WO1999036617A1

WO1999036617A1 - Paper having a three-dimensional pattern

Info

Publication number: WO1999036617A1
Application number: PCT/SE1998/002455
Authority: WO
Inventors: Holger Hollmark; Lennart Reiner; Hans-Jürgen Lamb; Hans Wallenius
Original assignee: SCA Hygiene Products AB
Current assignee: Essity Hygiene and Health AB
Priority date: 1997-12-30
Filing date: 1998-12-29
Publication date: 1999-07-22
Anticipated expiration: 2000-06-30
Also published as: DE29824270U1; SE9704909L; AU2194599A; SE9704909D0

Abstract

An impulse-dried paper having a three-dimensional pattern of alternating raised and recessed portions which is given the paper in connection with the impulse drying. The paper contains at least 10 % by weight, calculated on the dry fiber weight, of a lignin containg high yield pulp, such as mechanical, thermomechanical (TMP) and chemothermomechanical (CTMP) pulp. It is supposed that the softening temperature of the lignin in the high yield pulp is reached during the simultaneous impulse drying and the embossing and when the paper is cooled the lignin will become stiff again and contributes in locking the three-dimensional structure that has been conveyed to the paper.

Description

Paper having a three-dimensional pattern

Technical field

The present invention refers to an impulse dried paper having a three-dimensional pattern of alternating raised and recessed portions which is conveyed to the paper in connection with the impulse drying.

Background of the invention

Moist paper webs are usually dried against one or more heated rolls. A method which is commonly used for tissue paper is so called Yankee drying. At Yankee drying the moist paper web is pressed against a steam-heated Yankee cylinder, which can have a very large diameter. Further heat for drying is supplied by blowing of heated air. If the paper to be produced is soft paper the paper web is usually creped against the Yankee cylinder. The drying against the Yankee cylinder is preceded by a vacuum dewatering and a wet pressing, in which the water is mechanically pressed out of the paper web.

Another drying method is so called through-air-drying (TAD). In this method the paper is dried by means of hot air which is blown through the moist paper web, often without a preceding wet pressing. The paper web which enters the through-air-dryer is then only vacuum dewatered and has a dry content of about 25-30% and is dried in the through- air-dryer to a dry content of about 65-95%. The paper web is transferred to a special drying fabric and is passed over a so called TAD cylinder having an open structure. Hot air is blown through the paper web during its passage over the TAD cylinder. Paper produced in this way, mainly soft paper, becomes very soft .and bulky. The method however is very energy-consuming since all water that is removed has to be evaporated. In connection with the TAD drying the patterned 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-3,301,746. Impulse drying of a paper web is disclosed in e g SE-B-423 118 and shortly involves that the moist paper web is passed through the press nip between a press roll and a heated roll, which is heated to such a high temperature that a quick and strong steam generation occurs in the interface between the moist paper web and the heated roll. The heating of the roll is e g accomplished by gas burners or other heating devices, e g by means of electromagnetic induction. By the fact that the heat transfer to the paper mainly occurs in a press nip an extraordinarily high heat transfer speed is obtained. All water that is removed from the paper web during the impulse drying is not evaporated, but the steam on its way through the paper web carries along water from the pores between the fibers in the paper web. The drying efficiency becomes by this very high.

In EP-A- 0 490 655 there is disclosed the production of a paper web, especially soft paper, where the paper simultaneously with impulse drying is given an embossed surface. This embossment is made by pressing a pattern into the paper from one or both sides against a hard holder-on. This gives a compression of the paper and by this a higher density in certain portions just opposite the impressions and a lower density in the intermediate portions.

The object and most important features of the invention The object of the present invention is to provide a method of producing an impulse dried paper having a three-dimensional pattern, e g a soft paper intended as toilet paper, kitchen rolls, paper handkerchiefs, table napkins and the like, and where the paper is produced from a pulp especially suitable for impulse drying and the high stresses in the form of high pressures and temperatures that the paper is exerted to. Besides the paper should have a high bulk and a high absorption capacity and the three-dimenisional structure should be maintained in dry as well as in wet condition. This has according to the invention been solved by the fact that the paper contains at least 10% by weight, calculated on the dry fiber weight, of a lignin containing high yield pulp, such as mechanical, thermomechanical (TMP) and chemothermomechanical (CTMP) pulp. It is supposed that the softening temperature of the lignin in the high yield pulp is reached during the simultaneous impulse drying and shaping and when the paper then is cooled the lignin is stiffened again and contributes in permanenting the structure that has been given the paper. This is only a theory to explain the results obtained by the invention, which is not limited to this theory. The three-dimensional structure that has been given the paper is substantially maintained also in its wet condition, which strongly improves the absorption properties of the paper.

The paper preferably contains at least 30% by weight .and more preferably at least 50% by weight high yield pulp.

Description of the drawings

The invention will in the following be closer described with reference to some embodiments shown in the accompanying drawings. Fig. 1-3 are a schematic side views of impulse drying devices according to some different embodiments.

Fig. 4 a-c show in the form of bar charts dry and wet bulk of impulse dried paper produced from different types of pulp.

Fig. 5 shows in the form of bar charts the effect of temperature on bulk and absorption of impulse dried paper made from different types of pulp.

Fig. 6 shows in the form of bar charts the effect of temperature on the strength properties of impulse dried paper made from different types of pulp.

Description of the invention Fig. 1 shows schematically a device for performing impulse drying of a paper web.

The wet paper web 10 which is dewatered over suction boxes (not shown) is supported by a compressible press felt 11 and is brought into a press nip 12 between two rotatable rolls 13 and 14, at which the roll 13 which is in contact with the paper web is heated to a temperature which is sufficiently high for providing drying of the paper web. The surface temperature of the heated roll can vary depending on such factors as the moisture content of the paper web, thickness of the paper web, the contact time between the paper web and the roll and the desired moisture content of the completed paper web. The surface temperature should of course not be so high the paper web is damaged. An appropriate temperature should be in the interval 100-400°C, preferably 150-350°C and most preferably 200-350°C.

The paper web is pressed against the heated roll 13 by means of the roll 14. The press device may of course be designed in many other ways. Two and more press devices may also be arranged after each other. The holder-on 14 may also be a press shoe. It is also possible that the paper web 11 is passed into the press nip unsupported, i e not supported by any wire or felt.

A very rapid, violent and almost explosive steam generation takes place in the interface between the heated roll 13 and the moist paper web, at which the generated steam on its way through the paper web carries away water. For a further description of the impulse drying technique reference is made to the above mentioned SE-B-423 118 sand e g to

EP-A- 0 337 973 sand US-A-5,556,511.

The paper is after drying wound on a wind-up roll 16. If desired the paper can be creped before winding. It is however noted that the need for creping the paper in order to impart softness and bulk which is aimed at for soft paper, is reduced when using the impulse drying method according to the invention, since the paper by the strong steam expansion in the paper web is imparted bulk and softness and besides a three- dimensional structure.

The paper web can before it is brought into the impulse dryer either can be only dewatered over suction boxes or besides slightly pressed according to a conventional process.

Simultaneously with the impulse drying the paper is given a three-dimensional structure. This can be made as shown in Fig. 1 and 2 by the fact that the heated roll 13 is provided with an embossing pattern consisting of alternating raised and recessed areas. In Fig. 1 there is shown an example of such an embossing pattern where the raised portions 17 consist of a protruding relief and the recessed portions of milled grooves. This structure is substantially maintained also in a later wetted condition of the paper, since it has been imparted the wet paper web in connection with drying thereof. Since the term embossing is normally used for a shaping performed on dried paper we have in the following used press moulding for the three-dimensional shaping of the paper that occurs simultaneously with the impulse drying. By this press moulding the bulk and absorption capacity of the paper is increased, which are important characteristics for soft paper.

The paper can be pressed against a non-rigid surface, i e a compressible press felt 11. The roll 14 can also have an elastically yielding surface, e g am envelope surface of rubber. The paper is herewith given a three-dimensional structure the total thickness of which is greater than the thickness of the unpressed paper. By this the paper is imparted a high bulk and by that a high absorption capacity and a high softness. Besides the paper will be elastic. At the same time a locally varying density is obtained in the paper.

The paper can also be pressed against a hard surface, e g a wire 11 and/or a roll 14 having a hard surface, at which the pattern of the heated roll 13 is pressed into the paper web under a heavy compression of the paper opposite the impressions, while the portions therebetween are kept uncompressed.

The embodiment shown in Fig. 2 differs from what is shown in Fig. 1 by the fact that under the wire 11 there is arranged a felt 17, which extends around the roll 14. The function of the felt 17 is to improve the dewatering effect and extend the press nip.

According to the embodiment shown in Fig. 3 the paper web 10 is during the drying supported by a wire 11 having a pattern, which is press moulded into the paper web when this passes through the press nip 12 between the rolls 13 and 14. The roll 13 can either be smooth, as is shown in Fig. 3, or have an embossing pattern. In the case the roll 13 is smooth the press moulded paper will have one smooth surface and one surface with impressions. In the case the roll 13 has an embossing pattern this will also be pressed into the paper, which thus on one side will have a pattern corresponding to the structure of the wire 11 and on the opposite side having a pattern corresponding to the embossing pattern of the roll. The pattern may but need not coincide and/or be the same or different.

Paper can be produced by a number of different pulp types. If one disregards recovery pulp, which today is used to a great extent mainly for toilet paper and kitchen rolls, the most commonly used pulp type for soft paper is chemical pulp. This is produced by impregnating wood chips with chemicals and then boil it so that the lignin and the hemicellulose is transferred to the liquid. After finished boiling the pulp is screened and washed before it is bleached. The lignin content in such pulp is practically zero and the fibers, which mainly consist of pure cellulose, are relatively thin and flexible. Chemical pulp can be both of long- and short fiber type depending on the wooden raw material used, and can be of sulphate- or sulphite type depending on the composition of the boiling liquid. Chemical long fiber pulp (softwood), especially of sulphate type, has a favourable effect on the strength properties of the soft paper, both dry- and wet strength.

Chemical pulp is a low yield pulp since it gives a yield of only about 50% calculated on the wooden raw material used. It is therefore a relatively expensive pulp. It is therefore common to use cheaper so called high yield pulps, e g mechanical or thermomechanical pulp, in soft paper as well as in other types of paper, e g newsprint paper, cardboard etc. Mechanical pulp is produced by grinding or refining and the principle for mechanical pulp production is that the wood is mechanically disintegrated. The entire wood material is utilized and the lignin is thus left in the fibers, which are relatively short and stiff. The production of thermomechanical pulp (TMP) is accomplished by refining in a disc refiner at an increased steam pressure. Also in this case the lignin is left in the fibers. Chemomechanical pulp (CMP) or chemothermomechanical pulp (CTMP) are terms for a thermomechanical pulp which has been modified by the addition of small amounts of chemicals, usually sulphite, which is added before the refining. One effect of the chemical treatment is that the fibers are freed more easily. A chemomechanical or chemothermomechanical pulp contain more complete fibers and less shives (fiber aggregates and fiber fragments) than a mechanical or thermomechanical pulp. The properties of CMP and CTMP approaches those for the chemical pulps, but there are essential differences depending among other things on that in CMP and CTMP the fibers are coarser and can contain a high amount of lignin, resins and hemicellulose. The lignin and the resins gives the fibers more hydrophobic properties and a reduced ability ro form hydrogen bonds. The addition of a certain amount of chemothermomechanical pulp in soft paper has due to the reduced fiber-fiber bonding a positive effect on properties like bulk .and absorption capacity.

A special variant of chemothermomechanical pulp (CTMP) is so called high temperature chemothermomechanical pulp (HT-CTMP), the production of which differs from the production of CTMP of conventional type mainly by using a higher temperature for impregnation, preheating and refining, preferably no lower than 140°C. For a more detailed description of the production method for HT-CTMP reference is made to WO 95/34711. Characterizing for HT-CTMP is that it is a long fibrous-, easily dewatered- and bulky high yield pulp with a low shives content and low fines content.

It has according to the invention been found that high yield pulp is especially suitable for impulse drying since it is pressure insensitive, easily dewatered and has an open structure which admits the generated steam to pass through. This minimizes the risk for the paper to be overheated and destroyed during the impulse drying, which is performed at considerably higher temperatures than in other drying methods. The pressure insensitivity and the open structure depends on that the fibers in high yield pulp are relatively coarse and stiff as compared to the fibers in chemical pulp. A further advantage is that the three-dimensional structure that has been given the paper is substantially maintained also in wet condition of the paper, since it has been conveyed to the wet paper web simultaneously with the drying thereof. Impulse drying is further performed at a considerably higher temperature than e g Yankee-drying or through-air-drying, and according to a theory, to which however the invention is not bound, the softening temperature of the lignin in the high yield pulp is reached during the simultaneous impulse drying and press moulding. When the paper then cools down the lignin becomes stiff again and contributes to permanent the three-dimensional structure that has been conveyed to the paper. This structure is therefore substantially maintained also in the wet condition of the paper, which strongly improves the bulk and absorption characteristics of the paper.

The amount of high yield pulp should be at least 10 % by weight calculated on the dry fiber weight, preferably at least 30 % by weight and most preferably at least 50 % by weight. Admixture of a certain amount of other pulp with good strength properties, such as chemical pulp, preferably long-fibrous sulphate pulp, or recovery pulp, is an advantage if a high strength of the finished paper is aimed at.

Common additives such as wet strength agents, softening agents, fillers etc. may of course be used in the paper.

Trials have been made in an experimental equipment in which a paper web having a dry content of about 35 % by weight without previous pressing was exerted to impulse drying at temperatures varying between about 200-300°C and a pressure of about 4 MPA. The impulse drying time was between 3 and 20 msek. The pulp types that were tested were 100% unbeaten chemical sulphate pulp, 100% HT-CTMP and 50/50 unbeaten chemical sulphate pulp/HT-CTMP.

In Fig. 4 a-c the results of measurements performed with respect to dry and wet bulk of impulse dried paper containing the above pulps are shown. Measurements have been made on unembossed (plane pressed) as well as on embossed (press moulded) paper. Besides measurements have been made on paper with and without addition of KYMENE®, a polyamide-amine-epichlorhydrine resin (PAE). The wet strength agent should be added to the furnish or to the paper web before press moulding, since it has proved that the wet strength agent contributes in permanenting the three-dimensional structure which has been given the paper in connection with the press moulding. The added amount should be at least 0,05 % by weight calculated on the dry fiber weight.

From the results it is seen that impulse dried paper which has been press moulded according to the invention has a high dry and wet bulk. Especially good results were obtained for those papers that contained a high yield pulp in the form of HT-CTMP.

Press moulded paper containing HT-CTMP shows also in wet condition a high bulk of 7 c Vg and above. A clear improvement of the wet bulk was achieved when the paper contained a wet strength agent.

In order to investigate the importance of the temperature under the impulse drying, i e the surface temperature of the heated roll, tests were performed on the three different pulp types 100% unrefined chemical sulphate pulp, 100% HT-CTMP and a 50/50 mixture of unrefined chemical sulphate pulp at two different temperatures 200 and 300°C. The dry and the wet bulk and the relative absorption were measured. The results are shown in Fig. 5.

From these results it can be seen that a raising of temperature from 200 to 300° C did not have any real effect on these characteri.stics for paper containing 100% chemical sulphate pulp besides a small increase of the dry bulk. For the papers containing 100% HT-CTMP and the 50/50 mixture chemical sulphate/HT-CTMP did on one hand the dry bulk decrease somewhat, but on the other hand did the wet bulk and the relative absorption increase at a raising of temperature from 200 to 300°C. The increase was most significant for the paper containing 100%ι HT-CTMP. These results support the theory that the lignin in the high yield pulp softens during the simult-aneous impulse drying and the press moulding (embossing), which contributes in permanenting the three-dimensional structure when the paper is cooled. The higher temperature the more lignin will have time to soften during the impulse drying.

The temperature influence of the strength properties of the paper was also tested. The dry and the wet tensile index (TI) respectively for paper the had been impulse dried at

200 and 300°C respectively were measured. The results are shown in Fig. 6.

For paper containing 100% chemical sulphate pulp a lower tensile index was obtained at the higher temperature. The wet tensile index was essentially the same after drying at both temperatures.

For paper containing 100%) HT-CTMP however there was a significant increase of both the dry and the wet tensile index at an increase of the impulse drying temperature from 200 to 300°C (surface temperature of the roll 13). An explanation could be that a higher degree of softening is achieved at the higher temperature, which contributes in an increased bonding of the fiber structure. For the mixture 50/50 chemical sulphate pulp/HT-CTMP the dry tensile index was essentially unchanged while the wet tensile index increased somewhat at the higher temperature.

The invention is of course not limited to the embodiments described above and shown in the drawings, but may be varied within the scope of the claims. The lignin containing high yield pulp can as previously mentioned be of many different kinds such as mechanical pulp, thermomechanical, chemomechanical and chemothermomechanical pulp and comprise virgin fibers as well as recovery fibers. The admixture of a certain amount of other pulp with good strength properties, such as chemical pulp, preferably long-fibrous sulphate pulp is an advantage if high strength of the finished paper is aimed at. Also other pulps including recovery pulp can be contained in the paper.

The paper web can after the impulse drying be exerted to different types of treatments which per se are known such as addition of different chemicals, further embossing, lamination etc. Such a treatment may be that the paper web after it has been given the three-dimensional pattern is compressed in a subsequent roll nip which has a temperature which is lower than that of the heated roll, by means of which the paper has been given the three-dimensional pattern. Possibly a further pattern may be pressed into the paper web during this compression. The compression involves a decreased bulk of the paper, which saves space during transport and storing. The deformation of the paper web that takes place during this compression is maintained by means of fiber-to-fiber bonds that are not constant in wet condition. The paper will in contact with water or aqueous liquids recover its three-dimensional structure that was given to it at the impulse drying, at which by the expansion of the paper an increased water absorption capacity is obtained.

Claims

1. An impulse-dried paper having a three-dimensional pattern of alternating raised and recessed portions which is conveyed to the paper in connection with the impulse drying, characterized in that the paper contains at least 10% by weight, calculated on the dry fiber weight, of a lignin containing high yield pulp, such as mechanical, thermomechanical and chemothermomechanical (CTMP) pulp.

2. Paper as claimed in claim 1 , characterized in that it contains at least 30% by weight and preferably at least 50% by weight, calculated on the dry fiber weight, of the lignin containing high yield pulp.

3. Paper as claimed in claim 1 or 2, characterized in that the high yield pulp is chemothermomechanical pulp (CTMP).

4. Paper as claimed in any of the preceding claims, characterized in that the paper is absorbent soft paper.

5. Paper as claimed in any of the preceding claims, characterized in that it contains at least 0,05%> by weight of a wet strength agent calculated on the dry pulp fiber weight.

6. Method for producing .an impulse dried paper having an embossing pattern of alternating raised and recessed portions, which has been conveyed to the paper in connection with the impulse drying, characterized in that it contains at least 10%> by weight, preferably at least 30% by weight and most preferably at least 50% by weight, calculated on the dry fiber weight, of a lignin containing high yield pulp and that the impulse drying is performed at a temperature of at least the lower part of the softening temperature interval of lignin.