The inherent problems of vegetable oils, such as poor oxidation and low-temperature properties, can be improved by attaching functional groups at the sites of unsaturation through chemical modifications. In this article, you will see how functionalization helps overcome these disadvantages.

Five branched ester structures were prepared from commercially available methyl oleate and common organic acids. These branched esters are characterized as alpha-hydroxy ester derivatives of methyl oleate. Pour-point and cloud-point measurements have shown that this derivatization improved low-temperature properties over olefinic oleochemicals. The derivatization also increased thermo-oxidative stability, measured using Pressurized Differential Scanning Calorimetry (PDSC).

Tribological behaviors were evaluated as additives in hexadecane and polyalphaolefin, using four-ball and ball-on-disk configurations. These derivatives have good anti-wear and friction-reducing properties at relatively low concentrations under all test loads. Overall, the data indicates that some of these derivatives have significant potential to be used as lubricating base oils or additives.

Vegetable oils, or their derivatives, are a good alternative to petroleum oils as lubricants or lubricant additives in environmentally sensitive industrial applications. In many industries, around 40 percent of a lubricant can be lost to the environment. With the petroleum prices at a record high, development of economically feasible new industrial products using soybean oil is highly desirable. Though soybean oil and its derivative oleochemicals show superior lubricity, vegetable-oil-based lubricants have a lower oxidative stability and poor cold flow properties at low temperatures. One potential way to improve oxidation and low-temperature property is to modify it by attaching some functional groups at the site of unsaturation.

In this article, we report the viscosity, cold flow properties, thermo-oxidative stability and tribological behavior of a series of branched fatty esters — propionic ester of methyl hydroxy-oleate (PMO), the levulinic ester (LMO), the hexanoic ester (HMO), the octanoic ester (OMO) and the 2-ethylhexyl ester (EHMO) synthesized using oleochemical epoxides and carboxylic acids.

The ring-opening reaction of epoxidized methyl oleate (EMO) using propionic, levulinic, hexanoic, octanoic or 2-ethylhexanoic acids give respective alpha-hydroxy ester derivatives of methyl oleate as shown below. 

These derivatives were characterized for their kinematic viscosities at 40 and 100 degrees C (ASTM D445-95); oxidative stability (onset temperature) using pressure differential scanning calorimetry (PDSC); low-temperature flow property using pour point (ASTM D-5949) and cloud points (ASTM D-5773); boundary lubrication properties (coefficient of friction) as additives using ball-on-disk configuration; and anti-wear properties (wear scar diameter) as additives using four-ball test configuration.

Kinematic viscosity and low-temperature properties of hydroxy-ester products are shown below. Both viscosities are significantly larger for LMO than the other products, which may be due to the more polar structure of LMO compared to others, resulting in stronger intermolecular interaction. 

Except LMO, the viscosities increase with increasing chain length of ester branching due to overall increase in the molecular weights of the products. Attachment of an ester side chain with optimum length at the 9-10 position of the fatty acid chain improves the PP significantly as shown for OMO. All other products (except LMO) have PPs in the range of -15 to -33 degrees C and CP in the range of -8 to -31 degrees C.  It can be assumed that the presence of a large branching point on the fatty acid ester creates a stericbarrier around the individual molecules and inhibits crystallization, resulting in lower pour and cloud point.

 

An important property of lubricants is their ability to maintain a stable lubricating film at the metal contact zone.  Under lubricated conditions, the hydroxy and ester group of the products offers active oxygen sites that bind to the metal surface. The friction-reducing property of hydroxy-ester products as additives in hexadecane (0.01 M) is shown in Figure A. Under high load, all of the hydroxy-ester products show excellent reduction in CoF at 0.01 M concentration. The CoF values of all products are less than 0.15, a considerable improvement over the value of 0.4 observed for neat hexadecane. The esters are more effective than methyl oleate (0.19) or EMO (0.22) as additives in hexadecane at the same concentrations.

The four-ball tests were done in three different base oils — hexadecane (HD), soybean oil (SBO) and polyalphaolefin (PAO) — to demonstrate their effectiveness in petroleum, bio and synthetic base oils. In order to simplify results, the base oils used did not contain any other additives.

Hydroxy-ester products at 5-percent concentration in PAO lowered the wear scar diameter compared to PAO alone. Overall, the addition of any of the hydroxy-ester products caused a considerable reduction in wear in either PAO or hexadecane lubrication fluid.   

A possible explanation for improved tribological properties of hydroxy-ester products is the presence of two extra polar functional groups apart from the ester group of fatty acid ester. Oxygen functionalities, like the hydroxy and ester at the 9-10 position on the fatty acid, help the compounds adhere to the metal surface and reduce friction, especially under excessive load.

 

 

 

 

 

 

PDSC is an effective way of measuring the oxidative tendency of lubricant base oils, vegetable oils and oleochemicals in an accelerated mode.  A high onset temperature would suggest a high oxidation stability of the oil.  The OT for PMO is highest (175 degrees C) among this series of hydroxy-ester products, followed by EHMO (166 degrees C), LMO (162 degrees C) and OMO (160 degrees C). The data shows that oxidative stability decreases with increases in chain length of the ester side chain, may be because the longer side chains are more susceptible to oxidative cleavage than short ones.

The results show that hydroxy-ester products have better lubricant properties under experimental conditions, which enable them to outperform their methyl-ester analogues. Specifically, they have good anti-friction and anti-wear properties even as additives. Also, OMO demonstrates excellent low-temperature properties, while others show good low-temperature fluidity compared to their methyl ester analogs. Apart from being bio-based, these products remain a viable option for application in the lubricant industry.