GB2099829A

GB2099829A - Curable polymer compositions

Info

Publication number: GB2099829A
Application number: GB8215178A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1981-05-28
Filing date: 1982-05-25
Publication date: 1982-12-15
Also published as: GB2099829B

Abstract

Polymer compositions, especially conductive compositions for use in screening electric cables, have as essential ingredients (1) a polymer, cross-linkable by hydrolysis and condensation, having a hydrocarbon backbone with pendant ester groups in the structure <IMAGE> in at least some of which pendant ester groups R is an organic radical containing a silicon atom having attached to it at least two pendant groups that are hydrolysable to form silanol groups, and (2) an inert polymer that is susceptible to degradation by free radicals and is of limited compatibility with the cross-linkable polymer so that it reduces its adhesion to polyethylene.

Description

SPECIFICATION Polymer compositions This invention relates to compositions including at least one polymer of the type that has pendant ester groups and more specifically to those polymers containing the structure -C-CO-OR, I as distinct from the structure I -C-O-CO-R.

I Of these polymers, the homopolymers and copolymers of the alkyl acrylates and the alkyl methacrylates are probably the most important.

More particularly, it relates to cross-linkable compositions containing in addition to this polymer an inert polymer that is liable to degradation if exposed to free radicals.

The invention relates especially, though not exclusively, to such compositions rendered electrically conductive, as by incorporation of carbon black of a suitable grade, and to the use of conductive compositions for screening electric cables.

Electric cables for moderately high voltages (say about 6 kV to 100 kV) may be insulated with suitable polymeric materials provided that they are protected from discharge at the inner and outer surfaces by adequately adhering conductive polymeric screens; however such screens must not adhere too strongly because at least the outer screen must be stripped cleanly from a portion of the insulation at each joint or termination.

When the material is thermoplastic, the strength of adhesion to a range of cable insulating materials, such as polyethylene and ethylene-vinyl acetate copqlymers, can be regulated by incorporation of a proportion of polypropylene or of a semicrystalline copolymer of propylene, and it is desirable to extend this technique to cross-linked compositions, However, this presents serious problems when conventional cross-linking techniques are used, as they entail the use of peroxides or other free-radical generators either to induce cross-linking directly or to graft on silane side-chains for subsequent hydrolysis to form silanol groups and through them effect cross-linking by a catalytic condensation reaction. Not only is polypropylene liable to be degraded as a side-effect of cross-linking the base polymer, but also it requires (especially if exposed to copper) the addition of stabilisers.These stabilisers are free-radical scavengers and so reduce the efficiency of the cross-linking or grafting process and necessitate the use of more free-radical generator; and carbon blacks also tend to absorb free-radical generators and/or free radicals, with similar results.

We have now discovered certain compositions that can be made and cross-linked without the use of any free-radical generator, so avoiding these problems altogether.

The essential ingredients of the compositions of the present invention are: (1) a polymer cross-linkable by hydrolysis and condensation having a hydrocarbon backbone with pendant ester groups of the type -C-CO-OR I in at least some of which pendant ester groups R is an organic radical containing a silicon atom having attached to it at least two pendant groups that are hydrolysable to form silanol groups, and 2) an inert polymer that is susceptible to degradation by free radicals and is of limited compatibility with the cross-linkable polymer so that it reduces its adhesion to polyethylene.

In addition, the compositions will in most cases include also stabilisers (anti-oxidants and copper inhibitors in particular) and, besides conductive carbon black already referred to they may optionally incorporate fillers, pigments, lubricants, processing aids, plasticisers and other customary additives (except free-radical generators.) Except when the number of pendant ester groups in the cross-linkable polymer is very small, it is preferable that only a small proportion of the R's are silicon-containing groups of the kind defined above; the remainder may be simple organic radicals such as ethyl or butyl radicals.

Preferred crosslinkable polymers can be made by catalytic transesterification or other exchange reactions between a simple pendant-ester polymer and a suitably-reactive silane. The technique has been established for alkylene alkylacrylate copolymers and sitanes of the formula X(CH2)aSiY3, where X is OH, SH, NH2 or is R.Co.O-- wherein R is a C1 to Cis alkyl group or CH2=CR', R1 being H or a C1 to C18 alkyl group, a is from 1 to 18, and each Y is H, a C, to C18 hydrocarbon group, a C, to C18 alkoxy group o r --OO-(CH,)),,-O-(CH,) provided that not all the Y's are H.Within these ranges, organotitanates are suitable catalysts, as described in published European patent application 0 004 752, to which reference should be made for full practical details. It can be reasonably assumed that routine development work will shortly lead to the discovery of alternative catalysts and to the generalisation of the reaction to other polymers containing the appropriate pendant ester groups, such as homopolymers and other copolymers of the acrylates and alkacrylates and the acrylate elastomers (such as the copolymer of ethylene, methyl acrylate and a minor ingredient providing free carboxylic groups that forms the base of the product sold under the trademark Vamac), and to some other silane reagents.

The most useful inert polymers are the homopolymers and semicrystalline copolymers of propylene; poly 1-butene; poly iso butylene; and butyl rubbers.

Conductive compositions in accordance with the invention are especially suitable for use in screening electric cables in which the insulation is of polyethylene or the polymer cross-linked by the process entailing free-radical silane grafting and subsequent hydrolysis and condensation, either in the original form described in the specification of UK Patent No. 1286460 (Dow Corning) or in the improved form of UK Patent No. 1 526398 (BICC Ltd and Etablissements Maillefer S.A.).

The invention includes the cross-linked products resulting from hydrolysis and condensation of the compositions described.

Example 1 A silane-grafted ethylene-ethyl acrylate copolymer was prepared by the technique of Examples 1-10 in the specification of European Patent specification 0 004 752 and the reagents of Example 1 thereof. This reactive grafted copolymer was compounded to the following recipe:: Parts by weight The grafted copolymer 100 Polypropylene 30-70 (as required to control adhesion) Conventional antioxidant 0.1-3 (preferably 0.5-1) (Santonox R) Copper inhibitor 0.1-3 (preferably 0.5-1) (Irganox MD-i 024) High conductivity carbon black 8-25 (Ketjenblack EC) Condensation catalyst 0.05-0.1 (dibutyi tin dilaurate) Mixtures in this range were extruded to form inner and outer screening layers (conductor screen and dielectric screen) on a cable made as described in Example 1 of UK Patent No. 1 526398.

Example 2 This was similar to Example 1 but the high conductivity carbon black was replaced by a conventional conductive black (Vulcan P) in amounts ranging from 25 to 60 parts by weight.

Claims

1. A cross-linkable composition free of free radical generators comprising: (1) a polymer, cross-linkable by hydrolysis and condensation, having a hydrocarbon backbone with pendant ester groups in the structure --CC-CCO-OR in at least some of which pendant ester groups R is an organic radical containing a silicon atom having attached to it at least two pendant groups that are hydrolysable to form silanol groups, and (2) an inert polymer that is susceptible to degradation by free radicals and is of limited compatibility with the cross-linkable polymer so that it reduces its adhesion to polyethylene.

2. A cross-linkable composition free of free-radical generators comprising: (1) a polymer, cross-linkable by hydrolysis and condensation, having a hydrocarbon backbone with pendant ester groups in the structure -C-CO-OR I in at least some of which pendant ester groups R is an organic radical containing a silicon atom having attached to it at least two pendant groups that are hydrolysable to form silanol groups, and (2) an inert polymer that is susceptible to degradation by free radicals and is of limited compatibility with the cross-linkable polymer so that it reduces its adhesion to polyethylene.

(3) a conductive carbon black in an amount to render the composition electrically conductive.

3. A composition as claimed in Claim 1 or Claim 2 in which a small proportion of the R's contain silicon atoms as defined and the remainder are simple organic radicals.

4. A composition as claimed in any one of the preceding claims in which the inert polymer is a homopolymer or a semicrystalline copolymer of propylene; poly 1 -butene; poly isobutylene; or butyl rubber.

5. A cross-iinked composition being the product formed by hydrolysis and condensation from the composition defined in any one of the preceding claims.

6. A curable or cured composition substantially in accordance with the recipe in either of the Examples.

7. An electric cable having polymeric insulation and at least one screening iayer made of an electrically conductive composition claimed in Claim 5.

8. A cable as claimed in Claim 7 in which the insulation is cross-linked by a process entailing free radical silane grafting and subsequent hydrolysis and condensation.

9. An electric cable substantially as described in either of the Examples