Polyurethanes /
Polyurethanes / Elastomers Polyurethanes from orange peel and CO 2 by Rolf Mülhaupt and Moritz Bähr Freiburg Materials Research Center (FMF) and Institute for Macromolecular Chemistry University of Freiburg Freiburg, Germany The Freiburg Materials Research Center (FMF) of the University of Freiburg, jointly with Volkswagen, has developed novel families of 100% renewable resource based polyurethanes derived from natural terpene oils and the greenhouse gas carbon dioxide (CO 2 ). In contrast to the conventional polyurethanes, neither hazardous isocyanate resins nor fossil resources are required. Produced by a great variety of plants as essential oils, terpenes are exclusively recovered from bio-wastes and do not compete with food production. Prominent terpene raw material for the production of non-isocyanate polyurethanes is the citrus oil limonene, obtained from orange peel as a waste product in the manufacturing of orange juice. Based upon limonene and the chemical fixation of carbon dioxide recovered from the exhausts of power plants and as a by-product of liquid air production, a very versatile and cost-effective molecular toolbox has been developed at FMF for tailoring rigid and flexible polyurethanes with diverse applications ranging from automotive parts to textiles, rubbers, foams, coatings, sealants, and adhesives. Stimulated by the expected skyrocketing costs of crude oil and growing public awareness of global warming, the lean and clean production of renewable resource based plastics with a low carbon footprint has gained high priority [1]. Going well beyond the traditional scope of renewable polymers, bio-based intermediates supplied by biorefineries and the chemical fixation of carbon dioxide offer attractive opportunities for tailoring environmentally benign polyurethanes (PU). Conventional PU technology requires very strict health and safety precautions, owing to the severe health hazards upon exposure to toxic isocyanate monomers. In contrast, non-isocyanate polyurethanes (NIPU) are formed without using hazardous isocyanate resins at any point in the production process. Key NIPU intermediates are non-toxic polyfunctional cyclic carbonate monomers, which are readily produced by chemical conversion of epoxy resins with carbon dioxide [2, 3]. When cured with amines the cyclic carbonates undergo ring opening, thus forming poly(N-hydroxyethylurethanes). In contrast to the highly moisture-sensitive isocyanates, cyclic carbonate resins tolerate humidity and can be cured on wet substrates without foaming. Tedious drying of fillers is not required. NIPUs (Green Polyurethane TM ) based upon fossil resources and conventional epoxy resins are commercially available as zero VOC coatings with improved adhesion and better resistance to chemical degradation, corrosion, organic solvents, and wear [4, 5]. Most attempts towards the development of 100 % renewable resource based NIPU make use of epoxidized soybean and linseed oils, which are converted with carbon dioxide into the corresponding bio-based cyclic carbonate resins [6, 7, 8]. However, the ester groups of plant oil carbonates are partially cleaved during amine cure. These side reactions can cause undesirable emission problems and impair NIPU properties due to plasticization of the NIPU matrix. Therefore, at FMF an innovative generation of 100% renewable resource based NIPU has been produced from novel ester-fee cyclic carbonate resins derived from terpenes [9]. The limonenebased NIPU process is illustrated on the next page. Terpenes represent highly unsaturated, ester-free, natural hydrocarbons. Typical members of the terpene family include limonene, camphene, vitamin A, steroids, carotenoids and natural rubber. More than 300 plants produce limonene. For example, orange peel contains up to 90 wt.-% of limonene, which is readily recovered on a commercial scale using the waste products from orange juice production. The colorless viscous oil limonene dioxide, produced by oxidation of 40 bioplastics MAGAZINE [05/12] Vol. 7
Polyurethanes / Elastomers CO 2 C CH 3 H 3 C H 3 C O O O C C C CH 2 H 3 C CH 2 H 3 C CH 2 H 3 C O O O Limonene C + H 2 N O O H 3 C OH O C O NH Limonene dicarbonate O C C O C H 3 C OH H 2 bioplastics MAGAZINE [05/12] Vol. 7 41 NH NIPU limonene, is commercially used as component of epoxy resins. The FMF research has succeeded in reacting terpene oxides quantitatively with carbon dioxide, thus producing novel and cost-effective families of terpene carbonates. This chemical carbon dioxide fixation is highly effective. Around 34 wt-% carbon dioxide is incorporated into limonene dicarbonate! As a function of their stereoisomer compostion, limonene dicarbonates can be obtained as viscous liquid or white crystalline solid. The limonene dicarbonate reacts with a great variety of amines, producing multifunctional urethanes. Reaction with amines and amino-alcohols affords cycloaliphatic polyols useful as intermediates in conventional PU synthesis. As a chain extender of oligomeric polyamines and amino-alcohols limonene dicarbonate incorporates hard limonene segments into flexible curing agents. This approach has been used to produce new families of reactive prepolymers which can be functionalized in numerous ways. Upon curing with polyamines, e.g. using bio-based diamines or novel aminoamides derived from citric acid, 100% renewable and even 100% citrus-based NIPU are made available. In contrast to the rather soft soybean-oilbased NIPU, the mechanical properties of limonene-NIPU can be varied over a very wide range from highly rigid and stiff to rubbery and ultrasoft. Applications include casting resins, rubbers, thermoplastic elastomers, foams, coatings, sealants and adhesives. As illustrated using the example of limonene, this NIPU strategy can be applied to a very large variety of terpenes. Terpene carbonates are also attractive as components and non-toxic solvents for numerous other applications, going well beyond the scope of bio-based NIPU. www.fmf.uni-freiburg.de [1] R. Mülhaupt: “Green polymer chemistry and bio-based plastics – dreams and reality”, Macromol. Chem. Phys., accepted, in press [2] O. Figovsky, L. Shapovalov. F. Buslov: Ultraviolet and thermostable non-isocyanate polyurethane coatings“,Surface Coatings International Part B: Coatings Transactions 88, B1, 1-82 (2005) [3] B. Ochiai, S. Inoue, T. Endo: “One-Pot Non-Isocyanate Synthesis of Polyurethanes from Bisepoxide, Carbon Dioxide, and Diamine”, Journal of Polymer Science: Part A: Polymer Chemistry 43, 6613–6618 (2005) [4] www.hybridcoatingtech.com, accessed Sept. 08, 2012 [5] www.nanotechindustriesinc.com/GPU.php, accessed Sept. 08, 2012 [6] Ivan Javni, Doo Pyo Hong, Zoran S. Petrovi, Soy-Based Polyurethanes by Nonisocyanate Route, Journal of Applied Polymer Science, Vol. 108, 3867–3875 (2008) [7] B. Tamami, S. Sohn, G. L. Wilkes: “Incorporation of carbon dioxide into soybean oil and subsequent preparation and studies of nonisocyanate polyurethane networks”, J. Appl. Polym. Sci. 92, 883-891 (2004) [8] M. Bähr, R. Mülhaupt: “Linseed and soybean oil-based polyurethanes prepared via the non-isocyanate route and catalytic carbon dioxide conversion”, Green Chem. 14, 483–489 (2012) [9] M. Bähr, A. Bitto, R. Mülhaupt: “Cyclic limonene dicarbonate as a new monomer for non-isocyanate oligo- and polyurethanes (NIPU) based upon terpenes”, Green Chem., 14, 1447–1454 (2012)
