An insight into different phenomena involved in continuous extrusion foaming of biodegradable poly(lactic acid)/expanded graphite nanocomposites Seyed Mohammad Hassan Khademi, Farkhondeh Hemmati ⁎, Mohammad Ali Aroon ⁎ Caspian Faculty of Engineering, College of Engineering, University of Tehran, P.O. Box 43841-119, Guilan, Iran abstract article info Article history: Received 11 March 2020 Received in revised form 7 April 2020 Accepted 18 April 2020 Available online 28 April 2020 Keywords: Poly(lactic acid) Nanocomposite Foam The continuous extrusion foaming of poly(lactic acid) (PLA) has several critical drawbacks that crucially limit the substitution of the renewable foams for the extruded foams based on synthetic plastics. In this work, the foamability of PLA melt through a twin-screw extrusion process was improved by using expanded graphite (EG) nanoplatelets having different aspect ratios and loadings along with an organic peroxide. Morphological ob- servations demonstrated the beneficial influences of adding nanofiller, which resulted in the formation of microcellular foams with larger void content and cell densities. Different phenomena, which are involved in the extrusion foaming of PLA melt, are considerably affected by the presence of EG including rheological behavior, PLA crystallization, thermal chain scission of the matrix, chain extension function of the peroxide and thermal de- composition of foaming agent. To correlate the phenomena affected by EG nanofiller with foam morphology, the linear viscoelastic responses, molecular weight, crystallization kinetics and structural properties of PLA and PLA/ EG nanocomposites were evaluated. © 2018 Published by Elsevier B.V. 1. Introduction In last decade, much effort has focused on the replacement of syn- thetic polymers from fossil fuels by renewable and biodegradable poly- mers from natural resources owing to rising concerns about environmental degradation [1,2]. Poly(lactic acid) (PLA) is a renewable biopolymer produced from corn, rice, wheat, potato, sugar cane and ba- gasse [3,4]. As a thermoplastic aliphatic polyester, PLA has attracted great attention for biomedical applications including tissue engineering and drug delivery due to the superior biocompatibility, hydrolysis abil- ity and the absence of toxic gas emission during the synthesis process [5,6]. Additionally, PLA has been employed in packaging applications because of its good processability. It can be shaped into films and foams through conventional plastic processing techniques such as ex- trusion and injection molding [3,7]. PLA foams can potentially replace the common polyethylene and polystyrene foams in packaging indus- tries owing to the accessibility of raw materials, economical synthesis process, good mechanical performance and excellent biodegradability [8]. Polymer foaming is carried out through both physical and chemical foaming processes. In physical foaming, a supercritical gas such as nitrogen or carbon dioxide is inserted into the PLA melt, whereas the chemical foaming prerequisite is the incorporation of an exothermic or endothermic foaming agent into the polymer matrix. The thermal de- composition of the foaming agent is accompanied by the evolution of gas molecules that are involved in the bubble formation. Commercially, chemical foaming processes are applied more frequently owing to the considerable cost of a high pressure gas reservoir and pressure control difficulties in the physical foaming processes [9–11]. In spite of the advantages of PLA foams, there are some drawbacks that must be overcome to permit the commercialization of these biode- gradable and renewable foams. Poor melt strength, slow crystallization kinetics and low operating temperatures are included. In the foaming process, the low melt strength of PLA has an adverse effect on cell growth and often causes cell wall rupture. One of the commonly used solutions is the addition of a chain extender [12]. In the molten state, the chain extender additive reacts with the hydroxyl or carboxyl end groups of PLA chains and transforms the linear PLA chains to long chain branched macromolecules having higher molecular weight and melt strength. The common chain extender additives have functional groups such as amines, anhydride, carboxylic acid, isocyanate and epoxy groups [13–15]. The resultant branched chain structures give the melt higher strength in the foaming process, wherein the melt is ex- posed to a biaxial extensional deformation in the cell growth stage [16]. The processing aids based on epoxide groups have high efficiency in the International Journal of Biological Macromolecules 157 (2020) 470–483 ⁎ Corresponding authors. E-mail addresses: f.hemmati@ut.ac.ir (F. Hemmati), maaroon@ut.ac.ir (M.A. Aroon). https://doi.org/10.1016/j.ijbiomac.2020.04.127 0141-8130/© 2018 Published by Elsevier B.V. Contents lists available at ScienceDirect International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac