CN1069038A - Ethylene polymer with high melt strength and its preparation method and use - Google Patents

Abstract

The present invention discloses normally solid, high molecular weight, gel-free. dense Irradiated ethylene polymer with a density of 0.89 to 0.97g/cc, which characterized by the development of high melt strength due to strain hardening and considered Strain hardening is caused by the free terminal long chains of the molecular chains that make up the polymer branch formed.

The invention also discloses a method for preparing polymers by By irradiating normal solids with high energy in a reducing active oxygen environment, high Molecular weight ethylene polymers, the irradiated material in the environment Hold for a specific period of time and then deactivate the free radicals in the material.

Description

The present invention relates to chemicals. In particular it relates to chemicals made from synthetic resins derived from alpha-olefins or 1-olefins, and in particular it relates to synthetic resins formed by the polymerization of ethylene alone or with other olefins.

The synthetic resin formed by polymerizing ethylene as the only monomer is called polyethylene. Although the term "polyethylene" has been used many times in the prior art for copolymers comprising ethylene and a small amount of another monomer such as 1-butene, the term is used here to exclude such a meaning.

Commercially available polyethylene, such as low-density polyethylene (LDPE) and high-density polyethylene (HDPE) and copolymers of ethylene and C _{3 to 10} α-olefins [commonly known as linear low-density ethylene (LLDPE)], usually Solid, soft thermoplastic polymers formed by polymerizing specific monomers by various methods known in the art. Such polymers are prepared, for example, by free-radical polymerization at high pressure or by low-pressure methods such as fluidized bed, gas phase techniques, and with molybdenum-based catalysts, chromium-based catalysts or Ziehler-Natta catalyst systems . High pressure processes produce polymers with long chain branches, and low pressure processes produce substantially linear polymers with controlled short chain branches. In the Ziegler-Natta (Ziegler-Natta) catalyst system, the catalyst is an inorganic compound of metals from Groups I to III of the Periodic Table (for example, aluminum alkyl) and a transition metal compound of Groups IV-VIII of the Periodic Table (for example, titanium halides) to prepare. Using the method of Wunderlick & Guar, J. Phys. Chem. Ref. Data, Vol. 10, No. 1 (1981), the typical degree of crystallinity is about 21-75% by weight. The typical melt index of the ethylene homopolymer or copolymer is 0.2-50 g/10 minutes (measured according to ASTM1238, condition E). Furthermore, the melting point of the crystalline phase of commercially available normally solid polyethylene is about 135°C.

While commercially available linear polyethylenes have many desirable properties, they lack melt strength. When molten, they are free of strain hardening (increased tensile resistance during stretching of the melt material). In this way, linear polyethylene has many disadvantages of melt processing, including the sudden formation of edge ripples when high-speed extrusion coating on paper or other substrates, matrix sagging and local thinning in hot melt molding, and the occurrence of thinning in laminated structures. Fluid instability in coextrusion. As a result, their potential applications in areas such as extrusion coating, blow molding, profile extrusion, and thermoforming are limited.

Some efforts have been made in the prior art to overcome the lack of melt strength of commercially available polyethylene.

The prior art has irradiated polyethylene, but this type of irradiation is initially irradiated on products made of polyethylene, such as films, fibers and sheets, and at high doses (i.e. greater than 2 Mrads to make polyethylene Cross-linking. For example, U.S. Patent 4,668,577 discloses cross-linking of polyethylene filaments, and U.S. Patents 4,705,714 and 4,891,173 disclose differential crosslinking of sheets made of high-density polyethylene. Crosslinking. Polyethylene crosslinked by these methods has been reported to have improved melt strength, reduced solubility and melt flow. However, this crosslinking unnecessarily reduces the elongation of the polyethylene melt, thereby limiting For stretch lengths with special requirements for film or fiber applications.

In U.S. Patent No. 3,563,870, attempts are made to improve the melt strength and melt elongation of polyethylene by exposing linear polyethylene to low dose levels of high energy radiation, i.e., 0.05 to 0.3 Mrad.

European Patent Application 047171 discloses irradiating ethylene polymer particles by thermally aging ethylene polymer particles at a dose of less than 1.5 Mrads by pretreating them with a steam atmosphere to reduce the oxygen content of the particles. The treated polymer is irradiated, and the irradiated polymer is then vaporized.

British Patent No. 2,019,412 relates to irradiation of linear low density polyethylene (LLDPE) films in the range of 2 to 80 Mrads to increase their elongation at break.

U.S. Patent Nos. 4,586,995 and 4,598,128 relate to a method for preparing ethylene polymers with long "Y" branches by heating the ethylene polymers under non-gelling, non-oxidative conditions to produce Formation of unsaturated ethylene end groups in ethylene polymers without unsaturated end groups, or increased unsaturated ethylene end groups in polyethylene containing unsaturated end groups, irradiating the heat-treated The ethylene polymer is then gradually or rapidly cooled to produce an irradiated polymer.

U.S. Patent No. 4,525,257 discloses irradiating narrow molecular weight, linear, low density ethylene/C _{3 ~ 18} α-olefin copolymers with a radiation dose of 0.05 to 2 Mrad (Mrad) to produce no glue. Coagulated cross-linked copolymers having a sufficient degree of cross-linking to provide the copolymers with higher extensional viscosities and substantially equivalent high-shear viscosities than corresponding uncross-linked polyethylenes. The irradiated copolymers were not deactivated to reduce or eliminate free radical intermediates.

One aspect of the present invention includes a generally normal solid, high molecular weight, gel-free irradiated polyethylene having a density of 0.89 to 0.97 g/cc, a molecular chain having a large number of free terminal long chain branches, branched When the conversion ratio is less than 1, it has sufficient strain-hardening extensional viscosity.

More broadly, the present invention includes a normally solid, gel-free, high molecular weight, irradiated ethylene polymer material having a branching ratio of less than 1 and having sufficient strain hardening extensional viscosity.

Polymers of the present invention have a reduced melt index (demonstrated by _I2 at 190°C, the decrease in melt index demonstrates an increase in molecular weight, and an increase in the melt index ratio ( _I10 / _T2 ) indicates a broadening of the molecular weight distribution and improved The melt tension and strain hardening elongation viscosity are guaranteed, and the branching ratio is less than 1.

The ethylene polymers of the present invention are prepared by low dose irradiation under controlled conditions.

The term "ethylene polymer material" as used herein means a random copolymer of (a) ethylene homopolymer, (b) ethylene and an α-olefin selected from the group consisting of C _3-10 α-olefins , having a polymerized alpha-olefin content of about 20 wt%, preferably about 16 wt%, (c) random terpolymers of ethylene and said alpha-olefin, provided that the polymerized alpha-olefin content is at a maximum of about 20 wt% (preferably about 16% by weight), the ethylene polymer material selected from the group thus formed. C _3-10 α-olefins include linear and branched α-branches. Such as propylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3,4-dimethyl-1-butene, 1-heptene, 3- Methyl-1-hexene, 1-octene and the like.

When the ethylene polymer is a homopolymer of ethylene, its typical density is 0.960 g/cm ³ or greater, and when the ethylene polymer is a copolymer of ethylene and C _{3 to 10} α-olefin, its density is typically 0.91 g /Cm ³ or more, but less than 0.94 g/Cm ³ . Suitable ethylene copolymers include ethylene/butene-1, ethylene/hexene-1, ethylene/octene-1 and ethylene/4-methyl-1-pentene. The ethylene copolymer can be high-density polyethylene (HDPE) or short-chain branched linear low-density polyethylene (LLDPE), and the ethylene homopolymer can be high-density polyethylene (HDPE) or low-density polyethylene (LDPE). Typical linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) have a density of 0.910g/cm ³ or as large as 0.940g/cm ³ , while high-density polyethylene (HDPE) has a density greater than 0.940g /Cm ³ , usually 0.950g/Cm ³ or greater. In general, ethylene polymer materials having a density of 0.89 to 0.97 g/cc are suitable for use in the present invention. Preferred ethylene polymers are linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) having a density of 0.89 to 0.97 g/cc.

In this patent application, the term "high molecular weight" means an average molecular weight of at least about 50,000.

The branching ratio quantifies the degree of long chain branching and in preferred embodiments the branching ratio is less than about 0.9, and most preferably about 0.2 to 0.8. It is defined by the following equation:

${\mathrm{<span\; class="notranslate">\; g</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; IV</span>}<span\; class="notranslate">\; )</span>}_{\mathrm{<span\; class="notranslate">\; Br</span>}}}{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; IV</span>}<span\; class="notranslate">\; )</span>}_{\mathrm{<span\; class="notranslate">\; the\; lin</span>}}}<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; mw</span>}$

Wherein g ' is the branching ratio, (Ⅳ) _Br is the intrinsic viscosity of the branched ethylene polymer material, and (Ⅳ) _Lin is the corresponding ethylene polymer material (that is, usually a normal solid, basically having the same average molecular weight The intrinsic viscosity of ethylene polymer materials, for copolymers and terpolymers, the corresponding molecular ratio of the elastomer unit is basically the same.

Intrinsic viscosity is known to define the viscosity number, which is generally considered to be a measure of the ability of a polymer molecule to increase the viscosity of a solution. It depends on the particle size and shape of the polymer being dissolved. Thus, a nonlinear polymer has the configuration of a nonlinear polymer molecule compared to a linear polymer having substantially the same average molecular weight. Indeed, the above ratio of intrinsic viscosities is a measure of the degree of branching of a nonlinear polymer. A method for determining the intrinsic viscosity of ethylene polymers is described in J. App. Poly Sci., 21, pp. 3331-3343 (1977).

The average molecular weight can be determined by various methods. The method used here, however, is laser light scattering photometry, as described by McConnell in Am.Lab, May 1978, entitled "Polymer molecular weight and polymer molecular weight distribution dispersed by low-angle laser light scattering" .

Extensional viscosity is the resistance of a fluid or semifluid substance to stretching. It is the melt property of thermoplastic materials, which is measured by the instrument when the tensile strain is applied at a certain rate, the stress and strain of the sample in the molten state. Such an apparatus is described in Munstedt, J. Rheology, 23(4), 421-425 (1979) and is shown in Fig. 1 . A similar commercially available instrument is the Rhonetrics RER-9000 Extensional Rheometer. When a molten, high molecular weight ethylene polymer material is elongated or stretched at a certain rate from a fixed point, it shows that its elongational viscosity tends to increase with the stretching rate over a certain distance, and then after it thins to no - the so-called ductile failure or necking failure followed by rapid reduction. On the other hand, the molten ethylene polymer material of the present invention is substantially the same as the average molecular weight of the molten high molecular weight ethylene polymer material as a contrast, and under the conditions of substantially the same test temperature, it is substantially the same from a fixed point. Stretching at a higher rate of elongation shows that its elongational viscosity tends to increase over longer distances and break or fail due to fracture - so-called brittle failure or elastic failure. These features indicate strain hardening. In fact, the more long chain branched ethylene polymer materials of the present invention have a greater propensity for an increase in extensional viscosity when stretched close to failure. This tendency is most pronounced when the branching ratio is less than about 0.8.

Melt tension is also indicative of the melt strength of the material. Melt tension is measured with a Gottfert Rheotens melt tensiometer from the Gottfert Company, which measures the tension (centitons) of molten ethylene polymer strands as follows: The tested polymer was extruded through a 20 mm long and 2 mm diameter capillary at 180°C; the wire was stretched using a stretching system with a continuous acceleration of 0.3 cm/ ^s2 . Measure the tension (centitons) produced by the stretching described above. Higher melt tension indicates larger melt strength values, which also indicate the strain hardening ability of a particular material.

Another aspect of the present invention is to provide a method for converting a normally solid, high molecular weight, ethylene polymer material into a normally solid, gel-free ethylene polymer material with a branching ratio of less than 1 and sufficient strain hardening elongation viscosity.

The method includes

(1) Irradiate the ethylene polymer material as described

(a) in said environment where the active oxygen concentration is maintained at less than 15% by volume of the environment, and

(b) high energy ionizing radiation at a dose rate of about 1 to ¹⁰⁴ Mrad/min for a period of time up to 2.0 Mrad, but not sufficient to cause the material to gel;

(2) maintaining the irradiated material so obtained in such an environment for a sufficient time to allow sufficient long chain branching to form; and

(3) The irradiated material is then treated in such an environment to deactivate any free radicals present in the irradiated material.

The ethylene polymer material treated according to the method of the present invention can be any normally solid, high molecular weight ethylene polymer material. Generally speaking, the intrinsic viscosity of the starting ethylene polymer material can indicate the molecular weight, and the intrinsic viscosity is generally 1-25, preferably 1-6, so that the intrinsic viscosity of the final product is 0.8-25 (compared to Preferably 1 to 3). However, ethylene polymer materials having intrinsic viscosities greater or less than typical are also within the broader scope of the invention. Ethylene polymer materials having a melt index of about 0.01 or higher can be used.

The ethylene polymer material treated by the process of the present invention in its broadest sense may be in any physical form, for example, microparticles, granules, pellets, films, sheets and the like. In preferred embodiments of the process of the present invention, ethylene polymer material in the form of granules or pellets gives satisfactory results. Spherical particles having an average diameter greater than 0.4 mm are preferred.

The active oxygen content of the environment in the three method steps is a critical factor. "Active oxygen" here means oxygen in a form capable of reacting with the irradiated material, preferably with free radicals in the material. It includes molecular oxygen (which is the oxygen normally found in air). The required active oxygen content requirement in the process of the present invention can be achieved by using a vacuum or by replacing some or all of the air in the environment with an inert gas such as nitrogen.

As-fabricated ethylene polymer materials are generally free of active oxygen at that time. Thus, the polymerization and polymer processing steps of the process according to the invention (when the ethylene polymer material is not exposed to air) are within the concept of the invention. However, since the ethylene polymer material is stored in the air or due to some other reason, the ethylene polymer material contains active oxygen in most cases. It follows that, in a preferred embodiment of the method of the present invention, the finely divided ethylene polymer material is first treated to reduce its active oxygen content. The preferred mode is to place the material in a bed filled with nitrogen in which the active oxygen content is equal to or less than 0.004% by volume. The residence time of the material in the bed should generally be at least 5 minutes to ensure effective removal of active oxygen from the interstices of the material particles, preferably long enough to allow the material to reach equilibrium with the environment.

Between the preparation and irradiation steps, the prepared ethylene polymer material should be kept in an environment with an active oxygen concentration of less than 15%, preferably less than 5%, and most preferably 0.004% by volume in a gas delivery system. ). Additionally, the temperature of the ethylene polymer material should be maintained above the glass transition temperature of the amorphous portion of the material, but not above about 70°C, preferably about 60°C.

During the irradiation step, the active oxygen concentration of the environment is preferably less than 5% by volume, more preferably less than 1% by volume. The best active oxygen concentration should be less than 0.004% (volume).

During the irradiating step, the source of ionizing radiation should have sufficient energy to penetrate the irradiated ethylene polymer body to the desired extent. This energy must be sufficient to ionize molecular structures and excite atomic structures, but not enough to bombard atomic nuclei. The ionizing radiation can be of any kind, the most practical include electron beam and gamma rays. Preferable is an electron stream emitted from an electron generator having an accelerating potential of 500 to 4,000 kV. For ethylene polymer materials, with about 0.2-2.0 Mrad, preferably 0.3～less than 2.0 Mrad, preferably 0.5～1.5 Mrad's low dose of ionizing radiation, generally about 1～2.0 Mrad A dosage rate of 10,000 Mrad/minute, preferably about 18 to 2,000 Mrad/minute is delivered, and satisfactory results are thus obtained.

The term "rad" is generally defined as the value of ionizing radiation that results in the absorption of 100 ergs of energy per gram of irradiated material, independent of the source of the radiation. For the purposes of the present invention, the energy absorbed by the ethylene polymer material when irradiated is generally variable. Energy absorption in ionizing radiation is prepared in general practice with a well-known conventional dosimeter (a measuring device in which a strip of fabric containing a radiation-sensitive dye is the energy absorption sensitive element). Thus, the term "rad" as used in this specification means the dose per gram of rad that results in placement on the surface of an ethylene polymer material, whether in the form of a bed or layer of granules or film or sheet, that is irradiated. The strip of fabric absorbs the amount of ionizing radiation equivalent to 100 ergs of energy.

The second step of the method of the present invention is generally carried out within about 1 minute to 1 hour, preferably within about 2 to 30 minutes. A minimum amount of time is required to ensure that the ethylene polymer segments partially migrate to radical sites and recombine to form complete chains, or to form long branches on the chain. Radical migration times of less than 1 minute, eg about half a minute, are within the broader aspects of the invention, but are not preferred since the resulting number of long chain branches with free ends is rather low.

The final step of the process, the radical deactivation or quenching step, can be carried out by heating, generally at least 60°C to 280°C, or by adding additives which have the effect of trapping free radicals, such as methyl mercaptan.

In an example method using heat, it includes melt extruding an irradiated ethylene polymer material. As a result, quenching of free radicals is essentially complete. In this instance, the irradiated ethylene polymer can be blended with other polymers, and additives such as stabilizers, pigments, fillers, if desired, before extruding or melting the composition. Alternatively, such additives may be added as a side stream component to the extruder for blending.

Another example of the process of the present invention using heat is by placing the irradiated ethylene polymer material in a fluidized bed or staged fluidized bed system where the fluidizing medium is such as nitrogen or other inert gas. The average residence time of the irradiated ethylene polymer in the fluidized bed is from about 5 minutes to about 120 minutes, preferably 20-30 minutes.

The resulting product is usually a normally solid, high molecular weight, gel-free ethylene polymer material which is characterized by strain hardening. The material is also characterized by a melt index ratio greater than 10.

Although the process of the present invention can be carried out batchwise, it is preferably carried out continuously. In the case of continuous operation, the finely divided ethylene polymer material is layered on a conveyor belt in the desired environment, with or without a preparation step, depending on the active oxygen content of the material. The thickness of the layer depends on the desired degree of penetration of the ionizing radiation into the layer and the proportion of irradiated ethylene polymer material in the final product. The speed of the conveyor belt is selected so that the layer of finely divided ethylene polymer material passes through the beam of ionizing radiation at a rate at which the desired dose of ionizing radiation is received. After the desired dose of ionizing radiation has been obtained, the irradiated layer is allowed to rest on the conveyor belt of said environment for a period of time to allow its free radicals to migrate and combine, and then exits the belt and enters the extruder, where Processing is carried out at the melting temperature of the irradiated material, or, in another particular embodiment, into a heated bed or staged heated bed, with nitrogen or other inert gas to fluidize the particles of the irradiated material. In another implementation, after deactivation of at least substantially all of its free radicals, such irradiated materials are placed at atmospheric pressure and rapidly cooled to room temperature. In another example, the irradiated ethylene polymer material is transferred from the belt under the desired environment into a storage container, the interior of the storage container has the desired environment, and the material is preserved in the container to free radicals. The time required for the migration to complete. The irradiated material then enters an extruder and is processed at the melting temperature of the irradiated material, or the ethylene polymer material pellets are placed in a heated, inert gas fluidized bed or staged fluidized bed to quench free After the base, the irradiated polyethylene was released into the atmosphere.

Another aspect of the invention includes the use of strain hardening of the ethylene polymer material of the invention in dilatant flow. Dilated flow occurs when an ethylene polymer material in a molten state is stretched in one or more directions at a rate faster than the normal flow rate. Expanded flow occurs in extrusion coating operations in which molten coating material is extruded onto a substrate of a moving base such as paper or metal sheet, the movement of the extruder or substrate The rate is higher than the extrusion rate. Dilated flow occurs in film production when molten film is extruded and then stretched to the desired thickness. Expanded flow exists in the heat-setting operation where the molten sheet is clamped on a plug die, a vacuum is applied and the sheet is pressed into the mold. This occurs in the manufacture of foamed articles in which molten ethylene polymer material expands with a blowing agent. The strain-hardened ethylene polymer material of the present invention is subjected to these or other melt processing (such as in fiber melt spinning) with part (such as as little as 0.5 wt%, as much as 95 wt%) or substantially all of the molten plastic material. Profile extrusion) are particularly useful when making useful articles.

The invention is further elucidated by the accompanying drawings and the following examples, which form part of the description.

Figure 1 is a flow diagram of a preferred example of a continuous process for converting, for example, normally solid polyethylene to normally solid, gel-free polyethylene with strain hardening.

Fig. 2 is the graph that the extensional viscosity of two samples of the high-density polyethylene of free terminal long-chain branch and high-density polyethylene reference thing made by the inventive method are plotted to elongation time.

The meanings of the symbols in Figure 2 are as follows:

DOW4352-0.05秒-1 ◆DOW4352-0.1秒-1

DOW4352-0.5秒-1 ◇DOW4352-1.0秒-1 ■DOW4352-2.0秒-1 □0.5MRAD-0.05秒-1 ▲0.5MRAD-0.1秒-1 △0.5MRAD-0.5秒 ■0.5MRAD-1.0秒-1 +0.5MRAD-2.0秒-1 1.5MRAD-0.05秒-1 ×1.5MRAD-0.1秒-1

1.5MRAD-0.5秒-1

1.5MRAD-1.0秒-1 ■1.5MRAD-2.0秒-1

DOW4352-0.05 seconds-1 DOW4352-0.1 seconds-1

DOW4352-0.5 seconds-1 ◇DOW4352-1.0 seconds-1 DOW4352-2.0 seconds-1 0.5MRAD-0.05 seconds-1 ▲0.5MRAD-0.1 seconds-1 △0.5MRAD-0.5 seconds 0.5MRAD-1.0 seconds-1 +0.5MRAD-2.0s-1 1.5MRAD-0.05sec-1 ×1.5MRAD-0.1sec-1

1.5MRAD-0.5sec-1

1.5MRAD-1.0s-1 1.5MRAD-2.0s-1

Figure 3 is a graph plotting extensional viscosity versus elongation time for free terminal long chain branched low density polyethylene samples and linear low density polyethylene (LLDPE) controls prepared by the process of the present invention.

The meanings of the symbols in Figure 3 are as follows:

LLDPE-0.5 sec-1

◆LLDPE-1.0 sec-1

LLDPE-2.0秒-1

LLDPE-2.0 sec-1

◇1.5 Mrad-0.5 sec-1

■1.5 Mrad-1.0 sec-1

□1.5 Mrad-2.0 sec-1

In more detail, Figure 1 depicts a fluidized bed unit 10 of conventional construction and conventional operation, into which finely divided, high molecular weight polyethylene is introduced through line 11, and nitrogen, active oxygen-free high molecular weight polyethylene, is introduced through line 13. The polyethylene is discharged through a solid discharge pipe 15 equipped with a solid flow rate controller 16 , and the solid discharge pipe 15 leads to a conveyor belt feed hopper 20 .

The conveyor belt feed hopper 20 is a covered structure of conventional design. During operation, its interior is filled with nitrogen. It has a solids outlet at the bottom through which polyethylene particles are removed and layered at the top level of the continuously running endless conveyor belt 21 .

The conveyor belt 21 is generally horizontal and runs continuously under normal operating conditions. It is contained in the irradiation chamber 22 . The chamber completely seals the conveyor belt and operates with a nitrogen atmosphere filled and maintained inside.

Connected to the irradiation chamber 22 is an electron flow generator 25 of conventional design and operation. Under normal operating conditions, the generator sends a beam of high energy electrons to the layer of polyethylene particles on the conveyor belt 21 . Below the discharge end of the conveyor belt there is a solids collector 28 which collects irradiated polyethylene particles falling from the conveyor belt 21 as the conveyor belt turns and moves in the opposite direction. The irradiated polyethylene particles are removed from the solids collector 28 by a rotary valve or star wheel 29 and transferred to a solids transfer line 30 .

The conveying line 30 leads to a gas-solid separator 31 . The unit is of conventional construction, often a cyclone type, in which the separated gas is removed through a gas discharge pipe 33 while the separated solids are discharged through a rotary valve or star wheel 32 into a solids discharge line 34 . Solids discharge line 34 may lead directly to extruder hopper 35 .

Extruder hopper 35, which feeds extruder 36, is of conventional construction and conventional operation. It is also enclosed within a structure that fills and maintains a nitrogen atmosphere inside. Extruder 36 is of conventional construction and operates in the normal manner. The solids in the extruder hopper 35 are fed to the extruder, which is operated at an extrusion rate such that the time period between irradiation of the polyethylene and its entry into the extruder is sufficient to form a sufficient amount of long-chain branches at the free ends. Therefore, when necessary, the volume of the extruder hopper 35 should be selected to ensure that the storage time of the hopper reaches the required value, thereby satisfying this condition. The extruder 36 is designed (extruder barrel and screw length) and operated so that the free radical-containing polyethylene has sufficient time therein to allow all of the Free radicals are essentially deactivated.

The thus treated, finely divided polyethylene is characterized by being substantially gel-free, having a density of 0.89-0.97 g/cc, and being well branched by free terminal long chains of ethylene units. It can be used as is or directly into the pelletizing and cooling unit 37 and out through the solids delivery line 38 as solid pellets which can be stored for later use or used without storage.

Similar results were obtained in other specific examples of processing high molecular weight, ethylene polymer materials according to the continuous process described above.

The following examples illustrate the high molecular weight polyethylenes of the present invention and preferred examples of methods for their preparation using the foregoing.

Melt indices, I ₂ and I ₁₀ , are measured according to ASTM D-1238, the melt index ratio being determined by dividing I ₁₀ by I ₂ . This ratio indicates the molecular weight distribution, the higher the ratio, the broader the molecular weight distribution.

Example 1

Soltex T50-200 polyethylene pellets have a melt index MI of 2 and a density of 0.95. They are placed in a closed irradiation chamber 22 and passed through nitrogen until the oxygen content is 40 ppm.

The material was distributed on a moving stainless steel conveyor belt 21 to form a bed of polyethylene powder 1.3 cm high and 15 cm wide. The powder bed is passed by a conveyor belt 21 through an electron beam from a 2 MeV Van de Graff generator operating at a current of 50 mA. The resulting absorbed surface dose was 0.40 Mrad. In addition, the active oxygen content in the environment or atmosphere in the closed irradiation chamber 22 and the remainder of the system consisting of the irradiated polyethylene transfer line 30, the gas-solid separator 31 and the separator discharge line 34 should be maintained. less than 140ppm.

After irradiation, the polyethylene falls from the end of the conveyor belt 21 , enters the conveyor belt discharge collector 28 , and enters the conveyor line 30 through the rotary valve 29 . After the gas has been separated from the irradiated polymer, the polymer is passed through the discharge line 34 of the separator into a bag filled with nitrogen.

The irradiated polyethylene was kept in the absence of oxygen at room temperature for 30 minutes. The material was placed into a nitrogen-filled extruder hopper and then extruded at 260°C in a 2.5" single screw extruder.

The properties of the gel-free final product of Example 1 are summarized in Table 1 .

Embodiment 2-4

Examples 2-4 were manufactured using the method and components of Example 1, except that the absorbed doses were 0.6, 0.9, and 1.0 Mrad in sequence. In the closed chamber 22, the irradiated polyethylene delivery line 30, the gas-solid separator 31 and the separator discharge line 34, the oxygen content is kept below 60 ppm. The properties of the gel-free final products of Examples 2-4 are summarized in Table I.

Embodiment 5-8

Except that the melt index MI of Dow04052N polyethylene pellets is 4, the density is 0.952, and the absorbed dose is successively 0.5, 0.7, 0.8 and 1.0 Mrad, the method and components of Example 5-8 are used to manufacture . The oxygen content in the enclosed irradiation chamber 22, irradiated polyethylene transfer line 30, gas-solid separator 31 and separator discharge line 34 was maintained below 100 ppm. The properties of the gel-free final products of Examples 5-8 are summarized in Table I.

Comparative Examples 1 and 2

Comparative Examples 1 and 2 are samples of unirradiated Soltex T50-200 and Dow 04052N polyethylene pellets, respectively.

Table Ⅰ

Example dose I ₁₀ I ₂ I ₁₀ /I ₂

(Mrad)

C-1 0 4.1 1.8 2.27

1 0.4 10.9 1.0 10.4

2 0.6 10.0 0.89 11.2

3 0.9 8.7 0.67 13.0

4 1.0 7.9 0.64 12.5

C-2 0 31.5 4.30 7.32

5 0.5 15.6 1.27 12.3

6 0.7 9.70 0.682 14.2

7 0.8 8.48 0.521 20.7

8 1.0 6.49 0.333 19.5

Embodiment 9 and comparative examples 3～8

Corresponding to the irradiation step (1) of the method, it is the critical state showing the irradiation atmosphere, corresponding to the second step (2) of the method, which is the free radical intermediate (RIA) at an oxygen content less than Aging in a controlled environment of 15% corresponds to the last step of the process according to the invention, which is the radical intermediate deactivation step (RID), with the following exceptions, Example 9 and Comparative Example 4 ～8 manufactures with the method for embodiment 1:

- replace Soltex T50-200 high-density polyethylene pellets with high-density polyethylene spherical pellets (produced by Italy Heymont (HIMONT) Co., Ltd.) with a melt index MI of 6.90;

- an absorbed dose of 1.1 Mrad;

-The processing environment of contrasting examples 3～8 changes as shown in Table II; And

- Radical deactivation in a 3/4" Brabender extruder at about 260°C.

The results are shown in Table II below

Table II

Ⅰ ₂ @ Ⅰ ₂ @ Ⅰ ₂ @

Example IA RIA RID 1 day 3 days 13 days

C-3 - - - 6.90 - -

C-4 N ₂ Air Air 3.56 3.13 3.23

C-5 N ₂ - - 2.44 3.67 4.58

C-6 Air Air Air 4.62 4.30 4.51

C-7 Air N ₂ N ₂ 4.39 4.14 4.13

C-8 Air - - 4.56 5.90 9.40

9 N ₂ N ₂ N ₂ 2.86 2.63 2.60

Comparative Examples 5 and 8 showed poor stability of melt flow rate after 13 days compared with Example 9 of the present invention. Comparative Examples 4, 6 and 7 have slightly better melt flow stability than Comparative Examples 5 and 8, but their melt flow rate is not as low as that of Example 9.

Embodiment 10-15 and comparative example 9 and 10

Embodiment 10-15 is manufactured with the method and component of embodiment 1, and wherein different place is: the ethylene polymer used among the embodiment 10-15 enters the 3/ 3/ that is filled with nitrogen through separator discharge line 34 In the hopper of the 4″Brabender extruder, 210°C in the extruder is the first zone, 215°C is the second zone, and the die head is 220°C, and then granulated by conventional methods. A Maddoch mixer is equipped The pellets were secondary extruded with 0.1% Irganox 1010 stabilizer on a 1-1/4" Killian single screw extruder at 100 rpm and 420F on a flat profile. The hold time between irradiation and extrusion averaged about 15 minutes. The properties of the final products of Examples 10-15 are summarized in Table III, and Examples 10, 12 and 14 and Comparative Examples 9 and 10 are illustrated in Figures 2 and 3 at 180°C and strain rates of 0.05-2.0 sec ^-1 The following is the extensional viscosity measured with Rheometrics RER 9000 extensional rheometer.

(1) Use the Gottfert Rheotens melt tensiometer produced by Gottfert Company to measure the tension (cN) of the molten ethylene polymer strand as follows: the polymer to be tested passes through a 20mm long and 2mm diameter capillary at 180°C While extruding; the strand is stretched using a stretching system with a constant acceleration of 0.3 cm/ ^sec . The higher the tension, the better the viscoelasticity and thus the better the processability of the polymer in the molten state.

(2) Intrinsic viscosity measured in decalin at 135°C.

(3) The average molecular weight measured by laser scattering with a Wyatt Dawn laser scattering instrument produced by Wyatt Technology Co., Ltd. in trichlorobenzene at 135°C.

(4) Branching ratio, [IV] _Lin = 2.77×10 ^-4 Mw ^0.25 .

(5) High density polyethylene containing 0.5% of propylene monomer units, manufactured by Dow Chemical Company, which is a pellet having a melt index (MI) of 3.9.

(6) Linear low-density polyethylene (LLDPE) contains 6.1% butene-1 monomer unit, which is mixed with 200 parts/percent of Irganox1076 stabilizer. It is a pellet with a melt index MI of 7.9, produced by Italy Monte (HIMONT ) Co., Ltd. production.

The results in the table above show that the ethylene polymers of the present invention have increased ratios of melt index indicating increased molecular weight distribution, increased melt tension, increased melt strength and branching which indicates formation of long chain branches.

The results shown in Figures 2 and 3 show that the extensional viscosity of the ethylene polymers of the present invention is indicative of the strain hardening capability of the material. The trend is that the strain hardening elongational viscosity increases with decreasing branching ratio, ie with increasing degree of long chain branching.

The free terminal long chain branched ethylene polymers of the present invention can be used in melt processing to form useful articles such as foamed and thermoformed articles such as foam sheet and thermoformed sheet, extrusion coated articles and fibers, etc. . In fact, the strain hardened ethylene polymer materials of the present invention are useful in all melt processing applications where a high molecular weight, high melt strength ethylene polymer material is desired.

The foregoing disclosure makes other features, advantages, and examples of the invention apparent to those skilled in the art. In this regard, now that specific examples of the invention have been described in detail, changes and modifications may be made to these specific examples without departing from the spirit and scope of the specification and claims.

The term "consisting essentially of" in this specification excludes such unstated substances in concentrations sufficient to substantially affect the essential characteristics and properties of the compositions defined herein, however it allows one or Presence of unrepresented quantities of such substances in concentrations insufficient to materially affect the stated essential properties and characteristics.

Claims (16)

CLAIMS 1. A normally solid, high molecular weight, gel-free irradiated ethylene polymer material having a branching ratio of less than 1, characterized in that it has a strain hardening elongational viscosity. 2. A normally solid, high molecular weight, gel-free irradiated ethylene polymer material having a branching ratio of less than 1 and a density of 0.89 to 0.97 g/cc, characterized in that it has a strain hardening elongational viscosity. 3. The ethylene polymer material according to claim 2, characterized in that the material consists essentially of a high molecular weight ethylene polymer having a density greater than 0.94 g/cc and a branching ratio of less than 0.9. 4. The ethylene polymer material of claim 3, wherein the branching ratio is about 0.2 to 0.8. 5. The ethylene polymer material according to claim 2, characterized in that the material consists essentially of a high molecular weight ethylene polymer having a density of less than 0.94 g/cc and a branching ratio of less than 0.9. 6. The ethylene polymer material according to claim 5, wherein the branching ratio is 0.2-0.8. 7. The ethylene polymer material of claim 2 wherein the ethylene polymer is in particulate form. 8. A normal solid, high molecular weight, gel-free, irradiated ethylene polymer material with a density of 0.89 to 0.97 g/cc and no strain hardening elongation viscosity A method for an ethylene polymer material, characterized in that the method comprises: (1) Irradiate the ethylene polymer material as described (a) in said environment where the active oxygen concentration is maintained at less than 15% by volume of the environment, and (b) high energy ionizing radiation at a dose rate of about 1 to 1 x ¹⁰⁴ Mrad/min for a period of time up to 2.0 Mrad, but not sufficient to cause the material to gel; (2) maintaining the irradiated material so obtained in such an environment for a sufficient time to allow sufficient long chain branching to form; and (3) The irradiated material is then treated in such an environment to deactivate any free radicals present in the irradiated material. 9. The method of claim 8, wherein said ethylene polymer material is maintained in said reducing active oxygen environment prior to irradiation. 10. The method of claim 8 wherein the active oxygen content of said environment is less than about 0.004% by volume. 11. The method according to claim 8, wherein the absorbed dose of high-energy ionizing radiation is 1-12 Mrads. 12. The method of claim 8, wherein the time of step (2) is about 1 minute to 1 hour. 13. A thin film composition consisting essentially of a normal solid, high molecular weight, gel-free, irradiated ethylene polymer with a density of 0.89 to 0.97 g/cc and a branching ratio of less than 1, characterized in that The material has a strain hardening extensional viscosity. 14. A thin film substantially composed of normal solid, high molecular weight, gel-free, density 0.89-0.97 g/cc, branching ratio less than 1 irradiated ethylene polymer, characterized in that the material Has a strain hardening extensional viscosity. 15. A process for the preparation of blown film by extruding an ethylene composition into a tube which is subsequently blown into a bubble, said composition consisting essentially of a normally solid, high molecular weight, gel-free, density of 0.89 ~ 0.97g/cc, branching rate of less than 1 irradiated ethylene polymer material, characterized in that the material has a strain hardening extension viscosity. 16. A useful article consisting of a composition of ethylene polymers consisting of a mass of normally solid, high molecular weight, gel-free, irradiated ethylene polymer material, characterized in that the material has a strain hardening extensional viscosity.

Images