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

Transcription

1 Physical and Mechanical Properties of Sugarcane Rind and Mixed Hardwood Oriented Strandboard Bonded with PF Resin Guangping Han 1, Qinglin Wu 2 and Richard Vlosky 2 1 Associate Professor, College of Material Science and Engineering, Northeast Forestry University, Harbin 154, China. guangpingh@hotmail.com 2 Professors, School of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA 783, USA. s: wuqing@lsu.edu, vlosky@lsu.edu Keywords: Comrind, anatomical features, tensile strength, PF resin, OSB, physical and mechanical properties ABSTRACT 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 7% to 85% flakes for different panels were aligned within 3 to 3 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 4% 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

2 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 5%, 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 8 o C for 1 day before undergoing scanning electron microscopy (SEM). The cross surfaces of the rind and wood were observed with an Edward S15 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 137 (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.

3 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, 13.5%, 22.5%, 33.75%, to 1%. 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 19 o C for 4 min with an additional 4 seconds press closing. The board dimension was mm with a target density of 72kg/m 3. Besides, boards with 1% WCM (i.e., panel type A ) were also made under the same condition for comparison. Two replicates were used for each condition, and totally 2 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 Pure Wood Mixed Wood- Rind Pure Rind Panel Type Layering and Material Amount a A Face WFM 55% Core WCM 45% A Face WFM 55% Core WFM 45% A Face WCM 55% Core WCM 45% B Face WFM 55% Core CRD 13.5% & WCM 31.5% B Face WFM 55% Core CRD 13.5% & WFM 31.5% C Face - WFM 55% Core - CRD 22.5% & WCM 22.5% C Face - WFM 55% Core - CRD 22.5% & WFM 22.5% D Face - WFM 55% Core - CRD 33.75% & WCM 11.25% D Face - WFM 55% Core - CRD 33.75% & WFM 11.25% E Face - CRD 55% Core - CRD 45% Overall Rind Content % 13.5% 22.5% 33.75% Overall WCM Level Overall WFM Level 45% 55% % 1% 1% % 31.5% 55% % 86.5% 22.5% 55% % 77.5% 11.25% 55% % 66.25% 1% % % a WCM Wood core material, WFM Wood face material, and CRD Comrind. All panels were made with 55% and 45% of the materials in the face and core layers respectively. Panel Testing and Data Analysis Flake alignment level Flake alignment levels were evaluated from board surfaces by randomly selecting 15 flakes on each side of the board and taking a digital image of board surface The alignment angle of each strand was

4 measured from -9 to 9 o with o set as the principle machine direction using Sigmascan software and excel macro. Strand alignment was described by a percent alignment (PA) proposed by Geimer (1976). The statistical cumulative distribution of strand alignment was done for each board based on a uniform interval of 1 degree. Density profile Density profile through the thickness of the specimen (5? 5 mm) was evaluated using a Quintek Density Profile QDP-1X. The maximum, average, and minimum densities for each board were recorded. Six replicates were used, and the result was averaged for each group. Bending, IB, TS and LE Un-sanded boards were evaluated according to the ASTM D137 (ASTM 1999). The tests included modulus of rupture (MOR) and modulus of elasticity (MOE), internal bond (IB), thickness swelling (TS), and linear expansion (LE). Two samples from each board, mm, were cut along each of the two principal directions for static bending test (i.e., MOR and MOE). Six specimens of mm for each condition were tested for IB strength at a testing speed of.98 mm/min. TS test was carried out on four specimens of mm at each condition after being soaked in water for 24 hours at 2 o C. Eight samples (4 for parallel and 4 for perpendicular) of mm were prepared for LE test. All specimens were initially oven-dried for 24 h, and then vacuum-pressure-soaked in the water at 2 o C for 3 hours (1hour vacuum at 686 mm Hg and 2 hours at.69 MPa pressure). Sorption isotherm Samples for sorption test, 2? 2 12 mm, were prepared from mixed hardwoods and pure comrind OSBs. Test samples were conditioned over saturated salt solutions to reach equilibrium at relative humidity (RH) of 33, 66, 76, 81, and 93%, respectively, after oven dried at 7 o C for 2 days. Equilibrium moisture content (EMC) of each specimen was calculated based on the oven-dry weight. Experimental data of EMC at various RH levels were fit to the sorption isotherm model proposed by Nelson (1983). Properties of Comrind RESULTS AND DISCUSSION Basic anatomical feature Figure 1 shows the scanning electron micrographs of comrind and southern pine. It is observed from the transverse section that comrind stem consists of outer layer, rind fibers, and inner pith (Figure 1 A). The stem s outermost layer is composed of epidermal cells that contain a waxy layer called cutin. This outer waxy layer is hydrophobic. It has been reported that this hydrophobic slippery surface has significant implications for the bondability between particle elements and urea-based resin adhesives (Han et al. 1999). Similar to wood, the rind fibers contain parenchyma cells and the vascular bundles. However, rind fiber cell walls are thicker than those of wood, which is especially true for the outer surface region. Compared to southern pine (Figure 1 C), rind has a less uniform cell structure. There are distinct regions where fibers with thick cell wall are surrounded with thin cell wall fibers (Figure 1 B). These anatomical features of comrind will affect its physical and mechanical properties, and the interaction with adhesives due to the waxy layer.

5 A B C Figure 1. SEM images of comrind (A and B) and southern pine (C) strands. Tensile strength The tensile strength data are plotted in Figure 2. It was found that comrind strands with waxy layer had the largest tensile strength among the various materials tested. The tensile strength value of natural comrind (114 MPa) was more than two times higher than that of willow strands (31.5 MPa). The significantly higher tensile strength of comrind is attributed to its typical anatomical features where rind fiber cell walls are thicker than those of wood especially in the outer surface region. The high tensile strength of comrind can be used to help improve bending properties of structural wood composites. Tensile Strength (MPa) RDW RDNW SP YP WL Figure. 2. Comparison of tensile strength of comrind and different types of wood strands. RDW Rind with wax layer, RDNW-Rind no wax layer, SP- Southern pine, YP- Yellow poplar, and WL-Willow. It was concluded from the above results that comrind with comparable properties of wood strands has potential to be successfully used as a substitute material in manufacturing structural composites. Properties of Comrind and Mixed Hardwood OSB Flake alignment The cumulative distributions of alignment angles for the boards with WCM-CRD combinations in the core layer are shown in Figure 3. Typical strand alignment distribution curves are observed for all evaluated panels. About 7% to 85% flakes for different panels were aligned within 3 to 3 degrees from the panel principal direction. Pure rind OSB had superior alignment distribution compared with other mixed rind and wood panels. This is due to more regular strand shape and uniform width of comrind strands, which made it easy to control the strand orientation during hand-forming process. Cumulative Distribution (%) 1 9 Panel Type 8 A B 7 C D 6 E Alignment Angle (degree) Figure 3. Cumulative distribution of flake alignment angles for three-layer mixed hardwoods and comrind OSB. Refer to Table 1 for legend explanations.

6 Density profile Typical density profiles of the test panel are shown in Figure 4. In general, the density profiles of all tested boards had M-shapes, indicating regular density gradient through panel thickness. Generally, density gradients increased with decreased rind contents in the core layer. Boards made of pure wood large flakes had significant density gradient compared to the boards with different rind contents. Pure rind board showed small density gradient. This was mainly caused by the considerable thickness spring back of the comrind board after it was released from hot pressing (a) A 1 (b) Density (kg/m 3 ) A" C D B Density (kg/m 3 ) A' C' E B' D' Position (mm) Position (mm) Figure 4. Density profile across panel thickness for three-layer mixed hardwoods and comrind OSB. (a) WCM-CRD combination in core layer. (b) WFM -CRD combination in core layer. Refer to Table 1 for legend explanations. Bending Properties Bending properties are plotted in Figure 5. Using rind to replace part of wood fines can help reduce the difference on MOR and MOE in parallel and perpendicular directions, contributing a better property balance in these two directions. For both types of boards, there was a decreasing trend of MOR and MOE with increase of comrind contents. The bending properties were still comparable to commercial OSB as using 22.5% of rind in core layer. This is a promising result with PF-bonded mixed 6 5 Parallel Perpendicular 1 8 Parallel Perpendicular MOR (MPa) R1 = RL R 2 =.61 R2 = RL R 2 =.45 MOE (GPa) E1 = RL R 2 =.48 E2 = -.18 RL R 2 = Figure 5. Bending strength (MOR) and modulus (MOE) at various rind content levels for three-layer mixed hardwoods and comrind OSB with WFM -CRD combination in core layer.

7 hardwoods and rind OSB. Pure rind board had the lowest MOR and MOE values. This was due to the poor bonding between rind flakes caused by outer waxy layer of rind and the effect of large internal voids among the comrind strand. The used of thinner and better processed strand can help improve the properties. IB strength Figure 6 shows the IB strength of the panels as a function of rind content. Using rind to replace part of the wood material in the core layer lowered the IB strength of the panel. This was due to poor bonding between wood and rind strands, and among the rind strand themselves. However, for a rind content level up to 22.5%, the mean IB values of the board with WCM-CRD were about.28 MPa, which is comparable to the IB values of most commercial OSB (Wu and Piao 1999) (a).5 (b) IB Strength (MPa) IB = -.15 RL +.31 R 2 =.41 IB Strength (MPa) IB = -.16 RL +.32 R 2 = Figure 6. Internal bond (IB) strength at various rind content levels for three-layer mixed hardwoods and comrind OSB. (a) WCM-CRD combination in core layer. (b) WFM-CRD combination in core layer. Linear expans ion Linear expansion data are shown in Figure 7. The difference between LE parallel and perpendicular values was reduced with increase of rind. At a rind level of 33.75%, the LE perpendicular value was reduced to less than.5% from an original value of 1.2%. This means that using rind in the core layer to replace part of the WCM can effectively balance LE parallel and perpendicular values. Thus, rind can be surely used to control perpendicular LE properties of OSB. OSB made of 1% rind had the smallest and similar LE values in both directions. The superior LE of rind board is attributed to the inherent waterproof characteristics of rind material, which is related to the waxy layer and non-polar extractives on the outer surfaces, and the typical micro-structure of this material (Han and Wu 24, Nguyen and Johns 1979). Thickness swelling Figure 8 shows the effect of rind content on 24-hour water soaking TS of the panels. The TS values dramatically decreased when using rind to replace part of the wood material in the core layer. Thus, rind can be used to improve the thickness swelling properties of OSB. In addition, TS of the board with WFM-CRD in the core layer was reduced significantly compared with panels made with WCM-CDR in the core layer. This indicates that wood fines had an impact on panel s dimensional stability. The TS of pure rind board was less than 1%, indicating the excellent dimensional stability of

8 rind board (a) Parallel Perpendicular 1. (b) Parallel Perpendicular.8.8 LE (%).6 LE2 = -.48RL +.59 R 2 = LE1 = -.16 RL +.24 R 2 = LE (%) LE2 = -.79RL R 2 =.84 LE1 = -.17RL +.23 R 2 = Figure 7. Linear expansion (LE) at various rind content levels for three-layer mixed hardwoods and comrind OSB. (a) WCM-CRD combination in core layer. (b) WFM-CRD combination in core layer. 5 5 Thickness Swelling (%) (a) TS = -.26 RL R 2 =.81 Thickness Swelling (%) TS = -.34 RL R 2 =.8 (b) Figure 8. Thickness swelling (TS) at various rind content levels for three-layer mixed hardwoods and comrind OSB. (a) WCM-CRD combination in core layer. (b) WFM-CRD combination in core layer. Sorption Isotherm Figure 9 shows typical sorption isotherms for mixed hardwoods and comrind OSB. At lower RH levels, comrind OSB showed lower EMC values compared to wood OSB for both adsorption and desorption. This may be due to the higher wax content in the outer layer of comrind strand, which prevents moisture from transmitting into the panel through comrind outer surface. While comrind OSB showed higher EMC value than wood OSB as RH was above 8%. This is because the higher content of hemicellulose in comrind, which play a major important role on moisture transmission at higher RH levels. A sorption hysteresis was observed for both tested materials. The symbols and lines in the Figure show the measured and predicted sorption values, respectively. It was found that Nelson's sorption model reproduced accurately the experimental data for both wood and rind OSB.

9 4 4 EMC (%) Wood OSB - Adsorption Wood OSB - Desorption Wood OSB - Model EMC (%) Rind OSB - Adsorption Rind OSB - Desorption Rind OSB - Model (a) 5 (b) RH ( %) RH (%) Figure 9. Comparison of sorption isotherms for three-layer mixed hardwoods (a) and pure comrind (b) OSB. CONCLUSIONS SEM observation showed that comrind stem consists of outer 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. Comrind with comparable properties of wood strands has potential to be used as a substitute material in manufacturing structural composites. Properties of comrind and mixed hardwood OSB indicated that linear expansion and thickness swelling were significantly improved by using rind to replace part of the wood material in the core layer. As rind content increased up to 22.5% (i.e., up to 5% rind in core layer), bending properties reduced little for the board with face material in the core layer, and internal bond strength was still comparable to commercial OSB. It was concluded that comrind flakes can be successfully combined with wood flakes to produce three-layer OSB with desired properties. REFERENCES American Society for Testing and Materials (ASTM) Standard test methods for evaluating properties of wood-based fiber and particle panel materials. West Conshohocken, PA. Atchison, J. E., and D. E. Lengel Rapid growth in the use of bagasse as a raw material for reconstituted panelboard. Proc. of the19th WSU International Particleboard/Composite Materials Symposium. Pullman, WA. pp Berchem, A Personal communication on development of comrind composite boards. Geimer, R. L Flake alignment in particleboard as affected by machine variables and particle geometry. Res. Paper 275. Madison, WI: U. S. Dept. of Agric., Forest Prod. Lab. Han, G., and Q. Wu. 24. Comparative properties of comrind and wood flakes. Forest Prod. J. (In press). Han, G., K. Umemura, S. Kawai, and H. Kajita Improvement mechanism of bondability in UF-bonded reed and wheat straw boards by silane coupling agent and extraction treatments. J. Wood Sci. 45(4): Moeltner, H. G Personal communication on evaluation of Tilby cane separator.

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