In-plane Dimensional Stability of Oriented Strand Panel: Effect of Processing Variables1

Single-layer oriented strand panels were fabricated under a combination of three alignment levels, four densities, and two resin contents. Flake orientation, density gradient across panel thickness, linear expansion (LE), and bending properties were measured. Flake orientation was characterized with the von Mises distribution using mean flake angle and concentration parameter. It was shown that the shape of the LE-moisture content change curve varied with alignment level and test direction. The variation was attributed to whether the LE of a panel was controlled by transverse or longitudinal wood swelling along a particular test direction. Total LE from oven-dry to water-soak condition, modulus of elasticity (MOE), and modulus of rupture (MOR) varied significantly with flake orientation distribution and density. Effects of resin content at the levels used on LE, MOE, and MOR was relatively small and was niore diversified. The LE, MOE, and MOR were correlated with the concentration parameter, density. resin content, and moisture content using a power form equation. The experimental data form a database of layer properties for modeling three-layer, crosslaminated oriented strandboards (OSBs) manufactured under hot pressing. Kevword.~: Flakeboard, linear expansion. modeling, stiffness, strength. INTRODUCTION ognized importance of thickness stability, inOriented strandboard (OSB) is a structural reconstituted panel that consists of wood strands glued with an exterior-type, waterproof resin. The physical and mechanical properties of the board are enhanced by layering and alignment of wood flakes. In the last decade, OSB has gained significant growth in the structural wood-based panel market. Oriented strandboard capacity reached about 1 8.4 million m7 in 1997, with production at about 15.5 million m7 (Spelter et al. 1997). It has become a promising substitute for structural plywood. Oriented strandboard swells significantly when the product is exposed to high relative humidity (RH) conditions andlor in direct contact with water (Geimer 1982; Wu and Suchsland 1996, 1997). In addition to the well-recI This paper (No. 98-22-0139) is published with the approval of the Director of the Louisiana Agricultural Experiment Station. plane swelling, known as linear expansion (LE), can be a significant factor in structural applications. This is so because the swelling can greatly affect the state of stress that exists in the material. The in-plane movements can cause high internal stresses due to the restraint offered by fastening such as nails. These stresses may be large enough to cause buckled panels, pushed-out nails, and separation of the panel from the structure (Lang and Loferski 1995; Wu and Suchsland 1996). The in-plane dimensional change of OSB is a direct result of complex interactions among different layers across panel thickness in a three-layer, cross-laminated board (Bryan 1962; Xu and Suchsland 1997). Many processing parameters affect the dimensional change, the most important being flake orientation, shelling ratio (i.e., flake weight ratio between face layer and core layer), degree of bonding, flake geometry, density, and density W O < J ~ ( 0 1 < 1 I !I>< t 5, ,<,,,< c , 3 l l 11, 1'1~1'1. lpp. 28 40 c 1494 h) the Suclut) ot W<x,d Sc~encr and Technolirg~ Wti-IN-PLANE STABILITY OF ORIENTED STRAND PANELS 29 T A B L . ~ I . Bourd ,fabrication data-single-liryer uniform densiq oriented strand panels. Target hoard Revn l v u ~ ~ l b r r Number Boat d d r n w content" of of board\ 1y PC' ( g/crn ' 1 ( % i rcpliiatlon made HAL 0.55, 0.75, 0.95, 1.15 4, 6 2 16 LAL 0.55,0.75,0.95,1.15 4 . 6 : ? 16 RAL 0.55, 0.75, 0.95 4, 6 2 12 , ' H A L = High allgnnlent level: I.AL : 1.0% al1fn111r111 Ie\t.l, RAl. = Ran<lc,rn a l ~ g r ~ ~ ~ ~ r n t Irbrl. " B a d on o\cn-dry flake wstpht gradient (Kelly 1977; Geimer 1982). Variation in these variables among different products has led to a large variability of I,E values in commercial OSBs (Wu and Suchsland 1996). Efforts to reduce LE in these products require a quantitative understanding of the role each variable plays in controlling the in-plane movement. The study reported here represents the first part of a comprehensive study aimed at examining dimensional stability and its effect on durability of OSB. The objective of this work was to investigate to what extent in-plane stability of oriented strand panels (single-layer) is related to processing variables, namely, flake alignment level (AL), density, and resin content (RC) at various levels of nloisture content (MC). Since the in-plane movement and strength properties are closely related, data on bending modulus of elasticity (MOE) and modulus of rupture (MOR) were also presented and discussed. MATERIALS AND METHOLE