EP1808867A1 - Thermally stable fluoride composition - Google Patents

Abstract

Extrudable thermally stable composition comprises a fluorinated polymer, titanium dioxide and other inorganic fillers to neutralize the acid formed during extrusion of the composition, where the fillers have an average particle size of less than 100 nm, preferably less than 60 nm. An independent claim is included for an electrical conductor coated with an insulating layer obtained from the composition.

Description

The present invention relates to an extrudable, thermally stable composition used in the insulation of electrical cables and an electrical conductor coated with such an extruded composition.

It typically applies to the fluoropolymer-based insulation of copper wires in the field of LAN or Local Area Network (LAN) cables and, more generally, to data transmission cables.

In an exemplary embodiment, this composition is used to isolate part of the cables of category 5, 5e, 6 and 6e, in particular the white part of these cables. Indeed, these cables are formed by the assembly of four twisted pairs, where each twisted pair comprises a copper wire insulated with a white-colored insulator associated with a copper wire insulated with a colored insulator.

The document WO01 / 80253 proposes a high-speed extruded electrical cable insulation mixture composed of a pigmented fluoropolymer. The pigment used is titanium oxide coated with at least one layer of silicone and aluminum oxides. However, this composition does not provide acceptable thermal stability during the extrusion step. Thus, the thermal degradation generated by this step results in an imperfect surface appearance and a non-homogeneous internal structure of the insulation. The sheath thus formed has insulation defects with risks of electrical malfunctions and areas of blackening or discoloration.

The invention aims to solve the problems of the prior art by proposing a fluorinated composition, having improved thermal stability, in particular, during the thermal transformation steps of the composition, making it possible to obtain an insulating material free from any defect in the composition. insulation.

For this purpose, and according to a first aspect, the subject of the invention is an extrudable, thermally stable composition comprising a fluorinated polymer, titanium dioxide and, in addition, an inorganic filler capable of neutralizing the acid formed during extrusion step of the composition, having an average particle size of less than 100 nm, and preferably less than 60 nm.

Thanks to the invention, the thermal degradation of the polymer during the extrusion process is significantly limited.

In an advantageous embodiment, the inorganic filler is selected from zinc oxide, magnesium oxide or calcium oxide.

In particular, the preferred filler is zinc oxide. Zinc oxide is the most suitable scavenger for neutralizing hydrofluoric acid, especially during reactions taking place at high temperature such as the extrusion steps of fluoropolymers.

In an exemplary embodiment, the concentration of the filler is less than 5% by weight of the composition, and preferably less than 2%.

According to another characteristic, the concentration of the titanium dioxide is less than 30% by weight of the composition, and preferably between 5% and 15%.

Said concentrations of the filler and titanium dioxide thus limit the changes in the rheological properties of the fluorinated composition.

In addition, the fluoropolymer is a fluorinated ethylene propylene (FEP) copolymer.

Preferably, the FEP is chosen from the tetrafluoroethylene / hexafluoroisopropene copolymer or the tetrafluoroethylene / hexafluoroisopropene / perfluoroalkylvinylether copolymer. This type of copolymer is most commonly used in the isolation of twisted pairs of LAN cables.

According to a second aspect, the invention relates to an electrical conductor coated with at least one insulating layer, said layer being obtained from the composition according to the invention.

The advantage of such an insulator is to obtain a homogeneous protection of the entire electrical conductor to avoid, for example, electrical breakdowns and to have a uniform appearance uncolored.

The invention will be better understood from the description which follows with reference to the annotated drawings and illustrating various embodiments.

FIG. 1 represents a comparison of the thermal stability of extruded fluorinated compositions according to the invention and according to the prior art.

During the extrusion stages, the fluoropolymers undergo different stresses, in particular shear stresses and thermal stresses, with a temperature profile varying between 250 ° C. and 400 ° C.

The inevitable consequence of these two types of constraints is the degradation of the ends of the carbonyl chains such as -COF or -COOH groups and intramacromolecular chain defects, for example from thermally unstable hexaflurorisopropylene (HFP) diads.

These sites are therefore precursors of the thermal degradation process of fluoropolymers.

This thermal degradation will cause the formation of hydrofluoric acid which will thus feed the degradation process of the extruded fluorinated composition.

Titanium dioxide is used to whiten some of the cable sheaths for data transmission, including LAN cables.

Titanium dioxide is the most used white pigment in the coloring of plastics because of its coloring power higher than other white pigments such as zinc sulphide or lithopone white.

There are two types of titanium dioxide: rutile mesh and anatase mesh. In the case of coloring plastics, the commonly used grades are rutile type, because of their better durability.

It is known that titanium dioxide, in the fluorinated compositions, reacts with the hydrofluoric acid released during the extrusion phase to form titanium tetrafluoride.

In a particular example, when the titanium dioxide is covered with alumina in order to improve its dispersion in the plastic material, another metal halide is formed in the presence of hydrofluoric acid such as aluminum trifluoride.

Therefore, these metal halides will accelerate the chain splitting reactions and thus accelerate the thermal degradation process of the extruded fluorinated composition.

The sheath obtained undergoes an irreversible degradation of its chemical structure, translated by the appearance of microbubbles, by the formation of aggregates and, ultimately, by the blackening of the initially white sheath.

The invention therefore aims to limit the thermal degradation process of the extrudable compositions based on fluorinated polymer and comprising titanium dioxide.

For this, the invention proposes the addition in said composition of an inorganic filler, capable of neutralizing the acid formed during the extrusion step of the composition, having an average particle size of less than 100 nm, and of preferably less than 60 nm.

This average size of particles makes it possible to obtain an effective action on the stabilization process of pigmented fluorinated compositions. Indeed, the smaller the average particle size, the greater the specific surface area of the filler, since the specific surface area is inversely proportional to the average particle size. It is necessary so that a maximum surface of the load is in contact with said composition.

Said filler is a solid material which does not interact chemically with the polymer matrix and is intended to neutralize the hydrofluoric acid as it is formed.

The formation of metal halides is thus substantially avoided, which has the effect of considerably slowing down the kinetics of degradation of the extruded polymer resin.

The insulating material thus formed is homogeneous in its chemical structure and presents no insulation defects or aggregates.

As a result, the white color of the insulation is persistent and no trace of blackening is visible on its surface.

In a particular embodiment, the inorganic, acid scavenger is zinc oxide.

Among the various "acid scavenger" fillers such as metal stearates and hydrotalcite and hydrocalumite grades, zinc oxide is the most thermally stable filler.

Zinc oxide is thus particularly suitable for the stabilization of systems whose processing temperatures are high, particularly in the case of the extrusion step of fluoropolymers.

In a particular example, the composition comprises nanometric zinc oxide, the average particle size of which is between 35 and 55 nm.

In another embodiment of the invention, the filler is selected from dehydrated alkaline fillers such as magnesium oxide or calcium oxide.

In a particular example, the composition comprises nanoscale magnesium oxide or nanoscale calcium oxide, the average particle size of which is approximately 30 or 40 nm, respectively.

The concentration of the filler is less than 5% by weight of the composition, and preferably less than 2%.

The advantage of using a filler having a large specific surface makes it possible to stabilize the composition with a low concentration of said filler relative to the concentration of titanium dioxide and thus not to modify the rheological properties of the pigmented fluorinated composition.

According to another characteristic of the invention, the concentration of the titanium dioxide is less than 30% by weight of the composition, and preferably between 5% and 15%.

The concentration of the pigment in the extruded fluorinated composition and the dilution of this composition in a virgin fluorine resin during the insulation phase of an electrical conductor, in particular a copper wire, are the two parameters that govern the coloring. final insulation.

If the concentration of the pigment is greater than 30%, problems of rheology appear: the quality of the dispersion of the pigment in the fluorinated composition is impaired by the formation of aggregates and agglomerates of pigments. This results in a final non-homogeneous coloration of the insulation.

If the concentration of the pigment is less than 5%, a much larger proportion of extruded fluorinated composition should be used in addition to the virgin fluorine resin.

The fluorinated polymer may be chosen in particular from tetrafluoroethylene copolymers such as fluorinated ethylene propylene copolymer (FEP), ethylene tetrafluoroethylene (ETFE) copolymer, ethylene and fluorinated ethylene-propylene terpolymer ( DAIKIN EFEP or DYNEON HTE), copolymer of tetrafluoroethylene and perfluoromethylvinylether (MFA) and copolymer of tetrafluoroethylene and perfluoropropylvinylether (PFA), as well as homopolymers, copolymers and terpolymers containing vinylidene fluoride (PVDF, THV).

In a particular embodiment of the invention, the composition comprises a fluorinated ethylene propylene (FEP) copolymer, especially tetrafluoroethylene (TFE) / hexafluoroisopropene (HFP) copolymer.

Modified FEP copolymers can also be used. The modification is made by the addition, in a minor amount, of a third monomer during the synthesis of the modified FEPs, which may be a perfluoroalkylvinyl ether (PAVE) monomer of the perfluoropropylvinyl ether (PPVE) or perfluoroethylvinyl ether (PEVE) type. .

These monomers are used in the synthesis of FEP not only to provide better control of the molar distribution of the fluoropolymer, but their presence significantly improves the mechanical properties of this type of composition and in particular the resistance to fatigue under stress. However, the perfluoroalkylvinyl ether units are much more sensitive to metal halides than the TFE or HFP units, which makes the macromolecular chain more vulnerable.

The main but non-limiting application of the invention lies in the field of data transmission cables. It concerns, in fact, electrical conductors coated with at least one insulating layer obtained from the composition according to the invention.

In order to show the advantages obtained with the compositions according to the invention, an experimental protocol was developed in order to place the fluorinated composition under temperature and shear conditions close to those encountered in the body of a single-screw extruder.

The stability of the material can be evaluated in two ways. The first is to follow the evolution of the Newtonian viscosity of the fluorinated composition at different stages of thermal degradation: the composition having the lowest viscosity drop as a function of the treatment time is the most thermally stable. The second analysis consists in progressively comparing the evolution of the white color of the composition towards a gray discoloration, consequence of its thermal degradation.

Samples were prepared by mixing the FEP resin, pigments and fillers with a Berstorff Corotative Bi-Vis extruder.

The inorganic filler used is nanometric zinc oxide, the average particle size of which is between 35 and 55 nm and whose specific surface area is of the order of 20-30 m ² / g.

Table 1 gives the compositions of the samples of the various mixtures studied in the context of the invention. Table 1 Sample Reference Fluoropolymer TiO ₂ % Mass Inorganic charge % Mass Average Size of Particles μm AT TFE / HFP copolymer ⁽¹⁾ - - - B TFE / HFP / PAVE Copolymer ⁽²⁾ - - - VS TFE / HFP / PAVE Copolymer ⁽²⁾ 15% - - D TFE / HFP / PAVE Copolymer ⁽²⁾ 14.8% 0.2% ZnO 0.035 to 0.055 E TFE / HFP / PAVE Copolymer ⁽²⁾ 14% 1% ZnO 0.035 to 0.055 (1) MFI (Melt Flow Index) = 22, (2) MFI = 26.

The extruded fluorinated composition is then cooled in a tray of water, then dried and granulated.

The granules obtained are placed in the sheath of a capillary rheometer at 400 ° C. They are extruded through a die (L / D = 30) under a shear of 1500 s ^-1 after 60s, then reduced to 50 s ^-1 after a period of 150s. A viscosity measurement is recorded every 30 seconds between 180 seconds and 900 seconds.

For each sample analyzed, the evolution of the log of the viscosity of the samples A, B, C, D and E as a function of the aging time of said samples makes it possible to evaluate their thermal stability.

This thermal stability is determined by the calculation of the slopes of the curves thus obtained.

By assigning a coefficient equal to 100 for the sample whose degradation rate at 400 ° C is the slowest (sample A) or, in other words, corresponding to the highest thermal stability, it is thus possible to represent the thermal stabilities relative to each sample relative to the reference sample A, as illustrated in FIG.

Example 1

Sample A was aged according to the procedure described above and has the highest thermal stability among the samples tested in the study.

Example 2

Sample B was aged according to the procedure described above. Its thermal stability decreases by 4.1% compared to that of the standard sample A.

Example 3

Sample C was prepared according to the procedure described above by mixing the TFE / HFP / PAVE copolymer with 15% by weight of TiO ₂ .

The capillary rheometry analysis of the sample shows that the thermal stability drops by 22.6% compared to the standard sample A.

Example 4

Sample D was prepared according to the procedure described above by mixing the TFE / HFP / PAVE copolymer with 14.8% by weight of TiO ₂ and 0.2% by weight of nanometric ZnO, the average particle size of which is between 0.035 and 0.055 μm.

The capillary rheometry analysis of the sample shows that the thermal stability drops by 19.1% compared to the standard sample A.

Example 5

Sample E was prepared according to the procedure described above by mixing the TFE / HFP / PAVE copolymer with 14% by weight of TiO ₂ and 1% by weight of nanometric ZnO, the average particle size of which is between 0.035 and 0.055 μm.

The capillary rheometry analysis of the sample shows that the thermal stability drops by 13.7% compared to the standard sample A.

Thus, in view of the results obtained, the addition of zinc oxide in a fluorinated composition containing titanium dioxide makes it possible to obtain an extruded mixture which is more thermally stable.

Indeed, as shown in FIG. 1, it clearly appears that the presence of zinc oxide in samples D and E limits the thermal degradation of the extruded fluoropolymer in comparison with a composition containing only the TFE / HFP / PAVE copolymer and titanium dioxide (sample C).

The presence of 0.2% of nanometric ZnO (sample D) and 1% of nanometric ZnO (sample E) makes it possible to improve the thermal stability of the mixture by 4.2% and 10.3%, respectively, compared to sample C.

A second simple and effective means, not shown, for evaluating the thermal stability of extruded fluorinated compositions is to observe the evolution of the discoloration of the material, in particular its blackening, as a function of the thermal aging time at 400 ° C.

It is actually observed that the sample C begins to darken after 5 minutes in the sleeve of the capillary rheometer.

In comparison, samples D and E, having undergone the same type of thermal aging, begin to be discolored only after 10 minutes in the sleeve of the capillary rheometer.

The invention is not limited to the preceding embodiments.

Other fillers such as synthetic hydrocalcites or hydrocalumites can also be used.

Finally, the limit values of the given intervals are not to be considered in the strict sense and may vary within customary tolerances well known to those skilled in the art.

Claims (7)

An extrudable, thermally stable composition comprising: a fluoropolymer, and - titanium dioxide, characterized in that it further comprises an inorganic filler, capable of neutralizing the acid formed during the extrusion step of the composition, having an average particle size of less than 100 nm, and preferably less than 60 nm. Composition according to Claim 1, characterized in that the filler is chosen from zinc oxide, magnesium oxide or calcium oxide, preferably zinc oxide. Composition according to Claim 1 or 2, characterized in that the concentration of the titanium dioxide is less than 30% by weight of the composition, and preferably between 5% and 15%. Composition according to any one of Claims 1 to 3, characterized in that the concentration of the filler is less than 5% by weight of the composition, and preferably less than 2%. Composition according to any one of Claims 1 to 4, characterized in that the fluorinated polymer is a fluorinated ethylene propylene (FEP) copolymer. Composition according to Claim 5, characterized in that the FEP is chosen from the tetrafluoroethylene / hexafluoroisopropene copolymer or the tetrafluoroethylene / hexafluoroisopropene / perfluoroalkylvinylether copolymer. Electrical conductor coated with at least one insulating layer, characterized in that said layer is obtained from the composition according to any one of Claims 1 to 6.

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