*2.2. Mineral Wool*

Mineral wool lamella boards code MW-EN 13162-T5-DS(70,-)-DS(70,90)-CS(10\Y)40-TR80- WS-WL(P)-MU1, i.e., with perpendicular tensile strength ≥80 kPa, compression strength ≥40 kPa, and other properties described in Table 1, was used for sample preparation. The apparent density of MW was 73 ÷ 88 kg/m3, with an average of 82 kg/m3. The dimensions of the boards were 1000 mm × 500 mm × 50 mm. They had no coatings or facing in the form of fabric, veil, foil, etc., (Figure 2a), and 200 mm × 200 mm × 50 mm samples were cut from them.

**Figure 2.** Preparation of samples for bond strength tests (**a**) MW prepared for adhesion, (**b**) application of adhesive to MW, and (**c**) bonding.

#### *2.3. Substrates*

The substrates used were boards designed for load-bearing elements that can be used in wet conditions, including wall sheathing for timber frame buildings. Substrates used:


300 mm × 600 mm samples, with their thicknesses maintained, were cut from them. What is more, 50 mm thick concrete slabs, made at a water to cement ratio of 0.45:0.48, perpendicular tensile strength ≥1.5 MPa and moisture content ≤3% of the total weight, complying with EAD 040083-00-0404 [5], were used as a reference substrate.

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

The morphology of the polyurethane bonds' cross-section surface was examined with a field-emission scanning electron microscope (SEM) Sigma 500 VP (Carl Zeiss Microscopy GmbH, Köln, Germany) that renders high-resolution images at low accelerating voltage. The samples were gold-coated before scanning to provide an electrically conductive surface. The accelerating voltage was 10 kV to avoid degradation of the sample. The observations were carried out at 100× magnification.

The study was carried out on a material with 8 mm thick bonds of OSB/23/50/8, OSB/25/30/8, OSB/5/-/8, OSB/25/90/8 and 15 mm OSB/23/50/15 (Table 3). The microstructures of polyurethane bonds were observed on samples cut out perpendicularly to the bonds surface. The samples were cut out with a scalpel at room temperature.


**Table 3.** Conditions of sample preparation for bond strength testing.

<sup>1</sup> resulting (ca. 30 ± 5%).

#### *2.5. Apparent Density*

The test was carried out on cured polyurethane adhesive samples obtained from adhesive bonds made with the OSB described in point 2.3. Five series of 100 mm × 100 mm samples were prepared—in laboratory conditions (23 ± 2 ◦C, 50 ± 5%), at high temperature and low relative humidity (25 ± 2 ◦C, 30 ± 5%), at high temperature and high relative humidity (25 ± 2 ◦C, 90 ± 5%), and at low temperature (5 ± 2 ◦C) and the resulting RH. The OSB were stored for 24 h in the conditions determined for the adhesive application. The adhesive itself was stored for 24 h in laboratory conditions, which resulted from the manufacturer's recommendations on the storage conditions. The adhesive was applied to one of the boards in 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 or 15 ± 1 mm thick (only in laboratory conditions). The sets were cured for 48 h; 50 mm × 50 mm × 6 mm samples were cut out with a knife afterwards. The apparent density of the cured polyurethane adhesive was tested according to ISO 854:2006 [43]. The samples were weighed on analytical scales and measured with Vernier calliper. The apparent density of the polyurethane adhesive was calculated as an average mass/volume ratio.
