MXPA00006826A - Method for treating fibrous webs - Google Patents

Abstract

The present invention concerns a method for treating the surface of a fibrous web by mechanical grinding. According to the invention, the grinding is performed at substantially dry state by removing only the higher parts of the paper surface without substantially increasing the density of the web. By reducing the roughness of the surface by a maximum of 90%, the strength properties of the web remain essentially unchanged or they are even improved. Thus, when the roughness of the surface is reduced by about 40 to 60%, the tear strength increases with more than 5%in comparison to an untreated web. Papers and boards treated by the present method can be used for printing, packaging and wrapping.

Description

IETOD TO TREAT FIBROUS PAPER ROLLS The present invention relates to the finishing of fibrous paper rolls. Particularly, the invention relates to a method according to the preamble of claim 1 to increase the smoothness of the paper and the cardboard rolls through a mechanical treatment. Normally paper is made by the wet method. According to that method the fibers are suspended in water to form a fibrous material and a wet roll is formed therefrom in a wire cloth. Then, the roll is dried step by step using different thermal and mechanical systems in a preselected drying state. In conventional technology, the fibrous material is maintained in a turbulent state prior to roll formation to avoid orientation of the fibers. However, as a result of the turbulence, fluffs will form on the roll with a fiber density greater than that of the surrounding parts of the roll. For all printing operations, the surface of the paper should have the greatest smoothness and / or homogeneity possible. This applies both to papers coated with a layer of mineral particles and to latex adhesion materials. Therefore, the paper (base) often goes through a calendering process prior to its coating and also the paper that contains mineral fillers is treated with a calender to achieve a smoother surface. The calendering is particularly necessary to achieve certain qualities in the paper because of the aforementioned lint formation. There are several types of calenders, but all of them level the surface by means of mechanical pressing and sliding forces. Conventional calendering is hindered by some considerable disadvantages. After rewetting, a surface smoothed by the calendering process will partially or completely return to its original shape. It is also known that papers lose from 35 to 40% of their strength properties and from 25 to 35% of their original opacity as a result of calendering. Additionally, the original tenacity of the paper roll will decrease considerably. In view of the above problems related to calendering, great efforts have been made to avoid such formation of laps and to find some different methods for smoothing the surface. U.S. Patent No. 2,349,704 discloses a method for polishing the surface of a paper roll with a roll of smoothing fabric. The surface of the roll contains a powder abrasive that sticks to the surface with the help of an adhesive. The purpose is to press and smooth the paper to the same degree achieved by the supercalendering process, and according to a patent specification, the density of the treated paper will be the same as after a process of supercalendered and gloss, which will be of 10 points higher than before the treatment according to the measurement provided by a Bausch & Lomb. U.S. Patent No. 5,533,244 describes another method, somewhat similar to that mentioned above, for smoothing the paper with a woven belt that slides at a different speed on the paper roll than the roll itself, producing a frictional action. A malleable calendering device that acts as a rubbing device on the surface of the paper is described in U.S. Patent No. 4,089,738. The device will smooth the surface of the paper in the same way as the original supercalenders. None of the methods of the prior art will satisfactorily remove the high density smears from the paper surface. Additionally, apparently the paper strength properties deteriorate during the application of the known methods. Therefore, one purpose of the present invention will be to eliminate the disadvantages of the prior art and to provide a novel method for treating the surface of a fibrous roll., particularly a paper or cardboard surface in order to improve its smoothness while substantially maintaining the mechanical properties of the roll. The present invention is based on the surprising discovery that the surface of many fibrous rolls can be smoothed by grinding only the most protruding parts of the roll through a grinding medium, such as a grinding belt, a vibratory grinding device or a cylinder of rotary grinding to achieve a smoothed surface without altering or even improving the properties of mechanical strength. Particularly, the present invention comprises grinding in the dry state ("dry grinding") of only the upper portions of the fibrous web (in cross section) by pressing the surface against the grinding surface with such a small pressure that it can not be find an increase in the density of the roll. More specifically, the invention is characterized mainly by what is stated in the feature part of claim 1. The present invention provides several advantages. Surprisingly it has been found that for example the ground paper showed a better tensile strength as well as a better breaking strength than the original paper. Although we do not wish to commit ourselves to any particular theory, it seems that this phenomenon is based on the forces within the coil in tension that is distributed more evenly when the resistance of the parts that have the greatest resistance decrease. Initially, because of the poor uniformity (formation) of the paper roll, the forces are not as strong on the thinnest part of the paper. However, the grinding will re-distribute the adhesion forces within the roll matrix. Another possible explanation is that the fines generated obviously during the grinding process as well as the fibrils, one end of which adheres to the original fiber, are reattached to the surface. During the grinding process of the present invention, very limited amounts of loose fibers and dust are formed. This is probably because the grinding friction of the present invention will release some water vapor from the surface which will condense on the paper leaving the machinery part of the grinding process. This condensed water will again stick the fines to the surface. The invention will then be examined in more detail with the aid of the following detailed description and with reference to a practical example. Within the scope of the present invention, the terms "cellulosic" and "lignocellulosic" are used to designate materials derived from cellulose and lignocellulosic materials respectively. Particularly "cellulosic" refers to a material that can be obtained from the process of reduction to wood pulp and other raw materials from plants. Therefore, a roll containing "cellulosic fibers" is manufactured for example from organosol, sulfite or kraft pulp. "Lignocellulosic" refers to a material that can be obtained from wood and other raw materials of plants from defiberization, for example an industrial refining process, such as reduction to mechanical pulp by refiner (RMP), reduction to pulp mechanics by pressurized refiner (PRMP), reduction to thermomechanical pulp (TMP), defibrated wood (GW) or pressurized defibrated wood (PGW), mechanical pulp reduction chemo-thermo-mechanical (CTMP) or any other method to manufacture a fibrous material that can be formed in a roll and coated. The terms "paper" and "cardboard" refer to products formed from sheets containing cellulose or lignocellulosic fibers. "Cardboard" is synonymous with "cardboard". The grammage of paper or cardboard could vary within wide ranges of approximately 30 to 500 g / m2. The roughness of the roll to be treated in approximately from 01. To 30 μm, preferably from approximately 1 to 15 μm. The present invention could be used to treat any roll of paper or cardboard that is desired. In practice, the term "paper" or "paper roll" is used herein to designate both "paper" and "cardboard" and "paper roll" and "cardboard roll", respectively. "fines", "fibrils" and "fibers" denote finely divided material with a transverse diameter of less than about 10 μm, commonly within the range of 0.001 to 2 μm and the "fibrils" and "fibers" are materials with a length and a radius of transverse diameter of more than about 6. The "roughness" of the roll to be coated is generally given in "microns" (μm) .The roughness of the printing surface at 1000 Kpa can be measured according to, for example, ISO 8791- 4: 1992 (E).

Commonly the roughness of the paper rolls is within the range of 8 to 2 microns. As set forth and shown below in the active examples, by holding the surface of a roll of paper or cardboard to a grinding treatment according to the invention, it is possible to reduce the roughness of the roll by at least about 20% preferably from 40 to 60% while simultaneously maintaining the mechanical properties of the roll. The present invention comprises the steps of forming a wet roll from a fibrous material in a wire cloth. Then, the roll is dried in a paper or cardboard machine to a preselected drying state. At any desired point of drying, but preferably after the roll has been dried to sufficient dryness to impart reasonable mechanical strength in the roll, the roll is subjected to a dry grinding operation as explained in detail below. The grinding can be carried out between the unwinding and winding of the roll. After grinding and probable smoothing, the treated roll may be coated with suitable coating colors known per se. According to the invention, the grinding is carried out by contacting the surface of the paper roll with a grinding medium. According to a preferred embodiment of the present grinding process, it is carried out by means of grinding grains fixed to a movable grinding strip or to a vibrating plate which produces a non-glossy surface but opaque or matt. The preferable size of the grains of grinding media will be approximately 5-20 microns, obviously depending on the quality of the surface and the weight of the surface of the paper or paperboard. The surface of the grinding medium will be essentially dry (the moisture content will be less than about 50%, preferably less than 20% and in particular less than 10%) and preferably no water will be introduced between the roll and the grinding medium during the same. According to the present invention, it is essential that the highest points, i.e. The "mounds" are rectified on the surface of the paper and to achieve this, the back supports of the grinding strips and the paper supports must be constructed so that only the parts of the highest levels of the paper surface are removed. Generally, the roughness of the surface, measured in microns, is reduced by 10 to 90%, preferably approximately 40 to 60% after grinding. During grinding, the roll is subjected to a grinding energy of 700 to 14,000 J / m2, preferably approximately 2,000 to 8,000 J / m2. According to a particularly preferred embodiment, the roll is subjected to 2,000-3,000 J / m2 grinding / smoothness power in the roll microns. As mentioned above, the mechanical properties of paper or cardboard will remain unchanged - by grinding according to f. present invention. Even more grinding could improve them as explained above. Therefore, when the roughness of the surface is reduced by a maximum of 90% the properties of the roll resistance will remain essentially unchanged or will improve. When the roughness of the surface is reduced by approximately 40 to 60% the tear strength will increase by at least 5% (preferably by 10%) compared to an untreated roll. A visual inspection of the paper treated by means of the present grinding method reveals that the opacity of the paper does not change significantly when 40 to 60% of the mounds and similar irregularities on the surface have been subjected to grinding. Simultaneously, the mechanical strength of the paper is excellent. The pressure exerted on the roll could vary within a wide range provided there is no significant compression of the paper. Otherwise, this would weaken the mechanical strength of the paper. Generally, the grinding surface pressure should be about 0.01 to 20 kPa, preferably about 1 to 10 kPa. After grinding it is advisable to re-wet the surface of the treated surface and press it lightly against a very smooth surface or against a moving smooth surface to get all loose fibers and fines back to the surface. This treatment will also smooth the rectified surface. For wetting, water vapor may be used as well as spray containing small droplets evenly distributed e.g. by means of an ultrasonic treatment, and that can adhere to the surface by means of ionization methods. In an article entitled "Friction in Wood Grinding" (Paper and Timber, Vol. 72 (1997) No. 4) the grinding of wood with a grinding stone is discussed in detail. The authors argue that a grinding speed of less than 7 m / s is totally inefficient and that only at speeds of 10 to 30 m / s can the grinding stone release some fibers from the wood. A lower velocity will only result in unwanted fibrillation on contact with the wood surface. The present invention is based on the opposite concept: we do not want to release the complete fibers from the surface of the paper or cardboard roll, but only the fibrils and the loose parts of the fibers. Therefore, the difference in speed may be within the range of 1 to 10 m / s according to the present invention and still achieve satisfactory results. However, according to another modality, the greater the speed difference between the band and the paper or the cardboard to be rectified, the better the result will be. The best way to do this is to place the band and the roll so that they run in the same direction but with different speeds. This causes an efficient dust removal. A higher grinding speed will be advantageous for two different reasons: first, will prevent dust and fines from accumulating in the band and second, at high speeds the surface pressure can be kept low and the melting of resins, lignin, etc. it will not be carried out on the surface and therefore the grinding band or other surface of grinding medium will not be blocked. The critical speed depends on the quality of the pulp or wood from which the paper or paperboard has been made and also on the quality of the grinding particles on the surface of the grinding media. Grinding speed and pressure should be maintained at a level that does not result in local heating to the extent of softening the resins and lignins. If this occurs, the grinding medium will be clogged with fibers, resins, lignins and loose powder from the roll. According to a preferred embodiment, where a grinding band comprising a dry band of a polymeric material is used, the fibrous web is electrified by means of friction as a result of grinding. Therefore, the fibrils and fine particles released by the roll due to grinding are attracted to the surface by electrostatic forces between the fibrils and the network. The roll is not filled with dust. The electric charge of the surface can be made before grinding to increase the electric charge of the same. By treating the fibrous roll with cationized starch or a similar cationic material conventionally used to improve the retention of pigments or fines in the cable of a paper or cardboard machine, the cationic material will bond the fibrils that have been released during the process of grinding to the surface efficiently. According to a further preferred embodiment, the rectified surface, which as mentioned above, remains opaque or matte after grinding, can gain shine by wetting it slightly with steam and pressing it against a smooth surface. A paper or paperboard treated in accordance with the present invention may optionally be coated or used as such after being polished with a conventional calender, or preferably as explained above, after wetting. A polymer layer, barrier layer, lacquer or normal coating colors can be applied to coat the paper. T papers and cartons are particularly suitable for printing and printing with inkjet. Non-treated glossy quality products are also suitable for wrapping and bag making. The following non-limiting examples illustrate the invention: Example Test samples of a paper that is kept dry at a relative humidity of 50% with a surface weight of 114 g / m2 and a thickness of 0.16 mm which was subjected to the grinding action of a band with a roughness of 15 micras that ran at different speeds. The results are summarized in Table 1.

Table 1. Tersura and mechanical properties of a rectified paper with a band running at different speeds Tersure Resistance to Micron Resistance tearing rotu ra kPa kN / m Example-O 7.5 5.49 178 Speed of 3.2 6.05 250 band 3.5 m / s Speed of the 3.0 5.89 240 band 5.3 m / s Another group of test samples with the same paper quality was subjected to grinding through a vibratory medium at an average speed of 0.35 m / s. The results appear in Table 2.

Table 2. Tissue and mechanical properties of a rectified paper through a vibratory media Medium Ter Res Resistance to grinding Mic ics tear tear kPa size kN / m particles Example-O 7.5 5.49 1 78 18.5 micras 3.5 5.66 235 1 5 microns 3.2 5.66 240 12 microns 3.1 5.56 246 The vibration grinding quickly showed the worsening of the grinding plate. However, the grinding band did remain clean for extended periods. When grinding both sides of the paper samples 1 or 2 times with a grinding band for particles of 15 microns, the following results were obtained: Table 3. Grinding of a paper sample on both sides 1 and 2 times; speed of the band 5.3 m / s; Particle size 15 microns 1 time 2 times 2 times + steam humidification + pressure with 600 N / m Resistance to 6.85 6.48 6.95 tearing kN / m Tersura, micras 6.0 5.0 6.5 By reference, the tear strength of the untreated paper was 5.55 kN / m with a 9.0 micron sieve.

Claims (19)

1. CLAIMS 1. Method for treating the surface of a fibrous roll, where it is subject to mechanical rectification and characterized in that the grinding is carried out in a substantially dry state by removing only the highest parts of the paper surface without substantially increasing the density of the roll. The method according to claim 1, wherein the roughness of the surface is reduced to a maximum of 90% while the strength properties of the roll remain essentially unchanged or improve. The method according to claim 2, wherein the roughness of the surface is reduced by about 40 to 60% to increase the tear strength by at least 5% compared to an untreated roll. 4. The method according to any of the preceding claims, wherein both sides of the roll are subject to a grinding treatment. The method according to any of the preceding claims, wherein the roll comprises paper or paperboard with a surface weight of 30 to 500 g / m2 and a roughness of 1 to 15 μm. The method according to any of claims 1 to 5, wherein the grinding is carried out by contacting the surface of the roll with a grinding device selected from the grinding bands, grinding plates and rotating grinding cylinders. 7. The method according to claim 6, wherein the paper roll is subjected to a grinding power within the range of 700 to 14,000 J / m2, preferably from approximately 2,000 to 8,000 J / m2. The method according to claim 6 or 7 wherein the paper roll is subjected to a grinding power within the range of 2,000 to 3,000 J / m2 per meter of reduced roughness. The method according to any of the preceding claims, wherein the paper roll is subjected to a surface pressure of about 0.01 to 20 Kpa, preferably about 1 to 10 kPa, during grinding. The method according to any of claims 6 to 9, wherein the grinding device comprises the grains comprising the grinding medium with a size of 2 to 50 micrometers, preferably about 5 to 20 micrometers. 11. The method according to any of claims 1 to 10, wherein the roll comes into contact with the grinding means, the speed difference between the roll and the surface of the grinding means will be less than 10 m / s . The method according to any of claims 1 to 100. Where the roll comes into contact with a grinding strip comprising a polymer as web material to increase the electrical charge of the roll surface during grinding. 13. The method according to any of claims 1 to 12, wherein the roll is treated with a cationic material to provide a cationic surface in the roll joining the material released during grinding. The method according to any one of claims 1 to 13, wherein the rectified surface is slightly wetted with uniformly distributed steam or droplets and the wetted surface comes into contact with a smooth surface to further increase the smoothness of the roll surface of fibers. 15. The method according to claim 14, wherein the wetted surface is pressed against or slides along said smooth surface. 16. The method according to claim 1, wherein the surface is coated with material released during grinding of the surface. 17. The method according to claim 16, wherein the material comprises fines and / or fibers. 18. The method according to claim 1, wherein the speed of the grinding device is so high that the resin or lignin or any similar compound that melts on the removed surface or on the treated surface does not melt. 19. The method according to any of the preceding claims, wherein the paper or cardboard is used to print, pack or wrap.