EP2788548B1

EP2788548B1 - Embossing roller, embossing unit and method for embossing cellulosic plies and embossed cellulosic web material

Info

Publication number: EP2788548B1
Application number: EP12808513.1A
Authority: EP
Inventors: Mauro Gelli
Original assignee: Engraving Solutions Srl
Current assignee: Engraving Solutions Srl
Priority date: 2011-12-06
Filing date: 2012-11-30
Publication date: 2018-01-10
Anticipated expiration: 2032-11-30
Also published as: WO2013084127A1; BR112014013473A2; ITFI20110261A1; EP2788548A1; BR112014013473B1; BR112014013473A8

Description

Technical Field

The present invention relates to the field of the production of the so-called "tissue" paper, or creped paper, in particular toilet paper, kitchen towels, paper napkins or handkerchiefs and the like. More in particular, the invention relates to improvements to the embossing units and methods for tissue paper mechanical processing.

State of the Art

Tissue or creped paper is used for producing various articles for home, professional, as well as industrial use. In particular, tissue paper is used to produce toilet paper, kitchen towels and other sheet or roll products. In many applications, tissue paper undergoes a mechanical process of embossing. Substantially, the embossing process consists of passing a single or multiple ply of tissue paper through a nip defined between an embossing roller and a pressure roller. The embossing roller is provided with protuberances co-acting with the pressure roller surface. In some embodiments both the embossing roller and the pressure roller are made of hard material, for instance steel, and the protuberances of one roller engage with the cavities of the other.
In other more widely used embodiments, the embossing roller has protuberances penetrating inside an elastically yielding coating layer of the pressure roller and deforming it with respect to the substantially cylindrical shape this layer has when the pressure roller is not working and is not in contact with the embossing roller. The two rollers are pressed against each other so that the protuberances penetrate into the pressure roller side surface due to the compression deformation of the yielding coating of the pressure roller. The cellulosic material forming the web material and passing through the nip between the two rollers is permanently deformed and embossing projections are thus formed having a pattern corresponding to that of the protuberances of the embossing roller.
The embossing process entails great mechanical stress on the paper ply and localized breakage of its cellulosic fibers.
Embossing is used for aesthetic reasons, to decorate the paper ply, as well as, and above all, for technical functional reasons, for instance to create gluing areas for gluing more plies to one another thus forming a multi-ply web material. Glue is applied on the outer surfaces of the embossing projections, it is therefore applied on limited areas. Embossing is also used to modify, change or improve tissue paper specific features such as thickness, softness and absorption capacity.
The ply cellulosic fibers are subject to great stress in the areas where the ply is deformed through embossing; therefore, shapes, dimensions and arrangement of the embossing protuberances cannot be chosen at will, and specific criteria must be taken into account, limiting the choice of the embossing pattern. In designing a new embossing pattern, a compromise must be therefore achieved between the technical functional requirements and the need for the paper not to be subject to an excessive stress; since otherwise, localized breakages or an excessive weakening of the cellulosic material could occur.
To generate the embossing protuberances, the cylindrical surface of the embossing rollers is engraved. Earlier engraving techniques provided for mechanically processing through chip removal. Recent engraving techniques are based on laser and chemical etching. Using embossing rollers engraved through old techniques (forming, for instance, truncated pyramid shaped protuberances), deformation of the cellulosic material ply is uniformly distributed. On the contrary, weakening of the ply is not homogeneous due to the non isotropic arrangement of the cellulosic fibers, as explained below.
Using current engravings (obtained through chemical etching and of various shape) the embossing pattern aspect is significantly better, but the ply is not uniformly deformed; this results not only in heterogeneous weakening, but also in heterogeneous elongations, and consequently in creases, localized looseness and displacements of the ply under processing.
As it is well known to those skilled in the art, tissue paper is usually produced through wet or water processes. In a refiner, a pulp of water and cellulosic fibers is produced with a dry matter percentage below 5%, typically in the order of 2-4%. If necessary, additives, such as moisture resistant resins, dyes, or the like are added; then the pulp is delivered by head-boxes to a forming fabric or felt. Water is then gradually drained from the pulp layer formed on the forming fabric or felt, so as to increase the dry matter percentage inside the pulp layer. As the dry matter percentage is such that the cellulosic material layer has an adequate mechanical strength, the fiber layer is transferred to drying means, for instance a Yankee cylinder, drying rollers or the like.
Even if fibers in the pulp have a completely random orientation, once the pulp is distributed on the forming fabric most fibers are arranged in the machine direction, i.e. parallel to the forming fabric feeding direction, thanks to the way in which they have been distributed on the forming fabric, through a continuous flow of pulp fed by the linear nozzles of the head-box according to the feeding direction of the forming fabric. As a result, the finished cellulosic material has non-isotropic features of mechanical strength, and more precisely lower strength in cross-machine direction and greater strength in longitudinal direction, i.e. in machine direction. The material has substantially lower tensile strength in cross-machine direction than in machine direction. This further limits shape and orientation of the embossing protuberances and projections that will be generated on the cellulosic material, and shall be taken therefore into account when designing a new engraving pattern. In fact, embossing causes tensile stress and elongation on the cellulosic material.
Moreover, embossing obtained with protuberances of simple geometry (typically truncated pyramid or truncated cone) on one hand reduces mechanical strength and, on the other hand, increases the difference between the mechanical strength in longitudinal direction (machine direction) and that in cross-machine direction, increasing the anisotropic behavior of the embossed cellulosic material.
This typically occurs in the embossing units where the embossing roller cooperates with a roller coated with a layer of elastically yielding material with substantially cylindrical surface, i.e. without cavities in which the embossing protuberances are inserted. In this case, in fact, a pressure is exerted between the two rollers, causing the embossing protuberances partially to penetrate the coating made of elastically yielding material, with a consequent localized elongation of the web or ply material in correspondence of the sides of each protuberance, where the web or ply material is pressed between the protuberance and the yielding surface of the pressure roller.
US-A-2005/0173085 discloses an embossing unit and an embossing method for processing a cellulosic material so as to degrade it in the machine direction more than in cross-machine direction. This known embossing unit uses embossing rollers engraved with protuberances and cavities, i.e. male and female embossing rollers. The embossing rollers cooperate so that the protuberances of a male embossing roller penetrate into the cavities of a female embossing roller. The use of three distinct embossing rollers of this type, defining two sequentially arranged embossing nips, is aimed at obtaining a cellulosic ply processing that degrades the mechanical features of the ply in machine direction while preserving the strength thereof in cross-machine direction.
EP-A-2353858 discloses a unit for embossing wrapping or packaging sheets, in particular for cigarette filters. Processing of plastic films is in particular discloses. The embossing unit described therein comprises a pair of engraved embossing rollers cooperating by meshing with each other. In some embodiments described in that prior art document, to transmit motion between the embossing rollers the protuberances of the two rollers are generated by engravings of different depth, so that the sides of the protuberances in machine direction are longer than the sides in cross-machine direction. The embossing protuberances have therefore a variable height, as their bases lie not on a cylindrical surface but on a stepped surface, deeper in correspondence of the side oriented in machine direction and less deep in correspondence of the side in cross-machine direction. The resulting greater extension of the sides oriented in machine direction increases the torque transmitted from a motorized embossing roller to an embossing roller driven into rotation. Neither the problem of the anisotropic mechanical features of the cellulosic material nor the way to modify the mechanical strength of the embossed ply is mentioned therein.
US-A-2005/0138981 discloses embossing patterns for tissue paper embossing rollers. In some embodiments described therein, the embossing protuberances have an inclination which varies along their extension. No specific relationship is described between the change in the protuberance inclination and the machine or cross-machine direction of the embossing roller, and the problem of the difference in the cellulosic ply mechanical strength between the machine direction and the cross-machine direction is not mentioned.
DE-A-20 2007 011 885 discloses an embossing roller with embossing protuberances, along the sides of which discontinuities in the form of projections or cavities are provided. The shape of the embossing protuberance is aimed at increasing the compressive strength of the embossed ply. The problem of the difference in mechanical strength between machine direction and cross-machine direction of the cellulosic ply before embossing is not tackled. No measures are indicated therein to solve this problem.

Summary of the Invention

According to one aspect, to wholly or at least partially solve at least one of the prior art problems, an embossing roller is provided with a rotation axis and a substantially cylindrical outer surface provided with a plurality of embossing protuberances, each of which has a base, a side surface and a head surface, wherein the bases and the head surfaces of the embossing protuberances are delimited by closed lines and wherein, in plan view, the head surface is smaller than the base surface and is contained in this latter. In some embodiments each protuberance has a longer dimension and a shorter dimension, whose ratio is not greater than 20. The longer and shorter dimensions can be alternatively referred to the base, or to the head surface, or to both. In fact, as it will be clearly apparent below with reference to some embodiments, the protuberances may have a round base and an elongated head surface, or vice versa. Alternatively, both the base and the head may be elongated, although in a different manner. For instance, if the base is round shaped, the head surface is elongated and has therefore a longer and a shorter dimension. The head surface may be for instance a rectangle, and in this case it has a longer side (longer dimension) and a shorter side (shorter dimension). In other embodiments, the head surface may be elliptical, and in this case the longer dimension is that of the major axis of the ellipse and the shorter dimension is that of the minor axis of the ellipse. The same can occur when the base has an elongated shape. In general, when the base and/or the head surface have an elongated shape, a longer dimension and a shorter dimension can be identified.
To obtain a more uniform stress on the cellulosic web material embossed by the embossing roller, and therefore a more uniform strength of the embossed product, the side surface of at least some of the embossing protuberances has a variable inclination along the perimeter extension of the protuberance.
The side surface inclination can be defined at each point of the closed line delimiting, i.e. surrounding, the base of the embossing protuberance, hereunder also referred to as base line. The inclination of the surface of the protuberance side can be defined at any point of the base line as follows. Consider the tangent to the base line at the considered point. Then, consider the geometrical plane orthogonal to the tangent and passing through the considered point of the base line. This plane intersects the protuberance side surface along a line passing through the considered point. If this intersection line is a rectilinear segment, the angle formed by it with the protuberance base defines the inclination of the side surface of the protuberance at the considered point. If in the considered area the side surface is concave or convex, the intersection line will be then a curved line, with an outwardly or inwardly facing concavity. In this case an average inclination can be however identified, calculated for instance by the average of the angles formed by the protuberance base and the tangent lines of this curved line. Alternatively, consider the point on the base line and the intersection point of the considered plane with the edge line of the protuberance head surface; the inclination of the side surface at the considered point can be defined as the angle formed between the protuberance base and the line passing through said two points. The protuberance base is on a cylindrical surface of the embossing roller. However, the base dimension is significantly smaller than the diameter of the cross section of the embossing roller, and therefore the protuberance base may be assimilated to, or approximated to, a flat surface, with respect to which the inclination of the above mentioned line can be defined. The inclination of the side surface of at least some protuberances changes in a continuous or substantially continuous manner, i.e. gradually. Gradual, continuous or substantially continuous means a change without discontinuity and therefore wherein the side surface of the protuberance has no sharp corners. In this way a more delicate processing of the embossed ply is obtained, without stress concentration and with a smoother surface, more pleasant to the touch.
In some embodiments the side surfaces with variable inclination have a substantially conical, non-planar shape.
In advantageous embodiments, at least one of the closed lines, which delimit the base and the head surface of the protuberances having side surface with variable inclination, is round. In some embodiments one of the two closed lines delimiting the base and head of the protuberance has an elliptical or ovoid shape. In general, the head surface is preferably delimited by a curve line, without sharp corners, so as to obtain a smoother and softer embossed product, thus avoiding also stress concentration and tear or perforation risks.
In some embodiments, the closed line delimiting the base is round and the closed line surrounding the head surface is elliptical or ovoid, with the major axis shorter than the diameter of the round base line. In other embodiments the closed line delimiting the base is elliptical or ovoid and the closed line surrounding the head surface is round, and the minor axis of the line delimiting the base is longer than the diameter of the round line surrounding the head surface.
The closed line surrounding the protuberance head surface may be defined by an edge formed between a substantially planar head surface (or, more precisely, a cylindrical head surface, with radius corresponding to the embossing roller radius) and the protuberance side surface. In other embodiments these two surfaces (head and side) may be joined by a rounded surface portion, i.e. by a rounded edge.
Using such a designed embossing roller, the same features characterize the embossed projections generated on the web material. The structural and morphological features described above with reference to the roller protuberances are therefore intended herein also with reference to the embossed projections of the embossed material.
In general, not all the embossing protuberances of the embossing roller shall have a side surface with a variable inclination. On the contrary, it is possible that only some of these embossing protuberances have a variable angle, the others having a fixed angle. For instance, some embossing protuberances may be shaped like a truncated cone with a round cross-section, or like a pyramid with a square base, and in this case the side surface inclination is constant along the whole perimeter of the embossing protuberances. In general, at least one part, preferably more than 50%, more preferably more than 70% of the embossing protuberances of the embossing roller have a side surface with variable inclination.
The change in the inclination of the side surface of the embossing protuberance determines the way in which the embossed web material is deformed. The greater the inclination with respect to the base surface of the embossing protuberance, the greater the embossing stress. In fact, embossing causes a localized permanent deformation of the cellulosic material forming the web material. This deformation results in an elongation due to the penetration of the embossing protuberances in an elastically yielding coating of a pressure roller defining, together with the embossing roller, an embossing nip, through which the web material to be embossed is made pass. Therefore, in correspondence of the embossing protuberance the cellulosic material is elongated by an entity depending upon the penetration entity of the embossing protuberance into the elastically yielding coating of the pressure roller. The greater the inclination of the embossing protuberance side, i.e. of the side surface of the embossing protuberance, the more concentrated the elongation deformation and therefore the tensile stress on the web material. On the contrary, the lower the inclination, i.e. the smaller the angle formed at a given point by the side surface and the base of the embossing protuberance, the greater the length of web material on which the permanent deformation is distributed, and the lower thus the tensile stress.
It should be noted that due to the effect of the production technique, the cellulosic fibers of a cellulosic ply or of a web material have a preferred orientation in machine direction, i.e. in the direction in which the material is produced, corresponding also to the machine direction in the subsequent embossing processing. The cellulosic material has therefore a greater strength in machine direction and lower strength in cross-machine direction. Consequently, by imposing, during embossing, a greater deformation in machine direction and a lower deformation in cross-machine direction it is possible to obtain a more uniform mechanical tensile strength of the finished product. In fact, the greater embossing deformation is imposed in a direction parallel to the machine direction, i.e. in the direction in which the product is more resistant. Lower stress and lower deformation is imposed in cross-machine direction.
This results in a so-called "square" material, i.e. in which the mechanical strength features in the two directions (machine direction and cross-machine direction) are substantially comparable to one another, or anyway less dissimilar than in a traditional product. Substantially, to obtain this, at least some embossing protuberances must have a side surface with greater inclination in machine direction and lower inclination in cross-machine direction. It is not necessary that all the protuberances are shaped in this way. It is sufficient that at least some of them are designed in this way, while others have a constant inclination, or even an inclination variable in opposite way (greater in cross-machine direction and lower in machine direction). In this latter case, it is necessary that most embossing protuberances have side surface inclination lower in cross direction and greater in machine direction, while the remaining embossing protuberances have greater inclination in cross-machine direction and lower inclination in machine direction.
With reference to the single embossing roller, cross-machine direction is the direction parallel to the roller axis, whilst machine direction is orthogonal thereto, i.e. it is the direction of the tangent to the circumference of the cross section of the embossing roller.
In some embodiments, an embossing roller is provided, wherein the inclination of the side surfaces of the embossing protuberances changes so that the overall embossing stress of a ply of cellulosic material, embossed by the embossing roller, is lower in a direction parallel to the rotation axis of the embossing roller, i.e. in cross-machine direction, and greater in a direction orthogonal to the rotation axis of the embossing roller, i.e. in machine direction.
In some embodiments, at least some embossing protuberances have greater inclination in a direction parallel to the roller rotation axis and lower inclination in a direction orthogonal to the roller rotation axis.
The density of the protuberances may be for instance comprised between 10 and 200 protuberances/cm² and preferably between 15 and 180 protuberances/cm². Density may be in particular comprised between 20 and 100, and more in particular between 30 and 90 protuberances/cm². In general, density is preferably greater than 30 protuberances/cm²; above this value reference is made to micro-embossing.
In some embodiments, the bases of the embossing protuberances have a longer dimension and a shorter dimension and the ratio between longer and shorter dimension is greater than 1 and equal to or lower than 10, comprised preferably between 1.1 and 7, and more preferably between 1.5 and 5. It should be understood that the present disclosure also includes any intermediate value between those indicated above and any sub-interval comprised in the wider interval defined above.
In some embodiments the head surfaces of the embossing protuberances may have a substantially round extension. The base surfaces may have a nearly elliptical extension.
Most embossing protuberances are preferably oriented with the longer dimension of their elliptical base in a direction parallel to the embossing roller axis and with the shorter dimension in a direction orthogonal to the roller axis.
In some embodiments the protuberances may be designed differently and different series of protuberances may be provided. For instance, a first series of embossing protuberances may comprise protuberances directed with their longer dimension in a direction parallel to the embossing roller axis, and a second series of embossing protuberances may comprise protuberances directed with their shorter dimension in a direction parallel to the embossing roller axis. In this case, the number of embossing protuberances of the first series may be greater than the number of embossing protuberances of the second series. The ratio between the number of embossing protuberances of the first series and the number of embossing protuberances of the second series may be for instance greater than 1 and equal to or lower than 10, for instance comprised between 1.2 and 5, preferably between 1.5 and 4, and more preferably between 1.8 and 2.5, being understood that the present disclosure also includes any intermediate value and any sub-interval comprised in the wider interval defined above.
In some embodiments, the arrangement of the protuberances of the above mentioned two series may be such that each embossing protuberance of the second series is surrounded by a plurality of embossing protuberances of the first series, thus forming a cluster of protuberances. Cluster of protuberance means, in general, a set of protuberances arranged according to a repeated geometric pattern. Protuberances are therefore arranged on a surface of the embossing roller according to a pattern given by the repetition of a base pattern, which is in turn defined by a set or cluster of protuberances arranged according to the above mentioned geometric arrangement. In some embodiments, each cluster of protuberances comprises six embossing protuberances of the first series surrounding one corresponding protuberance of the second series. Some protuberances of the first series may be shared with adjacent clusters.
In some embodiments, to reduce vibrations, stresses and/or wear, the clusters of protuberances are aligned according to helical lines extending on the cylindrical surface of the embossing roller. The helical lines may have an inclination comprised for instance between 1° and 10°, preferably between 2° and 8° and more preferably between 3° and 7°, for instance about 5°, with respect to the embossing roller axis. It should be understood that the present disclosure also includes any intermediate value and any sub-interval comprised in the wider interval defined above.
In some embodiments the bases of the embossing protuberances, or at least some of them, may have a substantially elliptical extension. Elliptical bases may be combined with round head surfaces, or with elliptical front surfaces, provided that dimension and shape of the base and front surfaces are such that the inclination of the side surface of the protuberance is variable along the protuberance extension.
In some embodiments, the front surfaces of the embossing protuberances have a longer dimension and a shorter dimension and the ratio between longer and shorter dimension is greater than 1 and equal to or lower than 10, preferably comprised between 1.1 and 7, and more preferably comprised between 1.5 and 5, being understood that the present disclosure also includes any intermediate value and any sub-interval comprised in the wider interval defined above. The front surfaces may be for instance elliptical. At least some embossing protuberances may have an approximately round base.
In this case, at least most embossing protuberances are directed with their shorter dimension substantially parallel to the embossing roller axis. In some embodiments, all embossing protuberances are directed with the longer dimension of the front surface in a substantially parallel direction to the embossing roller axis.
In some embodiments, the embossing roller comprises a first series of embossing protuberances arranged with the longer dimension of the front surface oriented in a direction parallel to the embossing roller axis and a second series of said embossing protuberances oriented with the shorter dimension of the front surface parallel to the embossing roller axis. The number of embossing protuberances of the first series is preferably greater than the number of embossing protuberances of the second series. The ratio between the number of embossing protuberances of the first series and the number of embossing protuberances of the second series may be for instance greater than 1 and equal to or lower than 10, for instance comprised between 1.2 and 5, preferably between 1.5 and 4 and more preferably between 1.8 and 2.5. It should be understood that the present disclosure also includes any intermediate value and any sub-interval comprised in the wider interval defined above.
In some embodiments each embossing protuberance of the second series is surrounded by a plurality of embossing protuberances of the first series, thus forming a cluster of protuberances. Each cluster of protuberances comprises for instance six embossing protuberances of the first series surrounding one protuberance of the second series. Adjacent clusters may share some protuberances of the first series. Clusters may be aligned according to helical lines extending on the cylindrical surface of the embossing roller. Inclination of these helical line or lines of cluster alignment may be comprised between 1° and 10°, preferably between 2° and 8° and more preferably between 3° and 7° with respect to the embossing roller axis. Inclination may be for instance nearly 5°. It should be understood that the present disclosure also includes any intermediate value and any sub-interval comprised in the wider interval defined above.
The embossing roller may also have additional embossing protuberances, differently shaped than said embossing protuberances. For instance, additional embossing protuberances may be provided with a decorative shape, or with simple conic or truncated pyramid shape, with a constant, or substantially constant, inclination of the side surface with respect to the radial direction.
According to a further aspect, the present invention also relates to an embossing unit comprising at least one first embossing roller and one first pressure roller defining therebetween a first embossing nip and at least one first path for a first web material, extending across said embossing nip, characterized in that the first embossing roller is an embossing roller as described above.
In some embodiments, the embossing unit may comprise a second embossing roller and a second pressure roller, defining therebetween a second embossing nip and at least one second path for a second web material, extending across the second embossing nip, said second embossing roller being an embossing roller as described above. The embossing unit may be a tip-to-tip embossing unit. In this case, the first embossing roller and the second embossing roller form a lamination nip, through which the paths of the web materials pass. The distance between the two embossing rollers is such to cause lamination of the two or more web materials in the lamination nip. In other embodiments, the embossing rollers may be arranged so as to obtain a nested embossing, and in this case a laminating roller may be provided downstream of the nip between the two embossing roller, cooperating with one of the two embossing rollers to emboss the web materials. In both cases at least one glue dispensing unit may be provided to apply glue on embossing projections generated by one or the other embossing roller before laminating the web materials.
In general, a non-engraved pressure roller, i.e. with a substantially smooth surface, is associated with the embossing roller.
The pressure roller associated with the embossing roller advantageously has a coating of elastically yielding material. When the embossing unit is working, the pressure roller and the embossing roller are arranged with such a center-to-center distance that the embossing protuberances of the embossing roller penetrate inside the elastically yielding layer of the pressure roller, elastically deforming it. The web of cellulosic material is deformed due to the effect of the penetration of the embossing protuberances inside the elastically yielding coating of the pressure roller.
Usually one pressure roller for each embossing roller is provided. It is however also possible to use the same pressure roller to cooperate with two or more embossing rollers arranged for instance in sequence along the circumference extension of the pressure roller.
It is also possible to use more pressure rollers cooperating with the same embossing roller.
The invention also relates to a cellulosic web material, comprising at least one ply of embossed cellulosic material with embossed projections, each embossed projection having a base, a side surface and a head surface, said base and said head surface being delimited by closed lines. According to the invention, the side surface of at least some embossed projections has a variable inclination along the perimeter extension of the projection. The web material may also comprise further embossed projections, differently shaped than those with variable inclination, for instance projections having side surfaces with constant inclination.
In some embodiments, the web material is processed so that the inclination of the side surface of the projections is variable so that the embossing entity of the web material in machine direction is greater than the embossing entity of the web material in cross-machine direction. When referring to the product, machine direction is the direction according to which the web material is fed in the production or processing machines. In general, machine direction can be identified by the direction of most cellulosic fibers, as the latter are mainly or predominantly directed in machine direction. When the web material has an indefinite length, i.e. for instance a product in roll, machine direction is also the direction of longitudinal extension of the product, i.e. the length direction. Cross-machine direction is oriented at 90° with respect to machine direction.
Embossing entity means the permanent deformation to which the product or web material is subject due to embossing. Embossing entity is therefore proportional to, or anyway connected with the percent elongation of the material, i.e. the elongation per unit of length.
The web material may also have more than one ply, for instance two plies. The two or more plies may be bonded by means of a tip-to-tip or a nested bonding or the like. In general, the web material may have embossed projections with shapes corresponding to that of the embossing protuberances of the embossing roller that have generated them. The features of shape, density, arrangement, direction, etcetera, of the protuberances defined with reference to the embossing roller according to the invention may be therefore intended also as features of the embossed projections of the web material.
According to a further aspect, a method is provided for uniforming the mechanical strength of an embossed cellulosic web material, for instance tissue paper, having a cellulosic fiber distribution preferentially directed in machine direction. The method comprises the following steps:

arranging at least one first embossing roller and one first pressure roller, provided with an elastically yielding coating layer, defining therebetween a first embossing nip and at least one first path for a ply of web material, said path extending across the first embossing nip;
providing, on a cylindrical side surface of the embossing roller, a plurality of embossing protuberances, at least some of which have variable inclination along the perimeter extension of the embossing protuberance, at least some of said embossing protuberances having greater inclination in a direction orthogonal to the rotation axis of the roller and lower inclination in a direction parallel to the rotation axis of the roller;
embossing the web material ply in the embossing nip by means of the embossing protuberances, the variable inclination of the side surface of said at least some embossing protuberances generating an overall embossing stress of the ply of embossed cellulosic material lower in a direction parallel to the rotation axis of the embossing roller (cross-machine direction) and greater in a direction orthogonal to the rotation axis of the embossing roller (machine direction).

Substantially, according to the method described herein the mechanical strength of a ply of cellulosic material, which is formed with the fibers preferentially oriented in machine direction, is uniformed by imposing an embossing deformation through a unit with embossing roller provided with protuberances and smooth pressure roller (i.e. without protuberances) having an elastically yielding coating. The mechanical strength is made more uniform by stressing the cellulosic material more in machine direction and less in cross-machine direction. This is possible by making the protuberances penetrate in the elastically yielding coating of the pressure roller and providing that the sides of at least some protuberances of the embossing roller have variable inclination. The change in inclination is such that at least some embossing protuberances have a steeper side surface in machine direction than in cross-machine direction. When the embossing protuberance penetrates in the elastically yielding thickness of the pressure roller, greater inclination in machine direction causes, along the side surface of the protuberance of the embossing roller, a deformation of the cellulosic ply, which is greater than the deformation of the ply along the cross-machine direction, on the less steep side.
When the protuberance with variable inclination penetrates in the elastically yielding layer, the ply of cellulosic material is subject to a deformation due to traction along the side surface of the protuberance. As it is pressed between the smooth elastically yielding surface of the pressure roller and the head surfaces of the protuberances of the embossing roller, the ply of cellulosic web material is deformed locally, with traction along the sides of the protuberances of the embossing roller. Where the side surface of the protuberance is steeper, deformation is more concentrated and therefore a greater stress is applied on the cellulosic ply. This occurs in machine direction. In cross-machine direction, where the inclination of the protuberance side is lower, deformation is distributed on a greater length and therefore the deformation percentage (and therefore the mechanical stress) is lower.
The final result that can be thus achieved in a surprisingly simple manner is a more uniform mechanical strength of the embossed ply.
With respect to the embossing systems using groups of three male and female embossing rollers, a greater isotropy of the mechanical features of the ply is achieved in a very simpler manner. This is due both to the lower number of rollers used, as well as to the need of having only one engraved roller for each individually embossed ply, instead of three engraved rollers that must cooperate all together to emboss a single ply.
Using an embossing roller cooperating with a smooth (non-engraved) pressure roller with an elastically yielding coating, it is furthermore possible to obtain micro-embossing, i.e. embossing for instance with a density of 30 protuberances/cm² or greater. Micro-embossing, i.e. such high densities of protuberances per surface unit, cannot be obtained in embossing units using male and female embossing rollers.
Furthermore, the devices and methods described herein allow embossing that makes the embossed web material more uniform also by using standard embossing units, for instance tip-to-tip, simply using an embossing roller engraved with an embossing pattern as described above. This allows a tissue paper manufacturer, already having one or more traditional embossing units, to improve production by simply replacing the embossing rollers whilst maintaining the other features of the embossing unit unchanged. This does not occur in other known systems, that attempt to obtain greater uniformity of the mechanical strength by degrading the web material mainly along the machine direction and preserving it in cross-machine direction, and that require special embossing units, wherein nature and structure of the embossing and pressure rollers as well as their number and arrangement are specifically studied to obtain this result. These embossing units are more expensive than a traditional embossing unit.

Brief description of the drawings

The present invention will be better understood by means of the description below and the attached drawing, which shows a non-limiting embodiment of the invention. More in particular, in the drawing:

figure 1 is a side view if a first embossing unit in which the invention may be embodied;
figure 1A is an enlargement of the detail 1A of figure 1;
figure 2 is a side view if another embossing unit in which the invention may be embodied;
figure 2A is an enlargement of the detail 2A of figure 2;
figure 3 schematically shows a plane development of a portion of an embossing roller usable in one of the embossing units described above;
figures 3A and 3B show sections according to IIIA and IIIB of figure 3;
figure 3C shows a schematic enlargement of a protuberance and the geometric elements thereof defining the inclination of the side surface of the protuberance at a generic point of the base line of the protuberance;
figure 4 shows a plan development of an embossing roller usable in the embossing units illustrated above in a plane development;
figures 4A, 4B, 4C and 4D show sections according to IVA-IVA, IVB-IVB, IVC-IVC and IVD-IVD of figure 4;
figure 5 shows a plane development of a portion of an embossing roller in a further embodiment;
figure 6 shows a schematic enlarged cross-section of a two-ply web material obtained by means of an embossing unit as in figure 1 and a series of protuberances as in figure 4;
figure 7 is a section according to VII-VII of figure 6;
figure 8 shows an enlarged schematic section of a web material embossed through an embossing unit as in figure 1 with an embossing roller having embossing protuberances as in figure 3; and
figure 9 shows a local section according to IX-IX of figure 8.

Detailed description of an embodiment

Figure 1 schematically shows an embossing unit in which the invention may be embodied. In the illustrated example the embossing unit is a tip-to-tip unit, i.e. the embossing rollers are arranged so as to laminate the plies forming the web material in the nip between the same embossing rollers, in which at least some protuberances of an embossing roller are pressed against corresponding protuberances of the other embossing roller.
These embossing units are known in se; an example of this kind of embossing unit is disclosed for instance in US 6032712 . Herein only the essential elements of the embossing unit will be mentioned. More in particular, the embossing unit of figure 1, indicated as a whole with number 1, comprises a first embossing roller 3 cooperating with a first pressure roller 7. The pressure roller 7 has an elastically yielding coating, for instance made of rubber, indicated with 7B.
A second embossing roller 5 cooperates with the first embossing roller 3. A lamination nip 6 is defined between the embossing rollers 3 and 5. The second embossing roller 5 cooperates with a second pressure roller 9, whose cylindrical outer surface is coated with an elastically yielding material indicated with 9B, for instance rubber. The two embossing rollers 3 and 5 have embossing protuberances schematically indicated with P3 in the enlargement of figure 1A. The shape of these protuberances may vary; some examples will be described below with reference to figures 3 and the following.
With the first embossing roller 3 co-acts a gluing unit 11 that applies glue on the embossed projections formed by the embossing protuberances P3 on a first ply V1 fed in the embossing nip formed between the embossing roller 3 and the pressure roller 7 and across the lamination nip 6. A second ply V2 is fed around the pressure roller 9, in the embossing nip defined between this latter and the embossing roller 5, and is then glued to the ply V1 in the lamination nip 6.
At the exit of the lamination nip 6 a web material is obtained, schematically indicated with N.
Each ply V1 and V2 may be in turn composed of more layers. In some embodiments, not shown, between the two plies V1 and V2 also a third ply may be fed, which can be smooth or pre-embossed in a separate embossing unit.
Figure 2 schematically shows a nested embossing unit. These embossing units are known in se. An example of nested embossing unit is disclosed in US patent No. 3867225 .
Schematically, the embossing unit 21 of figure 2 comprises a first embossing roller 23 cooperating with a first pressure roller 25 provided with a side surface coated with an elastically yielding material 25B, for instance rubber. A glue dispenser 27 cooperates with the first embossing roller 23. The embossing unit 21 comprises a second embossing roller 29 cooperating with a second pressure roller 31 coated with an elastically yielding material 31B.
A first ply V1 is fed around the first pressure roller 25, and is embossed in the embossing nip formed between the pressure roller 25 and the first embossing roller 23. The glue dispenser 27 applies glue on the embossed projections generated on the first ply V1; in this way the embossed ply V1 is glued to the ply V2 by lamination in a lamination nip 36 formed between the first embossing roller 23 and a laminating roller 33. The second ply V2 is fed around the second pressure roller 31 and embossed in the embossing nip formed between the second pressure roller 31 and the second embossing roller 29.
The embossing rollers 23 and 29 are provided with protuberances schematically indicated with P23 in figure 2A, showing an enlargement of the area IIA of figure 2. The protuberance shape will be described in greater detail below with reference to some embodiments. The arrangement of the embossed projections formed on the plies V1 and V2 is such that the projections formed on the ply V1 are inserted, i.e. nested, in the spaces formed among the embossed projections of the ply V2 or vice versa, as known to those skilled in the art.
A possible embodiment of the embossing protuberances of one or the other embossing roller described above will be illustrated in detail below with reference to figures 3, 3A and 3B.
Figure 3 shows a plane development of a portion of the cylindrical surface of one embossing roller of the embossing unit 1, 5, 23, or 29. Figure 3 substantially shows part of the embossing roller surface, as it would be if the cylindrical surface of the roller would lie on a planar surface; the embossing protuberances indicated generically with P3 are therefore represented in a front view, i.e. orthogonal to the head surface of the protuberances.
In this embodiment each protuberance P3 of the embossing roller has a base delimited by a substantially round base line 41. D indicates the diameter of the base of the protuberance P3.
Each protuberance P3 has a head surface indicated with 43. In this embodiment the head surface 43 has a substantially elliptical shape with a longer dimension d1 and a shorter dimension d2. The protuberances are directed so that the longer dimension d1 of the head surface 43 is substantially parallel to the machine direction indicated with MD in figure 3, while the shorter dimension d2 is substantially parallel to the cross-machine direction indicated by the arrow CD in figure 3. As known, the cross-machine direction CD is substantially parallel to the roller axis, while the machine direction MD is orthogonal to the roller axis and therefore parallel to the feed direction of the ply material processed by the embossing roller. The machine direction MD corresponds to the longitudinal extension of the web material and therefore of the ply V1 or of the ply V2. The machine direction MD corresponds therefore also to the preferential direction of the cellulosic fibers forming the plies V1 and V2, due to the fact that this is also the direction according to which the pulp of cellulosic fibers is fed in the paper machine.
As mentioned in the introduction, the cellulosic ply has greater mechanical strength in machine direction MD and lower in cross-machine direction CD as a result of the preferential direction of the cellulosic fibers according to the machine direction MD with respect to the cross-machine direction CD.
Due to the fact that the head surface 43 of each protuberance P3 has an elliptical shape with a longer dimension d1 and a shorter dimension d2, while the base surface delimited by the base line 41 has a round shape with a constant dimension (diameter D), each protuberance P3 has a side surface 45 whose inclination is variable along the side surface around the central axis of the protuberance. This can be easily understood by observing figures 3A and 3B, showing two cross sections according to orthogonal planes IIIA-IIIA and IIIB -IIIB of figure 3 respectively.
Practically, the section plane IIIA-IIIA is orthogonal to the axis of the embossing roller and therefore to the cross-machine direction CD, while the section plane IIIB-IIIB is a radial plane, i.e. containing the roller axis.
α1 and α2 indicate the angles representing the side surface inclination, i.e. the side of the protuberance P3 in section respectively along a plane parallel to machine direction MD (figure 3A) and along a direction parallel to the cross-machine direction CD (figure 3B). Practically, these angles are formed by the line obtained by the intersection between the section plane and the side surface of the protuberance with respect to the protuberance base surface, which can be assimilated to a flat surface, due to the protuberance small dimensions relative to the embossing roller radius.
The angle α1 is greater, i.e. the inclination of the side surface 45 of the protuberance P3 is greater in machine direction, while the angle α2 is lower, in cross-machine direction CD.
Due to this different inclination, the portion of cellulosic material embossed by the single protuberance P3 is subject to greater deformation percentage in machine direction MD and lower in cross-machine direction CD. This is due to the fact that the side is steeper in machine direction than in cross-machine direction. Consequently, as the elongation of the cellulosic ply forming the web material is, in absolute terms, equal in both the directions and is practically defined by the degree of penetration of said protuberance inside the elastically yielding material coating the corresponding pressure roller, this deformation is distributed on a greater length in cross-machine direction and on a smaller length in machine direction so that practically the elongation percentage will be greater in machine direction and lower in cross-machine direction.
Substantially, the embossed cellulosic web material or ply will be consequently subject to greater stress and % elongation in machine direction, i.e. the direction in which it is more resistant, than in cross-machine direction CD, i.e. the direction in which it is less resistant. As a result, a cellulosic material is obtained with more uniform overall strength features in the various directions. The different embossing degree, i.e. the different deformation percentage, compensates the different strength of the cellulosic ply in the two directions CD and MD.
Figure 3 schematically shows an enlargement of a generic protuberance P3 and some geometric elements, upon which it is possible generally to define, in general terms, the inclination of the side surface 45 of the protuberance P3 at a generic point 1P of a base line 41 surrounding the base of the protuberance P3. At the generic point 1P along the base line 41 delimiting the base of the protuberance P3 the line "t" is drawn, tangent to the base line 41. Then the plane Pp is drawn passing through the point 1P and orthogonal to the tangent t. This plane Pp intersects the side surface 45 of the protuberance P3 along an intersection line "L". In most cases this line L is a straight line. Inclination of the side surface 45 of the protuberance P3 at the point 1P is given by the angle α formed between the line L and the base surface surrounded by the base line 41 that can be approximated to a plane. In the more general case, wherein the intersection line L of the plane Pp with the side surface 45 is not a straight line or cannot be approximated to a straight line, it will be possible to define on this line a point 2P on the edge line of the front or head surface 43. Consider now the line or line segment passing through the points 1P and 2P. This line forms with the protuberance base surface the angle α defining the (average) inclination of the side surface 45 at the point 1P.
In figures 3A and 3B, H indicates the protuberance height. The height H is the distance between the protuberance head surface and base. As the dimension of the radius of the embossing roller is significantly greater (of some orders) than the dimension (height and width) of the protuberances, the head surface 43 and the base surface are very small relative to the roller diameter (both on the head and on the base of the protuberances); these surfaces can be therefore approximated to planar surfaces. The height H is then the distance between the two flat surfaces. Practically, the closed line delimiting the base of a single protuberance is on a cylindrical surface that can be approximated, for the above mentioned reasons, to a planar surface. What illustrated above applies both to the protuberances of figures 3A to 3C and to protuberances with different shapes, for instance those described below.
Figure 4 shows, similarly to figure 3, a plane development of a portion of an embossing roller in a different embodiment.
In this case the protuberances P3 having side surface with a variable inclination are combined with a second series of protuberances P3X shaped like a truncated cone and with round base, that have therefore a constant inclination of the side surface along all the protuberance extension. Preferably, as shown in figure 4, the protuberances P3 are significantly more than the protuberances P3X. In particular, each protuberance P3X is surrounded by six protuberances P3. In other embodiments the protuberances P3X may be omitted and, if necessary, replaced with protuberances P3.
A similar configuration, with truncated cone shaped protuberance P3X may be also provided in combination with protuberances P3 designed as in figure 3.
The change of the inclination of the sides of the protuberance P3 of figures 4, 4A and 4B is obtained with a geometry different from that represented in figures 3, 3A and 3B.
More in particular, the protuberances P3 of figure 4, 4A, 4B have a base surface delimited by an elliptical curve 51. The front surface of the protuberances P3, indicated with 53, has a round extension. D1 indicates the diameter of the front surfaces 53, while D2 and D3 indicate respectively the longer and shorter dimension of the elliptical bases of the protuberance P3, delimited by the curve 51.
As shown in the two sections of figures 4A and 4B, also in this way an inclination is obtained greater according to the angle α1 and lower according to the angle α2 of the side surface 55 of the protuberances P3 in this embodiment. The protuberances are directed so that the longer dimension D2 of the base of the protuberances P3 is substantially parallel to the cross-machine direction CD, while the shorter dimension D3 is parallel to the machine direction MD due to the same reasons described in detail with reference to figures 3, 3A and 3B.
Therefore, in this embodiment again, through the protuberances P3 with variable inclination a deformation percentage, i.e. a % elongation, through embossing of the cellulosic ply is obtained, greater in machine direction and lower in cross-machine direction.
As mentioned, the protuberances P3X have, vice versa, sides with constant inclination, as schematically represented in the two sections of figures 4C and 4D obtained according to orthogonal planes, parallel to the cros-machine direction and the machine direction respectively.
Figure 5 shows, in a plane representation similar to that of figures 3 and 4, a further embodiment of an embossing roller according to the invention. MD and CD indicate the machine direction and the cross-machine direction again.
In this embodiment the embossing roller has two series of protuberances indicated with P3 and P3Y, with different shape and orientation. The protuberances P3 are designed like the protuberances P3 of figure 4 and of figures 4A and 4B. The protuberances P3Y are similar to the protuberances P3, but directed differently with the longer dimension of the elliptical base parallel to the machine direction and the shorter dimension parallel to the cross-machine direction. Moreover, in the embodiment of figure 5, as far as the protuberances P3Y are concerned the difference between the diameter dimension of the head surface and the two maximum and minimum dimensions of the base is lower than that of the protuberances P3. In other embodiments the protuberances P3Y may differ from the protuberance P3 only in the direction, having identical geometric shape to the protuberances P3.
It is important to note that the number of protuberances P3Y is substantially lower than the number of protuberances P3, so as to maintain the greater deformation percentage in machine direction than in cross-machine direction. In the illustrated embodiment, each protuberance P3Y is surrounded by six protuberances P3, arranged with the centers of the round head surfaces according to the vertices of a hexagon that, in this example, is slightly irregular due to the effect of the elliptical shape of the bases of the protuberances P3. These protuberances P3 surrounding a protuberance P3Y form a cluster of protuberances.
As shown in figure 5, the clusters of protuberances are arranged according to alignments, which are slightly inclined with respect to machine direction MD and cross-machine direction CD. More in particular, the alignment directions of the clusters, indicated with L1 and L2 in figure 5, form angles of about 5° with respect to the cross-machine direction CD and the machine direction MD respectively. In this way a more uniform wear of the pressure rollers and fewer vibrations in the machine are obtained.
Figures 6 to 9 schematically show cross-sections of portions of web material that can be obtained with an embossing unit designed as in figure 1 and with embossing protuberances designed as described above. More in particular, figure 6 shows a portion of a web material N formed by embossed plies V1 and V2 with embossed projections S1 and S2 glued in correspondence of the front surfaces, facing towards the inside of the web material and indicated with SF. Figure 7 shows a cross-section according to VII-VII of figure 6, where the embossed projections S1, S2 on the plies V1 and V2 have corresponding shape to that of the embossing protuberances of figures 4A and 4B. The cross-sectional planes of figures 6 and 7 are directed according to the machine direction and the cross-machine direction respectively.
The dimension of the head surface is equal in the two orthogonal directions of the sections of figures 6 and 7, as in this case they are round surfaces, while the base surfaces of the projections S1 and S2 have variable dimensions between a maximum (in the section of figure 7, according to cross-machine direction) and a minimum (section of figure 6, according to machine direction).
Figures 8 and 9 show, vice versa, local sections according to orthogonal planes of a web material N formed again by plies V1 and V2 with embossed projections S1 and S2, whose shape corresponds to that of the embossing protuberances of figures 3, 3A and 3B, with elliptically shaped front surfaces SF and round shaped base surfaces. In both cases the inclination of the embossed projections S1, S2 changes in machine direction (section of figure 8) and cross-machine direction respectively (section of figure 9).
In general, the shapes of the embossed projections of the web material obtained with an embossing roller or an embossing unit according to the invention will correspond to the shapes of the protuberances of the embossing roller or rollers of the embossing unit. The height of the embossed projections is usually different than the height of the embossing protuberances and depends upon the hardness of the elastically yielding material coating the pressure rollers and upon the linear pressure with which the pressure rollers press against the embossing rollers.
The features of the embossing protuberances are therefore the same of the embossed projections formed on the web material plies.
A cross-machine direction and a machine direction can be defined on the finished product, which will be usually wound in the form of a roll. When the product is wound in a roll, machine direction is that of the longitudinal development of the web material, and cross-machine direction is the one parallel to the roll winding axis. Also when the web material is cut in sheets to form for instance folded products such as napkins, handkerchiefs or the like, on the finished product a machine direction can be detected as the direction of preferential orientation of the cellulosic fibers forming the plies forming the web material, while the cross-machine direction is that orthogonal to the machine direction.
The following table shows comparison experimental values obtained by embossing paper material through a traditional embossing roller and embossing rollers according to what described herein. Data refer to the processing of a product subsequently wound in a roll and are useful to compare the parameters of mechanical strength in machine direction MD and cross-machine direction CD in the various cases, as well as to verify the achievement of given apparent thickness and winding volumes. In the left column some useful parameters are shown of the used products. In the following columns the values of the parameters are shown for the various types of materials. More precisely, the four tested materials are the following:

B/S: non-embossed ply of virgin paper
1(6386): traditional embossing pattern, with conical protuberances, density 44.86 protuberances/cm², embossing pressure 2.8 bar. Embossing pressure is the air pressure of the cylinder-piston actuators used to press the pressure roller and the embossing roller against each other. This parameter is significant in comparing and evaluating the various examples.
11(6984): embossing pattern as in figure 5 with 45.32 protuberances/cm², embossing pressure and winding length as above;
111(6984): embossing pattern as in figure 5 with 45.32 protuberances/cm², embossing pressure 3.5 bar and winding length 40m;

	B/S:	I(6386)	II(6984)	III(6984)
Weight (g/m2)	16,1	14,9	14,7	14,8
Thickness (mm for 12 plies)		2,85	1,87	2,45
% elongation direction MD	15,7	10,9	10,4	9,9
Strength (N/m) in MD direction - two plies	250	203	220	206
% elongation direction CD	4,5	7,8	6,8	7,4
Strength (N/m) in CD direction - two plies	128	68	80	80
Diameter of the wound roll (mm)		100	85	100
Ratio between strength in CD direction and MD direction	51,2	33,5	36,4	38,8

From the table below it is apparent that for virgin paper (B/S) strength in machine direction (MD) is nearly double than in cross-machine direction (CD). The % elongation-to-break in machine direction (MD) is equal to 15.7% while in cross-machine direction (CD) is equal to 4.5%. This confirms that virgin paper, even before embossing, has a significantly lower tensile strength in cross-machine direction than in machine direction. The second column shows how the data on strength and % elongation are strongly degraded by embossing. In fact, strength in machine direction decreases from 250 to 203 N/m and in cross-machine direction from 128 to 68 N/m. The ratio between the strength values decreases from 51.2% to 33.5%.
This means that embossing decreases the overall tensile strength and increases the anisotropic behavior of paper, i.e. the feature thereof of lower resistance in cross-machine direction than in machine direction.
The third sample shows that, by embossing with protuberances according to the description above, degradation is lower. In particular, strength in machine direction decreases only up to 220 N/m, while in cross-machine direction only to 80 N/m. The ratio of the breakage resistance values decreases from 51.2 (non-embossed paper) to 36.4 (embossed paper); this value is greater than 33.5 obtained with a traditional embossing.
The fourth sample has been obtained with the same embossing pattern but with higher embossing pressure. This was necessary to achieve a volume of the embossed material equivalent to that of the material of the second sample (standard embossing). Volume is given by the diameter of the wound roll. In the case of the second sample (standard embossing) with 40 m of wound cellulosic material a roll of 100 mm diameter has been obtained, while with the third sample only a diameter of 85 mm has been obtained. Consequently, in the fourth sample a higher embossing pressure has been used, generating higher embossed projections with a consequent stronger degradation of the tensile strength. In fact, the resistance in machine direction passes from 220 N/m to 206 N/m, while in cross-machine directions is still 80 N/m. The ratio between the breakage resistance in cross-machine direction and in machine direction improves, passing from 36.4 of the third sample to 38.8 of the fourth sample.
In conclusion, embossing through protuberances having sides with variable inclination allows therefore to obtain a cellulosic web material with breakage elongation less heterogeneous in the two directions MD and CD with respect to an embossed material with a traditional embossing pattern comparable as regards dimensions of the protuberances and therefore density and volume of the finished product (diameter of the roll with equal length of wound web material).
It is important to note also that degradation of tensile strength in cross-machine direction is degraded by embossing in all cases. However, with an embossing pattern as described above, with protuberances having side surfaces with variable inclination, this degradation is significantly less and does not increase when the embossing pressure increases beyond a given limit.
The examples above show different shapes of embossing protuberances and corresponding different embossed projections. In general it should be noted that the shape of the protuberances may be chosen according to the virgin paper features. In other terms, protuberances of different shapes may be combined at will to increase the resistance in a first direction, without however jeopardizing strength in the orthogonal direction, or for aesthetic reasons.
In fact, the tests conducted have proven that to each shape of protuberance (or of set or cluster of protuberances) corresponds a different reaction of the processed paper, i.e. different results are obtained for the tensile strengths according to the two main directions (CD and MD).
In the illustrated examples reference has been always made to protuberances with round or elliptical shaped base or head, but it is understood that other shapes may be used, for instance protuberances with quadrangular section (rhombs, rectangles, squares) or polygonal section with more than four sides, with sharp corner or differently joined, obtained through chemical etching or mechanical chip removal, provided that the protuberance sides are angled differently between adjacent sides of the same protuberance, i.e. between consecutive areas along the side surface of the same protuberance, so as to deform the paper in the manners indicated above.
It is understood that the drawing only shows an example provided by way of a practical demonstration of the present invention, which can vary in forms and arrangements without however departing from the scope of the claims. Any reference numbers in the appended claims are provided for the sole purpose of facilitating reading of the claims in the light of the description and the drawing, and do not in any manner limit the scope of protection represented by the claims.

Claims

An embossing roller (3, 5, 23, 29) with a rotation axis and a substantially cylindrical outer surface provided with a plurality of embossing protuberances (P3), each embossing protuberance having a base, a side surface (45; 55) and a head surface (43; 53), said base and said head surface being delimited and surrounded by closed lines (41; 51), the distance between base and head surface defining a height of the protuberance, wherein the side surface (45; 55) of at least some of said embossing protuberances (P3) has variable inclination with respect to the closed line delimiting the base of the embossing protuberance and along the perimeter extension of the protuberance, said inclination changing continuously along the perimeter of the embossing protuberance, so that the side surface of the protuberance has no sharp corners, whereby the overall embossing stress of a ply of cellulosic material (V1, V2), embossed by said embossing roller (3, 5, 23, 25), is lower in a direction (CD) parallel to the rotation axis of the embossing roller (3, 5, 23, 25) and greater in a direction (MD) orthogonal to the rotation axis of the embossing roller, for at least some of said embossing protuberances (P3) said inclination being greater in a direction (MD) orthogonal to the rotation axis of the roller and lower in a direction (CD) parallel to the rotation axis of the roller.
Embossing roller according to claim 1, characterized in that the base of said embossing protuberances (P3), delimited and surrounded by said closed line (41; 51), is on a cylindrical surface of the embossing roller, each embossing protuberance having constant height along its perimeter.
Embossing roller according to claim 1, or 2, characterized in that said protuberances (P3) are distributed according to a density comprised between 10 and 200 protuberances/cm².
Embossing roller according to one or more of the previous claims, characterized in that the bases of said embossing protuberances (P3) have a longer dimension and a shorter dimension, and that the ratio between the longer dimension and the shorter dimension is greater than 1 and equal to, or lower than 10, preferably comprised between 1.1 and 7, and more preferably between 1.5 and 5.
Embossing roller according to claim 4, characterized in that the head surfaces have a substantially round extension.
Embossing roller according to claim 4 or 5, characterized in that most said embossing protuberances (P3) are directed with their longer dimension in a direction (CD) parallel to the embossing roller axis.
Embossing roller according to claim 4 or 5, characterized in that a first series of said embossing protuberances (P3) are directed with their longer dimension in a direction (CD) parallel to the embossing roller axis, and a second series of said embossing protuberances (P3Y) are directed with their shorter dimension in a direction (CD) parallel to the embossing roller axis; and in that the number of embossing protuberances (P3) of the first series is greater than the number of embossing protuberances (P3Y) of the second series.
Embossing roller according to claim 7, characterized in that the ratio between the number of embossing protuberances (P3) of the first series and the number of embossing protuberances (P3Y) of the second series is comprised between 1.2 and 5, preferably between 1.5 and 4 and more preferably between 1.8 and 2.5.
Embossing roller according to claim 7 or 8, characterized in that each embossing protuberance (P3Y) of said second series is surrounded by a plurality of embossing protuberances (P3) of the first series, thus forming a cluster or set of protuberances; wherein each cluster of protuberances preferably comprises six embossing protuberances (P3) of said first series of embossing protuberances.
Embossing roller according to claim 9, characterized in that said clusters of protuberances (P3) are aligned according to helical lines extending on the embossing roller cylindrical surface, said helical lines preferably having inclination comprised between 1° and 10°, more preferably between 2° and 8° and even more preferably between 3° and 7° with respect to the embossing roller axis.
Embossing roller according to one or more of claims 4 to 10, characterized in that the bases of said embossing protuberances (P3) have a substantially elliptic shape.
Embossing roller according to one or more of claims 1 to 3, characterized in that the head surfaces of the embossing protuberances have a longer dimension and a shorter dimension, and that the ratio between the longer dimension and the shorter dimension is greater than 1 and equal to, or lower than, 10, preferably comprised between 1.1. and 7, and more preferably comprised between 1.5 and 5.
Embossing roller according to claim 12, characterized in that at least some of said embossing protuberances (P3) have an approximately round base, and wherein preferably at least most said embossing protuberances (P3) are directed with their shorter dimension substantially parallel to the axis of the embossing roller; and wherein preferably all said embossing protuberances (P3) are directed with their longer dimension substantially orthogonal to the axis of the embossing roller.
Embossing roller according to one or more of the previous claims, characterized by comprising additional embossing protuberances (P3X), differently shaped than said embossing protuberances; wherein said additional embossing protuberances preferably have a side surface with substantially constant inclination with respect to the radial direction.
An embossing unit comprising at least one first embossing roller (3; 5; 23; 29) and one first pressure roller (7; 9; 25; 31), provided with an elastically yielding coating layer (7B; 9B; 25B; 31B), defining therebetween a first embossing nip and at least one first path for a first ply (V1) of web material, extending across said embossing nip, characterized in that said first embossing roller (3; 5; 23; 29) is an embossing roller according to one or more of the previous claims.
Embossing unit according to claim 15, comprising a second embossing roller (3; 5; 23; 29) and a second pressure roller (7; 9; 25; 31), provided with an elastically yielding coating layer (7B; 9B; 25B,;31B), defining therebetween a second embossing nip and at least one second path for a second ply (VI; V2) of web material, extending across said embossing nip, said second embossing roller being an embossing roller according to one or more of claims 1 to 14.
A cellulosic web material (N) made of tissue paper, comprising at least one ply (VI; V2) of embossed cellulosic material with embossed projections (SI; S2) and having cellulosic fibers preferentially oriented in machine direction (MD), wherein each embossed projection comprises a base, a side surface and a head surface, said base and said head surface being delimited and surrounded by closed lines, wherein the side surface of at least some of said embossed projections has variable inclination with respect to the closed line delimiting the base of the embossed projection and along the perimeter extension of the projection, said inclination changing continuously along the perimeter of the embossed projection, so that an embossing entity and thus the percent elongation of the web material in said machine direction is greater than the embossing entity and thus the percent elongation of the web material in cross-machine direction.
A method for uniforming the mechanical strength of an embossed cellulosic web material (N) made of tissue paper and having a preferential cellulosic fiber distribution in machine direction (MD); said method comprising the steps of:
arranging at least one first embossing roller (3; 5; 23; 29) according to one or more of claims 1 to 14 and one first pressure roller (7; 9; 25; 31), provided with an elastically yielding coating layer (7B; 9B; 25B; 31B), defining therebetween a first embossing nip and at least one first path for a ply (VI; V2) of web material, said path extending across the first embossing nip;

providing, on a cylindrical side surface of the embossing roller (3; 5; 23; 29), a plurality of embossing protuberances (P3; P3Y), at least some of which have side surface (45; 55) with a variable inclination along the perimeter extension of the embossing protuberance, at least some of said embossing protuberances (P3; P3Y) having greater inclination in a direction orthogonal to the rotation axis of the roller and lower inclination in a direction parallel to the rotation axis of the roller;

embossing the ply (VI; V2) of web material in the embossing nip through the embossing protuberances (P3; P3Y) by making said embossing protuberances to penetrate in the elastically yielding coating layer (7B; 9B; 25B; 31B) of the pressure roller (7; 9; 25; 31), the variable inclination of the side surface (45; 55) of said at least some embossing protuberances (P3; P3Y) generating an overall embossing stress of the ply of embossed cellulosic material lower in a direction (CD) parallel to the rotation axis of the embossing roller and greater in a direction (MD) orthogonal to the rotation axis of the embossing roller.