Non-stretched polypropylene-based film

Abstract

The polypropylene copolymer according to the present invention has a low melting point and also is excellent in the low temperature heat sealing effect, transparency and strength, and the film prepared therefrom can be effectively used as a sealing layer of the non-stretched polypropylene-based film.

Claims

1. A polypropylene-based film comprising a propylene-ethylene-1-butene terpolymer which has a melting point (Tm) of 125 to 135 C., a molecular weight distribution (Mw/Mn, PDI) of 2.3 to 3.5, and a xylene soluble (Xs) content of 2.0 wt % or less, wherein the terpolymer has a crystallization temperature (Tc) of 75 to 87 C., and wherein, when heat-sealed at 134 C. and 0.2 MPa for 1 second, a sealing strength of the terpolymer is 300 to 500 g/15 mm.

2. The polypropylene-based film according to claim 1, which comprises a sealing layer comprising the terpolymer.

3. The polypropylene-based film according to claim 2, which further comprises a polypropylene-based core layer and a skin layer which are sequentially formed on the sealing layer.

4. The polypropylene-based film according to claim 1, wherein the terpolymer is prepared by copolymerizing propylene, ethylene and 1-butene in the presence of a catalyst comprising a metallocene compound represented by the following Chemical Formula 1: ##STR00005## in Chemical Formula 1, X is halogen which is the same as or different from each other, R.sub.1 is C.sub.6-20 aryl substituted by C.sub.1-20 alkyl, R.sub.2, R.sub.3 and R.sub.4 are each independently hydrogen, halogen, C.sub.1-C.sub.20 alkyl, C.sub.2-C.sub.20 alkenyl, C.sub.1-C.sub.20 alkylsilyl, C.sub.1-C.sub.20 silylalkyl, C.sub.1-C.sub.20 alkoxysilyl, C.sub.1-C.sub.20 ether, C.sub.1-C.sub.20 silylehter, C.sub.1-C.sub.20 alkoxy, C.sub.6-C.sub.20 aryl, C.sub.7-C.sub.20 alkylaryl, or C.sub.7-C.sub.20 arylalkyl, A is carbon, silicon, or germanium, R.sub.5 is C.sub.1-20 alkyl substituted by C.sub.1-20 alkoxy, and R.sub.6 is hydrogen, C.sub.1-C.sub.20 alkyl, or C.sub.2-C.sub.20 alkenyl.

5. The polypropylene-based film according to claim 4, wherein X is chloro.

6. The polypropylene-based film according to claim 4, wherein R.sub.1 is phenyl substituted by tert-butyl.

7. The polypropylene-based film according to claim 4, wherein R.sub.2, R.sub.3 and R.sub.4 are hydrogen.

8. The polypropylene-based film according to claim 4 wherein A is silicon.

9. The polypropylene-based film according to claim 4, wherein R.sub.5 is 6-tert-butoxy-hexyl, and R.sub.6 is methyl.

10. The polypropylene-based film according to claim 4, wherein the compound is a compound represented by the following Chemical Formula: ##STR00006##

11. The polypropylene-based film according to claim 2 wherein the sealing layer further comprises a propylene-based elastomer.

Description

DETAILED DESCRIPTION OF THE INVENTION

Technical Problem

(1) In order to solve the above-mentioned problems, an object of the present invention is to provide a terpolymer that has a low melting point and also is excellent in the low-temperature heat sealing effect, transparency and strength, and a non-stretched polypropylene-based film comprising the same.

Technical Solution

(2) In order to achieve the above object, the present invention provides a non-stretched polypropylene-based film comprising a propylene-ethylene-1-butene terpolymer which has a melting point (Tm) of 125 to 135 C., a molecular weight distribution (Mw/Mn, PDI) of 2.3 to 3.5, and a xylene soluble (Xs) content of 2.0% be weight or less, wherein when heat-sealed at 134 C. and 0.2 MPa for 1 second, a sealing strength is 300 to 500 g/15 mm.

(3) The non-stretched film is prepared without stretching process, and this is usually called CPP (cast polypropylene) film. Since this is prepared by T-Die process, the thickness is uniform, the transparency and gloss are excellent, and the printability is also excellent. Therefore, it is widely used as a material for flexible packaging such as food packaging at a high market price. The CPP film is generally composed of a skin layer, a core layer and a sealing layer. As a double sealing layer among them, polypropylene is most commonly used.

(4) The sealing layer is adhered between layers by applying heat, and this is called heat sealing. To increase the production rate during packaging, the time required for heat-sealing should be decreased. Therefore, it is necessary to use a polymer that can heat-seal at high temperature or allow sufficient heat sealing at a low temperature. Polypropylene is widely used as a sealing layer because the low-temperature heat sealing is possible, but the polypropylene-based resin prepared by Ziegler-Natta catalyst is sticky due to a high xylene soluble (Xs) content and thus, there is a problem that defects in a process are frequently generated.

(5) Thus, according to the present invention, a propylene-ethylene-1-butene terpolymer which can lower a xylene soluble content and exhibit high sealing strength at low temperature is used.

(6) The terpolymer according to the present invention is characterized by having a melting point (Tm) of 125 to 135 C. The melting point is associated with the temperature that is applied during the heat sealing. When the melting point is less than 125 C., there is a difficulty in the production process of terpolymers, and when it exceeds 135 C., the heat sealing characteristics are decreased.

(7) Also, the terpolymer according to the present invention is characterized in that it has a crystallization temperature (Tc) of 75 to 87 C.

(8) Also, the terpolymer according to the present invention is characterized by having a molecular weight distribution of (Mw/Mn, PDI) of 2.3 to 3.5. The molecular weight distribution is associated with the transparency and unique odor, and it can exhibit the optimum physical properties for use as packaging material in the range of the molecular weight distribution.

(9) In addition, the terpolymer according to the present invention is characterized in that the xylene soluble (Xs) content is less than 2.0% by weight. The xylene soluble (Xs) content is a value representing an amount of atactic components in the entire terpolymer. The lower this content refers to the lower that sticky level of the terpolymer. Since the terpolymer according to the present invention has a low xylene soluble content, a possibility of occurring defects in the processing and heat sealing procedures is extremely low. Preferably, the xylene soluble (Xs) content is 1.5% by weight or less, and more preferably, the xylene soluble (Xs) content is 1.0% by weight or less.

(10) The terpolymer according to the present invention is characterized by having excellent heat-sealing strength. The heat-sealing strength may be measured in accordance with ASTM F1921. Specifically, two films are sealed with a certain width (e.g., 15 mm) at a certain temperature, pressure and time, and then a force required for peeling out the two films is measured. At this time, the heat-sealing strength is measured by a value where the measured force is divided by the width of the film.

(11) Further, the terpolymer according to the present invention is characterized by having MFR.sub.2.16 (g/10 min, measured at 230 C. according to ASTM1238) of 5 to 7.

(12) Further, the terpolymer according to the present invention can be prepared by copolymerizing propylene, ethylene and 1-butene.

(13) Preferably, the weight ratio of the monomers (propylene:ethylene:1-butene) is 94-98: 1-5:1, preferably 96-99: 1-4:1.

(14) The terpolymer according to the present invention can be prepared by copolymerizing propylene, ethylene and 1-butene in the presence of a catalyst comprising a metallocene compound represented by the following Chemical Formula 1.

(15) ##STR00001##

(16) in Chemical Formula 1,

(17) X is halogen which is the same as or different from each other,

(18) R.sub.1 is C.sub.6-20 aryl substituted by C.sub.1-20 alkyl,

(19) R.sub.2, R.sub.3 and R.sub.4 are each independently hydrogen, halogen, C.sub.1-C.sub.20 alkyl, C.sub.2-C.sub.20 alkenyl, C.sub.1-C.sub.20 alkylsilyl, C.sub.1-C.sub.20 silylalkyl, C.sub.1-C.sub.20 alkoxysilyl, C.sub.1-C.sub.20 ether, C.sub.1-C.sub.20 silylehter, C.sub.1-C.sub.20 alkoxy, C.sub.6-C.sub.20 aryl, C.sub.7-C.sub.20 alkylaryl, or C.sub.7-C.sub.20 arylalkyl,

(20) A is carbon, silicon, or germanium,

(21) R.sub.5 is C.sub.1-20 alkyl substituted by C.sub.1-20 alkoxy, and

(22) R.sub.6 is hydrogen, C.sub.1-C.sub.20 alkyl, or C.sub.2-C.sub.20 alkenyl.

(23) Preferably, X is chloro.

(24) Also preferably, R.sub.1 is phenyl substituted by tert-butyl. More preferably, R.sub.1 is 4-tert-butyl-phenyl.

(25) More preferably, R.sub.2, R.sub.3 and R.sub.4 are hydrogen.

(26) Also preferably, A is silicon.

(27) Also preferably, R.sub.5 is 6-tert-butoxy-hexyl, and R.sub.6 is methyl.

(28) Representative examples of the compound represented by Chemical Formula 1 are as follows:

(29) ##STR00002##

(30) Further, the present invention provides a method for preparing a compound represented by Chemical Formula 1 as represented by the following Reaction Scheme 1:

(31) ##STR00003##

(32) Step 1 is a step for preparing a compound represented by Chemical Formula 4 by reacting a compound represented by Chemical Formula 2 with a compound represented by Chemical Formula 3. In the above reaction, it is preferable to use alkyl lithium (for example, n-butyllithium), and the reaction temperature is 200 to 0 C., more preferably 150 to 0 C. As the solvent, toluene, THF and the like can be used. In this case, after separating the organic layer from the product, a step for drying the separated organic layer under vacuum and removing excess reagent can be further performed.

(33) Step 2 is a step for preparing a compound represented by Chemical Formula 1 by reacting a compound represented by Chemical Formula 4 with a compound represented by Chemical Formula 5. In the above reaction, it is preferable to use alkyl lithium (for example, n-butyllithium), and the reaction temperature is 200 to 0 C. and more preferably 150 to 0 C. As the solvent, ether, hexane and the like can be used.

(34) The compound represented by Chemical Formula 1 can, in itself or together with a cocatalyst as a catalyst precursor, be used as a catalyst for polymerization of the terpolymer according to the present invention.

(35) The catalyst for polymerization of the terpolymer may be a catalyst supported on a support. The support can be used without particular limitation as long as it is conventional in the art, but preferably, one or more supports selected from the group consisting of silica, silica-alumina and silica-magnesia can be used. On the other hand, when supporting the catalyst on a support such as silica, the supporting is made by a chemical bonding of the silica support and the functional group of the compound represented by Chemical Formula 1. Thereby, there is almost no catalyst which is liberated from the surface in the polymerization step of the terpolymer. Therefore, when preparing the terpolymer by slurry or gas phase polymerization process, fouling which aggregates or adheres on a reactor wall or between polymer particles is not generated.

(36) Further, the terpolymer that is prepared in the presence of a catalyst comprising such silica support has excellent particle shape and apparent density of the polymer and thus it can be suitably used for the conventional slurry or gas phase polymerization process. Therefore, preferably it is possible to use the support that is dried at high temperature to have a siloxane group which is greatly reactive on the surface.

(37) Specifically, the silica, silica-alumina and the like dried at a high temperature can be used, and they may usually contain oxide, carbonate, sulfate, nitrate components such as Na.sub.2O, K.sub.2CO.sub.3, BaSO.sub.4, or Mg(NO.sub.3).sub.2.

(38) Further, the catalyst for the polymerization of the terpolymer may further comprise a cocatalyst consisting of alkylaluminoxane. In the case of using such a cocatalyst, it can be used in the form of a catalyst in which X bonded to a metal element of the compound represented by Chemical Formula 1 is substituted by alkyl group, for example, C.sub.1-20 alkyl.

(39) The cocatalyst can be used without particular limitation as long as it can be commonly used in the technical field to which the present invention pertains. However, preferably one or more cocatalysts selected from the group consisting of silica, silica-alumina, and an organic aluminum compound can be used.

(40) Here, the polymerization of the terpolymer may be carried out by reacting at a temperature of 25 to 500 C. and a pressure of 1 to 100 kgf/cm.sup.2 for 1 to 24 hours. At this time, the polymerization temperature is preferably 25 to 200 C. and more preferably 50 to 100 C. Further, the polymerization pressure is preferably 1 to 70 kgf/cm.sup.2 and more preferably 5 to 40 kgf/cm.sup.2. The polymerization time is preferably 1 to 5 hours.

(41) The polymerization process can adjust the molecular weight range of the polymer product finally produced, depending on the conditions of the addition or non-addition of hydrogen. In particular, the high molecular weight terpolymer can be prepared under the conditions of not adding hydrogen. If hydrogen is added, the low molecular weight terpolymer can be prepared even with low hydrogen content. At this time, the amount of hydrogen to be added to the polymerization process can be in the range of 0.07 L to 4 L under reactor condition of 1 bar, or hydrogen can be supplied at a pressure of 1 bar to 40 bar or it can be supplied at 168 ppm to 8,000 ppm in the range of hydrogen molar content compared to monomer.

(42) In addition, the present invention provides a non-stretched polypropylene-based film which is prepared by the terpolymer and which has a sealing strength of 300 to 500 g/15 mm when heat-sealed at 134 C. and 0.2 MPa for 1 second. The above-described film can be used as a sealing layer of the non-stretched film and it can be prepared by T-Die process.

(43) The non-stretched polypropylene-based film may further comprise a polypropylene-based core layer and a skin layer which are formed sequentially on the sealing layer.

(44) The adhesion between the sealing layers can be made by applying heat to the sealing layer of the non-stretched polypropylene-based film. As described above, the terpolymer according to the present invention allows sufficient heat sealing at a low temperature, increases the production rate upon packaging, allows the use of less energy, thereby increasing the process efficiency. In addition, due to the low xylene soluble content, it is possible to prevent the process defects caused by the sticky properties of the sealing layer.

(45) Preferably, the sealing layer of the non-stretched polypropylene-based film may further comprise a propylene-based elastomer, for example, an elastomer in the form of propylene-ethylene copolymer. As such propylene-based elastomer, all propylene-based elastomers, prepared by methods previously known, or prepared by existing Ziegler-Natta catalyst or metallocene catalyst which are commercially available, can be used without particular limitation. As the propylene-based elastomer is added, the sealing strength of the film can be more increased. The propylene-based elastomer is included preferably in an amount of 3-7% by weight relative to the weight of the film.

Advantageous Effects

(46) The polypropylene copolymer according to the present invention has a low melting point and also is excellent in the low temperature heat sealing effect, transparency and strength, and the film prepared therefrom can be effectively used as a sealing layer of the non-stretched film.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(47) Hereinafter, preferred examples are provided for better understanding of the invention. However, these examples are for illustrative purposes only and the invention are not intended to be limited by these examples.

Preparation Example 1

(48) ##STR00004##

Step 1) Preparation of (6-t-butoxyhexyl)(methyl)-bis(2-methyl-4-tert-butyl-phenylindenyl)silane

(49) 2-methyl-4-tert-butylphenylindene (20.0 g, 76 mmol) was dissolved in toluene/THF=10/1 solution (230 mL) to which n-butyllithium solution (2.5 M, hexane solvent, 22 g) was slowly added dropwise at 0 C., and then stirred at room temperature for a day. Thereafter, (6-t-butoxyhexyl)dichloromethane silane (1.27 g) was slowly added dropwise to the mixed solution at 78 C., stirred for about 10 minutes and then stirred at room temperature for a day. Thereafter, the organic layer was separated by adding water, and the solvent was distilled under reduced pressure to obtain (6-t-butoxyhexyl)(methyl)-bis(2-methyl-4-tert-butyl-phenylindenyl)silane.

(50) .sup.1H NMR (500 MHz, CDCl.sub.3, 7.26 ppm): 0.20-0.03 (3H, m), 1.26 (9H, s), 0.50-1.20 (4H, m), 1.20-1.31 (11H, m), 1.40-1.62 (20H, m), 2.19-2.23 (6H, m), 3.30-3.34 (2H, m), 3.73-3.83 (2H, m), 6.89-6.91 (2H, m), 7.19-7.61 (14H, m)

Step 2) Preparation of [(6-t-butoxyhexylmethylsilane-diyl)-bis(2-methyl-4-tert-butylphenylindenyl)]zirconium dichloride

(51) (6-t-butoxyhexyl)(methyl)-bis(2-methyl-4-tert-butyl-phenylindenyl)silane prepared in the step 1 was dissolved in toluene/THF=5/1 solution (95 mL) to which n-butyl lithium solution (2.5 M, hexane solvent, 22 g) was slowly added dropwise at 78 C. and then stirred at room temperature for a day. To the reaction solution, bis(N,N-diphenyl-1,3-propanediamido)dichlorozirconium bis(tetrahydrofuran) [Zr(C.sub.5H.sub.6NCH.sub.2CH.sub.2NC.sub.5H.sub.6)Cl.sub.2(C.sub.4H.sub.8O).sub.2] was dissolved in toluene (229 mL) and then slowly added dropwise at 78 C., followed by stirring at room temperature for a day. After cooling the reaction solution at 78 C., HCl ether solution (1 M, 183 mL) was slowly added dropwise and then stirred at 0 C. for 1 hour. Thereafter, the mixture was filtered and dried under vacuum to which hexane was added and then stirred to precipitate a crystal. The precipitated crystal was filtered and dried under vacuum to obtain [(6-t-butoxyhexylmethylsilane-diyl)-bis(2-methyl-4-tert-butylphenylindenyl)]zirconium dichloride (20.5 g, of 61%).

(52) .sup.1H NMR (500 MHz, CDCl.sub.3, 7.26 ppm): 1.20 (9H, s), 1.27 (3H, s), 1.34 (18H, s), 1.20-1.90 (10H, m), 2.25 (3H, s), 2.26 (3H, s), 3.38 (2H, t), 7.00 (2H, s), 7.09-7.13 (2H, m), 7.38 (2H, d), 7.45 (4H, d), 7.58 (4H, d), 7.59 (2H, d), 7.65 (2H, d)

Step 3) Preparation of a Supported Catalyst

(53) 3 g of silica was pre-weighed into a shrink flask to which 52 mmol of methylaluminoxane (MAO) was added and then reacted at 90 C. for 24 hours. After precipitation, the upper layer was removed and washed two times with toluene. 180 mol of ansa-metallocene compound [(6-t-butoxyhexylmethylsilane-diyl)-bis(2-methyl-4-tert-butylphenylindenyl)]zirconium dichloride synthesized above was dissolved in toluene and then reacted at 70 C. for 5 hours. After completion of the reaction, the upper layer solution was removed and the residual reaction product was washed with toluene and again washed with hexane and then dried under vacuum to obtain 5 g of silica-supported metallocene catalyst in the form of a solid particle.

Example 1

Step 1

(54) The polypropylene polymer was prepared by using the metallocene supported catalyst prepared in Preparation Example 1, by the following method.

(55) First, a 2 L stainless steel reactor was dried under vacuum at 65 C. and then cooled, to which 1.5 mmol of triethylaluminum was added at room temperature, 0.37 L of hydrogen was added, and 1.5 L of propylene were added, sequentially. Then, after stirring for 10 minutes, the metallocene supported catalyst prepared in Preparation Example 1 was added to the reactor under nitrogen pressure. Then, the reactor temperature was raised up to 70 C. within 5 minutes, followed by polymerization for 1 hour. After completion of the reaction, unreacted propylene was vented.

Step 2

(56) In order to minimize the deformation and damage of the resin at a high temperature, 500-1000 ppm of calcium stearate (neutralizing agent), 500-1000 ppm of Irganox 1010 (primary antioxidant), 1000-1500 ppm of Irganox 168 (secondary antioxidant), 1000-1500 ppm of Erucamide (slip agent) and SiO.sub.2 (anti-blocking agent) as additives were added based on the amount of the polypropylene polymer powder sample prepared in the step 1, and strands were drawn at 5-15 kg/h at 180-220 C. Then, the prepared strands were pelletized by a pelletizer at 500-900 rpm to prepare a pellet.

Example 2

(57) C.sub.3 elastomer (VM3020FL) was mixed to the polymer prepared in the step 1 of Example 1, and the pellet was prepared by the following method.

(58) Specifically, in order to minimize the deformation and damage of the resin at a high temperature, 500-1000 ppm of calcium stearate (neutralizing agent), 500-1000 ppm of Irganox 1010 (primary antioxidant), 1000-1500 ppm of Irganox 168 (secondary antioxidant), 1000-1500 ppm of Erucamide (slip agent) and 1000-1500 ppm of SiO.sub.2 (anti-blocking agent) as additives were added based on the amount of the polypropylene polymer powder sample prepared in the step 1 of Example 1, and then mixed with 5 wt % of C3 elastomer (VM3020FL) having the characteristics of low-temperature crystallization region in order to improve the heat-sealability of the resin. Strands were drawn at 5-15 kg/h at 180-220 C. Then, the prepared strands were pelletized by a pelletizer at 500-900 rpm to prepare a pellet.

Comparative Example 1

(59) WINTEC product (available from JPP Corporation) was used as Comparative Example 1.

Comparative Example 2

(60) T3450L (available from LG Chem, Ltd.) was used as Comparative Example 2.

Experimental Example

(61) The physical properties of the respective copolymers prepared in Examples and Comparative Examples were measured by the following method.

(62) (1) Melting point (Tm) of the polymer: The melting point of the polymer was measured using Differential Scanning Calorimeter (DSC, device name: DSC 2920, manufacturer: TA instrument). Specifically, the polymer was heated up to 220 C. and then maintained at the same temperature for 5 minutes. After cooling to 20 C., the temperature was again increased. At this time, the increasing speed and the lowering speed of the temperature was adjusted to 10 C./min, respectively.

(63) (2) Crystallization temperature (Tc) of the polymer: the crystallization temperature was set from a DSC curve that appears while reducing the temperature under the same conditions as the melting point.

(64) (3) Melt Index (MI): measured at 230 C. under a load of 2.16 kg in accordance with ASTM D1238, and expressed as weight (g) of the polymer obtained by melting for 10 minutes.

(65) (4) Sealing strength: measured in accordance with ASTM F1921. Specifically, after preparing two films with a width of 15 mm, the films were sealed at 128-140 C. and 0.2 MPa for 1 second, and then the force required for peeling out the two films were measured. At this time, the sealing strength was measured by a value where the measured force was divided by the width of the polymer.

(66) (5) Tensile strength and elongation: the film test sample that measured the thickness was fixed to UTM equipment (ZWICK Roell Inc.), to fill the cross-sectional area. MD and TD directions of the test sample were measured at a speed of 200 mm/min, respectively. The tensile strength (kg/cm.sup.2) was confirmed by dividing the elongation (%), the yield load (Kgf) and load at break (kg) of the respective samples by the cross-sectional area (cm.sup.2).

(67) (6) Xylene soluble: Xylene was added to the sample, heated at 135 C. for 1 hour, and then cooled for 30 minutes, followed by pre-treatment. Xylene was flowed at a rate of 1 mL/min for 4 hours with OminiSec (Viscotek Corporation, FIPA) equipment. When the base line of RI, DP and IP was stabilized, the concentration and the injection amount of the pre-treated samples were filled and measured, thereby calculating the peak area.

(68) The results are shown in Tables 1 and 2 below.

(69) TABLE-US-00001 TABLE 1 Xylene MI soluble Sealing strength (g/15 mm) Tm ( C.) Tc( C.) (g/10 min) (wt %) 128 C. 131 C. 134 C. 137 C. 140 C. Comparative 123.9 87.7 6.6 1.1 49 74 99 242 x.sup.1) Example 1 Comparative 128.3 82.1 7.6 10.2 102 256 496 x x Example 2 Example 1 125.4 84.6 5.8 0.9 59 122 321 x x Example 2 125.1 85.3 6.2 1.0 85 170 420 x x .sup.1)Not peeled when measured up to 500 g/15 mm

(70) TABLE-US-00002 TABLE 2 MD TD Tensile Tensile Tensile Tensile Strength Strength Strength Strength @ @ @ @ Yield Break Elon- Yield Break Elon- (kg/ (kg/ gation (kg/ (kg/ gation cm.sup.2) cm.sup.2) (%) cm.sup.2) cm.sup.2) (%) Comparative 203 370 487 157 370 >700 Example 1 Comparative 197 543 494 164 394 >700 Example 2 Example 1 203 444 481 156 429 >700 Example 2 188 281 372 154 385 >700