polymers Article Rotational Rheology of Wood Flour Composites Based on Recycled Polyethylene Antonella Patti 1, * , Gianluca Cicala 1 and Stefano Acierno 2, *   Citation: Patti, A.; Cicala, G.; Acierno, S. Rotational Rheology of Wood Flour Composites Based on Recycled Polyethylene. Polymers 2021, 13, 2226. https://doi.org/10.3390/ polym13142226 Academic Editor: José António Covas Received: 17 June 2021 Accepted: 2 July 2021 Published: 6 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Civil Engineering and Architecture (DICAr), University of Catania, Viale Andrea Doria 6, 95125 Catania, Italy; gianluca.cicala@unict.it 2 Department of Engineering, University of Sannio, Piazza Roma 21, 82100 Benevento, Italy * Correspondence: antonella.patti@unict.it (A.P.); stefano.acierno@unisannio.it (S.A.) Abstract: In this paper, we study the effect of the addition of wood flour as a filler in a recycled polyethylene (r-PE) in view of its potential applications in 3D printing. The composites, prepared by melt mixing, are characterized with torque measurements performed during the compounding, dynamic rotational rheology, and infrared spectroscopy. Data show that the introduction of wood results in increased viscosity and in sensible viscous heating during the compounding. The r-PE appear to be stable at temperatures up to 180 ◦ C while at higher temperatures the material shows a rheological response characterized by time-increasing viscoelastic moduli that suggests a thermal degradation governed by crosslinking reactions. The compounds (with wood loading up to 50% in wt.) also shows thermal stability at temperatures up to 180 ◦ C. The viscoelastic behavior and the infrared spectra of the r-PE matrix suggests the presence of branches in the macromolecular structure due to the process. Although the addition of wood particles determines increased viscoelastic moduli, a solid-like viscoelastic response is not shown even for the highest wood concentrations. This behavior, due to a poor compatibility and weak interfacial adhesion between the two phases, is however promising in view of common processing technologies as extrusion or injection molding. Keywords: wood-polymer composites (WPC); recycled thermoplastics; torque measurements; rheo- logical properties; dynamic mechanical analyses 1. Introduction Plastics are one of the main components of products of everyday life and industrial ac- tivities, such as packaging, agriculture, automotive, and biomedical applications, becoming an essential element for the way of life. As a consequence, an increase in plastic production and plastic products has been verified in recent times. Conventional plastics are materials with (relatively) high strength and durability and requiring hundreds of years to break down under normal ambient conditions. This represents an important disadvantage from the perspective of environmental impact and aspects of pollution [1]. As reported by Singh and Sharma [2], over 300 million metric tons of plastic are produced each year and half of that amount is spent on disposal applications, i.e., activities lasting less than one year: the product is used and thrown away one year after purchase. The accumulation of solid plastic waste in the environment has become an increasingly important worldwide problem to consider and deal with [3]. Over the past decades, environmentalists have devoted more and more effort to the impact of chemical and industrial processes and, as a consequence, in a number of countries, governments have promoted rules and laws to protect the quality of the environment for the future [4]. In this context, chemical industries have been pushed to adopt non-polluting chemical processes and materials, reduce the use of hazard chemicals, efficiently use raw materials, and reduce emissions and wastes. This has translated into a growing interest in biodegradable renewable systems, such as composites reinforced with plant fibers [5]. Polymers 2021, 13, 2226. https://doi.org/10.3390/polym13142226 https://www.mdpi.com/journal/polymers