Developing a new type of antioxidant

Developing a new type of antioxidant

Canter, Neil

In today’s competitive environment, lubricants are under increasing stress and need to be able to operate over longer time frames under more difficult operating conditions. Antioxidants have gained in importance, in both automotive and industial applications, as additives needed to achieve this goal.

The regulatory push to remove zinc dialkyldithiophosphates (ZDDPs) from motor oils has prompted compound/-blenders to compensate for the reduction in the level of this antioxidant/antiwear agent. This has prompted the increasing use of other antioxidants that will not create problems with an automobile’s emissions system. In the industrial lubricant sector, antioxidants play a critical role in controlling the level of sludge, acid and deposits that can be generated during use and lead to premature failure.

There are two main types of antioxidants that function in different manners. Primary antioxidants donate hydrogen atoms by neutralizing the formation and propagation of reactive radicals. Antioxidant molecules are sacrificed as part of this process and, therefore, become depleted.

Examples of primary antioxidants used in the lubricant industry are phenolics and aminics. The latter include alkylated diphenylamines and alkylated phenylalpha-naphthylamines (APANAs).

Secondary antioxidants represent the other category and are utilized mainly to decompose peroxides into stable products. This antioxidant type is primarily used in plastic resins, such as polyolefins.

The lubricant industry has found that phenolic antioxidants operate most effectively at temperatures below 120 C while aminics work best above 120 C. Compound/blenders have used the two types synergistically to maximize performance.

But there is a need to develop more effective antioxidants that can be used over a wider temperature range and at a lower treat rate. The latter is most important, as the lubricant industry is moving toward greater use of more highly refined base oils such as those in the Group II and III categories. These basestocks can present additive solubility challenges, which mean that a lower additive concentration can mean greater compatibility over a longer operating period.

Macromolecular antioxidants

A new class of antioxidants has been developed that display very promising characteristics as compared to the existing technology. These additives are known as macromolecular antioxidants and have been marketed by Polnox Corp. of Lowell, Mass., since 2003.

Dr. Ashok Cholli, chief scientific officer of Polnox Corp., was responsible for the development and is in the process of commercializing the macromolecular antioxidants.

He says, “The development of the antioxidants originated from phenolic research we were conducting at the University of Massachusetts Lowell. This work involved the transformation of monomeric phenolicbased compounds into macromolecular phenolics through the use of various catalyst systems.”

However, Cholli indicated that Polnox Corp. is not limited to just phenolic-based molecules as other functionalities, such as aminics, are used in combination with them. He adds, “Most of our products are phenolic-based, but we utilize other technologies. The molecular weight of the macromolecular antioxidant can be low, medium or heavy, depending upon the application.”

Cholli has evaluated his products in Group 1, Group II, Group III and a synthetic ester basestock vs. commercially available phenolic and aminic antioxidants. The test procedure used was ASTM D 3895, which involves the use of Differential Scanning Calorimetry (DSC).

In the test method, the sample is heated under a nitrogen atmosphere up to a temperature between 180 C and 250 C in order to melt a solid sample if needed. Cholli says, “The choice of the specific temperature is not critical, though every 10 C increase in temperature will cause oxidation to double.”

Once the sample has equilibrated at the desired temperature for one minute, oxygen is introduced and measurement of the Oxygen Induction Time (OIT) commences. Sample degradation will start as an exothermic process that leads to a rapid decrease in the curve.

The onset of degradation is defined as the OIT for the particular sample. There is a direct correlation between a higher OIT and antioxidant performance.

Macromolecular antioxidant performance

In Group II base oil, significant performance differences were observed at 180 C with testing of antioxidants at a treat rate of 200 ppm. A Polnox material known as PNX 5B 107 displayed an OIT of 117 minutes. In contrast the OIT for the phenolic, 2,6-di-tertbutyl phenol was only three minutes.

Aminic antioxidants were not much better. An APANA exhibited an OIT of 20 minutes and di-octyldiphenylamine’s OIT was only 40 minutes.

Cholli also determined what treat rates were needed to achieve an equivalent OIT for one of his antioxidants and the commercial phenolic, BHT. An OIT of 5.5 minutes can be obtained with BHT at a concentration of 0.2% vs. a Polnox antioxidant at the much lower treat rate of 0.005%. This leads to performance efficiency for the Polnox material of 4000%.

Similar results were observed in both Group III and eater-based basestocks. CholIi later observed a nonlinear increase in OIT performance in a Group III base oil for PoInox 5B107, as the concentration of the antioxidant climbs from 0 ppm to 3,000 ppm.

Testing of Polnox vs. a commercial phenolic antioxidant in soybean oil was particularly significant. Cholli says, “We chose to evaluate this basestock at a temperature of 190 C for 30 hours. The temperature was selected, as it is appropriate for evaluating our antioxidants for use in both food and lubricant applications. The control with a phenolic antioxidant displayed a higher viscosity and was darker than the sample containing the Polnox antioxidant.”

A picture showing the samples tested is provided in Figure 1 (see page 11).. Soybean oil contains a large percentage of a conjugated double-bond fatty acid known as linoleic acid. This material is very susceptible to oxidation that leads to the formation of varnish, sludge and discoloration. All of these factors can hamper the operating life of a bio-based lubricant.

Polnox antioxidants act as primary antioxidants. In assessing the performance differences between Polnox antioxidants and commercial technology, Cholli believes that it goes beyond a greater concentration of phenolic hydroxyls per kilogram of antioxidant.

He says, “The key performance criterion is the ability of the antioxidant to donate a proton to kill a radical. We have molecularly engineered our macromolecular antioxidants so that the strength in the oxygenhydrogen bond of the hydroxyl is weak. This enables the antioxidant to readily donate hydrogens.”

Cholli is developing a product line of antioxidants designed to specifically work in the food, plastics and lubricant applications. Future work will involve further customizing and promoting of this product line.

Initial data shows that the Polnox antioxidants have promise as being more effective at a lower treat rate than existing technology. Further information on Polnox can be obtained at the company’s website, www. polnox.com.

By Dr. Neil Canter

Contributing Editor

Copyright Society of Tribologists and Lubrication Engineers Aug 2005

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