Resistance of Untreated and Torrefied Medium-Density Fiberboard (MDF) Residues to Xylophage Fungi

Abstract

The manufacture of wood panels generates a large amount of waste. This material can be an option for renewable energy generation. However, long-term storage, exposure to moisture and contact of these panels with the soil facilitate colonization by xylophagous organisms. Torrefaction, a heat treatment between 200 and 300 °C in an oxygen-free atmosphere, is a process that decreases hygroscopicity while increasing carbon content, energy efficiency and resistance to fungal attack. This work aimed to evaluate the resistance of MDF panel residues. The MDF panels were produced using eucalyptus wood and bonded with thermosetting synthetic resin, under high temperature and pressure, torrefied at 300 °C for 20, 30 and 40 min and exposed to the xylophagous fungi of the white rot, Irpex lacteus (Fr.) Fr. (1828) and Trametes versicolor, and that of the brown rot, Postia placenta. After the 12-week evaluation period under fungal exposure, the mass loss of the samples attacked by T. versicolor and P. placenta was similar between treatments, except the MDF untreated, which had greater mass losses from the fungus Irpex lacteus. The torrefaction process increased the material resistance to deterioration by fungi, with an inverse correlation between the torrefaction period and the mass losses by fungal attack of the MDF panel residues.

1. Introduction

The world production of wood panels (medium- and high-density fiberboards) was 103.8 million m³ in 2020 [1]. Brazil produced 4.3 million m³ of medium-density fiberboard (MDF), an increase of 2.8% compared to the previous year [2]. The residues generated by the use of MDF panels are a problem, but they can be used as biomass to produce composites [3,4,5,6,7] and energy [8,9,10].

Torrefaction, a heat treatment at controlled and high temperatures in an oxygen-free environment, aims to concentrate carbon and lignin in the wood [11,12,13]. Torrefaction of MDF panels reduces the emission of volatile materials formed by condensable liquids, organic products, lipids, and non-condensable components or permanent gaseous fractions, mainly CO and CO₂ [14]. MDF torrefaction eliminates some nitrogen, such as HCN and NO_x, during combustion, and NH₃, HNCO and HCN during pyrolysis, in a closed gas burning system. Compounds such as urea, melamine and formaldehyde can be released from varnishes and adhesives, common in wood panels [15,16].

The torrefaction of lignocellulosic materials increases energy density, reduces hygroscopicity and facilitates storage in the industry [13,17], in addition to reducing attractiveness to xylophage fungi [18]. The utilization of alternative fuels increased the possibility of using torrefied waste to replace pulverized mineral coal in energy production. The use of torrefied biomass can mitigate pollutants and toxic gas emissions into the atmosphere, increasing the economic viability of these materials [19].

Long-term storage of wood panel residues exposed to moisture and contact with the soil facilitate colonization by xylophagous organisms, including rotting fungi modifying the physicochemical properties and reducing the mass and energy potential of these materials [20].

Rotting fungi are generally the first xylophagous organisms colonizing wood with economic damage to timber [21,22]. These fungi cause the brown, soft and white rot and degrade the cell wall, changing the physicochemical properties of the wood [23,24]. The MDF resistance to deterioration by rotting fungi varies with its adhesive content and composition and the characteristics of the raw material used to produce them [25]. The fiber properties, including those of naturally more resistant woods, affect the durability of wood panels to brown and white rot fungi [26,27,28]. The intensity of fungal attack varies according to the wood treatment. Torrefaction increased the resistance of wood chips to attack by Trametes sp. and P. ostreatus, reducing the mass loss from 1.63 and 2.78% to 0%, respectively [20]. Torrefaction of eucalyptus, pine and coffee husk at 290 °C decreased the mass loss against Trametes versicolor and Coniophora Puteana attack by at least 18% and 22%, respectively [29].

The large amount of waste generated in the production of MDF demands an alternative for its use [30]. Studies for energy generation focus on parameters such as calorific value and energy efficiency; however, in the storage of this material, it is subject to fungal attack, which can make its use unfeasible. This topic is understudied. This work aimed to evaluate the resistance of MDF panels, untreated and torrefied, to biological attack by the xylophage fungi of the white rots Irpex lacteus and Trametes versicolor, and the brown rot Postia placenta.

2. Material and Methods

The MDF panel used in this work was produced in Brazil, with eucalyptus wood bonded with thermosetting synthetic resin, which melts with the action of temperature together with pressure. Samples of MDF residues, untreated and torrefied, were subjected to an accelerated rot test against xylophagous fungi. The tests followed the protocols of the American Society for Testing and Materials-ASTM D-2017 [31].

2.1. Torrefaction Process

The first stage of the work was to prepare samples. The MDF panels were cut to 18 × 18 × 18 mm samples, and submitted to drying in an oven at 103 ± 2 °C at 0% humidity. Then, the samples were conditioned in a reactor for torrefaction. This device has three essential systems: (I) transport; (II) heating; and (III) cooling, according to Silva et al. [12], allowing the temperature to increase in conditions of absence of oxygen. The MDF panel samples were torrefied at 300 °C for 20, 30 and 40 min (Figure 1).

2.2. Chemical Characterization

The behavior during the combustion of untreated and torrefied samples was carried out through a thermogravimetric analysis, using a DTG60H simultaneous measuring instrument (Shimadzu, Kyoto, Japan) under a nitrogen atmosphere, following the norms of the American Society for Testing and Materials. The samples of 2 g of material were degraded, with an initial temperature of 100 °C and a final temperature of 600 °C, with a 10 °C/min⁻¹ heating rate. Additional chemical analysis of all treatments (

Table 1. Residue properties of untreated MDF and after torrefaction for 20, 30 and 40 min used in the test to evaluate damage by xylophage fungi.

Properties	Torrefaction Time
	Untreated	20 min	30 min	40 min
EMC (%)	9.66	7.69	8.79	8.49
Holocelluloses (%)	55.28	56.85	52.22	51.35
Total lignin (%)	32.06	32.83	40.23	42.36
Extractives (%)	12.66	10.32	7.54	6.27
Ash (%)	0.27	0.26	0.29	0.27
GY (%)	100	99.43	95.78	90.47
ODD (g.cm−3)	0.68	0.66	0.64	0.61

EMC = equilibrium moisture content; GY = gravimetric yield; ODD = oven dry density.

) are available in the article of de Castro et al. [30]. The rate of biodeterioration by fungi is related to the chemistry of the material; Table 1 helps explain the results.

2.3. Evaluation of Fungal Attack

The untreated and torrefied samples were subjected to a mass loss test due to fungal attack by white rot fungi Irpex lacteus and Trametes versicolor and brown rot fungus Postia placenta, according to ASTM D-2017 [31]. The MDF samples tested were subjected to drying at 103 °C, and the soil, glassware and other equipment autoclaved to avoid any contamination. The tests used containers of 600 mL, each one filled with 83 mL of distilled water and 300 g of sterilized clayed soil with a pH of 6.8. Fungal colonization tests were performed during the 12 weeks, and after the end of the period, the samples were oven-dried and weighted. The resistance class and the decay susceptibility index of the material were evaluated by the ratio between the mass of the sample after and before the test, as described in ASTM D-2017 [31].

2.4. Statistical Analysis

The results of biological assay in relation to the torrefaction period of the MDF samples were analyzed in a completely randomized design with four treatments (untreated and three torrefaction times) and three fungi with 12 replications. The means of the treatments were submitted to the Tukey test (p ≤ 0.05).

3. Results

The thermogravimetric curves (TG/DTG) of the MDF residues, for each torrefaction period in the temperature range, varied between 100 and 600 °C (Figure 2).

The thermal degradation rate drastically increased between 250 and 400 °C with all residence periods for torrefaction (

Table 2. Mass losses (%) due to thermal decomposition of untreated × torrefied MDF residues according to temperature ranges.

Temperature (°C)
100 to 150	150 to 250	250 to 350	350 to 400	400 to 450	450 to 500	Residual Mass (%)
Mass loss (%)
untreated
0	3.688	43.805	17.983	1.844	0.922	31.757
20 min
2.073	3.109	49.222	17.617	2.591	1.554	23.834
30 min
2.073	3.109	48.705	18.135	2.591	1.554	23.834
40 min
0.485	1.942	42.718	17.476	1.942	1.942	33.495

). Peaks of hemicelluloses and cellulose decomposition occur in this temperature range [10,32]. The hemicelluloses and cellulose undergo complex degradation reactions with an increase in devolatilization with temperature [33], and with great mass losses, such as the 60% for MDF biomass [8,34].

The resistance of the material to deterioration by fungi increased and the mass loss decreased with an increase in torrefaction period (

Table 3. Mass losses (%) of MDF residues, untreated and torrefied, when resisting attack of xylophagous fungi.

Fungus	Torrefaction Time
	Untreated	20 min	30 min	40 min
Irpex lacteus	47.54 ± 5.09aA	47.54 ± 10.72aA	6.10 ± 1.55bA	4.32 ± 0.94bA
Trametes versicolor	14.14 ± 4.26aB	6.04 ± 1.01bB	4.19 ± 0.95bA	2.62 ± 0.80bA
Postia placenta	6.21 ± 2.22aC	5.01 ± 0.93aB	4.75 ± 1.25aA	3.13 ± 0.65aA

Means followed by the same capital letter, per column, or lowercase letter, per line, do not differ by the Tukey test (p > 0.05).

), with lower values for 40 min. After 30 min, the mass losses of torrefied material were lower than the natural ones, but with similar resistance to the different xylophagous fungi.

4. Discussion

Mass loss by increasing the temperature was low between 100 and 250 °C (Figure 2)—mainly attributed to the degradation of polar extractives, which have a high oxygen content and therefore low resistance against thermal degradation [17]. This class of extractives is not well-represented in eucalyptus wood, with contents below 5% [11]. The degradation of MDF residues was greater from 250 to 400 °C. This temperature range is capable of degrading representative compounds of wood, such as cellulose and hemicellulose, the last one almost completely degraded at 400 °C [17]. The compounds added in the manufacture of panels, such as coating and adhesives, are also degraded in this temperature range. The chemical safety information sheet refers to melamine resin coating and paint of the MDF, and chemical components such as xylene, nitrocellulose, n-butyl acetate and glycol in MDP (medium-density particleboard) panels commonly used in furniture factories volatilize in the range of 300 °C, thus contributing to greater thermal degradation of residues from MDP panels in this temperature range [35]. Lignin has a high carbon content and strong bonds that unite its monomers, so it has high thermal resistance, being degraded between 225 and 900 °C [36], showing mass loss lower than that of cellulose, hemicelluloses and extractives [37,38]. With the application of torrefaction, the degradation of oxygen-rich components, such as hemicellulose, cellulose and some extractives, increases the concentration of lignin derivatives, increasing the biomass potential for energy generation.

The thermal decomposition of biomass starting from 450 °C differs between residues untreated or torrefied for 20 min, but was similar for the torrefaction periods of 30 and 40 min. The greater residual mass losses from 500 °C are due to the high thermal stability of the biomass with more lignin after the degradation of the hemicelluloses in torrefied biomass, as the lignin is the most thermally stable component of the biomass [37].

Greater resistance to deterioration with increased torrefaction time can be associated with chemical changes in carbohydrates increasing mass losses of the samples [39], which decreases the attractiveness of MDF panels to rotting fungi after longer torrefaction periods. The gradual reduction in the mass losses of the samples submitted to the decay fungi is due to an increase in the lignin content without the removal of certain extractives by the torrefaction process [40,41] with increased resistance to brown and white rot fungi.

Torrefaction increased the resistance and reduced the mass losses of the MDF samples by the brown rot Postia placenta and the white rot Trametes versicolor and Irpex lacteus fungi with lower mass losses (2.62%) in the treatment with 40 min of torrefaction (Table 3). The increased resistance of MDF samples to degradation by xylophage fungi after torrefaction may be due to a reduction in the holocellulose content, which are sources of fungi food and contribute to the generation of compounds with fungicidal activity [22,41]. In addition, torrefaction changes the chemical structure of the remaining carbohydrates, reducing their degradation by enzymes of xylophage fungi [42,43]. The phenolic groups in the lignin structure increase its resistance to xylophagous agents and, therefore, the content of this compound contributes to the resistance to these agents in the samples treated at 300 °C for 40 min.

Changes in wood due to torrefaction limit fungal attack. The degradation of cellulose and hemicellulose releases compounds that reduce the pH and limit the water adsorption capacity of the wood, reducing the equilibrium moisture content [44,45,46]. Fungi depend on the water in the wood to transport enzymes and nutrients; in addition, they operate in a very specific pH range, and changing these conditions limits their growth. Finally, a minimal change in the cellulose and hemicellulose molecules makes it unfeasible to feed the fungi, which prevents their development in the torrefied material [13,22,47]. The torrefaction process concentrated the lignin; this molecule has phenolic groups in its composition, which have fungicidal and insecticidal properties, limiting mass losses by xylophagous organisms [40,48].

The greater mass losses in the untreated material and in the shorter torrefaction periods can also be associated with the production of wood panels produced in refining processes with the defibrillating of the wood by mechanical forces and heat [49]. This process exposes the surface of the wood fibers, increasing their susceptibility to decay fungi. The combination of refining processes and hot pressing can expose polysaccharide components, such as segments of hemicelluloses or cellulose, providing nutrients for decay fungi [49,50].

The mass losses of the samples caused by T. versicolor and P. placenta was similar between treatments, except in the MDF untreated, with greater values from the fungus Irpex lacteus (Table 3). The mass losses caused by the fungi T. versicolor and P. placenta were similar, while that of Irpexlacteus was greater, due to the greater quantity of oxidoreductase enzymes secreted by the latter fungus [51]. The ligninolytic enzymes, namely, Mn²⁺ oxidizing peroxidases (MnPs), and laccases, are the main enzymes secreted by most white rot fungi, including Irpex lacteus, and degrade lignin and phenolic compounds [52]. In addition, they oxidize aromatic amines and other compounds, using oxygen from the air and releasing water as the only by-product [51]. The difference between the mass losses from each fungus in the fresh treatments relative to that of 30 min torrefaction is due to the nutritional requirements and the more selective enzymes of these xylophagous fungi [53].

5. Conclusions

The TG/DTG curves allowed us to obtain the thermal stability zone (between 100 and 250 °C); the peaks of thermal decomposition of cellulose and hemicelluloses and the degradation of volatile materials found in the MDF coating (between 250 and 400 °C); and the thermal stability zone of the biomass due to the high lignin concentration (above 500 °C). The intensity of torrefaction increased the resistance of the MDF residue to rot fungi, as the losses caused by the attacks from the fungi Irpex lacteus and Postia placenta were lower in the material torrefied for 30 and 20 min, respectively. Torrefaction is an effective technique to reduce the incidence of fungi and can be used to increase the storage time of biomass without loss by xylophages.