WO1997045465A1

WO1997045465A1 - Unsaturated ethylene copolymers and preparation thereof

Info

Publication number: WO1997045465A1
Application number: PCT/SE1997/000913
Authority: WO
Inventors: Bill Gustafsson; Jussi Laurell
Original assignee: Borealis A/S
Priority date: 1996-05-31
Filing date: 1997-05-28
Publication date: 1997-12-04
Also published as: AU3111497A; SE507045C2; SE9602125D0; SE9602125L

Abstract

The invention concerns an unsaturated ethylene copolymer, a method for producing it, and the use thereof for producing cross-linked structures, such as insulating layer material, semiconductor layer material and sheath material for electric cables. The unsaturated ethylene copolymer comprises a polymer obtained by radical polymerisation through a high-pressure process of ethylene and at least one monomer which is copolymerisable with ethylene and includes a diunsaturated comonomer of the formula (I): H2C = CH-O-R-CH = CH2 wherein R = -(CH2)m-O-, -(CH2CH2O)n-, or -CH2-C6H10-CH2-O-, m = 2-10 and n = 1-5. Preferably, the comonomer of formula (I) is 1,4-butanediol divinyl ether.

Description

UNSATURATED ETHYLENE COPOLYMERS AND PREPARATION

THEREOF

The present invention relates to an unsaturated ethy- lene copolymer and a method for the production thereof. More specifically, the invention concerns an unsaturated ethylene copolymer having an increased degree of unsatura- tion and being produced by radical polymerisation through a high-pressure process. Normally, polyethylene produced by radical polymeri¬ sation, so-called LDPE, has a low degree of unsaturation in the order of 0.1 double bonds/1000 carbon atoms. In many situations, it is desirable to use polymers having a higher degree of unsaturation, which may serve as seat for chemical reactions, such as the introduction of func¬ tional groups into the polymer molecule or the cross- linking of the polymer. It is known that an increased level of double bonds can be obtained in polyethylene produced by organometallic catalysis, i.e. involving a coordination catalyst, by introducing as comonomers com¬ pounds having several double bonds, in which case only one bond is used for polymerising the comonomer into the polymer chain. EP 0 008 528 and JP 0 226 1809, for instance, disclose such prior-art techniques. Further, EP 0 260 999 relates to copolymers of ethylene and dienes having 4-18 carbon atoms, such as 1, 4-hexadiene, in which case polymerisation is performed by means of a so-called metallocene catalyst at a high pressure. Mention may also be made of W0 91/17194 which concerns copolymers of cc-olefins, such as ethylene, and α,ω-dienes having

7-30 carbon atoms, preferably 8-12 carbon atoms, such as 1, 9-decadiene, in which case polymerisation is coordina¬ tion-catalysed. Moreover, US 3,357,961 discloses the pro¬ duction of a copolymer of ethylene and 1, 5-hexadiene by coordination-catalysed low-pressure polymerisation. One may further mention Chemical Abstracts, Vol. 116, No. 4, 27th January 1992, p. 15, Abstract 21674b (JP 0 322 1508, published on 30th September 1991); Chemical Abstracts Vol. 101, No. 12, 17th September 1984, p. 42, Abstract 92065e (JP 595 6412 published on 31st March 1984); and Chemical Abstracts, Vol. 69, No. 74, 9th December 1968, Kiti, Itsuo: "Ethylene-1,4-hexadiene copolymers" p. 9135, Abstract 97310m. These abstracts relate to copolymers of ethylene and non-conjugated dienes, such as 1,4-hexa¬ diene, 1,7-octadiene and 1, 9-decadiene, and involve the use of coordination-catalysed polymerisation. As already mentioned, the above references relate to coordination-catalysed polymerisation. Coordination-cata¬ lysed polymerisation and radical-initiated polymerisation are two fundamentally different types of polymerisation, resulting in different types of polymers. While coordina- tion-catalysed polymerisation essentially yields unbranch- ed linear polymer molecules, radical-initiated polymerisa¬ tion yields heavily branched polymer molecules with long side chains. Consequently, polymers produced by the two processes have different properties. For instance, poly- mers produced by coordination-catalysed polymerisation have a higher density than those produced by radical-ini¬ tiated polymerisation at equivalent comonomer contents. They also have a higher melt viscosity at the same melt index, which means that the polymers produced by a radi- cal-initiated high-pressure process are, in general, easier to process.

It should be emphasised that the fact that coordina¬ tion-catalysed polymerisation and radical-initiated poly¬ merisation are two fundamentally different processes means that no conclusions about one process can be drawn from the other. If, in coordination-catalysed polymerisation involving the addition of diene, only one double bond of the diene reacts, one may thus not conclude that this is also the case in radical-initiated polymerisation. Whether the diene reacts or not in coordination-catalysed polyme¬ risation depends on the action produced by the coordina¬ tion catalyst employed. Since radical-initiated polymeri- sation does not involve any such catalyst, there is no reason to assume that the diene will react in the same way in radical-initiated polymerisation.

On the contrary, in FR 2,660,660, for instance, non- conjugated dienes are used as chain-transfer agents in radical-initiated polymerisation of ethylene. The use of non-conjugated dienes as chain-transfer agents according to FR 2,660,660 is contrary to the prior-art technique in coordination-catalysed polymerisation described by way of introduction, and thus emphasises the difference between radical-initiated polymerisation and coordination-cata¬ lysed polymerisation.

Further, the published International Patent Applica¬ tion WO 91/07761 discloses a cable sheathing composition prepared by radical-initiated high-pressure polymerisation and containing ethylene, 30-60% by weight of a monofunc- tional ethylenically unsaturated ester, preferably vinyl acetate or methyl acrylate, and 1-15% by weight of a mul¬ tifunctional ethylenically unsaturated termonomer having at least two ethylenically unsaturated groups. The polymer has a melt index of 0.1-10, and the composition further contains a filler, a cross-linking agent and a stabiliser. The multifunctional termonomer is a doubly unsaturated molecule containing -0- or C=0. Preferably, the termonomer is obtained by esterification of a glycol and acrylic acid or a homologue thereof. It is most preferred that the ter¬ monomer is ethylene glycol dimethacrylate (EDMA). Unlike aliphatic diene hydrocarbons, this acrylate-containing polyunsaturated termonomer is very reactive, and all the unsaturation of the termonomer will thus react in the polymerisation of the polymer. Consequently, polymerisa¬ tion does net yield any unsaturated polymer product, and the termonomer serves to adjust, i.e. lower, the melt index of the product, which it does by cross-linking pairs of polymer chains. It may here be mentioned that the art embraces the knowledge that increased double-bond contents can be obtained also in polyethylene produced by radical poly¬ merisation through a high-pressure process, by adding propylene as chain-transfer agent, which has the limita¬ tions mentioned above with regard to FR 2,660,660. (This is described in, inter alia, Encyclopedia of Polymer Science and Technology, Rev. Ed., Vol. 6 (1986), p. 394, last par. - p. 395, first par.) The level of double-bond content thus achieved in LDPE of about 0.3-0.4 double bonds/1000 carbon atoms is, however, insufficient in many contexts.

Finally, it is known from WO 93/08222 how to prepare unsaturated copolymers of ethylene and a polyunsaturated comonomer by radical polymerisation and to use such unsa¬ turated copolymers in order to produce cross-linked struc¬ tures, such as insulating layer material, semiconductor material or sheath material for electric cables. The poly¬ unsaturated comonomer according to WO 93/08222 has a straight carbon chain, which is free from heteroatoms and has at least 8 carbon atoms and at least 4 carbon atoms between two non-conjugated double bonds, of which at least one is terminal. The polyunsaturated comonomer is suitably an α,ω-alkadiene having 8-16 carbon atoms, preferably selected from 1,7-octadiene, 1, 9-decadiene and 1,13-tetra- decadiene.

It has now surprisingly been found that unsaturated ethylene copolymers also may be produced by radical poly¬ merisation of ethylene and certain divinyl ethers, more particularly divinyl ethers of the general formula (I ) defined below.

The present invention thus provides an unsaturated ethylene copolymer, which is characterised in that it comprises a polymer obtained by radical polymerisation through a high-pressure process of ethylene and at least one monomer which is copolymerisable with ethylene and includes a diunsaturated comonomer of formula ( I ) H₂C = CH-O-R-CH = CH₂ (I) wherein

^{R =} -^(CH2⁾m-°-' -^(CH2^CH2⁰⁾n-' ^or -^CH ₂-^C6^H10-^CH2-°-' m = 2-10 and n = 1-5. The present invention also provides a method for pro¬ ducing an unsaturated ethylene copolymer, characterised by polymerising, at a pressure of about 100-300 Mpa and a temperature of about 80-300°C and under the action of a radical initiator, ethylene and at least one monomer which is copolymerisable with ethylene and includes a diunsatu¬ rated comonomer of formula ( I )

H₂C = CH-O-R-CH = CH (I) wherein R = -(CH₂)_m-0-, -(CH₂CH₂0)_n-, or -CH₂-C₆H₁₀-CH₂-0-, m = 2-10 and n = 1-5.

The present invention further relates to the use of an unsaturated ethylene copolymer as defined above for producing cross-linked structures, such as insulating layer material, semiconductor layer material or sheath material for electric cables.

Other distinctive features of the invention will appear from the following description and the appended claims. In comparison with the prior art discussed above, the diunsaturated comonomer of formula ( I ) is somewhat similar to the multifunctional termonomer of WO 91/07761. Accord¬ ingly, it would have been expected that, in the polymeri¬ sation with ethylene, both the double bonds of the diunsa- turated comonomer of formula ( I ) would react like the ter- monomomer of WO 91/07761. Surprisingly, this is not the case, and only one of the double bonds reacts in the poly¬ merisation of the polymer, while the other double bond remains intact to give a polymer of enhanced unsaturation. The present invention with the diunsaturated comono¬ mer of formula (I ) is in contrast to the teachings of WO 93/08222, which require that the polyunsaturated como- nomer has a straight carbon chain which is free from heteroatoms.

Contrary to what would have been expected, the diun¬ saturated comonomer of formula ( I ) thus performs very well according to the present invention in giving unsaturated ethylene copolymers. Although the reason for this is not known, one theory is that the absence of allylic hydrogen atoms may contribute to the unexpected behaviour of the comonomer of formula (I ) . An advantage of the comonomer of formula (I ) accord¬ ing to the invention as compared with known polyunsaturat¬ ed comonomers, such as 1, 9-decadiene of WO 93/08222, is that it does not have such a strong odour, which means that it is less problematic from the environmental point of view for the personnel handling the comonomer.

The fact that the comonomer of formula (I ) contains vinyl ether groups means that the polymer ought to be sen¬ sitive to acid hydrolysis. It has, however, suprisingly been found that the unsaturated ethylene copolymer accord- ing to the invention does not exhibit any high sensitivity to acid hydrolysis, which is a further advantage of the invention.

As indicated in the foregoing, R in formula (I) may, inter alia, stand for -(CH^) -0-, wherein m = 2-10. When m = 2, formula (I) signifies ethylene glycol divinyl ether, and when m = 4, 6, 8 and 10, formula (I) signifies 1,4-butanediol divinyl ether, 1, 6-hexanediol divinyl ether, 1, 8-cctanediol divinyl ether and 1, 10-decanediol divinyl ether, respectively. Being commercially available at a low cost, 1,4-butanediol divinyl ether is especially preferred.

Further, R in formula (I) may also stand for -(CH₂CH₂0) -, wherein n = 1-5. When n = 1, formula (I) signifies ethylene glycol divinyl ether as above, and when n = 2, 3, 4 and 5, formula (I) signifies diethylene glycol divinyl ether, triethylene glycol divinyl ether, tetra ethylene glycol divinyl ether and pentaethylene glycol divinyl ether, respectively.

When R in formula (I) stands for -CH₂-C₆H-₀-CH₂-0-, formula ( I ) signifies cyclohexane dimethanol divinyl ether.

Among the above examples of possible significations of formula (I), 1,4-butanediol divinyl ether is currently the most preferred.

The content of the diunsaturated comonomer is such that the unsaturated copolymer contains 0.2-3% by weight thereof, preferably 0.2-1.5% by weight, which corresponds to an unsaturation of, respectively, 0.2-3 and 0.2-1.5 double bonds/1000 carbon atoms for 1, 4-butanediol divinyl ether. Apart from ethylene and at least one diunsaturated comonomer, the ethylene polymer according to the invention may contain up to 40% by weight of some other monomer which is copolymerisable with ethylene. Such monomers are well-known to the expert and need not be accounted for in greater detail here. Mention may, however, be made of vinyl-unsaturated monomers, such as C„-C_Q α-olefins, e.g. propylene, butylene, and so forth; and vinyl-unsaturated monomers containing functional groups, such as hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups and ester groups. Such monomers may, for instance, consist of (meth)acrylic acid and alkyl esters thereof, such as methyl-, ethyl-, and butyl(meth)aerylate; and vinyl-unsa¬ turated hydrolysable silane monomers, such as vinyl tri- methoxy silane, and so forth. Propylene and higher α-olefins may be regarded as a special case, since they also act as chain-transfer agents and create terminal unsaturation in the polymer (cf. the foregoing regarding the creation of increased double-bond contents by adding propylene as comonomer (Encyclopedia of Polymer Sciences and Technology, Rev. Ed., Vol. 6 (1986), pp 394-395). Using propylene (or some other higher α-ole- fin) as comonomer in addition to the polyunsaturated como- 8 nomer defined above thus makes it possible to further increase the degree of unsaturation of the produced copo- lymer in a comparatively simple and inexpensive manner. As stated above, the unsaturated ethylene polymer according to the invention is produced by high-pressure polymerisation with free-radical initiation. This poly¬ merisation process, which is well-known in the art and thus need not be accounted for in more detail here, is generally performed by reacting the monomers under the action of a radical initiator, such as a peroxide, a hydroperoxide, oxygen or an azo compound, in a reactor, e.g. an autoclave or a tube reactor, at a high pressure of about 100-300 MPa and an elevated temperature of about 80-300°C. When the reaction is completed, the temperature and the pressure are lowered, and the resulting unsatu¬ rated polymer is recovered. Further details of the pro¬ duction of ethylene polymers by high-pressure polymerisa¬ tion with free-radical initiation can be found in the Encyclopedia of Polymer Science and Engineering, Vol. 6 (1986), pp 383-410, especially pp 404-407.

As mentioned by way of introduction, the copolymers according to the invention are intended for use when a polymer with reactive sites in the form of ethylenic unsa¬ turation is to be produced. Ethylenic unsaturation may be used for introducing functional groups, such as hydroxyl and carboxyl, in the polymer by a reaction with compounds containing such groups. The ethylenic unsaturation may also be used for cross-linking the polymer, which perhaps is its primary use. The cross-linking of polyethylene is of interest in many contexts, such as extrusion (e.g. of tubes, cable insulating material or cable sheathing), blow moulding, rotational moulding, etc.

This technique is of special interest in cable tech¬ nology, for which reason this will be discussed in more depth here. In the extrusion of e.g. a power cable, the metallic conductor generally is first coated with a semi¬ conductor layer, then with an insulating layer, then with another semiconductor layer optionally followed by water barrier layers, and finally with a sheath layer.

At least the insulating layer and the outer semi¬ conductor layer normally consist of cross-linked ethy- lene homopolymers and/or ethylene copolymers. Cross-link¬ ing substantially contributes to improve the temperature resistance of the cable, which will be subjected to con¬ siderable temperature stress when in operation. Cross- linking is brought about by adding free-radical-forming agents, mostly of peroxide type, to the polymer materials in the above layers prior to extrusion. This radical- forming agent should preferably remain stable during the extrusion but decompose in a subsequent vulcanisation step at an elevated temperature, thereby forming free radicals which are to initiate cross-linking. Premature cross-linking during extrusion will show as "scorch", i.e. as inhomogeniety, surface uneveness and possible discolouration, in the different layers of the finished cable. Consequently, the polymer material and the radi- cal-forming agent must not, in combination, be too reac¬ tive at the temperatures prevailing in the extruder (about 125-140°C).

After the extruder, the cable is passed through a long multi-zone vulcanising tube where cross-linking should take place as rapidly and completely as possible; initiated by the heat emitted in one or more heated zones of the tube. A nitrogen-gas pressure is also applied in the tube, and contributes to prevent oxidation processes by keeping away the oxygen of the air and to reduce the formation of microcavities, so-called voids, in the poly¬ mer layers by reducing the expansion of the gases result¬ ing from the decomposition of the radical-forming agent. It is desirable that cross-linking is rapid but requires as little free-radical-forming agent as possible, since this reduces the risk of scorch in the extruder, results in minimum formation of microcavities as mentioned above, and is economically advantageous, peroxide being an expen- sive additive. Thus, the polymer material to be cross- linked should be as reactive as possible in the vulcanis¬ ing step. As illustrated in Example 19 below, the present invention substantially contributes to such reactive pro- perties.

It appears from the foregoing that the unsaturated ethylene copolymer according to the invention can be used as material for semiconductor layers, insulating layers and/or sheath layers of electric cables. The following non-restricting Example is meant to further elucidate the invention. Example 1

A 200-ml reactor for batch polymerisation was flushed with ethylene and then connected to a vacuum pump which generated a negative pressure in the reactor. This nega¬ tive pressure was used for drawing 20 ml of a mixture of polymerisation initiator, diene and isodecane (solvent) into the reactor. Then, ethylene was pumped into the reac¬ tor at a pressure of about 130 MPa ( isothermic condi- tions). At this point, the temperature was about 20-25°C. Thereafter, the reactor was heated to about 160-170°C, the pressure in the reactor rising to about 200 MPa and the polymerisation reaction began, which was indicated by a further increase in temperature to about 175°C. No ethy- lene was supplied to the reactor in the course of the reaction. The test continued until the reactor temperature had passed a maximum value and started to drop, which indicated that the polymerisation reaction was completed. The reactor was then cooled to room temperature, blown off and opened for removal of the polymer formed, which was present in an amount of about 5-15 g.

In this Example, 1,4-butanediol divinyl ether was used as diunsaturated comonomer of formula ( I ) according to the invention. The amount of comonomer of formula (I) made up 1.9% by weight of the copolymer. In order to illustrate the unsaturation of the re¬ sulting polymer according to the invention and compare it with that of polyethylene prepared without the addition of a polyunsaturated comonomer (Reference polyethylene, RefPE), the following test was performed.

The enhanced degree of unsaturation of the copolymer according to the invention makes it more reactive in cross-linking, and a smaller amount of cross-linking cata¬ lyst (peroxide) is consequently required in order to achieve a certain degree of cross-linking than is the case in the cross-linking of Reference polyethylene. Inversely, a certain amount of cross-linking catalyst results in a higher degree of cross-linking in the unsaturated ethy¬ lene copolymer according to the invention than in Refe- rence polyethylene. The degree of cross-linking is measur¬ ed with the aid of a Gόttfert elastograph measuring the changes of the shear modulus of cross-linkable polyethy¬ lene when the peroxide cross-linking catalyst is decom¬ posed and cross-links the polymer chains at 180°C after 10 min. This testing method corresponds to ISO-6502. The following polyethylene polymers were used.

Polymer composition

A. Ethylene/1,4-butanediol divinyl ether (1.9% by weight of comonomer).

B. Ethylene (Reference polyethylene containing 0.12 vinyl groups per 1000 carbon atoms).

To each ethylene polymer was then added 0.2% by weight of Santonox (4,4' -thio-bis(2-tert-butyl-5-methyl phenol) ) as stabiliser by compounding on a Buss kneader of the type PR46B-11D/H1.

The Reference polyethylene was then divided into three batches, to each of which was added a cross-linking catalyst (dicumyl peroxide; "dicup" ) in varying concentra¬ tions ranging from 1.4% by weight to 2.1% by weight. To the unsaturated copolymer according to the invention was added 1.5% by weight of dicumyl peroxide. Pellets were made from the ethylene polymers, and plates were then made from the pellets by preheating at 120°C for 2 min and compacting at 9.3 MPa for 2 min.

The resulting plates were then tested in a Gottfert elastograph, and the results appear from Table 1 below. It is evident from the Table that the peroxide content required to attain a certain degree of cross-linking is substantially reduced for the unsaturated ethylene copo¬ lymer according to the invention.

Table 1 Gottfert elastograph at 180°C, 10 min. Stabilised base resins.

Dicup- Gottfert content elastograph

Sample [%] (max) [Nm]

A+0.2% Sx 1.5 0.71

B+0.2% Sx 1.4 0.44

B+0.2% Sx 1.7 0.56

B+0.2% Sx 2.1 0.71

Claims

1. An unsaturated ethylene copolymer, c h a r a c ¬ t e r i s e d in that it comprises a polymer obtained by radical polymerisation through a high-pressure process of ethylene and at least one monomer which is copolymerisable with ethylene and includes a diunsaturated comonomer of formula ( I )

H₂C = CH-O-R-CH = CH₂ (I)

wherein

R = -(CH₂2)m-0-, -(CH₂CH₂0)n-, or -CH₂-C₆H₁₀0-CH₂-0-, m = 2-10 and n = 1-5.

2. An unsaturated ethylene copolymer according to claim 1, c h a r a c t e r i s e d in that R is

3. An unsaturated ethylene copolymer according to claim 1 or 2, c h a r a c t e r i s e d in that m = 4 or 6.

4. An unsaturated ethylene copolymer according to claim 3, c h a r a c t e r i s e d in that the comono¬ mer of formula (I) is 1,4-butanediol divinyl ether.

5. An unsaturated ethylene copolymer according to any one of claims 1-4, c h a r a c t e r i s e d in that it has been obtained by polymerisation of ethylene, a monomer of formula (I), and a monovinyl-unsaturated monomer.

6. An unsaturated ethylene copolymer according to claim 5, c h a r a c t e r i s e d in that the mono¬ vinyl-unsaturated monomer is a C~-C_R alpha-olefin or con¬ tains at least one functional group selected from hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups, and ester groups.

7. A method for producing an unsaturated ethylene copolymer, c h a r a c t e r i s e d by polymerising, at a pressure of about 100-300 MPa and a temperature of about 80-300°C and under the action of a radical initia¬ tor, ethylene and at least one monomer which is copolymer¬ isable with ethylene and includes a diunsaturated como¬ nomer of formula (I)

H₂C = CH-O-R-CH = CH₂ (I)

wherein

R - -(CH₂)_m-0-, -(CH₂CH₂0)_n-, or -CH^CgH^-CH^O-, m = 2-10 and n = 1-5.

8. A method according to claim 7, c h a r a c ¬ t e r i s e d by utilising a diunsaturated comonomer of formula (I)⁷,' wherein R is -(CH„2)'m-0- .

9. A method according to claim 8, c h a r a c ¬ t e r i s e d by the diunsaturated comonomer of formula (I) being 1,4-butanediol divinyl ether.

10. A method according to any one of claims 7-9, c h a r a c t e r i s e d by polymerising ethylene, a monomer of formula (I), and a monovinyl-unsaturated mono¬ mer.

11. A method according to claim 10, c h a r a c ¬ t e r i s e d by the monovinyl-unsaturated monomer being a C₃-C₈ alpha-olefin or containing at least one functional group selected from hydroxyl groups, alkoxy groups, carbo- nyl groups, carboxyl groups, and ester groups.

12. The use of the unsaturated ethylene copolymer according to any one of claims 1-6 in compositions for producing cross-linked structures.

13. The use according to claim 12 in compositions for insulating layer material, semiconductor layer material or sheath material for electric cables.