Laser-markable polytetrafluorethylene tape

The present invention relates to a tape polytetrafluoroethylene (PTFE) formed by at least two layers of polytétafluoroéthylène of different colors, suitable for laser marking.

Remember that the laser marking is to achieve markings on the tape by irradiating the tape with the aid of a beam laser, especially a CO laser₂. By adjusting appropriately the intensity of the laser beam is carried out perforations in the layer superior ribbon, revealing in the perforated zones color of the underlying layer, thereby forming the desired marking.

Multi-layered ribbons known PTFE suitable for marking laser and mainly used for sheathing electric cables.

The tape is used in this case an isolation function as ribbons of traditional PTFE and is adapted to receive, by labeling laser, identification of the cable entries.

Known tapes, suitable for laser marking, include usually a plurality of very thin layers of baked PTFE between which are arranged layers of another fluororesin such as FEP and / or PFA, the tape itself is coated with FEP and / or PFA.

The presence of these other fluororesins is necessary for sufficient adhesion tape layers together and the strip on the support on which it is wound, in particular a cable electric.

The manufacture of such a tape having a large number of layers requires the implementation of a process by wet complex and cost, of depositing successive layers of resins fluorinated dispersion, the method involving steps of drying and firing at temperatures higher than the temperature gelling of PTFE.

The product obtained is therefore particularly expensive.

In addition, it is found, during the marking by a beam laser, the formation of bubbles and blisters in the band, due to retaining, in the layers of tape, gaseous reaction products induced by irradiation.

in U.S. patent it has been proposed US-A-4,652,528 a ribbon having a PTFE layer and a fluororesin layer PFA met by high temperature rolling.

This tape is intended for automotive parts by laser marking for their identification during assembly on vehicles.

The present invention proposes to provide a PTFE tape avoiding the disadvantages mentioned above, can be manufactured as particularly simple and economical, suitable for cable wrapping Power to be marked by laser.

The tape according to the present invention, having laser marking is formed from at least two layers of PTFE of different colors whose lower layer at least consists of raw PTFE, which layer greater at least has a density less than 1.0.

By raw PTFE is meant PTFE which has never been submitted by baking at a temperature called "gelling" or "sintering" that is about 350 ° C.

In a preferred embodiment of the invention, the tape comprises only two layers of PTFE, namely an upper layer low density, less than 1.0, a first color, eg white PTFE natural, raw or cooked, possibly with the addition of pigments white, and a bottom layer of a second color made of raw PTFE charged with colored pigments, for example black pigments.

In another embodiment of the invention, the tape comprises three layers of PTFE, namely an upper layer of a first color, for example white baked natural PTFE, possibly the addition of white pigments, an intermediate layer made of PTFE cooked charged pigments of a second color, for example pigments black, and a bottom layer made of raw PTFE charged with pigments a third color different from the second, especially pigments red, the top layer and the intermediate layer having a low density of less than 1.0.

One of the laser beam intensity setting can drill selectively or only white top layer is also the intermediate layer, which allows marking in two colors different color corresponding to each of the layer intermediate and the lower layer.

Each of the PTFE layers of the tape of the invention can be carried out by the known method of lubricated extrusion which provides a tape having perfectly controlled characteristics, unlike wet processes, such as those used for ribbons capable known laser marking.

Remember that the lubricated extrusion process involves mixing the PTFE powder with a lubricant to produce a preform compact which is subjected to spinning to the press in which a piston represses PTFE through a die so as to form such a profile a flat tape or a cylindrical rod.

This profile is then subjected to a calendering operation between two cylinders to obtain a thin ribbon.

The lubricant is then removed, especially by drying, or optionally using a solvent.

If desired, the resulting tape can be subjected to dedensification step by drawing, for example by passing the tape successively driven rollers at peripheral speeds which are linear in the report of the extension that is to be given ribbon, or by subjecting the ribbon to the action of a device tension exerted on the tape a force corresponding to the deformation that one wishes to communicate.

Where appropriate stretching of the tape may be applied after a heating thereof.

In a first strip embodiment according to the invention, implements the lubricated extrusion process followed by a calendering and if appropriate, a dedensification drawing separately on preforms cylindrical or prismatic made one of natural raw PTFE, possibly filled with white pigments or the other preforms being made of raw PTFE charged with colored pigments.

Is thus obtained separately in the form of ribbons, different layers intended to constitute the tape of the invention. Layers intended to form the upper layer of the tape and the layer intermediates can then be cooked by being subjected for a sufficient time at a temperature above the sintering temperature.

These different layers thus formed can be associated with or without binder.

Alternatively, one can apply a complete layer obtained by the lubricated extrusion process described above on an outgoing layer calendering rolls and from which the lubricant has not been eliminated.

During drying after calendering, a portion of lubricant evaporating from the layer which still contains the remainder between two layers of tape and achieves an intimate connection between membership layers.

In another variation, the extruded layers are separately calendered sets the formed tape then being dried to remove lubricant and optionally subjected to a step of dedensification.

In another embodiment of the tape according to the invention, implements the lubricated extrusion process followed by a calendering and if appropriate, a dedensification drawing on a composite preform, in particular cylindrical or prismatic, having longitudinal areas consisting of a raw mixture of PTFE and lubricant, any of the areas containing natural PTFE, optionally charged with white pigments or colored and or other areas containing PTFE filled with pigments colored in different colors.

For example to realize a tape two layers, the preform may comprise two semi-cylindrical areas containing mixed with a lubricant, one of the natural raw PTFE, possibly loaded white or colored pigments, the other raw PTFE charged with colored pigments different colors.

To achieve such a preform is placed in a mold cylindrical preform, a semicylindrical mandrel, it fills the space remainder of the mold with a first mixture of lubricant and of natural PTFE loaded if any white or colored pigments, is forced using a piston this mixture, after which the mandrel is removed and is placed in the free part of the mold another mixture of lubricant and PTFE containing color pigments of different colors, e.g. black pigments and discharge is carried out by using a piston compacting the two mixtures different content in the mold.

For making preforms with more than two mixtures to making tapes over two layers, the operations are performed above successively by providing one or if appropriate a plurality of mandrels of appropriate sections.

Besides the color pigments, raw PTFE layers of tape according the invention may include other additives to give the tape particular properties, for example carbon black if desired, giving the band an additional anti-static function.

Other advantages and features of the invention will on reading the following description given with reference to the accompanying drawing in which : <ul><li>1a and 1b illustrate the different steps of out the lubricated extrusion process to obtain a layer of the tape according to the invention from a preform,</li><li>2 shows a preform for the production by coextruction lubricated a tape two layers, and</li><li>Figures 3a to 3d illustrate the steps of producing the preform of Figure 2.</li></ul>

We firstly refer to figures 1a and 1b.

Figure 1a illustrates the extrusion phases and lubricated calendering for producing a raw PTFE tape layer, while Figure 1b illustrates the phase, optionally implementation of dédensification by stretching.

A preform, for example of cylindrical shape, is obtained by compacting a mixture of PTFE raw fine powder and lubricant.

The preform 1 is introduced into an extrusion press 2 where a piston 3 presses the raw PTFE powder and lubricant mixture through a die 4 which, in the illustrated example, slit-shaped, of so as to obtain a flat tape 5. Alternatively, the sector could be circular so as to obtain a cylindrical rod.

The tape 5 is then passed between two cylinders calendering 6 and 7 rotating in opposite directions so as to obtain a ribbon 8 thin.

In a subsequent step, not illustrated, the tape 8 is dried to remove the lubricant which was used to facilitate extrusion and calendering. Depending on the lubricant used, the drying step can be replaced by a passage of the tape in a solvent to remove the lubricant.

If the density of thus obtained tape 8 is preferably greater than the desired value, one proceeds to a drawing step illustrated in Figure 1b.

The tape 8 is fed via guide rollers 9, 10, a drum 11 which is rotated at constant speed around its axis in the direction of the arrow F1.

If necessary, the drum 11 comprises a heating device internal to maintain its periphery at a constant temperature adjustable.

After having wound on the periphery of the drum 11, the tape raw PTFE 8 may be wound on a core 12 associated with a device drive not shown, arranged so that the traction force exerted 2 on the tape can be adjusted at will.

Alternatively, it may be provided downstream of the drum 11, another drum driven at a peripheral speed greater than the speed the drum device 11 so as to cause elongation of the strip from it comes out of the drum.

The method illustrated in Figures 1a and 1b is implemented separately to achieve each of the layers to form the ribbon according to the invention. The ribbon for forming the upper layer may be then baked.

The combination of the different layers manufactured separately can be carried out according in one of the embodiments set out above.

Alternatively, it is possible directly by the tape the invention by coextrusion lubricated. For example to realize a tape two layers are used, instead of the preform 1 shown in Figure 1a, a preform 1 'illustrated in Figure 2 and formed of two mixtures M1 and M2 different. The mixture M1 consists of fine powder of raw PTFE natural, if applicable the addition of white pigments, and a lubricant, and the M2 layer is a mixture of a fine powder of raw PTFE added colored pigments, for example black pigments, and a lubricant.

The preparation of such a preform is illustrated in Figures 3a to 3d.

a cylindrical mold is used 13 in which slides a piston 14.

The upper mold part is selectively closed by a lid 15 firmly held on the mold 13 by support members 16. is put in place in the mold form a semicylindrical mandrel 17 and is fills the rest of the mold cavity of a first mixture M1 and is obturated with the cover 15 the top of the mold in alignment with the area containing the mixture M1.

Is actuated the piston 14 so as to deliver the mixture M1 after What we remove the mandrel 17 and it fills the semi-cylindrical space free mold of a second mixture M2. After completely closed the upper end of the mold is again actuates the piston 14 of so as to cause compaction of the mixtures M1 and M2. After removing the closure lid, the preform 1 'is obtained illustrated in Figure 2 which comprises two semicylindrical longitudinal zones containing respectively M1 and M2 mixtures thereof. By using multiple cores on can make preforms formed of three or more mixtures for make a ribbon consisting of three or more layers.

We will describe below two examples of suitable marking tapes by laser according to the invention.

example 1

By the method illustrated in Figures 1a and 1b are manufactured layer top of the natural PTFE tape, the resulting layer having a thickness 35 .mu.m and a density of 0.75.

By implementing the method shown in Figure 1a, we realizes the lower layer of the tape using raw PTFE charged from 1% by weight black pigments. It does not perform the step of dédensification illustrated in Figure 1b.

The lower layer has a thickness of 50 microns and a density of 1.60.

The two layers are combined by pressure.

The bilayer tape obtained was used in the preparation of a cable wrapped, white dedensified layer being disposed outside.

After curing in situ the tape, it is found that the layer outer tape has good opacity, the cable appearing white.

It was able to achieve a satisfactory manner, without bubbles or blisters, print using a CO laser₂ conventional with an energy density of between 8 and 15 J / cm².

example 2

There is provided a tape by coextrusion by performing first such a preform illustrated in FIG 2 using a mixture M1 of raw PTFE and lubricant containing 2% by weight of white pigment and a PTFE mixture M2 and raw lubricant containing 2% by weight of pigments black.

After coextrusion lubricated, calendering and drying, were obtained ribbon bilayer of a thickness of 75 .mu.m.

The top layer pigmented white has a thickness of 25 microns.

Was dedensified this tape by drawing so as to reduce its density from 1.58 to 0.80.

There was also a very good ability of the ribbon A laser marking on a cable wrapped after firing the tape in situ.

example 3

a bilayer tape was carried out by combining a PTFE tape expanded natural cooked with a density of 0.5 and a thickness of 30 .mu.m to a raw PTFE tape pigmented black with a density of 1.65 and a thickness of 50 microns.

The obtained tape has a very good ability to laser marking.

Although the invention has been described in association with modes of achieving individuals, it is obvious that it is not limited and we can bring him various changes and modifications without departing or its scope or its spirit.