WO1991008262A2

WO1991008262A2 - Cross-linkable polymer blends

Info

Publication number: WO1991008262A2
Application number: PCT/GB1990/001857
Authority: WO
Inventors: David Cyril Varrall; Wai Keung Wong
Original assignee: Exxon Chemical Limited; Exxon Chemical Patents Inc.
Priority date: 1989-12-01
Filing date: 1990-11-29
Publication date: 1991-06-13
Also published as: GB8927174D0; WO1991008262A3

Abstract

A blend of (a) a first linear low density polyethylene (LLDPE) and (b) one or more of (i) low density polyethylene (LDPE), (ii) polyethylene (PE) wax and (iii) a second LLDPE of melt index greater than that of the first LLDPE, the first LLDPE and optionally one or more of the polymers (b) possessing silane groups capable of crosslinking, is relatively rapidly crosslinked on exposure to moisture, whilst having good processability, and is therefore useful in an extrusion and subsequent crosslinking process to form a crosslinked coating for a cable.

Description

CROSS-LINKABLE POLYMER BLENDS

The present invention relates to a cross-linkable blend of polymers and its use in cable coating applications, a cross-linked polymer composition, a shaped coating formed from the crosslinked composition, and processes for their production.

Polymeric compositions are often used to coat wires and cables, particularly electric or optical cables. A -protective coating is formed around the cable, insulating it to prevent leakage of e.g., electricity and to protect the cable from attack by its surroundings. The coating is shaped by extruding a polymer, or blend of polymers, to form a solid coating of the desired dimensions.

The polymer or blend of polymers used as the coating composition may be thermoplastic or cross-linked. Crosslinking the composition may be advantageous; for example it may improve the resistance of the coating to thermal degradation, improve its resistance to environmental stress cracking and improve its resistance to fluids such as oil and water. Mechanical properties may also be improved e.g., tensile strength, elongation at break, tear strength and abrasion resistance.

Traditionally in the cable industry crosslinking has been achieved by thermochemical means, for example, by adding peroxide crosslinking agents and heating to form chemical crosslinks. However, this method is relatively expensive in terms of the equipment needed and the energy required to achieve the necessary level of crosslinking. Moreover, the extrusion and crosslinking steps are sequential; as the composition is extruded it then passes through the apparatus which causes crosslinking. In industrial applications the extrusion output is limited by the rate at which the extruded coating may be crosslinked. Faster, easier and less expensive methods of crosslinking are therefore sought.

More recently attempts have been made to "decouple" the crosslinking operation from the extrusion process so that the rate of extrusion is not controlled by the rate of crosslinking. The "decoupled" crosslinking has been particularly developed in systems using low energy crosslinking techniques, which have especially been used with polyolefin-based compositions.

Using this technique a polyolefin, such as low density polyethylene (LDPE) containing silane groups, is extruded and then crosslinked through the silane groups. The crosslinking is carried out using moisture to hydrolyse the silane groups, and then carrying out a condensation reaction in the presence of a condensation catalyst, to link silane groups between adjacent polyolefin chains.

The most widely used polymer in such applications is LDPE. However, it has the practical limitation that a relatively long time is required to achieve crosslinking, particularly for thicker coatings (thicker gauges) . For practical purposes the thickest gauge which may be obtained approximately 8 mm.

The present applicants have surprisingly found that fast rates of crosslinking may be obtained using a blend polyethylene polymers, whilst still maintaining the practic requirement that the blend is easy to process.

The present invention provides a blend prepared by blendi

(a) a first Linear Low Density Polyethylene (LLDPE) and (b) o or ^■ more of (i) Low Density Polyethylene (LDPE) , (i Polyethylene (PE) wax and (iii) a second LLDPE of melt ind greater than of the first LLDPE, said blend being grafte subsequent to or during blending to incorporate silane group capable of crosslinking.

It might be expected that the higher crystallinity of linea low density polyethylene compared to normal LDPE would result i a lower rate of penetration of moisture into the material giving a slower crosslinking rate. However, significan improvements in crosslinking rate have been achieved using blend of silane-grafted LLDPE with LDPE (b) (i) nad /or PE wa

(b) (ii) and/or the second LLDPE (b) (iii) , any one or more o which components (b) may optionally also be silane-grafted.

LLDPE polymers are copolymers of ethylene and an unsaturate ethylenic monomer. Examples of this second monomer ar 1-butene, 1-hexene, 4 methyl-1-pentene and 1 octene. The LLDP used as blend component (a) preferably has a melt index (MI measured at 190°C/2.16 kg of 0.2 to 10, more preferably 0.5 to 5. Preferably the first LLDPE (a) and/or the second LLDPE (b) (iii) has a density of 0.90 to

₃ 0.94 g/cc (0.0009 to 0.00094 kg/m ) more preferably 0.915 to

0.925 g/cc (0.000915 to 0.000925 kg/m³).

The properties of LLDPE and LDPE are well known to those of skill in the art. LLDPE polymers have shorter and fewer branches to the polyethylene chains than LDPE. Most preferably the LLDPE used as component (a) in the present blend has a narrow distribution of random short chain branching, and/or a narrow molecular weight distribution and/or a low level of terminal unsaturation. By narrow distribution of random short chain branching is meant that all the molecules in the polymer have an about equal number of short chain branches, and for each molecule these are randomly (uniformly) distributed along the molecular chain. Such LLDPE has been found to give improved crosslinking and processing compared with similar polymers in which the branches are non-randomly (non-uniformly) distributed along each molecular chain; or in which there is a broad distribution of branches, ie., some molecular chains have many branches and some have few.

A preferred blend according to the invention is one wherein LLDPE (a) has a narrow molecular weight distribution as represented by a ratio of weight average molecular weight to number average molecular weight of less than 5, more preferably less than 3, and/or a low unsaturation level as represented by less than 0.5 unsaturated carbon-carbon bonds per 1000 carbon atoms, as measured by Fourier Transform Infrared Spectrometry (FTIR) . This measurement is carried out by compression moulding the sample at about 150 C to form a film 200-300 μm thick. FTIR is used to measure the absorption peak areas of the film in the infra-red region owing to carbon-carbon unsaturated bonds. The unsaturated bonds are of three types: (1) vinyl terminal (2) trans internal and (3) vinylidene pendant methylene. The absorption wavenumbers (in cm ) for each type of unsaturation are (1) 910, (2) 965 and (3) 888.

The total unsaturation, S, a measurement of unsaturated carbon-carbon bonds per 1000 carbon atoms is related to the sum of the absorbances due to each type of unsaturation. This may be expressed by the equation: ^s - Z_ ^fi ^ai

where a. is the absorbance per cm thickness for each type of unsaturation, and

_* f. is the multiplying factor for each type of unsaturation.

The multiplying factor is a factor from 0-1 calculated for each type of unsaturation. It may be calculated from information obtained by analysing monomeric compounds of known unsaturation, or by reference to literature such as "Identification and Analysis of Plastics" by Hasborn, Willis and Squirrel. The factors for the three types of unsaturation which occur in the present blend are ( 1 ) 0. 13 , ( 2 ) 0. 18 and ( 3 ) 0 . 12.

Whilst LLDPE has been found to have a particularly fast rate of crosslinking when exposed to moisture its molecular structure makes it very difficult to process. For practical purposes therefore LLDPE (a) is blended with a polymer which, whilst not being so rapidly crosslinkable, has better processabuity characteristics. The blend combines good processabuity with a rate of crosslinking which is significantly faster than the previously used polymer, LDPE.

The polymer which is combined with LLDPE (a) to give the blend better processabuity is (b)(i) LDPE and/or (b) (ii) PE wax and/or (b) (iii) a second LLDPE which has a melt index greater than that of the first LLDPE. Grades of LDPE which may be used in the blends include those which have previously been used in the absence of LLDPE as cable coating. Preferably the LDPE has an MI at 190°C/2.16 kg of

0.5 to 0.2 and a density of 0.910 to 0.930 g/cc (0.00091 to

3 0.00093 kg/m ). For ease of processing it is preferred that the LDPE and LLDPE (a) used in a blend have melt indexes and densities in the same range.

Typically a blend of LLDPE (a) and LDPE (b) (i) and/or LLDPE (b) (iii) will contain 10 to 90% by weight, more preferably 30 to 70% by weight of LLDPE and 90 to 10% by weight, preferably 70 to 30% by weight of the balance component(s) (b) .

The second LLDPE (b) (iii) which may be employed preferably has a melt index at 190°C/2.16 kg of from 20 to 50, more preferably from 25 to 30. This functions to confer improved processabuity on the blend also containing the first LLDPE (a), which contains crosslinkable silane groups.

LLDPE (a) may also be combined with a PE wax (b)(ii). Typical PE waxes which may be used include homopolymer waxes which have a Brookfield viscosity at 121 C of 500 to 20,000, more preferably 600 to 3,000. Generally it is preferred to use a wax with a density of from 0.88 to -

₃ 0.96 g/cc (0.00088 to.0.00096 kg/m ) . It is preferred that in any one blend the density of the PE wax be in the same range

-as the density of the LLDPE (a) .

A typical blend of PE wax and LLDPE (a) will comprise 2 to 10% by weight, more preferably 3 to 5% by weight of PE wax and 98 to 90% by weight, more preferably 97 to 95% by weight of LLDPE (a) .

In the blends of the present invention at least LLDPE component (a) possesses silane groups capable of crosslinking. Preferably one or more of the other polymers (b) also possess such groups. Silane groups capable of crosslinking include any alkoxy silane- or chlorosilane- containing group which is capable of forming an -SiOH group on hydrolysis. The silane groups are preferably vinyl silane groups. They may be introduced into the polyolefin by grafting a silane e.g., a vinyl silane, preferably a vinyl trialkoxy silane such as vinyl tri ethoxy silane, onto the polyolefin backbone using known grafting methods, for example employing a peroxide initiator. The polymers may be blended together using known techniques e.g., by melt mixing or extrusion blending. If the blending is carried out before or during silane grafting, then more than one of the polymer components will possess crosslinkable silane groups. Alternatively the polymers may be separately silane grafted and then blended; or only the first LLDPE component (a) may posses grafted silane groups such that it is the only polymer in the blend which is crosslinkable.

Preferably the blend has a silane group content of 0.5 to 1.5-wt% based on the total weight of the blend of components (a) and (b) .

The silane group-containing blend of the present invention may be cured using moisture, typically water at 60 to 90°C, preferably about 80°C, in the presence of a catalyst such as a dibutyl tin dilaurate. Industrially, such curing may be performed for example in a hot sauna or a steam environment; alternatively the blend may simply be left for an extended period of time in moist air.

The blends of the present invention may contain additives which are typically used in coating compositions for cables e.g., fillers such as aluminium trihydrate, calcium carbonate, carbon black; colourants; and other additives such as anti-oxidants. They are generally incorporated at the polymer blending stage.

The blends may be extruded using known methods e.g., a single screw extruder to produce a crosslinkable coating of the necessary dimensions. The extruded coating is then exposed to moisture to crosslink it. Accordingly, the present invention provides a polymeric composition comprising a blend as described above which is crosslinked through the silane groups; and a process for producing such a composition comprising performing a crosslinking reaction by hydrolysing and condensing the silane groups of the above described blends.

The present invention also provides a shaped coating formed from such a composition; a cable when coated with such a composition; and the use of the above-described blends in cable coating applications.

Samples of polymers and blends to illustrate the invention were prepared by extrusion grafting using a single screw extruder. The 2mm thick samples produced all contained 1.5 weight % of vinyl trimethoxy silane. Each sample also contained 0.15 weight % of peroxide and 0.05 weight % of dibutyltindilaurate.

The curing properties of the blends are measured herein in terms of their crosslinking rate, defined herein as the crosslinking time in H₂0 at 80 C which is required to produce sufficient crosslinking in the sample to give an elongation of 100% in the hot set test. The hot set test is a widely accepted method to monitor the state of crosslinking

2 in PE. The specimen is put under a certain load (2.5 kg/cm ) in a 200°C oven. After 15 minutes the deformation (% elongation) of the specimen is measured. The formation of a good crosslinking network in the sample prior to the hot set test will result in a small % elongation and vice versa. The result from the hot set test reveals the crosslink density of the specimen. Herein, the hot set test is used to obtain kinetic data about the moisture curing process. Similar specimens of the same sample are cured in hot water (80°C) for various periods. After curing they are subjected to the hot set test. The slope of the plot of hot elongation versus curing period provides information about the rate of curing. The curing period required for the sample to exhibit a hot set elongation of 100% can be used to compare the curing samples, and is termed herein the crosslinking rate.

Preferred blends of the invention have a crosslinking rate of less than 100 minutes, preferably less than 50 minutes. These have significant commercial advantage compared with polymers hitherto used in the coated cable production industry. The processing rate of the blends of the invention is generally comparable with, if not better than, the processing rate of LDPE under the same conditions. The processing rate is a measure of the length of coating which may be produced per minute.

Using LDPE of MI of 0.2-2 typical process conditions are as follows:

A single screw extruder having a diameter of 120mm, a length/diameter ratio of 30, and a temperature profile of 150°C at the feed to 210°C at the die, is used to produce a coating of 0.8mm gauge to coat 1.5m of conductor. The processing rate is about 600 m/min (lO /sec) with such an LDPE. The blends of the invention have a processing rate under the same conditions of at least 500 m/min, more preferably more than 600 m/min.

Table 1 shows the properties of various LLDPE's, LDPE's and PE waxes some of which were then mixed to produce blends in accordance with the invention.

Table 2 shows the properties of blends of LLDPE with LDPE or a PE wax, compared with the properties of LLDPE and LDPE alone. Samples 3, 4 and 5 illustrate the invention.

TABLE 1

* than LLDPE 1

TABLE 2

SAMPLE NO. WT% WT% WT% WT% CROSSLINKING EASE OF LLDPE 1 LDPE 1 PE WAX 1 PE WAX 2 RATE (MINUTES) PROCESSING

100 10 Difficult

100 150 Good

50 50 52 Good

95 10 Good

It can been seen from this that Sample 2 containing only LDPE has adequate processability but a crosslinking rate of 150 minutes. On the other hand Sample 1 containing only LLDPE has a much faster crosslinking rate of 10 minutes but is difficult to process. A 50/50 blend of these two polymers (Sample 3) is easy to process, but has a crosslinking rate which is 52 minutes and is therefore much more useful than either of the polymers on its own.

It can also been seen from Table 2 that the addition of as little as 5 weight % of PE wax renders the LLDPE (component (a)) capable of being processed, but does not have a detrimental effect on the crosslinking rate. A 95/5 weight % blend of LLDPE as component (a) and a PE wax as component (b) (Sample 4 or 5) has a crosslinking rate of 10 minutes which is comparable with the curing rate of pure LLDPE, and yet the blend has much better processability than pure LLDPE.

Using the blends of the invention cable coatings may be extruded and crosslinked rapidly so that the overall output of cable coating may be increased.

Claims

1. A blend prepared by blending (a) a first Linear Lo Density Polyethylene (LLDPE) and (b) one or more of (i) Lo Density Polyethylene (LDPE) , (ii) Polyethylene (PE) wax an (iii) a second LLDPE of melt index greater than that of th first LLDPE, said blend being grafted subsequent to or durin blending to incorporate silane groups capable of crosslinking.

2. A blend as claimed in claim 1 in which the first LLDP has a melt index at 190°C/2.16 kg of 0.2-10.

3. A blend as claimed in claim 1 or 2 in which the firs LLDPE and/or the second LLDPE has a density of 0.90.94 g/cc.

4. A blend as claimed in any one of the preceding claim comprising 10-90 weight % of the first LLDPE and 90-10 weight of LDPE and/or second LLDPE.

5. A blend as claimed in claim 4 in which the LDPE has melt index at 190°C/2.16 kg of 0.5-2.0 and a density -o 0.910-0.930 g/cc.

6. A blend according to any one of claims 1 to 4 wherei the second LLDPE has a melt index at 190°C/2.16 kg of from 20 t 50, preferably from 25 to 30.

7. A blend as claimed in any one of claims l to comprising 90-98 weight % of the first LLDPE and 10-2 weight of PE wax.

8. A blend as claimed in claim 7 in which the PE wax is a homopolymer of polyethylene, which has a Brookfield viscosity at 121°C of 500-20000 and density of 0.88-0.96 g/cc.

9. A blend as claimed in any one of the preceding claims in which the silane groups are vinyl silane groups.

10. A blend according to any one of the preceding claims in which the silane group content is from 0.5 to 1.5 weight %, based on the total weight of (a) and (b) components.

11. A blend according to any one of the preceding claims wherein the first LLDPE has a narrow molecular weight distribution as represented by a ratio of weight average molecular weight to number average molecular weight of less than 5, preferably less than 3, and/or a low unsaturation level as represented by less than 0.5 carbon-carbon double bonds per 1000 carbon atoms, as measured by Fourier Transform Infrared Spectroscopy.

12. A blend according to any one of the preceding claims which has a processing rate (under the conditions hereinbefore defined) of at least 500 m/min, preferably at least 600 m/min.

13. A blend according to any one of the preceding claims which has a crosslinking rate (as hereinbefore defined) of less than 100, preferably less than 50 minutes.

14. A polymeric composition comprising a blend as claimed in any one of the preceding claims crosslinked through the silane groups.

15. A process for producing a composition as claimed in claim 14 which comprises performing a crosslinking reaction by hydrolysing and condensing the silane groups of a blend as claimed in any one of claims 1 to 13.

16. A shaped coating formed from a composition as claimed in claim 14.

17. A process for producing a shaped coating as claimed in claim 16 comprising extruding a blend as claimed in any one of claims 1 to 13 and cross-linking through the silane groups.

18. Cable when coated with a polymeric composition according to claim 14.

19. The use of a blend according to any one of claims 1 to 13 in cable coating applications.