New type of polyalkylene glycol

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2010

These lubricant basestocks are soluble in mineral oil and have the potential to be widely used in industrial applications.

KEY CONCEPTS
• A new series of PAGs are readily soluble in mineral oil.
• Oil soluble PAGs display excellent deposit control characteristics when blended as a co-basestock with Group I mineral oils.
• Oil soluble PAGs are best used as a basestock when combined with other basestocks such as mineral oil or PAO and as an additive to contribute friction-reducing properties to a lubricant.

Polyalkylene glycols (PAGs) have been used as basestocks in synthetic lubricants for more than 50 years. Applications using PAGs have been primarily compressor fluids, gear oils and hydraulic fluids.

STLE-member Dr. Govind Khemchandani, senior technical specialist for The Dow Chemical Co. In Freeport, Texas, says, “PAGs have provided good low-temperature properties, high viscosity indexes, excellent control of deposits and hydrolytic stability. The fact that every third atom of a PAG is an oxygen atom provides the polarity needed to prevent varnish formation.” 

STLE-member Dr. Martin Greaves, technology leader for Dow Europe in Horgen, Switzerland, discusses the synthetic versatility of PAG chemistry. He says, “As derivatives of ethylene oxide and propylene oxide, there are virtually a limitless number of possible combinations that can be developed.” 

In a previous TLT article, PAGs were used to boost the performance of biodegradable lubricants through the preparation of blends with high oleic canola oil (1). Sludge and varnish formation were significantly reduced with these blends as compared to high oleic canola oil without sacrificing biodegradability.

But PAGs have faced some challenges that limit their use in lubricant applications. Khemchandani says, “Three of the largest challenges in using PAGs are miscibility with mineral oil basestocks, hygroscopicity and compatibility with elastomers.” Greaves adds, “The inability of conventional PAGs to be compatible with mineral oil basestocks makes it very difficult, time-consuming and costly for lubricant end-users to convert from mineral oil-based fluids to a lubricant based on PAGs.” 

Development of a PAG that is soluble in mineral oil could facilitate the wider use of this lubricant basestock in additional applications and enable conversions to be done more efficiently. Such a PAG type has not been available until now.

OIL SOLUBILITY
Dow has just introduced a new series of PAGs that are readily soluble in mineral oil. They are known as OSP™ PAGs and are available in viscosity grades ranging from ISO 32 to ISO 680. Greaves says, “The oil soluble PAGs are based on downstream derivatives of butylene oxide as one of the precursors.” 

Khemchandani says, “The use of higher alkylene oxides such as butylenes oxide increases the ratio of carbon to oxygen in the PAG, which boosts oil solubility.” 

Solubility testing was conducted with the ISO 46 oil soluble PAG in a wide range of mineral oil basestocks including Group I, II, III and naphthenics. Synthetic basestocks such as esters, polyalphaolefins (PAOs) and PAG-propylene oxide homopolymers were also evaluated.

Testing was done at -10 C, ambient temperature and 80 C for 168 hours at the following basestock ratios: 10:90, 50:50 and 90:10. In all cases, except for 40 cSt PAO, complete miscibility is seen at all temperatures.

Aniline point is a measure of the effectiveness of the solubility of a basestock. This parameter is measured by checking the solubility of aniline in the basestock and measuring the temperature at which the solution turns hazy. A desired result is as low a temperature as possible.

Khemchandani says, “The aniline point for the oil soluble PAGs is less than -30 C. In contrast, the aniline point for a typical trimethylolpropane-based polyol ester is 8 C, naphthenic oil is 71 C and an 150 SN paraffinic oil is 96 C.” The low value of the oil soluble PAGs is an indication of how effective this basestock is as a solvent for oil soluble materials as well as additives.

A series of performance tests have been conducted to determine the performance of oil soluble PAGs when used in conjunction with other oil soluble basestocks. The ability of an ISO 46 oil soluble PAG to act as an additive in reducing the friction of an 8 cSt PAO was determined through use of a mini-traction machine in which a load is applied on a steel ball as it rolls on a steel disc.

Under a contact pressure of 0.9 gigapascal, a slide roll ratio of 10% and a temperature of 80 C, the coefficient of friction for a blend of 10% of the oil soluble PAG in the PAO dropped to less than 0.1 and remained at that level for the 30-minute duration of the test. The coefficient of friction for straight PAO rose steadily until it reached a value above 0.3 at the end of the test. Greaves says, “The polarity of the PAG enables it to reach the metal surface and significantly reduce the coefficient of friction.”

Deposit control characteristics of the oil soluble PAGs were evaluated by blending 20% of the ISO 46 grade in a Group I mineral oil and evaluating it for 70 days using a modified version of ASTM D2893. Greaves says, “Air is blown through a vessel containing the blend of oil soluble PAG and Group I mineral oil at a fixed rate at 120 C.” 

As shown in Figure 1, no deposits are detected in the PAG Group I mineral oil mixture while deposits are seen with the Group I mineral oil. Khemchandani says, “Degradation of mineral oil produces incompatible polar byproducts that form insoluble deposits. But the polarity of oil soluble PAGs enables them to solubilize these deposits.”


Figure 1. Deposits are eliminated when 20% of an oil soluble PAG is blended with a Group I mineral oil. Air was blown through samples containing the PAG/mineral oil blend and Group I mineral oil at a fixed rate using a modified version of ASTM D2893 for 70 days. (Courtesy of The Dow Chemical Co.)

Greaves adds, “The complete absence of insolubles means there is good potential for using oil soluble PAGs in applications such as turbine oils, which have been plagued with deposit problems.”

Further data generated indicates that oil soluble PAGs exhibit excellent deposit control characteristics down to treat rates as low as 5%. This means that a small increment of an oil soluble PAG added to a lubricant will minimize the generation of insolubles.

The hygroscopicity of conventional PAGs is of concern because they can absorb water into a lubricant that can lead to fluid degradation over time. Under conditions of 80% relative humidity and 50 C, ISO 46 oil soluble and conventional PAGs were evaluated for water absorption. After 16 days, the oil soluble PAG absorbs much less water than its conventional counterpart.

Khemchandani says, “This study is even more significant because no demulsifier was used with the PAGs.”

Both Greaves and Khemchandani believe that oil soluble PAGs have the potential to be used as basestocks in lubricant products, especially when combined with other basestocks such as mineral oil or PAO to upgrade them, and as an additive to impart friction-reducing characteristics to a finished lubricant.

This versatility should enable oil soluble PAGs to be used in a variety of new applications. Further information can be found in a recent presentation given at STLE’s 2010 Annual Meeting & Exhibition (2) or by sending an e-mail to OSP@dow.com. 

REFERENCES
1. Canter, N. (2009), “Upgrading Biodegradable Lubricant Performance,” TLT, 65 (6), pp. 14-15.
2. Greaves, M., Voorst, R., Carn, C., Khelidj, N., Meertens, R. , Johnson, L. and Hoozemans, E. (2010), “Oil Soluble Polyalkylene Glycols,” Presented at the STLE 2010 Annual Meeting & Exhibition, May 16-20, Las Vegas, Nev.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.