These benefits in a finished lubricant depend on the chemistry of the polymer, the base oil and other lubricant additives. Blends of two or more polymers in the same formulation can behave differently than individual polymers. And so a simple raw material substitution can become a project, and a new product development project can turn into a time-consuming ordeal.

Are polymers a hopeless conundrum for lubricant formulators?

According to Erik Willett, Andrew DeVore and Daniel Vargo of Functional Products Inc. in Macedonia, Ohio, polymers can indeed be complicated, but extensive experimental studies are yielding helpful insights regarding their use in lubricants. Willett gave an update on their findings and offered formulation guidelines at the Society of Tribologists and Lubrication Engineers annual meeting in May.

Viscosity

When a single polymer molecule is added to base stock, it forms a flexible coil. Depending on its size, a polymer coil can block or hinder movement of oil molecules, which thickens the oil and raises the viscosity, Willett explained at the Minneapolis gathering.

As more polymer molecules are added, they block more oil molecules, and the fluids viscosity increases-provided the coils are separated in a dilute solution. At a higher concentration (semi-dilute), polymer coils come into contact with each other and contract like crowded passengers in a bus or subway train. When the polymer concentration is even higher, coils can overlap and entangle.

Is it possible to predict which polymer, and how much, is needed to reach a target oil viscosity?

Willett explained that viscosity is higher when more oil molecules are blocked by polymer coils. Thus, lubricant viscosity depends on the concentration and size of coils in oil. Longer polymers with higher molecular weight naturally form larger coils than shorter polymers.

Further, a polymer will form a larger coil in compatible oil that is a good solvent for the polymer than in incompatible oil where the polymer folds upon itself to reduce contact with the oil.

As temperature rises, a coil tends to expand and block more oil molecules, counteracting the tendency of oil molecules to move faster and viscosity to decrease when the oil is heated. This is the principle behind formulating lubricants with polymers to balance the tendency of oil viscosity to decrease at high temperatures.

Viscosity Index

Viscosity index refers to the degree to which base stock or lubricant viscosity changes with temperature. The V.I. of an oil or lubricant is determined from its kinematic viscosity at 40 degrees Celsius and 100 C according to ASTM D2270. The higher the V.I., the smaller the difference between KV40 and KV100 and the less viscosity decreases as temperature rises.

Formulators use polymer additives called viscosity index improvers to increase V.I. The polymer coil expands with heat and boosts KV100 more than KV40. As a result, the difference between KV40 and KV100 is smaller than for the oil alone.

Willett explained that oil with V.I. 100 has a kinematic viscosity at 100 C that is roughly one-tenth of its kinematic viscosity at 40 C, and an oil with V.I. 200 has a KV100 that is about one-fifth of its KV40.

Viscosity index improvers are essential for lubricants used over wide temperature ranges, including multigrade motor and gear oils for vehicles and mobile equipment. Additionally, V.I. improvers are used to modify lower-viscosity base stocks to replace higher-viscosity oils. For example, the American Gear Manufacturers Associations 9005 guidelines for lubricating spur, helical and bevel gears specify three options: ISO 220 lubricant with V.I. 90, ISO 150 lubricant with V.I. 120 to 160 or ISO 100 lubricant with V.I. 240.

When temperature decreases, oil becomes thicker because its molecules are less mobile. At low enough temperatures, waxy molecules such as long, linear alkanes (C18 to C30) in mineral oil pack together and form microscopic crystallites. At the pour point temperature (ASTM D97), enough crystallites are present to trap smaller oil molecules; the fluid becomes a gel that cannot flow. Formulators use polymers called pour point depressants to modify the size and shape of crystallites and lower the pour point to prevent lubricants from clogging injectors and pumps in refrigerators, wind turbines and mining equipment.

Chemistry Conundrum

According to Willett, Viscosity index improvers are low- to high-molecular-weight (1,000-250,000 grams per mole) polymers that are added to oil to counteract the effect of oil thinning as temperature increases. Polymers are large molecules prepared from connecting many individual repeat units, called monomers, into a single structure. This structure may be linear like a chain or highly branched like a tree. The structure and choice of monomers determines the behavior of a polymer.

He continued, Polybutene and polymethacrylate polymers and olefin copolymers are common V.I.I.s used in gear oils, hydraulic fluids and crankcase oils, respectively. Some V.I.I.s perform better in one base stock than another. And there are situations where formulators blend two V.I.I.s in a formulation in order to use less of a V.I.I. that is more expensive or sensitive to shear. Blending two V.I.I.s with different chemistry or molecular weight sometimes results in synergy, or performance better than each V.I.I.

But using the wrong V.I.I. or blend of V.I.I.s can decrease viscosity index, increase pour point and worsen lubricant performance. Without an understanding of V.I.I. behavior, trial and error has been the only option for formulators. So we undertook a massive study to look for trends that can help us understand the behavior of polymers and how to formulate them in lubricants.

Willett prepared 81 model formulations for PB-OCP, OCP-PMA and PB-PMA combinations using 5 percent, 10 percent or 20 percent of each product (10 to 40 percent total) in API Group II, Group III or polyalphaolefin base oils. Pour point depressant was used at 0.2 percent by weight in the mineral oils. These model lubricants viscosity ranged from ISO 46 to 460 and from SAE 20 to 250.

Willett investigated simple questions: How do different V.I.I.s perform in the main classes of base stocks? How do blends of V.I.I.s differ? Which V.I.I.s affect pour point?

Formulations were compared in groups of three with low, medium and high (5, 10 and 20 percent) treat rates of one V.I.I., varied to demonstrate trends in viscosity and viscosity index with the combination of a second V.I.I. Blending different classes of V.I.I.s produced complex trends in viscosity index with treat level. Willett expected that adding more polymer would always increase V.I. Instead, he was surprised when his viscosity index data fell into four groups.

In Case 1, adding both V.I.I.s continuously increases viscosity and viscosity index. More coils block the movement of more oil molecules, and these coils expand with heat to boost KV100 more than KV40, which improves viscosity index. Case 1 is ideal, and it occurs when both V.I.I.s are soluble in oil and dilute (not crowded) so the coils expand easily when temperature rises.

In Case 2, adding V.I.I.s increases viscosity index at low or medium treat rate. As more polymer is added, viscosity index decreases or remains constant. Case 2 occurs when coils become crowded at higher concentrations (semi-dilute), which limits their ability to expand with temperature. The higher the molecular weight, the lower the treat rate at which coils crowd and cannot increase viscosity, so viscosity index suffers.

Willett observed a decrease in viscosity index when he added high-molecular-weight (10,000 to 100,000 g/mol) OCP at 4 to 5 percent and medium-molecular-weight (10,000 g/mol) PMA at 5 to 20 percent. This behavior occurs when the V.I.I. concentration changes from dilute to semi-dilute.

Case 2 can also occur when a low-molecular-weight V.I.I., such as PB, solvates a higher-molecular-weight V.I.I. instead of the oil molecules. The low-molecular-weight V.I.I. limits the expansion of the higher-molecular-weight V.I.I. with temperature. The net result is that oil viscosity decreases with temperature because the V.I.I.s are ineffective, and viscosity index worsens.

In Case 3, adding polymers has little or no effect on viscosity index because the molecular weight is very low and the polymer coils are small. For example, PB 1,000 had no measurable effect on base oil with viscosity index of 120, but PB 2,000 raised viscosity index from 120 to 145.

In Case 4, adding polymer decreases viscosity index because the polymer is incompatible with the oil, or the polymer concentration is too high and the coils collapse. The behavior of the oil itself dominates as temperature increases. Willett observed this behavior for PBs in PAO.

Viscosity index improvers can also influence lubricant pour point. Willett observed three cases: In Case A, adding V.I.I. has little or no effect on pour point. In Case B, adding V.I.I. has little or no effect on pour point up to a point, but then it increases as more of the polymer is added. In Case C, adding V.I.I. continuously increases pour point, a highly undesirable effect.

Willetts 81 formulations fell into 12 combinations of viscosity index behavior cases (1-4) and pour point behavior cases (A-C). The best combinations improved viscosity index without increasing pour point. Case 1 with Case A, Case 1 with Case B and Case 2 with Case A, all with low treat rate (5 percent) of PB along with PMA or OCP, gave the best performance.

He noted that in this study, Case 1 combined with Case A was only observed in API Group III oil.

The major source of poor performance (decreased viscosity index, increased pour point) was with V.I.I. at semi-dilute concentrations, incompatible with the oil or affected by a second V.I.I. that limited solubility and the expansion of coils with temperature.

Willett suggested several guidelines for formulators seeking to improve viscosity index: Use a lower treat rate of V.I.I. in a higher-viscosity base oil; use a higher-molecular-weight V.I.I. or higher-viscosity base oil; or use a lower treat rate of V.I.I. in a higher-
viscosity base oil and increase base oil solvency by blending in a co-base oil such as an ester or alkylated naphthalene.

To improve pour point, Willett recommended using V.I.I.s that do not contain waxy components and reducing treat rates by using base stocks with higher viscosity.

Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at mmmoon@ix.netcom.com or (+1) 267-567-7234.