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D6116-18 – Standard Test Method Technical Guide
ASTM D6116-18 (Reapproved 2023) provides a standardized, rigorous protocol for evaluating the resistance of finished leather surfaces to blocking—the undesirable adhesion or sticking of surfaces when subjected to specified conditions of temperature, humidity, and static pressure. This method is essential for quality assurance in applications where finished leathers are stacked or stored, ensuring they do not suffer finish damage. The test is conducted in SI units and is explicitly not applicable to wet blue leather.
🧪 Specimen Preparation and Humidity Conditioning
The test requires four specimens per sample, each measuring 2.5 cm by 10 cm. To account for orientation, two specimens are cut parallel to the backbone, and two perpendicular. If material is limited, one specimen per direction is suitable. Each specimen is perforated 2 mm from one end and hung on a standard paper clip. Up to three clips, each holding a maximum of five specimens, can be placed into the indentations of an American Medical Museum Jar (16 cm × 10 cm × 8 cm). The jar is filled with distilled/deionized water at 23 °C ± 2 °C to a level no higher than 1.25 cm from the bottom of the hanging specimen. The lid is sealed, and the specimen is conditioned for exactly 4 hours in the controlled high-humidity atmosphere.
| 📏 Specimen & Conditioning Parameter | 🎯 Standard Specification |
|---|---|
| Total Specimens per Sample | 4 (2 parallel, 2 perpendicular) |
| Standard Specimen Dimensions | 2.5 cm × 10 cm |
| Conditioning Medium | Distilled / Deionized Water |
| Water Temperature | 23 °C ± 2 °C |
| Conditioning Duration | 4 hours |
| Maximum Clearance from Water | 1.25 cm |
⚙️ Blocking Test Procedure (Method 1 – Petri Dish)
After the 4-hour conditioning period, the specimen is removed and immediately folded finish to finish to form a 2.5 cm by 5 cm rectangle. If the leather is too thick or stiff to be folded upon itself, it may be cut into two 2.5 cm by 5 cm sections and placed finish to finish. The folded (or sandwiched) specimen is placed into an inverted (rim up) petri dish cover (100 mm × 15 mm). The bottom plate of the dish is placed concentrically within the cover to cover the specimen. A 2000 g weight is placed on top. The whole assembly is then placed in a forced circulating hot air oven at 80 °C ± 3 °C for 2 hours. After heating, the assembly is removed, the weight and cover plate are taken off, and the specimen is allowed to cool for 30 minutes before the finished surfaces are separated and evaluated for blocking.
⚠️ Critical Compliance Note: The standard mandates a forced circulation oven for uniform temperature distribution. While the American Medical Museum Jar is specified, other glass containers of similar size may be used. For applying the compressive load, an alternative loading frame—such as the AATCC perspirometer tester described in AATCC TM15—is explicitly permitted, provided it is validated to yield equivalent results.
| 🟦 Blocking Test Condition | 📐 Method 1 (Petri Dish) Value |
|---|---|
| Applied Static Load | 2000 g |
| Over Temperature | 80 °C ± 3 °C |
| Heating Duration | 2 hours |
| Post-Test Cooling Time | 30 minutes |
| Final Test Piece Size | 2.5 cm × 5 cm (folded or stacked) |
🔬 Understanding Apparatus Flexibility
ASTM D6116 acknowledges that specific standard apparatus, while preferred, has commercially available alternatives. The American Medical Museum Jar (featuring three indentations on the 8 cm sides for holding clips) can be substituted with any glass container of similar dimensions that provides identical results. Similarly, the compression loading frame required for static pressure can be substituted with the AATCC perspirometer tester as standardized in AATCC TM15, providing laboratories with practical flexibility while maintaining test integrity.
💡 Technical Best Practice: The 4-hour conditioning step is crucial for moisture equilibrium. Ensure the distilled water is precisely at 23 °C ± 2 °C at the start, and strictly maintain the 1.25 cm clearance. This ensures the specimen is conditioned by the high-humidity atmosphere without being wetted by direct water contact, which would invalidate the surface blocking test.
❓ Frequently Asked Questions
🔍 What is blocking in the context of leather testing?
Blocking is the unwanted adhesion or sticking of finished leather surfaces when pressed together under specific conditions of heat, humidity, and pressure. ASTM D6116 objectively measures a leather finish’s resistance to this damage.
💡 Why is the 4-hour conditioning step in the jar so important?
This step standardizes the specimen’s moisture content. By suspending the leather above a precisely controlled water level (23 °C), the specimen absorbs consistent humidity before the heat and pressure of the blocking test are applied, ensuring reproducible results across different laboratories and specimens.
⚡ What are the exact environmental conditions for Method 1?
The loaded specimen assembly (with the 2000 g weight) is placed in a forced-circulation oven at 80 °C ± 3 °C for 2 hours. Following this, the weight is removed and the specimen must cool for exactly 30 minutes at room conditions before the blocked surfaces are separated and evaluated.
📌 Can this standard be used for all types of leather?
No. Section 1.1 of the standard specifically states that this test method does not apply to wet blue (chrome-tanned, unfinished leather). It is designed exclusively for finished leather where the surface coating might be susceptible to sticking.