U.S. patent application number 10/304856 was filed with the patent office on 2003-07-17 for use of ozone for increasing the wet strength of paper and nonwoven.
Invention is credited to Jaschinski, Thomas.
Application Number | 20030131958 10/304856 |
Document ID | / |
Family ID | 26974267 |
Filed Date | 2003-07-17 |
United States Patent
Application |
20030131958 |
Kind Code |
A1 |
Jaschinski, Thomas |
July 17, 2003 |
Use of ozone for increasing the wet strength of paper and
nonwoven
Abstract
A treatment sequence includes ozonation under acidic conditions
followed by an acidic wash for enhancing the wet strength of a
cellulosic fibrous material. Preferably the ozone treatment and the
acidic wash are followed by a second ozonation step under acidic
conditions. The fibrous cellulosic material obtained by this
treatment sequence has a breaking length of at least 100 m. The wet
strength of the cellulosic fibrous material is increased without
the use of additives, such as wet strength agents. This use of
ozone is very simple and efficient, and leads to highly pure
products. The use of ozone as the only treatment chemical in
particular avoids the introduction of so-called "non-process
elements" (NPE) into the treatment system, for instance metal
oxides such as MgO, which are frequently used in the oxidative
treatment of pulps.
Inventors: |
Jaschinski, Thomas;
(Ladenburg, DE) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Family ID: |
26974267 |
Appl. No.: |
10/304856 |
Filed: |
November 27, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60334092 |
Nov 30, 2001 |
|
|
|
Current U.S.
Class: |
162/65 ; 162/60;
162/82; 162/83; 162/95; 162/97 |
Current CPC
Class: |
D21C 9/02 20130101; D21C
9/153 20130101; D21C 9/004 20130101 |
Class at
Publication: |
162/65 ; 162/83;
162/82; 162/95; 162/97; 162/60 |
International
Class: |
D21C 009/02; D21C
009/153 |
Claims
1. Method of enhancing the wet strength of a fibrous cellulosic
material, which comprises the following treatment sequence:
subjecting the fibrous cellulosic material to an ozone treatment
under acidic conditions to obtain an ozonated material; and
subjecting the ozonated material to an acidic wash of the ozonated
material to obtain a fibrous cellulosic product having enhanced wet
strength.
2. The method according to claim 1, wherein the ozone treatment is
preceded by a further acidic wash.
3. The method according to claim 1, wherein the acidic wash is
followed by a second ozonation step under acidic conditions.
4. The method according to claim 3, wherein the second ozonation is
followed by a further acidic wash.
5. The method according to claim 3, wherein the first ozone
treatment is preceded by a further acidic wash.
6. The method according to claim 3, further comprising maintaining
acidic conditions after the last ozonation step of the treatment
sequence.
7. The method according to claim 1, wherein the treatment sequence
comprises at no point in time an alkaline wash.
8. The method according to claim 1, wherein the entire treatment
sequence is performed under acidic conditions.
9. The method according to claim 1, wherein the cellulosic material
is selected from sulfite pulps, bisulfite pulps, Kraft pulps,
cotton linters, or fibers from Esparto Grass, bagasse, hemp, flax
and agricultural crops.
10. The method according to claim 1, wherein the cellulosic fibrous
material to be treated has an initial kappa number of less than
50.
11. The method according to claim 10, wherein the initial kappa
number is more than 13.
12. The method according to claim 1, wherein the ozone-treated
cellulosic product has a final kappa number of less than 15.
13. The method according to claim 1, wherein the ozone treatment
leads to a Kappa reduction of at least 10.
14. The method according to claim 1, wherein ozone is used as the
only oxidant in the treatment of a cellulosic pulp material
obtained by a pulping process.
15. The method according to claim 1, wherein prior to the ozone
treatment no bleaching or other oxidizing step is employed.
16. The method according to claim 1, wherein the initial pH of the
ozone treatment is 5 or lower.
17. The method according to claim 1, wherein an inorganic acid,
carboxylic acid having 1 to 4 carbon atoms or dicarboxylic acid
having 2 to 4 carbon atoms is used for acidifying.
18. The method according to claim 17, wherein the inorganic acid is
sulfuric acid.
19. The method according to claim 1, wherein the consistency of the
cellulosic fibrous material to be ozonated is at least 30% per
weight.
20. The method according to claim 1, wherein the ozone is wetted
before contact with the cellulosic fibrous material.
21. The method according to claim 1, wherein the cellulosic fibrous
material is fluffed immediately before subjecting to the ozone
treatment.
22. The method according to claim 1, wherein the ozone-treated
cellulosic product has a final kappa number of less than 10.
23. The method according to claim 1, wherein the ozone treatment
leads to a Kappa reduction of at least 15.
24. A fibrous cellulosic material obtained by the method according
to claim 1.
25. A fibrous cellulosic material obtained by the method according
to claim 2.
26. A fibrous cellulosic material obtained by the method according
to claim 3.
27. The fibrous cellulosic material according to claim 24,
containing no effective amount of wet strength-enhancing
additives.
28. The fibrous cellulosic material according to claim 24 having a
wet breaking length of at least 100 m.
29. Paper or nonwoven comprising the fibrous cellulosic material of
claim 24.
30. Paper according to claim 29, wherein the paper is a tissue
paper.
Description
[0001] The present invention relates to the use of ozone for
enhancing the wet strength of a cellulosic fibrous material, e.g.
pulp, and paper or nonwoven made from this cellulosic material.
BACKGROUND ART
[0002] Papers and paper products are often exposed to extremely
varied strength requirements in the wet and dry states. For
instance, it must be ensured in the case of packaging paper that it
also retains its strength at least for a specific period of time
when exposed to rainwater. On the other hand, toilet paper should
dissolve in water--some time after use--in order to prevent the
sewage systems from clogging up. At the same time, toilet paper
must not immediately lose its strength properties during use, i.e.
whenever it has just briefly come into contact with the moisture
from excrement.
[0003] Similarly, kitchen paper should not lose their strength
properties when moistened with water and other primarily aqueous
liquids, which frequently happens during their use in the
kitchen.
[0004] To describe the strength properties of paper, the prior art
therefore often draws a distinction between a paper's "dry
strength", "initial wet strength", "temporary" and "permanent" wet
strength. This also applies to tissue paper and tissue
products.
[0005] Dry strength is generally determined in a similar manner, in
the case of paper usually based on DIN EN ISO 1924-2, Paper and
Board, Determination of properties under tensile load. Part 2:
method at a constant rate of elongation, April 1995, (ISO 1924-2:
1994). In the case of tissue paper and tissue products, tests are
performed on the basis of DIN EN 12625-4, Tissue Paper and Tissue
Products--Part 4: determination of width-related breaking strength,
elongation at break and the tensile energy absorption, January
1999.
[0006] The initial wet strength originally characterized the
strength after sheet formation, and particularly refers to the
strength of the initially formed moist paper web at the time of the
first free transfer, e.g. from the screen section to a subsequent
press section.
[0007] More recent prior art defines initial wet strength more
broadly than earlier prior art. This definition essentially acts as
a parameter for characterizing the strength behavior of remoistened
paper, paper products, tissue paper and tissue products. It is
ascertained as the tensile strength of paper soaked over a specific
period of time.
[0008] In this way, WO 97/36052 and WO 97/36054 do indeed define
the initial wet strength by means of the normal wet strength
determination employed in comparable measuring techniques. Yet the
so-called initial wet strength here corresponds to the wet strength
of a sample (test strip) from a test sheet exhibiting a
predetermined basis weight and produced under standardized
conditions, calculated--after previously soaking the test
strip--using a standardized tensile testing device under
standardized test conditions.
[0009] In addition to the initial wet strength, the aforementioned
documents introduce and use the terms "temporary" and "permanent"
wet strength as further criteria for evaluating the strength of a
product after it has been remoistened (wet strength) and hence as
criteria for its suitability in everyday practice (for example the
dissolving properties of toilet paper after it has been used in
order to avoid clogging up the pipes). The soaking duration and
decrease in wet strength over time are used in these documents as
criteria to differentiate between initial, temporary and permanent
wet strength.
[0010] In these documents, a rate of decrease is mathematically
determined on the basis of measured values as a criterion for the
evaluation of temporary wet strength in that the difference is
formed from the so-called initial wet strength as the wet strength
after 5 s soaking duration and the wet strength after 30 min
soaking duration for samples that were somehow pretreated e.g. by
addition of a wet-strength agent or by modification of the fibrous
material in order to increase wet strength. The difference of the
corresponding measurements for untreated samples is calculated in a
similar way. The difference of the strengths of the treated samples
is then placed in proportion to the difference of the strengths of
the untreated samples and expressed as a percentage.
[0011] In simplified terms, this means that temporary wet strength
should be defined as the drop in strength of a sheet of paper or
tissue paper or of a tissue product; after remoistening the paper,
tissue paper or tissue product after expiry of an interval of the
action of moisture (soaking) to be specified by definition, this
drop can be determined in terms of measurement technology by means
of a standard test method. In contrast, the permanent wet strength
should be defined as the maintenance of strength even after
moisture has exerted its influence for a fairly long time upon
remoistening, e.g. for a period of 30 min.
[0012] In the present invention, the term "wet strength" refers to
remoistened samples which were soaked into water for only a few
seconds as described in the section "test methods". In the case of
paper samples, the soaking period before tensile testing was fixed
as 30 sec. From the breaking strength (N/15 mm) obtained thereby,
it is possible to calculate the wet breaking length (in m), and the
relative wet strength, which is the ratio of breaking length
(wet)/breaking length (dry).
[0013] The same applies to nonwovens and products made thereof.
[0014] A paper of an untreated cellulose-containing fibrous
material usually loses 95% to 97% of its dry strength when
saturated with water, so that it normally cannot be used in the
moistened or wet states. This is due to the fact that the paper
and/or paper products to an extent develop a dry strength as a
result of inter-fiber hydrogen bonds. If the paper is moistened,
the water breaks up the hydrogen bonds and therefore reduces the
strength of the paper.
[0015] There are three important techniques for increasing the wet
strength of paper that have already been in use for some time.
[0016] The first technique prevents the water from reaching and
breaking up the hydrogen bonds, e.g. by applying a water-repellent
material to the fibers.
[0017] The second approach is to provide the paper with additives
or reagents that promote the formation of inter-fiber bonds during
production itself by addition into the substance.
[0018] To increase the wet strength according to the second
technique, poly(ethylene imines), polyamide epichlorohydrin resins
and urea or melamine formaldehyde condensates are for example used
as wet-strength agents. The use of such synthetic resins results in
permanent wet strength. On the other hand, however, enhanced wet
strength can also be achieved by addition of water-soluble starches
or starch derivatives. This effect is nevertheless only temporary
and decreases as soon as the starch derivative dissolves. Apart
from the aforementioned additives, modified soluble cellulose
derivatives are used as wet-strength agents. In this way, for
example, the addition of carboxymethyl cellulose is usual as an
additive besides the aforementioned polyamide epichlorohydrin
resins.
[0019] To bond cellulose fibers together according to the second
technique, thereby increasing the strength, U.S. Pat. No. 5,873,979
teaches the reaction of the cellulose's hydroxy functions with a
C2-C9 dicarboxylic acid.
[0020] The techniques for increasing the strength of paper in the
wet state as taught in the applications: WO 97/36051, WO 97/36053,
WO 97/36037, WO 97/36054 and WO 97/36052 primarily make use of the
second technique by incorporating wet strength increasing additives
to the pulp. However, WO 97/36052 modifies this technique insofar
as the short chain polysaccharide portion of pulps, typically
referred to as hemicelluloses, is oxidized to generate aldehyde
groups, which are capable of reacting with a water-soluble polymer
used as additive.
[0021] Specifically WO 97/36052 describes a paper product
exhibiting initial wet strength and comprising
[0022] (a) cellulosic fibers having free aldehyde groups
originating from cellulosic fibers that include a polysaccharide
(preferably galactose and/or mannose) in which the OH groups of
least a part of the recurrent units are OH groups in this position,
in combination with
[0023] (b) a water-soluble polymer having functional groups that
can react with the aldehyde groups.
[0024] Since cellulose exhibits OH groups in trans position, the
hemicellulose portion of pulps that have a high proportion of
hemicellulose is to be oxidized and the oxidation product used as a
"binder". Hemicelluloses are derived from (poly)saccharides with OH
groups in cis position (e.g. galactose, mannose) that can be
rapidly oxidized into aldehyde groups and which can then form
(hemi)acetal bonds in accordance with the teaching of this
document, such bonds holding the paper product together.
[0025] In example 1 of this document various pulps are oxidized
with ozone in one step under slightly alkaline or slightly acidic
conditions. Handsheets having a basis weight of 18 lb/3000 ft.sup.2
(29,3 g/m.sup.2) or 26 lb/3000 ft.sup.2 (42,3 g/m.sup.2) are
prepared from the ozonated pulps and measured with respect to their
initial wet strength, dry strength and wet strength after 30 min.
Sulfite pulp is oxidized at a low consistency (0.9 to 1.3 wt. % of
fibers) at an initial pH value of 7.88 and leads to an initial wet
strength of 30 g/inch (0.00181 N/15 mm) measured with a handsheet
of 29.3 g/m.sup.2. For handsheets with about 80 g/m.sup.2 the wet
strength value thus should be in the order of 0.003 N/15 mm. The
highest value measured (for a chemi-thermo mechanical pulp) is 450
g/inch, which corresponds to 0.027 N/15 mm.
[0026] WO 97/36052 does not teach that paper made from these
ozonated pulp displays wet and dry strength properties being
sufficient for practical use. It rather emphasizes that the
addition of a water-soluble polyhydroxy polymer, such as guar gum,
is necessary to achieve a product having desirable properties. This
document further does not disclose any washing steps prior to or
after the ozonation.
[0027] EP 0 685 593 A2 is concerned with a surface modification of
cellulose fiber, leading to paper products having a combination of
good strength in both wet and dry applications. The modification
involves the steps of:
[0028] (a) oxidizing cellulose fiber with an oxidizing agent to
form aldehydo cellulose; and
[0029] (b) sulfonating the oxidized cellulose with a sulfonation
agent to form sulfonated cellulose.
[0030] The oxidizing agent may be selected from various chemical
compounds including ozone, whereby periodate is preferred and also
used in the examples. This document does not teach that the use of
ozone per se improves wet strength of papers.
[0031] WO 00/50462 further discloses that the dry strength or wet
strength of paper can be improved by generating C6-aldehyde
functions within the cellulosic chain.
[0032] The majority of prior art techniques for improving the wet
strength of pulp or paper made therefrom is disadvantageous insofar
as it requires the use of additives, such as wet strength agents or
other low or high molecular additives, being capable of
crosslinking modified or unmodified cellulose.
[0033] Techniques for modifying the pulp per se are often
cumbersome, since they require multi-step procedures of different
chemicals or chemicals which are not readily available.
[0034] The technical object of the present application is therefore
to provide a process (use) for improving the wet strength of
cellulosic fibers material, in particular pulp, and paper or
nonwoven made therefrom, which does not require the use of further
additives, such as wet strength agents or water-soluble polyhydroxy
polymers.
[0035] It is a further object of the present invention to provide
this process on the basis of a simple and efficient chemical
treatment system.
[0036] It is a further object of the present invention to provide a
cellulosic material, in particular pulp, and paper or nonwoven made
therefrom, in particular tissue paper, which have an excellent wet
strength and further a suitable balance of other relevant
properties.
[0037] Further advantages may become apparent from the following
specification.
SUMMARY OF THE INVENTION
[0038] The present inventors have found that ozone, which is
traditionally employed as bleaching and delignification chemical in
the pulp industry, can be used under specific conditions to
increase the wet strength of pulp as well as paper or nonwoven, in
particular tissue paper, made therefrom.
[0039] The present invention thus relates to the use of a treatment
sequence comprising an ozonation under acidic conditions followed
by an acidic wash for enhancing the wet strength of a cellulosic
fibrous material, as well as the corresponding process.
[0040] Preferably the ozone treatment and the acidic wash are
followed by a second ozonation step under acidic conditions.
[0041] The present invention further concerns a fibrous cellulosic
material obtainable by this treatment sequence which preferably has
a breaking length of at least 100 m.
[0042] In the context of the present invention the phrase "wet
strength of a cellulosic material", or the corresponding
parameters, such as the breaking length refer to measurements with
paper test sheets under conditions which are explained in the
section "test methods".
[0043] The present invention thus allows increasing the wet
strength of a cellulosic fibrous material (e.g. pulp), paper or
nonwoven without the use of additives, such as wet strength agents.
This use of ozone is very simple and efficient, and leads to highly
pure products. The use of ozone as the only treatment chemical in
particular avoids the introduction of so-called "non-process
elements" (NPE) into the treatment system, for instance metal
oxides such as MgO, which are frequently used in the oxidative
treatment of pulps.
[0044] The inventive use of ozone thus leads to a high quality
cellulosic material (e.g. pulp), paper or nonwoven having wet
strength.
DETAILED DESCRIPTION OF THE INVENTION
Use of Ozone to Enhance Wet Strength and Corresponding Process
[0045] According to the use or process according to the invention a
fibrous cellulosic material is ozonated under acidic conditions
followed by an acidic wash of the ozonated material.
[0046] "Cellulosic" means that the fibrous material contains as
main component cellulose.
[0047] Cellulose is defined here, in accordance with the common
understanding in the art, as the long-chain fibrous portion
insoluble in 10% (wt. %) NaOH (R.sub.10 portion) and which is also
known in older literature as .alpha.-cellulose (to determine the
R.sub.10 value see ASTM Method D1695, Annual Book of ASTM
standards, Section 15, Vol. 15.04, American Society for Testing and
Materials, Philadelphia 1983 and "Cellulose Chemistry and its
Applications", edited by T. P. Nevell and S. H. Zeronian, Ellis
Harwood Pub., West Sussex, England 1985, p. 16 et seq.).
[0048] The cellulose portion (R.sub.10 value) is preferably at
least 50%, particularly at least 80%, relative to the total weight
of the oven-dried fibrous material (Hereinafter, the term
"oven-dried" refers to the determination of the dry content of
fibrous material/pulp samples corresponding to DIN EN 20638).
Greater preference is given to values of at least 83%, particularly
of at least 86%.
[0049] Cellulose is present in the cells, particularly of lignified
plants, in a proportion of up to 50% of the mass, whereas
hemicelluloses and lignin account for the remaining 50% of the mass
of lignified plant, depending on the particular variety in
varyingly large proportions (see Dietrich Fengel and Gerd Wegener,
Chemistry, Wood, Ultrastructure, Reactions, Walter de Gruyter
(1984)).
[0050] Measurements have indicated that the weight content of mono-
and oligosaccharides other than glucose of the fibrous material to
be used is preferably less than 20%, more preferably less than 16%,
in particular less than 13% by weight. It is assumed that that
these values also apply to the content of all hemicelluloses which
may include minor proportions of glucose-derived mono- and
oligosaccharides. It is also possible to perform the invention with
non-wood derived fibrous cellulosic material, such as cotton
linters, which contain essentially no hemicelluloses.
[0051] The fibrous cellulosic material is preferably selected from
wood based-pulps or other non-wood derived fiber sources. If
non-wood derived cellulosic material, such as cotton linters is
used as a raw material, no further pulping steps are usually
needed. Due to the morphological structure, the cellulose already
exists in an open state. The pulp may be a primary fibrous
materials (raw pulps) or a secondary fibrous materials, whereby a
secondary fibrous material is defined as a fibrous raw material
recovered from a recycling process.
[0052] In the production of paper, chemical pulp and mechanical
pulp are differentiated.
[0053] According to DIN 6730, chemical pulp is a fibrous material
obtained from plant raw materials from which most non-cellulose
components have been removed by chemical pulping without
substantial mechanical post-treatment. In case of chemical pulping
processes such as the sulfite or sulfate (Kraft) process, primarily
the lignin components and the hemi-cellulose components are
dissolved from the wood to varying degrees depending on the field
of application of the chemical pulp. The result is a fibrous
material consisting primarily of cellulose.
[0054] Mechanical pulp is the general term for fibrous materials
made of wood entirely or almost entirely by mechanical means,
optionally at increased temperatures. Mechanical pulp is subdivided
into the purely mechanical pulps (groundwood pulp and refiner
mechanical pulp) as well as mechanical pulps subjected to chemical
pretreatment: chemo-mechanical pulp (CMP), such as
chemo-thermomechanical pulp (CTMP). Synthetic cellulose-containing
fibers can also be used.
[0055] Chemical and mechanical pulp are also known by the general
designation pulp.
[0056] According to the invention preference is given to the use of
pulp from plant material, particularly wood-forming plants, more
particularly softwood-forming plants. Fibers of softwood (usually
originating from conifers, such as spruce or pine), hardwood
(usually originating from deciduous trees) or from cotton linters
can be used for example. Fibers from esparto (alfa) grass, bagasse
(cereal straw, rice straw, bamboo, hemp), kemp fibers, flax and
other woody and cellulosic fiber sources can also be used as raw
materials. The corresponding fiber source is chosen in accordance
with the desired properties of the end product in a manner known in
the art. For example, the fibers present in softwood which are
shorter than those of hardwood lend the final product a higher
stability on account of the higher diameter/length ratio. If the
softness of the product is to be promoted, which is important e.g.
for tissue paper, eucalyptus wood is particularly suitable as a
fiber source.
[0057] With regard to the softness of the products, the use of
chemical raw pulps is preferred. The chemical raw pulps suitable
according to the invention include, inter alia, sulfite pulps,
kraft pulps (sulfate process), soda pulps (cooking with sodium
hydroxide), pulps from high-pressure cooking with organic solvents
(e.g. Organosolv, Organocell, Acetosolv, Alcell) and pulps from
modified processes (e.g. ASAM, Stora or Sivola process). Among the
kraft pulps, it is possible to use those which were obtained in
continuous cooking systems (MCC (modified continuous cooking), EMCC
(extended modified continuous cooking) and ITC (isothermal
cooking)). The products of discontinuous kraft processes (e.g. RDH
(rapid displacement heating), Superbatch and Enerbatch) are also
suitable as a starting product. The sulfite processes include the
acidic sulfite/(bi)sulfite processes, (bi)sulfite process, "neutral
sulfite semi-chemical pulping" (NSSC) process and alkaline sulfite
processes such as processes in which in addition to aqueous alkali,
sulfite and/or anthraquinone in combination with organic solvents
such as methanol were used for cooking, e.g. the so-called ASAM
process (alkali sulfite anthraquinone methanol). The major
difference between the acidic and neutral or alkaline sulfite
processes is the higher degree of delignification in acidic cooking
processes (lower kappa numbers). The NSSC process provides
semi-chemical pulps which are advantageously defibered in
downstream mechanical fibrillation before they are used according
to the invention for the purpose of oxidation. The sulfite and
kraft pulps considerably differ in terms of their fibrous material
properties. The individual fiber strengths of sulfite pulps are
usually much lower than those of kraft pulps. The mean pore width
of the swollen fibers is also greater in sulfite pulps and the
density of the cell wall is lower compared to sulfate pulps, which
simultaneously means that the cell-wall volume is greater in
sulfite pulps. For this reason, there are also obvious differences
regarding water absorption and swelling behavior of the cellulosic
fibrous materials, which must also be taken into consideration when
selecting the raw material for oxidation.
[0058] According to the invention it is preferred to use among
chemical pulps, in particular pulp derived from processes using
sulfate ("Kraft pulp"), sulfite or (bi)sulfite ("sulfite pulp" or
"bisulfite pulp"), preferably under acidic conditions.
[0059] The cellulosic material to be ozonated preferably has a
residual lignin content (kappa number as determined according to
DIN 54357, August 1978) of not more than 50, 40, 30 or 20 with
increasing preference. Typically the pulps to be ozonated have
kappa values above 10, in particular above 13.
[0060] The ozonation treatment of the invention does not only lead
to an increase in wet strength, but is typically accompanied by a
decrease of the kappa number, which is preferably in the order of
at least 10, in particular at least 15.
[0061] Depending on the cellulosic material used, the final kappa
number lies below 15, 10, 5, 3, or 1,5 with increasing
preference.
[0062] It is also preferred that before and/or after ozonation,
preferably after ozonation, the chemical pulp should undergo
additional surface treatment (beating) which has a favorable effect
on the strength properties of the obtained paper or nonwoven. This
may be preferably brought about within the pulp refinement system
of a paper/tissue paper machine. In another preferred embodiment,
such surface treatment (beating) occurs as part of pulp production,
i.e. while it is still at the pulp plant. A refiner is particularly
suitable for this purpose. Fibrillation of the surface occurs
during mechanical treatment of the pulp/water suspension. This
treatment affects the static and dynamic strength properties.
Fibrillability of the fiber crucially depends on the fiber's
swelling capability. In this way, it is known that due to a low
polyuronic acid content, kraft pulps produced according to the
sulfate process are less readily beatable. Effects of beating in
accordance with e.g. the specific edge load, total energy
expenditure etc. are discussed in detail by the following authors:
Lothar Gottsching, Stofftechnologie--Mechanische Faserbehandlung;
Wochenblatt fur Papierfabrikation 23/24, (1998), 1194; M. L. Wild,
Festigkeitsentwicklung von Holz--und Deinkstoff aus
Zeitungsdruckpapier mit niedriger spezifischer Kantenbelastung;
Wochenblatt fur Papierfabrikation 23/24, (1998), 1218.
[0063] Fibrillation of the fibers during beating occurs either by
the fibers themselves or by the refiner knives. During beating, the
fibers are subjected to a variety of physical loads. Axial and
tangential shearing and compressive forces acting upon the fiber
play a particular role as regards fiber reforming. This leads to a
change in fiber morphology. In this way, the outer primary wall is
the first to be separated. The associated change in fiber
morphology can be described as follows:
[0064] a) tearing open and removing the fibrous material's outer
wall layer designated as the primary wall;
[0065] b) exposing the fibrils and fibrillation out of the wall
layers designated as S1 and S2;
[0066] c) partially shortening the total fiber unit or producing
accepts by shearing off fibrils.
[0067] The influences of the cutting angle of the ribs and grooves
attached to the beating unit in relation to the change in
characteristic of the fibrous material are described in PTS
Research Report: G. Br, Faserstoffoptimierung durch modifizierten
Mahlprozess PTS-FB 19/98, 1st edition, (1998).
[0068] Depending on the refiner's operating mode, the fibers are
shortened (cut) or are fibrillated, which includes the separation
of the outer layers of the fiber wall, this latter process
substantially increasing the surface and bonding capacity of the
fibers. The refiner operating mode that accompanies fibrillation is
therefore preferred (to simplify matters, this process step will
also be designated as beating in the following). Beating is
particularly used in the case of chemical pulps.
[0069] The "acid wash" used in the inventive treatment sequence has
been found to increase the efficiency of the ozone treatment and
the wet strength properties of the cellulosic material
obtained.
[0070] According to a preferred embodiment of the invention, the
(first) ozone treatment is preceded by a further acidic wash
("pre-wash") which also has a beneficial effect on the strength
properties of the pulp obtained after ozonation.
[0071] Further it was found that the use of at least two separate
ozonation steps further increases the strength properties of the
pulp obtained. Therefore it is advantageous if ozonation and acidic
wash are followed by a second ozonation step under acidic
conditions.
[0072] If more than one ozonation step is employed, it is also
preferred that each ozonation is followed by an acidic wash. In
this case, too, it has proven beneficial to employ an acidic wash
as first step of the entire treatment, i.e. prior to the first
ozonation step.
[0073] Moreover, it is preferred to maintain acidic conditions
after the last ozonation step of the treatment sequence. Similarly
it is advantageous to use at no point in time an alkaline wash.
According to a more preferred embodiment, the entire treatment
sequence is performed under acidic conditions.
[0074] It is also possible to perform more than one acid wash prior
to and/or after the ozonation step(s).
[0075] "Washing" means in the context of the present invention that
the cellulosic material is contacted with an acid aqueous medium,
accompanied or followed by removing this medium at least partially
from the cellulosic material. The medium can be removed by
filtration, at ambient pressure or by means of applying a vacuum to
the filtrate. To remove the medium as far as possible without
mechanically damaging the fibers, it can further be preferred to
press the pulp with devices typically used for this purpose (e.g.
screw press or (double) wire press). It is further possible to
combine washing and pressing steps in a washing press.
[0076] Each washing step can take from a few seconds to several
minutes or even hours. Typical treatment times range from 1 sec to
10 hours, preferably from 10 sec to 1 hour.
[0077] In one embodiment of the washing step, the acidic washing
medium is in motion with respect to the fibers. Preferably a
filtration device is used for this purpose. Usually shorter contact
times, e.g. from 1 sec to 5 min, more preferably from 10 sec to 1
min, provide a sufficient washing effect. It is believed that the
washing effect is primarily based on replacing the aqueous medium
(film) surrounding the fibers by the fresh washing medium (being
acidic in line with the invention). This washing mechanism is
referred to as "W.sub.R" (R=replacement) in the following. It can
be performed with filtration devices known in the art, for instance
with a Buchner funnel on a lab scale or a trommel screen (rotary
screen) on an industrial scale.
[0078] In an alternative embodiment, fibers and washing medium are
left to stand for a longer period of time or are both stirred. It
is desirable to select contact times allowing for the formation of
an equilibrium at the fiber surfaces, e.g. from 5 min to 10 hour,
preferably 10 min to 1 h, more preferably from 15 minutes to 30
min. With longer treatment times, it is believed that the aqueous
medium (film) surrounding the fibers is primarily diluted by the
fresh (acidic) washing medium. This washing mechanism will be
referred to "W.sub.D" (D=dilution) in the following. Preferably a
washing press as known in the art is used for this step.
[0079] It is also possible to combine both techniques (W.sub.R and
W.sub.D) in one washing step.
[0080] It should be noted that in both cases (W.sub.R and W.sub.D)
the combination of both effects or any other mechanism may also
contribute to the observed washing effect being beneficial for the
desired wet strength increase in the following ozonation step. It
should also be understood that the above explanations are given
without being bound to theory.
[0081] The "dilution wash" step ("W.sub.D") is preferably conducted
prior to at least one "W.sub.R" step. In the "W.sub.D" step, the
fibrous material is left for a longer period of time, for instance
5 min to 10 hours, preferably 10 min to 1 h, more preferably from
15 min to 30 min in contact with an aqueous acidic medium, e.g.
acidified water, before removing the medium, preferably by
pressing. It is possible to perform this washing/acidification step
("W.sub.D") within a wide concentration range for the acid and
consistency range for the cellulosic material. It is preferred to
adjust the consistency of the fibrous material to values from 0,01
to 10 weight %, preferably 0,1 to 5 weight %, based on the
oven-dried material.
[0082] It is preferred to perform an acid wash, be it of "W.sub.R"
and/or "W.sub.D" type, with an aqueous medium, in particular
acidified water, having a pH below 7, more preferably at most 6, in
particular at most 5. The lower pH limit is preferably at least 2,
more preferably at least 3.
[0083] According to the invention it is possible to use commercial
pulps and subject the same to an ozonation under acidic
conditions.
[0084] In preferred embodiments, typically moist pulp as obtained
from a pulping process is employed. "Pulping process" means here
the afore-mentioned chemical and/or mechanical digestion of
ligno-cellulosic material, preferably wood, most preferably the
chemical digestion of wood, in particular Kraft or (bi)sulfite
processes. Under these circumstances an "acid wash" prior to the
first ozonation step can substantially remove the "carry-over" from
the cooking liquor which has been found to contribute to strength
properties of the pulp after ozonation.
[0085] It is further preferred that prior to the ozone treatment,
the cellulosic material has not been subjected to any bleaching or
other oxidizing steps.
[0086] It is thus also preferred to use ozone as the only oxidant
in the post-pulping process.
[0087] Further it is possible to use ozone as the only oxidant
during the entire processing from the raw material, in particular
wood, to the final fibrous cellulosic material, in particular
pulp.
[0088] The advantage of using ozone as the only oxidant in the
post-pulping process, or even during the entire pulp production, is
the possibility of decreasing the amount of so-called "non-process
elements" (NPE), such as metal oxides (e.g. MgO, which is often
used with other oxidants, such as hydrogen peroxide). The term
"NPE" further covers metal impurities being present in the raw
material which are often difficult to remove during the pulping
process. If so-called NPEs are introduced or present in the
process, they tend to enrich in recycled process streams and
therefore should be reduced as far as possible.
[0089] Consequently, the inventive process (use of ozone) can lead
to very pure fibrous cellulosic materials, in particular pulps.
[0090] Having regard to the purity and the wet-strength properties
of a desirable cellulosic material, in particular pulp, it is
preferred to adjust the TOC value of the filtrate obtained from the
acid wash(es) to TOC values (total organic carbon, DIN ISO 8245
(1991)) from 10-10000 mg/ml, preferably 10-500 mg/ml by using
corresponding amounts of washing medium. Ozone is less consumed due
to side reactions, when the TOC level is reduced as much as
possible. The less ozone is consumed by dissolved organic
compounds, the more efficient the ozonation is.
[0091] For the same reason it is further desirable to adjust the
CSB values ("Chemischer Sauerstoffbedarf") of the said filtrate(s)
within the range of from 10 to 10000 mg/ml, in particular 10 to 600
mg/ml. The conductivity of the filtrate is preferably from 50 to
5000 .mu.S, in particular 500 to 1600 .mu.S.
[0092] Furthermore, it is preferred to work countercurrent, i.e. to
wash the completely ozonated fibrous material with clean(er) water
and to use preferably the resulting wash water (filtrate), if
necessary after acidification, for "an acid wash" in the sense of
the present invention. Finally, the wash water (filtrate) from the
"acid wash" after the first ozonation step is preferably used for
washing the pulp following the pulping process. It is possible in
the cases described above to use fresh water in addition to the
wash water that is led in countercurrent.
[0093] In addition, the use of ozone is favorable, since the
increase of wet strength is achieved under simple process
conditions. The inventors have also found that the use of ozone
under acidic conditions, rather than alkaline conditions, can lead
to an improved wet strength.
[0094] "Acidic conditions" means in this context that at least part
of the ozonation step(s) is performed at a pH below 7. It is
further preferred that the initial pH is also below 7. More
particularly, the pH of the entire ozonation step(s) should be
below 7.
[0095] It is preferred to use in the ozonation step(s) a pH of at
most 6, in particular at most 5. As regards the lower pH limit,
values of at least 2, more preferably at least 3 are most
suitable.
[0096] It is preferred to use anorganic (for instance sulfuric
acid) or organic acids for adjusting an acid pH value. Preferably,
the anorganic acid is a carboxylic acid having 1 to 4 carbon atoms
or dicarboxylic acid having 2 to 4 carbon atoms, in particular,
formic acid, acetic acid or oxalic acid.
[0097] The same acids can also be employed for adjusting the
aqueous washing medium (to be used in one of the washing steps
after and/or prior to the ozonation) to a suitable acidic pH.
[0098] Preferably, ozone is applied in a concentration as high as
possible. For practical reasons, a concentration of about 0.01 to
20% wt.-% ozone based on the ozone/O.sub.2-mixture prepared by the
ozone generator is typically used. It is further possible to apply
condensed ozone as a liquid phase.
[0099] The amount of ozone to be consumed by the fibers is adjusted
according to the desired increase in wet strength, whereby
typically pulps having high kappa values require higher amounts of
ozone to develop a sufficient wet strength. The weight ratio ozone
based on the oven-dried fibrous material can suitably be from 0,1
to 20 wt.-%, preferably from 0,5 to 5 wt.-%. If multi-step
ozonation processes are performed, the ozone consumption of each
step is preferably smaller, more preferably at least 30% smaller
than that of the preceding step.
[0100] It is possible to use ozone at low consistency of the
fibrous material (typically less than 3%, based on the dry weight
of the fibers), medium consistency (from 3 to less than 30%,
typically around 10%) and high consistency (at least 30%) whereby
the %-values refer to the weight proportion of the fibrous material
(oven-dried) with respect to the weight of the aqueous reaction
medium. Preferably, the consistency of the fibrous material is at
least 30%, or even at least 35%. A consistency of about 35 to 45%
has shown to give good results. Working at high consistency
requires dewatering the cellulosic material before it is contacted
with ozone. This can be done by pressing a moist fibrous material,
as it is typically obtained from the pulping process, or by
adjusting the moisture content of a dry material. Dewatering can be
achieved with any pressing equipment, i.e. with a screw press or a
(double)wire press. It is also possible to combine the acid
pre-treatment (explained below) and the pressing step by using a
washing press.
[0101] If the ozone treatment is performed at high consistencies, a
pseudo two-phase system, consisting only of fibers and gas is
generated. Thus, there are less or no problems regarding the
miscibility of the reaction gas with a liquid phase. The cell walls
and luminae of the fibers are saturated with water and only a thin
water film surrounds the fibers. In order to expose the surface of
the fibers as much as possible, a fluffing step (explained below)
further can be advantageous, though not necessary.
[0102] Typically, the oxidant is dried before it reaches the ozone
generator. In order to increase the affinity of the ozone towards
the fibers material, the oxidant/ozone mixture obtained from the
ozone generator is preferably rewetted before being introduced in
the fibrous material. Wetting of the ozone or ozone/oxygen gas
mixture can be achieved by leading the gas through water in any
kind of equipment used for gas-washing applications.
[0103] Temperatures of 0 to 120.degree. C., or even lower can be
used for the ozonation step(s), whereby a more favorable
temperature range is from 10 to 40.degree. C., in particular, 20 to
30.degree. C. It can become necessary to cool the reaction mixture,
since the reaction with ozone is exothermal, in order to keep the
temperature in the reaction vessel within the above-stated
ranges.
[0104] The reaction can be performed in any kind of vessel or
reactor or closed system, which is inert towards ozone.
[0105] It is preferred, though not necessary, to fluff the
cellulosic material before the at least one ozonation step. More
preferably each ozonation step is preceded by fluffing (in the
following designated by letter "F"). Fluffing typically leads to a
wadding-like material having no multi-fiber aggregates and a high
surface, which can be exposed to the ozone and increases its
effectiveness.
[0106] The fibrous material can be fluffed per hand by
disintegrating fiber assemblies. It is preferred to use a hammer
mill or a refiner, for instance a disk refiner. However, in
contrast to the beating treatment described above, fluffing, as a
rule, does not open and remove the outer wall layer of the fibrous
material, nor shortens or fibrillates the fibers. Therefore, this
fluffing treatment should also not alter the water retention
behavior of the fibrous material to a major extent. To achieve
this, the hammer mill or the refiner can be provided with more
coarse units acting upon the cellulosic material, for instance with
larger ribs and grooves. "Fluffing" is a process known in the art
and commercially available devices offered for this purpose can be
used in the invention. Another example of a suitable "fluffer" is
disclosed in EP 0 492 040 A1.
[0107] It is further preferred to stir or mix the fibrous material
when being in contact with ozone, in order to increase the
efficiency of the reaction. It is also possible to use down-flow
tubes or towers in which fluffed or separated pulp fibers are
treated with ozone. Any technique to improve the penetration and
permeability of the fibrous material towards the ozone gas is in
the scope of the process conditions.
[0108] According to the invention any cellulosic material is
preferably subjected to the following treatment sequences
comprising one ozonation step:
[0109] F-Z-W or W-F-Z-W,
[0110] or two ozonation steps:
[0111] F-Z-W-Z, Z-W-F-Z, F-Z-W-F-Z,
[0112] F-Z-W-Z-W, Z-W-F-Z-W, F-Z-W-F-Z-W,
[0113] W-F-Z-W-Z, W-Z-W-F-Z, W-F-Z-W-F-Z,
[0114] W-F-Z-W-Z-W, W-Z-W-F-Z-W, or W-F-Z-W-F-Z-W,
[0115] wherein "W" means acidic wash, "F" means fluffing and "Z"
ozone treatment.
[0116] From the above preferred embodiments it is seen that during
the inventive treatment, alkaline conditions are preferably
avoided.
[0117] Without being bound by theory, it is believed that the
following effects contribute to the wet strength increase, observed
for the inventive use of ozone. It would appear that the ozonation
of cellulosic fibrous material primarily leads to the formation of
carbonyl functional groups (ketone groups) resulting from the
attack at C2 and/or C3 hydrogen atoms of the anhydroglycose unit of
the cellulose chain, whereas aldehyde and carboxy functions are
generated to a lesser extent. C. Chirat and V. de la Chapelle
demonstrate in a recent publication ("Heat and Light induced
Brightness Reversion of bleached Pulps", Journal of Pulp and Paper
Science, vol. 25, No 6, June 1999, 201-205) that during ozonation
at low consistency (3.5% at room temperature) the aldehyde and
carboxy level remains almost constant (aldehyde below 20 meq/100 g
and carboxy below 60 meq/100 g cellulose), whereas the ketone level
constantly increases. It is believed that at least similar ketone
levels (above 80 meq/g, in particular above 100 meq/g cellulose, as
measured according to C. Chirat) can also be reached with the
present invention.
[0118] Presumably, it is the formation of ketone groups which
enhances the formation of inter-fiber bonds in a wet stage,
possibly by hydrogen bonds or even via (semi)ketal formation. The
formation of these bonds could thus contribute to the strength of a
cellulosic web in a wet state.
Cellulosic Material Having Wet Strength
[0119] If the present specification makes reference to the strength
properties (dry, wet) of pulp, we do not mean the mechanical
strength of the individual fibers, but rather the strength of paper
test sheets made form the pulp. More specifically, these test
sheets have a basis weight of about 80 g/m.sup.2, are formed and
conditioned in line with the procedure described in the section
"test methods", and are subjected to the measurements explained
therein.
[0120] A fibrous cellulosic material, in particular pulp treated in
line with the present invention, shows excellent wet strength
properties and preferably has a breaking length (wet) of at least
100 m, more preferably at least 200 m, in particular at least 300
m; for instance 600 m. These values were measured, in line with the
section "test methods", on test sheets having a basis weight of
about 80 g/m.sup.2 prepared from unbeaten cellulosic material
(beating degree of about 12 to 14.degree. SR, DIN-ISO 5267/1).
Higher beating degrees can further increase the wet strength. To
achieve this, a beating degree of at least 15, and preferably from
18 to 25.degree. SR can be used.
[0121] The above wet strength parameter can be reached without
adding effective amounts of wet strength enhancing additives, which
increase the initial, temporary or permanent wet strength of
modified or unmodified fibrous cellulosic material. For instance,
it is possible to attain these wet strength properties without
adding a water-soluble polymer having functional groups capable of
reacting with aldehyde groups (e.g. guar gum) as disclosed in WO
97/36052.
[0122] The inventive use of ozone does not have a major impact on
the dry-breaking length of the cellulosic fibrous material
obtained, or paper or nonwoven made therefrom. The dry-breaking
length of the cellulosic material (as measured with a 80 g/m.sup.2
test sheet under the condition described in the section "test
methods") preferably is at least 2500 m, more preferably at least
3000 m, in particular at least 4000 m.
[0123] Further, the ozone treatment does not appear to lead to a
major change in terms of tearing resistance of the cellulosic
fibrous material, or paper or nonwoven made therefrom. The tearing
resistance of the cellulosic material (as measured with a 80
g/m.sup.2 test sheet under the condition described in the section
"test methods") thus is preferably at least 300 m, in particular at
least 500 m, more preferably at least 1000 m, in particular at
least 1500 m.
[0124] The kappa number of the inventive cellulosic material
(measured according to DIN 54 357) is preferably less than 15, 10,
5, 3, 1.5 with increasing preference.
[0125] Further, it was observed that the ozone treatment of the
present invention, as a side effect, can contribute to the
brightness of the fibrous material (e.g. pulp) obtained, which is
typically at least 80% ISO and may reach values as high as at least
85% ISO, in particular at least 90% ISO.
Paper or Nonwoven
[0126] The present invention also relates to paper or nonwoven
comprising the ozone-treated cellulosic fibrous material according
to the invention, preferably in the amount of at least 50% by
weight, in particular at least 80% by weight, relative to the dry
weight of the finished product.
[0127] The paper can be a packaging paper, a graphic paper or
tissue paper. Preferably, the paper is a tissue paper.
[0128] In the following, depending on the context, the term "paper"
or "nonwoven" does not only refer to the raw material as obtained
from the paper/nonwoven machine, but also covers the corresponding
further processed products, since often there is often no strict
borderline to distinguish the same. Further, it should be
understood that the term "paper" or "nonwoven", in particular
"tissue paper", as used in the claims, extends to the corresponding
products which make use of raw paper, in particular raw tissue
paper or raw nonwoven.
[0129] The German terms "Vlies" and "Vliesstoffe" are applied to a
wide range of products which in terms of their properties are
located between the groups, paper, paperboard, and cardboard on the
one hand and the textile products on the other, and are currently
summarized under the term "nonwovens" (see ISO 9092-EN 29092). The
invention allows the application of known processes for producing
nonwovens, such as what are called air-laid and spun-laid
techniques, as well as wet-laid techniques.
[0130] Nonwovens may also be called textile-like composite
materials, which represent flexible porous fabrics that are not
produced by the classic methods of weaving warp and weft or by
looping, but by intertwining and/or by cohesive and/or adhesive
bonding of fibers which may for example be present in the form of
endless fibers or prefabricated fibers of a finite length, as
synthetic fibers produced in situ or in the form of staple fibers.
The nonwovens according to the invention may thus consist of
mixtures of synthetic fibers in the form of staple fibers and the
fibrous material according to the invention.
[0131] "Papers" are also planar materials, albeit essentially
composed of fibers of a plant origin and formed by drainage of a
fibrous-material suspension on a wire or between two continuously
revolving wires and by subsequent compression and drainage or
drying of the thus produced fibrous mat (cf. DIN 6730, May 1996).
The standard restricts the range of mass per unit area (basis
weight) for paper to a maximum of 225 g/m.sup.2.
[0132] Depending on the type of paper, the production process
comprise also a sizing and/or smoothing step, along with the
typical process steps of sheet formation, pressing, and drying
described above.
[0133] Based on the underlying compatibility of the production
processes (wet laying), "tissue" production is counted among the
paper making techniques. The production of tissue is distinguished
from paper production by its extremely low basis weight of normally
less than 40 g/m.sup.2 and its much higher tensile energy
absorption index. (In processing inventive fibrous material to
tissue paper, one generally selects a basis weight of 10 to 40
g/m.sup.2 per ply. The total basis weight of multiple-ply tissue
products is preferably equal to a maximum of 65 g/m.sup.2.) The
tensile energy absorption index is arrived at from the tensile
energy absorption in which the tensile energy absorption is related
to the test sample volume before inspection (length, width,
thickness of sample between the clamps before tensile load). Paper
and tissue paper also differ in general with regard to the modulus
of elasticity that characterizes the stress-strain properties of
these planar products as a material parameter.
[0134] A tissue's high tensile energy absorption index results from
the outer or inner creping. The former is produced by compression
of the paper web adhering to a dry cylinder as a result of the
action of a crepe doctor or in the latter instance as a result of a
difference in speed between two wires ("fabrics"). This causes the
still moist, plastically deformable paper web to be internally
broken up by compression and shearing, thereby rendering it more
stretchable under load than an uncreped paper. Most of the
functional properties typical of tissue and tissue products result
from the high tensile energy absorption index (see DIN EN 12625-4
and DIN EN 12625-5).
[0135] One example of papers and paper products is represented by
hygiene papers, particularly tissue papers and hygiene products
(tissue products) made therefrom and which are e.g. used in
personal grooming and hygiene, the household sector, industry, the
institutional field in a wide variety of cleaning processes. They
are used to absorb fluids, for decorative purposes, for packaging
or even as supporting material, as is common, for example, in
medical practices or in hospitals.
[0136] Hygiene paper primarily includes all kinds of dry-creped
tissue paper, as well as wet-creped paper and cellulose or pulp
wadding.
[0137] The one-ply intermediate products originating from the
paper-making machine and made of lightweight paper usually
dry-creped on a yankee cylinder by means of a crepe doctor are
generally described as "tissue paper" or more accurately raw tissue
paper. The one-ply raw tissue may be built up of one or a plurality
of layers respectively.
[0138] All one-ply or multi-ply final products made of raw tissue
and tailored to the end user's needs, i.e. fabricated with a wide
variety of requirements in mind, are known as "tissue
products".
[0139] Typical properties of tissue paper include the ready ability
to absorb tensile stress energy, their drapability, good
textile-like flexibility, properties which are frequently referred
to as bulk softness, a high surface softness, a high specific
volume with a perceptible thickness, as high a liquid absorbency as
possible and, depending on the application, a suitable wet and dry
strength as well as an interesting visual appearance of the outer
product surface. These properties allow tissue paper to be used for
example as cleaning wipes (paper wipe, windscreen cleaning wipe,
kitchen paper), sanitary products (e.g. toilet paper), paper
handkerchiefs, household towels, towels, cosmetic wipes (facials),
as serviettes/napkins, bed linen or garment.
[0140] If tissue paper is to be made out of the fibrous material
according to the invention, the process essentially comprises
[0141] a) forming that includes the headbox and the wire
portion,
[0142] b) the drying portion (TAD (through air drying) or
conventional drying on the yankee cylinder) that also usually
includes the crepe process essential for tissues,
[0143] c) as a rule, the monitoring and winding area.
[0144] Paper can be formed by placing the fibers, in an oriented or
random manner, on one or between two continuously revolving wires
of a paper making machine while simultaneously removing the main
quantity of water of dilution until dry-solids contents of usually
between 12 and 35% are obtained.
[0145] Drying the formed primary fibrous web occurs in one or more
steps by mechanical and thermal means until a final dry-solids
content of usually about 93 to 97%. In the case of tissue making,
this stage is followed by the crepe process which crucially
influences the properties of the finished tissue product in
conventional processes. The conventional dry crepe process involves
creping on a usually 4.5 to 6 m diameter drying cylinder, the
so-called yankee cylinder, by means of a crepe doctor with the
aforementioned final dry-solids content of the raw tissue paper
(wet creping can be used if lower demands are made of the tissue
quality). The creped, finally dry raw tissue paper (raw tissue) is
then available for further processing into the paper product or
tissue paper product according to the invention.
[0146] Instead of the conventional tissue making process described
above, the invention gives preference to the use of a modified
technique in which an improvement in specific volume is achieved by
a special kind of drying within process section b and in this way
an improvement in the bulk softness of the thus made tissue paper
is achieved. This process, which exists in a variety of subtypes,
is termed the TAD (through air drying) technique. It is
characterized by the fact that the "primary" fibrous web (like a
nonwoven) that leaves the sheet making stage is pre-dried to a
dry-solids content of about 80% before final contact drying on the
yankee cylinder by blowing hot air through the fibrous web. The
fibrous web is supported by an air-permeable wire or belt and
during its transport is guided over the surface of an air-permeable
rotating cylinder drum. Structuring the supporting wire or belt
makes it possible to produce any pattern of compressed zones broken
up by deformation in the moist state, resulting in increased mean
specific volumes and consequently leading to an increase in bulk
softness without decisively decreasing the strength of the fibrous
web.
[0147] Another possible influence on the softness and strength of
the raw tissue lies in the production of a layering in which the
primary fibrous web to be formed is built up by a specially
constructed headbox in the form of physically different layers of
fibrous material, these layers being jointly supplied as a pulp
strand to the sheet making stage.
[0148] When processing the raw fibrous web or raw tissue paper into
the final product, the following procedural steps are normally used
individually or in combination: cutting to size (longitudinally
and/or cross cutting), producing a plurality of plies, producing
mechanical ply adhesion, volumetric and structural embossing,
chemical ply adhesion, folding, imprinting, perforating,
application of lotions, smoothing, stacking, rolling up.
[0149] To produce multi-ply tissue paper products, such as
handkerchiefs, toilet paper, towels or kitchen paper, an
intermediate step preferably occurs with so-called doubling in
which the raw tissue in the finished product's desired number of
plies is usually gathered on a common multiply master roll.
[0150] The processing step from the raw tissue that has already
been optionally wound up in several plies to the finished tissue
product occurs in processing machines which include operations such
as repeated smoothing of the tissue, edge embossing, to an extent
combined with full area and/or local application of adhesive to
produce ply adhesion of the individual plies (raw tissue) to be
combined together, as well as longitudinal cut, folding, cross cut,
placement and bringing together a plurality of individual tissues
and their packaging as well as bringing them together to form
larger surrounding packaging or bundles. The individual paper ply
webs can also be pre-embossed and then combined in a roll gap
according to the foot-to-foot or nested methods.
EXAMPLES
Test Methods
[0151] The following test methods were used to evaluate the
ozonated fibrous materials according to the invention as compared
to fibrous materials which correspond, but which have not been
modified by ozonation.
[0152] 1) Producing the Test Sheets
[0153] The test sheets (having a basis weight of approx. 80
g/m.sup.2) were made in accordance with the Rapid Kothen method
(DIN 54 358-1, February 1981; see also ISO 5269-2: 1980). Before
being tested in terms of physical properties e.g. by means of the
tensile test, the thus obtained test sheets were always conditioned
for a duration of at least 12 hours in a standard climate at a
temperature of (23.+-.1) .degree. C. and a relative humidity of
(50.+-.2) % in accordance with DIN EN 20187, Paper, Cardboard and
Pulp, a standard climate for pretreatment and testing and a method
of monitoring the climate and pretreatment of samples, November
1993 (see ISO 187: 1990).
[0154] 2) Initial Wet Strength (Width-Related Breaking Strength
(Wet)) and Breaking Length (Wet)
[0155] The wet strength according to DIN ISO 3781, Paper and
Cardboard, tensile test, determination of the width-related
breaking strength after immersion in water, October 1994 (identical
to ISO 3781: 1983), is herewith defined as initial wet strength of
the fibrous material networks according to the invention, e.g.
paper/tissue paper/nonwoven.
[0156] When experimentally checking the invention, the tensile test
was accordingly performed by means of an electronic tensile test
apparatus (Model 1122, Instron Corp., Canton, Mass., USA) with a
constant rate of elongation of 10 mm/min using a Finch device. The
width of the test strips was 15 mm. The strip length was about 180
mm. The free clamping length when using the Finch clamp was about
80 mm. The test strip was secured with both ends in a clamp of the
test apparatus. The other end (loop) formed in this way was placed
around a pin and treated at 20.degree. C. with distilled water
until complete saturation. The soaking period of the samples before
tensile testing was fixed at 30 s. Six test strips at a time were
measured, the result being indicated as an arithmetic mean.
[0157] To ensure that the wet strength of the samples has fully
developed, the samples to be tested were always artificially aged
before conducting the tensile test. Aging was effected by heating
the samples in an air-circulating drying cabinet to (125.+-.1)
.degree. C. for a period of 10 min.
[0158] A similar approach applies to paper/tissue paper/nonwoven
products, modified only to the extent that the test strips to be
examined were taken from the finished product itself or from the
product made thereof and that they do not originate from a
laboratory test sheet.
[0159] As regards tissue paper and tissue products, DIN ISO 3781 is
replaced by DIN EN 12625-5 Tissue Paper and Tissue Products--Part
5: determination of width-related wet load at break, January 1999.
The strip width is then 50 mm, the free clamping length is
shortened to about 50 mm, the depth of immersion of the loop formed
by the test strip is at least 20 mm. The soaking duration
(immersion time) is shortened to 15 s, the rate of elongation is
set to a constant (50.+-.2) mm/min, the measurement of the breaking
strength is performed on the sample immersed in distilled
water.
[0160] Six test strips at a time were measured, the result being
indicated as an arithmetic mean.
[0161] The breaking length (wet) was calculated from the
width-related breaking strength in accordance with the following
formula (see TAPPI 494-96, Comment 9):
RL=102000(T/R)
[0162] where T is the initial wet strength in kN/m and
[0163] R is the basis weight in g/m.sup.2 (in a standard
climate)
[0164] 3) Dry Strength (Width-Related Breaking Strength (Dry)) and
Breaking Length (Dry)
[0165] The dry strength was determined according to DIN EN ISO
1924-2, Paper and Cardboard, determination of properties under
tensile load. Part 2: Method at a constant rate of elongation,
April 1995, (ISO 1924-2: 1994).
[0166] In the case of tissue paper and tissue products, the test is
performed in accordance with DIN EN 12625-4, Tissue Paper and
Tissue Products--Part 4: Determination of width-related breaking
strength, elongation at break and tensile energy absorption,
January 1999.
[0167] The breaking length (dry) was calculated from the
width-related breaking strength in accordance with the following
formula (see TAPPI 494-96, Comment 9):
RL=102000(T/R)
[0168] where T is the tensile strength in kN/m and
[0169] R is the basis weight in g/m.sup.2(in a standard
climate)
[0170] 4) Relative Wet Strength
[0171] The relative wet strength (WS) was calculated as
follows:
rel. WS=BS.sub.wet/BS.sub.dry
[0172] where BS.sub.wet is the width-related breaking strength of
the wet sample strip and BS.sub.dry is the width-related breaking
strength of the dry sample strip, and these values were ascertained
in the manner described above.
[0173] 5) Kappa Number
[0174] The kappa number is determined according to DIN 54357
(August 1978; "Examination of pulp, determination of the kappa
number")
[0175] 6) Brightness
[0176] The degree of brightness (in percent) was determined
according to ISO following scan S11 1975.
[0177] 7) Air Permeability
[0178] The air permeability was determined according to Bendtsen
(Zellcheming-Merkblatt V26/75).
[0179] 9) Tearing Resistance (Dry-Tear Strength) According to
Elmendorff
[0180] The tearing resistance was determined according to
Elmendorff using a test sheet according to the above item 1
following a process described in DIN 53128.
[0181] 10) Bursting Strength (kPa)
[0182] The bursting strength was determined according to
"Zellcheming-Merkblatt V12/57".
[0183] 11) Beating Degree
[0184] The beating degree was measured by means of the freeness
value (in .degree. SR) according to DIN-ISO 5267/1; March 1999.
[0185] 12) Dry Content
[0186] The wording "oven-dried" refers to the determination of the
dry content of fibrous material/pulp samples corresponding to DIN
EN 20638.
[0187] In the examples, %-values always refer to the weight, if not
indicated otherwise.
EXAMPLE 1
[0188] Beech sulfite pulp stemming from an acid sulfite pulping
process and having a brightness (% ISO) of 61.0 and a kappa number
of 13.9 was subjected to the following treatment steps.
[0189] a) After thoroughly washing the pulp with water, it was
treated over approximately 20 minutes with an acid aqueous
solution, which was adjusted to pH 3 with sulfuric acid, at a
consistency of 0.5 to 3% (typically 1 to 1.5%), based on the
oven-dried material. The slurry obtained thereby was filtrated over
a Buchner funnel and then further dewatered in a centrifuge to a
consistency of about 40%.
[0190] b) The pulp obtained from the acid wash was disintegrated
per hand into individual fibers.
[0191] c) A laboratory rotary evaporator was charged with about 30
g disintegrated pulp having a pH of 3 and a consistency of 40%.
This vessel was cooled in order to keep the temperature at
approximately room temperature during the reaction. A re-wetted
ozone/O.sub.2-mixture was introduced in the rotating vessel by
means of a glass frit being submerged into the pulp dispersion. The
reaction was stopped after a consumption of 1.0% ozone, based on
the weight of the pulp sample. The consumption was measured by
introducing small samples of the ozone/O.sub.2-mixture into an
aqueous KJ-solution having a predetermined content of KJ and
calculating the ozone concentration in the mixture from a
titrimetric measurement of J.sub.2 formed. Based on the assumption
that ozone immediately and completely reacts with pulp at suitable
low flow rates, the reaction time could be calculated from the
ozone concentration and the flow rate of the ozone/O.sub.2
mixture.
[0192] d) After completion of the reaction, the pulp was washed
again with a sulfuric acid solution (pH 3) over approximately 20
minutes at a consistency of 0.5 to 3% (typically 1 to 1.5%), based
on the oven-dried material. The slurry obtained thereby was
filtrated over a Buchner funnel and then further dewatered in a
centrifuge to a consistency of about 40%.
[0193] Then handsheets were produced from the ozonated and washed
pulp in the manner described above.
[0194] The breaking length (dry), breaking length (wet), air
permeability and tearing resistance (dry) of the handsheets were
determined in the manner explained above.
[0195] Further, the brightness and kappa number of the
ozone-treated pulp was measured in accordance with the section
"test methods."
[0196] The results obtained are shown in the following Table 1.
1TABLE 1 Beech Sulfite Pulp Properties untreated after
O.sub.3-treatment Pulp (unbeaten) Brightness, % ISO 61, 0 94, 5
Kappa number 13, 9 1, 0 Handsheet Breaking length (dry), m 2700
2900 Breaking length (wet), m, (rel. %) 70 (2, 7) 117 (4, 0) Air
permeability, ml/min 2800 2800 Tearing resistance (dry), mN
.multidot. m/m 300 400 O.sub.3-treatment carried out at room
temperature, pH 3, 1.0% O.sub.3.
EXAMPLE 2
[0197] Spruce sulfite pulp stemming from an acid sulfite pulping
process and having a brightness (% ISO) of 67.1 and a kappa number
of 18.1 was subjected to the same treatment steps (a)-(d) as in
Example 1, before handsheets were produced therefrom in the manner
described above.
[0198] The breaking length (dry), breaking length (wet), air
permeability and tearing resistance (dry) of the handsheets were
determined in the manner explained above.
[0199] Further, the brightness and kappa number of the
ozone-treated pulp was measured in accordance with the section
"test methods".
[0200] The results obtained are shown in the following Table 2.
2TABLE 2 Spruce Sulfite Pulp (61A_00) Properties untreated after
O.sub.3-treatment Pulp (unbeaten) Brightness, % ISO 67, 1 92, 1
Kappa number 18, 1 1, 0 Handsheet Breaking length (dry), m 3500
3200 Breaking length (wet),m, (rel. %) 89 (2, 6) 224 (7, 2) Air
permeability, ml/min 2200 2100 Tearing resistance (dry), mN
.multidot. m/m 1300 1200 O.sub.3-treatment carried out at room
temperature, pH 3, 1.0 % O.sub.3.
EXAMPLE 3
[0201] Spruce sulfite pulp stemming from an acid sulfite pulping
process and having a brightness (% ISO) of 64.0 and a kappa number
of 19.1 was subjected to the treatment steps a-d) as explained in
Example 1.
[0202] e) The pulp obtained from the acid wash was disintegrated
again under the same conditions given above under item (b).
[0203] f) The fluffed (disintegrated) pulp was finally subjected to
a second ozonation step using the same condition as under (c) with
the sole difference that the ozonation was stopped after an ozone
consumption of 0.4%, based on the weight of the oven-dried pulp
sample (total consumption 1,4% by weight).
[0204] g) After completion of the reaction, the pulp was washed
again with a sulfuric acid solution (pH 3) as explained under item
d) of Example 1, before handsheets were produced therefrom in the
manner described above.
[0205] The breaking length (dry), breaking length (wet), air
permeability and tearing resistance (dry) of the handsheets were
determined in the manner explained above.
[0206] Further, the brightness and kappa number of the
ozone-treated pulp was measured in accordance with the section
"test methods".
[0207] The results obtained are shown in the following Table 3.
3TABLE 3 Spruce Sulfite Pulp (02A_01) Properties untreated after
O.sub.3-treatment Pulp (unbeaten) Beating degree, .degree. SR 14 15
Brightness, % ISO 64, 0 89, 0 Kappa number 19, 1 1, 3 Fiber length
l.w., mm 2, 61 2, 54 Handsheet Breaking length (dry), m 4300 4200
Breaking length (wet),m, (rel.%) 98 (2, 3) 314 (7, 5) Air
permeability, ml/min 1850 1650 Bursting strength, kPa 193 237
Tearing resistance (dry), mN .multidot. m/m 1400 1500 2-step
O.sub.3-treatment carried out at room temperature, pH 3, 1, 4 %
O.sub.3 and intermediate acid washing.
[0208] From the above Table 3, it is seen that a 2-step ozone
treatment with an intermediate acid wash considerably improves the
wet-strength properties of paper produced with the ozonated
pulp.
* * * * *