polymers Article Excellent Fire Retardant Properties of CNF/VMT Based LBL Coatings Deposited on Polypropylene and Wood-Ply Zeeshan Ur Rehman, Atif Khan Niaz, Jung-Il Song and Bon Heun Koo *   Citation: Ur Rehman, Z.; Niaz, A.K.; Song, J.-I.; Heun Koo, B. Excellent Fire Retardant Properties of CNF/VMT Based LBL Coatings Deposited on Polypropylene and Wood-Ply. Polymers 2021, 13, 303. https://doi.org/10.3390/ polym13020303 Received: 25 November 2020 Accepted: 24 December 2020 Published: 19 January 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/). College of Mechatronic Engineering, Changwon National University, Changwon, Gyeongsangnam-do 51140, Korea; Zeeshan.physics@gmail.com (Z.U.R.); atif_khan_niaz@yahoo.com (A.K.N.); jisong@changwon.ac.kr (J.-I.S.) * Correspondence: bhkoo@Changwon.ac.kr Abstract: In this report, layer by layer (LBL) fire retardant coatings were produced on wood ply and Polypropylene Homopolymer/Flax fiber composites. FE-SEM and EDAX analysis was carried out to analyze the surface morphology, thickness, growth rate and elemental composition of the samples. Coatings with a high degree of uniformity were formed on Polypropylene composite (PP/flax), while coatings with highest thickness were obtained on wood ply (wood). FTIR and Raman spectroscopy were further used for the molecular identifications of the coatings, which confirmed the maximum deposition of the solution components on the wood substrate. A physiochemical analysis and model was proposed to explain the forces of adhesion between the substrate and solution molecules. Fire protection and thermal properties were studied using TGA and UL-94 tests. It was explored, that the degradation of the coated substrates was highly protected by the coatings as follows: wood > PP/flax > PP. From the UL-94 test, it was further discovered that more than 83% of the coated wood substrate was protected from burning, compared to the 0% of the uncoated substrate. The flammability resistance of the samples was ranked as wood > PP/flax > PP. Keywords: cellulose nano-fibrils; fire retardant coatings; DLVO theory; FTIR; UL-94; LBL 1. Introduction Every year, around the globe, fire causes massive loss to human life and property. The reason behind such tragic losses is the increasing use of polymers-based products, wood and other highly flammable goods. Although initiation of fire cannot be stopped due to the nature of the pyrolysis reaction, the speed of the fire, intensity of the fire and its flammability can be retarded using novel bulk and surface coating techniques, in order to provide sufficient time for human evacuation and emergency response measures [1]. Polypropylene and wood ply used in this study has widespread use in automo- tive, electrical, packaging, transport and commercial goods, chemical tanks and medical applications, due to its low cost, easy processing, low corrosion, excellent mechanical properties, durability, and fatigue properties [2–4]. However, the inherent flammability and melt dripping problems particularly with PP have restricted its widespread applica- tions [5,6]. Previously, various efforts have been made to improve the fire vulnerability of the PP. For instance, when 25% wt% loading of pentaerythritol (PER) and ammonium polyphosphate (APP) were added to the PP matrix, a vertical burning test V-0 rating was achieved [6]. However, such high quantity loadings causes significant negative shift in the mechanical properties and migration of additives in the PP matrix. Furthermore, Zuo et al., incorporated poly (2-morpholinyl-4-pentaerythritol phosphate-1,3,5-triazine) PMPT, a novel intumescent fire retardant (IFR) into PP, achieving improved thermal stability and flame retardancy of PP with lesser effects on the mechanical properties [7]. Lai et al. introduced another IFR, poly(-ethanediamine1,3,5- triazine-o-bicyclic pentaerythritol phos- phate) (PETBP) and achieved high durability and thermal stability [8]. However, precursors Polymers 2021, 13, 303. https://doi.org/10.3390/polym13020303 https://www.mdpi.com/journal/polymers