# Effectiveness of Bonding Steel Elements with Polyester-Coated Paint
## 2 Materials and Methods
### Adhesive Joints Technology  
The process of preparing the adhesive joints included the following stages: (i) the preparation of the adherends' surface, (ii) the preparation of the adhesive (in the case of the epoxy adhesive), (iii) the application of the adhesive, (iv) the creation of one-lap adhesive joints, and (v) curing.  
1. In the case of the reference samples, prior to applying the paint coating, the surfaces of the samples were degreased with technical acetone (Aned, Nowa Sucha, Poland) to remove any impurities.
2. In the case of the two-component epoxy adhesive, it was necessary to carry out the adhesive preparation step. For this purpose, the appropriate amounts of adhesive components (epoxy resin and polyamide curing agent) were measured using a TP-2/1 balance (Fawag, Lublin, Poland), and then the epoxy resin and the amide curing agent were mixed in a stoichiometric resin/curing agent ratio of 100:80 (part by weight). In order to mix the ingredients, a plate mixer and a polymer cup were used using the special adhesive mixing station. The mixing speed was 460 rpm, and the mixing time was 2 min. After mixing the adhesive components, the adhesive was degassed at a special station equipped with a container and a vacuum pump for 2 min. Regarding the polyurethane adhesive, there was no need to carry out the adhesive preparation step.
3. After preparing the epoxy adhesive, a layer of the adhesive was applied using a polymeric spatula. The same operation was carried out for the polyurethane adhesive.
4. After the surface and epoxy adhesive were prepared and the individual adhesives were applied, one-lap adhesive joints were created.
5. The curing process was carried out in one step at ambient temperature under a load of 0.02 MPa, and the curing time was 7 days. The thickness of the adhesive layer was controlled by pressing and using a special holder for fixing and pressing the joined samples. According to the information presented in [44], one-part polyurethaneadhesives can react with moisture to polymerize and cure. Since the curing of a one-component adhesive and moisture-curing polyurethanes depends on the diffusion of moisture through the polymer, the maximum depth of cure that can be achieved in a reasonable amount of time is limited to nearly 9.5 mm. Taking this fact into account, they assumed the same thickness of the adhesive joint as in the case of the epoxy adhesive in order to ensure that the curing process took place over the entire thickness of the adhesive joint.  
The treatment of the surfaces, the epoxy adhesive preparation process, and the curing of the adhesive joints (using two types of adhesive) were conducted at ambient temperatures and a air humidity of 33-35%.  
A summary of the number of adhesive joints made with various types of steel samples with paint coating and adhesives is presented in Table 3, and a few examples of the adhesive joints are shown in Figure 3. Comparative samples in the tests were reference samples that did not contain the paint coating; they were only degreased with acetone (as mentioned at the beginning of Section 2.5).