U.S. patent application number 10/025603 was filed with the patent office on 2002-09-19 for fully bleached sulfite chemical pulp, a process for the production thereof and products derived therefrom.
This patent application is currently assigned to SCA Hygiene Products Gmbh. Invention is credited to Geisenheiner, Andreas, Kordsachia, Othar, Patt, Rudolf, Reinhard, Armin.
Application Number | 20020129912 10/025603 |
Document ID | / |
Family ID | 26699958 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020129912 |
Kind Code |
A1 |
Reinhard, Armin ; et
al. |
September 19, 2002 |
Fully bleached sulfite chemical pulp, a process for the production
thereof and products derived therefrom
Abstract
A process for preparing a bleached sulfite chemical pulp
includes delignification of chips of a lignocellulosic material in
a sulfite pulping process until the defibration point of the
material is reached, and bleaching of the fibrous material thus
obtained by a chlorine-free bleaching sequence which includes at
least one first bleaching step with a chlorine-free oxidant in the
presence of a base. The chemical pulp prepared according to this
process and paper or nonwoven made from such pulp have a high
degree of brightness (at least 83% ISO) and high strength.
Inventors: |
Reinhard, Armin;
(Schriesheim, DE) ; Geisenheiner, Andreas;
(Mannheim, DE) ; Kordsachia, Othar; (Oststeinbek,
DE) ; Patt, Rudolf; (Reinbek, DE) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Assignee: |
SCA Hygiene Products Gmbh
Mannheim
DE
|
Family ID: |
26699958 |
Appl. No.: |
10/025603 |
Filed: |
December 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60257162 |
Dec 22, 2000 |
|
|
|
Current U.S.
Class: |
162/78 ; 162/76;
162/82; 162/90 |
Current CPC
Class: |
D21C 9/1042 20130101;
D21C 3/10 20130101; D21C 3/06 20130101; D21C 9/163 20130101 |
Class at
Publication: |
162/78 ; 162/76;
162/82; 162/90 |
International
Class: |
D21C 003/04; D21C
009/16; D21C 003/06; D21C 003/02 |
Claims
1. A process for preparing bleached sulfite chemical pulp,
comprising the following steps: delignifying chips of a
lignocellulosic material in a sulfite pulping process until a
defibration point of said lignocellulosic material is reached, and
a fibrous material is obtained; and bleaching the fibrous material
with a bleaching sequence which is performed exclusively with
chlorine-free reagents and comprises at least a first bleaching
step with a chlorine-free oxidant in the presence of a base.
2. The process according to claim 1, wherein the fibrous material
has a kappa value ranging between 50 and 75.
3. The process according to claim 1, wherein the sulfite pulping is
conducted under acidic conditions.
4. The process according to claim 1, wherein the sulfite pulping is
conducted in the presence of magnesium in an acidic
environment.
5. The process according to claim 1, wherein the sulfite pulping is
conducted with an aqueous solution containing magnesium bisulfite,
optionally an excess of SO.sub.2, at a temperature of 130.degree.
to 165.degree. C. and at a pH of 1.5 to 4.
6. The process according to claim 1, wherein the oxidant is
hydrogen peroxide.
7. The process according to claim 1, wherein the reaction sequence
for chlorine-free bleaching of pulp comprises at least one
additional bleaching step with a chlorine-free oxidant in the
presence of a base.
8. The process according to claim 7, wherein the oxidant used in
the additional bleaching step is hydrogen peroxide or peractic
acid.
9. The process according to claim 1, wherein prior to treatment
with an oxidant, treatment with a metal ion complexing agent is
conducted.
10. The process according to claim 1, wherein the reaction sequence
for chlorine-free bleaching of pulp comprises a final bleaching
step with a reducing bleaching agent.
11. The process according to claim 10, wherein the reducing
bleaching agent is selected from the group consisting of a
water-soluble dithionite salt, hydroxylamine, thio urea,
thioglycolic acid, borohydride hydride, and formamidinosulfinic
acid.
12. A chemical pulp, obtainable by a sulfite pulping process and
subsequent chlorine-free bleaching, wherein said chemical pulp has
a degree of brightness of at least 83% ISO and a test sheet
produced from said chemical pulp without beating has a strength,
expressed as breaking length (measured according to Zellcheming
V/12/57), of at least 6 km.
13. The chemical pulp according to claim 12, wherein said chemical
pulp has a kappa number of between 10 and 30.
14. The chemical pulp according to claim 12, wherein said chemical
pulp is subjected to a beating step following bleaching.
15. A paper or nonwoven prepared from a chemical pulp according to
claim 12.
16. A paper according to claim 15 which is tissue paper.
17. A product prepared from nonwoven or paper according to claim
15.
18. The process according to claim 6, wherein the reaction sequence
for chlorine-free bleaching of pulp comprises at least one
additional bleaching step with a chlorine-free oxidant in the
presence of a base.
19. The process according to claim 18, wherein the oxidant used in
the additional bleaching step is hydrogen peroxide or peractic
acid.
20. The process according to claim 6, wherein prior to treatment
with an oxidant, treatment with a metal ion complexing agent is
conducted.
21. The process according to claim 6, wherein the reaction sequence
for chlorine-free bleaching of pulp comprises a final bleaching
step with a reducing bleaching agent.
22. The process according to claim 10, wherein the reducing
bleaching agent is selected from the group consisting of a
water-soluble dithionite salt, hydroxylamine, thio urea,
thioglycolic acid, borohydride hydride, and formamidinosulfinic
acid.
Description
[0001] The present invention relates to a completely bleached
sulfite chemical pulp, a process for the production thereof and
products made of this pulp (nonwovens or paper, especially tissue
paper).
BACKGROUND ART
[0002] In the production of paper, we differentiate between
chemical pulp and mechanical pulp.
[0003] 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.
[0004] 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).
[0005] Chemical and mechanical pulp are also known by the general
designation pulp.
[0006] The strength of papers made from chemical pulps (hereinafter
abbreviated as "strength of the chemical pulp") is substantially
determined by the pulping degree of the wood used as starting
material. The binding ability of the fibers first increases as more
and more lignin is removed and reaches a maximum at a lignin
content of about 10%. At this residual lignin content, the fibrous
material has good stiffness and resistance to tearing along with a
high binding ability. As delignification proceeds, the overall
strength of the chemical pulp decreases due to the strong attack of
most of the pulping chemicals on the carbohydrate portion
(cellulose and hemicellulose) of the wood.
[0007] However, such extensive delignification is accompanied by
losses in yield so that, for many years, there have been efforts to
develop pulps having higher contents of residual lignin:
[0008] high-yield chemical pulps, according to DIN 6730 a chemical
pulp primarily pulped chemically, but subsequently subjected to
mechanical defibration;
[0009] semi-chemical pulp, a pulp according to DIN 6730 obtained
from plant raw materials by chemical pulping from which only part
of the non-fibrous components have been removed and which is
subjected to mechanical post treatment; and
[0010] chemi-mechanical pulps (CMP, CTMP, as defined above).
[0011] In the meantime, the recovery of primary fibers has focused,
on the one hand, on the production of mechanical pulps in very high
yields by addition of 1% to 5% maximum of sodium sulfite such that
hydrophilizing occurs by introducing sulfonic acid groups in
conjunction with a low dissolving rate of the wood components, and
on the other hand, on the production of full chemical pulps
according to the sulfate process.
[0012] If chemical pulp recovered by the sulfate process (called
"Kraft pulp" or "sulfate chemical pulp") is to be used for
producing products with a high degree of brightness, the lignin
must be removed completely, since even small amounts of residual
lignin are discolored due to chemical changes in the lignin
structure (primary condensation). This is largely carried out in a
pulping process. Any residual lignin still present is then removed
in several bleaching steps.
[0013] The native lignin is less discolored by the acidic sulfite
process. For this reason, it was possible to use high-yield
chemical pulps made by acidic or semi-acidic sulfite processes in
unbleached form for special applications. If a higher degree of
brightness was required, such chemical pulps were also bleached in
a delignifying manner with chlorine and bleaching agents containing
chlorine. However, these products were unable to satisfy the
ever-increasing optical demands. In addition, environmental
concerns about bleaching agents containing chlorine and chemical
pulps containing chlorine led to a situation where industrial-level
production of high-yield chemical pulps was not pursued.
Furthermore, less expensive recycling paper has increasingly been
used in paper production.
[0014] However, delignifying bleaching of chemical pulps often has
the disadvantage that chemicals used for bleaching also attack
cellulose fibers to a considerable extent, thus reducing the
strength of chemical pulps. Therefore up until now, it has been
thought that pulps of sufficient strength and of a high degree of
brightness could only be obtained by pulping to low kappa values
followed by bleaching. This also applies to the sulfite pulping
process.
[0015] A summary of the development of acidic sulfite pulping
processes for preparing semi-chemical pulps is found in R. Runkel
and K. F. Patt, "Halbzellstoffe" (Semichemical Pulps),
Gunther-Stalb Verlag, Biberach 1958, pages 35 - 37 and pages 95 -
96. The production of high-yield chemical pulps, semichemical pulps
and chemi-mechanical pulps according to the sulfite process is also
described in "S. A. Rydholm, Pulping Processes, Interscience
Publishers, New York, London, Sydney, 1965, pages 418 - 420".
[0016] In addition, G. Jayme, L. Broschinksi, W. Matzke (in Das
Papier 18, 1964, pages 308 through 314) present a general survey of
high-yield chemical pulps and give a detailed description of rapid
pulping in the vapor phase with magnesium bisulfite at a maximum
temperature of 180.degree. C. over a period of 8 to 20 minutes.
[0017] DE-A-1- 517 219 relates to the preparation of a (high-yield)
sulfite chemical pulp. Wood raw material is pulped with an aqueous
solution containing sulfite and/or bisulfite ions as well as
sodium, potassium, magnesium, or ammonium ions. The pH of the
solutions at onset of pulping is 3.0 to 7.0, preferably 3.7 to 5.0.
The maximum pulping temperature is 140.degree. to 190.degree. C.,
preferably 150.degree. to 170.degree. C. The entire pulping process
takes more than 400 minutes. The residence time at the maximum
temperature is 30 to 200 minutes. Pulping is carried out at a
chlorine number of the finished chemical pulp in the range of 15 to
32, the pulped material then being subjected to controlled
defibration and/or defibration/refinement. After that, fines are
removed in an amount of 0.2 to 7 % of the amount of the chemical
pulp.
[0018] In unbeaten form (freeness value .degree.SR =14.5 -15) the
material thus obtained has a breaking length of 6.3 km. The
chemical pulp is not bleached.
[0019] U.S. Pat. Nos.4,634,499 and 4,734,162 each relate to
processes for preparing a chemical pulp from hardwood which is
especially suitable for the preparation of tissue papers. Pulping
is carried out with ammonium sulfite, first at less than
110.degree. C., then at a maximum temperature of 140.degree. C. to
155.degree. C. at a pH of about 2 to 3. The chemical pulp is not
subjected to an additional bleaching step.
[0020] EP 0 287 960 A relates to a process for preparing a
hemicellulose hydrolysate and a special chemical pulp by a two-step
process, wherein
[0021] a first step comprises pre-hydrolysis of the
ligno-cellulosic materials, for example, with water, a mineral
acid, sulfur dioxide, sulfite pulping liquor, and sulfite waste
liquor, at a temperature of 100.degree. to 180.degree. C. and over
a hydrolysis period of 10 to 200 minutes, and
[0022] a second step in which the lignin contained in the
pre-hydrolysed material is dissolved occurs by means of neutral
sulfite pulping with addition of anthraquinone as the catalyst, the
initial pH being at least 10. The temperature is preferably
160.degree. to 180.degree. C. and the treatment time 100 to 200
minutes.
[0023] Following the second step, the resulting ligno-cellulosic
material is mechanically defibrated and optionally bleached.
[0024] Yields obtained in this manner range from 37.0 % to 45.7 %
based on the wood used, the kappa number ranges from 17.2 to 48. 1,
and the degree of brightness (ISO value) at 48.2 to 87.1.
[0025] GB-1,546,877 B relates to a CTMP (chemo-thermomechanical
pulp) which is suitable for absorbant products such as tissue
paper, and, additionally, has a light color. Yields at 85 % to 95 %
are well above the common values for high yield pulps. The
production process comprises the following steps:
[0026] washing of a ligno-cellulosic material with water
[0027] impregnation and pulping with a liquid containing sulfur
dioxide at a temperature of 100.degree. to 170.degree. C.,
preferably at a pH of 5 to 11.
[0028] partial defibration of the resulting material by mechanical
means with simultaneous bleaching.
[0029] Suitable bleaching chemicals are alkali metal peroxides such
as sodium peroxide or hydrogen peroxide, or reducing bleaching
agents such as dithionite, hydroxyl amine, thiourea, or
thioglycolic acid.
[0030] An essential feature of the resulting chemical pulp is a
content of at least 10 % by weight of sulfonated fiber bundles,
each consisting of two to four individual fibers.
[0031] This British patent does not address the strength properties
of the chemical pulp obtained. The description of the patent
further points to non-satisfactory degrees of brightness of the
resulting chemical pulp.
[0032] EP 0 446 110 A describes a process for bleaching chemical
pulps (yield ranges approx. 85 to 90 %), which are obtained
mechanically (mechanical pulp), optionally combined with chemical
methods (chemomechanical pulp) and/or thermal methods. This
bleaching process comprises the following steps:
[0033] pretreatment of (mechanical or chemomechanical) pulp with a
complexing agent for metal ions followed by a washing step
[0034] treatment with sulfite and a more electronegative reducing
agent at a pH between 7 and 12.5 followed by a washing step
[0035] bleaching treatment with hydrogen peroxide in an alkaline
environment.
[0036] According to example 1, this process produces degrees of
brightness of 83.9% ISO.
[0037] However, all processes known from the prior art for
preparing pulps having a high degree of brightness have
disadvantages. In particular, insufficient resistance against fiber
collapse and poor strength of individual fibers is observed in the
pulp obtained. At low degrees of beating, known pulps often have
strength values insufficient for preparation of tissue
products.
[0038] Bleached mechanical pulps and bleached chemomechanical pulps
have the further disadvantage of an unstable degree of brightness.
This is due to the fact that the bleaching chemicals convert
chromophoric groups of the lignin into non-chromophoric groups,
which however are unstable. This type of reduction in the degree of
brightness (discoloration) may be induced either by light or
heat.
[0039] A disadvantage of known chemical pulps is their chlorine
content and/or formation within the bleach of soluble reaction
products containing chlorine, which is environmentally
undesirable.
[0040] Therefore, it is the object of the present invention to make
available a pulp that has further properties in addition to a high
and durable optical quality, making it particularly suitable for
the production of nonwovens and paper (products), in particular
tissue products.
[0041] A further aspect of the invention is to make available a
process for producing such pulp.
[0042] Finally, a further object of the present invention is to
make available pulp based paper or nonwovens and/or products which
are characterized by both resistance to mechanical stress and high
quality of appearance.
[0043] It is a further object of the invention to make available an
environmentally compatible process for producing environmentally
compatible pulp and derivative paper and/or nonwoven products.
SUMMARY OF THE INVENTION
[0044] These objects are achieved by:
[0045] a process for producing a chlorine-free bleached sulfite
chemical pulp comprising the following steps:
[0046] delignification of chips (commonly woodchips) from
ligncellulosic material by a sulfite pulping process, in
particular, in the presence of magnesium until the defibration
point has been attained, and
[0047] bleaching of the resulting fibrous material in a
chlorine-free bleaching sequence comprising at least one bleaching
step using an oxidizing agent in the presence of a base;
[0048] chlorine-free bleached chemical pulp obtainable by a sulfite
pulping process followed by chlorine-free bleaching, characterized
in that it has an ISO degree of brightness of at least 83 % and a
strength, expressed as breaking length, of at least 6 km (measured
according to Zellcheming V/12/57), determined with a test sheet
(ISO 5269-2, August 1998) produced from the pulp without beating
the same, and
[0049] paper and nonwovens obtainable from this pulp.
DETAILED DESCRIPTION OF THE INVENTION
Process
[0050] In the first step of the process according to the invention,
chips (commonly woodchips) from ligno-cellulosic material are
partially delignified in a sulfite pulping process, in particular
in the presence of magnesium, until the point of defibration of the
material is attained.
[0051] The point of defibration is the point in time during the
pulping at which the fibers subjected to chemical delignification
may readily be separated into individual fibers without mechanical
defibration. Once the point of defibration has been reached, the
individual fibers are collected generally by pumping off the pulped
lignocellulosic material out of the cooker. The kappa value of the
resulting pulp depends on the type of wood, the degree of pulping,
as well as on the chemical system that was applied. Preferably, the
partially delignified fibrous material obtained from the first step
has a kappa value (according to DIN 54357, August 1978) of 50 to
75, in particular 60 to 70. This is the case, in particular, for
spruce or pine.
[0052] One advantage of the sulfite process used according to the
invention is that the point of defibration is obtained at
considerably higher residual lignin contents and yields than in the
sulfate pulping process. This is due to the fact that in the
initial pulping phase, sulfite solutions prefer to dissolve lignin
in the middle lamella of the wood cells, and thereby the bonding
between the wood cells is lost. This results in a fiber surface of
relatively low residual lignin content. The largest part of the yet
remaining lignin is located in the central layer of the cell wall
(S2) (on the structure of a wood cell, see e.g. FIG. 20.2, page 509
in T. P. Nevell and S. Haig Zeronian, Cellulose Chemistry and Its
Applications, John Wiley & Sons). In contrast to the residual
lignin content of sulfate pulp, the residual lignin content of the
pulp resulting from the sulfite process, in particular the acidic
sulfite process has not only relatively few chromophoric groups,
but it is also less condensed and therefore more reactive.
[0053] The term "ligno-cellulosic material" includes all materials
containing cellulose and lignin as main components, typically wood.
In (dry) wood, the lignin content is generally at least 20 wt.-%
(hardwood about 22 wt. %, conifers 27 - 33 wt.-%). Usually, the
cellulose content of wood is at least 40 wt.-%, generally 40 to 50
wt.-%. Both softwood from conifers and hardwood from deciduous
trees may be used in the process of the invention. Examples for
suitable kinds of wood comprise spruce, pine, aspen, beech, birch,
maple, poplar, and oak. In addition, eucalyptus is especially
suitable as a source of fibers for tissue papers. The use of spruce
and beech is especially preferred.
[0054] Prior to pulping, the ligno-cellulosic material used is
roughly chopped into chips. The size of the chips may vary and
ranges for example from 1 to 5 cm in breadth and length, with a
thickness of up to 1 cm.
[0055] Pulping (first step) is carried out with an acidic sulfite
solution in a known manner, e.g. with an aqueous solution
containing sulfite and/or bisulfite ions as well as sodium,
potassium, magnesium, or ammonium ions. Suitable methods are
described in depth in G. A. Smook, M. J. Kocurek, Handbook for Pulp
and Paper Technologists, Tappi, Atlanta, 1982, pages 58 through 65.
Preferably, the chemical pulping occurs in a single step, i.e.
without sudden (as contrast to gradual) changes in conditions of
the process such as pH. The entire pulping process does not require
any additional mechanical means.
[0056] Chips of lignocellulosic material may be pulped in alkaline,
neutral, or acidic conditions, however, preferably in an acidic
environment with sulfite. Pulping at a pH of 1 to 5, especially 1.2
to 4.0 is particularly preferred.
[0057] In a particularly preferred embodiment, pulping is performed
with an aqueous solution containing sulfite and/or bisulfite ions
as well as magnesium ions. One advantage of this variant is the
possibility of recovering MgO and SO.sub.2 from the pulping waste
liquors by thermal decomposition and recycling them to the pulping
process. The pulping temperature preferably ranges from 130.degree.
to 165.degree. C., in particular 135.degree. to 150.degree. C.
Until a maximum temperature of 130.degree. to 150.degree. C. is
attained, heating should preferably be carried out over a period of
30 to 120 minutes.
[0058] Then, the temperature is maintained at the maximum
temperature preferably for a time period of 120 to 300 minutes. A
time period of 30 to 60 minutes is preferred for cooling from the
maximum temperature (T.sub.max) to room temperature.
[0059] Total pulping time (heating +pulping at T.sub.max+cooling)
ranges preferably from 180 to 480 minutes.
[0060] The SO.sub.2 content of the pulping solution relative to the
dry weight (see Examples for its determination) of the
lignocellulosic material used preferably equals 5 to 30 % by
weight, in particular 15 to 24 % by weight. The proportion of
magnesium if present, relative to the dry weight of the
lignocellulosic material, expressed as MgO, preferably equals 4 to
10 % by weight, in particular 6 to 7.5 % by weight.
[0061] According to the invention, addition of further pulping
chemicals such as, for example, anthraquinone is not required, but
may be used in alkaline sulfite pulping processes.
[0062] The lignocellulosic material-to-liquor ratio preferably
ranges from 1:3 to 1:5, especially from 1:3.5 to 1:3.7, relative to
the dry weight of the lignocellulosic material.
[0063] The first step of the pulping process according to the
invention results in an unbleached chemical pulp (hereinafter also
referred to as "fibrous material"), which is then transferred to
the bleaching sequence.
[0064] The yield of unbleached chemical pulp relative to the dry
weight of the ligno-cellulosic material used is greater than 50 %
by weight, preferably at least 55 % by weight, for example, about
60 % by weight.
[0065] The degree of brightness of the material thus obtained is
usually equal to 35 to 60 % ISO, preferably 40 to 55 % ISO.
[0066] Preferably, the unbleached chemical pulp has the following
strength parameters, each measured according to ISO 5269-2; August
1998, on test sheets having a basis weight of about 80 g/cm.sup.2
using a standard climate according to DIN EN 20187 (see description
in the Examples). All of the following values are based on unbeaten
chemical pulp which corresponds to a freeness value (.degree.SR,
measured according to DIN-ISO 5267/1; March 1999) of approx. 12 to
15:
[0067] a breaking length (dry, measured according to Zellcheming
V/12/57) of at least 9 km, more preferably at least 10 km,
especially 10 to 11 km, and
[0068] a tear strength (dry, see Examples, measured according to
Elmendorff; DIN 53128) relative to 100 g/m.sup.2, of at least 70
cN, more preferably at least 75 cN, especially 85 to 100 cN.
[0069] The sulfite pulping process used as the first step of the
process of the invention has the advantage over the sulfate process
of producing a pulp having a relatively light color even at higher
residual lignin contents.
[0070] This intermediate, which likewise is a part of the
invention, as well as the process steps producing it, is then
subjected to bleaching in one or more steps.
[0071] Prior to bleaching, the chemical pulp resulting from the
first stage (delignification) is separated from the cooking liquor
in a known manner, e.g. it may be filtered and optionally washed
(usually with water).
[0072] The common feature of each bleaching sequence used according
to the invention is bleaching with a chlorine-free oxidant, in the
presence of a base in a so-called "first bleaching step". The
entire bleaching sequence is carried out with chlorine-free agents.
The terms "chlorine-free bleaching sequence" and "chlorine-free
bleaching chemicals" mean that the bleaching chemicals contain no
chlorine, neither elemental chlorine, nor bonded chlorine such as,
e.g. in chlorine dioxide.
[0073] The entire bleaching sequence is preferably carried out
without additional mechanical pulping means, i.e. defibration
means.
[0074] The base used in the first bleaching step is preferably the
same base that is used in pulping. This makes it easier to close
water cycles, since the filtrate from the washer after the first
bleaching step may be used for washing in the washer after the
pulping step. Furthermore, by using the same base, preferably one
containing magnesium, the resulting bleach waste liquors may be
disposed together with the cooking waste liquors or at least
partially recycled.
[0075] Sodium hydroxide, magnesium oxide (MgO), and/or magnesium
hydroxide (Mg(OH).sub.2) are preferred bases. Hydrogen peroxide
(H.sub.2O.sub.2) is a preferred oxidant. The amount of oxidant is
preferably 35 to 60 kg/t, in particular 40 to 55 kg/t, relative to
the dry weight of the fibrous material used. The amount of base
used generally ranges from 10 to 20 kg/t relative to the dry weight
of the fibrous material used.
[0076] The first bleaching step is preferably conducted at a
temperature of 60.degree. to 80.degree. C. The preferred pH range
is from 8.5 to 9.5. The duration of this bleaching step is
preferably 240 to 420 minutes.
[0077] The effect of this first bleaching step may be intensified
by using oxygen. Preferably oxygen is used in an amount of 0.5 % to
3 % relative to the dry weight of the fibrous material used.
[0078] This first bleaching step is preferably combined with at
least one other (so-called "second") bleaching step which is also
conducted with an oxidant, in the presence of a base. In addition
to the oxidant mentioned above (hydrogen peroxide), the use of
peracetic acid (PAA), preferably in a pH range from 7 to 9, is
especially suitable for this second optional bleaching step. The
preferred base is sodium hydroxide (NaOH) or magnesium oxide (MgO)
(where hereinafter the term "Magnesium oxide (MgO)" also includes
magnesium hydroxide (Mg(OH).sub.2). Both oxidant and base may be
used in the same amounts set forth above. Duration and temperature
of this bleaching step may also correspond to those of the first
bleaching step.
[0079] The advantage of a peracetic acid step (magnesium oxide as
base) as the second optional bleaching step is the fact that
additional waste water cycles may be closed in order to minimize
waste.
[0080] In particular, if using peroxides or peracids in the first
or second (optional) bleaching step, bleaching abilities may be
improved by first treating the fibrous material with a complexing
agent. According to the invention, examples of suitable complexing
agents are nitrogen-containing organic compounds, in particular
nitrogen-containing polycarboxylic acids, nitrogen-containing
polyphosphonic acids, and nitrogen-containing polyalcohols.
Examples of nitrogen-containing polycarboxylic acids are
diethylenetriamine pentaacetic acid (DTPA), ethylenediamine
tetraacetic acid (EDTA), and nitrilo-triacetic acid (NTA).
Diethylenetriamine pentamethylenephosphonic acid (DTPMPA) and
diethylenetriamine pentaphosphonic acid are examples of
nitrogen-containing poylphosphonic acids. Furthermore, one may also
use other complexing agents such as polycarboxylic acids, e.g.
oxalic acid, citric acid or tartaric acid, or phosphonic acids.
Preferred complexing agents are: DTPA, DTPMPA, and EDTA.
[0081] Preferably, complexing agent may be added in the amount of
0.5 to 3 kg/t relative to the fibrous material used. Preferably,
the complexing treatment is performed at approx. 60.degree. C.
[0082] In one embodiment, the complexing treatment is carried out
in a weakly acidic environment (hereinafter referred to as
"Q").
[0083] In order to activate the residual lignin content of the
fibrous material, one may also use an oxidant in an acidic
environment along with the complexing agent. For this purpose,
peracetic acid (hereinafter referred to as "Q/PAA") and ozone are
particularly suited.
[0084] In an additional, particularly preferred embodiment
(referred to as an acidic wash or "A.sub.Q"), one sets the pH of
the pulp suspension at a value of 3 or less, e.g. by addition of
H.sub.2SO.sub.4. In this manner, one may dissolve MgO that has
precipitated onto the pulp or into the suspension. After setting
the pH value, one adds the complexing agent to the suspension. This
form of complexing treatment is particularly suited as a first step
of the bleaching sequence, or if it has been preceded by at least
one bleaching step in the presence of MgO as base.
[0085] The second (optional) bleaching step may be followed by
additional oxidizing bleaching steps with chlorine-free oxidants in
the presence of base. After treatment with peracetic acid, a third
oxidizing bleaching step is preferably performed using hydrogen
peroxide in the presence of sodium hydroxide as base. This third
oxidizing bleaching step is preferably performed under the same
conditions as those described in the first and second bleaching
steps.
[0086] The oxidizing bleaching steps are preferably combined with a
so-called "reducing bleaching step" using a reducing bleaching
agent. In a preferred embodiment, a reducing bleaching step
concludes the entire bleaching sequence. In another preferred
embodiment, the bleaching sequence comprises a first oxidative
bleaching step, followed by reducing bleaching step, and then an
additional oxidative bleaching step. Bleaching agents that are
suitable for the reducing bleaching step include water-soluble
dithionite salts, hydroxylamine, thiourea, thioglycolic acid,
borohydride (e.g. sodium borohydride), or formamidinosulfinic acid.
Especially preferred is the use of formamidinosulfinic acid or
dithionite, particularly sodium dithionite.
[0087] The amount of reducing agent used in the reducing bleaching
step preferably ranges from 5 to 15 % by weight relative to the
amount of fibrous material used. This reaction is preferably
performed at a pH ranging from 9 to 11. Preferably, sodium
hydroxide or magnesium oxide is used to set the pH. Magnesium oxide
has the advantage that the waste of this step may be disposed
together with pulping waste liquor. The preferred temperature for
the reaction is in the range of 80.degree. to 95.degree. C. The
reaction preferably takes from 60 to 90 minutes.
[0088] Selection and sequence of the bleaching steps may be varied,
where with multi-step bleaching sequences it is preferred to begin
the bleaching sequence with an oxidative bleaching of the type
P.sub.MgO(bleaching step with hydrogen peroxide in the presence of
MgO as base), optionally in the presence of oxygen (OP.sub.MgO),
and to conclude the bleaching sequence with a reducing bleaching
step.
[0089] The total residence time for all oxidizing and/or reducing
bleaching steps (including complexing treatments) ranges preferably
from 700 to 1200 minutes.
[0090] Following each individual step of the bleaching sequence
(including complexing treatments), the fibrous material is
preferably separated from the bleaching solution, e.g. by
filtration, and washed. In this manner, the consumption of
chemicals in bleaching may often be reduced. By washing upon
completion of the bleaching sequence, the purity of the pulp
according to the invention may be increased. Furthermore, it is
preferred to work countercurrent, i.e. to wash the completely
bleached fibrous material with clean water and to use the resulting
wash water for pulp of the previous bleaching step or steps.
Finally, the wash water from the washing following the first
bleaching 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.
[0091] Preferred bleaching sequences are as follows:
[0092] 1) A.sub.Q - (P.sub.MgO or OP.sub.MgO) - Q -P.sub.NaOH
-(FAS.sub.NaOH or Y),
[0093] 2) A.sub.Q - (P.sub.MgO or OP.sub.MgO) -Q -PAA.sub.NaOH
-P.sub.NaOH -(FAS.sub.NaOH or Y),
[0094] 3) Q - (P.sub.MgO or OP.sub.MgO) - Q -PAA.sub.MgO
-P.sub.NaOH -(FAS.sub.NaOH or Y),
[0095] 4) A.sub.Q - (p.sub.NaOH or OP.sub.NaOH) - Q - P.sub.NaOH -
(FAS.sub.NaOH or Y),
[0096] 5) Q/PAA - (P.sub.MgO or OP.sub.MgO) - Q/PAA - FAS.sub.MgO -
P.sub.NaOH, or
[0097] 6) Q - P.sub.MgO - A.sub.Q - P.sub.NaOH - FAS.sub.NaOH
[0098] 7) Q - P.sub.MgO - Q - P.sub.NaOH - FAS.sub.NaOH
[0099] symbols meaning the following:
1 A.sub.Q acid wash with complexing treatment Q complexing
treatment Q/PPA complexing treatment with concurrent activation of
fiber surface with peracetic acid P.sub.NaOH/MgO bleaching step
with hydrogen peroxide in presence of NaOH or MgO as base
OP.sub.NaOH/MgO bleaching step with hydrogen peroxide in the
presence of oxygen and NaOH or MgO as base PAA.sub.NaOH/MgO
bleaching step with peracetic acid in the presence of NaOH or MgO
as base FAS.sub.NaOH/MgO reducing bleaching step with
formamidinosulfinic acid in the presence of NaOH or MgO. Y reducing
bleaching step with dithionite.
[0100] Based on the most recent knowledge, bleaching sequences 6
and 7 represent the best mode for practicing the invention.
Pulp
[0101] In the manner described above, one may obtain bleached
chemical pulp according to the invention. This pulp has the
properties of a chemical pulp that is obtained by a sulfite pulping
process, in particular a magnesium (bi)sulfite process, and that is
then bleached without the use of chlorine or chlorine-containing
chemicals. It has a brightness of at least 83% ISO and is further
characterized by a breaking length (dry) of at least 6 km,
preferably at least 7 km, more preferred at least 8 km, in
particular at least 9 km, e.g. 10 km [measured (according to
Zellcheming V/12/57) on a test sheet made from unbeaten pulp
(freeness of 12 to 15.degree. SR measured according to DIN-ISO
5267/1; March 1999) basis weight of approx. 80 g/m.sup.2, produced
according to ISO 5269-2; August 1998, in a standard climate
according to DIN EN 20187; November 1993)].
[0102] The degree of brightness is preferably at least 84 % ISO,
particularly at least 85% ISO. For example, the degree of
brightness may be 86 % ISO.
[0103] One advantage of the pulp according to the invention is that
in contrast to mechanical pulp or chemomechanical pulp, its degree
of brightness does not significantly decline during further
processing.
[0104] The pulp according to the invention is substantially free of
chlorine and/or chloride Preferably, the pulp has an OX content of
less than 30 mg/kg, or in particular, is free of OX. The OX content
relates to the residual halogen organic compounds in the pulp,
which may essentially be formed during bleaching and are measured
in accordance with DIN 38414/18 and PTS-RHO12/90.
[0105] Pulp that is low in OX or is OX-free, and/or products
derived from such pulp are more environmentally compatible than
pulp and pulp-derived products containing OX. This also applies to
the production process. In order to close water cycles as much as
possible, it is likewise preferred that only chlorine-free
chemicals be used in pulp production, because that way a build-up
of chlorine, chloride, and/or chlorine-containing substances may be
avoided.
[0106] These properties of the pulp make it especially suitable for
the production of tissue paper.
[0107] Preferably, (bleached, unbeaten) pulp according to the
invention has
[0108] a tear strength (dry, measured according to Elmendorff (DIN
53128) on test sheets described above under breaking length)
relative to 100 g/m.sup.2 of at least 90 cN, more preferably at
least 95 to 105 cN.
[0109] The (bleached) pulp according to the invention preferably
has a very low fiber bundle content of bundles having at least two
fibers, i.e. preferably less than 5 % by weight, in particular less
than 1 % by weight relative to the dry weight. It is more preferred
for it to be free of such bundles.
[0110] In a preferred embodiment, the pulp is ground following the
bleaching sequence. Preferably, it has a freeness value of more
than 15, in particular more than 15 and simultaneously not more
than 18 .degree.SR. With an increased freeness value there is an
increase in breaking length.
[0111] This additional surface treatment (beating), which has a
favorable effect on the strength properties of the resulting
paper/tissue paper, may preferably be 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 influences the static and dynamic
strength properties.
[0112] 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 associated with fibrillation is
therefore preferred.
[0113] The residual lignin content of the bleached pulp according
to the invention (measured according to DIN 54357, August 1978)
ranges preferably from 10 to 30, more preferred from 15 to 25, in
particular from 18 to 22. One may estimate the residual lignin
content by multiplying the kappa numbers by 0.15. A kappa number of
20, which lies within the inventive range, thus corresponds to a
residual lignin content of approx. 3 % by weight.
[0114] The process according to the invention thus makes it
possible to a large extent to delignify lignocellulosic material
without the aid of mechanical pulping means.
[0115] The average fiber length of the inventive bleached pulp
ranges preferably from 1.8 to 2.2 mm (measured according to Kajaani
using a Kajaani machine FS-200).
[0116] The water retention capacity of the pulp according to the
invention ranges preferably from 130 to 150 %, in particular from
140 to 145 % (measured according to Zellcheming IV/33/57, as
described in the Examples).
Paper or Nonwoven (Product)
[0117] The present invention also relates to paper or nonwoven
comprising the bleached pulp 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.
[0118] The paper can be a packaging paper, a graphic paper or
tissue paper. Preferably, the paper is a tissue paper.
[0119] The tissue paper or nonwoven may be one-ply or
multiple-ply.
[0120] 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.
[0121] 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
pulp according to the invention.
[0122] "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.
[0123] 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.
[0124] 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 pulp to tissue paper,
one generally selects a basis weight of 8 to 65 g/m.sup.2,
especially 10 to 40 g/m.sup.2. 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.
[0125] 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).
[0126] 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.
[0127] Hygiene paper primarily includes all kinds of dry-creped
tissue paper, as well as wet-creped paper and cellulose or pulp
wadding.
[0128] 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.
[0129] 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".
[0130] 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 cloths, sanitary products (e.g. toilet paper),
paper handkerchiefs, cosmetic wipes (facials) or as
serviettes/napkins.
[0131] If tissue paper is to be made out of the pulp according to
the invention, the process essentially comprises
[0132] a forming that includes the headbox and the wire
portion,
[0133] 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,
[0134] c the monitoring and winding area.
[0135] 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.
[0136] 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.
[0137] 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.
[0138] 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.
[0139] When processing the raw fibrous web or raw tissue paper into
the final product (third process section), 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, ply adhesion, folding, imprinting,
perforating, application of lotions, smoothing, stacking, rolling
up.
[0140] To produce multi-ply tissue paper products, such as
handkerchiefs, toilet paper, towels or kitchen towels, 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.
[0141] 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
[0142] In the description of the invention and in the following
examples the following test methods were used to evaluate the
chemical pulps according to the invention.
1) Producing the Test Sheets
[0143] The test sheets (having a basis weight of approx. 80
g/m.sup.2) were made in accordance with ISO 5269-2: August 1998).
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; November
1993, 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).
2) Breaking Length (Dry)
[0144] The breaking length was determined using a dry test sheet
produced according to 1) following a process described in
Zellcheming V/12/1957.
3) Tear Strength (Dry) according to Elmendorff
[0145] Tear strength was determined according to Elmendorff using a
test sheet produced according to 1) following a process described
in DIN 53128.
4) Kappa Number
[0146] The kappa number is determined according to DIN 54357
(August 1978)
5) WRV (Water Retention Value)
[0147] The WRV is determined in the following way.
[0148] The principle of determining the WRV is based on
centrifuging swollen pulp samples at room temperature in a
special-purpose centrifuge beaker with 3000-fold gravitational
acceleration. The WRV in percent (mass portions in percent)
indicates the portion of water in the sample that cannot be
centrifuged.
[0149] The implementation followed the German Zellcheming Code of
Practice IV/33/57 (issued on Jan. 1, 1957)
[0150] A fibrous material treated with excess water as a swelling
agent is taken.
[0151] The tubes for the centrifuge inserts are filled to about 2/3
of the volume with the moist pulp sample without pressing
firmly.
[0152] A triple determination is to be performed each time.
[0153] The tubes are inserted into the centrifuge beaker.
[0154] Centrifuging conditions: 15 min at 4800 rpm
[0155] After the centrifuge comes to a standstill, the tubes are
taken out and the centrifuged substance immediately transferred
with the aid of pincers as completely as possible into the weighing
bottles previously dried at 103.degree. C. and tared at room
temperature where they are weighed (remove glass spheres
beforehand)
[0156] The samples are now dried for at least 6 hours in the
circulating-air drying cabinet, immediately seal when taking them
out and allow them to cool in the desiccator. Now reweigh.
[0157] The calculation is made on the basis of the following
equation:
WRV =.sup.(W-D)x 100
[0158] D
[0159] where W =the weight of the moist substance, D represents the
dry weight of the substance; and W minus D =the weight of the
absorbed water.
6) Dry Weight
[0160] The weight values that are given as "dry weight" relate to
dried material, lignocellulosic material, pulp, paper, or nonwoven
(product) that was dried at 105.degree. C. until constant weight
was achieved.
7) Freeness Value
[0161] The freeness value (in .degree.SR) was measured according to
DIN-ISO 5267/1; March 1999.
8) Degree of Brightness
[0162] The degree of brightness (in %) was determined according to
ISO following SCAN C11:1975.
Example 1
[0163] Pine chips of a width of 10 to 25 mm, a length of 13 to 35
mm and with a thickness of 1 to 10 mm were pulped in a magnesium
bisulfite solution (free SO.sub.2 as hydrogen sulfite or
sulfite-bound SO.sub.2) having a total SO.sub.2 content of 23.6 %
by weight/wood (dry weight) and a magnesium content measured as MgO
of 7 % by weight/wood (dry weight) at an initial pH of 1.7 and
temperature of 138.degree. C. The wood-to-liquor ratio was 1:3.5.
The warming to the maximum temperature of 138.degree. C. took 30
minutes. The temperature was maintained at the maximum level for a
total of 4.5 hours (270 minutes). Once the defibration point was
reached, cooling to room temperature required 60 minutes. The
resulting pulp was separated from waste liquor, washed with water,
and dried, yielding 58.2 % (dry weight) relative to the wood used,
at a kappa number of 52.2 and a degree of brightness of 46.1 % ISO.
The breaking length of the unbeaten (=freeness value of 14.degree.
SR), unbleached pulp was 10.3 km at a tear strength according to
Elmendorff of 87 cN relative to 100 g/m.sup.2.
[0164] The washed pulp subsequently was subjected to complexing
treatment, being contacted at a temperature of 60.degree. C. for a
period of 30 minutes with 2 kg/t of complexing agent DTPA at a
density of 3 % (unless otherwise stated, the data given in kg/t in
Examples 1 and 2 consistently refers to the dry weight of pulp).
The pH value was set at 5.2 for the complexing treatment with
H.sub.2SO.sub.4. Then the pulp was bleached in aqueous solution
containing 50 kg/t of hydrogen peroxide and 15 kg/t MgO (at a
density of 10 %) for a reaction time of 360 minutes and at a
reaction temperature of 70.degree. C. This was followed by an
additional complexing treatment under the same conditions as
described earlier, and subsequently by a second oxidizing bleaching
with 50 kg/t hydrogen peroxide and 22.5 kg/t sodium hydroxide (at a
density of 10 %). A total of 50 kg/t of peroxide was consumed in
the two steps.
[0165] Finally, the resulting pulp was bleached for the last time,
reduced with formamidinosulfinic acid (FAS). In the FAS step, 5
kg/t formamidinosulfinic acid was used at a temperature of
90.degree. C. and density of 10 % for a period of 60 minutes.
[0166] Following treatment with complexing agents and each
bleaching step, the pulp was washed with water.
[0167] The degree of brightness of the resulting pulp was recorded
at 85.9 % ISO. The yield (dry weight) of the pulp obtained after
the bleaching sequence was 95.2 % by weight relative to the dry
weight of the pulp subjected to the bleaching sequence. The
bleached pulp had a breaking length of 9 km and a tear strength
according to Elmendorff of 102.4 cN relative to 100 g/m.sup.2 in
the unbeaten state (freeness value =15.degree.SR).
Example 2
[0168] Pine chips, 10 to 25 mm wide, 13 to 35 mm long, and 2 to 10
mm thick, were pulped in a magnesium bisulfite solution containing
16 % by weight total SO.sub.2/wood (dry weight) and 6.2 % by weight
MgO/wood (dry weight). The wood(dry weight)-to-liquor ratio was
1:3.7. Warming to the maximum temperature of 150.degree. C. took 60
minutes, and the pulping at this temperature lasted 270 min (4.5
h). Cooling to room temperature occurred for a period of 60
minutes. The initial pH of the cooking liquor was 3.5. The yield of
pulp after separation from the waste liquor, washing with water,
and drying was 58.9 %, at a kappa number of 60.2 and degree of
brightness of 47.8 % ISO. The unbeaten (freeness value
=14.5.degree. SR), unbleached pulp had a breaking length of 10.8 km
and a tear strength according to Elmendorff of 96.8 cN relative to
a weight of 100 g/m.sup.2.
[0169] This pulp was subjected to the same bleaching sequence as
that in Example 1, and attained a degree of brightness of 83.3%
ISO. A total of 38 kg/t peroxide was consumed in this bleaching
sequence. The yield (dry weight) after the bleaching sequence
relative to the yield of pulp (dry weight) after pulping was 95.6
%. In the unbeaten state (freeness value 15.degree.SR), the
bleached pulp had a breaking length of 9.4 km and a tear strength
according to Elmendorff of 99.5 cN relative to 100 g/m.sup.2.
[0170] The pulp, pulped and bleached according to the invention,
and/or any paper produced from such are characterized by an
excellent combination of degree of brightness and strength
properties. In contrast to other pulp having similar residual
lignin contents, the pulp according to the invention has a high
stability in degree of brightness.
* * * * *