Veneer-Reinforced Particleboard for Exterior Structural Composition Board

Two experiments were performed to determine the physical and mechanical characteristics of panels consisting of a veneer face and a particleboard core composed of mixed wood particles/powdered-recycled polyethylene (PE) bag waste (MWP) using urea-formaldehyde (UF) resin as a binder. The addition of 25 percent powdered-recycled PE bag waste to the MWP panels did not adversely affect nonaged bonding strength but did result in substantial improvement in internal bond (IB) retention after a 24-hour water soak and improved dimensional stability. Average MWP panel IB retention was more than 300 percent higher than the IB retention of wood particle (WP) panels and MWP thickness swell and linear expansion were 70 and 44 percent lower, respectively, than the values for WP panels. For the veneer overlay composite, the mean modulus of rupture (MOR) parallel to the surface grain veneer (MORjj) was lowest (3,668.2 pounds per square inch [psi]) for panels with two veneers cross-laminated on each face over a WP core. Conversely, MORjj was greatest (8,535.6 psi) for panels with single 1/8-inch veneers on each face over an MWP core. However, the large percentage of shear failure when stressed parallel to face veneer grain hindered an accurate determination of true MOR. As expected, all specimens tested in bending parallel to the surface grain of the veneers resulted in higher modulus of elasticity (MOE) than those tested perpendicular to the grain. For a single veneer overlay on each face, it is interesting to note that thinner veneers (i.e., 1/8 in.) resulted in higher MOE than thicker veneers (i.e., 3/16 in.). Particleboard is used widely in the manufacture of furniture, cabinets, and underlayment. The use of industrial grade particleboard as a core stock for wood veneer overlays (i.e., composite panels) is one of its prime applications. While the smoothness, surface integrity, uniform thickness, uniform mechanical properties, ease of layup, and ability to stay flat of particleboard make it an ideal core material, the decorative quality, originality, and the look and feel of real wood of the veneer provide the performance characteristics of veneered particleboard construction. Structurally, a veneered particleboard beam bends with the face veneers carrying direct compression and tension loads and the particleboard core carrying shear loads. It was shown that 1/36-inch walnut veneer overlaid particleboard composite increased the modulus of elasticity value in bending to more than 50 percent (Chow 1972), suggesting the potential for particleboard panels to develop structural applications. However, problems associated with hygroscopicity and dimensional stability limit their application. Thus, developing new particleboard-based composites with structural exterior grade performance will not only enhance their competitive capability but also create new markets. As a result, many exterior structural composite products were under development by Forest Industries, Potlatch Corporation, Elmendorf Research, and others in the early 1970s (Countryman 1975). A new composite product with strand board core and veneer faces combination, generally known as composite plywood, has been developed by Potlatch The authors are, respectively, Principal Wood Scientist, Southern Research Station, USDA Forest Serv., Pineville, Louisiana (chse@fs. fed.us [corresponding author]); Professor, Louisiana Forest Products Development Center, School of Renewable Natural Resources, Louisiana State Univ. Agric. Center, Baton Rouge (tshupe@ agcenter.lsu.edu); Assistant Professor, Dean Lee Research Sta., Louisiana State Univ. Agric. Center, Alexandria (hpan@agcenter. lsu.edu); and Professor, Research Inst. of Wood Industry, Chinese Academy of Forestry, Beijing ([email protected]). This paper was received for publication in September 2011. Article no. 11-00111. Forest Products Society 2012. Forest Prod. J. 62(2):139–145. FOREST PRODUCTS JOURNAL Vol. 62, No. 2 139 (McKean et al. 1975). With a modulus of rupture (MOR) of approximately 8,000 pounds per square inch (psi) and modulus of elasticity (MOE) of over 1,000,000 psi, the structural properties of the composite plywood are similar to conventional plywood in that the two types can be used interchangeably. Similar composite panels with various core and face materials and construction methods were studied by many other researchers. For instance, Biblis and Mangalousis (1983) and Biblis (1985) evaluated the physical and mechanical properties of composite plywood with southern pine (Pinus tadea) veneer faces and cores of various wood species, Chow and Janowiak (1983) and Chow et al. (1986) determined the effects of accelerated aging on panel strength retention of hardwood composite panels, Hse (1976) studied composite panels with southern pine veneer and cores of southern hardwood flakes, and Koeningshof et al. (1977) evaluated the possibility of producing house framing and structural panels with particleboard cores and veneer facings. These studies have shown that application of exterior grade adhesives significantly improved panel durability and that the addition of a veneer to various wood-based cores resulted in greater mechanical properties than those achieved from similar panels without veneer overlay. Discarded plastic bag waste has long been considered an environmental problem: plastic bags litter the landscape, clog waterways, and endanger wildlife. The problem is further escalated by the increasing rate of the bag use, the nonbiodegradable property of the bags and corresponding slow degradation period, and the low rate of recycling. Thus, the need is urgent and the interest has risen for the development of plastic bag waste recycling. One unique property of the bag is its hydrophobic nature, which is only minimally affected by atmospheric humidity. Thus, the combination of wood particles and the powdered plastic bag waste in the fabrication of particleboard could result in the improvement of water resistance properties of the products, if low-cost adhesives or coupling agents can provide a satisfactory glue bond between the hygroscopic/hydrophobic interfaces. In the present study, we manufactured particleboards containing powdered polyethylene (PE) bag waste by means of a method currently used in the particleboard industry, and we used the most cost-effective urea-formaldehyde (UF) resin. The primary objective of this study was to investigate the possibility of making particleboards with a combination of powdered PE bag waste and wood particles using UF resin as binder. The final objective was to apply the particleboard as a corestock to develop southern pine veneer overlay composite panels for structural applications. Some potential advantageous applications for this panel type include mobile homes and industrial work floors. Materials and Methods Wood particles and PE powder Wood particles, classified in the mill as core furnish, were obtained from the dry end of a local particleboard plant. The particles were stored in plastic bags, placed in a drum, and used without further preparation. Average moisture content of the wood particles was 4 percent based on ovendry weight. Low-density PE plastic bags were shredded, chilled in a freezer, and then reduced with a disc definer into powder. The sieve analysis of wood particles and PE powder are shown in Table 1. The two types furnished for the manufacture of particleboards are shown in Figure 1. The study was conducted in a sequence of two experiments: (1) fabrication of particleboard and (2) manufacture of veneer overlay composite panel. All panels were prepared in the laboratory with three replicates. The data were analyzed in SAS (2008) by using analysis of variance (ANOVA). Also, means were separated by Duncan’s multiple range tests at the 0.05 level of probability. First experiment: Manufacture of particleboard The first experiment evaluated the bonding strength, mechanical strength, and dimensional stability of the particleboard. The construction variables consisted of (1) two types of particleboards (based on the particle material mix)—a panel with 100 percent wood particles (WP) and a panel with 75/25 (wt/wt) mixture of wood particles/PE plastic bag powder (MWP), and (2) two UF resins—a UF resin (UF-I) prepared in the laboratory, which was formulated with 51 percent resin solids reacting at pH 5.1 with a molar ratio of formaldehyde to urea of 1.2, and a commercial UF resin (UF-II) with a 64.8 percent resin solid, which was used as a control. The general conditions used for manufacture of these panels were as follows: Panel density: 50 pounds per cubic foot (pcf; 0.8 g/mL) based on ovendry weight and volume at 4 percent moisture content. Resin content: 6 percent based on the ovendry weight of materials. Panel size and panel thickness: 36 by 36 inches (91.4 by 91.4 cm) and 3/4 inch (1.9 cm). Hot press temperature and time: 3408F (1718C) and 9 minutes total. To fabricate the panels, PE powder and wood particles were weighed according to the designated weight percentage to yield a panel density of 50 pcf (0.8 g/mL) and placed in a rotating drum–type blender. Resins (6% based on the ovendry weight of materials) were sprayed on the tumbling wood particle and plastic powder through an air-atomizing nozzle to get a fine dispersion of resin over the materials. The particles, after blending, were carefully felted on a caul plate in a 36 by 36-inch (91.4 by 91.4-cm) forming box. The formed mat was transferred immediately to a 40 by 40-inch (101.6 by 101.6-cm) single-opening hot press at 3408F (1718C). Sufficient pressure (about 400 psi; 2.75 Mpa) was applied so that the plates closed to 3/4-inch stops in approximately 45 seconds. Closed press time was 8 minutes 15 seconds. After hot pressing, all boards were conditioned in a chamber at 50 percent relative humidity and 808F Table 1.—Particle size distribution of wood particles and polyethylene powder by the sieve analysis.