Base Oils: Just feel that base!

Ever more stringent performance standards for finished lubricants are driving an increase in demand for synthetic base stocks and formulated lubricants derived from them

Newer engine designs that run at much higher temperatures, higher lubricant performance requirements from original equipment manufacturers (OEMs), more demanding government fuel efficiency standards and user expectations of longer engine/equipment lifetimes are all factors influencing the choice of ­lubricant.

Copyright: Rex Features

In many cases, conventional lubricants based on mineral oils cannot provide the needed level of performance, but the use of synthetics can provide a route to the highest performance levels.

Synthetic lubricants are formulated with base oils that have been prepared via chemical transformation of simpler starting materials (petrochemicals or others, including hydroprocessing of mineral oils). Unlike conventional mineral oil base stocks, which are isolated from crude oil and typically contain a complex mixture of compounds, synthetic base oils have controlled compositions and often increased functionality, both of which result in higher performance in lubricant formulations.

According to the 1999 ruling by the Better Business Bureau’s National Advertising ­Division in the US, API Group III, IV and V base stocks are considered to be synthetic base stocks, and lubricants made with them can be marketed as ­synthetic lubricants. Selected Group II base stocks may also be described as synthetic.

Semi-synthetic lubricants, meanwhile, consist of blends of conventional and synthetic base oils and are intended to provide some of the improved performance characteristics of synthetic lubricants at a lower price point. It should be noted, however, that no formal definition of synthetic finished lubricants has been agreed by the major industry trade groups, and the terms synthetic and semi-synthetic should be considered only as marketing ­descriptions.

The synthetic base oil category includes a broad range of chemistries, including Group III oils produced using gas-to-liquids technology and other crude-derived hydrocarbons, Group IV polyalphaolefins (PAOs) and Group V alkylated naphthalenes (ANs) and organic and inorganic esters and other 
functional-group-containing molecules (polyalkylene glycols [PAGs], silicones, halogenated ethers, etc) – except in Germany, where only PAOs are classified as synthetics.

PAOs are hydrogenated olefin oligomers manufactured via the polymerisation of linear alphaolefins, ANs via the alkylation of naphthalene with linear alphaolefins, and organic esters via various routes, but commonly through the reaction of carboxylic acid or acid derivatives with alcohols.

VARIETY OF APPLICATIONS
As a general rule, synthetic oils are used in lubricants for more demanding applications where good response to antioxidant additives (rather than direct oxidative stability), low volatility, clean breakdown pathways with fewer deposits, and higher viscosity indices all enhance lubricant performance, according to Brian Crichton, director of Crichton ­Consulting, and a moderator for the pre-­conference seminar on synthetics at the 19th ICIS Base Oils & Lubricants Conference.

The applications for synthetic oils/­lubricants are as varied as their chemistry, but generally they require lubricating oils to perform reliably under extreme conditions or duties, according to Shell base oils technology manager Wei Song. “Examples include the use of synthetic esters in aviation turbine engine oils and synthetic hydrocarbon oils in high-performance, downsized modern passenger car engines to cope with the high temperatures encountered when lubricating turbocharger bearings,” he says.

PAO base stocks serve as a common type of base stock in synthetic lubricants for passenger car ­engines, heavy-duty diesel engines, transmissions and gearboxes and ­various industrial equipment, while ANs are well-suited for high-moisture environments, such as food applications.

Esters, meanwhile, are used in automotive, textile, industrial, aviation, turbine and compressor lubricants as both stand-alone base stocks or in combination with other fluids. Notably, in addition to their high thermal and oxidative stability, low volatility, lubricity and solvency, many esters have the advantage of being inherently biodegradable and thus support the formulation of finished lubricants that may be used in environmentally sensitive applications.

On the other hand, according to Damon Davis, vice president of ExxonMobil Chemical’s global synthetic fluids business, ester-based lubricants can suffer from hydrolysis, particularly on rotary equipment where ­intimate mixing of the oil and air occurs, and thus PAO/AN mixtures may offer a ­better solution in high-moisture applications. Davis also points out the biodegradability attributes of esters only belong to the base stocks and do not necessarily translate to finished lubricants formulated with these base stocks.

There are, however, trade-offs with synthetic oils that must be considered when formulating lubricants for specific applications. First, PAO and Group III base oils tend to have poorer solvency than mineral base stocks. Of course, they do run cleaner so fewer deposits are generated.

Second, a lubricant formulated with a synthetic/semi-synthetic oil will have better performance attributes than a similar mineral oil-based lubricant, but at a higher cost.

Third, according to Fran Lockwood, senior vice president of R&D for Valvoline, in some cases lack of molecular diversity leads to phenomena that can be unanticipated by formulators that are not familiar with them. In fact, fully synthetic fluids often need ­balancing with other base stocks, such as esters with alkylated aromatics, to provide solvency and additive compatibility, according to Crichton.

Mineral oil-based lubricants are still widely used today in applications where there is less demand on the performance of the lubricant. However, advancement in engine and equipment design to improve fuel economy, operating efficiency and to extend the life of the engines and equipment are driving the need for improved lubricant performance, according to Bruce Marley, senior vice president of sales and marketing for Biosynthetic Technologies.

It is also important to make sure that the appropriate synthetic oil is formulated for the right application. Synthetic base stocks play a major role. For example, phosphate esters as used in aviation lubricants would provide little benefit if deployed in motor oils.

BETTER PERFORMANCE – WITH A CAVEAT
Synthetic oils tend to have a higher viscosity index, better oxidation stability and better cold-temperature performance, according to Richard Dixon, technology manager at Shell Motor Oils. He does note, however, that not all synthetic oils are created equal, and the additives used in lubricant formulations greatly affect their performance.

In fact, performance additives are used in all lubricants and serve to protect the base oil, enhance the base oil properties, and ­protect surfaces from damage. Lubricant additives provide a number of functions, including anti-wear, viscosity adjustment, antioxidant, compatibility, seal swell, dispersant, detergent, and extreme pressure tolerance.

Some lubricant formulations require only one additive, while others may require many. Each additive package is designed specifically for the base oil and end-use ­application, according to Marley. With ­synthetics, though, according to Dixon, the formulator can better identify the primary ­effect of an additive, because fewer unwanted side effects and interactions occur, which enables the development of better-­performing lubricants.

“Given the crucial role of additives, it is very important that all fluids be judged on their performance, not their composition,” notes Crichton. “Just because a synthetic base stock is used in a formulation is no guarantee of quality or performance. The oil/lubricant/fluid must be properly formulated with regard to additives and physical/chemical characteristics and constitution,” he ­asserts.

MORE DEMANDING REQUIREMENTS
Because synthetic base oils have many ­advantages over mineral oils, they are used in lubricant applications where the performance requirements cannot be met by mineral oils, according to Miles Oberton, global PAO business manager at Chevron Phillips Chemical. “PAOs, for example, offer lower friction, lower volatility, better oxidative ­stability, better heat transfer capabilities and better low-temperature viscometrics,” he ­observes.

Because synthetic oils enable superior low- and high-temperature engine oil performance, adds Lockwood, they are widely employed in low-viscosity engine oils, engine oils for high performance and extended drain, and long drain or fill for life transmission and differential fluids.

“Lubricants are no longer expected to simply do one thing well, rather they need to perform multiple, and sometimes conflicting, functions well over their service lifetimes,” states Dixon. Mineral oil-based lubricants were designed to meet the performance requirements of older engines and applications, and most do not have the capability to meet the higher demands of newer technologies, according to Marley. “Additives can boost performance to some degree, but their effect is limited,” he adds.

Dixon notes that this needed combination of performance parameters can only be satisfied by using synthetic oils, which can be designed to deliver particular combinations of properties, enabling them to outperform conventional mineral oils. Song adds that the ability of synthetic oils to withstand stress, be it thermal or oxidative, is a key feature that enables these oils to work harder.

Shell’s PurePlus base oil, for example, is produced from natural gas using a gas-to-liquids process that is designed to provide consistent molecules with suitable high purity, low viscosity, low volatility and low friction, plus the necessary oil film strength for the protection of components.

The Pearl plant in Qatar use a gas-to-liquids process to make Group III base oil Copyright: Rex Features

In automotive applications, according to Davis, incremental benefits brought by PAO base stocks versus mineral oils include better oxidative and thermal stability for long service life and higher viscosity index for improved protection and low-temperature fluidity, along with reduced contaminants. When used as co-base stocks, esters and ANs offer seal swell and additive solubility, improved lubricity and improved cleanliness.

For industrial applications, not only do lubricants based on synthetic oils resist breakdown under severe conditions, which helps to improve productivity through longer oil-drain intervals, their physical properties also allow safer operations through higher flash, fire and auto-ignition points, notes Davis, while low volatility helps to reduce oil consumption, oil carry over and deposit formation.

MEETING NEW STANDARDS
According to market research firm Kline & Company, the global demand for lubricants was 38.7m tonnes in 2012 and is growing at approximately 1%/year. The market for ­synthetic lubricants, on the other hand, is ­expected to grow at 4-5%/year for the next five years.

This healthy growth can be attributed to several factors, including increasingly ­demanding OEM specifications (eg ILSAC GF-6 and API PC-11) necessary for ­withstanding the more extreme operating conditions for light- and heavy-duty vehicle ­engines; reduced emissions standards (ie the International Council on Clean Transportation CO2 standard); stricter fuel economy regulations (eg, the US Corporate Average Fuel Economy or CAFE standards); and the desire of equipment owners to increase ­efficiency and reduce cost through improved reliability and extended service lifetimes.

“Automotive emission and fuel economy standards, regulations and the desire for lubricants that offer improved energy efficiency, longer drain intervals and the ability to work at higher stress operating conditions are general factors driving the use of higher-­performance, synthetic base oils,” Oberton observes.

In fact, the increasing demand for fuel economy is driving the use of “thinner” lubricants, with SAE J300 0W-16 oils the latest available viscosity grade under the API system, according to Song. “Synthetic oils can be designed to offer this lower viscosity, lower volatility and low friction while maintaining engine durability to deliver maximum benefit,” he notes.

PUSH FOR SUSTAINABILITY
In the future, synthetic lubricants will not only be necessary to meet the requirements of regulators and OEMs, they will also be required to meet environmental performance expectations, according to Marley. Important examples of recent US government actions affecting lubricant used include the US 
BioPreferred programme and the 2013 revised US Environment Protection Agency’s Vessel General Permit (VGP).

The BioPreferred programme is managed by the US Department of Agriculture and ­requires the mandatory purchase by federal agencies of bio-based products in 97 ­categories, including lubricants.

The VGP, which came into effect in December 2013, requires that vessels entering US waters use environmentally acceptable lubricants (EALs; lubricants that meet specific standards for biodegradability, toxicity and bioaccumulation potential) in all oil-to-sea ­interfaces on any vessels greater than 79 feet (27 metres) in length.

As a result, there is a growing interest in biolubricants that can meet both performance and cost expectations. Biosynthetic Technologies has developed estolides, which are bio-based esters prepared via the chemical modification of vegetable oils (high oleic soya, palm, canola, etc) that have ­performance properties equal to or better than other synthetic oils with high ­biodegradability and an acceptable cost, ­according to Marley.

Dixon stresses, however, that the assessment of a lubricant product, including those based on bio-derived materials, for improved sustainability and environmental profile should be based on the entire product and application life-cycle, rather than considered solely on the starting material.

“Key aspects such as compatibility with original equipment, demonstrable and consistent performance edges, availability and affordability should all be part of the evaluation matrix to arrive at comparisons based on a sound baseline,” he states.