NO324458B1 - Electrical wiring insulation - Google Patents

Abstract

An electrical wire or cable having insulation comprising (i) at least a first layer of a polyolefin-based formulation, of which at least 20 %, preferably at least 40 %, more preferably at least 60 % or very preferably at least 80 % of the weight of the polymeric portion of the said formulation consists of a carbonyl-containing polymer (homopolymer or copolymer or terpolymer), of which polymer the or at least one constituent monomer is a carboxylic acid ester, preferably an acrylate or acetate, especially an alkyl acrylate (preferably methyl acrylate, ethyl acrylate, propyl acrylate or butyl acrylate), the said monomer itself constituting at least 5 %, preferably at least 9 %, more preferably at least 15 % by weight of the said co-, or ter- polymer when used, and the remainder or the majority of the remainder of the said co-, or ter- polymer preferably being derived from olefinic monomer, preferably ethylene; in contact with (ii) at least a second layer of another material formulation, containing at least 10 %, more preferably at least 50 %, very preferably at least 90 %, especially 100 %, by weight of the second layer, of polyvinylidine fluoride (PVDF), or especially preferably a copolymer based on VDF with a partially or fully fluorinated co-monomer, most preferably a copolymer of VDF and hexafluoropropylene (HFP); wherein the said layers (i) and (ii) whilst in contact with each other have been subjected to cross-linking reaction, preferably by radiation, more preferably ionising radiation, sufficient to prevent delamination of the two layers during a 1 hour acetone immersion test at 23 DEG C, or to increase the peel bond strength between the said layers to at least 5N according to the ASTM B1876-95 method described below preferably increasing the bond strength by at least 50 %, more preferably by at least 100 %, especially by at least 500 % or 1000 %, compared to that between the uncrosslinked layers.

Description

This invention relates to insulation for electric wire or cable (hereinafter "wire") where a strong bond is achieved at the interface between a layer of polyolefin-based material and a layer of polyvinylidene fluoride-based material. The invention also relates to a method for producing such an electric wire or cable. The invention is particularly applicable to a multi-layer insulation on electrical cables and it makes it possible to achieve high-quality bonds between layers of such materials while maintaining an acceptable balance in the complex contexts with other usage requirements for the cable, which are special and different from the criteria for other types of articles, such as molded articles or packaging films.

The following abbreviations will henceforth be used:

PJ = Primary mantle; pro-rad = crosslink accelerator; TMPTM = trimethylolpropane trimethacrylate; ASTM = American Society for Testing and Materials; PVDF = polyvinylidene fluoride; VDF = vinylidene fluoride; HFP = hexafluoropropylene; HDPE = high density polyethylene; EEA = ethylene/ethyl acrylate; EMA = ethylene/methyl acrylate; EVA = ethylene/vinyl acetate; EA = ethyl acrylate; MA = methyl acrylate; VA = vinyl acetate.

Double conductor insulation comprising an inner layer of polyolefin (core) and with polyvinylidene fluoride (PVDF) in the outer layer (primer jacket, PJ) has been commercially available for over 30 years and is available from many different manufacturers. These products all have a negligible adhesion between the inner layer (polyolefin) and the outer layer (PVDF), and these are consequently easily separable. It has been necessary to accept certain disadvantages as a result of this lack of bonding, which limits the robustness of the construction. For example, the outer layer can crack and peel off from the inner layer if the cord is subjected to mechanical stress, exposure to certain fluids, contact with sharp objects, or impact. The insulation's resistance to wear and bending fatigue, as well as resistance to forming wrinkles when bending (which can cause difficulties when sealing the wire or when it is fed into cable glands or connectors) is also devastatingly affected by having two easily separable layers of insulation. It has been assumed that it has not been possible to bond together two layers of such different material classes as polyolefins and PVDF on one wire at a commercially acceptable cost and with acceptable production efficiency. Available bonding techniques can also unacceptably affect the service characteristics of the wire. The conventional approach to bonding polyolefins and PVDF has been to use a bonding layer material (see, for example, US Patent No. 5,589,028), but such materials tend to be expensive, and when applied to a wire can set other properties at risk, such as heat ageing. The manufacturing process of forming the extra layer will also be more complex. The bonding materials can also have a limited effect in terms of the bond strength achieved.

It has now been found according to the present invention that the various insulating materials in a polyolefin-based core and a polyvinylidene fluoride-based PJ on an electric wire or cable can be bonded together with a significant level of adhesion, that this bonding tends to reduce or eliminate the aforementioned problems with the cable's robustness, and that, contrary to what has been expected, it is possible to achieve this bond without unacceptable impacts on the resistance to crack propagation, costs or the general balance of the cable's performance characteristics.

In the wire or cable insulation according to the present invention, a significant bond strength has unexpectedly been achieved with a combination comprising a particular formulation of a polyolefin-based layer in contact with a polyvinylidene fluoride-based layer, and a cross-linking reaction which is preferably carried out by using radiation , especially ionizing radiation.

The invention therefore provides an electric wire or cable which has an insulation which is characterized by the fact that it comprises

(i) at least a first layer of a polyolefin-based material comprising at least 20% by weight of a carbonyl-containing homopolymer, copolymer or terpolymer, where the monomer or at least one of the monomers that make up the polymer is an acrylic, an acetate or another carboxylic acid ester, and this monomer constitutes at least 5% by weight of the co- or terpolymer, and the remainder of the co- or terpolymer is formed from ethylene or another olefinic monomer, in contact with (ii) at least a second layer of a material containing at least 10% by weight, based on the entire material mixture, of polyvinylidene fluoride (PVDF) or a copolymer based on vinylidene fluoride (VDF) and a partially or fully fluorinated comonomer, wherein the layers (i) and (ii) when in contact with each other, have been subjected to a cross-linking reaction sufficient to increase the delamination strength between the layers, when determined for bonded strips of the two materials used to form the layers, to at least 5 N measured with the test method ASTM Bl876-95. According to another aspect of the invention, an electric wire is provided with an insulation which is characterized in that it comprises (i) at least a first layer of a polyolefin-based formulation where at least 20% by weight of the polymer part in the formulation consists of a carbonyl-containing homopolymer, copolymer or terpolymer, where the monomer or at least one of the monomers that make up the polymer is an acrylate, acetate or another carboxylic acid ester, and this monomer makes up at least 5% by weight of the co- or terpolymer, and the remaining part, or the majority of the remaining part, of co- or the terpolymer is preferably formed from ethylene or another olefinic monomer, in contact with (ii) at least a second layer with a different material formulation, where at least 10% by weight of the second layer is of polyvinylidene fluoride (PVDF) or a copolymer of vinylidene fluoride

(VDF) and hexafluoropropylene (HFP), or another copolymer based on VDF and a partially or fully fluorinated comonomer,

wherein the layers (i) and (ii), while in contact with each other, have been subjected to a cross-linking reaction by irradiation or another cross-linking reaction sufficient to prevent delamination of the two layers by immersion in acetone for 1 hour at 23° C, or sufficient to increase the delamination strength between the two layers to at least 5 N, determined for bonded strips of the two materials used to form the layers, and measured by test method ASTM Bl876-95, or sufficient to increase the delamination strength by at least 100% relative to the delamination strength between corresponding non-crosslinked layers.

Preferably, the crosslinking reaction has increased the bond strength by at least 500% or 1000% of the bond strength between the non-crosslinked layers.

The invention also provides a method for producing a wire or cable according to the invention, where the method is characterized by the fact that it comprises the steps of bringing the layers (i) and (ii) in contact with each other on an electrical conductor, and then exposing the layers, while in contact with each other, for a crosslinking reaction.

Preferably, the respective layers are brought into contact with each other at a temperature above the melting point or the softening point of the polymer material in at least one of the layers. Thus, maximum contact will be achieved at the interface and thereby possibly make it easier to form adhesion-improving crosslinks at the interfaces during the subsequent crosslinking reaction.

The polyolefin-based layer (i) can, in addition to the polymer in the formulation, which must meet the requirements stipulated above, contain what is otherwise necessary with regard to additives, such as per se known antioxidants, pigments, fillers, flame -inhibiting agents, etc, to achieve the desired mechanical, thermal, electrical, etc, properties of the polymer.

The polyvinylidene fluoride-based layer (ii) can also contain other additives known per se to achieve the necessary properties in addition to bonding.

Advantages of achieving a strong bond according to the invention include:

- wear resistance of the surface layer, and the insulation as a whole, can increase if it (the surface layer) is bonded to a substrate material,

- improved resistance to delamination, especially when one of the layers is damaged/perforated,

- improved resistance to blisters in the two layers in case they are exposed to heat,

- improved resistance to delamination/cooling/wrinkling between the two layers, for example due to mechanical stress or chemical exposure to e.g. solvents, - achievement of reduced wrinkling when bending the wire and improvement of the features above, while maintaining satisfactory resistance to cutting through and crack propagation, the latter being unexpected for the strongly cohesive layers would normally be expected to tear all the way through the innermost layer when there is a cut or chip in the outermost layer.

The bond strength described in this application can be measured as delamination strength between bonded strips of the two materials in question. A standard method that can be used for such a test is ASTM 1876-95. With this definition, a significant bond strength will have a delamination strength of over 5 N, and a strong bond will have a delamination strength of over ION. The bond strength between layers (i) and (ii) applied to a wire is conveniently determined by placing a wire sample of total length 60 mm in acetone (for example AR certified acetone grade from Fischer Scientific UK) so that 70% of the length of the wire sample is in acetone at 23 °C ± 3 °C for a period of 1 hour. Cables with negligible bonding between the insulation layers will have an extension of the PVDF in the PJ, in the length of the cable, which is independent of possible extension of the polyolefin primary insulation, and/or wrinkling of the PJ so that it delaminates from the primary insulation in places. When this occurs, the above extension of the PJ will typically result in a PJ "tube" extending 1 mm or more beyond the cut end of the lead sample as a result of the test above. Cables with significant bonding between the insulation layers will have the same extension of the primary insulation and PJ, without separation, beyond the cut end of the conductor in the longitudinal direction of the cable, and/or wrinkling of the primary insulation and PJ layers together, without delamination. Any such wrinkling of primary insulation and PJ together can only be distinguished from wrinkling of PJ by microscopic examination of a cross-section through the wrinkles.

Methods of making the wire may include any process that involves intimate contact between the above-mentioned layers (i) and (ii). Examples include coating one material on a pre-formed layer of the other material, double or multi-wall extrusion to form insulating layers containing one and the other of the above two material classes respectively. The olefin-based material (i) preferably constitutes the innermost layer and the PVDF-based layer (ii) preferably constitutes the outermost layer of the wire. The layers made of the two different materials can be co-extruded, tandem-extruded, extruded in several passes or coated in other ways. Known wire insulation processes, such as pulling down extruded hose, can be used to form one or more layers. Pressure extrusion as it is known is preferred for optimal adhesion between a pre-formed underlying layer and the second, and possibly additional, insulation layers to be applied.

The wire insulation is exposed to a cross-linking reaction which may involve chemical reagents such as peroxides, but this is preferably carried out with irradiation, especially with a source that provides ionizing radiation with the ability to form free radicals and thus cross-link the polymers. Some of the free radicals should preferably be formed in the interface area between the two materials. It is desirable that the radiation penetrates the material at least to the interface, although it is not necessarily essential if the ion or radical mobility, for example, means that the molecular reactions can continue at or near the interface after the irradiation process. The radiation source can be, for example, a radioisotope or an X-ray source, or possibly a non-ionizing radical-forming source, for example a UV source, but electron radiation is preferred, more preferably electron radiation which gives a radiation dose of more than 2 Mrad, preferably of at least 5 Mrad, more preferably at least 10 Mrad, very preferably at least 15 Mrad inside the material.

It has been found that the bond strength between the interfaces can be improved by using certain additives. Preferred additives include a cross-linking accelerator ("pro-rad") in the polyolefin-based material and/or in the PVDF-based material. Known cross-linking materials can be used, for example methacrylate/acrylate-based materials, and very preferably materials of the type trimethylolpropane trimethacrylate (TMPTM) are used in the polyolefin material and/or in the PVDF-based material.

Test results:

All results referenced in the tables below were obtained by testing pressed sample sheets of two materials prepared by common, known polymer handling techniques. The test plates were pressed together to bond to each other surface to surface, and the bonded assembly was irradiated as indicated. These demonstration tests were carried out with test plates instead of wires because it is relatively easy to measure the bond strength of test plates. The test conditions were as follows: Dimensions of test plates: 150 mm x 150 mm x 0.85 mm

Temperature during pressing: 200 °C

Time for pressing: 2 minutes preheating, 1 minute pressing

Pressing pressure: 20-40 tonnes on a 300 mm x 300 mm metal plate Cooling conditions: 2 minutes between water-cooled 300 mm x 300 mm metal plates at a pressure as above.

Example of the effect of radiation dose on the bond strength between suitable polyolefin material and PVDF-based material

Example of the effect comonomer content in the ethylene copolymer material has on the bond strength between it and a suitable PVDF-based material after cross-linking with electron irradiation Example of the effect the copolymer content in a polyolefin mixture has on the bond strength between it and a suitable PVDF-based material after cross-linking with electron irradiation

Example of the effect the type of PVDF-based material has on the bond strength between it and a suitable polyolefin-based material after cross-linking with electron irradiation Example of the effect the addition of pro-rad in an olefin material has on the bond strength between it and a suitable PVDF-based material after cross-linking by electron irradiation

Examples of wire construction

An electrical wire in which the insulation according to the present invention consisted of two polymer layers bonded together was produced as follows: The innermost layer of the insulation (i.e. closest to the conductor of the wire) was a polyolefin-based material consisting mainly of (a) an EEA copolymer containing 15 wt% EA and (b) HDPE in a weight ratio of approx. 8:2 of copolymer:HDPE, with common additives present in small proportions, including cross-linking accelerators, stabilizers, antioxidants, pigments and processing aids in a total amount of 24% by weight. This layer was pressure extruded onto the metal conductor.

The outermost layer of the insulation consisted mainly of PVDF/FIFP copolymers containing 10% by weight of HFP, which in this example contained a cross-linking accelerator and other known additives such as pigments, plasticizers, stabilizers, antioxidants and processing aids used in normal conditions and with a total of 7, 5% by weight. This outermost layer was press-extruded in a separate operation onto the already formed innermost layer. This coated wire product was then passed through an electron beam and was given a radiation dose of 20 Mrad.

In a second example, a wire was prepared as above, but where the crosslinking accelerator in the innermost layer was 4% TMPTM, and the outermost layer of insulation comprised only PVDF/HFP copolymer containing 10% by weight HFP. This coated wire product was then passed through an electron beam and was given a radiation dose of 20 Mrad. This wire was subjected to the acetone immersion test, which confirmed that the insulation layers were firmly bonded together.

In a third example, a wire of the same construction as in the second example was produced by tandem pressure extrusion of the innermost and outermost insulation layers. This coated wire product was then passed through an electron beam and was given a radiation dose of 20 Mrad. This wire was subjected to the acetone immersion test, which confirmed that the insulation layers were firmly bonded together.

Demonstration of improved properties of wires constructed as in the second example above, compared to the wires that are in the trade today.

A wire designed and manufactured as above (designated wire A) and a market-leading commercially available polyolefin/PVDF double-insulated wire (designated wire B) of the same dimensions were compared for the wire's robustness in a series of tests at conditions relevant to the end use. The following results were obtained.

Example of improving wear resistance

Method: Equipment = conventional type scraper for wire, wire dimension 0.75 mm2 (cross-sectional area of the wire), blade of flat type, width 3.5 mm held perpendicular to the wire, with rounded edges of 0.05 mm radius on each side, applied load 1 .8 kg, stroke length 10 cm, 55 cycles/minute.

Example of improving impact resistance in the cold

Method: wire dimension 6 mm (conductor cross-sectional area), impact weight 800 g, drop height 275 mm against anvil with impact surface against wire with dimensions 7 mm x 2 mm that beveled out to 3.4 mm at 45° on each side, ambient temperature 5 °C.

Crack propagation in the insulation was detected visually.

Example of improving solvent resistance

Procedure: wire dimension 0.75 mm, wire length 60 mm, 75% of the wire was immersed in acetone, immersion time 1 hour, temperature 23 °C

Claims (20)

1. Electric wire or cable with insulation, characterized in that the insulation comprises (i) at least a first layer of a polyolefin-based material comprising at least 20% by weight of a carbonyl-containing homopolymer, copolymer or terpolymer, where the monomer or at least one of the monomers that make up the polymer is an acrylic, an acetate or another carboxylic acid ester, and this monomer constitutes at least 5% by weight of the co- or terpolymer and the remainder of the co- or terpolymer is formed from ethylene or another olefinic monomer, in contact with (ii) at least a second layer of a material containing at least 10% by weight, based on the entire material mixture, of polyvinylidene fluoride (PVDF) or a copolymer based on vinylidene fluoride (VDF) and a partially or fully fluorinated comonomer, wherein the layers (i) and (ii) when in contact with each other, have been subjected to a cross-linking reaction sufficient to increase the delamination strength between the layers, when determined for bonded strips of the two materials used to form the layers, to at least 5 N measured with the test method ASTM Bl876-95. 2. Electric wire or cable or wire with insulation, characterized in that the insulation comprises (i) at least a first layer of a polyolefin-based formulation where at least 20% by weight of the polymer part in the formulation consists of a carbonyl-containing homopolymer, copolymer or terpolymer, where the monomer or at least one of the monomers that make up the polymer is an acrylate, a acetate or another carboxylic acid ester, and this monomer constitutes at least 5% by weight of the co- or terpolymer, and the remaining part, or the majority of the remaining part, of the co- or terpolymer is preferably formed from ethylene or another olefinic monomer, in contact with (ii) at least a second layer with a different material formulation, where at least 10% by weight of the second layer is of polyvinylidene fluoride (PVDF) or a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), or another copolymer based on VDF and a partially or fully fluorinated comonomer, wherein the layers (i) and (ii), while in contact with each other, have been subjected to a cross-linking reaction by irradiation, or another cross-linking reaction, sufficient to prevent delamination of the two layers by immersion in acetone for 1 hour at 23 °C, or sufficient to increase the delamination strength between the two layers to at least 5 N, determined for bonded strips of the two materials used to form the layers, and measured by test method ASTM Bl876-95, or sufficient to increase the delamination strength by at least 100% in relation to the delamination strength between corresponding non-crosslinked layers. 3. Wire or cable according to claim 1, where the layers (i) and (ii), while in contact with each other, have been subjected to a cross-linking reaction to a sufficient extent to prevent delamination of the two layers by immersion in acetone for 1 hour at 23 °C. 4. Wire or cable according to any one of the preceding claims, where the crosslinking reaction has increased the bond strength by at least 500% or 1000% of the bond strength between corresponding non-crosslinked layers. 5. Wire or cable according to any of the preceding claims, where the respective layers have been brought into contact with each other before cross-linking of any of the layers and at a temperature above the melting point or softening point of the polymer material in at least one of the layers. 6. Wire or cable according to any of the preceding claims, where the polyvinylidene fluoride-based layer comprises a copolymer of VDF and hexafluoropropylene (HFP), and this copolymer constitutes the largest part by weight, preferably mainly all, of the material in the layer. 7. Wire or cable according to any one of the preceding claims, where the polyvinylidene fluoride-based layer comprises a copolymer of VDF and hexafluoropropylene (HFP) with an HFP content of 8-12% by weight. 8. Wire or cable according to any one of the preceding claims, wherein the polyolefin-based layer comprises a mixture of polyethylene and the carbonyl-containing polymer. 9. Wire or cable according to any one of the preceding claims, wherein it comprises an inner layer of the polyolefin-based material and an outer layer of the polyvinylidene fluoride-based material. 10. Wire or cable according to claim 9, where the outer layer has been pressure extruded onto the inner layer. 11. Wire or cable according to any one of the preceding claims, where the cross-linking reaction has been carried out by ionizing radiation. 12. Wire or cable according to any one of the preceding claims, where it comprises several alternating layers of the materials that make up the layers (i) and (ii). 13. Wire or cable according to any of the preceding claims, where the material in one or both of the layers (i) and (ii) contains at least one cross-linking accelerator. 14. Wire or cable according to any of the preceding claims, wherein the crosslinking accelerator is trimethylolpropane trimethacrylate (TMPTM) or another multifunctional acrylate or methacrylate ester. 15. Wire or cable according to claim 13 or 14, where the cross-linking accelerator has been added only to the material in layer (i). 16. Wire or cable according to any one of the preceding claims, wherein the polyvinylidene-based layer (ii) is transparent and contains substantially only the PVDF or VDF copolymer. 17. Method for producing a wire or cable according to any of the preceding claims, characterized in that it comprises the steps of providing on an electrical conductor the layers (i) and (ii) in contact with each other, and then subjecting the layers, while in contact with each other, to a cross-linking reaction. 18. Method according to claim 17, where the respective layers are brought into contact with each other (a) before cross-linking of some of the layers and (b) at a temperature above the melting point or softening point of the polymer material in at least one of the layers. 19. Method according to claim 17 or 18, where layer (i) is pressure-extruded onto the conductor and/or layer (ii) is pressure-extruded over layer (i). 20. Method according to claims 17-19, where the layers (i) and (ii) are coextruded or tandem extruded on a wire which makes a single pass from an unwinding device to a winding device in an extrusion process.