EP3707305B1

EP3707305B1 - Roll with anti-adhesive top layer and method for applying such a layer

Info

Publication number: EP3707305B1
Application number: EP18822482.8A
Authority: EP
Inventors: Sammy Lasseel; Dirk Vidts
Original assignee: Hannecard NV
Current assignee: Hannecard NV
Priority date: 2017-11-10
Filing date: 2018-11-12
Publication date: 2022-01-05
Anticipated expiration: 2038-11-12
Also published as: BE1025227B1; WO2019092663A1; EP3707305A1

Description

TECHNICAL FIELD

The present invention relates to rolls suitable for industrial applications, such as for example rolls for use as drying drums in the production of paper.

BACKGROUND

Rolls are used in demanding industrial environments where they are subject to long-term use under specific conditions. A particular application area for rolls is in producing a web, preferably a fibrous web such as paper, from pulp. In the guidance-based contact that the roll makes with the web, the roll has to support the pulp mass. A problem here is caused by stickies. Stickies are sticky contaminants present in the pulp and stem from, among other things, recycled material present in the paper pulp. The adhesion of stickies to drying drums disturbs the drying process, and also leads to irregularities in the produced paper. Such irregularities represent a loss of quality of the end product and are therefore undesirable. Improved anti-adhesion of the surface of the roll can help to prevent such problems.
EP 2 438 234 discloses a coating of a wear part for use in paper production. However, EP 2 438 234 discloses no measures adapted to contact with a web, since EP 2 438 234 is aimed at wear parts that make contact with the paper pulp, even before it has been processed into a web.
US 2010/00173167 discloses a coating which can be used to prevent stickies in paper production by providing the coating on the relevant parts of the paper production installation. However, the coating according to US 2010/00173167 is related to material choices which are insufficiently adapted to achieve the desired anti-adhesion of the surface of the roll.
US 2016/0340506 describes a protective layer which can be used to prevent stickies. However, US 2016/0340506 relates to substances such as paper machine coating, with therefore little relevance to the present invention.
US 2013/266808 A1 discloses a roll suitable for guiding a web, comprising a rigid and cylindrical roll core and a mantle comprising a coating with an anti-adhesive top layer comprising a fluoropolymer.
WO 2008/155453 A1 discloses the use of inorganic-organic hybrid sol-gel coatings for improving the wear and abrasion resistance as well as dirt resistance of metallic surfaces.
There is a need for a better device and a better method for rolls with an anti-adhesive surface. There is also a need for a simple and reliable way to apply such an anti-adhesive surface to such rolls. The present invention comprises an improved roll and an improved method for applying a top layer to a roll which provide a solution to at least one of the aforementioned drawbacks in known rolls.

BRIEF SUMMARY OF THE INVENTION

In a first aspect, the invention relates to a roll according to claim 1.
Inorganic-organic hybrid sol-gels are known from the development of new optical and bioactive materials, but not for use in rolls. Here, the presence of fluoropolymers in the sol-gel contributes to the anti-adhesive character of the roll.
Important advantages of such a roll are the advantageous mechanical properties of the roll when used for guiding a web, preferably the drying guiding of a web. The web can be a fibrous web such as in paper production from pulp, but can also have a substantially non-fibrous structure, such as for instance plastic foil. In the case of paper production from pulp, the roll according to the present invention can advantageously be used as a drying drum to dry the web when guiding the web, i.e. when drying guiding the web. The water-repellent or hydrophobic character of the top layer is extremely advantageous in this case. After all, the pulp contains stickies, which mainly occur in the wet part of the pulp, as a result of which the hydrophobic character contributes to the prevention of the adhesion. Fluoropolymers play an important role here because of their specific anti-adhesive and friction-reducing properties, which are very well adapted to the requirements set by an application on drying drums. However, such a roll can also be applied more widely, such as with heat exchangers and/or arrangements where the roll must make a guiding contact with a web or material to be guided without adhering to this material. The invention is also advantageous in such applications, since the top layer also has excellent water-repellent and anti-corrosive properties, is stable at high temperatures, has excellent chemical resistance to acids, bases and solvents, and at the same time offers great wear resistance. This wear resistance is further enhanced in a preferred embodiment wherein said coating comprises, under said top layer, an adhesion layer for proper adhesion of the coating to said jacket, said adhesion layer being provided over at least 80% of a core jacket, most preferably over at least 95%, and wherein said adhesion layer comprises a carbide, preferably tungsten carbide. The advantages of such an embodiment are further explained in detail in this document.
The anti-adhesive property of the top layer according to the present invention is based on an advantageous balance of free surface energy, surface structure and surface roughness. Due to the presence of sol-gel in the top layer of the roll, the surface becomes smoother on a micro scale, which helps to prevent contaminants such as stickies from attaching. Thanks to the sol-gel the roll becomes dirtrepellent in humid conditions, and a hydrophobic character also becomes possible. The presence of a fluoropolymer promotes the anti-adhesive character. Preferably, this is one of the fluoropolymers having a known anti-adhesive effect, such as PTFE, FEP or PFA. The fluoropolymer further contributes to a hydrophobic or superhydrophobic character for the top layer and the top layer jacket. A further advantage of the present invention is the advantageous combination of sol-gel and the fluoropolymer belonging to the sol-gel, wherein the anti-adhesive and hydrophobic character of both the sol-gel as such and the fluoropolymer contribute to superior performance of the roll in the prevention of stickies. In a preferred embodiment, said sol-gel comprises silica, more preferably hydrophobic silica. Silica, and in particular hydrophobic silica, appears to be an advantageous choice here because of its hydrophobic character, combined with insensitivity to acids, bases and solvents. The presence of silica also contributes to good mechanical properties for the resulting top layer.
A further advantage of a roll according to the present invention is the advantageous manner of applying the top layer on the roll. The nature of the top layer allows it to be applied and cured at relatively limited temperatures. In a preferred embodiment this is done at a temperature lower than 320°C and preferably lower than 300°C, and this for less than 60 minutes. This is advantageous because many anti-adhesive top layers according to the prior art require a temperature of 320°C for at least 30 minutes to cure properly. The use of a lower temperature is advantageous because it allows practical problems to be prevented. Excessive heating leads to excessive expansion of the roll, often leading to deformation and sometimes even breakage of the roll. Another advantage of a lower curing temperature is that it can be avoided that the roll has to be removed before the curing from the installation of which it is part. Indeed, in many cases the further installation is not resistant to strong heating, but heating according to the present invention is possible without having to remove the roll from the installation. The specific formulation of the top layer according to the present invention is advantageous because it can preferably be cured at temperatures lower than 300°C, and more preferably even at a temperature of only 180°C. In the prior art there are anti-adhesive top layers which allow such a low temperature of curing, but these do not exhibit the same high-quality anti-adhesive and mechanical properties of the top layer, and moreover have a completely different composition. In a further and most preferred embodiment, said top layer is cured during application at said temperature lower than 190°C and higher than 170°C, preferably about 180°C, for less than 40 minutes and more than 20 minutes, preferably about 30 minutes.
A further advantage of the roll according to the present invention is that the top layer retains its anti-adhesive functionality under grinding and polishing, and this thanks to the high homogeneity in the mass of the top layer. This offers advantages in applying the top layer, but also offers advantages during use. This means that the possible abrasion of portions of the surface will not disturb the proper functioning. This is made possible by the specific composition of the top layer, which comprises a sol-gel. These after all allow various properties of the roll to be improved without compromising the homogeneity of the top layer. In a preferred embodiment, the roll according to the present invention comprises a homogeneity in the top layer for maintaining its functionality under abrasion, grinding and polishing. Preferably said roll comprises said homogeneity in the top layer over at least 50% of the volume of the top layer, more preferably over at least 60%.
In a second aspect, the present invention offers a method for applying a top layer on a roll according to the present invention, wherein said method comprises following successive steps:

(a) providing a rigid and substantially cylindrical roll core belonging to said roll;
(b) optionally, applying an adhesion layer on a jacket of said roll core, preferably an adhesion layer comprising a carbide, more preferably tungsten carbide;
(c) applying a top layer over an outer jacket of said roll, preferably over said adhesion layer applied in step (b), said top layer comprising an inorganic-organic hybrid sol-gel comprising a fluoropolymer, and further preferably comprising silica, more preferably hydrophobic silica;
(d) allowing said top layer to cure at a temperature lower than 300°C for less than 60 minutes, preferably with said temperature lower than 190°C and higher than 170°C, more preferably about 180°C, and this preferably during less than 40 minutes and more than 20 minutes, preferably about 30 minutes.
(e) grinding down said outer surface of said roll, wherein a transverse dimension of said coating is preferably not more than 400 µm and not less than 20 µm, more preferably not more than 150 µm and not less than 50 µm, for obtaining said roll for guiding, preferably drying guiding of a web, preferably a fibrous web.

The advantages of such a method are similar to those of a roll according to the present invention. Moreover, this method advantageously allows the roll to be heated to no more than 300°C when applying the top layer, which limits the risk of damage to the roll during the application of the top layer. In addition, this method advantageously takes into account the known material properties of sol-gels and of coatings on a rigid roll core. For sol-gels it is advantageous that the thickness is limited to a dimension in the interval 1 to 400 µm, preferably 5 to 100 µm, more preferably between 8 and 50 µm, most preferably between 10 and 30 µm, because a greater value can lead to cracks and top layer defects resulting from trapped organic components in the top layer during curing.
In a preferred embodiment of said method, said step (b) comprises applying said adhesion layer to a jacket of said roll core; wherein said adhesion layer exhibits considerable roughness; wherein said step (c) comprises applying said top layer over said adhesion layer applied in step (b); and wherein said grinding down in step (e) takes place such that a top layer jacket comprises at least 60% portions of the top layer and at least 10% portions of the adhesion layer, preferably approximately 80% portions of the top layer and about 20% portions of the adhesion layer. Said roughness here concerns the roughness of the upper surface of the adhesion layer with which the top layer makes contact. Furthermore, said top layer jacket refers to the jacket of the roll as it is obtained in the finished state. The relative proportions of portions of the top layer and the adhesion layer, expressed as a percentage, therefore relate to the top layer jacket as outer surface of the roll, and not to the relative proportions in a cross section or any other form of section of the roll. Such a design leads to an economical and practical balance between free surface energy, surface structure and surface roughness. This results in a substantially cylindrical top layer surface which consists of portions of the adhesion layer surrounded by portions of the top layer. This is advantageous because the portions of the top layer provide anti-adhesion and thus help prevent stickies, for example, while the portions of the adhesion layer help to guide the web, preferably a fibrous web such as paper, the guiding being guiding while drying.
In a third aspect, the present invention provides a use of the roll according to the present invention in the production of paper from paper pulp, wherein a fibrous web, in particular paper, is pressed against said roll as it passes along said roll for the drying guiding of said paper.
In a fourth aspect, the present invention provides a use of an inorganic-organic hybrid sol-gel in a method for applying a top layer on a roll according to the present invention, wherein said sol-gel comprises a fluoropolymer, and further preferably comprises silica, more preferably hydrophobic silica.
In a fifth aspect, the present invention provides a use of an inorganic-organic hybrid sol-gel in an outer surface of a drying drum for the production of paper from paper pulp, wherein said sol-gel comprises a fluoropolymer, and further preferably comprises silica, more preferably hydrophobic silica.
Further details of the preferred embodiments are elaborated in the detailed description and in the dependent claims.

DETAI LED DESCRIPTION

In this document, 'a' and 'the' refer to both the singular and the plural, unless the context presupposes otherwise. For example, 'a roll' means one or more than one roll.
Quoting numerical intervals by the endpoints comprises all integers, fractions and/or real numbers between the endpoints, these endpoints included. When the term 'about' is used in this document with a measurable quantity, a parameter, a duration or moment, and the like, then variations are meant of approx. 20% or less, preferably approx. 10% or less, more preferably approx. 5% or less, even more preferably approx. 1% or less, and even more preferably approx. 0.1% or less than and of the quoted value, insofar as such variations are applicable in the described invention. However, it must be understood that the value of a quantity used where the term 'about' or 'around' is used, is itself specifically disclosed.
Stickies refer to sticky contaminants present in paper pulp. Stickies often come from glues, ink binders and coating binders. An important part of these contaminations is therefore due to the use of recycled paper, which may be laminated or highly glossy for example, and which may therefore contain a proportion of plastic-like and/or glue-like constituents. A lot of recycled paper also brings with it glue from the binding of the paper, which yields sticky impurities, which may or may not be plastic-like. By extension, a part of the wood extracts, such as the resins present in softwood, can also contribute to the sticky structure of these impurities. Typically, stickies passing through a sieve of 100 or 150 µm (depending on the standard) are referred to as 'micro stickies', while the particles that remain are called 'macro stickies'. Dissolved and colloidal stickies are referred to as 'secondary stickies' and are typically smaller than 1-5 µm. Secondary stickies typically agglomerate during sudden changes in the wet processing steps ('wet end chemistry') or the process conditions. In practice, it is found in paper processing that sudden changes in temperature, charge or pH can cause the accelerated formation of stickies, with in some cases an outbreak of stickies.
In a preferred embodiment, the web relates to a fibrous web such as in paper production from pulp. In an alternative embodiment, the web has a substantially non-fibrous structure, such as for instance plastic foil. In such a case the roll according to the present invention is responsible for guiding this plastic foil, for example when manufacturing plastic foil. Also in such a case the occurrence of adhesion of plastic particles is important, and such adhering plastic particles, and/or by extension the entire plastic foil, are equivalent to the stickies as known from paper production.
The roll according to the present invention comprises an inorganic-organic hybrid sol-gel which comprises a fluoropolymer. The fluoropolymer promotes the anti-adhesive character. Preferably, it relates to one of the fluoropolymers having a known anti-adhesive effect, such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP) and perfluoroalcoxy polymer (PFA). The fluoropolymer further contributes to the hydrophobic or superhydrophobic character of the top layer and the top layer jacket. The physical properties of fluoroorganic compounds are related to two main factors: the combination of high electron negativity with moderate size, on the one hand; and the excellent match between the 2s or 2p orbitals of fluorine (F) with the corresponding orbitals of carbon, and the resulting extremely low polarisability of fluorine, on the other hand. Characteristic of fluoroorganic compounds is the stability of the carbon-fluorine compound (C-F), which increases with the number of fluorine substituents bonded to the same carbon atom. This increased stability is reflected in the lengths of the C-F bonds (24). Fluoropolymers have excellent chemical resistance to acids, bases and solvents; high temperature stability; and loosening and anti-adhesive properties. In non-stick formulations, i.e. formulations with anti-adhesive properties, fluoropolymers are mixed with other polymers that act as binders. Such binders are preferably present in a primer and/or the adhesion layer and/or the top layer where they improve the adhesion to the underlayer. These polymers are commonly used as 52-60 percentage by weight solids dispersion in water, in other cases, micropowders are dispersed in a solvent-based formulation. The degree of adherence strongly depends on the surface preparation (roughness, etc.), the ratio between fluoropolymer and binder, and the degree of fluoropolymer stratification. Stratification is an effect caused by high temperature, where e.g. PTFE separates from other components and especially from the binder. This phenomenon is due to the very low surface tension of, for example, PTFE, combined with the high viscosity of for example PTFE (10¹¹-10¹³ Pa·s). In order to ensure the regularity of the top layer and the top layer jacket according to the present invention, the coating is cured at high temperatures, so that polymers soften/soak and form a uniform layer. At high temperatures, some compounds can evaporate or decompose (e.g. acrylics), and this facilitates the flow of the fluoropolymers to the surface, driven by the thermodynamic tendency to minimise the surface energy. This leads to a stratification of compositions on the surface. The degree of fluoropolymer stratification is not easy to control, and in a preferred embodiment is monitored and/or controlled on the basis of the melt viscosity of the fluoropolymer and/or the fluoropolymer mixtures related to the sol-gel, as well as on the basis of the curing conditions such as the length of time and the curing temperature.
The roll according to the present invention comprises a sol-gel. Sol-gels are suitable for use in a wide variety of applications through applicability at low temperatures, cost-effectiveness and versatility of possible substrates and complex geometries. Examples of applications are automotive parts, thermal barrier layers and piezoelectric coatings. A discussion of the production method can be found in Fauchais, Pierre L.: 'Thermal Spray Fundamentals: From Powder to Part.'; Springer; 2014. An important aspect here is densification, a strengthening process consisting of the heat treatment of a gel at elevated temperatures which is aimed at reducing the number of pores and increasing the connectivity by viscous phase sintering. According to a preferred embodiment, this allows a greater degree of hydrophobicity to be obtained for the top layer of the roll. The adsorption of water and liquids is generally due to the presence of Si-OH on the surface. Replacement of the H from the Si-OH groups by the hydroelectrically stable Si-O-R (where R = alkyl or aryl) groups prevents, thanks to the oxygen compounds, the adsorption of water and therefore results in hydrophobic silica surfaces.
The sol-gel according to the present invention relates to an inorganic-organic hybrid sol-gel. The low temperature of the sol-gel process makes it possible to obtain an organic-inorganic material whose composition is well controlled, with a low molecular weight and oligomeric/polymeric organic molecules. Temperatures and humidity have been controlled so that organic molecules can survive the process. According to various embodiments of the present inventions, the sol-gel is obtained by one of these methods.

Use of organoalkoxysilanes as precursors for the sol-gel reaction, in which organic groups are introduced within an inorganic network by the =Si-C bond.
Through the co-condensation of functionalised oligomers or polymers with metal alkoxides in which chemical bonding is established between inorganic and organic phases.
Synthesised by the in situ formation of inorganic species within a polymer matrix. Inorganic species, in the form of particles, can be generated in situ in the polymers. This allows various inorganic particles with a very homogeneous particle size to be prepared in elastomeric or plastic matrices.
By either infiltrating previously formed oxide gels with polymerisable organic monomers or mixing polymers with a single or mixture of metal alkoxides in a common solvent.
Impregnated or enclosed in inorganic gel matrices.
Made by interpenetrating networks and simultaneous formation of inorganic and organic phases.

In a preferred embodiment, the sol-gel according to the present invention comes about by impregnation or inclusion in an inorganic gel matrix. This type of hybrid sol-gel is known from the development of new optical and bioactive materials in the application of photophysical, electrical, biotechnical and non-linear optical (NLO) devices, but is not known for use in rolls. The addition of organic groups in an inorganic network leads to new structural characteristics in terms of flexibility, electronic properties and optical properties.
The anti-adhesive character of the top layer according to the present invention is related to the hydrophobic character of the top layer. A related measure is the water contact angle, which describes the angle (in degrees, between 0 and 180°) that forms a drop of water with the surface: the larger the angle, the more hydrophobic. An angle of less than 90° corresponds to a hydrophilic surface, an angle between 90° and 150° with a hydrophobic surface, and an angle greater than 150° to a superhydrophobic surface. The water repellence is related to the surface roughness; this relationship is described by Cassie-Baxtel and Wenzel.
In another preferred embodiment, the top layer is obtained according to a sol-gel process which leads to the formation of the inorganic network by starting with liquid precursors such as metal alkoxides and organic oligomers, preferably with suitable reactive groups. Here, the covalent bond between organic and inorganic components contributes to the formation of a cross-linked structure in which the organic and inorganic parts are phase-separated on a micro or nanoscale, but the resulting material is macroscopically uniform.
In a preferred embodiment, the mixed properties of the SiO₂/fluoropolymer inherent in the sol-gel lead to a top layer which exhibits good scratch resistance. More preferably, the sol-gel comprises barium sulphate (BaSO₄) for this purpose, which further increases the scratch and wear resistance. Barium sulfate is advantageous because it shows no chemical activity when exposed to alkali or acid. Furthermore, it has a moderate Mohs hardness of 3-3.5 and excellent wear stability.
In a preferred embodiment, the inorganic-organic hybrid sol-gel comprises traces of barium, sulphur, chromium, iron and copper, which can serve as firming hard phases added to the top layer.
In another preferred embodiment, the top layer has an arithmetic mean roughness R_a (arithmetic surface roughness) of the estimated profile between 2 and 7 µm, more preferably between 4 and 6 µm, most preferably about 5 µm.
In a preferred embodiment, the top layer has a water contact angle between 100° and 125°, more preferably between 110° and 120°, most preferably about 112°.
In a preferred embodiment of the roll, a hollow or full metal roll core is started from. On the jacket of this roll core a top layer is applied which belongs to a coating. In a preferred embodiment, the coating according to the present invention consists of an accretion on the roll core, by optional deposition of the adhesion layer, followed by deposition of the top layer. On the other hand, the top layer can be present or applied to a pre-produced roll jacket arranged on the roll core, for example by providing a high air pressure at the jacket of the roll core via air holes and sliding a roll jacket expanded under the influence of this air pressure over the roll core.
In an embodiment of the present invention, the roll is provided with a rigid cylindrical hollow or solid roll core, provided at least at one of its ends with bearing journals, surrounded by a top layer, in a more or less equal thickness radially with respect to the axis of the roll core, the ratio of the diameter of the roll core to the length of the roll core being preferably between 0.02 and 0.50, more preferably between 0.04 and 0.2, most preferably between 0.05 and 0.15. Under the said ratio of the diameter of the roll core with respect to the length of the roll core, a self-supporting roll core at one end is possible.
In a preferred embodiment, said sol-gel comprises hydrophobic silica and/or a silica different from said hydrophobic silica. Silica and in particular hydrophobic silica are advantageous because of their hydrophobic character, combined with insensitivity to acids, bases and solvents. The presence of silica also contributes to good mechanical properties for the resulting top layer.
In a preferred embodiment, said jacket of said roll core comprises metal, preferably cast iron, wherein said jacket preferably consists of metal, more preferably consists of cast iron. Such an embodiment is advantageously adapted to the needs of paper production, wherein the roll functions as drying drum. In an alternative embodiment, said roll core consists of steel, and said jacket preferably also consists of steel.
In a most preferred embodiment, said top layer is cured during application at said temperature lower than 190°C and higher than 170°C, preferably about 180°C, for less than 40 minutes and more than 20 minutes, preferably about 30 minutes. This is advantageous because the low temperature can prevent thermal problems related to the roll as much as possible due to a low curing temperature. In another and alternative preferred embodiment, said top layer is cured during application at said temperature lower than 270°C and higher than 250°C, preferably about 260°C, and preferably less than 40 minutes, more preferably during more than 20 minutes and less than 40 minutes, preferably about 30 minutes. Such an embodiment is advantageous with rolls which are thermally resistant to temperatures in the range of 200-300°C. In such a situation it may be advantageous to use a curing temperature of about 260°C, which is higher than the 180°C which is preferred. Indeed, this allows to use proven methods of curing at tested temperatures with a top layer according to the current version. It can also allow the curing time to be shortened. In another embodiment, even lower temperatures of curing are possible, possibly in combination with a shorter time period for curing. Examples are a temperature of at least 100°C and no more than 170°C, preferably at least 120°C and not more than 150°C, most preferably about 135°C, and this for a period of curing time of 30 minutes. Still further examples are a temperature of at least 120°C and not more than 200°C, preferably at least 150°C and not more than 170°C, most preferably about 160°C, and this for a period of time of curing of less than 30 minutes, preferably less than 25 minutes, most preferably about 20 minutes.
In a preferred embodiment, said coating under said top layer comprises an adhesion layer for proper adhesion of the coating to said jacket, said adhesion layer being provided over at least 80% of a core jacket, most preferably over at least 95%, and wherein said adhesion layer comprises a carbide, preferably tungsten carbide. Such an embodiment is advantageous because an adhesion layer with carbides enables very good adhesion with both metal roll cores and roll cores from other rigid materials. Moreover, such an adhesion layer provides a good substrate for adhering a top layer according to the present invention. Furthermore, an adhesion layer with carbides, and certainly an adhesion layer with tungsten carbides, shows a high hardness. In a preferred embodiment, said adhesion layer exhibits a hardness of at least 550 Vickers, more preferably at least 825 Vickers, most preferably about 1100 Vickers. This allows a durable and robust use of the roll. It also ensures an extremely stable substrate for adhering the top layer according to the present invention. In preferred embodiments of the present invention, portions of the adhesion layer also form part of the actual surface of the roll. In such a case, the great hardness of the adhesion layer offers the advantage that the web is reliably carried by these portions of the adhesion layer. In a preferred embodiment with sufficiently high hardness, the adhesion layer also fulfils the functionality of 'reinforcement', as explained elsewhere in this document. In this respect, the adhesion layer is also an anti-abrasion layer, and the terms 'adhesion layer' and 'anti-abrasion layer' are interchangeable.
In a further preferred embodiment, said top layer covers at least 20% of the adhesion layer, preferably at least 40%, most preferably at least 60%. Such an embodiment is advantageous because the anti-adhesive effect can be ensured in this way, in particular when the top layer covers the majority of the adhesion layer.
According to another preferred embodiment, an outer surface of the adhesion layer exhibits considerable roughness; and a top layer jacket comprises at least 60% top layer portions and at least 10% adhesion layer portions, preferably about 80% top layer portions and about 20% adhesion layer portions. Said considerable roughness can be interpreted here in relation to the intended thickness of the total coating in the interval 20-400 µm. Substantial roughness of the outer surface of the adhesion layer means that this outer surface exhibits 'mountains and valleys' on a submillimetre scale, i.e. irregularities corresponding to undulations in the order of at least 10 µm and preferably not more than 500 µm, although irregularities that are too big can still be ground down if required. Also, the considerable roughness implies that these undulations appear to be sufficiently concentrated on the outer surface, with for example at least 1 such undulation per cm² outer surface, preferably at least 1 undulation per mm² outer surface. Said top layer jacket comprising at least 60% portions of the top layer and at least 10% portions of the adhesion layer offers an advantageous balance between free surface energy, surface structure and surface roughness. This results in a substantially cylindrical top layer surface which consists of portions of the adhesion layer surrounded by portions of the top layer. This is advantageous because the portions of the top layer provide anti-adhesion and thus help prevent stickies, for example, while the portions of the adhesion layer help to carry the fibrous web, preferably paper. This embodiment can advantageously be combined with an embodiment wherein said adhesion layer exhibits a hardness of at least 550 Vickers, more preferably at least 825 Vickers, most preferably about 1100 Vickers. In such a case, the relatively harder portions of the adhesion layer present in the top layer surface can protect the relatively softer portions of the top layer. From this perspective, the top layer surface can be regarded as a composite of soft material of the top layer combined with hard material of the adhesion layer. Here, the hard material of the adhesion layer provides the functionality of reinforcement, while the softer material of the top layer gives the roll its advantageous anti-adhesive properties.
In a preferred embodiment, a transverse dimension of said coating is not more than 400 µm and not less than 20 µm, preferably not more than 150 µm and not less than 50 µm. This transverse dimension (thickness) advantageously takes into account the known material properties of sol-gels and of coatings on a rigid roll core. For sol-gels it is advantageous that the thickness is limited to a dimension in the interval 1 to 100 µm, preferably 5 to 50 µm, most preferably about 10 µm, because a greater value can lead to cracks and top layer defects resulting from trapped organic components in the top layer during curing.
In a preferred embodiment, a roll according to the present invention has superior UV resistance. This is related to the advantageous properties of optical reflection typical of sol-gels, which help to reduce the possible damage caused by UV light.
In a preferred embodiment, the top layer, and by extension the coating, is characterised in that it resiliently springs back after deformation, and that it is resistant to liquids with any corrosive effect, such as conventional water and solvent-based inks and other liquids present in, for example, paper pulp.

Claims

A roll suitable for guiding, preferably drying guiding, of a web, preferably a fibrous web, wherein the roll is provided with a rigid and substantially cylindrical roll core which is provided at its jacket with a coating comprising an anti-adhesive top layer for preventing adhesion of stickies, said top layer comprising a fluoropolymer, characterised in that said top layer comprises an inorganic-organic hybrid sol-gel comprising said fluoropolymer, and further preferably comprises silica, more preferably hydrophobic silica.
The roll according to claim 1, characterised in that said sol-gel comprises hydrophobic silica and/or a silica other than hydrophobic silica.
The roll according to claims 1 and 2, characterised in that said jacket of said roll core comprises metal, preferably cast iron, wherein said jacket preferably consists of metal, more preferably consists of cast iron.
The roll according to claims 1-3, characterised in that said top layer is cured at a temperature lower than 300°C for less than 60 minutes during application.
The roll according to claim 4, characterised in that said top layer is cured during application at said temperature lower than 190°C and higher than 170°C, preferably about 180°C, for less than 40 minutes and more than 20 minutes, preferably about 30 minutes.
The roll according to claim 4, characterised in that said top layer is cured during application at said temperature lower than 270°C and higher than 250°C, preferably about 260°C, for less than 40 minutes and more than 20 minutes, preferably about 30 minutes.
The roll according to claims 1-6, characterised in that said coating comprises, under said top layer, an adhesion layer for proper adhesion of the coating to said jacket, said adhesion layer being provided over at least 80% of a core jacket, most preferably over at least 95%, and wherein said adhesion layer comprises a carbide, preferably tungsten carbide.
The roll according to claim 7, characterised in that said adhesion layer exhibits a hardness of at least 550 Vickers, more preferably at least 825 Vickers, most preferably about 1100 Vickers.
The roll according to claims 7-8, characterised in that said top layer covers at least 20% of the adhesion layer, preferably at least 40%, most preferably at least 60%.
The roll according to claim 9, characterised in that an outer surface of the adhesion layer exhibits considerable roughness; and a top layer jacket comprises at least 60% top layer portions and at least 10% adhesion layer portions, preferably about 80% top layer portions and about 20% adhesion layer portions.
The roll according to claims 1 to 10, characterised in that a transverse dimension of said coating is not more than 400 µm and not less than 20 µm, preferably not more than 150 µm and not less than 50 µm.
A method for applying a top layer on a roll for guiding a web according to claims 1 to 11, wherein said method comprises the following successive steps:
(a) providing a rigid and substantially cylindrical roll core belonging to said roll;

(b) optionally, applying an adhesion layer on a jacket of said roll core, preferably an adhesion layer comprising a carbide, more preferably tungsten carbide;

(c) applying a top layer over an outer jacket of said roll, preferably over said adhesion layer applied in step (b), said top layer comprising an inorganic-organic hybrid sol-gel comprising a fluoropolymer, and further preferably comprising silica, more preferably hydrophobic silica;

(d) allowing said top layer to cure at a temperature lower than 300°C for less than 60 minutes, preferably with said temperature lower than 190°C and higher than 170°C, more preferably about 180°C, and this preferably during less than 40 minutes and more than 20 minutes, preferably about 30 minutes.

(e) grinding down said outer surface of said roll, wherein a transverse dimension of said coating is preferably not more than 400 µm and not less than 20 µm, more preferably not more than 150 µm and not less than 50 µm, for obtaining said roll for guiding, preferably drying guiding of a web, preferably a fibrous web.
The method according to claim 12, characterised in that said step (b) comprises applying said adhesion layer on a jacket of said roll core; wherein said adhesion layer exhibits considerable roughness; wherein said step (c) comprises applying said top layer over said adhesion layer applied in step (b); and wherein said grinding down in step (e) takes place such that a top layer jacket comprises at least 60% portions of the top layer and at least 10% portions of the adhesion layer, preferably approximately 80% portions of the top layer and about 20% portions of the adhesion layer.
Use of a roll according to any of preceding claims 1 to 11 as drying drum in the production of paper from paper pulp, wherein a fibrous web, in particular paper, is pressed against said roll as it passes along said roll for the drying guiding of said paper.
Use of an inorganic-organic hybrid sol-gel in a method for applying a top layer on a roll for guiding a web according to claims 12 to 13, wherein said sol-gel comprises a fluoropolymer, and further preferably comprises silica, more preferably hydrophobic silica.
Use of an inorganic-organic hybrid sol-gel in an outer surface of a drying drum for the production of paper from paper pulp, wherein said sol-gel comprises a fluoropolymer, and further preferably comprises silica, more preferably hydrophobic silica.