Lubricants: Mineral or synthetic?

The issue of switching from mineral to synthetic oils is debated regularly. Mineral oil, obtained from the separation of crude oil, is often seen as unreliable due to its natural and unpredictable structure, the fact that irregularities in its molecules can generate friction, and its tendency to form sludge at high temperatures – but they are not without their benefits. Generally cheaper than synthetic oils, mineral oils are more straightforward to dispose of or reuse, and are compatible with seals, metals and coatings which may be damaged by synthetic alternatives. 

Synthetic oils are generally specified for machinery operating at high temperatures; to cope with low start-up temperatures; for their low volatility and flammability; and for the reduced risk of residue build-up and evaporation loss. However, in machinery working at standard temperatures with little risk of fire, synthetic oils may be an unnecessary expense as mineral oils will fulfil requirements. 

The decision to adopt synthetic oils should not be taken therefore on a plant-wide basis, but after detailed evaluation of each piece of equipment. An efficient lubrication management system is tailored to each machine based on its operating conditions and so, in some cases, a mineral oil will still be an appropriate option. 

If, however, the application demands a lubricant that exceeds the performance properties offered by mineral oils, there are various synthetic options available.

Synthetic oils, derived from chemicals, offer more predictable behaviour due to their highly engineered nature and are available in several different classifications for varying applications. 

Suitable for gear lubrication, compressors, gas turbines and automotive engines, synthetic lubricants based on polyalphaolefins offer stability at high temperatures, a low temperature fluidity and low volatility. The high natural viscosity index offered by this class of synthetic lubricant offers a high resistance to shearing, and if treated with a small amount of antioxidant material, are less likely to form potentially problematic deposits. They can also be treated with small amounts of ester-based fluids to combat their tendency to shrink seals.

Ester-based fluids are a blend of acids and alcohols with a chemical structure which attracts them to metallic surfaces – resulting in highly effective boundary lubrication. Phosphate esters offer excellent fire resistance and the ability to perform under extreme pressures, making them suitable for aviation and specialist engines. In fact, oils in the ester class offer the lowest volatility of all the synthetics and are safe in hazardous applications. Organic esters meanwhile have a high thermal stability which contributes to engine cleanliness and reduced deposits. 

Originally designed as a replacement for castor oil as a brake fluid, polyalkylglycols have also found favour as circulating or compressor oils, and as hydraulic fluid in fire-resistant applications. With a very low friction coefficient, lubricants in this class are also ideal for gears with a high sliding percentage as well as aviation, steel production and mining thanks to their fire resistant properties. As a water-soluble oil, they are easily washed off fabric and are therefore favoured as gear lubricants by the textile industry. Polyalkylglycols also reduce residue left on machine surfaces when compared with mineral oils, due to the fact that they burn clean and actively dissolve deposits. However, they may cause damage to seals and can dissolve paints and other coatings, meaning that compatability testing is vital. 

Generally, synthetic lubricants are a suitable choice for all types of gears, including worm, hyphoid, spur and helical gears, as synthetics have a lower friction coefficient than minerals. This reduces tooth-related friction, increasing the efficiency of the gear and reducing the operating temperature, reducing replacement and repair costs. 

A further benefit of synthetic oil is the extended working life span – which can offset the higher initial cost of the lubricant. The oil change intervals of synthetic products may be three to five times longer than for mineral oils in an equivalent application, offering reduced maintenance costs. 

In selecting the correct classification of lubricant, and managing the storage, supply and disposal of multiple oil products, the services of a lubrication management partner can offer significant benefits.