Structural Application of Eco-Friendly Composites from Recycled Wood Fibres Bonded with Magnesium Lignosulfonate

: The pulp and paper industry generates substantial amounts of solid waste and wastewater, which contain waste ﬁbres. The potential of using these recycled wood ﬁbres for producing eco-friendly composites that were bonded with a formaldehyde-free adhesive (magnesium lignosulfonate) and their use in structural applications was evaluated in this study. Fibreboards were produced in the laboratory with a density of 720 kg · m − 3 and 15% magnesium lignosulfonate gluing content, based on the dry ﬁbres. The mechanical properties (bending strength, modulus of elasticity and internal bond strength), physical properties (thickness swelling and water absorption) and formaldehyde content were determined and compared with the European Standards requirements for wood-based panels. In general, the laboratory-produced panels demonstrated acceptable mechanical properties, such as bending strength (18.5 N · mm − 2 ) and modulus of elasticity (2225 N · mm − 2 ), which were higher than the minimum requirements for type P2 particleboards and equal to the requirements for MDF panels. The moisture properties, i.e., thickness swelling (24 h) and water absorption (24 h) signiﬁcantly deteriorated. The free formaldehyde content of the laboratory-produced composites (1.1 mg / 100 g) reached the super E0 grade ( ≤ 1.5 mg / 100 g), which allowed for their classiﬁcation as eco-friendly, low-emission wood-based composites. The L-type corner joints, made from the developed composites, demonstrated signiﬁcantly lower bending capacity (from 2.5 to 6.5 times) compared to the same joints made from MDF panels. Nevertheless, the new eco-friendly composites can be e ﬃ ciently utilised as a structural material in non-load-bearing applications.


Introduction
The depletion of fossil fuels and the increasing concern about the negative effects of global warming on the environment and human health are the main driving forces for the development of a bio-economy [1]. In this respect, the resource efficiency optimisation and the valorisation of lignocellulosic biomass in different high-value products from renewable sources as an alternative to their petroleum-based equivalents are one of the key objectives for implementing the circular economy principles in the wood-based panel industry [2][3][4]. Significant amounts of non-hazardous solid waste and sludge are generated annually at pulp and paper facilities worldwide, which require further such as furniture, interior structures and claddings. For this purpose, the bending strength capacity of L-type corner joints, constructed from the developed composites using four different connection techniques for assembling, i.e., dowels, Confirmat, Minifix and screws, was determined.

Materials and Methods
The residual fibre mass, composed of the species Norway spruce (Picea abies Karst.) and Scots pine (Pinus silvestris L.) and oven-dried to 12% moisture content, was provided by the pulp and paper factory Mondi Stambolyiski EAD. The bulk density of the factory waste fibres was 44.86 kg·m −3 . The waste fibres had lengths from 500 to 1000 µm and 7% reduced lignin content.
Magnesium lignosulfonate at 15% gluing content (based on the dry weight of the fibres) was used as a binder. The lignosulfonate additive had the following characteristics: total solids content-51.2%, magnesium content-6%, reduced sugars-7% and sulfate content-2%. European beech (Fagus sylvatica L.) veneers, made using the centric peeling process with an average thickness of 1.13 mm and a moisture content after drying and conditioning of approximately 7%, were supplied by the factory Welde Bulgaria AD, and used for veneering the laboratory-produced fibreboards.
Under laboratory conditions, the fibreboards were produced with a 16 mm thickness and a 720 kg·m −3 target density. Residual fibres were mixed with the magnesium lignosulfonate in a high-speed laboratory adhesive mixer at 850 min −1 . The hot pressing process was carried out using a single opening hydraulic press (PMC ST 100, Italy). The press temperature was 210 • C. The following four-stage pressing regime was used: in the first stage, the pressure applied was 4.5 MPa for 1 min; in the second stage, the pressure was steadily decreased to 2.23 MPa for 3 min; in the third stage, the pressure was decreased to 0.74 MPa for 10 min. The fourth pressing stage was performed at a pressure of 1.78 MPa for 2 min. The fabricated composites, after pressing, were conditioned for 10 days at 20 ± 2 • C and 60-70% relative humidity.
The veneer sheets with dimensions of 400 × 400 mm were cut. After that, the fibreboards were veneered using magnesium lignosulfonate at the content of 80 g·m −2 as an adhesive. A hand roller was used to apply the binder to the veneers in order to achieve a uniform adhesive layer. The veneering press factor used was 1 min·mm −1 of board thickness; the pressure applied was 0.6 MPa at 200 • C.
The physical and mechanical properties of the fabricated panels ( Figure 1) were tested according to the European Standards EN 310, EN 317, EN 322 and EN 323 [73][74][75][76]. A precision laboratory balance Kern (Kern & Sohn GmbH, Balingen, Germany) with an accuracy of 0.01 g was used to determine the mass of the test specimens. The dimensions of the test pieces were measured using digital callipers with an accuracy of 0.01 mm. The physical properties (water absorption and thickness swelling) were measured after 24 h of immersion in water. The thickness swelling was assessed using the differences between the initial and final panel thicknesses, and the water absorption was determined using the difference in weight. The mechanical properties of the panels were determined using a universal testing machine Zwick/Roell Z010 (Zwick/Roell GmbH, Ulm, Germany).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 12 such as furniture, interior structures and claddings. For this purpose, the bending strength capacity of L-type corner joints, constructed from the developed composites using four different connection techniques for assembling, i.e., dowels, Confirmat, Minifix and screws, was determined.

Materials and Methods
The residual fibre mass, composed of the species Norway spruce (Picea abies Karst.) and Scots pine (Pinus silvestris L.) and oven-dried to 12% moisture content, was provided by the pulp and paper factory Mondi Stambolyiski EAD. The bulk density of the factory waste fibres was 44.86 kg•m −3 . The waste fibres had lengths from 500 to 1000 μm and 7% reduced lignin content.
Magnesium lignosulfonate at 15% gluing content (based on the dry weight of the fibres) was used as a binder. The lignosulfonate additive had the following characteristics: total solids content-51.2%, magnesium content-6%, reduced sugars-7% and sulfate content-2%. European beech (Fagus sylvatica L.) veneers, made using the centric peeling process with an average thickness of 1.13 mm and a moisture content after drying and conditioning of approximately 7%, were supplied by the factory Welde Bulgaria AD, and used for veneering the laboratory-produced fibreboards.
Under laboratory conditions, the fibreboards were produced with a 16 mm thickness and a 720 kg•m −3 target density. Residual fibres were mixed with the magnesium lignosulfonate in a high-speed laboratory adhesive mixer at 850 min −1 . The hot pressing process was carried out using a single opening hydraulic press (PMC ST 100, Italy). The press temperature was 210 °C. The following fourstage pressing regime was used: in the first stage, the pressure applied was 4.5 MPa for 1 min; in the second stage, the pressure was steadily decreased to 2.23 MPa for 3 min; in the third stage, the pressure was decreased to 0.74 MPa for 10 min. The fourth pressing stage was performed at a pressure of 1.78 MPa for 2 min. The fabricated composites, after pressing, were conditioned for 10 days at 20 ± 2 °C and 60-70% relative humidity.
The veneer sheets with dimensions of 400 × 400 mm were cut. After that, the fibreboards were veneered using magnesium lignosulfonate at the content of 80 g•m −2 as an adhesive. A hand roller was used to apply the binder to the veneers in order to achieve a uniform adhesive layer. The veneering press factor used was 1 min•mm −1 of board thickness; the pressure applied was 0.6 MPa at 200 °C.
The physical and mechanical properties of the fabricated panels ( Figure 1) were tested according to the European Standards EN 310, EN 317, EN 322 and EN 323 [73][74][75][76]. A precision laboratory balance Kern (Kern & Sohn GmbH, Balingen, Germany) with an accuracy of 0.01 g was used to determine the mass of the test specimens. The dimensions of the test pieces were measured using digital callipers with an accuracy of 0.01 mm. The physical properties (water absorption and thickness swelling) were measured after 24 h of immersion in water. The thickness swelling was assessed using the differences between the initial and final panel thicknesses, and the water absorption was determined using the difference in weight. The mechanical properties of the panels were determined using a universal testing machine Zwick/Roell Z010 (Zwick/Roell GmbH, Ulm, Germany). The formaldehyde emission of the laboratory-produced panels was tested in the laboratory of Kronospan Bulgaria EOOD (Veliko Tarnovo, Bulgaria) on four test specimens in accordance with the standard perforator method [77]. The formaldehyde emission of the laboratory-produced panels was tested in the laboratory of Kronospan Bulgaria EOOD (Veliko Tarnovo, Bulgaria) on four test specimens in accordance with the standard perforator method [77].
For the evaluation of the bending strength of the joints made of the veneered eco-friendly composites from recycled wood fibres, L-shape test samples were prepared with the following dimensions: a length L 1 = L 2 = 106.4 mm, a width b = 100 mm and a thickness δ 1 = δ 2 = 18 mm, according to the test method [78] and as given in Figure 2.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 12 For the evaluation of the bending strength of the joints made of the veneered eco-friendly composites from recycled wood fibres, L-shape test samples were prepared with the following dimensions: a length L1 = L2 = 106.4 mm, a width b = 100 mm and a thickness δ1 = δ2 = 18 mm, according to the test method [78] and as given in Figure 2. Four types of end corner joints were selected, which are among the most used in furniture and interior constructions, where one was fixed with an adhesive and three with dismountable joints. Dowels with the dimensions 8 × 30 mm and glued with polyvinyl acetate emulsion adhesive (PVA) represented the fixed joints. The following connectors were used in this study as dismountable joints: Minifix with connecting bolts made of steel B 34 for direct screwing (Figure 3a), Confirmat one-piece connector 7 × 50 mm for screwing in holes Ø 5 mm (Figure 3b) and mounting screws for wood 3.5 × 50 mm (Figure 3c).  Four types of end corner joints were selected, which are among the most used in furniture and interior constructions, where one was fixed with an adhesive and three with dismountable joints. Dowels with the dimensions 8 × 30 mm and glued with polyvinyl acetate emulsion adhesive (PVA) represented the fixed joints. The following connectors were used in this study as dismountable joints: Minifix with connecting bolts made of steel B 34 for direct screwing (Figure 3a), Confirmat one-piece connector 7 × 50 mm for screwing in holes Ø 5 mm (Figure 3b) and mounting screws for wood 3.5 × 50 mm (Figure 3c).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 12 For the evaluation of the bending strength of the joints made of the veneered eco-friendly composites from recycled wood fibres, L-shape test samples were prepared with the following dimensions: a length L1 = L2 = 106.4 mm, a width b = 100 mm and a thickness δ1 = δ2 = 18 mm, according to the test method [78] and as given in Figure 2. Four types of end corner joints were selected, which are among the most used in furniture and interior constructions, where one was fixed with an adhesive and three with dismountable joints. Dowels with the dimensions 8 × 30 mm and glued with polyvinyl acetate emulsion adhesive (PVA) represented the fixed joints. The following connectors were used in this study as dismountable joints: Minifix with connecting bolts made of steel B 34 for direct screwing (Figure 3a), Confirmat one-piece connector 7 × 50 mm for screwing in holes Ø 5 mm (Figure 3b) and mounting screws for wood 3.5 × 50 mm (Figure 3c).  Seventeen test specimens were made using dowels, Minifix and Confirmat joints, and nine specimens were made with screw joints. All specimens were tested under compression bending loading according to the test scheme presented in Figure 4 [78].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12 Seventeen test specimens were made using dowels, Minifix and Confirmat joints, and nine specimens were made with screw joints. All specimens were tested under compression bending loading according to the test scheme presented in Figure 4 [78]. The ultimate bending moment was determined according to the formula: where: Fmax is the maximum value of the loading force (N). l is arm of bending (m).
The results from a previous study [79] were used to compare the results obtained in the present one, where the same types of joints, made from beech veneered MDF panels with a thickness of 18 mm were tested under compression bending loading.
A statistical analysis of the results was done with XLSTAT (version 2020.2.3, Addinsoft Inc., New York, NY, USA). One-way ANOVA was performed on the results for the bending strength of the Ltype corner joints for the analysis of variance at a 95% confidence interval (p < 0.05). The statistical differences between the mean values were evaluated using the post hoc Tukey HSD (honestly significant difference) test.

Formaldehyde Content
The free formaldehyde content of the fabricated composites, tested in accordance with the standard EN ISO 12460-5, was determined to be 1.1 ± 0.1 mg/100 g, i.e., the super E0 emission grade (≤1.5 mg/100 g) was achieved. This value was remarkably low and can be considered as a zero formaldehyde content [13,77]. Taking into account the fact that natural wood releases low but still measurable amounts of formaldehyde [80], which is formed by its main components and extractives [81][82][83] at approximately 0.5 to 2 mg/100 g [13,37], our result allowed for defining the produced panels as eco-friendly composites.

Mechanical and Physical Properties
The results for the mechanical and physical properties of the panels, which consisted of industrial waste fibres bonded with magnesium lignosulfonate, are presented in Table 1. The density of the laboratory-produced panels varied from 681 to 772 kg•m −3 , which was rather close to the The ultimate bending moment was determined according to the formula: where: F max is the maximum value of the loading force (N). l is arm of bending (m).
The results from a previous study [79] were used to compare the results obtained in the present one, where the same types of joints, made from beech veneered MDF panels with a thickness of 18 mm were tested under compression bending loading.
A statistical analysis of the results was done with XLSTAT (version 2020.2.3, Addinsoft Inc., New York, NY, USA). One-way ANOVA was performed on the results for the bending strength of the L-type corner joints for the analysis of variance at a 95% confidence interval (p < 0.05). The statistical differences between the mean values were evaluated using the post hoc Tukey HSD (honestly significant difference) test.

Formaldehyde Content
The free formaldehyde content of the fabricated composites, tested in accordance with the standard EN ISO 12460-5, was determined to be 1.1 ± 0.1 mg/100 g, i.e., the super E0 emission grade (≤1.5 mg/100 g) was achieved. This value was remarkably low and can be considered as a zero formaldehyde content [13,77]. Taking into account the fact that natural wood releases low but still measurable amounts of formaldehyde [80], which is formed by its main components and extractives [81][82][83] at approximately 0.5 to 2 mg/100 g [13,37], our result allowed for defining the produced panels as eco-friendly composites.

Mechanical and Physical Properties
The results for the mechanical and physical properties of the panels, which consisted of industrial waste fibres bonded with magnesium lignosulfonate, are presented in Table 1. The density of the laboratory-produced panels varied from 681 to 772 kg·m −3 , which was rather close to the targeted value. The difference in this main characteristic of the panels was significantly below 5%; thus, it did not have an effect on the mechanical and physical properties. This study followed our previous research work [44], in which the mechanical and physical properties of the fabricated panels, produced from waste fibres and magnesium lignosulfonate as a binder, were investigated in more detail and compared with the European Standard requirements for common engineered wood panels [84,85]. Generally, the laboratory-produced panels demonstrated acceptable mechanical properties in terms of the bending strength (MOR) (18.5 N·mm −2 ) and modulus of elasticity (MOE) (2225 N·mm −2 ), which were greater than the lowest requirements for type P2 particleboards and equal to the minimum requirements for MDF panels. The deteriorated dimensional stability, i.e., water absorption (24 h) and thickness swelling (24 h), was the main drawback of the fabricated composites.

Bending Strength of the L-type Corner Joints Made of the Fabricated Eco-friendly Composites
The results of the bending strength are given in Table 2 and the boxplot graphic in Figure 5. The performance of the joints made of the eco-friendly composites with a Minifix connector was unsatisfactory. In practice, they cannot be used as connecting elements due to the fact that the bolt of Minifix reacts with a limited part of the panel and the pullout strength of this part of the connector is significantly less. The rest of the tested joints showed sufficient bending capacity. The highest bending capacity was demonstrated by the joints with 8 × 30 mm dowels (8.02 N·m), followed by the Confirmat ø7 × 50 mm (6.95 N·m) and screws for wood ø3.5 × 50 mm (4.94 N·m). According to Tukey's test, a significant difference was observed only between the screw joints and the group of dowels and the Confirmat joints. The low strength of the joint with wood screws was due to the inhomogeneity of the eco-friendly panels fabricated from residual wood fibres. In contrast, the corner joints made from MDF panels exhibited an almost equal strength to the joints with the Confirmat and screws due to the homogeneity of the panels. This is proof that the small diameter of the screws had an adverse effect on the strength of the joint. Dowel joints constructed from eco-friendly composites showed almost 2.5 times lower bending capacity compared with the same joints made from MDF panels, while the joints with the Confirmat had a difference of 5 times and the joint with screws had a difference of 6.5 times.   From the statistical analysis of the one-way ANOVA test and the pairwise comparison performed with the Tukey HSD test, a significant difference of α = 0.05 at a confidence level of 95% was found in four groups between the obtained bending strength of the L-type end corner joints constructed from eco-friendly composites and MDF panels. The groups are presented in Table 3. All joints made of the eco-friendly composites fell in group "D," together with Rafix in MDF. The dowel joints in the eco-friendly composites were in one group with Minifix in MDF. Table 3. Tukey honestly significant difference (HSD) analysis of the differences between the groups with a confidence interval of 95% of the bending capacity of joints of two materials of all pairwise comparisons.  From the statistical analysis of the one-way ANOVA test and the pairwise comparison performed with the Tukey HSD test, a significant difference of α = 0.05 at a confidence level of 95% was found in four groups between the obtained bending strength of the L-type end corner joints constructed from eco-friendly composites and MDF panels. The groups are presented in Table 3. All joints made of the eco-friendly composites fell in group "D," together with Rafix in MDF. The dowel joints in the eco-friendly composites were in one group with Minifix in MDF. Table 3. Tukey honestly significant difference (HSD) analysis of the differences between the groups with a confidence interval of 95% of the bending capacity of joints of two materials of all pairwise comparisons.

Conclusions
Eco-friendly fibreboards with satisfactory mechanical and physical properties according to EN standards were produced from recycled fibres from the pulp and paper industry and bonded with magnesium lignosulfonate at 15% gluing content. The MOR and MOE values of the laboratory-produced composites were quite acceptable in comparison with the standard requirements for type P2 particleboards and almost equal to the minimum requirements for MDF panels [84,85].
The produced composites demonstrated a remarkably low formaldehyde content. The emission values, measured in accordance with the standard perforator method, were ≈1.1 mg/100 g, i.e., the composites reached the super E0 emission class, which allowed for their classification as eco-friendly, low-emission composites [77].
The main disadvantage of the laboratory-produced composites was their deteriorated dimensional stability, i.e., water absorption and thickness swelling (24 h). Hence, future investigations should focus on optimising the technological parameters and improving the lignosulfonate formula by adding suitable crosslinkers or catalysts.
The eco-friendly composites produced in this work can be used as a structural material with a limited application only, i.e., when applying minimal loads to the construction or as decorative panels. The Minifix connector was not suitable for joining the eco-friendly laboratory-produced composites. The tested L-type corner joints made of the fabricated eco-friendly composites demonstrated 2.5 to 6.5 times lower bending capacity compared to the same type of joints made of MDF panels with the same connectors. A higher bending capacity of the joints was achieved by improving the homogeneity of the composites and edgebanding of the structural elements.