CA1103871A - Colour-coded cellular insulation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method is provided for producing a cable con-ductor wire insulated with a colour-coded, cellular thermo-plastic electrically insulating polymer material in extruded form, in which a thermoplastic electrically insulating polymer material and a predetermined amount of a pelletized material comprising predetermined amounts of a particulate pigment composition and of a particulate, solid blowing agent in a polymer matrix compatible with said insulating polymer material are introduced into a screw extruder, the mixture is heated and sheared in the extruder to melt the polymer material and form an intimate mixture of the pig-ment composition and the blowing agent in the polymer mate-rial; the blowing agent thermally decomposes to a gaseous product, and the molten mixture is forced from the extruder and about a moving conductor wire to form an extruded coating, the extruded coating expands a predetermined amount, to form the conductor wire insulated with colour-coded, cellular thermoplastic electrically insulating polymer material; a plurality of such conductor wires can be formed into a multi-conductor communication cable in which the colour-coded conductors are dielectrically matched.

Description

11~)3871 This invention relates to dielectrically matched colour-coded insulating materials, more especially the illvention is concerned with methods for producing a colour-coded, cellularly insulated cable conductor wire and a multiconductor communication cable.
This application is a division of Canadian Patent Application, Serial No. 265,922 filed November 17, 1976.

Communication cables generally comprise a plurality of insulated conductors enclosed within a waterproof sheath, such cables may consist of groups of two or more such insulated conductors twisted together, for example, a twisted telephone pair or a quad. The cable may consist of a plurality of such twisted pairs or quads twisted together.
Frequently the conductors are inqulated with a cellular insulation and such insulated conductors may be pro-duced by extruding around a conductor wire a thermoplastic polymer containing gas bubbles, produced by the thermal decom-position of a solid blowing agent or by direct introduction of a gas into the polymer in the presence of a nucleating agent and allowing the gas bubbles to expand thus producing the cellular structure.
It is frequently necessary to employ readily identifiable conductor wires within a cable, and this ready identification may be achieved by the employment of different colours for different wires, so that the wires are colour-coded, By the employment of different colour combinations individual pairs or quads within a group can be readily identified, for example, one pair might comprise a red wire and a blue wire, a second might comprise a red wire and a white wire, a third might comprise a red wire and a green wire and so on.

~ 3~

The different colours are obtained by the employment of appropriate amounts of pigment. It is found, however, that different pigments have different effects on the electrical characteristics of the extruded insulating coatings and in particular different pigments effect the dielectric constant of an insulatin~ polymer to diEferent degrees. Further, the impairment in -the dielectric constant is not necessarily linear with respect to the degree of pigmentation.
Differently pigmented polymers are not suitable for insulation of conductors in communication cables if they are electrically mismatched, since, for example, in a telephone cable any mismatching results in crosstalk which is difficult to eliminate.
Systems of electrically matched colour-coding com-positions have been developed to overcome this problem of mis-matching, which compositions employ particular concentrations of particular pigments. One such system which has found wide acceptance is that described in Canadian Patent 616,767 of John B. Howard and Vincent L. Lanza, issued March 21, 1961.
Canadian Patent 616,767 describes eleven colour-coded com-positions for producing multiconductor communication ca~les in which different coloured insulating coatings are dielectrically matched. One of the colour compositions, namely the gold, has since become obsolete and there are now conventionally employed ten colour-coded compositions based on the com-positions described in the aforementioned Canadian Patent.
Thus there are conventionally employed ten visually distinctive colour compositions namely blue, orange, green, brown, slate, white, red, black, yellow and purple.
The present invention is concerned with improvements in the employment of dielectrically matched colour-coded com-87~

positions such as those described in the aforementioned Canadian patent which permits the production of colour-code~, dielectrically matched cellular insulations.
The aforementioned Canadian patent was concerned primarily with solid insulation coatings, however, with the increasing popularity o~ cellular insulating coatings in commllnicakion cables the same colour-coded systems have been employed in the production of cellular insulations.
In conventional practice the blowing agent and the polymer for the insulation coating are premixed in predeter-mined amounts having regard to the desired cellular structure and formed into pellets which are added to the screw extruder separately from the pigment.
In the present invention a colour-coded pellet concentrate comprising a predetermined amount of the pigment and of the blowing agent in a polymeric matrix, is added directly to the sc~ew extruder with the separate addition of the polymer for the insulation coating. The polymer which comprises the polymeric matrix of the pellets should be compatible with t~e polymer employed for forming the insulation coating and preferably the polymeric matrix comprises the same polymer material as is employed for the insulation coating.
The colour-coded pellets containing both the appropriate amount of pigment and blowing agent permit the cable manufacturer to employ any insulating polymer mate~ial to produce colour-coded cellular insulating coating without relying on the supplier of the polymer to incorporate the blowing agent therein~
In practice the blending of polymers and blowing agents is not an easy matter and conventionally employs an 3~

intensive mixer such as a Banbury,* the mixing being conducted under carefully controllecl temperature conditions, to render the polymer su~ficientl.y molten to permit the mixin~ ~hile at the same time avoidiny p~emature thermal decomposition of the b].owing acJent. The cable manufactureris thus largely reliant on the~ suppl:i.er o:E polymer and -the problern arises that different batches of polymcr-containing blowing agent may differ.

The presen-t invention does not suffer this problem since a concentrate in pellet form of the blowing agent and pigment is lntroduced directly with pellets of the polymer to the screw extruder, and this considerably simplifies the extrusion process and permits greater con-trol by the cable manufacturer, as wel.l as the production of uniform dielectri-cally matched colour-coded insulating coatings. The concen-trate is made by a relatively simple process.
The concentrate can be produced in a process in which a comminution of the particulate pigment and blowing agent solids and a simultaneous intimate blending which is facilitated by the fine comminution is employed. There is no such comminution or grinding when a conventional Banbury* is employed in blending the polymer and blowing agent~ The Banbury*depends entirely on the high viscosity of the media to produce adequate shear and the material is neither ground nor is the physical particle size changed, On the contrary a mixer such as a Banbury* often cause the formation of large agglomerates of the ingredients rather than a refinement of the size of the materials being dispersedO

In particular a pelletized masterbatch comprising a pigment and a solid blowing agent in a polymeric matrix for use in the manufacture of dielectrically matched colour-coded insulating materials is produced in a process which *trade mark comprises, mixing a predetermined amount of a particulate pig-ment composition and a predetermined amount of a solid parti-culate blowiny ayent in an organic vehicle, ef~ective to wet the particulate materials, to form a paste-like or powdery mass, comminuting and intimately blending the paste-like or powder~ mass to a riable mass, mixing a predetermined amount oE the friable mass with a predetermined amount of a solid thermoplastic, electrically insulating polymer, at a temperature at which the insulating polymer is able to wet said friable mass, to form an intimate blend of said polymer and the particulate materials of said friable mass, and pelletizing the intimate blend.
The pellets are suitably provided as a set of colour-coded pellets for use in the manufacture of dielectrically matched colour-coded insulating matarials comprising a plura-lity of separately packaged coloured pellets, each package containing pellets of a single colour, each pellet comprising a particulate pigment and a particulate solid blowlng agent in predetermined amounts in a matrix of a thermoplastic polymer, the pellets contained in each individual package being sub-stantially uniform in composition and the pellets ln each pack-age being dielectrically matchable with the pellets in the other packages of said plurality.
The present invention is more particularly con-cerned with the use of such concentrate or pellets to produce a cable conductor wire insulated with a colour-coded, cellular thermoplastic electrically insulating polymer material in extruded form. In accordance with the invention there is pro-vided a method o~ producing such an insulated wire which com-prises providing a solid thermoplastic electrically insulat-ing polymer material and a predetermined amount of a pelletized material comprising predetermined amounts of a particulate pigment and a particulate, solid blowing 7~

agent in a polymer matrix compatible with said insulating polymer material, introducing said polymer material and said pelletized material into a screw extruder barrel, containing a screw mounted for rotation therein, to form a mixture passing the ~ixture through said barrel, heating and shearing thc rnixture in said barrel to melt said polymer material and form an intimate mixture of said pigment and said blowing agent in said polymex material, allowing said blowing agent to thermally decompose to a gaseous product, forcing the resulting molten mixture through an extrusion head and about a moving conductor wire to form an extruded coating thereon, allowing the extruded coating to expand a predetermined amount, and collecting the conductor wire insulated with colour-coded, cellular, thermoplastic electrically insulating polymer material.
According to another aspect of the invention there is provided a method of producing a multiconductor com-munication cable containing a plurality of colour-coded di-electrically matched conductors which comprises producing a plurality of conductor wires insulated with cellular thermo-plastic electrically insulatlng polymer material by the methcd of the invention as described in the preceding paragraph, in which the pelletized material of each extruding operation is employed in a predetermined amount so that the resulting plurality of colour~coded conductors are dielectrically matched, and forming said plurality o~ conductors into a multiconductor communication cable.
Pelletizlng Process In the pelletizing process a particulate pigment com-position which suitably comprises amounts of a particular pig-ment, opacifier and antioxidant according to the teaching of the ~3~7~

aforementioned Canadian patent 616,767 is first mixed in a predetermined amount w.ith a predetermined amount of a particulate blowing agent in the presence of an organic vehicle, able to wet the particulate materials~
The amounts of pigment composition and blowing agent are pred~termined so that the pellets contain the appropriate relative amounts of pigment and blowing agent such that the addition to the screw extruder of a given predetermined weight of the final pellets will provide the required amount o:E blowing agent for the cellular structure desired as well as the required amount of pigment to colour the insulation coating so that the colour is visibl~ discernible for identification purposes and so that insulation coatings of different colours and containing different pigments will be dielectrically matched.
The percentage amounts of different pigment com-positions necessary ~or dielectric matching are already described in the aforementioned Canadian patent in relation to the polymer insulating material, so that it requires only a simple calculation and measuring techniques to determine the quantity of pellets to be employed relative to the quantity of polymer material to obtain the required pi~ment content. This having been es-tablished it similarly requires a simple calculation to determine the amount of blowing agent required in the pellets so that the same aforementioned quantity of pellets contains the required amount of blowing agent for the desired cellular structure.
The organic vehicle is employed in an amount sufficient to wet the particulate materials and form a paste-like or powdery mass, the organic vehicle is suitably one which is compatible both physically and electrically with the polymer which is sub~

sequently to form the matrix of the pellet, and the polymer 8~

which is to form the cellular insulation. A particularly suit-able organic vehicle is a low molecular weigh-t polybutene fluid having a molecular weight of about 300 to about 1900. Poly-butene fluid is especially suitable when the polymer insulation ancl polymer matri~ is a polyethylene, since the properties of polybutene fluid approach those of polyethylene particularly in respect oE electrical equivalency, polybutene fluid having a very low specific inductance capacity (S.I.C.), a very low power factor and a very high volume resistivity.
Particularly low moLecular weight fluid polybutene has an S.I,C~ of about 2.25 whereas low molecular weight polyethylene has an S.I.C. of from about 2.2 to 2O3~
Suitable blowing agents include azodicarbonamide, 4,4'-oxybis(benzenesulphonylhydrazide), N-aminophthalamide and ~,N'-dinitropentamethylenetetramine, however, azodicarbonamide is most commonly emplo~ed and represents the preferred blowing agent for the present invention.
The blowing agent is suitably employed in an amount ~y weight of 0.5 to 1.2, preferably 0.5 to 1.0, more preferably 0.75 parts by weight per 100 parts by weight of polymer material in the insulating coating. The amount of blowing agent in the pellets is thus preferably determined so that a given weight of pellets will provide an amount of blowing agent in the aforementioned range per 100 parts of polymer material.
As described in a first step predetermined amounts of pigment and blowing agent are mlxed with the organic vehicle when polybutene fluid is employed as the vehicle it is found to be adequate to employ from about 25 to 50%, by weight, of polybutene fluid to form the paste-like or powdery mass based on the total weight of the paste-like or powdery mass.

3~7~

In the formation of the paste-like or powdery mass the dry particulate materials are wetted with the vehicle and a rough mechanical mixing is accomplished, this is suitably achievecl in a mixing device 5uch as a Mi~mulle~.
The ~i~lny may be carriecl out at an elevatecl temperature in order to lower the viscosity of the vehicle so as to facilitate the mixing. Caution is required since the temperature should not be so high as to initiate thermal decom-position of the blowin~ agen-t, elevated temperatures further adversely affect many of the pigments employed, particularly the organic pigments, consequently the temperature should not be so high as to affect the pigment.
Generally when polybutene fluid is employed as the vehicle a temperature of about 150F is quite adequate to facilitate the mixing without adversely affecting the blowing agent and pigment.
It is desirable too that the materials not be exposed to such temperatures for prolonged periods, since prolonged e~posure even to temperatures below 150F mày adversely affect the materials. Generally when ~i~ing at a temperature of 150F, the mixing time should not exceed 30 minutes, and preferably should not be longer than 20 minutes.
The thus obtained paste-Like or powdery mass is subjected to a comminuting and blending process to produce a friable mass. In this stage the blowing agent and pigment in the paste-li]se or powdery mass is comminuted to a fine p~
ticle size and is intimately blended with t'ne organic vehicle to pro~uce tiny agglomerates in which the particles are coated with the organic vehicle. The agglol-nerates generally have a particle size of about 15 -to 30 millimicrons and preferably about 20 millimicrons.
* trade mark 38~L

This fine comminution and blending may conveniently be achieved in a three roll ink mill of the kind employed in the manufacture of ink compositions. The rolls of the ink mill have a very small clearance typically of the order of 1 to 2 thousandths of an inch and this results in a fine grinding of the particles of blowing agent and pigment. The material is passed over the in]c mill rolls in a succession of passes suit-ably for up to about 10 minutes to produce a highly-sheared intimately blended combi~a~ion of the two original powders.
In this stage the comminuting and blending may suitably be carried out at an elevated temperature, typically the same temperature as employed in the first stage. In, this respect the three rolls of the ink mill are conveniently internally cooled or heated, as necessary to obtain a desired temperature for -the comminuting and blending.
~le resulting friable mass is carefully weighed and added to a weighed amount of a solid thermoplastic electrically insulating polymer and the friable mass and polymer are intimately blended. This operation is carried out at a temperature at which the polymer is molten, preferably at a temperature which is just above the solid melt temperature of -the polymer.
The preferred polymer is a low density, low molecular weight polyethylene having a molecular weight of from about 2500 to 10,000, which melts in the range of about 150F to 170~F, and which has a melt flow index of about 300 to about 400 gms/
min, since this can be readily employed at temperatures which will not adversely affect the blowing agent and pigment.
I'he polymer is suitably employed in an amount such that the pellets contain 50 to 75%, preferably 50 to 65%, by weight of the polymer, the ratio of the organic vehicle, for example, polybutene fluid, in the pellets to the polymer in ~3~

the pellets suitably may be from 20:80 to 50:50~
This operation like the previous one is conveniently carried out in the three roll ink mill, wherein the very small clearance between the rolls produces an intimate blending. A
thin sheet of the blended material is thus obtained from the ink mill, The sheet is, after cooling, sLit inko convenient widths and subjected to a pelletiziny operation.
l'he pelletizing operation may comprise, for example, feeding the widths obtained from the sheet through an extruder equipped with a breaker plate and 100 mesh strainer screens and from there into a spinneret having a 1/8 inch diameter opening and thence through a water cooled fly cutter where the e~trudate from the spinneret is chopped into 1/8 inch lengths.
In this way there can be obtained uniformly sized cylindrical pellets having a diameter of about l/8 inch and a length of about 1/8 inch. The pellets are suitably air-dried for packaging, for ~xample, in a cyclone.
Packaqinq of Colour-Coded Pellets The colour-coded pellets may be packaged in a variety of ways ready for use as will be evident. E'or example, pellets of one colour may be packaged in strong paper sack, labelled to fully identify them, a separate sack being provided for each colour of pellets. Alternatively pellets of different colours might be packaged in a compartmentized container having two or more separated compartments and ernploying a separate compart-ment for each colour of pellets, For example, a two-compart-ment carton or drum might be employed for the separate packaging of red and blue pellets.

It will be recognized that all such modes of packaging the pellets so that pellets of the same colour are packaged together and separately from pellets of a different colour are envisaged in this invention.

~3~7~

Cable Manufacture In the manufacture of a cable conductor wire insulated with a colour-coded insulation, the pellets of a particular colour are introduced into the feed hopper of a screw extruder together with a solid thermoplastic electrically insulat-ing pol~mer m~terial, which polymer material may suitably com~
prise a single homopolymer or a mixture of homopolymers. Poly-olefins including polyethylene, polypropylene and mixtures thereof are especially preferred, with polyethylene in parti-cular being widely employed in the cable industry. When theinsulating polymer is a polyethylene, a medium density or high density polyethylene is employed. The polypropylenes are high molecular weight, high melting point materials. ~ixtures of a polyethylene with a polypropylene represent a material of intermediate physical characteristics.
The quantity of pellets added is carefully controlled so as to provide the requisite amount of blowing agent and pig-ment for the amount of polymer material added.
The pellets and polymer are forced through the screw extruder by rotation of the screw and are subjected to shearing and heating in the screw extruder barrel to melt the polymer material and intim~tely mix the polymer and the ingredients of the pellets. At the temperatures employed the blowing agent decomposes during its passage through the barrel to produce small gas bubbles which are maintained in a compressed state by the pressure generated in the extruder barrel. The molten mass is forced through an extruder head and about a moving conductor wire to forrn a coating on the wire, Simultaneously-the gas bubbles in the extruded coating expand to produce the cellular structure' the degree of expansion being suitably controlled by means of a moving water trough, for example, as ~3~

described in U. S. Patent 3,896,19~, Atushi Ut~ni et al issued July 22, 1975.
In producing a communication cable for example, a telephone cable, from a plurality of colour-coded conductor wires the conductor wires are suitably twisted into groups, for example, pairs or quads in distinctive colour combinations. The wires may be conveniently twisted together employing the method and apparatus described in Canadian patent 963,361 of Glen R.
Forester issued February 25, 1975. The latter Canadian patent describes a total method for the manufacture of a communication cable. There may similarly be employed procedures such as those described in Canadian patent 982,804, Shirley Beach, issued February 3, 1976, in which a plurality of colour-coded conductors is enclosed in a longitudinally wrapped sheath, for example, of aluminium tape, which forms a complete envelope, an extruded jacket being formed around the sheath.
It will be recognized that methods of assembling insulated conductors into communication cables are well known and the present invention does- not reside in the assen~ly method per se.
The invention is further illustrated by reference to the accompanying drawing in which FIGURE 1 is a flow diagram illustrating the steps of producing a pellet and cable according to the invention~
With further reference to Figure 1, there is illustrated in a si~lified form a process of producing pellets, the en~loyment of the pellets in a method of pro-ducing a colour-coded insulated conductor wire and the production of a cable from insulated wire combinations A, B, C and D each of which comprises a twisted pair of a distinct colour con~ination.

` ~.~i38~7~

The invention is further illustrated by reference to the following examples illustrating particular embodiments of the invention, Example 1 ~__ In a MixmulLcr* the~e was introcluce~d in parts by W~ t:
wh:Lte pigment (rutile Tio2) 11.5 parts blowing agent (azodicarbonamide) 15 parts organic vehicle (polybutene fluid) 25 parts The ingredients were mixed together at room tempera-ture in the Mixmuller*for 15 minutes at which time the particles were fully wetted by the polybutene and the mixture is a paste-like mass.
The paste-like mass was then fed through a three roll ink mill at room temperature for 10 minutes to produce a friable mass.
The friable mass was mixed in an amount of 40 parts by weight with 60 parts by weight of AC8* low density poly-ethylene having a melt flow index of about 330 gms/min. The mixture was passed through a three roll ink mill for 10 minutes at a temperature of about 180F to produce a thin sheet material.
The thin sheet material was cut into strips and fed into a screw extruder, where the strips were worked under shearing action at a temperature of about 180F.
The mixture was extruded as a continuous cylinder haviny a diameter of 1/8 inch which was fed into a water cooled fly cutter where the water evolved extrudate was chopped into 1/8 inch length.
The resulting pellets were dried in a cyclone.

* trademark , . , ~38 7~

Example 2 ~ n this Example a conductor wire with a white coded cellu]ar insulation was produced employing the pellets obtained in Example 1 and a host resin comprising a high density poly-e-thylene having a solid melting point of about 260F.
I'he objective was to produce a cellularly insulated conductor in which the cellular insulation has 55% of its volume occupied by cells, The host resin and the pellets were carefully fed into a screw extruder in proportions such that the host resin contained 1,2%, by weight, of azodicarbonamide, The pellets and host resin were sheared in the extruder at a temperature of 385F, at which temperature the azodicarbonamide decomposed, l~e resulting mixture was extrùded around a moving conductor wire and there was thus obtained a-white cellular insulated conductor wire having a coaxial capacitance of 52 pico farads.
By similar procedures and employing pigment compo-sitions such as those described in the afore-mentioned Canadian pateht 616,767, there was obtained a pluralit~ of colour-coded cellular insulated conductor wires all having a coaxial capacitance of 52 pico farads from which telephone cable could be produced in which the different colour-coded wires were dielectrically matched, The cellular colour-coded insulating coatings pro-duced are somewhat lighter in colour than the ~olid colour-coded insulating coatings produced according to ~he afore-mentioned Cana~ian patent, however, they are visi~ly dis-cernible to the eye.

387~
SUPPLEMENrrARY DISCLOSURE

This disclosure relates to colour-coded insulating materials which can be dielectrically matched and their manuEacture, more especially the disclosure is concerned with a particulate concentrate and a process for preparing it for the manufacture of insulating materials, with a set of separately packaged colour-coded particulate concentrates and with methods for producing a colour-coded, cellularly insulated cable conductor wire and a multi-conductor cable, more e~pecially a communication cable.
The principal disclosure refers particularly to a pelletized masterbatch for use in the manufacture of a colour-coded, cellulary insulated cable conductor wire, particularly for a multiconductor communication cable.
The invention is, however, also applicable to cables other than communication cab]es, for example power cables, and to particulate concentrates in general, not necessarily pellets.
The particulate concentrates, including the pellets, may be produced by a direct or an indirect me-thod. The principal disclosure is primarily concerned with a direct method, and indeed direct methods are generally preferred, however, satisfactory particulate concentrate can also be prepared by the indirect method.
The polymer-containing blowing agent employed in the prior art, as described in the principal disclosure, is prepared by an outside supplier and i5 termed a ~Ifully let-down" material, clearly a fully let-down material of this type is only available in a limited ratio of blowing agent to polymer and the cable manufacturer cannot readily alter the ratio.

~3387~

Such difficulties are now overcome in that a con-centrate of the blowing agent and pigment is introduced directly with pellets of the polymer to a screw extruder, which considerably simplifies the extrusion process and per-mits yreater control by the cable manufacturer, as well as -the production of uni:Eorm dielectrically matched colour-coded in-sulating coatings. The concentrate is made by a relatively slmple process~
No premature thermal decomposition of the blowing agent has occurred at the time that it is introduced with the polymer pellets into the screw extruder. Thus the cable manufacturer is able to exercise greater control over extru-sion conditions, and to vary the ratio of blowing agent to polymer as desired, thus providing versatility in the degree of blow and in the density of the extruded insulation coating, as well as permitting the ready production of uniform di-elect~ically matched, colour coded insulating coatings~ .
The particulate concentrate comprising a pigment and a solid blowing agent in a polymeric matrix for use in admixture with a thermoplastic electrically insulating poly-mer in the manufacture of colour-coded insulating materials, particularly dielectrically matched colour-coded insulating materials can be produced in a process which comprises: mixing a predetermined or weighed amount of pigment composition and a predetermined or weighed amount of a blowing agent composition with a predetermined or weighed amount of a solid, thermo-plastic, electrically insulating polymer to form an intimate blend of the polymer and the compositions, the predetermined or weighed amount of pigment composition being selected to provide a colour-coded insulating material, particularly to provide a dielectric matching amount in a colour-coded insul-ating material~

In a particular embodiment the intimate blend is pelletized to form a pelletized concentrate.
The concentrate is suitably provided as a set of colour-co~ed particulate concentrates, for example pellets for u~e in the manufacture of co:Lo~lr-coded insulating materials, especially dielectrically matched materials comprising a plurali.ty of separately packaged coloured concentrates, each package containing concentrate of a single colour, each con-centrate comprising a particulate pigment and a particulate solid blowing agent in predetermined or weighed amounts in a matrix of a thermoplastic polymer, the concentrate contained in each individual package being substantially uniform in composition, and the concentrates in each package belng di-electrically matchable with the concentrates in the other pac~-ages of said pluralit~.
The present invention is more particularly con-cerned with the use of such a concentrate to produce a cable conductor wire insulated with a colour-coded, cellular thermo-plastic electrically insulating polymer material in extruded form. In accordance with the invention there is thus provided a method which comprises providing a thermoplastic electri-cally insulating polymer material and a predetermined or weighed amount of a particulate concentrate, ~or example as pelletized material comprising predetermined or weighed amounts of a pa.rticulate pigment composition and o~ a particulate, solid chemical bl~wing agent materiaL in a polymer matrix compatible with the insulating polymer material, ,` ~,,f~

in-troducing the polymer material and the particulate con-centrate into a scre~ extruder barrel, containing a screw mounted for rotation therein, to ~orm a mixture, passing the mixture through the barrel, heating and shearing the mixture in the barrel to melt the polymer material and form an intimate mi.xture of the pi~ment composition and the blowing ag~t material in the polymer material, allowin~
the blowing agent to thermally decompose to a gaseous product, forcing the resulting molten mixture through an extrusion head and about a moving conductor wire to form an extruded coating, thereon, allowing the extruded coating to expand a predetermined amount, and collecting the conductor wire insulated with colour-coded, cellular, thermoplastic electrically insulating polymer material4 In another aspect of the invention there is provided a method of producing a multiconductor cable containing a plurality of colour-coded insulated con-ductors which comprises producing a plurality of conductor wires insulated with cellular thermoplastic electrically insulating pol~mer material by the method of the invention as described in the preceding paragraph, and forming said plurality of conductors into a multiconductor cable.
In a particular embodiment of this latter aspect of the invention the method is applied to the production of a multiconductor communication cable containing a plurality of colour-coded, dielectrically matched, insulated conductor, in which the particulate concentrate of each extruding operation is employed in a weighed amount so that the resulting plurality of colour-coded, insulated conductors is dielectrically matched, the plurality of conductors ~3~

being formed into a multiconductor communication cable.
CONCENTRATE MANUFACTURE
i) Direct Method In the manufacture oE the particulate concentrate composition which sultahly comprises amounts of a particular pi~ment, opacifier and antioxidant according to the teaching of the aforementioned Canadian Patent 616,767 a pigment composition is first mixed in a predetermined amount with a predetermined amount of a particulate blowing agent in the presence of an organic vehicle.
The amounts of pigment composition and blowing agent are predetermined so that the concentrate contains the approprlate relative amounts of pigmen-t and blowing agent, such that the addition to the screw extruder of a given predetermined weight of the particulate concentrate will provide the required amount of blowing agent for the cellular structure desired as well as the required amount of pigment to colour the insulation coating so that the colour is visibly discernible for identification purposes and so that insulation coatings of different colours for communication cables, containing different pi~ments, will be dielectrically matched, The percentage amounts of different pigment com-positions necessaryfor dielectric matching are a~ready describ-ed in the aforementioned Canadian Patent in relation to the polymer insulating material so that is requires only a simple calculation and measuring techniques to determine the quantity of concentrate to be employed relative to the quantity of polymer material to obtain the required pigment content. This having been established it similarly requires ~ 20 -a simple calculation to determine the amount of blowing agent required in the concentrate so that the same aforementioned quantity of concentrate contains the required amount of blowing agent for the desired cellular structure.
~ccording to this method predetermined amounts of pigmen-t composition and blowing agent material are mixed in a first step, with the organic vehicle to form a powdery mass.
In the formation of the powdery mass the particu-late materials are wetted with the vehicle and a mechanical mixing is accomplished; this is suitably achieved in a high intensity or low intensity mixer.
The m.ixing may be carried out at an elevated temperature in order to lower the viscosity of the organic vehicle so as to facilitate -the mixing. However, caution is required since the temperature should no be so high as to initiate thermal decomposition of the blowing agent and because elevated temperatures can adversely affect many of the pigments employed, particularly the organic pigments.
The organic vehicle is employed ln an amount sufficient to wet the particulate materials and ~orm a powdery mass; the organic vehicle is suitably one which is compatible both physically and electrically with the polymer which is subsequently to form the matrix of the particulate concentrate, and the polymer which is to form the cellular insulation.
A temperature of up to about 150C can be employed to facilitate the mixing with the organic vehicle without adversely affecting the pigment and without significant decomposition of the blowing agent.

33~7~L

It is desirable that the materials not be exposed to high temperatures for prolonged periods, since prolonged exposure even to temperatures below 150C may adversely affect the materials particularly the blowing agent.
Pre-ferably the mixiny temperature should not be more than 120C and more preferably not more than 105C. Generally when mixing at a temperature of 105C, the mixing time should not exceed 30 minutes, and preferably should not be longer than 10 minutes.
In the case in which the organic vehicle is a viscous, liquid polybutene a temperature of about 66C
(150F) is adequate to facilitate the mixing without adversely affecting the pigment and without siynificant decomposition of the blowing agent.
The thus obtained dry powdery mass is subjected to a comminuting and blending or dispersing process to produce a friable mass. In this stage agglomerates of blowing agent material and pigment composition in the dry powdery mass are comminuted and are intimately blended or dispersed with the organic vehicle to produce thoroughly wetted particles in which the pigment composition and blowing agent ma~erial are coated with the organic vehicle.
This fine comminution and dispersion may conven-iently be achieved in a three roll ink mill of the kind employed in the manufacture of ink compositions. The rolls of the ink mill have a very small clearance, typically of the order of 1 to 20, more especially 1 to 2 thousandths of an inch and this results in a fine uniform grinding and dispersion of the agglomerates. The material is suitably passed over the ink mill rolls in succession of pases suitably for up to about 10 minutes to produce a highly-~3~

sheared intimate blending and dispersion of the powdersin the organic vehicle.
In this stage the comminuting and dispersion may suitably be carried out a-t an eleva-ted temperature, typically the same temperature as employed in the first stage. In this respect ~he three rolls of the ink mill are conveniently internally coole~ or heated, as necessary to obtain a desired temperature to effect optimum comminuting and disper-sion.
The resulting friable mass of wetted particles is carefully weighed and added to a weighed amount of a thermoplastic electrically insulating polymer as the matrix material and the friable mass and polymer are intimately blended. The blending may suitably be carried out in a high intensity or low intensity blender to blend or disperse the blowing agent and pigment in the insulating polymer.
The blending may also be carried out in a three roll ink mill of the kind described previously.
This operation is suitably carried out at a temperature at which the insulating polymer is flowable or plastic and preferably molten, particularly at a temperature which is just above the melt temperature of the insulating polymer.
The insulating polymer is suitably employed in an amount such that ~he concentrate contains 20 to 75%, preferably at least 400/O, particularly 40 to 65%, more prefer-ably 40 to 55% by weight of the polymer, the ratio of the organic vehicle in the particulate concentrate to the insu-lating polymer in the concentrate suitably may be from 10:90 to 30:70, more particularly 20:80 to 50:50.

3~7~

The resulting powdered concentrate consisting of the blowing agent, pigment, or~anic vehicle and insulating polymer may be used as such or may be subjected to a pelleti-zing operation to provide the concentrate in the form of pellets.
The pelletizing may comprise feeding the blended mixture through an extruder equipped with a breaker plate and stainer screens, for example, 100 mesh screens, and from there into a die or spinneret having an opening of, for example, about 1/8 inch diameter and thence through a water cooled fly cutter or pelletizer cutter where the extrudate from the die is chopped in~o, for example about 1/8 inch lengths. In this way there can be obtained uniformly sized cylindrical pellets of a desired diameter and length, for example, a diameter of about 1/8 inch and a length of about 1/8 inch. The pellets may be air-dried for packaging, for example, in a cyclone.
Orqanic Vehicle The organic vehicle is suitably one which is both physically and electrically compatible with the insulating polymer, which is subsequently to form the matrix of the particulate concentrate, and the insulating polymer which is to form the cellular insulation.
The organic vehicle should be flowable and can be a liquid vehicle including a viscous liquid or a cold flowable or plastic solid vehicle which flows when subjected to shear forces, so that it will wet the particulate blowing agent and pigment.
Suitable organic vehicles include viscous hydro-carbon polymer liquids, for example, polybutenes, having a ~ 24 -3~7~

number average molecular weight of about 300 to 1900 and a viscoslty ranging from about 150 cps to about 5000 cps at 72F, hydrocarbon waxes, particularly polyethylene waxes, which melt at about 75C to about 120C, preferably 85C to 120C, have a Elow index of about 300 to 400 rn/min, and have a molecular weight o~ about 2,500 to about 12,000; and low density polyethylenes, which melt or flow at about 120C to about 150C, and which have a molecular weight 50,000 to about 200,000.

The organic vehicle may comprise one or more of these three classes of material~
A particularly suitable organic vehicle as, indicated above, is a low molecular weight viscous polybutene.
Polybutene i9 especially suitable when the polymer insulation and polymer matrix are both polyethylenes, since the proper-ties of poly~utene approach those of polyethylene particu-larly in respec-t of electrical equivalency, polybutene having a very low specific inductance apacity (S.I.C.), a very low power factor and a very high volume resistivity.
Particularly the low molecular weight polybutenes have an S.I.C. of about 2.25 whereas low molecular weight polyethylene has an S.I.C. of from about 2.2 to 2.3~
In an especially preferred embodiment the organic vehicle comprises a polybutene and a polyethylene wax, as described above in a weight ratio of 0:50 to 20:30.
An especially suitable polybutene is that available under the trademarkIndopol L-14 available from Amoco Chemicals Corp. In addition the polyethylene waxes and low density polyethylenes are suitable especially when the polymer insu-lation is a polyethylene. A preferred polyethylene wa~ is ~3~37~

Polye-thylene AC6~ available from Allied Chemical Company.
Blowing Agent The solid chemical blowing or foaming agents employed in the invention are solid organic compounds which thermally decompose with liberation of gas. Suitable blowing agents include aæodicarbonamide, ~,4'-oxybis(benzenesulphonylhydra-zide), N-aminophthalamide and N,N'-dinitrosopentamethylene-tetraamine.
The most commonly employed solid, thermally decom-posable blowing agent for the production of cellular insulated coatings particularly in the communication cable industry is azodicarbonamide, the commercial grade of azodicarbonamide used by, for example, the communication cable industry may include a hydrated silica additive having a controlled water content which acts as an anti-plateout agent. The hydrated silica evolves water on heating which reacts with isocyanic or cyanic acid, produced in the thermal decomposi-tion of azodicarbonamide, to prevent formation and decomposi-tion of solid cyanuric acid and cyamelide which would other-wise build-up on the surface of the screw and die during extrusion, in the form of a white powder or pasty substance, which interferes with the satisfactory operation of the extrusion' also these substances act as contaminants affecting the electrical properties of the cellular insulationO
Commercially available blowing and foaming agent materials in general are compounded to eliminate or minimize plate-out.
The blowing agent, as described in the principal disclosure, is suitably employed in an amount by weight of 0.5 to 1.2, preferably 0.5 to 1.0, more preferably 0~75 8~

parts by weight per 100 parts by weight of polymer material in the insulating coating. The amount of blowing agent in the particulate concentrate is thus preferably determined so that a given weight of pellets will provide an amount of blowing agent in the aforementioned range per 100 parts of total polymer material, in the insulating coating to be formed.

Piqment The pigment employed is suitably a particulate pig- -ment composition comprising amounts of a particular pigment, an opacifier and an antioxidant according to the teaching of the aforementioned Canadian Patent 616,767. ~owever, other pigments may be employed.
Organic pigments are usually used to produce the blue, red, green, yellow, orange and purple and brown colours for the insulation. The black colour .is provided by carbon black, and the white coLour by titanium dioxide, the slate composition is suitably a mixture of the black and white compositions.
The pigment composition is determined so that visibly discernible colour coded cellular ins~1lations can be produced employing the particulate concentrate of the invention.
In the case of particulate concentrates, used in the manufacture of-communication cables, the pigment composi-tion is additionally determined so that dielectrically matched colour-coded insulations can be produced. ~y dielectrically ~atched is meant that the colour-coded insu-lated materials have substantially the same dielectric constant, the maximum dielectric mis-match being no greater than about 2%, preferably no greater than 1%, and more preferably no more than 0.7%, as taught in the aforementioned Canadian Patent.

- 27 ~

~3~7~

Insulating Polymer Coatin~
The insula-ting polymer employed with the particulate concentrate is selected from those generally employed as insulation coatings Eor electrically conductive wires. In par-ticular the polymer may be a low density polyethylene, a medium density polyethylene, a high density polyethyler-e, polypropylene, ethylene vinyl acetate copolymer or plasticized polyvinyl chloride.
For communication cable insulating coatings the polymer is preferably one of the olefin polymers, particularly a low density polyethylene, which melts or flows at about 120C to about 150C, and high density polyethylene which has a density of about 0.9~ to about 0.95 and melts at a temperature of 270F to 360F.
Insulatinq Polymer Matrix of Concentrate The insulating polymer for the matrix of the parti-culate concentrate is suitably a solid flowable or plastic polymer, which flows when subjec-ted to shear forces. The polymer matrix should also be physically and electrically compatible with the insulating polymer to be extruded with the concentrate to form the e~truded coating. An especially preferred insulating polymer for the matrix is low density polyethylene which melts or flows at about 120C to about 150C.
ii) Indirect Method In another embodiment of the particulate concentrate manufacture a concentrate of the solid blowing agent is formed containing a known amount of the blowing agent, and a predetermined amount of the concentrate is mixed with a predetermined amount of a pigment concentrate having a known pi~nent composition content, and a predetermined amount of a ~3137~

solid particulate thermoplasticelectrically insulating polymer material.
The ingredients are mixed and blended until a uniforml~ blended mi~ture is obtained. This may be achieved in a blender, for example, a Pony mixer or a Ribbon blender.
The resulting blended mixture may suitably be extruded to produce uniform strands or rods, which may be cut with a suitable cutter to yield pellets of a size appropriate or use in dispensing equipment used in the wire and the cable industry in the production of cellular insu-lation.
The extrusion is similar to that described above in connection with the Direct Method.
The amounts of pigment composition and blowing agent are predetermined as in the case of the Direct Method so that the particulate concentrates contain the appropriate relative amounts of pigment and blowing agent such that the addition to the screw extruder of a qiven predetermined weight of the final concentrate with the insulating polymer will provide the required amount of blowing agent for the cellular structure desired as well as the required amount of pigment to colour the insulation coating so that the colour is visibly discernible for identification purposes and so that insulation coatings of different colours and containing different pigments will be, when necessary dielectrically matched.
Blowinq Aqent Concentrate In one step of the process a concentrate of a chemical blowing agent or foaming agent is produced.
The chemical blowing or foaming agents employed are as described in connection with the Indirect ~ethod.

~3~

In the preparation of the blowing agent concentrate a predetermined or weighed amount of blowing agent material having a known content of blowing agent is mixed wi.th the organic vehicle, which includes a solid particulate, thermo-plastic,electrically insulating polymer material. The mixing is continued until a uniform powder oE agglomerates i~s obtained.
The blowing agent is suitably employed in an amount by weight of 0.5 to 1.2, preferably 0.5 to 1.0, more preferably 0.75 parts by weight per 100 parts by weight of polymer material in the insulating coating~ The amount of blowing agent in the blowing agent concentrate is thus preferably determined so that a given weight of final particulate concentrate will provide an amount of bl.owing agent in the aforementioned range per 100 parts of total polymer material, in the insulating coating to be formed.
The unifo~m powder of agglomerates is comminuted and intimately blended in a milling or grinding device, for example, a three roll ink mill of the kind described in connection with the Direct Method. A fine uniform grinding of the agglomerates is achieved to produce a dry, flaky powder in which the blowing agent material is wetted by the organic vehicle.
The dry, flaky powder comprises particles of the blowing agent material surrounded by the organic vehicle.
The particles of the blowing agent concentrate typically have an average particle size of 0.5 to 25, preferably 1 to 5 and more preferably about 3 microns.
The blowing agent concentrate contains 10 to 70 parts by weight of the blowing agent material and 30 to 90 parts by weight of the organic vehicle. In particular the -7~

blowing agent concentrate may comprise 25 to 40, especially 30 to 40 parts of the solid polymer and 0 to 50, especially 5 to 50 parts by weight of a viscous liquid vehicle such as a polybuterle, to a total of 100 parts.
In a particular embodiment the organic vehicle is added to a weic3hed amount of the particulate blowing agent material, for example, a non-plating azodicarbonamide compo-sition, while mixing, and a weighed amount of the solid polymer is added. These ingredients are mixed until a uniform mix is obtaine~.
When the organic vehicle is used, it is suitably employed in an amount sufficient to wet the par-ticulate blowing agent material mass' the organic vehicle is suitably one which is compatible both physically and electrically with the polymer which is subseqently to form the matrix of the par-ticulate concentrated pellet, and the polymer which is to form the cellular insulation.
A particularly suitable organic vehicle is a low molecular weight polybutene having a number average molecular weight of about 300 to about 1900 and a viscosity of from 150 cps -to 5000 cps at 72F. Polybutene is especially suitable when the polymer insul~tion and polymer matxix both comprise a polyethylene as indicated above in connection with the Direct Method.
~he polymer is suitably employed in an amount such that the concentrate contains 20 to 75%, preferably 40 to 75%, more preferably 40 to 55%, by weight o~ the polymer, the ratio of viscous liquid organic vehic~e, for example, polybutene fluid, in the concentrate to the polymer in the concentrate is suitably from 0:50 to 20:30.

3~ -87~
Pi~ment Concentrate A pigment concentrate is formed which comprises a particulate pigment cornposition suitably comprising amounts of a particular pigment, an opacifier and an antioxidant according to the teaching of the aforementioned Canadian Patent 616,767.
In Eor~ing the pigment concentrate, a desired pig-ment composition in powder form is mixed with an organic vehicle and the polymer carrier, the ingredients being mixed until a uniform mixture is obtained.
The mi~ture is comminuted and dispersed in a manner similar to that employed for the blowing agent concentrate.
In this way there is formed a dry flaky powder of the pigment composition. The size of the flakes depends on the piigment composition.
The pigments which produce the blue, red, ~reen, yellow, orange and purple and brown colours for the insula-tion are typically organic pigmqnts which consist of fine particles in the pigment concentrate typically having an average dimension of 0.025 to 0.125 microns, and preferably about 0.05 microns. The black colour which is usually provided by carbon black is present as particles in the pigment concentrate havi.ng an average dimension of 0.1 to 50, preferably about 20 millimicrons. The white colour provided by titanium dioxide forms tiny agglomerates in the dry flaky powderhavingan average dimension of 0.1 to 50 microns and preferably an average dimension of about 5 microns.
The slatecomposition is usually a mixture of the black and white compositions.
The composition of the pigment concentrate is determined with the samerequirements as described with reference to the ~irect Method.

~3~7~

The percentage amounts of different pigment compositions necessary for dielectric matching are described in the aforementioned Canadian Patent in relation to the polyme~ insulating material, so tha-t it requires only a simple calculation and use of conventional measuring ~echniques -to deterrnine the quantity of pellets to be employed relative to the quantity of polymer material to ob-tain the required pigment content. This having been established it similarly requires a simple calculation to determine the amount oE blowing agent required in the pellets so that the same aforementioned quantity of pellets contains the required amount of blowing agent for formation of the desired cellular structure.
The preparation of the blowing agent concentrate and the pigment concentrate may suitably be carried out at.
room temperature. However, an elevated temperature may be employed in order to lower the viscosity of the organic vehicle, so as to facilitate the mixing. In this case caution is required since the temperature should not be so high as to initiate a premature thermal decomposit,ion of the blowing agent. Elevated temperatures may also adversely affect many of the pigments employed particularly the organic pigments, consequently the temperature should not be so high as to affect the pigment.
Packaginy of Colour-Coded Concentrate The colour-coded concentrates, which preferably are pellets may be packaged in a variety of ways as described in the principal disclosure.
Cable Manufacture = _ . ~. = , . . .__ _.

The manufacture of a cable conductor wire insulated 3~

with a colour-coded insulation using the particulate concen-trate is carried out in the manner described in the principal disclosure for pellets, the pellets being introduced with a thermoplastic electrically insulating polymer material into the feed hopper of a screw extruder.
The concentrate and polymer are forced -through the screw extruder by rotation of the screw and are subjected to shearing and heating in the screw extruder ~arrel to mel-t the polymer material and intimately mix the polymer and the ingredients of the concentrate. At the temperatures employed the blowing agent begins to decompose during its passage through the barrel. When the mixture is extruded through a die of an extruder head, gas bubbles generated from the decomposing blowing agent begin to form and expand rapidly in the extruded material at the die exit. The molten mass is forced through the extruder head and about a moving con-ductor wire to form a coating on the wire. Simultaneously the gas bubbles in the extruded coating expand to produce the cellular structure, the degree of expansion being suit-ably controlled by means of a moving water trough.
In addition to communication cable described in theprincipal disclosure other types of insulating conductors can be made using the procedures, including power cable.
Cables which can be produced utilizing the procedures of the invention include telephone cable, CATV cable and Micro-coaxial cable.
It will be recoginzed that methods of assembling insulated conductors into communication cables are well known and the present invention does not reside in the assembly method per se.

~3~

The invention is further illustrated by reference to the following examples illustra-ting particular embodiments of the invention.
Example 3 In a Pony mixer there was introduced in par-ts by weight:
white pigment (ru-tile TiO2) 25 parts blowing agent (non-plating type-azodicarbonamide) 35 parts organic vehicle (polybutene fluid) 15 parts polymeric vehicle ~polyethylene wax) 25 parts The ingredients were mixed together at room temperature in the Pony mixer for 10 minutes at which time the particles were fully blended with the vehicle and the mixture is a uniform powder blend~
The powder blend was then fed through a three roll ink mill at room temperature for 10 minutes to produce a uniform flaky powder.
The flaky powder was then blended in a Pony mixer in an amount of ~0 parts by weight with ~0 parts by weight ~of a low density polyethylene having a melt flow index number of 2 and a density of 0.925, to form a uniform granular powder concentrate.
The resultant free flowing powder concentrate was fed into a screw extruder, where the concentratewas worked under shearing action at a temperature of about 180F.
The mixture was extruded as a continuous strand having a diameter of about 1/8 inch which was fed into a pelletizer cutter where extrudate was chopped into about 1/8 inch lengths.

Claims (22)

The embodiments of the invention in which an exclusive pro-perty or privilege is claimed are defined as follows:-

1. A method of producing a cable conductor wire insulated with a colour-coded, cellular thermoplastic electric-ally insulating polymer material in extruded form, which comprises:
providing a solid thermoplastic electrically insu-lating polymer material and a predetermined amount of a pelletized material comprising predetermined amounts of a particulate pigment composition and of a particulate, solid blowing agent in a polymer matrix compatible with said insulating polymer material, introducing said polymer material and said pelletized material into a screw extruder barrel, containing a screw mounted for rotation therein, to form a mixture, passing the mixture through said barrel, heating and shearing the mixture in said barrel to melt said polymer material and form an intimate mixture of said pigment composition and said blowing agent in said polymer material, allowing said blowing agent to thermally decom-pose to a gaseous product, forcing the resulting molten mixture through an extrusion head and about a moving conductor wire to form an extruded coating thereon, allowing the extruded coating to expand a predeter-mined amount, and collecting the conductor wire insulated with colour-coded, cellular thermoplastic electrically insulating polymer material.

2. A method according to claim 1, in which said polymer matrix and said polymer material are both low molecular weight polyethylene having a melt temperature in a range of about 150°F to about 170°F.

3. A method according to claim 1 or 2, wherein said blowing agent comprises azodicarbonamide.

4. A method according to claim 1 or 2, wherein said predetermined amount of blowing agent is selected to provide an amount of 0.5 to 1.2 parts by weight per 100 parts by weight total of said polymer matrix and said insulating polymer material.

5. A method according to claim 1, wherein said pelletized material further comprises an organic vehicle electrically and physically compatible with said insulating polymer, effective to wet the particulate materials to form a paste-like mass containing 25 to 50%, by weight, of said vehicle.

6. A method of producing a multi-conductor communica-tion cable containing a plurality of colour-coded dielectric-ally matched conductors comprising:
producing a plurality of conductor wires insulated with colour-coded cellular thermoplastic insulating polymer material according to the method of claim 1, the pelletized material of each extruding operation being employed in a predetermined amount such that the resulting colour-coded plurality of conductors is dielectrically matched, and forming said plurality of conductors into a multiconductor communication cable.

7. A method according to claim 6, wherein said plurality comprises colour-coded insulated conductor wires of at least two different colours.

8. A method according to claim 6, in which said polymer matrix and said polymer material are both low molecular weight polyethylene having a melt temperature in a range of about 150°F to about 170°F.

9. A method according to claim 6, 7 or 8, wherein said blowing agent comprises azodicarbonamide,

A method according to claim 6, 7 or 8, wherein said predetermined amount of blowing agent is selected to provide an amount of 0.5 to 1.2 parts by weight per 100 parts by weight total of said polymer matrix and said insulating polymer material.

11. A method according to claim 6, 7 or 8,wherein said predetermined amount of blowing agent is selected to provide an amount of 0.5 to 1.2 parts by weight per 100 parts by weight total of said polymer matrix and said insulating polymer material said blowing agent comprises azodicarbonamide.

12. A method according to claim 6, 7 or 8, including the steps of wrapping said dielectrically matched, insulated conductor wires in a tape effective to form an electrical shield and extruding a jacket of insulating material around the electric shield.

13. A method according to claim 6, 7 or 8 wherein said pelletized material further comprises an organic vehicle electrically and physically compatible with said insulating polymer, effective to wet the particulate materials to form a paste-like mass containing 25 to 50% by weight, of said vehicle.

CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE

14, A method of producing a cable conductor wire insulated with a colour-coded, cellular thermoplastic electric-ally insulating polymer material in extruded form, which comprises:
providing a solid thermoplastic electrically insu-lating polymer material and a predetermined amount of a particulate concentrate comprising predetermined amounts of a particulate pigment composition and of a particulate, solid chemical blowing agent material in a polymer matrix compatible with said insulating polymer material, introducing said polymer material and said particu-late concentrate into a screw extruder barrel, containing a screw mounted for rotation therein, to form a mixture, passing the mixture through said barrel, heating and shearing the mixture in said barrel to melt said polymer material and form an intimate mixture of said pigment composition and said blowing agent material in said polymer material, allowing said blowing agent to thermally decompose to a gaseous product, forcing the resulting molten mixture through an extrusion head and about a moving conductor wire to form an extruded coating thereon, allowing the extruded coating to expand a predeter-mined amount, and collecting the conductor wire insulated with colour-coded, cellular thermoplastic electrically insulating polymer material.

15, A method according to claim 14, in which said polymer matrix and said polymer material are both low density polyethylene having a melt temperature in a range of about 120°C to about 150°C.

16. A method according to claim 14, wherein said blowing agent material comprises azodicarbonamide.

17. A method of producing a multi-conductor communication cable containing a plurality of colour-coded dielectrically matched conductors comprising:
producing a plurality of conductor wires insulated with colour-coded cellular thermoplastic insulating polymer material according to the method of claim 14 the particulate concentrate of each extruding operation being employed in a weighed amount such that the resulting colour-coded plurality of conductors is dielectrically matched, and forming said plurality of conductors into a multiconductor communication cable.

18. A method according to claim 17, wherein said plurality comprises colour-coded insulated conductor wires of at least two different colours.

19. A method according to claim 17, in which said polymer matrix and said polymer material are both low density polyethylene having a melt temperature in the range of about 120°C to about 150°C.

20. A method according to claim 17, wherein said blowing agent material comprises azodicarbonamide.

21. A method according to claim 17, wherein said particulate concentrate is in pellet form.

22. A method according to claim 21, including the steps of wrapping said dielectrically matched, insulated conductor wires in a tape effective to form an electrical shield and extruding a jacket of insulating material around the electric shield.