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D2856-94 – Standard Test Method Technical Guide
Rigid cellular plastics are characterized by their cellular structure, which profoundly influences their thermal insulation, mechanical compliance, and fluid permeability. ASTM D2856‑94 (Reapproved 1998) provides a standardized porosity determination method for measuring the open‑cell volumetric content of these materials using an air pycnometer. Because the specimen cutting operation inevitably fractures surface cells, the standard provides three distinct procedures to manage the resulting measurement artifacts and ensure accurate porosity characterization.
🔬 Core Concepts and Terminology
The standard defines an open cell as a cell not totally enclosed by its walls and hence interconnecting with other cells. A closed cell is one totally enclosed by its walls. The measurement determines the accessible cellular volume. The “volume of closed cells” defined by the method is an aggregate of solid polymer volume (cell walls and struts), filler volume, intact closed cells, and small groups of cells interconnected by ruptured walls that remain inaccessible to the gas. The corrected volume of open cells represents the true internal porous volume, while the uncorrected volume of open cells includes the irregular volumes generated at the cut‑cell surface of the test specimen.
| 🔍 Term | 📖 Definition per ASTM D2856‑94 |
|---|---|
| Open Cell | A cell not totally enclosed by its walls, interconnecting with other cells. |
| Closed Cell | A cell totally enclosed by its walls. |
| Corrected Open Cell Vol. | The internal porous volume accessible to the pycnometer gas. |
| Uncorrected Open Cell Vol. | Aggregate of internal porous volume and cut‑surface irregularities. |
⚙️ Test Procedures and Selection Criteria
Specimens are conditioned per Practice D 618. Procedure A is designed to correct for cells opened during sample preparation by measuring the cell diameter (per Test Method D 3576), calculating, and allowing for the surface volume. Procedure B also corrects for cut cells, but does so by cutting and exposing a new surface area equal to the surface area of the original sample dimension. Procedure C does not correct for cells opened during sample preparation; it provides good accuracy on highly open‑celled materials, but accuracy decreases as closed‑cell content and cell size increase.
| 📏 Feature | 🟦 Procedure A | 📐 Procedure B | 🎯 Procedure C |
|---|---|---|---|
| Correction Method | Cell diameter measurement (D 3576) | Cutting new surface equal to original | No correction applied |
| Ideal Material | General purpose, high‑closed‑cell foam | When cell diameter is impractical | Predominantly open‑celled materials |
| Accuracy | Depends on cell size uniformity | Depends on cutting precision | Decreases with higher closed‑cell content and larger cell size |
⚡ Critical Note on Sample Preparation: It is explicitly recognized that “a fraction of the closed cells will be opened during sample preparation.” Procedure C does not compensate for this artifact and will overestimate open‑cell content for materials with significant closed‑cell structure. Procedures A and B are specifically designed to mathematically neutralize this error.
📊 Interlaboratory Precision and Reference Standards
Practice E 691 is referenced for conducting interlaboratory studies to determine the precision of the test method. The standard also states (Note 1) that while it shares the same basic principles with ISO 4590‑1981, the experimental details are “significantly different,” making direct substitution invalid without careful cross‑validation. Safety concerns are addressed in Notes 2, 4, and 8 of the published document, which users must consult. Values stated in SI units are the primary standard; inch‑pound values are provided for information only.
© 2026 TNLab — This article is a technical interpretation for reference only. The original standard as published by ASTM International takes precedence.