1.What is the core purpose of solidification?
Film formation and curing: This process transforms the liquid coating from a "coated state" (perhaps a wet film after roller coating) into a solid, continuous film that adheres tightly to the substrate surface.
Property imparting: This process uses chemical reactions (such as resin cross-linking) to impart desired coating properties, such as hardness (scratch resistance), flexibility (flexibility without cracking), and chemical resistance (acid and alkali resistance).
Volatilization: Coatings typically contain solvents (such as organic solvents and water) or low-boiling-point additives, which must be evaporated during the curing process to prevent defects such as pinholes and bubbles in the coating.
2.How to remove volatiles during the preheating stage to avoid coating "bubbling"?
A freshly applied coating is in a "wet" state, containing 20%-50% volatiles (solvent or water, the exact ratio depending on the coating type; for example, solvent-based coatings contain more organic solvents, while water-based coatings contain more water). If directly heated to high temperatures, these volatiles will rapidly boil and expand, potentially breaking through the uncured coating, causing bubbles, pinholes, and even cracks. Therefore, a "preheating" process is necessary to slowly remove the volatiles.
• Controlled Conditions: The temperature is typically between 80-150°C (adjusted based on the boiling point of the coating's volatiles. If water-based coatings contain water, the preheating temperature can be slightly lower; if solvent-based coatings contain high-boiling-point solvents, the temperature can be slightly higher). The duration is short (usually 1-3 minutes, depending on substrate thickness and coating speed). • Key Function: Allowing the volatiles to evaporate slowly, maintaining a "semi-dry" coating (still somewhat fluid but without noticeable liquid solvent), preparing for the subsequent curing reaction.
3.Different coatings have different film-forming substances. What are the differences in the curing reaction mechanisms?
Cross-linking (most common): The resin in the coating (such as polyester, epoxy, and fluorocarbon resins) contains reactive groups (such as hydroxyl and carboxyl groups). These cross-linking reactions occur with curing agents (such as isocyanates and amino resins) at high temperatures. The linear structure of the resin molecules transforms into a three-dimensional network, like a "tight web of molecules," hardening and strengthening the coating.
For example, polyester coatings typically use amino resins as curing agents. At temperatures of 180-220°C, the hydroxyl groups of the polyester cross-link with the ether bonds of the amino resin, forming a stable coating.
Oxidative polymerization or thermal curing: Some coatings (such as alkyd coatings) polymerize by reacting with oxygen in the air or simply by subjecting the resin molecules to self-polymerization at high temperatures (without the need for an additional curing agent). However, this type of curing is less common in pre-coated steel sheets; cross-linking is more common. Condition Control: Temperature is critical and must match the coating's curing temperature (for example, polyester coatings are typically cured at 180-230°C, while fluorocarbon coatings, due to their higher temperature resistance requirements, may require 250-300°C). Duration must be sufficient (usually 2-5 minutes) to ensure a complete crosslinking reaction. Incomplete reaction can lead to a softened coating, poor adhesion, and even later tackiness.
Atmosphere Control: Most coatings cure in air, but some specialized coatings (such as certain high-temperature curing coatings) may require controlling the oxygen content within the oven to prevent oxidation and discoloration. This is typically achieved by adjusting the oven's air intake and exhaust (introducing fresh air and removing volatiles and reaction gases).
4.How to stabilize coating performance and avoid thermal damage during the cooling stage?
After primary curing, the coating has solidified. However, the substrate and coating are still very hot (near the curing temperature). If exposed to air or contact with cold objects, the rapid cooling may cause:
The coating may crack or wrinkle due to uneven thermal expansion and contraction;
The adhesion between the substrate and the coating may decrease due to thermal stress;
The coating may be easily scratched at high temperatures (although the coating is cured, its toughness is slightly reduced at high temperatures).
Therefore, a cooling stage is required to slowly cool the substrate and coating to near room temperature through a "gradient cooling" process.
Common cooling methods include air cooling (using a fan to blow cold air) or water cooling (using water-cooled rollers or sprays, suitable for rapid cooling). Continuous production lines often use a combination of air and water cooling (initial air cooling followed by water cooling to room temperature).
End-stage state: After cooling, the coating temperature typically drops below 50°C. At this point, the coating's properties are stable (hardness, adhesion, etc. meet standards), and it can proceed to subsequent processes (such as winding and slitting).
5.What are the key factors affecting the curing effect?
Temperature Accuracy: The temperature within the curing oven must be uniform (temperature difference ≤ ±5°C) and strictly match the coating's requirements. If the temperature is too low, the crosslinking reaction will be incomplete, resulting in an "under-cured" coating (manifested by low hardness, easy scratching, and even the ability to be removed with a fingernail). If the temperature is too high, the coating may be "over-cured" (manifested by discoloration, brittleness, and cracking when bent). Light-colored coatings (such as white and beige) are particularly susceptible to yellowing due to high temperatures.
Curing Time: In relation to the temperature, the "temperature-time product" must be met (i.e., sufficient time is required for the reaction to complete at a given temperature). If the production line speed is too high (the substrate stays in the oven for a short time), even if the temperature meets the specified standard, the reaction may not be complete. Conversely, excessive curing may result in over-curing.
Volatile Emissions: Volatiles must be removed promptly during the preheating phase. If the oven is not properly vented, volatile accumulation can cause "pinholes" (small pits) and "bubbles" (bulges that rupture and leave scars) on the coating surface, and may even affect the crosslinking reaction (volatiles may interfere with the reaction between the resin and the curing agent). Substrate surface condition: If the substrate is not thoroughly pre-treated (there is oil or rust on the surface), even if the curing conditions are suitable, the coating may have poor adhesion due to the "unclean substrate" and easily fall off after curing (this is the influence of pre-treatment, but it will be reflected in the final effect after curing).