Oxidation vs. thermal decomposition
February 6, 2024
Technical articles
Are high temperatures a critical factor for lubricants? The correct application of lubricants requires a basic understanding of oxidation and thermal decomposition. Martin Ruch, lubricant expert and Head of Technical Support at LAEMMLE Chemicals AG, explains exactly how this works.
Marketing: What can you tell us about the temperature-dependent performance of a lubricant?
Martin Ruch: When it comes to the minimum and maximum temperature resistance of a lubricant, there are different perspectives and assessment criteria; the answer to this question also always depends on the specific application. In addition, the composition of the base oil and the additive technology have a direct influence on the temperature resistance of the lubricant.
Marketing: How is the ideal operating temperature of a lubricant defined, and what other influencing factors need to be taken into account?
Martin Ruch: The temperature range within which a lubricant can be used is primarily determined by the lubricant’s viscosity-temperature behaviour. The viscosity limits are usually specified by the manufacturer of the machine, plant or component. For example, the minimum operating viscosity and the maximum start-up viscosity, or the required ISO or SAE viscosity grade, are specified. In addition, factors such as oil ageing (oxidation) and the thermal decomposition of the lubricant as a function of temperature must be taken into account.
Marketing: Are ambient/storage temperatures of up to 50 °C critical? How long can a lubricant be stored, and what storage conditions d LAEMMLE Chemicals AG, specify?
Martin Ruch: A lubricant can be stored for several years in its unopened original container, provided it is protected from the elements. Ambient temperatures of up to 50 °C do not generally damage the lubricant. It should be noted that temperature fluctuations, particularly when stored outdoors, can lead to the formation of condensation inside the sealed container.
Marketing: What effects does the formation of condensate have on the lubricant?
Martin Ruch: Condensate – that is, water – in lubricants is a hazard that is often underestimated and sometimes goes unrecognised. I would like to refer you to our information sheet ‘Water in Hydraulic Systems’ in this regard.
Marketing: How does the lubricant react to conventional operating temperatures in the range of 50 to 100 °C?
Martin Ruch: The ageing (oxidation) of the lubricant is accelerated at temperatures of 50 °C and above. Based on the ‘Arrhenius equation’, the rule of thumb is that for every 10 °C increase in temperature, the rate of ageing (oxidation) doubles, and consequently, the oil change interval is halved. Accordingly, the tank temperature – or, more precisely, the temperature of the lubricant – is the decisive factor in the rate of oxidation.
Marketing: What happens to the lubricant at elevated operating temperatures of 100 °C and above?
Martin Ruch: At operating temperatures of 100 °C and above, accelerated ageing (oxidation) is increasingly overshadowed or replaced by the thermal decomposition of the lubricant.
Marketing: What are the consequences of this process?
Martin Ruch: Thermal decomposition breaks down the basic bonds of the hydrocarbons, which are mainly used in lubricants. Compared to oxidation, thermal decomposition proceeds much more rapidly; consequently, even brief periods during which the lubricant is exposed to these peak temperatures (hot spots) are sufficient to cause damage.
Marketing: What are the typical differences between oxidation and thermal decomposition?
Martin shows us a diagram.
Marketing: Are there any applications where thermal decomposition is a factor?
Martin Ruch: Depending on the application, very high lubricant temperatures during operation are unavoidable. Heat transfer fluids are sometimes operated at consistently high temperatures (above 200 °C); thermal decomposition is normal in these applications and is taken into account in the oil formulation, operating parameters and oil change intervals.
Marketing: In which applications do unavoidable hot spots occur?
Martin Ruch: This applies, for example, to engine oils in internal combustion engines; localised, short-term temperature peaks (hot spots) of over 250 °C are unavoidable, and so thermal decomposition must be taken into account alongside oxidation when determining the oil change interval.
Marketing: Can oil change intervals be extended at will and at one’s own discretion?
Martin Ruch: No. The manufacturers’ prescribed oil change intervals must always be adhered to. However, if an extension of the oil change interval is sought, this can generally only be implemented in conjunction with appropriate oil analysis, for example by the LTC LAEMMLE Tec Centre.
Marketing: Which products with above-average oxidation stability would you personally recommend?
Martin Ruch: ROXOR TERRA CIRCULAR HV, ROXOR HLP PLUS and ROXOR MARVIS HYD SGM are examples of lubricants which, thanks to specially selected base oils and additives, possess exceptionally high oxidation stability and, when used in conjunction with oil analysis, are suitable for extended oil change intervals.
Marketing: Martin, thank you very much for the interesting in-depth look at the topic of oxidation and thermal decomposition. We value your expertise and the insights you have shared with us. We are already looking forward to further insightful discussions with you.
Interview: Marketing and Martin Ruch, Head of Technical Support
Figure 1: Close-up of the laemmle-chemicals ROXOR drum