Zinc Sulphate Hydrate Conversion Layer for Epoxy Adhesion
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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 issues in the automotive industry.
Innovation Solution
A surface treatment process that forms a zincsulphate-based conversion layer on a steel substrate by controlling the air drying temperature, strip velocity, wet film thickness, initial strip temperature, and air flow rate to exclude zinc hydroxysulphate, free water molecules, and free hydroxyl groups, favoring the formation of stable zincsulphate monohydrate, tetrahydrate, or heptahydrate, thereby enhancing adhesion with epoxy-based adhesives without compromising corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conversion layer based on zinc hydroxysulphate is formed on the zinc-based coating, then pre-lubrication performance is improved, but adhesion to epoxy-based adhesives deteriorates due to reactive hydroxyl groups
Solution Approach 1:
The invention extracts and removes the harmful zinc hydroxysulphate compound from the conversion layer composition. By controlling the drying process to favor zinc sulphate hydrates over zinc hydroxysulphate formation, the reactive hydroxyl groups that cause adhesion problems are eliminated while preserving the beneficial 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 conditions (temperature, time, air flow) to prevent the formation of zinc hydroxysulphate and instead promote the formation of stable zinc sulphate hydrates. This parameter control eliminates the adhesion problem while maintaining lubrication performance.
2Strength
If the conversion layer is dried to remove free water molecules and free hydroxyl groups, then adhesion to epoxy-based adhesives is improved, but the drying process complexity increases
Solution Approach 1:
The invention performs preliminary action by optimizing the drying process parameters in advance to ensure complete removal of free water molecules and free hydroxyl groups before adhesive application. By establishing proper drying conditions (temperature, time, air flow) as a preliminary step, the subsequent adhesion is guaranteed without requiring complex additional treatments.
Solution Approach 2:
The invention replaces complex mechanical drying systems with a simplified thermal drying process. By using controlled heat treatment at moderate temperatures (50-150°C) with proper air circulation, the invention achieves complete drying and hydroxyl group elimination without requiring sophisticated equipment, making the process industrially viable.
3Strength
If zinc hydroxysulphate is excluded from the conversion layer, then adhesion to adhesives is improved, but the formation of zinc hydroxysulphate is difficult to prevent as it precipitates when pH reaches 7
Solution Approach 1:
The invention applies preliminary action by controlling the drying process to occur rapidly after the aqueous solution application, before the pH can rise to 7 and trigger zinc hydroxysulphate precipitation. By completing the water removal and hydrate formation in the zinc sulphate phase before pH change occurs, the harmful compound is prevented from forming.
Solution Approach 2:
The invention uses the skipping principle by rapidly proceeding through the drying phase immediately after solution application, rushing through the critical time window before pH reaches 7. This rapid processing prevents the chemical transformation that would lead to zinc hydroxysulphate formation, making the process easier to control.
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
The process ensures strong adhesion with epoxy-based adhesives while maintaining the corrosion resistance and drawability of the metal sheets, as demonstrated by improved lap shear test results and resistance to degradation over time.
Implementation Method 1
the presence of zinc hydroxysulphate itself in the conversion layer led to the weak adhesion... as soon as the pH reaches 7 due to the metallic coating oxidation
Implementation Method 2
the conversion layer comprises at least one of the compounds selected from among zincsulphate monohydrate, zincsulphate tetrahydrate and zincsulphate heptahydrate
Implementation Method 3
these free water molecules and/or free hydroxyl groups can be present in the conversion layer even when it is apparently dry
Implementation Method 4
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 to avoid the formation of zinc hydroxysulphate
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.