Note: Descriptions are shown in the official language in which they were submitted.
PRODUCTION OF HIGH STRENGTH CELLULOSE
FIBER USING 7INC CHLORIDE, ORGANIC
SOLVENTS AND AQUEOU~ SOLUTION
FIELD OF THE INVENTION
The present invention relates to the
production of high tensile strength, solvent-spun
cellulose fiber which is stable in alkaline
solutions. More particularly, the invention relates
to a process in which cellulose is solubilized in a
zinc chloride solution then extruded into a
coagulation medium in which the cellulose fibers
form. The fibers are removed from the coagulation
medium, treated to remove the coagulation medium,
stretched, and recrystallized in water, thus forming
the high tensile strength, solvent-spun cellulose
fibers of the present invention.
BACXGROUND OF THE INVENTION
Cellulose, the most abundant polymer on
earth, is a straight-chain polymer of anhydroglucose
with beta 1-4 linkages. Cellulose fiber in its
natural form comprises such materials as cotton and
hemp, while solvent-spun fiber comprises products
such as rayon.
The process most frequently used to
produce solvent-spun fiber is known as the viscose
process. In the viscose process cellulose xanthate,
a cellulose derivative, is solubilized then spun
into a coagulation bath where the fiber forms. The
resulting ~iber is a product known as viscose.
A second solvent-spun fiber product,
cellulose acetate, is produced when a cellulose
derivative is solubilized in an organic solvent,
~ ~ A ~
then srlln ; n~n water or alcohol where it coagulates
to form fiber. The fiber may be regenerated from
its derivative form to true (nonderivatized~
cellulose using an alkaline solution, but such a
regeneration step is rarely performed.
In addition to the production of viscose
and cellulose acetate, a third embodiment of the
solvent-spun process involves the dissolution of
true cellulose in a solvent which is then spun into
lo a coagulation bath in which fiber formation occurs.
Due to the high processing costs and the
generally inferior properties of the fiber products
formed when nonderivatized cellulose is employed in
the solvent-spun process, derivatized cellulose such
15 as that used in the viscose process is generally
employed when producing solvent-spun fibers.
Approximately 20 years ago it became
apparent that the production of solvent-spun fiber
by methods such as the viscose process was becoming
20 disadvantageous due to the high capital costs and
environmental considerations associated with their
use. For this reason, modified or alternative
methods for producing solvent-spun fiber were
sought.
Several cellulose solvents were tested for
use in a modified or alternative solvent-spun
process. A few achieved favorable results in
solubilizing the cellulose, but were ultimately
deemed to be impractical for other reasons.
Specifically, solvents comprising
solutions of S0~/NH" and S0l/(CH,)~HN were tested and
found to form good cellulose solutions ~i.e.,
solutions having reasonable viscosities and
$
practical degrees of poly~eriza ~on~.
Unfortunately, it was impractical to regenerate
cellulose fiber, and to recover the solvent from the
coagulation medium.
Similarly, a 85% H3P0, solution was tested
for use as a cellulose solvent and was found to
dissolve cellulose well, however, the resulting
solution contained gels and fibers which made
filtration very difficult. Additionally, when
phosphoric acid was tested, the cellulose went into
solution well, but the phosphoric acid could not be
washed from the resulting fiber. D.M. MacDonald,
The S~innina of Unconventional Cellulose Solutions
in Turbak et al., "Cellulose Solvent Systems" ACS
Sym. Seri. 58 (1977).
Solutions of 52.5% Ca(CNS)2, DMF/N20" and
DMSO/para-formaldehyde were also tested. These too
proved unsuccessful for use, for while the solutions
were found to be acceptable cellulose solvents, they
either formed weak fibers or were difficult to
recover from the coagulation medium once fiber
formation occurred. Hudson, S.M., Cuculo, L.A.,
J. Macromolecular Science Rev., Macromolecular
Chemistry (1980) C18(1) p. 64.
In addition to the solvents listed above,
MacDonald (supra) also reported testing a 64% ZnCl2
solution. As with the previous solvents, the
results were unacceptable. In this case, the
solubilized cellulose could not be spun and the
coaqulated fibers were noncohesive.
In view of these results and until the
present invention, the use of ZnCl2 as a cellulose
solvent has only been successfully utilized in a
limited number of processes.
In one such process, the carbon fiber
procèss, cellulose is solubilized in a ZnCl2/~Cl
solution then extruded into a methanol bath wherein
the cellulose coagulates to form fibers. These
fibers are usually weak, and while they can
generally be handled with tweezers, they are not
usually strong enough to permit spinning.
Following coagulation, the ZnCl2/HCl is
removed from the cellulose by prolonged soaking in
the methanol bath. The fibers are then carbonized.
In addition to the carbon fiber process,
the production of "vulcanized fiber" or nonwoven
mats also involves the use of a ZnCl2 cellulose
solvent. See, eg. Young and Miller, Formation and
Pro~erties o~ Blended Nonwovens Produced bY
Cellulose-Cellulose Bondinq in Gould et al. "Blended
Nonwovens," ACS Symp. Ser. 10 (1975).
In producing vulcanized fiber, cellulose
is swollen and softened into a gel using a
concentrated ZnCl2 solution. The gel is then pressed
into sheets which are leached with water in order to
extract the ZnC12 from the cellulose. This results
in the formation of a tough, rigid, nonwoven plastic
sheet.
Prior to the present invention, the use of
ZnCl2 as a cellulose solvent has had limited
application as described above. Moreover, its use
in a solvent-spun process has until now proven
impractical.
s I
The present invention is advantageous,
there~ore, for it teaches the use of ZnCl2 as a
cellulose solvent in a solvent-spun process, as well
as teaching the production of a high tensile
strength, solvent-spun cellulose fiber resulting
therefrom. The use of ZnCl2 in the present invention
is further beneficial, for ZnCl2 is nontoxic, less
corrosive than preYiously employed solvents and is
easily recoverable for reuse. Further advantages of
the present invention are set-forth below.
Absent the teachings of the present
invention, it is generally preferred that the
cellulose starting material employed in solvent-
spun processes have a high degree of polymerization
(hereinafter "DP"), i.e. a DP preferably above 600,
but at least above 300 (hereinafter "high DP
cellulose"), when strong fiber products are desired.
Unfortunately, this usually means that the cellulose
starting material must be obtained from
conventional pulping processes. While the DP of
such pulp is normally high, the conventional
processes are relatively expensive to operate and
their pulp products are generally costly.
Unlike the solvent-spun processes
previously discussed, the solvent-spun process of
the pre~ent invention enables one to use both high
DP cellulose and cellulose with a DP ranging from
about 100 to 300 (hereinafter "low DP cellulose").
In fact, the cellulose starting material of the
present invention may range in DP from about 100 to
3000. Therefore, pulp obtained from a non-
conventional, acid pulping process (a potentially
cheap and efficient process yielding dissolving
grade cellulose havi~g ? n~ of 400 or less) which is
generally unsuitable for use in existing solvent-
spun fiber processes, may be used herein according
to the teachings of the present invention. For
purposes of the present invention, ~dissolving
grade" cellulose comprises substantially lignin-
free cellulose.
In addition to the above, the ability to
use low DP cellulose in the process of the present
invention is further advantageous because low DP
cellulose is theoretically both cheap and abundant,
potentially derivable from both municipal and
agricultural wastes such as used paper, corn stalk,
and sugar cane bagasse.
As a further advantage, the process of the
present invention may be employed (with only slight
modifications described infra) to form cellulose
~ibers and films suitable for use in food and
pharmaceutical applications. In such instances,
~ood-grade cellulose and food-grade ZnC1~ are taught
for use herein.
In addition to the DP of the cellulose
starting material being determinative of the
strength of the cellulose fibers of the existing
solvent-spun processes, it has also been observed
that fiber strength is dependent upon the
arrangement of the cellulose crystals within the
fiber as well.
To clarify, cellulose may exist in
amorphous or crystalline form. In fact, both
amorphous and crystalline regions form within the
cellulose fiber upon coagulation. The ratio and
orientation of these regions vary, but in the
~ ;3~,~
existing solvent-spun processes both a~~ ~t~rmined
during the coagulation step.
By way of example, when cellulose is
coagulated in a typical solvent-spun process, some
molecules randomly orient themselves in a
crystalline matrix, the degree of crystallization
being determined, generally, by the presence and
amount of water in the coagulation medium. Neither
the ratio nor the orientation of the crystalline
regions can be controlled in such a process, because
the crystallization occurs simultaneously with the
coagulation of the fiber.
In view this, it was theorized by the
present inventor that by separating the steps of
coagulation and crystallization in a solvent-spun
process, the ratio of crystalline and amorphous
regions in the fiber could be controlled. Moreover,
by applying tension to the fiber after coagulation
but before crystallization, the fiber could be
stretched thereby orienting the amorphous and
crystalline regions therein. This would result in a
solvent-spun cellulose fiber having high tensile
strength.
Based on this theory, and in view of the
fact that ZnCl~ was known to be a good cellulose
solvent, a process for producing high tensile
strength, solvent-spun cellulose fiber was
developed.
Additionally, a second embodiment of the
present process was found to produce a low tensile
strength cellulose fiber particularly suitable for
food and pharmaceutical applications when food-grade
starting materials and reagents were employed.
SUMMARY OF THE INVENTION
In accordance with the present invention,
high tensile strength, alkaline stable, solvent-
spun cellulose fiber may be produced from dissolvinggrade cellulose having a DP in the range of from
about 100 to 3000.
The present invention generally comprises
a process in which cellulose is mixed, heated and
solubilized in solvent comprising a solution of
ZnCl2. The cellulose/ZnCl2 solution is then extruded
into a coagulation medium comprising one or more
organic solvents wherein the cellulose coagulates to
form fibers. The resulting fibers are then treated
under tension to stretch and orient the fibers, as
well as to remove any solvent or coagulation medium
therefrom. The fibers are then placed in a water
bath where recrystallization is fully achieved. The
fibers may then be dried or otherwise treated for
packaging, shipment or use.
More specifically, the process of the
present invention comprises the steps of:
a) adding a solvent to cellulose to form
a mixture, the solvent comprising a solution of zinc
chloride (ZnCl,), and the resulting mixture
comprising cellulose/ZnCl,;
the mixture having a final cellulose
concentration in the range of from about 5% to about
45% (w/v), and a final ZnCll concentration in the
range of from about 55% to about 80% (w/w), more
preferably from about 62% to about 76%,
concentrations at the upper and lower extremes of
this range are sugge~ted for producing the strongest
cellulose fibers;
b) stirring and heating the cellulose/
ZnCl2 mixture at a temperature ranging from about
40C to about 120C, more preferably from about 40C
to about 100C, and most preferably at about 65C
until the cellulose dissolves and the mixture
becomes clear, thereby forming a cellulose/ZnCl2
solution;
c) extruding the cellulose/ZnCl2 solution
into a coagulation medium wherein the cellulose in
the cellulose/ZnCl2 solution coagulates to form
fiber, the fiber not being fully crystallized;
the coagulation medium comprising one or
more alcohols or ketones, the alcohols generally
being selected from the group consisting of straight
or branched chain C, to C4 alcohols such as methyl,
ethyl, propyl and isopropyl-alcohol and the ketones
generally being selected from the group consisting
of C, to C, ketones such as acetone or
methylethylketone (MEK);
d) removing the fiber from the
coagulation medium and treating the fiber to remove
residual solvent and coagulation medium therefrom;
e) applying tension to the fiber
sufficient to stretch the fiber and orient the
molecules therein;
the above treatment may comprise
evaporation or other conventional means known to
those skilled in the art;
f) submerging the fiber in a bath
containing wster to fully oryst~llize the f1ber;
., .
g) removing the fiber fro~ ~he water bath
for drying and/or further treatment.
Several optional manipulations may be
performed at step a), for instance:
i) pre-wetting the surface of the
cellulose with water (optionally containing one or
more of the chlorides as listed below) prior to its
dissolution in the ZnCl2 solution, thereby
facilitating the rapid formation of a homogenous
cellulose/ZnCl2 solution low in solid particles and
having a chemical and physical structure suitable
for extrusion;
ii) adding chlorides o~ magnesium,
calcium, lithium or aluminum directly to the
cellulose, or to the cellulose/ZnCl2 mixture, thereby
aiding in the mixing, dissolution and the extrusion
of the cellulose/ZnCl2 by lowering its viscosity
As disclosed in sub-step i) above,
pre-wetting the cellulose reduces the number of
solid particles normally encountered when a solution
of zinc chloride is added directly to dry cellulose.
The presence of solid particles is generally
disadvantageou~ in solvent-spun procesqes, for the
solid particle~tend to clog the narrow openings of
the spinnerettes which are frequently employed in
these processes. Furthermore, occasionally the DP
of the cellulose in the cellulose/ZnCl2 solution
decreases prior to extrusion. Pre-wetting the
cellulose prior to its mixture with the ZnCl2 solvent
appears ~o alleviate this problem as well.
In regard to coagulation step c), it
should be noted that the coagulation medium serves
to remove the ZnCl2 and other salts present in the
~ ?'3,~
extruded cellulose by dissolving and diluting them
in the medium. This results in a decreased
concentration of ZnCl2 in the extruded cellulose/
ZnCl2 solution so as to enable the cellulose to
coagulate to form fibers.
It is also important to note that in
keeping with the inventor's goal to separate the
steps of coagulation and crystallization in the
present process, the water content in the
coagulation medium should be kept to a minimum in
order to prevent premature recrystallization of the
fiber prior to stretching and orientation.
Likewise, it should also be noted that
when extruding the cellulose/ZnCl2 solution into the
coagulation medium, the solution may be extruded
directly or indirectly into the medium. Direct
extrusion entails the introduction of the cellulose/
ZnCl7 solution into the coagulation medium through a
nozzle or spinnerette immediately at or below the
surface of the medium, while indirect extrusion may
be achieved, for example, by extrusion of the
solution into air then into the coagulation medium.
While indirect extrusion methods may be
employed in the process of the present invention,
such methods are not preferred for they tend to form
fibers of non-uniform dimension and strength.
The above description provides the basic
steps in the formation of the high tensile strength
cellulose fiber of the present invention. It will
be apparent to those skilled in the art that
modifications and variations can be made in this
process without departing from the scope or spirit
of the invention. For example t the fibers may be
finished a~or~ing to common practice, and/or
various modifying agents may be added to the
solvent, the coagulation medium or the
crystallization bath.
Additionally, by slight modification to
the present process, a cellulose fiber or film
suitable for use in food and pharmaceutical
applications may be produced.
For example, a food-grade cellulose fiber
may be formed which can be woven into a netting or
binding for use as a packaging on food products such
as meats. Similarly, when the cellulose/ZnCl2
solution of the present invention is extruded as a
film, it may be used as an edible film or casing on
sausages and the like.
To form products such as those listed
above, food-grade reagents must be employed; namely,
food-grade cellulose and a food-grade solvent. The
present invention is important, therefore, for it
teaches the use of ZnCl2 as a cellulose solvent in a
solvent-spun process, and it teaches a solvent-spun
process in which the zinc chloride may be
successfully and practically used to form a fiber
suitable for the uses envisioned and disclosed
~5 herein.
The use of ZnCl2 is advantageous in the
present process for it is nontoxic and is available
in food-grade quality. Furthermore, while the use
of food-grade quality reagents in most processes is
prohibitively expensive, ZnCl2 is recoverable from
the coagulation medium of the present process for
reuse, thus significantly lowering its cost per use.
In the food applicet~onc listed above, the
need for a fiber having a high tensile strength is
not always great, therefore, after dissolution and
extrusion the fibers (or film) may be removed from
the coagulation medium and transferred directly into
the crystallization bath. Alternatively, the fibers
or film may also be coagulated and recrystallized
simultaneously. This would result in fibers with
tensile strengths of about 1.6 to 1.9 g/den ~see
Example 5 below).
DETAILED DESCRIPTION OF INVENTION
Reference will now be made in detail to
the preferred embodiments of the present invention,
examples of which are set forth below:
EXAMPLE I
Cellulose (Avicel PH 101) was pre-wet by
mixing with water. A 76% zinc chloride solution
(w/v) was added to the pre-moistened cellulose and
the mixture was stirred immediately and continuously
at 60e for about 15 minutes, by which time the
cellulose had dissolved.
When pre-wetting the cellulose, the amount
of water added to each sample was controlled so that
upon the addition of the 76% ZnCl2 solution thereto,
the final ZnC12 concentration in the resulting
cellulose/ZnCl2 mixtures ranged from 66~ to 74.6% as
depicted below in Table 1. The final cellulose
concentration in each sample was 10% (w/v).
14
TABLE 1
FIBER ¦ 76% I H20 I CELLULOSE I FINAL CONC. ITENSILE
No. ¦ ZnCl2 l l I ZnCl2 ISTRENGTH
5' (ml) '(ml) ~ (g) ' (%) ~ (g/den)
1 1 2 ~ 0.23l 0.223 1 74.4 1 4.1
2 1 2 1 0.441 0.244 1 72.7 1 3.3
3 1 2 1 0.68l 0.268 1 70.8 1 3.0
4 , 2 , 0.98l 0.298 , 68.6 1 2.8
1 ' I
1 2 1 1.351 0.335 1 66.0 1 4.5
Without cooling, the cellulose/ZnCl2
solutions were extruded by syringe through a 22
gauge hypodermic needle into acetone which served as
the coagulation media. After about 15 minutes,
cellulose fibers about three feet in length were
removed from the media. The ends of the fibers were
fixed to a table and the fibers were then allowed to
air dry. The drying caused the fibers to shrink.
Due to their attachment to the table, this shrinkage
exerted tension on the fibers causing them to
stretch.
After the fibers were completely dried and
stretched, they were submerged in a water, i.e.
crystallization bath for about ten minutes. The
fibers were then removed and dried in an oven at
60C.
The resulting fibers had tensile strengths
as depicted in Table 1.
2~J ~
EX~MPT~F' T T
A cellulose/ZnCl~ mixture containing 15%
cellulose (w/v) was prepared according to the
procedure described in Example I. The final zinc
chloride concentration in the mixture was 67% (w/w).
After heating and dissolution, the
cellulose/ZnCl2 solution was extruded into six baths
individually containing the following coagulation
media: acetone, ethyl alcohol, acetone:water (2:1)
acetone:water (5:1), ethyl alcohol:water (5:1) and
acetone:ethyl alcohol (1:1). The resulting fibers
were then treated as described in Example I. The
tensile strengths of the 6 resulting fibers are
listed in Table 2.
TABLE 2
COAGULATION BATH I TENSILE STRENGTH
SOLVENT '(g/den)
Acetone ' 5.1
Ethyl Alcohol , 5.5
Acetone:water (2:1) 1 2.4
Acetone:water (5:1) ¦ 3.8
Alcohol:water (5:1) ' 2.7
Acetone:Alcohol* (1:1) 1 3.0
I _
* For purposes herein, "alcohol" comprises ethyl alcohol.
16 ~ d ~ J
EXAMPLE III
A cellulose fiber was prepared from a
cellulose/ZnCl2 mixture as described in Example II
using ethyl alcohol as the coagulation medium. The
procedure set-forth in Example I was followed. The
resulting fiber had the following characteristics:
tensile strength - 5.7 g/den and fiber elongation -
13%. Solubility of the fiber in an alkaline
solution was undetectable and the zinc content in
the fiber was less than 0.4% by weight.
EXAMPLE IV
A cellulose fiber was formed using
alpha-cellulose (DP - 400, Sigma Chemical Co.) as
the cellulose starting material and zinc chloride
solution as the cellulose solvent. The final ZnCl2
concentration in the cellulose/ZnCl2 mixture was 67%
(w/w). The cellulose concentration was 10% (w/v).
The procedure of Example I was followed,
and acetone or ethyl alcohol were used as the
coagulation media. The characteristics of the
resulting fibers were compared and the results
achieved are as follows:
1) coagulation medium - acetone, tensile
strength - 6.2 gtden, % elongation - 15%;
2) coagulation medium - ethyl alcohol, tensile
strength - 3.6 g/den, % elongation - not calculated.
EXAMPLE V
Cellulose tAvicel PH101) was dissolved in
a solution of zinc chloride and the mixture was
prepared according to the procedure described in
Example I. The final ZnCl~ concentration in the
~ ~ 9.il~
17
celluloseJZn~12 mixture was 67% (w/w) and th~
cellulose concentration was 10% (w/v).
The mixture was heated and the resulting
solution extruded into a bath containing ethyl
alcohol as the coagulation medium. Three fibers
were formed. One fiber was removed from the bath,
air dried and stretched. The second fiber was
transferred from the coagulation medium and placed
directly into a water bath without drying or
stretching, while the third fiber was removed from
the coagulation medium, dried and stretched and
recrystallized in a water bath as described in
Example I.
The tensile strengths of the three fibers
were 1.6, 1.9 and 5.2 g/den, respectively.
From consideration of the specification
and examples, and practice of the invention as
disclosed herein, other embodiments of the invention
will be apparent to those skilled in the art It is
intended that the specification and examples be
considered as exemplary only, with the scope and
spirit of the invention being indicated by the
following claims.
