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Issue 04/2017

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Materials Limonene based

Materials Limonene based polycarbonate A by-product of the juice industry can replace BPA in polycarbonate From your phone case to airplane windows, polycarbonates are everywhere. Several million tonnes of polycarbonate are produced every year around the world. However, worries about the dangers of this material are increasing because of the toxicity of its precursors, especially bisphenol-A, a potential carcinogen. Now, a team of chemists led by Arjan Kleij, group leader at the ICIQ (Institute of Chemical Research of Catalonia, Tarragona, Spain) and professor at the ICREA (Catalan Institution for Research and Advanced Studies, Barcelona, Spain), developed a method to produce polycarbonates from limonene and CO 2 , both abundant and natural products. Besides, limonene is able to replace a dangerous building block currently used in commercial polycarbonates: bisphenol-A (also known as BPA). Although BPA has been repeatedly classified as a safe chemical by American and European agencies, some studies point out that it is a potential endocrine-disruptor, neurotoxic, and carcinogen. Some countries like France, Denmark and Turkey have banned the use of BPA in the production of baby bottles. “BPA is safe, but still causes concerns and is produced from petroleum feedstock,” Kleij pointed out. “Our approach replaces it with limonene, which can be isolated from lemons and oranges, giving us a much greener, more sustainable alternative,” he added. Limonene, or more correctly the isomer (+)-limonene, is an oil-like substance present in the peel of citrus fruits, such as grapefruits, lemons and, in particular, oranges. Its other optical isomer, (-)-limonene, is found in the volatile substances released by oaks and pines. For potential applications, (+)-limonene has received by far more attention, since its availability exceeds that of (-)-limonene by far. By-product from citrus-fruit juice processing (+)-Limonene is isolated as a by-product from citrus-fruit juice processing. In addition it comes from a non-edible feedstock, in contrast to certain biobased materials made from edible plant parts. The extraction of (+)-limonene is either done by cold-pressing of the peel or by steam distillation. Here, steam is passed through the solid waste of juice production, breaking up the peel and taking the oil with it. Depending on the fruit, a (+)-limonene content of about 95% is reached in the extract. Because fully replacing BPA for limonene can be complicated for most industries at this moment, Kleij explained that BPA can increasingly take over. “We can start adding small quantities of limonene, then progressively substituting BPA,” he commented. “Step by step, the adaptation process could lead to new limonene derived biomaterials with similar, or even enhanced and novel properties.” Economics Being asked about availability and cost, Arjan Kleij told bioplastics MAGAZINE: “The issue of prices might indeed be one of the limiting factors for a broader application. The market price of (+)-limonene was experiencing significant fluctuations over the recent years. Its supply is 34 bioplastics MAGAZINE [04/17] Vol. 12

Materials Typical Polycarbonate applications strictly coupled to the orange production, mainly in Brazil, China and the USA, being intrinsically volatile (affected by droughts, storms etc.). At the same time, the global demand for (+)-limonene was constantly rising. Thus, it is very likely that the prices will increase in the next years when the total scale of production stays the same. However, since there is an increasing interest for more sustainable polymers and growing concerns over the massive plastic emissions”, it can be expected that further growth of (+)-limonene and other biobased monomers will take place.” In order to efficiently tackle cost-issues, (+)-limonene should serve as the basis for bio-renewable high performance materials, with excellent thermal and mechanical properties. The ground-breaking work of Coates et al. from 2004 [1] showed that concerning thermal properties poly(limonene carbonate) are comparable to that of commercially available, fuel-based polystyrene. Greiner et al. [2] then addressed mechanical properties such as hardness, showing that also in this area poly(limonene carbonate) is competitive. However, when compared to Bisphenol A (BPA) based polycarbonate still a better performance with respect to the glass transition temperature (Tg) is desired. This temperature is decisive for potential applications and reaches 148°C for BPA based polycarbonates. The limonene-derived polymer has the highest glass transition temperature ever reported for a polycarbonate. Since the Tg is influenced by the flexibility of the polymer chain, the researchers envisioned to install more bulky side chains to rigidify the polymer backbone. Thus, the dangling double bond was used to post-modify the whole polymer. After epoxidation, it was possible to incorporate another carbon dioxide molecule, yielding a pendant cyclic carbonate. This strategy has several advantages: 1. The carbon dioxide content could be even further increased; 2. The bulkiness of the cyclic carbonate gives rise to a Tg as high as 180°C; 3. In principle also other post-modifications of the double bond could be used to adjust thermal properties to the customer’s needs and applications. Having a high glass transition temperature also has other implications: the new plastics require higher temperatures to melt, which make them safer for everyday use. Moreover, this new polymer can also offer a myriad of new applications for polycarbonates and block copolymers using appropriate material formulations. Outlook Kleij and co-workers are currently negotiating with plastic producers to further advance the industrial manufacture of limonene-derived biomaterials. MT References and further reading: [1] Byrne, C. M.; Allen, S.D.; Lobkovsky E.B; Coates G.W.: Alternating Copolymerization of Limonene Oxide and Carbon Dioxide, J. Am. Chem. Soc., 2004, 126 (37), pp 11404–11405 [2] Hauenstein, O.; Reiter, M.; Agarwal, S.; Rieger, B.; Greiner, A. : Bio-based polycarbonate from limonene oxide and CO2 with high molecular weight, excellent thermal resistance, hardness and transparency, Green Chem. 2016, 18, 760-770 [4] Pagliaro et al., Chem. Comm. 2014, 50, 15288-15296 [5] Thompson et al., Crit. Rev. Biotechnol. 2010, 30, 63-69. [6] Kindermann, N.;Cristòfol, a.; Kleij, A.W.: Access to Biorenewable Polycarbonates with Unusual Glass- Transition Temperature (Tg) Modulation (http://pubs.acs. org/doi/abs/10.1021/acscatal.7b00770) bioplastics MAGAZINE [04/17] Vol. 12 35

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