Note: Descriptions are shown in the official language in which they were submitted.
CA 02296554 2006-06-08
STRUCTURAL BOARD OF STRAW
Field of the Invention
This invention relates generally to a structural board made of straw.
Background of the Invention
In the past, straw was not considered a suitable structural material. Unlike
wood, straw has
not been considered for its strength and has not commonly been considered as a
building
material. Current trends in the use of straw for construction involve straw
bales where dense
packing and size provide necessary strength and structural support. In fact,
in many
countries, the use of straw for construction is not permitted due to a common
conception that
straw is a poor building material.
In the description that follows the term cereal straw is to encompass other
lignocellulosic
material that is cereal straw-like in structure, such as rice straw and
bamboo. Heretofore a
thin panel of compressed non-woody lignocellulosic material (i.e. straw) has
been made by
mixing short straw pieces with a binder. Disclosure of this thin panel is
found in U.S. patent
number 5,498,469 in the name of Howard et al. issued March 12. 1996. The panel
is used as
a core layer or core stock in a plywood laminate; thus a thin layer of straw
panel, is
sandwiched between two layers or sheets of plywood. Although this thin panel -
0.10 inches
appears to perform its intended function, the thin panels do not have
sufficient strength as
structural boards. The panels were incorporated with stronger wood laminate
layers for the
production of plywood.
Other references relating to fiber panel methods of manufacture and devices
for making such
panels are: United States Patent 5,730,830 in the name of Hall, issued March
24, 1998;
United States Patent 5,729,936 in the name of Maxwell, issued March 24, 1988;
and, United
States Patent 5,728,269 in the name of Kuno et al., issued March 17, 1988.
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It is an object of this invention, to provide a structural board that does not
require
expensive laminations forming wood/straw composites.
It is an object of this invention to provide a structural board comprised of
straw and
5 having a strength that far exceeds the strength of the straw panel described
by Howard
et al.
In accordance with this invention, a board or panel is provided wherein the
majority
of strands of straw are substantially oriented in a parallel fashion. The
strands are
!o combined with a binder.
In accordance with another aspect of the invention a board is provided wherein
strands of straw are oriented in a predetermined fashion or wherein at least
straw
strands within at least a layer are oriented in a substantially predetermined
fashion.
15
In accordance with the invention, there is provided, a panel, board, or beam,
comprising:
a compressed straw elongate material having a plurality of strands, a
plurality of the
strands being substantially split longitudinally to allow a binder to contact
some of the
2o inside of the strands; and binder for binding the straw into a solid panel,
board or
beam.
In accordance with the invention, a panel, board, or beam, is provided
comprising:
a compressed straw elongate material having a plurality of split strands, a
plurality of
25 the split strands being oriented in a predetermined manner; and
isocyanate binder for binding the straw into a solid panel, board or beam.
In accordance with another aspect of the invention, there is provided, a
method of
fabricating a panel, board or beam comprising:
3o providing a plurality of strands of cereal straw;
splitting the cereal straw;
orienting the cereal straw such that a plurality of strands are substantially
parallel;
and,
adding binder to the cereal straw.
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WO 99104943 PCTlCA98100700
In accordance with the invention, there is provided, a panel, board, or beam,
comprising:
a core of
a compressed straw elongate material having a plurality of strands, a
plurality
of the strands being substantially split to allow a binder to contact some of
the
inside of the strands; and
binder for binding the straw into a solid panel, board or beam; and,
outer layers comprised of compressed strands of lignocellulose material other
than
~ o straw.
In accordance with the invention a device for splitting straw is provided
comprising
two closely spaced shear rollers, said rollers being substantially the same
size and
having a diameter of substantially about 200 mm - 800 mm.
In accordance with yet another aspect of the invention, a straw panel is
provided
bonded with MDI (DiphenylMethane Diisocyanate) resin and preferably, wherein a
DPMA (DipropyleneGlycolMonomethylEtherAcetate) extender is used.
2o The strands preferably have a length of about 1 Omm or greater, and
preferably, are
50-100 mm long.
Structural, board, beams or panels can be fabricated in accordance with the
teachings
of this invention.
Brief Description of the Drawings
Exemplary embodiments of the invention will now be described in conjunction
with
the drawings, in which:
Fig. 1 is a graph comparing the bending ratio of a random oriented straw
strand board
(ROSSB) and an oriented straw strand board (OSSB);
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Fig. 2 is a graph of the modulus of rupture, and modulus of elasticity versus
the
average split straw strand length;
Fig. 3 is a graph depicting the modulus of elasticity of waferboard made from
split
5 wheat straw;
Fig. 4 is a graph depicting the bending strength of OSSB and waferboard made
from
split wheat straw;
Fig. 5 is a graph depicting internal bond strength of straw panels bonded with
MDI
resin extended with DPMA;
Fig. 6 illustrates a side view of a straw sputter according to the invention;
~5 Fig. 7 illustrates a side view of the opposite side of the straw splitter
according to the
invention shown in Fig. 6;
Fig. 8 illustrates a plan view of the straw splitter according to the
invention shown in
Fig. 6;
20
Fig. 9 illustrates a detailed view of the surface structure of the shear
rollers shown in
Fig.6;
Fig. 10 illustrates the splitting process of the straw splitter according to
the invention;
25
Fig. 11 illustrates the groove orientation on the surface of the shear rollers
shown in
Fig.6;
Fig. 12 illustrates a side view of a split straw orienter according to the
invention;
30
Fig. 13 illustrates a cross sectional view of the split straw orienter
according to the
invention; and
Fig. 14 illustrates a plan view of the split straw orienter according to the
invention.
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Detailed Description
Referring now to Fig. 1 graphs are shown comparing the bending ratio of a
random .
oriented straw strand board ROSSB and an oriented straw strand board OSSB,
wherein orientation of the strands is purposeful, and the strands are oriented
so as to
be substantially parallel with one another. The relative bending ratio of OSSB
to
ROSSB in the 'parallel direction. is shown in these figures to be
approximately 2:1, but
could be as small as 1.05:1.00 and still be useful. The length of the split
wheat strands
used was Smm to 100 mm.
0
Fig. 2 shows the bending properties of composite straw boards made with
different
longitudinally split wheat straw strand length. It can be seen that as the
length of the
straw increases, the bending strength and stiffness increases as well.
~5 Fig. 3 is a graph depicting the modulus of elasticity of waferboard made
from split
wheat straw, wherein the solid shaded columns illustrate the minimum property
requirement in the Canadian code (CSA 437) for "wood-based" oriented and
random
oriented sectional panels.
2o Fig. 4 is a graph depicting the bending strength of OSSB and waferboard
made from
split wheat straw, wherein the solid shaded columns illustrate the minimum
property
requirement in the Canadian code (CSA 437) for "wood-based'' oriented and
random
oriented sectional panels.
25 Fig. 5 is a graph depicting internal bond strength of straw panels bonded
with MDI
resin extended with DPMA.
In order to obtain maximum strength, the straw should be split, to ensure that
the
exterior and interior surfaces of the hollow straw stem core can be coated
with a
3o binder prior to hot pressing.
In addition to splitting the straw, it can be treated in such a manner as to
at least
partially strip the wax on the waxy outside stem by using a solvent.. After
removing
the wax and splitting the straw, it becomes easier to glue and requires less
glue to be
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WO 99!04943 PCTICA98100700
used. More importantly, the finished board has greater internal bond strength.
The
preferred binder is MDI Isocyanate resin such as ICI's "Rubinate 1840", or
Dow's
"PAPI-94". Phenolic resin normally used for wood panel does not bond well to
straw.
5 Transverse cutting or chopping of the straw can be accomplished by using a
forage
harvester.
Longitudinal straw cracking/splitting and node crushing can be accomplished by
using one of
to a) grooved rollers, for example a grain roller mill or a hay macerator
b) a sander having shear action provided by equipment with rolling shear
c) a CAE 6/36 disk waferizer ("feeding" with straw bundles rather than small
logs as
it is designed to be used)
d) a CAE 12/48 ringflaker (6") with small compressed bales of straw or a
t 5 e) a hammermill.
The combination of (a) and (e), a roller mill and a hammermill has the best
results.
The use of (b) and (c) is not preferred. A ringflaker (d) is useable but it
has the
limitation that it does not crush the node of the straw adequately.
20
Fines removal from split straw can be achieved by screening or air or
fractionation.
Once the straw has been split and separated, the strands must be oriented such
that the
strands are substantially parallel.
25 For strands longer than 1.0 mm moderate straw strand orientation can be
achieved
with minor modifications to commercially available equipment for orienting
wood
strands for OSB. This can also be accomplished by vibrating the strands on a
corrugated panel, preferably tilted at approximately 20 degrees, or
alternatively the
straw strands can be dropped on parallel-aligned vertical bars placed in the
form of a
30 spaced grid with a distance less than the strand length. Shaking will then
allow the
straw to fall through.
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For strands less than 1.0 mm, letting the strands fall between vertical
oppositely
charged electric condenser panels will align the straw. The dipole on the
falling straw
particles will align the particles parallel to the electric field.
Structural panel, boards, and beams can be made in this manner, by ensuring
that the
longitudinal axes of the straw are aligned.
The panel, board, or beam in accordance with an aspect of this invention
consists
essentially of longitudinal split straw and resin binder such as MDI, wherein
the straw
i o has been oriented such that the longitudinal axes of the straw pieces are
substantially
parallel. It has been found that the use of DPMA (DOwANOLTM) extends the
coverage
of MDI applied.
In another embodiment of this invention, a straw panel board is comprised of
oriented
t5 strand wood board having a straw core. This embodiment has the advantage of
providing a core made of Iignocellulose material other than wood, where wood
reserves are low, or the availability of wood is limited, while not
sacrificing the
structural integrity of the board. Furthermore, in some instances the
appearance of
wood on the outside faces of a panel board is of a commercial importance, and
this
2o embodiment meets this requirement.
In yet another embodiment of this invention, an oriented straw panel comprised
of
cementitious materials up to 50% (by weight), has been made. This embodiment
has
the advantage of providing a high degree of fire resistance combined with
mechanical
25 properties that exceeds the minimum strength requirement for wood based
structural
panels.
Referring to Fig. 6, a side view of a device 100 for longitudinally splitting
straw for
use in making a panel, board or beam according to the invention is shown. The
device
30 100 comprises a supporting bench 1, a feed table 5 and two machine grooved
shear
rollers 2 and 3 oppositely driven at different rotational speeds by an
electric motor 4.
The straw is fed generally parallel to the roller axes using the feed table ~
angled
downwardly towards the two shear rollers 2 and 3 where it is split
longitudinally due
to the shear action between the two shear rollers. Of course, alternatively,
the roller's
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CA 02296554 2006-06-08
diameters can be varied such that they are driven at the same rotational speed
but have
substantially different peripheral velocity. The term peripheral velocity is
used to indicate
that the rollers are of the same size and are driven at different rotational
speeds, or that the
rollers are of different sizes and are driven at the same or different
rotational speeds.
The upper shear roller 2 is affixed to the supporting bench 1, while the lower
shear roller 3 is
affixed to a supporting arm 6 pivoted to the bench 1 at the joint 7. The
clearance between the
two shear rollers is adjusted using an adjustment mechanism comprising an
elevating screw 8
and a tension spring 9. Other embodiments for adjusting the clearance between
the two shear
rollers may be envisaged, such as a ratchet gear or rack hoisting gear. The
adjustment
mechanism is also used for lowering the shear roller 3 in case some material
is stuck between
the shear rollers or for cleaning purposes.
The upper shear roller 2 is driven counterclockwise at approximately 500 rpm
to 1500 rpm by
the speed - controlled electric motor 4 using a V - belt drive or other such
drive means. The V
- belt drive comprises a V - belt 10 and V - belt pulleys 11 and 12 being axed
to the axis of
the electric motor 4 and the upper shear roller 2 respectively. The V - belt
10 is tightened
using the primary tension lever 13. For overload protection of the electric
motor 4 the V -
belt pulley 11 comprises an overload clutch such as a slipping clutch. .An
emergency shut off
is preferably also included.
Both shear rollers are made of hardened steel as a hollow cylinder of
approximately S00 mm
to 2000 mm in length and 200 mm to 800 mm in diameter. The shear roller
surfaces comprise
parallel cutting edges oriented at angles between 0° to 45 ° to
the shear roller axis, seen in
Figs. 9 and 11. Cutting edges are machined on the exterior surface of the
cylinders.
The lower shear roller 3 rotates in the opposite direction to the upper shear
roller 2 at a
substantially lower speed, i.e. approximately 50 rpm to 150 rpm. In order to
reverse the
direction of rotation and to reduce the speed a chain sprocket drive is used.
The first portion
of the drive comprises a V - belt pulley 14 affixed to the axis of the upper
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shear roller, a V - belt pulley 15 affixed to the axis of the lower shear
roller and a V -
belt 16. To reduce the speed the driving V - belt pulley 14 has a
substantially smaller
diameter than the driven V - belt pulley 15. The V - belt 16 is tightened
using the
secondary tensioning lever 17.
Referring to Fig. 7 the opposite side view of the device 100 is shown. The
second
portion of the drive comprises a sprocket 20 affixed to an axis driven by the
V - belt
pulley 15, a sprocket wheel 21 affixed to the axis of the lower shear roller
and two
supporting sprocket wheels 22 and 23. To further reduce the speed the driving
1 o sprocket wheel 20 has a substantially smaller diameter than the driven
sprocket wheel
21. In order to reverse the direction of rotation the chain 24 is driven by
the sprocket
wheel 20 on its inside and drives the sprocket wheel 21 on its outside. The
sprocket
wheel 22 ensures the contact between the chain 24 and a substantial part of
the
circumference of the sprocket wheel 21, whereas the sprocket wheel 23 keeps
the
lower portion of the chain 24 from contacting the upper portion.
There are numerous other. embodiments for driving the two shear rollers in
opposite
directions and at different speeds such as: a chain drive directly driven by
the electric
motor; two mating gears; two smaller electric motors each driving one shear
roller
2o using a V - belt drive; two smaller electric motors each driving directly
one shear
roller axis using a jaw clutch; or other arrangements obvious to persons of
skill in the
art.
Two shear rollers having a substantially different diameter may be used to
ensure the
different relative peripheral velocities of the shear rollers driven with the
same
rotational speed. Because the relation of the two diameters is directly
proportional to
the relation of the two surface speeds needed for the shear action this
embodiment is
limited by the feasibility of the combination of shear rollers with large
differences in
diameter.
Referring to Fig. 8 a plan view showing the top of the device 100 is shown.
The feed
table S is angled downwardly ending at the lower shear roller 3, which then
transports
the straw to the shear roller 2 for splitting. Seen more clearly in Figs. 6
and 7, the feed
table 5 is supported by a linkage 18 to the support atrn 6 to follow the lower
shear
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CA 02296554 1999-12-10
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roller 3 through all height adjustments. Seen more clearly in Fig. 9, the feed
table 5 is
directed towards the surface of the shear roller 3 ending very close to it for
depositing
the straw on the shear roller surface. The two shear rollers 2 and 3 are
driven by the
electric motor 4 using the V - belt drives on the one side of the device 100
and the
5 chain drive on the other side. The cutting edges of the upper shear roller 2
are
sharpened as required by holding a grind stone 25 to the surface of the upper
shear
roller 2 as it rotates in a direction opposite to which it is used. The grind
stone 25 is
advanced longitudinally using the adjustment mechanism 30 to contact and
sharpen
the cutting edges 41 along the full length of the shear roller 2. Sharpening
of the lower
~ o shear roller is achieved by moving the sharpening assembly Z5, 30 to the
opposite
side of the roller 2 on the underside of the supporting bench 1.
In front of the upper shear roller 2 a turbulence control mechanism comprising
a ledge
26 having the length of the shear roller is affixed to the supporting bench 1.
This
15 ledge 26 assists in preventing the straw from being unduly blown about.
Fig. 9 shows a detailed view of the surface structure of the two shear rollers
2 and 3
rotating in opposite direction at different speeds. The straw is fed generally
parallel to
the axes of the shear rollers using the feed table 5. The area where the two
shear
2o rollers are closest together is enlarged to show the surfaces in detail.
The clearance 40
between the two shear rollers is approximately 0.1 mm to 0.3 mm. Both shear
rollers
have parallel grooves 46 cut in their surfaces. These grooves have a
triangular shape
comprising a cutting edge 41 normal to the surface of the shear rollers
whereas the
opposite side 42 is at an angle of 45° to the surface of the plateau
ridge 43. The
25 groove spacing 44 is about 1.5 mm and the groove depth 45 is approximately
0.5 mm
to 1.5 mm. The groove spacing 44 and the groove depth 45 are dimensioned such
that
they are smaller than an unsplit straw to ensure that substantially all the
straw is split.
The grooves 46 on shear roller 2 are arranged at an angle to the grooves 46 on
shear
roller 3. Numerous different shapes of the grooves may be envisaged such as
the
30 opposite side 42 of the cutting edge 41 being at an angle to the surface
other than 45°
or being curved. Alternatively the cutting edge 41 may have a different angle
to the
surface or be curved. The various shapes may also be combined differently for
the
two shear rollers. The cutting edge 41 of the upper shear roller 2 faces in
the direction
of the rotation, indicated by arrow A, and moves at about ten times the speed
of the .
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CA 02296554 1999-12-10
WO 99104943 PCT/CA98100700
cutting edge 41 of the lower shear roller 3 which faces against the direction
of the
rotation, indicated by arrow B, of the lower shear roller 3.
Fig. 10 shows an unsplit straw 50 after being fed on the lower shear roller
and being
transported towards the opening 53 between the two shear rollers. The lower
portion
of the straw is sitting in a groove 46 of the lower shear roller 3, while the
upper
portion is caught by the cutting edge 41 of the upper shear roller 2. Due to
the
different orientation of the cutting edges and the different speed of the
shear rollers
the straw 50 is caught by the two cutting edges 41a and 41b. Consequently an
upper
to portion 51 of the straw is cut off by the cutting edge 41a due to the shear
action
between the two cutting edges 41a and 41b. The remaining part of the straw 50
is
further transported towards the opening 53 and is then caught by the cutting
edge 41c.
When the lower portion 52 of the straw 50 is cut off the remaining part of the
straw
50 is then caught by the cutting edge 41d. This process is repeated until the
straw 50
t 5 has passed through the opening 53 between the two shear rollers 2 and 3.
The split
portions of the straw are transported through the opening 53 and then
released.
Fig. 11 shows the orientation of the parallel grooves on the surface of the
shear
rollers. The grooves are oriented between 0° and 45 ° to the
shear roller axis. Having
2o a different orientation of the cutting edges for the lower 3 and the upper
shear roller 2
ensures a scissor-like action to split the straw longitudinally.
Advantageously, this
provides long fiber pieces. Fig. 1 I shows cutting edges parallel to the
roller axis for
the lower shear roller 3 to transport the straw 50 and cutting edges at an
angle of 45°
to the roller axis for the upper shear roller 2 to ensure a scissor-like
longitudinal
z5 cutting. Grooves 46 parallel to the lower shear roller axis 3 allow the
straw 50 which
is generally aligned to the shear roller axes to be transported in the grooves
46 of the
lower shear roller 3 without losing their orientation. The straw is arranged
for cutting
supported on its whole length by the cutting edge 41b. The straw 50 is then
split by
the cutting edges of the upper shear roller 2. Less force is needed for
cutting the straw
30 50 if the cutting edge is at an angle to the shear roller axis. Cutting
edges at an angle
of 45° ensure splitting of the straw 50 into long fiber pieces while
needing less force
which translates into less power needed to drive the shear roller 3. Various
different
orientations of the grooves may be envisaged such as both shear rollers having
grooves at an angle of 45 ° or any combination of angles between
0° to 45 ° on each
it
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CA 02296554 1999-12-10
WO 99104943 PCTICA98I00700
of the rollers. A preferred combined angle is 45 ° on the top roller
and 30 ° on the
bottom roller.
Referring to Fig. 12 a side view of a split straw orienter 200 according to
the
5 invention is shown. For making a panel, board or beam according to the
invention the
split straw strands must be aligned prior to pressing. The randomly oriented
split
straw strands 203 are deposited onto a board 201 having a corrugated surface.
The
board is vibrated transversely. Due to the vibration the split straw strands
are
substantially aligned accumulating at the bottom of the grooves. Tilting the
board
l0 ensures the movement of the split straw strands while being processed.
The split straw orienter 200 as shown in Fig. 12 comprises a board 201 having
a
corrugated surface and being tilted at an angle of approximately 10° to
45° . The
board 201 is sufficiently long to assure proper alignment of the split straw
strands,
15 approximately 1500 mm to 4000 mm. Raised lateral edges or walls 202 contain
the
split straw within the device 200 while being processed. The randomly oriented
split
straw strands 203 are deposited onto the board 201 at the elevated end. The
split straw
orienter 200 is vibrated transversely. The transverse vibration may be
realized using
an electric motor and an eccentric. The aligned split straw strands 204 leave
the
20 device 200 at the lower end and may be fed on a transport belt or other
means to
maintain the alignment.
Fig. 13 shows a cross sectional view of the split straw orienter 200. The
board 201
comprises a corrugated surface of a sine like shape having a distance 205
between two
25 consecutive ridges of approximately 25 mm to 100 mrn and a depth 206 of the
grooves of approximately 20 mm to 100 mm. Alternatively, different shapes of
the
corrugated-like surface may be envisaged such as a triangular shape or
spiked/upright
walls. Affixed to the board 201 are raised lateral edges or walls 202 to
contain the
split straw strands within the split straw orienter 200 during the process of
aligning
30 the split straw strands.
Referring to Fig. 14 a plan view of the split straw orienter 200 is shown. The
randomly oriented split straw 203 is deposited onto the board 20I at the
elevated end.
Due to the transverse vibration of the corrugated surface and gravitational
action the
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split straw strands are accumulating in the grooves of the corrugated surface
being
aligned by the groove walls 207, seen in Fig. 13. The tilting of the board 201
ensures
the movement of the split straw strands during the aligning process to the
lower end
of the board 201 by gravitational action.
13
