EP0093052B1 - Paper or cardboard for offset printing - 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

The present invention relates to paper or cardboard based on cellulosic fibers and more particularly to a new process for manufacturing paper or cardboard having good suitability for offset printing and in particular for rotary offset printing, also referred to as web offset.

The invention also relates to a device for implementing the method as well as the paper or cardboard (to simplify only the term paper will be used later) obtained by the implementation of said method.

Offset printing and in particular rotary offset printing is one of the most current printing methods and one of those which knows the highest percentage of increase. This is why we are trying to manufacture papers suitable for rotary offset printing, of increasingly varied types, with very different grammages, and with matt or glossy aspects, and in the latter case glossy higher and higher.

Papers suitable for offset printing are coated papers, that is to say papers coated on one or both sides, using a composition containing pigments and binders. After coating, the papers are dried to a determined water content, for example of the order of 6% by weight relative to the weight of the paper for papers of low grammages, that is to say less than 100 g / m ² . This drying takes place before any calendering.

This water content is determined in particular by the destination of the paper, namely rotary offset printing or offset format printing.

Indeed, during rotary offset printing and after passing through the presses, the printed paper is dried quickly, in order to evaporate the volatile materials contained in the inks and to harden them. During this rapid drying, carried out by passing through an oven brought to a temperature generally higher than 200 ° C., the paper is brought to a temperature higher than 100 ° C. and the water which it contains tends to evaporate. Evaporation is however more or less blocked by the coating layers and especially by the inks used for this type of printing because the water vapor is trapped by these layers; blisters are formed there and the paper can even burst. The printed paper is then unusable. These incidents occur mainly when the water content of the paper is high enough and especially when it is about papers coated with several coating layers and papers of high grammage: we noticed for example that they appear for a quantity of water greater than approximately 5 grams of water per m ² of paper for a porosity of approximately 120 seconds. This porosity is measured by the BEKK device which measures the duration of the passage of 10 cm ³ of air through 1 cm ² of paper under a vacuum of 0.5 atmosphere. Of course for a more porous paper, that is to say with a porosity of less than 120 seconds, these incidents may occur for water contents greater than 5 grams per m ² of paper. In contrast to a less porous paper, a lower water content can be the cause of the incidents cited.

In the case of offset format printing, the drying after printing is generally longer and is carried out without significant contribution of calories and blistering or bursting of the paper not occurring.

In order to avoid any risk of blistering, it was therefore necessary to dry the paper sufficiently after coating and before any calendering, that is to say to dry to a low water content, compatible with offset printing. rotary.

This significant drying shows several drawbacks.

It applies to the whole of a paper tape which can have several destinations for which the required degree of drying can vary. Indeed, it is common to form at the slitting station, from a reel of paper of great width or width, several coils of lower widths. It is thus possible, for example, to form from a reel of 3.70 m in width, three reels of 1 m in width intended for rotary offset printing and a spool of 65 cm in width intended for offset format printing.

In this case, the paper intended for offset format printing has the same water content as that of paper intended for rotary offset printing, that is to say a limited water content, not essential for printing. offset format. This is a significant amount of energy wasted.

In addition, the low water content of the papers before calendering is unfavorable for obtaining a paper with a very shiny appearance. This brilliance is due to several factors. A first factor is the coating composition used, which comprises mineral pigments such as kaolin, satin white, calcium carbonate, having grain sizes and suitable shapes. A second factor to influence the gloss of the papers is the calendering which by the action of pressure and temperature provides a good smoothness to the papers, smoothness which is accompanied by a gloss. However, a higher gloss can only be obtained by taking into account a third factor which is the water content of the papers before calendering. Water contents of less than 6% before calendering allow only a low gloss, that is to say of the order of 50, value measured with the GARDNER device, which measures the flux as a percentage luminous reflected after projection on the paper of an incident luminous flux of value 100, at an angle of incidence of 75 degrees.

To obtain a very shiny paper with a value of 70 for example, it is necessary for the paper to have, before calendering, a water content greater than about 6% expressed by weight relative to the weight of the paper. However, it was noted previously that a paper with a high water content was incompatible with rotary offset printing.

It has been proposed to modify the drying operation after offset printing by substituting for brutal drying, milder drying, in order to obtain a slow and gradual evaporation of the water contained in the paper. In this case, the printing speed is greatly slowed down and / or the drying oven has to be extended and occupies a considerable area; moreover, the energy consumed by this type of heating is greater.

The German patent publication DE-A-2 147 912 describes various treatments of a paper tape and in particular several successive calendering and drying independently of any coating.

The invention overcomes the aforementioned drawbacks and proposes a new process for the production of paper (or cardboard) which makes them suitable for offset printing both in rotary offset printing and in offset format printing, a process which saves money. energy and which is applicable to matt papers as well as to glossy and very glossy papers.

• - on prépare de façon connue en soi un papier comprenant une couche support à base de fibres cellulosiques,
• - on enduit au moins une des deux faces, à l'aide d'au moins une composition de couchage comprenant des pigments et des liants,
• - on sèche de façon limitée la papier après couchage pour lui laisser une teneur élevée en eau,
• - on soumet le papier couché à un calandrage, un refendage, un rebobinage ou un traitement similaire,
• - on sèche une seconde fois, après ce traitement, pour réduire la teneur en eau jusqu'à la valeur désirée.

In accordance with the invention, a paper suitable for offset printing is prepared by the succession of the following steps:

• a paper is prepared in a manner known per se comprising a support layer based on cellulose fibers,
• - at least one of the two faces is coated, using at least one coating composition comprising pigments and binders,
• - the paper is dried to a limited extent after coating to leave it with a high water content,
• - the coated paper is subjected to calendering, slitting, rewinding or a similar treatment,
• - dried a second time, after this treatment, to reduce the water content to the desired value.

The first drying, before calendering, is a limited drying which leaves the paper with a water content greater than about 6% by weight and of the order of 7%, a content which makes it possible to obtain, if necessary, a very shiny paper. In addition, this limited drying is generally sufficient for a paper intended for offset format printing.

The second drying according to the invention located after any calendering, that is to say after the paper has passed over the last pressing roller, has the aim of reducing the water content of the paper to the desired value, ie that is, the determined value from which there is no risk of blistering or bursting of the paper during the rotary offset printing process. In the case of a paper coated on both sides with a porosity of 120 seconds, said determined value of the water content is approximately 5 g / m ² . This value can be higher for more porous paper and lower for less porous paper.

Advantageously, the second drying is applied only to part of the ribbon, for example the part intended for rotary offset printing.

Preferably, the second drying is carried out after slitting and advantageously at the slitting station itself, just after cutting the paper tape into several widths to form spools which can have different destinations, namely spools for rotary offset printing. and reels for offset format printing. In these cases, this second drying is carried out only on specific webs, in particular those intended for rotary offset printing.

In the context of this second drying, it is also possible to dry in a different manner and / or to different degrees, each of the widths or groups of widths, depending on their subsequent use.

The first drying situated after coating and before calendering can be carried out in a conventional manner by passing, for example, ribbon coated over heated cylinders.

The second drying in accordance with the invention, after possible calendering, can be carried out for example by radiation such as infrared radiation, or using a microwave device or by conduction, for example by passing the part of the paper tape to be dried on at least one heated cylinder, or by convection by passage through a hot air oven or by other known heating means.

The invention applies to all coated papers capable of blistering at the time of rotary offset printing, whether they are matt or glossy. It is particularly applicable to glossy coated papers which generally have several coating layers and especially to papers having grammages greater than 100 g _{/ m} ² .

The invention is particularly applicable to glossy coated papers intended for rotary offset printing in which the papers are covered by inks on their two faces and / or by several layers of these inks, that is to say papers which after printing are very slightly permeable to water vapor.

The invention also relates to a device for implementing the method. This device further comprises conventional drying means after coating, arranged before the calender or the similar treatment device, additional drying means arranged after this calender, that is to say after the last pressure roller, and preferably mounted on the slitter, just after cutting the coated and possibly calendered paper tape, which has been transformed into several widths to form spools for rotary offset printing and possibly into other widths to form spools for printing format offset printing. These heating means can be arranged only above and / or below the ribbons forming the spools intended for rotary offset printing.

They can be formed of at least one infrared ramp arranged near the passage of the strip of paper to be dried. It can also be a microwave heating device. In a variant, these heating means consist essentially of at least one heated metal cylinder on which the strip of paper to be dried is applied at the time of its passage. In another variant, these heating means consist of a hot air oven or oven through which the paper to be dried passes.

The following examples illustrate the invention. The first two are controls while the following are in accordance with the invention.

Example 1

A coated paper with a grammage of 110 g / _M ² is prepared in a conventional manner, by pouring an aqueous composition containing cellulosic fibers, then after drying and coating of the two faces of the cellulosic layer formed using a coating composition known for offset printing, capable of providing a very shiny paper, the paper is dried after coating by passing over heated rollers. The coated paper is then subjected to sizing while it still contains about 7% of its weight in water. Calendering is carried out at a temperature of approximately 50 ° C and under a linear pressure of approximately 150 kg per centimeter. After calendering, the water content is slightly more than 6% by weight and the paper tape has a gloss of 70 measured using the GARDNER device. The paper ribbon, which is 3.70 m wide, is then split to form 3 ribbons of 1 m wide which are spooled for rotary offset printing and a ribbon 0.65 cm wide than the 'reel for offset format printing. The first three reels are used for rotary offset printing. After printing, the paper has many blisters in different places, in particular those corresponding to the printing of several layers of ink and this on each of the two sides of the paper.

Example 2

A paper for offset printing of the same grammage as in Example 1 is produced by operating in the same way, except that the calendering is carried out on a paper with low water content, of the order of 4.5% by weight. . At the outlet of the calendering machine, the paper has a water content of the order of 4% and a gloss which, measured as above, only reaches the value of approximately 50. The paper is used after slitting for rotary offset printing. After printing, which is carried out under the same conditions, in particular speed, as in Example 1, the printed paper does not show blistering.

Example 3

We make a paper for offset printing of the same grammage as in Example 1 by operating in the same way, but, at the slitting station when the main ribbon has been cut into 4 widths as before, we submit the papers of the three widths of 1 m wide, rapid drying by passing between infrared ramps. This drying has the effect of reducing the water content to a value of approximately 4.5% by weight, which corresponds to approximately 4.95 g of water per m ² of paper.

The paper obtained is wound up. Used for rotary offset printing, it does not exhibit, after having been printed, the defects of the printed paper of Example 1. The very glossy printed paper is free from blisters or bursts, even in the places corresponding to several layers of ink, on the same side of the paper or even on both sides.

Example 4

A paper for offset printing is manufactured as in Example 3, except that the second drying of the only webs intended for rotary offset printing is carried out by passing at a speed of the order of 600 m / min in an oven d '' a length of approximately 8 meters, with hot air brought to approximately 210 ° C and circulating at a speed of the order of 40 m / s. This drying has the effect of reducing the water content to a value of approximately 4.4% by weight, which corresponds to approximately 4.85 g of water per m ² of paper.

Used in rotary offset printing under the same conditions as above, the printed paper does not have any defects such as blistering or bursting.

Example 5

A coated paper for offset printing with a grammage of 135 is produced in the same way as in Example 1. Before calendering, the paper has a water content of approximately 6.5%, which decreases to 6%. after calendering. The paper obtained, very shiny, is split as above and undergoes rapid drying by contact on a metal cylinder heated to approximately 250 ° C. until its water content is approximately 3.5% by weight of paper, which corresponds to approximately 4.7 g of water per m ² of paper. The dried paper is wound up. It is then used for rotary offset printing under the same conditions as above. As in Example 3, the printed paper is free from defects such as blistering or bursting.

Example 6

A matt coated paper is produced for offset printing with a grammage of 135 by pouring an aqueous composition containing cellulose fibers, then after drying, coating of the two faces of the cellulosic layer formed using a coating layer known for offset printing and likely to provide matte paper. After coating, the entire paper tape is dried to a water content of approximately 7%, by passing the tape over heated rollers. The ribbon is then wound up and the spool split (or the ribbon is split directly) in several widths. After slitting, only the webs intended for rotary offset printing pass through an oven heated to approximately 200 ° C. and their water content decreases to a value of approximately 5% by weight of the paper. The dried paper is wound up. It is then used for rotary offset printing under the same conditions as above. After printing, the paper is free from defects such as blisters or bursts.

Claims (12)

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 calan- dered 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 reducing the water content to a desired value.

2. A method according to claim 1, characterised in that the second drying is carried out only on the part of the ribbon intended for rotary offset printing.

3. A method according to claim 1 or 2, characterised in that the second drying is carried out only on one or more selected widths of the cut ribbon.

4. A method according to claim 3, characterised in that the second drying is carried out at the cutting station just after cutting of the ribbon.

5. A method according to claim 4, characterised in that the second drying is different from one width to another or from one group of widths to another group.

6. A method according to one of claims 1 to 5, characterised in that the first drying is limited to leave the paper with a water content greater than 6% and of the order of 7%.

7. A method according to one of claims 1 to 6, characterised in that the second drying reduces the water content of the paper to a value less than 5 g/m² for a paper having a porosity of about 120 seconds.

8. A method according to one of claims 1 to 7, characterised in that it is applied to a very glossy paper.

9. Apparatus for carrying out a method according to any one of claims 1 to 8, that is to say a method of making paper or cardboard suitable for 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 surface and is then dried and optionally calan- dered before being wound, characterised in that it comprises first drying means arranged after the coating and before the calander or similar treatment device and second drying means arranged after the calander or similar device.

10. Device according to claim 9, characterised in that the drying means arranged after the calender are at the cutting station.

11. Paper or cardboard suitable for offset printing, such as rotary offset printing, obtained by use of a method according to any one of claims 1 to 8.

12. Printed paper or cardboard, obtained from paper or cardboard according to claim 11.