Note: Descriptions are shown in the official language in which they were submitted.
1~)848Z9
LIGNI~-BASED S~NTH~TIC RESIN
The present invention relates to a novel method of
treating lignin, primarily lignin obtained from cellulose indus-
trial waste liquors, lignosulphonates and sulphate lignin to
enable said lignin to be used as a synthetic resin, particularly
as a binding agent in the manufacture, for example, of chipboard
and plywood.
Lignin can be said to be the natural adhesive substance
in wood and consequently it is not surprising that lignin in
different forms has been tested as a binding agent in wood-based
~roducts of different types.
Lignin in the form of lignosulphonic acids, most often
in the form of sulphite liquor which has been modified to a greater
or lesser degree, has been tested, for example, in the so-called
Pedersen Process (Danish Patent 100,984). According to this
method, which provides weather-resistant chipboard of very high
quality, the pH of the liquor is lowered with an acid, whereafter
the liquor is sprayed on the chips. Curing is effected in two
stages, first hot pressing and then hardening for a relatively
long period of time (1 - 2 hours) in an autoelave. The proeess
is thus eomplieated and time-eonsuming when earried out on an
industrial seale. The aeidity of the binder also ereates serious
eorrosion problems. A similar process whieh utilises the aeidi-
fieation of sulphite liquor is described in Forest Products
Journal 24 (1975) 2, 3~. According to this process, which is
applied in the manufacture of so-called wafter-board, the chip
material is first sprayed with a 15 to 20% sulphuric acid solution.
The acidified ehip material is then admixed with sulphite waste
liquor powder. The process is said to afford an economical àdvan-
tage and to impart good mechanical strength to the board material,although the acidity of the material (pH approx. equal to 2.5)
means that the process and the material, from the aspect of cor-
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rosion, and the long-term properties of the material must be held
in serious doubt.
A large number of methods for modifying lignin so that
it is more suitable for use as an adhesive has been tested and
patents applied for.
The German Patent Specification 1,223,534 proposes the
admixture of melamine-formaldehyde resins, while Roffael and Rauch
(Holzforschung 25 (1971) 5, 149 and 26 (1972) 6, 197) proposes
the admixture of different forms of phenol resins. The disadvan-
tage with these methods, which provide high quality board, is
that large quantities of synthetic resin are still required in
order to obtain material properties equal to those which would
be obtained solely with synthetic resins. What has been achieved
is that approximately 25% of the synthetic resin has been replaced
with lignosulphonic acids with no appreciable adverse effect on
the properties of the material.
Lignin has also been used as a replacement for phenol
in the manufacture of phenol resins to a greater or lesser extent.
In this case the ability of the lignin, similar to that of phenol,
of reacting with formaldehyde has been utilised. In this way
there is obtained a copolymer of lignin-phenol-formaldehyde which
can be used as an adhesive.
According to U.S. Patent Specification No. 3,597,375
lignosulphonate is first reacted with phenol under alkaline con-
ditions, whereafter the reaction mixture is reacted with formal-
dehyde.
According to U.S. Patent Specification No. 3,227,667,
lignin is first reacted with phenol under acid conditions, where-
after the xeaction product is reacted with formaldehyde under
alkaline conditions. Swedish Patent No. 371,645 discloses a
method in which a quantity of formaldehyde is added in the first
stage.
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The disadvantage with these methods is the relatively
complicated method of manufacture and the large quantity (more
than half dry weight) of synthetic resin components (phenol +
formaldehyde) required. Another disadvantage is that, from the
adhesion aspect, the lignin molecules, which are from the start
slightly too large, are made even larger. The result of this is
that large quantities of lignin-phenol-formaldehyde resin must
be used in order to obtain the desired properties of physical
strength of the board material produced. In actual fact, with
some of the aforementioned methods, the quantity of phenol +
formaldehyde in the binding agents used has been greater than
in board produced solely with pure phenol resins.
To illustrate the background of the present Application,
the molecular size of an effective wood adhesive must be discussed.
It is important that the molecules of the binding agent are neither
too large nor too small. If the molecules are too small, then they
penetrate the surface of the wood to an excessive extent and the
proportion of the binding agent which remains between the two sur-
faces to be joined is too small for the purpose. When, for ;
example, a mixture of phenol and formaldehyde (phenol-formaldehyde
solution) which has not been precondensed is used as an adhesive,
practically no binding effect is obtained at all. On the other
hand, if the molecules are too large, the jointing surfaces are -
not sufficiently penetrated whereby the adhesion therebetween is
not that desired. Consequently, the molecular weight of phenol
adhesive for wood-glueing purposes lies between 200 - 300, while
the sulphate lignin lies between 1000 and 2000. The mean molecular
weight of lignosulphonates obtained from coniferous (softwood)
wood is about 10,000.
A further undesirable property of the unmodified lignin
with regard to adhesion is the tendency of the lignin molecules to
lump together to form agglomerates of relatively low plasticity
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and the relatively few groups possessed by the lignin capable of
cross-linking.
It has now been found that when the lignin, which is
not directly suitable for use as a wood adhesive, is mixed with
chemicals oflow molecular weight which (1) are capable of swelling
the agglomerate of lignin molecules, (2) impart additional cross-
linking possibilities to the adhesive, (3) when pressing the glued
joint surfaces together are capable of being pressed from the
lignin to a certain extent and to penetrate the wood material at
the joint surfaces and there with form a bridge between wood sub-
stance and adhesive, there is obtained a fully satisfactory adhesive.
One such chemical is the previously mentioned mixture
of phenol and formaldehyde, which in itself is completely unusable
as an adhesive. The surprising fact is that if, for example, pre-
cipitated and air-dried sulphate lignin and a phenol-formaldehyde
solution, which in itself affords no adhesive effect, are mixed
together there is obtained an adhesive which exhibits excellent
adhesion properties.
Accordingly the invention relates, in general, to a
synthetic resin product which can be used, in particular, for the
glueing of wood products and for the manufacture of shaped objects
said synthetic resin product comprising lignin and a low-molecular
weight liquid substance which is capable of penetrating and swell-
ing lignin and which can be reacted with lignin to form a hardened
product, the liquid substance being physically absorbed in the
lignin. Hardening of the lignin may be effected, for example, by
heating the same to a suitable temperature or by admixing said
lignin with a hardener which will cause hardening to take place
at room-temperature or a moderately raised temperature.
In the present context the term "lignin" is not used
solely with reference to lignin as such, but also, for example,
with reference to lignosulphonates and similar lignin derivatives
i~84~329
which possess substantially the same properties as lignin and
which can react in the same manner with the liquid substance used
in accordance with the invention. The lignin may be present in
the form of a purified product, although crude lignin containing
sugar and similar compounds may be used, for example, spray-dried
sulphite waste liquor.
As previously indicated, a suitable liquid substance
for use in accordance with the invention is a solution of formal-
dehyde in phenol, optionally in combination with an acid or
alkaline hardener. Such a solution may be readily prepared by
melting phenol and adding formaldehyde (for example in the form
of paraformaldehyde). The mixing ratio of phenol to formaldehyde
may vary as desired, depending upon the desired properties of the
finished product, although this ratio generally lies between 3:1
and 1.5:1, preferably between 2.2:1 and 1.8:1.
The hardener used for the phenol-formaldehyde solution `~
may be known acid and alkaline hardeners normally used for phenol
aldehyde resins. Examples of acid hardeners include ammonium
chloride and paratoluene sulphonic acid, while examples of alka-
line hardeners include sodium carbonate, sodium hydroxide and
ammonium hydroxide. The hardener may be dissolved in the phenol
formaldehyde solution or may be admixed with the lignin separately
at the same time as the mixing of the solution. The solution is
used generally in quantitiesof between 0.1 - 5%, preferably 1 - 2%
calculated on the weight of the total synthetic resin product.
The synthetic resin product is prepared by simple mixing
processes, suitably at room-temperature or a temperature slightly
higher than room-temperature, in any event at a temperature
which is so low that no substantial reaction takes place between
the lignin and the solution. The lignin can be used in powder
form, i.e. in the form of a purified precipitate from black liquor
or in the form of a lignosulphonate recovered from sulphite waste
liquor.
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4829
The lignin and the phenol-formaldehyde solution is mixed
in proportions which may vary between 1:10 and 100:1, preferably
- between 1:1 and 10:1, depending upon the desired properties of
the product. In the case of an adhesive, the mixing ratio is con-
veniently selected so as to obtain a paste-like consistency or a
viscous consistency reminiscent of conventional glue. Suitable
mixing proportions can be readily established empirically. In-
stead of ordinary phenol (i.e. hydroxybenzene) other phenols
such as m- or p-cresol can be used. Similarly it is possible to
substitute the formaldehyde with other aldehydes, such as furfural.
Instead of phenol and aldehyde, it is also possible to
use other liquid substances capable of absorbing lignin and of
reacting therewith, i.e. liquid substances which have the afore-
mentioned properties. -
When used as an adhesive, the synthetic resin product
according to the invention is applied to the joint surfaces in
the normal manner. When applying the product to chipboard, the
adhesive is applied to the chips prior to the chips being pressed, ~ -
in the form of a powder, a water-free liquid or a water-dispersion.
The product is then heated, conveniently under pressure, to
harden same. During heatin~ of the product, the added liquid sub-
stance reacts with the lignin, and when, for example, a phenol-
aldehyde solution is used, the components naturally react with
each other. In addition, during the pressing operation part of
the liquid substance will apparently be pressed out of the lignin
and penetrate into the bonded surfaces, e.g. wood surfaces, there-
by to form a bridge between said surfaces and the lignin, which
results in the formation of a strong joint.
The product can also be used as a moulding compound for
the production of moulded objects by pressure-moulding and similar
methods, normally used for the manufacture of products from thermo-
setting resins.
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The invention will now be illustrated by means of an
example.
EXAMPLE
Sulphate lignin was precipitated and purified in the
following manner: The lignin was precipitated from black liquor
with H2SO4 and filtered at 60C. The precipitate (lO0 g) was
purified from salts precipitated simultaneously therewith by
slurrying the precipitate in water (lO0 ml) stirring and centri-
fuging the mixture and air-drying the same to a light-brown powder
having a moisture content of 3.6~. The lignin thus produced had
an ash content of 3.7% (absolutely dry).
A phenol-formaldehyde solution was prepared in the ~ !
following manner: lO0 g phenol were melted and heated to approxi-
mately 80C. 50 g paraformaldehyde were added whilst stirring.
After about 30 minutes all the paraformaldehyde had decomposed
and dissolved in the phenol. I'he mixture was maintained at 80C
for a further 3 - 5 hours. In this way there was obtained a harden-
able solution of low viscosity of formaldehyde in phenol at a
mixing ratio of 1:2, said solution being clear even at room-
temperature.
A lignin product which could be used as an adhesive was
prepared in the following manner: 10 g of the sulphate lignin
prepared in accordance with the above was admixed with 5 g of
phenol-formaldehyde solution and 0.5 g NH4Cl. This mixture was
stirred for some minutes at room-temperature and obtained a con-
sistency reminiscent of a normal plywood adhesive. The adhesive
thus produced was used to glue a 2 mm pine veneer to a lO mm chip-
board. The pressing operation was maintained for 5 minutes at a
temperature of 150C and a pressure of 5 kg/cm2. A good glue
joint was obtained. When subjected to splitting tests a practi-
cally 100% wood fracture was obtained. Two pieces of veneer were
tested with respect to their transverse strength in accordance with
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SIS 234801. In both bases fracture occurred at 5.44 and 5.72
kg/cm2, respectively.
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