Appearance: colorless or light yellow uniform transparent liquid
Metal content (%): 22±0.2
Oil solubility: fully soluble
Solution stability: uniform without precipitates
Flash point ℃: ≥30
Color number (iron-cobalt colorimetric): ≤4
DIN 55945 defines adhesion strength as: the paint resists the mechanical force of separating from the substrate. This adhesion strength must not only meet the initial adhesion of the coating film after curing, but also consider whether the durability and adhesion strength of the coating film in the environment will be weakened. For example: the absorption of moisture by the coating film and the accumulation of water vapor at the coating film/substrate interface will quickly destroy the adhesion durability. Therefore, in water or high humidity environments, its wet adhesion strength must be able to resist the coating from the substrate. Separate. Another example: the paint around the fuel tank of a transportation tool must have good gasoline resistance. After the paint film has been tested for resistance to gasoline, in addition to no obvious changes, the most important thing is to ensure that the adhesion strength is maximized. Therefore, when we are talking about adhesion, we should not only look at the adhesion of the coating to the substrate after curing, but also consider the adhesion of the coating film on the protected substrate and the environment in which it is attached. Including exposure to humidity, high temperature or low temperature environment, or long-term corrosion of chemicals can still maintain good adhesion.
Adhesion enhancement and interface force
Improved substrate wetting and adhesion
According to W.A. Zisman, only when the surface energy of the coating is lower than or equal to the critical surface tension of the substrate can the adhesion be effectively formed. Table 21-1 shows the critical surface tension of various substrates, comparing the wetting of water-based paints and solvent-based paints [?]. It shows that before we talk about adhesion, we must first ensure that the coating liquid and coating film can wet the substrate or primer. If they cannot be wetted, then adhesion and adhesion cannot be discussed. For example, in Table 21-1, solvent-based coatings cannot wet oily substrates, and water-based coatings cannot wet polyurethane plastic substrates.
Therefore, in theory, as long as the coating can fully wet the surface of the substrate before curing, increase the effective contact area for adhesion and enhance the bonding force between the interfaces, better adhesion can be obtained. The concept of fully wetting the substrate has been introduced. As mentioned earlier, the bonding work W mentioned in the Dupre formula = the sum of the surface energy between solid and liquid-the interface energy can be used to explain that increasing the effective contact area for adhesion can also improve adhesion. As shown in Figure 21-1, for rough substrate surfaces, such as wood, cement walls, wax emulsion for paper, and surfaces that are generally primed first, the coating liquid can penetrate into these rough cavities or pores, and can pass through the contact area. Increase and form a rivet-like mechanical bonding force to firmly connect the coating film and the substrate.
Broadly speaking, all that can increase the physical contact between the substrate and the paint film, enhance the wetting of the substrate, and reduce the surface tension of the liquid film, all have the effect of enhancing adhesion.
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