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D5235-18 – Standard Test Method Technical Guide
📜 Test Method Scope and Summary
ASTM D5235-18 (Reapproved 2024) provides a standardized test method for measuring the dry film thickness (DFT) of coatings applied to wood and wood-based products. This method is applicable to smooth, textured, or curved rigid substrates. The test method covers specimen preparation, imaging using a calibrated microscope system, and analysis of the coating cross-section. It is essential for calibration adjustment and accuracy verification of coating thickness measuring instruments, and it is often referenced in product warranties.
Note: This standard is issued under fixed designation D5235, with the year of original adoption or revision indicated. The values stated in either SI units or inch-pound units are to be regarded separately as standard.
🔬 Specimen Preparation and Measurement Procedure
The test method involves cutting a specimen of coated wood or wood-based product to a convenient size. The edge face, which is a plane perpendicular to the surface showing the coating cross-section, is prepared by polishing or cutting with a sharp blade. The prepared edge is then imaged using a calibrated imaging system with known magnification. The dry film thickness is measured using an image analysis computer program. The method also offers suggestions for interpreting DFT on porous wood materials where “soak in” may occur.
📏 Key Terminology and Data Interpretation
Understanding the specific terms defined in this standard is crucial for accurate measurement. The following table summarizes key definitions:
| 🟦 Term | 📖 Definition | 🎯 Application |
|---|---|---|
| Dry Film Thickness (DFT) | The thickness of dried, cured coating film on the substrate surface, which may include attached fibers but excludes free fibers encapsulated in the layer. | Used for calibration and warranty compliance. |
| Edge Face | The part of the specimen that is a plane perpendicular to the surface, showing a cross-section of the coating and substrate. | Prepared for microscopic measurement. |
| Soak In | Refers to coating penetration into the fiber structure of wood or wood-based material, often due to porous nature. | Important for interpreting measurements on porous substrates. |
Important: Wood and wood-based products are generally porous, and coating absorption can be non-uniform, influenced by localized surface density differences or wood pore size. This phenomenon, known as “soak in,” must be considered when measuring DFT.
❓ Frequently Asked Questions
🔍 What is the scope of ASTM D5235-18?
It covers the microscopic measurement of dry film thickness of coatings on wood and wood-based products, including specimen preparation and analysis.
💡 How is dry film thickness defined in this standard?
DFT is defined as the thickness of the dried, cured coating film on the substrate surface, including attached fibers but excluding free fibers.
⚡ What does “soak in” mean?
It describes the penetration of coating into the porous fiber structure of wood, which can affect the measured thickness.
📌 Why is this test method significant?
It is used for calibration adjustment and accuracy verification of coating thickness measuring instruments and is referenced in product warranties.