Characterization of lubricants: Perfluoropolyethers

Perfluoropolyethers (PFPEs) are liquid polymers with good lubricating properties at room temperature. They have high viscosities and boiling points as well as low surface energies. PFPEs are used in almost all areas of industry, e.g. the electronics industry, aerospace and the semiconductor industry. For example, hard disks in computers are coated with a thin layer of perfluoropolyether to prevent friction losses or welding when the read/write head comes into contact with the hard disk surface. Or: Greases based on perfluoropolyethers are used to lubricate highly loaded ball bearings in conveyor belts in paint shops and ball bearings in ovens.

The physical properties of the individual PFPE products (trade names: e.g. Krytox, Demnum, Fomblin) depend on the structure of the polymer backbone, the molecular weight and the width of the molecular weight distribution.

These values can be easily determined using the TOF-SIMS method (see figure). In addition, the method enables trace detection and the exact identification of PFPEs in the area of defect analysis. The TOF-SIMS therefore plays an important role in the search for the source of PFPEs in painting processes that lead to defects (e.g. craters) in paint layers.

Figure 1: Positive TOF-SIMS spectrum of a thin Krytox film on silver substrate. An average molecular weight of approx. 7000 u corresponding to n = 47 repeat units is measured (for an explanation of the individual mass signals, see H. Feld et al., Analytical Chemistry, 65 (1993) 1947-1953).

The TOF-SIMS method enables the highly sensitive detection of perfluoropolyethers (sub-ppm range) even in microscopically small areas, the exact determination of the masses of their repeating units (R) and polymer end groups as well as the width and average molecular weight of the oligomer distribution (polymer dispersion).