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D5795-16 – Standard Test Method Technical Guide
The D5795-16 (Reapproved 2024) test method establishes a standardized procedure for measuring the liquid water absorption of coated wood and wood-based products using the Cobb ring apparatus. This method quantifies the amount of water that permeates a paint film and is absorbed by the underlying substrate, providing critical data for evaluating coating performance in moisture-prone environments.
📐 Specimen Geometry and Test Apparatus
The test utilizes rings constructed from metal or plastic, which are adhered or clamped to the flat, coated surface of the test specimen. Rings must form a watertight seal and have a defined internal diameter to ensure consistent surface area exposure. The standard specifically references a ring with an inside diameter of 100 mm (4 in), corresponding to an internal radius of 50 mm (2 in). The specimen substrate must be representative of wood or wood-based products, including engineered composites such as MDF, particleboard, or plywood.
| 🟦 Component | 📏 Specification | 📐 Value |
|---|---|---|
| Ring inside diameter | 100 mm (4 in) | Standard test ring size |
| Ring inside radius | 50 mm (2 in) | Used for area calculation |
| Ring material | Metal or plastic | Corrosion-resistant, non-reactive |
| Test water | Reagent water per D1193 | Distilled or deionized |
⚙️ Test Procedure and Calculation
The test involves filling the ring with distilled water and maintaining contact with the coated surface for 24 hours. After exposure, the unabsorbed water is removed, and the absorbed water weight is measured. The Cobb unit (C.U.) is calculated using the formula: C.U. Factor = 64516 / (π × r²) for SI units, where r is the ring radius in mm. The C.U. is expressed as grams of water per 645 cm² (100 in²) of surface area per 24-hour period.
| 🎯 Parameter | 📐 Formula | ⚡ Example (100 mm ring) |
|---|---|---|
| Ring area (mm²) | π × r² | 3.14 × (50)² = 7850 mm² |
| C.U. Factor (SI) | 64516 / area | 64516 / 7850 ≈ 8.22 |
| Cobb Unit (C.U.) | Weight × C.U. Factor | Calculated per test run |
💡 Technical Note: The Cobb unit factor is dimensionless and specific to the ring size used. For a 100 mm diameter ring, multiply the measured weight gain (in grams) by 8.22 to directly obtain the Cobb unit value in g/645 cm²/24h.
📊 Key Measured Properties
The primary property measured is the Cobb unit (C.U.), which represents the liquid water absorption capacity of the coated substrate. This value directly correlates with the protective performance of the coating system. Lower Cobb units indicate better water resistance. The standard also provides guidance on precision and bias in accordance with practices E177 and E691, ensuring reproducibility across laboratories.
⚠️ Important Consideration: Ensure that the test surface is flat and free of defects. The water temperature should be controlled, and the test environment must comply with standard conditions for wood product testing to avoid influencing absorption rates.
❓ Frequently Asked Questions
🔍 What is the Cobb unit (C.U.)?
The Cobb unit (C.U.) is the weight of distilled water absorbed by the wood substrate in grams per 645 cm² (100 in²) of surface area over a 24-hour period. It quantifies the water absorbed after passing through the coating.
💡 How is the Cobb unit factor determined?
The Cobb unit factor is a dimensionless term calculated as 64516 divided by the ring area in mm² (for SI units). For the standard 100 mm diameter ring, this factor is 8.22. Multiplying the water weight gain by this factor yields the Cobb unit value.
⚡ What ring sizes are permitted?
The standard describes alternative techniques, but the primary example is a ring with an inside diameter of 100 mm (4 in). The Cobb unit factor must be recalculated for any different ring size using the formula provided.
📌 What types of wood substrates are applicable?
This test method covers coated wood and wood-based products, including engineered wood products like particleboard, fiberboard, and plywood. The substrate must be flat for proper ring adhesion or clamping.