Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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APPARATUS FOR BLENDING WOOD STRANDS WITH A LIQUID ~ESIN
The present invention relates to the produc-
tion of particleboard. More specifically the present
invention provides an apparatus for coating wood
strands with a liquid resin~
The term "wood strands" includes particles,
flakes, wafers, chips used in the production of a
particleboard such as waferboard. The wood strands are
coated with an adhesive resin and then formed into a
mat for compression and curing to produce a board.
The most common adhesive used in the prep~
aration of waferboard is phenol formaldehyde. Most
waferboard mills apply this adhesive in a powder resin
form by mixing the resin with the wafers in a rotary
drum blender. Phenol formaldehyde is also available in
liquid form which is less expensive than the powder.
Considerable savings may be achieved by waferboard
manufacturers with liquid resin on an equivalent resin
solids basis while still ensuring that the adhesive
coating is applied evenly on wafer surfaces.
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The main problem with using liquid resin to-
day is that when low levels of application, i.e. 2% to
4%, are attempted for economic reasons, the drum type
blender does not provide even distribution of the resin
on the wafer surfaces. Thus, some surfaces or portions
of some surfaces are left uncoated. The result of this
uneven resin distribution is that the resulting par-
ticleboard does not achieve its maximum strength
potential. Attempts have been made to overcome this
uneven distribution of the liquid resin by increasing
the number of spray nozzles in a blender, or by using a
rotating disc to apply the resin instead of spray
nozzles. However, none of these approaches have been
effective to date in attaining even distribution of the
resin on the wafer at low enough resin levels to be
cost-effective. A disadvantage of the use of multiple
spray nozzles is that invariably one or more become
blocked and are often inaccessible for maintenance
purposes.
The drum type blenders are not suited for
continuous operation with liquid resin because the
flights that create the curtain of wood strands within
the drum tend to become clogged with a buildup of
wafers and resin. If these flights in the drum are not
able to perform their function, then the curtain of
wafers becomes too thick which results in some of the
wafers being screened from the resin spray. Further-
more in drum type blenders, the spray nozzles are
invariably placed in the curtain of wood strands which
contributes to the blocking of the nozzles.
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The present invention overcomes the problems
of liquid resin blending described above by providing a
multiple pass blender including a series of falling
curtains of separated wood strands and providing spray
nozzles away from the falling curtains, to spray liquid
resin droplets from both sides of the curtains. The
sprays extend through the curtain but not far beyond
it and mixing of the wafers between each stage may be
provided to assure random distribution of wafer sur-
faces exposed to the liquid resin spray. This multiple
pass operation with remixing between passes, ensures
the coating of sufficient wood strands to give good
particleboard strength properties with liquid resin.
The horizontal cross-section of the falling
curtain of strands has "length" which is the largest
distance along the boundaries of the curtain, and
"width" which is the distance between the two boundar-
ies of the curtain.
The multiple pass operation may be used with
the addition of a wax coating sprayed onto the wood
strands. A combination of liquid and powder resin may
be applied to the wood strands, with a mixing of the
strands between each falling curtain. Furthermore, the
multiple pass operation permits two or more types of
resin, chemical additives such as wax, fire retardants,
preservatives and the like, to be sprayed onto the wood
strands.
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The present invention provides an apparatus
for continuously blending wood strands with a liquid
resin comprising means for forming a substantially
constant flow of wood strands into a first falling
curtain of separated wood strands, the first curtain
having a predetermined substantially constant width;
first liquid spraying means including at least one
spray nozzle located on each side of the first falling
curtain of separated wood strands away from the first
falling curtain, adapted to spray liquid resin droplets
for the width of the first curtain, but not substanti-
ally beyond; collector means for collecting the first
sprayed wood stands and means for forming the first
sprayed wood strands into a second falling curtain of
separated wood strands, the second curtain having a
predetermined width; and second liquid spraying means
including at least one spray nozzle located on each
side of the second falling curtain of separated wood
strands away from the second falling curtain, adapted
to spray liquid resin droplets for the width of the
second curtain but not substantially beyond.
In embodiments of this invention additional
means of forming the wood strands into further falling
curtains are provided with additional liquid spraying
means to spray either side of the further curtains for
the width of curtain but not substantially beyond. In
another embodiment the spray nozzle on one side of the
curtain may be placed at a different elevation to the
spray nozzle on the other side of the curtain. In
still a further embodiment the initially sprayed wood
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strands may be mixed together before being formed into
the next falling curtain.
The present invention also provides a process
for continuously blending wood strands with a liquid
resin, comprising the steps of advancing a substanti-
ally constant flow of wood strands to form a first
falling curtain of separated wood strands, the first
curtain having a predetermined substantially constant
width; spraying liquid resin droplets on each side of
the first curtain, the spraying extending from both
sides of the width of the first curtain, but not sub-
stantially beyond, collecting the first sprayed wood
strands and advancing the first sprayed wood strands
to form a second falling curtain of separated wood
strands, the second curtain having a predetermined
substantially constant width; and spraying liquid resin
droplets on each side of the second curtain, the spray-
ing extending from both sides of the width of the
second curtain, but not substantially beyond.
In a further embodiment of this process at
least one further falling curtain of separated wood
strands is provided, and liquid resin droplets are
sprayed on both sides of the further curtain for the
width of the curtain but not substantially beyond. In
another embodiment the wood strands are mixed between
each spraying step. Phenol formaldehyde is disclosed
as a preferred liquid resin and the number of spraying
steps is preferably sufficient to minimize the proba-
bility that two uncoated surfaces are adjacent to each
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other in the particleboard and thus reduce the internal
bond strength.
In drawings which illustrate the embodiments
of the invention
- Figure 1 is an elevational view of one embodiment of
the multiple pass bender according to the present
invention.
- Figure 2 is a partial elevational view showing one
embodiment of an interior of a curtain forming and
spraying system for use in a multiple pass blender.
Referring now to Figure 1, a three pass
blender is illustrated. Wood strands are fed through
an entrance 10 to form a pile 11 of strands resting on
an apron belt conveyor 12. The level of the pile 11 is
controlled by a rake back conveyor 13. A number of
spike rolls 14 are arranged in a substantially vertical
plane with slight slope back towards the rake back
conveyor 13 to pick individual strands from the pile 11
and deposit them into a first falling curtain 16 of
separated wood strands. The rotational speed of the
spike rolls 14 and the speed of the apron belt conveyor
12 may be individually varied to control the flow of
strands to the first curtain 16. The rotational speed
of spike rolls 14 also controls the width of the cur-
tain 16 of strands which fall in front of the spray
nozzles 17, at least one on each side of the curtain
16. Whereas two nozzles are illustrated one on each
side it will be understood that there may be three or
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four nozzles along the length of the curtain depending
on the overall length of the curtain of strands. A
common length of curtain 16 is four feet, and it is
found that two nozzles 17 on each side are satisfactory
for this distance. The nozzles 17 produce a spray of
liquid resin particles which are directed through to
the other side of the curtain 16 but not far beyond the
curtain and certainly not as far as the housing. Thus
a cloud of liquid resin droplets is produced through
which the curtain 16 of wood strands fall. There is
little or no wastage of liquid resin as it all settles
on the falling strands.
As illustrated in dotted lines in Figure 1, a
baffle or deflector 19 is positioned beneath the first
curtain 16 so that the wood strands which have been
sprayed once with the liquid resin are mixed. The mix-
ing action deflects the outside wood strands in towards
the center, and deflects the strands in the center to-
wards the outside. The first sprayed strands are then
deposited on a belt conveyor 20 which delivers them to
a spike roll 21 for picking individual strands off the
belt conveyor 20 and forms them into a second curtain
22 of separated wood strands. The width of the second
curtain 22 may be controlled by the rotational speed of
the spike roll 21. Spray nozzles 23 on each side of
the second curtain 22 spray liquid resin droplets onto
the second curtain and through the curtain but not far
beyond and certainly not so far as the housing. The
second curtain 22 falls through a cloud of liquid resin
droplets as in the first spray curtain 16 and then onto
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a second baffle or deflector 24 to mix the wood strands
before depositing them onto another conveyor belt 25.
This conveyor belt 25 is similar to the first conveyor
belt 20 and has a spike roll 26 positioned at the end
to pick up the strands off the belt 25 and form a third
curtain 27 of separated wood strands. The width of this
third curtain is controlled by the speed of ~he spike
roll 26. The third curtain 27 falls past two spray
nozzles 28 which provide a cloud of resin droplets as
in the first and second spray curtains. The wood
strands are deposited onto an output conveyor 29 which
conveys them to the next step in the preparation of a
particleboard.
Another embodiment of a curtain spray system
is illustrated in Figure 2 wherein an apron feed belt
12 feeds a pile of wood strands 11 towards a number of
spike rolls 14 which pick out individual strands to
form a first curtain 16 of separated wood stranas to
drop past a first spray nozzle 40 mounted on the far
side of housing 41 which sprays a cone shaped cloud 42
of liquid resin droplets and then past a second spray
nozzle 43 located below the first spray nozzle 40 and
on the near side of the housing 41 which sprays a sec-
ond cone shaped cloud 44. Both cone shaped clouds of
liquid resin droplets extend almost as far as the oppo-
site wall of housing 41 but do not reach the housing
wall thus the liquid resin droplets are sprayed onto
the wood strands passing in the curtain 16 or fall with
the wood strands. Little or no resin is deposited on
the sides of the housing 41. The wood strands in the
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curtain drop onto the conveyor 45 which passes to a
second spray curtain or in the case of the last spray
curtain to a further processing step. ~ baffle or
deflector is not illustrated in this embodiment, but
may be included as shown in Figure 1.
By using a multiple pass blending system the
exposure time of the strands to the liquid resin is
increased and thus the resin distribution on the wood
strands is improved. Whereas a multiple pass blending
apparatus is disclosed in the drawing, in certain
instances it is feasible to recirculate the strands
more than once through a single spray curtain system.
It is preferred to mix the strands between the spraying
stages to ensure that the strands do not fall in the
same pattern from one pass through the spray curtain to
the next.
The liquid resin may be applied using an air
spray system, an airless system, or a rotating disc
system; it being important that the resin does not
spray onto the far wall of the housing surrounding the
curtain. The control of the resin spray is at least
partially achieved by controlling the density of the
curtain. The wafer flow rate which is controlled by
the speed of the apron belt conveyor 12, and the width
of the curtain which is controlled by the speed of the
spike rolls 14, prevent the resin spraying on the far
wall of the housing.
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To test the concept of the multiple pass
blender a factorial experiment was designed based on
the number of passes made by the wood strands through
the blender. A preferred coverage of the surface area
of the wood strands results when the probability of two
uncoated surfaces being adjacent to each other in the
particleboard was minimized. Practical evidence that
this minimum had been achieved was demonstrated when
three or four passes produced a high internal bond.
Further passes do not result in significant increases
in this strength parameter.
For the preparation of waferboard, a 22 inch
wide experimental blender was constructed to produce a
wood wafer flow rate of 90 lb/min. An airless spray
system was mounted on either side of a wafer collection
box. Samples of wood wafers were passed through the
liquid resin blender and waferboards were produced for
conditions with one, two, three and four passes. The
adhesive resin application of 2.5% at 45% resin solids
was applied. For the four pass run, the resin was di-
luted to 35% resin solids in order to have a sufficent
quantity of liquid to spray onto the wood strands.
Boards were produced for each condition and the inter-
nal bond was determined for each of the waferboards.
The results of two separate trials in Table 1 show the
effect of the number of passes on the internal bond
with a wafer flow rate of 90 lb/min.
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TABLE 1
No. of Internal Bond, psi*
Passes Trial 1 Trial 2
1 31-5 36.8
2 49.6 59.8
51.2 58.1
3 56.8 59.8
4 66.5
57.3
60.1 63.5
*Adjusted to an average specific gravity of 0.700.
Variation of the apron belt speed affects the
wafer flow rate and controls the penetration of resin
into the curtain. For example, if the wafer flow rate
is too heavy causing a dense curtain, strands are
screened from the resin spray. If the curtain is too
light, resin passes between the strands hitting the
wall of the collection box.
A series of trials at different wafer flow
rates demonstrated the effect of curtain density on
blender efficiency. At the wafer flow rate of 90 lb/
min, the resin was used as supplied at 45% resin solids
and applied a resin content of 2.5%, while at lower
wafer rates the resin was first diluted to 35% resin
solids before being used. The results in Table 2 show
the effect of wafer flow rate on the internal bond.
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TABLE 2
Wafer Flow Rate No. of Internal Bond
lb/min Passes _ psi*
2 58.9
57.2
2 61.2
2 53.9
53.1
3 61~3
54.4
55.2
53.9
3 65.3
64.6
65.5
71.7
3 72.5
*Adjusted to an average specific gravity of 0.693.
The multiple pass blender may be used with a
mixture of liquid resin and powder resin. Furthermore
wax or other additives may be sprayed onto the curtain
of falling strands, either in a separate spray curtain
or at one or more of the spray curtains where liquid
resin is applied. Different types of liquid resin may
be applied in multiple stages. A two component liquid
resin, for example, can be applied at two separate
stages.
Various changes may be made to the scope of
the present invention which is limited only by the
following claims.
