Note: Descriptions are shown in the official language in which they were submitted.
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1 BACKGROUND OF THE INVENTION
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1. Field of the Invention
This invention relates to a method of producing insulated
wires and cables, in particular, coaxial cables having excellent
high frequency characteristics, comprising the steps of extruding
a mixture of a polyolefin, a nucleating agent and a volatile
liquid using an extruder and tightly coating a conductor therewith.
2. Description of the Prior Art
Polyolefin foams, in particular, polyethylene foams have
been used by preference as the insulation material for use with
communication cables.
However, the foaming ratio in these prior art polyolefin
foams }s at most about two because of the difficulty encountered
in e production techniques for highly expanded foams. There-
fore, for applications such as coaxial cables which require a
higher ratio (e.g., 3 or more), polyolefin is partially replaced
with highly expanded polystyrene which is capable of higher expan-
sion although it is less preferred from the standpoint of elec-
trical properties.
However, since highly expanded polystyrene is by natureinferior in flexibility and has no elasticity, it has the fatal
defect that it tends to break readily when repeatedly fle~ed during
production or coating as in the case of wires and cables.
Special insulation composites, such as balloon type,
~' helical type, disk type, nob-supported type, etc., are also used
but these composites are not highly expanded foams in a strict
sense but have an effect equivalent thereto. None of these types
can be, however, produced efficiently as is apparent from the
structure thereof. Present methods also-result in the signifi-
cant defect of poor water-proofing property.
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1 Although production methods per se of various hiyhly
expanded polyolefin foams using various volatile li~uids are
already known, various skills are required for polyolefin foams
which are obtained ~rom conventional processes in order to apply
them to insulated wires and cables, because they are not at all
suitable for practical use if used as they are. The properties
of foams required for wires and cables are different from those
of foams for cushion materials, and special characteristics such
as impedance, attenuation, SRL (Structural Return Loss) and the
like are required. Therefore, a production technique is required
which is quite different from the conventional production process
for extruding highly expanded foams. Since this technique is
extremely difficùlt, highly expanded polyolefin insulated wires
and cables have not yet been put to practical use.
After much research and development on highly expanded
polyolefin insulated wires and cables, previously considered
` difficult to obtain, to overcome the foregoing defects, we have
succeeded in providing highly expanded polyolefin insulated wires
and cables which have sufficient properties for practical uses.
SUMMARY OF THE INVENTION
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An object of this invention is to provide a process for
producing highly expanded polyolefin insulated cables and wires.
Another object of this invention is to provide a process
for producing highly expanded polyethylene from a mixture of low
density polyethylene and high density polyethylene using a volatile
liquid.
Another object of this invention is to provide a process
for providing foamed products having a uniform cell size by com-
pounding a solid blowing agent as a nucleating agent and a decom-
position accelerator each separately into a polyethylene matrix,
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1 and then blending thes~ separate polyethylene compounds in a proper
ratio on extruding.
Another object of this invention is to provide a process
for molding while expanding of various crystalline plastics in
which a volatile liquid has hitherto been considered unsuitable.
nother object of this invention is to provide a process
~or feeding a foamable compound to an extruder while preventing
the volatilization of t~e volatile liquid contained therewith.
Another object of this invention is to provide a process
for molding by extrusion of expanded foams in a tubular form
(hereinafter tubular foam~ and simultaneously insulating con-
ductors, for example, the inner conductor or inner conductors of
a cable therewith,while controlling the size of the outer diameter
and the configuration of the tubular foams by controlling the
foaming conditions in the tubular foams at the time of molding the
foams by extrusion.
Another object of this invention is to provide a process
for producing highly expanded foam insulated wires and cables
having excellent properties in which t~e shape of the cells in
the highly expanded insulation layer is controlled during foaming
so as to prevent longitudinal shrinking of the insulation layer.
A further object of this invention is to provide a
process for producing highly expanded foam insulated coaxial cables
in which the insulation layers of highly expanded foam are tightly
adhered to the inner conductor and/or the outer conductors.
The invention provides a process for producing a highly
expanded polyolefin insulated cable comprising the steps of feed-
ing a compound of polyolefin, nucleating agent and volatile liquid
to an extruder, heat extruding the compound through the extruder
onto an inner conductor passing through the extruder to cover the
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1 outer periphery of the inner conductor thereby forming an insula-
tion layer o~ highly expanded polyolefin foam having a foaming
ratio in excess o~ 2.5 times (60~ expansion degree) tightly adhered
to said inner conductor. As embodiments the invention also includes
additionally the applying of a pressure reduction around the inner
conductor up to an area near the contacting point of the highly
expanded polyolefin foam insulation layer and the inner conductor,
the applying of an adhesive layer onto said inner conductor prior
to forming the highly expanded polyolefin insulated layer onto the
1~ adhesive layer on the inner conductor, the feeding of the compound
to the extruder from a hopper with the temperature of the
com~ound at the exit of the hopper being higher than the temper-
ature of the compou`n~ in the zone of the extruder near the inlet
to the extruder, the temperature of the compound linearly and
zonally decreasing as the distance from the exit of the hopper
increases, the extruding and forming initially of a highly ex-
panded polyolefin foam insulation layer on the inner conductor
and then sizing the outer surface of the insulation layer by pass-
ing the inner conductor having thereon the foam insulation layer
through a sizing die to provide a highly expànded uniform poly-
olefin insulation layer of a predetermined size, and the forming
of an outer conductor around the highly expanded polyolefin foam
insulation layer coated on the inner conductor by welding or sink-
ing the outer conductori wherein "sinking" as used throughout this
application shall mean drawing down the diameter of a pipe or the
like using a die. -
BRIEF DESCRIPTION OF THE ACCOMPANYING DR~WINGS
In the drawings:
Figure 1 is a graph representing the relation between
the degree of swelling for actually measured values for pellets
of varying composition with respect to two different types of
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1 polyolefins in accordance with this invention and the arithmetic
average values calculated ~rom each polyolefin density;
Figure 2 is a graph representing a relation between the
foaming ratio and the extrusion tempera-ture with respect to the
pellets used in accordance with this invention as actually measured
for each of high density polyethylene and low density polyethylene
as well as that actually obtained for the blend of these poly-
ethylenes and the expected arithme-tic value of the blend of these
polyethylenes;
. 10 Figure 3 is a schematic view showing the manner in which
the compound used in foaming is supplied to the extruder hopper
according to one embodiment of this invention;
Figure 4 is a schematic view showing a method of coating
an inner conductor with tubular expanded foams using a conventional
extruder;
Figure 5 and Figure 6 are views illustrating one embodi-
ment of the method of coating an inner conductor with tubular
expanded foams according to this invention showing a heating and
cooling adaptor, a vacuum means, and as means for heating the con-
ductor upon which a foamed polyolefin is coated;
Figures 7(a) - (e) are cross section views of various
insulated wires showing the effects of the cooling and the heating
of the adaptor and the heating of a conductor on foaming and
adhering conditions in accordance with the embodiments of Figures
5 and 6;
Figure 8 is a schematic view of one embodiment of this
invention of molding expanded foams using a vacuum method in
accordance with this invention;
- Figure 9 is a schematic view of another embodiment of
this invention for molding expanded foams using a sizing die;
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1Figure 10 is a cross section view of the wire produced
by an embodiment of the process according to this invention of
Figure 9;
Figure 11 is a graph showing the relation between the
ratio of extrusion rate and the length of shrinking of the foamed
insulation layer of coaxial cables wherein the ordinate represents
the length of shrinking and the a~scissa represents the ratio of
the extrusion rate (the calculated value of the extrusion rate of
the highly expanded foam insulation layer per the extrusion rate
of the conductor);
Figure 12 is a schematic view of one embodiment of this
invention for applying an adhesive layer coating on a conductor;
Figure 13 is a perspective view of a coaxial cable prior
:to employment of an embodiment of this invention in which a weld-
ing and sin~ing operation is applied to an outer conductor surround-
ing an inner conductor having a highly expanded foam insulation
layer thereon;
Figure 14 is a schematic cross section view illustrating
the operations of welding and sinking for the coaxial cable shown
in Figure 13; and
:Figure 15 is a schematic view illustrating the test for
the strength of adhesion between the highly expanded foam insulation
:layer and the outer conductor of a coaxial cable using a tension
tester.
DETAILED DESCRIPTION OF THE INVENTION
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This invention provides foaming materials, methods of
compounding the same, a method of extrusion and a method of improv-
ing the properties of wires and cables in the production of highly
expanded polyolefin insulated wires and cables. For brevity the
term conductor or inner conductor will be used throughout this
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1 sp2cification to designate a conductor(s) which is(are) coated in
accordance with this invention and the embodiments thereof since
a major application of this invention is use of such in cables.
Suitable polyolefins which can be employed herein include
l;nown high density polyethylene (e.g., polyethylene having a density
p of 0.940 < p < 0.960 g/cm3 and a melt index (MI) of about 0.1 to
2.0, preferably 0.1 to 1.0), medium density polyethylene (e.g.,
polyethylene having a density p of from 0.925 to 0.940 g/cm3 and
a melt index (MI) of from about 0.1 to 2.0, preferably 0.1 to 1.0),
low density polyethylene (e.g., polyethylene having a density P
of 0.915 < p < 0.925 g/cm3 and a melt index (MI) less than about
10 to 0.05, preferably 0.1 to 2), polypropylene and mixtures
thereof, with high density polyethylene or mixtures of high density
and low density polyethylenes being preferred, for example at
about 80 to 20~, preferably 50 to ~0%, by weight of high density
polyethylene with the low density polyethylene being the balance
of the composition
Suitable blowing agents which can be employed include
those having a decomposition point above 110C and generating
heat upon decomposition, for example, azodicarboamide and p,p'-oxy
: bis-benzenesulfonyl hydrazide, as disclosed, for example, in US
Patent 3,251,911. The preferred nucleating agent used herein is
a solid blowing agent. When a solid blo~ing agent as a nucleating
agent and a decomposition accelerator such as t~e metal salts of
fatty acids, for example, zinc stearate, calcium stearate and barium
stearate, as disclosed in US Patent 3,017,371, are usea together,
it is particularly desirable to compound these agents into separate
portions of the polymer respectively and blend them in a predeter-
mined ratio on using. ~enerally the nucleating agent and the
decomposition accelerator are em~loyed in an amount of about 0.1
` to 5 phr, preferably 0.3 to 1.0 phr.
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1 Known volatile liquids can be used herein but various
liqueried gases, e.g., gases liquefied at temperatures less than
20C and 1 atmosphere, such as the fluorinated hydrocarbons, e.g.,
Freon*12, 13 and 14 or liquids having a boiling point at about
ambient temperature, e.g~, about 20C, such as fluorinated hydro-
carbons, e.g., Freon 11, 112 and 113, are preferred.
The volatile liquid can be compounded into polymers in
known manners, e.g., the swelling process or liquid injection pro-
cess as disclosed in US Patent 3,253,065. In the swelling process,
it is advantageous to subject the polymer to a pre-expansion before
swelling and then swell the polymer using a volatile liquid to
assist the swelling. It is also effective for the convenience of
operation to swell the polymer in a volatile liquid vapor, as
disclosed for example in US Patents 2,885,738 and 3,020,248.
In supplying the swelled compound to a hopper of an
extruder, it is desirable to use a sealed multiple hopper, and a
front end drive extruder, as disclosed in the Twenty-first Inter-
national ~ire and Cable Symposium pp~ 173-181, is preferred as
the extruder in order to retàin the sealing.
The temperature of the extruder is preferably
highèr nearer the hopper than near the die of the
extruder.
For tightly insulating a conductor, on which the foaming
polyolefin is coated, a pressure reduction is applied around the
conductor just prior to its contacting the foaming polyolefin.
The design for the nipple of the extruder die is of
importance in this invention, and it is particularly desirable
that the rate at which the diameter of the die is reduced be as
low as possible in order to reduce the heat shrinkage in the
foamed body.
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The extruded material does not uniformly foam when
blown, resulting in an extre~ely uneven product. Accordingly,
it must be subjected to a sizing using various types of sizing
, dies.
,~ The insulation la~er thus coated onto an inner conductor
is fragile and easily compressed as it is highly expanded. It is
necessary to prevent the deformation of the expanded product as
much as possible by reducing the resistance of a pay-out capstan
for the inner conductor and by lining the inner side of a take-up
10 capstan with sponge.
While a conventional water cooling trough can be used in
the prior process for low density polyethylene, water should be
~' strictly avoided in the process of this invention as a highly
expanded product is used. The cooling should be effected in
an atmosphere of gases such as air, carbon dioxide gas and the
' like.
HLghly expanded foam insulated wires and cables of
excellent quality can thus be obtained with this invention, but
' when the insulated wires and conductors are coated with an outer
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conductor to form co-axial cables, it is observed that
the foamed product shrinks to a great extent. In order to avoid
' this, it is necessary to tightly adhere the foamèd product further ,
to the outer conductor or inner conductor using an adhesive.
This invention will now be described in greater detail
as to the components used therein and preferred embodiments,thereo~.
x Foaming Compound
One object of this invention is to provide a process for
producing highly expanded polyethylene from a mixture of low
30 density polyethylene (polyethylene of a density of less than
,~ 0.925 g/cm3) and high density polyethylene (polyethylene of a
` density of more than 0.94 g/cm3) using a volatile liquid.
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1 Japanese Patent Publication No. 390~/1972 discloses a
process for producing a ~oamed body having highly uniform cells
incorporated therein by addin~ a solid blowing a~ent and a
decomposition accelerator to a c:ompound o~ low density poly-
ethylene and high density polyethylene. ~Iowever, this process
has the disadvantages that the operation can not be effected at h~h
temperatures since the solid blowing agent sometimes decomposes
on blending the mixture of polyethylene and the blowing agent,
that uniform blending is difficult to attain, and that a con-
siderably high temperature, that is a temperature somewhat
higher than the melting point of polyethylene, is required to
completely decompose the solid blowing agent when extruding the
blend. This causes a reduction in the viscosity of the molten
polyethylene thereby preventing the formation of highly expanded
polyethylene.
Processes for extruding expanded polyethylene using low
density polyethylene employing a volatile liquid are widely
practiced but a process using high density polyethylene has not
yet been practiced industrially at all.
The inventors have succeeded in developing a process for
producing highly expanded polyethylene from a mixture of low
density polyethylene and high density polyethylene, as raw materials,
using a volatile liquid.
One embodiment of this invention provides a process for
producing expanded polyethylene which comprises melting poly-
ethylene having a density less than 0.925 g/cm3 and polyethylene
having a density more than 0.94 g/cm3,compounding and then pellet-
izing the same, immersing the compounded pellets into a volatile
liquid so as to swell the polymers, and foaming the swelled product
at a relatively lower temperature, preferably at temperatures not
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1 exceeding by 20C the arithmetic average of the melting points of
the two types o~ polyethylene. The foaming is effected herein
simultaneously with the extrusion molding.
The desired compounding ratio between the low density
polyethylene and the high density polyethylene used in the process
of this invention comprises less than S0 percent by weight of low
density polyethylene and more than 50 percent by weight of high
density polyethylene. Since it is extremely difficult to compound
a major amount of low density polyethylene with a minor amount of
0 high density polyethylene uniformly, since the resulting product
molded therefrom does not have a smooth surface, and since the
- physical properties of the products are substantially the same
as those of the low density polyethylene alone, there is no
significant advantages in compounding with high density poly-
ethylene where the low density polyethylene is used in relatively
large amounts.
In the process according to this invention, the two types
of polyethylene in the above-described compounding ratio are
thoroughly compounded at temperatures above the melting point of
the high density polyethylene in a roll mixer, Banbury mixer, etc.
and then formed into pellets. Then, the pellets are immersed in
a volatile liquid such as a halogenated hydrocarbon, for example,
methylene chloride, fluorinated hydrocarbons and aliphatic
hydrocarbons, for example, pentane, hexane, heptane, etc.,an~
then swelled to yield the foamable compound.
The degree of swelling of the pellets is observed to
exceed the arithmetic average of the individual degrees of swelling
of each type of polyethylene, since the degree of swelling of the
low density polyethylene is much higher than that of the high
density polyethylene and, therefore, the volatile liquid can more
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1 easily permeate into the center oE the pellets to increase the
degree of swelling when the low density polyethylene is uni-
forml~ compounded with the high density polyethylene. The above
analysis is also confirmed by examinin~ the changes in the degree
of swelling relative to the passaqe oE time ~herein the time
required for the maximum degree o~ swelling ~or the low densi-ty
polyethylene alone is substantially the same as that of the
mixture of the low density polyethylene and the high density
polyethylene. In contrast a much longer time is necessary with
high density polyethylene alone.
The relationship in the degree of swelling is shown in
Figure l, wherein 1 is a curve showing saturation degrees of
swelling in which high density polyethylene is immersed in Freon*
11 (trichloromonofluoromethane) as a volatile liquid at normal
temperatures e.g.r20 to 30C, and 2 is the curve showing the
saturation degrees of`swelling obtained from the arithmetic
average of those with each density of polyethylene.
In producing foamed products from the foamable compound
- thus obtained, the compound can be ~oamed simultaneously with
the molding in a conventional extruder and the like. Highly
expanded products can be obtained in this process at a relatively
lower extrusion temperature but it is usually preferred to employ
temperatures-not exceeding by 20C the arithmetic average of the
` melting point of each of the polyethylene. Since higher extru-
sion temperatures are desired for the extrusion processability
and lower temperatures are desired for the foaming properties r
- there is an optimum temperature in the extruding step of this
process and this optimum temperature is in a somewhat lower range
in this process as described above.
In the pxesent process, it has been experimentally con-
firmed that the foaming ratio can easily be controlled by adjust-
ing the extruding temperatures since the foaming ratio changes
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1 slowly relative to changes in the extrusion temperature, although
the th~oretical reason for this is not at present known.
The relationship between the extrusion temperature and
the foaming ratio is shown in Figure 2, wherein 3 is an experi-
mentally determined curve obtained from the foaming compound
produced by swelling the pellets, which are prepared by compound-
ing the two types of polyethylene in an equivalent amount, usîng
Freon* 11, to a degree of swelling of 1~ wt.% (polyethylenej. 4
is an experimentally determined curve obtained from the foaming
compound produced by swelling pellets consisting of low density
polyethylene alone and 5 is an experimentally determined curve `
obtained from the foaming compound produced by swelling the pellets
consisting of high density polyethylene alone in a similar manner.
Curve 6 represents the arithmetic average of above curves 4 and
5 when the two polyethylene are used in an equivalent amount. As
is apparent from Figure 2, since the slope of the curve 3 is
less than that of 4, 5 or 6, control of the foaming ratio is
easier, and products with a higher foaming ratio can be obtained
at a lower temperature with the blend of low and high density
polyethyleneS-
Although the advantageous effects in blending poly-
ethylene of different densities have been described ~ith the swell-
ing method using a volatile liquid for the sake of convenience,
it can readily be understood that the advantage of easy control
of the foaming ratio can also be attained using a liquid injection
method. It will further be easily understood by those skilled in
the art that the blènding of polymers having different melting
points or melting viscosities instead of blending polyethylenes
of different densities is advantageous in the control of the
foaming ratio.
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1 The use of a solid blowing agent as a nucleating agent
will next be described. It is known that solid blowing agents
are effective for controllin~ the cell size but it is necessar~
that the solid blowing a~ent decompose at a relatively low tem-
perature since the extrusion tempe:rature .is low in the present
process as long as a volatile liqu:id is used.
Incorporation of a decomposition accelerator to accel-
erate the decomposition of the blowing agent is known but it is
difficult to compound them uniformly and the accelerator is de-
composed in the compounding step in the mixer.
The present invention, therefore, in one embodiment
; provides a process for providing foamed products having a uniform
cell size by compounding each of the blowing agent and the acce-
; lerator separately into a polyethylene matrix and blending these
separately compounded polyethylenes in an appropriate ratio.in
extruding.
When, for example, azodicarboanamide is used as a solid
blowing agent and zinc stearate is used as a decomposition
accelerator, the azodicarbonamide is compounded with the.low
density polyethylene having a lower melting point and the zinc
stearate is compounded with the high density polyethylene having
a higher melting point.
It is also effective to incorporate the solid blowing
agent into the polyethylene having the higher melt flow index
and generating less heat in melt compounding,and the decomposition
accelerator into the polyethylene having lower melt flow index and
generating more heat. The use of the blended polymer results,
as stated above, in the advantage of easier control of the foam-
ing ratio as compared with the use of only a single type of -
polymer~
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1 It is, of course, possible ~o compound the solid blowing
agent into a portion of a pol~mer and decomposition accelerator
to another portion of the s~me polymer when a blend polymer is
not used.
~ en the blended polymer is used and an accelerator is
not used, the solid blowing agen-t can be compounded only with the
lower density or lower melt flow index polymer.
Method of Swelling
:
Referring now to more specific details of the volatile
liquid, the volatile liquid used herein is not per se novel, is
known and includes volatile liquids such as the fluorinated
hydrocarbons. A feature of this invention resides in the process
of compounding the volatile liquid into the polymer, particularly
in the swelling process.
A foaming compound produced by swelling polystyrene
beads with a volatile Iiquid is conventionally used in various
industrial applications. Althoug~ there are no difficulties in
using them for expansion molding under high pressure such as
compression molding using molds, the cell size developed in the
molded products becomes too large when they are used for expansion
molding such as extrusion molding under relatively low pressures.
Thus, the incorporation of finely divided organic or inorganic
particles as nucleating agents is required to avoid such defects.
However, since both the foaming compound and the
nucleating agent are solid, it is difficult to mix them uniformly
in the polymer,and the volatile liquid impregnated in the poly-
styrene beads partially volatilizes in the mixing operation.
Moreover, the s~elling of polystyrene with the volatile liquid
requires a longer time, and care must be taken in selecting a
volatile liquid which is compatible with the polystyrene beads.
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1 Another embodiment of this invention, therefore, provides
a process capable of overcoming the foregoing disadvantages
thoroughly and which is also applicable to a process for expansion
molding of various crystalline plastics such as polyethylene and
polypropylene for which the use oE a volatile liquid has hitherto
been considered unsuitable.
The present invention comprises molding while expanding
in the usual manner the foaming compound which is obtained by
compounding a solid blowing agent and, if desired, a decomposition
accelerator with a polyolefin and foaming and pelletizing the
same at temperatures lower than the decomposition point of the
solid blowing agent and then swelling the pellets with a volatile
liquid.
The solid blowing agents which can be compounded with
the polyolefin include the conventionally used organic or inorganic
blowing agents such as azodicarbonamide, p,p'-oxybisbenzene-
sulfonylhydrazide, sodium bicarbonate, etc., and those having a
nucleating effect at the expansion stage are particularly preferred.
If desired, a decomposition accelerator, such as a metal
soap, urea and the like can be added to and compounded with the
polyolefin and the solid blowing agent and then this compound
expanded and pelletized at a temperature above the decomposition
point of the solid blowing agent. The foaming ratio of the
pelletized products can appropriately be controlled by varying
the expansion temperature and pelletizing temperature used.
Since a decomposition accelerator is used for the foaming of the
pellets so that they foam smoothly, it can be selected depending
on the combination of the plastic material and the solid blowing
agent.
The foaming pellets thus prepared are immersed in vola-
tile liquids such as aliphatic hydrocarbons, for example, pentane,
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1 he~ane, etc. and halogenated hydrocarbons, for example, methylene
chloride and Freon* etc. to swell the cells in the pellets with
; the volatile liquid. The volatile liquid used herein can be
selected in accordance with the types of plastic materials employed
and depending on the latent heat of volatilization, the critical
temperature, gas permeability, etc. The degree of swelling by
the volatile liquid is determined of itself by setting the expan-
sion and pelletizing temperature o~ the foaming pellets depending
on the desired foaming ratio of the final products because the
degree of the swelling of the volatile liquid is due to the
foaming ratio of the foaming pellets.
The foaming compound thus obtained is subjected to foam-
ing in the extrusion process, when the solid blowing agent remain-
ing in the foaming compound provides a nucleating e~fect to render
the cell size uniform.
In the process of this invention, as described above,
the plastic materials are extremely easily swelled by the volatile
liquid since they are in the form of foamed pellets, and excellent
results are aiso obtained in using the process in the production o
crystalline plastics (polyolefin~ as well, with which the use of
the volàtile liquid has hitherto been considered unsuitable.
Morèover, the degree of swelling of the volàtile liquid
can easily be controlled by adjusting the foaming ratio of the
pellets. In addition, the swelling time can be shortened as
compared with the conventional volatile liquid method, and the
foaming ratio of the extruded products can be increased.
Further, since the foaming pellets entrap the volatile
liquid in the cells thereof while swelling, the volatile liquid
is not easily volatilized and the products can be stored for a
long time without difficulties thereby stabilizing the foaming
ratio in the extrusion processs.
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1 This swelling method is also effective for other appli-
cations such as in the polymer blending described in the nucleat-
ing agent compounding method.
In blending the polyethylene of lower melt flow index
with the polyethylene of higher melt flow index, for e~ample, two
types of volatile liquids which swell the polyethylene to dif~erent
extents are used, the higher melt flow index polyethylene being
compounded with the volatile liquid swelling polyethylene to a
higher degree and lower melt flow index polyethylene being com-
pounded with the volatile liquid swelling polyethylene to a lowerdegree. Each are swelled respectively and then the two are
compounded in a desired ratio, and fed to an extruder. Then, the
difference in the melt viscosities is reduced as compared with
those before the swelling to thereby facilitate uniform compound-
ing and, therefore, the production of uniformly foamed products.
This concept is also applicable to the blending o~ poly-
ethylenes having different densities, wherein the volatile liquid
swelling polyethylene to a greater extent is used for the higher
density polyethylene and the volatile liquid swelling polyethylene
to a lower extent is used for the lower density polyethylene.
The polymer need not always be immersed completely into
the volatile liquid in the swelling and sufficient swelling can
be attained even in the vapor of the volatile liquid.
In this swelling process, it is not always necessary
for the volatile liquid to swell all of the materials, For
example, a compound which contains ingredients which may be eluted
by the volatile liquid can be used by incorporating this compound,
which has not been swelled, into another compound which has been
swelled.
In using polymers having different densities and vis-
cosities together, it is only necessary to swell the higher density
or the higher viscosity polymer to provide satisfactory results.
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1 ~ethod of Extrusion
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Referring to the extrusion method, a process for supply-
ing the foaming compound to the extruder using the volatile liquid
swelling method will be initially explained.
This embodiment of this invention provides a process for
feeding the foaming compound to an extruder while preventing the
volatilization of the volatile liquid.
Recent foaming agents for polyethylene and polystyrene,
etc. include highly volatile liquids such as halogenated hydro-
carbons, for example, methylene chloride, Freon*, etc. and aliphatichydrocarbons, for example, pentane, hexane, etc. Particularly,
with respect to polystyrene, pellets (beads) swollen using such
highly volatile liquid are commercially available as a foaming
compound.
However, these foaming compounds have the disadvantages
that the volatile liquid included in the pellets partially vola-
tilizes on storage or in the hopper, thereby resulting in a
variation in the degree of swelling of the individual pellets
(beads) and resulting in a non-u~iform foaming condition in the
molded foam products or reducing the foaming efficiency.
This embodiment overcomes these disadvantages and pro-
vides a process for supplying the ~oaming compound to the extruder
while preventing the volatilization of the volatile liquid by
.: .
providing a closed system between the swelling step of the pellets
(beads) and the supplying step of the swollen pellets to the
extruder by advantageously utilizing the vapor pressure of the
volatile liquid.
This embodiment of the present invention provides a pro-
cess for supplying the foaming compound to the extruder and an
apparatus therefor with the embodiment comprising swelling the
*Trade Mar~
.
-- 19 --
.A
pellets (or beads) with the volatile liquid in a swelling tank,
introducing the thus swelled foaming compound together with the
volatile liquid into one or more storage (also drying) tank or
tanks, thereafter, separating the volatile liquid therefrom, then
circulating a gas such as air or nitrogen containing the volatile
liquid vapor in the storage (drying) tank or tanks onto the
surface of the swelled polymer to remove the volatile liquid
adhered on the surface of the foaming compound, and then filling
the storage (drying) tank or tanks with the saturated vapor of
the volatile liquid, and thereafter, supplying the foaming com-
pound to a previously pressurized hopper of the extruder in a
~ closed system extending fro~ the point at which the pellets (beads)
-~ are swelled to the point at which the swollen pellets (beads) are
supplied to the extruder.
` This embodiment will be more clearly understood from the
descriptions in detail referring to the accompanying drawings.
: In Figure 3 showing an embodiment of this invention, a
swelling tank 7, a storage (drying) tank 8, extruder 9, a hopper
10 of the extruder, a motor 11, turn-over valves 12, 12', pipes
13, 14, 15 and 16, filter 17 provided in the pipe 16 adjacent
a turn-over valve 12', level switches 18, 18', swelled pellets
(foaming compound) 19 and volatile liquid 20.
The pellets (foaming compound) 19 swelled by the volatile
. liquid 20 in the swelling tank 7 are introduced together with the
-. volatile liquid into the storage (drying) tank 8 through the pipes
13 and 15 by means of the motor 11. The volatile liquid 20 is
separated using the filter 17 and returned to the swelling tank
7 through the pipe 16. ~hen the storage (drying) tank 8 is filled
completely with the foaming compound 19, the level switch 18 is
actuated to switch the turn-over valve 12 thereby closing the
passage in the pipe 13.
- 20 _
:', ~ - '
l~JL~3yl 7~
1 Then, the saturated vapor of the volatile liquid 20
filled in the swelling tank 7 is circulated through pipes 14, 15
and 16 using the motor 11 into the storage (drying) tank 8 to
remove the volatile liquid 20 adhered to the surface of the foaming
compound 19 and, thereafter, the turn-over valve 12' is sw.itched
to close the passage in the pipe 16 thereby filling the storage
(drying) tank 8 with the saturatea vapor of the volatile liqui.d
20. When the foaming compound is :not used immediately, the turn-
over valve 12 is switched to close the passages in the pipes 13
and 14 and the inner pressure of the storage (drying) tank 8 is
kept above the vapor pressure, preferably, above the critical
pressure of the volatile liquid at the temperature in the tank 8.
The foaming compound can thus be stored for a long time with no
changes in the degree of swelling thereof due to aglng. Large
quantities of the foaming compound can be stored by providing
two or more storage (drying) tanks 8 connected to each other.
In using, the stored foaming compound 19 is supplied to
the extruder by switching the turn-over valve 12' to open the
passage for the hopper 10 and introducing the foaming compound
into thè hopper 10, previously pressurized with the saturated
; vapor of the volatile liquid. The pressure applied to the hopper
10 serves to prevent volatilization of the volatile liquid 20
incorporated in the foaming compound 19 due to the effect of
the heat generated in extruding, and it can be higher than the
vapor pressure, preferably the critical pressure of the volatile
liquid at the melting temperature of the foaming compound 19.
In order to avoid any changes in pressure on introducing
the foaming compound 19 from the storage (drying) tank 8 into the
hopper 10, it is desirable to equalize the pressure in the storage
(drying) tank 8 to that in the hopper 10 directly before the .intro-
duction of the foaming compound 19. Such changes in pressure can
- 21 -
'7'~
1 be prevented by connecting a pressure pump to the storage (drying)
tank 8 and the hopper 10.
When the hopper 10 is filled with the foaming compound
19, the level switch 18' is actuat:ed to turn over the valves 12
and 12' and, thereafter, the same operations as described above
are repeated.
~- According to this embodiment, as above described, the
system from the preparing step of the foaming compound to the
supplying step for the extruder is completely closed and, in
addition, the foaming compound is always kept in the volatile
liquid or under the saturated vapor thereof. Accordingly, no
changes on aging occur at all in the degrees of swelling of the
individual pellets (beads), and the degassing in the extrusion
step caused by the operation of the extruder can be prevented
thereby enabling the production of uniformly expanded products
with excellent foaming efficiency and stability. Moreover, this
embodiment of the invention can be applied also to the production
of the highly expanded products and provide excellent products
easily that have hitherto required difficult operations.
In the process of this invention, sufficient effects can
not be achieved if the hopper portion is closed where the screw
axis portion is incompletely sealed In order to avoid this and
keep the system closed completely, it is advantageous to use a
conventional front-end drive type extruder.
Referring now to the method of setting the temperature
for the extruder, the temperature condition is very important in
the extrusion of highly expanded foams and, particularly, a non-
uniform temperature directly causes non-uniform foaming.
One of the features of this invention is to set the
temperature of the extrusion cylinder toward the hopper connection
- 22 -
: - - ; ; - . . .
1 higher than the temperature of the cylinder at the extruding side.
The polymer is thus melted at the hopper side of the extruder
primarily by the heat from the cylinder and this prevents local-
ized over heating caused by the sh~ar of the screw, keeps the
resin temperature constant and avoids the degassiny of the volatile
liquid. Since the polymer is melted early at the side of the
hopper, the passage for the gas generated through the degassing
of the volatile liquid is inhibited from communicating to the
hopper and almost all of the volatile liquid can be utilized
effectively. It is particularly surprising that highly expanded
foams can be obtained by setting the temperature higher at the
hopper side in supplying the compound swelled by the volatile
- liquid to the hopper.
Adhesion of Foams and Conductors
-- . .
The foregoings describe the embodiment for extruding
highly expanded foams using a volatile liquid. The importance
in using highly expanded foams for the insulation of wires and
cables lies in considering how to coat the inner conductors
tightly with this foam as an insulation. If they should be
extruded without providing a particular extruding means, the
coated products cannot be used as wires and cables because of
a considerable unevenness in both the inner and outer diameters,
and because the foam does not adhere tightly to the inner con-
ductors at all since they are highly expanded.
The process for tightly adhering the highly expanded
foam onto the inner conductors to provide useful highly expanded
foam insulated wires and cables according to this invention will
be described hereinafter.
The vacuum method for improving the tight adherence
between the highly expanded foam and the inner conductors will be
described as initially explained.
; ~ - 23 -
- ^ .
1 If a foaming compound is formed in a tubular shape,
the general tendency is the format:ion of cells in various direc-
tions and, particularly, in the direction along which the inner
and outer diameter of the tube increase. This tends to provide
a non-uniform outer diameter and an unstable configuration, and
the tendency becomes more signific:ant as the foaming ratio in-
creases. Accordingly, although the production of wires and cables
using molding by extrusion of a tubular foamed body and a simul-
-` taneous coating of the same onto inner conductors has been hitherto
practiced with the lower expanded foam, highly expanded foam
insulated wires and cables have not yet been produced because of
the difficulty of tightly adhering the expanded foams to the
inner conductors.
This embodiment of the invention provides a process for
molding by extruding an expanded foam in a tubular shape and
simul~neously insulating the inner conductors therewith while
regulating the size of the outer diameter and the configuration
of the tubular foams by controlling the foaming conditions of
the tubular foam at the time of molding the foam by extrusion.
This embodiment of the present invention provides a
process for~extruding tubular foamed products and insulating
inner conductors therewith in which an extruder provided with an
adapter connected to a vacuum pump at the inside of a nipple is
used, and the inner conductors, previously heated if desired,
are supplied through thus adapter and insulated with an extruded
tubular foam. In the process the inner pressure of the tubular
foam is reduced to adjust the foaming conditions therein using
this adapter. Further in the process an extrusion head having a
nipple in which the adapter, provided with cooling or heating
means, is used and the inner pressure of the tubular foam is
- 24 -
reduced simultaneously with a cooling or heating to adjust the
foaming ratio in coating the inner conductors by extrusion as
above.
The advantages thus ob-tained Erom this embodiment of
this invention can be summarized as follows:
(1) Since the inner pressure of the tubular foams is
reduced using a vacuum pump to aid the foaming (in the direction
toward the conductor) and the inner conductors are preheated if
desired:
a) the contact areas are increased to improve and tightly adhere
the foam in the melting,
b) the condition of foaming in 'he tubular foams can freely be
controlled by adjusting the pressure reduction by the vacuum
- pump, and
- c) the depth of the interior layer (i.e., the material which is
~ solid closest to the conductor) of ~he tubular foams can appro-
; priately be varied by adjusting the heating temperature in the
inner conductors.
t2~ Since the inner pressure of the tubular foams is
reduced using the vacuum pump to facilitate the foaming (inwardly)
and the inside thereof is simultaneously cooled or heated by the
cooling and heating adapter:
d) the foaming ratio can be controlled more precisely and over
a wider range by aiternatively adjusting the degree of pressure
reduction by the vacuum pump and the temperature of the adapter,
e) by heating the adapter, the foamed products can be tightly
adhered in melting to the inner conductors without preheating,
and ~-
f) in using the foaming compound to be molded at a higher tem-
perature, the decrease in the adhering power caused by the rapid
cooling can be prevented by heating the adapter.
25 -
3~ 7J~
1 This ~mbo~iment of this invention is to be described
with reference to the accompanying drawings, particularly to
Figure ~, which illustrates the co;nventional process ~or molding
a tubular expanded product havin~ a foamin~ ratio of more than 3
and simultaneously insulating the inner conductors therewith using
a conventional extruder, wherein die 21, nipple 22 for supporting
- and carrying the inner conductor, molding compound 23 capable of
foaming, molded foam 23', and the inner conductor 24 are shown.
The foaming compound 23' effected the foaming in the direction
that increases the inner diameter and the size and the configu-
ration of the inner diameter of the foamed polyethylene are uneven -
to inhibit tight adherence to the inner conductors 24.
Figures 5 and 6 illustrate this embodiment of the
invention for extruding tubular foams having a foaming ratio of more
than 3 and simultaneously insulating the inner conductors there-
with by using an extruder. The extruder shown includes the
; addition of an adapter and an inner conductor heating means of ;
this invention to the conventional extruder as shown in Figure 4.
In Figure 5, an adapter 25 is provided inside a nipple 22 for
supporting and carrying the inner conductor 24. This adapter 25
is connected to a vacuum pump 26, and a heating means 27 for the
inner conductor is provided just before the point at which an
inner conductor 24 is introduced to the adapter 25. In Figure
6, an adapter 25 o~ a heat conductor such as a metal is heat
insulated using a heat insulation material 28 such as fluoro
resin at the inside of a nipple 22 for supporting and carrying
the inner conductor shown in Figure 5 and the adapter is provided
with a cooling or heating means 29 such as pipes, etc. and the
adapter is connected to a vacuum pump 26.
The process of this invention is carried out by extrud-
ing and molding the tubular foams 23',simultaneously reducing the
26 -
3~7~
1 pressure inside thereof to a predetermined level using vacuum
pump 26 and supplying therein inner conductor 24 which is heated,
if desired, using heating means 27 through the adapter 25, or
by cooling or heating the inside of the foams 23' to a pre-
determined temperature and supplying therein the inner conductor
24 which is heated if desired. It: is desired to adequately
select the degree of pressure reduction, generally a pressure
reduction of about 5 mmHg to about 200 mmHg, preferably less than
140 mmHg in the vacuum pump, and the temperature of the adapter
and conductor, depending on the end use applications intended and
the types of foaming compound used. It is also preferred to set
the cooling temperature necessary for the cooling of the adapter
below that of the foaming point of the foaming compound and the
heating temperature necessary for heating the adapter in the
vicinity of the melting point of the foaming compound.
Figures 7a and 7b are cross sections of tubular foam
insulated wires and cables illustrating the effects of this
embodiment of the invention in which the effects of the degree
of pressure reduction, cooling and heating of the adapter, and
the heating of the inner conductor on the foaming conditions and
on the close adhesion to the inner conductors (as set forth in
detail in the table below), wherein gas cell 30, solid layer 31
. and void 32 are shown.
- 27 -
lU'~3~3 t'~,
... --:" ~ o .'
~ ~ ~ a)~ ~ o~ ~ ~ o
C h O a~ O O ~ t)
o la ~ ~1 ~1 S :~ ~ ~ ~ n~ ~
~n o o ~ ~ ~ ~ ~ o
Q)
~:: o o ~ a) ~ L~ ~ ~ t~ ~ U~
O ~ ~ ~ ~ ~ ~ ~ ~ ~ U~
,1 ~ ~ (1~ ~ l~ ~ ~1 r
u~ h ~ ~ h
a) ~ ~ ~ ~ Q) ~ :~ ~ S~
U~ ~ ~ ~ ~ ~ ~1 ~ ~q ,1
~1 u~ rl h .~ ~ ~ ~ O
¢ a) ~ ol ul a) ul S O O
~ h ~1 h S h ~ h ~1 ~} h ~
s ~ o o ~ Q~ ~ a) ~ ~ a) ~:
,~ o a~ ~ ~ ~ ~: ~ :: ~ ~ ~
E~ ~ ~: .~ (~ ,1 ~: .,1 h ~ ~ O
UaJ rl ~11 a~ a) ~ h ~1
Q~ -1 C~ ~ S ~ ~ ~ ~ ~}
a) s~ ~ s~ ~ ~ O ~
Oh E3 O ~ ~ h o u~
tl~ C) Q) ~: ~ ~ ~ ra ~ ~ E~ ~ ~: h
~~ ~ O ~ O X O ~ ~ ~ ~ O
~1 h 0-~1 ~: a) ~ u~ O ~ ,1 ~
~; a) h ~ 5-1 h ~q ,~ ~3 Ul O
~ ~: t~ o a~ ~ Q~ u~ ~ ~ ~ tJ) Il~
O ~ ~ Q~ ~1 ~ ~,1 ~ ~ O
1:'~ H rl h U h a~ h O h O ~1 0 4~
E~ ~ ~IJ-rl 11~ ~ a) ~ o o :`
S O r-l ~1 ~ ~I X ~1 l~i ~n E3~ . `
. O ~ ~ ~: h ~ ~ O a~ h ~
41 1~ 0 a)-~l h a) O O ~ ~I S O ta h
.~ ~ ~ h O h ~ h ,1 ~ ~I h a~
.` O ~ S u~ h ~ u~ a) O ~ O ~ ~ rl rl
-; ~ ~ E; o ~ 1~ ~ ~; ~ ~ I
~1 ~ ~ ~ ~t ~d h a) ,1
:; ~ o ~ o o a~ o o h ~ 1 O O ,~
~, . ~ O q I ~ U ~1 ~ ~ ~ ~ S:: U
~;
,., . .
_ -.
:- .
-.` h rù ~ ~ .
a~ ~ ~ ~ ~ ~
Q ~1 O hO td O al .
:-: 1~ o ~ ~ h ~ ~1 ~
,., ~ O ~ O t~ O ~ O
,.......... , ,,.¢ u o Q s I u ~ ~
~ _
h O R
.~ ~ t) ~ ~ h ~: ~ ~ .
~ a) ~ a) ~ ~ ~ ~ a~
~ H ~ ~ ~ ~) tJ) ~:
.,
~ o ~ ~ ~s ~ o a~ o a~ O
.~ ~: O ~ S
d a) O
:i h ,1 ~ ~ ~ 'U
:' ~1 ~ Q) a) aJ ~J a)
.. ~n o u u ~ u u u
.~ tn
Q)'a ~ ~ ~ ~ '
., ~ a~ Q~ a) ~ ~ ~ o a
~ ~ ~; h ~ ~1 h h ~:
'.' O -- ... ___
U
8 ~ R U U _
C~
. ~ I~ I~ ~` l_
~
2 8
. .
.: , ,
11~4~7~
1 Foaming Compound ... high density polyethylene (d = 0.95 g/cm3
and MI = 0.3)
Freon 11 ... 13 phr by weight
Heating Temperature of
; Inner Conductor .... a: 120; b: 120; c: 250C;
Degree of Pressure Reduction ... a: 4 mmHg; b: 4 mmHg;
c: 4 mmHg; d: 4 mmHg;
Cooling Temperature for Adapter ... a: 60; d: 60 C;
Heating Temperature for Adapter ... c': 135 C
According to this embodiment of the invention, insulated
wire and cables having preferred electric properties can be
obtained by increasing the degree of the pressure reduction in
the adapter and stopping the cooling of the same to increase the
~oaming ratio inside the adapter and simultaneously increasing
the heating temperature for the inner conductor to increase the
depth of the non-forming layer thereby tightly adhering the foams
to the inner conductor, or by heating the adapter approximately
to the melting point of the foaming compound and increasing the
degree of the pressure reduction thereby eliminating the pre-
heating of the inner conductor.
` As is apparent from the foregoing, the tight adhesion
of the highly expanded foams to inner conductors can be attained
by employing a suitable process such as applying a vacuum from
the side of the inner conductors and the like. However, the foams
produced do not always have a smooth outer surface but, on the
contrary, the surface is considerably uneven, and they are not
suitable for use as wire and cables in this condition.
This embodiment of this invention of a method of control-
ling the size of the surface thereof will, therefore, be described
referring at first to the vacuum method.
- 29 -
.. . .
1 This embodiment provides a pressure adjus-ting adapter
for recovering the excess ~olatile liquid which volatilizes in
the vicinity of the exit of the Eoaming compound from the extruder
and controlling the foaming ratio and the outer diameter thereof
in the extruder for the foam.
Recently, halogenated hydrocarbons such as methylene
chloride, Freons, etc., and aliphatic hydrocarbons such as pPntane,
hexane, etc. have been used as volatile liquids for polyethylene,
polystyrene, and the like. However, since halogenated hydro-
carbons are expensive and toxic, and aliphatic hydrocarbons areinflammable and explosive, such conventional volatile liquids
should be recovered when they volatilize in the vicinity of the
exit for the foaming compound.
According to this embodiment, to achieve the above
purpose, the excess volatile li~uid volatilizing in the vicinity
of the exit for the foaming compound can be recover~d by providing
in a conventional extruder an adapter connected to a vacuum pump
and to a pressure pump. A device for adjusting the foaming
ratio and the outer diameter of the foams is provided.
More specifically, a pressure adjusting adapter for use
with the foam extruder is provided according to this invention.
This comprises providing an adapter at the exit for the foaming
product and/or on the extruder head, connecting a liquefying
(storage) tank to the adapter, and connecting a vacuum pump alone
or in combination with a pressure pump to the adapter by way of
the liquefying (storing) tank.
` This embodiment will now be described in detail with
reference to, but not restricted to, an extruder for wires and
cables consisting of tubular foams and inner conductors insulated
therewith.
*Trade Mark
.
- 30 -
, ~ .
.
~ , . . .
1 Figure 8 is a view illustrating the structure and
o~eration of an extruder for the highly expanded foam insulated
~ires and cables according to this invention, wherein inner
conductor 33, foaming compound 3~ for insulating the inner con-
ductor and containing the volatile liquid therein and tubular
foams 34' molded and coated on the inner conductor are shown.
35 is a die, 36 indicates a nipple for supporting and passing the
conductor 33 therethrough, 37 and 37` are adapters mounted near
the exit of the tubular foams 34' and in the extruder head, 38
1~ is a liquefying (tank) connected to adapters 37, 37', 39, 39' are
vacuum pumps connected by way of the liquefying tank 38 to the
adapters 37, 37', 40 is a valve and 41 is a sealing die which is
- provided on the inside of both ends of the adapter 37 and can
also serve as the sizing die for the foams.
The apparatus according to this invention collects the
volatile liquid which volatilizes on the outer and the inner
surfaces of the tubular foams under vacuum evacuation by way of
the vacuum pumps 39, 39', and introduces the same into the
liquefying (storage) tank 38 for liquefication and storage
therein. The foaming ratio and the outer diameter of the foams
is controlled by the effect of the sizing die 41 for the foam
and by the suction controlling effect of the vacuum pumps 39, 39'.
If the vacuum pump 39 is replaced with a pressure pump, the inner
pressure of the adapter 37 is increased by the blowing agent in
the gaseous or liquid state, and the foaming ratio on the outer
side of the foams is suppressed. The inner pressure of the
adapter 37' is redu~ed by vacuum pump 39' to facilitate the
foaming inside of the foam thereby improving the close a &esion
power of the foams to the inner conductors.
Coo ing and heating devices can be provided on
the adapters 37, 37' in order to enhance the ability to adjust
31 -
.. ;` ` ~ .
7~
1 the foaming ratio and the outer diameter. These cooling and
heating devices are also effective for the recovery of the
volatilizing liquid. -
As described above, the apparatus of this inven-tion,
provides excellent advantages in view of material economy and the
safety of operation since the expensive volatile liquid which is
highly toxic, inflammable and explosive can be recovered and
reused. This also increases the working efficiency and the value
of the products since the foaming ratio and outer diameter of
lG the foamed products can be adjusted easily and close adhesion of
the foams to the inner conductors can be improved.
Another embodiment for the surface sizing process of
this invention is described below.
This embodiment provides a process for producing tubular
foams having a uniform outer diameter.
Various sizing processes in producing tubular foams
using an extruder for controlling the outer diameter of the final
products are known, such as the Selka~ process (Modern Plastics
Jan. 1969, p 106-107) and the processes disclosed in Japanese
Patent Publication ~os. 7556/1969 and 32318/1969. However, these
processes use a sizing die and only the outer diameter of the
tubular foams can be controlled. Therefore, the inner diameter
thereof remains uneven and the foaming condition of the entire
foams car. not be controlled appropriately. Moreover, the processes
described in the two patent publications immediately above are not
applicable to the product of highly expanded polyolefin insulated
wires and cables, since the diameter of the sizing die is gradually
incre~sed in these processes, which tends to result in cells in
- the outer portion and to increase the inner diameter thereby
causing unevenness.
*Trade Mark
- 32 -
,A
. ~ . .
1 The inventors have also developed an embodiment of this
invention for sizing the inner diameter by controlling the foaming
ratio through a cooling and a pressure reduction inside of the
tubular foams in extruding and coating the tubular foams.
This embodiment provides the capabili~y of more easily
controlling the inner diameter and also the outer diameter of the
tubular foams. The present invention provides a process for
insulating the inner conductor with the tubular foams using a
mold extruder for tubular foams which comprises foaming and
extruding the foaming compound by passing the same between a die
having a parallel land of a same diameter as the outer diameter
of the tubular foams and a nipple having a tapered portion in the
parallel land of the die, and appropriately coolin~ the outer
periphery of the tubular foams, if desired.
The above process according to the invention will be
described referring to the accompanying drawings, in particular,
to Figure 9 showing an embodiment of this invention for insulat-
ing the inner conductors, wherein 42 indicates a die having a
parallel land 42' of substantially the same diameter as the outer
diameter of the tubular foams and a tapered portion 42", 43 is
; a nipple having a tapered portion 43' in the parallel land 42'
and 44 is a cooling device incorporated in the parallel land 42'.
Inner conductor 48, heat insulation material 45, foaming compound
46, foamed product 46', and the vacuum apparatus 47 are also
shown.
It is desirable that the parallel land 42' of the die
42 be as long as possible, and the angle of the tapered portion
43' of ~he nipple 43 is preferably selected appropriately depend-
ing on the foaming rate of the foaming compound so that the angle
30is decreased as the foaming rate is reduced. The angle of the
- 33 -
` ' ' .
~ O ~
1 tapered portion 42" of the die 42 is also selected appropriately
relative to that of the tapered portion 43' of the nipple 43.
The changes in the pressure in the die at the time of foaming
- (compression --- opening) is produced by varying the cross
section of the passage for the resin just before and after the
foaming initiation point. Whether the pressure changes are rapid
- or slow is determined by the foaming rate. If the relationship
- between them is not appropriately controlled, various inconve-
niences result such as cells of a non-uniform diameter and a
non-uniform configuration in the final products.
The outer diameter of the foaming compound 46' is con-
trolled as the outer periphery thereof is gradually cooled in the
parallel land 42' of the die 42 in molding, where the parallel
land 42' can be cooled while adjusting the cooling temperature
depending on the properties of each foaming compound used and the
properties of the tubular foams desired.
Figure 10 shows a cross section of the insulated wires
and cables obtained by applying the process of the present
invention, in which the cells increase gradually in the tubular
2~ foam 46' toward the inner conductor 48 to adhere to the conductor
tightly under compression, and the outer diameter of the foamed
product is uniform and stable. When the outer surface is rapidly
cooled to form a solid layer, it is most suited as a protecting
insulation of the foams.
Since a die having an elongated parallel land of sub-
stantially the same diameter as the outer diameter of the tubular
foams and capable of being cooled, if desired, in the process of
this invention is used, the tubular foams are extruded after the
foaming in the outer periphery of the foaming compound has been
thoroughly completed and, therefore, free foaming which is dis-
advantageous, does not occur again outside of the extruder and
- 34 -
1 the outer diameter of the tubular foam can easily be controlled.
In addition, the use of a nipple having a tapered portion
in the area of the parallel land of the die causes foaming along
the tapered portion of the nipple to be gradual and this keeps
the inner diameter always in a uniform shape (circular). Moreover,
by the vacuum attraction of the tip of the nipple, the inner
diameter can be controlled and adhered closely to the inner con-
ductors.
When this process is applied to a foaming compound in-
corporating volatile liquid therein, the molding proceeds moresmoothly and better results are obtainable because the volatile
liquid serves also as a lubricant.
Although there are other known processes for shaping
the surface which use various other types of dies, all of these
processes have the defect that the foamed product is difficult
to pull back toward the conductor because of the resistance
exerted by the sizing dies. Therefore, the foam should be pulled
back by the vacuum method while tightly and closely adhered to
the inner conductor. As will ke described in detail hereinafter,
the method is also extremely effective in which adhesives are
used between the inner conductor and the foam. It will readily
be understood that the molding can further be effected easily
by the combination of the use of the vacuum method and adhesives.
Expanded foams having a uniform outer diameter can thus
be extruded onto the surface of the inner conductor to insulate
the conductor as described above. However, attention should be
paid to factors which degrade the properties of the expanded
foams. One of them is the water absorption caused by cooling
and the other is the deformation (collapse~ of the expanded foams
by a take-up capstan.
1 Referring now to the cooling, it can be performed
usually by filling a cooling trough with water as used in the
conventional expansion of foams of a lower degree of expansion.
For highly expanded foams, however, since the continuous foaming
tends to occur and the significan~ heat shrinkage of the foams
occurs at the time of cooling, water deeply permeates into the
foams to hinder easy drying. This severely degrades the electrical
properties thereof as wires and cables and therefore water cool-
ing is strictly avoided,th~s necessitating gas cooling with inert
gases such as air and the like. If the air is insufficient for
cooling, dry ice, etc. must be used to assist the cooling. A
significant advantage can sometimes be obtained, particularly
- from the standpoint of production, by using an atmosphere having
a higher gas permeability to the polymer than the volatile li~uid.
It is further preferred to use such gases in a heated atmosphere,
because this results also in the annealing of the expanded foams
Y without producing a great degree of shrinkage,thereby providing
wires and cables having electrical properties show1ng fewer changes
at the time of use.
The take-up capstan used herein should be of the Cater-
pillar type and constructed, for example, so as to be pressed by
a belt lined with sponge in order to prevent deformation of the
expaned foams. The provision of the supply capstan for the inner
conductor ]ust before the extruder head is advantageous to reduce
back-tension and decrease the pressure of the capstan. The driving
power source for the supply and take-up capstans, extruding screw
and the like can be provided in common so as to prevent abrupt
changes in indiv:idual power thereby avoiding abnormal tensions
in them and a deEormation of t~e foams, and the desired wires
and cables having stable properties can thus be produced.
* Trade Mark
.
- 36 -
7-~
1 In producing a coaxial cable , covering the outer
periphery of the highly expanded polyolefin insulated cables
and wires thus obtained with an outer conductor such as aluminum
and the li~e, there is a serious problem that the expanded foams
shrink significantly when they are subjected to heat cycling
comprising hea~ing and cooling.
It is essential for a coaxial cable that the properties
thereof do not change at all even over a long period of time of
use, and any shrinking of the foam on the insulation etc. must
1~ be avoided by all means since suc~ changes apparently cause a
degradation in the properties. The method of preventing such
shrinking is described below using a method in which
the diameter of the extrusion die is decreased as much as possible.
This embodiment of the invention provides a process for
producing a highly expanded foam insulatea wires and cables in
` which longitudinal shrinking of the highly expanded foam insula-
tion layer is prevented.
,
In a highly expanded foam insulated ~ire and cabler the
heat insulation layer generally tends to shrink in the longitu-
~ dinal direction when subjected to heat cycling, and this tendencybecomes even greater as the foaming ratio of the insulation layer
increases. Particularly, in cables of a structure in which the
outer periphery is confined by metals such as in a coaxial cable
having an outer conductor, remarkable shrinking occurs therein
causing various difficulties after extended use such as a peeling
of the insulation layer from the conductor, etc. A method of
adhering the highly expanded insulation layer to the conductor
is employed to prevent such difficulties,but with the defect
that buckling can sometimes occur,if the diameter of the inner
conductor is small in the adhesion between the inner conductor
and the insulation layer because the highly expanded insulation
- 37 -
.
1 layer shrinks to a gxeat extent. The inner conductor may shift
resulting in a break in the adhesion between the outer conductor
; and the insulation layer. Further, an entanglement with the inner
conductor can be caused when a soft outer conductor is used such
as metallic shaled wire. In addition to the foregoing defects,
adhesives having better adhesion properties also have higher
dielectric constants and dielectric losses thereby degrading the
electrical properties. Moreover, the method necessarily involves
the additional adhesive application step in the production of
cables and also a removal step thereof on use, which extremely
reduces the efficiency of the operation.
This invention overcomes the foregoing defects and
provides a process for producing highly expanded foam insulated
wires and cables of excellent property having none of the above
defects in which the shape of foaming cells in the highly expanded
insulation layer is controlled so as to prevent longitudinal
shrinking of the insulation layer.
The inventors have discovered that when the foaming
cells in the highly expanded foam insulation layer of the wire
20 and cables take the form of an ellipse having its major axis ;
aligned in the direction of the radius of the wires and cables,
the cells shrink in the direction of this major axis by which the
surface area thereof is reduced at the time of the cooling and,
therefore, the foamed insulation layer shrinks radially and not
longitudinally, and that insulated wires and cables with higher
compression strength can be obtained by controlling the outer
diameter of the insulation layer using a sizing die and the like,
so the radial shrinkage of the wire and cables can be prevented.
The inventors further discovered that the high expanded foam
insulation layer having such a cell shape can be produced by
setting the extrusion velocity of the highly expanded foam insula-
tion layer (the velocity at the exit of the die before the foaming,
- 38 -
7 f~
1 that is, the velocity just after contact of the foaming material
~ith the inner conductor) higher than the velocity of the inner
conductor. Since the foaming compound is compressed in the
direction of extrusion, the foaming in this direction (longitu-
dinal direction) is suppressed and the foaming in a radial
direction is facilitated and thus the cells are produced in the
shape of an ellipse having its major axis aligned in the direction
of the radius of the inner conductor.
In this inVentiQn, the relationship between the extru-
sion velocity ratio of the highly expanded insulation layer versusthat of the inner conductor (referred to simply as extrusion
velocity ratio hereinafter) and the length of shrinking of the
highly expanded insuiation layer is hyperbolic. The
extrusion velocity ratio can appropriately be selected based on
this relationship in accordance with the end-use applications o~
the cables, the number of heat cycles, the type of foaming com-
pounds and the like.
Figure 11, plotted on the basis of the data obtained
from the examples described below, shows the hyperbolic relation-
ship which exists wherein the ordinate represents the length ofshrinking and the abscissa represents the extrusion velocity
ratio (calculated value of the extrusion velocity of the highly
expanded foam insulated wires and cables of the inner conductor).
As is apparent from Figure 11, the shrinking approaches zero as
the extrusion velocity ratio increases and a practical upper limit
of this ratio is preferably up to 1 : 3 because of the roughing
of the surface of the expanded foams, cellular disturbance, and
pressure increase, etc.
Cells of a desired shape can be obtained in this inven-
tion also by varying the diameter of the die. As the diameter of
- 39 -
'..~
,:'',, ~ '
t r~ '
1 ~e die decreases, the flow rat~ of the foaming compound becomes
higher than that of the inner conductor, and this produces a
longitudinal compressional stress and radial stretching stress
thereby resulting in elliptically shaped cells whose major axis
is aligned in the direction of the radius. Changes in the flow
rate of the foaming compound in this case do not affect the size
of the outer diameter of the highly expanded foam insulation
layer and expanded foams of the same uniform diameter can be
obtained.
The same effect of this invention can also be attained
by varying the extrusion amount or the feed velocity of the
inner conductors.
- This embodiment of the invention is applicable to the
production of coaxial cables to provide highly expanded foam
insulated wires and cables of extremely h~h quality.
The advantages provided according to this embodiment
of the invention are summarized below.
(1) Since the highl~ expanded insulation layer per se does
not shrink, this layer need not be adhered to the conductors
using adhesives.
(2) The absence of the adhesives prevents a degradation of
the electrical properties of the wires and cables and provid~s
excellent operational efficiencies both in production and on use.
~ 3) Since the insulation layer does not shrink, this inven-
tion can effectively be applied not only to wires and cables with
inner conductors of a large diameter but also to thin or soft
- inner conductors.
(4~ Since the foaming occurs in the radial direction, highly
expanded foam insulated wires and cables can be provided having
high compression strength.
- 40 -
" ~
~ t~ ~
1 For further and complete prevention of shrinking, tight
adhesion of the inner or outer conductors and the highly expanded
oam insulation layers is advanta~eous.
The adhesion of the highly expanded insulation layer to
the inner conductors will be next clescribed.
This embodiment of the invention provides a process for
extremely easily producing highly expanded foam insulated wires
and cables having a highly expanded foam insulation layer and
the inner conductor tightly adhered to each other and possessing
0 preferred electrical properties.
It is generally known to provide an adhesive layèr onto
an inner conductor for closely adhering the insulation layer to
; the inner conductor in highly expanded foam insulated wires and
cables. Since the polarity of the adhesives and thus the di-
electric constant and dielectric losses thereof increases as the
adhesivity increases, the adhesive layer should be as thin as
possible. However, it is difficult to provide an extremely thin
adhesive layer using conventional methods such as the extrusion
method, the solution coating method, the fluidizing bed method and
the like, since problems exist with each of these methods.
In the extrusion method, the formation of a thin film is difficult
since the film formed tends to be discontinuous, the depth of the
film is not uniform due to the difficulty in aligning the centers
of the die and the nipple, and the residual stress is high because
` a non-uniform film is stretched per se. In solution coating,
several application steps are required to obtain a uniform thickness
of film because the solution is highly diluted and has a low
viscosity, and the solvent used therein inhibits a thorough pre-
heating of the inner conductors creating undesirable safety
and health problems. The fluidizing bed method results in a film
'' ' ~
- 41 -
, . :: .
r7 ~6
1 having many pin holes since a powder is coated in this method;
the film has an uneven surface and a poor film strength since
the coating is effected only by dissolving the powder.
This embodiment of the invention provides a process for
producing highly expanded foam insulated wires and cables having
a tightly adhered insulation layer and an inner conductor and
excellent electrical properties in a conventional extruder which
comprises: applying an extremely thin adhesive film (abbreviated
hereinafter as thin film) onto the inner conductor using an
extremely simple facility and operation without using an extruder
while advantageously applying the "self-centering" ability of
a 'Ifloating die" which is one of the concepts for providing a
uniform plastic coating on an inner conductor and providing a
further coating of a highly expanded foam insulation layer by
extrusion thereon.
This embodiment of this invention comprises keeping in
a squeeze die the material which is melted by heating above the
heat flowing temperature, passing the inner conductor therethrough,
coating a thin film thereon using the drag stress of the inner
; conductor due to the film adherence and then providing on this
thin film a coating of highly expanded foam insulation compound
` by extrusion.
In the conventional process, the floating die is mounted
on the extruder in a floating manner and freely moves in accordance
with the changes in the extrusion to effect self-centering. On
the contrary in the present process, the self-centering can be
attained by dragging the inner conductor without floating the
die particularly. On dragging the inner conductor, the passing
point of the inner conductor can freely be shifted in accordance
with the changes in the heat flow condition of the thin film
material in the die to thereby effect the self-centering. In
.
~ - 42 -
3l~)'~;;~.3'7~
I addition, in the present process, the thin film material in the
die is dragged out by the drag stress of the inner conductor
causing an inherent stirring and compounding in the die, and
the provision o~ the squeeze die applies a shear stress at the
exit of the die to provide strength to the thin film.
Figure 12 illustrates an embodiment of the process of
this invention for providing a coating of a thin film material
on the inner conductor, wherein a squeeze die 49, die exit 49',
- a heating device 50 for the die 49, a preheater 51 for the inner
conductor 52, thin film material 53 and the coated thin film 53'
on the inner conductor 52. The inner conductor 52, preheated in
the preheater 51, is introduced into the squeeze die 49 in which
. the heat flowing thin film 53 is kept, uniformly coated with the
thin film 53' using the drag stress of the inner conductor 5~,
and the thin film 52' thus coated is provided with strength due
to the shear stress exerted thereon at the exit 49' of the die
49. The thickness of the thin film is determined depending on
the electrical property of the material used and generally a
: film thickness of less than 0.3 mm is preferred. The thin film
material used herein includes homopolymers and copolymers of
ethylene, vinyl type polymers or other series which have a heat
sealing property with respect to the highly expanded foam
insulation layer.. They are preferably used in a particulate
form in view o~ the.ease of melting. To provide a heat flow- :
ability to material in such a form, they can be heated to a
temperature above the melting point of the thin film but below
the decomposition point thereof. To improve the tight adhesion
between the thin film and the inner conductor, higher temperatures
are preferred for the preheating of the inner conductor and for
the thin film material, and on the contrary, lower temperatures
are desirable for a uniform coating of the thin fiim. Therefore,
A - 43 -
.
. - - . , -- : ,
10 ~ '`t;C~
1 the heating temperature can be selected appropriately depending
on the particular end-use application intended. The angle of
the squeeze die can appropriately be selected depending on the
characteristics required such as the strength of the thin film,
etc.
The relationship between the angle of the inside of the
die and the characteristics requi~ed for the thin film is set
forth in the table below.
1 Die Inside Thin Film Smoothness on De-gassing Retained Stress
Angle Strength Film Surface Effect on in Thin Film
Film For- (shrinkage)
mation
.
large high high great great
small low low small small
.. . .
The inner conductor thus coated with the thin film is
introduced into the extruder and~coated with the highly expanded
foam insulated material.
Since the coating of the adhesive layer can be effected
simultaneously with the coating of the highly expanded foam
insulation layer by extrusion according to this invention using
the facility as shown in the drawings, the cost of the equipment
is reduced and the operation is greatly simplified. Moreover,
since the adhesive layer can be provided in an extremely thin and
` uniform thickness, lowering of the electrical properties advanta-
geously can be avoided. In addition, since the inner conductor
is not buckled if the highly expanded foam insulation layer
shrinks, this process provides even more significant effects when
applied to coaxial cables comprising an inner conductor having a
relatively large diameter and a highly expanded foam insulation
layer.
- 44 -
.. ,
:....... - : . -
1 The invention will now be described with respect to
another embodiment in which the outer conductor is adhered to
the highly expanded foam insula-tion layer.
This embodiment of the invention provides a process
for producing highly expanded foam insulated coaxial cables in
which the insulation layers of the highly expanded ~oam and
the outer conductors are tightly adhered.
It is known to adhere tightly the inner conductor and
the insulation layer using adhesives in a highly expanded ~oam
insulated coaxial cable comprising a highly expanded foam insu-
lation layer and the outer conductor sequentially on the inner
conductor . The tight adhesion of the outer conductor to the
insulation layer using adhesives is also known.
This embodiment provides a method of tightly adhering
the outer conductor consisting of a metal having a smooth surface
- to the highly expanded foam insulation layer usin~ a nonpolar
polymeric adhesive material, etc. on the basis of the development
of a production process for a highly expanded foam insulated
coaxial cable which advantageously utilizes the step of sinking
in the production of the coaxial cable.
This e~bodiment of the invention provides a process .
for producing a highly expanded foam insulated coaxial cable in
which a highly expanded foam insulation layer and an outer con-
ductor are sequentially provided on the inner conductor by pro-
viding an adhesive layer between the highly expanded foam insula-
tion layer and the outer conductor and, thereafter, providing the
outer conductor cover and sinking the same.
Figures 13 and 14 illustrate an example of coating and
sinking the outer conductor, wherein a tape-like outer conductor
56 is molded onto the highly expanded foam insulation layer 55
- 45 -
~ .
. . - ~ : .
using a sizing roller into circular body having diameter slightly
larger than that of the layer 55 coated onto the inner conductor
54. The butted portion 57 of the tape-like outer ~onductor 55
is welded by a welding machine 59 and this assembly is subjected
to a sinking by passing through the sinking die 60 and the highly
expanded foam insulation layer and the outer conductor are adhered
to each other tightly.
It has been found and utilized in this invention that
the cells contained in the outer portion of the highly expanded
foam insulation layer 55 are collapsed in this sinking and any
uneveness in the surface of the insulation layer is smoothed.
- According to this invention, the collapsed portions are filled
with adhesives thereby enabling tight adhesion bet~een the highly
expanded foamed insulation layer 55, which is diff:icult to
adhere, and the outer conductor having a smooth surface.
The outer conductor used in this process can include
plain tape or tubular aluminum, copper metal and a composite of
copper and adhesive plastics.
The adhesives which can be used herein include various
types such as solution types, two-component types, tape types,
heat setting types, heat fusing types, and the like. As shown
- in the drawing, the adhesive layer 58 of such an adhesive is
provided between a highly expanded foam insulation layer 55 and
an outer conductor 56, for example, by applying a coating of the
adhesive on the inner surface of the outer conductor 56 opposite
the butted portion 57 at the time of coating the highly expanded
foam insulation layer 55 with the outer conductor 56 in the form
of a plain tape while forming the tape into a circular cross-
section. The adhesive layer 58 thus provided covers the entire
surface to tightly adhere the insulation layer 55 and the outer
- 46 -
.
lS~
1 conductor 56 in the sinking. In using two-component type adhesives,
each component is applied separately and mixed to react at the time
of the sinkiny. In the case of the heat setting type and the heat
fusing type, of adhesive the adhesives are cured by the heat pro-
duced in the sinking and, if desirea, with additional external
heating to tightly adhere the insulation layer and the outer
conductor. Moreover, the adhesives are embeded in the cells
collapsed in the sinking or in the recesses in the surface of
the insula-tion layer to increase t~e adhes~on strength together
by an anchoring effect.
The process of this invention provides more advantageous
effects in comparison with the conventional process in which the
inner conductor is adhered to the insulation layer, and an
excellent coaxial cable can be obtained by employing a composite
~tape) as an outer conductor and adhering the same also to the
protection layer thereon. It can easily be understood that the
process of this invention is also applicable with more advantageous
effects to the production of a corrugated type coaxial cable in
which the surface of the outer conductors are corrugated.
.. .
This invention will now be further described by xeference
to the following non-limiting examples t~ereof. Unless otherwise
,.~. .
indicated, all parts and percents, etc. are b~ weight.
.
Example 1
, .
` 50% by weight of low density polyethylene having a
- density of 0.92 g/cm3 and a melt index of 1.0 and 5Q~ by weight
of high density polyethylene having a density of 0.95 g/cm3 and
melt index of 0.3 were uniformly compounded in a roll mixer at a
-` temperature of 160C and then pelletized. The pellets were
~~ 30 immersed in Freon*ll (monofluoro trichloromethane) at 20C and
swelled to a degree of swelling of 18~ (by weight).
*Trade Mark
' .:
- 47 -
.': i--~ .
;::
;-
,: - .- . . . : ~ -
~Q~ 5 7~
1 The swelled pellets were extruded using an extruder
~ith an inner diameter of 38 mm si2e at an extrusion temperature
of 130C to form an expanded foam body having a foaming ratio of
12.
The relationship between the extrusion temperature and
the foaming ratio was examined and it was found that, as shown
by curve 3 of Figure 2 that the foaming ratio can be con-trolled
more easily as compared with the curves 4 and S in which low
density polyethylene and high density polyethylene were respectively
used alone.
Example 2
The high density polyethylene used in Example l was
formulated with 1 part by weight of azodicarbonamide (this for-
mulation hereafter referred to as A) and with 1 part by weight
of zinc stearate (similarly referred to as B), respectively, and
blended uniformly in a mixer at a temperature of 160C and then
pelletized.
The Formulations A and B were blended in a ratio of
1:1 and then extruded after the swelling in the same manner as
described in Example 1 except that the extrusion temperature
was set at 145C to produce a finely and uniformly expanded foam
body.
For comparison, 0.5 parts by weight of each of azodi-
`~ carbonamide and zinc stearate were formulated with the high
density polyethylene to form Formulation C and compounded in a
roll mill at a temperature of 160C. The azodicarbonamide
was almost decomposed and the yellowish color characteristic
to azodicarbonamide completely disappeared. Compound C was
swelled and extruded as described above to result in an expandedfoam body containing coarse cells therein which were not applicable
to practical use also from the stand point of appearance.
- 48 -
Example 3
1 part by weight of azodicarbonamide was formulated with
high density polyethylene (density 0.95 g/cm3 : MI 0.3) and
pelletized at a temperature of 200C to form pelle-ts having a
foaming ratio of 1.3.
The pellets are immersed in Freon*ll and subjected to
two heat cycles of 60C and 23C to prepare a foaming compound
having a degree of swelling of 18% by weight. The time required
` for saturating the swelling was 50 hours.
The foaming compound was extruded to form an expanded
foam body (D) having a foaming ratio of 15.
Control Example
The high density polyethylene used in Example 1 (not
foamed) was immersed in Freon*ll under similar conditions as used
in the above examples. The swelled product thus obtained had a
degree of swelling of only 14~ by weight.
The foaming compound was extruded using the same con-
` ditions as described in the above examples and the foaming ratio
; of the resulting foam body (E) was proved to be only 8. This
ratio was about half that obtained with the foam body of the above
examples.
A foaming compound (D) prepared according to the Example
3 and a foaming compound (E) prepared in the same way as the
Control Example were left in air at room temperature tabout
~0-30C), and the time required for a reduction in the degree of
swelling to 10% was measured. The foaming compound (E) required ~ -
6 hours and the foaming compound (F) required about 3 hours.
.
: Example 4
The degree of swelling of polyolefîns by the volatile
~ liquid was measured. Each polyolefin was immersed in Fxeon 11*
: *Trade Mark
- 49 -
. . .
and Freon*113 and the saturated degree of swelling (increase in
weight ratio) was measured. The results obtained are set forth
below.
Polyolefln MP** Density MI*** Degree of Swelling
(C) (g/cm3) (wt. ~)
Freon 11 Freon 113
. _ _
Polypropylene 168 0.90 1 0 ~6.0 36.0
230C
1 0 .
Polyethylene 111 0.92 1.0 23.0 11.0
... ~ _._ ~_ .
.. 126 0.95 0.3 12.5 3.5
_ _ . . .. _
L _ ¦ 138 l ¦o.01
~* Melting Point
*** Melt Index
Example 5
The saturated degree of swelling was measured in the
same way as described in Example 4 except that the swelling was
effected in Freon*vapor. The results obtained are set forth below.
Polyolef1n Density MI Degree of Swelling
tC) (g/cm3) (wt. ~)
Freon*ll Freo~ 113
. . _ . _ . ... .. _ .. _
~ Polypropylene168 0~90 (130C)at 36.5 28.0
__ ___ _ _ _ ._ . ~
Polyethylene 111 0.92 1.0 2~.5 11.5
. .. _ .. _
ll 126 0.95 0.3 13.0 - 3.5
. ___ __ _ ._ _ . _
;~ 30 138 than 12.5 3 0
- *Trade Mark
: - S O -
: ~'
1 E~ample 6
A low density polyethylene having density of 0.92 g/cm3
and a MI of 1.0 was swelled with Freon*113 at 20C to a degree
of swelling of 10~ by weight and then extruded using the apparatus
as shown in Figure 3 with the pressure in the storage (drying)
tank and the hopper at 8 Kg/cm2 and at a temperature of 100 C
to obtain a uniformly expanded foam body having a foaming ratio
of 5.
When the foaming compound formed by similarly s~elling
these pellets was supplied to the conventional open t~pe extruder
and extruded at a temperature of 100C, the resulted expanded
: foam had a foaming ratio of only 3.
'
ExamDles 7 and 8
. . .. __, _ . _ .......... ... _._ _ :"
. ExamF le 7 Examp Le 8
Pellet Volatile Pellet . Volatile
Liquid Liquid
_ ... . . ._. _ . . :
Density: 3 Freon* Density: Freon*
. 0.92 g/cm 11 0.95 g/cm3 113 :~
MI : 1.0 . MI : 0.3 ~ :
.~ . .. ~ _ ._ ._ .................. .,
Degree o~ Swelling 22% w at 20 C 5% w at 0C
Pressure in Storage 2 . .
(drying) Tank & 10 Kg/cm 10 Kg/cm2
Hopper
Extrusion 100C 130C
Temperature .
Foaming Ratio of . .
the Expanded 21 (15**) 20 ~15**)
Foams Obtained . .
. .. _ . _ __ .
** Foaming ratio obtained with expanded foams in
a conventional open type hopper
*Trade Mark
- 51 -
~ 9'
:: , . .. .. . ..
Æxam?le 9 1~ J~
In this example, the relationship between the temperature
conditions used in the extrusion and the foaming ratio was examined
for the foaming polyeth~lene used in Example 2.
The extruder used was of a full flight type, 50 mm in
size and provided with a screw having a compression ratio of 1:3.
The extrusion temperature was linearly varied in the four divided
zones in the extruder. The temperature of the cross-head die was
set constant after C4. The results are set forth below.
: Temperature (C) Foaming Ratio .
~ . ._.. .. , _. ..
` Cl C2 C3 C4
. 200 180 160 140 .10.5
. 170 160 150 140 . 8.5
., 140 140 140 140 7.0
110 120 130 140 6.0
9S 110 125 140 5.0
_ .
Example 10
The foaming polyethylene used in Example 1 was extruded
for coating in the apparatus shown in Figure 6 at a resin temper-
ature of 130C and with a reduced pressure of 5 mmHg without
cooling the adapter and applied onto an inner conductor heated
at 120C. An insulated wire was obtained having the cross
section as shown in Figure 7(b). The foaming ratio of the expanded
foam was 10~5.
Example 11
. . .
The foaming compound used in Example 10 was extruded for
coating onto an inner conductor heated at 250C at the same resin
1~ .
- 52 -
`~ 3~
temperature and a degree of pressure reduction as in Example 10,
and without a cooling adapter. An insulated wire was obtained
having the cross section as shown in Figure 7(c). The foaming
ratio was 9Ø
Control_Example
The foaming compound used in Example 10 was extruded for
;~ coating at a resin temperature of 130C without effecting a p~ss~e
reduction, a cooling of t~e adapter and a heating of the inner
conductor. An insulated wire having the cross section as shown
in Figure 7(e) was obtained. The foaming ratio was 9.5.
~' , .
- Example 12
The foaming compound used comprises a mixture of low
density polyethylene (density : 0.92 g/cm , MI : 1.0), Freon~ll -
as a foaming agent, and p,p'-oxybisbenzenesulfonylhydrazide as a
nucleating agent in a ratio of 100:10:1 by weight, respectively.
The extruder shown in Figure ~ was used provided with a
die of a diameter of 12.5 mm, a parallel land of a length of
125 mm, a nipple with tapered angle of 30 and a fluorine resin
insulation material, and cooling was effected only with the die
portion using water at 20C. The foaming compound was extruded
for coating onto an inner conductor of copper wire having a
diameter of 25 mm and supplied from the nipple at the rate of - -
5 m/min. The pressure was adjusted using a vacuum pump to about
4 mmHg.
The product obtained had the cross section as shown in
Figure 10, which has a solid layer on the outer surface thereof
and the same outer diameter size of 12.0 mm as the die diameter.
The foaming ratio was 6.
In the comparison runs in which a vacuum method or a
water cooling of the die was not effected, the expanded foams
can not be applied to the inner conductor at all.
*Trade Mark
- 53 -
.
.. . . .
-;. ' . :
: - .
~V'l;~
1 The preferred adhesion between the expanded foams and
the inner conductor can be achiev~ed in the manner as shown in
Figure 8. Particularly, the best result was obtained with die
portion 41 which is radially extended at the contacting die 35
side approximately corresponding to the expanding configuration
of the expanded foams.
' Example 13
The polyethylene used in Example 2 was extruded for
coating onto an inner conductor of 4.4 mm a diametex of using a
50 mm extruder while varying the die diameter and the ratio of
the extrusion rate (calculated value per extrusion rate of the
inner conductor) as shown in the table below to produce an
expanded foam insulation layer of an outer diameter of 17 mm
having a foaming ratio of about 7. A sizing die (molding die)
was used to smoothen the surface of the èxpanded foams.
A coaxial cable was prepared by applying, on the
expanded foam layer, an aluminum outer conductor with an inner
diameter of 17 mm. Specimens were cut in a length of 50 cm from
the coaxial cable and repeatedly subjected to heat cycling for
ten times each comprising cooling for 1 hour at -20C and 1 hour
heating at +60C to measure the longitudinal shrink in the
expanded foams. The results are set forth together with the
relation between the ratio of the extrusion rate and the length
-~ of shrink in the expanded foams in the table below:
._. . .. . .__ .... ~
Die Diameter Length of Ratio of the Extrusion Rate
(mm) Shrink between the Inner Conductor
(mm) and the Foaming Polyethylene
. . . .
5.5 1 1 : 3.5
6.0 4 1 : 2.2
2.8 16 1 : 1.0
2.0 21 1 : 0.6
_ _ . _ . . _ _ ..
- 54 -
~t~
Example 14
Particulate low density polyethylene (density : 0.92
g/cm3; 2.0) was supplied to a squeeze die (inlet diameter : 20 mm,
exit diameter : 4.8 mm, and 30 mm in length) heated to 200C in
the apparatus shown in Figure 12 and rendered flowable by heat,
and an inner conductor (4.4 mm~ copper) preheated to 100C was
introduced therein at the rate of 5 m/min. Onto the inner con-
ductor thus insulated with this film in a thickness of about
0.15 mm, foaming polyethylene with foaming ratio of 6.5 is
extruded for coating in a thickness of 6.2 mm and an aluminum
outer conductor is further applied thereon for coating and
subjected to sinking thus to result in a highly expanded foam
insulated coaxial cable having an outer conductor of an inner
diameter of 17.0 mm.
The coaxial cable is suited for practical applications
having the following characteristics
:
.
Characteristic Impedance 750 ohms at 10 MHz
Attenuation 25 db/Km at 250 MHz
S R L 1.1 at 10 - 300 MHz
,
The measurement of the strength required to pull out and
separate the inner conductor and the highly expanded foam is
impossible because the expanded foams are collapsed at the time
of measuring due to the strong adhesion present between them.
The heating shrinkage of the coaxial cable insulator is
scarcely observed, when tested, at temperature below the melting
point of polyethylene.
The capacitance varies from 12 pF to 20 pF or more per
0.25 m when cooled through a water cooling trough at the time of
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.:
': '
1 extrudin~ foaming polyethylene, but the capacitance variation is
~liminated by o~itting the wa-ter cooling.
When a conventional caterpillar capstan is used for
the take-up capstan, deformation occurs in ~he outer diameter
of the expanded foams in more than ~ 1 mm. This deformation can
be reduced to less than ~ O 5 mm hy applying polyurethane foam
lining on the caterpillar capstan.
Example 15
A copper wire of a diameter of 4.4 mm as an inner
conductor was coated with foaming polyethylene having a foaming
ratio of 7 to produce a highly expanded foam insulated wire of
a diameter of 17.5 mm. Then, a 20% solution of rubber solution
type SEIKA Olefin MA-E 30 (Trade mark: available from Seitetsu
Kagaku Kogyo Co. Ltd.) in toluene was continuously applied as
an adhesive on the inner surface of an aluminum sheet tape of
a thickness of 0.9 mm when the insulated wire was coated with
the aluminum as an outer conductor while being shaped into a
circular body through a forming die having a diameter of about
20.0 mm. T~en, the butted portion of the aluminum tape was
longitudinally welded and sinking was effected using-a sinking
die of a diameter of 18.8 mm at a temperature of about 150C and
with a feeding velocity of about 10 m/min to produce a coaxial
cable having an outer conductor of an outer diameter of 17.0 mm.
Example 16
The same outer conductor as used in Example 15 was
applied in the same way onto the same highly expanded foam
insulated wire as described in Example 15 while longitudinally
providing on both sides a cohesive tape NP 111 (rubber type
adhesive trade name: available from Sony Chemical Co. Ltd.~ as
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~ .
- ~
~U'~3~
1 a tape type of adhesive along the aluminum tape and then perform-
ing welding and sinking as described in Example lS to produce a
coaxial cable having an outer conductor of a diameter o~ 17.0 mm.
Example 17
: .
The same outer conductor as described in Example 15 was
applied in the same way onto the same highly expanded foam insu-
lated wire as described in Example 15 while continuously applying -
an epoxy series Bond-quick set (rubber type adhesive trade name:
available from Konishigisuke Co.) as a two-component type adhesive
. 10
on the inner surface of the aluminum tape. Then, the welding
and sinking were effected as described in Example 15 to produce a
coaxial cable having an outer conductor of a diameter of 17.0 mm.
. .
Example 18
The same outer conductor as described in Example 15 was
applied in the same way onto the same highly expanded foam insu-
lated wire as described in Example 15 while longitudinally pro-
viding a copolymer resin Bond-fast tape (rubber type adhesive
trade name: Sumitomo Chemical Co., ~td.) as a heat fusing type
adhesive along the aluminum tape. The welding and sinking were
performed in the same way as described in Example 15 to produce
a coaxial cable having an outer conductor of a diameter of 17.0 mm.
Reference Example
., :
The strength required to pull the inner conductor from
the highly expanded foam of the coaxial cables in Examples 15 to
1~ were tested using a tensile tester in the manner as shown in
Figure 15, and the adhesion strength between the highly expanded
foam insulation layer and the outer conductor was measured using
this pull-out strength. In Figure 15, an inner conductor 54, an
j 30 insulation layer 55 of expanded polyethylene, an outer conductor
.: .. . . , . , ' ~
~3'~3~
1 (aluminum) 56, a butt strap 61 having an aperture a little larger
than the outer diameter of the insulation layer and a chuck 6~
for uniformly clamping the entire periphery of the insulation layer.
The arrow indicates the direction of the pull-out. The length of
a sample was 50 cm and the pull-out velocity was 100 mm/min. The
test results obtained are set forth in the table below compared
with the data obtained from a cable in which an adhesive layer was
not provided between the highly expanded foam insulation layer and
the outer conductor.
._ .. . ~
Sample Pull-out Strength
. _ ._ _ _ _ ___ . _ . .. _ . _
Control Sample 20 Kg
(without adhesive layer)
Example 15 more than 30 Kg (expanded
polyethylene insulation
layer ruptured)
Example 16 .. "
Example 17
Example 18
.,
When welding and sinking the outer conductor of the
samples of Examples 15 - 18 and the Comparison Example, the outer
surfaces of the outer conductors are water cooled in order to
avoid deformation in the expanded foams.
While the invention has been described in detail and
with reference to specific embodiments thereof, it will be
apparent to one skilled in the art that various changes and
modifications can be made therein without departing from the
spirit and scope thereof.
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