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News

Refined in Europe, Rotational Casting of Polyurethane Coverings Now Ready for U.S., According to Bayer Corporation

Bayer MaterialScience AG : 12 April, 2001  (Company News)
Rollers with elastomeric coverings are widely used in a variety of industries. While the majority of these rollers are covered with rubber, polyurethanes are often chosen when special properties, such as abrasion resistance, tear resistance, high load bearing with high hardness and solvent resistance are required.
According to a technical paper by Bayer Corporation, rotational casting of polyurethanes has been developed and refined to the point where rollers can be made for almost any application. The paper, 'Rotational Casting Of Polyurethane Roll Coverings,' was presented by John Perry, a Technical Services Specialist with the Elastomers Business Group of Bayer's Polyurethanes Division, at the Epoxy Resin Formulators Spring 2001 Conference, held April 1-3 in Toronto, Ontario, Canada.

Perry explains that until recently, the most common way to make polyurethane rollers was to pour cast using conventional polyurethane prepolymers, a potentially difficult, time-consuming, energy- consuming and resource-consuming process.

Perry said rotational casting has a number of advantages over the traditional method, including: no need for molds; streamlined production; minimal loss of material; no use of release agents; energy savings; multi-module coverings; ability to cast coverings ranging from 1/8 inch to nearly 1 inch in a single pass; and, no post-finishing requirements.

'The technology for rotational casting of polyurethane rolls has become established in Europe, and the technology is ready for use in the United States,' said Perry. 'The processing and chemistries seem to be rather complicated at first glance, but because of the long application history, both parameters have been proven and are easily transferable.'

A Balancing Act

Simply put, rotational casting is a process where the liquid polyurethane is poured continuously onto the rotating roller core until the desired thickness of elastomer is achieved. 'It is very much like wrapping string on a spool, wrapping rubber onto a conventional rubber roll, or even filament winding,' explained Perry.

Though in principle quite simple, there are many chemical and physical factors that influence the overall process. 'To successfully use this method, you must be able to balance all parameters,' said Perry.

On the chemistry side of the equation, Perry explains that polyurethane systems for rotational casting can be based on a polyether polyol, polyester polyol or polycarbonate polyol, depending on the application.

For example, the polyether system is designed for light to medium demanding applications in the steel, mining and paper industries; the polyester system is primarily used in the steel and mining industries; and the polycarbonate system has been designed to meet the demands of the papermaking industry. In his paper, Perry discusses typical mix ratios, physical properties and possible applications for the different polyurethane systems.

On the equipment side, he points out that the urethane casting machine should accurately meter the four components of each of the three different systems over the complete range of component ratios. Other important considerations include the roller drive, mix head support and nozzle selection.
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