Typically, perfluoropolyether lubricants are chemically inert, thermally stable, moisture repellant compositions that possess relatively low surface tension, good lubricity and low volatility. As a result, they are effective and long-lasting lubricants for a variety of substrates such as magnetic media, precision mechanical instruments, and electrical contacts. Because thick coatings on a recording or reading surface of a data hard disk may lead to output losses, lubricant coatings on such substrates are generally applied to the substrate in the form of a dilute solution. After application, the coating is dried to yield a thin, uniform lubricant coating.
Various perfluoropolyether lubricants have been described (see, for example, U.S. Pat. No. 4,721,795 (Caporiccio et at.)) and many are commercially available (e.g., from Montedison S.p.A of Milan, Italy under the "FOMBLIN" tradename and from E. I. DuPont de Nemours & Company under the "KRYTOX" tradename). Many perfluoropolyether lubricants are a mixture of perfluoropolyether molecules of various molecular weights and structures. Some perfluoropolyether lubricants contain perfluoropolyether molecules having end groups that are thought to become fixed to, or immobilized on, the substrate during deposition and use.
One particularly preferred solvent commonly used with perfluoropolyether lubricants is 1,1,2-trichloro-1,2,2-trifluoroethane. It is a good perfluoropolyether solvent and relatively volatile, so it can be readily removed from the coating by drying the coating. However, chlorofluorocarbon liquids are no longer favored solvents for such uses because such compounds are believed to deplete ozone in the earth's atmosphere. See, for example, Chem. & Eng. News, Nov. 15, 1993, pages 12-18.
Certain perfluorinated alkane liquids have been used instead of chlorofluorocarbon liquids as perfluoropolyether solvents. For example, U.S. Pat. No. 5,049,410 (Johary et al.) states that "FG-72" from Minnesota Mining and Manufacturing (St. Paul, Minn.) is a suitable liquid carrier for perfluorinated lubricants. This FLUORINERT.TM. perfluorinated fluid (more appropriately referred to as "FC-72") and Performance Fluid PF-5060 are acylic perfluorinated alkanes available from Minnesota Mining and Manufacturing. They are good solvents for many perfluoropolyether lubricants; however, it has been determined that they will not dissolve all components of some perfluoropolyether lubricants, particularly lubricants containing perfluoropolyether molecules having hydrogen-containing end groups, e.g., terminal neutral planar groups such as those described in U.S. Pat. No. 4,721,795 (Caporiccio et al. ). The limited solubility of such perfluoropolyether lubricants in acylic, perfluorinated alkanes results in hazy lubricant coating compositions and nonuniform coatings upon application to a substrate.
Thus, what is needed are solvents that are useful for a wide variety of perfluoropolyether lubricants, particularly those containing perfluoropolyether molecules having hydrogen-containing end groups, and are more environmentally acceptable.