Volume 41, Issue 2 (March 2013)
Advances in the Understanding of Gelation in the Framework of the Test ASTM D5133
This work presents a theoretical survey that sheds light on the physical meaning of the gelation index, that is, the main parameter involved in ASTM D5133. More specifically, it is proved that the pulse-shaped region of the gelation curve can be explained as the relief of the supersaturation of the solution, which is a nucleation process that finishes when the saturation ratio takes a value equal to 1. Comparison between experiments and the theoretical approach put forward in this work reveals qualitative agreement. This work also confirms the poor reproducibility of ASTM D4684, which is an alternative test for the evaluation of gelation in engine oils. Particularly, it is shown that the poor reproducibility of this test can be attributed to a systematic error made during the measurements. The lack of accuracy of the temperature, in particular the final test temperature, could be the main cause for discrepancies between laboratories. Finally, it is shown that in order to predict gelation within ASTM D5133, not only must the gelation index be considered, but the gelation index temperature also plays an important role.