Carbohydrate Polymers 114 (2014) 149–156 Contents lists available at ScienceDirect Carbohydrate Polymers j ourna l ho me page: www.elsevier.com/locate/carbpol Controlled graft copolymerization of lactic acid onto starch in a supercritical carbon dioxide medium Kouroush Salimi a , Mehmet Yilmaz a , Zakir M.O. Rzayev a , Erhan Piskin a,b,∗ a Hacettepe University, Chemical Engineering Department and Bioengineering Division, 06800 C ¸ ankaya, Ankara, Turkey b Biyomedtek/NanoBiyomedtek, 06800 C ¸ ankaya, Ankara, Turkey a r t i c l e i n f o Article history: Received 3 March 2014 Received in revised form 8 July 2014 Accepted 29 July 2014 Available online 13 August 2014 Keywords: Graft copolymerization Starch Lactic acid Supercritical carbon dioxide a b s t r a c t This work presents a new approach for the synthesis of a starch-g-poly L-lactic acid (St-g-PLA) copolymer via the graft copolymerization of LA onto starch using stannous 2-ethyl hexanoate (Sn(Oct) 2 ) as a catalyst in a supercritical carbon dioxide (scCO 2 ) medium. The effects of several process parameters, including the pressure, temperature, scCO 2 flow rate and reaction time, on the polymerization yield and grafting degree were studied. Amorphous graft St-g-PLA copolymers with increased thermal stability and processability were produced with a high efficiency. The maximum grafting degree (i.e., 52% PLA) was achieved with the following reaction conditions: 6 h, 100 ◦ C, 200 bar and a 1:3 (w/w) ratio of St/LA. It was concluded that these low cost biobased graft biopolymers are potential candidates for several environment-friendly applications. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Conventional polymers/plastics are indispensable materials of modern life, but in a short time, the after-use fates of these products has led to a waste disposal problem (Bertuzzi, Armada, & Gottifredi, 2007). The development of environment-friendly materials based on natural and renewable resources (i.e., biobased) have arisen as the most attractive and popular approach to overcome this prob- lem (Petersen et al., 1999). Starch is the most abundant renewable biopolymer in the world, and it is relatively inexpensive (Lawal, Lechner, Hartmann, & Kulicke, 2007; Xie, Zhang, & Liu, 2011). Com- mercial granular starch has been used in polyolefins as a filling agent and blended with synthetic polymers to produce biodegrad- able materials with desired properties (Otey & Westhoff, 1982). Earlier publications (Bhattacharya, Vaidya, Zhang, & Narayan, 1995; Vaidya, Bhattacharya, & Zhang, 1995) have indicated that blends of functional anhydride polymers and starch could lead to products with useful end properties. Research on blending polysaccharides (e.g., starch and cellulose) and synthetic polymers (e.g., polyolefins) has a long history, but poor compatibility limits the production of these blends. To improve the poor compatibility, many approaches ∗ Corresponding author at: Hacettepe University, Chemical Engineering Depart- ment and Bioengineering Division, 06800 C ¸ ankaya, Ankara, Turkey. Tel.: +90 5327079468; fax: +90 312 2363657. E-mail addresses: salimi@hacettepe.edu.tr (K. Salimi), ylmzmehmet@yahoo.com (M. Yilmaz), rzayevzmo@gmail.com (Z.M.O. Rzayev), piskin@hacettepe.edu.tr (E. Piskin). have been proposed, such as the chemical modification of both starch (Kiatkamjornwong, Thakeow, & Sonsuk, 2001; Thakore, Desai, Sarawade, & Devi, 2001) and polyolefins (LDPE) (Chandra & Rustgi, 1997) and/or the introduction of compatibilizer (Bikiaris, Prinos, & Panayiotou, 1997; Yoo et al., 2002) into the blends of starch and polyethylene (PE). Grafting hydrophobic thermoplastic polymers with more hydrophilic monomers (e.g., maleic anhydride (MA) and dioctyl maleate (DOM)) has been presented as an effec- tive approach to fabricate compatible and biodegradable polymer materials (Rzayev, 2010, 2011). Zhang and Sun (2004) have used MA as a nontoxic reactive compatibilizer to improve the compat- ibility and properties of poly(lactic acid) (PLA)/starch blends for extrusion. According to Heinze and Liebert, starch in a granular state was modified by chemical deviation grafting processes that improve the properties of the starch (Heinze & Liebert, 2001). In the starch modification by grafting, acrylic monomers (e.g., methyl acrylate) were grafted onto the starch backbone leading to modified starch that can be injected or extruded into films (Willett, Jasberg, & Swanson, 1994). In the conventional methods, to produce grafted starch mate- rials, organic solvents such as dimethylsulfoxide (DMSO) and pyridine have been employed, and that is a major drawback of these materials, especially when they are used as food packag- ing. Moreover, this bottleneck creates environmental problems and obstructs the industrialization and commercialization of the pro- duction method (Harris, Jureller, Kerschner, Trzasko, & Humphreys, 1999). Well-known techniques utilize highly toxic organic sol- vents as mediums in polymerization reactions. This problematic issue forced researchers to propose alternative approaches and http://dx.doi.org/10.1016/j.carbpol.2014.07.077 0144-8617/© 2014 Elsevier Ltd. All rights reserved.