JPS6140176B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing polyolefin film. More specifically, the present invention relates to a method for producing a low-temperature heat-sealing polyolefin film with excellent transparency and rigidity. In recent years, crystalline propylene-α-olefin copolymers mainly composed of propylene, such as ethylene-propylene random copolymer, ethylene-propylene-butene-1 copolymer, and propylene-butene-1 copolymer, have been developed with excellent properties. Taking advantage of its properties such as transparency, impact resistance, and low-temperature heat sealability, it is used in a wide range of film applications, including lamination, general packaging, etc. As the content of the comonomer component in the crystalline propylene-α-olefin copolymer increases, the crystalline melting point of the copolymer decreases, and the low-temperature heat-sealability and high-speed heat-sealability of the film improve, but at the same time, the rigidity of the film increases. descend. For example, an unstretched film has a crystalline melting point of 150 to 140°C, compared to a crystalline polypropylene film whose crystalline melting point is 164°C.
The stiffness of crystalline ethylene-propylene random copolymers at 20 to 40% is reduced at 20 to 40%. Therefore, increasing the comonomer content in the copolymer for the purpose of speeding up bag-making operations and automatic packaging by improving heat-sealing properties can reduce wrinkles during film formation due to a decrease in rigidity, and tightening during winding. This not only causes poor flatness of the film, such as thickening of both ends, but also causes problems such as sagging and elongation of the film, which reduces workability in printing, bag making, automatic filling, etc. This stiffness becomes more important as a processing characteristic as the thickness of the film becomes thinner, and the decrease in stiffness is a major obstacle to making films thinner in order to save resources. As a result of various studies aimed at improving the rigidity of crystalline propylene-α-olefin copolymer without impairing its excellent transparency and low-temperature heat sealability, the present inventors found that crystalline propylene-α-olefin copolymer -We have completed the present invention by finding that the above object can be achieved by forming a film under specific conditions from a composition in which an olefin copolymer is mixed with a specific high-density polyethylene in a specific ratio. That is, in the present invention, the polypropylene is a crystalline propylene-α-olefin copolymer having a crystal melting point of 150° C. or less and a propylene component content of 70% or more, and the polyethylene is a high-density copolymer having a density of 0.945 or more and a flow ratio of 15 or less. 96 to 85% by weight of polypropylene and 4 to 85% by weight of polyethylene, which is polyethylene and has a ratio of melt index (MI) of the polyethylene to melt flow rate (MFR) of the polypropylene of (MI/MFR)≧1.2.
15% by weight of the composition is melt-extruded at 180 to 250°C and rapidly cooled to below 50°C. The polypropylene used in the present invention is crystalline propylene containing 70% by weight or more of a propylene component.
α-olefin copolymer with crystal melting point
Must be 150 or less. The propylene component
If it does not reach 70% by weight, the obtained film is likely to cause blocking, and if the crystal melting point exceeds 150°C, low temperature heat sealability and high speed heat sealability will be significantly reduced, both of which are unfavorable. Such polypropylene consists of the main component propylene and the comonomer ethylene or carbon atoms of 4 to 4.
It can be obtained by random copolymerization of α-olefin No. 8 by a known method using, for example, a Ziegler-Natsuta catalyst. Examples of such copolymers include ethylene-propylene copolymer, propylene-butene-1 copolymer, ethylene-propylene-butene-1 terpolymer, and the like. In the present invention, the crystal melting point (hereinafter abbreviated as Tm) refers to the temperature associated with the melting of the crystal obtained by heating a sample at a rate of 10°C/min in a nitrogen atmosphere using a scanning differential calorimeter. Refers to the peak temperature of the endothermic curve. Tm is 105 to 112 for low density polyethylene
℃, 124-132℃ for high-density pothyethylene and 162-165℃ for crystalline polypropylene. In addition, in the crystalline propylene-α-olefin copolymer,
As the content of comonomer components increases, Tm decreases,
In the case of ethylene-propylene random copolymer,
If the ethylene component in the copolymer exceeds 2.5% by weight, Tm will be 150°C or less. Incidentally, the Tm of a block copolymer such as an ethylene-propylene block copolymer is generally 150 DEG C. or higher, and although the resulting film has high rigidity, its transparency is extremely poor, so it cannot be used in the present invention. The polyethylene used in the present invention has a density of 0.945
The above is high-density polyethylene with a flow ratio of 15 or less. If the density of polyethylene is less than 0.945, the effect of improving rigidity will be insufficient and the flow ratio will decrease.
When it exceeds 15, the transparency of the film decreases, which is not preferable. In order to effectively carry out the present invention, high-density polyethylene having a density of 0.955 or more and a flow ratio of 8 to 12 is particularly preferably used. In the present invention, density refers to density measured based on JIS K6760 (polyethylene test method). In addition, the flow ratio (hereinafter abbreviated as FR) is the value obtained using a load of 10.8 kg (MI ₁₀ ) in the measurement method of melt index (hereinafter abbreviated as MI) of JIS K6760. It is the quotient obtained by dividing by the value (MI) obtained using a load of Kg. That is, FR=MI ₁₀ /MI. Polyethylene with a higher FR tends to have a broader molecular weight distribution and better fluidity, but if the FR exceeds 15, the resulting film will have poor transparency and cannot be used in the method of the present invention. The polypropylene and polyethylene used in the present invention have a ratio of polyethylene MI to polypropylene melt flow rate (hereinafter abbreviated as MFR) of 1.2 or more, that is, MI/MFR≧1.2,
In addition, the mixing ratio must be in the range of 96 to 85% by weight of polypropylene and 4 to 15% of polyethylene. If the ratio of MI to MFR does not reach 1.2, the transparency of the film will decrease, which is not preferable. If the mixing ratio of polyethylene is less than 4%, the effect of improving rigidity is insufficient,
If it exceeds 15%, the film becomes brittle and the transparency decreases significantly, both of which are unfavorable. In the present invention, MFR is measured based on JIS K6758 (polypropylene test method). The composition comprising the above-mentioned polypropylene and polyethylene used in the present invention may include stabilizers, processing aids, antistatic agents, flame retardants, fillers, etc. commonly used for polyolefins, as well as ethylene-propylene rubber, It is possible to add other polymers such as ethylene-vinyl acetate copolymer, etc., within a range that does not impair the object of the present invention. As a method for forming a film using the above composition, any of the known T-die method and tubular method can be used.
The film must be formed under conditions such that the composition melt extruded at ~250°C is rapidly cooled to below 50°C.
If the melt extrusion temperature does not reach 180℃, the transparency of the film will be poor, and if it exceeds 250℃, there will be frequent fissures due to polyethylene gelation, and the film moldability will become unstable due to the difference in fluidity between the two resins. Both are unfavorable. In addition, the cooling temperature after melt extrusion is 50°C or less, preferably 40°C or less,
If the temperature exceeds 50°C, the film becomes cloudy due to crystallization of polyethylene, the transparency is significantly reduced, and the film becomes brittle, which is not preferable. The film obtained by the method of the present invention has excellent transparency, low-temperature heat-sealing properties, and high-speed heat-sealing properties, and has significantly superior rigidity compared to conventionally known films. Not only does it have a wide range of uses as a film, mainly for packaging, but it is also extremely useful from the perspective of resource conservation, as it can be made 20 to 50% thinner than conventional films for the same purpose. The present invention will be specifically explained below using examples. The methods for measuring and determining film characteristics used in the examples are shown below. (1) Transparency: Turbidity according to ASTM D1003, % (2) Rigidity: Young's modulus (1% modulus) according to ASTM D882, Kg/cm (3) Heat sealability: Seal temperature at which a seal strength of 500 g/15 mm is obtained, °C Seal Conditions: Toyo Seiki thermal gradient seal temperature measuring device, seal pressure 1Kg/cm ²・G, 1 second Tensile test: Sample width 15mm, tensile speed 300mm/m
in (4) Impact strength: Punching method according to ASTM D781, Kg・
cm (5) Film winding appearance: The surface flatness of the roll obtained by winding a 1000 m film, winding wrinkles, thickening at the end of the roll, etc. were observed. (6) Fishery eye: Using "Fishyu eye counter TPLS type" manufactured by Yaskawa Electric Manufacturing Co., Ltd.
^1000m2 of fish eye with a diameter of 0.1mm or more
The number of hits was measured. Examples 1 to 8, Comparative Examples 1 to 10 Various propylene-α-olefin copolymers (containing 0.1% phenolic antioxidant, 0.1% calcium stearate, and 0.1% erucamide) and various polyethylenes (containing 0.1% phenolic antioxidant, 0.1% erucamide) (containing 0.1% antioxidant and 0.2% calcium stearate) in various proportions to obtain the compositions shown in Table 1. Using the above composition, a film with a thickness of 30 μm and a width of 300 mm was formed by the T-die method at a melt extrusion temperature of 220°C and a cooling roll temperature of 30°C,
1000m of each was wound onto a paper tube. The properties of the obtained film and the winding form of the film are also listed in Table 1.

[Table] The following is clear from the results in Table 1. (1) Tm of propylene-α-olefin copolymer
When the ratio becomes low, the heat sealability improves, but the rigidity decreases significantly and the winding appearance of the film becomes poor. (2) The films produced under the conditions specified in the present invention (Examples 1 to 8) have excellent properties in terms of transparency, rigidity, impact strength, heat sealability, and film winding form. However, films outside the limited range of the present invention (Comparative Examples 1 to 10) have a defect in at least one of the above properties and are not preferred in practice. Examples 9 to 16, Comparative Examples 11 to 15 Ethylene content 2.8%, butene-1 content 3.3%,
Tm144℃, MFR7.0 ethylene-propylene-butene-1 terpolymer (phenolic antioxidant)
0.1%, calcium stearate 0.1%, erucamide 0.1%, silica 0.1%) to 93%,
Film forming conditions (melt extrusion temperature and cooling roll The thickness was 25 mm using the T-die method by changing various temperatures (temperature).
A film with a width of 400 mm was obtained. Table 2 shows the film forming conditions and the properties of the obtained film. From the results in Table 2, it is clear that films with excellent properties can be obtained by the method of the present invention.

【table】

Claims (1)

[Claims] 1. The polypropylene is a crystalline propylene-α-olefin copolymer having a crystal melting point of 150° C. or less and a propylene component of 70% or more, and the polyethylene is high-density polyethylene with a density of 0.945 or more and a flow ratio of 15 or less, and polypropylene 96 such that the ratio of the melt index (MI) of the polyethylene to the melt flow rate (MFR) of the polypropylene is (MI/MFR) ≧1.2.
An easily heat-sealable polyolefin film with excellent transparency and rigidity, characterized by melt-extruding a composition consisting of ~85% by weight and 4-15% by weight of polyethylene at 180-250°C and rapidly cooling it to 50°C or less. manufacturing method.