Polyethylene resin composition and film thereof

Abstract: A polyethylene resin composition comprised of (A) 30 to 50% by weight of a linear low density polyethylene resin wherein (i) the MFR 190°C is 0.3 to 3.0/10 min, (ii) the density is 0.900 to 0.918 g/cc, (iii) the MI₂₀/MI₂ is 40 or less, (iv) there are one or more endothermic peaks of the melting point determined by a differential scanning calorimeter, wherein, when there are two or more peaks, the highest melting point (T m1 ) is 118 to 125°C and the ratio T m1 H/T m2 H between the endothermic peak height (T m1 H) of the highest melting point and the next low temperature side endothermic peak height (T m2 H) of 1.2 or less, and (v) the comonomer is an α-olefin having four or more carbon atoms; (B) 20 to 40% by weight of a high density polyethylene resin wherein (i) the MFR 190°C is 0.01 to 0.2 g/10 min, (ii) the density is 0.935 to 0.950 g/cc, and (iii) the /MI₂₀/MI₂ is 80 or more, (C) 10 to 40% by weight of a high pressure low density polyethylene resin wherein (i) the MFR 190°C is 0.1 to 2.0 g/10 min, (ii) the density is 0.915 to 0.924 g/cc, and (iii) the melt tension is 5 g or more, wherein the polyethylene resin composition is characterized in that (i) the MFR 190°C is 0.2 to 1.0 g/10 min, (ii) the density is 0.918 to 0.935 g/cc, and (iii) the melt tension is 5 g or more.

Description: The present invention relates to a polyethylene resin composition having an excellent balance of strength, rigidity, and gloss and excellent moldability (or shapability), heat sealability of through contaminant and low temperature resistance properties. More particularly it relates to a polyethylene resin composition and film thereof, suitable for use as heavy duty packaging bags such as for organic fertilizer. At the present time, industrial heavy duty bags comprised of polyolefin resin film are being used as packaging bags for items such as organic fertilizer because they have superior moisture-resistance and air-tight seals can be formed by heat sealing the openings. Such polyolefin resin film heavy duty packaging bags are produced by shaping high pressure low density polyethylene resin manufactured by the high pressure method, or high pressure low density polyethylene resin copolymerized with about 3 to 6 mole% of vinyl acetate, to a thickness of about 200 µm. These high pressure low density polyethylene films are inferior in strength, rigidity, and gloss compared with linear low density polyethylene resin film manufactured by the medium and low pressure methods. On the other hand, in recent years, to conserve on resources, various types of packaging materials have been made thinner. Due to the above-mentioned characteristics, the use of linear low density polyethylene resin with large strength and rigidity is being increased. However, linear low density polyethylene resin is difficult to extrude and places a large load on the motor of the extruder, in particular in inflation molding, and the bubble stability is poorer than the case of high pressure low density polyethylene resin. In the manufacture of a thick film, due in part to the insufficient cooling of the bubbles, the bubbles become unstable and formation of film becomes difficult. Further, the film surface easily becomes rough. Therefore, film-forming machines using linear low density polyethylene resin as materials are given larger lip clearance of the dies and make use of screws with small compression ratios, motors with large capacities, and devices for strengthening the cooling. It is difficult to form a film by machine for high pressure low density polyethylene with a small lip clearance and small motor capacity. Therefore, in consideration of the moldability, the method has been used of mixing high pressure low density polyethylene resin into the linear low density polyethylene resin material, but in this case, the rigidity and strength decline. Further, to alleviate the above-mentioned problems in the use of linear low density polyethylene resin as the sole material, it is necessary to broaden the distribution of molecular weight and increase the molecular weight. A film obtained in this way can satisfy requirements of rigidity, but is inferior in strength and gloss. Accordingly, the objects of the present invention are to eliminate the above-mentioned disadvantages of the prior art and to provide a polyethylene resin composition capable of easily forming a thin film with an excellent balance of strength, rigidity, and gloss and excellent in heat sealability of through contaminant and low temperature resistance properties by a molding machine with a small lip clearance and small motor capacity and of a film provided with the above-mentioned superior properties. In accordance with the present invention, there is provided a polyethylene resin composition comprising: (A) 30 to 50% by weight of a linear low density polyethylene resin having (i) an MFR 190°C of 0.3 to 3.0 g/10 min, (ii) a density of 0.900 to 0.918 g/cc, (iii) an MI₂₀/MI₂ of 40 or less, and (iv) one or more endothermic peaks of the melting point determined by a differential scanning calorimeter, wherein, when there are two or more peaks, the highest melting point (T m1 ) is 118 to 125°C and the ratio T m1 H/T m2 H between the endothermic peak height (T m1 H) of the highest melting point and the next low temperature side endothermic peak height (T m2 H) of 1.2 or less, and containing (v) an α-olefin having 4 carbon atoms or 4-methylpentene-1 as a comonomer; (B) 20 to 40% by weight of a high density polyethylene resin having (i) an MFR 190°C of 0.01 to 0.2 g/10 min, (ii) a density of 0.935 to 0.945 g/cc, and (iii) an MI₂₀/MI₂ of 80 or more; and (C) 10 to 40% by weight of a high pressure low density polyethylene resin having (i) an MFR 190°C of 0.1 to 2.0 g/10 min, (ii) a density of 0.915 to 0.924 g/cc, and (iii) an melt tension of 5 g or more, wherein the polyethylene resin composition has (i) an MFR 190°C of 0.2 to 1.0 g/10 min, (ii) a density of 0.918 to 0.935 g/cc, and (iii) a melt tension of 5 g or more and providing a film having a gloss of 50% measured with an angle of incidence of 60°. In accordance with the present invention, there is also provided a polyethylene resin film of a thickness of 50 to 250 µm obtained by molding by the air cooling inflation method a polyethylene resin composition comprised of (A) 30 to 50% by weight of a linear low density polyethylene resin having (i) an MFR 190°C of 0.3 to 3.0 g/10 min, (ii) a density of 0.900 to 0.918 g/cc, (iii) an MI₂₀/MI₂ of 40 or less, and (iv) one or more endothermic peaks of the melting point determined by a differential scanning calorimeter, wherein, when there are two or more peaks, the highest melting point (T m1 ) is 118 to 125°C and the ratio T m1 H/T m2 H between the endothermic peak height (T m1 H) of the highest melting point and the next low temperature side endothermic peak height (T m2 H) of 1.2 or less, and containing (v) an α-olefin having 4 carbon atoms or 4-methylpentene-1 as a comonomer; (B) 20 to 40% by weight of a high density polyethylene resin having (i) MFR 190°C of 0.01 to 0.2 g/10 min, (ii) a density of 0.935 to 0.945 g/cc, and, (iii) an MI₂₀/MI₂ of 80 or more; and (C) 10 to 40% by weight of a high pressure low density polyethylene resin having (i) an MFR 190°C of 0.1 to 2.0 g/10 min, (ii) a density of 0.915 to 0.924 g/cc, and (iii) a melt tension of 5 g or more, said polyethylene resin composition having (i) an MFR 190°C of 0.2 to 1.0 g/10 min, (ii) a density of 0.918 to 0.935 g/cc, and (iii) a melt tension of 5 g or more, wherein the film is characterized by (i) a gloss of 50% or more, (ii) a Young's modulus of 3500 kg/cm² or more, and (iii) a dart impact strength of 40 kg/cm or more. The linear low density polyethylene resin (A) usable in the present invention is a copolymer of ethylene and a comonomer comprised of α-olefins having 4 carbon atoms such as 1-butene, or 4-methylpentene-1. This linear low density polyethylene resin is synthesized at 0 to 100 atmospheres (gage pressure) using a coordination catalyst i.e., is manufactured by the so-called medium and low pressure method. The content of the comonomer in the copolymer is 0.5 to 40 mole%, preferably 0.5 to 30 mole%. Such a linear low density polyethylene resin (A) has an MFR 190°C (melt flow rate: ASTM D 1238, (E)) of 0.3 to 3.0 g/10 min, preferably 0.5 to 2.5 g/10 min. When the MFR 190°C is smaller than 0.3 g/10 min, surface roughness and inferior extrusion occur, while when larger than 3.0 g/10 min, the bubble becomes unstable, so these are not preferable. Further, the density is 0.900 to 0.918 g/cc, preferably 0.900 to 0.915 g/cc. When the density is smaller than 0.900 g/cc, the rigidity becomes insufficient, while when greater than 0.918, the strength becomes insufficient, so these are not preferable. The MI₂₀/MI₂ is 40 or less, preferably 15 to 35. Here, the MI₂₀ is the value of the MFR 190°C at a load of 21.6 kg and MI₂ is the value of the MFR 190°C at a load of 2.16 kg. MI₂₀/MI₂ is the ratio of the same. Further, there are one or more endothermic peaks of the melting point determined by a differential scanning calorimeter (DSC). When there are two or more peaks, the highest melting point (T m1 ) is 118 to 125°C, preferably 118 to 123°C, and the ratio T m1 H/T m2 H between the endothermic peak height (T m1 H) of the highest melting point and the next low temperature side endothermic peak height (T m2 H) is 1.2 or less, preferably 1.0 or less. At this time, the measurement by the differential scanning calorimeter adopts the method of giving identical histories to the samples at all times by raising the measurement samples once to 200°C, then lowering them to 47°C and then measuring the endothermic peaks. The amount of mixture of the linear low density polyethylene resin is 30 to 50% by weight, preferably 35 to 50% by weight. The high density polyethylene resin (B) is comprised of ethylene homopolymer or an ethylene and α-olefin copolymer. This high density polyethylene resin is synthesized by the low pressure method using a Ziegler-Natta catalyst, the Phillips method and other medium pressure methods. The α-olefins may include those with 3 to 20 carbon atoms, for example, propylene and 1-butene. The content of α-olefins is 0 to 10 mole%, preferably 0.2 to 7 mole%. This high density polyethylene resin (B) has an MFR 190°C of 0.01 to 0.2 g/10 min, preferably 0.03 to 0.1 g/10 min. When the MFR 190°C is less than 0.01 g/10 min, extrusion defects occur, while when it is larger than 0.2 g/10 min, the strength becomes insufficient, so these are not preferable. The density 0.935 to 0.945 g/cc. When the density is less than 0.935 g/cc, the rigidity becomes insufficient, while when it is greater than 0.950 g/cc, the strength becomes insufficient, so these are not preferable. The MI₂₀/MI₂ is 80 or more, preferably 90 or more. When less than 80, the bubbles become unstable, so this is not preferable. The amount of mixture of the high density polyethylene resin is 20 to 40% by weight, preferably 20 to 35% by weight. The high pressure low density polyethylene resin (C) is a homopolymer of ethylene or a copolymer with vinyl acetate. This high pressure low density polyethylene resin is synthesized by the high pressure method where synthesis is performed at 1000 to 2000 atmospheres and 200 to 300°C by radical polymerization. The amount of the vinyl acetate is 2 to 10 mole%, preferably 2 to 8 mole%. This high pressure low density polyethylene resin (C) has an MFR 190°C of 0.1 to 2.0 g/10 min, preferably 0.1 to 1.0/10 min. When the MFR 190°C is less than 0.1 g/10 min, extrusion defects occur, while when larger than 2.0 g/10 min, the bubbles become unstable, so these are not preferable. The density is 0.915 to 0.924 g/cc, preferably 0.918 to 0.924 g/cc. When the density is smaller than 0.915 g/cc, the rigidity becomes insufficient, while when larger than 0.924 g/cc, the strength becomes insufficient, so these are not preferable. The melt tension is 5 g or more, preferably 7 g or more. The amount of the high pressure low density polyethylene resin mixed in is 10 to 40% by weight, preferably 20 to 35% by weight. The resin composition comprised of the above-mentioned components (A) to (C) may be mixed by, for example, dry blending using a Henschel mixer and/or melt blending using an extruder. The polyethylene resin composition according to the present invention has an MFR 190°C of 0.2 to 1.0 g/10min, preferably 0.3 to 0.8 g/10min., a density of 0.918 to 0.935 g/cc, preferably 0.920 to 0.930 g/cc, and a melt tension of 5 g or more, preferably 6 g or more. The polyethylene resin composition obtained in this way may be shaped or molded into a film by the air-cooling inflation method. The physical properties of the film obtained at this time are a gloss of 50% or more, preferably 60% or more, Young's modulus of 3500 kg/cm² or more, preferably 3800 kg/cm² or more, and a dart impact strength of 40 kg/cm or more, preferably 45 kg/cm or more. A film with a thickness of 50 to 250 µm with an excellent balance of strength, rigidity, and gloss and superior in heat sealability of through contaminant and low temperature resistance properties can be obtained. As mentioned above, the polyethylene resin composition of the present invention is comprised of the components (A) to (C). Whereas it was previously necessary to use a film at least 90 µm thick, the desired qualities may now be obtained by using a film of the present invention which is thinner and enables shaping or molding and has an excellent balance of strength, rigidity, and gloss The film is superior in heat sealability of through contaminant and low temperature resistance properties and may be made by a molding machine with a small lip clearance and a small motor capacity. EXAMPLES The present invention will now be further illustrated by, but is by no means limited to, the following Examples. The measurements in the Examples were performed by the following methods: (1) MFR 190°C : Measured according to ASTM D 1238 (D) at 190°C. (2) Density: Measured according to ASTM D 1505. (3) /MI₂₀/MI₂: MI₂₀ is value of MFR determined by ASTM D 1238 at 190°C with load of 21.6 kg, while MI₂ is similarly value of MFR at load of 2.16 kg. The ratio of the same was calculated. (4) Melting point (T m ): Measured by differential scanning calorimeter (made by Perkin Elmer Co.) Samples of 4 to 5 mg were weighed and raised in temperature to 200°C at a rate of temperature rise of 10°C/min, held there for 10 minutes, then lowered to 47°C at a rate of temperature descent of 20°C/min. After this, they were raised at a rate of temperature rise of 10°C/min and the endothermic peaks were recorded on a chart. When there was one endothermic peak at this time, this was expressed as T m . When there were two or more peaks, the endothermic peak of the highest melting point was expressed as T m1 H and the next lower temperature side endothermic peak was expressed as T m2 H. (5) Melt tension: Measured by melt tension tester made by Toyo Seiki Co. Nozzle used: L = 8.000 mm, D = 2.095 mm Test temperature: 190°C Extrusion speed: 15 mm/min Takeup speed: 2 m/min (6) Gloss: Measured according to ASTM D 523 with angle of incidence of 60 degrees. (7) Dart impact strength: Value measured according to ASTM D 1709 B divided by thickness. Dart measurement value (kg)/Film thickness (cm) (8) Young's modulus: Measured by tensile tester of fixed crosshead travel speed type (made by Instron Co.) Sample: JIS K 6781 (dumb-bell) Ambient temperature: 23°C Tensile speed: 500 mm/min Chart speed: 200 mm/min Measurement was made under the above conditions in the MD and TD directions and the modulus was calculated by the following formula from the resultant chart. The values found were averaged out to give the Young's modulus (E). E₀ = R₀(L₀/A) wherein, E₀ is Young's modulus in various directions, R₀ is the initial gradient, L₀ is the distance between chucks, and A is the minimum area during preparation of a sample. At this time, R₀ is calculated by the formula: R₀ = F₁/L₁ wherein, F₁ is the load at any point on the initial tangent, and L₁ is the elongation corresponding to F₁ on the tangent. (9) Sealing strength: The samples were sealed by a New Long HS-33D top sealer, then tested by a tensile tester of the fixed crosshead travel speed type and the strength at break measured. (10) Low temperature bag dropping strength: Ten bags were prepared to be filled with 20 kg of material sealed at the top and bottom using a New Long HS-33D top sealer under conditions of a heater gap of 150 percent and a cooler gap of 200 percent. Each of the bags was dropped 10 times from its side surface in a -10°C environment from a height of 1.2 m and the number of ruptured bags was determined. Examples 1 to 5 The linear low density polyethylene resin (LLDPE), high density polyethylene resin (HDPE), and high pressure low density polyethylene resin (HPLDPE) shown in Table 1 were mixed by a Henschel mixer and pelletized by an extruder. The characteristics of the resultant compositions are shown in Table 1. Using the resultant compositions, further, film was formed under the following molding conditions by the air-cooling inflation method to produce film having a thickness of 150 µm and a width of 450 mm. The physical properties of the same were measured by the above-mentioned measurement methods. The results are shown in Table 1. Shaping machine: Placo LM65⊘ inflation shaping machine (made by Placo Co., HPLD specifications) Screw: L/D = 28, C·R = 2.8, with intermediate mixing Dies: 200 mm⊘ (diameter), 1.2 mm (lip clearance) Air ring: 2 gap type Shaping temperature: 210°C Takeup speed: 10 m/min Comparative Example 1 As the polyethylene resin, ethylenevinyl acetate (EVA) with an MFR 190°C of 0.6 g/10 min prepared by copolymerizing 4 percent by weight of vinyl acetate (VA) with HPLDPE was used. This was molded and measured in the same way as in Examples 1 to 5, except that the molding temperature was made 180°C. The result are shown in Table 1. Comparative Examples 2 to 6 As the polyethylene resin, those of the compositions shown in Table 1 were used. These were molded and measured in the same way as Examples 1 to 5, except that the molding temperature of Comparative Example 6 was made 180°C. The results are shown in Table 1. From the results of Table 1, the polyethylene resin composition according to the present invention is superior in sealing strength and bag performance compared with the Comparative Examples and further is superior in the Young's modulus and gloss compared with EVA (Comparative Example 1) comprised of VA copolymerized with HPLDPE and is superior in the dart impact strength and Young's modulus compared with a resin comprised of HPLDPE (Comparative Example 6). Further, it is superior in moldability compared with a resin composition comprised of HPLDPE and LLDPE (Comparative Examples 2 and 3) and a resin comprised of LLDPE (Comparative Example 4) and is superior in gloss compared with a resin composition comprised of LLDPE and HDPE (Comparative Example 5). In this way, in the Examples, it was learned that it is possible to obtain a polyethylene resin film with a thickness of 150 µm with an excellent balance of strength, rigidity, and gloss and superior in moldability, heat sealability of through contaminant, and low temperature resistance properties.

Claims: A polyethylene resin composition comprising: (A) 30 to 50% by weight of a linear low density polyethylene resin having (i) an MFR 190°C of 0.3 to 3.0 g/10 min, (ii) a density of 0.900 to 0.918 g/cc, (iii) an MI₂₀/MI₂ of 40 or less, and (iv) one or more endothermic peaks in the melting point determined by a differential scanning calorimeter, wherein, when there are two or more peaks, the highest melting point (T m1 ) is 118 to 125°C and the ratio T m1 H/T m2 H between the endothermic peak height (T m1 H) of the highest melting point and the next low temperature side endothermic peak height (T m2 H) of 1.2 or less, and containing (v) an α-olefin having 4 carbon atoms, or 4-methylpentene-1 as a comonomer; (B) 20 to 40% by weight of a high density polyethylene resin having (i) an MFR 190°C of 0.01 to 0.2 g/10 min, (ii) a density of 0.935 to 0.945 g/cc, and (iii) an MI₂₀/MI₂ of 80 or more; and (C) 10 to 40% by weight of a high pressure low density polyethylene resin having (i) an MFR 190°C of 0.1 to 2.0 g/10 min, (ii) a density of 0.915 to 0.924 g/cc, and (iii) a melt tension of 5 g or more, said polyethylene resin composition having (i) an MFR 190°C of 0.2 to 1.0 g/10 min, (ii) a density of 0.918 to 0.935 g/cc, and (iii) a melt tension of 5 g or more. A composition according to claim 1 comprising 35% to 50% by weight of the linear low density polyethylene resin (A), 20% to 35% by weight of the high density polyethylene resin (B), and 20% to 35% by weight of the high pressure low density polyethylene resin (C). A composition according to claim 1 or claim 2 wherein the content of the comonomer (v) in the linear low density polyethylene resin (A) is 0.5 to 40 mole%. A composition according to any one of claims 1 to 3 wherein the linear low density polyethylene resin (A) has (i) an MFR 190°C of 0.5 to 2.5 g/10 min., (ii) a density of 0.900 to 0.915 g/cc, (iii) an MI₂₀/MI₂ of 15 to 35, and (iv) a highest melting point (T m1 ) of 118 to 123°C and the ratio T m1 /T m2 H of 1.0 or less. A composition according to any one of claims 1 to 4 wherein the high density polyethylene resin (B) comprises at least one ethylene homopolymer or copolymer of ethylene with up to 10 mole% of a α-olefins having 3 to 20 carbon atoms. A composition according to any one of claims 1 to 5 wherein the high density polyethylene resin (B) has (i) an MFR 190°C of 0.03 to 0.1 g/10 min, (ii) a density of 0.935 to 0.945 g/cc, and an MI₂₀/MI₂ of 90 or more. A Composition according to any one of claims 1 to 6 wherein the high pressure low density polyethylene resin (C) comprises at least one ethylene homopolymer or copolymer of ethylene with 2 to 10 mole% of vinyl acetate. A composition according to any one of claims 1 to 7 wherein the high pressure low density polyethylene resin (C) has (i) an MFR 190°C of 0.1 to 1.0 g/10 min., (ii) a density of 0.918 to 0.924 g/cc, and (iii) a melt tension of 7 g or more. A composition according to any one of claims 1 to 8 having (i) an MFR 190°C of 0.3 to 0.8 g/10 min., (ii) a density of 0.920 to 0.930 g/cc, and (iii) a melt tension of 6 g or more. A polyethylene resin film of a thickness of 50 to 250 µm obtained by molding by the air cooling inflation method a polyethylene resin composition according to any one of claims 1 to 9 the film having (i) a gloss of 50% or more measured with an angle of incidence of 60°, (ii) a Young's modulus of 3500 kg/cm² or more, and (iii) a dart impact strength of 40 kg/cm or more.

Inventors: MATSUNAGA TAKASI, NISIMURA TOSHIRO, MATSUNAGA, TAKASI, NISIMURA, TOSHIRO
Applicants: MITSUI PETROCHEMICAL IND, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1989
Main classification: C08L23
IPC codes: C08L23, C08J5