U.S. patent number 3,668,298 [Application Number 04/883,973] was granted by the patent office on 1972-06-06 for multiconductor communications cable.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to Walter L. Hawkins.
United States Patent |
3,668,298 |
Hawkins |
June 6, 1972 |
MULTICONDUCTOR COMMUNICATIONS CABLE
Abstract
A multiconductor communications cable includes a hydrophobic
material filling the space between primary insulated conductors.
Compatible filler-insulation materials designed for long-life
stabilization are described. Insulation materials include various
ingredients which stabilize the insulation and resist extraction by
the filler.
Inventors: |
Hawkins; Walter L. (Montclair,
NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
25383697 |
Appl.
No.: |
04/883,973 |
Filed: |
December 10, 1969 |
Current U.S.
Class: |
174/23C;
174/113R |
Current CPC
Class: |
H01B
7/28 (20130101); H01B 3/441 (20130101); H01B
11/00 (20130101) |
Current International
Class: |
H01B
7/28 (20060101); H01B 3/44 (20060101); H01B
7/17 (20060101); H01B 11/00 (20060101); H02g
015/20 () |
Field of
Search: |
;260/45.9,95
;174/116,23R,23C,113 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldberg; E. A.
Claims
1. In a communications cable comprising a plurality of insulating
conductors, said conductors including, as primary insulation, a
plastic composition consisting essentially of a polymer at least 75
percent of which may be considered as having been polymerized from
propylene, the interstices of which are substantially filled by a
hydrophobic substantially hydrocarbon filler which includes
petroleum jelly as a primary ingredient, said primary insulation
contains at least one modifier each from the groups of chain
terminators, peroxide decomposers and metal deactivators, the chain
terminator being selected from the group consisting of elemental
sulfur, 4,4' thiobis(2 tert. butyl-5 methyl phenol), 2,2' thiobis(6
tert. butyl-4 methyl phenol), thiobis beta naphthol; the peroxide
decomposer being selected from the group consisting of dinaphthyl
disulfide, tetramethyl thiuram sulfide and elemental sulfur and
polymers containing repeating units meeting the foregoing
requirements; and the metal deactivator being selected from the
group consisting of Di-2-pyridyl oxamide, Di-p-tolyl oxamide,
Di-o-chlorophenyl oxamide, Di-p-ethoxyphenyl oxamide, Dibenzyl
oxamide, Dimethyl oxamide, Di-p-chlorophenyl oxamide, Di-allyl
oxamide and Di-n-hexyl oxamide, in which each of the said modifiers
is contained in an amount of from 0.1 percent to 1.0 percent by
weight of the total composition and in which the ratio of peroxide
decomposer to chain terminator is from 1:1 to 4:1.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is concerned with multiconductor communication cables
in which water penetration is minimized by inclusion of a
hydrophobic material filling the space between insulated
conductors.
2. Description of the Prior Art
In Vol. 47, Bell Laboratories Record, p.70 (Mar. 1969), there is
described a cable structure in which water penetration is minimized
by use of a filler material in the interstices between primary
insulated conductors. As indicated in that article, conventional
cable structures in which interstices are occupied by air or other
gases, while suitable for many purposes, are subject to various
types of malfunction due to water penetration. This water
penetration which is most significant in underground installations,
while not particularly significant when present locally, may,
through capillary action, fill exceedingly long lengths of cable.
The ultimate effect is somewhat dependent upon the frequency of
which the cable operates. In high frequency operation, the mere
presence of the water between unbroken insulated sections adversely
modifies the dielectric properties of the cable. In ordinary voice
frequency installations, significant difficulty is ordinarily
encountered only upon actual penetration of the water through the
insulation. This, however, is not an unusual occurrence in long
insulation lengths. Punctures may be due to manufacturing defects
or to mishandling, particularly in the very thin layers (10 mils)
in prevalent use.
Recognizing this difficulty, there has been a widespread effort
directed toward alleviation of the problem. A significant
development which shows some promise involves the use of a
water-repellent material filling the air spaces between insulated
conductors. The cited reference outlines this approach. In
accordance with the article, the hydrophobic filler material, which
may include petroleum jelly as a primary ingredient, is introduced
in a heated fluid form. Upon cooling, the filler becomes
sufficiently rigid so as to be retained in position.
Such filler materials, while eminently satisfactory for water
exclusion have introduced concomitant difficulties. For example,
where utilized with ordinary polyethylene primary insulation, the
filler has produced swelling and also stabilizer extraction.
Attempts to overcome this difficulty have included substitutions of
polypropylene insulator compositions. While this substitution has
largely avoided the swelling problem, it is observed that
stabilizer impairment is still present. This difficulty is the more
pronounced due to the inherent instability of polypropylene which,
as is well known, has required larger concentrations of oxidation
inhibitors for given life expectancy. In fact, it is this known
susceptibility to oxidation which has limited the use of
polypropylene for many long-term applications.
SUMMARY OF THE INVENTION
In accordance with the invention, a long-life cable structure of
the type described in the preceding section utilizes critical
specified primary insulation compositions which remain
satisfactorily stabilized in contact with filler. As in some
preceding structures, the base polymer contains at least 75 percent
of polypropylene which may be present as a constituent in a blend
or as an equivalent amount of homopolymer based on the initial
amount of monomer in a copolymer.
The advantage of the inventive structure resides primarily in the
nature of the constituents which are designed to protect against
oxidative degradation. It is convenient to consider stabilization
against oxidation degradation in terms of three categories, and
ordinarily compositions utilized in accordance with the invention
contain at least one ingredient for protection in each category. In
one embodiment, however, a single ingredient serves simultaneously
in each of two categories.
The first stabilizing ingredient is sometimes thought of as a chain
terminator. This is characteristically a labile hydrogen donor
which contributes its hydrogen to terminate an oxidative chain.
The second category is concerned with the hydroperoxides which are
intermediate decomposition products. These intermediate products,
if not inactivated, take part in further autocatalytic oxidation of
the polymer. It is the function of stabilizers in this category to
decompose such peroxides and thereby render them inert.
The third category is designed to inhibit the catalytic effect that
certain metals, such as copper, are known to have on oxidation
processes.
In accordance with the invention, ingredients in each category are
so selected that their extraction by normal hydrophobic filler
materials is minimal. Selection of appropriate ingredients in each
category is extremely critical and, accordingly, acceptable
categories are set forth with considerable specificity.
In addition to the stabilizing ingredients which are of primary
concern, usual design objectives required certain additional
ingredients. In general, selection of such additional ingredients
has not posed a problem aggravated by use of a hydrophobic filler.
Such additional ingredients include pigments for color code
identification, and processing aids, serving mainly as lubricants
during extrusion.
A significant additional ingredient is concerned with economics and
contemplates dilution of the primary insulation with a cheaper
material. Ingredients serving this function by introduction of gas
include nucleating agents for providing bubble sites and blowing
agents for providing the gaseous expander.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a cross-sectional view of a cable containing a
hydrophobic filler and utilizing a primary insulation composition
in accordance with the invention; and
FIG. 2, on coordinates of oxygen uptake in cubic centimeters per
gram and time in hours, is a plot including several curves
representative of accelerated test data for compositions of the
invention. Also included is a commercial composition for comparison
purposes.
DETAILED DESCRIPTION
1. drawing
The cable structure 1, depicted in FIG. 1, is composed of an outer
sheathing 2 which is ordinarily predominantly polyethylene and
which encloses a metallic layer 3. This metallic layer, which may
be aluminum, is designed to lend structural rigidity, lightning
protection and a means of dissipating heat which may be generated
locally. Region 4 is a core wrap. This generally takes the form of
an overlapping spiral wrapping of a suitable material. Region 5,
contained within wrap 4, includes the primary conductors 6 which
may be of copper or other suitable metal, each of which is
electrically insulated by primary insulation 7. Interstitial
positions 8, shown in shaded cross section, are, in the concerned
structure, filled with a hydrophobic filler material.
The structure depicted is merely illustrative and may take other
forms. Under specific environmental conditions, there may be
additional metallic layers to protect against animal life as well
as additional plastic sheathing layers. Other forms may be designed
for high frequency use and, to this end, may contain one or more
conductor pairs of coaxial configuration.
2. Concerned Compositions
a. The Primary Insulation
The nature of the primary insulation, as unmodified, has been
briefly discussed. Since the problem to which the invention is
addressed concerns extraction of modifying ingredients from
polypropylene, it is required that the primary insulation contain a
sufficient amount of this polymer for the extraction problem to be
significant. It has been stated that the equivalent amount of
polypropylene in the primary insulation, at least at the surface
exposed to the hydrophobic filler, must be a minimum of 75 percent
by weight polypropylene. Since both simple blends and copolymers
are contemplated, the 75 percent figure has reference either to a
distinguishable amount of polypropylene or to the amount of
propylene monomer required to produce a copolymer within the
specified composition range.
Additional ingredients may be polymers or monomers of ethylene,
vinyl monomers such as vinyl acetate, butene-1, acrylic acid,
methacrylic acid, and their corresponding esters as well as similar
vinyl monomers.
b. Insulation Modifiers
The first category of modifiers is conventional. These include
colorants, processing aids, blowing agents, etc. While some
colorants are subject to extraction by the hydrophobic filler,
materials now in use do not appear to suffer significantly from
this problem. Extraction of processing aids and of blowing agents
is of little concern since they are intended to serve no function
in the completed product.
In the main, disadvantages in earlier structures have been
concerned with the stabilizer systems, and systems considered
appropriate in accordance with the invention are now discussed in
some detail under the three categories noted above.
Chain Terminators
This ingredient is ordinarily included in the amount of from 0.1 to
1.0 percent by weight. Chain termination is dependent upon one or
more functional hydroxyl groups. However, while there is a large
category of materials which owe their effectiveness to this
grouping, only a selected few have been shown to be appropriately
included in the inventive structure. The basic chain terminators
found suitable are classified as thiobis phenols. Typical examples
include:
4,4' thiobis(2 tert. butyl-5 methyl phenol);
2,2' thiobis(6 tert. butyl-4 methyl phenol); and thiobis beta
naphthol.
Isomers of any of the above materials as well as modifications in
which cyclic moieties contain additional hydrocarbon or other
substituents are also effective. Generally, hydrocarbon
substituents, where present in the named examples or where added in
any modification, should result in a total of no more than about 10
carbons in each moiety, it being observed that further increase
results in a significant loss in protection.
Peroxide Decomposers
Peroxide decomposers are all dependent upon either free sulfur or
compounds containing two or more bonded sulfurs. Interestingly,
most monosulfides have been determined to be ineffective for this
use in a long-life product. Exemplary disulfides are listed:
1. Tetramethyl thiuram disulfide
2. Tetraethyl thiuram disulfide
3. Dinaphthyl disulfide
Several variations are permitted. For example, any of the listed
compounds may contain simple alkalyl substituents although, again,
where such substituents are present on a cyclic moiety, the total
number of carbons in the substituents should not exceed 10. Other
substituents either on cyclic or aliphatic compounds may include
elements known to be harmless to the polymer. These include nitro
groups, halogens, carboxyls and various esters. Generally, the
aliphatics have been found to be of decreasing effectiveness as the
chain length of substituents increases and, for this purpose, a
limit of 8 carbons is imposed. Substituents may also be cyclic and
the basic cyclic grouping in the moiety may be heterocyclic as well
as hydrocarbon. An example is morpholine disulfide. An additional
class of materials useful in this category is the polymers
including repeating units of which meet the requirements set forth.
An example is the polymer formed from 1, 10 dimercapto-decane.
Permissible amounts of peroxide decomposers are, again, in the
range of from 0.1 to 1.0 percent by weight based on total
composition.
Elemental sulfur may serve both as a chain terminator and peroxide
decomposer and, if unsupplemented in either function, may be
included in amounts of up to 2 percent by weight.
Copper Deactivators
The preferred material in this category is oxanilide (diphenyl
oxamide) although other oxamide derivatives may be substituted.
Such derivatives include:
Di-2-pyridyl oxamide
Di-p-tolyl oxamide
Di-o-chlorophenyl oxamide
Di-p-ethoxyphenyl oxamide
Dibenzyl oxamide
Dimethyl oxamide
Di-p-chlorophenyl oxamide
Di-allyl oxamide
Di-n-hexyl oxamide
Inclusion is within the range of from 0.1 to 1.0 weight
percent.
c. The Hydrophobic Filler
Filler materials must fulfill certain physical requirements. For
example, they must have significantly low viscosity to fill the
interstices at extrusion temperatures which are ordinarily in the
range of from 165.degree. to 250.degree. C, and must be
sufficiently rigid at operating temperatures to be retained in
position. It has been previously determined that a composition
suited to this end consists of a blend of 80-89 percent petroleum
jelly, remainder a polyolefin having a molecular weight of the
order of 20,000 or greater. Suitable alternatives include blends of
higher molecular weight polymers which may contain lesser amounts
of high molecular additives, and also low molecular weight polymers
such as, for example, polyethylene having a molecular weight of
less than 5,000.
3. Test Data
An extensive series of tests has been conducted to determine the
effectiveness of the inventive compositions. The basic accelerated
procedure is now well established and has been in use for many
years. It involves heating the sample under test in an
oxygen-containing atmosphere and measuring oxygen uptake per unit
volume of sample. The trend of such data as represented on
rectilinear coordinates of oxygen uptake and time is familiar.
Representative curves may be regarded as being made up of two
essentially straight line portions, the first commencing at the
origin and of small slope while the second and subsequent section
is of substantially steeper slope. As for typical antioxidant
protected plastics, the break in slope represents the commencement
of autocatalytic breakdown; and for many purposes, this point (at
which the slop changes) is considered to represent the termination
of useful life of the plastic. For usual purposes, the precise
value considered is that of the intercept of the autocatalytic
portion of the curve with the abscissa. The period from the origin
to the time so defined is commonly referred to as the "induction
period." Curves on FIG. 2 corresponding with inventive compositions
show this general form. While precise equivalent operating
temperature lifetime under real conditions corresponding with the
test data presented cannot be specified due in part to the
variation in conditions under which cables operate, the induction
periods for all inventive compositions represented on FIG. 2 and
for all compositions otherwise defined in the specification
correspond with minimum useful life of the order of 20 years or
greater. For most purposes, this is considered adequate. The
commercial sample, in accordance with the data presented, does not
result in a sufficiently long protected life to be useful in
accordance with most cable standards.
The test procedure is presented:
Test compositions were milled in an atmosphere of nitrogen. The
antioxidant mixtures were added to 35 grams of unprotected
polypropylene with mixing at 180.degree. to 190.degree. C for 5
minutes at a speed of 50 rpm. Copper dust was then added to the
concentration of 1.4 percent and mixing was continued for 5
minutes. The composition was then removed and 10 mil sheets were
molded on an electric press. Test samples were cut from these
sheets.
Test samples weighing between 0.6 and 0.9 grams were covered with
an equal weight of the filler mixture by smearing over one surface.
The samples, in contact with filler, were then heated at 70.degree.
C for 40 hours. After removal from the oven, the filler was wiped
off with paper towels, and samples were cut from the treated
material for oxygen-uptake measurements.
Samples weighing approximately 0.1 grams were oxidized in an
atmosphere of pure oxygen at 120.degree. C. The oxidations were
carried out in an aluminum block with duplicate samples for each
composition. The rate of reaction with oxygen was measured through
the period of autocatalysis, and the steady-state rate of oxidation
was extrapolated back to the time axis in the conventional method
for determining the induction period.
4. FIG. 2
Each of the curves represents oxygen uptake data conducted in
accordance with the generalized procedure set forth above.
Conditions for each set of data were identical. Based on prior
experience, it has been determined that the stabilizer most
significantly impaired by extraction is the peroxide decomposer.
The data in FIG. 2 shows a variation in the character and amount of
this particular ingredient. Five curves are presented.
Curve 10
Curve 10 corresponds with an undesignated proprietary composition
prepared by a commercial compounder specifically for use in the
filled structure which is the subject for this application. The
stabilizer system is known to contain at least three and possibly
four members, and it is considered as the best commercially
available composition for this purpose. From curve 10, it is seen
that the induction period after exposure to filler is of the order
of 2 hours.
The following curves, for comparison purposes, utilize 4,4' thiobis
(2 tert. butyl-5 methyl phenol) which at this time is probably the
most prevalently used chain terminator for communications cable
insulation. The metal deactivator, probably the least affected of
the stabilizer ingredients, is oxanilide. For the purpose of this
figure, experimental data utilizing three peroxide decomposers was
used. Additionally, composition variations within one particular
system are represented.
Curve 11
The peroxide decomposer is tetramethyl thiuram disulfide. The
amounts by weight of each of the three stabilizer ingredients is
0.5 percent. The induction period is approximately 70 hours.
Curve 12
Again, utilizing 0.5 percent by weight of each of the three
stabilizer ingredients, however, substituting elemental sulfur as
the peroxide decomposer, it is seen that the induction period is
now 94 hours.
Curve 13
The peroxide decomposer is dinaphthyl disulfide. Amounts of the
three members are 0.8 percent of peroxide decomposer, 0.2 percent
of chain terminator, and 0.5 percent of metal deactivator. The
induction period is 60 hours.
Curve 14
Still utilizing dinaphthyl disulfide as the peroxide decomposer but
with a sample containing 0.8 percent of the peroxide decomposer and
0.2 percent of the chain terminator, it is seen that the induction
period has risen to 100 hours. While the difference in induction
periods is experimentally significant and while data represented by
curve 14 is certainly definitive of a preferred embodiment, all
induction periods for curves 11 through 14 are considered adequate
for commercial installation. The preferred embodiment represented
by data of which curve 14 is exemplary may be represented in terms
of the preferred peroxide decomposer to chain terminator ratio of
from 1:1 to 5:1.
* * * * *