1.What are the experimental methods applicable to all types of color coated coils?
The cross-cut test is to cut the coating to the substrate and observe the degree of coating shedding to determine the adhesion level. The steps and evaluation criteria are as follows:
Test tools
Single-blade or multi-blade scribing tool (blade spacing is 1mm, 2mm, and 3mm, selected according to coating thickness: 1mm for coating thickness ≤60μm, 2mm for 60μm-120μm, and 3mm for >120μm);
Soft brush (to remove cutting debris);
3M 600 or equivalent tape (stable adhesion, width 25mm).
Test steps
Draw a 10mm×10mm square grid on the surface of the color-coated coil. The grid lines must be even and vertical, and the depth must penetrate the coating to reach the substrate;
Use a brush to brush the grid diagonally 5 times to remove debris;
Stick the tape flat on the grid, press with your fingers to ensure there are no bubbles, let it stand for 90 seconds, and then quickly tear off the tape at an angle of 60° to the surface;
Observe the shedding of the coating inside the grid.
Rating level (0-5, the higher the level, the worse the adhesion)
Level 0: The cutting edge is completely smooth, without any coating shedding;
Level 1: A small amount of coating shedding occurs at the cutting intersection, with a shedding area of ≤5%;
Level 2: Coating shedding occurs at the cutting edge or intersection, with a shedding area of 5%-15%;
Level 3: Part of the coating shedding occurs at the cutting edge, or coating shedding occurs in the grid, with an area of 15%-35%;
Level 4: A large amount of coating shedding occurs at the cutting edge, or coating shedding occurs in the grid, with an area of 35%-65%;
Level 5: Shedding area > 65%, or coating shedding occurs over a large area.
2.What experimental methods are suitable for thin or flexible coatings?
The scratch test uses a circular scratching knife to scratch a circle on the coating surface and evaluates the coating shedding on the inner side of the circle. It is mainly used in scenarios that require higher flexibility (such as color-coated coils for processing and molding).
Test tools
Circle adhesion tester;
Magnifying glass.
Test steps
Fix the sample, place the scratching needle perpendicular to the surface, apply a specified load, and scratch a circle with a diameter of about 7.5mm at a speed of 20mm/s. The scratch must penetrate the coating to the substrate;
Observe the peeling of the coating inside the circle.
Rating level (1-7, the lower the level, the better the adhesion)
Level 1: No coating peeling, the scratch is smooth;
Level 2: A very small amount of peeling occurs at the edge of the scratch, not exceeding 5% of the circle area;
Level 3: Shedding area 5%-15%;
Level 4: Shedding area 15%-35%;
Level 5: Shedding area 35%-65%;
Level 6: Shedding area 65%-85%;
Level 7: Shedding area＞85%.
3.What are other auxiliary assessment scenarios?
Adhesion after processing: For color-coated coils that need to be bent or stamped (such as automotive parts), a cross-cut test is required after processing (such as 180° bending). The coating must not fall off at the bending point, otherwise it is considered that the adhesion after processing is unqualified; Adhesion after wet heat/salt spray: Some high-demand scenes (such as outdoor buildings) will first conduct wet heat tests (40℃, relative humidity 95%, 168h) or salt spray tests, and then test adhesion to evaluate the bonding stability of the coating in harsh environments. Usually, the grade is required to remain unchanged.
4.How does the quality of substrate surface treatment affect the adhesion of color coated coils?
Surface cleanliness: If there is oil, rust, scale or dust on the surface of the substrate, a physical isolation layer will be formed, which will hinder the chemical bonding between the coating and the substrate.
Surface roughness: Appropriate roughness can increase the contact area between the coating and the substrate ("mechanical anchoring" effect), but insufficient roughness (such as too smooth) will reduce the bonding force; conversely, excessive roughness (such as surface scratches and pitting) will cause uneven local thickness of the coating, and even pinholes will appear, which will affect the adhesion.
Chemical conversion treatment: Pretreatment of the substrate surface (such as phosphating, passivation, silane treatment) can form a chemical conversion film to enhance the chemical bonding between the coating and the substrate. For example, after chromate passivation, the adhesion of the primer can be significantly improved on the galvanized sheet; if the conversion film is too thin or missed, it will cause a decrease in local adhesion.
5.How do coating process parameters affect coating adhesion?
Coating thickness: When the coating is too thin, it is difficult to form a continuous and uniform film layer, and local defects may lead to insufficient adhesion; when it is too thick, the internal stress during curing will be too large (resin shrinkage), resulting in peeling force between the coating and the substrate, especially during bending processing. For example, the thickness of the primer is usually controlled at 5-10μm, and the topcoat is 15-25μm. If the total thickness is too thick (such as more than 60μm), a higher performance resin must be matched.
Curing temperature and time: The coating needs to reach the specified temperature (such as the curing temperature of polyester topcoat 180-220℃) and time to ensure that the resin is fully cross-linked to form a film. If the curing is insufficient (the temperature is too low or the time is too short), the resin will not react completely, the mechanical properties of the coating will be poor, and the adhesion will decrease; if it is overcured (the temperature is too high), the resin will degrade, causing the coating to become brittle, which also affects the adhesion.
Coating equipment accuracy: Uneven roller surface pressure and mismatched speed of coating machines (such as roller coaters) will lead to uneven coating thickness or local coating omissions; poor atomization of spray guns will lead to uneven distribution of powder coatings (such as powder color coating coils), forming "orange peel" or pinholes, and reducing adhesion stability.