WO2013060736A1

WO2013060736A1 - Process

Info

Publication number: WO2013060736A1
Application number: PCT/EP2012/071074
Authority: WO
Inventors: Ernst Zopf; Pablo Ivan Aguayo ARELLANO; Sandeep Tyagi; Antti Tynys; Juliane Braun; Yi Liu; Markus Gahleitner; Saeid Kheirandish; Velichko Hristov; Henrich KAMINSKI
Original assignee: Borealis Ag
Priority date: 2011-10-26
Filing date: 2012-10-24
Publication date: 2013-05-02
Also published as: EP2771399B1; RU2014117926A; IN2014DN03286A; EP2771399A1; CN103906806A; RU2609029C2; CN103906806B; EP2771399B2

Abstract

A high density polyethylene blend, comprising (A) 55 to 95 wt% of a high density multimodal polyethylene copolymer component having a density of at least 940 kg/m³, and (B) 5 to 45 wt% of an ultra-high molecular weight polyethylene homopolymer having an intrinsic viscosity of at least 6 dl/g and an MFR₂₁ of less than 0.5 g/10m in (UHMW polyethylene); and wherein said blend has an MFR₂₁ of 1 0.0 g/10min or less and a density of at least 940 kg/m³.

Description

Process

This invention concerns a multimodal polyethylene composition. In particular, the invention relates to a polyethylene composition comprising a high density multimodal polyethylene component and an ultrahigh molecular weight polyethylene homopolymer component. The invention also covers articles, preferably pipes, made from the multimodal polyethylene composition.

Multimodal polyethylene polymers are well known in the art. A multimodal polyethylene system typically comprises a high molecular weight (HMW) and a low molecular weight (LMW) component. The HMW component confers good mechanical properties to the system, whilst the LMW component provides good processability. Multimodal polyethylene systems have a wide range of useful practical applications, such as in the production of blow moulded articles films or pipes. Improved mechanical properties can be achieved by increasing the molecular weight of the HMW component. This, however, is usually at the cost of a loss in homogeneity resulting from an increase in the viscosity ratio between the HMW and LMW components, which can in turn actual ly be detrimental to the mechanical properties attained.

Further improved mechanical properties are possible by including an ultra high molecular weight (UHMW) fraction into a multimodal polyethylene system. There are serious compatibility problems however when such a high Mw species is added. For example. Ogunniyi et al (Journal of Applied Polymer Science. 2005, 97. 413-425) and Vadhar et al (Journal of Applied Polymer Science. 1986, 32, 5575- 5584) both report the need for long blending times of the order of 1 5 minutes in a batch mixer w hen UHMW polyethylene was added to other polyethylenes.

The incorporation of UHMW polyethylene into a polyethylene composition as a copolymer is also known and is reported in, for example, WO 2007/042216, WO 96/18677 and WO 2006/092378.

The inclusion of UHMW polyethylene into HDPE via extrusion has also been investigated and has been carried out using a co-rotating twin screw ext ude by Huang and Brown ( Polymer, 1992. 33, 2989-2997). However, although the UHMW polyethylene particles were found to be wel l bonded in the matrix and this helped to slow down the rate of crack propagation, w hen analysed under SEM, the UH W polyethylene was found to remain in large separate domains with no evidence of "melting" into the HDPE matrix. For these reasons, the amount of UHMW polyethylene is l imited to low loadings.

In WO94/28064, polyethylene compositions are reported comprising a

UHMW component and a unimodal HDPE component.

In order to maximise the advantage gained by including an UHMW

polyethylene in a bimodal polyethylene blend, there remains a need for the generation of polymers with increased weight contents of this useful component.

It is an object of the present invention to provide a novel multimodal polyethylene blend which has desirable mechanical properties and processabil ity. Specifical ly, it is desired to produce a blend of an UHMW polyethylene into a multimodal polyethylene matrix at increased loadings than was previously possible, allowing advantage to be taken of the resulting enhanced mechanical properties. Improvements in sagging behav iour and impact strength, without an associated loss in tensile properties of the resultant polymer are desired.

The present inventors have found that the combination of a particular high density mul timodal polyethylene polymer with a particular homopolymer ultra high molecular weight component can result in a blend which prov ides the necessary properties. These components can surprisingly be blended together to give homogeneous blends which possess excellent impact strength and strain at break without losses of tensile modulus. The blends also exhibit excellent sag resistance making them ideal for pipe formation.

11 m ma of Invention

Thus view ed from a first aspect the invention prov ides a high density polyethylene blend, comprising

(A) 55 to 95 wt% of a high density multimodal polyethylene component having a density of at least 940 kg m , and (B) 5 to 45 wt% of an ultra-high molecular weight polyethylene

homopolymcr having an intrinsic viscosity of at least 6 dl/g and an MFR₂i of less than 0.5 g/lOmin (UHMW polyethylene);

and wherein said blend has an MFR₂i of 10.0 g/lOmin or less and a density of at least 940 kg/m³.

Viewed from another aspect the invention provides a high density polyethylene blend, comprising

(A) 55 to 95 wt% of a high density multimodal polyethylene component having a density of at least 940 kg/m^" , and

(B) 5 to 45 wt% of an ultra-high molecular weight polyethylene

homopolymcr having a nom inal viscosity molecular weight Mv of at least 800,000 g/mol and an MFR₂i of less than 0.5 g Ί Omin (UHMW polyethylene);

and wherein said blend has an MFR₂i of 1 0.0 g/lOmin or less and a density of at least 940 kg/m³.

The homogeneous polymer blends of the current invention are well suited for use in pipes for various purposes, such as fluid transport, e.g. transport of liquids or gases such as water or natural gas is known. It is common for the fluid to be pressurised in these pipes.

Thus v iewed from a further aspect, the invention prov ides an article, preferably a pipe, comprising the polymer blend as hereinbefore described.

Viewed from another aspect the invention prov ides a process for the preparation of a blend as hereinbefore defined comprising mi ing

(A) 55 to 95 wt% of a high density multimodal polyethylene component having a density of at least 940 kg/m , and

(B) 5 to 45 wt% of an ultra-high molecular weight polyethylene

homopolymcr having a nominal viscosity molecular weight Mv of at least 800,000 g/mol. and an MFR₂i of less than 0.5 g 1 Omin (UHMW polyethylene):

and extruding or kneading the same so as to form a blend hav ing an MFR₂i of 1 0.0 g l Omin or less and a density of at least 940 kg/m .

View ed from another aspect the invention provides the use of the blend as hereinbefore defined in the manufacture of an article, especially a pipe. Detailed Description of Invention

The tests for any claimed parameter are given in the "analytical tests" section of the text which precedes the exampl es.

Wherever the term "molecular w eight Mw" is used herein, the weight average molecular weight is meant. Whereever the term "molecular weight Mv" is used herein, the nominal v iscosity molecular weight is meant.

The polyethylene lend of the invention comprises at least two components: a high density mul timodal polyethylene component, and an ultra-high molecular weight polyethylene homopolymer component. Taken together these form the polyethylene blend of the invention. In al l embodiments, the blend is an H DPE, i.e. one having a density of at least 940 kg m .

Blend Properties

The properties of the blend are reported below. The parameters w hich follow may be measured in the presence of standard additives that are inherently present in commercial polymers which may be used to manufacture the blends of the invention.

The polyethylene blend of the invention preferably has a density according to ISO 1 183 at 23°C of at least 940 kg/ nr . preferably at least 945 kg/nv , more

3 ^

preferably at least 950 kg m , especially at least 952 kg m . The upper l imit for

3 3 3

density may by 980 kg/m , preferably 975 kg/m , especially 970 kg m . A highly

3 3

preferred density range is 950 to 965 kg/m , especially 952 to 961 kg m .

The MF ₂₁ according to ISO 1 133 of the polyethylene blend of the invention is preferably in the range of 0.05 to 1 0 g / 1 0 min, preferably 0. 1 to 8 g l Omin, especial ly 0.2 to 5 g l Omin.

The polyethylene blend preferably has an MFR₅ of less than 1 .0 g l Omin, preferably less than 0.5 g/lOmin.

The polydispcrsity index (PI) of the polyethylene blends of the invention is preferably in the range 0.5 to 4 Pa , more preferably 0.8 to 3.4 Pa ¹. Highly preferably the PI is in the range 1 to 3. 1 Pa ' . The notched impact strength of the polyethylene blends of the invention is

2 2

preferably greater than 30 kJ/m. , more preferably greater than 50 kJ/m , even more preferably greater than 1 00 kJ/m , when measured at 0 °C.

The notched impact strength of the polyethylene blends of the invention is preferably greater than 1 5 kJ/m when measured at -30 °C, more pref erably greater than 30 kJ/m².

The tensile modulus of the blends of the invention is preferably higher than 85% of the value of component (A) on its own, especially higher than 90%. The tensile modulus of the blends of the invention may therefore be at least 950 MPa, such as at least 1000 MPa, preferably at least 1 050 Pa, especially at least 1 100

MPa.

The strain at break of the blends of the invention is preferably greater than the strain at break of component (A) on its own, e.g. at least 10% more.

The blends of the invention resist sagging. This can be shown with reference to Eta 747. Values of at least 1 ,000 kPas are preferred, preferably more than 3,000 kPa are possible.

The blends aiso show very low white spot values indicating low levels of gels and hence good homogeneity. White spots can be determined using

microscopy. It is preferred if the biend.s of the invent ion have minimal white spots.

High Densit Multimodal Polyethylene Component

The blend of the invention includes a high density multimodal polyethylene component, i.e. one having a density of at least 940 kg m . The term "multimodal" means herein, unless otherwise stated, multimodal ity with respect to molecular weight distribution and includes therefore a bimodal polymer. Usually, a polyethylene composition, comprising at least two polyethylene fractions, hich have been produced under different polymerization conditions resulting in different (weight average) molecular weights and molecular weight distributions for the fractions, is referred to as "multimodal^". The prefix "muiti" relates to the number of different polymer fractions present in the polymer. Thus, for example, multimodal polymer includes so called "bimodal" polymer consisting of two fractions. The form of the molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as a function of its molecular weight, of a multimodal polymer will show two or more maxima or is typically distinctly broadened in comparison with the curves for the individual fractions. For example, if a polymer is produced in a sequential multistage process, utilizing reactors coupled in series and using different conditions in each reactor, the polymer fractions produced in the different reactors will each have their own molecular weight distribution and weight average molecular weight. When the molecular weight distribution curve of such a polymer is recorded, the indiv idual curves from these fractions form typical ly together a broadened molecular weight distribution curve for the total resulting polymer product.

Component (A) of the blend of the invention is a high density multimodal polyethylene and is preferably present in an amount of 55 to 95 wt%, such as 58 to 90 wt%, preferably 60 to 87 wt%, more preferably 60 to 80 wt % of the blend.

The multimodal polyethylene component (A) of the invention preferably has a density according to ISO 1 183 at 23°C of at least 940 kg/m³, preferably at least

3 3

945 kg/m^" . The upper limit for density may by 980 kg/m , preferably 975 kg/m ,

3 3 especially 970 kg/m . A highly preferred density range is 945 to 965 kg/m .

The MFR₂] according to ISO 1 133 of the multimodal polyethylene of the invention is preferably in the range of 1 to 20 g/lOmin, preferably 2 to 1 5 g 10 min. Preferably the multimodal polyethylene component (A) has an MFR₂₁ of 3 to 1 2 g l Omin.

The MFR₅ according to ISO 1 133 of the multimodal polyethylene component (A) of the invention is preferabl y less than 1 .0 g l Omin.

Component (A) preferably has a M_w of at least 70.000 g/mol, more preferably at least 1 20,000 g/mol. The Mw of the Component (A) should be less than 400,000 g/mol, preferably less than 300,000 g/mol .

The Mw/Mn of component (A) may be at least 4, such as at least 10, such as 10 to 30.

in all embodiments of the invention, it is preferable if component (A) is a multimodal polyethylene comprising at least (i) a lower weight average molecular weight (LMW) ethylene homopolymer or copolymer component, and (ii) a higher weight average molecular weight (HMW) ethylene homopolymer or copolymer component. Preferably, at least one of said LMW and HMW components is a copolymer of ethylene with at least one comonomer. It is preferred that at least said HMW component is an ethylene copolymer. Alternatively, if one of said

components is a homopolymer, then said LMW is the preferably the homopolymer.

Said LMW component of multimodal polymer preferably has a MFR₂ of at least 5 g/10 min, preferably at least 50 g 10 min, more preferably at least 100 g/lOmin.

The density of LMW component of said multimodal polymer may range from 950 to 980 kg^'nr , e.g. 950 to 970 kg/m³.

The LMW component of said multimodal polymer may form from 30 to 70 wt%, e.g. 40 to 60% by weight of the multimodal polymer with the HMW component form ing 70 to 0 wt%, e.g. 60 to 40% by weight. In one embodiment said LMW component forms 50 wt% or more of the multimodal polymer as defined above or below . Typical ly, the LMW component forms 45 to 55% and the H MW component forms 55 to 45% of the blend.

The HMW component of said multimodal ethylene polymer has a lower MFR₂ than the LMW component.

The multimodal ethylene polymer of the invention may be an ethylene homopolymer or copolymer. By ethylene homopolymer is meant a polymer w hich is formed essential ly only ethylene monomer units, i.e. is 99.9 wt% ethylene or more. It will be appreciated that minor traces of other monomers may be present due to industrial ethylene containing trace amounts of other monomers.

The multimodal ethylene polymer of the invention may also be a copolymer (and is preferably a copolymer) and can therefore be formed from ethylene with at least one other comonomer, e.g. C_3-20 olefin. Preferred comonomers are alpha- olefins, especially with 3-8 carbon atoms. Preferably, the comonomer is selected from the group consisting of propene, 1 -butene, 1 -he ene, 4-methyl- l -pentene, 1 - octene, 1 .7-oetadienc and 7-methyl- 1 ,6-octadiene. The use of 1 -hexene or 1 -butene is most preferred.

The multimodal ethylene polymer of the invention can comprise one monomer or two monomers or more than 2 monomers. The use of a single comonomcr is preferred. If two comonomcrs are used it is preferred if one is an Cs_8 aipha-oiefin and the other is a diene as hereinbefore defined.

The amount of comonomcr is preferably such that it comprises 0-3 mol%, more preferably 0. 1 -2.0 mol% and most preferably 0. 1 - 1 .5 mol% of the ethylene polymer. Values under 1 .0 moi% are also envisaged, e.g. 0. 1 to 1 .0 mol%. These can be determined by NMR.

It is preferred however if the ethylene polymer of the invention comprises a LMW homopolymer component and a HMW ethylene copolymer component, e.g. an ethylene hexene copolymer or an ethylene butene copolymer.

For the preparation of the multimodal ethylene polymer of the present invention polymerisation methods well known to the skil led person may be used, it is within the scope of the invention for a multimodal, e.g. at least bimodal, polymers to be produced by blending each of the components in- situ during the polymerisation process thereof (so called in-situ process) or, alternatively, by bl endi ng mechanically two or more separatel y produced components in a manner known in the art.

Polyethy!encs useful in the present inv ention is preferably obtained by in-situ blending in a multistage polymerisation process. Accordingly, polymers are obtained by in-situ blending in a multistage, i.e. two or more stage, polymerization process including solution, slurry and gas plia.se process, in any order. Whilst it is possible to use different single site catalysts in each stage of the process, it is preferred if the catalyst employed is the same in both stages.

Ideally therefore, the polyethylene polymer used in the blend of the invention are produced in at least two-stage polymerization using a single site catalyst or Ziegler Nana catalyst. Thus, for example two slurry reactors or two gas phase reactors, or any combinations thereof, in any order can be employed. Preferably howev er, the polyethylene is made using a slurry polymerization in a loop reactor followed by a gas pha.se polymerization in a gas phase reactor.

A loop reactor - gas phase reactor system, is well known as Borealis technology, i.e. as a BORSTAR™ reactor system. Such a multistage process is disclosed e.g. in EPS 17868. The conditions used in such a process are well known. For slurry reactors, the reaction temperature will general ly be in the range 60 to 1 10°C, e.g. 85-1 10°C, the reactor pressure will generally be in the range 5 to 80 bar, e.g. 50-65 bar, and the residence time will general ly be in the range 0.3 to 5 hours, e.g. 0.5 to 2 hours. The diluent used will generally be an aliphatic hydrocarbon having a boil ing point in the range -70 to +100°C, e.g. propane. In such reactors, polymerization may if desired be effected under supercritical conditions. Slurry polymerisation may also be carried out in bulk where the reaction medium is formed from the monomer being polymerised.

For gas phase reactors, the reaction temperature used will generally be in the range 60 to I 1 5 C, e.g. 70 to I I O C, the reactor pressure will generally be in the range 10 to 25 bar, and the residence time will generally be 1 to 8 hours. The gas used wil l commonly be a non-reactive gas such as nitrogen or low boiling point hydrocarbons such as propane together with monomer, e.g. ethylene.

The ethylene concentration in the first, preferably loop, reactor may be around 5 to 1 5 mol%, e.g. 7.5 to 1 2 mol%.

In the second, preferably gas phase, reactor, ethylene concentration is preferably much higher, e.g. at least 40 mol% such as 45 to 65 mol%, preferably 50 to 60 mol%.

Preferably, the first polymer fraction is produced in a continuously operating loop reactor where ethylene is polymerised in the presence of a polymerization catalyst as stated above and a chain transfer agent such as hydrogen. The di luent is typical ly an inert al iphatic hydrocarbon, preferably isobutane or propane. The reaction product is then transferred, preferably to continuously operating gas phase reactor. The second component can then be formed in a gas phase reactor using preferably the same catalyst.

The multimodal poiyethyienes of the invention are commercial products and can be purchased from various suppliers.

UHMW Component The blend of the invention further comprises component (B) an UHMW polyethylene homopolymer component in an amount of 5 to 45 wt%. Preferably, this UHMWPE comprises 10 to 42 wt%, such as 13 to 40 wt% of the blend, ev en more preferably 20 to 40 wt%. It will be appreciated that the UHMW component is different to the (A) component of the blend.

The UHMW polyethylene component of the blends of the invention preferably has a nominal viscosity molecular weight ( Mv ) of at least 800,000 g/moi, preferably at least 850,000 g'mol, especially at least 950,000 g/mol . In al l embodiments, it is preferred if the UHMW polyethylene has a Mv of less than 2,000 .000 g/moi, even more preferably less than 1 ,500,000 g/moi. In some embodiments, it is preferred if the Mv of the UH MW polyethylene is in the range of 800,000 to 1 .000.000. It has been found that when using a polyethylene having such an Mv that the impact strength of the blend can be very high. Moreover, this high impact strength can be achieved without any reduction in tensi le modulus. It is often the case that improvements in impact strength lead to a reduction in stiffness due to a reduction in crystal I in ity. This is not observed in the present invention, in particular w here the UH MW polyethylene has an Mv in the range of 800,000 to 1 ,000,000.

The UHMW polyethylene of the invention is an ethylene homopolymer. The UHMW component is also preferably unimodal. This means that in has a single peak on GPC. Ideally it is formed from a single component and is therefore produced in a single manufacturing step.

The UHMW polyethylene of the invention can be prepared by conventional processes. Preferably, the UHMW polyethylene is prepared using a Ziegler-Natta catalyst. These UHMW polymers are commercially available polymers.

The density of the UHMW component can be in the range 920 to 960 kg/m , preferably 930 to 950 kg/m³.

This component has a very low M FR, such as an MFR₂i of less than 0.5 g 1 Omin, especially MFR₂i of less than 0. 1 gT Omin, more especial ly less than 0.05 g/lOmin.

The intrinsic viscosity of the UHMW component is at least 6 dl/g, preferably at least 7 dl/g such as at least 8 dl/g. The intrinsic viscosity of the UHMW component should preferably not exceed 20 dl/g. It will be appreciated that intrinsic v iscosity is a measure of molecular weight in this field.

Preparation of Blend

The blends of the invention m ay be prepared simply by mixing the components but to ensure homogeneity and hence no white spots, it will be appreciated that the components have to be compounded. This can be achiev ed by any conventional method known to those skil led in the art, e.g. extrusion or kneading.

Where e trusion is used to prepare the blends of the invention, a second extrusion step may optionally be employed, e.g. under the same conditions as the first. It has been found that the use of two extrusion steps can improve

homogeneity. The strain at break and possibly also sagging properties can thus be improved. In the context of UHMW polyethylene which has an Mv in the range of

800,000 to 1 ,000,000, it is believed that a single extrusion step is sufficient to achieve the improved impact and stiffness (tensile modulus) properties discussed above.

The use of extrusion to homogenise the compositions is preferred, in particular the use of a co-rotating twin extruder, such as ZS 18 or ZSK 40.

The use of kneading (e.g. the use of a Haake kneader) causes a very large uplift in sagging properties (measured as Eta 747 ). Sagging properties can be measured via the Eta 747. Values of Eta 747 of at least 3,000 kPa are preferably achievable in the present case.

It will be appreciated that prior to forming the blend of the invention, the two polymer components of the invention may be blended with standard additives and adjuvants known in the art. It may also contain additional polymers, such as carrier polymers of the additive masterbatches. The properties of the components of the blend and the blend itself can be measured in the absence of or in the presence of any addit ives. It will be preferred if any additives are present however when properties are determined. Suitable antioxidants and stabilizers are, for instance, sterically hindered phenols, phosphates or phosphonites, sulphur containing antioxidants, alkyl radical scavengers, aromatic amines, hindered amine stabilizers and the blends containing compounds from two or more of the above-mentioned groups.

Examples of sterical ly hindered phenols are, among others, 2.6-di-tert-butyl -

4-methyl phenol (sold, e.g., by Degussa under a trade name of lonol CP), pcntaerythrityl-tetrakis(3-(3 ' ,5 ' -di-tert. butyl -4-hydroxyphenyl )-propionate (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irganox 1010 ) octadecyl- 3-3(3 '5 '-di-tert-butyi-4'-hydroxyphenyi)propionate (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irganox 1076 ) and 2,5,7,8-tetramethyl-

2(4',8',12'-trimethyltridecyi)chroman-6-oi (sold, e.g., by BASF under the trade name of Alpha -To co p h e ro 1 ) .

Examples of phosphates and phosphonites are tris ( 2,4-di-/-butylphenyl ) phosphite (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irgafos 168), tetrakis-(2,4-di- )utylpheiiyl )-4,4 ^"-biphenylen-di-phosphonite (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irgafos P-EPQ) and tris- (nony phenyl )phosphate (sold, e.g., by Dover Chemical under the trade name of Doverphos Hi Pure 4 ).

Examples of sulphur-containing antioxidants are dilaurylthiodipropionate (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irganox PS 800), and d i s t e a r y It h i o d i p ro p i o n a t e (sold, e.g., by Chemtura under the trade name of Lowinox DSTDB).

Examples of n i t rogen-con tain i n g antioxidants are 4,4'-bis(l ,l '- d i meth y lbenzy I )d i phenyl am i ne (sold, e.g., by Chemtura under the trade name of Naugard 445 ), polymer of 2,2,4-trimethyl- 1 ,2-dihydroquinoline (sold, e.g., by

Chemtura under the trade name of Naugard EL- 1 7 ), / -(/ -toluene-suHbnyiamido)- diphenyiamine (sold, e.g., by Chemtura under the trade name of Naugard SA ) and A^',.V^"-diphcnyl-/ -phenylene-diaminc (sold, e.g., by Chemtura under the trade name of Naugard J).

Commercially available blends of antioxidants and process stabilizers are also available, such as Irganox B225, Irganox B2 1 5 and Irganox B561 marketed by Ciba-Specialty Chemicals. Suitable acid scavengers are, for instance, metal stearates, such as calcium stearate and zinc stearate. They are used in amounts generally known in the art, typically from 500 ppm to 10000 ppm and preferably from 500 to 5000 ppm.

Carbon black is a general ly used pigment, which also acts as an UV- screener. Typical ly carbon black is used in an amount of from 0.5 to 5 % by weight, preferably from 1.5 to 3.0 % by w eight. Preferably the carbon black is added as a masterbateh where it is prcmixed w ith a polymer, preferably high density polyethylene (HDPE), in a specific amount. Suitable masterbatches are, among others, HD4394, sold by Cabot Corporation, and PPM I SOS by Poly Piast Muller. Also titanium oxide may be used as an UV-screener.

Components (A) and (B) in the polymer blend of the invention can be further blended with any other polymer of interest or used on its own as the only olefin ic material in an article. Thus, the ethylene polymer of the invention can be blended w ith known HDPE, MDPE, LDPE, LLDPE polymers. Ideal ly however any article made from the ethylene polymer blend is the invention consists essentially of the polymer blend, i .e. contains the multimodal polyethylene component and the UHMWPE component.

Applications

The blends of the invention can be used to make ail manner of articles such as cabl e sheathings, fibres, fi lm s and moulded articles. They are of primary interest in the formation of pipes. Pipes can be manufactured using various techniques such as RAM extrusion or screw extrusion.

It will be appreciated that the preferred features of the polymers of the invention as described herein can ail be combined with each other in any way.

The invention wil l now be described with reference to the fol low ing non limiting examples and figure. Figure 1 is a microscopic section of the pellets of blend 3 showing some w hite spots and figure 2 is a microscopic section of the pellets of blend 4 showing minimal white spots. Figure 3 is a plot of impact strength vs stifness for the polymer blends of table 3. Analytical tests Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1 133 and is indicated in g/10 min. The MFR is an indication of the melt viscosity of the polymer. The MFR is determined at 190°C for polyethylene. The load under which the melt flow rate is determined is usually indicated as a subscript, for instance MFR₂ is measured under 2.16 kg load, MFR₅ is measured under 5 kg load or MFR₂i is measured under 21.6 kg load.

Density

Density of the polymer was measured according to ISO 1 183 / 1 872-2 B. Molecular weight

M_w, M_n and MWD are measured by Gel Permeation Chromatography (GPC) according to the following method:

The weight average molecular weight M_w and the molecular weight distribution (MWD = Μ_ν,/Μ,, wherein M_n is the number average molecular weight and M_w is the weight average molecular weight ) is measured according to ISO 1 6014-4:2003 and ASTM D 6474-99. A Waters GPCV2000 instrument, equipped with refractive index detector and online viscosimeter was used with 2 x GMHXL-HT and 1 x

G7000HXL-HT TSK-gel columns from Tosoh Bioscience and 1 ,2,4- trichlorobenzene (TCB, stabilized with 250 mg/L 2.6-Di tert-butyl-4-methy!-phenol ) as solvent at 140 °C and at a constant flow rate of 1 mL min. 209.5 μΐ , of sample solution were injected per analysis. The column set was calibrated using universal cal ibration (according to ISO 1 6014-2:2003 ) with at least 1 5 narrow MWD polystyrene ( PS ) standards in the range of 1 kg mo I to 1 2 000 kg mo I . Mark

Houwink constants were used as given in ASTM D 6474-99. All samples were prepared by dissolving 0.5 - 4.0 mg of polymer in 4 mL (at 140 °C) of stabilized TCB (same as mobile phase) and keeping for max. 3 hours at a maximum temperature of 160 °C with continuous gentle shaking prior sampling in into the GPC instrument.

As it is known in the art, the weight average molecular weight of a blend can be calculated if the molecular weights of its components are known according to:

Mw_b = ^ W:^■ Mw_i

i

where Mwi, is the weight average molecular weight of the blend,

Wj is the weight fraction of component "i" in the blend and

Mwi is the weight average molecular weight of the component "i".

The number average molecular weight can be calculated using the wel l-know n mixing rule:

1 _ y ^wi

Mn_b i Mn_;

where M¾ is the weight average molecular weight of the blend,

wj is the weight fract ion of component "i" in the blend and

Mn; is the weight average molecular weight of the component "i".

Nominal v iscosit molecular weight (Mv) is calculated from the intrinsic viscosity [rj] according to ASTM D 4020 - 05

Mv = 5.37 x 10⁴ x [rj] ^{1 7}

Rheological polydispersity index

Rheological polydispersity inde (rheological PI ) was calculated as 10⁵/G_C w here G_c stands for the cross-over modulus.

Crossover Modulus G_c

The cross-over modulus is related to the rheological polydispersity index by the equation:

PI = 10⁵/G_C The cross over modulus G_c is the value of G' (storage modulus) and G" (loss modulus) at the frequency where the two moduli are equal. That is where the curves of G'(co) and G"(co ) cross. Fitting the points on both curves in the vicinity of the cross over point with cubic splines permits an objective identification of the cross over modulus.

Rheologv

Rheological parameters such as complex viscosity are determined by using a rheometer, preferably a Anton Paar Physica MCR 300 Rheometer on compression moulded samples under nitrogen atmosphere at 190°C using 25 mm diameter plates and plate and plate geometry with a 1.8 mm gap according to ASTM 1440-95. The oscillatory shear experiments were done within the l inear viscosity range of strain at frequencies from 0.05 to 300 rad/s (ISO 6721-1). Five measurement points per decade were made. The method is described in detail in WO 00/22040.

The values of storage modulus (G ), loss modulus (G ^" ) complex modulus (G* ) and complex viscosity (η*) were obtained as a function of frequency (ω).

Eta 747

The viscosity of the polymer at this shear stress is determined at a temperature of 190°C and has been (bund to be inversely proportional to the gravity flow of the polymer, i.e. the greater the v iscosity the lower the gravity flow.

The determination is made by using a rheometer, preferably a Boh I in CS

Melt Rheometer. Rheometers and their function have been described in

"Encyclopedia of Polymer Science and Engineering", 2nd Ed., Vol. 14, pp. 492-509. The measurements are performed under a constant stress between two 25 mm diameter plates (constant rotation direction ). The gap between the plates is 1.8 mm.

An 1.8 mm thick polymer sample is inserted between the plates.

The sample is temperature conditioned during 2 min before the measurement is started. The measurement is performed at 190°C. After temperature conditioning the measurement starts by applying the predetermined stress. The stress is maintained for 1800 s to let the system approach steady state conditions. After this time the measurement starts and the viscosity is calculated.

The measurement principle is to apply a certain torque to the plate axis via a precision motor. This torque is then translated into a shear stress in the sample. This shear stress is kept constant. The rotational speed produced by the shear stress is recorded and used for the calculation of the viscosity of the sample.

Intrinsic viscosity is measured according to DIN EN ISO 1628 (1998 ) in Decalin at

135 °C.

Melting temperature T,„. crystallization temperature T... is measured with Mettler TA820 differential scanning calorimetry ( DSC ) on 5-10 mg samples. Both crystallization and melting curves were obtained during 10 °C/min cool ing and heating scans between 30 °C and 200 °C. Melting and crystallization temperatures were taken as the peaks of endotherms and exotherms.

Also the melt- and crystal l ization enthalpy ( H m and He) were measured by the DSC method according to ISO 1 1357-3.

Quantification of comonomer content by FTI R spectroscopy

The comonomer content is determined by quantitative Fourier transform infrared spectroscopy (FTIR) after basic assignment cal ibrated via quantitative C nuclear magnetic resonance (NMR) spectroscopy in a manner well known in the art. Thin films are pressed to a thickness of between 100-500 μηι and spectra recorded in transmission mode.

Specifically, the ethy lene content of a polyp ropylene-co-ethylene copolymer is determined using the basel ine corrected peak area of the quantitative bands found at 720-722 and 730-733 cm ¹. Specifically, the butene or hexene content of a polyethylene copolymer is determined using the baseline corrected peak area of the quantitative bands found at 1377-1379 cm^"1. Quantitative results are obtained based upon reference to the film thickness. nIS Charpy impact strength was determined according to ISO 1 79:2000 on V-notched samples of 80x10x4 mm³ at 0 °C (nIS, 0°C) and -30 °C (n IS -30°C). Samples were cut from plaques of 4 mm thickness prepared by compression molding according to

ISO 293 :2004 using the conditions defined in chapter 3.3 of ISO 1872-2:2007.

Tensile Properties:

Tensile modulus

As a measure for stiffness, the tensile modulus (E -modulus) of the compositions was measured at 23°C on compression moulded specimens according to ISO 527- 2: 1993. For samples prepared with the co-rotating twin screw extruder, the specimens were cut from plaques of 4 mm thickness prepared by compression molding according to ISO 1872-2: (type: 1B4 4mm ), while those prepared with the kneader, the specimens were cut from plagues of 2 mm thickness (type: S2/5A 2mm). The modulus was measured at a speed of 1 mm/min.

Stress at Yield:

Stress at yield (in MPa) was determined on the same samples according to ISO 527- 2. The measurement was conducted at 23 °C temperature with an elongation rate of

50 mm/min.

Stress and Strain at Break:

Stress at break (in MPa) and Strain at break (in%) were determined on the same samples according to ISO 527-2. The measurement was conducted at 23 °C temperature with an elongation rate of 50 mm/min.

White spots

A pellet section was viewed under a microscope. The number of white spots was observed.

Examples LJH W PE1 homopolymcr was purchased from Jingchem Corporation. It has a narrow, quite wel l defined Mv of 1 .1 50,000 g/mol by ASTM 4020-81 (denoted in material info from the supplier).

UHMW PE2 homopolymcr was purchased from Jingchem Corporation. It has a narrow, quite well defined Mv of 880,000 g/mol by ASTM 4020-81 (denoted in material info from the supplier).

HE3490-LS-H is a commercial bimodai HOPE.

Table 1 : properties of UHMW-PE and H E3490-LS-H

Intr. Eta 747

Tm Tc MF ₂i density

viscosity (190°C)

UHMW-PE1 8.4 di/g 1 3 1 °C 1 17 °C 0.03 934 kg/m³ 428,820 kPas

UH W-PE 2 7.7 dl/g 13 1 °C 1 1 7 °C 933 kg/m³ Not measured

HE3490-LSH 130 °C 1 1 7 °C 9 959 kg/m³ 557.000 kPas

Example 1

Various blends of the two starting materials were prepared using extrusion or a batch mixer (kneader). The batch mixing conditions employed were 230°C, 40 rpm for 8 mins. Conventional extrusion conditions using a co-rotating twin screw extruder were employed. Extrusion was carried out at 230 to 240°C and 120 rpm.

Blends 1 , 2 and 4 were extruded twice using the same conditions. Blends 3 and 6 to

8 are extruded only once.

For all blends, the impact strengths are up to ten times higher for the

inventive blends.

Table 2. Summary of Blend Properties (UHMW-PE 1)

Nm not measurable

^♦Relative to the reference HE3490-LS-H

# i.e. Reference is 100% HE3490-LS-H

Table 3. Summary of Blend Properties (UHMW-PE2)

Extrusion

UHMWPE2 1 11 - 3490- ZSK18, Tensile Relative* nlS, Relative* content LS-I I co- modulus tensile o°c impact

(wt%) (wt%) rotating (MPa) modulus (kJ/m²) strength

TSE

Ref 0 1 00 1 -pass 1 1 6 1 100% 21.9 100%

Blend 6 1 0 90 1 -pass 1 188 102% 36.2 165%

Blend 7 20 80 1 -pass 1215 105% 55.0 251% Blend 8 40 60 1 -pass 1 197 103% 1 16.5 532%

The "lower" Mv UHMWPE-2 was used i only 1 -pass extrusion and the impact strength of the blends is again very high. Moreover, this is achieved without any loss in tensile modulus, i.e. the toughness of HE3490-LS-H is improved without even marginal loss in the stiffness. These results are summarised in figure 3.

Claims

1 . A high density polyethylene blend, comprising

(A) 55 to 95 wt% of a high density multimodal polyethylene copolymer component having a density of at least 940 kg/m , and

(B) 5 to 45 wt% of an ultra-hi h molecular weight polyethylene homopolymer having an intrinsic viscosity of at least 6 dl/g and an MFR₂i of less than 0.5 g/ l Omin (UHMW polyethylene);

and wherein said blend has an MFR₂i of 10.0 g/ 1 0m in. or less and a density of at least 940 kg/m³.

2. A high density polyethylene blend, comprising

(A) 55 to 95 wt% of a high density multimodal polyethy lene copolymer component having a density of at least 940 kg/m , and

(B) 5 to 45 wt% of an ultra-high molecular w eight polyethylene homopolymer having a nominal viscosity molecular weight Mv of at least 800,000 and an MFR₂i of less than 0.5 g/ 1 0m in. (UHMW polyethy lene);

and w herein said blend has an MFR₂i of 1 0.0 g/ 1 0m in. or less and a density of at least 940 kg/m³.

3. A blend as defined in claim 1 or 2, wherein component (A) comprises a bimodai HOPE.

4. A blend as defined in any preceding claim, wherein component (A) is a copolymer hav ing 0. 1 to 2 mol% comonomcr.

5. A blend as defined in any preceding ciaim, w herein component (A) has an MFR₂i of 1 -20 g/ 10min.

6. A blend as defined in any preceding claim, wherein component (A) comprises an ethylene homopolymer component and a butene or hexene ethylene copolymer component.

7. A blend as defined in any preceding claim, wherein component (A) is present in an amount of 58 to 90 wt% and component (B) in an amount of 10 to 42 wt%.

8. A blend as claimed in any preceding claim wherein the MFR₅ of the blend is less than 1 .0 g 1 Omin.

9. A blend as claimed in any preceding claim wherein the density of the blend is 952 to 961 kg/m \

10. A blend as claimed in any preceding claim wherein component (A) has an M /Mn of at least 1 0.

1 1 . A blend as claimed in any preceding claim wherein component (B) has an intrinsic viscosity of 7 dl/g or more.

12. A blend as claimed in any preceding claim wherein component (B) has a density of 930 to 950 kg/m³.

13. A blend as defined in any preceding claim, wherein component (B) is prepared using a Ziegler- atta catalyst.

14. A blend as claimed in any preceding claim having an MFR₂i of 0. 1 to 8 g/ l Om in.

1 5. An article, preferably a pipe, made from the polyethylene blend as defined in any preceding claim.

16. A process for the preparation of a blend as hereinbefore defined comprising mi ing (A) 55 to 95 wt% of a high density multimodal polyethylene copolymer component having a density of at least 940 kg 'm , and

(B) 5 to 45 wt% of an ultra-high molecular weight polyethylene

homopolymer having a nominal viscosity molecular weight Mv of at least 800,000 and an MFR₂₁ of less than 0.5 g l Omin (UHMW polyethylene);

and extruding, optionally twice, or kneading the same so as to form a blend having an MFR₂i of 1 0.0 g l Omin or less and a density of at least 940 kg/nr .

1 7. A process as claimed in claim 16 wherein the blend components are extruded together twice.