EP4013913B1 - Transfer paper for dye sublimation printing processes, and method for producing transfer paper - Google Patents

Description

The invention relates to a method for producing transfer paper for thermal sublimation printing processes, a transfer paper for thermal sublimation printing processes and a method for printing textiles.

Transfer printing processes are primarily used to apply printed images to objects that cannot be printed directly or only to a very limited extent or with great effort. In a transfer printing process, a (negative) print image is first printed onto a transfer paper and then transferred to a substrate with heating and, if necessary, pressure. The printing inks used in transfer printing processes include sublimable color components, which can be transferred from the transfer paper onto or into the substrate that is actually to be printed by heating and printing. The transfer paper can generally be printed contactlessly, for example using inkjet printing, or initially onto the transfer paper using contact printing methods, such as rotary screen printing. Transfer printing is usually used to print on substrates that have, for example, a polyester coating, or on textiles suitable for this purpose.

Since in transfer printing the printed image is first printed on a transfer paper and only then transferred to the substrate that is actually to be printed, the transfer paper has special requirements that cannot be met or can only be met very inadequately by conventional printing paper intended for printing. In addition to good printability, transfer paper is also required to transfer the printed image to the substrate to be printed as completely and as qualitatively as possible. On the one hand, a high transfer efficiency for the transfer of the printing ink(s) is desired, but also a transfer of the printed image applied to the transfer paper that is as contrasting or true to the contour as possible. In other words, a transfer paper should, on the one hand, be easy to print on, but on the other hand, an applied printed image or the sublimation dyes used for this purpose should be released or transferred as completely and accurately as possible to the substrate to be printed when heat and pressure are used. Proposals have already been disclosed in the past, which relate in particular to transfer paper with improved transfer efficiency. In the US 2005/0186363 A1 For example, a transfer paper with a coating based on carboxymethyl cellulose, polyvinyl alcohol, alginate or gelatin with a certain porosity and kaolin or talc as a filler is disclosed. A similar transfer paper is also available, for example US 2019/0001728 A1 known. In the WO 2018/139925 For example, a transfer paper with a coating based on a cationic polyamine or polyimine is disclosed.

In the KR 20110106507 A sublimation transfer paper with a barrier layer is also disclosed.

Despite the solutions proposed in the past for the design of transfer paper for sublimation transfer printing processes, there is still a need for improvement in terms of transfer efficiency and, above all, the contrast or contour-accurate transfer of a printed image from a transfer paper to a substrate to be printed. Furthermore, the methods for producing the transfer paper known from the prior art are only economically useful to a limited extent, so that the transfer papers produced in this way have correspondingly high costs. In particular, the methods specified are only suitable to a limited extent for the production of large quantities of transfer paper.

The object of the present invention was, on the one hand, to provide a method for producing a transfer paper for thermal sublimation printing processes, by means of which transfer paper with good transfer printing properties can be produced in high quality but still cost-effectively in large quantities. A further object of the invention was to provide a transfer paper for thermal sublimation printing processes with improved properties, as well as a method for printing textiles using a corresponding transfer paper.

The object of the invention is achieved on the one hand by a method for producing a transfer paper according to the corresponding claims.

1. a) Bereitstellung eines Zellstoffes oder einer wässrigen Zellstoff-Suspension,
2. b) Herstellung einer Suspension des Zellstoffes in Wasser mit einem Wassergehalt von mindestens 90 Gew.%,
3. c) optional Beimengung von Additiven, Füllstoffen oder Prozesschemikalien zu der wässrigen Suspension,
4. d) Entwässerung der Suspension auf einen Wassergehalt von 60 Gew.% bis 85 Gew.% mittels einer Siebpartie,
5. e) Trocknung der entwässerten Suspension zu einer Papierbahn mit einem Wassergehalt von 30 Gew.% bis 55 Gew.% mittels einer Pressenpartie,
6. f) weitere Trocknung der Papierbahn auf einen Wassergehalt von 1 Gew.% bis 10 Gew.% mittels einer Trockenpartie,
7. g) Beschichtung zumindest einer der Oberflächen der Papierbahn mittels einer Beschichtungsvorrichtung durch Auftragen einer wässrigen Lösung einer modifizierten Stärke, welche Stärke kationisch modifiziert ist und einen Substitutionsgrad durch kationische Gruppen von 0,03 bis 0,6 mol/mol aufweist,
8. h) Nachtrocknung der beschichteten Papierbahn mittels einer weiteren Trockenpartie bzw. Nachtrockenpartie auf einen Wassergehalt von 3 Gew.% bis 7 Gew.%,
9. i) Konfektionierung der beschichteten und nachgetrocknteten Papierbahn.

The process for producing transfer paper for dye sublimation printing processes includes the process steps

1. a) providing a pulp or an aqueous pulp suspension,
2. b) producing a suspension of the pulp in water with a water content of at least 90% by weight,
3. c) optional addition of additives, fillers or process chemicals to the aqueous Suspension,
4. d) dewatering the suspension to a water content of 60% by weight to 85% by weight using a sieve section,
5. e) drying the dewatered suspension to form a paper web with a water content of 30% by weight to 55% by weight using a press section,
6. f) further drying of the paper web to a water content of 1% by weight to 10% by weight using a drying section,
7. g) coating at least one of the surfaces of the paper web using a coating device by applying an aqueous solution of a modified starch, which starch is cationically modified and has a degree of substitution by cationic groups of 0.03 to 0.6 mol/mol,
8. h) post-drying of the coated paper web using a further drying section or post-drying section to a water content of 3% by weight to 7% by weight,
9. i) Preparation of the coated and post-dried paper web.

Through the specified process measures or steps, high-quality transfer paper can be produced continuously in a continuous process sequence that makes economic sense and in large quantities. A separate step for applying a coating, such as by means of doctor blades as stated in the prior art, can be dispensed with here. In particular, the method for producing transfer paper can be intended for thermal sublimation printing processes in which the transfer paper is printed using digital inkjet printing.

Because the coating is carried out in-line in the process and the backing paper or the paper web with the applied coating is treated together in the process, in particular dried to a desired water content, a composite material with particularly good adhesive properties of the layers to one another can also be produced , and are manufactured with particularly good quality of the coating. Delamination phenomena or the formation of defects in the coating, which can, for example, have a negative impact on the print image quality, can be effectively prevented.

As it has turned out, the specified coating comprising the modified starch is particularly advantageous with regard to the requirements placed on a transfer paper.

As will be shown in detail below using exemplary embodiments, a transfer paper produced using the specified process measures is characterized by particularly good properties when used in thermal sublimation printing processes. Due to the special coating, the transfer paper has, on the one hand, good printability, i.e. good absorption of the sublimation printing dyes or pigments, but, surprisingly, on the other hand, above all, good release properties in the course of a thermal sublimation printing process, i.e. when transferring the printing dyes or pigments to a substrate by sublimation , on. In particular, the coated transfer paper can prevent unwanted penetration of the sublimation dyes onto the back of the transfer paper and the associated loss of sublimation dyes for a sublimation printing process, during and after printing on the transfer paper and also during the transfer printing process. The coating can, for example, consist of over 90% by weight of the modified starch, but can, for example, contain small amounts of, for example, manufacturing or other impurities, as well as small amounts of additives common in paper technology.

In principle, starch of any kind or obtained from different sources can be used, for example potato, wheat or corn starch can be used for the aqueous solution.

On the one hand, the cationic modification can ensure good solubility of the starch in water. In addition, the cationic modification of the starch can achieve an improved coating of the paper web and, above all, good adhesion of the coating to the paper web can be provided. This ultimately contributes to particularly good properties of the composite, including good mechanical properties and also high resistance to damage, even during the high load in a thermal sublimation printing process. The product Licocat P from Südstarke GmbH should be mentioned as an example of a suitable, cationically modified starch.

Overall, it has been shown that an improved transfer paper can be provided by using the cationically modified starch for the coating as well as the specific sequence of process steps. In particular, the targeted influence on the water content of both the paper substrate or the paper web should also be possible The targeted adjustment of the water content of the coated paper web at the end of the process may be essential.

In process step b), a suspension of the pulp in water with a water content of 90% by weight up to 99.5% by weight can be produced, which is then successively formed into a paper web with the optimal water content for the coating in process step g) according to the specified process sequence. is dried or converted. The coated paper web is then specifically dried to the specified water content of 3% by weight to 7% by weight. In this way, a transfer paper can be produced which is particularly suitable for thermal sublimation printing, as will be explained in more detail below using exemplary embodiments.

In a further development of the process, it can be provided that in process step g) an aqueous solution of a starch cationically modified with quaternary ammonium groups is used.

A starch modified in this way has proven to be particularly suitable for producing the coating of the transfer paper. Furthermore, a transfer paper with such a coating has particularly good properties when used in thermal sublimation printing processes.

In a process variant, it may be expedient if in process step f) the paper web is dried to a water content of 2.5% by weight to 10% by weight, and in process step g) the aqueous solution of the cationically modified starch is dried using a size press to at least a surface of the paper web is applied.

This represents a simple but nevertheless efficient measure for coating at least one of the surfaces of the paper web, whereby in principle both surfaces of the paper web can be coated with the aqueous solution of the modified starch using a size press. The coating can be done on a paper web with a high water content.

In this context, it can preferably be provided that the aqueous solution of the cationically modified starch is not applied to both surfaces of the paper web, but rather that the aqueous solution of the modified starch is applied in process step g). one surface of the paper web is applied and an aqueous solution containing 5% by weight to 15% by weight of an enzymatically degraded starch is applied to the other surface of the paper web.

In this case, the surface of the transfer paper intended for thermal sublimation printing is formed by the surface which is coated with the cationically modified starch. That surface of the transfer paper to which the aqueous solution of the enzymatically degraded starch was applied represents the back of the transfer paper. In other words, the surface of the transfer paper coated with the cationically modified starch is printed, and is also this in the course of a thermal sublimation printing process Surface in contact with the substrate to be printed, for example textile. The advantage of this process measure is, among other things, that cost-effective production of the transfer paper can be provided without having to accept significant losses when used for thermal sublimation printing processes. Furthermore, transfer paper produced in this way is characterized by good durability and good mechanical properties.

In a process variant, it can also be provided that in process step f) the paper web is dried to a water content of 1.2% by weight to 2.0% by weight, and in process step g) the aqueous solution of the cationically modified starch is dried using a Film press is applied to at least one surface of the paper web.

In this case, the aqueous solution of the modified starch is not applied directly to at least one surface of the paper web, but can be applied in advance to one or two application rollers of the film press in the desired layer thickness. The layer can then be transferred from the applicator roller(s) onto the paper web. The application or coating using such a film press is particularly suitable for high throughput of the paper web, and is also advantageous in terms of controlling the layer thickness.

Even if a film press is used, it can preferably be provided that in process step g) the aqueous solution of the cationically modified starch is applied to one surface of the paper web and an aqueous solution which contains 5% by weight to 15% by weight is applied to the other surface of the paper web .% of an enzymatically degraded starch is applied.

Furthermore, an embodiment of the method may be useful in which, in method step g), an aqueous solution having a concentration of the cationically modified starch of 5% by weight to 20% by weight is applied to at least one surface of the paper web.

An aqueous solution with a concentration of the modified starch in the specified weight percent range has proven to be particularly suitable for coating at least one surface of the paper web. This is at least largely independent of the type of coating. In particular, very homogeneous coatings can be produced using aqueous solutions in the specified weight percent range, and the formation of defects or microcracks in the coating, in particular during drying in process step h), can be prevented.

As a further consequence, it can be advantageous if an amount of 0.5 g/m ² to 20 g/m ² of the aqueous solution () of the cationically modified starch is applied to at least one surface of the paper web.

In this way, the coating, which comprises the cationically modified starch, can be given a layer thickness that is particularly suitable for thermal sublimation printing processes. On the other hand, an upper limit for the layer thickness is specified, which is still useful in terms of economically viable production of the transfer paper.

Finally, in the process it can also be provided that in process step h) a water content of the coated paper web of 4% by weight to 6% by weight is set.

With a water content in this specified range, the best test results were achieved with regard to the absorption capacity and also the release capacity of sublimation dyes.

The object of the invention is also achieved by a transfer paper for thermal sublimation printing processes. The corresponding transfer paper can in particular be produced according to a process for producing transfer paper for thermal sublimation processes according to the process measures specified above.

The transfer paper comprises a paper substrate with a basis weight of 40 g/m ² to 160 g/m ² . As is known, the paper substrate can predominantly comprise cellulose fibers, although other components, such as lignin or hemi-celluloses, or additives and fillers common in paper technology can also be contained in the paper substrate.

At least one surface of the paper substrate is provided with a coating consisting at least predominantly of a modified starch, which starch is cationically modified and has a degree of substitution by cationic groups of 0.03 to 0.6 mol/mol. The transfer paper has a water content of 3% to 7% by weight.

As already explained above in connection with the process for producing a transfer paper and shown in detail below using exemplary embodiments, a transfer paper produced using the specified process measures is characterized by particularly good properties when used in thermal sublimation printing processes. In particular, the special coating has, on the one hand, good printability, i.e. good absorption capacity of the sublimation printing dyes or pigments, and additionally also good release properties in the course of a thermal sublimation printing process. The water content in the specified range also has an advantageous effect. In particular, the transfer paper can be intended for thermal sublimation printing processes in which the transfer paper is printed using digital inkjet printing.

When developing the transfer paper, it can be advantageous if the coating consists at least predominantly of a starch cationically modified with quaternary ammonium groups.

On the one hand, this is because starch modified in this way has proven to be particularly easy to process. In addition, a transfer paper with a coating comprising such a modified starch has particularly good properties when used in thermal sublimation printing processes.

Furthermore, in one embodiment of the transfer paper it can be provided that the coating has a basis weight of 0.2 g/m ² to 4 g/m ² .

A basis weight or a layer thickness in the specified range has proven to be particularly suitable for thermal sublimation printing processes. Above all, a layer thickness selected from this range appears to be suitable in order to prevent excessive amounts of sublimation printing dyes or pigments from penetrating into the underlying paper substrate. On the other hand, the specified range specifies an upper limit for the basis weight, which is still useful in terms of economically viable production of the transfer paper.

In particular, it can be provided that the transfer paper has a water content of 4% by weight to 6% by weight.

In this way, a transfer paper can be provided which has further improved properties with regard to a thermal sublimation printing process.

In addition, an embodiment of the transfer paper that has a water absorbency according to Cobb according to ISO 535 of 20 g/m ² to 35 g/m ² can be useful.

Such a transfer paper has proven to be particularly suitable with regard to the absorption of sublimation printing dyes or pigments, but also with regard to their re-release in the course of a thermal sublimation process.

Finally, the object of the invention is also achieved by a method for printing textiles. The process includes producing a negative printed image on a transfer paper using sublimation printing dye(s) and transferring the printed image to a textile substrate using a thermal transfer press. A transfer paper is used here, which is designed as described above and below.

Using such a transfer paper, textiles with print images of particularly high quality, in particular high contour fidelity, high contrast and good color density, can be provided. Preferably, in the method for printing textiles it can be provided that the negative print image is produced on the transfer paper using digital inkjet printing.

For a better understanding of the invention, it will be explained in more detail using the following figures.

Fig. 1

Ein Ausführungsbeispiel für einen Verfahrensschritt d) zur apparativen Entwässerung einer Suspension eines Zellstoffes mittels einer Siebpartie;

Fig. 2

Ein Ausführungsbeispiel für einen Verfahrensschritt e) zur weiteren, apparativen Entwässerung der Suspension zu einer Papierbahn mittels einer Pressenpartie;

Fig. 3

Ein Ausführungsbeispiel für einen Verfahrensschritt f) zur weiteren, apparativen Trocknung der Papierbahn mittels einer Trockenpartie;

Fig. 4

Ein Ausführungsbeispiel für einen Verfahrensschritt g) zur Beschichtung der getrockneten Papierbahn mittels einer Beschichtungsvorrichtung;

Fig. 5

Ein weiteres Ausführungsbeispiel für einen Verfahrensschritt g) zur Beschichtung der getrockneten Papierbahn mittels einer Beschichtungsvorrichtung;

Fig. 6

Ein Ausführungsbeispiel für einen Verfahrensschritt i) für eine Konfektionierung einer beschichteten und getrockneten Papierbahn;

Fig. 7

Ein Ausführungsbeispiel für ein Transferpapier für ThermosublimationsdruckProzesse;

Fig. 8

Ein Ausführungsbeispiel für ein Verfahren zum Bedrucken von Textilien.

They show in a highly simplified, schematic representation:

Fig. 1

An exemplary embodiment of a process step d) for the dewatering of a suspension of a pulp by means of a sieve section;

Fig. 2

An exemplary embodiment of a method step e) for further dewatering of the suspension into a paper web using a press section;

Fig. 3

An exemplary embodiment of a method step f) for further drying of the paper web using a drying section;

Fig. 4

An exemplary embodiment of a process step g) for coating the dried paper web using a coating device;

Fig. 5

A further exemplary embodiment of a method step g) for coating the dried paper web using a coating device;

Fig. 6

An exemplary embodiment of a method step i) for making up a coated and dried paper web;

Fig. 7

An exemplary embodiment of a transfer paper for thermal sublimation printing processes;

Fig. 8

An exemplary embodiment of a method for printing textiles.

As an introduction, it should be noted that in the differently described embodiments, the same parts are provided with the same reference numbers or the same component names, whereby the disclosures contained in the entire description can be transferred analogously to the same parts with the same reference numbers or the same component names. The position information selected in the description, such as top, bottom, side, etc., is also related to the figure directly described and shown and, in the event of a change in position, these position information must be transferred accordingly to the new position.

In the process for producing transfer paper for thermal sublimation printing processes, the provision of a pulp or an aqueous pulp suspension is provided in a first process step a). The pulp can be used as is known per se and in the case of printable papers, it usually consists predominantly of cellulose, although small amounts of other components, for example lignin or hemicelluloses from the production of the cellulose, or additives and impurities may also be contained. In principle, a pulp can be dried or provided as an aqueous suspension.

In a further process step b), a suspension of the pulp in water with a water content of at least 90% by weight, the water content of this pulp suspension can be up to 99.5% by weight. This is based on 100% by weight of the suspension of the pulp in water. In an optional process step c), additives, fillers or process chemicals can also be added to the aqueous pulp suspension. In particular, additives common in paper technology can be added to the aqueous pulp suspension.

Like in the Fig. 1 illustrated, the next process step d) is dewatering of the suspension 1 by means of a sieve section 2. In the case in which Fig. 1 In the highly schematic and only incompletely illustrated embodiment, such a sieve section 2 can have a circumferential sieve 3, with the aqueous pulp suspension 1 being carried on a surface 4 of the sieve 3. The other surface 5 of the sieve 3 can be guided over dewatering means 6 of the sieve section 2 during each revolution of the sieve 3. Such drainage means can be formed, for example, by drainage and/or suction strips. Dewatering in a sieve section can generally be carried out using gravity. In addition, it can also be done as in the Fig. 1 shown, the dewatering of the suspension 1 can be supported by applying a negative pressure to the surface 5 of the sieve 3 facing away from the pulp suspension 1 by means of a negative pressure device 7. The aqueous suspension of the pulp in water is in the Fig. 1 illustrated process step d) adjusted to a water content of 60% by weight to 85% by weight or dewatered.

In a subsequent process step e), the dewatered suspension 1 is dried to form a paper web 8 by means of a press section 9, as is also shown very schematically in the Fig. 2 is illustrated. For this purpose, the pulp suspension 1 pre-dewatered in process step d) can be passed between at least two rollers 10 of the press section and dried under high pressure between the rollers 10 to form the paper web 8. In principle, the press section can also have additional pairs of rollers. Additionally, as in the Fig. 2 is shown, the drying of the suspension 1 to the paper web 1 is additionally supported by means of absorbent support material, such as in the Fig. 2 illustrated felt mats 11. By drying using the press section 9 in process step e), the aqueous suspension 1 from process step d) is dried into a paper web 8 with a water content of 30% by weight to 55% by weight.

Subsequently, in a further process step f), further drying of the paper web 8 from process step e) is carried out by means of a drying section 12, as in the Fig. 3 is again illustrated very schematically. Such a drying section 12 can, as shown, comprise numerous rotating drying cylinders 13 over which the paper web 8 can be guided. The drying cylinders can be heated directly. For example, in the Fig. 3 Heating channels, not shown, can be formed in the drying cylinders 13, through which a heating medium, for example water vapor, can be passed. Alternatively or additionally, electrical resistance heaters are also fundamentally conceivable. A temperature of the drying cylinders 13 can be used, for example, in the Fig. 3 illustrated embodiment from left to right, i.e. in the direction of passage of the paper web 8, rise successively. In addition, the drying section 12 can also have other aids, such as screen webs 15, 16 guided over deflection rollers 14. Such screen webs 15, 16 can prevent direct contact of the paper web 8 with the hot drying cylinders 13. In any case, in process step f), the paper web 8 is dried by means of the drying section 12 to a water content of 1% by weight to 10% by weight.

In the process, in a process step g), at least one of the surfaces 17 of the paper web 8 dried in process step f) is coated by means of a coating device 18. Two alternative solutions for coating at least one of the two surfaces 17 of the paper web 8 are in the Fig. 4 and Fig. 5 shown, and will be explained in more detail below. Regardless of how the coating is carried out, in process step g) the coating is carried out by applying an aqueous solution (19) of a modified starch. The starch is cationically modified and has a degree of substitution by cationic groups of 0.03 to 0.6 mol/mol. For example, cationically modified potato, wheat or corn starch can be used for the process.

In particular, it can be provided that in process step g) an aqueous solution 19 of a starch cationically modified with quaternary ammonium groups is used.

In Fig. 4 A first exemplary embodiment for carrying out the coating of the paper web 8 is shown very schematically. In this embodiment variant, in process step g), the aqueous solution 19 of the cationically modified starch can be applied to at least one surface 17 of the paper web 8 using a size press 20. For this exemplary embodiment of the method, provision can then be made for the paper web 8 to be dried to a water content of 2.5% by weight to 10% by weight in method step f), which is carried out directly in advance. In the case of coating, the paper web 8 can be coated using a size press 20 as in the Fig. 4 illustrated between two applicator rollers 21, whereby the aqueous solution 19 of the modified starch can be introduced for the purpose of coating between the at least one surface 17 of the paper web 8 and the applicator roller 21 directly assigned to this surface 17.

How further from the Fig. 4 As can be seen, it can be provided in particular that in process step g) the aqueous solution of the cationically modified starch is applied to one surface 17 of the paper web 8 and to the other surface 22 of the paper web 8 an aqueous solution 23, which is 5% by weight. up to 15% by weight of an enzymatically degraded starch is applied. For this purpose, the aqueous solution 23 of the enzymatically degraded starch can be introduced between the surface 22 and the application roller 21 directly assigned to this surface 22. Corresponding enzymatically degraded starches as well as the methods for obtaining such starches are known per se and can be obtained, for example, by cooking wheat or corn starch in an aqueous solution which contains corresponding decomposition enzymes, as for example in Be-Miller, JN, & Whistler, R.L. (2009). Starch: Chemistry and Technology (3rd Edition). Academic Press is described. The enzymatic degradation primarily serves to reduce the molecular weight of the starch and the associated reduction in viscosity in order to make the starch easier to process. As an example of a degradation enzyme, α-amylase is known.

In the Fig. 5 An alternative exemplary embodiment for carrying out the coating of the paper web 8 is shown. In this embodiment variant, in process step g). aqueous solution 19 of the cationically modified starch can be applied to at least one surface 17 of the paper web 8 using a film press 24. For this exemplary embodiment of the method, provision can then be made for the paper web 8 to be dried to a water content of 1.2% by weight to 2.0% by weight in method step f), which is carried out directly in advance. In the case of coating, the paper web 8 can be passed between two stripping rollers 25 using a film press 24. In this case, how from the Fig. 5 As can be seen, the aqueous solution 19 of the cationically modified starch is applied in advance in a specifically adjusted layer thickness to the stripping roller 25 assigned to the at least one surface 17 of the paper web 8, and transferred from this stripping roller 25 to the at least one surface 17 of the paper web.

Also with the one in the Fig. 5 Illustrated exemplary embodiment, it can be provided that in process step g) the aqueous solution of the cationically modified starch is applied to one surface 17 of the paper web 8 and an aqueous solution 23, which contains 5% by weight to 15% by weight, is applied to the other surface 22 of the paper web 8. % of an enzymatically degraded starch is applied. The aqueous solution 23 of the enzymatically degraded starch can be used as from the Fig. 5 can be seen first applied to the stripping roller 25 directly assigned to this surface 22 of the paper web 8 and then transferred to the surface 22.

Regardless of the type of application of the aqueous solution 19 of the cationically modified starch or the method used for this purpose, it can be provided that in process step g) an aqueous solution 19 having a concentration of the cationically modified starch of 5% by weight to 20% by weight is applied to at least one surface 17 of the paper web 8.

Furthermore, regardless of the coating method, an amount of 0.5 g/m ² to 20 g/m ² of the aqueous solution 19 of the cationically modified starch can be applied to at least one surface 17 of the paper web 8.

In principle, it can of course also be provided that an aqueous solution 19 of the cationically modified starch is applied to both surfaces 17, 22 of the paper web 8. However, it can also be generally provided that in process step g) the aqueous solution 19 of the modified starch is only applied to one surface 17 of the paper web is applied, and only water or no fluid at all is applied to the other surface 22 of the paper web 8.

Following the coating of the paper web 8 in process step g), in a further process step h), the coated paper web 8 is post-dryed by means of a further drying section 12 or post-drying section 12. Such post-drying of the coated paper web 8 by means of a post-drying section 12 can in principle be analogous to the one already in the Fig. 3 illustrated drying by means of the drying section 12 shown, or a corresponding post-drying section 12 can be designed analogously or at least similarly to the drying section 12 used in method step f). A further description of a drying process using a drying section 12 or post-drying section 12 can therefore be unnecessary at this point and will be discussed below Fig. 3 and the associated description. The only important thing is that in process step h) the coated paper web 8 is dried to a water content of 3% by weight to 7% by weight. In particular, it can be provided that in process step h) a water content of the coated paper web 8 of 4% by weight to 6% by weight is set.

Finally, in the process, i) assembly of the coated and post-dried paper web 8 is carried out in one process step. One possibility or an exemplary embodiment for assembling the paper web 8 is in the Fig. 6 illustrated. As shown, the coated and dried paper web 8 can, for example, first be smoothed using a calender 26, and then be applied, for example, to a take-up roll 27 for transporting the paper web 8 or for further, external packaging. As an alternative to this packaging, it is of course also conceivable to divide the paper web, not shown in more detail, both in the longitudinal direction to form paper strips and to divide it in the transverse direction to form sheets.

The ones in the Fig. 1 - 6 Illustrated process steps d) to i) can be carried out in direct succession in a paper machine. What is important here is the successive dewatering of the suspension 1 of the pulp and the drying of the paper web 8 in the process sequence, including the joint drying of the paper web 8 with the coating or the subsequent drying of the coated paper web in process step h). This corresponds to the specified ranges for water contents for the aqueous pulp suspension 1 or the paper web 8. If water contents in the individual process steps in question are outside the upper and lower limit values specified, the resulting products or transfer papers can have increasingly poor properties, with varying water contents, but which are within the respective limits for the respective ones Limit values specified in the process steps have only minor or hardly any negative influences on the properties desired with regard to a transfer paper for thermal sublimation printing processes.

In the Fig. 7 a transfer paper 28 for thermal sublimation printing processes is shown in cross section. Such a transfer paper 28 can in particular according to a method as above using the exemplary embodiments in the Fig. 1 to 6 described. The transfer paper 28 is in the Fig. 7 shown in sections and not to scale, and can in principle have any dimensions suitable for an intended thermal transfer printing process. For example, standardized dimensions for papers can be provided, such as Din A4 or Din A3.

Like in the Fig. 7 is shown, the multilayer transfer paper 28 comprises, on the one hand, a paper substrate 29. The paper substrate 29 has a basis weight of 40 g/m ² to 160 g/m ² . At least one surface 30 of the paper substrate 29 is provided with a coating 31. This coating 31 consists at least predominantly of a modified starch. In particular, the coating can consist of over 90% by weight of the modified starch, but can, for example, contain small amounts of manufacturing or other impurities, as well as small amounts of additives common in paper technology. In particular, the coating 31 contains a water content set in the course of producing the transfer paper 28, the transfer paper 28 and thus also the coating 31 having a water content of 3% by weight to 7% by weight. The transfer paper 28 and thus also the coating 31 can preferably have a water content of 4% by weight to 6% by weight. The modified starch is cationically modified and has a degree of substitution by cationic groups of 0.03 to 0.6 mol/mol. In particular, it can be provided that the coating 31 consists at least predominantly of a starch cationically modified with quaternary ammonium groups.

The coating 31 can preferably have a basis weight of 0.2 g/m ² to 4 g/m ² . Furthermore, it can preferably be provided that the transfer paper 28 has a Cobb water absorbency according to ISO 535 of 20 g/m ² to 35 g/m ² .

In principle, only one surface 30 of the paper substrate 29 can be provided with a coating 31 comprising the modified starch. In principle, both surfaces 30, 32 of the paper substrate 29 can also be provided with a coating 31 comprising the modified starch.

In a preferred embodiment of the transfer paper 28, as in the Fig. 7 illustrates a surface 30 of the paper substrate 29 to be provided with the coating 31 comprising the cationically modified starch, and the surface 32 of the paper substrate 29 opposite this surface 30 to be provided with a further, different coating 33. In particular, it can be provided that the further coating 33 consists at least predominantly of an enzymatically degraded starch. A description of a corresponding degraded starch has already been made based on the description of the process and is referred to here.

In the Fig. 8 Finally, the process for printing textiles is shown roughly schematically. The process is based on a thermal sublimation printing process. Like from the Fig. 8 As can be seen, in the method for printing textiles, a negative print image is first produced on a transfer paper 28 using one or more sublimation printing dyes 34. In principle, this can be carried out using conventional printing processes, whereby both contact printing methods such as rotary screen printing or contactless printing processes such as inkjet printing can be used. Of course, in the printing process it can be provided that the negative printed image is monochrome or multi-colored, with multi-colored printed images of course being able to use several sublimation dyes or pigments of different colors, as is known per se.

At the one in the Fig. 8 The transfer paper 28 shown is a paper that is coated at least on one side, as shown above with reference to Fig. 7 was described. Here, the side 35 of the transfer paper 28, which is provided with the coating 31 consisting at least predominantly of the cationically modified starch, is intended for printing with the thermal sublimation dye(s) 34. As in Fig. 8 is shown at the method for printing textiles, the printed side 35 of the transfer paper 28, which is as in Fig. 7 shown is provided with the coating 31, then brought into contact with a textile substrate 36 to be printed.

The thermal sublimation dye(s) or the printed image formed by the thermal sublimation dye(s) are/are subsequently made by means of one in the Fig. 8 Only partially and very schematically shown thermal transfer printer 37 is at least partially transferred from the transfer paper 28 to the textile substrate 36. In a conventional thermal transfer printer 37, as is known per se, the sublimation dye(s) or the sublimation pigment(s) are/are transferred into the gas phase by heating and, if necessary, transferred into the textile substrate with the support of pressure. Subsequently, the transfer paper 28 can be removed from the textile substrate 36, as in the Fig. 8 indicated a residual amount of sublimation dye(s) can remain in the transfer paper 28.

For a method for printing textiles or for a thermal sublimation printing process, it is important, on the one hand, that the transfer paper 28 can be easily printed, i.e. that it can absorb and temporarily store the sublimation printing dyes as well as possible. On the other hand, it is also important to have the best possible transfer efficiency to the textile that is actually being printed.

The properties of the transfer paper produced using the method described above or the transfer paper for thermal sublimation printing processes also described above will now be characterized using exemplary embodiments.

Example 1:

Several transfer papers were produced as test papers A, B, C, each with different coating weights, at least predominantly from cationically modified starch. The production took place according to process steps a) - h) of the process described above. The water contents of the aqueous suspension of the pulp and the paper web formed in the course of the process fluctuated slightly in the course of the production of the individual transfer papers, but were within the ranges specified for the relevant process steps. No significant influences of slightly different water contents on the properties of the transfer papers produced could be determined become. For all transfer or test papers A, B, C, the aqueous solution of the cationically modified starch was applied to one surface of the paper web using a film press, and an aqueous solution containing 10% by weight of an enzymatically degraded starch was applied to the other surface

1. A: 0,25 g/m²;
2. B: 0,5 g/m²;
3. C: 1 g/m².

The coating made of cationically modified starch had the following basis weights in the transfer or test papers produced:

1. A: 0.25 g/m ² ;
2. B: 0.5 g/m ² ;
3. C: 1 g/m ² .

• D: herkömmliches Druckpapier, 50 g/m²;
• E: kommerziell erhältliches Transferpapier mit einer Beschichtung basierend auf Carboxymethylcellulose.

These transfer papers A, B, C, as well as comparison papers D and E, were printed with a simple test pattern in the form of filled circular areas of individual sublimation dyes of different colors. The following were used as comparison papers:

• D: conventional printing paper, 50 g/m ² ;
• E: commercially available transfer paper with a coating based on carboxymethyl cellulose.

The identical print images applied to the transfer papers A, B, C and to the comparison papers D, E were each transferred to a commercially available polyester textile using a thermal transfer press, type Hotronix STX11. The transferred print images were then characterized individually for each printing color using densitometry with regard to color density.

Two test series 1 and 2 were carried out, with two different, commercially available sublimation inks, each with four base colors or dyes for inkjet printing, being used to print the test papers in test series 1 and 2. The printing of the test papers before transferring them to the polyester textile was carried out using inkjet printing.

To measure the color density of the thermal transfer prints of the individual sublimation colors or inks of test series 1 and 2, the relevant prints on the polyester textile were measured using an X-rite eXact advanced densitometer in reflection mode. As is known, a respective color density D is calculated as D = log ₁₀ (1/reflection) or -log ₁₀ (i ₀ /i); The respective color density was determined according to ISO 13655:2017-07.

From the color densities determined in this way, an arithmetic mean D _m for the color density was formed for each test paper from test series 1 and 2 from all color-specific measured values or from the color densities determined for all dyes of an ink from test series 1 and 2.

Table 1 below shows the mean values D _m1 and D _m2 of the color densities for both test series 1 and 2 for the printed images transferred to the polyester textiles using test papers A, B, C, D, E. With regard to the transfer efficiency of the sublimation printing dyes to the test textile, the highest possible color density values in Table 1 are desirable. Table 1: Measurement results for polyester textile Test paper Color density D _m1 Color density D _m2 A 1.84 1.64 b 1.93 1.58 C 1.88 1.69 D 1.79 1.58 E 1.84 1.70

Example 2:

In order to further characterize the suitability of a transfer paper coated according to the invention with the cationically modified starch, thermal sublimation printing processes were also carried out with the test papers, in which a conventional printing paper with a basis weight of 50 g / m ² was placed as backside test paper. The respective thermal sublimation printing processes were otherwise carried out completely analogously to those described under exemplary embodiment 1.

In exemplary embodiment 2, after the thermal sublimation prints had been carried out, the back side test papers were checked for penetration of dyes from the inks used in test series 1 and 2 onto the back of a respective test paper, i.e. undesirable penetration of the dyes onto the back of the respective test paper and Transfer of the dyes to the backside test paper is visually examined by a trained test person and rated according to the school grade principle, i.e. from 1 to 5. As a key point, back-side test papers that do not contain any visible traces of color are rated with a grade of 1, whereas back-side test papers that have a print image that has almost completely passed through or transferred to the back of the test paper are rated with a grade of 5. A reference folder with samples of different quality serves as the basis for the assessment or grade grading for comparison or as a reference point.

Furthermore, the print images on the backside test papers for the ink from test series 1 were evaluated electronically. For this purpose, the backside test papers were scanned with a high-resolution scanner and evaluated using the Print Target evaluation software from the company Prübau. An evaluation was carried out once with the software setting Low Sensitivity and once with the software setting High Sensitivity.

Both the grades for the visual evaluation of the backside test papers with the inks of test series 1 and 2, as well as the values for the electronic evaluation of the backside test papers in the low sensitivity and high sensitivity modes are marked accordingly in Table 2. For Table 2, lower values for the visually evaluated and electronically evaluated print images on the back side test papers are desirable, as this represents a lower loss of dye sublimation dyes due to passage over the back side of a test paper. Table 2: Measurement results for back side test paper Test paper Visual 1 Visual 2 Electronic 1 Low sensitivity Electronic 1 High sensitivity A 2 3 14 189 b 2 2 10 33 C 1 2 3 19 D 2 3 33 2255 E 3 4 233 655

As can be seen from the exemplary embodiments, the paper produced as described with the coating consisting at least predominantly of the cationically modified starch is well suited as transfer paper for thermal sublimation printing processes. Above all, unwanted bleed-through or penetration of thermal sublimation dyes onto the back of such a transfer paper can be effectively prevented.

The exemplary embodiments show possible embodiment variants, whereby it should be noted at this point that the invention is not limited to the specifically illustrated embodiment variants, but rather various combinations of the individual embodiment variants with one another are possible and this variation possibility is based on the teaching on technical action through the subject invention Skills of the specialist working in this technical field.

The scope of protection is determined by the claims. However, the description and drawings must be used to interpret the claims. Individual features or combinations of features from the different exemplary embodiments shown and described can represent independent inventive solutions in their own right. The task underlying the independent inventive solutions can be found in the description.

All information on value ranges in this description should be understood to include any and all sub-ranges, e.g. the information 1 to 10 should be understood to include all sub-ranges, starting from the lower limit 1 and the upper limit 10 , that is, all subranges begin with a lower limit of 1 or greater and end with an upper limit of 10 or less, for example 1 to 1.7, or 3.2 to 8.1, or 5.5 to 10.

For the sake of order, it should finally be pointed out that in order to better understand the structure, elements have sometimes been shown out of scale and/or enlarged and/or reduced in size.

List of reference symbols

1 suspension 31 Coating 2 Sieve section 32 surface 3 Sieve 33 Coating 4 surface 34 Sublimation printing dye 5 surface 35 Page 6 Dehydrators 36 Textile substrate 7 Negative pressure device 37 Thermal transfer printer 8th paper web 9 Press section 10 roller 11 Felt mat 12 drying section 13 Drying cylinder 14 pulley 15 Sieve track 16 Sieve track 17 surface 18 Coating device 19 Solution 20 Size press 21 Applicator roller 22 surface 23 Solution 24 Film Press 25 stripper roller 26 smoothing work 27 Recording roll 28 Transfer paper 29 Paper substrate 30 surface

Claims (15)

1. Method for producing transfer paper for dye sublimation printing processes comprising the process steps:

   a) providing a cellulose or an aqueous cellulose suspension,

   b) producing a suspension (1) of the cellulose in water with a water content of at least 90 % by weight,

   c) optionally adding additives, fillers or process chemicals to the aqueous suspension (1),

   d) dewatering the suspension (1) to a water content of 60 % by weight to 85 % by weight by means of a wire section (2),

   e) drying the dewatered suspension (1) to a paper web (8) with a water content of 30 % by weight to 55 % by weight by means of a press section (9),

   f) further drying of the paper web (8) to a water content of 1 % by weight to 10 % by weight by means of a dryer section (12),

   g) coating at least one of the surfaces (17) of the paper web (8) by means of a coating device (18) by applying an aqueous solution (19) of a modified starch, which starch is cationically modified and has a degree of substitution by cationic groups of 0.03 to 0.6 mol/mol,

   h) post-drying of the coated paper web (8) by means of a further dryer section (12) to a water content of 3 % by weight to 7% by weight,

   i) finishing of the coated and post-dried paper web (8) .
2. Method according to Claim 1, characterized in that in process step g) an aqueous solution (19) of a starch cationically modified with quaternary ammonium groups is used.
3. Method according to Claim 1 or 2, characterized in that in process step f) the paper web (8) is dried to a water content of 2.5 % by weight to 10 % by weight, and in process step g) the aqueous solution (19) of the modified starch is applied to at least one surface (17) of the paper web (8) by means of a size press (20) .
4. Method according to Claim 3, characterized in that in process step g) the aqueous solution (19) of the modified starch is applied to one surface (17) of the paper web (8) and an aqueous solution (23) which contains 5 % by weight to 15 % by weight of an enzymatically degraded starch is applied to the other surface (22) of the paper web (8).
5. Method according to Claim 1 or 2, characterized in that in process step f) the paper web (8) is dried to a water content of 1.2 % by weight to 2.0 % by weight, and in process step g) the aqueous solution (19) of the modified starch is applied to at least one surface (17) of the paper web (8) by means of a film press (24).
6. Method according to Claim 5, characterized in that in process step g) the aqueous solution (19) of the modified starch is applied to one surface (17) of the paper web (8) and an aqueous solution (23) which contains 5 % by weight to 15 % by weight of an enzymatically degraded starch is applied to the other surface (22) of the paper web (8).
7. Method according to one of the preceding claims, characterized in that in process step g) an aqueous solution (19) having a concentration of the modified starch of 5 % by weight to 20 % by weight is applied to at least one surface (17) of the paper web (8).
8. Method according to Claim 7, characterized in that an amount of 0.5 g/m² to 20 g/m² of the aqueous solution (19) of the modified starch is applied to at least one surface (17) of the paper web (8).
9. Method according to one of the preceding claims, characterized in that in process step h) a water content of the coated paper web (8) of 4 % by weight to 6 % by weight is set.
10. Transfer paper (28) for dye sublimation printing processes, in particular produced using a method according to one of Claims 1 to 9, comprising a paper substrate (29) having a basis weight of 40 g/m² to 160 g/m², wherein at least one surface (30) of the paper substrate (29) is provided with a coating (31), which coating (31) consists at least predominantly of a modified starch, which starch is cationically modified and has a degree of substitution by cationic groups of 0.03 to 0.6 mol/mol, and wherein the transfer paper (28) has a water content of 3 % by weight to 7 % by weight.
11. Transfer paper according to Claim 10, characterized in that the coating (31) consists of a starch modified cationically with quaternary ammonium groups.
12. Transfer paper according to Claim 10 or 11, characterized in that the coating (31) has a basis weight of 0.2 g/m² to 4 g/m².
13. Transfer paper according to one of Claims 10 to 12, characterized in that it has a water content of 4 % by weight to 6 % by weight.
14. Transfer paper according to one of Claims 10 to 13, characterized in that it has a Cobb water absorbency according to ISO 535 of 20 g/m² to 35 g/m².
15. Method for printing textiles, comprising production of a negative print image on a transfer paper (28) by means of sublimation printing dye(s) (34), transferring the print image to a textile substrate (36) by means of a thermal transfer printer (37), characterized in that a transfer paper (28) according to one of Claims 10 to 14 is used.

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