U.S. patent application number 09/998283 was filed with the patent office on 2002-05-30 for lyocell fibers produced from kraft pulp having low average degree of polymerization values.
This patent application is currently assigned to Weyerhaeuser Company. Invention is credited to Jewell, Richard A., Luo, Mengkui, Neogi, Amar N., Roscelli, Vincent A., Sealey, James E. II.
Application Number | 20020064654 09/998283 |
Document ID | / |
Family ID | 27556024 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020064654 |
Kind Code |
A1 |
Luo, Mengkui ; et
al. |
May 30, 2002 |
Lyocell fibers produced from kraft pulp having low average degree
of polymerization values
Abstract
The present invention is directed to lyocell fibers having a
high hemicellulose content of at least 5%, a low lignin content as
measured by a kappa number less than 2.0 and including cellulose
that has a low average degree of polymerization (D.P.) in the range
of 200 to 1100. Further, the lyocell fibers of the present
invention have enhanced dye-binding properties and a reduced
tendency to fibrillate.
Inventors: |
Luo, Mengkui; (Tacoma,
WA) ; Roscelli, Vincent A.; (Edgewood, WA) ;
Neogi, Amar N.; (Seattle, WA) ; Sealey, James E.
II; (Federal Way, WA) ; Jewell, Richard A.;
(Bellevue, WA) |
Correspondence
Address: |
PATENT DEPARTMENT CH2J29
WEYERHAEUSER COMPANY
P.O. BOX 9777
FEDERAL WAY
WA
98063-9777
US
|
Assignee: |
Weyerhaeuser Company
|
Family ID: |
27556024 |
Appl. No.: |
09/998283 |
Filed: |
October 31, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09998283 |
Oct 31, 2001 |
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09768741 |
Jan 23, 2001 |
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09768741 |
Jan 23, 2001 |
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09256197 |
Feb 24, 1999 |
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09256197 |
Feb 24, 1999 |
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09185423 |
Nov 3, 1998 |
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09185423 |
Nov 3, 1998 |
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09039737 |
Mar 16, 1998 |
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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: |
428/364 |
Current CPC
Class: |
Y10T 428/2933 20150115;
D21C 3/02 20130101; D21C 11/0007 20130101; Y10T 428/249926
20150401; D01D 1/02 20130101; D21C 9/004 20130101; Y10T 428/249921
20150401; Y10T 428/29 20150115; D01D 5/098 20130101; D01F 2/00
20130101; D21C 9/10 20130101; Y10T 428/2913 20150115; Y10T
428/249925 20150401; D01D 5/18 20130101; Y10T 428/2965
20150115 |
Class at
Publication: |
428/364 |
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 Kraft pulp comprising: (a)
at least 5% by weight hemicellulose; (b) cellulose having an
average degree of polymerization of from about 200 to about 1100;
and (c) a kappa number of less than 2.0.
2. The fiber of claim 1 having a hemicellulose content of from 5%
by weight to about 27% 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
1000.
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
1000.
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 1
100.
9. The fiber of claim 1 further comprising cellulose having an
average degree of polymerization of from about 400 to about
700.
10. The fiber of claim 1 wherein said cellulose has a unimodal
distribution of degree of polymerization values.
11. The fiber of claim 1 having a copper number of less than about
2.0.
12. The fiber of claim 11 having a copper number of less than about
1.1.
13. The fiber of claim 11 having a copper number of less than about
0.7.
14. The fiber of claim 1 having a total transition metal content of
less than 20 ppm.
15. The fiber of claim 14 having a total transition metal content
of less than 5 ppm.
16. The fiber of claim 1 having an iron content of less than 4
ppm.
17. The fiber of claim 1 having a copper content of less than 1.0
ppm.
18. The fiber of claim 1 having a pebbled surface.
19. The fiber of claim 1 having a reflectance of less than about
8%.
20. The fiber of claim 1 having a natural crimp of irregular
amplitude and period.
21. The fiber of claim 20 wherein said crimp amplitude is greater
than about one fiber diameter and said crimp period is greater than
about five fiber diameters.
22. The fiber of claim 1, said fiber having an enhanced
dye-absorptive capacity.
23. The fiber of claim 1, said fiber having a substantially reduced
tendency to fibrillate.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 09/768,741, filed Jan. 23, 2001, which in turn is a
continuation-in-part of U.S. application Ser. No. 09/256,197, filed
Feb. 24, 1999, now U.S. Patent 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, which is a continuation-in-part of U.S. application
Ser. No. 09/039,737, filed Mar. 16, 1998, now U.S. Patent 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 of 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 lyocell fibers. In
particular, the present invention is directed to a lyocell fiber
having a high hemicellulose content, a low lignin content, and
having cellulose with a low average degree of polymerization.
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 ammoniacal 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, Acordis, Ltd. presently manufactures and sells a lyocell
fiber called Tencel.RTM. fiber.
[0008] Currently available lyocell fibers suffer from one or more
disadvantages. One disadvantage of some lyocell fibers made
presently is a function of their geometry which tends to be quite
uniform, generally circular or oval in cross section and lacking
crimp as spun. In addition, many current lyocell fibers have
relatively smooth, glossy surfaces. These characteristics make such
fibers less than ideal as staple fibers in woven articles since it
is difficult to achieve uniform separation in the carding process
and can result in non-uniform blending and uneven yarn.
[0009] In addition, fibers having a continuously uniform cross
section and glossy surface produce yams tending to have an
unnatural, "plastic" appearance. In part to correct the problems
associated with straight fibers, man-made staple fibers are almost
always crimped in a secondary process prior to being chopped to
length. Examples of crimping can be seen in U.S. Pat. Nos.
5,591,388 or 5,601,765 to Sellars et al. where a fiber tow is
compressed in a stuffer box and heated with dry steam. Inclusion of
a crimping step increases the cost of producing lyocell fibers.
[0010] Another widely-recognized problem associated with prior art
lyocell fibers is fibrillation of the fibers under conditions of
wet abrasion, such as might result during laundering. Fibrillation
is defined as the splitting of the surface portion of a single
fiber into smaller microfibers or fibrils. The splitting occurs as
a result of wet abrasion caused by attrition of fiber against fiber
or by rubbing fibers against a hard surface. Depending on the
conditions of abrasion, most or many of the microfibers or fibrils
will remain attached at one end to the mother fiber. The
microfibers or fibrils are so fine that they become almost
transparent, giving a white, frosty appearance to a finished
fabric. In cases of more extreme fibrillation, the microfibers or
fibrils become entangled, giving the appearance and feel of
pilling, i.e., entanglement of fibrils into small, relatively dense
balls.
[0011] Fibrillation of lyocell fibers is believed to be caused by
the high degree of molecular orientation and apparent poor lateral
cohesion of microfibers or fibrils within the fibers. There is
extensive technical and patent literature discussing the problem
and proposed solutions. As examples, reference can be made to
papers by Mortimer, S. A. and A. A. Pguy, Journal of Applied
Polymer Science, 60:305-316 (1996) and Nicholai, M., A. Nechwatal,
and K. P. Mieck, Textile Research Journal 66(9):575-580 (1996). The
first authors attempt to deal with the problem by modifying the
temperature, relative humidity, gap length, and residence time in
the air gap zone between extrusion and dissolution. Nicholai et al.
suggest crosslinking the fiber but note that "at the moment,
technical implementation [of the various proposals] does not seem
to be likely". A sampling of related U.S. Pat. Nos. includes those
to Taylor, 5,403,530, 5,520,869, 5,580,354, and 5,580,356; Urben,
5,562,739; and Weigel et al. 5,618,483. These patents in part
relate to treatment of the fibers with reactive materials to induce
surface modification or crosslinking. Enzymatic treatment of yams
or fabrics is currently the preferred way of reducing problems
caused by fibrillation; however, all of the treatments noted have
disadvantages, including increased production costs.
[0012] Additionally, 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.
[0013] For example, in the Kraft process a mixture of sodium
sulphide 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 in the bleach plant. In order to
prepare dissolving type pulps by the Kraft process, it is necessary
to give the chips an acidic pretreatment before the alkaline
pulping stage. A significant amount of material, on the order of
10% of the original wood substance, is solubilized in this acid
phase pretreatment. 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. Primary delignification also occurs during the Kraft
cook.
[0014] 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 temperature. 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 to a range of 3 to 4.
[0015] While the prehydrolysis can be carried out in a continuous
digester, typically the prehydrolysis is carried out in a batch
digester. As pulp mills become larger and the demand for dissolving
grade pulp increases, more batch digesters will be needed to
provide prehydrolyzed wood. The capital cost of installing such
digesters and the costs of operating them will contribute to the
cost of dissolving grade pulps. Further, prehydrolysis results in
the removal of a large amount of wood matter and so pulping
processes that incorporate a prehydrolysis step are low yield
processes.
[0016] Moreover, a relatively low copper number 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 degradation during and after dissolution in an amine
oxide solvent. The copper number is an empirical test used to
measure the reducing value of cellulose. Further, 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 degradation of cellulose and NMMO in the lyocell
process.
[0017] Thus, there is a need for relatively inexpensive, low alpha
pulps that can be used to make lyocell fibers, for a process for
making the foregoing low alpha pulps, and for lyocell fibers from
the foregoing low alpha pulp. Preferably the desired low alpha
pulps will have a low copper number, a low lignin content and a low
transition metal content. Preferably it will be possible to use the
foregoing low alpha pulps to make lyocell fibers having a decreased
tendency toward fibrillation and a more natural appearance compared
to presently available lyocell fibers.
SUMMARY OF THE INVENTION
[0018] As used herein, the terms "composition(s) of the present
invention", or "composition(s) useful for making lyocell fibers",
or "composition(s), useful for making lyocell fibers," or "treated
pulp" or "treated Kraft pulp" refer to pulp, containing cellulose
and hemicellulose, that has been treated in order to reduce the
average degree of polymerization (D.P.) of the cellulose without
substantially reducing the hemicellulose content of the pulp. The
compositions of the present invention preferably possess additional
properties as described herein.
[0019] Accordingly, the present invention provides compositions
useful for making lyocell fibers, or other molded bodies such as
films, having a high hemicellulose content, a low lignin content
and including cellulose that has a low average D.P. Preferably, the
cellulose and hemicellulose are derived from wood, more preferably
from softwood. Preferably, the compositions of the present
invention have a low copper number, a low transition metal content,
a low fines content and a high freeness. 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 a
high hemicellulose content, a low lignin content and including
cellulose that has a low average D.P. The present invention also
provides lyocell fibers containing cellulose having a low average
D.P., a high proportion of hemicellulose and a low lignin content.
The lyocell fibers of the present invention also preferably possess
a low copper number and a low transition metal content. In one
embodiment, preferred lyocell fibers of the present invention
possess a non-lustrous surface and a natural crimp that confers on
them the appearance of natural fibers. Further, the preferred
lyocell fibers of the present invention have enhanced dye-binding
properties and a reduced tendency to fibrillate.
[0020] Compositions of the present invention can be made from any
suitable source of cellulose and hemicellulose but are preferably
made from a chemical wood pulp, more preferably from a Kraft
softwood pulp, most preferably from a bleached, Kraft softwood
pulp, which is treated to reduce the average D.P. of the cellulose
without substantially reducing the hemicellulose content.
Compositions of the present invention include at least 7% by weight
hemicellulose, preferably from 7% by weight to about 30% 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 about200 to about 1100, preferably from about300 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
proprietary Weyerhaeuser sugar content assay. Further, compositions
of the present invention 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 T236om85.
[0021] 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, i.e., 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. Preferably the
compositions of the present invention have a reduced fines content,
a freeness that is comparable to untreated pulp, and a
length-weighted percentage of fibers, of length less than 0.2 mm,
of less than about 4%.
[0022] Additionally, compositions of the present invention
preferably have a copper number of less than about 2.0, more
preferably less than about 1.1, most preferably less than about 0.7
as measured by Weyerhaeuser Test Method PPD3. Further, compositions
of the present invention preferably have a carbonyl content of less
than about 120 .mu.mol/g and a carboxyl content of less than about
120 .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.
[0023] 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.
[0024] 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 6 herein. 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 40X to 70X.
[0025] The compositions of the present invention may be in a form,
such as a sheet, a roll or a bale, that is adapted for convenient
and economical storage and/or transportation. In a particularly
preferred embodiment, a sheet of a composition of the present
invention has a Mullen Burst Index of less than about 2.0 kN/g
(kiloNewtons per gram), more preferably less than about 1.5 kN/g,
most preferably less than about 1.2 kN/g. The Mullen Burst Index is
determined using TAPPI Test Number T-220. Further, in a
particularly preferred embodiment a sheet of a composition of the
present invention has a Tear Index of less than 14 mNm.sup.2/g,
more preferably less than 8 mNm.sup.2/g, most preferably less than
4 mNm.sup.2/g. The Tear Index is determined using TAPPI Test Number
T-220.
[0026] 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 and
cellulose having an average degree of polymerization of from about
200 to about 1100.
[0027] 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 kappa number less than two and 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.
[0028] 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 and a copper number less than 0.7.
[0029] A fourth 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, an iron content less than 4 ppm and a copper content less
than 1.0 ppm.
[0030] A fifth 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 less than 1100, and a lignin content of about 0.1
percent by weight.
[0031] In another aspect, the present invention provides lyocell
fibers including at least about 5% by weight hemicellulose,
preferably from about 5% by weight to about 27% 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, and 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. 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 0.7 as measured by
Weyerhaeuser Test Number PPD3. Further, preferred lyocell fibers of
the present invention have a carbonyl content of less than about
120 .mu.mol/g and a carboxyl content of less than about 120
.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. Lyocell fibers of the present invention have
a kappa number of less than 2.0, preferably less than 1.0.
[0033] In preferred embodiments lyocell fibers of the present
invention have a pebbled surface and a non-lustrous appearance.
Preferably the reflectance of a wet-formed handsheet made from
lyocell fibers of the present invention is less than about 8%, more
preferably less than 6%, as measured by TAPPI Test Method
T480-om-92.
[0034] Additionally, lyocell fibers of the present invention
preferably have a natural crimp of irregular amplitude and period
that confers a natural appearance on the fibers. Preferably the
crimp amplitude is greater than about one fiber diameter and the
crimp period is greater than about five fiber diameters. Preferred
embodiments of lyocell fibers of the present invention also possess
desirable dye-absorptive capacity and resistance to fibrillation.
Further, preferred embodiments of the lyocell fibers of the present
invention also possess good elongation. Preferably, lyocell fibers
of the present invention possess a dry elongation of from about 8%
to about 17%, more preferably from about 13% to about 15%.
Preferably, lyocell fibers of the present invention possess a wet
elongation of from about 13% 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.
[0035] A presently preferred lyocell fiber of the present invention
includes cellulose from treated Kraft pulp having at least 5% by
weight hemicellulose, cellulose having an average D.P. of 200 to
1100 and a kappa number of less than two.
[0036] 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 a
first embodiment, the present invention provides a process that
includes contacting a pulp comprising cellulose and hemicellulose
with an amount of a reagent 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. This D.P. reduction treatment occurs after the pulping
process and before, during or after the bleaching process, if a
bleaching step is utilized. The reagent is preferably at least one
member of the group consisting of acid, steam, alkaline chlorine
dioxide, the combination of at least one transition metal and a
peracid, preferably peracetic acid, and the combination of ferrous
sulfate and hydrogen peroxide. Preferably the copper number of the
treated pulp is reduced to a value less than about 2.0, more
preferably less than about 1.1, most preferably less than about
0.7. The copper number is measured by Weyerhaeuser test PPD3.
[0037] Presently the most preferred acid is sulfuric acid. The
acid, or combination of acids, is preferably utilized in an amount
of from about 0.1% w/w to about 10% w/w in its aqueous solution,
and the pulp is contacted with the acid for a period of from about
2 minutes to about 5 hours at a temperature of from about
20.degree. C. to about 180.degree. C.
[0038] When the reagent is steam, the steam is preferably utilized
at a temperature of from about 120.degree. C. to about 260.degree.
C., at a pressure of from about 150 psi to about 750 psi, and the
pulp is exposed to the steam for a period of from about 0.5 minutes
to about 10 minutes. Preferably the steam includes at least one
acid. Preferably, the steam includes an amount of acid sufficient
to reduce the pH of the steam to a value within the range of from
about 1.0 to about 4.5.
[0039] When the reagent is a combination of at least one transition
metal and peracetic acid, the transition metal(s) is present at a
concentration of from about 5 ppm to about 50 ppm, the peracetic
acid is present at a concentration of from about 5 mmol per liter
to about 200 mmol per liter, and the pulp is contacted with the
combination for a period of from about 0.2 hours to about 3 hours
at a temperature of from about 40.degree. C. to about 100.degree.
C.
[0040] When the reagent is a combination of ferrous sulfate and
hydrogen peroxide, the ferrous sulfate is present at a
concentration of from about 0.1 M to about 0.6 M, the hydrogen
peroxide is present at a concentration of from about 0.1% v/v to
about 1.5% v/v, and the pulp is contacted with the combination for
a period of from about 10 minutes to about one hour at a pH of from
about 3.0 to about 5.0.
[0041] Preferably the yield of the first embodiment of a process
for making compositions 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.
[0042] In another aspect of the present invention a process for
making lyocell fibers includes the steps of (a) contacting a pulp
including cellulose and hemicellulose with an amount of a reagent
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; and (b) forming fibers from the
pulp treated in accordance with step (a). The copper number of the
treated pulp is preferably reduced to a value less than 2.0 prior
to fiber formation. 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
[0043] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
becomes better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0044] FIG. 1 is a block diagram of the presently preferred process
for converting pulp, preferably Kraft pulp, to a composition of the
present invention useful for making lyocell molded bodies.
[0045] FIG. 2 is a block diagram of the steps of the presently
preferred process of forming fibers from the compositions of the
present invention;
[0046] FIG. 3 is a partially cut away perspective representation of
centrifugal spinning equipment useful with the present
invention;
[0047] FIG. 4 is a partially cut away perspective representation of
melt blowing equipment useful with the present invention;
[0048] FIG. 5 is a cross sectional view of an extrusion head that
is preferably used with the melt blowing apparatus of FIG. 4;
[0049] FIGS. 6 and 7 are scanning electron micrographs of
commercially available Tencel.RTM. lyocell fiber at 200X and
10.000X magnification respectively;
[0050] FIGS. 8 and 9 are scanning electron micrographs at 100X and
10,000X magnification of a melt blown lyocell fiber produced from a
dope prepared, as set forth in Example 10, from treated pulp of the
present invention;
[0051] FIG. 10 is a graph showing melt blowing conditions where
continuous shot free fibers can be produced;
[0052] FIG. 11 is a scanning electron micrograph at 1000X of
commercially available Lenzing lyocell fibers showing fibrillation
caused by a wet abrasion test;
[0053] FIG. 12 is a scanning electron micrograph at 1000X of
commercially available Tencel.RTM. lyocell fibers showing
fibrillation caused by a wet abrasion test;
[0054] FIGS. 13 and 14 are scanning electron micrographs at 100X
and 1000X, respectively, of a lyocell fiber sample produced from
compositions of the present invention as set forth in Example 10
and submitted to the wet abrasion test;
[0055] FIG. 15 is a drawing illustrating production of a self
bonded nonwoven lyocell fabric using a melt blowing process (the
equipment and process illustrated in FIG. 15 can also be utilized
to make individual fibers);
[0056] FIG. 16 is a drawing illustrating production of a self
bonded nonwoven lyocell fabric using a centrifugal spinning process
(the equipment and process illustrated in FIG. 16 can also be
utilized to make individual fibers); and
[0057] FIG. 17 is a graph showing solution thermal stability of
acid-treated pulps of the present invention having either low or
high copper number.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0058] 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. The presently
preferred starting material in the practice of the present
invention is a chemical wood pulp, preferably a Kraft wood pulp,
more preferably a bleached Kraft wood pulp. 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.
[0059] 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, 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 cellulose
without substantially reducing the hemicellulose content), the
latter will be referred to as "composition(s) of the present
invention", or "composition(s) useful for making lyocell fibers",
or "composition(s), useful for making lyocell fibers," or "treated
pulp" or "treated Kraft pulp."
[0060] 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).
[0061] 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 intramolecular and intermolecular hydrogen bonding.
[0062] 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.
[0063] Lignin is a complex aromatic polymer and comprises about 20%
to 40% of wood where it occurs as an amorphous polymer.
[0064] 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.
Acid hydrolysis of the carbohydrate components of wood then ensues,
with a lesser hydrolysis of lignin. Hemicelluloses are especially
susceptible to acid hydrolysis, and most 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.
[0065] 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.
[0066] Conventional pulping procedures, such as sulfite pulping or
alkaline pulping, can be used to provide a wood pulp that is
treated in accordance with the present invention to provide a
composition useful for making lyocell fibers. An example of a
suitable alkaline pulping process is the Kraft process, without an
acid prehydrolysis step. When utilized as a starting material in
the practice of the present invention, Kraft pulps are not subject
to acid prehydrolysis. By avoiding the acid pretreatment step prior
to alkaline pulping, the overall cost of producing the pulped wood
is reduced. Further, current industry practice utilizes batch
pre-hydrolysis treatments whereas continuous pulping systems are
increasingly being employed to produce pulp. Consequently, batch
pre-hydrolysis treatments may limit the rate of pulp production in
an otherwise continuous pulping system.
[0067] Characteristics of 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; and a
lignin content of from 0% to about 20% by weight. 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.
[0068] The pulp may be subjected to bleaching by any conventional
bleaching process utilizing bleaching agents including, but not
limited to, chlorine, chlorine dioxide, sodium hypochlorite,
peracids and hydrogen peroxide.
[0069] As shown in FIG. 1, in the practice of the present
invention, once starting material, such as softwood, has been
converted to pulp, such as a Kraft pulp, containing cellulose and
hemicellulose, it is subjected to treatment whereby the average
D.P. of the cellulose is reduced, without substantially reducing
the hemicellulose content, 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%. 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 occurs 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 bleaching step, if
utilized, occurs before treatment to reduce the average D.P. of the
cellulose. 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.
[0070] 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 30% 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.
[0071] A presently preferred means of treating the pulp in order to
reduce the average D.P. of the cellulose without substantially
reducing the hemicellulose content is to treat the pulp with acid.
Any acid can be utilized, including, but not limited to:
hydrochloric, phosphoric, sulfuric, acetic and nitric acids,
provided only that the pH of the acidified solution can be
controlled. The presently preferred acid is sulfuric acid because
it is a strong acid that does not cause a significant corrosion
problem when utilized in an industrial scale process. Additionally,
acid substitutes can be utilized instead of, or in conjunction
with, acids. An acid substitute is a compound which forms an acid
when dissolved in the solution containing the pulp. Examples of
acid substitutes include sulfur dioxide gas, nitrogen dioxide gas,
carbon dioxide gas and chlorine gas.
[0072] Where an acid, or acid substitute, or a combination of acids
or acid substitutes, is utilized to treat the pulp, an amount of
acid will be added to the pulp sufficient to adjust the pH of the
pulp to a value within the range of from about 0.0 to about 5.0;
preferably in the range of from about 0.0 to about 3.0; most
preferably in the range of from about 0.5 to about 2.0. The acid
treatment will be conducted for a period of from about 2 minutes to
about 5 hours at a temperature of from about 20.degree. C. to about
180.degree. C.; preferably from about 50.degree. C. to about
150.degree. C.; most preferably from about 70.degree. C. to about
110.degree. C. The rate at which D.P. reduction occurs can be
increased by increasing the temperature and/or pressure under which
the acid treatment is conducted. Preferably the pulp is stirred
during acid treatment, although stirring should not be vigorous.
Additionally, acid treatment of pulp in accordance with the present
invention results in a treated pulp having a low transition metal
content as more fully described herein.
[0073] Another means of treating the pulp in order to reduce the
average D.P. of the cellulose, without substantially reducing the
hemicellulose content, is to treat the pulp with steam. The pulp is
preferably exposed to direct or indirect steam at a temperature in
the range of from about 120.degree. C. to about 260.degree. C. for
a period of from about 0.5 minutes to about 10 minutes, at a
pressure of from about 150 to about 750 psi. Preferably, the steam
includes an amount of acid sufficient to reduce the pH of the steam
to a value within the range of from about 1.0 to about 4.5. The
acid can be any acid, but is preferably sulfuric acid. The exposure
of the pulp to both acid and steam permits the use of lower
pressure and temperature to reduce the average D.P. of the
cellulose compared to the use of steam alone. Consequently, the use
of steam together with acid produces fewer fiber fragments in the
pulp.
[0074] Another means of treating the pulp in order to reduce the
average D.P. of the cellulose, but without substantially reducing
the hemicellulose content, is to treat the pulp with a combination
of ferrous sulfate and hydrogen peroxide. The ferrous sulfate is
present at a concentration of from about 0.1 M to about 0.6 M, the
hydrogen peroxide is present at a concentration of from about 0.1%
v/v to about 1.5% v/v, and the pulp is exposed to the combination
for a period of from about 10 minutes to about one hour at a pH of
from about 3.0 to about 5.0.
[0075] Yet another means of treating the pulp in order to reduce
the average D.P. of the cellulose, but without substantially
reducing the hemicellulose content, is to treat the pulp with a
combination of at least one transition metal and peracetic acid.
The transition metal(s) is present at a concentration of from about
5 ppm to about 50 ppm, the peracetic acid is present at a
concentration of from about 5 mmol per liter to about 200 mmol per
liter, and the pulp is exposed to the combination for a period of
from about 0.2 hours to about 3 hours at a temperature of from
about 40.degree. C. to about 100.degree. C.
[0076] Yet other means of treating the pulp in order to reduce the
average D.P. of the cellulose, but without substantially reducing
the hemicellulose content, is to treat the pulp with alkaline
chlorine dioxide or with alkaline sodium hypochlorite.
[0077] With reference again to FIG. 1, once the pulp has been
treated to reduce the average D.P. of the cellulose, preferably
also to reduce the transition metal content, without substantially
reducing the hemicellulose content of the pulp, the treated pulp is
preferably further treated to lower the copper number to a value of
less than about 2.0, more preferably less than about 1.1, most
preferably less than about 0.7, as measured by Weyerhaeuser Test
Number PPD3. A low copper number is desirable because it is
generally believed that a high copper number causes cellulose
degradation during and after dissolution. 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. The copper number of the treated pulp of the present
invention can be reduced, for example, by treating the pulp with
sodium borohydride or sodium hydroxide, as exemplified in Example 2
and Example 3, respectively, or by treating the pulp with one or
more bleaching agents including, but not limited to, sodium
hypochlorite, chlorine dioxide, peroxides (such as hydrogen
peroxide) and peracids (such as peracetic acid), as exemplified in
Example 17.
[0078] Again with reference to FIG. 1, once the copper number of
the treated pulp has been reduced, 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. If the treated pulp is washed
and dried, it is preferably formed into a sheet prior to drying.
The dried sheet can then be formed into a roll or into a bale, if
desired, for subsequent storage or shipping. In a particularly
preferred embodiment, a sheet of a treated pulp of the present
invention has a Mullen Burst Index of less than about 2.0 kN/g
(kiloNewtons per gram), more preferably less than about 1.5 kN/g,
most preferably less than about 1.2 kN/g. The Mullen Burst Index is
determined using TAPPI Test Number T-220. Further, in a
particularly preferred embodiment a sheet of a treated pulp of the
present invention has a Tear Index of less than 14 mNm.sup.2/g,
more preferably less than 8 mNm.sup.2/g, most preferably less than
4 mNm.sup.2/g. The Tear Index is determined using TAPPI Test Number
T-220. A sheet of dried, treated pulp having Mullen Burst Index and
Tear Index values within the foregoing ranges is desirable because
the sheets made from treated pulp can be more easily broken down
into small fragments thereby facilitating dissolution of the
treated pulp in a solvent such as NMMO. It is desirable to use as
little force as possible to break down the treated pulp sheets
because the application of a large amount of crushing or
compressive force generates sufficient heat to cause hornification
of the treated pulp, i.e., hardening of the treated pulp at the
site of compression thereby generating relatively insoluble
particles of treated pulp. Alternatively, the treated, washed pulp
can be dried and broken into fragments for storage and/or
shipping.
[0079] 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, or less than, those of the
untreated pulp. The ability to form the treated pulp of the present
invention into a sheet, which can then be formed into a roll or
bale, is largely dependent on the integrity of the cellulose fiber
structure. Thus, for example, the fibers of pulp that has been
subjected to extensive steam explosion, i.e., treated with high
pressure steam that causes the fibers to explode, in order to
reduce the average D.P. of the cellulose, are extensively
fragmented. Consequently, to the best of the present applicants'
knowledge, steam exploded pulp cannot be formed into a sheet or
roll in a commercially practicable way. Steam treatment of pulp
according to the practice of the present invention is conducted
under relatively mild conditions that do not result in significant
damage to the pulp fibers.
[0080] 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.
[0081] 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.
[0082] The solubility of treated pulps of the present invention in
a tertiary amine oxide solvent, such as NMMO, can be measured by
counting the number of undissolved, gelatinous particles in a
solution of the pulp. Example 7 herein shows the total number of
undissolved, gelatinous particles in a sample of treated pulp of
the present invention as measured by laser scattering.
[0083] 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 6 herein. 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 40X to 70X.
[0084] Further, compositions of the present invention preferably
have a carbonyl content of less than about 120 .mu.mol/g and a
carboxyl content of less than about 120 .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.
[0085] 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.
[0086] 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. 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. Ser. No. 5,277,857
to Nicholson.
[0087] 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.
Here the treated pulp is wet in a nonsolvent mixture of about 40%
NMMO and 60% water. The ratio of treated pulp to wet NMMO is about
1:5.1 by weight. The mixture is mixed in a double arm sigma blade
mixer for about 1.3 hours under vacuum at about 120.degree. C.
until sufficient water has been 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 (December 1980) for laboratory
preparation of cellulose dopes in NMMO water solvents.
[0088] The dissolved, treated pulp (now called the dope) is forced
through extrusion orifices into a turbulent air stream rather than
directly into a regeneration bath as is the case with viscose or
cuprammonium rayon. Only later are the latent fibers
regenerated.
[0089] One example of such a technique is termed centrifugal
spinning. Centrifugal spinning has been used to form fibers from
molten synthetic polymers, such as polypropylene. Centrifugal
spinning is exemplified in U.S. Pat. Nos. 5,242,633 and 5,326,241
to Rook et al., and in U.S. Pat. No. 4,440,700 to Okada et al. A
presently preferred apparatus and method for forming lyocell fibers
of the present invention by centrifugal spinning is set forth in
U.S. patent application Ser. No. 09/039,737, incorporated herein by
reference. FIG. 3 is illustrative of a presently preferred
centrifugal spinning equipment used to make lyocell fibers of the
present invention. With reference to FIG. 3, in a typical
centrifugal spinning process the heated dope 1 is directed into a
heated generally hollow cylinder or drum 2 with a closed base and a
multiplicity of small apertures 4 in the sidewalls 6. As the
cylinder rotates, dope is forced out horizontally through the
apertures as thin strands 8. As these strands meet resistance from
the surrounding air they are drawn or stretched by a large factor.
The amount of stretch will depend on readily controllable factors
such as cylinder rotational speed, orifice size, and dope
viscosity. The dope strands either fall by gravity or are gently
forced downward by an air flow into a non-solvent 10 held in a
basin 12 where they are coagulated into individual oriented fibers.
Alternatively, the dope strands 8 can be either partially or
completely regenerated by a water spray from a ring of spray
nozzles 16 fed by a source of regenerating solution 18. Also, they
can be formed into a nonwoven fabric prior to or during
regeneration. Water is the preferred coagulating non-solvent
although ethanol or water-ethanol mixtures are also useful. From
this point the fibers are collected and may be washed to remove any
residual NMMO, bleached if desired, and dried. The presently
preferred centrifugal spinning process also differs from
conventional processes for forming lyocell fibers since the dope is
not continuously drawn linearly downward as unbroken threads
through an air gap and into the regenerating bath.
[0090] Another example of a technique useful for forming the
lyocell fibers of the present invention is referred to as melt
blowing wherein dope is extruded through a series of small diameter
orifices into a high velocity air stream flowing generally parallel
to the extruded fibers. The high velocity air draws or stretches
the fibers as they cool. The stretching serves two purposes: it
causes some degree of longitudinal molecular orientation and
reduces the ultimate fiber diameter. Melt blowing has been
extensively used since the 1970s to form fibers from molten
synthetic polymers, such as polypropylene. Exemplary patents
relating to melt blowing are Weber et al., U.S. Pat. No.3,959,421,
Milligan et al., U.S. Pat. No. 5,075,068, and U.S. Pat. Nos.
5,628,941; 5,601,771; 5,601,767; 4,416,698; 4,246,221 and
4,196,282. Melt-blowing typically produces fibers having a small
diameter (most usually less than 10 .mu.m) which are useful for
producing non-woven materials.
[0091] In the presently preferred melt-blowing method, the dope is
transferred at somewhat elevated temperature to the spinning
apparatus by a pump or extruder at temperatures from 70.degree. C.
to 140.degree. C. Ultimately the dope is directed to an extrusion
head having a multiplicity of spinning orifices. The dope filaments
emerge into a relatively high velocity turbulent gas stream flowing
in a generally parallel direction to the path of the latent fibers.
As the dope is extruded through the orifices the liquid strands or
latent filaments are drawn (or significantly decreased in diameter
and increased in length) during their continued trajectory after
leaving the orifices. The turbulence induces a natural crimp and
some variability in ultimate fiber diameter both between fibers and
along the length of individual fibers. The crimp is irregular and
will have a peak to peak amplitude that is usually greater than
about one fiber diameter with a period usually greater than about
five fiber diameters. At some point in their trajectory the fibers
are contacted with a regenerating solution. Regenerating solutions
are nonsolvents such as water, lower aliphatic alcohols, or
mixtures of these. The NMMO used as the solvent can then be
recovered from the regenerating bath for reuse. Preferably the
regenerating solution is applied as a fine spray at some
predetermined distance below the extrusion head.
[0092] A presently preferred method and apparatus for forming
lyocell fibers by melt blowing is set forth in U.S. patent
application Ser. No. 09/039,737, incorporated herein by reference.
The overall preferred meltblowing process is represented by the
block diagram presented in FIG. 2. FIG. 4 shows details of the
presently preferred melt blowing process. A supply of dope is
directed through an extruder and positive displacement pump, not
shown, through line 200 to an extrusion head 204 having a
multiplicity of orifices. Compressed air or another gas is supplied
through line 206. Latent fibers 208 are extruded from orifices 340
(seen in FIG. 5). These thin strands of dope 208 are picked up by
the high velocity gas stream exiting from slots 344 (FIG. 5) in the
extrusion head and are significantly stretched or elongated as they
are carried downward. At an appropriate point in their travel the
now stretched latent fiber strands 208 pass between two spray pipes
210, 212 and are contacted with a water spray or other regenerating
liquid 214. The regenerated strands 215 are picked up by a rotating
pickup roll 216 where they continuously accumulate at 218 until a
sufficient amount of fiber has accumulated. At that time, a new
roll 216 is brought in to capture the fibers without slowing
production, much as a new reel is used on a paper machine.
[0093] The surface speed of roll 216 is preferably slower than the
linear speed of the descending fibers 215 so that they in essence
festoon somewhat as they accumulate on the roll. It is not
desirable that roll 216 should put any significant tension on the
fibers as they are accumulated. Alternatively, a moving
foraminiferous belt may be used in place of the roll to collect the
fibers and direct them to any necessary downstream processing. The
regeneration solution containing diluted NMMO or other solvent
drips off the accumulated fiber 220 into container 222. From there
it is sent to a solvent recovery unit where recovered NMMO can be
concentrated and recycled back into the process.
[0094] FIG. 5 shows a cross section of a presently preferred
extrusion head 300 useful in the presently preferred melt-blowing
process. A manifold or dope supply conduit 332 extends
longitudinally through the nosepiece 340. Within the nosepiece a
capillary or multiplicity of capillaries 336 descend from the
manifold. These decrease in diameter smoothly in a transition zone
338 into the extrusion orifices 340. Gas chambers 342 also extend
longitudinally through the die. These exhaust through slits 344
located adjacent the outlet end of the orifices. Internal conduits
346 supply access for electrical heating elements or steam/oil
heat. The gas supply in chambers 342 is normally supplied preheated
but provisions may also be made for controlling its temperature
within the extrusion head itself.
[0095] The capillaries and nozzles in the extrusion head nosepiece
can be formed in a unitary block of metal by any appropriate means
such as drilling or electrodischarge machining. Alternatively, due
to the relatively large diameter of the orifices, the nosepiece may
be machined as a split die with matched halves 348, 348" (FIG. 5).
This presents a significant advantage in machining cost and in ease
of cleaning.
[0096] Spinning orifice diameter may be in the 300-600 .mu.m range,
preferably about 400-500 .mu.m. with a L/D ratio in the range of
about 2.5-10. Most desirably a lead in capillary of greater
diameter than the orifice is used. The capillary will normally be
about 1.2-2.5 times the diameter of the orifice and will have a L/D
ratio of about 10-250. Commercial lyocell fibers are spun with very
small orifices in the range of 60-80 .mu.m. The larger orifice
diameters utilized in the presently preferred melt-blowing
apparatus and method are advantageous in that they are one factor
allowing much greater throughput per unit of time, e.g.,
throughputs that equal or exceed about 1 g/min/orifice. Further,
they are not nearly as susceptible to plugging from small bits of
foreign matter or undissolved material in the dope as are the
smaller nozzles. The larger nozzles are much more easily cleaned if
plugging should occur and construction of the extrusion heads is
considerably simplified. Operating temperature and temperature
profile along the orifice and capillary should preferably fall
within the range of about 70.degree. C. to about 140.degree. C. It
appears beneficial to have a rising temperature near the exit of
the spinning orifices. There are many advantages to operation at as
high a temperature as possible, up to about 140.degree. C. where
NMMO begins to decompose. Among these advantages, throughput rate
may generally be increased at higher dope temperatures. By
profiling orifice temperature, the decomposition temperature may be
safely approached at the exit point since the time the dope is held
at or near this temperature is very minimal. Air temperature as it
exits the melt blowing head can be in the 40.degree.-100.degree. C.
range, preferably about 70.degree. C.
[0097] The extruded latent fiber filaments carried by the gas
stream are preferably regenerated by a fine water spray during the
later part of their trajectory. They are received on a take-up roll
or moving foraminous belt where they may be transported for further
processing. The take-up roll or belt will normally be operated at a
speed somewhat lower than that of the arriving fibers so that there
is no or only minimal tension placed on the arriving fibers.
[0098] Fibers produced by the presently preferred melt blowing
process and apparatus of the present invention possess a natural
crimp quite unlike that imparted by a stuffer box. Crimp imparted
by a stuffer box is relatively regular, has a relatively low
amplitude, usually less than one fiber diameter, and short
peak-to-peak period normally not more than two or three fiber
diameters. In one embodiment, preferred fibers of the present
invention have an irregular amplitude usually greater than one
fiber diameter and an irregular period usually exceeding about five
fiber diameters, a characteristic of fibers having a curly or wavy
appearance.
[0099] FIGS. 6 and 7 are scanning electron micrographs at
200.times. and 10,000.times. magnification, respectively, of
commercially available Tencel.RTM. lyocell fiber. These fibers are
of quite uniform diameter and are essentially straight. The surface
seen at 10,000X magnification in FIG. 7 is remarkably smooth. FIG.
8 and FIG. 9 are scanning electron micrographs of a melt blown
lyocell fiber of the present invention at 100X and 10,000X
magnification respectively. The fibers shown in FIG. 8 and FIG. 9
were produced from treated pulp as described in Example 10. As seen
especially in FIG. 8, fiber diameter is variable and natural crimp
of the fibers is significant. The overall morphology of the
melt-blown fibers of the present invention is highly advantageous
for forming fine, tight yams since many of the features resemble
those of natural fibers. As shown in FIG. 9, the surface of the
melt-blown fibers is not smooth and is pebbled.
[0100] The presently preferred melt-blowing method is capable of
production rates of at least about 1 g/min of dope per spinning
orifice. This is considerably greater than the throughput rate of
present commercial processes. Further, the fibers have a tensile
strength averaging at least 2 g/denier and can readily be produced
within the range of 4-100 .mu.m in diameter, preferably about 5-30
.mu.m. A most preferred fiber diameter is about 9-20 .mu.m,
approximately the range of natural cotton fibers. These fibers are
especially well suited as textile fibers but could also find
applications in filtration media, absorbent products, and nonwoven
fabrics as examples.
[0101] Certain defects are known to be associated with melt
blowing. "Shot" is a glob of polymer of significantly larger
diameter than the fibers. It principally occurs when a fiber is
broken and the end snaps back. Shot is often formed when process
rates are high and melt and air temperatures and airflow rates are
low. "Fly" is a term used to describe short fibers formed on
breakage from the polymer stream. "Rope" is used to describe
multiple fibers twisted and usually bonded together. Fly and rope
occur at high airflow rates and high die and air temperatures. "Die
swell" occurs at the exit of the spinning orifices when the
emerging polymer stream enlarges to a significantly greater
diameter than the orifice diameter. This occurs because polymers,
particularly molecularly oriented polymers, do not always act as
true liquids. When molten polymer streams are held under pressure,
expansion occurs upon release of the pressure. Orifice design is
critical for controlling die swell.
[0102] Melt blowing of thermoplastics has been described by R. L.
Shambaugh, Industrial and Engineering Chemistry Research
27:2363-2372 (1988) as operating in three regions. Region I has
relatively low gas velocity similar to commercial "melt spinning"
operations where fibers are continuous. Region II is an unstable
region which occurs as gas velocity is increased. The filaments
break up into fiber segments. Region III occurs at very high air
velocities with excessive fiber breakage. In the presently
preferred melt blowing process, air velocity, air mass flow and
temperature, and dope mass flow and temperature are chosen to give
operation in Region I as above described where a shot free product
of individual continuous fibers in a wide range of deniers can be
formed. FIG. 10 is a graph showing in general terms the Region I
operating region to which the present preferred melt-blowing
process is limited. Region I is the area in which fibers are
substantially continuous without significant shot, fly or roping.
Operation in this region is important for production of fibers of
greatest interest to textile manufacturers. The exact operating
condition parameters such as flow rates and temperatures will
depend on the particular dope characteristics and specific melt
blowing head construction and can be readily determined
experimentally.
[0103] A technique known as spun bonding can also be used to make
lyocell fibers of the present invention. In spun bonding, the
lyocell fiber is extruded into a tube and stretched by an airflow
through the tube caused by a vacuum at the distal end. In general,
spun bonded fibers are continuous, while commercial melt blown
fibers tend to be formed in discrete, shorter lengths. Spun bonding
has been used since the 1970s to form fibers from molten synthetic
polymers, such as polypropylene, and the numerous, art-recognized
techniques for spun bonding synthetic fibers can be readily
modified by one of ordinary skill in the art for use in forming
lyocell fibers from a dope formed from pulp treated in accordance
with the present invention. An exemplary patent relating to spun
bonding is U.S. Pat. Ser. No. 5,545,371 to Lu.
[0104] Another technique useful for forming lyocell fibers is dry
jet/wet. In this process, the lyocell filament exiting the
spinneret orifices passes through an air gap before being submerged
and coagulated in a bath of liquid. An exemplary patent relating to
dry jet/wet spinning is U.S. Pat. Ser. No. 4,416,698 to McCorsley
III.
[0105] 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.
[0106] The lyocell fibers of the present invention exhibit numerous
desirable properties. For example, the lyocell fibers of the
present invention exhibit a high affinity for dye stuffs. While not
wishing to be bound by theory, it is believed that the enhanced
affinity for dyestuffs exhibited by the fibers of the present
invention results, at least in part, from the high hemicellulose
content of the fibers.
[0107] Additionally, the lyocell fibers of the present invention
have a substantially reduced tendency to fibrillate. As described
more fully in the Background of the Invention, the term
fibrillation refers to the process whereby small fibrils peel away
from the surface of lyocell fibers, especially under conditions of
wet abrasion such as occur during laundering. Fibrillation is often
responsible for the frosted appearance of dyed lyocell fabrics.
Further, fibrillation also tends to cause "pilling" whereby the
fibrils that peel away from the surface of the lyocell fibers
become entangled into relatively small balls. Fibrillation thus
imparts a prematurely aged appearance to fabrics made from lyocell
fibers. While treatments that reduce the tendency of lyocell fibers
to fibrillate are available, they add to the cost of manufacturing
the fibers.
[0108] While there is no standard industry test to determine
fibrillation resistance, the following procedure is typical of
those used. 0.003 g to 0.065 g of individualized fibers are weighed
and placed with 10 mL of water in a capped 25 mL test tube
(13.times.110 mm). Samples are placed on a shaker operating at low
amplitude at a frequency of about 200 cycles per minute. The time
duration of the test may vary from 4-80 hours. The samples shown in
FIGS. 11-14 were shaken 4 hours.
[0109] FIGS. 11 and 12 are scanning electron micrographs at
1000.times. of fibers from each of two commercial sources showing
considerable fibrillation when tested by the foregoing test for
fibrillation resistance. FIG. 11 shows a Lenzing lyocell fiber
subjected to the wet abrasion test, and FIG. 12 shows a Tencel.RTM.
lyocell fiber subjected to the wet abrasion test. Considerable
fibrillation is evident. In comparison, FIGS. 13 and 14 are
scanning electron micrographs at 100.times. and 1000.times.,
respectively, of a melt-blown fiber sample produced from treated
pulp as set forth in Example 10 and similarly submitted to the wet
abrasion test. Fibrillation is very minor. While not wishing to be
bound by theory, it is believed that the fibers of the present
invention have somewhat lower crystallinity and orientation than
those produced by existing commercial processes. The tendency to
acquire a "frosted" appearance after use is almost entirely absent
from the fibers of the present invention.
[0110] 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.
Examples of the properties possessed by lyocell fibers produced by
a dry jet wet process from treated pulp of the present invention,
include: denier of 0.3 to 10.0; tensile strength ranging from about
10 to about 38 cN/tex dry and about 5 cN/tex wet; elongation of
about 10 to about 25% when dry and about 10 to about 35% when wet;
and initial modulus less than about 1500 cN/tex when dry and about
250 to about 40 cN/tex when wet. The fibers were produced by means
of a proprietary dry jet wet spinning process performed by
Thuringisches Institut fur Textil-und Kunstoff Forschunge. V.,
Breitscheidstr. 97, D-07407 Rudolstadt, Germany.
[0111] FIG. 15 shows one method for making a self bonded lyocell
nonwoven material using a modified melt blowing process. A
cellulose dope 450 is fed to extruder 452 and from there to the
extrusion head 454. An air supply 456 acts at the extrusion
orifices to draw the dope strands 458 as they descend from the
extrusion head. Process parameters are preferably chosen so that
the resulting fibers will be continuous rather than random shorter
lengths. The fibers fall onto an endless moving foraminous belt 460
supported and driven by rollers 462, 464. Here they form a latent
nonwoven fabric mat 466. A top roller, not shown, may be used to
press the fibers into tight contact and ensure bonding at the
crossover points. As mat 466 proceeds along its path while still
supported on belt 460, a spray of regenerating solution 468 is
directed downward by sprayers 470 (although a sprayer positioned
close to dope strands 458 is also effective). The regenerated
product 472 is then removed from the end of the belt where it may
be further processed, e.g., by further washing, bleaching and
drying.
[0112] FIG. 16 is an alternative process for forming a self bonded
nonwoven web using centrifugal spinning. A cellulose dope 580 is
fed into a rapidly rotating drum 582 having a multiplicity of
orifices 584 in the sidewalls. Latent fibers 586 are expelled
through orifices 584 and drawn, or lengthened, by air resistance
and the inertia imparted by the rotating drum. They impinge on the
inner sidewalls of a receiver surface 588 concentrically located
around the drum. The receiver may optionally have a frustoconical
lower portion 590. A curtain or spray of regenerating solution 592
flows downward from ring 594 around the walls of receiver 588 to
partially coagulate the cellulose mat impinged on the sidewalls of
the receiver. Ring 594 may be located as shown or moved to a lower
position if more time is needed for the latent fibers to self bond
into a nonwoven web. The partially coagulated nonwoven web 596 is
continuously mechanically pulled from the lower part 590 of the
receiver into a coagulating bath 598 in container 600. As the web
moves along its path it is collapsed from a cylindrical
configuration into a planar two ply nonwoven structure. The web is
held within the bath as it moves under rollers 602, 604. A takeout
roller 606 removes the now fully coagulated two ply web 608 from
the bath. Any or all of rollers 600, 602 or 604 may be driven. The
web 608 is then continuously directed into a wash and/or bleaching
operation, not shown, following which it is dried for storage. It
may be split and opened into a single ply nonwoven or maintained as
a two ply material as desired.
[0113] 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. Ser. No.
5,277,857 to Nicholson.
[0114] 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
Acid hydrolysis
[0115] The average D.P. of the cellulose of Kraft pulp NB416 (a
paper grade pulp with DP of about 1400) was reduced, without
substantially reducing the hemicellulose content, by acid
hydrolysis in the following manner. Two hundred grams of
never-dried NB416 pulp was mixed with 1860 g of a 0.51% solution of
sulfuric acid. The NB416 pulp had a cellulose content of 32% by
weight, i.e., cellulose constituted 32% of the weight of the wet
pulp, an average cellulose D.P. of about 1400 and a hemicellulose
content of 13.6%.+-.0.7%. The sulfuric acid solution was at a
temperature of 100.degree. C. prior to mixing with the NB416 pulp.
The pulp and acid were mixed for 1 hour in a plastic beaker which
was placed in a water bath that maintained the temperature of the
pulp and acid mixture within the range of 83.degree. C. to
110.degree. C. After 1 hour, the acid and pulp mixture was removed
from the water bath, poured onto a filter screen and washed with
distilled water until the pH of the treated pulp was in the range
of pH 5 to pH 7. The average D.P. of the cellulose of the
acid-treated pulp was 665, the hemicellulose content was
14.5.+-.0.7% and the copper number was 1.9.
EXAMPLE 2
Reduction of Copper Number By Treatment With Sodium Borohydride
[0116] The average D.P. of a sample of never-dried NB416 Kraft pulp
was reduced by acid hydrolysis and the copper number of the
acid-treated pulp was subsequently reduced by treatment with sodium
borohydride in the following manner. Four hundred and twenty two
grams of never-dried NB 416 pulp were placed in a plastic beaker
containing 3600 grams of a 2.5% solution of sulfuric acid that was
preheated to a temperature of 91.degree. C. The pulp had a
cellulose content of 32% by weight, the average D.P. of the pulp
cellulose was 1400 and the hemicellulose content of the pulp was
13.6%.+-.0.7%. The copper number of the NB 416 was about 0.5. The
mixture of acid and pulp was placed in an oven and incubated at a
temperature of 98.degree. C. for two hours. After two hours the
mixture of acid and pulp was removed from the oven and placed at
room temperature to cool to a temperature of 61.degree. C. and was
then washed with distilled water until the pH of the treated pulp
was in the range of pH 5 to pH 7. The average D.P. of the cellulose
of the acid-treated pulp was 590, and the hemicellulose content of
the acid-treated pulp was 14.1%.+-.0.7%. The copper number of the
acid-treated pulp was 2.4.
[0117] The acid-treated pulp was dried after washing with distilled
water and the dried pulp was treated with sodium borohydride in
order to reduce the copper number. One hundred grams of the dry,
acid-treated pulp was added to distilled water containing one gram
of dissolved sodium borohydride. The total volume of the pulp mixed
with the sodium borohydride solution was three liters. The pulp was
stirred in the sodium borohydride solution for three hours at room
temperature (18.degree. C. to 24.degree. C.). The pulp was then
washed with distilled water until the pH of the pulp was in the
range of pH 5.0 to pH 7.0, and the pulp was then dried. The average
D.P. of the cellulose of the borohydride-treated pulp was 680, and
the copper number of the borohydride-treated pulp was 0.6. Copper
number was determined using Weyerhaeuser Test Number PPD3.
[0118] Although, in the present example, the acid-treated pulp was
dried before borohydride treatment, a never-dried pulp can be
treated with sodium borohydride in order to reduce the copper
number. Other process conditions, such as pH, temperature and pulp
consistency can be adjusted to give desirable results.
EXAMPLE 3
Reduction of Copper Number By Treatment With Sodium Hydroxide
[0119] Sixty grams of the dry, acid-treated pulp of Example 1 was
mixed with a 1.38% aqueous solution of sodium hydroxide. The volume
of the pulp and sodium hydroxide mixture was two liters. The pulp
and sodium hydroxide mixture was incubated in an oven at a
temperature of 70.degree. C. for two hours and then washed with
distilled water until the pH was in the range of pH 5.0 to pH 7.0.
The copper number of the sodium hydroxide-treated pulp was 1.1. The
copper number of the acid-treated pulp, before sodium hydroxide
treatment, was 1.9.
EXAMPLE 4
Steam Treatment of Pulp
[0120] The average D.P. of the cellulose of never-dried Kraft pulp
NB 416 was reduced, without substantially reducing the
hemicellulose content, by steam treatment in the following manner.
The average cellulose D.P. of the starting NB 416 pulp was about
1400 and the hemicellulose content was 13.6%. Three hundred and
fifty grams of never-dried NB 416 Kraft pulp was adjusted to pH 2.5
by adding sulfuric acid. The consistency of the acidified pulp was
25% to 35%, i.e., 25% to 35% of the volume of the acidified pulp
was pulp, and the rest was water. The acidified pulp was added to a
steam vessel. The steam pressure was increased to between 185 to
225 p.s.i.g within two seconds and the pulp was maintained within
that pressure range for two minutes. After steam treatment the
viscosity, as measured by the falling ball test, was 23 cP
(centipoise) which corresponds to an average D.P. of the pulp
cellulose of about 700. The yield of the steam-treated pulp was
99%.+-.0.1 %. The extremely high yield of the foregoing steam
treatment process indicates that almost no pulp material (less than
1.1%), including hemicellulose, was lost during steam
treatment.
EXAMPLE 5
Carboxyl Content of Pulp Treated with Acid
[0121] 422 grams of never-dried NB 416 pulp were acid hydrolyzed in
5% sulfuric acid at 93.degree. C. for three hours, according to the
procedure set forth in Example 2. The acid-hydrolyzed pulp was
treated with sodium borohydride as described in Example 2. The
carboxyl content of the treated pulp was 11.1 .mu.mol/g, and the
Cuen viscosity was 315 ml/g. Both carboxyl content and viscosity
were measured by means of proprietary assays performed by
Thuringisches Institut fur Textil-und Kunstoff Forschunge. V.,
Breitscheidstr. 97, D-07407 Rudolstadt, Germany.
EXAMPLE 6
Dissolution Time In Tertiary Amine Solvent of Pulp Treated with
Acid or Steam
[0122] The effect of acid or steam treatment on the rate of
dissolution of NB 416 pulp in NMMO was assessed in the following
manner. Two and a half kilograms of dried NB 416 were mixed with a
5.3% stock solution of sulfuric acid to yield a total volume of
13.5 liters. The average cellulose D.P. of the starting NB 416 pulp
was about 1400 and the hemicellulose content was 13.6%. The acid
was preheated to 92.degree. C. and the acid plus pulp mixture was
heated to 90.degree. C. before being incubated in an oven at
73.degree. C. to 91.degree. C. for two hours. The acid-treated pulp
was then washed until the pH of the treated pulp was in the range
of pH 5.0 to pH 7.0. The copper number of the treated pulp was
reduced by treatment with sodium borohydride. The copper number of
the acid-treated pulp was 2.45 which was reduced to 1.2 by
borohydride treatment. The average D.P. of the treated pulp
cellulose after acid and borohydride treatment was 570.
[0123] The dissolution time of the steam-treated pulp of Example 4
was also measured. The viscosity of the steam treated pulp was 23
cP. The acid-treated and steam-treated pulps were separately
dissolved in NMMO at 80.degree. C. to 100.degree. C. to yield a
0.6% solution of cellulose without minimum stirring. The time for
complete dissolution of the pulps was observed by light microscopy
at a magnification of 40.times. to 70.times.. The times taken for
complete dissolution of the acid-treated and steam-treated pulps
are set forth in Table 1. For comparison, Table 1 also shows the
dissolution time of untreated NB 416 (NB 416).
1 TABLE 1 Pulp Time for Complete Dissolution NB 416 >1.6 hour
Acid treated NB 416 15 minutes Steam treated NB 416 pulp 1 hour
EXAMPLE 7
Average Number of Gelatinous Particles Found in Pulp Treated with
Acid
[0124] The number of gelatinous particles present in the dissolved,
acid-treated pulp prepared as described in Example 6 was measured
using a proprietary laser scattering assay performed by
Thuringisches Institut fur Textil-und Kunstoff Forschunge. V.,
Breitscheidstr. 97, D-07407 Rudolstadt, Germany. The results of the
assay are presented in Table 2.
2TABLE 2 Total Particle Content of Acid-Treated Pulp 10-104 ppm
Percentage of Particles Having Diameter Less Than 20-50% 12 Microns
Percentage of Particles Having Diameter in the Range 40-50% of
12-40 Microns Percentage of Particles Having Diameter Greater 3-20%
Than 40 Microns
EXAMPLE 8
Physical Properties of Acid-Treated Pulp
[0125] NB 416 Kraft pulp was acid hydrolyzed as set forth in
Example 2. Table 3 discloses various physical properties of the NB
416 pulp, and sheets made from the NB 416 pulp, before and after
acid treatment. The analytical methods are proprietary Weyerhaeuser
test methods.
3TABLE 3 Analytical NB416, Acid Method Property NB416 Treated
P-045-1 Basis weight (g/m.sup.2) 64.79 65.59 P-045-1 Caliper (mm)
0.117840 0.11046 P-360-1 Density (kg/m.sup.3) 549.916 593.973
P-360-1 Bulk (cm.sup.-3/g) 1.81879 1.68409 P-076-0 Mullen Burst
index (KN/g) 2.1869 1.1095 P-326-4 Tear index, single 14.484 3.0500
ply (mNm.sup.2/g) P-340-4 Fiber length (mm) 1.27/2.64/3.32
1.09/2.47/3.15 W-090-3 Fines, Length-weighted (% 4.1 3.0 of fibers
having length <0.2 mm) W-090-3 Coarseness (mg/100 23.1 22.2
meters) W-090-3 Fiber/g (X 10.sup.6) 3.5 4.2 W-105-3 Freeness (ml)
735 760
[0126] The data set forth in Table 3 show that when pulp treated
with acid in accordance with the present invention is formed into a
sheet, the sheet has a substantially lower Mullen Burst Index and
Tear Index compared to the untreated pulp. Consequently, the sheets
made from acid-treated pulp can be more easily broken down into
small fragments, thereby facilitating dissolution of the treated
pulp in a solvent such as NMMO. It is desirable to use as little
force as possible to break down the treated pulp sheets because the
application of a large amount of crushing or compressive force
generates sufficient heat to cause homification of the treated
pulp, i.e., hardening of the treated pulp at the site of
compression thereby generating relatively insoluble particles of
treated pulp that may clog the orifices through which the
dissolved, treated pulp is expressed to form lyocell fibers.
[0127] Fiber length is represented by a series of three values in
Table 3. The first value is the arithmetic mean fiber length value;
the second value is the length-weighted average fiber length value,
and the third value is the weight-weighted average fiber length
value. The data set forth in Table 3 show that fiber length is not
substantially reduced by acid-treatment.
[0128] The fines content is expressed as the length-weighted
percentage value for the percentage of pulp fibers having a length
of less than 0.2 mm. The data set forth in Table 3 demonstrate that
acid treatment of pulp in accordance with the present invention
generates a treated pulp having a fines content that is comparable
to that of the untreated pulp. A low fines content is desirable
because the acid-treated and washed pulp drains more quickly when
spread on a mesh screen prior to formation into a sheet. Thus,
there is a saving of time and money in the sheet-forming process.
It is also desirable to produce an acid-treated pulp, having a
lowered cellulose D.P., without substantially reducing the pulp
fiber length because it is difficult to make a sheet from treated
pulp if the fiber length has been substantially reduced compared to
the untreated pulp.
EXAMPLE 9
Transition Metal Content of Acid-Treated Pulp of the Present
Invention
[0129] Acid treatment of pulp according to the practice of the
present invention results in a treated pulp having a low transition
metal content, as exemplified herein. Two and a half kilograms of
dried FR-416 pulp (a paper grade pulp manufactured by Weyerhaeuser
Corporation) pulp were deposited in a plastic beaker containing
sixteen liters of a 1.3% solution of sulfuric acid that was
preheated to a temperature of 91.degree. C. The pulp had an average
cellulose D.P. of 1200 and the hemicellulose content of the pulp
was 13.6%.+-.0.7%. The copper number of the FR 416 was about 0.5.
The mixture of acid and pulp was placed in an oven and incubated at
a temperature of about 90.degree. C. for two hours. After two hours
the mixture of acid and pulp was removed from the oven and was then
washed with distilled water until the pH of the treated pulp was in
the range of pH 5 to pH 7. The wet, acid-treated pulp was then
treated with 0.5% sodium borohydride for about three hours and
washed with water until the pH was in the range of pH 5 to pH 7.
The average D.P. of the cellulose of the acid-treated,
borohydride-reduced pulp was 690, and the hemicellulose content of
the acid-treated, borohydride-reduced pulp was 14.1%.+-.0.7%. The
copper number of the acid-treated, borohydride-treated pulp was
0.9.
[0130] The copper and iron content of the treated pulp was measured
using Weyerhaeuser test AM5-PULP-1/6010. The copper content of the
acid-treated, borohydride-reduced pulp was less than 0.3 ppm and
the iron content of the acid-treated, borohydride-reduced pulp was
less than 1.3 ppm. The silica content of the acid-treated,
borohydride-reduced pulp was 6 ppm as measured using Weyerhaeuser
test AM5-ASH-HF/FAA.
EXAMPLE 10
Formation of Lyocell Fibers of the Present Invention by Melt
Blowing
[0131] A dope was prepared from a composition of the present
invention in the following manner. Two thousand three hundred grams
of dried NB 416 Kraft pulp were mixed with 1.4 kilograms of a 5.0%
solution of H.sub.2SO.sub.4 in a plastic container. The consistency
of the pulp was 92%. The average D.P. of the never-dried NB 416
prior to acid treatment was 1400, the hemicellulose content was
13.6% and the copper number was 0.5. The pulp and acid mixture was
maintained at a temperature of 97.degree. C. for 1.5 hours and then
cooled for about 2 hours at room temperature and washed with water
until the pH was in the range of 5.0 to 7.0. The average D.P. of
the acid-treated pulp was about 600, as measured by method ASTM D
1795-62 and the hemicellulose content was about 13.8% (i.e., the
difference between the experimentally measured D.P. of the
acid-treated pulp and that of the untreated pulp was not
statistically significant). The copper number of the acid-treated
pulp was about 2.5.
[0132] The acid treated pulp was dried and a portion was dissolved
in NMMO. Nine grams of the dried, acid-treated pulp were dissolved
in a mixture of 0.025 grams of propyl gallate, 61.7 grams of 97%
NMMO and 21.3 grams of 50% NMMO. The flask containing the mixture
was immersed in an oil bath at about 120.degree. C., a stirrer was
inserted, and stirring was continued for about 0.5 hours until the
pulp dissolved.
[0133] The resulting dope was maintained at about 120.degree. C.
and fed to a single orifice laboratory melt blowing head. Diameter
at the orifice of the nozzle portion was 483 .mu.m and its length
about 2.4 mm, a L/D ratio of 5. A removable coaxial capillary
located immediately above the orifice was 685 .mu.m in diameter and
80 mm long, a L/D ratio of 116. The included angle of the
transition zone between the orifice and capillary was about
118.degree.. The air delivery ports were parallel slots with the
orifice opening located equidistant between them. Width of the air
gap was 250 .mu.m and overall width at the end of the nosepiece was
1.78 mm. The angle between the air slots and centerline of the
capillary and nozzle was 30.degree.. The dope was fed to the
extrusion head by a screw-activated positive displacement piston
pump. Air velocity was measured with a hot wire instrument as 3660
m/min. The air was warmed within the electrically heated extrusion
head to 60-70.degree. C. at the discharge point. Temperature within
the capillary without dope present ranged from about 80.degree. C.
at the inlet end to approximately 140.degree. C. just before the
outlet of the nozzle portion. It was not possible to measure dope
temperature in the capillary and nozzle under operating conditions.
When equilibrium running conditions were established a continuous
fiber was formed from each of the dopes. Throughputs were varied
somewhat in an attempt to obtain similar fiber diameters with each
dope but all were greater than about 1 g of dope per minute. Fiber
diameters varied between about 9-14 .mu.m at optimum running
conditions.
[0134] A fine water spray was directed on the descending fiber at a
point about 200 mm below the extrusion head and the fiber was taken
up on a roll operating with a surface speed about 1/4 the linear
speed of the descending fiber.
[0135] A continuous fiber in the cotton denier range could not be
formed when the capillary section of the head was removed. The
capillary appears to be very important for formation of continuous
fibers and in reduction of die swell.
[0136] It will be understood that fiber denier is dependent on many
controllable factors. Among these are solution solids content,
solution pressure and temperature at the extruder head, orifice
diameter, air pressure and other variables well known to those
skilled in melt blowing technology. Lyocell fibers having deniers
in the cotton fiber range (about 10-20 .mu.m in diameter) were
easily and consistently produced by melt blowing at throughput
rates greater than about 1 g/min of dope per orifice. A 0.5 denier
fiber corresponds to an average diameter (estimated on the basis of
equivalent circular cross section area) of about 7-8 .mu.m.
[0137] The melt blown fibers were studied by x-ray analysis to
determine degree of crystallinity and crystallite type. Comparisons
were also made with some other cellulosic fibers as shown in the
following Table 4.
4TABLE 4 Crystalline Properties of Different Cellulose Fibers
Lyocell of Present Fibers Invention Tencel .RTM. Cotton
Crystallinity Index 67% 70% 85% Crystallite Cellulose II Cellulose
II Cellulose I
[0138] Some difficulty and variability was encountered in measuring
tensile strength of the individual fibers so the numbers given in
the following table (Table 5) for tenacity are estimated averages.
Again, the fibers of the present invention are compared with a
number of other fibers as seen in Table 5.
5TABLE 5 Fiber Physical Property Measurements Melt So. Blown Fibers
Cotton Pine Rayon.sup.(1) Silk Lyocell.sup.(2) Tencel Typical 4
0.35 40 >104 Con- Variable Length, tinuous cm Typical 20 40 16
10 9-15 12 Diam., .mu.m Tenacity, 2.5-3.0 -- 0.7-3.2 2.8-5.2 2-3
4.5-5.0 g/d .sup.(1)Viscose process. .sup.(2)Made with 600 D.P.
acid-treated pulp of Example 10.
EXAMPLE 11
Formation of Lyocell Fibers of the Present Invention by a Dry
Jet/Wet Process
[0139] Dope was prepared from acid-treated pulp of the present
invention (hemicellulose content of 13.5% and average cellulose
D.P. of 600). The treated pulp was dissolved in NMMO and spun into
fibers by a dry/jet wet process as disclosed in U.S. Pat. Ser. No.
5,417,909, which is incorporated herein by reference. The dry
jet/wet spinning procedure was conducted by Thuringisches Institut
fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407
Rudolstadt, Germany. The properties of the fibers prepared by the
dry jet/wet process are summarized in Table 6 which also discloses
the properties of the following types of fibers for comparison:
lyocell fibers made by meltblowing (made from the dope of Example
10); rayon and cotton.
6TABLE 6 Structure and Properties of Dry Jet Wet Fibers Lyocell
Lyocell Lyocell Property Centrifugal Meltblowing (dry jet wet)
Rayon Cotton Tencel .RTM. Crystallinity 67% 67-73% -- 35-40% 85%
70-78% Index Orientation 0.039 0.026-0.04 -- 0.026-0.032 0.044
0.046-0.051 (Birefringence) Strength (g/d) 2.1 2-3 37.5 0.7-3.2
2.5-3.0 4.5-5.0 cN/tex Dry -- 10% 14.0% 20-25% 10% 14-16%
Elongation Water 115% 72% Imbibition
EXAMPLE 12
Average D.P. of Cellulose of Meltblown Lyocell Fibers of the
Present Invention
[0140] Meltblown lyocell fibers were prepared according to Example
10, from the acid-treated pulp of Example 10, and the average D.P.
of the cellulose of the meltblown fibers was measured using Test
ASTM D 1795-62. The data set forth in Table 7 shows that the
average D.P. of the lyocell fiber cellulose is approximately 10%
less than the average D.P. of the treated pulp cellulose.
7TABLE 7 Average D.P. of Cellulose of Meltblown Lyocell Fibers
Average D.P. Cellulose Treated Pulp 600 Fibers 520
EXAMPLE 13
Hemicellulose Content of Meltblown Lyocell Fibers of the Present
Invention
[0141] Meltblown lyocell fibers were prepared according to Example
10, from the acid-hydrolyzed NB 416 pulp of Example 10, and the
hemicellulose content of the meltblown fibers was measured using a
proprietary Weyerhaeuser sugar analysis test. The data set forth in
Table 8 shows that the hemicellulose content of the lyocell fiber
is approximately 20% less than the hemicellulose content of the
pulp cellulose.
8TABLE 8 Hemicellulose of Lyocell Fibers Wt. % Hemicellulose
Treated Pulp 13.0 Fibers 10.0
EXAMPLE 14
Reflectance of Lyocell Fibers of the Present Invention
[0142] The pebbled surface of the preferred fibers of the present
invention produced by melt blowing and centrifugal spinning results
in a desirable lower gloss without the need for any internal
delustering agents. While gloss or luster is a difficult property
to measure the following test is exemplary of the differences
between a melt blown fiber sample made using the acid-treated dope
of Example 10 and Tencel.RTM., a commercial lyocell fiber produced
by Courtaulds.
[0143] Small wet formed handsheets were made from the respective
fibers and light reflectance was determined according to TAPPI Test
Method T480-om-92. Reflectance of the handsheet made from meltblown
lyocell fiber of the present invention was 5.4% while reflectance
of the handsheet made from Tencel.RTM. was 16.9%.
EXAMPLE 15
Dye-Absorptive Capacity of Lyocell Fibers of the Present
Invention
[0144] The fibers of the present invention have shown an unusual
and very unexpected affinity for direct dyes. Samples of the melt
blown fibers made from the acid-treated dope of Example 10 were
carded. These were placed in dye baths containing Congo Red, Direct
Blue 80, Reactive Blue 52 and Chicago Sky Blue 6B, along with
samples of undyed commercial lyocell fibers, Tencel.RTM. fibers and
Lenzing Lyocell fibers. The color saturation of the dyed, melt
blown fibers was outstanding in comparison to that of Tencelg
fibers and Lenzing Lyocell fibers used for comparison. It appears
that quantitative transfer of dye to the fiber is possible with the
fibers of the invention.
EXAMPLE 16
Yarn made from Melt Blown Lyocell Fibers of the Present
Invention
[0145] Fiber made from the 600 D.P. acid-treated dope of Example 10
was removed from a take-up roll and cut by hand into 38-40 mm
staple length. The resultant fiber bundles were opened by hand to
make fluffs more suitable for carding. The tufts of fiber were
arranged into a mat that was approximately 225 mm wide by 300 mm
long and 25 mm thick. This mat was fed into the back of a full size
cotton card set for cotton processing with no pressure on the crush
rolls. Using a modified feed tray the card sliver was arranged into
12 pieces of equal lengths. Since the card sliver weight was quite
low, this was compensated for on the draw frame. Two sets of draw
slivers were processed from the card sliver. These sets were broken
into equal lengths and placed on the feed tray. This blended all
the sliver produced into one finish sliver. A rotor spinning
machine was used to process the finish sliver into yarn. The rotor
speed was 60,000 rpm with an 8,000 rpm combing roll speed. The yarn
count was established as between 16/1 and 20/1. The machine was set
up with a 4.00 twist multiple. The yarn was later successfully
knitted on a Fault Analysis Knitter with a 76 mm cylinder.
EXAMPLE 17
Reduction of Copper Number by Treatment with Bleaching Agents
[0146] The copper number of acid-treated pulp of the present
invention was reduced by treatment with bleaching agents as
described herein. Two and a half kilograms of air dried, new NB416
pulp (hemicellulose content of 15.9% as determined using a
proprietary Weyerhaeuser sugar analysis test) was mixed with 14
liters of 5% H.sub.2SO.sub.4 and incubated at 89.degree. C. for 3
hours, and then cooled down to about 60.degree. C. The acid-treated
pulp (hemicellulose content of 15.4% as determined using a
proprietary Weyerhaeuser sugar analysis test) was then washed until
the pH was within the range of pH 5-7. The acid-treated pulp had an
average DP of 399 (as determined using Tappi method T230) and a
copper number of 3.3 (as determined by Weyerhaeuser test number
PPD-3). The copper number of samples of the foregoing, acid-treated
pulp was reduced using three different bleaching agents as
described herein.
[0147] The aforedescribed acid-treated pulp (having a copper number
of 3.3 and an average DP of 399) was oven dried and 13 grams of the
oven dried, acid-treated pulp were mixed with a solution of 1.0%
NaOCl (sodium hypochlorite) and 0.5% NaOH at a temperature of
45.degree. C. for 3 hours. The NaOCl treated pulp had a copper
number of 1.6, and an average DP of 399 (as determined using Tappi
method T230).
[0148] Fifty grams of the air-dried, acid-treated pulp of Example 6
(having a copper number of 2.2 and an average DP of about 520) were
mixed with 500 ml of a solution of 1.6% borol at a temperature of
60.degree. C. for 2 hours. Borol is a 50% NaOH solution containing
12% sodium borohydrate. The borol-treated pulp had a copper number
of 0.86, while the average DP of the pulp was about 600 (cellulose
D.P. was measured using Tappi method T230).
EXAMPLE 18
Solution Thermal Stability of Pulp With or Without NaBH.sub.4
Treatment
[0149] The effect of reducing the copper number of acid-treated
pulp of the present invention on the thermal stability of a
solution of the acid-treated pulp in NMMO was investigated in the
following manner. Acid-treated pulp from Example 17, having a
copper number of 3.3, was treated with 1% NaBH.sub.4 according to
Example 2. The copper number of the borohydride-treated pulp was
1.0 (as measured using Weyerhaeuser test number PPD-3), and the
average D.P. of the borohydride-treated pulp was 418. A 4.6%
solution of the borohydride-treated pulp (having a copper number of
1.0) was prepared in NMMO. Similarly, a 4.5% solution of the
acid-treated pulp (having a copper number of 3.3) from Example 17
was prepared in NMMO. In both cases, the solutions were prepared at
98.degree. C. No antioxidant was added to the solutions.
[0150] The solution viscosity of each of the two pulp solutions was
measured using a Brookfield viscometer for a period of about 3-hour
(shear rate: 100 rad/minute). The curves depicting solution
viscosity versus dissolution time for each of the two pulp
solutions are shown in FIG. 17 and reveal that borohydride-treated
pulp (upper graph shown in FIG. 17) has higher thermal stability
than the same acid-treated pulp without borohydride treatment
(lower graph shown in FIG. 17).
[0151] These results demonstrate that reducing the copper number of
acid-treated pulp of the present invention, prior to dissolving the
treated pulp in NMMO to form a dope, improves the thermal stability
of the dope.
[0152] While the preferred embodiment of the invention has been
illustrated and described, it will be appreciated that various
changes can be made therein without departing from the spirit and
scope of the invention.
* * * * *