WO2000036212A2

WO2000036212A2 - Aerodynamic method for making tissue paper

Info

Publication number: WO2000036212A2
Application number: PCT/US1999/024732
Authority: WO
Inventors: Viktor Mikhailovich Drobosyuk
Original assignee: Pragmatic Vision, Inc.
Priority date: 1998-12-15
Filing date: 1999-10-25
Publication date: 2000-06-22
Also published as: RU2159304C2; ATE260359T1; DE69915116D1; WO2000036212A3; CA2350479A1; AU1220700A; EP1141479B1; EP1141479A2; DE69915116T2

Abstract

An aerodynamic method of making tissue paper comprises the steps of preparing an aerosuspension out of cellulose fibers, forming a layer of fibers (8) on a forming wire (7), moistening the formed layer of fibers and pressing and drying of said formed layer. The step of moistening the layer of fibers is performed concurrently with the step of pressing, for which purpose the layer of fibers is placed between the profiling (3) and moistening (4) belts. The surface of the profiling belt (3) comprises protruding elements (14), wherein a distance between two mutually-adjacent protruding elements doesn't exceed an average length of the fibers. A wire with smoothed nodes (14) of interweaving threads can be used as the profiling belt, while fine-mesh wire can be used as a moistening belt. Selective moistening of fibers only in the areas being pressed eliminates moistening of the entire layer, and drying of the paper web requires significantly less expenditures of time and energy. Shrinkage of the paper web is also minimized because the non-pressed areas of fibrous layer don't practically get moistened.

Description

AERODYNAMIC METHOD FOR MAKING

TISSUE PAPER

FIELD OF THE INVENTION

The present invention relates to the field of papermaking, and more

particularly, to a method of manufacturing tissue paper featuring enhanced

absorbency.

BACKGROUND OF THE INVENTION

One of the main problems that one encounters when making tissue

paper by an aerodynamic method without using binders is the ability to

provide a tissue paper having both a high absorbency (hygroscopicity) and

sufficient strength. This is so because when the aerodynamic method is

used without binders, the bonding of fibers is obtained from hydrogen

bonds formed as a result of pressure processing and subsequent drying of

the moistened layer of fibers produced from aerosuspension. Pressing of

the fibrous layer is necessary to provide a greater area of inter-fibrous

contact, while a drying is required to remove water molecules and form the

above-mentioned hydrogen bonds between the fibers. Thus, the greater

the pressure, the stronger the tissue produced and the lower its

absorbency, and vice versa.

One conventional aerodynamic method of paper making comprises

forming of a layer of cellulose fibers out of aerosuspension, impregnating this layer by a liquid reagent, and subsequently pressing and drying of this

layer. See, for example, USSR Author's Certificate 1594237, IPC 5 D21 H

23/00, 21/18, published August 23, 1990. This method is characterized by

the use of 2 - 8% water solution of resorcin that provides much better

swelling of fibers than water. Penetration of water into intercrystalline

space of pulp fibers facilitates enhancement of their plasticity, which results

in more complete contact of fibers during pressing and drying, and, hence,

enhances the strength of fiber bonding. Since molecules of resorcin form

bonds of a "cellulose-resorcin-cellulose" type, resorcin also performs the

function of a binder, which also facilitates the enhancement of the

produced tissue strength. Thus, when using a water/resorcin solution for

moistening the fibrous layer, one can decrease the pressure applied at the

stage of pressing, thereby improving tissue absorbency while preserving

tissue strength. However, introduction of chemical additives makes tissue

paper production more expensive.

Another conventional method for making high-absorbency products

out of fibrous materials comprises forming of a multi-layer structure of thin

paper layers and a layer of fibers produced out of aerosuspension and

placed between paper layers. All the layers arranged in the above manner

are pressed between rolls, one of which has a patterned surface. See, for

example, USA patent 3908653, IPC 2 A61F 13/16, A61L 15/00, published

September 30, 1975. Final formation of the product proceeds in the following manner: two structures obtained in the above-described way are

folded together facing each other with patterned surfaces and then the

edges are jet-molded or glued together. Since the filler in the product

comprises substantially non-pressed fibers, the final product offers high

hygroscopicity, but the manufacturing cost of such products is very high.

One conventional aerodynamic method of papermaking, which is

believed to be the closest to the present invention, comprises preparation

of aerosuspension of cellulose fibers, forming a fibrous layer on a moving

forming wire, moistening the moving fibrous layer with water, the amount of

which constitutes 20 - 60 % of fiber weight, and subsequently pressing and

drying of said fibrous layer. See, for example, USA patent 3949035, IPC 2

B29C 17/04, published April 6, 1976 - prototype. Pressing is performed

between two rotating rolls, one of which has a patterned surface made in

the form of ridges with flat faces of round (or circular) shape, and the

distance between ridges doesn't exceed the average length of the cellulose

fibers. During pressing, compaction of the fibrous layer and formation of

greater contact area between the fibers take place in the ridge areas, while

no compaction occurs in the areas between the ridges (i. e., in valleys). As

a result, the final product obtained after drying has two types of areas:

areas of a pressed fibrous layer that determine the strength of the tissue

paper, and areas of non-pressed fibrous layer that determine tissue absorbency. Thus, this method enables one to produce tissue paper, the

structure of which concurrently provides tissue strength and hygroscopicity.

At the same time, to provide the formation of the above-mentioned

inter-fibrous bonds, the formed fibrous layer should be moistened with a

substantial amount of water. Besides, to provide better penetration of

water into the fibrous layer, this moistening is accompanied by rarefaction

of air produced underneath the wire carrying the fibrous layer. Such a

moistening process requires amounts of water that are excessive

compared to the amounts needed for the formation of inter-fibrous bonds.

In addition, it takes an extra time to moisten the entire fibrous layer. All the

above results in extra expenditures of energy (mainly expended on

subsequent drying of the tissue paper web) and slows down the process of

tissue production. Besides, removal of great amounts of water through

drying leads to the shrinkage of the non-pressed part of the fibrous layer,

which results in a decrease in absorbency of the produced tissue paper.

SUMMARY OF THE INVENTION

The present invention advantageously decreases the costs related to

the manufacture of tissue paper and increases of the quality of the tissue

paper.

According to an embodiment of the present invention, a method of

making tissue paper comprises the following steps: preparing an aerosuspension of fibrous material; forming a layer of fibers on a forming

wire; transferring the layer of fibers to a profiling belt having a pressing

surface containing protruding elements for impressing first areas of the

fibrous layer in contact therewith; contacting the layer of fibers disposed on

the pressing surface of the profiling belt with a moistening belt; and

pressing the layer of fibers between the profiling belt and the moistening

belt. In addition, the moistening belt has a lower sorption capacity than a

sorption capacity of the first areas of the fibrous layer being impressed by

the protruding elements and higher than a sorption capacity than second

areas of the fibrous layer that are not impressed by the protruding

elements.

According to the present invention, a distance between protruding

relief elements on the pressing surface does not exceed an average length

of the fibers. The formed layer of fibers can be placed on a profiling belt

that has a pressing surface that faces the layer of fibers. Moistening of the

formed layer of fibers is performed concurrently with the pressing step, an

utilizes an additional belt such as, for example, a moistening belt. The

moistening belt is disposed such that a pressing force is exerted

concurrently on the profiling belt, the moistening belt, and the layer of fibers

located therebetween. The moistening belt is preferably made of a

material having a sorption capacity that is lower than a sorption capacity of

those areas of the layer of fibers that are pressed due to the protruding relief elements, but higher than the sorption capacity of the areas of the

layer of fibers that are not pressed by the relief elements. The moistening

belt can be saturated with an appropriate fluid such as. for example, water,

in an area that is outside the pressing zone.

In one embodiment of the claimed invention, the fibrous layer placed

between the profiling and moistening belts during the step of pressing. In

the course of pressing, the sections of fibrous layer that are in the areas of

protruding relief elements get compacted, which results in an increase of

absorbency of the fibrous layer, due to the increase in the pressure of

capillary absorption. When the fibrous layer absorbency reaches a value

equal to the value of the same parameter of the moistening belt, the

sections of the fibrous layer being compacted begin to absorb water from

the moistening belt surface. With further compaction of the fibrous layer

the excess water is squeezed out from the compacted sections into the

non-compacted sections, and due to the difference in capillary absorption

pressures, this water returns to the moistening belt. Part of the water

returned will subsequently be absorbed by new sections of the layer of

fibers being compacted. The moistening belt receives water required for

moistening outside the pressing zone, for example, absorbing it when

being passed through a tub with water.

The present invention provides concurrent moistening and pressing of

the fibrous layer in order to minimize the amount of water required at the pressing stage. Selective moistening of fibers only in the sections being

compacted eliminates the requirement for moistening the entire fibrous

layer, and excessive moistening. As a consequence, drying of the tissue

paper web after the pressing step requires significantly lower expenditures

of time and energy. Additionally, shrinkage of the tissue paper web is

eliminated because the non-pressed sections of the fibrous layer are not

moistened.

A wire made by means of interweaving threads can be used as the

profiling belt. In this instance interweaving nodes represent the protruding

relief elements of wire surface, and the shape of flat areas can be endowed

through the use of smoothing. This approach can significantly reduces the

cost of the process of the present invention.

Another simple and inexpensive embodiment of a moistening belt

comprises a fine-mesh wire. In this instance sorption properties of the

moistening belt are determined by surface properties of the material of the

wire, as well as by relative sizes and geometrical configurations of threads

and openings of the wire.

Further, longitudinal twisting of fibers that significantly decreases the

area of contact between fibers in the areas pressed can be prevented if the

prepared aerosuspension has a moisture content sufficient for causing

saturation of fibers' walls with moisture. BRIEF DESCRIPTION OF THE DRAWINGS

Fig.1 is a process diagram illustrating the process of moistening and

pressing of a fibrous layer according to an embodiment of the present

invention.

Fig.2 is a top plan view illustrating a profiling belt made in the form of

wire, having smoothed surfaces of nodes produced by intersections of

threads.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Fig. 1 , a pressing unit of a paper-making machine that

uses the method of the present invention comprises a roller press having

two pressure rollers 1 and 2, a profiling belt 3, a moistening belt 4, and a

tub 5 with water into which a drum 6 is submerged. The drum 6 is intended

for transporting the moistening belt 4 through the tub 5. Fig. 1 also shows

a forming wire 7 on which fibrous layer 8 is formed, a drying drum 9, and

wire-driving rollers and take-up suction rolls 10 and 11.

The profiling belt 3, a fragment of which is shown in Fig. 2, can be

made out of wire comprising interweaving threads 12 and wefts 13 of round

(circular) cross-section. The nodes of this wire on the side contacting the

fibrous layer are smoothed to such an extent that flat pressing surfaces 14

of elliptic shape are produced, and said flat pressing surfaces 14 determine

the fibrous layer sections to be pressed. Geometric size of the wire and the surfaces 14 are selected so that a distance between the surfaces 14 of

juxtaposed nodes of the wire doesn't exceed an average length of the

fibers. The strongest paper retaining good absorbing properties can be

produced when this distance is approximately equal to half of the average

length of the fibers. In this instance the individual fibers interconnect with

each other and transmit mechanical stresses arising in the conditions of

paper break from one pressed area to another.

Fibrous layer 8 formed out of aerosuspension (forming process is not

shown in Fig.1) through the use of the forming wire 7 and the profiling belt

3 is supplied to a zone of the suction roll 10 where the forming wire 7

breaks away, and the side of the fibrous layer 8 that has just lost contact

with the forming wire is covered by the moistening belt 4. Such a forming

belt can be made, for instance, in the form of a fine-mesh wire. The fibrous

layer 8, now positioned between the profiling belt 3 and the moistening belt

4, is then fed to a pressing operation between rolls 1 and 2. Pressing of

the fibrous layer 8 proceeds as described in the Summary of the Invention

above. Subsequent to pressing, the belt 4 breaks away in the area of a

take-up suction roll 11 , and the pressed fibrous layer is fed to the drying

drum 9, from which the finished paper web is subsequently obtained.

The possibility of implementing the claimed method was

experimentally tested in a following manner. Samples of tissue paper with specific weight of 40 - 45 g/m³ were

produced. Softwood and hardwood sulfate bleached pulp (with the

average length of fibers 2.7mm and 1.4 mm, respectively) were used as

fibrous semi-finished material.

An aerosuspension of fibers was prepared out of fibrous semi-finished

material that has been moistened in advance to a moisture content of 50%.

This aerosuspension was fed to the forming wire where a fibrous layer

having a specific weight of 15 - 20 kg/m³ was formed. A wire of interwoven

synthetic threads 0.25mm in diameter and with the distance of 0.25 mm

between core threads and 0.3 mm between weft threads was used as

forming wire.

The formed fibrous layer (the moisture content of which was 30 - 35%

at this technological stage) was transported to the profiling belt represented

by standard metal wires #1 or #2 of simple interweaving generally used in

papermaking industry. Wire #1 is woven out of flat threads, and the shape

of its meshes is square. There are eight threads per 1 cm of running

length, thread width is equal to 0.6 mm; thread thickness = 0.15 mm;

distance between threads = 0.65 mm, and the area of threads constitutes

70% of the total wire area.

Wire #2 is woven out of threads of round section and of diameter 0.5

mm, and the shape of its meshes square. There are eight threads per 1 cm of running length, and the distance between threads is 0.75 mm. One

surface of wire #2 was smoothed up to a depth of 0.25mm. Flat areas of

elliptical shape, the total area of which constitutes 40% of the total wire

area, were produced as a result of the smoothing. The fibrous layer was

placed on the smoothed wire surface at the pressing stage.

Two fine-mesh wires joined together were used as a moistening belt.

Each fine-mesh wire is made by simple interweaving of threads 0.25 mm in

diameter, and there are 24 threads per 1 cm of running length. Moisture

capacity defined as the amount of water retained by capillary forces in the

wire of area 1 m² constitutes 0.08 kg/m².

The formed fibrous layer placed between the profiling wire (#1 or #2)

and moistening wires was fed into the gap between the rolls of a roller

press developing a force of 10 or 18 kg on 1cm of layer width. Subsequent

to pressing, the moistening wires were taken off the fibrous layer, while the

layer being held by profiling wire was fed to drying unit, the surface of

which was heated to the temperature of 115 C.

Results of experimental testing are given in the Table below.

The experiments performed confirm the possibility of implementing

the method of the present invention and verified the above-indicated

results. Using this method, it is possible to make tissue paper offering such

strength and hygroscopic properties that correspond to the current

specifications for tissue paper making. It should be pointed out that the

amounts of water expended with this method are significantly less

compared to the amounts spent when using other known methods. It can be seen from the above Table that moisture content of the fibrous layer fed

to drying subsequent to pressing varies only slightly compared to the

moisture content of fibrous layer on the forming wire, which significantly

reduces the cost of drying and decreases the shrinkage of paper web

during drying.

Results of the experiments also indicate how parameters of

technological process and equipment units exert influence on the final

result. For example, when using wire #1 which is made out of flat threads

and which has "shallow" relief formed by the interwoven nodes of threads,

greater pressure should be applied to obtain required strength properties of

the final product.

On the other hand, to obtain a required strength of tissue paper, quite

high pressure is also needed when using wire #2 that is made of round

threads and that has lesser area of pressing zones compared to wire #1.

However, it is just the lesser area of pressing zones that makes it possible

to obtain tissue paper offering greater absorbency than the tissue paper

produced using wire #1.

Results of experiments given in the above Table in the 3^rd, 4^th, and 5^th

lines confirm that initial moisture content of a fibrous layer fed to a pressing

operation also exerts impact on the strength of tissue paper being

manufactured. The greater the moisture content of the fibers, the softer and less twisted are they. Therefore, the contact area of such fibers is

greater in the course of pressing. This fact results in the formation of inter-

fibrous bonds on greater area, and, hence, in stronger tissue paper, while

absorbency of such tissue paper remains the same.

While this invention has been described in connection with what is

presently considered to be the most practical and preferred embodiments,

it is to be understood that the invention is not limited to the disclosed

embodiments, but, on the contrary, is intended to cover various

modifications and equivalent arrangements included within the spirit and

scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. An aerodynamic method of making tissue paper comprising the steps:

(a) preparing an aerosuspension of fibrous material;

(b) forming a layer of fibers on a forming wire;

(c) transferring the layer of fibers to a profiling belt having a pressing

surface containing protruding elements for impressing first areas of

the fibrous layer in contact therewith;

(d) contacting the layer of fibers disposed on the pressing surface of

the profiling belt with a moistening belt; and

(e) pressing the layer of fibers between the profiling belt and the

moistening belt;

wherein the moistening belt has a lower sorption capacity than a

sorption capacity of the first areas of the fibrous layer being

impressed by the protruding elements and higher than a sorption

capacity of second areas of the fibrous layer that are not

impressed by the protruding elements.

2. The method of Claim 1 , wherein the step of transferring comprises a

step of transferring the layer of fibers to a profiling belt having a

pressing surface containing protruding elements for impressing first

areas of the fibrous layer in contact therewith, and a distance between mutually-adjacent protruding elements is not greater than an average

length of individual fibers of the layer of fibers.

3. The method of claim 1 , wherein the step of transferring is performed

using a profiling belt that comprises a wire made of threads

interwoven such that nodes formed by said interwoven threads form

the protruding elements of the pressing surface, and the protruding

elements having relatively flat surface areas contacting the layer of

fibers.

4. The method of claim 1 , wherein the step of contacting comprises a

step of contacting the layer of fibers disposed on the pressing surface

of the profiling belt with a moistening wire.

5. The method of claim 1 , wherein the step preparing an aerosuspension

comprises a step of preparing an aerosuspension of fibrous material

having a moisture content that provides saturation of walls of the

fibers.

. An apparatus for making tissue paper by an aerodynamic method, the

apparatus comprising:

a forming wire for receiving an aerosuspension of fibers and forming a

layer of fibers thereon;

a profiling belt having a pressing surface comprising protruding

elements configured and arranged for contacting first areas of

said layer of fibers, thereby impressing the first areas of said layer

of fibers;

a moistening belt comprising a material having a sorption capacity

lower than a sorption capacity of the first areas of said layer of

fibers, and higher than a sorption capacity of second areas of said

layer of fibers that are not contacted by said protruding elements;

and

a pressing assembly for impressing the layer of fibers between the

profiling belt and the moistening belt.

7. The apparatus of claim 6, wherein said pressing assembly comprises

a pair of pressure rollers for exherting a force on the layer of fibers,

the profiling belt, and the moistening belt.

8. The apparatus of claim 6, wherein a distance between mutually-

adjacent protruding elements of the pressing surface of said profiling

belt is not greater than an average length of individual fibers of the

layer of fibers.

9. The apparatus of claim 6, wherein the protruding elements of said

pressing surface have elliptical profiles.

10. An aerodynamic method for tissue paper making comprising the

steps: preparing an of aerosuspension out of cellulose fibers or other

fibrous material, forming of a layer of fibers on a moving forming wire;

moistening the formed layer of fibers; and

and pressing and drying said formed layer;,

wherein the pressing is performed by a pressing means having a pressing surface, for contacting with said layer of fibers, which is made as a relief surface and the distance between the protruding relief elements on the pressing surface doesn't exceed the average length of fibers; and,

wherein the method is distinguished by the fact that during pressing, the

formed layer of fibers is placed on an additional profiling felt, the

surface of which facing said layer of fibers represents said pressing

surface, and moistening of the formed layer of fibers is performed

concurrently with pressing for which purpose an additional moistening

felt is used, and said moistening felt is accommodated in such a way that pressing action is exerted concurrently on profiling and

moistening felts and on the layer of fibers located between said felts,

and such a material is used as a moistening felt the sorption capacity

of which is lower than sorption capacity of those areas of said layer of

fibers that are pressed due to the protruding relief elements, and at

the same time the sorption capacity of said material is higher than the

areas of said layer of fibers that are non-pressed by said relief

elements, and saturation of moistening felt with water is performed

outside the pressing zone.