WO2011073534A1

WO2011073534A1 - A coating for use in a component of a fiber web machine

Info

Publication number: WO2011073534A1
Application number: PCT/FI2010/051053
Authority: WO
Inventors: Jani Turunen; Jan Paasonen
Original assignee: Metso Paper, Inc.
Priority date: 2009-12-18
Filing date: 2010-12-17
Publication date: 2011-06-23
Also published as: FI20096355A0

Abstract

The present invention relates to a coating for use in a component of a fiber web machine, comprising polyurethane elastomer and 0.1 – 9.0 wt-% of surface modified particles selected from the group consisting of high density polyethylene, ultra high molecular weight polyethylene and mixtures thereof. The invention also relates to a cover, a roll, a rod bed and a manufacturing or treatment apparatus for a cellulosic fiber web comprising such coating.

Description

A COATING FOR USE IN A COMPONENT OF A FIBER WEB MACHINE

FIELD OF THE INVENTION

The present invention relates to a coating for use in a component of a fiber web machine, such as for a roll of a pulp, paper, board, tissue or finishing machine or a rod bed of a sizer. The invention also relates to a cover for a roll, a roll and a roll bed using said coating as well as to a manufacturing or treatment apparatus for a cellulosic fiber web comprising a roll and/or a roll bed using said coating.

BACKGROUND OF THE INVENTION In fiber web machines such as pulp, paper, board, finishing and tissue machines several different types of rolls are used, such as press rolls, suction rolls, guide rolls, calender rolls, coater rolls, sizer rolls and reeling spools or other rolls used in reelers. It is known that these rolls, which are typically made of metal, can be covered by polymers. In certain positions, for example in the press section or in the reel spools, the polymer covered rolls may have a surface texture like grooving or perforation to receive and transport water from the web or air carried by the web.

One type of polymers used in the manufacturing of roll covers is polyurethane elastomers which are also used for manufacturing rod beds for sizer.. Polyurethane has excellent physical and dynamic properties, as well as satisfactory wear resistance, but has some drawbacks with respect to wear resistance, hydrolysis resistance and tear strength.

Rotating surfaces in the fiber web machine are subjected to abrasive conditions due to a contact with the web or a felt or a counter roll. Thus, especially with the softer polyurethanes, there is still room for improvement in their wear resistance, as wear shortens the service life of the product and thus lessens its customer value. Another problem of polyurethane elastomer coatings is their lower resistance to hydrolysis, when compared to rubber. When polyurethane coatings are subjected to water, which is the prevailing environment in the wet end of a fiber web machine, the bonds between polymer molecules are broken. As a result not only dynamic properties of the cover are degraded, but also diffusion of water through the coating occurs. Water diffusion degrades the adherence of the cover layers to each other and/or to the roll body resulting finally, at its worst, in peeling-off or delamination of the cover. This problem has been approached by attempts to select ingredients of the polyurethane composition such that more hydrolysis resistant components as well as more appropriate activators and catalysts are chosen. Other preferred properties of the cover are however thus compromised..

A yet further problem encountered in the coating of rolls and rod beds in paper machinery is the tear resistance, which is typically directly proportionate to the impact resistance and affects also the risk of delamination. Usually this problem has been addressed by the choice of the raw materials also.

The problem of wear resistance of polyurethanes has been the subject of research for some time and has been approached by the selection of appropriate filler materials improving the wear resistance. Inorganic particles have been proved to be good in improving wear resistance of polyurethane covers. An example of appropriate inorganic filler material is aluminium oxide (see EP 487 477), which is however only useful in even surfaces, as these particles tend to be too hard for machining tools, such as grooving or drilling tools as well as grinders.

It is known that polymer particles can be used as fillers in rubber covers. Rubber covers with polyethylene fillers are described in document US 6,328,681 in which ultra high molecular weight polyethylene (UHMWPE) is used as one filler ingredient in an amount of 20-50 parts per hundred parts of rubber and in document US 6,918,865 in which UHMWPE is used in an amount of 10-24 parts per hundred part of rubber (phr). The elastomer in both of the documents above is rubber, thus the recipies in the examples are given in phr, but when converted to weight-% basis, these amounts of UHMWPE are calculated to correspond to 20- 33.3 and 9.1 -19.4 weight-% (wt-%) of the total weight of the elastomer (i.e. excluding additional fillers), respectively. It is to be noted that in the examples of the above mentioned US patent documents UHMWPE is not the only filler in the rubber composition but that other fillers are also used in addition to it, such as furnace black filler, anhydrous aluminium silicate, titanium dioxide filler, hydrated silica reinforcement in US 6,328,681 and black/mineral mix, which is used in an amount as high as 63.25 phr in all the examples of US 6,918,865.

Further, document US 7,014,604 discloses a rubber cover with high density polyethylene (HDPE) particles in the rubber composition in an amount of 10-80 phr, which corresponds to 9.1 -44.4 wt-% of the total weight. In spite of the extensive research done in the field, there is still need to improve the properties of the polyurethane elastomeric covers.

Polyurethane coatings of rolls are typically manufactured by ribbon flow method. In this process, the roll is rotating around its longitudinal axis and the polyurethane is applied to it by using a nozzle. In order to ensure that the polyurethane remains in the position that is has been applied (instead of flowing or dripping), the choice of activators and catalysers is essential. This can be also influenced by choosing additives that modify the viscosity of the raw materials in the reaction temperature.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to at least partly overcome the problems existing in the prior art, and to provide a cost-effective roll cover with improved properties.

An object of the invention is thus to provide a solution to the problem of wear resistance, resistance to hydrolysis and tear strength. Especially, an object of the invention is to provide a solution to these problems that is as cost-effective as possible and easy to manufacture. An object is then also to improve the processability of the polyurethane elastomer material in the manufacture of rolls and rod beds for component of a fiber web machine. A yet another object of the invention is to lower the surface energy of the elastomeric polyurethane cover or coating. Another further object of the invention is to provide improved rolls and rod beds for component of a fiber web machine.

The present invention relates to a coating for use in a component of a fiber web machine, comprising polyurethane elastomer and 0.1 - 9.0 wt-% of surface modified particles selected from the group consisting of high density polyethylene, ultra high molecular weight polyethylene and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 schematically shows a device for manufacturing a cover according to the present invention.

Figure 2 shows the wear resistance of some coatings according to the present invention.

Figure 3 shows the DIN abrasion of some coatings according to the present invention. Figure 4 shows the tear resistance of some coatings according to the present invention.

Figure 5 shows the wear resistance of some coatings according to the present invention.

Figure 6 presents the tear strength of some coatings according to the present invention.

Figure 7 shows the total surface energy of some coatings according to the present invention.

Figure 8 shows the contact angle of water on some coatings according to the present invention. Figure 9 shows the wear resistance of some further coatings according to the present invention. Figure 10 shows the tear strength of some further coatings according to the present invention.

Figure 1 1 the abrasion values of some further coatings according to the present invention. Figure 12 shows the tear strength of some yet further coatings according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Now it has been surprisingly found out that even significantly low amounts of polymer particles can be used in polyurethane covers and still obtain the beneficial effects on the final properties of the rolls and rod beds.

An advantage of the present invention is thus that it allows obtaining a more resistant coating for use in a component of a fiber web machine in a more cost- effective way than previously, as it is possible to use lower amounts of the particles improving the properties of the coating. The rolls and the roll covers as well as the rod beds according to the present invention can be used in any kind of fiber web machines, including pulp, paper, tissue and board machines as well as off-line finishing machines such as off-line coating machines or calenders. Moreover, when fiber web is mentioned in this text, it is clear that webs of pulp, paper board, tissue and other webs comprising mainly cellulosic fibers are covered by this expression, which is only used for simplicity of wording.

Moreover, when polyethylene particles are mentioned in this description, surface modified particles selected from the group consisting of high density polyethylene, ultra high molecular weight polyethylene and mixtures thereof are meant. According to a preferred embodiment of the invention, no other filler material is present in the coating according to the invention. By filler material, it is here understood materials that are conventionally used as fillers, and typically materials in particle form, which particles have an average diameter of over 5 μιτι, typically 10-300 μιτι.

The coating according to the present invention can be used for both even and structured surfaces, such as grooved or perforated surfaces, as the particles are not incompatible with machining tools.

The particles improve the tear strength of the final coating, as they stop the development and progress of a tear on the surface.

According to an embodiment of the invention the amount of surface modified particles in the coating is 0.5 - 8.5 wt-% of the total weight of the coating. According to another embodiment of the invention the amount of surface modified particles is 3 - 7 wt-%. When an amount in this description is given in wt-%, weight percentage of the total weight is meant unless otherwise specified.

The amount of surface modified particles in the coating can be for example from 0.1 , 0.5, 1 .0, 1 .2, 1 .5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0 or 9.5 wt-% up to 0.5, 1 .0, 1 .2, 1 .5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5 or 9.0 wt-%.

As the amount of polyethylene particles in the finished coating is low, they do not have any significant effect on the dynamic properties of the polyurethane or to the functionality of the coating. The average particle size of the polyethylene particles is for example 5-300 μιτι, typically 10-100 μιτι, preferably 20-50 μιτι.

According to one embodiment of the invention, the average molecular weight of the high density polyethylene is 50 000-900 000 g/mol, the average molecular weight being the weight average molecular weight. A preferable range for the average molecular weight of HDPE is 100 000-300 000 g/mol.

According to another embodiment of the invention, the average molecular weight of the ultra high molecular weight polyethylene is over 1 000 000 g/mol. The average molecular weight (also the weight average molecular weight) may also be over 3 000 000 g/mol.

According to an embodiment of the invention, the surface treatment of the particles is plasma treatment or reactive gas oxidation treatment. These treatment methods are known to a person skilled in the art and such treated particles are also available commercially, for example from the company Inhance/Fluoro-Seal Ltd under the tradenames HD1800, UH1250, UH1700 and UH1900.

The coating of the invention may also contain other additives and process aid agents such as polymerization initiators, activators and accelerators, plasticizers, thermal stabilisers, antioxidants, antiozonants, pigments, air release agents, filler wetting and dispersing agents, levelling agents, defoaming agents, dispersion stabilisers, polyols etc. for promoting the process and improving physical properties of the cover.

The present invention also relates to a roll cover for a roll for a fiber web machine comprising as its outermost layer a coating according to the present invention. A typical roll for a fiber web machine comprises a cylindrical roll body. The roll body is typically made of metal, such as cast iron or hardened steel, or fiber reinforced plastic. The cover is typically made of one or more polyurethane layers. In addition to polyurethane layer or layers, the roll cover may comprise other layers beneath the polyurethane surface. These other layers include but are not limited to for example a bottom layer located on the roll body, which typically comprises glass fibers polyester fibers and/or and/or para-aramide fibers and/or carbon fibers, and a polymer matrix which typically is an epoxide, and an intermediate layer or layers between the bottom layer and the coating according to the present invention. A typical thickness of a polymeric cover in this kind of rolls is 12-20 mm.

The present invention further relates to a roll for a fiber web machine, comprising a roll body and a cover arranged on said roll body, said cover comprising as its outermost layer a coating according to the present invention. The invention yet further relates to a rod bed for a coater or sizer of a fiber web comprising as its outermost layer a coating according to the present invention, as well as to a manufacturing or treatment apparatus for a cellulosic fiber web comprising a roll according to the present invention and to a manufacturing or treatment apparatus for a cellulosic fiber web comprising a rod bed according to the present invention.

The coating according to the invention is manufactured by mixing a prepolymer part consisting essentially of the polyurethane elastomer precursors and the other components together immediately before the coating is applied. The particles according to this invention can be added either to the prepolymer part or to the other components, and are preferably added to the mixture of other components or admixed beforehand with one or more other components. The effect of the particles to the viscosity of the coating materials during coating is achieved irrespective of the part in which the particles are added. Preferably the particles are admixed with the cross linking component as the effect on the viscosity of the polyurethane before its gelling is found to be the highest. Moreover, this also allows decreasing the amount of activators and catalysers, which typically have a negative effect on the resistance to hydrolysis of the finished coating. The polyurethane coating on a roll or a rod bed according to the present invention can be manufactured by any suitable method known, such as by ribbon flow casting. Typically, the coating is post-cured in an oven, followed by polishing or grinding of the final surface.

All the embodiments and characteristics disclosed in this specification apply to all the different aspects of the invention, i.e. the embodiments listed for the coating apply mutatis mutandis to the cover, roll, rod bed and a manufacturing or treatment apparatus for a cellulosic fiber web.

In this context, the term "comprising" is to be construed as an open term, i.e. that other features may be included in addition to those listed. Moreover, the reference numbers are not to be construed as limiting the scope of protection of the claims.

DETAILED DESCRIPTION OF THE DRAWING

Figure 1 schematically shows a device for manufacturing a cover according to the present invention. The roll body 1 is rotated around its longitudinal axis while the coating material is supplied from a mixer 2 via a conduit 3 to a nozzle 4. The nozzle 4 advances along the length of the roll body 1 to form a coating 5.

Figures 2-12 are explained in more detail below in the Experimental part.

EXPERIMENTAL PART The present invention was tested in a series of experiments, where coatings comprising polyurethane with surface treated particles of HDPE or UHMWPE were compared to coatings comprising polyurethane but no such particles.

Examplel

A polyurethane top stock was prepared by mixing different amounts of Inhance HD-1800 surface modified high density polyethylene particles (by Inhance/Fluoro- Seal Ltd) to Neuthane 3100 polyurethane prepolymer (by Notedome Ltd), which is a high performance methylene di-isocyanate polytetramethylene ether glycol (MDI- PTMEG) based urethane, prior to mixing 1 ,4-butanediol into it. The formulations are shown in Table 1 (compositions A-E). Material temperatures were 70 °C for Neuthane 3100D and 25 °C for butanediol. The mold-cast specimens (mold temperature 70 °C) were post-cured at 130 °C for 20 hours. Hardness of the resulting unfilled polyurethane was measured to be 23 P&J. P&J-hardness refers to Pusey and Jones-scale, which is typically used in roll covers.

Table 1

Figure 2 shows the wear resistance test results which were measured by abrasion method DIN 53516 and the results given as loss mm³. It is evident that the abrasion resistance of the polyurethane is improved when even less than 1 wt-% of surface modified high density polyethylene particles are added to the polyurethane. In addition, when the HDPE content is increased to 18 wt-% the abrasion loss value increases again, indicating lower HDPE content to be optimal.

Example 2 A polyurethane top stock was prepared by mixing different amounts of Inhance HD-1800 surface modified high density polyethylene or UH-1250 ultra high molecular weight polyethylene particles in the curative side, formulations being depicted in Table 2 (compositions F-K). The polyethylene particles and butanediol were premixed with Terathane 1000 curative, which is a PTMEG-polyether diol (at 50 °C, by Invista), Adiprene RFA-1000 (at 40 °C, by Chemtura), which is a reaction product of a polyether with diphenylmethane diisocyanate (MDI) was mixed with the curative and the mixture was cast to molds at 70 °C. After gelation the polyurethane samples were postcured at 130 °C for 20 hours. Hardness of the resulting unfilled polyurethane was measured to be 40 P&J. Table 2

Figure 3 shows the DIN abrasion values of compositions presented in Table 2, the values being measured by method DIN 53516 and the results given as loss mm³. Like in the case presented in Example 1 , the abrasion resistance of polyurethane is improved when even less than 1 wt-% of surface modified high density polyethylene particles are added in the polyurethane. With 18 wt-% HDPE content the abrasion resistance remained the same as with 9.0 wt-% HDPE. Therefore, it can be concluded that 9 wt-% or less of HDPE particles in the polyurethane composition of Table 2 is sufficient for improving the wear resistance to its maximum. The use of UH-1250 does not produce as good improvement in wear resistance as HD-1800 but nevertheless improvement is still observed.

Tear resistance was determined for three compositions shown in Table 2 (F, G and I) and the results are presented in Figure 4, in N/mm. The measurements were made according to ISO 34-1 , method B, procedure (b). Even 0.5 wt-% HD1800 particles in the polyurethane markedly improve the tear strength. Example 3

Soft top stock polyurethane samples were prepared from commercial raw materials for ribbon flow casting. The compositions are depicted in Table 3 (compositions L and M). Temperature of the RFA-1001 MDI-PTMEG prepolymer was 70 °C and that of the RFB-1060 curative 35 °C which resulted in potlife time of approximately 8 seconds. The samples were postcured at 70 °C for 20 hours and resulted in hardness of 68 P&J.

Table 3

Figure 5 shows the results on wear resistance (in mm³) of the 68 P&J hardness polyurethane unfilled and filled with 3 wt-% of HDPE particles (compositions L and M). It can thus be concluded that especially in the case of a soft polyurethane elastomer, the improvement in wear resistance is significant.

Figure 6 presents the results in tear strength (given in N/mm) of the same compositions L and M. Again, a clear improvement of tear strength is achieved by using the filler.

Figure 7 shows the total surface energy of a 68 P&J-hardness polyurethane unfilled and filled with 3 wt-% of HDPE particles (compositions L and M). The total surface energy has been measured by using a KSV Cam 200 contact angle device. Measurements were made with three liquids: water, ethylene glycol and methylene iodide. The surface energy values were calculated according to the theory of Wu. (WU, S., J.Polym. Sci, 34 (1971 ) 19). Figure 8 shows the contact angle of water on these compositions. The decrease in surface energy and increase in the contact angle of water means that the surface will be less likely to collect dirt once the device coated with the present coating is used in a paper machine or the like.

Example 4 A polyurethane top stock was prepared by mixing different amounts of Inhance UH-1250 (by Inhance/Fluoro-Seal Ltd.) surface modified ultra high molecular weight polyethylene particles to the Neuthane 3100D (by Notedome Ltd.) polyurethane prepolymer, prior to mixing 1 ,4-butanediol in it. The formulation is shown in Table 4. Material temperatures were 70 °C for Neuthane 3100D and 25 °C for 1 ,4-butanediol. The mold-cast specimens (mold temperature 70 °C) were postcured at 130 °C for 20 hours. Hardness of the resulting unfilled polyurethane was measured to be 23 P&J.

Table 4

Figure 9 shows the wear resistance test results which were measured by abrasion method DIN 53516 and the results given as loss mm³. It is clearly evident that the abrasion resistance of the polyurethane is improved when even 3 wt-% of surface modified UHMWPE particles (Composition Q) are added in the polyurethane. Figure 10 shows improvement in the polyurethane's tear strength (in N/mm²) when UHMWPE particles are added in the material. Example 5

A polyurethane top stock was prepared by mixing different amounts of Inhance UH-1250 (by Inhance/Fluoro-Seal) surface modified ultra high molecular weight polyethylene particle in the curative side, formulations being depicted in Table 5. The polyethylene particles and 1 ,4-butanediol were premixed with the Terathane 1000 at 50 °C, RFA-1000 (40 °C) was mixed with this curative and the mixture was cast in molds at 70 °C. After gelation the polyurethane samples were postcured at 130 °C for 20 hours. Hardness of the resulting unfilled polyurethane was measured to be 40 P&J.

Table 5

Figure 1 1 shows the DIN abrasion values of compositions presented in Table 5, the values being measured by method DIN 53516 and the results given as loss mm³. Clearly addition of even 3 w-% of UHMWPE (Composition V) improves abrasion resistance, the effect of higher concentrations being even more significant.

The tear strength (in N/mm²) was determined for three compositions (Compositions U, V and X) shown in Table 5 and the results are presented in Figure 12. The measurements were made according to IS034-1 , method B, procedure (b). Figure 12 shows that with this type of polyurethane there is a marked improvement when UHMWPE content is increased to up to 3 w-% (Composition V), then the effect levels down.

Claims

1 . A coating for use in component of a fiber web machine, comprising

- polyurethane elastomer and

- 0.1 - 9.0 wt-% of surface modified particles selected from the group consisting of high density polyethylene, ultra high molecular weight polyethylene and mixtures thereof.

2. A coating according to claim 1 , characterised in that the amount of surface modified particles is 0.5 - 8.5 wt-%.

3. A coating according to claim 2, characterised in that the amount of surface modified particles is 3 - 7 wt-%.

4. A coating according to any of the preceding claims, characterised in that the average molecular weight of the high density polyethylene is 50 000 - 900 000 g/mol, preferably 100 000-300 000 g/mol.

5. A coating according to any of the preceding claims, characterised in that the average molecular weight of the ultra high molecular weight polyethylene is over

1 000 000 g/mol.

6. A coating according to any of the preceding claims, characterised in that the surface treatment of the particles is plasma treatment or reactive gas oxidation treatment.

7. A coating according to any of the previous claims, characterised in that it further comprises additives.

8. A coating according to any of the preceding claims, characterised in that the average particle size is 5-300 μιτι.

9. A roll cover for a roll of a fiber web machine comprising as its outermost layer a coating according to any of the claims 1 -8.

10. A roll for a fiber web machine, comprising a roll body and a cover arranged on said roll body, said cover comprising as its outermost layer a coating according to any of the claims 1 -8.

1 1 . A rod bed for a coater or a sizer of a fiber web comprising as its outermost layer a coating according to any of the claims 1 -8.

12. A manufacturing or treatment apparatus for a cellulosic fiber web comprising a roll according to claim 9.

13. A manufacturing or treatment apparatus for a cellulosic fiber web comprising a rod bed according to claim 1 1 .