FI81158B - PAPPER ELLER PAPP AVSEDDA FOER OFFSETTRYCKNING. - Google Patents

Abstract

1. A method of making paper or cardboard suitable for offset printing, for example rotary offset printing, in which the paper or cardboard in the form of a ribbon is subjected to at least one coating operation on at least one of its surfaces and then dried and optionally calandered before being wound, characterised in that drying after coating is carried out in two stages, the first stage before calandering, cutting, rewinding or similar treatment being limited so that the paper is left with a high water content, and the second stage after this treatment reducting the water content to a desired value.

Description

1 81158

Offset printing paper or paperboard

The invention relates to cellulosic fiber-based papers or paperboards and, more particularly, to a new method for producing papers or paperboards which are well suited for offset printing and in particular for offset rotary and, more specifically, offset roll rotary printing.

The invention also relates to an apparatus for carrying out the process and to papers or cardboard (for the sake of simplicity, only the term "paper" is used hereinafter) produced by said process.

Offset printing, and in particular offset rotary printing, is one of the most current printing methods and also one of those with the highest growth rate. Therefore, the aim is to produce an increasing variety of papers suitable for offset rotary printing, with the most different basis weights and which are either opaque or glossy, and in the latter case only more glossy.

Papers suitable for offset printing are coated papers, that is, papers coated on one or both sides with a compound containing dyes and binders. After coating, the papers are dried to a certain water content, for example 6% by weight of the weight of the paper in papers with a low basis weight, i.e. less than 100 g / m 2. This drying takes place before possible calendering.

This water content is determined in particular by the purpose for which the paper is intended, i.e. either for offset rotogravure printing or offset flatbed printing.

In offset rotary printing, after the paper has arrived from the printing press, the printed paper is quickly dried to evaporate the volatiles in the inks and to cure the inks. During this rapid drying, which takes place by using paper in an oven, which usually has a temperature above 200 ° C, the temperature of the paper rises above 100 ° C and the water it contains tends to evaporate. However, evaporation is prevented by the coating layers and, above all, by the inks used in this type of printing, since water vapor remains inside these layers; it forms blisters there and the paper may even burst. The printed paper is then unusable. This is mainly the case when the water content of the paper is relatively high, and above all when paper is used which is coated with several layers of coating and has a high basis weight: for example, it has been found that this occurs when the amount of water is greater than about 5 grams of water / m paper with a porosity of about 120 seconds. This porosity is measured with a BEKK device, which measures how fast 10 cm 2 of air passes through 2 1 cm of paper at 0.5 atmospheric vacuum. Naturally, when the paper is more porous, that is, when its porosity is less than 120 seconds, this damage may occur when the 2 water content is greater than 5 grams / m of paper. Conversely, if the paper is less porous, even a lower water content may be the cause of this damage.

In offset flatbed printing, drying after printing usually takes longer and takes place without large thermal calipers and without blistering and bursting of the paper.

Thus, in order to avoid any blistering, it has been decided to dry the paper sufficiently after coating and before possible calendering, i.e. to dry it to such a low water content as is necessary for suitability for offset rotation weight.

There are several disadvantages associated with this vigorous drying.

Drying is applied to the entire paper web, which may be intended for multiple uses, for which the required degree of drying 3 81158 may vary. Usually, several narrower rolls are formed from one wide roll of paper at the splitting point. Thus, for example, three rolls of 1 m wide for offset rotation printing and one roll of 65 cm wide for offset flat weight can be formed from a paper roll 3.70 m wide.

In this case, the water content of the paper going to the offset flatbed printing is the same as that of the paper intended for offset rotary printing, i.e. a small water content which is not necessary in the offset flatbed printing. This wastes quite a significant amount of energy.

In addition, the low water content of the papers prior to calendering is disadvantageous for obtaining highly glossy paper. Gloss is caused by several different factors. The first factor is the composition of the coating compound used, comprising rock dyes such as kaolin, glossy white or calcium carbonate, the grain composition and shape of which are suitable for use. Another factor that affects the gloss of papers is calendering, which under the influence of compression and temperature gives the papers a good smoothness with associated gloss. However, a stronger gloss can only be achieved if the third factor, which is the water content of the papers before calendering, is also taken into account. If the water content is less than 6% before calendering, only a low gloss is obtained, i.e. only a gloss of about 50, measured by a Gardner device which measures the percentage of light reflected from the surface of the paper and the light directed at the surface with an input power of 100 , at a setting angle of 75 °.

In order to obtain a high-gloss paper with a gloss of, for example, 70, the water content of the paper must be greater than about 6% by weight of the paper before calendering. But it has already been stated above that paper with a high water content is not well suited for offset rotary printing.

4,81158

However, it has been proposed to change the drying step after offset printing by replacing the sudden drying with a gentler drying so that the water contained in the paper evaporates slowly and gradually. In this case, however, the printing speed is slowed down and / or the drying oven has to be extended, which takes up considerable space; in addition, this type of drying consumes more energy.

The present invention avoids said disadvantages and proposes a new method of making papers (or paperboards) which makes them suitable for offset printing, both offset rotogravure printing and offset flatbed printing, and which saves energy and is as suitable for opaque surfaces as glossy and highly glossy papers.

According to the invention, a paper suitable for offset printing is produced by successively carrying out the following steps: - preparing a paper comprising a cellulosic fiber-based paper base in a manner known per se, - coating at least one side of the paper with at least one coating compound containing dyes and binders, remains large, - the coated paper is optionally calendered, - dried a second time, after possible calendering, to reduce the water content to the desired value.

The first drying prior to calendering is a limited drying that leaves the water content of the paper in the range of greater than about 6% by weight and about 7% by weight, resulting in a highly glossy paper if necessary. In addition, this limited drying is usually sufficient for paper intended for offset flatbed printing.

The purpose of the drying according to the second invention, which is carried out after possible calendering, i.e. after the paper has come from the last press roll, is to reduce the water content to the desired value, i.e. to a certain value, after which there is no risk of blistering or bursting of the paper. during. Said water content of coated paper having a porosity of 120 seconds on both sides has a value of about 5 g / m 2. This value may be higher if the paper is more porous and lower if the paper is less porous.

Preferably, the second drying is performed on only a part of the paper web, for example the part intended for offset rotary printing.

Preferably, the second drying is performed after splitting, and preferably at the splitting site itself, immediately after the paper web has been cut lengthwise into several widths to form paper rolls for various uses, namely rolls for offset rotary printing and offset flat weight. In this case, the second drying is performed only for certain roll widths, especially those intended for offset rotary printing.

Within the framework of the second drying, the drying of each width or group of widths can be carried out in different ways and / or to different degrees, depending on their subsequent use.

The first drying, which takes place after coating and before calendering, can be carried out in a conventional manner by passing a coated paper web between the hot cylinders.

The second drying according to the invention, which takes place after possible calendering, can be carried out, for example, by irradiation such as infrared irradiation or hyperfrequency or by a conduction method, for example by passing the part to be dried through at least one hot cylinder, or by convection using a hot air oven or with a known heating device.

The invention is applicable to all types of coated papers where blistering may occur during the offset rotogravure printing process, regardless of whether they are opaque or glossy. It is particularly suitable for coated glossy papers, which usually have several layers of coating, and above all for papers with a basis weight of more than 100 g / m 2.

The invention is particularly suitable for coated glossy papers intended for offset rotary printing, in which ink and / or several layers of these inks are applied to the surface of the papers on both sides thereof, i.e. papers which are not very permeable to water vapor after printing.

The invention also relates to a device for carrying out the method. This device comprises, in addition to the drying devices located after the conventional coating and before the calendering means, additional drying means arranged after the calender, i.e. after the last press cylinder and preferably flush with the diameter immediately after the step of cutting the coated and possibly calendered paper web to form flat paper rolls.

These heating devices can be arranged only above and / or below the paper webs intended for the offset rotary printing paper roll.

The heating device may be at least one infrared series arranged close to the path of the paper web to be dried. The heating device can also be a hyperfrequency heating device. In one variation, these heating devices consist essentially of at least one metallic hot cylinder through which the paper web to be dried passes. In another embodiment, the heating device is a hot air oven through which the paper to be dried passes.

The following examples illustrate the invention. The first two are comparative examples, while the following are examples according to the invention.

Example 1

A coated paper having a basis weight of 110 g / m 2 is prepared in a conventional manner by applying an aqueous compound containing cellulosic fibers to a paper web and then drying the paper web and coating the cellulosic fibrous layer on both sides with a known coating paper after passing it through hot cylinders. The coated paper is then calendered when it still contains about 7% by weight of water. The calendering takes place at a temperature of about 50 ° C and a linear compression of about 150 kg / cm. After calendering, the water content is just over 6% by weight and the gloss of the paper web, measured with a GARDNER, is 70. The paper web, which is 3.70 m wide, is then split into 3 1 m wide webs which are rolled for offset rotation weight and one into 0.65 cm wide track that is rolled for offset flat weight. The first three rolls of paper are used for offset rotary printing. After printing, the paper has a lot of blisters at different points, especially where the printing has been performed with several layers of ink and this is the case on both sides of the paper.

Example 2

An offset printing paper having the same basis weight as in Example 1 is prepared in the same manner as above, except that the calendering is performed on paper having a low water content of 8,81158 in the order of 4.5% by weight. The water content of the paper from the calender is about 4% by weight and the gloss, measured in the same way as above, is only about 50. The paper is used for offset rotary printing after splitting. After printing, which is performed under the same conditions, in particular at the same speed as in Example 1, the printed paper is free of blisters.

Example 3

An offset printing paper having the same basis weight as in Example 1 is prepared in the same manner as above except that in the splitting step, when the main web is cut to 4 widths as above, three 1 m wide webs are rapidly dried by passing them through an infrared heating ramp. The purpose of this drying is to reduce the water content to about 4.5 2% by weight, which corresponds to approximately 4.95 g water / m paper.

The resulting paper is rolled. When this paper has been used for offset rotary printing, it does not have the disadvantages of the printed paper of Example 1 after printing. The printed paper is highly glossy and does not exhibit blistering nor onset, not even at those points where there are multiple layers of ink, paper and the same side of either side.

Example 4

Paper for offset printing is prepared as in Example 3, except that the second drying, which is performed only for offset rotation widths, is performed by passing the paper web at a speed of 600 m / min through an oven about 8 m long with hot air at 210 ° C for about 40 m. / s speed. As a result of this drying, the water content is reduced to approximately 4.4% by weight, which corresponds to about 4.85 g water / m paper.

When this paper is used for offset rotary printing under the same conditions as above, the printed paper does not show

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9 81158 defects such as blisters or bursts.

Example 5

An offset printing paper having a basis weight of 2 g / m 2 is prepared in the same manner as in Example 1.

Before calendering, the water content of the paper is about 6.5%, which decreases to 6% after calendering. The resulting high-gloss paper is split as above and dried rapidly by contact with a metal cylinder heated to about 250 ° C until it has a water content of about 2 3.5% by weight of the paper, corresponding to about 4.7 g of water / m paper. The dried paper is rolled. It is then used for offset rotary printing under the same conditions as above. As in Example 3, the printed paper is free of defects such as blisters or punctures.

Example 6

An opaque coated coated paper for offset printing with a basis weight of 135 g / m 2 is prepared by applying an aqueous compound containing cellulosic fibers to a wire and then, after drying the cellulosic layer, coating it on both sides with a known offset printing paper compound. After coating, the entire paper web is dried to a water content of about 7% by passing the web through hot cylinders. The paper web is then rolled and the roll is split (or the web is split directly) into several widths. After splitting, the widths for offset printing only are passed through an oven heated to about 200 ° C and their water content is reduced to about 5% by weight of the paper. The dried paper is rolled. It is then used for f rotogravure printing under the same conditions as above. After printing, the paper is free of defects such as blisters or punctures.

Claims (10)

10 81158 A method for producing papers or paperboard suitable for offset printing, in particular for offset rotary printing, in which the paper or paperboard in the form of a paper web is coated at least once on at least one side, then dried and optionally calendered before being rolled, characterized in that drying is performed after coating, regardless of the secondary drying in calendering, in two steps, the first limited pre-calendering drying, which leaves the paper with a high water content of more than 6%, and a second post-calendering drying, which reduces the water content to the desired value. Method according to Claim 1, characterized in that the second drying is carried out only on that part of the paper web which is intended for offset rotary printing. Method according to Claim 1 or 2, characterized in that the second drying is carried out after splitting on only one or more specified widths of the cut web. A method according to claim 3, characterized in that the second drying is performed at the splitting point immediately after cutting the web. Method according to Claim 4, characterized in that the second drying is different for different widths or groups of widths. Method according to one of Claims 1 to 5, characterized in that the first drying before possible calendering is limited so that it leaves the water content in the order of 7%. n 81158 Method according to one of Claims 1 to 6, characterized in that the second drying reduces the water content of the paper to a value of less than 5 g / m 2 for paper with a porosity of about 120 seconds. Method according to one of Claims 1 to 7, characterized in that it is suitable for high-gloss paper. Apparatus for carrying out the method according to one of Claims 1 to 8, characterized in that it comprises drying means which are arranged after the calendar, i.e. after the last press roll. Device according to Claim 9, characterized in that the drying elements arranged downstream of the calender are at a splitting point.