1.What are the principles and standards for water vapor permeability testing?
Principle: Use the "cup method" (weight method) or "electrolytic sensor method" to measure the amount of water vapor passing through the coating per unit time, in units of g/m²·24h.
Test standard: Refer to ASTM E96 (weight method) or ISO 15106-3 (electrolytic method).
Operation steps: Seal the color-coated coil sample on a moisture permeable cup containing a desiccant (such as calcium chloride), place it in a constant temperature and humidity environment (such as temperature 38°C, relative humidity 90%), and weigh it regularly to calculate the water vapor permeation.
Result interpretation:
Excellent dense coating (such as PVDF fluorocarbon coating): Water vapor permeability is usually <5g/m²・24h (molecular structure contains fluorine, low polarity, and water vapor is difficult to penetrate).
Medium dense coating (such as silicon-modified polyester SMP): Permeability 5-15g/m²・24h.
Coatings with poor density (such as ordinary acrylic acid): permeability> 15g/m²・24h (low resin cross-linking degree and large molecular gap).
2.What are the principles of gas permeability testing and how to interpret the results?
Principle: The permeation rate of gases such as oxygen and carbon dioxide in the coating is measured by a gas permeometer to reflect the coating's ability to block gas media.
Result differences:
The oxygen permeability of fluorocarbon (PVDF) coating is less than 10cm³/m²・24h, which is suitable for high-demand scenarios;
The oxygen permeability of ordinary polyester (PE) coating can reach 30-50cm³/m²・24h, and gas is easily permeable.
3.What are the principles and application scenarios of liquid (such as salt water) penetration testing?
Principle: One side of the coating sample is exposed to 3% NaCl solution, and the other side is monitored by a conductive detection device to see if there is liquid penetration, and the liquid penetration time is recorded.
Application scenario: Evaluate the ability of color-coated coils to resist corrosion medium penetration in a high salt spray environment. A penetration time of >1000h is excellent.
4.How to observe coating cross-section using scanning electron microscope (SEM)?
Operation: Cut the color-coated coil sample and perform gold spraying. Use SEM to observe the internal structure of the coating:
Dense coating: There are no obvious pores in the cross section. The resin and pigment particles are tightly combined without gaps;
Loose coating: Tiny pores or gaps formed by pigment agglomeration can be seen in the cross section. Such defects are "channels" for medium penetration.
5.What are the coating pinhole detection methods and standard requirements?
Method: Use an "electric spark pinhole detector" (voltage 500-3000V), scan the electrode on the coating surface, if there is a pinhole (diameter > 0.01mm), the current will break through the defect to form a spark, and record the number of pinholes (unit: /m²).
Standard requirements: The number of pinholes in color-coated coils for construction (such as exterior wall panels) must be ≤1 /m², otherwise it is easy to become an intrusion point for corrosive media.