Towards circular economy by valorization of waste upholstery textile fibers in fibrous wood-based composites production

The aim of the research was to utilize textile waste by adding upholstery fabric fibers with different content (0, 5, 10, and 20% by weight referred to dry wood fibers) to high density fiberboards (HDF) and analyze selected physical and mechanical properties of the obtained boards. Tests of mechanical (modulus of elasticity in bending and bending strength, surface soundness, internal bond, screw withdrawal resistance) and physical (density profile, swelling in thickness after immersion in water, water absorption) properties were performed. The results show that the increasing content of textile fibers in HDF panels has the strongest negative influence on mechanical properties, especially internal bond, and screw withdrawal resistance. Modulus of elasticity and modulus of rupture also decreased, but still fulfilled the requirements of European standards. No significant influence of rising content of textile fibers on HDF thickness swelling and water absorption has been found. It can be concluded that, depending on further application of HDF, it is possible to utilize the recovered upholstery textile fibers as a reasonable addition to wood fibers when producing HDF panels. It is also a step toward carbon storage extension, which is fixed in textile fibers.


Introduction
Wood-based panels are materials that are a good alternative to solid wood. Although they contain raw material wood in the form of veneers, particles, or fibers, they have better properties such as dimensional stability or uniformity and improved strength parameters, compared to solid wood. In addition, wood-based composites are more cost-effective, for example when used in furniture manufacturing. Panels made of wood fibers (and synthetic resins) include high-density fiberboard or HDF for short. Physical and mechanical properties in fiberboard production are influenced by factors such as binder content, bonding agents, type of additive, board density, duration and temperature of heat treatment (Nazerian et al. 2015). In addition to natural raw materials, like wood and cotton, artificial materials are also used in the furniture industry. An example is upholstery fabrics. Unfortunately, huge numbers of residues are generated during production, classified as so-called textile waste. They are primarily sent for recycling, while the unused ones are sent to landfills (Kowalska and Kijeńska 2009). Such landfills, apart from having a negative impact on the environment, are also dangerous-they can cause fires, as it was in Kamieniec (Poland), where as many as 130 firemen fought to extinguish a fire caused by the ignition of a large pile of textiles (MŁ/ PAP 2021). The problem is especially significantly important, due to the fact that in Poland, which is one of the biggest producers of furniture in Europe, about 30.7% of total furniture production is upholstery furniture (Orlikowska 2020). When it comes to state of the art, in one of the studies (Nemli et al. 2019), a way to use the residue from the textile production process in the form of textile dust was to use it in the middle layer of three-layer particleboard. It was considered that its 20% by weight could be accepted as an alternative to wood materials in particleboard production, but it had no significant impact on mechanical properties: internal bond (IB), modulus of rupture (MOR), and modulus of elasticity in bending (MOE), while it had a negative influence on them at 30% by weight. In order to observe the changes in physical and mechanical properties by modifying with another fibrous material in the production of the boards, tests were performed in which these additives were included in different amounts. An example is the preparation of hybrid composite panels from wood debris and coconut fiber (Yao et al. 2012). This treatment improved sound absorption and thermal insulation (due to the coconut fiber additive). The results showed that it could also increase the modulus of rupture values and maintain the internal bond, with a sufficiently large amount of additive. Another study (Borysiuk et al. 2020) used different content (0, 10, 20, and 30%) of sunflower seed husk addition to the middle layer in 3-layer particle boards. Their average density was 650 kg m −3 while the addition did not significantly affect it as well as the course of the density profile or their thermal conductivity. Strength properties, on the other hand, decreased with an increase in the proportion of scales, but the swelling in thickness increased and the water absorption was worse. Another way to modify, this time composite layered panels, was to add fiber mats of different weights (Radziejewicz et al. 2018). These increased the density and bending strength of the boards. In an experiment where partial replacement of wood fiber in MDF boards with mineral filler (10, 20, and 30%) was performed, a decrease in mechanical properties above 10% share was shown (Ozyhar et al. 2020). The reduction of the amount of waste from HDF is also needed. One way may be to add recycled HDF, in the form of recycled fibers, to the wood fibers during HDF production. This method was presented in a study by Sala et al. (2020), which shows that producing HDF with 10% recovered high-density fiberboards is possible in industrial-scale production. In order to utilize the waste, in the presented study, an attempt was made to use fibers from upholstery fabrics for the production of HDF boards, and checking the influence of the addition of various content of textile fibers, on selected physical and mechanical properties of the products. The present work intends to prove the potential of upcycling the waste upholstery textile fibers by elaborating on MDF-type (Medium Density Fiberboard) boards as a high value-added alternative to the current management of this kind of textile waste; this represents cascading and added-value of this fibrous material in a circular economy, as it generates new products.

Materials and methods
The raw materials listed below were used to make the test material: virgin pine (Pinus sylvestris L.) debarked round wood from Polish State Forests (Podlaskie voivodeship, Poland) was used to produce the fiberboards. Virgin fibers were produced on industrial Metso Defibrator EVO56 (Metso, Helsinki, Finland) with a 2.5 m diameter disc with ten knives. The moisture content of the fibers was 4%. The commercial melamine-urea-formaldehyde (MUF) resin with a melamine content of 9%, molar ratio of 0.89, and solid content of 66.5% has been used. The resination was set at 12% of dry resin calculated on dry fibers with 3.0% of ammonium nitrate hardener, both calculated regarding dry resin content. The curing time of the adhesive mass composed as mentioned above at 100 °C was about 88 s. No further hydrophobic agents have been added. The length of upholstery fabric, used in research, obtained by manual-mechanical shredding of the fabrics, was about 10 ± 2 mm. The technical specification of the textile is as follows: braided type, 100% polyethylene composition, 260 g m ± −2 5% by weight, abrasion resistance (Martindale test) 49000, mud and pilling resistance 4. The contact angle on the wood fibers and textile fibers was measured with the use of distilled water using the PHOENIX 300 Goniometer (Surface Electro Optics Co., Ltd., Suwon, Korea) by 5 repetitions on every type of raw material.
The test material was laboratory-made MDF-type boards with an aimed density of 800 kg m −3 , 320 × 320 mm −3 (width and length), and a nominal thickness of 3 mm, 3 replicates per every panel type. The following variants of the panels were produced: reference panels and panels with various textile fibers content (5, 10 and 20% w/w) added at the production stage. Reference boards were made without the addition of upholstery fabric fibers. Upholstery textile fibers were added to the inner zone of the boards, accounting for 68% of the board's weight (the outer layers of the board with a weight share of 32% were left without the upholstery textile fibers). The panel pressing parameters were temperature 200 °C, pressing factor 20 s mm −1 of nominal panel thickness, and unit pressing pressure 2.5 MPa. After production, the panels were conditioned at 20 °C with a 65% relative moisture content to constant weight (EN 323 1993).

Physical, chemical, and mechanical properties
Physical and mechanical properties were determined in accordance with European Standards: density (EN 323 1993), modulus of rupture (MOR) and modulus of elasticity (MOE) (EN 310 1993), internal bond (IB) (EN 319 1993), screw withdrawal resistance (SWR) (EN 320 2011), water absorption (WA) and thickness swelling (TS) after 2 and 24 h of immersion (EN 317 1993). The tests for the mechanical properties were conducted using an INSTRON 3369 universal testing machine (Northwood, USA). The constant load speed of 10 mm/min was applied when testing screw withdrawal resistance (SWR). For each mechanical and physical parameter test, as many as 12 samples of each panel type were used. The infrared spectra of composites and textile have been taken on JASCO FT/IR-4700 Fourier Transform Infrared Spectrometer (JASCO International Co., Ltd., Tokyo, Japan) with a wavenumber resolution of 0.964 cm −1 . To determine the density profile (DP), all samples were cut into 50 × 50 mm test specimens and analyzed in a Grecon DA-X measuring instrument (Alfeld, Germany) with direct scanning X-ray densitometry across the panel thickness with an incremental step of 0.02 mm. As many as 3 samples of each tested variant were measured, but for further evaluation, one representative density profile per panel type was selected.

Statistical analysis
Analysis of variance (ANOVA) and t-tests calculations were used to test (α = 0.05) for significant differences between factors and levels using the IBM SPSS statistic base (IBM, SPSS 20, Armonk, NY, USA). A comparison of the means was performed when the ANOVA indicated a significant difference by applying the Duncan test. Where applicable, the mean values of the investigated features and the standard deviation indicated as error bars, were presented on the plots as error bars.

Results and discussion
In Figs. 1 and 2, respectively, the dependence between modulus of rupture and modulus of elasticity and textile fibers content is presented. It can be seen that with increasing textile fibers content, the MOR and MOE decrease. The lowest MOE value is for the highest textile fibers content -2969 N mm −2 for 20% content. The highest MOE (3403 N mm −2 ) is for the reference sample (0% content). For the reference sample, the MOR value is 47.2 N mm −2 and for 20% textile fibers content it is 34.7 N mm −2 . However, even with  (EN 622-5 2010). The only statistically significant differences were found between 0 and 20%, for both, MOR and MOE. The same dependency was shown by Nemli et al. (2019), where for 10% textile dust content in the particleboard, the MOR and MOE were 13.0 and 1814 N mm −2 , respectively and for 20% textile dust content, the MOR and MOE values were 12.9 and 1755 N mm −2 , respectively. There is a noticeable decrease in the value of mechanical properties with increasing additive content.
The results of the surface soundness (SS) are presented in Fig. 3a. These results show a declining trend with increasing textile fiber content. Between the lowest content of textile fibers (reference panel 0%) and the highest (20%), the difference in surface soundness values is 0.41 N mm −2 which is 53% lower than the 20% textile fibers content. It is the biggest difference from mechanical properties in the research performed here. There were no statistically significant differences in average SS between 0 and 5%, as well as between 5 and 10%. A similar decreasing tendency was found in the case of internal bond shown in Fig. 3b. As the textile fiber content increases, the internal bond value of HDF decreases. When comparing the strength values at 5 and 10% textile fiber content, a decrease of 0.17 N mm 2 was found. Up to 5% content, the value only decreased by 0.07 N mm −2 so it is not a significant change. The most significant difference is seen when the content of textile fibers is 20%, namely a decrease in strength value of 0.53 N mm −2 , which is about 50% lower than for the reference panel. The only panels not meeting the standard requirements (EN 622-5 2010) were those with 20% textile fibers content. However, it can be estimated that the maximum textile fibers content in HDF panel, due to the IB criterion, is about 15.6%. All the achieved differences between average values of IB were statistically significant. In a study, where the addition of recovered HDF fibers (HDF-r) has been tested during HDF production (Sala et al. 2020), for the 10% mass share of HDF-r, the IB value was 0.61 N mm −2 and SS value was 1.02 N mm −2 . Compared to IB and SS values in the reference panel, IB decreased by nearly 50% because for 0% HDF-r mass share, the IB value was 1.15 N mm -2 . The SS value also decreased. For 0% textile fibers mass share, it was 1.09 N mm −2 .
In the resistance to withdrawal of a screw there is also a declining trend which is seen in the results presented in Fig. 4a. The decline between the value for a board with 5 and 10% textile fibers content is 32 N mm, which is the same as for content between 10 and 20%. As the textile fibers content increases, the screw withdrawal resistance decreases. For 20% textile fiber content, it is 85 N mm and for 0% it is 152 N mm. The only statistically insignificant difference between mean SWR values was found between 0 and 5% panel type.
The density profile study visualized in Fig. 4b shows that with higher textile fibers content, the density in the outer layers of HDF boards decreases, while in the inner layers it increases. The density value in the middle layer increased to about 760 kg m −3 at 20% textile fibers content, while for 0% it is about 720 kg m −3 . In the outer layer, when the textile fibers content increases to 5%, the density decreases from about 940 kg m −3 to about 915 kg m −3 . As the proportion of textile fibers increases, the difference in density between the outer and inner layers of the board becomes smaller.
After 2 h of soaking, the intensity of the thickness swelling results shown in Fig. 5a was more visible for increasing textile fibers content than for 24 h of soaking where textile fibers content has no significant influence. After two hours, for 0% textile fibers content, thickness swelling value was 31.3 and for 20% content it was 32.1%, so it increased by 0.8% After 24 h of soaking, for 0% content, thickness Fig. 3 Influence of the upholstery textile fibers' content on the surface soundness a and internal bond b of HDF panels swelling value was 34.9 and for 20% content -34.3% (the difference was only 0.6%). It can be seen that the maximum thickness swelling of the tested panels, excluding the 10% panel, is near the limit given by the European standard (35%) (EN 622-5 2010). It can also be concluded that with increasing textile fibers content in the panels, the intensity of thickness swelling rises. The results show that the water absorption after 2 and 24 h of soaking, depending on the textile fibers content presented in Fig. 5b, has the same relationship. It can be concluded that there is no significant influence of the textile fibers content on water absorption of tested panels. There were no statistically significant differences between average values of TS and WA for both, 2 and 24 h soaking time. However, the rising textile fibers content in the tested panels inhibits the intensity of water uptake by MDF-type composites. In the research by Borysiuk et al. (2020), where three-layer particleboards with different content of sunflower hulls added to the middle layer were produced, the thickness swelling after 2 and 24 h increased by 12.4 and 30.6%, respectively. Water absorption value was only significant after 24 hours of soaking in water, and a decrease was noticed with increasing additive content. It should be noted here that none of the hydrophobic agents has been added to the panels with different textile fibers content during production in this research.
The plot of the FTIR spectra of HDF with 20% of textile fibers and textile is shown in Fig. 6. In case of HDF panel, there are clearly visible vibrations of amine groups (3332.39 and 1022.09 cm −1 ) due to the presence of melamine-urea-formaldehyde resin as a binder. For textile, the

Conclusion
The present work aimed to prove the potential of upcycling the waste upholstery textile fibers by incorporation of these into HDF boards. The results show that the increasing content of textile fibers in HDF panels has no strong and significant influence on the physical properties including density profile, thickness swelling after immersion in water, and water absorption, even if neither hydrophobic agent nor water-resistant resin was applied. The highest impact is on mechanical properties, in particular internal bonding and screw withdrawal resistance. Even the lowest values of modulus of elasticity and modulus of rupture, achieved here with 20% by weight of upholstery textile fibers, meet the requirements of European standards. It can be concluded that with a not excessively high fiber content from textile waste, taking into account the subsequent use of the HDF produced, it is possible to utilize the recovered upholstery textile fibers as an addition to wood fibers when producing the MDF-type panels. It can help to reduce the amount of textile waste and is a promising result regarding circular economy rules, waste upcycling, and carbon capture and storage (CCS) policy.