Resin Efficiency in Particleboard As Influenced by Density, Atomization and Resin Content

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1 Resin Efficiency in Particleboard As Influenced by Density, Atomization and Resin Content William F. Lehmann Abstract Resin efficiency in particleboards was studied using a Douglas-fir flake-particle mixture. Two levels of board density and resin atomization and three levels of resin content were studied. All variables were found to have controlling influences on board properties. Bending strength approached optimum levels between 4 and 8 percent resin content, while internal bond strength continued to increase with increasing resin content. Equations showed that savings in raw materials, either wood particles or resin, could be attained by use of proper resin application techniques. A simple technique using a reflection photometer to estimate coverage of wood particles with dyed resin was presented. RESIN EFFICIENCY has been defined by Burrows (1) as the application of a minimum quantity of adhesive to wood particles, with resulting optimum physical properties in a pressed board. Theoretically, as pointed out by Marra (12), resin needs to be applied only to those portions of the particles which will come into contact with other particles. Realizing the impracticality of this, two alternatives are advocated. As suggested by Marian (13) and Kollmann (7), the resin should be applied in the form of a continuous film over the entire surface of the particle.. The alternative method suggested by Klauditz (5) and Marra (12) is to apply the resin as small droplets to the extent that each particle will be uniformly covered and the resin droplets will be sufficient to act upon the entire contact area of the particles. Klauditz and Ulbricht (6) and Meinecke (14) have found it possible to achieve a continuous film by spraying small amounts of resin onto glass plates and then subjecting them to heat and pressure which causes The author is Technologist, USDA Forest Service, Forest Products Laboratory, Madison, Wis. This study was completed while the author was Assistant Professor of Wood Technology, Forest Research Laboratory, School of Forestry, Oregon State University, Corvallis, Oreg. Grateful appreciation is expressed to all who aided with this project, especially the various manufacturers representatives who supplied raw materials, adhesives, wax emulsions, and valuable suggestions. Personnel at the Oregon State University Forest Research Laboratory deserving special thanks are R. A. Currier, L. E. Knowles, D. A. Carlson, K. Hori, and A. H. Doerksen. This paper was received for publication in August the resin to flow into a very thin film. Both of these phenomena have been shown to occur in the flakeboards in a study by Lehmann (10) and were dependent upon the degree of resin dispersion and distribution. Other researchers, Carroll and McVey (2), Fehn (41) and Marian (13) caution against loss of resin efficiency through over-penetration of resin into the wood. Marian (13) also emphasized the importance of undamaged particle surfaces. In relation to particle geometry, Klauditz (5) states: The thinner the chips and consequently the greater the total surface to be coated, the greater care to be given to thorough blending. The importance of particle surface area and its relation to resin distribution also has been studied and reported by Carroll and McVey (2), Maloney (11), and Schwarz, et al. (15). A study by Suchsland (16) revealed that with woods of specific gravity less than 0.5 and when spraying resin on veneer under favorable conditions, glue solids spreads as low as 0.3 gram per square foot would produce joints stronger than the wood. Assuming resin specific gravity of 1.26 and solids content of 66 percent, this would produce a uniform layer of liquid glue only 3.88 microns or about inch thick. Klauditz and Ulbricht (6) have calculated that the surface area of 100 grams of spruce particles of inch average thickness was approximately 15.4 square feet. Assuming a 6 percent resin solids content, theoretically this resin could be applied at a spread of 0.39 gram of solids per square foot under ideal conditions. They also varied the degree of resin binder atomization and found a 15 percent strength increase due to finer atomization. This is corroborated in the study by Burrows (1) on resin efficiency in flakeboard which indicated that fine atomization should be advocated, particularly at the low resin contents used in the particleboard industry. A study by Engels (3) concerning spraying efficiencies of conventional air nozzles, an airless system, and a centrifugal disc demonstrated the importance of spraying methods, pressures, distance of raw material from spray nozzles, volume sprayed per time unit, and percent solids of the binder. He also pointed out that without proper chip movement during the blending 48 NOVEMBER 1970

2 operation, resin efficiency will be low no matter how well these previous factors are coordinated. A previous study by Lehmann (9) explored several variables: method of spraying, resin solids content, resin temperature, degree of atomization, and resin application time. Uniform distribution of resin produced by fine atomization and a suitable application time were found to be the controlling factors in determining optimum properties at a given resin content. The factors of board density and resin content were, however, held constant in this earlier study. Experimental Procedure Manufacture of the boards followed commercial practice as closely as possible. Variables investigated were board density, degree of atomization of resin, and resin content. Boards were tested for strength and dimensional stability. Also, resin distribution and actual resin content of particles were examined. The results of the study were analyzed by statistical and graphical means. In the factorial design used, levels of variables were as follows: degree of atomization, fine and coarse; resin content, 2, 4, and 8 percent solids based on ovendry (OD) weight of wood; and density, 0.65 and 0.75 g/cm 3, (40.6 and 46.8 lb./cu. ft.) based on volume at test and OD weight. Two boards were produced at each experimental combination for a total sample size of 2 x 3 x 2 x 2 = 24 boards. Raw Material The furnish used in the study was a uniform mixture of Douglas-fir flakes and particles such that a screen analysis showed almost equal proportions in the fractions separated by a 9-mesh Tyler screen. The ureaformaldehyde resin used as binder was WW-59M, supplied by the Borden Chemical Company at 66 percent solids. The wax emulsion used was Paracol 404N, supplied by Hercules, Inc., at 50 percent solids. A fluorescent red dye, Rhodamine B, made by Antara Chemicals, was added to the resin-wax mixture at 0.6 percent by weight of resin solids. Preparing the Boards The boards were manufactured in a random order by the following procedure. The blender was charged with slightly more than enough particles to make one board. Water was sprayed on the particles to adjust the moisture content to the proper level so that moisture content of the furnish would be as desired for pressing after the resin and wax were applied. The resin, wax emulsion, and dye were mixed together and applied by an air spray gun to the tumbling particles. After blending, moisture content was determined and the proper weight of furnish was formed into a homogeneous mat and hot-pressed. Excess furnish was saved for further study. The following factors were held constant for all boards : Species Douglas-fir Particle type Flake-particle mixture Board size by 18 inches Thickness /16 inch, sanded to 5/8 inch Wax content Atomization air pressure /4 percent Fine spray, 60 psi; coarse spray, 20 psi Moisture content into press ± 0.5 percent Press closing time to 30 sec. to stops Press cycle min. at 310 F. After pressing, boards were cooled and conditioned at 70 F. and 65 percent relative humidity (RH) to practical equilibrium. Testing the Boards After conditioning, the boards were sanded to 5/8- inch thickness, trimmed to 16 by 16 inches, and sawed into five 3 by 16-inch strips. Two strips from each board were tested for strength and two for dimensional stability. In all tests of strength, procedures outlined in ASTM Standard D , Methods of Evaluating the Properties of Wood-Base Fiber and Particle Panel Materials, were followed as closely as possible. Strips for tests of dimensional stability were conditioned at 90 F. and 30 percent RH for at least 3 weeks before testing. Strength and Related Properties Modulus of rupture (MOR) and modulus of elasticity (MOE) were determined from specimens tested on a 15-inch span at 0.30 inch per minute. Specimens for measuring internal bond (IB), 2 by 2 inches in size, were cut from the specimens tested in static bending, glued to metal blocks with epoxy resin, and tested at 0.05 inch per minute. Density (D), based on volume at test and OD weight, was determined from weight and dimensions of the remaining half of the static bending specimen. Dimensional Stability Properties After conditioning at 90 F. and 30 percent RH, specimens were exposed to 90 F. and 90 percent RH to determine linear expansion and thickness swelling. With the exception of thickness swelling (TS), however, malfunction of equipment limited the period of measurement to 21 days. Other Properties Because the resin added was dyed, differences could be seen in the various board furnishes. An attempt was made to correlate these differences with the relative coverage of particle surfaces by resin binder. This was done with a photoelectric reflection photometer (ELREPHO), manufactured by the Carl Zeiss Company, and normally used for color determinations of pulp and paper. The photometer essentially determines differences in the intensity of light reflected from surfaces under study. For brevity in this report, the term reflectance is used to refer to the measurement of these differences in reflected light intensity based on readings obtained from the percent reflectance scale of the instrument. The following procedure was followed: The machine was calibrated in filter position 8 (457 mµ), which is the TAPPI standard brightness filter, by the use of the ELREPHO glass standard. By a series of preliminary measurements, filter position 4 (540 mµ) was found to be most sensitive to FOREST PRODUCTS JOURNAL Vol. 20, No

3 small visual differences between the various mixtures of particles containing the dyed resin. For the reflectance determinations, a small randomly selected sample of flakes and particles was fixed between glass plates and placed on the specimen holder. The circular area measured in each determination was 1.41 square inches. Three random measurements were taken in each group of particles for a total measured area of 4.23 square inches. Reflectance was determined from furnishes that had 2, 4, or 8 percent of dyed resin solids applied as fine or coarse spray. To determine the maximum possible range between reflectance readings, particles with Figure 1. Modulus of rupture as influenced by density, resin content, and resin atomization. Figure 2. Internal bond as influenced by density, resin content, and resin atomization. 50 NOVEMBER 1970

4 no resin and others that had been soaked in dyed resin were measured. Finally, furnishes were separated into two fractions with a 9-mesh screen and reflectance was measured for each of these groups. From these measurements, the percentage of particle surface covered with resin was estimated by comparing the reflectance reading of the furnishes under test with the difference between readings for particles without resin and particles saturated with resin. Also, nitrogen content was measured by a Kjeldahl procedure on the final group of furnishes to correlate reflectance measurements with actual resin content. Analyses were performed on furnishes at all resin contents applied as fine or coarse spray, on the particles with no resin, and on particles completely covered with resin. Each furnish was analyzed in both screen fractions to determine the relative amount of resin on each fraction. Analysis of Data Data were analyzed graphically as well as statistically. Analyses of variance were made on MOR, MOE, IB, TS, and reflectance. Linear regressions were determined for possible relations between reflectance and strength. Multiple regressions were computed to determine relations between strength, resin content, and density. Results and Discussion Strength Properties The strength properties are summarized in Table 1 and Figures 1 and 2. Values for MOR and MOE increased rapidly with increase in resin content from 2 percent to 4 percent and reached optimum levels between 4 and 8 percent resin content (Fig. 1). More notable was the effect of resin atomization and density. The fine sprays, at all but the highest resin content and greatest density, produced boards stronger in bending than those made with coarse spray. Also, the bending properties showed more actual numerical improvement with fine spray at the higher density than at the lower density. This was caused by better bonding from improved particle-to-particle contact due to the increased pressure at the higher density. For internal bond, the same effects were evident as resin content and density increased, except that the leveling-off tendency was less apparent and fine spray produced higher internal bonds than did coarse spray at all resin contents. The three strength properties all were highly significantly affected by the three primary variables: resin atomization, resin content, and density (Table 2). Also, all were highly significantly affected by the resin content-density interaction. The correlation coefficients from multiple regression equations emphasize the precision of these prediction equations with the type of furnish used (Table 3). An example is included in the table to demonstrate the use of the equations to predict required amounts of resin or density levels to obtain boards of desired properties. Further expansion of data from these formulas would allow preparation of simple graphs similar to Figures 1 and 2, from which necessary resin content or density levels could be obtained at a glance, In the example shown in Table 3, the differences between fine and coarse atomization of resin are markedly evident. When resin content was maintained at 6 percent, board density could be decreased 3.2 percent, 4.7 percent, or 16.7 percent to produce boards of the given values for MOR, MOE, or IB with fine atomization as compared to coarse atomization. Conversely, if FOREST PRODUCTS JOURNAL Vol. 20 No

5 density was maintained at 0.65 g/cm 3, fine atomization would produce boards of equal quality with 19.6 percent, 19.6 percent, or 27.9 percent less resin than with coarse atomization. These relations are valid only for the data presented, but a similar set of data could be easily developed by a manufacturer for use with his own process. Dimensional Properties Thickness swelling in 21 days at 90 F. and 90 percent RH is summarized in Table 1 and Figure 3 and the results of the analysis of variance are shown in Table 2. Leveling-off in thickness swelling as resin content increased was especially evident, as illustrated in Figure 3. At low resin contents, low-density boards were most stable, but at high resin contents the reverse was true, even though differences were not large. Also, fine atomization allowed less dimensional change than coarse atomization. In the analysis of variance (Table 2), resin atomization and resin content were highly significant, but density was not a significant factor in determining thickness swelling, although there was a highly significant interaction between resin content and density. Reflectance Measurements and Resin Content Analyses In the analyses of variance of the data from color measurements and resin content, resin atomization and resin content had highly significant effects, but, although not expected because of similar conditions of resin application, density also was significant. Two interactions, resin atomization-resin content and resin contentdensity, were highly affected by the reflectance readings. Figure 3. - Thickness swelling after 21 days at 90 F. and 90% RH as influenced by density, resin content, and resin atomization. Figure 4. - Relation of color reading with the photometer to internal bond strength. The atomization-resin content interaction would be expected. The very high significance of resin content in combination with the much lower significance of density probably resulted in the significant interaction between resin content and density. The relation of reflectance readings to estimated coverage of wood particles by resin from the data in Table- 1 was as follows: Estimated coverage (percent) = reflectance The linear regressions obtained in an attempt to predict strength properties from reflectance readings were not well correlated except in the case of internal bond, which is the test most sensitive to differences in bonding efficiency. This relationship is as follows and is illustrated in Figure 4: IB (psi) = reflectance; R 2 = Determining differences in reflectance of furnish by use of a photometer was found to be a rapid means of estimating resin efficiency and board quality. The number 4 (540 mµ) filter was most suitable with the red-dyed resin used, but other dyes would probably require a different filter for greatest precision. Where colored furnish may be a disadvantage, a, colorless fluorescent dye may be added, although such a dye would require an ultraviolet light source for the photometer. The influences of resin atomization and resin content on reflectance readings and estimated coverage on the two screen fractions are shown in Table 4 and Figures 5 and 6. The larger particles always received less resin than the fraction with smaller particles. The average estimated surface coverage of the larger particles was 37.4 percent with fine atomization and 25.8 percent with coarse atomization. Overall, with fine atomization, the particles had an average estimated coverage of NOVEMBER 1970

6 1) Resin efficiency is highly affected and controlled by degree of atomization of resin, by resin content, and by board density. The first two factors primarily control the distribution and coverage of wood particles by the resin binder, and density contributes to the efficiency of bonding. 2) Regardless of resin efficiency, in the density range studied, static bending and thickness swelling approach optimum levels below 8 percent resin content, but internal bond strength continues to increase with increasing resin content. Figure 5. Relation of actual resin content of particles to color readings with the photometer as influenced by resin atomization. percent, but with coarse atomization the average estimated coverage was only 31.5 percent. There were rather large differences between estimated coverages with fine and coarse atomization at the 8 percent resin level, but small differences in strength. As the coverage of the wood particle surfaces by resin droplets increased, however, so did the probability that resin droplets on adjacent wood particles would coalesce during the pressing cycle to produce either thicker or more continuous glue lines, or both. And thus, as coverage approached an optimum level, the resulting boards would approach the optimum strength as determined by the given density range. Conclusions The following conclusions are offered based upon the conditions of this study and data obtained therefrom: Figure 6. Estimated coverages of particles with resin as influenced by resin content and resin atomization. FOREST PRODUCTS JOURNAL Vol. 20, No

7 3) 4) Multiple regression equations are valuable in selecting levels of important variables to obtain desired properties. In addition, the equations showed that at a density of 0.65 g/cm 3, boards of desired strength levels could be obtained with from 19.6 percent to 27.9 percent less resin with fine atomization than with coarse atomization. The technique for estimating coverage of wood particle surfaces with resin using a photoelectric reflection photometer was simple and precise for estimating resin efficiency in terms of resin distribution. Literature Cited 54 NOVEMBER 1970