Rigid polyurethane foam for refrigerated vehicles and containers

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2. Insulation for the most demanding applications Refrigerated containers and vehicles are generally run under conditions which demand both optimum thermal insulation and high mechanical strength. These properties have to be maintained throughout the service life of the product, which often exceeds 15 years. The specific requirements of shipping lines, leasing companies, etc., which result from a number of different influences, are also an important factor and include: • type of freight transported • loading and unloading conditions • quality of handling in different areas • requirements of the International Standards Organization (ISO regulations) • personal preferences and company policy • national transport legislation. In addition, production costs, durability, corrosion resistance, low repair and maintenance costs (lifecycle costs) have to be taken into consideration when producing refrigerated containers and vehicles. Bayer MaterialScience AG works in close cooperation with its customers here: all our polyurethane raw materials are subjected to a lengthy internal quality assurance procedure in compliance with strict DIN EN ISO 9001/2000 standards. A typical refrigerated container Page 4 of 6 File No.: PU21019-0410 en Issue 2004-10-11 Reinforcing sandwich panels The design of the frame, the floor and the roof of insulated containers and vehicles is critical, the strength of the structure being dependent on the mechanical properties of the reinforcing sandwich panels. These panels consist of a rigid polyurethane foam core with metal facings which are bonded over the entire surface of the polyurethane core layer to strengthen the container. The constant vibrations and shocks encountered during road transportation make long-term stability an essential requirement, and one which the elasticity of the rigid polyurethane foam satisfies. In addition to the mechanical requirements, the original high insulation value of the rigid polyurethane foam has to be maintained over long periods to avoid aging. This is achieved by using facings which prevent the diffusion of water even at internal temperatures below - 25 °C. The insulation thickness ranges from 50 to 200 mm at densities of between 45 and 100 kg/m³. However, if internal temperatures are only 0 °C to 5 °C, thinner insulating layers down to a minimum of 20 mm are used. The facings can consist of aluminum, steel, stainless steel or glass fiber reinforced plastics laminates and are from 0.4 to 2.0 mm thick. The sandwich panels are produced in lengths of 3 to 16 m using specially designed temperaturecontrolled foam presses and are either welded or riveted to the frame, depending on the materials used and the frame construction. Insulation Technical Information
Cavities remaining after the panels have been fixed are filled with foam to provide uniform insulation with excellent physical properties. About 300 kg of rigid polyurethane foam are required to insulate a standard refrigerated container. The formulations used and the methods of production have been continuously developed and improved in recent years. This guarantees that the polyurethane sandwich panels produced have uniformly high strength and elasticity as well as optimum insulating values. 3. New-generation blowing agents Up until the end of 2003, the use of the HCFCbased blowing agent R141b in the production of rigid polyurethane foam in the transport sector was still sanctioned under the Kyoto Protocol. Nowadays, however, blowing agents having a GWP (global warming potential) > 0 must be used. New foam systems based on alternative blowing agents have accordingly been developed. The most promising alternatives Since the early 1990s, Bayer has been researching new R11-free polyurethane foams to replace the former systems. Numerous chemical and physical tests yielded three alternative foam systems with the blowing agents R141b, cyclopentane and carbon dioxide. The Kyoto Protocol states that an alternative must now also be found for the blowing agent R141b. Meanwhile, cyclopentane, isopentane, n-pentane and mixtures of these have enjoyed much success with European polyurethane manufacturers, and there is also the new generation of HFC blowing agents, 365mfc and 245fa. The mechanical properties of all the foams tested easily surpassed the minimum requirements for such applications. The production parameters could also largely be adapted to previous values, resulting in minimal production changes. Proven insulation efficiency The insulation value of rigid polyurethane foam is also important in refrigerated containers. The heat transmission capacity of an insulated container, that is the total energy required to maintain a constant internal temperature against defined external temperatures, determines the power requirement of the refrigerating unit. Page 5 of 6 File No.: PU21019-0410 en Issue 2004-10-11 Heat losses through rigid polyurethane foam can be calculated purely theoretically on the basis of its γ value. On the other hand, data relating to the complete container can only be determined by testing the finished article. The thermal properties of the rigid polyurethane foam as well as any thermal bridges resulting from structure-related factors or insufficient integrity are critical. These parameters were investigated by subjecting a batch of 20 containers to 12-hour tests in a refrigeration chamber with substantially constant temperature variations and air speeds, to determine heat transmission. Each type of container was examined by measuring five identical containers in succession to provide statistically accurate results. Test results When measured, the diathermancy of the HFC blowing agents 245fa and 365mfc/227ea 94/6 was approx. 9.5 % higher than that of R141b. The insulating effect of the foams blown by the representatives of the pentane family was reduced by approx. 8 % in the case of cyclopentane, approx. 11 % in the case of 70/30 cyclopentane/isopentane, and approx. 14 % in the case of iso-pentane and npentane, compared with R141b-blown foam. Cyclopentane or the 70/30 c/i-pentane mixture currently looks set to establish itself in Europe as the norm for large-scale production. The 94/6 HFC 365mfc/227ea mixture will for the time being retain its place in the market as an alternative for smallscale or specialized production. Bayer Material- Science AG has fully-developed systems to cover all of these developments. The loss in insulating efficiency compared to HCFC R141b will, however, have to be compensated by an increase in insulation thicknesses and possibly by improved construction and refrigeration units. Insulation Technical Information
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