Citation: Uddin, M.; Alabbad, M.; Li, L.; Orell, O.; Sarlin, E.; Haapala, A. Novel Micronized Mica Modified Casein–Aluminum Hydroxide as Fire Retardant Coatings for Wood Products. Coatings 2022, 12, 673. https://doi.org/10.3390/ coatings12050673 Academic Editors: Huirong Le and Marko Petric Received: 9 March 2022 Accepted: 11 May 2022 Published: 14 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/). coatings Article Novel Micronized Mica Modified Casein–Aluminum Hydroxide as Fire Retardant Coatings for Wood Products Mezbah Uddin 1 , Maitham Alabbad 2 , Ling Li 2 , Olli Orell 3 , Essi Sarlin 3 and Antti Haapala 1, * 1 School of Forest Sciences, University of Eastern Finland, FIN-80100 Joensuu, Finland; mezbu@uef.fi 2 School of Forest Resources, University of Maine, Orono, ME 04473, USA; maitham.alabbad@maine.edu (M.A.); ling.li@maine.edu (L.L.) 3 Department of Materials Science and Environmental Engineering, Tampere University, FIN-33014 Tampere, Finland; olli.orell@tuni.fi (O.O.); essi.sarlin@tuni.fi (E.S.) * Correspondence: antti.haapala@uef.fi Abstract: Sustainable coating solutions that function as a fire retardant for wood are still a challenging topic for the academic and industrial sectors. In this study, composite coatings of casein protein mixed with mica and aluminum trihydroxide (ATH) were tested as fire retardants for wood and plywood; coating degradation and fire retardancy performance were assessed with a cone calorimeter, and a thermogravimeter was used for the thermal stability measurement. The results indicated that casein–mica composites were beneficial as coatings. The heat release rate (HRR) and the total heat released (THR) of the sample coated with casein–mica composite were reduced by 55% and 37%, respectively; the time to ignition was increased by 27% compared to the untreated sample. However, the TTI of the sample coated with the casein–mica–ATH composite was increased by 156%; the PHR and THR were reduced by 31% and 28%, respectively. This is attributed to the yielded insulating surface layer, active catalytic sites, and the crosslink from mica and endothermic decomposition of ATH and casein producing different fragments which create multiple modes of action, leading to significant roles in suppressing fire spread. The multiple modes of action involved in the prepared composites are presented in detail. Coating wear resistance was investigated using a Taber Abrader, and adhesion interaction between wood and a coated composite were investigated by applying a pull-off test. While the addition of the three filler types to casein caused a decrease in the pull-off adhesion strength by up to 38%, their abrasion resistance was greatly increased by as much as 80%. Keywords: composite coating; flame retardancy; thermal analysis; wear testing; wood 1. Introduction Wood is a traditional building material in the forms of dimension lumber and wood- based composites (e.g., oriented strand board and plywood) which has been dominantly used in light wood-frame residential and commercial buildings for a century. In the past decades, emerging new mass timber products like cross-laminated timber have been on a rapid growth trajectory in mid/high-rise buildings to replace steel and concrete due to un- paralleled environmental benefits such as low embodied energy and carbon negativity [1,2]. However, untreated solid wood is rated as a combustible and fire-hazardous material [3]. The design and construction of wood buildings shall meet the requirements of fire safety and fire prevention by complying with a specific fire code; e.g., the 2018 international fire code (IFC) [4]. Effective fire protection measures can provide wood with the ability to withstand flame and its surface propagation, preventing the free access of oxygen which promotes wood destruction and accelerates the burning process [5]. Similar risks have been identified for furniture uses [6] where the chemical treatments of frames and upholstery are now mandatory. One strategy is to shield the timber structures with non-flammable materials like gypsum boards, leaving the timber only for encased framing. Alternatively, waterborne Coatings 2022, 12, 673. https://doi.org/10.3390/coatings12050673 https://www.mdpi.com/journal/coatings