New antiwear and EP additive technology

Tribology & Lubrication Technology, Mar 2008 by Canter, Neil

The tremendous demands placed on automotive and industrial lubricants to provide superior performance over longer operating periods and to also act in an environmentally friendly fashion is accelerating the search for newer additive technologies that can meet these two critical criteria.

One additive segment that has provided traditional chemistries with strong performance features over many years is antiwear and extreme pressure additives. Unfortunately, many of these chemistries have been scrutinized due to environmental concerns.

Zinc dialkyldithiophosphates (ZDDPs) have been the main antiwear additive type used in automotive (such as passenger car engine oils, heavy-duty diesel engine oils, etc.) and industrial (antiwear hydraulic fluids, industrial gear oils, etc.). The mechanism of how ZDDPs provide such strong performance was discussed previously in TLT.1 We are all aware that the excellent film-forming properties of ZDDPs are the key to its strong antiwear characteristics, but it can also lead to poisoning emissions catalysts, which is of concern to the automotive industry.

Another additive that has been used to provide extreme pressure properties is molybdenum disulfide. This compound is used as a solid lubricant and can be dispersed into oil for use in industrial lubricant applications such as grease. The rapidly rising cost of molybdenum disulfide is of concern and is prompting the need for evaluating alternative technologies that exhibit better cost performance.

Engine oils

A new technology known as TechroBond(TM) has just been introduced by Dallas-based Platinum Research Organization and has the capability of providing antiwear and extreme pressure properties for many lubricant applications. STLE member Pranesh Aswath, professor of materials science and engineering at the University of Texas at Arlington, says, "TechroBond represents a family of close to 20 products that are prepared from a combination of organo-thiophosphates, fluorinated hydrocarbons and other chemistries. The fluorinated hydrocarbons are functionalized derivatives of polytetrafluoroethylene."

Researchers found that treatment of commercial ZDDPs with ferric fluoride at elevated temperatures can lead to the incorporation of fluorine on the backbone of this antiwear additive. Aswath says, "Nuclear magnetic resonance (NMR) studies clearly show that there has been a change in the structure of the initial ZDDP molecule used in our experiments. Further support for this finding comes from elemental analysis of the resulting product that shows the sizeable presence of fluorine."

The impact of fluorination is to reduce the decomposition temperature of ZDDP, which influences its antiwear performance and effect on the emissions catalyst. A modified ball-on-cylinder test was conducted to evaluate the performance of fluorinated ZDDP vs. conventional ZDDP. The experimental setup is shown in Figure 3.

Aswath explains, "The traditional procedure for using the ball-on-cylinder is to completely coat the sample with the test lubricant. We decided to limit the amount of lubricant used to 50 microliters at the contact point to boost the severity of the test."

Test results show that the use of the fluorinated ZDDP in a commercial engine oil, with a phosphorus content of 100 ppm, exhibits superior wear protection as compared to the conventional ZDDP used in the same engine oil at a phosphorous content of 750 ppm.

Aswath says, One of the reasons the fluorinated ZDDP produced better antiwear results is that incorporation of the fluorine enables the additive to decompose at a lower temperature. The resulting film produced adheres more effectively to the metal surface and is 40% to 50% thicker, as measured by focused ion beam and nanoindentation testing."

These results indicate that fluorinated ZDDP is a more effective antiwear additive than conventional ZDDP. Aswath adds, "We believe that fluorinated ZDDP increases the efficiency of conventional ZDDP. With the industry looking to reduce phosphorus levels in engine oils, fluorinated ZDDP can provide good performance to meet the needs of the automotive industry."

One other benefit of less ZDDP in an engine oil formulation is reduction of the concentration of deposits on the catalytic converter, extend its operating life and facilitating the reduction of automotive emissions. Future work will involve verification of this factor and also an evaluation of fluorinated ZDDP-based automotive lubricants in a wide range of engine tests.

No incompatibilities have been found with fluorinated ZDDP and the other additives used in conventional engine oils.

Greases

The organo-thiophosphate, fluorinated hydrocarbon technology also has been evaluated as an alternative for molybdenum disulfide in greases. Some initial work in testing this technology in a railroad grease was described in an previous TLT article.2 For the current series of testing, Aswath says, "We initially evaluated our product in a lithium-based grease at a comparable treat rate to molybdenum disulfide."