EP0229562B1 - Double-layered paper product for printing and process for its manufacture - Google Patents

Abstract

1. Papermaking product comprising two layers based upon cellulosic fibres, a support layer and a covering layer : - the support layer comprises unbleached and/or bleached, recycled or not recycled cellulosic fibres, - the covering layer comprises mineral fillers which impart to it a white pigmentation, in a proportion of from 25 to 50% by weight of dry material in the layer, the remainder being white, long and short cellulosic fibres, with 25% at least by weight of long fibres of a length exceeding 4 mm, - the covering layer has a weight per unit area of from 25 g/m**2 to 70 g/m**2 , - the support layer has a weight per unit area such that the papermaking product has a total weight per unit area of from 120 g/m**2 to 300 g/m**2 , - the covering layer has a degree of whiteness, determined in accordance with the standard NFQ 03.039, of at least 70, - the covering layer has a suitability for flexographic printing determined by measuring optical density by means of a MACBETH RD 914 densitometer on a product printed in blue with a strong screening greater than 0.5.

Description

The present invention relates to the manufacture of papers for packaging having good printability. It aims in particular to supply papers in fields of application where such a property is required without correlatively excellent mechanical properties being required.

So-called high-end papers are known which have a very good surface condition, good printability, and high mechanical resistance characteristics. These are, for example, papers intended for offset or gravure printing. Papers are also known, the surface condition of which is not very uniform and the mechanical properties are high, and which at the same time have poor printability. These are, for example, papers of the kraft paper type intended to serve as various packaging, in particular carrying boxes.

The papers used in the packaging field are printed by flexography. According to this process, ink is added to a plate using a cylinder, the plate being used in turn to print the paper. In this type of process, it may happen that the ink migrates into the paper instead of remaining on the surface, which causes dimensional instability of the paper and leads to poor printing. Generally this characteristic does not harm the applications envisaged.

However, in the field of packaging or covering cardboard boxes, in particular, there is currently an effort to present packaging that is pleasing to the eye in order to attract more customer attention. It has been found that white packaging with all kinds of printing patterns is particularly appreciated. To achieve this result, a good printing impression of the packaging is necessary. However, a paper used in such an application need not necessarily have exceptional mechanical characteristics. As an example, we can cite advertising displays for food products by which we seek to capture the attention of customers; However, the only products currently available on the packaging market are those indicated above, of the printing and writing paper type, which are particularly expensive. There is currently no paper known having both a very good surface condition, good printability, and low mechanical properties, while retaining a white color.

Known from the document JA Bristow et al "Multilayer Structures in printing papers", Zellstoff papier 32, n ° 6: 248-253 multilayer papers for printing comprising coating layers containing a high percentage of pigment and which are manufactured from a mixture of short and long fibers.

The document Tappi Journal vol 68, n ° 3, March 1985, describes a two-layer paper formed by a support layer and a coating layer containing 40% by weight of pigment, and which has a whiteness index of about 60.

The invention aims to provide a paper which has further improved whiteness characteristics. By paper is meant according to the invention any paper product such as paper, cardboard ...

The paper according to the invention comprises a support layer obtained from unbleached and / or bleached cellulosic fibers, recycled or not, and a coating layer having a mineral filler content giving it a white pigmentation of between 25 and 50% by weight of dry matter of the layer, the rest being long and / or short white cellulosic fibers with at least 25% by weight of long fibers of length greater than 4 mm, the coating layer having a grammage of between 25 g / m2 and 70 g / m2, the support layer having a grammage such that the paper has a total grammage between 120 g / m ² and 300 g / m ² , the coating layer having a degree of whiteness, determined in accordance with the standard NF Q 03 039 at least equal to 70, as well as a suitability for flexographic printing, determined by measuring optical density using a Macbeth RD 914 densitometer on paper printed in blue with a strong screen, upper at 0.5.

The pulp used to form the support layer can be obtained from all kinds of recovery materials, from old recovery cardboard or from cardboard, i.e. boxes made from old paper, or from old recovery logs.

To form the coating layer, all known types of white fibers can be used such as eucalyptus fibers, pine fibers, etc., these fibers can be long or short.

The short fibers tend to provide surface homogeneity characteristics, while the long fibers tend to improve the breakdown characteristics at the folds of the paper. By long fibers is meant fibers of length at least equal to 4 mm. This fold characteristic is defined as follows: it is the tensile force necessary to break a test piece of paper at its fold on a metal edge. Good fold-through characteristics are particularly advantageous in the packaging field.

To obtain even better whiteness characteristics, it is advisable, according to another characteristic of the invention, to add an optical brightener as an ingredient in the manufacture of the paper product according to the invention. This addition can be carried out at a rate of approximately 0.2 to 0.5% by weight of dry matter of the composition of the coating layer.

Among all the mineral fillers capable of imparting white pigmentation to the product, mention may, for example, be made of talc, kaolin, titanium dioxide, etc. The nature and content of the fillers can be chosen and adapted according to the needs of the invention.

The mineral filler content corresponds to a compromise between two requirements: on the one hand, to give the product suitable whiteness characteristics, it must have a minimum value. On the other hand, it is difficult to manufacture a product meeting the characteristics of a paper product when the content of fillers is excessive. During the manufacturing process, the loads may clog the manufacturing fabrics, which results in poor drainage of the sheets. Even if we manage to put the dough in sheets, we do not obtain sufficient properties, in particular of surface resistance.

To further satisfy this compromise, an additional characteristic of the invention consists in forming the coating layer so that it has a mineral filler content of between 35 and 45% by weight of dry matter of the layer.

The choice of mineral fillers depends on the desired properties of the paper. Advantageously, talc will be chosen to obtain a paper whose surface hold is further improved. Kaolin is also good.

To further improve, if necessary the surface hold of the paper, it is advantageous to introduce into the coating layer synthetic or natural binders which help to maintain its internal cohesion. These binders can be chosen from latex, polyvinyl alcohol, acrylates, starch. Preferably, they are present in the coating layer at a content of less than 10% by weight of dry matter of the layer, and preferably between 5 and 8%. Starch is particularly suitable because, as will appear in the following description, it also plays an advantageous role in the implementation of the papermaking process.

The invention also relates to a method of manufacturing a paper product in accordance with the object of the invention.

According to the method, a first layer is formed by pouring a jet from a primary box onto a manufacturing fabric, a second layer is formed by pouring a second jet from a secondary box, the layers are pressed together and heats to a temperature sufficient to obtain a structure having good cohesion.

Various variants of the process can be envisaged: according to a first variant, the coating layer is first formed, then the support layer above the coating layer. According to a second preferred variant, the support layer is first formed and then the coating layer above the support layer. This variant is preferred because if the coating layer is first formed, it is more difficult to maintain the mineral charges in the coating layer during the draining operation. Subsequently, the cohesion of the paper may, while being satisfactory, be less good.

The second layer can be formed on top of the first layer in several ways.

According to a variant, the support layer is first formed, then on the same manufacturing fabric as that used for the formation of the support layer, the coating layer.

According to another variant, the support layer is first formed on a first fabric, the coating layer being formed on a fabric of additional manufacture, then brought to the support layer.

For the implementation of the method, a conventional paper machine is used. This machine comprises a manufacturing table provided with at least two boxes, each of which provides a jet for the formation of a layer, the two boxes being spaced from each other over the same canvas. , or being above two different fabrics, as well as a press section, a drying section and a calender.

We have previously indicated the advantage of the presence of a binder, in particular starch, in the coating layer. This addition of starch is particularly advantageous in the implementation of the process with a single manufacturing fabric.

Indeed, in this variant, the jet of the coating layer is poured onto the support layer. If the fillers do not retain the cellulose fibers sufficiently due to the lack of internal cohesion of the cast layer, the fibers tend to adhere to the presses at the time of pressing. This results in a linting phenomenon of the presses which can eventually lead to their deterioration, and therefore require a more frequent replacement of the pressing members.

The presence of binder therefore improves the implementation of the process by promoting the retention of the fibers in the coating layer. Such a problem does not generally arise in the variant with two manufacturing fabrics since the coating layer is already sufficiently formed when it is brought to the support layer.

The binder can be introduced in two ways: according to a first variant, it is sprayed uniformly on the support layer while it is not completely formed, before the coating layer is poured.

According to another preferred variant, the binder is introduced into the composition jet of the coating layer. This variant makes draining the layers easier, eliminating any risk of blockage of the drip rollers.

Other characteristics and advantages of the invention will appear during the following detailed description of exemplary embodiments of the invention.

The properties of the papers according to the invention are specified with the aid of a certain number of standardized tests which reveal characteristics conforming to the subject of the invention.

1-Wax test

This test, which enables the surface resistance of the paper to be determined, is established in accordance with standard T 459 om 83. Numbered waxes are applied to the surface of the paper. These waxes have adhesion capacities to the paper which increase correlatively with the number assigned to them, and they are torn off to then observe the surface condition of the paper. It is the highest number in a series of waxes which does not alter the surface of the paper which gives the result of the test.

2 - Burst index

It is established in accordance with standard NFQ 03.053. It is the quotient of the maximum pressure distributed uniformly, supported by a test piece of paper, perpendicular to its surface, by the grammage of the conditioned paper.

3 - Bendtsen roughness

It is determined using the air flow method at constant pressure in accordance with standard NFQ 03.049. A paper test piece is applied under a pressure defined by a metal crown against a smooth, flat and hard surface. The air arrives at constant pressure in the center of the crown. Bendtsen roughness is the flow of air passed between the crown and the surface of the paper.

4 - Claauaae with folds

It is the tensile force necessary to break a test piece of paper at its fold on a metal edge.

5 - ISO whiteness (degree of whiteness)

It is determined in accordance with standard NFQ 03.039. The degree of whiteness is the measure of the diffuse reflectance factor in the blue of the paper layer studied, using the perfect reflective diffuser.

6 - Aptitude for flexograic imoression

To measure it, the surface of the paper coating layer is printed in blue by the flexography process. A photopolymer plate is inked using an ANILOX steel cylinder. The face of the paper is printed with water-based ink. The optical density is measured on a paper printed with a strong screen, that is to say having a high density of ink dots, using a densitometer sold under the name MACBETH RD 914.

All the measurements of the characteristics of the paper are carried out under an air-conditioned atmosphere at 20 _° C. at 65% humidity.

• - kaolin : kaolin de grade C commercialisé par la société E.E.C. PAPER CLAYS
• - talc : talc standard O commercialisé par la société des talcs de LUZENAC avec un diamètre moyen de 10 micromètres et délité à 50 %
• - azurant optique : commercialisé par la société BAYER sous l'appellation BLANCOPHOR
• - amidon : amidon de maïs cru commercialisé sous l'appellation ORISOL

Unless otherwise indicated, the following products will be used:

• - Kaolin: grade C kaolin sold by the company EEC PAPER CLAYS
• - talc: standard talc O marketed by the talcs company of LUZENAC with an average diameter of 10 micrometers and 50% disintegrated
• - optical brightener: marketed by BAYER under the name BLANCOPHOR
• - starch: raw corn starch sold under the name ORISOL

The attached tables 1 and 2 group together the characteristics and properties of the paper products obtained.

EXAMPLE 1:

We make a paper with a grammage of 160 g / m ² formed from a support layer with a grammage of 115 g / m ² from old paper and a coating layer with a grammage of 45 g / m2 based on white fibers and mineral fillers.

The pulp used to form the support layer is a pulp made from 100% old recycled kraft paper. It is poured from a primary box onto a manufacturing fabric in the form of an aqueous dispersion at 5 g / l. The fabric advances at a speed of 180 m / minute.

The composition constituting the second jet is poured from a secondary box onto a second fabric. It is an aqueous dispersion at 8 g / l which comprises a mixture of long and short white fibers, the long fibers representing a third of the fibers. It also includes 44% kaolin, expressed by weight of dry matter (which corresponds in the paper obtained to 20 g / m ² of kaolin), as well as the usual papermaking additives.

The paper obtained after pressing, drying at a temperature above 90 _° C. suitable for the correct extraction of the condensates, and calendering, has the characteristics indicated in table 2.

The most significant results concern the degree of whiteness, the value of waxes and the Bendtsen roughness. These last two values make it possible to characterize a good surface condition and to predict a good aptitude for flexographic printing.

The Bendtsen roughness is 480. For comparison, that of dappled paper is 1000 and that of kraft paper is 1800.

EXAMPLE 2:

• - 10 % de vieux papiers kraftés,
• - 20 % de papiers recyclés des caisseries françaises,
• - 70 % de journaux recyclés.

. The procedure is as in Example 1, except that the composition of the support layer is modified, it comprises:

• - 10% of old kraft paper,
• - 20% recycled paper from French cash registers,
• - 70% of newspapers recycled.

In addition, 0.4% by weight of dry matter of an optical brightener is added to the composition of the coating layer.

The presence of the brightener shows an improvement in whiteness and in the ability to flexographic printing.

EXAMPLE 3:

The procedure is as in Example 1, except that starch is added to the composition of the coating layer, at a rate of 6.5% by weight of dry matter to improve the fibrous cohesion of the layer.

There is an improvement in the value of the waxes compared to Example 1, which highlights the role of starch in the fibrous cohesion of the paper.

EXAMPLE 4:

The procedure is as in Example 2, except that the kaolin is replaced by talc.

Talc is 40% by weight of dry matter. The optical brightener has a content of 0.3%.

The results expressed in Table 2 show that by using talc as a filler, and by not using a binder, characteristics comparable to those obtained with kaolin as a filler and starch as a binder are obtained.

EXAMPLE 5:

We make a paper with a grammage of 140 g / m ² formed of a support layer at 85 g / m ² and a coating layer at 55 g / m ² as follows: we cast on a canvas manufacturing running at a speed of 160 m / min., from a primary box, an aqueous dispersion at 8 g / I comprising 100% of old kraft paper.

• - un mélange de fibres blanches longues et courtes dans un rapport tel qu'il y ait un tiers de fibres longues
• - 40 % en poids de matière sèche (22 g/m² dans la couche de revêtement) de kaolin
• - 6,5 % en poids de matière sèche d'amidon

The coating layer is then formed by pouring onto the support layer from a secondary case of an aqueous dispersion at 8 g / l which comprises:

• - a mixture of long and short white fibers in a ratio such that there is a third of long fibers
• - 40% by weight of dry matter (22 g / m ² in the coating layer) of kaolin
• - 6.5% by weight of starch dry matter

The results obtained show that compared to the process with two fabrics with the same compositions, the degree of whiteness and the value of waxes are slightly lowered.

EXAMPLE 6

The procedure is as in Example 5 except that the kao lin is replaced by talc. It is found that the degree of whiteness and the value of wax are better with talc than with kaolin.

EXAMPLE 7:

The procedure is as in Example 6, except that an optical brightener is added. It is found, as with the two-web process, that the presence of a brightener improves the whiteness properties and the ability to flexographic printing.

COMPARATIVE EXAMPLES EXAMPLE 8:

In this comparative example, the paper is made according to the 1-canvas method under the conditions described above in Example 7 except that no filler or binder is used. The total grammage is 140 g including 65 g for the coating layer.

The degree of whiteness obtained is 65 and the printability of 0.45.

These results show a marked deterioration in the whiteness and aptitude characteristics for flexographic printing.

EXAMPLE 9:

The procedure is as in Example 8, except that 6% of charges are put. A degree of whiteness of 67 is obtained and a printability of 0.49, which is less good than for the papers according to the invention.

Claims (13)

1. Papermaking product comprising two layers based upon cellulosic fibres, a support layer and a covering layer:

- the support layer comprises unbleached and/or bleached, recycled or not recycled cellulosic fibres,

- the covering layer comprises mineral fillers which impart to it a white pigmentation, in a proportion of from 25 to 50% by weight of dry material in the layer, the remainder being white, long and short cellulosic fibres, with 25% at least by weight of long fibres of a length exceeding 4 mm,

- the covering layer has a weight per unit area of from 25 g/m² to 70 g/m²,

- the support layer has a weight per unit area such that the papermaking product has a total weight per unit area of from 120 g/m2 to 300 g/m2,

- the covering layer has a degree of whiteness, determined in accordance with the standard NFQ 03.039, of at least 70,

- the covering layer has a suitability for flexographic printing determined by measuring optical density by means of a MACBETH RD 914 densitometer on a product printed in blue with a strong screening greater than 0.5.

2. Product according to Claim 1, characterized in that the covering layer comprises said mineral fillers in a proportion of from 35% to 45% by weight of dry material of the layer.

3. Product according to Claim 2, characterized in that talc is used as mineral filler.

4. Product according to one of Claims 1 to 3, characterized in that the covering layer comprises an optical blueing substance in a proportion of 0.2% to 0.5% by weight of dry material in the layer.

5. Product according to one of Claims 1 to 4, characterized in that the covering layer comprises a synthetic or natural binder.

6. Product according to Claim 5, characterized in that said binder is present in a proportion of less than 10%, and preferably of from 5 to 8% by weight, of dry material in the layer.

7. Product according to Claim 6, characterized in that said binder ris starch.

8. Method of making a papermaking product according to one of Claims 1 to 7, according to which a first layer is formed by pouring, onto a forming wire, a jet applied by a primary headbox, a second layer is formed from a second jet applied by a secondary headbox, the layers are pressed together, and heating is carried out to a temperature sufficient for obtaining a structure having good cohesion.

9. A method according to Claim 8, characterized in that the support layer is first formed and then the covering layer over the support layer.

10. A method according to Claim 9, characterized in that the covering layer is formed by pouring onto the same production wire as was used for the forming of the support layer.

11. A method according to Claim 9, characterized in that the covering layer is formed by pouring onto a supplementary production wire, and is then brought onto the support layer.

12. A method according to one of Claims 9 to 11, characterized in that, for making a product according to one of Claims 5 to 7, said binder is atomized uniformly onto the support layer while it is not yet entirely formed.

13. A method according to one of Claims 10 to 12, characterized in that, for making a product according to one of Claims 5 to 7, said binder is introduced into the composition jet for the covering layer.