Modified low density polyethylene resin and process for its preparation

Abstract

If the starting LDPE resin and the irradiation level are properly selected, irradiation of a low density polyethylene resin with electron beams produces a modified polyethylene resin having significantly improved melt strength and retaining a useful melt index, and having little crosslinked gel.

Description

Technical Field

[0001] This disclosure relates to low-density polyethylene resin and methods for modifying it to improve its physical properties. Background Technology

[0002] Low-density polyethylene (LDPE) resin and its preparation methods are well known and described in numerous publications, such as the following patents: US 7,741,415 B2; US 8,415,442 B2; US 8,729,186 B2; US 8,871,887 B2; US9,120,880 B2; US10,435,489 B2; US 10,465,024 B2; US10,494,457 B2; WO 2010 / 144784; WO2011 / 019563; WO 2012 / 082393; WO 2009 / 114661; US ​​8,916,667; US 9,303,107; and EP2239283B1.

[0003] LDPE resin (also known as "high-pressure ethylene polymer" or "hyperbranched polyethylene") is an ethylene polymer prepared by free radical polymerization under high pressure (≥100 MPa (e.g., 100 MPa-400 MPa)). The density of LDPE resin is typically around 0.915 g / cm³. 3 Up to 0.935 g / cm 3 Within the range.

[0004] Two distinct types of high-pressure radical-initiated polymerization processes are known. In the first type, a stirred autoclave vessel with one or more reaction zones is used. The autoclave reactor typically has several injection points for initiator or monomer feed, or both. In the second type, a jacketed tube is used as a tubular reactor with one or more reaction zones. Suitable, but not limiting, reactor lengths can range from 100 m to 3000 m or from 1000 m to 2000 m. The start of the reactor's reaction zone is typically defined by a lateral injection of the initiator, ethylene, chain transfer agent (or telomer), comonomer, or any combination thereof. The high-pressure process can also be carried out in an autoclave or tubular reactor with one or more reaction zones, or in a combination of an autoclave and a tubular reactor, each comprising one or more reaction zones.

[0005] Chain transfer agents can be used to control molecular weight. In a preferred embodiment, one or more chain transfer agents (CTAs) can be added to the polymerization process. Typical CTAs include, but are not limited to, propylene, isobutane, n-butane, 1-butene, methyl ethyl ketone, acetone, and propionaldehyde.

[0006] LDPE resin is used in many common thermoplastic manufacturing processes to produce useful articles, including single-layer and multi-layer films; molded articles, such as blow-molded, injection-molded, cast, or rotationally molded articles; coatings; fibers; and woven or nonwoven fabrics. Films include extruded coatings, food packaging, consumer goods, industrial (applications or films), agricultural (applications or films), laminated films, fresh-cut manufacturing films, cast films, blown films, thermoformed films, meat films, cheese films, confectionery films, transparent shrink films, finishing shrink films, stretch films, silage films, greenhouse films, fumigation films, liner films, stretch covers, heavy-duty transport bags, pet food, interlayer bags, sealing layers, and diaper backing sheets.

[0007] LDPE resin can also be used in wire and cable coating operations, sheet extrusion and molding of articles for vacuum forming operations (including using injection molding, blow molding or rotational molding processes), soft-touch articles (such as appliance handles), gaskets and profiles, automotive interior parts and profiles, foam articles (open-cell and closed-cell), and as an impact modifier for other thermoplastic polymers (such as high-density polyethylene).

[0008] LDPE resins used in these applications advantageously possess high melt strength, high shear thinning, and a relatively low melt index to provide good processability. Particularly in blends, LDPE typically increases the flexibility and processability of the blend, while HDPE or LLDPE increases stiffness and strength.

[0009] For example, blown film production lines are often limited by bubble stability in terms of output. Blending LDPE with linear low-density polyethylene (LLDPE) increases bubble stability, partly due to the higher melt strength of LDPE. LDPE resins with higher melt strength can be used in smaller quantities in extrusion blends and / or allow for faster film production. However, excessively high melt strength can lead to gelation and poor film quality. Furthermore, some high melt strength LDPE resins often have low melt index and very low shear thinning, making them more difficult to process. Therefore, new ethylene-based polymers, such as LDPE, are needed where the balance of melt strength, melt index, and rheological properties is optimized.

[0010] Numerous post-processing methods are known to induce crosslinking or the formation of long branches in polyethylene (HDPE, LLDPE, and LDPE). Examples of known post-processing techniques include treatment with oxygen, free radical initiators, high-energy electromagnetic radiation, and electron beams. Examples of post-processing techniques are described in the following U.S. patents and patent applications: US 4,586,995; US 7,094,472 B2; US 7,892,446 B2; US 10,844,210 B2; US2014 / 0342141A1; US2019 / 0100644A1; and are described in the following PCT disclosure: WO 2010 / 009024A2, and in the following paper: Ono et al., Gamma Irradiation Effects in Low Density Polyethylene, 2011 International Nuclear Atlantic Conference (October 24-28, 2011). In the case of LDPE resins, post-treatment research has focused more on crosslinking than on long-chain branching, because LDPE resins are already highly long-chain branched. The aim is to identify specific resins and treatment options that can provide improved properties for the intended use of the resin. Summary of the Invention

[0011] We have found that, with proper selection of the starting LDPE resin and irradiation level, electron beam irradiation of low-density polyethylene (LDPE) resin produces modified polyethylene resins with significantly improved melt strength, while maintaining a useful melt index, good processability, and minimal crosslinking gel.

[0012] One embodiment of the present invention is a method for modifying LDPE resin, the method comprising the following steps:

[0013] a. Providing a starting LDPE resin, the starting LDPE resin having:

[0014] i.0.91g / cm 3 Up to 0.94 g / cm 3 The density;

[0015] ii. Melt index (I2) from 5 dg / min to 18 dg / min; and

[0016] iii. At least 6 conventional molecular weight distributions (M w(Conv) / M n(Conv) );as well as

[0017] b. Irradiating the initiating polyethylene resin with an electron beam to provide a dose that effectively delivers the modified polyethylene resin, which has the following properties:

[0018] i. A melt index (I²) of at least 1 dg / min; and

[0019] ii. A conventional molecular weight distribution of at least 10 (M w(Conv) / M n(Conv) );as well as

[0020] iii. Melt strength of at least 15 cN; and

[0021] iv. At least 95% GPC quality recovery rate.

[0022] A second embodiment of the present invention is an LDPE resin having the following characteristics:

[0023] a.0.91g / cm 3 Up to 0.94 g / cm 3 The density;

[0024] b. Melt index (I2) from 1.5 dg / min to 6 dg / min; and

[0025] c. Typical molecular weight distribution from 10 to 20 (M w(Conv) / M n(Conv) );as well as

[0026] d. Melt strength of at least 25 cN; and

[0027] e. At least 95% GPC quality recovery rate.

[0028] A third embodiment of the present invention is an article comprising a modified polyethylene formulation.

[0029] This application describes several characteristics of starting and modified LDPE resins, such as density, melt index, conventional and absolute molecular weight, and various branching and rheological measurements. In each case, the described characteristics were measured using the test methods listed in the "Test Methods" section of this application. References to measured characteristics should be considered as meaning those measured using the listed test methods. Alternative test methods may sometimes produce different results. Detailed Implementation

[0030] In the method of the present invention, the starting LDPE resin is subjected to electron beam irradiation.

[0031] The starting LDPE resin and modified LDPE resin in this invention are polyethylene polymers. As used herein, the term "polymer" refers to a polymer compound prepared by polymerizing monomers of the same or different types. The general term polymer therefore includes both homopolymers and interpolymers as defined below. Polyethylene homopolymers contain repeating units derived almost exclusively from ethylene, where it should be understood that small amounts of impurities (such as chain transfer agents) may be incorporated into the polymer structure. The impurities preferably constitute less than 1% by weight of the homopolymer, more preferably less than 0.5% by weight, and most preferably less than 0.3% by weight. Polyethylene interpolymers are polymers prepared by polymerizing ethylene monomers with at least one monomer of a different type. The general term interpolymer includes ethylene copolymers (used to refer to polymers prepared from ethylene and one other comonomer) and polymers prepared from ethylene and two or more comonomers. Polyethylene interpolymers may also contain small amounts of impurities, such as chain transfer agents, which may be incorporated into the polymer structure. In the polyethylene interpolymers used in this invention, preferably at least 50% by weight of the repeating units are derived from ethylene monomers. The starting LDPE resin and modified LDPE resin are more preferably ethylene homopolymers.

[0032] Starting LDPE resin

[0033] The initial LDPE resin has the following characteristics: (i) 0.91 g / cm³ 3 Up to 0.94 g / cm 3 (ii) density; (iii) melt index (I2) of 5 dg / min to 18 dg / min; and (iv) conventional molecular weight distribution of at least 6. w(Conv) / M n(Conv) ).

[0034] The initial density of LDPE resin was 0.91 g / cm³. 3 Up to 0.94 g / cm 3 The density of the initial LDPE resin is preferably at least 0.912 g / cm³. 3 More preferably at least 0.915 g / cm³ 3 And the optimal value is at least 0.917 g / cm³. 3 The initial density of the LDPE resin is preferably at most 0.935 g / cm³. 3 More preferably up to 0.930 g / cm³ 3 And the optimal value is at most 0.925 g / cm³. 3 .

[0035] The melt index (I2) of the initial LDPE resin component is in the range of 5 dg / min to 18 dg / min. The melt index is preferably at least 6 dg / min, more preferably at least 6.5 dg / min, and most preferably at least 7 dg / min. The melt index is preferably at most 17.5 dg / min, and more preferably at most 17 dg / min.

[0036] The molecular weight of LDPE resin can be measured using two different methods: (1) the “conventional” or “relative” GPC method; and (2) the “absolute” method. For polymers with a high degree of long-chain branching, such as LDPE resin, the absolute method typically yields a larger molecular weight than the conventional method.

[0037] The conventional number-average molecular weight (M) of the starting LDPE resin n(conv) Preferably, it is at least 7,000 g / mol, more preferably at least 12,000 g / mol, and most preferably at least 13,000 g / mol. The conventional number-average molecular weight (M) of the starting LDPE resin... n(conv) Preferably, the concentration is up to 30,000 g / mol, more preferably up to 25,000 g / mol, more highly preferably up to 18,000 g / mol, and most preferably up to 16,000 g / mol.

[0038] The conventional weight-average molecular weight (M) of the starting LDPE resin w(conv) Preferably, the molecular weight is at least 35,000 g / mol, more preferably at least 45,000 g / mol, and most preferably at least 100,000 g / mol. The conventional weight-average molecular weight (M) of the starting LDPE resin... w(conv) Preferably, the concentration is up to 300,000 g / mol, and more preferably up to 180,000 g / mol.

[0039] The starting LDPE resin has a conventional molecular weight distribution of at least 6 (M w(conv) / M n(conv) The conventional molecular weight distribution of the starting LDPE resin is preferably at least 7, more preferably at least 7.5, and most preferably at least 8. The conventional molecular weight distribution of the starting LDPE resin is preferably at most 13, more preferably at most 12, and most preferably at most 11.

[0040] The absolute weight-average molecular weight (M) of the starting LDPE resin w(Abs) The absolute weight-average molecular weight (M) of the starting LDPE resin is preferably at least 100,000 g / mol, and more preferably at least 270,000 g / mol. w(Abs)) Preferably, the concentration is up to 750,000 g / mol, more preferably up to 500,000 g / mol, and most preferably up to 450,000 g / mol.

[0041] For the starting LDPE resin, the ratio of the absolute molecular weight of the starting LDPE resin to the conventional molecular weight (M) w(Abs) / M w(Conv) Preferably at least 1.5, more preferably at least 2.0, and most preferably at least 2.2. The ratio of the absolute molecular weight of the starting LDPE resin to the conventional molecular weight (M...) w(Abs) / M w(Conv) The value is preferably up to 5, more preferably up to 3.5, and most preferably up to 2.8.

[0042] Long-chain branching in polymers is also characterized by several different measurements. Branching measurements described in the following test methods include: branching index (g'), long-chain branching frequency (LCBf), and GPC branching index (gpcBR).

[0043] The long-chain branching frequency (LCBf) of the starting LDPE resin is preferably at least 0.5, more preferably at least 1.0, and most preferably at least 1.2. The long-chain branching frequency (LCBf) of the starting LDPE resin is preferably at most 5.0, more preferably at most 3.5, and most preferably at most 3.0.

[0044] The GPC branching index (gpcBR) of the starting LDPE resin is preferably at least 0.5, more preferably at least 1.5, and most preferably at least 1.6. The GPC branching index (gpcBR) of the starting LDPE resin is preferably at most 6, and more preferably at most 4.

[0045] The melt strength of the initial LDPE resin at 190°C is preferably at most 20 cN, more preferably at most 10 cN, and most preferably at most 8 cN. The ratio of the melt strength (in cN) to the melt index (in dg / min) of the initial LDPE resin is preferably at least 0.1. The ratio of the melt strength (cN) to the melt index (dg / min) of the initial LDPE resin is preferably at most 10, more preferably at most 5, and most preferably at most 1.

[0046] Preferably, the gel content in the starting LDPE resin has been minimized. (The gel is a cross-linked polymer insoluble in trichlorobenzene, decahydronaphthalene, or xylene, or a highly entangled high molecular weight polymer chain that is not easily dissolved). The gel content is conveniently measured by measuring the amount of resin recovered via gel permeation chromatography (GPC recovery), as described in the test method. A higher resin recovery rate corresponds to a lower gel content. The GPC recovery rate of the starting LDPE resin is preferably at least 95 percent, more preferably at least 97 percent, more highly preferably at least 99 percent, and most preferably at least 99.5 percent. There is no maximum preferred GPC recovery rate; the GPC recovery rate can be substantially 100 percent.

[0047] The viscosity ratio is the ratio of the resin viscosity under low shear conditions (0.1 rad / s) to the resin viscosity under high shear conditions (100 rad / s), both at a temperature of 190°C. The viscosity ratio of the initial LDPE resin is preferably at least 1, more preferably at least 4, and most preferably at least 5. The viscosity ratio of the initial LDPE resin is preferably at most 20, more preferably at most 10, and most preferably at most 9.

[0048] The tangent of the phase angle δ (tan-δ) is a viscoelastic measurement indicating the ratio of the loss modulus (G”) to the storage modulus (G’) at 190°C under low shear conditions (0.1 rad / s). The tan-δ of the starting LDPE resin at 0.1 rad / s is preferably at least 3, more preferably at least 5, and most preferably at least 6. The tan-δ of the starting LDPE resin is preferably at most 50, more preferably at most 10, and most preferably at most 8.

[0049] The starting LDPE resin can be a single polymer or a blend of two or more polymers. If it is a blend, the preferred embodiments described above apply to the individual polymer components. Preferably, the starting LDPE resin is a single polymer. The starting LDPE resin may optionally contain common additives such as antistatic agents, color enhancers, dyes, lubricants, fillers, pigments, primary antioxidants, secondary antioxidants, processing aids, UV stabilizers, nucleating agents, slip agents (such as erucamide), anti-blocking agents (such as talc), and combinations thereof. Preferably, the additives in the starting LDPE resin do not interfere with the formation of long-chain branches. More preferably, the starting LDPE resin is substantially additive-free.

[0050] The starting LDPE resin is commercially available or can be prepared by known methods as described above in the Background section. The starting LDPE resin is preferably prepared by free radical polymerization of ethylene monomers and optionally comonomers at temperatures from 180°C to 350°C and pressures from 14,500 psi to 58,000 psi (100–400 MPa). The polymerization is initiated by common free radical initiators, such as organic peroxide initiators. The chain length can be controlled by adding chain transfer agents, such as butane, isobutane, butene, propylene, propionaldehyde, or methyl ethyl ketone. The reactor system may contain one or more autoclaves or high-pressure tubular reactors. However, the starting LDPE resin of this invention has a wider molecular weight distribution than is common for LDPE resins, and LDPE resins with a wide molecular weight distribution are more typically prepared in autoclave reactors. If a tubular reactor system is used, the conditions should be adjusted to produce the desired molecular weight distribution.

[0051] The starting LDPE resin is preferably in powder, granule, or pellet form, and more preferably in pellet form. The pellets typically have 10–60 pellets / gram.

[0052] Electron beam modification

[0053] In the method of this invention, the starting LDPE resin is modified by electron beam irradiation. In an unconstrained manner, we hypothesize that when the electron beam enters the polymer, it ionizes and excites the molecules, leading to the displacement of hydrogen atoms and the formation of free radicals. The combination of two free radicals forms long branches. These additional long branches increase melt strength. We further hypothesize that the radiation dose should be high enough to initiate long-chain branching, but low enough to avoid the formation of a highly cross-linked network (which is a gel).

[0054] Electron beam radiation sources are known and commercially available. The electron beam is preferably emitted from a linear electron beam accelerator. Typically, the electron beam is emitted from a heated cathode filament (usually tungsten). In a linear accelerator, electrons emitted from the cathode are accelerated in an electric field applied between the cathode and anode. The energy gain of the electron beam is proportional to the accelerating voltage. Energy is measured in electron volts (eV), and accelerators up to 12 MeV are commercially available. The electron beam dose absorbed by matter is measured in megarads (MRad, 1 Rad = 0.01 Gy = 0.01 J / kg).

[0055] The irradiation level should be selected to achieve the following results:

[0056] i. A melt index (I²) of at least 1 dg / min; and

[0057] ii. A conventional molecular weight distribution of at least 10 (M w(Conv) / M n(Conv) );as well as

[0058] iii. Melt strength of at least 20 cN; and

[0059] iv. 95% or higher GPC quality recovery rate

[0060] The starting LDPE resin preferably accepts an average dose of at least 0.2 MRad, more preferably at least 0.25 MRad, more highly preferably at least 0.4 MRad, and most preferably at least 0.45 MRad. The starting LDPE resin preferably accepts an average dose of at most 1.25 MRad, more preferably at most 1 MRad, and most preferably at most 0.8 MRad.

[0061] Typically, if the irradiation is too low, the desired melt strength cannot be achieved in the modified LDPE resin. If the irradiation is too high, the melt index of the modified LDPE resin will be too low and the GPC recovery rate will be too low.

[0062] To achieve the desired irradiation level, a linear electron beam accelerator preferably has the following characteristics.

[0063] The linear electron beam accelerator preferably operates at a beam energy range of at least 2 MeV, more preferably at least 3 MeV, and most preferably at least 4 MeV. The linear electron beam accelerator preferably operates at an energy range of up to 12 MeV, more preferably up to 8 MeV, and most preferably up to 5 MeV.

[0064] The electron beam power depends on the beam energy and beam current. The electron beam power is preferably at least 20 kW, more preferably at least 30 kW, and most preferably at least 75 kW across the entire energy range. The electron beam power is preferably at most 350 kW, more preferably at most 200 kW, and most preferably at most 175 kW across the entire energy range.

[0065] The electron beam penetration depth of the initial LDPE resin during irradiation is preferably shallow enough to allow all initial LDPE resins to receive a uniform and desired dose of electron beam radiation. Without being bound by any theory, the electron beam penetration depth depends on the density of the LDPE and the beam energy (MeV). For example, it is preferable to use a 4.5 MeV beam to irradiate the initial LDPE resin to a penetration depth of up to 6 cm, more preferably up to 4.5 cm, and most preferably up to 3.5 cm, to ensure that all initial LDPE resins receive sufficient and uniform radiation exposure.

[0066] Irradiation is preferably carried out in a vacuum, air, or inert atmosphere. Irradiation is more preferably carried out in air. Electron beam irradiation can be carried out in batch or continuous processes. A continuous process is preferred, in which the initial LDPE resin is transported on a belt and exposed to an electron beam curtain.

[0067] The optimal irradiation time depends on the intensity of the electron beam source (beam energy, current, and beam power). A person of ordinary skill can easily determine the optimal irradiation time experimentally based on the polymer and the equipment used with it.

[0068] It is hypothesized that similar results could be obtained by irradiating with higher-energy electromagnetic radiation (such as X-rays or gamma rays) at the same dose level. However, this process has not been investigated due to practical difficulties in obtaining suitable sources.

[0069] Modified LDPE resin

[0070] The product of irradiation is modified LDPE resin. The irradiation process does not substantially alter the following characteristics of the starting LDPE resin, and therefore the following limitations and preferred embodiments of the modified LDPE resin after irradiation are the same as those of the starting PE resin: density, monomer and comonomer content, single polymer or polymer blend, additive content, and physical form (powder, granules, or pellets).

[0071] The melt index (I2) of the modified LDPE resin is preferably at least 1.0 dg / min, more preferably at least 1.5 dg / min, and most preferably at least 2 dg / min. The melt index of the modified LDPE resin is preferably at most 10 dg / min, more preferably at most 6 dg / min, and most preferably at most 3 dg / min.

[0072] The melt index (I2) of the modified LDPE resin is preferably at least 10% and more preferably at least 20% of the melt index of the starting LDPE resin. The melt index (I2) of the modified LDPE resin is preferably at most 60% of the melt index of the starting LDPE resin, and more preferably at most 30% of the melt index of the starting LDPE resin.

[0073] Conventional number-average molecular weight (M) of modified LDPE resin n(conv) Preferably, the number-average molecular weight (Mn) of the modified LDPE resin is at least 7,000 g / mol, more preferably at least 9,000 g / mol, and most preferably at least 13,000 g / mol. n(Conv) Preferably, the concentration is up to 30,000 g / mol, more preferably up to 19,000 g / mol, and most preferably up to 15,500 g / mol.

[0074] Conventional weight-average molecular weight (M) of modified LDPE resin w(conv) The preferred molecular weight is at least 45,000 g / mol, more preferably at least 100,000 g / mol, and most preferably at least 120,000 g / mol. The conventional weight-average molecular weight of the modified LDPE resin is preferably at most 400,000 g / mol, more preferably at most 300,000 g / mol, and most preferably at most 265,000 g / mol.

[0075] Conventional molecular weight distribution of modified LDPE resin (M w(Conv) / M n(Conv) The molecular weight distribution of the modified LDPE resin is preferably at least 12, and more preferably at least 14. The molecular weight distribution of the modified LDPE resin is preferably at most 25, more preferably at most 20, and most preferably at most 18.

[0076] Absolute weight-average molecular weight (M) of modified LDPE resin w(Abs) The molecular weight of the modified LDPE resin is preferably at least 100,000 g / mol, more preferably at least 200,000 g / mol, and most preferably at least 350,000 g / mol. The absolute weight-average molecular weight of the modified LDPE resin is preferably at most 2,500,000 g / mol, more preferably at most 1,700,000 g / mol, and most preferably at most 1,250,000 g / mol.

[0077] For modified LDPE resins, the ratio of absolute weight-average molecular weight to conventional weight-average molecular weight (M... w(Abs) ) / (M w(Conv) Preferably, it is at least 1.6, more preferably at least 1.8, and most preferably at least 3.5. For modified LDPE resins, the ratio of absolute weight-average molecular weight to conventional weight-average molecular weight (M... w(Abs) ) / (M w(Conv) Preferably, the number is at most 12, more preferably at most 8, and most preferably at most 5.

[0078] The long-chain branching frequency (LCBf) of the modified LDPE resin is preferably at least 0.6, more preferably at least 1.0, more highly preferably at least 3.5, and most preferably at least 5. The long-chain branching frequency (LCBf) of the modified LDPE resin is preferably at most 10, more preferably at most 8.0, and most preferably at most 7.6.

[0079] The preferred objective of the modification process is to increase the long-chain branching in the starting LDPE resin. The long-chain branching frequency (LCBf) of the modified LDPE resin is preferably at least 20% higher than that of the starting LDPE resin, more preferably at least 50% higher, and most preferably at least 100% higher. The long-chain branching frequency (LCBf) of the modified LDPE resin is preferably up to 300% higher than that of the starting LDPE resin.

[0080] The GPC branching index (gpcBR) of the modified LDPE resin is preferably at least 0.6, more preferably at least 0.8, more highly preferably at least 2.0, and most preferably at least 3.5. The GPC branching index (gpcBR) of the modified LDPE resin is preferably at most 12, more preferably at most 10, and most preferably at most 8.

[0081] The modified LDPE resin preferably has a melt strength of at least 15 cN, more preferably at least 20 cN, and most preferably at least 25 cN at 190°C. The melt strength is preferably at most 35 cN, and more preferably at most 32 cN.

[0082] One objective of the modification process is to increase the melt strength of the starting LDPE resin. The melt strength of the modified LDPE resin at 190°C is preferably at least 10 cN higher than that of the starting LDPE resin, more preferably at least 15 cN higher, more preferably at least 20 cN higher, and most preferably at least 25 cN higher. The melt strength of the modified LDPE resin is preferably up to 45 cN higher than that of the starting LDPE resin, more preferably up to 35 cN higher, and most preferably up to 30 cN higher. Typically, tubular reactor systems can produce LDPE resin at higher capacities and ethylene conversion rates, but LDPE resin prepared in autoclave reactor systems has higher melt strength. In one embodiment of the invention, the starting LDPE resin is a product of a tubular reactor system; however, this modification process can impart to it a melt strength similar to or even better than that of conventional LDPE resin prepared in an autoclave reactor system.

[0083] The viscosity ratio of the modified LDPE resin at 190°C is preferably at least 5, more preferably at least 9, and most preferably at least 12. The viscosity ratio is preferably at most 30, more preferably at most 25, and most preferably at most 18. The viscosity ratio of the modified LDPE resin is preferably at least 10% higher than that of the starting LDPE resin, more preferably at least 20%, and most preferably at least 25%. This change in viscosity ratio indicates that the modified LDPE resin can form a more stable film at a higher production rate in blown film production.

[0084] The tan-δ of the modified LDPE resin at 190°C and 0.1 rad / s is preferably at least 1, and more preferably at least 2. The tan-δ is preferably at most 10, more preferably at most 5, and most preferably at most 3. The tan-δ of the modified LDPE resin is preferably at most 65 percent of the tan-δ of the starting LDPE resin, and more preferably at most 50 percent. A lower tan-δ of the modified LDPE resin indicates improved elasticity.

[0085] The ratio of melt strength (cN) to melt index (dg / min) of the modified LDPE resin is preferably at least 1, more preferably at least 3, and most preferably at least 10. The ratio of melt strength (cN) to melt index (dg / min) of the modified LDPE resin is preferably at most 30, and more preferably at most 20.

[0086] One objective of the modification process is to limit gel formation in the modified LDPE resin. The gel content of the modified LDPE resin is preferably less than 3% by weight, more preferably less than 2.8% by weight, more highly preferably less than 2% by weight, and most preferably less than 1% by weight. In many cases, the gel content of the modified LDPE resin can be substantially 0% by weight; the measured gel content can be less than or equal to the usual confidence limit of the test.

[0087] Based on the weight of the modified LDPE resin, the GPC recovery rate of the modified LDPE resin is preferably at least 95 percent, more preferably substantially 100 percent. For clarity, electron beam modification is not expected to reduce gel content, but modification conditions are preferably selected to avoid or minimize the formation of additional gel.

[0088] One preferred embodiment of the modified LDPE resin has the following characteristics:

[0089] a.0.91g / cm 3 Up to 0.94 g / cm 3 The density;

[0090] b. Melt index (I2) from 1.5 dg / min to 6 dg / min; and

[0091] c. Typical molecular weight distribution from 10 to 20 (M w(Conv) / M n(Conv) );as well as

[0092] d. Melt strength of at least 25 cN; and

[0093] e. 95% or higher GPC quality recovery rate.

[0094] If the modified polyethylene resin is irradiated as powder or granules, it is preferable to extrude it to form pellets. The pellets may optionally include additives such as antistatic agents, color enhancers, dyes, lubricants, fillers, pigments, primary antioxidants, secondary antioxidants, processing aids, UV stabilizers, nucleating agents, slip agents (such as erucamide), anti-blocking agents (such as talc), and combinations thereof; preferably, they do not include substantial amounts of additives.

[0095] Powders, granules, or pellets can be blended and / or co-extruded with other resins, such as HDPE, LLDPE, or another LDPE, to prepare resin blends. It is well known to select and blend polyethylene resins with chosen properties to impart desired properties to the entire blend.

[0096] Powders, granules, pellets, or blends can be extruded to prepare extruded single-layer or multi-layer films and sheets, extruded coatings, and extruded blow-molded articles and other products. This technology is well known and is briefly described in the background section. Preferred uses of modified LDPE resins and blends containing them include blow-molded and cast single-layer and multi-layer films, stretched single-layer and multi-layer films, and extruded single-layer and multi-layer coatings.

[0097] Test methods

[0098] Throughout the specification and appended claims, references to the physical and chemical properties of LDPE resin refer to properties measured by the following test methods.

[0099] density Density is measured according to ASTM D792 Method B.

[0100] Melt index Melt index or I2 is measured according to ASTM D1238 at 190°C and 2.16 kg. Results are reported in decigrams per minute (dg / min).

[0101] melt strength Melt strength was measured at 190°C using a Goettfert Rheotens 71.97 (Goettfert Inc.; Rock Hill, SC). The melt was fed using a Goettfert Rheotester 2000 capillary rheometer equipped with a 30mm long and 2mm diameter planar inlet angle (180 degrees). Pellets were fed into a hopper (L = 300mm, diameter = 12mm), compressed, and melted for 10 minutes, then extruded at a constant piston speed of 0.265mm / s, corresponding to 38.2s for a given die diameter. -1 The wall shear rate. The extrudate passes through the Rheotens wheel located 100 mm below the die exit and is driven by the wheel at a rate of 2.4 mm / s. 2 The acceleration pulls downwards. The force applied to the wheel (in cN) is recorded as a function of the wheel's velocity (mm / s). Melt strength is reported as the plateau force (cN) before strand breakage or significant tensile resonance.

[0102] Irradiation level The electron beam is calibrated using dosimetry film and by measuring color changes. The irradiation level can then be calculated based on the electron beam energy, current, and band velocity.

[0103] gel contentThe gel content (insoluble fraction) resulting from crosslinking is determined by extraction with the solvent decahydronaphthalene. This method is applicable to crosslinked ethylene plastics of all densities, including those containing fillers, and provides correction for inert fillers present in some of the compounds. See ASTM D2765-16, Standard Test Methods for Determination of Gel Content and Swell Ratio of Crosslinked Ethylene Plastics, ASTM International, West Conshohocken, PA, 2016, www.astm.org.

[0104] Vinyl content The vinyl content of LDPE was determined by 1H NMR spectroscopy, which is described in Busico, V. et al., Macromolecules, 2005, 38, 6988 and U.S. Patent 8,916,667, column 11, lines 35-12, line 15.

[0105] Samples were prepared as follows: In a Norell 1001-7 10 mm NMR tube, approximately 0.1 to 0.2 g of the sample was added to 3.25 g of 50 / 50 (by weight) 1,1,2,2-tetrachloroethane-d2 / perchloroethylene (TCE / PCE) containing 0.001 M Cr(AcAc)3 and approximately 75 ppm butylated hydroxytoluene (BHT). The sample was purged of oxygen by bubbling N2 through the solvent for approximately 3 minutes using a pipette inserted into the tube. The tube was capped, sealed with a Teflon tape, and then heated and vortexed at 115 °C to dissolve and ensure homogeneity.

[0106] 1H NMR was performed at a sample temperature of 120 °C on a Bruker AVANCE 600 MHz spectrometer equipped with a Bruker high-temperature CryoProbe. Spectra were acquired using ZG pulses, 1.8 s AQ, 64 or 128 scans with a 14 s relaxation delay.

[0107] The spectrum references the residual proton signal of the TCE at 6.0 ppm. The total polymer integral from approximately -0.5 ppm to 2.5 ppm is set to an arbitrary value, such as 2000. Corresponding integrals are obtained for unsaturated regions (approximately 5.40 ppm to 5.60 ppm for cis and trans vinylenes, approximately 5.16 ppm to 5.35 ppm for trisubstituted vinylenes, approximately 5.0 ppm to 5.15 ppm for vinylenes, and approximately 4.75 ppm to 4.85 ppm for vinylenes). The BHT-OH signal at approximately 4.9 ppm is not included in the integration region.

[0108] The integral of the entire polymer is divided by 2 to obtain the total polymer carbon, which is 1000 in this example. The integral of the unsaturated groups is divided by the corresponding number of protons contributing to that integral, representing the number of moles of each type of unsaturation per 1000 total polymer carbon molecules. This is referred to as the unsaturated groups per 1000 carbons.

[0109] Nuclear magnetic resonance (13C NMR for branching)

[0110] 13C NMR samples were prepared as follows: Approximately 3 g of 1,1,2,2-tetrachloroethane (TCE) containing 25 wt% TCE-d2 and 0.025 M Cr(AcAc)3 was added to approximately 0.25 g of the polymer sample in a 10 mm NMR tube. Oxygen was removed from the sample by purging the top space of the tube with nitrogen. The tube and its contents were then heated to 120–140 °C using a heating block and vortex mixer to dissolve and homogenize the sample. Each dissolved sample was visually inspected to ensure homogeneity. The sample was thoroughly mixed before analysis and was not allowed to cool before being inserted into the heated NMR sample holder.

[0111] All data were collected using a Bruker 600MHz spectrometer equipped with a 10mm cryostat. 13C data were acquired at a sample temperature of 120°C using a 7.8-second pulse repetition delay, a 90-degree tilt angle, and inverse gating decoupling. All measurements were performed on non-rotating samples in locked mode. Samples were allowed to equilibrate for seven minutes before data acquisition. The 13C NMR chemical shift internal reference is 30.0 ppm for the EEE terpolymer. Table 1 lists the peak assignments used for branching measurements in LDPE. The “C6+” value is a direct measure of C6+ branches in LDPE, where long branches are not distinguished from “chain ends.” The “32.2 ppm” peak, representing the third carbon from the end of all chains or branches with six or more carbons, was used to determine the “C6+” value.

[0112] Table 1: Branching type and 13C NMR integration range for quantification

[0113]

[0114] Used to measure conventional molecular weight (Mw) conv absolute molecular weight (Mw) Abs ), Long chain branching frequency (LCB) f ) and gpcBR Gel permeation chromatography (GPC) .

[0115] The chromatographic system consisted of a PolymerChar GPC-IR (Valencia, Spain) high-temperature GPC chromatograph equipped with an internal IR5 infrared detector, a Precision Detector (now Agilent Technologies) 2-angle laser scattering (LS) detector 2040, and an internal 4-capillary viscometer. A 15-degree angle was used for all light scattering measurements.

[0116] The systematic method for determining multi-detector bias is performed in a manner consistent with that published by Balke, Mourey et al., using PolymerChar GPCOne. TM The software optimized the standard from the wide homopolymer polyethylene (M) w / M n >2.7) triple detector logarithmic (MW and IV) results compared with narrow standard column calibration results from narrow standard calibration curves. As used herein, “MW” refers to molecular weight, and MWD refers to molecular weight distribution.

[0117] Columns and Calibration: The columns in the GPC chromatograph are four Agilent “Mixed A” 30cm 20µm linear mixed-bed columns and a 20µm pre-column. The autosampler oven chamber is set to 160°C, and the column chamber is set to 150°C.

[0118] The GPC column assembly was calibrated using 21 polystyrene standards with narrow molecular weight distributions, ranging from 580 g / mol to 8,400,000 g / mol, arranged in a six-cocktail mixture with individual molecular weights spaced at least ten times apart. These standards were purchased from Agilent Technologies. For molecular weights equal to or at least 1,000,000 g / mol, 0.025 g of polystyrene standard was prepared in 50 mL of solvent, while for molecular weights less than 1,000,000 g / mol, 0.05 g of polystyrene standard was prepared in 50 mL of solvent. The polystyrene standards were dissolved by gentle stirring at 80°C for 30 minutes. The peak molecular weights of the polystyrene standards were converted to polyethylene molecular weights using Equation 1 (as described in Williams and Ward, J. Polym. Sci., Polym. Let., 6, 621 (1968)).

[0119] MW 聚乙烯=A x(Mw 聚苯乙烯 ) B (1)

[0120] Where MW is the molecular weight, A is 0.4315 and B is 1.0.

[0121] A fifth-order polynomial was used to fit the calibration point for the corresponding polyethylene equivalent. (A small adjustment to A (approximately 0.3950 to 0.440) was made to correct for column resolution and band broadening effects, resulting in linear homopolymer polyethylene standards at 120,000 Mw).

[0122] Total plate counts were performed on the GPC column assembly using decane (prepared as 0.04 g in 50 mL of TCB). Plate counts and symmetry were measured using 200 μL injections according to the following equations (Equation 2 and Equation 3):

[0123]

[0124] Where RV is the retention volume in milliliters, peak width is in milliliters, peak value is the maximum height of the peak, and 1 / 2 height is 1 / 2 height of the peak value.

[0125] as well as

[0126]

[0127] Where RV is the retention volume in milliliters, and the peak width is in milliliters, the peak value is the position of the maximum peak, the one-tenth height is 1 / 10 of the height of the peak value, the later peak refers to the tail of the peak in a slightly later retention volume compared to the peak value, and the earlier peak refers to the front of the peak in a slightly earlier retention volume compared to the peak value.

[0128] The plate count of the chromatographic system should be at least 20,000, and the symmetry should be between 0.98 and 1.22.

[0129] LDPE sample preparation and separation LDPE sample preparation was as follows: The chromatographic solvent was 1,2,4-trichlorobenzene containing 200 ppm butylated hydroxytoluene (BHT). The solvent source was bubbled under nitrogen. Sample preparation was performed semi-automatically using PolymerChar Instrument Control software, with a target sample weight of 1 mg / mL. The solvent was added to a pre-bubbled, septum-capped vial via a PolymerChar high-temperature autosampler. The sample was dissolved at 160°C for 2 hours under low-rotational oscillation. The injection volume into the column was 200 μL, and the flow rate was 1.0 mL / min.

[0130] To monitor flow rate deviations over time, a flow rate marker (decane) was introduced into each sample via a micropump controlled by a PolymerChar GPC-IR system. This flow rate marker (FM) was used to linearly correct the pump flow rate (nominal flow rate) for each sample by comparing the retention volume (RV) of the corresponding decane peak within the sample (RV(FM sample)) with the retention volume (RV(FM calibration)) of the decane peak within the narrow standard calibration. It was then assumed that any variation in the decane marker peak time was linearly related to the flow rate (effective flow rate) over the entire run. To achieve the highest accuracy in RV measurement of the flow marker peak, a least-squares fitting procedure was used to fit the peak values ​​of the flow marker concentration chromatogram to a quadratic equation. The first derivative of the quadratic equation was then used to solve for the true peak position. After calibration based on the flow marker peak, the effective flow rate (relative to the narrow standard calibration) was calculated according to Equation 4. The flow marker peaks were measured via PolymerChar GPCOne. TM Software processing. Acceptable flow rate correction ensures that the effective flow rate is within + / -1% of the nominal flow rate.

[0131] Flow rate (effective) = Flow rate (nominal) * (RV (FM calibrated) / RV (FM sample)) (4)

[0132] Data Analysis

[0133] The standard molecular weight and GPC recovery rate are calculated from data from the internal IR5 detector (measurement channel).

[0134] Based on equations 5-6, use PolymerChar GPCOne. TM The software calculates Mn(conv) and Mw(conv) based on the baseline-subtracted IR chromatograms at each equidistant data collection point (i) and the polyethylene equivalent molecular weight obtained from the narrow standard calibration curve at point (i) according to Equation 1.

[0135]

[0136]

[0137] GPC recoveries were determined using the total signal area of ​​the sample eluted via an IR5 wide-filter detector using a GPC method and adjusted using a mass constant determined as specified by the vendor-recommended polyethylene homopolymer standard, in a manner consistent with that used in the PolymerChar GPCOne software. The mass recovery was calculated using the expression M-REC = 100 × [(initial analyte - filtered analyte) / initial analyte], using the analyte mass value obtained in the PolymerChar SoGPC test. It should be understood that polymers with internal crosslinks form insoluble gels, which can be quantitatively detected by analysis with low mass recoveries.

[0138] Absolute (abs) molecular weight data were obtained using PolymerChar GPCOne. TM The software is available in a manner consistent with the following publications: Zimm (Zimm, BH, J. Chem. Phys., 16, 1099 (1948)) and Kratochvil (Kratochvil, P., Classical Light Scattering from Polymer Solutions, Elsevier, Oxford, NY (1987)).

[0139] Absolute weight-average molecular weight (M w (Abs) is (using GPCOne) TM The area (calculated from the light scattering constant) of the integrated light scattering (LS) chromatography is obtained by dividing the mass recovered from each elution volume by the mass constant and the mass detector (IR5) area. The total injection concentration used to determine the molecular weight is obtained from the mass detector area and the mass detector constant, derived from one of suitable linear polyethylene homopolymers or polyethylene standards with a known weight-average molecular weight. The mass detector response (IR5) and light scattering constant (using GPCOne) are also considered. TM The determination was performed using linear polyethylene standards with a molecular weight exceeding approximately 50,000 g / mol. The calculated molecular weight (using GPCOne) was determined. TM The light scattering constant and refractive index concentration coefficient dn / dc of 0.104 were obtained using one or more polyethylene standards mentioned below.

[0140] Typically, viscometer calibration (using GPCOne) TMThe determination can be performed using the method described by the manufacturer, or alternatively, by using published values ​​of a suitable linear standard (such as Standard Reference Material (SRM) 1475a), which are available from the National Institute of Standards and Technology (NIST). The viscometer constant is calculated using GPCOne. TM The specific viscosity area (DV) and injection mass of the calibration standards obtained are related to their intrinsic viscosity (IV). It is assumed that the chromatographic concentration is low enough to eliminate the effect of the second virial coefficient (the effect of concentration on molecular weight). Absolute weight-average molecular weight (M0) w (Abs) is (using GPCOne) TM The molecular weight and intrinsic viscosity responses are obtained by dividing the area of ​​the light scattering (LS) integral chromatography (calculated from the light scattering constant) by the mass constant and the mass recovered from each elution volume and the area of ​​the mass detector (IR5). Extrapolation of the molecular weight and intrinsic viscosity responses is performed at the chromatographic ends where the signal-to-noise ratio decreases (using GPCOne). TM ).

[0141] Viscometer calibration (using GPCOne) TM The determination can be performed using the method described by the manufacturer, or alternatively, by using published values ​​of a suitable linear standard (such as Standard Reference Material (SRM) 1475a), which are available from the National Institute of Standards and Technology (NIST). The viscometer constant is calculated (using GPCOne). TM The specific viscosity area (DV) and injection quality of the standard obtained will be related to its intrinsic viscosity (IV).

[0142] Comparison of branching (gpcBR) :

[0143] The gpcBR branching index method for characterizing long-chain branching is described in Yau, Wallace W., “Examples of Using 3D-GPC—TREF for Polyolefin Characterization,” Macromol. Symp., 2007, 257, 29-45.

[0144] The gpcBR branching index was determined using data from light scattering, viscosity, and concentration detectors as previously described. Baselines were subtracted from the light scattering, viscometer, and concentration chromatograms. An integration window was set to ensure integration of all low molecular weight retention volume ranges in the light scattering and viscometer chromatograms, which indicate the presence of detectable polymers from the infrared (IR5) chromatogram.

[0145] Mark-Houwink constants for polyethylene and polystyrene were established using linear polyethylene standards. After obtaining the constants, these two values ​​were used to construct two linear references for polyethylene molecular weight and intrinsic viscosity as a function of elution volume, as shown in equations (7) and (8):

[0146]

[0147] In the case of 3D-GPC, the intrinsic viscosity of the sample is also obtained independently using Equation (9). This area calculation provides higher accuracy because, as the total sample area, it is less sensitive to variations caused by detector noise and 3D-GPC settings for baseline and integration limits. More importantly, the peak area calculation is unaffected by detector volume shifts. Similarly, a high-precision intrinsic viscosity (IV) of the sample is obtained using the area method shown in Equation (9):

[0148]

[0149] Where η spi This represents the specific viscosity obtained from the viscometer detector.

[0150] To determine the branching index of gpcBR, the light scattering elution area of ​​the sample polymer was used to determine the molecular weight of the sample. The viscosity detector elution area of ​​the sample polymer was used to determine the intrinsic viscosity (IV or [η]) of the sample.

[0151] Initially, based on equations (10) and (11), the molecular weight and intrinsic viscosity of linear polyethylene standard samples such as SRM1475a or equivalents were determined as a function of elution volume using conventional calibration values ​​(“cc”) of both molecular weight and intrinsic viscosity:

[0152]

[0153] Equation (11) is used to determine the branching index of gpcBR:

[0154]

[0155] Where: [η] is the measured intrinsic viscosity, [η]cc is the intrinsic viscosity from routine calibration, Mw is the measured weight-average molecular weight, and Mw,cc is the weight-average molecular weight from routine calibration.

[0156] The weight-average molecular weight determined by light scattering (LS) is usually referred to as the "absolute weight-average molecular weight" or "M." w(Abs) According to Equation (6), the Mw,cc obtained using the conventional GPC molecular weight calibration curve (“conventional calibration”) is usually referred to as the “polymer main chain molecular weight”, “conventional weight-average molecular weight”, and “Mw(conv)”.

[0157] All statistical values ​​with the subscript "cc" were determined using their respective elution volumes, corresponding to the conventional calibration and concentration (Ci) as described above. Unsubscripted values ​​were based on measurements from the mass detector, LALLS, and viscometer area. The value of KPE was iteratively adjusted until the gpcBR measurement for the linear reference sample was zero. For example, in this case, the final values ​​of α and Log K for determining gpcBR were 0.725 and -3.391 for polyethylene, and 0.722 and -3.993 for polystyrene, respectively. Once the K and α values ​​were determined using the previously discussed procedure, the procedure was repeated using branched samples. The final Mark-Houwink constant obtained from the linear reference was used as the optimal "cc" calibration value for analyzing branched samples.

[0158] For linear polymers, the gpcBR calculated by equation (11) will be close to zero because the values ​​measured by LS and viscometry will be close to the standard calibration. For branched polymers, the gpcBR will be higher than zero, especially for high levels of long-chain branching, because the measured polymer molecular weight will be higher than the calculated Mw,cc, and the calculated IVcc will be higher than the measured polymer IV. The gpcBR value represents the fractional change in IV due to the molecular size shrinkage effect as a result of polymer branching. A gpcBR value of 0.5 or 2.0 means that the molecular size shrinkage effect of IV is at the level of 50% and 200% respectively relative to the equivalent linear polymer molecules.

[0159] LCB frequency (LCB) f ) calculation

[0160] The LCB of each polymer sample was calculated using the following procedure. f ( LCB1000C) (Based on long chain branches of 1000 carbon atoms):

[0161] 1) Calibrate the light scattering, viscosity, and concentration detectors using NBS1475 homopolymer polyethylene (or an equivalent linear reference).

[0162] 2) Calibrate the light scattering and viscometer detector offsets relative to the concentration detector as described in the calibration section above (see Mourey and Balke's references).

[0163] 3) Subtract the baseline from the light scattering, viscometer, and concentration chromatograms, and set an integration window to ensure that all low molecular weight retention volume ranges in the light scattering chromatogram that can be observed from the refractometer chromatogram are integrated.

[0164] 4) Establish a Mark-Houwink reference line for linear homopolymer polyethylene by injecting standards with a polydispersity of at least 3.0, calculate the data file (according to the calibration method above), and record the intrinsic viscosity and molecular weight of the mass constant calibration data from each chromatographic slice.

[0165] 5) Analyze the LDPE sample of interest, calculate the data file (according to the calibration method above), and record the intrinsic viscosity and molecular weight from the mass constant, as well as the corrected data for each chromatographic slice. At lower molecular weights, it may be necessary to extrapolate the intrinsic viscosity and molecular weight data so that the measured molecular weight and intrinsic viscosity asymptotically approach the linear homopolymer GPC calibration curve.

[0166] 6) The linear reference intrinsic viscosity of the homopolymer is shifted at each point (i) by the following coefficient: IVi = IVi * 0.946, where IV is the intrinsic viscosity.

[0167] 7) The following coefficient is used to shift the linear reference molecular weight of homopolymers: MW = MW * 1.57, where MW is the molecular weight.

[0168] 8) Calculate g' for each chromatographic slice according to the following equation:

[0169] g' = (IV(LDPE) / IV(linear reference))

[0170] At the same molecular weight (MW), IV (linear reference) is calculated by fitting a fifth-order polynomial to a reference Mark-Houwink plot, where IV (linear reference) is the intrinsic viscosity of the linear homopolymer polyethylene reference (with the addition of a certain amount of SCB (short-chain branching)) to explain the backbiting at the same molecular weight (MW) via Equations 5) and 6). The IV ratio is assumed to be the ratio for molecular weights less than 3,500 g / mol to explain natural scattering in the light scattering data.

[0171] 9) Calculate the number of branches at each data slice according to Equation 12 (as described in Zimm, Stockmayer J. Chem. Phys. 17, 1301 (1949)): (12)

[0173] 10) According to Equation 13, calculate the average number of LCBs across all slices (i):

[0174]

[0175] Example

[0176] The following starting LDPE resins were obtained from commercial raw materials in pellet form: LDPE 722, LDPE 4016, and AGILITY. TM EC 7080, LDPE 780E, LDPE 993I, and LDPE 955I. All resins were available from Dow, Inc. The resins were additive-free, except for LDPE 993I, which contained a slip agent. The initial properties of each resin were measured using the test methods described above, and the results are listed in Table 2.

[0177] Table 2

[0178]

[0179]

[0180] The modified LDPE was produced by irradiating each resin with the doses listed in Table 3 using the following procedure: the starting LDPE was irradiated with a predetermined dose (up to 1.15 MRad) in air using a DYNAMITRON linear electron beam accelerator. The operating parameters of the linear electron beam accelerator were: an energy range of 4.5 MeV, a beam power of 150 kW across the entire energy range, a beam energy distribution of + / - 10 percent, and an average current of 30 mA.

[0181] After irradiation, the properties of each resin were measured again. In addition, the same properties were measured on three unirradiated commercially available resin samples: LDPE 621I from Dow Inc., LDPE 662I from Dow Inc., and LDPE1I2-A from Sinopec. The results are shown in Tables 3A and 3B. In Tables 3A and 3B, IE1-IE5 are examples of the present invention. CE1-CE13 are comparative examples. The properties of Example 4 and Comparative Example 5 of the present invention, as well as their base resins, were measured. TM The density of EC 7080 was found to be 0.919 g / cm³. 3 This result is consistent with our experience that irradiation at the levels used in this invention does not substantially change the density of the resin.

[0182] Vinyl content and NMR branching analysis, such as for base resins LDPE 722, LDPE 4016, and AGILITY. TM The tests were performed as described in EC7080 and the test methods for IE1 to IE5 and CE3 to CE4. The results are shown in Table 4.

[0183] Table 3A

[0184]

[0185]

[0186] Table 3B

[0187]

[0188]

Claims

1. A method for modifying low-density polyethylene (LDPE) resin, the method comprising the following steps: a) Providing a starting LDPE resin, the starting LDPE resin having: i) 0.91 g / cm 3 Up to 0.94 g / cm 3 The density; ii) Melt index I2 from 5 dg / min to 18 dg / min; as well as iii) At least 6 conventional molecular weight distribution M w (Conv) / M n (Conv) ;as well as b) Irradiating the starting LDPE resin with an electron beam, using a linear electron beam accelerator to efficiently provide the strength and time required to modify the LDPE resin, wherein the modified LDPE resin has: i) A melt index I2 of at least 1 dg / min; ii) At least 10 conventional molecular weight distribution M w (Conv) / M n (Conv) ; iii) Melt strength of at least 15 cN; iv) At least 95% GPC quality recovery rate; as well as v) A long-chain branching frequency (LCBf) of at least 3.

5. The average dose received by the initial LDPE resin is 0.45 MRad to 1.25 MRad, and the linear electron beam accelerator operates in a beam energy range of at least 2 MeV.

2. The method of claim 1, wherein the starting LDPE resin receives an average dose of 0.45 MRad to 1.0 MRad; and / or the linear electron beam accelerator operates in a beam energy range of at least 3.0 MeV to at most 12 MeV.

3. The method of claim 1, wherein the starting LDPE resin receives an average dose of 0.45 MRad to 0.8 MRad; and / or the electron beam power of the linear electron beam accelerator is at least 20 kW to at most 350 kW over the entire energy range.

4. The method according to any one of claims 1 to 3, wherein the starting LDPE resin has a melt index I2 of 5 dg / min to 17 dg / min.

5. The method according to any one of claims 1 to 3, wherein the conventional molecular weight distribution M of the modified LDPE resin is... w (Conv) / M n (Conv) It is 140% to 200% of the molecular weight distribution of the starting LDPE resin.

6. The method according to any one of claims 1 to 3, wherein the melt strength of the modified LDPE resin is at least 10 cN higher than the melt strength of the starting LDPE resin.

7. The method according to any one of claims 1 to 3, wherein the melt index of the modified LDPE resin is at least 20% of the melt index of the starting LDPE resin.

8. The method according to any one of claims 1 to 3, wherein the starting LDPE resin is a product from a tubular reactor system.

9. A modified LDPE resin formed by the method according to any one of claims 1-8, said modified LDPE resin having the following characteristics: a) 0.91 g / cm 3 Up to 0.94 g / cm 3 The density; b) Melt index I2 from 1.5 dg / min to 6 dg / min; c) Typical molecular weight distribution of 10 to 20 M w (Conv) / M n (Conv) ; d) Melt strength of at least 25 cN; e) At least 95% GPC quality recovery rate; and f) A long chain branching frequency of at least 3.5 LCBf.

10. The modified LDPE resin according to claim 9, wherein the modified LDPE resin has a gpcBR of at least 3.

75.

11. The modified LDPE resin according to claim 9 or 10, wherein the modified LDPE resin has an M of at least 3. w (Abs) ) / (M w (Conv) Compare.

12. The modified LDPE resin according to claim 9 or 10, wherein the modified LDPE resin has a viscosity ratio of at least 9 at 190°C.

13. The modified LDPE resin according to claim 9 or 10, wherein the ratio of the melt strength in cN to the melt index in dg / min of the modified LDPE resin is 2 to 25.

14. A polymer blend comprising (a) a modified LDPE resin according to any one of claims 9 to 13 and (2) a high-density polyethylene resin, a linear low-density polyethylene resin or another low-density polyethylene.

15. An article comprising the modified LDPE resin according to any one of claims 9 to 13, wherein the article is an extruded single-layer or multi-layer film and sheet, an extruded coating or foamed article.

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