GB2205846A - Crosslinkable polymer - Google Patents

Abstract

A water crosslinkable thermoplastic random interpolymer prepared by radical interpolymerisation, the interpolymer comprising (A) 78 to 98 weight % of units derived from ethylene. (B) 1.0 to 20 weight % of units derived from ethyl acrylate and/or butyl acrylate and (C) 1.0 to 2.0 weight % of units derived from an ethylenically unsaturated hydrolysable silane compound e.g. vinyltrimethoxy silane, a vinyl methyl dimethoxy silane. s

Description

CROSSLINKABLE POLYMER The present invention relates to crosslinkable thermoplastic organic polymers and to compositions based thereon. More particularly the present invention is concerned with crosslinkable copolymers which are prepared by copolymerising unsaturated organic monomers with unsaturated silane compounds containing hydrolysable groups.

It is known that organic polymers containing hydrolysable silane groups can be crosslinked by the action of water, preferably in the presence of a so-called silanol condensation catalyst. A number of methods are known for producing such crosslinkable organic polymers. One method comprises copolymerising unsaturated organic monomers with unsaturated silane compounds containing hydrolysable groups. Examples of this method are described in GB-A-2028831 and GB-A-2039513 which disclose the preparation of crosslinkable copolymers of ethylene and an ethylenically unsaturated silane compound by copolymerising the monomers at relatively high temperatures and pressures in the presence of a radical polymerisation initiator.Another example of this copolymerisation method is described in GB-A-1415194 which discloses the preparation of crosslinkable copolymer by contacting ethylene, optionally with other olefinically unsaturated comonomer, and a terminally unsaturated silane compound with certain defined Ziegler catalysts under polymerisation conditions which preferably employ relatively low temperatures and pressures.

It is an object of the present invention to provide a crosslinkable thermoplastic polymer having improved properties.

Accordingly the present invention provides a crosslinkable thermoplastic random interpolymer prepared by radical interpolymerisation, the interpolymer comprising (A) 78 to 98 weight Z of units derived from ethylene, (B) 1.0 to 20.0 weight Z of units derived from ethyl acrylate and/or butyl acrylate and (C) 1.0 to 2.0 weight Z of units derived from an ethylenically unsaturated hydrolysable silane compound.

The present invention further provides a process for preparing a crosslinkable thermoplastic random interpolymer comprising contacting a mixture of monomers comprising a major proportion of (D) ethylene and minor proportions of (E) ethyl acrylate and/or butyl acrylate and (F) an ethylenically unsaturated hydrolysable silane compound, with a free radical initiator at a temperature in the range 150 to 400etc and at a pressure in the range 1000 to 4000 bars, the proportions of monomers being such as to provide an interpolymer comprising 78 to 98 weight Z units derived from ethylene, 1.0 to 20.0 total weight Z units derived from ethyl and/or butyl acrylate and 1.0 to 2.0 units derived from the silane compound.

Ethylenically unsaturated silane compounds suitable for use as the monomer (F), or from which the units (C) are suitably derived in the present invention are compounds having the general formula R4SiR5nY3n wherein R4 represents an ethylenically unsaturated hydrocarbyl or hydrocarbyloxy group; R5 represents an aliphatic saturated hydrocarbyl group; Y represents a hydrolysable organic group; and n represents zero, 1 or 2. R4 can be, for example, vinyl, allyl, isopropenyl, butenyl, cyclohexenyl or gamma-methacryloxypropyl. Y can be, for example, methoxy, ethoxy, formyloxy, acetoxy, propionyloxy, alkylamino or arylamino. R5 can be, for example, methyl, ethyl, propyl, hexyl, octyl, decyl or phenyl. R4 is preferably a vinyl group, Y is preferably methoxy, ethoxy or acetoxy.

Examples of ethylenically unsaturated silane compounds suitable for use as the monomer (F), or from which the units (C) are derived in the present invention, are vinyl trimethoxy silane, allyl triethoxy sibane, gamma-methacryloxypropyl trimethoxy silane, vinyl triethoxy silane, vinyl triacetoxy silane, vinyl tripropionyloxysilane, vinyl dimethoxy methyl silane and vinyl diethoxy ethyl silane. Vinyl trimethoxy silane, vinyl triethoxy silane and vinyl triacetoxy silane are preferred.

The interpolymers of the present invention preferably comprise 86.0 to 97.0 weight I of units (A) derived from ethylene, 2.0 to 12.0 weight Z of units B derived from ethyl acrylate and/or butyl acrylate and 1.0 to 2.0 weight 2 of units (C) derived from the silane.

The polymerisation process of the present invention is suitably performed using apparatus and process conditions similar to those employed in the manufacture of low density polyethylene by the well-known high pressure free radical initiated process. The polymerisation temperature employed is preferably in the range 200 to 300it and the pressure is preferably in the range 2000 to 4000 bars.

The free radical initiator employed can be, for example molecular oxygen, an organic peroxide or an azo compound. Examples of suitable initiators are oxygen gas, lauroyl peroxide, benzoyl peroxide, ditertiary butyl peroxide, tertiary butyl peroxyisobutyrate, and azobisisobutyronitrile.

If desired, a chain transfer agent can be employed in the polymerisation process to control the average molecular weight of the produced interpolymer. Examples of suitable chain transfer agents are propane, butane, propylene, methyl ethyl ketone and toluene.

The polymerisation process can be carried out under batch or continuous conditions, for example in reactors of the autoclave or tubular type.

When the process of the present invention is carried out under batch conditions, for example in an autoclave, it is generally found necessary to employ an excess of ethylene over the "stoicheiometricn quantity desired in the final polymer to allov for the fact that a considerable quantity of unreacted ethylene is generally associated with the final product, whereas the polymerisation of the acrylate and silane monomers tends toward completion. In the case of continuous polymerisation processes, the ethylene monomer isolated in the pressure let-down stages is generally repressurised and fed back to the polymerisation, but the quantities of acrylate and silane monomers present in the product are generally too small to justify their separation and recovery.

The interpolymer obtained from the process of the present invention is suitably granulated, for example pelleted, together with conventional additives if desired, or alternatively fed as a melt for direct conversion into shaped articles.

A further aspect of the present invention provides a crosslinkable thermoplastic composition comprising the crosslinkable thermoplastic interpolymer and a silanol condensation catalyst.

Examples of compounds suitably employed as the silanol condensation catalyst include carboxylates of metals such as tin, zinc, iron, lead and cobalt, and organic bases, organic acids or inorganic acids, for example dibutyl tin dilaurate, dibutyl tin diacetate, dibutyl tin dioctoate, dibutyl tin maleate, stannous acetate, stannous caprylate, lead naphthenate, zinc caprylate, cobalt naphthenate, ethylamine, pyridine, acetic acid and maleic acid. The silanol condensation catalyst is preferably a dihydrocarbyl tin carboxylate wherein the hydrocarbyl groups are preferably C1 to C12 alkyl groups and the carboxylate is preferably provided by a mono or dicarboxylic acid. Particularly preferred are dibutyl tin dilaurate, dibutyl tin dipalmitate and dibutyl tin maleate.

The quantity of silanol condensation catalyst employed in the composition of the present invention is suitably 0.003 to 0.05 moles per mol of silyl units present in the interpolymer. The content of silanol condensation catalyst on a weight basis is preferably 0.03 to 3Z.

The composition comprising the interpolymer and the silanol condensation catalyst can be prepared for example by direct blending of the two components, or by masterbatch techniques. Masterbatch techniques are preferred. Suitable masterbatches can be prepared for example by compounding the silanol condensation catalyst, for example dibutyl tin dilaurate with a thermoplastic polymer to form a concentrate and the latter is then blended vith the interpolymer.

Examples of thermoplastic polymers suitable for preparing the masterbatch are LDPE, LLDPE, ethylene/ethyl acrylate copolymer and ethylene vinyl acetate copolymer. The masterbatch concentrate can also contain other additives which it is desired to incorporate into the final composition.

The composition of the present invention can contain additives conventionally employed in the art. Examples of such additives are antioxidants, fillers, metal deactivators (e.g. salicylaldehyde oximes), lubricants, water-tree inhibitors, foaming agents, flame retardants and pigments. Additives of this type are conventionally incorporated into the composition either directly or by a masterbatching technique. The composition can also be blended with other compatible polymeric materials, for example, polyethylene, polypropylene, ethylene/ethyl acrylate copolymer and ethylene/l-olefin copolymer.

The composition of the present invention can be used to manufacture crosslinked products using the technology known in the manufacture of articles from conventional silane grafted polymers or silyl copolymers of this type. For example, the composition can be used in blow-moulding, injection-moulding, film-blowing, calendering, extrusion, roto-moulding and extrusion coating techniques. The composition is particularly preferred for wire and cable coating applications. Wire and cable insulation produced by extrusion coating using the composition of the present invention generally exhibits good surface finish.

If desired, the surface finish of extruded articles, for example, wire and cable insulation, can be improved by including in the composition; a fluorohydrocarbon polymer, for example in accordance with the methods disclosed in EP-A-0169069; or a defined phosphorous or antimony compound as disclosed in EP-A- 0149903; or by presupporting the silanol condensation catalyst on a finely divided support material as disclosed in EP-A-0150595.

Articles fabricated from the composition of the present invention can be crosslinked by the techniques known in the art for crosslinkable polymers containing hydrolysable silane groups. For example crosslinking can be achieved by exposing the shaped article to water, steam or moist air. Crosslinking can be accelerated by the use of ultrasonics or microwaves if desired.

The invention is illustrated with reference to the following Examples.

Example 1 Ethylene - vinyltrimethoxy silane - ethyl acrylate terpolymer was continuously produced by feeding ethylene, vinyl trimethoxy silane, ethyl acrylate, propylene chain transfer agent and peroxide polymerisation initiators into a tubular reactor polymerising at a pressure of about 2400 kg/cm2 and a temperature of about 200 to 250*C.

The terpolymer produced was blended on a roll mill with 5 Z by weight of a masterbatch comprising 0.75Z by weight dibutyl tin maleate, 92 Z by weight of low density polyethylene and 6 Z antioxidant at a temperature of about l5OC. This composition was then formed into a 1.5 mm thick pressed sheet in a steam heated press at 1800C. The sheet was then immersed in water at 80"C for two hours to effect cross-linking of the terpolymer.

Three grade of terpolymers were produced having the following characteristics:1. Melt flow index (ASTM D1238) in the range 2.7 to 3.8. The vinyl trimethoxy silane content of the terpolymer was from 1.25 to 1.5 Z by weight and the ethyl acrylate content was from 8 to 8.5. The heat elongation (IEC 540) of the crosslinked material measured at 2000C, under 20 N cam~2, 15 mins was from 50 to 100%.

2. Terpolymer in which the ethyl acrylate content was varied from 2.5 to 8.5 % by weight. The melt flow index increased from 2 to 8.6 as ethyl acrylate content increased. The heat elongation varied from 50 to 120 Z. The vinyl trimethoxy silane content was from about 1.25 to about 1.5S.

Terpolymer having an ethyl acrylate content of about 4 % by weight. The melt flow index was about 0.5 and the vinyl trimethoxy silane content was about 1.3S.

ExamPle 2 Example 1 was repeated except that vinyl methyl dimethoxy silane was used in place of VTMS. Ethylene-ethyl acrylate-vinyl methyl dimethoxy silane terpolymer was produced which had a melt flow index of from 0.18 to 0.23 a vinyl methyl dimethoxy silane content of about 1.3 S by weight and the heat elongation of the cross-linked material was about 50 %. The composition was easy to extrude.

Claims (9)

Claims

1. A crosslinkable thermoplastic random interpolymer prepared by radical interpolymerisation, the interpolymer comprising (A) 78 to 98 weight Z of units derived from ethylene, (B) 1.0 to 20.0 weight % of units derived from ethyl acrylate and/or butyl acrylate and (C) 1.0 to 2.0 weight % of units derived from an ethylenically unsaturated hydrolysable silane compound.

2. A crosslinkable thermoplastic random interpolymer as claimed in claim 1 in which the ethylenically unsaturated hydrolysable silane compound is selected from the group comprising vinyl trimethoxy silane, vinyl triethoxy silane and vinyl triacetoxy silane.

3. A crosslinkable thermoplastic random interpolymer as claimed in either claim 1 or claim 2 which comprises 86 to 97 weight Z of units (A) derived from ethylene, 2.0 to 12.0 weight X of units (B) derived from ethyl acrylate and/or butyl acrylate and 1.0 to 2.0 weight Z of units (C) derived from the silane.

4. A crosslinkable thermoplastic composition comprising a crosslinkable thermoplastic interpolymer as claimed in any one of claims 1 to 3 and a silanol condensation catalyst.

5. A crosslinkable thermoplastic composition as claimed in claim 4 in which the silanol condensation catalyst is dibutyl tin dilaurate, dibutyl tin dipalmitate or dibutyl tin maleate.

6. A crosslinked composition prepared by exposing a crosslinkable composition as claimed in claim 4 or claim 5 to water, steam or moist air.

7. A process for preparing a crosslinkable thermoplastic random interpolymer comprising contacting a mixture of monomers comprising a major proportion of (D) ethylene and minor proportions of (E) ethyl acrylate and/or butyl acrylate and (F) an ethylenically unsaturated hydrolysable silane compound, with a free radical initiator at a temperature in the range 150 to 400it and at a pressure in the range 1000 to 4000 bars, the proportions of monomers being such as to provide an interpolymer comprising 78 to 98 weight % units derived from ethylene, 1.0 to 20.0 weight X units derived from ethyl and/or butyl acrylate and 1.0 to 2.0 units derived from the silane compound.

8. A crosslinkable thermoplastic random interpolymer substantially as described with reference to Examples 1 and 2.

9. A process for preparing a crosslinkable thermoplastic random interpolymer substantially as described with reference to Examples 1 and 2.