for aspen and southern pine using: (1) Urea resin with 0, 1, 2, and 4 percent of catalyst B. (2) The fast-curing phenolic resin with no additional catalyst additive. Press Temperatures and Closing Times Finally, using the best catalyst system observed and a mat moisture distribution pre- 2 viously found effective in shortening presstimes (steam shock effect),- minimum presstimes using Douglas-fir flakes were determined at various levels of pressing temperatures and closing speeds. Mat moisture contents were 5 percent with an additional 5 percent moisture sprayed on the surfaces. Variables were as follows: FPL 208-2-
Urea-resin.--2 percent catalyst B. (1) 325 F. and 300 pounds per square inch (p.s.i.) closing pressure. (2) 325 F., 400 p.s.i. closing pressure. (3) 350 F., 300 p.s.i. closing pressure. (4) 350 F., 400 p.s.i. closing pressure. (5) 375 F., 15-second closing time (no pressure limit). Standard phenolic resin.--4 percent catalyst E. (1) 350 F., 400 p.s.i. closing pressure. (2) 375 F., 15-second closing time (no pressure limit). Data Recorded For each test panel, the following data were recorded: (1) Board pressure and centerline mat temperatures throughout the press cycle (measured on one board per series). (2) Moisture content and thickness, out of press. (3) Modulus of rupture (MOR), modulus of elasticity (MOE), and internal bond strength values obtained by standard test procedures as described in ASTM D 1037 3 (five per panel). Unsanded panels were conditioned at 80 F., 65 percent relative humidity (RH) and then cut into specimens. RESULTS AND DISCUSSION Urea-Bonded (Interior) Boards Preliminary data (table 1) showed that both catalysts A and B produced similar gel time results, reducing the time for urea resin to gel from 1-1/2 minutes to less than 1-1/4 minutes. Gel times in this case did serve as a fair indicator of results obtained during hot pressing. Data for the urea-bonded (interior) boards are summarized in table 2. Both catalyst types reduced presstime by about 1 minute. Minimum presstime (internal bond > 100 p.s.i.) for the noncatalyzed board can be estimated at 3-1/2 minutes while that of a catalyzed board was 2-1/2 minutes. Increasing the level of catalyst above 1 percent did not achieve a further reduction of presstimes. The differences in strengths of boards made at various catalyst levels were slight; however, there was a noticeable drop in final pressing pressures as catalyst levels rose from 1 to 4 percent. Addition of 2 percent of catalyst B produced boards having high strength values along with low final pressures and this combination was chosen as the most promising. 3 American Society for Testing and Materials. Standard Methods of Evaluating the Properties of Wood-Base Fiber and Particle Panel Materials. ASTM Desig. D 1037-72. 1972. -3-
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As long as the centerline temperature increased at the same rate, minimum presstime depended on cure time. In this discussion cure time is referred to as the time that centerline temperatures are at or above 220 F. Thus cure time, in addition to being a function of the resin and catalyst reaction rate, also is dependent on such factors as the buffering capacity of the wood, moisture quantity, and internal pressures. Minimum time above 220 F. necessary to bring about cure of the catalyzed resin in a Douglas-fir "standard" board is between 0 and 1/4 minute. Cure time curves as shown in figure 1 represent the temperature rise in the center of the board and are dependent on variables which influence heat transfer such as press temperatures, closing speed, board thickness, and mat moisture distribution. The time needed to raise the centerline temperature to 220 F. is the value of x (minimum presstime) when y (cure time) is 0. The curves are drawn on a 1:1 slope; i.e., a 1-minute reduction in presstime is accompanied by a 1-minute reduction in time above 220 F., as would occur if the boards were similar. Location of the points along the curve therefore represent additional time needed to cure the resin. A portion of the variation from the curve can be attributed to addition of the catalyst. This is to be expected since the heat of resin polymerization accounts for as much as 20 percent of the total heat supplied 4 and an increase in cure rate serves to accelerate the exothermic reaction. Note that species and density differences do not essentially affect the rate at which internal temperature rises, but do increase cure time (i.e., necessary time that centerline temperatures are above 220 F.). Species.--The use of catalyzed resin in boards made with aspen and southern pine reduced by 1 minute the time necessary to cure the resin, and allowed for a comparable reduction in presstime. Heat transfer was much the same as with the standard Douglas-fir board, but perhaps due to the buffering capacity of the wood as shown in figure 2, minimum presstimes were 1/2 minute longer. Catalyzing above 2 percent did little other than to lower the final pressure. Maximum temperature in the southern pine panels was considerably less than that reached in the Douglas-fir or aspen boards, probably because of the greater permeability of panels made with the higher density species. Initial temperature rise was identical to boards constructed of other species, however. Thickness and density.--addition of catalyst at any level shortened presstime of the thicker boards, again by 1 minute as shown in table 2. Minimum time above 220 F. was very close to that of a standard board--around 1/4 minute for a catalyzed resin. The pressure cycle for thicker boards followed 1/2-inch boards. Minimum presstime, however, was much longer-- 4-1/2 minutes for a 3/4-inch board and 6 minutes for a 1-inch board. The extension of presstime was directly attributed to a significant shift in the rate of initial temperature rise (figs. 1 and 3) and was not affected by pressure. Catalyst addition to the higher density boards resulted in the same 1-minute reduction in minimum presstime experienced with other types of urea-bonded boards.- 5 Initial temperature rise was similar to that of a standard board as shown in figure 3. Consequently an increase in the presstime required over that of a 40 p.c.f. board could be attributed to the increase in moisture content or steam pressure or both in the center of the boards. Under conditions involving high moisture content or high internal pressure or both, it was not definite whether the longer cure time was needed because of retardation of resin cure rate or because more time was needed to relieve the high internal pressures. As shown in figure 4, only increased 4 Bowen, M. E. Heat Transfer in Particleboard During Hot Pressing. Ph. D. Thesis, Colorado State Univ., Ft. Collins. 115 pp. 1969. 5 Noncatalyzed control board data available for the 50 p.c.f. series only. FPL 208-8-
density resulted in increases in pressure required to close the press and even though final pressures were higher, adequate boards could be produced with shorter cycles than for thicker boards. This again emphasizes the importance of rapid temperature rise as a primary factor con trolling press times. It should be noted that in order to achieve presstimes as low as 4 minutes with a 50-pound board, the pressure was relieved slowly during the last 20 seconds of the pressing cycle. To determine the effects of catalyst distribution within a mat, duplicate boards were made of the thickness and density series in which the catalyst ratio was changed from 2 percent throughout the boards to 1 percent in the face resin and 3 percent in the core resin. Because face and core weight ratios were 1:2:1, the total amount of catalyst used remained the same. As shown in table 2, no consistent differences could be noted between boards using different catalyst distribution. Closing time and pressure.--boards were constructed in this phase of the investigation using a moisture content distribution of 5 percent in the mat plus another 5 percent sprayed equally 2 over the two faces of the mat. Previous work had shown that with this type of moisture distribution initial heat rise was rapid and presstimes could be reduced accordingly. Increasing press temperatures and reducing closing time affect initial heat transfer similarly. This is illustrated in figures 1 and 3. Minimum cure time varied, with the temperature-pressure combination used, from 0 to 0.75 minute. It would appear that with the attainment of maximum catalytic action, minimum presstime was largely a function of initial heat transfer. Where it was impossible to make a board in 1 minute at 350 F. and 300 p.s.i. closing pressure using the standard 7/32-inch-thick aluminum cauls, a highly acceptable board was produced in 0.75 minute using thin aluminum foil for caul plates. Limitations caused by internal pressure as observed in the denser (50 p.c.f.) boards also occurred in the board pressed at 375 F. and 15-second closing time. Under these conditions, the board was blown at 3/14 minute presstime, a good board was manufactured in 1 minute, and a partial blow occurred at 1-1/2 minutes. In this case, resin cure was essentially complete after the centerline reached temperatures in excess of 220 F. for 1/2 minute. The total time necessary for this to occur was 1 minute. Due to the high press temperature used, internal steam pressure continued to rise and surpassed the internal bond strength of the board sometime between 1 and 1-1/2 minutes. Even though the resin was cured, additional presstime was needed until sufficient moisture had escaped from the board and internal pressures had dropped below that of the board strength. In general, the catalyzing system was independent of the other variables controlling presstime and shortened the curing time of the resin by about 1 minute. Other factors being favorable, minimum presstimes were controlled by the rate of heat transfer and can be associated with the time the centerline of the board was exposed to temperatures of 220 F. or greater. This is approximately 1/4 minute for a catalyzed urea resin. Once cure was started, catalyzed urea resin cured at a very rapid rate. Consequently, minimum presstimes were rather definite and internal bond strengths dropped abruptly as the minimum time was reached. -9-
M 140 921 Figure 1.--Centerline cure times for urea-bonded (interior) flakeboards. Numerals by symbols indicate catalyst levels. FPL 208-10-
M 140 923 Figure 2.--Curves of ph and buffering capacity for Douglas-fir, aspen, and southern pine wood used in study. -11-
M 140 924 Figure 3.--Centerline temperatures during pressing of urea-formaldehydebonded Douglas-fir flakeboards containing 2 percent catalyst. FPL 208-12-
M 140 925 Figure 4.--Pressure profiles during pressing of catalyzed urea-formaldehydebonded Douglas-fir flakeboards at 325 F. -13-
Phenolic-Bonded (Exterior) Boards 6,7 Catalyst types and levels.--existing patents indicate that both lower organic peroxy compounds and alkali-soluble chromium compounds have a catalytic effect on phenol-aldehyde resins, making possible decreases in cure time by as much as 25 to 50 percent. Investigation showed that methylethyl ketone hydroperoxide (catalyst C) reduced gel times by 12 percent, whereas potassium chromate (catalyst D) lowered gel times by as much as 64 percent (table 1). Substantial reductions in time (56+ pct.) were also gained by use of a small amount of a phenol-resorcinol resin (catalyst E). In practice, none of the catalyst systems under consideration proved to be effective in reducing presstines for the phenolic-bonded boards (table 3 and fig. 5). Addition of certain catalysts (C and D) increased the bending strength of the boards but these results did not apply to all catalysts used and were not consisrent throughout the experiment. Use of a "fast-curing" resin in combination with 8 percent catalyst E had no advantage over the catalyzed standard phenolic resin, also proving to be ineffective in reducing presstimes. The combination of 4 percent catalyst E and the standard phenolic resin was the only system suggesting a reduction in presstime. At 3 minutes (1 min. less than minimum presstime for the control board), the panel emerged from the press without blowing, although it did have a low-density center core. Curing rate with phenolic resins was less rapid than that experienced with urea resins. Consequently, amount of cure could be detected by internal bond measurements. Minimum presstime was 4 minutes in all cases and centerline temperatures had to remain above 220 F. for about 1-3/4 minutes to effect a cure. Species.--The fast-curing phenolic was used in an uncatalyzed state to determine species effect on resin performance. Neither aspen nor southern pine produced acceptable boards which could be pressed in less time than that required (5 min.) for the Douglas-fir board. Note (table 1) that the fast-curing phenolic which had gel times less than one-third of a standard phenolic resin, required an additional 1 minute presstime to form a Douglas-fir board. Longer presstimes in the case of the phenolic-bonded aspen and southern pine boards were, therefore, directly attributed to resin type whereas the longer presstimes needed for the urea-bonded aspen and southern pine boards were attributed to the wood furnish. Presstime reduction was possible only at the expense of a reduction in internal bond strength. The fact that southern pine boards with internal bond strengths as low as 32 p.s.i. could be produced without blowing is attributed to the lower internal steam pressures, as indicated by reduced maximum temperatures. Higher internal bonds were obtained in equivalent times with the aspen board. This could be attributed to one of several factors such as less damage to the wood during flaking, higher specific pressure during pressing due to the greater bulk of aspen flakes, or an increased cure due to the more basic characteristic of the furnish (fig. 2). Thickness and density.--catalysts did not reduce presstimes in 3/4- and 1-inch-thick boards. The apparent reduction of 1 minute for the 3/4-inch board as shown in table 3 was attributed to an experimental error in which the board made with catalyzed resin was at a higher initial 6 Stevens, M. P. Phenol-aldehyde Wood Bonding Adhesives Incorporating Organic Hydroperoxides and Having Improved Curing Rates. U.S. Patent No. 3,459,708. Aug. 5, 1969. 7 Cone, C. N. Phenol-aldehyde Adhesive Containing a Chromium Compound. U.S. Patent No. 2,612,481. Sept. 30, 1952. FPL 208-14-
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M 140 926 Figure 5.--Centerline cure times for phenolic-bonded (exterior) flakeboards. Numerals by symbols indicate catalyst levels. -17-
moisture content leading to early mat consolidation and faster heat transfer. Cure time remained about the same (1-1/2 min.) for both catalyzed and noncatalyzed boards. Minimum time for the 3/4-inch board was 5-1/2 minutes while that for a 1-inch board was 7 minutes. In the density series, minimum presstimes for 45 p.c.f. boards were 5 minutes both without and with a catalyst. Considerably longer times were required for the 50 p.c.f. boards with 8 and 9 minutes required without and with a catalyst. The seemingly inconsistent data (minimum presstimes without a catalyst, and lower internal bond strengths with a catalyst table 3) suggest that phenolic resins are much more sensitive to moisture in the curing zone than are urea resins. Temperature and closing pressure.--boards were made for this series using 5 percent mat moisture content with another 5 percent water sprayed evenly on the surfaces. Presstimes were decreased to 2 minutes in boards pressed at 350 F., 400 p.s.i. closing pressure (30 sec. closing time) and also at 375 F. with no limit on closing pressure (15 sec. closing time). The reduction of presstime is primarily due to the change in mat moisture distribution Any advantage gained by catalyzing the resin was slight. SUMMARY AND CONCLUSIONS (1) Minimum pressing time of a particleboard primarily depends on heat transfer, which in turn varies with thickness, press temperature, closing rate, and mat moisture distribution. When high internal steam pressures are involved, presstimes necessary to prevent blows depend on such factors as resin type, density, press temperature, and total moisture content. (2) Except where controlled by high internal pressures or excessive moisture contents, the time required to maintain centerline temperatures above 220 F. (cure time) varied only slightly between board types and pressing conditions, and served as a good indicator of minimum presstime. Minimum time above 220 F. averaged 0.30 minute for the boards made with catalyzed urea resins and was 1.6 minutes for the phenolic-bonded boards. (3) The effects of catalysis were found to be independent of other variables controlling presstime and shortened the presstime of urea-bonded boards by 1 minute. None of the catalysts studied had any appreciable effect in reducing presstimes of phenolic-bonded boards. (4) Catalyzing urea resins above a 2 percent level was ineffectual in reducing presstimes but did reduce final pressures. (5) Final pressure, although a good indicator of minimum presstime within a board type varied considerably as board variables changed. (6) The curing rate of urea resins is much faster than that of phenolic resins; consequently, undercured urea boards are often characterized by a "blow" or drastically reduced internal bond strength while in a phenolic-bonded board gradual reductions in internal bond strength appear as minimum presstime is approached. (7) Phenolic resins appear to be more sensitive to internal moisture conditions during pressing than urea resins. (8) Use of catalyst, higher press temperature, fast closing, and steam shock produced adequate 1/2-inch-thick flakeboard in 1 and 2 minutes' presstime for urea and phenolic resins, respectively. FPL 208-18-
M 140 922 Figure 6.--Centerline temperatures during pressing of phenol-formaldehydebonded Douglas-fir flakeboards containing 4 percent catalyst. -19-
APPENDIX A "Standard" FPL Particleboard (Revised April 1968) The "standard" FPL particleboards are prepared using the following materials and procedures. Thus, in research work, panels may be prepared using modifications of these materials or procedures and deviations may be simply stated as "aspen," "35 p.c.f.," "10 percent resin," etc. "Standard" Particleboard Species Douglas-fir heartwood. Particle type and size: Flakes, 0.015 inch by 1 inch by random width. Panel Density size Rough--1/2 by 24 by 28 inches. Trimmed--1/2 by 22 by 26 inches. 40 p.c.f. (based on weight and volume at 6 pct. moisture content). Resin solids content Resin Catalyst Additive Mat moisture content 6 percent resin solids (based on ovendry weight of wood). 65 percent solids liquid urea-formaldehyde (UF) or 44 percent solids liquid phenol-formaldehyde (PF). None. 1 percent wax emulsion (solids bases) (PF boards only). 10 percent (ovendry basis). Press cycle UF--5 minutes at 325 F. PF--10 minutes at 350 F. Press closing time 1 minute to thickness. FPL 208-20- U.S. Government Pringint Office 754-546/29 3.5-21-6-73