Improvement of the tribological behaviour of palm biodiesel via partial hydrogenation of unsaturated fatty acid methyl esters

Partial hydrogenation of unsaturated fatty acid methyl esters, the so-called H-FAME, has been developed, particularly for application in warm climates, to improve the oxidation stability of biodiesel. During the H-FAME process, polyunsaturated ester molecules, which are a primary cause of biodiesel induced oxidation, can be reduced. In this paper, the lubrication properties and mechanisms have been studied when high quality biodiesel from palm was upgraded to H-FAME. Lubricity tests were performed using a high-frequency reciprocating rig (HFRR). After the lubricity tests, wear scar and deposits on the tested specimens were analyzed using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectrometry (EDS). It was found that the partial hydrogenation upgrading process reduced polyunsaturated molecules and also transformed cis-monounsaturated molecules (C18:1 cis) into trans-monounsaturated molecules (C18:1 trans). This increased the stability of the lubricating film when C18:1 trans molecules were present, and so improved the lubricity of the H-FAME. An observation of wear damage suggested that the H-FAME was able to form dense and stable tribo-films on the steel test surfaces, protecting them from abrasive damage such as scoring and scratching. As a result of the reduction of polyunsaturated ester molecules, the lubricity of the H-FAME was less sensitive to the changes in humidity. Moreover, there was evidence of a reduction in both corrosive wear and large agglomerations of deposits for the H-FAME upgraded palm biodiesel due to its higher oxidation stability.