OBSERVATION OF THICKENER STRUCTURE IN GREASE Michitaka Yoshihara Tsutomu Moriuchi Kyodo Yushi Co,. Ltd.
1. Introduction
Lubricating grease was defined in ASTM-D288 as “a solid-to-semi fluid product of dispersion of a thickening agent in a liquid lubricant” [1]. Grease manufacturing process typically consists of the following three steps: reaction where thickener molecules are formed by reaction of ingredients in base oil, development where the thickener molecules develop to become fibrous, and dispersion where thickener fibers are dispersed and homogenized. Although it is important to know the thickener state of being in grease in order to understand its function as well as the rheological behavior of grease, thickener structure in grease still remains to be explained. This study aims to make direct observations of thickener structure in grease using a confocal laser fluorescence microscope, CLFM.
2. Traditional observation techniques
The observation has traditionally been made using a transmission electron microscope, TEM or a scanning electron microscope, SEM. An atomic force microscope, AFM has recently been used as an alternative. A TEM offers a magnified and focused image of thickener structure by transmitting a beam of electrons through a specimen under vacuum (Figure 1). It requires grease sample processing such as dilution, dispersion, placement of a thin film of the sample on a screen and oil extraction. Accordingly, TEM observations have the problem of thickener being present less densely compared to that in grease in the atmosphere. Using a SEM, observation images are formed by
collecting secondary electrons from the specimen scanned with a beam of electrons (Figure 2). The thickener structure can be observed without dilution or dispersion but still with oil extraction under vacuum. In addition, electron microscopic observations are frequently associated with vapor deposition process to obtain sharply outlined images and to avoid excess electron absorption. Despite being reported that oil extraction does not affect thickener fiber shape [2], these are like looking at “dried-up shell of grease” and far from a direct observation of thickener structure in grease. With an AFM, an untreated sample can be observed by gathering physical information including surface topology and hardness. A previous study examined grease structure by comparing AFM with SEM based on another older observation technique[3]. However, AFM can only provide near-surface observations which do not allow a detailed study of thickener network structure in grease. In short, researchers have so far had to recognize the object observed by these techniques of limited choice as thickener structure. To solve these issues on the traditional observation techniques, this study examines the potential of a confocal laser fluorescence microscope, CLFM as an alternative technique capable of making direct observations of thickener structure in grease.
3. Observations
3.1 CLFM Configuration Figure 3 shows the appearance of the CLFM used in this study (Nicon’s A1RSi+).
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