New Sustainable Intumescent Coating Based on Polyphenols Obtained from Wood Industry Waste

Abstract

The global proliferation of Pinus radiata, known for its rapid growth and wood density, has led to an environmental challenge—significant waste production, especially bark, without a clear valorization route. This waste poses ecological concerns, and despite the crucial role of forest resources in structural applications, their limited fire resistance requires the use of coatings. However, traditional coatings lack an eco-friendly footprint. Addressing this challenge, this study aims to develop an intumescent coating with tannins extracted from waste bark, offering a sustainable alternative. This not only repurposes waste on a global scale but also aligns with the imperative for environmentally friendly materials, contributing to sustainable practices in the construction and wood treatment industry. This study achieved an eco-friendly FRR15 (fire resistance ratio 15) fire resistance classification with a 15% equivalence of low-molecular-weight tannins, presenting a sustainable alternative to commercial products. Characterization showed low-molecular-weight tannins comparable to conventional charring agents, with high hydroxyl content and oil absorption, while high-molecular-weight tannins exhibited lower viability. A reference coating achieved FRR30 fire resistance, aligning with commercial strength. The mechanical properties of tannin-based coatings matched commercial standards, with increased abrasion resistance and adhesion and decreased flexibility. Intumescent coatings with higher tannin content significantly reduced wood substrate charring and mass loss in flame response assessments.

1. Introduction

Structural steel is widely used in construction due to its high strength-to-weight ratio and ductility. However, its mechanical resistance decreases above 500 °C in a fire, necessitating improvements [1,2]. Technological solutions, mainly foams and coatings, have been proposed [3,4,5]. Fire-resistant coatings, including fire retardant and intumescent types, protect metal structures and wood substrates [6,7]. These coatings consist of a carbon-forming material, a mineral acid catalyst, a blowing agent, and a binder resin. During combustion, these components form a honeycomb carbon structure that insulates the steel and creates a protective barrier. In wood, the coating reduces pyrolysis gases and retains more fuel as insulating char [8,9]. The protection mechanism in fireproof coatings slows flame spread, while intumescent coatings release NH₃ and organic acids during combustion, triggering an esterification reaction. This leads to carbonization and the formation of an intumescent foam or layer. Developing ecological and sustainable fire-resistant coatings is a pressing challenge. Natural source products are gaining attention for their potential to create insulating carbon layers on substrates [9,10].

The carbon source of an intumescent coating is essential for the formation of the carbonaceous layer and the coating’s performance in protecting the metal substrate from fire. Currently, non-renewable sources are the primary raw materials used as carbon sources; however, their processing is harmful to the environment, making it necessary to study and develop new, more ecologically sound alternatives [11,12,13]. Cellulose and lignin are the most abundant natural resources, with lignin being the only renewable aromatic resource found in nature that can enhance flame-retardant behavior [14,15]. In response to the eco-friendly challenge of coating development, materials like tannins emerged, capable of being extracted from natural sources, such as tree bark, currently considered waste [16]. This strategy allows not only the utilization of abundant waste, such as bark, but also the creation of environmentally friendly coatings. The valorization of these wastes contributes to mitigating the ecological footprint and represents a step towards more sustainable practices in the construction and wood treatment industry.

Tannins are non-toxic, economical, and abundant polyphenolic oligomers found in biomass. Among all types of tannins (hydrolyzable, complex, and condensed), condensed tannins represent 90% of global production. These tannins exhibit high chemical and thermal stability, as well as low thermal conductivity, due to their specific aromatic structure, making them suitable for the preparation of thermal insulating and flame-retardant materials for various applications. Due to their non-toxic, plant-based, and water-soluble characteristics, condensed tannins are a more environmentally friendly and acceptable option compared to synthetic flame retardants [17]. The global availability of Pinus radiata is increasing due to its short juvenile period and the brief interval between seed harvests, which promotes rapid population growth [18,19]. Its seeds are highly adaptable to wind dispersal, resulting in a notable capacity for propagation and a global presence in timber plantations. Despite being considered a significant producer of waste [20], its rapid growth and moderate wood density make it a highly attractive resource [21]. However, its expansion poses a significant environmental challenge due to the substantial production of waste, particularly in the form of bark [22]. An example of this reality can be observed in Chile, where 60% of the surface area of native vegetation resources is occupied by Pinus radiata, primarily for cellulose production, which necessitates the extraction of bark. Unfortunately, the bark currently lacks a clear valorization route, becoming a waste with no significant purpose and generating thousands of tons annually [9,19,22,23].

The literature reports several studies on the use of tannins in intumescent coatings, particularly highlighting the work of M.R. da Silveira et al. [24]. In their research, they utilized black wattle tannin as a carbon source in the formulation of these coatings. They incorporated concentrations of 5% and 10% tannin into a novolac resin. Subsequently, the coating was applied to a steel plate, and thermal protection was evaluated by exposing the sample to a flame for 30 min. The results indicated that the tannin compound could be effectively used as a carbon source for intumescent coatings, as the temperature of the samples containing 10% tannin was nearly 300 °C lower compared to the uncoated steel plate.

Additionally, literature reviews [25] are presented that analyze the application of tannic acid as a flame retardant in various materials, including polymers, textiles, and wood products. These reviews highlight the effectiveness of tannic acid in enhancing fire resistance and its biodegradability. In this context, the mechanisms of action of tannic acid are discussed, including the formation of a carbonaceous layer during combustion, which acts as a barrier to limit the spread of fire. Furthermore, the advantages of using tannic acid compared to synthetic flame retardants are analyzed, including its lower toxicity and reduced environmental impact. However, the challenges in implementing tannic acid as a flame retardant are also addressed, such as its cost and the need to improve its effectiveness in certain applications.

In the previously described context, the main objective of this study is to develop an intumescent coating formulation that incorporates tannins as a carbonizing agent and natural additive, thereby valorizing waste derived from Pinus radiata on a global scale. Additionally, a comprehensive analysis of the mechanical properties and fire resistance will be conducted in accordance with international standards. The novelty of this work lies in the innovative use of tannins, both for their carbonization properties and for their potential to repurpose pine waste worldwide.

2. Materials and Methods

2.1. Extraction and Characterization of High (TH)- and Low (TL)-Molecular-Weight Tannins

TH and TL from Pinus radiata were used, which were extracted from bark waste according to the procedure described by Montoya et al. [26]. The tannins were obtained by liquid–liquid extraction using polar solvents. For this, the dried and ground Pinus radiata bark was placed in contact with a 3:1 (v/v) ethanol/water mixture for 2 h at 120 °C. Subsequently, the volatile solvent was removed by room temperature evaporation under 5.0 kPa using a vacuum system and dried by lyophilization. To determine the composition of the extracts, the Reverse-Phase High-Performance Liquid Chromatography (RP-HPLC) (Agilent Technologies 1100, Palo Alto, CA, USA) technique was employed, utilizing a Diode Array Detector (DAD) and a mass spectrometer (MS). The analysis time and the gradient of the mobile phase were adjusted to suit both extracts. An RP-HPLC-DAD-MS system equipped with a binary pump and a Kinetex™ C18 core–shell column (100 mm × 4.6 mm × 2.6 µm) was used. Initially, 10 µL of the sample was filtered through a 15 mm Phenex-RC injection filter. Then, 2 mL were injected in triplicate using a mobile phase of 1% acetic acid (phase A) and acetonitrile (phase B) at a flow rate of 0.8 mL/min. The mobile phase was scheduled according to the author’s methodology.

The compounds in the extracts were identified using UV and MS data and quantified with a DAD detector. Quantification was performed by comparing calibration curves of catechin (0.10–1.00 g/L) and taxifolin (0.06–1.00 g/L).

Tannin samples (TH) obtained from Pinus radiata bark were characterized for hydroxyl groups following the requirements of ASTM D4274-21. To achieve this, a composite sample was acetylated at a ratio of 2.4 mL acetic anhydride to 17.6 mL pyridine and 1 g of high- and low-molecular-weight tannins. Twenty milliliters of this solution were used in Erlenmeyer flasks and heated to 98 °C for at least 2 h. Subsequently, the sample was allowed to cool to room temperature for a period of 24 h. During characterization, 1 mL of phenolphthalein indicator was added to the acetylation agent solution, and titration was initiated with 0.5 N NaOH solution, ensuring constant agitation. Titration should be continued until the first color change to pink occurs, with maintenance for at least 15 s. The concentration of hydroxyl groups is calculated from the volume difference using titration experiments with samples of the formulation containing tannins and blanks, employing Equation (1).

$$\mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{l} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}={\displaystyle \frac{\left[\left(\mathrm{B}-\mathrm{A}\right)\mathrm{N}\times 56.1\right]}{\mathrm{W}}}$$

(1)

where A corresponds to the NaOH required for the titration of the sample in mL, B corresponds to the NaOH required for the titration of the blank in mL, N is the NaOH normality, and W corresponds to the amount of sample used in grams.

2.2. Intumescent Coating Formulation

To assess the performance of tannin in intumescent coatings, seven formulations were prepared. Five of these were designed to evaluate tannin’s potential as a substitute for a carbonizing agent due to its polyphenolic properties, high carbon atom content, and esterification capability. On the other hand, the remaining two formulations were used to examine the feasibility of tannin as an additive to enhance the mechanical properties of the paint while simultaneously serving as a supplementary carbonizing agent. The assessment of tannin as a carbonizing agent was conducted by utilizing the equivalence ratio between the reactive hydroxyl groups of the carbonizing agent pentaerythritol and tannin. This ratio was used to establish the substitution proportion between both compounds, adjusting the formulation to maintain an appropriate relationship between the reactive hydroxyl groups present in the formulation and the other components of the intumescent chemical reaction. Using this method, it was determined that 1.82 g of low-molecular-weight tannin is equivalent to 1 g of pentaerythritol. Evaluations were carried out considering five different percentages of tannin substitution, which were 0%, 15%, 35%, 65%, and 90%. The formulation with 0% tannins was considered the reference formulation.

The formulations with tannin as an additive were developed by keeping the percentages of the compounds involved in the intumescent chemical reaction the same as those in the reference formulation. Tannin was then added at fixed percentages of 5% and 10% of the total weight, determined based on preliminary results from flame tests, which indicated improved intumescent responses at lower levels of tannin. The compositional breakdown by weight of the various components comprising the intumescent coating formulations can be observed in

Table 1. Intumescent formulations based on low-molecular-weight tannin.

Raw Material (%w/w)	Ref.	15% Eq.	35% Eq.	65% Eq.	90% Eq.	5% Addit.	10% Addit.
Water	17.6	17.8	17.5	17.2	17.0	17.0	16.7
Sodium polyacrylate	1.5	1.5	1.5	1.5	1.5	1.5	1.5
Dowanol DPBN	1.6	1.6	1.6	1.6	1.6	1.6	1.6
Technical Propylene Glycol	1.3	1.3	1.3	1.3	1.3	1.3	1.3
Proclear 756	0.2	0.2	0.2	0.2	0.2	0.2	0.2
Soluble Tannin	0.0	2.7	6.9	10.7	14.3	5.0	10
Titanium Dioxide	7.8	7.8	7.8	7.8	7.8	7.8	7.8
Nonyl Phenol	1.5	1.7	2.0	2.3	2.5	2.0	2.3
Pentaerythritol	10	8.4	5.7	3.2	0.9	9.0	7.8
Melamine	9.2	9.0	8.6	8.3	7.9	8.1	7.1
Monoammonium Phosphate	25.6	25.2	24.1	23.1	22.2	22.7	19.9
Mowilith DM 230	23.4	22.6	22.6	22.6	22.6	23.4	23.4
Proxel 109	0.2	0.2	0.2	0.2	0.2	0.2	0.2
Total	100	100	100	100	100	100	100

.

The oil absorption capacity per pigment was determined using the Gardner–Coleman method associated with ASTM D1483-12 standards. For this, 10 g of pigment was weighed in a beaker, and oil was added from a burette at a rate of one drop per second while continuously folding the pigment with a spatula during the addition. This process was continued until an excess of oil was observed on the beaker’s wall. Subsequently, the absorption percentage was calculated using Equation (2):

$$\mathrm{A}={\displaystyle \frac{\mathrm{M}\times 0.93}{\mathrm{P}}}\times 100$$

(2)

where A is the percentage of oil absorption, M corresponds to the millimeters of oil consumed, and P is the mass of pigment in grams.

2.3. Fire-Resistant Behavior

The comparison process among coatings formulated with tannins extracted from Pinus radiata followed the guidelines outlined in ASTM D3806-19a. This approach yielded insights into the protection the coating provides to its substrate and comparative burning characteristics by assessing flame spread on the surface under controlled conditions in an inclined tunnel with a deposition surface of 645 mm× 245 mm, inclined at 28°, and elevated by 185 mm. Tests were conducted over 240 s intervals with direct flame exposure using a burner on the surface protected with the intumescent coating, placed at a distance of 57 mm. Flame progression was recorded at 10 s intervals. Finally, measurements of mass, intumescence dimensions, and carbonization were taken at the conclusion of each experiment. The inclined tunnel is based on ASTM D3806-19 standards. The comparison of formulated paints on wood substrate involved tests based on ASTM E119-20 standards. The flame response was characterized under controlled conditions by exposing a substrate consisting of 9 mm thick plywood and 1 mm thick galvanized steel typical of construction. The intumescent coating formulated was applied to one side of each substrate.

Based on the above, the temperature of the non-exposed face was evaluated under a limit of 190 °C, flame tightness, and the emission of flammable gases. Control of the contact temperature with the coating was carried out using a muffle furnace, maintaining a constant temperature of 700 °C for 15 min on substrate surfaces measuring 170 mm × 170 mm. The criterion for the temperature of the non-exposed face was measured using a UNI-T model PRO UT309 infrared thermometer located on the non-exposed side to record temperature on the substrate.

2.4. Evaluation of the Mechanical Properties of the Coatings

The resistance to abrasion of the organic coatings was evaluated using the Taber 5135 Abraser (Taber Industries, North Tonawanda, NY, USA) following ASTM 4060-19 standards for this test, a uniform layer of intumescent coating was applied to metal specimens measuring 100 mm × 100 mm with a 6.5 mm diameter hole in the center. The mass and thickness of the specimens were calculated at four different points before each test, and the mean of these thicknesses was used as the reference value. During the test, the test heads of the Taber 5135 Abraser equipment were rotated over the surface with a 1 Kg load at both ends until paint wear was observed or 1000 cycles were reached. The mass and thickness were measured every 250 cycles. At the end of the test, the mass loss was measured to assess the resistance of the organic coating to abrasion.

The study of adhesion resistance of the formulated coatings was conducted according to ASTM 4541-22 standards, using a portable PosiTest AT-A DeFelsko adhesion tester (Defelsko Corporation, Ogdensburg, NY, USA), and the coating was taken to detachment by applying a normal load to the surface. This way, the plane of the limit resistance within the system was determined. Additionally, the manner in which fracture occurs in the material plane was analyzed, considering the percentage of failures that occurred in the adhesion and cohesion of the coating, as well as in the interfaces and layers involved in the process. Fractures were classified as cohesive when occurring within a coating layer, adhesive when taking place in the contact zone between layers, and cohesion failure when there is a visible separation of the adhesive within the layer.

Flexibility tests were conducted following ASTM D522/522IM-17 standards (Elcometer 108, Worcestershire, UK). The coated samples underwent bending using a conical mandrel, and the cracking or detachment of the coating was analyzed. The crack length after the test was considered as the cracking resistance value. Ball impact tests were conducted on metal plates following ASTM E643-15 standards (TABER 5135 Friction Tester, Taber Industries, North Tonawanda, NY, USA), thereby assessing the resistance to detachment or cracking when gradually deformed under biaxial forces. This was achieved by using a ball punch on the substrate surface opposite to the coating, generating deformation until the coating ruptured. This determined the material’s ability to withstand deformation without experiencing detachment or cracking. All the aforementioned tests were performed in triplicate. For analysis, the mean value was used, which is the result shown in each table.

3. Results

3.1. Characterization of the Tannin Extracts Obtained from the Bark of Pinus radiata

The results obtained from the acetylation and potentiometric titration process can be observed in

Table 2. Summary of hydroxyl group concentration results.

Sample	Number of Hydroxide
Ref	-
LMW Tannin	558.20
HMW Tannin	224.20
Pentaerythritol	1014.0

Note: low molecular weight (LMW); high molecular weight (HMW).

.

Through the obtained results, the relationship between the number of equivalent hydroxides present in pentaerythritol, low-molecular-weight tannins, and high-molecular-weight tannins was determined. These results are presented in

Table 3. Equivalence ratio of hydroxyl number pentaerythritol–tannins.

Pentaerythritol/HMW Tannin ratio	4.52
Pentaerythritol/LMW Tannin ratio	1.82

. On the other hand, the phenol composition and content of the Pinus radiata bark extract, along with the average molecular weight (Mn) of pine extracts at various sample development percentages, as determined by GPC, are presented in Tables S1 and S2.

The results presented in Table 3 indicate the quantity of tannins needed to achieve the same charring effect as pentaerythritol. These results are significantly valuable for the industry in adapting to the ISO 14001 standard by incorporating the concept of eco-design governed by ISO 14006 and reducing waste through the optimization of the life cycle of forest products following the guidelines of ISO 14044. In this way, there is a positive impact on industries seeking sustainable and environmentally friendly alternatives in the formulation and manufacturing of intumescent coatings. The results associated with the oil absorption percentage for low and high-molecular-weight tannins are presented in

Table 4. Summary of oil absorption results in tannins.

Materials	Sample Mass (g)	Oil Absorption (%)
LMW Tannin	1.25	109.0
HMW Tannin	1.02	55.0

. The results indicate that tannins have a high oil absorption capacity.

3.2. Evaluation of the Coating Properties

The mechanical response of the formulated coatings to adhesion can be observed in

Table 5. Summary of results obtained in the adhesion test.

Formulation	Maximum Stress (MPa)	Failure Type	% Fracture
Ref.	0.50	Cohesive	80
15% Eq.	0.52	Cohesive	75
35% Eq.	0.57	Cohesive	91
65% Eq.	0.82	Cohesive	90
90% Eq.	0.84	Cohesive	81
5% Addit.	0.49	Cohesive	80
10% Addit.	0.63	Cohesive	76
Firewall 200	0.50	Cohesive	60
Ak 7000	0.79	Cohesive	80

. The results indicate that all fracture failures occurred as cohesive, which is not ideal for coatings. This failure happens when the paint layer separates or breaks internally due to weakness in the bond between coating material molecules or particles. This indicates that the coating’s cohesion is deficient because the components composing the film are not properly bonded, leading to peeling or detachment issues on the coating surface. The findings in this study are consistent with those observed by Solis-Pomar et al. in eco-friendly intumescent coatings with condensed tannins [9]. However, it is not an exclusive issue of tannin formulations, as this effect has also been found in commercial paints with lower magnitude tensile stresses. The performance of cohesive failure can be improved by reducing the coating thickness, enhancing substrate adhesion, and controlling moisture to prevent potential cracking and delamination damage on the surface, creating non-adhesive zones [27,28].

It can also be observed in Table 5 that as the tannin content in the formulation increases, the maximum tensile stress increases, suggesting that higher concentrations of tannins improve adhesion between the coating and the substrate. These results indicate that the formulation with 90% tannin equivalence is 68% better than the reference formulation and surpasses commercial coatings Firewall 200 and Ak 7000 by 68% and 6.3%, respectively. In summary, these findings suggest that tannins are an additive with great potential to promote better interaction between the coating and the substrate, resulting in efficient and uniform adhesion.

The results of the ball impact, abrasion, and flexure tests can be observed in Figure 1, Figure 2, and Figure 3, respectively. In Figure 1, it is shown how samples containing tannin exhibit low resistance to fracture under biaxial stresses. However, they remain within the range of resistance of the commercial coating Firewall 200. These findings indicate that tannins generate high susceptibility to cracking under deformation conditions in coatings, mainly due to the plasticizing properties they possess. For this reason, the coating that showed the best results is the reference one that does not contain tannins and includes pentaerythritol as a carbonizing agent. Unlike tannin, pentaerythritol has high plasticizing properties, allowing the coating surface to withstand greater deformation.

The results of the abrasion test are shown in Figure 2. These findings indicate that tannins generate high wear resistance compared to commercial intumescent coatings and the reference coating. This resistance is consistent with the percentage of tannin equivalence in the formulation, with the formulation containing 90% tannin equivalence exhibiting the highest wear resistance, showing the least mass loss. It outperforms the reference formulation by 228% and by 815% and 108% compared to the commercial intumescent coatings Ak 7000 and Firewall 200, respectively. It is important to highlight that all coatings endured 1000 cycles without cracking, except for the commercial coating Ak 7000, which lasted only 467 cycles. These results indicate that tannins in the formulation strengthen the surface, making it more resistant to abrasion and wear, preventing premature damage and delamination that could compromise the effectiveness of fire protection.

The results of the flexural or flexibility tests can be observed in Figure 3. It is important to highlight that the reference formulation showed no cracking, indicating high adaptability and corroborating the results obtained from the ball impact test. These findings in the reference formulation are attributed to the substantial plasticizing capacity of pentaerythritol. In the same context, it is observed that the inclusion of tannins in the formulation leads to high susceptibility to cracking because they cannot adapt under significant deformations, reducing their ability to maintain integrity. However, when comparing the results obtained by formulations with tannins and commercial intumescent coatings, it is evident that they are, on average, below the fracture length generated by the commercial coating Firewall 200. These results indicate that the tannin-containing coating exhibits characteristics similar to currently available intumescent coatings in the market, and its performance will depend on the substrate it is applied to or the environment it aims to protect. For example, the coating could be applied indoors as these surfaces are not subject to significant deformations. The comparative results of the evaluation of the mechanical properties of the coatings are shown graphically in Figure 4.

3.3. Evaluation of the Fire-Resistant Properties of the Coatings

The results associated with fire resistance can be observed in Figure 5. A similar overall behavior is noticeable in all formulations, characterized by a sudden increase in temperature until reaching approximately 95 °C. After reaching this temperature, there is stabilization and plateauing, indicating that the coating has reached the necessary energy to initiate the intumescent reaction and generate the carbonaceous foam that acts as an insulating barrier, releasing cooling gases in its initial stage and reducing heat transfer to the substrate. Subsequently, a change in the heat transfer rate is observed due to the carbonaceous barrier, but with a constant temperature increase rate in all tannin-containing formulations. This temperature increase rate is closely related to the protective quality, which includes the thickness of the carbonaceous layer, its stability, and the area of coverage on the substrate. The best performances in this stage were achieved by the commercial Firewall 200 coating, which reached 40 min without reaching the temperature of 190 °C, and the reference coating, which reached 34.6 min to reach 190 °C. The tannin-containing coating that showed the highest performance in this category was the 15% equivalence compared to the commercial Ak 7000 coating, thus achieving a fire-resistance rating (FRR15). This indicates that formulations with a lower content of low-molecular-weight tannin in their composition exhibit higher fire resistance. These findings suggest that tannins do not have good properties as a charring agent despite their high hydroxyl group content. The reasons explaining this phenomenon from the obtained results are detailed below.

The first reason involves the difficulty of access due to the long carbon chains present in tannin, which can create a voluminous structure [5,8,9,29]. This structure may hinder the access of reactants to hydroxyl groups and the formation of chemical bonds at specific locations in the molecule. Another reason could be the low carbon content upon decomposition. This is due to its condensed structure, resulting in a lower amount of volatile materials and a higher amount of carbon compared to hydrolyzable tannins [8,9]. This reason might indicate that, despite their lower reactivity and hydroxyl group content compared to hydrolyzable tannins, condensed tannins may have a greater capacity to create carbonaceous layers due to their lower mass loss when exposed to high temperatures, leading to increased intumescence. However, further studies are required to explore this aspect [29].

In Figure 6, it is concluded that the formulations exhibit remarkable resistance to mass loss under elevated temperature conditions. These values are comparable or even superior to those of the examined commercial coatings, suggesting that intumescent coatings formulated with tannins can maintain their physical integrity and resist decomposition at high temperatures, thus preserving their heat resistance properties. In both tests, the formulation with the lowest mass losses was identified as Firewall 200. This formulation showed values practically identical to the reference formulation, with minimal differences of 0.35% and 0.44% in muffle furnace and inclined tunnel tests, respectively. Additionally, the formulation with 15% tannin equivalence also demonstrated outstanding performance, differing only by 0.96% and 0.03% from the aforementioned formulation. This pattern suggests that weight losses are lower in formulations with low levels of tannin. It should be noted that the uncoated sample experiences mass losses of 27.38% and 3.89% in muffle furnace and inclined tunnel tests, respectively. When comparing the test results, it is observed that the values obtained in the inclined tunnel experiment are significantly lower due to the shorter duration and lower temperature of the experiment. However, these results reflect a similar behavior, showing greater mass preservation in formulations with low levels of tannin. These findings align with the fire resistance values, suggesting that the formulations yielding superior results are the reference formulation and the 15% tannin equivalence formulation.

As seen in Figure 7, the highest intumescence height and a more stable carbonaceous reaction surface were primarily observed in the commercial Firewall 200 coating (see Figure 7c). Secondly, the best intumescence reaction was achieved by the reference formulation (see Figure 7b), which reached a height of 19.86 mm, surpassing the commercial Ak 7000 coating by 74.5%. Similarly, a clear relationship can be observed in the tannin-containing formulation described earlier. It can be seen that as the tannin equivalence increases from Figure 7e–j, the intumescence reaction decreases, becoming increasingly similar to the substrate without intumescent coating (see Figure 7a).

Figure 8 shows the photographic results of the intumescence reaction presented in graph 6 for the inclined tunnel condition. It can be clearly observed that there is a direct correlation with the findings in the muffle furnace but on a smaller scale due to the thermal energy flow presented in both tests.

4. Conclusions

This study analyzed the influence of tannins extracted from radiata pine bark waste on the intumescent reaction capacity to produce environmentally friendly coatings. The following are the most relevant results. The formulation with a 15% equivalence of low-molecular-weight tannins has enabled achieving a fire resistance classification of FRR15. This advancement represents an eco-friendly alternative compared to commercially available products. The characterization of samples of low- and high-molecular-weight tannins obtained from Pinus radiata bark revealed that low-molecular-weight tannins have a high content of hydroxyl groups, comparable to conventional charring agents like pentaerythritol and high oil absorption. On the other hand, high-molecular-weight tannins exhibited lower hydroxyl group content, suggesting they are less viable options as charring agents, and also showed lower oil absorption than low-molecular-weight tannins. The formulation of a reference intumescent coating using conventional materials achieved a fire resistance classification of FRR30, demonstrating comparable strength to commercial products. The mechanical characterization of the tannin-based intumescent coatings investigated in this study indicated that abrasion resistance and adhesion increased with tannin concentration while flexibility and indentation decreased. It is noteworthy that the values of the mechanical properties for all developed formulations were comparable to those reported for commercial products. The assessment of the effectiveness of intumescent coatings on wood substrates through flame response revealed a significant reduction in the charring index and mass loss of substrates with increasing tannin content.