Note: Descriptions are shown in the official language in which they were submitted.
- 2 ~ 1068426
This invention relates to an adhesive for the manufacture
of wood products.
The Canadian Patent application 181,020, filed 14.9.1973
discloses an adhesive for preparing plywood, particle
boards, fiber boards, and similar products which adhesive
contains lignosulfonates and phenol-formaldehyde resins,
and the invention is characterized by the fact that most
lignosulfonates used have high molecular weights. The
invention is thus based on the conception that the ad-
hesive properties of highmolecular-weight lignin derivates
are better than those of low-molecular-weight derivates.
The adhesive according to the said application is based
on lignosulphonates obtained from the sulfite process of
wood pulping. It has now been discovered, however, that
lignin derivatives obtained from alkali wood pulping
processes can be advantageously used. These derivatives
are called alkali lignins.
In the alkali pulping processes wood chips are heated
under pressure with an aqueous solution of sodium hydroxide.
The most widely used alkali pulping process is the sulfate
or kraft process in which the pulping liquor additonally
contains sulfide and hydrosulfide ions. The spent liquor
from the kraft process, often called black liquor, contains
the wood lignin in the form of derivatives soluble in
alkaline solutions. These particular alkali lignins
are known as sulfate lignin. It has been discovered, as
Schulerud, C.F. and Doughty, J.B. point out in their
article "Reactive Lignin-Derived Products in Phenolic
High-Pressure Laminates", ~appi 44 (1961), 823-30, that
sulfate lignin, reacts poorly with phenol-formaldehyde
resins. Therefore, there have been efforts to produce
a lignin product with better reactivity, e.g. by chemically
modifying black liquor or sulfate lignin obtained therefrom.
United States Patent 3,185,654 dislcoses a method in which
sulfate lignin is precipitated from black liquor by acid,
and the thus obtained sulfate lignin in acid form is dissolved
in phenol-formaldehyde resin. A volatile liquid i.e.
B ~ ~
- 3 - 1068426
methanol, ethanol, or propanol, is used as solvent, and the
pH of the adhesive solution is maintained between 3 to 7.
The patent points out that sulfate lignin can be fractionated
by precipitating by acid. These fractions were found to
have slightly different characteristics. The inventors
conclude that all these fractions are usable as long as
the lignin is in the free acid form.
According to the present invention there is provided an
adhesive for the manufacture of wood products, comprising
an alkali lignin and a phenol-formaldehyde resin, at least
50~ by weight of said alkali lignin having a molecular weight
of over 5000. It has been unexpectedly discovered that the
molecular weight of the alkali lignin is of critical
significance as far as the adhesive characteristics of the
product is concerned and at least 50% by weight of the
alkali lignin should have a molecular weight of over 5000.
When using the present adhesives, the same manufacturing
conditions, such as pressing times and temperatures, can
be used as when using commercial phenol-formaldehyde resin
in the manufacture of plywood. Since the plywood thus
manufactured fulfills requirements set for exterior grade
plywoods, and the lignin derivatives used in the adhesive
are considerably cheaper than phenol-formaldehyde resins,
the use of the present adhesive is economically advantageous.
Due to the moderate alkalinity of the adhesive, it does
not cause corrosion or hydrolytic defects in veneer as may
result when using acid adhesives.
Weatherproof particle board can be manufactured using the
pesent adhesive. This is a noteworthy advantage, because
a board glued with phenol formaldehyde is not weatherproof.
The advantage of the manufacturing method as compared with
other processes employing spent sulfite liquor lignosulfonates
include the one-phase hardening process of boards and the
short pressing time needed in it. The corroding and
hydrolyzing effect of acid adhesives has also been avoided.
The above-mentioned advantages of the invention have an
extremely beneficial effect upon the economic aspects of
the manufacturing method of an industrial scale.
12 3
- 4 - 10~8~Z6
The alkali lignin can be sulfate lignin from alkaline
pulping processes, for example the soda process, wherein
the pulping liquor contains sodium hydroxide, or the sulfate
process, wherein the pulping liquor contains both sodium
hydroxide and sodium sulfide and hydrosulfide, or the oxygen-
alkali pulping process, where pulping is performed with
sodium hydroxide in the presence of oxygen.
The molecular weight distribution of the lignin derivatives
is such that at least 50~ by weight of the alkali lignin has
a molecular weight of over 5000.
In order to obtain the required molecular weight distribution
of the lignin from alkaline processes, ultrafiltration can
be used in a manner analogous to the separation of ligno-
sulfonates.
The present adhesive is preferably obtained by preparing an
aqueous solution containing the alkali lignin and phenol-
formaldehyde resin. The two basic classes of phenol-
formaldehyde resins are resoles and novolacs and both types
can be used in the invention. Resoles are prepared using
an alkaline catalyst with the weight ratio of phenol to
formaldehyde generally being in the range of 1:1 to 1:5.
Novolacs can be prepared using either an acid or alkaline
catalyst, although an acid such as oxalic acid is preferred.
The phenol to formaldehyde ratio is generally in the range
of 1:0.8 to 1:1. Novolacs require the addition of a curing
agent, such as paraformaldehyde, hexamethylenetetramine
or furfuraldehyde.
The preferred weight ratio of the alkali lignin to the
phenol-formaldehyde resin when used for plywood manufacture
is 70:30 to 20:80, and for particle board and corresponding
products, 90:10 to 20:80. The water content of the adhesive
as used may be 30 to 70% by weight and the viscosity can be
altered by regulating the water content or by heating the
adhesive. The suitable range of viscosity is generally
100-2,000cP measured at 23C.
The adhesive may also include an aldehyde as a hardener,
the aldehyde being selected from formaldehyde, paraformaldehyde
B and furfuraldehyde.
1068~'Z6
- 4a -
Tradional extenders, such as chalk, wood flour and wheat
flour, may be used in the adhesive.
When manufacturing particle board using the present
adhesive, for instance paraffin may be added to the
adhesive or chips separately so as to improve the board's
swelling properties.
Example 1 (Comparative Example)
The following example illustrates the suitability of two
black liquor products for preparing the binder.
B
10~84;~6
One agent was black liquor, evaporated to a solids content of
37.5 % by weight, the other was sulfate lignin obtained from black
liquor by precipitating with acid. This sulfate lignin had been
dissolved into a NaOH-solution after filtration and washing. (See
Table I).
Table I
With acid precipitated
Black Sulfate Lignin Dissolved
liquor into NaOH-solution
Solids Content (by weight) 37,5 % 37,5 %
pH 12,7 12,2
Viscosity measured by Brook-
field RVT-viscosimeter, 23C 43 cP 3,336 cP
Weight distribution according
to molecular weight
Mw ~ 5000 25 % 32 %
Mw ~10000 14 % 18 %
Mw ~15000 9 % 11 %
The binders were prepared as follows:
450 g of a NaOH-solution of acid-precipitate sulfate lignin was
mixed with 550 g of a commercial phenol formaldehyde resole resin
with a solids content of 46 %.
84 g of a composition containing 30 parts by weight of wood powder,
50 parts by weight of chalk, and 20 parts by weight of wheat flour
was added to the solution.
The binder was spread on birch veneers 1,5 mm thick with a moisture
content of about 3 % by weight. The glue spread on each side of
the veneer was 150 g/m . 3-ply panels were prepared. Prepressing
pressure was 7 kp/cm and time 7 minutes. Hot pressing temperature
was 135C, pressure 16 kp/cm2, and time 4 minutes and 2 minutes.
6 10684Z6
Properties of the prepared panels as determined according to
Finnish standard O.IV.l are illustrated in Table II.
Table II
_.
Dry Wet
Pressing Sh`ear Wood Shear Wood
Time Stress Failure Stress Failure
Min kp/cm2 kp/cmZ %
Black liquor 2 Z6.4 57 16.0 8
4 28.S 73 16.8 12
~cid-precipitated 2 26.4 68 16.0 23
sulfate lignin _ 28.1 76 18.0 27
Finnishstandard _20.9 _14.1 _50
Table II indicates that, by using both adhesive compositions, it
was possible to prepare plywood the shear stress values of which
meet the requirements set by Finnish standard O.IV.l. ~ood failure,
which is of primary importance considering the weather proofness
of the product, did not, however, meet the requirements. As shown
in Table II these values could not be improved even by doubling the
pressing time.
Example 2
Preparation of plywood using sulfate lignin in an adhesive having
a molecular weight distribution as required by the invention.
Sulfate lignin which was separated from black liquor and had
the following properties was used for the preparation of the
binder:
~ - 6-
10684~6
6a
Table III
Sulfate lignin derivative sepa-
rated from black liquor dissolved
into NaOH solution
..
Solids Content (by weight) 37,5 %
. . .. ._ ._
pH 11,5
. _ ..
Viscosity measured by
Brookfield RVT-viscosimeter, 45,480 cP
23C
.. _ ._
Weight distribution according .
to molecular weight:
Mw ~ 5000 58
Mw > 10000 38 ~
Mw >15000 24 %
450 g of a NaOH-solution of the above mentioned sulfate lignin
derifative was mixed into 550 g of a phenol formaldehyde resole
resin with a solids content of 46 % by weight. Viscosity of the
mixture was 388 cP at 23C. 84 g of a composition containing 30
weight parts wood powder, 50 weight parts chalk, and 20 weight
parts wheat flour was added to this solution. After this addition,
the viscosity was 1000 cP at 23C.
The manufacturing conditions of plywood were the same as in
Example 2.
The properties of the plywood boards appear in the following table
IV:
6a
7 1068426
Table IV
.
Dry Wet
Shear Wood Shear Wood
Pressing Time Stress Failure Stress2 Failure
Min kp/cm2 % kp/cm %
2 `30.1 89 17.3 80
4 3~.0 91 20.0 96
Finnish standard ~20.9 > 50 ~14.1 ~ 50
___ .. ___ .
The panels completely met the requirements of Finnish standard
O.IV.l for exterior grade plywood.
Example 3
Preparation of particle boards.
The binder was prepared using the same sulfate lignin as in Example
2. 540 g of a 46 ~ by weight aqueous solution containing 210 g
of sulfate lignin and 40 g NaOH was mixed into 460 g of a
commerical phenol formaldehyde resole resin solution with a solids
content of 46 % by weight. The viscosity of the adhesive was 1040
cP at 23C and the pH was 12Ø
184 g of paraffin emulsion (solids content 50 % by weight) was
mixed into the adhesive, the viscosity thereafter being 850 cP
t23C, measured by Brookfield RVT-viscosimeter).
3-ply particle board containing 30 % outer layer chips and 70 %
inner layer chips was prepared. The dry matter of the adhesive
sprayed amounted to 12 % of the dry surface chips and 10 % of the
dry inner layer chips. Prior to gluing the moisture content of
the chips was approximately 1 % and after the gluing about 11 %.
10684Z6
7a
The board thickness was 15 mm and the volume weiqht 750 kg/m3.
Pressing temperature was 210C, pressure 27 kp/cm2, and time 60
sec/mm. The board properties appearing in the following table V
were determined according to German standard DIN 52360-65 and 68761.
Table V
_ Bending Tensile Thickness Swelling Telsine
Stren~th Stren~th 2 h 24 h Strength
kp/cm~ kp/cm~ % 96 aft~r Boil-
kp/cm21
Binder of the 208 6.21.9 12.9 2.5
Invention
Requirements ~ 180 '3.576.0 ~ 15.0 ~1.5
according to DIN
As it appears from Table V, the b~nder of the invention can be
used in preparation of particle boards which meet the West German
Standards (DIN) for weatherproof particle boards.
~. .
_7 ~
8 1068426
` .ious modes of carrying out the invention are contemplated as
being within the scope of the following claims particularly pointing
out and distinctly claiming the subject matter which is regarded
as the invention~
