PU: A high-standard material
You sleep on a cold foam mattress, after breakfast you walk on synthetic shoe soles to your car where you sit on a flexible polyether foam seat. Plastics from the polyurethane (PU) family of products are our constant companions throughout the day. Manufacturing them requires specific know-how.
Polyurethanes are versatile, practically omnipresent plastics. They feature – usually in the form of cold or warm foam – in mattresses, shoe soles, seals, hoses, floors, insulation materials, coatings, adhesives, sealants, skis, vehicle seats, athletics tracks, vehicle dashboards and many other products. The source materials used to make them are liquids – diols and polyisocyanates.
One particular characteristic is vital for the production of PU: “When a polyurethane facility is started up, the formulation must be exactly right straightaway,” Thomas Neuhaus, liquids expert at Brabender Technologie, explains. “The ongoing process cannot be stopped or adjusted, the extruder would then have to be emptied and the batch would be rejected.”
Proprietary control technology
This delicate property necessitates a sophisticated control system. Brabender Technologie uses its own M-CON multi-component system together with the Congrav® control module, which can be connected to an overall process control system (host / PLC). Visible consequences of this are the relatively large control cabinets that are part of every Brabender-designed PU feeder system. “In contrast, what you cannot see are most cables, which we conceal in the hollow sections. Many have plug connections in key places to make replacing them as easy as possible.”
Because the cabling involved in the many control loops is complex, it is assembled by Brabender Technologie in the factory. Neuhaus: “A PU feeder station is practically delivered like a plug-and-play device. After being connected to the extruder, it is immediately ready to use.”
Sensitive to temperature fluctuations
Since the chemicals have to be processed within a narrow temperature range of +/-5 degrees Celsius, the entire feeder system basically has to be electrically heated. For this purpose, all modules and flexible tubes are wrapped in tailor-made heating sleeves, which are closed using Velcro fasteners (see picture on page 9). Each is fitted with an individual pluggable control loop, in order to make replacement as easy and convenient as possible and to facilitate precision control.
An additional agitator in the hopper ensures the residues are constantly mixed with new liquid, so that the temperature remains as constant as possible. If the liquid is not being conveyed, it flows through a bypass in the loop back into the hopper. “These source materials must always remain in motion, otherwise they sediment and clog the system,” Adrian Baron explains.
Motors with absolute constant speed
The liquid feeding expert highlights another specialty of PU plants: “We basically use powerful servo motors for these systems. They are actually oversized for what they are intended to do, but therefore operate at an absolute constant speed. Pressure fluctuations have absolutely no impact and the gear pump can operate with absolute precision.”
The source chemicals and their catalyst are mixed in a static mixer that can be incorporated in a heat exchanger. Here the chemicals flow through double-walled heat pipes until they have reached the ideal processing temperature and are ready for mixing and extrusion. “Brabender Technologie is able to plan not only the feeding system itself but also the peripheral equipment with all the necessary connections etc. as well as automation, if customers request us to do so,” Thomas Neuhaus relates. This provides customers with a worry-free turnkey package for their PU production facilities.