Reconstituted Wood Products
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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
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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
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These wood adhesives are heavily dependent on petroleum-based chemicals.
Performance of Thermosetting Wood adhesives
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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)
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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)
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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
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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)
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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)
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Rising cost for petroleum-based chemicals forces the wood industry to
reconsider manufacturing wood adhesives from renewable materials
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It is a challenge to manufacture wood adhesives from renewable materials to
match the performance of petroleum-based wood adhesives
Plywood Manufacturing
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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
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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.
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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.
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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.