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D1986-14 – Standard Test Method Technical Guide
🔬 Apparatus and Specimen Geometry
ASTM D1986-14 (Reapproved 2021) specifies a small-volume, coaxial (concentric cylinder) rotational viscometer for this test. The essential instrumentation required for compliance includes a drive motor capable of applying unidirectional rotational displacement at a rate of 0.05 to 6.0 rad/s (0.5 to 60 r/min), constant to ±1%. A force sensor measures the torque developed by the specimen up to 0.6 mN·m. A precision temperature bath maintains the specimen at 140 °C throughout the analysis. The standard prescribes a coaxial spindle with specific dimensions detailed in the standard’s Fig. 1, ensuring a fixed shear geometry between the drive shaft and the stationary position.
💡 Instrument Integrity: The stand must support and level the drive motor precisely. A dedicated data collection device is required to acquire and display the critical output signals: torque, rotational speed, temperature, and time.
⚙️ Test Procedure and Speed Selection
The test is conducted at an isothermal temperature of 140 °C, which is above the maximum melt point of the materials in scope. The rotational element is immersed in the molten wax, and a unidirectional rotational displacement is applied. Because many polyethylene waxes are non-Newtonian, the choice of rotational speed (between 0.5 and 60 r/min) will directly influence the measured apparent viscosity. The resulting torque is measured and expressed in millipascal-seconds (mPa·s) or centipoises (cP), where 1 cP equals exactly 1 mPa·s. Results must always be reported with the specific spindle and rotational speed used.
📊 Key Material Properties and Test Parameters
The standard specifically defines the material scope for this test method. Polyethylene wax is defined as polymerized ethylene with specific physical characteristics. The tables below list the critical material specifications and the exact test parameters required by the standard.
| 🧪 Polyethylene Wax Property | 📐 Specified Range |
|---|---|
| Molecular Weight | 2,000 to 10,000 g/mol |
| Density | 0.9 to 1.0 g/cm³ |
| Melt Point | Lower than 140 °C |
| ⚙️ Critical Test Parameter | 📏 Specified Value |
|---|---|
| Test Temperature | 140 °C |
| Rotational Speed Range | 0.5 to 60 r/min |
| Speed Stability | ±1% |
| Torque Measurement Range | Up to 0.6 mN·m |
⚠️ Apparent Viscosity Variance: The standard deliberately uses the term “apparent viscosity” because the value may vary with the shear rate (rotational speed) selected. Many polyethylene waxes exhibit non-Newtonian behavior, making strict adherence to the test conditions critical for meaningful quality control comparisons.
❓ Frequently Asked Questions
🔍 What is the scope of ASTM D1986-14?
This test method describes the determination of the apparent viscosity of polyethylene wax using a small-volume, concentric cylinder, rotational viscometer at a controlled temperature of 140 °C.
💡 How is apparent viscosity defined in this standard?
Apparent viscosity is the viscosity determined by this specific test method and is expressed in millipascal-seconds (mPa·s) or centipoises (cP). The standard notes that 1 cP = 1 mPa·s and that the value may vary with the spindle and speed selected.
⚡ What are the primary apparatus requirements?
The standard requires a coaxial rotational viscometer with a drive motor (0.5 to 60 r/min, ±1% stability), a torque sensor (up to 0.6 mN·m), a temperature bath for 140 °C, and a data collection device.
📌 What materials are covered by this test?
The standard covers polyethylene waxes with a molecular weight of 2,000 to 10,000 g/mol, a density of 0.9 to 1.0 g/cm³, and a melt point lower than the test temperature of 140 °C.