**5. Conclusions**

The analysis of the CTAs adhesion results performed by 19 laboratories participating in 2 consecutive ILC/PT editions showed that the ILC/PT objective was achieved. Laboratories participating in two successive editions maintain a similar level of quality of performed measurements. In the case of the initial adhesion strength measurement in the eleventh edition of the ILC, 89.5% of laboratories obtained the result "satisfactory" under the ISO 13528 criteria. A year later, the result was slightly better—94.7% of laboratories received a satisfactory result. Regarding the measurement of tensile adhesion strength after water immersion, all 19 laboratories were classified as "satisfactory" in the eleventh edition. A year later, one of the laboratories obtained results classified as "questionable". The analysis of the results in the field of the predominant mode of failure showed more significant differences. The goal has been achieved from the perspective of the purpose of the ILC/PF. However, the results obtained from the perspective of the manufacturer whose product is being assessed by the supervisory authorities are not satisfactory. The differences between the results obtained between individual laboratories are significant.

The manufacturer of the product, knowing the acceptance criteria and being aware of the "imperfections" of the measurement method, must take all of that into account in their risk analysis. They must add to the value of the acceptance criterion all the possible variations, including the MU resulting from the measurement method. In their risk analysis, they must assume a variant in which all potential volatilities coincide. The ILC/PT results analyzed in this study indicate that the measurement method, with its imperfections, is an integral part of the final value of the acceptance criterion. In a situation where, when assessing a product by market surveillance authorities, the simple acceptance rule is applied, which does not take into account the variability resulting from the MU, the risk analysis must additionally take this circumstance into account.

The obtained ILC/PT results, although they are satisfactory in terms of the requirements for this type when assessing the competence of measurement laboratories, indicate significant imperfections of the measurement method. At the same time, the obtained results signal to the product manufacturer that they must pay special attention when determining the constancy of performance of their product. Otherwise, it can happen that their product will not meet the acceptance criteria (threshold value).

The results obtained in ILC/PT also indicate that the authors of EN 12004 may analyze possible revision of the standard. In the light of the results described in this study, it seems that it would be reasonable to introduce the obligatory requirement to determine the measurement uncertainty by providing the tensile adhesion strength result in EN 12004. Moreover, the provision that due to the specificity of the tensile adhesion strength measurements, it seems inappropriate for the assessment of CTAs application of the simple acceptance rule.

**Author Contributions:** Conceptualization, C.S. and J.M.; methodology, C.S.; validation, C.S.; formal analysis, C.S. and J.M.; investigation, C.S. and J.M.; resources, C.S.; data curation, C.S.; writing original draft preparation, J.M.; writing—review and editing, C.S. and J.M.; visualization, C.S. and J.M.; supervision, C.S. and J.M.; project administration, J.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** J.M. was partially supported by the Smart Growth Operational Programme 2014–2020 (Project number POIR.02.01.00-00-0350/16).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.

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

#### **References**

