Effect of Expandable Graphite Flakes on the Flame Resistance of Oak Wood

Abstract

One of the strategies to improve the fire resistance of wood is to use flame retardants. It would be best to find an ecological, nonhalogenated flame retardant to improve the fire protection properties. In this work, oak wood (Quercus robur L.) samples were treated with an aqueous solution of sodium silicate and expandable graphite flakes, which were applied to different parts of the samples: only on the top, on the sides and together on the top and sides of samples. The fire characteristics of samples were studied by a non-standard test method—a radiant heat source test which is used to determine the mass loss and ignition time of the tested samples (50 mm × 40 mm × 10 mm), and the measurement was carried out using a visual recording of a thermal camera. The results of the laboratory test method showed a significant positive effect of the application of the retardant treated only on the top and together on the top and the sides of the samples in terms of decreasing the mass loss and the course of temperature. When we treated only the sides of the sample, the results were closer to the untreated samples, so there was more than 80% weight loss and a significant temperature increase. The results demonstrated that the appropriate modification of the wood using sodium silicate and expandable graphite flakes has the potential to reduce the loss of mass by 79% and reduce the rise in temperature on the surface of the sample.

1. Introduction

Wood and wood materials are widely used in the interior and exterior: structural building elements, tiles, floors, furniture, etc. Wood is a renewable, environmentally friendly material with many excellent mechanical, physical and aesthetic properties. On the other hand, dry wood does burn quite easily when heated above its ignition temperature. The research and development of a new flame retardant for wood protection are given great attention from an ecological, economical and legislative point of view. There are various types of retardants that are based on different principles, e.g., inorganic (silica, titania and zinc oxide) and organic (graphene, graphite, graphene nanoplatelet, phosphorus-modified wheat starch, modified nanocellulose and lignin nanoparticles) [1,2,3,4,5]; however, the most hopeful are intumescent flame retardants (IFR). The IFR mechanism works by creating a char layer, due to heat exposure, between the heat source and the protected material [6,7,8].

One of the strategies to enhance the fire protection of wood is to treat it with flame retardants. The goal of flame retardants is to delay or prevent the ignition and to diminish the effects of combustion. We can expand the use of wood products in construction precisely by improving its behavior in the case of fire [9]. Three of the most common fire retardancy mechanisms are known: (I) the gas phase inhibition mechanism, where the FRs react with the polymer under combustion in the gas phase with hydroxyl or oxygen agents at the molecular level and extinguish the combustion (e.g., halogenated and phosphorous compounds); (II) hydrated minerals decompose in an endothermic reaction, using a cooling mechanism with the release of water and by diluting or removing the flammable fuels and oxygen; (III) char-forming polymers undergo an endothermic decomposition reaction at an elevated temperature that causes the coating to swell and form into a highly porous, thick and thermally stable char layer that has a very low thermal conductivity (e.g., cellulose or carbon family retardants). These IFRs can be used instead of the halogen-containing flame retardants, which are free of halogen and have a relatively high efficiency [6,8,10]. Furthermore, densified wood effectively reduces heat transfer and combustion rates when exposed to flames, resulting in improved fire retardant and heat-insulating properties during combustion [11].

The expandable graphite (EG) flake is an environmentally friendly nonhalogenated fire retardant for materials to improve fire-protection characteristics. Exposing the graphite flake to concentrated sulfuric acid in combination with other strong oxidizers, such as nitric acid, potassium permanganate, hydrogen peroxide and sodium chromate, is achieved by EG through flake graphite intercalation [6]. The EG structure consists of layers of hexagonal carbon structures within which a sulfuric acid can be embedded. When exposed to a source of heat, sulfuric acid decomposes into gaseous products (water and sulfur dioxide).

More details on the mechanism of action of EG can be found in Wang et al. [12] and Tomiak et al. [13]. Generally, EG begins to expand in the temperature range from 140 to 230 °C, with an expansion rate usually between 100 and 400 mL/g, depending on the carbon content, particle size and reaction conditions. Blowing agents intercalated between the graphene layers react when a critical temperature is reached. They form a voluminous and thermally stable char-residue with a typically worm-like structure. The layer creates an insulating layer to prevent the transfer of heat and flammable gas flows [13].

Due to its solely physical effect, EG can be used in various polymeric systems, e.g., polyethylene—PE, polypropylene—PP, polystyrene—PS, polyvinylchloride—PVC, acrylonitrile butadiene styrene—ABS, polyamide 6—PA6 [14].

It is also used to protect wood–plastic composites (WPC) of wood fiber/flour and thermoplastic(s) (including PE, PP, PVC, etc.). The types of plastics normally used in WPC formulations have higher fire hazard properties than wood alone, as plastic has a higher chemical heat content and can melt [15].

Wood is a mixture of polymers composed of partially crystalline cellulose microfibrils and large amorphous hemicelluloses and lignin molecules. In lignin, phenyl propane units create chains which are crosslinked in a three-dimensional, amorphous structure and linked to the cellulose fibrils via hemicelluloses [16].

However, because of the inherent flammability of wood and wood products, they often contribute to unwanted fires, causing numerous injuries and fatalities. The use of wood is, therefore, regulated by various safety requirements and regulations pertaining to its flammability and the spread of fire characteristics. Timber products are commonly treated with fire retardants with the intention to improve the reaction to fire [17].

Oak is an attractive species for various interior and exterior applications. It is an ideal choice for frameworks, doors and gates as well as parquets and flooring. As a construction material, oak wood is particularly well suited for structures with high demands for mechanical properties. However, wood is a flammable material and it is therefore necessary to protect it from ignition. Oak is classified as very heavily treatable, and impregnation is therefore challenging; for example, Franke and Volkmer [18] treated oak wood with a new fire retardant based on in situ calcium oxalate deposition. Other authors used exfoliated and reassembled graphite on the wood surface [19], mixing wood particles with EG [20] and EG with an intumescent flame retardant in wood flour-polypropylene composites [15]. However, the application of EG to flame-retardant coatings for wood and other lignocellulosic materials is still not reported adequately, although some works on this issue have been published [6,21]. The objective of this work was therefore to evaluate three different variants of applying sodium silicate (an aqueous solution) and EG flakes to a sample to enhance the fire resistance of oak wood. Based on the obtained results, the influence of EG on flame-retardant coating was evaluated.

2. Materials and Methods

2.1. Wood Treatment

Specimens of European oak (Quercus robur L.) had dimensions of 10 × 40 × 50 mm (thickness × width × length). The specimens were conditioned in a climate chamber (20 ± 2 °C, 65 ± 3% relative humidity (RH)) for 3 weeks to reach a moisture content of approximately 12%. The samples were divided into four groups, each group consisting of five samples (untreated = reference; top + side = treated the top and sides of the samples with a 50% aqueous solution of sodium silicate (water glass—WG) and EG flakes (+50 mesh; >300 μm; expansion ratio (X:1): 270 to 325; supplied by Sigma-Aldrich (Saint Louis, MI, USA), linear formula: C₂₄(HSO₄)(H₂SO₄)₂); top = treated the top of the samples with a 50% aqueous solution of WG and EG flakes; side = treated the sides of the samples with a 50% aqueous solution of WG and EG flakes). The samples were coated with a 50% aqueous solution of WG by brush, on which we then sprinkled EG flakes. Both WG and EG were weighted before application, and the ratio of WG:EG was 1:1. The amount of retardant used was 175 g·m⁻². The treated samples were then dried to a constant weight at room temperature.

2.2. Sample Analyses

The Radiant Heat Source Test

We used a non-standard test method with a ceramic thermal infrared heater (Ceramicx, Cork, Ireland) with an electric power of 1000 W. The duration of action was 600 s. The distance of the samples from the surface of the heater was 40 mm. and we tested five samples for each group. In the experiment, we used Radwag PS 3500.R2 electronic scales (Radom, Poland); the mass loss was recorded every 10 s (using the RLAB program). Any ignition of the samples was visually checked with a time record if this phenomenon occurred. Subsequently, we calculated the relative mass loss of wood from the measured values [22].

2.3. Surface Temperature Measurement

With the thermal camera Testo 871 (Testo SE & Co. KGaA, Lenzkirch, Germany) (resolution of the infrared detector 240 × 180 points), images were taken every 10 s during the test with a radiant heat source, from which we then monitored the course of temperatures for 600 s using the Testo software IRSoft2 at specified sides—A, B and C (Figure 1). Side A was on the top of the sample, side B was on the longer side of the sample and side C was on the front, shorter side of the sample. All samples had the same orientation in relation to anatomical directions. The temperature values in places A, B and C represent the average values of the temperatures measured on the respective surface.

2.4. FTIR Spectroscopy

The FTIR spectra of the expandable graphite before (EG) and after thermal treatment (EG-TT) were recorded using the KBr technique at wavenumbers from 400 to 4000 cm⁻¹ on a Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) for 32 scans with a resolution of 4 cm⁻¹. Samples EG and EG TT (2 mg) and KBr (300 mg) were thoroughly mixed and pressed to pellets before the measurement.

3. Results and Discussion

Research on the flame retardants of wood is being geared towards green and bio-derived compounds which simultaneously improve the innate properties of wood to enhance sustainability [23]. That is also why, using the test method described in Section 2, a series of experiments were performed to monitor the mass loss and time to ignition of the test specimens after exposure to the thermal infrared heater (Figure 2). With a thermal camera, we monitored the temperature course of the samples at selected locations. The results of the monitored evaluation criteria are shown in Figure 3, Figure 4, Figure 5 and Figure 6 and

Table 1. Ignition time of the tested samples (s).

	Samples
	1	2	3	4	5	Mean ± SD
side	281	188	291	168	136	213 ± 69
top	-	-	-	-	-	-
top + side	-	-	-	-	-	-
untreated	169	188	157	139	136	158 ± 22

.

From the resulting measurements, we recalculated the resulting mass loss of wood for all three types of treatment and for the untreated samples (Figure 3). The relative mass loss was similar for samples treated on the top and on the top and sides simultaneously, with approximately 0.7% less weight loss observed for samples treated only on the top of the sample. We noticed a more significant difference compared to the other two treatments for samples treated only on the sides, which lost up to 81% of their original weight. The highest mass loss was in untreated samples, which lost up to 86% of their original weight. The treatment of samples only on the sides seems to us to be insufficient, since the difference in the weight loss of untreated samples and samples treated only on the sides is approximately 5%. If we compare the samples based on their mass loss, the best results were achieved by the samples treated on the top, closely followed by the samples treated on the top + side; worse results were achieved by the samples treated on the side, and the worst were achieved by the untreated samples. The mass loss in the treated samples is probably caused by the decomposition of some wood components (extractives and partly thermally labile parts of the main wood components) and due the fact that the intercalation compound (H₂SO₄) decomposes into gaseous products (SO₂ and H₂O) [12].

The ignition time was another evaluation criterion (Table 1). In the case of samples treated on the top and on the top + side, their ignition did not occur during the experiment. Side-treated specimens ignited at 212th s, while untreated specimens ignited approximately 55 s earlier at 157th s. None of the samples sustained flame burning until the end of the experiment (600 s).

The temperature trend on side A (on the top of the sample) is approximately the same for the samples treated on the top and sides and the samples treated on the top, where the maximum temperature was 445.2 °C in the 240th s when the sample was treated on the top, while when treating the sample on the top and sides, a maximum temperature of 410.5 °C was measured in the 300th s of the test. As for the untreated samples, there was an increase in temperature in the 180th s due to the ignition of the sample. The maximum temperature depth was reached at 657.8 °C in the 500th s. The temperature trend for the samples treated on the sides is the same as that for the untreated samples, albeit with a time shift, where the temperature started to rise sharply at the 270th s when it ignited. The maximum value in this case was 655.7 °C in the 570th s of the test duration.

The temperature delay of the side-treated samples compared to the untreated ones is due to the protective layer formed by EG and WG. However, the effect of this modification was negligible and did not protect the sample from burning.

The temperature trend on side B (on the longer side of the sample) is approximately the same for the samples treated on the top and sides and the samples treated on the top, where the maximum temperature was 276.8 °C in the 280th s when the sample was treated on the top, while when treating the sample on the top and sides, a maximum temperature of 201.7 °C was measured in the 310th s of the test. As for the untreated samples, there was an increase in temperature in the 190th s due to the ignition of the sample. The maximum temperature depth was reached at 618.0 °C in the 500th s. In the case of samples treated on the side, the temperature first increased in the 270th s and then again in the 500th s, after which point the sample subsequently ignited and the maximum temperature was reached in the 540th s, namely, 585.3 °C.

The protective layer on the side of the sample was effective in delaying the increase in temperature on side B. The sharp rise in temperature in about 520 s was caused by the complete flame burning of the entire sample (top + bottom).

The temperature trend on side C (on the front, shorter side) is the same for the samples treated on the top and sides and the samples treated on the top, where the maximum temperature was 304.8 °C in the 280th s when the sample was treated on the top, while when treating the sample on the top and sides, a maximum temperature of 285.2 °C was measured in the 440th s of the test. As for the untreated samples, there was an increase in temperature in the 170th s. The maximum temperature depth was reached at 529.8 °C in the 500th s. In the case of samples treated on the side, the temperature increased in the 270th s, and the maximum temperature was reached in the 500th s, namely, 631.8 °C.

Therefore, if we compare the course of temperatures for individual samples, all samples reached their maximum temperatures precisely on side A—at the top of the sample. The lowest maximum temperature values were reached by the samples on side B—on the longer side of the sample, for all samples, except for the untreated samples, for which it was on side C—on the front, shorter side. The maximum temperatures, except for the measurement of the temperature course on side C, were reached by the untreated samples.

The FTIR spectrum of EG before heat treatment (Figure 7) shows the characteristic peaks at 3428 cm⁻¹ (stretching vibration of –OH groups), 2923 cm⁻¹ and 2848 cm⁻¹ (asymmetric and symmetric CH₂ stretching) [24,25,26]. The peaks at 1740 cm⁻¹ and 1637 cm⁻¹ were assigned to C=O groups in carbonyl and carboxylates, respectively [27,28]. The peak at 1401 cm⁻¹ contributed to –OH bonding. Peaks in the range of 1000 to 1100 cm⁻¹ might be the sum peak of S–O and C–O. In a lower wavenumber range, peaks in the range of 923–463 cm⁻¹ are related to C–O stretching vibration [29,30]. The spectrum of heat-treated EG (EG-TT) (Figure 7) shows changes at all characteristic peaks when compared to the untreated EG. The increase in intensities at about 3428 cm⁻¹ is associated with an increase in hydroxyl groups. In addition, the strength of all the peaks related to oxygen-containing functional groups increases after thermal decomposition. Similar trends have been observed in the thermal treatment of the EG and various synthetic polymer mixtures [30,31]. The higher intensities of the peaks at 2923 and 2848 cm⁻¹ indicate the formation of methylene bridges in the EG TT molecule.

In the FTIR spectra of water glass, the broad absorption band at around 3500 cm⁻¹ is due to OH, and other broad absorption bands between 1050 and 1200 cm⁻¹ and at around 700 cm⁻¹ are due to Si–O–Si asymmetric stretching, symmetric stretching and bending vibrations, respectively. The broad absorption peak at 1100 cm⁻¹ corresponds to the Si–O–Si bonding, and the peak at 1630 cm⁻¹ represents the Si–OH stretch vibrations [32]. In the water glass used in our experiment (not shown), peaks at 3444, 1652, 1200, 1024, 775 and 440 cm⁻¹ were observed. These peaks are mostly covered by other functional groups, with the exception of the peak at about 440 cm⁻¹. Its intensity decreases during thermal treatment, probably as a result of the increase in the volume of EG.

Currently, the effect of EG, alone or in combination with various other substances, on improving the thermal resistance of polyurethane foams is mainly being examined. Several authors have dealt with this effect. Chao et al. [33] studied the flame-retardant properties of the flexible polyurethane foam (FPUF) added with borax, EG and EG/Borax as a flame retardant. They found that most of the parameters were reduced by the treatment, and the best results were with the EG/Borax treatment. Wang et al. [34] studied the flame-retardant rigid polyurethane foams (RPUFs) containing pentaerythritol phosphate (PEPA) and EG. Their results demonstrated that the PEPA/EG system can reduce the maximum release rate of RPUFs and increase the LOI values. Strąkowska et al. [35] investigated reducing the flammability of rigid polyurethane foams. They found out that the addition of EG in the presence of ionic and silica liquid leads to reduced flammability and improved mechanical properties.

Tomiak et al. [13] studied the EG type as a flame retardant additive in PA6. The fire behavior was characterized by a cone calorimeter using external heat fluxes of 35, 50 and 65 kW·m⁻², LOI and UL-94 burning tests. The PA6/EG formulations provided excellent results in the cone calorimeter and LOI test, although they showed less efficiency in the UL-94 tests. Tomiak et al. [14] also assessed EG and graphite (G) as multifunctional additives that improve both flame retardancy and thermal conductivity in highly filled, thermally conductive polymeric materials based on PA6 using the same methods.

4. Conclusions

Three different treatments were studied to enhance the fire resistance of oak wood with an aqueous solution of sodium silicate and EG flakes (top, side, top + side of samples). The best results in terms of mass loss were achieved by the samples treated on the top, closely followed by samples treated on the top + side; considerably worse results were achieved by the samples treated on the side, and the worst results were achieved by untreated samples. The difference between the mass loss of the untreated samples and that of the samples treated on the top is 79%. From the point of view of the course of temperatures, the maximum temperatures are reached at the top of the sample (on side A), in contrast to the temperatures measured on sides B and C. The results showed that proper wood modification using WG and EG has the potential to improve flame retardant properties.