Renewable Footwear Systems Based on Polyols Derived from Waste Carbon Dioxide

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1 Renewable Footwear Systems Based on Polyols Derived from Waste Carbon Dioxide JASON M ANDERSON and SCOTT ALLEN RAFAEL CAMARGO and DANIEL ROSENVASSER Novomer Inc. Huntsman Polyurethanes 6 Trapelo Road 29 Executive Hills Boulevard Waltham, MA 245 Auburn Hills, MI USA USA ABSTRACT Over the past few years, a number of companies have explored renewable alternatives to petroleum-based polyols for a number of polyurethane applications. Huntsman and Novomer have taken a different approach to develop sustainable flexible footwear foams using an entirely new class of CO 2 -based polyols. Novomer has developed a unique route to high performance renewable polyols using a proprietary catalyst system which transforms waste carbon dioxide (CO 2 ) into valuable polymers. This technology delivers renewable polyols that are up to 5% CO 2 by weight; have a 3-9x carbon footprint advantage versus existing petroleum-based materials and are cost competitive versus commodity polyols. The flexible platform technology enables polymer structure and molecular weight to be tailored for specific applications. Huntsman has leveraged this technology, along with its deep polyurethanes formulations expertise and product development capabilities, to explore the use of these materials in conventional footwear foams. These sustainable foams exhibit interesting performance characteristics versus traditional footwear foams made exclusively with petroleum-based polyols.. INTRODUCTION Increased concerns for sustainable chemicals and reduction of oil consumption in recent years, has driven a large number of companies to investigate the possibility of producing raw materials from sources that are not based on fossil feedstock. Consumers and leading finished product companies in many markets are increasingly involved in actively seeking raw material options that rely in sources such as recycling or products derived from renewable resources []. An alternative approach is to develop products that are based on waste stream or sustainable materials not based on oil. The chemical industry, in general, and the polyurethane industry in particular, have increased R&D dollars dedicated to the development of intermediates and final products based on renewable resources. In an earlier work we reported on the use of bio-based,3 propane diol (PDO) as a new chemical intermediate in polyurethane elastomers and of bio-derived succinic acid as raw materials for the production of polyurethane elastomers [2, 3]. Through that work, we demonstrated that using these intermediates, it was possible to formulate elastomers with similar performance and properties to equivalent formulations used already commercially derived from conventional oil based intermediates. The typical bio-renewable carbon content in this approach was as high as 65 percent, as measured by ASTM D-6866 [4]. To further investigate non traditional feedstock alternatives to footwear elastomers, Huntsman and Novomer, Inc. have teamed up to develop new alternatives. Novomer has pioneered the approach to other sustainable sources by developing novel catalyst technologies that can convert gases such as carbon dioxide or carbon monoxide into value added chemicals for a number of applications [5]. This proprietary catalysts technology allows the insertion of a carbon dioxide into a propylene or ethylene oxide molecule to produce a number of hydroxyl terminated polycarbonates that can be tailored in molecular weight and composition as polyols for the formulation of polyurethane products. In this work we have evaluated a series of polypropylene oxide based polyols of different molecular weight in typical microcellular elastomer formulations. These have been characterized primarily using a dynamic mechanical analysis and compared to conventional elastomers based on polyester polyols. EXPERIMENTAL Materials and Preparation The core of Novomer s technology platform is a highly active and selective cobalt-based catalyst system [6]. This advanced catalyst technology maximizes CO2 incorporation and delivers polycarbonates with perfectly alternating CO2-

2 epoxide backbones and thus zero ether linkages. Novomer developed and obtained the global foundational intellectual property [7] on the use of chain transfer technology to produce CO2-based polyols with low molecular weights (<, to ca., g/mol). This enables the production of highly precise diols, triols, or multifunctional polyols with essentially perfect OH functionality and very narrow polydispersity indices (<.4, typically <.2). This highly flexible polyol technology platform can thus be applied to a wide range of thermoplastic or thermoset applications including coatings, foams, adhesives, elastomers, and TPUs. In this work three different propylene oxide based diols were chosen with nominal molecular weights of 75, 85 and 6 to produce a series of elastomers using conventional footwear prepolymers and formulation approaches. The three polycarbonate polyols, PPC, are designated as PPC-64, PPC-32, and PPC-85, where the numbers represent the nominal hydroxyl value of each polyol. Two series of specimens were investigated. In the first series, Series A, systems were formulated using a target high performance polyester elastomer formulation based on SUPRASEC 2543 MDI isocyanate prepolymer. In this series, the conventional polyester polyol was replaced by the new polycarbonate polyols chosen. The level of chain extender was varied to modify the hardness of the elastomer. No attempt, however, was made to maintain the hard segment content of these elastomers constant. The target molded density was.5 g/cm³. All additives such as surfactants, catalysts, etc. are typical and were not changed. Water was used as the only blowing agent. All samples were mixed using an isocyanate and polyol temperature of 45 C, except for the high molecular weight PPC based systems where a 6 C component temperature was used to reduce the very high viscosity of the polyol blends. Mold temperature was maintained at 45 C. The mold was made of machined aluminum (mold dimensions were x 25 x mm). All plaques were prepared manually. Due to variations in reactivity and the small-scale nature of this work, molded densities had larger than expected variation and no attempt to reduce this variation was made (see Table below). A second set of samples, Series B, was prepared using as a reference a polyester formulation based on SUPRASEC 298, with a target molded density of.4 g/cm³. In this series, a control formulation was included, and for each polycarbonate polyol two blends were prepared, one with all the polyol coming from the polycarbonate materials, and the second using a 5:5 blend by weight of the PPC diol and the reference polyester, DALTOREZ P-775, abbreviated hereafter as P-775, a slightly branched ethylene glycol/diethylene glycol adipate, liquid at room temperature, with a nominal molecular weight of 2,5 g/mol, which has a Tg of -52 C, as measured by DSC. Molding conditions were similar to those employed in the first series of samples. A summary of all samples, along with their final molded densities is shown in Table. Thermal Analysis Liquid polyol samples were tested using differential scanning calorimetry (DSC). DSC measurements were performed in a TA Instruments Q2 model DSC instrument. Heating rate was ºC/min. The samples were in a heating and cooling cycle from -8 to 2ºC and from 2 to -8ºC. A dwell time of minutes was used at -8 C for all samples. Dynamic Mechanical Thermal Analysis All measurements were performed on rectangular bars in torsion mode in an Anton Paar Physica MCR 3 compact rheometer. Temperature was ramped at 3ºC/minute from -7ºC to about the crossover temperature of the storage modulus, G, and the loss modulus, G. Samples were run only as received. The frequency of the test was 6.28 rad/sec ( Hz). The samples were run at a constant strain of. percent. Typical dimensions of the bars were approximately 4x5x5 mm (between clamps). Fourier Transform Infrared Spectroscopy Full mid-infrared spectra of the bio-based molded samples were obtained in the ATR mode using a Nicolet 67 analytical FTIR spectrometer from Thermo Fisher Scientific. Each spectrum was compounded from 32 scans using a resolution of 4 cm - at room temperature. 2

3 Table. Summary of Samples SAMPLE ISOCYANATE POLYOL INDEX HARD BLOCK (%) MOLDED DENSITY (kg/m³) SERIES A PPC-64- SUPRASEC 2543 PPC PPC-64-2 SUPRASEC 2543 PPC-64 8* PPC-32- SUPRASEC 2543 PPC-32 2* PPC-32-2 SUPRASEC 2543 PPC PPC-85- SUPRASEC 2543 PPC-85 * 4 55 PPC-85-2 SUPRASEC 2543 PPC SERIES B PE/PPC-64 SUPRASEC 298 P-775/PPC PPC-64-3 SUPRASEC 298 PPC NM PE/PPC-32 SUPRASEC 298 P-775/PPC PPC-32-3 SUPRASEC 298 PPC PE/PPC-85 SUPRASEC 298 P-775/PPC PPC-85-3 SUPRASEC 298 PPC PE SUPRASEC 298 P * These samples were run at an optimum index (see results section) Mechanical Properties All mechanical properties were tested in accordance to established ASTM methods. Only the second set of samples, i.e. those based on SUPRASEC 298, Series B, was used for these tests. RESULTS AND DISCUSSIONS Dynamic Mechanical Thermal Analysis. As discussed in the past, DMTA data is very useful to understand the elastomeric behavior of many polyurethanes foams [8]. It is based on the assumption, as established about 45 years ago [9], that these materials are made of distinct chain segment compositions that segregate themselves into at least two phases, a flexible one provided mainly by the polyol and a more rigid or crystalline one made up mainly of the alternative sequences of the diisocyanate, MDI in this case, and the extender. The presence of a glass transition and a medium modulus rubbery plateau gives a good indication of the two phase morphology. This combination is critical in the elastomeric behavior of these materials. Also, the location of the glass transition temperature, the breadth of the transition and the flatness of the mid region plateau provide an indication about the quality of the elastomeric behavior as function of temperature. The loss factor, Tan δ, provides an indication of the elastic response of the material. Broadly speaking, Tan δ values above. indicate a material with low elastic response, or low resilience. This in general correlates to dead elastomer or viscoelastic foam feeling. Ideally, the elastomeric behavior should remain stable over the range of temperatures in which the material is used. As molded the specimens of Series A, displayed either a rigid, but tough behavior or a very viscoelastic behavior. Examination of the DMTA data confirmed that all samples had glass transition temperatures, as measured from the maximum in Tan δ, near or above room temperature (see Table 3 later). In this respect, elastomers made using the PPC polyols of this study do not behave similarly to other polyurethane elastomers. In other words, they display a behavior of mixed hard and soft phases. Figure shows three representative samples with Tg values ranging from 2 C (PPC-32-) to 45 C (PPC-85-). All samples of Series A had Tg values in this range. Part of this behavior can be attributed to the low molecular weight of two of the PPC polyols. Previous studies of polyurethane elastomers have shown that for polyol molecular weights below,, there exists a high degree of phase mixing [-]. The data for PPC-64- and PPC-64-2, however, is not consistent with this explanation. At a molecular weight of,75, the elastomers made from this polyol behaved differently than elastomers made with traditional polyether or polyester polyols of the same molecular weight. As shown in Figure 2, the Tg of these samples is not only high, but it varies significantly with index. It is important to indicate at this point that, for this particular system, the optimum index obtained from minimum penetration of a probe in a cup test was unusually high at 8. Most elastomers of this type have optimum index around. We believe that this was caused by an uncertain value of the polyol hydroxyl value, as the nominal value used, 64 mg KOH/g, could not be confirmed through standard methods used at Huntsman which included pyridine as a solvent. Novomer and its contract R&D labs use ASTM D4274 methods C and D and have selected THF as the preferred solvent. This issue was not further investigated in 3

4 this work. Figure 3, showing the DMTA data of the two samples of the Series A made using PPC-32 confirms the high phase miscibility of these systems. As the hard block content increases from 32% to 37%, Tg increases from 2 C to 39 C. Such a large shift is not normally observed for well phase separated elastomers, where an increase in hard segment content results mainly in a higher plateau modulus with little change in the loss factor..e+9.e+8 PCP-32-2 PCP-64-2 PCP E+7.2 G', Pa.E+6.8 Tan δ.e+5.6.e e Temperature, C Figure. DMTA data, storage shear modulus (G, solid curves) and loss factor (Tan δ) of microcellular elastomers based on propylene carbonate diols of different molecular weight..e+9.e+8 PCP-64- (I=2) PCP-64-2 (I=8) E+7.2 G', Pa.E+6.8.E+5.6.E E Temperature, C Figure 2. Comparison of DMTA data (G and Tan d) for an elastomer system based on PPC-64 at two different indexes DMTA data for the Series B samples shows a behavior similar to Series A samples. Elastomers made from only PPC diols show a Tg above room temperature and consequently, high stiffness. Blending these polyols with a saturated polyester, P-775, does reduce significantly the Tg and makes the materials more flexible. Elastic response, however, is reduced. This is consistent with the values of the loss factor above Tg which are typically above.. In contrast, the all polyester control material shows a classical microcellular elastomer behavior with low Tg (-23 C) and a loss factor below. in the temperature range 5 C to over C. It can be observed that in many of the PPC based samples, Tan d increased rapidly at temperatures below 2 C. These results are illustrated in Figures 4 and 5 below. Differential Scanning Calorimetry. In order to understand better the behavior of PPC diols, the Tg of the unreacted materials was determined using DSC. In addition, blends of PPC and polyester diol, P-775, were prepared at 25% and 75% by weight of the PPC diol. The blends were observed to be fully miscible. By contrast, blends of the PPC polyols and a typical ethylene oxide capped propylene oxide polyol were not compatible. The DSC results confirmed these observations for the PE/PPC blends. One single Tg was observed at all times. The results are shown in Table 2, along with values predicted from the Fox Equation for compatible polymer blends [4]. With the exception of one sample, all the predicted values were in good agreement with the measured ones as shown in Figure 6. This deviation could ve been caused by experimental error due to the highly viscous nature of PPC-64. Figure 7 shows a typical DSC heating cycle for three samples of different composition. All Tg values are given in Table 2. 4

5 .E+9.6.E+8.E+7 PPC-32-2 PPC G', Pa.E+6.E+5.E Tan δ.e Temperature, C Figure 3. Comparison of DMTA data for two elastomers based on PPC-32 at different hard segment concentrations: 32% (----) and 37% (----).E+9.E+8 Polyester System (PE) PE/PPC-64.8.E+7.6 G', Pa.E+6 Tan (δ).4.e+5.e+4.2.e Temperature (ºC) Figure 4. Comparison of DMTA characteristics for a polyester elastomer and an elastomer based on a blend of polyester and PPC diol..e+9 2.E+8 PE/PPC-85 PPC E+7.2 G', Pa.E+6.8 Tan δ.6.e e Temperature, ºC Figure 5. Comparison of DMTA characteristics for a polyester/polycarbonate elastomer and an elastomer based only on a PPC polycarbonate diol. 5

6 -25 Tg predicted - Fox equation, C Tg Measured, C Figure 6. Experimental versus predicted glass transition temperature for compatible blends of an aliphatic saturated polyester and PPC diols. Predictions according to the Fox Equation [2] -. PPC-64/P-775 (5/5) PPC-64/P-775 (25/75) PPC-64 Heat flow, mw/g Temperature, C Figure 6. Glass transition region in typical DSC scans for pure PPC-64 and blends of PPC-64 and aliphatic polyester, P-775. Table 2. Summary of Tg data in polyols SAMPLE Tg ( C) PE P PPC-64 7 PPC-32-2 PPC PPC-64/P-775 (25/75) -42 PPC-64/P-775 (5/5) -32 PPC-32/P-775 (25/75) -43 PPC-32/P-775 (5/5) -35 PPC-85/P-775 (25/75) -36 PPC-85/P-775 (5/5) -46 Fourier Transform Infrared. Another very valuable technique for the study of phase separation in polyurethane elastomers is infrared spectroscopy. The principles of the interpretation are based on frequency shifts of the carbonyl stretching absorption depending on whether the groups are hydrogen bonded or not [2, 5]. Free carbonyl groups will show an absorption band at ca. 73 cm -, while the hydrogen bonded ones will show at absorption at ca. 75 cm -. Unfortunately, the value of this technique in the case of polycarbonate polyols is limited since the carbonyl region is dominated by the carbonyl groups coming from the carbonate linkage. Thus the degree of hydrogen bonding is not easy to detect, as seen from Figure 9 for a typical sample, where the bonded carbonyl region (ca. 75 cm-) appears only as a weak shoulder. No further attempt to use this technique was made in this work. 6

7 Sample PPC-32-3 Absorbance Wavenumber (cm-) Figure 7. Typical mid-infrared absorbance of the carbonyl region in a PPC based elastomer. Hydrogen bonded carbonyls have an absorbance at ca. 75 cm Mechanical properties. The summary of mechanical properties for Series B, is provided in Table 3. While a direct comparison of properties is not straight forward given the variations in density obtained for the different samples, some trends are clear. Those samples with Tg above room temperature display, as expected, higher hardness and higher tensile strength than samples with Tg values below room temperature. These samples also have lower elongations at break of the order of %, while those samples with Tg below room temperature have elongations of 3% or higher. This increase in elongation with lower Tg is well understood for many thermoplastics [4]. As observed upon demolding, despite the much higher modulus of the samples with Tg above room temperature, tear strengths confirm that these samples are not brittle but they behave more like tough plastics. Thus, the elastomers based on PPC polyols alone are very different from other high rigidity polyurethane foams such as structural foams, where brittle behavior is more common. In this respect, this new family of polyols provides a different set of functionalities not easily achievable with other raw materials, and new applications can be explored. Table 3. Physical Properties PROPERTY METHOD PE PPC-85-3 PE/PPC-85 PPC-32-3 PE/PPC-32 PE/PPC-64 Density, kg/m³ ASTM D3574 Test A Hardness, Shore A ASTM D Tensile, Mpa ASTM D-3574E Elongation, % ASTM D-3574E Tear Strength, Die C, N/mm ASTM D Tg, C CONCLUSIONS This new family of polycarbonate diols obtained by the polymerization of carbon dioxide and propylene oxide creates urethane systems with unique properties when compared with conventional footwear elastomers. For the most part, straight use of these polyols without other polyols in the formulation leads to high strength microcellular foams that can be both soft and high dampening or rigid and very tough, which does not make them suitable as drop-in substitutes in traditional midsole applications. However, the increased hardness, tensile strength, and tear strength of PPC-based foams indicate that these polyols have interesting potential as a performance enhancing component of a polyol formulation in foams requiring these characteristics, including certain footwear foams and elastomers. Given the high dampening behavior of some formulations, these materials are also worth exploring in energy absorbing-type applications including vibration dampening foams and viscoelastic foams. While it is clear that these new polyols are not drop-in replacements for existing polyol systems, they do offer a unique set of functionalities that are not easily achievable with existing raw materials and thus warrant additional exploration in formulated foam systems. ACKNOWLEDGEMENTS The authors would like to thank Huntsman International LLC and Novomer, Inc. for supporting and allowing this work to be published. We would like to acknowledge the support of other Huntsman colleagues, in particular Carlos Faraon for sample 7

8 preparation and Ms. Tianshen Hu for all the characterization work. We also thank the personnel in the physical testing lab in Auburn Hills, MI for providing valuable support to the project. While the information and recommendations in this publication are, to the best of our knowledge, information and belief, accurate at the date of publication, NOTHING HEREIN IS TO BE CONSTRUED AS A WARRANTY, EXPRESS OR OTHERWISE. IN ALL CASES, IT IS THE RESPONSIBILITY OF THE USER TO DETERMINE THE APPLICABILITY OF SUCH INFORMATION AND RECOMMENDATIONS AND THE SUITABILITY OF ANY PRODUCT FOR ITS OWN PARTICULAR PURPOSE. NOTHING IN THIS PUBLICATION IS TO BE CONSTRUED AS RECOMMENDING THE INFRINGEMENT OF ANY PATENT OR OTHER INTELLECTUAL PROPERTY RIGHT, AND NO LIABILITY ARISING FROM ANY SUCH INFRINGEMENT IS ASSUMED. NOTHING IN THIS PUBLICATION IS TO BE VIEWED AS A LICENCE UNDER ANY INTELLECTUAL PROPERTY RIGHT. Except where explicitly agreed otherwise, the sale of products referred to in this publication is subject to the general terms and conditions of Huntsman International LLC or of its affiliated companies. Huntsman Polyurethanes is an international business unit of Huntsman International LLC. Huntsman Polyurethanes trades through Huntsman affiliated companies in different countries such as Huntsman International LLC in the USA and Huntsman Holland BV in Western Europe. SUPRASEC, and DALTOPED, are registered trademarks of Huntsman LLC or any affiliate thereof, in one or more countries, but not all countries. Copyright 22 Huntsman LLC or an affiliate thereof. All rights reserved. REFERENCES. A. H. Tullo, Bioplastics Gathering, Chemical and Engineering News, July, 2, 89(28), D. Rosenvasser, J. McCloud, L. Cao and R. Camargo, New Polyurethane Elastomers with Increased Bio-renewable Content for Footwear Applications, 2, CPI Polyurethanes Technical Conference, Houston, Texas. 3. R. Camargo, T. Hu, D. Rosenvasser and J. McCloud, Further Advances in Footwear Elastomers with Increased Biorenewable Content, 2 CPI Polyurethanes Technical Conference, Nashville, TN. 4. ASTM International, ASTM D Standard Test Methods for Determining the Biobased Content of Solid, Liquid and Gaseous Samples Using Radiocarbon Analysis, West Conshohocken, Pennsylvania ( 5. Novomer, Inc., Low Molecular Weight Polyols, WO 2/22388, can be found at 7. WO2/28362A, can be found at 9. P. Mackey, Introduction to Elastomers, The Polyurethanes Book, Chapter 9, D. Randall and S. Lee, editors, 22, John Wiley and Sons, Inc.. S. L. Cooper and A. V. Tobolsky, Properties of Linear Elastomeric Polyurethanes, 966, J. of Applied Polymer Science,, R. J. Zdrahala, S. L. Hager, R. M. Gerkin and F. E. Critchfield, Polyether Based Thermoplastic Polyurethanes: Effect of Soft Segment Molecular Weight, 98, J. of Elastomers and Plastics, 2, C. G. Seefried, Jr., J. V. Koleske and F. E. Critchfield, Thermoplastic Urethane Elastomers. I. Effect of Soft Segment Variations, 975, J. of Applied Polymer Science, 9,

9 . S. W. Seymour, G. L. Estes and S. L. Cooper, Infrared Studies of Segmented P.U. Elastomers. I. Hydrogen Bonding, 973, Macromolecules, 6, P.C. Hiemenz and T.P. Lodge, Polymer Chemistry, 2nd Ed., 27, Boca Raton, FL, CRC Press 5. R. E. Camargo, C. W. Macosko, M. Tirrell and S. T. Wellinghoff, Hydrogen Bonding in Segmented Polyurethanes: Band Assignment for the Carbonyl Region, 983, Polymer Communications, 24, 34 BIOGRAPHIES Rafael E. Camargo Rafael Camargo is the CoreScience USA Manager for the Polyurethanes Division of Huntsman. He is responsible for novel science and materials developments at the Huntsman Advanced Technology Center in The Woodlands, Texas. Dr. Camargo has over 25 years of experience in polyurethane applications. Dr. Camargo has a chemical engineering degree from Universidad Pontificia Bolivariana in Colombia and a PhD in chemical engineering and materials science from the University of Minnesota. Daniel Rosenvasser Daniel Rosenvasser is currently Commercial Manager for Huntsman Polyurethanes for the Footwear and ACE Business, based in Auburn Hills, Michigan. In 986, Dr. Rosenvasser joined Huntsman, then ICI Polyurethanes, as Technical and Commercial Manager in Argentina, with responsibility over Chile and Brazil and relocated to the US in 22. He holds a PhD in chemistry from the Universidad de Buenos Aires, and an MBA from IDEA in Buenos Aires, Argentina. Scott Allen Scott Allen is Vice President of Catalyst Development for Novomer. In 24 Dr. Allen earned a Ph.D. in organic chemistry working with Dr. Coates at Cornell. His Ph.D. work focused on the design of catalysts for the controlled ring-opening polymerization of heterocycles. He co-founded Novomer in 24 and leads the company's research and polymer commercialization efforts. Scott was recently awarded the 22 Statistics in Chemistry Award. Jason Anderson Jason Anderson leads Polymer Business Development for Novomer where he is responsible for identifying and commercializing applications for Novomer polymers and polyols. Prior to Novomer, Jason was an Engagement Manager with McKinsey & Company and earlier in his career was Product Manager for BASF s Polyalcohols business in North America. Jason has a B.S. in Chemical Engineering from The University of Oklahoma and an M.B.A. from Harvard Business School. 9

10 This work is protected by copyright. This paper and all data and information contained in it, are owned and protected by the ACC through its Center for the Polyurethanes Industry. Users are granted a nonexclusive royalty-free license to reproduce and distribute this paper, subject to the following limitations: () the work must be reproduced in its entirety, without alterations; and (2) copies of the work may not be sold. Copyright Polyurethanes Technical Conference, American Chemistry Council.