UNCOATED PAPER AND BOARD PRODUCTS

Abstract: A new quality of uncoated paper and board products, such as liquid board or white top liner, of one or more layers with a top layer of bleached chemical pulp is described. The top layer surface has a gloss value of 20-50 %, a coefficient of gloss variation in the wavelength region 3-30 mm of less than 5 %, and a surface roughness (PPS-10) of 2 - 5 mu m. The good printability of the disclosed products makes them useful in the production of packages. The disclosed products are obtainable by a paper making process wherein the calendering of the paper or board is performed with an extended soft nip calender which has a heated, e.g. internally induction heated, roll with a temperature at the surface of 250 to 350 DEG C, and the pressure against the paper board of 5-40 MPa. The calendering operation is optionally preceded by application of steam onto the top of the paper or board web.

Description: Uncoated paper and board products The present invention relates to a new quality of uncoated paper and board products with a top layer of bleached chemical pulp. The characteristic features of the new quality are low surface roughness, high gloss, and minimal gloss variations. The paper and board products of the invention have a top surface that is highly suitable for printing, and the products are preferably used in the production of different kinds of packages. The invention also relates to the production of the new products. A characteristic feature of the top layer surface is that it resembles a coated rather than an uncoated product. Background In the field of package production there is a high demand of good printability of the paper or board products used for packages. A suitable surface for printing of paper or board products is normally achieved by reduction of the surface roughness by calendering and/or by applying a pigmented coating layer to the surface and thereby obtaining a finer surface pore structure and surface topography compared to the uncoated surface. In most cases a calendering operation is also performed after the coating operation to further improve the surface and increase the gloss. For resource saving and improved economics it is advantageous to reduce the amount of non-renewable raw materials needed for the production of a specific type of product (e.g. inorganic coating pigments and organic coating binders from oil-based raw materials). However, savings of non-renewable raw material should desirably be achieved without having to compromise with the quality requirements of the product, such as adequate bulk, stiffness, surface gloss and surface for printing. It would be desirable to be able to produce uncoated paper and board products with good surface for printing without compromising with other quality requirements. In the production of paper and board products with a surface suitable for printing, the paper web is compressed in one or more roll nips, in most cases after the drying or the coating, in a calendering operation. The thickness reduction, and the bulk deformation caused by the calendering are in the most cases regarded as drawbacks. It is therefore desirable to localise the deformation to the surface regions and minimise the deformation of the bulk structure. The general idea is to create a surface layer which is softer and thus more deformable than the interior structure of the paper. A stiffness gradient in the thickness direction is then obtained and the compressive stress necessary to achieve a certain surface property, e.g. gloss, can then be reduced. A reduced compressive stress will partly retain the bulk structure (fibres and fibre network). The most generally known possibility to obtain such a stiffness gradient in the thickness direction is to thermally soften the paper surface. The paper is then compressed between two rolls, of which either one or both are heated to a high temperature (> 120 °C). The contact time in the nip is short, which results in surface regions of higher temperature than the bulk material. The surface material (e.g. fibres or coating binders) will therefore be comparatively more compressible. The principal technique is known as temperature-gradient calendering (Kerekes and Pye 1974, Crotogino 1982 and Nreeland 1986). For uncoated paper products, the softening of the paper surface region during temperature-gradient calendering can be referred to the thermal softening of the wood polymers; lignin, hemi-cellulose and cellulose. However, localising the deformation to the surface layer to a greater extent, by the use of the temperature-gradient calendering, may lead to a more non-uniform densification of the surface by introducing differences between the surface pore structure of the fibre floe areas and the areas in between the fibre floes, i.e. in-plane variations in a macro-scale. If the stresses are concentrated to the denser part of the paper structure, unwanted inhomogeneities in the surface properties and mechanical properties may be introduced due to the calendering. Therefore the use of the temperature-gradient calendering in combination with comparatively hard nips is limited. In other words, the compression of fibre floes and the areas in between the fibre floes when calendering need to be balanced. The use of soft nips and especially extended soft nips provides a possibility to obtain such balance. The advantages of soft calendering nips, compared to hard nips, have often been described in terms of a more gentle compression where the stresses not only are localised to the thicker parts of the paper structure. Due to the more even compression, soft calendering causes less porosity variations and therefore the tendency for calender blackening and print mottle is reduced (Stevens et al. 1989). Compared to conventional soft nips, the local stress concentrations in the calender nip is substantially reduced with the extended soft nip. The required smoothening of the paper surface can therefore be obtained with a minor or none increase of the local variations of the surface properties (Wikstrδm et al. 1997) when coated paper webs are calendered. Soft nips have been used in combination with high roll temperatures for a rather long time. For instance, Brecht and Mϋller (1968) soft calendered coated board and found that an increased roll temperature, up to 200 °C, improved the gloss and smoothness at the same bulk substantially. However, an extended soft nip calendering technique with substantially higher calendering temperature than normally used, has not previously been disclosed to produce uncoated paper and board products with a surface layer of bleached chemical pulp with low surface roughness, high gloss, and minimal gloss variations enabling the products to be used in the production of high-quality printed packages. Description of the invention The present invention provides uncoated paper and board products with a top layer of bleached chemical pulp and with a good surface for printing without compromising with other quality requirements. The invention further provides the use of uncoated paper and board products in the production of packages. The invention also provides a new step in a process of producing uncoated paper and board products to obtain the products of the invention. The uncoated paper and board products of the invention are obtainable by a paper making process wherein a high degree of surface deformation on woodfree uncoated paper and board products are obtained at the same time as the uniformity of the paper structure is preserved. The paper making process includes an extended soft nip (also named long nip) calendering technique with substantially higher calendering temperature than normally used, especially in the context of uncoated paper and board products with a surface layer of bleached chemical pulp. More precisely, the use of the extended soft calendering nip in combination with roll temperatures of 250 °C or more provides a possibility to achieve unique absolute values of surface properties (such as paper gloss, and PPS surface roughness) on uncoated paper and board products in only one nip passage, especially since the uniformity of the paper structure is preserved (low gloss variations, no blackening etc.). Thus, one aspect of the invention is directed to an uncoated paper or board product of one or more layers with a top layer of bleached chemical pulp, wherein the top layer surface has a gloss value of 20 - 50 % measured according to Tappi T480, a coefficient of gloss variation in the wavelength region 3-30 mm of less than 5 %, and a surface roughness (PPS- 10) of 2 - 5 μm, preferably 3 - 5 μm, measured according ISO 8791-4. The coefficient of gloss variation may be measured by a method described by Bryntse and Norman (1976). In two preferred embodiments of the products of the invention are a liquid board and a white top liner. Another aspect of the invention is directed to the use of an uncoated paper or board product according to the invention in the production of packages. Yet another aspect of the invention is directed to the paper making feature that in a process of producing an uncoated paper or board product according to the present invention, the calendering of the paper or board is performed with an extended soft nip calender which has a heated roll pressing against the top layer of the paper or board and has a surface temperature of 250 to 350 °C, and the pressure against the paper board is adjusted to 5 - 40 MPa depending on the line load, the properties of the calender belt and the length of the extended soft nip. In preferred embodiments of this aspect of the invention the heated roll is an internally induction heated metal roll and steam is applied to the paper or board surface immediately before the nip, on the side of the web that will be contacted by the heated roll. The invention will now be illustrated by the description of embodiments, but it should be understood that these embodiments do not limit the scope of protection defined by the claims. Description of embodiments The paper making process used to produce the products of the invention comprehends an extended soft nip calender wherein the metal roll can be heated to more than 250 °C by induction heating. With this arrangement a calendering result can be obtained that is more or less unique for an uncoated woodfree product, especially when considering that the web passes only one calendering nip. The reasons for why it is believed at present that the achieved surface properties most likely only can be produced by using the extended soft nip in combination with a high roll temperature can be divided into some principal features: • Paper products of bleached woodfree pulp is comparatively difficult to soften (or plastisize) compared to wood-containing or non-bleached pulp (Salmen and Back 1980). The extended soft nip provides an enhanced temperature pulse due to the long nip dwell time, i.e. with shorter nips the heat transfer is not sufficient without a substantial reduction of the machine speed. • A more uniform pressure due to that the extended soft nip deforms locally to a greater extent over the thicker and denser regions of the paper, i.e. fibre floes. Large stress concentrations are then avoided and the nip pressure produces an elastic-plastic deformation in all regions of the paper. The paper surface replicates the smooth surface of the rigid heated roll, whereas the deformable backing produces a flexible support, leading to more even density and porosity distributions of the paper structure. As a result, the average gloss level can be increased with the extended soft nip with only a limited increase of the gloss variations. The gloss variations remain more or less at the same level as for the uncalendered paper or increase only slightly when the extended soft nip is used, whereas using conventional soft nip or hard nips enhanced the gloss variations substantially at the same time as the average gloss level increases. Figure 1 illustrates these findings, where uncoated bleached liquid board was calendered with an extended soft, conventional soft and hard nips. The same behaviour has been noted for the other grades tested. The gloss variations are characterised with a method described by Bryntse and Norman (1976). Description of the drawing Figure 1 is a diagram wherein the gloss distribution (i.e. the local variations in gloss) is given as the coefficient of variation (wavelength region 3-30 mm) in gloss. The average gloss (Tappi T-480) is also indicated (the unfilled circles). The trial points represent different calendering conditions for the same set of uncoated bleached liquid board. Notations: »Base» - base paper, »H« - hard nip, »S« -conventional soft nip, »LN« -extended soft nip, »m« -steam application immediately before the calender nip and the figures represent the roll temperature, °C. From figure 1 it can be seen that a rather remarkable increase in the absolute values of gloss is obtained with the new calendering concept. When the uncoated bleached liquid board was calendered with an extended soft nip at a roll temperature of 250 °C and steam applied to the web immediately before the nip, the gloss was increased from 6 to 41 %. At the same time the PPS- 10 surface roughness decreased from 8 to 3 μm. The visual impression of the board surface is that it reminds more of coated than an uncoated product. Test print results confirmed the good printing properties of the products of the invention. The print was less mottled compared to uncoated references calandered in conventional ways with soft nips, probably due to the homogeneous surface structure and the decreased surface porosity. It is of great importance that the pore structure is rather homogeneous in order to avoid local variations in the ink absorption that causes print mottle. Moreover, the print gloss was higher compared to the references, which probably can be referred to a higher ink-holdout due to a more closed surface (decreased surface porosity). It was also noticed that the achievements with the new calendering concept could in some cases be further improved by application of steam immediately before the calender nip. The presence of a temperature gradient may be especially effective when it is combined with a moisture gradient, since the moisture reduces the softening temperature of the wood polymers (Salmon and Back 1980). Equipment used for the calendering operation The following describes the equipment that had been used to perform the new calendering concept. Two different categories of extended soft calendering nips have been used in the development work. The targeted calendering result, i.e. products according to the invention, can be achieved with both types. The shoe-belt nip described in detail elsewhere (e.g. Turtinen and Tani 1998, Nahass and Crawford 1999). The major part of the device consists of a stationary hydraulically-loaded concave press shoe and an endless polymer belt. To prevent the friction heat developed between the stationary press shoe and the mobile polymer belt from becoming too high, an intermediate layer in the form of an oil film, which dissipates the pressure force, is used . The length and shape of the press shoe is the dominating factors determining the nip length. The roll-belt configurationis the second type of extended soft calender nip used (e.g. Neider and Rudt 1995). The polymer belt is here supported by a routable steel roll instead of a stationary press shoe. The extended nip length is determined mainly by the belt thickness and the compressive deformation behaviour of the polymer belt, which is significantly more deformable than a conventional backing roll cover. A roll that stretches the belt and an alignment roll that controls the CD-position of the belt are other necessary components of this concept. The static nip length is estimated to be about 35 mm with the extended soft nip. Calendering rolls are normally heated by oil, water or steam flowing in peripherally drilled pipes through the shell of the roll. However, in order to achieve surface temperatures of the heated roll of 250 °C and more in these experiments, internal induction heating has been used. The heating is then generated by an electromagnetic field generated by inductions coils mounted inside the roll shell (Okamoto 1989 Tokuden, Kyoto, Japan). Combinations of oil heated rolls and external induction heating may be an alternative way to obtain a roll surface temperature of 250 °C and more in this context. This possibility has however not been used in the above disclosed experiments. References Brecht W. anάMuller G. (1968): "Test performed with a gloss calender", Tappi, 51:2, 61A. Bryntse G. and Norman B. (1976): "A method to measure variations in surface and diffuse reflectance of printed and unprinted samples", Tappi, 59:4, 102. Crotogino R. H. (1982): "Temperature-gradient calendering", Tappi J., 65:10, 97. Kerekes R. J. and Pye I. T. (1974): "Newsprint calendering: an experimental comparison of temperature and loading effects", Pulp Paper Can., 75:11, T379. Nahass P. and Crawford K. E(1999): "Belt calendering: A new concept for sheet property enhancement", Proc. TAPPI Papermakers Conference, TAPPI PRESS, Atlanta, USA, p. 757. Neider T. M. and Rudt R. J. (1995): "Apparatus for finishing a continuous sheet of paper", US patent no. 5 400 707, in Swedish). Okamoto K. (1989): "New aspects on temperature gradient calendering", Proc. TAPPI Finishing and Converting Conference, TAPPI PRESS, Atlanta, USA, p. 168. Salm n L. and. BackE. L. (1980): "Moisture dependent thermal softening of paper, evaluated by its elastic modulus", Tappi, 63:6, 117. Stevens R. K., Mihelich W. G. and. Neill M. 7 (1989): "On-line soft calender for uncoated groundwood grades", Proc. TAPPI Coating Conference, TAPPI PRESS, Atlanta, USA, p. 191. Turtinen P. and Tani M. (1998): "OptiDwell - The new bulk preserving calendering method", Proc. PITA Ann. Conference, p. 53. VreelandJ. H. (S.D. Warren Co.) (1986): "Method of finishing paper utilizing substrata thermal molding", US Patent no. 4,624,744. Wikstrδm M., Nylund T. and Rigdahl, M. (1997): "Calendering of coated paper and board in an extended soft nip", Nordic Pulp Paper Res. J., 12:4, 289.

Claims: Claims 1. Uncoated paper or board product of one or more layers with a top layer of bleached chemical pulp c h a r a c t e r i z e d in that the top layer surface has a gloss value of 20 - 50 % measured according to Tappi T480, a coefficient of gloss variation in the wavelength region 3-30 mm of less than 5 %, and a surface roughness (PPS-10) of 2 - 5 μm measured according to ISO 8791-4. 2. Uncoated paper or board product according to claim 1, wherein the surface roughness (PPS-10) is 3 - 5 μm measured according to ISO 8791-4. 3. Uncoated paper or board product according to claim 1 or 2, wherein the product is a liquid board. 4. Uncoated paper or board product according to claim 1 or 2, wherein the product is a white top liner. 5. Use of a paper or board product according to any one of claims 1 - 4 in the production of packages. 6. In a process of producing an uncoated paper or board product according to claim 1, the calendering of the paper or board is performed with an extended soft nip calender which has a heated roll pressing against the top layer of the paper or board and has a surface temperature of 250 to 350 °C, and the pressure against the paper board is adjusted to 5 - 40 MPa depending on the line load, the properties of the calender belt and the length of the extended soft nip. 7. In a process according to claim 6, the heated roll is an internally induction heated metal roll. 8. In a process according to claim 6 or 7, steam is applied to the paper or board surface immediately before the nip, on the side of the web that will be contacted by the heated roll.

Inventors: HAAKANSSON SVEN, WIKSTROEM MAGNUS, HAAKANSSON, SVEN, WIKSTROEM, MAGNUS
Applicants: HAAKANSSON SVEN, KORSNAES AB, WIKSTROEM MAGNUS, HAAKANSSON, SVEN, WIKSTROEM, MAGNUS
Application year: 2000
Main classification: D21H25
IPC codes: D21G1, D21H11, D21H25