U.S. patent application number 11/628961 was filed with the patent office on 2007-10-25 for pulp bleaching processes.
Invention is credited to Kazuhiro Kurosu, Shoichi Miyawaki, Takashi Ochi.
Application Number | 20070246176 11/628961 |
Document ID | / |
Family ID | 35503100 |
Filed Date | 2007-10-25 |
United States Patent
Application |
20070246176 |
Kind Code |
A1 |
Miyawaki; Shoichi ; et
al. |
October 25, 2007 |
Pulp Bleaching Processes
Abstract
The present invention aims to further advance the acid treatment
or irradiation technology for pulp to develop a bleaching process
using smaller amounts of chlorine chemicals with higher efficiency
as compared with conventional bleaching processes. It also aims to
provide an excellent high brightness pulp having a low
environmental impact and no discoloration as well as a paper
containing it. The present invention provides a chlorine-free
bleaching process with very high efficiency by irradiating a pulp
washed after an acid treatment with UV light and/or visible light
at a wavelength of 100-400 nm under alkaline conditions, preferably
in a pH range of 10-13. The acid treatment can be performed under
conditions of pH 1-6 and a temperature of 80.degree. C. or more.
The irradiation treatment can be performed in the presence of at
least one compound selected from the group consisting of reducing
agents, peroxides, and hydrogen-donating organic compounds. The
present invention also provides a high brightness chemical pulp
having an ISO brightness of 88% or more and a brightness loss of
1.0% or less in a specific fading test.
Inventors: |
Miyawaki; Shoichi; (Tokyo,
JP) ; Kurosu; Kazuhiro; (Tokyo, JP) ; Ochi;
Takashi; (Tokyo, JP) |
Correspondence
Address: |
ROYLANCE, ABRAMS, BERDO & GOODMAN, L.L.P.
1300 19TH STREET, N.W.
SUITE 600
WASHINGTON,
DC
20036
US
|
Family ID: |
35503100 |
Appl. No.: |
11/628961 |
Filed: |
June 8, 2005 |
PCT Filed: |
June 8, 2005 |
PCT NO: |
PCT/JP05/10521 |
371 Date: |
December 8, 2006 |
Current U.S.
Class: |
162/9 ; 162/202;
162/50; 422/186.3 |
Current CPC
Class: |
D21C 9/1005 20130101;
D21C 9/1084 20130101; D21C 9/16 20130101 |
Class at
Publication: |
162/009 ;
162/202; 162/050; 422/186.3 |
International
Class: |
D21C 9/10 20060101
D21C009/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 8, 2004 |
JP |
2004-169367 |
Sep 30, 2004 |
JP |
2004-289267 |
Mar 31, 2005 |
JP |
2005-102615 |
Mar 31, 2005 |
JP |
2005-102558 |
Claims
1. A pulp bleaching process characterized in that a pulp washed
after an acid treatment is irradiated with UV light and/or visible
light at a wavelength of 100-400 nm under alkaline conditions.
2. The pulp bleaching process of claim 1 characterized in that the
alkaline conditions comprise a pH range of 10-13.
3. The pulp bleaching process of claim 1 characterized in that the
acid treatment is performed under conditions of pH 1-6 and a
temperature of 80.degree. C. or more.
4. The pulp bleaching process of claim 1 characterized in that the
acid treatment is performed in the presence of ozone under
conditions of pH 1.0-8.0 and a temperature of 25-95.degree. C.
5. The pulp bleaching process of claim 1 characterized in that the
irradiation treatment is performed in the presence of at least one
compound selected from the group consisting of reducing agents,
peroxides, and hydrogen-donating organic compounds.
6. The pulp bleaching process of claim 1 characterized in that the
irradiation source of the UV light and/or visible light consists of
multiple light sources having different wavelength
characteristics.
7. The pulp bleaching process of claim 1 characterized in that the
irradiation treatment is repeated multiple times.
8. A pulp bleaching process characterized in that a pulp washed
after an acid treatment is irradiated with UV light and/or visible
light at a wavelength of 100-400 nm in the presence of ozone.
9. The pulp bleaching process of claim 8 characterized in that the
irradiation treatment with UV light and/or visible light is
performed under acidic conditions of pH 2-4 or under alkaline
conditions of pH 10-13.
10. The pulp bleaching process of claim 8 characterized in that the
acid treatment is performed under conditions of pH 1-6 and a
temperature of 80-180.degree. C.
11. The pulp bleaching process of claim 8 characterized in that the
acid treatment is performed in the presence of ozone under
conditions of pH 1-8 and a temperature of temperature 25-95.degree.
C.
12. The pulp bleaching process of claim 8 characterized in that the
ozone is generated by irradiating air or oxygen or a mixture
thereof with UV light.
13. The pulp bleaching process of claim 8 characterized in that the
ozone concentration during the irradiation treatment is 0.5-100
ppm.
14. A pulp bleacher characterized in that air or oxygen or a
mixture thereof is supplied to the surroundings of an irradiation
source of UV light and/or visible light to generate ozone and a gas
containing the ozone is fed to a pulp.
15. A pulp bleacher comprising an ozone-generating chamber having
an irradiation source for emitting UV light and/or visible light
therein and a pulp slurry chamber, wherein the ozone-generating
chamber has a gas injection port and a discharge port, and the
discharge port communicates with the pulp slurry chamber.
16. A totally chlorine-free (TCF) bleaching process for chemical
pulp characterized in that an oxygen-delignified pulp is
acid-treated and then bleached by a bleaching process used in
totally chlorine-free bleaching, and the resulting pulp is further
subjected to a light bleaching treatment with UV light and/or
visible light at a wavelength of 100-400 nm under alkaline
conditions, followed by alkaline hydrogen peroxide bleaching.
17. A totally chlorine-free bleaching process for chemical pulp
characterized in that an oxygen-delignified pulp is acid-treated
and then bleached by a bleaching process used in normal TCF
bleaching to an ISO brightness of 70-75%, and the resulting pulp is
further subjected to a light bleaching treatment with UV light
and/or visible light at a wavelength of 100-400 nm under alkaline
conditions to an ISO brightness of 75-80%, followed by alkaline
hydrogen peroxide bleaching to give a pulp having an ISO brightness
of 84% or more.
18. The bleaching process for chemical pulp of claim 16
characterized in that the light bleaching treatment is performed in
a pH range of 10-13.
19. The bleaching process for chemical pulp of claim 16
characterized in that the acid treatment is performed under
conditions of pH 1-6 and a temperature of 80.degree. C. or
more.
20. The bleaching process for chemical pulp of claim 16
characterized in that the light bleaching treatment is performed in
the presence of at least one compound selected from the group
consisting of oxidizing agents, reducing agents, and peroxides.
21. A high brightness chemical pulp having an ISO brightness of 88%
or more and a brightness loss of 1.0% or less in the following
fading test: a hand-made paper is prepared according to JIS P 8222
and irradiated with a xenon lamp at an intensity of 67 W/m.sup.2
for 30 minutes in an atmosphere at 30.degree. C. according to
fading test method B of J. TAPPI No. 21 Paper and Paperboard (using
a xenon arc lamp light fastness tester) and then the ISO brightness
is measured and a loss from the ISO brightness before treatment is
determined.
22. The high brightness chemical pulp of claim 21, which has been
treated with UV light and/or visible light and has an ISO
brightness of 88% or more.
23. A paper containing the high brightness chemical pulp of claim
21.
24. A process for preparing a high brightness chemical pulp
characterized in that a bleached chemical pulp is treated with UV
light and/or visible light.
25. A process for preparing a high brightness chemical pulp
characterized in that a bleached chemical pulp having an ISO
brightness of 80% or more is treated with UV light and/or visible
light to an ISO brightness of 88% or more.
26. The bleaching process for chemical pulp of claim 17
characterized in that the light bleaching treatment is performed in
a pH range of 10-13.
27. The bleaching process for chemical pulp of claim 17
characterized in that the acid treatment is performed under
conditions of pH 1-6 and a temperature of 80.degree. C. or
more.
28. The bleaching process for chemical pulp of claim 17
characterized in that the light bleaching treatment is performed in
the presence of at least one compound selected from the group
consisting of oxidizing agents, reducing agents, and peroxides.
Description
TECHNICAL FIELD
[0001] The present invention relates to a bleaching process for
pulp characterized in that a pulp washed after an acid treatment is
irradiated with UV light and/or visible light under alkaline
conditions; a bleaching process characterized in that a pulp washed
after an acid treatment is treated by a combination of irradiation
with UV light and/or visible light and ozone feeding, thereby
promoting pulp bleaching; and a bleaching process for chemical pulp
by which a high brightness chemical pulp can be obtained (herein
collectively referred to as the first invention); as well as a high
brightness chemical pulp with greatly improved discoloration and a
paper containing it, more specifically a novel high brightness
chemical pulp with greatly improved discoloration obtained by
further treating a bleached chemical pulp with UV light and a paper
containing it (herein collectively referred to as the second
invention).
BACKGROUND ART
[0002] The background art of the first invention described below is
as follows.
[0003] To meet the growing concern about the influence of waste
from bleaching processes of paper pulp plants on the environment,
chlorine-free (ECF) bleaching or even totally chlorine-free (TCF)
bleaching is becoming global and mainstream in place of
conventional bleaching techniques mainly using chlorine or
chlorine-based chemicals or combinations thereof. Under such
circumstances, there is a tendency to use limited chemicals such as
chlorine dioxide, hydrogen peroxide, oxygen and ozone in ECF
bleaching or TCF bleaching. However, pulp quality, especially
brightness obtained by bleaching with only such chemicals is
limited even if they are used in combination, or large amounts of
expensive chemicals must be used to attain sufficient quality. It
would be desirable to develop chlorine-free chemicals having
unprecedentedly excellent bleaching performance or novel bleaching
techniques to solve these problems.
[0004] It has been known that various metals from pulp promote
decomposition of oxygen-based bleaching chemicals, thereby wasting
the oxygen-based bleaching chemicals. Thus, techniques for removing
these metals to improve the bleaching efficiency of oxygen-based
bleaching chemicals were proposed, such as acid treatments at
relatively low temperatures or chelator treatments or combinations
thereof. Such acid treatment techniques previously disclosed
include a process for delignifying a pulp prepared from a
lignocellulose material by oxygen bleaching, comprising first
adding a nitrite and an acid to the pulp to pretreat the pulp,
followed by oxygen bleaching, or a bleaching process comprising
subjecting a cooked chemical pulp to an acid treatment followed by
delignification with a peroxide and pressurized oxygen in an
alkaline medium (e.g., patent document 1, patent document 2).
Another bleaching process was also disclosed, comprising subjecting
a cooked chemical pulp to an oxygen bleaching treatment at high
temperature and high pressure and then an acid treatment or a
chelator treatment followed by delignification/bleaching with a
peroxide or hydrogen peroxide and oxygen in an alkaline medium
(e.g., patent document 3).
[0005] It has become known from recent findings that not only
lignin and modified lignin but also hexenuronic acid are
responsible for discoloration of ECF or TCF bleached pulp. This
hexenuronic acid is produced by demethylation from methylglucuronic
acid in hemicellulose during the cooking process. This hexenuronic
acid is said to be responsible for discoloration of the pulp. A
method proposed to remove the hexenuronic acid was an acid
treatment technique carried out at a relatively high temperature.
This method comprises treating unbleached pulp at a high
temperature under acid conditions, thereby acid-hydrolyzing the
hexenuronic acid and modified lignin to remove them. For example, a
technique was disclosed wherein a suspension of a cellulose pulp
prepared by the sulfate process or alkaline process is heated and
treated at about 85 to 150.degree. C. and a pH of about 2 to 5 to
remove at least about 50% of hexenuronic acid in the cellulose
pulp, thereby decreasing the Kappa number of the pulp by 2 to 9
units (see patent document 4).
[0006] Bleaching techniques using irradiation were also disclosed,
such as techniques involving UV irradiation in hydrogen peroxide
bleaching of unbleached kraft pulps (e.g., see non-patent document
1, or patent document 5), or UV irradiation in oxygen bleaching of
unbleached kraft pulps (e.g., see non-patent document 2). UV
irradiation in the presence of a peroxide as a pretreatment for
promoting normal alkaline hydrogen peroxide bleaching was also
disclosed (e.g., see patent document 6).
[0007] Other disclosed techniques include a pulp bleaching process
using a reducing agent involving irradiation with UV light or
visible light or a combination thereof (see patent document 7), or
a process involving irradiation with UV light or visible light or a
combination thereof in the presence of an organic peroxide
represented by ROOR' as an oxidizing agent (see patent document
8).
[0008] Recently, water pollution caused by industrial wastewater or
domestic wastewater is worsening and water environment pollution
has become social concerns. Under such circumstances, water
environment protection technologies such as activated carbon
treatment, membrane treatment, ozone treatment, UV treatment,
biological treatment, etc. are under active development. Among
them, the advanced oxidation technology combining ozone and UV
light (see patent document 9) is a promising comprehensive
treatment capable of improving efficiency of decomposition and
deodorizing, effects of decoloring and sterilizing as well as
ensuring clarification treatment without producing secondary
waste.
[0009] Patent document 1: Japanese Patent No. 2895977.
[0010] Patent document 2: JPA No. Hei 6-101186.
[0011] Patent document 3: JPA No. Hei 6-158573.
[0012] Patent document 4: JPA No. Hei 10-508346.
[0013] Patent document 5: JPA No. 2002-88673.
[0014] Patent document 6: JPA No. Hei 6-128890.
[0015] Patent document 7: JPA No. 2002-88671.
[0016] Patent document 8: JPA No. 2002-88672.
[0017] Patent document 9: JPA No. 2004-97992.
[0018] Non-patent document 1: B. Marccia, et al. J34-J39, JOURNAL
OF PULP AND PAPER SCIENCE: Vol. 17, No. 2, March 1991.
[0019] Non-patent document 2: J. Abbot, et al. p 198-202, Appita
Vol. 46, No. 3, May 1993.
[0020] The background art of the second invention described below
is as follows.
[0021] Paper products mainly made from chemical pulp, especially
communication papers such as inkjet papers and thermal transfer
papers or photographic base papers are required to have high
brightness. Normally, in order to increase brightness of unbleached
kraft pulp, materials responsible for coloration such as lignin or
polysaccharides remaining in the unbleached pulp are removed by
multistage bleaching with chemicals such as chlorine,
hypochlorites, chlorine dioxide, oxygen, hydrogen peroxide, ozone,
etc. The ISO brightness of pulp obtained by chlorine bleaching with
chlorine gas or more environmentally friendly ECF bleaching with
reduced production of organic chlorine compounds using chlorine
dioxide is normally 82-86%. High brightness pulp bleached to a
higher brightness level than normal is typically prepared by
applying harsher cooking and/or bleaching conditions or by using
easy-to-cook and bleach wood species having low contents of
phenolic extract components.
[0022] As a prior technique for preparing high brightness pulp, a
process for preparing a high brightness pulp was disclosed, for
example, characterized in that a pulp bleached in a sequence
including at least one chlorine bleaching stage is treated with
xylanase and further bleached in a bleaching sequence including a
hypochlorite stage and a chlorine dioxide stage (see patent
document 10). Another process for preparing a high brightness pulp
was disclosed, comprising further bleaching a bleached pulp derived
from a lignocellulose material via a continuous sequence of a
hypochlorite bleaching stage under high temperature and highly
alkaline conditions and a chlorine dioxide bleaching stage,
characterized in that the chlorine dioxide bleaching stage is
performed at a chlorine dioxide concentration of 1-3% by weight (on
the basis of the bone dry weight of the pulp) at a high temperature
of 91.degree. C. or more and less than 100.degree. C. (see patent
document 11). A photographic base paper was also disclosed,
characterized in that it uses a chemical pulp bleached in a
bleaching sequence of oxygen bleaching--ozone bleaching--alkali
extraction--hydrogen peroxide bleaching--chlorine dioxide
bleaching--hydrogen peroxide bleaching--chlorine dioxide bleaching
and that the chemical pulp is bleached with 0.1-1.0% by weight of
ozone on the basis of the bone dry weight of the pulp during the
ozone bleaching stage (see patent document 12). A process for
preparing a high brightness pulp with improved discoloration for
use in photosensitive materials was also disclosed, characterized
in that an unbleached kraft pulp having a kappa number of 23 or
less is bleached with oxygen at a delignification degree of 40% or
more and then bleached with ozone at a pulp consistency of 25% or
more and then adjusted to a PN number of 2.8 or less followed by
multistage bleaching including hydrogen peroxide bleaching and
chlorine dioxide bleaching (see patent document 13). However, it
was difficult to avoid brightness loss with time or so-called
discoloration phenomenon in high brightness pulps obtained by the
conventional processes because very small amounts of potential
coloring matters liable to be darkened by heat or UV light remain
in them.
[0023] Patent document 10: JPA No. Hei 6-101185.
[0024] Patent document 11: JPA No. Hei 9-105091.
[0025] Patent document 12: JPA No. 2002-62622.
[0026] Patent document 13: JPA No. 2003-41494.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0027] The first invention aims to further advance the acid
treatment or irradiation technology for pulp as described above to
develop a bleaching process using less chlorine chemicals with
higher efficiency as compared with conventional bleaching
processes, and to develop a TCF bleaching process capable of
achieving a final ISO brightness of 84% or more while greatly
shortening the irradiation period at the light bleaching stage by
inserting an acid treatment and a bleaching stage used in normal
TCF bleaching before the light bleaching stage and an alkaline
hydrogen peroxide bleaching stage after the light bleaching
stage.
[0028] The second invention aims to overcome the drawbacks of the
conventional technology and to provide a high brightness pulp
having a low environmental impact and no discoloration as well as a
paper containing it.
Means to Solve the Problems
[0029] As a result of careful studies, we found a chlorine-free
bleaching process with very high efficiency by irradiating a pulp
washed after an acid treatment with UV light and/or visible light
at a wavelength of 100-400 nm under alkaline conditions, preferably
in a pH range of 10-13, and accomplished a first aspect of the
first invention.
[0030] We also found a pulp bleaching process with very high
efficiency by irradiating a pulp, that is washed after an acid
treatment, with UV light and/or visible light at a wavelength of
100-400 nm in the presence of ozone, and accomplished a second
aspect of the first invention.
[0031] We also found a very efficient totally chlorine-free (TCF)
pulp bleaching process wherein an oxygen-delignified pulp is
acid-treated and then bleached by a bleaching process used in
normal TCF bleaching, and the resulting pulp is further subjected
to a light bleaching treatment with UV light and/or visible light
at a wavelength of 100-400 nm under alkaline conditions, followed
by alkaline hydrogen peroxide bleaching (a third aspect of the
first invention).
[0032] As a result of careful studies to overcome the drawbacks of
conventional technology and to attain a pulp with such a high
brightness and low discoloration, we found that a pulp having high
brightness and no discoloration as described above and also having
high paper strength can be prepared by further treating a bleached
pulp with UV light, and accomplished the second invention.
Accordingly, the second invention relates to a high brightness
chemical pulp having an ISO brightness of 88% or more and a
brightness loss of 1.0% or less in the following fading test:
[0033] a hand-made paper is prepared according to JIS P 8222 and
irradiated with a xenon lamp at an intensity of 67 W/m.sup.2 for 30
minutes in an atmosphere at 30.degree. C. according to fading test
method B of J. TAPPI No. 21 Paper and Paperboard (using a xenon arc
lamp light fastness tester) and then the ISO brightness is measured
and a loss from the ISO brightness before treatment is
determined.
BRIEF EXPLANATION OF THE DRAWINGS
[0034] FIG. 1 shows an example of a system using an irradiation
reactor in the present invention.
[0035] FIG. 2 shows an irradiation reactor with an internal light
source used in the present invention.
[0036] FIG. 3 shows an irradiation reactor with an internal light
source used in the examples of the present invention.
[0037] FIG. 4 shows the relationship between pH and brightness in
irradiation treatments using a hardwood pulp.
[0038] FIG. 5 shows the relationship between irradiation period and
brightness in irradiation treatments using a hardwood pulp.
[0039] FIG. 6 shows the relationship between pH and brightness in
irradiation treatments using a softwood pulp.
[0040] FIG. 7 shows an example of an experimental apparatus for UV
bleaching test.
REFERENCE NUMERALS
[0041] 10: irradiation target material conditioning tank;
[0042] 11: irradiation reactor feeding pump;
[0043] 12: irradiation reactor;
[0044] 20: irradiation reaction chamber;
[0045] 21: quartz glass tube;
[0046] 22: irradiation source;
[0047] 23a, 23b: three-way valve;
[0048] 24: diffuser, diffuser tube;
[0049] 25: pulp slurry inlet;
[0050] 26: pulp slurry outlet;
[0051] 27: stirrer.
PREFERRED EMBODIMENTS OF THE INVENTION
[0052] Embodiments of the first invention are explained below.
[0053] The first and second aspects of the first invention are
applied to pulps washed after an acid treatment. Especially, they
are applied to kraft pulps (KPs) and are well suitable for not only
unbleached KPs but also oxygen-delignified KPs, ozone-bleached KPs,
etc. The bleaching process according to the third aspect of the
first invention is applied to kraft-cooked and oxygen-delignified
chemical pulps. The pulp materials used in the first invention and
the second invention (chemical pulp) are not specifically limited
and include hardwood and softwood as well as other plants such as
kenaf, flax, rice, bagasse, bamboo, etc. In the pulp bleaching
process according to the second aspect of the first invention, the
oxygen-delignified pulp is acid-treated and then bleached by a
bleaching process used in normal TCF bleaching, and the resulting
pulp is further subjected to an irradiation treatment with UV light
and/or visible light at a wavelength of 100-400 nm under alkaline
conditions, followed by alkaline hydrogen peroxide bleaching.
[0054] The type of the acid used in the acid treatment of the
present invention may be inorganic or organic. Inorganic acids that
can be used include mineral acids such as sulfuric acid,
hydrochloric acid, nitric acid, sulfurous acid, nitrous acid,
phosphoric acid, and residual acids in chlorine dioxide generators.
Sulfuric acid is preferred. Organic acids that can be used include
acetic acid, lactic acid, oxalic acid, citric acid, formic acid,
etc. The pH during the acid treatment is in the range of 1.0-6.0,
preferably 1.0-5.0, more preferably 2.0-5.0, most preferably
2.5-3.5. If the pH is less than 1.0, hexenuronic acid or the like
and metal ions are sufficiently removed, but the viscosity greatly
decreases because of excessive acidity. If the pH exceeds 6.0,
however, the acid concentration is so low that hexenuronic acid or
the like and metal ions are insufficiently removed. Hardwood pulps
rich in hexenuronic acid can be acid-treated at lower temperatures
in a pH range of 2.5-3.5, which leads to the advantage that the
acid treatment cost can be reduced.
[0055] The acid treatment can be performed at an atmospheric
pressure or under pressure and at a temperature of 80.degree.
C.-180.degree. C., preferably 80.degree. C.-130.degree. C.
Temperatures of 30.degree. C. or higher and lower than 80.degree.
C. are effective for removing metals but not effective for removing
hexenuronic acid or the like. Temperatures lower than 100.degree.
C. are advantageous in terms of equipment costs because no
pressure-tight reaction chamber is required.
[0056] The pulp consistency during the acid treatment is in the
range of 0.1-50% by weight, preferably 1.0-30% by weight, more
preferably 2.0-20% by weight.
[0057] The extent to which hexenuronic acid or the like and harmful
metals are removed depends on the pH, reaction temperature and
reaction period during the acid treatment. Thus, the reaction
period is determined as appropriate depending on the other two
conditions, but typically the reaction period is 1.5-6 hours at a
reaction temperature of 90.degree. C., the reaction period is 50
minutes-5 hours at a reaction temperature of 95.degree. C., the
reaction period is 30 minutes-4.5 hours at a reaction temperature
of 100.degree. C., and the reaction period is 5-50 minutes at a
reaction temperature of 120-130.degree. C.
[0058] Ozone bleaching under acidic conditions is also a form of
the acid treatment contemplated by the present invention, and
normal acidic ozone bleaching conditions can be applied. In this
connection, typical acidic ozone bleaching conditions regarded as
suitable include using ozone gas at an ozone concentration of 1-20%
by weight at pH 1.0-8.0 (the first and third aspects of the first
invention) or pH 1-7 (the second aspect of the first invention) at
a pulp consistency of 0.1-50% by weight and a temperature of
25-95.degree. C. The pressure here is not specifically limited from
a negative pressure state to a pressurized state.
[0059] A further greater bleaching reaction promoting effect is
obtained in the irradiation treatment by using chelating agents
such as EDTA, DPTA in combination with the acid treatment.
[0060] Although the reason why the acid treatment promotes the
bleaching effect in the subsequent irradiation treatment with UV
light and/or visible light is not clear, lignin and metal ions,
especially iron ions remaining in pulp form metal complexes, which
become colored by the irradiation treatment. Thus, it is assumed
that the bleaching effect of the irradiation treatment is improved
by removing metal ions by the acid treatment.
[0061] In the present invention, the pulp can be dehydrated and/or
washed by using a known dehydrator and/or washing after the acid
treatment including ozone bleaching. Washing can be performed by
using not only fresh water but also wastewater generated from
bleaching processes after the acid treatment or wastewater
generated from papermaking processes.
[0062] In the present invention, the acid-treated pulp is
irradiated with UV light and/or visible light at a wavelength of
100-400 nm under alkaline conditions, preferably at pH 10-13.
Especially, hardwood pulps are preferably treated at pH 10-12, and
softwood pulps are treated at pH 11-13. In the second aspect of the
first invention, the acid-treated pulp is irradiated with UV light
and/or visible light at a wavelength of 100-400 nm under alkaline
conditions or acidic conditions. Preferably, the alkaline
conditions are in a pH range of 10-13 and the acidic conditions are
in a pH range of 2-4. The alkalis used for this pH adjustment can
be normal alkaline chemicals, but especially preferred are sodium
hydroxide, potassium hydroxide, sodium silicate and sodium
carbonate because of easy handling.
[0063] The pulp consistency during the irradiation treatment
according to the present invention is preferably 0.1-12% by weight.
Consistencies lower than 0.1% by weight are not preferred because
energy efficiency decreases though bleaching reaction efficiency
increases. Consistencies higher than 12% by weight are not
preferred because the rheology of pulp slurry in the irradiator
decreases so that bleaching reaction efficiency decreases.
[0064] The temperature of pulp slurry during this irradiation
treatment is preferably 20-95.degree. C., and temperatures lower
than 20.degree. C. are not preferred because bleaching reaction
efficiency is low while temperatures exceeding 95.degree. C. are
not preferred, either, because pulp quality may deteriorate or the
pressure in the reactor may exceed atmospheric pressure so that the
reactor must be designed to resist pressure.
[0065] The wavelength of the light emitted in the irradiator of the
present invention is preferably 100-400 nm, especially 200-360 nm
(180-360 nm in the second aspect of the first invention).
Wavelengths shorter than 100 nm are not preferred because
photodegradation of cellulose is promoted so that pulp strength is
greatly lowered, while wavelengths exceeding 400 nm are not
preferred, either, because photosensitive coloring matters are
insufficiently photoexcited so that light bleaching performance is
greatly lowered.
[0066] Irradiation sources that can be used include those emitting
light in a wavelength range of 100-400 nm, specifically xenon short
arc lamps, ultra high pressure mercury lamps, high pressure mercury
lamps, low pressure mercury lamps, deuterium lamps, metal halide
lamps, etc., [see Kinoshita: "UV irradiators", Adhesion (2002, vol.
46, No. 7) pp. 20-27; or Sugimori Akira: "Photochemistry, Chapter 8
Experimental methods of photochemistry I" (Shokabo Publishing Co.,
Ltd. 1998) pp. 126-136], which can be used alone or by combining
two or more methods.
[0067] The degree of irradiation to which pulp is exposed in the
irradiation reactor can be adjusted at will by controlling the
retention time of pulp in the irradiation reactor or controlling
the power of the irradiation source. Specifically, the retention
time of pulp in the irradiation reactor can be controlled by
diluting the pulp consistency in the irradiation reactor with water
or by blowing air or an inert gas such as nitrogen into pulp slurry
to control the pulp consistency. These conditions can be selected
as appropriate depending on the pulp quality goals (brightness,
etc.) after the irradiation reaction.
[0068] The second aspect of the first invention is characterized in
that the irradiation treatment with UV light and/or visible light
at a wavelength of 100-400 nm is performed in the presence of
ozone. In the case of irradiation with light in a wavelength range
of 135-242 nm, ozone is normally generated due to the presence of
air in the gas layer around the light source. In the present
invention, air is continuously supplied to the surroundings of the
light source while continuously extracting the generated ozone and
injecting it into the irradiation target material, whereby the
ozone can be used as a reaction promoter without supplying ozone
from any external source. In addition, a larger amount of ozone can
be obtained by supplying air to the gas layer around the light
source. Naturally, the generated ozone can be used not only as a
promoter during the irradiation reaction but also for regular ozone
bleaching. Thus, another great advantage of the present invention
is that the ozone by-produced in the irradiation reactor can be
used. Ozone can also be generated by supplying such air or oxygen
to the pulp to be treated but not to the surroundings of the light
source. Moreover, ozone obtained from an external source by a
method not relying on such effect of UV light can also be used as a
reaction promoter.
[0069] The concentration of the ozone by-produced in the
irradiation reactor is 0.5-100 ppm, depending on the manner of
supplying air or oxygen or the concentration of oxygen. A major
feature of the second aspect of the first invention is that a high
bleaching efficiency can be obtained with even such
low-concentration ozone by combination with the irradiation
treatment.
[0070] This concept can be further expanded to select and use
multiple light sources having a characteristic wavelength range
within 100-400 nm and different characteristic wavelengths as light
sources in the irradiation reactor. Specifically, a light source
having a narrow wavelength characteristic of 135-242 nm providing a
high ozone-generating efficiency and a light source having an
evenly distributed wavelength range of 100-400 nm suitable for
irradiation reaction can be combined, for example, whereby a
further higher bleaching efficiency can be obtained.
[0071] In the irradiation treatment of the present invention, the
irradiation reaction efficiency can be increased by using a
combination of additives such as reducing agents (e.g., NaBH.sub.4,
hydrazine, hydrogen), oxidizing agents (e.g., oxygen, ozone),
peroxides (e.g., hydrogen peroxide, peracetic acid, Na
percarbonate, Na perborate), hydrogen-donating organic compounds
(e.g., alcohols; linear amines such as ethyl amine, diethylamine;
cyclic amines such as tetramethyl piperidine), acetyl-containing
organic compounds (e.g., a-acetyl-.gamma.-butyrolactone, acetol,
acetone).
[0072] In the third aspect of the first invention, ozone bleaching,
hydrogen peroxide bleaching and the like can be used alone or in
combination as TCF bleaching for attaining an ISO brightness of
70-75% before the light bleaching stage.
[0073] In the third aspect of the first invention, ozone bleaching
can be performed under normal ozone bleaching conditions. That is,
ozone bleaching conditions may include using ozone gas at an ozone
concentration of 1-20% by weight at pH 1-8, a pulp consistency of
0.1-50% by weight, and a temperature of 25-95.degree. C. The
pressure during ozone bleaching is not specifically limited from a
negative pressure state to a pressurized state.
[0074] In the third aspect of the first invention, hydrogen
peroxide bleaching can be performed under normal alkaline hydrogen
peroxide bleaching. That is, alkaline hydrogen peroxide bleaching
can be performed with hydrogen peroxide at a concentration of
0.1-2.0% by weight based on the pulp, pH 11-13, a pulp consistency
of 0.1-50% by weight, and a temperature of 50-95.degree. C.
[0075] In the third aspect of the first invention, the ISO
brightness has been improved to 70-75% in advance by normal TCF
bleaching after the acid treatment, whereby the treatment period
for light bleaching can be greatly shortened, and as a result,
electric power costs required for light bleaching can be greatly
reduced. Moreover, the ISO brightness is improved by 5% or more to
reach 75-80% by light bleaching followed by hydrogen peroxide
bleaching at the final bleaching stage, whereby the hydrogen
peroxide bleaching reaction is promoted so that a pulp having an
ISO brightness of 84% or more can be efficiently obtained. In terms
of bleaching costs and pulp quality, it is not preferable to
increase the brightness over 75% by TCF bleaching before light
bleaching because larger amounts of chemicals are required under
harsher reaction conditions (high temperature, long period). If the
ISO brightness before light bleaching is higher than 80%, only very
small amounts of coloring components are contained in the pulp so
that the reaction efficiency of light bleaching significantly
deteriorates. Thus, the brightness is preferably improved by
hydrogen peroxide bleaching rather than light bleaching in terms of
bleaching costs. If the brightness after TCF bleaching is lower
than 70%, large amounts of coloring components are contained in the
pulp so that a long period is required for the light bleaching
treatment to achieve a final brightness of 84% or more, which
invites an increase in electric power costs. If the brightness
after light bleaching is lower than 75%, it is difficult to achieve
a final brightness of 84% or more by hydrogen peroxide bleaching
alone because of relatively large amounts of hard-to-bleach
coloring components remaining in the pulp.
[0076] An example of a system using an irradiation reactor in the
present invention is shown in FIG. 1.
[0077] An acid-treated pulp is received in an irradiation target
material conditioning tank (10), where it is conditioned to a
temperature, pH and pulp consistency suitable for irradiation
reaction while stirring. The conditioned irradiation target
material 1 is sent to an irradiation reactor (12) via an
irradiation reactor feeding pump (11). Before that, an additive
such as a reducing agent in the irradiation reaction is added as a
chemical solution, if desired. The additive can be added at the
site indicated in FIG. 1 or the irradiation target material
conditioning tank (10) or both at will depending on the properties
of the additive or irradiation reaction conditions, but
fast-reacting additives or highly decomposable additives are
preferably added immediately before the entry into the irradiation
reactor (12), i.e. the site indicated in FIG. 1.
[0078] If desired, a gas can be supplied before the entry into the
irradiation reactor (12). This allows the pulp consistency in the
irradiation reactor (12) to be controlled (in terms of % by volume
here because of the low gas density), whereby the retention time of
the pulp in the irradiation reactor (12) or the irradiation
reaction period can be controlled at will. The type of the gas used
here is preferably air or an inert gas such as nitrogen, and such
gas is used as fine bubbles dispersed in a pulp slurry. When a gas
such as hydrogen, oxygen or ozone is used among photoreaction
promoters, it can also be supplied to a site indicated in the
figure in the same manner.
[0079] Then, the pulp having reached quality goals and leaving the
irradiation reactor (12) after the irradiation reaction exits the
irradiation reaction and is sent to the subsequent step (C1: pulp
after irradiation reaction 1). The pulp falling short of pulp
quality goals is recycled to repeat the irradiation reaction (C2:
pulp after irradiation reaction 2). The ratio of C1 to C2 can be
determined at will in response to pulp quality goals.
[0080] The irradiation reactor basically consists of an irradiation
source section and a pulp slurry container section, and the present
invention is not specifically limited to either the internal
irradiation type in which the irradiation source section exists
inside the pulp slurry container section or the external
irradiation type in which the irradiation source section exists
outside the pulp slurry container section [see Sugimori Akira:
"Photochemistry, Chapter 8 Experimental methods of photochemistry
I" (Shokabo Publishing Co., Ltd. 1998) pp. 126-136]. It is
necessary to provide a barrier against gases such as air normally
existing around the light source section from which light is
emitted to a pulp slurry. In this regard, the choice of the barrier
material is important in order that light energy may permeate
through the barrier without being attenuated.
[0081] In the present invention, a hard glass barrier can be used
in combination with light at a wavelength longer than e.g., 300 nm,
while a quartz glass barrier is used in combination with light at a
wavelength shorter than 254 nm. The materials of parts of the pulp
slurry container not involved in light transmission reaction can be
selected from suitable materials less sensitive to the light
wavelength used.
[0082] An example of an irradiation reactor is shown in FIG. 2. An
acid-treated pulp is conditioned to a temperature, pH and pulp
consistency suitable for irradiation reaction and optionally
combined with additives such as reducing agents if desired, and
then injected as slurry (a1) into a reaction chamber (20) via (25).
The injected pulp slurry undergoes irradiation reaction with light
generated from an irradiation source (22) and having passed through
the barrier (21: quartz glass tube) while it flows within the
reactor (20), after which it is discharged from an outlet (26) of
the reactor.
[0083] If desired, a gas can be supplied via a diffuser (24) fitted
to the irradiation reactor (20). This allows the pulp consistency
in the irradiation reactor (20) to be controlled (in terms of % by
volume because of the low gas density), whereby the retention time
of the pulp in the irradiation reactor (20) or the irradiation
reaction period can be controlled at will. The type of the gas used
here is preferably air or an inert gas such as nitrogen, and such
gas is used as fine bubbles dispersed in a pulp slurry.
[0084] When a gas such as hydrogen, oxygen or ozone is used among
photoreaction promoters, it can also be supplied via this diffuser
(24).
[0085] When light at a wavelength range of 135-242 nm is used as an
irradiation source and air or oxygen is injected as a light source
cooling gas (b1) as shown in FIG. 2, ozone exists in the gas
discharged from the irradiation section. This discharged gas
containing ozone can be injected into a pulp slurry in the
irradiation reactor (20) via diffuser (24), whereby the ozone can
be used as a reaction promoter without supplying ozone from any
external source. The generated ozone can be used not only as an
irradiation reaction promoter but also for normal ozone bleaching.
It can also be used in combination with a gas effective as an
irradiation reaction promoter such as hydrogen, oxygen or ozone
injected from an external source. The use of these gases can be
selected at will by providing a three-way valve (23a, 23b).
[0086] The irradiation reactor can be fitted at will with accessory
equipments such as temperature/pH controller, gas concentration
detector, etc., if desired.
[0087] The irradiation treatment of the present invention can be
repeated one or more times in any manner that can be selected as
appropriate depending on a situation such as bleaching efficiency,
pulp quality goals (brightness), or the relationship with other
bleaching processes used in combination. Examples of manners of
repeating the irradiation treatment one or more times are as
follows. (1) Two or more irradiators shown in FIG. 1 can be
provided. In this case, they can be arranged in a series or in
parallel. (2) Multiple irradiation sources (which may have
identical or different characteristics) can be placed in the
irradiator shown in FIG. 1. (3) Pulp can be recycled in the system
shown in FIG. 1.
[0088] The bleaching process of the present invention can be
combined with any other known chlorine or chlorine-free bleaching
process. Specifically, another bleaching process can be followed by
the bleaching process of the present invention, or the bleaching
process of the present invention can be followed by another
bleaching process. Especially, bleaching of the present invention
is preferably followed by a hydrogen peroxide treatment. These
sequences can be repeated multiple times, and a washing stage can
be inserted between different bleaching processes. The bleaching
sequence incorporating an irradiation system can also be repeated
multiple times. When the irradiation treatment is performed
multiple times, the irradiation treatment is preferably followed by
washing.
[0089] Embodiments of the second invention are explained below.
[0090] The present invention relates to a high brightness chemical
pulp having an ISO brightness of 88% or more and a brightness loss
of 1.0% or less in the fading test described below. That is, our
studies revealed that the discoloration of paper evaluated by the
fading test using UV light as described below shows a good
correlation with the actual discoloration, rather than the
conventional thermal fading test.
[0091] A hand-made paper is prepared according to JIS P 8222 and
irradiated with a xenon lamp. at an intensity of 67 W/m.sup.2 for
30 minutes in an atmosphere at 30.degree. C. according to fading
test method B of J. TAPPI No. 21 Paper and Paperboard (using a
xenon arc lamp light fastness tester) and then the ISO brightness
is measured and a loss from the ISO brightness before treatment is
determined. The chemical pulp used in the present invention can be
obtained by using known cooking processes such as kraft cooking,
polysulfide cooking, sodium hydroxide cooking or alkaline sulfite
cooking, among which kraft cooking is preferred in terms of pulp
quality and energy efficiency. Kraft cooking processes include
known modified cooking processes such as MCC, EMCC, ITC, and
Lo-solids processes, any of which can be applied to the present
invention without limitation. Wood can be kraft-cooked under known
conditions, for e.g., under conditions described below. The
sulfidity of the cooking liquor is 7-75%, preferably 15-45%, the
effective alkali content is 5-30% by weight, preferably 10-25% by
weight on the basis of the bone dry weight of wood, and the cooking
temperature is 140-170.degree. C. The cooking process may be in a
continuous mode or a batch mode, and the type of cooker is not
specifically limited.
[0092] The chemical pulp used in the present invention is obtained
by washing an unbleached chemical pulp obtained by a known cooking
process, passing it through coarse screening and fine screening
steps and then subjecting it to an oxygen delignification
treatment. The oxygen delignification can be performed under known
conditions. In the case of hardwood chemical pulps, the kappa
number after oxygen delignification is typically in the range of
5-15, preferably 7-15, more preferably 8-12. This oxygen
delignification treatment is performed by a known, medium
consistency process or high consistency process. For example,
typical reaction conditions for the medium consistency process
include a pulp consistency of 10-18% by weight, a temperature of
100-110.degree. C., a reaction period of 60-120 minutes, and a
pressure in the reactor of 3-6 kg/m.sup.2, and the sodium hydroxide
content and the oxygen content are adjusted depending on the target
kappa number.
[0093] In the preparation of the high brightness chemical pulp of
the present invention, it is preferable to use a pulp having
undergone oxygen delignification followed by an acid treatment. The
type of the acid used in the acid treatment of the pulp may be
inorganic or organic. Inorganic acids that can be used include
mineral acids such as sulfuric acid, hydrochloric acid, nitric
acid, sulfurous acid, nitrous acid, phosphoric acid, and residual
acids in chlorine dioxide generators. Sulfuric acid is preferred.
Organic acids that can be used include acetic acid, lactic acid,
oxalic acid, citric acid, formic acid, etc. Hardwood pulps are
desirably acid-treated in a pH range of 1.5-6.0, preferably
1.0-5.0, more preferably 2.0-5.0, most preferably 2.5-3.5. If the
pH is less than 1.0, hexenuronic acid and metal ions are
sufficiently removed, but the pulp viscosity greatly decreases
because of excessive acidity. If the pH exceeds 6.0, however, the
acid concentration is so low that hexenuronic acid and metal ions
are insufficiently removed. Hardwood chemical pulps can be
acid-treated at lower temperatures in a pH range of 2.5-3.5, which
leads to the advantage that the acid treatment cost can be
reduced.
[0094] The acid treatment can be performed at an atmospheric
pressure or under pressure. For example, the reaction temperature
during the acid treatment at an atmospheric pressure is in the
range of 80.degree. C. or higher and lower than 100.degree. C.
Preferably, it is 80-95.degree. C., more preferably 80-90.degree.
C. Temperatures of 30.degree. C. or higher and lower than
80.degree. C. are effective for removing metals but not effective
for removing hexenuronic acid.
[0095] After the acid treatment, the pulp is continuously bleached
by a multistage bleaching process. Chemicals used include those
consisting of known bleaching agents such as atomic chlorine (C),
sodium hydroxide (E), hypochlorites (H), chlorine dioxide (D),
oxygen (O), hydrogen peroxide (P), ozone (Z), sulfuric acid (A),
and organic peracids in combination with bleaching additives, and
any combination appropriately selected from the above list is used
as bleaching chemicals. The bleaching sequence is not specifically
limited, and examples that can be used include sequences including
atomic chlorine and chlorine bleaching chemicals such as
C/D-E/O-H-D; ECF bleaching sequences free from atomic chlorine such
as D-E-D, Z-E/O-D; and TCF bleaching sequences totally free from
chlorine chemicals such as Z-E-P, A-Z-E/O-P.
[0096] The high brightness chemical pulp of the present invention
is preferably prepared by a process further comprising irradiating
the bleached chemical pulp obtained by the process described above
with UV light and/or visible light. The bleached chemical pulp
before light treatment preferably has been bleached to an ISO
brightness of 80% or more, preferably 86% or more. For example,
very high brightness pulp can be easily obtained by introducing a
peroxide bleaching stage after the light treatment stage.
[0097] The irradiation treatment with UV light and/or visible light
is preferably performed under alkaline conditions. The alkaline
conditions are preferably in a pH range of 10-13. The alkalis that
can be used for this pH adjustment include normal alkaline
chemicals, among which sodium hydroxide is preferred. The acids
that can be used for the pH adjustment include normal acidic
chemicals, among which sulfuric acid is preferred.
[0098] The pulp consistency during the irradiation treatment with
UV light and/or visible light is preferably 0.1-12% by weight.
Consistencies lower than 0.1% by weight are not preferred because
energy efficiency decreases though bleaching reaction efficiency
increases. Consistencies higher than 12% by weight are not
preferred because the rheology of pulp slurry in the bleacher
decreases so that bleaching reaction efficiency decreases.
[0099] The temperature during the irradiation treatment with UV
light and/or visible light is not specifically limited, either, but
preferably 20-95.degree. C. Temperatures lower than 20.degree. C.
are not preferred because bleaching reaction efficiency is low
while temperatures exceeding 95.degree. C. are not preferred,
either, because pulp quality may deteriorate or the pressure in the
reactor may exceed atmospheric pressure so that the reactor must be
designed to resist pressure.
[0100] The irradiation period of UV light and/or visible light can
be determined as appropriate by taking into account the structures
or concentrations of potential coloring matters contained in the
material pulp.
[0101] The UV light and/or visible light used in the present
invention is not specifically limited, but it is desirable to use
UV light and/or visible light at a wavelength of about 100-400 nm,
preferably 200-360 nm. UV light at a wavelength shorter than 100 nm
is not preferred because photodegradation of cellulose is promoted
so that pulp strength and brightness are greatly lowered, while UV
light at a wavelength exceeding 400 nm is not preferred, either,
because coloring matters are insufficiently photoexcited so that
light bleaching performance is greatly lowered.
[0102] Irradiation sources that can be used include conventional
light sources such as low-pressure mercury lamps, high-pressure
mercury lamps, and xenon lamps as well as various excimer lamps and
various lasers, but when a large amount of pulp is to be treated,
it is desirable to use a high power ozone-generating low-pressure
mercury lamp. Ozone-generating UV lamps mainly emit UV light at a
wavelength of 254 nm and also include UV light at a wavelength of
185 nm and visible light. The irradiation intensity of UV light at
a wavelength of 185 nm is not influenced by temperature, but the
intensity of UV light at a wavelength of 254 nm is
temperature-dependent and reaches its maximum at 20-40.degree. C.
Thus, high power ozone-generating lamps having a high surface
temperature are cooled with air, and at the same time, ozone gas is
generated from oxygen in the air by UV light at a wavelength of 185
nm. In the case of wastewater treatment, this ozone gas decomposes
by UV light at a wavelength of 254 nm to produce a highly active
oxygen species which remarkably promotes decomposition of coloring
components. In contrast to wastewater treatment in which the
treatment efficiency improves as the ozone concentration increases,
excessive ozone not only blocks UV light at a wavelength of 254 nm,
which is the most effective UV light for pulp bleaching, to invite
bleaching efficiency loss but also accelerates damage to cellulose
fibers by a lot of active oxygen species generated from high
concentration ozone to invite significant paper strength loss.
Thus, an optimum amount of ozone should be supplied, and such an
amount is controlled as appropriate depending on the structures or
amounts of coloring matters in the pulp.
[0103] In the present invention, all of the known reducing agents,
oxidizing agents and hydrogen-donating organic compounds can be
used as light bleaching promoters. Such reducing agents include,
for example, hydrosulfite and borohydride compounds, etc.;
oxidizing agents include hydrogen peroxide, sodium percarbonate,
peracetic acid, etc.; and hydrogen-donating organic compounds
include primary alcohols such as ethanol. Additives in the present
invention may be used alone without using solvents, but should
desirably be used as dispersions or solutions in solvents
transparent to UV/visible light. Different additives can also be
used as mixtures. Such solvents include water, alcohols, linear or
cyclic alkanes, ethers, etc. as single solvents or mixed solvents
thereof, preferably water. The amount of each additive to be used
is not specifically limited so far as it is at or below the
saturated concentration of the additive in the solvent, but a
suitable amount is preferably 0.01-40% by weight, more preferably
0.1-20% by weight in the solvent.
[0104] Although the reason why the high brightness chemical pulp
obtained by the present invention shows very little discoloration
is not clear, it is assumed that discoloration is not induced by
relatively weak UV light emitted from the lamp used in the fading
test because materials responsible for coloration involved in
discoloration remaining in the pulp have been preliminarily
decomposed by a very intense UV light at 254 nm and removed.
[0105] Papers containing the high brightness chemical pulp of the
present invention can be used as not only book papers but also
offset printing papers, relief printing papers, gravure printing
papers, newsprint papers, electrophotographic papers, or base
papers for coated papers, inkjet recording papers, thermosensitive
recording papers, pressure sensitive recording papers or the
like.
[0106] In addition to the high brightness chemical pulp of the
present invention, papers containing the high brightness chemical
pulp of the present invention may use other raw pulps such as
chemical pulps, mechanical pulps and deinked pulps alone or in
admixture at any ratio. The pH during the papermaking process may
be acidic or neutral or alkaline.
[0107] Papers containing the high brightness pulp of the present
invention can contain paper strength enhancers. Examples of paper
strength enhancers include starches, modified starches,
polyacrylamide, polyvinyl alcohol, polyamide-polyamine resins,
urea-formalin resins, melamine-formalin resins, polyethylene
imines, etc. The paper strength enhancers are preferably contained
in an amount of 0.1% by weight or more and 2% by weight or less on
the basis of the bone dry weight of the pulp.
[0108] Papers containing the high brightness pulp of the present
invention can contain fillers. Fillers that can be used include
known fillers such as white carbon, talc, kaolin, clay, ground
calcium carbonate, precipitated calcium carbonate, titanium oxide,
synthetic resin fillers, etc.
[0109] Papers containing the high brightness pulp of the present
invention can further contain aluminum sulfate, sizing agents,
yield improvers, freeness improvers, colorants, dyes, antifoaming
agents, bulking agents, fluorescent whitening agents or the like,
if desired.
[0110] Papers containing the high brightness pulp of the present
invention may not be coated or may be coated with a pigment-free
finishing agent. Non-coated papers are desirably coated with a
finishing agent based on a water-soluble polymer for the purpose of
improving surface strength or sizing performance. Suitable
water-soluble polymers include commonly used finishing agents such
as starches, modified starches, polyacrylamide, polyvinyl alcohol,
etc. alone or as mixtures thereof. In addition to the water-soluble
polymers, the finishing agents can also contain paper strength
enhancers designed to improve water resistance or surface strength
and external sizing additives designed to provide sizing
performance. The finishing agents can be applied with coaters such
as two-roll size press coaters, gate roll coaters, blade metering
coaters, rod metering coaters, etc. The finishing agents are
preferably applied in an amount of 0.1 g/m.sup.2 or more and 3
g/m.sup.2 or less per side.
EXAMPLES
[0111] The following examples further illustrate the present
invention in detail without, however, limiting the invention
thereto.
[0112] <Determination of Physical Properties of Pulp>
[0113] Determination of kappa number: performed according to JIS P
8211.
[0114] Determination of pulp brightness: Pulp was defibrated and
then formed into a sheet having a basis weight of 60 g/m.sup.2
according to Tappi test method T205os-71 (JIS P 8222), and measured
for pulp brightness according to JIS P 8148.
[0115] <Experimental Apparatus>
[0116] The experimental apparatus used in the examples below is
shown in FIG. 3.
[0117] An irradiation reaction chamber (1) consists of a 3 L glass
cylinder (100 mm f.times.620 mm H). This irradiation reaction
chamber (1) is equipped with a temperature controller and a pH
meter in addition to a stirrer (4) and a diffuser tube (5) shown in
the figure. An irradiation source (16 W low-pressure mercury lamp,
AY-1 from Photoscience Japan Corporation) is placed in a quartz
glass tube (45 mm f.times.470 mm H, thickness 2 mm) in such a
manner that air can be injected around the irradiation source.
Example 1
[0118] An oxygen-delignified hardwood kraft pulp available from
Nippon Paper Group, Inc. (kappa number 11.6, ISO brightness 45.6%)
was used.
[0119] An acid treatment was performed under the following
conditions to give a pulp having a kappa number of 5.5 and a
brightness of 47.5%.
[0120] Acid treatment conditions: pulp consistency 10% by weight,
pH 3.0 (adjusted with sulfuric acid), temperature 95.degree. C.,
treatment period 180 minutes. After the treatment was completed,
the pulp was washed with water.
[0121] A 5 g (bone dry weight) portion of the acid-treated pulp
thus obtained was collected and diluted to a pulp consistency of
0.25% by weight and then prepared into pulp slurries at pHs over an
acidic to alkaline range using NaOH and H.sub.2SO.sub.4. These
slurries were injected into the experimental apparatus shown in
FIG. 3, and subjected to an irradiation reaction while stirring
under conditions of a temperature of 25.degree. C. for a treatment
period of 120 minutes using a low-pressure UV lamp having a
dominant wavelength at 254 nm. After the reaction was completed,
the pulps were washed and then formed into sheets and measured for
brightness. The results are shown in FIG. 4 and Table 1.
Example 2
[0122] The same oxygen-delignified hardwood-kraft pulp as used in
Example 1 was treated with ozone under the following conditions to
give a pulp having a kappa number of 3.0 and a brightness of
56.6%.
[0123] Ozone treatment conditions: pulp consistency 10%, ozone feed
7 kg/ADTP, temperature 50.degree. C., treatment period 30 seconds,
pH 2.5 (adjusted with sulfuric acid).
[0124] A 5 g (bone dry weight) portion of the ozone-treated pulp
thus obtained was collected and prepared into pulp slurries at pHs
over an acidic to alkaline range and subjected to an irradiation
reaction under similar conditions to those of Example 1, and the
resulting pulps were measured for brightness. The results are shown
in FIG. 4 and Table 1.
Comparative Example 1
[0125] A 5 g (bone dry weight) portion of the same
oxygen-delignified hardwood kraft pulp as used in Example 1 was
collected and prepared into pulp slurries at pHs over an acidic to
alkaline range and subjected to an irradiation reaction under
similar conditions to those of Example 1, and the resulting pulps
were measured for brightness. The results are shown in FIG. 4 and
Table 1.
[0126] [Table 1] TABLE-US-00001 TABLE 1 ISO brightness (%) pH
Example 1 Example 2 Comparative example 1 2.84 60.24 67.84 4.69
57.95 71.41 7.77 58.75 69.26 9.48 60.3 74.67 55.58 10.43 66.15
77.41 58.83 11.75 71.19 82.36 61.85 12.61 67.7 77.83 62.9
Example 3
[0127] A 5 g (bone dry weight) portion of the same acid-treated
pulp as used in Example 1 was collected and diluted to a pulp
consistency of 0.25% by weight and then prepared into a pulp slurry
at pH 11.5 (adjusted with NaOH and H.sub.2SO.sub.4). This slurry
was injected into the experimental apparatus shown in FIG. 3, and
subjected to an irradiation reaction while stirring under
conditions of a temperature of 25.degree. C. for varying treatment
periods using a low-pressure UV lamp having a dominant wavelength
at 254 nm. After the reaction was completed, the pulps were washed
and then formed into sheets and measured for brightness. The
results are shown in FIG. 5 and Table 2.
Example 4
[0128] A 5 g (bone dry weight) portion of the same ozone-treated
pulp as used in Example 2 was collected and subjected to an
irradiation reaction under similar conditions to those of Example 3
for varying treatment periods and measured for brightness. The
results are shown in FIG. 5 and Table 2.
Comparative Example 2
[0129] A 5 g (bone dry weight) portion of the same
oxygen-delignified hardwood kraft pulp as used in Comparative
example 1 was collected and subjected to an irradiation reaction
under similar conditions to those of Example 3 for varying
treatment periods and measured for brightness. The results are
shown in FIG. 5 and Table 2.
[0130] [Table 2] TABLE-US-00002 TABLE 2 UV irradiation ISO
brightness (%) period (h) Example 3 Example 4 Comparative example 2
0 47.5 56.57 47.5 0.5 55.57 70.97 1 62.87 79.05 2 71.19 82.36 61.85
4 78.59 85.5 66.75 8 85.69 85.86 79.59
Example 5
[0131] An oxygen-delignified softwood kraft pulp available from
Nippon Paper Group, Inc. (kappa number 9.1, ISO brightness 33.3%)
was used.
[0132] An acid treatment was performed under the following
conditions to give a pulp having a kappa number of 9.1 and a
brightness of 34.3%.
[0133] Acid treatment conditions: pulp consistency 10% by weight,
pH 3.0 (adjusted with sulfuric acid), temperature 95.degree. C.,
treatment period 180 minutes. After the treatment was completed,
the pulp was washed with water.
[0134] A 5 g (bone dry weight) portion of the acid-treated pulp
thus obtained was collected and diluted to a pulp consistency of
0.25% by weight and then prepared into pulp slurries at pHs over an
acidic to alkaline range using NaOH and H.sub.2SO.sub.4. These
slurries were injected into the experimental apparatus shown in
FIG. 3, and subjected to an irradiation reaction while stirring
under conditions of a temperature of 25.degree. C. for a treatment
period of 120 minutes using a low-pressure UV lamp having a
dominant wavelength at 254 nm. After the reaction was completed,
the pulps were washed and then formed into sheets and measured for
brightness. The results are shown in FIG. 5 and Table 3.
Comparative Example 3
[0135] A 5 g (bone dry weight) portion of the same
oxygen-delignified softwood kraft pulp as used in Example 5 was
collected and prepared into pulp slurries at pHs over an acidic to
alkaline range and subjected to an irradiation reaction under
similar conditions to those of Example 5, and the resulting pulps
were measured for brightness. The results are shown in FIG. 6 and
Table 3.
[0136] [Table 3] TABLE-US-00003 TABLE 3 ISO brightness (%) pH
Example 5 Comparative example 3 10.43 44.52 41.1 11.75 54.89 48.02
12.61 57.24 47.56 13.1 54.31 48.23
Example 6
[0137] An oxygen-delignified hardwood kraft pulp available from
Nippon Paper Group, Inc. (kappa number 9.5, ISO brightness 47.5%)
was used.
[0138] An acid treatment was performed under the following
conditions to give a pulp having a kappa number of 5.5 and a
brightness of 48.6%.
[0139] Acid treatment conditions: pulp consistency 10% by weight,
pH 3 (adjusted with sulfuric acid), temperature 85.degree. C.,
treatment period 180 minutes. After the treatment was completed,
the pulp was washed with water.
[0140] A 5 g (bone dry weight) portion of the acid-treated pulp
thus obtained was collected and diluted to a pulp consistency of
0.5% by weight and then prepared into a pulp slurry at pH 11.5 with
NaOH. This slurry was injected into the experimental apparatus
shown in FIG. 3, and subjected to an irradiation reaction while
stirring under conditions of a temperature of 25.degree. C. for a
treatment period of 120 minutes using a low-pressure UV lamp having
a dominant wavelength at 254 nm. After the reaction was completed,
the pulp was washed and then formed into a sheet and measured for
brightness. The results are shown in Table 4.
Comparative Example 4
[0141] A 15 g (bone dry weight) portion of the same acid-treated
pulp as used in Example 6 was collected and diluted to a pulp
consistency of 0.5% by weight and then injected into the
experimental apparatus shown in FIG. 3, and subjected to an ozone
treatment while stirring at a temperature of 25.degree. C., pH 11.5
for 120 minutes (without using an irradiation source). The
accumulated ozone feed to the pulp during the time was 0.7% by
weight. The results are shown in Table 4.
Example 7
[0142] A 15 g (bone dry weight) portion of the same acid-treated
pulp as used in Example 6 was collected and diluted to a pulp
consistency of 0.5% by weight and then injected into the
experimental apparatus shown in FIG. 3, and subjected
simultaneously to an ozone treatment and an irradiation reaction
while stirring at a temperature of 25.degree. C., pH 11.5 for 120
minutes using a low-pressure UV lamp having a dominant wavelength
at 254 nm. The accumulated ozone feed to the pulp during the time
was 0.7% by weight. The results are shown in Table 4.
Example 8
[0143] The same oxygen-delignified hardwood kraft pulp as used in
Example 1 (kappa number 11.6, ISO brightness 45.6%) was treated
with ozone under the following conditions to give a pulp having a
kappa number of 3.0 and a brightness of 56.6%.
[0144] Ozone treatment conditions: pulp consistency 10%, ozone feed
7 kg/ADTP, temperature 55.degree. C., treatment period 30 seconds,
pH 2.5.
[0145] A 15 g (bone dry weight) portion of the ozone-treated pulp
thus obtained was collected and diluted to a pulp consistency of
0.5% by weight and then injected into the experimental apparatus
shown in FIG. 3, and subjected to an irradiation reaction while
stirring at a temperature of 25.degree. C., pH 11.5 for 120 minutes
using a low-pressure UV lamp having a dominant wavelength at 254
nm. The results are shown in Table 4. TABLE-US-00004 TABLE 4 ISO
brightness Gain Treatment after after Pulp method treatment
treatment Example 6 After acid Irradiation 60.4% 11.8% treatment
treatment with heating alone Comparative After acid Ozone 51.7%
3.1% example 4 treatment treatment with heating alone Example 7
After acid Combination 72.2% 23.6% treatment of ozone with heating
treatment and irradiation treatment Example 8 After ozone
Irradiation 74.7% 18.1% treatment treatment
[0146] <Evaluation of Physical Properties of Pulp>
[0147] Determination of kappa number: performed according to JIS P
8211.
[0148] Determination of pulp brightness: Pulp was defibrated and
then formed into a sheet having a basis weight of 60 g/m.sup.2
according to JIS P 8222, and measured for pulp brightness according
to JIS P 8148.
[0149] <Experimental Apparatus>
[0150] The experimental apparatus used in the examples below is
shown in FIG. 3. An irradiation reaction chamber (20) consists of a
4 L glass cylinder (100 mm f.times.620 mm H). This irradiation
reaction chamber (20) is equipped with a temperature controller and
a pH meter in addition to a stirrer (27) and a diffuser tube (24)
shown in the figure. An irradiation source (16 W low-pressure
mercury lamp, AY-1 from Photoscience Japan Corporation) is placed
in a quartz glass tube (25 mm f.times.470 mm H, thickness 2 mm) in
such a manner that air can be injected around the irradiation
source. In the examples below, two such irradiation sources were
used.
Example 9
[0151] An oxygen-delignified hardwood kraft pulp available from
Nippon Paper Group, Inc. (kappa number 11.6, ISO brightness 45.6%)
was acid-treated at a pulp consistency of 10% by weight, pH 3.0
(adjusted with sulfuric acid), at a temperature of 95.degree. C.
for a treatment period of 180 minutes. After the acid treatment was
completed, the pulp was washed with water to give a pulp having a
kappa number of 5.5 and a brightness of 47.5%.
[0152] A 15 g (bone dry weight) portion of the acid-treated pulp
was collected and prepared into a pulp slurry having a consistency
of 0.5% by weight with water, and then the pulp slurry was adjusted
to pH 11.5 with NaOH. This pulp slurry was injected into the
experimental apparatus shown in FIG. 3, and subjected to an
irradiation reaction while stirring under conditions of a
temperature of 25.degree. C. for a treatment period of 120 minutes
using a low-pressure UV lamp having a dominant wavelength at 254
nm. Simultaneously, the air injected into the quartz glass tube
shown in FIG. 3 to cool the light source was discharged via the
diffuser and ozone was fed. After the treatment was completed, the
pulp was washed and then formed into a sheet and measured for
brightness. The results are summarized in Table 5.
Example 10
[0153] Irradiation treatment and ozone feeding were performed under
similar conditions to those of Example 9 except that the pulp
slurry was adjusted to pH 2.9 with sulfuric acid, after which the
resulting pulp was measured for brightness. The results are
summarized in Table 5.
Example 11
[0154] Irradiation treatment and ozone feeding were performed under
similar conditions to those of Example 1 except that the pulp
slurry was treated at pH 6.6 without pH adjustment, after which the
resulting pulp was measured for brightness. The results are
summarized in Table 5.
Comparative Example 5
[0155] Irradiation treatment was performed under similar conditions
to those of Example 9 except that ozone feeding was omitted (i.e.,
air was not injected into the quartz glass tube shown in FIG. 3 but
directly introduced into the diffuser to feed the pulp slurry),
after which the resulting pulp was measured for brightness. The
results are summarized in Table 5.
Comparative Example 6
[0156] Reaction was performed under similar conditions to those of
Example 9 except that irradiation treatment was omitted (i.e., the
light source was not turned on) and the ozone (concentration 50
ppm) generated from an ozone generator (ozone spray, NS-3 available
from Kankyo Kogaku Co., Ltd.) was fed via the diffuser shown in
FIG. 3, after which the resulting pulp was measured for brightness.
The results are summarized in Table 5. TABLE-US-00005 TABLE 5
Irradiation Ozone feeding pH Brightness (%) Example 9 Yes Yes 11.5
73.8 Example 10 Yes Yes 2.9 70.9 Example 11 Yes Yes 6.6 63.3
Comparative Yes No 11.5 60.4 example 5 Comparative No Yes 11.5 53.3
example 6
[0157] As shown in Table 5, pulps with higher brightness can be
prepared in Examples 9-11 involving irradiation with UV light or
visible light or both in the presence of ozone. However, brightness
was somewhat lower in Example 11 wherein irradiation was not
performed under acidic conditions of pH 2-4 or under alkaline
conditions of pH 10-13.
[0158] <Determination of ISO Brightness of Pulp>
[0159] Determination of pulp brightness: Pulp was defibrated and
then formed into a sheet having a basis weight of 60 g/m.sup.2
according to JIP P 8222, and measured for ISO brightness according
to JIS P 8148.
[0160] <Pulp>
[0161] An oxygen-delignified hardwood kraft pulp (ISO brightness
45.6%, available from Nippon Paper Group, Inc.) was further
subjected to acid treatment--ozone bleaching under the following
conditions and the resulting pulp was used in the examples and
comparative examples below.
[0162] Acid treatment: The oxygen-delignified hardwood kraft pulp
was acid-treated at a pulp consistency of 10% by weight, pH 3.0
(adjusted with sulfuric acid), at a temperature of 95.degree. C.
for a treatment period of 180 minutes. After the treatment was
completed, the pulp was washed with water. At this stage, the ISO
brightness of the pulp was 47.5%.
[0163] Ozone bleaching: The acid-treated pulp was bleached with
ozone at a pulp consistency of 10%, ozone feed 7 kg/(1 t of
air-dried pulp), at a temperature of 50.degree. C. for a treatment
period of 30 seconds at pH 2.5 (adjusted with sulfuric acid). After
the treatment was completed, the pulp was washed with water. At
this stage, the ISO brightness of the pulp was 59.7%.
Example 12
[0164] The ozone-bleached pulp was further bleached in a bleaching
sequence of hydrogen peroxide bleaching 1--light
bleaching--hydrogen peroxide bleaching 2 under the following
conditions.
[0165] Hydrogen peroxide bleaching 1: pulp consistency 10% by
weight, pH 11.5 (adjusted with sodium hydroxide), temperature
75.degree. C., treatment period 90 minutes. After the treatment was
completed, the pulp was washed with water. At this stage, the ISO
brightness of the pulp was 75.0%.
[0166] Light bleaching conditions: A 5 g (bone dry weight) portion
of the pulp bleached with hydrogen peroxide was collected and
diluted to a pulp consistency of 0.25% by weight and then prepared
into a pulp slurry at pH 11.5 with sodium hydroxide. This slurry
was injected into a 2 L glass cylinder, and subjected to an
irradiation reaction while stirring at a temperature of 25.degree.
C. for a treatment period of 15 minutes, using a 16 W low-pressure
UV lamp having a dominant wavelength at 254 nm (AY-1 from
Photoscience Japan Corporation). After the treatment was completed,
the pulp was washed with water. At this stage, the ISO brightness
of the pulp was 78.5%.
[0167] Hydrogen peroxide bleaching 2: The light bleached pulp was
treated under the same conditions as those of hydrogen peroxide
bleaching 1 above. The final ISO brightness was 85.0%.
Example 13
[0168] Bleaching treatment was performed under the same conditions
as those of Example 1 except that the treatment period of light
bleaching was 30 minutes. The ISO brightness after light bleaching
was 80.0%. The resulting light bleached pulp was treated under the
conditions of hydrogen peroxide bleaching 2 above. The final ISO
brightness was 86.1%.
Example 14
[0169] Treatment was performed under the same conditions as those
of Example 1 except that the treatment period of hydrogen peroxide
bleaching 1 was 45 minutes. The ISO brightness after hydrogen
peroxide bleaching 1 was 71.0%. The ISO brightness after light
bleaching was 74.5%. The resulting pulp was treated under the same
conditions as those of hydrogen peroxide bleaching 2 above. The
final ISO brightness was 84.1%.
Example 15
[0170] The ozone-bleached pulp was further bleached in a bleaching
sequence of light bleaching--hydrogen peroxide bleaching. Light
bleaching was performed under the same conditions as those of
Example 12 except that the treatment period was 60 minutes. The
brightness after light bleaching was 75.4%. The resulting pulp was
treated under the same conditions as those of hydrogen peroxide
bleaching 2 of Example 12. The final ISO brightness was 84.3%.
Example 16
[0171] Treatment was performed under the same conditions as those
of Example 4 except that the treatment period of light bleaching
was 120 minutes. The ISO brightness after light bleaching was
81.8%. The resulting pulp was treated under the same conditions as
those of hydrogen peroxide bleaching 2 of Example 12. The final ISO
brightness was 85.2%.
Example 17
[0172] Treatment was performed under the same conditions as those
of Example 12 except that the treatment period of hydrogen peroxide
bleaching 1 was 30 minutes. The ISO brightness after hydrogen
peroxide bleaching 1 was 68.2%. The ISO brightness after light
bleaching was 72.3%. The resulting pulp was treated under the same
conditions as those of hydrogen peroxide bleaching 2 of Example 12.
The final ISO brightness was 81.7%.
Comparative Example 7
[0173] The ozone-bleached pulp was further bleached in a bleaching
sequence of hydrogen peroxide bleaching 1--hydrogen peroxide
bleaching 2. Bleaching treatment was performed under the same
conditions as those of Example 12 except that light bleaching was
omitted. The final ISO brightness was 79.3%.
Comparative Example 8
[0174] The ozone-bleached pulp was further bleached in a bleaching
sequence of hydrogen peroxide bleaching 1--light bleaching. Light
bleaching was performed under the same conditions as those of
Example 12 except that the treatment period was 60 minutes.
Hydrogen peroxide bleaching 1 was performed under the same
conditions as those of Example 12. The final ISO brightness was
83.3%.
Comparative Example 9
[0175] The ozone-bleached pulp was further bleached in a bleaching
sequence of hydrogen peroxide bleaching 1--light
bleaching--hydrogen peroxide bleaching 2. Bleaching treatment was
performed under the same conditions as those of Example 1 except
that light bleaching was performed at pH 4.0 (adjusted with
sulfuric acid). The brightness after light bleaching was 75.9%. The
resulting pulp was treated under the same conditions as those of
hydrogen peroxide bleaching 2 of Example 12. The final ISO
brightness was 82.6%.
[0176] The results of Examples 12-17 and Comparative examples 7-9
are shown in Table 6. TABLE-US-00006 TABLE 6 ISO brightness ISO
brightness (%) after ISO brightness (%) after hydrogen (%) after
hydrogen peroxide light peroxide bleaching 1 bleaching bleaching 2
Example 12 75.0 78.5 85.0 Example 13 75.0 80.0 86.1 Example 14 71.0
74.5 84.1 Example 15 -- 75.4 84.3 Example 16 -- 81.8 85.2 Example
17 68.2 72.3 81.7 Comparative 75.0 -- 79.3 example 7 Comparative
75.0 83.3 -- example 8 Comparative 75.0 75.9 82.6 example 9
[0177] As shown in Table 6, high brightness pulps were obtained by
treating the ozone-bleached pulp in a bleaching sequence of
hydrogen peroxide bleaching 1--light bleaching--hydrogen peroxide
bleaching 2. However, the final brightness was somewhat low in
Example 6 wherein a pulp having an ISO brightness of less than 70%
before the light bleaching treatment. In the sequence of
Comparative example 2 wherein hydrogen peroxide bleaching was
omitted after light bleaching, the light treatment period had to be
greatly extended to obtain an ISO brightness of 80% or more. In
Comparative example 9 wherein the pH after light bleaching was
acidic, both brightness after light bleaching and final brightness
were lower than those of Example 12.
Example 18
[0178] A 200 g (bone dry weight) portion of a bleached hardwood
pulp (ISO brightness 85.6%) obtained by the chlorine bleaching
process of plant A of Nippon Paper Group, Inc. was collected and
diluted to a pulp consistency of 1% and then adjusted to pH 11.5
with sodium hydroxide. This slurry was injected into the
experimental apparatus shown in FIG. 7, and subjected to a UV light
bleaching treatment while stirring at a temperature of 25.degree.
C. for a treatment period of 120 minutes. When the treatment was
completed, the pulp was washed and then formed into a sheet and
measured for brightness. The sheet measured for brightness was then
used in a fading test. A sheet was prepared from the pulp after
defibration and measured for breaking length. The methods for these
evaluations are described below, and the results are shown in Table
1.
[0179] Determination of freeness: A pulp slurry having a
consistency of 10% was treated at 6000 rev in a PFI mill and then
measured for freeness (CSF) according to JIS P 8121.
[0180] Determination of pulp brightness: Pulp was defibrated and
then formed into a sheet having a basis weight of 60 g/m.sup.2
according to JIP P 8222, and measured for ISO brightness according
to JIS P 8148.
[0181] Determination of breaking length: Pulp was defibrated and
then formed into a sheet having a basis weight of 60 g/m.sup.2
according to JIP P 8222, and measured for breaking length according
to JIS P 8113.
[0182] Fading test: performed with a xenon lamp weatherometer.
Samples were irradiated with UV light generated from a xenon lamp
for 30 minutes and then measured for ISO brightness (JIS P 8148).
The fading test was performed at a temperature of 30.degree. C. and
an intensity of 67 W/m.sup.2. In Table 1, .DELTA. brightness and
brightness loss are defined as follows:
[0183] .DELTA. Brightness=ISO brightness after fading test-ISO
brightness before fading test.
[0184] Brightness loss=.DELTA. brightness/ISO brightness before
fading test.
[0185] UV light bleaching experimental apparatus: The experimental
apparatus used in the example is shown in FIG. 7. An
ozone-generating low-pressure UV lamp (95 W, 18 mm (f).times.1100
mm (H), SUV110D from Sen Light Corporation) is fixed at the center
of a UV light irradiation reaction chamber in the form of a glass
cylinder of 72.1 nm (f).times.1180 mm (H) (effective capacity 2.64
L), and the ozone gas generated (540 mg/h) is introduced from the
bottom of the reaction chamber and flows upward with pulp slurry in
the reaction chamber. After light bleaching, the pulp slurry can be
recycled to the reaction chamber by a pump via a stock tank
(capacity 30 L).
Example 19
[0186] A 200 g (bone dry weight) portion of a bleached hardwood
pulp (ISO brightness 84.9%) obtained by the ozone ECF bleaching
process [acid treatment (oxygen-delignified pulp consistency 10% by
weight, pH 3 (adjusted with sulfuric acid), temperature 85.degree.
C., treatment period 180 minutes), ozone treatment (pulp
consistency 10% by weight, pH 2.5 (adjusted with sulfuric acid),
ozone feed 7 kg/1 t of air-dried pulp, temperature 55.degree. C.,
treatment period 30 seconds)] of plant B of Nippon Paper Group,
Inc. was collected and diluted to a pulp consistency of 1% and then
adjusted to pH 11.5 with sodium hydroxide. This slurry was injected
into the experimental apparatus shown in FIG. 7, and subjected to a
UV light bleaching treatment while stirring at a temperature of
25.degree. C. for a treatment period of 120 minutes. When the
treatment was completed, the pulp was washed and then formed into a
sheet and measured for brightness. The sheet measured for
brightness was then used in the fading test. A sheet was prepared
from the pulp after defibration and measured for breaking length.
The results are shown in Table 7.
Example 20
[0187] A 200 g (bone dry weight) portion of a bleached hardwood
pulp (ISO brightness 84.3%) obtained by the ECF bleaching process
(chlorine dioxide treatment--hydrogen peroxide treatment--chlorine
dioxide treatment) of plant C of Nippon Paper Group, Inc. was
collected and diluted to a pulp consistency of 1% and then adjusted
to pH 11.5 with sodium hydroxide. This slurry was injected into the
experimental apparatus shown in FIG. 7, and subjected to a UV light
bleaching treatment while stirring at a temperature of 25.degree.
C. for a treatment period of 120 minutes. When the treatment was
completed, the pulp was washed and then formed into a sheet and
measured for brightness. The sheet measured for brightness was then
used in the fading test. A sheet was prepared from the pulp after
defibration and measured for breaking length. The results are shown
in Table 7.
Comparative Example 10
[0188] A 100 g (bone dry weight) portion of a bleached hardwood
pulp (ISO brightness 86%) obtained by the chlorine bleaching
process of plant A of Nippon Paper Group, Inc. was collected and
diluted to a pulp consistency of 10% and then adjusted to pH 11.5
with sodium hydroxide. This slurry was bleached with 3.0 kg of
hydrogen peroxide/1 t of air-dried pulp at a temperature of
50.degree. C. for a treatment period of 180 minutes. When the
treatment was completed, the pulp was washed and then formed into a
sheet and measured for brightness. The sheet measured for
brightness was then used in the fading test. A sheet was prepared
from the pulp after defibration and measured for breaking length.
The results are shown in Table 7.
Comparative Example 11
[0189] A bleached hardwood pulp (ISO brightness 89.3%) obtained by
a commercially available chlorine dioxide ECF bleaching process was
used to form a sheet and measured for brightness. The sheet
measured for brightness was then used in the fading test. A sheet
was prepared from the pulp after defibration and measured for
breaking length. The results are shown in Table 7.
Comparative Example 12
[0190] A bleached hardwood pulp (ISO brightness 85.6%) obtained by
the chlorine bleaching process of plant A of Nippon Paper Group,
Inc. was used to form a sheet and measured for brightness. The
sheet measured for brightness was then used in the fading test. A
sheet was prepared from the pulp after defibration and measured for
breaking length. The results are shown in Table 7.
Comparative Example 13
[0191] A bleached hardwood pulp (ISO brightness 84.9%) obtained by
the chlorine bleaching process of plant B of Nippon Paper Group,
Inc. was used to form a sheet and measured for brightness. The
sheet measured for brightness was then used in the fading test. A
sheet was prepared from the pulp after defibration and measured for
breaking length. The results are shown in Table 7. TABLE-US-00007
TABLE 7 ISO brightness (%) Brightness CSF after Breaking Before
After .DELTA. Brightness loss defibration length fading test fading
test (%) (%) (mL) (km) Example 18 90.5 90.0 0.5 0.55 370 5.99
Example 19 89.0 88.4 0.6 0.67 372 5.86 Example 20 88.6 88.0 0.6
0.68 376 5.90 Comparative 89.4 86.5 2.9 3.20 375 5.62 example 10
Comparative 89.3 86.7 2.6 2.90 365 5.18 example 11 Comparative 85.6
82.5 3.1 3.60 385 5.58 example 12 Comparative 84.9 82.0 2.9 3.40
380 5.64 example 13
* * * * *