Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
STEAM PRE-HEATING IN ORIENTED STRAND BOARD PRODUCTION
Technical Field
The invention is generally related to oriented strand board and, more
particularly,
to improving the production efficiency and product quality in oriented strand
board
manufacturing.
Backiaround Art
Oriented strand board is commercially available from a number of companies
including J. M. Huber Corporation, Georgia-Pacific corporation, Louisiana-
Pacific, and a
number of other sources. This material has multiple layers of wood "flakes" or
"strands"
bonded together by a binding material such as phenol formaldehyde resin or
isocyanate
resin together with sizing materials such as paraffinic waxes. The flakes are
made by
cutting thin slices with a knife edge parallel to the length of a debarked
log. The flakes
are typically 0.01 to 0.5 inches thick, although thinner and thicker flakes
can be used in
some applications, and are typically, less than one inch to several inches
long and less
than one inch to a few inches wide. The flakes typically are longer than they
are wide. In
the fabrication of oriented strand board, the flakes are first dried to remove
water, and are
then coated with a thin layer of binder and sizing material. The coated flakes
are then
spread on a conveyor belt in a series of alternating layers, where one layer
will have the
flakes oriented generally in line with the conveyor belt, and the succeeding
layer of
flakes oriented generally perpendicular to the conveyor belt, such that
alternating layers
have flakes oriented in generally perpendicular to one another. The word
"strand" is used
to signify the cellulosic fibers which make up the wood, and, because the
grain of the
wood runs the length of the wood flake, the "strands" in the oriented strand
board are
oriented generally perpendicular to each other in alternating layers. The
layers of
oriented "strands" or "flakes" are fmally subjected to heat and pressure to
fuse the strands
and binder together. The resulting product is then cut to size and shipped.
Typically, the
resin and sizing comprise less than 10% by weight of the oriented strand board
product.
The fabrication of oriented strand boards is described in U.S. Patent
5,525,394 to Clarke
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et al. Oriented strand board has been used in sheathing walls, wooden I-beam
structural
supports, and in roofs and floors where strength, light weight, ease of
nailing and
dimensional stability under varying moisture conditions are the most important
attributes.
Oriented strand board is sold at a substantial discount compared to structural
grades of soft
plywood.
Several other patents describe the production of oriented strand board. U.S.
Patent 5,635,248 to Hsu et al. describes a process for producing a smooth hard
finish on
products such as oriented strand board. In Hsu, a foamed polymerized latex
emulsion is
applied to the surface and dried. After drying, the emulsion is crushed, and
then cured to
form the coating, with post-cure heat treatments being found to improve the
hardness of
the coating. U.S. Patent 5,554,429 to Iwata et al. discloses an oriented
strand board
flooring material which is indicated to have significant moisture resistance.
In Iwata, the
oriented strand board is fabricated with the surface layers having strands
with longer
average length values and wider average width values than the centrally
located layers.
Iwata uses a foaming urethane resin and a non-foaming aqueous emulsion-type
phenol
resin in combination to join the wood strips together. Iwata also contemplates
attaching a
decorative sheet of material (e.g., ciak sheets) to the oriented strand board
surface using
an aqueous polymeric isocyanate adhesive, and subsequently overcoating the
decorative
sheet with a polyurethane, thus producing high gloss, decorative, wood
flooring which
has the appearance of oak.
Recently, efforts have been made to produce oriented strand board on a
continuous basis. Prior production systems had utilized a batch press
operation to fuse
the mat of flakes together. These systems required the use of a saw and
separation
procedures to isolate individual lengths of matted flakes. Advances were made
in presses
and shuttling sub-systems which allowed multiple lengths of matted flakes to
be
simultaneously pressed and fused together. However, despite these advances, it
was
envisioned that the use of continuous presses would be advantageous in
oriented strand
board production since their use would eliminate cutting and separating
procedures prior
to pressing, thereby increasing production capacity. Examples of continuous
belt presses
for making oriented strand board can be found in U.S. Patent 5,520,530 to
Siempelkamp
and U.S. Patent 5,596,924 to Gerhardt, and an example of a continuous
production
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process to form particleboard or fiberboard is described in U.S. Patent
5,538,676 to
Biefeldt.
In oriented strand board production processes that utilize continuous belts to
move materials through the pressing step, a conveyor moves the mat through two
opposing, closely spaced belts which press the flakes together. A pair of
heated plates or
a heated moveable ram and an opposing table are positioned behind the
opposing,
closely spaced belts, and provide heat. and additional pressing forces. Thus,
as the mat is
moved through the opposing, closely spaced belts, the wood fibers are pressed
together
both by the belts and by the plates or ram, and the binder and filler are
heated to a point
where the individual wood flakes or "strands" are fused to form a continuous
flake board
or "strand" board product.
Disclosure of the Invention
According to the invention, isocyanate-based binder materials, such as
methylene
diphenyl diisocyanate (MDI) and polymeric methylene diphenyl diisocyanate
(pMDI),
are used in a continuous oriented strand board or "flake board" manufacturing
process. A
mat of wood flakes or "strands" is produced which includes alternating layers
of strands
oriented generally perpendicular to each other. The strands are coated with an
isocyanate
binder, which is preferably pMDI, as well as filler material. The typical
filler material is
a paraffin based wax; however, wood fines, dyes, flours, and the like may also
be
included. The mat is carried on a conveyor to a continuous press. Just prior
to entering
the continuous press, the mat is subjected to a steam treatment which both
raises the
temperature and the moisture content of the mat. Steam treatment softens the
wood fibers
and lowers the Tg of lignin to start lignin flow. Due to the use of isocyanate
binder
materials, the moisture provided by the steam as well as the moisture
emanating from the
wood material itself chemically reacts with the isocyanate to produce
polyureas. In
addition, the isocyanate binder materials chemically react with hydroxyl
moieties on the
cellulose and hemicellulose constituents of the wood material to produce
urethane bonds.
Heretofore, isocyanate and, in particular,, pMDI was not used as the resin or
binder material in methods of producing oriented strand board which included
the step of
pre-heating wood material to obtain desirable moisture levels. This was due to
the belief
that the presence of pMDI during the pre-heating step would create a condition
known as
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pre-curing whereby the resin would set before the wood material was
compressed.
However, in the process disclosed herein such pre-curing does not take place.
Further,
the reactivity of the isocyanates with water and hydroxyl moities allows
curing of the
binder and fusion with the wood material to be achieved under relatively mild
press
operating conditions (e.g., temperature, pressure, and time of exposure) as
well as at a
faster production rate. This, in turn, reduces the amount of volatile organic
compound
(VOC) emissions produced. Furthermore, the oriented strand board emanating
from the
continuous press has a relatively high moisture content which allows it to
withstand
thickness swelling under humid conditions.
Brief Description of the Drawings
The foregoing and other objects, aspects and advantages will be better
understood
from the following detailed description of the preferred embodiments of the
invention
with reference to the drawings, in which:
Figure 1 is a cut-away isometric view of an oriented strand board showing the
general orientation of wood flakes in the pressed, composite product;
Figure 2 is a schematic drawing showing the reaction chemistry of pMDI and
phenol-formaldehyde (PF) resin systems;
Figures 3a and 3b are a schematic drawings showing the chemical and
mechanical bonding of pMDI resin to wood material in oriented strand board,
and the
mechanical bonding of PF resin to wood material in oriented strand board,
respectively;
and
Figure 4 is a schematic side view of a continuous oriented strand board
manufacturing system and method according to the present invention.
Best Mode of Carrying Out Invention
With reference to Figure 1, it can be seen that oriented strand board 10 is
comprised of multiple layers 12, 14, and 16, of wood "flakes" or "strands"
which are
preferably oriented generally perpendicular to each other in adjacent layers.
However, it
should be noted that the layers of strands may be oriented parallel to one
another or
oriented in a variety of other ways. The size of the strands can vary and the
number of
layers in the oriented strand board can vary to meet a wide range of design
requirements.
In addition, the size of strands in different layers may also vary. As
discussed above, the
strands are held together by a binding material, and the oriented strand board
typically
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includes a wax material for sizing. The oriented strand board 10 can be
produced by a
variety of techniques; however, common to all fabrication processes is a step
of
subjecting layers 12, 14, and 16 to high temperature and pressure to fuse and
bind them
together using the binding material.
In the practice of this invention the binding material is an isocyanate, and
most
preferably a polyisocyanate such as pMDI. Figure 2 compares the cure reaction
of pMDI
to the PF resin systems described in U.S. Patent 5,538,676 to Biefeldt. It is
noted that
with pMDI, urethane bonds are formed between hydroxyl moieties on the wood
surface,
such as those which occur along cellulose and hemicellulose chains. In
addition, in the
presence of moisture, urea bonds are created between pMDI monomers. In sharp
contrast, in phenol-formaldehyde resins, the monomer sub-units react via a
condensation
reaction and release water as a reaction byproduct. Thus, the presence of
excessive
moisture inhibits curing of PF resin systems. Figure 3a shows that pMDI is
chemically
and mechanically bonded to wood material, while PF resin systems are most
likely
mechanically bonded to wood material in oriented strand board production.
During
fabrication of an oriented strand board, a substantial amount of energy is
used to drive
off water molecules prior to and during curing of the resin. When an
isocyanate resin
such as pMDI is used, chemical reactions proceed with the water molecules and
hydroxy
moieties because they are thermodynamically favored. The net result is that an
isocyanate resin system such as pMDI will achieve a stronger fusion with the
wood
material under reduced temperature and exposure time conditions compared to PF
resin
systems.
Figure 4 illustrates a continuous oriented strand board forming process
according
to the present invention. A mat 18 of wood flakes progresses from left to
right on a
conveyor 20 through a continuous press 22 to produce a continuous sheet of
oriented
strand board 24. The conveyor is preferably coated with a release agent to
facilitate the
releasing of the board from the press without delamination or blistering.
Typical release
agents are wax-based release agents such as Blackhawk Specialty Chemical's EX-
24 or
soap-based release agents such as Houghton International's #8315.
As discussed in conjunction with Figure 1, the wood flakes or "strands" are
positioned on the conveyor 20 as alternating layers where the "strands" in
adjacent layers
are oriented generally perpendicular to one another. The number of layers will
vary
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depending on the application and desired thickness of oriented strand board to
be
produced. Typically, the mat 18 will be 1 to 20 inches thick. The individual
strands in the
mat will be pre-coated with isocyanate binder, sizing, such as paraffin wax,
and/or other
materials such as dyes, etc., using conventional processes. The preferred
isocyanate
binder is pMDI, and it is commercially available from the ICI Polyurethanes
Group of
New Jersey (as Rubinate pMDI), and from other commercial sources. Preferably,
the
isocyanate binder comprises about 1.5 to about 8% by weight of the mat, and
the sizing
materials comprise about 0.5% to about 4% by weight of the mat. Prior to
depositing the
wood flakes on the conveyor, they are dried. The moisture content of the mat
18 is
preferably 2% to 20% by weight. Since pMDI and other isocyanates beneficially
react
with water, the moisture content need not be as strictly controlled or be as
low as that
which is employed when PF binder resins are used.
Prior to entering the continuous press, the mat 18 of wood material is exposed
to
a steam treatment by steam sources 26. The steam sources may be positioned on
opposite
sides of the mat 18. In the preferred embodiment, the conveyor 20 will be made
of
porous wire material such that steam can penetrate through to the bottom of
the mat 18.
The steam functions to soften the wood fibers. Steam also lowers the glass
transition
temperature (Tg) of lignin, and thereby enhances lignin flow in the wood
material in the
mat 18. As explained above in conjunction with Figures 2 and 3, water enhances
the cure
reaction of isocyanates thereby allowing relatively lower temperatures and
reduced press
times to be used in the press than if phenol-formaldehyde resin systems were
utilized. In
addition, an oriented strand board product 24 having higher moisture content
has
improved resistance to thickness swelling caused by exposure to humidity. The
amount
of steam can vary depending on the thickness of the mat, or the desired
characteristics of
the end product. It is expected that in most applications the steam will raise
the
temperature in the mat 18 from about 50 to about 95 C and the moisture content
from
about 6 to about 24%.
Because isocyanates such as pMDI react with water molecules and hydroxyl
moities at relatively low temperatures, the temperature and pressure and time
of exposure
conditions in the continuous press 22 can be reduced compared to when PF
resins are
used in order to achieve curing.
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Thus, no additional pre-heat stations are required prior to pressing as is the
case
in U.S. Patent 5,538,676 to Biefeldt. However, also because of the reaction,
it is
important that the mat 18 proceed directly into the press 26 so that the
binder can fuse
with the wood material. In prior art systems, pMDI has been used as a binder
for oriented
strand board production, but it has not been used in combination with steam
pre-
treatment. These prior systems employed a batch/shuttle mechanism in
combination with
a multi-port press such that multiple oriented strand boards would be
simultaneously
produced in a single pressing; however, the separated strand board sections
would need
to be stacked and shuttled into position prior to pressing. If steam pre-
treatment were
employed in these systems, the reaction of the pMDI with water may proceed too
rapidly
prior to pressing, thus limiting chemical reactions with the wood material and
possibly
hindering mechanical joining of the isocyanate to the wood (as shown in Figure
3).
Hence, prior to this invention, it was believed by those of skill in the art
that steam pre-
treatment could not be employed in oriented strand board manufacturing which
utilized
an isocyanate resin such as pMDI. However, because continuous presses 22 can
receive
and process steam treated mat 18 on a continuous basis, it has been discovered
that
isocyanates can be used as the binder and that steam pre-treatment can be
advantageously employed to achieve benefits in cure/press conditions and
benefits in the
physical properties of the oriented strand board 24 produced in oriented
strand board
manufacturing.
The continuous press 22 can be similar to those described in U.S.
Patents 5,520,530, 5,538,676, and 5,596,924; however, a wide variety of
continuous
presses can be used in the practice of this invention. The chief requirement
for the
continuous press 22 is that it be able to continuously take in mat 18 and
press and heat
the mat 18 to fuse the isocyanate binder to the wood material, and
continuously output
oriented strand board 24. A continuous press 22 will typically have a pair of
closely
spaced, opposing conveyors 28, and internal, heated press plates 30 which can
be
progressively and repetitively moved toward each other. Instead of heated
press
plates 30, one moveable plate or "ram" and one stationary plate can be used.
The heated
press plates are responsible for exerting a pressure on the mat material at a
temperature
which both cures the resin binder and fuses the wood and binder together. The
press
plates 30 will typically move closer together than the gap between the
opposing
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conveyors 28, and the distance between the press plates 30 can be varied to
accommodate the production of oriented strand board 24 of differing
thicknesses.
The temperature employed in the press 22 can vary depending on the application
and properties of the oriented strand board to be produced, as well as the
time period to
traverse the press 22. In most applications employing isocyanate resin
binders, the
temperature of the belts in the press 22 will range from about 120 to about
260 C. When
pMDI is used as the binder, the preferred temperature for the belts in press
22 ranges
from about 175 to about 227 C. It should be apparent to those skilled in the
art that the
temperature can be varied to achieve similar end product results by varying
the pressure
and/or residence time in the press 22.
The pressure exerted by the press plates 30 can be varied in a similar manner
to
the temperature. In most applications in the practice of this invention the
maximum
pressure will range from about 300 to about 900 psi. Likewise, the residence
time in the
press 22 can be varied and is dependent on the length of the press 22, the
speed of the
conveyor 20, and the thickness of the panel. In most applications in the
practice of this
invention the residence time will range from 0.5 to 10 minutes. The residence
time in the
press and product properties may also be modified through the addition of
catalysts or
polyols to the isocyanate or pMDI binder, or to the wood strands.
It is preferred that the temperature, pressure, and time in the press 22 be
selected
to allow for complete curing of the isocyanate resin and fusion with the wood
material.
These operational specifications for the press, as well as the moisture
content of the
mat 18 as adjusted by the steam generators 26 can be varied to achieve the
continuous
production of oriented strand board 24 of desired moisture content.
Experiments have
shown that higher moisture content oriented strand board 24 is more resistant
to
thickness expansion and linear expansion resulting from exposure to humidity.
In the
preferred embodiment, the parameters used in the press and the steam generator
will
produce oriented strand board 24 having a moisture content ranging from about
4 to
about 12% by weight.
While the invention has been described in terms of its preferred embodiments,
those skilled in the art will recognize that the invention can be practiced
with
modification within the spirit and scope of the appended claims. By way of
example,
and not limitation, the steam pre-heating process described above may be
utilized in the
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production of other engineered wood products such as wood composite lumber,
rim
board, webstock, particleboard and fiberboard.
