Citation: Lubis, M.A.R.; Labib, A.; Sudarmanto; Akbar, F.; Nuryawan, A.; Antov, P.; Kristak, L.; Papadopoulos, A.N.; Pizzi, A. Influence of Lignin Content and Pressing Time on Plywood Properties Bonded with Cold-Setting Adhesive Based on Poly (Vinyl Alcohol), Lignin, and Hexamine. Polymers 2022, 14, 2111. https://doi.org/10.3390/ polym14102111 Academic Editors: Francesco Paolo La Mantia and Charles Frihart Received: 4 April 2022 Accepted: 20 May 2022 Published: 22 May 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 Influence of Lignin Content and Pressing Time on Plywood Properties Bonded with Cold-Setting Adhesive Based on Poly (Vinyl Alcohol), Lignin, and Hexamine Muhammad Adly Rahandi Lubis 1,2, * , Ahmad Labib 3 , Sudarmanto 1 , Fazhar Akbar 1 , Arif Nuryawan 3, * , Petar Antov 4 , Lubos Kristak 5 , Antonios Nikolaos Papadopoulos 6, * and Antonio Pizzi 7 1 Research Center for Biomass and Bioproducts, National Research and Innovation Agency of Indonesia, Cibinong 16911, Indonesia; sudarmantokm46@gmail.com (S.); fazharbio46@yahoo.co.id (F.A.) 2 Research Collaboration Center for Biomass and Biorefinery between BRIN and Universitas Padjadjaran, National Research and Innovation Agency of Indonesia, Cibinong 16911, Indonesia 3 Department of Forest Products Technology, Faculty of Forestry, Universitas Sumatera Utara, Kwala Bekala Campus, Medan 20355, Indonesia; alabib602@gmail.com 4 Faculty of Forest Industry, University of Forestry, 1797 Sofia, Bulgaria;p.antov@ltu.bg 5 Faculty of Wood Sciences and Technology, Technical University inZvolen, 96001 Zvolen, Slovakia; kristak@tuzvo.sk 6 Laboratory of Wood Chemistry and Technology, Department of Forestry and Natural Environment, International Hellenic University, GR-661 00 Drama, Greece 7 LERMAB, Faculte des Sciences, University of Lorraine, Blvd. des Aiguillettes, 54000 Nancy, France; antonio.pizzi@univ-lorraine.fr * Correspondence: marl@biomaterial.lipi.go.id (M.A.R.L.); arif5@usu.ac.id (A.N.); antpap@for.ihu.gr (A.N.P.) Abstract: The sustainability, performance, and cost of production in the plywood industry depend on wood adhesives and the hot-pressing process. In this study, a cold-setting plywood adhesive was developed based on polyvinyl alcohol (PVOH), high-purity lignin, and hexamine. The in- fluence of lignin content (10%, 15%, and 20%) and cold-pressing time (3, 6, 12, and 24 h) on co- hesion, adhesion, and formaldehyde emission of plywood were investigated through physical, chemical, thermal, and mechanical analyses. The increased lignin addition level lowered the solids content, which resulted in reduced average viscosity of the adhesive. As a result, the cohesion strength of the adhesive formulation with 10% lignin addition was greater than those of 15% and 20% lignin content. Markedly, the adhesive formulation containing a 15% lignin addition level exhibited superior thermo-mechanical properties than the blends with 10% and 20% lignin content. This study showed that 10% and 15% lignin content in the adhesive resulted in better cohesion strength than that with 20% lignin content. However, statistical analysis revealed that the addition of 20% lignin in the adhesive and using a cold-pressing time of 24 h could produce plywood that was comparable to the control polyurethane resins, i.e., dry tensile shear strength (TSS) value of 0.95 MPa, modulus of rupture (MOR) ranging from 35.8 MPa, modulus of elasticity (MOE) values varying from 3980 MPa, and close-to-zero formaldehyde emission (FE) of 0.1 mg/L, which meets the strictest emission standards. This study demonstrated the feasibility of fabricating eco-friendly plywood bonded with PVOH–lignin–hexamine-based adhesive using cold pressing as an alternative to conventional plywood. Keywords: adhesion; bio-based adhesives; cold pressing; cohesion; formaldehyde emission; hexamine; lignin; PVOH; wood-based panels; plywood 1. Introduction The sustainability, performance, and cost of production in the plywood industry depend on wood adhesives and the hot-pressing process [1,2]. Plywood is produced dominantly using urea-formaldehyde (UF) resins, with an estimated global consumption Polymers 2022, 14, 2111. https://doi.org/10.3390/polym14102111 https://www.mdpi.com/journal/polymers