GB2055854A - Electrical tree and water tree resistant polymer compositions - Google Patents

Abstract

The use as a water tree-resisting and electrical tree-resisting ingredient in polymeric electrical insulating compositions of a compound of formula A wherein R, R1, R2 and R3 are selected independently from class D (defined herein as consisting of C1 to C8 alkyl, C1 to C8 alkoxy, C1 to C8 acyloxy, C6 to C18 aryloxy and substituted aryloxy, C6 to C18 aryl and substituted aryl, hydrogen, halogen epoxy-containing radicals, C2 to C8 alkenyl, nitrogen-containing radicals, carboxy-containing radicals, mercapto-containing radicals, and ether-containing radicals), with at least one (preferably three or all) of R, R1, R2 and R3 being a group which contains in its chain at least one electron-donating atom at a position other than adjacent to the silicon atom; or formula E wherein R6 is selected from the said class D and R<1>, R<2> and R<3> are selected from the said class D and Y<1> (CnH2n) Y<2>R<6>; Y<1> and Y<2> are selected independently from O, S and N; Z is Si, Sn, Ti, P or B; a is zero or 1 according to the identity of Z; and n is an integer of 1 to 8. <IMAGE>

Description

SPECIFICATION Electrical tree and water tree resistant polymer compositions This invention relates to polymeric compositions having resistance to electrical treeing and water treeing said compositions being useful as insulation for electrical cables.

Polymeric compositions are well-known and are used extensively as insulation materials for wire and cable. As an insulator, it is important that the composition have various physical and electrical properties, such as resistance to mechanical cut through, stress crack resistance and dielectric failure.

Recent publications have indicated that water tree growth and electrical tree growth in the insulation are particularly important problems since they are associated with, though not necessarily totally responsible for, dielectric failure.

An important application for an insulation material is in high voltage transmission and distribution cable, especially in direct buried underground service, and three types of tree have been observed in power cables, to wit, electrical trees, water trees and electrochemical trees. It is generally believed that electrical trees are generated by corona discharges causing fusion and breakdown of the polymer, whereas water trees are usually observed in cables buried in wet locations and have a different appearance from to the electrical trees. The electrochemical trees are similar to the water trees but are characterized by the presence of metal ions in the trees.

U.S. Patent No. 4,144,202 granted to Ashcraft et al. relates to inhibiting the electrical breakdown of insulation by water treeing in dielectric materials based on ethylene polymers. This patent discusses electrical failures which are due to treeing and explains the concept of treeing and some of the causes fo treeing. In general, as the polymeric composition breaks down the damage progresses through the insulator, or dielectric, in a path that looks something like a tree. Treeing usually is a slow type failure anc may take years to cause a failure in the insulation. As disclosed in the patent, water treeing is inhibited in the ethylene polymer compositions by employing therein certain organo silane compounds. In particular, the organo silane is a silane containing an epoxy containing radical.Suitable polymers, adjuvants and processing procedures for preparing the composition are described in the patent, which patent is hereby incorporated by reference.

German Offenlegungsschrift 2,737,430 discloses that certain alkoxysilanes added to polyolefin insulation prevent water-tree formation. Several trimethoxy and triethoxy silanes are said to be useful.

No alkoxyalkoxy silanes are taught or suggested as having both water treeing and electrical treeing inhibiting properties.

U.S. Patent No. 3,553,348 granted to Betts, British Patent No. 1,248,256 granted to General Electric Company and British Patent No.1,277,378 granted to General Electric Company relate to mineral filled polymer compositions useful as electrical wire and cable insulation. The mineral filter is treated with an organosilane such as an alkyl alkoxysilane or a vinyl alkoxysilane to decrease the porosity of the composition. None of these patents teach or suggest that addition of an organosilane to an unfilled polymer composition will beneficially enhance the water treeing and electrical treeing resistance of the polymer composition.

Unfortunately, however, the prior art has not provided an insulation composition having both increased resistance to water treeing and electrical treeing. As noted in U.S. No. 4,144,202, supra, intrinsic electric breakdown, failure by corona, electrical treeing and water treeing are different and the mechanisms for each are different and a different solution is required to effect an improvement in a dielectric material for each mode of failure involved. Thus, the problem of providing a single composition capable of resisting both electrical treeing and water treeing is a formidable one confronting the art.

It has now been unexpectedly discovered that polymeric compositions containing an effective amount of organic compound selected from formulae A and E below can exhibit resistance to both water treeing and electrical treeing. Some such compositions may be cured to provide crosslinked compositions having further improved properties for certain applications.

Formula A represents the silanes:

wherein R, R,, R2 and R3 are selected independently from class D (defined herein as consisting of C1 to C8 alkyl, C, to C8 alkoxy, C, to C8 acyloxy, C6 to C,8 aryloxy and substituted aryloxy, C6 to C,8 aryl and substituted aryl, hydrogen, halogen, epoxy-containing radicals, C2 to C8 alkenyl, nitrogen-containing radicals, carboxy-containing radicals, mercapto-containing radicals, and ether-containing radicals), with at least one (preferably three or all) of R, RX, R2 and R3 being a group which contains in its chain at least one electron-donating atom at a position other than adjacent to the silicon atom. The said electrondonating atom may be, for example, oxygen, nitrogen, sulphur or the like.Oxygen is preferred because of its demonstrated effectiveness. A highly preferred group has the said electron-donating atom separated from the silicon atom by three atoms.

Polymeric compositions according to the invention suitably include about 0.1 to 10 parts by wt. of formula A silane per hundred parts by wt. of polymer, i.e. about 0.1 to about 10 phr. of formula A silane.

A preferred composition contains about 0.5 to about 5 phr silane component, most preferably about 1 to about 3 phr.

Formula E represents the organic compounds:

wherein R1, R2 and R3 are selected independently from the said class D and Y'(CnH2n)Y2R6; and Re is selected from the said class D; Y1 and Y2 are selected independently from 0, S and N; Z is Si, Sn, Ti, P or B; a is zero or 1 according to the identity of Z; and n is an integer of 1 to 8.

Polymeric compositions according to the invention suitably include about 0.1 to about 10 phr of formula E organic compound. A preferred composition contains about 0.5 to about 5 phr of formula E compound component, most preferably about 1 to about 3 phr.

Compositions according to the invention may contain a total of two or more compounds selected from either or both of formula A and E; the above phr ranges apply in each case to the total amount of formula A compound plus formula E compound present.

Polymeric compositions according to the invention may be used for coating electrical conductors.

The compositions find particular utility in high voltage transmission and distribution cables but are useful in other electrical applications where a combination of water treeing and electrical treeing resistances is needed.

In general, the polymers suitable for the practice of this invention include any normally solid synthetic organic polymeric thermoplastic resin. Included are olefin homopolymers and copolymers including olefin-olefin, olefin-vinyl and olefin-allyl copolymers, vinyl polymers, polyamides, polyacrylics, polystyrenes, cell ulosics, polyesters and polyflurocarbons.

The polyolefins include normally solid polymers of olefins, particularly mono-alpha-olefins of from two to six carbon atoms, e.g., polyethylene, polypropylene, polybutene, polyisobutylene, poly(4-methylpentene), and the like. Preferred polyolefins are polyethylene and polypropylene. Polyethylene is especially preferred. An especially preferred polyethylene because of its demonstrated effectiveness is termed NA 310 and is sold by National Distillers and Chemical Corporation.

Copolymers of ethylene with other compounds interpolymerizable therewith such as butene-1, pentene-1, styrene and the like may be employed. In general the copolymer will contain about 50 to < 100 weight % ethylene.

Suitable vinyl polymers include polyvinyl chloride, polyvinyl acetate, vinyl chloride/vinyl acetate copolymers, polyvinyl alcohol and polyvinyl acetal.

Suitable olefin-vinyl copolymers include ethylene-vinyl acetate, ethylene-vinyl propionate, ethylene-vinyl isobutyrate, ethylene-vinyl alcohol, ethylene-methyi acrylate, ethylene-ethyl acrylate, ethylene-ethyl methacrylate, and the like. In general the ethylene constitutes at least about 25% of the copolymer by weight.

Olefin-allyl copolymers include ethylene-allyl benzene, ethylene-allyl ether, ethylene-acrolein, and the like.

A number of suitable groups of said Class D is shown on page 143 of "Chemicals and Plastics Physical Properties, 1978-80" published by Union Carbide Company and to which attention is directed for detail. Exemplary are those of the following List I: chloro, methyl, ethyl, methoxy, ethoxy, phenyl, hydrogen, chloropropyl, vinyl 2-methoxyethoxy, gam ma-methacryloxypropyl, beta (3,4epoxycyclohexyl)-ethyl, gam ma-glycidoxypropyl, acetoxy, gamma-mercaptopropyl, gammaaminopropyl, bis-hydroxyethyl-gamma-amino-propyl, bis-acrylic acid gamma-amino-propyl, Nbeta(aminoethyl)-gamma-amino-propyl, and methyl [2(gamma-trimethoxysilypropylamino)ethylamino] 3 propionate.

As noted hereinabove, at least one of the R, R,, R2 and R3 groups of formula A has an electron donating atom such as oxygen, nitrogen or sulphur in the chain and separated from the silicon atom, preferably by three atoms. A preferred group has the following formula: (OR40Rs) wherein R4 is a C, to C6 group and R5 is C1 to C8 alkyl, hydrogen, C, to C8 alkoxy or C2 to C8 alkenyl. A particularly preferred group is 2-methoxyethoxy which has the formula, (OC2H4OCH3) A preferred compound is sold under the name A-1 72 by Union Carbide Company and is chemically defined as vinyltris (2-methoxyethoxy) silane.Other R, R,, R2 and R3 groups include gammamethacryloxy-propyl, gamma-glycidoxypropyl, gamma-aminopropyl, bis-hydroxy-ethyl-gammaaminopropyl and N-beta(aminoethyl)-gamma-amino-propyl.

The R1, R2 and R3 groups of formula E when Z is silicon suitably include groups discussed above in connection with the Union Carbide Company pubiication, particularly when Y'(CnH2n)Y2R6 is an alkoxyalkoxy group. Among the useful silanes of formula E are gamma-methacryloxypropyl-tris(2methoxyethoxy) silane, tetrakis-(2-methoxyethoxy) silane, methyl-tris (2-methoxyethoxy) silane, phenyltris (2 methoxyethoxy) silane, vinyl-tris (2-phenoxyethoxy) silane, vinyl-tris (2-methylthioethoxy) silane and vinyl-tris (2-methoxyethoxy) silane with the latter being particularly preferred. Replacing the silicon with such atoms as tin, titanium, phosphorous or boron provides other useful compounds which find utility in the invention.Thus, such compounds as tris (2-ethoxyethyl) phosphite, tris (2-n-butoxyethyl) phosphite, tetrakis (2 methoxyethoxy) titanium and the like may be employed and are included with the scope of this invention.

In the preferred organic compounds of formula E, R', R2, and R3 are each selected from Y(CnH2n)Y2R6, alkyl, alkoxy, acyloxy, aryl or alkenyl, B6 is alkyl or aryl Y' and Y2 are 0 and Z is Si or P. Of course, when Z is Si, a is 1 and when Z is P, a is zero.

When it is desired to use a polymeric composition which can be crosslinked, crosslinking can be accomplished by any of the known procedures such as chemical means including peroxide cross-linking; radiation using electron accelerators, y-rays, high energy radiation, such as X-rays, microwaves etc.; or thermal crosslinking. The basic procedures for crosslinking polymers are extremely well known to the art and need not be described here in detail.

Conventional crosslinking agents such as organic peroxides may suitably be employed. Typical organic peroxide free radical generators include dicumyl peroxide; 2,5-bis (tert.-butylperoxy)-2,5 dimethylhexane; di-t-butyl peroxide; benzoyl peroxide; a,a'-bis(t-buWl peroxy) diisopropyl benzene and the like, as discussed in U.S. Patent No. 3,287,312. The amount of organic peroxide, when employed, may e.g. be from about 0.5 to 5.0% by weight based on the total weight of the composition, or about 0.5 to 10 phr, preferably 3 to 6 phr.

While the formula A and E compounds described hereinabove are useful for both thermoplastic and cured polymeric compositions, for compositions to be cured it is preferred that one of the substituent groups be an organo functional group, e.g. a vinyl group, which provides the composition with enhanced curing properties.

Minor amounts of other additives may also be employed in conventional amounts to obtain the desired results. Conventional antioxidants such as the hindered phenols, polyquinolines and the like may be employed. Other ingredients that may be included are plasticizers, dyes, pigments, heat and light stabilizers, antistatic agents and the like.

The preferred compositions of this invention are "unfiiled" polymer compositions, meaning compositions which contain less than 10 wt.% of conventional polymer filler. For certain applications and to meet particular specifications the compositions herein contain no filler. The compositions of this invention thus preferably contain no filler or less than 10 wt.% filler, e.g. mineral filler.

The polymer compositions of this invention can be prepared by mixing the various ingredients.

When the formula A and/or E compound(s) and the polymeric component are mixed together, they are homogeneously dispersed in each other. The order of mixing and specific procedure employed are not crtical except to the extent that from the time the peroxide is added, if employed, the temperature is less than about 1300 C. in order to prevent premature curing of the composition. This precaution, however, is conventional in the art.

The components may be mixed on a variety of apparatus including multi-roll mills, screw mills, continuous mixers, compounding extruders and Banbury mixers.

After being extruded onto wire or cable, or other substrate, the crosslinkable compositions are vulcanized at elevated temperatures, e.g., above about 1 800C. using conventional vulcanizing procedures.

In order to determine the utility and effectiveness of polymeric compositions of the present invention in resisting water treeing and electrical treeing, the compositions were evaluated by the use of accelerated tests.

Electrical tree tests were performed using a method similar to that in IEEE Conference Paper No.

C73, 257-3 1973 by E. J. McMahon and J. R. Perkins. Strips of material approximately 1" wide were cut from a 1/4" thick compression molded plaque. The block was machined to give a strip having parallel edges 1" apart. The strip was then cut into 1" square blocks. A blunt needle and a sharp needle were inserted into opposite parallel edges, at elevated temperatures, so that the points were 1/8" apart. Needle insertion and cooling of the sample was performed slowly to avoid inducing thermal or mechanical stresses in the specimen. The sharp needle has a tip diameter to about 0.0002" while the diameter of the blunt needle is 0.002". Eight specimens were prepared and tested simultaneously for each composition.The electrical tree test was performed by energizing the sharp needle at 1 5 KV using a frequency of 60 Hz; the blunt needle was connected to ground. The time required for each of the eight specimens to fail by tree growth and subsequent electrical short was recorded. The time required for 50% of the samples ot fail was employed to characterize the effectiveness of the tree retardant being evaluated.

The water tree test is performed using a procedure similar to that described in U.S. Patent 4,144,202. A compression molded disc about 1 50 millimeters (mm.) in diameter having 24 conical depressions was prepared for each composition. The geometry of the disc and dimensions of the depressions are substantially the same as shown in USP 4,144,202. The base of the disc is sprayed with silver paint which serves as the ground electrode. An acrylic tube 6" long is clamped to the upper face forming a test cell. About 1 50 ml. of 0.01 N sodium chloride solution was poured into the cell and the air bubbles trapped on the surface of the sample were removed. A platinum wire ring was then immersed in the electrolyte and connected to the electrical supply which provides 5KV at a frequency of 3KHz.Samples were energized for 22 hours after which time they were removed from the test cell and washed with distilled water. The ten central depressions were cut from the disc and stained to make the water trees more visible. Thin sections were obtained with a microtome, which were then examined microscopically (at 200X) and the tree size measured. Normally four discs were made for each sample so that the average tree size is calculated from forty individual measurements. In evaluating different tree retardants, the relative tree size was determined by comparing the average tree size obtained on a standard thermoplastic high voltage insulation material containing no tree retardant additives.

Various embodiments of the present invention will now be illustrated by reference to the following specific examples. It is to be understood, however, that such examples are presented for purposes of illustration only, and the present invention is in no way to be deemed as limited thereby. All parts and percentages are by weight and temperatures in degrees Fahrenheit unless otherwise noted.

EXAMPLE I The compositions were prepared by milling a commercial grade of polyethylene (NA 310) and the treeing additive (2% by weight) on a 2-roll mill at about 3O00F. for about 10 minutes to obtain a homogeneous dispersion. The crepe obtained was then used to prepare the samples for electric tree and water tree testing using the procedures described hereinabove. The test results are shown in Table 1. All the compositions have the same formulation except for the "treeing" additive as noted in Table 1 and comprise a commercial grade of polyethylene having a Melt Index of 0.20 to 0.35 g/1 0 min. and a density of about .917 grams/cubic centimeter (g/cc). The control sample does not contain a "treeing" additive.

TABLE I Double Needle Test Time to Water Tree Sample 50% Failure (Relative No. Treeing Additive (minutes) Tree Size) A Vinyl-tris-(2-methoxy ethoxy)silane > 12,700 (no failures) 0.23 B Gamme-glycidoxypropyl trimethoxysilane 2,800 0.34 1 Control (No Additive) 80 1 2 Vinyl triethoxy silane 30 0.29 3 Beta-(3,4-epoxycyclo hexyl)-ethyltrimethoxy silane 620 0.34 4 Dodecanol 127 0.34 The results clearly show the improvement in both water treeing gnd electrical treeing properties of formulations prepared in accordance with the present invention. Thus, comparing Samples A and B, of the invention, with Samples 1-4, which are outside the invention, the improvement is readily apparent.Comparing Sample A with the control, Sample 1, shows the vast improvement in properties when vinyl-tris (2-methoxyethoxy) silane is employed. Similarly, a comparison of Sample A with Sample 2 shows the importance of utilizing a silane having an electron donor atom in the chain of the groups attached to the silicon atom. A comparison of Sample A with Sample B shows the advantage of employing three electron donating group radicals attached to the silicon atom.

EXAMPLE II In the same fashion as in Example I, a number of organic compounds were evaluated as "treeing" additives. In all instances the additive was incorporated in the polyethylene at a concentration of 1.5%.

The results of the electrical tree and water tree testing are shown in Table II.

TABLE II Double Needle Test Water Tree Test Sample Time to 50% Failure (Relative Tree No. Treeing Additive (minutes) Size)

Structure Name 5 X Control 75 1.00 6 (CHOCH2CH2O)3 Si CH=CH2 Vinyl-tris(2-methoxyethoxy) > 6000 0.32 7 (C2H5O)3 Si CH=CH2 Vinyltriethoxysilane O 0:42 0 II 8 (CH3CO)3 Si CH=CH2 Vinyltriacetoxysilane 186 0.68 9 C:> CH2 oD Si CH3 Methy l-tris(2-methoxyethoxy)si lane > 4800 0.37 COD (CH3 OCH2 CH2 t Si Tetrakis(2-methoxyethoxy)s lane 2820 0.33 me (CH3O)a A O) 12 (C2H30)3 Si C8 H17 n-Octyltriethoxysilane 690 0.56 0 CH3 III 13 (CH3O)3SiCH2CH2CH2OC C--OH2 gamma-Methaoryloxyprop'l- 200 0.38 O OH3 x n III Z (CH3OCH2CH2O)3 Si CH2OH2CH2OO = ,% D C AE m a, > r a, ,r n lane 0 15 ~ Si É CH 2CH2CH < |5 E CD O708 0.32 trimethoxysi lane c > cS > S ~ .,C, O G) ' > , cn g > > > E F s I - O a, > O o IN O OIN I O O IX N ~ oz ~ O I Q .~ ô N N N N to 0) N 0 4) 0 = O O O O UX O IN so I oN I I I N I Ô O O u) so s co a) o r CQ n t u > r TABLE II (Continued) Double Needle Test Water Tree Test Sample Time to 50% Failure (Relative Tree No.Treeing Additive (minutes) Size)

Structure Name a, c; o: o OH2CH2CHOH2OHOH beta-(3,4-Epoxycyclohexyl)eThyl- o 0.32 o o C30 00 OH2OH2 OH cOv cOo Cq b CH=OH2 Vinyl-tris(tert-butyl peroxy)silane 180 0.85 cu C (C2Hs0)3 Si OH2OH2OH2NH2 gamma-Aminopropyl triethoxysi lane 80 a) A a, a, E 20 mrb Si OH2OH2SH Mercaptoethyltrimethoxysilane 125 0.53 21 (?H30)a Si OH2OH2OH2SH Mercaptopropyltrimethoxysilane 265 0.60 22 (OH0O2 v, ,E Si Pheny l-tris(2-methoxyethoxy)s i lane 3600 0.35 23 > = Si OH " OH2 Vi nyl-tris(2-phenoxyethoxy)si lane < ;xh 0.22 t V.E E E P tris(2-Ethoxyethyl) phosphite > 20,000 = Cq x ~ 25 (O4H90O2H40)3 P tris(2-n-butoxyethyl) phosphite 3000 0.30 26 (OH3 0C2H4 I\I Ti tetrakis (244ethoxyethoxy) titanium 930 O E &verbar; I I U I IN I F d " X X co N O N N " I I I O ^ " ' O - O O O Ow 4 0 0 0 0 s W V I I N I I I I r 1 N I" I CD s 0D C5) 0 v CQ v Cx J N The silanes evaluated demonstrated a superiority in both water tree and electrical tree resistance for those silanes having alkoxyalkoxy substituents. (Samples 6, 9, 10, 14, 22 and 23). This can be seen by comparing inter alia the silane pairs of samples 6 and 7, 9 and 11 and 13 and 14.It also appears that there is an optimum number of alkoxy alkoxy substituents-compare samples 6, 9 and 10. The effect of a vinyl substituent as compared to an alkyl or aryl substituent is evident from comparing samples 6, 9 and 22. The location of a particular substituent, viz, an aryl group, can influence the inhibition properties of the organic compound as seen from samples 22 and 23.

Samples 24 and 25 show that organic phosphites are effective in both water and electrical tree inhibition while sample 26 shows similar effectiveness for an organic titanium compound.

While the invention has been directed principally to the use of silanes, it will be understood by those skilled in the art that other compounds containing a multivalent atom such as titanium, tin, phosphorous, and the like may be employed.

When one or more of R1, R2 and R3 in formula E is a Y'(CnH2n)Y2R6, the R6's of the compound need not be the same.

Claims (33)

1. A polymeric composition comprising a mixture of a polymeric component and, as water treeing and electrical treeing inhibitor for the composition, at least one compound of the formula E:

R1, R2 and R3 are selected independently from Y1(CnH2n)Y2R6 and the class D as defined herein and R6 is selected from the class D as defined herein; Y1 and Y2 are selected independently from 0, S and N; Z is Si, Sn, Ti, P or B; a is O or 1 according to the identity of Z; and n is an integer of 1 to 8.

2. A composition according to claim 1 containing at least one compound of formula E wherein R1, R2 and R3 are selected independently from Y1(CnH2n)Y2R6, alkyl, alkoxy, acyloxy, acyloxy, aryl and alkenyl; R6 is alkyl or aryl; andY1 andY2 are each 0.

3. A composition according to claim 2 wherein Z is Si and a is 1.

4. A composition according to claim 3 wherein R1 is vinyl, R2 and R3 are each Y1(CnH2n)Y2R6, R6 is methyl, and n is 2.

5. A composition according to claim 3 wherein R1 is methyl, R2 and R3 are each Y1(CnH2n)Y2R6, R6 is methyl, and n is 2.

6. A composition according to claim 3, wherein R1, R2 and R3 are each Y1(CnH2n)Y2R6, R is methyl, and n is 2.

7. A composition according to claim 3 wherein R1 is gamma-methacryloxypropyl, R2 and R3 are each Y1(CnH2n)Y2R6,R3is methyl, and n is 2.

8. A composition according to claim 3 wherein R1 is phenyl, R2 and R3 are each Y1(CnH2n)Y2R6, R6 is methyl, and n is 2.

9. A composition according to claim 3 wherein R1 is vinyl, R2 and R3 are each Y1(CnH2n)Y2R6, R6 is phenyl, and n is 2.

10. A composition according to claim 2 containing at least one compound of formula E wherein Z is P and a is zero.

11. A composition according to claim 10 wherein R1 and R2 are each Y1(CnH2n)Y2R6,R6is ethyl, and n is 2.

12. A composition according to claim 10 wherein R1 and R2 are each Y1(CnH2n)Y2R6,R6is n-butyl, and n is 2.

13. A composition according to claim 2 containing at least one compound of formula E wherein Z is Ti and a is 1.

14. A composition according to claim 13 wherein R1, R2 and R3 are each Y1(CnH2n)Y2R6,R6is methyl, and n is 2.

15. A composition according to claim 1 wherein said inhibitor comprises vinyl-tris(2methylthioethoxy) silane.

16. A composition according to any of claims 1 to 14 wherein Y1(CnH2n)Y2R6 is alkoxyalkoxy.

17. A polymeric composition comprising a mixture of a polymeric component and, as water treeing and electrical treeing inhibitor for the composition, at least one silane of the formula A:

wherein R, R1, R2 and R3 are selected independently from the class D as herein defined, with at least one of R, R1, R2 and R3 being a group which contains in its chain at least one electron-donating atom at a position other than adjacent to the silicon atom.

18. A composition according to claim 17 wherein R, R1, R2 and R3 are selected independently from List I herein.

19. A composition according to claim 18 wherein said inhibitor comprises gamma-glycidoxypropyl trimethoxysilane.

20. A composition according to any of claims 1 to 1 6 and according to any of claims 1 7 to 19.

21. A composition according to any of claims 1 to 20 wherein the polymeric component comprises an ethylene polymer.

22. A composition according to any of claims 1 to 21 containing by weight a total of from 0.1 to 10 parts of said inhibitor per hundred parts by weight of polymer.

23. A composition according to claim 1 or 8 containing a compound of formula A or E from Example I or il herein.

24. A composition according to claim 1 or 8 and substantially as hereinbefore described in any Sample of Example I or II.

25. A composition according to any preceding claim including at least one additive selected from crosslinking agents, antioxidants, plasticisers, stabilisers and antistatic agents.

26. A composition according to any preceding claim including pigment and/or dye.

27. A composition according to any preceding claim containing from 0 to 10 wt.% filler.

28. A composition according to any preceding claim in which the polymer has been cross-linked.

29. An electrical conductor coated with a composition according to any preceding claim.

30. A method of making a polymeric composition which comprises forming a homogeneous mixture of uncured polymer and at least one compound selected from those of formula A and formula E as defined herein.

31. A method according to claim 1 and substantially as hereinbefore described in any Sample of Example I or II.

32. A method according to claim 31 including coating the homogeneous mixture onto an electrical conductor.

33. A method according to claim 32 including crosslinking polymer of the coating.