"EMBOSSING ROLLERS WITH SURFACE TREATMENT, METHOD FOR THE PRODUCTION THEREOF AND EMBOSSING UNIT COMPRISING SAID ROLL¬ ERS"

DESCRIPTION Technical Field The present invention relates to innovations in the field of embossing roll¬ ers, in particular although not exclusively to produce embossing rollers intended for processing paper, especially tissue paper, intended for the production of toilet paper, kitchen towel, handkerchiefs and the like. Specifically, the invention relates to embossing rollers, methods for the pro¬ duction thereof and embossing units comprising said rollers. Prior Art In the processing of web or sheet materials, an embossing process is fre¬ quently provided, wherein the material is subjected to mechanical processing causing breakage of the fibers and permanent deformation of the material to ob¬ tain special effects both of an aesthetic and functional nature. In particular, in the field of converting of the so-called tissue paper, intended to produce rolls of toilet paper, rolls of kitchen towel, paper handkerchiefs and napkins or the like, emboss¬ ing is used as an operation to decorate the product and to improve its physical properties, in particular to increase volume, softness and capacity to absorb liq¬ uids. Embossing units are frequently embossing-laminating units, wherein the sheet materials are not only embossed, but also bonded to one another to obtain multi-ply products. Bonding is usually obtained by glue applied to the protrusions of the embossed product. The embossing units can have various configurations and can be more or less complex. Simpler embossing units are characterized by a cylinder made of a hard material, typically steel, with a cylindrical surface provided with protrusions or protuberances. This cylinder cooperates with a smooth cylinder coated in a yield- ing material, typically rubber or the like. The web material, composed of one or more plies, is fed through two rollers pressed against each other so that the pro¬ trusions or protuberances of the steel cylinder penetrate the yielding coating of the pressure roller and consequently cause deformation of the web material fed into the nip between the rollers. In some cases the two cylinders or rollers cooperating with each other are both made of hard material, such as steel, and mesh with each other, in the sense that the protuberances produced on one cylinder enter cavities produced , on an opposed cylinder. Also in this case the web material fed into the nip between the two cylinders is permanently deformed. However, contrary to what occurs when using rubber cylinders, in this case the two cylinders can cooperate without being pressed against each other, as deformation of the web material is obtained by in¬ terlocking, that is, by geometrical interpenetration of the protrusions in the cavities of the opposed cylinder. In more complex embossing units, more than two rollers are provided to emboss two or more plies and join them to one another by applying glue. Embossing laminating units of this type have, for example, two embossing cylinders each cooperating with a respective pressure roller coated in a yielding material, typically rubber, with a smooth surface. Two plies are embossed sepa- rately between each embossing roller and the corresponding pressure roller. In the nip between the embossing rollers two embossed plies are joined and laminated after having applied a glue to the protuberances produced on one or other or on both plies. In some cases lamination takes place downstream of the nip between the embossing rollers, pressing the two plies against each other against the protu- berances of one of the embossing cylinders by means of a laminating roller. Other embossing-laminating devices are in any case configured to perform embossing operations on several plies and lamination by means of gluing. Examples of embossing units in various configurations are described in the US patent n.6.578.617, in US patent n.6.470.945, in European patent n.1.075.387, in European patent n.370.972, in publication WO-A-99/41064, in the publication WO-A-99/44814, and in numerous other patents pertaining to the same techno¬ logical field. When processing paper, especially tissue paper, a great deal of dust is pro¬ duced, formed of fibers which detach from the base material. This dust tends to soil the embossing rollers, especially when they also operate in combination with gluing units used to apply glue for reciprocal joining of the plies forming the mate¬ rial delivered from the embossing unit. Therefore, embossing rollers require periodic cleaning operations which can also be particularly complex when the paper dust adheres to the roller as a result of glue seeping through the paper and soiling the surface of the roller to cause ad¬ hesion of the paper fibers and dust. Objects and summary of the invention The object of the present invention is to produce an embossing roller which overcomes or reduces the aforesaid drawbacks. Another object of the invention is to provide a procedure to produce an em¬ bossing roller which overcomes or reduces the aforesaid drawbacks. Yet another object of the invention is to produce an embossing unit with rollers which are less subject to becoming soiled and consequently reduce or solve the aforesaid problems relative to the need for frequent cleaning operations. In substance, according to a first aspect, the invention relates to an em¬ bossing roller comprising an essentially cylindrical surface provided with a plurality of protuberances, characterized by a non-stick surface treatment. With this treat¬ ment adhesion of any glue that may seep through the paper (or other material to be embossed), treated by the embossing roller is made more difficult and this con¬ sequently reduces the tendency of paper dust or fibers to adhere to the surface of the roller. According to a particularly advantageous aspect of the invention, the em¬ bossing roller has an anchoring layer of the non-stick treatment, combined with a non-stick material. The anchoring layer advantageously has a porous structure. In this way the non-stick material is anchored to the surface of the embossing roller filling the surface porosities of the anchoring layer. In this way, a stable non-stick surface treatment is obtained, which is consequently also resistant to long operat¬ ing times of the roller. According to an advantageous embodiment of the invention, an anchoring layer comprising or composed of a ceramic material is applied to the surface of the embossing roller provided with the embossing protuberances. This ceramic layer can be a ceramic oxide of a metal, such as chromium oxide, titanium oxide, alumi¬ num oxide, zirconium oxide or a combination of the aforesaid materials. This an- choring layer can be applied to the base material forming the thickness in which the protuberances of the embossing roller are engraved according to a suitable technique. Alternatively to metal oxide, the anchoring layer may be composed of or comprise a metal carbide, such as chromium carbide, tungsten carbide or mixtures thereof. Alternatively, the anchoring layer can be composed of a metal superalloy, based on chrome, nickel, cobalt or equivalent materials. However, it has been found that layers based on ceramic metal oxide and in particular based on aluminum oxide are particularly advantageous, in view of the high porosity of these materials, and consequently the particularly effective an¬ choring exerted thereby on the non-stick material. The non-stick material can be a fluoropolymer based resin, teflon®, a prod¬ uct based on silicon oils, or other suitable materials having a non-stick effect. According to an advantageous embodiment the anchoring layer is applied to the cylindrical surface of the roller so that the thickness thereof is greater in the cavities than on the frontal surface and on the sides of the protuberances pro¬ duced on said cylindrical surface. For example and in particular, the thickness can be greatest in the cavities and minimum on the sides. For example, the thickness of the anchoring layer can be in the range of 40-60 micrometers on the bottom of the cavities, in the range of 40-50 micrometers on the frontal, or radially outermost, surface of the protuberances of the roller. The embossing roller can be produced in different materials, and can also have a composite structure. For example, it can be composed of machining steel, nitridable stainless steels, composite materials based on resin and carbon fiber or glass fiber; plastic or rubber materials, such as ebonite, PVC, polyethylene or the like. It would also be possible to use composite rollers with a core and an inter¬ changeable outer sleeve made of steel, composite materials, plastics or rubbers as indicated above. In this case the nonrstick treatment is applied to the cylindrical surface of the interchangeable sleeve. It would also be possible to produce rollers with a core or body faced with a layer made of alloys using spray metallizing processes or the like, such as copper, nickel, zinc, steel and alloys thereof. It would also be possible to use materials ap¬ plied by submerged arc welding or other procedures. In general, the invention relates to an embossing roller with surface protu- berances produced in any way, with any base material of the roller, provided that it has suitable characteristics of mechanical strength for the use for which it is in¬ tended and is susceptible to receive said non-stick treatment. The protuberances provided on the roller and to which the non-stick treat¬ ment is applied can be obtained with various techniques, such as engraving by chip-forming, .machining, using a hob or the like. Alternatively, or in combination, engraving techniques using mechanical pressure (mechanical engraving), chemi¬ cal etching, photographic exposure or laser paint removal and subsequent chemi¬ cal removal of the base material of the surface to be engraved could also be used. Engraving with laser systems according to drawing are also possible. In general, the invention is not limited to the type of engraving technique used to produce the protuberances, as said technique can be chosen according to the nature of the material of the embossing cylinder or roller and as a function of the forms and dimensions of the protuberances to be produced. For example, pro- tuberances with a simple geometric form can be easily obtained by engraving by means of chip removal' or mechanical pressure, while protuberances of complex form, intended in particular for operations to decorate the paper material, can be more easily obtained by means of laser engraving systems or chemical etching. In general, according to a particular aspect, the invention relates to a method to produce an embossing roller wherein embossing protuberances are produced on an essentially cylindrical surface, characterized in that a non-stick treatment is applied to the essentially cylindrical surface provided with protuber¬ ances. According to a particular embodiment of the method according to the inven- tion, an anchoring layer is applied to the surface engraved and provided with pro¬ tuberances, and a non-stick material is subsequently applied to said anchoring layer. The anchoring layer can be composed of one of the aforesaid materials and have a more or less porous structure to guarantee stable anchoring of the non¬ stick material which fills, entirely or in part, the surface porosities of the anchoring layer. According to a particular embodiment of the method of the invention, the engraved surface of the roller is subjected to an activation phase to improve the grip of the anchoring layer. The activation phase can comprise a treatment to in¬ crease surface roughness, such as a sandblasting treatment. According to a particular embodiment of the method of the invention, the anchoring layer (which can also have anti-wear characteristics) is applied by means of a plasma spray procedure or by means of a combustion spray proce¬ dure. The non-stick material can advantageously be applied to the anchoring layer by means of spraying, coating or another suitable method. Further advantageous characteristics and embodiments of the roller accord¬ ing to the invention, of the embossing unit employing said roller and of the produc¬ tion method are indicated in the appended claims. Brief description of the drawings The invention shall be better understood by following the description and accompanying drawing, which shows a non-limiting practical embodiment of the finding. In the drawing: Figures 1 and 2 show possible configurations of embossing-laminating units in which the present invention can be advantageously employed; and Figures 3A-3C schematically show the succession of processes in a greatly enlarged schematic section of a protuberance of an embossing roller. Detailed, description of the invention Figures 1 and 2 schematically show two embossing-laminating units of dif- ferent configuration wherein rollers with a non-stick treatment according to the in¬ vention can be used. It must be understood that the embossing-laminating units in Figures 1 and 2 are only two examples of possible machinery to which embossing rollers treated according to the invention can be advantageously applied. With reference to Figure 1 , the embossing-laminating unit, indicated as a whole with 1 , comprises a first embossing roller 3 and a second embossing roller 5 with parallel axes forming therebetween a nip. The embossing rollers 3 and 5, which are provided on the surface thereof with protuberances obtained by means of engraving using any of the available techniques, each cooperate with a corre¬ sponding pressure roller 7 and 9. The pressure rollers 7 and 9 are coated in a yielding material, such as plastic or rubber, and have a smooth outer surface. Two plies V1 and V3 of web material, such as tissue paper, are fed into the nips between the rollers 3 and 7 and between the rollers 5 and 9 respectively. In this way the two plies V1 and V3 are embossed as a result of the pressure exerted by the embossing rollers 3 and 5 against the yielding surfaces of the rollers 7 and 9. Glue is applied to the protuberances produced on the ply V1 by the protrusions of the embossing roller 3, by means of a gluing unit 11, of known type and repre¬ sented schematically in the figure. The two plies are laminated to each other in the nip between the embossing rollers 3 and 5 by pressing them at the level of corre¬ sponding protrusions or protuberances of the rollers 3 and 5. Alternatively, the two rollers 3 and 5 can be disposed with their protuberances staggered, that is, with the protuberances of the roller 5 opposing the cavities between the protuberances of the roller 3, or distanced from one another and in any case without reciprocal contact between the points. In this case the two plies V1 and V3 are not laminated between the rollers 3 and 5, but between the roller 3 and a secondary laminating roller 13. Notwithstanding the embossing and laminating technique used, a multi-ply web material N is delivered from the embossing unit, composed of the two plies V1 and V3, embossed and glued to each other. Each ply can be in turn composed of several layers. In the embodiment in Figure 2 the embossing-laminating unit, indicated herein with 21, comprises a main embossing roller 23 which cooperates with a pressure roller 25 with a coating in a yielding material, such as rubber, and having an essentially smooth surface. Also cooperating with the embossing roller 23, pro- vided with protrusions 23P, is a laminating roller 27 with a smooth surface, rigid or yielding, in any case advantageously more rigid than the cylindrical surface of the pressure roller 25. A first ply of web material V1 is fed about the pressure roller 25 through the nip formed by this roller and by the embossing roller 23. Before passing through this nip the web material V1 passes through a secondary embossing unit, indi¬ cated as a whole with 29, and composed of an embossing cylinder or roller 31 provided with protuberances or protrusions 31 P and cooperating with a pressure roller 33 coated in rubber or another yielding material. A second ply of web material V3 is fed into the nip Between the embossing roller 23 and the laminating roller 27. In this nip the ply V3 is glued against the embossed ply V1, to which, by means of a gluing unit 30, a glue has been applied on the protrusions or protuberances produced by the protuberances 23P of the embossing roller 23. The ply V3 can also advantageously have been pre-embossed by means of an embossing roller 41 provided with protrusions 41 P and cooperating with a pressure roller 43 coated in a yielding material, such as rubber or plastic. The configurations of these embossing-laminating units illustrated by way of examples are described in greater detail in WO-A-99/41064 and WO-A-99/44814, which should be referred to for further details. The embossing rollers 3, 5, 23 and optionally also the embossing rollers 31 and 41 can be produced with a non-stick treatment according to the invention. Figures 3A-3C schematically show a greatly enlarged local section of a pro¬ tuberance of any one of these embossing rollers prior to the non-stick treatment, in an intermediate phase of the treatment (Figure 3B) and after completion of the treatment (Figure 3C). In practice, the non-stick material (Teflon® or the like) which is applied to the rough surface of the anchoring layer fills the microporosities of the anchoring layer (which also has the function of increasing the resistance to wear of the roller) and also forms a continuous coating layer or film. However, dur- ing the first hours of use of the roller the layer of non-stick material in excess is gradually removed, especially at the level of the surface portions with the most ex¬ posures, although the microporosities of the anchoring layer remain filled, provid¬ ing the roller with an adequate non-stick effect. As can be seen by comparing Figures 3A to 3C, the side surface of the em- bossing roller, generically indicated in these figures- with 3 and provided with pro¬ trusions generically indicated with 3P, after treatment (Figure 3C) is provided with an anchoring layer 51 which coats the entire surface of the protuberances and of the cavities between adjacent protuberances. More specifically, the anchoring layer 51 coats the bottom of the cavities between adjacent protuberances 3P, the sides of the protuberances 3P and the frontal surfaces of said protuberances. The thickness of the anchoring layer 51 is advantageously variable in the various sur¬ face areas of the roller. More specifically, this thickness is greater on the bottom of the cavities between protuberances 3P and lesser on the sides and on the frontal surfaces of the protuberances 3P. The anchoring layer 51 is produced, for example, in chromium oxide, which is a ceramic material with high porosity. The porosity is filled at least partially with a non-stick material 53, applied for example by spraying and subsequently fixed by baking in the oven, or by any other suitable means, to the anchoring layer 51. The non-stick material 53 also forms a coating layer on the entire surface of the an- choring layer 51. By penetrating the porosities of the latter, the non-stick material 53 remains fastened to the surface of the embossing cylinder or roller 3 and with¬ stands the action of abrasion exerted thereon by the material being embossed. The roller 3, as previously indicated, can be made of any material suitable to withstand embossing stresses. These can be extremely high, especially when . 9 the roller operates in an embossing-laminating unit of the type illustrated in Figures 1 and 2. However, it would also be possible for the stresses on the roller to be -greatly reduced, for example, when it is part of a simple embossing unit, such as a roller which is part of a pair of male and female rollers, wherein it cooperates with the opposed roller provided with cavities and wherein the two rollers mesh with each other without particular reciprocal pressure stresses. In a possible embodiment, the overall treatment cycle, with a steel emboss¬ ing cylinder or roller 3, is schematically as follows: manufacture of the steel roller by means of known technique; - any preliminary heat treatments, followed by any grinding or pre-grinding treatments; engraving and creation of the protuberances 3P with a known technique, such as chip removal using a hob, laser engraving, chemical etching, me¬ chanical engraving or with a combination of these methods; - softening of the engraving, with consequent rounding of the engraving edges, for example by brushing; elimination of processing residues, residues of oils or other treatment liquids by sandblasting processes, pickling of various type, treatment in the oven or other processes; - surface activation by means of sandblasting or other procedure to increase surface roughness; application of the ceramic layer by plasma spraying or the like; application, by spraying, painting or other methods, of the non-stick material, typically a fluoropolymer based resin (teflon®); - oven baking; any finish according to drawing. It must be understood that the processing cycle indicated above is provided purely by way of example and can vary according to the type of material used, es¬ pecially to produce the base cylinder. The embossing roller thus treated, provided with a non-stick layer anchored by means of the ceramic layer, for example based on ceramic metal oxides, dras¬ tically reduces the tendency of glue and/or paper debris to accumulate on the ac¬ tive surface of the embossing roller, eliminating or greatly reducing the need for cleaning operations during processing or between processing cycles ~of the" em- bossing or embossing-laminating device in which the roller is inserted. It is understood that the drawing merely shows an example provided purely as a practical embodiment of the invention, which may vary in forms and arrange¬ ments. without however departing from the scope of the concept on which the in- vention is based.