Evaporation Behavior of Lubrication Oils

We shall consider the practical relevance of the Noack test of evaporation loss (DIN 51 581). An oil sample is held at 250 °C for 60 minutes in a defined flow of air, and weighed before and after this procedure. We cannot say whether the weight loss is based only on evaporation of parts of the original sample, or whether parts of the sample have been oxidized and then evaporated. The repeatability is + 5 % and the reproducibility 10 %, each for the mean value. ASTM D 972-86 and IP 183/79 use an instrument of different geometric construction and permit a free choice of temperature between 99 °C and 150 °C. An air flow of 21/min is applied and the test time is 22 h. The main difference of the ASTM 2595-85 lies in the extension of the temperature range fi om 99 °C to 316 °C. A reproducibility of 15 % is quoted for these methods. 

Blends of standard lubrication oils have been produced for a systematic investigation of the application of thermal methods of analysis. Starting from four vacuum distillates (samples FVA1-FVA4), seven base oils were blended (samples MI-MVII). The subsequent addition of 8 wt% of a performance additive package (Orogil XOA 560, Orogd 

Thermogravimetry in inert gas of the vacuum distillates and base oils shows that the molecule size governs the evaporation behavior. The evaporation start temperatues T % and T5 % are linear functions of the average molecular weight M (Fig. 4-124) and of the chain length of the paraffinic molecule part, KLMAX (Fig. 4-125). Also, the peak maximum temperature of the DTG increases linearly with increasing molecular weight (Fig. 4-126) whereas the evaporation rate decreases (Fig. 4-127). There is a clear linear 

Thermogravimetry in air of the vacuum distillates is no different to that in argon for temperatures up to 200 °C (Table 4-166). Above 200 °C the cuiwe of the TGA in air is shifted to higher temperatures as shown by the value of AG300. Low-volatile oxidation products are probably present. 

Of some interest is the question whether the Noack evaporation loss test could be replaced by isothermal gravimetry. In contrast to the Noack test, which only delivers one value after one hour test time, the weight loss versus the test time is recorded by the thermobalance. The important information is whether the evaporation has already reached its final stage. But there is a significant difference in the ratios of surface to volume S V between the cup of the Noack tester and the pan of the thermobalance (Stanton-Redcroft TG 750)  

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