Dispersants

Dispersants

Dispersants are a vital component for lubricant formulators. They are widely used in a variety of lubricant applications and may make up 60% of the additives in a crankcase lubricant package. Usage has grown for many decades, with variations of chemistry based on their purpose. Why are they so important, how have they evolved over time, and will they continue to be a major formulation component in the future?

Ashless dispersants are used primarily in passenger car, motorcycle and heavy-duty diesel crankcase oils plus automatic transmission fluids. To a lesser extent, they may be used in other such applications as natural gas engine oils, other transmission oils, and marine and industrial lubes.

Dispersants keep insoluble materials and soot suspended in the oil so they can be removed at the next oil change. They prevent waste materials from agglomerating, causing deposits around the engine and reducing operational efficiency.

In gasoline engines, sludge formation under low speed, low temperature, stop-go conditions can be an issue. Oil-insoluble polar materials and contaminants build up in the oil. If unchecked, they cause sludge and varnish in cooler areas of the engine, creating operational problems.

Dispersants control viscosity increase caused by soot formation, primarily in HDD engines but also in some gasoline direct injection engines. The soot is created during the combustion process and makes its way into the bulk oil, causing the oil to thicken.

Natural gas engines can be very sensitive to lubricant ash levels. They come in many designs and operate on a wide variety of fuel sources. Lubricants with no or very low ash rely on ashless dispersants to provide the maximum control of insolubles and deposits.

Conventional dispersants are organic materials consisting of an oil-soluble polymeric tail, usually polyisobutylene, and an attached polar group. The polar group consists of a bridging group, usually maleic anhydride, and a functional group normally based on nitrogen.

The most common conventional dispersant type uses PIB as the oil-soluble group. The molecular weight is a key variable for dispersancy properties. PIB is made by oligomerization of isobutylene and is available in a variety of molecular weights from a few hundred to tens of thousands.

The polymeric group has to be oil soluble, and the polar group has to attach itself to the waste material in the oil so that it remains in the oil solution. If the alkyl polymer group is too small, the dispersant is not capable of keeping the insoluble material dispersed.

To convert PIB into dispersant, it is grafted with maleic anhydride (bridge) to form polyisobutylene succinic anhydride. The reaction with maleic anhydride can be “thermal,” using highly reactive PIB (HR-PIB) or facilitated with chlorine gas. More than one maleic anhydride can be added to a molecule of PIB to maximize the functionality per molecule.

PIBSA then reacts with an amine to give functionality. The type and nitrogen level of the amine is a further variable, and in many dispersants this is a polyamine. Other modifications, like adding boron, can be made to amend the properties.

Dispersant viscosity modifiers are used in some engine oil formulations. They do not use a PIB polymer, but instead a standard polymer, like olefin copolymer, which reacts with maleic anhydride to become functional. These have much longer chain lengths than conventional dispersants.

Dispersant poly-methacrylates use an alkyl methacrylate monomer to create an oil soluble polymeric group. The carboxylic acid group in the monomer is used as a bridge to add the nitrogen-containing functional groups. The bridge group and functionality are regularly attached along the polymer chain.

Dispersant PMA properties can be varied through the choice of the methacrylate-based monomer, the polymer molecular weight as well as the type and nitrogen level of the functional amine group. They combine the process of enhancing the viscosity characteristics of the fluid with dispersancy control. PMA VM technology is used in transmission fluids because of its very good low-temperature fluidity properties relative to other VM types. PMAs can be combined with the rest of the additive package into a single stable transmission performance package.

Current drivers for new crankcase lubricants include reducing emissions and improving fuel economy. Dispersants have no significant impact on emission control hardware, like exhaust catalysts and particulate filters, and no contribution to the chemical constraints of sulfated ash, sulfur and phosphorus. Hence, they are beneficial components in formulations constrained for emissions. The drive to lower-viscosity oils to improve fuel economy is a challenge for dispersants, as they are a significant thickening contributor to low-temperature viscosity. Researchers are seeking to maintain the benefits of sludge, varnish and soot control while reducing the polymeric contribution to viscosity thickening.