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Article

Effect of Combining Fungal and Flame-Retardant Coatings on the Thermal Degradation of Spruce and Beech Wood Under Flame Loading

by
Bohuš Leitner
1,
Stanislava Gašpercová
1,
Iveta Marková
1,* and
Ivana Tureková
2
1
Department of Fire Engineering, Faculty of Security Engineering, University of Žilina, Univerzitná 8215/1, 010 01 Žilina, Slovakia
2
Department of Technology and Information Technologies, Faculty of Education, Constantine the Philosopher University in Nitra, Tr. A. Hlinku 1, 949 74 Nitra, Slovakia
*
Author to whom correspondence should be addressed.
Fire 2024, 7(12), 463; https://doi.org/10.3390/fire7120463
Submission received: 9 October 2024 / Revised: 21 November 2024 / Accepted: 3 December 2024 / Published: 6 December 2024
(This article belongs to the Section Fire Research at the Science–Policy–Practitioner Interface)
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Versions Notes

Abstract

:
Compliance with fire safety standards for wood is crucial for its application in the internal applications of buildings. This article focuses on monitoring the quality of protective coatings for wood under thermal loading conditions. The examined samples of spruce (Picea abies L. Karst.) and beech wood (Fagus sylvatica L.) were treated with selected fungicidal coatings based on dimethylbenzyl ammonium chloride. Following this, they were soaked in a ferric phosphate-based flame-retardant solution. Additionally, a portion of the samples was treated solely with the flame retardant. The effectiveness of the protective coatings was assessed through experimental thermal loading of the prepared samples. The testing method adhered to according to selected standards, which evaluate the ignitability of building materials when subjected to a small flame source. The experimental results, including the mass loss, mass loss rate, and time–temperature curves of the thermally loaded samples, demonstrated a significant influence of the selected coatings on thermal degradation. Notably, the fungicidal coating exhibited protective properties. Samples treated only with the flame retardant showed higher mass losses compared to those treated first with the fungicidal coating followed by the retardant. Additionally, differences were observed between the wood types, with beech samples exhibiting greater mass losses and higher mass loss rates than spruce.

1. Introduction

Wood is a versatile building material characterized by a unique combination of advantageous properties [1]. It offers a high strength-to-weight ratio [2], is easily machinable and joinable, ranks among the most cost-effective materials [3], maintains environmental sustainability [4], and possesses significant aesthetic appeal [5,6]. Despite these benefits, the use of wood in structural applications presents challenges, particularly due to its considerable variability in physical properties, which are highly dependent on factors such as moisture content and the duration of loading. Additionally, wood is a highly anisotropic material, with its mechanical strength being significantly lower perpendicular to the fibers compared to the direction parallel to the fibers [7,8].
The degradation of wood properties can be exacerbated by biological and environmental factors, including insect infestation [9,10,11], wood-decaying fungi, and thermal exposure [12,13,14]. Moreover, wooden structures are vulnerable to complete destruction by fire [15,16,17].
The macroscopic structure of wood is defined by a series of features discernible to the naked eye or under magnification, forming characteristic patterns on the wood surface. These features include the presence, frequency, and condition of macroscopic inhomogeneities such as knots, reaction wood (compression and tension wood), heartwood, and resin channels [7,17,18].
The microstructure of wood, encompassing its anatomical and chemical composition, governs the movement of fluids in various anatomical directions. These processes have been extensively documented in the literature [8,19]. Thanks to the characteristics presented above, it is possible to apply wood surface treatment in order to increase its resistance (either to heat, or to biotic factors, or to mechanical damage). Surface treatment is carried out with protective coatings.
Once a tree is felled and sawn, it loses all the protective mechanisms that were active during its growth, becoming susceptible to degradation by water. Water creates a conducive environment for the growth of molds and fungi, which are eventually accompanied by insects [20].
The once tough, flexible, and resilient wood gradually loses its beneficial properties, eventually breaking down into organic waste that serves as a nutrient source for bacteria and plants [20,21]. Therefore, wood requires protection from external influences, fungi, and insects [20].
Maintaining the stability of wood involves proper drying followed by appropriate surface treatments [19]. An effective method is the application of protective coatings [3,22,23,24,25]. Among these, fungicidal coatings are a primary form of wood protection [26,27,28,29,30]. This paper aims to explore combinations of fungicidal coatings, particularly in conjunction with flame retardants, to enhance the quality of wood surfaces.
Wood can be treated to reduce its flammability through a process known as retardation [31]. Retardation treatments are economically demanding and must adhere to various requirements, ensuring that they do not negatively impact other technical, hygienic, or aesthetic properties [32,33,34,35]. In the process of reducing flammability, the combustion process is primarily altered by interfering with heat-removal capabilities, degradation processes, or the self-sustaining combustion stage [36]. Flame retardants are substances widely used across various industrial sectors [37,38,39].
The classification of retardants based on their action is as follows [40]:
  • Retarders releasing non-flammable gases;
  • Heat-accumulating retardants;
  • Intumescent retardants;
  • Mechanical-type retardants.
The classification of retardants according to chemical composition is as follows:
  • Inorganic salts;
  • Alkaline earth metal hydroxides;
  • Bromine compounds.
Inorganic salt-based retardants release non-flammable gases in the same thermal range as flammable gases. Non-flammable gases also mix with air, making the initiation process more difficult [40]. Alkaline earth metal hydroxide-based retardants decompose in a fire and release water. Another effect is the formation of a protective layer by the carbonation of the carbon residue from the polymer matrix by metal oxides [41]. Bromine compound-based retardants block combustion by forming volatile hydrogen bromide in the presence of hydrocarbons [42].
Their significance lies in their ability to reduce the flammability of materials [43,44]. Most flame retardants function as synergists, enhancing the fire-protective benefits of the materials to which they are applied [39].
The aim of this paper is to experimentally assess the effectiveness of protective coatings on wood, specifically on samples of Norway spruce (Picea abies L. Karst.) and European beech (Fagus sylvatica L.), under flame exposure. The research is distinctive in its focus on the combined application of protective coatings, beginning with an initial fungicidal treatment followed by the application of a flame-retardant coating. The study involves monitoring the mass loss of both untreated and surface-treated samples subjected to a flame. The experimental results, including the mass loss and mass loss rate, reveal the significant influence of the selected coatings on the thermal degradation of the samples and the burning process.

2. Materials and Methods

2.1. Experimental Samples of Spruce and Beech

The experimental samples studied were Norway spruce (Picea abies) and European beech (Fagus sylvatica). These species are among the most commonly used in the production of fiberboard, particleboard, plywood, veneers, and as timber for building and construction in both indoor and outdoor environments (Table 1) [45,46].
  • Norway Spruce (Picea abies (L.) Karst.)
Spruce wood is a significant domestic raw material in the Slovak Republic, with its physical and mechanical properties and its widespread availability in local forests. It is predominantly used as construction timber, especially in above-ground buildings. Spruce wood is extensively employed in the production of roof structures for residential, agricultural, and specialized buildings. It is also a well-established semi-finished product for manufacturing windows, exterior and interior doors, balconies, staircases, and other building and carpentry products. Additionally, spruce sawn timber is an integral component of wooden frame structures [47,48].
  • European Beech (Fagus sylvatica L.)
The European beech (Fagus sylvatica L.) is the most widespread and economically significant deciduous tree species in Europe [49]. Beech wood is classified as a medium hardwood, yet it is relatively easy to split [49]. It exhibits favorable properties for impregnation and staining [50], although it demonstrates limited resistance to fungal and insect infestation [8,51]. By contrast, lower-grade or waste beech wood is processed into particleboard, which is subsequently employed in the construction industry for flooring applications. Additionally, technical beech veneers are used in the production of plywood for both flat and molded boards. The primary application of beech wood is in the furniture industry, where it is utilized in the production of structural frames for upholstered furniture, cut furniture and bentwood seating [8].
Fifty-four samples, each measuring 250 × 90 × 25 mm, were prepared from each type of wood. The moisture content of the individual samples was determined gravimetrically according to [52]. The density of the wood samples was determined according to [53].
Table 1. Fire-technical characteristics, and physical and mechanical properties of spruce wood at 15% and 12% moisture contents [8,54,55,56].
Table 1. Fire-technical characteristics, and physical and mechanical properties of spruce wood at 15% and 12% moisture contents [8,54,55,56].
ParametersSpruceBeech
Fire-Technical Characteristics
Flash point [°C]The stated values are valid at 15% moisture.350 ÷ 360360–370
Ignition temperature [°C]390 ÷ 400400–410
Specific heat (calorific value)19.919.8
Mass loss rate [kg m−2]0.0560.066
Physical properties
Wood density [kg m−3]In absolutely dry state300–640490–910
Fresh740990
Experimentally determined weight [kg] 295.1 ± 11.2421.17 ± 7.6
Mechanical properties
Tensile strength [MPa]Parallel to the fibers90.0135.0
Perpendicular to the fibers2.710.7
Compressive strength [MPa]Parallel to the fibers50.056.3
Perpendicular to the fibers4.011.4

2.2. Experimental Samples—Protective Coatings

Three protective coatings were chosen: two fungicidal coatings and one fire-retardant coating, intended to protect wood used in both interior and exterior applications. They are applied using a coating method [57], which is eco-friendly and water-soluble. Fungicidal coatings, in particular, are non-washable from wood and are weather-resistant [58].
The retardant FR used in the experiment belongs to the group of retardants based on inorganic salts releasing non-flammable gases. The main advantage is good solubility of FR in water and easier application to surfaces. The disadvantage is the rapid loss of protective properties. In the case of the exposure of retardant-treated materials to weather conditions, especially water and snow, these materials lose the required ratio of necessary retardant substances. FR has a relatively short lifespan and therefore needs to be treated more often [40].

2.3. Preparation of Samples for Flame Initiation Monitoring

The preparation of samples for monitoring flame initiation was conducted in the Fire Laboratory of the Department of Fire Engineering at FBI UNIZA. The steps were carried out in accordance with HSE precautions [58] as follows:
  • Conditioning and Drying: From 1 July 2020 to 15 July 2020, spruce (marked as SP) and beech (marked as BCH) samples were conditioned and then dried (drying plant, Airtechno, Nová Dubnica nad Váhom, Slovakia). The sample mass was measured using digital scales with an accuracy of 0.001 g (Mettler Toledo, Columbus, OH, USA).
  • Sample Division and Coating: After mass stabilization, the samples were divided into three groups: the first group consisted of untreated samples, the second group was coated with a flame retardant (designated as FR), and the third group was successively treated with fungicidal coatings. The coatings, labelled FR, Bio, and Fun (Table 2), are water-dilutable (Table 3) and were applied in two coats. The drying time for the coatings was determined according to the manufacturers’ guidelines.
  • Storage: All samples were stored for 10 days.
  • Application of Flame Retardant: After the storage period, two additional coats of flame retardant were applied within 48 h, following the manufacturer’s instructions, to both the treated spruce (SMBio, SMFun) and beech (BCHBio, BCHFun) samples (see Table 2 and Table 4).
  • Long-Term Storage: The samples were Then stored for 8 months, both indoors (lumber storage) and outdoors (wood storage shed), from August 2020 to March 2021.
  • Mass Loss Rate Testing: Experimental determination of the mass loss rate began on 1 April 2021. In Table 4 are intentionally highlighted samples that were actually exposed to the flame; SP (spruce) wood samples are green and BCH (beech) wood samples are blue.
The sample treatment process presented above served to increase the resistance of wooden samples to heat. At the same time, a different method of storing the samples was chosen. Samples placed outdoors can lose their protective coating faster due to external factors (such as humidity, changes in weather conditions, or the action of biology) than samples in interiors. The extent to which this fact manifests itself can be assessed by testing the flammability of the samples with a small ignition initiator.

2.4. Experimental Determination of Mass Loss Rate Using a Small Ignition Source—Bunsen Burner

The experiments (Figure 1) were conducted in the Fire Laboratory at the Department of Fire Engineering, FBI UNIZA, under the specified conditions listed in Table 5.
A detailed description of the construction and modification of the apparatus (Figure 1a) is provided by [56]. The prepared apparatus simulates the placement of the sample in a vertical position, subjected to a flame with a length of 50 mm at an angle of 45°. The selected flame length of 50 mm was determined based on the established minimum distance of the chimney structure from wooden parts of the roof frame, according to the Slovak technical standard Reg. 401/2007 Col. [62]. Propane–butane gas was used as fuel for the Bunsen burner. The duration of the experiment was 30 min. The primary outputs of the experiment were as follows:
  • Mass loss of the sample;
  • Thickness of the charred layer;
  • Temperature curve (temperatures were recorded every 30 s) of the sample mass loss as a function of the exposure time.
During the experiment, the mass was continuously recorded at 30 s intervals. The accuracy of the mass measurements was ensured by scales (Mettler Toledo MS1602S/M01, Greifensee, Switzerland) with an accuracy to the hundredths of a gram. Accurate measurement times at specified intervals and automatic mass recording were facilitated by BalanceLink 4.2.0.1 (Mettler Toledo, Greifensee, Switzerland). Each sample was exposed to the flame for 30 min, with measurements repeated three times to ensure consistency.

3. Results

The wood samples were exposed to a 50 mm flame at an angle of 45° and a distance of 40 mm. During the 30 min experiment, both ignition and combustion processes occurred. Subsequently, the samples were extinguished, and the formation of a charred layer was observed. The differences between the samples can be quantified by comparing the obtained time–mass curves, mass loss rates, and changes in mass during the experiment. A significant difference was noted in the depth of charring, as evidenced by the thickness of the charred layer (Table 5, Figure 2).
The parameters of mass loss rates were calculated using the following Equations (1)–(3) (the results are shown in Table 6):
Δ m ( τ ) = m τ − m ( τ + τ )
where:
Δm(τ)—mass loss [g];
m(τ)—mass at time (τ) [g];
m(τ + Δτ)—mass at time (τ + Δτ) [g].
δ m ( τ ) = m τ − m ( τ + τ ) m τ
ν ( τ ) = δ m ( τ ) Δ τ
where:
ν(τ)—mass loss rate [%].
Table 6. Results of maximum mass loss (Δm, %), average mass loss rate (v, %·s−1), and average thickness of the charred layer (R, mm) for spruce and beech samples.
Table 6. Results of maximum mass loss (Δm, %), average mass loss rate (v, %·s−1), and average thickness of the charred layer (R, mm) for spruce and beech samples.
Sample TreatmentSamples
SpruceBeech
DesignationΔm (g)v
(%.s−1)		R (mm)DesignationΔm (g)v
(%.s−1)		R (mm)
UntreatedSP15.1300.008914.34BCH19.0860.011519.67
Treated with fire retardant FR, stored indoorsSPFRIn11.7460.006812.33BCHFRIn15.5020.009217.34
Treated with fire retardant FR, stored outdoorsSPFREx12.0790.007013.00BCHFREx16.0780.009616.34
Treated with fungicide coating Bio and fire-retardant FR, stored indoorsSPBioFRIn12.8590.006414.34BCHBioFRIn15.7400.009318.00
Treated with fungicide coating Bio and fire-retardant FR, stored outdoorsSPBioFREx10.0810.009414.54BCHBioFREx19.4610.011813.34
Treated with fungicide coating Fun and fire-retardant FR, stored indoorsSPFunFRIn11.220.006510.30BCHFunFRIn15.6710.009315.00
Treated with fungicide coating Fun and fire-retardant FR, stored outdoorsSPFunFREx13.5680.007911.67BCHFunFREx19.4810.011815.34

3.1. Results of the Thermal Degradation and Combustion Process of Spruce Samples

All spruce wood samples charred gradually during the experiment, regardless of the application of protective coatings (Figure 2).
Figure 3 compares the mass loss of all spruce samples during the experiment. The untreated sample showed the steepest slope, with the highest mass loss of 15.132% (Table 5). By contrast, the spruce samples treated only with the retardant displayed the lowest mass loss slope.
Since wood is a heterogeneous material, each sample exhibited distinct mass characteristics. Table 3 presents the average values calculated from all samples. Consequently, drawing meaningful conclusions from Figure 3 is not feasible, as it lacks sufficient interpretive value; therefore, a new quantifier was deemed necessary. The mass loss of each sample throughout the experiment was selected as the primary quantifier (Figure 4).
The mass loss of beech samples (Figure 4) exhibits a similar trend across all samples, except for the untreated sample. Specifically, the lowest mass loss is observed in the samples SPFunFRIn and SPBioFREx, indicating no significant influence of the storage location (indoors vs. outdoors). Notably, the combination of Bio fungicide and retardant (SPBioFRIn) demonstrates better thermal resistance compared to the conventional FR retarder coating (Figure 4).
The next highest mass loss (after the untreated sample) is recorded for SPFunFREx, while SPBioFREx demonstrates the best overall performance but exhibits the highest mass loss rate of 14.54% s−1 (Figure 5, Table 5).
The graph of Figure 5 presenting the mass loss rates of spruce samples (Figure 5) is particularly noteworthy for identifying deviations from the mean values. Of interest are the spruce samples treated solely with the retardant, which exhibit the lowest mass loss rates.
Spruce wood is used as a building material. Fire protection research is more focused on finding a suitable retarding agent [63]. Kmeťová et al. [64] used beams of spruce wood (Picea abies (L.) H. Karst.) that were treated with three different retardants: expandable graphite in combination with water glass, Bochemit Antiflash, and Bochemit Pyro. The samples were exposed to radiant heat, and the evaluation parameters included the sample mass and mass loss rate. The combination of expandable graphite with water glass yielded the best results, with a maximum mass loss (Δm) of 10% and a rate of <0.05%·s−1.

3.2. Results of Thermal Degradation and Combustion Process of Beech Samples

An illustration of the mass loss of the beech samples during the experiment is depicted in Figure 6. As expected, the untreated beech sample exhibits the steepest decline.
The mass loss data for beech samples (Figure 7) indicate a significant influence of storage conditions on the samples. The curves depicting mass loss for the BCHFunFRIn and BCHBioFREx samples closely resemble those of the untreated sample. After eight months of storage, the samples treated with both coatings exhibit similar behavior to the untreated samples, as evidenced by the formation of a charred layer and the mass loss rate.
The BCHFunFRIn sample exhibited the lowest mass loss, demonstrating that the combination of fungicide and fire retardant provides superior thermal resistance compared to the standard fire-retardant coating (Figure 7). The performance of the BCHBioFRIn samples is nearly identical to that of the samples treated solely with the fire retardant. This suggests that the additional protection provided by the fungicide coating beneath the fire retardant does not significantly enhance the thermal resistance of the beech samples. This conclusion is further supported by other parameters (Table 6).
The results of the combined treatment (fungicide + fire retardant) are comparable across samples but clearly highlight the impact of environmental conditions. Notably, the BCHBioFREx samples show the poorest performance, with a mass loss (Δm) of 19.461% and a mass loss rate (v) of 0.0118%·s−1, values that are comparable to those of the untreated BCH sample.
A visual representation of the behavior of beech samples is demonstrated in Figure 8.
The sample BCH FR Ex exhibited specific behavior, including deformation and cracking. This occurred particularly at 16 min (1000 s), which affected the mass loss rate (Figure 8b and Figure 9, with the sample indicated by the light-blue triangle).
A comparison of untreated beech samples (BCH) and beech samples treated with fire retardant (BCH FR) (Figure 9) demonstrates the effectiveness of the fire retardant up to 5 min (300 s).
The BCHFRFunIn sample demonstrated the best performance when exposed to flames (Figure 7). The combination of the Fun fungicide coating with the fire retardant resulted in the least mass loss. Repič et al. [65] confirmed the benefits of combining different wood protection methods. This combination is based on the thermal treatment of wood samples followed by mineralization [66,67]. Repič et al. [68] used European beech (Fagus sylvatica) to evaluate the effects of thermal modification and mineralization through the in situ formation of CaCO3. The sample prepared with this combination of methods was tested for the decay resistance, fire reaction, and mechanical properties of the wood. They confirmed that this combination provided the best response to fire as well as resistance to fungi.

4. Discussion

Based on the results of the conducted experiments, significant differences were sought. The first aspect examined was the impact of the wood species on the mass loss rate, mass loss, and thickness of the charred layer (Figure 10 and Table 6). In all the parameters observed, the beech sample exhibited inferior properties (Figure 11).
The thickness of the charred layer was determined after the end of the experiment by cutting the sample along its length into two equal halves. The thickness to which the wood was thermally degraded (black charred layer) was designated as the R thickness of the charred layer (in mm). An example of the formation of a charred layer is shown in Figure 10.
This fact, specifically the inferior parameters of beech, is also confirmed by Figure 12. The beech samples exhibit higher mass loss, and the results do not reveal any dependence on the different storage conditions (indoors and outdoors) (Figure 12a). The comparison of the charred layer indicates that the Fun fungicide coating is weaker (Figure 12b), as the charred layer is thicker than that of the Bio coating. Furthermore, comparing the mass loss rates (Figure 12c), the Fun fungicide coating demonstrates higher values than the Bio coating for both the spruce (SP) and beech (BCH) samples. The Bio and Fun coatings have relatively comparable chemical compositions (Table 3), with the Bio sample being enriched with the component “Alkyl (C12-16) dimethylbenzyl ammonium chloride”.
Our assumption regarding the superior performance of the Bio coating was not substantiated by the results. The authors of [69] conducted a similar study in which they combined fungicidal and fire-retardant coatings on isolated lignin. Coatings were applied by impregnation. In this study, sulphate lignin was modified with ammonium dihydrogen phosphate (ADP) and urea to achieve the phosphorylation and carbamylation of lignin isolated from Scots pine (Pinus sylvestris L.). Their goal was to protect wood from biological influences and fire. For their research, they employed DSC and TGA analyses and reported a slowing of combustion and an increased residue of charred wood at elevated temperatures. The modified wood exhibited excellent resistance to fungal and decay attacks under laboratory conditions.
All samples were intentionally stored for 8 months. The storage locations selected represented an interior (lumber storage) and an exterior (wood storage shed). The assessment of thermal resistance through a small ignition initiator, after eight months of storage, shows slight differences. In all cases (Figure 13a), samples stored outdoors exhibit higher mass loss rates (Figure 13a) and poorer thermal resistance (except for the SP Bio FR Ex sample). If mass loss is considered a critical factor (Figure 13b) indicating thermal resistance, the results are noteworthy. The beech sample demonstrated consistent behavior. The mass losses of the samples stored indoors are similar (Figure 13b). However, outdoors, the samples with the fungicide plus retarder coatings exhibit the highest mass losses (15.00 and 15.34 mm). The spruce samples display varied results (Figure 12b). Tracking the thickness of the charred layer (Figure 12b) does not allow for drawing relevant conclusions (Figure 12c).
The experimental results show differences in the thermal degradation of untreated and treated samples. The difference was also shown for various selected protective coatings. Of course, the difference is based on the chemical structure of the protective coatings. The choice of Bio and Fun was based on the practical use of the mentioned substances in practice. Both substances have a very similar chemical composition and basis of active substances: N-(3-aminopropyl)-N-dodecylpropane-1,3-diamine and propiconazole (hydrocarbon derivatives containing nitrogen atoms) (Table 3). All active substances are based on hydrocarbon derivatives with nitrogen and chlorine elements. By the number of presented substances, Fun has a richer mixture, which may also be reflected in better thermal resistance of treated samples (Figure 12).
The technological modification was the gradual application of protective coating solutions. Fungi coatings were applied first. After they dried, a retarding coating was applied. It is difficult to assume whether there was an interaction or reaction of the above chemicals, but in any case the retarder coating on the fungi coating was solid and non-washable. However, there are studies [69] that use iron orthophosphate as the integration of additive flame retardants for the purposes of preparing new epoxy resins. Thanks to the implementation of UL-94 tests (small ignition initiators) [70], the authors proved the reduction of pyrolysis products together with the formation of a dense and stable charred layer (22.0 wt.% reduction in total smoke production (by 36.3%) [69].
Our experiment only identified the depth of the char layer (Figure 11b and Figure 12c). The char layer was the largest in the untreated samples. Interestingly, the samples treated with only fungicide had the second-highest value after the untreated samples. It was the combination of fungi + retarder that had a more significant effect on reducing the char thickness than the applied retarder alone.

5. Conclusions

Based on the obtained experimental results, it is possible to draw conclusions from several perspectives, namely from the assessment of the influence of the wood, the influence of the protective coating, the influence of the combination of fungi and retarder, and last but not least, the influence of the place of storage of the samples. The results are as follows:
  • Impact of wood species: This has been confirmed. In all observed parameters, the beech samples exhibited worse results, including a higher mass loss, greater mass loss rates, and an increased charred depth. For spruce, the best variant was the combination of SPBioFRIn, while for beech, it was BCHFunFRIn.
  • Impact of fire-retardant coating: Untreated samples demonstrated the fastest thermal degradation with the highest charred layer. The fire-retardant treatment (FR) reduced both the mass loss and charred layer thickness in spruce and beech samples. The combination of the Bio coating and FR improved the protective characteristics of the surface in beech wood.
  • Impact of the combination of fungicidal coating and retardant: The results for the combination of FR + Fun were consistently higher, although a definitive conclusion cannot be drawn (Figure 12).
  • Storage time: The differences were minimal; samples placed outdoors exhibited behavior and quantitative results (Δm and v) similar to those of the untreated samples.
  • Location of use (outdoor and indoor): Better results were achieved by samples stored indoors, except for spruce samples treated with the Bio + FR coating. The assumption of the influence of external conditions on the quality (resistance to heat) of the protective coating was confirmed.
The aim of this article was to experimentally verify the quality of protective coatings for wood, specifically on samples of common spruce (Picea abies L. Karst.) and European beech (Fagus sylvatica L.), through exposure to a flame. This study serves as an initial probe into the potential combinations of protective coatings for wood to enhance protection against insects and heat exposure. The chemical substances used as protective coatings are widely represented in practical applications. We believe that the right combination for a suitable wood sample will create an effect of increased thermal resistance, and we plan to continue this research.

Author Contributions

Conceptualization, S.G. and I.M.; methodology, S.G.; software, B.L.; validation, I.M., B.L. and I.T.; formal analysis, I.T.; investigation, I.M.; resources, S.G.; data curation, B.L.; writing—original draft preparation, I.M.; writing—review and editing, S.G.; visualization, I.T.; supervision, B.L.; project administration, S.G.; funding acquisition, I.M. All authors have read and agreed to the published version of the manuscript.

Funding

This article was supported by the internal support system of the University of Žilina, No. 12716: Evaluation of fire-technical characteristics of natural and synthetic (including recycled) organic materials used in transport.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are openly available in Iveta Marková at https://orcid.org/0000-0001-9424-2024, reference number [7006353032].

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) 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) [56]; (b) demonstration of measuring spruce modified with the retardant SPFREx at the 25th minute of the experiment.
Figure 1. (a) 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) [56]; (b) demonstration of measuring spruce modified with the retardant SPFREx at the 25th minute of the experiment.
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Figure 2. Spruce samples after the experiment in all combinations: (a) untreated spruce stored in the SPIn interior; (b) spruce treated with HR retardant, stored in the SPFRIn interior; (c) spruce treated with Bio fungicide coating and FR retarder, stored in the SPBioFRIn interior; (d) spruce treated with Fun fungicide coating and FR retardant, stored in the SPFunFRIn interior; (e) untreated spruce stored outdoors (SPEx); (f) spruce treated with FR retardant, stored outdoors (SPHREx); (g) spruce treated with Bio fungicide coating and FR retardant, stored outdoors (SPBioFREx); (h) spruce treated with Fun fungicide coating and FR retardant, stored outdoors (SPFunFREx).
Figure 2. Spruce samples after the experiment in all combinations: (a) untreated spruce stored in the SPIn interior; (b) spruce treated with HR retardant, stored in the SPFRIn interior; (c) spruce treated with Bio fungicide coating and FR retarder, stored in the SPBioFRIn interior; (d) spruce treated with Fun fungicide coating and FR retardant, stored in the SPFunFRIn interior; (e) untreated spruce stored outdoors (SPEx); (f) spruce treated with FR retardant, stored outdoors (SPHREx); (g) spruce treated with Bio fungicide coating and FR retardant, stored outdoors (SPBioFREx); (h) spruce treated with Fun fungicide coating and FR retardant, stored outdoors (SPFunFREx).
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Figure 3. Actual mass loss of spruce samples during the experiment. Legend: black square with a red frame: SP—untreated spruce sample; dark-blue triangle: spruce treated with retardant and stored indoors (SPFRIn); light-blue triangle: spruce treated with retardant and stored outdoors (SPFREx); yellow rhombus: spruce treated with Bio fungicide coating and FR retardant, stored indoors (SPBioFRIn); green rhombus: spruce treated with Bio fungicide coating and FR retardant, stored outdoors (SPBioFREx); red rhombus: spruce treated with Fun fungicide coating and FR retardant, stored indoors (BCHFunFRIn); red circle: spruce treated with Fun fungicide coating and FR retardant, stored outdoors (SPFunFREx).
Figure 3. Actual mass loss of spruce samples during the experiment. Legend: black square with a red frame: SP—untreated spruce sample; dark-blue triangle: spruce treated with retardant and stored indoors (SPFRIn); light-blue triangle: spruce treated with retardant and stored outdoors (SPFREx); yellow rhombus: spruce treated with Bio fungicide coating and FR retardant, stored indoors (SPBioFRIn); green rhombus: spruce treated with Bio fungicide coating and FR retardant, stored outdoors (SPBioFREx); red rhombus: spruce treated with Fun fungicide coating and FR retardant, stored indoors (BCHFunFRIn); red circle: spruce treated with Fun fungicide coating and FR retardant, stored outdoors (SPFunFREx).
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Figure 4. Mass losses on spruce samples. Legend: black square with red frame: SP—untreated spruce sample; dark-blue triangle: spruce treated with retardant and stored indoors (SPFRIn); light-blue triangle: spruce treated with retardant and stored outdoors (SPFREx); yellow rhombus: spruce treated with Bio fungicide coating and FR retardant, stored indoors (SPBioFRIn); green rhombus: spruce treated with Bio fungicide coating and FR retardant, stored outdoors (SPBioFREx); red rhombus: spruce treated with Fun fungicide coating and FR retardant, stored indoors (BCHFunFRIn); red circle: spruce treated with Fun fungicide coating and FR retardant, stored outdoors (SPFunFREx).
Figure 4. Mass losses on spruce samples. Legend: black square with red frame: SP—untreated spruce sample; dark-blue triangle: spruce treated with retardant and stored indoors (SPFRIn); light-blue triangle: spruce treated with retardant and stored outdoors (SPFREx); yellow rhombus: spruce treated with Bio fungicide coating and FR retardant, stored indoors (SPBioFRIn); green rhombus: spruce treated with Bio fungicide coating and FR retardant, stored outdoors (SPBioFREx); red rhombus: spruce treated with Fun fungicide coating and FR retardant, stored indoors (BCHFunFRIn); red circle: spruce treated with Fun fungicide coating and FR retardant, stored outdoors (SPFunFREx).
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Figure 5. Comparison of mass loss rates of spruce samples. Legend: black cube with red frame (BCH): untreated spruce sample; dark-blue triangle: spruce treated with retardant and stored indoors (BCHFRIn); light-blue triangle: spruce treated with retardant and stored outdoors (BCHFREx); yellow diamond: spruce treated with fungicide coating (Bio) and retardant FR, and stored indoors (BCHBioFRIn); green diamond: spruce treated with fungicide coating (Bio) and retardant FR, and stored outdoors (BCHBioFREx); red diamond: spruce treated with fungicide coating (Fun) and retardant FR, and stored indoors (BCHFunFRIn); red circle: spruce treated with fungicide coating (Fun) and retardant FR, and stored outdoors (BCHFunFREx).
Figure 5. Comparison of mass loss rates of spruce samples. Legend: black cube with red frame (BCH): untreated spruce sample; dark-blue triangle: spruce treated with retardant and stored indoors (BCHFRIn); light-blue triangle: spruce treated with retardant and stored outdoors (BCHFREx); yellow diamond: spruce treated with fungicide coating (Bio) and retardant FR, and stored indoors (BCHBioFRIn); green diamond: spruce treated with fungicide coating (Bio) and retardant FR, and stored outdoors (BCHBioFREx); red diamond: spruce treated with fungicide coating (Fun) and retardant FR, and stored indoors (BCHFunFRIn); red circle: spruce treated with fungicide coating (Fun) and retardant FR, and stored outdoors (BCHFunFREx).
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Figure 6. Actual mass loss of beech samples during the experiment. Legend: black cube with a red frame (BCH): untreated beech sample, dark-blue triangle: beech treated with a fire retardant and stored indoors (BCHFRIn); light-blue triangle: beech treated with a fire retardant and stored outdoors (BCHFREx); yellow diamond: beech treated with a Bio fungicide coating and a fire retardant FR, stored indoors (BCHBioFRIn); green diamond: beech treated with a Bio fungicide coating and a fire retardant FR, stored outdoors (BCHBioFREx); red diamond: beech treated with a Fun fungicide coating and a fire retardant FR, stored indoors (BCHFunFRIn); red circle: beech treated with a Fun fungicide coating and a fire retardant FR, stored outdoors (BCHFunFREx).
Figure 6. Actual mass loss of beech samples during the experiment. Legend: black cube with a red frame (BCH): untreated beech sample, dark-blue triangle: beech treated with a fire retardant and stored indoors (BCHFRIn); light-blue triangle: beech treated with a fire retardant and stored outdoors (BCHFREx); yellow diamond: beech treated with a Bio fungicide coating and a fire retardant FR, stored indoors (BCHBioFRIn); green diamond: beech treated with a Bio fungicide coating and a fire retardant FR, stored outdoors (BCHBioFREx); red diamond: beech treated with a Fun fungicide coating and a fire retardant FR, stored indoors (BCHFunFRIn); red circle: beech treated with a Fun fungicide coating and a fire retardant FR, stored outdoors (BCHFunFREx).
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Figure 7. Mass loss in beech samples. Legend: black cube with a red frame (BCH): untreated beech sample; dark-blue triangle: beech treated with a fire retardant and stored indoors (BCHFRIn); light-blue triangle: beech treated with a fire retardant and stored outdoors (BCHFREx); yellow diamond: beech treated with Bio fungicide coating and a fire retardant FR, stored indoors (BCHBioFRIn); green diamond: beech treated with Bio fungicide coating and a fire retardant FR, stored outdoors (BCHBioFREx); red diamond: beech treated with Fun fungicide coating and a fire retardant FR, stored indoors (BCHFunFRIn); red circle: beech treated with Fun fungicide coating and a fire retardant FR, stored outdoors (BCHFunFREx).
Figure 7. Mass loss in beech samples. Legend: black cube with a red frame (BCH): untreated beech sample; dark-blue triangle: beech treated with a fire retardant and stored indoors (BCHFRIn); light-blue triangle: beech treated with a fire retardant and stored outdoors (BCHFREx); yellow diamond: beech treated with Bio fungicide coating and a fire retardant FR, stored indoors (BCHBioFRIn); green diamond: beech treated with Bio fungicide coating and a fire retardant FR, stored outdoors (BCHBioFREx); red diamond: beech treated with Fun fungicide coating and a fire retardant FR, stored indoors (BCHFunFRIn); red circle: beech treated with Fun fungicide coating and a fire retardant FR, stored outdoors (BCHFunFREx).
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Figure 8. Beech samples after the experiment in all combinations: (a) untreated beech, stored indoors (BCHIn); (b) spruce treated with fire retardant (FR), stored indoors (BCHFRIn); (c) spruce treated with Bio fungicide coating and fire retardant (FR), stored indoors (BCHBioFRIn); (d) spruce treated with Fun fungicide coating and fire retardant (FR), stored indoors (BCHFunFRIn); (e) untreated spruce, stored outdoors (BCHEx); (f) spruce treated with fire retardant (FR), stored outdoors (BCHFREx); (g) spruce treated with Bio fungicide coating and fire retardant (FR), stored outdoors (BCHBioFREx); (h) spruce treated with Fun fungicide coating and fire retardant (FR), stored outdoors (BCHFunFREx).
Figure 8. Beech samples after the experiment in all combinations: (a) untreated beech, stored indoors (BCHIn); (b) spruce treated with fire retardant (FR), stored indoors (BCHFRIn); (c) spruce treated with Bio fungicide coating and fire retardant (FR), stored indoors (BCHBioFRIn); (d) spruce treated with Fun fungicide coating and fire retardant (FR), stored indoors (BCHFunFRIn); (e) untreated spruce, stored outdoors (BCHEx); (f) spruce treated with fire retardant (FR), stored outdoors (BCHFREx); (g) spruce treated with Bio fungicide coating and fire retardant (FR), stored outdoors (BCHBioFREx); (h) spruce treated with Fun fungicide coating and fire retardant (FR), stored outdoors (BCHFunFREx).
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Figure 9. Comparison of mass loss rates in beech samples. Legend: black cube with a red frame (BCH): untreated beech sample; dark-blue triangle: beech treated with a fire retardant and stored indoors (BCHFRIn); light-blue triangle: beech treated with a fire retardant and stored outdoors (BCHFREx); yellow diamond: beech treated with Bio fungicide coating and a fire retardant FR, stored indoors (BCHBioFRIn); green diamond: beech treated with Bio fungicide coating and a fire retardant FR, stored outdoors (BCHBioFREx); red diamond: beech treated with Fun fungicide coating and a fire retardant FR, stored indoors (BCHFunFRIn); red circle: beech treated with Fun fungicide coating and a fire retardant FR, stored outdoors (BCHFunFREx).
Figure 9. Comparison of mass loss rates in beech samples. Legend: black cube with a red frame (BCH): untreated beech sample; dark-blue triangle: beech treated with a fire retardant and stored indoors (BCHFRIn); light-blue triangle: beech treated with a fire retardant and stored outdoors (BCHFREx); yellow diamond: beech treated with Bio fungicide coating and a fire retardant FR, stored indoors (BCHBioFRIn); green diamond: beech treated with Bio fungicide coating and a fire retardant FR, stored outdoors (BCHBioFREx); red diamond: beech treated with Fun fungicide coating and a fire retardant FR, stored indoors (BCHFunFRIn); red circle: beech treated with Fun fungicide coating and a fire retardant FR, stored outdoors (BCHFunFREx).
Fire 07 00463 g009
Fire 07 00463 g010
Figure 10. Examples of creating the thickness of the charred layer of selected spruce’s samples. (a) sample SPFunFRIn at 22 min ex-perimental time; (b) sample SPFunFRIn at 20 min experimental time. Legend: red arrows show the thickness of the char layer created durring experiment.
Figure 10. Examples of creating the thickness of the charred layer of selected spruce’s samples. (a) sample SPFunFRIn at 22 min ex-perimental time; (b) sample SPFunFRIn at 20 min experimental time. Legend: red arrows show the thickness of the char layer created durring experiment.
Fire 07 00463 g010
Fire 07 00463 g011
Figure 11. Mutual comparison of the observed parameters “ν”, “Δm”, and “R” between spruce and beech samples.
Figure 11. Mutual comparison of the observed parameters “ν”, “Δm”, and “R” between spruce and beech samples.
Fire 07 00463 g011
Fire 07 00463 g012
Figure 12. Mutual comparison of the observed parameters “ν”, “Δm”, and “R” between the Bio and Fun fungicidal coatings. (a) Δm(τ)—mass loss (g); (b) R—charring layer (mm); (c) ν(τ)—mass loss rate (%.s−1).
Figure 12. Mutual comparison of the observed parameters “ν”, “Δm”, and “R” between the Bio and Fun fungicidal coatings. (a) Δm(τ)—mass loss (g); (b) R—charring layer (mm); (c) ν(τ)—mass loss rate (%.s−1).
Fire 07 00463 g012
Fire 07 00463 g013aFire 07 00463 g013b
Figure 13. Mutual comparison of the observed parameters “ν”, “Δm”, and “R” between storage locations (indoor and outdoor). (a) ν(τ)—mass loss rate (%.s−1); (b) Δm(τ)—mass loss (g); (c) R—charring layer (mm).
Figure 13. Mutual comparison of the observed parameters “ν”, “Δm”, and “R” between storage locations (indoor and outdoor). (a) ν(τ)—mass loss rate (%.s−1); (b) Δm(τ)—mass loss (g); (c) R—charring layer (mm).
Fire 07 00463 g013aFire 07 00463 g013b
Table 2. Description of the methods of application of protective coatings.
Table 2. Description of the methods of application of protective coatings.
Sample NameCoating CompositionTreatment of Protected Coating
SpruceSPUntreatedUntreated (conditioning and drying during 15th day)
Coated spruce
(next 15th day)		SP FR
SP FR In
SP FR Ex		Treatment with retardantFirst coat of paint layer FR and after 48 h continued with second coat of paint layer of FR
Then stored for 8 months indoors
Then stored for 8 months exterior
SP Bio
SP Bio FR
SP Bio FR In
SP Bio FR Ex		Treatment with Fungicide—Bio
Treatment
with Fungicide Bio
+ Retardant FR		First coat of paint layer and after 48 h continued with second coat of paint layer Bio
First coat of paint Bio and after 48 h continued with second coat of paint Bio, followed by 10 days of drying and was made First coat of paint FR and after 48 h continued with second coat of paint FR
Then stored for 8 months indoors
Then stored for 8 months exterior
SP Fun
SP Fun FR
SP Fun FR In
SP Fun FR Ex		Treatment with Fungicide 2—Fun
Treatment with Fungicide Fun
+ Retardant FR		First coat of paint layer and after 48 h continued with second coat of paint
First coat of paint Fun and after 48 h continued with second coat of paint Fun, followed by 10 days of drying and was then applied first coat of paint FR and after 48 h continued with second coat of paint FR
Then stored for 8 months indoors
Then stored for 8 months exterior
BeechBCHUnteratedUntreated (conditioning and drying during 15th day
Coated beech
(next 15th day)		BCH FR
BCH FR In
BCH FR Ex		Treatment with retardantFirst coat of paint layer FR and after 48 h continued with second coat of paint layer
Then stored for 8 months indoors
Then stored for 8 months exterior
BCH Bio
BCH Bio FR
BCH Bio FR In
BCH Bio FR Ex		Treatment with Fungicide 1—Bio
Treatment with Fungicide Bio
+ Retardant FR		First coat of paint layer and after 48 h continued with second coat of paint layer Bio
First coat of paint Bio and after 48 h continued with second coat of paint Bio, followed by 10 days of drying and was made First coat of paint FR and after 48 h continued with second coat of paint FR
Then stored for 8 months indoors
Then stored for 8 months exterior
BCH Fun
BCH Fun FR
BCH Fun FR In
BCH Fun FR Ex		Treatment with Fungicide 2—Fun
Treatment with Fungicide Fun
+ Retardant FR		First coat of paint layer and after 48 h continued with second coat of paint layer Fun
First coat of paint Fun and after 48 h continued with second coat of paint Fun, followed by 10 days of drying and was then applied first coat of paint FR and after 48 h continued with second coat of paint FR
Then stored for 8 months indoors
Then stored for 8 months exterior
Table 5. Conditions of the experiment with a small ignition source.
Table 5. Conditions of the experiment with a small ignition source.
Atmospheric ConditionsExperimental Parameters
Air temperature [°C]20Flame angle [°]45
Moisture [%]47Flame length [mm]50
Pressure [hPa]980Duration of flame action [min]30
Table 3. Parameters of selected protective coatings.
Table 3. Parameters of selected protective coatings.
ParametersFungicidal Protective CoatingsFire-Resistant Coating
SourceBio [59]Fungi [60]FR [61]
Chemical compositionAlkyl (C12-16) dimethylbenzyl ammonium chloride, N-(3-aminopropyl)-N-dodecylpropane-1,3-diamine, propiconazole, tebuconazole and fenoxycarbN-(3-aminopropyl)-N-dodecylpropane-1,3-diamine, lactic acid, 2,2′-oxydiethanol, amines, coco alkyldimethyl, N-oxides, propiconazole, cypermethrin.Iron orthophosphate, citric acid, octadecan-1-ol, ethoxylated.
AppearanceColorless transparent liquidColorless transparent liquidColorless transparent liquid
Relative density at 20 °C (g/m3)1.00–1.031.01.1 ± 0.05
pH (at 20 °C)10.0–11.57.0–7.22.5
Non-volatile substances (%hm)5.6-10
Water solubility (%)100Yes, but the SDS does not list it.100
Notes Reaction to fire [7]
B-s1, d0
Table 4. Designation of test samples.
Table 4. Designation of test samples.
Sample Preparation ProcedureSample Designation
SpruceBeech
UntreatedSPBCH
Treatment with fungicide preparation 1 (Bio)SP BioBCH Bio
Treatment with fungicide preparation 2SP FunBCH Fun
Treatment with retardant (FR)SP FRBCH FR
Storage for 10 days
Fungicide treatment 1 (Bio)SP BioBCH Bio
Fungicide treatment 2 (Fun)SP FunBCH Fun
Fire retardant application and storage for 8 months
Storage locationInterior (lumber storage)Exterior (wood storage shed)Interior (lumber storage)Exterior (wood storage shed)
Spruce/beech retardant treatmentSP FR InSP FR ExBCH FR InBCH FR Ex
Spruce/beech treatment with retardant and fungicide BioSP Bio FR InSP Bio FR ExBCH Bio FR InBCH Bio FR Ex
Spruce/beech treatment with retardant and fungicide FunSP Fun FR InSP Fun FR ExBCH Fun FR InBCH Fun FR Ex
Note: wood samples SP (spruce) highlighted in green were tested with flame and wood samples BCH (beech) highlighted in blue were tested with flame.
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MDPI and ACS Style

Leitner, B.; Gašpercová, S.; Marková, I.; Tureková, I. Effect of Combining Fungal and Flame-Retardant Coatings on the Thermal Degradation of Spruce and Beech Wood Under Flame Loading. Fire 2024, 7, 463. https://doi.org/10.3390/fire7120463

AMA Style

Leitner B, Gašpercová S, Marková I, Tureková I. Effect of Combining Fungal and Flame-Retardant Coatings on the Thermal Degradation of Spruce and Beech Wood Under Flame Loading. Fire. 2024; 7(12):463. https://doi.org/10.3390/fire7120463

Chicago/Turabian Style

Leitner, Bohuš, Stanislava Gašpercová, Iveta Marková, and Ivana Tureková. 2024. "Effect of Combining Fungal and Flame-Retardant Coatings on the Thermal Degradation of Spruce and Beech Wood Under Flame Loading" Fire 7, no. 12: 463. https://doi.org/10.3390/fire7120463

APA Style

Leitner, B., Gašpercová, S., Marková, I., & Tureková, I. (2024). Effect of Combining Fungal and Flame-Retardant Coatings on the Thermal Degradation of Spruce and Beech Wood Under Flame Loading. Fire, 7(12), 463. https://doi.org/10.3390/fire7120463

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