Laser-markable polytetrafluorethylene tape.

p0001The present invention relates to a tape of polytetrafluoroethylene (PTFE) formed by at least two polytetrafluoroethylene layers of different colors, suitable for laser marking.

p0002Remember that the laser marking is to make inscriptions on the tape by irradiating the tape with a laser beam, including a CO₂ laser. By adjusting suitably the intensity of the laser beam is carried out perforations in the upper layer of the tape, leaving the perforated zones appear in the color of the underlying layer, thereby forming the desired marking.

p0003We know PTFE multi-layered ribbons suitable for laser marking and used primarily for sheathing electric cables.

p0004The tape is used in this case an insulation function like traditional PTFE tapes and is adapted to receive, by laser markings, cable identification inscriptions.

p0005Known tapes suitable for laser marking, usually comprise a plurality of very thin layers of baked PTFE between which are disposed layers of another fluorinated resin such as FEP and / or PFA, the tape itself being coated FEP and / or PFA.

p0006The presence of these other fluororesins is necessary to allow satisfactory adhesion of the tape layers together and the tape on the support on which it is wound, in particular an electric cable.

p0007The manufacture of such a tape having a large number of layers requires the implementation of a method by complex and costly wet of depositing successive layers of fluorinated resins in dispersion, the method involving steps of drying and cooking at temperatures above the gelling temperature of PTFE.

p0008The product obtained is therefore particularly expensive.

p0009In addition, it is found, during the marking by a laser beam, the formation of bubbles and blisters in the band, due to restraint, in the layers of tape, gaseous products of the reaction induced by the irradiation.

p0010The present invention proposes to provide a PTFE tape avoiding these disadvantages and can be manufactured more particularly easily and economically.

p0011The tape according to the present invention, having laser marking is formed from at least two layers of PTFE of different colors and is essentially characterized in that the lower layer at least consists of raw PTFE, and that the upper layer at least has a density less than 1.0.

p0012By raw PTFE is meant PTFE which has never been subjected by cooking at a temperature called "gelling" or "sintering" is of the order of 350 ° C.

p0013In a preferred embodiment of the invention the tape comprises only two layers of PTFE, namely an upper layer of low density, less than 1.0, a first color, for example white natural PTFE raw or cooked, optionally with the addition of white pigments, and a lower layer of a second color made of raw PTFE charged with colored pigments, for example black pigments.

p0014In another embodiment of the invention, the strip comprises three layers of PTFE, namely an upper layer of a first color, for example white cooked in natural PTFE, optionally with the addition of white pigments, an intermediate layer made of PTFE cooked charged pigments of a second color, for example black pigments, and a bottom layer made of raw PTFE charged with pigments of a third color, different from the second, including red pigments, the upper layer and the intermediate layer with a low density of less than 1.0.

p0015One of the laser beam intensity setting for selectively pierce or only the white upper layer, being also the intermediate layer, which allows marking in two different colors respectively corresponding to the color of the intermediate layer and the lower layer .

p0016Each of the layers of PTFE tape of the invention can be carried out by the known method of lubricated extrusion which allows to obtain a ribbon having perfectly controlled characteristics, in contrast to wet processes such as those used for get the tapes suitable for marking by laser known.

p0017It is recalled that the lubricated extrusion process involves mixing the PTFE powder with a lubricant to produce a compact preform which is subjected to spinning to the press in which a piston presses the PTFE through a die to form a section such as a flat tape or a cylindrical rod.

p0018This profile is then subjected to a calendering operation between two rolls to obtain a thin ribbon.

p0019The lubricant is then removed, especially by drying, or optionally using a solvent.

p0020If desired, the resulting tape can be subjected to a step of de-densification by stretching, for example by passing the tape successively on rollers driven at linear peripheral speeds that are in the ratio of the elongation that is wishes to give to the strip, or by subjecting the ribbon to the action of a tensioning device which exerts on the tape a force corresponding to the deformation that is desired to communicate.

p0021Where appropriate stretching of the tape can be performed after heating thereof.

p0022In a first mode of realization of tape according to the invention, it implements the lubricated extrusion process followed by a calendering and optionally a dedensification drawing separately on cylindrical or prismatic preforms constituted one of PTFE raw natural, possibly filled with white pigments or the other preforms are made of raw PTFE charged with colored pigments.

p0023Is thus obtained separately in the form of ribbons, individual layers intended to constitute the tape of the invention. The layers intended to form the upper layer of the ribbon and the intermediate layer or layers can then be cooked being subjected for a sufficient time at a temperature above the sintering temperature.

p0024These different layers thus formed may be associated with or without binder.

p0025Alternatively, one can apply a complete layer obtained by the lubricated extrusion process described above on the outgoing layer of the calendering rolls and from which the lubricant has not been eliminated.

p0026During drying after calendering, a portion of the lubricant is evaporated from the layer which still contains the remainder between the two tape layers and achieves an intimate bond of adhesion between the layers.

p0027In another variation, the extruded layers are calendered separately sets the formed tape then being dried to remove the lubricant and optionally subjected to a step of dedensification.

p0028In another embodiment of the tape according to the invention, it implements the lubricated extrusion process followed by a calendering and optionally a dedensification drawing on a composite preform, in particular cylindrical or prismatic, having longitudinal zones made of a raw mixture of PTFE and lubricant, one of the regions containing natural PTFE, optionally charged with white or colored pigments and the other zone containing PTFE filled with colored pigments of different colors.

p0029For example, to achieve a tape two layers, the preform may comprise two semicylindrical regions containing, mixed with a lubricant, one of the natural raw PTFE, optionally loaded with white or colored pigments, the other raw PTFE charged with colored pigments of different colors.

p0030To produce such a preform is placed in a cylindrical preform mold, a semicylindrical mandrel, the remaining space is filled the mold of a first mixture of lubricant and of natural PTFE filled if any white or colored pigments which can displaces with 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, for example, pigments and black is carried out by displacement with the piston compaction of the two mixtures different content in the mold.

p0031For making preforms with more than two mixtures, to produce tapes to more than two layers, the aforementioned operations are successively carried out by providing one or if appropriate several appropriate sections of mandrels.

p0032Besides the color pigments, raw PTFE layers of the tape of the invention may include other additives to give the tape particular properties, for example carbon black if you want give the tape a further function anti- static.

p0033Other advantages and characteristics of the invention appear on reading the following description given with reference to the accompanying drawings wherein: <ul><li>1a and 1b illustrate the various stages of implementation of 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 lubricated by coextruction a tape two layers, and</li><li>Figures 3a to 3d illustrate the steps of making the preform of Figure 2.</li></ul>

p0034We firstly refer to figures 1a and 1b.

p0035Figure 1a illustrates the phases lubricated extrusion and calendering for producing a raw PTFE tape layer, while Figure 1b shows the phase, optionally implementation, dedensification by stretching.

p0036A preform, for example of cylindrical shape, is obtained by compacting a raw PTFE mixture into a fine powder and lubricant.

p0037The preform 1 is introduced into an extrusion press 2 wherein a piston 3 presses the raw PTFE powder and lubricant mixture through a die 4 which, in the illustrated example, slit-shaped, so as to obtain a flat tape 5. Alternatively, the die may be circular so as to obtain a cylindrical rod.

p0038This tape 5 is then passed between two calender rolls 6 and 7 rotating in opposite directions so as to obtain a thin ribbon 8.

p0039In 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 may be replaced by a passage of the tape in a solvent to remove the lubricant.

p0040If the density of thus obtained tape 8 is still greater than the desired value, one proceeds to a drawing step illustrated in Figure 1b.

p0041The tape 8 is fed, via deflecting rollers 9, 10, a drum 11 which is driven in rotation at constant speed about its axis in the direction of the arrow F1.

p0042If necessary, the drum 11 has an internal heater for maintaining its periphery at an adjustable constant temperature.

p0043After having wound on the periphery of the drum 11, the raw PTFE tape 8 may be wound on a core 12 associated with a not shown driving device, arranged so that the tensile force exerted on the ribbon 2 can be adjusted at will.

p0044Alternatively, it may be provided downstream of the drum 11, another drum driven at a higher peripheral speed than the peripheral speed of the drum 11 so as to cause elongation of the tape as soon as it leaves the drum.

p0045The method illustrated in Figures 1a and 1b is implemented separately to realize each of the layers to constitute the tape of the invention. The ribbon for forming the upper layer can then be subjected to baking.

p0046The membership of the separately produced layers can be performed in one of the implementation methods set out above.

p0047Alternatively, one can directly produce the tape of the invention by coextrusion lubricated. For example to realize a tape with two layers, is used instead of the preform 1 shown in Figure 1a, a preform 1 'illustrated in Figure 2 and formed of two different mixtures M1 and M2. The mixture M1 is composed of fine powder of natural raw PTFE, optionally the addition of white pigments, and a lubricant, and the M2 layer is a mixture of a fine powder of raw PTFE added with colored pigments, e.g. black pigments, and a lubricant.

p0048The preparation of such a preform is illustrated in Figures 3a to 3d.

p0049a cylindrical mold 13 is used in which a piston 14.

p0050The upper mold part is selectively closed by a lid 15 firmly held on the mold 13 by supporting members 16. It sets up in the mold a semi-cylindrical shape of mandrel 17 and is filled the rest of the cavity mold of a first mixture M1 and is sealed with the lid 15, the upper mold part plumb with the zone containing the mixture M1.

p0051Is actuated the piston 14 so as to deliver the mixture M1 is removed after which the mandrel 17 and it fills the free space of the semi-cylindrical mold with a second mixture M2. After completely sealed the upper end of the mold is again actuates the piston 14 so as to cause compaction of the mixtures M1 and M2. After removing the end cover, the preform 1 'is obtained illustrated in Figure 2 which comprises two semicylindrical longitudinal zones respectively containing the mixtures M1 and M2. Using a plurality of mandrels can be achieved preforms formed of three or more mixtures for making a ribbon consisting of three or more layers.

p0052We will describe below two examples of suitable laser marking tapes of the invention.

example 1

p0053By the method illustrated in Figures 1a and 1b is made the upper layer of natural PTFE tape, the resultant layer having a thickness of 35 .mu.m and a density of 0.75.

p0054By implementing the method shown in Figure 1a, is carried out the lower layer of the tape using raw PTFE charged at a rate of 1% by weight black pigments. Is not carried to the dedensification step illustrated in Figure 1b.

p0055The lower layer has a thickness of 50 .mu.m and a density of 1.60.

p0056The two layers are combined by pressure.

p0057The bilayer tape obtained was used in the preparation of a covered cable, white dedensified layer being disposed outside.

p0058After curing in situ of the tape, it is found that the outer layer of the tape has good opacity, the cable appearing white.

p0059It was able to achieve a satisfactory manner, without bubbles or blisters, print using a laser to conventional CO₂ with an energy density of between 8 and 15 J / cm².

example 2

p0060a tape by coextrusion was carried out by first producing such a preform illustrated in FIG 2 using a raw PTFE and lubricant mixture M1 containing 2% by weight of white pigment and a raw PTFE mixture M2 and lubricant containing 2% by weight black pigments.

p0061After lubricated coextrusion, calendering and drying, there was obtained a two-layer tape with a thickness of 75 .mu.m.

p0062The top layer pigmented white has a thickness of 25 .mu.m.

p0063Was dedensified this tape by drawing so as to reduce its density from 1.58 to 0.80.

p0064There was also a very good ability of the tape to the laser marking on a cable wrapped after firing the tape in situ.

example 3

p0065a bilayer tape was carried out by combining a natural expanded PTFE tape 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 .mu.m.

p0066The obtained tape has a very good ability to laser marking.

p0067While the invention has been described in connection with particular embodiments, it is obvious that it is not limited and can bring him various changes and modifications without departing from its scope or not of his mind.