*2.7. Shear Strength and Shear Modulus*

For shear strength and shear modulus test, OSB, FGB and CPB, as described in point 2.3, were used. 140 mm × 100 mm samples were cut from them. For each type of substrate, three series of samples were prepared—each under different thermal and moisture conditions (Table 4). The high temperature and low relative humidity (25 ± 2 ◦C, 30 ± 5%) and high temperature and high relative humidity (25 ± 2 ◦C, 90 ± 5%) were used, as well as a low temperature (5 ± 2 ◦C) at the resulting RH.

OSB, FGB and CPB boards were stored for 24 h under the conditions programmed for the adhesive application. The adhesive itself was stored for 24 h in laboratory conditions. The adhesive was applied to one of the plates, creating a serpentine pattern. At 180 ± 10 s, the boards were joined together to form sets with distance pieces and screw clamps in which the adhesive bond was 8 ± 1 mm thick. The samples were cured for 48 h, and then

the excess adhesive in the form of spouts was cut off with a knife, forming adhesive bonds with dimensions of 100 mm × 100 mm × 8 mm (Figure 4).


**Table 4.** Conditions for preparation of samples for shear strength and shear modulus test.

<sup>1</sup> resulting (ca. 30 ± 5%).

The shear strength test consisted of determining the maximum stresses with the force acting in the plane of the bond, using the test technique according to EN 12090 [45]. Tests were conducted under laboratory conditions immediately after the samples were cured under specific conditions (Table 4). The test was carried out using a computercontrolled class 1 testing machine (Instron, Darmstadt, Germany), with a constant speed of 3 ± 0.5 mm/min. The shear force value until damage and the force-displacement curve was recorded. In each series, ten samples were tested, and the total was 90 samples. Shear strength *τ* was calculated according to (2), expressed in kPa.

$$
\pi = \frac{F\_{\tau \text{max}}}{A} \tag{2}
$$

where: *Fτmax*—maximum shear force, in kN; *A*—a cross-section of the adhesive bond, in m2.

Shear modulus G was calculated according to (3), expressed in kPa.

$$G = \frac{d \cdot \tan\alpha}{A} \tag{3}$$

where: *d*—adhesive bond thickness, in m; *A*—a cross-section of the adhesive bond, in m2; and *tanα-* tangent of the angle of inclination of the straight-line segment of the curve showing the force-displacement relation, in kN/m.

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

#### *3.1. Scanning Electron Microscopy (SEM)*

The microstructure is one of the most notable factors that may affect polyurethane adhesive foam properties [40]. The structure, especially cell size and type, depends on the process parameters such as temperature, humidity and viscosity of mixture during foaming [40,46,47]. In general, foam's physical and mechanical properties depend not only on the rigidity of the polymer matrix but are also related to the cellular structures [36,40]. The closed cells' size and shape are essential parameters of the foam's cellular structure, directly affecting the polyurethane foam's mechanical properties [36].

The microstructure of the polyurethane adhesive foam was analysed by SEM to determine the effect of simulated use conditions on its morphology. The SEM micrographs showed in Figure 5a,c,d revealed a homogeneous microstructure with a closed cell porosity well distributed within the polyurethane matrix. The adhesive structure in 8 mm thick bonds, at the adhesive application in laboratory conditions (Figure 5a), at high temperature and low humidity (Figure 5c) and at low temperature (Figure 5d) is characterised by relatively homogenous pore size. Single pore cells have regular tetraoval shape and comparable size in all three series. Although cells with a diameter lower than 300 μm prevail, individual cells with 500 μm diameter were observed as well. The adhesive cells applied at high temperature and high humidity were noticeably larger (Figure 5e,f). The predominant cells were about 450 μm and larger in diameter. The above can be attributed to the differences in the foaming conditions of polyurethane. An increase in the substrate's water content intensifies the foaming process, leading to increased cell size [20]. Deformed cell structures were also observed.

The microstructure of the 15 mm thick bond, developed in laboratory conditions (Figure 5b), was also observed. The increase in the bond thickness from 8 mm to 15 mm leads to a loss in the structure homogeneity. The images show a fraction of small pores surrounding individual large cells. Both the cell's size and shape are important for polyurethane foam's mechanical properties [40]. A non-homogeneous structure with inclusions of large pores may significantly decrease the mechanical properties.
