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Influence of fiber alignment on stiffness and dimensional stability of high-density dry-formed hardboard

1974; Forest Products Society; Volume: 24; Issue: 5 Linguagem: Inglês

2376-9637

P.E. Steinmetz, Charles W. Polley,

Forest Biomass Utilization and Management

The effects of aligning fibers and of their placement within the fiber mat on strength, elastic modulus, and dimensional stability of high-density, dry-formed hardboard were investigated. This approach was taken to produce hardboards with increased strength for use as structural components. Four sets of oriented fiber configurations and one set of random-formed control boards 3/16 inch thick were prepared from aspen and from Douglas-fir. For a given species, homogeneous, highly oriented fiber boards showed the greatest improvement in strength, elastic modulus, and dimensional stability. These properties varied in direct proportion to the percent of the total fibers in the one direction. Some loss in cross-direction strength and in stability accompanied these fiber-direction improvements. With the highly oriented boards, the elastic modulus was in the range of that of many clear lumber species. The linear movement in the fiber direction was also comparable to that for wood in the grain direction. The bending strength and stiffness were the only properties that benefited significantly from concentrating the oriented fibers on the surface of the mat. Tensile strength and both tensile modulus of elasticity and bending modulus of elasticity were improved by orienting alternate layers of fibers perpendicular to the preceding layer but not to the extent of the other oriented boards. The dimensional stability, however, was comparable to that for random-formed control boards in either fiber or cross-fiber direction. The authors are, respectively, Mechanical Engineer and Physical Science Technician, USDA Forest Service., Forest Prod. Lab., Madison, Wis. This paper was received for publication in October 1973. W O O D F I B E R B A S E H A R D B O A R D , a well-known product , has been used for years . However , i ts use for structural members has been limited. With the growing shortage of high-quality knotfree timber and related products, the need for improved wood fiber-base structural products that are uniformly strong and that can be better engineered for the particular end use becomes apparent. Although chemical binders have been employed successfully to improve many of the properties, they have not provided certain properties, especially stiffness and linear stability, comparable to those of many wood products. The dry-formed process now makes it possible to produce thick fiber-base members economically. It has been shown that a fiber is stronger and more dimensionally stable in the longitudinal direction than in the cross-fiber direction. To take advantage of these inherent characteristics, this work was undertaken to establish the benefits of forming hardboard with the majority of the fibers in one direction. Experimental Procedure Fiber Preparation and Refining Quaking aspen (Populus tremuloides Michx.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) France) fibers were prepared in a MacMillan Fiberizer in which wood bolts are passed over rapidly rotating drums fitted with metal needles that reduce the wood to a coarse fiber. The fiber from this process is particularly suited for orientation because of its relative straightness. Exploratory experiments were conducted with fibers prepared by other processes including pressurized refiners, but these fibers. were more difficult to orient (Fig. 1). Eight percent by weight, based on the fiber, of a 30-percent solution of an alkaline-catalyzed phenolformaldehyde resin of the type commonly used for dry formed hardboard was sprayed on the fiber in a rotating Marks, R. E. 1973. The relationship between fiber modulus and S2 angle. Tappi 56(4) : 164-167. Cave, I. D. 1968. The anisotropic elasticity of plant cell walls. Wood Sci. and Tech. 2:268-278. F O R E S T P R O D U C T S J O U R N A L V o l . 2 4 , N o . 5 45 drum. The fiber-resin mixture was allowed to air-dry to about 8 percent moisture content. This was followed by one pass through a 12-inch-diameter disk mill equipped with pyramid-type plates to disperse the large cluster of fiber bundles formed during resin treatment and drying. Boardmaking. – A method devised at the Forest Products Laboratory by the authors was used to orient the fibers and form the mat. The separated fibers were deposited by air conveyance from a height of 18 inches or more onto a vibrating corrugated plate with flutes 11/32 inch wide. These oriented fibers were then transferred to a flat caul plate, and the process repeated until the desired mat thickness was obtained. The mats were then pressed between platens of a hot press at a platen temperature of 385°F for 5 minutes using “stops” to control board thickness. The boards were given a further heat treatment in a circulating-air oven at 200°F for 3 hours. Boards, 12 inches by 12 inches by 3/16 inch, were fabricated with the following configurations (Fig. 2). Highly oriented, homogeneous (A). – Most of the fibers were aligned in the same general direction (90 percent orientation) . Partially oriented, homogeneous (B). – One-half of the fibers by weight were oriented, and distributed throughout the thickness of the mat in 17 equal-weight layers alternating oriented and random-formed layers (70 percent orientation) . Partially oriented, surface (C). – One-half of the fibers by weight were oriented and divided equally on the two surfaces of the mat with the middle half of the mat consisting of randomly formed fibers (70 percent orientation) . Alternate layers oriented (D). – Nineteen equalweight oriented layers of fiber were formed with the fibers in the adjacent layer perpendicular to those in the preceding layer (50 percent orientation) . Random oriented (E). – Control boards with all fibers randomly oriented as in conventional hardboard (50 percent orientation). 3 Three boards of each configuration and of each species were fabricated.