Cellulose sheet packaging material with transparency obtained by coating

Abstract

A sheet packaging material comprising a cellulose layer covered with a coating layer that renders the cellulose layer transparent, the cellulose layer having a basis weight of between 18 g / m² and 75 g / m², containing between 10% and 60% short fibers, having a transverse tear strength of at least 138 mN, and a forward tear strength of at least 129 mN. The coating layer comprises vegetable wax, animal wax or fat, mineral paraffin, or a mixture of at least two of these. A method and apparatus for coating a cellulose-coated paper strip to form the packaging material. A bag and a packaging box comprising such a material.

Description

[0001] The present invention relates to the manufacture and use of sheet materials having a certain transparency that is at least partially localized, more particularly intended for the field of packaging, and more specifically for the packaging of food products. The paper which falls within the scope of the invention is mainly made of cellulose, the transparency then being obtained by coating. The invention also relates to a corresponding coating method, a device for coating paper strips with a view to constituting the materials according to the invention, as well as packaging made of said material.

[0002] The invention responds to a growing need expressed by the food industry, primarily large-scale retailers, to provide visibility to bagged food products. The idea, of course, is for consumers to have informative visual access to what they are purchasing. Moreover, with growing awareness of environmental issues, both customers and manufacturers want food packaging that is environmentally friendly.

[0003] However, the current range of bagged packaging that leaves the product at least partially visible offers either plastic bags or cellulose packaging with a plastic window. However, in the environmental logic of reducing the share of plastic in packaging advocated nowadays, the trend is to replace fossil plastic windows with windows made of bio-plastic or made of glassine-type materials, also known as crystal paper.

[0004] However, the use of these materials is expensive, due to the economic value of the material itself, which is two to three times more expensive to purchase than simple paper, but also because of the costs associated with the manufacturing of packaging, which requires combining it with conventional paper in the manufacturing processes. The need to set up separate supply chains also contributes to the higher price of packaging.

[0005] The aim of the invention is to create a packaging material made entirely of paper, therefore mono-material, with the possibility of a transparent area on at least a fraction of the surface of the packaging. Such a product therefore meets new demands due to the renewable nature of cellulose allowing complete recyclability of the packaging. In other words, it is a question of creating a cellulose-based material which is capable of presenting total or partial transparency, on at least part of the packaging. The uniqueness of the material gives packaging enormous added value in practice, particularly but not exclusively in the food sector.

[0006] It should be noted that the coating of paper to obtain a more or less successful transparency is already known, for example described in document CN112853816A which discloses the manufacture of a transparent paper used to manufacture food packaging bags, or to make windows to be placed on food packaging boxes, from kraft paper coated with a preferably edible oil. CN203753586U discloses a glassine paper bag with a window formed by locally making transparent an area of ​​a sheet of paper from which said bag is made. It should be noted that the base material of the bag is already translucent, and that the more transparent window is obtained by applying a wax coating to the glassine paper.

[0007] JP19890130188 describes the coating of a specific area of ​​an opaque or translucent base paper with an oily solvent of the paraffin type or with a silicone-based solution, so as to transform this area into a transparent surface. FR628956A discloses a process for manufacturing transparent papers intended in particular for the packaging of food products, according to which the paper to be made transparent is coated with a gelatin solution or the like. The process consists more precisely in coating the paper with a gelatin solution which, after drying, then becomes transparent. Depending on the degree of transparency required, the gelatinous solution can be applied to one or both sides of the paper.The use of glassine or crystal paper for the same purposes also results from the description of GB323302, the paper then being impregnated with a solution of castor oil or the like and a solvent of the methylated spirit type containing an aromatic hydrocarbon such as benzol.

[0008] The principle of impregnating or coating paper to make it transparent has therefore been known for a long time, since most of these documents are old. However, in some of the above cases, the paper used is glassine or crystal paper, the inherent cost of which, on the one hand, and the need to combine it with other types of paper for the manufacture of the final packaging, on the other, constitute prohibitive obstacles to the invention.

[0009] Document FR 3 008 392 does disclose a paper consisting of a coated cellulose layer, but this invention is more particularly focused on the coating, in this case the weight of the induction layer and the fact that it comprises a vegetable oil, the characteristics highlighted for the cellulose layer being limited to the proportions of long and short fibers. However, the most important part of the transparentized sheet materials of the field of the invention obviously remains the paper layer, which is the main substrate secondarily supporting the coating layer.

[0010] However, in order to present a good transparency / solidity compromise, essential for the many current applications, the appropriate papers must be selected in such a way as to present suitable technical properties, particularly mechanical ones. Before any work on the coating, it is therefore necessary to propose a paper which presents a combination of technical characteristics making it certainly suitable for being coated, but above all presenting an adequate solidity / transparency / flexibility compromise.

[0011] This is the objective of the present invention, which proposes in particular to optimally combine the technical characteristics of the elements entering into the composition of the packaging materials, so that they have on the one hand a solidity compatible with their use as packaging for example of food products and on the other hand that they can be given a sufficient degree of transparency so that the packaged products are clearly visible.

[0012] To this end, the invention firstly relates to a sheet packaging material comprising a cellulose layer covered with a coating layer covering said cellulose layer to make it transparent, characterized in that the cellulose layer: has a grammage of between 18 g / m 2< and 75 g / m 2<; contains between 10% and 60% short fibers; has a tear strength in the cross direction of between 138 mN and 250 mN; and has a tear strength in the running direction of between 129 mN and 220 mN.

[0013] In terms of strength, a low-grammage paper is certainly less strong than a high-grammage paper, but it is also less opaque. The choice of grammage, with the aforementioned compromise in mind, is therefore important. Similarly, papers with long fibers are certainly stronger than papers with short fibers, but the ratio between long and short fibers has an impact on the opacity of the paper, and it is therefore not conceivable to do without short fibers, at the risk of sacrificing transparency for strength.

[0014] To find the paper most suitable for becoming transparent through coating without neglecting its suitability for transporting food or other goods, it is important to also consider certain mechanical properties of the paper, such as its resistance to tearing in the running and cross directions, its resistance to bursting, or its resistance to elongation in the running and cross directions. Running and cross directions in practice refer to the continuous web production of the paper, the web being then wound into reels for storage purposes. In this perspective, the running direction is the unwinding direction of the reel ('long grain' in English), i.e., the axial direction of the web. The cross direction refers to a perpendicular, rather transverse, direction.

[0015] Furthermore, preferably, the cellulose layer should have a cloudy sheen, the corresponding structure of the paper being particularly suitable for obtaining transparency where the coating layer is applied. Sheen designates an aspect of the structure of the paper observable by transparency, well known to those skilled in the art. Sheen can thus be described as regular or melted, irregular or cloudy, depending on the more or less homogeneous distribution of the fibers: the qualifier attributed to sheen is immediately recognizable to the person skilled in the art who examines it. It is cloudy when the distribution is irregular and flaky. In other words, a cloudy sheen means that the internal aspect of the cellulose layer is cloudy when observed by transparency in front of a light source.

[0016] According to the invention, the coating layer comprises vegetable wax, or animal wax or fat, or mineral paraffin, or a mixture of at least two of them. Unlike many known configurations, where the coating is used to improve the preservation characteristics of the packaging, its essential function - within the framework of the invention - is to provide transparency to all or part of said packaging.

[0017] Preferably, the coating layer has a weight of between 1 g / m 2 and 10 g / m 2, more preferably between 4 g / m 2 and 8 g / m 2.

[0018] The invention also relates to a method for manufacturing sheet packaging material as presented above, i.e. essentially consisting of a cellulose layer coated with a coating layer, the method being such that the temperature for depositing said coating layer on the cellulose layer is between 50°C and 200°C or, preferably, between 60°C and 180°C.

[0019] In reality, the application temperature depends on the nature of the coating material (wax, paraffin, grease or oil). It can vary to ensure good transparency and usability of the packaging at room temperature, or even at a temperature slightly higher than normal.

[0020] In order to set the coating layer, according to the method of the invention, the cellulose layer covered with the coating layer is cooled at least once to between 0°C and 10°C, preferably between 2°C and 6°C. This cooling is carried out by means of a cooling system comprising one or more successive cooling stages, making it possible to maintain the bond between the coating layer and the cellulose base layer.

[0021] A device is also provided, according to the invention, for coating a continuous strip of paper consisting of a layer of cellulose, characterized in that it comprises: two coating rollers between which the strip is driven for applying the coating liquid to one side of the strip, one of said coating rollers being partially immersed in a tank of vegetable wax, or animal wax or fat, or mineral paraffin, or a mixture of at least two of them, brought to a temperature between 50°C and 200°C; at least one cooling roller placed downstream of the coating rollers, in the direction of travel of the strip, said roller being brought to a temperature between 0°C and 10°C.

[0022] The known devices of the prior art do not disclose this configuration, but propose very different structures. Thus, GB910341 and US3194706 describe installations which require total immersion of the paper web, so that both sides of said web are coated. Furthermore, in the configuration of GB910341, the two guide rollers are partially immersed in the coating liquid, unlike the present invention. Finally, GB894426 describes a system in which the web is not driven between two rollers, at least one of which is immersed, since none of the rollers between which the web is driven dips into a tank.

[0023] According to one possible configuration, a printing stage for the paper web can be arranged upstream of the coating rollers. The cellulose coating is generally carried out after the total or partial printing of the cellulose layer.

[0024] Finally, the invention also relates to bag-type packaging made of packaging material according to the characteristics described above, which comprise at least one portion covered with a coating layer forming a transparent area. The manufacture of bags can then be carried out in different forms, for example by gluing a panel of the packaging material as described, that is to say in fact at least one layer of cellulose coated with the coating layer of the invention on another panel of the material, coated or not, so as to form a bag, said bag of course comprising at least one opening.

[0025] Alternatively, but not limited to, the packaging concerned may be a cardboard frame box comprising at least one wall made of sheet packaging material according to the characteristics described above, which is then such that said at least one wall comprises at least one portion covered with a coating layer forming a transparent zone.

[0026] The invention as described has the overriding advantage of allowing manufacturing without the need for transparent paper supply chains, which leads to a significant reduction in production costs. The technical characteristics of the two layers forming the packaging material are also such that the degree of transparency achieved can exceed 60%, reach 80%, or even 90% for certain combinations of characteristics. In most cases, only a portion of the packaging is made transparent, that is to say, the coating of the covering layer is applied by precisely delimiting it, so as to create a window in a bag or in a package which otherwise remains opaque.

[0027] The invention, as a whole, eliminates the need to separate a polymer window from the cellulose fiber part for the recycling and / or composting of bags and packaging, which greatly facilitates these operations.

[0028] Other aims and advantages of the present invention will appear during the description which follows relating to embodiments which are given only as indicative and non-limiting examples.

[0029] Understanding of this description will be facilitated by referring to the attached drawings, for which:

[0030] There figure 1 represents a schematic view of a production line for a paper packaging material comprising a cellulose layer which is coated with an additional layer as defined above, giving it at least localized transparency allowing consumers to see the packaged product;

[0031] There figure 2 shows a schematic view of a bag according to the invention, one of the faces of which is provided with a transparent window obtained according to the invention.

[0032] The device appearing in figure 1 comprises successive rollers which guide and drive the cellulose strip 1 towards a tank 2 containing a coating product 3 brought to a sufficient temperature so that it is in liquid form. This coating temperature varies according to the nature of the coating product, which may be of vegetable, animal or mineral origin and, more particularly, may consist of wax, paraffin, fat, oil etc. As indicated by the cartridge present in the tank 2, said temperature may vary between 50°C and 200°C. It is ideally between 60°C and 180°C, in order to guarantee good transparency of the two-layer material, and consequently good usability of the bags, boxes, or other packaging at room temperature or a little higher.

[0033] In practice, two rollers 4, 5 ensure both the application of the coating liquid on one side of the continuous cellulose strip 1 - since only the roller 4 is immersed in said coating liquid - and the guidance of said strip 1.

[0034] In order to perpetuate the coating, that is to say to guarantee that the coating layer remains well fixed on the cellulose layer, a cooling system is necessary. This system is in this case composed of two cooling rollers 6, 7 brought to a temperature between 0 and 10°C (see in particular the cartridges on said rollers 6, 7) and, more particularly, to a temperature between 2 and 6°C.

[0035] Input rollers 10, 11 and output rollers 20, 21 complete the device, and allow the cellulose strip 1 to be guided respectively towards the input tray 2, and the two-layer strip towards the output and possibly other processing stations, for example a station for cutting out sections of paper adapted to the type of packaging to be manufactured, and / or a gluing station. Note that if a printed decoration must be applied to the paper, and / or information printing, this is done upstream of the coating rollers 4, 5, at or upstream of the input rollers 10, 11.

[0036] Bag 30 of the figure 2 , obtained according to the characteristics of the invention, takes the form of a flattenable pouch which can be expanded due to the existence of lateral gussets 31, 32 and a base 33 of substantially the same width, which can be folded against one of the two main panels 34, 35 forming said bag 30. One of these two panels 34, 35 comprises a window 36 obtained by coating with a layer of wax or varnish according to the invention.

[0037] It should be noted that the characteristics inherent in the present invention do not alter the standard characteristics of packaging or bags intended to contain food products, such as that presented in figure 2 , namely: burst strength test: at least 70 kPa; permeability test: at least 14s / 100ml; humidity resistance: from 0 to 50% as appropriate.

[0038] Ultimately, the present invention makes it possible to make visible by transparency all or part of the contents of a packaging made of a 100% cellulose-based material such as for example a bag, or which includes a part with such a material, or a box of which a panel is made of it. In other words, it makes it possible to produce, for example, bags or boxes of which at least a portion has a visibility approaching that of a plastic packaging, that is to say which has a degree of opacity of less than 40%, which can be lowered to 20%, knowing that it is even possible to reach 10%, which is moreover the preferred objective. In such packaging, the polymers are completely eliminated, which makes them easily recyclable or compostable. This type of packaging leads to a reduction in packaging waste due to its mono-material nature.

[0039] The invention described is not limited to the examples given, in particular with reference to the figures, but it includes in particular the variations in shape made for example to the bag of the figure 2 .

Claims

1. Sheet packaging material comprising a cellulose layer covered by a coating layer that coats said cellulose layer to render it transparent, characterized in that the cellulose layer: - has a grammage of between 18 g / m2 and 75 g / m2, - contains between 10% and 60% short fibers, - has a tear resistance in the short grain direction of between 138 mN and 250 mN, and - has a tear resistance in the long grain direction of between 129 mN and 220 mN.

2. Sheet packaging material comprising a cellulose layer covered by a coating layer according to claim 1, characterized in that the cellulose layer has a cloudy formation.

3. Sheet packaging material comprising a cellulose layer covered by a coating layer according to any of the preceding claims, characterized in that the coating layer comprises vegetable wax, or animal wax or fat, or mineral paraffin, or a mixture of at least two thereof.

4. Sheet packaging material comprising a cellulose layer covered by a coating layer according to the preceding claim, characterized in that the coating layer has a weight of between 1 g / m2 and 10 g / m2.

5. Sheet packaging material comprising a cellulose layer covered by a coating layer according to claim 3, characterized in that the coating layer has a weight of between 4 g / m2 and 8 g / m2.

6. Paper coating method for manufacturing sheet packaging material consisting of a cellulose layer coated with a coating layer according to the preceding claims, characterized in that the temperature at which said coating layer is deposited on the cellulose layer is between 50°C and 200°C.

7. Paper coating method consisting of a cellulose layer coated with a coating layer according to the preceding claim, characterized in that the temperature at which the coating layer is deposited on the cellulose layer is between 60°C and 180°C.

8. Paper coating method for manufacturing sheet packaging material consisting of a cellulose layer coated with a coating layer according to one of claims 6 and 7, characterized in that the cellulose layer coated with the coating layer is cooled at least once to between 0°C and 10°C.

9. Paper coating method for manufacturing sheet packaging material consisting of a cellulose layer coated with a coating layer according to claim 8, characterized in that the cellulose layer coated with the coating layer is cooled at least once to between 2°C and 6°C.

10. Device for coating a continuous paper web (1) consisting of a cellulose layer according to any of the preceding claims, characterized in that it comprises: - two coating rollers (4, 5) between which the web (1) is driven in order to apply the coating liquid to one face of the web, one of said coating rollers (4, 5) being partially immersed in a tank (2) of vegetable wax, or animal wax or fat, or mineral paraffin, or a mixture of at least two thereof, brought to a temperature of between 50°C and 200°C; - at least one cooling roller (6, 7) placed downstream of the coating rollers (4, 5), in the direction of travel of the web (1), said roller (6, 7) being brought to a temperature of between 0°C and 10°C.

11. Device for coating a continuous paper web (1) consisting of a cellulose layer according to the preceding claim, characterized in that a printing stage for the paper web (1) is arranged upstream of the coating rollers (4, 5).

12. Bag-type packaging made of sheet packaging material according to any of claims 1 to 5, characterized in that it comprises at least one portion covered by a coating layer forming a transparent zone.

13. Cardboard frame box comprising at least one wall made of sheet packaging material according to any of claims 1 to 5, characterized in that said at least one wall comprises at least one portion covered by a coating layer forming a transparent zone.

Images