Evaluation of reaction-to-fire behaviour of wood chipboard: A comparative study of treatments and coatings

2.1. Sample preparation and conditioning

All samples were conditioned for one week before testing in a controlled environment at 23°C and 50% relative humidity to ensure consistency. The equilibrium condition was verified through weight stabilization measurements over consecutive days. The moisture content of each specimen was measured using a calibrated moisture meter before testing, ensuring uniform initial conditions of approximately 7–8%. One sample per treatment type was prepared. Samples coated with polyurethane varnish or water-based intumescent paint were applied uniformly with a brush to achieve a film thickness of 27 μm, and then allowed to dry.

2.2. Materials

A series of wood agglomerate boards, cut to 9 cm x 9 cm with a nominal thickness of 16 mm, were used in this study to assess their reaction to fire. The treatments applied included: (A) unprotected wood chipboard, (B) Formica-coated chipboard, (C) varnish-coated chipboard, (D) intumescent paint-coated chipboard, and (E) fire retardant chipboard (Fimapan IGN E-Z - Finsa). The base material for samples A–D consisted of wood particles bonded with urea-formaldehyde resin, while sample E included phosphorus-based fire-retardant additives. Technical specifications for all chipboards are summarized in Table 1, with detailed properties for Fimapan provided in Table 2.

2.3. Experimental setup and testing procedure

The experiment utilized an electric radiator (Figure 1) designed for reaction-to-fire testing as per UNE 23-725-90. The equipment includes:

Before testing, the radiator was calibrated following UNE 23-729. Each sample was placed on the specimen holder and exposed to the radiant heat source for 5 minutes. The incident heat flux on the surface was estimated at approximately 50 kW/m², significantly lower than that of standard cone calorimeter tests (ISO 5660-1), which typically range from 25 kW/m² to 75 kW/m².

2.4. Weight measurement and loss calculation

To evaluate the fire reaction of each sample, the following parameters were recorded:

Where:

WL = Weight loss (g)

IW = Initial weight (g)

FW = Final weight (g)

These data provide a comparative analysis of different treatments in terms of their reaction-to-fire performance, not structural fire resistance.

3.1. Ignition behavior and combustion analysis

The ignition point of a combustible material is defined as the set of physical conditions (pressure, temperature) necessary for the substance to begin burning and for the flame to sustain itself without external heat. Figure 2 illustrates the behavior of the intumescent paint-treated chipboard (D) in front of the fire, showing a protective mass formed on the fire-exposed face, indicating rapid fire suppression and good fire reaction performance (2121. Fu F, Lin L, Xu E. 2017. Functional pretreatments of natural raw materials. In: Advanced high strength natural fibre composites in construction. Oxford (UK): Elsevier; p. 87-114. https://doi.org/10.1016/B978-0-08-100411-1.00004-2). Figures 3–7 graphically represent the ignition patterns of each sample over time, providing a clear view of their fire behavior across different treatments, obtained via the UNE 23-725-90 test.

The unprotected chipboard (A) showed significant ignition activity (Figure 3), with 14 ignitions recorded over a 5-minute period. The horizontal axis (X) represents the number of generations (0 to 10), while the vertical axis (Y) indicates the frequency of ignitions in percentage (0% to 100%). Blue points mark specific ignition frequency values, connected by a green line showing the trend. The frequency started low (~10%), peaked near 80% in generation 3, sharply declined in generation 4, gradually increased to ~70% by generation 7, and settled around 30% by generation 10. The longest ignitions occurred early, indicating high combustibility due to exposed wood particles and urea-formaldehyde resin (Section 2.2).

The Formica-coated chipboard (B) exhibited 12 ignitions characterized by prolonged times and higher flames (Figure 4), with the first ignition at 1 minute 4 seconds, indicating a longer response time compared to other samples. The figure presents time in seconds on the horizontal axis (X) and ignition intensity on the vertical axis (Y), with an orange curve connecting points representing ignition moments. Observations indicate that these 12 ignitions occurred over an extended period, with higher flames compared to other coatings, likely due to gas release during degradation (88. Stevens R, Van Es DS, Bezemer R, Kranenbarg A. 2006. The structure-activity relationship of fire retardant phosphorus compounds in wood. Polym Degrad Stab. 91(4):832-841. https://doi.org/10.1016/j.polymdegradstab.2005.06.014).

The varnish-coated chipboard (C) exhibited 17 ignitions over 5 minutes (Figure 5), with ignition durations remaining relatively consistent but increasing toward the end (last two ignitions at ~23 seconds each). The figure displays the ignition number on the horizontal axis (1 to 17) and duration in seconds on the vertical axis, showing an upward trend in duration toward the end, indicating longer ignitions in the final trials.

The intumescent paint-treated chipboard (D) exhibited four short-duration ignitions (Figure 6): at 14 seconds (3 seconds), 43 seconds (2 seconds), 70 seconds (3 seconds), and 79 seconds (7 seconds), with no reignition after 1 minute 23 seconds. The figure presents time in seconds on the horizontal axis (X) and ignition intensity on the vertical axis (Y), with points marking ignition moments. Observations indicate that, although four ignitions occurred, they did not persist for a prolonged period and were accompanied by significant smoke emission.

The fire-retardant chipboard (E) exhibited four ignitions (Figure 7): at 230 seconds (24 seconds), 270 seconds (16 seconds), 290 seconds (4 seconds), and 300 seconds (2.5 seconds), with durations decreasing over time. The figure shows time in seconds on the horizontal axis (0 to 250 seconds) and the number of ignitions on the vertical axis (0 to 12). No ignitions occurred from 0 to 200 seconds. The sequential behavior included: (1) bubbling in the first seconds, (2) white smoke emission due to water vapor, and (3) momentary ignitions.

4.1. Weight loss analysis

The weight loss data (Table 3) show that the untreated chipboard (A) experienced a weight loss of 14.69%, while the chipboard with varnish showed a slightly lower loss of 11.71%. These results suggest that the varnish offers marginal fire protection compared to the untreated chipboard, which is consistent with previous studies indicating that varnish is not an effective fire retardant (99. Arao Y, Nakamura S, Tomita Y, Takakuwa K, Umemura T, Tanaka T. 2014. Improvement on fire retardancy of wood flour/polypropylene composites using various fire retardants. Polym Degrad Stab.100(1):79-85. http://dx.doi.org/10.1016/j.polymdegradstab.2013.12.022). In contrast, the chipboard with intumescent paint showed a weight loss of 7.08%, indicating a significant improvement in fire resistance. This finding is supported by literature highlighting the effectiveness of intumescent paints in fire protection (2323. Chen SN, Li PK, Hsieh TH, Ho KS, Hong YM. 2021. Enhancements on flame resistance by inorganic silicate-based intumescent coating materials. Materials. 14(21):6628. https://doi.org/10.3390/ma14216628).

The Formica-coated sample exhibited the lowest weight loss of 4.39%. These results should be evaluated against relevant fire safety standards and building regulations, such as those set by the European Union’s EN 13501-1 standard (2222. Comité técnico CTN 23 Seguridad contra incendios. 2019. UNE-EN 13501-1:2019. Clasificación en función del comportamiento frente al fuego de los productos de construcción y elementos para la edificación. Madrid (Spain): UNE. Available from: https://www.une.org/encuentra-tu-norma/busca-tu-norma/norma?c=N0062154), to ensure compliance. Ensuring that construction materials meet these standards is crucial for enhancing building safety and mitigating fire-related risks.

Surprisingly, the fire-retardant treated sample (E), despite its protective treatment, exhibited the highest weight loss of 19.55%. This behavior aligns with its Euroclass B-s2,d0 rating (Table 2), which prioritizes low flammability and smoke production over mass retention (1212. Wang F, Wang Q, Wang X. 2010. Progress in research on fire retardant-treated wood and wood-based composites: A chinese perspective. For Prod J. 60(7-8):668-678. https://www.doi.org/10.13073/0015-7473-60.7.668). The higher density of Fimapan IGN E-Z (710 kg/m³, Table 1) compared to other chipboards (650–680 kg/m³) may contribute to increased material degradation under radiant heat (Section 2.2.1).

The uniform initial moisture content of 7–8% (Section 2.1) ensured consistent testing conditions, minimizing variability in combustion behavior.

4.2. Ignition pattern analysis

The ignition frequency of the unprotected chipboard (A) suggests significant fluctuations, possibly due to external factors or material characteristics, reflecting its high combustibility due to exposed wood particles and urea-formaldehyde resin (Section 2.2). The Formica-coated chipboard (B), despite its low weight loss (4.39%, Table 3), showed prolonged ignition times and higher flames, indicating sustained combustion due to gas release (88. Stevens R, Van Es DS, Bezemer R, Kranenbarg A. 2006. The structure-activity relationship of fire retardant phosphorus compounds in wood. Polym Degrad Stab. 91(4):832-841. https://doi.org/10.1016/j.polymdegradstab.2005.06.014, 2424. Yew MC, Ramli Sulong NH. 2012. Fire-resistive performance of intumescent flame-retardant coatings for steel. Mater Des. 34:719-724. http://dx.doi.org/10.1016/j.matdes.2011.05.03). These findings are crucial for material selection in construction projects and for meeting fire safety standards like EN 13501-1 (2222. Comité técnico CTN 23 Seguridad contra incendios. 2019. UNE-EN 13501-1:2019. Clasificación en función del comportamiento frente al fuego de los productos de construcción y elementos para la edificación. Madrid (Spain): UNE. Available from: https://www.une.org/encuentra-tu-norma/busca-tu-norma/norma?c=N0062154).

The varnish-coated chipboard (C) exhibited limited fire resistance, consistent with studies indicating varnish is not an effective fire retardant (99. Arao Y, Nakamura S, Tomita Y, Takakuwa K, Umemura T, Tanaka T. 2014. Improvement on fire retardancy of wood flour/polypropylene composites using various fire retardants. Polym Degrad Stab.100(1):79-85. http://dx.doi.org/10.1016/j.polymdegradstab.2013.12.022). The intumescent paint-treated chipboard (D) acted quickly to suppress the fire, supported by literature (2121. Fu F, Lin L, Xu E. 2017. Functional pretreatments of natural raw materials. In: Advanced high strength natural fibre composites in construction. Oxford (UK): Elsevier; p. 87-114. https://doi.org/10.1016/B978-0-08-100411-1.00004-2). The fire-retardant chipboard (E) aligns with its Euroclass B-s2,d0 rating (Table 2), which prioritizes low flammability and smoke production (1212. Wang F, Wang Q, Wang X. 2010. Progress in research on fire retardant-treated wood and wood-based composites: A chinese perspective. For Prod J. 60(7-8):668-678. https://www.doi.org/10.13073/0015-7473-60.7.668).

4.3. Treatment comparison

The experimental findings indicate significant differences between intumescent paint and fire-retardant treatments in their effectiveness at enhancing fire resistance in wood agglomerates.

The use of intumescent paint on wood samples, as demonstrated in Figure 6, shows that although there were multiple ignitions, each was brief and accompanied by considerable smoke. This indicates that intumescent paint acts quickly to suppress fire. The paint’s mechanism of forming an insulating char layer upon exposure to heat effectively slows down heat transfer, delaying the substrate’s ignition and structural weakening.

In contrast, the fire retardant-treated samples, shown in Figure 7, delayed ignition significantly, with no ignitions observed in the first 200 seconds of the test. However, once ignited, the fires persisted for longer durations compared to those treated with intumescent paint. This underscores the need for complementary fire management strategies when using fire retardants to ensure comprehensive fire protection (88. Stevens R, Van Es DS, Bezemer R, Kranenbarg A. 2006. The structure-activity relationship of fire retardant phosphorus compounds in wood. Polym Degrad Stab. 91(4):832-841. https://doi.org/10.1016/j.polymdegradstab.2005.06.014, 1616. LeVan SL, Jerrold. EW. 2007. Effects of fire-retardant treatments on wood strength: a review. Wood Fiber Sci. 22(1):113-131. Available from: https://wfs.swst.org/index.php/wfs/article/view/2074/2074).

From a practical perspective, the choice between these two treatments should be informed by the specific fire safety requirements of the application. Intumescent paint provides rapid fire suppression by forming a protective char layer, making it ideal for applications requiring high levels of structural integrity. This is particularly suitable for critical infrastructure and high-rise buildings were maintaining structural integrity and allowing safe evacuation are crucial. The quick action of intumescent paint makes it effective in slowing heat transfer and preventing rapid structural weakening, although it generally comes at a higher cost compared to some fire retardant treatments (99. Arao Y, Nakamura S, Tomita Y, Takakuwa K, Umemura T, Tanaka T. 2014. Improvement on fire retardancy of wood flour/polypropylene composites using various fire retardants. Polym Degrad Stab.100(1):79-85. http://dx.doi.org/10.1016/j.polymdegradstab.2013.12.022).

Fire retardant treatments, on the other hand, are effective in delaying initial ignition and are suitable for a wide range of applications, including residential buildings, furniture, and textiles. These treatments are typically less expensive than intumescent paints and provide long-lasting protection as the chemicals are integrated into the material. However, they may not prevent material degradation once combustion occurs, and some chemical treatments, particularly those containing halogenated compounds, pose environmental and health risks (1212. Wang F, Wang Q, Wang X. 2010. Progress in research on fire retardant-treated wood and wood-based composites: A chinese perspective. For Prod J. 60(7-8):668-678. https://www.doi.org/10.13073/0015-7473-60.7.668).

Environmental and health considerations are also crucial in selecting fire protection treatments. Intumescent paints, generally free from halogenated compounds, are considered more environmentally friendly and safer for indoor air quality. In contrast, some fire retardants, especially those containing halogenated compounds, have been linked to environmental and health risks. For instance, certain fire retardants can release toxic gases during combustion, posing additional hazards to occupants and first responders (1010. Haurie L, Giraldo MP, Lacasta AM, Montón J, Sonnier R. 2019. Influence of different parameters in the fire behaviour of seven hardwood species. Fire Saf J. 107:193-201. https://doi.org/10.1016/j.firesaf.2018.08.002).

The Formica-coated chipboard (B) showed the lowest weight loss (4.39%, Table 3) but had 12 ignitions, indicating less effective ignition performance compared to intumescent paint (D) and fire-retardant treatments (E) (88. Stevens R, Van Es DS, Bezemer R, Kranenbarg A. 2006. The structure-activity relationship of fire retardant phosphorus compounds in wood. Polym Degrad Stab. 91(4):832-841. https://doi.org/10.1016/j.polymdegradstab.2005.06.014, 2424. Yew MC, Ramli Sulong NH. 2012. Fire-resistive performance of intumescent flame-retardant coatings for steel. Mater Des. 34:719-724. http://dx.doi.org/10.1016/j.matdes.2011.05.03). The varnish-coated chipboard (C) exhibited 17 ignitions, confirming its limited protective capability, consistent with literature (99. Arao Y, Nakamura S, Tomita Y, Takakuwa K, Umemura T, Tanaka T. 2014. Improvement on fire retardancy of wood flour/polypropylene composites using various fire retardants. Polym Degrad Stab.100(1):79-85. http://dx.doi.org/10.1016/j.polymdegradstab.2013.12.022).

4.4. Practical implications and future directions

These findings provide practical guidance for material selection in construction projects, emphasizing the need for balancing fire protection, structural integrity, and environmental impact. While intumescent paint and fire-retardant treatments are most effective at preventing ignition and delaying fire spread, Formica coatings minimize weight loss and surface degradation, making them suitable for applications prioritizing material preservation.

Given the limitations of this study, future research should explore standardized fire testing methods, such as the cone calorimeter (ISO 5660-1), to facilitate comparisons with other studies. Additionally, assessing the environmental impact of emissions from treated wood during combustion is recommended to ensure compliance with safety and sustainability regulations.

Overall, this study contributes to the ongoing development of fire-safe construction materials, offering insights into the advantages and limitations of different fire protection strategies for wood agglomerate boards.

The results align with EN 13501-1 standards, which classify materials based on flammability, smoke production, and flaming droplets (2222. Comité técnico CTN 23 Seguridad contra incendios. 2019. UNE-EN 13501-1:2019. Clasificación en función del comportamiento frente al fuego de los productos de construcción y elementos para la edificación. Madrid (Spain): UNE. Available from: https://www.une.org/encuentra-tu-norma/busca-tu-norma/norma?c=N0062154). Fimapan’s Euroclass B-s2,d0 rating indicates low flammability, moderate smoke production, and no flaming droplets, but its high weight loss (19.55%, Table 3) suggests mass retention is not prioritized, unlike Formica or intumescent paint. These results guide compliance with building regulations, optimizing safety and performance (2222. Comité técnico CTN 23 Seguridad contra incendios. 2019. UNE-EN 13501-1:2019. Clasificación en función del comportamiento frente al fuego de los productos de construcción y elementos para la edificación. Madrid (Spain): UNE. Available from: https://www.une.org/encuentra-tu-norma/busca-tu-norma/norma?c=N0062154).