Effects of Steam and Acetylated Fiber Treatment, Resin Content, and Wax on the Properties of Dry-Process Hemlock Hardboards

Transcription

1 In: Wang, S.Y.; Tang, R.C., eds. Proceedings of the 1990 joint international conference on processing and utilization of low-grade hardwoods and international trade of forest-related products; 1990 June 11-13; Taiwan. Taiwan: National Taiwan University; 1990: Effects of Steam and Acetylated Fiber Treatment, Resin Content, and Wax on the Properties of Dry-Process Hemlock Hardboards John A. Youngquist, Roger Rowell, Nancy Ross, and Andrzej M. Krzysik 1 and Poo Chow 2 [Abstract] This paper reports the physical and mechanical properties of steam-pretreated or acetylated western hemlock dry-process hardboards prepared with two levels of resin and wax contents. Both heat pretreatment and acetylation improved dimensional stability. Properties measured included modulus of elasticity, modulus of rupture, tensile strength parallel to and perpendicular to board surface, thickness swell, water absorption, and linear expansion. Both 24-h water-soak and 2-h water-boil tests were conducted to determine the potential use of dry-process hardboards as structural components under high moisture content conditions. INTRODUCTION tion of adhesive-like wood components and the formation of hydrogen bonds. This reduces or eliminates the need for resin Hardboard, a panel material with a specific gravity rangadhesives. However, the treatment, recovery, and disposal of ing from 0.5 to 1.45, is made from wood that is first reduced the process water is an important and difficult problem. to fibers or fiber bundles and then manufactured into pan- In the dry process, air is the conveying and distributing els of relatively large size and moderate thickness. In their medium; without water, the conditions for natural bonding final form, these panels retain some properties of the origdo not develop. The development of mechanical and other inal wood, but because of the manufacturing methods and board properties relies entirely on added adhesives. the option of using modified fibrous materials, the panels also Control of moisture penetration is an important considgain new and different properties. Because hardboard panels eration in hardboard manufacture. One technique commonly are manufactured, they can be and are tailored for specific used with hardboards is to cover the surfaces of the individual end-uses. fibers with wax. This process reduces the surface energy of Hardboards are manufactured by either wet- or drythe fiber, thus making it more hydrophobic and less susceptiprocess technology. In both processes, wood is reduced to ble to moisture influences in high humidity conditions. fibers and formed into rigid sheets by recombination and This paper discusses research conducted on dry-process consolidation. hardboards. One major advantage of dry-process fiberboards In the wet process, large quantities of water serve as the is that their production requires much less process water, a conveying and distributing medium for the fibers and profeature that is particularly important because of the increasmote the development of natural bonding; that is, the activaingly stringent regulations that govern the quality of plant effluents (USDA, 1987). The use of dry-process technology 1 U.S. Department of Agriculture, Forest Service, Forest is limited because the hydrogen bond is eliminated, the op- Products Laboratory, Madison, WI. The Forest Products portunity for lignin bonding is substantially reduced, and the Laboratory is maintained in cooperation with the Univer- dimensional stability of dry-process boards is inferior to that sity of Wisconsin. This article was written and prepared by of wet-process boards (USDA, 1986). Recently, several treat- U.S. Government employees on official time, and it is there- ments have been developed that alter the physical properties fore in the public domain and not subject to copyright. of wood and thus affect its strength, stability, stiffness, and 2 Department of Forestry, University of Illinois, Urbana, IL water repellency (Rowell and Konkol, 1987) Stamm and others (1960) reported that heating wood -253-

2 in a vacuum at high temperatures caused lignin flow and hemicellulose decomposition, which produces water-insoluble polymers. This treatment increased dimensional stability, but decreased strength. More recently, Hsu and others (1988) reported that simple steam pretreatment of wood fibers caused partial hydrolysis of hemicellulose in both hardwoods and softwoods, which markedly increased the compressibility of the wood. This, in turn, significantly reduced the buildup of internal stresses in the composites during hot pressing. Rowell (1990) reported that wood flakes, strands, and fibers can be chemically modified using an acetylation process. When these materials are converted into panel products, they possess greatly improved dimensional stability properties and reduced susceptibility to biodegradation by decay fungi. The purpose of the research reported here was to determine if two fiber treatments, steam pretreatment and acetylation, could improve the mechanical and dimensional stability properties of dry-process fiberboards made from western hemlock at two levels of wax and phenolic resin content. EXPERIMENTAL DESIGN AND ANALYSIS The experiment was a full-factorial test of boards made from untreated (control), acetylated, or steam-pretreated wood fibers sprayed with 3 or 7 percent resin and 0 or 0.5 percent wax. Each treatment combination was considered a replicated set, consisting of 5 individual boards, for a total of 60 boards for the experiment. MATERIALS The western hemlock (Tsuga heterophylla) wood fibers, obtained from Canfor, Ltd. (Vancouver, British Columbia, Canada), were produced from 100 percent pulp-grade chips, steamed for 2 min at 7.59 MPa, disk refined, and flash dried at 160 C in a tube drier. The fibers were sent through a fan to separate them prior to board manufacture. The steam-pretreated wood fibers were prepared at the Forintek Corp. (Ottawa, Canada). The phenolic resin, obtained from Georgia Pacific Co. (Atlanta, GA), had a solids content of 51 percent, viscosity of 75 to 125 cps at 25 C., and ph of 9.5 to Two levels of resin content were used: 3 and 7 percent (based on resin solids content and ovendry fiber weight). The wax had a solids content of 50 percent, viscosity of 300 to 500 cps at 25 C, and ph of 6 to 6.5. The wax used was an unrefined paraffin slack wax obtained from Hercules, Inc. (Wilmington, DE). PROCESSING Steam Pretreatment The fibers were placed in a closed-ring system and subjected to saturated steam at a pressure of 1.55 MPa for 10 min. This steam pretreatment causes partial hydrolysis of the hemicellulose component of the wood, and it markedly increases the compressibility of the wood. The amounts of xylan, mannan, and galactan in the wood were decreased without any apparent change in the cellulose or lignin content (Hsu and others, 1988). Acetylation The aceylation reaction was carried out in a large stainless steel reactor using acetic anhydride, with a small amount of acetic acid generated during the reaction. The acetic anhydride was preheated in a holding tank to 110 C, and the reaction vessel was preheated to 120 C. The fibers (5.45 kg) were placed in a stainless steel wire-mesh basket in a stainless steel reactor, the reactor sealed, and acetic anhydride introduced; the fibers were reacted while totally submerged in acetic anhydride for 6 h at 120 C. The fibers were then ovendried at 105 C to remove excess acetic acid and acetic anhydride. This procedure resulted in an acetyl weight gain of 23 percent. Adhesive Application The wax and resin were sprayed on the hemlock fibers as they tumbled in a rotating drum. Tumbling was continued for 5 min after wax and resin addition to enhance dispersion of the adhesive on the fibers. The blended furnish was then transferred to an atmospheric single-disk refiner equipped with knobby plates set 2.5 mm apart to break up the fiber and resin-wax clumps. This technique greatly improved the uniformity of the adhesive distribution on the wood fibers. Board Manufacture Five hardboards with a specific gravity of 1.0 and a target thickness of 3.2mm were produced for each condition. Mats were formed on a 254- by 254-mm vacuum forming box where the fiber was passed through a 1.3-mm screen and onto a 1.6-mm screen. The mat was then flipped over onto a caul for pressing. All boards were pressed on a manually controlled, steam-heated press at 190 C for 8 min at a maximum pressure of 7.24 MPa for steam-pretreated and control boards, and at MPa for acetylated boards. After pressing, the 279- by 279-mm panels were trimmed to 254 by 254 mm. TESTING Static bending and tensile properties, and water absorption and thickness swell properties after a 24-h water soak were all determined according to ASTM D1037 (1987) standards. Thickness swell and water absorption after a 2-h water-boil test were determined according to Can M78 (CSA, 1975). RESULTS AND DISCUSSION The results are presented in Tables 1 to 4; values in parentheses are coefficients of variation in percent. Each data point represents an average of at least 10 test results. Physical Properties The target board thickness was 3.2 mm and the specific gravity 1.0. The data presented are very close to these values (Table 1), as evidenced by the low coefficients of variation for these two parameters. The moisture content values of the test and control boards before testing for static bending properties show no discernible trends related to wax or resin content. The moisture content values of the acetylated boards were approximately one-half that of the control and steam-pretreated boards. However, the coefficients of variation for moisture contents for the acetylated boards were somewhat higher than those for the control and steam-pretreated boards. Mechanical Properties Static Bending Properties-The minimum required modulus of rupture (MOR) values in the ANSI/AHA A Basic Hardboard Standard (AHA, 1982) is 31.0 MPa for standard hardboard and 41.4 MPa for tempered hardboard (Table l). All the boards made with 7 percent resin and either 0 or 0.5 percent wax easily met these minimum property standards for both standard and tempered boards. Control boards had the highest MOR values at both resin and wax levels; acetylated and steam-pretreated boards performed at -254-

3 Table 1. Static bending propertics of test and control boards. a,b a ANSI/AHA values for hardboard are as follows: thickness, 2.9 to 3.9 mm; specific gravity, 0.50 to 1.45; moisture content, 2 to 9%; modulus of rupture, 31.0 MPa for standard board and 41.4 MPa for tempered board (minimum requirements). b Values in parentheses are coefficients of variation (%). c Based on air-dry thickness and ovendry weight. d At 50% relative humidity and 20 C. Table 2. Tensile Strength of Test and Control Boards a,b Table 3. Dimensional Stability of Test and Control Boards After 24-h Water Soak a,b a ANSI/AHA values are as follows: tensile strength parallel to surface, 15.2 MPa (standard hardboard) and 20.7 MPa (tempered hardboard); tensile strength perpendicular to surface, 0.6 MPa (standard) and 0.9 MPa (tempered). b Values in parentheses are coefficients of variation (%). c Internal bond a ANSI/AHA values for standard and tempered hardboard are <35% and <25% water sorption, respectively, and <25% and <20% thickness swell, respectively. b Values in parentheses are coefficients of variation (%).

4 Table 4. Dimensional Stability of Test and Control Boards After 2-h Water-Boil Test a a Values in parentheses are coefficients of variation (%). about equal levels, but the values were lower than those of the control boards. For control boards, modulus of elasticity (MOE) values were higher when no wax was present at both resin levels. Increasing the resin content at both levels of wax content increased MOE values. Acetylated boards without wax had higher MOE values at both resin levels; increasing the resin content from 3 to 7 percent increased MOE values. Steampretreated boards performed exceptionally well at both wax and resin levels. At all treatment levels, control boards performed better than acetylated boards but not as well as steam-pretreated boards. Tensile Properties-The minimum tensile strength values parallel to board surface required by the ANSI/AHA 1982 standard are 15.2 and 20.7 MPa for standard and tempered hardboard, respectively; tensile strength values perpendicular to board surface (internal bond) are 0.6 and 0.9 MPa, respectively. All the treatments at all wax and resin levels equaled or exceeded these minimum tensile strength requirements (Table 2). Control and steam-pretreated boards exhibited parallelto-surface tensile strength values that were somewhat higher than those of the acetylated boards. Perpendicular-to-surface tensile strength values were approximately equivalent for all the treatments at all wax and resin levels. Dimensional Stability Properties Depending upon the end use of the product, dimensional stability in water, especially in the thickness direction, can be a great problem in composites made from a high percentage of wood fibers, like those tested in our study. The composites undergo not only normal (reversible) swelling but also swelling caused by the release of residual compressive stresses imparted to the product during pressing. A 24-h water-soak test (Table 3) and a 2-h water-boil test (Table 4) were used to measure thickness swell properties of test and control boards. Control boards with 3 or 7 percent resin and without wax had higher water absorption values than those allowed by the ANSI/AHA standard (Table 3). Water absorption values of control boards with 3 or 7 percent resin and 0.5 percent wax were about equal to the maximum allowable values. Steampretreated boards exhibited values in excess of this maximum standard level, regardless of the amount of resin or wax used. Acetylated boards made with 3 percent resin and without wax had 25 percent water absorption; boards with 0.5 percent wax had 19 percent water absorption. At the 7-percent resin level, water absorption levels were 19 and 17 percent for the 0- and 0.5-percent wax levels, respectively; these values are well below the standard maximum of 25 percent required for tempered hardboard. Maximum thickness swell properties for standard and tempered hardboard are 25 and 20 percent, respectively. Control boards without wax had 33 percent thickness swell at both resin levels; thickness swell of boards with 0.5 percent wax was 23 percent at 3 percent resin and 17 percent at 7 percent resin. Thickness swell of acetylated boards ranged from 0 to 3.5 percent for all combinations of wax and resin. Thickness swell of steam-pretreated boards ranged from 13 to 19 percent for all combinations of wax and resin. For the 24-h water-soak test, the addition of wax to the control boards improved water absorption and thickness swell at both resin levels. Similar results were obtained with the steam-pretreated boards at the 7-percent resin level; however, the addition of wax at the 3-percent resin level increased water absorption and thickness swell. For acetylated boards with wax at both resin levels, water absorption and thickness swell were further reduced from the already low values found with acetylated boards without wax. After the 2-11 water-boil test, water absorption values for all combinations of resin and wax levels were highest for the control boards, somewhat lower for the steam-pretreated boards, and substantially lower for the acetylated boards (Table 4). The same trends occurred for total volume expansion and thickness swell measurements. Adding wax to the control and steam-pretreated boards at both resin levels did not improve water absorption, total volume expansion, or thickness swell. Wax added to acetylated boards at both resin levels improved dimensional stability in all tests. CONCLUDING REMARKS 1. Bending strength (MOR) of all control boards was higher than that of steam-pretreated or acetylated boards. 2. Bending stiffness (MOE) was improved by steam pretreatment,compared to no treatment or acetylation. 3. Tensile strength values parallel to and perpendicular to board surface for all treatments and resin and wax combinations were relatively close. In all cases, values were substantially above the minimum requirements of the ANSI/AHA standard. 4. For the 24-h water-soak test, increased resin and wax contents generally improved water absorption and thickness swell for all treatments. Acetylation substantially improved both of these dimensional stability properties. Wax added to the acetylated boards further improved these properties. 5. For the 2-h water-boil test, increasing the resin content from 3 to 7 percent improved water absorption, total volume expansion, and thickness swell for all treatments. Although addition of wax to control and steam-pretreated boards did not improve these properties, the opposite effect was noted for acetylated boards. In general, dimensional stability of boards was somewhat improved by steam pretreatment and greatly improved by acetylation

5 LITERATURE CITED AHA American National Standard. Basic hardboard. ANSI/AHA A American Hardboard Association, Palatine, IL. ASTM Standard methods of evaluating the properties of wood-base fiber and particle panel materials. ASTM D In: 1987 Annual Book of ASTM Standards, Vol , Sec. 4. American Society for Testing and Materials, Philadelphia, PA. CSA Mat-formed wood particleboard. Can M78. Canadian Standards Association, Ontario. Hsu, W.E., W. Schwald, J. Schwald, and J.A. Shields Chemical and physical changes required for producing dimensionally stable wood-based composites. Wood Science Technology, 22: Rowell, R.M Chemical modification of wood: its application to composite wood products. In: Proceedings, 1990 IUFRO World Congress, August, 1990, Montreal, Canada. In press. Rowell, R.M. and P. Konkol Treatments that enhance physical properties of wood. U.S. Department of Agriculture, Forest Service. General Technical Report FPL- GTR-55. Madison, WI. Stamm, A.J., H.K. Burr, and A.A. Kline Heat stabilized wood - Staybwood. Rep. No U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI. USDA Fiberboard manufacturing practices in the United States. Agriculture Handbook 640. U.S. Department of Agriculture, Washington, DC. USDA Wood handbook: Wood as an engineering material. Agriculture Handbook 72. U.S. Department of Agriculture, Washington, DC. Printed on recycled paper