U.S. patent application number 09/986708 was filed with the patent office on 2002-07-25 for method of treating cellulose fibers and products obtained thereby.
Invention is credited to Chihani, Thami, Eichhorn, Stephan, Lindstrom, Eva, Sandberg, Sussan.
Application Number | 20020096274 09/986708 |
Document ID | / |
Family ID | 26074215 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020096274 |
Kind Code |
A1 |
Lindstrom, Eva ; et
al. |
July 25, 2002 |
Method of treating cellulose fibers and products obtained
thereby
Abstract
A method for reducing the level of or removing wood extractives
from cellulose fibers and products having cellulose fibers, by the
use of a supercritical or near supercritical fluid in order to
increase the absorption rate of such cellulose fibers and products
having cellulose fibers. Also disclosed are cellulose fibers and
products having cellulose fibers, such as cellulose tissue paper,
sanitary napkin or towel, nonwoven industrial wipes, baby diaper,
incontinence garments, or pulp having improved qualities, such as
improved absorption rate.
Inventors: |
Lindstrom, Eva; (Sundsvall,
SE) ; Eichhorn, Stephan; (Gernsheim, DE) ;
Sandberg, Sussan; (Onsala, SE) ; Chihani, Thami;
(Gernsheim, DE) |
Correspondence
Address: |
Ronald L. Grudziecki
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
26074215 |
Appl. No.: |
09/986708 |
Filed: |
November 9, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60247056 |
Nov 13, 2000 |
|
|
|
Current U.S.
Class: |
162/72 ; 162/100;
162/90 |
Current CPC
Class: |
D21H 27/30 20130101;
D21C 9/08 20130101 |
Class at
Publication: |
162/72 ; 162/90;
162/100 |
International
Class: |
D21C 003/20; D21C
003/02; D21F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 10, 2000 |
EP |
00850190.0 |
Claims
What is claimed is:
1. A method for reducing the level of or substantially removing
wood extractives from fibers, the method comprising the steps of:
contacting the fibers with a supercritical or near supercritical
fluid to create an extract of the wood extractives; and removing
the extract from the fibers so as to obtain improved cellulose
fibers with a reduced content of extractives.
2. The method according to claim 1, wherein the supercritical or
near supercritical fluid is selected from the group consisting of
carbon dioxide, ethane, ethylene, propane, propylene, cyclohexane,
isopropanol and chlorotrifluoromethane.
3. The method according to claim 1, wherein the supercritical or
near supercritical fluid is carbon dioxide.
4. The method according to claim 1, wherein the contacting of the
cellulose fibers with the supercritical or near supercritical fluid
is made under conditions where the temperature is at least T.sub.C
for said supercritical or near supercritical fluid and the pressure
is at least P.sub.C for said supercritical or near supercritical
fluid.
5. The method according to claim 1, wherein the contacting of the
cellulose fibers with the supercritical or near supercritical fluid
is made under conditions where the temperature is at least T.sub.C
for said supercritical or near supercritical fluid.
6. The method according to claim 1, wherein the contacting of the
cellulose fibers with the supercritical or near supercritical fluid
is made under conditions where the pressure is at least P.sub.C for
said supercritical or near supercritical fluid.
7. The method according to claim 4, wherein the temperature is at
least 1.5 times T.sub.C.
8. The method according to claim 4, wherein the temperature is at
least 2 times T.sub.C.
9. The method according to claim 4, wherein the pressure is at
least 2 times P.sub.C.
10. The method according to claim 4, wherein the pressure is at
least 3 times P.sub.C.
11. The method according to claim 4, wherein the pressure is at
least 4 times P.sub.C.
12. The method according to claim 4, wherein the pressure is at
least 5 times P.sub.C.
13. The method according to claim 4, wherein the pressure is at
least 6 times P.sub.C.
14. The method according to claim 1, wherein the wood extractives
in the extract are substantially hydrophobic substances.
15. The method according to claim 14, wherein the wood extractives
in the extract comprise hydrophobic substances selected from the
group consisting of fatty acids, resin acids, sterylesters and
triglycerides.
16. The method according to claim 1, wherein the wood extractives
in the extract comprise substances selected from the group
consisting of lignin or sterols.
17. The method according to claim 1, wherein the extracted wood
extractives are collected in an organic solvent.
18. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.5.
19. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.55.
20. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.6.
21. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.65.
22. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.7.
23. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.75.
24. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.8.
25. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.85.
26. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.9.
27. The method according to claim 1, wherein the ratio calculated
between the total extract content and the extract content obtained
by a standard method SCAN C 7:62 is at least about 0.95.
28. Cellulose fibers obtainable by the method claimed in claim
1.
29. Cellulose fibers having a reduced level of or being
substantially free from wood extractives, which impart--due to
ageing--decreased absorption rate to the cellulose fibers.
30. A method of producing a fibrous web, said method comprising the
step of using cellulose fibers according to claim 28 for the
production of the fibrous web.
31. A method of producing a fibrous web, said method comprising the
step of using cellulose fibers according to claim 29 for the
production of the fibrous web.
32. A method of producing an absorbent product, said method
comprising the step of using cellulose fibers according to claim 28
for the production of the absorbent product.
33. A method of producing an absorbent product, said method
comprising the step of using cellulose fibers according to claim 29
for the production of the absorbent product.
34. An improved fibrous web comprising cellulose fibers according
to claim 28.
35. An improved fibrous web comprising cellulose fibers according
to claim 29.
36. The fibrous web according to claim 34, wherein the improved
fibrous web at least after an initial period of time of at least
about two days, has a 5% higher liquid absorption rate than the
parent cellulose fibers.
37. The fibrous web according to claim 34, wherein the improved
fibrous web at least after an initial period of time of at least
about two days, has a 12% higher liquid absorption rate than the
parent cellulose fibers.
38. The fibrous web according to claim 34, wherein the improved
fibrous web at least after an initial period of time of at least
about two days, has a 20% higher liquid absorption rate than the
parent cellulose fibers.
39. The fibrous web according to claim 34, wherein the improved
fibrous web at least after an initial period of time of at least
about two days, has a 30% higher liquid absorption rate than the
parent cellulose fibers.
40. The fibrous web according to claim 34, wherein the improved
fibrous web at least after an initial period of time of at least
about two days, has a 50% higher liquid absorption rate than the
parent cellulose fibers.
41. A fibrous web comprising cellulose fibers, which have a reduced
level of or is substantially free from wood extractives, which
impart--due to ageing--decreased absorption rate to the cellulose
fibers.
42. A method of producing an absorbent product, said method
comprising the step of using the fibrous web according to claim 34
for the production of the absorbent product.
43. A method for reducing the level of or substantially removing
wood extractives from cellulose fibers, the method comprising the
steps of: contacting the cellulose fibers with a supercritical or
near supercritical fluid to create an extract with the wood
extractives; and removing the extract from the cellulose fibers so
as to obtain improved cellulose fibers with a reduced content of
extractives; wherein an improved fibrous web of said cellulose
fibers is prepared and wherein the improved fibrous web at least
after an initial period of time of at least about two days, has a
5% higher liquid absorption rate than the parent cellulose
fibers.
44. The method of claim 43, wherein the improved fibrous web has a
12% higher liquid absorption rate.
45. The method of claim 43, wherein the improved fibrous web has a
20%higher liquid absorption rate.
46. The method of claim 43, wherein the improved fibrous web has a
30% higher liquid absorption rate.
47. The method of claim 43, wherein the improved fibrous web has a
50% higher liquid absorption rate.
48. An absorbent product comprising cellulose fibers according to
claim 28.
49. An absorbent product comprising cellulose fibers, which have a
reduced level of or is substantially free from wood extractives,
which impart--due to ageing--decreased absorption rate of the
absorbent product.
50. Absorbent product according to claim 48, wherein the product is
a cellulose tissue paper, sanitary napkin or towel, nonwoven
industrial wipe, baby diaper, or incontinence garment.
51. Absorbent product according to claim 49, wherein the product is
a cellulose tissue paper, sanitary napkin or towel, nonwoven
industrial wipe, baby diaper, or incontinence garment.
52. Stratified absorbent product comprising cellulose fibers
according to claim 28, comprising at least one outer layer and
possibly at least one middle layer, the outer layers having a high
amount of improved cellulose fibers.
53. A fibrous web according to claim 34, comprising at least one
outer layer and possibly at least one middle layer, the outer
layers having a high amount of improved cellulose fibers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the priority of EP
00850190.0, filed at the European Patent Office on Nov. 10, 2000,
and claims the benefit of U.S. Provisional Application, serial No.
60/247,056, filed in the United States on Nov. 13, 2000. The entire
contents of EP 00850190.0 and U.S. Provisional Application, serial
No. 60/247,056, are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] The present invention relates to a method using a
supercritical or near supercritical fluid for removal of wood
extractives from cellulose fibers and products comprising cellulose
fibers to improve the qualities thereof. The present invention
further relates to cellulose fibers and products comprising
cellulose fibers having improved qualities.
[0004] 2. Description of Related Art
[0005] Kitchen towels and household towels are products which
should have good wetting and absorption properties such as
absorption rate and absorption capacity, especially with respect to
absorption of a liquid such as an aqueous medium. Also very
important is the rate for the uptake of a liquid. This means that
when wiping liquid from a surface it is necessary for the towel to
absorb the liquid as quickly as possible, or in other words to
remove the liquid from the surface by in some way binding it to the
towel by, e.g., an absorption process.
[0006] There are several possibilities of obtaining a towel with
good absorption properties, such as the absorption rate, for
example by adding an absorption booster as an additive during the
production of towels. Examples of such absorption boosters may be
surfactants. The addition of a surfactant to the pulp, i.e.,
chemically or mechanically fiberised wood, results in a pulp having
improved absorption properties such as a faster absorption rate for
liquid uptake.
[0007] The disadvantage of using an absorption booster is that in
general it is regarded as leading to a decrease in the strength
properties of the towel and the capillary pressure of the fibrous
web. Another disadvantage with absorption boosters like, e.g.,
surfactants, is that they tend to loose their wettability property
with time. It is also important to mention that chemicals should be
avoided, as much as possible, in hygiene paper due to problems with
toxicity and allergic reactions.
[0008] It is known that cellulose fibers and products comprising
cellulose fibers normally are subject to ageing phenomena, i.e.,
their behaviour with respect to, e.g., wettability and absorption
rate change during storage. After a certain period of time it is
generally found that the wetting and absorption properties of the
products have significantly decreased as compared to the wetting
and absorption properties directly after production. Other
properties like color and strength of the cellulose fibers and
products comprising cellulose fibers may also be affected. One
reason for this phenomenon is the outflow of so called "wood
extractives", i.e., some wood extractives will migrate from the
inner side of the cellulose fibers or the inner side of the
cellulose fiber product to the outer or outermost surface of the
cellulose fibers or cellulose fiber product and more or less
homogeneously coat or cover the outer surface with such wood
extractives. As such wood extractives generally are of a
hydrophobic nature this means that the outer surface of the
cellulose fibers or the outer surface of the cellulose fiber
product, i.e., that surface which is to be exposed to the liquid to
be absorbed, becomes more hydrophobic and, accordingly, it will
tend to repel an aqueous based liquid. The result is that the
wettability properties decrease.
[0009] The "wood extractives" are contemplated to be fiber
compounds, such as, e.g., low molecular fatty acids. Suggested, as
a theory, is that after migration from the interior of the fibers,
these compounds covers the fibers and make them more hydrophobic.
As a result the surface tension is reduced which in turn leads to a
decrease in the absorption properties and the tissue product
becomes less wettable.
[0010] One possibility to remove these hydrophobic compounds is to
extract them from the pulp used. Extraction procedures are well
known and organic solvents are often used. One of the preferred
best nonflammable solvents to be used for removal of unpolar
substances is dichloromethane (DCM). However, the use of DCM is not
altogether harmless since it contains chlorine and should for this
reason be avoided. Furthermore, dichloromethane is an organic
solvent that should be avoided for environmental reasons.
[0011] From U.S. Pat. No. 5,009,746 it is known how to remove
sticky contaminants from secondary cellulose fibers by the use of
supercritical carbon dioxide. The sticky contaminants are all
additives added to the paper to make it suitable for its purpose,
such as adhesives and the like.
[0012] From the fragrance industry it is known to use supercritical
carbon dioxide in an extraction procedure to remove nonpolar
chemicals (Caragay, A. B. Perfume and Flavorist 1981, 6:43-55;
Schultz, E.g., and Randall, J. N. Food Tech. 1970, 24:94-98).
OBJECTS AND SUMMARY
[0013] The need for a method for treating cellulose pulp and
cellulose fibers in order to prevent/slow down ageing processes of
the cellulose fibers is evident from reasons described above.
Furthermore, there is a need for a method and products to meet
increasing economical and environmental demands as well as a method
for the production of such a product in a large scale.
[0014] It is an object of the invention to provide a method for
reducing the level of, or substantially removing, wood extractives
imparting--due to ageing--a decreased absorption rate from
cellulose fibers comprising contacting the fibers with a
supercritical or near supercritical fluid to extract the wood
extractives and removing the extract from the fibers so as to
obtain improved cellulose fibers with a reduced content of
extractives which impart decreased absorption rate. The use of such
a method during manufacturing of cellulose fibers, or an absorbent
product containing cellulose fibers, will prevent or slow down
ageing processes and lead to an increased quality of the cellulose
fibers or the product thereof, especially with respect to increased
absorption properties such as, e.g., an increased absorption rate
of a liquid to the cellulose fibers or to a product comprising
cellulose fibers due to the removal of the ageing substances in an
early phase.
[0015] It is also an object of the invention to provide a method
for reducing the level of or removing wood extractives from
cellulose fibers, such method being adapted for large scale
production, cost-efficient, simple and which does not negatively
affect the fibers treated.
[0016] Furthermore, it is an object of the invention to provide
cellulose fibers and/or products comprising cellulose fibers having
improved qualities, e.g., absorption properties such as the
absorption rate.
[0017] Specifically, it is an object of the invention to provide a
stratified absorbent product comprising the above mentioned
improved cellulose fibers which results in a relatively cheap
product having improved absorption properties such as the
absorption rate due to the previous removal of wood
extractives.
[0018] To achieve these and other objects, the present invention
relates to a method for reducing the level of or removing wood
extractives from cellulose fibers, wherein the fibers are brought
into contact with a supercritical or near supercritical fluid for a
period of time sufficient to reduce the level of or to
substantially remove the wood extractives.
[0019] The invention also refers to the use of a supercritical
fluid or near supercritical fluid for reducing the level of or
substantially removing wood extractives from cellulose fibers.
[0020] In a further aspect the invention refers to extracted
cellulose fibers or products produced thereof, such as a fibrous
web, tissue paper, sanitary napkin, baby diaper, incontinence
garment, non-woven industrial wipes, comprising reduced levels of
or being substantially free from wood extractives.
[0021] In a specific aspect, the invention provides a method for
reducing the level of, or substantially removing, a more specific
group of wood extractives compared to the relatively unspecific
extraction of wood extractives which is obtained by using
well-known methods employing organic solvent extraction. Thus, by
use of a method according to the present invention, the composition
of wood extractives removed from the cellulose fibers is different
compared with the composition of wood extractives obtained after
extracting cellulose fibers with dimethylchloromethane (DCM). By
use of a method according to the invention, the most hydrophobic
compounds such as triglycerides and sterylestes are extracted in a
more efficient way, while the less hydrophobic compounds, such as
lignin, are extracted to a decreased degree and retained in the
fibers. However, if desired, by adjusting the extraction parameters
(increased pressure, addition of co-solvent e.g., methanol) lignin
can be extracted to various extents.
[0022] In still a further aspect, the invention relates to a
stratified product comprising cellulose fibers comprising at least
one outer layer and possibly at least one middle layer, wherein at
least one of the outer layers has a high amount of extracted virgin
cellulose fibers or pulp.
[0023] Use of the method according to the invention will increase
the qualities of the resulting absorbent product, such as the
absorption rate of the final product comprising cellulose fibers by
preventing or minimizing the disadvantages generally observed by
ageing of such product. Further, the method considers environmental
aspects, since a supercritical fluid like, e.g., carbon dioxide,
CO.sub.2, is less toxic and not flammable compared to known
alternatives like, e.g., DCM. As a further advantage, a method
according to the invention is much cheaper compared to conventional
extraction methods due to the use of less expensive fluids, e.g.,
the use of CO.sub.2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic view of an extractor that can be used
in accordance with the present invention.
[0025] FIG. 2 is a phase diagram of carbon dioxide showing the
critical point and the supercritical region of the fluid.
[0026] FIG. 3a is a table (table 2) showing the amount of
extractives obtained by the present method compared to the amount
obtained by means of a known solvent extraction method.
[0027] FIG. 3b is a diagram showing the composition of wood
extractives in the extracts from Example 2.
[0028] FIG. 4 is a table (table 3) showing the results from
measurement of absorption in Example 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] As revealed above, the present invention relates to the use
of a supercritical, or near supercritical, fluid (SF) for removing
wood extractives from cellulose fibers and absorbent products
comprising improved cellulose fibers. In the present context, the
term "improved cellulose fibers" is used to denote any kind of
cellulose fibers that have been subjected to the method according
to the present invention, i.e., improved cellulose fibers are
cellulose fibers which have been subject to a purification
procedure in which an amount of impurities like wood extractives
have been removed from the fibers.
[0030] The term "cellulose fiber" is intended to mean any natural
cellulose containing fibers, e.g., wood fiber, cotton fiber or hemp
fiber, preferably virgin cellulose fibers or substantially virgin
fibers. The major component in pulp is the natural cellulose
fiber.
[0031] The term "virgin" with cellulose fiber is intended to mean
not reclaimed. Virgin pulp according to this definition is made
from such cellulose fibers and may be the result of mechanical,
chemical (e.g., sulphate or sulphite), or chemimechanical
pulping.
[0032] The term "wood extractives" is intended to mean compounds,
such as aliphatic compounds (mainly fats and waxes), terpenes,
terpenoids and phenolic compounds, such compounds being inherent in
and extractable from the cellulose. Wood extractives mainly include
hydrophobic compounds, e.g., fatty acids, resin acids, fats
(glycerol esters, e.g., triglycerides) and waxes (esters of other
alcohols, e.g., sterylesters) defined according to an analytical
procedure for quantitative determination of wood extractives and
lipins (rs.ang., F. and Holmbom, B., J Pulp and Paper Sci. 1994,
20:J361-J365).
[0033] The term "supercritical or near supercritical fluid" is
intended to mean any fluid presented under supercritical or near
supercritical pressure and temperature conditions, within narrow
ranges for the specific fluid wherein still most of the fluid
behaves supercritical. Herein, the critical temperature and the
critical pressure is abbreviated T.sub.c and P.sub.c,
respectively.
[0034] In the supercritical state, substances behave partly as a
gas and partly as a liquid. Their densities are liquid-like but in
respect of diffusivity or penetration capabilities they resemble
gases. The density of a supercritical fluid may be continuously
changed without phase separation by changes in pressure and/or
temperature. Density dependent properties such as solvent power
also undergo corresponding changes.
[0035] As discussed above, the present invention uses a
supercritical or a near supercritical fluid, such as, e.g., carbon
dioxide, to remove an amount of wood extractives from cellulose
fibers. The critical temperature and pressure for pure carbon
dioxide are 31.1.degree. C. and 73.8 bar, respectively. Of course,
as used in the present invention, carbon dioxide (or any other
solvent employed) is not operating in a simple, pure state, but as
a part of a multi-component system. It is possible, therefore, that
depending upon the particular system, the advantages of the
invention can be achieved even though the pressure and/or
temperature used are not above the critical values for pure carbon
dioxide (or any other solvent employed). It is in this sense that
the term "near supercritical" is used herein.
[0036] Wood extractives can be divided into three subgroups:
aliphatic compounds (mainly fats and waxes), terpenes and
terpenoids, and phenolic compounds. A large variety of aliphatic
compounds exist in the resin. The fatty acids occur mostly as
esters (glycerol esters) present as di- or triglycerides. Compounds
which are, according to the invention, of specific interest to
remove are hydrophobic compounds, such as fats, i.e., esters of
glycerol, e.g., tri- and diglycerides; waxes, i.e., esters of
alcohols, e.g., sterylesters, fatty acids and resin acids. These
compounds give rise to problems in pulping and paper making, and
one effect is a decrease in absorption properties. The content of
wood extractives and their composition vary greatly among different
wood species and also within the different parts of the same tree.
The concentration of extractives in pulp is, however, mainly
dependent on which process that has been used for producing the
pulp where mechanical and chemimechanical contain the highest
amounts and sulphite pulp contain high amounts of extractives after
pulping.
[0037] In other words, the term "wood extractives" denotes a
relatively large and unspecific group of compounds, namely such
compounds contained in the wood or cellulose fibers that are
extractable by means of an organic solvent. The amount and
composition of the wood extractives extracted from specific
cellulose fibers depend on i) the specific organic solvent used,
ii) the conditions employed during extraction such as, e.g., the
temperature, the flow conditions and the number of extractions, and
iii) the specific type of cellulose fibers or wood fibers employed.
In the present context, the wood extractives which are of main
interest to remove from the cellulose fibers are those which give
rise to an unwanted ageing phenomenon, namely a decrease in the
wetting and absorption properties, strength and/or color of the
fibers. The improved cellulose fibers obtained according to the
invention may of course still contain wood extractives of any of
the above-mentioned types. The main point is that some of the
unwanted wood extractives have been removed to a degree that is
sufficient to obtain improved properties of the cellulose fibers,
cf. above.
[0038] The cellulose fibers used according to the invention may be
any cellulose fiber or product comprising cellulose fibers.
However, preferably virgin cellulose fibers are used, since such
fiber includes a high content of wood extractives. During storage,
cellulose fiber is aged due to outflow of so called "wood
extractives". Virgin fibers and pulps contain wood extractives,
although the contents vary greatly among different wood species and
also due to the pulping and bleaching conditions used in the
production of different pulps. Even low levels of wood extractives,
such as the level found in bleached chemical pulps, can affect the
absorption properties of the fibers in a negative way.
[0039] In reclaimed pulps, the extractives consist mainly of
additives originating from the paper making process, printing and
other converting processes to make the paper suitable for its
purpose, so called extractable stickies. The wood extractives are
only a minor part of the extractable stickies in such pulps.
[0040] The cellulose fibers to be used according to the invention
may be selected from the group consisting of Kraft (sulphate) pulp,
sulphite pulp, mechanical pulp and HTCTMP pulp (High Temperature
Chemi Thermo Mechanical Pulp).
[0041] According to the invention, the level of these wood
extractives is reduced or substantially removed compared to
conventional extraction methods using DCM according to standard
SCAN C 7:62. In one embodiment herein, the ratio is calculated
between the total extract content--measured according to Example
1--and the extract content obtained by a standard method SCAN C
7:62 is at least about 0.5, or alternatively, at least about 0.55,
0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.90 or about 0.95.
[0042] Particularly, the removal of triglycerides and/or
sterylesters is beneficial and their removal increases the
absorption rate in the final cellulose fibers and products
comprising cellulose fibers. The relative amount of sterylesters
and/or triglycerides in the extract is higher, .about.2 times for
sterylesters and .about.3 times for triglycerides, using
supercritical fluid CO.sub.2 as the extraction method compared to
solvent extraction using DCM (FIG. 3). Furthermore, by means of the
present method, the amount of lignin is retained in the cellulose
fibers to a high degree. The amount of lignin extracted can be
varied to some extent by adding a solvent modifier, such as
methanol, to the CO.sub.2 and by adjusting pressure and
temperature. According to a specific embodiment of the present
invention, the lignin content of the extract is .ltoreq.5%.
[0043] The method according to the invention for reducing the level
of, or substantially removing, wood extractives from cellulose
fibers implies the use of a supercritical or near supercritical
fluid. Examples of such fluids are CO.sub.2, ethane and ethylene.
Further examples of fluids to be used according to the present
invention are given in table 1 below. Preferably CO.sub.2 is used,
due to the fact that it is readily available and rather
inexpensive, as well as not toxic and not flammable.
1Table I Critical temperature, Critical pressure Solvent T.sub.c
(.degree. C.) P.sub.c (bar) Carbon dioxide 31.1 73.8 Ethane 32.2
48.8 Ethylene 9.3 50.4 Propane 96.7 42.5 Propylene 91.9 46.2
Cyclohexane 280.3 40.7 Isopropanol 235.2 47.6
Chlorotrifluoromethane 28.9 39.2
[0044] In principle, the cellulose fibers could be treated at any
stage such as before or after bleaching. Experience shows that the
extraction result is better if fresh fibers are used, but also
older fibers can be extracted.
[0045] The supercritical or near supercritical fluid is used at a
temperature and pressure optimal for the fluid, with pressures
above from about 70 bar, preferably from 140 and most preferably
from 200 bar to about 500 bar and temperatures above about
25.degree. C., preferably in the range from about 31.degree. C. to
150.degree. C. and most preferably in the range from 31.degree. C.
to 90.degree. C. An example of a phase diagram for CO.sub.2, with
the supercritical fluid region marked, is shown in FIG. 2.
[0046] In one embodiment, the cellulose fibers are contacted with
the supercritical or near supercritical fluid under conditions,
where the temperature is at least the critical temperature
(T.sub.c) for the intended supercritical fluid, or alternatively at
least about 1.5 or 2 times higher than T.sub.c and/or the pressure
is at least the critical pressure (P.sub.c) for the intended
supercritical fluid, or alternatively at least about 2, 3, 4, 5, or
6 times higher than P.sub.c.
[0047] The extraction apparatus used may be the one shown in FIG.
1. With reference to FIG. 1, there is shown an extractor (1) in the
form of a heated oven (2) with a sample cell (3) included, having a
supercritical fluid supply tank (4), a supercritical fluid pump
(5), a modifier pump (6), extract collection tube or tubes (7), all
connected in a tubing system (8).
[0048] The process according to the invention may be as follows. A
pulp sample is prepared. Then the pulp sample is homogenized and
packed into a sample cartridge, further placed into the sample cell
(3). CO.sub.2 is supplied from the supply tank (4) pumped and
controlled via (5) and (6) into the heated oven (2) and over the
sample cell (3). The sample is extracted by carbon dioxide at a
specified temperature and pressure. The resulting extract is then
collected (7) by trapping the extract into a vial filled with an
organic solvent, e.g., DCM.
[0049] The present invention also refers to extracted cellulose
fibers, preferably based on, or substantially based on, virgin
cellulose fiber, which fibers have reduced levels of, or are
substantially free of, wood extractives. An improved fibrous web
made of cellulose fibers has, at least after an initial period of
time of at least about two days, or alternatively at least about
10, 90 or about 180 days, a higher liquid absorption rate, such as
at least about 5, 12, 20, 30 or about 50% higher, than a fibrous
web made of parent cellulose fibers as evidenced by the method
described in Example 3.
[0050] The present invention also refers to a fibrous web
comprising the improved cellulose fibers described above. In one
embodiment, the fibrous web comprises cellulose fibers which have a
reduced level of, or is substantially free from, wood extractives
which impart--due to ageing--decreased absorption rate of the
cellulose fibers. In still another embodiment the fibrous web
described above has a liquid absorption rate at almost the same
order of magnitude during storage for such as at least about 2, 10,
30, 90 or 180 days. Further, such fibrous web is used for the
production of an absorbent product.
[0051] The cellulose fibers according to the present invention are
useful for preparing absorbent products such as tissue paper,
nonwoven and absorbent articles such as baby diapers, incontinence
garments and sanitary napkins. The tissue paper can be used in
kitchen towels, household towels, toilet paper, handkerchiefs and
facials. Apart from cellulose fibers, the nonwoven comprises
polymeric fibers, such as regenerated cellulose (viscose, rayon),
polypropylene, polyester, polylactide, or polyamide. The nonwoven
can be bonded by any of the regular types of bonding mechanism,
e.g., by thermo-bonding, by entangling or by hydro entangling. This
type of nonwoven is mostly used for industrial and offset wipes.
Regarding the absorbent articles, the fibers according to the
invention are used mainly in the surface and acquisition material
and in the absorption core. The tissue paper, nonwoven and surface,
acquisition and core materials of the absorbent articles should
comprise at least 10%, preferably at least 30% and most preferably
at least 50% by weight, of the extracted cellulose fibers.
[0052] In a further aspect, the present invention refers to a
stratified absorbent product such as tissue or nonwoven based on
the above extracted cellulose fibers. The stratified absorbent
product of the invention comprises at least one outer layer and at
least possibly one middle layer, at least one of the outer layers
having at least about 10%, or at least about 30% or at least about
50%, by weight, of the dried sheet of the extracted cellulose
fibers. By such an absorbent product it is possible to reduce costs
and at the same time provide a product having good absorption
properties, such as a high absorption rate.
[0053] It is also advantageous to make a multi-ply tissue paper or
nonwoven absorbent product. At least one outer surface having at
least about 10%, at least about 30% or at least about 50% by weight
of the dried sheet of the extracted cellulose fibers. This
absorbent product could be built up using two plies being
stratified as above or not being stratified. The absorbent product
could further have three or more plies whereby at least one of the
outer plies should contain extracted pulp. The outer plies may be
multi- or single-layered.
[0054] The tissue product may also comprise man-made polymeric
fibers, such as regenerated cellulose (viscose, rayon),
polypropylene, polyester, polylactide, or polyamide. The tissue may
contain up to 50% by weight of polymeric fibers. Different amounts
of polymeric fibers can be used in different layers and/or
plies.
[0055] As used herein a stratified or multi-layered product means
an absorbent product where the layers have been brought together
when they are still wet, either through the use of a multi-layered
headbox or by couching together several wet webs. This gives a
stratified or multi-layered web (ply) where the layers are very
hard to separate.
[0056] As used herein a multi-ply product means an absorbent
product where two or more plies are laid together when dry to work
as one unit. The layers can be plybonded although it is not
necessary. The plybonding can be achieved through mechanical
plybonding, e.g., through embossing or knurling. It can also be
achieved through adhesive bonding of the plies, e.g., in a dot
pattern, as a continuous network or with adhesive applied all over
the plies.
EXAMPLES
Example 1
Supercritical Fluid (SF) Extraction of Pulp
[0057] This example describes, without limiting the invention, the
extraction of pulp with CO.sub.2 as the supercritical fluid.
[0058] Definition
[0059] Wood extractives are defined as the amount of material,
which can be extracted from a virgin fiber pulp sample with neutral
organic solvents (e.g., hexane, dichloromethane). The method
described within this example illustrates a procedure to extract
such components by means of supercritical carbon dioxide being as
efficient as conventional solvent extraction methods.
[0060] Principle
[0061] The pulp sample is homogenized and packed into a sample
cartridge. The cartridge is placed in the extractor (1) in the
sample cell (2) where it is extracted by carbon dioxide at a
specific temperature and pressure. The resulting extract is trapped
and collected in a vial (6) filled with dichloromethane (DCM) (FIG.
1).
[0062] Reagents
[0063] Carbon dioxide, CO.sub.2 (Air Liquide, SFC quality)
pressurised with 110 bar helium and DCM (Riedel deHan, p.a.
quality). The pulp used in this particular example is bleached
sulphite pulp "Excellent" from SCA Hygiene Products GmbH in
Mannheim.
[0064] Apparatus
[0065] A principal drawing of an extractor to use in this invention
is illustrated in FIG. 1. However, the invention is not limited to
the disclosed embodiments.
[0066] The equipment includes: ISCO SF 3560 with two pumps and
automated extractor (FIG. 1); Sample cartridge for use at high
temperature (crystalline polymer); Collection vials 20 ml;
[0067] Balance with an accuracy of 0.0001 g;
[0068] Vials 4 ml.
[0069] Sample Preparation
[0070] Dry the collection vials at 100.degree. C. The vials are
cooled in a vacuum dessicator after drying and weighed to the
nearest 0.0001 g. The dry matter of the pulp is determined
according to ISO 638:1979. An exact amount of the sample is weighed
to the nearest 0.0001 g in the sample cartridge (a rod can be used
for this purpose).
[0071] Extraction
[0072] The sample cartridges are placed in the extractor. The
weighed collection vials are also placed in the extractor. A
computer file describing the method for extraction of fiber pulp is
loaded from the control unit of the extractor.
[0073] This method file uses the following extraction
parameters:
2 Extraction chamber temperature 80.degree. C. Extraction chamber
pressure 448 bar Restrictor temperature 80.degree. C. Collection
temperature 30.degree. C. Restriction flow (CO.sub.2) 1.5 ml/min
Extraction time 40 min Collector solvent Dichloromethane
[0074] After completed, extractions in the collection vials are
evaporated under nitrogen gas flow to dryness. The vials are
subsequently dried in a vacuum drying oven at 40.degree. C. for
three hours. The vials with the dried extract are then weighed to
the nearest 0.0001 g.
[0075] Extract Content
[0076] The extract content is calculated according to formula A
below 1 Extract content % = ( We - Wc ) .times. 100 Ws .times. DM .
A
[0077] Results are shown in FIG. 3a. The results show that the
method according to the invention is at least as sufficient as the
DCM method.
Example 2
[0078] Comparison of Supercritical CO.sub.2 Extraction and Solvent
Extraction With Dichloromethane.
[0079] This example is a comparison between super critical CO.sub.3
extraction and solvent extraction with dichloromethane (DCM) of
different pulp materials. For the DCM extraction a standard method
(SCAN C 7:62) was used where the pulp is refluxed with DCM in a
soxhlet apparatus for six hours followed by a graviometric
determination of the extract content.
[0080] The pulp samples are prepared according to the invention
with the method described in Example 1. The total extract content
and the composition of the extractives in the extracts are shown in
FIG. 3.
[0081] From the results shown in FIG. 3a, it appears that the
SF-CO.sub.2 extraction is as effective as the solvent extraction
with respect to the total extract content. From FIG. 3b it appears
that certain compositions are preferably extracted with SF-CO.sub.2
compared to the conventional method using DCM. In particular, the
low level of extracted lignin after using the SF-CO.sub.2 method
shows that the method is very selective in removing wood
extractives.
[0082] Also seen in FIG. 3 is the increase in percentage extracted
sterylesters and triglycerides out of the total percentage
extractives, viz. .about.55% triglycerides when using SF-CO.sub.2
compared to .about.15% when using DCM, i.e., more than three times
more when using SF-CO.sub.2. Also, conventional methods extract
large amounts of less defined compounds, about 40% in this
particular example with DCM as extraction fluid. Since highly
hydrophobic compounds like sterylesters and triglycerides effect
the absorption rate in a negative way, an increased removal of such
compounds is of high value for the present invention.
[0083] Results are shown in FIG. 3b.
Example 3
[0084] Measurement of Absorption Rate Before and After Extraction
With CO.sub.2
[0085] This example describes the absorption rate in sheets
prepared from extracted or unextracted sulphite pulp.
[0086] Definition
[0087] Absorption rate is the mean velocity with which a liquid
drop of a defined volume is fully absorbed into a paper sheet. The
mean velocity is calculated in that the defined volume is divided
by the time needed for the liquid drop of the defined volume to be
fully absorbed into the paper. The absorption time needed is
dependent on both the wettability, i.e., surface chemistry and the
basis weight, i.e., network structure of the cellulose fibers
used.
[0088] Principle
[0089] A tissue sheet made from a dynamic sheet former called
Formette was used in a lab scale equipment for the manufacturing of
Formette sheets. The absorption was measured using a high-speed
video system.
[0090] Material
[0091] Extraction of sulphite pulp as in Example 1. Fibers were
extracted twice, but no significant amount of extractives was
removed in the second extraction. Formette sheets of unextracted or
extracted sulphite pulp had a basis weight of 36.7 and 40.8
g/m.sup.2, respectively. No additives such as wet strength agents
were added and the materials are uncrept. Liquid to be absorbed in
this example is water.
[0092] Apparatus
[0093] High speed video, such as MotionScope, Redlake Imageing,
model PCI 500 S was used. A Plexiglass plate is used as a sample
support.
[0094] Procedure
[0095] The drop volume is 5 RI and the number of drops measured is
10. The distance between drop and sample surface before application
of drop is 5 mm and each drop is applied and measured on an
individual and well separated spot.
[0096] Analysis
[0097] The absorption rate is measured using the high-speed video
system. The measurements are not made under climatized conditions.
The temperature is normally varying between 20-25.degree. C. while
the relative humidity normally varies between 30-75%. Therefore one
should pay most attention to the difference between materials
measured at the same occasions, i.e., under the same conditions
rather than comparing absolute numbers of absorption time and rate
from different occasions. Due to the variation in basis weight,
normalized absorption times have also been calculated, assuming a
linear relationship with decreasing values between the basis weight
and the absorption time, i.e., the higher the basis weight, the
lower the absorption time.
[0098] Normalisation of absorption times:
T.sub.measured* (W.sub.measured/W.sub.aimed)
[0099] where W=basis weight (g/m.sup.2)
[0100] (W.sub.aimed is in this case 40 g/m.sup.2)
[0101] Calculation of relative change in absorption time:
((T.sub.extracted-T.sub.unextracted)/T.sub.unextracted)*100
[0102] where T=absorption time (ms)
[0103] Calculation of absorption rate:
r=V/T
[0104] where V=volume of a drop
[0105] (in this case 5 .mu.l)
[0106] Calculation of relative change in absorption rate:
((r.sub.extracted
r.sub.unextracted)/r.sub.inexttacted)*100=T.sub.unextrac-
ted(1/T.sub.extracted-1/T.sub.unextacted)*100
[0107] Results (average value of IO drops) are shown in FIG. 4. The
difference between the unextracted and extracted samples obtained
the same day are all significant (95% confidence interval), where
the extracted samples show a faster absorption time. As seen in
FIG. 3, the relative decrease in absorption time as well as the
relative change in absorption rate is improved already after two
days. Also, the results show an improvement over time. Due to
variations in temperature and relative humidity, one should pay
most attention to the difference between materials measured at the
same occasions, i.e., under the same conditions rather than
comparing absolute numbers of absorption time and rate from
different occasions.
Example 4
[0108] Method for the Determination of Wood Extractives in
Papermaking
[0109] This procedure refers to a published method (rs.ang., F and
Holmbom, B. J Pulp and Paper Sci. 1994, 20:J361-J365). This
analytical procedure has been developed to enable a quantitative
determination of wood extractives and lignins in papermaking
processes. As used in present invention, extraction with
supercritical carbon dioxide replaces the published extraction step
with methyl tert.butyl ether (MTBE). Sililation and gas
chromatography (GC) for the determination of extractives follow the
extraction procedure. A short thin-film capillary column enables
direct determination of free fatty and resin acids, sterols, steryl
estes, triglycerides and lignanes. All samples are determined
quantitatively relative a standard.
[0110] Although only preferred embodiments are specifically
illustrated and described herein, it will be appreciated that many
modifications and variations of the present invention are possible
in light of the above teachings and within the purview of the
appended claims without departing from the spirit and intended
scope of the invention.
* * * * *