A method for preparing biaxially oriented LLDPE film resin

Abstract

The invention discloses a method for preparing a biaxially oriented LLDPE film resin, comprising the following steps: polymerizing ethylene and alpha-olefin in a single reactor in the presence of hydrogen, oxygen, an inert gas and a supported chromium catalyst, wherein the supported chromium catalyst comprises a carrier and chromium oxide modified by one or more of titanium, fluorine and vanadium. The molecular chain of the biaxially oriented film resin prepared by the method provided by the invention has a unique structure: a wide molecular weight distribution, a unique short chain branch and a small amount of long chain branch structure, a wide melting range and a double melting point are formed on the DSC curve, and multiple melting peaks are formed on the SSA curve, and the content of each peak is normally distributed.

Description

Technical Field

[0001] The invention belongs to the technical field of film resins, and in particular relates to a method for preparing a biaxially stretched LLDPE film resin. Background Art

[0002] Biaxially oriented polyethylene (BOPE) film is a kind of high-performance film material produced by flat film biaxially oriented step-by-step stretching process using PE resin with special molecular structure as raw material. At present, biaxially oriented step-by-step stretching process is mainly used for the production of polypropylene (PP) film, nylon (PA) film and polyester (PET) film. Compared with the above films, after the BOPE film is processed and formed by the biaxial stretching process, its macromolecular chain and aggregated structure are highly oriented, so it has the advantages of high tensile strength, good transparency, low temperature resistance, puncture resistance, impact resistance, easy tearing, etc. It can be used for agricultural greenhouse film, composite bag making, low-temperature packaging, medical packaging, etc., and has broad market prospects. Compared with similar polyethylene films, when the thickness is reduced by 50%, the key performance can still reach or even exceed the level of other polyethylene films, which can greatly reduce the production cost and provide favorable support for solving the energy crisis and environmental pollution problems caused by plastic films. Biaxially oriented polyethylene film is a technological breakthrough for polyethylene packaging film. The biaxially oriented resin currently developed is a special structure of LLDPE.

[0003] Traditional LLDPE is a copolymer of ethylene and α-olefin. The introduction of α-olefin monomers makes the polymer contain a considerable number of branches, and these branches of varying lengths directly affect the performance of the polymer. The LLDPE molecular chain is linear, with highly branched short branches, so the crystallinity is low and it has certain stretchability, but the crystallization rate is fast and the crystallinity is high. Traditional LLDPE cannot meet the needs of biaxial stretching.

[0004] During the production process, when the resin changes from the molten state to the glassy state, it begins to crystallize within a certain temperature range below the melting point. Since the crystal nuclei are randomly generated and the spherulites grow under natural conditions, the spherulites are large in size and unevenly distributed. Large-sized spherulites reflect light on the surface of the LLDPE film, which deteriorates the transparency of the film. In addition, due to the difference in refractive index between the crystalline region and the amorphous region, irregular light scattering and reflection occur at the interface, resulting in poor transparency of the LLDPE resin. Therefore, by controlling the crystallization properties of the LLDPE resin and reducing the difference between the crystalline phase and the amorphous phase, the transparency of the LLDPE film can be improved.

[0005] In terms of processing performance, LLDPE is less sensitive to stress, that is, under the high shear stress of extrusion processing, the melt viscosity is much higher, requiring a larger torque, higher melt temperature and die pressure, and is prone to melt rupture; but it does not harden during bubble stretching. In addition, LLDPE has poor melt strength and is prone to film rupture during film stretching.

[0006] In recent years, synthetic high biaxially oriented LLDPE has attracted the attention of many researchers. High biaxially oriented high-performance resin products are developed on the basis of ordinary LLDPE. The products combine the advantages of LDPE and LLDPE. They have both the excellent physical and mechanical properties of LLDPE products and the processing and transparency properties of LDPE products. They have irreplaceable advantages over other products in terms of strength, stability, low shrinkage, crack resistance, etc., and are widely used in the production of various film products by biaxially oriented processes.

[0007] CN1338477A introduces a carrier catalyst system for preparing LLDPE and a method for preparing LLDPE. The method provides a bifunctional catalyst system for synthesizing linear low-density polyethylene from ethylene, wherein the dimerization catalyst is an alkoxy compound catalyst, and the copolymerization catalyst is composed of a supported metallocene catalyst. Alkyl aluminum or boron compounds are used as co-catalysts. In the same polymerization system, ethylene is directly dimerized and a copolymerization reaction occurs in situ to produce LLDPE.

[0008] CN1124034A introduces a highly processable LLDPE polymer composition, a polymer composition based on linear low-density polyethylene (LLDPE), which comprises (A) 75-95% (by weight) of a copolymer of ethylene and α-olefin, and (B) 5-25% (by weight) of a copolymer of propylene, ethylene and α-olefin. The copolymer (B) is characterized by high insolubility in xylene. Compared with conventional LLDPE, the polymer composition of the present invention has improved processability and mechanical properties. The polymerization reaction is carried out in two or more fluidized bed or mechanically stirred bed reactors connected in series. The order of the reactors is not limited and the catalyst used is the same.

[0009] CN1145082A introduces a LLDPE resin blend. A blend of LLDPE and low-density high-pressure polyethylene resin prepared using a metallocene catalyst can be extruded to form a film with improved optical properties and impact strength.

[0010] CN1183105A introduces a method for producing LLDPE polymers, which is a method for producing ethylene copolymers in particle form by polymerizing ethylene and a small amount of C3-C6 α-olefin in the presence of an ethylene polymerization catalyst in a slurry reactor. According to the present invention, the polymerization is carried out in a propane diluent by using a metallocene catalyst activated with aluminoxane.

[0011] CN1217343A introduces a bimetallic catalyst for synthesizing LLDPE and a preparation method thereof. The method relates to a bimetallic catalyst system for synthesizing linear low-density polyethylene from ethylene. The dimerization catalyst is composed of a tungsten-based main catalyst and a halogenated alkylaluminum co-catalyst, and the copolymerization catalyst is composed of a titanium-based catalyst and an alkylaluminum. The bimetallic catalyst system composed of these two catalysts can directly dimerize ethylene and simultaneously undergo an on-site copolymerization reaction in the same polymerization system to produce LLDPE, and the copolymerization activity is very high.

[0012] Ye Zhibin et al. successfully synthesized LLDPE using the oligomerization catalyst (η5-C5H4CMe2C6H5)TiCl3 / modified methylaluminoxane (MMAO) and the copolymerization catalyst [(η5-C5Me4)SiMe2(tBuN)]TiCl2 / MMAO in the process of studying the long branching and rheological properties of ethylene-1-hexene copolymers synthesized using an in-situ polymerization catalytic system. The results of the small amplitude dynamic vibration test showed the typical rheological properties of long-chain branched polymers (such as the enhancement of zero shear viscosity, the improvement of shear thinning, the increase of dynamic modulus, and complex thermorheological properties), which fully demonstrated the presence of long branches in the synthesized LLDPE.

[0013] Galland et al. found in their research on the synthesis of long-branched PE using Fe and Zr catalysts that Fe catalysts can synthesize α-olefins with Me close to 2000 g / mol. The α-olefins polymerize with ethylene at Zr activation points to synthesize PE with long chain branches, in which the long chain branches account for more than 40% of the total branching.

[0014] Farley et al. used a transition metal catalyst system and a gas phase fluidized bed reactor to synthesize LLDPE. Compared with the narrow MWD metallocene product, TM At similar MFR, comonomer type and density, this LLDPE resin can be easily extruded into film products through cast or blown film processing at lower motor load, higher throughput and reduced head pressure. At similar MFR, this LLDPE resin is better than EXCEED TM The resin has a higher weight average molecular weight and a wider MWD, the film tearing performance is balanced in both directions, and the dart impact strength is greater than 19.7g / um.

[0015] Michie et al. used an in-situ blending method, a dual reactor, and a Mg / Li catalyst to produce bimodal LLDPE. The polyolefin produced in the high molecular weight reactor has a low MFR (0.01-30.00 g / 10 min) and a density of 0.86-0.94 g / cm 3The MFR of polyolefins produced in low molecular weight reactors is 5-30 g / 10 min and the density is 0.900-0.979 g / cm 3 The film prepared in this way not only has good tensile, impact and puncture resistance, but also greatly improves the problem of difficult processing of traditional LLDPE film.

[0016] The shrink film produced by Myhre et al. using wide / bimodal LLDPE (MWD of 10-35) has better mechanical properties and shrinkage properties than traditional shrink films. During the shrinkage process, the crystallization rate of the low molecular weight component in the bimodal LLDPE is greater than that of the high molecular weight component, which effectively shrinks and deforms the film and improves the mechanical properties of the film. During the film formation process, the high degree of entanglement of the polymer chains enhances the rigidity of the film and forms a highly oriented structure that improves shrinkage.

[0017] Myhre et al. invented a bimodal PE composite breathable film. The bimodal PE component provides high strength and excellent processing performance for the film, and can prepare films with low quantitative content. Among them, the low relative molecular weight component can help improve the processing performance, with an MFR of 0.1-4.0 g / 10min and a density of about 0.918-0.935 g / cm 3 ; High relative molecular mass components give it certain mechanical properties. High relative molecular mass components are ethylene and C4-C l0 Copolymers of olefins with higher comonomer content. For a given low molecular weight component content and component ratio, the produced bimodal PE has the desired MFR and density.

[0018] Cheng Song et al. introduced long chain branches into LLDPE by relatively low electron beam radiation. There was little or no crosslinking in the irradiated LLDPE, and the MWD became wider. At low shear rates, the melt stiffness of long-branched LLDPE increased; at high elongation at break, the melt strength increased. The long chain branches make the irradiated LLDPE have rheological behaviors such as relaxation resistance and strain hardening, which improves the processing properties of LLDPE. Grafting treatment of LLDPE can also improve the processing properties of LLDPE.

[0019] The above technologies for producing LLDPE products have certain deficiencies in terms of product quality uniformity, production cost and comprehensive product performance. At present, there is a lack of a biaxially oriented film base resin with a wide molecular weight distribution, a unique short-chain branch structure and a small amount of long-chain branch structure, and a film made from the resin has good transparency and processability, good comprehensive performance, uniform quality and low production cost. Summary of the invention

[0020] The purpose of the present invention is to provide a method for preparing a biaxially oriented LLDPE film resin, so as to solve the problems that the prior art is difficult to meet in terms of biaxial stretching, film transparency, processability, etc.

[0021] To achieve the above object, the present invention provides a method for preparing a biaxially oriented LLDPE film resin, comprising the following steps:

[0022] Ethylene and alpha-olefin are polymerized in a single reactor in the presence of hydrogen, oxygen, inert gas and a supported chromium catalyst, wherein the supported chromium catalyst comprises a carrier and chromium oxide modified with one or more of titanium, fluorine and vanadium.

[0023] In the method for preparing the biaxially oriented LLDPE film resin of the present invention, the α-olefin is 1-butene and / or 1-hexene.

[0024] In the method for preparing the biaxially stretched LLDPE film resin of the present invention, the carrier is silicon dioxide, aluminum oxide, zirconium oxide or thorium oxide.

[0025] The method for preparing biaxially oriented LLDPE film resin of the present invention comprises the following steps: the polymerization is gas phase polymerization, the polymerization temperature is 80-90°C, preferably 83-86°C; the polymerization pressure is 1.8-2.5MPa, preferably 2.0-2.3MPa; the circulation gas velocity is 0.60-0.82m / s, preferably 0.64-0.74m / s; and the residence time is 1-8h, preferably 4-6h.

[0026] In the method for preparing biaxially oriented LLDPE film resin of the present invention, the molar ratio of α-olefin to ethylene is 0.02:1-0.1:1, preferably 0.028:1-0.045:1; the molar ratio of hydrogen to ethylene is 0.001:1-0.01:1, preferably 0.003:1-0.006:1, and the concentration of oxygen in the reactor is 10-140 ppb, preferably 30-90 ppb.

[0027] In the preparation method, α-olefin is used to adjust the density of polyethylene, oxygen and temperature are used to adjust the molecular weight of polyethylene, and the inert gas is nitrogen. The method can be carried out in the form of slurry polymerization or solution polymerization.

[0028] The method for preparing the biaxially oriented LLDPE film resin of the present invention has the structural formula of:

[0029]

[0030] Among them, -(CH2) m-CH3 represents a long chain branch, m is greater than or equal to 100; n is greater than or equal to 8000, the average spacing between branching points is 20 to 25 ethylene sequence lengths, the long chain branches are randomly distributed on the main chain of the polyethylene molecule, and the average spacing between the long chain branching points is 500 to 5000 ethylene sequence lengths.

[0031] The resin has a wide molecular weight distribution, a unique short chain branch and a small amount of long chain branch structure, forming a wide melting range and double melting points on the DSC curve, and multiple melting peaks on the SSA curve, and the content of each peak is normally distributed.

[0032] The branching distribution of the short chain branches of the resin in the polyethylene molecular chain is discontinuous, and the long chain branches are randomly distributed on the polyethylene molecular main chain, and the polyethylene molecular main chain is of different lengths. The comonomer inserted in the polyethylene molecular main chain is discontinuously distributed in the molecular chain, and the average spacing between the branching points is 20 to 25 ethylene sequence lengths, that is, the average sequence length of ethylene between the branching points is 20 to 25. The long chain branches are randomly distributed on the polyethylene molecular main chain, and the branching distribution in the molecular chain is randomly changed, and the average spacing between the branching points is 500 to 5000 ethylene sequence lengths, that is, the average sequence length of ethylene between the branching points is 500 to 5000.

[0033] The molecular chain structure of the resin of the present invention has obvious heterogeneity, and the heterogeneity of the molecular chain structure includes heterogeneity caused by different molecular weights and distributions and heterogeneity caused by different lengths, contents and distributions of branches, which has a great influence on the comprehensive information of polyethylene products. The resin has a wide molecular weight distribution, a unique short branch chain and a small amount of long branch chain structure, so that many unfolded branches pass through the non-scenic areas between crystal layers and enter other crystal regions, so that a large number of tie molecules exist between crystal layers, thereby improving the mechanical properties of the polymer; at the same time, these branches contain more long branches, which improves the rheological properties and processing performance.

[0034] The LLDPE film resin has a special wide molecular weight distribution, a unique short chain branch structure and a small amount of long chain branch structure, so that the film has fine crystals, low crystallinity and good transparency. The haze of traditional LLDPE on the market is less than 15%, while the haze of the film resin is less than 5%.

[0035] The method for preparing the biaxially oriented LLDPE film resin of the present invention has a melt index of 0.1-10.0 g / 10 min and a density of 0.916-0.928 g / cm 3; The relative molecular mass range is 6000-280000, preferably 50000-240000; the number average molecular mass is 6000-60000, preferably 30000-50000; the weight average molecular mass is 80000-280000, preferably 100000-240000; the relative molecular mass distribution is 9.0-15.0, preferably 10.0-12.5, and the melt flow ratio is 50-90, preferably 65-75.

[0036] The processing torque of the LLDPE film resin prepared by the method of the present invention is 30 to 60 Nm, preferably 35 to 55 Nm, which is significantly lower than that of traditional LLDPE processing torque on the market; the larger the melt flow ratio of the product and the wider the molecular weight distribution, the smaller the processing torque of the product, the less processing energy consumption of the product, the better the stability and product quality during processing, and the better the comprehensive performance of the product.

[0037] Beneficial effects of the present invention:

[0038] The molecular chain of the biaxially oriented film resin prepared by the method provided by the present invention has a unique structure: a wide molecular weight distribution, a unique short chain branch and a small amount of long chain branch structure, forming a wide melting range and a double melting point on the DSC curve, forming multiple melting peaks on the SSA curve and the content of each peak presents a normal distribution. The wide molecular weight distribution resin, in which the low relative molecular weight part can reduce the apparent viscosity of the melt, plays an internal plasticizing role, improves the shear thinning sensitivity, and improves the processability of the product. A small amount of long chain branch structure can also improve the shear thinning sensitivity and improve the processing performance of the product. The heterogeneity caused by the different molecular weight size and distribution and the heterogeneity caused by the different length, content and distribution of the branch determine the crystallization morphology of the product, forming a wide melting range and a double melting point on the DSC curve, forming multiple melting peaks on the SSA curve and the content of each peak presents a normal distribution, and this unique crystallization morphology makes the resin show unique physical properties. The processing torque is greatly reduced compared with traditional LLDPE on the market; the larger the melt flow ratio of the product and the wider the molecular weight distribution, the smaller the processing torque of the product, the less processing energy consumption of the product, the better the stability and product quality during processing, and the better the comprehensive performance of the product. The film made from this resin has good transparency and processability, and its good comprehensive performance can be used for biaxially oriented film products with special needs. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the GPC characterization spectrum of the biaxially oriented film resin prepared in Example 1 of the present invention;

[0040] Figure 2 This is a DSC characterization spectrum of the biaxially oriented film resin prepared in Example 1 of the present invention;

[0041] Figure 3This is the SSA characterization spectrum of the biaxially oriented film resin prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0042] The present invention is described in detail below by way of examples. It is necessary to point out that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Those skilled in the art in this field can make some non-essential improvements and adjustments to the present invention based on the above content of the present invention.

[0043] Preparation of catalyst:

[0044] 2 g of inorganic carrier SiO2 was immersed in a mixed solution of tetrabutyl titanate, (NH4)2SiF6, ammonium metavanadate and water in a mass ratio of 1:1:1:20 for 5 h at a temperature of 50°C, then dried at 150°C for 12 h and calcined at 800°C for 6 h to obtain titanium, fluorine and vanadium modified silica gel.

[0045] Bistriphenylsilane chromate is the active component of the catalyst, and the catalyst was prepared by the impregnation method. 60 mg of bistriphenylsilane chromate was accurately weighed and placed in a three-necked bottle filled with nitrogen and protected from light. 50 ml of n-hexane solvent that had been dried, dehydrated and degassed with molecular sieves was added to dissolve it, and then 2 g of silica gel carrier modified with titanium, fluorine and vanadium (chromium loading was 0.25%) was added. Under nitrogen protection and anhydrous conditions, the loading and adsorption was carried out for 10 hours. After the impregnation equilibrium was reached, the catalyst was dried under reduced pressure until all the n-hexane was removed to obtain a loaded chromium catalyst, and finally stored under nitrogen protection for use.

[0046] Example 1

[0047] Ethylene, 1-butene, hydrogen, oxygen, nitrogen and a supported chromium-based catalyst were added into a single gas-phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours at a 1-butene / ethylene molar ratio of 0.10:1, a hydrogen to ethylene molar ratio of 0.001, an oxygen concentration of 140 ppb, a polymerization temperature of 80°C, a polymerization pressure of 1.8 MPa, and a circulating gas velocity of 0.60 m / s.

[0048] The prepared resin was subjected to melt flow ratio, haze, torque, molecular weight distribution characterization (GPC), differential scanning calorimetry (DSC) and continuous self-nucleation annealing thermal classification (SSA) characterization. The results are shown in Table 1 and Figure 1 to Figure 3 shown.

[0049] Depend on Figure 1 It can be seen from the GPC curve that the relative molecular mass distribution of the biaxially oriented film resin provided by the present invention is relatively wide, the relative molecular mass difference between the resins is relatively large, the uniformity of the molecular chain length is not good, and the product processing performance and transparency are improved.

[0050] Depend on Figure 2 It can be seen from the DSC curve that a wide melting range and double melting points are formed on the DSC curve, indicating that the product has poor structural heterogeneity between and within molecules. On the one hand, the uniformity of molecular chain length is poor, and on the other hand, the uniformity of comonomer distribution on the molecular chain is poor.

[0051] Depend on Figure 3 It can be seen from the SSA curve that the melting peaks on the SSA characterization curve are basically normally distributed, the content of each fraction is basically equivalent, and the fraction distribution is different from other resins (in the prior art, most of the fractions of the melting peaks of resins are relatively concentrated, and the fraction content at the highest melting temperature is relatively large, and is much larger than that of other fractions), which indicates the heterogeneity of the molecular chain structure in the resin.

[0052] Example 2

[0053] Ethylene, 1-butene, hydrogen, oxygen, nitrogen and a supported chromium-based catalyst were added into a single gas-phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours at a 1-butene / ethylene molar ratio of 0.10:1, a hydrogen to ethylene molar ratio of 0.002, an oxygen concentration of 120 ppb, a polymerization temperature of 82°C, a polymerization pressure of 1.9 MPa, and a circulating gas velocity of 0.62 m / s.

[0054] Example 3

[0055] Ethylene, 1-butene, hydrogen, oxygen, nitrogen and a supported chromium-based catalyst were added into a single gas-phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours under the process conditions of a 1-butene / ethylene molar ratio of 0.10:1, a hydrogen to ethylene molar ratio of 0.003, an oxygen concentration of 80 ppb, a polymerization temperature of 83°C, a polymerization pressure of 2.0 MPa, and a circulating gas velocity of 0.64 m / s.

[0056] Example 4

[0057] Ethylene, 1-butene and 1-hexene, hydrogen, oxygen, nitrogen and a supported chromium catalyst were added into a single gas phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours under the process conditions of a polymerization temperature of 84°C, a polymerization pressure of 2.1 MPa and a circulating gas velocity of 0.66 m / s, with a 1-butene and 1-hexene / ethylene molar ratio of 0.08:1, a 1-butene / 1-hexene molar ratio of 1:1, a hydrogen to ethylene molar ratio of 0.004 and an oxygen concentration of 60 ppb.

[0058] Example 5

[0059] Ethylene, 1-butene and 1-hexene, hydrogen, oxygen, nitrogen and a supported chromium catalyst were added into a single gas phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours under the process conditions of a polymerization temperature of 85°C, a polymerization pressure of 2.2 MPa and a circulating gas velocity of 0.68 m / s, with a molar ratio of 1-butene and 1-hexene / ethylene of 0.08:1, a molar ratio of 1-butene / 1-hexene of 1:1, a molar ratio of hydrogen to ethylene of 0.005 and an oxygen concentration of 50 ppb.

[0060] Example 6

[0061] Ethylene, 1-butene and 1-hexene, hydrogen, oxygen, nitrogen and a supported chromium catalyst were added into a single gas phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours under the process conditions of a polymerization temperature of 86°C, a polymerization pressure of 2.3 MPa and a circulating gas velocity of 0.72 m / s, with a molar ratio of 1-butene and 1-hexene / ethylene of 0.06:1, a molar ratio of 1-butene / 1-hexene of 1:1.5, a molar ratio of hydrogen to ethylene of 0.006 and an oxygen concentration of 40 ppb.

[0062] Example 7

[0063] Ethylene, 1-butene and 1-hexene, hydrogen, oxygen, nitrogen and a supported chromium catalyst were added into a single gas phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours under the process conditions of a polymerization temperature of 87°C, a polymerization pressure of 2.4 MPa and a circulating gas velocity of 0.74 m / s, with a 1-butene and 1-hexene / ethylene molar ratio of 0.06:1, a 1-butene / 1-hexene molar ratio of 1:2, a hydrogen to ethylene molar ratio of 0.007 and an oxygen concentration of 30 ppb.

[0064] Example 8

[0065] Ethylene, 1-hexene, hydrogen, oxygen, nitrogen and a supported chromium-based catalyst were added into a single gas-phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours under the process conditions of a 1-hexene / ethylene molar ratio of 0.09:1, a hydrogen to ethylene molar ratio of 0.008, an oxygen concentration of 20 ppb, a polymerization temperature of 88°C, a polymerization pressure of 2.5 MPa, and a circulating gas velocity of 0.76 m / s.

[0066] Example 9

[0067] Ethylene, 1-hexene, hydrogen, oxygen, nitrogen and a supported chromium-based catalyst were added into a single gas-phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours under the process conditions of a 1-hexene / ethylene molar ratio of 0.09:1, a hydrogen to ethylene molar ratio of 0.008, an oxygen concentration of 10 ppb, a polymerization temperature of 89°C, a polymerization pressure of 2.4 MPa, and a circulating gas velocity of 0.78 m / s.

[0068] Comparative Example 10

[0069] Ethylene, 1-hexene, hydrogen, nitrogen and a supported titanium-based catalyst were added into a single gas-phase fluidized bed reactor, and the polymerization reaction was carried out for 2 hours at a 1-hexene / ethylene molar ratio of 0.3:1 and a hydrogen to ethylene molar ratio of 0.01 under the process conditions of a polymerization temperature of 88°C, a polymerization pressure of 2.4 MPa and a circulating gas velocity of 0.72 m / s.

[0070] Polymerization tests were carried out according to the preparation methods of Examples 1-9 and Comparative Example 10, and the biaxially oriented linear low-density polyethylene resins obtained by polymerization were collected and subjected to physical property tests. The results are listed in Table 1.

[0071] Table 1 Physical properties of polymer products tested

[0072]

[0073] As can be seen from Table 2, the biaxially oriented film resin prepared by the method provided by Examples 1-9 of the present invention has good transparency and processability, the haze of the film products is less than 5%, the processing torque is low at 30-60Nm, and only a small amount of comonomer and hydrogen are needed to directly polymerize in a single reactor, the polymerization operating conditions are mild, the process flow is short, and the existing equipment can be produced by simple modification, which has good application prospects. However, the resin prepared by Comparative Example 10 using a traditional supported titanium catalyst has a narrow molecular weight distribution, a small melt flow ratio, and poor product transparency and processability.

[0074] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, technicians familiar with the field may make various corresponding changes and deformations based on the present invention, but these corresponding changes and deformations should all fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a biaxially oriented LLDPE film resin, characterized in that: The following steps are involved: Ethylene and alpha-olefins are polymerized in a single reactor in the presence of hydrogen, oxygen, an inert gas and a supported chromium catalyst, wherein the supported chromium catalyst comprises a support and chromium oxide modified with titanium, fluorine and vanadium; The polymerization is gas phase polymerization, the polymerization temperature is 80-90°C; the polymerization pressure is 1.8-2.5MPa; the circulation gas velocity is 0.60-0.82m / s; the residence time is 1-8h; The molar ratio of α-olefin to ethylene is 0.08:1~0.1:1; the molar ratio of hydrogen to ethylene is 0.001:1~0.01:1; and the concentration of oxygen in the reactor is 10~140ppb.

2. The method for preparing a biaxially oriented LLDPE film resin according to claim 1, characterized in that: The α-olefin is 1-butene and / or 1-hexene.

3. The method for preparing a biaxially oriented LLDPE film resin according to claim 1, characterized in that: The carrier is silicon dioxide, aluminum oxide, zirconium oxide or thoria.

4. The method for preparing a biaxially oriented LLDPE film resin according to claim 1, characterized in that: The polymerization is gas phase polymerization, the polymerization temperature is 83-86° C., the polymerization pressure is 2.0-2.3 MPa, the circulation gas velocity is 0.64-0.74 m / s, and the residence time is 4-6 h.

5. The method for preparing a biaxially oriented LLDPE film resin according to claim 1, characterized in that: The molar ratio of hydrogen to ethylene is 0.003:1~0.006:1, and the concentration of oxygen in the reactor is 30~90ppb.

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