FIELD OF THE INVENTION

The present invention relates to a machine for making tissue paper in which a Yankee drying cylinder with a steel shell is used. The invention also relates to a method of making tissue paper in which method the inventive machine is used.

BACKGROUND OF THE INVENTION

A machine for making tissue paper typically includes a Yankee drying cylinder which, during operation, is heated internally by hot steam. In the following, the term“Yankee” will be used for Yankee drying cylinders. A wet fibrous web that is transferred to the Yankee is dried on the outer surface, the“drying surface” of the Yankee, and subsequently creped from the drying surface (the outer surface) of the Yankee by a creping doctor. During the twentieth century, Yankees were usually made of cast iron but in recent years, Yankees have been made in welded steel. A steel Yankee weighs less than a cast iron Yankee having the same required strength and steel Yankees are thus seen as advantageous. An example of a steel Yankee is disclosed in EP 2126203 B 1. As mentioned above, a creping doctor is used to crepe the ready-dried fibrous web from the surface of the Yankee. In practice, this means that the surface of the Yankee will be subjected to pressure from the doctor blade. A cast Yankee has a surface with a relatively high hardness since cast iron may typically have an HB hardness (i.e. Brinell hardness) of 220 - 260. For this reason, the surface is not so easily worn out by contact with the doctor blade that is pressed against its surface. However, experience has showed that even Yankees made of cast iron are subjected to a considerable degree of wear. For this reason, it has long been a practice to metallize the surface of Yankees in order to obtain a harder surface. In the context of this patent application,“to metallize” and“metallization” refer to a process in which the outer surface (the drying surface) of the Yankee is coated with a hard layer which may be based on an element or an alloy or a mixture of metal powder and at least one carbide or nitride or possibly other elements, or a metal matrix containing at least one carbide or nitride or possibly other elements. The coating may typically be applied to the surface of the Yankee by spraying. An example of this technology is disclosed in US patent No. 4,064,608. Other examples are disclosed in US patent No. 5,123,152; US patent No. 6,171,657 and US patent No. 10,240,291. It should be noted that many different compositions for the hard layer have been proposed. For example, the hard layer may be an iron alloy containing from about 20 to about 47 weight per cent chromium, about 2.5 to about 6.5 weight per cent boron, about 1.7 to about 2.7 weight per cent silicon, and less than about 8 weight per cent molybdenum as suggested in the above-mentioned US patent No. 6,171,657. In the context of Yankees, it should be noted that the term“coating” may also be used to refer to liquid coating that is continuously sprayed onto the surface of the Yankee during operation which is fundamentally different from the hard coating that is applied as a part of the manufacturing process. When discussing application of a hard layer in order to increase hardness and resistance to wear, a technical term often used by persons skilled in the art is“metallization” and this term and“to metallize” (except when used in reference to other documents that are cited herein and that use the term in a different meaning) will be used in this patent application and all patents that may be granted based on it to refer to the application of a hard and wear-resistant layer permanently joined to the steel drying surface of the Yankee, regardless of the exact composition of that layer. Such a layer will be called“metallization layer” in the following. The term “coating” as used henceforth in this patent application will refer to the application of a temporary liquid coating during operation of a tissue paper making machine (except when used in reference to other documents that are cited herein and that use the term in a different meaning).

Steel used for making steel Yankees has a hardness which is markedly lower than that of cast iron. A typical value for the hardness of such steel may be on the order of about 140 HB or, possibly, in the range of 120 - 170 HB.

Since steel has a hardness (on the order of 140 HB) that is substantially lower than that of cast iron (220 - 260 HB), it has been a requirement that Yankees made of steel be metallized. Different metallization layers for steel Yankees have been proposed. In a paper from 2007, Mr. Jorg Baubock discussed a steel Yankee with a“metallic coating” with a surface hardness which was said to be double that of cast iron (“Application of a Steel Yankee in Tissue Machines”, TAPPI Yankee Dryer Safety Committee,

Jacksonville 10/23/07) although no details were given about the exact composition. In a 2008 paper by Mr. Luca Mignani (“Advances with Steel Yankee Dryers”, TISSUE

WORLD, Asia 2008), Mr. Mignani suggested that a steel Yankee dryer can be provided with a“metal coating” (i.e. metallization layer) and that the“metal coating” may be for example an alloy with a high content of Cr and Ni and have a hardness of 60 HRC. Although hardness measured according to HRC does not correspond directly to Brinell hardness, the value of 60 HRC can be said to correspond to about 600 HB. In the same paper, Mr. Mignani also suggests that INFINIKOTE® can be used. Infinikote® is a trade mark owned by Valmet Corporation and refers to metallization of Yankees with thermal spraying and which, at least in one version thereof, is free from chromium. The paper by Mr. Mignani gives the hardness value in this case as“50 to 60 HRC”, i.e. on the order of about 500 - 600 HB.

As an alternative to metallization, it has been suggested that the shell of a steel Yankee be hardened by laser and such a solution is disclosed in AT 519996 A2. According to that document, it is possible to obtain a hardness value of up to 400 HB.

However, both currently known technologies for metallization and possible hardening by laser represent a further step in the manufacturing process of a Yankee which makes the manufacturing process more complicated.

With regard to metallized steel Yankees, the inventors of the present invention have also noted that streaks can occur in the paper that has been dried on such dryers.

The inventors have also seen that the metallization layer of a Yankee may crack and/or delaminate which makes it necessary to halt production and shut off the machine. When this occurs, production is seriously disturbed and the costs for this may be very high.

Therefore, it is an object of the present invention to provide a machine using a steel Yankee that is properly protected against surface wear and which can yet be manufactured in a simple way. A further object of the invention is to prevent the occurrence of streaks in the paper that is died on the machine. It is also an object to provide a method of manufacturing tissue paper which reduces wear on the dying surface of the Yankee and which reduces the risk of disturbances in the production.

BRIEF SUMMAY OF THE INVENTION

The invention relates to a machine for making tissue paper. The machine comprises a Yankee having a shell which has a circular cylindrical shape and a heatable outer surface (i.e. the drying surface) which heatable outer surface is a steel surface. The Yankee is arranged to be rotatable about a longitudinal axis of rotation in a direction of rotation. The machine further comprises an imprinting fabric that forms a continuous loop and which is arranged to carry a fibrous web to the Yankee. At least the web- contacting side of the imprinting fabric has a texture such that a three-dimensional pattern can be imprinted into the fibrous web if the imprinting fabric is pressed into the fibrous web. A press roll is arranged inside the loop of the imprinting fabric and forms a non-dewatering transfer nip against the Yankee. A creping doctor is arranged to act against the outer surface of the shell to crepe a dried fibrous web from the outer surface of the shell. A coating device which, in the direction of rotation of the Yankee, is located after the creping doctor but before the transfer nip is arranged to be capable of applying a liquid coating onto the outer surface of the shell. The shell is a non- metallized steel shell.

Since the shell is a non-metallized steel shell, the outer surface of the shell is formed by steel and the creping doctor will thus act against a surface formed by steel instead of acting against a surface formed by a metallization layer. Strictly speaking, the creping doctor (or to be precise, the blade of the creping doctor) will act against a surface that is formed by steel but has a layer of liquid coating that protects the steel surface from the creping doctor.

Preferably, the outer surface of the shell is formed by a steel material having a Brinell hardness in the range of 120 HBW - 170 HBW, preferably in the range of 130 - 160 HBW. In_this context, the HBW hardness value for the outer surface of the shell of the Yankee should be understood as the value as verified according to ISO 6506-1, HBW 10/3000. While the indicated range of hardness values is a preferred range, embodiments are conceivable in which the outer surface of the shell has a different value for hardness, for example up to 180 HBW.

In preferred embodiments, the machine may optionally comprise a supply system for a coating fluid which supply system is connected to the coating device such that it can supply coating to the coating device. The supply system and the coating device may be dimensioned such that, when the machine is running with a machine speed of 1600 m/minute, the coating device can deliver coating to the outer surface of the shell in a quantity in the range of 15 mg/m ² - 120 mg/m ² and preferably such that, at a machine speed of 2000 m/minute, the coating device can deliver coating to the outer surface of the shell in a quantity up to 120 mg/m ² . In this context, the term“machine speed” is to be understood as the peripheral speed of the outer surface (the drying surface) of the shell of the Yankee.

In embodiments of the invention, a cleaning doctor may be arranged to act against the outer surface of the shell at a location which, in the direction of rotation of the Yankee, is located after the creping doctor but before the coating device.

According to a first embodiment, the machine may additionally comprise at least one through- air drying cylinder. In this embodiment, the imprinting fabric will be an air permeable fabric which is arranged to carry a fibrous web over a part of the circumference of the at least one through-air drying cylinder and to carry the fibrous web from the at least one through-air drying cylinder to the transfer nip formed between the press roll and the Yankee.

According to a second embodiment of the invention, the imprinting fabric is an air permeable fabric and the machine also comprises an endless belt that forms a loop and has an outer side/outer surface which is covered by polyurethane. The machine further comprises a felt forming a loop and a press roll is located inside the loop of the felt. Another press roll is located inside the loop of the endless belt and forms a dewatering nip with the press roll inside the loop of the felt. The endless belt is arranged to carry a fibrous web that has been dewatered in the dewatering nip from the dewatering nip to an intermediate transfer point located between the dewatering nip and the non-dewatering transfer nip formed against the Yankee. A suction pick-up roll is placed inside the loop of the imprinting fabric at the intermediate transfer point and is arranged to pick up the fibrous web from the endless belt and transfer it to the imprinting fabric. The imprinting fabric then is arranged to carry the fibrous web from the intermediate transfer point to the transfer nip formed against the Yankee.

In a variation of the second embodiment, a nip roll may optionally be located inside the loop of the endless belt and form a transfer nip against the suction roll within the loop of the imprinting fabric such that the intermediate transfer point is a transfer nip.

In a third embodiment, the imprinting fabric may be an endless substantially

impermeable belt having a web-contacting surface defining a multitude of depressions. In this embodiment, a dewatering nip is formed between a press roll within the loop of the imprinting fabric and a counter roll located within the loop of a water-absorbing felt.

The press roll that forms a transfer nip with the Yankee may advantageously be a roll with a cover in a polymeric material and the thickness of the cover is in the range of 15 mm - 35 mm, preferably 20 mm - 25 mm, and a hardness in the range of 20 - 40 P&J (Pusey & Jones hardness according to standard test method D531-00). The cover may be a cover with grooves, but it can also be a cover with a plain outer surface without grooves.

However, it should be understood that the press roll in the transfer nip against the Yankee may very well take other forms than a roll with such a cover. For example, it may conceivably be a shoe roll.

The creping doctor may have a blade which has a free length in contact with the outer surface of the shell which is in the range of 12 mm - 25 mm. The invention also relates to a method for making tissue paper. The inventive method comprises forming a fibrous web in a forming section and carrying the newly formed fibrous web on one or several fabrics of which at least one is an imprinting fabric that forms a loop. At least the web-contacting side of the imprinting fabric has a texture. The fibrous web is carried on the imprinting fabric to a Yankee which rotates about a longitudinal axis of rotation in a direction of rotation and has a shell with a circular cylindrical shape and an outer surface. The fibrous web is transferred from the imprinting fabric to the Yankee in a non-dewatering transfer nip formed between the Yankee and a press roll arranged inside the loop of the imprinting fabric such that the textured web-contacting side of the imprinting fabric imprints a three-dimensional pattern into the fibrous web in the transfer nip. The fibrous web is then dried on the outer surface of the shell of the Yankee. A creping doctor is used to crepe the dried fibrous web from the outer surface of the shell. A liquid coating is applied on the outer surface of the shell at a location which, in the direction of rotation of the Y ankee, lies after the creping doctor but before the non-dewatering transfer nip. The shell is a non- metallized steel shell such that the outer surface of the shell is formed by non-metallized steel. Preferably, the non-metallized steel is untreated apart from the manufacturing mechanical processes such as welding, turning, milling, grinding, polishing and the like.

The liquid coating is applied preferably by means of a coating device which, in the direction of rotation of the Yankee, is located after the creping device but before the transfer nip. The method may then further comprise the step of causing a cleaning doctor to act against the outer surface of the shell at a location which, in the direction of rotation of the Yankee, is located after the creping doctor but before the coating device.

In embodiments of the inventive method, the method may comprise applying a linear load on the creping doctor which is in the range of 4.0 kN/m - 10 kN/m. For example, the linear load may be in the range of 4.0 kN/m - 7.5 kN/m or in the range of 8.0 kN/m - 10 kN/m.

The liquid coating may preferably be applied onto the outer surface of the shell in a quantity of 15 mg/m ² - 120 mg/m ² , more preferably 15 mg/m ² - 80 mg/m ² and even more preferably in a quantity of 30 mg/m ² - 75 mg/m ² . However, the coating may also be applied in quantities even higher than 120 mg/m ² . For example, the liquid coating may conceivably be applied onto the outer surface of the shell in quantities of as much as 120 mg/m ² - 150 mg/m ² . In extreme cases, quantities of even more than 150 mg/m ² may be considered if wear on the Yankee and/or the doctor blade is still perceived as a problem. The coating that is applied may optionally comprise 50 - 65 % by weight polyvinyl alcohol (PVOH), an adhesive that constitutes 15 - 30 % by weight of the coating, a modifier/release agent that constitutes 5 - 30 % by weight of the coating and preferably also phosphate. Phosphate used for such coating may be, for example, mono- ammonium phosphate, Di-ammonium phosphate, trisodium phosphate or tetra

phosphate. Instead of the above-mentioned phosphates (or in combination with one or several of them), phosphoric acid may be used in the coating.

In the transfer nip against the Yankee, a linear load may optionally be applied which is in the range of 40 kN/m - 100 kN/m, preferably in the range of 45 kN/m - 90 kN/m. The inventive method may take the form that the imprinting fabric is an air permeable fabric that carries the fibrous web over a part of the circumference of at least one through-air drying cylinder and then carries the fibrous web from the at least one through-air drying cylinder to the transfer nip formed between the press roll and the Yankee.

Alternatively, the method may take such a form that the imprinting fabric is an air permeable fabric while the method also comprises carrying the web on a felt that forms a loop to a dewatering nip formed between a press roll inside the loop of the felt and a counter roll within the loop of an endless belt having an outer web-contacting side which is covered by polyurethane. The fibrous web is then dewatered in the dewatering nip and transferred to the endless belt. The fibrous web is carried on the endless belt away from the dewatering nip and the fibrous web is then picked from the endless belt and transferred to the imprinting fabric by a suction device arranged inside the loop of the imprinting fabric. This is followed by carrying the fibrous web to the transfer nip formed against the Yankee by the press roll which is arranged inside the loop of the imprinting fabric. Preferably, the imprinting fabric runs with a speed that is lower than the speed of the endless belt.

Alternatively, the method may be performed with an imprinting fabric which is an endless substantially impermeable belt having a web-contacting surface defining a multitude of depressions. The method then comprises the step of dewatering the fibrous web in a dewatering nip which is formed between a press roll within the loop of the imprinting fabric and a counter roll located within the loop of a water-absorbing felt. The fibrous web is then carried on the imprinting fabric to the transfer nip and transferred to the outer surface of the shell of the Yankee. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic cross-sectional representation of a Yankee.

FIG. 2 is a schematic side view of the Yankee during operation.

FIG. 3 is a schematic side view of a creping doctor acting against a Yankee.

FIG. 4 is a schematic side view of an embodiment of the inventive tissue paper making machine.

FIG. 5 is a schematic side view of a second embodiment of the inventive tissue paper making machine.

FIG. 6 is a schematic side view of a third embodiment of the inventive tissue paper making machine.

Fig. 7 is a schematic cross-sectional view of a press roll used in the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to Fig. 1, the Yankee 2 which is used in the present invention has a shell 3 with an outer surface 4 arranged between two end walls 29. The shell 3 is preferably welded to the end walls 29, for example as disclosed in EP 2920360 B1 or in EP 2126203 Bl. In Fig. 1, the welds are indicated by the reference numeral 31 and they are preferably circumferential welds. The end walls 29 may optionally be provided with thermal insulation, for example as disclosed in EP 2475819 Bl or in WO 2016/026662 Al. The Yankee 2 has journals 32 and, during operation, the Yankee 2 is intended to rotate about an axis of rotation A which is the longitudinal axis of symmetry of the Yankee 2. During operation, the Yankee 2 will be heated from the inside by steam. The hot steam will transfer heat to the shell 3 such that the shell 3 is heated and can serve to evaporate water from a fibrous web when the fibrous web comes into contact with the outer surface 4 (i.e. drying surface 4) of the shell 3. Therefore, the Yankee is designed such that steam can be delivered into the Yankee 2. The inner surface of the shell 3 may be provided with circular grooves in which condensate is collected as disclosed in EP 2126203. In preferred embodiments, the Yankee may also be designed such that condensate can be removed from the interior of the Yankee 2 and the Yankee may thus have a condensate removal device. For example, the Yankee 2 may have a condensate removal device as disclosed in US patent No. 5,090,135 or in EP 2614182 Bl. With reference to Fig. 2, the Yankee 2 is represented rotating about its axis of rotation A in the direction of arrow R, i.e.“clockwise” in Fig. 2. As can be understood from Fig. 2, the shell 3 has a circular cylindrical shape. The Yankee 2 is so arranged in the tissue paper making machine that it is rotatable about the axis of rotation A. The Yankee 2 may optionally be provided with a Yankee drying hood 30, for example such a Yankee drying hood as is disclosed in EP 2963176 B1 but other Yankee drying hoods may also be considered and the inventive machine may also operate without a Y ankee drying hood.

An imprinting fabric 5 that forms a continuous loop is arranged to carry a fibrous web W to the Yankee 2. At least the web-contacting side 6 of the imprinting fabric 5 has a texture such that a three-dimensional pattern can be imprinted into the fibrous web 5 when the imprinting fabric 5 is pressed into the fibrous web W. A press roll 7 is arranged inside the loop of the imprinting fabric 5 and forms a non-dewatering transfer nip TN against the Yankee 2. It should be understood that, during operation, the press roll 7 will act with a certain linear load against the shell 3 of the Yankee 2 and the linear load may suitably be in the range of 40 kN/m - 100 kN/m, preferably in the range of 45 kN/m - 90 kN/m. This means that the texture of the imprinting fabric 5 will be pressed into the fibrous web W while it is still relatively wet. During operation, the textured web-contacting side 6 of the imprinting fabric 5 will thus imprint a three-dimensional pattern into the fibrous web W in the transfer nip TN. The fibrous web W is then dried on the outer surface 4 of the shell 3 of the Yankee 2. In Fig. 2, it can be seen how a creping doctor 8 is arranged to act against the outer surface 4 of the shell 3 to crepe the ready-dried fibrous web W from the outer surface 4 of the shell 3 as is known as such in the art to which the invention pertains. As seen in the direction of rotation R of the Yankee 2, a coating device 10 is located after the creping doctor 8 but before the transfer nip TN. The coating device 10 is arranged to apply a liquid coating onto the outer surface 4 of the shell 3. The coating is dissolved in water and may be supplied from a system/arrangement 11 for supplying an aqueous solution of a liquid coating to the coating device 10. The coating itself (i.e. apart from the water) suitably contains 50 - 65 % by weight polyvinyl alcohol (PVOH), adhesive at an amount of 15 - 30 % by weight and a modifier/release agent which may constitute 5 - 30 % by weight. In addition, the coating may contain phosphate.

For processes in which an imprinting fabric is used to deliver the fibrous web W to the Yankee, the inventors have found that more coating should be applied than in other processes for manufacturing tissue paper. In a more conventional process in which a felt is used to convey the fibrous web to the Yankee, coating is typically applied in an amount of 4 - 10 mg/m ² (not including the water in which the coating is dissolved). However, the inventors have found that, in processes in which an imprinting fabric 5 delivers the fibrous web W to the outer surface 4 of the shell 3, a larger amount of coating should be applied. If the layer of coating is too thin, there is a considerable risk that the imprinting fabric 5 will damage the web W instead of giving it the desired three-dimensional pattern that increases the bulk of the fibrous web. The inventors have found that, for such processes, liquid coating should be applied in a quantity of at least 15 mg/m ² and suitably even higher quantities. The precise amount that is required depends on the process but there are cases where quantities up to 120 mg/m ² may be considered. In many realistic cases, the amount of coating may be within the range of 15 mg/m ² - 80 mg/m ² , 30 mg/m ² - 75 mg/m ² , or 30 mg/m ² - 55 mg/m ² .

The inventors have found that, when coating is applied in such quantities, the coating itself can act as sufficient protection for the outer surface 4 of the shell 3 of the Yankee 2. This makes it possible to use a steel shell 3 with an outer surface 4 that is entirely non-metallized and the coating is then applied directly onto the steel drying surface (i.e. onto the outer surface 4 of the shell 3). Thereby, the complicated process of metalizing the shell 3 can be entirely avoided. Preferably, the non-metallized steel is not treated with any mechanical processes apart from the manufacturing process such as welding, turning, milling, grinding polishing and the like.

Moreover, the inventors have established that the hard, outer layer of a metallized

Yankee entails a disadvantage. While the metallization protects against wear, there is a risk that cracks will occur in the hard layer and the hard layer of metallization may even delaminate· This can damage the doctor blade which is intended to be even and very sharp. When a crack catches the doctor blade, the doctor blade can be deformed, and the inventors have found that this is a source of streaks in the paper. Therefore, the

inventors have concluded that elimination of the metallization layer is actually desirable from a process point of view.

The inventors have also seen that streaks in the paper is a phenomenon that very often precedes such a major collapse of the metallization layer that the entire machine must be shut off such that the Yankee can be repaired or replaced. Without wishing to be bound by theory, it is believed that streaks in the paper is a warning sign that a major collapse of the metallization layer is imminent.

While it is currently unknown to the inventors if a laser hardened surface may entail a risk that the doctor blade is damaged, they have concluded that such a hardening is actually not necessary at all if coating is applied in a sufficient quantity and thus eliminating the laser treatment results in a less complicated manufacturing process.

With reference to Fig. 2, it can be seen that a cleaning doctor 12 is arranged to act against the outer surface 4 of the shell 2 at a location which, in the direction of rotation R of the Yankee 2, is located after the creping doctor 8 but before the coating device 10. The cleaning doctor 12 removes remaining fibres and coating. While the cleaning doctor 12 is advantageous, it is not an absolute requirement for practicing the invention and it is thus optional.

With further reference to Fig. 3, it can be seen that the creping doctor 8 has a blade 9 that acts against the outer surface 4 of the Yankee shell 3. The blade 9 is held in a blade holder 33. From the blade holder 33, the blade 9 has a free length L in contact with the outer surface 4 of the shell 3 which is in the range of 12 mm - 25 mm. The expression “free length” should here be understood as the distance from the blade holder 33 to the surface 4 of the Yankee shell 3. When the doctor blade 9 acts against a surface 4 onto which coating has been supplied in such large quantities (i.e. 15 mg/m ² and higher), the linear load of the creping doctor 8 must be higher than what would otherwise have been the case. For processes in which the fibrous web W is not transferred to the Yankee by an imprinting fabric, in particular processes in which the fibrous web W is carried to the Yankee by a felt, the linear load may be on the order of 1.5 kN/m - 3.5 kN/m. However, in processes using an imprinting fabric and in which larger quantities of coating is used, the linear load may be from 4 kN/m - 10 kN/m. Such higher linear loads can be used to prevent the coating film from building up to a thickness that would be detrimental to the process or even make continued operation impossible. Since the use of an imprinting fabric makes it desirable to use more liquid coating than what is the case with processes in which a felt carries the fibrous web to the Yankee, higher linear loads than otherwise are used to control the layer of liquid coating, i.e. to prevent that the layer of liquid coating builds up and becomes too thick. With such higher linear loads, the free length L should preferably be smaller than what would be the case in a more conventional process. With a shorter free length L, the blade 9 can take the higher linear loads better. In such processes where the linear load is lower, the free length of the blade may be in the range of 30 mm - 40 mm.

The creping doctor 8 may optionally be arranged such that it can be moved toward or away from the outer surface 4 of the shell 3. This may be the case when, for example, the blade 9 is to be exchanged. For example, the doctor can be pivoted away from the outer surface 4 (or pivoted toward the outer surface 4). A possible embodiment of the invention will now be explained with reference to Fig. 4. In this embodiment, the inventive machine 1 includes a through- air drying cylinder 13. In the embodiment of Fig. 4, only one through-air drying cylinder 13 is used. However, it should be understood that embodiments of the invention with two or even mote such through-air drying cylinders are conceivable. In the embodiment of Fig. 4, a fibrous web W is formed in a forming section 25 with a headbox 26 and the fibrous web is formed between two fabrics 5, 34 of which the fabric 34 is used as a forming fabric. The fabric 5 is also used as a forming fabric but also serves as the imprinting fabric. The fibrous web W is formed when stock is ejected into the forming gap 37 between the fabrics 5, 34 and passed over the forming roll 36 as is known in the art. In this embodiment, the imprinting fabric 5 is a through-air drying fabric which carries the fibrous web W over a part of the circumference of the through-air drying cylinder 13. The through-air drying fabric (i.e. the imprinting fabric 5) then carries the fibrous web W to the transfer nip TN between the Yankee 2 and the press roll 7. In this embodiment, the imprinting fabric 5 is thus an air permeable fabric. An example of such a fabric which may be used in the embodiment of Fig. 4 is disclosed in US patent No.

7,878,223. Another example of an imprinting fabric that may be used in the

embodiment of Fig. 4 is disclosed in US patent No. 9,062,414. When the fibrous web W has been transferred to the Yankee 2, it will be creped off from the outer surface 4 of the Yankee shell as explained with reference to Fig. 2 and coating will be applied by the coating device 10. During creping, the creping doctor 8 will act against the non- metallized outer surface of the shell 3 and the coating device will supply liquid coating which, in addition to its other functions, also serves to protect the non- metallized steel drying surface of the shell 3 of the Yankee 2. As can be seen in Fig. 4, the liquid coating is applied to the outer surface 4 of the shell 3 at a location which, in the direction of rotation R of the Yankee 2, lies after the creping doctor 8 but before the non-dewatering transfer nip TN. The outer surface 4 of the shell 3 is formed by a steel material that may typically have a Brinell hardness (HBW) in the range of 120 - 170, preferably in the range of 130 - 160. A typical value for the hardness may be about 140 HBW or 145 HBW.

The doctor (or strictly speaking the doctor blade 9) will thus act against a steel surface with a layer of liquid coating that protects the steel surface. It should be noted that the layer of liquid coating also means protection for the doctor blade 9. The liquid coating forms a thin layer or film on the outer surface of the shell 3 of the Yankee 2. The film is normally very thin. The rate at which the liquid coating is applied corresponds to a thickness which can be expected to be less than 1 micrometre. Over time, the thickness of the film will build up, but the built-up film will still be on the order of only about 0.1 mm, i.e. 100 micrometres. However, this thin film is still sufficient to protect the outer steel surface 4 of the shell 3.

The steel (normally rolled steel) used in the shell may be a grade known as EN P355 NH (a steel grade following the standard EN10028-3). This steel grade is suitable for pressure vessels such as Yankees. Other steel grades that could be used include grades according to ASME SA 516 standard and an example of a steel grade that can be used is ASME SA 516 grade 70. Steel grades according to ASME SA 36 may also be considered as well as steel grades according to the standard Q345R (Chinese Guobiao standard).

When the fibrous web W has been creped off from the outer surface 4 of the Yankee shell 3, it can be sent to a reel-up (not shown in Fig. 4). In Fig. 4, the fabrics 5, 34 are supported by rolls 35 and the fabrics 5, 34 are running in the direction indicated by arrows P. The overall machine direction is indicated by the arrow MD. It should be understood that the precise arrangement of various components in Fig. 4 is only an example and variations of the configuration are conceivable. For example, the imprinting fabric 5 is not necessarily used as a forming fabric in the forming section 25 but could be arranged to receive the fibrous web W from a separate forming fabric. Although only a creping doctor 8 is shown in Fig. 4, it should be understood that the embodiment of Fig, 4 could very well include a cleaning doctor 12 as indicated in Fig.

2. While a Yankee drying hood 30 is indicated in Fig. 4, it should be understood that, while this may be advantageous, it is an optional feature. In the embodiment of Fig. 4, the coating device 10 may supply liquid coating containing PVOH to the outer surface 4 of the Yankee shell in a quantity which may suitably be in the range of 30 mg/m ² - 120 mg/m ² , preferably 30 mg/m ² - 75 mg/m ² and even more preferred 30 mg/m ² - 50 mg/m ² . In some cases, the liquid coating may conceivably be supplied in quantities down to 20 mg/m ² . The linear load for the creping doctor may suitably be in the range of 8 kN/m - 10 kN/m but other values are also conceivable. As explained with reference to Fig. 2, the linear load applied in the transfer nip TN against the Yankee 2 may be in the range of 40 kN/m - 100 kN/m, preferably in the range of 45 kN/m - 90 kN/m.

Yet another embodiment will now be explained with reference to Fig. 5. In the embodiment of Fig. 5, a fibrous web W is formed in the forming section 25 which has a head box 26 for injecting stock between the fabrics 34, 15 of which the fabric 15 is a water-receiving felt. The reference numeral 36 indicates a forming roll and the fabrics are supported in their path by rolls 35. The newly formed fibrous web W is carried by the felt 15 to a dewatering nip 18. The dewatering nip is formed between a press roll 16 located within the loop of the felt 15 and a press roll 17 which is located within the loop of an endless belt 14. The reference numeral 38 indicates an optional suction turning roll that can contribute to dewatering. The endless belt 14 has an outer side/surface 40 which is the web-contacting surface of the endless belt 14. The outer side 40 of the endless belt 14 is covered by polyurethane and is much smoother than the web contacting surface of the felt 15 such that the fibrous web will be easily transferred to the endless belt 14. The endless belt 14 carries the fibrous web W from the dewatering nip 18 to an intermediate transfer point ITP located between the dewatering nip 18 and the non-dewatering transfer nip TN formed against the Yankee 2. When the fibrous web reaches the intermediate transfer point ITP, it is transferred to the imprinting fabric 5 by means of a suction device 19 located within the loop of the imprinting fabric 5 which is an air permeable fabric having a texture such that a three-dimensional pattern can be imprinted into the still wet fibrous web W. The suction device 19 may be a suction box or a suction pick-up roll or the like. The suction device 19 picks up the fibrous web W from the endless belt 14 and transfers it to the imprinting fabric 5. The imprinting fabric 5 then carries the fibrous web W from the intermediate transfer point ITP to the transfer nip TN formed against the Yankee 2 and the fibrous web W is transferred to the Yankee 2, dried on the hot drying surface 4 of the Yankee and creped away from the Yankee 2 by the creping doctor 8. During creping, the creping doctor 8 will act against the non- metallized outer surface of the shell 3 and the coating device will supply liquid coating which, in addition to its other functions, also serves to protect the non- metallized steel shell 3 of the Yankee 2. The outer surface 4 (the drying surface) of the shell 3 is formed by a steel material that may typically have a Brinell hardness in the range of 120 HBW - 170 HBW, preferably in the range of 130 - 160. A typical value for the hardness may be about 140 HBW or 145 HBW.

After creping, the fibrous web W will be sent to a reel-up schematically indicated by the reference numeral 39. The reel-up 39 may be, for example, such as reel-up as disclosed in US patent No. 5,901,918 but any reel-up suitable for tissue paper may be considered. The coating device 10 supplies liquid coating containing PVOH to the outer surface 4 of the Yankee shell in a quantity which may suitably be in the range of 30 mg/m ² - 120 mg/m ² , preferably 30 mg/m ² - 75 mg/m ² and even more preferred 30 mg/m ² - 50 mg/m ² . In some cases, the liquid coating may conceivably be supplied in quantities down to 20 mg/m ² . As can be seen in Fig. 5, the liquid coating is applied to the outer surface 4 of the shell 3 at a location which, in the direction of rotation R of the Yankee 2, lies after the creping doctor 8 but before the non-dewatering transfer nip TN. The linear load for the creping doctor may suitably be in the range of 8 kN/m - 10 kN/m but other values are also conceivable. Although no cleaning doctor is shown in Fig. 5, it should be understood that a cleaning doctor could optionally be present. Although no Yankee drying hood is shown in Fig. 5, it should likewise be understood that the machine 1 may optionally have such a Yankee drying hood. As explained with reference to Fig. 2, the linear load applied in the transfer nip TN against the Yankee 2 may be in the range of 40 kN/m - 100 kN/m, preferably in the range of 45 kN/m - 90 kN/m.

If the suction transfer device 19 is a suction roll, a nip roll 20 may optionally be placed within the loop of the endless belt 14 and form a transfer nip with the suction roll. The intermediate transfer point ITP will then be a transfer nip.

Preferably, but not necessarily, the imprinting fabric 5 runs with a linear speed that is lower than the linear speed of the endless belt 14. In the embodiment of Fig. 5, the endless belt may be running with a linear speed that is 5 % - 25 % higher than the linear speed of the imprinting fabric 5. In this way, a rush transfer is achieved that improves the caliper of the fibrous web W. For further understanding of the embodiment of Fig.

5, reference is made to US patent No. 8,871,060 which may serve as an additional source of information for how the embodiment of Fig. 5 can be implemented.

Yet another embodiment will now be explained with reference to Fig. 6. A fibrous web W is formed in a forming section 25 where a head box 26 is arranged to inject stock in a gap formed between a forming fabric 34 and a water- absorbing (water-receiving) felt 15. The felt 15 and the forming fabric run in their loops supported by lead rolls 35. The felt carries the newly formed fibrous web W to a dewatering nip formed between a press roll 22 within the loop of the water-receiving felt 15 and a press roll 21 (or counter roll 21). The press roll 21 is located within the loop of the imprinting fabric 5. The press roll

22 may optionally be a shoe roll. In the embodiment of Fig. 6, the imprinting fabric 5 is an endless substantially impermeable belt having a web-contacting surface 6 defining a multitude of depressions. Thereby, a three-dimensional pattern can be imprinted into the fibrous web W, both in the dewatering nip 28 and in the transfer nip TN formed by the press roll 7 and the Yankee 2. The imprinting fabric 5 in the embodiment of Fig. 6 may comprise a back layer (which does not contact the web) and a web-contacting layer that forms the web-contacting side 6 of the imprinting fabric 5. The web-contacting layer (side) has a multitude of depressions that may be uniformly distributed or distributed in a non-uniform way. The web-contacting side 6 may have flat or arched portions situated between the depressions. A coating of a polymer may be applied to the side of the fabric that does not come into contact with the fibrous web W. An example of a possible imprinting fabric 5 for the embodiment of Fig. 6 is disclosed in US patent No.

6,547,924 but the imprinting fabric 5 may also take other forms. However, the embodiment of Fig. 6 is intended for a substantially impermeable imprinting fabric, unlike the imprinting fabrics for the embodiments of Fig. 4 and Fig. 5. After having passed through the dewatering nip 28, the fibrous web is carried by the imprinting fabric 5 to the transfer nip TN and transferred to the Yankee 2 and dried on the Yankee 2 while the Yankee rotates in the direction of arrow R. The fibrous web W is then creped off from the outer surface of the Yankee shell 3 by the creping doctor 8. During creping, the creping doctor 8 will act against the non-metallized outer surface of the shell 3 and the coating device will supply liquid coating which, in addition to its other functions, also serves to protect the non-metallized drying surface 4 of the steel shell 3 of the Yankee 2. As can be seen in Fig. 6, the liquid coating is applied to the outer surface 4 of the shell 3 at a location which, in the direction of rotation R of the Yankee 2, lies after the creping doctor 8 but before the non-dewatering transfer nip TN. The outer surface 4 of the shell 3 is formed by a steel material that may typically have a Brinell hardness in the range of 120 HBW - 170 HBW, preferably in the range of 130 HBW - 160 HBW.

A typical value for the hardness may be about 140 HBW or 145 HBW.

In the embodiment of Fig. 6, the linear load used for the creping doctor 8 may suitably be in the range of 4 kN/m - 7.5 kN/m and liquid coating may preferably be supplied in a quantity of 15 mg/m2 - 30 mg/m2 although other linear loads and other quantities of liquid coating may also be considered.

The Yankee drying hood 30 that is shown in Fig. 6 may be, for example, a Yankee drying hood as disclosed in EP 2963176 B1 but other Yankee drying hoods may also be considered and the inventive machine may also operate without a Yankee drying hood.

While a cleaning doctor 12 is shown in Fig. 6, it should be understood that this cleaning doctor 12 is optional, even if it is an advantageous feature. When a cleaning doctor 12 is used, it will be caused to act against the outer surface 4 of the shell 3 at a location which, in the direction of rotation R of the Yankee 2, is located after the creping doctor 8 but before the coating device. This applies to all embodiments.

In all embodiments of the invention, the press roll 7 that forms a transfer nip TN with the Yankee 2 may be a roll with a cover 24 having a thickness in the range of 15 mm - 35 mm, preferably 20 mm - 25 mm, and a hardness in the range of 20 - 40 P&J, see Fig. 7. The cover is made of or comprises a polymeric material. The cover 24 may have a plain outer surface without grooves, alternatively it could also be provided with groves.

The press roll 7 that is used in the transfer nip against the Yankee may conceivably take other forms than a roll with a cover 24. For example, it may conceivably be a shoe roll with a design as disclosed in US patent No. 7,527,708; US patent No. 9,885,153 or EP 2085513 Bl. As another example, a suction roll could also be considered in this position.

In all embodiments of the invention, in particular in all embodiments of the inventive method, the linear load in the transfer nip TN, i.e. may be in the range of 40 kN/m - 100 kN/m, preferably in the range of 45 kN/m - 90 kN/m. Other linear loads may also be considered but practical experience has demonstrated to the inventors that the linear loads indicated are best suited if a three-dimensional pattern is to be imprinted into the fibrous web W.

It should be understood that, in all embodiments of the invention, the machine 1 would normally have a supply system 11 for a coating fluid which supply system 11 is connected to the coating device 10 such that it can supply coating to the coating device. The supply system 11 and the coating device 10 must be so dimensioned that they can deliver the required quantity of liquid coating. Preferably, the supply system 11 and the coating device 10 should be dimensioned such that, when the machine is running with a speed of 1600 m/minute, the coating device 10 can deliver liquid coating to the outer surface 4 of the shell 3 in a quantity in the range of 15 mg/m ² - 120 mg/m ² . Even more preferred, they should be so dimensioned that, even at a machine speed of 2000 m/minute, the coating device 10 can deliver coating to the outer surface of the shell 3 in a quantity of up to 120 mg/m ² , for some applications possibly even more.

In all embodiments, the liquid coating that is applied contains a substantial amount of polyvinyl alcohol (PVOH) and suitable compositions may comprise 50 - 65 % by weight polyvinyl alcohol (PVOH), an adhesive that constitutes 15 - 30 % by weight of the coating, a modifier/release agent that constitutes 5 - 30 % by weight of the coating and preferably also phosphate.

In all embodiments of the invention, the tissue paper making machine may be running at a speed which is in the range of 1200 m/min - 2000 m/rnin. However, higher speeds may also be considered and machine speeds in the range of 2000 m/min - 2300 m/min may also be used or possibly even speeds up to 2400 m/min. In all embodiments of the invention, the Yankee may have a diameter in the range of, for example, 3 m - 7 m. For example, the diameter may be 3.5 m; 3.66 m; 4.88 m or 5.5 m. The width of the Yankee 2 may be, for example, in the range of 1.5 m - 7 m. For example, the width could be 3 m or 5 m. However, the Yankee could also have other dimensions than the ones given above.

The end walls of the Yankee may be provided with thermal insulation to reduce heat losses. For example, the end walls may be provided with thermal insulation as disclosed in EP 2475819 Bl.

In the context of this patent application and any patent that may be granted based on this application, the term“tissue paper” should be understood as referring to paper with a basis weight which is equal to or greater than 12 g/m ² and equal to or less than 45 g/m ² , in particular equal to or greater than 12 g/m ² and equal to or less than 30 g/m ² .

The invention is based on the surprising finding that the application of a hard and wear- resistant layer that is permanently joined to the outer surface of the Yankee can be entirely avoided in such processes in which a sufficient amount of liquid coating is applied. Thanks to the inventive solution of using liquid coating as a way of protecting the Yankee shell 3, it is possible to avoid the complicated metallization process and manufacturing of the Yankee becomes easier. The blade of the creping doctor can thus act against a surface formed by steel instead of acting against a hard and wear-resistant layer permanently joined to the steel drying surface of the Yankee.

Moreover, since the Yankee does not have a layer of metallization, the invention significantly reduces the risk of deformation to the doctor blade due to faults in the metallization layer. Thereby, the risk of streaks in the tissue paper that is manufactured can be reduced.

Elimination of the metallization layer also reduces the risk that the machine must be shut down because of damages to the drying surface of the Y ankee.

Generally speaking, more liquid coating normally means better protection for the shell 3 and for the doctor blade 9. If it is noted that there appears to be unwanted wear on the shell of the Yankee, it is advisable to try increasing the supply of liquid coating.

However, an excessive use of liquid coating will be expensive and should be avoided.

While the invention has been discussed above in terms of a machine and a method, it should be understood that these categories merely reflect different aspects of one and the same invention. The method may thus include any step that would be the natural result of using the machine in its various embodiments and the machine may include means for performing any functional step of the method.