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Article

Experimental Investigation of Fire—Technical Characteristics of Selected Flame Retardants for the Protection of Wooden Structures

by
Patrik Mitrenga
*,
Miroslava Vandlíčková
and
Milan Konárik
Department of Fire Engineering, Faculty of Security Engineering, University of Žilina, 010 26 Žilina, Slovakia
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(2), 193; https://doi.org/10.3390/coatings15020193
Submission received: 30 November 2024 / Revised: 13 January 2025 / Accepted: 27 January 2025 / Published: 6 February 2025
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Abstract

:
This study evaluates selected flame retardants on the basis of their influence on the change of fire-technical parameters of soft and hard woods (spruce and oak) during exposure to a flame heat source. The parameters evaluated were mass loss, mass loss rate and depth of the charred layer. The experiments were carried out on simple test equipment on which the samples were exposed to direct flame while their mass was monitored. The measured data and their statistical evaluation showed a significant dependence of the mass loss on the type of retardant used (inorganic salt-based flame retardant—IS and intumescent flame retardant—IFR) and on the type of wood species. In spite of the same reaction to fire class specified by the manufacturers for both types of retardants studied, significant differences were observed in the parameters monitored. The mass loss, mass loss rate and charred layer reached much lower values when using IFR retardant, whose efficiency was higher in the order of tens of percent compared to the use of IS retardant. The use of IFR flame retardant reduced the depth of the charred layer on oak samples by up to 84% compared to untreated samples, indicating its high effectiveness and potential to increase the fire resistance of wooden structures. These results show that IFRs are more effective in the parameters studied compared to ISs despite their equal class of reaction to fire, which may have wider implications for the construction industry and highlight the need for a thorough evaluation of flame retardants based on their performance under real-world conditions.

1. Introduction

Wood is a renewable raw material with a wide range of applications and is one of the oldest materials used by humans [1]. The wood industry accounts for about one-fifth of all manufacturing enterprises in Europe [2]. Wood is also often used in the construction industry due to its excellent properties, such as high strength and low thermal conductivity [3], as evidenced by the world statistics on the total production of lumber, which is often used in buildings [4].
The main problem that this study tries to address is wood flammability [5,6,7,8,9,10,11]. Wood combustion can be described as a process of pyrolysis and slow heterogeneous oxidation of carbon [12]. Exposure to high temperatures first causes the evaporation of chemically bound water, followed by the decomposition of the chemical structure of the wood (mainly cellulose, hemicellulose and lignin) [13], releasing flammable gases. When the desired temperature is reached (approximately above 115 °C), flaming combustion occurs if there is a sufficient concentration of released combustibles in the air. At temperatures above 280 °C, a charred layer forms on the surface of the wood [14,15,16]. These negative properties of wood limit its use in construction [17,18,19].
Many previous studies have focused on reducing the flammability of wood and increasing its fire resistance using various methods, especially the application of flame retardants [20,21,22]. Their use can improve various fire-technical properties of wood such as time to ignition, flammability, smoke emission or flame spread over the surface [23,24,25]. Flame retardants generally operate on different principles, with some using several at once [26]. Flame retardants are most commonly applied to wood products by coating or impregnation [23,27]. Commonly used flame retardants generally include water-based or solvent-based salt solutions [28]. Recently, however, water-based solutions have been more widely used because they are more environmentally friendly. However, the use of some preparations may be limited due to their instability to external influences, so top protective coatings, so-called topcoats, are used to ensure long-term durability [29].
However, the studies lack a more detailed comparison of the effectiveness of the different types of flame retardants available on the basis of their selected fire performance characteristics. Many studies concentrate on performing tests to classify materials into classes of reaction to fire. Manufacturers of commercial flame retardants list the class of reaction to fire as a basic characteristic in terms of the fire safety of the product. However, they do not sufficiently address the overall ability of fire retardants to improve the fire performance of wood. There is no in-depth analysis of how the different types of fire retardants affect parameters such as mass loss, mass loss rate and charring layer thickness.
From a broader perspective, it is important to understand the differences between different types of flame retardants in order to select the most appropriate type for specific applications. In order to select the appropriate type of flame retardant for the protection of timber structures, it is important to be familiar with the building regulations and fire safety standards of the country concerned, which determine the required class of reaction to fire and fire resistance of the materials according to their end use in buildings. Untreated solid wood products are classified as having a reaction to fire class D-s2,d0 with a minimum wood element density of 380 kg·m−3 and a minimum thickness of 22 mm [30], i.e., as combustible with a moderate contribution to fire, high smoke production and no drops or burning drops during fire [31]. By treating wood elements with flame retardants, we can improve the fire reaction class; for most flame retardants applied to both softwood and hardwood, manufacturers declare that they can achieve a fire reaction class of B-s1,d0. However, if structures are to be protected effectively against fire, the selection of a suitable flame retardant should not be based solely on classification into a reaction to fire class but should take into account its overall ability to improve the fire performance of the element. An improved understanding of how different flame retardants affect the fire performance of wood can lead to the development of more effective and reliable solutions.
The aim of this paper is to investigate the selected flammability characteristics of two different types of flame retardants with the same reaction to fire class and to observe their fundamental differences in the protection of wooden elements. Two representative types of flame retardants, namely, inorganic salt-based flame retardant and intumescent flame retardant, were selected for this study. This study focuses on key parameters such as mass loss, mass loss rate, charred layer thickness and fire spread rate to understand their different mechanisms of action. By comparing these parameters, this study aims to provide insight beyond simple classifications of fire response classes. Continuous measurement data of the mass loss rate provides valuable information on the phases of action of individual flame retardants beyond the simple determination of total mass loss. The addition of the “fire spread rate” parameter has provided interesting information on fire spread. The results of this study are then complemented by the charred layer, which is crucial for determining the fire resistance of timber structural elements.
The findings of this study will contribute to a better understanding of how different flame retardants affect wood behavior during fire. This knowledge can lead to the selection of appropriate flame retardants for specific applications, resulting in improved fire safety of timber building structures.

2. Materials and Methods

To investigate the problem, an experiment was conducted on a device that monitors the actual mass of test wood samples treated with flame retardants during exposure to a flame heat source. It is a modification of the method according to STN 73 0862 [32] used in the past in Slovakia for the classification of materials into flammability classes. The primary parameters monitored are mass loss and mass loss rate. This method assesses changes in the above parameters with sufficient sensitivity [33,34,35] and is suitable for comparing even minor differences in the behavior of samples during testing. It is designed to mimic real fire conditions [36]. Another key parameter that has been evaluated is the thickness of the charred layer. This is an important parameter that directly affects the fire resistance of wood elements [37]. On the one hand, the charred layer shows a weaker thermal conductivity than undegraded wood, and the heat propagates more slowly into the intact wood layers [38,39,40]. On the other hand, the depth of the charred layer directly affects the resistance of the element to fire loads [37,41]. The charring rate depends on several factors, such as wood type, wood density and moisture content, heat load, etc. [38], and it is the use of flame retardants that has a significant effect on the resulting charring depth of pure wood. The effectiveness of flame retardants has also been evaluated by the parameter ’fire spread rate,’ which is the ratio of the maximum mass loss rate to the time taken to reach this value. The authors Kačíková and Makovická-Osvaldová [42] consider this parameter as important from the point of view of fire spread through the building element.

2.1. Experimental Samples

Samples of spruce (Picea abies) and oak (Quercus robur) wood were selected for the experiment so that changes in the tested parameters could be evaluated separately on both softwood and hardwood. Fifteen samples of each species were used. The dimensions of the samples were based on STN 73 0862 [32], while their thickness was 10 mm wider as some specimens were overburned in previous experiments. All samples were 200 × 100 × 20 mm. Prior to the application of flame retardants, the samples were placed in a laboratory drying oven for 2 h at a drying temperature of 100 °C. After two days, an attempt was made to measure the moisture content of the samples using a VOLTCRAFT FM-300 (Conrad Electronic SE, Hirschau, Germany) instrument. However, the instrument failed to measure the moisture content of the samples because it was below the lower limit of its measuring range, i.e., less than 6%. The density of the wood samples was determined according to [43] and ranged from 331 to 449 kg·m−3 for the spruce wood samples and from 592 to 774 kg·m−3 for the oak wood samples (Table 1).
Spruce wood is one of the most commonly used woods in the construction industry because of its strength, flexibility and availability. It is not only used as building elements in the form of beams, columns, floors, etc., both indoors and outdoors but also in the production of particleboard, fibreboard, plywood and veneers [44]. In this experiment, spruce wood represents the softwood representation.
Oak wood is mainly used for furniture production [45]. However, it is also used in the construction industry for the production of beams, columns, girders and other structural elements. Due to its high hardness, durability and ability to resist wear, it is popularly used for flooring. Oak wood is classified as a hardwood with a bulk density of over 500 kg·m−3.

2.2. Used Flame Retardants for the Experiment

Experiments were carried out with two different types of flame retardants from different manufacturers. These are commercially available flame retardants, namely, the flame retardant based on inorganic salts “HR Prof” from the manufacturer COLOR COMPANY (Color Company, s.r.o., Dubnica nad Váhom, Slovakia) and the intumescent flame retardant “Flamgard” from the manufacturer STACHEMA (STACHEMA Bratislava a.s., Rovinka, Slovakia). In both cases, these are flame retardants from Slovak manufacturers. These flame retardants were chosen because of their different properties, effects of action and composition, and also because they are frequently used flame retardants in Slovakia.
HR Prof is a fire-retardant product designed to limit the ignition and propagation of flames of various wood-based materials, including wooden structures, cassette ceilings, wooden floors, and other cellulose-based products. This product is suitable for both interior and exterior applications. Upon application and drying, HR Prof forms a durable, non-washable coating. Importantly, it does not release toxic fumes at high temperatures, and its burned residue does not disperse harmful particles into the surrounding environment. When exposed to temperatures exceeding 1700 °C, the treated surface turns black, effectively halting the spread of fire [46]. The preparation consists of an aqueous solution of ferric phosphate (30%), citric acid (1%) and special additives (0.5%) [47].
Flamgard is a fire-resistant coating designed to protect wooden structures (columns, beams) against fire. It protects wood and lignocellulosic materials against the action of fire and radiant heat. It is used exclusively for indoor and dry environments (up to a maximum relative humidity of 80%). It is suitable for embedded wood with a maximum relative humidity of 20%. The service life of the coating is 15 years [48]. Flamgard is a one-component, water-thinnable, foamable coating of a suspension nature (contains dispersed solid particles). It consists of a coke-forming component, a mineral acid source component (phosphoric acid), a foaming component, binders, fillers, additives and auxiliaries modifying the performance properties of the formulation. The coating does not contain asbestos, toxic pigments or chlorinated additives. The hardened layer produces a (grey) white matt coating with an acknowledged granular texture [48].
Both coatings achieve the same reaction to fire class B-s1,d0 on both softwood and hardwood. To achieve this, a coating quantity of 300 g·m−2 for HR Prof or 500 g·m−2 for Flamgard should be applied. Other properties of the coatings are given in Table 2.
In general, inorganic salt-based flame retardants (ISs) exhibit high water solubility and facile penetration into the superficial layers of wood. These compounds endothermically decompose to release non-combustible gases, thereby diluting the oxygen concentration and suppressing the generation of flammable volatiles. Concurrently, they form a carbonaceous char layer, which acts as an effective thermal barrier [49,50,51]. Intumescent flame retardants (IFRs) are aqueous dispersions of carbon-rich polymers and resins with the addition of an inorganic acid acting as a catalyst for the formation of carbone and amine/amide compounds as a gas source. Upon exposure to fire, the coating undergoes thermal decomposition, yielding a carbonaceous char. Concurrently, non-combustible gases, released from the amine/amide compounds, induce foaming of the char, resulting in a porous, insulating layer [52]. This expanded char acts as a barrier against heat transfer and combustion [49,50,51].

2.3. Application of Flame Retardants

Each type of flame retardant was applied by brush coating to five spruce and five oak samples according to the manufacturers’ recommendations. The samples with applied flame retardants are shown in Figure 1. The total amount of coating on the exposed side of the specimen was standardized to 500 g·m−2 for both flame retardants, although for HR Prof a smaller amount of coating was sufficient. The amount of coating was checked for each sample separately by weighing the sample before and after coating. Depending on the type of retardant and the wood samples, the samples were coated two to three times to achieve the required amount of coating. The required time interval between the application of each coating was maintained according to the technical specifications supplied by the manufacturers [47,48]. The average coating amounts for each group of samples are shown in Table 1. After applying the required amount of coating, the samples were left in a room with a humidity of 40% and a temperature of 21 °C to allow the coatings to cure naturally.

2.4. Test Equipment and the Procedure

The experimental procedure consists of exposing the samples to a direct flame and monitoring their actual mass using a device assembled as shown in Figure 2.
A constant gas flow of 30mbar was maintained by a flow regulator (3) on the gas cylinder. The burner, equipped with a gas flow regulator (7), allowed for precise adjustment of the flame height, which was set to 10cm in this experiment. Samples (10) were secured in a holder (9) at 45° to the horizontal plane. This configuration set the conditions for flame propagation and the development of combustion.
During the experiment, the samples were exposed to a flame heat source applied from below, with a 1cm flame directly contacting the sample surface. Propane–butane gas with a flame temperature of 1950 °C and a calorific value of 44 MJ·kg−1 served as the fuel source [54].
A Mettler Toledo MS1602S/M01 scale (Greifensee, Switzerland) was employed to record the mass of the samples at 15 s intervals. The scale, capable of measuring to the hundredth of a gram, ensured accurate measurements. The BalanceLink 4.2.0.1 program (Mettler Toledo, Greifensee, Switzerland) automated the mass recording process at predefined intervals. The total duration of the experiment was 10 min. Following this period, mass recording was terminated.

2.5. Evaluation and Calculation

From the measured data obtained on the mass changes of the samples over time, it was possible to calculate important fire-technical parameters. The main evaluation criterion in terms of fire resistance is the progression of the percentage mass loss over time δmp and the total mass loss after the end of the experiment δ, which were calculated according to relations (1) and (2). In terms of fire spread assessment, the main criterion is the dependence of the mass loss rate vr on the time of sample exposure to the flame source, calculated according to (3). The fire spread rate parameter Rfs, calculated according to Formula (4), which represents the ratio of the maximum value of the mass loss rate (the first peak of the mass loss rate reached) and the time to reach this value was used as an additional criterion for the fire spread assessment.
Another very important piece of data affecting the fire resistance of wooden elements is the thickness of the charred layer. To calculate this, all samples were cut lengthwise down the middle after testing. The charred layer was then removed with a dull knife and the intact part of the specimens at the narrowest point was measured with a vernier caliper. The thickness (depth) of the charred layer was determined as the difference between the original thickness of the sample (accurately measured with the vernier caliper before coating) and the thickness of the smallest intact portion of the sample after the experiment (Formula (5)).
δ m p τ = m − m τ m · 100
δ = m − m 600 m · 100
v r = m τ − m τ + Δ τ m τ · Δ τ · 100
R f s = v r max τ v r max
d = d 1 − d 2
where δmp(τ) is the mass loss at time (τ) [%], m is the original mass of the sample before the experiment [g], m(τ) is the mass of the sample at time (τ) [g], m(600) is the mass of the sample at testing time 600 s [g], vr is the relative mass loss rate [%·s−1], m(τ + Δτ) is the sample mass at a time (τ + Δτ) [g], Δτ is the mass reading time interval [s] (15 s in our case), Rfs is the fire spread rate [%·s−2], vr max is the first maximum mass loss rate [%·s−1], τ(vr max) is the time to reach the first peak of the maximum mass loss rate, d is the charred layer, d1 is the original thickness of the sample and d2 is the thickness of the smallest intact portion of the sample after the experiment.

3. Results and Discussion

3.1. Results and Evaluation of Mass Loss

For evaluating the overall effectiveness of flame retardants, data on the mass loss of the samples during the experiment and the overall mass loss of the samples after the experiment provide valuable information. At a glance, we can see, in Figure 3 and Figure 4, the differences in the untreated spruce and oak samples.
The untreated spruce wood samples show a higher percentage increase in mass loss than the untreated oak wood samples almost from the beginning of the experiment (from the testing time of 30 s), which they maintain until the end of the experiment. The average total mass loss of spruce wood samples (Table 3) was 17.27%, while that of oak wood samples was only 11.76%. Thus, the difference in their mass loss is 5.5%, which proves the greater thermal stability and fire resistance of oak wood. This is also confirmed by the study of Gašpercová et al. [55], who investigated the effect of fungicidal coatings on the resulting retardant effect of the HR Prof coating, conducting the study on different species of softwoods and hardwoods. The results of their study show that oak, among the studied wood species, has the lowest flame spread and the lowest mass loss.
At the same time, the dispersion of mass losses (Figure 5) shows a higher variability of the measured data for untreated samples of both wood species. Since wood is a heterogeneous material, such results were expected. The cause of the greater dispersion may be the different densities of the individual samples (Table 1). Extreme values of high mass losses are caused by spontaneous combustion of these samples, which can be influenced by several factors, such as the formation of cracks in the samples, increased resin content, etc. Significantly lower variability of mass losses is observed for samples with the HR Prof (SH, OH) coating with a variation range of up to 1.3%. A stable effect is observed on samples applied with the Flamgard flame retardant, where the variation range does not even represent 0.6% of mass loss on spruce samples (SF) and 0.3% on oak samples (OF).
From the point of view of the effect of the tested flame retardants, it can be stated that both retardants had an impact on the change in mass loss on both spruce and oak wood. The samples applied with the HR Prof flame retardant achieved a total mass loss of 7.8% and 6.5% (on spruce and oak samples, respectively), which is 9.5% and 5.3% less than in the untreated samples. However, in the samples with the Flamgard flame retardant applied, we observe an even more significant effect on mass loss, which is 13.9/10.5% lower than in the untreated spruce/oak samples. This represents an increase in its effectiveness by 46% on spruce samples and 97% on oak samples compared to the samples applied with the HR Prof flame retardant. Therefore, a significant difference in the effectiveness of the studied flame retardants can be confirmed, despite their identical fire reaction class B-s1,d0.
To determine the influence of flame retardant type and wood species on mass loss, we conducted a multivariate analysis of variance (MANOVA) using R statistical software (version 4.1.2). The findings of this analysis are presented in Table 4.
The MANOVA test confirmed the statistical significance of the mass loss from the type of flame retardant used and also from the type of wood. The flame retardant used can have significantly different effects despite seemingly similar properties (for example, the same class of reaction to fire). This dependence is evidenced by the value p = 2.48 × 10−15, which indicates a very high influence. As follows from the analysis, the retarding effect also depends on the type of wood, which has a value of p = 1.48 × 10−8 in relation to the mass loss. A weak effect of the interaction between the variables flame retardant and type of wood (p = 0.04) is also observed, which indicates that the influence between mass loss and the type of flame retardant can also be slightly affected by the type of wood. This is also observed in Figure 3 and Figure 4 in the more significant differences in the retarding effect of the investigated coatings on oak samples compared to spruce ones.

3.2. Results and Evaluation of Mass Loss Rate

The mass loss rate is an important parameter for assessing the behavior of samples during testing, and it also indicates the ability of the material to spread fire. The application of flame retardants significantly reduced the mass loss rate on both oak and spruce wood (Figure 6 and Figure 7). For untreated samples, two peaks of the maximum mass loss rate occurred by the testing time of approximately 120 s. We assume that the first peak, which also represents the maximum mass loss rate, was caused by the degradation of the surface layers of the samples due to the action of a direct flame on the samples and the production of flammable gases, which increased the burning intensity. Subsequently, a charred layer begins to form, limiting the transfer of heat and flame to lower layers, and the mass loss rate begins to decrease.
The effect of the HR Prof retardant can be divided into two steps. In the first step, after exposure to a heat source, part of it is consumed for its own decomposition and further forms non-flammable gases diluting the oxygen concentration [49]. This achieved a significant reduction in the maximum mass loss rate by almost half compared to untreated samples. In the next step, the flame retardant promotes the formation of a charred layer, which acts as a barrier against the transfer of heat and flame into the lower layers of the wood [5,24,49], which is reflected in a lower mass loss rate until the end of the experiment.
An even more significant reduction in the mass loss rate was achieved with the flame retardant Flamgard. The maximum mass loss rate of spruce samples with this coating was 0.0167%·s−2 which is 75% less than untreated samples and 44% less than samples with HR Prof coating. The maximum mass loss rate of oak samples with the Flamgard coating was 0.008%·s−2, which is 83% less than untreated samples and 32% than samples treated with HR Prof. This effect is caused by the way in which the flame retardant works. Flamgard belongs to the group of intumescent flame retardants, which form a carbonaceous mass when exposed to heat. This foams due to the release of non-flammable gases (from amine compounds). This layer on the surface of the material then forms a protective barrier against the effects of radiant heat and burning [49,50]. The formed foamed carbon layer can be seen in Figure 8.
From the point of view of fire spread, the authors Kačíková and Makovická [42] consider not only the maximum mass loss rate to be important but also the time to reach this value. The proportion of these values is expressed by the parameter fire spread rate. The higher this parameter is, the more the material participates in fire spread [42]. The lowest average values of this parameter are again achieved by samples treated with the Flamgard retardant (Table 3).

3.3. Results and Evaluation of Charred Layer

The evaluation of the effectiveness of flame retardants based on the formation of a charred layer during a fire has two aspects. On the one hand, many flame retardants support the formation of a charred layer or accelerate it. Such a charred layer has good thermal insulation properties [56], protecting the underlying material. The second aspect is the formation of a charred layer of the pure wood itself, which is to be protected. The depth of charring, which depends on the rate of charring [57], expresses the resistance of materials to the effects of fire loads. The fire resistance of structures is crucial for maintaining the required functions of the building from the point of view of fire safety, which limits the use of unprotected wooden elements in buildings. The charred layer has no strength and has a significant impact on determining the fire resistance of wooden structural elements [38]. With increasing charring depth, the fire resistance of the element decreases, which can be determined based on the residual cross-section [37,58].
The char depth is considered to be the distance between the outer surface of the original element and the position of the char line [37]. The detailed procedure for measuring the char layer is described in the chapter “Materials and Methods”. Interesting findings result from the average values of the maximum charred layer. The average char depth of untreated spruce samples is 11 mm, which is 3.4 mm more than for oak samples. The HR Prof flame retardant is more effective in reducing the char depth of spruce samples (reducing the char depth by 32%), while the Flamgard flame retardant is more effective in oak samples (reducing the char depth by up to 84% compared to untreated samples) (Figure 9). Regardless of the type of wood, it can be confidently stated that the Flamgard flame retardant is more effective in reducing the char depth than the HR Prof flame retardant. The lowest char depth was measured for OF samples with an average value of 1.19 mm. This is the most pronounced effect compared to O samples with an average char depth of 7.61 mm. The reason is the formation of a thick char layer from the retardant itself, which acts as an insulating barrier [48,50]. Together with the properties of hardwood, such as its greater thermal stability and lower flammability compared to softwood, the Flamgard coating thus provides high protection against thermal degradation.
The results of this study show a significant influence of the type of flame retardant (inorganic salts and intumescent) on all the investigated characteristics. Despite their same fire reaction class B-s1,d0 on both soft and hardwoods, their fundamentally different retardation effect can be stated. The flame retardant Flamgard is more effective in reducing mass loss, reducing the maximum mass loss rate and reducing the depth of charring than the HR Prof retardant. Since the depth of the charred layer has a significant influence on determining the fire resistance of wooden structural elements [37], reducing it by using flame retardants could achieve an increase in their fire resistance. The manufacturer of the Flamgard coating even reports an increase in the fire resistance of wooden structural elements depending on the location of the element and the fire resistance of the unprotected element in some cases by up to 17 min [48]. The authors Gašparík et al. [59] investigated the effect of the flame retardant Flamgard Transparent on thermally modified oak wood. The product is similar to the Flamgard we tested, with the difference that the coating formed on the surface is transparent. After exposing the samples to direct flame, they monitored the weight loss, and the flame retardant studied achieved a weight loss reduction of more than 60% on thermally modified oak wood. Čabalová et al. [60] investigated the effect of thermal loading from a radiant heat source on spruce wood samples treated with the Flamgard retardant and concluded that the retardant significantly reduced material degradation. Kmeťová et al. [61] performed a flammability test according to EN ISO 11925-2 [62] on spruce wood samples treated with the HR Prof preparation and intumescent flame retardants. The test showed that the samples treated with the retardants did not ignite. However, the studies mentioned do not compare two types of flame retardants with the same reaction to fire class using the same methods. Our research demonstrates a significantly higher effectiveness of intumescent flame retardant compared to inorganic salt-based retardants, both on softwood and hardwood. Unlike inorganic salt flame retardants, which release non-flammable gases and form a carbon layer, IFR forms a bulky insulating barrier to counteract radiant heat and combustion. Such a foamed layer is then more effective in preventing the transfer of heat and flame than the thin carbon layer formed by the action of the IS retardant. This is the reason for the overall higher flame retardancy of IFR compared to IS.
However, it should be noted that this study focuses on two specific types of flame retardants and further research needs to be conducted to determine their effectiveness compared to other products. The limitations of the test method should also be taken into account. For more accurate and comprehensive information on the behavior of the different flame retardants, further studies should be carried out using different methods to include large-scale tests, radiant heating, other types of wood used in construction and monitoring of other important parameters such as smoke emission, etc. However, in view of the results of this study, despite the limitations mentioned above, similar performance characteristics of other intumescent flame retardants can be assumed on other types of wood, the effectiveness of which has been demonstrated in many studies [63,64,65,66,67].

4. Conclusions

The following conclusions can be summarized from the experiments performed:
-
Samples without treatment showed a high variability of the measured total mass loss compared to samples treated with flame retardants. The lowest variability in mass loss was observed for the samples treated with Flamgard (max 0.6% of mass loss).
-
Both flame retardants investigated have a significant effect on mass-loss reduction. The mass loss of samples treated with flame retardants was significantly lower for oak wood.
-
The retardants tested showed different effects on the testing parameters investigated. The Flamgard retarder was more effective in reducing mass loss and maximum mass loss rate compared to the HR Prof retarder, regardless of wood species. In terms of the mass loss criterion, its efficiency is 46% and 97% higher on spruce and oak samples, respectively, compared to HR Prof treated samples.
-
A significant dependence of the mass loss on the flame retardant used and the species of wood was confirmed.
-
The HR Prof flame retardant is more effective in reducing charring depth in spruce samples (32% reduction in charring depth), while the Flamgard flame retardant is more effective in oak samples (up to 84% reduction in charring depth compared to untreated samples). In spite of the above, the Flamgard retardant reduces the charring depth more effectively than HR Prof.
Finally, it should be noted that the flame retardants investigated have a variety of applications. While HR Prof is aesthetically suitable for exposed timber structures (it creates a transparent layer on the surface), the Flamgard retardant creates a white matt coating on the surface and is, therefore, more suitable for application to structures in non-habitable spaces, such as closed attics. However, there are also transparent versions of intumescent flame retardants that combine the aesthetic and performance benefits of such coatings. The decisive criterion for the selection of a suitable flame retardant should therefore not only be the class of reaction to fire declared by the manufacturers but also the ability of the retardant to effectively reduce other fire-technical parameters or to increase the fire resistance of the protected element.
Based on the findings of this research, there is more to consider when selecting a flame retardant than just the class of reaction to fire. The use of intumescent flame retardants may be recommended for the protection of timber structures, but their aforementioned limitations of application must be taken into account. This study raises awareness of the importance of the correct selection of flame retardants, which can significantly increase the fire resistance of timber structures. Further research in this area should focus on investigating the effectiveness of other types of flame retardants under different fire conditions in order to find suitable solutions for different applications. In doing so, the focus should be on more environmentally friendly flame retardants.

Author Contributions

Conceptualization, P.M. and M.V.; formal analysis, M.K.; funding acquisition, P.M.; methodology, P.M. and M.V.; project administration, P.M.; resources, P.M. and M.V.; software, P.M.; supervision, M.V. and M.K.; validation, M.K.; visualization, P.M.; writing—original draft, P.M.; writing—review and editing, M.V. and M.K. All authors have read and agreed to the published version of the manuscript.

Funding

This article was funded by the Grant System of the University of Zilina for the project Experimental Determination of Fire-technical Parameters of Alternative Building Materials and Evaluation of its Fire Safety. Project No. 16961.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. Samples before the experiment (untreated/treated with flame retardants).
Figure 1. Samples before the experiment (untreated/treated with flame retardants).
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Figure 2. Schematic diagram of the test equipment (description: 1—gas cylinder, 2—shut-off valve, 3—flow regulator, 4—gas intake tube, 5—burner holder, 6—burner, 7—gas flow regulator on the burner, 8—scales, 9—sample holder, 10—sample, 11—connection between scales and computer, 12—computer) [53].
Figure 2. Schematic diagram of the test equipment (description: 1—gas cylinder, 2—shut-off valve, 3—flow regulator, 4—gas intake tube, 5—burner holder, 6—burner, 7—gas flow regulator on the burner, 8—scales, 9—sample holder, 10—sample, 11—connection between scales and computer, 12—computer) [53].
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Figure 3. Average mass loss of spruce samples untreated/treated with retardants over time with confidence intervals at the 95% confidence level (indicated by dotted lines).
Figure 3. Average mass loss of spruce samples untreated/treated with retardants over time with confidence intervals at the 95% confidence level (indicated by dotted lines).
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Figure 4. Average mass loss of oak samples untreated/treated with retardants over time with confidence intervals at the 95% confidence level (indicated by dotted lines).
Figure 4. Average mass loss of oak samples untreated/treated with retardants over time with confidence intervals at the 95% confidence level (indicated by dotted lines).
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Figure 5. Box plots of total mass loss of sample groups with outliers indicated (red dots).
Figure 5. Box plots of total mass loss of sample groups with outliers indicated (red dots).
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Figure 6. Average mass loss rate of spruce samples untreated/treated with retardants over time.
Figure 6. Average mass loss rate of spruce samples untreated/treated with retardants over time.
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Figure 7. Average mass loss rate of oak samples untreated/treated with retardants over time.
Figure 7. Average mass loss rate of oak samples untreated/treated with retardants over time.
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Figure 8. Samples after the end of the experiment.
Figure 8. Samples after the end of the experiment.
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Figure 9. Average values and standard deviations of the depth of the charred layer of sample groups.
Figure 9. Average values and standard deviations of the depth of the charred layer of sample groups.
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Table 1. Samples data and designation of sample groups.
Table 1. Samples data and designation of sample groups.
Designation of Sample GroupsWoodFlame RetardantAmount of Coating [g·m−2]Density of Wood Samples [kg·m−3]Moisture Content of Wood Samples [%]
SHspruceHR Prof502.58 ± 2.28398.76 ± 45.16<6
SFspruceFlamgard506.89 ± 10.02344.17 ± 12.00<6
SspruceUntreated-422.63 ± 20.07<6
OHoakHR Prof500.90 ± 2.47729.59 ± 44.19<6
OFoakFlamgard503.93 ± 5.87697.56 ± 71.59<6
OoakUntreated-654.39 ± 58.08<6
Table 2. Physical and chemical properties of the flame retardants used [46,47,48].
Table 2. Physical and chemical properties of the flame retardants used [46,47,48].
Flame RetardantAppearanceBoiling Point
[°C]		SolubilitiesDensity
[g·cm−3]		pHReaction to Fire ClassRequired Amount of Coating [g·m−2]
HR Proflight brown translucent liquid170100% water soluble1.1 ± 0.052.5B-s1, d0min 300
Flamgardgray–white suspensionunspecified100% water soluble1.15 ± 0.064–6B-s1, d0min 500
Table 3. Average values of total mass loss, maximum mass loss rate, fire spread rate and maximum depth of charred layer.
Table 3. Average values of total mass loss, maximum mass loss rate, fire spread rate and maximum depth of charred layer.
SamplesTotal Mass Loss (δ)
[%]		Max Mass Loss Rate (vr max)
[%·s−1]		Fire Spread Rate (Rfs)
[%·s−2]		Depth of Charred Layer
[mm]
SH7.82 ± 0.540.0324 ± 0.00450.00050 ± 0.000157.41 ± 0.71
SF3.40 ± 0.210.0234 ± 0.00750.00037 ± 0.000104.37 ± 0.81
S17.27 ± 3.150.0731 ± 0.02330.00130 ± 0.0001711.00 ± 2.96
OH6.48 ± 0.430.0262 ± 0.00760.00028 ± 0.000045.82 ± 0.77
OF1.33 ± 0.100.0080 ± 0.00020.00007 ± 0.000051.19 ± 0.31
O11.75 ± 2.690.0520 ± 0.01260.00067 ± 0.000137.61 ± 1.07
Table 4. Multivariate analysis of variance (MANOVA) of total mass loss.
Table 4. Multivariate analysis of variance (MANOVA) of total mass loss.
DfSum SqMean SqF ValuePr (>F)Influence
Flame retardant1114.29114.29858.7182.48 × 10−15Strong
Wood114.5614.56109.3971.47 × 10−8Strong
Flame retardant: Wood10.670.674.9990.04Low
Residuals162.130.13
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Mitrenga, P.; Vandlíčková, M.; Konárik, M. Experimental Investigation of Fire—Technical Characteristics of Selected Flame Retardants for the Protection of Wooden Structures. Coatings 2025, 15, 193. https://doi.org/10.3390/coatings15020193

AMA Style

Mitrenga P, Vandlíčková M, Konárik M. Experimental Investigation of Fire—Technical Characteristics of Selected Flame Retardants for the Protection of Wooden Structures. Coatings. 2025; 15(2):193. https://doi.org/10.3390/coatings15020193

Chicago/Turabian Style

Mitrenga, Patrik, Miroslava Vandlíčková, and Milan Konárik. 2025. "Experimental Investigation of Fire—Technical Characteristics of Selected Flame Retardants for the Protection of Wooden Structures" Coatings 15, no. 2: 193. https://doi.org/10.3390/coatings15020193

APA Style

Mitrenga, P., Vandlíčková, M., & Konárik, M. (2025). Experimental Investigation of Fire—Technical Characteristics of Selected Flame Retardants for the Protection of Wooden Structures. Coatings, 15(2), 193. https://doi.org/10.3390/coatings15020193

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