The Effect of Particles from Rotten Spruce Logs and Recycled Wooden Composites on Changes in the Bio-Resistance of Three-Layer Particleboards Against the Decaying Fungus Coniophora puteana and Mixture of Moulds

Abstract

Wood-based particleboards (PBs) are widely used in construction and interior applications, yet their durability, particularly against biological degradation, remains a challenge. Recycling wood and incorporating degraded particles from rotted wood can potentially enhance PB sustainability and align with circular bioeconomy principles. This study investigates the biological resistance of the three-layer, laboratory-prepared PBs with varied amounts of particles, from sound spruce wood to particles, and from spruce logs attacked by brown- or white rot, respectively, to particles from recycled wooden composites of laminated particleboards (LPBs) or blockboards (BBs), i.e., 100:0, 80:20, 50:50, and 0:100. The bio-resistance of PBs was evaluated against the brown-rot fungus Coniophora puteana, as well as against a mixture of moulds’ “microscopic fungi”, such as Aspergillus versicolor BAM 8, Aspergillus niger BAM 122, Penicillium purpurogenum BAM 24, Stachybotrys chartarum BAM 32, and Rhodotorula mucilaginosa BAM 571. PBs containing particles from brown-rotten wood or from recycled wood composites, particularly LPBs, had a partly enhanced decay resistance, but their mass loss was nevertheless more than 30%. On the other hand, the mould resistance of all variants of PBs, evaluated in the 21st day, was very poor, with the highest mould growth activity (MGA = 4). These findings suggested that some types of rotten and recycled wood particles can improve the biological resistance of PBs; however, their effectiveness is influenced by the type of wood degradation and the source of recycled materials. Further, the results highlight the need for improved biocidal, chemical, or thermal modifications of wood particles to enhance the overall biological durability of PBs for specific uses.

1. Introduction

The increasing trend of forest degradation, particularly among coniferous species, has become a critical issue in the Slovak Republic. In 2021, the proportion of coniferous trees exhibiting defoliation levels exceeded 25%—reaching 54% of the total forest area. Additionally, 87.0 ± 5.7 million cubic metres of dead wood, including standing logs, stumps, and lying thick and thin wood, were recorded in Slovak forests [1]. The accelerated decline in forest health is further complicated by decay agents, such as brown- and white-rot fungi, which contribute to the breakdown of lignin–saccharide wood components.

Simultaneously, the ever-increasing demand for wood products, driven by rising consumer consumption patterns and growth in the wooden building and furniture industries, has led to the generation of substantial volumes of wood waste. Common sources of wood waste “recyclates” include old furniture, pallets, or packaging materials, as well as by-products from wood processing, such as sawdust or wood chips. In Slovakia alone, 12,711 thousand tons of waste were generated in 2021, comprising 2,767 thousand tons of municipal waste, 474 thousand tons of hazardous waste, and 9,470 thousand tons of other waste. Recycling rates for municipal waste reached 50.05%, with 44.95% being materially recovered and 5.10% used for energy recovery. Despite these efforts, 20.30% of the waste was still landfilled in 2021 [1].

In response to increasing raw material shortages, recent legislation and growing environmental concerns have prioritized the efficient and cascading use of wood resources, posing significant challenges to the wood-based panel industry [2]. Particleboards (PBs) prepared from wood and/or various plant particles, glue, and additives have emerged as an innovative solution, allowing for the effective utilization of wood and/or plant resources by incorporating the agricultural, forest, and industrial lignocellulosic waste as the basic entrance material. This approach has been explored by several studies [3,4,5,6,7,8,9,10].

PB production facilitates the use of various wood sources, including low-quality timber and wood waste. The produced PBs are homogeneous panels with large dimensions, offering advantages over solid wood, such as the elimination of wood defects (e.g., knots, juvenile wood, and grain slope). By controlling production parameters, such as glue content, particle geometry, and pressing conditions, PBs can be tailored to exhibit specific biological, physical, and mechanical properties. Additionally, regional availability often dictates the choice of the lignocellulosic materials used in PBs [11]. The quality of the final product is directly influenced by the characteristics of the input materials [12,13,14,15,16,17].

Despite their advantages, PBs are susceptible to degradation by wood-deteriorating agents under appropriate moisture and temperature conditions. Common wood-decay organisms, including insects—mainly termites—and fungi, can compromise the structural integrity of PBs [18,19,20,21,22,23]. Several intrinsic and extrinsic factors contribute to the susceptibility of PBs and other lignin–polysaccharide-based composites to fungal decay—such as wood or plant species and their possible thermal and chemical modifications—furthering wood/plant particle geometry and distribution, wood/plant moisture content, and the type and amount of glue and other additives used in PB.

The natural durability of wood species used in the production of PBs significantly influences their bio-resistance. Softwoods, such as spruce, fir, and pine, are commonly used in PBs, but they are less resistant to decay compared to some hardwoods, such as oak and teak [18]. Extractives present in certain wood species, such as phenolic compounds and tannins, can inhibit fungal growth, but these natural protective compounds are often diluted or lost during particleboard production [19,24,25,26]. Thermally modified wood and chemically modified wood are increasingly being used in PB production to enhance fungal resistance. Thermal modification involves heating wood at high temperatures (160–240 °C) in a low-oxygen environment, which reduces its hygroscopicity and alters the chemical structure of hemicelluloses, making it less digestible to decaying fungi [27,28]. Chemically modified wood, such as acetylated wood, undergoes reactions that reduce water absorption and improve durability [29]. Incorporating thermally or chemically modified particles in PBs has shown significant improvements in resistance to brown-rot and white-rot fungi [7].

According to Choupani Chaydarreh et al. [30], particle size plays a major role in particleboard manufacturing. Different particle sizes could provide different properties based on the type of raw material, meaning that particle size and geometry optimization are required for any newly used lignocellulosic materials that may be a competitive alternative to wood species in particleboard manufacturing. Although, the porosity of a particleboard influences its properties, especially those related to physical, thermal, and acoustic properties [31]. The size, shape, and distribution of wood particles in PBs affect their porosity, which in turn influences moisture absorption and fungal colonization. Smaller particles and greater compaction during the production of PBs can lead to their higher density, reducing voids and limiting fungal infiltration. However, high-density panels may also have lower permeability, making it more difficult for the wood particles to dry out once moisture is absorbed, creating a favourable environment for fungal growth.

Fungal growth is primarily dependent on moisture availability. For most fungi, wood moisture content exceeding 20% is necessary for their growth [19,20]. PBs and other composites often absorb moisture from the environment due to capillary action, especially in poorly protected or uncoated panels. Elevated moisture levels can also occur in areas of water intrusion, such as in construction sites or in external applications where panels are exposed to rain or humidity. Fungal growth is influenced by a range of environmental and biological factors beyond water, which play critical roles in determining whether fungi proliferate or remain dormant. The key factors impacting fungal growth include temperature, pH and oxygen levels, light, substrate composition, the presence of antagonistic organisms, chemical inhibitors or preservatives, and coatings [19,20]. Each of these elements can significantly affect fungal activity, either promoting growth or limiting fungal spread, depending on the specific environmental conditions and species involved.

The type of glue used in PB production plays a critical role in fungal resistance. Urea-formaldehyde (UF) glue, commonly used in PBs due to its low cost, is not inherently resistant to fungal degradation, whereas phenol–formaldehyde glues offer superior resistance due to their chemical stability and hydrophobicity [32]. The inclusion of biocides, preservatives, or fungicidal additives in PB formulations can improve decay resistance by inhibiting fungal colonization [33]. However, environmental regulations restricting the use of certain preservatives (e.g., creosote, CCA) have prompted the development of more eco-friendly alternatives, such as borates and copper-based compounds [34]. A variety of polymer coatings may be used for the protection of wood-based materials. Carboxyl-containing water-soluble polymers improve wood’s resistance to water and environmental damage. pH-sensitive coatings enhance durability by adjusting polymer composition. Multi-layer polymer structures, formed through layer-by-layer self-assembly, provide robust protection against environmental factors. These coatings significantly boost wood’s resistance to moisture, photodegradation, and biological threats, thereby extending its lifespan and preserving its appearance [35].

The study aims to provide a comprehensive understanding of how different ratios of sound and degraded/recycled particles influence PB durability, with a focus on resistance to both brown-rot and common moulds. This approach represents a novel contribution by integrating multiple decay sources, wood types, and recycled components, offering insights into the bio-resistance potential of PBs produced from a broader range of lignocellulosic waste. The biological resistance of PBs were tested against fungi—one brown-rot and a mixture of moulds.

2. Materials and Methods

2.1. Particleboard Variants

Three-layer particleboards (PBs), with the dimension of 400 × 300 × 16 mm, were produced under laboratory conditions from five different types of particles:

• Control–sound spruce wood particles (C).
• Brown-rotten (BR) wood particles: particles from spruce logs decayed by Fomitopsis pinicola (brown-rot fungus).
• White-rotten (WR) wood particles: particles from spruce logs decayed by Armillaria ostoyae (white-rot fungus).
• Recycled particles from laminated PBs (LPBs) sourced from waste laminated PBs.
• Recycled particles from waste blockboards (BBs): sourced from waste blockboards.

Each variant of PB (BR, WR, LPB, and BB) was produced with different amounts of rotten or recycled particles in the particleboard mixture, corresponding to the following weight ratios (w/w) of 0%, 20%, 50%, and 100%. The particles were bonded with UF glue containing additives “hardener and paraffin” in the recipe used in the KRONOSPAN s. r. o, Zvolen, Slovakia. The glue mixture was applied to the particles in a rotary mixing machine (VDL, TU, Zvolen, Slovakia). The particles layered with glue were then cold-pressed in a low-temperature machine at a pressure of 1 MPa in a CBJ 100-11 press (TOS, Rakovník, Czech Republic) according to a three-stage pressing diagram—at maximum pressing plate temperature of 240 °C, a maximum specific pressing pressure of 5.23 MPa, and with a pressing factor of 8 s·mm⁻¹. In total, 78 PBs were manufactured (six from each of the 4 types, and with 4 w/w combinations). Figure 1 and

Table 1. Different variants of prepared PBs—13 variants in total.

PB Variant	Abbreviation	Amount of Sound Spruce Particles in PB [%]				Amount of Rotten/Recycled Particles in PB [%]				Density 1 [kg·m3]
		0	50	80	100	0	20	50	100
Control PB	PB-C				✗	✗				656 ± 23
PB-BR	20 BR			✗			✗			638 ± 21
PB manufactured from	50 BR		✗					✗		630 ± 23
brown-rotten particles 2	100 BR	✗							✗	636 ± 23
PB-WR	20 WR			✗			✗			642 ± 21
PB manufactured from	50 WR		✗					✗		645 ± 23
white-rotten particles 3	100 WR	✗							✗	640 ± 25
PB-LPB	20 LPB			✗			✗			646 ± 27
PB manufactured from recycled	50 LPB		✗					✗		648 ± 28
particles from waste laminated PBs 4	100 LPB	✗							✗	643 ± 21
PB-BB	20 BB			✗			✗			657 ± 20
PB manufactured from recycled	50 BB		✗					✗		654 ± 18
particles from waste blockboards 5	100 BB	✗							✗	649 ± 21

The symbol “✗” indicates the combination of the “Amount of Sound Spruce Particles in PB [%]” and the “Amount of Rotten/Recycled Particles in PB [%]”. ¹ The density of the PBs was measured according to the EN 323 [36] as described by [37]. ² The rotten particles were sourced from standing Norway spruce (Picea abies Karst. L.) trees affected by the brown-rot fungus Fomitopsis pinicola/Sw./P. Karst., as described by [38]. ³ The rotten particles were sourced from standing Norway spruce (Picea abies Karst. L.) trees affected by the white-rot fungus Armillaria ostoyae/Romang./Herink, as described by [38]. ⁴ The laminated PBs were sourced from 60-year-old furniture. ⁵ The blockboards were sourced from 50-year-old furniture.

summarize the particleboard variants, their compositions, and densities.

2.2. Mycological Test with the Brown-Rot Fungus Coniophora puteana

The decay resistance of the PBs, using samples with dimensions of 50 × 50 × 16 mm, was assessed by a standardized mycological test following the EN 113-3:2023 [39] for basidiomycete monocultures. The decay resistance class of the PBs was assigned according to EN 350:2016 [18]. The Coniophora puteana isolate was identified based on DNA-based molecular genetics methods [40]. Scots pine (Pinus sylvestris L.) and Norway spruce (P. abies Karst. L.) wood samples measuring 25 × 15 × 50 mm were used to assess the virulence of the fungal isolate (Figure 2).

Before mycological testing, the edges of the PB samples were sealed with epoxy resin (CHS-Epoxy 1200) mixed with amino hardener (P11) at a weight ratio of 11:1 (Stachema, Mělník, Czech Republic). The epoxy resin was applied at amount of 200 ± 10 g/m². The samples were dried at 103 °C for 48 h and weighed to determine the initial weight (m₀). Then, the PB samples were sterilized twice for 30–45 min under UV light and soaked in demineralized water for 240 min to achieve a moisture content above 20%. Such prepared PB samples were placed into 1 L Kolle flasks on stainless steel grids over active fungal mycelium growing on an agar-malt soil. The mycological test lasted for 16 weeks at 22 ± 2 °C and 65 ± 5% RH. After the test, the samples were cleaned, weighed to determine the wet weight (m_{w/Fungally-Attacked/}), and dried to determine the final dry weight (m_{0/Fungally-Attacked/}). The mass loss (Δm) and moisture content (w_decayed) of PB samples were calculated using Equations (1) and (2).

$$\mathsf{\Delta }m={\displaystyle \frac{m_0-m_{0/\mathrm{F}\mathrm{u}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{y}-\mathrm{A}\mathrm{t}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{e}\mathrm{d}/}}{m_0}}\times 100\left(\mathrm{\%}\right)$$

(1)

$$w_{decayed}={\displaystyle \frac{m_{\mathrm{w}/\mathrm{F}\mathrm{u}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{y}-\mathrm{A}\mathrm{t}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{e}\mathrm{d}/}-m_{0/\mathrm{F}\mathrm{u}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{y}-\mathrm{A}\mathrm{t}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{e}\mathrm{d}/}}{m_{0/\mathrm{F}\mathrm{u}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{y}-\mathrm{A}\mathrm{t}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{e}\mathrm{d}/}}}\times 100\left(\mathrm{\%}\right)$$

(2)

2.3. Mould Resistance of Particleboards

The mould resistance of PBs was evaluated against a mixture of microscopic fungi (Aspergillus versicolor BAM 8, Aspergillus niger BAM 122, Penicillium purpurogenum BAM 24, Stachybotrys chartarum BAM 32, and Rhodotorula mucilaginosa BAM 571) following a modified EN 15457:2022 [41]. PB samples (50 × 50 × 16 mm) were sterilized with UV light twice for 30 min, and then the air-conditioned PB samples were placed into 180 mm Petri dishes on Czapek-Dox agar (HiMedia Laboratories Pvt. Ltd., Mumbai, India) and inoculated with a water suspension of mix-mould spores. Incubation occurred over 21 days at 24 ± 2 °C and 90–95% RH of air.

Mould growth activity (MGA) on the top PB surface was assessed on days 7, 14, and 21. A scale according to EN 15457:2022 [41] from 0 to 4 was used to evaluate mould coverage: 0 = no mould; 1 = mould on up to 10%; 2 = mould on up to 30%; 3 = mould on up to 50%; 4 = mould on more than 50% (Figure 3).

2.4. Statistical Analyses

The statistical software STATISTICA 12 (Stat Soft, Inc., Tulsa, OK, USA) was used to analyze the experimental data. Descriptive statistics (median, mean, standard deviation) were calculated for the measured properties. Simple linear correlation analysis was performed to evaluate the relationships between the mass loss as well as other measured parameters of prepared PBs and the material compositions of PBs, with the coefficient of determination (r²) used as the primary metric: r² = 0.00–0.19 indicates a very weak correlation; r² = 0.20–0.39 indicates a weak correlation; r² = 0.40–0.59 indicates a moderate correlation; r² = 0.60–0.79 indicates a strong correlation; r² = 0.80–1.00 indicates a very strong correlation [42].

3. Results

3.1. Decay Resistance of Particleboards to the Brown-Rot Fungus Coniophora puteana

The results of biological resistance, both to the decaying fungus Coniophora puteana and the mixture of moulds, of particleboards (PBs) manufactured with brown-rot (PB-BR) or white-rot (PB-WR) particles, as well as with recycled laminated PB (PB-LPB) or blockboard (PB-BB) particles, are summarized in

Table 2. The biological resistances of the PBs manufactured from brown-rotten particles (PB-BR): (Ⅰ) decay resistance based on mass loss (Δm) caused by C puteana; (Ⅱ) mould resistance against mixture of microscopic fungi based on mould growth activity on surfaces of PBs (MGA from 0 to 4).

Biological Resistance of PBs-BR		Amount of Brown-Rotten Particles in PB w/w [%]
		0	20	50	100
Decay attack by C. puteana
Δm [%]	Median	49.47	38.77	36.87	30.55
	Average (SD 1)	50.09 (2.98)	39.69 (2.09)	35.44 (5.11)	30.45 (1.02)
wdecayed [%]		67.54 (2.85)	49.26 (2.92)	51.26 (3.32)	45.81 (3.95)
Durability class (EN 350 [18])		5	5	5	5
Attack by mixture of interior moulds—MGA (0–4)
7th day		1	1	1	0
14th day		3	3	3	2
21st day		4	4	4	4

¹ SD—Standard deviation.

,

Table 3. The biological resistance of the particleboards manufactured from white-rotten particles (PB-BR): (Ⅰ) decay resistance based on mass loss (Δm) caused by C puteana; (Ⅱ) mould resistance against a mixture of microscopic fungi based on the mould growth activity on the surfaces of PBs (MGA from 0 to 4).

Biological Resistance of PB-WR		Amount of White-Rotten Particles in PB w/w [%]
		0	20	50	100
Decay attack by C. puteana
Δm [%]	Median	49.47	35.42	36.43	40.45
	Average (SD 1)	50.09 (2.98)	35.23 (1.42)	36.36 (1.88)	39.86 (4.39)
wdecayed [%]		67.54 (2.85)	49.91 (2.42)	48.60 (2.56)	46.72 (4.38)
Durability class (EN 350 [18])		5	5	5	5
Attack by mixture of interior moulds—MGA (0–4)
7th day		1	1	1	1
14th day		3	3	3	3
21st day		4	4	4	4

¹ SD—Standard deviation.

,

Table 4. The biological resistance of the particleboards manufactured from recycled particles from laminated PBs (PB-LPB): (Ⅰ) decay resistance based on mass loss (Δm) caused by C puteana; (Ⅱ) mould resistance against a mixture of microscopic fungi based on the mould growth activity on the surfaces of PBs (MGA from 0 to 4).

Biological Resistance of PB-LPB		Amount of Recycled Particles from Laminated PBs in PB w/w [%]
		0	20	50	100
Decay attack by C. puteana
Δm [%]	Median	49.47	43.72	42.06	32.96
	Average (SD 1)	50.09 (2.98)	44.20 (2.35)	41.07 (5.47)	33.84 (3.36)
wdecayed [%]		67.54 (2.85)	48.81 (2.07)	50.06 (4.88)	48.26 (8.39)
Durability class (EN 350 [18])		5	5	5	5
Attack by mixture of interior moulds—MGA (0–4)
7th day		1	0	0	0
14th day		3	1	2	1
21st day		4	4	4	4

¹ SD—Standard deviation.

and

Table 5. The biological resistance of the particleboards manufactured from recycled particle blockboards (PB-BB): (Ⅰ) decay resistance based on mass loss (Δm) caused by C puteana; (Ⅱ) mould resistance against a mixture of microscopic fungi based on the mould growth activity on the surfaces of PBs (MGA from 0 to 4).

Biological Resistance of PB-BB		Amount of Recycled Particles from Blockboards in PB w/w [%]
		0	20	50	100
Decay attack by C. puteana
Δm [%]	Median	49.47	47.10	44.93	42.96
	Average (SD 1)	50.09 (2.98)	47.51 (4.35)	45.82 (3.09)	43.07 (2.36)
wdecayed [%]		67.54 (2.85)	53.23 (4.13)	53.69 (4.70)	50.06 (5.46)
Durability class (EN 350 [18])		5	5	5	5
Attack by mixture of interior moulds—MGA (0–4)
7th day		1	1	1	0
14th day		3	2	3	1
21st day		4	4	4	4

¹ SD—Standard deviation.

. The comparison of decay resistance and mould resistance across different PB compositions reveals significant differences in performance based on the type and amount of rotten or recycled particles used. The control particleboard (C-PB = 0% of rotten or recycled particles) exhibited the highest mass loss across all variants, with median of 49.47%, which placed it in durability class 5 (very low durability) under EN 350 [18].

The results of the decay resistance test for PBs produced with different proportions of brown-rotten (BR) particles are summarized in Table 2. The mass loss caused by C. puteana demonstrated a decreasing trend with an increasing amount of BR particles in the PB composition (Figure 4a). As the proportion of BR particles increased to 20%, the mass loss decreased to 38.77%, showing improved decay resistance. This trend continued with PBs containing 50% and 100% BR particles, which had median mass losses of 36.87% and 30.55%, respectively, making PB-BRs the most decay-resistant of all tested variants. These findings indicate that while the incorporation of BR particles did not confer significant durability to PB (as all samples fell within durability class 5 under EN 350 [18]), the inclusion of wood particles with a lower amount of polysaccharides somewhat reduced their susceptibility to further brown-rot decay.

The moisture content (w) of the PBs also varied with the proportion of BR particles. PBs with 0% BR particles, i.e., those theoretically with the highest amount of polysaccharides, had the highest w = 67.54%, while PBs with 100% BR particles had the lowest w = 45.81% (Table 2). The moisture content of PB-BR after mycological testing decreased with increasing amounts of BR particles (Figure 4b). The lower moisture content in PBs with a higher proportion of BR particles may have contributed to the observed improvements in their decay resistance.

The decay resistance of PBs manufactured from white-rot (WR) particles decreased only slightly with increasing amounts of white-rotten particles (Table 3 and Figure 5a). The lowest mass loss (35.42%) was observed in PBs with 20% WR particles, but as the WR particle content increased to 50% and 100%, the mass loss increased to 36.43% and 40.45%, respectively. This indicates that a higher proportion of white-rot particles already weakened the decay resistance of PBs.

The moisture content of PBs manufactured from white-rotten particles followed a similar trend (Table 3 and Figure 5b) as was followed for the mass loss. Control PBs had a moisture content of 67.54%, while PBs with 100% white-rotten particles had a reduced moisture content of 46.72%. However, compared to PB-BR samples, the PB-WR samples showed slightly higher moisture retention across all particle ratios, reflecting the more extensive degradation of lignin by white-rot fungi. Since white-rot fungi attack both lignin and polysaccharides, the remaining structure retains more water, making it more susceptible to further fungal decay despite the pre-rotting process.

PB-LPB manufactured from recycled particles from waste laminated PBs showed a low improvement in decay resistance when the amount of recycled particles increased. The mass loss decreased progressively from 43.72% (20% recycled LPB) to 32.96% (100% recycled LPB) (Table 4 and Figure 6a). This suggests that laminated PB particles contributed positively to the fungal decay resistance of newly manufactured PBs.

The PBs made from recycled laminated PB particles demonstrated more consistent moisture content across different particle ratios (Table 4 and Figure 6b). Control PBs had a moisture content of 67.54%, whereas PBs with 100% laminated particles had a moisture content of 48.81%. The laminated structure and presence of melamine formaldehyde resins in the laminated surfaces of recycled PBs likely contribute to the reduced moisture absorption in the new PBs.

PBs made from recycled blockboards (BB) exhibited lower decay resistance, with mass losses ranging from 47.10% for PBs with 20% (BB) particles to 42,96% with 100% BB particles (Table 5 and Figure 7a). The BB are usually made from layers of solid wood, which may still be prone to decay even after recycling to particles. However, glues presented in BB could provide some protection to PB-BB against decaying fungi.

The PB-BB after the decay test exhibited lower moisture content (w) than the control C-PBs (Table 5 and Figure 7b), gradually with the blockboard particles highest proportion in PBs. After decay tests the w-values decreased from 67.54% in C-PBs to 50.06% in PB-BB with 100% blockboard particles. The recycled BBs, composed of solid wood layers glued together, may offer some degree of moisture resistance due to the glue used. However, BBs typically absorb more moisture than LPBs, which may explain the relatively higher moisture content and susceptibility to decay observed in PB-BB compared to PB-LPB.

3.2. Mould Resistance of PBs

All PB variants were tested against a mixture of common interior moulds. The mould resistance, evaluated using the mould growth activity (MGA) scale, revealed only minimal or none differences between PB variants, particularly in the final 21st day of test when all PB variants had an MGA of 4—indicating extensive mould coverage on the top surfaces of PBs. The resistance of individual variants of PBs to moulds in the 7th, 14th, and 21st day is summarized in Table 2, Table 3, Table 4 and Table 5.

PB-BR samples showed quite uniform mould growth across all particle ratios on the 7th day (MGA = 1, or 0 for 100% w/w), as well as on the 14th day (MGA = 3, or 2 for 100% w/w); see Table 2. So, the presence of BR particles did not significantly reduce mould growth.

Similarly, PB-WR samples (Table 3) exhibited no significant improvement in mould resistance. WR degradation, which affects both lignin and polysaccharides, does not appear to inhibit mould growth, as the remaining wood structure is still favourable to mould growth.

In contrast, PB-LPB showed improved mould resistance, particularly in the early stages of the test (Table 4). On the 7th day, PBs with 20% and 50% recycled laminated particles had an MGA of 0, indicating no mould growth, while control PBs already exhibited MGA of 1. On the 14th day, PBs with higher proportions of recycled particles from LPB continued to show reduced mould growth compared to the control PBs, with MGA values of 1 or 2. The presence of resin layers in these recycled particles likely inhibited moisture uptake, thereby slowing mould colonization. However, by the 21st day, all PB-LPB exhibited an MGA of 4, suggesting that the protection offered by laminated particles is limited over prolonged exposure periods.

PB-BB also showed improved early-stage mould resistance compared to control PBs (Table 5). On the 7th day, PBs with 100% BB particles exhibited no mould growth (MGA of 0), while control PBs had an MGA of 1. By the 21st day, however, all PB variants, including those with 100% BB particles, exhibited significant mould growth (MGA of 4). The improved mould resistance of BB particles in the early stages may be attributed to the glues between wood substance, which provide some protection against moulds.

4. Discussion

The findings of this study support with previous research showing that the incorporation of recycled materials can enhance the decay resistance of particleboards (PBs) to varying degrees [6,7]. However, the impact on mould resistance of PBs is more questionable, considering only PBs containing recycled particles from laminated PBs showed a moderately better resistance to the growth of moulds in the first days of test. These results suggest that while the degraded and recycled wood particles can specifically improve the biological resistance of PBs, their effectiveness is influenced by the type of degradation (e.g., brown-rot vs. white-rot) and the source of recycled materials (e.g., laminated PBs vs. blockboards).

The brown-rot fungus C. puteana, a known aggressive wood-decaying organism, readily degrades various wood materials, including PBs made from low-quality particles, recycled LPBs, and BBs [6,19,20,43,44]. The median mass loss (alternatively average mass loss) across all variants of PBs exceeded 30%, indicating a high susceptibility of tested PBs to decay.

The effectiveness of combining wood particles and adhesive in particleboards (PBs) for preventing rot, moisture, and mould depends on factors such as the type of wood, adhesive formulation, and specific additives used. In a previous study [45], it was showed that wood previously damaged by one species of decay fungus remains susceptible to subsequent fungal attacks, even after sterilization. The severity of this secondary decay may vary due to the depletion of structural components like hemicelluloses and other easily digestible elements in the initially degraded wood. The fungus C. puteana was able to maintain its decay activity at comparable levels even after prior fungal exposure.

Using naturally durable wood species as particle sources is an effective strategy to improve PB durability. The study [46] evaluated the resistance of oriented strandboards (OSB) made from wood with different natural durability against fungi [18]. Eucalyptus OSB demonstrated the highest resistance to fungal decay, while pine OSB was the most susceptible. Conversely, the study [47] reported significant mass losses (aver 41%) in PBs exposed to decay by Gloeophyllum trabeum, particularly in boards made from non-durable sugarcane bagasse, supporting our findings. Similarly, the study [48] observed a mass loss exceeding 15% in untreated OSB panels due to the degradation of Fomitopsis palustris and Trametes versicolor. These studies emphasize that PBs made from non-durable wood species or recycled materials, without chemical treatments, remain highly susceptible to fungal decay. Another study on fungal resistance in PBs made from agro-industrial waste [49] also reported significant mass losses upon fungal exposure, with no notable differences among the material variants. In PBs containing eucalyptus sawdust and macadamia nut carpel, white-rot fungi resulted in mass losses of 30%–50%, regardless of the proportion of recycled or deteriorated particles. All treated PBs challenged with Brunneoporus malicola (similar to G. trabeum) and T. versicolor were classified as only moderately resistant or not resistant.

Traditional adhesives such as urea–formaldehyde (UF) and phenol–formaldehyde (PF) are widely used in PB manufacturing. Research suggests that PF adhesives generally offer greater fungal resistance than UF, thereby reducing vulnerability to rot and mould [32]. In study [46], the resistance of OSBs made from pine, eucalyptus, and cypress was investigated, using UF and PF adhesives in different concentrations. The findings revealed that higher concentrations of PF resin significantly enhanced durability, with eucalyptus OSB treated with 8% PF resin showing the highest resistance.

Appropriately selected particle modification also contributes to the better resistance of PBs to rot. In study [49], the effects of a combination of thermal modification and adhesive levels on the wettability and biological resistance of PBs made from sugarcane residue and bamboo particles was explored. A thermal treatment at 220 °C for approximately 3.5 h altered the chemical composition of the particles, reducing levels of lignin, extractives, and holocellulose. These treated PBs exhibited higher contact angles, indicating reduced wettability, and were categorized as non-wettable to partially wettable. This thermal modification also improved biological resistance, making the boards highly resistant to Rhodonia placenta, resistant to G. trabeum, and moderately resistant to T. versicolor. These findings suggest the potential for using thermally modified sugarcane and bamboo particles to produce durable, decay-resistant PBs. Similar improvements through chemical modification were observed in our previous research [7].

Moreover, adding specific treatment additives, such as nanoparticles or fungicide, to adhesives or on particles can also substantially improve decay resistance against both white-rot and brown-rot fungi. In studies [50,51], PBs treated with UF adhesive containing different nano-ZnO concentrations were investigated. Their results demonstrated that higher nano-ZnO levels significantly enhanced decay resistance, highlighting its efficacy as a preservative in minimizing fungal-induced degradation and weight loss.

While decay resistance remains a primary concern, the susceptibility of the tested PBs to mould growth is also notable. The mould growth on the particleboard surfaces was rated as severe (MGA = 4) across all treatments by the 21st day. PBs are often susceptible to mould growth due to several inherent properties. Primarily, PBs tend to absorb and retain moisture, creating an ideal environment for mould, especially in humid and warm conditions [51,52,53]. The wood particles in PBs provide nutrients that moulds can easily utilize to proliferate [54,55,56]. Additionally, the type of adhesive may impact mould susceptibility [57]. As UF resins break down, they increase the PB’s porosity, creating more sites for mould colonization. Untreated PBs also lack fungicidal agents, which would otherwise inhibit mould growth. Without such treatments, PBs are vulnerable to mould, particularly in fluctuating humidity levels. The small wood particles used to create PBs result in a porous structure that traps moisture, creating microenvironments conducive to mould [31]. PBs made with recycled materials can be especially prone to mould, since recycled particles may have undergone prior fungal degradation. This prior exposure can weaken the wood’s structure, making it more susceptible to mould re-infestation.

In the current study, PBs with higher proportions of rotten or recycled particles generally retained less moisture after fungal exposure, likely contributing to the enhanced decay resistance observed in boards with brown-rot particles and particles from recycled laminated PBs. Despite this, all variants showed significant mould growth. Various researchers [3,4,5,6,7,8,9,12,23,24,25,27,32,33,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62] also noted the connection between the water sorption properties of wood-based materials and their susceptibility to mould growth or decay by wood-decaying fungi, emphasizing that hygroscopic natural fibres tend to support biological attacks due to moisture content fluctuations. The ability of different particle types to resist moisture plays a key role in their biological durability. Natural fibres are hygroscopic because their cell walls contain high amounts of water sorption sites (hydroxyl groups) and can expand to accommodate the water [60]. Moulds have been shown to appear in succession on a material as the moisture content of the material fluctuates, according to their minimum moisture demands of the mould [53,61,62]. The results suggest that particles from brown-rotten wood and recycled laminated PBs offer the best combination of decay resistance and reduced moisture content, while white-rot particles and blockboard particles, though still effective, may retain more moisture, making them more susceptible to biological attacks over time.

In summary, while different treatments involving recycled particles from laminated PBs, blockboards and rotten wood showed no significant difference in fungal decay resistance; the results collectively underscore the need for more effective treatments of PBs determined for specific applications, e.g., by material modifications to improve their decay and mould resistance. Also, the comparisons with the above studies reinforce the view that recycled and low-quality wood materials, even when used in particleboard production, are highly susceptible to biological degradation if they are not treated with preservatives or modified. The challenge remains in finding cost-effective and sustainable treatments that align with circular bio-economy principles, such as natural biocides or environmentally friendly chemical additives.

5. Conclusions

The study reinforces the importance of addressing both the fungal decay and mould resistance of particleboards for specific uses. While recycled particles, particularly those from laminated PBs, improved the decay resistance of newly prepared PBs, the mould susceptibility of all prepared PB variants remained high.

Ultimately, the ability of different particle types in PBs to resist moisture plays a key role in their biological durability. Brown-rotten wood particles and particles from recycled laminated PBs offer the best combination of decay resistance and reduced moisture content, while white-rotten wood particles and blockboard particles, though effective, may retain more moisture and thus become more susceptible to biological attack over time.

Further research should focus on optimizing treatments to enhance both decay and mould resistance in particleboards, ensuring sustainability and durability in their applications.