As additives, small amounts of specialty polymers can significantly benefit the performance of most lubricating greases, providing a straightforward and useful way to differentiate the grease for applications and customers.
Specialty polymers benefit the spectrum of grease types: aluminum complex, calcium sulfonate, polyurea, organic, and the most popular lithium and lithium complex. There are tradeoffs, however. Issues like low temperature mobility in calcium sulfonate grease and very limited compatibility in inorganic silica greases compel formulators to test and optimize various types of polymer in their products.
Potential benefits of polymer additives include:
Greater cohesive tack. Polymers reinforce the greases fibrous structure and reduce grease deterioration under adverse conditions such as high temperature or mechanical working.
Enhanced water resistance. Better adhesion results in better water wash-out and water spray-off properties. This is typically the main reason why polymers are added to grease.
Greater adhesive tack. Specific polymers improve adhesion to the surface of equipment. The grease stays where the work is being done.
Shear resistance. Roll stability can be improved by 60 percent in certain greases.
Reduced bleeding. Polymers can trap more oil in the grease and increase the time for oil separation.
Added yield. Polymers stiffen the grease, so more oil may be contained in the modified thickener, increasing the yield and lowering costs.
A polymers performance in a grease depends on the lubricants base fluid as well as its soap structure. The polymer must be compatible with the base oil. Due to the difference in chemical structure between mineral oils and vegetable oils, the compatibility of polymer with base fluid varies.
The same polymer that is compatible with the base oil must also form an interpenetrating network with the grease soaps fibrous structure. This interpenetrating network is largely irreversible under typical operating temperatures.
There are at least eight major families of polymers relevant to grease production. The chemical structure of the polymer itself also influences the final product. A fatty acid may contain up to 20 carbon atoms, while a polymer chain may be as long 200,000 carbon atoms. The ordering and size of these chains will govern how effective the polymeric additive is in providing the final product with viscosity, tack and water spray-off resistance.
In manufacturing, it is necessary to understand when and at what temperature to combine the polymer with the grease. Polymers form an interpenetrating network with the grease soap through several different mechanisms depending on the type of polymer. This networking mechanism governs when it is best to blend the polymer with the grease during the manufacturing process.
For non-reactive polymers, in batch operations, generally the polymer is added with the cooling oil at temperatures of 120 degrees Celsius or lower for a period of about two hours or until the polymer is fully dissolved.
If the polymer chemistry is reactive, it is best to add it at the initial step of manufacturing. If a grease plant prefers to add a non-reactive polymer at the beginning of the reaction, it is likely necessary to increase the treat rate as temperatures around 200 C may degrade polymers. This could result in grease darkening and inferior performance.
Most importantly, be patient in adding a polymer when preparing a manufacturing batch or a lab batch. Experimentation and optimization is needed.
A Good Fit
The compatibility between a polymer and the lubricant oil in grease is a key factor determining the performance of the polymer additive in the final product.
Like dissolves like. Generally speaking, but with exceptions, base oils with double bonds (unsaturated) are compatible with unsaturated polymers; base oils with no double bonds (saturated) are compatible with saturated polymers.
Base fluids in grease can be petroleum-derived (including polyalphaolefins), synthetic (esters, ethers, silicones, others) or extracted from plants or animals. Conventional mineral oils-hydrocarbons that tend to have single bonds-are classified as paraffinic oils based on alpha-alkanes, naphthenic oils based on cycloalkanes or aromatic oils based on aromatic hydrocarbons. Semi-crystalline ethylene/propylene copolymer (OCP) and styrene/ethylene/butylene copolymer (SEBS), as well as anhydride grafted OCPs, can significantly improve the water resistance of mineral oil based grease due to formation of interpenetrating networks.
Biobased oils, biodegradable oils and certain esters have unique tribological properties. The major component in vegetable oils is triglycerides, which is an ester derived from glycerol and a mixture of saturated and unsaturated fatty acids. The ester structure and unsaturated double bonds are what make vegetable oil biodegradable. Polyisoprene, styrene-butadiene and anhydride-grafted OCP polymers can improve the water resistance of vegetable oil based grease. On the other hand, OCP grafted with amide/anhydride can form intermolecular hydrogen bonds and cause the grease to gel.
Networking
Polymeric additives form a three-dimensional network with the grease thickener structure by one of several mechanisms: physical crosslinking via a crystalline phase; a less soluble hard phase; or by hydrogen bonding, chemical reactions or a more simple long-chain entanglement. Different families of polymers will structure themselves in the grease network through one or more of these processes.
During chain entanglement, the long chains in a polymer provide viscosity due to individual chains tangling up with one another. The grease soap and polymer networks then entangle to form an interpenetrating network. When the grease and polymer are heated, the polymer chains diffuse throughout the grease network and remain dispersed after cooling. Viscosity modification begins at treat rates of up to 10 percent with polymers of 200,000 to 400,000 molecular weight. Imparting tack to grease requires much longer chains (1 million to 6 million MW) but treat rates as low as 0.5 percent.
Another type of network is a polyolefin structure. Olefin copolymers contain a blend of ethylene, propylene and/or butylene units within a single chain. Polymers with an overabundance of one unit type will partially crystallize in oil, as observed with paraffin waxes. Small crystalline regions dispersed throughout the oil bind the polymer chains together into a strong network around the soap. Varying the ratio of olefin units can be used to tailor this network.
In hydrogen bonding, hydrophilic sites on polar esters, amides and anhydrides form strong interactions capable of binding polymer chains together into a network. This effect is particularly strong when the polymer is dispersed into hydrophobic oil. Polar groups may be included on a monomer during polymerization or chemically grafted to the polymer post-production.
Block copolymers are another way networks are formed. Most polymers of two or more monomer types contain a random distribution of those units along the chain, but block copolymers are designed with one or more of a single monomer together. Separating the oil soluble and insoluble monomers along the chain causes the different regions to bind together when dispersed into oil. This is similar to how fatty acid soaps form their structure. For example, styrene-olefin and styrene-butadiene copolymers are produced as block copolymer systems with styrene as the oil insoluble monomer.
Passing the Test
Functional Products has studied and tested a range of polymer chemistries to determine how each might enhance a lubricants performance. In our investigation, we looked at cone penetration (ASTM D217), water spray-off (ASTM D4049) and low temperature mobility. Study data reflect successes and failures, but when the polymer is optimized (with only a few exceptions such as silica grease), the additives confer material benefits to grease.
Results of the cone penetration test showed that a polymer network stiffens the grease and improves shear stability. The polymers reinforced the grease soap matrix, lessening the amount of breakdown as the grease was being worked. Additionally, certain polymers retained the greases base oil for a period, which reduced bleeding.
The water spray-off test evaluates a greases ability to adhere to a metal panel when subjected to direct, intense water spray. Results showed that the existence of a polymer network structure improved the water resistance of grease.
Certain polymers also improved the adhesiveness and tack of the grease, allowing it to stay in place for a longer period. Tackifiers have high cohesive and adhesive forces. High cohesive forces allow the tackifier to remain a single mass while high adhesive forces cause it to remain on the surfaces being lubricated. The reference grease, a calcium complex product, had predominantly adhesive tack. Greases formulated with 1 to 1.5 percent polymer tackifier displayed a significant amount of cohesive tack. At a treat rate of 2 percent tackifier, the grease displayed a significant amount of both adhesive and cohesive tack.
For lithium complex grease and aluminum complex grease, low temperature mobility results at -1 degree C showed higher final pressures because grease does not flow as well at low temperatures. This was not materially different from the reference grease, with the exception of the SEBS polymer, which stiffened and became hard to pump. For calcium sulfonate grease, the polymers dramatically raised the pressure differential between the reference grease and the treated grease. With polymer added, all calcium sulfonate greases did not flow as well as lithium complex grease. z
David DeVore is president of Functional Products Inc. While others helped shape this article, Dan Vargo, Shanshan Wang and Erik Willett of Functional Products contributed extensively. For the complete paper with specific citations and references, contact dvargo@functional
products.com.