**Walnut and Hazelnut Shells: Untapped Industrial Resources and Their Suitability in Lignocellulosic Composites**

**Marius Cătălin Barbu 1,2 , Thomas Sepperer <sup>1</sup> , Eugenia Mariana Tudor 1,2,\* and Alexander Petutschnigg <sup>1</sup>**


Received: 24 August 2020; Accepted: 10 September 2020; Published: 11 September 2020

**Abstract:** Walnut and hazelnut shells are agricultural by-products, available in high quantities during the harvest season. The potential of using these two agricultural residues as raw materials in particleboard production has been evaluated in this study. Different panels with either walnut or hazelnut shells in combination with melamine-urea formaldehyde or polyurethane at the same level of 1000 kg/m<sup>3</sup> density were produced in a laboratory hot press and mechanical properties (modulus of elasticity, bending strength, and Brinell hardness) and physical properties (thickness swelling and water absorption) were determined, together with formaldehyde content. Although Brinell hardness was 35% to 65% higher for the nutshell-based panels, bending strength and modulus of elasticity were 40% to 50% lower for the melamine-urea formaldehyde bonded nutshells compared to spruce particleboards, but was 65% higher in the case of using polyurethane. Water absorption and thickness swelling could be reduced significantly for the nutshell-based boards compared to the spruce boards (the values recorded ranged between 58% to 87% lower as for the particleboards). Using polyurethane as an adhesive has benefits for water uptake and thickness swelling and also for bending strength and modulus of elasticity. The free formaldehyde content of the lignocellulosic-based panels was included in the E0 category (≤2.5 mg/100 g) for both walnut and hazelnut shell raw materials and the use of polyurethane improved these values to super E0 category (≤1.5 mg/100 g).

**Keywords:** hazelnut; walnut; shells; lignocellulosic composites; UF; PUR; formaldehyde content

#### **1. Introduction**

The continuous interest in the efficient use and reuse of resources in the wood and agricultural sector for upcycled applications [1] is of great interest nowadays [2,3] in the context of the circular economy [4].

Steered by the paucity of non-renewable resources, the interest for wood could exceed its sustainable supply within the next few decades [5]. The wood demand has increased steadily, not only in the industry or for energy production (more than 50%), while the supply of wood is limited in specific regions of the world [6]. This leads to the need for substitutes for wood in engineered wood products (e.g., particleboards, PB). A promising alternative for wood in composites is provided by agricultural residues. A lot of research on agricultural waste has been carried out for PB based on wheat straw [7,8], rice straw [9,10], rapeseed [11], hemp shives [12,13], cotton dust [14], or sunflower stalks [15] and topinambour [16]. Brewer's spent grain is also a raw material for PB [17] together

with tree bark [18–20]. The advantages of these agricultural and forestry by-products include reduced costs, ample availability, biodegradability, and renewability, followed by an enlarged flexibility and sound insulation [21,22]. Some drawbacks in using agricultural residues in lignocellulosic composites include unequal availability over the year, the manufacture of products with these raw materials cannot run year-round, no industrialized processing yet, big storage facilities, and different necessary pre-treatments [23–25]. Walnut and hazelnut shells are agricultural residues available in high quantities, but despite their thermal utilization [26], with no industrial use yet. Nut shells can exhibit high hardness and toughness [27]. Worldwide walnut production in 2019/2020 was roughly 965,400 tons and 528,070 tons for hazelnut [28]. Taking in account that roughly 67 % of the total fruit weight is comprised of the shell leads to 646,818 tons of walnut shells and roughly 353,807 tons of hazelnut shells each year [29]. Especially in Iran and Turkey research on the solely use of nutshells and nutshells in combination with wood has been done. The authors in [30] studied the properties of particleboard from hazelnut husks and combined with European black pine (*Pinus nigra* Arnold) [31,32]. Walnut shells were studied by [33,34]. Other research refers to particleboards manufactured with peanut hulls mixed with European black pine [35], peanut shell flour [36], and almond shells [37–39].

The aim of this study was to compare and evaluate the influence of the nutshell type (hazelnut and walnut) and resin (bonded with melamine urea formaldehyde (MUF) and polyurethane (PUR) 10% each) on the mechanical and physical properties and on the free formaldehyde content of particleboards produced solely from the above-mentioned nutshells. Other studies have dealt with PB bonded only with UF, so this study brings a novel process of gluing the nut shell panels with MUF and PUR as the properties of these boards including the Brinell hardness and formaldehyde content have not been reported yet.

#### **2. Materials and Methods**

The raw materials used for the particleboard production consisted of walnut and hazelnut shells. The hazelnut (*Corylus avellana* L.) shells were provided by the Faculty of Forestry at the University of Zagreb (Croatia). The walnut (*Juglans regia* L.) shells were provided by a family-owned walnut cracking company in Carinthia (Maria Rojach, Austria). Melamine urea formaldehyde (MUF) resin (Prefere 10G268) was provided by metaDynea (Krems, Austria) and polyurethane (PUR 501.0) was provided by Kleiberit Klebchemie (Kleiberit Klebchemie M. G. Becker GmbH & Co. KG, Weingarten, Germany).

A total of 40 kg walnut shells and 30 kg hazelnut shells were available for the project. The shells were at first manually cleaned from impurities and afterward shredded in an R40 industrial four shaft shredder at Untha Company (Kuchl, Austria) using an 8-mm screen. The shredded nutshells were dried in a Brunner-Hildebrand High VAC-S, HV-S1 (Hannover, Germany) kiln dryer for three days to reach a moisture content of about 5%. After drying, the particles were sorted into three main size classes (fine-grained, middle-grained and coarse-grained) using a sieve shaker Retsch AS 200 (Haan, Deutschland). Particles in the middle-sized (3–6 mm) and fine (<3 mm) fraction were used for the board production. The percentage of particles in each group and nutshell type is listed in Table 1.


**Table 1.** Weight distribution according to the fraction size of the raw material.

A total of twelve boards (two for each group) was produced, of which the compositions are listed in Table 2.



The 320 mm × 320 mm boards were produced with a thickness of 10 mm and a density of 1000 kg/m<sup>3</sup> for nutshells and 700 kg/m<sup>3</sup> for the spruce control boards. The density of 1000 kg/m<sup>3</sup> could not be reached with wood particles due to the technical limitations of the hydraulic press Höfer HLOP 280. The walnut and hazelnut boards were composed of 70% middle-sized and 30% coarse-grained particles.

For all panels, the particles were mixed with resin manually and then formed into a mat and pre-compressed. This mat was pressed afterward in a hydraulic laboratory press (Höfer HLOP 280, Taiskirchen, Austria) with a pressure of 3 N/mm<sup>2</sup> . For the UF and MUF bonded boards, 10% resin based on weight was used including 1% ammonium sulfate as a hardener. These boards (Figure 1) were pressed at 160 ◦C for 6 min. The PUR bonded boards were also produced with 10% adhesive, but these were pressed at 60 ◦C for 20 min. After pressing, the panels were conditioned at 20 ◦C and 65% relative air humidity for one week before cut to test specimen size, which was done according to [40].

**Figure 1.** Hazelnut and walnut 10 mm thick samples bonded with PUR; 1000 kg/m<sup>3</sup> density; composition: 70% fine-grained particles (<3 mm) and 30% coarse-grained particles (3–6 mm).

In terms of mechanical properties, bending strength (MOR) and modulus of elasticity (MOE) according to [41] and Brinell Hardness [42] were tested.

For the bending properties [41], the three-point bending test was employed.

The physical properties such as thickness swelling and water absorption after 24 h water immersion [43] and density [44] were also measured.

To determine the formaldehyde content of the panels, 250 mm × 250 mm boards with fine-grained particles (<3 mm) and 10 mm thickness and a density of 1000 kg/m<sup>3</sup> were manufactured with walnut and hazelnut shells. Moisture content (m.c.) was measured for each type of board (Table 3).


**Table 3.** Moisture content of the nutshell boards prior to determining the formaldehyde content with the perforator method.

Each board was cut into 2.5 × 2.5 mm samples after cooling. The test specimens were placed in airtight bags and delivered to the Kaindl Company, Wals, Salzburg, Austria, where formaldehyde content was measured according to [45]. This method is recommended for nonlaminated and uncoated wood-based panels.

#### **3. Results**

For all boards described earlier, the mechanical and physical properties were evaluated according to the corresponding standard and statistically analyzed using mean separation tests and ANOVA.

#### *3.1. Mechanical Properties*

In terms of mechanical properties, MOE, MOR and Brinell hardness have been evaluated. The mean value, standard deviation, and minimal and maximal values are listed in Table 4. Results of ANOVA are indicated by letters a–e and u–z. The first letter refers to the different raw material (for values with the same letter, the raw material has no significant influence), the second letter refers to the used adhesive (again, and for values with the same letter, the adhesive has no influence).


**Table 4.** Mechanical properties of the hazelnut, walnut, and spruce particle boards.

a,b,c,d,e values with the same letter were not significantly different (raw material),u,v,w,x,y,z values with the same letter were not significantly different (adhesive).

#### 3.1.1. MOE and MOR

For the MUF bonded boards, the walnut shell ones (B) performed better in terms of MOE (1.51 GPa) and a MOR of 5.20 N/mm<sup>2</sup> compared to boards made from hazelnut shells (A) with an MOE of 1.30 GPa and a MOR of 4.25 N/mm<sup>2</sup> (Table 4). Although ANOVA showed that the material had a statistically significant influence in terms of MOE, it did not have an influence on the MOR. When PUR was used as an adhesive, it did not make a difference whether the boards were made of hazelnut (D) or walnut (E) shells. MOE was roughly 1.3 GPa for both, MOR 7.43 and 8.86 N/mm<sup>2</sup> for hazelnut and walnut shell boards, respectively. Furthermore, ANOVA showed that for walnut shell boards, it did not make a difference if MUF or PUR (both at a concentration of 10 w.t. %) was used as an adhesive when it came to MOE, while it did influence the MOR. When hazelnut shells were used as a raw material, the adhesive had a statistically significant influence on both MOE and MOR. The reference boards made from spruce and MUF (C) achieved the highest values for MOE and MOR (2.57 GPa and 13.26 N/mm<sup>2</sup> , respectively). Surprisingly, the PUR bonded spruce boards (F) performed weaker than the nutshell panels (0.87 GPa for MOE and 5.47 N/mm<sup>2</sup> for MOR). The results for both hazelnut and walnut panels bonded with PUR were similar to the values obtained by [34], namely a MOR between 6 and 9 N/mm<sup>2</sup> and an MOE between 0.8 and 1.3 GPa. When the same amount of hazelnut shells was mixed with wood particles (50% + 50%), the values of the MOE and MOR increased significantly [32].

In terms of MOE (Table 4), the walnut shell boards showed higher values than the ones produced by [46], which reached 1.15 GPa on average compared to 1.51 GPa and might have been caused by the adhesive used. The study by [46] applied UF, while for these boards was utilized a less brittle MUF [47]. Another reason might be that [46] produced boards with a density of 700 kg/m<sup>3</sup> while those boards had a target density of 1000 kg/m<sup>3</sup> [48].

The results for MOE and MOR of the nutshell boards were expected to range below the results of spruce particleboards. This can be traced back to the fact that the mechanical properties are strongly influenced by different properties of the raw material like density, chemical composition, and particle size. A low-density raw material allows a higher compression in the panel, leading to improved performance in bending tests [49]. Given that the density of the used nutshells was very high (between 700 and 1030 kg/m<sup>3</sup> depending on the fraction) and spruce particles have a density of 450 kg/m<sup>3</sup> , the lower performance of the MUF bonded nutshell boards can be traced back to a lower compression rate [50]. The weak behavior of the PUR bonded spruce boards can be traced back to unsatisfying bonding, caused by an insufficient moisture content of the raw material (3%) for the adhesive to react [51].

#### 3.1.2. Brinell Hardness

Boards made from hazelnut shells and MUF (A) showed the highest average Brinell hardness with 62.5 N/mm<sup>2</sup> while the lowest hardness value was achieved by PUR bonded spruce particles with only 18 N/mm<sup>2</sup> (Table 4). The highest value was achieved by hazelnut with 82.6 N/mm<sup>2</sup> while the lowest for those panels was 39.9 N/mm<sup>2</sup> . The big difference between the highest and lowest value and the high standard deviation can be explained as a result of the different measuring spots and the manual scattering of raw material in the press mold. This showed that the hardness was very high when measured on a big piece of the nutshells, while it was much lower when determined on the fine-grained particles that are filling the gaps between the coarse ones. This was not only valid for MUF, but also for PUR. The highest Brinell hardness value for a hazelnut board glued with PUR (D) was 70.59 N/mm<sup>2</sup> while the lowest was 27.82 N/mm<sup>2</sup> . The mean was determined at 54 N/mm<sup>2</sup> . Walnut panels showed similar results this time, PUR bonded boards (E) performed a little better with an average Brinell hardness of 55.5 N/mm<sup>2</sup> , while MUF boards (B) only reached 43.7 N/mm<sup>2</sup> . The highest value for the walnut boards was 63.9 N/mm<sup>2</sup> for those produced with PUR. ANOVA showed that when it came to PUR bonded boards, the nutshell type had no influence. There was also no statistically significant difference for hazelnut boards concerning the adhesive. Compared to spruce particleboards that showed an average result of 18 N/mm<sup>2</sup> for PUR and 28 N/mm<sup>2</sup> for MUF, the nutshells were much harder. However, the results for the wood-based boards were closely distributed to the mean value, with a standard deviation of only 3 N/mm<sup>2</sup> . This means that the spot where the force is applied is less relevant compared to the nutshell boards.

#### *3.2. Physical Properties*

In terms of physical properties, thickness swelling (TS) and water absorption (WA) both after 24 h water immersion were evaluated.

Thickness swelling after 24 h water immersion for MUF bonded hazelnut husk panels (A) was determined with 17.5% while it was a little lower for walnut shells (B) with 13.2% (Table 5). Compared to the spruce reference board (C) with an average of 52%, the increase in thickness was much lower. The results for hazelnut (A) glued with MUF were a little better compared to those obtained by [31] and [35] (29.3% TS with a standard deviation of 3.5%), while the results for walnut (B) were higher compared to [34], who evaluated the thickness swelling with 10.2%.


**Table 5.** Physical properties of the hazelnut, walnut, and spruce particle boards.

a,b,c,d,e values with the same letter were not significantly different (raw material), u,v,w,x,y,z values with the same letter were not significantly different (adhesive).

Thickness swelling after 24 h was significantly reduced when PUR was used to produce the boards. This means that it was only 9.7% for hazelnut (D) and 8.4% for walnut (E). The spruce boards showed an increase in thickness of nearly 52% for MUF and still 44.5% for PUR. The nutshells in combination with PUR performed better compared to the spruce particles. It was found that when MUF was used as the adhesive, there was a significant difference between the walnut and hazelnut shells, while the material did not have a big influence when bonded with PUR.

The results for water absorption after 24 h water submersion were similar to those for thickness swelling after 24 h (Table 5). This means that the nutshell boards glued with MUF had a higher water uptake, namely 27% for hazelnut (A) and 25% for walnut (B), compared to the PUR bonded ones with 12.5% (D) and 11.5% (E), respectively. The values were much lower compared to the results of spruce particles with 64% for MUF (C) and almost 90% for PUR (F). Regarding water uptake over 24 h, there was no influence of the used nutshell type in combination with either MUF or PUR.

The same trend of blocking and reduction of TS and WA was observed in the case of using hazelnut shells combined with wood particles [32], with a mean of 20% for TS and 70% for WA and from [34], in the case of walnut shells with the same value for TS as reported by [32] and lower WA of 37%.

It was expected that thickness swelling and water absorption will be lower, compared to the spruce particleboards. The water uptake of lingo-cellulosic materials is strongly influenced by the number of free hydroxyl groups where water is bonded to. These hydroxyl groups are mainly present in the natural polymer cellulose. Hemicellulose is amorphous and has a hydrophilic character that is additionally increasing the water uptake. Lignin, however, is totally hydrophobic, which means that water cannot be absorbed within [52]. The high amount of lignin in the walnut shells (49.1%) compared to roughly 35% in softwood indicated a decreased amount of water absorption. Furthermore, hazelnut husks contain almost 42% of lignin and only 55% of holocellulose compared to 65% in softwood.

PUR is more hydrolytically stable than MUF, which explains the big differences for thickness swelling and water absorption when these two adhesives are compared. The high value for water absorption for the PUR glued spruce particleboards was again caused by the low (3%) moisture content of the raw material and a non-complete reaction of the adhesive.

#### *3.3. Formaldehyde Content*

The corrected values of free formaldehyde content varied depending on the type of adhesive formulation for the board (Figures 2 and 3).

**Figure 2.** Free formaldehyde content for both the measured and corrected perforator values (EN 120:2011) of samples of walnut and hazelnut shell boards compared with larch bark panels from [53].

**Figure 3.** Free formaldehyde content measured according to EN 120:2011 of 10 mm walnut and hazelnut panels bonded with UF and MUF and compared with the values of larch bark boards glued with the same adhesives from [53].

The values for walnut and hazelnut shell panels were compared with similar 10 mm boards manufactured with the same adhesives reported by [53]. The lowest formaldehyde content of 0.07 mg/100 g oven dry was measured for the board with larch bark bonded with PUR (PUR\_bark). This is included in the super E0 classification (≤1.5 mg/100 g). In the same category were comprised the values of the walnut (PUR\_WN) and hazelnut (PUR\_HN) shell boards glued with PUR and the larch bark board bonded with UF. To the E0 class (≤2.5 mg/100 g) belonged the values for the nutshell boards bonded with UF (UF\_HN and UF\_WN). These results are consistent with those obtained by [54] regarding formaldehyde release from low-emission wood-based panels using the perforator method, which ranged from 0.71 to 2.99 mg/100 g and was slightly lower than that of [34].

The role of bark to decrease the formaldehyde content of wood-based composites was also studied by [55,56]. It was expected that the level of free formaldehyde content of the panels glued with PUR would be significantly lower than that of the boards bonded with UF, but the combination of both adhesives and raw materials resulted in panels that reached at least the E0 classification.

#### **4. Conclusions**

This study has shown that the nutshells can be used for the manufacture of PBs with improved performances in terms of physical properties (dimensional stability), but also with higher Brinell hardness compared to the spruce particleboards.

MOR and MOE are lower by roughly 50% and cannot meet the requirements for P1 or P2 PBs, according to EN 310:2005. Future research should consider the improvement potentials by choosing the proper percentage of nutshells chips, combining the coarse-grained particles with the fine-grained ones, in order to fill voids in-between, combined with a proper bonding system.

This study has also shown that, when MUF is used to bond the boards, the raw material has no influence on MOR and water absorption, when PUR is used, the raw material has no influence on MOE, but with an improvement in terms of MOR (35% higher compared to spruce particles), Brinell hardness (up to 67%), thickness swelling (65–75% lower), and water absorption (58–87% lower). There is no statistically significant influence of the adhesive for hazelnut shells in terms of MOE and Brinell hardness.

The values of the formaldehyde content are all included in the E0 emission class. In the case of the panels bonded with PUR, the measured values were less than 1.5 mg/100 g, which means that the super E0 category was reached. All these perforator results for the panels manufactured with hazelnut and walnut shells recommend them as low-emissions lignocellulosic composites.

The nutshells, utilized only for energy purposes [57], could be considered as a raw material in particleboard production. Further research can focus on the properties of improved formulations for adhesives and lignocellulosic particles, eventually with reinforcements [58] to achieve similar properties to P1 and P2 [59].

**Author Contributions:** Conceptualization, T.S.; Methodology, T.S. and E.M.T. Validation, E.M.T.; Formal analysis, A.P. and M.C.B.; Investigation, T.S.; Resources, T.S.; Writing—original draft preparation, T.S. and E.M.T.; Writing—review and editing, T.S. and E.M.T.; Visualization, T.S., and M.C.B.; Supervision, A.P. and M.C.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank Frederick Kamke from Oregon State University (Oregon, USA) for making the project possible and his support throughout the project; Danijela Domljan from the University of Zagreb (Croatia) for providing the hazelnut shells used for the board production in this project and to Kaindl Co. in Salzburg for supporting this research with the determination of the formaldehyde content of the walnut and hazelnut samples.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Impact of Structural Defects on the Surface Quality of Hardwood Species Sliced Veneers**

#### **Vasiliki Kamperidou 1,\* , Efstratios Aidinidis <sup>2</sup> and Ioannis Barboutis <sup>1</sup>**


Received: 20 August 2020; Accepted: 7 September 2020; Published: 9 September 2020

**Abstract:** The surface roughness constitutes one of the most critical properties of wood and wood veneers for their extended utilization, affecting the bonding ability of the veneers with one another in the manufacturing of wood composites, the finishing, coating and preservation processes, and the appearance and texture of the material surface. In this research work, logs of five significant European hardwood species (oak, chestnut, ash, poplar, cherry) of Balkan origin were sliced into decorative veneers. Their surface roughness was examined by applying a stylus tracing method, on typical wood structure areas of each wood species, as well as around the areas of wood defects (knots, decay, annual rings irregularities, etc.), to compare them and assess the impact of the defects on the surface quality of veneers. The chestnut veneers presented the smoothest surfaces, while ash veneers, despite the higher density, recorded the highest roughness. In most of the cases, the roughness was found to be significantly lower around the defects, compared to the typical structure surfaces, probably due to lower porosity, higher density and the presence of tensile wood. The results reveal that the presence of defects does not affect the roughness of the veneers and increases neither the processing requirements of the veneer sheets before finishing, nor the respective production cost of veneers and the veneer-based wood panels. The high utilization prospects of the examined wood species in veneer production, even those bearing various defects, is highlighted.

**Keywords:** chestnut; decay; defect; density; knot; roughness; surface; texture; quality; veneer

#### **1. Introduction**

The surface of each material or final product consists of a miniature of peaks and valleys, the size and distribution of which determine the surface properties of the material, such as roughness, texture, etc. The surface roughness is estimated in order to predict the surface behavior of the material during its application in various uses. As regards wood, roughness greatly affects its aesthetics and the structures in which it participates, and should be in line with the criteria and requirements of consumers in terms of quality. As regards wood veneer sheets, rough surfaces of veneers not only negatively affect the appearance of the finished products, but also affect manufacturing processes such as coatings and adhesion appliance, and adhesion strength, since they reduce the contact between them, resulting, according to the literature, in weak interactions between glue and wood and, therefore, low-strength properties of laminated veneer lumber, plywood and several other wood-based composites [1–3].

Due to its structure and anatomical features, the wood surface is a multidimensional and complex substrate, and its roughness is influenced by various factors such as the wood species (hardwood versus softwood), wood density and porosity (denser wood corresponds to lower porosity and smoother surfaces), annual rings' width, ratio of early wood to late wood, the log temperature during

slicing/peeling and wood storage conditions (temperature, relative humidity), the moisture content, wood anisotropy, structure and types of cells and the kinetics of liquids–gases into its mass, as well as several mechanical and machine processing operations (sawing, sanding, planning, etc.) parameters, such as the cutting means type, knife angle and marks per centimeter, cutterhead speed, tool wear, cutting direction (longitudinal, radial and tangential), etc. [1,3–8]. Tanritanir et al. [9] revealed that steaming for 20 h is an ideal pre-treatment of veneers to provide smooth surfaces of both heartwood and sapwood. In general, the use of coarse-grained veneers can reduce the bonding quality by 1/3, compared to smooth surface veneers [10–12]. Less rough wood surfaces exhibit better performance in the application of finishing agents, more uniform distribution of adhesive, require much lower amounts of paint/dye to cover the whole surface, while the phenomenon of resin bleeding through the face veneer is avoided [11,12]. Furthermore, according to the literature, the surface roughness of wood material decreases as the grit number of sandpaper increases from 60 to 240 [13]. Usually, the veneer production industries apply a sanding of 80–100 grit number, to keep the production cost at low levels, while the woodworkers and manufacturers further apply additional sanding processes to the veneer-based panels, once or twice, using 180 or 220 sandpapers.

The defects generally affect the appearance of the veneer sheets, making them usually less preferable for face-side application in furniture and structures [3]. A high number of defects makes the veneer be categorized as low-value and it is usually applied in back-side applications. However, the wood defects correspond to the natural appearance of wood and, especially in recent years, there has been a phenomenon of asking for artificially aged or intensely rough furniture, precisely because they refer to and remind the customer of something special and unique. Furthermore, in the recent years in which the wood of high quality has been in short supply, the rational utilization of woody biomass, even the low-value wood species bearing a high number of defects, as well as the high-quality raw-lumber saving strategy, seems to be of crucial importance.

Currently, there is no comprehensive information available concerning the way that several different wood defects affect the smoothness and surface quality of sliced veneer sheets manufactured from different hardwood species. Therefore, the purpose of this study is to examine the surface roughness parameters values of sliced veneers made of five different species significant for veneer production European hardwood species (ring-porous, semi-ring porous and diffuse porous), are investigated, for the first time according to the literature, in terms of how their roughness level is influenced by the presence of various structural defects in the mass and surface of veneers, such as knots, irregularities of annual rings structure (spiral grain), decay, discoloration etc., compared to typical structure surfaces of each wood species' veneers. Additionally, it is investigated how the surface roughness of sliced veneer sheets, continuous in row and successively cut, differentiates in different wood depths, observing the evolution of the whole defect as it is encountered in the trunk, in areas of typical and non-typical wood structure.

#### **2. Materials and Methods**

The raw material of this experimental work consisted of logs of five European hardwood species of Greek and Balkan origin of large diameter (350 mm mean diameter). Specifically, one log was examined per examined forest species, which were oak (*Quercus robur* L.), chestnut (*Castanea sativa*), ash (*Fraxinus excelsior* L.), hybrid poplar (*Populous* spp.) and wild cherry (*Prunus avium*). Only the poplar wood was of Greek origin, while the rest of the species used were obtained from 3 different Balkan countries (Romania, Croatia and Serbia), and they were all commercially converted into decorative sliced veneer sheets, using the veneer slicing method, applied in the infrastructures of a Greek industry of sliced veneer production, located in central Greece (Chalkida, Evia), so that the processing conditions, the cutting means and the slicing method applied would be common for the five wood species. The veneer production fulfilled the requirements of the industrial sliced veneer production standards of this certified company, as regards the absence of thickness inequalities, defects attributed to mechanical processing failures, like burning, etc. The trunks were initially peeled

to remove the bark, cut into logs, and then steamed under the same conditions (duration, temperature, pressure) followed by the industry, prior to the slicing process. The cutting machine used for the slicing of veneers was of horizontal operation, since this is suitable for the veneer slicing of hardwood species and they were all cut into plain cut (flat cut) veneers. After a visual assessment of the defects on the produced veneers, for each case of wood species and defect species, a package of 10 sliced veneer sheets, continuous in a row and successively cut, was obtained, aiming to observe the evolution of the whole defect as it is encountered in the trunk mass. The veneer sheets produced were of 0.55 ± 0.1 mm thickness, mainly cut from the heartwood part of trunks and for the purposes of this experiment, they were cut in our laboratory in smaller dimensions (350 mm length × 250 mm width) to be easily handled, and were left to be conditioned at 20 ± 2 ◦C and 65 ± 5% relative humidity, until constant weight. All veneer samples were conditioned to equilibrium moisture content (EMC), which ranged at low levels (5.5–10%) [14]. Twenty days before the roughness measurements' implementation, the veneers were anchored tightly on flat surfaces and, subsequently, the veneer sheets' surfaces were slightly sanded with 80-grit sand paper for 15 s. under the same laboratory conditions, since the sanding process creates a new and fresh surface by removing the material and, therefore, can improve the surface quality of veneers before finishing [15]. Afterwards, the veneers were removed from the abovementioned flat surfaces and left for approximately three weeks to be conditioned at 20 ± 2 ◦C and 65 ± 5% relative humidity, until constant weight. At the end of the conditioning duration, the EMC was measured again by applying the drying method of the veneer samples [14] and recording similar EMC values with those prior to the sanding and conditioning processes (<10%). The mean density of the veneers was also measured after their conditioning process (calculated as dry mass/ wet volume, with volume measured in the state of the EMC), following the respective international standard process [16], with the only difference that specimens of different dimensions were measured (20 mm × 20 mm × 0.5 mm). For the dry mass measurement, a weight of high accuracy (of 4 decimals) was used, and for the volume determination, a digital caliper was used. The density of oak wood was found to be 0.742 g/cm<sup>3</sup> , of chestnut wood was 0.554 g/cm<sup>3</sup> , of ash wood was 0.705 g/cm<sup>3</sup> , of poplar wood was 0.385 g/cm<sup>3</sup> and of cherry wood was 0.627 g/cm<sup>3</sup> .

On the veneers, 10–12 measurements of roughness parameters were randomly implemented on the surface of typical wood structure areas and, respectively, another 10–12 measurements of roughness were conducted on non-typical wood structures of veneer surfaces in the peripheral area of each defect (10–30 mm radius around the defect). The defects were different for each wood species, including knots (Figure 1), tensile wood, irregular annual rings, deflection of wood fibers (spiral grain), discoloration (Figure 2), decay (Figure 3) etc., since, as with the material of wood, its defects, as well, are unique. The number of 3 to 10 different veneer sheets were measured from each veneer's package and for each wood species, in order to investigate the potential differentiation of roughness as a function of different wood depths on the defects' development areas.

**Figure 1.** Dead knots in oak wood sliced plain-cut veneer (**a**), and in chestnut wood sliced veneer (**b**).

(**a**) (**b**)

**Figure 2.** Ash wood sliced veneer with discoloration and irregularities of annual rings (**a**), and live internal knot with spiral grain around the knot (**b**).

**Figure 3.** Poplar wood sliced plain-cut veneer sheet with live knot and decay (**a**), and cherry wood veneer with irregularities of annual rings (**b**).

The roughness parameters of the prepared veneer surfaces were evaluated using a fine stylus type profilometer, Mitutoyo Surftest SJ-301 (Figure 4), with the profile tracing method using the diamond stylus of the device, according to ISO 4287:1997 [17]. The stylus technique was determined to be used, since compared to the other methods, such as pneumatic, laser, and acoustic emission, it is accurate, practical, and repeatable [5]. The measuring speed, the diameter of the pin and the upper angle of the pin tool were 10 mm / min, 4 µm, and 90◦ , respectively. The sampling length was of 2.5 mm, and the evaluation length was of 12.5 mm (five times of the sampling length). The values of the surface roughness parameters were determined to be within ±0.01 µm. The measurements were implemented in a direction perpendicular to the direction of grain orientation.

**Figure 4.** Roughness test using the Mitutoyo Surftest SJ-301 profilometer (**a**); an example of the roughness spectrum on a poplar sliced veneer sheet of typical structure (**b**).

The roughness measurement points were randomly selected by marking them on the surface of the samples, in order to cover the whole area. Three roughness parameters, the mean numerical deviation from the midline profile along the entire length of the stylus movement (mean arithmetic deviation of profile—Ra), the average height between the peak-valley derived from five identical lengths of the profile (mean peak-to-valley height—Rz), and the distance between peak and valley points of the profile, which can be used as an indicator of the maximum defect height within the assessed profile (maximum roughness—Ry), have been widely used in previous studies [18–20], where detailed information about these roughness parameters has been presented. These parameters, employed also in the current study, have also been used previously in the quantification of surface quality of veneers [4,21], and other wood composites, and they are defined by the respective roughness standards [17]. Prior to each measurement, the instrument was calibrated and the roughness measurements were performed at room temperature (20 ± 2 ◦C) [22,23].

For the processing and statistical analysis of the test results, the statistical package SPSS Statistics PASW 18 was used to determine the variability of the roughness parameters' mean values, and the effect of two different independent variables, "Veneers" (referring to the different veneers from V1 to V10, obtained continually and successively cut/produced as it is found in the trunk), and "Structure" (referring to the typical and non-typical structure of wood), and the potential interaction of these two factors upon the dependent variable of roughness parameter Ra (chosen as the most significant one and representative), using two way analysis of variance (ANOVA) with a significance level of 0.05 (*p* < 0.05).

#### **3. Results and Discussion**

#### *3.1. Veneer Surfaces of Typical Wood Structure*

According to the results (Figure 5), all the three surface roughness parameters (Ra, Rz, Ry), measured on typical wood structure surfaces, follow similar routes concerning the different wood species. The veneers of ash wood, despite their high wood density (0.705 g/cm<sup>3</sup> ), exhibited the highest surface roughness parameter values among the five species examined in this study, presenting statistically significant differences from the respective roughness values of all the other wood species veneers. The lowest roughness parameter values, and therefore, the smoother surfaces, were observed in veneers' surfaces of chestnut, whose roughness parameters' mean values were also found to differ significantly from the rest of wood species' respective values. Oak, poplar and cherry veneers recorded similar values of surface roughness parameters, even though they are characterized by much different wood density (oak 0.742 g/cm<sup>3</sup> and poplar 0.385 g/cm<sup>3</sup> ) and different structure (ring-porous/diffuse-porous/semi-ring-porous).

**Figure 5.** Mean values of surface roughness parameters Ra, Rz and Ry (µm), measured on typical wood structure areas of the sliced veneer sheets of the five different species studied.

#### *3.2. Oak Veneers*

In the case of oak wood, the roughness parameters of three veneers from the oak veneers package that corresponds to the evolution of the whole knot, were chosen to be measured (Figure 6). According to the results, all the three surface roughness parameters (Ra, Rz, Ry) were found to be, from the statistical analysis point of view, significantly lower in the case of the area around the knot, indicating a smoother area around the knot, compared to the typical structure of wood surface veneers. Generally, oak wood is a ring-porous hardwood species, whose structure, as expected, results in higher roughness levels. The lower roughness of the wood areas around the dead knot that had been felled, could be attributed to their higher density, lower porosity and the presence of tensile wood that was also visually detected. As it is widely known, tensile wood is formed on hardwoods on the upper side of logs and branches in places that are under tension. Tensile wood is characterized by lighter color and fibers that have thick walls and very small cavities. The cell walls in tensile wood areas are characterized by the presence of a gelatinous layer, which consists of concentrated microfiber substrates arranged almost parallel to the fiber axis, and can be deposited on the layer S<sup>3</sup> or even on layer S2, causing the cell walls to be of higher thickness and the surface of tensile wood to become glossy [24]. Therefore, the tensile wood areas were easily recognized on the sliced veneers of this study.

According to the statistical analysis of the results, in all cases examined, the Levene tests revealed that the null hypothesis that the error variance of the dependent variable (Ra) is equal across the groups, was accepted (6th requirement of a successful ANOVA), recording a significance level > 0.05 (0.12–0.278). Investigating the roughness in different depths and areas around the knot (radius of 30 mm from the knot), going from veneer V1 to veneer V10, it is apparent (Table 1), that the roughness level records a slight decrease, marginally not statistically significant. The tests of Between-Subjects effects revealed that the factor of "veneers", referring to this progress of the different cutting depths from V1 to V10, affected the Ra variance by 16.4%. In addition, 83.4% of Ra variance is attributed to the factor of "Structure", referring to the typical and non-typical structure of wood (the latter around the knot), while the interaction between the factors "Veneers" and "Structure" affects the Ra variance by 21.4%.

**Figure 6.** Surface roughness parameters Ra, Rz and Ry (µm) of oak wood veneers (V1, V5 and V10) on typical wood structure areas (Typ) and areas around dead knot (Knot).


**Table 1.** Descriptive statistics of dependent variable Ra measured on oak veneers of typical and non-typical structure around the knot.

#### *3.3. Chestnut Veneers*

The results of roughness measurements carried out on veneers of the ring-porous hardwood species of chestnut wood (V1–V8) (Figure 7, Table 2), reveal that in six of the eight veneers studied, the roughness around the defect area was found to be lower than the respective mean roughness parameter values of typical wood structure surfaces, with only three of them corresponding to statistically significant differences. Only in the case of V6, the area around the dead knot was found to be of higher roughness than typical wood structure surfaces, but without marking a statistically significant difference. Investigating the roughness in different depths and areas around the dead knot, going from veneer V1 to veneer V10, it is evident that the roughness level records a gradual, though statistically significant, increase.

**Figure 7.** Surface roughness parameters Ra, Rz and Ry (µm) of chestnut wood veneers (V1–V8) on typical wood structure areas and areas around dead knot.

The tests of Between-Subjects effects demonstrated that the factor of "veneers", referring to this progress of the different depths from V1 to V8, affected the Ra variance by 29.9%. In addition, 15.2% of Ra variance is attributed to the factor of "Structure", referring to the typical and non-typical structure of wood (around the knot), while the interaction between the factors "Veneers" and "Structure" affects the Ra variance by 47.5%.

195


**Table 2.** Descriptive statistics of dependent variable Ra measured on chestnut veneers of typical and non-typical structure around the knot.

#### *3.4. Ash Veneers*

Ash is a wood species with a ring-porous wood structure, with the apertures of vessels to potentially increase the roughness of the wood surfaces [3]. The results from the roughness parameter measurements on ash veneers' surfaces (Figure 8, Table 3) demonstrate that the surface areas with irregularities of annual rings, in four of the total five cases examined (veneers V2–V5), exhibited significantly lower roughness parameters and smoother surfaces than typical ash wood surfaces, while in only one case (veneer V1), the typical and non-typical wood structure surfaces displayed similar roughness values. The factor of "veneers", referring to this progress of the different depths from V1 to V8, statistically significantly affected the Ra variance by 23.7%. In addition, 80.8% of Ra variance is attributed to the factor of "Structure", referring to the typical and non-typical structure of wood around and on the irregularities of annual rings, while the interaction between the factors "Veneers" and "Structure" affects the Ra variance by 56.6%.

**Figure 8.** Surface roughness parameters Ra, Rz and Ry (µm) of ash wood veneers (V1–V5) on typical wood structure areas and areas of irregular annual rings and discoloration.



In the case of ash veneers obtained from the area near the trunk base, bearing discoloration and eccentric annual rings (spiral grain), statistically significant differences were not recorded between the roughness of typical and non-typical structure areas on the surface of veneers (Figure 9, Table 4). The factor of "veneers", referring to this progress of the different depths from V1 to V10, statistically significantly affected the Ra variance by 41.4%. In addition, 15.9% of Ra variance is attributed to the factor of "Structure", referring to the typical and non-typical structure of wood around and on the irregularities of annual rings, while the interaction between the factors "Veneers" and "Structure" affects the Ra variance by 28.3%.

**Figure 9.** Surface roughness parameters Ra, Rz and Ry (µm) of ash heartwood veneers (V1, V5, V10) on typical wood structure areas and in areas of eccentric annual rings (spiral grain) of the wood near the trunk base, also bearing discoloration.



#### *3.5. Poplar Veneers*

Poplar is a fast-growing diffuse-porous hardwood species of high availability, whose utilization is restricted by its low density [3]. As regards the poplar wood veneers (Figure 10), the surface roughness parameters in the areas around the decay were found to be, in most of the cases (veneers V1, V3, V4 and V5), of lower surface roughness, compared to the wood surface of typical structures. Specifically, in the case of veneers V1 and V5, the differences were found to be statistically significant, with the roughness values of areas around decay to be the lowest ones (Table 5). The tests of Between-Subjects effects revealed that the factor of "veneers", referring to this progress of the different depths from V1 to V5, significantly affected the Ra variance, by 39%. In addition, 52.4% of Ra variance is attributed to the factor of "Structure", referring to the typical and non-typical structure of wood (around the decay area), while the interaction between the factors "Veneers" and "Structure" significantly affects the Ra variance, by 59.1%.

**Figure 10.** Surface roughness parameters Ra, Rz and Ry (µm) of poplar wood veneers (V1–V5) on typical wood structure areas and around areas of decay.



Concerning the roughness parameter values measured on the surface of poplar wood veneers (Figure 11, Table 6), around typical and non-typical wood structure areas, it was also revealed that the areas around the live knot detected were found to be, in each case, of significantly lower surface roughness compared to those of typical structure. The factor of "veneers", referring to this progress of the different depths from V1 to V5, significantly affected the Ra variance, by 70.4%. In addition, 60.9% of Ra variance is attributed to the factor of "Structure", referring to the typical and non-typical structure of wood around and on the irregularities of annual rings, while the interaction between the

#### *Appl. Sci.* **2020**, *10*, 6265

factors "Veneers" and "Structure" was not found to be statistically significant and it affects the Ra variance only by 3.2%.

**Figure 11.** Surface roughness parameters Ra, Rz and Ry (µm) of poplar wood veneers (V1–V5) on typical wood structure areas and areas around live knots.


**Table 6.** Descriptive statistics of dependent variable Ra measured on poplar veneers of typical wood structure areas and areas around a live knot.

#### *3.6. Cherry Wood Veneers*

Furthermore, the roughness parameters of three veneers (V1, V5, V10) made of the semi-ring porous cherry wood were investigated (Figure 12, Table 7), revealing in each case lower surface roughness in the areas of annual rings with irregularities, compared to surface areas of typical wood structures. Nevertheless, only in the case of veneer V1, the difference between the typical structure's wood surface and irregular annual rings' surface was found to be statistically significant. Statistically significant differences between the roughness parameter values of the defect areas of different veneers (going from veneer V1 to V10) were not found. More specifically, the tests of Between-Subjects effects revealed that the factor of "veneers", referring to this progress of the different depths from V1 to V5, insignificantly affected the Ra variance, by 5%. The factor of "Structure" (typical and non-typical structure) around and on the irregular annual rings, affects the Ra variance significantly by 62.4%, while the interaction between the factors "Veneers" and "Structure" does not significantly affect the Ra variance (14.1%).

**Figure 12.** Surface roughness parameters Ra, Rz and Ry (µm) of cherry wood veneers (V1, V5, V10) on typical wood structure areas and around areas of irregular rings.

**Table 7.** Descriptive statistics of dependent variable Ra measured on cherry veneers of typical wood structure areas and areas around areas of irregular rings.


*3.7. Di*ff*erences between Roughness of Typical Wood Structure and Non-Typical Structure of Defects Veneer Surfaces*

Concerning the veneers of the five hardwood species examined, their roughness was found to be lower in each case in the areas around the defects, compared to typical structure wood areas (Figure 13). Even though oak veneers did not present the highest level of surface roughness among the species examined, they demonstrated the highest difference of roughness parameters, recorded between areas of typical structure and non-typical wood structure (mean decrease of 38.84% compared to the reference material). Ash wood veneers, which exhibited the higher roughness values among the five species, recorded a quite high difference between the roughness levels of typical and non-typical wood structure areas (mean decrease of 31.59%). Chestnut veneers, which generally recorded the lowest surface roughness among the five species studied, also presented the lowest difference between the roughness parameters of the typical and non-typical structure areas (mean decrease in roughness of 7.11% compared to control). Poplar and cherry wood veneers recorded a medium level decrease in roughness in areas around the defects in relation to the typical structure areas that ranged between 18.77% and 21.88%.

**Figure 13.** Percentage values depicting the differences between roughness parameters Ra, Rz and Ry of typical wood structure veneer surfaces and non-typical structure areas around the defects (decreased around the defects).

#### **4. Conclusions**

In this study, sliced veneers of five different European hardwood species of high significance were commercially produced and conditioned under the same conditions, to investigate the surface roughness of them in areas of typical wood structure and non-typical wood structure, in areas around defects. The chestnut species presented the lowest surface roughness among the five species studied, demonstrating the smoother surfaces, while ash wood veneers recorded the highest roughness, despite the high wood density. Although the chestnut wood studied in this experimental work was of lower density, its veneers presented smoother surfaces, compared to the other species, and this fact reveals the potential of utilizing this valuable species even more intensively in veneer production. Since the veneers were processed and conditioned under the same conditions, it is indicated by the results that density is a significant factor, but not the only one, affecting the smoothness of the veneer surfaces. Other morphological characteristics of wood, as well as the slightly different EMC of the different wood species veneers, probably have more influence on the smoothness and surface quality.

According to the results of the surface roughness parameter measurements, almost all the areas around the different defects recorded lower surface roughness values compared to typical structure areas, which could be possibly attributed to the different structure, lower porosity, higher density, presence of tensile wood, etc., in the areas around the defects. The smoothness of these surface areas around the defects indicates that the defects increase neither the roughness of surfaces, nor the processing requirements of the veneers, and therefore, do not increase the cost of veneer production. The veneers bearing several defects should not be considered as low-value and useless, but equally valuable, since they can be utilized in a wide range of applications, applying them on the backside of

furniture and structures, or after cutting and removing the area of the defect and substituting it with another one of typical structure, or maintaining the unique appearance of the defect in the structure if possible. These data and findings were obtained through our first experimental attempt in this wide scientific field and the respective preliminary tests conducted in the frame of a project, while further studies will certainly follow from our research team, as well as the research community, in the near future, clarifying the impact of defects on the surface quality of sliced veneers and contributing to the comprehensive understanding of such veneers' final application and the respective manufactured veneer-based structures' and panels' performances. In the future, it is proposed that the effect of a single type of defect on the roughness and surface quality of a single wood species will be thoroughly investigated, examining several different logs.

**Author Contributions:** Conceptualization, E.A.; methodology, E.A. and I.B.; experimental work/measurements, V.K.; resources, E.A.; data curation, V.K.; writing—original draft preparation, V.K.; writing—review and editing, E.A. and I.B.; supervision, E.A. and I.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Utilization of Partially Liquefied Bark for Production of Particleboards**

**Wen Jiang <sup>1</sup> , Stergios Adamopoulos 1,\* , Reza Hosseinpourpia <sup>1</sup> , Jure Žigon <sup>2</sup> , Marko Petriˇc <sup>2</sup> , Milan Šernek <sup>2</sup> and Sergej Medved 2,\***


Received: 13 July 2020; Accepted: 28 July 2020; Published: 30 July 2020

**Abstract:** Bark as a sawmilling residue can be used for producing value-added chemicals and materials. This study investigated the use of partially liquefied bark (PLB) for producing particleboard with or without synthetic adhesives. Maritime pine (*Pinus pinaster* Ait.) bark was partially liquefied in the presence of ethylene glycol and sulfuric acid. Four types of particleboard panels were prepared with a PLB content of 4.7%, 9.1%, 20%, and 33.3%, respectively. Another five types of particleboard panels were manufactured by using similar amounts of PLB and 10 wt.% of melamine–urea–formaldehyde (MUF) adhesives. Characterization of bark and solid residues of PLB was performed by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and automated vapor sorption (AVS). Mechanical and physical properties of the particleboard were tested according to the European standards EN 310 for determining modulus of elasticity and bending strength, EN 317 for determining thickness swelling after immersion in water, and EN 319 for determining internal bond strength. The results showed that the increase in PLB content improved the mechanical strength for the non-MUF boards, and the MUF-bonded boards with up to 20% of PLB met the requirements for interior uses in dry conditions according to EN 312. The non-MUF boards containing 33.3% of PLB and the MUF-bonded boards showed comparable thickness swelling and water absorption levels compared to the reference board.

**Keywords:** bark; bonding; partial liquefaction; MUF adhesives; water vapor sorption; thickness swelling; wood-based panels

#### **1. Introduction**

Particleboard is a panel product made from wood particles, originating from low value wooden raw material (e.g., chips and shavings) or other lignocellulosic materials, bonded by synthetic adhesives and pressed at high pressures and temperatures [1,2]. Particleboard is a low-cost panel product with adequate strength for furniture and interior applications and have been widely applied in flooring, wall and ceilings, flat-pack furniture, cabinets, and work surfaces such as speaker boxes, sewing machine tops, etc. [3,4]. Adhesives are an important element in the wood-based panel industry. Particleboard is traditionally produced with wood adhesives such as urea–formaldehyde (UF), melamine–urea–formaldehyde (MUF), and isocyanate-based adhesives. It is estimated that the adhesives used for particleboard production in Europe are split among UF (92%), MUF (7%), and isocyanates (1%) [5]. All these existing commercial adhesives are petroleum-based, and thus not sustainable [6]. At the same time, the concern about formaldehyde emissions from wood-based

panels, especially in indoor applications, is currently the most important driving factor for wood panel manufacturers to move away from using formaldehyde-based synthetic adhesives [7]. Therefore, the development of natural binders and bio-based adhesives for wood panel production is needed. A common problem for bio-based adhesives is their industrialization, which requires stable qualities and quantities of raw materials and final products. Adhesives based on a variety of natural materials such as starch, proteins, lignin and tannin have been proved to be less reactive than their formaldehyde-based counterparts, and this leads to much longer press times and considerably higher production costs. Thus, the particleboard industry has not yet been able to use natural binders in the production and the penetration of such adhesive systems in the market is rather small [8].

Recently, the utilization of bio-based adhesives from liquefied biomass has received considerable attention. Liquefaction is a method that converts wood and other lignocellulosic biomass into liquids for obtaining oils, chemicals, and other value-added materials [9]. Liquefaction of biomass includes two methods: hydrothermal liquefaction (HTL) and moderate acid-catalyzed liquefaction (MACL). HTL is usually carried out in the water or organic solvents at a temperature of 200–400 ◦C and pressure of 5–20 MPa and MACL takes place at a lower temperature of 120–250 ◦C under atmospheric pressure with the assistance of acid catalysts [9–13]. The primary products are chosen by the different liquefaction methods and the liquefying conditions. HTL produces bio-oil as main products while MACL mainly produces bio-polyol or phenolated compounds depending on the solvents that are used [9]. Liquefied wood (LW) from MACL with polyhydric alcohols and phenols has been used in different adhesive systems, such as polyurethane, UF, MUF, phenol-formaldehyde, and epoxy systems [14–21]. LW has high reactivity with other adhesives precursors and reactive sites due to a large amount of phenolic and alcoholic hydroxyl groups in their compositions [16].

Kunaver et al. [16] produced particleboard from melamine–formaldehyde (MF) or MUF adhesives with added LW, where spruce was liquefied in glycerol–diethylene glycol mixture as a solvent and *p*-toluenesulfonic acid as a catalyst for 3 h at 180 ◦C. The results showed that the addition of 50% LW to the MF and MUF adhesives did not influence the mechanical properties of particleboard but significantly reduced the formaldehyde emissions. Cuk et al. [ ˇ 22] bonded particleboard with MF that was partially substituted by LW. Adhesive mixtures containing 20% of LW had the largest improvements in the mechanical properties of the particleboard compared to the reference boards. Substitution of MF by LW of up to 30% resulted in a comparable mechanical strength to board bonded only with MF, while significantly reducing formaldehyde emissions. LW worked as a plasticizer and increased the mobility of the MF resin molecules. However, the thermal stability of MF substituted by LW was reduced because LW prolonged the curing time of the final adhesive, decreased the cross-linking degree, and accelerated the thermal degradation. Janiszewska et al. [23] produced particleboard with a mixture of UF and LW (ratio 4:1) and investigated the influence of different liquefying solvents, e.g., a mixture of solvents from the polyhydroxy alcohol group, including glycerine, ethylene glycol, propylene glycol, diethylene glycol, and dipropylene glycol, on the chemical structure, physical and mechanical properties of the particleboard. For all liquefying solvents, boards exhibited comparable properties to the ones produced without LW.

Bark, as an industrial residual material, with heterogeneous structure and diverse chemical composition, can be used for the production of a variety of and bio-composites and bio-compounds such as tannin and polyphenols, bio-oil, antioxidants, and bio-based adhesives [24–31]. Particleboard based on larch bark has been reported by Tudor with a much lower formaldehyde emission content than the wood-based panels, which means bark work as formaldehyde scavenger in the particleboard [31]. Limited publications can be found related to the utilization of liquefied bark in wood adhesives and wood-based composites, while, all of them followed a complete liquefaction process. Janiszewska [24] liquefied bark in polyhydric alcohols and *p*-toluenesulfonic acid at 120 ◦C for 2 h, and then prepared three-layer particleboard by using an adhesive mixture of 80% MUF and 20% liquefied bark (LB) with the addition of 1 M NaOH or 25% ammonium hydroxide for neutralizing the adhesive mixtures. The boards made with MUF-LB adhesives had 10–20% lower modulus of elasticity, 22–29% lower

bending strength, and 25% lower tensile strength in comparison with the ones made with commercial MUF adhesives. It was also determined that replacing of MUF with LB led to a slight reduction in formaldehyde content. Lee and Liu [32] prepared particleboard bonded by LB-based resol adhesives, which was prepared from LB of Taiwan Acacia (*Acacia confusa*) and China fir (*Cunninghamia lanceolata*) with two types of catalysts, i.e., sulfuric acid and hydrochloric acid. The results showed that the resol type adhesives prepared from bark, liquefied with sulfuric acid, had a higher viscosity compared to those made from liquefied bark with hydrochloric acid. Adhesives from liquefied China fir had a higher viscosity than those from liquefied Taiwan acacia with the same acid catalyst. The thermal analysis showed that the adhesives based on hydrochloric acid-catalyzed liquefied wood had a higher maximum temperature, a greater height of exothermic peak, and a larger quantity of exothermic heat at thermosetting than the adhesives based on sulfuric acid-catalyzed liquefied wood. Particleboard made with resol adhesives based on liquefied Taiwan acacia bark catalyzed by sulfuric acid had the best mechanical properties and the lowest thickness swelling among all particleboard panels.

A complete moderate-acid catalyzed liquefaction process of biomass in alcohols or phenols takes approximately 90–120 min under constant stirring and heating [9,12]. The obtained compounds require further purification stage to retrieve liquids for applying in adhesive formulations, which is time- and energy-consuming. Therefore, present work explores a novel approach at the production of partially liquefied bark (PLB) and its further utilization in manufacturing particleboard panels. Changes in the chemical characterization structure, thermal stability, and water vapor sorption behavior of bark due to partial liquefaction process were determined by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and automated vapor sorption (AVS) apparatus, respectively on the solid residues of PLB. The effect of PLB on the physical, mechanical, and microscopic structure of particleboard made with or without MUF adhesives were then analyzed. It was hypothesized that bark was partially solvolyzed after liquefaction generating PLB, which is a chemical-activated particle that can be incorporated for better compatibility with wood particles, and can thus enhance the physical and mechanical properties of the particleboard.

#### **2. Materials and Methods**

#### *2.1. Materials*

Bark of maritime pine (*Pinus pinaster* Ait.) was used for liquefaction. Bark was purchased from BVB Substrates (De Lier, The Netherlands), and then milled by a Condux mill CSK 360/N1 (Hanau, Germany) to particles. Chemicals used for liquefaction were ethylene glycol (EG) (Honeywell, Charlotte, NC, USA) as a solvent and 96% sulfuric acid (SA) (KEMIKA d.d., Zagreb, Croatia) as a catalyst. 1,4-dioxane, purchased from Honeywell GmbH (Seelze, Germany) was used for purification after liquefaction. Fresh wood particles from spruce (*Picea* spp.) were collected from a local sawmill in Ljubljana, Slovenia. Melamine–urea–formaldehyde (MUF) adhesives H97 was provided by Melamine Koˇcevje d.d. (Koˇcevje, Slovenia). Ammonium sulfate with 20% solid content was used as an adhesive hardener at a content of 3% in the adhesive formulations.

#### *2.2. Partial Liquefaction Process*

Oven-dried bark particles (103 ◦C for 24 h) together with the solvent and catalyst were added into a three-neck glass reactor submerged in an oil bath equipped with a mechanical stirrer and a water condenser. A weight ratio of 3:1 was used for solvent and bark. The catalyst concentration was 3% (*w*/*w*) based on the solvent mass. The liquefaction process was initiated at 180 ◦C with constant stirring under ambient atmosphere. After 30 min, the liquefaction was stopped by removing the reactor from the oil bath and transferring the liquefied bark to a clean beaker for cooling down to room temperature.

Figure 1 shows the bark, bark mixed with solvent and catalyst before liquefaction, and PLB after liquefaction. PLB (Figure 1c) used for the production of particleboard is a wet material with a high solid content (un-liquefied bark), unreacted EG and SA, and liquefaction liquid intermediates. PLB had a solid content of 41% (oven-dry mass of solids divided by total PLB mass). For further characterization, PLB was oven-dried at 103 ◦C for 24 as the solvent-containing solid residue of PLB. Purified PLB as a solvent-free solid residue of PLB was prepared by first dissolving wet PLB in a mixture solvent of 1,4-dioxane and water at a mass ratio of 4:1. Then the wet PLB and solvent mixture was centrifuged at 1000 rpm for 10 min by removing the residual solvents and the intermediate chemicals. The obtained solids as purified PLB were dried in the oven at 103 ◦C for 24 h. Bark, oven-dried PLB, and purified PLB were milled to powders with a size of 2 mm for subsequent analysis.

**Figure 1.** Bark (**a**), mixture of bark with the solvent and catalyst (**b**), and partially liquefied bark (**c**).

#### *2.3. Particleboard Production*

− Single-layer particleboard panels were manufactured with a target thickness of 8 mm by following standard procedures that simulate industrial production in the laboratory. Wet PLB was mixed with dry wood particles (less than 4% moisture content) at different loading levels of 4.7%, 9.1%, 20%, and 33.3%, and then the corresponding panels were labelled as I, II, III, and IV. It should be mentioned that the PLB content was measured based on the total mass of the mixture of PLB and wood particles. Five panels were manufactured by adding 10 wt.% of MUF adhesives and PLB at a loading level of 0, 4.7%, 9.1%, 20%, and 33.3% to dry wood particles, and these panels were labelled as V, VI, VII, VIII, and IX. For MUF, 3% (*w*/*w* of dry adhesives) of hardener was used. The mats were then manually formed into frame dimensions of 500 × 500 mm<sup>2</sup> . The hot-pressing temperature was set as 190 ◦C. The pressing speed was set to 52 s·mm−<sup>1</sup> (pressing time including closing and opening was 420 s). Such as long-pressing time was needed due to high mat moisture content due to the usage of wet PLB, hence there is a degassing stage in the middle of the pressing schedule (Figure 2). The final density and thickness of the particleboard are shown in Table 1. −

**Figure 2.** Pressing diagram.


**Table 1.** Density and thickness of the manufactured particleboard.

\* as a reference board made from melamine–urea–formaldehyde (MUF) adhesives and wood particles.

#### *2.4. Characterizations*

#### 2.4.1. Fourier Transform Infrared (FTIR) Spectroscopy

The chemical structure of bark, oven-dried PLB, and purified PLB powders was analyzed with a Fourier Transform Infrared Spectrometer (Alpha FTIR spectrometer, Bruker, Karlsruhe, Germany) with a versatile high throughput ZnSe ATR crystal. The FTIR analysis was conducted in a wavelength region from 4000 to 800 cm−<sup>1</sup> at room temperature, accumulating 64 scans with a resolution of 4 cm−<sup>1</sup> .

#### 2.4.2. Thermogravimetric Analysis (TGA)

The thermal stability of bark, oven-dried PLB, and purified PLB powders were analyzed using a NETZSCH STA 409PC instrument (Netzsch, Selb, Germany). Approximately 5 mg of dried samples (24 h at 105 ◦C) were heated from 30 to 800 ◦C at a rate of 10 ◦C/min under a flowing nitrogen atmosphere.

#### 2.4.3. Automated Vapor Sorption (AVS)

The water vapor sorption behavior of bark, oven-dried PLB, and purified PLB powders was determined using an automated vapor sorption (AVS) apparatus (Q5000 SA, TA Instruments, New Castle, DE, USA) as reported previously [33,34]. Approximately 5 mg of grounded samples, passed through a 10-mesh sieve, were exposed to the relative humidity (RH) from 0 to 90% in step sequences of 15% and then continued to reach 95% at a constant temperature of 25 ◦C. The instrument maintained a constant target RH until the mass change in the sample (dm/dt) was less than 0.01% per min over a 10 min period. The equilibrium moisture content (EMC) for each sample was assessed based on their equilibrium weight at each given RH step throughout the adsorption run.

#### 2.4.4. Mechanical Properties of Particleboard

The bending test for measuring the modulus of elasticity (MOE) and modulus of rupture (MOR) of the particleboard panels manufactured with PLB was performed according to EN 310:1993 [35] by using a universal testing machine (Zwick/Roell Z005, Zwick/Roell GmbH, Ulm, Germany). Six samples per board measuring 210 × 25 mm<sup>2</sup> were tested using a span of 160 mm and a cross-head speed of 7 mm min−<sup>1</sup> (time to break was between 45 and 75 s). MOE was determined between 10 and 40% maximum load.

Six samples measuring 210 × 25 mm<sup>2</sup> (width × length) were cut from each panel and tested for their tensile strength parallel to the surface by using a universal testing machine (Zwick/Roell Z005, Ulm, Germany).

The internal bond (IB) strength test was conducted following EN 319:1993 [36]. Six samples per particleboard measuring 50 × 50 mm<sup>2</sup> were bonded with hot-melt glue and the test perpendicular to their surfaces were performed using a universal testing machine (Instron 4466, Darmstadt, Germany). A loading speed of 0.75 mm·min−<sup>1</sup> was used for testing (time to break was between 50 and 70 s).

#### 2.4.5. Thickness Swelling and Water Absorption of Particleboard

Samples measuring 50 × 50 mm<sup>2</sup> were cut from the particleboard and immersed in water at 20 ± 2 ◦C for 2 h and 24 h. Thickness swelling (TS) was evaluated by the difference between the final and initial thickness, and water absorption (WA) was evaluated by the weight difference. Four samples per panel were used for the determinations according to EN 317:1993 [37].

#### 2.4.6. Scanning Electron Microscopy (SEM)

The formed bonds between wood particles, PLB and MUF adhesives were studied on the cross-section of the particleboard with a scanning electron microscope (SEM, FEI Quanta 250, FEI, Hillsboro, OR, USA), equipped with the energy dispersive X-ray spectrometer (EDX, AMETEK Inc., Berwyn, PA, USA). Before observations, the surfaces of the selected area of samples were evened by cutting on a Leica SM2010R microtome (Leica, Wetzlar, Germany). The SEM micrographs were taken with large field detector (LFD) at 100×, 500× and 1000× magnifications in a low vacuum (50 Pa), at a voltage of 5.0 kV, a spot size of 3.0, and a beam transition time of 45 µs.

#### *2.5. Statistical Analysis*

The SPSS version 25.0 statistical software package (IBM Corp., Armonk, NY, USA) was used for the statistical analysis. One-way ANOVA was performed on the mechanical and water-related results for the analysis of variance at a 95% confidence interval (*p* < 0.05). The statistical differences between mean values were assessed by using the Tukey's honestly significant difference (HSD) test.

#### **3. Results and Discussion**

#### *3.1. Characterization of Bark and Solid Residues of PLB*

FTIR spectroscopy analysis detected the chemical structure of bark and solid residues of PLB (oven-dried PLB and purified PLB) (Figure 3). Raw bark showed a strong absorption peak at 3350 cm−<sup>1</sup> , which corresponds to -OH stretch vibration in cellulose, lignin, and hemicelluloses. Bark and oven-dried PLB illustrated two distinct peaks at 2918 and 2845 cm−<sup>1</sup> , which are assigned to -CH stretch vibration in aromatic methoxyl groups and aliphatic methyl and methylene groups [38,39]. The spectra for oven-dried PLB appears similar to that of purified PLB. A slight decrease in the -OH and -CH bonds were observed in oven-dried PLB and purified PLB compared to bark. This might be related to the degradation and dehydration of bark, and the formation of alcohol-soluble intermediates [12,40–42]. A peak representing the C-O linkage of alcohol or ether that are typical polyol products from the complete liquefaction was found at around 1112 cm−<sup>1</sup> in oven-dried PLB and purified PLB (Figure 3). This peak verified the production of polyols through the partial liquefaction [43]. The absorption band at 1030 cm−<sup>1</sup> corresponding to -CO stretching for both PLB and purified PLB was weakened due to the cleavage of β-O-4 bonds of lignin [42]. The broad band between 1266 and 1030 cm−<sup>1</sup> in the spectrum for oven-dried PLB and purified PLB indicates -CO stretching in primary alcohol, secondary alcohol, ethers, and esters. The changes in the absorbance of -CO groups in bark and PLB confirmed that the liquefaction has occurred with the formation of the above intermediates. The intensive vibration at 1727 and 1605 cm−<sup>1</sup> in bark, oven-dried PLB and purified PLB correspond to -C=O in hemicelluloses and lignin, and to -C=C- stretching in lignin, which indicates the high content of lignin remained in the bark structure after partial liquefaction.

Thermal degradation behavior of bark, PLB, and purified PLB was examined by TGA (Figure 4a) and derivative thermogravimetric (DTG, Figure 4b) analyses of the samples after drying in the oven at 105 ◦C for 24 h. There were apparent differences in the thermal degradation pattern of bark after partial liquefaction and purification compared to raw bark. Figure 4a shows the weight loss of the three tested samples up to a maximum of 40–60% of their initial weight. It was previously reported by Yang et al. (2007) that cellulose pyrolysis occurs in a higher temperature range than lignin and hemicelluloses, and lignin is the most difficult polymer to decompose with a solid residue of 45.7 wt.% [44]. As shown

in Figure 4b and Table 2, decomposition of bark (Tonset) initiated at 219 ◦C and reached a maximum mass loss (Tmax1) at 361 ◦C, thus representing typical pyrolysis of lignocellulosic material [45,46]. The respective Tonset of PLB and purified PLB were, respectively, 152 and 148 ◦C and were both lower than that of bark. The decomposition of PLB and purified PLB started earlier than bark, which might be attributed to a large number of hemicelluloses and amorphous part of cellulose that decomposed in the liquefaction due to the effect of the acid catalyst [42]. The decomposition of the intermediate products such as alcohols and esters in the PLB and purified PLB caused a shifted second maximum degradation temperature Tmax2. As shown in Table 2, the Tonset, Tmax1, and Tmax2 of PLB and purified PLB were very close to each other but considerably lower than bark, which can be related to the lower content of volatiles in PLB and purified PLB [47].

**Figure 3.** Fourier transform infrared (FTIR) spectrum of raw bark, oven-dried partially liquefied bark (PLB), and purified PLB.

**Figure 4.** Mass loss (**a**) and first derivative (DTG) (**b**) of bark, PLB, and purified PLB.

**Table 2.** Thermal degradation properties of bark, PLB, and purified PLB.


The water vapor sorption isotherms of bark, PLB and purified PLB are presented in Figure 5a. PLB and purified PLB showed considerably lower EMC than bark in the RH range of 0% to 75%. A strong upward bend was observed in the EMC of PLB and purified PLB from 75% to 95% and surpassed bark at 95% RH. PLB showed an EMC of 33% at 95% RH, while the EMC values of purified PLB and bark at this RH level were 30% and 24%, respectively. Moisture increment (MI) of bark was found to vary little over the entire RH range (Figure 5b), as it was slightly decreased from 15% to 45% RH, then gradually increased from 45% to 90% RH, and then decreased from 90% to 95% RH. PLB and purified PLB, however, showed a different MI trend. The MI of PLB gradually increased with increasing the RH from 15% to 75% and then sharply increased from 75% to 95%. MI was decreased in purified PLB at the RH range of 15% to 60% but then increased greatly after 60% RH. PLB and purified PLB illustrated a more hydrophilic behavior than bark at the higher RH range over 75%. This can be related to more accessible hygroscopic hydroxyl sites after partial liquefaction from the degradation of cellulose due to alcohol hydrolysis catalyzed by strong acids [48,49]. It was reported previously that liquefaction of lignocellulosic materials in polyhydric alcohols initially hydrolyzes the glucoside linkage of the cellulose to produce glucoside monomers, which further decomposed to levulinic acid esters [41,50]. During the liquefaction process, the polymeric structure of lignin was degraded, and the obtained lignin monomers reacted with ethylene glycol to form a condensed lignin-based polymeric material with predominant aromatic hydroxyl groups, which can enhance the hydrophilicity of PLB [51]. The higher moisture sorption of PLB as compared to purified PLB can be attributed to the remaining ethylene glycol that provided extra hydroxyl groups in the PLB. Some part of these hydroxyl groups may have also been removed in purified PLB by the solvent during the purification step.

**Figure 5.** (**a**) Equilibrium moisture content (EMC) of bark, PLB and purified PLB samples exposed to increasing water vapor from relative humidity (RH) of 0 to 95%; (**b**) moisture increment during adsorption of bark, PLB, and purified PLB samples.

#### *3.2. Performance of the Particleboard Containing PLB*

The mechanical properties (i.e., MOE, MOR, IB, and tensile strength) of the particleboard panels containing PLB in the presence of MUF or not are presented in Figure 6. The non-MUF bonded boards containing 4.7% and 9.1% PLB (boards I and II) did not show any cohesion, and, thus, easily decomposed during cutting as illustrated in Figure 7. The MOR, MOE, and tensile strength values of the non-MUF boards were improved by increasing the PLB content. The particleboard containing wood particles and the highest amount of PLB (board IV) exhibited the highest values. A significant increment in the MOE, MOR and tensile strength of the non-MUF boards occurred when the PLB content increased from 9.1% to 20%. At least 20% of PLB was required for the non-MUF panels to be able to perform the IB test. It should be noted that the mechanical property values of all the non-MUF boards (I, II, III, and IV) were very low and did not meet the requirement of particleboard for interior uses in dry conditions according to EN 312:2010.

**Figure 6.** Average MOE (**a**), MOR (**b**), IB (**c**), and tensile strength (**d**) of particleboard containing partially liquefied bark, with or without MUF adhesives. Values labelled with the same letter (small for MUF boards, and capital for non-MUF boards) are not statistically different from each other (ANOVA, Tukey's HSD test, *p* < 0.05). Error bars represent standard deviations.

**Figure 7.** Particleboard samples containing 4.7%, 9.1%, 20%, and 33.3% (I–IV in order) partially liquefied bark without using MUF adhesives.

The mechanical properties of the particleboard were apparently changed by the synergistic effect of MUF adhesives and PLB. The MOE, MOR, and IB strength of the boards bonded with MUF adhesives increased by adding 4.7% and 9.1% PLB (boards VI and VII), and they were significantly higher than the reference one, board V. However, the differences between MOE, MOR, and IB values of the boards with 4.7% and 9.1% of PLB, i.e., boards VI and VII, were not statistically significant. Further, increasing the PLB content to 20% and 33.3% drastically decreases the MOE, MOR, and IB strength of the boards. This might be attributed to the decreasing proportion of the wood particles in the board content, which led to a decrease in the density (as shown in Table 1). The higher PLB amount may have also increased the inhomogeneity of the boards, and thus disturbed the equal distribution of the applied stresses during the mechanical tests. The results of mechanical properties showed that up to 20% of PLB can be used in the particleboard content to produce boards that meet the minimum requirements for interior use in dry conditions according to EN 312:2010, in terms of MOR, MOE, and IB values that are, respectively, 13, 1800, and 0.40 N/mm<sup>2</sup> . The tensile strength parallel to surface was negatively affected by increasing the PLB content in the MUF-bonded boards. When over 20% PLB was applied, the tensile strength of the boards (VIII, IX) was lower than half of the strength of the reference (V). As a conclusion, 9.1% of PLB loading should be allowed for producing particleboard with good overall mechanical properties.

Thickness swelling (TS) and water absorption (WA) of the particleboard after 2 and 24 h immersion in water are shown in Figure 8. The boards I and II exhibited remarkably high TS and WA after 2 h immersion in water, and they were decomposed after 24 h. This can be related to the very low internal bond strength between wood particles and PLB. Low interaction of PLB and wood particles may also be an additional reason, which caused poor interfaces. The increase of the PLB level in the non-MUF panels from 4.7% to 33.3% decreased the TS and WA suggesting that the PLB protected the wood particles against water. Similar trends were observed in TS of the boards made with MUF adhesives. This might be attributed to the increasing amount of compact PLB particles in the particleboard mat, which resulted in a reduction of liquid water penetration into the board. The lowest TS value obtained in the MUF-bonded boards containing the highest amount of PLB (33.3%), which was 3.15% after 2 h and 4.04% after 24 h immersion in water. According to EN 312:2010, the maximum thickness swelling within 24 h of particleboard for non-load bearing applications in humid conditions is 17%. Therefore, the boards produced of the highest amount of PLB (33.3%) without MUF adhesives (panel IV) as well as the panels manufactured with MUF adhesives and 9.1, 20, and 33.3% PLB (panels VII, VIII, and IX) fulfilled the standard requirement. The WA values of the particleboard manufactured with MUF adhesives and 4.7–33.3% PLB (VI, VII, VIII, and IX) were statistically lower than the reference (board V). The current result from TS and WA test indicated that PLB acted as an excellent water-resistant reagent in the particleboard.

The overall results from the mechanical and water-related tests confirmed the hypothesis of this study that PLB particles provided an activated surface that can enhance its compatibility with wood particles. The SEM micrographs provided a visual explanation of the mechanical properties and TS changes due to the incorporation of different PLB levels in the particleboard. In detail, the SEM micrographs displayed that PLB caused a compact region with good interaction with wood particles (Figure 9). The compact PLB provided a less porous structure than wood particles and resulted in a reduction of water penetration in the particleboard. The PLB itself showed a homogeneous compact structure that can facilitate the transfer of stresses from the surface layers through the board [22]. An equal distribution of applied stresses at the PLB-wood particles interfaces could be achieved at a PLB level of 9.1% in the particleboard. Moreover, PLB has an advantage of modifying the surfaces of wood particles with the presence of unreacted solvent, acid and intermediates from partial liquefaction. Pressing of particleboard took place under the same temperature of 180 ◦C as partial liquefaction helps to transfer the liquid phases of PLB, containing remaining polyhydric alcohol and strong acids, to the wood surface for creating chemical bonding and self-adhesion ability. It can be seen from Figure 9b–d that cell walls of wood near the interface between PLB and wood particles are densified that can be caused by the chemical-active components of PLB.

**Figure 8.** Results of thickness swelling and water absorption of particleboard without (**a**,**c**) and with MUF (**b**,**d**) after 2 and 24 h of testing. Values labelled in the same colored letter (small blue for 24 h, and capital red for 2 h) are not statistically different from each other (ANOVA, Tukey's HSD test, *p* < 0.05).

**Figure 9.** Scanning Electron Microscopy (SEM) micrographs showing the interaction of partially liquefied bark (PLB) and wood particles in the particleboard: (**a**) reference; (**b**) with 20% PLB and no MUF; (**c**) with 9.1% PLB and 10% MUF; and (**d**) with 20% PLB and 10% MUF.

#### **4. Conclusions**

Bark as an abundant and easily accessible industrial waste has not been economically and significantly used. The current paper provides a new method for developing bio-based panels from

β

pine bark with the partial liquefaction technique. In this study, pine bark was partially liquefied and then used to produce single-layer particleboard. Four particleboard panels were produced (I–IV) with wood particles and different load levels of PLB (4.7%, 9.1%, 20%, and 33.3%), and five boards (V–IV) were manufactured with wood particles, 10% MUF adhesives and different load levels of PLB (0, 4.7%, 9.1%, 20%, and 33.3%).

Characterization of bark and solid residues of PLB (oven-dried PLB and purified PLB) revealed that the cleavage of the glucoside linkage of cellulose and the β-O-4 bonds of lignin occurred during the partial liquefaction for forming intermediates. PLB and purified PLB decomposed faster than bark during the thermogravimetric analysis because the bark was degraded in the partial liquefaction. PLB and purified PLB were more hydrophilic than bark at higher RHs, meaning that PLB has more hydroxyl groups accessible to water. Excessive hydroxyl groups can be attributed to the bonding between particles and PLB.

Production of particleboard by replacing wood particles with PLB was possible with or without adding MUF adhesives. However, the boards made without MUF adhesives exhibited inferior mechanical properties as compared to the ones with MUF. The mechanical properties and thickness swelling of the non-MUF boards were considerably improved by increasing the content of PLB from 9.1% to 20%. The best mechanical properties were obtained in the particleboard with MUF adhesives and a PLB content of 9.1%. However, up to 20% of PLB content can be used for manufacturing particleboard for meeting the standard requirements. The boards manufactured with 4.7% and 9.1% PLB (I and II) without MUF adhesives showed very poor water resistance, while further addition of PLB at 20% and 33.3% in boards III and IV significantly enhanced the hydrophobicity of the boards. The boards bonded with MUF and different load levels of PLB from 4.7% to 33.3% exhibited significantly lower TS and WA values than the reference board V. The above results indicated that PLB acted as a good water-resistant substance in the particleboard.

**Author Contributions:** Conceptualization, W.J., S.A., M.P., M.Š., and S.M.; methodology, S.M. and R.H.; software, W.J.; validation, W.J. and S.M.; formal analysis, W.J.; investigation, W.J., R.H., and J.Ž.; resources, M.P. and M.Š.; data curation, W.J.; writing—original draft preparation, W.J.; writing—review and editing, S.A., R.H., and J.Ž.; visualization, W.J.; supervision, S.A. and S.M.; project administration, S.A.; funding acquisition, S.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Formas project 942-2016-2 (2017-21) titled "Utilization of renewable biomass and waste materials for production of environmental-friendly, bio-based composites".

**Acknowledgments:** The authors would like to thank Ove Eklund from Ikea Industry AB, Hultsfred, Sweden for his technical support in testing internal bond strength.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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