Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF MANUFACTURING LIGHT COLORED COMPOSITE
PANELS WITH PHENOL FORMALDEHYDE BINDERS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for treating furnish mat in
composite panel
manufacturing, such as in the manufacture of particleboard, fiberboard,
oriented strandboard and
the like and compositions suitable for the practice of the process. More
particularly, prior to
pressing the furnish mat, which has been previously blended with phenol
formaldehyde resins,
into panels, the process uses a buffer or weak acid treatment to prevent the
darkening of the
panel surfaces which generally occurs during and after resin curing in a hot
press.
2. Description of Related Technology
Phenol-formaldehyde resins are widely used as adhesives and binders in many
products,
including wood panel products such as plywood, particleboard, fiberboard,
hardboard and
oriented strandboard. Other adhesives like urea formaldehyde, melamine
formaldehyde,
melamine urea formaldehyde, and diisocyanate binders are also used in wood
panel industry.
However, phenolic resoles, either in liquid or powder form, have been the
predominant adhesives
for wood panel products for exterior application.
In general, the manufacturing process of particleboard, fiberboard, and
oriented
strandboard include furnish preparation, furnish drying, resin blending, mat
formation, hot
pressing, and post pressing treatment.
In the furnish preparation, wood and/or other lignocellulosic materials are
reduced to
either fibers or flakes or chips or strands of desired dimensions. The furnish
then passes through
a drier to remove excessive moisture. Undersized and/or oversized particles
are removed either
before or after the drying process. The dried furnish is blended with an
adhesive in blenders for
particleboard and oriented strandboard, or in blowlines for dry process
fiberboard. The resinated
furnish is formed into mats with desired orientations for orient strandboard,
or with desired
particle size differentials between core and surface layers for particleboard.
The mats are
conveyed to and pressed in a multi-opening press or a continuous press at
elevated temperature
and pressure to make the panels. The pressed panels are trimmed, cut to size,
and may also be
hot-stacked, sanded, conditioned, or edge sealed, and finally labeled and
packaged for shipment.
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When a phenolic resole adhesive, which is referred to PF resin hereafter, is
employed in
wood panel manufacturing, the top and bottom surfaces of pressed panels tend
to have medium-
brown or dark-brown color, which is undesirable for certain applications of
these panels. Urea
formaldehyde resins are capable of producing light colored wood panels, but
only of interior
grade. Melamine urea formaldehyde resins and diisocyanate binders also produce
light colored
panels of exterior grade, however at much higher production costs compared
with using regular
liquid or powder PF resins. Demands for light colored wood panels made from
regular PF resins
have therefore emerged from the panel industry.
3. Background of the Invention
PF resins bear reddish colors due to the absorption of visible light by
phenolates
generated in the presence of alkaline catalysts, most commonly sodium
hydroxide. The resin's
color is further darkened during the resin synthesis and storage due to
oxidation of the PF resin.
Using other catalysts such as amines or lithium hydroxide may produce lighter
colored PF resins.
A more sophisticated method for preparation of light colored PF resins was
disclosed in Fl
patent application 961331, in which a lighter resin color has been achieved by
conducting resin
synthesis, storage, and application under inert atmosphere such as nitrogen.
Even made with a light colored PF resin, the color of panel surfaces still
turns to medium
brown or dark-brown, and in some cases purplish brown after hot pressing.
Adding anti-oxidants
to a PF resins may slightly lighten the color of finished panels but not to
the desired extent in
most cases, and a high cost and/or incompatibility with the resin are often
associated with
addition of such strong anti-oxidants.
The most effective modification of regular PF resins to achieve light colored
panels is to
reduce the pH of the resin to 9.0 or below. By doing so two difficulties
inevitably arise:
insufficient curing speed and poor stability of the resin. A significantly
reduced amount of the
catalyst or the use of a weaker catalyst is required for low pH PF resins,
which often reduces the
curing speed of the resin so much that the resin is no longer suitable for
today's fast press cycles
in most panel mills. The viscosity of such low pH resins will increase rapidly
at ambient
temperature, causing problems in storage and transportation. To compensate for
the lost curing
speed due to low pH, the resin has to be cooked further to a markedly higher
molecular weight.
The trouble with conventional solution type of low pH PF resins is that the
resin precipitates
before reaching the desired molecular weight advancement.
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An answer to the precipitation of highly advanced low pH PF resins is to
convert the
resin from solution type to emulsion or suspension form. A method for
manufacturing light
colored inorganic fiber panels was disclosed in 1993 (JP 95 18158) that
described the synthesis
and application of such emulsion PF resin. Another two patents filed in 1999
(Fl 990 674 and Fl
990 675) described the synthesis and production of emulsion PF resins and
powder PF resins
spray dried from the emulsion resins, respectively. These two patents claimed
that the emulsion
PF resins and powders resulted from emulsion PF resins produced colorless or
light colored glue
line or panel surface when cured.
While emulsion PF resin technology provides a viable method of producing light
colored
panels, the complexity of emulsion production and the extra cost associated
with the significant
amount of emulsifier and/or protective colloids needed for emulsification
certainly reduces its
cost efficiency. The solid content of emulsion resins is usually significantly
lower than
conventional solution type of PF resins in order to keep the viscosity in a
workable range of 50 -
300 cps. The presence of emulsifiers and/or protective colloids reduces the
effective phenol
formaldehyde content in the resins and also tends to lower the yields of the
spray drying process
in the case of power resin. All of the above further increase the cost of
using emulsion PF resins
for panel manufacturing.
In general, regardless of the exact composition and method of application, PF
resins
produce oxidized phenolic polymers upon curing that give a dark color to the
finished panels.
Many consumers find the resulting dark appearance of some of composite panel
products,
particularly wood panels such as particleboard, fiberboard, and oriented
strandboard,
undesirable. The development of a simple and inexpensive method to produce
light colored
panels with traditional liquid or powder PF resins is therefore warranted.
SUMMARY OF THE INVENTION
The invention follows conventional practice in many respects concerning the
manufacture of composite panels, which utilize PF resins as the adhesive or
binder. The present
invention modifies conventional practice by treating the furnish, before or
after formation into
mats, with a buffer or acid system before the mat is hot pressed in a
conventional manner. This
treatment provides a low pH environment on the surfaces of the furnish mat,
which is to be hot
pressed to make panels, and generally prevents the oxidation of the resin that
darkens the color
of the finished panels during and after the hot press.
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The method in this invention utilizes the neutralization effects of the buffer
or acid system
on the PF resins existing on the outer surfaces of the furnish mat to provide
a low pH
environment, specifically to maintain the pH on the outer surfaces within the
range of 2.0 to 9.0,
preferably 6.0 to 8Ø This environment is believed to significantly slow down
the oxidation of
PF resins during and after hot pressing and virtually eliminated the
deprotonated oxidized phenol
species that are believed to be the main source of dark color of finished
panels.
The method of this invention reduces or eliminates the darkening of the color
of the
composite panels that occurs in methods that utilize phenolic resins as
adhesives or binders.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a process for the bonding of
lignocellulosic materials
and other natural or artificial fibers in the manufacture of composite panels
which utilize PF
resins as the adhesive or binder, where the use of a buffer or acid system
provides a low pH
environment for the phenolic resins, and therefore reduces or prevents color
darkening of the
panels during and/or after the hot press.
The said lignocellulosic materials include, but are not restricted to wood
strands, chips,
veneers, wafers, fibers, particles, saw dust, and mixtures thereof. The said
other natural and
artificial fibers include, but are not restricted to, fibers from agricultural
products or byproducts,
glass and mineral fibers, synthetic fibers, and mixtures thereof.
The said composite panels include wood panels and panels made from other
natural or
artificial fibers or mixtures thereof. The panels utilize PF resins as the
thermosetting adhesive
and the color of panel surfaces darkens during and/or after hot press. Wood
panels applicable to
this invention are oriented strandboard, particleboard, fiberboard, and the
like.
The said composite panels are manufactured by using a PF resin as either the
sole
adhesive or one component in a mixed adhesive system co-used with other
binders, which
include, but are not restricted to diisocyanate, urea formaldehyde resins,
melamine formaldehyde
resins, melamine urea formaldehyde resins, tannin resins.
The said PF resins are phenol formaldehyde resole resins with pH equal to or
greater than
9.0, either in liquid or powder form or a mixture of each, with or without
additives. The said
additives include, but are not restricted to substituted phenols and phenol
derivatives, organic or
inorganic curing accelerators, urea, and other amino compounds, lignin and
derivatives, organic
or inorganic fillers and extenders.
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The said pH is the pH of the resin deposited on the surfaces of furnishes,
regardless of
whether the resin is applied as one-component system or two-component system.
The said buffer and acid system can be either in liquid form or solid form.
The liquid
form is a buffer solution or acid solution; the solid form is either a solid
mixture of buffer
components that constitute a buffer solution when dissolved in water or a
solid acid, both
preferably exist in fine powders. Because larger amounts of reagents are
required for the powder
buffer or acid systems than corresponding solution systems, solution systems
are preferred.
The said acid system contains an organic or inorganic acid or acidic compounds
preferably a weak acid or acidic compound, or mixture thereof. The said acids
refer to organic or
inorganic proton acids, which are also called Bronsted acids. The said acidic
compounds refer to
organic or inorganic acidic salts, esters, anhydrides and compounds which
react with water to
release acids. The pH of said acid solution is between 2.0 and 9.0, preferably
between 6.0 and
8Ø The said acids and acidic compounds include, but are not restricted to,
hydrochloric acid,
sulfuric acid, sulfurous acid, substituted sulphonic acids like
phenolsulphonic acid and
naphthalenesulfonic acid, phosphoric acid, phosphorous acid, sulfamic acid,
boric acid,
tetraboric acid, carbonic acid, sillcic acids, formic acid, acetic acid and
substituted acetic acids,
acrylic acid, propionic acid, butyric acid, cinnamic acids, citric acid,
nicotinic acids, fumaric
acids, furancarboxylic acid, furoic acid, gallic acid, itaconic acid, lactic
acid, lutidinic acid,
malefic acid, malic acid, malonic acid, naphtholic acids, oxalic acid,
succinic acid, tataric acids,
toluic acids, uric acid, mono- and di-nitrophenols, ammonium chloride and
substituted amine
chlorides, ammonium sulfate and substituted amine sulfates, ammonium
phosphates and
substituted amine phosphates, acetyl acetate, propylene carbonate, triacetin,
acetic anhydride,
malefic anhydride, succinic anhydride, and the like.
The said buffer system is either an organic or inorganic buffer solution with
a pH range
of 4.5 - 9.0, preferably 6.0 - 8.0 or a powder mixture of compounds that
constitute a buffer
solution when dissolved in water. Suitable buffer systems include, but are not
restricted to,
sodium dihydrogen phosphate with sodium hydroxide or their potassium
equivalent,
tris(hydroxymethyl) aminomethane with hydrochloric acid, borax with
hydrochloric acid, borax
with boric acid, boric acid with sodium hydroxide, boric acid with potassium
hydroxide, citric
acid with its salts, potassium hydrogen phthalate with dipotassium phthalate,
sodium dihydrogen
phosphate with disodium phosphate, acetic acid with sodium acetate, benzoic
acid with sodium
benzoate, sodium hydrogen oxalate with disodium oxalate, propionic acid with
sodium
propionate, ammonia with hydrochloric acid, ammonia with ammonium chloride,
sodium
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hydrogen sulfite with disodium sulfite, sodium hydrogen carbonate with
disodium carbonate, and
the like.
Controlling the pH in the preferred range of 6.0 - 8.0 is important because of
the concern
about the corrosion of the equipment if lower pH systems are used. Therefore,
buffer or weak
acid systems are preferred.
The buffer or weak acid system can be used alone or in combination with other
furnish
mat surface treatment system such as, but not restricted to, water spray on
the mat surfaces in the
manufacturing processes of particleboard and oriented strandboard. The buffer
or weak acid
system may be applied onto the mat surfaces before of after the mat is formed,
preferably after.
In the application of this invention to wood panel manufacturing, the buffer
or weak acid system
may be applied onto the surfaces of the lignocellulosic furnishes when the mat
is being
transported from the forming station to the hot press. A set of nozzles may be
used to evenly
distribute the acid or buffer solution to completely cover the outer surfaces
of the furnish on the
upper and bottom surfaces of the mat. Methods other than nozzle spray can also
be used to
deliver the buffer or weak acid solution. The amount of the buffer or weak
acid solution is
dependent on several parameters: the concentration and/or the buffer capacity
of the solution, the
amount of the PF resin on the furnish surfaces, the alkalinity of the PF
resin, the type of the PF
resin, and the minimum volume needed to cover the entire mat surfaces. For
instance, the
required spray coverage of a buffer or weak acid solution on the wood strands
in oriented
strandboard manufacturing is at least 20 g/m2, preferably 25 - 40 g/m2 when a
liquid PF resin is
used as the adhesive and preferably 40 - 70 g/mz when a powder PF resin is
used.
Various procedures are known in the art for the manufacture of lignocellulosic
material-
containing products. The choice of raw material for the lignocellulosic
component is based
mainly on availability and cost. As is common in board manufacturing
operations, the wood
from which particles are produced may be in the form of logs that are
unsuitable for conversion
into lumber or plywood because they are too small, crooked or knotty, or the
like. When such
logs are reduced to small particle form, defects are screened out.
The invention is useful in the production of panels that are made from
homogeneous
lignocellulose material or from mixtures of different kinds of such material.
A panel may be
made, for example, completely from wood particles, or completely from wood
flakes, or from
fibers, planer shavings or the like, or from mixtures of these. Similarly, a
panel may be formed
with multiple layers, with fine surface flakes and a core of coarse flakes, or
it may have a coarse-
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flaked core with an overlay of fibers on each of its surfaces. Other
combinations may also be
produced.
Wood flakes or strands are generally made by a machine that shaves off flakes
or strands
of the wood in a direction such that the length of each flake or strands is
parallel to the wood
grain. A normal flake or strand has dimensions from 1/a" by 1" up to 1" by 6",
with a thickness in
the range from about 0.005" to about 0.075", depending upon the intended end
use.
The lignocellulosic material may also be in the form of wood fibers. In the
production of
such fibers, wood chips are generally mechanically reduced to fiber form in an
atrition mill. The
fibers so produced are either directly used as the furnish in dry process
fiberboards
manufacturing or placed in the form of a pulp slurry containing from about 1 %
to 2% by weight
of fiber for wet process.
The wood pieces employed in making the composite panel have some affinity for
water
and a tendency to absorb it. Water entering a composite panel tends to weaken
it, may cause
some swelling of surface fibers, and increases the dimensional instability of
the composition
panel. To prevent this tendency to absorb water, a wax may be applied to the
wood furnish to
provide a built-in resistance of the panel to water absorption. The wax
employed may be any
wax, either slack wax or emulsified wax, that will suffice. It is applied,
generally, at a rate of
from about 10% by weight to about 30% by weight of the binder, and preferably
about 20% by
weight, dry solids basis. When expressed in terms of oven-dried furnish
solids, the amount of
wax ranges from about 0.3 % to about 3.0% by weight of wax to wood.
The amount of phenol formaldehyde resin used generally will depend upon the
characteristics required in the final product. For oriented strandboard, the
amount of PF resins
are generally 1.8 - 3.5% for powder resins and 2.8 - 4.5 for liquid resins.
Some specialty
oriented strandboard such as rim-board and webstock may require significantly
higher dosage of
PF resins. For a high performance grade of particleboard, the amount of resin
should be
sufficient to provide from about 3% to about 8% dry resin solids based on the
weight of the
furnish for the composite panel. In a multi-layered board, a lesser amount of
resin will often be
used in the core than is used for the surface layers, such as, for example, 3%
of resin solids for
the core, and 8% of resin solids in the two surface layers. The added amount
of resin in the
surface layers imparts added strength and resistance to swelling as compared
to the core. More
resin can be used, but a greater amount presently may not be cost-efficient.
Hot pressing conditions will depend upon the thickness of the board as well as
on resin
characteristics. A representative press cycle for the production of a 23/32"
thick phenolic bonded
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particleboard would be about 4.5-6.5 minutes at a press platen temperature of
about 380° - 420°
F. For instance 23/32" three layer oriented strandboard typically requires a
pressing time of 5
minutes.
PF resins deposited on furnish surfaces without the treatment described in
this invention
are in an alkaline environment. The pH of PF resins is usually higher than
9.0, often 10.0 or
above. At such a high pH environment, the phenol nuclei in the resins exist
mainly in their
deprotonated forms, namely phenolates. Phenolates are far more electron
negative and easier to
be oxidized comparing with their phenol counterparts. When the furnish mat is
hot pressed at
elevated temperature, the oxidation of these phenolates occurs, resulting in
the formation of
oxidized phenol derivatives which remain in deprotonated form, further
darkening the color of
the finished panels. When a buffer or weak acid system is applied on to the
mat surfaces prior to
the hot pressing, the phenolates in the PF resin are protonated back to
corresponding phenol
forms, which lessens the degree of oxidation during hot-pressing and renders
the oxidized phenol
species also in the protonated forms. Therefore, the color darkening of the
finished panel is
significantly reduced or virtually eliminated.
EXAMPLES
Example 1
Buffer #1 (1M boric acid + 0.1 M borax, pH 6.4) was sprayed onto the top
surface of a 12"x12"
Aspen oriented strandboard mat prior to hot pressing at a coverage of 25 g/m2
and 50 g/m2. A
55% solids liquid resin with a total caustic content of 3.8 - 4.0% (pH of 10.0
- 10.2) and a
viscosity of between 40 and 90 cps was used throughout the board at 4.0% by
weight of resin
solid to wood. The blended moisture content of the furnish was 5.8%. Mats were
pressed for a
total of 240 seconds at 410°F to obtain a final thickness of 7/16" and
a density of 39 lb/ft3. The
panels which had the buffer spray applied prior to pressing were significantly
lighter (whitish
yellow) in color compared to those without the treatment (dark brown).
Thickness swelling and
water absorption were not significantly different between the treated and
untreated panels.
Example 2
Buffer #1 (1M boric acid + 0.1 M borax, pH 6.4) was sprayed onto the top
surface of a 12"x12"
Southern Yellow Pine oriented strandboard mat prior to hot pressing at a
coverage of 50 g/m2.
Powder resin #1 with a total caustic content of 8.0 - 8.4% was used throughout
the board at 4.0%
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by weight of resin to wood. Mats were pressed for a total of 260 seconds at
400°F to obtain a
final thickness of 7/16" and a density of 44 lb/ft3. The panels which had the
buffer spray applied
prior to pressing were significantly lighter (whitish yellow) in color
compared to those without
the treatment (dark brown). Thickness swelling and water absorption were not
significantly
different between the treated and untreated panels.
Example 3
Buffer #1 (1M boric acid + 0.1 M borax, pH 6.4) was applied onto the top and
bottom surfaces of
a 12"x 12" Southern Yellow Pine oriented strandboard mat prior to hot pressing
at a coverage of
50 g/m2. Powder resin #1 with a total caustic content of 7.0 - 7.4% was used
throughout the
board at 4.0% by weight of resin to wood. Mats were pressed for a total of 260
seconds at 400
to obtain a final thickness of 7/16" and a density of 44 lb/ft3. The panels
which had the buffer
spray applied prior to pressing were significantly lighter (whitish yellow) in
color compared to
those without the treatment (dark brown). Thickness swelling and water
absorption were not
significantly different between the treated and untreated panels.
Example 4
Acid #1 (1M boric acid solution) was applied onto the top and bottom surfaces
of a 12"x12"
Aspen oriented strandboard mat prior to hot pressing at a coverage of 25 g/m2
and 50 g/m2,
respectively. A 55% solids liquid resin with a total caustic content of 3.8 -
4.0% (pH of 10.0 -
10.2) and a viscosity of between 40 and 90 cps was used throughout the board
at 4.0% by weight
of resin solid to wood. Mats were pressed for a total of 240 seconds at
410°F to obtain a final
thickness of 7/16" and a density of 39 lb/ft3. The panels which had the acid
solution applied prior
to pressing were significantly lighter (whitish yellow) in color compared to
those without the
treatment (dark brown).
Example 5
Acid #2 (1M formic acid solution) was sprayed onto the top surface of a
12"x12" Aspen
particleboard mat prior to hot pressing at a coverage of 35 g/m2. A 55% solids
liquid resin with a
total caustic content of 3.8 - 4.0% (pH of 10.0 - 10.2) and a viscosity of
between 40 and 90 cps
was used throughout the board at 5.0% by weight of resin solid to wood. Mats
were pressed for a
total of 300 seconds at 410°F to obtain a final thickness of 7/16" and
a density of 44 lb/ft3. The
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panels which had the buffer spray applied prior to pressing were significantly
lighter (whitish
yellow) in color compared to those without the treatment (dark brown).
Example 6
Acid #3 (1 M powdered citric acid) was applied onto the top and bottom
surfaces of a 12"x12"
Aspen oriented strandboard mat prior to hot pressing at a coverage 50 g/m2. A
55% solids liquid
resin with a total caustic content of 3.8 - 4.0% (pH of 10.0 - 10.2) and a
viscosity of between 40
and 90 cps was used throughout the board at 4.0% by weight of resin solid to
wood. Mats were
pressed for a total of 240 seconds at 410 to obtain a final thickness of 7/16"
and a density of 39
lb/ft3. The panels which had the buffer spray applied prior to pressing were
significantly lighter
(whitish yellow) in color compared to those without the treatment (dark
brown).
Each of the references referred to herein is expressly incorporated by
reference. The
invention being thus described, it will be clear that the same may be varied
in many ways. Such
variations are not to be regarded as a departure from the spirit and scope of
the invention, and all
such modifications as would be obvious to one skilled in the art are intended
to be included
within the scope of the following claims.
