Rotational casting of PU covered rolls.

Rolls with elastomeric coatings are widely used in a variety of industries, especially in the paper, lumber, printing, metals and textile industries.

The majority of these rolls are covered with rubber. Rolls covered with polyurethane elastomers, because of the economics involved, are only used where special properties are required, such as:

* Abrasion resistance;

* Tear resistance;

* High load bearing with high hardness;

* Solvent resistance.

Rotational cast polyurethanes have been under development over the past several years. They have recently been introduced commercially, primarily in the European market area. What is the major attraction of rotational cast polyurethanes? Why is there an increasing desire to explore their possibilities? The answer becomes obvious by viewing a comparison of the processes.

Process and equipment

As can be seen from figures 1 and 2, the processing advantages of the rotational casting roll covering system are extremely significant versus the conventional roll covering system. Basically, the conventional 13-step process is reduced to a 7-step process.

The polyurethane reaction mixture is metered through a movable mixing head traveling at a controlled speed in the axial direction along the controlled rotating roller core. The mixing head is maintained at a specified distance from the roller surface. The polyurethane mixture solidifies in a few seconds while building up a layer with a controlled thickness from 4-25 mm depending on the parameter settings. Usually the layers are built up with thicknesses of 4-5 mm, applied sequentially until the, desired roll thickness is achieved. On each required pass of the mixing head, the head indexes to the proper distance to adjust for thickness build-up. After a relatively short curing time (approximately 12 hours at ambient temperature), the roll covering can be machined to the final dimensions for subsequent shipment.

On the basis of the knowledge assembled, it has been possible to construct units for suitably covering rolls of different sizes and shapes oil an industrial scale. One of the most important parts of such a unit is the mixing head support which can be moved independently in the x, y and z directions. A second mixing head has turned out to be very useful by providing greater versatility. With this additional mixing head, special polyurethane systems can be applied along with the standard system, and the time between the casting of two consecutive layers can be shortened. In certain applications moisture is a potential problem because it migrates toward the steel core; therefore, a special hard barrier system has been developed for the initial covering of the core. This protective barrier system or base layer has curing properties which require the subsequent layer of material to be applied within a very short period of time. The additional mixing head allows this to be done. When used with conventional rotational casting materials, the additional mixing head allows for an even shorter start-to-finish time.

The turning device for the roll is another essential part of the covering system. It must have an infinitely variable drive and be connected to the program controlling the material delivery system.

Reviewing the overall rotational casting process as shown in figure 2 , the following details help to provide an understanding of the system:

* The old covering must be completely removed; this is usually done using a lathe.

* Thorough cleaning of the metal core surface is then done via degreasing and shot blasting. Several good methods are available; however, consistency is very important. This step is essentially the same for both the rotational method and the conventional method.

* Dust is removed with a vacuum system.

* To prevent oxidation of the cleaned metal surface, the adhesion promoter should be applied immediately. The recommended adhesion promoter is a specially developed two-part polyurethane system. The system is applied by either roller brushing or spraying.

* The polyurethane layers are applied within approximately two hours. The covering is produced in several layers, with the time between two consecutive layers not exceeding 40 minutes in order to assure good adhesion.

* The overall time needed for applying the covering depends very much on the size of the roll. For example, the material for a roll with a length of 6 m, a diameter of 400 mm and a covering thickness of 15 mm is applied in about one hour.

* Post-curing takes place at room temperature.

* Machining and grinding can take place after approximately 12 hours.

* The finished elastomer attains its final properties after approximately three, days.

Chemicals (polyurethane raw materials)

The chemical system for producing the rotationally cast elastomeric roll covering consists normally of four components. There are two polyol blends: one for 73 Shore A hardness, and one for 95 Shore A hardness. The polyols can be mixed in any ratio to cover the hardness range in between. The polyols are mixtures of long-chained polyethers and short-chained extenders, containing highly reactive hydroxyl and amine groups. The isocyanate component is a prepolymer based on diphenylmethane diisocyanate (MDI) and polyether. The catalyst is metered as a separate component in order to adjust the gel time of the reaction mixture to varying conditions. Rotational casting requires formulations with very short gel times of about 3-6 seconds. The viscosity of the components is in the range of 2,000-3,000 mPa-s at 23 [degrees] C. The solidifying material passes through a waxy phase and starts to get elastic after 20 to 40 minutes depending on the hardness.

Mechanical properties

As shown in table 1, the mechanical properties of the finished elastomers show the typical variations as a function of hardness. The level of these properties is distinctively higher than that of other cold-curing polyether formulations, but is somewhat lower than that of hot-curing systems based on polyesters or . These elastomers exhibit good resistance to oil, grease, certain types of solvents and chemicals, and microbes.

Table 1 - mechanical properties(*) of elastomer

Hardness Shore A       73     85     95
Modulus (100%) MPa    2.9    4.5    8.6
Tensile MPa            11     18     28
Elongation %          500    500    400
Tear strength kN/m     14     20     40
Resilience %           47     41     37
Abrasion cubic mm     100     86     65
Density g/cubic cm   1.07   1.08   1.10

(*) These items are provided for informational use only.
They are approximate values and are not part of the production
specification.

Advantages of rotational casting of rolls

Previously, roll covering with polyurethane elastomers was carried out mainly by the conventional cast-in-mold technique. The hot-curing polyurethane system, metered and mixed by a low-pressure machine, flows into the narrow gap between the roller core and the mold. Obviously, a long gel time is needed for a homogeneous filling, which consequently results in a long demolding time. A large oven is necessary to preheat the roller core and mold, and then to post-cure the covered roller.

Some of the advantages of rotational casting of roller covers include:

* Molds are not needed.

* Mold release is not needed, thereby avoiding the danger of contaminating the roll core.

* Lower energy costs result from the room temperature cure of the systems.

* Overall processing time is shortened.

* Quick adjustment can be made when the production schedule calls for different roll sizes and/or hardness to follow each other.

* Rolls with a large difference in size can be covered by one piece of equipment.

* Emergency roll replacement can be handled with less potential downtime for the customer.

* Hardness can be varied from layer to layer providing unique design options.

* Less overpour is needed.

* Defect caused by air entrapment which sometimes occur during conventional mold pouring are minimized.

* Less inventory of cores, rolls and molds is needed.

* Less floor space is required.

Developments

In a relatively short time, roll coverings produced by rotational casting have gained a remarkable share of the market for polyurethane elastomer coverings. In some cases even less expensive rubber coverings can be replaced because rotational casting offers lower production costs. In order to extend the scope of applications, chemical and technical development is continuing. Expanded ranges of achievable hardness and resistance to specific environments are key targets for future development.