*2.2. Methods*

2.2.1. Preparation of Test Specimens

The test specimens consisted of a base plate, adhesive and a metal target of a circular shape with a bonding area of 2500 mm<sup>2</sup> for the tensile tests and a metal strip of <sup>100</sup> × 25 mm with a bonding area of 625 mm2 for the shear tests. The base plate consisted of a layer that corresponded to the actual composition used in facades with contact insulation. The soft and easily deformable thermal insulator was replaced by a solid cement-bonded particleboard for laboratory purposes. The intention was to measure only the deformation of the tested adhesive without the undesired influence of any deformation of the thermal insulation. It was assumed that the adhesive would be torn off either in the plaster or in the screed. The cement-bonded particleboards were cut into 100 × 100 mm squares from a production size of 3350 × 1250 mm, and then cement screed with a thickness of 3 mm was applied to them in two layers. A fibreglass fabric was fitted between the first and second layers of the cement screed. After the screed had cured the surface was coated with a primer, and a final 1.5 mm thick layer of acrylic plaster, was applied. The composition of the test specimen for the tensile test is shown in Figure 1, and in Figure 2 the test specimen for the shear test can be seen. Close attention was paid to the production of the bonded joint, where a constant adhesive thickness of 3 mm had to be maintained. Inaccuracies of this basic parameter would adversely affect the measurement results [11].

**Figure 1.** Test specimen for tensile test.

**Figure 2.** Test specimen for shear test.

### 2.2.2. Adhesion Tests

The tests were carried out on a LaborTech E.2 measuring device fitted with L06 vice jaws under normal laboratory conditions. The adhesion tests included both tensile and shear stresses. The basis of both the chosen test methods was the recording of the force course required to tear the adhered metal component of the substrate as a function of the elongation of the adhesive under testing. The loading was carried out at a rate of 5 mm per minute until failure, defined by a drop in force to 5% of the maximum achieved force for the tensile test and 30% for the shear test. The most important values for the evaluation were the maximum achieved force *F* (in N) and the corresponding elongation Δ*l* (in mm). The tensile test was based on the procedure defined by the Czech technical standard CSN ˇ 732,577 [12]. The principle of the test was to measure the force required to break away from the adhering body by tension applied perpendicular to the substrate. The tensile strength *σadh* (in MPa) was subsequently calculated according to the following relation:

$$
\sigma\_{\text{all}} = \frac{F}{A} = \frac{F}{2500} \tag{1}
$$

The shear test was based on the procedure defined by the European technical standard EN 1465 [13]. The principle of the test was to measure the tensile force acting parallel to the surface of the bonded joint. The shear adhesion (in MPa) was calculated according to the formula:

$$
\pi = \frac{F}{A} = \frac{F}{625} \tag{2}
$$

The elongation (in %) was subsequently calculated from the increment of the length Δ*l* (in mm) to the original length *l* (in mm):

$$
\varepsilon = \frac{\Delta l}{l} \ast 100 \tag{3}
$$

#### **3. Results**

The results were used to generate a number of charts showing the dependence of the elongation Δ*l* on the applied force *F*. An example of a chart for the tensile test is shown in Figure 3, and an example of a chart for the shear test is in Figure 4.

**Figure 3.** Example of tensile test results for measuring adhesive deformation on aluminium targets.

**Figure 4.** Example of shear test results for measuring adhesive deformation on galvanised strips.

The data from the deformation diagrams were tabulated in a clear way, with the decisive point for the evaluation being the maximum tensile or shear force and the corresponding elongation. Based on this data, the tensile adhesion *σadh* was calculated according to Formula (1), the shear adhesion τ was calculated according to Formula (2) and the elongation ε for both tensile and shear adhesion was calculated according to Formula (3). Statistical evaluation was performed for all the calculated parameters in the form of calculating the standard deviation sx and the coefficient of variation vx. The tensile test results can be seen in Table 1, and the shear test results are presented in Table 2.

**Table 1.** Results of tensile adhesion *σadh* (in MPa) and elongation ε (in %), standard deviation sx (in MPa for σadh, in % for ε) and coefficient of variation vx (in %) <sup>1</sup> for aluminium alloy (Al), galvanised steel (FeZn) and copper alloy (Cu) targets compared to results of adhesive alone (AD) on aluminium targets.


<sup>1</sup> The average values are a sum of six measurements conducted for each tested combination.

A comparison of the adhesion and elongation in both tests can be seen in Figure 5. Possible types of destruction are shown in Figure 6. The most common type of destruction in the adhesion tests was the tearing of the metal target or strip, including the adhesive from the plaster surface. The second most common type of destruction was the tearing of the plaster, including the covering layer of the screed to the layer of the reinforcing fibreglass fabric. The least common destruction type presented only in copper strips in the shear test was the peeling of the adhesive from the copper material.

**Table 2.** Results of shear adhesion τ (in MPa) and elongation ε (in %), standard deviation sx (in MPa for τ, in % for ε) and coefficient of variation vx (in %) <sup>1</sup> for aluminium alloy (Al), galvanised steel (FeZn) and copper alloy (Cu) targets compared with the results of adhesive alone (AD) on aluminium strips.


<sup>1</sup> The average values are a sum of six measurements conducted for each tested combination.

**Figure 5.** Comparison of test results: (**a**) *σadh* (in MPa) for tensile test (blue) and τ (in MPa) for shear test (orange); (**b**) ε (in %) for tensile test (blue) and shear test (orange).

**Figure 6.** Possible types of destruction: (**a**) tearing of the metal strip including the adhesive from the plaster surface; (**b**) tearing of the plaster including the covering layer of the screed from the layer of reinforcing fibreglass fabric; (**c**) peeling off the adhesive from the copper strip.
