For. 202 Wood Utilization, Fall 2012

For. 202 Wood Utilization, Fall 2012

3rd Review Reconstituted Wood Products

I-1 Raw Materials: Hundreds of useful products can be produced from different types of wood elements, individually in homogeneous fashion or in combinations of these wood elements. Typical wood elements used for producing reconstituted wood products are lumber, thin lumber (or thick veneer), veneer, long flakes (or short veneers), chips, flakes, excelsior, strands, particles (saw dust), fibers, and wood flour.

I-2 Adhesives: Most of the important wood adhesive resins used for manufacturing reconstituted wood products are of the thermosetting type, in that heat is used to accelerate the setting of adhesive resins to bond wood elements together. These thermosetting wood adhesive resins mainly are manufactured from petroleum-derived chemicals.

Urea-formaldehyde resin (UF)—This resin is manufactured by reacting urea with formaldehyde. This liquid resin usually contains 60% to 65% solids in water. About 4.2 billion pounds are consumed annually in the North America for manufacturing products for interior uses. This resin and products bonded with it have the following characteristics:

--A low-cost resin, about $0.25 per dry pound.

--UF requires a low curing temperature (~130 oC) and short press time (1 minute to 5 minutes depending upon thickness of products)

--UF adhesive bonds are water-resistant but not waterproof, and these bonds tend to degrade under elevated temperature and humid environments, causing separation of bonded wood elements and emission of formaldehyde gas.

Melamine-Urea-formaldehyde resin (MUF)—This resin is manufactured from urea, melamine, and formaldehyde. Melamine-formaldehyde (MF) itself is a very good (waterproof) but expensive adhesive resin. Melamine is used in manufacturing MUF as a supplement to UF resin to overcome the shortcomings of UF resin. The melamine-to-urea should be at least 15% by weight so that the adhesive bonds are more wet-durable and reduce formaldehyde gas emission.

--More costly than UF.

--MUF’s curing temperature and press time is higher and longer than those for UF.

--MUF adhesive bonds are near waterproof, and therefore reduce formaldehyde gas emission.

--MUF resin is used in other countries where PF resin is more expensive to produce

waterproof reconstituted wood products.

Phenol-formaldehyde resin (PF)—This is the most important adhesive resin in North America to manufacture exterior construction grade building materials. About 2.99 billion pounds of PF are consumed annually in North America. Commercial PF resin usually contains 50% solids in water.

--The current market PF price is over $0.50 per pound.

--PF requires a higher curing temperature (~ 175 oC) and/or a long press time (twice as

long) than UF.

--PF adhesive bonds are waterproof and PF-bonded products do not emit formaldehyde gas.

Phenol-resorcinol-formaldehyde resin (PRF)—This resin is manufactured from phenol, resorcinol and formaldehyde, usually containing 50% phenol and 50% resorcinol. Resorcinol is much more expensive than phenol, and it is used to promote curing (or setting) of the resin. PRF can be cured at the ambient temperature but requires several hours (or overnight) of curing time. PRF is used for manufacturing large glue-laminated beams (glulams) where hot pressing is not practical or impossible.

--PRF is more expensive than PF.

--Best used as a cold-setting resin.

--PRF adhesive bonds are waterproof and do not emit formaldehyde gas.

Isocyanates—The most used isocyanate wood adhesive resin is polymeric methyl-diphenyl isocyanate (pMDI). This is a relatively new resin and only about 80 million pounds are consumed annually in North America.

--MDI adhesive bonds are very durable (waterproof).

--MDI contains no water, and therefore it is not necessary to dry wood elements

to a low moisture content.

--MDI is about $0.70 per pound.

--Requires special release agents applied to surfaces to prevent adhesion of the products

to press surfaces.

Polyvinyl alcohols (PVA) and Polyvinyl acetate (PVAc)—These are most common thermoplastic adhesives used in woodworking. Thermoplastic adhesives are those can be fluidized or plasticized at high temperatures. PVA (Elmer’s glue) and PVAc (Wood Glue) contain about 50% solids in water. When used as wood glues, PVA and PVAc loss moisture (drying) and are solidified to form a tight bond between two wood surfaces. These types of adhesive bonds are water resistant but not waterproof.

II Plywood

Plywood is produced either as panels for interior uses (paneling and furniture parts) or for construction purposes. Interior plywood is normally manufactured using hardwood veneer as the surface ply and using UF resin as the glue. Exterior plywood is manufactured with softwood rotary-cut veneers bonded with PF resin. Veneer logs are softened by heating in 85 oC (or higher) water for several days. Softening of logs before veneer cutting not only reduces energy requirement during rotary peeling but also increase veneer quality. A slicer is used to produce high-grade thin veneers from fine hardwoods, and softwood veneer is generally rotary-cut by a lathe to produce construction grade plywood. A powered-backup roll is used in the production of rotary-cut veneer to increase yield, especially from small-diameter logs. The backup roll provides a support in the mid-span to prevent chuck slippage and log breakage when the log is reduced to a small diameter during the peeling process, it also helps turning of the logs during peeling. Green veneers are dried to about 4% moisture content in a jet drier where the veneer surfaces are impinged with jets of hot air.

PF resin is used for construction (exterior) grade plywood and UF is used for interior plywood. Plywood PF or UF glues are formulated (thickened) with either wheat or corn flour with some filler (such as nutshell flour or wood flour) so that the glue would not be squeezed out or squeezed in (into wood structure beneath the joint) to avoid starved joints (poor bonding). Various types of plywood construction are possible. Plywood assemblies are pressed with multi-opening presses.

Softwood plywood grades, such as A-B, C-C, and C-D, refer to the grades on the two faces. C-DX (X for exterior glue) plywood is most common softwood for roof sheathing and floor decking for residential light frame constructions. Hardwood plywood is mainly used for paneling, furniture, and door skins. Refer to Chapter 14 (textbook) for detailed description of plywood grades.

III Particleboard:

--Particle Generation: Sawmill residues, especially sawdust, is the typical raw material but low quality logs are sometimes used. Raw materials are hammer-milled and/or disk-refined, dried to about 4% moisture content, and screen-classified for particle size. Agricultural residues, such as straws and baggasse (sugar cane residue), also are used in particleboard production in recent years.

--Resin Application: UF resin is used for production of wood particleboard for interior uses. For producing agricultural particleboard, MDI is used in order to bond waxy surfaces. The cuticle layer lining the surfaces of agricultural materials cannot be bonded with synthetic resins except MDI. The most used method of resin application is done with a tube resin mixer, in which resin is metered into the tube and a high speed mixing (rubbing) action achieves a uniform distribution of resin. Resin is applied to fine and coarse wood furnishes separately at different rates, about 8% resin for fine furnish and about 6% resin for coarse furnish.

--Forming Mats: Resin-coated particles are blown from a bin to a moving conveyor to form a mat of uniform and pre-determined thickness. The mat is formed in such as manner that coarse furnish is sandwiched between surface layers of fine furnish as illustrated in Figure. Mat thickness control is important as it determines density of the panels.

--Hot Pressing: Formed mats are hot pressed with either a multi-opening press or a continuous press to a predetermined thickness. Press temperature and press time are dependent on resin used and thickness of the mat. Boards bonded with UF resin are quickly cooled down (usually with a fan) to void resin breakdown due to prolonged heating. Boards are then cut to size and sanded to the final thickness.

IV Fiberboard:

Fiberboard based on density can be classified into insulation board (less than 31 lbs/ft3), medium-density fiberboard (31 lbs/ft3 to 50 lbs/ft3 MDF), and hardboard (>50 lbs/ft3). Insulation board is normally manufactured by the wet-formed process and MDF usually is produced by the dry-formed process. Hardboard can either be produced by the wet- or dry-formed process. In manufacturing insulation board, no resin and other additives are necessary, fiber-to-fiber bonding is achieved by hydrogen bonding. Dry-formed boards require adhesive resin for bonding.

--Fiber Generation: Thermomechanical pulps (TMP) is produced by the pressurized disk- refining process, in which wood chips about 50% in moisture content are digested with 150 psi steam (temperatures over 180 oC) for about 4 minutes, followed by feeding the chips through a gap of about 0.004” between two rotating refining disks. Refined fibers for dry-formed process are flashed with high-temperature air (over 400 oC) and blown to destinations in a large tube. Fibers are usually dried to about 4% moisture content. In the wet-formed process, refined fibers are suspended in a large volume of water, followed by draining water through a screen to form a wet mat before hot pressing.

--Resin Application: In the dry-formed process, about 8% to 12% resin (solids) can be applied during the hot-air drying stage (called blow-line application) or it can be sprayed onto fibers after drying. If the wet-formed process is used, PF resin is added to the fiber slurry with a vigorous mixing action, followed by precipitation of the PF resin onto fiber surfaces by adjusting pH to 4.5. Only about 1% to 2% of resin is needed in the wet-formed process because resin applied in such manner is very efficient and uniform.

--Forming Mats: In the wet–formed process, the fiber slurry flows onto a moving screen where most of water is removed. The wet-formed process is currently less favorable than the dry-formed process because it is costly to dispose or treat large amount of wastewater. Dry-forming fiber mats is done much the same way as discussed in the particleboard manufacture, except it is not necessary to form the mat in different layers as there is no “particle size” difference in the fiber furnish.

--Hot-pressing: A wet-formed mat requires a long press time and a screen where the mat is rested on to facilitate vapor escape during hot-pressing. The finished board has a matted bottom surface and is called S1S (Smooth on 1 Side). Dry-formed mat requires a shorter press time and does not need a screen to vent vapor. A dry-formed board also is called S2S (Smooth on 2 Sides).

V Oriented Strand Board (OSB): Strands are also called flakes or wafers, and therefore boards made from these wood elements also are called flakeboard or waferboard. OSB, best manufactured by using low-density hardwoods such as aspen, cottonwood and yellow poplar, gradually replaces plywood for sheathing and for roof and floor decking. Because of load-bearing requirements for these uses, OSB is manufactured by using exterior grade, waterproof adhesive resins, PF or MDI resins.

--Flake Generation: A disk flaker or a ring flaker may be used to produce flakes. A disk flaker generally uses low-quality logs as the raw materials and produces high-quality flakes. A ring flaker uses smaller wood scraps or chunks as raw materials and generally produces flakes of less predictable geometry and contains a high proportion of fines. Flakes are flash-dried in rotating drums to about 4% moisture content and screened to remove fines, if necessary.

--Resin Application: About 3.5 to 5.0% PF or MDI (3.5% maximum) resin is sprayed onto flake surfaces in a blender. There is a major U.S. company producing OSB uses a combination of PF and MDI resins to produce OSB. Flakes sprayed with 3% MDI are formed in the core and flakes sprayed with 5% PF resin are formed as surface layers of a mat. By this arrangement, the use of special release agent to prevent sticking of board to the press is avoided.

--Forming Mats: In forming an OSB mat, the long axis of flakes is aligned in the machine direction. Flake aligning usually is done by mechanical means.

--Hot Pressing: the mat usually is pressed at 175 oC for different duration, depending upon the thickness of OSB. OSB usually is made at 3/8”, 4/8”, or 5/8” in thickness. OSB usually is not sanded because OSB surfaces often are coated with greasy release agents that are water repellent.

VI Glued-Laminated Timber (Glulam):

Large and often arched and shaped beams (Figure) are manufactured by gluing small and thinner pieces of lumber with phenol-resorcinol-formaldehyde (PRF) resin. PRF resin is the choice because it is capable of cold set. Advantages of laminated lumber include (1) higher utilization of wood, (2) size and length of structural members are not limited by length of tree, (3) strength-reducing characteristics, such as knots and other defects can be controlled, (4) individual laminated can be easily and thoroughly dried and treated before lamination, and (5) architectural pleasing effects are obtained with glued-laminated design.

Laminated Veneer Lumber (LVL): Veneer sheets are spread with PF resin or other exterior grade adhesives such as MDI and hot-pressed into thick (1.5 inches) and long (16’, 32’ or 64’) pieces. Then, these large pieces can be cut into 2 by lumber pieces, such as 2 by 2, 2 by 4, 2 by 6, etc. A most common used of LVL is the 2 by 4 pieces used as fringes of I beams (or I-joists).

Parallel Strand Lumber (PSL): Veneer sheets are clipped into 0.75-inch wide long strand, and these long strands are coated with PRF resin and parallel assembled, press with a continuous press to a uniform density, followed by curing the PFR resin with microwaves. Just as LVL, PSL can be cut into 2 by lumber pieces and used as framing lumber.

Laminated Strand Lumber (LSL): Large flakes (about 1 to 2 inches in width and over one foot in length) and veneer strips in similar sizes are coated with exterior adhesive resins (PF or MDI) and pressed into large pieces, followed by cutting these large pieces into dimension lumber. This type of product is sold in the market as TimberStrand.