Polyurethane, roll cover and production method

Abstract

The invention relates to polyurethane for producing roll covers, wherein a peroxide-based cross-linking agent is added during the production of the mixture and in addition both 4,6-bis (octylthiomethyl)-o-cresol and 4,4'- bis(phenylisopropyl) diphenylamine are added.

Description

Polyurethane, roller cover and manufacturing process The invention relates to a peroxide-crosslinked polyurethane for use in a roll cover, a roll cover comprising such a polyurethane, and a production process for a polyurethane and such a roll cover. Such roller covers can be used, for example, on rollers for machines for the production or processing of fibrous webs. Industrial rollers, especially those used in plants for the production or processing of paper webs or other fibrous webs, often consist of a completely or largely cylindrical roller core, usually made of metal. A single- or multi-layer roller cover is then applied to this roller core. This roller cover can consist of one or more polymers. By appropriately selecting the structure, composition, and manufacturing process of these covers, the properties of the roller can be adapted and optimized for the intended application. Polyurethanes (PU) are often used to manufacture these roller covers. Polyurethanes (PU, DIN abbreviation: PUR) are plastics or synthetic resins resulting from the polyaddition reaction of diols or polyols with polyisocyanates. The urethane group is characteristic of polyurethanes. - NH - CO - O - PU rollers can be manufactured with very different hardnesses, from hard rollers (up to 3 P&J or less) to very soft rollers with 100 P&J or softer. PU covers are typically manufactured using a casting process. It is known that polyurethane covers produced using this process generally have excellent mechanical properties and very good hydrolysis stability. However, there is a significant decline in these excellent properties, particularly with comparatively soft covers in the range > 40 P&J (Pusey & Jones) and especially in the range above 60 P&J. Furthermore, these covers are difficult to equip with so-called functional fillers. Therefore, optimal adaptation of the roller cover's properties is not always guaranteed. Therefore, DE 101 51 485 proposes avoiding the casting process and producing polyurethane roll covers using an extrusion process or calendered sheets. However, DE 101 51 485 provides only vague guidance to the skilled person regarding the optimal design of the polyurethanes used. It is generally known, for example from US 6,008,312, that in addition to cast polyurethanes there are also thermoplastic polyurethanes and so-called “millable” polyurethanes. These millable PUs are advantageous for the processes described in DE 101 51 485, since they can be processed using conventional rollers, presses or other equipment known from rubber processing, and in particular can also be applied by extrusion. The object of the present invention is to further develop the teaching of DE 101 51 485. It is also an object of the invention to propose a material for a roll cover with good mechanical properties, good hardness stability, and / or good equilibrium roughness. Hardness stability refers, among other things, to the fact that the hardness of the polyurethane, and thus in particular the hardness of the roll cover, does not change, or only changes to a small extent, during roll operation. This is desirable because the The hardness of the rollers is usually selected or optimized for their respective application position. In many areas it has proven useful to describe a roll cover as 'hardness stable' if its hardness (measured in Pusey & Jones (P&J)) changes by a maximum of 10% over the service life of the roll. According to this definition, a hardness-stable roll that has a hardness of 50 P&J when installed will have a hardness between 45 and 55 P&J at the end of its service life. The roll cover should also exhibit a reduced tendency to mark. In particular, the object of the present invention is to propose a class of polyurethanes that can be processed on machines commonly used in the rubber industry (kneader, rolling mill, strainer, extruder, etc.). A roller cover can be produced from millable polyurethane (“millable PU”) using an extrusion process, whereby the viscosity of the millable polyurethane can be adjusted so that, due to the excellent flow properties of the millable polyurethane, no disturbing striped markings occur, as described in DE 101 51 485 A1.

[0015] described. It has been shown that peroxide crosslinking can achieve significantly better hardness stability than, for example, sulfur crosslinking. However, peroxide crosslinking is negatively affected by the addition of conventional anti-aging agents. Surprisingly, the inventors have found that a mixture of 4,6-bis(octylthiomethyl)-o-cresol (e.g. "Irganox 1520L" from BASF) and 4,4'-bis(phenylisopropyl)diphenylamine (e.g. "Naugard 445" from ChemPoint) has little influence on the peroxidic crosslinking and yet retains its effect, in particular as an ageing inhibitor, even under the conditions existing in a paper machine. Thus, the task posed at the beginning will be solved by using a peroxide crosslinker together with a combination of 4,6-bis(octylthiomethyl)-o-cresol (e.g. “Irganox 1520L” from BASF) and 4,4'-bis(phenylisopropyl)diphenylamine. Dosage tests have shown that 4,4'-bis(phenylisopropyl)diphenylamine (Naugard 445) can be added at a standard dosage, for example, 0.5 wt% based on the polymer. A range of, for example, 0.4 wt% to 0.6 wt% may also be possible. However, in the case of 4,6-bis(octylthiomethyl)-o-cresol (Irganox 1520 L), it has been shown that the dosage of 0.3 wt% commonly used for this stabilizer, which is also recommended by the manufacturer, is often too low. A significantly higher proportion of Irganox 1520L between 0.8 wt% and 1.2 wt%, especially 1 wt% based on the polymer, leads to noticeably improved roll cover properties, particularly better hardness stability. A polyurethane as described above can be used as a polymer material for use in a roller cover. However, the polymer material can also contain other components besides such a polyurethane. For example, the addition of fillers, especially fine-particle polymers or minerals, has proven advantageous. This can further improve the mechanical properties and abrasion resistance of the polymer material or roll cover. A variety of polymers can be used for these polymer fillers. HDPE (high-density polyethylene) has proven particularly effective. This material has a high density (0.94-0.97 g / cm 3 ) and, with its comparatively strong and dense structure, is well suited as a polymer filler, which UHMWPE (ultra-high molecular weight PE) is also well suited as a molecular filler due to its high wear resistance. A combination of HDPE and UHMWPE fillers is also conceivable. For these fillers, especially for the polymers, particles are preferably used which have a particle size of less than 100 pm, in particular less than 80 pm. For fillers made of polyethylene powder, for example, plasma treatment can improve compatibility with the polymer matrix. In particular, significantly improved dispersion and bonding to the matrix material (the polyurethane) can be achieved. Alternatively or additionally, it is possible to add further fillers. For example, carbon black and / or precipitated silica and / or fumed silica can be added. This can contribute to further structural reinforcement. Alternatively or additionally, fillers can also be added to influence the surface properties of the final roller. For example, the addition of powdered pumice can influence the wear of doctor rods or blades that come into contact with the roller surface during operation for cleaning or dosing application media. It may further be provided that at least one diisocyanate is added during polymer production. The diisocyanate may be selected from the group consisting of methylenediphenyl isocyanates (MDI), naphthylene-1,5-diisocyanates (NDI), isophorone diisocyanates (IPDI), toluene-2,4-diisocyanates (TDI), or H12MDI. Alternatively or additionally, it may be provided that allyl groups are added during polymer production, in particular in the form of glyceryl alpha-allyl ether or trimethylolpropane monoallyl ether. In machines for producing or processing fibrous webs, roller shells typically reach operating temperatures between 50°C and 80°C during operation, and almost always remain below 100°C. Many polymer materials can be used for these temperature ranges. However, during continuous operation of the systems, exceptional situations such as web breaks, spray nozzle failure, or improperly adjusted parameters often occur. Until the damage is repaired, a roller may reach significantly higher temperatures than normal operating temperatures for a certain period of time. In advantageous embodiments, it can be provided that the polymer material has a thermal conductivity of more than 0.24 W / mK or more than 0.25 W / mK or more than 0.26 W / mK, in particular between 0.25 W / mK and 0.8 W / mK, particularly preferably between 0.26 W / mK and 0.5 W / mK. In further advantageous embodiments, it can be provided that the polymer material has a specific heat capacity of more than 1.95 MJ / m 3 K or more than 1.98 MJ / m 3 K or more than 2.0MJ / m 3 K, in particular between 1 .98 MJ / m 3 K and 2.3 MJ / m 3 K, particularly preferably between 2.05 MJ / m 3 K and 2.13 MJ / m 3 K. The thermal conductivity and capacity are measured according to ISO 22007-2 A comparison between a conventional cast polyurethane ("Standard") and a polymer material according to one aspect of the invention based on a rollable polyurethane ("Sample 1") is shown in the graph of Figure 1. There, the course of the loss factor "tan δ" is shown over the temperature. The tangent of the phase angle δ of the sample is a relative measure of the viscous and elastic properties of a material. It ranges from 0 for a perfectly elastic material to infinity for a completely viscous material with a value of 1 when the storage modulus = loss modulus Technically speaking, it is the ratio of the loss modulus over the storage modulus and thus describes the tendency of a material to “lose energy” (i.e., more viscous) or “store energy” (i.e., more elastic). It can be seen that the curve of "Sample 1" is slightly higher than the curve of the reference sample in the typical operating range between 50 and 90 °C, but continuously decreases with increasing temperature. These curves intersect in the range of approximately 140 °C. While the loss factor of "Sample 1" continues to decrease continuously with increasing temperature, the loss factor of the reference sample increases significantly. In the exceptional situation described above, this behavior of the "standard" material is very disadvantageous. If the temperature of the roll shell rises above a certain value, e.g. due to a cooling system failure, the massive increase in the loss factor leads to increased dissipation of energy in the roll covering, which accelerates the temperature rise. This temperature increase can quickly lead to destruction of the roll covering. In contrast, the continuous decrease in the loss factor tan ö in Sample 1 has a dampening effect on the temperature rise, since less energy is dissipated at higher temperatures. This at least significantly increases the period before the roll covering is destroyed in the event of an accident. There are many fields of application for roll covers that comprise or consist of a rollable polyurethane or a corresponding polymer material according to one of the aspects described. In the paper machine sector, examples include: Coater backing rolls. These rolls in coating machines typically have a hardness between 40 and 70 P&J (Pusey & Jones). • Rollers for film presses. In these coating units, which are typically used to apply starch to the paper web, the roller covers usually have a hardness between 10 and 75 P&J. • Lump breakers. These are rollers that usually have very soft roller covers. Hardnesses from 50 P&J to 100 P&J or even 150 P&J are possible. Generally, using a rollable polyurethane according to one of the described aspects, roller covers can be produced in a hardness range between 10 P&J and 150 P&J.

Claims

Patent claims 1. Polyurethane for the production of roller covers, wherein a peroxide-based crosslinking agent is added during the preparation of the mixture, characterized in that both 4,6-bis(octylthiomethyl)-o-cresol and 4,4'-bis(phenylisopropyl)diphenylamine are also added during the preparation of the mixture.

2. Polyurethane according to claim 1, characterized in that the added amount of 4,4'-bis(phenylisopropyl)diphenylamine is between 0.4 wt% and 0.6 wt%.

3. Polyurethane according to one of claims 1 or 2, characterized in that the added amount of 4,6-bis(octylthiomethyl)-o-cresol is between 0.8 wt% and 1.2 wt%.

4. Polyurethane according to one of the preceding claims, characterized in that allyl groups are added during the reaction or polymerization, in particular in the form of glyceryl alpha-allyl ether or trimethylolpropane monoallyl ether, 5. Polymer material for use in a roll cover, wherein the polymer material comprises or consists of at least one polyurethane according to one of the preceding claims.

6. Polymer material according to claim 5, characterized in that the polymer material comprises fillers, in particular in the form of polymer particles and / or minerals.

7. Polymer material according to claim 6, characterized in that the polymer particles consist entirely or partially of a polyethylene, in particular of a HDPE or UHMWPE.

8. Polymer material according to one of claims 6 or 7, characterized in that the fillers have a particle size of 100 pm or less, in particular of 80 pm or less.

9. Polymer material according to one of claims 5 to 8, characterized in that the polymer material has a thermal conductivity of more than 0.24 W / mK, preferably more than 0.25 W / mK, in particular between 0.25 W / mK and 0.8 W / mK.

10. Polymer material according to one of claims 5 to 9, characterized in that the polymer material has a specific heat capacity of more than 1.95 MJ / m 3 K, preferably more than 1.98 MJ / m 3 K.

11. A roll cover for a roll, particularly for use in a plant for producing or processing a fibrous web, characterized in that the roll cover is constructed entirely or partially from a polymer material according to one of claims 5 to 10.

12. Coating device, in particular a film press for a machine for producing or processing a fibrous web, comprising a first and a second roller which are arranged so that they form a treatment nip for the fibrous web, characterized in that at least the first roller, in particular both rollers, have a roller cover which is designed according to claim 11.

13. Coating device according to claim 12, characterized in that the roll cover of the first roll and / or the roll cover of the second roll has a hardness between 10 P&J and 75 P&J.

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