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D6022-19 – Standard Test Method Technical Guide
ASTM D6022-19 provides a standardized methodology for calculating the Permanent Shear Stability Index (PSSI) of a lubricant additive. By comparing viscosities before and after a defined shearing procedure, this practice allows formulators to quantify the viscosity loss contributed specifically by a single component within a finished lubricant blend. The calculation is presented in a general form to accommodate data from a wide range of test apparatus and product chemistries.
📐 Scope and Essential Terminology
Per Section 1 of the standard, this practice is strictly focused on the procedure for calculating PSSI for a single additive component. The calculated index can then be used to estimate the effects of that additive on finished lubricant blends. The standard relies on a clear distinction between its key metrics:
- Permanent Shear Stability Index (PSSI): A calculated property for a single additive component. It measures the irreversible decrease in viscosity contributed by that additive under shear.
- Viscosity Loss (VL): A property measured directly on a finished oil, representing its overall decrease in viscosity. VL and PSSI are not interchangeable.
- Degree of Thickening (DT): The ratio of an oil’s viscosity with an additive to the oil’s viscosity without the additive. DT is the basis for the PSSI calculation.
Technical Note: A high PSSI value indicates an additive whose thickening contribution is highly susceptible to permanent mechanical shear. Selecting the appropriate PSSI is critical for meeting specifications like SAE J300 viscosity grade retention.
⚙️ Referenced Shearing Procedures and Data Sources
D6022-19 intentionally separates the calculation method from the shearing procedure. The practice explicitly references several established ASTM and CEC standard test methods that serve as valid data sources for the viscosities required in the PSSI calculation. Shearing typically induces molecular scission of the polymer additive by passing the fluid through an orifice, gears, or bearings.
| 🟦 Standard | ⚙️ Test Procedure | 📐 Typical Application |
|---|---|---|
| ASTM D6278 | European Diesel Injector Apparatus | Polymer-containing engine oils (single pass) |
| ASTM D7109 | European Diesel Injector Apparatus (Multi-Pass) | Severe shear conditions (30/90 cycles) |
| CEC L-45-99 | KRL Tapered Roller Bearing | Transmission and gear lubricants |
| ASTM D2603 | Sonic Shear Stability Test | Sonic energy induced shear for screening |
📊 Application of PSSI Data
PSSI is derived directly from the Degree of Thickening (DT) measured before and after the shearing procedure. The calculation normalizes the loss of viscosity against the initial thickening contribution of the additive.
Caution: Since shearing severity varies widely between methods (e.g., Injector vs. KRL), PSSI values calculated from different sources are not directly comparable. The calculation is strictly valid only for the specific conditions under which the base viscosities were obtained.
| 🎯 Parameter | 📐 Base Oil Only | ➕ Oil + Additive A | ➕ Oil + Additive B |
|---|---|---|---|
| Viscosity @ 100°C (cSt) – Unsheared | 10.0 | 15.0 | 19.0 |
| Degree of Thickening (DT) – Unsheared | 1.00 (Baseline) | 1.50 | 1.90 |
| Viscosity @ 100°C (cSt) – Sheared (per D6278) | 10.0 | 11.6 | 12.7 |
| Degree of Thickening (DT) – Sheared | 1.00 | 1.16 | 1.27 |
| Calculated PSSI (%) | N/A | 68% | 70% |
Note: PSSI = (DT_initial − DT_final) / (DT_initial − 1) × 100%. In this example, Additive A and B show similar permanent shear loss despite different thickening efficiencies.
❓ Frequently Asked Questions
🔍 How does PSSI differ from Viscosity Loss (VL)?
Per Section 3 of the standard, PSSI is a calculated property for a single component representing the irreversible viscosity loss contributed by that additive. VL is a measured property of a finished oil as a whole. PSSI helps formulators target performance during blending; VL validates the final product.
💡 Is the PSSI value dependent on the specific test method used?
Yes. The PSSI value is entirely dependent on the shearing method and conditions used to obtain the sheared viscosity. A sonic shear test (D2603) and a KRL bearing test (CEC L-45-99) will impose different mechanical stresses, yielding different PSSI values for the same additive. The method used must always be stated alongside the PSSI.
⚡ What is the significance of a high PSSI percentage?
A higher PSSI indicates a greater susceptibility to permanent shear. For example, an additive with a PSSI of 80% loses 80% of its original thickening contribution under the test conditions. Selecting Viscosity Modifiers (VMs) with a PSSI appropriate for the target application (e.g., lower PSSI for HD engine oils) is critical for durability.
📌 Can PSSI be used to estimate the sheared viscosity of a finished oil blend?
Yes. As stated in Scope Section 1.2, the PSSI calculated for a single component can be used to estimate the effects of that component on finished lubricant blends. By applying the PSSI to the additive’s thickening contribution in the blend equation, the final sheared viscosity of the formulation can be projected, though finished oil testing remains the definitive measure.