HK1126031B

HK1126031B - Electric cable comprising a foamed polyolefine insulation and manufacturing process thereof

Info

Publication number: HK1126031B
Application number: HK09104452.4A
Authority: HK
Inventors: M．弗里杰里奥; F．卡西尔格齐; V．克里希; G．格拉塞利; J-L．庞斯; A．巴莱基
Original assignee: 普雷斯曼电缆及系统能源有限公司
Filing date: 2005-12-22
Publication date: 2012-08-03

Description

Cable with foamed polyolefin insulation and method for making same

Background

The present invention relates to electrical cables.

Furthermore, the invention relates to a method for manufacturing said cable.

Prior Art

Cables for power transmission are generally provided using a metal conductor surrounded by an insulating coating.

The cable may be provided with a sheath at a radially outward position relative to the insulating layer. The sheath is provided to protect the cable from mechanical damage.

US 4789589 relates to insulated conductive wires in which the insulation surrounding the conductor wire comprises a polyolefin compound and an inner layer of honeycomb structure, and an outer layer of uncured and non-curable polyvinyl chloride.

WO03/088274 relates to a cable with an insulating coating comprising at least two insulating layers, such that the insulating coating comprises at least one insulating layer made of unfoamed polymer material and at least one insulating layer made of foamed polymer material in a radial direction from the inside to the outside of the cable. In fact, foamed insulation layers exhibit discontinuities (i.e., voids within the polymeric material that are filled with air or gas) and do not work properly in the space surrounding the conductor where the electric field is most relevant.

It is reported, for example, in US4591606, that cross-linked polyolefin foams are produced by using chemical blowing agents that decompose on heating and generate gaseous nitrogen, such as azodicarbonamide. Crosslinking is usually effected with the aid of free-radical formers, for example dicumyl peroxide. The crosslinking reaction is also effected with the aid of heat. A process for the production of crosslinked polyethylene foams has also been developed, but in this case crosslinking is effected with the aid of radiation. The product of this process has a very low density and therefore applications where strength and stiffness are not required can be considered. When organic peroxides are used as crosslinking agents, the control of the process is difficult, since both the foaming and crosslinking processes are temperature dependent.

US3098831 relates to crosslinked and foamed polyethylene materials which are particularly useful as electrical insulators. The polyethylene material is said to have a density of no greater than 0.32g/cm³(20 lbs/ft)³). Examples of polyethylenes having a degree of foaming of 90-95% are provided. Foamed polyethylene is prepared by subjecting crosslinked polyethylene containing a rubber blowing agent to elevated temperatures at which the blowing agent decomposes and thereby causes the polyethylene to foam. The polyethylene starting material can be crosslinked, for example, by means of organic peroxides, wherein the amount of crosslinking agent used can generally vary from 0.002 to 0.01mol per 100g of polyethylene. Among the blowing agents, azodicarbonamide is exemplified, and about 2-15 parts by weight of the blowing agent is used, based on 100 parts of the polyethylene material.

Generally, cables for building wiring and/or industrial applications should be installed within a wall, and the installation process requires the constraint of the cable passing through the wall, or more commonly, the cable being pulled through a conduit in which the cable is permanently constrained.

In order to be correctly installed with simple and quick operation, it is necessary that the cable is particularly flexible so that it can be inserted into the passage of the wall and/or into the duct of the wall and bent along the installation path without being damaged.

During consumer installation, building cable is often prone to tearing or scrap from rough edges and/or surfaces due to buckling of the installation path and friction during towing operations.

Increasing the flexibility of the cable may allow for a reduction in damage caused by the tearing or scrapping action. As disclosed in WO03/088274 cited above, it is possible to advantageously increase the flexibility of the cable and to obtain advantageous results in its installation process by providing the cable with a foamed insulating layer.

Increased flexibility can be provided by the foamed insulation layer due to the "sponge" nature of the material. In particular, when the insulating layer consists of a single layer of foamed material, the flexibility of the cable can be maximized.

In addition, the presence of the foamed coating within the cable reduces the weight of the cable, which is advantageous in its transport and installation.

Nevertheless, the foamed insulating layer may cause problems such as the following:

the discontinuity of the foam material when in contact with the conductor may impair the insulating properties of the layer;

the foamed material in the foamed layer should have a degree of foaming that is sufficiently high to provide the desired flexibility, but this does not result in an undesirable weakening of the coating from the point of view of mechanical properties.

Another important aspect that the cable should meet is the simple and rapid stripping of the cable.

The stripping performance of cables, for example for building wires, is a widely felt requirement in the market, since stripping of cables is an operation performed manually by technicians. For this reason, it is required that the operation be carried out easily and quickly by the operator, while also taking into account that it is generally carried out in narrow spaces and under rather uncomfortable conditions.

Typically, the cable sheath is made of a mixture based on polyvinyl chloride (PVC) and including, inter alia, a plasticizer. Plasticizers tend to migrate out of the PVC skin into the insulation, thereby changing its composition. During the accelerated ageing tests, the applicant observed that this effect is significant in the case of an unfoamed insulating layer. As a result, the composition has impaired electrical (insulation) properties, impairs mechanical characteristics, and can cause premature aging of the cable, in view of the polar nature of the plasticizer.

Summary of The Invention

The applicant believes that when the polyolefin material is both foamed and crosslinked, the foamed polyolefin material may be advantageous as an insulating layer for a cable. The concurrent crosslinking and foaming provides polyolefin materials with improved flexibility and easy exfoliation without compromising the mechanical properties of the layers formed therewith.

The applicant has observed that if it is attempted to foam and crosslink polyolefins, the degree of foaming is generally impossible to control, either excessive or insufficient.

In the present invention, however, the applicant has found that suitably foamed and crosslinked insulating layers can be obtained by silane-based crosslinking systems and exothermic blowing agents. The insulation layer thus obtained has a degree of foaming that advantageously provides a cable having the above-mentioned characteristics.

In particular, the applicant has found that a polymer foamed/crosslinked insulating layer improves the aging stability of a cable with an outer sheath.

This result is considered to be due to the fact that: such an insulating layer has a better compatibility with respect to the skin material.

Definition of

For the purposes of the present description and of the claims which follow, unless otherwise indicated, all numbers expressing quantities, amounts, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Moreover, all ranges include any combination of the maximum and minimum points disclosed, and include any intermediate ranges therein, which may or may not be specifically listed herein.

In the description of the present invention, the expression "cable core" refers to a structure comprising at least one conductor and respective electrically insulating coatings arranged at positions radially outward of said conductor.

For the purposes of the present description, the expression "unipolar cable" means a cable having a single core as defined above, while the expression "multipolar cable" means a cable having at least one pair of said cores. In more detail, when a multipolar cable has a number of cores equal to 2, said cable is technically defined as a "bipolar cable", if there are three cores, said cable is called a "tripolar cable", and so on.

In the present description the term "stripping of the cable" is used to indicate that all cable layers are removed radially outwards of the conductor so that it is exposed for electrical connection to e.g. a conductor or an electrical device of a further cable.

In this specification, the expression "low voltage" refers to a voltage of less than about 1 kV.

In the present description and in the subsequent claims, by "conductor" is meant a conductive element of elongated shape, and preferably a metallic material, such as aluminum or copper.

"insulating coating" or "insulating layer" means a coating consisting of an insulating constant (k)_i) A coating or layer made of a material greater than 0.0367MOhmkm (according to IEC 60502).

In the present description and claims, "silane-crosslinked" refers to a polyolefin material having siloxane bonds (-Si-O-Si-) as crosslinking elements.

In the present description and claims, by "expanded polyolefin material" is meant a material having a certain percentage of free space (i.e. space not occupied by the polymeric material but by gas or air) inside the material, said percentage being expressed in terms of "degree of expansion" (G) defined as follows:

wherein d is₀Is the density of the unfoamed polymer, and d_eIs the apparent density measured on the foamed polymer.

According to the italian standard rule CEI EN 60811-1-3: 2001-06, the apparent density was measured.

In the present description and claims, the term "sheath" is intended to mean a protective outer layer of a cable having the function of protecting the cable from accidental impacts or abrasions. According to the foregoing, the cable sheath is not required to provide the cable with specific electrical insulation properties according to the above-mentioned terminology.

In the present description and claims, by "silane-based crosslinking system" is meant a compound or a mixture of compounds containing at least one organosilane.

In the present description and claims, by "foaming system" is meant a compound or mixture of compounds containing one or more blowing agents, at least one of which is an exothermic blowing agent.

In the present specification and claims, an "endothermic blowing agent" refers to a compound or a mixture of compounds that is thermally unstable and causes heat absorption while generating gas and heat at a predetermined temperature.

In the present description and claims, an "exothermic blowing agent" refers to a compound or mixture of compounds that is thermally unstable and decomposes at a predetermined temperature to generate gas and heat.

In the present description and claims, the "drawdown ratio" refers to the ratio of the thickness of the extruder die opening to the final thickness of the extruded product.

In a first aspect, the present invention relates to a process for preparing a cable comprising at least one core comprising a conductor and an insulating coating surrounding said conductor, comprising the steps of:

-providing a polyolefin material, a silane-based crosslinking system and a foaming system comprising at least one exothermic foaming agent, said exothermic foaming agent being used in an amount of 0.1 to 0.5% by weight relative to the total weight of the polyolefin material;

-forming a blend with a polyolefin material, a silane based cross-linking system and a foaming system;

-extruding the blend onto a conductor to form an insulating coating.

"polyolefin material" means a polymer selected from the group comprising: polyolefins, copolymers of various olefins, copolymers of olefin/unsaturated ester, polyesters, and mixtures thereof. Preferably, the polyolefin material is: polyethylene (PE), in particular low density PE (ldpe), medium density PE (mdpe), high density PE (hdpe) and linear low density PE (lldpe); ethylene-propylene elastomeric copolymers (EPM) or ethylene-propylene-diene terpolymers (EPDM); ethylene/vinyl ester copolymers, such as ethylene/vinyl acetate (EVA); ethylene/acrylate copolymers; ethylene/alpha-olefin thermoplastic copolymers; and copolymers or mechanical blends thereof.

More preferably, the polyolefin material of the present invention is selected from the group consisting of: polyethylene (PE), especially low density PE (ldpe), medium density PE (mdpe), high density PE (hdpe) and Linear Low Density PE (LLDPE), more preferably LLDPE, optionally in blends with EPDM or olefin copolymers.

When the polyolefin material of the invention is a blend of a polyethylene material and a copolymer material, the latter is advantageously present in an amount of from 5 to 30 phr.

Preferred silanes which can be used are those having at least one double bond (C)₁-C₄) Alkoxysilanes, and especially vinyl-or acryloyl- (C)₁-C₄) An alkoxysilane; compounds suitable for the purposes of the present invention may be gamma-methacryloxypropyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyldimethoxyethoxysilane, vinyltris- (2-methoxyethoxy) silane, and mixtures thereof.

The silane-based crosslinking system used in the process of the present invention comprises at least one peroxide. Preferably, peroxides which can advantageously be used are di (tert-butylperoxy) propyl- (2) -benzene, dicumyl peroxide, di-tert-butyl peroxide, benzoyl peroxide, tert-butylcumyl peroxide, 1-di (tert-butylperoxy) -3, 3, 5-trimethylcyclohexane, 2, 5-bis (tert-butylperoxy) -2, 5-dimethylhexane, 2, 5-bis (tert-butylperoxy) -2, 5-dimethylhexyne, tert-butyl-3, 5, 5-trimethylhexanoate, ethyl 3, 3-di (tert-butylperoxy) butyrate, butyl 4, 4-di (tert-butylperoxy) valerate and tert-butylperbenzoate.

Preferably, the silane-based crosslinking system used in the process of the present invention comprises at least one crosslinking catalyst selected from those known in the art; preferably, it is convenient to use an organic titanate or metal carboxylate. Particular preference is given to dibutyltin Dilaurate (DBTL).

Advantageously, the silane crosslinking system is used in an amount to provide a blend having from 0.003 to 0.015mol silane per 100g polyolefin material. Preferably, the amount of silane used is from 0.006 to 0.010mol silane per 100g polyolefin material.

Optionally, the foaming system of the process of the invention comprises at least one endothermic blowing agent, preferably in an amount equal to or less than 20% by weight with respect to the total weight of the polyolefin material.

Advantageously, the exothermic blowing agents used in the process of the present invention are azo compounds, such as azodicarbonamide, azobisisobutyronitrile and bisazaaminobenzene. Preferably, the exothermic blowing agent is azodicarbonamide.

Preferably, the exothermic blowing agent is used in an amount of 0.15 to 0.24% by weight relative to the total weight of the polyolefin material.

Advantageously, the foaming system is added to the polyolefin material in the form of a masterbatch comprising a polymeric material, preferably an ethylene homo-or copolymer, such as ethylene/vinyl acetate copolymer (EVA), ethylene-propylene copolymer (EPR) and ethylene/butyl acrylate copolymer (EBA). The masterbatch comprises a blowing agent (exothermic blowing agent, and in some cases endothermic blowing agent) in an amount of 1 to 80% by weight, preferably 5 to 50% by weight, more preferably 10 to 40% by weight, relative to the total weight of the polymeric material.

Advantageously, the foaming system further comprises at least one activator (a.k.a.kicker). Preferably, suitable activators for the foaming system of the invention are transition metal compounds.

Optionally, the foaming system of the process of the present invention further comprises at least one nucleating agent. Preferably, the nucleating agent is an active nucleating agent.

Advantageously, the process of the invention is carried out in a single-screw extruder.

Preferably, the step of extruding the blend onto a cable conductor to provide an insulation layer for such conductor comprises the steps of:

-feeding the conductor into an extruder;

-depositing an insulating layer by extrusion.

Advantageously, the step of extruding the blend is carried out by means of a die having a reduced diameter, i.e. a "draw down ratio" (DDR) of less than 1, preferably less than 0.9, more preferably less than 0.8.

Optionally, the manufacturing method of the present invention further comprises the step of providing an outer skin layer at a position radially outward with respect to the circumference of the at least one conductor coated with the relevant insulating layer. This step is carried out by extrusion.

In another aspect, the invention relates to a cable comprising at least one core consisting of a conductor and an insulating coating surrounding and in contact with said conductor, said insulating coating consisting essentially of a layer of a foamed silane-crosslinked polyolefin material having a degree of foaming of from 3 to 40%.

Preferably, the cable of the present invention has three cores as described above.

The cable of the invention is preferably a low voltage cable.

More preferred according to the invention are polyolefin materials selected from the following: polyethylene (PE), in particular low density PE (ldpe), medium density PE (mdpe), high density PE (hdpe) and linear low density PE (lldpe); more preferred is LLDPE, optionally in blend with EPDM or an olefin copolymer.

More preferably, the degree of foaming of the insulating coating for cables according to the invention is comprised between 5 and 30%, even more preferably between 10 and 25%.

Advantageously, the insulating coating of the cable of the invention exhibits a foaming characterized by a specific average cell diameter.

In particular, the insulating coating of the cable of the invention advantageously has an average cell diameter equal to or less than 300 microns, preferably equal to or less than 100 microns.

Advantageously, the insulating coating of the present invention is free from foaming in the circumferential portion in contact with and/or in the vicinity of the conductor, i.e., substantially free from cells therein.

Preferably, the cable of the invention has an outer sheath, preferably in contact therewith, in a position radially outward with respect to the insulating layer.

Preferably, the outer skin layer is made from a complex comprising polyvinyl chloride (PVC), a filler (e.g. chalk), a plasticizer (e.g. octyl, nonyl or decyl phthalate), and an additive.

In a further aspect, the present invention relates to a method for improving the aging stability of a cable comprising a conductor, an insulating layer and a sheath, wherein the insulating layer comprises a silane-crosslinked polyolefin material having a degree of foaming of 3 to 40%.

Brief Description of Drawings

Further features and advantages of the invention will become apparent in view of the following description of some preferred embodiments of the invention.

The following description refers to the accompanying drawings in which:

figure 1 shows a right-hand section of an example of a cable according to the invention;

figure 2 is a photograph of a sample of the insulation layer of a comparative cable 17;

figure 3 is a photograph of a sample of the insulation layer of the cable 19 of the invention;

figure 4 is a photograph of a sample of the insulation layer of the cable 20 of the invention.

Detailed description of the preferred embodiments

Figure 1 shows a cross section of a cable for power transmission according to the invention at low voltage.

The cable 10 is of the tripolar type (with three cores) and comprises 3 conductors 1 each covered by a foamed and crosslinked polymeric insulating coating 2. The three conductors 1 with the associated insulating coatings are surrounded by a sheath 3.

Insulation constant k of the electrically insulating barrier 2_iSuch that the required electrical insulation properties are in accordance with standards (e.g. IEC 60502 or its other equivalents). For example, the insulation constant k of the electrically insulating layer 2 at 90 ℃_iEqual to or greater than 3.67MOhm km.

The foaming degree of the insulating layer for the cable is 3-40%. In particular, the applicant observed that a degree of foaming lower than 3% does not provide a cable having significant advantages in terms of flexibility and weight reduction. On the other hand, when the degree of foaming is higher than 40%, mechanical characteristics of the cable, such as tensile strength, are impaired to an unacceptable degree as required for installation.

Fig. 1 shows only one of the possible embodiments in which the cable of the invention can be advantageously used. Thus, any suitable modification may be made to the above-mentioned embodiments, for example using a multipolar type of cable or a conductor of sector cross-section.

According to the invention, in order to impart suitable mechanical resistance to the insulating coating without reducing the flexibility of the cable, the foamed polyolefin material is obtained from a polyolefin material having a flexural modulus at room temperature of from 50MPa to 1000MPa, measured according to ASTM standard D790-86, before foaming. Preferably, the flexural modulus at room temperature is not more than 600MPa, more preferably it is from 100MPa to 600 MPa.

The cable of fig. 1 can be produced, for example, by a process carried out in an extrusion apparatus having a single-screw extruder with a diameter of 60-175mm and a length of about 20D-30D, wherein these characteristics are selected in view of the diameter and/or the desired production speed of the resulting cable.

Suitably, the screw may be a single flight screw with barrier flights optionally present in the deflector zone; preferably no mixer device is employed along the screw.

The extrusion device is advantageously fed by a multi-component dosing system of the gravity type or preferably of the volumetric type. The dosing system can feed the ingredients (polyolefin material, silane-based crosslinking system and foaming system).

If it is desired to color the cable (either fully colored or with a colored skin coat), a pigment masterbatch may be used.

The ingredients mentioned above are advantageously fed in pellet form into the feed throat of the extruder and dosed in the desired percentages by gravity or a volume control system. The preliminary mixing of the ingredients off-line or in the hopper above the feed inlet can advantageously improve the dispersion of the components and the quality of the final product.

Optionally, the crosslinking system (typically obtained in liquid form) is introduced into the extruder by injection at low pressure (lbar) at the bottom of the extruder hopper (top of the feed inlet); wherein the percentage of the crosslinking system introduced can be measured by gravity or volume.

For example, the ingredients listed above are fed into the extruder port, heated, melted and mixed by screws along the extruder, and finally metered into the extrusion cross head.

Along the extruder, the silane groups grafted onto the polymer chains were chemically activated and the crosslinking process started.

The foaming of the polyolefin material used for the insulating coating of the invention can be achieved by means of specific foaming agents. Such blowing agents are advantageously chosen from exothermic blowing agents, in particular azo compounds, such as azodicarbonamide, azodiisobutyronitrile and diazoaminobenzenes. Azo compounds are preferred blowing agents because of their chemical inertness with respect to the reactants used in the preparation of the insulating coating, in particular with respect to the crosslinking system.

The foaming system is blended with the other ingredients and starts to decompose at a predetermined temperature. After the reaction, the gas generated by the foaming system remains dispersed inside the blend.

The blend after passing through the filtration unit is fed, for example, to a crosshead where it is distributed around the conductor in a text configuration relative to the extruder. In the die area, the conductor is coated by the blend, and after the die, when the pressure is released, the blend begins to foam. After a length of, for example, 1m in which the coated conductor is exposed to the environment, it is immersed into a cooling bath, where it is cooled by swirling water or other similar cooling liquid. The cooling bath may be of the single pass or multiple pass type.

As soon as the melt has cooled down, the foaming phase of the extruded insulation layer is terminated immediately, and should therefore take place in a short time.

At the end of the cooling unit, the insulated conductor is dried, for example by using a gas-jet system or heating, and then collected on a rotating drum.

At this stage, the crosslinking of the insulating coating optionally continues with the aid of water and temperature. The time delay to complete the crosslinking phase can be reduced by placing a rotating drum with an insulated conductor inside the curing chamber (sauna).

The step of extruding the blend can be carried out by means of a die having a diameter decreasing according to the "draw down ratio" (DDR) in order to increase the compression on the molten compound and to obtain a foam having an improved regularity and size of the cells.

According to the above, in the process of the invention, the exothermic blowing agent is used in an amount of 0.1 to 0.5% by weight relative to the total weight of the polyolefin material. Amounts below 0.1% by weight give negligible foaming of the polyolefin material, while on the other hand the accompanying examples show that amounts above 0.5% by weight give such high foaming, with consequent impairment of the mechanical characteristics of the product.

The foaming system of the invention may further comprise at least one activator, such as zinc-, cadmium-or lead-compounds (oxides, salts (usually fatty acid salts) or other organometallic compounds), amines, amides and glycols.

The foaming system of the process of the present invention may further comprise at least one nucleating agent. Nucleating agents provide nucleation sites where physical blowing agents can flow out of solution during foaming expansion; the nucleation site refers to the starting point at which the cells of the foam begin to grow. If the nucleating agent can provide a higher number of nucleation sites, more cells will be formed and the average cell size will be smaller.

Two types of nucleating agents can be used in the process of the present invention: inactive (or deactivated) and active nucleating agents. Inactive nucleating agents include solid materials having a particle size such as talc, clay, diatomaceous earth, calcium carbonate, magnesium oxide, and silica. These materials act as nucleating agents by providing interference within the system as the blowing agent begins to bubble out of solution. The efficiency of these materials is affected by the shape and size of the particles. Chemical blowing agents, which generate gases upon decomposition, such as azodicarbonamide, may also act as active nucleating agents. Nucleation of systems that directly generate gas using chemical blowing agents is referred to as "active nucleation". Active nucleating agents are preferred over inactive nucleating agents because of their higher efficiency and provide smaller and more uniform cells.

The silane crosslinking system is used in an amount to provide a blend having from 0.003 to 0.015mol silane per 100g polyolefin material. Silane amounts below 0.003mol do not provide sufficient crosslinking of the polyolefin material, while amounts above 0.015mol, in addition to large excesses, can cause screw slippage within the extruder.

Example 1

Low voltage cables of the invention and not of the invention were prepared according to the cable design shown in fig. 1.

Cable conductor 1 made of copper and having a cross-section of about 1.5mm²。

Main extruder size: 150/26D

A tip die head: 1.38mm

An annular die head: 2.70mm

Foaming mb quantitative feeding system: maguiire (gravity type)

Temperature profile (. degree. C.):

Z1	Z2	Z3	Z4	Z5	Z6	H1	H2	H3	H4
										160	180	190	200	210	220	220	230	240	240

linear velocity: 1500m/min

Main extruder speed: 48rpm

Current: 65A

Pressure: 380bar

Diameter of the thermal cable: 2.9mm

Diameter of the cold cable: 2.9mm

Each insulating coating has a thickness of about 0.6mm, 0.7mm, according to the italian standard CEI-UNEL 35752 (2 nd edition-2 months 1990).

Each cable was then cooled in water and wound onto a storage reel.

The degree of foaming of each polymer blend is also listed in table 1.

TABLE 1

N.b. -mol and% w/w refer to the content of silane or blowing agent, respectively.

The cables marked with asterisks are comparative cables.

LL 4004EL LLDPE (ExxonMobil chemical) having an MFL of 0.33g/10min at 190 ℃ under a load of 2.16kg

BPD 3220＝LLDPE(BP)

Sil/perox ═ LUPEROX 801(Arkema) plus DYNASYLAN VTMO (Degussa)

Silfin 06 ═ a mixture of vinyl silane, peroxide initiator and crosslinking catalyst (Degussa)

Hostatron ═ PV22167 foaming system based on azodicarbonamide foaming agents (Clariant)

Hostatron 50% ═ 50% of PV22167 foaming system based on azodicarbonamide foaming agent in EVA masterbatch (Clariant)

Hydrocerol ═ BIH 40, foaming system based on a mixture of citric acid and soda lye as foaming agent (Clariant)

The composition of the blend (expressed in parts by weight per 100 parts by weight of base polymer) is shown in table 1.

The% w/w of blowing agent refers to the amount of blowing agent added.

Cables 1 and 3 (no blowing agent used) were provided as a reference for calculating the degree of foaming, and for electrical testing of cables having crosslinked and foamed insulation layers.

The cables 15-17 concerned were insulated by polymer blends foamed with endothermic blowing agents (Hydrocerol).

Cables 11 and 14 are insulated by polymer blends foamed with an amount of exothermic foaming agent outside the preferred ranges. In the case of the cable 11, the degree of foaming is substantially 0, so this cable has no advantage in terms of flexibility and peelability over cables having a non-foamed insulating coating. On the other hand, the cable 14 shows an insulating coating with too high a degree of foaming and a loss of mechanical properties, as can be seen from example 3.

Example 2

According to the Italian standard rule CEI EN 60811-2-1: 1999-05, the cable produced as in example 1 was tested to evaluate the degree of crosslinking of its insulating coating. The results are listed in table 2.

TABLE 2

The cables marked with asterisks are comparative cables.

Considering that the limit specified by the above-mentioned requirements is at most 175%, the cable 16 is shown to be out of standard (out of scale), i.e. the polyolefin is not sufficiently crosslinked, and this negatively affects the resistance to thermal stresses. The cables 17 break up due to the excessive average cell diameter and irregular cell distribution within the foamed polyolefin, as shown in figure 2. The two failure cases reported in table 2 are due to the use of an endothermic blowing agent as the sole blowing agent in the process for producing crosslinked and foamed polyolefin materials. Endothermic blowing agents may interact negatively with silane-based crosslinking systems.

Example 3

According to the Italian standard rule CEI EN 60811-1-1: 2001-06, the cable produced as in example 1 was tested to measure its mechanical properties, requiring a tensile strength of at least 12.5 MPa. The results are listed in table 3.

TABLE 3

The cables marked with asterisks are comparative cables.

Cable 14 was insulated by using polymer blends foamed with the inventive exothermic blowing agent in amounts outside the selected range (higher) to provide a degree of foaming (48.0%) non-inventive insulating coating. Such cables exhibit unsuitable mechanical characteristics.

Cable 15 is insulated by a polymer blend foamed with a endothermic blowing agent and has an insulating coating with a degree of foaming within the range of the invention (34.0%), but always shows poor mechanical characteristics. This is due to the use of endothermic blowing agents which give unsatisfactory degrees of foaming, from a qualitative point of view.

Example 4

In table 4 below, the mechanical properties and heat-set as well as the average cell diameter of two cables according to the invention and one comparative cable were evaluated together.

The average cell diameter was evaluated as follows. The foamed portion of the insulating coating was randomly selected and cut perpendicular to the longitudinal axis. The cut surface was observed by a microscope, and an image was formed on the photograph. The major diameters of 50 randomly selected cells were measured (considering that the cells may not be perfectly round). The arithmetic mean of the 50 measured diameters represents the average cell diameter.

For each cable, two samples were tested. All cables differ from those of the previous embodiments only in that the cross-section of the conductor 1 is about 2.5mm²。

The insulation coatings of cables 17 and 19 were extruded at DDR ═ 1, and the insulation coating of cable 20 was extruded at DDR ═ 0.7.

The draw down ratio is calculated by comparing the cross-sectional area of the die to the cross-sectional area of the extrusion. The following formula is adopted:

wherein DDR is the draw ratio

D_dInner diameter of ring-type die head

D_mOuter diameter of tip die

D_tPipeline with outer diameter

D_bPipeline with inner diameter

TABLE 4

TS tensile Strength

Elongation at break EB ═

The cables marked with asterisks are comparative cables.

It was found that a reduction in the average cell diameter would improve the mechanical characteristics of the insulation layer, such as heat-set and tensile strength.

The insulation layer of cable 17 has a similar degree of foaming as the cable of the invention, but the average cell diameter is higher. The cables 17 have a high average cell diameter and are accompanied by non-uniform foaming, as can be seen in figure 2.

The cables 19 and 20 of the invention have improved mechanical properties with respect to the comparative cable 17. In particular, cable 20 had the same degree of foaming as cable 19, but the average cell diameter was lower due to lower extrusion DDR, and had excellent tensile strength. Fig. 3 and 4 show the cables, respectively.

Example 5

To measure the ease of stripping the insulating coating material from the conductor, the same cable as in example 4 was tested and compared to unfoamed cable 3.

For each cable, 6 samples 120mm long were provided. Each sample was peeled off beforehand to a degree of 40mm so that an 80mm sample was used in the test conducted according to MIL-W-22759.

The results are listed in table 5 below.

TABLE 5

The force used to peel off the inventive cable is less than the reference cable 3 with an unfoamed insulation. The maximum load is the force employed to initiate exfoliation.

Example 6

Three cables produced according to example 1 and sheathed with PVC containing decyl phthalate as plasticizer (sheath thickness 1.56mm) were tested to evaluate the mechanical characteristics after 7 days at 100 ℃ (ageing test according to EN 60811). According to the experimental requirements, the maximum variation of the tensile strength should not exceed ± 25%. The results are listed in table 6.

TABLE 6

The cables 4-6 of the present invention pass this test, whereas the reference cable 3 with an unfoamed insulation layer does not.

The presence of the foamed insulating layer improves the mechanical properties after the compatibility test, reducing the negative effects of the migration of the plasticizer present in the cable sheath.

Claims

1. A process for preparing a cable comprising at least one core comprising a conductor and a foamed and crosslinked insulating coating surrounding and in contact with said conductor, the process comprising the steps of:

-extruding the blend onto a conductor to form an insulating coating.

2. The method of claim 1 wherein the polyolefin material is selected from the group consisting of polyolefins, copolymers of olefins/unsaturated esters, and mixtures thereof.

3. The method of claim 1 wherein the polyolefin material is selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, linear low density polyethylene, ethylene-propylene elastomeric copolymers, ethylene-propylene-diene terpolymers, ethylene/vinyl ester copolymers, ethylene/acrylate copolymers, ethylene/alpha-olefin thermoplastic copolymers, and mechanical blends thereof.

4. The method of claim 3, wherein the polyolefin material is selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, linear low density polyethylene, and blends thereof with ethylene-propylene-diene terpolymers or olefin copolymers.

5. The method of claim 4 wherein the polyolefin material is selected from the group consisting of linear low density polyethylene and blends thereof with ethylene-propylene-diene terpolymers or olefin copolymers.

6. The process of claim 1 wherein the silane-based crosslinking system comprises at least one double bond selected from (C)₁-C₄) At least one silane of the alkoxysilanes.

7. The method of claim 6, wherein the at least one silane is selected from the group consisting of vinyl-and acryl- (C)₁-C₄) An alkoxysilane.

8. The method of claim 7, wherein the at least one silane is selected from the group consisting of gamma-methacryloxypropyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyldimethoxyethoxysilane, vinyltris- (2-methoxyethoxy) silane, and mixtures thereof.

9. The method of claim 1, wherein the silane-based crosslinking system comprises at least one peroxide.

10. The process of claim 9 wherein the at least one peroxide is selected from the group consisting of di (t-butylperoxy) propyl- (2) -benzene, dicumyl peroxide, di-t-butyl peroxide, benzoyl peroxide, t-butylcumyl peroxide, 1-di (t-butylperoxy) -3, 3, 5-trimethylcyclohexane, 2, 5-bis (t-butylperoxy) -2, 5-dimethylhexane, 2, 5-bis (t-butylperoxy) -2, 5-dimethylhexyne, t-butyl-3, 5, 5-trimethylhexanoate, ethyl 3, 3-di (t-butylperoxy) butyrate, butyl 4, 4-di (t-butylperoxy) valerate and t-butyl perbenzoate.

11. The method of claim 1, wherein the silane-based crosslinking system comprises at least one crosslinking catalyst.

12. The method of claim 11, wherein the at least one crosslinking catalyst is selected from the group consisting of organic titanates and metal carboxylates.

13. The method of claim 12, wherein the at least one crosslinking catalyst is dibutyl tin dilaurate.

14. The method of claim 1, wherein the silane crosslinking system is added in an amount to provide a blend having from 0.003 to 0.015mol silane per 100g polyolefin material.

15. The method of claim 14 wherein the silane crosslinking system is added in an amount to provide a blend having from 0.006 to 0.010mol silane per 100g polyolefin material.

16. The method of claim 1, wherein the foaming system comprises at least one endothermic blowing agent.

17. The process according to claim 16, wherein the at least one endothermic blowing agent is used in an amount equal to or less than 20% by weight with respect to the total weight of the polyolefin material.

18. The method of claim 1, wherein the exothermic blowing agent is an azo compound.

19. The method of claim 18, wherein the azo compound is selected from the group consisting of azodicarbonamide, azobisisobutyronitrile, and bisazo aminobenzene.

20. The method of claim 19, wherein the azo compound is azodicarbonamide.

21. The process according to claim 1, wherein the amount of exothermic blowing agent ranges from 0.1 to 0.5% by weight relative to the total weight of the polyolefin material.

22. The process of claim 21, wherein the amount of exothermic blowing agent used is between 0.15 and 0.24 wt% relative to the total weight of the polyolefin material.

23. The method of claim 1, wherein the foaming system is added to the polyolefin material in the form of a masterbatch containing the polymer material.

24. The method of claim 23, wherein the masterbatch of polymeric material is selected from the group consisting of ethylene homopolymers and ethylene copolymers.

25. The method of claim 24, wherein the masterbatch of polymeric material is selected from the group consisting of ethylene/vinyl acetate copolymer, ethylene-propylene copolymer, and ethylene/butyl acrylate copolymer.

26. The method of claim 23, wherein the masterbatch comprises the blowing agent in an amount of 1 to 80 wt% relative to the total weight of the polymeric material.

27. The method of claim 26, wherein the blowing agent is present in an amount of 5 to 50 wt% relative to the total weight of the polymeric material.

28. The method of claim 27, wherein the blowing agent is used in an amount of 10 to 40 wt% relative to the total weight of the polymeric material.

29. The method of claim 1, wherein the foaming system comprises at least one activator.

30. The method of claim 29, wherein the at least one activator is selected from transition metal compounds.

31. The method of claim 1, wherein the foaming system comprises at least one nucleating agent.

32. The method of claim 31, wherein at least one nucleating agent is an active nucleating agent.

33. The process of claim 1, wherein the step of forming a blend having the polyolefin material, the silane-based crosslinking system, and the foaming system is performed in a single screw extruder.

34. The method of claim 33, wherein the extruder is fed by a volumetric type multi-component dosing system.

35. The method of claim 1, wherein the step of forming the blend having the polyolefin material, the silane-based crosslinking system, and the foaming system follows the step of off-line mixing the polyolefin material, the silane-based crosslinking system, and the foaming system.

36. The process of claim 1 wherein the step of extruding the blend onto a cable conductor to provide an insulation layer for such conductor comprises the steps of:

-feeding the conductor into an extruder;

-depositing an insulating layer by extrusion.

37. The process of claim 1, wherein the step of extruding the blend is performed with the aid of a die having a drawdown ratio of less than 1.

38. The process of claim 37, wherein the draw ratio is less than 0.9.

39. The process of claim 38, wherein the draw ratio is less than 0.8.

40. The method of claim 1, comprising applying to the substrate a coating of a dielectric material having a dielectric coating of interest

The step of extruding the outer skin layer at a location radially outward of the circumference of the at least one conductor.

41. A cable comprising at least one core consisting of a conductor and an insulating coating surrounding and in contact with said conductor, said insulating coating consisting essentially of a layer of a foamed silane-crosslinked polyolefin material having a degree of foaming of from 3 to 40%.

42. The cable of claim 41 which is a low voltage cable.

43. The cable of claim 41 comprising three cores.

44. The cable according to claim 41, wherein the polyolefin material is selected from the group consisting of polyolefins, copolymers of olefins/unsaturated esters, and mixtures thereof.

45. The cable of claim 44 wherein the polyolefin material is selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, linear low density polyethylene, ethylene-propylene elastomeric copolymers, ethylene-propylene-diene terpolymers, ethylene/vinyl ester copolymers, ethylene/acrylate copolymers, ethylene/α -olefin thermoplastic copolymers, and copolymers or mechanical blends thereof.

46. The cable according to claim 45 wherein the polyolefin material is selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, linear low density polyethylene, and blends thereof with ethylene-propylene-diene terpolymers or olefin copolymers.

47. The cable according to claim 46 wherein the polyolefin material is selected from the group consisting of linear low density polyethylene and blends thereof with ethylene-propylene-diene terpolymers or olefin copolymers.

48. The cable according to claim 46, wherein the polyolefin material is a blend of a polyethylene material and a copolymer material, wherein the latter is present in an amount of 5-30 phr.

49. A cable according to claim 41 wherein the insulating coating has a degree of foaming of from 5 to 30%.

50. The cable according to claim 49, wherein the insulating coating has a degree of foaming of 10 to 25%.

51. The cable according to claim 41 in which the insulating coating has an average cell diameter equal to or less than 300 microns.

52. The cable according to claim 51 in which the insulating coating has an average cell diameter equal to or less than 100 microns.

53. The cable of claim 41 wherein a circumferential portion of the foamed insulating coating contacting the conductor is unfoamed.

54. The cable of claim 41 having an outer sheath layer positioned radially outwardly relative to the insulation layer.

55. A method of improving the aging stability of a cable comprising a conductor, an insulating coating and an outer sheath, the method comprising providing around the conductor an insulating coating in contact therewith, wherein the insulating coating consists essentially of a layer of foamed silane-crosslinked polyolefin material having a degree of foaming of from 3 to 40%.