Note: Descriptions are shown in the official language in which they were submitted.
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LIGNIN-BASED ADHESIVES FOR PARTICLE BOARD MANUFACTURE
The present invention relates to the manufacture of particle boards and
similar wood-based
products comprising Iignocellulosic particles, fibers or flakes mixed with and
bonded
together with an adhesive binder. In particular the present invention concerns
navel lignin
and/or carbohydrate -based adhesives and a preparation process thereof as well
as particle
boards manufactured using the adhesives.
The rapid increase in the production of particle boards and fiber boards,
especially medium
density fiber boards (in the following also abbreviated MDF boards), during
the last decades
has created a demand for adhesives that are inexpensive, available in large
quantities, and
independent of crude oil. Lignin meets well these requirements, and it does
not contain any
formaldehyde, which traditionally been considered a serious problem with
conventional
urea-formaldehyde (LJF) adhesives. As a major wood component, native lignin is
neither
hygroscopic nor soluble in water. However, during technical pulping lignin
becomes soluble
in water, due to degradation and chemical changes.
The use of spent sulphite liquor (SSL) as an adhesive for paper: wood and
other iigno-
ceIlulosic materials is well-known in the art, and a large number of patent
applications has
been filed during the last three decades for the use of lignin products as
adhesives for
particle boards, pivwood and fiber boards instead of conventional PF or OF
adhesives.
Reference is made to DE Patents Nos. 3 037 992, 3 621 218: 3 933 279, 4 020
969, 4 204
793 and 4 306 439 and PCT Applications published under Nos. WO 93125622, WO
94/01488, WO 95123232 and WO 96/03546.
The main drawback of using SSL as an adhesive for fiber board manufacture is
its
r hygroscopicity. For this reason it cannot realty compete with otner natural
or synthetic
adhesives. When SSL is applied as an adhesive, it can be convened to an
insoluble
substance by curing=. Chemically the curine of lignin is a cross-linking
process, which leads
to new carbon-carbon and ether bonds between different lignin molecules or
within one
macromolecule. Inter- as well as intramolecular cross-linking reactions
decrease the
solubility and swelling of lignin. Cross-links in lignin can be achieved
either by condensation
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or by radical coupling reactions. For the condensation reactions, either high
temperatures
and long heating times or mineral acids are required, which cause structural
changes or
charring in the wood particles. Recently, cross-linking of the Iignosulphonate
molecules by
radical combinations has been developed. Inmost cases additional cross-linking
agents for
lignin are necessary, such as epoxides, polyisocyanates, polyols, poly(acryl
amides,
polyethylene imine) and aldehydes.
Further, it has been shown that iaccase enzymes and other peroxidases can be
used as
polymerization or curing catalysts of lignin (DE Patent No. 3 037 992, WO
96/03546).
However, the enzymes for creating radical reaction have shown limited success
so far.
Fibers and wood chips used in the production of fiber boards contain 5 - 20 %
water and
the laccases used need some water to effectively catalyze the polymerization
reaction
needed for extensive bonding of the fiberboard. Kraft lignin like native
lignin to its major
part is; however, insoluble in water and thus two solid phases are formed on
the production
line. An uneven distribution of the solids cause spotting and major failure in
the strength
properties of the board formed in the pressing stage.
A further problem relating to the use of isolated lignin is the high price of
kraft lignin) which
is near the limit for economical production of particle boards.
For the above mentioned reasons, lignin-based board production processes have
not, so far,
led to any major practical applications.
The present invention aims at eliminating the problems relating to the prior
art. In particular
it is an object of the present invention to provide a novel lignin-based
adhesive for use with
particle boards and similar wood-based products. It is another object of the
present
invention to provide new particle boards and similar wood-based products. It
is a third
object to provide a method for manufacturing the adhesive.
These and other objects, together with the advantages thereof over known
lignin-based
adhesives and processes for the preparation thereof, which shall become
apparent from the
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3
specification which follows, are accomplished by the invention as hereinafter
described and
claimed.
According to the present invention, lignin or lignin polymerized with
oxidative enzymes or a
soluble fraction of carbohydrates obtained from mechanical refining of
lignocellulosic
materials, is added to and mixed with the fibers or chips or flakes used as
lignocellulosic
raw material of the wood-based product in the form of a foam to provide an
even dispersion
of the solids.
Foamed LTF and PF adhesives are known in the art. Said foams are used for
improving
process performance and product quality especially in adhesives with high
solid contents.
CA Patent Application No. 2,114,258 describes particle board production by
using foamed
mixtures ofLTF resins and animal blood. Also DE 3,644,067 describes the use
offoamed
materials in binding of fibers and flakes to produce a homogeneous adhesive
application of
fine particles (fibres or powders) on the furnish. T. Sellers describes in
Forest Prod. J. 38
(1988) p. 55-56 the superior performance of foamed OF and PF resins in
particle board
manufacture especially at higher solids content of the adhesive. However, none
of the prior
art references mentions foaming of lignin-containing adhesive compositions.
According to
these references, the composition of the gas is not important either.
The lignin-based adhesive composition according to the present invention
comprises an
aqueous suspension of lignin andlor carbohydrates obtained from a process of
pulping
Iignocellulosic materials. The suspension has been foamed to I .OS to 10,
preferably 1.1 to 8,
in particular to 1.2 to 5 times the volume of the liquid mixture. It is
preferred to use
polymerized kraft lignin as the lignin component. Furthermore) according to
the present
invention, natural lignin in wood fibers is used to replace a substantial part
of the lignin in
._ the adhesive binder intended for particle board production. Thus, the
present adhesive
binders comprise an aaueous mixture of fibers together with lignin andlor
carbohydrates
obtained from a process of pulping lignocelluiosic raw materials. The fibers
in the adhesive
binder as well as the added lignin and/or other lignocellulosic material
radical polymerize
due to the effect of the enzyme in an even oxygen-containing foam matrix.
instead of lignin,
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the use of native lignin -containing fibers of MDF or TMP types leads to
savings in adhesive
use, and, however, to a corresponding strength.
The foam will homogenize the solid phase and the liquid phase, increase the
viscosity of the
mixture and prevent the sedimentation of the solids. The oxygen-containing
foamed
structure will also provide more oxygen for the oxygen-dependent oxidase
catalyzed
ligninlcarbohydrate polymerization: Because of its large volume, the foam will
cover more
homogeneously the wood fibers and wood particles in the preparation of the
boards. This
will lead to better strength values and better control of the use of the
adhesive in the
production.
The invention will now be explained in more detail with the aid of the
following detailed
description and with reference to a number of working examples.
Within the context of the present invention, the terms "adhesive", "adhesive
binder" and
"resin" designate a chemical composition which, in the wet stages of the
manufacture of,
e.g. particle and fiber boards, provides adhesion between the particles or
fibers. After heat
compression during board manufacture, the composition containing polymerized
resin
works as a binder which keeps the particles or fibers or flakes bonded
together.
The term "wood-based product" denotes any Ggnocellulose-based product, such as
particle
boards; fiber boards (including high and medium density fiber boards, i.e.
hard boards and
N>DF boards), flake boards, plywood and similar products constituted by
particles, fibers or
flakes of vegetable origin, in particular derived from wood or annular or
perennial plants
mixed with and bonded together with adhesive binders. According to a
particularly
preferred embodiment, the present invention is used for preparing particle
boards and
similar wood-based products in which wood particles can be crosslinked and
joined
together with the fibers of the adhesive composition, which will be attached
to the particles
by the lignin-based adhesive.
The term "fibers" encompasses organic and inorganic fibers of any suitable
material having a
length-to-thickness ratio of more than 6, preferably more than 10, in
particular more than
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20. It is preferred to use lignin-containing fibers, because the lignin of the
fibers will be
oxidized in the presence of oxidases and work as an adhesive in mixtures of
wood particles
and the adhesive foam. Particularly preferred iignocellulosic fibers are
derived from wood
or annular or perennial plants. Such fibers can be obtained from mechanical
refining of
wood or plants by refiner mechanical pulping, pressurized refiner mechanical
pulping,
thermomechanical pulping, groundwood or pressurized groundwood pulping, or
cherni-
thermomechanical pulping. Furthermore, fibers from the preparation of fiber
boards (e.g.
medium density or high density fiber boards) can be used in the preparation of
particle
boards.
In addition to the cellulosic fibers mentioned above, other natural fibers,
such as cotton
fibers, Abaca hemp fibers, sisal fibers, ramie fibers, flax fibers or jute
fibers, can be used.
Inorganic fibers such as glass fibers, carbon fibers, gypsum fibers, etc. can
also be
employed.
For polymerizing lignin and carbohydrates of soluble wood fractions, oxidative
enzymes
capable of catalyzing oxidation of phenolic groups can be used. These enzymes
are
oxidoreductases, such as peroxidases and oxidases. "Peroxidases" are enzymes
which
catalyze oxidative reaction using hydrogen peroxide as their substrate,
whereas "oxidases"
are enzymes which catalyze oxidative reactions using molecular oxygen as their
substrate.
Phenoioxidases (EC 1.10.3.2 benzenediol:oxygen oxidoreductase) catalyze the
oxidation of
o- and p-substituted phenoiic hydroxyl and amino/amine groups in monomeric and
polymeric aromatic compounds. The oxidative reaction leads to the formation of
phenoxy
radicals and finally to the polymerization of lignin and possibly the
carbohydrate matter. In
the method of the present invention, the enzyme used may be any of the enzymes
catalyzing
the biological radical formation and secondary chemical polymerization of low
molecular
weight lignins, such as laccase, tyrosinase, peroxidase or oxidase.
As specific examples of oxidases the following can be mentioned: laccases (EC
1. I 0.3.2),
catechol oxidases (EC 1.10.3 .1 ), tvrosinases (EC 1.14.18.11 and bilirubin
oxidases (EC
1.3.3.5). iaccases are particularly preferred oxidases. They can be obtained
from bacteria
and fungi belonging to, e.g., the following strains: Aspergillus. Neurospora,
Podospora,
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Botrytis, Lentinus) Polyporus, Rhizoctonia, Coprinus, Coriolus, Phlebia,
Plerrrotus,
Fusarium and Trametes.
Suitable peroxidases can be obtained from plants or fungi or bacteria.
Preferred peroxidases
are those which originate from plants, in particular horseradish peroxidase
and soy bean
peroxidase.
The terms "surfactant" or "surface active agent" are synonymously used to
designate
compounds which have affinity to water and to hydrophobic (e.g. fatty)
materials, thus
aiding the hydrophobic materials to suspend in water.
Adhesive com op vent
As mentioned above, lignin of different origins can be used as the adhesive
component of
the present foamed adhesive binders. In particular, isolated lignin of, e.g.,
the sulphate,
sulphite, ORGANOSOLV and MILOX processes can be used.
However, in addition to lignin or instead of it, various soluble wood
fractions can also be
employed. Thus, it is known in the art that during mechanical refining of
chips, a part of the
compounds of the fibrous raw material is dissolved (about 1 % of the fiber
weight). This
fraction contains primarily the same chemical components as the fibers
(carbohydrates,
extractives and lignin): To mention 'an example: the soluble fraction of
softwood chips
contains some 40 to 70 % carbohydrates, 10 to 25~% lignin and 1 to 10 %
extractives,
whereas hardwood pulping process water contains some 20 to 60 % carbohydrates,
10 to
25 % lignin and 10 to 40 % extractives.
It has been found that this soluble lignin/carbohydrate fraction obtainable
from mechanical
or chemimechanical pulping is particularly useful as an additive or adjuvant
for gluing of
particles boards, fiber boards and other similar wood-based composite
products.
Particularly good gluing is achieved if this fraction (as is the case with
lignin) is polymerized
with laccase (or similar) oxidase enzyme(s). The results are on the same level
as those
obtainable with conventions! phenol or urea formaldehyde resins.
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It should be noticed that a similar soluble fraction can be obtained by
treating lignocellulosic
raw material, such as wood fibers or saw dust, with hydrofases, e.g.
cellulases,
hemicellulases and pectinases.
~s4~.=f~rtare of adhesive~inder foam
The present invention relates to foam-based technology for providing good
dispersion of
the different solids. Thus, efficient contact during economical production of
particle board is
achieved using adhesive binders based on oxidase-activated lignin and/or
carbohydrates
together with wood fibers or other lignocellulosic fibers. 0.1 to 50 %,
preferably 0.5 to 30
°ro and in particular about 1 to 10 % by weight of the kraft lignin or
similar lignin is replaced
by fibers in the foam-based, oxidase-activated adhesive formulation.
According to the preferred embodiment of the present invention, in which wood-
derived
1 S fibers are used, lignin and the fibers are activated by oxidases are mixed
with the chips in the
form of a foam that provides a good viscose suspending medium to form an even
dispersion
of the solids and hence improves substantially the quality of the particle
board formed. The
use of the foamed adhesives as described in this invention leads also to
better control of the
application and to substantial savings in adhesive use. The foam is produced
by the use of a
surface active agent.
The foamed, activated fiber and lignin/carbohydrate dispersion can be produced
separately
from the chips, which are mixed into the foam by known mixing technology, e.g:
by
extruding or spraying the foam onto the fcbers or chips. The foam can also be
produced
simultaneously by mixing the fibers and chips with the foam chemicals, the
lignin/carbohydrates and the laccase.
According to the present invention, the foam is produced by dispersing the
mixture of iignin
and fibers into water to form a suspension and bubbling a gas through the
suspension to
form bubbles having a medium diameter of 0.001 to 1 mm, in particular about
0.0I to 0.1
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The foam is produced by the use of a surface active agent that can be anionic,
cationic or
non-ionic. Thus, the surfactant can be selected from the group consisting of
alkylsulfonate
or alkyl benzene sulfonate, Tween~ and other commercial compounds, fatty acid
soaps,
lignosulfonates, sarcosinates, fatty acid amines or amines or poly(oxyetylene
alcohol)s and
wood and plant extractives. Foam stabilizers and solid surfactants, such as
CMC, gelatin,
pectin, wood extractive and similar compounds, can be used to produce and
enhance the
foam stability. A smali amount of the surface active compounds i5 needed, i.e.
about 0.01 to
%, in particular about 0.05 to 5 %.
10 The foam can be produced by foaming in a static foamer or in a turbulent
foam cell by using
known mixing technology. Although any gas can be used for foaming, it is
preferred to use
oxygen-containing gases, such as air, oxygen enriched air, oxygen or
pressurized systems of
these. The importance of using oxygen-containing gases is discussed in further
detail below.
The foam produced is essentially stable during handling, storage, transport
and manufacture
of wood-based products and it has a density in the range of 0.1 to 0.9 kg/1,
in particular
about 0.2 to 0.7 kg/l) and a medium foam bubble diameter in the range of 0.005
to 0.1 mm,
in particular about 0.01 to 0.1 mm, preferably about 0.02 mm.
The method according to the present invention can be used for all oxidase-
catalyzed,
previously unsuccessfully suggested enzyme-catalyzed glueing processes using
oxidases.
The enzyme used can be any of the enzymes catalyzing the oxidation and
polymerization of
aromatic compounds or lignins, such as laccase, tyrosinase) or other oxidases,
as mentioned
above. The amount of enzymeused varies depending on the activity of the enzyme
and on
the amount of dry matter content of the composition. Generally, the oxidases
are used in
amounts of 0.001 to 10 mg protein/g of dry matter, preferably about 0,1 to 5
mg protein/g
of dry matter. The activity of the oxidase is about 1 to 100,000 nkatlg,
preferably over 100
nkat/g.
In connection with the present invention it has been found that oxygen plays a
decisive role
in the enzymatic polymerization of lignin of any origin. This is important in
particular far the
production of adhesives for the manufacture of fiber boards, particle boards
and flake
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boards and other similar wood-based products. Thus, in addition to the
aromatic reactant,
also oxygen is needed. The oxidative reaction leads to the formation of
phenoxy radicals
and finally to the polymerization of lignin.
In the known methods discussed above, crosslinking was only partially achieved
because of
apparent linutations on the availability of oxygen. The limitation of the
reaction by oxygen
manifests itself in the long reaction times used, and in the poor strength
properties obtained,
thus impairing the result of the enzyme-aided polymerization.
Oxygen can be supplied by various means, such as efficient mixing, air
enriched with
oxygen or introducing oxygen supplied by enzymatic or chemical means to the
solution.
However, according to the present invention, by foaming the suspension with an
oxygen-
containing gas it is possible to provide the oxygen needed for full
polymerization of the
lignin. At the same time, the foamed structure gives enough strength to the
adhesive so as
to prevent sedimentation of water-insoluble lignin or the polymerized lignin
from the
adhesive thus producing failure in the production and in the board product.
The present invention provides considerable advantages. Thus, the foamed
structure can be
utilized in the preparation of particle boards of lignin andlor carbohydrates
or polymerized
lignin by utilizing the adhesive properties of the lignin or polymerized
lignin) whereas the
fibers work as crosslinkers between the particles. The foam will homogenize
the
inhomogenous solid phase and the liquid phase, increase the viscosity of the
mixture and
prevent the sedimentation of the solids and the fibers. The oxygen-containing
foamed
structure will also provide more oxygen for oxygen-enhanced oxidase catalyzed
Iigninlcarbohydrate polymerization. Because of its large volume the foam will
cover more
homogeneously the wood fibres and wood particles in the preparation of the
boards. The
adhesive binder is evenly distributed throughout the wood-based product, about
0.02 ~
0.002..Ø2 t 0.02 glcm3, preferably 0.04 to 0.08 glcm3 in any randomly
selected volume
unit of the product.
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This leads to better strength values and better control of the use of the
adhesive in the
production.
The following non-limiting working examples illustrate the invention.
Example 1
Manufacture of MDF fibers
MDF fibers were prepared from hardwood (Swedish birch) by refining birch chips
in a
10 conventional TMP process at a refiner pressure of 4 bar.
The fiber quality was tested using both Bauer McNett analysis and a Pulmac
shive analysis.
The results are shown below:
Table 1. Fiber characteristics
Bauer McNett
>16(>1.19mm 18.6
16-30 (1.19-0:595 mm) 25.1
- 30-50 (0.595-0.297 mm) 12.4
50-100 (0.149 mm 29.0
<100 (<0.149 mm) 15.0
Puimac shive analysis
Screen late 0.15 mm 7.0
Screen late 0.25 mm 0.8
Example 2
Manufacture of particles boards
This Example describes the preparation of a foamed adhesive consisting of
kraft lignin,
iaccase, wood fibre and the manufacture of particle boards from the adhesive.
The test has
been carried out on laboratory scale.
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Kraft lignin Indulin AT and 1 - 10 % of its weight substituted with the MDF
fibers of
Example 1 were used as a binder mixture in particle board test panel
manufacture.
3.6 - 3.96 g of the lignin was vigorously mixed with 0.04 - 0.4 g of MDF
fibers and aerated
S for 30 min with 4.0 g of laccase concentrate (activity 4,000 nkat/g) in 2.O
g of 2 M sodium
acetate buffer (pH 4.5). 1.4 g of the mixture was mechanically mixed with 4.4
g of chips.
The chips had already been treated with 1 % of wax (Mobilex 54, 60 % emulsion)
of the
dry weight of the fibers. The reference tests were performed without iaccase
(water was
used instead) and using commercial ureaformaldehyde resins.
For strength tests particle board panels of the size of 50 rnm x 50 mm x 2 mm
(weight about
5 g) were prepared by pressing 2 min in 30 kp/cm2 of pressure and 190
°C of temperature.
After pressing, the panels were then cut into four pieces (50 mm x 12 mm x 2
mm). These
pieces were tested for parallel tensile strength with Zwick tensile strength
testing
equipment.
Table
2.
Results
of
glueing
tests
for
particle
boards
Foamed binder Tensile strength
MPa
Water 3.0 0.3
Indulin AT ( 10 %) + 8. 5 0.4
MDF fibers
{90 %)
Indulin AT ( 10 %) + 11. 9 0. 7
MDF fibers
(90 %) + iaccase ,
Induiin AT 7.2 0.4
indulin AT + laccase 12.7 0.6
Reference OF resin 12.5 0.7
As apparent from the results of Table 2, up to 10 % of laccase-activated kraft
lignin can be
substituted as an adhesive by MDF fibers without any significant loss in
tensile strength.
The strength properties of particle boards prepared according to the present
invention are
quite comparable to those of conventional OF resin glued boards.
