184.108.40.206.3 Composite Roll Covers
Two basic composite designs for the functional layer of roll covers are used in the paper industry: Composite covers with a fiber reinforced functional layer and com¬posite covers with a cast functional layer (resin covers).
220.127.116.11.3.1 Fiber Reinforced Functional Layer
Composite covers with fiber reinforcement in the functional layers are mainly used in calender applications. The use of fibers significantly improves the mechan¬ical and thermal robustness of the cover. High performance materials like Ara¬mide fibers are used, where a high damage tolerance is required. Compared with structural composites, where the fiber characteristics dominate the mechanical properties, the fiber volume content of the functional layer of roll covers is rela¬tively low. The major physical properties like hardness, compressive modulus or strength of the covers are mainly dominated by the particle filled resin matrix.
The base-layers of the multilayer designs are mostly glass fiber reinforced, in these layers the fiber content is higher, and therefore the fiber properties dominate the properties of the composite.
For soft calendering different hardnesses – or more accurately different Young’s moduli – of the covers are required in order to ensure the required paper proper¬ties. This can be achieved primarily by varying the amount and combination of different fillers in the fiber reinforced composite.
The surface of the covers must be as smooth as possible, the wear resistance excellent. These properties can be influenced for instance by the modulus of the fibers, particle size distribution and hardness combinations of the fillers and, to a minor extent, by the properties of the resin. Temperature resistance and low heat build up of the cover under dynamic load is mainly a function of the resin matrix system. Operating conditions of calenders such as 25 Hz load frequency, 90 °C surface temperature and 50 MPa nip pressure as well as expected load cycles of >109 make the elastic calender covers the most demanding applications of roll covers. Figure 6.13 shows an elastic composite cover in a multi-nip calender.
For lower demand applications composite covers with conventional fiber rein¬forcements (glass or polyester) are used. These covers are for applications such as guide rolls or special press positions.
18.104.22.168.3.2 Cast Functional Layer (Resin Covers)
Composite covers with cast functional layers are applied in the press section or in calenders. Typically, mineral filled resin systems are applied onto a fiber reinforced base layer. The advantages of the cast resin systems are extreme homogeneity of the functional layer and good abrasion resistance. These properties are important for applications such as soft calender covers or center press roll positions. Special formulations of the resin/filler composite allow the adjustment of sheet release properties as well as wear resistance. Limitations for this type of cover are the sensitivity to mechanical damage and thermal shocks due to the relatively brittle functional layer.
For the manufacture of composite covers wet impregnation processes are pri¬marily used. The reinforcement fibers (tapes, rovings or nonwovens) are impreg¬nated in formulated resin mixtures and wound onto the roll body. Multilayering with different fiber reinforcements and different fiber orientations is applied. The winding angle of the reinforcement material mainly governs the reinforcement direction of the fibers, creating anisotropic (direction dependent) material proper¬ties. These special material properties are used for design purposes, e. g. to influ¬ence the strength, thermal expansion behavior or modulus of the composite structure.
Particle filled functional layers are applied by casting processes similar to polyu¬rethane covers.
The curing of the resin systems which are primarily epoxy-based, is done by infrared heating and/or by heated curing ovens. Mechanical processes are similar to those used for the other cover types. Final surface grinding requires special techniques to reach the desired smooth surface properties.
22.214.171.124.4 Thermal Coatings
Thermally sprayed coatings are gaining wider use in modern paper machines due to their outstanding wear resistance. Even thin coatings of 0.2–0.7 mm provide both excellent resistance and long lifetime. Different thermal spraying processes
(e. g. plasma, HVOF, flame/arc spraying) allow the use of a large variety of mate¬rials.
126.96.36.199.4.1 Ceramic Coatings
Oxide ceramic coatings based on Al, Ti, Cr Oxides or combinations, are used in hard press roll positions. The outstanding wear resistance of the ceramic surface as well as excellent sheet release make these coatings suitable for high speed paper machines. Due to the well defined porosity structure of the ceramic, a hydrophilic coating surface is created. This results in excellent sheet release and a low ten¬dency for deposition of hydrophobic stickies on the roll surface. The surface topog¬raphy of the coating, an important factor for sheet release, is kept constant over its lifetime, even under doctored conditions.
The coating itself is usually a two or three layer design. The functional layer made of ceramics is, by its nature, porous. To ensure a corrosion resistant coating either chemical sealing of the pores is carried out or corrosion resistant nonporous underlayers are applied. The main applications are for hard press rolls/center press rolls in fast running paper machines.
188.8.131.52.4.2 Hard Metal Coatings
Hard metal coatings are carbides, nitrides and borides of transition metals. Car¬bides of the Cr-group are mainly used for thermal coatings. These wear resistant layers are used for grooved or drilled press rolls providing increased grinding intervals. Optimized formulations of these multiphase coatings are used to coat heated calender rolls. These coatings can be polished to extremely fine surface smoothness. The durability of these systems allows continuous doctoring of the coatings.
184.108.40.206.4.3 Metals and Alloys
Metals and/or alloys are sprayed mainly as base or underlayers in combination with oxide ceramics and hard metal coatings. The difference in thermal expansion of the oxide layer and the metallic roll is partly compensated by these layers. The major features of these materials are ductility and toughness, improving the over¬all coating performance.
Thermal spraying is the build up of a coating on a substrate from particles sprayed at defined temperature and kinetic energy onto the roll surface. The ther¬mal energy is required to melt the powder particles or the wire while the gas flow is necessary for the particle acceleration. Energy sources can be either electrical (arc, plasma) or chemical: H2, propane or kerosene (flame spraying). The melted and accelerated particles hit the surface, re-solidify and build up the coating layer by layer (approx. 10–20 mm/pass).
Arc and plasma spraying can be performed either in vacuum (VPS) or under atmospheric (APS) conditions, but only the latter is applicable for roll coatings. Differences in flame spraying mainly concern the particle velocity. High velocity spraying can be continuous (HVOF) or discontinuous (D-Gun). The plasma coat¬ing of a press roll is shown in Fig. 6.14.
220.127.116.11.5 Chromium Coatings
Galvanic chromium coatings are used in special applications for coating the hard calender rolls. The supreme surface finish of an extremely dense homogeneous coating stays is balanced by its sensitivity to damage and limited doctorability. This is why thermal coatings compete in these applications.
18.104.22.168.6 Thermoplastic Covers, Sleeves and Coatings
Pure thermoplastic materials are applied only in niche applications. Cast or extruded polyamide tubes are used as counter rolls in marking presses,
e. g. for cigarette papers, thin PTFE sleeves are applied as hoses on bow rolls. High performance thermoplastic covers are applied in niche calendering applications. Limitations of thermoplastic materials are their moderate wear resistance and the dimensional instability of thermoplastics under load.
Combinations of thermoplastic coatings with an extremely open layer of a hard metal coating combine the properties of both material classes. Release coatings of PTFE sintered on thermally sprayed hard metal coatings for good release proper¬ties are examples of these hybrid coatings. The hard metal underlayer ensures wear resistance, the thermoplastic PTFE layer fills the pores and creates the good release. This type of coating is applied e. g. in the dryer section for the first drying cylinders after the press or size press, as well as for critical guide roll applica¬tions.
22.214.171.124.7 Granite Rolls and Calender Paper Shafts
Granite rolls and paper or cotton filled paper shafts are not roll covers or coatings in the usual sense. Here the material which is providing the function is also part of the load bearing structure of the roll itself.
Certain properties of granite rolls or of cotton filled paper bowls were of great advantage to the paper maker and these were also targets for the steadily improved coatings and covers which in turn could provide additional advantages. Granite rolls, for instance, have become history, regardless of their excellent release charac¬teristics, due to the operational risks at high machine speeds or the technical and economic problems encountered when building this equipment for modern paper machines of 10 m width or more. The release properties in the meantime were matched by ceramic coatings or synthetic composite or rubber covers.
In existing supercalenders the competition from composite covers is increasing due to the limited marking resistance of paper bowls.
These roll types have been replaced in most cases by rolls with modern coatings and covers.