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Impressive performance even at extreme temperatures

Bayer MaterialScience AG : 07 January, 2005  (Company News)
When it comes to insulation, rigid polyurethane foams are considered by experts to be world class. With rigid polyurethane foams, insulating layers can be much thinner than those made of most competing products in general use and yet still deliver the same performance.
With their high proportion of fine closed cells filled with gas – a poorer conductor of heat than the surrounding air – their thermal conductivity values are lower than those of conventional polystyrene, and much lower than those of the heavier mineral wool. At especially high or low temperatures, however, the insulating material is subject to additional stress. With long-term exposure to temperatures of 150 °C or more – such as are found in pipelines carrying steam – rigid polyurethane foam insulation materials can become brittle and lose their shape over time. If they are repeatedly cooled to extremely low temperatures such as those of liquid gases for instance, there is a risk of cracks that may cause thermal bridges.

Strengthening network for high temperature resistance

Bayer researchers have succeeded in making rigid polyurethane foam perform even under these hostile conditions. “To make polyurethanes highly resistant to heat, a surplus of isocyanate can be added to them. When the foam hardens, the surplus isocyanate molecules react further to produce a more densely cross-linked polyisocyanurate (PIR for short),” says Dr. Jacqueline Kušan-Bindels, polyurethane expert at Bayer MaterialScience. “The polyisocyanurate backbone stabilizes the rigid foam from the inside, making it heat resistant.” One of the key tasks for Bayer researchers was to find suitable polyols that would not make the PIR foams too brittle. The new high-temperature (HT) rigid foams provide just as good insulation as standard rigid polyurethane foams, but can withstand temperatures up to 250 °C for short periods, or long-term exposure to temperatures of up to 200 °C at densities of between 40 kg/mł and 130 kg/mł. They also keep their shape better at these temperatures than foams produced using conventional formulations and, even with pentane as a propellant and no flame retardants, still achieve a B2 flammability classification.

“Tank trucks transporting hot goods can now be made lighter while at the same time providing more capacity,” says Dr. Kušan-Bindels. The high-temperature insulating materials are already being used to insulate floors onto which hot asphalt mastic is poured. The high temperatures of this material widely used in the construction industry would destroy conventional insulation materials.

Important applications in the Middle East

Energy losses during energy distribution are becoming an increasing cause for concern. Economical combined heat and power production requires efficient transport of heat energy over large distances, mostly in the form of hot water or even steam. The oil industry needs to pump crude oil at high temperatures in order to keep its viscosity low and save pumping energy. “When used in pipelines at temperatures up to 200 °C or with short-term exposure to temperatures up to 250 °C, PIR rigid foam has clear advantages over mineral wool, because the insulating layer takes up less space and insulating the pipe with this rigid polyurethane foam is much more cost-effective than ´stuffing´ it with mineral wool,” said Joachim Kleser, heat insulation expert at Bayer MaterialScience, recently at a talk during the Gulf Construction Conference Week 2004 in Dubai, United Arab Emirates. An added advantage is the fact that, in the event of a leak in the insulation, the material absorbs less moisture and above all no flammable liquids. Unlike mineral wool, rigid polyurethane foam does not act like a wick. “Oil-exporting countries, including those in the Middle East, are a key market for these products, to ensure that the energy really does reach us,” said Joachim Kleser.

Giant refrigerators on the high seas

For the other end of the temperature scale, Bayer researchers had to find a different solution – “built-in” PIR reinforcement is not suitable for this application. “In the low-temperature range,” says Dr. Kušan-Bindels, “we have achieved very successful results by combining tough rigid polyurethane foams with glass fibers. Just like in high-tech composites, these strengthening materials prevent the polyurethane foam from becoming fatigued and eventually tearing as a result of repeated heating and cooling.” The researchers see a possible application for this special version of the polyurethane foam Baymer® in membrane tankers transporting liquid gas across the oceans. Currently, these giant tankers that can carry up to 200,000 cubic meters of liquid gas are mostly insulated with densely packed pearlite beads that take up much more space than sheets of rigid polyurethane foam because they have lower insulating capacity. “With rigid polyurethane foam that is stable at low temperatures, ocean-going loads can be increased, thereby enhancing cost-effectiveness,” says Dr. Kušan-Bindels, summing up the advantages of the new insulation material.
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