Coating processes – Nonuniform coating – Edge or border coating
Patent
1997-09-10
2000-08-29
Beck, Shrive
Coating processes
Nonuniform coating
Edge or border coating
427533, 427551, 427566, 427384, 427421, B05D 500, H05H 100
Patent
active
061105325
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with a method of coating cutting edges, more particularly razor blade cutting edges, with polytetrafluoroethylene (PTFE).
For many years razor blade cutting edges have been coated with PTFE, an early disclosure of the use of such coatings being, for example, British Specification 906005. Such coatings have been shown to improve the shaving effectiveness of the blade edge by reducing the force required to cut through the hair and thus reduce the pull on the hairs of the shaving area which the shaver experiences.
It has been known for some time that for most PTFE-coated razor blades the force required to cut hair with an unused blade, that is the first shave force, is significantly higher than the force required in the immediately following shaves, say the second to fifth shaves, with the same blade edge. It has been postulated that this phenomenon is due to the removal of much of the PTFE coating during the first shave, the difference between the first shave force and that for, say, the second to fifth shaves representing the force required to remove the "excess" polymer.
A number of processes for forming PTFE coatings on razor blade cutting edges have been described (for example, in Specification 906005 already referred to). One process which has been widely used commercially comprises spraying the blade edges with a 1% by weight dispersion of PTFE telomar (having a molecular weight of less than 100,000, for example 5000) in a chlorofluorocarbon and then sintering the PTFE coating formed. As a production process, this has been very satisfactory because it can readily be incorporated into a continuously operated razor blade production line and gives uniform results. However, there is a need to phase out the use of chlorofluorocarbons in industrial processes and, if possible, to use only water as the dispersion vehicle.
We have now developed a method of coating razor blade cutting edges with PTFE which does not require the use of a chlorofluorocarbon or other volatile organic solvent.
According to the present invention, there is provided a method of forming a PTFE coating on a razor blade cutting edge, which comprises spraying the cutting edge with an aqueous dispersion of PTFE having a molecular weight of at least 500,000 to form a coating of the PTFE on the edge subjecting the PTFE coating to ionising radiation in the presence of an oxygen-containing gas to obtain a radiation dose of up to about 60 Mrads (megarads), and then sintering the PTFE coating.
It is possible by the method of the invention to obtain PTFE coatings which do not exhibit the phenomenon, referred to above, of the first shave force being significantly greater than the force required for the second to fifth shaves.
The PTFE starting material preferably has a molecular weight of from 1,000,000 to 2,000,000. This material is conventionally produced by aqueous polymerisation and is conventionally used for forming non-stick coatings on articles, such as cookware. It will be appreciated that at no stage in the production of the PTFE-coated razor blades of the invention, that is neither during the production of the PTFE polymer nor during the formation of the coatings, is a chlorofluorocarbon or other volatile organic solvent necessary. The process is intended to be carried out entirely without the use of such materials so that it is environmentally acceptable throughout. The invention does not, however, exclude the use of such materials.
It is neither required nor desired that PTFE telomers, that is polymers with a molecular weight below about 100,000, should be formed before the actual coating process.
The aqueous dispersion used to form the initial coating preferably contains from 0.15 to 0.5% by weight, more preferably approximately 0.25% by weight of PTFE. The dispersion may contain one or more surfactants to assist dispersion of the PTFE particles.
The spray coating operation may otherwise be carried out in the same way as the spray coating step of the conventional process using a chlorofluorocarbon dispersion
REFERENCES:
patent: 3071856 (1963-01-01), Fischbein
patent: 3203829 (1965-08-01), Seyer et al.
patent: 3518110 (1970-06-01), Fischbein
patent: 3658742 (1972-04-01), Fish et al.
patent: 3838512 (1974-10-01), Sanderson
patent: 4029870 (1977-06-01), Brown et al.
patent: 4220511 (1980-09-01), Derbyshire
patent: 5263256 (1993-11-01), Trankiem
Causton Brian Edward
Glasson Edwin Lloyd
Beck Shrive
Chen Bret
The Gillette Company
Williams Stephan P.
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