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D3285-93 – Standard Test Method Technical Guide
📐 Apparatus and Specimen Geometry
The Cobb test apparatus includes a metal ring with an inside diameter of 11.28 ± 0.02 cm (100 cm² area), 2.5 cm high, and about 0.6 cm thick. This ring is clamped to a base plate (15 × 15 cm) using a cross bar (17 × 2.5 × 0.6 cm) with wing nuts. A rubber mat on the base plate seals the specimen. Additional equipment includes a brass metal roller with a 20 cm face, weighing 10.0 ± 0.5 kg, a timer, a 100 mL graduated cylinder, and a balance with 0.01 g accuracy.
| 🟦 Component | 📏 Specification |
|---|---|
| Ring inside diameter | 11.28 ± 0.02 cm |
| Cross-sectional area | 100 cm² |
| Ring height | 2.5 cm |
| Metal roller weight | 10.0 ± 0.5 kg |
| Balance accuracy | 0.01 g |
⚙️ Test Procedure and Conditions
Conditioned specimens are clamped over the rubber mat. Distilled or deionized water (100 mL) is poured onto the specimen for a specified time (typically 60 or 120 seconds). After soaking, water is removed by tilting the apparatus, and the specimen is blotted with standard blotting paper using a metal roller. The blotting paper must have an oven-dry weight of 250 ± 10 g/m², thickness 0.495–0.521 mm, and absorbency ≤25 s for 1.0 mL water (Test Method D 824).
⚠ Note: Metal parts should be brass or corrosion-resistant material. The clamping mechanism may affect the time required to remove the specimen from the apparatus.
📊 Water Absorption Measurement
Water absorptiveness is calculated as weight gain per area (g/m²). This test is applicable for nonbibulous paper and paperboard with minimum thickness 0.1 mm. It indicates surface and internal sizing but is not a water-resistance test nor suitable for writing papers.
| 📐 Parameter | 🎯 Standard Value |
|---|---|
| Minimum specimen thickness | 0.1 mm |
| Blotting paper weight (oven-dry) | 250 ± 10 g/m² |
| Blotting paper thickness | 0.495 – 0.521 mm |
| Blotting paper absorbency (1.0 mL water) | ≤25 seconds |
💡 Tip: This method is useful for comparing relative penetration rates of aqueous solutions like adhesives or coatings. Ensure clean specimen surfaces and consistent blotting for reproducible results.
❓ Frequently Asked Questions
🔍 What is the purpose of the Cobb test? It measures the quantity of water absorbed by nonbibulous paper and paperboard in a specified time, providing insight into sizing, porosity, and fiber structure.
💡 What materials are suitable? This test is applicable to sized paper and paperboard with a minimum thickness of 0.1 mm. It is not recommended for bibulous or writing papers.
⚡ What apparatus is essential? Key equipment includes a metal ring (100 cm² area), base plate, rubber mat, timer, graduated cylinder (100 mL), balance (0.01 g accuracy), and a metal roller (10.0 ± 0.5 kg).
📌 How is water absorption calculated? The result is expressed in g/m², determined by the weight gain of the specimen after the test divided by the exposed area (100 cm²).