Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1
METHOD OF JOINING WOOD BASED MATERIALS
The present invention relates to a method of joining wood based materials
wherein a curable adhesive composition and a composition comprising a gel
forming
substance are separately applied onto wood based materials. It also relates to
wood
based products made by the method.
Background of the invention
Some processes of gluing wood based materials include a pre-pressing step
followed by a hot-pressing step. This is particularly the case in gluing
processes, such as
manufacture of plywood and laminated veneer lumber, where a thermosetting
adhesive is
applied onto veneers, which are assembled and usually pre-pressed at room
temperature
forming an assembly of veneers. The assembly is then usually hot-pressed where
the
curing of the adhesive takes place. It is important that the veneers are
sufficiently
adher ed to e?aci i oii ier after ii ie pr e-pl esslng step s~v ti i~ai ii ie
a$S~mbiy i$ not di~ur i,ipied
thereby destroying its shape. If the veneers are insufficiently adhered to
each other, the
assembly may also get stuck in the hot-pressing equipment. Consequently, bad
cold-
adhesion will lead to that some material cannot be used after pre-pressing,
thus forming a
waste.
When formulating a typical curable adhesive, various additives are usually
mixed
into a curable resin. The additives usually affect the pot life, which can be
reduced from
being up to several weeks for the resin alone, to only a few days or shorter
for the
formulated curable adhesive composition. Some additives lead to very short pot
life which
causes handling problems. Thus, problems with too short pot life are
frequently present
when formulating adhesive compositions.
In order to improve the cold-adhesion, additives such as cereal based
materials
including wheat flour, maize flour etc. are commonly used as components in
curable
adhesive compositions. However, cereal base additives can only be used up to a
certain
amount without thickening the adhesive composition too much. Also, different
natural or
synthetic polymers have been tested, but not giving any satisfactory
improvement. Thus,
the performance of the known additives for improving cold-adhesion is
insufficient and
there is a need for improving the cold-adhesion when joining .wood based
materials
without causing problems with too short pot life.
When gluing together two surfaces, there exists a certain maximum assembly
time available, i.e. the maximum length of time between application of the
adhesive
components and joining of the parts in, for example, a press. The assembly
time must be
sufficient long to enable assembling and adjusting of the parts to be glued in
a practical
way.
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WO 89/05221 discloses a method for the production of wood products where
hardener is applied in a zone along edges of a veneer layer. However, due to
the rapid
hardener used, the assembly time before pre-pressing, will be very short.
Also, the
method according to WO 89/05221 may lead to variations of the quality of the
adhesive
bond between different parts of the adhesive bond, and more waste.
US Patent No. 4,824,896 discloses an adhesive for bonding wood pieces of high
moisture content, which optionally includes borax. US Patent No. 2,889,241
discloses a
phenolic resin composition comprising a boron oxygen compound, such as boric
acid.
EP 0259042 A2 discloses the use of esters as curing agents for a phenol-
formaldehyde
resin. However, none of US 4,824,896, US 2,889,241 or EP 0259042 A2 discloses
anything about cold-adhesion.
It is an object of the present invention to provide a method for joining wood
based materials which gives excellent cold-adhesion, minimises pot life
problems, and
C~.IV~f1'S SUIII(ICIIL Cls'GIIIIJI~/ Llllles. T le Illel IVd belllg sUltable
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curable adhesive composition is used comprising a pre-pressing step and a hot-
pressing
step. It is a further object of the present invention to provide wood based
products made
by the method.
The invention
According to the invention it has surprisingly been found possible to achieve
the
above mentioned objects by a method of joining two or more pieces of wood
based
materials, comprising the steps of:
(a) providing at least two pieces of wood based material;
(b) separately applying an adhesive composition (A), comprising a curable
resin,
and a composition (B), comprising a gel forming substance, onto at least one
of
said at least two pieces;
(c) joining the two pieces together to form an assembly having the
compositions (A)
and (B) situated between the pieces;
(d) pre-pressing the obtained assembly, with no curing of the resin; and then
(e) hot-pressing the assembly to cure the resin.
According to the invention it has also surprisingly been found possible to
achieve
the above mentioned objects by a method of joining two or more pieces of wood
based
materials, comprising the steps of:
(a) providing at least two pieces of wood based material;
(b) separately applying an adhesive composition (A), comprising a curable
resin,
and a composition (B), comprising a gel forming, non-curing, substance, onto
at
least one of said at least two pieces;
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(c) joining the two pieces together to form an assembly having the
compositions (A)
and (B) situated between the pieces;
(d) pre-pressing the obtained assembly; and then
(e) hot-pressing the assembly to cure the resin.
By "a gel forming substance" is herein meant a substance, which, by itself or
by
its derivatives, is able to make the adhesive composition (A) according to the
invention
form a gel, but not to make any resin in adhesive composition (A)
significantly cure at the
relatively low temperatures during pre-pressing, for example, at room
temperature, 20-
25°C, which is an example of a temperature range during pre-pressing.
By "curing" the resin is herein meant transforming the curable resin into a
hardened, cross-linked, suitably solid, state.
By "pre-pressing", is herein meant pressing at a temperature below the
required
curing temperature of a th eriiio~eiiiiig Qd Ie5lVe Co111pojItIUl1 appIICd, 3~
t hat i~o
significant curing takes place during the pre-pressing time.
By "cold-adhesion" is herein meant the adhesion after pre-pressing between two
surfaces having a thermosetting adhesive layer between the surfaces.
By "hot-pressing" is herein meant pressing at a temperature higher than the
temperature at pre-pressing, at or over the required curing temperature of a
thermosetting adhesive composition applied, so that the adhesive is cured.
A suitable curable adhesive composition (A) according to the invention include
any curable adhesive composition requiring an elevation in temperature to cure
sufficiently fast. The adhesive composition (A) may comprise more than one
resin.
Suitably, the adhesive composition (A) comprises a phenolic resin or a
combination of an
amino resin and a non-curable resin.
Phenolic resins are condensates of different phenolic compounds and
aldehydes. A phenolic compound can be phenol itself, polyhydric phenols and
alkyl
phenols such as resorcinol, alkyl resorcinol, cresols, ethyl phenol and
xylenoi, and
phenolic compounds of natural origin such as tannins. Examples of suitable
aldehydes
include formaldehyde, acetaldehyde, glutaraldehyde, propionaldehyde, n-
butyraldehyde,
isobutyraldehyde and furfural. A phenolic resin can exist as a solution in
water, or alcohol
such as ethanol. Suitably, the phenolic resin exists as an aqueous solution
with varying
dry content of resin. A phenolic resin used in the adhesive composition (A)
according to
the invention is suitably a formaldehyde-based phenolic resin such as a phenol-
formaldehyde, a resorcinol-formaldehyde and a phenol-resorcinol-formaldehyde
resin.
Preferably, the adhesive composition (A) comprises a phenol-formaldehyde
resin. The
dry resin content in the phenolic resin is suitably from about 20 to about 80
weight %,
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preferably from about 30 to about 60 weight %, most preferably from about 40'
to about
50 weight %, the remaining part of the resin preferably substantially being
water. The
molar ratio formaldehyde to total amount phenolic compounds (one or both of
phenol and
resorcinol) when making the resin is suitably from about 0.1 to about 4, for a
phenol-
s resorcinol-formaldehyde resin preferably from about 0.5 to about 1. For a
phenoi-
formaldehyde resin, the molar ratio phenol to formaldehyde is preferably from
about 1 to
about 3.
Amino resins are condensates of carbonyl compounds, such as aldehydes, with
compounds containing amino, imino or amide groups such as~urea, melamine,
thiourea,
guanamines, ethylene urea, cyanamide, dicyanodiamide and guanidine. Mostly,
amino
resins refer to condensates of formaldehyde and amine-containing compounds
among
which the most common ones are urea and melamine giving urea-formaldehyde and
melamine-formaldehyde, and melamine-urea-formaldehyde. An amino resin used in
the
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or a melamine-formaldehyde resin. The dry resin content in the amino resin is
suitably
from about 30 to about 90 weight %, preferably from about 45 to about 80
weight %, most
preferably from about 55 to about 75 weight %, the remaining part of the resin
preferably
substantially being water. The non-curable resin is suitably a polymer
comprising polar
groups, preferably a polyvinylester or therefrom derived polymers, most
preferably
polyvinylacetate or polyvinylalcohol. The amount of non-curable resin in the
adhesive
composition (A) is suitably from about 0.1 to about 40 weight %, preferably
from about
0.5 to about 30 weight %, more preferably from about 1 to about 15 weight %,
most
preferably from about 2 to about 10 weight %, calculated as dry matter non-
curable resin
in the whole, non-dry, adhesive corriposition (A). The amino resin is suitably
cured by
using conventional curing agents at elevated temperatures, suitably above
50°C,
preferably above 80°C.
The adhesive composition (A) can be formulated in any suitable way according
to the state of the art. Suitably, the adhesive composition (A) may comprise
various
additives, such as curing catalysts, and fillers, such as nutshell flour, or
other starch-
containing substances known in the art. The suitable viscosity of the adhesive
composition (A) at application onto a surface of a wood based material depends
on the
application method used. The adhesive composition (A) has suitably a
Brookfield
viscosity, at 25°C, of from about 100 to about 150000 cP, preferably
from about 300 to
about 50000 cP.
The gel forming substance is suitably a non-curing substance, by which is
herein
meant that a curable adhesive composition comprising the substance should not
irreversibly cross-link at 20°C even after several hours. Examples of
suitable gel forming
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substances according to the invention include inorganic and organic gel
forming
substances. The inorganic gel forming substances include boron-containing
compounds
and metal ions. A boron-containing compound according to the invention
preferably
contains oxygen, most preferably -the boron-containing compound is selected
from the
5 group of boric acid, borate salts, or a combination thereof. By a borate
salt is herein
included all types of borate salts such as metaborates, triborates,
tetraborates etc. The
boron-containing compound is suitably dissolved in water or organic solvents
such as
glycerol, ethylene glycol, diethylene glycol, methanol or ethanol. Preferably,
the boron-
containing compound is dissolved in water. The metal ions are preferably
belonging to
group 3A, or, are transition metal ions belonging to any of groups 3B to 8B in
the periodic
table of elements, such as zinc, zirconium and chromium. The metal ions are
suitably
dissolved in water. If one or more inorganic gel forming substances are used,
composition (B) comprises the one or more gel-forming substances suitably in
an amount
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suitably from about 0.01 to about 50 weight %, preferably from about 0.1 to
about 30
weight %, more preferably from about 0.5 to about 25 weight %, most preferably
from
about 1 to about 20 weight %.
The organic gel forming substances according to the invention include
compounds having an ester functional group. An organic gel forming substance
is
suitable when a phenolic resin is used in the adhesive composition (A).
Suitably, such a
gel forming substance is selected from the group consisting of esters,
lactones, organic
carbonates, or mixtures thereof. Suitable esters include, but are not limited
to, methyl
formate, methyl acetate, ethyl acetate, N-butyl acetate, butylene glycol
diacetate,
ethylene glycol diacetate and glycerol triacetate (triacetetin). Also, other
aliphatic C~-C4
monoesters can be used, such as methylformate, ethylformate, propylformate,
butylformate, methylacetate, ethylacetate, butyiacetate, propylacetate,
methylpropionate,
ethyl propionate, propylpropionate, butylpropionate, methylbutyrate,
ethylbutyrate,
propylbutyrate, propylbutyrate, methylpentanate, ethylpentanate,
propylpentanate,
butylpentanate. Also multiesters can be used such as ethylene glycol di-
alkylester,
diethylene glycol di-alkyl ester, propylene glycol di-alkyl ester, butylene
glycol di-alkyl
ester, glycerol alkyl ester, 1,3-propanedioi alkyl ester, 1,3-butanediol alkyl
ester, 1,4-
butanediol alkyl ester, where alkyl is C~-C4. Suitable organic carbonates
include, but are
not limited to, propylene carbonate, ethylene glycol carbonate, glycerol
carbonate, 1,2-
butanediol carbonate, 1,3-butanediol carbonate, 1,2-pentanediol carbonate, 1,3-
pentanediol carbonate. Preferably, propylene carbonate is used. Lactones
include, but
are not limited to, gamma-butyrolactone, beta-propiolactone, beta-
butyrolactone, beta-
isobutyrolactone, beta-isopentylactone, gamma-isopentylactone, and delta-
pentylactone.
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If an organic compound, such as a compound having an ester functional group,
is used
as the gel forming substance, composition (B) suitably comprises from about 1
to about
100 weight %, preferably from about 10 to about 40 weight %, of gel forming
substance.
The remaining part of composition (B) is suitably substantially water.
Suitably, the Brookfield viscosity of an adhesive composition (A) gelled by
the
gel forming substance at 20°C should decrease when the gel is
moderately heated, for
example, to 50°C. This indicates that the gelling is reversible and
that no significant
irreversible cross-linking in the adhesive composition (A) has taken place.
The weight ratio of applied adhesive composition (A) to applied composition
(B)
is suitably from about 100:1 to about 2:1, preferably from about 50:1 to about
5:1, most
preferably from about 30:1 to about 10:1, calculated per unit area.
The application of the adhesive composition (A) and composition (B) can be
made in optional order of application. Preferably, the adhesive composition
(A) is applied
vi~iv' a Suiiac2 b~fvr~ OOmp.~a~tiOn (B). The appliratlnn of tile adheSIVe
COmpOSI'tl0n (A)
and composition (B) onto a wood based material can be made by using any
suitable
method known in the art, such as spraying, brushing, extruding, roll-
spreading, curtain
coating etc. forming shapes such as droplets, one or several strands, beads or
a
substantially continuous layer. Preferably, the adhesive composition (A) is
applied by
using extruding, roll-spreading or curtain-coating methods, while composition
(B) is
preferably applied by spraying.
Suitably, from about 20 to about 100 % of tile surface subjected to
applicafiion of
adhesive composition (A) is subjected to application of composition (B),
preferably from
about 50 to about 100%, most preferably from about 80 to about 100 %. By
"surface
subjected to application" is herein meant the surface exposed to any
application method,
with no requirement of a certain degree of coverage of any of composition (A)
or (B).
In the method according to the invention, two or mare pieces of wood based
materials are joined forming an assembly of pieces, which is subjected to a
pre-pressing
step, suitably at ambient temperature, preferably at from about 0 to about
40°C, most
preferably at from about 5 to about 30°C. The pre-pressing step
according to the
invention is suitably performed at a pressure of from about 1 to about 50
kg/cm2,
preferably from about 2 to about 25 kg/cm~, most preferably from about 4 to
about 15
kg/cm~. The pre-pressing time depends on the resin used and the type of
assembly to be
pressed and is suitably from about 0.1 to about 30 minutes. For a phenolic
resin, the pre-
pressing time is preferably from about 0.1 to about 15 minutes, even more
preferably
from about 1 to about 10 minutes, most preferably from about 3 to about 5
minutes. For
an amino resin, the pre-pressing time is preferably from about 5 to about 30
minutes,
most preferably from about 8 to about 15 minutes.
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The assembly of pieces is further subjected to a hot-pressing step, which is
suitably at a temperature at or above the curing temperature of the adhesive
composition
(A). The hot-pressing step according to the invention is suitably perFormed at
a pressure
of from about 1 to about 50 kg/cm2, preferably from about 2 to about 25
kg/cm2, most
preferably from about 4 to about 15 kg/cm2. The temperature in the hot-
pressing step
depends on the resin used and is suitably from about 50 to about 200°C.
For a phenolic
resin, the hot-pressing temperature is suitably from about 80 to about
200°C, preferably
from about 120 to about 170°C. For an amino resin, the hot-pressing
temperature is
. suitably from about 50 to about 150°C, preferably from about 90 to
about 130°C. The hot
pressing time depends on the resin and the thickness of the assembly, and is
suitably
from about 0.01 to about 5 minutes per mm thickness of the assembly,
preferably from
about 0.1 to about 2 minutes per mm thickness of the assembly, most preferably
from
about 0.2 to about 1 minutes per mm thickness of the assembly.
In nr,ea r,rcfcrrcrl embodiment of the inyPntinn~ a C:~_lrablP__ ~C~hesive
C~Qr~']t~p$i~ion
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comprising a phenolic resin is applied onto veneers of wood whereupon a
composition
comprising boric acid is applied onto the applied adhesive composition. Two or
more
veneers are pre-pressed forming an assembly, which is thereafter hot-pressed,
thereby
curing the adhesive composition.
In another preferred embodiment of the invention, a curable adhesive
composition comprising a combination of an amino resin and a polyvinylester is
applied
onto veneers of wood whereupon a composition comprising boric acid is applied
onto the
applied adhesive composition. Two or more veneers are pre-pressed forming an
assembly, which is thereafter hot-pressed, thereby curing the adhesive
composition.
The wood based materials according to the method of the invention can be of
any kind that can be joined by an adhesive system. Preferably, the wood-based
materials
are veneers or other wood based sheets.
The wood based products obtainable by the method according to the invention
can be of any kind where a pre-pressing step is suitable, examples of such
products are
plywood, laminated veneer lumber, and flooring materials.
The invention will now further be described in connection with the following
examples which, however, not should be interpreted as limiting the scope of
the
invention.
Examples
Example 1:
The method according to the invention was tested by gluing veneers into a two-
ply panel with a standard adhesive composition made of 100 parts by weight of
phenol-
formaldehyde resin, 1 part of sodium carbonate, 9 parts of wheat flour and 5
parts of
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water. The resin had a dry content of 45 weight %. The formulated adhesive
composition
had a Brookfield viscosity at 25°C of about 3000 cP. The veneers tested
were Radiata
Pine of 3 mm thickness. The adhesive composition was applied by extrusion into
strands
in an amount of 220 g/m~. A composition (B) according to the invention was
made
comprising about 5 weight % sodium tetraborate decahydrate and about 95 weight
% of
water. Composition (B) was sprayed onto the applied adhesive composition in an
amount
of about 10 g/m2. The pre-pressing was made at a pressure of 10 kg/cm~ and at
a
temperature of 21-25°C for 60 seconds. Three identical test panels were
made and
tested. The amount of surface on top and bottom of the two-ply panel that had
separated
("released") from each other was recorded after 30 minutes. Release values of
100%
means total separation while 0 % means that the veneers are still sticking
together.
Table 1.
~afi~pieR~iease at
30 iriiir
(%)
1 . 0
2 10
3 5
Average:5
Comparative examcle:
A test was made as in Example 1, but without spraying composition (B) onto the
applied adhesive.
Table 2.
Sample Release at
30 min
(%)
1 95
2 100
3 . 100
Average:98
It is concluded that separate application of the components according to the
invention gives a significant improvement in cold-adhesion.
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Example 2:
Samples of a PF resin, and a combination of a UF- resin and a polyvinylalcohol
(PvAI) were tested with a gel forming substance according to the invention,
and a citric
acid hardener (for UF) according to WO 89/05221. 15 g of a composition
comprising
either 10 weight % gel forming substance or 20 weight % hardener according to
WO 89/05221, was mixed with 180 g of an adhesive comprising either a PF resin
or a
UF/PvAI resin system. The UF/PvAI system also comprised a conventional
hardener, and
the relations of the components were 100 parts UF, 6 parts PvAI (17 weight %),
and 20
parts hardener. Thereafter, the mixture was allowed to rest for 1 minute at
20°C after
which the appearance and viscosity were noted. Thereafter, the temperature was
quickly
raised to about 50°C and the appearance and the viscosity of the sample
were again
noted.
Table 3.
Brookfield
viscosity
and/or
appearance
Added substancePF 20C PF 50C UF/PvAI 20C UF/PvAI 50C
Sodium tetraborate30000 cP 3000 50000 cP 5000 cP
cP
decahydrate (gel) (fluid) (gel) (fluid)
+
boric acid (1:1)
Citric acid * * >500000 cP hard/brittle
+
aluminium sulphate (hard gel)
(WO 89/05221)
~' combination not relevant to test
It is concluded that the gel forming substances according to the invention do
not
irreversibly cross-link the resins at room temperature. Furthermore, a
combination of a
citric acid/ aluminium sulphate hardener, according to WO 89/05221, with
UF/PvAI gives
a hard gel already after 1 minute at 20°C, and which does not soften
when heated,
indicating irreversible curing.
