REPLACEMENT OF A FILM/PAPER COMPOSITE BY SOLELY FIBER-BASED PAPER

Description

TECHNICAL FIELD

The invention relates to a printable paper.

BACKGROUND

Bags or sacks are nowadays often made from a film-paper composite, whereby the composite is folded during bag or sack production.

In such a composite, the paper comprises the strength optimized for the respective purpose and also bears the overprint essential for marketing. In order to achieve the highest possible quality of the printed image, such a paper comprises a so-called “coating”. This coating is usually an application of a viscous ink that dries to a thin layer and consists primarily of a high proportion of minerals (coating pigments). The coating defines the color of the unprinted paper and forms the surface on which the overprint is printed.

In this composite, the plastic film that covers the overprint has a protective function for the overprint. The film prevents damage to the overprint caused by physical and/or chemical environmental influences, e.g. within the scope of its capabilities. It also ensures that the coating cracking often observed when folding coated paper, i.e. during bag or sack production, is prevented. Coating cracking is understood in specialist circles to mean the cracking of the coating along or in the vicinity of the fold or the fold edges, which leads to the visibility of the typically white paper in the overprint at the cracking points. This is why coating cracking is also known as stress whitening. Coating cracking is easily visible and gives the viewer of the bag or sack an impression of inferior quality.

Against this background, the invention has set itself the task of creating an improved paper, in particular for bag or sack production.

SUMMARY OF THE INVENTION

This task is solved by a paper according to claim 1. The object of the invention is therefore a paper, in particular a purely fiber-based paper, the surface of which comprises a coating on one or both sides, characterized in that a water-soluble primer layer is applied to the coating.

This task is further solved by a paper packaging according to claim 6. The invention therefore relates to a paper packaging made from a paper according to the invention.

This task is further solved by a finishing method according to claim 9. The invention therefore relates to a finishing method for a paper, in particular purely fiber-based paper, which comprises the following process steps, namely: provision of a paper comprising a coating on one or both sides, and application of a water-soluble primer layer to the coating.

This problem is further solved by a use according to claim 14. The invention therefore relates to a use of a water-soluble primer layer on a coating of a paper, in particular a purely fiber-based paper, on the one hand to provide a water-based separability of the fiber material of the paper from the primer layer and on the other hand to at least reduce the tendency of the coating to break when the paper is used folded.

In a finishing method for paper, one or more desired properties are imparted to a paper through appropriate process steps. The paper with these properties can thus be referred to as finished paper. The finishing method is therefore a manufacturing process in which a paper is converted from an initial state with original properties to an intermediate state or final state with target properties.

The measures according to the invention are accompanied by the advantage that the most ecologically compatible recyclability possible is ensured for the paper because the primer layer as such is water-soluble and can therefore be removed or separated from the fiber material of the paper in the most environmentally compatible manner possible, i.e. preferably without the use of ecologically problematic chemicals, in particular preferably exclusively by means of water. This makes a decisive contribution to ensuring that both the paper as such as well as end product (e.g. bag or sack) made from the paper are recognized as having a high degree of recyclability. The primer layer therefore does not impair fiber recovery.

At the same time, the primer layer, which is also known as a pre-primer, ensures that the coating is protected and at the same time the conditions for further coating application are improved. The primer layer therefore has a dual effect here, namely on the one hand preventing the coating from cracking during the processing of the paper or at least preventing it to such an extent that the coating cracking is not visually perceived as disturbing, and on the other hand creating optimized conditions for a further layer build-up on the primer layer. In relation to the further layer structure, the primer layer thus forms a mechanical interface between the coating and a further layer or layer sequence to be established on the primer layer.

All in all, the water-soluble primer layer has the combined effect of making the paper easier and more environmentally friendly to recycle, on the one hand, and preserving the visually desirable appearance of the end product made from the paper, on the other.

Further, particularly advantageous embodiments and further developments of the invention result from the dependent claims and the following description.

The water-soluble primer layer preferably comprises the property that it covers and binds the surface of the coating after its application and thus stabilizes the coating, in particular its surface, during further processing with respect to the otherwise existing tendency to crack. It is therefore particularly advantageous that the water-soluble primer layer exhibits such plastic properties after its application that the primer layer itself resists fracture without cracking during the expected deformation caused by further processing of the paper, to which the primer layer is naturally also subjected. This also ensures the two-dimensional cohesion of the other layer applied to it, namely so effectively that any fine cracks and the like that may occur are no longer perceived as disturbing by an observer.

These properties can be achieved with a wide variety of polymer-based coatings. However, it has proven to be particularly advantageous for the water-soluble primer layer to be a water-soluble polyvinyl alcohol varnish. Polyvinyl alcohol is usually abbreviated to PVAL or sometimes PVOH. Analogous to the water-soluble property, the positive ecological effect also comes into play here in that water and not an organic solvent is used as a solvent to produce the water-soluble polyvinyl alcohol varnish ready for processing. In addition, the use of the water-soluble polyvinyl alcohol varnish enables a desirable oxygen barrier effect to be achieved in the paper itself, which has a positive effect on the shelf life of food (or other goods that react with oxygen) that are packaged using packaging made from the paper.

To produce the water-soluble polyvinyl alcohol varnish, PVOH granules are processed with water as the solvent, whereby in the following discussion the proportion of PVOH granules and the proportion of water always add up to 100 parts. In tests carried out by the applicant, a ratio of 11 parts (%) PVOH granules and 89 parts (%) water proved to be optimal for processability and prevention of coating cracking. These tests also showed that the desired effect, namely the prevention of coating cracking, can also be achieved for a range of 9 to 11 parts (%) PVOH granules.

With regard to the lower limit of the range, it was observed that too low a proportion of PVOH granules, i.e. less than 9 parts, has too little of an effect preventing coating cracking. This is due to the fact that the primer layer itself becomes too unstable to withstand the stresses during folding.

With regard to the upper limit of the range, it was observed that too high a proportion of PVOH granules, i.e. a proportion of more than 11 parts, does not allow any improvement in the anti-coating-cracking effect because the varnish can no longer be processed efficiently and reliably due to its reduced viscosity.

The water-soluble polyvinyl alcohol varnish is produced by filling the required amount of water (e.g. ideally 89 parts as stated) into a heating boiler in which a stirrer is inserted, activating the stirrer, activating a heating device (a heating rod or similar) and heating the water to 90° Celsius. As soon as the temperature of the water reaches 90° Celsius, the required amount of PVOH granules (e.g. ideally 11 parts as stated) is added to the water. As soon as the total quantity of PVOH granules has been added, a boiler lid is closed and a timer, which controls the operation of the heating device, is set to a heating time of 90 minutes. After this heating time has elapsed, the heating device is deactivated by the timer control. In the subsequent cooling phase, the stirrer should continue to run with the boiler lid still closed to prevent the formation of a skin on the surface of the liquid water-soluble polyvinyl alcohol varnish.

The varnish produced as described above has a limited shelf life. Depending on the storage temperature, it should be processed within 14-20 days. Therefore, the required quantity should only be produced shortly before the planned production. If a quantity of varnish is to be stored temporarily for a short period of time, the corresponding containers must also be sealed with a lid. Tin cans must not be used for this, as they are susceptible to rust.

The disposal of the residual quantities or the rinsing water can be carried out via the normal sewage system, as the polyvinyl alcohol varnish is neither toxic nor hazardous to water and is biodegradable.

During further processing of the water-soluble polyvinyl alcohol varnish produced as discussed above, attention must first be paid to the application quantity or this must be determined. In tests conducted by the applicant, a range of 1.5+/−0.5 g/m² was determined to be ideal. The processing viscosity of the water-soluble polyvinyl alcohol lacquer produced as discussed above is in the range of 20-25 seconds, determined in accordance with DIN 53211, using a standardized discharge cup with a 4 mm nozzle and measuring the time until the originally filled discharge cup is emptied.

This also favors the production of the paper because the polyvinyl alcohol varnish can be easily applied to the coated paper on a large scale in its liquid or viscous consistency.

Particularly advantageously, this application of the water-soluble primer layer takes place “inline” in the finishing method. This application therefore takes place in a particularly advantageous way “inline” in the manufacturing process of the finished paper. The application process step is therefore integrated as efficiently as possible into the overall paper manufacturing process.

Various techniques can be used to apply the primer layer, which can be integrated “inline” into the manufacturing process. Preferably, the water-soluble primer layer can be applied using a first application device; in particular, the water-soluble primer layer is applied by means of an application device designed for gravure printing. This means that no fundamentally new tools are required in the production process for this process step. Instead, a process technology that is available per se is used, which in the present case is not used to produce an overprint as before, but to apply the water-soluble primer layer.

In order to further improve the surface quality of the applied water-soluble primer layer, the further process step of smoothing has proven to be advantageous. This can be done, for example, after drying by polishing or the like. However, it has proven to be particularly advantageous to smooth the applied water-soluble primer layer after the gravure printing process, in particular before it dries, with the aid of a smoothing device, in particular by means of a smoothing device designed for roller smoothing. This measure ensures that any surface effects present in the surface of the primer layer and caused by the printing cylinder used in the gravure printing process are smoothed, i.e. the surface of the primer is optimized for the subsequent application of the actual overprint.

In connection with the additional surface smoothing, the surprising effect was also found that the overprinting behavior of the printing ink improved significantly, especially in the halftones or color gradients.

In a preferred embodiment, the water-soluble primer layer serves as a print carrier that bears an overprint, such as a motif or written information. The print is usually applied over the entire surface, with the motif and written information being printed together with a colored background, where applicable. The conditions created by the above measures result in a high-quality printed image, whereby this printed image is retained with essentially unimpaired quality even after folding.

It has proven to be particularly advantageous for a protective layer to be applied to the print carrier. This protective layer also helps to prevent coating cracking because it also mechanically stabilizes the layer structure on the paper. Preferably, the protective layer can even be formulated.

The protective coating can be a non-water-soluble (water-insoluble) protective coating, preferably a solvent-based two-component varnish. This measure helps to ensure that the end product made from the paper, in which the outer side is formed by the non-water-soluble protective layer, can also be used in a humid environment without degradation.

However, the protective layer can also be a water-based protective layer, preferably a water-based varnish, provided that the water solubility is unproblematic or negligible during further use of the paper. This increases the degree of recyclability in an ecological sense.

The protective layer is preferably transparent or translucent in such a way that the overprint is essentially visible with all its details unhindered. However, the optical property of the protective layer can also be realized matt, for which a matt varnish is used to replace an otherwise required matt lamination with a matt film.

The varnish forming the protective layer is applied using a printing process, preferably the gravure printing process already mentioned, in particular inline.

In general, the measures discussed at the beginning have proven to be particularly suitable for use of the paper in applications with folds. Therefore, according to a preferred embodiment, the paper is kraft paper, preferably for the production of folded paper packaging. Kraft paper is the type of paper with the highest strength, which is required, for example, for the production of paper bags.

The paper according to the invention is used in the manufacture of a wide variety of paper packaging which is free of plastic film. By way of example and not exhaustively, such paper packaging can be a paper sack or paper bag, particularly preferably a block bottom bag. Such paper packaging can also be a paper outer wrapper, in particular a paper outer wrapper for coffee.

Since there is no standardized measurement method for assessing the resistance to coating cracking, the applicant has defined a suitable method for determining the resistance to coating cracking under folding stress, which can be carried out both manually and mechanically and provides reproducible results. In this method, a 15 mm wide test strip of the paper produced as discussed above is subjected to a fold-friction stress at three test points evenly distributed over its length.

When implementing the procedure manually, the index finger and thumb are slightly moistened. The test strip is then bent with the printed side facing inwards and the printed paper sections, which are now lying on top of each other, are pressed between the thumb and index finger at the test point and the thumb and index finger are moved back and forth so that the paper sections lying on top of each other are moved relative to each other at the test point. It should be noted that only paper moves against paper and that there is no relative movement between paper and finger. The resulting stress on the paper surface corresponds approximately to that which occurs during the manufacturing process of folded packaging using the paper.

This stressing of the paper surface is then repeated on the test strip at two further test points.

The number of back-and-forth movements can be varied depending on the degree of stress. For comparative testing, this number of back-and-forth movements must of course be the same at the different test points.

As the procedure provides for three test points per test strip, a total of nine stressed test points are available for evaluation.

The following procedure is used to evaluate the stressed test points. In principle, each of the three test strips is evaluated. If the three test points of a test strip show different appearances, only the test point whose appearance is “in the middle” is evaluated. So if a first test point shows a maximum coating cracking and a second test point shows a minimum coating cracking, only the third test point is evaluated, which shows a coating cracking between the extremes mentioned.

The evaluation scheme used for the assessment comprises five categories, marked with the integral numbers 0 to 4, with each category defined by visually perceptible criteria:

• • 0=100% color coverage in the area of the test point;
  • 1=the test point appears colored with very occasional white spots or stripes;
  • 2=the test point appears colored with occasional white stripes;
  • 3=the white and colored stripes are approximately equally distributed in the area of the test point;
  • 4=the test point appears white with very occasional colored areas or stripes.

This evaluation scheme is based on the fact that the examiner determines his visual impression of the respective test point under the criteria specified and selects the appropriate category, i.e. classifies it or, in other words, assigns the respective test point to one of the five categories. In this context, the examiner's work can also be supported by a template to further facilitate the application of the test scheme and to further reduce the influence of subjectivity. This template can show typical images of the five categories. A template can also be used to determine a defined view strip that extends to the left and right along the paper, which is unfolded along the crease line, and limits the view width perpendicular to the crease line. A defined view window can also be defined with the aid of a template, which in addition to the restricted view width perpendicular to the crease line also restricts the view width along the crease line. These measures serve to limit the view width during the visual examination in a defined manner, in particular to the area that was actually subjected to the stress. A defined type of lighting can also be used for the visual examination of the test points. In principle, the exact specification of the lighting is not important. Rather, the examiner should assess the various test points under identical lighting conditions.

Moreover, the visual examination can be carried out without optical aids (e.g. a magnifying glass or a microscope) because the visual impression that packaging makes in the course of normal business transactions, e.g. on a shelf or in a store, is usually determined by the consumer without such optical aids. For the purposes of a more precise classification, a visual classification with an optical aid can be carried out in addition to the visual classification without optical aids. Optionally, the classification can also be carried out entirely with an optical aid.

When classifying the test points, the specified integral numbers (0, 1, 2, 3, 4) are always used. In the case of test points that cannot be clearly classified, half numbers can also be assigned as individual values if necessary.

The final determination of the resistance to coating cracking under folding stress for the paper concerned is made by specifying the median of the individual evaluation results of the maximum of nine test points determined in accordance with the above evaluation scheme. The median determined indicates where the paper in question is classified along the scale between minimum resistance to coating cracking (category 4) and maximum resistance to coating cracking (category 0).

However, this process for determining the resistance to coating cracking under folding stress can also be carried out mechanically or automatically. In this case, folding and stressing is carried out mechanically, after which the paper is unfolded flat again and then subjected to a computer-aided optoelectronic examination, e.g. with the aid of an image-capturing device, in particular a still camera, with computerized classification of the digital images of the individual test points generated and automated calculation of the median for the paper concerned.

According to a preferred embodiment example, the measures according to the invention provide a paper consisting of a base paper coated with the coating, the water-soluble primer layer covering the coating, the overprint applied to the primer layer, and the protective layer covering the overprint in a protective manner. In the production of this paper, it is advantageous to completely omit the integration of a film. This significantly improves the recyclability compared to known film-paper composite materials.

In the recycling process, a method known as “de-inking” is used to remove the overprint, including the protective layer if necessary. To separate the fiber mass from the remaining composite, which consists of the base paper, the coating and the water-soluble primer, water is used in an ecologically advantageous way.

In summary, the invention thus provides a paper for folded packaging with sharp creased edges, the appearance of which is not or only negligibly affected by folding and which nevertheless makes a decisive contribution to achieving good recyclability and the associated disposal in the paper recycling stream.

These and other aspects of the invention are shown in the figures discussed below.

SHORT DESCRIPTION OF FIGURES

The invention is explained in more detail below with reference to the attached figures using examples of embodiments, to which the invention is not limited, however. In the various figures, identical components are provided with identical reference signs. The figures show schematically:

FIG. 1 a paper according to the invention,

FIG. 2 a method according to the invention,

FIG. 3 a device for carrying out the method,

FIG. 4 a comparison between a bag made from the paper according to the invention and a bag made from conventional paper,

FIG. 5 a microscopic view of a fold line of the bag made with conventional paper,

FIG. 6 a microscopic view of a fold line of the bag made with the paper according to the invention.

DESCRIPTION OF THE DESIGN EXAMPLES

FIG. 1 shows a cross-section through a portion of a paper 1 according to the invention, which consists of a base paper 2, a coating 3 applied to one side of the first surface of the base paper 2, a water-soluble primer layer 4 consisting of a water-soluble polyvinyl alcohol varnish applied to the coating 3, an overprint 5 applied to the primer layer 4 and a non-water-soluble protective varnish layer 6 covering the overprint 5. In FIG. 1, the base paper 2 with the coating 3 applied to it is identified as coated base paper 7.

This paper 1 was produced according to the manufacturing or finishing method 8 visualized in FIG. 2.

According to this method 7, the first block I can comprise all process steps for production of the base paper 2 including application of the coating 3, i.e. the provision of a base paper 2 which comprises the coating 3 on one side. In the simplest case, block I concerns the provision of the coated base paper 7 on a first storage reel, from which it is unwound for further finishing.

In block II, the water-soluble primer layer 4 is applied to the coating 3 using a gravure printing process.

After the primer layer 4 has been applied, it is still in a liquid or viscous state. Before it dries completely, the surface is smoothed, preferably by roller smoothing, in a block III.

Subsequent to the drying of the primer layer 4, which is preferably carried out by means of a hot-air dryer, the overprint 5 is applied to the primer layer 4 in a block IV by means of a printing process. Following the drying of the overprint 5, which in turn is preferably carried out using a hot-air dryer, the overprint is coated with the protective varnish layer 6 in a block V and then this protective varnish layer 6 is also dried, for which in turn a hot-air dryer is preferably used.

A further block VI summarizes all subsequent measures for further processing of the finished paper 1, such as storing the paper 1 on a second storage roll for further transport, separating it into processing units or “inline” further processing of the paper 1 into a bag or sack, possibly up to filling it with a filling material and closing the bag or sack.

The method 7 described is carried out by means of a device 9 visualized in FIG. 3, which is a paper finishing system, and which is described in detail below.

FIG. 3 shows a first storage reel 10 on which the coated base paper 7 is wound and from where it is unwound for further finishing. The coated base paper 7 passes through the device 9 in a web-like manner and is guided by a number of guide or deflection rollers 11 and processed as it passes through three processing sections 13, 14 and 15 until it is wound onto a second storage roll 12 (rewind roll) as the paper 1 and made available for further processing.

The three processing sections 13-15 are essentially identical in design. They comprise a printing unit 16, with the aid of which a liquid or viscous application medium, which is stored in a container 17, is applied to the coated base paper 7. Here, an application roller 18 rotates, which picks up the application medium and applies it to the web of coated base paper 7 running past it synchronously, whereby the coated base paper 7 is pressed against the application roller 18 by a pressure roller 19. A squeegee 20 removes excess application medium.

In the first processing section 13, the liquid polyvinyl alcohol varnish 21, which was prepared according to the general description and, when cured, forms the water-soluble primer layer 4 of the paper 1, is used as the application medium.

In the second processing section 14, a liquid ink 22, which forms the cured overprint 5 of the paper 1, is used as the application medium. It should be mentioned here that, of course, several colors can also be applied, for which the second processing section 14 can then comprise a number of sub-sections (not shown) corresponding to the number of colors.

In the third processing section 15, a liquid solvent-based two-component varnish 23, which forms the protective layer 6 of the paper 1 when cured, is used as the application medium.

The first processing section 13 also comprises a rotating squeegee roller for smoothing the still wet polyvinyl alcohol varnish 21 applied to the coated base paper 7 in order to create an optimum surface for subsequent application of the overprint 5.

The device 9 also comprises at least one hot-air dryer 25 per processing section 13-15 for drying the respective application medium 21, 22 and 23, while the coated base paper 7 with the respective application medium 21, 22 or 23 as the uppermost layer moves past the respective hot-air dryer 25.

FIG. 4 shows a comparison of the side gusset with its fold zones of a bag 26 made from conventional paper and a bag 27 made from the paper 1 according to the invention.

The coating cracking along the fold lines 28 can already be clearly seen with the naked eye on the left bag 26. The fold line 28 appears here as a white stripe, which is delimited by its edges 29 and 30 and which separates the otherwise homogeneously colored areas of the side fold.

Clearly, no line break can be seen at all along the fold lines 28 on the right-hand pouch 27 with the naked eye. Therefore, no edges 29 and 30 are visible in this view. The fold line 28 appears here as a sharp fold edge that separates homogeneously colored areas of the side gusset from one another.

FIG. 5 shows an enlarged (microscopic) image of a first section area 31 of the fold line 28 and its immediate surroundings for the bag 26 shown on the left in FIG. 4. The coating cracking along the fold line 28 is clearly shown here by massive white zones between the edges 29 and 30. Adjacent to this, massive disturbances in the originally homogeneous coloring of the conventional paper can be seen on the left side of the edge 29 and on the right side of the edge 30.

If the evaluation scheme discussed in the general description is used to evaluate section area 31, i.e. section area 31 is defined as the test point, the test point can at best be rated as number 4. The resistance to coating cracking of this conventional paper can therefore be classified as poor.

In comparison to FIG. 5, FIG. 6 shows an enlarged (microscopic) image of a second section area 32 of the fold line 28 and its surroundings for the bag 27 shown on the right in FIG. 4. In this view, relatively small, but nevertheless still visible coating cracking can now be seen, which extends clearly visible along the fold edge between the now perceptible, but relatively close edges 29 and 30. Adjacent to this, on the left side of edge 29 and on the right side of edge 30, hardly any disturbances can be seen in the originally homogeneous coloring of the paper 1 according to the invention. These areas of the paper 1 therefore appear homogeneously colored throughout, even in the enlarged view, with smaller brightened zones which, however, are not noticeable in the normal view (of FIG. 4).

If the evaluation scheme discussed in the general description for evaluating the section area 32 is now also applied to the bag 27, i.e. the section area 32 is defined as the test point, this test point can be evaluated with the number 1. The resistance to coating cracking is therefore sufficient here to produce a bag 27 for premium demands using the paper 1 according to the invention.

For the sake of completeness, it should be mentioned that, for the sake of comparability of the evaluation, the section areas 31 and 32 were chosen to be the same size and in the same place on the respective bags 26 and 27. Bags 26 and 27 were also folded on one and the same machine using identical processing parameters.

Finally, it is pointed out once again that the figures described in detail above are only examples of embodiments, which can be modified by the skilled person in various ways without leaving the scope of the invention. For the sake of completeness, it is also pointed out that the use of the indefinite articles “a” or “one” does not exclude the possibility that the features in question may also be present more than once.