Method for calendering paper and paper manufactured by the method

Abstract: The invention relates to a method for calendering paper, especially with so-called glassine calender­ ing. The calender comprises press rolls arranged successively in the paper advancing direction where­ by, at least between some of rolls, there are form­ ed press nips for passing paper therethrough. Each press nip is preferably formed by a pair of rolls, one of which is hard and the other elastic. Paper is advanced into the calender at an initial moisture X within the range of 12 % ≦ X ≦ 25 %, at least one of the press nips having a calculable line pressure arranged higher than 250 kN/m. In operating con­ ditions, the ratio X/LP between paper moisture X and calculable line pressure LP is maintained in the press nips within certain limits in a manner that the maximum value of ratio X/LP divided by the minimum value of ratio X/LP gives a reading Y, which is Y ≦ 2.15, whereby X refers to the moisture percentage of paper and LP is a line pressure at a particular press nip expressed as a quantity kN/m (kilonewton/meter). The invention relates also to a paper manufactured according to the method.

Description: The present invention relates to a method for calendering paper as described in the preamble of claim 1. Glassine calendering is a special application of traditional supercalendering. For instance, the British Patent Specification 1.327.433 describes a super-calendering process, and proposes the use of a combination of high pressure, high temperature and substantial initial water content of the paper in order to obtain a high density paper with high surface smoothness and high gloss, suitable for photographic paper. For glassine calendering, paper is generally advanced into a calender at a relatively high initial moisture and the line pressures applied are substantially highher than in traditional calendering. The number of rolls and hence the total number of nips in a glassine calender exceeds that used in traditional supercalendering since, at least in some applications, such calender acts as "a drying machine" at the same time. The final moisture of paper is typically circa 6 %. In other respects the principle of glassine calendering is similar to traditional supercalendering. This means that every second roll is soft, e.g. a paper roll (hardness less than 95° Shore D) and every second roll is an internally heated metal roll. However, the demands set on a final product are clearly different in glassine calendering to those in supercalendering. The objective in glassine calendering is to provide a paper having a uniform thickness, high transparency and high density and having a high hold-out capacity for surface solvents. Generally in prior known supercalenders, especially in those that are constructed from vertically superimposed rolls, the lead-in of paper into a calender or the unwinding of a roll of paper is positioned in the top section of a supercalender and a paper web is always brought out of a supercalender at the bottom section where the rewinding takes place. Thus, the paper web travels from top nip to bottom nip. This running mode does not produce best possible results especially in glassine calendering since the highest line pressures are not applied to paper until the paper has almost reached its final moisture, at which time the resistance to shaping in paper is at its peak. The present invention seeks to put the potential of glassine calendering to better use in developing the characteristics of a final product. In order to achieve this object, a method of the invention is mainly characterized by the characterizing features of claim 1. Herein, X refers to the moisture percentage of paper (moisture, moisture percentage or final moisture percentage refers in this context to the relative share of water of the total paper pulp.) LP is a line pressure in a given nip as expressed in the quantity kN/m (kilonewton/meter), (X/LP)max is the ratio at its highest and (X/LP)min is the ratio at its lowest in a particular operating situation. The method is capable of achieving paper having characteristics clearly superior to those obtained on the same base paper by traditional calendering techniques. It is obvious that a method of the invention can be utilized with various outset values, e.g. in a manner that the initial moisture of a paper web is reduced to e.g. X circa 12 % for increasing the output of a calender used in the method as a result of a lesser drying demand. A paper produced with a method of the invention is technically ready for use, with the exception of moisture, as quickly as after a few press nips which is why the desired result can be achieved with a number of press nips fewer than in traditional glassine calendering. The method can be utilized by reducing the line pressure in press nips as compared to traditional methods whenever the object is to produce a glassine type of release paper grade of conventional quality. An advantage gained this way is that the paper rolls have a longer service life or a longer time span between grinding and resurfacing operations, resulting in cost savings. The gist of a method of the invention can be summarized as follows: By the application of a method of the invention it is possible to produce a special paper, particularly a so-called release paper, by means of equipment involving substantially less investment costs than the present-day equipment, the capacity of available production machinery can be increased and costs per unit can be decreased. The above features make it possible to mount an apparatus for the application of a method of the invention in connection with a paper-making machine as one of its operating units. A paper manufactured with a method of the invention has a combination of characteristics which is definitely novel over the prior known special grade papers, particularly so-called release paper. A combination of characteristics essentially inherent of the paper is set forth in the characterizing clause of an independent claim directed to a paper. The following table 1 illustrates by way of an example and in a tabulated fashion the values of the moisture and calculable line pressure in a calender consisting of sixteen nips. The numbering of nips increases in the web advancing direction. Nip No. (running order) Paper moisture (%) Calculable line pressure LP (kN/m) Ratio moisture/line pressure (x 10⁻²) Ratio X/LP max/min 1 19.5 450 4.3 4.3/2.1 = 2.05 (i.e. lower than 2.15) 2 15.3 439 3.5 3 13.4 425 3.2 4 11.5 414 2.8 5 10.4 402 2.6 6 9.3 391 2.4 7 8.7 380 2.3 8 7.9 369 2.1 9 7.7 359 2.1 10 7.3 348 2.1 11 7.1 337 2.1 12 6.9 326 2.1 13 6.6 316 2.1 14 6.3 305 2.1 15 6.2 293 2.1 16 6.0 280 2.1 As indicated in the table, the ratio of moisture/line pressure (X/LP) in the first nip is at its maximum and has a reading 4.3 x 10⁻². After the eighth nip, the ratio of moisture/line pressure (X/LP) became nearly constant and was of the order of 2.1 x 10⁻². The ratio X/LP remains within certain limits in a manner that the highest ratio divided with the lowest gives a reading which is lower than or equal to 2.15. After a certain minimum number (in practice 6-8 nips), it was also found that the nip number had no effect other than on final moisture and, to some degree, on caliber. The following table 2 shows a comparison of characteristics, the comparative products being the same base paper calendered with a traditional method described in the beginning of this specification and with a method of the invention. Characteristics of paper Traditional method Method of the invention IGT stain length (cm) 13.5 ± 0.5 > 14 Elrepho transparency (%) - 100 % bleached pulp 50 ± 2 > 55 - partly 1/2-bleached pulp 47 ± 2 > 52 Unger oil absorption (g/(m² x 1 min)) 0.9 ± 0.2 0.5 thickness (um) 57 ± 2 58 ± 3 grammage (g/m²) (at circa 6 % moisture) 65 ± 1 65 ± 1 It can be concluded from table 2 that there is no characteristic required of a special grade paper, especially release paper, that would deteriorate but, instead, the combined effect of characteristics produces a highly favourable end result. (The characteristics of a highly dense paper for a relatively bulky paper). The paper grade used in a method of the invention is manufactured from a chemical, semi-bleached and/or bleached softwood and/or hardwood pulp. It is surface-sized, preferably with a polyvinyl alcohol (PVA) or a carboxy-methyl cellulose (CMC) material or a mixture thereof. The paper making has been effected by using conventional chemicals and additives for improving certain properties. After the application of the method, paper moisture is within the range of 3 % ≦ X ≦ 8 %. Transparency (Elrepho Transparency) is determined with the method: SFS 3517. IGT-value (IGT stain length) is determined with the method: Al-pendulum device for IGT. Unger-value (Unger oil absorption) is determined with the method: SCAN-P:37.

Claims: A method for calendering paper, especially with socalled glassine calendering, the calender comprising press rolls arranged successively in the paper advancing direction with press nips being formed between at least some of the rolls for passing paper therethrough, each press nip being preferably formed by a pair of rolls, one being hard and the other elastic, wherein the paper is advanced into a calender at an initial moisture X of at least 2%; characterized in that the initial moisture X of the paper is within the range of 12% ≦ X ≦ 25%; that the calculable line pressure of at least one of the nips is made higher than 250 kN/m; and that in the operating conditions the ratio X/LP between paper moisture X and calculable line pressure LP in the nips is maintained within certain limits in a manner such that the maximum value (X/LP) max divided by the minimum value (X/LP) min produces a reading Y which is lower than or equal to 2.15, said X referring to the moisture percentage of paper and LP being a line pressure in a particular press nip, expressed as a quantity kN/m (kilonewton/meter). A method as set forth in claim 1, characterized in that the ratio X/LP is adapted to reduce at least in the first press nips of a calendering operation, preferably up to about half-way through calendering in a manner that the ratio X/LP at a nip upstream in the paper advancing direction is higher than at the next press nip downstream of said press nip. A method as set forth in claims 1 and 2, characterized in that towards the final stage of calendering, preferably over the downstream half of calendering, the ratio X/LP is maintained substantially constant. A method as set forth in claim 1, characterized in that the final paper moisture X is selected within the range of 3% ≦ X ≦ 8%, preferably 6%.

Inventors: SARANTOLA JAAKKO, TAIPALE JYRKI, SARANTOLA, JAAKKO, TAIPALE, JYRKI
Applicants: YHTYNEET PAPERITEHTAAT OY
Application year: 1989
Main classification: D21G1
IPC codes: D21G1