Zinc Sulphate Hydrate Coating for Epoxy Adhesion
Solution Overview
Problem
The existing surface treatment methods for zinc-coated metal sheets, particularly those involving zinc hydroxysulphate, result in inadequate adhesion with epoxy-based adhesives due to the reactivity of hydroxyl groups and the presence of free water molecules, which degrades the interfacial bonds and leads to adhesion problems in the automotive industry.
Innovation Solution
A surface treatment method that excludes zinc hydroxysulphate and free water/hydroxyl groups by forming a zincsulphate-based layer comprising zincsulphate monohydrate, tetrahydrate, or heptahydrate, achieved through controlled air drying temperatures and process parameters such as strip velocity, wet film thickness, initial strip temperature, and air flow rate, ensuring the layer is perfectly dry within 4 seconds to prevent zinc hydroxysulphate formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If zinc hydroxysulphate conversion layer is formed on zinc-coated metal sheet, then pre-lubrication performance is improved, but adhesion to epoxy-based adhesives deteriorates due to reactivity of hydroxyl groups
Solution Approach 1:
The invention extracts and removes the harmful zinc hydroxysulphate component from the conversion layer while retaining the beneficial zinc sulphate hydrate compounds. By controlling the drying process to form only zinc sulphate monohydrate, tetrahydrate, or heptahydrate, the patent eliminates the hydroxyl groups that cause adhesion problems while preserving the pre-lubrication properties of the zinc sulphate-based layer.
Solution Approach 2:
The invention changes the chemical composition parameters of the conversion layer by controlling the drying temperature and time to prevent formation of zinc hydroxysulphate. By maintaining specific temperature ranges and limiting contact time to below 4 seconds, the process transforms the conversion layer composition to exclude harmful hydroxyl-containing compounds while maintaining beneficial lubrication properties.
2Ease of manufacture
If zinc hydroxysulphate is present in conversion layer, then conversion layer forms readily through oxidation, but adhesion deteriorates due to hydroxyl group reactivity with epoxy system
Solution Approach 1:
The invention converts the harmful effect of oxidation into a beneficial process by controlling it to form zinc sulphate hydrates instead of zinc hydroxysulphate. The oxidation process is maintained to ensure conversion layer formation, but the drying conditions are controlled to direct the chemical reaction toward forming only zinc sulphate monohydrate, tetrahydrate, or heptahydrate, which lack the harmful hydroxyl groups.
3Productivity
If air drying temperature is increased to dry conversion layer faster, then productivity is improved, but formation of zinc hydroxysulphate and free water molecules increases
Solution Approach 1:
The invention optimizes the drying parameters by establishing specific temperature ranges and time limits that prevent zinc hydroxysulphate formation while ensuring complete drying. The contact time is controlled to remain below 4 seconds, and temperature is maintained within ranges that favor formation of stable zinc sulphate hydrates without creating free water molecules or hydroxyl groups.
4Manufacturing precision
If contact time of aqueous solution is extended to ensure complete coating, then manufacturing precision is improved, but zinc hydroxysulphate formation increases leading to adhesion problems
Solution Approach 1:
The invention uses a rapid drying process that completes the coating and drying operation in less than 4 seconds. This rushed process prevents the chemical transformation from zinc sulphate hydrate to zinc hydroxysulphate, ensuring that the coating is applied completely and uniformly while the harmful conversion has not yet occurred.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly improves the adhesion of epoxy-based adhesives to the metal sheets while maintaining the corrosion resistance and drawability properties, as evidenced by the absence of zinc hydroxysulphate in the zincsulphate-based layer and stable sulphate hydrates, which prevents decomposition and enhances the surface treatment's effectiveness.
Implementation Method 1
as soon as the pH reaches 7 due to the metallic coating oxidation
Implementation Method 2
the air drying temperature in the dryer was carefully controlled... the contact time of the aqueous solution on the metallic coating between the application of the solution and the end of the dryer had to be below 4 seconds
Data Source
Figure 1~2
Figure 3a~3b
Figure 3c~3d
AI summary
The invention relates to a steel substrate coated on at least one of its faces with a metallic coating based on zinc or its alloys wherein the metallic coating is itself coated with a zincsulphate-based layer comprising at least one of the compounds selected from among zincsulphate monohydrate, zincsulphate tetrahydrate and zincsulphate heptahydrate, wherein the zincsulphate-based layer comprises neither zinc hydroxysulphate nor free water molecules nor free hydroxyl groups, the surface density of sulphur in the zincsulphate-based layer being greater than or equal to 0.5 mg/m². The invention also relates to the corresponding treatment method.