Technical field
[0001] The present invention refers to a method of making a multi-layer paper web. It further
refers to a multi-layer paper web having a three-dimensional structure. Especially
it refers to the production of tissue products such as toilet and kitchen paper, paper
towels, handkerchiefs, wiping material and the like.
Background of the invention
[0002] It is very common to laminate two or more tissue paper plies in order to produce
the final tissue product. Herewith a more flexible and softer tissue product is obtained
as compared to if one single ply with a corresponding thickness and basis weight had
been produced as for the laminated product. The absorbent capacity and the bulk are
moreover improved.
[0003] The lamination of two or more tissue plies is often made by means of gluing. A mechanical
embossing of the plies is also often performed before they are glued together. It
is further known to laminate two plies only by means of a mechanical embossing, at
which a mechanical joining of the plies occurs in the embossing sites.
[0004] Through for example EP-A-796 727 it is known to first emboss two paper plies in a
three dimensional structure with alternating raised and recessed portions, after which
glue is applied to one of the plies and the two plies are joined in a press nip between
two embossing rolls, so that the raised portions of the respective plies are glued
to each other. A similar embossing procedure is shown in EP-A-738 588, according to
which the glue also has a colouring effect.
[0005] US 3,414,459 discloses laminating of tissue plies by a combined embossing and gluing
procedure. The embossing is of so called foot-to-foot type according to which the
raised protuberances of the embossed tissue plies are joined together. In US 3,555,907
and US 3,867,225 there are also disclosed a combined embossing and gluing process,
but where the embossing is of so called nested type according to which the raised
projections of one tissue ply will rest in and be joined to the depressions of the
opposite Ply.
[0006] There would however from a manufacturing point of view be a general advantage to
be able to make a single ply tissue product. The downtime of the converting line would
be considerably reduced and the speed of the converting machine could be increased.
There are however difficulties to envisage process solutions for single ply tissue
that would render the desired product properties in terms of softness and absorbency.
[0007] One way of achieving a single ply tissue product with possibly acceptable properties
would be to join two or more individual layers in the wet state in the paper machine
before the paper is pressed and dried, while striving to optimize the fibre structure
and fibre properties in the individual layers. Two layers can be joined together in
the wet state through several operations. One is to use a so called multilayer headbox,
another is to form a second layer on top of a previously formed first layer and a
third possibility is to couch together two separately formed layers.
[0008] There is however considerable doubt that any of the above mentioned methods of joining
the wet layers would produce the required softness and absorbency to replace a multi-ply
tissue. The reason is that there are difficulties to maintain the void volumes between
the layers that seem to be necessary in order to provide the required flexibility
of the material and the required pore volume for absorption.
[0009] The term "multi-ply" in this respect means that two or more paper sheets have been
combined outside the paper machine in the converting line, such as by embossing and/or
gluing, while the term "multi-layer" refers to that two or more sheets of paper have
been combined in the paper machine in a wet or moist state, so that papermaking bonds
are formed between the layers.
[0010] It is further known to impart a three-dimensional pattern in a moist paper web while
drying the web. This can be done by so called through-air-drying (TAD), in which hot
air is blown through the moist paper web while it is carried by a patterned drying
fabric or belt. In connection with the TAD drying the pattern structure of the drying
fabric is transferred to the paper web. This structure is essentially maintained also
in wet condition of the paper, since it has been imparted to the wet paper web. A
description of the TAD technique can be found in e g US-A-3,812,000.
[0011] Through for example WO 99/34055 it is known to impart a three-dimensional pattern
in a moist paper web while drying the web by so called impulse drying and impulse
embossing. The wet paper web is passed through a press nip comprising a rotatable
roll which is heated and the paper web is given said three-dimensional pattern when
passing through the press nip, either by means of a patterned wire and/or by the fact
that the heated roll is provided with a pattern intended to be pressed into the paper
web against a non-rigid holder-on, such as a felt. The three-dimensional pattern is
in this case essentially maintained in wet condition of the paper, since it has been
imparted to the wet paper web in connection with drying thereof.
Summary of the invention
[0012] An object of the present invention is to provide a process of joining two or more
sheets of paper in wet or moist condition and obtain a final product which possess
properties such as flexibility and absorption capacity which are comparative to those
of a multi-ply product. The product could be defined as a single-ply multi-layer product.
[0013] This has according to the invention been provided by separately forming and drying
at least two paper sheets to a dry content of no more than 80 % by weight, imparting
a three-dimensional pattern of alternating raised and recessed portions in the paper
sheets during drying, combining the at least two paper sheets having a dry content
of no more than 80% by weight into a multilayer web, in which void volumes are created
between the raised and recessed portions of the combined sheets, drying the multilayer
web.
[0014] In order to maintain a high bulk it is preferred that drying of the multilayer web
is made without any considerable compression of the multilayer web.
[0015] The term "without any considerable compression" in this respect means that the drying
technique used will not cause a compression or bulk reduction of the multilayer web
of more than about 25%.
[0016] Examples of non-compressible drying techniques are through-air-drying (TAD) and infrared
(IR) drying.
[0017] According to one embodiment of the invention drying and imparting said three-dimensional
pattern in the separately formed paper sheets is made by impulse drying and impulse
embossing, wherein the wet paper sheet is passed through a press nip comprising a
rotatable roll which is heated so that the paper sheet when passing through the press
nip is given said three-dimensional pattern either by means of a patterned wire and/or
by the fact that the heated roll is provided with a pattern intended to be pressed
into the paper sheet against a holder-on. The holder-on preferably has a non-rigid
surface.
[0018] In an alternative embodiment drying and imparting said three-dimensional pattern
in the separately formed paper sheets is made by through-air-drying (TAD) wherein
the wet paper sheet is carried by a patterned wire or belt.
[0019] In a further embodiment an additional component, such as an absorbent material and/or
spacing means is applied between the paper sheets before combining them.
[0020] The invention further refers to a multilayer paper web having a three-dimensional
structure, said multilayer paper web comprising at least two paper sheets each of
which has a three-dimensional pattern of alternating raised and recessed portions,
said paper sheets being joined together point- or spotwise by papermaking bonds forming
bonding sites, while leaving void volumes between the sheets and between the bonding
sites.
Description of drawings
[0021] The invention will now be described more in detail with reference to embodiments
shown in the accompanying drawings.
Fig. 1 is a schematic view of a machine configuration for the method according to
the invention.
Fig. 2 shows on a larger scale an impulse drying unit.
Fig. 3 shows the press nip of the impulse drying unit.
Fig. 4 illustrates a two-layer paper web according to the invention.
Fig. 5 illustrates a two-layer web containing an additional component in the void
volumes between the layers.
Description of embodiments
[0022] Fig. 1. shows schematically a machine configuration for making a two-layer paper
web, and having a first twin-wire forming unit comprising a first headbox 10 delivering
a fibre suspension jet into a nip created by a pair of tensioned forming wires or
felts 11 and 12 both of which wrap around a rotating forming roll 13, and a second
twin-wire forming unit comprising a second headbox 14 delivering a fibre suspension
jet into a nip created by a pair of tensioned forming wires or felts 15 and 16 both
of which wrap around a rotating forming roll 17. The forming rolls 13 and 17 may have
a solid or open surface. In the case the forming rolls 13 and 17 have open surfaces,
they may be supported with a vacuum. According to one embodiment the inner part of
the twin wire nip is a felt 12; 16 and the outer part is a wire 11; 15.
[0023] The wet sheets supported by the wires or felts 12 and 16 may be further dewatered
over suction boxes (not shown) and are each brought into an impulse drying nip between
two rotatable rolls 18, 19 and 20, 21 respectively, at which the rolls 18 and 20 which
are in contact with the paper sheets are heated to a temperature which is sufficiently
high for providing drying of the paper sheet. The surface temperature of the heated
rolls 18 and 20 can vary depending on such factors as the moisture content of the
paper sheet, thickness of the paper sheet, the contact time between the paper sheet
and the roll and the desired moisture content of the completed paper product. The
surface temperature should of course not be so high that the paper sheet is damaged.
An appropriate temperature should be in the interval 100-400°C, preferably 150-350°C
and most preferably 200-350°C.
[0024] The paper sheet is pressed against the heated roll 18; 20 by means of the respective
wire or felt 12; 16 and the backing roll 19; 21, which may or may not be provided
with a soft non-rigid surface layer, e g rubber or another resilient material. A very
rapid steam generation takes place in the interface between the heated roll 18; 20
and the moist paper sheet, at which the generated steam on its way through the paper
sheet carries away water.
[0025] Instead of the backing roll 19; 21 a press shoe or the like may be used as a holder-on
in the impulse drying press nip. The backing roll 19; 21 is in the embodiment shown
a suction pressure roll with a vacuum 22; 23 such that the paper web is retained on
the felt or wire 12; 16.
[0026] Simultaneously with the impulse drying the paper sheet 24 is given a three-dimensional
structure by the fact that the heated roll 18; 20 is provided with an embossing pattern
in the form of alternating raised and recessed portions 25, 26. This structure is
substantially maintained also in a later wetted condition of the paper, since it has
been imparted the wet paper sheet in connection with drying thereof.
[0027] The paper sheet 24 may be pressed against a non-rigid surface, e g a compressible
press felt 12; 16. The backing roll 19; 21 may also have a flexible surface, e g an
envelope surface of rubber. This is however not necessary in order to accomplish the
materials and processes of the present invention. The paper sheet 24 is herewith given
a three-dimensional structure having a total thickness greater than that of the impressed
paper. By this high bulk, high absorption and high softness of the paper are achieved.
The paper further becomes elastic. At the same time there is obtained a locally varying
density in the paper.
[0028] The paper sheet 24 may also be pressed against a hard surface, e g a wire 12; 16
and/or a roll 14 having a rigid surface, at which the pattern of the heated roll 18;
22 is pressed into the paper sheet under heavy compression of the paper just opposite
the impressions, while the portions there between are kept uncompressed.
[0029] The three-dimensional pattern imparted in the paper sheet may also be provided by
a pattern wire or belt wrapping the heated cylinder 18; 20, which in this case does
not need to be patterned.
[0030] The three-dimensional patterns imparted to the paper sheets in the two impulse drying
and embossing stations may be the same or different.
[0031] The paper sheets are not dried to the final dryness in the impulse drying and embossing
stations, but have a moisture content of at least 20% by weight, preferably at least
30% by weight, when leaving the impulse drying and embossing stations.
[0032] After the impulse drying and embossing stations the two paper sheets 24a and b are
joined by letting one sheet be picked up by the other sheet while this is still on
its felt 16 wrapping the backing roll 21 with an extended suction zone 23. After that
the two joined sheets are picked up from the felt 16 by a wire over a roll 26 having
a suction zone 27. Further dewatering may, if necessary, occur by suction box 28.
[0033] The combined web is then dried by being passed over a drying drum 29 having an air
hood 30 mounted there over to direct heated air against and through the web on the
drum surface 29. This drying technique is often called through-air-drying (TAD) and
is a non-compressive drying producing high bulk and absorbent tissue.
[0034] Other non-compressive drying techniques that may be used instead of TAD is for example
IR dryers. The dryness of the paper web after the non-compressive drying should be
at least 90% or preferably a dryness, which is in equilibrium with the ambient air.
[0035] It would also be possible to use another drying and imprinting technique than impulse
drying and embossing for the drying and imprinting of the individual sheets. Such
an alternative technique is TAD using an imprinting fabric or belt.
[0036] The combined paper sheets 24a, b are when dried on the heated cylinders 18;20 joined
together point- or spotwise by papermaking bonds forming bonding sites 31, while leaving
void volumes 32 between the sheets 24a, b and between the bonding sites 31. This is
illustrated in Fig. 4. The void volumes 32 between the sheets contribute in providing
the required absorbency and flexibility of the final product.
[0037] These void volumes 32 may according to one embodiment of the invention, illustrated
in Fig. 5, contain an additional component 33, such as an absorbent material and/or
spacing means. The absorbent material may for example be a so called superabsorbent
material, and the multilayer web containing the superabsorbent material may be used
as an absorbent layer in an absorbent article such as sanitary napkins, panty liners,
diapers and incontinence guards or as highly absorbent wiping material.
[0038] Spacing means may be a porous moisture insensitive material such as a polymeric foam
or synthetic fibrous material, which helps maintaining the bulk of the multilayer
web when wet.
[0039] The invention is not limited to the embodiments described and shown but several modifications
are possible within the scope of the claims.
1. A method of making a multilayer paper web,
characterized in
separately forming and drying at least two paper sheets (24a,b) to a dry content of
no more than 80 % by weight,
imparting a three-dimensional pattern of alternating raised and recessed portions
(25,26) in the paper sheets during drying,
combining the at least two paper sheets having a dry content of no more than 80% by
weight into a multilayer web, in which void volumes (32) are created between the raised
and recessed portions (25,25) of the combined sheets,
drying the multilayer web (24).
2. A method of making a multilayer paper web as claimed in claim 1,
characterized in
that drying of the multilayer web (24) is made without any considerable compression of
the multilayer web.
3. A method of making a multilayer paper web as claimed in claim 2,
characterized in
that the non-compressible drying of the multilayer web (24) is made by through-air-drying
(TAD) or infrared (IR) drying.
4. A method of making a multilayer paper web as claimed in any of the preceding claims,
characterized in
that drying and imparting said three-dimensional pattern in the separately formed paper
sheets (24a,b) is made by impulse drying and impulse embossing, wherein the wet paper
sheet is passed through a press nip comprising a rotatable roll (18;20) which is heated
and that the paper sheet when passing through the press nip is given said three-dimensional
pattern either by means of a patterned wire and/or by the fact that the heated roll
(18;20) is provided with a pattern intended to be pressed into the paper web
against a holder-on (12,19 ;16,21).
5. A method of making a multilayer paper web as claimed in claim 4,
characterized in
that that the holder-on (12,19;16, 21) has a non-rigid surface.
6. A method of making a multilayer paper web as claimed in any of claim 1-3,
characterized in
that drying and imparting said three-dimensional pattern in the separately formed paper
sheets (24a,b) is made by through-air-drying (TAD) wherein the wet paper sheet is
carried by a patterned wire or belt.
7. A method of making a multilayer paper web as claimed in any of the preceding claims,
characterized in
that an additional component (33), such as an absorbent material and/or spacing means
is applied between the paper sheets (24a,b) before combining them.
8. A multilayer paper web having a three-dimensional structure,
characterized in
that said multilayer paper web comprises at least two paper sheets (24a,b) each of which
having a three-dimensional pattern of alternating raised and recessed portions (25,26
), said paper sheets being joined together point- or spotwise by papermaking bonds
forming bonding sites (31), while leaving void volumes between the sheets (24a,b)
and between the bonding sites (31).
9. A multilayer paper as claimed in claim 8,
characterized in
that an additional component (33), such as an absorbent material and/or spacing means
is contained in the void volumes (32) between the sheets (24a,b).