Note: Descriptions are shown in the official language in which they were submitted.
~S3Z~4
METHOD ~F BONDING WOO~ PLIES TO ~RM PLYWOOD PANELS
This invention relates to plywood and more particu-
larly to a method of bonding wood plies to form plywood panels
utili:zing phenol formaldehyde and lignin formaldehyde, referred
to as methylolated kraft lignin (MKL~.
The most common bonding agent used in the preparation
of exterior grade plywood is phenol formaldehyde resin (herein-
after referred to as PF). This resin is derived from
non renewable petrochemical sources, and is day by day be-
GOming more expensive. Resins produced from wood chemicals
have been considered as a replacement for PF and other petro-
chemical resins However, no satisfactory resin has yet been
produced from wood chemicals which has the same bonding properties
as PF resins at the same low resin solids content used in PF
bonding of plywood.
In plywood, the bonding is dependent on two criteria,
1~ controlled wetting and penetration of the wood by the ad-
hesive, and 2) the development of sufficient strength in the
adhesive after curing. Adhesive penetration is largely influenced
by the molecular weight of the resin and the association of resin
molecules. If the effective size of the resin molecule is too
small, it is rapidly absorbed by the capillary structure of the
wood, producing an adhesive starved joint. If the resin mole-
cular size is too large, penetration of the resin into the wood
is retarded, resulting in a shallou, weak bond. After hot press-
ing the plywood assembly, the strength of the cured adhesive must
exceed the strength of the wood itself. When this condition is
not fulfilled failure of the adhesive joint occurs in the glue
line instead of in the wood.
Many attempts have been made with kraft or soda black
liquor to form adhesives as substitutes for PF resin. In general,
the adhesives produced by these attempts have been limited
because in order to obtain the same adhesive properties as ob-
11~32~
tained with PF it has been necessary to use large quantities ofthe lignin derivsd materials which reduces the advantages of this
type of adhesive. Attempts have been made to modify kraft lignin
with PF to cause cross linking of the lignin molecules. Clarke
and Dolenko in U.S. Patent 4,113,675 combined ~KL with a liquid
PF resin in the pH range of 4 - 7. The resulting dispersion was
used as a wet adhesive for plywood. In this adhesive MKL acted
both as an extender and as a source of resin solids. However, it
was found that to achieve a satisfactory bond in plywood required
nearly twice as much lignin containing adhesive as compared to
the level of PF resin normally used.
In another example, Herrick et al, in U.S. Patent
3,454,508, describe a thermosetting phenolic resin in which an
alcohol soluble polymethylol phenol was condensed with an acid
precipitated alkali lignin in a fugitive base such as ammonia.
In another example, Ball et al in U.S. Patent 3,185,654 suggested
a phenolic resin in which an A stage resole was reacted in methanol
with an alkali lignin. Neither Herrick nor Ball showed sufficient
crosslinking between free methylol groups of the resole and
hydroxyl groups in the lignin. These methods failed to produce
satisfactory bonds in plywood.
Marton et al in the publication entitled "Lignin
Structure and Reactions", Advances in Chemistry Series 59,
American Chemical Society, Washington, D.C., Pages 125 - 144,
(1966) reported a method of improving the reactivity o~ kraft
~ lignin by introducing methylol groups into the lignin structure
via an alkali catalyzed condensation with formaldehyde. Abe in
a publication of Hokkaido Forest Product Institute ~o. 55 (1970)
also extensively examined the mechanism of the lignin formalde-
hyde reaction in alkaline media, and prepared plywood adhesivesbased on the co-condensation of lignin with phenol and formalde-
hyde. Both these adhesives gave poo:r water resistance and wet
llS;~
strength properties in the bonded product. Enkvist, in U.S.
Patent 3,864,291 describes a process for manufacture of an ad-
hesive in which kraft or soda black liquor was methylolated with
formaldehyde in the presence of alkali and subsequently condensed
with phenol or cresol formaldehyde. However, plywood panels glued
with this adhesive showed unsatisfactory wet strength after boil-
ing.
One purpose of the present invention is to provide a
method of bonding wood plies to form plywood panels. The adhesive
used for this bonding replaces a portion of the more commonly
used PF resin adhesives and provides glue bond properties that
meet the requirements of CSA standards for plywood. CSA 0151
(1974) lays down the minimum glue bond requirements for plywood
panels. Glue bond is measured by wood failure tests. The ad-
hesive is applied in either a one step or a two step application.
In the present invention, methylolated kraft lignin (MXL)
is prepared from black liquor first by methylolation wherein
formaldehyde is added to strong black liquor, followed by the
addition of acid to the methylolated liquor. Suspension, dilution
and filtration of the precipitate produces a filter cake of MKL
solids. An aqueous alkaline solution of the MKL organic solids
is then prepared from the filter cake,
The composltion of an adhesive mix is gener~lly
expressed in terms of percentage by weight of the total adhesive
solids in the mix In the case of PF the total adhesive solids
` comprisesthe resin solids which is the portion of the adhesive
taking part in the condensation of the adhesive during the curing
step. In the present text the term "organic solids" is used
throughout, especially in association with ~IKL. In a simple system
such as PF, organic solids and resin solids mean the same thing,
however, in the case of MKL the organic solids may include other
organic matter coprecipitated with the MKL.
-- 3 --
li53;~4
Plywood is made in a variety of different constructions
wlth varying numbers and types of plies The bonding surfaces of
the pli.es refer to all the surfaces of the plies except the exter-
ior surfaces of the two outer plies. In the present application
the outer plies in a sheet o~ plywood are referred to as face
plies, and the inside plies are referred to as inner plies.
Although only a three-ply panel with a single in~er ply is de-
scribed herein, it will be apparent to th~ e skilled in the art
that plywood panels with any desirable number of plies may be
made by the process of the present invention.
The present invention provides a method of bonding wood
plies to form plywood panels comprising the steps of, applying
water soluble PF resole resin in liquid form to at least one bond-
ing surface between the plies, wherein the PF resole resin in
liquid form has a pH of at least about 9, applying MKL organic
solids in liquid form to the at least one bonding s~r~ace between
the plies, wherein the MKL organic solids in liquid form have a
pH of at least about 10, and wherein the MKL organic solids are
up to about 60% by weight of the total PF resole resin and MKL
organic solids applied to the at least one bonding surface between
the plies, sandwiching the plies together, and subjecting the plies
to pressure and heat simultaneously to bond the plies together to
form plywood panels,
In one method of bonding wood plies referred to as the
one-step method, water soluble PF resole resin in liquid form
having a pH of at least about 9 is mixed with MKL organic solids in
liquid form, having a pH of at least about 10 such that the NKL
organic solids are up to about 30% b~ weight of the total PF resole
resin and MKL organic solids, applying the mixture to at least one
bonding surface between the plies, sandwiching the plies together,
and subjecting the plies to pressure and heat simultaneously to bond
the plies together to form plywood panels,
In another method referred to as the two-step method
-- 4 --
11S3;~4
a first layer of water soluble PF resole resin in liquid form
is applied to insLde surfaces of two face plies, and both sur-
faces of at least one inner ply, wherein the PF resole resin in
liquid form has a pH of at least about 9, a second layer is
applied of ~L organic solids in liquid form to both surfaces of
the inner ply, wherein the MKL organic solids in liquid form
- have a pH of at least about 10, and wherein the MKL organic solids
are up to about 60% by weight of the total PF resole resin and
MKL organic solids supplied to the surfaces of the plies. A
layup is then made by sandwiching the inner ply between the two
face plies and the layup is subjected to pressure and heat simul-
taneously to bond the plies together to form plywood panels. In
one variation of the two-step method, the first layer is a mix-
ture of PF and MKL, wherein the weight ratio of MKL to PF is up
to about 30 to 70, that is the mixture should not contain more
than about 30 parts MKL. In another variation of the two step
method, the second layer is a mixture of MKL and PF in any pro-
portion. Any combination of these variations may be used so as to
give the ratio of total MKL to total PF required in the finished
product. In the two step method, up to about 18 hours may pass
between the application of the first layer and the application of
the second layer, In one embodlment the first layer is applied
by spraying, and the second layer is applied by roll spr0ading.
In preferred embodiments, the water soluble PF resole
resin in liquid form has a resin solids content range by weight
of from about ~5 - 40%. The MKL organic solids in liquid form has
an organic solids content preferably in the range of from about
15 - 35% by weight, and the solution contains an alkali metal
base such as NaOH or sodium carbonate, In the case of NaOH, the
ratio of NaOH to ~KL organic solids is preferably in the range of
about 0.15 - 0.30 to 1 by weight,
The invention will now be described in more detail,
MKL organic solids in liquid form are made by mixing filter cake,
115;~2~?~
prepared in the manner described, with water and an alkali
metal base such as sodium hydroxide The MKL organic solids in
liquid form has a pH of at least about 10, preferably in the
range of 11 - 13. The sodium hydroxide mixed with this MKL
organic solids in liquid form is in the ratio of NaOH to MKL in
the range of about 0.15 - 0.30 by weight. The PF resol~ resin
may be a commercially available resin with a pH of at least about
9 when in liquid form. Preferably the range is about 11 to 13.
The MKL organic solids in liquid form has an organic solids con-
tent range of from about 15 - 35 percent by weight, and the PF
resole resin in liquid form has a resin solids content range by
weight of from about 15 - 40 percent. In a preferred embodiment
and to form a plywood meeting CSA standards, it is preferable that
the ratio of MKL to PF should be not greater than about 30 to 70.
The MKL/PF adhesive mixture is applied to at least one of the
bonding surfaces between the plies by any suitable method, such
as spraying, curtain coating, or roll spreading. In one embodiment
when making a three-ply panel, the mixture is applied to both
surfaces of the inner ply. The plies are then sandwiched together~
and in that form subjected to pressure and heat simultaneously
so that the plies bond together toform plywood panels
In the two-step process, a PF resole resin adhesive ~n
liquid form is first prepared as in the one-step process. The
PF resole resin in liquid form has a pH of at least about 9, and
in a preferred embodiment the resin solid content range is from
- about 15 - 40 percent by weight of the liquid adhesive mixture
A first layer of the PF resole resin in liquid form is applied to
the inside surfaces of the face plies and to both surfaces of one
or more inner plies The coating may be applied by any suitable
method such as spraying, curtain coating or roll spreading It
has been found that the PF resole resin in liquid form may be
applied to the plies several hours in advance of the application
of the second layer. In fact, tests have shown that about 18
1153~4
hours may elapse between the f~rst and second layer application
of adhesive.
The first layer may comprise 100% P~ or may contain
MKL in the weight ratio of up to about 30 to 70, MKL to PF. The
mixture should not contain more than about 30 parts MKL to 70
parts PF.
The second layer may comprise a 100% MKL organic
solids in liquid form, having a pH of at least about 10. Pre-
ferably the organic solids in the liquid form has a content range
of about 15 - 35 percent by weight. Alternatively, the second
layer may be a prepared mixture of PF resole resin in liquid form
having a pH of at least about 9 and MKL organic solids in liquid
form having a pH of at least about 10. Any proportion of PF may
be added to the MKL, depending only on the ratio of P~ to MKL
desired in the final plywood product. The second layer is only
applied to the surfaces of the inner plies such that there is
always a first layer between the second layer and the surface of
the wood. There should not be a second layer which is predomi-
nately MKL dir~ctly adjacent to the wood surface. The plies are
then sandwiched together between the two face plies, and are
sub~ected to pressure and heat simultaneously to bond the plies
together to form the plywood panels. Excellent plywood bonds
were obtained with formulations wherein the MKL organic solids
were up to between about 40 to 60 percent by weight of the total
PF and MKL organic solids applied to the surfaces of the plies,
; with assembly times as long as about 50 minutes. The second
layer may be applied by any suitable method such as spraying,
curtain coating, or roll spreading. Durlng adhesive mixing the
necessary extenders for application may be included.
The MKL adhesive mix is prepared from MKL resin and
water J and includes preferably an alkali such as sodium hydroxide
to give an alkaline mix. The alkaline aqueous solution of MKL
-- 7 --
~1532~4
organic solids in liquid form has a pH of at least about 10. A
preferred embodiment provides a pH in the range of about 11 - 13.
The MKL organic solids in liquid form have an organic solid con-
tent range of from about 15 - 35 percent by weight. If the mix
is to be sprayed onto the plies, then the organic solids content
is generally in the lower level to avoid plugging of the sprays.
However, in some cases it is preferred to apply the MKL organic
solids in liquid form by roll spreader in which case a higher
organic solids content in the mix may be used. Plywo~d bonded
wi$h adhesives according to the two-step process wherein MKL
organic solids range up to about 60% of the total organic solids
applied to the plywood exhibited similar behaviour to panels
bonded with 100~ PF resole resin adhesive. This was determined
after tests for accelerated aging for one boil/dry/boil cycle,
in accordance with CSA Standard 0151 - 1974 for Canadian soft-
wood plywood.
EXAMPLE: 1
MKL organic solids were prepared by stirring an alka-
line solution of kraft lignin with formaldehyde at room tempera-
ture for 72 hours.
ConstituentsParts by total solution
wei~ht for 22,4%
resin sollds
Kraft lignin (95% total
solids~ 0.4~ ash) 2328
Formaldehyde (37%) 1646
Sodium hydroxide (flake) 398
Water 5450
A series o~ plywood adhesive mixes was prepared inwhich a portion of the water soluble PF resole resin (35. lZ resin
solids) was replaced by an equivalent amount of MKL organic solids.
The MKL/PF adhesives were formed with initial viscosity maintained
in the range of 1.0 - 3.0 Pa.s by reducing the amount of extender
added with increasing proportion of MKL replacement. All other
additives were held constant.
-- 8 --
11532~(~
Parts by total solution
weight for 21.0 - 21.2%
resiD sollds
Formulation (MKL/PF) 20/80 40/60 60/4080/20
Constituents
_
PF Resin 3728 2796 1864 932
MKL 1445 2889 43345778
Extender 750 650 - 600 300
Wheat flour 160 150 150 150
Soda ash 150 150 150 150
Water 1479 1069 603 400
3-ply 5/16" Douglas fir plywood panels were manufactured by a
single stage application of the MKL/PF formulation Qn both surfaces
of the inner ply using a roll spreader as follows:
Adhesive spread 56 lb/MDGL
Press temperature 177C (350F)
Press time 9 minutes (2 panels per opening)
Pressure 1380 kPa (200 psi)
Closed Assembly time 10 minutes, 30 minutes
8 panels per condition
Sample chips were tested after one boil/dry/boil cycle (CSA 0151-1974
-
1~53294
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_ 10 --
115~z~4
Table 1 shows that up to 20% of the PF resole resin
solids in an adhesive mix could be replaced by MKL while still
achieving satisfactory plywood bond quality. Beyond this re-
placement level, wood failure showed an inverse dependence on the
degree of MKL substitution. The shear strength of plywood ad- -
hesive joints bonded with MKL/PF adhesives were acceptable even
at 60% MKL substitution.
EXAMPLE: 2
. . .
A plywood adhesive mix was prepared in uhich 20~ of the
PF resole resin solids were replaced by an equivalent amount of
organic solids derived from MKL. The caustic and extender con-
tent in this formulation was maintained at the same level as the
control glue mix.
A series of 3-ply 5/16" Douglas fir plywood panels was
manufactured by a single stage application of the adhesive mixture
using a roll spreader OD both surfaces of the inner ply, as follows:
Glue Spread 56 lb/MDGL
Closed Assembly Time 0, 10, 20, 30 minutes
Press Temperature 149C (300F); 166~C (330F)
Press Time 4, 4~ minutes (2 panels per
opening)
~ Pressure 1,380 kPa (200 psi)
;~ 20
8 panels per condition
Sample chips were tested after vacuum pressure treatment. Bond
quality data is summarized in Table 2.
The plywood panels bonded with PF resole resins modified
with 20% MKL give an average wood failure in excess of 80~ as re-
quired by CSA 0151 over the complete range of pressing conditions
tested. ,
liS3Z~
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: - 12 -
1153294
EXAMPLE 3
MKL organic solids were prepared by reacting formalde-
hyde with strong black liquor at 120 - 130F (49 - 54C) for 12
hours. This mixture was then acidified with concentrated H2S04
to pH 5. The precipitated MKL was collected by filtration and
washed.
Analysis
% Total solids 59.4 of moist filter cake
% Ash 8.5
% Lignin (sPectrophotometric) 79.2
% Organics (100~ - % ash) 91.5
An aqueous solution of PF resole resin (16.7% resin s~lids) for
spray application was prepared by diluting 4 parts of the standard
PF mixed glue (20,9% resin solids) with one part water.
Constituents ofParts by total solution weight
Standard PF Gluefor 20.9% resin solids
Resin 4660
; Primary Extender750
Wheat Flour 150
Soda Ash 150
Water 1990
Two formulations of alkaline aqueous solutions of MKL organic
:20 solids were prepared, one having 20~ of the MKL organic solids
~: rsplaced with PF ~,
Parts by total solution wei~ht
for 20.9~ organic solids
For~ulation (MKL/PF) 80/20 100/0
Constituents
.
PF Resin 932
MKL 2434 3041
Primary Extender 600 600
Wheat Flour 150 150
Soda Ash 150 150
Sodium hydroxide (flake)336 419
Water 3260 3538
The adhesive solutions were applied to the plies in
two stages as follows: first, all four wood surfaces in a 3-ply
construction namely the inside surfaces of the two face plies,
~lS3Z~4
and both surfaces of the inner ply were sprayed with PF solution
and allowed to stand open for 60 minutes in order to achieve pene-
tration of the PF into the wood, and secondly the MKL solution
was applied to the core using a roll spreader.
Control
MXL/PF (Overall) 40/60 6.0/40 0/100
Formulation MKL/PF (to inner
ply surfaces)80/20100/080/20 100/0 0/100
PF to all bonding surfaces
lb/MSGL17.6 21.2 8.8 14.0 --
MKL/PF to inner ply surfaces
lb/MDGL28.4 22.9 42.7 34.4 56.0
PROCESSING CONDITIONS
2 Step Adhesive System Control
Press temperature 177C (350F) 149C (300F)
Press time (2 panels per
opening) 9 minutes 4~ minutes
Pressure 1380 kPa (200 psi) 1380 kPa
(200 psi)
Open assembly time 60 minutes
(PF treated plies) ---
Closed assembly time 0, 10,20,30 minutes 0,10,20,30
minutes
8 panels per condition
Samples were tested a~ter one cycle o~ the boil/dry/boll condl-
tionlng treatment (CSA 0161 - lg74),
~ .
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- 14 -
1153Z~4
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llS~Z94
The two step adhesive application showed a considerable
improvement over the conventional one step application system.
(Compare Table 3 with Table 1) E~cellent plywood bonds were
obtained with both the 80/20, and 100/0 MKL/PF formulations at
the total replacement level of 40% MKL solids.
EXAMPLE: 4
Example 3 was repeated using the 80/20 MKL/PF formu-
lation for application to the inner ply. Plies sprayed with PF
resole resin mixed adhesive were allowed to stand open at room
temperature for 18 hours prior to application of the MKL/PF ad-
hesive to the inner ply. The results are shown in Table 4.
TABLE 4
.,
MKL/PF (Overall) 40/60 60/40
MKL/PF to
; inner ply 80/20 80/20
Closed
Assembly Shear Shear
Time Strength Strength
Minutes % WF psi % WF psi
O 84 102 80 122
86 110 70 123
89 11~ 80 128
89 117 87 112
. _
Average 87 110 79 121
Mixed PF resole resin solutions may be sprayed onto the plies
several hours in advance of application of the MKL/PF solution
to the inner ply. This method of adhesive application allows
- replacement of between 40 - 60% of the PF solids in the total
adhesive mix, while ~till maintaining plywood bond quality.
EXAMPLE: 5
Example 3 was repeated with 80/20 and 100/0 ~L/PF
formulations applied to the inner ply. The overall ratios were
- 16 -
115~Z~4
40/60 and 60/40 MKL/PF and the results were compared with a
100% PF control. Table 5 shows the results.
,
3~
_ 17 -
- ~153Z~4
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l~S3Z~4
The two step adhesive application system produced
superior plywood bonds with both the 80/20 and 100/0 MKL/PF
formulations, at an overall replacement level of 40 and 60% PF
resole resin solids.
In a further series of tests the closed assembly time
was extended to 50 minutes, excellent results were obtained as
shown in Table 6.
TABLE 6
MKL/PF _
(Overall) 60/40
MKL/PF
to inner ply 80/20 100/0
Closed
Assembly Shear Shear
Time Strength Strength
Minutes % WF psi% WF psi
152 82 114
EXAMPLE: 6
Plywood panels were manufactured using the two step
adhesive application system described in Example 3, so that 70%
and 80% respectively of the PF resole resin solids were replaced by
MKL. Analysis of MKL
% Total solids 61.8 of moist filter cake
~ % Ash 8.5
: % Organics (100% - % ash)91.5
Preparation of MKL Mixed Adhesives
-
Formulation (MKL/PF - 100/0) Parts by total solution weight
for 20.5% organic solids
Constituents
MKL 2843
Primary Extender 600
Wheat flour 150
Soda Ash 150
Sodium hydroxide (flake)419
Water 3686
- 19
ilS32~?4
MKL and PF were applied to the plies as a two step system (as
described in Example 3)
MKL/PF (Overall) 70/30 80/20
Formulation MKL/PF ~to inner ply) 100/0 100/0
PF to lnnerply surfaces lb/MSGL 10.5 7.0
PF to core lb/MDGL 40.1 45.8
Processing Conditions
Press temperature 177C -(350F)
Press time (2 panels per opening) 9 minutes
Pressure 1390 kPa (200 psi)
Open assembly time 60 minutes (PF treated
plies)
Closed assembly time 0,10,20,30,50 minutes
8 panels per condition
Samples were tested after one cycle of the boil/dry/
boil conditioning treatment (CSA 0151-1974)
TABLE 7
MKL/PF (Overall) 70/30 80/20
MXL/PF
To inner ply 100/0 100/0
Closed
Assembly Shear Shear
Time Strength Strength
: Minutes % WF psi % WF psi
_
O 78 117 64 101
126 74 135
: 20 79 128 68 128
119 66 la7
126 73 130 ~ -
_
Average 178 123 69 124
The results in Table 7 show that when 70% or more of the PF resole
resin solids were replaced with MKL, the wood failure fell below
the CSA specification of 80
- 20 -
