Definitions. Forestry 240 Wood Science

Transcription

1 Definitions Forestry 240 Wood Science Chapter 15 Nonstructural Panels Panel products made from wood-based veneer and particle products that are not suitable for structural applications. Used for Decorative plywood Particleboard Fiberboards Cement-bonded panels Decorative Plywood Wood veneers are glued together so that the grain direction of some veneer runs at right angles and others run parallel to the long axis of the panel. Cross-banding gives good strength and dimensional stability. Appearance-graded hardwood face veneers and interior resin types are used. Core plies are from lower-value wood species. Paneling Manufacturing Similar to plywood manufacturing except: Veneers both peeled and sliced Greater attention to grading face plies Veneers may be matched by grain book matching Drying done at lower temperatures to preserve wood quality Layup often done manually Interior resins used Particleboard Made by compressing small particles of wood while simultaneously bonding them with an adhesive. Great variation based on how particle type: Shaving higher valued Flake Chip Sawdust Least valued Sliver Strand Wafer Excelsior long curly slender slivers Particleboard Often made from mill residues planer shavings, etc. Panels made from flakes of uniform thickness produce a superior product. Allows for better utilization of smaller logs. 1

2 Particleboard Resins Urea-formaldehyde (UF) most common and least expensive Not suitable for structural subflooring or sheathing. Diphenylmethane-diisocyanate (MDI) Used less, more expensive, Greater water resistance. Resin quantity is major factor that determines strength and properties. Normally, resins are 6-10% of board by weight. Resins make up 25% of manufacturing costs. Particles should have uniform size, species, and moisture content. Refiners, hammermills, and flakers are used to produce the desired furnish. Ring flakes cut thin particles from chips. Disk refiners are used to prepare small, fiber-like particles, grinding them into fiber bundles. Particles are then dried to 2-5% MC. Next particles are screened to remove fines, which can absorb too much resin, lowering board strength. Furnish is then blended with the resin and wax. Furnish is then deposited on the forming mat. The mats then move to the pre-press, the press loader, and finally to the hot press. 2

3 After the panels are unloaded from the press, they are allowed to equalize while the resin finishes curing. They are then trimmed, sanded, and prepared for transport. Raw Materials Board strength is determined by glue bond quality, not wood strength. Lower-density species are preferred, because: Lighter-weight species have more particles per kg furnish. Require higher pressures for proper densification. Achieve better glueline contact. Reduced density variation within mat. Raw Materials Species with higher extractive content can result in: Problems with resin curing. Blows internal rupture of the board resulting from internal gas pressure as press opens. Results from volatilized extractives or excessive MC. Agriculture residues like bagasse, kneaf, flax shivers, and wheat straw can be used for particle boards. There is no real environmental benefit to using agricultural-based fibers because the environmental impact of agriculture crops is much higher than silviculuture. Nonwood raw materials is limited to the waste streams of agriculture crops. About half of the particleboard made is used for home or office manufacture in the form of core stock. MOE, MOR, and screw-holding strength are the most important properties, in addition to creep. Particleboard Use Hardboard High density wood fiber product. Developed by William H. Mason Masonite Used in case-good furniture, cabinets for electronic components, drawer bottoms, backing, tabletops, wall paneling door faces, pegboard. 3

4 Hardboard Manufacture Lignin is retained during the pulping process to help with fiber to fiber bonding. Resin and wax are added after the pulping process or during drying of the fiber. Furnish is then formed and pressed. Panels may also be tempered using oil soaks to increase cross-linking between cellulose and other polymers. Insulation/Acoustical Board Insulation board low density ceiling tiles to higher-density wall sheathing board used under siding in light-frame construction. Instead of hot pressing the mat is brought to a desired thickness using a press roll and dried. Ligneous bonding is not achieved. Fiber to fiber linkages may be enhanced by starch or asphalt additives. Insulation/Acoustical Board Forming is done by a single or double cylinder Fourdrinier. Medium-Density Fiberboard (MDF) Made from wood that has been reduced to individual fibers and fiber bundles with a binder added to enhance board strength. Pressurized refiners produce pulp of a very low bulk density. MDF Manufacture Similar to hardboards chips are thermomechanically pulped to produce fibers. Fibers are then formed and pressed similar to particleboard. MDF Uses Kitchen cabinets edges do not have to be banded like particleboard. Can be finished to a smooth surface and grain printed, eliminating the need for surface laminates and veneers. 4

5 Cement Fiber Board Portland cement is used as an inorganic binder for the fibers. Fewer problems with water damage, termites, fire, and decay (higher ph). Once sealed, a paintable long-weathering surface is produced, great for siding (~30% market share and growing fast). Cement Fiberboard Other Fiberboards Wool wood 25-30% excelsior fibers, remainder Portland cement. Well suited for developing areas, uses simple forming and binding methods. Thermoplastic composites 5