Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR HARDENING WOOD
FIELD OF THE INVENTION
The present invention is concerned with an efficient method for increasing
wood hardness of hard-to-treat hardwoods and softwoods without changing the
original grain or colour of the wood. The proposed method involves very high
pressure impregnation (upwards of 3000 psi) of wood using a new impregnating
cylinder mechanism. Thus, the present invention is also concerned with a new
impregnation composition and apparatus. The impregnated composition contains
monomers and a proposed aliphatic organic acid such as propionic, palmitic or
stearic acid to improve upon properties of traditional hardened woods.
The monomer composition is catalysed using conventional means such as heating
or gamma-ray irradiation.
BACKGROUND OF THE INVENTION
Wood hardening begins with a high-pressure impregnation of the wood
with a mixture of vinyl monomers, catalysts and additives. Following this
impregnation, the wood is heated and hot-pressed. This heating causes a
dissociation of the catalysts into free-radicals which initiate the
polymerisation
reaction of the monomers. The end-result is a hardened wood, containing in its
pores a polymer which confers to the hardened wood increased resistance to
impact, humidity and wood destroying insects and fungus.
US Patent number 5,683,820 (Shannon et al) discloses wood products
impregnated with from 30 to 80% of a polymerizable monomer selected from the
group consisting of hexanediol diacrylate and hexanediol dimethacrylate having
excellent indent resistance. The wood products are impregnated by placing the
wood under a vacuum to remove air, contacting the wood with a monomer
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solution containing a polymerizable hexanediol diacrylate and/or hexanediol
dimethacrylate monomer and from 0.5 to about 2 percent of a thermally
activated
free radical source as a polymerization initiator for a period sufficient to
achieve
the desired polymer loading. The wood is then heated under pressure to
polymerize or cure the monomer.
US Patent number 5,679,407 (Mansikkamaki et al) discloses a method for
the hardening of wood material by impregnating it with a polymerizable
substance
and by polymerizing the polymerizable substance by means of heat. Heat
transfer
and cleaning in the polymerization have been improved through the impregnated
wood material being in contact with water during the polymerization. The water
is preferably hot and pressurized and serves at the beginning of the
polymerization
as a heating medium and during the polymerization as a cooling and cleaning
medium.
US Patent number 5,605,767 (Shannon et al) discloses wood products
impregnated with from 30 to 80% of a polymerizable monomer selected from the
group consisting of hexanediol diacrylate and hexanediol dimethacrylate having
excellent indent resistance. The wood products are impregnated by placing the
wood under a vacuum to remove air, contacting the wood with a monomer
solution containing a polymerizable hexanediol diacrylate and/or hexanediol
dimethacrylate monomer and from 0.5 to about 2 percent of a thermally
activated
free radical source as polymerization initiator for a period sufficient to
achieve the
desired polymer loading. The wood is then heated under pressure to polymerize
or
cure the monomers.
US Patent number 4,883,719 (Brebner) discloses a method of surface
impregnation of wood articles with a plastic and to the resulting wood
articles.
The method uses a diallyl o- phtalate monomer-prepolymer solution and is
deemed
to improve upon wood properties.
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US Patent number 4,568,564 (Witty discloses a wear-resistant wood
product such as a wood flooring material having a white or bleached appearance
which is obtained by impregnating wood with a solution of a polymeric resin
whitening agent in a polymerizable monomer such as methylmethacrylate and,
after uniform distribution of the monomer solution throughout the wood is
obtained, the monomer is polymerized and the whitening agent precipitated in
situ
in the polymer matrix as a material Having a refractive index sufficiently
different
from the refractive index of the matrix to impart a bleached or white
appearance
to said product.
None of the above patents discloses a method for increasing wood
hardness of hard-to-treat hardwoods and softwoods without changing the
original
grain or colour of the wood using high-pressures (higher than 1000 psi) for
impregnation, and using an aliphatic organic acid such as propionic, palmitic
or
stearic acid.
SUMMARY OF THE INVENTION
The object of the present invention is to improve upon traditional wood-
polymer composites by producing a hardened wood wherein the original color of
the untreated wood is preserved, and wherein the wood is treated uniformly in
sapwood and heartwood, and does not warp during the polymerisation of the
polymer inside the wood.
In a first aspect, the present invention provides an impregnation composition
for maintaining the original color of the untreated wood after treatment. It
has
been found that low molecular weight monomers having a molecular weight
comprised between 300 and 400, such as methyl acrylate or methyl methacrylate,
produce less discoloration during polymerisation than high molecular weight
monomers such as isodecyl acrylate or styrene. It has also been found that by
adding as ari additive to the monomers an aliphatic organic acid having the
formula
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CH3-(CH2)n-COOH, n being a number between 1 and 17, such as propionic,
palmitic or stearic acid, one may improve greatly the retention of the
original color
of the wood.
It is important to note that the additives do not contain unsaturated carbon-
carbon bonds which would make the end-product more susceptible to UV
discoloration. In fact, the addition of these acids seems to provide some UV-
stabilisation effect.
In another aspect, the present invention provides a method for hardening
wood, in which the wood is impregnated with an impregnating composition such
as one described hereinabove, at a pressure higher than 1000 psi. It has been
found that by simply increasing the pressure of impregnation at a pressure
higher
than 1000 psi and preferably up to 3000 psi while keeping low viscosity
formulation, one may achieve a more uniform impregnation .
In a further aspect of the invention, there is provided an apparatus for
performing the above-mentioned impregnation method. The apparatus for
impregnating wood articles at pressures higher than 1000 psi comprises an
impregnating cylinder for receiving a cartridge which embodies an untreated
wood,
two hydraulic cylinders on which the impregnating cylinder is mounted on a
pair
of hydraulic cylinders for sealing the impregnating cylinder and pumping a
hydraulic fluid into the impregnating cylinder.
Hence it is made possible to do very high pressure impregnations (3000 psi)
on an industrial scale by using a cylinder mechanism in which the doors can be
rapidly opened and the interior of the impregnating cylinder easily accessed.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig.1 is a side view of a preferred embodiment of an impregnation apparatus
5 according to the invention;
Fig.2 is a side view of the impregnation apparatus in a ready-to-be-closed
position.
Fig. 3 is a side view of the impregnation apparatus in a closed position,
where the doors are sealed.
Fig. 4 is a top view of the impregnation apparatus.
DETAILED DESCRIPTION OF THE INVENTION
As mentioned hereinabove, the impregnation composition according to the
invention comprises low molecular weight monomers. These monomers have a
molecular weight varying between 300 and 400. The monomers intended for use
in the impregnation composition are selected from the group comprising:
acrylamide, 4-hydroxybutyl acrylate, butyl acrylate, ethyl acrylate, isodecyl
acrylate, lauryl acrylate, octadecyl acrylate, stearyl acrylate,
acrylonitrile, divinyl
benzene, ethylene glycol dimethacrylate, furfuryl alcohol, methacrylamide,
allyl
methacrylate, butyl methacrylate, ethyl methacrylate, hydroxyethyl
methacrylate,
hydroxypropyl methacrylate, isodecyl methacrylate, lauryl methacrylate, methyl
methacrylate and chlorure de vinylidene. The composition may preferably
comprise an aliphatic organic acid having the general formula of CH3-(CHZ)n-
COOH, n being a number between 1 and 17.
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A typical formulation used to produce original-color hardened wood could
be:
MMA lmethyl methacrylate) 5%
VAZO-52 0.5
S Palmitic acid 10%
where VAZO-52 is pentanenitrile 2,4-dimethyl 2,2 azobis.
The impregnation method for hardening wood in which the wood is
impregnated with an impregnating composition at pressures higher than 1000 psi
comprises essentially the steps of:
-loading an impregnating cylinder with a cartridge containing an untreated
wood;
-pumping hydraulic fluid under pressure into the hydraulic cylinder;
-impregnating the wood under pressure for approximately 5 minutes
at ambiant temperature; and
-releasing the pressure and removing the impregnating mixture.
More precisely, the method comprises:
STEP I IcLding
As can be seen in Fig. 1, small hydraulic cylinders (7) hold the impregnation
cylinder (3) up while the wood cartridge (1 1 ) is put inside. The
impregnating
cylinder (3) is located at the level of the conveyors (13), above the
hydraulic
cylinders 15) and a cartridge (11 containing the wood to be impregnated is
loaded
into the impregnating cylinder (3).
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Once the wood is inside the cylinder, the impregnating cylinder is lowered and
aligned with its doors (9) and hydraulic cylinders (5).
STEP 2 imn~egmation/r~ressurization
Once the impregnating cylinder (3) is lowered, hydraulic fluid is pumped under
pressure into the hydraulic cylinders (5) using the pistons to move and the
doors
to seal the cylinder. Since typical hydraulic pressures are upwards of 3000
psi,
it is now possible to increase the pressure inside the impregnation cylinder
(3) to
very high pressures. If the pressure remains steady for a significant period
of
time, such as 1 to 10 minutes, it means that the impregnation reached all the
zones of the wood, and that the impregnation step is finished.
STEP 3 unloading_
Once the impregnation at high pressure is finished, the pressure inside the
impregnation cylinder (3) is released, excess impregnation solution and
pressure
inside the hydraulic cylinders (5) are removed, the doors are pulled (9), the
impregnating cylinder (3) is lifted and the wood cartridge (11 ) is removed.
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