Challenges and Opportunities in Recycling Upholstery Textiles: Enhancing High-Density Fiberboards with Recycled Fibers

Abstract

Recycling upholstery textiles is challenging due to the complexity of materials, which often include a mix of fabrics, foams, and adhesives that are difficult to separate. The intricate designs and layers in upholstered furniture make it labor-intensive and costly to dismantle for recycling. Additionally, contaminants like stains, finishes, and flame retardants complicate recycling. Despite these difficulties, recycling upholstery textiles is crucial to reducing landfill waste and conserving resources by reusing valuable materials. It also helps mitigate environmental pollution and carbon emissions associated with producing new textiles from virgin resources. The presented research aimed to establish the feasibility of incorporating textile fibers from waste artificial leather fibers from the upholstery furniture industry into the structure of high-density fiberboards. The bulk density of samples with wood fiber was 28.30 kg m⁻³, while it was 25.77 kg m⁻³ for textile fiber samples. The lowest modulus of elasticity (MOE) was 2430 N mm⁻², and it was 3123 N mm⁻² for the reference sample. The highest bending strength (MOR) was 42 N mm⁻², and the lowest was 27.2 N mm⁻². Screw withdrawal resistance decreased from 162 N mm⁻¹ in the reference sample to 92 N mm⁻¹ with 25% artificial leather fibers. The internal bond (IB) strength ranged from 1.70 N mm⁻² (reference) to 0.70 N mm⁻² (25% of artificial leather fibers content). Water absorption ranged from 81.8% (1% of artificial leather fibers) to 66% (25% of artificial leather fibers content). It has been concluded that it is possible to meet the European standard requirements with 10% addition of the artificial leather fiber content. This approach positively contributes to carbon capture and storage (CCS) policy and mitigates the problem of such waste being sent to landfills. The research shows that while selected mechanical and physical parameters of the panels decrease with a rising content of recycled textile fibers, it is possible to meet proper European standard requirements by adjusting technological parameters such as nominal density.

1. Introduction

The wood panel industry is characterized by an ever-increasing scarcity of resources and raw materials, resulting in the need for suitable alternatives [1]. An excellent way to solve this problem is to use traditionally used sources combined with newly discovered ones. Researchers often rely on recycled additives. This is an added advantage when we can use the waste in production. It is sometimes found that additives positively affect the tested material’s physical and mechanical properties. For example, Kilani et al. [2] found that banana and orange peels hold substantial potential for reinforcing concrete. Similar findings have been provided by [3] in the case of concrete reinforcing by recycled aramid textile nonwoven fabric. What is more, according to [4], the decisions regarding waste fibers application in fiber cement boards made based solely on economic and some of environmental indicators may lead to solutions with lower sustainability performance than what is required, optimal, or expected. Another study where recycled waste was used is described by Borysiewicz and Kowaluk [5], where the addition of high-density polyethylene (HDPE) can benefit medium density fiberboard (MDF) parameters. In this way, highly innovative solutions can be invented.

The textile industry ranks among the world’s top polluters [6]. It is responsible for approximately 8% of the global carbon emissions and 20% of the pollution in industrial water [7]. Almost 85% of textile waste is incinerated in landfills, leading to significant environmental pollution and substantial resource wastage, highlighting the need for alternative fabric production methods [8]. The same authors suggest the green production of textiles with a focus on additive manufacturing, 3- and 4-dimension printing, recycling textile waste, and synthetic and natural fibers. The entire lifecycle of textiles, from fiber production to disposal, poses numerous environmental challenges. Textile manufacturing relies heavily on chemicals and uses large amounts of water at various stages [9]. This results in a significant impact not only on the environment but also on the economy. Each year, USD 500 billion is lost due to clothing needing to be utilized more and the lack of recycling [10]. In the past twenty years, the textile industry has doubled its production, and the average global annual consumption of textiles has increased from 7 to 13 kg per person, reaching 100 million tons [11]. Artificial fiber woven products, such as those made from nylon and polyester, are known for their durability but present significant environmental challenges due to their slow decomposition rates [12]. Polyester, a widely used synthetic fiber, can decompose anywhere from 20 to 200 years [13]. Reusing and recycling textile waste are the most effective disposal methods, offering the lowest environmental impacts [14]. According to Juanga-Labayen et al. [15], the technologies for reusing and recycling textile waste, such as anaerobic digestion, fermentation, composting, fiber regeneration, and thermal recovery, are crucial for waste management. However, challenges persist in enhancing collection systems, automating sorting processes, and developing new recycling technologies. Implementing extended producer responsibility (EPR) policies and a circular economy model requires a collaborative approach and consensus among key stakeholders. European Union regulations [16] mandate that by January 2025, the textile waste recycling rate in Europe should be increased [17]. Increasing the textile recycling rate can reduce the negative environmental impact caused by landfill use and the production of new textiles. Recycled textile fibers can be a valuable secondary material. In their study, Zach et al. [18] obtained a raw material to produce medium, higher, and high-density acoustic insulators from used filters and edges from carpet manufacturing. An essential aspect of using textile waste in producing fiberboards is its impact on the bonding process of these boards. Some methods allow for assessing the chemical effects on gelation, such as the Acid Buffering Capacity (ABC) coefficient. In the development of technology for producing such boards, especially when there is significant variability in the properties of textile waste, ABC can be utilized in the manufacturing process [19]. In the research of Suchorab et al. [20] the upholstery furniture fabric waste fibers have been applied in various ratios (0, 5, 10, and 20% w/w) to high-density fiberboards. The results indicate that an increased content of textile fibers in high-density fiberboards (HDFs) has the most pronounced negative impact on mechanical properties, particularly internal bond strength and screw withdrawal resistance. Although the modulus of elasticity and modulus of rupture decreased, they still met the requirements of European standards. No significant effect was observed on HDF thickness swelling or water absorption with the higher textile fiber content. Therefore, depending on the intended application of the HDF, incorporating recovered upholstery textile fibers as an additive to wood fibers in HDF production is feasible. This approach also contributes to extending carbon storage, as carbon is sequestered in textile fibers.

This research contributes significantly to the field by addressing environmental challenges posed by artificial leather waste and its recycling potential in the wood panel industry. Unlike prior studies that focused on conventional additives like banana peels, recycled textiles, or polyethylene to enhance material properties, this work explores the innovative integration of artificial leather fibers into HDF. This study highlights a feasible solution to reduce the environmental footprint of the textile industry, known for its significant waste and pollution issues, by repurposing artificial leather waste. Compared to earlier works, this study demonstrates the potential of artificial leather fibers for improving carbon storage and offering sustainable alternatives in HDF manufacturing. It addresses mechanical limitations by adjusting production parameters, paving the way for broader applications of recycled textile-based materials.

This research aimed to assess the feasibility of incorporating textile fibers from artificial leather waste originating from the upholstered furniture industry into the structure of high-density fiberboards and to verify if the tested alternative raw material of recycled origin would have a positive impact on the physical and mechanical properties of the produced panels. This is also an attempt to extend the period of carbon fixation in the textile and produced HDF boards, positively contributing to CCS policy instead of energetic recycling and carbon emission.

2. Materials and Methods

2.1. Materials

The following raw materials were used to create the tested fiberboards:

• Virgin pine (Pinus sylvestris L.) debarked round wood from Polish State Forests (Podlaskie voivodeship, Orla, Poland) was used to produce the fibers; the fibers have been made in industrial conditions on Metso (Valmet Oyj, Espoo, Finland) defibrator EVO 56, moisture content about 4%;
• Commercial urea-formaldehyde (UF) resin (Silekol Sp. z o.o., Kędzierzyn-Koźle, Poland) of about 66.5% of dry content with formaldehyde to urea (F:U) molar ratio of 0.89, pH of 9.6, and viscosity of 470 mPa s was used;
• Ammonium nitrate hardener, 3.0%, calculated regarding the dry resin content;
• Distilled water;
• The textile fibers mixture containing 75% polyester and 25% cotton (w/w) coming from the mechanical recycling of eco-leather upholstery fabric particles about 30 mm × 30 mm (thickness 1.2 mm, grammage 400 g m⁻²) covered by polyurethane surface layer (foamy brown zone in Figure 1), that has been removed during milling. The fibers produced this way were free from the polyurethane surface layer and were separated from each other, giving single fibers of about 30 mm in length and about 0.1 mm in thickness (Figure 2).

2.2. Production of the Panels

The material tested was high-density dry-formed panels with dimensions 320 mm × 320 mm, 3 mm thick, and a target density of 900 kg m⁻³. Boards were made with artificial leather fibers at 1%, 5%, 10%, and 25% w/w, or board weight. The synthetic fibers were only added to the core layer, constituting 68% w/w of the panel weight. The reason for adding the artificial fibers to the core zone only was to avoid the presence of these fibers on the panel surface, which could affect further panel finishing. Different natures of artificial and wood fibers can influence the lacquer or paint spread and/or uptake, leading to worsening of the finished surface roughness. The face layers, 2 × 16% w/w, have been created without artificial fiber addition. The reference variants were produced without incorporating artificial leather fibers. The adhesive compound had a curing time of approximately 88 seconds at 100 °C. The resination was set at 12% of dry resin calculated on dry fibers. The wood fibers were placed in a drum mixer. Artificial leather fibers were gradually added during mixing, and the glue mixture was sprayed with an air gun. The hot pressing parameters (hydraulic press AKE, Mariannelund, Sweden) were a temperature of 200 °C, a maximum unit pressure of 2.5 MPa, and a pressing factor of 20 s mm⁻¹ of the nominal board thickness. No hydrophobic agent was used to produce the boards. Before testing, all samples were conditioned in atmospheric air pressure at 20 °C and 65% relative humidity to achieve constant weight.

2.3. Methods

The bulk density of the fibers used in the research was tested [21]. At least three individual measurements were carried out for each listed type. The bending elasticity (MOE) and bending strength (MOR) tests were carried out using a computer-controlled universal testing machine in accordance with the EN 310 (1993) [22] standard. Screw withdrawal resistance (SWR) was tested according to EN 320 [23]. At least eight samples from each board variant were used for testing. Tensile strength perpendicular to the plane of the board (IB) was tested according to EN 319 [24]. Water absorption (WA) and thickness swelling (TS) tests were conducted following EN 317 [25] guidelines after 2 h and 24 h of water immersion, using at least eight samples per variant. Surface water absorption (SWA) was measured according to EN 382-2 [26] after 2 h on two samples from each variant. The density profile of the samples was analyzed using a DA-X measuring instrument (GreCon in Alfeld, Germany). The measurement was carried out with a speed of 0.05 mm/s across the panel thickness with a sampling step of 0.02 mm. Samples were cut into 50 mm × 50 mm nominal dimensions. Three samples of each composite type were tested to determine the density profile. Then, the representative profile per every tested variant was selected for further evaluation and presentation in the plot. The density was tested in accordance with EN 323 [27] standards.

Analysis of variance (ANOVA) and t-test calculations were conducted to identify significant differences (α = 0.05) between factors and levels when applicable, using the IBM SPSS statistic base (IBM, SPSS20, Armonk, NY, USA). The homogenous groups are indicated in

Table 1. The statistical assessment results of mean values.

Test Type	Alternative Raw Material Fibers Share [%]
	0	1	5	10	25
MOE	a 1	a, b	b	b	b
MOR	a	a	b	b	b
IB	a	b	c	d	e
SWR	a	a	a	b	c
TS 2 h	a	a, b	b	a, b	a
TS 24 h	a	a, b	b	a, b	b, c
WA 2 h	a	b	a	a	a
WA 24 h	a	b	a	c	c
SWA	a	a	a	a	a

¹ a, b … homogeneous group.

. The results shown in the graphs represent mean values and standard deviation as error bars.

3. Results and Discussion

3.1. Bulk Density

The bulk density of the samples made with the addition of wood fiber was 28.30 kg m⁻³. This was marginally higher than the bulk density of the samples with textile fibers as an additive (25.77 kg m⁻³). Sala et al. [28] validated that the density distribution in HDF surface layers changes after adding raw materials with different bulk densities. The low bulk density of hemp shavings (90 kg m⁻³) in the Savov et al. [29] study positively affected physical and mechanical strengths.

3.2. Modulus of Elasticity in Bending and of Bending Strength

The results of the measurement of the modulus of elasticity in bending are visualized in Figure 3. The lowest result for the sample was 25% of the content of artificial leather fibers and was 2430 N mm⁻². As can be seen from the chart, no sample with artificial leather fibers for MOE has higher values than the reference sample, whose value is 3123 N mm⁻². The reference sample and the sample with a 1% addition of synthetic leather met the requirements of the European standards according to EN 622-5 [30]. However, when the trend line is analyzed, it can be concluded that the panels with 10% artificial leather fiber content still meet the requirements of the mentioned standard. In a study on the production of eco-friendly, formaldehyde-free high-density fiberboard (HDF) panels from hardwood fibers bonded with urea-formaldehyde (UF) resin and ammonium lignosulfonate (ALS), Antov et al. [31] obtained MOE values ranging from 3197 N mm⁻² to 4114 N mm⁻².

The following graph (Figure 4) shows the test samples’ rupture modulus. As with the modulus of elasticity, the MOR decreases as the addition of artificial leather fibers increases. The reference sample had the highest bending strength, 42 N mm⁻². The lowest result obtained from the sample with the highest synthetic leather content was 27.2 N mm⁻². As far as MOR is concerned, all samples obtained results in conformance with European standards according to EN 622-5 [30]. Very similar results were obtained in the research by Nicewicz and Monder [32], where the highest modulus of rupture value was 41 N mm⁻² and the lowest was 29 N mm⁻². In their study, Barbu et al. [33] also observed decreased MOR and MOE values by adding waste skin. The same relationship was observed by Pásztory et al. [34] in their study in which MOE and MOR values decreased linearly with poplar bark content. Research confirms that the mechanical strength of artificial leather can depend on the material from which it is made or the conditions under which it is produced. Therefore, depending on the type of synthetic leather, it may or may not affect the mechanical properties of wood-based materials [35]. Nemli et al. [36] found that 10% textile dust content in particleboard resulted in an MOR and MOE of 13.0 and 1814 N mm⁻², respectively, while 20% textile dust content resulted in MOR and MOE values of 12.9 and 1755 N mm⁻², respectively.

Increasing additive content leads to a notable drop in mechanical characteristics. The size of the wood and synthetic leather fibers also influences strength properties [37] and replacing traditional wood raw materials with different sources can impact wood composites’ mechanical resistance and water interaction [28]. One of the reasons for the mechanical parameters’ drop could be fiber composition and quality. Artificial leather fibers are typically made from synthetic materials like polyurethane or polyvinyl chloride [38], which have different physical and chemical properties compared to natural wood fibers. These synthetic fibers may not bond as effectively with the resin used in HDF production, resulting in weaker internal bonding and reduced mechanical strength [39]. Another reason can be that the properties of artificial leather fibers—such as flexibility, elongation, and density—may not match those of wood fibers, leading to inconsistencies in the panel structure [40]. This mismatch can reduce the overall strength and durability of the HDF. There are also several processing challenges. During the panels’ manufacturing process, artificial leather fibers may behave differently under the heat and pressure applied to form HDF. If they do not compact or bond properly during the process, it can lead to weaker spots in the final product [41]. Artificial leather scraps, as well as the leather itself, exhibit insulating properties. Therefore, one should not only view the results negatively but also look for potential applications of the produced composites as insulating materials, such as for sound insulation [42]; for this purpose, bamboo fibers and leather fibers [43] and denim shoddy and waste jute fibers were used [44].

3.3. Screw Withdrawal Resistance and Internal Bonding

The results of the measurement of internal bonding and screw withdrawal resistance are presented in Figure 5. For screw withdrawal resistance, the average values show a decreasing trend as the leather particle content increases. Upon analyzing the values, it is evident that the reference variant samples exhibited the highest resistance (162 N mm⁻¹). The sample with 25% artificial leather fiber content revealed a resistance of 92 N mm⁻¹, representing the lowest result in the test. A similar study was conducted by Bartoszuk and Kowaluk [45], describing the effect of adding natural leather fibers on the production of HDF. Despite the comparable material used for the study, their results were surprisingly different. For example, screw withdrawal resistance showed an increasing trend with the addition of fibers. In their test, a sample with 10% added natural leather fibers achieved a result of 156 N mm⁻¹. In our test, a sample with the same fiber content achieved a result of 115 N mm⁻¹. Similar studies on HDF panels with the addition of upholstery fabrics have shown that the internal bond (IB) decreases as the amount of fibers increases. It was estimated that the maximum content of upholstery fabrics can be approximately 15.6% to meet the standard requirements for IB [20].

Figure 6 shows the results of tensile strength perpendicular to the plane of the board. The graph shows that all samples have a lower tensile strength perpendicular to the surface than the reference sample. The highest internal bond strength is 1.70 N mm⁻², as shown in the reference sample. The sample with 25% artificial leather fibers as an additive revealed a tensile strength of 0.70 N mm⁻², representing the lowest internal bond strength test result. The example of the sample with 25% artificial leather fibers content after the IB test is shown in Figure 7. It can be seen that there are single textile fibers not connected to the board structure, that can explain the lowering of the IB. According to the requirements of EN 622-5 [30], for MDF-type boards with a thickness of 2.5 mm to 4 mm, the minimum IB value is 0.65 N mm⁻². All tested samples achieved values in accordance with the European standard. Wood damage was observed in most of the samples examined. In the case of Kryńska and Kowaluk’s [46] study about using soy starch as a binder in HDF technology, internal bond values decreased as the proportion of soy starch increased. This is similar to the relationship obtained in this study, but in the case of artificial leather fibers, all samples obtained results per European norms. Antov et al. [47] described that replacing UF resin with PF resin resulted in an increase in IB strength values from 0.75 to 1.16 N mm⁻², i.e., an overall improvement of that property by 1.56 times.

When evaluating Figure 3, Figure 4, Figure 5 and Figure 6, it can be found that the achieved results of MOE are less dependent on the artificial leather fibers content since the R² coefficient is 0.70, and the value of this coefficient increases for MOR (0.80), SWR (0.90) and the highest, 0.93 is for IB. The reason for the lowest R² for MOE and MOR could be the procedure of the test, where during bending, the largest contribution comes from the face layers of the sample, and the bending properties are less influenced by the core layers, where the artificial leather fibers are located. During IB measurement, the sample breaks in the zone with the lowest density, which correlates with the location of the artificial leather fibers. Thus, the change in artificial leather fiber content is more related to the IB values.

3.4. Thickness Swelling, Water Absorption, and Surface Water Absorption

The graph shown in Figure 8 displays the influence of artificial leather fibers on the thickness swelling. Samples soaked in water for 2 h and 24 h exhibit a gradual reduction in swelling with increased leather addition. The swelling decreases with higher artificial leather fiber content from the sample with 1% leather after two h 27.7% and after 24 h 31.7% to sample with 25% leather after two h 24% and 24 h 25.8%. The reference sample achieved 26.4% after two h soaking and 29.7% after 24 h. In accordance with EN 622-5 [30], MDF boards with a thickness ranging from 2.5 mm to 4 mm should have a maximum TS value of 35%. Research confirms that depending on the type of fiber, a given material may react differently to water [20]. Polypropylene fibers, for example, are lightweight, dry (because of low water retention), and have high strength properties [48]. On the other hand, natural materials are often more prone to water absorption [49]. The hygroscopic nature of natural fibers has a negative impact on the mechanical properties of composites. Hence, it is vital to understand and control water absorption in these materials [50]. All tested samples obtained results that were in accordance with European standards. In a study by Antov et al. [51] aimed at reducing formaldehyde emissions, the addition of ammonium lignosulphonate (ALS) as an eco-friendly additive to urea-formaldehyde resin for manufacturing HDF panels had a positive effect on TS results. Values ranged from 33.4% to 23.3%, with only the reference sample failing to achieve a result compliant with EU regulations.

The results of the measurement of water absorption are displayed in Figure 9. Similar to TS, absorption decreases with higher artificial leather fiber content. The sample obtained the highest result with 1% synthetic leather fibers. After two hours, this sample achieved a score of 81.8%; after 24 hours, it gained 86.3%. The lowest result obtained in the test was 66% after two hours and 69.2% after 24 h, soaking the sample with 25% leather. The reference sample scored 70.9% after two hours and 77.3% after 24 h. A downward trend in thickness swelling and water absorption was obtained by Dasiewicz and Kowaluk [52] in their study on the manufacture of HDF using rice starch as a binder. Similar to our study, their research found that the samples with the highest filler content had the lowest value (not including reference samples).

Figure 10 shows the results of surface water absorption for boards containing varying amounts of leather particles. As can be seen from the graph, the results for all samples tested are almost identical. Based on the results, it can be concluded that adding artificial leather fibers has no effect on water absorption on the surface. Averaging gives a result of 3142 g m⁻². This is also the result that the reference sample achieved. In their study, Gumowska and Kowaluk [53] observed a decrease in SWA as the biopolymer binder content increased. In Rosa and Kowaluk’s [54] study of MDF production using a plant binder, TS, WA, and SWA values decreased with increasing resination.

3.5. Density Profile

The results of the density profile measurement are displayed in Figure 11. The graph shows the results of one sample from each variant, as the analysis revealed that the values obtained across the series of tests were similar. All samples have a slightly higher density in the face layers than in the core layer. The most varied results were obtained for samples with 5% of artificial leather fibers as an additive, as they have a slightly higher value in the middle layer. The sample obtained the lowest value in the inner layer with 10% of the artificial leather fibers as an additive (880 kg m⁻³). In comparison, the sample received the highest value in this layer, with 5% of the artificial leather fibers as an additive (998 kg m⁻³). As for the outer layers, the highest density was obtained by a sample with 1% of the artificial leather fibers as an additive (1070 kg m⁻³). In a study by Badin et al. [55], adding bark and increasing the percentage of hardwood fibers while reducing the rate of softwood in the production of HDF had a negative effect on density. The study by Henke et al. [56] shows that density is not a determining factor for the surface roughness of HDF boards. The appearance of the attached density profile is influenced by the size of the fibers used, both wood and artificial leather fibers [57]. The pressing temperature also affects the final appearance of the density profile [58].

4. Conclusions

Based on the conducted research and the analysis of the results obtained, the following conclusions and remarks can be made:

• As the addition of artificial leather fibers increases, the bending strength and modulus of elasticity decrease.
• Of all the tested samples, the variant with 5% artificial leather fibers as an additive differed most from the others.
• The average values for screw withdrawal resistance indicate a downward trend as the content of artificial leather fibers increases.
• In the case of thickness swelling and water absorption, a profitable decreasing trend can be observed with an increase in synthetic leather fibers. Even without adding any hydrophobic agent, the panels meet the requirements of the European standard in the case of thickness swelling.
• The bulk density of textile fibers is slightly lower than that of wood fibers.

The study results show that adding synthetic leather fibers to produce HDF boards does not show properties that would improve the boards’ mechanical and physical quality. Despite this, most tested samples achieved results aligned with European standards. It can be concluded that the maximum content of the artificial leather fibers content in HDF boards produced, as described in the paper, analyzing the trend lines of the observed changes, should be at most 10% w/w due to the modulus of elasticity limitation.

Future studies can improve the mechanical properties of high-density fiberboards with recycled artificial leather fibers through various methods. Chemical treatments like alkali, silane, or adhesion promoters enhance fiber–matrix bonding. Surface treatments (plasma, corona, or resin coating) improve compatibility. Optimizing adhesive types, ratios, and pressing conditions strengthens bonding and density. Adding reinforcements like nanocellulose or glass fibers enhances strength. Hybrid fiber blends balance performance and sustainability. Nanomaterials, fillers, and plasticizers improve stiffness and flexibility. Thermal pre-treatments or pyrolysis remove contaminants. These approaches address property limitations while maintaining environmental benefits, aligning with circular economy goals for sustainable materials.