The worldwide polyurethane cast elastomer market is. Naphthalene-1.5-Diisocyanate as a Building Block for High Performance Polyurethane Elastomers

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1 Naphthalene-1.5-Diisocyanate as a Building Block for High Performance Polyurethane Elastomers C. PROLINGHEUER and J. J. LINDSEY Mobay Corporation Polyurethane Division Mobay Road Pittsburgh, Pennsylvania H. KLEIMANN Bayer AG Polyurethane Division D-5090 Leverkusen Federal Republic of Germany ABSTRACT For many years naphthalene-1.5-diisocyanate (NDI) has been used in the polyurethane elastomer VULKOLLAN. NDI differs from the widely used diphenylmethane diisocyanate (MDI) and tolulene diisocyanate (TDI) with regard to its rigid structure and high melting point, resulting in significantly higher melting points of the corresponding NDI/glycol hard segments. Combined with special polyadipate diols and crosslinked with glycols, NDI has proven to be the key building block to ensure the high property profile of the VULKOLLAN elastomer system. The NDI elastomer combines the elasticity of an ether based material with the toughness of an ester based elastomer. In Europe the NDI elastomer is the oldest commercial cast polyurethane system, being used for more than 35 years now. It is also by far the biggest product in the European hot cast polyurethane market. The reason that the NDI based material has kept its leading position in Europe versus MDI and TDI prepolymer systems is the combination of dynamic performance and wear resistance of the elastomer. Results from dynamic mechanical analysis, flex fatigue tests and static properties are being discussed in comparison to elastomers based on other isocyanates. Moreover processing information of NDI and application areas for this material are given. Beside the glycol crosslinking water is used as another crosslinker for the NDI prepolymer. The resulting products can be a compression molded solid elastomer or a cellular material, the latter widely being used for shock absorbers and spring elements in the automotive industry. For these applications the cellular NDI system has shown its superiority versus all other used materials, like rubber or cellular MDI elastomers. Corresponding static and dynamic properties are being discussed. INTRODUCTION The worldwide polyurethane cast elastomer market is comprised of materials which are based mainly on TDI, MDI and NDI. In the U.S. predominately diamine extended TDI prepolymers are used for elastomeric polyurethanes. TDI prepolymers amount to approximately 70% of the prepolymer systems in the U.S. cast elastomer market. The main portion of these TDI prepolymers have polytetramethylene glycol (PTMEG) backbones. The European cast elastomer market is quite different. Here, NDI/polyester adipate diols crosslinked with glycols are preferably used. The NDI based elastomer is not only one of the biggest products in the European hot cast elastomer market, but also the oldest commercially available cast polyurethane system. The main reason that the NDI elastomers have kept their predominant position in Europe for almost 4 decades now is the performance of these elastomers in particularly demanding applications. The superior performance of NDI elastomers has also been recognized in the American market, as finished products based on NDI have been imported to the U.S. for many years, e.g. for high dynamic applications in the textile and automotive industry. Currently, there is a trend toward domestic production, which will increase significantly over the next years. Nevertheless the NDI based elastomer is still relatively unknown in the U.S. Therefore, this paper will review processing, properties and applications. Properties of the solid elastomer are compared to the predominant TDI based products. In the case of microcellular elastomers, comparisons will be drawn between systems based on NDI and MDI (rather than TDI), since these represent the major alternatives. We will refer to recent results, utilizing modern physical methods, to characterize the performance of these elastomers and to confirm and explain the advantages found in practice. CHEMISTRY OF NAPHTHALENE-1.5- DIISOCYANATE (NDI) Naphthalene-1.5-diisocyanate (NDI) is a solid diisocyanate having a solidification point of 127 C (Figure 1). NDI is supplied in the form of white flakes. The comparatively high melting point of NDI is the reason that the processing conditions are quite different from MDI or TDI based materials. REPRINTED FROM POLYURETHANES 88 PROCEEDINGS OF THE SPI 31 sr ANNUAL TECHNICAL/MARKETING CONFERENCE

2 NDI Equivalent weight: 105 Solidification point: 127 C OCN Figure 1. Naphthalene-1.5-diisocyanate. < 100 I * 'Shrxe A ' Shore 0 BO (/) "I 70 <= Due to the high melting point of NDI the melting points of the corresponding NDI-glycol hard segments are significantly higher than the corresponding MDI or TDI hard segmenta The NDI/butanediol hard segment does not melt up to 320 C, while the corresponding MDI and TDI hard segments with butanediol melt in the range of 230 C and 210 C respectively. PROCESSING OF NDI The use of NDI for solid and cellular elastomers requires special processing conditions. While MDI and TDI prepolymers show good storage stability, even at elevated temperatures, NDI prepolymers must be used within 30 minutes to a few hours after production. Therefore, NDI prepolymers cannot be supplied by prepolymer manufacturers but rather have to be produced at the custom molder. Batch sizes, which are comparatively small, depend on the production rate during crosslinking with glycol or water. For the production of the NDI prepolymer, special VULKOLLAN polyesters or polycarbonates are used. These backbones consist of 2000 molecular weight ethylene adipates or ethylene butylene adipates. A neopentylglycol-hexanediol adipate and a hexane diol based polycarbonate are recommended for improved hydrolysis resistance. Figure 2 shows the possible processing steps for NDI elastomers. The glycol extended prepolymer results in a cast elastomer. Water crosslinking of the prepolymer gives a cellular material, which is used mainly for bumpers and spring elements in the automotive industry. A special variant is the water-crosslinked, compression molded NDI elastomer. This solid elastomer shows unique properties due to the NDI/urea structure of the hard segment IB IS IS 1« pbw NDI/100 pbw Vulkollan ester Figure 3. Hardness range of NDI elastomers. In the following the property profiles and application areas for the solid and cellular NDI elastomers are discussed. 1. SOLID NDI ELASTOMER, GLYCOL CROSSLINKED Solid NDI elastomers are used in a broad hardness range, reaching from 65 Shore A to 76 Shore D. The hardness is directly related to the ratio NDI/polyester polyol (Figure 3). Soft grades (< 83 Shore A) are available by replacing part of the Butanediol-1.4 crosslinker by TMP (trimethylolpropane). To ensure reasonable processing times (pot lives) high hardness elastomers with an NDI/polyol weight ratio of >2:5 are crosslinked preferably with a secondary glycol, e.g RC crosslinker 0312 (Wyrough and Loser). The production of prepolymer in small batches (e.g. 15 to 20 kg) and the exclusive use of 2000 mol. wt. backbones has the advantage that the grade can be changed from batch to batch. Thus, a broad range in the elastomer hardness is available by only changing the ratio of two materials, the NDI and the polyester polyol. Ethylene adipate and ethylene butylene adipate polyesters are predominantly used for the solid elastomer. The ethylene adipate shows advantages regarding tear strength; the ethylene butylene adipate gives better resilience and improved low temperature properties. 45 Glycol _L NOI/ester Prepolymer Casting SoNd NOI Elastomer VULKOLLAN Polyester J Open Foaming ' Milling 1 Compression Molding Solid NOI Elastomer Figure 2. Processing of NDI elastomers. Properties Table 1 shows the static physical properties of the NDI/ethylene butylene adipate elastomer in comparison to TDI/ethyleneadipate and TDI/PTMEG systems extended with methylene-bis-(2-chloroaniline). It is obvious that the overall property profile of the NDI material exceeds those of the TDI based elastomers. In general the NDI systems combine the toughness and tear strength of ester based materials with the elasticity of PTMEG based systems. Moreover, low compression set values and excellent abrasion resistance are characteristic for NDI elastomers. The resilience as measured by ball rebound of NDI/ester elastomers is unexpectedly high and even surpasses TDI/PTMEG/MBOCA elastomers. As shown in Table 1 the NDI/ethylene butylene adipate has a resilience value of 52% compared to 49% in the case of TDI/PTMEG material. An elastomer based on MDI and a 2000 mol. wt. ethylene butylene adipate shows a resilience value of 41% at a hardness of 90 Shore A. Prolingheuer, Lindsey, Kleimann / 395

3 Table 1. Typical physical properties of NDI and TDI elastomers. Physical Property Hardness (Shore A) Die C Tear Strength (kn/m) Tensile Strength (MPa) Elongation at Break (%) Tensile Modulus, 100% (MPa) Tensile Modulus 300% (MPa) Compression Set (%) Resilience (% Rebound) Taber Abrasion (mg) NDI/ester BD TDI/PTMEG MBOCA TDI/ester MBOCA NDI/ester elastomers are widely used for dynamic applications, like tires, rollers, couplings etc. Low damping behavior, corresponding to high resilience and resulting in low heat build up under dynamic load, is the key requirement in these applications. Results of dynamic mechanical analysis and flex fatigue tests confirm experience with NDI elastomers in practice. Figure 4 shows the storage moduli G' of different NDI/ethylene butylene adipate elastomers. In the most commonly used hardness range of Shore A the modulus is constant over a broad temperature range. Figures 5 to 7 show the dynamic mechanical analysis of the NDI/ethylene butylene adipate/butanediol elastomer in comparison to the TDI/PTMEG/MBOCA system and the TDI/ethylene adipate/mboca system. All three elastomers have the same Shore hardness of 90 Shore A. In the service temperature range of the PUR elastomers, the NDI system shows the highest storage modulus values G' over the broadest temperature range (Figure 5). This indicates a higher and less temperature dependent stiffness of the NDI elastomer. In the temperature range of approximately 10 C to +150 C the shear loss modulus G" of the NDI elastomer is lower than the G" values of the TDI systems (Figure 6). This implies a lower heat build up under dynamic load and longer life time of the NDI based material in dynamic applications. In general, NDI elastomers show lower damping values over a wide temperature range (Figure 7). The broad damping curves of the TDI elastomers and the higher tan 5 values are also typical for MDI based materials. The dynamic mechanical analysis shows why NDI elastomers often outlast TDI or MDI based systems in many dynamically demanding applications, like tires and wheels, rolls, vibration damping elements etc. The results of the dynamic mechanical analysis were confirmed by exposing test specimens to a flexometer test and measuring energy absorption and permanent set. Cylindrical elastomer samples (height 24mm, diameter 20mm) were compressed at a frequency of 10 Hz, a preload of 1200 N and a variable load of 800 N. Table 2 shows that the energy absorption of the NDI elastomer was much lower than the values of the TDI elastomers. After running for 30 minutes the TDI/ester elastomer had long been destroyed. The TDI/PTMEG elastomer looked much better, but compared to the NDI elastomer it was absorbing more energy per deflection. Moreover, the set measured on the TDI/PTMEG elastomer was higher. Similar results were observed by testing the elastomers at 2 Hz and 1 million cycles. Test specimens (height 20mm, diameter 20mm) were compressed at ambient temperature and 80 C with a deflection of 20%. The set values measured directly after testing and 100 hours after testing are shown in Table 3. The results clearly show the superiority of the NDI elastomer. While elastomers with ester backbones show much better oil and UV resistance than ether materials, sensitivity to hydrolysis sometimes generates problems. To overcome these problems and still to keep the advantages of ester backbones, carbodiimids as acid scavengers are recommended. The ester based NDI elastomer is often protected against hydrolysis attack by adding 1 to 2% by weight Stabaxol 1 (Wyrough and Loser) to the prepolymer before crosslinking. Table 4 shows the improvement of the tensile half life values of specimens containing Stabaxol 1. Even better results are achieved by using a 2000 MW polycar- Storage Modulus G' (MPa) NDi/Ester Elastomer* 11 Short A Short A * Short A/43 Short D 1C2 1(1' I Temperature (*C) Figure 4. Storage modulus G' of NDI/ester elastomers. 396 / Prolingheuer, Lindsey, Kleimann

4 Storage Modulus G' (MPa)~ 103 NDI/E«tof/BO... TM/PTMEQ/MBOCA.. TDI/Estaf/MBOCA 10* u.-, _ Temperature ( C) Figure 5. Storage modulus G' of NDI and TDI elastomers (Hardness 90 Shore A). bonate backbone. The NDI/polycarbonate/butanediol based elastomer clearly surpasses TDI/PTMEG/ MBOCA elastomers regarding hydrolysis resistance. Applications The most common application of NDI/ester based VULKOLLAN elastomers is in tires, wheels and rollers. Tires and support wheels on fork lift trucks and rollers on electric carts are examples of applications, where high dynamic load bearing capacity combined with good wear and tear resistance are required. Another example for the successful use of NDI elastomers are heavy duty wheels for the transport of pendulum sludge scrapers in waste water treatment plants. With dimensions of 41.5 cm diameter and 21.5 cm width, each of these wheels carries a load of more than 6000 kg. Similar to the forklift truck tires, cable lead rollers on lifts and cable cars can carry increased loads when made from VULKOLLAN products. Advantageous is also the excellent resistance to oil, ozone and UV radiation of the ester based material. NDI/ester elastomers play an important role in the textile industry. Friction rollers for the drive of spindles on spinning frames have to run with approx. 40,000 rpm in order to drive the spindle up to 600,000 to 800,000 rpm. In similar applications, rollers are used to stretch the spun threads. High stress is applied to the rollers by the circulating thread. Rollers for these applications show prolonged life time if they are based on NDI. VULKOLLAN parts for the textile industry have been imported to the Loss Modulus G (MPa) NDVEsterfBD TN/PTMEG/MBOCA TM/E*«W*MBOCA Temperature (*C) Figure 6. Loss modulus G" of NDI and TDI elastomers (Hardness 90 Shore A). Prolingheuer, Lindsey, Kleimann/397

5 Table 2. Flexometer testing of NDI and TDI elastomers (Hardness 90 Shore A). Elastomer Type TDI/PTMEG/MBOCA TDI/Ester/MBOCA NDI/Ester/BD 0' Energy Absorption (N/mm) after 20' Test Specimen: Cylinder height 24mm, diameter 20mm Conditions: Frequency 10 Hz Preload 1200 N, Variable load 800 N Elastomer Type 30' Set (%) after 30' destroyed Table 3. Flexometer testing of NDI and TDI elastomers (Hardness 90 Shore A). TDI/PTMEG/MBOCA TDI/Ester/MBOCA NDI/Ester/BD Set after testing (%) 22 C 80 C Test Specimen: Cylinder height 20mm, diameter 20mm Conditions: Frequency 2 Hz, Deflection 20% 1 million cycles, temp. 22 C and 80 C System Set 100 Hours after testing (%) 22 C 80 C Table 4. Tensile strength half life of polyurethane elastomers after immersion in water (no reconditioning before tensile measurements). NDI/polycarbonate diol/bd NDI/polyester adipate diol/bd NDI/polyester adipate diol/bd + Stabaxol 1 TDI/PTMEG/MBOCA Tensile Strength Half Life (days) Conditions Water at 100 C Water at 80 C U.S. for many years since MDI and TDI based elastomers did not meet the severe requirements. NDI elastomers are widely used in couplings, belts, hydrocyclones, squeegees, snow plow blades and milling disks. Solid VULKOLLAN bumpers are used for hopper cars in the mining industry. Seals and O-rings tolerate the highest hydraulic pressure (up to 400 bar) of all nonreinforced seal materials. 2. SOLID NDI ELASTOMER, WATER-CROSSLINKED The water-crosslinked solid NDI/ester elastomer is known as VULKOLLAN 18W. The NDI urea structure of this elastomer results in unique properties, making VULKOLLAN 18W particularly suitable for extreme wear and abrasion applications. The production of this elastomer is quite different from a normal casting process and includes a compression molding operation (Figure 8). The first production step is comparable to the processing of the glycol crosslinked solid elastomer and involves the production of the NDI/ester prepolymer. In the case of the water crosslinked elastomer preferably 18 PBW NDI are reacted with 100 PBW polyester (Mol. wt. 2000). The prepolymer is rapidly mixed with water and the resulting foam is heated in an oven until tack free. After being rolled into sheets, preforms cut from the rolled mats are compression molded in order to complete the crosslinking and to convert the cellular material into a solid elastomer. The production of VULKOLLAN 18W is labor and time intensive, but excellent performance in high demanding applications has proven this material to outlast all other PUR elastomers of comparable hardness range. Properties Table 5 shows the typical physical properties of VULKOLLAN 18W. At a hardness of 70 Shore A, the tensile and tear values show high values. The material has a low compression set and offers excellent abrasion resistance. As already observed in the case of the glycol crosslinked NDI elastomer, the resilience is very high, considering the ester-based backbone. Table 5. Typical physical properties of VULKOLLAN 18W. Hardness, Shore A Die C Tear Strength (kn/m) Tensile Strength (MPa) Elongation at Break (%) Tensile Modulus, 100% (MPa) Tensile Modulus, 300% (MPa) Compression Set (%) Resilience (% Rebound) Taber Abrasion (mg) Applications VULKOLLAN 18W is widely used for pump membranes with extremely high flex life. The membranes isolate the moving parts of the pump from abrasive slurries. For this application, the elastomer has been imported from West Germany to the United States for many years. In 1987 the first U.S. manufacturer began the domestic production of VULKOLLAN 18W for pump membranes. The pumps are used in the mining, construction and other industries. In the construction industry e.g., the pumps are used for backfilling cavities and for pressure-grouting mortar and cement sludge. The membranes based on the water crosslinked NDI elastomer perform so well due to their high elasticity and wear and tear resistance. 398 / Prolingheuer, Lindsey, Kleimann

6 tan 5 NOVEstw/BO...». TOMTMEOflWOCA.,*. TDVEMMrtMOCA ' _,_ Temperature ( C) Figure 7. Damping (tan d) of NDI and TDI elastomers (Hardness 90 Shore A). 3. CELLULAR NDI ELASTOMER, WATER-CROSSLINKED The cellular NDI elastomer supplements the solid elastomer, allowing a higher degree of volume compressibility without significant bulging. It is mainly used for applications where vibration damping and shock absorption are the key requirements. As in all NDI elastomer processing, the first step of producing an NDI/ester prepolymer is the same. The foaming is accomplished by reacting the prepolymer with water, often in the presence of additives like emulsifiers and catalysts. Unlike the cast elastomer process, the foaming reaction and final compression of the expanding foam is done in closed molds. Depending on the amount of reaction mixture poured into a given mold, a broad range of densities is available, reaching from approximately 250 kg/m3 to 1000 kg/m3. The densities directly influence the load bearing capacity and energy absorption of the cellular material. Properties As already mentioned, in the area of urethane elastomers cellular NDI materials mainly compete with cellular MDI based elastomers. Therefore, whenever product comparisons are shown in the following, NDI is compared to MDI material. Figures 9 and 10 show the dynamic mechanical analysis of the cellular NDI/ethylene butylene adipate elastomer at different densities. The storage moduli (Figure 9) are constant over a wide tempeature range and the damping curves tan 6 = /XT) (Figure 10) show low values in the range of usage temperature. Compared to these curves cellular MDI materials look quite different. The cellular MDI elastomer used for comparison is also based on a 2000 mol. wt. polyester and has a similar density to the NDI elastomer. Figure 11 shows that there are significant differences in storage modulus and tan 6 between the NDI and MDI material. The dynamic mechanical analysis indicates a higher degree of damping of the MDI elastomer resulting in increased heat build-up under dynamic load. Moreover, it is evident that the stiffness (and by implication other properties) of the MDI material is more sensitive to the service temperature. The compression load/deflection diagrams (Figures 12 and 13) show much higher loads for the NDI material to give similar deflections as the MDI elastomer. In other words, Table 6. Flexometer testing of microcellular NDI and MDI elastomers. edyn = 40% CDYN = 60% System Permanent Tmax ATmax Set (%) ( C) ( C) Permanent Set (%) Tmax ( C) ATmax ( C) A B Test Conditions Figure 8. Production of the water-crosslinked solid NDI elastomer. System A: NDI Elastomer System B: MDI Elastomer Density: About 500 kg/m3 Temperature: Frequency: Cycles: Deflection CDYN 23 C 2 Hz % and 60% Prolingheuer, Lindsey, Kleimann/399

7 Stereo* (MPl) G Density 910 DetMlty SM Density 409 kg/m*. Density 301 kg/nrt. Density 1011 kg/m* Density 742 kg/in* Density 51S kg/in* : 10"' Iwnpwwttm (*C) Figure 9. Storage modulus G' of microcellular NDI elastomers at different densities. under a given load and at a similar density the MDI elastomer is compressed to a larger extent than the NDI elastomer. This obviously does affect heat build-up and lifetime of the part. As already indicated by the shear modulus curves, CLD measurements made at higher temperature show that the MDI elastomer softens more significantly. Figure 13 shows the test results at 80 C test temperature. A dynamic fatigue test run at 2 Hz for 106 cycles at different deflections shows increased permanent set values and higher heat build-up of the cellular MDI elastomer vs. the cellular NDI elastomer (Table 6). The results of the various tests are clearly confirmed by the experience from the practice. Applications Microcellular NDI elastomers are widely used as components in automobile jounce bumpers. In MacPherson strut suspension systems (Figure 14) the cellular polyurethane spring supplements the cushioning effect of the steel coil spring. The linear coil spring deflection curve is transformed to a progressive spring characteristic, increasing the riding comfort and the general stability of the automobile. Cellular NDI elastomers show several significant advantages in comparison to rubber and cellular MDI materials also being used for this application. In general the cellular polyurethane elastomers can be compressed to a much higher degree than rubber up ta 80% of the original height or twice as much as possible with a rubber bumper. Moreover the increase of the crosssection diameter of the bumper under a given deflection is much smaller with the cellular PUR elastomer. This results in significant space savings and also weight savings, since the cellular material has a lower density (approx. 500 kg/m3) than the rubber part. The ester backbone of the cellular PUR elastomer contributes to an excellent oil and grease resistance of the spring or bumper. tan Density 910 kg/m1 Density 1011 kg/m* Density 599 kg/m* 0.1 Density 742 kg/m* 0.01 tan 8 Density 301 kg/m* Density 409 kg/m* DeneMy SIS kg/m* T«mp«ratur«*C Figure 10. Damping (tan 5) of microcellular NDI elastomers at different densities. 400 / Prolingheuer, Lindsey, Kleimann

8 Storage Modulus G (MP«)r~ 10* NCM system >.. MtM aystem I tan 10 Compression Load 6000 MPa Temperature (*C) Figure 11. Dynamic mechanical analysis of microcellular NDI and MDI elastomers (density: 500 kg/m3) As seen in the diagrams above and proven in practice, the microcellular NDI system is the best elastomeric material used for automotive springs and bumpers. It clearly outperforms MDI based elastomers with regard to much lower heat build-up under dynamic load, lower permanent set values, higher load bearing capacity at given deflections and densities, and better stiffness retention over a broad temperature range. Therefore, the cellular NDI system is used worldwide in cars built in the U.S., Europe and Japan. Besides the use in strut suspension systems, cellular NDI elastomers are used in applications like crane buffers, seals and bridge pads. In seals, the NDI/ester elastomer combines the oil and Compression Load MPa NDI Elastomer MDI Elastomer /0 Deflection Figure 13. Compression load/deflection of microcellular NDI and MDI elastomers at 80 C (density: 505 kg/m3). grease resistance of the ester backbone with the low permanent set values typical for NDI systems. Due to the partially open cell structure of this elastomer, it can be saturated with oil or grease and can serve as lubricant storage for prolonged lubricating intervals. Bridge pads are increasingly made of the cellular NDI elastomer instead of rubber. Again, advantages are high load bearing capacity and only slight permanent set. This special characteristic is also of high importance for mountings of precision tooling machines. Vibration-free installation of these machines can be achieved by pads of cellular NDI elastomers. A typical application is the vibration damping of a large surface grinder. The total weight, consisting of foundation, machine and material to be polished is 88,000 kg. This weight is distributed to the building foundation via cellular NDI elastomer pads. The machine is also cushioned in horizontal direction by four similar elements cellular NDI elastomer NDI Elastomer _ Deflection MDI Elastomer... Figure 12. Compression load/deflection of microcellular NDI and MDI elastomers at 23 C (density: 505 kg/m3). Figure 14. MacPherson strut suspension system with microcellular NDI elastomer spring. Prolingheuer, Lindsey, Kleimann 1 401

9 CONCLUSION For many years, solid and microcellular elastomers based on naphthalene-1.5-diisocyanate have proven their superiority over MDI and TDI based systems in many applications. The combination of NDI with special polyester adipate diols results in a high performance elastomer which shows the following characteristics: high melting point of the NDI/Butanediol hard segment constant shear modulus over broad temperature range low damping values tan 5, resulting in low heat build-up under dynamic load low permanent set values high tear and wear resistance high resilience, in spite of ester backbone excellent oil and grease resistance Besides these advantages, the process gives flexibility in covering a very broad hardness range by only changing the ratio of two materials, the NDI and the polyester. The domestic production of NDI elastomers in the U.S. will provide advantageous solutions for many applications where MDI or TDI based materials have not worked satisfactorily or failed prematurely. Many other applications where the special characteristic of NDI can be utilized, such as high temperature polyurethane elastomers or greases are being evaluated. ACKNOWLEDGEMENTS The authors would like to thank all of our colleagues at Bayer AG and Mobay Corporation who participated in the work reported in this paper. Special thanks to M. Barnes, P. Henrichs, H. D. Ruprecht and B. Stelte for their contributions. BIOGRAPHIES E. Christoph Prolingheuer Dr. E. Christoph Prolingheuer joined Bayer AG's headquarters in Leverkusen, West Germany, in 1979 after receiving his Ph.D. degree in Organic Chemistry from the University of Bochum. He began his career in the Rigid Foam Group of the Polyurethane Applications Development Department, where he was responsible for the development of insulating systems for the appliance industry. In 1984 he was transferred to the Elastomer Group of Mobay Corporation in Pittsburgh. As a Section Manager he is in charge of the development of polyurethane elastomer technology. J. J. Lindsey Dr. J. J. Lindsey is presently Manager of Specialties Application Development in the Polyurethane Division of Mobay Corporation. He joined Mobay in 1974 immediately after earning the Ph.D. degree in Organic Chemistry at Carnegie Mellon University in Pittsburgh. During his career with the Bayer organization, he has been involved with developing PU technologies in the fields of flexible molded foams for seating (including an assignment at Bayer, Leverkusen), RIM elastomers, and RIM structural materials, as well as the cast and cellular elastomers which are the subject of this paper. Helmut Kleimann Dr. Helmut Kleimann joined Bayer AG in 1966 after receiving his Ph.D. degree in Organic Chemistry from the University of Stuttgart. He spent two years in the Central Research Laboratory working on peptide chemistry. Then he was assigned to the Polyurethane Applications Development Department, where he was responsible for the development of Baydur structural foam (RIM). Since 1981 H. Kleimann has been Manager of the Elastomer Group in the Applications Development Department at Bayer AG's headquarters in Leverkusen, West Germany. He is responsible for such products as Vulkollan, Baytec, Baysport, Desmoflex and Urepan. Since 1987 he has also been responsible for the Slabstock Foam Group at Bayer AG. 402 / Prolingheuer, Lindsey, Kleimann