COATING CUTTING EDGES WITH FLUOROCARBON POLYMERS This invention is concerned with a method of coating cutting edges, more particularly razor blade cutting edges, with fluorocarbon polymers. For many years razor blade cutting edges have been coated with polyfluorocarbons, more particularly polytetrafluoroethylene (PTFE) ; see, for example, British Specification 906005. The polyfluorocarbons used for this purpose are telomers, that is they are polymers having a relatively low molecular weight. Thus the PTFE conventionally used for the coating of razor blade cutting edges has a molecular weight of about 25000, as compared with PTFE which is conventionally used for the formation of non-stick coatings which has a molecular weight of 3-6 million.

The reason for using fluorocarbon telomers rather than high molecular weight materials for coating cutting edges is that the former give improved blade-to-blade uniformity and coatings of improved durability.

Polyfluorocarbons, such as PTFE, are conventionally made by polymerization of the monomer in an aqueous dispersion and this gives rise to the high molecular weight material referred to above. Fluorocarbon telomers are made by polymerization in a chlorofluorocarbon, such as trichlorotrifluoroethane. However, in view of

mounting pressure on industry for environmental reasons to eliminate the use of CFCs in manufacturing processes and in products, there is a need to use an alternative process for making fluorocarbon telomers.

We have now found that fluorocarbon telomers, more particularly, tetrafluoroethylene telomers, made by subjecting high molecular weight polyfluorocarbons in dry powder form to ionizing irradiation can be used to form excellent polyfluorocarbon coatings on razor blade cutting edges, that is coatings having properties equivalent to those obtained with fluorocarbon telomers made by polymerization in a chlorofluorocarbon. For the purpose of forming such coatings the telomer formed by irradiation is dispersed in a volatile organic solvent, such as isopropanol, the dispersion is sprayed on to the cutting edges and heated to remove the solvent and sinter the telomer, the steps of spraying and heating being carried out under generally conventional conditions.

According to the present invention, there is provided a method of forming a polyfluorocarbon coating on a razor blade cutting edge, which comprises subjecting a fluorocarbon polymer having a molecular weight of at least 1,000,000 in dry powder form to ionizing irradiation to reduce the molecular weight of the polymer, forming a dispersion of the irradiated polymer in a volatile organic liquid, spraying the dispersion on to a razor blade cutting edge and heating the coating obtained to sinter the polyfluorocarbon.

The radiation dose is preferably from 20 to 80 megarad and the ionizing radiation is preferably by γ-rays from a Co ⁶⁰ source.

The polyfluorocarbon is preferably

polytetrafluoroethylene and irradiation is preferably effected to obtain a telomer having a molecular weight of about 25,000.

For the purpose of forming the dispersion which is sprayed on to the cutting edges, the irradiated polyfluorocarbon should have a fine particle size, preferably an average particle size of not more than 100 microns. The powdered polyfluorocarbon starting material will normally be available as a coarser material than this and it may be ground to this fineness either before or after the irradiation step, preferably the latter.

A large number of volatile organic liquids are suitable for use as the liquid medium of the dispersion; it is currently preferred to use isopropanol. The dispersion preferably contains not more than 1% by weight of the fluorocarbon telomer, for example about 0.5 to 0.7% by weight. The good results obtained by the method according to the invention which, as stated above, are equivalent to those obtained with fluorocarbon telomers made by polymerization in chlorofluorocarbons - the materials which are currently used to make coatings on razor blade cutting edges - is surprising since we have found that equivalent results cannot be obtained by subjecting high molecular weight polyfluorocarbons present in liquid dispersions to ionizing irradiation, and then using such irradiated dispersions to form the coating. While polyfluorocarbon coatings can be formed by the latter procedure, they do not have the properties looked for in such coatings on razor blade cutting edges. The reason for the significance of carrying out the irradiation with the polyfluorocarbon in the form of a dry powder is not known.

In order that the invention may be more

fully understood, the following example is given by . way of illustration only. Example

Polytetrafluoroethylene ("Teflon 60", Trade Hark, supplied by E.I. du Pont de Nemours & Co.) having a molecular weight in excess of 1,000,000 and in the form of a dry powder, was placed in ampoules of heat-resistant glass and the ampoules were exposed to Y -irradiation from a Co ⁶⁰ source at room temperature (25'C.) in air, the dose rate being 3 megarads/hour. After the polymer had received a radiation dose of about 25 megarads, radiation was stopped and the powder was taken out of the ampoules. No discoloration of the powder was observed. Air was blown through the irradiated material to remove the hydrogen fluoride generated by the irradiation of the polytetrafluoroethylene. The irradiated powder was then ground in a grinder to reduce the particle size and passed through a 100 micron sieve.

A dispersion containing 0.7% by weight of the ground and sieved powder in isopropanol was prepared and homogenized with an ultrasonic stirrer. Stainless steel razor blade cutting edges were then sprayed with the dispersion. After drying, the coating on the blade edges was sintered in nitrogen at 650*F. (343*C.) for 35 minutes. The blades so treated exhibited greater blade-to-blade uniformity, better blade performance and the same coating durability as similar blades which had been treated in exactly the same way, but omitting the irradiation step.