Marking fluorocarbon surfaces.

This invention relates to a method of marking fluorocarbon surfaces of high-performance electric cables and other articles, and to the products of the method.

The insulated conductors (and sometimes composite cores or complete multiconductor cables) need to be marked for the purpose of identifying conductors and circuits, designating cable types, recording the supplier and/or date of manufacture, or for other purposes. Laser printing (which is used for some other cables) is attractive on account of operating without inks or pigment, without physical contact and at high speed; but it has hitherto been found unworkable when the surface to be marked was of fluorinated ethylene propylene copolymer (FEP) or polytetrafluoroethylene (PTFE): a large proportion of the incident radiation appeared to be reflected and/or diffused, and no clearly visible mark could be obtained, except perhaps at unacceptably high intensities.

In accordance with the present invention, the surface to be coated is either formed of or coated with a reflective flurocarbon polymer composition; a light-transmitting flurocarbon polymer composition comprising an ethylene/tetrafluoroethylene copolymer or an ethylene/chlorotrifluoroethylene copolymer is applied as a surface coating over the surface of the reflective composition; and the coated surface is then marked by laser printing.

The reflective fluorocarbon surface is preferably a white pigmented one, more especially one made white by conventional amounts of titanium dioxide pigment. If required however, coloured reflective surfaces can be used, though pastel shades are naturally more satisfactory as background for dark indicia.

By a "light-transmitting" composition is meant one that is translucent or transparent to the extent that at least 20% of incident radiation of appropriate wavelength (eg about 1000nm if an infra-red laser is to be used) will penetrate the thickness of the surface coating, with or without being scattered , to be reflected by the underlying reflective surface.

Surprisingly, the presence of a substantially transparent coating one of the specified copolymers, which might have been expected to make no difference to the result, allows legible markings to be produced with both infra-red and ultra-violet lasers. Better results are however obtained when the coating is translucent so that a significant fraction of the incident light (say 10-50%) is scattered at least once within the light-transmitting layer: this results in markings with significantly sharper edges. Optimum translucency appears to be that achievable by incorporation into the copolymer of about 5-6% by weight of a titanium dioxide pigment of mean particle size 0.8nm.

When, as is preferred, ethylene-tetrafluoro ethylene copolymer (ETFE) as the light transmitting coating and is applied directly to an underlying reflective surface of polytetrafluoroethylene (PTFE) or of fluorinated ethylene/propylene copolymer (FEP), adhesion is not satisfactory, but we have found that this difficulty can be overcome by applying a primer coating of mixed ETFE/PTFE or ETFE/FEP as appropriate in dispersion form before the PTFE is sintered; suitable mixtures comprise between 0.1 and 1 parts by weight of ETFE for each part of PTFE or FEP. Alternatively, a suitable mixture of polymers can be used to form the light-transmitting outer layer, but this may produce markings less legible than can be achieved with a layer in which ETFE or ECTFE is the sole polymer. Satisfactory adhesion could also be obtained by etching the pigmented surface (e.g. with a solution of sodium in liquid ammonia), but this is not recommended for products in which the marked surface will be subject to significant electrical stress.

Example 1

A 19 x 0.20mm nickel-plated copper wire strand was coated with a white PTFE composition comprising 0.5% titanium dioxide pigment by a convention extrusion process to form a dielectric of radial thickness 0.25mm. Prior to sintering a 16% solids mixed aqueous dispersion comprising only 1 part dispersion grade ETFE and 5 parts dispersion grade PTFE was applied to form a mixed fluorocarbon primer coating about 0.005mm thick.

The primed insulated wire was then dried and heated to 375-400°C for half a minute to sinter both the dielectric and the primer. Next a translucent coating 0.02mm thick was applied by dipping six times in a 35% total solids aqueous dispersion of ETFE with 5% titanium dioxide pigment of mean particle size 0.8nm, each coat being dried and fused at 300°C for half a minute. The resulting coated wire was then laser printed using a 1064nm wavelength YAG laser at nominal beam power of 7W, which formed clearly visible lettering without detriment to electrical properties.

It was noted that the coated wire was also receptive of ink jet markings, making it suitable for use also when coloured markings are needed.

Example 2

A 19 x 0.20mm nickel plated copper wire strand was lapped with two "Kapton 616" tapes ("Kapton" is a trademark) each comprising a polyimide film 0.025mm thick coated on each side with 0.0025mm of FEP, applied each with 50% overlap and with opposite hand to form a layer with a radial thickness of 0.12mm. A white coating with a radial thickness of 0.025mm thick was applied to the surface of the tape layer from a 35% total-solids PTFE dispersion containing 12% of the same titanium dioxide pigment. A translucent coating was applied and printed exactly as in Example 1.

Example 3

This was the same as example 2 except that the dispersion used to form the white coating was based on FEP instead of PTFE, the total solids content being 35% as before.

Example 4

Another 19 x 0.20mm nickel plated copper wire strand was lapped with a single Kapton 616 tape applied with 75% overlap and then a white pigmented PTFE tape 0.051mm thick applied with 50% overlap. The PTFE tape was primed with a single application of a 35%-solids mixed dispersion containing equal weights of PTFE and FEP, each of dispersion grade, and the tape and coating sintered simultaneously by heating for 30 seconds at about 375-400°C. After sintering (during which significant shrinkage of the PTFE tape occurs) the radial thickness of the coated tape layer was 0.18mm. A translucent coating was applied and printed exactly as in Example 1.

Example 5

Two 19 x 0.20mm nickel plated copper wire strands were each lapped with a Kapton 616 tape applied with 50% overlap and then with a pigmented PTFE tape 0.051mm thick applied with 50% overlap. The pigmented tapes were respectively red and blue. After sintering (as in Example 4) the tape layers had a radial thickness of 0.12mm. The red and blue covered strands were twisted together to form a pair. This was lapped with a single Kapton 616 tape applied with 50% overlap. This was coated with white FEP, and primed with a 16% solids mixed aqueous dispersion comprising only 1 part dispersion grade ETFE and 5 parts dispersion grade FEP. After sintering, a translucent coating was applied exactly as in Example 4, and was easily printed as in the other examples despite its twisted non-circular shape.

Example 6

This was exactly the same as Example 3, except that the translucent layer was based on ECTFE instead of ETFE.