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Article

Investigation of Pressure Vacuum Impregnation Using Inorganic, Organic, and Natural Fire Retardants on Beech Wood (Fagus sylvatica) and Its Impact on Fire Resistance

by
Tomáš Holeček
,
Přemysl Šedivka
,
Lukáš Sahula
,
Roman Berčák
,
Aleš Zeidler
and
Kateřina Hájková
*
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(8), 318; https://doi.org/10.3390/fire8080318
Submission received: 23 July 2025 / Accepted: 7 August 2025 / Published: 11 August 2025
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Abstract

This article investigates the effects of pressure vacuum impregnation using inorganic, organic, and natural flame retardants on enhancing the fire resistance and chemical composition of structural beech wood (Fagus sylvatica). The study examines fire resistance characteristics such as the limiting oxidation number and heat of combustion, which indicate the effectiveness of the flame retardants used. Chemical changes in the beech wood were characterized through various analyses, including changes in chemical composition, FTIR spectra, DSC thermograms, and SEM images. The relationships between combustion characteristics and chemical changes were assessed using multiple methods. The results demonstrate that using 5% potassium acetate achieved a lower heat of combustion compared to 15% sodium phosphate, and it was significantly lower than the heat of combustion observed with 5% arabinogalactan or the reference sample of beech wood. However, neither potassium acetate nor diammonium phosphate significantly affected the macromolecular structures of the wood when compared to the reference sample. Low concentrations of flame retardants reduce environmental release and environmental impact while increasing fire resistance, which could be used for structural solutions made of hardwoods.

1. Introduction

Wood is one of the natural materials that are suitable for structural applications due to their mechanical properties [1,2]. Owing to climate change, the species composition of economic forest stands is changing [3,4]. Mixed forest stands, more resistant to drought and bark beetle infestation, are increasingly planted in place of spruce monocultures [5]. Timber from these species-diverse stands can find large-scale applications in high-value-added construction products [1,2,3].
As a result of climate change, changes in the supply of raw materials to the manufacturing industry can be expected in favour of broadleaved species. As the market share of broadleaf beech wood increases, its use for applications will also increase. One of the main drawbacks of the wood, and therefore an obstacle to its broader use in construction, is its easy degradation [6]. Beech wood has an advantageous ratio of density and mechanical-physical properties, making it a suitable substitute for spruce wood in construction [1,2,3]. On the other hand, beech wood is at risk of limitations in terms of fire resistance.
Wood treatability is affected by many factors, primarily physical and chemical properties, as well as differences in wood anatomy [7]. Wood (hardwood/softwood) kind is very critical for predicting impregnation treatability. A combination of factors, including wood structure, responds to great variation. Particles size also play an important role in the process [8]. Wood is a porous material, and it can be impregnated with a wide range of impregnating agents. To ensure that the impregnation process is successful, the size of the particles used must be smaller than the diameter of the pores. Hardwoods are characterized by the presence of vessels, which are the most important pathways for fluids [9]. The diameter of cell lumens is typically in the range of 15–400 μm [10].
European beech (Fagus sylvatica) is one of Europe’s most important deciduous woods. Due to its high strength and hardness, it is becoming increasingly important in the construction industry. Beech is representative of the diffuse porous woods in terms of wood structure and, therefore, has a strong wood structure and good workability compared to other groups such as spruce or oak. In the case of beech wood, the heartwood does not occur naturally, and the vessels are not blocked by thyloses. Therefore, wood is much easier to impregnate and treat compared to timbers featuring occurrence of the heartwood [11]. The diameter of vessels in beech wood reaches up to 85 μm [12], which allows easy and fast penetration. Therefore, its impregnation is much easier than in softwoods, as Tondi et al. [13] reported.
Pressure vacuum impregnation is advantageous for effective impregnation of wood throughout the cross-section. Thus, the active impregnating agents penetrate deep into the wood structure, which provides long-term protection against biodegradation, unlike surface treatments [8]. This is because surface treatment leads to a possible reactivity of the impregnating agents with the surrounding environment. By synergistic effect, it is also possible to apply effective flame retardants by modifying the impregnation solution and simultaneously increasing the fire reaction class [14].
Fully soluble impregnating agents are suitable for pressure vacuum impregnation. For biodegradation protection, these can be impregnating agents based on synthetic salts [15] or natural substances [16]. For fire protection, these impregnating agents be further modified with flame retardants, of which there are many. Synthetic retardants may be non-ecological, but, on the other hand, they are effective [17]. Natural ones can be cheap but relatively ineffective for applications [18]. Therefore, further systematic research on these natural impregnating substances is needed to make the impregnation process environmentally friendly [19].
Impregnating agents should not be easily leachable or excessively volatile and, most importantly, should have a suitable viscosity for pressure vacuum impregnation [20,21]. Inorganic diammonium phosphate is highly efficient when used in lignocellulosic materials containing carboxyl groups [22,23]. Phosphates act as radical scavengers, thereby inhibiting heat transfer from the combustion zone and at the same time exhibiting low toxicity [18,24]. Alkaline salts formed from organic acids provide a limited number of anionic types that are stable below 400 °C, mainly sodium acetate- or potassium-based flame retardants [25]. Combining natural arabinoxylan and galactomannan can provide wood treatments with excellent fire retardant and mechanical properties [26,27,28].
This study was based on a previous publication [29] where the authors dealt with fir wood, and the treatment was both coating and vacuum pressure impregnation. Based on the research of [29], this research focused only on impregnation. According to [30,31], concentration was chosen, applying 5%. The research [32] focused on impregnating expanded polystyrene and sawdust with a concentration of 20%. Therefore, 15% was the second concentration chosen.
Therefore, this study focused on deciduous wood and its impregnation, specifically beech wood. The first aspect investigates the chemical changes, starting from the chemical representation of the primary wood components and utilizing FTIR spectra, DSC thermograms, or scanning electron microscope (SEM) images, which can provide insight into changes in the chemical structure. The second aspect evaluates the fire retardants’ properties, such as limiting oxygen number and heat of combustion of treated and reference beech wood samples.

2. Materials and Methods

2.1. Materials

Beech wood (Fagus sylvatica) was used as input material as sawn timber. Test specimens of 100 × 10 × 20 mm were made from this wood for vacuum pressure impregnation with fire retardants. Before impregnation, the samples were dried to constant mass, and their density was determined simultaneously.
The substances for fire resistance selected were diammonium hydrogen phosphate (DAP) CAS 7783-28-0 (Penta Chemicals, Pardubice, Czech Republic), potassium acetate (PAc) CAS 127-08-2 (Carl Roth, Karlsruhe, Germany), and arabinogalactan (AG) CAS 9036-66-2 (Merck SA, Darmstadt, Germany). The retardants were selected based on previous research [29], and they are also retardants with minimal toxicity, making them suitable for various industrial applications.

2.2. The Pressure Vacuum Impregnation

The impregnation of beech wood was carried out using 5% and 15% solutions of diammonium hydrogen phosphate (DAP), 5% and 15% solutions of potassium acetate (PAc), and 5% solutions of arabinogalactan (AG). The beech wood samples were placed in an impregnation vessel together with the given impregnation agent in the form of solutions and then placed in a VTIZ 0.5 × 2 pressure impregnation chamber (VYVOS, spol. s r.o., Uherský Brod, Czech Republic). Vacuum pressure impregnation was carried out according to Bethell. The Bethel method is based first on a vacuum of –70 kPa; this process takes 15 min. Subsequently, the impregnation was carried out at a pressure of 750 kPa for 2 h. Both parts of the impregnation were carried out at a laboratory temperature of 21 °C. The samples were then removed and left at laboratory temperature 21 °C for 72 h. After air drying, the samples were completely dried to the specified coating weights of the given retardants. The percentage of impregnating agent was calculated based on the weight and density of all samples (20 samples from each variant) before and after impregnation.

2.3. Chemical Analysis and Microscopy

2.3.1. Analysis of the Chemical Components

A chemical analysis of the wood components was performed on the impregnated samples and the reference sample of beech wood. The proportion of mineral inorganic substances was determined in the form of ash, according to Tappi T 211 om-02 [33]. Extraction into an ethanol–toluene mixture was performed by Tappi T 280 wd-06 [34], both to remove unwanted substances and to determine the amount of organic extractive substances. Lignin was determined using sulfuric acid by the Klason method by Tappi T 13 wd-73 [35]. Cellulose was determined by the Seifert method [36]. Hemicelluloses were determined by calculation as the difference between holocellulose according to Wise [37] and Seifert cellulose. Chemical analysis was conducted on five samples for each impregnation variant, including the reference sample. The arithmetic mean and standard deviation were calculated from the results.

2.3.2. Fourier Transform Infrared Spectroscopy (FTIR)

For analysis using Fourier transform infrared spectroscopy on the device, pellets with a diameter of 1.2 cm had to be produced. The pellets were produced on a Tempos TIRATEST 2850 device (TIRA GmbH, Schalkau, Germany) by pressing a weighed sample at a pressure of approximately 50 MPa to a thickness of 1 mm. Pellets were created in triplicate from disintegrated material smaller than 0.5 mm for each type of impregnation and reference.
The pellets were analyzed using Nicolet iS20 Fourier transform infrared spectrophotometry (Thermo Fisher Scientific Inc., Waltham, MA, USA). The infrared spectrum was obtained by accumulating 128 interferograms with a resolution of 4 cm−1 in absorption mode at wavelengths ranging from 400 to 4000 cm−1. The total crystallinity index can be calculated from the individual spectra, which, after correction of all samples to the baseline and assignment of the C-O vibration in cellulose, is calculated as the ratio of the absorbance at 1368 cm−1 to the absorbance at 2894 cm−1.

2.3.3. Differential Scanning Calorimetry (DSC)

Using differential scanning calorimetry on a Mettler Toledo DSC 3+ (Mettler Toledo GmbH, Greifensee, Switzerland), the effect and response of impregnated retardants were studied in comparison to untreated samples at a temperature range of 0–550 °C at a heating rate of 10 °C·min−1 in three repetitions.

2.3.4. Scanning Electron Microscopy (SEM)

The retardant-treated and reference samples were mounted on sample stems and sputter-coated with gold in a Q150 machine (Quorum Technologies Ltd., Lewes, UK). After application of sputter gold, they were examined with a Mira3 Tescan scanning electron microscope (Tescan, Brno, Czech Republic) at 15 kV. After scanning each sample, elemental analysis for selected elements was determined using AZtec program (Oxford Instruments, Abingdon, UK).

2.4. Fire Resistance Characteristics

2.4.1. Heat of Combustion

An isoperibol calorimeter type 6400 (Parr Instrument Company, Moline, IL, USA) was used to determine the heat of combustion. This experimental determination involves the oxidation of samples in a highly enriched oxygen atmosphere, resulting in complete conversion to the final oxidation products. The sample for determination was first ground to a fine powder, homogenized, and compressed into pellets weighing approximately 1 g. After pelletization, the sample was dried to achieve an absolute absence of moisture, thus improving the measurement result. The prepared sample was placed in a calorimetric steel tray so that a cotton filament could be placed underneath to allow controlled ignition using an electric resistance wire. During the measurement, the amount of heat released under constant pressure conditions of the thermodynamic system was monitored by ASTM D5865/D5865M-19 [38] in three repetitions.

2.4.2. Limiting Oxygen Index (LOI)

Samples for LOI determination were adjusted to a size of 100 × 10 × 10 mm. The analysis was performed in triplicate for each type of treated and reference sample. Before determination, the samples were conditioned at 23 °C and 50% relative humidity. The limiting oxygen number measurements were performed on an FTT0077-01 (Fire Testing Technology, East Grinstead, UK) by ISO 4589-2 [39] in three repetitions for each impregnated sample and reference.

3. Results and Discussion

Table 1 shows the retention of retardant for impregnated wood samples compared with reference beech wood samples. The amount of retardant retention impregnated into the sample was calculated as the difference between the absolutely dry weights before and after impregnation.
The highest retention retardant was potassium acetate (PAc), and retention was also high at a concentration of 5% arabinogalactan (AG). Unfortunately, at 5% diammonium phosphate, it was less than half of the highest achieved value. The amount of retention retardant is closely related to the intensity of hydrogen bonding [40]. This increase is associated with more oxygen-containing groups forming during impregnation, and water decomposition occurred during curing, which caused a change in the molecular structure. A concentration of 5% was chosen based on research work involving the application of phosphate retardants [30,31] and 15% based on impregnation with expanded polystyrene [32]. We did not want to increase these concentrations in order to avoid a decrease in retention, as was the case with beams treated with a solution of inorganic salts [41].

3.1. Analysis of the Chemical Components

Table 2 describes the percentage of chemical components of wood such as ash, extractives, cellulose, lignin, and hemicelluloses for both untreated and impregnated beech wood samples.
Chemical analysis of the untreated sample confirmed the expectation that the amount of extractives and ash in beech wood is low. The low content of extractives in untreated beech is suitable for applying retardant. This is because extractives are predominantly high-calorific substances compared to the macromolecular compounds contained in wood [42,43], which very easily dissolve in water [44]. Nevertheless, applying selected retardants increased these substances, especially potassium acetate, which is leachable.
The cellulose and hemicellulose content are the same for most retardants after application, but in the case of arabinogalactan, this amount increases. However, Čermák et al. [45], who studied the impregnation of beech wood, report that modification reduces cellulose content and increases hemicellulose content. The lignin content is the same for almost all retardants used. In the case of beech wood modification, the amount was almost identical to the reference, but the lignin content in the case of melamine formaldehyde resin treatment was found to be 22.3%, and in the case of acetylation, it decreased to 15.9% [45].

3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Figure 1 shows the measured FTIR spectra for the retardant-impregnated and reference samples. As expected, the highest absorbance in the region of –OH valence vibrations of polymers (3600–3000 cm−1) was achieved at a concentration of 5% for arabinogalactan. This broad, strong absorption band is related to the vibrations of –OH bonds resulting from hydrogen bonds between polysaccharide molecules [46]. In the next region, 2900 cm−1, there are –CH valence vibrations in aliphatic groups [47]; this region was the highest for the measured samples for 15% impregnation PAc.
The region is around 1700 cm−1, the region of aromatic functional groups. Beech wood dominates in this region, but all retardants’ absorbance values are almost identical. The methoxyl follows this –OCH3 region around 1430–1470 cm−1, where 15% potassium acetate impregnation again dominates for both woods.
Another significant peak is in the OH and CH bond bending region, the deformation vibration of CH2 bonds in cellulose and hemicelluloses (1330 and 1360 cm−1). The highest absorbance was again achieved for 15% PAc in the case of beech wood. The same applies to the band around 1300 cm−1, which indicates the crystallinity of cellulose. Similarly, in the region around 1250 cm−1, C–O–C and C–O bonds vibrate, causing the stretching of CO3 in lignin and xylan [48]. The highest peak was achieved for C–O stretching and deformation in the 1060 cm−1 region and was again dominated by potassium acetate at a concentration of 15%.
Below the wave number value of 900 cm, compounds based on C–H and C–C bonds, deformation in cellulose and hemicelluloses, are shown [48]. However, these values were very similar for all treated beech wood samples, corresponding to the individual samples’ chemical composition.

3.3. Differential Scanning Calorimetry (DSC)

DSC thermograms for the reference and impregnated beech wood samples in the temperature range 0–550 °C are shown in Figure 2.
In the case of DAP and PAc treatments, an endothermic reaction occurs first, during which mainly physically bound water evaporates. It is clear that when using DAP or PAc impregnation, evaporation occurs at a temperature of around 100 °C, while in the reference sample it occurs at a temperature of around 80 °C. However, AG-treated samples differ from this because natural substances crystallize first, rather than water evaporating as in other samples, similar to spruce impregnated with natural organic acids, where this reaction occurred at a temperature of 30 °C [49,50]. This temperature was reduced due to the formation of a new ordered structure that reduced the energy required for curing [51].
This is followed by exothermic reactions resulting from dehydration and condensation of the sample [20]. Impregnation is beneficial in this area because the decomposition of hemicellulose occurs at around 220–230 °C, compared to the reference sample, where this reaction occurs at around 180 °C. At this stage, 15% PAc behaves differently from the others, similar to AG, so these decompositions occur at a heat flux almost 10 mW higher.
In the final phase of violent decomposition, when cellulose degradation occurs, PAc at a concentration of 15% and AG at a concentration of 5% are manifested, requiring a significantly higher energy supply for decomposition than for the reference or DAP-treated samples. The effect of 5% AG and 15% PAc is suitable for the treatment of beech wood compared to the reference sample.

3.4. Scanning Electron Microscopy (SEM)

Using scanning electron microscopy, as seen in Figure 3, the elemental analysis was determined, as shown in Figure 4. Those elements that should be present in the samples were selected for this analysis.
Figure 3a–e show the visible structures of the retardant in comparison with Figure 3f (reference). SEM images demonstrate the presence of retardants, even at a magnification of 6000×. Inorganic DAP is characterized by crystal formation, but at a concentration of 15%, it begins to fill the wood’s pores. Organic PAc fills the pores at a concentration of 5%, but at 15%, it creates an almost uniform interconnected structure. Natural AG appears as crystals on wood fibres.
The AZtec program (energy dispersive X-ray spectroscopy integrated into SEM) was used to determine the presence of individual elements in impregnated wood samples (Figure 4a–e) compared to a reference sample (Figure 4f). In samples using 5% and 15% DAP, the nitrogen content (red colour) increased compared to the reference, and at higher concentrations, the phosphorus content (green colour) also increased slightly. We expected these two substances due to the composition of the retardant. However, the amount of silicon (yellow colour) also increased, which is almost absent in the reference sample, which may be due to the preparation of beech wood samples for impregnation with the retardant. In beech wood treated with PAc (Figure 4c,d), we can see an extreme increase in potassium (blue colour) and oxygen (pink colour), as expected. Figure 4e shows the use of 5% AG in this sample, where in addition to oxygen and carbon, as expected, there was also an increase in phosphorus, potassium, and especially nitrogen compared to the original sample.

3.5. Heat of Combustion

The calorimetric method was used to characterize the fuel properties, specifically to determine the heat of combustion and then calculate the higher heating value. The analysis of the heat of combustion shows that different retardants, such as impregnation substances, have different effects on the overall heat of combustion of beech wood (Figure 5).
Figure 5 shows a box plot of the distribution heat of combustion. The distribution patterns correspond to the oxygen index results (Table 3). In this data set, it is important to note that a 15% potassium acetate solution remains the most effective treatment, resulting in an approximately 6% reduction in calorific value compared to the reference samples.
The heat of combustion of the beech reference sample is almost identical to that of beech logs (18.56 MJ·kg−1 [52]) and poplar sawdust (18.1 MJ·kg−1 [53]). Compared to fir wood from previous research [29], there was a minor decrease when using 15% DAP from the reference value of 18.6 MJ·kg−1 to 18.3 MJ·kg−1, but with 5% DAP it went to 17.4 MJ·kg−1. Similar values were found for PAc 5%, 17.8 MJ·kg−1, and 15%,16.5 MJ·kg−1, for fir wood. However, in the case of arabinogalactan, a greater effect on the heat of combustion was demonstrated for deciduous wood than fir wood.

3.6. Limiting Oxygen Index (LOI)

Determining the limiting oxygen index (LOI) is a key assessment of the combustibility of the material, with elevated LOI values indicating reduced combustibility. The empirically obtained data (Table 3) is then processed using a reduction matrix, and the final LOI calculation is performed in strict accordance with the methodologies defined in the standards.
Limiting oxygen index (LOI) testing yielded clear conclusions regarding the reduction in flammability achieved by various treatment methods (Table 3). Pressure impregnation with 15% PAc showed the most significant improvement, with LOI increasing by nearly 37% compared to untreated reference samples. 5% PAc also improved flammability, albeit to a lesser extent, with approximately half the effectiveness of the 15% solution. Pressure impregnation with 15% DAP yielded positive results and was rated the second-best treatment. Compared to the values for the impregnation of black poplar (Populus nigra L.) with calcium hydroxide, where it increased from 26.75 after 60 min of impregnation and a concentration of 5% to 31.23 [54], which is a similar increase to that of PAc impregnation.
The limiting oxygen index of beech wood (Fagus orientalis Lipsky) was studied by Tan [55], who applied borax and mussel shell at concentrations of 10% and 15%. The LOI of the reference sample was 29.4 and increased to only 29.7 with 15% mussel shell but 41.7 with borax. They also tried applying a mixture where the LOI was 33.3, which confirms that mussel shell is also not a suitable natural retardant.

4. Conclusions

Although the characteristics of beech wood for current areas of use are well known, information on its performance in structural applications, in terms of fire protection, is limited. Therefore, this study investigated the heat of combustion and limiting oxidation number for beech wood impregnated with various retardants. The results show that 15% potassium acetate applied by vacuum pressure impregnation is the most suitable retardant for beech wood. This finding was demonstrated in terms of fire protection properties and changes in chemical structure, which is evident in DSC thermograms and FTIR spectra. This study examined the fire resistance of beech wood using combustible heat and the limiting oxygen index. Therefore, the presence of retardants was demonstrated using SEM microscopy, including changes in chemical structure. Potassium acetate is the most suitable retardant. However, natural arabinogalactan does not have suitable fire-retardant properties, so we will continue to investigate other natural retardants in future research. Finding a suitable natural retardant and appropriate applications could lead to new ways of treating building materials and, at the same time, reduce the burden on the environment.

Author Contributions

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

Funding

This article was funded by the Faculty of Forestry and Wood Sciences, CULS Prague, excellence project “The impact of fires on the wood quality of Central European climax tree species”.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Švajlenka, J.; Pošiváková, T. Innovation potential of wood constructions in the context of sustainability and efficiency of the construction industry. J. Clean. Prod. 2023, 411, 137209. [Google Scholar] [CrossRef]
  2. Santi, S.; Pierobon, F.; Corradini, G.; Cavalli, R.; Zanetti, M. Massive wood material for sustainable building design: The Massiv-Holz-Mauer Wall System. J. Wood Sci. 2016, 62, 416–428. [Google Scholar] [CrossRef]
  3. Nevins, M.T.; D’Amato, A.W.; Foster, J.R. Future forest composition under a changing climate and adaptive forest management in southeastern Vermont, USA. For. Ecol. Manag. 2021, 479, 118527. [Google Scholar] [CrossRef]
  4. Vacek, Z.; Prokůpková, A.; Vacek, S.; Bulušek, D.; Šimůnek, V.; Hájek, V.; Králíček, I. Mixed vs. monospecific mountain forests in response to climate change: Structural and Growth Perspectives of Norway Spruce and European Beech. For. Ecol. Manag. 2021, 488, 119019. [Google Scholar] [CrossRef]
  5. Jandl, R.; Spathelf, P.; Bolte, A.; Prescott, C.E. Forest adaptation to climate change-is non-management an option? Ann. For. Sci. 2019, 76, 48. [Google Scholar] [CrossRef]
  6. Reinprecht, L. Wood Deterior. Prot. Maintenance; Wiley-Blackwell: Hoboken, NJ, USA, 2016; p. 384. [Google Scholar]
  7. Ayanleye, S.; Udele, K.; Nasir, V.; Zhang, X.; Militz, H. Durability and protection of mass timber structures: A review. J. Build. Eng. 2022, 46, 103731. [Google Scholar] [CrossRef]
  8. Augustina, S.; Dwianto, W.; Wahyudi, I.; Syafii, W.; Gérardin, P.; Marbun, S.D. Wood impregnation in relation to its mechanisms and properties enhancement. BioResources 2023, 18, 4332–4372. [Google Scholar] [CrossRef]
  9. Tarmian, A.; Tajrishi, I.Z.; Oladi, R.; Efhamisisi, D. Treatability of wood for pressure treatment processes: A Literature Review. Eur. J. Wood Wood Prod. 2020, 78, 635–660. [Google Scholar] [CrossRef]
  10. Yin, J.; Song, K.; Lu, Y.; Zhao, G.; Yin, Y. Comparison of changes in micropores and mesopores in the wood cell walls of sapwood and heartwood. Wood Sci. Technol. 2015, 49, 987–1001. [Google Scholar] [CrossRef]
  11. Sandberg, D.; Kutnar, A.; Karlsson, O.; Jones, D. Wood Modification Technologies: Principles, Sustainability, and the Need for Innovation; CRC Press: Boca Raton, FL, USA, 2021; p. 442. [Google Scholar]
  12. Wagenführ, R. Holzatlas; Fachbuchverlag: Leipzig, Germany, 2007; p. 816. [Google Scholar]
  13. Tondi, G.; Thévenon, M.F.; Mies, B.; Standfest, G.; Petutschnigg, A.; Wieland, S. Impregnation of Scots pine and beech with tannin solutions: Effect of Viscosity and Wood Anatomy in Wood Infiltration. Wood Sci. Technol. 2013, 47, 615–626. [Google Scholar] [CrossRef]
  14. 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, 196. [Google Scholar] [CrossRef]
  15. Wen, M.Y.; Kang, C.W.; Park, H.J. Impregnation and mechanical properties of three softwoods treated with a new fire retardant chemical. J. Wood Sci. 2014, 60, 367–375. [Google Scholar] [CrossRef]
  16. Wang, K.; Meng, D.; Wang, S.; Sun, J.; Li, H.; Gu, X.; Zhang, S. Impregnation of phytic acid into the delignified wood to realize excellent flame retardant. Ind. Crops Prod. 2022, 176, 114364. [Google Scholar] [CrossRef]
  17. Marney, D.C.O.; Russell, L.J. Combined fire retardant and wood preservative treatments for outdoor wood applications—A Review of the Literature. Fire Technol. 2008, 44, 1–14. [Google Scholar] [CrossRef]
  18. Liang, Y.; Jian, H.; Deng, C.; Xu, J.; Liu, Y.; Park, H.; Wen, M.; Sun, Y. Research and application of biomass-based wood flame retardants: A Review. Polymers 2023, 15, 950. [Google Scholar] [CrossRef]
  19. Blanchet, P.; Pepin, S. Trends in chemical wood surface improvements and modifications: A Review of the Last Five Years. Coatings 2021, 11, 1514. [Google Scholar] [CrossRef]
  20. Huang, Y.; Zheng, Z.; Chen, C.; Guo, C.; Wang, X.; Zhou, Q.; Tu, D. Effective strategy for fabricating surface impregnated and unilaterally densified wood with furfuryl alcohol/flame retardants for enhanced mechanical performance and flame retardancy. Eur. J. Wood Wood Prod. 2024, 82, 731–745. [Google Scholar] [CrossRef]
  21. Liu, J.; Kong, X.; Wang, C.; Yang, X. Permeability of wood impregnated with polyethylene wax emulsion in vacuum. Polymers 2023, 281, 126123. [Google Scholar] [CrossRef]
  22. Brahmia, F.Z.; Zsolt, K.; Horváth, P.G.; Alpár, T.L. Comparative study on fire retardancy of various wood species treated with PEG 400, phosphorus, and boron compounds for use in cement-bonded wood-based products. Surf. Interfaces 2020, 21, 100736. [Google Scholar] [CrossRef]
  23. Stevens, R.; Es, D.S.; Bezemer, R.; Kranenbarg, A. The structure–activity relationship of fire retardant phosphorus compounds in wood. Polym. Degrad. Stab. 2006, 91, 832–841. [Google Scholar] [CrossRef]
  24. Yi, X.; Cao, S.; Hao, X.; Wang, Z.; Huang, Y.; Li, L.; Guo, C. Dimensionally stable, flame-retardant, and leach-resistant furfurylated wood prepared by incorporating ammonium polyphosphate and nano-silica. Polym. Adv. Technol. 2023, 34, 2501–2514. [Google Scholar] [CrossRef]
  25. Hazlewood, F.J.; Rhodes, E.; Ubbelohde, A.R. Melting mechanisms and melt properties of alkali acetates. Trans. Faraday Soc. 1966, 62, 3101–3113. [Google Scholar] [CrossRef]
  26. Hosseinashrafi, S.K.; Hosseinihashemi, S.K.; Gorji, P.; Akhtari, M. Environment-friendly waterborne fire retardants for protection of wood and bark against fire flames. BioResources 2023, 18, 7681–7699. [Google Scholar] [CrossRef]
  27. Leong, W.I.; Lo, O.L.I.; Cheng, F.T.; Cheong, W.M.; Seak, L.C.U. Using recombinant adhesive proteins as durable and green flame-retardant coatings. Synth. Syst. Biotechnol. 2021, 6, 369–376. [Google Scholar] [CrossRef] [PubMed]
  28. Hiremath, V.S.; Reddy, D.M.; Mutra, R.R.; Sanjeev, A.; Dhilipkumar, T.; Naveen, J. Thermal degradation and fire retardant behaviour of natural fibre reinforced polymeric composites—A Comprehensive Review. J. Mater. Res. Technol. 2024, 30, 4053–4063. [Google Scholar] [CrossRef]
  29. 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. [Google Scholar] [CrossRef]
  30. Suardana, N.P.G.; Ku, M.S.; Lim, J.K. Effects of diammonium phosphate on the flammability and mechanical properties of bio-composites. Mater. Des. 2011, 32, 1990–1999. [Google Scholar] [CrossRef]
  31. Gaan, S.; Sun, G. Effect of phosphorus flame retardants on thermo-oxidative decomposition of cotton. Polym. Degrad. Stab. 2007, 92, 968–974. [Google Scholar] [CrossRef]
  32. Terzi, E.; Kartal, S.N.; White, R.; Shinoda, K.; Imamura, Y. Fire performance and decay resistance of solid wood and plywood treated with quaternary ammonia compounds and common fire retardants. Eur. J. Wood Prod. 2011, 69, 41–51. [Google Scholar] [CrossRef]
  33. Tappi T 211 om-02; Ash in Wood, Pulp, Paper and Paperboard: Combustion at 525 °C. Tappi Test Methods: Atlanta, GA, USA, 2007.
  34. Tappi T 280 wd-06; Acetone Extractives of Wood and Pulp. Tappi Test Methods: Atlanta, GA, USA, 2015.
  35. Tappi T 13 wd-74; Lignin in Wood. Tappi Test Methods: Atlanta, GA, USA, 2015.
  36. Seifert, K. Uber ein neues Verfahren zur Schnellbestimmung Der Rein-Cellulose. Das Pap. 1956, 10, 301–306. [Google Scholar]
  37. Wise, L.E.; Murphy, M.; D’Addieco, A.A. Chlorite holocellulose, its fractionation and bearing on summative wood analysis and on studies on the hemicelluloses. Pap. Trade J. 1946, 122, 35–43. [Google Scholar]
  38. ASTM D5865/D5865M-19; Standard Test Method for Gross Calorific Value of Coal and Coke. American Society for Testing and Materials: Philadelphia, PA, USA, 2019.
  39. ISO 4589-2; Plastics—Determination of burning Behaviour—Oxygen Index Method. International Organization for Standardization: London, UK, 2017.
  40. Li, X.; Yue, K.; Tang, Z.; Lv, C.; Li, Q.; Wu, P.; Zhang, Z. Improvement in fire resistance of glulam beams containing modified laminae by thermal treatment, inorganic impregnation and compression in the fire-side tension zone. Constr. Build. Mater. 2024, 416, 135138. [Google Scholar] [CrossRef]
  41. Kačík, F.; Šmíra, P.; Kačíková, D.; Veľková, V.; Nasswettrová, A.; Vacek, V. Chemical alterations of pine wood saccharides during heat sterilisation. Carbohydr. Polym. 2015, 117, 681–686. [Google Scholar] [CrossRef]
  42. Domingos, I.; Ayata, U.; Ferreira, J.; Cruz-Lopes, L.; Sen, A.; Sahin, S.; Esteves, B. Calorific Power Improvement of Wood by Heat Treatment and Its Relation to Chemical Composition. Energies 2020, 13, 5322. [Google Scholar] [CrossRef]
  43. Rhén, C. Chemical Composition and Gross Calorific Value of the Above-Ground Biomass Components of Young Picea Abies. Scandnavian J. For. Res. 2004, 19, 72–81. [Google Scholar] [CrossRef]
  44. Bonfatti, J.E.A.; de Barros, J.M.R.; Silva, G.F.; Lengowski, E.C. A Comparative Analysis of Tannin and Commercial Fire Retardants in Wood Fire Protection. Forests 2024, 15, 951. [Google Scholar] [CrossRef]
  45. Čermák, P.; Baar, J.; Dömény, J.; Výbohová, E.; Rousek, R.; Pařil, P.; Oberle, A.; Čabalová, I.; Hess, D.; Vodák, M.; et al. Wood-water interactions of thermally modified, acetylated and melamine formaldehyde resin impregnated beech wood. Holzforschung 2022, 76, 437–450. [Google Scholar] [CrossRef]
  46. Yuan, J.M.; Feng, Y.R.; He, L.P. Effect of thermal treatment on properties of ramie fibers. Polym. Degrad. Stab. 2016, 133, 303–311. [Google Scholar] [CrossRef]
  47. Hong, T.; Yin, J.Y.; Nie, S.P.; Xie, M.Y. Applications of infrared spectroscopy in polysaccharide structural analysis: Progress, Challenge and Perspective. Food Chem. 2021, 12, 100–168. [Google Scholar] [CrossRef]
  48. Feng, X.; Chen, J.; Yu, S.; Wu, Z.; Huang, Q. Mild hydrothermal modification of beech wood (Zelkova schneideriana Hand-Mzt): Its Physical, Structural, and Mechanical Properties. Eur. J. Wood Prod. 2022, 80, 933–945. [Google Scholar] [CrossRef]
  49. Grzybek, J.; Paschová, Z.; Meffert, P.; Petutschnigg, A.; Schnabel, T. Impregnation of Norway spruce with low melting-point binary fatty acid as a phase-change material. Wood Mater. Sci. Eng. 2023, 18, 1755–1764. [Google Scholar] [CrossRef]
  50. Du, W.; Fei, H.; He, Q.; Wang, L.; Pan, Y.; Liu, J. Preparation and properties of capric acid–stearic acid-based ternary phase change materials. RSC Adv. 2021, 11, 24938–24948. [Google Scholar] [CrossRef] [PubMed]
  51. Song, F.; Xu, X.; Jiang, P.; Chen, Z.; Jia, H.; Yu, W.; Wang, L. Preparation of wood scrimber with low hygroscopicity and high flame retardancy through impregnation of silicon-boron inorganic salt. Constr. Build. Mater. 2025, 467, 140384. [Google Scholar] [CrossRef]
  52. Wang, J.; Wang, G.; Zhang, M.; Chen, M.; Li, D.; Min, F.; Chen, M.; Zhang, S.; Ren, Z.; Yan, Y. A comparative study of thermolysis characteristics and kinetics of seaweeds and fir wood. Process Biochem. 2006, 41, 1883–1886. [Google Scholar] [CrossRef]
  53. Pappa, A.A.; Tzamtzis, N.E.; Statheropoulos, M.K.; Parissakis, G.K. Thermal analysis of Pinus halepensis pine needles and their main components in the presence of (NH4)2HPO4 and (NH4)2SO4. Thermochim. Acta 1995, 261, 165–173. [Google Scholar] [CrossRef]
  54. Beram, A. Enhancing Surface Characteristics and Combustion Behavior of Black Poplar Wood through Varied Impregnation Techniques. Appl. Sci. 2023, 13, 11482. [Google Scholar] [CrossRef]
  55. Tan, H. Crushed Mussel Shell Powder and Optional Borax in Surface Char Layers to Protect Four Wood Species Against Fire. BioResources 2022, 17, 5319–5334. [Google Scholar] [CrossRef]
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Figure 1. FTIR spectra of beech wood.
Figure 1. FTIR spectra of beech wood.
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Figure 2. DSC thermogram of beech wood.
Figure 2. DSC thermogram of beech wood.
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Figure 3. Scanning electron microscope (SEM) micrographs of beech wood ((a) 5% DAP; (b) 15% DAP; (c) 5% PAc; (d) 15% PAc; (e) 5% AG; (f) reference).
Figure 3. Scanning electron microscope (SEM) micrographs of beech wood ((a) 5% DAP; (b) 15% DAP; (c) 5% PAc; (d) 15% PAc; (e) 5% AG; (f) reference).
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Figure 4. Elemental analysis using SEM of beech wood ((a) 5% DAP; (b) 15% DAP; (c) 5% PAc; (d) 15% PAc; (e) 5% AG; (f) reference).
Figure 4. Elemental analysis using SEM of beech wood ((a) 5% DAP; (b) 15% DAP; (c) 5% PAc; (d) 15% PAc; (e) 5% AG; (f) reference).
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Figure 5. Heat of combustion analysis of beech wood.
Figure 5. Heat of combustion analysis of beech wood.
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Table 1. Amount of retardants.
Table 1. Amount of retardants.
Application of RetardantsRetention, %
Impregnation 5% DAP 1.21 (0.09)
Impregnation 15% DAP2.29 (0.23)
Impregnation 5% PAc2.09 (0.17)
Impregnation 15% PAc2.72 (0.11)
Impregnation 5% AG2.32 (0.11)
Standard deviation values are in parentheses.
Table 2. Chemical analysis of beech wood.
Table 2. Chemical analysis of beech wood.
Wood Sample/Chemical
Components		Ash,
%		Extractives,
%		Cellulose,
%		Lignin,
%		Hemicelluloses, %
Reference0.52 (0.01)1.21 (0.03)33.81 (1.36)21.17 (0.17)31.19 (0.47)
Impregnation 5% DAP 0.90 (0.03)1.51 (0.04)33.96 (1.07)21.23 (0.70)31.03 (2.28)
Impregnation 15% DAP1.26 (0.04)1.65 (0.43)33.89 (0.70)21.91 (0.91)31.11 (0.34)
Impregnation 5% PAc2.95 (0.08)2.41 (0.10)33.56 (1.25)20.15 (1.68)31.44 (1.71)
Impregnation 15% PAc4.88 (0.15)8.54 (0.38)33.61 (0.72)21.15 (0.29)31.39 (1.54)
Impregnation 5% AG0.72 (0.03)0.84 (0.02)37.87 (1.13)20.60 (1.68)37.13 (0.83)
Standard deviation values are in parentheses.
Table 3. Limiting oxygen index of beech wood.
Table 3. Limiting oxygen index of beech wood.
5% DAP15% DAP5% PAc15% PAc5% AGReference
23.65
(0.76)		27.84
(0.38)		24.43
(0.20)		28.73
(0.15)		22.55
(0.76)		20.98
(0.13)
Standard deviation values are in parentheses.
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MDPI and ACS Style

Holeček, T.; Šedivka, P.; Sahula, L.; Berčák, R.; Zeidler, A.; Hájková, K. Investigation of Pressure Vacuum Impregnation Using Inorganic, Organic, and Natural Fire Retardants on Beech Wood (Fagus sylvatica) and Its Impact on Fire Resistance. Fire 2025, 8, 318. https://doi.org/10.3390/fire8080318

AMA Style

Holeček T, Šedivka P, Sahula L, Berčák R, Zeidler A, Hájková K. Investigation of Pressure Vacuum Impregnation Using Inorganic, Organic, and Natural Fire Retardants on Beech Wood (Fagus sylvatica) and Its Impact on Fire Resistance. Fire. 2025; 8(8):318. https://doi.org/10.3390/fire8080318

Chicago/Turabian Style

Holeček, Tomáš, Přemysl Šedivka, Lukáš Sahula, Roman Berčák, Aleš Zeidler, and Kateřina Hájková. 2025. "Investigation of Pressure Vacuum Impregnation Using Inorganic, Organic, and Natural Fire Retardants on Beech Wood (Fagus sylvatica) and Its Impact on Fire Resistance" Fire 8, no. 8: 318. https://doi.org/10.3390/fire8080318

APA Style

Holeček, T., Šedivka, P., Sahula, L., Berčák, R., Zeidler, A., & Hájková, K. (2025). Investigation of Pressure Vacuum Impregnation Using Inorganic, Organic, and Natural Fire Retardants on Beech Wood (Fagus sylvatica) and Its Impact on Fire Resistance. Fire, 8(8), 318. https://doi.org/10.3390/fire8080318

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