Physical and Mechanical Properties of Sugarcane Rind and Mixed Hardwood Oriented Strandboard Bonded with PF Resin

Anatomical and tensile strength properties of sugarcane rind (i.e., comrind) and wood strands from southern pine, yellow poplar, red oak, and willow were investigated. Scanning electron microscopy (SEM) observation showed that the comrind stem consists of an outer waxy layer, rind fibers, and inner pith. Rind fiber cell walls are thicker than those of wood. Comrind had the largest tensile strength among the various materials tested. Three-layer mixed comrind and hardwoods oriented strandboard (OSB) was manufactured using phenol formaldehyde (PF) resin with comrind used in the core layer. The effects of comrind and wood contents on panel properties were examined. About 70% to 85% flakes for different panels were aligned within –30 to 30 degrees from the panel principal direction. Pure rind OSB had superior alignment distribution compared with other mixed rind and wood panels. The density profiles through panel thickness revealed that boards with 45% wood fines in the core layer had a significant density gradient. Generally, density gradients increased with increased wood fines contents in the core layer. Linear expansion and thickness swelling were significantly improved by using comrind to replace part of the wood material in the core layer. Internal bond strength showed little decrease as rind content increased up to 22.5% for boards made of core material and rind combination in the core layer. Bending properties of the boards with wood face material and rind combination in the core layer reduced little when the rind content was below 22.5%. At lower relative humidity levels, pure rind OSB showed lower equilibrium moisture content values compared to wood OSB for both adsorption and desorption. Nelson’s sorption model provided an excellent fit to the sorption data for both panel types. It was concluded that comrind with comparable properties of wood strands has potential to be used as a substitute material in manufacturing structural composites. Comrind flakes can be successfully combined with wood flakes to produce three-layer OSB with desired properties. INTRODUCTION Sugarcane is an important agricultural crop in the southern United States with an annual yield of 15 million tons in Louisiana alone, accounting for more than 40% of the total U.S. production (Rowell 1995). Large quantities of outer rinds (i.e., comrind) are produced after the inner pith that contains most of sucrose is separated from sugarcane. Comrind with a high content of lignocellulosic fibers is a potential raw material for structural composite manufacturing (Atchison and Lengel 1985). Currently wood-based strand composites are widely used as sheathing, flooring, and I-joist materials in construction. With the recent growth of manufacture and consumption of the composites and increasing wood cost, the development of new materials as a substitution of wood fibers becomes increasingly important (Rowell 1998). The use of comrind can help reduce the wood consumption, lower manufacturing cost, and improve panel performances. Comrind contains a lower lignin and a higher hemicellulose compared with wood. The cellulose content of comrind is comparable with wood (Hurter 1997 and Rowell 1996). Comrind has its typical anatomical features. Similar to other agricultural residues, the waxy layer on the outer surface of comrind has a significant impact on the bondability when it is bonded with phenol and urea formaldehyde resins (Han et al. 1999). Various types of rind composite panels including strandboard and waferboard were made in early studies (Moeltner 1978). The results showed that these rind-based structural boards had competitive strength properties with plywood. Previous work has indicated that even when standard particleboard was made with this material in random formation, the resulting board was superior to normal wood-based particleboard (Berchem 1978). However, very little research work has been reported with comrind flakes used as raw materials for OSB so far. The objectives of the study were to develop comparative properties of comrind and wood strands for structural composite manufacturing; to investigate the physical and mechanical properties of three-layer mixed comrind and hardwoods OSB bonded with phenol formaldehyde (PF) resin. MATERIALS AND METHODS Raw Material Selection Sugarcane rinds were prepared through the Tilby cane separation process (Atchison and Lengel 1985). During the process, the cane stalk was separated into rind, pith, sugar juice, and epidermis (wax) fractions. The rind was about 45-cm in length and had been air-dried. The bundles of rind were band-sawn into pieces of 11-cm in length. For comparison purpose, wood strands of willow, red oak, southern pine, and yellow poplar cut by an 882-mm disc flaker were prepared for strand property test. For board manufacturing, two types of mixed hardwood flakes – large wood face material (WFM) and small wood core material (WCM) were also prepared. All flakes were kiln-dried to about 3% moisture content (MC) prior to board fabrication. Commercial PF resin and wax with solid contents of 55% and 50%, respectively, were used. SEM Analysis Samples of comrind were prepared both from stalk (i.e., internodal section) and node. The surface for observation was finished by a microtome. Wood samples of willow and southern pine strands were prepared in the same way. All samples were dried at 80C for 1 day before undergoing scanning electron microscopy (SEM). The cross surfaces of the rind and wood were observed with an Edward S150 scanning electron microscope. Tensile Strength Testing Comrind strands without cracks were selected for tensile strength test. They were hand-cut into samples with width varying from 6 to 12 mm. The samples were notched in the center part to ensure the breakage in the middle section of the samples. Wood samples of southern pine, yellow poplar, and willow were also prepared in the same way. All samples were tested according to the ASTM D 1037 (ASTM 1999) using an INSTRON machine at a loading speed of 4mm/min. Fifty specimens for each material type were tested, and the results were averaged. Panel Manufacturing Three-layer OSB with controlled alignment level was manufactured with pure comrind, pure mixed hardwoods, and a mixture of both using PF resin in combination with wax. Two groups of panels were made in this study. Table 1 shows the panel design. All three-layer boards were made with 55% of WFM in the face layer and 45% WCM-CRD or WFM-CRD combinations in the core layer. The rind was used in the core layer only except for the pure rind boards, and the overall rind content varied from 0, 13.5%, 22.5%, 33.75%, to 100%. The PF resin and wax were added to the flakes with content levels of 4.5% and 1%, respectively, based on the oven-dried weight of the strands. Mats were formed using a specially designed forming box for aligning purpose to control the strand alignment level. Boards were manufactured by hot pressing at 190C for 4 min with an additional 40 seconds press closing. The board dimension was 500 × 500 × 12 mm with a target density of 720kg/m. Besides, boards with 100% WCM (i.e., panel type A”) were also made under the same condition for comparison. Two replicates were used for each condition, and to tally 20 boards were manufactured. The panels were trimmed and conditioned for 2 weeks under room condition before testing. Table 1 –Experimental design of pure wood, comrind, and mixed wood and comrind OSB Material Type Panel Type Layering and Material Amount a Overall Rind Content Overall WCM Level Overall WFM Level A Face – WFM 55% Core – WCM 45% 45% 55% A’ Face – WFM 55% Core – WFM 45% 0% 100% Pure Wood A’’ Face – WCM 55% Core – WCM 45% 0%