NO323580B1 - Coated roll printing paper suitable for coldset offset inks and use thereof - Google Patents

Abstract

A coated roll printing paper suitable for printing with cold-set offset printing inks is based on paper containing paper fibres and mineral filler and has a coating containing ground calcium carbonate (CaCO3) pigment in a synthetic binder. The novel features are that: (a) not less than 50 wt.% of the pigment is a natural ground CaCO3; (b) all the pigment has a fineness of not less than 80% being less than 2 mu m; and (c) the dry binder fraction is less than 13 wt.% with respect to the pigment. Also claimed is a method of making the paper.

Description

The present invention relates to a coated nil printing paper suitable for printing with cold-set offset printing inks according to the preamble of claim 1. The invention further relates to the use of such paper.

Today, newspapers are predominantly printed using offset methods, and more specifically using so-called coldset inks, which, in contrast to heatset offset printing inks, are not exposed to heat for drying, but which dry already by the emulsion water of the printing ink, as well as the in this, the oil contained in the printing medium, namely the paper, is knocked off as quickly as possible, whereby the color carrier of the printing ink must remain on the paper surface. In addition, oxidative drying takes place here.

Admittedly, the print quality improves with increasing smoothness of the printing medium and furthermore the ink consumption is lower, higher smoothness of a paper generally results in a lower absorption capacity, whereby the removal rate for the color emulsifiers is lower, which in turn leads to smearing on the guiding elements of the printing machine and to deposition in false apparatus and in specimen pile. On the other hand, an absorption capacity that is too high causes an increased penetration of the printing colors into the paper, which leads to a lack of expression, i.e. poor point quality and to see-through of the printed image on the back. Ordinary newsprint paper meets the conditions for a sufficiently fast drying of coldset colours, however, as is well known, the quality of the printed image on newsprint is limited. Standard newsprint paper is an uncoated, so-called natural paper. The pore volume is consequently not covered by a coating, which could prevent the printing ink emulsifiers from being knocked out.

Rotary presses for newspaper printing are usually only fully utilized for a limited part of the day. Consequently, it would be appropriate to be able to use such expensive facilities with other printing assignments in the dead time available, especially assignments in the accident area. However, for such assignments, whether they are project documents or the like, a higher quality of the printed image is required, especially in multi-colour printing, than is possible on normal newsprint paper. Consequently, there has been no shortage of attempts and deliberations to provide so-called improved newsprint paper for the aforementioned purpose, which penetrates into the quality range of SC paper (super calendered), namely high-satin paper used for headset offset printing, or even as coated paper within the range of LWC paper (low weight coated). In the attempts to date, however, it has not succeeded in achieving market acceptance. EP-A-0 377 983 describes a coated newsprint paper whose coldset suitability is to be achieved by the coating pigment containing a certain minimum proportion of oily pigments and in total must at least exhibit a certain oil absorption value. It is not yet known that such paper has found its way onto the market. Acicular pigments, such as satin white, precipitated calcium carbonates and delaminated structured kaolins, which are proposed in the publication, are generally very expensive. Furthermore, due to their structure, they require a high binder content, which significantly increases production costs.

The invention was based on the task of providing an economically manufacturable, coated nil printing paper which is suitable for printing with coldset offset colors, which stands out optically from the normal newspaper printing papers and which can also be processed at print speeds that are common in offset newspaper printing , respectively on the machines provided for this purpose.

This task is solved in principle, starting from the subject matter of EP-A-0 377 983 by a gravure printing paper with the characterizing features from claim 1.

It has surprisingly been found that the printability of a coated nil printing paper with coldset inks can be achieved when the coating exhibits a sufficient microcapillarity and affinity for the emulsifiers of the coldset ink. These properties can be achieved when the coating pigment exhibits a certain minimum proportion, namely 50% of a natural, ground (not needle-shaped) calcium carbonate, the fineness of the coating pigment in total is such that at least 65% by weight of the pigment consists of particles of a size smaller than 2 um and the proportion of dry covered binder, relative to coating pigment, is less than 14% by weight. Sufficiently fine kaolin qualities are known which lead to a high coating porosity, but which, however, due to the high specific surface, entail a high binder requirement. This applies in particular to delaminated and otherwise pretreated kaolins. Calcium carbonate rather has a hydrophobic and inert surface and therefore requires less binder in coating.

From EP-A-0 377 983, it is true that the use of a ground calcium carbonate in coating is also known. However, the calcium carbonate is only used as a mixing pigment alongside the required needle-shaped pigments.

To ensure rapid drying of the coldset printing inks, the calcium carbonate used must have a medium to high degree of fineness, in order for the applied coating layer to have a high microcapillarity. The finer the capillaries, the higher the capillary pressure and the faster the phase separation of the more or less emulsified ink takes place. For high requirements, 90 to 100% by weight of a calcium carbonate with a degree of fineness of at least 80% less than 2 µm and at least 75% by weight to approx. 85% by weight of a calcium carbonate with a degree of fineness of approx. 90% less than 2 um. For a satisfactory drying of the coldset printing inks, the color removal time and especially the water absorption capacity of the paper surface seem to be decisive. This will be dealt with in more detail below. Admittedly, certain measurement value limits must be met for both properties in order to achieve a satisfactory print result, however, one property can be compensated to a certain extent by the other. With increasing fineness of the coating pigment, the value for the color rejection time does indeed fall, but the water absorption capacity increases. At the same time, with increasing degree of fineness, the specific surface of the coating pigment also increases and thus its binder requirement, which in turn reduces the favorable printing properties. With regard to the more precise relationship between the degree of fineness of the coating pigment and the proportion of binder used, the expert must find an optimal trade-off.

As a mixed pigment, aluminum hydroxide is particularly suitable due to its morphology and fineness. It can be used up to amounts of approx. 20% by weight of the coating pigment. Incidentally, kaolin with a degree of fineness of 65% by weight and more below 2 µm can also be used as a mixed pigment when the printability properties allow this. Talc, titanium dioxide and gypsum are likewise suitable in small quantities as mixed pigments, provided that the desired coating capillarity is not counteracted.

As a mixed pigment with a higher water absorption capacity, a sodium bentonite has also proven suitable, of which, limited by the effect on the coating color rheology, up to 25% by weight of a quality with a specific surface of 600 m<2>/g can be used in the coating pigment.

In order not to affect the cold-set suitability of the applied reinforcement with too much binder, highly active binders are preferably used. In paper coating, the following binder types are generally used, whereby the order indicates decreasing binding effect: Plastic dispersions (e.g. styrene-butadiene, acrylate, styrene-acrylate), polyvinyl alcohol, protein or casein, starch. In order to achieve a high binding effect with the lowest possible binder proportion, relative to the coloring pigment, plastic binders and polyvinyl alcohol (PVA) are preferably used according to the invention. In addition to its binding power, PVA also has the property that it irreversibly pulls up on surfaces that exhibit a relatively inert reactivity, such as calcium carbonate. It is therefore preferably used in combination with a plastic binder. Preferably, according to the invention, less than 12% by weight of binder is used, relative to the coating pigment, and in the above combination for the most part even less than 0.5% by weight, typically only approx. 6.5% by weight binder. If starch is used at the same time, the amounts of binder are rather in the upper limit range. Depending on the pigment mixture, a more or less hydrophilic binder is used to control the color rejection time, and more specifically, next to PVA, also starch or CMC.

An exemplary binder combination of consists of 1.0 to 4.0% by weight PVA and 4.5 to 5.5% by weight of a synthetic binder, especially a butadiene-styrene binder or a styrene-acrylate binder. In connection with the present description, the aforementioned plastic dispersions, also in combination with PVA, are referred to as highly active binders. For certain binders, the addition of a cross-linking agent may be required.

The coating color according to the invention can typically have the following components:

Otherwise, the coating colors used may contain common additives such as e.g. up to 1.5% by weight melamine-formaldehyde resin as a wet fixing agent, up to 0.4% carboxymethylcellulose (CMC) as a solution, optical brighteners and/or chemicals for pH value adjustment, such as NaOH.

Ground, natural calcium carbonates suitable for the invention and largely available on the market are, for example, the types "C60 HS", "C70" and "C90 HS" from the company ECC International. The quality "C60 HS" has a proportion of 63 ± 3% by weight < 2 µm, a maximum of 2% by weight > 10 µm and a maximum of 0.01% by weight > 45 µm. The quality "C90 HS" contains 90 ± 3% by weight < 2 µm, a maximum of 1% by weight larger than 10 nm and a maximum of 0.01% by weight > 45 µm. These qualities are delivered as seedings with a solids content of 78 ± 1% by weight. A further manufacturer of suitable calcium carbonate grades is the company Omya. The coating colors according to the invention are processed in an aqueous slurry at a solids content of 30-65% by weight dry covered mass. As application methods, scraper application methods such as "Inverted Blade", "Jet Flow" as well as roller application devices such as the Massey coater, and also film presses such as the film press from Jagenberg, speedsizer or "Metering Size Press" from Beloit are mentioned. The method according to the invention and the paper obtained according to it are consequently essentially independent of the type of coating application method, although one or the other application method may under certain conditions lead to a better result than the other. As is known, the squeegee coating method evens out the paper surface and therefore locally leads to different layer application, while the roller coating devices achieve a more even coating application, which can be positive for the color rejection behavior under certain conditions. A gentle layer drying can also be important, so that unwanted binder migration processes do not worsen the desired even microcapillarity of the coating application.

In the case of single-coated paper, according to the invention, dried, covered coating quantities with a surface-related mass of more than 4 g/m2 and side are applied to the raw paper. Surface-related mass of 7-12 g/m<2>and side is preferred, typically approx. 8 g/m<1>and page.

However, the invention is not limited to single coated paper. It can also be used on double coated paper. Double coatings are based on a surface-related mass of at least 15 g/m<2>and side, typically 20 g/m<2>and side, whereby the coating mass is distributed approximately evenly on both coating applications. Decisive for the properties of the paper according to the invention is the cover coating. When, within the scope of the present description, it is a question of a coating application without it being specified in more detail, then single coated paper means the only coating application and double coated paper usually the cover coating, but in some definitions also the overall coating. In the case of double coating, within the scope of the present description, the fiber coating is always expressly designated as such.

Decisive for the properties of a paper according to the invention with double coating is to a large extent the cover coating. It must therefore fulfill the requirements and conditions described above essentially when used on single coated paper. The coating does not necessarily have to exhibit the same fineness or micro capillarity as preferred embodiments of the previously mentioned coating application. But the pre-coating must also contain at least 50% by weight ground, natural calcium carbonate and also correspond to the conditions in the characterizing part of patent claim 1. For definition purposes, the minimum requirements of this claim are valid for single as well as double-coated paper. This is all the more so since in the case of a tire coating, which in its composition and degree of fineness constitutes a preferred embodiment, which goes beyond the minimum conditions in patent claim 1, the pre-coating alone must not necessarily fulfill the conditions in patent claim 1.

It may be appropriate to smooth the raw paper before the application of the only coating or pre-coating, for example in a machine smoothing plant at the end of the paper machine, which may also be equipped with a so-called soft-nip.

The invention is not limited to the use of a particular raw paper. Consequently, wood-free as well as wood-containing raw paper and those with a significant proportion of shredded recycled paper fibers can be used. Consequently, for example, a wood-free raw paper is suitable if input for paper production in dry, thatched portions contains approx. 78% chemical mass, approx. 20% mineral filler, divided into 15% calcium carbonate, 2.5% kaolin and 2.5% talc, approx. 1% starch and approx. 1% other aids.

For cost reasons alone, wood-containing raw paper is preferred, which also contains a proportion of shredded recycled paper fibres. In addition, wood-containing raw paper usually also exhibits printing technical advantages, for example a higher opacity. Fiber input for a raw paper containing wood or recycled paper can, for example, relative to the dry covered total fiber consist of approx. 20% chemical pulp, 20% wood pulp and 60% recycled paper material. Related to fiber material, the material input can further contain up to approx. 50% mineral filler, which roughly corresponds to a share of 1/3 of the material composition. As is well known, this amount of filler does not remain completely in the paper during the manufacturing process, but partially reaches the waste water.

When, within the scope of the present description, we are talking about wood substances as fiber components, then it can be all such substances that are usually understood under this term in paper technology, namely, for example, wood shavings, thermo-mechanical pulp (TMP), chemical-thermal-mechanical pulp (CTMP) etc.

A further important prerequisite for an acceptable print result when printing a paper with coldset inks is, in addition to satisfactory drying of the inks, the dimensional stability of the paper. As water penetrates the raw paper that carries the coating when the coldset inks are removed, this has an impact on the fiber bond to each other and thus affects the dimensional stability of the paper. This impact is stronger than normal newsprint natural paper, since in the case of a coated paper with a comparable surface-related mass, the raw paper as carrier paper for the coating adds a correspondingly lower mass proportion, i.e. the raw paper is thinner. The dimensional stability of a paper under the influence of moisture can be improved by additives, for example starch. Consequently, it is common to add a raw paper composition approx. 0.5% starch. For paper that is produced on open long-form paper machines or also on so-called hybrid formers, whereby an upper dewatering screen is only fed along with it after complete leaf formation on the long-form, and which, as a result of this production method, exhibits a relatively favorable fiber orientation ratio, a cross- to length ratio of approx. 1:2 to a maximum of 1:2.5, the dimensional stability is possibly sufficient for use in the coldset printing process without starch being added to the raw paper at all. Due to the predominant fiber orientation in the direction of production, i.e. in the longitudinal direction of the paper, the lack of dimensional stability essentially consists of a transverse contraction, which is further reinforced by the paper pull in the processing machines.

Pulp printing paper is today produced economically only on very fast paper machines, which, according to the current state of the art, exclusively use so-called "gap forms", whereby the sheet formation takes place in the confluence gap between two sieves. In the case of paper obtained on such modern machines, the cross-to-length ratio for the fiber orientation is significantly worse and ranges from approx. 1:3 to 1:4. This results in a significantly lower transverse stability for such paper. It has now been found, within the framework of the present invention, that the dimensional stability of raw paper produced on "gap formers" can be sufficiently influenced in a positive direction when adding the raw paper input more than 1%, to a maximum of approx. 2%, typically approx. 1.5% starch. The astonishing thing is not in the effect of the starch on the paper, but rather in the fact that a paper with such a high starch input can be produced at all on a "gap former", which was previously not considered possible. Within the scope of the present invention, this has been achieved with a modified and more precisely defined high cationic starch. The surprising effect was that by adding approx. 1.5% of this starch for the substance introduction was approx. 1.4% to be found in the raw paper, which indicates an astonishingly high retention of the starch during leaf formation, without which even higher amounts of starch additives in the material introduction remain without a significant effect on the raw paper and, at best, increase the waste water load and costs. In trials carried out with the highly cationic starch, the raw paper could be produced without reducing the machine speed at approx. 1220 m/min.

With regard to the suitability of a paper for coldset printing inks, extensive research was conducted to determine which measurable paper properties determine the suitability of a paper for the coldset printing process. It was thus found that certain limits in the color loss behaviour, or in particular in the surface wettability, are decisive. To determine the color rejection, a rejection test modified in the applicant's company is used using the multi-proof printing machine system Dr. Dumer from the company Prtifbau dr.ing. Herbert Dfirner in Peisenberg. The surface wettability is usually determined by the time-dependent reduction of the edge angle for a drop of liquid applied to the surface. For this purpose, the "FIBRO 1100 Dynamic Absorption Tester" from the company FTBRO-System ab in Stockholm was used. A brief description of the investigation procedure is attached as appendices A and B to the present description.

In the rejection test, a test print with a standard printing color is obtained under defined conditions, which after a defined time under pressure is brought into contact with a counter paper. The ink intensity printed on the backing paper is measured with a densitometer. Indicated in the following are the densitomer values of the back pressure after 30 seconds.

Distilled water was used for the edge angle measurements carried out to determine the surface wettability. Indicated below is the edge angle measured with the FIBRO tester after 2 seconds.

It has been shown that for a suitability in the coldset printing process the color rejection value is important and should in principle be < 1.1, preferably < 0.8. The edge angle determination with the FIBRO tester after 2 seconds should give a value < 70°, preferably < 55". Standard newsprint paper shows after 2 seconds, for example, a value of only approx. 42°. Such natural paper has a high wetting behavior. The two above mentioned the paper properties partially compensate each other. Consequently, an excellent printing result was achieved at an edge angle after 2 seconds of 45° and a knock-off value for both paper sides of 0.5. Paper with an edge angle of less than 50° and a knock-off value of < 0, 7 consequently exhibits an excellent suitability for the coldset printing process.

The papers according to the invention in question here, which exhibit the above-mentioned properties, are essentially unsatinized, respectively only very weakly satinized, paper with smoothness values according to Bekk between approx. 10 and 50 seconds. It is therefore about so-called matt qualities. A high satin finish of the paper according to the invention would, due to the relatively low proportion of binder in the coating, not only reduce the napping resistance of the surface necessary for printability, but also the required microcapillarity could be lost, which is required for the drying of the coldset printing inks.

The invention further includes the use of roll printing paper as mentioned above for printing by coldset offset printing process, in particular a rotary newsprint press.

Unless otherwise stated in the present description, percentages, even when this is not expressly mentioned, are always to be understood as percentages by weight. Furthermore, when not otherwise specifically stated, the percentages, like other quantities, are always related to the dry, thatched components. In this context, the indication "unrefined" also refers to an oven-dried state.

Embodiments of the invention follow.

Example 1

On a fast-running paper machine with a double-screen former (gap former), at a machine speed of approx. 1200 m/min. obtained a raw paper from the following material input:

Example 2

The raw paper according to example 1 was coated with a coating application of the following composition:

The coating application of the flat-related mass of approx. 8 g/m<2>and page. On finished paper, the following measurement data was obtained:

Coldset suitability, i.e. the adequate color drying of the paper in practical trials, was satisfactory.

Example 3

The raw paper according to example 1 was coated with a coating compound of the following composition:

Effort<kn>in<g>spipment

Binder op additives. relative to bestrvknin<g>spi<g>ment

The paper had the following surface properties:

The other test data corresponded to the paper according to example 2. The paper according to this example showed excellent coldset suitability in practical tests.

Example 4

The raw paper according to example 1 was coated with a coating color which only differed from the coating color according to example 3 in that the coating pigment was used instead with a polyacrylate as dispersant for a high-amine-containing cationic dispersant, for a cationic pigment slurry and a cationic plastic binder for coating color preparation. The following surface measurement values were obtained for the finished paper:

The printability of this paper in the coldset process was also very good.

Example 5 fCompari<g>nin<g>sexemD

The raw paper according to example 1 was coated with a coating color of the following composition: Binder and auxiliary agent, relative to coating pigment

The paper coated with this coating color had the following surface data:

The ink drying when printing with coldset inks was not in order. This was already expressed in the surface properties and was conditioned by the high proportion of binder.

A correspondingly poor print result was also obtained with a paper whose coating application only differed from that stated above in that instead of 20% kaolin, 20% Al(OH>3 with 98% < 2 um) was used in the coating pigment. Proportion and composition of binder was unchanged.

In particular, the test papers according to examples 3 and 4 could be processed on coldset printing machines at normal production speeds and led to a point-accurate image reproduction at normal printing ink drying conditions. At the same time, compared to newsprint paper, a lower ink consumption was achieved. The water flow could be reduced. Compared to normal newsprint paper, the higher whiteness led to a contrast-rich expression comparable to the print result for LWC matte qualities.

Appendix A

"Fibro 1100 DAT" is the designation for a device from the company Fibro System ab, Box 9081, S-12609 Stockholm, Sweden, from which a detailed prospectus is available. The letters DAT stand for "Dynamic Absorption Tester". The device is used to measure the wetting behavior of surfaces, a paper property which for further processing processes, such as coating, printing etc. must be measured exactly and set. It works according to the principle of contact angle measurement and is based on a method developed by the Swedish Paper Research Institute.

From the prospectus it appears that the device includes a medical dosing device and a CCD camera, that the droplet size is adjustable between 0.1 and 9.9 ul and that the characteristic change in the wetting behavior of the drop placed on the paper sample can be documented by video images that can be stored with a clock time of 20 milliseconds. The time course of the contact angle can be recorded, so that the wetting behavior of different paper samples can be easily compared with each other.

In the present case, distilled water was used for wetting and the chastening after 2 seconds was determined.

Appendix B

In a counter pressure test, also known as a degreasing test or rejection test, a defined amount of printing ink is applied to a paper strip, which is then rolled over with a counter test strip on average at pre-determined time intervals. The amount of color released on the counter sample strip is determined optically and allows conclusions regarding the color absorption behavior and stacking behavior of the sample strips.

Details regarding the test execution can be found in a detailed description of the universal proof printing machine from the company Prttfbau, dr.ing. Herbert Diimer, Aich 17-23, D-82380 Peisenberg/Munchen, of 26.9.1972, especially under numbers 10.5 and 14.2.

According to this, for coated paper, an amount of color of 0.3cm<3>, a rubbing time of 30 seconds in color work and 30 seconds for the printing form is recommended. The pressing pressure by pressure and counter pressure must in any case amount to 200 N/cm, i.e. 800 N at a pressure mold width of 4 cm. Rejection test color No. 52 0068 from color factory Michael Huber, Munich, must be used. The back pressure must be carried out after 30, 60, 120 and 240 seconds. As pressure speed, 0.5 m/sec is recommended. Standard paper with the designation APCO MI from the company Scheufelen is used as proof paper.

In the present case, the tests were carried out at twice the pressure rate, otherwise with the specified values. In particular, the color transfer on the counter test strips, which was achieved by counter pressure after 30 seconds, is judged.

Claims (18)

1. Coated roll printing paper suitable for coldset offset printing inks with a raw paper as carrier paper, which contains paper fiber material and mineral filler and with a coating application which contains a ground calcium carbonate in the coating pigment and a synthetic binder in the binder, characterized by the surface wettability, determined as the contact angle by the fibro test after 2 seconds, gives a value < 70°, the ink absorption test gives a value of 1.1 to 0.25 and the napping resistance meets the requirements for newsprint offset printing. 2. Coated roll printing paper according to claim 1, characterized in that the surface wettability, determined as the contact angle by the fibro test after 2 seconds, gives a value of <55°. 3. Coated roll printing paper according to claim 1 or 2, characterized in that the ink absorption test gives a value of 0.8 to 0.3. 4. Coated roll printing paper according to at least one of claims 1 to 3, characterized in that the smoothness according to Bekk is 10-50 seconds. 5. Coated roll printing paper according to at least one of claims 1-4, characterized in that the calcium carbonate in the coating pigment is ground calcium carbonate and constitutes a proportion of at least 50% by weight, preferably at least 70% by weight of the coating pigment. 6. Coated roll printing paper according to at least one of claims 1-5, characterized in that in the coating application, the proportion of the binder expressed by dry weight, based on the coating pigment, is less than 14% by weight. 7. Coated roll printing paper according to at least one of claims 1-6, characterized in that the coating pigment has up to 25% by weight of sodium bentonite. 8. Coated roll printing paper according to at least one of claims 1-7, characterized in that the coating pigment has proportions of up to 20% by weight Al(OH)3 and/or up to 50% by weight kaolin. 9. Coated roll printing paper according to at least one of claims 1-8, characterized by the composition of binder, in the form of dry weight in the coating application, based on the weight-% of coating pigment on 3-10% by weight synthetic binder 0-5% polyvinyl alcohol (PVA) 0-5% protein and/or casein 0-5% starch up to 0.4% CMC. 10. Coated roll printing paper according to at least one of claims 1-8, characterized in that the binder, based on coating pigment, essentially consists of 1.0 to 4.0% by weight PVA and 4.5 to 5.5% by weight of a synthetic binder , especially a butadiene-styrene binder, or a styrene-acrylate binder. 11. Coated roll printing paper according to claim 10, characterized in that the binder additionally contains 0-5% by weight of starch and/or up to 0.4% by weight of CMC. 12. Coated roll printing paper according to at least one of claims 1-11, characterized in that the weight per unit area of the coating application is more than 4 g/m<2>and page, especially 7-12 g/m<2>and page for single coated papers. 13. Roll printing paper according to at least one of claims 1-12, characterized in that the pulp of the base paper consists essentially of pulp (wood-free paper). 14. Roll printing paper according to at least one of claims 1-13, characterized in that the base paper contains up to 18% by weight of mineral filler, based on oven-dried pulp. 15. Roll printing paper according to at least one of claims 1-14, characterized in that the mineral filler is essentially calcium carbonate and/or kaolin. 16. Roll printing paper according to at least one of claims 1-15, characterized in that the base paper contains at least 1.0% by weight oven-dried, strong cationic starch. 17. Roll printing paper according to claim 16, characterized in that the starch content in the base paper is at least 1.3% by weight. 18. Use of roll printing paper according to at least one of claims 1-17, for printing by coldset-offset printing process, in particular a rotary newsprint press.