KraussMaffei technology enables addition of expandable graphite to PUR foam

KraussMaffei’s technology for expandable graphite metering makes the material’s use as a flame retardant an alternative or addition to liquid admixtures.


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Flame protection requirements for PUR foam components are increasing worldwide, both in the automotive and industrial applications sectors and also due to regulatory requirements. In response to this need, KraussMaffei (Munich, Germany) has announced it will present a complete system for the high-pressure processing of expandable graphite for high material and process efficiency and clean production at K 2019, the worlds leading plastics and rubber industry trade fair, taking place October 16-19 in Düsseldorf, Germany.

“Expandable graphite is an economical filler that offers clear advantages for many automation tasks,” explains Nicolas Beyl, president of KraussMaffei's Reaction Process Machinery Segment. “Unfortunately, the material is sensitive to mechanical stress during processing.”

KraussMaffei’s newly developed high-pressure mixing head with low-pressure bypass and a special premixing station for expandable graphite metering make its use as a flame retardant a particularly efficient alternative or addition to liquid admixtures. The fully automated process chain enables short cycle times and high overall system efficiency in component manufacture.

KraussMaffei says the advantages of high-pressure countercurrent injection mixing in the precise processing of highly reactive PUR foam systems can be put to use in applications with expandable graphite as a filler. This reportedly forms the basis for short cycle times and leads to efficiency in production. In this process, unlike processing in the low-pressure range, the self-cleaning mixing head reportedly eliminates the need for a flushing process after each shot. According to KraussMaffei, this saves material, production time and contributes to consistent product quality, while also eliminating costs for the provision and disposal of flushing material. In the high-pressure mixing process, a higher mixing energy is also achieved. This can be used to implement shorter cycle times.

A special expandable graphite mixing head is at the heart of the technology. The new mixing head is based on Krauss Maffei’s high-pressure mixing heads. The system has been equipped with a low-pressure bypass with an enlarged cross-section, specifically for the processing of expandable graphite. The mechanical stress imposed on the expandable graphite particles between the cycles during which the charged polyol is continuously circulated is thus minimized. The material circulates through the nozzle to build up pressure only just before pouring starts. The filler is therefore subject to minimum mechanical stress. With this technology, high filling levels are possible, according to requirements and depending on the raw material system of up to over 30% by weight in the polymer. The high flame retardant classes UL94-V0 can therefore be achieved.

According to KraussMaffei, the polyol-expandable graphite mixture is prepared in a special premixing station. A special agitator mixes the filler homogeneously with the liquid component. This is done in a gentle manner such that the structure and size of the expandable graphite particles are preserved. Metering takes place automatically and can be increased to 80% by weight in the polyol, providing consistent quality. In addition, production is made cleaner and more efficient because the steps of manual handling, weighing and filling are eliminated.

The weight and volume of the component parts can be optimized using the mixing proportions of expandable graphite and other components during pre-mixing, without sacrificing the flame retardant characteristics.