Thickeners&Viscosity Index Improvers

What Are Viscosity Index Improvers?

Viscosity index improvers (VIIs), also called viscosity modifiers, are oil-soluble polymers that make multi-grade lubricants possible. Base oil thins out as it heats up — a single-grade oil thick enough to protect an engine at 100°C would be too thick to crank at -25°C. VIIs solve this by expanding their molecular coil as temperature rises, compensating for the base oil's natural thinning and delivering a flatter viscosity-temperature curve.

The mechanics are straightforward: at low temperatures, the polymer chains contract into tight coils that contribute little to viscosity. As temperature increases, the coils expand — their hydrodynamic volume grows, and with it their contribution to the oil's resistance to flow. This allows a thin base stock (say, 5 cSt at 100°C) to perform like a 10W-30 or 5W-40 multigrade oil. At the CheMost factory in Jinzhou — 20+ reactors, 20,000 tons annual capacity, over 20 testing instruments — we supply four polymer chemistries spanning solid bales, liquid concentrates, and high-reactivity PIB for dispersant synthesis.

VIIs were first commercialized in the 1930s and today are the second-largest additive category by volume after detergents and dispersants. Modern passenger car engine oils typically contain 4–10 wt% VII polymer. The polymer type, molecular weight, and concentration directly determine the oil's viscosity grade, cold-cranking performance, and shear stability over the drain interval.

How Viscosity Index Improvers Work

VIIs control viscosity through two independent phenomena: thickening efficiency and shear stability.

Thickening efficiency (TE). This measures how much polymer is needed to reach a target kinematic viscosity. High-molecular-weight polymers are more efficient thickeners — less polymer for the same viscosity — but they also break down faster under mechanical stress. Thickening efficiency also increases with ethylene content in OCPs: a 80 mol% ethylene copolymer reaches target viscosity at lower concentration than a 60 mol% copolymer. Narrow molecular weight distributions give better TE than broad distributions.

Shear stability index (SSI). Under the high mechanical loads in journal bearings, piston rings, and gear meshes, polymer chains stretch and can break. This permanent viscosity loss is measured by SSI (ASTM D6278, Kurt Orbahn test) — the fraction of polymer-contributed viscosity lost during shear. A high-SSI polymer (SSI 50) delivers strong thickening with less material but loses more viscosity over time. A low-SSI polymer (SSI 20–25) requires more material but stays in grade longer. Commercial OCPs range from SSI 23 to 55; the choice depends on the target drain interval and the mechanical severity of the application.

Low-temperature performance is the other critical dimension. All multigrade oils must pass cold-cranking simulator (CCS, ASTM D5293) and mini-rotary viscometer (MRV, ASTM D4684) tests at their rated winter temperature. High-ethylene OCPs (above 60 wt% ethylene, called low-temperature OCPs or LTOCPs) partially crystallize at sub-zero temperatures, causing the polymer coil to contract and thus contributing less to cold viscosity — an advantage for meeting CCS limits. But these same ethylene sequences can interact with base oil wax, requiring careful pour point depressant (PPD) selection.

Viscosity Index Improver Polymer Types

Four polymer chemistries dominate commercial production, each with a distinct performance profile. CheMost supplies all four across solid and liquid forms, from cost-effective OCP bales to high-reactivity PIB for dispersant synthesis.

The prevailing strategy in modern formulations is OCP as the workhorse (60–80% of the VM package), with PMA or HSD blended in for low-temperature or shear-stability enhancements. Hybrid OCP+PMA systems are common: OCP provides cost-effective thickening while PMA improves cold-cranking (CCS) performance and pour point. PIB occupies a specialty niche where ashless clean-burn properties matter — two-stroke oils, greases, and as HR-PIB feedstock for PIBSI dispersant manufacturing.

Solid vs. Liquid VII — Which Form to Choose?

OCPs and PIBs are available in both solid and liquid forms. Solid bales or pellets (J0010, J0050, V220) offer the lowest cost per kilogram of active polymer — but require dissolution in hot oil (100–130°C) with mechanical agitation at the blending plant. Liquid concentrates (VII6000, VII8000) are pre-dissolved polymer-in-oil solutions at 500–1500 cSt, eliminating the dissolution step and reducing blending complexity. The trade-off: liquid concentrates cost more per unit of active polymer and include diluent oil that must be factored into the base oil blend calculations. Large blending operations tend to prefer solid bales for cost; smaller or faster-turnaround blenders prefer liquid concentrates for simplicity.

How to Select a Viscosity Index Improver

• Target viscosity grade (SAE J300). This is the starting point. A 5W-30 passenger car oil needs a VII that delivers good CCS performance at -30°C while maintaining HTHS above 2.9 cP at 150°C. For a 15W-40 HDEO, the CCS limit at -20°C is the binding constraint. Map your base oil CCS and KV100 against the grade limits first, then determine how much VII is needed to close the gap.
• Shear stability requirement. Long-drain diesel engines (50,000+ km) need low-SSI polymers (SSI 24–30) to stay in grade. Passenger car oils with 15,000 km intervals can use medium-SSI (SSI 35). Industrial applications with short drain intervals can use high-SSI (SSI 45–55) for maximum thickening at minimum cost. VII8000 (SSI 25) keeps oil in grade longer than VII6000 (SSI 35) but requires a higher treat rate.
• Low-temperature performance. If your formulation is CCS-limited (tight cold-cranking target), consider high-ethylene LTOCPs. They contribute less to CCS viscosity due to low-temperature coil contraction. Or blend in PMA to improve cold fluidity. But be aware: LTOCPs may require specific PPD matching to avoid MRV yield stress failures.
• Blending plant capability. Solid OCP bales require heated dissolution tanks and mechanical agitation. If your plant doesn't have this infrastructure, pre-dissolved liquid concentrates (VII6000, VII8000) allow direct blending with no dissolution step. The per-kilogram cost is higher, but the capex and blending time savings often outweigh it for smaller operations.
• Oxidation and deposit sensitivity. OCPs are non-polar hydrocarbons — they oxidize like base oil does. In high-temperature applications (turbocharged gasoline direct injection, heavy-duty diesel), consider functionalized OCPs (dispersant OCPs, antioxidant OCPs) that carry grafted dispersant or antioxidant groups on the polymer backbone. These reduce the detergent/dispersant load needed elsewhere in the formulation.

Applications of Viscosity Index Improvers