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Article

The Effect of Chemical Modification by Synthetic and Natural Fire-Retardants on Burning and Chemical Characteristics of Structural Fir (Abies alba L.) Wood

by
Kateřina Hájková
*,
Přemysl Šedivka
,
Tomáš Holeček
,
Roman Berčák
and
Lukáš Sahula
Faculty of Forestry and Wood Sciences, Czech University of Life Sciences Prague, Kamýcká 129, 165 00 Prague, Czech Republic
*
Author to whom correspondence should be addressed.
Fire 2025, 8(3), 116; https://doi.org/10.3390/fire8030116
Submission received: 18 February 2025 / Revised: 15 March 2025 / Accepted: 17 March 2025 / Published: 18 March 2025
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Abstract

The effect of a surface coating with an aqueous solution containing a synthetic diammonium hydrogen phosphate fire retardant and vacuum pressure impregnation with a synthetic diammonium hydrogen phosphate fire retardant, potassium acetate, and a natural polymeric retardant, arabinogalactan, to improve the fire resistance and selected properties of structural fir (Abies alba L.) wood was investigated in this article. The combustion characteristics were investigated, and the heat of combustion reflects the effect of the presence of fire retardants. Changes in the content of cellulose, hemicelluloses, holocellulose, lignin, and extractives characterize the chemical changes in wood caused by these factors. The relationship between the combustion characteristics and chemical changes in chemically modified wood as a consequence of the presence of flame retardants were assessed using Fourier transform infrared spectroscopy. The results showed that the effectiveness of the fire retardants against ignition and burning when applied by vacuum pressure impregnation was always higher than in the case of surface coating, even when using impregnation solutions of low concentrations. In the case of diammonium hydrogen phosphate, a low 5% concentration of retardant was sufficient to provide suitable flame retardancy. Further, degradation by depolymerization of cellulose occurred only at temperatures between 460 and 470 °C. Low concentrations of retardant limit the loss to the environment and consequent ecological impact.

1. Introduction

Wood is one of the few materials suitable for construction applications that is fully renewable [1,2,3]. Climate change is altering the composition of forest stands. Spruce monocultures are undergoing species transformation [4,5,6]. Fir wood is a possible alternative to spruce [7], and surface impregnation or coating [8,9] or vacuum pressure impregnation [10,11] may improve its properties for structural applications [12]. With climate change, fir wood may become an alternative to spruce wood [13]. However, inherently low bio-resistance and durability [14,15], as well as limitations in terms of fire resistance of wood in structures [16,17] require innovation to meet the prescriptive regulations for the intended applications.
Chemical modification of wood is the treatment of wood with an impregnating agent that diffuses into the cell walls. Subsequent polymerization can alter and significantly improve the desired wood properties [18]. Chemical modification of wood can be divided into two primary types [19]. The first is active modification, in which the chemical properties of wood are altered through the reaction of hydroxyl groups of the polymeric components of wood, mainly cellulose, hemicellulose, and lignin [20]. A typical example is the reaction of wood hydroxyl groups with anhydrides (e.g., acetic anhydride, maleic anhydride, succinic anhydride) [21,22,23,24,25], which block hydroxyl groups in the chemical structure of wood and thus improve the hydrophilic properties, making the wood surface more hydrophobic [26]. In the second type of passive modification, the lumen and cell walls are impregnated without any chemical reaction of the modifying agent with the wood polymers. These modifiers improve the hydrophilic properties, biological resistance, durability, and dimensional stability of the wood, but the resulting fire resistance remains similar to that of unmodified wood [27].
Wood flame retardants can be divided into those suitable for surface coating (surface impregnation) and those effective for vacuum pressure impregnation. The materials should not be easily leachable or excessively volatile and should be transparent for a surface coating [28]. For vacuum pressure, they must possess suitable viscosity and the absence of solids [29,30]. A magnesium dioxide-based retardant dissolved in water is considered a nontoxic agent for increasing the fire resistance class of wood at a favorable price. However, it shows relatively low effectiveness for fire resistance and is the most suitable for surface penetration of wood [31]. Phosphates as flame retardants exhibit high effectiveness in lignocellulosic materials containing carboxyl groups [32,33]. Phosphates promote devolution in hydroxyl-containing structures during thermal degradation, thus promoting carbon formation to form a protective glass barrier containing polyphosphates, while also acting as radical scavengers, thus inhibiting heat transfer from the combustion zone while displaying very low toxicity [34,35]. However, phosphate salts are highly leachable in contact with water; so, vacuum pressure impregnation of wood is the most suitable for outdoor applications [36]. Alkali salts of various monovalent organic acids provide only a limited number of anionic types, which form stable melts with melting temperatures below 400 °C. These form a class of liquids practically applicable for vacuum-pressure impregnation as flame retardants, particularly acetate-based (e.g., potassium or sodium) [37]. Combining arabinoxylan and galactomannan with inorganic agents can provide products with excellent mechanical, fire retardant, and barrier properties [38,39,40,41].
In addition to the mentioned fire retardants, polymer-based retardants are utilized in the production of composites such as WPC (wood–polymer composite) [42] and NFRC (natural fiber-reinforced composites) [43]; alternatively, polyvinyl chloride [44] is employed for the production of multilayer materials or graphene-based materials [45].
Increasing the flame resistance of structural wood through surface treatment and vacuum pressure impregnation with flame retardants is gaining popularity. Researchers have investigated the relationships between fire behavior to develop a fir wood product suitable for environmentally friendly structural building applications. This product is designed to have favorable characteristics in terms of fire performance, specifically focusing on properties such as the heat of combustion and limiting oxidation number. Furthermore, the study considers the choice of retardants, including their application and impact on chemical components, such as surface treatment methods, vacuum pressure impregnation techniques, and the use of natural or synthetic retardants, as well as any change in crystal structure.
Fire retardants have already been used to increase fire resistance in various types of wood, including spruce [46,47,48,49], pine [47,50], and Douglas fir [51]. Specifically, arabinogalactan and diammonium phosphate have been utilized as retardants for thermally modified meranti [41] and black locust [52] wood or epoxy resins [53]. The application of a 5% concentration of retardant is based on its use with natural fibers [54,55,56,57], while a concentration of 20% has been employed for expanded polystyrene and sawdust [58].
Determining the crystal structure of cellulose is important because it influences the swelling rate, chemical resistance, and impregnation of wood [59]. Several factors contribute to the increase in the crystalline region, including the presence of xylose and mannose [60], enhanced vibrational motion of the glucose ring [61], and the dehydration of amorphous carbohydrates [62]. The state of the crystal structure can be examined through X-ray crystallography [59] or by a simpler non-destructive method using FTIR spectroscopy [63]. Additionally, some retardants, such as lysine, can reduce the crystalline region [64].
This research investigates the effectiveness of synthetic and natural fire retardants applied at concentrations of 5% and 15% using two different methods: surface treatment and vacuum pressure impregnation for fir wood. The study focuses on two key aspects related to the application of treated fir wood. The first aspect examines changes in the chemical structure, specifically the total crystallinity index. The second aspect assesses the fire characteristics of treated fir wood, including the heat of combustion and the limiting oxygen index.

2. Materials and Methods

2.1. Materials

The input material for the analysis was white fir (Abies alba L.), timber in the form of sawn timber qualitatively classified in structural strength class C25 by standard EN 338 [40]. Test samples of 10 × 10 × 20 mm were prepared to apply retardants from this timber. The flame retardants selected were inorganic diammonium hydrogen phosphate (DAP), organic potassium acetate (PAc), and arabinogalactan (AG) as a natural polymeric retardant, Table 1. This choice was made in order to determine which has the greatest effect on fire retardant properties, whether inorganic, organic, or natural. At the same time, these are minimally toxic retardants, so they can be used in building construction where appropriate.

2.2. Application of the Retardants

The first method of retardant application was surface coating. For this, 5% and 15% diammonium hydrogen phosphate solutions were applied at 90–110 g∙m−2. According to the above concentration, the required amount was dissolved in water at 90 ± 3 °C. This was followed by three brush coatings on the prepared fir samples. After coating application, the samples were left in the laboratory at laboratory temperature for 72 h and then transferred to a drying oven, where they were dried to determine the amount of coating of the retardant by weighing. However, because the amount of retardant transferred was low, retardant was applied to the other samples by impregnation.
Impregnation was carried out with 5% and 15% diammonium hydrogen phosphate solution (DAP), 5% and 15% potassium acetate solution (PAc), and 5% arabinogalactan (AG) solution. The substance of a given concentration was dissolved in hot water, and the samples were placed in a VTIZ 0.5 × 2 pressure impregnation chamber (VYVOS, spol. s r.o., Uherský Brod, Czech Republic), where they were subjected to 2 h of vacuum pressure impregnation by Bethell. Subsequently, the samples were removed, left at laboratory temperature for 72 h, and again placed in the dryer as in the previous coating application. After the samples were dried, the weight of the coating of the retardants in question was determined. All tests (fire resistance, DSC, FTIR) were conducted on three samples before and after retardant treatment.

2.3. Chemical Analysis

2.3.1. Chemical Analysis of Primary Raw Material

In addition to the analyses on the retardant-treated samples, chemical analysis was also performed on the primary untreated fir sample. In order to determine the macromolecular components, it was necessary first to perform an extraction to remove unwanted substances. Extraction was conducted into a toluene–ethanol mixture for subsequent cellulose determination, and acetone was used to determine lignin representation. The determination of the extractives was by standards Tappi T 280 wd-06 [65] and Tappi T 6 wd-73 [66]. The determination of lignin was carried out by the Klason method, using sulfuric acid according to the standards Tappi T 222 om-01 [67] and Tappi T 13 wd-73 [68]. For the determination of cellulose, the Seifert method was used [69,70]. The last chemical analysis was the solubility of cellulose or determination of alpha-cellulose, beta-cellulose, and gamma-cellulose according to Tappi T 203 cm-09 [71].

2.3.2. FTIR Analysis

The crystallinity was determined using a Fourier transform infrared spectrophotometer (FTIR). Before the measurements were taken, pellets with a diameter of 1.2 cm had to be produced. The pellets were produced on a Tempos TIRATEST 2850 equipment (TIRA GmbH, Schalkau, Germany) by pressing the weighed sample at a pressure of about 50 MPa to a thickness of 1 mm.
The pellets were analyzed using a Nicolet iS20 Fourier transform infrared spectrophotometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). Infrared spectra were obtained by accumulating 64 interferograms with a resolution of 4 cm−1 in absorption mode at wavelengths ranging from 400 to 4000 cm−1.
All samples were baseline corrected, the C–O stretching vibration in cellulose was assigned to them, and the average of multiple spectra was used as the average from the 1032 cm−1 band. After deconvolution, several parameters, the total crystallinity index (TCI), index lateral ordering (ILO), and hydrogen bonding intensity (HBI), were determined according to the relations [72,73]. The total crystallinity index is proportional to the total degree of cellular crystallinity in the wood, and the lateral ordering index represents ordered regions perpendicular to the direction of the chain.
T C I = A 1368 A 2894 ,
I L O = A 1429 A 897 ,
H B I = A 3332 A 1320

2.3.3. DSC Analysis

The samples were analyzed using differential scanning calorimetry (DSC) to predict how they would react and how their properties would be affected when retardant was applied. The measurements were carried out over a temperature range from 0 to 550 °C and used equipment from Mettler Toledo DSC 3+ (Mettler-Toledo GmbH, Greifensee, Switzerland).

2.4. Analysis of Combustion Heat Using an Isoperibol Calorimeter

Within the quantification of the combustion heat of materials, an isoperibol calorimeter of type 6400 (Parr Instrument Company, Moline, IL, USA) was utilized. The experimental protocol involves the oxidation of samples in a highly enriched oxygen atmosphere, leading to their complete conversion into final oxidation products. During this exothermic process, the amount of heat released is monitored under conditions of constant pressure of the thermodynamic system. The initial phase includes the mechanical processing of the material, where it is first ground into a fine powder, then homogenized and compressed into pellets weighing approximately 1 g. This pelletization is critical to ensuring the uniformity of the test units. The pellets are further dried to achieve an absolute absence of moisture, thereby eliminating its potential impact on the measurement. Before placement into the calorimetric device, the pellets are weighed with high precision to the thousandths of a gram, contributing to the high accuracy of the results. To initiate oxidation, the pellet is placed in the calorimetric cup so that a cotton fiber can be placed beneath it, allowing controlled ignition through an electrical resistance wire.

2.5. Limiting Oxygen Index

The limiting oxygen index (LOI) was measured on the FTT0077-01 (Fire Testing Technology, East Grinstead, UK). The samples for measurement were adjusted to a size of 100 × 10 × 10 mm. Three samples for each retardant treatment and three samples for the reference untreated sample were used. Before determination, the prepared samples were conditioned at 23 °C and 50% relative humidity. BS 2782-0 [74] and ISO 4589-2 [75] were used to carry out the limiting oxygen index measurements.

2.6. Scanning Electron Microscopy (SEM)

Samples were mounted on specimen stubs, sputter-coated with gold in the Q150R (Quorum, Lewes, UK) under argon atmosphere, and examined by scanning electron microscopy using a Mira3 Tescan instrument (Tescan, Brno, Czech Republic) operating at 15 kV.

3. Results

In the following analysis, untreated fir wood is compared with treated wood. Table 2 shows the retention of the retardant transferred to the samples.
The increase in retention clearly indicates that the impregnation process transfers significantly more active substance into the fir wood compared to the coating treatment. Among the tested substances, potassium acetate (PAc) at a concentration of 15% proved to be the most effective in terms of quantity. Notably, even lower concentrations of PAc yielded better results than impregnation with arabinogalactan (AG) and diammonium phosphate (DAP).

3.1. Chemical Analysis of Primary Raw Material

The results of the individual chemical components are shown in Table 3.

3.2. FTIR Spectrophotometry

From the FTIR spectra, Figure 1, the crystallinity parameters were calculated. Parameters such as the total crystallinity index (TCI), index lateral ordering index (ILO), and hydrogen bonding intensity (HBI) listed in Table 4 were evaluated using FTIR spectra. The reference is fir wood without any treatment.
The broad absorption band for –OH valence vibrations of polymers is 3600–3000 cm−1 [76]. The highest absorbance in this region was achieved by 15% potassium acetate. In the next region, the band of spectra with a wavenumber of 2900 cm−1, the C–H valence vibration is found in methylene groups [63]. Another distinct peak occurs in the region around 1700 cm−1, which is the region of aromatic functional groups. In this region, the absorbance values are identical to the reference untreated fir wood sample. Around 1500–1560 cm−1, ketones and carbonyls occur, and the fir wood sample treated with 15% potassium acetate completely dominates this region. The methoxyl follows this –OCH3 region around 1430–1470 cm−1, again dominated by the 15% potassium acetate impregnation. The next prominent peak in fir wood is around 1400 cm−1, which is the region of OH and CH bond bending or strain vibration of CH2 bonds in cellulose and hemicelluloses. Again, the highest absorbances were obtained for 15% potassium acetate. The band around 1300 cm−1 indicates the crystallinity of cellulose. The highest absorbance was obtained using 15% potassium acetate in this wavenumber. In the region around 1200 cm−1, there is vibration of C–O–C and C–O bonds; again, the impregnation with 15% potassium acetate is dominant. The highest peak was obtained for C–O stretching and deformation, the 1060 cm−1 region, and was dominated by 15% potassium acetate. Below the 900 cm−1 wavenumber, compounds based on C–H and C–C bonds are shown, but these values were very similar for all samples.

3.3. DSC Thermograms

The following Figure 2 shows the thermograms obtained by measuring the samples for each type of retardant.
In Figure 2, for diammonium hydrogen phosphate, the initial reaction is the same for both concentrations of retardant solutions (5% and 15%); water evaporation occurs at temperatures ranging from 70 to 100 °C, and for the wood sample impregnated with 15% solution, the water evaporation temperature is up to 100 °C. Again, the decomposition of the hemicelluloses occurs around a temperature of approximately 190 °C; for samples where the fir wood has been impregnated, it reaches values of over 200 °C. As the temperature increases, it is seen that the samples that have been treated with the coating do not need this amount of energy to start decomposition. It is confirmed that this concentration is not entirely suitable for treatment. On the contrary, the impregnated samples push the depolymerization of cellulose to temperatures around 460 to 470 °C; so, it is also suitable to use diammonium hydrogen phosphate impregnation for fir wood. At the same time, the thermogram shows that impregnation with a 5% solution is sufficient for fir wood.
In Figure 2, for potassium acetate, it is possible to see the three stages of degradation of the samples: water evaporation, hemicellulose breakdown, and cellulose breakdown. Here, we notice that for the sample impregnated with a 5% solution, almost the same energy is required to reach all three phases. The impregnation with 15% solution has an exothermic reaction in the first phase from the beginning of the measurement. For this sample, we can observe a trend that more energy is required for the reaction with increasing temperature. The highest increase in energy required is between 260 °C and 300 °C. The decomposition of cellulose can be seen for the sample with 15% potassium acetate at about 300 °C. The thermograph shows that impregnation with a 5% potassium acetate solution is less efficient than for the sample impregnated with a 15% solution. It is, therefore, preferable to use a 15% potassium acetate solution in the case of fir wood.
Figure 2, for arabinogalactan, shows the effect of impregnation with a 5% solution of arabinogalactan on fir wood. In the first stage, it can be seen that the sample with the 5% solution has an earlier exothermic reaction at a temperature of about 75 °C to 110 °C. In contrast, the reference sample does not react until about 80 °C to 130 °C. Once the water has evaporated, there is an almost linear slight increase in the energy required. A small exothermic process occurs at a temperature of about 460 °C. A fir sample treated with a 5% solution of arabinogalactan by impregnation does not demonstrate its effectiveness as a wood fire retardant on the DSC. According to the thermograph, the result for the arabinogalactan treated sample is very similar to the reference sample. Although research tries to move from synthetic or petroleum-based materials to bio-based materials, it also ensures sustainability. It fits into the circular economy, but it cannot solve the main risk of all materials: their flammability [76], similar to our arabinogalactan, which is nature-based but does not achieve the results of synthetic retardants.

3.4. Combustion Heat Analysis

The analysis of the total heat of combustion demonstrates that different flame retardants exert diverse effects on the total combustible heat of materials, Figure 3.
Our experimental data reveal that a 15% potassium acetate solution exhibits the most significant reduction in combustible heat, indicating that this flame retardant efficiently consumes oxygen during its complete oxidation reaction. The consumption of oxygen is directly proportional to the amount of the impregnated retardant, illustrating that lower concentrations, such as a 5% potassium acetate solution, have a less pronounced impact on reducing the combustible heat compared to a 15% solution.
The oxidation reaction, where oxygen acts as the primary reactant, leads to its depletion during the combustion of the treated organic substrate, thereby slowing the spread of flames. In the case of dihydrogen phosphate, which requires energy, specifically heat (Δt), for its oxidation reaction, this retardant can absorb the generated energy and utilize it for its oxidation reaction, thus reducing the available energy for fire propagation. The method of application of this retardant is significant; our data show that the application of a surface coating has a limited effect on reducing combustible heat, whereas the method of deep impregnation is more effective.

3.5. Limiting Oxygen Index

The limiting oxygen index determines the flammability of the material. The higher the LOI, the less flammable the material. The results of the samples treated with retardants are shown in Table 5.
It is clear from these values that the untreated sample is the most flammable. The natural arabinogalactan-based retardant hardly increased its non-flammability. The diammonium phosphate coatings already marked a slight increase but did not match the impregnation values. The LOI results confirmed that the higher the concentration used for impregnation, the higher the LOI value.

3.6. Scanning Electron Microscopy

Scanning electron microscope samples are shown in Figure 4.
In Figure 4b,c, it is clear that, compared to Figure 4a, small structures of retardant are visible; in this case, a coating of 5 and 15% diammonium phosphate was used. In Figure 4d,e, we can already see the inorganic structure of the diammonium phosphate, which was applied by depth impregnation. To better see the inorganic structure and how it is bonded to the fir wood, a detailed Figure 5 is shown. Figure 4g and then immediately afterward Figure 4f appear to be the most suitable in terms of SEM images, where potassium acetate was applied by impregnation and fully bonded to the pores of the fir wood. Figure 4h indicates that the arabinogalactan is indeed natural, and, therefore, under 30,000× magnification, it appears almost identical to the untreated fir wood corroborated in Figure 4a.
Figure 5 shows the application of 15% diammonium phosphate by deep impregnation. The crystalline structure formed by the inorganic retardant can be seen in Figure 5. Therefore, the most suitable retardant in terms of its ability to bind to the wood appears to be organic potassium acetate, which, from the previous Figure 4f,g, forms an almost continuous structure with the fir wood.

4. Discussion

The chemical analysis of the reference fir wood materials confirmed expectations. In the case of extractives, fir wood contains resin or fatty acids and sterols; hence, there are more extractives in the case of acetone. Our results also confirmed that more lignin is contained in conifers than in deciduous trees. The cellulose content of fir is several units of percent higher than expected. This may be due to, for example, the location, the climate where the wood was harvested, or other aspects that affect the chemical composition of the wood [77].
For fir (Abies alba L.), higher values of extractives of 1.86% were obtained, which decreased with increasing temperature, which is supported by our results of the DSC spectra. The amount of lignin, which increased with temperature, was higher in the original, untreated sample at 31.06%. Research on fir wood [78] confirms that the carbohydrate abundance decreased with increasing temperature, which is consistent with the fact that hemicelluloses were degraded. Even at 280 °C, xylose, arabinose and galactose almost disappeared in the samples [78].
In the region 1800–800 cm−1, cellulose, lignin, and hemicelluloses have characteristic absorption peaks. The peak of the wood treated with 15% potassium acetate impregnation indicates a high intensity at 1600 cm−1 of the C=O stretching vibration in the O=C–OH group of the glucuronic acid unit, which belongs to the hemicelluloses. The characteristics of cellulose were a CH bending vibration at 1368 cm−1 and ring stretching vibration at 1100 cm−1. The intensities of the absorption bands decreased with temperature [79], confirming the degradation of celluloses and hemicelluloses in the DSC thermograms.
TCI, the total crystallinity index, is proportional to the total degree of cellulose crystallinity in the wood. The index lateral order ILO correlates with the overall degree of ordering in the cellulose [80]. In our case, it can be seen that the TCI value for all types of retardants decreased significantly compared to the reference sample for fir wood. This could be because the original sample is recrystallized in amorphous regions due to the reorientation of cellulose inside these regions, rather than the sample being exposed to retardants. Besides this effect, the higher hydrophobicity of the reference samples could also cause this.
There was an increase in ILO values for fir wood treated with retardant. This phenomenon indicates that the treated samples undergo crystallization, and these values imply greater susceptibility to degradation in the amorphous regions.
The hydrogen bonding intensity of HBI is closely related to the amount of water absorbed [81]. This increase may be related to the fact that more oxygen-containing groups were formed by impregnation and water decomposition, which caused a modification of the molecular structure to form new hydrogen bonds in the crystalline cellulose. However, fir wood’s HBI values were identical to the reference sample.
In the DSC curves, similar to the fir (M. glyptostriboides) [82], an endothermic peak that corresponds to the maximum peak in the mass loss rate and precedes the maximum exothermic peak was found. The endothermic peaks are related to the depolymerization process, and the exothermic peaks are due to charring [83]. The inorganic fraction contained in wood can also catalyze thermolysis, which can reduce the volatile temperature regime—devolatilization [84].
The DSC curves behave in a standard way. First, there is a significant reduction in wood weight related to the loss of bound water and partly to extractives. In the temperature range of around 200 °C, the lignin and carbohydrate content increase, due to the release of degradation products similar to the thermal modification of fir (Abies alba L.) wood [85].
The role of the concentration of the impregnated solution is crucial, as confirmed by both coating and impregnation methods. These conclusions are supported by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) analyses, which corroborate the significant impact of the chosen retardant and its application method on reducing the combustible heat and slowing the spread of flames.
The heat of combustion is significantly higher compared to poplar wood at 13.75 MJ·kg−1, dropping to 13.32 MJ·kg−1 when treated with furfuryl alcohol and even to 7.63 MJ·kg−1 when treated with a mixture of urea and phytic acid [85]. Similar values were achieved in comparison with beech logs 18.56 MJ·kg−1 [86], olive kernel pellets after oil pressing 17.7 MJ·kg−1 [87], or poplar sawdust 18.1 MJ·kg−1 [87]. Furthermore, they were slightly lower compared to other conifers such as Scots pine 19.61 MJ·kg−1 [88], Norway spruce 19.9 MJ·kg−1 [16], silver fir 21.16 MJ·kg−1 [88], and larch 18.1 MJ·kg−1 [89].
The limiting oxygen index of fir wood was studied by Cavdar [90], who applied various chemical retardants. When 5% ACQ (composed of component N-alkylbenzyldimethyl ammonium chloride, copper tetraamine hydrogen carbonate, and ammonium) was applied, an LOI of 26 was achieved, 68, for 5% Tan-E (copper carbonate ethanolamine complex and tebuconazole) LOI 27.40, for 5% CCB (copper sulfate, potassium dichromate, and boric acid) LOI 33.37, and for the untreated fir wood sample, 25.33. These values are almost identical to our values without using toxic copper compounds. Although the sample after application of 5% CCB showed the highest LOI, this variant would no longer be applicable because of the proven carcinogenicity of potassium dichromate.
The results of the heat of combustion and the limiting oxygen index indicate that treating fir wood with a 15% concentration of DAP and PAc through impregnation yields the highest limiting oxygen values. This suggests that wood treated in this manner is less flammable. Additionally, the heat of combustion values obtained with pressure vacuum impregnation using a 15% concentration of potassium acetate confirm that this retardant is the most effective for enhancing fire-retardant properties.
Scanning electron microscope figures show unequivocal evidence of the presence of retardants. The formation of crystals characterized the inorganic-based retardant; the organic potassium acetate filled the pores and thus allowed better fire resistance of the wood, and the natural arabinogalactan appeared similar to the wood under magnification.
In this study, the fire efficiency of fir wood was investigated using the heat of combustion and the limiting oxygen number. The presence of retardants was demonstrated by SEM microscopy, and potassium acetate appeared to be the best of all the samples, so it would be good to look into it further in the future.

5. Conclusions

Wood is used in various industries, from energy and construction materials to raw materials for everyday use, due to its mechanical properties, renewability, and environmental friendliness. However, the fire performance of wood is another issue that needs to be understood regarding its fire resistance. In this work, fir wood treated with different types of fire retardants was studied regarding the amount of combustion heat.
In terms of application, the vacuum pressure impregnation method achieved better results, since retardants applied only by coating did not have a proper significant effect on the properties of the samples. Of the retardants used, 15% potassium acetate achieved the best results. This is evident from the results of the heat of combustion and the DSC thermograms.
When examining the crystallinity of the fir wood samples, no positive effect on the HBI hydrogen bonding intensity was obtained. It can even be said that the value was comparable to the reference sample. The overall crystallinity index significantly decreased after applying retardants, whereas the lateral order values increased.
The results show that 15% potassium acetate applied by vacuum pressure impregnation is the most suitable retarder for fir wood. Therefore, it would be a good idea in future research to also focus on hardwoods and this particular retardant’s application (coating/impregnation). At the same time, it would be good to test wood, such as the representation of the primary wood components for the treated samples. These findings could lead to new ways of treating building materials as hardwoods, which are increasingly used in several countries in terms of their availability for construction purposes.

Author Contributions

Conceptualization, K.H.; methodology, K.H., P.Š. and T.H.; validation, K.H., P.Š., T.H. and L.S.; formal analysis, K.H., T.H., R.B. and L.S.; investigation, K.H. and P.Š.; writing—original draft preparation, K.H., P.Š. and T.H.; writing—review and editing, K.H. and T.H.; visualization, K.H. and P.Š.; supervision, K.H.; project administration, K.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available on request due to ethical restrictions. The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors are grateful for the support of the project “Optimization of core wood surface adhesion to increase the strength of the glued joint”, No. A_04_22 financed by the Internal Grant Agency of the Faculty of Forestry and Wood, Czech University of Life Sciences in Prague.

Conflicts of Interest

The authors declare no conflicts of interest.

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Fire 08 00116 g001
Figure 1. FTIR spectra of fir wood.
Figure 1. FTIR spectra of fir wood.
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Figure 2. Thermogram of fir wood.
Figure 2. Thermogram of fir wood.
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Figure 3. Combustion heat analysis.
Figure 3. Combustion heat analysis.
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Figure 4. Scanning electron microscope (SEM) micrographs ((a)—reference; (b)—5% DAP coating; (c)—15% DAP coating; (d)—5% DAP impregnation; (e)—15% DAP impregnation; (f)—5% PAc impregnation; (g)—5% PAc impregnation; (h)—5% AG impregnation).
Figure 4. Scanning electron microscope (SEM) micrographs ((a)—reference; (b)—5% DAP coating; (c)—15% DAP coating; (d)—5% DAP impregnation; (e)—15% DAP impregnation; (f)—5% PAc impregnation; (g)—5% PAc impregnation; (h)—5% AG impregnation).
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Figure 5. Scanning electron microscope (SEM) detail micrographs.
Figure 5. Scanning electron microscope (SEM) detail micrographs.
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Table 1. Specifications of retardants.
Table 1. Specifications of retardants.
Retardants/Properties
of Retardants		Diammonium Hydrogen
Phosphate (DAP)		Potassium Acetate (PAc)Arabinogalactan (AG)
Chemical formula(NH4)2HPO4CH3CO2KC20H36O14
Characterizationinorganic substancesorganic substancesnatural polymer
Molar mass [g∙mol−1]132.0698.15285.29
Viscosity [mPa∙s]0.0012–0.00180.0030–0.00340.0100–0.0120
Density [kg∙m−3]161915701200
Melting point (decomposes) [°C]155 ± 5292 ± 5181 ± 30
Appearancecolorless monoclinic
crystals		white crystalswhite powder
Table 2. Amount of retardants.
Table 2. Amount of retardants.
Application of RetardantsRetention, %
Coating 5% DAP0.20 (0.04)
Coating 15% DAP0.33 (0.06)
Impregnation 5% DAP1.36 (0.11)
Impregnation 15% DAP2.32 (0.16)
Impregnation 5% PAc2.31 (0.08)
Impregnation 15% PAc3.98 (0.19)
Impregnation 5% AG1.80 (0.15)
Table 3. Chemical analysis of untreated fir wood.
Table 3. Chemical analysis of untreated fir wood.
ComponentExtractives
Acetone, %		Extractives
Ethanol-
Toluene, %		Lignin, %Cellulose, %Alpha-
Cellulose, %		Beta-
Cellulose, %		Gamma-
Cellulose, %
Fir wood2.61 (0.08)1.21 (0.03)26.84 (0.78)47.67 (1.18)38.86 (0.96)19.96 (2.37)9.58 (1.36)
Table 4. Crystallinity and other parameters of untreated and retardant-treated fir wood.
Table 4. Crystallinity and other parameters of untreated and retardant-treated fir wood.
Type and Treatment with RetardantTCIILOHBI
Reference1.7855 (0.126)0.5728 (0.056)1.0387 (0.121)
Coating 5% DAP1.0268 (0.008)1.0636 (0.007)1.0075 (0.002)
Coating 15% DAP1.0173 (0.005)1.0798 (0.011)1.0082 (0.003)
Impregnation 5% DAP0.9957 (0.016)1.0848 (0.002)1.0010 (0.010)
Impregnation 15% DAP1.0208 (0.002)1.1107 (0.024)1.0167 (0.010)
Impregnation 5% PAc0.9922 (0.007)1.0688 (0.013)1.0145 (0.001)
Impregnation 15% PAc0.9637 (0.016)0.9713 (0.014)1.0032 (0.016)
Impregnation 5% AG1.0163 (0.007)1.0434 (0.030)0.9827 (0.010)
Table 5. Limiting oxygen index.
Table 5. Limiting oxygen index.
ComponentReferenceCoatingImpregnation
5% DAP15% DAP5% DAP15% DAP5% PAc15% PAc5% AG
Fir wood20.7522.5823.4923.5527.7524.4028.7021.88
(0.25)(0.08)(0.11)(0.15)(0.05)(0.40)(0.30)(0.08)
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Hájková, K.; Šedivka, P.; Holeček, T.; Berčák, R.; Sahula, L. The Effect of Chemical Modification by Synthetic and Natural Fire-Retardants on Burning and Chemical Characteristics of Structural Fir (Abies alba L.) Wood. Fire 2025, 8, 116. https://doi.org/10.3390/fire8030116

AMA Style

Hájková K, Šedivka P, Holeček T, Berčák R, Sahula L. The Effect of Chemical Modification by Synthetic and Natural Fire-Retardants on Burning and Chemical Characteristics of Structural Fir (Abies alba L.) Wood. Fire. 2025; 8(3):116. https://doi.org/10.3390/fire8030116

Chicago/Turabian Style

Hájková, Kateřina, Přemysl Šedivka, Tomáš Holeček, Roman Berčák, and Lukáš Sahula. 2025. "The Effect of Chemical Modification by Synthetic and Natural Fire-Retardants on Burning and Chemical Characteristics of Structural Fir (Abies alba L.) Wood" Fire 8, no. 3: 116. https://doi.org/10.3390/fire8030116

APA Style

Hájková, K., Šedivka, P., Holeček, T., Berčák, R., & Sahula, L. (2025). The Effect of Chemical Modification by Synthetic and Natural Fire-Retardants on Burning and Chemical Characteristics of Structural Fir (Abies alba L.) Wood. Fire, 8(3), 116. https://doi.org/10.3390/fire8030116

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