OF PARTICLEBOARD. U.S. Department of Agriculture Forest Service Forest Products Laboratory Madison, Wis. USDA FOREST SERVICE RESEARCH PAPER FPL 212

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1 WEATHERING CHARACTERISTICS OF PARTICLEBOARD USDA FOREST SERVICE RESEARCH PAPER FPL U.S. Department of Agriculture Forest Service Forest Products Laboratory Madison, Wis.

2 ABSTRACT Thirty-two types of boards were with varying components, levels of additives, constructed and pre-, post-, and surface treatments. The report includes previous unpublished data pertaining to a study of component variables, and describes the effects of weathering on steam post-treated boards.

3 WEATHERING CHARACTERISTICS OF PARTICLEBOARD BY ROBERT L. GEIMER, Technologist BRUCE G. HEEBINK, Engineer and F. V. HEFTY, Technician Forest Products Laboratoryr 1 Forest Service U.S. Department of Agriculture ---- Particleboards exposed to weathering undergo property changes just as solid woods do. Because the composition of these boards varies with construction and treatments, these products demonstrate a wide range of reactions to exterior exposure. In 1963, the Forest Products Laboratory began a series of studies designed to study the influence of component variables on the properties of particleboard. Some 32 types of boards were constructed with varying types of components, levels of additives, and pre-, post-, and surface treatments. 2 A paper published in 1966 by Gatchell et a1. presents initial strengths, stability after exposure to several laboratory temperature and humidity conditions, and thickness swell after 1 year of outdoor exposure. Outdoor conditioning of these boards was continued for periods of 2 and 5 years. The study was expanded to include eight other variables and to obtain strength retention on five board types after outdoor weathering. During this period one of the most promising methods of reducing thickness swelling proved to be post-treatment with steam. A separate series of boards was constructed to explore the effect of outdoor weathering on the strength retention and thickness swell of steam post-treated boards. It is the purpose of Part I of this paper to report the previously unpublished data pertaining to the component variables study and of Part II to describe the effects of weathering on steam post-treated boards. 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Catchell, C. J., Heebink, B. G., and Hefty, F. V. Component variables on properties of particleboard for exterior use. Forest Prod. J. 16(4): April

4 PART I. EFFECT OF COMPONENT VARIABLES ON PROPERTIES OF PARTICLEBOARDS EXPERIMENTAL DESIGN Variations of the components or manufacturing processes of the "standard FPL exterior particleboard" are the basis for property evaluations. Specifications for this standard board are as follows: Species--Douglas-fir Board size--1/2 by 24 by 28 inches, rough; 1/2 by 22 by 24 inches, trimmed Particle geometry by 1-inch random width flakes Specified density--40 pounds per cubic foot at 6 percent moisture content Moisture content of particles before resin application--6 percent Resin--6 percent (solids basis) liquid phenol formaldehyde Additives--1 percent (solids basis) wax Mat moisture content (before pressing)--12 percent Cold prepress--400 pounds per square inch for 1 minute Press temperature--350 F. Time to stops-3 minutes Total press time--15 minutes Sanding-1/32 inch from each face Table 1 shows the types of boards constructed. Variables were not combined but simply substituted for the standard level. Various board constructions or treatments are as follows : Slivers.--Half-inch Douglas-fir with a 1/4-inch screen. pulp chips were passed through a hammermill equipped Shavings.--Douglas-fir shavings were made on a planer with a 12-inch-diameter cutterhead; 3/32-inch depth of cut; eight knife marks per inch. Sheathing grade.--an unsanded board was made with 2-inch-long by inch-thick flakes and 2 percent phenolic resin content. Two-layer.--(Paintable one side). Fines (through 16-mesh screen) on one face only, 10 percent resin content in fines. Four percent resin content in flakes, 800 pounds per square inch prepress, unsanded. This construction was made with both Douglas-fir and redwood. Weight of the fines on the face equals one-fifth total weight of wood. Three-layer.--(Paintable two sides). Fines (through 16-mesh screen) on faces, 10 percent phenolic resin in fines. The weight of fines on each face equals one-fifth the total weight of wood. Standard with overlay.--phenolic-impregnated kraft paper was cold-pressed bonded to each face of a standard board with an acid-catalyzed phenolic resin at room temperature. Two-layer overlay.--this construction is similar to the two-layer board mentioned above and is overlaid on each side with a phenolic-impregnated kraft using an acid-catalyzed phenol resin. Bonding of the kraft to the board was done using a cold-pressing operation. "One shot" overlay.--a standard board was overlaid on both surfaces with phenolicimpregnated kraft paper containing a glueline and bonded to the board during hot pressing of the mat. FPL

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6 Water-repellent.--Specimens were submerged for 30 seconds in a commercial treatment containing 4.1 percent pentachlorophenol, 0.9 percent other chlorophenols, 85 percent petroleum solvents, and 10 percent resin and water repellent. Paint.--Two coats of an oil-based exterior white paint were applied to each face and all edges. Post-cured.--Post-curing was done by placing standard boards directly from the press into an oven heated to 160" F. for 16 hours. Steam (restrained).--boards were restrained under pressure and subjected to 350 F., 120 pounds per square inch steam. The time to reach 108 pounds per square inch was 5 minutes. After the desired hold time, pressure was released over a 1-1/2-minute period. Steam (autoclave).--boards were placed in an autoclave with no restraint and held under 350" F., 120 pounds per square inch steam, for 2 minutes. For a more detailed explanation of the steam post-treatment, see U.S. Forest Service Research Note FPL TESTING As explained more fully in an earlier report, 2 the following data were obtained: (1) Dimensional stability.-measurements give a direct indication of dimensional movement occurring during humidity changes. Thickness and lineal dimension measurements were made on samples conditioned at 80" F. and 30, 65, 80, and 90 percent relative humidity, and also after a 30-day water soaking. 4 (2) Springback/accelerated-aging (ASTM D ). --The data are indicative of the springback released and are not a value of the maximum thickness reached during exposure. The six-cycle, 12-day exposure included soaking, steaming, freezing, and drying. Thickness swelling was measured after reconditioning to 80 F., 65 percent relative humidity and reported as a percentage of the original thickness at that condition. (3) Springback/exposure site conditioning.--this is another indication of the springback released. Specimens were exposed at four outdoor sites: Fresno, Calif.; Madison, Wis.; Gulfport, Miss.; and Olympia, Wash. Samples exposed for periods of 1, 2, and 5 years were reconditioned to 80 F., 65 percent relative humidity, and measured for springback. 4 (4) Static bending and internal bond strength (ASTM D ). --Tests were performed on all board types in the unsanded and sanded stage, conditioned at 80" F. and 65 percent relative humidity. In addition, five board types were tested following exposure to accelerated aging and outdoor weathering conditions. Measurements were taken after reconditioning to equilibrium at 80 C., 65 percent relative humidity. 3 Heebink, B. G., and Hefty, F. V. Steam post-treatments to reduce thickness swelling of particleboard (exploratory study). U.S. Forest Serv. Res. Note FPL-0187, Forest Prod. Lab., Madison, Wis. March American Society for Testing and Materials. Standard methods of evaluating the properties of wood-base fiber and particle panel materials. ASTM Desig. D FPL 212 4

7 RESULTS Dimensional Stability Dimensional change from ovendry to equilibrium at four constant relative humidity conditions or after a water soak are given for the eight board variables not already reported on (table 2). Data are reported as percentage of thickness and length increase over ovendry conditions. With the exception of the steam post-treated boards, swelling values are similar or slightly higher than the standard. Steam post-treatment considerably reduces thickness swell at the higher moisture contents where springback is a major factor. As observed in other studies, 3 lineal expansion is not affected by steam post-treatment. Table 3 contains the outdoor exposure site and accelerated aging thickness swell data on all of the 40 board types included in the study. Figures given are in percent of thickness increase over original thickness, both measurements being made after equilibrating the sample to 80 F., 65 percent relative humidity. The data consequently represent springback release due to exposure. Exposure site index is an average of the percent springback experienced at the four exposure sites. For comparative information, the degree of thickness increase (measured after the same equilibration) obtained at each of the four sites is presented for the fifth year. Exposure site index values are presented graphically in figure 1. In most cases, 5-year performance could be roughly predicted by the swelling occurring during the first year. There is little correlation between springback due to accelerated aging and that experienced in outdoor weathering. The accelerated aging test is severe enough to overcome many of the factors preventing rapid swelling in outdoor exposures. Boards bonded with urea resins, which have an extremely high thickness swell value during accelerated aging, experienced average swelling for the first few years of outdoor exposure and then deteriorated rapidly. Painted samples which experienced normal thickness swell during accelerated aging withstood the first 2 years' outdoor exposure very well, but gained rapidly in thickness thereafter. The painted board weathering pattern shows the importance of surface smoothness and integrity in reducing the amount of free water entering a panel during a rainstorm. Increased thickness swelling occurred as soon as the protective coating of paint had weathered. A pronounced difference exists between accelerated aging and outdoor weathering thickness swell values of redwood. Apparently the redwood board has a substantial amount of potential springback; but because of the excellent weathering properties of the species, these forces are restrained under normal environmental conditions. Sheathing-grade panels did very poorly in all types of exposures. This is to be expected as the construction employed a combination of the worst variables in the groups; that is, 2 percent resin, 2- by inch flakes, and an unsanded condition. A slight reduction in springback after 5 years' exposure was obtained by the use of paper overlays. Exception to this was the two-layer overlay where springback was high. No advantage in reducing thickness swell was found in applying the overlay in a "one shot" operation. Treatment with water repellent had little effect. Of all the surface treatments tried, painting was the most effective in reducing springback until the paint lost its protective value. 5

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11 M Figure 1.--Influence of accelerated aging and outdoor weathering on thickness swell (based on measurements after equilibrating at 80 F. and 65 pct. relative humidity). 9

12 M Figure 1 (continued).--influence of accelerated aging and outdoor weathering on thickness swell (based on measurements after equilibrating at 80 F. and 65 pct. relative humidity). FPL

13 M Figure 1 (continued).--influence of accelerated aging and outdoor weathering on thickness swell (based on measurements after equilibrating at 80 F. and 65 pct relative humidity). 11

14 While post-treating with heat alone had no effect on thickness swelling, steam treatments were outstandingly effective. It will be noted that the no-restraint steam treatment in this case is limited to 2 minutes as opposed to the 10 minutes used in later studies. 3 Still, the antispringback efficiency was high. Strength Retention Static bending and internal bond tests were performed on all specimens in the sanded and unsanded state prior to exposure. Results for the eight boards not previously reported 2 on are given in table 4. Neither heat post-curing nor a 2-minute treatment with steam while under restraint showed any effect on initial strengths of the board. Longer periods of steaming (restrained) may be somewhat detrimental to bending strengths as indicated by the drop in modulus of elasticity and modulus of rupture values of the 5-minute steam (restrained) post-treated boards. Two minutes of steam treatment in an autoclave (unrestrained) was enough to lower modulus of rupture values of the unsanded boards by 20 percent. The apparent reduction in strength is due in part to the use of the increased thickness in computing the modulus of rupture values. True load-carrying capacity of the board was reduced approximately 5 percent by the treatment. The disadvantage of using modulus of elasticity and modulus of rupture values to make "strength" comparisons of a nonhomogeneous-type board when the physical size of the panel has changed is also apparent in relating unsanded to sanded board values. Sanding off a portion of a panel cannot increase its load-carrying capacity even though the modulus of rupture values have increased. Strength data after accelerated aging and outdoor weathering were obtained on five boards of selected construction: Standard, 8 percent phenolic, 10 percent phenolic, water-repellent treated, and 2 minutes' steam (restrained) post-treatment (tables 5 and 6). Modulus of rupture and modulus of elasticity are all based on the original thickness prior to exposure, permitting relative comparisons of original strength retention. Internal bond strengths, being partly a function of specific gravity, accordingly vary with thickness swelling. The 10 percent phenolic board and the steam post-treated board showed the least amount of thickness swell and retained the greatest percentage of internal bond strength after both 5-year weathering and accelerated aging. All board types retained the same degree (83 pct.) of original stiffness after 2 years. At the end of 5 years, modulus of elasticity retentions ranged from 73 to 83 percent with the higher resin content boards showing a slightly better performance. Results were very similar in regard to ultimate bending strengths. In this case the steam posttreated board retained the same percentage of strength, 82 percent, as that of the 10 percent phenolic-bonded board. Part I CONCLUSIONS Part I (1) In general, 5-year springback performance can be predicted by measurements taken after 1 year of outdoor exposure. Exceptions to this include the urea-bonded boards and painted specimens. Springback in the urea-bonded boards increased to a high level after 5 years due to the breakdown of the adhesive, whereas springback in the painted phenolic bonded board rose to a normal level after 5 years because of deterioration of the painted surface. FPL

15 (2) Accelerated aging springback measurements are not well correlated with a board's performance in outdoor exposures. Factors tending to prevent moisture movement into a panel when exposed to outdoor conditions are largely overcome by the severity of the accelerated aging test. (3) Paper overlays and water-repellent-treated boards showed small reductions in thickness swelling. Painting proved to be the best surface treatment and retarded thickness swelling in outdoor conditions until the paint deteriorated. Steam post-treatment was exceptionally effective in reduction of springback. 13

16 (4) Internal bond strength loss after 5-year exposure was directly related to degree of thickness swelling. About 20 percent of the original stiffness and bending strength was lost in 5 years. A short (2 min.) period of steam post-treatment (restrained) did not adversely affect bending strength. Longer periods (5 min.) of steaming under restraint and short periods (2 min.) of steaming with no restraint did adversely affect modulus of rupture values. FPL

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18 PART II. EFFECT OF STEAM POST-TREATMENT ON THE PROPERTIES OF PARTICLEBOARD Reduction in thickness swelling attributed to steam post-treatment was sufficient to encourage further study along these lines. Initial results on boards exposed to laboratory-controlled environments have been reported in a paper by Heebink and Hefty. 5 To determine the effect of outdoor exposure, a group of boards were constructed and tested for thickness swell and strength values after 1 and 2 years of exposure at the Madison, Wis., test site. EXPERIMENTAL DESIGN Two types of board constructions were used in this study, one a "standard" homogeneous flakeboard and the other a three-layer board, having a flake core and a fines face. With the exception of no prepress and a closing time of 1-1/2 minutes, the basic construction is the same as described in Part I of this report. Species investigated were Douglas-fir, redwood, and aspen for both types of board construction. Post-treatments were made by subjecting the particleboard specimens to saturated steam at 360 F. for 10 minutes in a closed chamber, with no restraint. All boards were sanded approximately 1/32 inch on each surface before initial testing and exposure. Thickness and strength (bending and internal bond) measurements were made on control specimens and on specimens exposed to accelerated aging or to 1 to 2 years of outdoor weathering. In all cases the samples were equilibrated at 80 F. and 65 percent relative humidity prior to measurements. Modulus of rupture and modulus of elasticity values were calculated using the control sample thickness. RESULTS Thickness Swelling Steam post-treatment using no restraint affects particleboard in much the same manner as the standard accelerated aging exposure. This is indicated by the close correlation existing between the initial thickness swelling of steam post-treated boards and the springback due to accelerated aging of the control samples (fig. 2). Three-layer boards in both cases swelled less than their homogeneous counterparts. Springback of steam post-treated boards when subjected to the accelerated aging cycle varied with the degree of previous post-treatment swelling and was dependent on species and board configuration. Within a species type, however, the total thickness increase caused by a combination of steam post-treatment and accelerated aging was a constant percentage of the springback due to accelerated aging of the untreated control board. This amounted to 75 percent for Douglas-fir, 82 percent for aspen, and 93 percent for redwood, and held true for both homogeneous and three-layer boards. Assuming that the accelerated-aging test serves to indicate the total possible springback inherent in a board, steam post-treatment of unrestrained boards (1) reduces the total springback level, varying from 25 percent with Douglas-fir to 7 percent with redwood and (2) alleviates some of the remaining springback by the process of swelling during treatment. This alleviation is dependent at least in part on species and board configuration, and varies from 65 to 90 percent of the remaining springback. 5 Heebink, B. G., and Hefty, F. V. Treatments to reduce thickness swelling of phenolic bonded particleboard. Forest Prod. J. 19(11): Nov FPL

19 M Figure 2.--Influence of accelerated aging, outdoor weathering, and steam post-treatment on thickness swell (based on measurements after equilibrating at 80 F. and 65 pct. relative humidity).

20 Exposure of the treated boards to outdoor weathering resulted in springback percentages of varying degrees; however, with the exception of redwood, it was correlated to a great extent on the remaining springback left in the board. The redwood reacted much the same as in the earlier study shown in this report. Accelerated aging gave high springback values but the board performed well in outdoor exposures. Swelling of the steam-treated boards was, of course, much less than that of the control boards. Antispringback efficiencies varied from between 74 to 92 percent and are shown in table 8. Best results were obtained with the aspen three-layer board. Maximum swelling in the outdoor exposures was reached by the end of 1 year, with the single exception of the aspen control board. Strength Retention Results of the strength tests following outdoor exposures and accelerated aging are given in table 7 and also shown as a percent of the original values in table 8. In general, 2-year exposure lowered internal bond values only slightly, the effect being more noticeable in three-layer boards and dependent on species. Redwood lost considerable internal bond strength in the accelerated aging cycle (between 25 and 42 pct.) but weathered well, losing very little of its original internal bond strength in 2 years' time. Internal bond strengths were not affected by steam post-treatments, either in original strength tests or after a period of outdoor weathering. Bending strength retention varied considerably between species and board types. Steam post-treatment generally reduced original bending strength by 20 percent and stiffness by 10 to 20 percent. Upon exposure to weathering conditions, treated boards continued to lose strength at about the same rate as that experienced by the control boards. This rate loss also varied considerably between board type, species, and time and type of exposure, ranging from an additional 5 to 20 percent loss in 2 years' time. Testing will be done on additional matched sets of exposed specimens as it becomes apparent that the effects of weathering have further changed their strength and springback character is tics. Part II CONCLUSIONS Part II (1) Steam post-treatment (with no restraint) reduced springback by reducing the potential swelling in a board and by causing initial swelling during treatment. The level by which the potential swelling was reduced was found to be species dependent, varying from a low of 7 percent with redwood to a high of 25 percent with Douglas-fir. Antispringback efficiencies varied between 74 and 92 percent depending on species and board configuration. (2) With the exception of the three-layer Douglas-fir boards, internal bond values were not appreciably lowered by 2 years of outdoor weathering. No significant difference exists between the internal bond values of steam-treated boards and those for the control samples. (3) Steam post-treatments of 10 minutes at 360 F. (with no restraint) reduced original bending load-carrying capacity by 20 percent and stiffness by 10 to 20 percent. Rate of loss thereafter was comparable to untreated boards, varying considerably between species and board types and averaging between 5 to 20 percent in 2 years' time. FPL

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