Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-05-04
2003-01-14
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S192000, C525S194000, C525S195000, C525S222000, C525S240000
Reexamination Certificate
active
06506841
ABSTRACT:
The present invention relates to an extrudable composition that can be cross-linked in air, and also to a method of cross-linking said composition.
BACKGROUND OF THE INVENTION
This composition is for use in the manufacture of sheathing material for medium and low-voltage power cables or for telecommunications cables. These cables carry direct current (DC) or alternating current (AC). Medium-voltage cables are generally constituted by a conductive core surrounded by an insulating structure that is coaxial thereabout. The structure comprises at least a semiconductive first layer placed in contact with the core of the cable, itself surrounded by an electrically insulative second layer, in turn covered by a semiconductive third layer. The outer layers are sheaths which serve to protect the cable. Low-voltage cables (voltage lower than 20 kilovolts (kV)) have a conductive core surrounded by an insulating structure which is coaxial thereabout and covered in a sheath. This composition is also usable as an insulating material.
In the event of a short circuit occurring in a cable, temperatures locally rising to 200° C. can be observed. At these temperatures and under stress, the material is subject to deformation or creep. This elongation when hot is evaluated using the so-called “hot set test” (HST) under conditions which are defined in standard NF EN 60811-2-1. Compositions are being sought which present good results in the “hot set test”.
Document WO-88/05449 proposes in its examples 11 and 12 an extrudable composition which comprises a mixture of a copolymer of an ethylenically-unsaturated compound and an unsaturated ester containing an acrylate such as ethylene butylacrylate (EBA) (example 11) or an ethylene methylacrylate (EMA) (example 12) with a hydrolizable silane compound having no ethylenic unsaturation, such as a 3-amino-propyltrimethoxysilane (MEAM) and also with a thermoplastic polymer having no functional groups such as a polyethylene homopolymer, e.g. low density polyethylene (LDPE).
The resulting material presents good adhesion on metals or polar substances and it is used for this purpose in cables. However that material is not cross-linkable.
Document EP-A-0 802 224 describes an extrudable composition that is cross-linkable, and that is constituted by a mixture comprising a thermoplastic polymer material, a hydrolizable silane compound, a non-cross-linked elastomer, and a cross-linking agent. The thermoplastic polymer material and the hydrolizable silane compound respectively carry mutually reactive functional groups so that they react together selectively. The elastomer is cross-linked dynamically and selectively by the cross-linking agent in the mixture. The thermoplastic polymer material carrying reactive functional groups is a commercially-available substance, preferably polyethylene grafted by maleic anhydride. The silane compound is preferably aminoalkoxysilane. The amine groups of the silane compound react selectively with the maleic anhydride groups of the polyethylene, and the alkoxy groups of the silane compound react with the surface hydroxyl groups of the filler. The elastomer is an ethylene vinyl acetate (EVA) copolymer which is cross-linked dynamically and selectively by the cross-linking agent.
The special feature of that composition is the choice of basic components in the mixture which have specific and different reaction kinetics, thereby enabling the composition to be made in a single stage. As a result, the choice of basic components is very restricted and the cost thereof is high.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to propose an extrudable composition that is cross-linkable in air to give a cross-linked material which is recyclable and which is simpler to manufacture, and which is of lower cost than known compositions.
In a first aspect, the present invention thus provides a composition that is extrudable and cross-linkable in air, comprising a mixture:
of a copolymer of an ethylenically-unsaturated compound and an unsaturated ester;
of a hydrolizable silane compound having no ethylenic unsaturation and containing functional groups; and
of a thermoplastic polymer;
wherein said functional groups of said ester are selectively reactive with functional groups of said silane compound, and wherein said thermoplastic polymer has no functional groups.
The invention is based on the discovery that under certain conditions it would appear that a selective reaction occurs between the functional groups of the ester of the copolymer and the reactive functional groups carried by the silane.
The composition of the invention comprises a thermo-plastic polymer that has no functional groups liable to interfere with the reaction between the silane and the copolymer involved in cross-linking the composition. This thermoplastic copolymer confers good mechanical properties on the composition of the invention since it contributes to reinforcing its structure.
The present invention makes it possible to use a vast range of silane compounds. In addition, it does not require recourse to polymers having special functional groups, and consequently it makes it possible to use a vast range of polymers, and in particular polymers that are the most widespread and the least expensive.
The present invention makes it possible to obtain a composition having good resistance to creep at high temperature.
In addition, a composition of the invention makes it possible to obtain finished products which can be recycled. It also makes it easy to obtain finished products that are very flexible, for example cables based essentially on (poly)vinyl acetate.
In a second aspect, the present invention provides a method of preparing a composition which enables the composition of the invention to be cross-linked, and including in particular a “dynamic cross-linking” step.
This step serves in particular to lengthen the chains of the polymer and of the copolymer and thus reduce the number of chains. In this way, this step makes it possible to improve compatibility between the polymer and the copolymer by promoting cross-linking of the composition.
In another aspect, the present invention provides a cable whose insulation and/or sheath contain the composition obtained by the method.
DETAILED DESCRIPTION OF THE INVENTION
Other characteristics and advantages of the invention are described below in detail in the following description which is given by way of non-limiting illustration.
Composition of the invention
A composition of the invention comprises in particular a copolymer of an ethylenically-unsaturated compound and an unsaturated compound and unsaturated ester.
The selective reaction between the silane and the ester on which the invention is based can be illustrated by the following scheme:
SiX
4
+4RCOOR′→Si(OR′)
4
+4RCOX
in which:
X represents a functional group carried by the silane SiX
4
; and
RCOOR′ represents a fragment of the copolymer of an ethylenically-unsaturated compound and an unsaturated ester.
In the invention, the unsaturated ester of the copolymer generally satisfies the formula R
1
COOR
2
in which:
R
1
is an alkyl group preferably having 1 to 4 carbon atoms; and
R
2
is an alkylene group preferably having 2 to 4 carbon atoms.
Advantageously, the functional groups of the ester of the invention can comprise acetates. Acetates promote the above-mentioned reaction, unlike other groups such as acrylates.
A composition of the invention preferably also comprises a cross-linking agent which can be a peroxide, a phenolic resin, a sulfur-containing derivative, or a mixture of these compounds, in particular. The composition of the invention generally contains 0.5 to 7 parts by weight and preferably 0.5 to 3 parts by weight of said cross-linking agent.
In the invention, the ethylenically-unsaturated compound is preferably ethylene.
As examples of copolymers entering into the composition of the invention, mention can be made of ethylene vinyl acetate copolymers (EVA), ethylene vinyl propionate copolymers, ethylene allyl
Fomperie Lionel
Prigent Madeleine
Nexans
Nutter Nathan M.
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