This study was conducted in an experimental field at the Federal University of Sergipe, in the municipality of São Cristóvão (SE), located in the northeastern region of Brazil. The climate of the municipality is characterized as megathermic humid and sub-humid, with an average temperature of 25 °C throughout the year, an annual average rainfall of 1333.4 mm, and a rainy season between March and August. The soil types in the area include red-yellow podzolic, eutrophic and dystrophic alluvial, slightly humid gray podzol, and mangrove indiscriminate soils, with the presence of Atlantic Forest, capoeira, and hygrophilous vegetation ().

The decay field was set up using pinewood stakes obtained from the Woodworking and Furniture Production Laboratory of the Department of Forest Sciences (DCF) at the Federal University of Sergipe (UFS). The pinewood samples, which were 15 years old and had a moisture content of 12%, were prepared through tangential cuts. 2.0 x 10.0 x 100 cm stakes (thickness, width, and length, respectively) were manufactured while following standards adapted from ASTM D143-94. The test specimens were then produced, measuring 46.6 cm in length, 5.0 cm in width, and 2.0 cm in thickness. Out of a total of 35 test specimens, 30 were subjected to preservative treatments using industrial waste, and five were reserved as a control group.

Two industrial types of waste were used as preservatives: residual soybean oil from restaurants, with a viscosity between 65 and 70 cp (centipoise); and used motor oil, with a SAE-10W viscosity. These materials were obtained from the Wood Technology and Machining Laboratory (DCF) of UFS.

The preservatives were applied using three distinct methods - note that, for the control group, five test specimens were not subjected to any preservative treatment:

1. 1. Immersion treatment. Five stakes were submerged in motor oil and five in soybean oil, where they remained for 24 hours. After this period, they were air-dried for 48 hours.
2. 2. Brushing treatment. Using industrial brushes, five stakes received three coats of motor oil, and another five received three coats of soybean oil, with intervals of 1 hour between applications. After the last coat, the stakes were left to dry for 48 hours, following an adaptation of the method by İlhan et al. (1976).
3. 3. Hot-cold bath treatment. Five stakes were immersed in motor oil and another five in soybean oil at temperatures between 90 and 100 °C for 1 hour. During the process, rigorous care was taken to avoid the ignition of the oils, given their flammability. After heating, the test specimens were immersed in containers with the respective oils at room temperature, where they remained for 2 hours before being air-dried for 48 hours, following an adaptation of the method by Santini (1988).

After the 48-hour drying period, the test specimens were weighed on a Col-BN3000-CLN precision balance (in grams) and installed in the decay field. The samples were fixed in the soil at a depth of 30 cm, with 16.6 cm remaining above the ground. The evaluation took place between July and October 2023.

The degree of deterioration of the stakes was assessed through visual and tactile inspection by six evaluators after removing the test specimens from the rotting field, according to the method by Lepage (1970). During this assessment, the stakes were visually inspected and individually scored based on the percentage of preservation, as presented in Table 1.

Table 1
Classification of wood deterioration index, as suggested by Lepage (1970)

To analyze the mass loss variation, the specimens were weighed after the application of the products and again after they were removed from the decay field. They were then placed in a ventilated environment for six days to dry, and they were weighed again to assess the weight variation. The mass loss caused by wood-degrading agents was determined for each wood sample using Equation (1).

where: PM = Mi = initial mass (g); Mf = final mass (g).

The data obtained from this study were electronically processed and analyzed via the Sisvar statistical software (Ferreira, 2011), in accordance with the research objectives. The mass loss (g) was analyzed using a completely randomized design (CRD) in a 2 x 3 factorial arrangement, with the factors being the products (at two levels) and the treatments (at three levels). Additionally, the degradation percentages of the specimens were determined, and the assessment was based on the degradation indices per treatment. All the data were subjected to an analysis of variance (ANOVA), followed by mean comparisons using the Scott-Knott test, with a significance level of 5% probability of error.

The results indicate a significant influence of the type of preservative and the different treatments on wood protection against mass loss. Therefore, it can be stated that the lower mass loss observed in the preservative treatments is influenced by the type of product used, the different types of treatments, and the combinations of these factors (Table 2).

Table 2
ANOVA for the variation in mass loss (g) of the test specimens regarding the preservative products

DF = degrees of freedom; SS = sum of squares; MS = mean square; ** = significant at a 5% level of error probability

For the samples treated with soybean oil residue, the immersion and brushing methods and the control showed no statistically significant differences, with all pine stakes exhibiting the highest average mass loss values, indicating the ineffectiveness of these treatments. In contrast, the hot-cold bath treatment with soybean oil demonstrated greater efficiency in protecting the wood, showing no statistical difference from the treatments with used motor oil (Table 3).

Table 3
Mass loss (g) of pine wood under three different treatments with two industrial residues

Means followed by the same uppercase letter in the rows and the same lowercase letter in the columns do not differ from each other according to the Scott-Knott test (p<0.05).

As shown in Table 3, the used motor oil treatments also showed no statistical differences between each other, displaying the lowest variations in mass loss and providing greater protection against deteriorating agents, which supports the literature’s recommended utilization of used motor oil for stakes, posts, and other wooden pieces intended for rural use (Owoyemiet al., 2020; Belchinskaya et al., 2021; Vani et al., 2022).

The immersion, brushing, and hot-cold bath treatments in burnt oil, as well as the hot-cold bath with soybean oil, significantly influenced the preservation index of the test specimens, with greater wood health observed in the samples subjected to these treatments (Figure 1).

Figure 1
Preservation index of wood treated with two different industrial residues. Columns followed by the same uppercase or lowercase letter do not differ statistically from each other at a 5% probability level, according to Fisher's LSD test.

Although there was no statistical difference between these preservative treatments, a decreasing trend was noted in the preservation index, starting from the hot-cold bath with burnt oil treatment (Figure 1).

Santini (1988) emphasizes that an effective wood preservative should exhibit high resistance to leaching, given that test specimens are subjected to adverse environmental conditions. In contrast to treatments involving brushing and immersion in soybean oil at room temperature, the hot-and-cold bath treatment, as presented in this analysis, demonstrated a significantly superior efficacy. Moreschi (2014) explains that the effectiveness of this treatment is due to the rapid contraction of heated air inside the wood, creating a vacuum that facilitates a deeper penetration of the preservative into the wood structure through absorption, a crucial phenomenon for enhancing the wood's resistance to degrading agents, as illustrated in Table 3.

The treatment of wood by immersion in vegetable oil at high temperatures has shown promising results (Zablonsky et al., 2017; Baar et al., 2021). Studies suggest that vegetable oils enhance the wood's resistance to xylophagous attacks more effectively than other preservation methods (; Bahmani & Schmidt, 2018; Sousa et al., 2019; Woźniak, 2022; Šimůnková et al., 2022).

Although not classified as a conventional preservative, the use of vegetable oil can be recommended for wood treatment in rural construction due to the significant resistance it imparts to pine wood in decay contexts (Mandraveli, 2023). Lee et al. (2018) observed that wood thermally treated with soybean oil is also suitable for flooring and external applications such as fences, garden furniture, and cladding, given its enhanced moisture resistance. However, it is important to note that, despite its high efficiency, the application of vegetable oil should be avoided in areas where the wood comes into direct contact with the soil. This, in order to prevent contamination.

The effectiveness of used oil can be attributed to its viscosity (SAE-10W), as demonstrated in studies by Souza et al. (2016). These studies investigated the penetration and retention capacity of oil with SAR 10 viscosity in Trattinnickia rhoifolia wood, confirming that this type of oil increases the wood's resistance to biological degradation due to its superior penetration capacity.

These findings are consistent with the results obtained by Souza et al. (2016), who found that used oil provided greater resistance to the biological decomposition of amescla wood. This protection was further confirmed by Gallon et al. (2014), who evaluated the deterioration resistance of Amazonian woods treated by simple immersion in used oil, attributing the efficacy of the treatment to its water-repellent properties. Eaton and Hale (1993) support this observation, noting that high moisture levels in wood promote the activity of xylophagous microorganisms, thereby compromising the physical properties of the wood.

Additionally, Mattos et al. (2013) concluded that, when comparing preservative treatments with CCB and used oil in eucalyptus wood, the latter exhibits a high capacity to retain more preservative, thereby increasing resistance and maintaining wood mass. Although not traditionally classified as a wood preservative, Ssemaganda et al. (2011) reported that used oil can provide protection against termite attacks. Nonetheless, as with the recommendations for soybean oil, in order to prevent contamination, the application of used oil should be avoided in areas where the wood comes into direct contact with the soil.

These results underscore the relevance of thermal and oil-based treatments in wood preservation, suggesting that, despite their limitations, these approaches offer effective and viable alternatives for protection against xylophagous agents, with significant practical implications for rural and external applications.

The results obtained for the wood preservation index, measured through a visual analysis followed by scoring (Figure 1), show that the treatments involving used oil and hot-cold baths with soybean oil are efficient due to their high penetration capacity, resulting in wood resistant to degradation. According to Kamdem et al. (2002) and Weiland and Guyonnet (2003), the durability of wood is related to the hydrophobic character, the production of extractives, and the modification of polymers related to the application of oils, providing greater resistance to the attack of xylophagous agents.

According to Moreschi (2014), a product's ability to penetrate and adhere to wood is directly linked to the depth of the sapwood, the anatomical directions, the vessels with tyloses, the resin canals, and the types of pits. For Temizet al. (2008), a promising strategy to enhance the cell wall penetration and adhesion capacity of preservatives and consequently increase the effectiveness of treatments is the incorporation of additives. The treatment of wood using hot-cold bath cycles with soybean oil led to high preservation percentages. This approach presents itself as a viable alternative for the wood preservative industry, as it employs an underutilized industrial residue.

* Corresponding author: Paulo Felix Bento, correo electrónico: pfelix964@gmail.com