Viscosity index improver additives are differentiated in the laboratory by three numbers: the shear stability index (SSI), the thickening efficiency (TE), and the kinematic viscosity of the polymer concentrate. Two products that look identical on paper — same chemistry, same name — can perform very differently in a finished oil because these three parameters diverge. This article explains what each number means, how they trade against each other, and how to read a data sheet like a formulator rather than a buyer.
The Three Numbers That Define a Viscosity Index Improver
Shear Stability Index (SSI)
SSI expresses the percentage of the polymer's thickening effect that is permanently lost under mechanical shear. It is measured by the Bosch injector method (ASTM D6278, 30 or 90 cycles) or the sonic shear method (ASTM D5621), and the test method must always be stated — 30-cycle and 90-cycle SSI values differ significantly for the same product.
- SSI below 20: suitable for gear oils, heavy-duty diesel oils, and long-drain applications.
- SSI 20–35: the mainstream engine oil range.
- SSI above 35: acceptable only where drain intervals are short or grade retention is tolerant.
Thickening Efficiency (TE)
TE is the viscosity increase delivered per unit weight of polymer, normally expressed either as the KV100 of a polymer solution at a defined concentration in a reference oil, or as the concentration required to reach a target viscosity. It is the direct driver of treat rate: a high-TE polymer reaches the grade window with less material, which usually means lower cost per liter of finished oil.
Kinematic Viscosity of the Concentrate
Liquid viscosity index improver additives are sold as concentrates — polymer dissolved in a carrier oil — with a declared KV100 and active-polymer content. The concentrate viscosity governs pumping, metering, and blending behavior. A concentrate that is too viscous to pump at ambient temperature adds handling cost regardless of its chemical performance.
How SSI and Thickening Efficiency Trade Off
The central tension in viscosity index improver selection is that the two most desirable properties pull in opposite directions. High molecular weight gives high thickening efficiency — fewer kilograms of polymer to hit the window — but long chains are exactly what breaks under shear, raising SSI. Short chains survive shear but thicken poorly, pushing up the treat rate.
| Polymer | Molecular weight | Thickening efficiency | SSI | Best use |
|---|---|---|---|---|
| High-MW OCP | High | High | 40–60 | Not grade-critical, short-drain oils |
| Standard OCP | Medium | Medium | 25–40 | Mainstream engine oils |
| Low-SSI OCP | Low–medium | Low–medium | 10–25 | HD diesel, gear, long-drain oils |
| PMA | Medium | Medium–high | 20–45 | ATF, cold-climate oils |
Manufacturers manage this trade-off by controlling molecular weight distribution: a polymer with a narrow distribution can deliver acceptable TE at a lower SSI than a broad-distribution product of the same average weight. When comparing two viscosity index improver additives, compare the full TE-vs-SSI curve, not the headline numbers.
Chemistry Still Matters
Within the same SSI and TE envelope, chemistry decides secondary behavior:
- OCP (ethylene-propylene copolymers) is robust across base oils and the most cost-effective mainstream choice.
- PMA (polyalkyl methacrylates) adds a genuine VI boost and the gentlest cold behavior, valuable in ATF and winter-grade oils.
- Styrene-diene copolymers offer high TE with good HTHS behavior, useful for fuel-economy formulations.
- PIB (polyisobutylene) is ash-free and shear-stable at low molecular weight, preferred for two-stroke and severe gear service.
For engine oil formulations, the practical benchmark is a low-SSI OCP that holds the grade after ASTM D6278 shear while keeping the treat rate economical. This is precisely the design target of the T613 and T614 OCP grades supplied by Minglan Chemical — documented molecular weight, SSI, and thickening data for reproducible compounding.
Reading a Data Sheet Like a Formulator
- Check the test method on the SSI (Bosch 30 vs 90 cycles, sonic). Do not compare across methods.
- Confirm the reference oil for TE claims. Thickening efficiency in a Group I reference is not the same as in Group III.
- Account for the diluent. A 25% active concentrate contributes its carrier to the finished KV100.
- Verify in your base oil. Solvency differences shift TE by 10–20% between base oil groups.
- Test the full package. SSI is measured on the polymer, but grade retention is measured on the finished oil.
FAQ
What is a good SSI for a viscosity index improver additive? For engine oils, SSI 20–35 (Bosch 30-cycle) is mainstream; below 20 suits heavy-duty diesel, gear, and long-drain oils; above 35 risks premature grade drift.
How is thickening efficiency measured? By blending a defined polymer concentration into a reference oil and measuring KV100, or by reporting the concentration needed to hit a target viscosity. Always note the reference oil.
Does a higher SSI mean a cheaper oil? Often yes, because high-SSI polymers are cheaper per unit of thickening. The trade-off is grade retention: the oil may shear out of its window before the drain interval.
Are OCP and PMA interchangeable viscosity index improver additives? Not directly. They deliver different cold behavior, VI contribution, and shear response. Many formulations blend both to combine OCP's economics with PMA's cold performance.
Conclusion
Viscosity index improver additives are bought and specified on three numbers — SSI, thickening efficiency, and concentrate viscosity — and the skill is in reading them together: the TE-vs-SSI trade-off defines the polymer, the test method defines the numbers, and the base oil defines the real-world result. Specify on the full curve, verify in your own blend, and you will avoid both overspending and grade drift. Minglan Chemical's T613/T614 OCP viscosity index improver additives ship with complete shear and thickening documentation — contact us for data sheets and blend support. For procurement guidance across the whole category, see our viscosity index improvers buying guide.