Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR TREATING CEI~I~UI~OSIC FIBRES
The present invention relates to a method for modifying
the surface of cellulose-containing fibres, to the surface-
modified fibres and to the products that can be made from
such fibres. These include fibrous structures (or fibrous
webs), including paper or non-woven articles, especially
hygienic products, such as tissue paper and non-woven
wipers. The fibrous structure can further be used for
making different kinds of paper, e.g. fine paper.
1o The present invention also relates to a method of
making such fibrous structures and the final structures
obtainable from such fibrous structures.
The wording "cellulose containing fibres" should be
understood according to the present invention as all kind
of fibres containing cellulose, either natural fibres or
synthetic fibres, e.g. pulp but also regenerated cellulose
fibres like rayon or viscose.
The wording "fibrous structure" should be understood
according to the present invention as covering intermediate
products, having one or more layers.
The terms "paper" and "non-woven" should be understood
according to the present invention aso covering final
products.
Depending on the kind of cellulose-containing fibrous
structures and applications considered, properties of wet
strength, wettability, adhesion, storage stability, or else
absorption kinetics and capacity will be particularly
looked for.
Fibrous structures comprising cellulose-containing
3o fibres have a wide range of use in products such as
printing paper, Kraft paper, packaging paper, tissue paper,
wipes for households use, as well as heavy duty in the
industry. The common feature for all these fibrous
structures is that they have to show at least some strength
when being wet.
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"Non-woven" is commonly understood as an independent
group of products. These composite materials are made up
from endless filaments, shorter staple fibres or micro-
fibres that have been bonded together to a mat, by e.g.
intertwining, by cohesive or adhesive bonding. The fibres
may be synthetic, natural fibres or blend of synthetic and
natural fibres. Examples of natural fibres are cotton and
cellulose pulp fibres (one can for example report to
standards DIN 61 210 T2 of October 1998 and IS 9092
EN29092).
Paper, cardboard and paperboard are essentially
composed of fibres of vegetable origin and formed by
drainage of a fibrous suspension on e.g. a screen and
subsequently drying of the formed web (one can for example
report to the standard DIN 6730, May 1996).
The range of the basis weight differs between the
various products of this group . for paper less than 225
g/m2, for cardboard the range being 150 g/m2 to 600 g/m2
and for paperboard above 225 g/m2.
2o Tissue paper is a sub group of paper having low basis
weights normally less than 40 g/m2. Tissue paper is porous,
absorbent paper and normally elastic due to foreshortening
of either the wet or the dry sheet, e.g. creping. In the
final products, tissue plies produced on a tissue paper
machine are often bonded to other tissue plies.
Although structures comprising cellulose-containing
fibres often have good strength properties in dry
conditions, they lose it when the fibres get wet. The
reason is that the fibres are held together with hydrogen
3o bonds formed between fibres and/or with inter-fibre
frictional forces. In water and other polar solvents, the
hydrogen bonds between the pulp fibres more or less
disappear and the strength of the material becomes very
dependent on the friction between the fibres.
This sensitivity to polar solvents can be reduced by
the addition of various binders. For non-woven of
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conventional type, the following components can for example
be used . latex based on ethylvinyl acetate, acrylates,
polyvinyl alcohol or styrene-butadiene. Wet strength
resins, for example polyamide- epichlorhydrin (PAE) resins,
urea formaldehyde (UF), polyethylene imine and different
starches can be used to increase the wet strength for non-
woven as well as paper.
Most of the wet strength resins contain some kind of
reactive groups, such as unsaturated groups (double or
so triple bounds), epoxy-, amine-, hydroxy- or carboxylic
groups. These reactive groups will react or interact with
chemical groups within other wet strength resin molecules
or chemical groups situated on cellulose containing fibres.
Reinforcement of structures comprising cellulose
containing fibres with the help of various binders and wet
strength resins can result in a number of problems of more
or less serious nature depending on where and how the
material is to be used. Certain chemical binders have poor
resistance to commonly occurring solvents, something which
is a significant drawback for structures that are used in
wiping clothes. The use of binders and wet strength resin
often leads to a stiffening of the structures, which is
also a significant drawback for hygienic and wiping
applications in which a soft and drapable structures are
asked for and/or required. Furthermore, the addition of a
binder is a chemical treatment which is often less
desirable from an environmental point of view.
In addition, it should be mentioned that the
incorporation of resins leads to a decrease of water
3o absorption kinetics and capacity, due to the hydrophobic
nature of many resins.
Another method of raising the wet strength in
structures comprising cellulose-containing fibres is by
thermo-bonding, which can be used where the structures also
contain thermoplastic fibres or particles. In such cases
the thermoplastic fibres in the materials are melted by
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means of raised temperature and if needed pressure. The
drawback with this method is that the structures become
stiffer and fused thermoplastic fibres can locally form
hard regions which can score delicate surfaces or the skin
of the user. A further drawback with the thermal bonding is
that the fibre recycling becomes more difficult.
In addition, the same remark (above mentioned)
concerning a decrease of water absorption kinetics and
capacity, due to the hydrophobic nature of such resins, can
be made here.
A number of methods for chemically-physically affecting
the surface of different materials have been developed.
Among these methods there can be mentioned methods using
ultraviolet light, plasma, or else corona discharge. One of
the advantages of these methods is that the material is
gently treated and no subsequent drying or post-treatment
is required.
"Plasma" should be understood as the general term for
gases in a state which comprises ions, electrons, free
2o radicals, and photons within the UV-range, but also
molecules and atoms. Plasma is electrically neutral and is
normally generated by an energy source, for example by an
electrical discharge, or else by microwaves.
Plasma treatment can be said to be a further
development of corona treatment and the primary difference
is that corona treatment takes place at s.tmospheric
pressure whilst so-called glow discharge in cold plasma
takes place at reduced pressure. Plasma treatment can be
executed in the presence of different gases depending on
3o which result is desired.
Plasma treatment is used nowadays, for example, to
provide plastic components with a coatable surface. It is
also used to chemically modify the surface on fibres with
an aim to increase the wettability of fibres as well as to
increase the adherence between fibres and filler.
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Corona treatment has for long been used to
morphologically and chemically modify the surface of
polymer films and in particular for the purpose of
improving the adhesion of printing inks or to perforate the
5 film. Apparatus for corona treatment is described in , for
example, US-A-4,283,291. It is also known from, for
example, US-A- 4,535,020 and EP-A-0,483,859 to treat
surface materials for absorbent products such as diapers
and sanitary napkins with corona at the same time as the
1o material is also treated with a surfactant to increase the
liquid permeability. Thanks to the corona treatment, an
improved permanent wettability is attained. In EP -A-
0,484,830 it is disclosed that wiping clothes of, for
example, meltblown material can be treated with corona to
provide the material with improved permanent absorption
properties.
Furthermore, WO 96/27044 proposes that a hydro
entangled non-woven material is subjected to a' plasma or
corona treatment to achieve a higher wet strength. The non
2o woven material comprises cellulose containing fibres.
An increase in the fibre-to-fibre friction, also when
the material is wet, is proposed in WO 96/27044 as a reason
for the wet strength increase. It is also known from SE-A-
9804294 that a tissue material that comprises fibres and
wet strength resin can be treated with corona to enhance
the initial wet strength of paper. This procedure makes it
possible to cut the curing time of the wet strength resin
to a minimum.
On the other hand, the document US-A-5,576,076
describes a method to treat a substrate, for example a
cellulose-containing substrate such as paper or paper
board, with electrical discharge in an atmosphere
containing silane, an oxidizing agent such as oxygen, and
an inert carrier gas. The European patent application
N°9840201.7 describes tissue treated with the
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aforementioned method. The aim of the treatment in both
documents is to get a better wettability of the product.
One of the objects of the present invention is to
propose a method for modifying the surface of cellulose
containing fibres in order - depending on the
specifications required by each final product- to enhance
strength properties, wettability, storage stability,
absorption kinetics and capacity.
A further object of the invention is a method of
modifying the surface of cellulose-containing fibres in
such a manner that they exhibit a higher density of
functional groups on the surface, such as carboxylated
groups or else that they exhibit new nitrogen-containing
groups.
l5 Another object of the invention is to provide surface
modified cellulose-containing fibres with improved strength
properties wettability and storage stability. The fibres
having been modified with the method according to the
invention.
2o A further object is to provide surface modified
cellulose-containing fibres showing increased number of
carboxylated and nitrogen-containing groups attached to the
surface of the cellulose containing fibres with chemical
bonds, e.g. covalent bonds. The fibres can be treated
25 either separately, before they have been formed to a
structure, or within a structure e.g. after they have been
formed to a web.
Another object of the invention is a fibrous structure
comprising the surface modified cellulose containing fibres
3o with carboxylated and nitrogen-containing groups attached
to the fibre surface.
A further object of the invention are products, such as
tissue paper and non-woven wipes, comprising surface
modified cellulose containing fibres with carboxylated and
35 nitrogen-containing groups attached to the fibre surface.
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The method according to the present invention, for
treating cellulosic fibres, according to which the fibre is
submitted to an electrical discharge, in presence of a
treatment gaseous mixture comprising a carrier gas, as well
as a reducing gas and/or an oxidizing gas, at the
atmospheric pressure, is characterized the following way .
- when the treatment gaseous mixture comprises an
oxidizing gas, the content of the oxidizing gas in the
mixture is in the range 50 to 2000 ppm vol,
- when the treatment gaseous mixture comprises a
reducing gas, the content of the reducing gas in the
mixture is in the range 50 to 30000 ppm vol.
The method according to the present invention may also
adopt one or more of the following characteristics
- when the treatment gaseous mixture comprises an
oxidizing gas, this oxidizing gas is COZ or N20 or H20 or a
mixture thereof.
- more carboxylated functional groups are introduced at
the surface of the fibres during the treatment (in
2o comparison with the surface state of the fibres before
treatment).
- at least nitrogen-containing functional groups are
introduced at the surface of the fibres during the
treatment.
- the treatment gaseous mixture comprises a carrier
gas, a reducing gas and an oxidizing gas, and R being the
ratio between said content of reducing gas and said content
of oxidizing gas, 0< R <15 .
- said content of oxidizing gas is in the range 100 to
1000 ppm vol. , the ratio R being in the range 0,55 R 58.
- said contents and said ratio R are controlled in
order to obtain a surface tension of a fibrous structure
including the fibres this way treated of 40 to 50 mN/m.
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- said contents and said ratio R are controlled in
order to obtain a surface tension of a fibrous structure
including the fibres this way treated higher than 50 mN/m.
- said contents and ratio R are controlled in order to
obtain a water contact angle on the fibres this way treated
between 0 and 50°.
- said contents and ratio R are controlled in order to
obtain a water contact angle on the fibres this way treated
between 0 and 30°.
- said reducing gas is hydrogen.
- said carrier gas is nitrogen, argon or helium or a
mixture thereof.
- the fibre has been, prior to said treatment,
submitted to a pre-treatment, by being submitted to an
l5 electrical discharge, in presence of a pre-treatment
gaseous mixture comprising air or an inert gas or a mixture
thereof.
- the inert gas of the pretreatment is advantageously
nitrogen, argon or helium or a mixture thereof.
The notion of "oxidizing gas" according to the
invention covers therefore oxygen or any other oxygen
containing gases such as, for instance, CO, C02, N0, H20,
NZO or N02 . As mentioned previously, it is preferred to
use according to the invention C0~ or NZO or H20 or a
mixture thereof.
The fibres can be treated individually, for example
mixed in a fluidising medium, e.g. in a gas . Fibres that
are treated individually can be modified to give a fibrous
structure a higher dry and wet strength. A non-exclusive
theory to the achieved increase of the strength properties
is that chemical active groups, e.g. carboxylate or
nitrogen- containing groups formed on the surface of fibres
during the modification will interact with other formed
active groups on other fibres or directly with the
untreated surface of other fibres. If the fibres are
treated with wet or dry strength agents to further increase
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the strength properties of a formed fibrous structure the
active groups on the fibres may interact with these
strength agents and so achieve a crosslinking between the
fibres. These interactions achieve more or stronger bonds
between the fibres and the strength agent than is common
when the fibres are untreated. The interaction described
above may be for example hydrogen bonds, covalent bonds or
ionic bonds. The interaction between different groups is
most probably established when the modified cellulose
1o containing fibres are formed to a fibrous structure e.g. by
one of the methods described hereinafter.
The fibres may also be modified after they have been
formed to a fibrous structure. If the fibres are treated
after the formation of a fibrous structure the modification
will mostly give rise to changes in wettability and storage
stability. According to the invention the fibrous structure
can be treated when it is still wet, e.g. on a machine
forming the fibrous structure or when it is dry e.g. during
converting of the fibrous structure to the final product.
2o During production of nonwoven fibrous structure fibres
are air- or wet- or foam-formed on a wire. Thereby are the
fibres dispersed in a gas, in a liquid or in a foamed
liquid respectively, subsequently placed on a wire whereby
any dispersing liquid or gas is drained by processes known
per se, see e.g. SE 9402470~A, CA 841,938. Thereafter the
fibres are hydro-entangled bonded to form a fibrous
structure. Other ways of bonding the fibres is using
bonding agents, thermobonding by melting some of the fibres
and part of fibres in the fibre mixture or by needling.
3o The fibres can be bonded by hydro-entanglement with an
energy input in the range 200-800 kWh/ton. The hydro-
entanglement takes place using conventional methods and
equipment. The hydro-entanglement of wet- or a foam-formed
fibre web can either take place in-line i. e. immediately
after the fibre has been drained on the wire or on a web,
which has been dried. A plurality of wet and/or dry fibrous
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structures can also be laminated together by hydro-
entanglement forming a layered fibrous structure or by
other bonding methods.
The invention is of particularly great significance for
5 wet and foam-formed nonwoven material where the choice of
fibres is more restricted since too long fibres are
difficult to disperse in liquid or foam. The problem with
sufficient wet strength is normally greater in material
containing short fibres.
10 During the production of paper webs the fibres are
dispersed in water and introduced onto a draining wire
through a headbox, the formed web is drained and dried.
Tissue paper is usually creped on a Yankee cylinder during
the drying. Other paper qualities can be sized and or
calandered if it necessary for the quality. The headbox may
have more than one nozzle for forming a layered fibrous
structure.
As already mentioned, the term "cellulose containing
fibres" is used for all fibres that contain cellulose,
being either of natural or synthetic nature.
The natural fibres are made from plant fibres from e.g.
hardwood, softwood or cotton. Fibres from esparto grass,
bagasse, (cereal straw, rice straw, bamboo, hemp), kemp
fibres flax or other woody and cellulosic fibres can also
be used as raw material. These fibres can be treated
according to different chemical and mechanical methods
before they are treated with the method according to the
invention. The fibres may further be primary (virgin)
fibres or secondary fibres (from waste paper).
The chemical methods for making a chemical pulp can be
the sulphite process the sulphate or Kraft process, the
soda process or process using organic solvents (e. g.
Organosolv, organocell, Alcell). Modified method of these
processes can also be used.
The mechanical pulp can be groundwood pulp or refiner
pulp. The refiner pulp can be further subdivided into
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thermo-mechanical pulp (TMP), and chemo- thermo-mechanical
pulp (CTMP and other subgroups thereof).
The fibres can further be bleached with known methods.
As the man skilled in the art knows , the different
ways of making the fibres have an influence on the surface
chemistry of the final fibres. A fibre produced by a
mechanical method will contain much more lignin and
hemicellulose than a bleached fibre made by some of the
chemical methods. These differences will give different
1o surface properties to the fibres and different reaction
behaviour when treated according to the invention.
Different types of synthetic cellulose containing
fibres may be fibres made from regenerated cellulose such
as rayon or viscose fibres.
The cellulose containing fibres can also be mixed with
other synthetic fibres, e.g. polyester, polyamide or the
like. Any mixture that still contains at least 10%, 30 0 or
500, by weight of cellulose containing fibres is according
the invention still a web comprising cellulose containing
fibres.
The cellulose containing fibres may preferably be
fibrillated before the treatment according to the
invention. During the fibrillation the surface layer of the
fibres is torn open and partially removed. This increases
the surface area and the bonding capacity of the fibres.
During the fibrillation some of the fibres are also
ruptured in to smaller pieces. The fibrillation changes the
static and dynamic strength properties of the fibres. The
fibres can be fibrillated in a refiner known in the state
of the art.
The fibrous structure of the invention comprises
surface modified fibres according to the invention. The
fibrous structure may also comprise cellulose-containing
fibres with a surface that has not been modified according
to the invention. The content such non-modified cellulose-
containing fibres may be according to the invention up to
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900, 70 % or 50 0, by weight based on the weight ,of the
structure.
A layer as described in this application is a fibrous
structure. A mufti-layer structure contains more than one
layer that are bonded together by covalent and/or hydrogen
bonds and/or by other conceivable bonds. The fibrous
structure may consist of one or more layers, preferably 1-
10, which cannot be separated from each other. The modified
fibres according to the invention may be present in all
layers in a layered structure or in just some of the
layers.
Th,e fibrous structure of the invention can also
comprise synthetic fibres, e.g. polyester, polyamide or the
like that are surface modified but also such synthetic
l5 fibres that are not modified. The amount of synthetic
fibres can be up to 10 0, 50 % or 80 o by weight based on
the weight of the structure. The synthetic fibres may be
present in all layers in a layered structure or in just
some of the layers.
The fibrous structure according to the invention may
have a basis weight in the range 8-3008 /m2.
Even if it is possible to make a fibrous structure that
has a good wet strength without wet strength agents
according to the invention, the addition of wet strength
agents may be appropriate in individual cases in order to
achieve a further improvement in strength properties.
Examples of suitable wet strength agents are:
carboxymethylcellulose, polyamide-epichlorhydrin (PAE),
polyacryl amide urea formaldehyde resins and pre-polymers,
melamine formaldehyde resins and pre-polymers as well as
phenol formaldehyde resins and pre-polymers.
Paper and nonwoven products according to the invention
comprise the fibrous structure of the invention. The
products can be made up by one or more plies. At least one
ply is made of the fibrous structure according to the
invention described above.
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A ply in the sense of the invention is a fibrous
structure as described above. Plies can be bonded together
with adhesives or embossing in such a way that it will be
possible to at least partially separate them from each
other. A ply comprises one or more layers with the same or
different compositions. One or more of the plies of a
multiply paper may consist of conventional non-modified
fibres, e.g. cellulose-containing fibres or synthetic.
Examples of tissue paper products and nonwoven products
are
- Wiping clothes for domestic or industrial use;
- Sanitary products like toilet paper, handkerchiefs,
facials and napkins/serviettes;
- Disposable garments or bed-linen.
The fibrous structures that may be one ply of a product
according to the invention are converted to a sellable
product. During the converting the fibrous product may be
embossed and/or printed and/or provided with an active
ingredient such as an softening or caring lotion or an
2o detergent. The paper/nonwoven product may exist as a sheet
or as a roll.
The paper/nonwoven according to the invention
preferably has a basis weight of 20 to 300 g/m2 , in the
case of tissue product the basis weight is usually 10 to 60
g/mz and for nonwoven 30 to 200 g/mz.
