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Article

A Comparative Analysis of Tannin and Commercial Fire Retardants in Wood Fire Protection

by
Eraldo Antonio Bonfatti Júnior
1,
Jeinna Michelly Rodrigues de Barros
2,
Gustavo Felizardo Silva
2 and
Elaine Cristina Lengowski
2,3,*
1
Department of Forest Engineering, Agrarian and Environmental Sciences Sector, Midwestern State University, Professora Maria Roza Zanon de Almeida Street, Irati 84505-677, PR, Brazil
2
Faculty of Forestry Engineering, Federal University of Mato Grosso, 2367 Fernando Corrêa da Costa Street, Cuiabá 78060-900, MT, Brazil
3
Faculty of Chemical Engineering, University of Waterloo, 200 University Avenue West Waterloo, Waterloo, ON N2L 3G1, Canada
*
Author to whom correspondence should be addressed.
Forests 2024, 15(6), 951; https://doi.org/10.3390/f15060951
Submission received: 31 March 2024 / Revised: 28 May 2024 / Accepted: 29 May 2024 / Published: 30 May 2024
(This article belongs to the Special Issue Advances in the Study of Wood Mechanical and Physical Properties)
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Abstract

:
In civil construction, one of the primary challenges associated with wood application is its high flammability and low durability during fires. Although chemical treatment with fire-retardant properties exists, they are expensive and of non-renewable origin. Tannin, a wood extractive, being a phenolic compound, holds promise for enhancing the thermal properties of wood. This study aimed to assess the efficacy of tannin as a fire retardant and compare it with a commercial product, as well as comparing different application techniques for these products. Wood samples from the Simarouba amara Aubl. species were utilized. Tannin and a commercial flame retardant were applied via immersion, vacuum impregnation, brushing, and a treatment of tannin incorporated into water-based wood varnish. Alongside the burning test performance, assessments of the wood’s physical properties, such as colorimetry, thermal stability, and mass retention, were conducted. The application of tannin altered the wood’s color and reduced the spread of fire; its presence significantly reduced the flame duration and maintained the wood’s structural integrity. However, tannin retention was lower compared to commercial flame retardant, leading to reduced fire retardancy. Among the methods tested, immersion proved to be the most effective in enhancing the wood’s resistance to flame contact.

1. Introduction

Wood has long been recognized as a traditional and well-established building material, owing to its remarkable strength-to-weight ratio [1], comparable to that of steel and concrete [2]. Its selection for construction purposes is a sustainable choice due to its reduced carbon footprint, renewable origin, versatility, and agility in construction. Additionally, wood offers thermal and acoustic insulation, along with aesthetic benefits owing to its inherent natural beauty [3].
In construction, wood finds application in building materials for beams, rafters, boards, doors, windows, floors, and finishes. However, due to its flammable nature, concerns regarding its safety in fire situations arise. While wooden components with a large cross-section may char and resist fire to some extent [4,5], pieces with smaller cross-sections require additional protection through coating with non-combustible materials [6] or treatment with fire-retardant compounds [6,7].
The performance of wood under fire exposure can be evaluated under various conditions, considering factors such as thermal degradation, ignition from heat sources, heat and smoke release, flame spread, and carbonization rates [7,8]. Fire induces depolymerization and carbonization processes in wood, which influence its properties, notably the mechanical strength of wooden structures and the carbonization of their surfaces [8,9].
Firewood degradation can be classified into five stages [10]. The first of these is slow pyrolysis, which occurs at temperatures up to 200 °C [10]. During this stage, gases and water vapor are emitted without ignition, accompanied by some exothermic reactions, leading to alterations in the color of the wood [10]. The second stage, fast pyrolysis, takes place between 200 and 280 °C, characterized by the intensification of chemical reactions and the release of gases [10]. Although primary exothermic reactions occur during this stage, flames are absent [10]. The subsequent stages are totally exothermic. The third stage appears in the temperature range between 280 and 380 °C, marked by the presence of flames and the production of charcoal alongside volatile gases [10]. The fourth stage, occurring in the range of 380 and 500 °C, is characterized by the reduction of gas emissions, giving rise to tar and condensable gases [10]. Finally, the fifth and final stage manifests at temperatures above 500 °C [10]. During this stage, carbonization ceases, and coal gasification begins; after consuming all the combustible material, combustion stops, the temperature drops, and approximately 0.2 to 1% of ash remains [11].
Certain products known as fire retardants offer thermal resistance to wood when subjected to intense heat [7,12]. These retardants are designed to function within the combustion temperature range, thereby protecting the wood from heat sources during the first and second stages of thermal degradation [8,13]. In situations involving combustion, these retardants have the capability to retard the flame [8,13].
Fire retardants based on aluminum hydroxide, magnesium, and other compounds such as boron primarily exert a physical action [8,14,15]. They function by absorbing heat from the environment or the exposed wood, hindering the formation of flammable gases and inhibiting the progression of combustion [8,14,15]. However, retardants composed of halogenated (bromine and chlorine) and non-halogenated (phosphorous) products exert a chemical action [16,17]. These compounds target free radicals, resulting in the generation of fewer combustible substances and rendering combustion more challenging. However, they do not absorb heat from the environment, resulting in the formation of toxic byproducts [16,17].
Certain natural substances, such as tannins, possess the potential to serve as fire retardants, offering the advantage of being more accessible, safe, and non-toxic [18,19,20,21].
Tannins are wood extractives that belong to the category of phenolic compounds [22,23,24]. They are commonly found in wood, barks, leaves, galls, fruits, and seeds [22,24,25]. They are heterogeneous chemical groups formed by structures containing 12–16 phenolic groups and 5–7 aromatic rings per 1000 Da [26], with the total molecular weights varying between 500 and 20,000 Da [27]. Tannins can be further categorized as condensable or hydrolysable types [22] as follows: condensable tannins consist of chains of polyhydroxy flavan-3-ol units [28], whereas hydrolysable tannins are polyesters [29].
Intumescent-type fire retardants typically comprise three components: an inorganic acid, a blowing agent, and a carbon source [18]. Tannins serve as a potential carbon source due to its abundance of phenols [20]. Furthermore, tannin is an ineffective heat conductor [22] and can act in the early stages of the thermal degradation of wood.
The Amazon Forest, situated in South America, stands as the largest equatorial forest globally, spanning an area of 5.5 million km2 across 9 countries. Brazil notably claims 60% of the Amazonian area [30]. Numerous species of towering trees with straight stems, many of which are endemic, thrive within the Amazon. It is estimated to harbor around 16,000 species of large trees [31]. In Brazil, regulated forest exploitation aims to drive economic, social, and environmental development in the region.
Brazil emerges as a key producer of tropical sawn timber, with the state of Mato Grosso playing a significant role in the sector revenue [32,33]. Following tree extraction, logs undergo mechanical processing, being refined in the timber industry into rectangular or square pieces primarily used in construction [34,35]. Notable among the Brazilian Amazon timber trade are Hymenolobium petraeum Ducke, Apuleia leiocarpa J.F. Macbr., Hymenaea courbaril L., Astronium lecointei Ducke., Goupia glabra Aubl., Peltogyne angustifolia Ducke., and Dipteryx odorata (Aubl.) Forsyth f. [35].
Categorizing timber based on its intended use allows for optimal applications, whether for indoor or outdoor settings, structural or otherwise, taking into account market value [36]. Simarouba amara (Aubl.) wood, among the species sourced from sustainable forest management, ranks among the most traded in the state. However, it is often utilized in less prestigious roles such as crates, plywood, and charcoal, owing to the limited knowledge of its properties.
Improving the fire resistance of wood increases its reliability and safety as a construction material [1,5,6]. Tannins, characterized by their renewable nature and low toxicity to human health, hold promise for this purpose [37]. Therefore, the study aimed to assess the efficacy of tannins as fire retardants in Simarouba amara Aubl. wood. For this purpose, three application techniques were employed, along with the addition of tannins to wood varnish. A commercial flame retardant served as a reference for comparison. The results were evaluated via the colorimetric analysis of the wood, thermogravimetric analysis, and a short exposure to a flame burning test.
The novelty of this work is the use of a wood species that has been little studied, the evaluation of several tannin application techniques on wood, comparisons with a commercial fire retardant, and the combination of the results of the flame exposure test with thermogravimetric analysis.

2. Materials and Methods

2.1. Materials

The wood utilized in this study is the heartwood of Simarouba amara Aubl, collected from a sawmill in the municipality of Colniza, Mato Grosso, Brazil. It is a tropical species with a low basic density of 0.359 g cm−3, rendering it highly porous (76.75%), with a moisture content of 12% (65% of environment moisture at 25 °C). Furthermore, when it comes to construction timber from Brazilian tropical forests, there is no commercialization of sapwood. Defect-free specimens with dimensions of 15 × 25 × 50 mm (transverse, radial, and tangential sections, respectively) were obtained (Figure 1).
The powdered tannin employed was the commercial product TANFLOC SG 1500 (TANAC S.A., Montenegro, Rio Grande do Sul, Brazil), which was diluted in water at a concentration of 50 g L−1. For comparison, the commercial flame retardant Osmoguard® FR100 (Montana Química Ltda., São Paulo, Brazil) was utilized. To formulate a varnish incorporating tannin, the water-based varnish, Nobile Lasur Incolor (Montana Química Ltda., São Paulo, Brazil), was employed in a ratio of 5 g of tannin to 100 mL of varnish, as well as 30 mL of distilled water. The varnish used provides an open-pore, water-based finish that penetrates the wood, highlighting its natural grains and designs. Its composition is acrylic resin, water, additives, and glycols. The flame retardant contains phosphorous and nitrogenous compounds, borates, surfactants, and water. The tannin was obtained through the aqueous leaching of Acacia mearnsii bark and basically consists of flavonoid structures with an average molecular weight of 1700 DA, as well a small portion of sugars, hydrocolloidal gums, and soluble salts.

2.2. Application of Fire Retardants

In this study, three techniques were used to apply the fire retardants to the wood as follows: immersion for 48 h, immersion in a vacuum system for 2 h (−40 kPa), and brushing in three coats, with an interval of 3 h between each coat. A total of nine treatments were conducted, with each treatment being replicated ten times, including the control treatment and varnish without the addition of tannin (Table 1). All samples were acclimatized for two weeks in a controlled environment at a 12% equilibrium moisture before performing the analysis.

2.3. Colorimetry

The color of the wood was assessed using a Konica Minolta CR-410 portable spectrophotometer (Konica Minolta, Ramsey, MN, USA), with adjustments made for the light source and an observation angle of 10°. Quantitative CIELAB color space was utilized for the quantitative analysis, and all colorimetric variables were computed according to ASTM D2244-21 [38]. The chromatic coordinates luminosity (L*), green-red (a*), and blue-yellow (b*) were directly obtained from the spectrophotometer, while the color saturation (C*) (Equation (1)), hue angle (h*) (Equation (2)), and total color variation (ΔE) (Equation (3)) were calculated following the set of equations below.
C * = a * 2 + b * 2
h * = t a n − 1 b * a *
Δ E = Δ L * 2 + Δ a * 2 + Δ b * 2
where C*—color saturation, a*—green-red coordinate, b*—blue-yellow coordinate, h*—hue angle, Δ E —total color variation, Δ L*—luminosity variation, Δ a*—green-red coordinate variation, and Δ b*—blue-yellow coordinate variation.

2.4. Mass Retention

The mass retention of each treatment was calculated based on the relationship between the mass of the specimen after the treatment and the mass of the specimen in its natural state, using Equation (4).
M R = ( t m − i m ) i m × 100
where MR—mass retention in percentage, im—initial mass of the sample in grams, and tm—mass of the treated sample in grams.

2.5. Thermal Stability

The thermal stability was evaluated through thermogravimetric analysis (TGA) and derivative thermogravimetric curve (DTG) analysis in accordance with ASTM E2550-21 standard [39]. TGA was conducted using a Shimadzu DTG-60H thermal balance (Shimadzu, Kyoto, Japan) in an oxidative atmosphere. Samples weighing 8 mg were utilized, with a gas flow of 100 mL min−1, with a heating rate of 10 °C min−1 starting from 25 °C to 1000 °C.
The products used (tannin, commercial flame retardant, varnish, and tannin + varnish), the treated wood samples, and the control treatment (IT, VT, BT, IO, VO, BO, BTV, BV, and CT) were evaluated. For the TGA analyses, three wood samples per treatment were reduced to a powder and mixed following the procedures established by the TAPPI T 257 sp-21 standard [40].

2.6. Burning Test with Short Exposure to Flame

The test was performed by reproducing the methodology developed by Tondi et al. [21]. The experiment was conducted in a fume hood with constant air flow of 6 m3/min. The wood samples were exposed on their largest side (tangential) to a 100%-oxidizing flame produced by a Bunsen burner for 120 s.
In this test, the ignition time, flame time, and ember time were recorded in seconds using a digital chronometer. The ignition time is the time required for a visible flame to occur in the sample, the flame time is the duration of the flame after exposure to the Bunsen burner flame ceases, and the ember time is the time required for the complete extinction of the ember, characterized by the absence of any incandescence and smoke emission from the sample (Figure 2).
Following exposure to the flame, the masses of the burned samples were determined to calculate the residual mass (Equation (5)).
R = b m o m × 100
where R—residual mass after the burning test in percentage, bm—mass of the sample after burning in grams, and om—mass of wood before burning test in grams.
The burned samples were sectioned perpendicular to their longitudinal axis (cross-section) to facilitate the visualization of the burning progression within the samples.

2.7. Statistical Analysis

The experiments were conducted using a completely randomized design (CRD). The following tests were performed for statistical analysis: Grubbs test for outliers, Shapiro–Wilk test for data normality, Levene’s test for homogeneity of variance, and analysis of variance (ANOVA). Subsequently, when the equality hypothesis was invalidated, Tukey’s mean comparison test was performed. The data were analyzed with the support of R software version 4.3.2 [41], using the package ExpDes.pt 1.2.2 [42], at a 5% probability.

3. Results

3.1. Colorimetry

The application of fire-retardant products induces changes in the color of the wood (Table 2). The tannin treatments caused the most intense changes and significantly darkened the wood (chromatic coordinate L*). Both the tannin and commercial flame retardant reddened (chromatic coordinate a*) and yellowed (chromatic coordinate b*) the wood. However, tannin treatments induced more pronounced reddening, while commercial flame-retardant treatments were chiefly responsible for more intense yellowing. In addition to these modifications, the treatments increased the color saturation (C*), with the hue angle (h*) remaining unchanged by the application of pure varnish (BV); all h* values were between 0° and 90°, that is, within the first quadrant.
Regarding the total color variation, the commercial flame-retardant treatments exhibited the lowest values, although the wood underwent comparatively less modification than with the varnish treatment (BV) (Figure 3).

3.2. Mass Retention

The mass retention is influenced by the application method and the type of fire retardant employed (Figure 4). The tannin exhibited low retention rates, with no discernible difference between immersion for 24 h (IT) and vacuum immersion (VT) for 2 h. Brushing (BT) proved very inefficient, failing to achieve even 1% retention. In contrast, immersion for 24 h resulted in retention of over 35% of the commercial flame retardant (IO). When immersed for 2 h under vacuum, the retention was 22.47% (VO). Notably, brushing with the commercial flame retardant (BO), although the least efficient method, led to a greater retention than all treatments with tannins. The use of varnish (BV) and varnish mixed with tannin (BTV) also promoted a greater mass retention than tannin treatments, but fell short of surpassing those involving commercial flame-retardant treatments.

3.3. Thermal Stability

The chemical composition of the wood and the compounds used as flame-retardant agents interfere with its thermal stability. The wood of S. amara showed a peak before 100 °C resulting from moisture loss, resulting in an 8% reduction in the initial mass (Figure 5a). The second stage of mass loss due to thermal degradation commenced at 245 °C, peaking at 320 °C, with a loss of 49.37% of its initial mass. Between 400 °C and 500 °C, two degradation peaks emerged, one at 420 °C and the other at 440 °C, corresponding to a degradation of 43.37%. The residual mass at 500 °C was 6%.
Similar to wood, the tannin lost 11.26% of its mass in moisture up to 100 °C (Figure 5b). However, its degradation occurred across different temperature ranges. The first degradation peak occurred between 150–300 °C with a peak at 250 °C, with a loss of 30% of mass. A third peak was observed around 450 °C, with a mass loss in the range of 300–500 °C of 35%. The last peak of mass loss occurred between 500 °C and 600 °C, with a loss of 21% of mass. At 500 °C, the residual mass was around 37%.
The mixture of varnish and tannin altered the thermal stability, with only one peak of mass loss being observed with a peak temperature at 382 °C, and a residual mass of 15% at the end of its extension at 407 °C (Figure 5c). Beyond 500 °C, only 7% of the residual mass was identified. At around 600 °C, one curve can be observed from the TGA; this curve appears in some lignocellulosic materials, indicating that ignition occurred in this range.
The commercial flame retardant also showed a mass loss of 8% due to humidity up to 100 °C (Figure 5d). Three degradation peaks were observed as follows: the first degradation peak occurring at a lower temperature when compared to other materials, at 214 °C, but with a mass loss of 16% between the initial and final degradation range. The second peak occurred at 294 °C, resulting in a mass loss of 31% of the mass, while the third peak appeared at 495 °C, with a residual mass of 23.88%. The residual mass up to 500 °C was 30%, decreasing to 15% after 600 °C.
For pure varnish, two degradation peaks were observed (Figure 5e) as follows: the first at 349 °C was attributed to a 6% moisture loss in the sample, with no significant reaction occurring within this range. In the range of 260–420 °C, the mass loss resulting from the first peak was 77%, and, from 420–510 °C, it was 11%. The residual mass after 600 °C was only 6%.
In general, the mass loss behavior for the tannin treatment was consistent, regardless of the application method used (Figure 6). There was an improvement in mass loss from 300 °C to 500 °C. The immersion treatment preserved the most mass above 350 °C. Comparatively, the immersion treatment (IT) increased the residual mass by 10% at 320 °C, where the peak of greatest degradation occurred in the control sample. The samples from the brushing treatment (BT) showed a similar behavior to those from the immersion treatment (TI), preserving 61% of their mass at this temperature. All treatments increased the temperature at which the maximum mass loss occurred relative to the control sample, with the IT and BT treatments providing a 15 °C increase in stability.
For the commercial flame-retardant immersion treatment (IO) samples, the first degradation peak occurred at 250 °C, 70 °C below that of the control sample (Figure 7a). However, at 320 °C, this treatment preserved 50% of the residual mass, being greater than that observed for the untreated sample, which was approximately 35%. After this temperature, this treatment proved to be the most efficient for reserving mass, retaining around 27% at 600 °C. No additional mass loss peaks were observed in the DTG curve (Figure 7b).
In the case of vacuum treatment (VO), the mass loss was less stable when compared to the immersion treatment (IO) (Figure 8). The first mass loss peak occurred at 132 °C, followed by a second peak at 200 °C, and a third at 350 °C. Beyond 350 °C, this treatment proved more efficient than brushing (BO) and the control sample (CT) in preserving residual mass. For the BO samples, the behavior closely resembled that of the control sample in the first stage of mass loss up to 300 °C, becoming more efficient beyond this temperature, increasing the residual mass up to 600 °C. The temperature of the maximum mass loss was approximately 20 °C higher for the second degradation peak and 320 °C for the first peak.
Considering a high-temperature fire scenario, the immersion and vacuum treatments would be more effective in preserving wood, as, at 500 °C, they retained a residual mass equal to 34% and 24%, respectively, exceeding those of the brushing treatment (6%) and the control sample (1.50%).
The application of the tannin varnish (BTV) did not significantly change the thermal stability when compared to the control sample. A modest increase of 10 °C in the temperature of the maximum mass loss was observed for the first peak with the application of varnish alone, using the brushing method (BV). However, the effect of both surface treatments was minimal in improving the mass loss and maximum temperature when compared to the control sample (CT). It is observed that, at high temperatures (500 °C), the residual mass values were comparable across treatments.

3.4. Burning Test with Short Exposure to Flame

The treatments significantly modified the resistance of wood to burning (Table 3). The best treatments were the application of a commercial flame retardant through immersion (IO) and vacuum (VO) methods, effectively preventing wood ignition, flames, and embers. In contrast, when this product was applied through brushing (BO), ignition and flames occurred, but with short durations and without ember formation. While the tannin treatment proved less efficient than commercial flame retardants, they demonstrated efficacy in reducing ignition time when applied via immersion (IT) or brushing (BT), as well as in reducing flame duration across all application techniques, notably decreasing the immersion time (IT).
All treatments led to increased residual mass post-flame exposure (Table 4), including varnish applications via brushing (BV), which is a treatment without a fire retardant. Notably, treatments with commercial flame retardants exhibited the best preservation of wood mass during burning, with the immersion treatment (IO) being particularly noteworthy. The application of tannins contributed to an increased residual mass of the wood, with the immersion (IT), vacuum (VT), and varnish mixture (BTV) yielding the best results.
Visually, commercial flame-retardant impregnation via immersion (IO) and vacuum (VO) were the ones that best preserved the wood sample, followed by tannin applications through immersion (IT), vacuum (VT), and brushing (BT). The application of varnish (BV) and tannin with varnish (BTV) resulted in a more degraded appearance than the other treatments, but was still better than the control. The cracks observed on the surfaces did not go through the entire piece.
Upon the examination of the cross-sections of the pieces, the control treatment (CT) displayed internal darkening, indicating the initiation of combustion in the innermost parts of the wood. In this aspect, again, the best treatments were those that used the commercial flame retardant; however, the presence of tannin conferred advantages over the control treatment.

4. Discussion

4.1. Colorimetry

The L* value serves as an indicator of wood lightness or darkness [43]; wood with L* greater than 56 is considered light [44]. The wood of S. amara is very light, with an L* value of 81.37 on a scale ranging from 0 to 100. The only treatment that did not significantly darken the wood was the application of a commercial flame retardant via vacuum (VO), whereas only the combination of varnish with tannin (BTV) no longer met the criterion for lightness (L* < 56).
As observed in the present study, it is expected that tannin reddens the wood [45], due to the presence of molecules with chromophore bonds which are responsible for light absorption, a characteristic common in phenolic compounds [46]. Specifically, tannins stabilize in red-orange colors, contributing to the observed reddening effect [47].
Saturation (C*) is influenced by the coordinates a* and b*, with both variables contributing to increased saturation values [43]. In all treatments, wood pigmentation was evident. However, the presence of tannins exerted a more pronounced influence on saturation due to its intense redness-inducing properties. Remarkably, the only treatment that did not result in a reduction in the hue angle (h*) was the application of brush varnish (BV), indicating that all products darkened the wood [48]. These results align with the observed variation in L*.
Total color variation values (ΔE) between three and six and between six and twelve are considered noticeable and very noticeable to human vision, respectively [44]. In this study, all color changes induced by the treatments, even the most subtle ones, were noticeable to wood users. However, new colors are still typical of wood materials, with no instances of bluing or greening identified [44].

4.2. Mass Retention

Tannin retention in the wood was minimal (2.57% for IT, 2.47% for TV, and 0.71% for BT) due to the ease of leaching caused by the high solubility of tannins in water, which damaged the reaction of tannin oligomers with the wood cell wall [49]. However, the scenario changed with the mixture of the water-based varnish (BTV). Although it altered the retention scenario, it was not a pure application of tannin, and the presence of varnish influenced other evaluations of the wood.
The high mass retention provided by the commercial flame retardants can be attributed to two factors as follows: firstly, the presence of catalyst compounds, such as boric acid [50], which allow the adhesion of substances to the cell wall [50]; and secondly, the high concentration of the solution used [51], which is very common in the production of commercial fire retardants for wood [52]. With the intention of the retention of any products in the wood, the solution of that product must be impregnated into the wood efficiently. Impregnation depends on the physical and anatomical characteristics of the wood and on the physical properties of the solution, such as the surface tension and viscosity [53]. In the present study, in which there is standardization in the wood samples, the differences in impregnation and mass retention are due to the intrinsic characteristics of the products used.

4.3. Thermal Stability

Due to its specific aromatic structure, tannins have high chemical and thermal stability above 300 °C, as well as low thermal conductivity [54]. Natural fire resistance may be conferred to aid tree survival using tannins, and this can be attributed mainly to their reactivity similar to that of phenols, since phenoxy radicals can quench free oxygen radicals when the polymer is broken down during heating [55].
Basically, the thermal breakdown of tannic acid can be categorized into three distinct phases. In the first phase, occurring between 20 °C and 182 °C, there is an increase in the rate of the thermal decomposition of tannic acid from 100 °C onwards, due to the decomposition of the volatile components [19,56]. In the temperature range between 182 and 328 °C, there is a significant loss of mass where the outer and inner benzene rings of tannic acid are converted into 3,4,5-trihydroxybenzoic acid (product 1) and 3,4-dihydroxybenzoic acid (product 2) [19]. Product 1, because it has more electron-absorbing groups, decarboxylates more easily, resulting in the production of CO2 and catechol in the appropriate temperature zone. In the third stage, at temperatures between 328 and 700 °C, the central glucose molecules are broken down by the COC, producing CO and CH4 [19,56]. Product 2 undergoes gradual decarboxylation, releasing CO2 and generating catechol. At high temperatures, catechol is deoxidized to H2O, while the rest of the benzene ring and its branched chains crosslink, promoting the formation of carbon [56].
The stability of the varnish after the addition of tannins resulted in intermediate stability between the components of the mixture. In contrast, the commercial flame retardant, characterized by its more crystalline structure, owing to its composition of salts, exhibited a greater stability and ash content at the end of the test. These inherent differences in the structures and chemical natures of these materials justify the observed variations in behavior [57].
The thermogravimetric curves observed post-treatments exhibit characteristic curves of lignocellulosic materials, consistent with those observed in the control sample. These curves indicate the decomposition of hemicellulose, cellulose, and lignin within a relatively narrow temperature range, partially overlapping [58]. Each wood macro component exhibited distinct pyrolysis behavior [57] as follows: The first to be degraded is hemicellulose, with pyrolysis occurring from 220–315 °C [57]; 197–380 °C [59,60]. Cellulose undergoes thermal degradation from 250–427 °C [57,59,60]. Lignin, on the other hand, has a wider range of thermal degradation ranging from150–900 °C, but is the most thermally stable component [57,59,60].
The DTG curves offer insights into the behavior of lignocellulosic materials, namely dehydration, active burning, and passive burning. In the first stage, the absorbed water is lost [61]. In the second stage (active pyrolysis), two peaks corresponding to the decomposition of cellulose and hemicellulose were observed [59]. Lignin decomposition occurs during both active and passive pyrolysis. The wide range of lignin decomposition temperatures did not allow the appearance of a characteristic peak, attributable to this component in the DTG curves of the wood [57,62]. The commercial flame retardant modified the behavior the most; it was possible to identify only one peak for the immersion treatment, attributed to the reactivity of the components with wood. Commercial flame retardants work on the three following fronts: providing thermal insulation to wood, absorbing surrounding heat through endothermic reactions, and increasing the thermal conductivity of wood to dissipate heat from the wood surface [63]. In essence, wood pyrolysis aims to produce charcoal and water, which are less volatile and serve as a blanket to prevent the release of flammable vapors and access to oxygen [8].
Among the methods employed, the highest efficiency was achieved through immersion and vacuum, likely due to the impregnation of the material, resulting in the highest percentages of mass retention among all the treatments evaluated. When impregnation occurred, the structure of the wood was viewed as similar to that of a sponge with cell cavities and cell walls. The immersion method allows the components to react with the cell wall, whereas the vacuum method causes the products to migrate to empty spaces and gradually infiltrate the cell wall. By removing air from the cavities, the vacuum treatment creates space for the fire-retardant solution, which is then forced deep into the wood under high pressure. In contrast, the application of varnish as a protective layer resulted in minimal penetration of the product throughout the sample [8]. The varnish layer was thin, failing to permeate the entire sample. Consequently, there was no thermal effect on the entire sample, as it was ground and homogenized for the analysis. Superficial treatments, such as paints, are often favored for their ease of application and comparatively low material requirements for fire protection [8].
The interaction of tannin with wood, explained via the use of the hydrogen bond assembly system, has been widely adopted in the construction of tannic assemblies due to its strong cohesion, simple synthesis process, flexibility for structural modifications, and predictable recognizability [15,63]. However, it is important to note that hydrogen bonding is an intermolecular force, and the materials resulting from this association often have poor thermal stability. To improve the thermostability of the matrix, it is possible to combine the hydrogen bonding system with highly efficient flame retardants [19,63].

4.4. Burning Test with Short Exposure to Flame

The expected ignition time for wood in this test is 9–15 s [21,49]. Treatments employing a commercial fire retardant with higher retentions rates (IO and VO) did not result in ignition (ignition time > 120 s). Conventionally, tannin alone does not suffice to prevent wood from igniting when exposed to heat [21,49]; this outcome is typically achieved only when tannin is combined with an additive [21].
When compared to untreated wood, the presence of tannin significantly reduced the flame time. The impregnated tannin (IT) treatment, in particular, exhibited exceptional efficacy, reducing the burning time by more than 90%. During combustion, oxidants and free radicals are formed, which are responsible for the decomposition of organic materials and the expansion of flames [64]. In addition to being poor heat conductors [65], tannins efficiently capture these oxidants and free radicals when exposed to fire [65,66], thus slowing the burning process and minimizing damage.
In the short flame exposure test, the residual mass of untreated wood (CT) was below 30%. However, treatments involving tannin application via immersion (IT) or in combination with varnish (BTV) was quite efficient, retaining over 50% of the wood mass. The presence of tannin facilitates the formation of char, which can form a protective layer that blocks heat transfer and the exchange of gases, such as oxygen and other flammable gases [55]. The greater the residual mass, the greater the conservation of the mechanical resistance of the wood, a critical factor in wooden construction to maintain the structure and ensure safety for firefighters attempting to control fires [66].
The surface of the wood that was in direct contact with the flame exhibited cracks; however, these cracks did not pass through the entire piece of wood and were restricted to the charred fraction. The pieces treated with the commercial flame retardant were the only ones that had their edges maintained; the outermost portions were consumed by fire, and white spots appeared in these samples, indicating the presence of ash, and thus signifying complete combustion [11], mainly in the control treatment (CT). Additionally, intense carbonization in untreated wood was evidenced by the darkening of the innermost portion of the wood, indicating unrestricted heat propagation.
Although the tannin method is efficient, its performance does not match that of a commercial flame retardant, which has demonstrated capabilities to prevent ignition, flame formation, and ember generation. The inclusion of varnish reduced the burning time and increased the residual mass.
Previous works show the effect of tannin as a fire retardant. The intumescent flame-retardant system is widely employed and typically comprises acid, carbon, and air sources [67]. The presence of carbon sources depends on both the carbon content and the number of active hydroxyl groups. Tannic proves to be a suitable carbon source for incorporation into intumescent flame-retardant systems. Flame retardants based on tannic acid have demonstrated their effectiveness in reducing flammability in plastics like polyurethanes, commonly used in construction and packaging. For instance, Xinyi Chen et al. [18] successfully developed self-expanding non-isocyanate polyurethanes based on glucose, utilizing condensed tannin as flame retardants. Tannins exhibit a high efficiency in producing char during combustion, with condensed tannins yielding 55% char and tannic acid yielding 28% char, thereby forming a protective layer which is capable of blocking heat, oxygen, and combustible gases [65]. Intumescent flame-retardant systems find applications in refractory coatings, extensively employed for safeguarding metal structures and, notably, wood substrates [68].
Tannin-boron wood preservatives intended for outdoor use exhibit notable mechanical strength and fire resistance. Their research highlighted that incorporating tannin as a flame retardant yielded consistently positive results across various fire tests. The inclusion of mimosa tannin in wood conferred a broad spectrum of beneficial flame-retardant effects. Overall, the ignition and flame duration were significantly reduced, and the weight loss was slower under continuous exposure to fire [21].

5. Conclusions

The utilization of tannins as a fire retardant causes significant changes in the color of wood, potentially influencing the perception of wood users; however, these color changes remain within the expected range for wood material.
Tannin was not expected to be superior to a commercial fire retardant, given its inability to completely prevent wood ignition. However, the presence of tannin reduced the spread of fire in burning conditions with short exposure to flame.
The concentration of the fire retardant influenced its performance. While tannins exhibit inferior mass retention when compared to commercial products, its efficiency for this purpose can be improved through strategies that increase its retention within the wood.
The highest thermal stability was achieved through wood immersion in the commercial flame retardant, followed by vacuum impregnation with the same product. Additionally, the presence of tannin enhances the thermal stability of wood in the temperature range of 300–500 °C. The brushing method was the least effective for improving the stability via thermogravimetry, likely due to the superficial treatment, and, when it went through the grinding process, the protective layer was broken.

Author Contributions

Conceptualization, E.A.B.J. and E.C.L.; methodology, E.A.B.J. and E.C.L.; software, E.C.L.; validation, E.A.B.J. and E.C.L.; formal analysis, J.M.R.d.B., G.F.S. and E.C.L.; investigation, J.M.R.d.B., G.F.S. and E.C.L.; resources, E.C.L.; writing—original draft preparation, E.A.B.J. and E.C.L.; writing—review and editing, E.A.B.J. and E.C.L.; visualization, E.C.L.; supervision, E.C.L.; project administration, E.C.L.; funding acquisition, E.C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior–Brasil (CAPES)–Finance Code 001.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors are grateful to Madeira Zanatta for donating the wood, to Montana Química Ltda for donating the flame retardant (Osmoguard FR100) and the wood coating (Nobile Lasur Incolor), and to TANAC SA for donating the tannin (TANFLOC SG 1500).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Wood sample used in the burning test with short exposure to flame.
Figure 1. Wood sample used in the burning test with short exposure to flame.
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Figure 2. Burning test with short exposure to flame. (a) Moment at which flame ignition begins (ignition Time). (b) Sample continues burning after the Bunsen burner flame stops (flame Time). (c) Presence of incandescence indicating that the sample is still burning (ember time).
Figure 2. Burning test with short exposure to flame. (a) Moment at which flame ignition begins (ignition Time). (b) Sample continues burning after the Bunsen burner flame stops (flame Time). (c) Presence of incandescence indicating that the sample is still burning (ember time).
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Figure 3. Wood samples and total color variation in wood. Numerical values represent the total color variation (ΔE).
Figure 3. Wood samples and total color variation in wood. Numerical values represent the total color variation (ΔE).
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Figure 4. Mass retention (%) in treated samples of S. amara wood. Same letters in the bars indicate no statistical differences according to the Tukey test at 95% probability, and the range bars represent the standard deviation.
Figure 4. Mass retention (%) in treated samples of S. amara wood. Same letters in the bars indicate no statistical differences according to the Tukey test at 95% probability, and the range bars represent the standard deviation.
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Figure 5. The thermogravimetric analysis (TGA) and derivative of the thermogravimetric curve (DTG) of the wood and products used. (a) S. amara wood; (b) Tannin; (c) Tannin + Varnish; (d) Commercial flame retardant, and (e) Varnish.
Figure 5. The thermogravimetric analysis (TGA) and derivative of the thermogravimetric curve (DTG) of the wood and products used. (a) S. amara wood; (b) Tannin; (c) Tannin + Varnish; (d) Commercial flame retardant, and (e) Varnish.
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Figure 6. (a) TGA of tannin treatments (IT, VT, and BT) and control (CT) with increasing temperature; (b) DTG of tannin treatments (IT, VT, and BT) and control (CT) with increasing temperature.
Figure 6. (a) TGA of tannin treatments (IT, VT, and BT) and control (CT) with increasing temperature; (b) DTG of tannin treatments (IT, VT, and BT) and control (CT) with increasing temperature.
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Figure 7. (a) TGA of commercial flame-retardant treatments (IO, VO, and BO) and control (CT) with increasing temperature; (b) DTG of commercial flame-retardant treatments (IO, VO, and BO) and control (CT) with increasing temperature.
Figure 7. (a) TGA of commercial flame-retardant treatments (IO, VO, and BO) and control (CT) with increasing temperature; (b) DTG of commercial flame-retardant treatments (IO, VO, and BO) and control (CT) with increasing temperature.
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Figure 8. (a) TGA of varnish treatments (BV and BTV) and control (CT) with increasing temperature; (b) DTG of varnish treatments (BV and BTV) and control (CT) with increasing temperature.
Figure 8. (a) TGA of varnish treatments (BV and BTV) and control (CT) with increasing temperature; (b) DTG of varnish treatments (BV and BTV) and control (CT) with increasing temperature.
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Table 1. Techniques for applying fire retardants to wood.
Table 1. Techniques for applying fire retardants to wood.
MaterialApplication TechniqueAcronym
Tannin48 h of immersionIT
Tannin2 h of immersion with vacuumVT
Tanninbrushing in three coatsBT
Commercial flame retardant48 h of immersionIO
Commercial flame retardant2 h of immersion with vacuumVO
Commercial flame retardantbrushing in three coatsBO
Tannin + Varnishbrushing in three coatsBTV
Varnishbrushing in three coatsBV
Control-CT
Table 2. Mean values of colorimetric parameters.
Table 2. Mean values of colorimetric parameters.
TreatmentL*a*b*C*h*
IT61.96 f (0.61)6.94 b (0.49)19.38 d (0.49)20.59 e (0.60)70.30° cd (1.11)
VT65.39 e (1.24)6.98 b (0.31)21.22 c (0.61)22.35 d (0.65)71.78° c (0.92)
BT62.80 f (1.04)8.36 a (0.32)22.47 b (0.40)23.98 b (0.31)69.58° d (0.95)
IO79.59 c (0.65)2.67 c (0.20)24.65 a (1.30)24.80 b (0.61)83.79° b (0.64)
VO80.79 ab (0.51)2.52 c (0.18)23.10 b (0.39)23.24 c (0.39)83.78° b (0.47)
BO80.57 b (0.33)2.56 c (0.14)21.71 c (0.27)21.86 de (0.25)83.28° b (0.42)
BTV54.98 g (0.94)12.78 a (0.30)23.25 b (0.58)26.53 a (0.42)61.18° e (1.10)
BV78.26 d (0.51)2.01 d (0.18)24.44 a (0.44)24.52 b (0.43)85.29° a (0.46)
CT81.37 a (0.40)1.86 d (0.15)18.89 d (0.40)18.98 f (0.40)84.38° a (0.47)
Means followed by the same letters in the same column indicate no statistical differences according to the Tukey test at 95% probability. Values in parentheses refer to the coefficient of variation in percent.
Table 3. Mean values of ignition time, flame time, and ember time.
Table 3. Mean values of ignition time, flame time, and ember time.
TreatmentIgnition Time (s)Flame Time (s)Ember Time (s)
IT16.56 b (3.20)119.49 b (8.46)68.98 c (6.89)
VT11.54 c (1.61)152.13 b (4.63)276.21 b (7.32)
BT15.95 b (2.84)161.77 b (4.18)285.09 b (3.74)
IO>120.0000
VO>120.0000
BO34.56 a (8.51)27.21 c (2.42)0
BTV9.97 c (2.71)151.15 b (4.43)411.01 b (4.46)
BV12.39 c (2.64)194.93 ab (4.92)357.85 b (7.58)
CT10.58 c (2.59)209.41 a (3.97)698.63 a (2.33)
Means followed by the same letters in the same column indicate no statistical differences according to the Tukey test at 95% probability. Values in parentheses refer to the coefficient of variation in percent.
Table 4. Mean values of residual mass following the burning test.
Table 4. Mean values of residual mass following the burning test.
TreatmentR (%)Sample after BurningCross-Section
IT57.25 cd (1.51)Forests 15 00951 i001Forests 15 00951 i002
VT45.87 de (2.58)Forests 15 00951 i003Forests 15 00951 i004
BT41.58 e (2.43)Forests 15 00951 i005Forests 15 00951 i006
IO82.02 a (2.58)Forests 15 00951 i007Forests 15 00951 i008
VO78.17 ab (1.45)Forests 15 00951 i009Forests 15 00951 i010
BO70.66 bc (1.73)Forests 15 00951 i011Forests 15 00951 i012
BTV52.52 de (2.64)Forests 15 00951 i013Forests 15 00951 i014
BV45.23 e (6.96)Forests 15 00951 i015Forests 15 00951 i016
CT28.48 f (7.04)Forests 15 00951 i017Forests 15 00951 i018
Means followed by the same letters in the same column indicate no statistical differences according to the Tukey test at 95% probability. Values in parentheses refer to the coefficient of variation in percent.
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Bonfatti Júnior, E.A.; de Barros, J.M.R.; Silva, G.F.; Lengowski, E.C. A Comparative Analysis of Tannin and Commercial Fire Retardants in Wood Fire Protection. Forests 2024, 15, 951. https://doi.org/10.3390/f15060951

AMA Style

Bonfatti Júnior EA, de Barros JMR, Silva GF, Lengowski EC. A Comparative Analysis of Tannin and Commercial Fire Retardants in Wood Fire Protection. Forests. 2024; 15(6):951. https://doi.org/10.3390/f15060951

Chicago/Turabian Style

Bonfatti Júnior, Eraldo Antonio, Jeinna Michelly Rodrigues de Barros, Gustavo Felizardo Silva, and Elaine Cristina Lengowski. 2024. "A Comparative Analysis of Tannin and Commercial Fire Retardants in Wood Fire Protection" Forests 15, no. 6: 951. https://doi.org/10.3390/f15060951

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