U.S. patent application number 09/975174 was filed with the patent office on 2002-03-21 for lyocell fiber made from alkaline pulp having low average degree of polymerization values.
This patent application is currently assigned to Weyerhaeuser Company. Invention is credited to Luo, Mengkui, Neogi, Amar N., Persinger, W. Harvey JR., Roscelli, Vincent A., Sealey, James E. II.
Application Number | 20020034638 09/975174 |
Document ID | / |
Family ID | 24296571 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020034638 |
Kind Code |
A1 |
Sealey, James E. II ; et
al. |
March 21, 2002 |
Lyocell fiber made from alkaline pulp having low average degree of
polymerization values
Abstract
The present invention provides lyocell fibers made from
compositions having a high hemicellulose content of at least 5% by
weight, a low kappa number of less than 2.0 and including cellulose
that has a low average degree of polymerization (D.P.) value of
about 200 to 1100, and a narrow molecular weight distribution (R)
of less than 2.8.
Inventors: |
Sealey, James E. II;
(Federal Way, WA) ; Persinger, W. Harvey JR.;
(Enumclaw, WA) ; Luo, Mengkui; (Tacoma, WA)
; Roscelli, Vincent A.; (Edgewood, WA) ; Neogi,
Amar N.; (Seattle, WA) |
Correspondence
Address: |
PATENT DEPARTMENT CH2J29
WEYERHAEUSER COMPANY
P.O. BOX 9777
FEDERAL WAY
WA
98063-9777
US
|
Assignee: |
Weyerhaeuser Company
|
Family ID: |
24296571 |
Appl. No.: |
09/975174 |
Filed: |
October 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09975174 |
Oct 10, 2001 |
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09574538 |
May 18, 2000 |
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09574538 |
May 18, 2000 |
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09256197 |
Feb 24, 1999 |
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6210801 |
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09256197 |
Feb 24, 1999 |
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09185423 |
Nov 3, 1998 |
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6306334 |
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09185423 |
Nov 3, 1998 |
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09039737 |
Mar 16, 1998 |
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6235392 |
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09039737 |
Mar 16, 1998 |
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08916652 |
Aug 22, 1997 |
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60023909 |
Aug 23, 1996 |
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60024462 |
Aug 23, 1996 |
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Current U.S.
Class: |
428/375 ;
428/393 |
Current CPC
Class: |
D01D 5/18 20130101; Y10T
428/2913 20150115; Y10T 428/2933 20150115; D21C 9/10 20130101; Y10T
428/29 20150115; Y10T 428/2965 20150115; D01F 2/00 20130101; Y10T
428/23964 20150401; D01D 5/098 20130101; D21C 3/02 20130101; D21C
9/004 20130101 |
Class at
Publication: |
428/375 ;
428/393 |
International
Class: |
D02G 003/00 |
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. Lyocell fiber comprising: a treated alkaline pulp comprising:
(a) at least 5% by weight hemicellulose; (b) cellulose having an
average degree of polymerization of from about 200 to about 1100;
(c) a kappa number of less than 2.0; and (d) a .DELTA.R less than
about 2.8.
2. The fiber of claim 1 having a hemicellulose content of from 5%
by weight to about 22% by weight.
3. The fiber of claim 1 having a hemicellulose content of from 5%
by weight to about 18% by weight.
4. The fiber of claim 3 further comprising cellulose having an
average degree of polymerization of from about 300 to about
1100.
5. The fiber of claim 1 having a hemicellulose content of from
about 10% by weight to about 15% by weight.
6. The fiber of claim 5 further comprising cellulose having an
average degree of polymerization of from about 300 to about
1100.
7. The fiber of claim 1 further comprising cellulose having an
average degree of polymerization of from about 300 to about
1100.
8. The fiber of claim 1 further comprising cellulose having an
average degree of polymerization of from about 400 to about
700.
9. The fiber of claim 1 wherein said cellulose has a unimodal
distribution of degree of polymerization values.
10. The fiber of claim 1 having a copper number of less than about
2.0.
11. The fiber of claim 10 having a copper number of less than about
1.1.
12. The fiber of claim 10 having a copper number of less than about
0.8.
13. The fiber of claim 1 having a total transition metal content of
less than 20 ppm.
14. The fiber of claim 13 having a total transition metal content
of less than 5 ppm.
15. The fiber of claim 1, wherein the .DELTA.R is less than about
2.0.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S.
application Ser. No. 09/574,538, filed May 18, 2000, now pending,
which in turn is a continuation-in-part of U.S. application Ser.
No. 09/256,197, filed Feb. 24, 1999, now U.S. Pat. No. 6,210,801,
which in turn is a continuation-in-part of U.S. application Ser.
No. 09/185,423, filed Nov. 3, 1998, now pending, which in turn is a
continuation-in-part of U.S. application Ser. No. 09/039,737, filed
Mar. 16, 1998, now U.S. Pat. No. 6,235,392, which in turn is a
continuation-in-part of U.S. application Ser. No. 08/916,652, filed
Aug. 22, 1997, now abandoned, and claims the benefit from U.S.
Provisional Application Nos. 60/023,909 and 60/024,462, both filed
Aug. 23, 1996.
FIELD OF THE INVENTION
[0002] The present invention is directed to lyocell fibers. In
particular, lyocell fibers made from compositions having a high
hemicellulose content, a low kappa number and including cellulose
having a low average degree of polymerization and a narrow
molecular weight distribution.
BACKGROUND OF THE INVENTION
[0003] Cellulose is a polymer of D-glucose and is a structural
component of plant cell walls. Cellulose is especially abundant in
tree trunks from which it is extracted, converted into pulp, and
thereafter utilized to manufacture a variety of products. Rayon is
the name given to a fibrous form of regenerated cellulose that is
extensively used in the textile industry to manufacture articles of
clothing. For over a century strong fibers of rayon have been
produced by the viscose and cuprammonium processes. The latter
process was first patented in 1890 and the viscose process two
years later. In the viscose process cellulose is first steeped in a
mercerizing strength caustic soda solution to form an alkali
cellulose. This is reacted with carbon disulfide to form cellulose
xanthate which is then dissolved in dilute caustic soda solution.
After filtration and deaeration the xanthate solution is extruded
from submerged spinnerets into a regenerating bath of sulfuric
acid, sodium sulfate, zinc sulfate, and glucose to form continuous
filaments. The resulting so-called viscose rayon is presently used
in textiles and was formerly widely used for reinforcing rubber
articles such as tires and drive belts.
[0004] Cellulose is also soluble in a solution of ammonia copper
oxide. This property forms the basis for production of cuprammonium
rayon. The cellulose solution is forced through submerged
spinnerets into a solution of 5% caustic soda or dilute sulfuric
acid to form the fibers, which are then decoppered and washed.
Cuprammonium rayon is available in fibers of very low deniers and
is used almost exclusively in textiles.
[0005] The foregoing processes for preparing rayon both require
that the cellulose be chemically derivatized or complexed in order
to render it soluble and therefore capable of being spun into
fibers. In the viscose process, the cellulose is derivatized, while
in the cuprammonium rayon process, the cellulose is complexed. In
either process, the derivatized or complexed cellulose must be
regenerated and the reagents that were used to solubilize it must
be removed. The derivatization and regeneration steps in the
production of rayon significantly add to the cost of this form of
cellulose fiber. Consequently, in recent years attempts have been
made to identify solvents that are capable of dissolving
underivatized cellulose to form a dope of underivatized cellulose
from which fibers can be spun.
[0006] One class of organic solvents useful for dissolving
cellulose are the amine-N oxides, in particular the tertiary
amine-N oxides. For example, Graenacher, in U.S. Pat. No.
2,179,181, discloses a group of amine oxide materials suitable as
solvents. Johnson, in U.S. Pat. No. 3,447,939, describes the use of
anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine
N-oxides as solvents for cellulose and many other natural and
synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and
4,196,282, deal with the difficulties of dissolving cellulose in
amine oxide solvents and of achieving higher concentrations of
cellulose.
[0007] Lyocell is an accepted generic term for a fiber composed of
cellulose precipitated from an organic solution in which no
substitution of hydroxyl groups takes place and no chemical
intermediates are formed. Several manufacturers presently produce
lyocell fibers, principally for use in the textile industry. For
example, Accords, Ltd. presently manufactures and sells a lyocell
fiber called Tencel.RTM. fiber.
[0008] It is believed that currently available lyocell fibers are
produced from high quality wood pulps that have been extensively
processed to remove non-cellulose components, especially
hemicellulose. These highly processed pulps are referred to as
dissolving grade or high alpha (or high .alpha.) pulps, where the
term alpha (or .alpha.) refers to the percentage of cellulose.
Thus, a high alpha pulp contains a high percentage of cellulose,
and a correspondingly low percentage of other components,
especially hemicellulose. The processing required to generate a
high alpha pulp significantly adds to the cost of lyocell fibers
and products manufactured therefrom.
[0009] For example, when the Kraft process is used to produce a
dissolving grade pulp, a mixture of sodium sulfide and sodium
hydroxide is used to pulp the wood. Since conventional Kraft
processes stabilize residual hemicelluloses against further
alkaline attack, it is not possible to obtain acceptable quality
dissolving pulps, i.e., high alpha pulps, through subsequent
treatment of Kraft pulp in the bleaching stages. In order to
prepare dissolving type pulps by the Kraft process, it is necessary
to give the raw material an acidic pretreatment before the alkaline
pulping stage. A significant amount of material primarily
hemicellulose, on the order of 10% or greater of the original wood
substance, is solubilized in this acid phase pretreatment and thus
process yields drop. Under the prehydrolysis conditions, the
cellulose is largely resistant to attack, but the residual
hemicelluloses are degraded to a much shorter chain length and can
therefore be removed to a large extent in the subsequent Kraft cook
by a variety of hemicellulose hydrolysis reactions or by
dissolution.
[0010] The prehydrolysis stage normally involves treatment of wood
at elevated temperature (150-180.degree. C.) with dilute mineral
acid (sulfuric or aqueous sulfur dioxide) or with water alone
requiring times up to 2 hours at the lower temperatures. In the
latter case, liberated acetic acid from certain of the naturally
occurring polysaccharides (predominantly the mannans in softwoods
and the xylan in hardwoods) lowers the pH below 4.
[0011] Moreover, a relatively low copper number, reflective of the
relative carbonyl content of the cellulose, is a desirable property
of a pulp that is to be used to make lyocell fibers because it is
generally believed that a high copper number causes cellulose and
solvent degradation, before, during, and/or after dissolution in an
amine oxide solvent. The degraded solvent can either be disposed of
or regenerated; however, due to its cost it is generally
undesirable to dispose of the solvent. Regeneration of the solvent
suffers from the drawback that the regeneration process involves
dangerous, potentially explosive conditions.
[0012] A low transition metal content is a desirable property of a
pulp that is to be used to make lyocell fibers because, for
example, transition metals accelerate the undesirable degradation
of cellulose and NMMO in the lyocell process.
[0013] In view of the expense of producing commercial dissolving
grade pulps it would be desirable to have alternatives to
conventional high alpha dissolving grade pulps as a lyocell raw
material. In addition, pulp manufacturers would like to minimize
the capital investment necessary to produce such types of pulps by
utilizing existing capital plants.
[0014] In order to control lyocell fiber properties, lyocell
manufacturers utilize dopes that comprise a blend of different
pulps having different ranges of average degree of polymerization
values. In view of this, there is also a need for pulp
manufacturers to produce pulps having an average degree of
polymerization within a relatively narrow band.
[0015] Thus, there is a need for relatively inexpensive, low alpha
(e.g., high yield) pulps that can be used to make lyocell fibers,
for a process of making the foregoing low alpha pulps using capital
equipment that is currently available to pulp manufacturers, and
for lyocell fibers from the foregoing low alpha pulp. Preferably,
the desired low alpha pulps will have a desirably low copper
number, a desirably low lignin content and a desirably low
transition metal content.
[0016] In the prior application Ser. No. 09/256,197, assigned to
the assignee of the subject application, various methods of
reducing D.P. values and copper number of a Kraft pulp are
described. Such methods include treating pulp with acid, or an acid
substitute, or a combination of acids and acid substitutes. Other
means of treating the pulp to reduce the average D.P. of cellulose
without substantially reducing the hemicellulose content described
in the prior application include treatment of the pulp with steam,
a combination of ferrous sulfate and hydrogen peroxide, at least
one transition metal and peracetic acid, an alkaline chlorine
dioxide treatment which ends acidic or a sodium hypochlorite
treatment which ends near neutral. Such processes are effective at
reducing the average degree of polymerization without substantially
reducing the hemicellulose content, however, such processes can be
expensive from a capital improvement standpoint if the existing
pulp mills in which such processes are to be used are not
configured to allow for the simple deployment of such processes. In
the prior application, additional steps are described in order to
reduce the copper number of the pulp which have been treated to
reduce its average degree of polymerization without substantially
decreasing the hemicellulose content. The need for this subsequent
copper number reducing step arose because the methods described in
the prior application for reducing the average degree of
polymerization for the cellulose resulted in an increase in the
copper number for the resultant pulp.
[0017] In view of environmental concerns, there has been a great
interest in using bleaching agents, which reduce the amount of
chlorocompounds that must be recovered from process streams. In
recent years, the use of oxygen as a delignifying agent has
occurred on a commercial scale. Examples of equipment and apparatus
useful for carrying out an oxygen stage delignification are
described in U.S. Pat. Nos. 4,295,927; 4,295,925; 4,298,426; and
4,295,926.
[0018] While the methods described in the prior application are
effective at reducing the average D.P. of cellulose without
substantially decreasing the hemicellulose content, a need exists
for a process that does not require a separate copper number
reducing step and which is readily adaptable to pulp mills that
include oxygen reactors, multiple alkaline stages and/or alkaline
conditions suitable for substantial D.P. reduction of bleached or
semi-bleached pulp.
SUMMARY OF THE INVENTION
[0019] As used herein, the terms "composition(s) of the present
invention", or "composition(s) useful for making lyocell fibers",
or "treated pulp" refer to pulp, containing cellulose and
hemicellulose, that has been treated under alkaline conditions in
order to reduce the average degree of polymerization (D.P.) of the
cellulose without substantially reducing the hemicellulose content
of the pulp or substantially increasing the copper number for the
pulp. The compositions of the present invention preferably possess
additional properties as described herein.
[0020] Compositions of the present invention are compositions
useful for making lyocell fibers, or other molded bodies such as
films, having a high hemicellulose content, a low copper number and
a narrow molecular weight distribution, including cellulose that
has a low average D.P. Preferably, the cellulose and hemicellulose
are derived from wood, more preferably from softwood. Additionally,
the compositions of the present invention exhibit a variety of
desirable properties including a low lignin content, and a low
transition metal content. Compositions of the present invention may
be in a form that is adapted for storage or transportation, such as
a sheet, roll or bale. Compositions of the present invention may be
mixed with other components or additives to form pulp useful for
making lyocell molded bodies, such as fiber or films. Further, the
present invention provides processes for making compositions useful
for making lyocell fibers having desirable hemicellulose content
and copper number, and including cellulose that has a desirable
average D.P. and molecular weight distribution.
[0021] The present invention also provides lyocell fibers
containing cellulose having a low average D.P., a high proportion
of hemicellulose and a low copper number, a narrow molecular weight
distribution, and a low lignin content. The lyocell fibers of the
present invention also preferably possess a low transition metal
content.
[0022] Compositions of the present invention can be made from any
suitable source of cellulose and hemicellulose but are preferably
made from an alkaline chemical wood pulp such as Kraft or soda, and
more preferably from a Kraft softwood pulp. Compositions of the
present invention include at least 7% by weight hemicellulose,
preferably from 7% by weight to about 25% by weight hemicellulose,
more preferably from 7% by weight to about 20% by weight
hemicellulose, most preferably from about 10% by weight to about
17% by weight hemicellulose, and cellulose having an average D.P.
of from about 200 to about 1100, preferably from about 300 to about
1100, and more preferably from about 400 to about 700. A presently
preferred composition of the present invention has a hemicellulose
content of from about 10% by weight to about 17% by weight, and
contains cellulose having an average D.P. of from about 400 to
about 700. Hemicellulose content is measured by a sugar content
assay based on TAPPI Standard T249 hm-85. Further, compositions of
the present invention preferably have a kappa number of less than
2, preferably less than 1. Most preferably compositions of the
present invention contain no detectable lignin. Lignin content is
measured using TAPPI Test T236 cm-85.
[0023] Compositions of the present invention preferably have a
unimodal distribution of cellulose D.P. values wherein the
individual D.P. values are approximately normally distributed
around a single, modal D.P. value, ie., the modal D.P. value being
the D.P. value that occurs most frequently within the distribution.
The distribution of cellulose D.P. values may, however, be
multimodal i.e., a distribution of cellulose D.P. values that has
several relative maxima. A multimodal, treated pulp of the present
invention might be formed, for example, by mixing two or more
unimodal, treated pulps of the present invention that each have a
different modal D.P. value. The distribution of cellulose D.P.
values is determined by means of proprietary assays performed by
Thuringisches Institut fur Textil-und Kunstoff Forschunge. V.,
Breitscheidstr. 97, D-07407 Rudolstadt, Germany.
[0024] Compositions of the present invention which have been
treated to reduce their D.P. without substantially reducing the
hemicellulose content of the pulp, exhibit a desirably narrow
molecular weight distribution as evidenced by a differential
between R.sub.10 and R.sub.18 values (.DELTA.R) of less than about
2.8, preferably less than about 2.0 and most preferably less than
about 1.5.
[0025] Additionally, compositions of the present invention
preferably have a relatively low carbonyl content as evidenced by a
copper number of less than about 2.0, more preferably less than
about 1.1, most preferably less than about 0.8 as measured by TAPPI
Standard T430. Further, compositions of the present invention
preferably have a carbonyl content of less than about 60 .mu.mol/g
and a carboxyl content of less than about 60 .mu.mol/g, more
preferably, a carbonyl content less than 30 .mu.mol/g and a
carboxyl content less than about 30 .mu.mol/g. The carboxyl and
carbonyl group content are measured by means of proprietary assays
performed by Thuringisches Institut fur Textil-und Kunstoff
Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany,
referred to below as TITK.
[0026] Compositions of the present invention also preferably
possess a low transition metal content. Preferably, the total
transition metal content of the compositions of the present
invention is less than 20 ppm, more preferably less than 5 ppm, as
measured by Weyerhaeuser Test Number AM5-PULP-1/6010. The term
"total transition metal content" refers to the combined amounts,
measured in units of parts per million (ppm), of nickel, chromium,
manganese, iron and copper. Preferably the iron content of the
compositions of the present invention is less than 4 ppm, more
preferably less than 2 ppm, as measured by Weyerhaeuser Test
AM5-PULP-1/6010, and the copper content of the compositions of the
present invention is preferably less than 1.0 ppm, more preferably
less than 0.5 ppm, as measured by Weyerhaeuser Test
AM5-PULP-1/6010.
[0027] Compositions of the present invention are readily soluble in
amine oxides, including tertiary amine oxides such as NMMO. Other
preferred solvents that can be mixed with NMMO, or another tertiary
amine solvent, include dimethylsulfoxide (D.M.S.O.),
dimethylacetamide (D.M.A.C.), dimethylformamide (D.M.F.) and
caprolactan derivatives. Preferably, compositions of the present
invention fully dissolve in NMMO in less than about 70 minutes,
preferably less than about 20 minutes, utilizing the dissolution
procedure described in Example 11 below. The term "fully dissolve",
when used in this context, means that substantially no undissolved
particles are seen when a dope, formed by dissolving compositions
of the present invention in NMMO, is viewed under a light
microscope at a magnification of 40.times. to 70 .times..
[0028] A first preferred embodiment of the treated pulp of the
present invention is a treated Kraft pulp including at least 7% by
weight hemicellulose, a copper number less than about 2.0,
cellulose having an average degree of polymerization of from about
200 to about 1100, and a .DELTA.R less than about 2.8.
[0029] A second preferred embodiment of the treated pulp of the
present invention is a treated Kraft pulp including at least 7% by
weight hemicellulose, a copper number less than two, cellulose
having an average degree of polymerization of from about 200 to
about 1100, the individual D.P. values of the cellulose being
distributed unimodally, and a .DELTA.R less than about 2.8.
[0030] A third preferred embodiment of the treated pulp of the
present invention is a treated Kraft pulp including at least 7% by
weight hemicellulose, cellulose having an average degree of
polymerization of from about 200 to about 1100, a kappa number less
than two, a copper number less than 0.8, and a .DELTA.R less than
about 2.8.
[0031] Lyocell fibers formed from compositions of the present
invention include at least about 5% by weight hemicellulose,
preferably from about 5% by weight to about 22% by weight
hemicellulose, more preferably from about 5% by weight to about 18%
by weight hemicellulose, most preferably from about 10% by weight
to about 15% by weight hemicellulose, cellulose having an average
D.P. of from about 200 to about 1100, more preferably from about
300 to about 1100, most preferably from about 400 to about 700, and
a lignin content providing a kappa number less than about 2.0 and
more preferably less than about 1.0. Additionally, preferred
lyocell fibers of the present invention have a unimodal
distribution of cellulose D.P. values, although lyocell fibers of
the present invention may also have a multimodal distribution of
cellulose D.P. values, i.e., a distribution of cellulose D.P.
values that has several relative maxima. Lyocell fibers of the
present invention having a multimodal distribution of cellulose
D.P. values might be formed, for example, from a mixture of two or
more unimodal, treated pulps of the present invention that each
have a different modal D.P. value.
[0032] Preferred lyocell fibers of the present invention have a
copper number of less than about 2.0, more preferably less than
about 1.1, most preferably less than about 0.8 as measured by TAPPI
Standard T430. Further, preferred lyocell fibers of the present
invention have a carbonyl content of less than about 60 .mu.mol/g
and a carboxyl content of less than about 60 .mu.mol/g, more
preferably a carbonyl content less than about 30 .mu.mol/g and a
carboxyl content of less than about 30 .mu.mol/g. The carboxyl and
carbonyl group content are measured by means of proprietary assays
performed by Thuringisches Institut fur Textil-und Kunstoff
Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany.
Additionally, preferred lyocell fibers of the present invention
have a total transition metal content of less than about 20 ppm,
more preferably less than about 5 ppm, as measured by Weyerhaeuser
Test Number AM5-PULP-1/6010. The term "total transition metal
content" refers to the combined amount, expressed in units of parts
per million (ppm), of nickel, chromium, manganese, iron and copper.
Preferably the iron content of lyocell fibers of the present
invention is less than about 4 ppm, more preferably less than about
2 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010, and the
copper content of lyocell fibers of the present invention is
preferably less than about 1 ppm, more preferably less than about
0.5 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010.
[0033] Preferred embodiments of the lyocell fibers of the present
invention possess desirable elongation properties. Preferably,
lyocell fibers of the present invention possess a dry elongation of
from about 8% to about 17%, more preferably from about 12% to about
15%. Preferably, lyocell fibers of the present invention possess a
wet elongation of from about 12% to about 18%. Elongation is
measured by means of proprietary assays performed by Thuringisches
Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr.
97, D-07407 Rudolstadt, Germany. Lyocell fibers produced from
treated pulps of the present invention have exhibited dry
tenacities on the order of about 40-42 cN/tex and wet tenacities on
the order of 30-33 cN/tex as measured by the proprietary assays
performed by Thuringisches Institut fur Textil-und Kunstoff
Forschunge. V., Breitscheidstr.
[0034] In another aspect, the present invention provides processes
for making compositions of the present invention that can, in turn,
be formed into lyocell molded bodies, such as fibers or films. In
this aspect, the present invention provides a process that includes
contacting an alkaline pulp comprising cellulose and at least about
7% hemicellulose under alkaline conditions with an amount of an
oxidant sufficient to reduce the average D.P. of the cellulose to
within the range of from about 200 to about 1100, preferably to
within the range of from about 300 to about 1100, more preferably
to within the range of from about 400 to about 700, without
substantially reducing the hemicellulose content or increasing the
copper number. Pulps which are to be treated according to the
present invention with an oxidant to achieve the D.P. reduction
without substantially reducing the hemicellulose content or
increasing the copper number as discussed above preferably have a
kappa number less than 40, more preferably less than 30 and most
preferably less than 25 when they are contacted for the first time
with the oxidant.
[0035] This D.P. reduction treatment can occur after the pulping
process and before, during or after the bleaching process, if a
bleaching step is utilized. The oxidant under alkaline conditions
is any oxidant containing a peroxide group such as hydrogen
peroxide, oxygen, chlorine dioxide and ozone. Preferably the
oxidant is a combination of oxygen and hydrogen peroxide, or
hydrogen peroxide alone.
[0036] Preferably the carbohydrate yield of the D.P. reducing step
of the present invention is greater than about 95%, more preferably
greater than about 98%. The process yield is the dry weight of the
treated pulp produced by the process divided by the dry weight of
the starting material pulp, the resulting fraction being multiplied
by one hundred and expressed as a percentage.
[0037] In another aspect of the present invention a process for
making lyocell fibers includes the steps of (a) after the pulping
process, contacting an alkaline pulp including cellulose and at
least about 7% hemicellulose with an amount of an oxidant
sufficient to reduce the average degree of polymerization of the
cellulose to the range of from about 200 to about 1100, preferably
to the range of from about 300 to about 1100, without substantially
reducing the hemicellulose content or increasing the copper number
of the pulp; and (b) forming fibers from the pulp treated in
accordance with step (a). In accordance with this aspect of the
present invention, the lyocell fibers are preferably formed by a
process selected from the group consisting of melt blowing,
centrifugal spinning, spun bonding and a dry jet/wet process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0039] FIGS. 1A-1C are block diagrams of the presently preferred
processes for converting pulp, preferably an alkaline pulp, to a
composition of the present invention useful for making lyocell
molded bodies, combinations of FIGS. 1A-1C could also be
employed;
[0040] FIG. 2 is a block diagram of the steps of the presently
preferred process of forming fibers from the compositions of the
present invention; and
[0041] FIGS. 3 and 4 are scanning electron micrographs at
100.times.and 10,000.times.magnification of a dry jet/wet lyocell
fiber produced, as set forth in Example 11, from treated pulp of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0042] U.S. application Ser. No. 09/574,538, is herein expressly
incorporated by reference. Starting materials useful in the
practice of the present invention contain cellulose and
hemicellulose. Examples of starting materials useful in the
practice of the present invention include, but are not limited to,
trees and recycled paper. The starting materials used in the
practice of the present invention, from whatever source, are
initially converted to a pulp using an alkaline pulping process,
such as the Kraft or soda process. The presently preferred starting
material in the practice of the present invention is an alkaline
chemical wood pulp, preferably an unbleached Kraft wood pulp, or a
bleached Kraft wood pulp containing cellulose and at least about 7%
hemicellulose, that has not been exposed to acid hydrolysis
conditions or any other heterogeneous mixture conditions (i.e.,
reaction time, temperature, and acid concentration), where
cellulose glycosidic bonds are broken. The discussion of the
preferred embodiment of the present invention that follows will
refer to the starting material as pulp or pulped wood, but it will
be understood that the specific reference to wood as the source of
starting material pulp in the following description of the
preferred embodiment of the present invention is not intended as a
limitation, but rather as an example of a presently preferred
source of hemicellulose and cellulose.
[0043] In order to distinguish between the pulp that is useful as a
starting material in the practice of the present invention (such as
a bleached or unbleached, alkaline Kraft wood pulp) and the
compositions of the present invention (that are produced by
treating the starting material, in order to reduce the average D.P.
of the starting material pulp without substantially reducing the
hemicellulose content or increasing the copper number of the
starting material pulp), the latter will be referred to as
"composition(s) of the present invention", or "composition(s)
useful for making lyocell fibers", or "treated pulp" or "treated
Kraft pulp."
[0044] In the wood pulping industry, trees are conventionally
classified as either hardwood or softwood. In the practice of the
present invention, pulp for use as starting material in the
practice of the present invention can be derived from softwood tree
species such as, but not limited to: fir (preferably Douglas fir
and Balsam fir), pine (preferably Eastern white pine and Loblolly
pine), spruce (preferably White spruce), larch (preferably Eastern
larch), cedar, and hemlock (preferably Eastern and Western
hemlock). Examples of hardwood species from which pulp useful as a
starting material in the present invention can be derived include,
but are not limited to: acacia, alder (preferably Red alder and
European black alder) aspen (preferably Quaking aspen), beech,
birch, oak (preferably White oak), gum trees (preferably eucalyptus
and Sweetgum), poplar (preferably Balsam poplar, Eastern
cottonwood, Black cottonwood and Yellow poplar), gmelina and maple
(preferably Sugar maple, Red maple, Silver maple and Bigleaf
maple).
[0045] Wood from softwood or hardwood species generally includes
three major components: cellulose, hemicellulose and lignin.
Cellulose makes up about 50% of the woody structure of plants and
is an unbranched polymer of D-glucose monomers. Individual
cellulose polymer chains associate to form thicker microfibrils
which, in turn, associate to form fibrils which are arranged into
bundles. The bundles form fibers which are visible as components of
the plant cell wall when viewed at high magnification under a light
microscope. Cellulose is highly crystalline as a result of
extensive intermolecular and intermolecular hydrogen bonding.
[0046] The term hemicellulose refers to a heterogeneous group of
low molecular weight carbohydrate polymers that are associated with
cellulose in wood. Hemicelluloses are amorphous, branched polymers,
in contrast to cellulose which is a linear polymer. The principal,
simple sugars that combine to form hemicelluloses are: D-glucose,
D-xylose, D-mannose, L-arabinose, D-galactose, D-glucuronic acid
and D-galacturonic acid.
[0047] Lignin is a complex aromatic polymer and comprises about 15%
to 30% of wood where it occurs as an amorphous polymer.
[0048] In the pulping industry, differences in the chemistry of the
principal components of wood are exploited in order to purify
cellulose. For example, heated water in the form of steam causes
the removal of acetyl groups from hemicellulose with a
corresponding decrease in pH due to the formation of acetic acid.
At elevated temperatures of about 150.degree. C.-180.degree. C.,
acid hydrolysis of the carbohydrate components of wood then ensues,
with a lesser hydrolysis of lignin. Hemicelluloses are especially
susceptible to this acid hydrolysis, and most of the hemicellulose
can be degraded by an initial steam, prehydrolysis step in the
Kraft pulping process, as described in the Background, or in an
acidic sulfite cooking process.
[0049] With respect to the reaction of wood with alkali solutions,
all components of wood are susceptible to degradation by strong
alkaline conditions. At the elevated temperature of 140.degree. C.
or greater that is typically utilized during Kraft wood pulping,
the hemicelluloses and lignin are preferentially degraded by dilute
alkaline solutions. Additionally, all components of wood can be
oxidized by bleaching agents such as chlorine, sodium hypochlorite
and hydrogen peroxide.
[0050] Pulping procedures, such as alkaline pulping, can be used to
provide an alkaline wood pulp that is treated in accordance with
the present invention to provide a composition useful for making
lyocell fibers. Examples of a suitable alkaline pulping processes
include the Kraft or soda process, without an acid prehydrolysis
step or exposure to other acidic heterogeneous mixture conditions
(i.e., reaction time, temperature and acid concentration) where
cellulose glycosidic bonds are broken through (1) the rapid
protonation of the glycosidic oxygen atom, (2) slow transfer of the
positive charge to C-1 with consequent formation of a carbonuins
ion and fusion of the glycosidic bond and (3) rapid attack on the
carbonium ion by water to give the free sugar. While a typical
Kraft bleaching sequence containing a chlorine dioxide state or
multiple chlorine dioxide stages involves a pH less than 4 and a
temperature greater than about 70.degree. C., the combined
heterogeneous mixture conditions of such stages are not suitable to
induce substantial DP reduction in cellulose. By avoiding an acid
pretreatment step prior to alkaline pulping, the overall cost of
producing the alkaline pulped wood is reduced. Further, by avoiding
the acid prehydrolysis the degradation of hemicellulose is reduced
and the overall yield of the pulping process can be increased.
Thus, as used herein the phrase alkaline pulp refers to pulp
containing cellulose and hemicellulose that has not been subjected
to any combination of acidic conditions or any other heterogeneous
mixture conditions (i.e., reaction time, temperature, and acid
concentration) that would result in breaking of the cellulose
glycosidic bonds before or during the pulping process wherein wood
chips or other biomass is converted to fibers.
[0051] Characteristics of alkaline pulped wood suitable for use as
a starting material in the practice of the present invention
include a hemicellulose content of at least 7% by weight,
preferably from 7% to about 30% by weight, more preferably from 7%
to about 25% by weight, and most preferably from about 9% to about
20% by weight; an average D.P. of cellulose of from about 600 to
about 1800; a kappa number less than about 40 preferably less than
30 and more preferably less than 25, and a copper number less than
about 2.0, preferably less than 1.0. As used herein, the term
"percent (or %) by weight" or "weight percent", or grammatical
variants thereof, when applied to the hemicellulose or lignin
content of pulp, means weight percentage relative to the dry weight
of the pulp.
[0052] As shown in FIGS. 1A-1C, in the practice of the present
invention, once starting material, such as softwood, has been
converted to an alkaline pulp containing cellulose and
hemicellulose, it is subjected to treatment in reactor whereby the
average D.P. of the cellulose is reduced, without substantially
reducing the hemicellulose content or increasing the copper number,
to provide the compositions of the present invention. In this
context, the term "without substantially reducing the hemicellulose
content" means without reducing the hemicellulose content by more
than about 50%, preferably not more than about 15%, and most
preferably not more than about 5% during the D.P. reduction step.
The term "degree of polymerization" (abbreviated as D.P.) refers to
the number of D-glucose monomers in a cellulose molecule. Thus, the
term "average degree of polymerization", or "average D.P.", refers
to the average number of D-glucose molecules per cellulose polymer
in a population of cellulose polymers. This D.P. reduction
treatment can occur after the pulping process and before, after or
substantially simultaneously with the bleaching process, if a
bleaching step is utilized. In this context, the term
"substantially simultaneously with" means that at least a portion
of the D.P. reduction step occurs at the same time as at least a
portion of the bleaching step. Preferably the average D.P. of the
cellulose is reduced to a value within the range of from about 200
to about 1100; more preferably to a value within the range of from
about 300 to about 1100; most preferably to a value of from about
400 to about 700. Unless stated otherwise, D.P. is determined by
ASTM Test 1301-12. A D.P. within the foregoing ranges is desirable
because, in the range of economically attractive operating
conditions, the viscosity of the dope, i.e., the solution of
treated pulp from which lyocell fibers are produced, is
sufficiently low that the dope can be readily extruded through the
narrow orifices utilized to form lyocell fibers, yet not so low
that the strength of the resulting lyocell fibers is substantially
compromised. Preferably the range of D.P. values of the treated
pulp will be unimodal and will have an approximately normal
distribution that is centered around the modal D.P. value.
[0053] In this application, the term "without substantially
increasing the copper number" means without increasing the copper
number by more than about 100%, preferably not more than about 50%
and most preferably not more than about 25% during the D.P.
reduction step. The degree to which the copper number changes
during the D.P. reduction step is determined by comparing the
copper number of the pulp entering the D.P. reduction step and the
copper number of the treated pulp after the D.P. reduction step. A
low copper number is desirable because it is generally believed
that a high copper number causes cellulose and solvent degradation
during and after dissolution of the treated pulp to form a dope.
The copper number is an empirical test used to measure the reducing
value of cellulose. The copper number is expressed in terms of the
number of milligrams of metallic copper which is reduced from
cupric hydroxide to cuprous oxide in alkaline medium by a specified
weight of cellulosic material.
[0054] The hemicellulose content of the treated pulp, expressed as
a weight percentage, is at least 7% by weight; preferably from
about 7% by weight to about 25% by weight; more preferably from
about 7% by weight to about 20% by weight; most preferably from
about 10% by weight to about 17% by weight. As used herein, the
term "percent (or %) by weight" or "weight percentage", or
grammatical equivalents thereof, when applied to the hemicellulose
or lignin content of treated pulp, means weight percentage relative
to the dry weight of the treated pulp.
[0055] Treated pulps of the present invention also exhibit a
desirably narrow molecular weight distribution as evidenced by a
differential between R.sub.10 and R.sub.18 values (.DELTA.R) of
less than about 2.8, preferably less than about 2.0, and most
preferably less than about 1.5. In contrast, pulps treated in
accordance with the teachings of U.S. application Ser. No.
09/256,197 prior to treatment to reduce its copper number exhibits
a .DELTA.R greater than about 2.8. After treatment to reduce the
copper number in accordance with this prior application, the
.DELTA.R for the pulps of the prior application can be reduced to
less than about 2.8. Sulfite pulps tend to exhibit a .DELTA.R on
the order of about 7.0 and prehydrolyzed Kraft pulps exhibit a
.DELTA.R that tends to be on the order of about 3.0. R.sub.10
refers to the residual undissolved material that is left after
attempting to dissolve the pulp in a 10% caustic solution. R.sub.18
refers to the residual amount of undissolved material left after
attempting to dissolve the pulp in an 18% caustic solution.
Generally, in a 10% caustic solution, hemicellulose and chemically
degraded short chain cellulose are dissolved and removed in
solution. In contrast, generally only hemicellulose is dissolved
and removed in an 18% caustic solution. Thus, the difference
between the R.sub.10 value and the R.sub.18 value represents the
amount of chemically degraded short chained cellulose that is
present in the pulp sample. Providing a pulp having a relatively
narrow molecular weight distribution is desirable from the
standpoint of being able to provide customers with pulp which can
be mixed with pulps of different molecular weight properties to
predictably tailor the molecular weight distribution in a dope used
to produce lyocell fibers. Another advantage of providing the pulp
having a relatively narrow molecular weight distribution is the low
concentration of short chain cellulose or hemicellulose molecules
present in such pulp. Such short chain oligomer material if
present, may complicate the lyocell solvent recovery process.
[0056] Without intending to be bound by theory, it is believed that
the chemical form of the hemicellulose in pulps treated in
accordance with the present invention is distinct from the chemical
form of hemicellulose in pulps that have been exposed to acidic
conditions or heterogeneous mixture conditions described above
which result in the breaking of cellulose glycosidic bonds, such as
the pulps described in prior application Ser. No. 09/256,197 and
commercially available dissolving grade pulps. This difference in
chemical form may be evidenced by the D.P. of the hemicellulose in
the pulp of the present invention compared to the D.P. of the
hemicellulose of the pulp of the prior application or commercial
dissolving grade pulps. This D.P. difference can be observed when
the respective pulps are derivatized (acetylated) and tested in the
accordance with the discussion by S. A. Rydholm in Pulping
Processes, Interscience Publishers, 1965. The higher D.P.
hemicellulose in treated alkaline pulps of the present invention
may be less likely to be extracted from lyocell filaments during
the filament formation process or post treatment of the formed
lyocell filament as compared to the hemicellulose of the pulps of
the prior application or commercially available dissolving grade
pulps.
[0057] A presently preferred method of treating pulp in order to
reduce the average D.P. of the cellulose without substantially
reducing the hemicellulose content of the pulp and without
substantially increasing the copper number of the pulp is to treat
the pulp under alkaline conditions in high consistency or medium
consistency reactor(s) where the pulp is contacted with an oxidant
containing a peroxide group such as oxygen, chlorine dioxide, ozone
or combinations thereof. Preferably the oxidant is a combination of
oxygen and hydrogen peroxide or hydrogen peroxide alone.
[0058] The treated pulps formed in accordance with the present
invention which have been treated in order to reduce their average
degree of polymerization values without substantially decreasing
the hemicellulose content or the copper number for the pulp can be
produced by contacting the pulp in reactor with an oxidant under
conditions suitable to achieve the desired results described above.
Suitable reactors include reactors conventionally used as oxygen
reactors in a Kraft process. Examples of reactors capable of
carrying out the contacting of the pulp with the oxidant are
described in U.S. Pat. Nos. 4,295,925; 4,295,926; 4,298,426;
4,295,927, each of which is herein incorporated by reference.
Unlike conventional oxygen reactors which are configured and
operated under conditions that preferably do not decrease the
average degree of polymerization of cellulose while at the same
time remove lignin applicants' invention is designed to operate a
reactor under conditions that reduce the average degree of
polymerization of the cellulose without substantially reducing the
hemicellulose content or increasing the copper number of the
cellulose. In accordance with the present invention, the reactor
can be a high consistency reactor wherein the consistency of the
feedstream to the reactor is greater than about 20% or it can be a
medium consistency reactor where the consistency ranges between
about 8% up to about 20%. The conditions under which a high
consistency reactor or a medium consistency reactor is typically
operated in order to achieve the desired results of the present
invention relate primarily to operation of the high consistency
reactor at a temperature that is slightly higher than the
temperature at which the medium consistency reactor can be operated
as described below in more detail.
[0059] The following describes particular conditions under which a
reactor can be operated in order to achieve reduction in average
degree of polymerization values for the pulp without substantially
decreasing the hemicellulose content or increasing the copper
number of the incoming pulp. It should be understood that
variations from the conditions described above can be made in order
to optimize the process to provide the desired product.
[0060] Examples of oxidants that can be employed have been
described above. Preferred oxidants include hydrogen peroxide alone
or a combination of oxygen and hydrogen peroxide. The amount of
oxidant employed should provide the desired D.P. reduction and
lignin removal given the time and temperature conditions employed.
Examples of suitable ranges for oxygen and hydrogen peroxide are
given below. Preferably, for a high consistency reactor, the oxygen
is present in an amount ranging from about 0 to the maximum
pressure rating for the reactor, preferably about 0 to about 85
psig, and more preferably, from about 40 to about 60 psig. The
hydrogen peroxide may be present in an amount ranging from greater
than about 0.75 weight percent up to about 5.0 weight percent, more
preferably about 1.0 to about 2.5 weight percent.
[0061] In medium consistency reactors, the oxygen can be present in
an amount ranging from about 0 to about 100 pounds per ton of the
pulp, more preferably, about 50 to about 80 pounds per ton of pulp.
The hydrogen peroxide may be present in an amount ranging from
greater than about 0.75 weight percent up to about 5 weight
percent, more preferably from about 1.0 to about 2.5 weight
percent.
[0062] The temperature at which the reactor is operated will in
part depend upon the concentration of the oxidants. When the
oxidants are used in amounts that fall within the ranges described
above, temperatures on the order of about 110.degree. C. up to
about 130.degree. C. are suitable. It should be understood that the
temperature in the reactor may vary over time as the reactions that
occur therein tend to be exothermic which will most likely result
in an increase of the temperature of the reactor. It should be
understood that temperatures and oxidant concentrations falling
outside the ranges described above may still provide suitable
results depending on the various permutations of the amounts of
oxidant used and the temperature.
[0063] In accordance with the present invention, the stage or
stages used to reduce the average degree of polymerization of the
pulp without substantially decreasing the hemicellulose content or
increasing the copper number of the pulp remains alkaline through
the stage or stages. Preferably, the pH of the stage or stages used
to achieve the D.P. reduction described above is greater than about
8.0 and more preferably greater than about 9 throughout the D.P.
reduction process. It should be understood that pHs above or below
the noted ranges may provide satisfactory results if the
temperature or concentration of oxidant is modified as
necessary.
[0064] In accordance with the present invention, it is preferred
that contact between the pulp and the oxidant occur prior to any
acid wash or chelation stage normally used to remove transition
metals. Unlike prior art processes which intentionally sought to
remove transition metals which were believed to result in
decomposition of hydrogen peroxide into cellulose-degrading
intermediates that negatively impacted the viscosity of the
cellulose, applicants have discovered that they can take advantage
of the presence of naturally occurring transition metals in the
wood to partially degrade the hydrogen peroxide to produce
intermediates that react with the cellulose to reduce its average
degree of polymerization without substantially decreasing the
hemicellulose content or increasing the kappa number. In addition,
unlike prior art processes that use magnesium sulfate as a means of
inhibiting the degradation of cellulose, applicants prefer not to
introduce magnesium sulfate into the reactor or upstream therefrom
so that the pulp is contacted with the oxidant(s) in the
substantial absence of an inhibitor to the degradation of the
cellulose by the oxidant. If magnesium sulfate is present in the
pulp prior to the reactor, it is preferred that the ratio of
magnesium to the transition metals be less than 50% on a weight
percent basis.
[0065] In addition to the oxidants, caustic is preferably contacted
with the pulp in the reactor as a buffering agent. The source of
caustic can be sodium hydroxide or other materials such as
unoxidized white liquor or oxidized white liquor. The amount of
caustic added will depend in part upon the kappa number of the
untreated pulp. Generally, as the kappa number increases, more
caustic is added. The amount of caustic introduced can vary
depending on process conditions, with an amount of 4 to 5 weight
percent or a greater being suitable.
[0066] When wood pulp containing cellulose and at least 7%
hemicellulose having a copper number of about 2 or less is
contacted with an oxidant under the conditions set forth above, a
treated pulp is produced having a D.P. ranging from about 200 to
about 1,100, containing at least 7% by weight hemicellulose, having
a copper number less than about 2 and a .DELTA.R of less than about
2.8. It should be understood that the description above of
particular conditions under which a bleached or unbleached wood
pulp can be contacted with an oxidant to reduce its average degree
of polymerization without substantially reducing the hemicellulose
content or increasing the copper number are exemplary and that
other conditions can provide suitable results and still fall within
the scope of the present invention. In addition, it should be
understood that in some situations, the pulp exiting the D.P.
reduction stage may be suitable for use in producing a dope for
manufacture of lyocell fibers; however, in other situations,
subsequent process stages such as bleaching stages may be desirable
provided that subsequent stages do not result in a significant
decrease in the hemicellulose content or a significant increase in
the copper number of the pulp. In addition, as noted above, in some
situations, it may be necessary or advantageous to subject the pulp
which has been exposed to an oxidant in a first stage to a second
or even third stage of contact with an oxidant in order to further
reduce the degree of polymerization of the cellulose without
substantially reducing the hemicellulose content or increasing the
copper number thereof.
[0067] Again with reference to FIG. 1, once the alkaline pulp has
been treated with oxidants in a reactor in accordance with the
present invention, the treated pulp can either be washed in water
and transferred to a bath of organic solvent, such as NMMO, for
dissolution prior to lyocell molded body formation, or the treated
pulp can be washed with water and dried for subsequent packaging,
storage and/or shipping. Alternatively, the treated, washed pulp
can be dried and broken into fragments for storage and/or
shipping.
[0068] A desirable feature of the treated pulps of the present
invention is that the cellulose fibers remain substantially intact
after treatment. Consequently, the treated pulp has a freeness and
a fines content that are similar to those of the untreated
pulp.
[0069] Another desirable feature of the treated pulps of the
present invention is their ready solubility in organic solvents,
such as tertiary amine oxides including NMMO. Rapid solubilization
of the treated pulp prior to spinning lyocell fibers is important
in order to reduce the time required to generate lyocell fibers, or
other molded bodies such as films, and hence reduce the cost of the
process. Further, efficient dissolution is important because it
minimizes the concentration of residual, undissolved particles, and
partially dissolved, gelatinous material, which can reduce the
speed at which fibers can be spun, tend to clog the spinnerets
through which lyocell fibers are spun, and may cause breakage of
the fibers as they are spun.
[0070] While not wishing to be bound by theory, it is believed that
the processes of the present invention utilized to reduce the
average D.P. of the cellulose also permeabilize the secondary layer
of the pulp fibers, thereby permitting the efficient penetration of
solvent throughout the pulp fiber. The secondary layer is the
predominant layer of the cell wall and contains the most cellulose
and hemicellulose.
[0071] Further, compositions of the present invention preferably
have a carbonyl content of less than about 60 .mu.mol/g and a
carboxyl content of less than about 60 .mu.mol/g, more preferably,
a carbonyl content of less than about 30 .mu.mol/g and a carboxyl
content of less than 30 .mu.mol/g. The carboxyl and carbonyl group
content are measured by means of proprietary assays performed by
Thuringisches Institut fur Textil-und Kunstoff Forschunge. V.,
Breitscheidstr. 97, D-07407 Rudolstadt, Germany. As an alternative
to determining the carbonyl content of the pulp using the
proprietary TITK assays, pulp samples and a thermal stable,
low-carbonyl group pulp can be analyzed FTIR and the differences in
the spectrums between the two samples can provide an indication of
the existence of carbonyl groups.
[0072] Additionally, the treated pulp of the present invention
preferably has a low transition metal content. Transition metals
are undesirable in treated pulp because, for example, they
accelerate the degradation of cellulose and NMMO in the lyocell
process. Examples of transition metals commonly found in treated
pulp derived from trees include iron, copper, nickel and manganese.
Preferably, the total transition metal content of the compositions
of the present invention is less than about 20 ppm, more preferably
less than about 5 ppm. Preferably the iron content of the
compositions of the present invention is less than about 4 ppm,
more preferably less than about 2 ppm, as measured by Weyerhaeuser
Test AM5-PULP-1/6010, and the copper content of the compositions of
the present invention is preferably less than about 1.0 ppm, more
preferably less than about 0.5 ppm, as measured by Weyerhaeuser
Test AM5-PULP-1/6010.
[0073] In order to make lyocell fibers, or other molded bodies,
such as films, from the treated pulp of the present invention, the
treated pulp is first dissolved in an amine oxide, preferably a
tertiary amine oxide. Representative examples of amine oxide
solvents useful in the practice of the present invention are set
forth in U.S. Pat. No. 5,409,532. The presently preferred amine
oxide solvent is N-methyl-morpholine-N-oxide (NMMO). Other
representative examples of solvents useful in the practice of the
present invention include dimethylsulfoxide (D.M.S.O.),
dimethylacetamide (D.M.A.C.), dimethylformamide (D.M.F.) and
caprolactan derivatives. The treated pulp is dissolved in amine
oxide solvent by any art-recognized means such as are set forth in
U.S. Pat. Nos. 5,534,113; 5,330,567 and 4,246,221. The dissolved,
treated pulp is called dope. The dope is used to manufacture
lyocell fibers, or other molded bodies, such as films, by a variety
of techniques, including melt blowing, spun-bonding, centrifugal
spinning, dry-jet, wet, and other methods. Examples of techniques
for making a film from the compositions of the present invention
are set forth in U.S. Pat. No. 5,401,447 to Matsui et al., and in
U.S. Pat. No. 5,277,857 to Nicholson.
[0074] One useful technique for making lyocell fibers from dope
involves extruding the dope through a die to form a plurality of
filaments, washing the filaments to remove the solvent, and drying
the lyocell filaments. FIG. 2 shows a block diagram of the
presently preferred process for forming lyocell fibers from the
treated pulps of the present invention. The term "cellulose" in
FIG. 2 refers to the compositions of the present invention. If
necessary, the cellulose in the form of treated pulp is physically
broken down, for example by a shredder, before being dissolved in
an amine oxide-water mixture to form a dope. The treated pulp of
the present invention can be dissolved in an amine solvent by any
known manner, e.g., as taught in McCorsley U.S. Pat. No. 4,246,221.
The treated pulp can be wet in a nonsolvent mixture of about 40%
NMMO and 60% water. The mixture can be mixed in a double arm sigma
blade mixer and sufficient water distilled off to leave about
12-14% based on NMMO so that a cellulose solution is formed.
Alternatively, NMMO of appropriate water content may be used
initially to obviate the need for the vacuum distillation. This is
a convenient way to prepare spinning dopes in the laboratory where
commercially available NMMO of about 40-60% concentration can be
mixed with laboratory reagent NMMO having only about 3% water to
produce a cellulose solvent having 7-15% water. Moisture normally
present in the pulp should be accounted for in adjusting necessary
water present in the solvent. Reference might be made to articles
by Chanzy, H. and A. Peguy, Journal of Polymer Science, Polymer
Physics Ed. 18:1137-1144 (1980), and Navard, P. and J. M. Haudin,
British Polymer Journal, p. 174 (Dec. 1980) for laboratory
preparation of cellulose dopes in NMMO water solvents.
[0075] The dissolved, treated pulp (now called the dope) is forced
through extrusion orifices to produce latent filaments or fibers
that are later regenerated.
[0076] FIG. 3 and FIG. 4 are scanning electron micrographs of a
dry-jet, wet lyocell fiber of the present invention at 100.times.
and 10,000.times. magnification respectively. The fibers shown in
FIG. 3 and FIG. 4 were produced in accordance with Example 11.
[0077] Owing to the compositions from which they are produced,
lyocell fibers produced in accordance with the present invention
have a hemicellulose content that is equal to or less than the
hemicellulose content of the treated pulp that was used to make the
lyocell fibers. Typically the lyocell fibers produced in accordance
with the present invention have a hemicellulose content that is
from about 0% to about 30.0% less than the hemicellulose content of
the treated pulp that was used to make the lyocell fibers. Lyocell
fibers produced in accordance with the present invention have an
average D.P. that is equal to, larger than or less than the average
D.P. of the treated pulp that was used to make the lyocell fibers.
Depending on the method that is used to form lyocell fibers, the
average D.P. of the pulp may be further reduced during fiber
formation, for example through the action of heat. Preferably the
lyocell fibers produced in accordance with the present invention
have an average D.P. that is equal to, or from about 0% to about
20% less than or greater than, the average D.P. of the treated pulp
that was used to make the lyocell fibers.
[0078] The lyocell fibers of the present invention exhibit numerous
desirable properties. For example, lyocell fibers prepared from
treated pulps of the present invention comprise at least about 5
weight percent hemicellulose, cellulose having an average degree of
polymerization from about 200 to about 1100, a copper number less
than about 2.0 and a .DELTA.R less than about 2.8. Preferably, such
fibers have a hemicellulose content ranging from about 5% by weight
to about 27% by weight and more preferably from about 5% by weight
to about 18%, most preferably from about 10 weight percent to about
15 weight percent. The average degree of polymerization of the
cellulose preferably ranges from about 300 to about 1000, more
preferably from about 300 to about 1100 and most preferably from
about 400 to about 700. These fibers exhibit a copper number of
less than about 2.0, more preferably less than about 1.1, and most
preferably less than about 0.8.
[0079] Lyocell fibers of the present invention formed from dopes
prepared from treated pulp of the present invention exhibit
physical properties making them suitable for use in a number of
woven and non-woven applications. Examples of woven applications
include textiles, fabrics and the like. Non-woven applications
include filtration media and absorbent products by way of
example.
[0080] Additionally, the treated pulp of the present invention can
be formed into films by means of techniques known to one of
ordinary skill in the art. An example of a technique for making a
film from the compositions of the present invention is set forth in
U.S. Pat. No. 5,401,447 to Matsui et al., and in U.S. Pat. No.
5,277,857 to Nicholson.
[0081] The following examples merely illustrate the best mode now
contemplated for practicing the invention, but should not be
construed to limit the invention.
EXAMPLE 1
[0082] Southern pine unbleached alkaline Kraft pulp with a kappa
number of 26.4 (TAPPI Standard T236 cm-85 and a viscosity of 302 cp
(TAPPI T230) (D.P. of 1593), a copper number of 0.6 and a
hemicellulose content of 13.5%.+-.2.0% was treated with oxygen in a
pressure vessel with high consistency mixing capabilities. The
mixture was stirred slowly for ten seconds every minute. The vessel
had been preheated before pulp addition to about 90.degree. C. An
amount of sodium hydroxide (NaOH) equivalent to 100 pounds per ton
of pulp was added to the alkaline pulp. The mixture was stirred for
20 seconds. The reaction vessel was then closed and the pressure
was increased to 60 psig by introducing oxygen into the pressure
vessel. The mixer was run for 60 minutes as described above. Water
was present in the vessel in an amount sufficient to provide a 25%
consistency.
[0083] After the 60 minutes, the stirring was stopped and the pulp
was removed from the pressure vessel and washed. The resulting
washed pulp viscosity was 46 cp (D.P. of 963). The treated pulp had
a copper number of about 0.5 measured by TAPPI standard T430, a
hemicellulose content of 13.5 percent .+-.2.0%, a kappa number of
10.6, and the .DELTA.R for the treated pulp was 0.4.
EXAMPLE 2
[0084] The procedure of Example 1 was repeated with the addition of
hydrogen peroxide after the addition of sodium hydroxide. The
pressure vessel was run for 60 minutes at a temperature of
115.degree. C. The peroxide was added in an amount of 20 pounds per
ton of pulp.
[0085] The treated pulp had a viscosity of 30 cp (D.P. 810), a
copper number of 0.3, and a hemicellulose content of 13.5.+-.2.0%.
The pulp exhibited a kappa number of 7.0.
EXAMPLE 3
[0086] The treated pulp of Example 1 was bleached to determine the
effect of bleaching on the D.P. of the treated pulp. The treated
pulp of Example 1 was subjected to a DED bleaching sequence
comprising a chlorine dioxide D1 stage, a sodium hydroxide/hydrogen
peroxide E stage and a chlorine dioxide D2 stage.
[0087] D1 Stage
[0088] The D1 stage treated pulp processed in accordance with
Example 1 by washing it three times with distilled water, pin
fluffing the pulp, and then transferring the pulp to a
polypropylene bag. The consistency of the pulp in the polypropylene
bag was adjusted to ten percent with the addition of water.
Chlorine dioxide corresponding to an amount equivalent to 28 pounds
per ton of pulp was introduced to the diluted pulp by dissolving
the chlorine dioxide in the water used to adjust the consistency of
the pulp in the bag. The bag was sealed and mixed and then held at
65.degree. C. for 15 minutes in a water bath. The pulp was removed
and washed with deionized water.
[0089] E Stage
[0090] The washed pulp was then placed in a fresh polypropylene bag
and caustic was introduced with one-half of the amount of water
necessary to provide a consistency of ten percent. Hydrogen
peroxide was mixed with the other one-half of the dilution water
and added to the bag. The hydrogen peroxide charge was equivalent
to 20 pounds per ton of pulp. The bag was sealed and mixed and held
for one hour at 88.degree. C. in a water bath. After removing the
pulp from the bag and washing it with water, the mat was filtered
and then placed back into the polypropylene bag and broken up by
hand.
[0091] D2 Stage
[0092] Chlorine dioxide was introduced to the pulp in an amount
equivalent to 20 pounds per ton of pulp with the dilution water
necessary to provide a consistency of 10 percent. The bag was
sealed and mixed, and then held for three hours at 80.degree. C. in
a water bath.
[0093] The resulting pulp was removed from the bag and dried. The
bleached pulp had a pulp viscosity of about 40 cp (D.P. of 914), a
TAPPI brightness of 88, a copper number of 0.6, a .DELTA.R of 1.4
and a hemicellulose content of 13.0%. The kappa number of the pulp
prior to the D.sub.1 stage was 10.6.
EXAMPLE 4
[0094] This example treats a pulp of Example 2 with the bleaching
sequence of Example 3. The resulting pulp exhibited a viscosity of
about 22 cp (D.P. of 697), a TAPPI brightness of 88.3, a copper
number of 0.6, a .DELTA.R of 2.0, and a hemicellulose content of
13.0%. The kappa number of the pulp prior to the D.sub.1 stage was
7.0.
EXAMPLE 5
[0095] Southern pine unbleached alkaline pulp was treated by the
process described in Example 1 with unoxidized Kraft white liquor
being used as caustic in place of sodium hydroxide. The unoxidized
white Kraft liquor was a synthetic white liquor with the following
strength:
[0096] Total Titratable Alkali (TTA) 108.5 Grams per liter as
Na.sub.2O
[0097] Active Alkali (AA) 106.9 Grams per liter as Na.sub.2O
[0098] Effective Alkali (EA) 91.5 Grams per liter as Na.sub.2O
[0099] Sulfidity 24.8 percent TTA and 28.8 percent AA
[0100] Specific gravity of the white liquor was 1.125
[0101] The resulting pulp had a viscosity of 30 cp (D.P. 810), a
kappa number of 7.0, a copper number of 0.3, and a hemicellulose
content of 13.0%.
EXAMPLE 6
[0102] Southern pine unbleached alkaline Kraft pulp was treated in
accordance with Example 2 except that the sodium hydroxide was
replaced with unoxidized Kraft white liquor as described in Example
5.
[0103] The resulting pulp had a viscosity of 42 cp (D.P. of 931), a
kappa number of 6.3, and a copper number of 0.3. The hemicellulose
content of the pulp was 13.0%.
EXAMPLE 7
[0104] Southern pine unbleached alkaline Kraft pulp of Example 5
was subjected to the DED bleaching sequence of Example 3.
[0105] The resulting pulp exhibited a viscosity of about 25 cp
(D.P. of 744), a TAPPI brightness of about 87.6, a copper number of
0.9, and a hemicellulose content of 13.0%.
EXAMPLE 8
[0106] This example illustrates the reduction of the degree of
polymerization without a significant increase in hemicellulose
content or copper number in a medium consistency reactor.
[0107] Southern pine unbleached alkaline Kraft pulp with a kappa
number 26.4 and a viscosity of 456 cp (D.P. of 1721) was placed in
a pulp basket of a bench scale medium consistency oxygen reactor.
One-half of the amount of water necessary to provide a 6 percent
consistency was poured into the top of the basket along with sodium
hydroxide in an amount equivalent to 100 pounds per ton of pulp.
The remaining half of the dilution water necessary to provide a 6
percent consistency was poured onto the top of the basket and
included hydrogen peroxide in an amount equivalent to 20 pounds per
ton of pulp. The top of the reactor was closed and oxygen gas was
introduced in an amount equivalent to 60 psig. The temperature of
the reactor was increased to 125.degree. C. over five to eight
minutes using a heated jacket and heating the recirculating fluid.
The temperature was held at 125.degree. C. for one hour. The
pressure was then released and the heating removed and the liquor
dumped. The basket with the treated pulp was removed and washed
with deionized water. The procedure was then repeated. Upon
completion of the second treatment, the pulp was processed in
accordance with the DED sequence of Example 7.
[0108] The resulting pulp had a viscosity of about 25 cp (D.P. of
744), a TAPPI brightness of 89.5, a copper number of 0.6, and a
.DELTA.R of essentially zero. The hemicellulose content of the
treated pulp was 13.0%.
COMPARATIVE EXAMPLE 9
[0109] This example reproduces the process of Example 3 with the
exception that rather than the final D stage in Example 3, a final
acid stage is provided as described below. Pulp from the E stage of
Example 3 was diluted to 25 percent consistency using deionized
water. The pH of the pulp was changed to 1.0 by adding sulfuric
acid. The resulting pulp was then cooked for 45 minutes at
70.degree. C. The pulp was then removed from the bag and washed
with deionized water.
[0110] The treated pulp exhibited a viscosity of 24 cp (D.P. of
729), a TAPPI brightness of 84.3, a copper number of 1.4, a
.DELTA.R of about -0.3.
[0111] In this comparative example, the copper number of the pulp
increases from 0.5 to 1.4 due to the bleaching process. In
comparison, the copper number of the pulp treated by the bleaching
sequence of Example 3 exhibited an ending copper number of 0.6.
COMPARATIVE EXAMPLE 10
[0112] This example illustrates the effects of using a hypochlorite
stage as the final stage in Example 3.
[0113] The pulp of Example 3 after the E stage was diluted to 25
percent consistency with water containing sodium hypochlorite at a
loading equivalent to 15 pounds per ton of pulp. Sufficient caustic
was introduced to provide a final pH of 8. The pulp was then heated
for 2 hours at 55.degree. C. The resulting pulp was removed from
the bag and washed with deionized water. The resulting pulp
exhibited a viscosity of about 26 cp (D.P. of 758), a TAPPI
brightness of 90.0, a copper number of 1.6 and a .DELTA.R of about
3.9.
[0114] In this comparative example, the copper number increased
from 0.5 to 1.6 due to the bleaching sequence described above. In
contrast, the bleached pulp of Example 3 exhibited a copper number
of 0.6.
EXAMPLE 11
Dry Jet Wet-Spun Fibers
[0115] The pulp of Example 4 was used to prepare a dope by
dissolving the treated pulp in NMMO. The dope was spun into fibers
by a dry jet wet-process as described in U.S. Pat. 5,417,909, which
is incorporated herein by reference. The dry jet wet-spinning
procedure was conducted by TITK. The properties of the fibers
prepared by the dry jet/wet process are summarized in Table 1
below.
1TABLE 1 FIBER PROPERTIES fiber fineness (dtex) 1.63 1.25 cellulose
content (%) 11.3 11.3 hemicellulose content (%) 13 13 tenacity dry
(cN/tex) 40.9 42.0 tenacity wet (cN/tex) 31.0 32.5 tenacity ratio
(%) 75.8 77.4 elongation dry @ break (%) 12.9 12.7 elongation wet @
break (%) 13.2 12.7 loop tenacity (cN/tex) 8.7 10.4 loop tenacity
ratio (%) 21.3 24.8 initial modulus (cN/tex) 787 766 wet modulus
(cN/tex) 191 213 fiber DP 462 462
* * * * *