EP1270196B1 - Method of making embossed tissue paper and embossing cylinder - Google Patents

Abstract

The method for making the sheet comprises embossing it on an embossing cylinder having a part of its surface provided with picots. The picot has a truncated base (14a) and a top portion (14b) having a flat (14c). Independent claims are included for a sheet obtained using the method and an embossing cylinder

Description

The invention relates to the field of paper for sanitary or domestic use, including a method of manufacturing a sheet of absorbent paper creped cellulose wadding, having an embossing pattern. The invention also relates to the product obtained by the process as well as the cylinder for carrying out the process.

Cellulose wadding, also referred to as "tissue paper" is a light weight tissue paper, between 15 and 35 g / m ² , which can be made to have a certain elongation capacity. A conventional technique is to apply and glue the still wet sheet to a drying cylinder, and then detach it by means of a doctor blade to create creping corrugations. Then, by the known technique of embossing, it can improve or at least change one or the other of its characteristics, such as softness, flexibility, absorption, thickness or appearance .

The embossing operation is performed either on paper with high humidity, ie on a paper machine in its wet part or on paper with low humidity, ie in transformation. partly dry. The invention relates to the embossing of paper with low humidity. The embossing pattern is most commonly constituted by protuberances or bosses in the form of pyramids with a square section or conical frustum with round or oval cross section, regularly or not distributed on the surface of the sheet. Note that a protuberance on one side of the sheet corresponds to a cavity on the other side.

In order to emboss the cellulose wadding, according to a first technique, a rigid roll is generally used having pimples on the surface. The sheet is applied to the roll and is pressed by means of another roll having a deformable coating, eg rubber. She thus marries the relief. For the same reason, we obtain a deformation more or less important according to the parameters we have chosen, such as the flexibility of the rubber, for its ability to deform and conform to the relief of the etching, and the embossing pressure.

According to another technique, the two cylinders have on the surface reliefs with complementary shapes, male and female.

The pins are made by engraving the cylinder, for example by means of a wheel itself engraved. Depending on the material used, the spikes can also be machined directly.

The pins have a generally frustoconical shape, circular section, oval or pyramidal square section. The half-angle at the top of the inscribed cone is usually between 25 ° and 35 °. Their size, their distribution, their density - that is to say their number on a given portion of surface - are chosen according to the desired performances for the product. Such pins are known from US-A-5,620,776.

For example, when it is desired to give the sheet a textile appearance and to soften it without particularly seeking to improve its absorption capacity, very small peaks are chosen: from 0.4 to 0.6 mm distributed according to a pattern of high density, more than 30, even 80 pins per cm ² . On the other hand, for a paper intended for wiping, including household paper towels, on the contrary, it is sought to significantly deform the sheet to create pockets of air between the plies after assembly. These pockets significantly increase the absorption capacity of the "tissue paper". The height of the pins can then be up to 2.5 mm for a low pattern density, less than 10 pins per cm ² . Thus an embossing pattern composed for the most part of frustoconical pimples is characterized both by the height of the latter and by its density.

By deforming the sheet beyond its limit of elastic deformation, some of the bonds between the fibers of the paper are always destroyed. Any embossing operation therefore leads to a decrease in its tear resistance. On the other hand, the number of links having decreased, the sheet is more flexible.

When wadding cellulose wadding by means of an embossing unit consisting of an engraved cylinder and a rubber cylinder, the deformation of the sheet, and therefore its thickness, depends on the pressure exerted by this material. latest. For the same reason, the loss of tear strength is therefore related to the thickness of the embossing obtained, and increases with it. This is especially true for maximum values. At a maximum thickness value is the maximum resistance loss.

The applicant has set a goal to improve the thickness of the embossing, for the same pattern and for the same embossing pressure. In other words, it sought to obtain the highest thickness possible by sacrificing the least possible loss of strength.

The Applicant has also set a goal of increasing the maximum embossing thickness that can be obtained for a given pattern.

Thanks to the manufacturing method according to the invention, these objectives are achieved. This method, according to which the sheet is embossed by means of an embossing roll with at least a part of its surface provided with pins, the so-called pins having a frustoconical base portion and a crown portion, is characterized by the fact that said crown portion forms a flat portion having a surface area of from 0.01% to 75% of the smallest portion of the base portion, and in that the profile of the crown portion between the base portion and the flat portion is a convex curve.

The flat is the distal surface of the pin, usually flat or with a large radius of curvature.

The surface of the crown portion joining the base portion to the flat has a curved profile so that there is no sharp edge between the trunk and the flat.

In particular, the width of the flat is less than 20%. Indeed, it has been found that the stresses applied to the sheet are better distributed when the surface of the flat surface is reduced, taking into account other technical requirements such as, in particular, the need to apply a film of adhesive to bind the sheet. to another sheet. According to a particular embodiment of the invention, the surface of the flat is reduced to one point only. This embodiment is suitable for a sheet which is not bound by the tops of the protuberances.

US Pat. No. 3,414,459 describes a sheet of paper with two embossed folds, arranged in such a way that the protuberances are placed tip to tip. In the figures of the patent, there is shown the pins engraved on the surface of the embossing rolls. These pins have a cylindrical shape of substantially rectangular section, with two parallel flat faces oriented in the machine direction. The top of the spikes is flat and has a rounded portion. It appears that the rays curvature as well walking direction as cross directions shown in the figures are different. It is observed that the parallelepipedic shape of the pins causes shearing forces on the sheet which contribute to a significant weakening of its resistance. The invention on the contrary by the optimal shape of the pin allows an improvement of the characteristics of the sheet.

The advantages provided by the process are also of an industrial nature. For the same installation, using the pins of the invention, up to 15% more thickness can be obtained in addition to equal embossing pressure. It follows an industrial exploitation easier because one can then reduce this pressure. Rubber wears less. Conversely, there is a reserve of greater thickness in case of need.

According to another characteristic, the density of the pin pattern is between 20 and 100 per cm ² , the height of the latter being between 0.1 and 1.5 mm or more particularly between 0.1 and 0.6 mm. . Indeed, the solution of the invention is particularly advantageous when considering the reasons of the micro-embossing type because the fine pins have a certain perforating effect. It is therefore possible to increase the embossing pressure without the risk of piercing the sheet.

The present invention also relates to the sheet obtained according to the process. In particular, a first embossed sheet is combined on pins having almost no flat surface with a second sheet having a flat of sufficient size to receive a film of glue. This solution makes it possible to reduce as much as possible on the first ply the loss of strength resulting from the embossing.

• La figure 1 représente schématiquement une installation de gaufrage permettant de mettre en oeuvre le procédé de l'invention.
• La figure 2a représente des picots conformes à l'invention, sur une partie de la surface du cylindre gravé, vus de dessus.
• La figure 2b est une vue selon la coupe AA de la figure 2a
• La figure 3 est une représentation en perspective d'un picot conforme à l'invention.
• La figure 4a représente l'empreinte du motif de l'invention sur une feuille sensible à la pression.
• La figure 4b représente l'empreinte d'un motif de l'art antérieur sur une feuille sensible à la pression.
• La figure 5 représente une installation permettant de réaliser une feuille multipli.

Other features and advantages will appear on reading the following description of an embodiment of the invention, with reference to the drawings in which:

• Figure 1 schematically shows an embossing installation for implementing the method of the invention.
• Figure 2a shows pins according to the invention, on a part of the surface of the engraved cylinder, seen from above.
• FIG. 2b is a view according to section AA of FIG. 2a
• Figure 3 is a perspective representation of a pin according to the invention.
• Figure 4a shows the imprint of the pattern of the invention on a pressure sensitive sheet.
• Figure 4b shows the impression of a pattern of the prior art on a pressure-sensitive sheet.
• FIG. 5 represents an installation making it possible to produce a multiplied sheet.

FIG. 1 diagrammatically shows an embossing installation 10 of a sheet 15. A first cylinder 12 is made of steel or other rigid material and has on its surface pins 14, the size of which is exaggerated in the figure. A rubber cylinder 11 or any material capable of deformation under pressure is rotatably mounted parallel to the first cylinder 12 with a certain tightening. A sheet of cellulose wadding to be embossed is drawn between the two rolls. The sheet 15 may be single-ply, but it may also be composed of two or more folds superimposed. Each of the plies is creped cellulose wadding, weight of between 15 and 35 g / m ² . The crepe rate is sufficient to allow the embossing treatment. At the level of the interval between them cylinders, the sheet is deformed by the pins of which it marries the form.

With reference to FIGS. 2 and 3, an exemplary embodiment of pins according to the invention is described below. Figures 2a and 2b show the pins 14 on the surface of the cylinder. Figure 2a is a top view, partial and developed, of the surface of the engraved cylinder 12. It shows the pins 14 arranged in this example in a staggered pattern of no direction and direction determined direction. For a very dense pattern, the steps are respectively 1.75 mm and 1.4 mm for a pin height of 0.5 mm. The pin 14 is shown in transparency in Figure 3; there are two parts: a base 14a and a crown portion 14b. The base 14a is frustoconical axis ab. The half angle at the top of the frustoconical surface is here 30 °. This portion 14a is extended by a crown portion 14b which forms a flat portion 14c at its top. Between the flat 14c and the base 14a, the portion 14b has a curved profile 14d. It may advantageously be an arc of a circle. Preferably the profile 14d is tangent to the truncated cone 14a along the virtual separation line 14e between the two parts 14a and 14b. According to the invention, the area of the flat 14c is at most equal to 75% of the area delimited by the closed line 14e. Preferably, it is less than 20% of the area delimited by 14e. he can even be advantageous to reduce the flat to a point, represented by the value 0.01%. Indeed, without being bound by this hypothesis, it is believed that by reducing the surface at the top, the rubber pressing the sheet is allowed to gradually deform along the pin, ensuring a more homogeneous deformation thereof. As a result, less areas of leaf weakness are formed. The loss of resistance is therefore lower.

In order to confirm this result, several attempts have been made.

• 80 picots /m²,
• 0,5 mm de hauteur,
• une portion de sommet en forme de dôme de rayon 0,2 mm.

A first visual test was carried out on a pilot machine with a cylinder carrying pins according to the example mentioned above, namely:

• 80 pins / m ² ,
• 0.5 mm high,
• a domed top portion of 0.2 mm radius.

Dynamical impressions were made at a machine speed of 50m / min. on a sheet of carbonless paper. This footprint is shown in Figure 4a. In comparison, impressions were made with an identical pattern with the difference that the peaks of the spikes were of standard type with a flat corresponding to that of the truncated cone, the characteristics of such a pattern are the following: 80 spikes / cm ² , 0.5 mm in height and truncated cone studs whose flat is a 0.2 mm radius disc. The imprint is shown in Figure 4b. Note that the fingerprints of the pins according to the invention are reduced to one point while those of the pins of the prior art are different. The imprints form a ring. The whole periphery of the summits seems crushed.

Other comparative tests with the same patterns were performed at several embossing pressures. At an embossing pressure corresponds a width of the rubber mark on the steel cylinder. Thus, the level of embossing pressure is generally defined in the technical field of the invention by the width of this mark, also known as an imprint. There are thus five imprints: 20, 25, 30, 35 and 39 mm. Then a ply thus embossed was glued with a ply of the same cellulose wadding but not embossed (column I1). By way of comparison, a product of the same structure but embossed with a pattern of the prior art (column A1) was manufactured. The table below shows that for the same footprint, ie for the same machine setting, a product (column I1) having a thickness greater than that of a product manufactured according to the art is obtained. previous (column A1). This implies, for example, that the load on the rolls can be reduced. It follows then a lower wear of the rubber in particular.

Moreover, at the maximum setting of the pressure, ie at a footprint of 39 mm, 10 to 15% more thickness is obtained with the pins of the invention. There is thus an additional thickness potential with these. In addition, for the same value of the cross-directional resistance, more thickness is obtained, in particular for the highest thicknesses. Finally, for equal thicknesses, a more resistant embossed paper is obtained. Footprint in mm A1 Thickness in mm A1 Cross-directional resistance ST in N / m I1 Thickness in mm I1 Cross-direction resistance ST in N / m 20 0.27 116 0.29 114 25 0.31 107 0.34 102 30 0.35 102 0.40 93 35 0.41 90 0.45 86 39 0.42 84 0.48 82

The same tests were carried out under the same conditions but this time by associating an embossed fold with a non-embossed fold by knurling. It can be seen from the table below that similar results are obtained: column A2 for the product of the prior art, column 12 for the product manufactured according to the invention. Footprint in mm. A2 Thickness in mm A2 Cross direction resistance STenN / m 12 Thickness in mm 12 Cross-directional resistance STenN / m 20 0.28 102 0.31 97 25 0.33 96 0.35 90 30 0.37 84 0.41 78 35 0.42 75 0.45 74 39 0.42 68 0.46 70

FIG. 5 shows an installation making it possible to manufacture a multi-ply sheet of which at least one ply is embossed according to FIG. method of the invention. There is provided a first embossing unit with an engraved cylinder 112 and a rubber-coated cylinder 111 and a second embossing unit 112 'and 111'. A gluing device applies the glue on the tops of the protuberances of one of the two units. We understand that we can equip one or both units of pins according to the invention. To make best use of the advantages of the invention, it is possible for example in the case of a household paper towel to adapt fully rounded peaks on the unit (111/112) which does not receive the glue and keep a certain surface in the flats of the unit 111 '/ 112' which receives the glue.

Claims (11)

1. Method for manufacturing a sheet of paper tissue comprising embossing said tissue using an embossing cylinder (12) having a surface of which at least a portion has embossing elements (14) that comprise a frustoconical base portion (14a) and a tip portion (14b), wherein said tip portion (14b) has a flat (14c) at a free end of the tip portion, the flat having a surface less than 75% of the smaller cross-sectional area of the base portion (L4a) and said tip portion has a contour between the base portion and the flat which is a convex curve.
2. Method as claimed in the preceding claim, wherein the flat (14c) has a surface less than 20% of the smaller cross-sectional area of the base portion.
3. Method as claimed in the preceding claim, wherein said contour is an arc of a circle.
4. Method as claimed in one of the preceding claims, wherein a half-angle at a top of the frustoconical surface of the base portion (14a) is between 25° and 35°.
5. Method as claimed in the preceding claim, wherein said tip portion has a contour of a semi-circle.
6. Method as claimed in any one of the preceding claims, wherein the top of the base portion is tangent to said tip portion.
7. Method as claimed in one of the preceding claims, wherein the base portion (14a) is circular in cross-section.
8. Method as claimed in one of the preceding claims, wherein the embossing elements (14) are present in a density of between 20 and 100/cm² and have a height of between 0.1 and 1.5 mm.
9. Method as claimed in preceding claim, wherein the embossing elements have a height of between 0.1 and 0.6 mm.
10. Method as claimed in any one of claims 1 to 7, wherein the embossing elements are present in a density of between 4 and 20/cm² and have a height of between 1 and 2.5 mm.
11. An embossing cylinder to implement the method claimed in any one of claims 1 through 10, wherein said cylinder comprises embossing elements (14) having a frustoconical base portion (14a) and a tip portion (14b) having a flat top (14c) with an area less than 75% and preferably less than 20% of the smaller cross-sectional area of the frustoconical base portion, the contour of the tip portion between the base portion and the flat being a convex curve.

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