Combined treatment of wood with thermosetting resins and phosphorous flame retardants

To determine the initial dry weight, all specimens were dried in an oven at 103 °C until a constant mass was achieved. The oven-dried specimens were then conditioned at 65% RH and 20 °C for two weeks. Aqueous solutions of thermosetting resins and a phosphorus polyol (PP) were prepared as described in Table 1. Sample preparation involved impregnation by vacuum (5 kPa, 1 h) followed by a pressure phase (1200 kPa, 2 h). After treatment, the specimens were air-dried for 1 week and then underwent a stepwise dry-curing process at 40 °C (24 h), 60 °C (24 h), 80 °C (24 h), 103 °C (24 h) and 120 °C (24 h) for sufficient curing without significant weight loss due to thermal degradation.

To calculate the weight percent gain (WPG, in %) and cell wall bulking (CWB, in %), wood specimens of 25 × 25 × 10 (ax.) mm³ were used (10 for each group). The WPG and CWB were determined based on the oven-dry weight (Sartorius CP 323 S, 0.001 g) and cross-sectional dimensions (Sylvac 100, 0.001mm) before (m_untreated, A_untreated) and after treatment (M_treated, A_treated), according to (1) and (2), respectively.

Table 2 presents the average and standard deviation values of weight percent gain (WPG) and cell wall bulking (CWB) of the treated specimens. Wood modification using DMEHEU, PF and MF resins led to a WPG of over 30% for all specimens. The incorporation of phosphorus polyol resulted in similar WPG percentages of approx. 50% for DMDHEU-PP, PF-PP, and MF-PP, indicating successful penetration of the phosphorus polyol into the wood specimens regardless of the resin type used. The considerable deposition of phosphate flame retardant has been shown to contribute to the improvement of the reaction to fire (Wu et al. 2023).

The higher CWB indicate more chemicals inside the wood cell wall, which correlates with many properties such as biological resistance and dimensional stability (He and Riedl 2004). Also, the flame retardant could be more leaching-resistant in the cell wall than in the lumen due to the enveloping by water repellent agent (Yalinkilic et al. 1999). The inclusion of phosphorus polyol (PP) in the impregnation solutions containing DMDHEU and PF resins resulted in an increase in the cell wall bulking (CWB) value, indicating the deposition of the PP within the cell walls. However, these resins exhibit distinct modes of action in wood. DMDHEU can either react with the hydroxyl groups of wood polymers or undergo auto-condensation reactions, resulting in the formation of a resin that is unable to leach due to its size (Emmerich et al. 2019). In contrast, PF resin exclusively undergoes self-condensation within the wood, leading to the deposition of a long-chain polymer (Shams and Yano 2011). In contrast, MF-PP exhibited comparable CWB as MF, with the lowest CWB compared with DMDHEU and PF treated wood. This may be attributed to a higher molecular weight or polymerization of MF (Lin et al. 2021), which inhibits the penetration of the resin and flame retardant into the wood cell wall.

The thermal stability of both untreated and treated wood was assessed using the thermogravimetric analysis (TGA) and the corresponding results are presented in Fig. 1. In the case of untreated Scots pine sapwood, the highest mass loss per unit time occurred at 383 ^oC, primarily attributed to cellulose dehydration and partially to lignin (Rowell 2012). The primary degradation temperature of DMDHEU is typically around 320 ^oC (Xie et al. 2014). Compared with untreated wood, the mass loss peak of DMDHEU treated wood shifted towards 371 ^oC. For MF-treated wood, the mass loss peak was observed at 364 ^oC, indicating the degradation of both MF resin and the wood itself (Girods et al. 2008). Similarly, the mass loss peak of PF-treated wood was found to shift to lower temperature (Xie et al. 2016), specifically 335 ^oC. Despite the minor change in the temperature corresponding to the maximum mass loss per unit time, treatment with any of the three thermosetting resins significantly increased the char residue. This increase can be attributed to the higher thermal stability of the thermosetting resin with chemically stable and cross-linked structure, when compared to untreated wood (Li et al. 2020; Wang et al. 2009; Dorieh et al. 2022). Notably, the PF resin treatment resulted in the highest char residue, reaching 31%, owing to the presence of thermally stable aromatic structures within the PF resin.

PF-PP and MF-PP treatments demonstrate notably higher final char residues, measuring 40% and 38% respectively, compared to DMDHEU-PP treatment which yielded a char residue of 34%. These findings were confirmed by higher thermal stability of PF and MF modified wood when compared to DMDHEU modified wood. Furthermore, all phosphorus polyol-treated wood samples exhibited lower onset degradation temperature (5% total weight loss) but significantly higher char residues than untreated wood, which only reached 18%. Additionally, in comparison to untreated wood, both the mass loss peak of DMDHEU-PP (287^oC), PF-PP (304^oC) and MF-PP (323^oC) were observed to shift towards lower temperatures. This decrease in decomposition temperature can be attributed to the catalyzed dehydration of acids resulting from dephosphorylation occurring at lower temperatures. The catalytic action facilitates the formation of a thermally stable char, particularly at elevated temperatures (Green 1992).

The Bunsen burner test is a quick and convenient method for assessing the flammability of wood. After conditioning for two weeks, the moisture of untreated wood reached approx. 9%, while resin treatment and resin combine PP treatment showed lower moisture content in a range from 6.24 to 7.83%. Variations in moisture content among different wood specimens can influence the vertical flame spread in larger wood specimens, while their impact on mass loss, flashover time, and heat release in a cone calorimeter may tend to be minor (Kraaijeveld and Log 2017). Figure 2 presents the mass loss and time to extinguish of each group. The mass loss of DMDHEU and PF treated wood was similar to the untreated wood, reaching approx. 70%. This indicates a high flammability and suggests that the resin treatment did not improve fire retardancy. The flammability test results for PF treatment contradict the positive TGA results. This discrepancy arises because the PF resin lacks oxidation resistance in high-temperature conditions (Gao et al. 1999). Moreover, treatment with these resins resulted in a longer time to extinguish compared to the untreated wood, which can be attributed to the increased density by resin treatment. In contrast, treatment with MF resin alone exhibited a significant improvement in fire resistance, with the mass loss of merely 20% and the ability to self-extinguish within 39 s. This is attributed to the release of nitrogen compounds (Ullah et al. 2014) and the formation of higher thermal stability cyameluric structure (Komatsu 2001). It is worth noting that even though the DMDHEU resin also contains nitrogen, it does not exhibit the same self-extinguishing effect. Further research is needed to fully understand the underlying mechanisms.

The incorporation of a phosphorus polyol into modified wood significantly enhanced the fire retardancy of the treated material. Among the tested combined treatments (resin + phosphorus polyol), the MF-PP collective displayed the most promising fire resistance, with the lowest mass loss at around 9% and a rapid self-extinguishing ability after the removal of the exterior flame. The flame retardancy mechanism of MF-PP can be partially attributed to its intumescent flame retardant system with N-P synergism, as suggested by Alongi et al. (2015). The MF resin, functioning as a nitrogen-based flame retardant, exhibits excellent compatibility with the phosphate flame retardant to synergistically improve the flame retardancy, as reported by Zhu and Xu (2020). The phosphorus polyol, containing polyol as a carbon source, acts as an acid source to facilitate the formation of a char layer. Simultaneously, the MF resin, functioning as a blowing agent, releases incombustible gas to prevent oxygen contact and dilute the combustible gas and heat. However, the combination of DMDEHU with a phosphate flame retardant did not exhibit the same N-P synergism in the thermogravimetric analysis (Holme and Patel 1982). Nevertheless, the char formation observed in the MF-PP system is not comparable to that of commercial intumescent systems, which generate a thick char layer. Instead, the formation of bubbles on the wood’s surface and within its structure acts as a barrier, providing effective flame retardancy.

The fire performance of the treated wood was evaluated using a mass loss calorimeter. The key parameters used to assess the reaction to fire are the heat release rate (HRR) and the total heat release (THR), which could be the acceptance criteria used in codes and national regulations (Östman 2023). The output from the horizontal exposure could predict the fire performance for mid-scale fire test method such as single burning item (Hakkarainen and Kokkala 2001). Figure 3 illustrates the HRR and THR curves, displaying the average heat release for each group, while Table 3 provides a summary of the corresponding data. All wood specimens exhibit the characteristic combustion pattern of wood, characterized by two distinct peaks (Chung 2010). The first peak represents the rapid release of heat immediately after ignition, while the second peak (referred to as the peak heat release rate or PHRR) corresponds to complete combustion as the flame propagates to the bottom of the wood (Xu et al. 2015). Following the PHRR, the HRR of all wood specimens gradually decreased until the flame eventually extinguished.

Both DMDHEU- and PF-treated wood exhibited significantly higher heat release peaks (251.3 kW/m² and 282.2 kW/m², respectively) than the untreated wood (173.4 kW/m²). Furthermore, the time to reach PHRR was notably reduced in both cases, with DMDHEU-treated wood requiring 520 s and PF-treated wood requiring 472 s, compared to untreated wood (687 s). The higher and earlier heat release observed in these treatments increases the fire risk, which does not satisfy the regulatory requirements (Östman 2023). Nevertheless, it is noteworthy that MF-treated wood displayed a heat release pattern similar to that of untreated wood. This contrasts with the flammability test results, where MF-treated wood successfully demonstrated fire inhibition. The discrepancy can be attributed to the high constant heat flux in the cone calorimeter test, which surpasses the fire-retardant capabilities of the MF resin (Steen-Hansen and Kristoffersen 2007).

After incorporating phosphorus polyol, DMDHEU-PP and PF-PP showed significant improvements in flame retardancy compared to DMDHEU and PF. Their peak heat release rates (PHRR) occurred at later times (DMDHEU-PP: 148.6 kW/m², 567 s; PF-PP: 151.7 kW/m², 570 s) due to char formation inhibiting combustion. The similar fire performance of DMDHEU-PP and PF-PP with untreated wood is due to the introduction of combustible DMDHEU and PF resin. The increases in the fire loading reduce the effectiveness of phosphorus polyol. MF resin combined with phosphorus polyol resulted in quick self-extinguishing due to the formation of a protective char layer (Kurkowiak et al. 2023). The PHRR (161.7 kW/m², 795 s) of MF-PP is due to the prolonged extinguishing process before the HRR peak emerges. The THR of MF-PP was only 8.6 MJ/m² (approximately 16% of untreated wood) within 600 s.