materials Article Bio-Based Polyurethane Networks Derived from Liquefied Sawdust Kamila Gosz 1 , Agnieszka Tercjak 2 , Adam Olszewski 1 ,Józef Haponiuk 1 and Lukasz Piszczyk 1, *   Citation: Gosz, K.; Tercjak, A.; Olszewski, A.; Haponiuk, J.; Piszczyk, L. Bio-Based Polyurethane Networks Derived from Liquefied Sawdust. Materials 2021, 14, 3138. https:// doi.org/10.3390/ma14113138 Academic Editor: Milena Ignatova Received: 22 April 2021 Accepted: 1 June 2021 Published: 7 June 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 Polymers Technology, Chemical Faculty, Gdansk University of Technology, G. Narutowicza Str., 11/12, 80233 Gdansk, Poland; kamila.gosz@pg.edu.pl (K.G.); adam.olszewski@pg.edu.pl (A.O.); jozhapon@pg.edu.pl (J.H.) 2 Group ‘Materials + Technologies’ (GMT), Department of Chemical and Environmental Engineering, Engineering School of Donostia-San Sebastian, University of Basque Country (UPV/EHU), Plaza Europa 1, 20018 Donostia-San Sebastian, Spain; agnieszka.tercjaks@ehu.eus * Correspondence: lukpiszc@pg.edu.pl Abstract: The utilization of forestry waste resources in the production of polyurethane resins is a promising green alternative to the use of unsustainable resources. Liquefaction of wood-based biomass gives polyols with properties depending on the reagents used. In this article, the liquefaction of forestry wastes, including sawdust, in solvents such as glycerol and polyethylene glycol was investigated. The liquefaction process was carried out at temperatures of 120, 150, and 170 ◦ C. The resulting bio-polyols were analyzed for process efficiency, hydroxyl number, water content, viscosity, and structural features using the Fourier transform infrared spectroscopy (FTIR). The optimum liquefaction temperature was 150 ◦ C and the time of 6 h. Comprehensive analysis of polyol properties shows high biomass conversion and hydroxyl number in the range of 238–815 mg KOH/g. This may indicate that bio-polyols may be used as a potential substitute for petrochemical polyols. During polyurethane synthesis, materials with more than 80 wt% of bio-polyol were obtained. The materials were obtained by a one-step method by hot-pressing for 15 min at 100 ◦ C and a pressure of 5 MPa with an NCO:OH ratio of 1:1 and 1.2:1. Dynamical-mechanical analysis (DMA) showed a high modulus of elasticity in the range of 62–839 MPa which depends on the reaction conditions. Keywords: liquefaction; liquefied wood; bio-polyols; polyurethane resin 1. Introduction Polyurethanes (PU) are one of the most important and most versatile classes of poly- meric materials for a wide range of applications, i.e., flexible or rigid foams [1–3], ther- moplastic elastomers [4], adhesives [5], coatings [6–8] and resins [9,10]. The production of polyurethanes is based on the polyaddition reaction between oligomeric polyols termi- nated with hydroxyl groups and diisocyanate. Due to the variety of oligomeric polyols, polyurethane structure and its end properties can be modulated. Polyols and isocyanate are the two main raw materials for polyurethane production and are currently obtained from fossil resources. The main disadvantage of petroleum products is that they are non- renewable and non-biodegradable. Due to human activity in the field of mass production, excessive consumption, and huge amounts of waste, environmental problems have been very serious since the beginning of the 20th century [11,12]. Therefore, the search for biomass-based alternatives has attracted a lot of attention. Particularly noteworthy is biomass obtained from forestry industry residues that are not fully utilized. It is also referred to as dry plant matter (i.e., waste wood). It consists of three main components, namely cellulose (30–50%), hemicellulose (15–35%), and lignin (20–35%), which contain two or more hydroxyl groups in the molecule and form a stable three-dimensional network structure [13,14]. Many types of agriculture and forestry industry biowastes have been used to create polyols. These biowastes include wood [2,9,15,16], wheat straw [4,17,18], Materials 2021, 14, 3138. https://doi.org/10.3390/ma14113138 https://www.mdpi.com/journal/materials