Note: Descriptions are shown in the official language in which they were submitted.
~ r~
METHOD FOR RECOVE~ING AND USING
~IGNIN IN ADHE~SIV~ RESINS
BACRGROUND OF T~E INVENTIO~
1. Field of the Invention
This invention generally relates to the extraction
of lignin, especially demethylated lignin, from
lignin-containing aqueous solutions such as, for
example, from the raffinate which is a by-product of
dimethyl sulfide production which is, in turn, a by-
product of wood pulping operations. This invention
also relates to the use of demethylated lignin in the
preparation of resol-type resins such as those
employed in the manufacture of wood adhesives used to
make structural wood products such as, for example,
plywood and particleboard, wood veneers and the like.
2. Description of the Prior Art
Adhesive resins such as those used in the
manufacture of structural wood products are either
aminoresins or phenolic resins. Aminoresins are
polymeric products of the reaction of an aldehyde with
compounds containing an amino group, particularly urea
and melamine. In virtually all aminoresins the
aldehyde component is formaldehyde and by far the
--2--
dominant aminoresin is urea-formaldehyde because of
its relatively low cost compared to other resins used
in wood adhesives. The major disadvantage of
aminoresins, especially urea-formaldehyde resin, is
that they are not totally water resistant, and
consequently their gluelines will eventually
delaminate. They also are known to release
formaldehyde during their slow water hydrolysis.
Phenolic resins are polymeric products of the
reaction of an aldehyde with compounds containing a
phenolic hydroxyl group. The phenolic component is
most often phenol, but may be cresol, resorcinol, or
catechol, or the like. The rate of reaction is highly
dependent on the relative reactivity of the phenolic
substance used. Resorcinol is much more reactive than
phenol and consequently is used in "cold-setting"
resin adhesives. Catechol is also more reactive than
phenol, but somewhat less reactive than resorcinol.
The phenolic component used in the manufacture of such
resins may also be a mixture of phenolic substances,
such as phenol-resorcinol resins. Formaldehyde is the
most common aldehyde component although others such as
furfural are occasionally used. Phenol and the other
phenolic substances are considerably more expensive
than urea; however, phenolic resins give weather-and
boil-proof gluelines. Moreover, phenolic resins do
not release formaldehyde.
The reaction between a phenolic substance and an
aldehyde can be either acid catalyzed or base
catalyzed. Under acidic conditions and with an excess
molar ratio of phenolic component an essentially
linear low-degree-of-polymerization "prepolymer" is
formed. This material is known as a novolak.
Polymerization can be continued to an infusible solid
through the addition of sufficient aldehyde. The
--3--
completely polymerized material, often referred to as
cured or hardened, is usually used in molded products.
Novolak prepolymers are usually not water soluble;
however, under sufficiently basic conditions the
novolak may form a salt and become water soluble.
Under basic conditions aldehyde can be added to the
mixture and a base catalyzed polymerization performed
at elevated temperatures.
Under basic conditions and with an excess molar
ratio of aldehyde a highly branched low-molecular-
weight "prepolymer" resin known as a resol is formed.
This material is usually water soluble because of salt
formation in the basic solution. No additional
aldehyde is required for continued polymerization to
an infusible water insoluble product. The
poiymerization is usually continued by elevating the
temperature of the reaction. Resol prepolymers are
the phenolic resins usually used for wood adhesives.
They are preferred over novolaks for several reasons:
(1) The resol adhesive is a single component entity in
that no additional aldehyde is required. (2) Thair
continued polymerization to achieve a highly water-
resistant glueline is easily controlled by elevating
the temperature. (3) The basic adhesive mixture does
not cause long term wood degradation adjacent to the
glueline as is the case with acid catalyzed material.
By far, the most common phenolic-type resin for
wood adhesives is phenol-formaldehyde resol resin.
Resorcinol-formaldehyde cold-setting adhesives are of
the novolak-type, but without an acidic catalyst due
to their high reactivity. These cold-setting
adhesives are two component systems requiring aldehyde
addition to the novolak prepolymer immediately before
use; otherwise there is no means of controlling the
polymerization to an infusible product.
Phenol, as well as the other phenolic substances
noted above are relatively expensive. Phenol is also
petroleum derived, l.e., it is a petrochemical. In
general, it would be regarded as highly advantageous
to utilize a less expensive, nonpetrochemical phenolic
material in such resins if this were possible. For
example, a biomass derived phenolic material which is
readily available to the forest products industry, and
which is presently under-utilized, would be an ideal
substitute for petrochemical derived phenol. Such a
substitution would imply that the industry using the
wood adhesives also would control the raw material
required in the preparation of the resin used in these
adhesives.
Lignin is the dominant phenolic substance in
biomass and it is grossly under-utilized. Most
lignin, especially that which is produced as a by-
product of the pulping industry, is used primarily as
an energy source. It has long been recognized that
this is a low value use of this lignin source;
consequently many technical proposals have been made
for upgrading the use of this material as well for
obtaining an economic advantage over the petroleum
derived phenol used for this purpose. The allure of
using a biomass lignin resource in wood adhesives has
produced patents and other publications which describe
the use of lignin in resins used for wood adhesives.
The majority of these publications describe the use of
various lignin sulfonates, by-products of the sulfite
pulping industry, although many other types of lignin
are also mentioned. This information is
comprehensively covered by H.H. Nimz in "Lignin-Based
Wood Adhesives", a chapter in "Wood Adhesives:
Chemistry and Technology" edited by A. Pizzi, Marcel
Dekker, Inc. publisher (1983).
. ,~ i,. . ~, i i 1.
--5--
There are other references made in the literature
which more particularly address the use of
demethylated lignin in resins for adhesives. Since
the methods of this patent disclosure also involve the
use of demethylated lignin, a critical review
emphasizing how the methods of this patent disclosure
differ from what is believed to be the most pertinent
of these references is in order. Such a review should
begin by first noting that kraft lignin is known to he
slightly demethylated during the pulping process.
That is to say that, by way of example, the methoxyl
content of softwood kraft lignin is 12.6%, whereas
that of the original lignin in the original softwood
wood is 14.5%. For the purposes of this patent
disclosure the term "demethylated lignin", regardless
of its source, should be taken to mean lignin which
has undergone some substantial degree of
demethylation, e.g., such that its methoxy content is
from about 11% to essentially 0% by weight of the
original lignin material. For purposes of this patent
application, the term demethylated lignin should also
be taken to mean other lignin derived materials such
as, for example, any pulping by-product lignin
regardless of process, and by-product lignin from
biomass processing which also have been subjected to
some degree of demethylation.
Regardless of terminology, it is well known that
only slight degrees of demethylation are insufficient
to adequately improve the resin forming properties of
kraft lignin; consequently many attempts have been
made to modify and then utilize kraft lignin, as well
as other lignin-containing sources, as substitutes for
petroleum derived phenol in the manufacture of wood
adhesives.
For example, the publication "Thermosatting Resins
-6- ~3 ~` '?, t
of Demethylated and Tall Oil Lignins" in Khim.Ispol'z
Lignina 1974:428-433 teaches a process wherein
demethylated lignin, which was a product of an
alkaline hydrolysis of kraft lignin, is employed.
However, it has been noted that an alkali, even at
high temperatures, is a poor demethylating agent.
Hence, little demethylation can be accomplished by
this process as was indicated by its need for
petrochemical derived phenol to produce its resin.
Moreover, the temperature rlequired for hardening in
this process is also high, compared to even a typical
phenol-formaldehyde resol resin. This indicates that
deactivation rather than activation takes place; this
would not occur with substantial demethylation.
A publication titled "Kraft Lignin Utilization in
Adhesives" found in Wood Science Technology 22:157-165
(1988), evaluated demethylated lignin in several
resins for adhesives. However, since kraft lignin is
insoluble in acid solutions such as those of the
disclosed acidic chromate solutions, the reaction must
be performed in a heterogeneous two-phase system
wherein only limited demethylation takes place. In
any event, the publication concluded that "Resins
prepared with demethylated lignin are not included [in
the estimated cost evaluations] due to their deficient
behavior in particleboard panels."
A publication by Gapta and Sehgal in Holzforschung
and Holzverwertung (1978) 30:85-87 discloses
demethylation by hydriodic acid treatment of kraft
lignin. However, because of numerous technical
difficulties, use of their demethylated lignin led to
only about a 10% reduction in the phenol requirement
in the adhesive. The difficulties generally involve
the fact that kraft lignin is insoluble in an acidic
reaction medium, isolation of the demethylated lignin
by precipitation and premature gelling of the
demethylated lignin.
An article by Enkvist et al, published in Tappi
45, 128(1962) suggests the possibility of using
demethylated lignins to produce condensation polymers
with formaldehyde. The material employed in this
process was degradation products of a demethylation
process used to produce simple compounds such as
catechol and related substances. The "demethylated
lignin", which was only a minor amount of the total
obtainable material, was evaluated for its novolak
properties, but not as a resol resin. Moreover, since
the isolated demethylated lignin of this reference is
somewhat insoluble under acidic conditions it would be
rather difficult to prepare a novolak with acceptable
properties. It should also be noted that novolaks are
not used in adhesives for structural wood products.
In analyzing the problems encountered by the prior
art in seeking to employ lignin in place of phenol in
the production of wood adhesives, it should also be
noted that another difficulty with using such lignin
as the phenolic component in resin or wood adhesives
is that lignin is less reactive than phenol itself.
This problem of lower reactivity when using lignin in
place of phenol is apparently the result of two
separate causes. The first is that most of the
necessary bonding sites for a phenolic-aldehyde-type
resin are not available in lignin due to the very
nature of the material. Th~ second problem derives
from the fact that when lignin is isolated by prior
art methods such as those noted above, only a fraction
of the original material is actually isolated.
Usually the lower molecular weight component is not
isolated but instead is lost during the recovery step.
Moreover, as a consequence of this reduced reactivity,
-8- f~
compared to phenol, a resol prepolymer prepared using
lignin requires a longer cure time at elevated
temperature and the resulting glueline does not
exhibit the same strength properties as a phenol-
formaldehyde resol. These conditions also areconsidered highly unacceptable to the wood products
industry. Consequently there are no processes which
are widely acceptable to the industry which enable
lignin to be substituted for phenol in the manufacture
of resol resins.
SUMM~RY OF T~E INVEN~ION
Applicant has found that the foregoing problems
can be overcome by the hereinafter described
extraction methods which are based upon
(1) demethylation of the lignin to provide the
necessary bonding sites required for production of a
phenolic-aldehyde-type resin, and (2) the use of
certain organic solvents for increasing the efficiency
of lignin isolation from a demethylated lignin source
such as raffinate and thereby make more material
(including the lower-molecular-weight lignin material
of the overall demethylated lignin source) recoverable
and (3) where necessary, acidification of the
demethylated lignin-containing source material prior
to solvent extraction. These advantages are obtained
through the use of organic solvents which are further
characterized by the fact that they are both polar and
at least partially water immiscible. They can be
employed as extracting agents for the removal of
demethylated lignin from various demethylated lignin-
containing aqueous solutions in general, and from
acidified aqueous solutions in particular. That is to
say that a subject demethylated lignin-containing
aqueous solution may already be acidified when it is
:
.
, ~ .
.~:i ? ;;i~
_9_
introduced as a raw material for the herein disclosed
processes or it may be acidified as part of the
overall disclosed process. Thus for example since
raffinate is alkaline, it would have to be acidified
to carry out the herein disclosed demethylated lignin
extraction process. In either case, however, the
chief object of this invention is to at least
partially or even completely replace the petroleum
derived phenolic component in prior art phenol-
formaldehyde type resins with a modified lignin
material, e.g., demethylated lignin obtained from
certain nonpetroleum, lignin-derived materials such as
pulping spent liquor, regardless of process, as well
as any by-product material from biomass processing
which also has undergone some degree of demethylation.
It is contemplated that all varieties of demethylated
lignin, regardless of origin, may be beneficiated with
respect to their resin forming properties by the
extraction methods described in this patent
disclosure. That is to say that the chemical methods
and processes used for accomplishing the demethylation
which the lignin source has previously undergone is
not a part of the overall inventive concept of this
disclosure; several such methods have been patented or
otherwise described in the literature. The most
common example of such a demethylated lignin source is
the raffinate which is a by-product of dimethyl
sulfide production which can be carried out in
conjunction with wood pulping operations.
Preferably the lignin used in the herein disclosed
processes will be demethylated to some predetermined
extent which will be dependent upon the increase in
reactivity toward phenolic-aldehyde resin formation
which is desired. Thus, for example, a demethylated
lignin of from about 9% to about 4% methoxy content
--10--
can be employed to produce resins having greater
reactivities than typical phenol-formaldehyde resins.
Regardless of its source, however, applicant has
established that any demethylated lignin can be much
5 more effectively extracted from a demethylated lignin-
containing aqueous solution by use of processes which
use an organic solvent to recover the lignin from the
aqueous solution once it is properly acidified.
Again, the organic solvent is most preferably a polar,
and at least a partially water-immiscible, substance.
Representative organic solvents having the attributes
of polarity, and at least partial water immiscibility,
would include esters such as, for example, ethyl
acetate (which is preferred) propyl acetate, butyl
acetate, amyl acetate and the like, ketones such as,
for example 2-butanone (methyl ethyl ketone, which is
preferred), 2-pentanone, 3-pentanone, 2-hexanone, 3-
hexanone and like polar, partially water-immiscible
organic solvents.
The use of these recovery methods produce recovery
yields of from about 68% to almost 100% of the
demethylated lignin present in the demethylated
lignin-containing solution are achievable. For
example, about 97% of the demethylated lignin in a
raffinate can be obtained by the herein disclosed
methods. Moreover, much of the low-molecular-weight
demethylated lignin material, which is generally not
recovered by prior art demethylated lignin recovery
methods, is made recoverable by this process. This
inability of the prior art methods to recover low-
molecular weight portions of the demethylated lignin
material (e.g., that demethylated lignin material
which is obtained at recovery yields in excess of
about 68%, and more preferably in excess of about 90%,
of the demethylated lignin in a lignin-containing
...
.,,
--11--
solution such as raffinate and/or wherein "low
molecular weight" is taken to mean demethylated
lignins having molecular weights of less than about
1000) follows from the fact that demethylation of
lignin results in catechol formation which, in turn,
considerably increases the water solubility of the
material, especially that of the low-molecular-weight
material. Again, isolation and/or extraction of
demethylated lignin by employment of prior art methods
for lignin isolation, which usually involve only
acidification of the solution and precipitation o~ the
lignin, are not nearly as effective. To some degree
this relative ineffectiveness also follows from the
fact that the required washing of the precipitated
demethylated lignin results in considerable loss of
material due to solubilization.
In any case, applicant has found that not only do
the methods of this invention produce demethylated
lignin yields much higher (e.g., 68% to virtually 100%
of the demethylated lignin present) than the prior art
recovery methods, but that resol resins made from
demethylated lignins so recovered, are capable of
being partially or exclusively used in place of
petroleum-derived phenolic materials in the production
of resol resins. That is to say that applicant has
found that when demethylated lignins are recovered at
yields greater than about 67%, but preferably greater
than about 90% of the lignin content of the lignin-
containing solution, then low molecular weight
demethylated lignins are recovered (again low
molecular weight lignins generally may be considered
as those demethylated lignins having molecular weights
less than about 1000) and that the presence of these
low molecular weight demethylated lignins in an
overall demethylated lignin formulation comprising
- , ~
-12-
both high molecular weight demethylated liynin and low
molecular weight lignin causes the overall
demethylated lignin formulation to produce, when
reacted with one or more appropriate aldehydes, resol
resins which are especially well suited for the
production of wood adhesives.
Applicant believes that these advantages follow
from the fact that when demethylated lignin having a
low molecular weight demethylated lignin component,
and especially demethylated lignin produced by the
methods of this patent disclosure, is used as a
supplement or replacement for petrochemical phenol in
resol resins, such demethylated lignin is more
reactive than the phenol itself, whereas in the case
of lignin, not demethylated, the reverse is true.
Applicant also believes that the presence of a low
molecular weight demethylated lignin component in the
bulk demethylated lignin also encourages much more
effective cross linking reactions with the aldehyde in
the production of resol resins. It should again be
noted that demethylation of lignin results in catechol
formation and it is well known that catechol is more
reactive than phenol. The catechol formation also
provides additional binding sites on the lignin needed
for resol formation. Moreover, it should again be
noted that the gel times of such demethylated lignin-
formaldehyde resol resins are shorter than those of
phenol-formaldehyde resol resins. Thus, this patent
disclosure specifically contemplates certain novel
compositions of matter (i.e., those resol resins made
from reactions of aldehydes with demethylated lignins
having a low molecular weight demethylated lignin
component) as well as the reactions from which such
compositions are made and the reactions from which
demethylated lignin may be extracted.
13
Stated in more detailed process terms the herein-
disclosed methods for extracting demethylated lignin
from a demethylated lignin-containing aqueous solution
comprise: (1) mixing a demethylated lignin-containing
solution, and especially a demethylated lignin-
containing solution which is acidified by use of
certain acids hereinafter more fully described tagain
those skilled in this art wi:ll appreciate that, by way
of example, raffinate is alkaline and would have to be
acidified in order to carry out the herein disclosed
demethylated lignin extraction process) with a polar,
partially water immiscible organic solvent to form a
demethylated lignin-containing aqueous
solution/organic solvent mixture; (2) allowing the
demethylated lignin-containing aqueous
solution/organic solvent mixture to form at least two
layers or phases wherein at least one of the two
phases is an aqueous phase and at least one of the
phases is an organic solvent phase which contains at
least a portion of the demethylated lignin contained
in the demethylated lignin-containing aqueous
solution; (3) separating the aqueous phase from the
organic solvent phase; and (4) recovering the
demethylated lignin contained in the organic solvent
phase.
The above-described general method of lignin
recovery contemplates several more specific versions
such as, for example, versions wherein: (1) the
demethylated lignin dissolved in the demethylated
lignin-containing aqueous solution is demethylated
kraft lignin; (2) the polar, partially water
immiscible organic solvent is selected from the group
consisting of esters such as ethyl acetate propyl
acetate, butyl acetate amyl acetate and ketones such
as 2-butanone (methyl ethyl ketone), 2-pentanone, 3-
f ~ ~;. r~
~ 1 4 ~ f~
pentanone, 2-hexanone and 3-hexanone and like polar,
partially water immiscible organic solvents; (3) the
demethylated lignin-containing aqueous solution i5
acidified to a pH of from about 5 to about 2; (4) the
separation is carried out in a temperature range of
from about 10 to about 80C and preferably at ambient
pressure conditions; (5) the demethylated lignin-
containing aqueous solution/organic solvent mixture is
acidified to a pH of from about 5 to about 2 by use of
an acid selected from the group consisting of a
mineral acid such as sulfuric acid, hydrochloric acid
and phosphoric acid, as well as by organic carboxylic
acids such as oxalic acid, formic acid, acetic acid
and lactic acid; (6) the demethylated lignin-
containing aqueous solution is a raffinate from theproduction of the dimethyl sulfide produced from the
spent liquor from wood pulping operations; (7) the
demethylated lignin-containing aqueous
solution/organic solvent mixture is produced by
mixing, preferably by mechanical agitation, about 1 to
about 5 parts by weight of a raffinate having from
about 5% to about 75% solids content, with about 1 to
about 5 parts by weight ethyl acetate (solids content
is of course determined by the amount of water added);
(8) the demethylated lignin-containing aqueous
solution/organic solvent mixture is acidified to a pH
of about 3.0 after the organic solvent is added to
demethylated lignin-containing aqueous solution;
(9) the separating of the aqueous phase from the
organic solvent phase is aided by counter current
solvent extraction or by centrifuging and partitioning
the aqueous solution/organic solvent mixture; (10)
the aqueous phase is extracted more than once with a
polar, partially water immiscible organic solvent;
(11) more than one aqueous phase is extracted by a
.~
~: .
-15- ," r ,~`?
polar, partially water immiscible organic solvent,
combined and washed with water before recoveriny the
demethylated lignin; and (12) the recovery of the
demethylated lignin from the organic solvent phase is
accomplished by evaporation of the organic solvent,
flash drying or a general drying of the resulting
demethylated lignin by other methods well known to the
art.
Also stated in more detailed process terms the
herein disclosed processes also contemplate methods
for employing a demethylated lignin dissolved in a
demethylated lignin-containing alkaline aqueous
solution (e.g., raffinate) to produce a resol resin,
(such as, for example, those resol resins used in the
manufacture of wood adhesives) which methods
preferably comprise~ ixing a demethylated lignin-
containing aqueous solution with a polar, partially
water immiscible organic solvent to form a
demethylated lignin-containing aqueous
solution/organic solvent mixture; (2) allowing the
lignin-containing aqueous solution/organic solvent
mixture to form two or more phases or layers wherein
at least one of the two or more phases is an aqueous
phase and at least one of the two or more phases is an
organic solvent phase which contains at least a
portion of the demethylated lignin contained in the
demethylated lignin-containing aqueous solution;
(3) separating the aqueous phase from the organic
solvent phase; (4) recovering the demethylated lignin
contained in the organic solvent phase; (5) dissolving
the recovered demethylated lignin in an alkaline
medium (preferahly, wherein the alkali used to produce
the alkaline medium is selected from the group
consisting of metal hydroxides such as sodium
hydroxide, potassium hydroxide and calcium hydroxide
"~. .
r )
--16--
to produce a reaction medium having a pH of from about
7.5 to about 13.5 (and more preferably from about 9 to
about 11); (6) reacting (at a temperature of about
~oac to about 105C, and preferably at ambient
pressure) the demethylated lignin with an aldehyde
source in said alkaline medium to form a resole resin;
and (7) recovering said resole resin by known (e.g.,
flash drying, evaporation, etc.) recovery methods.
Preferably the aldehyde source which yields
formaldehyde. Those skill~d in this art will also
appreciate that the resulting resol resin need not be
dried in order to use it in the further production of
adhesives. For example, resol resins having from
about 40% to about 60% solids content may be used in
conjunction with other ingredients to make the
adhesive.
The general method of resol resin production also
contemplates several specific versions such as, for
example, versions specifically adopted for wood
adhesive production wherein: (1) the aldehyde source
is capable of producing formaldehyde, such as, for
example, an aldehyde selected from the group
consisting of formaldehyde, paraformaldehyde,
hexamethylenetetramine, and also other aldehydes such
as acetaldehyde, and furfural; (2) the demethylated
lignin dissolved in the demethylated lignin-containing
al~aline aqueous solution is demethylated kraft
lignin; (3) the polar, partially water immiscible
organic solvent is selected from the group consisting
of esters such as ethyl acetate (which is preferred),
propyl acetate, butyl acetate and amyl acetate and
ketones such as 2-butanone (which is preferred), 2-
pentanone, 3-pentanone, 2-hexanone and 3-hexanone;
(4) the demethylated lignin-containing aqueous
solution is acidified to a pH of from about 5 to about
-17- . .
2; (S) the demethylated lignin-containing aqueous
solution/organic solvent mixture is acidified to a pH
of about 3 by use of a mineral acid selected from the
group consisting of sulfuric acid, hydrochloric acid,
phosphoric acid and/or an organic carboxylic acids
such as oxalic acid, formic acid acetic acid and
lactic acid; (6) the demethylated lignin-containing
aqueous solution is a raffinate from the production of
the dimethyl sulfide produced as a by-product from
wood pulping operations; (7) the demethylated lignin-
containing aqueous solution/organic solvent mixture is
produced by mixing, preferably by mechanical
agitation, about 1 to about 5 parts by weight of a
raffinate having from about 5% to about 75% solids
content, with about l to about 5 parts by weight ethyl
acetate (again, the solids content is of course a
function of the amount of water added); (8) the
demethylated lignin-containing aqueous
solution/organic solvent mixture is acidified to a pH
of about 3.0 after the organic solvent is added to
demethylated lignin-containing aqueous solution;
(9) the separation is carried out at a temperature
between about 10C and 80C, and preferably at ambient
pressure; (10) the separating of the aqueous phase
from the organic solvent phase is carried out by
counter current solvent extraction and/or by
centrifuging and partitioning the demethylated lignin-
containing aqueous solution/organic solvent mixture;
(11) the aqueous phase is extracted more than once
with one or more than one polar, partially water
immiscible organic solvent; (12) more than one aqueous
phase is extracted by a polar, partially water
immiscible organic solvent, combined and washed with
water before recovering the final demethylated lignin
product; (13) recovery of the demethylated lignin from
.
.
-18- ~ .?
the organic solvent phase is accomplished by
evaporation of the organic solvent under reduced
pressure and drying of the resulting demethylated
lignin under vacuum, or by flash drying; and (14) the
reaction of the demethylatecl lignin with the aldehyde
in the alkaline medium is carried out in conjunction
with a phenolic component, other than the demethylated
lignin, in order to form a :resole resin. That is to
say that the
resol resin production methods of this patent
disclosure also specifically contemplate reactions of
the demethylated lignin with an aldehyde in an
alkaline medium which are carried out in conjunction
with one or more phenolic components, other than the
demethylated lignin, in order to form a resole resin
and wherein said phenolic components is provided by
phenols selected from the group consisting of phenol,
resorcinol, cresol, catechol and the like; and
especially reactions of this type wherein a degree of
demethylation of the lignin in the demethylated
lignin-containing aqueous solution is determined by a
specified reactivity level of the resol resin desired.
DESCRIPTION QF THE PREFERRED EMBODIMENTS f
The following examples are presented for purposes
of illustration only, and are not intended to limit
the scope of the inventive concepts of this patent
disclosure. For example, the method of isolating
demethylated lignin using the herein disclosed solvent
extraction process should not vary substantially with
the original source of the lignin material, nor should
the use of such isolated demethylated lignin vary
substantially in the preparation of resol resins for
adhesives.
Example 1
Concentrated black liquor from a kraft pulping
process, which contains kraft lignin, was demethylated
using elemental sulfur by a known commercial process
for the production of dimethyl sulfide. This liquor,
known as the raffinate from dimethyl sulfide
production, was employed as a starting material for
the isolation of the resulting demethylated lignin.
A 320 gram sample of raffinate (specific gravity
at 50% solids content 1.40) was diluted with 250 ml.
of water and 750 ml. of ethyl acetate added. The
resulting mixture was stirred vigorously while 260 ml.
of 4 normal sulfuric acid was added dropwise until a
pH 3 was reached. The mixture was stirred for an
additional 20 minutes after cbmpletion of the sulfuric
acid addition, and it was then centrifuged and
partitioned. The aqueous layer was extracted three
additional times with 30 ml. of ethyl acetate each
time. The aqueous layer had changed from an original
black color to tan and since it is the lignin which is
responsible for the color this indicated almost
complete removal of the lignin from the aqueous layer
into the organic solvent. The combined organic layer
was washed three times with 150 ml. of water. The
organic layer was evaporated under reduced pressure at
40 degrees Celsius and dried under vacuum to afford 61
gms. of demethylated lignin as a dark brown solid,
yield 97% based on original lignin content of the
black liquor.
Example 2
A resol resin was prepared utilizing the
demethylated lignin isolated from the raffinate. The
lignin was assumed to be 60% demethylated, which would
mean a molecular weight of 171 for the monomeric unit.
The optimum molar ratio for resol formation was
'
~ ':
2 o--
determined as demethylated lignin, paraformaldehyde
and sodium hydroxide of 1:2.5:0.9, respectively. The
lignin was added first to the alkaline solution until
virtually all dissolved. The paraformaldehyde (or 37%
formalin can be used) is then slowly added to the
solution. The reaction conditions are 60 to 65
degrees Celsius for 1 1/2 to 2 hours and cooliny of
the mixture afterwards. Because of the higher
molecular weight of the demethylated lignin compared
to phenol, only 60~ as much formaldehyde is required.
This resol resin utilizing demethylated lignin is
more reactive than a commercial phenol-formaldehyde
resin. At 113 degrees Celsius, the lignin-containing
resin will gel in 10 minutes, whereas the commercial
resin require 19 minutes, and at the higher
temperature of 121 degrees Celsius, the values are 7
minutes and 1~.5 minutes, respectively. The
demethylated lignin-formaldehyde resol resin gave a
glueline which exceeds the British Standard 120~:1964
in both the cold soak and boil test by a considerable
margin. The British Standard values (see generally
B.S. 1204: 1964, Standards Information Services
401.01, National Bureau of Standards, Washington, D.C.
20234) are 500 psi and 325 psi, respectively, for cold
soak and boil test shear strength values.
Those skilled in this art will appreciate that
even though the most obvious use of the herein
disclosed methods of extracting lignins is in the
further production of resol resins, the methods
employed may be applied to other lignin extraction
processes. Moreover, the conditions employed in such
reactions will be those appropriate to some particular
version of such reactions. Moreover, the demethylated
lignins produced by such methods may have uses in
addition to their use in producing resol resins used
'
~. .
--2 1--
to manufacture wood adhesives. Thus, while this
invention generally has been described in terms of the
general discussions, specific examples and preferred
embodiments, none of these should be taken
individually as a limit upon the inventive concepts
which are set forth in the following claims.
,
- ' ,
.
