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ELANTAS Beck: Monomer-free unsaturated polyester resins in the electrical industry

In the past, unsaturated polyester resins replaced the solvent-containing varnishes used as secondary insulation for windings in, mainly, electrical motors. This helped to considerably reduce the emissions that occurred during curing, as the varnishes contain about 50% of solvents which have to be removed during the curing process. In addition to the technical disadvantages, such as bad filling of the windings with all the negative consequences this entailed for thermal and mechanical performance of the finished machine, the environment was affected by these emissions.

Unsaturated polyester resins consist of a base resin dissolved in a comonomer. This comonomer is essential for the curing of the unsaturated polyester and is incorporated into the network by chemical bonds. Theoretically, it is a system curing for 100% and therefore does not produce emissions when curing is carried out at ambient temperatures. For technical reasons, curing at ambient temperatures is not possible on a large scale because the pot life of the activated resin is limited at ambient temperature. Hence, hot curing systems have been developed which guarantee a long pot life at ambient temperature, but have a short curing time at elevated temperature. This property has become increasingly important because of the desire for short times of the objects involved in the application process. But because of the curing at elevated temperatures the typical properties of the solvent nature of the comonomer, the high vapour pressure leading to emissions became more and more important. Depending on object and curing cycle, such mainly emission-free curing systems can have emissions of up to 20% of the resin used. These emissions affect the environment, even when they are treated using waste air treating equipment. Another aspect is the economic loss due to the not to regain emissions.

Many efforts have been made in the past to substitute the normally used comonomers styrene and vinyl toluene by other comonomers that have, even at elevated temperatures, considerably lower vapour pressure and therefore lower emission rates during curing. Such systems have other disadvantages, especially in toxicology. Therefore ELANTAS Beck has looked for another principle, which can best be described as "taking out what is unwanted". Unsaturated polyesters have been developed which do not need a comonomer for curing. Such systems have several advantages over conventional unsaturated polyester resins. As they have a very low vapour pressure at curing temperatures, they do not have zero emissions but, depending on object and curing conditions, the emissions range between 1 and 5%. As they are polymers, they are not classed as dangerous goods and therefore there are no restrictions imposed on transport, storage and application. Because of the special technology employed in the impregnating process it is possible to accurately adjust the resin content in the windings and, for the first time, to impregnate a winding for nearly 100% - an advantage particularly for machine constructions with the risk of partial discharges.

On the other hand, the monomer-free unsaturated polyesters have their disadvantages. As they are polymers, they have a much higher application viscosity, and therefore new application technologies are necessary. The reactivity is lower, because, under the laws of physics, high molecular substances react slower than low molecular substances. Physical drying, as occurs with varnishes and conventional unsaturated polyesters through the evaporation of the solvent or comonomer from the surface of the objects, is not possible in principle - monomer-free resins dry only by curing. These disadvantages currently reduce the direct use to trickling and dip-roll application on existing impregnating equipment without changing the application process. Of course, the existing disadvantages are likely to be reduced by further research.

The monomer-free resins have been on the market for 8 years. In those cases where they have been applied, they have proved successful. In spite of the undeniable disadvantages that exist, ELANTAS Beck is convinced that this principle is the only effective way to reduce the emissions that occur during the impregnation process and the curing, effectively providing the electrical industry with the opportunity to reduce the emissions as required by the Göteborg Protocol, at least in one small area of technical applications.