U.S. patent number 3,861,029 [Application Number 05/287,444] was granted by the patent office on 1975-01-21 for method of making heater cable.
This patent grant is currently assigned to Raychem Corporation. Invention is credited to Robert Smith-Johannsen, Jack McLean Walker.
United States Patent |
3,861,029 |
Smith-Johannsen , et
al. |
January 21, 1975 |
METHOD OF MAKING HEATER CABLE
Abstract
Described herein are self-regulating conductive articles
comprised of an extruded length of polymeric material containing
not more than about 15 percent by weight conductive carbon black,
the resistivity of the extrudate following prolonged exposure to
temperatures in excess of the crystalline melting point or range of
the polymeric matrix in which the black content satisfies the
equation: 2 L + log.sub.10 R .ltoreq. 45. Wherein L is percent by
weight black and R is resistivity of the extrudate expressed in
ohm-cm. The articles exhibit room temperature resistivity in the
range from about 5 to 100,000 ohm-cm and may be employed, e.g., in
heat tracing and thermostating applications.
Inventors: |
Smith-Johannsen; Robert
(Portola Valley, CA), Walker; Jack McLean (Portola Valley,
CA) |
Assignee: |
Raychem Corporation (Menlo
Park, CA)
|
Family
ID: |
23102943 |
Appl.
No.: |
05/287,444 |
Filed: |
September 8, 1972 |
Current U.S.
Class: |
29/611; 219/528;
219/544; 219/552; 264/346; 392/468; 29/610.1; 219/549; 252/511;
338/214; 392/480 |
Current CPC
Class: |
H01C
7/027 (20130101); Y10T 29/49083 (20150115); Y10T
29/49082 (20150115) |
Current International
Class: |
H01C
7/02 (20060101); H05b 003/10 () |
Field of
Search: |
;338/31,22,28,211,212,214,213 ;252/511 ;29/610,611 ;264/346 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldberg; E. A.
Attorney, Agent or Firm: Lyon & Lyon
Claims
We claim:
1. A method of forming an electrically conductive self-regulating
article which comprises the steps of ( 1) extruding onto a pair of
elongate parallel electrodes held in spaced-apart relation an
electrode-interconnecting web of a composition consisting
essentially of a thermoplastic crystalline polymeric material
exhibiting overall at least about 20 percent crystallinity as
determined by x-ray diffraction and (b) conductive carbon black,
the percentage by weight (L) of carbon black based on the total
weight of said composition being not greater than about 15, the
resulting extrudate exhibiting room temperature resistivity (R,
ohm-cm) greater than about 10.sup.7, and (2) annealing the
extrudate at or above the melting temperature of said crystalline
polymeric material for a period of time sufficient to reduce R to
satisfaction of the equation
2 L + S log.sub.10 R .ltoreq. 45,
the value L being not more than about ten, annealing being
performed at a temperature of at least about 300.degree.F over a
period of not less than about 15 hours.
2. A method according to claim 1 wherein said polymeric material is
a blend of polyethylene and, in minor proportion relative thereto,
a copolymer of ethylene and a vinyl ester.
Description
BACKGROUND OF THE INVENTION
Electrically conductive thermoplastic compositions have previously
been achieved by the addition of conductive carbon black to a
polymeric base. In one category of such compositions, advantage has
been taken of a non-linear positive temperature resistivity
coefficient displayed by the particular material to obtain
self-regulating or current-limiting semiconductive articles. In
U.S. Pat. No. 3,243,753 to Kohler, one such composition is
described as containing from 25 to 75 percent carbon black about
which the polymeric matrix has been formed by in situ
polymerization. As the temperature of such a composition increases,
either through a rise in ambient temperature or by reason of
resistive heating occasioned by the passage of current
therethrough, the polymer matrix expands at a rate greater than
that of the carbon black particles which, in an interconnected
array of channels, impart the property of conductivity. The
resulting diminution in the number of current-carrying channels
decreases the amount of power generated by I.sup.2 R heating. This
self-limiting feature may be put to work in, eg, heat tracing pipes
in chemical plants for freeze protection, maintaining flow
characteristics of viscous syrups, etc. In such applications,
articles formed from the conductive composition ideally attain and
maintain a temperature at which energy lost through heat transfer
to the surroundings equals that gained from the current. If the
ambient temperature then falls, increased heat transfer to the
surroundings is met by increased power generation owing to the
resistivity decrease associated with the article's lowered
temperature. In short order, parity of heat transfer and power
generation is again attained. Conversely, where ambient temperature
increases heat transfer from the conductive article is reduced and
the resistivity rise resulting from increased temperature
diminishes or stops I.sup.2 R heating.
Self-regulating conductive compositions may, of course, be used in
employments other than resistive heating, for example, in heat
sensing and circuit-breaking applications. In every case, however,
the high carbon black content characteristic of most prior art
compositions is disadvantageous. High black loadings are associated
with inferior elongation and stress crack resistance, as well as
low temperature brittleness. In addition, high black loading
appears to adversely affect the current-regulating properties of
the conductive compositions. If a semi-conductive thermoplastic
composition is externally heated and its resistivity plotted
against temperature (on the abscissa) the resulting curve will show
resistivity rising with temperature from the low room temperature
value (Ri) to a point of "peak resistance" (Rp), following which
additional increase in temperature occasions a precipitous
resistivity drop associated with the melt phase of the polymer
matrix. To avoid resistance runaway with the concomitant
irreversible change in resistivity characteristics, the practice of
cross-linking the polymer matrix has grown up, in which event
resistivity levels off at the peak temperature and remains constant
upon further increase in ambient temperature. Cross-linked
semiconductive articles with high black loadings exhibit
undesirably low resistivity when brought to peak temperature by
exposure to very high or low ambient temperatures. In such
instances poor heat transfer characteristics can prevent
dissipation of I.sup.2 Rp generation, causing burnout.
It would accordingly be desirable to prepare semiconductive
self-regulating articles with substantially lower black contents,
with the objects, inter alia, of improving flexural and other
physical properties and substantially increasing the ratio Rp/Ri.
However, attainment of these goals has in large part been precluded
by the extremely high room temperature resistivities exhibited by
polymers with low black loadings. In Cabot Corporation's Pigment
Black Technical Resport S-8, entitled "Carbon Blacks for Conductive
Plastics" percent carbon-resistivity curves for various polymers
containing "Vulcan XC-72," an oil furnace black, show resistivities
of 100,000 ohm-cm or more, asymptotically increasing at black
loadings of about 15 percent. Others have reported similarly high
resistivities with low black loads. Recently resistivities
sufficiently low for freeze protection applications have been
achieved with low black loadings by resort to the special
deposition techniques, such as solvent coating, disclosed in
commonly assigned copending U.S. Pat. application S.N. 88,841,
filed Nov. 12, 1970 by Robert Smith-Johannsen, and now abandoned.
Self-limiting compositions have been extruded heretofore, eg, U.S.
Pat. No. 3,435,401 to Epstein, but when low black loading has been
attempted the extrudates have exhibited room temperature
resistivities of 10.sup.7 ohm-cm or higher, essentially those of
the polymer matrices themselves. Indeed, the patentees in G.B. Pat.
No. 1,201,166 urge the avoidance of hot melt techniques where
significant conductivities are desired with less than about 20
percent black.
SUMMARY OF THE INVENTION
We have now for the first time obtained self-limiting extrudates
advantaged by low black loading yet exhibiting room temperature
(hereafter, 70.degree.F) resistivities in the useful range from
about 5 to about 100,000 ohm-cm, the relation of the carbon black
loading and room temperature resistivity satisfying the
equation
2 L + 5 log.sub.10 R .ltoreq. 45
wherein L is the percentage by weight of the carbon black in the
extruded composition. After extrusion in conventional fashion, we
have learned, resistivity can be greatly reduced by subjection of
the yet uncross-linked article to thermal structuring according to
a time-temperature regime far more severe than that which
heretofore has been employed for strain relief or improved
electrode wetability, eg., exposure to 300.degree.F for periods on
the order of 24 hours. The resulting articles are suitable for
freeze protection and other self-limiting applications, exhibit
high Rp/Ri, and are otherwise advantaged by low black content. In
particular and unlike extrudates with high black content, their
resistivity-temperature properties are stable in storage and
unaffected by temperature cycling.
The manner in which these and other objects and advantages of the
invention are attained will become apparent from the detailed
description which follows and from the accompanying drawing in
which:
FIG. 1 is a cross-sectioned end-on view of one jacketed extrudate
formed according to the practice of this invention; and
FIG. 2 is a flow chart which depicts the steps of the preferred
manner of obtaining jacketed extrudates like those depicted in FIG.
1.
DETAILED DESCRIPTION OF THE INVENTION
In order to obtain self-limiting compositions, the polymeric matrix
in which conductive black is dispersed in whatever proportion must
exhibit overall an appropriately non-linear coefficient of thermal
expansion, for which reason a degree of crystallinity is believed
essential. Generally, polymers exhibiting at least about 20 percent
crystallinity as determined by x-ray diffraction are suited to the
practice of the invention. Among the many polymers with which the
invention may be practiced are polyolefins such as low, medium and
high density polyethylenes and polypropylene, polybutene-1,
poly(dodecamethylene pyromellitimide), ethylene-propylene
copolymers and terpolymers with non-conjugated dienes,
polyvinylidine fluoride, polyvinylidine
fluoride-tetrafluoroethylene copolymers, etc. As will be recognized
by those skilled in the art, limiting temperatures tailored to the
application intended (eg., freeze protection, thermostatting, etc.)
may be obtained by appropriate selection of polymeric matrix
material. For example, elements which self-limit at temperatures on
the order of 100.degree.F, 130.degree.F, 150.degree.F, 180.degree.F
and 250.degree.F may be produced with, respectively,
wax-poly(ethylene-vinyl acetate) blends, low density polyethylene,
high density polyethylene, polypropylene and polyvinylidene
fluoride. Other criteria of polymer selection will, in particular
instances, include desired elongation, environmental resistance,
ease of extrusibility, etc., as is well known.
Particularly preferred matrix materials are multi-component blends
in which black is mixed with a first blend component to form a
master batch which is in turn mixed with the principal polymeric
component. The first and second polymer blend components are chosen
such that they exhibit a positive free energy of mixing, one with
the other. Their attendant incompatibility apparently has the
effect of segregating contained black into generally delimited
regions of the polymer matrix, and such blends have proven
extremely stable in the face of temperature cycling in use. In the
case of single component matrices, cycling has occasionally had the
effect of requiring that successively higher temperatures be
attained to provide identical wattage values. Of course, even in
the case of single component matrices, the low black loadings
achieved according to this invention can result in satisfactory
stability to cycling. Typically, the minor polymeric blend
component is chosen for superior compatibility with carbon black
relative to the blend component present in major proportion, while
the latter component is selected for the particular physical
properties desired in the overall extrudate. The principal blend
component is preferably present in at least about 3:1 weight ratio
relative to the minor component with which the black is first
mixed. Presently, the blends most preferred have a polyethylene as
the principal component, the other being an ethylene-vinyl ester
copolymer, such as ethylene-vinyl acetate or ethylene-ethylacrylate
copolymers. An especially preferred extrudate contains about 70:20
polyethylene: ethylene-ethyl acrylate copolymer by weight.
The carbon blacks employed are those conventionally used in
conductive plastics, eg., high structure varieties such as furnace
and channels blacks. Other conventional addends such as
antioxidants, etc., may be employed provided only that their
quantities and characteristics do not subvert the objects of the
invention. An especially interesting class of beneficial addends,
it has been found, are materials such as waxes which, while
compatible with the predominant blend component, melt at lower
temperature. The result is to permit obtainment of a given wattage
at lower temperature, owing to a first peaking effect of the wax on
the resistivity-temperature curve. Compounding is conventional and
generally involves banburying, milling and pelletizing prior to
pressure extrusion of the self-limiting element from the melt.
In the preferred embodiment, as will appear from FIG. 1, the
black-containing matrix 1 is extruded onto a spaced-apart pair of
elongate electrodes 2 to form an element rod-shaped or, most
preferably, dumbell-shaped in cross-section, the extruded
thermoplastic both encapsulating and interconnecting the
electrodes. Thereafter, polymeric jackets 3 and 4 may be extruded
thereover, as in the fourth and sixth steps of the flow chart which
is FIG. 2.
Now, in the freeze protection applications in which self-limiting
elements are most commonly employed it is desirable that at least
about 4-8 watts per foot be available for transfer to ambient. With
commonly available voltages ranging from 120 to 480 volts,
resistivity values must be in the range from about 6,000 to 100,000
ohm-cm in order to generate 4-watts per foot and, of course, lower
at a particular voltage to obtain as much as 8 watts/foot. However,
we have found that following extrusion of compound containing not
more than about 15 percent by weight carbon, room temperature
resistivity is greater than about 10.sup.7 ohm-cm, and most
commonly on the order of the resistivity of the dielectric polymer
matrix itself. At such resistivities available wattage under power
is essentially zero. We have learned that enormous increases in
conductivity of such extrudates may be obtained by subjecting the
extrudate to temperatures above the melt for periods substantially
longer than those which heretofore have been employed to improve
electrode wetting, etc., when self-limiting articles were achieved
by other methods. By so doing, we having attained resistivities
ranging from 5 to about 100,000 ohm-cm with carbon contents not
greater than about 15 percent, and indeed have commonly achieved
room temperature resistivities well below 10,000 ohm-cm even at
black loadings less than about 10 percent. The thermal structuring
process apparently involves microscopic movement of carbon
particles of a sort not commonly associating with "annealing,"
although that term is employed herein for the sake of
convenience.
Annealing is performed at a temperature greater than about
250.degree.F, preferably at at least about 300.degree.F, and in any
case at or above the melting point or range of the polymeric matrix
in which the carbon black is dispersed. The period over which
annealing is effected will, it will be appreciated, vary with the
nature of the particular matrix and the amount of carbon black
contained therein. In any case, annealing occurs over a time
sufficient to reduce resistivity of the annealed element to
satisfaction of the equation 2 L + 5 log.sub.10 R .ltoreq. 45,
preferably .ltoreq. 40, and the time necessary in a particular case
may be readily determined empirically. Typically, annealing is
conducted over a period in excess of 15 hours, and commonly at
least about a 24 hour anneal is had. Where the element is held at
anneal temperature continuously throughout the requisite period, it
is advisable to control cooling upon completion of the anneal so
that at least about 1 1/2 hours are required to regain room
temperature. However, it has been learned that control of cooling
is substantially less important where the requisite overall
annealing residence time is divided into at least about three
roughly equal stages, and the element returned to room temperature
between each annealing stage.
Because the polymeric matrix of the black-containing extrudate is
in the melt during annealing, that extrudate is preferably supplied
prior to annealing, with an insulative extruded jacket of a
thermoplastic material which is shape-retaining when brought to the
annealing temperature. Jacketing materials suitable for the
preferred embodiments of this invention are set out in the Examples
which follow, and are discussed at length in the commonly assigned
application entitled SELF-LIMITING CONDUCTIVE EXTRUDATES AND
METHODS THEREFOR, Ser. No. 287,442 filed concurrently herewith, the
disclosure of which is incorporated herein by reference.
Upon completion of annealing and optional addition of a further
insulative jacket of, e.g., polyethylene, the self-limiting element
is desirably subjected to ionizing radiation sufficient in strength
to cross-link the black-containing core. Radiation dosage is
selected with an eye to achieving cross-linking sufficient to
impart a degree of thermal stability requisite to the particularly
intended application without unduly diminishing crystallinity of
the polymer matrix, i.e., overall crystallinity of the cross-linked
black-containing matrix less than about 20 percent is to be
avoided. Within those guidelines, radiation dosage may in
particular cases range from about 2 to 15 megarads or more, and
preferably is about 12 megarads.
The invention is further described in the following Examples of
preferred embodiments thereof, in which all parts and percentages
are by weight, and all resistivities measured at room temperature
and with a Wheatsone bridge unless otherwise indicated.
EXAMPLE 1
Seventy-six lbs. of polyethylene (density 0.929 gm/cc, 32 lbs. of a
mixture of 34 percent Vulcan XC-72 and ethylene ethyl acrylate
copolymer (density 0.930 gm/cc, 18 percent ethyl acrylate) were
loaded with 1 lb. of antioxidant into a Banbury mixer. The ram was
closed and mixing commenced. When temperature reached about
240.degree.-50.degree.F the batch was dumped, placed in a 2-roll
mill, and cut off in strips which were fed to a pelletizing
extruder. The pelletized compound was next extruded onto two
parallel tinned copper electrodes (20 AWG 19/32) to form an
extrudate generally dumbbell-shaped in cross-section. The
electrodes were 0.275 inch apart (center-to-center), the
interconnecting web being about 15 mils in thickness, at least 8
mils thickness of the semiconductive composition surrounding the
electrodes. Extrusion was performed in a plasticating extruder with
crosshead attachment (Davis-Standard 2 inches extruder, 24/1 L/D,
with PE screw. Thereafter, the same extruder was arranged to
extrude an 8 mil thick insulation jacket of polyurethane (Texin
591-A, available from the Mobay Corporation). For optional
geometric conformation, a conventional tube extrusion method was
employed in which a vacuum (eg., 5-20 in. H.sub.2 O) is drawn in
the molten tube to collapse it about the semiconductive core within
about 3 inches of the extrusion head. The jacketed product was next
spooled onto aluminum disks (26 inches dia) and exposed to
300.degree.F for 24 hours in a circulating air oven. Following this
thermal structuring procedure and cooling to room temperature oven
about 11/2 hours the resistivity of the sample was determined at
various temperatures. The following data was taken.
Table I ______________________________________ Resistivity Variance
with Temperature T, .degree.F R, ohm-cm
______________________________________ 60 4,800 80 5,910 100 9,600
120 20,950 140 69,900 160 481,500 180 6,150,000 200 > 2 .times.
10.sup.7 ______________________________________
EXAMPLES 2 - 9
Additional extrudates were prepared with various polymers and black
loadings following the procedure of Example 1 save where otherwise
indicated below. The polymeric matrices for the various examples
were as follows: (2) a 3:1 blend of low density polyethylene:
ethylene ethyl acrylate copolymer; (3) a 5:1 blend of low density
polyethylene: ethylene vinyl acetate copolymer; (4) polyvinylidene
fluoride; (5) a 3:1 blend of medium density polyethylene:
ethylene-ethyl acrylate copolymer; (6) a 3:1 blend of high density
polyethylene: ethylene-ethyl acrylate copolymer; (7)
ethylene/propylene copolymer (Eastman Chemical Company's
"Polyallomer"); (8) polybutene-1; and (9) polyvinylidene
fluoride/tetrafluoroethylene copolymer (Pennwalt Chemical Company's
"Kynar 5200"). In the case of each blend, carbon black was first
mixed with the minor component of the polymeric blend, and the
resulting masterbatch mixed with the other polymeric component. The
jacketed extrudate of each composition exhibited a non-linear
positive resistivity temperature coefficient. The data reported in
Table II was taken.
Table II
__________________________________________________________________________
Example % Carbon R(as extruded) R(annealed) Rp Annealing 2 L + 5
log R ohm-cm ohm-cm ohm-cm Regimen
__________________________________________________________________________
2 10 10.sup.9 5 .times. 10.sup.3 >10.sup.7 at 210.degree.F 24
hrs. 300.degree.F 38.5 3 10 10.sup.9 6050 2 .times. 10.sup.5 at
212.degree.F 18 hrs. 350.degree.F 38.9 4 13 10.sup.12 116 6 .times.
10.sup.3 at 325.degree.F 2 hrs. 450.degree.F 36.5 5 13 10.sup.11
393 2.82 .times. 10.sup.6 at 240.degree.F 15 hrs. 300.degree.F 39.0
6 5 10.sup.11 570 2.66 .times. 10.sup.6 at 280.degree.F 20 hrs.
300.degree.F 23.0 7 9 10.sup.12 5980 5.78 .times. 10.sup.6 at
220.degree.F 20 hrs. 400.degree.F 36.9 8 13 10.sup.10 434 1.59
.times. 10.sup.5 at 210.degree.F 5 hrs. 300.degree.F 39.2 9 13
10.sup.11 39.9 800 at 250.degree.F 4 hrs. 450.degree.F 34.0
__________________________________________________________________________
EXAMPLE 10
The procedure of Example 1 was repeated to obtain an identical
polyurethane--jacketed extrudate. Thereafter, the extrudate was
exposed to 300.degree.F for nine 3-hour periods separated by
intervals in which the article was permitted to cool to room
temperature. Thereafter, the annealed article was provided with a
final insulative jacket of polyethylene (12 mils in thickness) by
the tubing extrusion method and cross-linked throughout by exposure
to a 1-Mev electron beam for a total dose of 12 megarads. The strip
so produced exhibited the following resistivity values at the
temperatures given in Table III.
Table III ______________________________________ T.degree.F R
T.degree.F R ohm-cm ohm-cm ______________________________________
60 4800 140 69,900 80 5910 160 481,500 100 9600 180 6,150,000 120
20,950 200 >2 .times. 10.sup.7
______________________________________
* * * * *