Manufacturing and Testing the Panels with a Transverse Texture Obtained from Branches of Norway Spruce (Picea abies L. Karst.)

Abstract

As a result of the imbalances in the forestry market and the increased demand for wood products worldwide, the resource of branches resulting from the exploitation of a forest has attracted special attention from researchers, in order to use these secondary resources judiciously and obtain an added value superior to classic uses. In this context, the current research took into consideration the use of spruce branches to obtain panels with a transverse structure. The work methodology has focused on the process of obtaining panels with a transverse texture and on determining the physical–mechanical properties of the created panels. The results regarding the panel density (determined as a ratio between mass and volume of specimens) showed about 693 kg/m³, static bending resistance parallel to the face of 5.5 N/mm², resistance of adhesion of 5.6 N/mm², shear strength parallel to face of 4.1 N/mm², and screw pull-out resistance perpendicular to the face of 31.3 N/mm², highlighting that the properties were in accordance with the European standards and that the panels obtained were suitable for obtaining furniture products with a special aesthetic aspect. As a general conclusion of the research, it can be stated that spruce branches are a sustainable wood resource with great possibilities to add more value in the form of panels with a traverse texture.

1. Introduction

The lack or insufficiency of forest resources in recent decades has determined the search for new solutions to replace solid wood in the form of timber or logs. The branches obtained from the exploitation of a forest, be they coniferous or deciduous, has been considered for a long period of time as forest biomass [1], with exclusive uses in combustion and in the technology of composite boards made of chips and wood fibers [2,3,4]. In the last four decades, tree branches have been studied more and more [5,6,7,8] in order to better know their properties and implicitly understand their uses [9]. It was found that the branches have the same structure as the trunk wood [10], with the exception of the reaction wood, particularized in compression wood for softwoods or tension in the case of hardwoods, whose properties are slightly different from the trunk wood.

The large amount of wood from the branches of a tree, estimated at about 10% of the total volume of the trunk, shows that when exploiting a forest area, an impressive volume of wood is available, with physical and mechanical properties close to those of the trunk [11,12,13]. These branches must be used judiciously and economically efficiently in order to obtain the greatest possible added value, superior to that obtained in the case of use in the field of wood-based composites.

Regarding the design of a new product, the sustainability of the raw material must be taken into account as well as the fact that the products obtained should be cleaner and greener. In recent years, the importance of environmental aspects has increased [14], which is why the product eco-design has become one of the most important conditions for the development of new products. Some of the criteria mentioned in the research of some authors [15,16,17,18,19,20] are the acquisition of secondary resources such as woody biomass, the choice of materials with a low impact on the environment and the development of products with a high added value. Therefore, through this eco-design (or ecological product design), the design of the new product is brought into line with the environment, both during the product’s functionality and after its removal from use.

Wood-based panels are part of the boards category, including particleboard, fiberboard, oriented strand board (OSB), medium-density fiberboard (MDF), plywood, laminated veneer lumber (LVL), glue laminated products (Glulam), cross laminated timber (CLT), etc. That is why the tests for determining the physical–mechanical properties of wood-based panels are the same as those of the previously mentioned boards. Depending on their field of use of EN 13353: 2022, a wetting treatment via immersion was recommended before performing some tests [21] such as gluing resistance, modulus of rupture (MOR), and modulus of elasticity (MOE) in bending strength (EN 300: 2019 for OSB and EN 312: 2010 for particleboard) (

Table 1. Type and duration of the prior treatment depending on the environment of use.

Type	Conditions of Use Conform EN 13353: 2022	Type of Prior Treatment
T1	SWP/1 Dry medium (interior)	Immersion in water (20 °C, 24 h)
T2	SWP/2 Wet medium	Boiling, 6 h Cooling in water (20 °C, 1 h)
T3	SWP/3 Exterior medium	Boiling, 4 h Drying in oven (60 °C, 16–20 h) Boiling, 4 h Cooling in water (20 °C, 1 h)

).

The analysis of previous research in the field of the theme highlights the fact that they studied the properties of wood from branches. Based on these properties, some future uses are predicted. The use of wood from branches in the field of wood-based composite materials is the most common because they are reconstituted engineering products, which even out the structural inhomogeneity of this wood. Starting from these limitations, the current research aimed to highlight the special structure of the cross-sectional area of the wood from the branches by creating a panel that can be used in small furniture. The objectives of this study are to present the optimal technology for manufacturing panels with a transverse texture obtained from spruce (Picea abies L.) branches and test these panels. The research started from the hypothesis that the structural inhomogeneity of the wood from the branches will be mitigated by gluing the friezes with a suitable adhesive, and the adequate technology of the panel with a traverse structure will remove any cracks that may appear during the use of the finished product.

2. Materials and Method

Technology of manufacturing and plan of panel testing. The manufacture of panels with a transverse texture from branches includes several phases, such as sorting them in order to choose the appropriate branches, cutting the prisms, forming the blocks of prisms by gluing on the edge, cutting the block into transverse friezes, forming a panel from transverse friezes by applying adhesive on the edge and pressing them, panel processing via formatting and calibration, panel packaging via wrapping, and panel storage (Figure 1).

Due to the small dimensions of the 6-dimensional types of panels made of spruce branches (426 × 335 × 20 mm, noted as P1; 455 × 300 × 20 mm noted as P2, 430 × 314 × 20 mm noted as P3, 510 × 308 × 20 mm noted as P4, 465 × 412 × 20 mm noted as P5, and 420 × 394 × 20 mm noted as P6), the application of the adhesive was performed manually with a trowel. The adhesive specific consumption of 280 g/m² of finished panel was used. The panels were glued with two types of cold glue (vinyl and polyurethane Jowapur 687.40 (Rudolf Ostermann GmbH, Bocholt, Germany)).

The preliminary tests carried out for the choice of the adhesive concerned the following mechanical characteristics of the cross-textured panels made of spruce branches: gluing strength, static bending strength, and static bending modulus of elasticity. Regarding the adhesion resistance, the European norm DD CEN/TS 13354:2003 stipulates that before the test [22], the samples should be subjected to a preliminary treatment in cold water immersion t = 20 °C for 24 h, in accordance with the conditions of indoor use [23,24,25,26,27,28,29] defined by EN 1995-1-1:2004 and/or EN 335-2:1992. The static bending strength (MOR) and the modulus of elasticity (MOE) were also tested, according to EN 310:1996.

The establishment of the plan of physical–mechanical tests, to which the panels with transverse texture made of spruce branches were subjected, was carried out on the basis of the European standards and the descriptions from the specialized literature [7,12,13,19].

Table 2. Physical and mechanical tests to which the panels from the branches were subjected.

No.	Name of the Mechanical Test	Justification of the Choice	Norm	Number of Specimens
1.	Dimensions of the panels	It reflects the dimensional accuracy of panel manufacturing	EN 324-1:1993	6
2.	Flatness	The basic technological property of panels	Mantanis et al., 1994 [27]	6
3.	Bonding strength	The basic property of any glued product. It influences most of the mechanical properties	EN 13354:2008	minim 10
4.	Modulus of elasticity and resistance to static bending	Important for horizontal elements in furniture construction (boards, shelves, etc.)	EN 310:1996	6 EN 326-1:1996
5.	Shear resistance in the plane of the plate	Important for vertical or horizontal elements subject to shear (side walls, uprights, crossbars, etc.)	EN 13354:2008	8 EN 326-1:1996
6.	Shear resistance perpendicular to the face plate
7.	The pull-out resistance of the screws	Screw assemblies are frequently used in furniture construction	EN 13446:2004	6
8.	Determination of the swelling coefficient	It determines the stability of structures in various environmental conditions	Mantanis et al., 1994	14

shows the physical–mechanical tests.

Dimensions and flatness of the panels. The tests were carried out in the Manufacturing Precision Testing Laboratory in the Wood Industry, Transylvania University Brasov, Brasov, Romania. The testing methodology was in accordance with the requirements of EN 324-1: 1993. The moisture content of the panels at the time of the test was 9.1%, falling within the requirements of EN 13353: 2022, which makes the following specifications regarding moisture content: 8 ± 2% for use in dry environment, 10 ± 3% for use in a humid environment, and 12 ± 3% for use in an outdoor environment.

The measurements were carried out with an OPTOdesQ Measurement Table (Hecth Electronic AG, Stuttgart, Germany) equipped with a magnetic measurement system on the three axes, X, Y, and Z, with a precision of 0.01 mm. Both programming and data acquisition were performed electronically by means of the Hecht OptodesQ software package. The size measurement scheme according to EN 324–1: 1993 is shown in Figure 2.

The deviation was calculated as the difference between the actual size and the nominal size, expressed in mm with one decimal for the dimensional deviation and three decimals for the flatness deviation. The mean and standard deviation of the results were determined [29], according to EN 326-1 The calculation of the thickness deviation is based on measuring the thickness at the points set by the user and comparing it with the reference thickness g, the machine directly providing the positive or negative differences from it.

The gluing strength of panel with transverse texture. Bonding strength was determined in accordance with the DD CEN/TS 13354:2003 specifications. The tests were carried out on the Universal Testing Machine type ZDM 5t/510 (PPT Group UK Ltd., West Sussex, United Kingdom). The principle of the method consists of pre-treating the specimens according to the service class established according to EN 1995-1-1, and then in determining the maximum compressive shearing force of the bonding surface. Based on the maximum force and the bonding surface, the gluing strength was determined. The method of specimens cutting from the panel, as well as the shape and dimensions of the specimens, are shown in Figure 3. Sixteen specimens were tested.

Since the panels were designed for small pieces of furniture, used in indoor conditions, the samples, after cutting, were measured and then immersed in cold water (20 °C) for 24 h. After they were taken out of the water, they were measured at the same points as before the immersion.

The determination of the length and width of the shear area was made by using a Holzman digital caliper with a measuring range of 150 mm and a precision of 0.01 mm and a Holzman Rotating Digital Dial Gage (Holzmann maschinen GmbH, Haslach, Austria) with a measuring range of 0–12.5 mm and precision of 0.01 mm. Constant application of shearing force was made at a rate that produces maximum force (break) in approximately 60 ± 30 s. The shear strength of the gluing was calculated with the following relation Equation (1).

$$f_v=\frac{F}{l·g}[\mathrm{N}/\mathrm{m}{\mathrm{m}}^2]$$

(1)

where: f_v —the shear strength of the bond, in [N/mm²], F—the maximum force at which the break occurred, in [N], l—the length of the shear area, in [mm], g—the thickness of the specimen, in [mm].

Determination of resistance (MOR) and elasticity modulus (MOE) in static bending. The determination of the modulus of elasticity and the resistance to static bending was carried out according to EN 310:1996. The elastic modulus and static bending strength were calculated using Equation (2).

$${\sigma }_i=\frac{3{·F}_{max}·l_1}{2·b·t^2}\hspace{1em}\hspace{1em}{E}_m=\frac{l_1^3·(F_2-F_1)}{4·b·t^3·(a_2-a_1)}[N/m{m}^2]$$

(2)

where: σ_i—the static bending resistance, in [N/mm²], F_max—the maximum breaking force, in [N], b—width of the specimen, in mm, t—the thickness of the specimen, in [mm], l₁—the distance between the supports, equal to 20 times the thickness of the specimen, E_m—the modulus of elasticity in static bending, in [N/mm²], (F₁ − F₂)—the force increment on the linear portion of force–deformation (F₁ = 0.1·F_max, F₂ = 0.4·F_max), in [N], and (a₂ − a₁) represents the increment of the bending arrow corresponding to the force difference (F₂ − F₁), in [mm].

The number of specimens according to EN 326-1:1996 was six valid pieces. According to the requirements of EN 310:1996, the specimens had the following dimensions: a thickness equal to that of the panel from which it comes, a width of 50 ± 1 mm, and the length equal to 20 times the thickness plus 50 mm [25,29]. In order to study the influence of the joining method on the strength of the specimen, two types of specimens were cut, namely: specimens with a central longitudinal line of gluing (Figure 4a) and randomly cut specimens (Figure 4b).

The laboratory machine for testing wood-based boards and panel—model IB × 600—produced by IMAL-PAL Group (San Damaso, Italy) was used for the bending test. When the test was finished, the software of the IB × 600 machine calculated both the static bending strength and the modulus of elasticity.

Determination of shear strength parallel to the panel face. The test was determined in accordance with the requirements of EN 13354:2008. The compressive strength parallel to the faces was calculated as the ratio between the maximum force and the breaking surface of the specimen Equation (3). The scheme of the test is presented in Figure 5.

$${\tau }_{f_{II}}=\frac{F_{max}}{b·l}$$

(3)

where: τ_fII—the resistance to compression parallel to the faces, in [N/mm²], F_max—the maximum force, in [N], b—width of sample equal with 50, in mm, l—the breaking length of the specimen equal with 50, in mm².

Eight specimens were tested [29], in accordance with the requirements of EN 326-1:1996. The dimensions of the sample were: the thickness equal to that of the panel, width of 50 ± 1 mm, and length of 50 ± 1 mm (Figure 5). A Holzman digital caliper with a measuring range of 150 mm and a precision of 0.01 mm was used to determine the thickness of the specimen. A Holzman Rotating Digital Dial Gage (Holzmann maschinen GmbH, Haslach, Austria) with a measuring range of 0–12.5 mm and a precision of 0.01 mm was used to determine the width and length of the sample area. The tests were carried out on the ZDM 5t/51 Universal Testing Machine. To carry out the test in accordance with the requirements of EN 13354:2008, it was necessary to design and make two test devices. In parallel with the samples obtained from panels with a transverse texture of spruce branches, in order to be able to make a comparison, 6 specimens made of veneered chipboard with a thickness of 20 mm were tested for comparison under the same conditions.

Determination of shear resistance perpendicular to the panel plane. This test was determined in accordance with the requirements of EN 13354:2008. The shear strength parallel to the faces was calculated as the ratio between the maximum force and the breaking surface of the specimen, as in Equation (4). The scheme of determining the shear strength in the plane of the plate is shown in Figure 6.

$${\tau }_{f_{\perp }}=\frac{F_{max}}{2·A}$$

(4)

where: τ_f⊥—shearing resistance to perpendicular plane, in [N/mm²], F_max—maximum force [N], A—breaking surface of sample, as the product of thickness and 46 mm.

Eight samples were tested in accordance with the requirements of EN 326-1:1996. A Holzman Rotating Digital Dial Gage with a measurement range of 0–12.5 mm and a precision of 0.01 mm was used to determine the thickness of the specimen. To determine the width and length of the specimen, a Holzman digital caliper with a measuring range of 150 mm and a precision of 0.01 mm was used.

Before the test, for each sample, the thickness of the specimens was measured at two points in the direction of the shear planes, and the arithmetic mean of the two measurements was made. The tests were carried out on the ZDM 5t/51 Universal Testing Machine. To carry out the test in accordance with the requirements of EN 13354:2008, it was necessary to design and make the two supports and the force application device.

Determination of the pull-out resistance of screws. This test was determined in accordance with the requirements of EN 13446:2004 (Figure 7). The resistance to pulling out the screws was calculated as the ratio between the maximum force required to pull out the screw and the area of the screwing surface, as in Equation (5).

$${\tau }_s=\frac{F_{max}}{d·l_p}$$

(5)

where: τ_s—the resistance to pulling out the screws, in [N/mm²], F_max—the maximum force required to pull out the screws, in [N], d —the diameter of the screw given by the standard, in [mm], and l_p—the screwing length.

Three types of specimens were tested, depending on the structural characteristics of the panel (Figure 8). For each type of specimen, 6 specimens were tested in accordance with the requirements of EN 326-1:1996. The shape and dimensions of the specimens as well as the way of inserting the screws are shown in Figure 8. The screw insertion point can be at the intersection of four joint lines—on a joining line and inside the wood.

The tests were carried out on the FMPW—1000 Traction Machine. For this test, ST 4.2 screws according to ISO 1478 were used, with nominal dimensions of 4.2 × 38 mm and a thread pitch of 1.4 mm [30]. The samples were pre-drilled, the hole diameter being 2.7 ± 1 mm in accordance with EN 320:1993. The penetration depth of the screws is regulated by EN 13446:2004 and for the tested specimens the penetration depth was chosen for the faces—pierced [31,32], and, for the edges, 8 times the diameter, or 21.6 mm.

In the specialized literature, the expression of the resistance to the pulling out of the screws was calculated in accordance with the older norms (EN 13446: 2004), and the expression of the result was communicated in N/mm. In order to be able to compare the experimental data, the calculation of the resistance to the pulling out of the screws was made with Equation (6).

$${\tau }_{s1}=\frac{F_{max}}{h}$$

(6)

where: τ_s1—the pull-out resistance of the screws, in N/mm, F_max—the maximum force, in N, and h—the penetration depth, in [mm].

Determination of swelling in thickness. The principle of the test consisted in measuring the dimensions before and after immersing them for 24 h in cold water (t = 20 °C) and determining the swelling coefficient with Equation (7).

$$\beta =\frac{d_{max}-d_{min}}{d_{min}}·100[\%]$$

(7)

where: β_d—the lineal swelling coefficient, in [%], d_max—the maximum size after immersion, in [mm], and d_min—the minimum size before immersion, in [mm].

To determine the swelling coefficient, the same samples were used for determining the bonding strength. The initial moisture content of the specimens was 9.1% (determined using the gravimeter method EN 322:1993) and the density was 687 kg/m³, determined according to EN 323:1993.

Statistical analysis. All groups of values were processed statistically, obtaining the average of the values, the standard deviation, and the value for a confidence interval of 95% (or the elimination of 5% of the values). This statistical procedure (excluding 5% of the values) was based on a lot of standards and procedures, and has calculated the upper quantile of each group of values according to the arithmetic mean, standard deviation, and Student’s t-distribution with n + 1 degrees of freedom, where n represents the number of attempts of the respective test. The coefficient of variation was also introduced under the name “Variance”, determined with the Microsoft Excel program.

3. Results

3.1. Results Regarding Manufacturing Technology

The specimens from the panels glued with aqueous dispersion polyvinyl acetate (PVC) adhesive after immersion in water became unglued and could no longer be tested [11,12,13]. From this moment, the use of vinyl adhesive was abandoned. The results obtained by applying the polyurethane adhesive were the following: bonding strength of 5.6 N/mm² before immersion and 4.2 N/mm² after immersion, compared to 2.5 N/mm² as the minimum admissible value, a static bending resistance of 5.5 N/mm², compared to 5 N /mm² as the minimum admissible value, and the modulus of elasticity of 1027.3 N/mm², compared to 600 N/mm² as the minimum admissible value. It is observed that the bonding strength was 124.6% higher than the minimum admissible strength provided by EN 13353:2022, the static bending resistance (MOR) was 11.2% higher than the minimum admissible strength provided by EN 13353:2022, and the value of the bending elasticity modulus (MOE) by 71.2% higher than the minimum admissible figure provided by EN 13353: 2022.

3.2. Results Regarding Dimensional and Flatness Deviations

Length, width, and thickness deviations are presented in Figure 9.

In Figure 9, the linear deviations determined according to EN 324-1:1993 and the admissible deviations according to EN 13353: 2022 are compared. According to the data presented, it is found that the dimensional deviations of the panels with a transverse texture made of spruce branches fall within the admissible limits established by EN 324-1.

Table 3. Deviation (D) from flatness of panels with transverse texture obtained from branches.

No.	Panel Number
	Panel P1	Panel P2	Panel P3	Panel P4	Panel P5	Panel P6
Dmax mm	−0.2	0.23	0.05	0.16	−0.05	0.14
Dmin mm	−1.19	−0.09	−0.67	−0.51	−0.61	−0.51
Dmean mm	1.19	0.32	0.72	0.67	0.61	0.65
Variance	0.063	0.031	0.051	0.047	0.045	0.046

shows the deviations from flatness measured with respect to the adjacent plane in the 15 measurement points for each panel.

If both deviations are negative or positive, then the minimum value was taken, and if they are different, their sum was taken. For panels with a length of less than 1000 mm, which are part of the furniture, the admissible deviation from flatness is ±2 mm [12,33], and for boards made of wood chips, the maximum admissible deflection is 1.5–2 mm/m for normally pressed boards and 1.5 mm/m for extruded boards [34]. Eco-panels fit into the requirements stated above (Figure 9, Table 3).

3.3. Bonding Strength Results

The results of the measurements and the interpretation of the results are shown in

Table 4. The gluing resistance of panels.

No.	Dimension MC = 9.1%		Dimensions after Immersion in Water		Maxim Force [N]	Bonding Strength [N/mm2]	Time [s]
	g [mm]	l [mm]	g [mm]	l [mm]
Mean	19.746	25.023	20.040	25.738	2403	4.88	46.7
SD	0.089	0.120	0.104	0.259	120	1.40	2.33
Except 5%	19.707	24.970	19.995	25.624	2360	4.22	45.1
Variance	0.0045	0.0047	0.0051	0.0101	0.0499	0.0286	0.0498
Admissible					Minim 2.5 N/mm2 EN 13353 2022

.

In order to study the influence of moisture content on the gluing resistance, a set of tests was carried out, on a number of 14 specimens, without being previously treated via immersion in cold water; the results of these tests are shown in Figure 10 and are comparative with the water immersion ones.

The gluing resistance of panels with a transverse texture made of spruce branches, determined experimentally according to DD CEN/TS 13354: 2003, falls within the requirements of EN 13353:2022. According to the data from the specialized literature [11,13], the moisture content of the samples almost inversely proportionally influences the bonding strength. In the case of these panels, the gluing resistance after 24-h immersion in cold water (20 °C) was lower by 24.8% than the gluing resistance of the specimens at a 9.1% moisture content. The breakage of the specimens tested at U = 9.1% occurred exclusively in the wood, usually in a different area than the gluing area, and was accompanied by a specific level of noise. For the samples immersed for 24 h in cold water (20 °C), a very high elasticity was observed, and the breaking was performed without noise, usually in the gluing area.

3.4. The Modulus of Elasticity (MOE) and Resistance (MOR) to Static Bending

The results regarding the modulus of elasticity of about 1000 N/mm² and resistance to static bending of about 6.1 N/mm² are shown in

Table 5. Results of bending strength (MOR, MOE, and maximum arrow).

No.	Density [kg/m3]		MOR [N/mm2]		MOE [N/mm2]		Maximum Arrow [mm]
	T1	T2	T1	T2	T1	T2	T1	T2
Mean	693.2	692.3	6.09	6.10	1129.97	1006	7.15	7.58
SD [%]	14.16	3.1	1.12	0.41	102.58	44.17	1.24	0.59
Except 5%	682.8	657.7	5.27	5.79	1054.68	956.1	6.24	7.20
Variance	0.020	0.004	0.018	0.067	0.090	0.043	0.023	0.077

. The values determined for both types of specimens (T1—the gluing line in the middle of the test pieces, and T2—the gluing lines are arranged randomly on the face of the samples) fell within the requirements of EN 13353: 2022, which regulate the minimum admissible values of a modulus of elasticity over 600 N/mm² and resistance to static bending for solid wood panels over 5 N/mm². A maximum arrow of 7.58 mm was obtained. The samples were broken, producing a characteristic noise of solid wood. The breaking plan was after the tissues of minimum resistance, namely after the medullary rays and separations on the annual ring in the early wood area, meaning that the gluing strength was greater than that of the wood from the branches.

Regarding the influence of density on the static bending resistance (MOR), it could be observed that the resistance to static bending increased with increasing density up to a certain value of 705 kg/m³, then decreases (Figure 11a). The correlation between density and static bending strength is given using a polynomial equation of the second degree, having the density as the parameter and the resistance (MOR) as the dependent variable.

The correlation between density and the modulus of elasticity (MOE) in static bending is given by a second degree polynomial equation with a concave shape (Figure 11b) and with a minimal value of 685 kg/m³. The correlation between density and the maximum deflection in static bending is given by a second degree polynomial equation with a convex shape (Figure 11c) and with a maximal peak of 700 kg/m³.

3.5. Shear Resistance Parallel to the Panel Plane

The results of the tests on shear resistance parallel to the panel plane are shown in

Table 6. Shear resistance parallel to panel face (SD—standard deviation).

No.	Length, [mm]	Width, [mm]	Force, [N]	Shear Resistance Parallel to Face, [N/mm2]	Time, [s]
Mean	50.443	50.426	10541.25	4.141	46.212
SD	0.257	0.293	2036.04	0.781	16.472

. There were obtained an average resistance of 4.1 N/mm² and a 46.2 s time of breaking.

Shear strength parallel to the panel plane for other materials; for the 20-mm thick veneered chipboard, it was 1.17 N/mm², for the 20-mm non-veneered chipboard, it was 1.8 N/mm², and for the 16-mm thick high-density fiberboard, it was 1.5 N/mm². These values were inferior to cross-structured panels obtained from spruce branches (3.2 N/mm²). In the case of panels with a cross-section made of spruce branches, shear failure did not occur, as in the case of the chipboard specimens, because the compression with splitting occurred in the upper third of the specimen due to the tangential and normal unit stresses at the surface on the shear plane. The height at which the splitting had occurred varies between 3 and 18 mm, which represents between 6% and 36% of the height of the panel specimen.

3.6. Shear Resistance Perpendicular to the Panel Plane

The results of the tests regarding the shear strength in the plane of the panels are shown in

Table 7. Perpendicular-to-plate shear strength of cross-textured panels (SD—standard deviation).

No.	Thickness 1 [mm]	Thickness 2 [mm]	Mean Thickness	Maximum Force [N]	Shearing Resistance [N/mm2]	Breaking Time, [s]
Mean	19.788	19.782	19.785	2240	1.886	33.97
SD	0.049	0.051	0.046	488.6	0.411	2.374
Variance	0.002	0.002	0.0023	0.0217	0.0212	0.067

. There obtained are an average value of resistance of 1.886 N/mm² and a breaking time of 33.97 s. It can be seen that the breaking time is shorter than in the previous test, due to the fact that the resistance is also lower.

The breakage of the specimens started from the corner of the cuts, advancing in the direction of the medullary rays or in the direction of the annual ring through the early wood zone.

3.7. Results Regarding the Pull-Out Resistance of the Screws

The results obtained after the tests to pull out the screws allowed the formulation of the following conclusions. For screws inserted perpendicular to the plane of the panels, the best resistance to pulling out the screws is given by the situation in which the screw is inserted on a single joint line (type B samples), followed by the situation in which the screw is inserted at the intersection of four joint lines (type A samples), and lastly the situation where the screw was inserted into the wood (type C samples) (

Table 8. Screw pull-out resistance parallel to the face panels (SD—standard deviation).

Sample Type	No.	Screw Pull-Out, Parallel to the Panel Face
		lp, [mm]	d [mm]	Force, [N]	Time, [s]	τs [N/mm2]	τs1 [N/mm]
Type A	Mean	19.707	4.2	2624	6.308	31.704	133.158
	SD	0.071		347.4	0.932	4.189	17.596
Type B	Mean	19.675		2701	5.43	32.684	137.275
	SD	0.0302		261.1	1.445	3.168	13.306
Type C	Mean	19.675		2439	9.595	29.520	123.984
	SD	0.030		617.4	3.066	7.486	31.444
Total	Mean	19.685		2588	7.111	31.303	131.473
	SD	0.047		424.8	2.653	5.143	21.601

). The pulling out of the screws inserted perpendicular to the plane of the panel was performed silently, with the extraction of the wooden thread formed by the screw, usually without breaking the test piece, and only chips and sawdust in the area of the thread.

For screws inserted parallel to the plane of the panel (on edge), the highest resistance to pulling out the screws is given by the situation where the screw was inserted into the wood but also passes through a joint line (type B samples) followed by the situation where the screw was inserted on four joint lines (type A samples), and lastly the situation where the screw was inserted into the wood (type C samples).

The difference between the pull-out strength of the screws on the different edges was 30.7 N/mm² for edge 1 and type A, 33.1 N/mm² for type C, 22.9 N/mm² for edge 2 type A, and 28.5 N/mm² for type C. For the identical test cases (specimens of type A and C), it was found that the pull-out resistance of the screws from edge 2 was lower than edge 1 by 25.3% in the case of type A and 13.7% in the case of type C specimens (

Table 9. The pulling out resistance of the screws on the edge of panel (SD—standard deviation).

Sample Type	No.	Edge 1		Edge 2
		τs [N/mm2]	τs1 [N/mm]	F [N]	t [s]	τs [N/mm2]	τs1 [N/mm]
Type B	Mean	27.111	113.888	2482	5.442	27.358	114.907
	SD	14.942	62.758	474.462	4.01	10.335	43.410
Type C	Mean	33.090	115.817	962.001	9.223	28.549	119.907
	SD	5.989	25.154	474.462	0.992	4.134	17.365
Total	Mean	30.327	119.655	1843.333	7.332	26.300	105.001
	SD	9.926	46.306	665.001	3.072	7.330	30.218

). This difference was caused by the weakening of the strength of the test piece during the pulling out of the screw from the edge tested firstly.

3.8. Coefficient of Swelling on the Thickness and Width of the Panels

The swelling coefficient values were tabulated in

Table 10. The swelling coefficient after immersion (SD—standard deviation).

No.	Dimension MC = 9.1%		Dimension after Immersion in Water		Swelling Coefficient on Thickness, [%]	Swelling Coefficient on Width, [%]
	g, [mm]	l, [mm]	g, [mm]	l, [mm]
Mean	19.74	25.023	20.04	25.738	1.487	2.858
SD [%]	0.089	0.12	0.104	0.259	0.351	1.028
Except 5%	19.707	24.97	19.995	25.624	1.334	2.407
Variance	0.0045	0.0048	0.0052	0.0097	0.0237	0.0357

.

The value of the swelling coefficient in thickness, i.e., swelling along the fiber, was 1.48%, and the value of the swelling coefficient in width, i.e., tangential swelling, was 2.85%. The coefficients of swelling in thickness after immersion in cold water for 24 h, for the other materials, apart from traverse-structured panels [33,34,35], were limited to 12% for the chipboard, 12% for the medium-density fiberboard (MDF), 15% for the oriented strand board (OSB), 8% for the plywood, 12% for the softwood as spruce, and only 1.4% and 2.8% (on thickness and width) for the traverse-structured panel from spruce branches.

These values obtained experimentally are due to the high content of compression wood (33–66%), and they support the theory from the specialized literature [12,13], according to which compression wood has a high and irregular longitudinal contraction coefficient and a tangential shrinkage coefficient smaller than normal wood. Thus, the ratio of the two types of swelling was, in the case of the panel with the transverse structure of branches, only 1:2, in relation to about 1:20 for the solid wood [35,36].

4. Discussion

Table 11. Synthesis of the physical–mechanical and technological properties of panels with a transverse texture obtained from spruce branches (SD—standard deviation).

No.	Characteristics	Methodology According to which the Test was Carried Out	Value		SD	UM
1.	Deviation from nominal length	EN 324-1:1993; EN 324-2:1993; EN 13353:2022	Mean	0.03	0.001	mm
			Admissible	±2	-
2.	Deviation from the nominal width	EN 324-1:1993; EN 324-2:1993; EN 13353:2022	Mean	−0.62	−0.003	mm
			Admissible	±2	-
3.	Deviation from the nominal thickness	EN 324-1:1993; EN 324-2:1993; EN 13353:2022	Mean	0.47	0.023	mm
			Admissible	±1	-
4.	Deviation from flatness	[27]	Mean	1.304	0.065	mm
			Admissible	2	-
5.	Apparent density at MC = 9.1%	EN 323:1993	Mean	687.05	14.1	kg/m3
			Admissible	-	-
6.	Internal bond resistance at MC = 9.1%	EN 13354:2008	Mean	5.61	0.25	N/mm2
			Admissible	2.5	-
7.	Bonding strength after 24-h immersion	EN 13354:2008; EN 13353:2022	Mean	4.22	0.21	N/mm2
			Admissible	2.5
8.	Modulus of elasticity (MOE)	EN 310:1996; EN 13353:2022	Mean	1027.38	102.5	N/mm2
			Admissible	600	-
9.	Static bending resistance (MOR)	EN 310:1996 EN 13353:2022	Mean	5.57	1.12	N/mm2
			Admissible	5	-
10.	Shear resistance parallel to the panel plane	EN 13354:2008	Mean	4.14	0.78	N/mm2
			Admissible	-	-
11.	Shear resistance perpendicular	EN 13354:2008	Mean	1.88	0.41	N/mm2
			Admissible	-	-
12.	Pull-out resistance of screws perpendicular	EN 13446:2004	Mean	31.30	5.14
			Admissible	-	-	N/mm2
13.	Pull-out strength of screws parallel	EN 13446:2004	Mean	30.82	9.9	N/mm2
			Admissible	-	-
14.	Coefficient of swelling in thickness	[12,36]	Mean	1.33	0.35	%
			Admissible	-	-
15.	Coefficient of swelling in width	[12,36]	Mean	2.40	1.00	%
			Admissible	-	-

summarizes the tests to which the cross-textured panels from spruce branches were subjected, the results obtained, the admissible values, the test methodology, as well as the norm that regulates the admissible value. These values are used for comparing them with the results of authors and limitative values of European standards.

All dimensional and flatness deviations fell within the limits provided by the standard [37,38] or expressed by other authors and research in this field. For example, the dimensional deviation in thickness for panel number 2 was 0.061 mm compared to the upper limit of 2 mm according to EN 324-1:1993 and EN 13353:2022, which meant a deviation of the value of 32.7 times.

The apparent density of the panels corresponded to the species used (spruce, with an average wood density of the trunk of 425 kg/m³ and of 510 kg/m³ for the branches [12,36], due to the presence of compression wood), but it also increased significantly due to the Jowapur adhesive used and the pressure force used during the formation of the friezes and panels.

The average value for the resistance of the gluing for the panel made of branches of 5.61 N/mm² not only corresponded to the limiting value of the European standard (EN 13353: 2022) of 2.5 N/mm², but also had an increase of 141.3%. Additionally, these values were in accordance with those expressed by other authors [12,13].

The static bending strength of the traverse-structured panel was 10.71% higher than the minimum admissible value recommended by EN 13353:2022, 19.65% higher than the static bending strength of the chipboard with a 20-mm thickness, and 6.6% lower than the static bending strength of the high-density fiberboard (HDF) [33,34]. Regarding the influence of the cutting method of the samples from the panel on the strength and modulus of elasticity in static bending, the followings were observed.

The shear resistance parallel to the board plane was superior to wood-based boards [36], being higher by 56.53–71.65% than the shear resistance parallel to the board plane of the 20-mm thick chipboard, and with 63.77% higher than the shear strength parallel to the plate plane of the high-density fiberboard. The shear resistance perpendicular to the plane of the board was superior to boards based on fibers and wood chips. This was 78.8% higher than the shear resistance perpendicular to the plane of the chipboard with the 20-mm thickness and the same fiberboard [12].

Pulling out the screws inserted parallel to the plane of the panel was performed with noise, without extracting the wooden thread formed by the screw, and usually caused the specimen to break. When screws were inserted perpendicular to the panels, specimen type A had a resistance of 31.7 N/mm² and a breaking time of 6.3 s, specimen type B had 32.6 N/mm² resistance and breaking time of 5.4 s, and specimen type C had a resistance of 29.5 N/mm² and a breaking time of 9.5 s. The strength and pull-out time of the screws for the three cases of screw insertion parallel to the panel plane were for edge 2 and type A, 22.9 N/mm² and 5.5 s, for type B, 27.3 N/mm² and 5.4 s, and for type C, 28.5 N/mm² and 9 s. Screw strength determined according to the old method had different values for all composite materials. The values were 133.1 N/mm for panel type A on the face, 137.2 N/mm for panel type B on the front, and 123.9 N/mm for panel type C on the front, and were comparable with that of 80 N/mm for solid wood with a density of 600 kg/m³, of 100 N/mm for solid wood with a density of 650 kg/m³, and of 150 N/mm for solid wood with a density of 700 kg/m³ [13,36]. By comparing the data obtained with those from the specialized literature regarding the resistance to pulling out screws [35,36], it was concluded that cross-structured panels fall within the requirements imposed by the European norms [35,36].

Values for screw pull-out resistance for various types of materials [33] are: for screws on the plane of the plate (on the edge) with a chipboard density of 600 kg/m³, it was 75 N/mm, for a solid wood density of 500 kg/m³, it was 110 N/mm, and for a solid wood density of 700 kg/m³, it was 170 N/mm, compared to traverse-structured panels that had 280.1 N/mm for panel type A and edge 1, 114.9 N/mm for panel type B and edge 2, and 119.9 N/mm for panel type C and edge 2.

The pull-out force of the screw inserted perpendicular to the panel was quite different compared to the parallel-inserted ones. Comparative to the resistance of a medium-density fiberboard of 1000 N [33], a different force of 2692 N was obtained for panel type A front, 2701 N for the panel type B front, 2439 N for the type C front panel.

Referring to the coefficient of swelling in thickness and width, the fact that at the initial moisture content of 9.1% after immersion in water for 24 h resulted in a swelling coefficient of 1.33%, the theory according to which the wood from the branches has moisture saturation point of the fiber (FSP) approximative 9% [11,12,13] was disproved, despite the fact that the wood in the trunk has a fiber saturation point of around 30%. According to this theory, the panels should not have swelled over 9%, because the swelling and shrinking phenomena occur only in the moisture content range under the fiber saturation point (FSP).

The uses of this type of traverse-structured panel could be various, but it is recommended to use it only in indoor conditions. Even in indoor conditions, the designed panels can be used for small furniture products with dimensions up to 1–1.2 m. These types of panels were used for several types of small furniture, such as tables, coffee tables, desks, etc. (Figure 12).

5. Conclusions

The judicious use of spruce branches in the form of wooden panels leads to obtaining a new base of raw material for the furniture industry.

Highlighting the transverse structure of the spruce wood branches, this technology is considered a method to be taken into consideration by the producers of art furniture.

Even if the technology for obtaining these wooden panels with a transverse texture is not yet patented, this remains a superior valorization solution for the branches, and its viability remains to be demonstrated in the coming years.

Following all tests of research, the panels with a transverse texture from spruce branches could be considered one of the best methods for the high valorization of spruce branches.