1.How does adhesion, which combines "physical interlocking" and "chemical bonding," manifest itself?
Microscopic mechanical bonding: The coating liquid penetrates into the tiny irregularities and pores on the substrate surface, forming a physical anchor upon curing.
Interfacial chemical bonding: The coating undergoes molecular-level chemical bonding or strong polar adsorption with the treated substrate surface.
2.What damage will occur if the substrate has rust or oil stains?
Rust (oxides): Porous and porous, rust itself has weak adhesion to the solid metal substrate. Applying paint to rust is like "sticking it to a biscuit"-when the rust peels off the metal, the coating will come off with it.
Grease, dust, salt, and other contaminants: These form a weak interface layer between the substrate and the coating, acting like a "barrier film" that completely prevents direct contact between the coating and the metal, preventing both physical bonding and chemical adhesion from forming.
3.What is the standard pretreatment process for color-coated coil production?
Degreasing and Cleaning:
Purpose: To remove rolling oil, rust-preventive oil, and grease and dust acquired during transportation and storage.
Method: Spraying or immersion with an alkaline cleaning agent, followed by multi-stage rinsing with water.
Chemical Conversion Treatment (Core Step):
This is not "rust removal," but rather the formation of a dense, non-metallic protective film with excellent adhesion to the coating through a chemical reaction on a clean metal surface.
Common Methods:
Chromating: Forms a chromate conversion film with excellent corrosion resistance, but requires high environmental standards.
Chromium-Free Passivation: Such as zirconium-based and titanium-based conversion films, this is currently the mainstream environmentally friendly process.
Functions:
Enhanced Adhesion: The conversion film surface has numerous micropores and polar groups, enabling it to form strong chemical bonds with the coating.
Improved Corrosion Resistance: It has a passivating effect and can block the penetration of corrosive media.
4.What problems might arise when colored rolls leave the factory?
White rust: Basic zinc carbonate formed after the galvanized layer becomes damp; it must be removed.
Oil and dust: Originate from transportation and storage.
Slight oxide layer: A very thin oxide film that forms naturally and needs to be "reformed" through conversion treatment.
5.How to assess and ensure adhesion?
Required Testing Reports: When purchasing, request suppliers to provide reports for the "T-bend Test" and the "Cross-cut Adhesion Test."
T-bend Test (e.g., T2, T3, T4, T5): Evaluates the adhesion and crack resistance of the coating after severe bending deformation. Higher grades (lower numbers) indicate better adhesion.
Cross-cut Adhesive Test (Grades 0-5): After scoring with a knife, apply adhesive tape and peel off, observing the coating's peeling. High-quality color-coated steel sheets should achieve Grade 0 or 1 (no peeling or minimal peeling at the edges).
Assess Supplier Strength: Manufacturers with modern, complete pre-treatment lines and strict process control ensure more stable product quality.