Finished Lubricants

Giving Grease a Boost

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The use of synthetic ester base oils has been on the rise due to rapid industrialization and infrastructure development, the need for high temperature lubricants in applications where temperatures are constantly rising, increasing demand from the automotive and aerospace industries, as well as a growing market for environmentally friendly products.

Synthetic esters, which are made from carboxylic acids and alcohols, possess an ideal set of features when compared to traditional mineral oil base stocks, and grease manufacturers can exploit the technical performance of these fluids to make high-capability greases, noted Siegfried Lucazeau, product manager for industry and automotive lubricants at Paris-based Nyco.

There are scores of facilities that manufacture synthetic esters worldwide. Besides Nyco, noted producers include Croda, BASF, Cargill, Emery Oleochemicals, Lexolube (now part of Zschimmer & Schwarz) and Oleon, according to LubesnGreases Non-Conventional Base Stocks Guide. Production capacities for these facilities are unavailable because esters are usually manufactured in small batches of 1 to 20 metric tons.

Synthetic and semi-synthetic base stocks are a 2.9 million metric ton market, according to Parsippany, New Jersey-based market research firm Kline & Co. Of this volume, 11 percent are synthetic esters, and they are forcast to enjoy about 4.5 percent annual growth through 2019. The global synthetic ester lubricants market is expected to reach U.S. $2 billion by 2020, up from $1.3 billion in 2013, according to San Francisco-based consultancy Grand View Research Inc.

Synthetic esters are designed. They are not a result of the refining process. They can be built from a variety of building blocks, which means that their chemical structure may be selected to optimize some properties over other properties to best suit the requirements of the application, said Lucazeau during the annual conference of the National Lubricating Grease Institutes India Chapter in Varanasi, India.

Synthetic esters are available in a wide range of viscosity grades, from ISO VG 15 to ISO VG 10,000. Most synthetic esters are compatible with other base stocks. So they may be used as pure base fluids, co-base stocks or even additives in some cases, where they act as friction modifiers, he added.

Greases made with synthetic esters can be used in ultra-low temperature conditions in airframe lubrication, as well as ultra-high temperatures present in steel mills and furnaces, noted Lucazeau. Depending on the specific composition, these greases may be found in automotive applications such as bearings, in incidental contact areas of machinery used in the food industry, and in gears and other environmentally-sensitive areas in marine vessels.

Synthetic esters can be used in nonpolar, highly paraffinic fluid-based greases to improve thickener yield, resistance to oxidation, seal compatibility and evaporation while maintaining low temperature performance and ensuring solubility of additives, Lucazeau said. He added that the chemical structure of synthetic esters may be adjusted to maximize thermo-oxidative stability to make high temperature greases.

Obtaining Specific Properties

The chemical building blocks for synthetic esters, a family of products that includes monoesters, diesters, neopolyol esters and complex esters, are reacted under specific temperatures and pressures to produce the desired properties. Highlighting their unique performance profile, Lucazeau stated that they show lower volatility and higher flash points than other base stocks of equivalent viscosity, thanks to a stronger intermolecular attraction.

He added that synthetic esters are good polar solvents, helping to dissolve components such as additives in nonpolar base stocks. They also serve as seal swell agents in nonpolar media and show high affinity with positively charged metal surfaces, generating friction modification and wear protection.

Synthetic esters are composed of mainly one or several components. In addition to generating high flash points and low volatility, they also generate higher viscosity indices and lower pour points than the majority of traditional base stocks available, Lucazeau elaborated.

While some esters are prone to thermal degradation, Lucazeau said that neopentyl structures greatly improve thermal stability and are highly resistant to oxidation due to the strength of the carbonyl double bond. Synthetic esters tend to undergo clean decomposition, having low deposit formation propensity due to that specific structure and purity, he noted.

A comparison of various base fluids with synthetic esters, using the ASTM D4636 oxidation and corrosion test, revealed that synthetic esters have much higher resistance to oxidation and deposit formation than traditional base stocks. The results, as expected, were much better with neopolyol esters than with monoesters or diesters, Lucazeau said.

Neopolyol esters have a specific chemical structure that totally inhibits thermal degradation reactions called beta-elimination, not taking place with monoesters or diesters. As a result they do show exceptional thermo-oxidation stability, he later told LubesnGreases.

When focusing on specific properties of neopolyol esters, Lucazeau said their solvency power and low temperature behavior can be used in the formulation of polyalphaolefin based greases, while their resistance to thermo-oxidation, low propensity for deposit formation and low volatility makes them particularly suitable for the formulation of high temperature greases.

Lucazeau illustrated the benefits of synthetic esters by testing greases at different temperatures. In the first test, the grease was composed of a VG 15 base stock blend of PAO with 10 percent diester, thickened with 15 percent lithium complex soap. In tests, this grease functioned at an operating temperature range of -73 to 135 degrees Celsius and demonstrated outstanding performance at ultra-low temperatures while keeping excellent high temperature resistance. This makes it suitable for applications with high speed bearings as well as cold operations.

In the second test, a grease made with 90 percent neopolyol ester with VG 32 viscosity and thickened with chemically modified clay, showed high temperature stability with low evaporation results. It was usable at temperatures of up to 180 degrees C, said Lucazeau.

To further push the limits, an ultra-high temperature chain oil based on fully branched neopolyol esters was developed. The product used a neopolyol ester with branched acids (viscosity: 400 mm2/s) that showed an ideal profile for use in this high temperature application.

Combined with a high performance antioxidant system, neopolyol ester achieves outstanding behavior at high temperatures, Lucazeau said. He noted that temperatures of up to 300 degrees C may be sustained on high temperature oils using such technology.

Weighing the Costs

The technical performance and benefits of synthetic esters can be achieved in greases through adjustments in production and performance features, such as low temperature behavior, solvency, thermo-oxidative stability and evaporation, Lucazeau stated. In particular, ultra-high temperature capability may be achieved with greases that will cover a range of temperatures beyond where PAOs are effective.

Using synthetic esters in greases may require some process adjustments, Lucazeau cautioned. He noted that alkali should be added carefully and only gradually to avoid possible reaction with esters, and the use of water should be avoided. It can help to use oil dispersions, and pre-formed soaps can be considered. However, in most cases little changes are needed.

Lucazeau said he is not aware of thickeners that are incompatible with synthetic esters. However, thickening ester-based fluids may require some adjustments in the process, in the thickener content and sometimes chemical nature to achieve best results compared to traditional base fluids.

The cost of manufacturing synthetic ester based greases is certainly higher than conventional types, but depends on a number of factors. It is extremely dependent on the type of mineral base stock and especially on the type of esters, Lucazeau clarified. Besides, production cost is not the only parameter to look at-operating costs [of a facility] are a lot more relevant.

However, the price and availability of synthetic base stocks remains a challenge for the industry, more so for those seeking to formulate using synthetic esters. The availability of synthetic base stocks has nothing to do with that of mineral base stocks for a number of reasons. For instance, synthetic esters do rely partly on vegetable feedstocks, said Lucazeau. They have their limitations in terms of availability worldwide, and its pretty much the same thing for PAOs.

He noted theres an added cost to synthetic esters, but they also bring a number of interesting benefits for the formulation of high performance lubricants. Plus, the overall operating costs for consumers will be lower considering the benefits of friction reduction, fuel and energy efficiency as well as reduced downtime and maintenance costs.

It will be a very different picture and you will see an advantage, Lucazeau assured. But this has to be studied, demonstrated and conveyed to the market.

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