coatings Article Study of the Adhesion of Silicate-Based Coating Formulations on a Wood Substrate Arnaud Maxime Cheumani Yona 1,2, *, Jure Žigon 1 , Sebastian Dahle 1 and Marko Petriˇ c 1   Citation: Cheumani Yona, A.M.; Žigon, J.; Dahle, S.; Petriˇ c, M. Study of the Adhesion of Silicate-Based Coating Formulations on a Wood Substrate. Coatings 2021, 11, 61. https://doi.org/10.3390/ coatings11010061 Received: 21 December 2020 Accepted: 1 January 2021 Published: 7 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva ulica 101, SI-1000 Ljubljana, Slovenia; Jure.Zigon@bf.uni-lj.si (J.Ž.); Sebastian.Dahle@bf.uni-lj.si (S.D.); Marko.Petric@bf.uni-lj.si (M.P.) 2 Macromolecular Research Team, Faculty of Science, University of Yaoundé 1, P.O. 812 Yaoundé, Cameroon * Correspondence: ArnaudMaximeCheumani.Yona@bf.uni-lj.si Abstract: Silicate coatings are environmentally friendly inorganic-based products that have long been used for mineral substrates and protection of steel against corrosion. The development and acceptance of these coatings in the wood sector require some adjustments in formulations or special preparation of the surface to be coated to obtain durable finishes. In this work, the adhesion of various silicate-based formulations to a beech wood substrate (Fagus sylvatica L.), was assessed with the main objective to study relevant parameters and potential improvements. Adhesion strength was determined by pull-off and cross-cut tests. Other coating properties such as scratch, impact, and water resistance were also determined. Surface roughness and interface were analyzed using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM), and coating curing was studied by attenuated total reflection-infrared spectroscopy (ATR FTIR). The results showed that adhesion was highly dependent on formulation, penetration of the coatings into wood, and mechanical anchoring. Increasing the content of solid particles in the coating formulations or adding a polyol (glycerol, xylose), which probably acted as a coalescent, considerably decreased the adhesion strength, probably by blocking penetration into the wood by forming aggregates. Adhesion was improved by pre-mineralization of the surface, and substitution of a part of the potassium silicate binder with potassium methyl siliconate reduced the formation of cracks caused by dimensional instability of the wood. Keywords: coating; silicate; water glass; wood; potassium methyl siliconate; adhesion 1. Introduction Wood coatings are used extensively as domestic and industrial finishes for the decora- tion and surface protection of wood and other wood-based materials. To perform durably, these wood finishing products specifically require good adhesion, flexibility, water resis- tance, and resistance to other damage-causing parameters (photodegradation, scratching, and impact). The global wood coatings market is currently dominated by organic-based materials. However, there is a growing interest in the development of purely inorganic or inorganic-rich alternative solutions stimulated by new requirements and regulations such as the reduction of volatile organic compounds emissions or the demand for ultra- violet-resistant and fire-retardant products. A large number of research works have been published in recent years on surface coatings of wood by depositing thin layers of various inorganic nanoparticles (e.g., TiO 2 , SiO 2 , Al 2 O 3 , ZnO, CuO) or even metallic particles (e.g., Ag, Zn, Cu, Al) using techniques such as sol–gel impregnation, or physical and chemical vapor deposition [1–4]. Silicate-based coating systems are also potential inorganic alternatives that have the added advantage of being formulated for application by any end user with a minimum of experience and for wood renovation. Silicate coatings consist of water glass (e.g., potassium water glass) or silica sol (e.g., ethyl silicate) or mixtures thereof as the main binder, calcium carbonate, zinc ox- Coatings 2021, 11, 61. https://doi.org/10.3390/coatings11010061 https://www.mdpi.com/journal/coatings