Note: Descriptions are shown in the official language in which they were submitted.
CA 02682956 2009-11-17
LOW DENSITY ORIENTED STRAND BOARD
FIELD OF THE INVENTION
The present invention relates to low density oriented strand board and novel
methods for
making low density oriented strand board.
BACKGROUND OF THE INVENTION
The oriented strand board ("OSB") industry emerged in the late 1970s and soon
became a
major competitor to the plywood industry. By the year 2000, OSB had already
captured more
than half of the North American structural panel market. Although many efforts
have been
made by the OSB industry to improve their products' properties, several OSB
properties, such
as the strength-to-weight ratio, homogeneous density profile, and dimensional
stability, still
compare unfavourably with plywood.
OSBs are manufactured from wooden strands combined with a thermosetting resin
and
consolidated together under heat and pressure in a hot press. Typically, an
OSB panel
comprises a middle core layer and two outer face layers. In order to develop
adhesive bonds
between the wooden fiunish, it is necessary to produce adequate contact
between wood and
resin, and raise the temperature to cure the resin. Currently, common
commercial pressing
operations use a closing time (press platens ramp to the final position) in
the range of 25 to 90
seconds. With these durations, the temperature in the middle layer (core) of
the OSB is still
below the point necessary to soften the wooden furnish and cure the resin in
the core layer.
Additional time is necessary for the heat to transfer into the core to soften
the wooden furnish
and cure the resin. Because of this temperature gradient, strands in the
surface and bottom
layers of the mat that contacted to the hot platens first will be softer than
those in the middle
layer. When the pressure is applied to the mat, the outside layers compress
more than the
middle core layer. As a result, commercial OSBs typically have an "M" shape
vertical
density profile through the vertical direction (higher in the surface and
bottom, and lower in
the core) as shown in the prior art Figures 1 and 2.
Figure 1 is the vertical density profile of a commercial 23/32" OSB made with
Southern Pine
wood furnish. Although the average density of this panel is 44 1b/fl , the
density of the surface
CA 02682956 2009-11-17
and bottom layers can be as high as 571b/ft3 but the core layer is only
371b/ft3. Figure 2 is
another commercial 23/32" OSB made with Aspen which average panel density is
351b/ft3 .
The highest density in the outside layer is 451b/ft3 and the core layer is
only 291b/ft3. Both
panels have shown the typical "M" shapes in their vertical density profiles -
very high density
in the surface and bottom but a much lower density in the core. There is
currently no known
technique to overcome this issue and produce OSBs having a flatter vertical
density profile.
At the present time, the only way to make the adequate contact for the
consolidation is to use
a much higher average density to raise the density in the core layer. In
addition, when these
panels are exposed to water, they would have very high thiclmess swelling due
to the high
compression ratio in the surface and bottom layers.
Because of the uneven heating and resin setting nature of a convention of OSB
process, the
moisture content is regulated such that the moisture content of the outer face
layers is
significantly higher than the moisture content of the core layer. This is done
to promote heat
transfer into the core layer by heat conduction.
The average density of conventional OSB is between 35 to 45 lb/ft3 depending
on the wood
species used. When OSB manufacturers attempt to make a lower density OSB below
this
range, the first problem they will confront would be a very lower density core
with a porous
appearance, therefore, causing problems of low strength properties.
Therefore, there is a need in the art for a method of making low density OSB
having a
relatively homogenous vertical density profile which mitigates the
difficulties in the prior art.
SUMMARY OF THE INVENTION
The present invention is directed to methods of making low density OSB and the
resulting
low density OSB panels. Accordingly, in one aspect of the invention, the
invention
comprises a method of forming a low density OSB product, comprising the steps
of:
(a) combining wood strands with thermosetting resin;
(b) preheating the wood strands-resin mixture; and
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(c) pressing and heating the preheated wood strands-resin mixture until the
resin
sets to form OSB.
In another aspect of the invention, the invention comprises a method of
forming a low density
OSB product including a core layer and two outer face layers, comprising the
step of
controlling the moisture content of the core layer to be about equal to or
higher than the
moisture content of the outer face layers prior to pressing. Preferably, the
moisture content of
the core layer is between about 10% to about 30% while the moisture content of
the outer
face layers is below about 10%. More preferably, the moisture content of the
core layer is
about 20% while the moisture content of the outer face layers is about 8%.
In another aspect of the invention, the invention comprises a low density OSB
having a
homogenous vertical density profile.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of an exemplary embodiment with
reference to the accompanying simplified, diagrammatic, not-to-scale drawings.
In the
drawings:
Figure 1 is a vertical density profile of prior art commercial OSB made with
Southern
Pine.
Figure 2 is a vertical density profile of prior art commercial OSB made with
Aspen.
Figure 3 is a picture of a cross section of low density OSB made with the pre-
heated
wood furn.ish as described herein.
Figure 4 is a vertical density profile of a low density OSB made with long
Aspen
strands.
Figure 5 is a vertical density profile of a low density OSB made with short
Aspen
strands.
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Figure 6 is a vertical density profile of two low density OSBs of different
thicknesses
made with moisture content controlled wood furnish and without preheating.
Figure 7 is an example of a pressing cycle used to produce the low density OSB
of
Figure 6.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a method of making a low density OSB product by
preheating
the wood strands prior to consolidation. As used herein, "low density" refers
to OSB having
an average density of less than about 1.5 times higher than the density of the
wood used in the
OSB, and preferably less than about 1.4 times higher. With typical wood
species used in OSB
production, "low density" may refer to OSB products having an average density
of less than
about 40 lb/ft3, preferably less than about 351b/ft3, and more preferably
around 301b/ft3. For
example, the density of aspen log is typically in the range of 22 to 25
lb/ft3, therefore a
preferred "low density" aspen OSB product may have an average density of less
than about 35
lb/ft3 (25 x 1.4). The density of shortleaf Southern Pine is about 321b/ft3,
therefore a preferred
"low density" shortleaf Southern Pine OSB product may have an average density
of less than
about 451b/ft3 (25 x 1.4).
As used herein, "homogenous vertical density profile" refers to a density
profile similar to
that shown in Figures 4, 5 and 6, wherein the lowest core density is at least
about 75% of the
highest surface density. Preferably, the lowest core density is greater than
about 80% of the
highest surface density. More preferably, it is greater than about 85% and
most preferably it
is greater than about 90%.
A feature of the present invention is a pre-heating procedure used with a
conventional OSB
production line to raise the temperature of OSB strands before they are
consolidated into the
fmal product. The range of the pie-heating temperature may be from about 35 C
up to the
onset temperature of the particular adhesive or resin used in the product.
After the pre-heating
procedure, the softened strands in the core layer will be easier to densify.
With this invention,
OSB manufacturers no longer need to use a higher average panel density to
raise the core
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density. As the result, the relatively high density in the surface and core
can be reduced or
even eliminated.
The preheating process can be applied by any heat source, such as by
microwave, radio
frequency (RF) or high frequency in-ddiation, infrared irradiation, hot air,
or steam, to bring
up the strands' temperature. Any method of heat transfer, such as conduction,
convection or
radiation may be used. The preheating process can be applied in any location
in the
production line before the fmal consolidation, such as heating the strands
during the blending
process, heating the mat during the mat formation, heating the mat after the
mat formation but
before the consolidation, or heating the mat before the final stage of the
consolidation.
Because the target of the pre-heating step are the strands in the core layer,
the heating area of
the OSB strands or mat can be either the entire mat or only the core zone.
This invention adds the pre-heating process to current production lines in OSB
mills for
making low density OSBs but still with performances able to pass standard
requirements. In
addition to a significant reduction in density without significant loss of
strength and integrity,
the OSBs made in accordance with this invention may also have a homogeneous
vertical
density profile as compared to conventional OSB. Because of its low
compression ratio in the
surface and bottom layers, OSB products made with this invention also have an
excellent
dimensional stability and low thickness swelling.
In an alternative embodiment, the moisture content of the core layer and face
layers may be
manipulated to produce OSB products with a homogenous vertical density
profile, either with
or without the preheating step described herein. Preferably, the moisture
content of the core
layer is maintained between about 10% to about 30% while the moisture content
of the outer
face layers is below about 10%. More preferably, the moisture content of the
core layer is
between about 18% to about 22%, while the moisture content of the outer face
layers is about
8%. The mat with the controlled moisture contents is then pressed. Lower ramp
pressures
than conventional OSB pressing cycles may be used because the softer core.
Because of the
lower density, the mechanism of the heat transfer during the pressing cycle in
the low density
OSB is mainly dependent on heat convection rather than the heat conduction as
in the
conventional OSBs. In spite of the higher moisture content in the core layer
than in the
surface layers, the rate of the heat transfer into the core layer during the
pressing cycle is
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higher. As a result, the vertical density profile is more homogenous than with
conventional
prior art OSB methods.
In preferred embodiments, a method of the present invention may combine the
preheating
steps and the moisture content control steps referred to above. Heating by
microwave or RF
iiradiation works particularly well with higher moisture content in the core
layer as the
increased moisture causes greater heat production in the core layer upon
irradiation.
EXAMPLES
The following examples are intended to illustrate the claimed invention,
without limiting the
invention to the specific elements described in the examples.
Example 1
Aspen strands having a length within a range of 5- 5.75", a width within a
range of 0.5 -1 ",
and a thickness within a range of 0.015" - 0.020" were used as the raw
material. Seven
weight percent of MDI (diphenylmethane diisocyanate) was applied as the binder
to these
strands when they tumble with the blender by means of a spinning disc. After
blending, the
furnish was formed into a mat similar to conventional OSB 3-layer orientation.
The mat was
pre-heated by microwave irradiation for 28 seconds until the core temperature
in the mat was
raised to 53 C. The pre-heated mat then was pressed with the hydraulic press
at a temperature
of 200 C for a period of 6 minutes. The thickness of the panel was 0.72 inches
and the
average density was targetted at 30 lb/ft3.
Figure 4 is the view of the cross section of the final product. Although its
density is only 30
lb/ft (the actual density measured by the QMS density profiler, Model QDP-01X
is 29.2
lb/ft3), the sample has displayed a very smooth appearance without any porous
areas. Figure 3
shows the vertical density profile of the product. Unlike the "M" shape
commonly seeing in
conventional OSB products, the panel has a very flat or homogenous density
profile. Table 1
is the testing result of the product. With the density of 301b/ft3, the panel
has still passed all
standard requirements of CSA0437.0-9. The panel has an excellent dimension
stability which
thickness swelling is only 4.3%.
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Table 1. Testing results of the low density OSB
Testing Requirement Results
Units 0-2 Direction
Modulus of rupture psi 4200 Para 4430
1800 Perp 3020
Modulus of elasticity psi x 800 Para 919
1000 225 Perp 300
Internal bond psi 50.0 56.8
Thickness swell
- 24 h soak % 10 4.3
- Water Absorption % No Req. 21
Example 2
Shorter aspen strands having a length within a range of 4- 4.5", a width
within a range of 0.5
-1 ", and a average thiclcness of 0.022" were used as the raw material. Eight
weight percent of
MDI (diphenylmethane diisocyanate) was applied as the binder to these strands
as they
tumble with the blender by means of a spinning disc. After blending, furnish
was hand-
formed into a mat similar to the conventional OSB 3-layer orientation. The mat
was pre-
heated by a microwave oven for 22 seconds until the core temperature in the
mat was raised
to 55 C. The pre-heated mat then was pressed with the hydraulic press at a
temperature of
200 C for a period of 6 minutes. The target thickness was 0.72 inches and
density was 30
lb/ft3. Although its density is only 301b/ft3 (the actual density measured by
the QMS density
profiler, Model QDP-01X is 29.3 lb/W), the panel has passed all standard
requirements of
CSA0437.0-9 as shown in Table 2. Figure 4 is the vertical density profile of
the product in
which the "M" shape density profile has been reduced to minimum. The product
has a
homogeneous density profile.
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Table 2. Testing results of low density OSB (short strands)
Testing Requirement Results
Units 0-2 Direction
Modulus of ruptuee psi 4200 Para 4540
1800 Perp 2840
Modulus of elasticity psi x 800 Para 819
1000 225 Porp 355
Internal bond psi 50.0 74.7
Thickness swell
- 24 h soak % 10 5.7
- Water Absorption % No Req. 22.9
Example 3
Aspen strands having a length within a range of 5 - 5.75", a width within a
range of 0.5 -1 ",
and a thickness within a range of 0.015" - 0.020" were used as the raw
material. Seven
weight percent of MDI (diphenylmethane diisocyanate) was applied as the binder
to these
strands when they tumble with the blender by means of a spinning disc. After
blending,
fiunish was formed into the mat similar to conventional OSB 3-layer
orientation except that
the moisture content of the core furnish was adjusted to 20% while the
moisture content of
the face furnish was adjusted to 8%. The mat was not pre-heated. The mat then
was pressed
with the hydraulic press at a temperature of 200 C for a period of 3 minutes.
The thickness of
the panel was 0.72 (23/32") inches and the average density was targeted at 33
lb/ft3. A
similar panel was pressed under similar conditions to a thickness of 7/16" and
an average
density of 35 lb/ft . Figure 7 shows the pressing cycle conditions for each
panel.
Figure 6 shows the vertical density profile of the products. Unlike the "M"
shape commonly
seeing in conventional OSB products, the panels have a relatively flat or
homogenous density
profile. Table 3 includes the testing results of the product. With an average
density of 33 lb/ft3
and 35 lb/ft3 respectively, the panels have still passed all standard
requirements of
CSA0437.0-9. The panels have excellent dimension stability in which thickness
swelli.ng is
only 6.7% and 4.8% respectively.
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Table 3
(0437.1-93)
Group 1 Units 0-2 Dir'n
Requirement 7/16" 23/32"
Modulus of rupture psi 4200 Para 7620 4600
1800 Perp 4340 3420
Modulus of elasticity psi x 800 Para 1222 991
1000 225 Perp 396 401
Internal bond psi 50.0 98.2 45.7
Thickness swell
-24hsoak % 10.0 6.7 4.8
- Water Absorption % No Req. 21.7 17.7
As will be apparent to those skilled in the art, various modifications,
adaptations and
variations of the foregoing specific disclosure can be made without departing
from the scope
of the invention claimed herein.
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