GB2141719A

GB2141719A - Ethylene polymer compositions

Info

Publication number: GB2141719A
Application number: GB08415641A
Authority: GB
Inventors: Ludwig Binder
Original assignee: Unifos Kemi AB
Current assignee: Unifos Kemi AB
Priority date: 1983-06-20
Filing date: 1984-06-19
Publication date: 1985-01-03
Also published as: FI842442A; DE3422853A1; GB8415641D0; FR2548198A1; NL8401850A; DK301784A; IT8448412A0; NO842473L; FI842442A0; SE8303538D0; SE8303538L; DK301784D0; BE899942A

Abstract

Compositions of high molecular weight low pressure polyethylene having a very high molecular weight, a melt index of from 0.1 to 20g/10 min. measured with a 21.6 kg weight and ethylene- (meth)acrylate copolymers 1-25% weight of the polyethylene can be prepared and further treated at required high temperatures without giving off corrosive acid degradation products. They are particularly suitable for the production of pipes and cable materials, and especially for crosslinked pipes. Compositions containing additives, such as organic additives and fillers, are advantageously prepared using the ethylene-(meth)acrylate copolymer as carrier material for the additives.

Description

SPECIFICATION Ethylene polymer compositions The present invention relates to compositions of very high molecular weight linear polyethylene and copolymers of ethylene and alkyl acrylate or ethylene and alkyl methacrylate.

The compositions may also contain different kinds of additives. The invention further also relates to a method for the production of compositions of linear polyethylene, ethyleneacrylate copolymers and additives.

The use of high molecular weight low pressure polyethylene types, i.e. essentially linear polyethylene materials, increases more and more along with the availability of extruder types and extrusion methods which manage to form these polymers which have very high melt viscosities. One of the reasons which makes it desirable to use these types of polyethytene is the fact that most mechanical properties are improved with a higher moiecu- lar weight of the polymer. This is true for low pressure polyethylene independent of density or production method.

As an example it can be mentioned that low pressure polyethylene types prepared according to the Phillips-process, i.e. using a catalyst based on chromium oxide, have been used for the production of large drums and tanks of volumes up to 10000 litres. These types of ethylene polymers have a density between 940 and 963 kg/m3 and a melt index, measured with 21.6 kg weight, in the range of from 0.1 to 20. Low pressure polyethylene types having corresponding melt index and densities produced according to a gas-phase fluid bed process with organo-chromium compounds as catalyst or with catalysts of the Ziegler-Natta type in a solution process have found similar applications.

A melt index of 0.1 to 20, measured with a 21.6 kg weight, indicates a very high melt viscosity and thus a very high average molecular weight. As a comparison it can be mentioned that conventional high pressure polyethylene types have melt index values in the same range, but then measured with a 10 times lighter weight and their melt viscosity is thus considerably lower at the same temperature.

On account of the importance of the average molecular weight with respect to the mechanical properties high molecular weight linear polyethylene materials are also used in the production of high quality pipes, profiles, plaques, cable insulation and jacketing, and in recent years also for the production of thin films. The materials have also been very successfully used in the production of crosslinked products, and especially in the production of crosslinked pipes. A high average molecular weight makes it possible to achieve a complete crosslinking with very small amounts of crosslinking agent or a low radiation dose.

At compounding and final forming of as high molecular weight low pressure polyethylene as contemplated herein, very high temperatures are required, often above 1 50 C and more often than not above 200"C. This means that special considerations must be taken in order that compositions with these polymers can be compounded without the formation of degradation products which have a detrimental effect on processes, equipment or finished products. The use of stabilizers and other additives is especially essential for these types of ethylene polymers as they are used in fields where a long working life is assumed. Different types of additives must as a rule be incorporated in fairly high amounts and they must be well dispersed in the polymer material.

The present invention relates to compositions of high molecular weight low pressure polyethylene having a melt index within the range of 0.1 to 20 g/10 min., measured according to ISO 1133 with 21.6 kg weight (condition 7), and 1 to 25 per cent by weight, based on the low pressure polyethylene, of a copolymer of ethylene and alkyl acrylate or ethylene and alkyl methacrylate. The compositions can also advantageously comprise additives which are conventionally used for polyethylene such as stabilizers, fillers, processing aids, pigments etc.

High molecular weight low pressure polyethylene here designates essentially linear homopolymers of ethylene or copolymers of ethylene and alpha-olefins having 3 to 14 carbon atoms, which have a melt index in the above given range. These types of ethylene polymers are prepared by low pressure processes, i.e. processes in which the pressure as a rule does not exceed 5 MPa. The melt index for the polymers should suitably be within the range from 1 to 15 g/10 min., measured as above. The high molecular weight low pressure ethylene polymers can have low or high density, within the range of from about 917 to about 963 kg/m3. High molecular weight linear polyethylene of low density has been made available by the development in catalyst and polymerization technology in recent years.

The low density is obtained by copolymerization of ethylene and alpha-olefins having 3 to 14 carbon atoms, suitably 3 to 10 carbon atoms. Generally 1 to 20 per cent by weight of the comonomer is used and a higher amount results in a lower density.

The acrylate copolymers in the present compositions are copolymers of ethylene and alkyl acrylate or alkyl methacrylate wherein the alkyl group usually has 1 to 8 carbon atoms, such as methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, i-butyl acrylate, 2ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate. Copolymers of ethylene and two or more of the above mentioned acrylates or methacrylates can of course be used. The copolymer is preferably an ethylene-alkyl acrylate copolymer. The copolymer should suitable have a melt index of 0.1 to 300 g/10 min., measured according to ISO 11 33 with 2.1 6 kg weight (condition 4), and preferably the melt index is within the range of from 0.3 to 40 g/10 min.The amount of acrylate in the copolymer should be from 1 to 60 per cent by weight and suitable between 3 and 30 per cent by weight. In the compositions according to the invention the acrylate copolymers are present in an amount of 1 to 25 per cent by weight, based on the low pressure polyethylene, and suitably in an amount of 3 to 1 5 per cent by weight.

The compositions of high molecular weight low pressure polyethylene and ethylene-acrylate copolymer according to the invention can be produced at required high temperatures without giving rise to problems with acid degradation products which might lead to corrosion of equipment and of objects in contact with the finished products. The compositions of the invention can be prepared by compounding the copolymer and the high molecular weight low pressure polyethylene in conventional, batchwise or continuous, compounding equipment.

The present compositions of high molecular weight low pressure polyethylene and low molecular weight polar copolymers are particularly advantageous for use in the production of crosslinked pipes. If chemical agents are used a complete crosslinking can be obtained by very small amounts of the crosslinking agent, organic peroxides or azo compounds. A very small dose can be used at crosslinking with electron radiation and the polar copolymers prevents the formation of accumulated electric charges, which can cause defects in the form of holes of channels at a discharge. The compositions are also very suitable as cable material, for example for insulation and jacketing.Low pressure polyethylene has very low dielectric values, i.e. a low dielectric constant and low dissipation factor, and these can be increased by the addition of the polar ethylene-acrylate copolymers which makes the compositions particularly interesting for this application. The compositions can of course also be used in the other fields, mentioned earlier, where high molecular weight low pressure polyethylene is used.

The compositions can advantageously contain additives which are conventionally used for polyethylene such as organic additives and inorganic fillers and pigments. The additives can be mixed into the low pressure polyethylene material as such or, advantageously, by using the ethylene-acrylate copolymer as carrier. Organic additives, stabilizers, are usually always used in ethylene polymers.

The polar acrylate copolymers allows a much higher concentration of additive to be incorporated than when the additives are incorporated in a pure ethylene homopolymer or a copolymer of ethylene and non-polymer monomers such as alpha-olefins. Stabilizers, lubricants and other additives are easily exuded from granulates of ethylene homopolymers or copolymers of ethylene and non-polar monomers at storing and particularly at temperature cycles. If corresponding concentrations of additives are incorporated in a copolymer of ethylene and a polar monomer this exudation does not occur. Compositions of high molecular weight low pressure polyethylene, ethylene-acrylate copolymers and additional additives are particularly advantageous and the invention offers a method for producing these wherein the additives are incorporated using the ethylene-acrylate copolymer as a carrier.In contrast to the copolymers of ethylene and vinyl acetate, which are generally used in so-called masterbatches--concen- trates of additives in a carrier-the ethyleneacrylate copolymers can be used for this purpose according to the present invention at the high compounding temperatures which are required for the high molecular weight polyethylene material without giving rise to acid degradation products, as the ethylene-vinyl acetate copolymers do.

Organic additives such as antioxidants, light stabilizers, lubricants etc. are generally used in polyethylene and are as a rule incorporated in amounts of up to about 2 per cent by weight based on the final compounds. Inorganic additives such as carbon black, fillers and pigments are incorporated for special applications and generally in substantially higher amounts than the organic stabilizers. The amount of filler in a final compound can for example reach 40 per cent or higher. Desired stabilizers and inorganic additives can be incorporated in the compositions of the invention in required amounts. They are preferably incorporated in the form of a master batch with the ethylene-acrylate copolymer as carrier whereby the concentration of the additives in the masterbatch material is adjusted to give the desired concentration in the final compound. The additives will be very satisfactorily dispersed using this method. Prepared compounds, in the form of pellets or granules, can then be further treated, i.e. finally formed, at high temperatures.

As has been mentioned earlier, the present compositions are especially suitable for the production of crosslinked products and particularly for crosslinked pipes. A particularly preferred composition according to the invention contains, besides the linear ethylene polymer and the acrylate copolymer, also an organic crosslinking agent which is an azo compound. In contrast to most organic peroxides there are azo compounds which have a sufficiently high composition temperature to be in corporated in the high molecular weight ethylene polymers. Such azo compounds, which require a temperature of at least 200"C for the crosslinking reaction are described for example in the Swedish patent application 7710448-7.Compositions of the invention containing chemical crosslinking agents in the form of azo compounds also contain conventional stabilizers and they can also contain carbon black.

The azo compounds can advantageously be included in a masterbatch with the ethyleneacrylate copolymers and in this manner be incorporated in the present compositions. The amount of crosslinking agents will most often be in the range of 0.5 to 2.5 per cent by weight, based on the final compositions, and usually 1 to 1.5 per cent is used.

The invention is further illustrated in the following examples which, however, are not intended to limit the same.

Example 1 A commercially available high molecular weight type of low pressure polyethylene in powder form, prepared by the so-called Phillips dispersion process, having a melt index of 2 (ISO 1133, 21.6 kg weight) and a density of 958, containing 700 ppm of a commercial antioxidant (Irganox 1076) was compounded with 6 per cent by weight of a masterbatch of carbon black. This masterbatch consisted of 40 per cent by weight of carbon black of the type "furnace black", 59 per cent by weight of an ethylene-ethyl acrylate copolymer (MI2.16 = 4.0 and 18 per cent by weight of ethyl acrylate) and 1 per cent of a commercial antioxidant (Santonox R). The compounding was carried out in a medium sized equipment consisting of a batch mixer of the Banburytype having a mixer volume of 10 litres, a single-screw feeding extruder and an underwater pelletizing equipment.

To make it possible for the polyethylene melt to pass the compounding equipment, without motor failure due to a too high load, it was necessary to raise the mass temperature of the polyethylene to 230"C.

The obtained granular compound of high molecular weight low pressure polyethylene containing carbon black, ethylene-ethyl acrylate copolymer and antioxidants was used for producing crosslinked polyethylene pipes (degree of crosslinking 75 %) according to Engel process, i.e. with a small amount of organic peroxide as the crosslinking agent. The black pipes had quite satisfactory mechanical properties.

From the same granular compound containing the ethylene-ethyl acrylate copolymer 5 litre drums were blown. The drums showed normal mechanical properties and had a very uniform black colour which indicates that the carbon black had been well dispersed in the compound.

In the same manner as described above another compound was prepared with the difference that instead of the ethylene-ethyl acrylate copolymer in the carbon black mass terbatch an ethylene-vinyl acetate copolymer (Ml2 16 = 4.0, 18 per cent by weight of Villyl acetate) was used.

Pellets of both the produced compounds were treated in an equipment for heat treatment of polyethylene wherein acid degradation products were collected. In the equipment the compounds were treated at a temperature of 180"C and samples of acid degradation products were taken and analyzed by ion chromatography. Compounds with extra lene-vinyl acetate copolymer gave a very cieo reading for corrosive acetic acid. The detection limit for the chromatograph was 0.3 ppm. For compositions with the ethylene-ethyl acrylate copolymer no acetic acid or other acid degradation products were found.

Example 2 In the same manner as described in example 1 (with the difference that the mass temperature was about 200"C) a high molecular weight loss pressure polyethylene material (MI21.6 = 6 and density 942) in powder form, prepared according to the Union Carbide Unipol-process, i.e. by means of a gas-phase fluid bed process using an organo-chromium catalyst, were compounded with 10 per cent by weight of an ethylene-n-butyl acrylate copolymer (MI2.16 = 2.7, 7 per cent by weight of n-butyl acrylate). The copolymer contained 10 per cent by weight of a mixture of equal parts of a commercial antioxidant (Irganox 1010), a commercial light stabilizer of benzophenone type (Cyasorb 531) and a commercial process stabilizer (Irgafos 168).The copolymer was thus the carrier for one of the several commercial stabilizer masterbatches.

The granular compound, containing ethylene-n-butyl-acrylate copolymer, was extruded in a conventional pipe extruder to a pipe having an outer diameter of 25 mm and a thickness of 1 mm. The pipes were then crosslinked by electron radiation to a degree of crosslinking of 65 %. The crosslinked pipes had normal mechanical strength properties, they had a smooth surface and there were no holes or cracks, they had a smooth surface and there were no holes or cracks indicating discharge of accumulated free electrons.

Samples of the granular compound above and a corresponding compound where the ethylene-acrylate copolymer had been replaced by an ethylene-vinyl acetate copolymer (Ml2 16 = 2, 8 per cent of vinyl acetate) were treated in the equipment for heat treating as in example 1. The ion chromatograph did not give any indication of acid degradation products for compounds according to the invention but for compounds with ethylene-vinyl acetate copolymer it gave a clear reading of free acetic acid.

L-xarrple 3 A high molecular weight low pressure polyethylene material in powder form (Ml2I 6 = 8 and density 920) prepared by means of a Phillips catalyst in the Unipol-process of Union Carbide (gas-phase fluid bed polymerization) was compounded (according to example 1, mass temperature 190"C) with 5 per cent by weight of an ethylene-methyl acrylate copolymer (Ml2 16 = 20 and 28 per cent by weight of methyl acrylate).

The copolymer contained 20 per cent by weight of a commercial azo compound (Luazo AP) which is used for crosslinking polyethylene.

The granular compound, containing ethylene-methyl acrylate copolymer, was extruded in a conventional pipe extruder to pipes having an outer diameter of 63 mm and a thickness of 2 mm, and the pipes were heated in an autoclave under pressure to a degree of crosslinking of 65 %. The obtained pipes had a smooth surface, free from pores, and they had normal mechanical properties.

A sample of this compound and a sample of a corresponding compound wherein the ethylene-methyl acrylate copolymer (Ml2,6 = 20, 28 per cent vinyl acetate) were heat treated and analyzed with respect to acid degradation products. The compound with ethylene-vinyl acetate could not be treated at the high temperatures without releasing acetic acid which was detected by the ion chromatography.

However, no detectable acid products were released from the compound with the acrylate copolymer.

Example 4 A commercially available type of high molecular weight low pressure polyethylene {M1216 = 7, density 951) in powder form, prepared according to the Ziegler-Natta solvent process, containing about 700 ppm of a phenolic antioxidant was compounded with an additive masterbatch according to example 1 at a mass temperature of 200"C. The additivemasterbatch consisted of 90 per cent by weight of a copolymer of ethylene and 2ethylhexyl acrylate (MI2.16 = 7 and 1 7 per cent by weight of acrylate), 6.67 per cent by weight of a hindered amine light stabilizer and 3.33 per cent by weight of a metal stearate as lubricant. From the granular compound 10 litre drums were blown. The drums looked normal and had normal strength properties.

Compounds according to this example and a corresponding compound wherein the acrylate copolymer had been replaced by an ethylene-vinyl acetate copolymer with My216 = 7 and containing 1 8 per cent by weight of vinyl acetate were treated and analyzed as described previously. Again there was a clear reading of free acetic acid from the compound with the ethylene-vinyl acetate copolymer while compounds according to the invention could be treated at high temperatures without formation of products which can cause corrosion of equipment and materials.

Example 5 A high molecular weight low pressure polyethylene material in powder form with My21 6 = 1 3 and a density of 924, prepared by means of a Ziegler-Natta catalyst in the Unipol-process of Union Carbide, was compounded, according to example 1 at a mass temperature of 175"C, with 5 per cent by weight of yellow pigment masterbatch.

The pigment mastebatch contained 10 per cent by weight of yellow pigment and 1 per cent by weight of a phenolic antioxidant and was based on an ethylene-ethyl acrylate copolymer (my2 16 = 7, 18 per cent of ethyl acrylate).

A 0.025 mm thick yellow film was blown from the granular compound. The film had good strength properties and an attractive appearance indicating a uniform distribution of the yellow pigment.

A corresponding compound wherein the ethylene-ethyl acrylate copolymer had been replaced by an ethylene-vinyl acetate copolymer (my2 16 = 10, 20 per cent vinyl acetate) was prepared. Both compounds were heat treated and collected samples were analyzed by ion chromatography. Free acetic acid was detected in the compound containing the ethylene-vinyl acetate copolymer but no acid degradation products could be detected in the sample of the compound according to the invention.

Example 6 A low pressure polyethylene material in powder form, which could not be classified as high molecular weight, was prepared by means of a Ziegler-Natta catalyst in the Unipol-process of Union Carbide. The low pressure polyethylene had a Ml2, 6 of 50 and a density of 934.

It was possible to compound this polyethylene material according the process of example 1 at such a low mass temperature as 160"C with the master batch based on ethylene-vinyl acetate copolymer described in example 1.

Free acetic acid was not detected by the ion chromatography analysis which means that the concentration of free acetic acid was below the detection limit of 0.3 ppm. This shows that detectable amounts of free acetic acid occur only when high molecular weight low pressure polyethylene material, with M1216 below about 20, is compounded with ethylene-vinyl acetate copolymers, i.e. when the mass temperature at the compounding must be kept above 160"C.

Claims

1. Ethylene polymer composition, characterised in that it comprises high molecular weight low pressure ethylene polymer with a melt index within the range of 0.1 to 20 g/10 min. measured according to ISO 1133 with 21.6 kg weight, and 1 to 25 per cent by weight, based on the low pressure ethylene polymer, of a copolymer of ethylene and alkyl acrylate or alkyl methacrylate.

2. A composition according to claim 1, wherein the low pressure ethylene polymer has a melt index within the range of 1 to 1 5.

3. A composition according to claim 1 or 2, wherein the copolymer of ethylene and alkyl acrylate or alkyl methacrylate has a melt index within the range of from 0.1 to 300 g/10 min. measured according to ISO 1133 with 2.16 kg weight.

4. A composition according to claim 1, -2 or 3, which further comprises stabilizers and/ or inorganic additives.

5. A composition according to any preceding claim which further comprises a chemical cross-linking agent which is an azo compound.

6. An ethylene polymer composition as claimed in claim 1, substantially as described in any one of the foregoing Examples.

7. A method for the preparation of a composition which comprises high molecular weight low pressure ethylene polymer with a melt index within the range of 0.1 to 20 g/ 10 min. measured according to ISO 1133 with 21.6 kg weight, and 1 to 25 per cent by weight, based on the low pressure ethylene polymer, of a copolymer of ethylene and alkyl acrylate or alkyl methacrylate and which additionally contains one or more additives appropriate to ethylene polymers wherein the additives are incorporated in the high moelcular weight low pressure ethylene polymer in the form of a masterbatch using the copolymer of ethylene-alkyl acrylate or ethylenealkyl methacrylate as carrier for the additive(s).

8. A method as claimed in claim 7, for the preparation of an ethylene polymer composition, substantially as described in any one of the foregoing Examples.

9. An extrusion method for the production of ethylene polymer pipes or cable coverings which comprises extruding a composition as claimed in any one of claims 1 to 6.

10. A method as claimed in claim 9, which comprises cross-linking the polymer composition.

11. A method for the production of polymer pipes, substantially as described in any one of the foregoing Examples 1 to 3.

1 2. A polymer pipe or cable covering, which has been produced by the method claimed in any one of Claims 9 to 11.