WO2020096474A1

WO2020096474A1 - Adhesive polyethylene composition with improved long-term stability

Info

Publication number: WO2020096474A1
Application number: PCT/RU2018/000724
Authority: WO
Inventors: Alexey Mikhailovich VOLKOV; Irina Gennadievna RYZHIKOVA; Nikolay Alexandrovich BAUMAN
Original assignee: Public Joint Stock Company "Sibur Holding"
Priority date: 2018-11-06
Filing date: 2018-11-06
Publication date: 2020-05-14
Also published as: RU2768173C1

Abstract

The present invention relates to an adhesive polyethylene composition for use as an adhesive layer in multilayer articles. The adhesive polyethylene composition according to the invention comprises: A) from 50 to 80 wt.% of a linear low-density polyethylene (LLDPE) obtained by Ziegler-Natta catalyst systems, having a density in the range from 0.915 to 0.925 g/cm³ and a melt flow index (MFI_190/2.16) in the range from 2.5 to 8.0 g/10 min; B) from 15 to 30 wt.% of a high-density polyethylene (HDPE) grafted with a functional monomer, having a density of at least 0.955 g/cm³ and a melt flow index (MFI_190/21.6) in the range from 1.5 to 2.5 g/10 min; C) from 5 to 20 wt.% of an elastomer. The ratio of MFI_190/2.16 of the LLDPE to the HDPE grafted with a functional monomer is in the range from 1:0.2 to 1:0.9, preferably from 1:3 to 1:0.7. In addition to said components (A)-(C), the composition may further comprise optional additives. The invention also relates to a process for manufacturing such an adhesive composition, its use as an adhesive layer in multilayer coatings, in particular, as multilayer insulation of tubes for constructing main gas - and oil pipelines, and to a tube provided with such multilayer insulation comprising a layer of the claimed adhesive composition. The claimed composition maintains high peel strength in an aqueous environment for a long period of time and exhibits resistance of an adhesive peel to cathodic disbondment.

Description

ADHESIVE POLYETHYLENE COMPOSITION WITH IMPROVED

LONG-TERM STABILITY

FIELD OF THE INVENTION

The present invention relates to an adhesive polyethylene (PE) composition that is used for the production of an adhesive layer coated onto various metal surfaces to provide long-term protection, including anticorrosive protection, and to a process for manufacturing and using the same. In particular, the adhesive compositions of the present invention are used for insulating tubes, in construction of main gas- and oil pipelines to maintain their continuous, uninterrupted operation under climatic, mechanic, and electromagnetic environmental variables.

PRIOR ART

Patent US 6855432, 03.08.1990, [1], discloses polyethylene compositions that are suitable as adhesive coatings for protection of metal tubes. In particular, patent [1] discloses a composition used as an adhesive layer between a metal and an outer insulating material of a tube, comprising polyethylene or its copolymers with a-olefms. The known composition includes the following components: (A) 20 to 60 parts by weight of polyethylene; (B) 10 to 30 parts by weight of maleic anhydride grafted polyethylene; (C) 10 to 35 parts by weight of high impact polystyrene; (D) 10 to 25 parts by weight of an elastomer.

Patent EA 007577, 29.12.2006, [2], discloses a multicomponent polyethylene composition suitable for various uses, in particular, the known composition is used to form an adhesive layer between a metal and outer insulation of a tube. The composition according to invention [2] has the following formulation: A) non-elastomeric polyethylene comprising from 40 to 97 wt.% of the total weight of the composition, and B) an elastomer comprising an elastomeric copolymer of ethylene with polar comonomer moieties, wherein the component A) or the components A) and B) are grafted using a functional monomer (modifier).

Adhesive compositions used as coatings perform a protective function; a number of requirements are imposed thereupon; first and foremost, they must provide the firmest adherence of the composition to a surface of a protected material. Firm adherence as a property may be characterized by peel strength, i.e. tearing strength of a specific material, in particular, of a clean or thermosetting resin-primed metallic surface of a tube.

Patent US 8247053, 21.08.2012, [3], which was selected as a prototype, is the closest in terms of technical essence and discloses an adhesive polyethylene composition comprising from 1 to 40 wt.% of polyethylene or its copolymers with a- olefins (component A) grafted with a functional monomer, or a blend of polyethylene (component Al) with a second polymer (component A2) different from Al, in which the components Al and A2 are co-grafted with an unsaturated polar monomer selected from unsaturated carboxylic acids and anhydrides thereof. The content of the unsaturated polar monomer is from 0 to 10,000 parts per million (ppm). The composition described in prototype [3] comprises from 25 to 98 wt.% of a non-grafted polyethylene (component B) obtained by metallocene catalysis, having a density in the range from 0.910 to 0.930 g/cm³, from 1 to 35 wt.% of an elastomeric product (component C), which is either a copolymer of ethylene with a-olefms having a density in the range of 0.860 to 0.900 g/cm³, or ethylene-alkyl(meth)acrylate copolymers or ethylene-alkyl(meth)acrylates-maleic anhydride terpolymers, and a peroxide.

Reference [3] also discloses that lubricant additives are used to improve processability of the adhesive polyethylene composition. Stearamide, oleamide, erucinamide, calcium stearate, zinc stearate, aluminium stearate, magnesium stearate, and polyethylene wax in an amount from 0.01 to 1 wt.% are mentioned as said lubricants.

The process for manufacturing the adhesive composition according to prototype US 8247053 is a two-step process, comprising a step of blending and modifying one or more of polyethylenes A, then diluting the resulting blend with polymers B and the component C. The blending takes place in a twin-screw extruder. The so-obtained known composition is useful as an adhesive layer for protecting metal surfaces of a pipeline.

The disadvantageous feature of the invention described in [3] is limitations in the method of manufacturing the non-grafted polyethylene (component B), especially its obtainment through a metallocene catalytic system. There are also limitations in the manufacturing process of the adhesive composition as such, in particular, there is no guidance and conditions for using maleinized commercially available polyethylenes that would permit simplifying the technology by reducing the number of production steps. Furthermore, [3] is silent on any long-term tests of said composition supporting its stability and retention of the peel strength values obtained after exposing it to water, temperature, and cathodic polarization for a certain period of time, which are indicated in the known patent. The main object of all the above-referenced documents [1] to [3] is the achievement of the highest peel strength values possible. The required values of this parameter are ensured by a sufficient number of polar functional moieties grafted on polyethylene during its chemical modification, as stated above, by means of maleic anhydride (MA).

In view of the need of long-term operation of pipelines, quality assessment of adhesive compositions intended for protection of metal articles should depend on such external factors as an elevated ambient temperature and humidity. In addition to naturally occurring environmental variables, stability of peel strength of an adhesive peel in double and triple insulation of tubes is adversely affected by one of the methods that is widely utilized in contemporary metal pipeline structures, a so-called“active” anticorrosive protection, i.e. a cathodic protection system.

Therefore, another parameter of quality assessment of an adhesive coating is maintenance of high peelstrength and resistance to cathodic disbondment measured via a disbondment rate or a disbondment surface area during cathodic polarization.

Reference RU 2112004, 27.05.1998, [4], describes an insulating polymeric coating tested as to peelstrength stability in water by measuring initial peel strength formed after a day after applying a hot melt adhesive onto a surface of a metal plate, and measuring the peel strength after 1000 hours of exposure to water at 20°C. The tests of [4] have shown that the peel strength in water at 20°C decreases by mere 2%, and the disbondment rate during cathodic polarization measured under the same conditions is 3 cm²/hr. The results of the engineering solution of RU 2112004 cannot achieve the objects of the present invention, because the tests were performed under “soft” conditions that did not correlate to pipeline operation conditions.

The combination of the above-discussed factors affecting performance of an adhesive coating has called for further investigations into improvement of properties of an adhesive polyethylene composition. There is therefore a need to provide an adhesive polyethylene composition that maintains high peel strength in an aqueous environment for a long period of time, and has high resistance of an adhesive peel to cathodic disbondment.

It is an object of the present invention to provide an adhesive polyethylene composition maintaining high peel strength in an aqueous environment for a long period of time and resistance of an adhesive peel to cathodic disbondment.

The technical result of the present invention is the retention of peel strength values of a polyethylene composition to at least 55% of the initial peel strength values, when the adhesive peel is kept in an aqueous environment (for 1000 hours at 80°C). In addition, the claimed adhesive composition is characterized by resistance of an adhesive peel to cathodic disbondment, and the disbondment area under cathodic polymerization at 60°C for 30 days is not more than 13.5 cm². A further technical result is a decrease in the concentration of the functional monomer grafted polyethylene used in the composition. Yet another technical result is the simplification of the process for manufacturing the composition by running the same in a single step.

Meanwhile, a melt flow index (MFI190_/2_.16) of the composition is at least 3-4 g/lOmin, which is crucial for high processibillity of articles made from the polyethylene composition.

In order to achieve the desired technical result of the present invention, it is essential to use in the composition a blend of polyethylenes consisting of a linear low- density polyethylene (LLDPE) and a high-density polyethylene (HDPE) that considerably differ from one another by their density and melt flow indices, wherein it is crucial to use polyethylenes characterized by certain density and MFI within the interval of the claimed concentrations in the composition of polyethylenes. A polyethylene obtained in the presence of Ziegler-Natta catalyst systems is employed as the LLDPE according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The adhesive polyethylene composition according to the present invention comprises, relative to its total weight: A) from 50 to 80 wt.% of a linear low-density polyethylene (LLDPE) in the presence ofZiegler-Natta catalyst systems, having a density in the range from 0.915 to 0.925 g/cm³ and a melt flow index (MFI190/2.16) in the range from 2.5 to 8.0 g/lO min;

B) from 15 to 30 wt.% of a high-density polyethylene (HDPE) grafted with a functional monomer, having a density of at least 0.955 g/cm³ and a melt flow index (MFL90/2.I6) in the range from 1.5 to 2.5 g/lO min;

C) from 5 to 20 wt.% of an elastomer.

The composition may optionally contain further additives.

Said ratio of MFI190/2.I6 of the used linear low-density polyethylenes (LLDPE) to the HDPE grafted with a functional monomer is preferably in the range from 1 :0.2 to 1 :0.9, more preferably from 1:0,3 to 1 :0.7.

Copolymers of ethylene with an a-olefm comprising at least four carbon atoms are used as the linear low-density polyethylene (LLDPE) according to the invention. Compounds selected from the group consisting of butane- 1, hexane- 1, octane- 1 and similar a-olefms are used as the a-olefm. The content of the a-olefin copolymer in the LLDPE is from 2.5 to 8 wt.%, preferably from 3 to 6 wt.%, most preferably from 3.5 to 5 wt.%.

Meanwhile, use of a LLDPE obtained by Ziegler-Natta catalyst systems is essential for achievement of the technical result.

Said LLDPE has a density in the range from 0.915 to 0.925 g/cm³, preferably from 0.916 to 0.920 g/cm³. At that, the melt flow index (MFL90/2.16) is from 2.5 to 8 g/10 min, preferably from 3.0 to 6.0 g/lO min, most preferably from 3.0 to 4.5 g/10 min.

The content of the LLDPE, relative to the total weight of the composition, is from 50 to 80 wt.%, preferably from 60 to 80 wt.%, most preferably from 65 to 78 wt.%.

The high-density polyethylene (HDPE) grafted with a functional monomer is an ethylene monomer to the macromolecule of which the functional monomer is grafted. Exemplary functional monomers are unsaturated carboxylic acids and their derivatives. The unsaturated carboxylic acids according to the invention include acids comprising from 2 to 20 carbon atoms, such as, for example, acrylic acid, methacrylic acid, maleic acid, fumaric acid, and itaconic acid. Functional derivatives of these acids include, for example, ethers and anhydrides of unsaturated carboxylic acids. Exemplary unsaturated carboxylic acid esters are alkyl(meth)acrylates, wherein alkyls comprise up to 24 carbon atoms. Examples of a suitable alkylacrylate and alkyl(meth)acrylate are, particularly, methylmethacrylate, ethylacrylate, n-butylacrylate, isobutylacrylate, 2- ethylhexylacrylate. Examples of anhydrides of an unsaturated carboxylic acid are, particularly, maleic anhydride, itaconic anhydride, citraconic anhydride, tetrahydrophthalic anhydride. The most preferable functional monomer of the present invention is maleic anhydride.

The amount of the functional monomer grafted to HDPE is at least 1 wt.%, preferably from 1.0 to 1.5 wt.%, relative to the total weight of the HDPE grafted with a functional monomer.

A polyethylene obtained by anionic coordinate polymerization of ethylene at low pressure in Ziegler-Natta catalyst systems or metallocene catalyst according to the known techniques is used as the HDPE. A functional monomer is grafted on HDPE in conformity with the techniques that are well-known to those skilled in the art, by a periodic or continuous method, using a device for mixing the melt. According to the invention, the graft is effected by reactive extrusion in the presence or in the absence of a radical initiator. A functional monomer is preferably grafted in the presence of a radical initiator, such as an organic peroxide. Examples of organic peroxides include, but are not limited to, the following compounds: tert-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, di-tert-butyl hydroperoxide, dicumene peroxide, l,3-l,4-bis-(tert-butylperoxyisopropyl)benzene, acetyl peroxide, benzoyl peroxide, isobutyryl peroxide, bis-3,5,5-trimethylhexanoyl peroxide, methyl ethyl ketone peroxide, and other organic peroxides. Peroxides may be used in their pure form and may be coated onto a mineral or polymeric filler (peroxide concentrate). Examples of suitable peroxides are commercially available products marketed under such trademarks as Trigonox 301, Luperox DCP, Luperox DC40, Luperox DC40KE, Luperox DC40MG, Luperox DC40P-SP2, Luperox DI, Luperox DTA, Luperox F, Luperox F40, Luperox F40MG, Luperox MIX, Luperox 101, Luperox 101SIL45, Luperox 130XL45, Luperox 801. The amount of said pure initiator added to the composition is preferably in the range from 0.01 to 0.4 wt.%, most preferably from 0.05 to 0.15 wt.%, relative to the total weight of the composition. Said HDPE grafted with a functional monomer has a density of at least 0.955 g/cm³, preferably not less than 0.960 g/cm³. At that, the melt flow index (MFI190_/2_.16) of said HDPE is from 1.5 to 2.7 g/lO min, preferably from 2.0 to 2.5 g/lO min.

The adhesive composition according to the invention comprises the HDPE grafted with a functional monomer in the amount from 15 to 30 wt.%, preferably from 15 to 25 wt.%, most preferably from 17 to 25 wt.%, relative to the total weight of the composition. The content of the HDPE grafted with a functional monomer of less than 15 wt.% reduces the concentration of functional polar moieties in the interface zone in the vicinity of the protected material that contacts the adhesive composition and, as a consequence, results in a decrease of peel strength. If the content of the HDPE grafted with a functional monomer exceeds 30 wt.%, its compatibility with the LLDPE deteriorates considerably due to heterogeneity of their structures thereby causing adverse changes in morphology of said blend. The coarsely dispersed and heterogeneous HDPE phase in the LLDPE matrix worsens the structure and properties of the adhesive layer with a polar support (protected material), which deteriorates peel strength accordingly.

According to the present invention, an LLDPE having a density in the range from 0.915 to 0.925 g/cm³, preferably from 0.916 to 0.920 g/cm³, and a HDPE grafted with a functional monomer, having a density of at least 0.955 g/cm³, preferably at least 0.960 g/cm³, are used as polyethylenes in the adhesive composition. Otherwise, convergence of density values for the polyethylenes, less than 0.955 g/cm³ for the HDPE and more than 0.925 g.cm³ for the LLDPE, will hinder diffusion of the HDPE grafted with a functional monomer to a surface of a material with which the adhesive composition contacts, thereby deteriorating its peel strength.

Use of polyethylenes characterized by different melt flow indices (MFI190/2.16) in the composition is essential for obtainment of the technical result of the present invention. The melt flow index of the HDPE grafted with a functional monomer is from 1.5 to 2.7 g/ 10 min, preferably from 1.5 to 2.5 g/ 10 min, more preferably from 2.0 to 2.5 g/cm³, whereas the LLDPE has a melt flow index in the range from 3.0 to 8.0 g/lO min, preferably from 3.5 to 6.0 g/lO min. Meanwhile, the ratio of MFI190/2.16 of the used polyethylene, i.e. the LLDPE to the HDPE grafted with a functional monomer must be in the range from 1 :0.2 to 1:0.9, preferably from 1 :3 to 1 :0.7. While not wishing to be bound by any particular theory, the authors of the present invention believe that the aforementioned difference in the MFI values is necessary for reduction of interface diffusion of an elastomer toward the layer directly contacting the solid metal support and consisting of the HDPE grafted with a functional monomer in order not to decrease its cohesive strength, which latter influences decisively the peel strength of an adhesive peel on the whole.

The authors of the present invention have surprisingly discovered that stability of a bond strength index increases significantly when using LLDPE obtained in the presence of Ziegler-Natta catalyst systems.

Ziegler-Natta catalyst systems allow to obtain polymers of certain tacticity (stereoregularity). Such catalyst systems are complexes formed during a reaction of transition metal compounds with alkyls and alkyl halides of Group II-III metals. Highly active Ziegler-Natta catalyst systems in the form of supported titanium-magnesium complexes are the most preferable.

Said highly active supported titanium-magnesium complexes are titanium chlorides fixed on a surface of an“activated” magnesium and chlorine-containing support. Under polymerization conditions, in the presence of trialkylaluminum activator (AlR₃), the titanium-magnesium complexes are activated and active sites are formed.

Synthesis of LLDPE on heterogeneous (supported) Ziegler-Natta catalyst systems, such as, for example, polynuclear titanium-magnesium systems, is accompanied by simultaneous growth of polyethylene macrochains on several active sites located on a solid surface of a support. Thus, use of Ziegler-Natta catalyst systems characterized by high density of active sites fixed on a support surface to produce linear low-density polyethylenes suitable as a component of the adhesive composition according to the invention contributes to formation of a supramolecular LLDPE structure with an increased density of a network of physical entanglements of macrochains in the interface (intercrystalline) area being up to 10¹²-10¹⁴ entanglements per 1 mg of the polymer. The dense network of physical entanglements complicates elastomer diffusion through such a medium and, as a consequence, contributes to better retention of the structure and properties of an adhesive peel consisting of three basic layers: an LLDPE, an HDPE grafted with a functional monomer, and an elastomer positioned therebetween.

Use of an LLDPE obtained in the presence of said metallocene catalyst systems within the claimed composition results in higher, yet far less resistant to hostilities, peel strength values. The authors suggest that this deterioration of adhesive properties of the composition is due to lower barrier properties of such polyethylene compared to an LLDPE of an identical viscosity that was obtained in the presence of Ziegler-Natta catalyst system. During long exposure to high temperature, the most fusible part of an adhesive peel, the elastomer, will tend to diffuse into an LLDPE layer having a considerably lower density than the HDPE grafted with a functional monomer. The degree of elastomer diffusion and, consequently, loosening of the adhesive peel will depend on barrier properties of the LLDPE layer. The more friable structure of a metallocene LLDPE, as compared to a conventional LLDPE, will allow the elastomer flowing from the interface layer to diffuse more intensely therein.

Apart from the LLDPE and the HDPE grafted with a functional monomer, the composition also comprises an elastomer in an amount from 5 to 20 wt.%, preferably from 5 to 15 wt.%, most preferably from 5 to 10 wt.%, relative to the total weight of the composition. At that, the elastomer is used at a ratio to the HDPE grafted with a functional monomer of from 1 :1 to 1 :6, preferably from 1 :2 to 1 :3 parts by weight. The elastomer used within the claimed composition improves compatibility of the LLDPE and the HDPE grafted with a functional monomer and enhances plasticity of the adhesive layer thereby enhancing its strength under external influence.

Copolymers of ethylene with a-olefms comprising 4 to 8 carbon atoms (for example, Engage, Exact and the like) and/or copolymers of ethylene with unsaturated carboxylic acid esters (for example, those manufactured by Du Pont, Exxon Mobil, Dow Chem. under Elvaloy, Lotryl trademarks, and the like) are used as the elastomers according to the invention.

Copolymers of ethylene with unsaturated carboxylic acid esters, which are employed as the elastomers, are random copolymers comprising from 5 to 40 wt.% of an ester comonomer. Exemplary unsaturated carboxylic acid esters are alkylacrylate and/or alkylmethacrylate, wherein the alkyl contains up to 24 carbon atoms. Examples of the alkylarylate and alkyl(meth)acrylate are, particularly, methylmethacrylate, ethylacrylate, n-butylacrylate, isobutylacrylate, 2-ethylhexylacrylate.

The claimed composition may optionally comprise various additives, for example, antioxidants, UV -absorbers, antistatic agents, crystallization agents, fillers, lubricants, flame retardants, and other additives in an amount from 0 to 1 wt.%, preferably from 0.1 to 0.5 wt.%.

Examples of suitable antioxidants include 2,6-di-tert-butyl-p-cresol, tetrakis- [methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane, p-naphthylamine, and paraphenylenediamine derivatives.

Examples of UV-absorbers include 2,4-dihydroxybenzophenone, 2-(2’ -hydroxy- 3’5’ -di-t-butylphenyl)-5 -chlorobenzotriazole, 2-(2-hydroxy-3 -t-butyl-5 -methylphenyl)- 5-chlorobenzotriazole, and bis-(2,2’,6,6’-tetramethyl-4-piperidine)sebate.

Examples of antistatic agents include lauryl diethanolamine, palmityl, diethanolamine, stearyl diethanolamine, oleyl diethanolamine, behenyl diethanolamine, polyoxyethylene-alkylamines, stearyl-monoglycerides, and 2-hydroxy-4-n- octoxybenzophenone.

Examples of crystallization agents include aluminum p-tert-butylbenzoate, aluminum dibenzylidene-sorbitol, and aluminum hydroxy-di-p-t-butylbenzoate.

Fillers that can be used in the composition are, for example, glass fiber, carbon fiber, talc, clay, silica, calcium carbonate, barium sulfate, magnesium hydroxide, calcium hydroxide, and calcium oxide.

Lubricants are utilized to facilitate the manufacture of the composition, in particular, its extrusion. Examples of lubricants include, particularly, stearamides, oleamides, erucamides, calcium stearate, zinc stearate, aluminum stearate, magnesium stearate, polyethylene wax, petrolatum oil.

Examples of flame retardants are metal hydroxides, halogenating agents, antimony oxide, decabromodiphenyl ether, and bis-(3,5-dibromo-4- bromopropyloxyphenyl)sulfone.

The adhesive composition according to the present invention is obtained by blending all components, using the known thermoplastic blending techniques, for example, extrusion, or blending in mixers of various designs. Internal mixers with blades or rotors, single-screw extruders, counter-rotating or co-rotating twin-screw extruders may be used. In accordance with the present invention, the adhesive composition is manufactured by blending the components followed by meltcompounding the resultant blend by means of prior art equipment, for example, such mixing equipment as Banbury mixers, Brabender mixers, single-screw extruders, twin- screw extruders, and similar mixers. Blending is preferably performed in mixing equipment, while the so-obtained blend is subjected to further compounding in an extruder. At that, what is understood under compounding in the present invention is a process technology of melt-blending polymers and additives so as to produce a composition with homogeneously blended components.

The present invention also pertains to a process for manufacturing the aforementioned adhesive polyethylene composition, the process comprising blending the components A)-C) taken in the following amounts, relative to the total weight of the composition:

A) from 50 to 80 wt.% of a linear low-density polyethylene (LLDPE) obtained by Ziegler-Natta catalyst systems, having a density in the range from 0.915 to 0.925 g/cm³ and a melt flow index (MFI190/2.16) in the range from 2.5 to 8.0 g/lO min;

B) from 15 to 30 wt.% of a high-density polyethylene (HDPE) grafted with a functional monomer, having a density of at least 0.955 g/cm³ and a melt flow index (MFI190_/2_.16) in the range from 1.5 to 2.5 g/lO min;

C) from 5 to 20 wt.% of an elastomer, and

subsequently melt-compounding the so-obtained blend.

In conformity with the present invention, the composition is manufactured by a single-step process, which comprises blending the components (A), (B), and (C) and optional additives, and subsequently melt-compounding the resultant blend.

The component (B), taken in an amount from 15 to 30 wt.%, relative to the total weight of the composition, is blended with the component (A) taken in an amount from 50 to 80 wt.%, relative to the total weight of the composition, and with the component (C) taken in an amount from 5 to 20 wt.%, relative to the total weight of the composition. Optional additives taken in an amount from 0 to 1 wt.%, preferably from 0.1 to 0.5 wt.%, relative to the total weight of the composition, are also introduced into the composition. The components are blended for a period of time from 1 to 20 minutes, preferably from 2 to 10 minutes, at a temperature from 10 to l00°C, preferably from 10 to 50°C, more preferably from 20 to 50°C, even more preferably from 20 to 40°C. The so-obtained blend is then melt-compounded in a twin-screw extruder at a temperature in the range from l90°C to 240°C, preferably from 220°C to 230°C, most preferably at a maximum temperature in roller zones of 2lO°C and at a speed of rotation of the screw of about 250 min ¹. The so-obtained adhesive composition is granulated and used according to its intended purpose.

The adhesive polyethylene composition manufactured according to the present invention may be used as a co-extruded adhesive layer in multilayer coatings, preferably in insulation, in particular, for protection of tubes intended for construction of main gas- and oil pipelines.

The present invention also relates to an article that includes an adhesive layer comprising the adhesive composition described above. Such articles include, for example, tubes, cables, films, articles manufactured by extrusion coating the adhesive composition. In particular, the present invention relates to a multilayer tube, i.e. to a tube comprising multilayer insulation with two, three, four, five or more layers, the insulation including a polylefin layer and an adhesive layer adjacent to the polyolefin layer and containing the adhesive composition described above. The invention will be explained by examples below that are given for illustrative purposes and are not intended to limit the scope of the present disclosure.

Examples

The polyethylenes listed in Table 1 were used as the LLDPE.

Table 1. Characteristics of polyethylenes used in the examples of the invention

Additives: AO (antioxidant), PPA (polyethylene processing

TS (thermal stabiliser)

The commercially produced polyethylenes listed in Table 2 are used as the

HDPE grafted with a functional monomer.

Table 2. Characteristics of HDPE functionalized with maleic anhydride

Engage 8452 (copolymer of ethylene and octene-l) are used as the elastomers.

Tests on peel strength retention of the claimed composition are performed in conformity with GOST R. 52568 Annex A.

Tests on resistance of an adhesive peel to cathodic polarization are performed in conformity with GOST R. 51164 Annex B.

Example 1

A blend comprising 15 wt.% of a Fusabond 100 HDPE (see its properties in Table 2) grafted with a functional monomer, 80 wt.% of a PE 5118Q LLDPE (see its properties in Table 1), 5 wt.% of an elastomer, an Elvaloy AC3427 copolymer of ethylene and butylacrylate, is prepared in a blade mixer and mixed for 2 to 10 minutes at 10 to 50°C. The resultant blend of components is processed in a LTE -20-44 twin- screw extruder at a maximum temperature in roller zones of 2lO°C and at a speed of rotation of the screw of 250 min ¹.

The so-obtained adhesive composition is characterized by peel strength to steel of 30.5 N/mm at 23°C, maintains the peel strength of 73.1% and the cathodic disbondment area of 12.4 cm² after 1000 hours of exposure to water at 80°C.

Example 2

The composition according to Example 1 is provided, save that 10 wt.% instead of 5 wt.% of Elvaloy AC 3427 is introduced, and 75 wt.% instead of 80 wt.% of the PE 5118Q LLDPE is measured out. The so-obtained composition is characterized by peel strength to steel of 35.2 N/mm at 23°C, maintains the peel strength of 80.1% and the cathodic disbondment area of 11.2 cm² after 1000 hours of exposure to water at 80°C.

Example 3

The composition according to Example 1 is provided, save that 20 wt.% instead of 15 wt.% of Fusabond 100 is introduced, and 75 wt.% instead of 80 wt.% of the PE 5118Q LLDPE is measured out.

The so-obtained composition is characterized by peel strength to steel of 30.4 N/mm at 23°C, maintains the peel strength of 61.5% and the cathodic disbondment area of 12.2 cm² after 1000 hours of exposure to water at 80°C.

Example 4

The composition according to Example 1 is provided, save that 25 wt.% instead of 15 wt.% of Fusabond 100 is introduced, 10 wt.% instead of 5 wt.% of Elvaloy AC 3427 is introduced, and 65 wt.% instead of 80 wt.% of the PE 5118Q LLDPE is measured out.

The so-obtained composition is characterized by peel strength to steel of 34.4 N/mm at 23°C, maintains the peel strength of 83.7% and the cathodic disbondment area of 7.8 cm² after 1000 hours of exposure to water at 80°C.

Example 5

The composition according to Example 1 is provided, save that the Sabic 318B LLDPE instead of the PE 5118Q LLDPE is introduced.

The so-obtained composition is characterized by peel strength to steel of 29.2 N/mm at 23°C, maintains the peel strength of 59.6% and the cathodic disbondment area of 11.8 cm² after 1000 hours of exposure to water at 80°C.

Example 6 (comparative)

The composition according to Example 1 is provided, save that 35 wt.% instead of 15 wt.% of Fusabond 100 is introduced, 10 wt.% instead of 5 wt.% of Elvaloy 3427 is introduced, and 55 wt.% instead of 80% of the PE 5118Q LLDPE is measured out.

The so-obtained composition is characterized by peel strength to steel of 11.7 N/mm at 23°C, maintains the peel strength of 52.1% and the cathodic disbondment area of 16.3 cm² after 1000 hours of exposure to water at 80°C.

Example 7 (comparative) The composition according to Example 1 is provided, save that 15.7 wt.% instead of 15 wt.% of Fusabond 100 is introduced, 5.3 wt.% instead of 5 wt.% of Elvaloy AC3427 is introduced, and 78.9 wt.% of a Daelim XP 9400 LLDPE (see its properties in Table 1) instead of 80 wt.% of PE 5118Q is measured out.

The so-obtained composition is characterized by peel strength to steel of 44.6 N/mm at 23°C, maintains the peel strength of 18.4% and the cathodic disbondment area of 15.0 cm² after 1000 hours of exposure to water at 80°C.

Example 8

The composition according to Example 1 is provided, save that an Engage 8452 elastomer is used instead of Elvaloy AC 3427.

The so-obtained composition is characterized by peel strength to steel of 29.5 N/mm at 23°C, maintains the peel strength of 68.8% and the cathodic disbondment area of 11.5 cm² after 1000 hours of exposure to water at 80°C.

Example 9 (comparative)

The composition according to Example 1 is provided, save that 20 wt.% instead of 15 wt.% of Fusabond 100 is used, and the elastomer is excluded from the formulation of the composition.

The so-obtained composition is characterized by bond strength to steel of 33.6 N/mm at 23°C, maintains the peel strength of 40.1% and the cathodic disbondment area of 19.2 cm² after 1000 hours of exposure to water at 80°C.

Example 10 (comparative)

The composition according to Example 1 is provided save that a Bondyram 5108 HDPE grafted with a functional monomer is used instead of Fusabond 100.

The so-obtained composition is characterized by peel strength to steel of 19.2 N/mm at 23°C, maintains the peel strength of 39.6% and the cathodic disbondment area of 19.8 cm² after 1000 hours of exposure to water at 80°C.

Example 11 (comparative)

The composition according to Example 1 is provided save that a BYC Scona HDPE grafted with a functional monomer is used instead of Fusabond 100.

The so-obtained composition is characterized by peel strength to steel of 12.0 N/mm at 23°C, maintains the peel strength of 30.1% and the cathodic disbondment area of 21.4 cm² after 1000 hours of exposure to water at 80°C. Example 12 (comparative)

The composition according to Example 1 is provided, save that 3.0 wt.% instead of 5 wt.% of Elvaloy AC 3427 is used, and 82 wt.% instead of 80 wt.% of the PE 5118Q LLDPE is used.

The so-obtained composition is characterized by peel strength to steel of 33.5

N/mm at 23°C, maintains the bond strength of 38.2% and the cathodic disbondment area of 20.7 cm² after 1000 hours of exposure to water at 80°C.

Example 13 (comparative)

The composition according to Example 1 is provided, save that DOWLEX 5066 (see Table 1) is used instead of the PE 5118Q LLDPE.

The so-obtained composition is characterized by peel strength to steel of 21.2 N/mm at 23°C, maintains the peel strength of 44.3% and the cathodic disbondment area of 18.9 cm² after 1000 hours of exposure to water at 80°C.

Table 3 shows formulations of polyethylene compositions (Examples 1-12) and results of determining their peel strength during long-term tests and cathodic polarization.

Table 3. Formulations and properties of the adhesive polyethylene compositions according to Examples 1-12.

Continuation of Table 3.

As demonstrated by the experimental data above, the claimed adhesive polyethylene composition has long-term stability of peel strength values after exposure of an adhesive peel to an aqueous medium for 1000 hours at 80°C, with the initial peel strength values remaining in the range from 59.6% to 83,7%. Meanwhile, the disbondment area under cathodic polarization of the proposed polyethylene compositions within 30 days at +60°C varies in the permissible range from 7.8 cm² to 12.4 cm².

The advantages of the proposed composition are illustrated by the examples given in Table 3.

Examples 1-5 show a behaviour of the claimed compositions under a permissible variation of the dosing interval and selection of grades of basic components of a composition consisting of an HDPE grafted with a functional monomer, an LLDPE, and an elastomer.

The results of the experiments (Examples 1-5 and Example 8) fully support the technical result achievable by the present invention provided that the following conditions are satisfied:

1) densities of the used polyethylenes differ considerably from each other; the LLDPE density is in the range from 0.915 to 0.925 g/cm³, preferably from 0.916 to 0.920 g/cm³, whereas the density of the HDPE grafted with a functional monomer is at least 0.955 g/cm³, preferably at least 0.960 g/cm³;

2) polyethylenes having different melt flow indices (MFI190_/2.16) are used in the composition. The melt flow index of the HDPE grafted with a functional monomer is from 1.5 to 2.7 g/lO min, preferably from 2.0 to 2.5 g/lO min, the LLDPE has a melt flow index in the range from 3.0 to 8.0 g/lO min, preferably from 3.5 to 6.0 g/lO min;

3) the composition includes an LLDPE obtained by Ziegler-Natta catalyst systems; and

4) the optimal ratio of starting components is from 50 to 80 wt.%, most preferably from 56 to 75 wt.%, of the LLDPE, from 15 to 30 wt.%, most preferably from 15 to 25 wt.%, of the HDPE grafted with a functional monomer, and from 5 to 20 wt.%, most preferably from 5 to 10 wt.%, of the elastomer,.

The experimental data bear evidence that when using polyethylenes (LLDPE and HDPE grafted with a functional monomer) which ratio of densities extends beyond the density ranges of the claimed composition, peel strength retention values become worse (see Comparative Example 13). Comparative Example 10 demonstrates the influence of an HDPE grafted with a functional monomer having an undesirably high melt flow index (MFI190/2.16 of 8.0 g/lO min) on peel strength retention values, as well as on values of disbondment area under cathodic polarization. Comparative Example 11 shows the influence of an HDPE having an undesirably low melt flow index (MFI 1909/2. i6 of 0.35 g/lO min) on peel strength.

5) Comparative Example 7 demonstrates that the use of polyethylenes obtained in the presence of catalyst systems other than Ziegler-Natta catalysts, in particular, in the presence of metallocene catalysts, reduces peel strength retention values and drastically increases the disbondment area under cathodic polarization. Comparative Examples 6, 9, 12 show deterioration of peel strength retention values to 38.2%-52.l%, when the optimal ranges of the starting components extend or shrink.

Therefore, the examples given herein above fully support the production of the claimed adhesive polyethylene composition that maintains high peel strength in an aqueous environment for a long period of time, and has high resistance of an adhesive peel to cathodic disbondment.

Claims

1. An adhesive polymer composition comprising, relative to its total weight:

A) from 50 to 80 wt.% of a linear low-density polyethylene (LLDPE) obtained by Ziegler-Natta catalyst systems, having a density in the range from 0.915 to 0.925 g/cm³ and a melt flow index (MFIi9o_/2_.i6) in the range from 2.5 to 8.0 g/lO min;

B) from 15 to 30 wt.% of a high-density polyethylene (HDPE) grafted with a functional monomer, having a density of at least 0.955 g/cm³ and a melt flow index (MFI 190_/2_.16) in the range from 1.5 to 2.5 g/lO min;

C) from 5 to 20 wt.% of an elastomer.

2. The adhesive composition according to claim 1, wherein the content of the LLDPE, relative to the total weight of the composition, is from 60 to 80 wt.%.

3. The adhesive composition according to claim 2, wherein the content of the LLDPE, relative to the total weight of the composition, is from 65 to 78 wt.%.

4. The adhesive composition according to claim 1, wherein the content of the HDPE grafted with a functional monomer, relative to the total weight of the composition, is from 15 to 25 wt.%.

5. The adhesive composition according to claim 4, wherein the content of the HDPE grafted with a functional monomer, relative to the total weight of the composition, is from 17 to 25 wt.%.

6. The adhesive composition according to claim 1, wherein the content of the elastomer, relative to the total weight of the composition, is from 5 to 15 wt.%.

7. The adhesive composition according to claim 6, wherein the content of the elastomer, relative to the total weight of the composition, is from 5 to 10 wt.%.

8. The adhesive composition according to claim 1, wherein the weight ratio of the elastomer to the HDPE grafted with a functional monomer is from 1:1 to 1 :6.

9. The adhesive composition according to claim 8, wherein the weight ratio of the elastomer to the HDPE grafted with a functional monomer is from 1 :2 to 1 :3.

10. The adhesive composition according to claim 1, wherein the density of the LLDPE is from 0.916 to 0.920 g/cm³.

11. The adhesive composition according to claim 10, wherein the density of the HDPE grafted with a functional monomer is at least 0.960 g/cm³.

12. The adhesive composition according to claim 1, wherein the MFI190_/2.16 for the LLDPE is in the range from 3.0 to 6 g/ 10 min.

13. The adhesive composition according to claim 1, wherein the MFI190_/2.16 for the HDPE grafted with a functional monomer is in the range from 2.0 to 2.5 g/lO min.

14. The adhesive composition according to claim 1, wherein the ratio of MFI190_/2.16 of the LLDPE to the HDPE grafted with a functional monomer is in the range from 1 :0.2 to 1:0.9.

15. The adhesive composition according to claim 1, wherein the ratio of MFI 190_/2.16 of the LLDPE to the HDPE grafted with a functional monomer is in the range from 1 :0.3 to 1 :0.7.

16. The adhesive composition according to claim 1, wherein the LLDPE is a copolymer of ethylene with an a-olefm comprising at least four carbon atoms.

17. The adhesive composition according to claim 16, wherein the content of the a-olefm copolymer in the LLDPE is from 2.5 to 8 wt.%.

18. The adhesive composition according to claim 17, wherein the content of the a-olefm copolymer in the LLDPE is from 3 to 6 wt.%.

19. The adhesive composition according to claim 18, wherein the content of the a-olefm copolymer in the LLDPE is from 3.5 to 5 wt.%.

20. The adhesive composition according to claim 1, wherein the HDPE grafted with a functional monomer is a homopolymer of ethylene to the macromolecule of which the functional monomer is grafted.

21. The adhesive composition according to claim 1, wherein the functional monomer is selected from unsaturated carboxylic acids and derivatives thereof.

22. The adhesive composition according to claim 21, wherein the unsaturated carboxylic acids are acids containing from 2 to 20 carbon atoms.

23. The adhesive composition according to claim 21, wherein derivatives of the unsaturated carboxylic acids include ether derivatives and anhydrides of unsaturated carboxylic acids.

24. The adhesive composition according to claim 23, wherein the ether derivatives of the unsaturated carboxylic acids include, in particular, alkyl(meth)acrylate, wherein the alkyl comprises up to 24 carbon atoms.

25. The adhesive composition according to claim 23, wherein the anhydrides of the unsaturated carboxylic acids include maleic anhydride, itaconic anhydride, citraconic anhydride, tetrahydrophthalic anhydride.

26. The adhesive composition according to claim 1, wherein the functional monomer is maleic anhydride.

27. The adhesive composition according to claim 1, wherein the amount of the functional monomer grafted on the HDPE is at least 1 wt.%, relative to the total weight of the HDPE grafted with a functional monomer.

28. The adhesive composition according to claim 27, wherein the amount of the functional monomer grafted on the HDPE is from 1.0 to 1.5 wt.%, relative to the total weight of the HDPE grafted with a functional monomer.

29. The adhesive composition according to claim 1, wherein the Ziegler-Natta catalyst system is a complex obtained by reacting transition metal compounds with alkyls and alkyl halidesof Group II-III metals.

30. The adhesive composition according to claim 1, wherein the Ziegler-Natta catalyst system is selected from supported titanium-magnesium catalyst systems.

31. The adhesive composition according to claim 1, wherein the LLDPE is a polyethylene with an increased density of a network of physical entanglements of macrochains being up to l0¹²-l0¹⁴ entanglements per 1 mg of a polymer.

32. The adhesive composition according to claim 1, wherein the composition comprises optional additives.

33. The adhesive composition according to claim 32, wherein the optional additives are used in an amount from 0 to 1 wt.%, preferably from 0.1 to 0.5 wt.%.

34. The adhesive composition according to claim 32, wherein the optional additives comprise antioxidants, UV-absorbers, antistatic agents, crystallization agents, fillers, lubricants, and flame retardants.

35. A process for manufacturing an adhesive polymer composition, comprising: blending the following components taken in amounts, relative to the total weight of the composition:

A) from 50 to 80 wt.% of a linear low-density polyethylene (LLDPE) obtained by Ziegler-Natta catalyst systems, having a density in the range from 0.915 to 0.925 g/cm³ and a melt flow index (MFI190_/2_.16) in the range from 2.5 to 8.0 g/lO min; B) from 15 to 30 wt.% of a high-density polyethylene (HDPE) grafted with a functional monomer, having a density of at least 0.955 g/cm³ and a melt flow index (MFI 190/2.16) in the range from 1.5 to 2.5 g/lO min;

C) from 5 to 20 wt.% of an elastomer, and

subsequently melt-compounding the so-obtained blend.

36. The process for manufacturing an adhesive composition according to claim

35, wherein the content of the LLDPE, relative to the total weight of the composition, is from 60 to 80 wt.%.

37. The process for manufacturing an adhesive composition according to claim

36, wherein the content of the LLDPE, relative to the total weight of the composition, is from 65 to 78 wt.%.

38. The process for manufacturing an adhesive composition according to claim 35, wherein the content of the HDPE grafted with a functional monomer, relative to the total weight of the composition, is from 15 to 25 wt.%.

39. The process for manufacturing an adhesive composition according to claim 38, wherein the content of the HDPE grafted with a functional monomer, relative to the total weight of the composition, is from 17 to 25 wt.%.

40. The process for manufacturing an adhesive composition according to claim 35, wherein the content of the elastomer, relative to the total weight of the composition, is from 5 to 15 wt.%.

41. The process for manufacturing an adhesive composition according to claim 40, wherein the content of the elastomer, relative to the total weight of the composition, is from 5 to 10 wt.%.

42. The process for manufacturing an adhesive composition according to claim 35, wherein the elastomer is used at a weight ratio to the HDPE grafted with a functional monomer of 1 : 1 to 1 :6.

43. The process for manufacturing an adhesive composition according to claim 42, wherein the elastomer is used at a weight ratio to the HDPE grafted with a functional monomer of from 1 :2 to 1 :3.

44. The process for manufacturing an adhesive composition according to claim 35, wherein the density of the LLDPE is from 0.916 to 0.920 g/cm³.

45. The process for manufacturing an adhesive composition according to claim 44, wherein the density of the HDPE grafted with a functional polymer is at least 0.960 g/cm³.

46. The process for manufacturing an adhesive composition according to claim 35, wherein the MFI190/2.16 for the LLDPE is in the range from 3.0 to 6 g/lO min.

47. The process for manufacturing an adhesive composition according to claim 35, wherein the MFI190_/2.16 for the HDPE grafted with a functional monomer is in the range from 2.0 to 2.5 g/ 10 min.

48. The process for manufacturing an adhesive composition according to claim 35, wherein copolymers of ethylene with an a-olefin comprising at least four carbon atoms are used as the LLDPE.

49. The process for manufacturing an adhesive composition according to claim

48, wherein the content of the a-olefin comonomer in the LLDPE is from 2.5 to 8 wt.%.

50. The process for manufacturing an adhesive composition according to claim

49, wherein the content of the a-olefin comonomer in the LLDPE is from 3 to 6 wt.%.

51. The process for manufacturing an adhesive composition according to claim

50, wherein the content of the a-olefin comonomer in the LLDPE is from 3.5 to 5 wt.%.

52. The process for manufacturing an adhesive composition according to claim 35, wherein the HDPE grafted with a functional monomer is a homopolymer of ethylene to the macromolecule of which the functional monomer is grafted.

53. The process for manufacturing an adhesive composition according to claim 35, wherein the functional monomer is unsaturated carboxylic acids and derivatives thereof.

54. The process for manufacturing an adhesive composition according to claim 53, wherein the unsaturated carboxylic acids are acids containing from 2 to 20 carbon atoms.

55. The process for manufacturing an adhesive composition according to claim 53, wherein derivatives of the unsaturated carboxylic acids include ether derivatives and anhydrides of unsaturated carboxylic acids.

56. The process for manufacturing an adhesive composition according to claim 55, wherein the ether derivatives of the unsaturated carboxylic acids include, in particular, alkyl(meth)acrylate, wherein the alkyl comprises up to 24 carbon atoms.

57. The process for manufacturing an adhesive composition according to claim 55, wherein the anhydrides of the unsaturated carboxylic acids include maleic anhydride, itaconic anhydride, citraconic anhydride, and tetrahydrophthalic anhydride.

58. The process for manufacturing an adhesive composition according to claim 35, wherein the functional monomer is maleic anhydride.

59. The process for manufacturing an adhesive composition according to claim 35, wherein the amount of the functional monomer grafted on the HDPE is at least 1 wt.%, relative to the total weight of the HDPE grafted with a functional monomer.

60. The process for manufacturing an adhesive composition according to claim 59, wherein the amount of the functional monomer grafted on the HDPE is from 1.0 to 1.5 wt.%, relative to the total weight of the HDPE grafted with a functional monomer.

61. The process for manufacturing an adhesive composition according to claim 35, wherein complexes obtained by reacting transition metal compounds with alkyls and haloalkanes of Group II-III metals are used as the Ziegler-Natta catalyst systems.

62. The process for manufacturing an adhesive composition according to claim 35, wherein the Ziegler-Natta catalyst system is selected from supported titanium- magnesium catalyst systems.

63. The process for manufacturing an adhesive composition according to claim 35, wherein a polyethylene with an increased density of a network of physical entanglements of macrochains being up to 10¹²-10¹⁴ entanglements per 1 mg of a polymer is used as the LLDPE.

64. The process for manufacturing an adhesive composition according to claim 35, wherein the composition comprises optional additives.

65. The process for manufacturing an adhesive composition according to claim 64, wherein the optional additives are used in an amount from 0 to 1 wt.%, preferably from 0.1 to 0.5 wt.%.

66. The process for manufacturing an adhesive composition according to claim 64, wherein the optional additives comprise antioxidants, UV-absorbers, antistatic agents, crystallization agents, fillers, lubricants, and flame retardants.

67. The process for manufacturing an adhesive composition according to claim 35, comprising a step of blending the HDPE grafted with a functional monomer with the LLDPE and the elastomer for a period of time from 1 to 20 minutes, preferably from 2 to 10 minutes at a temperature from 10 to l00°C, preferably from 20 to 50°C.

68. The process for manufacturing an adhesive composition according to claim 35, wherein melt-compounding is carried out at a temperature from 190 to 240°C, preferably from 220 to 230°C.

69. The process for manufacturing an adhesive composition according to claim 35, wherein the blending and melt-compounding is carried out using mixing equipment and an extruder.

70. Use of the adhesive polyethylene composition according to any of claims 1- 34 as an adhesive layer in multilayer coatings.

71. The use according to claim 70, wherein the coating is, in particular, multilayer insulation of tubes for constructing main gas- and oil pipelines.

72. A tube provided with multilayer insulation comprising an adhesive layer including the adhesive polyethylene composition according to any of claims 1-34.