184.108.40.206.2 Polyurethane Roll Covers
Polyurethane roll covers were introduced to the paper industry at the end of the 1980s. The outstanding mechanical and dynamical properties make polyurethane the most suitable material for soft elastomeric covers in press positions. Depend¬ing on paper quality and press design, high open surfaces up to 45 % with grooves, blind drilled holes or suction holes, as well as combinations thereof are used. An example of surface design is shown in Fig. 6.11.
For these types of covers in modern machines only polyurethane elastomers are able to provide enough wear resistance to reach acceptable grinding intervals. High performance polyurethane formulations are used, giving the required me¬chanical strength and elasticity as well as outstanding hydrolytic stability.
Usually the cover has a multilayer design in order to ensure excellent bonding of the polyurethane functional layer to the core as well as safe running properties. State of the art are composite base-layers with endless fiber reinforcements, which allow a gradual adjustment of the mechanical and thermal properties of the cover in a radial direction. This results in a gradual adjustment of the shear forces cre¬ated by the nip load within the cover layers. Two layer designs with a harder polyu¬rethane underlayer are also used to provide bonding to the shell.
In fast running machines the dynamic heat build up in the cover limits the application of elastomeric materials. Polyurethane polymers, based on special for¬mulations with excellent dynamic properties, ensure good performance in these applications with minimal heat build up. An example of such an application is the substitution of grooved steel rolls by elastomeric covers in shoe presses.The functional layer of polyurethane covers is manufactured by casting proc¬esses, thus ensuring very homogeneous material properties.
Roll preparation is similar to that for rubber, chemical adhesives are used to bond the base layer. The base layer can be a fiber reinforced composite, which is applied in a wet impregnation process of the reinforcement material. The glass fiber reinforcement (tapes or rovings) is impregnated in a resin bath and then spirally wound onto the roll shell. Curing of the base layer with infrared heating and/or ovens and tooling to a defined geometry are the next steps.
The polyurethane elastomer itself is a reaction polymer, created by the mixing of prepolymers and chain extenders or hardeners. This reactive mixture, which is created in the desired mixing ratio by special casting machines, is cast into a mold built around the roll shell with the base layer. The casting procedure can be carried out horizontally or vertically. Bonding to the base layer is done chemically with adhesives or reactive layers. The polyurethane material itself is not normally filled, thus providing outstanding elastomeric material properties.
Another method for manufacturing polyurethane covers is the moldless rota¬tional casting process (Fig. 6.12). Here the reacting polyurethane mixture is cast as a spiral on the rotating roll, the fast reaction of the polyurethane mixture prevents the material dripping off the roll. Layers of different hardness can be applied easily with this method. Most polyurethane formulations need to be post-cured at elevated temperatures for optimum material properties. The final mechanical manufacturing steps are similar to those of rubber covers.