Design of Biobased Poly(Butylene Succinate) Foams by Single-Screw Extrusion: Identification of Relevant Rheological Parameters Controlling Foam Morphologies C. Duborper, 1,2,3 C. Samuel , 1,2 A.-C. Akue-Asseko, 1,2,3 C. Loux, 1,2 M.-F. Lacrampe, 1,2 P. Krawczak 1,2 1 Ecole Nationale Sup  erieure Mines T  el  ecom Lille Douai (IMT Lille Douai), Department of Polymers and Composites Technology & Mechanical Engineering, 941 rue Charles Bourseul, Douai 59508, France 2 Universit  e de Lille, Lille 59000, France 3 French Institute for Biobased Materials (IFMAS), Parc Scientifique de la Haute Borne, 60 Avenue du Halley, Villeneuve-d’Ascq 59650, France Key relationships between foam morphologies and melt- state rheological parameters are here exposed for biobased poly(butylene-succinate) (PBS) and standard petrobased polyethylene (PE) foams processed by single- screw extrusion. Scanning electron microscopy followed by image analysis revealed cell diameters and densities in the range of 250–700 mm and 3–4.10 4 cells/cm 3 , respec- tively. PBS and PE have similar morphologies except for cell diameters which are slightly higher for PBS foams. The melt index roughly controls foam microstructures but deeper insights are obtained through correlations with shear/extensional rheology experiments. In particular, the melt strength and the strain hardening control the cell growth mechanisms. Concerning the cell density, the shear viscosity in the die plays a key role and agreements with nucleation theories can be discussed based on simu- lated pressure–velocity profiles using finite element soft- ware. In our extrusion conditions, the residence time comes out to have a crucial role with distinct behaviors between PE and PBS indicating a potential modification of the polymer/CO 2 interfacial tension. Consequently, an accurate control of the foam morphology seems achiev- able via a careful selection of the polymer grades and PBS represents a promising alternative to PE for further developments of biobased foams. POLYM. ENG. SCI., 00:000–000, 2017. V C 2017 Society of Plastics Engineers INTRODUCTION For decades, a particular attention was paid within the plastic industry to the development of polymer foams and their related processing by continuous single-screw extrusion processes. Poly- mer foams historically found numerous applications in packaging and automotive for mechanical shock absorption and various commodity polymers such as polyethylene (PE), polystyrene (PS) or polyurethanes (PU) were used for these purposes [1, 2]. Nowa- days, new applications and challenges were found for polymer foams and a rising demand for high-performance foams with spe- cific and controllable properties is clearly observed in transporta- tion, building, electronic and medical markets. In this respect, various properties are actually implemented such as improved biodegradability [3], lower density without sacrificing mechanical strength/rigidity [3–5], lower thermal conductivity/higher thermal insulation [6], improved dielectric properties [7] and/or drug delivery properties [8, 9]. As a consequence, advanced applica- tions of polymer foams require new polymer matrices and an accurate control over the foam morphology in terms of foam den- sity, cell density and cell size and distribution. Polymer foams are readily obtained by single-screw extrusion processes as a low-cost and continuous production processes. Polyethylene (PE), polypropylene (PP), polycarbonate (PC), acrylonitrile-butadiene-styrene resins (ABS) foams can be extruded using either chemical or physical foaming agents [10]. For the chemical foaming process, endothermic chemical foam- ing agents (CFA) (sodium bicarbonate and/or citric acid) are now preferred to exothermic CFA such as azodicarbonamide for improved safety and reduced toxicity. Endothermic CFA are usually dry-mixed with polymer pellets before extrusion and, during extrusion, the CFA decomposes into various gases (nitro- gen, water, carbon dioxide, etc.) under the action of the shear and the temperature [11]. A homogenous gas/polymer mixture is formed at high pressure in the die and a pressure drop is applied to the polymer/gas mixture at the die exit to induce cell nucle- ation and growth followed by the solidification of the foamed structure under cooling. Closed-cell foams are mainly produced by single-screw extrusion and extrusion conditions can influence foam morphologies. The die temperature, the flow rate and the CFA concentration can roughly control the expansion ratio and the cell density [12, 13]. However, the pressure drop rate (PDR) in the extrusion die was early identified as a key factor of the cell formation with a specific control over the cell density. The cell density increases with PDR and cells densities ranging from 10 4 to 10 9 cells/cm 3 can be obtained [14, 15] with PDR up to 10 GPa/s. The influence of melt pressure in chemical foaming by extrusion process could be predicted by several nucleation theories where the homogenous polymer/gas phase undergoes a phase separation phenomenon with characteristic nucleation Additional Supporting Information may be found in the online version of this article. Correspondence to: C. Samuel; e-mail: cedric.samuel@imt-lille-douai.fr This study has been granted by the French State under the “Programme d’Investissements d’Avenir” Program supported by the Agence Nationale de la Recherche (ANR, France, contract n8ANR-10-IEED-0004-01) and by the French Institute for Biobased Materials (IFMAS, France). Authors also grate- fully acknowledge both the International Campus on Safety and Intermodality in Transportation (CISIT, France), the European Community (FEDER funds) as well as the Hauts-de-France Region (France) for co-funding C. Duborper’s PhD grant (contract n815000305). The authors thank H. Amedro and S. Marcille (Roquette, France) for their support and contribution. Authors gratefully thank the PMI-2016 organization committee for oral presen- tation acceptance within the Conference on Polymers and Moulds Innovations (PMI-2016), which was held in Ghent (Belgium) 21–23 of September 2016. DOI 10.1002/pen.24733 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2017 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2017