However, these compounds produce ash by-products when reacted in the lubricant, which are believed to poison emission control systems like three-way catalysts and cause risks to human health from potential inhalation, such as in vehicle exhaust gases when the additives are used in engine oil or through worker exposure when used in metalworking fluids. They can also lead to ground and water contamination when used in agricultural and marine applications, said Cinta Lorenzo-Martin, principal materials scientist at Argonne National Laboratory in Illinois.

Ongoing research in recent years has aimed to test the properties of additives that might stand in for organo-metallic ones (see Finding Alternatives to ZDDP in the November 2018 issue), but so far it has been difficult to find additives that are fully organic and that work as well as conventional additives, especially when we think about replacing ZDDP, Lorenzo-Martin told attendees at the Society of Tribologists and Lubrication Engineers annual meeting in Minneapolis.

ZDDP has proven optimal performance in several categories, which is why finding alternatives is so difficult. When you think about how ZDDP behaves, if you want to replace it, suddenly you have to figure out how you come up with a chemistry that is going to do everything that ZDDP does.

Due to the multi-functionality of ZDDP, replacing it with so-called ashless additives-which mostly have carbon, hydrogen, oxygen and nitrogen compounds-would likely require multiple additives, increasing potential chemical interactions, which Lorenzo-Martin noted leads to higher complexity in synergistic and antagonistic effects.

Formulations and Parameters

Supported by the United States Department of Energys Office of Vehicle Technologies, Lorenzo-Martin conducted a study to evaluate the performance of two ashless, biobased antiwear and friction modifier additives under boundary lubrication conditions. The polymeric vegetable oils have a three-dimensional network structure and are capable of forming a thick lubricating film on surfaces. These additives were formulated to be multifunctional as well as renewable, biodegradable and without negative health effects.

The approach consisted of testing two types of additives at 2 percent and 5 percent weight blended in an ultra-low-viscosity, 4 centistoke polyalphaolefin synthetic base oil and an API Group III mineral oil of the same viscosity.

The main difference between the biopolymer additives was their molecular weight. Additive 1 had a mass-average molecular weight of 6,209; Additive 2 was heavier at 10,250 Mw. Lorenzo-Martin later told LubesnGreases that, for example, smaller molecules are favored in metalworking fluids used in cutting applications, while for drawing and stamping operations, higher molecular weight additives provide better performance.

She noted that the additive supplier, Archer Daniels Midland Co., recommended the use of a dispersant to go along with the finished lubricant formulations, so she selected oleic acid to blend between 0.132 percent and 0.3 percent of weight into the samples. These [additive] molecules can differ insize, polarity and acid value. Therefore, in some cases the formulator needs to use a dispersant or additional surface wetting agents to adjust emulsion stability, emulsion particle size or surface tension, Lorenzo-Martin expanded.

In total, 10 PAO based lubricant samples were tested: PAO alone; PAO and oleic acid; PAO and 2 percent Additive 1 or Additive 2; PAO and 2 percent additive with oleic acid; PAO and 5 percent additive; and PAO and 5 percent additive with oleic acid. Similarly, she tested 10 mineral oil based samples with these same parameters for additives and oleic acid.

The fluids were tested for friction and wear performance with a 52100 steel ball on a mirror finish flat disc in reciprocating sliding configuration using a high frequency reciprocating rig. In this test, the ball remained stationary under static force while the flat disc slid back and forth. The test parameters were 15.6 Newtons of load, speed of 60 revolutions per minute at 100 degrees Celsius for a duration of 64 minutes.

Part of the evaluation included running a speed ramp procedure that consisted of two speed rounds going from 0 to 300 rpm, one at the beginning and one at the end of each test to compare how the lubricant behaves over the range of lubrication regimes and how this changes after you form [a tribofilm] on the surface during the one-hour test, Lorenzo-Martin added.

Friction Results

An initial test to measure friction evolution found that friction was noticeably higher and noisier for the base oils alone, while adding oleic acid to the PAO and the mineral oil led to quieter operation with less friction.

Both additive types reduced friction at 2 percent and 5 percent weight with and without oleic acid in both base fluids, she pointed out.

When compared to PAO alone, both types of additives reduced friction by at least half. Additives blended with oleic acid showed the most friction reduction, especially for the lower molecular weight Additive 1. This is because oleic acid is more surface active, and when you have a molecule, depending on the shape, they will attach to surfaces and form a tribofilm faster, Lorenzo-Martin explained.

Friction results for the additives in mineral oil were similar to those of the PAO based mixtures, with a 50 percent reduction compared to the baseline fluid. Oleic acid alone was also effective in reducing friction, though the fluids with additives generally performed better.

But it seems that polymeric vegetable oils are not the best cards for mirror finish surfaces in the anti-friction game. Further studies will be conducted on more representative surfaces, she added.

Lorenzo-Martin also conducted a test to compare the friction reduction capacity of the biobased additives against organo-metallic additives, in this case ZDDP and molybdenum dialkylthiocarbamate (MoDTC). Results favored the conventional additives more than the biobased ones. As a friction modifier, this [performance] is not as optimal as we were expecting, she lamented.

Wear Performance

The additives wear prevention properties show more promise.

Wear scar volumes were measured with white light interferometry. In PAO, both additives significantly decreased wear on the flat disc-over one order of magnitude. Those mixed with oleic acid were equally as effective in reducing wear when compared to the baseline PAO. However, oleic acid alone was just as effective as the additives in reducing flat wear over the pure PAO.

Ball wear results showed similar performance, with the additives producing a reduction of up to two orders of magnitude from the baseline PAO. But again, adding the oleic acid alone was extremely effective in reducing ball wear-even better than the additives in 5 percent concentration.

Compared to the traditional additives, Lorenzo-Martin pointed out that there was similar flat wear reduction between the fluids with 2 percent by weight of biobased additives and the oils containing ZDDP and MoDTC additives. But for fluids with 5 percent biobased additives, wear reduction was better than for the model fluids.

In ball wear tests, both biobased additive concentrations showed improved wear reduction over the traditional additives, even resulting in negative wear, meaning material actually built up on the surface rather than wearing away.

Where the Chips Fall

While friction performance was a bust in these tests, the researchers had more luck with the green additives wear protection, making them better suited to be antiwear additives than friction modifiers. Lorenzo-Martin attributed the wear prevention to formation of a protective film on the surface of the test pieces, noting the need to study the nature and dynamics of this tribofilm in order to optimize additive performance.

According to Lorenzo-Martin, biobased additives are already used commercially in lubricants, grease, metalworking fluids, gear oils and other industrial oils. She explained to LubesnGreases that the polymeric vegetable oil molecules should be durable because they are crosslinked and a lot of the double bonds are utilized, giving them oxidative stability.

Further, these particular additives do not stack up and solidify at low temperatures as fatty acids do. They are formed by polymerization above 300 degrees Fahrenheit, so polymerization does not continue below that temperature and the oils stay liquid. The additive supplier, Archer Daniels Midland Co., noted that the shelf life of the additives exceeds five years and that molecular weight does not change during storage.

The advantage of these molecules is their biodegradability. Although they are stable to oxidation and pretty resistant to hydrolysis, in the soil with the right bacteria and moisture conditions, they will degrade at a slower rate, said Lorenzo-Martin. Eighty percent degradation can be achieved in 25 to 30 days depending on the type of additive and molecular weight, she noted. To meet the U.S. Environmental Protection Agencys definition of readily biodegradable, a lubricant must show 60 percent degradation within 28 days.