Reconstituted Wood Products • Raw Materials (Wood elements): Many products can be produced by gluing different forms of wood elements. Nano Cellulose Adhesives for Reconstituted Products Year Adhesives Before 1960s 1960s 1970s 1980s 1990s • blood and soy glues casein glue urea-formaldehyde (UF) phenol-formaldehyde (PF) PRF (cold setting resin) PVAc (polyvinyl acetate) MDI MDI Products plywood wood gluing particleboard plywood, OSB Glulum wood glue OSB Ag. Particleboard Applications construction wood working furniture, decking house construction large wood beams wood wroking house construction furniture Current annual wood adhesive consumption in North America UF: 4.20 billion pounds PF: 2.99 billion pounds MDI: 0.25 billion pounds • These wood adhesives are heavily dependent on petroleum-based chemicals. Performance of Thermosetting Wood adhesives • Urea-formaldehyde resin (UF) --Adhesive bond is strong but only water resistant not waterproof --Resin cures at about 130 oC and cures quickly, efficient in production --Adhesive bonds deteriorate over time and releases formaldehyde gas --emits free formaldehyde, which is toxic --low cost (~$ 0.25/pound) • Phenol-formaldehyde (PF) --Adhesive bond is strong and water proof --Resin cures at 165 oC and twice as long as UF --Does not emit formaldehyde gas --Higher in cost (~$ 0.50/pound) • Methyl Diphenyl Isocyanate (MDI) --Adhesive bond is strong and waterproof --Resin cures at lower temperature and shorter time than PF resin --Does not emit formaldehyde gas --High cost (~$ 0.75/pound) Performance of Thermosetting Wood adhesives • PRF (phenol-resorcinol-formaldehyde resin) --Adhesive bond is strong and waterproof --Does not need heat to cure, a cold-setting resin --Does not emit formaldehyde gas --Very high cost (>$ 1.00/pound) • Melamine-Urea-Formaldehyde (MUF) --Strong adhesive bonds and somewhat waterproof --Cures at moderate temperature and duration --Emits litter formaldehyde gas --Costs a little more than PF (about $ 0.65/pound) • Rising cost for petroleum-based chemicals forces the wood industry to reconsider manufacturing wood adhesives from renewable materials • It is a challenge to manufacture wood adhesives from renewable materials to match the performance of petroleum-based wood adhesives Plywood Manufacturing • • Exterior Plywood: Made with rotary-peeled softwood veneers and glued with phenol-formaldehyde (PF) resin for constructions. Interior Plywood: Made with sliced hardwood veneers and glued with ureaformaldehyde (UF) resin for interior paneling and furniture. Rotary-peeling veneer Slicing veneer Plywood Manufacturing • • Veneer bolts or wood blocks (flitches) are softened by cooking in hot water (~85 oC) for days before veneer cutting to ensure veneer quality. After selecting and clipping, veneers are dried with hot-air jet drier to about 4% MC. • Veneer is spread with plywood glue (80 lbs DG/MFt) formulated from UF or PF resin with extender (wheat flour) and filler (wood flour), lay-up in the pre-determined construction and hotpressed. • Plywood grades: Veneers are graded into A, B, C and D grades. Plywood is graded based the quality of the two surfaces, such as A-B, C-D, etc., C-D X(exterior) is the grade most used for building construction. Veneer drying Multi-opening press Particleboard Manufacturing • • • Particle generation: raw materials can be saw-dust, other mill residues, or small logs. Saw-dust is screened into different particle sizes, and larger materials are first hammer-milled and refined in small particles, followed by screening. Coarser particles are used for the core, and fine particles are used on board surfaces. Particle drying: coarse and fine particles are usually dried separately with drum dryers, both are dried to about 4% MC. Adhesive resin application: UF is usually used for particleboard. The resin is metered (6% based on particle dry weight for coarse particles and 8% for fine particles) into a auger mixer where the resin is spread by rubber between particles. Particleboard Manufacturing • Mat forming: Remote sensing ensures quality of the mats, for example each mat is weighed to control the density of resulting board. Forming if coarse and fine particles are not processed separately Particleboard Manufacturing • • Pre-pressing the mats: This is done by rollers immediately after mat forming. The resin must be tacky to hold particles together after pre-pressing to keep the integrity of mats during conveyor transportation (to the press). Hot pressing (UF): Mats are hot-pressed with multi-opening press or continuous press at about 130 oC for 10 sec/mm (about 3 minutes for 0.75-inch thick board). Modern particleboard plants use continuous press. • Post pressing treatment: Pressed boards must be cooled down immediately because prolonged heating breaks adhesive bonds. Cooling is done by placing each board separately on a rotating rack. Continuous Press Fiberboard Manufacturing • Fiberboard Classification by Density --Low-density fiberboard (insulation board) ≤ 30 lbs/ft3 (pcf) --Medium-density fiberboard (MDF) > 30 pcf and < 50 pcf --High-density fiberboard (hardboard) > 50 pcf • Classification by Methods of Processing --Wet-formed Fiberboard: Fiber slurry is formed on screen into mats followed by pressing the mats to specific density resulting in boards with smooth surface only on one side (S1S). Insulation boards are exclusively manufactured in this manner. Some hardboard is made by mixing fibers with 1% to 2% PF resin before mat forming and pressing for making Masonite type sidings. --Dry-formed Fiberboards: Dry fibers (about 4% MC) are coated with about 10% UF or PF resin, forming into mats and pressed, resulting in boards with smooth surface on both sides (S2S). Medium-density fiberboard is manufactured only in this manner, and some hardboard siding also is made this way. Fiberboard Manufacturing • Fiber Generation: The process starts with 50% MC wood chips or other forms of mill residues (e. g., planer shavings) and exposing the chips to 150 psi steam at 200 oC for 4 minutes, followed by forcing heat-softened chips through 0.004’ gap between two rotating grinding plates to obtain fibers. The process is called pressurized disk refining, and the resulting pulp is called thermo-mechanical pulp or TMP. • Fiber drying : After leaving refining disk, fibers are blown with 400 oC air through a tube more than 200 ft long to dry down to about 4% MC. • Resin coating: UF resin, usually 10% to 12% based on dry weight of fiber, is sprayed onto fibers in the initial section of the drying tube. Resin also can be sprayed after fiber drying. • Mat forming and Pressing: Pressing usually is done with a continuous press. Particleboard and Fiberboard Quality Control • Particleboard and MDF are mainly used for furniture construction. One or both surfaces are overlaid and edges taped (edge banding) with veneer, resin-impregnated paper, or thin foil. MDF is a better product because of easy overlaying smooth surfaces and edges. • Important quality control points: MC and particle size of furnish; resin application (usage, uniform distribution), Mat forming (overall density and density uniformity), quality and precision of sanding. • Particleboard and MDF must comply to American National Standards Institute’s (ANSI Standards). For example, free-formaldehyde emission must be less than 0.3 ppm (parts per million). Oriented Strand Board (OSB) Manufacturing • • Flake Generation: preferably from low density species such as aspen with disk flaker or ring flaker. Flake thickness = 0.050” to 0.020”; width and length depend on products. --LSL (parallel strand lumber): 1” to 2” in width and about 2 ft in length --OSB: 0.5” to 1” in width and about 6” in length Disk flaker uses short bolts and ring flake uses scrapes for smaller flakes; Ring flaker generate too much fines which need to be screened. Disk Flaker Ring Flaker OSB Manufacturing • • • • Drying: Drying of Flakes (strands) is done in drum dryer to about 4% MC. Resin application: Waterproof resins, PF (3.5 to 5%) or MDI (no more than 3.5%) or both, must be used for construction applications such as sheathing, floor and roof decking and siding. When both PF and MDI are used together, PF is used as surface resin and MDI the core resin. Forming and Pressing: Flakes are mechanically oriented during forming, pressed with a multiopening press at 175 oC to predetermined thickness, and immediately hot-stacked after pressing. Resin Efficiency: A fixed amount of resin is sprayed to cover all flake surfaces as uniformly as possible. Glued-Laminated Timber • • • Preparation: Lumber or small wood laminates are dried, fabricated and treated. Resin application: Laminates are spread with PRF (phenol-resorcinol-formaldehyde resin. PRF is used because it is a cold setting resin. Clamping: Laminates are put together and clamped on a rack to size and certain shape as shown below. Glulam--Advantages • High utilization of wood and lumber • Size and length of structural members are not limited • Strength reducing defects such as knots can be controlled; (laminates without or less defects are placed on or near the surfaces and pieces with defects are placed in the interior) • Individual laminates can be more thoroughly dried or treated • Architectural pleasing effects LVL, PSL and LSL • LVL (laminated veneer Lumber): About 13 sheets of 1/8-inch-thick veneers are spread with PF resin and parallel laminated and hot-pressedinto 1.5-inch-thick and 32’ or 64’ long pieces. The best use of LVL is for manufacturing I-joists (used as the fringes) but also can be used as framing lumber. • PSL (Parallel Strand Lumber): Veneer is clipped into 0.75-inch-wide long strands, coated with PRF resin, fed into a continuous press and finally the resin is cured using microwaves. • LSL (Laminate Strand Lumber): The products uses large flakes (3- to 6-inches wide and more than 12-inches long), which are sprayed with MDI resin and hot pressed into 20-feet wide and 40-feet long pieces (billets). Then, the billets are cut into 2 by 4, 6, 8 or wider pieces of lumber.