Note: Descriptions are shown in the official language in which they were submitted.
2~3737~
METHOD FOR THE PRODUCTION OF FIBERBOARDS
This invention relates to a method for the production of fiberboards made from pieces of
wooden particles, and a polyurethane glue used as a binder in an amount of less than 10
percent by weight, in which the wooden particles are heated by pressurized steam and thus
refined into fibers, the fibers being subjected to a blow line, being dried in a drying device,
and then formed in the shape of a mat and pressed to form the fiberboards. The invention
also provides a fiberboard made from pieces of wooden particles refined to fibers with a
polyurethane glue added as a binder in an amount of less than 10 percent by weight. The
new fiberboard may be used in the production of furniture, as plates for wall panelling, or as
covering plates in rooms having high humidity.
A method for the production of fiberboards containing the steps described above is known
from EP-B-92 699. The wooden particles are treated with steam under pressure and refined
in a refiner into hot and wet fibers, the fibers subsequently being placed in a blowing device
containing a blow line. The hot and wet fibers are partly separated from the water by
condensing out a part of the steam within a cyclone in oredr not to pass all of the steam
through the drying device. The fibers are dried and formed into the shape of a mat. The mat
is pressed under the action of heat and pressure into the desired fiberboard. An organic
polyisocyanate is used as a binder and sprayed onto the hot and wet fibers, the organic
polyisocyanate used being a polyurethane glue. The polyisocyanate preferably comprises a
mixture of diphenylmethane-4,4'-diisocyanate and polymethylene polyphenyl
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polyisocyanates. The improvement in this patent comprises the addition of the binder in the
blow line with the result of a good distribution. Advantageously the pressed fiberboard does
not show spots of glue. The high amount of the polyisocyanate binder being in the range of
about 1 to 10 percent by weight, preferably in the range of 3 to 6 percent by weight, is a
disadvantage. According to the examples described therein, polyisocyanate binder in an
amount of 4 percent by weight is recommended, so it is clear that f1berboards with useful
technical properties cannot be produced with a binder of about 1 percent by weight. A
relatively high amount of binder is necessary because part of the binder cannot be used for
binding purposes because a preliminary reaction occurs in the pressing station. The relatively
high total bulk density of about 850 kg/m3 is a further disadvantage. The resulting fiberboards
are heavy, not easy to handle, made costly in production due to the high amount of binder
used, costly in the delivery due to the resulting weight of the fiberboards, especially in the
production of furniture where hinges and other fittings must be constructed as heavy duty
f1ttings adapted to carry the weight of parts of the furniture being made out of the
fiberboards.
A method for the production of fiberboards made from pieces of wooden particles and
isocyanate as a binder is described in DE-A- 41 22 842. The fibers are conveyed through a
blow line and under expansion into a drying device and subsequently dried therein. The
isocyanate is sprayed onto the fibers after leaving the blow line and prior to the drying. This
also gives a good distribution of the binder and the pressed fiberboard does not show spots
of glue. The amount of binder used is not mentioned, however, but it is known to be in the
amount between about 4 and 6 percent by weight. Thus the amount of binder being 1 to 10
percent by weight as described in EP-B- 92 699 is reduced to an amount of about 4 percent
by weight, this being useful for production of the fiberboard. In addition to the the
disadvantage of the high amount of binder used, the fiberboards produced according to DE-
A- 41 22 842 also have the disadvantage of having a high bulk density of about 770 kg/m3
to 800 kg/m3.
2137~71
From the pamphlet titled "GoldenEdge Liteboard (600 kg/m3), Manufactured at Nelson Pine
Industries Ltd New Zealand", distributed at the 23rd International Symposium on
Particleboard, in Pullman, Washington in 1989, it is known to manufacture light medium
dense fiberboards (MDF) containing urea formaldehyde resin as a binder. These light
fiberboards have a total bulk density of about 600 kg/m3 and are manufactured incontinuously running pressing devices. Thus, due to the low weight of the fiberboard, the
fittings needed in ~e production of furniture may be constructed accordingly smaller. The
weight of these fiberboards is 20 percent less than the weight of normally processed medium
dense fiberboards (MDF), and is about equal to the weight of particleboard. These
fiberboards have a good surface and there are no difficulties in applying a veneer, a coating,
or a lacquering to these fiberboards. Due to the reduced weight of these fiberboards, they
may be handled easier and thus the costs for delivery are lower. These fiberboards are
produced in a manner so as to create a profile of the bulk density over the thickness of the
board, having a bulk density in the outer layers of about 1100 kg/m3, and a bulk density of
about 530 kg/m3 in the middle layer. A super light fiberboard with a total bulk density of 460
kg/m3 is described, where the outer layers have a bulk density of about 1000 kg/m3. The
internal bond is 0.45 N/mm2, the module of rupture is 20 N/mm2, and the module of elasticity
is1600 N/mm2. The minimum properties of British Standards for fiberboards (BS 1142/1989)
are not reached. These light and super light fiberboards have the disadvantage of containing
a binder of urea formaldehyde resin in a high amount, amounts of 10 to 15 percent by
weight, necessary to guarantee mechanical properties which are acceptable. But there still are
the known disadvantages associated with the use of urea formaldehyde resin.
In Deppe/Ernst "Taschenbuch der Spanplattentechnik", 3. Auflage, 1991, pages 402 and 403,
fiberboards are described containing urea formaldehyde resin as a binder and having a bulk
density of about 680 kg/m3 to 840 kg/m3, the average is about 750 kg/m3. In
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\
picture 372 on page 402 the relationship between maximum bulk density in the outer layers
and the total bulk density for medium dense fiberboards with normal weight is shown . From
this it is understood that acceptable compression in the outer layers, resulting in good
properties in the surface of the outer layers, is reached only if the total bulk density is at least
about 750 kg/m3. According to the thinking of specialists in the furniture industry, the
properties in the surface of furniture pieces can be reached only if the total bulk density is
chosen as high as possible, and, of course, higher than 750 kg/m3. The higher the total bulk
density the better the properties in the surfaces.
It is the object of this invention to provide a method for the production of fiberboards of the
described kind where the f1berboards, which are a made of natural raw material, do not have
the disadvantages of natural wood, especially the disadvantages of inhomogenities of
anisotropy, knots, and so on. Grown wood has a bulk density of about 450 kg/m3 to 550
kg/m3.
The object of the invention is attained in a method with the steps described herein, wherein
the fibers are applied in an amount which results in the bulk density of the whole fiberboard
being approximately 550 kg/m3 to 650 kg/m3, where the binder is applied in an amount of 0.5
to 4.0 percent by weight, preferably in the range of 1.0 to 3.0 percent by weight, with respect
to dry wooden particles, and where the steam generated during heating is passed through the
drying device without having a portion of the steam separated out, the steam being used with
the binder to form a coating layer of polycarbamide to prevent the fibers from forming a
preliminary reaction with the binder.
Thus the bulk density of natural wood is nearly reached, but the fiberboards are substantially
lighter (20 %) than known medium dense fiberboards (MDF). Surprisingly, with this new
method it is possible to manufacture lightweight fiberboards which nonetheless have good
mechanical properties and good
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surface properties also. The intemal bond is acceptable. The fiberboards may be lacquered,
veneered, or coated. The may be treated by milling in the area of their rims and thus are
ideal for the production of fumiture. Due to the decreased arnount of.wood particles used the
manufacturing costs of the fiberboard are decreased and the manufacturing time is shorter,
especially the time needed for pressing. The reduced weight results in a better handling, in
a lower stress to fittings used therewith, in reduced costs for cargo handling, and in reduced
wear of the tools used during production of the fiberboard.
Surprisingly, and contrary to the thinking of experts, such lightweight fiberboards with the
properties described may be manufactured by using low amounts polyurethane glue as a
binder. The polyurethane glue may be an organic polyisocyanate alone, or used in a mixture
with polyoles or polyamines. As far as the method for the production of fiberboards is
concerned, it is the essential step of the invention, in spite of the high temperatures used, to
prevent a preliminary reaction of the binder and that even with the use of low amounts of
binder enough active binder is available while pressing the particles together. While up to
now the steam at the exit of the blow line has been separated for reasons of saving energy,
according to this invention the steam is maintained and used to create a covering layer out
of polycarbamide for the drops of the binder. There is a reaction in the boundary surface
between the hot drops of steam and the drops of the binder resulting in a covering layer of
polacarbamide protecting the drops of the binder. This covering layer around the drops of the
binder inhibits a prelimin~ry reaction of the binder. The covering layer is destroyed in the
press by pressing action, and the prior included binder is thus active for binding action.
With this new method fiberboards may be manufactured which are substantially made of a
natural raw material, because by using only 2 % of binder, for example, the fiberboard
contains 98 % wood. In addition it is an advantage of the new fiberboards that they may be
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also milled in a third direction in the direction of the thickness of the fiberboard plates. This
is essential, for example, if kitchen fumiture is produced containing relieved shaped edges.
Due to the good intemal bond in the fiberboard, the plates of fiberboard and thus pieces of
furniture manufactured from the fiberboard may be provided with hinges and other fittings.
According to the prior art, in produced plates of fiberboard the arnounts of wood and of
binder have been combined in a contrary manner. To produce lightweight plates it was
unavoidable to increase the amount of binder to reach acceptable properties, especially for
lacquering and internal bond. So in the prior art a decrease of the total bulk density from
about 800 to 600 kg/m3 resulted in the need to increase the amount of urea formaldehyde
resin from 10 to 15 percent by weight.
It is especially advantageous if the polyurethane glue is sprayed onto the fibers after leaving
the blow line and before entering the drying device. Thus the binder is added in the
production line at as late a time as possible. In addition, at the exit of the blow line the
binder and the steam are in the shape of drops in a fine distribution making it easy to
generate the protecting covering layer.
Pressure, temperature, and time during the production of the fiberboards are varied so that a
profile of the bulk density over the thickness of the fiberboard results in which the outer
layers have a bulk density of 750 kg/m3 to 950 kg/m3, and the middle layer has a bulk
density of 550 kg/m3 to 600 kg/m3. In spite of the low bulk density in the outer layers,
acceptable properties for veneering, lacquering, or coating are reached. The total bulk density,
even with thick outer layers, is not increased too much.
37371.
The fiberboard has a total bulk density of 550 kg/m3 to 650 kg/m3, and contains binder in an
amount of about 0.5 to 4.0 percent by weight, especially in the range of 1.0 to 3.0 percent by
weight. The new fiberboard plate approximates natural wood without having its
disadvantages. The total bulk density is slightly higher than that of grown wood, but the use
of such a low amount of binder is surprising. The high amount of wooden particles used in
the fiberboard is the reason to call this product a natural raw material. The further advantages
are the same as discussed in connection with the inventive method for production.
Each outer layer of the fiberboard has a thickness of about 10 percent of the total thickness
of the fiberboard, whereby a lean prof1le of the bulk density is formed. The middle layer,
with 80 percent of the board thickness, is relatively large. It is self explanatory, especially in
continuous production, that there is a transitional region between the middle layer and the
outer layers.
The fiberboard has a bulk density in the outer layers of more than 900 kg/m3, and a total
bulk density of less than 650 kg/m3. Such a fiberboard has a high compression within the
outer layer, which even can be increased by special steps during production. The fiberboard
is well suited to take a lacquer due to its dense surface.
The invention is described in further detail in the following table:
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Composition I II m IV V VI
Amount of binder, % 0.6 0.9 1.8 2.3 3.0 4.0
(PMDI)
waterproofing 2 2 1 1.5 - 0.5
agent, %
f1bers % 97.4 97.1 97.2 96.2 97.0 95.5
Properties I II m IV V VI
bulk density, kg/m3 634 603 610 581 599 603
internal bond, N/mm2 0.65 0.69 0.70 0.68 0.71 0.71
module of rupture, N/mm2 19.5 20.9 21.8 20.5 22.3 22.8
swelling (2h) % 4.0 3.5 3.3 3 3 3.2 3.2
The composition and the resulting properties are illustrated for six examples. The amount of
binder is varied between 0.6 to 4.0 percent, the examples in the middle showing the region
from 1.0 to 3.0 percent by weight. The densities are in the range from 550 to 650 kg/m3.
Low amounts of binder have been combined with high densities. The aim is to reach an
acceptable internal bond. The internal bond in the range of 0.7 N/mm2 is relatively high in
spite of the low amount of binder used. Thus standards are fulfilled and this is particularly
important for the use of hinges and other fittings in the production of furniture. The modules
of rupture, as known from lightweight fiberboards, are relatively low, but this is of no great
importance for the furniture industry.
In the following table the difference between the fiberboard of the invention and two
fiberboards of the prior art is illustrated. In the first column a plate of medium dense
fiberboard with urea formaldehyde resin is shown. The amount of binder is 10 percent by
weight, and the bulk density is 750 kg/m3. In the second column a plate of the prior art as
discussed in the
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beginning of the description is illustrated, being a lightweight fiberboard with urea
formaldehyde resin as a binder, with a bulk density of 600 kg/m3, and where the amount of
binder is 15 percent by weight.The internal bond is lower than for normal medium dense
fiberboards, but is still acceptable. A plate made according to the method of this invention
is shown in the third column. The essential difference of the subject of the invention is the
unexpected low amount of binder connected with a decreased density compared with normal
medium dense fiberboards. The new fiberboards are lightweight, due to the low arnount of
binder, low in costs, but nevertheless have a good internal bond and good mechanical
properties.
medium dense Golden invention
fiberboards Edge ureaformaldehyde
(MDF) 600 resin
amount of binder, % 10 15 2
bulk density, kg/m3 750 600 600
internal bond, N/mm2 0.80 0.70 0.70
