FIELD OF THE INVENTION

The present invention relates to coaxial cables, and more particularly to a coaxial cable resistant to contraction caused by high temperature gradients.

BACKGROUND

RG-59 coaxial cables enable high frequency electric signals transmission and are frequently used in car antennas. Since such antennas are exposed to many temperature changes, it is necessary to find a way to prevent for the coaxial cable isolating layer, also referred as insulation, to contract due to such temperature changes in order to prevent quality loss of electric signals transmission. For specific automotive assemblers, the maximum distance that the isolation member must be contracted on each cable end is of 5 mm on a distance of 2 m, the equivalent to 0.25%.

Attempts have been made to prevent the isolation member contraction by using different materials thin layers or "skins" along with adhesives. For example, the document JPH0428118 describes a coaxial cable that comprises a conductor, an isolation member, a polyester "skin", a coated adhesive layer, a metal mesh and a cover, wherein the polyester skin and the coated adhesive layer reduce the contraction caused by temperature changes which prevents a quality loss of electric signals transmission due to such contraction. Nevertheless, such document fails to demonstrate that within such broad temperature ranges a contraction is achieved, or that a maximum 0.25% contraction is achieved.

On the other hand, it has been attempted to prevent the isolation member contraction by using several adhesive layers between the conductor and the isolation member. For example, document WO2011115295 describes a coaxial cable with several adhesive layers in order to prevent a contraction caused by temperature changes. Flowever, it is necessary to find an alternative to the adhesives use since these might affect the quality of high frequency electric signals transmission, to decrease the environmental impact and to simplify the cable fabrication process.

In turn, documents JP2009224284 and JP2009110774 describe a coaxial cable that comprises a perfluoroalkoxy copolymers protective layer or PFA which firmly holds the rest of the layers conforming the coaxial cable, and in this way prevents the contraction during temperature cycles ranging from -20°C to 85°C. Nevertheless, it fails to prevent the contraction for temperature cycles under -40°C.

Pursuant to the above, it is an object to eliminate the drawback present on RG-59 coaxial cables presently used by developing a coaxial cable resistant to contraction caused by temperature changes that, apart from having a minimum contraction caused by temperature changes, can be easily manufactured in compliance to safety and quality standards. OBJECTS OF THE INVENTION

Taking into account the previous art defects, it is an object of the present invention to reduce the contraction occurring on coaxial cables by temperature changes.

Furhtermore, an RG-59/U type coaxial cable with a contraction minor to 0.25% caused by temperature changes of -40°C to 85°C is seeked to obtain.

These and other objects are achieved by means of the coaxial cable resistant to contraction caused by temperature changes according to the present invention.

SUMMARY OF THE INVENTION

A coaxial cable has been invented, which comprises: an internal conductor; an isolating layer surrounding the internal conductor; an external conductor; and a protective layer surrounding the external conductor; characterized by further comprising a thermoplastic olefin-base thin layer positioned between the isolating layer and the external conductor, allowing for the coaxial cable to have a higher resistance to contraction caused by temperature changes.

BRIEF DESCRIPTION OF DRAWINGS

The novel aspects, considered characteristic of the present invention, will be particularly set forth in the appended claims. However, some embodiments, features and some objects and advantages thereof, will be better understood from the detailed description read together with the appended drawings, wherein:

Figure 1 illustrates an internal front view of a specific embodiment of a coaxial cable according to the principles of the present invention.

Figure 2 illustrates another internal front view of a specific embodiment of a coaxial cable according to the principles of the present invention.

Figure 3 shows a perspective view of the thin layer surface of a specific embodiment of a coaxial cable according to the principles of the present invention obtained by any conventional extrusion method (30) and by a grooved extrusion die (40).

DETAILED DESCRIPTION OF THE INVENTION

It has been found that a coaxial cable comprising: an internal conductor; an isolating or dielectric layer surrounding the internal conductor; an external conductor or a metal mesh; and a protective layer surrounding the external conductor; characterized by further comprising a thermoplastic olefin-base thin layer or "skin" positioned between the isolating layer and the external conductor, allows for the coaxial cable to have a higher resistance to contraction caused by temperature changes.

In a preferred embodiment of the present invention, the coaxial cable is an RG-59 coaxial cable, and more preferably is an RG-59/U coaxial cable.

In addition, in a preferred embodiment of the present invention, the internal conductor is preferably made of at least a conductive metal, and more preferably is made of copper, aluminum, or copper coated steel also referred to as "copper clad steel".

In turn, the isolating layer surrounding the internal conductor is preferably made of polymer, and more preferable is made of fluoropolymer or polyolefin such as polyethylene or polypropylene.

Regarding the external conductor, in a preferred embodiment of the present invention is a mesh made of at least one conductive metal, wherein such conductive metal is preferably copper, tinned copper or aluminum.

In turn, the protective layer is preferably made of polymer, and more preferably is made of polyvinyl chloride (PVC).

In a preferred embodiment of the present invention, the thermoplastic olefin-base thin layer is preferably selected from a thermoplastic olefin of the thermoplastic rubber type known as "TPO", "TPV" or "TPE", polyethylene, polypropylene or mixtures thereof.

One of the advantages that the use of butadiene-base thermoplastic olefin or "rubber" provides is to improve the adhesion or "grip" since this material, by not being rigid at room temperature, gets softer as it gets warmer and this causes for the mesh to penetrate and clog, which in turn results in preventing the isolation member from contracting.

Additionally, the thermoplastic olefin-base thin layer has a preferred thickness of 0.20 mm to 0.40 mm.

In another preferred embodiment of the present invention, the thermoplastic olefin-base thin layer has a rough texture. Preferably, the rough texture is achieved by foaming the thin layer in order to obtain a foamed thin layer and/or by the use of a predetermined 3D shape of the thin layer.

Foaming creates a greater expansion of the thermoplastic olefin-base thin layer and as a result, the expansion pushes the isolating layer against the external conductor which increases the adhesion or "grip" and decreases the isolating layer contraction. That is, the rough texture of the thermoplastic olefin-base thin layer caused by foaming has an even more important effect on decreasing the isolating layer contraction since it causes that the external conductor provokes a greater friction force with the isolating layer and prevents it from moving.

Foaming the thin layer is achieved by using a foaming agent which is preferably selected from compounds based on azodicarbonamide (ADC), 4,4'-Oxibis, benzene sulfonyl hydrazides (OBSFI), para-toluene sulfonyl hydrazide (TSFI), benzene sulfonyl hydrazide (BSFI), para-toluene semicarbazide (TSS) and 5-phenyl tetrazole (5PT). The presence of foam or air bubbles in the thermoplastic polyolefin- base thin layer decreases contraction, and at the same time provides a thermic isolating layer that also contributes to decrease the contraction and protects the cable from temperature changes.

Preferably, the thermoplastic olefin-base thin layer roughness is achieved by a predetermined 3D shape obtained by extrusion by using a groove extrusion die in order to obtain a thermoplastic olefin-base thin layer with a more homogeneous texture that provides the desired roughness.

On the other hand, having in mind a better fastening of the isolating layer to the internal conductor, in an optional embodiment, the coaxial cable of the present invention comprises a coupling layer between these two layers, which comprises a polymer mixture with greater affinity to the internal conductor than the one of the material forming the isolating layer, thus improving the adherence without affecting the coaxial cable electric properties. Preferably, the polymers used are selected from thermoplastic olefins of the thermoplastic rubber type known as "TPO", "TPV" or "TPE", polyethylene, polypropylene or mixtures thereof. In a preferred embodiment of the present invention, the coupling layer is made of a low density polyethylene (LPDE) mixture with LDPE grafted with maleic anhydride, and more preferably is Orevac® 18302N.

Said coupling layer is formed as a thin layer located between the internal conductor and the isolating layer. Said coupling layer must be made of a polymer mixture which resists a thermal shock within a range of -40°C to 80°C and it does not detach from any of the adjacent layers in order to prevent or decrease the contraction. Preferably, the coupling layer has a thickness of 100 to 120 micrometers.

The main advantage that the present invention provides is that the thermoplastic olefin - base layer provides resistance to contraction caused by temperature changes, wherein the temperature changes performed preferably comprise up to 300 temperature cycles ranging from -40°C up to 85°C within a thermic shock chamber, remaining there at -40°C for 30 minutes and at 85°C for 30 minutes in order to complete 1 cycle.

In a preferred embodiment of the present invention, the maximum contraction of the coaxial cable after being subjected to the cycles of temperature is of 0.25% or 5 mm within a 2m cable length.

Making reference now to figure 1, this shows an inner front view of a specific embodiment of a coaxial cable (10) according to the principles of the present invention. As can be seen in said figure, the internal conductor (11) is covered by the isolating layer (12), which in turn is covered by the thermoplastic olefin-base thin layer (13) in order to prevent a contraction caused by temperature changes, followed by the external conductor (14); finally there is the protective layer, which for the illustrated case is made of "PVC" (15).

Making reference now to figure 2, this shows an inner front view of another specific embodiment of a coaxial cable (20) according to the principles of the present invention. As can be seen in said figure, the internal conductor (21) is covered by the coupling layer (26), which in turn is covered by the isolating layer (22); this is in turn covered by the thermoplastic olefin -base thin layer (23) in order to prevent a contraction caused by temperature changes; followed by the external conductor (24); finally there is the protective layer, which for the illustrated case is made of "PVC" (25).

The present invention will be better understood from the following examples, which are given only for illustrative purposes in order to allow a better understanding of the preferred embodiments of the invention, without implying that there are not other non-illustrated embodiments which can be taken into practice based on the above detailed description. EXAMPLES

Example 1

An assay was carried out to evaluate the contraction suffered by different embodiments of coaxial cables manufactured in accordance to the principles of the present invention.

As such, the 2 meters length analyzed coaxial cables, were subjected to 100 temperature cycles, wherein each cycle consists of 30 minutes at -40°C and 30 minutes at 85°C and the contraction of each of them was measured to demonstrate the resistance to contraction that the coaxial cable of the present invention shows. It has been observed that if the contraction is too low, less than 5 mm for the first 100 cycles, the cable will go through all 300 cycles without showing a contraction of the isolation greater than 5 mm, i.e., greater than 0.25% of the cable length.

The results that were obtained are shown in the following table, where the composition of two coaxial cables according to the invention can be evaluated as well as the observed contraction for the same.

TABLE 1

The foregoing results show that both coaxial cables manufactured according to the principles of the present invention have a total contraction less than 5 mm. Nevertheless, for practical purposes, coaxial cable No. 1 is preferred over coaxial cable No. 2 despite coaxial cable No. 2 presents a smaller contraction. In this respect, coaxial cable No. 2, which has the isolating layer and the thermoplastic olefin-base thin layer of Garaflex® O 9777-A2 (synthetic rubber-base thermoplastic polyolefin), has the disadvantage that both materials are very expensive and too soft, causing problems when cutting the cable, also being the cable too flexible and, when being compressed, it might lose transmission capability. On the other hand, coaxial cable No. 1, which combines a thermoplastic olefin- base thin layer of Garaflex O 9777-A2 and an LDPE isolating layer, shows a contraction within acceptable parameters and does not show the foregoing disadvantages in terms of cost and possible transmission loss due to a high flexibility of the material, being this the cable according to the principles of the present invention that has the more suiting characteristics for the industry. Example 2

An assay was carried out to evaluate the contraction suffered by an embodiment of the coaxial cable manufactured in accordance to the principles of the present invention regarding the foaming of the thermoplastic olefin-base thin layer.

As such, the 2 meters length analyzed coaxial cable, was subjected to 100 temperature cycles, wherein each cycle consists of 30 minutes at -40°C and 30 minutes at 85°C and the contraction on both ends of the cable was measured. The analyzed coaxial cable shows a foaming grade on the isolating layer in order to compare it with the coaxial cables from Example 1, which are not foamed. Cable No. 3 was foamed with DFNA-0012® at a concentration of 0.2%. The obtained results are shown in the following table:

TABLE 2

The foaming effect can be seen on the foregoing table. Coaxial cable No. 3 has a foaming with rough texture that is the result of using a relatively small amount of foamer. The advantage of having a rough texture is that the slightly uniform structure provided by such roughness improves the adherence, and in case of a smooth structure, its structure is more uniform due to the bubbles arrangement within the material, which is of no help for the adherence. According to this and to the obtained results, though coaxial cables No. 1 and 2 from Example 1 comply with the requirement of a contraction less than 5 mm for a 2 m coaxial cable without the use of a foamer, when a foamer is used to provide a rough texture to the isolating layer, a contraction even minor to the one of cable No. 1 from example 1 is achieved.

Example 3

An assay was carried out to evaluate the contraction suffered by different embodiments of coaxial cables manufactured according to the principles of the present invention regarding the use of a grooved extrusion die that provides a specific roughness to the thermoplastic olefin-base thin layer to conform the predetermined 3D shape.

As such, the 2 meters length analyzed coaxial cables which have a copper clad steel internal conductor and natural copper mesh external conductor, were subjected to 100 temperature cycles, wherein each cycle consists of 30 minutes at -40°C and 30 minutes at 85°C and the contraction suffered by each of them in both cable ends was measured. Such experiment was performed 4 times for each coaxial cable type.

The following table shows the obtained results:

TABLE 3

Even when a grooved extrusion die was implemented for the isolating layer of cable A, it did not comply with the minimal contraction requirements due to the fact that it does not comprises a thermoplastic olefin-base thin layer according to the principles of the present invention. Thus, even where a coupling layer and the grooved extrusion die were used, if no thermoplastic olefin-base thin layer according to the invention is used, a contraction minor to 5 mm in a 2 m coaxial cable is not achieved.

However, cables B, C and D manufactured according to the present invention, show very low contractions that comply with the minimum contraction requirements. Also, it is proved that the contraction decrease is even greater in relation to cables No. 1 and 2 from examples 1 and 2 due to the use of the grooved extrusion die on the cable thin layer. Example 4

An assay was carried out to evaluate the resistance to contraction by using different embodiments of coaxial cables manufactured in accordance to the principles of the present invention in relation to the use of a grooved extrusion die or foaming to form the thermoplastic olefin-base thin layer.

The 2 meters length coaxial cables which have a copper clad steel internal conductor and natural copper mesh external conductor, were subjected to 100 temperature cycles, wherein each cycle consists of 30 minutes at -40°C and 30 minutes at 85°C and the contraction suffered by each of them in both cable ends was measured. The following table shows the obtained results.

TABLE 4

In the table it can observed that by using a grooved extrusion die (Cable E) or a foamed thin layer (Cable F) according to the principles of the present invention, the contraction is minor to the one of cable No. 1 of Example 1 which does not use any of these two characteristics.

On the other hand, the adhesive percentages used on the thin layer of coaxial cables F, G and FI decrease the contraction suffered by such cables, i.e. the higher adhesive percentage, less is the contraction suffered by the cable. Nevertheless, based on the different illustrated examples, is clear that the adhesive use is unnecessary to achieve the desired effect of reducing the contraction.

It will be obvious for any expert on the art that the embodiments of the coaxial cable resistant to contraction caused by temperature changes as previously described and illustrated on the accompanying drawings, are just illustrative and do not limit the present invention since many possible changes are possible considering the details without departing from the scope of the invention.

Therefore, the present invention should not be considered as restricted except for what the prior art demands and the scope of the appended claims.