TECHNICAL FIELD
[0001] The present invention relates to a coaxial cable in which a cyclic olefin-based resin
is used.
BACKGROUND ART
[0002] In recent years, there are increasingly higher demands for broadband capability of
communications such as mobile phones, Internet, wireless LANs, and the like. Additionally,
for transmitting information of larger amounts at higher speed, shifting of electric
signal to a higher frequency range has been significantly in progress. Under such
circumstances, coaxial cables accompanied by lower attenuation and less signal delay
in a high frequency band have been desired.
[0003] Meanwhile, coaxial cables are constituted mainly with a center conductor, an insulator
layer provided thereon, and an outer conductor provided therearound. Importance has
been placed on lower attenuation in a high frequency band, and lowering of a dielectric
dissipation factor of the insulator layer is most effective for achieving a high-frequency
coaxial cable. Moreover, foaming is effective for further lowering the attenuation.
However, foaming may result in deterioration of lateral pressure resistance of the
insulating layer, and thus a problem of difficulty in maintaining the shape of the
foam may be caused.
[0004] It has been known that a relative permittivity of the insulating layer can be effectively
lowered by increasing an extent of foaming of the insulating layer. In addition, as
an insulating material having a low dielectric dissipation factor, cyclic olefin-based
resins may be exemplified. The cyclic olefin-based resins exhibit satisfactory foam
moldability, and also characteristics superior in the lateral pressure resistance
derived from high rigidity may be thereby expected.
[0005] In this respect, Patent Document 1 discloses a coaxial cable for high frequency transmission
in which a norbornene resin is used. Additionally, Patent Document 2 discloses an
insulating material that is superior in the lateral pressure resistance by blending
a cyclic olefin-ethylene copolymer with polyolefin or the like.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2000-297172
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2000-311519
DISCLOSURE OF THE INVENTION
Problems to be solved by the Invention
[0006] However, a coaxial cable accompanied by still lower attenuation in a high frequency
band has been desired, and improvement that allows a coaxial cable having a performance
higher than those of Patent Document 1 and Patent Document 2 to be provided has been
necessary. As is evident from Examples of Patent Document 1 and Patent Document 2,
the extent of foaming of the insulator layer is about 62% in the former document,
and about 72% in the latter document. Therefore, for achieving a coaxial cable having
a lower attenuation in a high frequency band, a method of improvement by increasing
the extent of foaming of the insulator layer may be conceived. However, in general,
when the extent of foaming increases, the lateral pressure resistance deteriorates
due to expansion of foam cells. Furthermore, breakage of the foam cells upon expansion
leads to failure in obtaining an insulating layer having a low relative permittivity.
Therefore, a coaxial cable having lateral pressure resistance which allows for satisfactory
use as a coaxial cable through keeping independency of the foam cells even though
the extent of foaming is increased has been desired.
[0007] The present invention was made in order to solve the problems as described above,
and an object of the invention is to provide a coaxial cable in which an insulator
layer has sufficient lateral pressure resistance which allows for satisfactory use
as a coaxial cable, and has a higher extent of foaming.
Means for Solving the Problems
[0008] The present inventors have thoroughly researched in order to solve the above-described
problems. As a result, it was found that the aforementioned problems can be solved
by foam molding of a resin composition including a cyclic olefin-based resin, a high
density polyethylene, and a low density polyethylene and/or a linear low-density polyethylene.
Accordingly, the present invention was achieved. More specifically, the present invention
provides the following.
[0009] According to a first aspect of the present invention, a coaxial cable is provided
which includes a layer, as an insulating layer, formed by foam molding of a resin
composition containing a cyclic olefin-based resin, a high density polyethylene, and
at least one of a low density polyethylene and a linear low-density polyethylene,
in which the extent of foaming of the insulating layer is from 80% to 90%.
[0010] According to the first aspect of the invention, the insulating layer in the coaxial
cable of the present invention includes a cyclic olefin-based resin. The cyclic olefin
resin has a low dielectric dissipation factor, a low relative permittivity and satisfactory
foam moldability, and improvement of the lateral pressure resistance of the foam molded
product may be expected by virtue of its features having an even higher modulus of
elasticity. Therefore, the coaxial cable of the present invention can be preferably
used as one for high frequency transmission since the insulating layer in the coaxial
cable of the present invention has these characteristics. On the other hand, the cyclic
olefin-based resins are disadvantageous in that they lack in flexibility. However,
the insulating layer in the coaxial cable of the present invention includes a low
density polyethylene and/or a linear low-density polyethylene being superior in flexibility.
Thus, the disadvantage of the cyclic olefin-based resin can be compensated, thereby
enabling formation of an insulating layer that is superior in flexibility.
[0011] In addition, the low density polyethylene and the like are preferably included also
in view of increase in the extent of foaming of the insulating layer since they also
have satisfactory foam moldability. Meanwhile, the low density polyethylene and/or
the linear low-density polyethylene are disadvantageous in that they have a comparatively
high dielectric dissipation factor. However, the insulating layer in the coaxial cable
of the present invention includes a high density polyethylene. Since the high density
polyethylene has a low dielectric dissipation factor, it can compensate for a disadvantageously
high dielectric dissipation factor that is a drawback of the low density polyethylene,
along with the cyclic olefin-based resin.
[0012] Although the high density polyethylene is also disadvantageous in that it is inferior
in foam moldability, both the cyclic olefin-based resin, and the low density polyethylene
and/or the linear low-density polyethylene can compensate for such disadvantages.
By taking advantage of the aforementioned three or four components, a coaxial cable
that is suitable for high frequency transmission can be obtained.
[0013] The extent of foaming indicates an extent of foam included in the insulating layer.
When the foam is included in a greater amount, the ratio of a gas having a low relative
permittivity occupying the insulating layer increases. Therefore, the higher extent
of foaming results in lowering of the dielectric dissipation factor and the relative
permittivity of the insulating layer in a coaxial cable, whereby a coaxial cable accompanied
by a lower attenuation even in a high frequency range can be obtained. The insulating
layer in the coaxial cable of the present invention can realize a higher extent of
foaming than conventional ones. Favorable lateral pressure resistance of the cyclic
olefin-based resin, and satisfactory foam moldability of the cyclic olefin-based resin
and the low density polyethylene and/or the linear low-density polyethylene account
for such advantages. When the foam moldability is favorable, foam cells can be present
independently even though the extent of foaming is increased. It should be noted that
the extent of foaming as used herein means an extent of foaming determined by the
following formula (1).
[0014] According to a second aspect, the resin composition in the coaxial cable of the first
aspect includes: from 15% by weight to 30% by weight of the cyclic olefin-based resin;
and from 70% by weight to 85% by weight of the high density polyethylene and at least
one of the low density polyethylene and the linear low-density polyethylene in terms
of the total amount.
[0015] According to the second aspect of the invention, by virtue of the content of the
cyclic olefin-based resin included in the insulating layer in a coaxial cable falling
within the above range, effects of the cyclic olefin-based resin such as satisfactory
foam moldability, a low relative permittivity and lateral pressure resistance can
be sufficiently exhibited. In addition, since the content of the cyclic olefin-based
resin falls within the above range, the disadvantage of some lack in flexibility can
be compensated enough with the polyethylene. Thus, by using the insulating layer described
above in a coaxial cable, a coaxial cable further suitable for high frequency transmission
can be obtained.
[0016] According to a third aspect, at least one of the low density polyethylene and the
linear low-density polyethylene is included in a total amount of from 20% by weight
to 40% by weight, in the coaxial cable of the second aspect.
[0017] According to the third aspect of the invention, the low density polyethylene and/or
the linear low-density polyethylene included in the insulating layer in the coaxial
cable in a total amount falling within the above range enables the disadvantage of
a high relative permittivity to be sufficiently compensated with the high density
polyethylene and the cyclic olefin-based resin, while satisfactorily exhibiting the
effect of the polyethylene being excellent in flexibility. Thus, a coaxial cable more
suited for high frequency transmission can be obtained.
[0018] According to a fourth aspect, the insulating layer in the coaxial cable of any one
of the first to third aspects has a compressive strength of no less than 800 N/cm
2, and an attenuation of no greater than 24 dB/100 m.
[0019] According to the forth aspect of the invention, even though the attenuation is kept
at a level of no greater than 24 dB/100 m through lowering the relative permittivity
of the insulating layer by increasing the extent of foaming of the insulating layer
in the coaxial cable to increase pores in the insulating layer, a coaxial cable having
sufficiently favorable mechanical strength can be obtained since the compressive strength
is no less than 800 N/cm
2. In addition, the insulating layer in the coaxial cable of the present invention
includes a low density polyethylene and/or a linear low-density polyethylene. Thus,
since all materials in the coaxial cable are excellent in flexibility, the insulating
layer becomes excellent in flexibility, whereby excellent flexibility is achieved
as a whole of the coaxial cable, which is preferable as a coaxial cable.
[0020] The compressive strength refers to a maximum stress at which a material can endure
against a compression load, and is one indicator that represents the lateral pressure
resistance of an insulating layer in a coaxial cable. Because an insulating layer
having a low compressive strength can be broken by a force imparted in production,
use and the like of the coaxial cable, it is not preferred as an insulating layer
for use in a coaxial cable for high frequency transmission. Since the insulating layer
used in the present invention has a compressive strength of no less than 800 N/cm
2, it can be suitably used as an insulating layer for a coaxial cable.
[0021] The compressive strength of the insulating layer in the coaxial cable of the present
invention is preferably no less than 800 N/cm
2. Since the compressive strength of no less than 800 N/cm
2 serves in attaining sufficient mechanical strength, such an insulating layer can
be suitably used as an insulating layer for a coaxial cable.
[0022] The insulating layer in the coaxial cable of the present invention preferably has
an attenuation of no greater than 24 dB/100 m. An attenuation of greater than 24 dB/100
m leads to large transmission loss, and thus correct operation of electronic instruments
may fail.
[0023] In general, high frequency is attenuated as the dielectric dissipation factor is
higher. In particular, dielectric dissipation factor greatly influences attenuation
in a high frequency band. Therefore, in order to lower the attenuation of a coaxial
cable in a high frequency band, it is necessary that the insulating layer has a low
dielectric dissipation factor.
[0024] According to a fifth aspect, the insulating layer has a moisture permeability of
no greater than 0.55 g/m
2·day·atm in the coaxial cable of any one of the first to fourth aspects.
[0025] According to the fifth aspect of the invention, the insulating layer in the coaxial
cable of the present invention is hardly permeable to water molecules, and has a high
relative permittivity that can be the cause of increase in attenuation; therefore,
it is more suitable for use in high frequency transmission. Moreover, corrosion of
the inner conductor due to permeation of moisture through the insulating layer can
be prevented. Thus, a coaxial cable which can be used for a longer period of time
can be obtained.
[0026] The insulating layer used in the coaxial cable of the present invention has a moisture
permeability of preferably no greater than 0.55 g/m
2·day·atm. When the moisture permeability exceeds 0.55 g/m
2·day·atm, the moisture is more likely to be attached, and conductive failure may be
caused through occurrence of corrosion in significant cases.
[0027] According to a sixth aspect, the insulating layer has a relative permittivity of
no greater than 1.20 in a frequency domain of from 1 GHz to 10 GHz in the coaxial
cable of any one of the first to fifth aspects.
[0028] According to the sixth aspect of the invention, delay time of the signal in transmission
in the coaxial cable is lessened by undergoing lowering of the relative permittivity
of the insulating material used in the coaxial cable of the present invention, and
thus a coaxial cable available in accelerated and increased capacity of communication
can be obtained.
[0029] The insulating layer in the coaxial cable of the present invention has a relative
permittivity in a frequency domain of from 1 GHz to 10 GHz of preferably no greater
than 1.2. The relative permittivity in the aforementioned frequency domain being no
greater than 1.2 is preferred since less delay of the signal is caused.
[0030] According to a seventh aspect, the cyclic olefin-based resin is a copolymer of cyclic
olefin and α-olefin, or a hydrogenated product thereof in the coaxial cable of any
one of the first to sixth aspects.
[0031] According to the seventh aspect of the invention, since the cyclic olefin-based resin
is constituted as described above, an effect having excellent balance of signal transmission
characteristics, flexibility, compressive strength , moisture permeable characteristics
and the like as a coaxial cable is achieved.
[0032] According to an eighth aspect of the present invention, the cyclic olefin-based resin
has a relative permittivity of no greater than 2.3 in a frequency domain of from 1
GHz to 10 GHz, a dielectric dissipation factor of no greater than 4 x 10
-4, and a flexural modulus at room temperature of no less than 2.0 GPa in the coaxial
cable of any one of the first to seventh aspects.
[0033] According to the eighth aspect of the invention, since the cyclic olefin-based resin
included in the insulating layer in the coaxial cable of the present invention has
a low dielectric dissipation factor, formation of an insulating layer having a low
dielectric dissipation factor is enabled when this cyclic olefin-based resin is used.
Low dielectric dissipation factor of the insulating layer results in lowering of the
attenuation of the coaxial cable. Accordingly, the coaxial cable having such an insulating
layer having a low dielectric dissipation factor is suited for high frequency transmission.
[0034] Furthermore, by using a cyclic olefin resin having a low relative permittivity, delay
time of the signal is lessened, and a characteristic excellent as a coaxial cable
for high frequency transmission is achieved.
[0035] The flexural modulus refers to an extent of deformation resistance of a material
against bending stress. Materials having a higher flexural modulus are preferred since
they have greater mechanical strength as they are more superior in resistance to bending
stress. The bending stress referred to herein is a flexural modulus measured according
to ISO 178. The insulating layer in the coaxial cable of the present invention includes
a material having flexibility, such as a low density polyethylene. Thus, it can be
prevented from becoming a fragile material.
[0036] The flexural modulus of the cyclic olefin-based resin is preferably no less than
2 GPa, and more preferably no less than 2 GPa and no greater than 3.5 GPa. When the
flexural modulus is less than 2 GPa, the lateral pressure resistance is deteriorated,
and the modulus of elasticity beyond 3.5 GPa may result in deterioration of flexibility
and may narrow the range of blending.
Effects of the Invention
[0037] According to the present invention, a coaxial cable can be obtained which is provided
with an insulator layer having sufficient lateral pressure resistance which allows
for satisfactory use as a coaxial cable, and having a higher extent of foaming of
the insulator layer. The higher extent of foaming of the insulating layer in the coaxial
cable results in lowering of the dielectric dissipation factor and the relative permittivity
of the insulating layer, whereby preferable use as a coaxial cable for high frequency
transmission is enabled.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038]
Fig. 1 shows a view illustrating an extrusion apparatus;
and
Fig. 2 shows a view illustrating a coaxial cable manufacturing apparatus.
EXPLANATION OF REFERENCE NUMERALS
[0039]
- 1
- First extruder
- 2
- Second extruder
- 3
- Hopper
- 4
- Foaming agent press-in port
- 5
- Conductor delivery unit
- 6
- Conductor heating unit
- 7
- Crosshead die
- 8
- Cooling system
- 9
- Drawing unit
- 10
- Rolling unit
- 11
- Inner conductor
- 12
- Electric wire
DETAILED DESCRIPTION OF THE INVENTION
[0040] Hereinafter, one embodiment of the coaxial cable of the present invention is explained
in detail, but the present invention is not in any way limited to the following embodiment.
The present invention can be realized with appropriate modifications within the scope
of the object of the invention. With respect to points the explanations of which overlap,
the description may be appropriately omitted, but the gist of the invention is not
limited thereto.
Cyclic Olefin-based Resin
[0041] Hereinafter, a cyclic olefin-based resin to be an essential component of the coaxial
cable of the present invention is explained. Since cyclic olefin-based resins have
properties such as a low dielectric dissipation factor, low relative permittivity,
foam moldability, low water absorbing capacity, and lateral pressure resistance, they
are preferred as a material to be included in an insulating layer for use in coaxial
cables. The cyclic olefin-based resin used in the present invention contains a cyclic
olefin component as a copolymer component, and is not particularly limited as long
as it is a polyolefin resin containing a cyclic olefin component in the main chain
thereof. For example,
(a1) an addition polymer of cyclic olefin, or a hydrogenated product thereof,
(a2) an addition copolymer of cyclic olefin and α-olefin, or a hydrogenated product
thereof, and
(a3) a ring-opening (co)polymer of cyclic olefin, or a hydrogenated product thereof
can be exemplified.
Moreover, the cyclic olefin-based resin containing a cyclic olefin component as a
copolymer component used in the present invention includes
(a4) any of the resins of the above (a1) to (a3) being grafted and/or copolymerized
with an unsaturated compound having a polar group.
[0042] The polar group may include, for example, carboxyl groups, acid anhydride groups,
epoxy groups, amide groups, ester groups, hydroxyl groups, or the like. Examples of
the unsaturated compound having a polar group include (meth)acrylic acid, maleic acid,
maleic anhydride, itaconic anhydride, glycidyl (meth)acrylate, (meth)acrylic acid
alkyl (1 to 10 carbon atoms) esters, maleic acid alkyl (1 to 10 carbon atoms) esters,
(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, and the like.
[0043] In the present invention, one kind alone or a mixture of two or more kinds of the
cyclic olefin-based resins containing the cyclic olefin component described above
(a1)-(a4) as a copolymer component may be used. In the present invention, the addition
copolymer of cyclic olefin and α-olefin, or a hydrogenated product thereof (a2) can
be preferably used.
[0044] In addition, a commercially available resin can be used for the cyclic olefin-based
resin containing a cyclic olefin component as a copolymer component which may be used
in the present invention. The commercially available cyclic olefin-based resins may
include, for example, TOPAS (registered trademark, manufactured by TOPAS Advanced
Polymers), Apel (registered trademark, manufactured by Mitsui Chemical Co.), ZEONEX
(registered trademark, manufactured by ZEON Corp.), ZEONOR (registered trademark,
manufactured by ZEON Corp.), ARTON (registered trademark, manufactured by JSR Corp.),
and the like.
[0045] The addition copolymer of cyclic olefin and α-olefin (a2) preferably used in the
composition of the present invention is not particularly limited. Particularly preferable
examples include copolymers containing [1] an α-olefin component having 2 to 20 carbon
atoms and [2] a cyclic olefin component represented by the following general formula
(I):
wherein, R
1 to R
12 may be the same or different from one another, and are each selected from the group
consisting of a hydrogen atom, a halogen atom and a hydrocarbon group; R
9 and R
10, and R
11 and R
12 may be combined to form a bivalent hydrocarbon group; R
9 or R
10 may form a ring with R
11 or R
12; and n represents 0 or a positive integer, and when n is two or more, R
5 to R
8 may each be the same or different, for each repeating unit.
[1] α-Olefin Component having 2 to 20 Carbon Atoms
[0046] The α-olefin having 2 to 20 carbon atoms preferably used in the present invention,
which serves as a copolymer component of the addition polymer that is formed by copolymerization
of the cyclic olefin component and other copolymer component such as ethylene, is
not particularly limited. For example, ethylene, propylene, 1-butene, 1-pentene, 1-hexene,
3-methyl-1-butene, 3-methyl-1-pentene, 3-ethyl-1-pentene, 4-methyl-1-pentene, 4-methyl-1-hexene,
4,4-dimethyl-1-hexene, 4,4-dimethyl-1-pentene, 4-ethyl-1-hexene, 3-ethyl-1-hexene,
1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene
and the like can be included. These α-olefin components may be used alone, or two
or more kinds thereof may be used simultaneously. Among these, use of ethylene alone
is most preferred.
[2] Cyclic Olefin Component Represented by the General Formula (I)
[0047] The cyclic olefin component represented by the general formula (I) preferably used
in the present invention, which serves as a copolymer component in the addition polymer
that is formed by copolymerization of the cyclic olefin component and other copolymer
components such as ethylene, are described.
[0048] R
1 to R
12 in the general formula (I) may be the same or different from one another, and are
each selected from the group consisting of a hydrogen atom, a halogen atom, and a
hydrocarbon group.
[0049] Specific examples of R
1 to R
8 may include, for example, a hydrogen atom; halogen atoms such as fluorine, chlorine
and bromine; lower alkyl groups such as a methyl group, an ethyl group, a propyl group
and a butyl group. These may be different from one another, partially different, or
entirely the same.
[0050] Specific examples of R
9 to R
12 may include, for example, a hydrogen atom; halogen atoms such as fluorine, chlorine
and bromine; alkyl groups such as a methyl group, an ethyl group, a propyl group,
an isopropyl group, a butyl group, an isobutyl group, a hexyl group and a stearyl
group; cycloalkyl groups such as a cyclohexyl group; substituted or unsubstituted
aromatic hydrocarbon groups such as a phenyl group, a tolyl group, an ethylphenyl
group, an isopropylphenyl group, a naphthyl group and an anthryl group; a benzyl group,
a phenethyl group, and aralkyl groups formed by substitution of an alkyl group with
an aryl group, and the like. These may be different from one another, partially different,
or entirely the same.
[0051] Specific examples of the case in which R
9 and R
10, or R
11 and R
12 are combined to form a bivalent hydrocarbon group include, for example, alkylidene
groups such as an ethylidene group, a propylidene group and an isopropylidene group,
and the like.
[0052] When R
9 or R
10 forms a ring with R
11 or R
12, the resultant ring may be either monocyclic or polycyclic, may be polycyclic having
crosslinking, may be a ring having a double bond, or may be a ring constituted with
any combination of these rings. In addition, these rings may include a substituent
group such as a methyl group.
[0053] Specific examples of the cyclic olefin component represented by the general formula
(I) include bicyclic cycloolefins such as bicyclo[2.2.1]hept-2-ene (common name: norbornene),
5-methyl-bicyclo[2.2.1]hept-2-ene, 5,5-dimethyl-bicyclo[2.2.1]hept-2-ene, 5-ethyl-bicyclo[2.2.1]hept-2-ene,
5-butyl-bicyclo[2.2.1]hept-2-ene, 5-ethylidene-bicyclo[2.2.1]hept-2-ene, 5-hexyl-bicyclo[2.2.1]hept-2-ene,
5-octyl-bicyclo[2.2.1]hept-2-ene, 5-octadecyl-bicyclo[2.2.1]hept-2-ene, 5-methylidene-bicyclo[2.2.1]hept-2-ene,
5-vinyl-bicyclo[2.2.1]hept-2-ene and 5-propenyl-bicyclo[2.2.1]hept-2-ene;
tricyclic cycloolefins such as tricyclo[4.3.0.1
2,5]deca-3,7-diene (common name: dicyclopentadiene), tricyclo[4.3.0.1
2,5]dec-3-ene; tricyclo[4.4.0.1
2,5] undeca-3, 7-diene or tricyclo[4.4.0.1
2,5]undeca-3,8-diene, or tricyclo[4.4.0.1
2,5]undec-3-ene that is a partially hydrogenated product (or an adduct of cyclopentadiene
and cyclohexene) thereof; 5-cyclopentyl-bicyclo[2.2.1]hept-2-ene, 5-cyclohexyl-bicyclo[2.2.1]hept-2-ene,
5-cyclohexenyl bicyclo[2.2.1]hept-2-ene, and 5-phenyl-bicyclo[2.2.1]hept-2-ene;
tetracyclic cycloolefins such as tetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene (also simply referred to as tetracyclododecene), 8-methyltetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene, 8-ethyltetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene, 8-methylidenetetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene, 8-ethylidenetetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene, 8-vinyltetracyclo[4,4.0.1
2,5.1
7,10]dodec-3-ene and 8-propenyl-tetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene;
polycyclic cycloolefins such as 8-cyclopentyl-tetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene, 8-cyclohexyl-tetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene, 8-cyclohexenyl-tetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene, 8-phenyl-cyclopentyl-tetracyclo[4.4.0.1
2,5.1
7,10]dodec-3-ene; tetracyclo[7.4.1
3,6.0
1,9.0
2,7]tetradeca-4,9,11,13-tetraene (may be also referred to as 1,4-methano-1,4,4a,9a-tetrahydrofluorene),
tetracyclo[8.4.1
4,70
1,10.0
3,8]pentadeca-5,10,12,14-tetraene (may be also referred to as 1,4-methano-1,4,4a,5,10,10a-hexahydroanthracene);
pentacyclo[6.6.1.1
3,6.0
2,7.0
9,14]-4-hexadecene, pentacyclo[6.5.1.1
3,6.0
2,7.0
9,13]-4-pentadecene, pentacyclo[7.4.0.0
2,7.1
3,6.1
10,13]-4-pentadecene; heptacyclo[8.7.0.1
2,9.1
4,7.1
11,17.0
3,8.0
12,16]-5-eicosene, heptacyclo[8.7.0.1
2,9.0
3,8.1
4,7.0
12,17.1
13,16]-14-eicosene; and tetramers of cyclopentadiene, and the like.
[0054] These cyclic olefin components may be used alone or in combinations of two or more
kinds thereof. Among them, use of bicyclo[2.2.1]hept-2-ene (common name: norbornene),
or tetracyclododecene is preferable.
[0055] The method for polymerizing [1] an α-olefin component having 2 to 20 carbon atoms
and [2] a cyclic olefin component represented by the general formula (I), and the
method for hydrogenating the resultant polymer are not especially limited, and can
be carried out according to publicly known methods. Although it may be carried out
by either random copolymerization or block copolymerization, random copolymerization
is preferable.
[0056] In addition, the polymerization catalyst that may be used is not particularly limited,
and the polymer can be obtained by using a conventionally well-known catalyst such
as a Ziegler-Natta series, metathesis series, or metallocene series catalyst according
to a well known process. The addition copolymer of cyclic olefin and α-olefin or the
hydrogenated product thereof that is favorably used in the present invention is preferably
manufactured by use of a metallocene series catalyst or a Ziegler-Natta series catalyst.
[0057] An exemplary metathesis catalyst may be a molybdenum or tungsten series metathesis
catalyst that is well-known as a catalyst for ring-opening polymerization of cycloolefin
(for example, as described in Japanese Unexamined Patent Applications, First Publication
Nos.
S58-127728,
S58-129013, etc.). In addition, the polymer obtained with the metathesis catalyst is preferably
hydrogenated using a transition metal catalyst supported on an inorganic support,
at a rate of no less than 90% of the double bond in the main chain, and at a rate
of no less than 98% of the carbon-carbon double bond in the aromatic ring of the side
chain.
Other Copolymer Component
[0058] The addition copolymer of cyclic olefin and α-olefin (a2), particularly preferably
used in the composition of the present invention, may contain, in addition to [1]
the α -olefin component having 2 to 20 carbon atoms and [2] the cyclic olefin component
represented by the general formula (I), other copolymerizable unsaturated monomer
component as needed within a range not to impair the object of the present invention.
[0059] The unsaturated monomer, which may be optionally copolymerized, is not particularly
limited, and for example, hydrocarbon based monomers including two or more carbon-carbon
double bonds in one molecule and the like may be exemplified. Specific examples of
the hydrocarbon based monomer including two or more carbon-carbon double bonds in
one molecule include: linear unconjugated diene such as 1,4-hexadiene, 1,6-octadiene,
2-methyl-1,5-hexadiene, 4-methyl-1,5-hexadiene, 5-methyl-1,5-hexadiene, 6-methyl-1,5-heptadiene
and 7-methyl-1,6-octadiene; cyclic unconjugated diene such as cyclohexadiene, dicyclopentadiene,
methyltetrahydroindene, 5'-vinyl-2-norbornene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene,
5-isopropylidene-2-norbornene, 6-chloromethyl-5-isopropenyl-2-norbornene and 4,9,5,8-dimethano-3a,4,4a,5,8,8a,9,9a-octahydro-1H-benzoindene;
2,3-diisopropylidene-5-norbornene; 2-ethylidene-3-isopropylidene-5-norbornene; 2-propenyl-2,2-norbornadiene;
and the like. Among them, 1,4-hexadiene, 1,6-octadiene, and cyclic unconjugated diene,
in particular, dicyclopentadiene, 5-ethylidene-2-norbornene, 5-vinyl-2-norbornene,
5-methylene-2-norbornene, 1,4-hexadiene, and 1,6-octadiene are preferable.
[0060] The content of the cyclic polyolefin based resin in the insulating layer is preferably
from 15% by weight to 30% by weight. When the content is less than 15% by weight,
signal transmission characteristics, compressive strength , moisture permeability
and the like for use in a coaxial cable may be inferior, and when the content is beyond
30% by weight, sufficient flexibility may not be attained.
Polyethylene
[0061] The high density polyethylene, the low density polyethylene, and the linear low-density
polyethylene to be essential components of the coaxial cable of the present invention
are explained below. Since the high density polyethylene has a low dielectric dissipation
factor, it is preferable as a material for use in the insulating layer in a coaxial
cable. The low density polyethylene and the linear low-density polyethylene are preferable
materials for use in the insulating layer in the coaxial cable since they have flexibility.
[0062] The high density polyethylene, the low density polyethylene, and the linear low-density
polyethylene have MFR defined in JIS K6922-1 of preferably from 1.0 g/10 min to 10.0
g/10 min, and more preferably from 2.0 g/10 min to 8.0 g/10 min. Although flexibility
can be attained when the MFR is lower than 1.0 g/10 min, the flowability can be deteriorated,
whereby favorable processability may not be provided. In contrast, although favorable
processability can be provided when the MFR exceeds 10.0 g/10 min, the flexibility
may not be attained.
[0063] Total content of the high density polyethylene, the low density polyethylene, and
the linear low-density polyethylene in the insulating layer of a coaxial cable is
preferably 70% by weight to 85% by weight.
[0064] In particular, total content of the low density polyethylene and the linear low-density
polyethylene is preferably 20% by weight to 40% by weight. When this content is less
than 20% by weight, sufficient flexibility may not be achieved, and when this content
is beyond 40% by weight, relative permittivity of the insulating layer may not be
kept at a low level.
Other Components
[0065] To the insulating layer in the coaxial cable of the present invention can be added
other thermoplastic resins, various compounding agents, and the like as needed, within
a range not impairing its characteristics. Illustrative examples of the other resin
include, for example, other polyolefin resins, polystyrene resins, fluorine resins,
and the like. These other resins may be used alone or in a combination of two or more.
Furthermore, when flexibility of the coaxial cable is required, an elastomer is preferably
added to the cyclic polyolefin based resin. The elastomer which may be added is not
particularly limited as long as it does not impair characteristics such as attenuation
of the coaxial cable, and for example, a polyolefin based elastomer and a styrene
based elastomer are preferred. Particularly, a polyolefin based elastomer is preferred
for balancing the attenuation and the flexibility. In addition, illustrative examples
of the compounding agent include stabilizers (antioxidant or anti-oxidizing agent,
heavy metal resistant stabilizer, ultraviolet ray absorbing agent, heat stabilizer
and the like), antistatic agents, fire retardants, retardant aids, colorants (dye,
pigment and the like), wetting agents, plasticizers, lubricants, mold lubricants,
crystal nucleating agents, dripping inhibitors, crosslinking agents, and the like.
Since the insulating layer of a coaxial cable will be in contact with a metal such
as copper that is a conductor, addition of a heavy metal resistant stabilizer is preferred.
Illustrative examples of the heavy metal resistant stabilizer include salicylic acid
derivatives (for example, trade name ADKSTAB CDA6), hydrazide derivatives (for example,
trade name Irganox MD1024), oxalic amide derivatives (for example, trade name Naugard
XL-1), sulfur-containing phosphite compounds (for example, trade name Hostanox OSP-1)
and the like, and the type of the heavy metal resistant stabilizer is not particularly
limited as long as characteristics of the coaxial cable are not impaired. Also, the
amount of the heavy metal resistant stabilizer added is not particularly limited,
and in general, the amount of addition of no greater than 0.3% by weight based on
the resin component is preferably employed. Although the addition method is not particularly
limited, it is more preferred to add beforehand to the cyclic polyolefin based resin,
the polyethylene resin, other added resin, or the like.
Coaxial Cable
[0066] Although the constitution of the coaxial cable is not particularly limited, the most
general examples include coaxial cables having an inner conductor, an insulating layer,
an outer conductor and a sheath. The phrase "having a layer obtained by foam molding
of a resin composition as an insulating layer" herein means to have an insulating
layer formed to cover an inner conductor. In typical coaxial cables, an outer conductor
is formed to cover the insulating layer for the purpose of electromagnetic shielding
and the like, and further a sheath is formed to cover thereon.
[0067] The inner conductor is not particularly limited as long as it has electric conductivity,
and for example, an electrically conductive metal such as copper or a copper alloy
may be exemplified. It should be noted that a stranded wire produced by twisting multiple
electrically conductive metal element wires may be used as the inner conductor.
[0068] The outer conductor is constituted as, for example, a conductor yarn braid produced
by knitting multiple conductor element wires to form a mesh structure. As the conductor
element wire for use in the outer conductor, for example, a copper wire or a copper
alloy may be used. Examples of the procedure other than constituting in the form of
a yarn braid include spiral winding, duplex winding and the like of a tape shaped
conductor.
[0069] The method of foam molding of the resin composition in the present invention is not
particularly limited as long as a desired extent of foaming can be achieved. A preferable
method of foam molding may be exemplified by gas foaming.
[0070] The gas foaming refers to a method which includes pressing-in of a foaming agent
into a melt extruder, covering a conductor with an insulating material, and allowing
for foaming concomitantly with extrusion. Examples of the foaming agent include inert
gases such as nitrogen, argon and carbon dioxide; and gases such as methane, propane,
butane, pentane, hexane and fluorocarbon. In addition, a foaming auxiliary agent may
be used in combination. Examples of the foaming auxiliary agent include urea, urea
based compounds, zinc white, zinc stearate, and the like. The foaming agent and the
foaming auxiliary agent are not limited to these exemplified compounds. Also, the
foaming agent and the like may be used either alone, or in combination of two or more
kinds thereof.
[0071] The foaming agent may be mixed with an organic polymer to be foamed beforehand, or
may be supplied into an extruder from a foaming agent supply port provided on a barrel
of the extruder.
[0072] The extent of foaming is preferably 80% to 90%. When the extent of foaming is less
than 80%, the relative permittivity and the dielectric dissipation factor of the insulating
layer may become so high that a characteristic as a high-frequency coaxial cable can
be insufficient. When the extent of foaming exceeds 90%, it is probable that sufficient
mechanical strength of the insulating layer may not be maintained.
Method for Manufacturing Coaxial Cable
[0073] A method for manufacturing the coaxial cable of the present invention is not particularly
limited, and general method can be employed. For example, manufacture of a coaxial
cable with an extruder may be included. With respect to the type of the extruder,
for example, a twin screw extruder or a single screw extruder may be used, or these
may be connected to impart functions of gas injection and covering.
[0074] In the manufacture of a coaxial cable, for example, extrusion foam molding on an
inner conductor is carried out using a foaming agent in an extruder, and a foam insulating
layer is formed to cover the outer periphery of the inner conductor. In covering the
inner conductor with a foam insulating layer, a covering device such as a crosshead
die is generally used. The coaxial cable can be manufactured without impairing the
characteristics even though introduction of the inner conductor into the covering
device is carried out in the air. In the case of manufacturing a coaxial cable having
a very low attenuation, improvement of the covering device, for example, by filling
a port for introducing the inner conductor with an inert gas such as nitrogen may
be preferred in attempts to stabilize the characteristics since oxidation of the resin
component which may result from the air can be inhibited. An outer conductor is further
formed by covering the foam insulating layer with a common method, and finally a sheath
is formed by covering the outer conductor by a common method.
EXAMPLES
[0075] The present invention is described in detail below with reference to Examples and
Comparative Examples, but the present invention is not to be limited thereto.
Various Materials
Cyclic Olefin Resin
Manufactured by TOPAS ADVANCED POLYMERS, trade name: TOPAS8007F-04; TOPAS6013S-04
and TOPAS6015S-04
Manufactured by ZEON Co., Ltd., trade name: ZEONOR 1060R
[0076] The modulus of elasticity, the specific gravity, the relative permittivity and the
dielectric dissipation factor of the cyclic olefin resins described above were measured.
The modulus of elasticity was measured in accordance with ISO178. Dielectric characteristics
(relative permittivity and dielectric dissipation factor) were determined using a
network analyzer 8757D manufactured by Agilent Technologies, Inc., and a cavity resonator
complex relative permittivity measurement apparatus manufactured by Kanto Denshi Co.,
Ltd., and the measurement of the relative permittivity was carried out at 1 GHz, 3
GHz and 10 GHz by a cavity resonator perturbation method at 23°C. Upon measurement,
the insulating layer was formed to have a predetermined shape (ϕ: 2.5 mm, and length:
80 mm), and inserted into the cavity resonator. Each of the measurement results is
shown in Table 1.
[Table 1]
|
1 GHz |
3 GHz |
10 GHz |
|
Modulus of elasticity |
Specific gravity |
Relative permittivity |
Dielectric dissipation |
Relative permittivity |
Dielectric dissipation |
Relative permittivity |
Dielectric dissipation factor |
TOPAS8007F-04 |
2.4 |
1.02 |
2.26 |
0.0004 |
2.26 |
0.0002 |
2.22 |
0.0002 |
TOPAS6013S-04 |
2.8 |
1.02 |
2.24 |
0.0003 |
2.23 |
0.0002 |
2.20 |
0.0002 |
TOPAS6015S-04 |
2.9 |
1.02 |
2.23 |
0.0002 |
2.22 |
0.0002 |
2.20 |
0.0002 |
ZEONOR 1060R |
2.1 |
1.01 |
2.30 |
0.0004 |
2.30 |
0.0003 |
2.30 |
0.0003 |
[0077] From Table 1, it was confirmed that the aforementioned cyclic olefins have sufficient
lateral pressure resistance and appropriate flexibility since each modulus of elasticity
was in the range of no less than 2.0 GPa and no greater than 3.5 GPa. In addition,
since the relative permittivity was no greater than 2.3, and the dielectric dissipation
factor was no greater than 4 x 10
-4, the relative permittivity of the insulating layer after foam molding can be prevented
from elevation due to the relative permittivity of the material portion.
Polyethylene
[0078] High density polyethylene; manufactured by Tosoh Corporation, trade name: Nipolon®
Hard 4010, MFR: 5.5 g/10 min
(JIS K6922-1)
[0079] Low density polyethylene; manufactured by Sumitomo Chemical Co., Ltd., trade name:
Sumikasen G401, MFR: 4.0 g/10 min (JIS K6922-1)
[0080] Linear low-density polyethylene; manufactured by Sumitomo Chemical Co., Ltd., trade
name: Sumikasen L GA401, MFR: 3.0 g/10 min (JIS K6922-1)
Measurement and Evaluation Method
[0081] The extent of foaming, the compressive strength , the moisture permeability, the
relative permittivity and the attenuation were measured with the resin compositions
shown in Table 2 below.
Measurement of Extent of Foaming
[0082] The extent of foaming was measured with a specific gravity method in a manufacturing
step of the coaxial cable explained below. The resin density before foaming, and the
density of the foam were measured, and the extent of foaming was determined using
the above formula (1).
Compressive strength
[0083] The resin composition shown in Table 2 was blended, and extrusion molding of a sheet
foamed with a nitrogen gas was conducted using an extrusion equipment in which the
temperature of a cylinder C was set to be 200°C, and the temperature of a die D was
set to be 195°C (Fig. 1). The blended composition was charged via a hopper A, and
a nitrogen gas was injected from a mixing unit B in a middle region of the cylinder.
The sheet was molded to have a thickness of 5 mm.
[0084] Thus obtained sheet having a thickness of 5 mm was cut into a piece of 30 mm x 100
mm, and a load was applied in the thickness direction to measure a compressive strength.
The measurement was conducted using TENSILON UTA-50KN manufactured by Orientec Co.,
Ltd., at a test speed of 1 mm/min.
Measurement of Moisture Permeability
[0085] The moisture permeability was measured in accordance with ISO 10156-1 (differential
pressure method) using a VAC-V2 Gas Permeability Tester for differential pressure
method manufactured by Labthink as a measurement device.
Method for Manufacturing Coaxial Cable
[0086] Using a coaxial cable manufacturing apparatus shown in Fig. 2, a foam insulating
layer was formed on an inner conductor (copper wire). First, an inner conductor insertion
port of a crosshead die 7 on the side of a conductor heating unit 6 was closed, and
each resin of a resin composition shown in Table 2 was charged into a hopper 3 of
a first extruder. Then, a nitrogen gas was pressed-in from a foaming agent press-in
port 4 while melt kneading of the resin is carried out, and the mixture was injected
into a second extruder 2.
[0087] The mixture further melt kneaded in the second extruder 2 was injected into the crosshead
die 7, and a foam insulator not including an inner conductor was obtained after passing
through a cooling system 8 and a drawing unit 9. The density of the foam insulator
was measured, and the pressure of the nitrogen gas was regulated such that a predetermined
extent of foaming was attained. Accordingly, foaming conditions of the foam insulator
were determined. With respect to present temperatures of the first extruder 1 and
the second extruder 2, the settings were: 200°C in the case of the resin blends containing
TOPAS8007F-04 and ZEONOR 1060R, and in the case of Comparative Examples; 215°C in
the case of the resin blend containing TOPAS6013S-04; and 230°C in the case of the
resin blend containing TOPAS6015S-04, respectively. With respect to adjustment of
the extent of foaming, setting of the extent of foaming of 80% to 90% was enabled
except for Comparative Example 1 in which the high density polyethylene alone was
used. In the case of Comparative Example 1, the extent of foaming could not be increased,
and thus the extent of foaming of 40% was employed. The foam insulator without including
an inner conductor obtained in this step was used for measurement of the relative
permittivity.
[0088] Next, the inner conductor insertion port of the crosshead die 7 on the side of a
conductor heating unit 6 was opened, and an inner conductor 11 (copper wire) having
a diameter of 1.4 mm was lead out from a conductor delivery unit 5, and placed sequentially
into the conductor heating unit 6, the crosshead die 7, the cooling system 8, and
the drawing unit 9. The inner conductor 11 was covered with a mixture extruded under
the same conditions as the extrusion conditions of the foam insulator determined in
the above procedure by way of the crosshead die 7, and then transferred sequentially
to the cooling system 8 and the drawing unit 9. The drawing speed was regulated such
that an electric wire 12 including the inner conductor 11 covered by the insulating
layer had an external diameter of 4.8 mm. After regulation, the electric wire 12 was
wound up to the rolling unit. Thereafter, the electric wire 11 was covered with corrugated
copper as an outer conductor, and further covered with a polyethylene sheath to obtain
a coaxial cable. The attenuation of the resulting coaxial cable was measured. When
the density of the foam insulating layer was measured after eliminating the inner
conductor 11 of the electric wire 12, it was confirmed to be the same as the density
of the foam insulator produced under the same extrusion conditions except that covering
with the inner conductor was omitted.
[0089] From Table 2, the extent of foaming falling within the range of 80% to 90%, the compressive
strength being no less than 800 (kPa), the moisture permeability being no greater
than 0.55 (g/m
2·day·atm), the relative permittivity being no greater than 1.20, and the attenuation
being no greater than 24 (dB/100 m) were ascertained in Examples 1 to 14. Therefore,
the coaxial cables of Examples 1 to 14 are coaxial cables suited for high frequency
transmission. To the contrary, Comparative Example 1 without containing a cyclic olefin
had a high relative permittivity due to a low extent of foaming, and as a result,
also the attenuation was higher. In addition, it was confirmed to be unsuited for
high frequency transmission since the moisture permeability was also elevated. Since
Comparative Examples 2 and 3 without containing a cyclic olefin similarly to Comparative
Example 1 had an extent of foaming of 80%, a low relative permittivity was attained.
However, they cannot be suitably used as coaxial cables for high frequency transmission
owing to the lowered compressive strength.