Sulphur in metal working no problem with naphthenic oils

An important choice in metalworking is high or low sulphur content. But with naphthenic base oils, you can choose freely which sulphur content you wish, dependent on the application.

An important choice in metalworking is high or low sulphur content. But with naphthenic base oils, you can choose freely which sulphur content you wish, dependent on the application.

Better cooling ability

Naphthenic base oils have long been used for metal cutting fluids on account of their good ability to dissolve lubricant-improving additives and their good emulsibility. In this article, we will discuss two further properties which are important when selecting a base oil for metalworking. In the first place, solubility of sulphur will be used to illustrate the higher solubility of additives in a naphthenic oil compared to a paraffinic oil. Moreover, the experiments that have been carried out show that the sulphur present in refined oils is not active as an additive to improve lubrication under extremely high pressure. The characteristic is above all important for rolling oils.

Another important property is better cooling ability which is dependent on the lower viscosity index that a naphthenic oil has.

Solubility of sulphur

Some end-users of straight rolling oils demand that the oil has a high sulphur content, between 1-2 percent sulphur, and in some cases higher. This demand is made so that the oil will have an EP function, it will prevent welding and pickup between plate and rollers when the pressure is so great that the actual oil is not capable of lubricating. One would think that Nynas naphthenic base oils could not be used in this connection since their sulphur content is low. The sulphur content in Nynas NS 30, for example, is less than 0.01 percent.

On the other hand, the requirement is sometimes that the sulphur content should be very low, so as not to cause discoloration of the rolled plate when it is heat-treated after rolling. The investigations we have carried out in the laboratory have shown that there is no problem in adapting Nynas naphthenic oils to meet both these demands.

We have investigated EP capacity for a number of different base oils without additives. They had a viscosity of between 20-30 mm/s and sulphur content between 0.01 and 1.3 percent. We used Shell’s 4-ball test to measure the EP function of the oil. The original sulphur content has obviously no effect at all on the EP function. Compare the paraffin base oil and the low-refined naphthenic oil with the T and NS qualities. This sulphur is thus not reactive but firmly fixed to the hydrocarbons in the oil.

The next experiment we conducted was to saturate the oils with elementary sulphur and then redo the 4-ball test. After an addition of elementary sulphur that is only dissolved in the oil, and not chemically fixed, the load ability increases considerably. The experiments show that the naphthenic oil can dissolve large amounts of sulphur, 1.3 percent for T22 compared to 0.6 percent for paraffin base, and thereby give a higher shear load than a paraffinic oil.

Another advantage with the solubility of naphthenic oils is that the oil gives good wetting of metal and thereby facilitates the transport of metal chips from the working area. This is an advantage in the case of, for example, deep hole drilling.

Viscosity index, cooling capacity, filtration

Naphthenic base oils have a lower viscosity index (viscosity is reduced more quickly when temperatures rise for oils that have a lower viscosity index) than paraffinic oils. This is often used as an argument against naphthenic oils. When it comes to motor oils of the multigrade type, and when the demand on flash point is placed very high in relationship to viscosity, naphthenic oils can fall short.

In certain applications, for example for straight roller oils, a low viscosity index is an advantage. The main gain is the better cooling capacity you get in the hot areas in the cutting. The warmer the oil gets, the lower the viscosity, and thereby the cooling capacity increases. Another advantage is that it can directly save costs arising with filtration of the oil. Often the temperature of the oil is raised before it goes to the filter installation to lower the oil's viscosity. Lower viscosity means quicker filtration. In other words, an oil with a low viscosity index doesn't need to be warmed as much as a paraffinic oil and thus does not incur such high heating costs.

Conclusions

  • Naphthenic, severely-refined base oil can be used for applications that, on the one hand, demand very low sulphur content when the requirement is that there should be no discoloration of the plate when heat-treated. They can also be used for applications with demands on very high sulphur content and the EP function after an addition of sulphur additive.
  • Naphthenic oil, above all the T grades, have – in addition to the fact that they give a cheaper end product through using less emulsifier – technical advantages like better cooling capacity, less discolouring and that they dissolve more EP additives that increase the lifetime of tools and oil. This can bring the added advantage that more environmentally friendly additives can be used in severe operations.
  • The naphthenic oils dealt with in this article have, apart from the low-refined one, low levels of polyaromatic hydrocarbons – under three percent according to IP 346.

 

Oil Type Viscosity at
40°C mm/s
Original
sulphur
weight %
Shearing 
4-ball/N
150 SUS Paraffinic base oil 30 0.6 125
NS 30 Naphthenic base oil 29 0.04 112
T22 Naphthenic base oil 22 0.01 112
Low refined Naphthenic base oil 26 1.3 125

 

Oil Type Dissolved viscosity 
at 40°C mm/s
Sulphur 
weight %
Shearing 
4-ball/N
150 SUS Paraffinic base oil 30 0.6 350
NS 30 Naphthenic base oil 29 0.9 450
T22 Naphthenic base oil 22 1.3 450
Low refined Naphthenic base oil 26 1.2 400

Author

Olle Livén


Olle Livén, Nynas Naphthenics product development for base oils and metal working, has taken a closer look at the properties of naphthenic oil as a base oil for metal cutting fluids.

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