  Citation: Bisneto, M.P.d.S.; Gouveia, J.R.; Antonino, L.D.; Tavares, L.B.; Ito, N.M.; dos Santos, D.J. Effects of Functionalized Kraft Lignin Incorporation on Polypropylene Surface Energy and Practical Adhesion. Polymers 2022, 14, 999. https://doi.org/10.3390/ polym14050999 Academic Editors: John Vakros, Evroula Hapeshi, Catia Cannilla and Giuseppe Bonura Received: 17 January 2022 Accepted: 27 February 2022 Published: 1 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- iations. Copyright: © 2022 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/). polymers Article Effects of Functionalized Kraft Lignin Incorporation on Polypropylene Surface Energy and Practical Adhesion Manuel Patricio da Silva Bisneto 1 , Julia Rocha Gouveia 1 , Leonardo Dalseno Antonino 1 , Lara Basílio Tavares 1 , Nathalie Minako Ito 1 and Demetrio Jackson dos Santos 1,2, * 1 Nanoscience and Advanced Materials Graduate Program (PPG-Nano), Federal University of ABC (UFABC), Santo Andre 09210-580, Brazil; manuel.bisneto@ufabc.edu.br (M.P.d.S.B.); juliargouveia@gmail.com (J.R.G.); leonardoantonino@hotmail.com (L.D.A.); lara.btavares@hotmail.com (L.B.T.); nathalie.minako@gmail.com (N.M.I.) 2 Center of Engineering, Modeling and Applied Social Sciences, Federal University of ABC (UFABC), Santo Andre 09210-580, Brazil * Correspondence: demetrio.santos@ufabc.edu.br Abstract: Polypropylene (PP) is a multifunctional and widely applied polymer. Nevertheless, its low energy surface and poor adhesion are well-known and might impair some prospective applications. Aiming to overcome these limitations, PP composites can be applied as a tool to enhance PP surface energy and then increase its practical adhesion. In this work, Kraft lignin (KL) was chemically modiﬁed and blended with PP. In short, KL was hydroxypropylated and further reacted with acetic anhydride (A-oxi-KL) or maleic anhydride (M-oxi-KL). Lignin modiﬁcations were conﬁrmed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). PP-composites with different lignin contents, as well as pristine PP, were characterized in terms of their thermal behavior, morphology, surface energy, and practical adhesion by DSC, scanning electron microscopy (SEM), contact angle measurement, and peeling tests, respectively. Lignin incorporation did not affect the PP degree of crystallization. The lignin modiﬁcations led to a better compatibility with the PP matrix and surface energies up to 86% higher than neat PP. Increases of up to 66% in the peel strength were veriﬁed. Composites with M-oxi- KL showed the best adhesion performance, conﬁrming the lignin functionalization is an efﬁcient approach to improve the practical adhesion of PP ﬁlms. Keywords: lignin; surface energy; wettability; practical adhesion 1. Introduction Polypropylene (PP) is a multifunctional and low-cost polymer widely used in indus- trial applications. Nevertheless, it presents low surface energy because of its non-polar chemically stable structure, which leads to adhesion hindrance, such as coating failures or ﬂexible laminated layers delamination. Polymer blending can be used as a strategy to alter the wettability of PP and increase the strength of PP adhesive joints. The combination of PP with renewable resource materials has been intensively investigated as a promising approach to reduce petroleum-based dependency, hence collaborating to a reduced environ- mental impact. The wettability and surface roughness of polypropylene can be improved when ﬁlled with wood ﬂour [1]. Cellulose, potato starch, and chitosan were also reacted with PP for membranes application, and the new materials changed their hydrophobicity, besides mechanical properties improvements [2]. Another strategy arises from a remarkable sustainable raw material that has been successfully applied as high-added-value for polymer blends and composites. Lignin, the second most abundant biopolymer on earth, can be found in all vascular plants and is an amorphous phenolic polymer, consisting of three main phenylpropane units (guaiacyl, syn- rigyl, and p-hidroxyphenyl) [3], that must be isolated from the other components, cellulose Polymers 2022, 14, 999. https://doi.org/10.3390/polym14050999 https://www.mdpi.com/journal/polymers