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Article

Flame-Retardant Coating on Wood Surface by Natural Biomass Polyelectrolyte via a Layer-by-Layer Self-Assembly Approach

by
Mengyun Weng
1,2,
Yanchun Fu
1,2 and
Wei Xu
1,2,*
1
College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037, China
2
Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Forests 2024, 15(8), 1362; https://doi.org/10.3390/f15081362
Submission received: 15 June 2024 / Revised: 28 July 2024 / Accepted: 2 August 2024 / Published: 4 August 2024
(This article belongs to the Section Wood Science and Forest Products)
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Abstract

:
In this study, environmentally friendly and low-cost biomass materials were selected as wood flame retardants. Three polyelectrolyte flame-retardant coatings made from chitosan (CS), tea polyphenols (TP), soybean isolate protein (SPI), and banana peel powder (BBP) were constructed on wood surfaces by layer-by-layer (LBL) self-assembly. The results of SEM-EDS and FT-IR analyses confirmed the successful deposition of CS-TP, CS-SPI, and CS-BPP on the wood surface, and the content of N element increased. The TG results showed that the initial decomposition temperature and the maximum thermal decomposition temperature of the coated wood specimens decreased, while the char residue increased significantly. This is due to the earlier pyrolysis of CS-TP, CS-SPI, and CS-BBP. This shows that the three polyelectrolyte flame-retardant coatings can improve the thermal stability of wood. The combustion behavior of the wood specimen was observed by exposure to combustion; the coated wood could self-extinguish within a certain period of time after ignition, and the flame-retardant performance was improved to a certain extent. SEM and EDS characterization analyses of the carbon residue after combustion showed that the coated wood charcoal layer was denser, which could effectively block heat and combustible gas.

1. Introduction

Wood is one of the most widely distributed durable natural materials and is a renewable and green material mainly composed of cellulose, hemicellulose, and lignin [1]. Wood has the advantages of a beautiful texture and color, light weight, high strength, easy processing, etc. It is widely used in human life, especially in making furniture and interior decoration materials [2,3]. However, wood is flammable and can easily cause fire once ignited, resulting in huge life or economic losses [4,5]. Therefore, it is of great significance to provide flame-retardant treatment to wood to effectively reduce the probability of fire [6].
Currently, the most common methods for the fire-retardant treatment of wood are impregnation and surface treatment. The impregnation method is to impregnate the wood with flame retardants, which usually include halogen salt, inorganic salt compounds containing nitrogen and phosphorus, and inorganic nanoparticles [7,8,9,10]. Wood impregnated with flame retardants has excellent flame-retardant properties, but also has potential to negatively impact the physical and mechanical properties of the wood [11]. Surface treatment methods include brushing [12,13], spraying [14], and sol-gel methods [15]. These methods can improve the flame retardancy of wood by constructing a flame-retardant coating on the surface of wood; however, these methods generally have the problems of complicated processes.
Layer-by-layer (LBL) self-assembly is a simple and highly promising surface modification method. Different polyelectrolytes will ionize positively or negatively charged ions in solution, and the polyelectrolytes with opposite charges will be deposited at the solid/liquid interface for layer-by-layer self-assembly by electrostatic action [16,17,18]. It has received much attention in both academic research and industrial applications. Renneckar et al. [19] first assembled polydiallydimethylammonium chloride (PDDA) and polyethylenimine (PEI) on the surface of wood materials by using the LBL self-assembly and prepared a nano-coating on the surface of wood, which realized the controllable adjustment of the wood surface chemistry. The LBL self-assembly process is simple and independent of the shape and size of the substrate, and it is now often applied to the flame-retardant treatment of fabrics, paper fibers, and foams [20,21,22]. In terms of wood flame retardancy, the construction of multilayer nano using LBL technology can be regarded as a new and effective technique for the flame-retardant treatment of wood surfaces. Zhou et al. [23] coated chitosan/sodium phytate/TiO2-ZnO nanoparticles (CH/SP/nano-TiO2-ZnO) on wood surfaces by LBL self-assembly, and the oxygen index of the modified wood increased by 8.4%. Zhao et al. [24] constructed a composite coating on the surface of wood with CS/SP/nano-MgO to improve the thermal stability and flame-retardant properties of wood. Yan et al. [25] also constructed a CS-GO-APP ternary flame-retardant coating system on wood by the LBL self-assembly method.
In recent years, people have gradually begun to pay attention to climate change and the emissions from chemical products such as coal and petroleum. Therefore, the adoption of bio-based renewable materials to replace petroleum-based raw materials has become an important need for sustainable development. Green, sustainable flame-retardant products are receiving more and more attention. There have been many research institutes that have found environmentally friendly and low-cost biomass materials that can be used as a source of green flame retardants, such as chitosan, lignin, and phytic acid [26,27]. Cheng et al. [28] synthesized a highly efficient, reactive, phosphorus-containing flame retardant, HPPHBTCA, with phytic acid, pentaerythritol, and 1,2,3,4-butanetetracarboxylic acid and applied it to the flame retardancy of wool fabrics. Yao et al. [29] confirmed that tea polyphenols are a potentially superior and environmentally friendly flame retardant by analyzing the heating and oxidation behavior of tea polyphenols (TP) and monitoring the thermal stability, heat evolution, and gas production of a sample undergoing linear heating in an aerial atmosphere. Chen et al. [30] prepared a biomass-based tannin-furanic-SPI flame-retardant foam. The ignition and limiting oxygen index (LOI) and cone calorimetry test results showed that the foams have outstanding non-flammability, a lower heat-release rate, and good smoke-suppression properties. Banana peel powder is a product of agricultural waste and is an abundant resource at a cheap cost. Kong et al. [31] prepared polylactic acid (PLA) bio-based composites from banana peel powder (BPP) and enhanced their flame retardancy by introducing silica-gel-microencapsulated ammonium polyphosphate (MCAPP). Tea polyphenol (TP), soybean isolate protein (SPI), and banana peel powder (BPP) are all from plant extracts, which have the advantages of low cost, environmental protection, and degradability. All of them have a high carbon content, which can be expected to be used as biomass carbon source. Therefore, in this study, positively charged CS and negatively charged TP, SPI, and BBP were LBL self-assembled on the wood surface to construct a surface flame-retardant coating. The morphology and microelements of the wood surface coatings were characterized and analyzed by scanning electron microscopy, elemental analysis, and infrared spectroscopy. The thermal stability and flame-retardant properties were further observed by thermogravimetric testing and exposed to combustion. The flame-retardant mechanism was also investigated by analyzing the morphology of the residual char. This study can provide a reference for the preparation of new biomass flame retardants.

2. Materials and Methods

2.1. Materials

Fast-growing poplar (Populus × euramericana cv. ‘74/76’) from Shijiazhuang, Hebei, China, was used. The specimens’ moisture content was 8%–12%, and the density was 0.40–0.45 g/cm3. Each group of experiments was used to prepare 10 wood samples, for a total of 150. Chitosan (CS) (Degree of deacetylation ≥ 95%, Viscosity 100–200 mPa·s) was purchased from Shanghai McLean Biochemical Co., Ltd. (Shanghai, China). Tea polyphenol (TP) (99%) was purchased from Shanghai McLean Biochemical Co., Ltd. (Shanghai, China). Soy protein isolate (SPI) was purchased from Shanghai McLean Biochemical Co., Ltd. (Shanghai, China). Banana peel powder (BPP) was purchased from Shaanxi Hengling Natural Biological Products Co., Ltd. (Xi’an, China). Acetic acid (Glacial acetic acid) was purchased from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Deionized (DI) water was used as solvent for the impregnation solutions.

2.2. Preparation of the Self-Assembly Solution

Initially, 2.5 g of CS was added to 500 mL of deionized water, and then acetic acid was added to adjust the pH of the solution to 3~4. The solution was magnetically stirred until the chitosan was completely dissolved, and 5 g/L of the chitosan solution was obtained. For the preparation of the TP solution, 2.5 g TP were dissolved in 500 mL deionized water and magnetically stirred until the tea polyphenols were completely dissolved to obtain 5 g/L TP solution. Similarly, a SPI solution and BPP solution with a concentration of 5 g/L were prepared respectively.

2.3. Construction of Flame-Retardant Coating on the Wood Surface

As shown in Figure 1, the wood specimens were impregnated in deionized water for 1 h before LBL self-assembly to remove the impurities on the surface of the wood specimens and at the same time activate the wood surface. The pretreated wood specimens were then dried in an oven at 60 °C until the mass of the specimens no longer changed.
Then, the specimens were alternately impregnated with CS solution and TP/SPI/BBP solution, respectively. The impregnation time was 1 h. The impregnation process was carried out at room temperature and atmospheric pressure. After each impregnation in one solution, the excess solution on the surface of the specimens was rinsed with deionized water and dried in an oven at 60 °C. The specimens were sequentially impregnated in CS solution and TP/SPI/BPP solution, which is the alternating deposition of positive and negative ions for one layer of adsorption (1BL). It was counted as (CS-TP)1, (CS-SPI)1, and (CS-BPP)1, respectively. The above process was cycled to assemble 1BL, 5BL, 10BL, 15BL, and 20BL.

2.4. Characterization and Measurements

The zeta potential of different polyelectrolyte solutions was detected by laser particle size analyzer (Zetasizer nano ZS). The surface morphology and elemental composition of wood coatings were characterized by a scanning electron microscope (QUANTA 200, FEI, Hillsboro, OR, USA) equipped with an energy-dispersive X-ray spectroscopy (EDS) system. Fourier transform infrared spectroscopy (FT-IR) analysis of the samples was performed using an ALPHA II infrared spectrometer (ALPHA II, Bruker, Fällanden, Switzerland) in the scanning region of 4000~400 cm−1. Thermogravimetric (TG209, Netzsch, Selb, Germany) analyses were used to measure the thermal stability of wood samples with a thickness of 0.5 mm before and after being in a nitrogen atmosphere. The heating rate was 10 °C/min, and the temperature range was 30~800 °C.
The exposed combustion test is used to expose wood specimens (120 × 10 × 4 mm3) to the external flame of the fire source, ignite them for 15 s, and then evacuate the fire source. The combustion status and the time required for complete extinguishment were recorded, and the combustion trend was observed. The whole process of each wood specimen after ignition was videotaped and timed, and 5 repetitions were carried out in each group. The most typical specimen was selected as the representative for discussion.

3. Results and Discussion

3.1. Zeta Potential Analysis

Figure 2 shows the zeta potential values of CS, TP, SPI, and BPP aqueous solutions. The pH of CS solution is in the range of 3~4, and the pH of the rest of the solutions is 7. The concentration of the solutions is 5 g/L. From the figure, it can be seen that the CS solution is positively charged, with a zeta potential of +39.2 mV. The TP, SPI, and BPP solutions are negatively charged, with a zeta potential of −22.3 mV, −17.5 mV, and −11.1 mV. The wood surface showed a negative charge in the aqueous solution. During impregnation with CS, positively charged CS molecules are adsorbed on the wood surface through electrostatic interactions, and the wood surface changes from negatively to positively charged. When impregnated with a negatively charged TP/SPI/BPP solution, the adsorbed negative charge on the wood surface turns negative again. According to the above self-assembly method, chitosan and tea polyphenol/soya bean isolate protein/banana peel powder were deposited on the wood surface to form a natural polyelectrolyte flame-retardant coating.

3.2. Morphology and Elemental Composition of the Flame-Retardant Coating on Wood

In order to observe the surface morphology and elemental changes of the self-assembled wood, SEM-EDS characterization, and macroscopic surface images of the control wood and surface self-assembled wood were carried out (Figure 3, Table 1). The SEM images and macroscopic surface images in Figure 3 were taken tangentially on the surface of the specimens. The CS-coated wood was the control wood after impregnation with chitosan solution for 10 h. The surface morphology of the self-assembled wood is shown in Figure 3a. The surface micro-morphology of the unassembled wood is shown in Figure 3a, which shows that the surface structure of the wood is rough and uneven. The surface of the wood impregnated in chitosan solution for 10 h did not change significantly, and the surface was still uneven (Figure 3b). The elemental content was not significantly different compared to unassembled wood (Table 1). The wood surfaces after self-assembly by different polyelectrolytes had different microscopic morphologies. Figure 3c shows the microscopic morphology of the wood surface after 20 BL of CS-TP assembly; some polymer-like substances can be seen on the surface of the wood, and the surface of the wood is relatively flat. The N element content of the wood surface increased. At the same time, the color of the wood surface becomes uniformly red (Figure 3(c1)). This indicates that CS-TP was successfully assembled on the wood surface. The surface morphology of CS-SPI after 20 BL on the wood surface is shown in Figure 3d. It can be observed that the SPI on the wood surface was in a near-spherical morphology with some clusters of particles [32]. The overall surface of the wood was smooth, and the N element content increased significantly compared with that of the unassembled wood, indicating that the CS-SPI coating was successfully assembled on the wood surface. Figure 3e shows the surface morphology of wood after 20 BL by CS-BPP. Compared with the unassembled wood, the surface became smooth and flat as a whole. Meanwhile, the content of C and N elements increased. The color of the wood surface coated by CS-SPI and CS-BPP self-assembled coatings did not change significantly, which may be related to the solution color of the polyelectrolyte.

3.3. FT-IR Analysis

Treated and untreated wood specimens were prepared in dimensions of 5 mm × 5 mm × 1 mm, and infrared spectroscopy was performed on their surfaces. The FT-IR spectra of uncoated and self-assembled coated wood specimens are shown in Figure 4. The characteristic absorption peak of uncoated wood at 3330 cm−1 is attributed to the -OH stretching vibration, the peak at 2920 cm−1 is attributed to the -CH2 stretching vibration, the peak at 1030 cm−1 is attributed to the C-O-C stretching vibration, and the peak value at 1730 cm−1 is attributed to the C=O stretching vibration. The FT-IR of the wood specimens only immersed in the CS solution show no significant differences from those of uncoated wood specimens, which may be attributed to the similar polysaccharide nature shared by CS and cellulose. Consequently, distinct characteristic absorption peaks of CS are not notably observed in the self-assembled coated wood specimens. After the construction of the (CS-TP)20 self-assembled coating, the stretching vibration peaks of the wood specimens at 2918 cm−1, 1730 cm−1, and 1590 cm−1 shift toward lower wavenumbers, specifically to 2909 cm−1, 1690 cm−1, and 1600 cm−1, respectively. The characteristic peaks at 1620 cm−1 and 1510 cm−1 of the self-assembled coated wood specimens were attributed to the bending vibration peaks of N-H. In wood coated with (CS-TP)20 and (CS-BPP)20, this may be due to hydrogen bonding between -NH2 in chitosan and oxygen-containing functional groups such as -OH in TP and BBP. Since SPI has an N-H bond, the characteristic peak strength at 1620 cm−1 and 1510 cm−1 in (CS-SPI)20-coated wood is enhanced.

3.4. Thermal Stability

Figure 5 shows the TG and DTG curves of uncoated wood and CS-TP-, CS-SPI-, and CS-BPP-coated woods, and Table 2 shows the corresponding data. As can be seen from the Figure 5, the pyrolysis of the wood specimens can be divided into three stages: 30~250 °C, 250~400 °C and 400~800 °C. The initial stage of pyrolysis is the drying and pre-decomposition stage of wood. The pyrolysis of uncoated wood started at 240.29 °C (T5%), the wood was further dehydrated and charred and decomposed rapidly at a T10% of 270.29℃, the pyrolysis rate reached its maximum at 371.14 °C (Tmax), and the char residue of the wood started to stabilize at 400 °C, with a char residue of 19.97%, and the char residue was 11.45% at 800 °C. The char residue and Tmax of wood modified by different polyelectrolyte self-assemblies were changed. The higher the number of LBL layers of the samples treated with CS-TP coating, the higher the char residue at 800 °C. The higher the number of LBL layers, the higher the char residue. This indicates that the higher the number of layers, the better the flame-retardant effect, and the thermal stability of the specimen is improved. It is noteworthy that the Rmax of (CS-TP)15 in Figure 5b is significantly higher. This may be due to the significant increase in thermal stability of (CS-TP) at 15 BL. The T5% and T10% of (CS-TP)20 decreased to 240.29 °C and 270.29 °C, and the char residue increased to 29.91% and 19.09% at 400 °C and 800 °C, which indicated that the CS-TP coating was preferentially pyrolyzed. In the study of self-assembly treatment of wood with zirconium phosphate (ZrP) and ammonium polyphosphate (APP) by Xu et al. [33]., the T10% was higher than that in this study, but the Tmax was lower than that in this study. This may be because the biomass polyelectrolyte in this study is preferentially pyrolyzed, but the pyrolysis rate is relatively slow. From Figure 5c, it is concluded that the char residue of CS-SPI-coated wood increased with the increase in the number of self-assembled layers, indicating that the more self-assembled layers, the better the flame-retardant effect. The T5% of (CS-SPI)20 decreased to 115.83 °C, which was 124.46 °C lower than that of unassembled wood, indicating that the (CS-SPI)20 coating pyrolyzed earlier. The char residue increased to 27.79% and 17.22% at 400 °C and 800 °C. The pyrolysis of (CS-BPP)20 started from 77.11 °C (T5%), which was 163.18 °C lower than that of the uncoated wood, probably due to the relatively low initial decomposition temperature of BBP. Similarly, the char residue of CS-BPP-coated wood increased with the increase in the number of assembled layers. The char residue were 27.01% and 16.52% at 400 °C and 800 °C, respectively. The Tmax of CS-TP-, CS-SPI-, and CS-BPP-coated wood was close to that of the material, and the trends of TG and DTG curves were consistent, which indicated that these three flame-retardant coatings had a good match with the wood substrate. Under the same conditions, CS-TP has the highest char residue and the lowest Tmax. This indicates that CS-TP has the best flame-retardant effect and the best thermal stability.

3.5. Exposure Combustion and Char Residue Characterization

All specimens were exposed to the fire source for 15 s. As shown in Figure 6a–e, the timing was started after removing the fire source and the burning behavior of the specimens was evaluated. As can be seen in Figure 6a, the uncoated control wood ignited rapidly upon exposure to the fire source, and the flame spread very quickly and burned vigorously. It continued to burn until the bottom of the specimen. The (CS-TP)20-, (CS-SPI)20-, (CS-BPP)20-coated wood specimen burned slowly and with less fire. (CS-TP)20 can self-extinguish after 17.4 s of ignition, (CS-SPI)20 self-extinguishes after 18.7 s, and (CS-BPP)20 self-extinguishes after 14.9 s. This shows that the (CS-TP), (CS-SPI), and (CS-BPP) flame-retardant coatings can delay the flame propagation and have a better flame-retardant effect.
SEM was used to analyze the char structure and morphology of the specimens after burning. The elemental composition of carbon residue is shown in Table 3. The surface of the uncoated wood was loose with holes, and a gray friable char layer was deposited on the surface. This charcoal layer structure did not effectively insulate oxygen and heat, resulting in the wood being easy to burn completely. The same phenomenon was found in the study of the self-assembly of ammonium polyphosphate and polyethyleneimine in particleboard by Wang et al. [34]. The EDS spectra show that the untreated wood has a high content of the N and O elements. This may be due to the fact that the wood itself contains traces of elemental N, which combines with other elements (C, O) to form nitrogen compounds or oxides of nitrogen during complete combustion, which then remain in the residual charcoal. The significant increase in the C element content of the coated wood indicates a higher density of char residual on the surface. And the decrease of N content is probably due to the fact that combustion of the surface coating produces non-combustible gases such as NH3 and N2. Figure 6f shows the morphology of the char residue of (CS-TP)20 coated wood; the surface of the char residue is dense, with many bubble-like small particles distributed.
The flame-retardant mechanism of CS-TP coating is shown in Figure 7a. TP has excellent antioxidant properties, and not only can it be used as a blowing agent to release non-combustible gases, mainly H2O and CO2, during the combustion process, but also its large number of phenolic hydroxyl groups can capture free radicals generated during the combustion process, slowing down or interrupting the combustion chain reaction. Meanwhile, the high carbon content of tea polyphenols can be used as a carbon source to form a dense char residue. It can be shown that CS-TP is an environmentally friendly flame retardant with superior and comprehensive functions. The morphology of the char residue after the burning of (CS-SPI)20-coated wood is shown in Figure 6g, where a large number of ruptured near-spherical SPIs and bubbles can be observed, and a protein carbon layer is formed on the surface. This may be due to the fact that before the wood substrate is heated and burned, the carbon-rich SPI will preferentially become char, forming a dense char layer on the wood surface and retarding the heat transfer. As shown in Figure 7b, SPI contains the N element, which generates refractory gases such as N2, NH3, and CO2 after thermal decomposition, which can effectively dilute the concentration of combustible gases and thus effectively inhibit wood combustion. Figure 6h shows the charcoal layer of (CS-BBP)20 after combustion; it can be seen that the charcoal layer is dense, and the internal structure of the wood is still relatively intact. The main components of BBP are cellulose (7~10%), hemicellulose (6~8%), and lignin (6~12%) [32], which are organic macromolecules with multi-hydroxy structure, and also contain high carbon content and can be used as a good source of carbon in the biomass, and the flame-retardant mechanism is shown in Figure 7c. In general, the three flame-retardant coatings preferentially decompose at the early stage of combustion to release non-combustible gases and play the role of gas-phase flame retardants. At the same time, after the pyrolysis of the flame-retardant coating, a dense charcoal layer can be formed quickly on the surface of the wood substrate to isolate the external heat and oxygen, thus delaying the pyrolysis, increasing the residual charcoal rate and playing the role of cohesive-phase flame retardant, which improves the thermal stability of the wood specimens.

4. Conclusions

In this study, the CS-TP, CS-SPI, and CS-BBP flame-retardant coatings were constructed on the surface of wood using the LBL method to improve the thermal stability and flame-retardant property of wood. The results of SEM-EDS and FT-IR analyses confirmed the successful construction of CS-TP, CS-SPI, and CS-BPP coatings on the wood. The TG results showed that the Tmax of the coated wood samples decreased to a lower temperature, indicating that the coated wood decomposed earlier. Rmax decreased significantly, and the thermal stability of surface modified wood was improved. The char residue of coated wood increased obviously. The exposed combustion results showed that the coating-modified wood could self-extinguish within a certain period of time after 15 s of ignition, indicating that it has a certain flame-retardant effect. In general, this work presents a feasible method to construct wood surface coatings with biomass polyelectrolyte, which has potential application value in the field of eco-friendly bio-mass flame retardants.

Author Contributions

Conceptualization, M.W. and Y.F.; methodology, Y.F.; software, M.W.; validation, M.W., Y.F. and W.X.; formal analysis, M.W.; investigation, M.W. adn Y.F.; resources, Y.F.; data curation, Y.F.; writing—original draft preparation, M.W.; writing—review and editing, M.W. and Y.F.; visualization, W.X.; supervision, W.X.; project administration, Y.F. and W.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the “Qinglan Project” of Jiangsu Universities; the International Cooperation Joint Laboratory for Production, Education, Research and Application of Ecological Health Care on Home Furnishing; Ministry of Education Industry University Cooperation Collaborative Education Project (202101148004); Natural Science Foundation of Jiangsu Province (BK20220426) and Natural Science Research of Jiangsu Higher Education Institutions of China (22KJB220004).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Construction process of flame-retardant coating on wood surface.
Figure 1. Construction process of flame-retardant coating on wood surface.
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Figure 2. Zeta potential of different polyelectrolyte solutions.
Figure 2. Zeta potential of different polyelectrolyte solutions.
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Figure 3. SEM and macroscopic surface images of different wood samples: (a) SEM image of uncoated wood; (a1) macroscopic surface images of uncoated wood; (b) SEM image of CS-coated wood; (b1) macroscopic surface images of CS-coated wood; (c) SEM image of (CS-TP)20-coated wood; (c1) macroscopic surface images of (CS-TP)20-coated wood; (d) SEM image of (CS-SPI)20-coated wood; (d1) macroscopic surface images of (CS-SPI)20-coated wood; (e) SEM image of (CS-BBP)20-coated wood; and (e1) macroscopic surface images of (CS-BBP)20-coated wood.
Figure 3. SEM and macroscopic surface images of different wood samples: (a) SEM image of uncoated wood; (a1) macroscopic surface images of uncoated wood; (b) SEM image of CS-coated wood; (b1) macroscopic surface images of CS-coated wood; (c) SEM image of (CS-TP)20-coated wood; (c1) macroscopic surface images of (CS-TP)20-coated wood; (d) SEM image of (CS-SPI)20-coated wood; (d1) macroscopic surface images of (CS-SPI)20-coated wood; (e) SEM image of (CS-BBP)20-coated wood; and (e1) macroscopic surface images of (CS-BBP)20-coated wood.
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Figure 4. FT-IR spectra of different wood samples.
Figure 4. FT-IR spectra of different wood samples.
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Figure 5. TG (a,c,e) and DTG (b,d,f) curves of different wood samples.
Figure 5. TG (a,c,e) and DTG (b,d,f) curves of different wood samples.
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Figure 6. (a–e) burning behavior of the specimens; (f) morphology of the char residue of (CS-TP)20 coated wood; (g) morphology of the char residue after the burning of (CS-SPI)20-coated wood; (h) charcoal layer of (CS-BBP)2020 after combustion.
Figure 6. (a–e) burning behavior of the specimens; (f) morphology of the char residue of (CS-TP)20 coated wood; (g) morphology of the char residue after the burning of (CS-SPI)20-coated wood; (h) charcoal layer of (CS-BBP)2020 after combustion.
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Figure 7. Schematic of possible flame-retardant mechanism of CS-TP (a), CS-SPI (b), CS-BBP, and (c) coated wood during combustion.
Figure 7. Schematic of possible flame-retardant mechanism of CS-TP (a), CS-SPI (b), CS-BBP, and (c) coated wood during combustion.
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Table 1. Elemental composition of different wood samples.
Table 1. Elemental composition of different wood samples.
Element SamplesC (%)O (%)N (%)
Uncoated wood39.560.50
CS-coated wood40.259.80
(CS-TP)20-coated wood38.259.22.6
(CS-SPI)20-coated wood42.749.47.9
(CS-BPP)20-coated wood44.351.83.9
Table 2. Thermal analysis parameters of different wood samples.
Table 2. Thermal analysis parameters of different wood samples.
SamplesT5% (°C)T10% (°C)Tmax (°C)Rmax (%/min)Char Residue at 400 °C (100%)Char Residue at 800 °C (100%)
uncoated wood240.29270.29371.1412.7219.9711.45
(CS-TP)20179.91255.98346.547.5629.9119.09
(CS-SPI)20115.83260.57366.3110.827.7917.22
(CS-BPP)2077.11240.28365.0211.5227.0116.52
T5%: decomposition temperature when the weight loss was 5 wt %; T10%: decomposition temperature when the weight loss was 10 wt %; Tmax: decomposition temperature when the weight loss was at its maximum; Rmax: the maximum mass loss rate.
Table 3. Elemental composition of char residue.
Table 3. Elemental composition of char residue.
Element SamplesC (%)O (%)N (%)
Uncoated wood27.544.228.5
(CS-TP)20-coated wood75.920.33.8
(CS-SPI)20-coated wood56.929.513.6
(CS-BPP)20-coated wood61.523.315.2
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Weng, M.; Fu, Y.; Xu, W. Flame-Retardant Coating on Wood Surface by Natural Biomass Polyelectrolyte via a Layer-by-Layer Self-Assembly Approach. Forests 2024, 15, 1362. https://doi.org/10.3390/f15081362

AMA Style

Weng M, Fu Y, Xu W. Flame-Retardant Coating on Wood Surface by Natural Biomass Polyelectrolyte via a Layer-by-Layer Self-Assembly Approach. Forests. 2024; 15(8):1362. https://doi.org/10.3390/f15081362

Chicago/Turabian Style

Weng, Mengyun, Yanchun Fu, and Wei Xu. 2024. "Flame-Retardant Coating on Wood Surface by Natural Biomass Polyelectrolyte via a Layer-by-Layer Self-Assembly Approach" Forests 15, no. 8: 1362. https://doi.org/10.3390/f15081362

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