JP5068407B2 - Non-coated paper or non-coated paperboard products - Google Patents

Description

[0001]
The present invention relates to a new quality of one or more layers of uncoated (uncoated) paper and uncoated paperboard products, including the top layer of bleached chemical pulp. Its new quality features are small surface roughness, high gloss (gloss), and minimal gloss variation. The paper and board of the present invention are very suitable for printing, and the product is suitably used for various types of packaging products. The invention also relates to the manufacture of new products. A feature of the surface of the top layer is that it is closer to a coated product than an uncoated product.
[0002]
In the field of background packaging products, there is a high demand for good printability of packaging paper or paperboard.
[0003]
A surface suitable for printing on paper or paperboard is usually achieved as follows. That is, surface roughness is reduced by calendering and / or by applying a colored coating layer to the surface, thereby obtaining a finer surface porous structure and surface topography than an uncoated surface. In many cases, a calendering operation is also performed after the coating operation to further modify the surface and increase the gloss.
[0004]
In order to save resources and improve economy, it is advantageous to reduce the amount of non-renewable raw materials required to produce certain types of products (eg, organic coating binders and inorganic coatings derived from petroleum-based raw materials) Pigment). However, savings in non-recycled raw materials should desirably be achieved without compromising in terms of product quality requirements such as sufficient bulk, stiffness, surface gloss and printing surfaces.
[0005]
In the production of uncoated or uncoated paperboard products with a good surface for printing, the paper web is compressed in one or more roll nips during the calendering operation (often after drying or coating). Is done. Bulk deformation and thickness reduction caused by calendering is often considered a drawback. It is therefore desirable to limit the deformation to the surface area and minimize the deformation of the bulk tissue. The general idea is to create a surface layer that is softer (and therefore more deformable) than the internal structure of the paper. Then, a stiffness gradient in the thickness direction is obtained, and the compressive stress required to achieve a certain surface characteristic, for example, gloss, can be reduced. By reducing the compressive stress, the bulk tissue (fiber and fiber network) will be partially retained.
[0006]
The most commonly known possibility for obtaining such a stiffness gradient in the thickness direction is to thermally soften the paper surface. Then, the paper is compressed between two rolls in which either one or both are heated to a high temperature (over 120 ° C.). The contact time at the nip is short, which causes the surface area to be hotter than the bulk material. Therefore, the surface material (for example, a fiber or a coating binder) is relatively more easily crushed. The principal technique is known as temperature gradient calendering (Kirks and Pie 1974, Crotosino 1982, and Breeland 1986). For uncoated paper products, softening of the paper surface area during temperature gradient calendering can be viewed as temperature (thermal) softening of wood polymers, lignin, hemi-cellulose and cellulose. .
[0007]
However, more limited deformation to the surface layer through the use of temperature gradient calendering will lead to a more uneven density of the surface. This is due to the difference between the surface porous structure of the fiber floc region and the region between the fiber flocs, i.e., in-plane variation on a macro scale. If the stress is concentrated in a denser portion of the paper tissue, calendering will create undesirable inhomogeneities in surface and mechanical properties. Therefore, the use of temperature gradient calendaring in combination with a relatively hard nip is limited. In other words, it is necessary to balance the compression of the fiber floc during calendering and the area between the fiber flocs. The use of soft nips, especially extended soft nips, offers the possibility of obtaining such a balance.
[0008]
The advantages of soft nip calendering (soft calendering) compared to hard nips are often described in terms of gentle compression such that stress is hardly concentrated in thicker parts of the paper tissue. Soft calendering results in smaller porosity fluctuations due to more even compression, thus reducing roll burn and print spot trends (Stevens et al. 1989).
[0009]
The extended soft nip substantially reduces local stress concentrations at the calendar nip as compared to conventional soft nips. Therefore, it is possible to obtain the required smoothness of the paper surface while suppressing or eliminating the increase in local surface characteristics (Victorstrom et al. 1997).
[0010]
Soft nips have been used in combination with high roll temperatures for a relatively long time. For example, Bletch and Mueller (1968) have shown that gloss and smoothness can be improved with substantially the same bulk by soft calendering the coated paperboard and raising the roll temperature to 200 ° C. I found it.
[0011]
However, non-coated paper or non-coated paperboard products with a top layer of bleached chemical pulp with low surface roughness, high gloss, and minimal gloss variation that can be used to produce high quality printed packaging A calendering technique with an extended soft nip at a calendering temperature higher than the normally used temperature for the production of has not been disclosed so far.
[0012]
DESCRIPTION OF THE INVENTION The present invention provides an uncoated paper or non-coated paperboard product comprising a top layer of bleached chemical pulp that has a surface for good printing without compromising on other quality requirements. To do. The present invention also provides the use of non-coated paper or non-coated paperboard products in the manufacture of packaging. The present invention also provides a new step in the production procedure for non-coated paper or non-coated paperboard products.
[0013]
The non-coated paper or non-coated paperboard product of the present invention is obtained by a papermaking process in which the non-coated (non-wood) paper or non-coated paperboard product is obtained and at the same time the uniformity of the paper structure is maintained. can get. This papermaking process is based on an extended soft nip (also referred to as a long nip) at a higher calendering temperature than is normally used, particularly in the context of uncoated paper or uncoated paperboard products that include an exposed top layer of chemical pulp. Includes calendaring technology.
[0014]
More precisely, by calendering with an extended soft nip in combination with a roll temperature of 250 ° C. or higher, only one nip passage (on paper gloss or PPS surface of non-coated paper or non-coated paperboard products) It offers the possibility of achieving absolute values that are unparalleled in surface properties (such as roughness). This is particularly due to the fact that the uniformity of the paper structure is maintained (low gloss fluctuation, avoidance of roll burn, etc.).
[0015]
Therefore, one aspect of the present invention is that in one or more layers of non-coated paper or non-coated paperboard product, including a top surface layer of bleached chemical pulp, the surface of the top layer is compliant with Tappi T480. A gloss value of 20 to 50% in the measurement, a gloss variation coefficient of less than 5% in the wavelength region of 3 to 30 mm, and a surface roughness (PPS of 2 to 5 μm, preferably 3 to 5 μm, measured according to ISO 8791-4. -10) and is directed to non-coated paper or non-coated paperboard products.
[0016]
The coefficient of variation of gloss can be measured by the method described by Blins and Norman (1976).
[0017]
Two preferred embodiments of the product in the present invention are a liquid board and a white top liner.
[0018]
Another aspect of the present invention is directed to the use of an uncoated paper or uncoated paperboard product according to the present invention in the manufacture of packaging.
[0019]
Yet another aspect of the present invention is that in the process of producing a non-coated paper or non-coated paperboard product according to the present invention, the paper or paperboard is calendared by an extended soft nip calendar, which is 250-350. A heating roll having a surface temperature of 0 ° C. pressed against the top layer of the paper or paperboard, the pressure on the paperboard depending on the line load, the characteristics of the calender belt and the length of the extended soft nip And directed to a feature on papermaking such as being adjusted to 5-40 MPa.
[0020]
In a preferred embodiment of such an aspect of the invention, the heating roll is an internal induction heating type metal roll, and the paper is on the side of the web to be contacted with the heating roll, just before the nip. Or steam is applied to the surface of the paperboard.
[0021]
The present invention will now be described by the following description of embodiments, which should be understood as not to limit the scope of protection defined by the claims.
[0022]
DESCRIPTION OF THE EMBODIMENTS The papermaking process used to produce the product of the present invention includes an extended soft nip calendar such that the metal roll can be heated to 250 ° C. or higher by induction heating. This design can provide somewhat unmatched calendering results for uncoated, non-wood products, especially when considering the web passing through only one calendering nip. .
[0023]
The reason why it is believed to be most reasonable that the surface properties so achieved can only be produced by the use of extended soft nips in combination with high roll temperatures is due to several principle features Can be divided into
[0024]
• Paper products of bleached non-wood pulp are relatively difficult to soften (or plasticize) compared to wood-containing pulp and unbleached pulp (Salmen and Bag 1980). The extended soft nip results in increased temperature pulsations due to the long nip dwell time. That is, a shorter nip results in insufficient heat transfer unless the machine speed is substantially reduced.
[0025]
-More uniform pressure due to the extended soft nip being deformed more locally on the thicker and denser area of the paper, i.e. the fiber floc. This avoids large stress concentrations and the nip pressure causes elasto-plastic deformation in all areas of the paper. While the deformable backing provides flexible support, the paper surface captures the smooth surface of the hard heated roll, which leads to a more even density and property distribution of the paper texture.
[0026]
As a result, the extended soft nip can improve the average gloss level with only a limited increase in gloss fluctuations. The use of conventional soft nips and hard nips improves average gloss levels and at the same time substantially increases gloss fluctuations, whereas using extended soft nips, gloss fluctuations are calendered. It is kept at the same level as paper that is not. FIG. 1 illustrates these findings. There, calendaring of non-coating exposed liquid packaging board using an extended soft nip and conventional soft nip and hard nip is performed. Similar behavior is shown for other grades of testing. Gross variation is characterized by the method described by Blins and Norman (1976).
[0027]
Description of Drawings FIG. 1 is a diagram showing a gloss distribution (that is, a local variation of gloss) as a coefficient of variation of the gloss (wavelength region 3 to 30 mm). Average gloss (Tapi T-480) is also shown (open circles). Each trial point represents a different calendering condition for a set of similar uncoated bleached liquid packaging paperboards. Notation: “Base”: Base (raw) paper, “H”: Hard nip, “S”: Conventional soft nip, “LN”: Extension soft nip, “m”: Steam application just before the calendar nip, Each number represents a roll temperature (° C.).
[0028]
From FIG. 1 it can be seen that the new calendering concept gives a somewhat noticeable increase in the absolute gloss value. When uncoated bleached liquid wrapping paper was calendered with an extended soft nip at a roll temperature of 250 ° C. and steam was applied just before the nip (LN 250 m), the gloss increased from 6% to 41%. At the same time, the PPS-10 surface roughness decreased from 8μ to 3μ. The visual impression of the paperboard surface is reminiscent of a coated product rather than an uncoated product.
[0029]
As a result of the test printing, the good printing properties of the product according to the invention have been demonstrated. Compared to the non-coated comparative example calendered in a conventional manner with a soft nip, the print has less speckles, probably due to a homogeneous surface texture and reduced surface porosity. In order to avoid local variations in ink absorbency that cause print spots, it is important that the porous structure is somewhat homogeneous. Furthermore, the print gloss is better (higher values) compared to the comparative example, probably due to the higher ink-holdout due to the more closed surface (reduced surface porosity).
[0030]
It is also noted that the benefits of the new calendering concept can be further improved by steaming just before the calendar nip. The presence of a temperature gradient will be particularly effective when it is combined with a humidity gradient. This is because humidity reduces the softening temperature of wood polymers (Salmen and Bag 1980).
[0031]
Equipment used for calendering operations Below we describe equipment that has been used to implement the new calendering concept.
[0032]
Two different categories of extended soft nips have been used in development work.
[0033]
Shoe belt nips are described in detail elsewhere (eg, Tutinen and Thani 1998, Neihas and Crawford 1999). The majority of the equipment consists of a stationary concave press shoe that is hydraulically loaded and an endless polymer belt. An intermediate layer in the form of an oil film that disperses the pressing force is used to prevent excessive frictional heat between the stationary press shoe and the moving polymer belt from becoming excessive. The length and shape of the press shoe are the dominant factors that determine the nip length.
[0034]
The second type of extended soft nip used is a roll belt shape (e.g., Nader and Ludo 1995). In that case, the polymer belt is supported by a routable steel roll instead of a stationary press shoe. The length of the extended soft nip is mainly determined by the compressive deformation behavior and belt thickness of the polymer belt which is much more deformable than conventional backing roll covers. The roll that tensions the belt and the alignment roll that controls the CD position of the belt are other components required for this concept. The static nip length is estimated at about 35 mm with an extended soft nip.
[0035]
The calendering roll is usually heated by oil, water or steam flowing through a pipe drilled in the periphery through the outer shell of the roll. However, in these experiments, internal induction heating was used to achieve a heated roll temperature of 250 ° C. or higher. Heat is generated by the electromagnetic field generated by the induction coil attached to the inside of the roll shell (Okamoto 1989, Tokuda, Kyoto, Japan). In this situation, a combination of oil heated rolls and external induction heating can be an alternative means for obtaining a roll surface temperature of 250 ° C. or higher. However, this possibility has not been used in the experiments disclosed above.
[0036]
References Brecht W. and Muller 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 RH (1982): “Temperature-gradient calendering”, Tappi J., 65: 10, 97
Kirkes RJ and Pye IT (1974): “Newsprint calendering: an experimental comparison of temperature and loading effects” , Pulp Paper Can, 75: 11, T379
Nahas P. and Crawford KT (1999): “Belt Calendering: New aspects on temperature gradient calendering”, Proc TAPPI, Papermakers Conference, TAPPI Press, Atlanta, USA, p. 757
Neider TM and Rudt RJ (1995): “Apparatus for finishing a continuous sheet of paper”, US Pat. No. 5,400,707. No., Swedish Okamoto K. (1989): “New aspects on temperature gradient calendering”, Proc. TAPPI, Finishing and Converting Conference ), TAPPI Press, Atlanta, USA, p. 168.
Sahmen L. and BackE. L. (1980): “Moisture dependent thermal softening of paper, evaluated by the elastic modulus of the paper. , evaluated by its elastic modulus) ", Tappi, 63: 6, 117
Stevens RK, Mihelich W. G. and Neill MT (1989): "Online for uncoated Grandwood grades On-line soft calender for uncoated groundwood grades ”, Proc. TAPPI Coating Conference, TAPPI Press, Atlanta, USA, p.191
Tutinen P. and Tani M. (1998): "OptiDwell-The new bulk preserving calendering method", Proc. PITA Ann. Meeting , p. 53
Vreeland JH (SD Warren Co.) (1986): “Method of finishing paper utilizing substrata thermal molding” Wikstreom M, Nylund T. and Rigdahl, M. (1997): "Calendaring of coated paper and coated paperboard in an extended soft nip" (Calendering of coated paper and board in an extended soft nip) ", Nordic Pulp Paper Res. J., 12: 4, 289
[Brief description of the drawings]
FIG. 1 is a graph showing a gross distribution (gross local variation) as a gross variation coefficient (wavelength region: 3 to 30 mm).

Claims (3)

In one or more layers of uncoated paper or uncoated paperboard products, including the top layer of bleached chemical pulp,
The surface of the upper surface layer has a gloss value of 20 to 50% as measured in accordance with Tappi T480, a gloss variation coefficient of less than 5% in a wavelength region of 3 to 30 mm, and 2 to 5 μm as measured in accordance with ISO 8791-4. A surface roughness (PPS-10) of a non-coated paper or a non-coated paperboard product,
The paper or paperboard is calendered by an extended soft nip calender, which has a heated roll that is pressed against the top layer of the paper or paperboard with a surface temperature of 250-350 ° C., The pressure on the paperboard is adjusted to 5-40 MPa depending on the line load, the characteristics of the calender belt and the length of the extended soft nip,
A method for producing a non-coated paper or a non-coated paperboard product.   The manufacturing method according to claim 1, wherein the heating roll is an internal induction heating type metal roll.   The manufacturing method according to claim 1 or 2, wherein steam is applied to the surface of the paper or paperboard immediately before the nip on the side of the web to be contacted with the heating roll.

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