VI Improvers: Reading the Spec Sheet Like an Engineer

VI improvers — the industry shorthand for viscosity index improvers — are the polymers that let one oil behave like a winter grade at -25°C and a summer grade at 100°C. In everyday formulation work the name rarely appears in full: engineers write "VI improver," data sheets print "VII," and blend sheets list the product under its commercial code. But the shorthand hides real specification work. This article translates the numbers on a VI improver data sheet into decisions you can make in the blend plant.

What the Abbreviations Mean

• VI improver / VII: viscosity index improver — the polymer that raises the viscosity index of the finished oil.
• SSI: shear stability index — the percentage of thickening effect permanently lost under mechanical shear (ASTM D6278 or D5621).
• TE: thickening efficiency — the viscosity increase per unit weight of polymer in a reference oil.
• Mw / Mn / MWD: weight-average and number-average molecular weight, and their ratio (polydispersity) — the structural numbers that drive SSI and TE.
• KV100: kinematic viscosity at 100°C — how the concentrate (or finished oil) is graded.

When a data sheet lists only "VI improver, OCP, KV100 600 cSt," it has told you almost nothing you can act on. The decision-grade information is SSI, TE, and Mw/MWD.

The Numbers That Drive Your Blend

Shear Stability Index First

SSI is the first filter because it is the hardest property to change after the fact. If you blend a 15W-40 with an SSI-45 polymer, no amount of adjustment elsewhere restores the grade retention — the oil will shear out of the 40 window in service. Decide the SSI class from the application:

• Gear and heavy-duty diesel oils: SSI ≤ 20–25.
• Mainstream engine oils: SSI 25–35.
• Short-drain, non-critical industrial oils: SSI up to 40 acceptable.

Thickening Efficiency Second

Once the SSI class is fixed, TE decides cost. Among polymers of the same SSI class, higher TE means a lower treat rate and lower cost per liter. Ask the supplier for the TE curve in a reference oil that matches your base oil type, then re-verify in your own base oil — solvency differences between Group I and Group III change TE by 10–20%.

Molecular Weight as the Root Cause

SSI and TE are both consequences of Mw and MWD. Long chains thicken efficiently and shear easily; short chains survive shear and thicken poorly. A supplier that controls MWD tightly gives you consistent blends; a supplier that does not gives you a new treat rate with every lot. Demand Mw/MWD on the certificate of analysis.

Common Mistakes When Specifying VI Improvers

• Comparing SSI across test methods. Bosch 30-cycle, Bosch 90-cycle, and sonic shear give different numbers for the same polymer. Match the method before comparing.
• Ignoring the diluent. A 20% active concentrate contributes its carrier oil to the finished KV100. Account for it in the base oil balance.
• Sizing the blend on KV100 alone. A blend that hits KV100 can still fail CCS or MRV in the cold. Always verify the cold window with the final package.
• Assuming one grade fits all base oils. Recalibrate per base oil; do not carry treat rates across projects.

A Quick Blending Workflow

1. Fix the SAE grade window and the application's SSI limit.
2. Select the VI improver grade whose SSI fits, then compare TE among candidates.
3. Build a 2–3 point dosing curve in your actual base oil (0.3/0.6/0.9 wt%).
4. Interpolate the treat rate that centers KV100 in the window.
5. Verify CCS, MRV, HTHS, and post-shear grade retention on the full package.
6. Lock the treat rate into the blend sheet with the lot's CoA values.

FAQ

Is a VI improver the same as a viscosity improver? Yes. "VI improver" and "viscosity improver" are shorthand for the same product family; "viscosity index improver" is the full term.

What SSI should I specify for a heavy-duty diesel oil? Below 25 (Bosch 90-cycle preferred for long-drain service), to hold the grade across extended drain intervals.

How much VI improver is in a typical engine oil? Usually 0.2–1.5 wt% of active polymer, depending on grade span, base oil, and the polymer's TE.

Why does the same VI improver give different results in Group I and Group III oils? Group I aromatics solvate polymers better, giving higher TE. The same treat rate undershoots in Group III — recalibrate per base oil.

Conclusion

VI improvers are specified on three numbers — SSI, TE, and molecular weight distribution — and the abbreviation on the blend sheet is no substitute for the data behind it. Filter by SSI for the application, compare TE at equal SSI, verify in your own base oil, and lock lot-to-lot consistency with CoA tracking. Minglan Chemical's T613/T614 OCP VI improvers ship with complete SSI, TE, and molecular weight documentation — contact us for data sheets and dosing-curve support for your base oils. For the flagship treatment of how these polymers work, see our viscosity index improver guide.