CN107987203A - A kind of preparation method of transparent LLDPE film resin - Google Patents

Abstract

The invention discloses a preparation method of a transparent LL DPE film resin, which is characterized in that ethylene and α -olefin are polymerized in a single reactor in the presence of hydrogen, oxygen, inert gas and a modified supported chromium catalyst, wherein the polymerization temperature is 60-90 ℃, the polymerization pressure is 1.8-2.5 MPa, &ttttranslation = alpha &tttα &ttt/t &ttt-olefin and ethylene are in a molar ratio of 0.01: 1-0.09: 1, the molar ratio of hydrogen to ethylene is 0.001: 1-0.01: 1, and the concentration of oxygen in the reactor is 10-140 ppb.

Description

A kind of preparation method of transparent LLDPE film resin

technical field

The invention relates to a film resin, in particular to a transparent and easy-to-process LLDPE film resin.

Background technique

LLDPE (linear low-density polyethylene) film resin is mainly used to produce agricultural film and packaging film. In recent years, with the development of my country's light industry export business, the consumption of film-grade LLDPE has grown steadily. In 2015, the apparent consumption of LLDPE film material was 4.8 million tons , the demand for high-strength and high-transparency LLDPE is 200,000 tons per year, mainly replacing LDPE materials on the market.

LLDPE is a copolymer of ethylene and α-olefin. The introduction of α-olefin monomer makes the polymer contain a considerable number of branched chains. These branched chains of different lengths directly affect the performance of the polymer. The molecular chain of LLDPE is linear and has highly branched short chains, so its crystallinity is low. Compared with LDPE, its properties such as stress crack resistance, tensile strength, tear strength and drop hammer impact strength have greater advantages. Improvement, the appearance is similar to LDPE, but the transparency is slightly worse, and the processing performance is not as good as LDPE.

In the process of blown film production, when the resin transitions from the molten state to the glass state, it begins to crystallize in a certain temperature range below the melting point. Since the crystal nuclei are randomly generated, the spherulites grow under natural conditions, resulting in 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 crystal region and the amorphous region, irregular light scattering and reflection occur at the interface, thus making LLDPE The transparency of the resin is poor. Therefore, by controlling the crystallization properties of LLDPE resin and reducing the difference between the crystalline phase region and the amorphous phase region, the transparency of the LLDPE film can be improved.

In terms of processing performance, LLDPE is poorly sensitive to stress, that is, under the high shear stress of extrusion processing, the melt viscosity is much higher, which requires larger torque, higher melt temperature and die head pressure, and is prone to melt body rupture; but no hardening occurs when the bubble is stretched. In addition, the melt strength of LLDPE is poor, and the film bubble is not resistant to the impact of high-speed cold air when it is cooled, and its stability is poor. It is necessary to strengthen the design of the cooling air ring.

In order to improve the transparency and processability of existing LLDPE products, the synthesis of highly transparent and easy-to-process LLDPE has attracted the attention of many researchers in recent years. High-transparency and easy-processing high-performance resin products are developed on the basis of ordinary LLDPE. Its molecules contain both long and short chain branches and have a wide molecular weight distribution. The product combines the advantages of LDPE and LLDPE, and has the excellent properties of LLDPE Physical and mechanical properties, combined with the processing and transparency properties of LDPE products, have irreplaceable advantages of other products in terms of strength, stability, low shrinkage, crack resistance, etc., widely used in various film products, can be used in the present Processing on LDPE processing equipment (mainly film blowing equipment) can more replace traditional LDPE in the terminal market.

The molecular structure difference between LLDPE and LDPE is manifested in: LLDPE has no or few long-chain branches, and LLDPE for traditional membranes is a narrow Mw/Mn polymer (Mw/Mn is 3 to 4). The chain length of short chain branches of LLDPE depends on the type of comonomer. The number of branches depends on the incorporated comonomer content. Only the copolymerization of olefins with a carbon number greater than or equal to 4 can effectively disrupt the close packing of chain molecules, thereby reducing the density to obtain LLDPE, and at the same time produce tie molecules connecting different crystal regions to improve the toughness and strength of the polymer. Therefore, LLDPE for film is a copolymer of ethylene and l-butene, l-hexene, l-octene or 3-methyl-pentene. The crystallization state of LLDPE depends not only on the type of comonomer, but also on the distribution of comonomer in the polymer and the frequency of distribution in the molecular chain. At the same density, the melting point of LLDPE is 10-15°C higher than that of LDPE. Observed with a high-power polarizing microscope, the diameter of LLDPE spherulites is 10-20 μm, while that of LDPE spherulites is only 2-3 μm.

There are three basic approaches to designing LLDPE polymer chains to improve transparency and processability: designing catalysts to produce long branched structures, bimodal or broad relative molecular weight distribution structures, introducing terpolymerization of a third (or fourth) comonomer Substances (or tetrapolymers), additives are added during post-processing.

At present, it is mainly achieved by improving traditional catalysts, and the most notable processes involved include Montell's SPHERILENE (terpolymer) process, Phillips' LLDPE (wide MWD) process, Univation's UNIPOL bimodal process, and Recently, Borealis' BORSTAR (double peak) process and NOVA chemical company's advanced SCLAIRTECH (double peak) process. More processable LLDPE has also been produced industrially using metallocene/single-site catalyst systems. These new technologies include Dow's long-chain branched (LCB) INSITE process, Mitsui's EVOLUE (double peak) process and Phillips' mPACT process.

At present, it is still difficult for LLDPE to completely replace LDPE. The main reason is the long-chain branched structure contained in LDPE. The so-called long-chain branching means that the molecular weight of the branched chain is greater than the minimum entanglement molecular weight of the molecular chain. For polyethylene, the minimum entanglement chain length is about 1000 g/mol. Long-chain branched polyethylene (LCB-PE) has excellent rheological properties and high strength, which is unmatched by other structural polyethylenes. When the branched chain length of LCB-PE exceeds the critical chain length of polyethylene, the branched chain will enhance the entanglement between the molecular chains, thus showing high melt strength at low shear rates; the branched chain will reduce the polymerization rate. The hydrodynamic volume of ethylene, which results in a high shear-thinning behavior at high shear rates. These characteristics are of great significance to the processing modification of polyethylene materials, especially high melt viscosity polyethylene (such as metallocene polyethylene, high molecular weight polyethylene, etc.). In addition, long chain branched polyethylene can significantly improve the performance of polyethylene film. Therefore, LCB-PE has long been concerned by the scientific and industrial circles. In recent years, LCB-PE is still one of the research hotspots in the field of polyolefins. LCB-PE is a high value-added product, and the development of this product is in line with the development goals of my country's polyolefin industry. The structure of LCBPE prepared by different methods is different, but in general, its rheological properties can be improved. What kind of structure of LCB-PE is the best to use, and what kind of synthesis method is used to prepare LCB-PE with the lowest cost? These problems have not yet been well resolved. Therefore, this topic attracts people to continue to explore and find new and more suitable synthesis methods to achieve the goal of large-scale and low-cost preparation of optimal structure LCB-PE.

At present, the long-chain branched structure catalysts can be produced as the second-generation transparent and easy-to-process metallocene catalysts and chromium-based catalysts, but due to the protection restrictions of major companies, the catalysts cannot be obtained.

The biggest feature of bimodal polyethylene is that its molecular weight distribution presents two peaks. Since the processability and mechanical properties of polyethylene resin are contradictory, increasing the molecular weight can make the product have better mechanical properties, but at the same time it will make the resin difficult to process. The bimodal polyethylene is composed of high molecular weight and low molecular weight. The high molecular weight part is used to ensure the physical and mechanical strength, and the low molecular weight part is used to improve the processing performance, which can effectively solve the processability of polyethylene resin. The problem of conflicting with mechanical properties enables the rigidity and toughness of the material to achieve a good balance. Therefore, the parts produced by bimodal polyethylene resin have higher strength, better stress cracking resistance and better molding performance than those produced by ordinary grade resin. Bimodal polyethylene also has a better market prospect because of these excellent properties.

Bimodal LLDPE not only has better physical properties than unimodal LLDPE, but also has a longer service life. The distribution and structure of the bimodal molecular weight can improve all properties in a balanced manner. This product first appeared in the European market, and has been widely used in the production of various daily-use products abroad, such as films, flexible packaging materials and blow molding.

At present, only Shanghai Petrochemical can produce such products in my country. The low melt index LLDPE produced by Beixing double peak PE technology is easy to process and has good rigidity. Compared with LDPE film materials, LLDPE film materials have similar processing properties, but the processed films have higher tear strength, so the film thickness can be reduced. Representative bimodal LLDPE uses include: industrial gaskets, heavy-duty packaging, refrigerated packaging, compression packaging, agricultural films, etc.

The production of bimodal products is mainly based on the reactor series process and bimodal catalysts for single reactors. For gas phase single reactor devices, bimodal products cannot be realized with existing single catalysts. In addition, due to the protection restrictions of major companies, suitable bimodal catalysts cannot be obtained.

Chromium-based catalysts are mainly used in the production of products with a wide distribution of molecular weight, and chromium-based catalysts can produce long-chain branched structures to improve rheological properties and improve processability. Due to the introduction of the third monomer, the ternary polymerization process has more branched chains compared with the binary copolymerization, and many unfolded branched chains pass through the amorphous region between the crystal layers and enter other crystal regions, so that there are a large number of branches between the crystal layers. The tether molecules of the polymer improve the mechanical properties of the polymer; at the same time, these branches contain more long chain branches, which improve the rheological properties and improve the processing performance. In addition, the addition of the third monomer has a certain influence on the crystal size and improves the transparency of the product to a certain extent.

Dow Chemical Company used Insite technology to synthesize metallocene PE (mPE) with narrow MWD and a small amount of long-chain branching. Among them, the macromonomer can be generated by the β-H elimination reaction in the process of ethylene homopolymerization or the chain transfer reaction to the monomer transfer. Using Constrained Geometric Configuration Technology (CGCT), the long-chain branches are introduced into the linear short-chain branched structure of the polymer in a controlled manner. The highly regular short-chain branches and a limited amount of long-chain branches make the polymers have both excellent physical properties and Has good processing performance.

Enable ^TM mPE series products of American Exxon mobil chemical company. This unique resin combines outstanding film processability with the excellent physical properties of high carbon alpha olefins. Enable mPE is an ethylene-1-hexene copolymer with a density of 0.920-0.927g/cm ³ and a melt flow rate (MFR) of 0.3-1.0g/10min. This product has extremely wide operability on both LLDPE and LDPE equipment, and can cope with variable processing conditions.

Univation uses a new composite carrier metallocene catalyst, gas phase polymerization, and synthesizes long-chain branched second-generation LLDPE in a single high-pressure reactor. It has a relatively high melt flow ratio and melt strength (MS), which is equivalent to LLDPE /LDPE blends [w (LDPE) 20% ~ 30%] processability.

Borealis uses the Borstar process to produce bimodal LLDPE, which has narrow MWD, less extractables, and good processability. Compared with traditional HDPE, it has higher tensile modulus and impact strength, and unique oil resistance, transparency and sealing. it is good.

The Evolue LLDPE copolymer produced by Japan's Mitsui Chemicals is superior to traditional LLDPE in terms of impact strength, adhesion resistance, low temperature heat sealability, and formability. Evolue LLDPE is synthesized by two gas-phase fluidized bed reactors. Its relative molecular weight has a bimodal distribution, and its processing performance is similar to that of LDPE. It can be processed by existing LDPE blown film equipment.

CN1338477A has introduced a kind of preparation LLDPE carrier catalyst system and the method for preparing LLDPE, and this method provides a kind of bifunctional catalyst system that is used for ethylene synthesis linear low density polyethylene, and dimerization catalyst is alkoxy compound catalyst, and copolymerization catalyst is made of The supported metallocene catalyst is composed of alkyl aluminum or boron compound as a cocatalyst, and in the same polymerization system, it directly dims ethylene and undergoes copolymerization reaction in situ to produce LLDPE.

CN1124034A has introduced a kind of highly processable LLDPE polymer composition, based on the polymer composition of linear low density polyethylene (LLDPE), this composition comprises (A) the copolymerization of 75～95% (weight) ethylene and alpha-olefin Material, (B) 5-25% (weight) copolymer of propylene, ethylene and alpha-olefin. 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 each reactor is not limited, and the catalyst used is the same.

CN1145082A has introduced a kind of LLDPE resin blend, a kind of LLDPE and low-density high-pressure polyethylene resin blend prepared by metallocene catalyst can be extruded and made into the film that has improved optical property and impact strength.

CN1183105A has introduced a kind of method of producing LLDPE polymer, and this method is a kind of ethylene copolymer by polymerizing ethylene and a small amount of C ₃ -C ₆ α-olefins in slurry reactor in the presence of ethylene polymerization catalyst to form ethylene copolymer Process, according to the invention, polymerization is carried out in propane diluent by using a metallocene catalyst activated by alumoxane.

CN1217343A has introduced a kind of synthesizing LLDPE bimetallic catalyst and its preparation method, and this method relates to a kind of bimetallic catalyst system that is used for synthesizing linear low density polyethylene by ethylene, dimerization catalyst is made of tungsten series main catalyst and halogenated alkylaluminum cocatalyst Composition, the copolymerization catalyst is composed of titanium catalyst and aluminum alkyl. The bimetallic catalytic system composed of these two catalysts can directly cause ethylene dimerization and in-situ copolymerization reaction in the same polymerization system to generate LLDPE, and copolymerize Very active.

Ye Zhibin et al. used an oligomerization catalyst (η ₅ -C ₅ H ₄ CMe ₂ C ₆ H ₅ ) in the study of the long branching and rheological properties of ethylene-1-hexene copolymers synthesized using an in-situ polymerization catalyst system LLDPE was successfully synthesized by TiCl ₃ /modified methylaluminoxane (MMAO) and copolymerization catalyst [(η ₅ -C ₅ Me ₄ )SiMe ₂ (tBuN)]TiCl ₂ /MMAO. The small-amplitude dynamic vibration test results showed the typical rheological properties of long-chain branched polymers (such as the enhancement of zero-shear viscosity, the improvement of shear thinning, the improvement of dynamic modulus, and the complex thermal rheological properties), which fully explained the synthesized There are long chain branches in LLDPE.

Galland et al found in the research on the synthesis of long branched PE by Fe, Zr catalyst that Fe catalyst can synthesize α-olefin with Me close to 2000g/mol. The α-olefin is polymerized with ethylene at the Zr activation point to synthesize PE with long chain branches, wherein the amount of long chain branches accounts for more than 40% of the total amount of branches.

Farley et al. used a transition metal-type catalyst system to synthesize LLDPE in a gas-phase fluidized bed single reactor. Compared with the narrow MWD metallocene product EXCEED, the LLDPE resin can be produced at a lower rate under similar MFR, comonomer type and density. It is relatively easy to extrude film products through casting or blown film processing processes under lower motor loads, higher throughput and reduced head pressure. Under similar MFR, the LLDPE resin has higher weight-average molecular weight and wider MWD than EXCEED ^TM resin, and the film tear performance is bidirectionally balanced (the aspect ratio is generally greater than 0.9), and the dart impact strength is greater than 19.7g/um .

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

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

Myhre et al. invented a bimodal PE composition breathable film. The bimodal PE component provides the film with high strength and excellent processability, and enables the production of films with low basis weight (25 g/ ^m2 or less). Among them, the low relative molecular weight components can help improve the processing performance, the MFR is 0.1-4.0g/10min, and the density is about 0.918-0.935g/cm ³ ; the high relative molecular weight components make it have certain mechanical properties. The high molecular mass component is a copolymer of ethylene and C ₄ -C ₁₀ olefins, with a higher comonomer content. For a given low molecular weight component content and component distribution ratio, the bimodal PE produced has the desired MFR and density.

Cheng song et al. introduced long branched chains into LLDPE by using a relatively low electron beam radiation method. There is little or no crosslinking in the irradiated LLDPE, and the MWD broadens. At low shear rates, the melt hardness of long-branched LLDPE increases; at high elongation at break, the melt strength increases. The long-chain branches make the irradiated LLDPE have rheological behaviors such as relaxation resistance and strain hardening, and improve the processing performance of LLDPE (such as blown film, foaming and hollow injection molding, etc.). Grafting of LLDPE can also improve the processing performance of LLDPE.

Yao Zhanhai et al. used capillary rheometer to study the rheological behavior of acrylic acid (AA) grafted LLDPE (LLDPE-g-AA). Under high shear stress, the apparent viscosity of LLDPE-g-AA is smaller than that of pure LLDPE; the apparent viscosity of LLDPE-g-AA decreases with the increase of AA, indicating that the AA grafted to the LLDPE molecular chain plays an internal role. The role of lubricants; the tensile strength, Young's modulus and elongation at break of the grafted product are basically the same as those of pure LLDPE.

Dynamar is a series of polymer processing aid products based on fluoropolymers developed by 3M Tailiang Company, which can be used to improve the processability of resins without affecting the physical and mechanical properties of matrix resins. At present, it has been successfully applied in the processing of LLDPE resin.

CN200910232637.2 relates to a special composite additive for high-transparency and low-density polyethylene film, which uses high-pressure polyethylene as a carrier, consists of 25% to 30% silicon dioxide blocking agent, 5% to 15% silicone flow aid, 2 ～8% organic silicon active crosslinking agent, 1～10% organic silicon antioxidant and the remaining high-pressure polyethylene resin are mixed by stirring, preheated ball milling at 75～85°C to disperse, and finally at 140°C～170°C, pass The granules extruded and granulated by the twin-screw granulator, and the low-density polyethylene film made by adding this compound additive, are odorless, highly transparent, and have good physiological safety, high opening, smoothness and extreme Good migration resistance. It can be especially used in food, medicine, business card, ID card lamination and advertising packaging, etc., which require high transparency, high safety and high opening of the film.

CN200710064958.7 relates to a method for preparing highly transparent linear low-density polyethylene, in particular to a method for preparing highly transparent linear low-density polyethylene by using polyethylene glycol monoacrylate as anti-reflection agent. First, the anti-reflection agent polyethylene glycol monoester is synthesized, which is based on acrylic acid, polyethylene glycol, resorcinol, p-toluenesulfonic acid, β-(3′,5′-ditertiary Butyl-4'-hydroxyphenyl) propylene octadecyl ester and distearyl pentaerythritol phosphite are raw materials; The weight ratio is 0.1-1.5:100, melt blending and extruding at 160°C-210°C to prepare highly transparent linear low-density polyethylene resin.

The above adopts the design catalyst to produce long branched structure, bimodal or wide relative molecular mass distribution structure, introduces the terpolymer (or tetrapolymer) of the third (or fourth) comonomer, and adds it in the post-processing process Additive blending modification, chemical modification and other technologies produce transparent and easy-to-process products, but there are certain deficiencies in product quality uniformity, production cost and product comprehensive performance. At present, there is a lack of a transparent and easy-to-process film base resin with a wide molecular weight distribution, a unique short-chain branched structure and a small amount of long-chain branched structure, and a preparation method. The film obtained by processing the resin has good transparency and processability. Film resin with good performance, uniform quality and low production cost.

Contents of the invention

The invention provides a method for preparing transparent LLDPE film resin. The resin produced by the method has a wide molecular weight distribution, a unique short-chain branch structure and a small amount of long-chain branch structure, has good transparency and processability, and can be used for special Film products such as packaging film and stretch film required.

A kind of preparation method of transparent LLDPE film resin provided by the invention, this method is that ethylene and α-olefin are polymerized in a single reactor in the presence of hydrogen, oxygen, inert gas and modified loaded chromium catalyst, wherein , the polymerization temperature is 60～90℃, the polymerization pressure is 1.8～2.5MPa, the molar ratio of α-olefin to ethylene is 0.01:1～0.09:1, the molar ratio of hydrogen to ethylene is 0.001:1～0.01:1, the reaction The concentration of oxygen in the device is 10-140ppb. The polymerization includes slurry polymerization, gas phase polymerization and solution polymerization, and the inert gas is nitrogen, and its function is mainly to maintain the pressure in the reactor.

The preparation method of the transparent LLDPE film resin of the present invention, wherein, the molar content of the α-olefin in the transparent LLDPE film resin is 0.5-5.0%.

The preparation method of the transparent LLDPE film resin of the present invention, wherein, the supported chromium catalyst refers to a catalyst in which a chromium compound is loaded on a carrier, the chromium compound is chromium oxide, and the carrier is silicon dioxide, aluminum oxide , zirconia or thorium oxide.

In the transparent LLDPE film resin of the present invention, the modified supported chromium catalyst is a catalyst obtained by modifying the supported chromium catalyst through aluminum, titanium, fluorine and vanadium.

The preparation method of the transparent LLDPE film resin of the present invention, wherein the α-olefin is 1-butene and/or 1-hexene.

The preparation method of the transparent LLDPE film resin of the present invention, wherein, the molar ratio of the α-olefin to ethylene is 0.03:1˜0.06:1.

The preparation method of the transparent LLDPE film resin of the present invention, wherein, the molar ratio of hydrogen to ethylene is 0.003:1˜0.006:1.

The preparation method of the transparent LLDPE film resin of the present invention, wherein the concentration of oxygen in the reactor is 30-90 ppb.

The preparation method of the transparent LLDPE film resin according to the present invention, wherein, the polymerization is gas phase single reactor polymerization, the polymerization temperature is 80-90°C, preferably 83-86°C, and the polymerization pressure is 1.8-2.5MPa, preferably 2.0- 2.3MPa, the circulating gas velocity is 0.60-0.82m/s, preferably 0.64-0.74m/s, and the residence time is 1-8h, preferably 4-6h.

The transparent LLDPE film resin of the present invention, wherein, in the modified supported chromium catalyst, the total load of chromium, vanadium, titanium and aluminum is 0.01wt% to 15wt% of the total weight of the catalyst, preferably 0.05wt% ～8wt%, more preferably 1wt%～6wt%, wherein, the molar ratio of chromium and vanadium is 0.1:0.9～0.9:0.1, preferably 0.25:0.75～0.75:0.25, more preferably 0.4:0.6～0.6:0.4, chromium and vanadium The molar ratio of titanium is 0.1:0.9-0.9:0.1, preferably 0.25:0.75-0.75:0.25, more preferably 0.4:0.6-0.6:0.4. The loading amount of fluorine is 0.01wt%～10wt% of the total weight of the catalyst, preferably 0.05wt%～8wt%, more preferably 1wt%～6wt%, wherein the molar ratio of chromium to fluorine is 0.1:0.9～0.9:0.1, preferably 0.25:0.75～0.75:0.25, more preferably 0.4:0.6～0.6:0.4.

Beneficial effects of the present invention:

1. The catalyst used in the preparation of film resin in the present invention is a chromium-based catalyst modified by aluminum, titanium, fluorine and vanadium, and the content of active components is strictly controlled. Aluminum modification makes the catalyst generate high molecular weight polyethylene The number of active sites increases, and at the same time inhibits the active sites in the catalyst that catalyze the formation of low-molecular-weight polyethylene, making the polymerization reaction more inclined to generate high-molecular-weight polyethylene; titanium modification not only increases the activity of the active sites in the catalyst, making The molecular weight of polyethylene is increased, and it can also reduce the insertion amount of branched chains in low molecular weight parts; fluorine modification greatly improves the activity of low-temperature activated catalysts, and can improve the insertion amount of α-olefins and reduce the content of low molecular weight polyethylene; Vanadium modification can also inhibit the formation of low molecular weight polyethylene and increase the content of high molecular weight polyethylene.

2. In addition to using the general molecular weight regulator hydrogen, the present invention also adds oxygen as a catalyst inhibitor (poison) into the system to strictly control the addition of oxygen to adjust product performance.

3. The film resin prepared by the present invention has a special wide molecular weight distribution, a unique short-chain branched structure and a small amount of long-chain branched structure, so that the crystallization is refined, the crystallinity is reduced, and the transparency is good. Compared with the traditional LLDPE haze on the market, it is <15% , The resin haze of this film is <13%.

Description of drawings

Fig. 1 is the GPC characterization spectrogram of transparent LLDPE film resin of the present invention;

Wherein, Mw represents weight average molecular weight, and W represents quality;

Fig. 2 is the NMR characterization spectrogram of transparent LLDPE film resin of the present invention;

Fig. 3 is the SSA characterization spectrogram of transparent LLDPE film resin of the present invention;

Fig. 4 is the TREF characterization spectrogram of transparent LLDPE film resin of the present invention;

Fig. 5 is the processing torque figure of transparent LLDPE film resin of the present invention;

Figure 6 is a rotational rheological diagram of a transparent LLDPE film resin.

Wherein, a curve represents the test result of embodiment 2, b curve represents the test result of embodiment 4, c curve represents the test result of embodiment 5, d curve represents the test result of embodiment 6, and e curve represents the test result of embodiment 8.

Detailed ways

The present invention is described in detail by the following examples. It is necessary to point out that this example is only used to further illustrate the present invention, and can not be interpreted as limiting the protection scope of the present invention. Make corresponding non-essential improvements and adjustments.

Catalyst preparation:

Use Al ₂ O ₃ , titanium and F to modify the porous inorganic carrier SiO ₂ carrier (the carrier can also be replaced by zirconia or thorium oxide), and impregnate the vanadium source on the porous inorganic carrier SiO ₂ on a silica gel carrier to obtain the desired modified catalyst. Immerse about 10-15g of porous silica gel modified by Ti, Al, F, V in basic chromium acetate aqueous solution, wherein the chromium load is 0.1-1wt% of the total weight of the catalyst, after continuous stirring for 3-8 hours , heat up and dry, then roast the silica gel carrier loaded with basic chromium acetate in a fluidized bed at high temperature, remove the physical water of the carrier at the low temperature section, and remove the hydroxyl groups on the silica gel surface at the high temperature section of 500°C-900°C, This high temperature section is maintained for 5-8 hours, and finally under the protection of nitrogen, the temperature is naturally lowered and cooled to obtain the modified catalyst required by the present invention.

The melt index range of the resin prepared by the method of the present invention is 0.3-1.0g/10min, and the density is 0.916-0.922g/cm ³ ; the relative molecular mass range of the resin is 6000-280000, and the number-average relative molecular mass range is 6000-60000, the weight-average molecular mass range is 80000-280000, and the relative molecular mass distribution is 9.0-15.0.

The transparent and easy-to-process film resin prepared by the method of the invention has a melt flow ratio of 50-90 and a processing torque of 30-60Nm.

Example 1

Add ethylene, 1-butene, hydrogen, oxygen, nitrogen, and modified supported chromium-based catalysts into a single gas-phase fluidized bed reactor. According to the 1-butene/ethylene molar ratio of 0.01:1, the ratio of hydrogen to ethylene The molar ratio is 0.001, the oxygen concentration is 140ppb, and the polymerization reaction is carried out under the process conditions of polymerization temperature of 80°C, polymerization pressure of 1.8MPa, circulation gas velocity of 0.60m/s, catalyst addition of 2g/h, and residence time of 2h .

Example 2

Add ethylene, 1-butene, hydrogen, oxygen, nitrogen, and modified supported chromium-based catalysts into a single gas-phase fluidized bed reactor. According to the 1-butene/ethylene molar ratio of 0.024:1, the ratio of hydrogen to ethylene The molar ratio is 0.002, the oxygen concentration is 120ppb, and the polymerization reaction is carried out under the process conditions of polymerization temperature of 82°C, polymerization pressure of 1.9MPa, circulation gas velocity of 0.62m/s, catalyst addition of 2g/h, and residence time of 2h. .

Example 3

Add ethylene, 1-butene, hydrogen, oxygen, nitrogen, and modified supported chromium-based catalysts into a single gas-phase fluidized bed reactor. According to the 1-butene/ethylene molar ratio of 0.028:1, the ratio of hydrogen to ethylene The molar ratio is 0.003, the oxygen concentration is 80ppb, and the polymerization reaction is carried out under the process conditions of polymerization temperature of 83°C, polymerization pressure of 2.0MPa, circulation gas velocity of 0.64m/s, catalyst addition of 2g/h, and residence time of 3h. .

Example 4

Add ethylene, 1-butene and 1-hexene, hydrogen, oxygen, nitrogen and modified supported chromium-based catalysts into a single gas-phase fluidized bed reactor, according to 1-butene and 1-hexene/ethylene The molar ratio is 0.03:1, the molar ratio of 1-butene/1-hexene is 1:1, the molar ratio of hydrogen to ethylene is 0.004, the oxygen concentration is 60ppb, the polymerization temperature is 84°C, the polymerization pressure is 2.1MPa, and the circulating gas The polymerization reaction was carried out under the technical conditions of a speed of 0.66m/s, a catalyst addition of 2g/h, and a residence time of 4h.

Example 5

Add ethylene, 1-butene and 1-hexene, hydrogen, oxygen, nitrogen and modified supported chromium-based catalysts into a single gas-phase fluidized bed reactor, according to 1-butene and 1-hexene/ethylene The molar ratio is 0.035:1, the molar ratio of 1-butene/1-hexene is 1:1, the molar ratio of hydrogen to ethylene is 0.005, the oxygen concentration is 50ppb, the polymerization temperature is 85°C, the polymerization pressure is 2.2MPa, and the circulating gas The polymerization reaction was carried out under the technical conditions of the speed of 0.68m/s, the catalyst addition amount of 2g/h, and the residence time of 4.5h.

Example 6

Add ethylene, 1-butene and 1-hexene, hydrogen, oxygen, nitrogen and modified supported chromium-based catalysts into a single gas-phase fluidized bed reactor, according to 1-butene and 1-hexene/ethylene The molar ratio is 0.035:1, the molar ratio of 1-butene/1-hexene is 1:1.5, the molar ratio of hydrogen to ethylene is 0.006, the oxygen concentration is 40ppb, the polymerization temperature is 86°C, the polymerization pressure is 2.3MPa, and the circulating gas The polymerization reaction was carried out under the technical conditions of the speed being 0.72m/s, the catalyst addition amount being 2g/h, and the residence time being 4.8h.

Example 7

Add ethylene, 1-butene and 1-hexene, hydrogen, oxygen, nitrogen and modified supported chromium-based catalysts into a single gas-phase fluidized bed reactor, according to 1-butene and 1-hexene/ethylene The molar ratio is 0.04:1, the molar ratio of 1-butene/1-hexene is 1:2, the molar ratio of hydrogen to ethylene is 0.007, the oxygen concentration is 30ppb, the polymerization temperature is 87°C, the polymerization pressure is 2.4MPa, and the circulating gas The polymerization reaction was carried out under the technical conditions of speed of 0.74m/s, catalyst addition of 2g/h, and residence time of 5h.

Example 8

Add ethylene, 1-hexene, hydrogen, oxygen, nitrogen, and a modified supported chromium-based catalyst into a single gas-phase fluidized bed reactor. According to the 1-hexene/ethylene molar ratio of 0.05:1, the ratio of hydrogen to ethylene The molar ratio is 0.008, the oxygen concentration is 20ppb, and the polymerization reaction is carried out under the process conditions of polymerization temperature of 88°C, polymerization pressure of 2.5MPa, circulation gas velocity of 0.76m/s, catalyst addition of 2g/h, and residence time of 6h. .

Example 9

Add ethylene, 1-hexene, hydrogen, oxygen, nitrogen, and a modified supported chromium-based catalyst into a single gas-phase fluidized-bed reactor. According to the 1-hexene/ethylene molar ratio of 0.055:1, the ratio of hydrogen to ethylene The molar ratio is 0.008, the oxygen concentration is 10ppb, and the polymerization reaction is carried out under the process conditions of polymerization temperature of 89°C, polymerization pressure of 2.4MPa, circulation gas velocity of 0.78m/s, catalyst addition of 2g/h, and residence time of 7h .

Example 10

Add ethylene, 1-hexene, hydrogen, oxygen, nitrogen, and a modified supported chromium-based catalyst into a single gas-phase fluidized-bed reactor. According to the 1-hexene/ethylene molar ratio of 0.06:1, the ratio of hydrogen to ethylene The molar ratio is 0.01, and the polymerization reaction is carried out under the technological conditions of polymerization temperature of 90°C, polymerization pressure of 2.4MPa, circulation gas velocity of 0.82m/s, catalyst addition of 2g/h, and residence time of 8h.

The polymerization test was carried out according to the preparation method of Examples 1-10, and the transparent and easy-to-process linear low-density polyethylene resin obtained by polymerization was collected, and its physical properties were tested. The results are listed in Table 1.

Table 1 embodiment 1-10 polymer product physical property test

The present invention characterizes the transparent and easy-to-process film resins prepared in five examples, and the characterization spectra are shown in Figures 1-6.

As can be seen from the GPC comparison curve of Fig. 1, the relative molecular mass distribution of the transparent and easy-to-process film resin provided by the present invention is relatively wide, the relative molecular mass difference between the resins is large, and the molecular chain length uniformity is not good, reflecting the ease of processing. The product is characterized by a broad molecular weight distribution.

The NMR spectrum in Figure 2 shows that the copolymer resins have the same peak positions on the spectrum, indicating that they have the same type of functional groups. After the peaks of the spectrum are assigned and the regions are divided, the calculated three-unit sequence distribution data is shown in Table 2 . (The three-unit sequence distribution is used to characterize the distribution of comonomers, where E represents a vinyl group, B represents a butene group, and H represents a hexene group, such as EBE, which represents the insertion of a butene in the middle of two ethylene groups. Structural units)

Table 2 Triad sequence distribution/%

From the comparison of the three-unit sequence distribution data, it can be seen that the content of comonomers in the molecular chains of the resins in each embodiment is equivalent.

At present, foreign studies on the heterogeneity of the molecular chain structure of transparent and easy-to-process polyethylene mainly focus on the use of solution pumping classification, temperature-rising dissolution classification and cross-classification methods to characterize the heterogeneity of transparent and easy-processing polyethylene. Work is mainly carried out in large petrochemical companies (such as DuPont, Philips, etc.). In colleges and universities, such as the University of Akron in the United States, the DSC multi-step crystallization classification method is mainly used to characterize the branching heterogeneity and its influence on the crystal structure. The present invention characterizes the molecular chain structure of the transparent and easy-to-process film resin provided by the present invention through two analysis methods of continuous self-nucleation annealing thermal classification (SSA) and temperature-rising leaching fractionation (TREF). The characterization spectrum is shown in Figure 3 ~ Figure 4.

The SSA characterization curve usually has multiple narrow melting peaks, and different melting peaks represent the melting results of lamellar crystals with different thicknesses, that is, lamellar crystals formed by chain structural units of different molecular sizes. This is because when the temperature is raised to the annealing temperature after the first melting and cooling, only a part of the wafer can be melted, and the unmelted part is the part with relatively perfect crystallization, and they are relatively thick lamellar crystals. At the second annealing temperature, another part of the lamellae was not melted. In this way, wafers of different thicknesses can be graded, and the formed wafers of different thicknesses are related to the structure of molecular chains. Among these melting peaks, the higher temperature peaks correspond to more structured molecules with thicker lamellae, while the lower temperature peaks correspond to less structured molecules with thinner lamellae , the comonomer content is relatively high. In this way, each melting peak on the heating curve after SSA classification basically represents a crystal formed by a class of molecules with very close branched chain contents.

On the SSA characterization curves of similar resins at home and abroad characterized in previous document retrieval and work, each melting peak is normally distributed substantially, the middle peak is high, and the peaks at both ends are low, indicating that most fractions are all more concentrated, but the present invention The fraction content of the resin at the highest melting temperature is relatively large (as shown in Figure 3), and it is much larger than other fractions, and the fraction distribution is different from other resins, which shows the heterogeneity of the molecular chain structure in the resin. The resin does have such a fraction, which is characterized by TREF, and the fraction distribution and crystallization state of the resin are further analyzed.

Referring to Figure 4, the TREF characterization results show that the transparent and easy-to-process film resin provided by the present invention obviously has two crystal forms, which shows the heterogeneity of the molecular chain structure in the resin, and the conclusion is consistent with the SSA analysis. The thickness of the lamella formed by crystallization is related to the branch chain distribution state in the molecular chain. The length of the chain end between the insertion points of the two comonomers is small and it is not easy to crystallize, so the thickness of the lamella formed is small; the molecular chain segment with less comonomer content, two The length of the chain end between the insertion points of the comonomer is large and the chain is regular, easy to crystallize, and the thickness of the formed lamellae is large. Therefore, the distribution of comonomers in the molecular segments of the transparent and easy-to-process resin is not consistent, and the distance between branch points of some molecular segments is relatively close, and the distance between branch points of some molecular segments is relatively far.

The long-chain branched structure of the polymer will strongly affect the processing properties of the polymer, such as increasing the melt viscosity of the polymer, strengthening the tensile hardening and shear thinning behavior, and so on. However, the content of long-chain branches in polymers is often very low, which leads to the failure of some traditional polymer structure characterization methods. For example, GPC-MALIS cannot detect low-level long-chain branches; NMR cannot distinguish between short-chain branches and long-chain branches. The rheological properties of polymers are very sensitive to low content of long-chain branched structures, so rheological testing has become an important method to characterize polymer long-chain branched structures. Fig. 5 is the torque curve of the sample containing a small amount of long-chain branches in the present invention. The processing torque is 30-60Nm, which is less than that of products without long-chain branches, indicating that the formation of long-chain branches improves the processing performance of LLDPE, and the content of long-chain branches The higher the value, the better the processing performance of the product, which is very beneficial to saving energy consumption in production and reducing side reactions in the extrusion process. Figure 6 is the rotational rheological diagram of the transparent and easy-to-process film resin of the present invention. The samples undergo shear thinning in the low frequency range, that is, they are more sensitive to shear, which further indicates that the samples have long chain branches.

And also can know from table 2, the polyethylene resin prepared by the present invention has broad molecular weight distribution (10～12), has less processing torque (32～48Nm) (compared with traditional LLDPE processing torque 50 in the prior art) ～90Nm is greatly reduced), larger melt flow ratio (59.2～70.7), which makes the polyethylene resin of the present invention have better stability and product quality, and better overall performance of the product.

The heterogeneity of molecular chain structure includes heterogeneity caused by different molecular weight and distribution and heterogeneity caused by different branch length, content and distribution, which has a great impact on the comprehensive information of polyethylene products. As can be seen from Table 2, the polyethylene resin prepared by the present invention has a wide molecular weight distribution (10-12), and due to the unique short-chain branched and a small amount of long-chain branched structures, many unfolded branched chains pass through non-crystalline layers. The crystal region enters other crystal regions, so that there are a large number of tie molecules between the crystal layers, thereby improving the mechanical properties of the polymer; at the same time, these branched chains contain more long chain branches, which improves the rheological properties and improves the processing performance. In addition, the special structure has a certain influence on the crystal size, improves the transparency of the product, and greatly improves the comprehensive performance of the product.

Certainly, the present invention also can have other various embodiments, without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and deformations according to the present invention, but these corresponding changes All changes and modifications should belong to the protection scope of the claims of the present invention.

Claims (10)

1. A preparation method for transparent LLDPE film resin is characterized in that, the method is that ethylene and α-olefin are polymerized in a single reactor in the presence of hydrogen, oxygen, inert gas and modified loaded chromium catalyst , wherein the polymerization temperature is 60-90°C, the polymerization pressure is 1.8-2.5MPa, the molar ratio of α-olefin to ethylene is 0.01:1-0.09:1, and the molar ratio of hydrogen to ethylene is 0.001:1-0.01:1 , The concentration of oxygen in the reactor is 10-140ppb. 2. The preparation method of the transparent LLDPE film resin according to claim 1, characterized in that, the molar content of the α-olefin in the transparent LLDPE film resin is 0.5-5.0%. 3. the preparation method of transparent LLDPE film resin according to claim 1 is characterized in that, described loaded chromium catalyst refers to the catalyst that chromium compound is loaded on the carrier, and described chromium compound is chromium oxide, and described carrier is Silica, alumina, zirconia or thoria. 4. The transparent LLDPE film resin according to claim 1, characterized in that, the modified supported chromium catalyst is a catalyst obtained by modifying the supported chromium catalyst through aluminum, titanium, fluorine and vanadium. 5. the preparation method of transparent LLDPE film resin according to claim 1 is characterized in that, described α-olefin is 1-butene and/or 1-hexene. 6. The preparation method of transparent LLDPE film resin according to claim 1, characterized in that, the molar ratio of the α-olefin to ethylene is 0.03:1 to 0.06:1. 7. The preparation method of transparent LLDPE film resin according to claim 1, characterized in that, the molar ratio of hydrogen to ethylene is 0.003:1 to 0.006:1. 8. The preparation method of transparent LLDPE film resin according to claim 1, characterized in that the concentration of oxygen in the reactor is 30-90 ppb. 9. The preparation method of the transparent LLDPE film resin according to claim 1, characterized in that, the polymerization is gas-phase single reactor polymerization, the polymerization temperature is 80-90°C, the polymerization pressure is 1.8-2.5MPa, and the circulation gas velocity It is 0.60~0.82m/s, and the residence time is 1~8h. 10. the preparation method of transparent LLDPE film resin according to claim 4 is characterized in that, in the supported chromium catalyst of described modification, the total load of chromium, vanadium, titanium and aluminum is 0.01wt of catalyst gross weight %～15wt%, wherein, the molar ratio of chromium and vanadium is 0.1:0.9～0.9:0.1, the molar ratio of chromium and titanium is 0.1:0.9～0.9:0.1; the load of fluorine is 0.01wt%～of the total weight of the catalyst 10wt%, wherein the molar ratio of chromium to fluorine is 0.1:0.9˜0.9:0.1.