Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method of treating cellulose fibres
The present invention relates to a method of treating cellulose fibres. The
invention further involves the production of paper from said treated fibres
and the paper
obtainable therefrom. The invention also relates to the use of acellulose
derivative as an
additive to an acidic bleaching stage.
Background of the invention
In the field of papermaking, various methods of improving the wet strength of
paper are known by retaining wet strength agents to the cellulosic fibres in
the pulp
suspension while forming the paper. The wet strength of a paper relates to its
ability to
maintain physical integrity and to resist tearing, bursting, and shredding
under use,
especially under wet conditions. A further important property of wet
strengthened paper is
the softness, especially for tissue paper or the like. The softness can be
described as the
tactile sensation perceived when holding or rubbing a paper across the skin.
WO01/21890 discloses a method of modifying cellulose fibres to provide high
wet strength to a paper. This method, however, involves adding electrolyte to
the pulp
suspension and treating it at a temperature of at least 100 C which restricts
flexibility and
the use of this process.
The present invention intends to provide an energy-efficient and simple method
for producing paper with increased wet strength and softness as well as other
advantageous properties imparted by the modification of the fibres. It is a
further object of
the present invention to provide a method which can be used with conventional
existing
equipment and machinery.
The invention
The present invention relates to a method of modifying cellulose fibres
comprising providing a pulp suspension of cellulose fibres, adding a cellulose
derivative
during the bleaching of said cellulose fibres to at least one acidic bleaching
stage.
Preferably, no electrolyte is added in conjunction with the addition of
cellulose
derivative, except for optional addition of acid or base to adjust the pH.
Addition of a pH
regulating base or acid may be made in an amount of about 0.001 to about 0.5 M
if the
electrolyte is univalent. Addition of e.g. Ca2' or other bivalent electrolyte
could in some
cases increase the risk of precipitation of calcium oxalate. Equipment used in
the
bleaching process may then become clogged with such precipitates derived from
electrolytes since the pulps may naturally contain e.g. oxalic acid. The
electrolyte,
however, does not significantly influence the modification of the fibres.
The pH of the pulp suspension in the acidic bleaching stage suitably is from
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about I to about 7, more preferably from about 2 to about 6, and most
preferably from
about 2 to about 4.
The temperature during the acidic bleaching suitably is from about 30 to about
95, preferably from about 60 to about 90 C.
Preferably, the dry content of cellulose fibres in the pulp suspension is from
about I to about 50, more preferably from about 15 to about 30, and most
preferably from
about 5 to about 15 wt%.
The bleaching suitably is performed for about 0.1 to about 10, preferably from
about I to about 5, and most preferably from about I to about 3 hours. The
acidic
bleaching stage at which the cellulose derivative is added may be duririg any
of the
stages where the pulp is treated with chlorine dioxide, ozone, peracid, or
other acidic
bleaching treatment stages, preferably during the treatment with chlorine
dioxide. In this
context, also acidic stages integrated in the bleaching process or sequences
of acidic
bleaching stages such as washing steps, acidification, or acidic chelating
stages are here
also meant to be included in the bleaching treatment during which a. cellulose
derivative
may be added.
It has been found that the adsorption of cellulose derivatives to cellulose
fibres,
particularly the adsorption of CMC to fibres results in significantly
increased surface
charge compared to non-CMC treated wood fibres.
This may be the explanation why the wet strength of a paper produced from the
CMC-treated pulp in which CMC was added to an acidic bleaching stage was
significantly
increased. as well as the relative wet strength when a wet strength agent
subsequently
was added to the paper furnish in a papermaking process.
The present method can thus also impart enhanced softness properties of the
produced paper. The softness of a paper sheet can be estimated, at least
indirectly, by
means of the relative wet strength value, which is defined as the ratio
between the wet
tensile index and the dry tensile index according to the formula RWS (in %) =
(WS/DS) =
100 , where RWS stands for the relative wet strength, WS is the wet tensile
index and DS
is the dry tensile index of a paper. RWS is often a good measure of the
softness of a
paper; the higher the RWS, the higher the softness of the paper.
The modification with a cellulose derivektive may also influence the effect of
any
subsequent addition of paper chemicals to the pulp furnish which in turn may
influence
both the necessary dosage of the paper chemicals to the pulp furnish as well
as the
quality of the obtained paper product.
It has also been seen that sizing, retention and dewatering can be improved as
a
result of the modified cellulosic fibres in papermaking processes.
Any further paper chemicals suitable in the production of paper may be added
to
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the pulp furnish containing the modified bleached cellulose fibres. Such
chemicals may
include e.g. dry strength, wet strength agents, retention agents, sizing
agents etc.
The cellulose fibres may be derived from any type of soft or hard wood-based
or
nonwood-based material, e.g. pre-bleached, half-bleached or unbleached
sulphite,
sulphate or soda pulps or unbleached, half-bleached or pre-bleached
mechanical,
thermomechanical, chemo-mechanical and chemo-thermomechanical pulps, and
mixtures thereof. As examples of nonwood materials can be mentioned e.g.
bagasse,
kenaf, grass fibres, sisal or the like.
The cellulose derivative, preferably an alkyl cellulose derivative, and most
preferably a carboxymethyl cellulose derivative, is water-soluble or at least
partly water-
soluble or water-dispersible, preferably water-soluble or at least partly
water-soluble.
Preferably, the cellulose derivative is ionic. The cellulose derivative can be
anionic,
cationic or amphoteric, preferably anionic or amphoteric. Examples of suitable
cellulose
derivatives include cellulose ethers, e.g. anionic and amphoteric cellulose
ethers, alkali
cellulose, cellulose metal complexes, graft copolymer cellulose preferably
anionic
cellulose ethers. The cellulose derivative preferably has ionic or charged
groups, or
substituents. Examples of suitable ionic groups include anionic and cationic
groups.
Examples of suitable anionic groups include carboxylate, e.g. carboxyalkyl,
sulphonate,
e.g. sulphoalkyl, phosphate and phosphonate groups in which the alkyl group
can be
methyl, ethyl propyl and mixtures thereof, suitably methyl; suitably the
cellulose derivative
contains an anionic group comprising a carboxylate group, e.g. a carboxyalkyl
group. The
counter-ion of the anionic group is usually an alkali metal or alkaline earth
metal, suitably
sodium.
Examples of suitable cationic groups of cellulose derivatives according to the
invention include salts of amines, suitably salts of tertiary amines, and
quaternary
ammonium groups, preferably quaternary ammonium groups. The substituents
attached
to the nitrogen atom of amines and quaternary ammonium groups can be same or
different and can be selected from alkyl, cycloalkyl, and alkoxyalkyl groups,
and one, two
or more of the substituents together with the nitrogen atom can form a
heterocyclic ring.
The substituents independently of each other usually comprise from I to about
24,
preferably from 1 to about 8 carbon atoms. The nitrogen of the cationic group
can be
attached to the cellulose or derivative thereof by means of a chain of atoms
which
suitably comprises carbon and hydrogen atoms, and optionally 0 and/or N atoms.
Usually the chain of atoms is an alkylene group with from 2 to 18 and
preferably 2 to 8
carbon atoms, optionally interrupted or substituted by one or more
heteroatoms, e.g. 0 or
N such as alkyleneoxy group or hydroxy propylene group. Preferred cellulose
derivatives
containing cationic groups include those obtained by reacting cellulose or
derivative
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thereof with a quaternization agent selected from 2, 3-epoxypropyl trimethyl
ammonium
chloride, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride and mixtures
thereof.
The cellulose derivatives of this invention cari contain non-ionic groups such
as
alkyl or hydroxy alkyl groups, e.g. hydroxymethyl, hydroxyethyl,
hydroxypropyl,
hydroxylbutyl and mixtures thereof, e.g. hydroxyethyl methyl, hydroxypropyl
methyl,
hydroxybutyl methyl, hydroxyethyl ethyl, hydroxypropyl and the like. In a
preferred
embodiment of the invention, the cellulose derivative contains both ionic
groups and non-
ionic groups.
Examples of suitable cellulose derivatives according to the invention include
carboxyalkyl celluloses, e.g. carboxymethyl cellulose, carboxyethyl cellulose,
carboxy-
propyl cellulose, sulphoethyl carboxymethyl cellulose, carboxymethyl
hydroxyethyl
cellulose ("CM-HEC"), carboxymethyl cellulose wherein the cellulose is
substituted with
one or more non-ionic substituents, preferably carboxymethyl cellulose
("CMC").
Examples of suitable cellulose derivatives and methods for their preparation
include
those disclosed in U.S. Pat. No. 4,940,785, which is hereby incorporated
herein by
reference.
The terms "degree of substitution" or "DS", as used herein, mean the number of
substituted ring sites of the beta-anhydroglucose rings of the cellulose
derivative. Since
there are three hydroxyl groups on each anhydroglucose ring of the cellulose
that are
available for substitution, the maximum value of DS is 3Ø According to one
preferred
embodiment of the invention, the cellulose derivative has a degree of
substitution of net
ionic groups ("DSNi") up to about 0.65, i.e. the cellulose derivative has an
average degree
of net ionic substitution per glucose unit up to about 0.65. The net ionic
substitution can
be net anionic, net cationic or net neutral. When the net ionic substitution
is net anionic,
there is a net excess of anionic groups (net anionic groups = the average
number of
anionic groups minus the average number of cationic groups, if any, per
glucose unit) and
DSNI is the same as the degree of substitution of net anionic groups ("DSNA").
When the
net ionic substitution is net cationic, there is a net excess of cationic
groups (net cationic
groups = the average number of cationic groups minus the average number of
anionic
groups, if any, per glucose unit) and DSNI is the same as the degree of
substitution of net
cationic groups ("DSNC"). When the net ionic substitution is net neutral, the
average
number of anionic and cationic groups, if any, per glucose unit is the same,
and DSN, as
well as DSNA and DSNc are 0. According to another preferred embodiment of the
invention, the cellulose derivative has a degree of substitution of
carboxyalkyl groups
("DSCA") up to about 0.65, i.e. the cellulose derivative has an average degree
of
carboxyalkyl substitution per glucose unit up to about 0.65. The carboxyalkyl
groups are
suitably carboxymethyl groups and then DSCA referred to herein is the same as
the
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degree of substitution of carboxymethyl groups ("DScM"). According to these
embodiments of the invention, DSNI, DSNA, DSNC and DScA independently of each
other
are usually up to about 0.60, suitably up to about 0.50, preferably up to
about 0.45 and
more preferably up to 0.40, whereas DSNI, DSNA, DSNc and DSGA independently of
each
5 other are usually at least 0.01, suitably at least about 0.05, preferably at
least about 0.10
and more preferably at least about 0.15. The ranges of DSNI, DSNA, DSNc and
DSCA
independently of each other are usually from about 0.01 to about 0.60,
suitably from
about 0.05 to about 0.50, preferably from about 0.10 to about 0.45 and more
preferably
from about 0.15 to about 0.40.
Cellulose derivatives that are anionic or amphoteric usually have a degree of
anionic substitution ("DSA") in the range of from 0.01 to about 1.0 as long as
DSNi and
DSNA are as defined herein; suitably from about 0.05, preferably from about
0.10, and
more preferably from about 0.15 and suitably up to about 0.75, preferably up
to about 0.5,
and more preferably up to about 0.4. Cellulose derivatives that are cationic
or amphoteric
can have a degree of cationic substitution ("DSc") in the range of from 0.01
to about 1.0
as long as DSNi and DSNc are as defined herein; suitably from about 0.02,
preferably from
about 0.03, and more preferably from about 0.05 and suitably up to about 0.75,
preferably
up to about 0.5, and more preferably up to about 0.4. The cationic groups are
suitably
quaternary ammonium groups and then DSc referred to herein is the same as the
degree
of substitut't.on of quaternary ammonium groups ("DSQN"). For amphoteric
cellulose
derivatives of this invention DSA or DSc can of course be higher than 0.65 as
long as
DSNA and DSNC, respectively, are as defined herein. For example, if DSA is
0.75 and DSc
is 0.15, then DSNA is 0.60.
The water-soluble cellulose derivatives suitably have a solubility of at least
85 %
by weight, based on total weight of dry cellulose derivative, in an aqueous
solution,
preferably at least 90 % by weight, more preferably at least 95 % by weight,
and most
preferably at least 98 % by weight.
The cellulose derivative usually has an average molecular weight which is at
least 20000 Dalton, preferably at least 50000 Dalton, and up to about 1000000
Dalton,
preferably up to about 50000 Dalton.
The cellulose derivative is suitably added in an amount of from about 0.5 to
about 50, preferably from about 5 to about 20, and most preferably from about
5 to about
10 kg/t dry cellulose fibres.
The invention also relates to a paper obtainable by a method comprising
dewatering on a wire a pulp furnish of modified bleached cellulose fibres
produced
according to a method as described herein and forming a paper of said
dewatered pulp
furnish.
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Diagram I
SR vs Wet Strength
30
c .. . . . . . . - - -~ =
= ..=..
CO
- = -Ref
10 --+l--Ads. CMC
= --~r--Stock CMC
5
14 16 18 20 22 24 26 28 30 32 34 36 38
SR
5 As can be seen from diagram 1, the wet strength is strongly increased when
CMC has
been adsorbed in a final chlorine dioxide stage compared to addition of CMC to
the stock
or with no CMC addition. Here, the increase in wet strength of the produced
paper was
up to 65%.
10 Diagram 2
Relative Wet Strength vs SR
38 ~ . -
- =-Ref
-l-Ads. CMC
34 -*-Stock CMC
26
22
=- õ~_.....----=---------- ----
18
14
14 16 18 20 22 24 26 28 30 32 34 36 38
SR
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The invention being thus described, it will be obvious that the same may be
varied in many ways. Such variations are not to be regarded as a departure
from the gist
and scope of the present invention, and all such modifications as would be
obvious to
one skilled in the art are intended to be included within the scope of the
claims. The
following examples will further illustrate how the described invention may be
performed
without limiting the scope of it. If not otherwise stated, all parts and
percentages refer to
parts and percent by weight.
Examples
The aim of the experiment was to adsorb CMC to fibres in a final acidic
bleaching stage which here was a chlorine dioxide stage. Even though not
necessary,
calcium chloride was used to enhance the adsorption. The used pulp was a five
stage
Elemental Chlorine Free bleached softwood pulp of full brightness having a
final
brightness of 90% ISO. A reference pulp was treated as the CMC modified pulp
according to the invention but without the charge of CMC. The final chlorine
dioxide stage
was performed at 80 C for 180 minutes at a 10 wt% pulp consistency. The
chemical
charges were: 10 kg/t chlorine dioxide, as active chlorine 7 kg/t, 18 kg/t
calcium chloride
calculated as CaZ+ based on the weight of dry pulp. The end pH of the chlorine
dioxide
stage was 2.8. The used CMC was Finnfix WRH from Noviant. The degree of
substitution
was 0.5 and the molecular weight 1=106. Wet strength agent Kenores XO was
added at a
charge of 15 kg/t dry pulp to the bleached pulp suspension. The strength
properties of the
CMC treated pulp were evaluated at different beating degrees ( SR). The
beating was
performed in a lab-scale PFI beater. The strength properties of beaten CMC-
treated pulp
were compared with the non-CMC treated beaten reference pulp and with a pulp
to which
CMC was added to the pulp suspension subsequent to the bleaching. The analyzed
pulp
had a final brightness of 90 % ISO that had been bleached with a final
chlorine dioxide
stage (in laboratory scale).
35
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In diagram 2, RWS (relative wet strength) is plotted versus SR for adsorbed
CMC in the
final acidic chlorine dioxide stage and addition of CMC to the stock in the
papermaking
process as well as a reference without addition of CMC. As is evident from
diagram 2, the
RWS is considerably increased in a paper obtained by addition of CMC to the
acidic
chorine dioxide stage.
