Environmentally friendly extreme pressure additive

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2011

Researchers develop new lignin-based additives for use in lubricant applications.

KEY CONCEPTS
• An environmentally friendly EP additive has been developed based on lignin, a naturally occurring organic polymer.
• The lignin-based EP additive displays better performance than molybdenum disulfide in four-ball EP and wear testing on aluminum complex greases.
• EP performance is due to the ability of the oxygen-containing, organic functionalities to form a natural barrier between metal surfaces under extreme pressure conditions.

Extreme pressure (EP) additives perform an important function in providing the needed lubricity to ensure that machinery does not suffer friction, wear and damage under severe operating conditions that typically involve high temperatures and pressures.

The classic EP additives are based on chlorinated, phosphorus, sulfur and overbased sulfonate additives. The first three react with the metal surface to form a chemical film at specific activation temperatures. Overbased sulfonates form a film that acts as a barrier between metal surfaces.

A previous TLT article focuses on these EP additives and provides information on how they function (1). One of the key drivers affecting the current and future use of EP additives is whether restrictions will be made on the use of chlorinated paraffins. Currently, chlorinated paraffin use has been under scrutiny primarily in the U.S. and Canada.

In exploring potential new technologies, one goal has been to develop more environmentally friendly EP additives that provide at least comparable performance to the existing portfolio of products. STLE-member Dr. Liwen Wei, president of Novitas Chem Solutions LLC in Bellaire, Texas, says, “We decided to examine the properties of the naturally occurring material known as lignin. The reason for our interest is that we were impressed with lignin’s aromatic and chemical functionalities of phenols, alkoxy and hydroxyl groups. Lignin is an ashless, naturally occurring material that is used in animal feeds and in food supplements, which means that it is environmentally friendly and safe.”

Wei points out that lignin is one of the most abundantly available organic polymers and is the main structural component of wood that comes from trees. Lignin is commercially produced as a byproduct in the manufacture of paper through several pathways, including the Kraft process. He adds, “The makeup of lignin is tied directly to its wood/plant species origins. The molecular weight of lignin can vary greatly from 200 to over one million.”

Further background information on lignin can be found in a previous paper written by Goring (2).

Development of lignin-based extreme pressure additives may provide an alternative technology that is also environmentally friendly. Such a series of additives are in the process of being prepared and evaluated for use in lubricant applications.

MULTIFUNCTIONAL PROPERTIES
Wei, in collaboration with STLE-member Dr. James King, president of Desilube Technology in Lansdale, Pa., has developed a new EP additive technology based on lignin. Wei says, “We have prepared a series of lignin-based additives that have demonstrated outstanding extreme pressure and antiwear characteristics. The product line is in the process of being commercialized and will be marketed under the Desilube AEP trade name.”

The lignin-based EP additives are solids, which led the researchers to evaluate them initially in greases. Initial evaluation testing was done by incorporating the lignin-based EP additives in lithium soap and aluminum complex greases.

Four-ball EP and wear testing was conducted on these greases. Initial testing that incorporated 1% and 3% of the lignin-based EP additive in a lithium soap grease showed four-ball weld load that increased from 160 kgf to initially 200 kgf at the 1% treat rate and then to 315 kgf when the additive is used at 3%.

Comparison testing with molybdenum disulfide was done in an aluminum complex grease. At a treat rate of 3%, the lignin-based EP additive displayed a four-ball EP weld load of 315 that is nearly double that of 3% molybdenum disulfide formulated in the same grease. The wear scar diameters for both greases are in the 0.6-mm range. Graphite used at a 3% treat rate also displayed a four-ball EP weld load that is half of the lignin-based EP additive.

Results from four-ball EP testing showed that a synergism is achieved between the lignin-based EP additive, a commercial EP additive and sulfurized esters. A weld load of 400 was obtained with a 1.5% treat rate of the lignin-based EP additive with the two other EP additives.

The phenolic functionality in lignin led the researchers to evaluate the antioxidant properties of the lignin-based EP additive. Using the same aluminum complex grease, the researchers evaluated the base lubricant vs. one treated with 3% of the lignin-based EP additive in the oxygen pressure method described in ASTM D942. After 100 hours, no pressure drop was seen with the grease containing the lignin-based EP additive. In contrast, the base grease generated a pressure drop greater than 50 psi after 100 hours. Further testing up to 500 hours led to a pressure drop of 19 psi for the grease containing the lignin-based EP additive.

The researchers prepared the lignin-based EP additive at particle sizes ranging from less than one micron to 100 microns. Wei says, “We observed that reducing the particle size of the grease will lead to an increase in the thickness of the grease and a change in the appearance of the grease.”

Figure 1 (a) shows a lithium soap grease that is prepared with lignin particles displaying a size of 65 microns. When the particle size is reduced to 1.5 microns, the resulting grease shown in Figure 1 (b) is a brighter brown color.


Figure 1. A reduction in the particle size of the lignin-based EP additive will lead to an increase in grease thickness and appearance. Figure 1 (a) shows a lithium soap grease prepared with lignin particles displaying a size of 65 microns. Reducing the particle size to 1.5 microns leads to the formation of the brighter grease shown in Figure 1 (b). (Courtesy of Novitas Chem Solutions LLC)

Wei believes this grease thickening effect is due to the interaction between the lignin and the soap thickener. He says, “Our hypothesis is that the hydroxyl groups in the lignin interact with the soap thickener through hydrogen bonding.”

The mechanism for how the lignin acts as an antiwear/EP additive is due to its oxygen functionalities producing a natural barrier between the metal surfaces under severe conditions, according to Wei. He explains, “The solid nature and rich content of oxygen-containing, organic functionalities built into lignin produce a natural barrier that enables the additive to function under extreme pressure conditions. We believe the lignin-based EP additive combines the boundary lubricity characteristics of organic compounds such as esters with the solid EP properties of such minerals as calcium carbonate.”

Current research efforts are moving to see if the lignin-based EP additive can be incorporated into a dispersion so that it can be used in oil-, water- and even paste-based lubricants. Additional information can be found in a presentation made at STLE’s 2011 Annual Meeting & Exhibition last May in Atlanta (3) or by contacting Wei at liwenw@novitaschem.com.

REFERENCES
1. Canter, N. (2007), “Trends in Extreme Pressure Additives,” TLT, 63 (9), pp. 10–18.
2. Goring, D. (1962), “The Physical Chemistry of Lignin,” Pure Appl. Chem., 5 (1-2), pp. 233–310.
3. Wei, L. and King, J. (2011), “Novel Lignin Based Grease Additives,” Presented at the STLE 2011 Annual Meeting & Exhibition, May 16-20, Atlanta, Ga.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.