JPH0720664B2 - Method for producing low density polyethylene biaxially stretched film - Google Patents

Description

The present invention relates to a method for producing a biaxially stretched film of low density polyethylene.

More specifically, the present invention relates to a method for producing a biaxially stretched film from a composition comprising a specific linear low density polyethylene and a branched low density polyethylene.

[Prior art]

Linear low-density polyethylene with less branching produced by copolymerization of ethylene and α-olefin is stronger in tensile strength, impact strength, rigidity, etc. than high-pressure low-density polyethylene produced by radical polymerization under high temperature and high pressure. It has excellent properties, such as environmental stress crack resistance (ESCR), heat resistance, and heat sealability, and has been used in various fields in recent years. Particularly in the field of film, the substitution of high-pressure low density polyethylene for linear low density polyethylene is rapidly advancing because of its superior physical properties.

However, the linear low-density polyethylene having the above-mentioned excellent properties has poor transparency, inferior commercial value in appearance, and low melt tension as compared with the high-pressure low-density polyethylene. At times, there are problems such as instability of bubbles and a phenomenon that the neck-in becomes large at the time of T-die film forming, and film forming conditions must be adjusted to narrow conditions. Further, in order to improve the transparency, an attempt to stretch the film is proposed, but in the uniaxial stretching treatment, the physical properties of the film are different from each other, particularly in the longitudinal direction (stretching direction).
There is still a problem with the tear resistance of. Therefore, it is conceivable to subject the raw film to biaxial stretching, but since the linear low-density polyethylene resin is essentially a crystalline polymer,
Like conventional high-density polyethylene resin, it is difficult to biaxially stretch, and even if it is strongly biaxially stretched, appearance problems such as stretch unevenness in the film and deterioration of transparency occur, and strength is also increased. In particular, a stretched film that can be put to practical use has not been obtained. Further, a method for improving the above-mentioned drawbacks of the biaxial stretching treatment, that is, a method for biaxially stretching an unstretched linear low-density polyethylene film under specific conditions has been proposed. (JP-A-58-90924) [Problems to be solved by the invention] However, the above-mentioned proposal also has poor drawing operability when biaxially drawing an unstretched film of linear low-density polyethylene, resulting in a stretched film to be obtained. However, it cannot be said that a biaxially stretched film having a sufficient commercial value can be provided because the thickness unevenness is large and the transparency is not sufficiently improved.

[Means for solving problems]

In view of such prior art, the present inventors have improved the unstretched film formability without impairing the above-mentioned various properties of the linear low-density polyethylene, and the stretching operability of the biaxially stretched film,
As a result of repeated intensive studies to obtain a biaxially stretched film with significantly improved transparency, an unstretched film was prepared using a mixture of linear low density polyethylene with a specific amount of branched low density polyethylene having specific physical properties. It was found that the object can be achieved by molding and then subjecting this to biaxial stretching under specific conditions, and completed the present invention.

That is, the gist of the present invention is that the melt index is 0.3 to 4
97 to 55 parts by weight of linear low-density polyethylene of g / 10 minutes, melt index of 0.3 to 4 g / 10 minutes, and molecular weight distribution index of 3 to 15
Of a branched low-density polyethylene of 3 to 45 parts by weight and a polyethylene resin composition having a molecular weight distribution index of 1 to 10 are used to form an unstretched film or sheet, and the film or sheet is then stretched in a longitudinal direction of 1.1 to 2 4 times in the horizontal direction
It exists in the manufacturing method of the low density polyethylene biaxially stretched film characterized by being biaxially stretched at a draw ratio of up to 7 times.

To explain the present invention in detail, the linear low-density polyethylene used in the present invention is a copolymer of ethylene and other α-olefin, and is a low-density polyethylene resin produced by a conventional high-pressure method. Is different. The linear low-density polyethylene is, for example, copolymerized with ethylene and other α-olefins such as butene, hexene, octene, decene, 4-methylpentene-1 in an amount of about 4 to 17% by weight, preferably about 5 to 15% by weight. It was produced using a Ziegler type catalyst or Phillips type catalyst used in the medium and low pressure method high density polyethylene production, and a conventional high density polyethylene with a short branched structure by a copolymerization component,
The density is also appropriately reduced by utilizing this short chain branching to 0.91 ~
It is about 0.95 g / cm ³ and is a polyethylene having a structure that is more linear than conventional low density polyethylene and more branched than high density polyethylene.

As the linear low density polyethylene used in the present invention,
Melt index is 0.3-4g / 10min, preferably 0.5-3
It is in the range of g / 10 minutes and the density is 0.915 to 0.935 g / c.
m ^3, preferably is preferably used in the range of 0.918~0.925g / cm ^3. If the melt index is less than 0.3 g / 10 min, the film tension becomes excessive at the start of the biaxial stretching operation by the tenter method, and it is necessary to raise the tenter temperature to near the melting point.In this case, part of the film melts in the tenter. As a result, the drawing operability is reduced. Further, if it is larger than 4 g / 10 min, the film is likely to be cut during stretching, the elongation from the center of the film becomes excessive even when stretched, and the uniform stretching property in the transverse direction (thickness unevenness) becomes poor, which is not preferable. .
Furthermore, if the density is less than 0.915 g / cm ³ , the melt index will be
The same phenomenon occurs as when it is larger than 4g / 10 minutes, and 0.9
If it is larger than 35 g / cm ^3, the same phenomenon as that of the case where the melt index is less than 0.3 g / 10 minutes occurs, which is not desirable.

As the branched low-density polyethylene to be blended with the linear low-density polyethylene, a normal high-pressure method polyethylene, that is, using a radical generator such as organic peroxide or oxygen, pressure 1000 to 3000 kg / cm ² , temperature 150 Those obtained by homopolymerizing ethylene or copolymerizing ethylene with another copolymerization component under the condition of up to 400 ° C are used. As the copolymerization component, propylene, butene, hexene, octene,
Examples include α-olefin such as 4-methylpentene-1 and vinyl compounds such as vinyl acetate, ethyl acrylate and methyl acrylate. The copolymerization amount of the copolymerization component is 0.5 to 18% by weight, preferably about 2 to 10% by weight.

The branched low density polyethylene has a melt index of 0.3 to 4 g / 10 minutes, preferably 0.53 g / 10 minutes, and a density of 0.915 to 0.930 g / cm ³ , preferably 0.918.
The range is from 0.930 g / cm ³ . Melt index is 0.3g
If it is less than / 10 minutes, high-temperature stretching is required, and in this case, a part of the film is melted in the tenter, which leads to a reduction in stretching operability. On the other hand, if it is larger than 4 g / 10 minutes, the film is likely to be cut during stretching, and even if it is stretched, the thickness unevenness in the transverse direction becomes excessive, and the film strength decreases, which is not preferable. Further, when the above-mentioned density is less than 0.915 g / cm ³ , a phenomenon similar to that with a melt index of less than 4 g / 10 min occurs, and when it is more than 0.930 g / cm ³ , the melt index is less than 0.3 g / 10 min. This is not desirable because it causes a phenomenon similar to that of the above.

Furthermore, as the branched low-density polyethylene described above,
The molecular weight distribution index (hereinafter abbreviated as α) represented by (value of melt index) × (value of melt tension) is 3 to 15
It is necessary that it be in the range of from the viewpoint of stretching operability. When the above α is less than 3, the molecular weight distribution becomes sharp, the uniform stretchability in the transverse direction (thickness unevenness) becomes poor at the time of stretching, and when it is more than 15, the stress at the time of stretching becomes excessive,
This is not desirable because the original film of the film is easily cut during the stretching in the transverse direction.

In the present invention, the melt index is a value measured at 190 ° C. according to JIS K6760 (g / 10 minutes), and the melt tension is 160 ° C. from a nozzle used for measuring the melt index in VIS K6760, a speed of 0.25 g / min. Extruded at 1.52m /
It is the tension (g) measured at a position 25 cm away from the nozzle when it was taken at a speed of min, and the density was JIS K67.
It is the value measured according to 60 (g / cm ³ ).

The blending ratio of the linear low-density polyethylene and the branched low-density polyethylene is 97 to 55 parts by weight of the linear low-density polyethylene, preferably 95 to 60 parts by weight, and the branched low-density polyethylene is 3 to 45 parts by weight, preferably Is used within the range of 5 to 40 parts by weight. When the branched low-density polyethylene is less than the above range, it is extremely difficult to draw at a predetermined draw ratio as well as uniform drawability, and when it is more than the above range, remarkable stretching unevenness (thickness unevenness) may occur. Is not preferable.

The polyethylene resin composition used in the present invention may be obtained by dry blending the above-described linear low density polyethylene and branched low density polyethylene with a blender, or after dry blending, an ordinary melt kneading method, for example, Banbury mixer, container. It can be obtained by melt-mixing with an intimate mixer, a mixing roll, a kneading extruder, etc. and pelletizing.

The polyethylene resin composition obtained as described above has a molecular weight distribution index (hereinafter abbreviated as α) represented by (value of melt index) × (value of melt tension) of 1
It is necessary to be in the range of -10 from the viewpoint of stretching operability. When the above α is less than 1, the molecular weight distribution becomes sharp and the uniform stretchability (thickness unevenness) in the transverse direction during stretching becomes poor. Is easy to cut, which is not desirable.

It should be noted that, if necessary, known additives generally used for polyethylene such as an antioxidant, an ultraviolet absorber, an antistatic agent and a lubricant may be added to the above polyethylene resin composition.

In the present invention, an unstretched film or sheet is molded using the polyethylene resin composition obtained above, and then the unstretched film or sheet is biaxially stretched under specific conditions to produce a biaxially stretched film. To do.

The unstretched film or sheet is formed by using a usual film or sheet forming apparatus and forming method, for example, an inflation forming method using a circular die, a T die forming method using a T die, and the like. 250
The molding is carried out under the molding conditions of ℃ and draft ratio of 2 to 100.

Inflation molding has a blow-up ratio of 1.
It is desirable to carry out under the conditions of 5 to 4.0 and a draft rate of 2 to 100.

In the present invention, the draft rate is obtained as follows.

In the formula, the symbols are as follows In the case of T-die molding, it is expressed as BUR = 1.

If the draft ratio is less than 2, the optical properties of the film-forming film will be poor, and if it is greater than 100, longitudinal tearing will tend to occur during stretching, such being undesirable.

The unstretched film obtained as described above is then biaxially stretched. The biaxial stretching treatment is carried out by subjecting an unstretched film obtained by a T-die or an inflation method as it is or slitting it into a predetermined width and sequentially biaxially stretching or simultaneously biaxially stretching it. Sequential biaxial stretching is carried out either in the longitudinal direction (film take-up direction) and then in the transverse direction (direction orthogonal to the film take-up direction) or vice versa. Further, in the simultaneous biaxial stretching, temporal distribution of stretching in the machine direction and the transverse direction is arbitrary, and for example, the stretching is gradually continued in the machine direction until the stretching in the machine direction is completed, or the start of stretching is in the machine direction. The horizontal direction is performed at the same time, but the vertical direction is completed first.

In the present invention, it is possible to employ a tenter method sequential biaxial stretching, a tubular method simultaneous biaxial stretching method, or the like, but particularly when the tenter method sequential biaxial stretching method is adopted, transparency is remarkably improved. Therefore, it is preferable.

The biaxial stretching treatment in the present invention is performed under the following conditions of stretching temperature, stretching speed and stretching ratio.

The stretching temperature is the melting point of the polyethylene resin composition −20 ° C.
The melting point range, preferably the mobility of the molecular chain is poor, so that the film is easily cut during stretching, and even if it can be stretched, the stretching ratio does not increase and a stretched film having excellent physical properties cannot be obtained. In addition, at a temperature higher than the melting point, the resin composition is partially melted and stretch orientation cannot occur, and even if apparently stretched, stretch unevenness is severe, transparency is also impaired, and the film has commercial value. Don't

Stretching speed is in the range of 2-40% / sec, preferably 10-20% /
It is in the range of seconds. If the stretching speed is slower than 2% / sec, the stretchability is apt to be hindered by oriented crystallization during stretching, and 40%
If it is faster than / sec, the deformation of the polymer cannot keep up with the stretching speed and the stretch will be broken.

The stretching ratio is 1. in the machine direction of the film in terms of stretching operability (ease of stretching) and physical properties of the obtained biaxially stretched film.
The range is 1 to 2 times and the range is 4 to 7 times in the lateral direction. If the stretching ratio in the machine direction is more than 2 times, the stretching operability is deteriorated, and a satisfactory stretched film cannot be obtained. Further, if the stretching ratio in the transverse direction is less than 4, the thickness unevenness of the obtained stretched film becomes large. A uniform stretched film cannot be obtained, and if it is more than 7 times, the stretch operability deteriorates,
It is not preferable because a satisfactory stretched film cannot be obtained.

〔Example〕

Example 1 Linear low density polyethylene (melt index (hereinafter,
Abbreviated as MI. ): 0.5 g / 10 minutes, density: 0.920 g / cm ³ , flow ratio: 20, copolymerization component: butene-1, copolymerization amount: 10% by weight)
After 85 parts by weight of 15 parts by weight of branched low-density polyethylene (high-pressure polyethylene, MI: 1.3 g / 10 minutes, density: 0.923 g / cm ³ ) was dry-blended, a single-screw extruder with a cylinder diameter of 40 mmφ at 190 ° C The mixture was melt-kneaded at the temperature of and extruded into pellets.

The obtained polyethylene resin composition (melting point: 120 ° C.) is 190 μm thick under the conditions of a resin temperature (die temperature) of 200 ° C., a cooling roll temperature of 15 ° C., a winding speed of 30 m / min, and a draft rate of 5 using a T-die film apparatus. An unstretched raw film of Sano was formed. This film stock is stretched at a temperature of 110 ° C. using a tenter sequential biaxial stretching device, and the stretching speed is 110 ° C. at a stretching speed of 10% using a tenter sequential biaxial stretching device.
/ Sec and a stretching ratio (1.25 times in the longitudinal direction and 5.0 times in the lateral direction), a biaxially stretched film having a thickness of 30 μm was produced. The obtained biaxially stretched film was subjected to a film physical property test by the following evaluation methods, and the results shown in Table 1 were obtained.

 Molding stability ○ Stable stretching can be continuously performed for 1 hour or more.

△ Can be stably stretched by connecting for 20 minutes or more.

C. Cutting (vertical tear) occurs at the start point of biaxial stretching.

Film thickness unevenness A dial gauge with a minimum scale of 1μ is measured at 3cm intervals in the film width direction, and the thickness unevenness relative to the average thickness is displayed as a percentage on the + and-sides. -67 Tear strength JIS P8116-73 vertical length Tear notched in the flow direction Tear strength Horizontal tear film Notched tear strength in the width direction Examples 2-9 and Comparative Examples 1-7 In Example 1 The same procedure as in Example 1 was repeated except that the physical properties and blending amounts of the linear low-density polyethylene and the branched low-density polyethylene, the unstretched film molding conditions, and the biaxial stretching conditions were changed as shown in Table 1. The results are shown in Tables 1 and 2.

Examples 10 to 12 and Comparative Examples 8 to 10 In Example 1, the physical properties and blending amounts of the linear low density polyethylene and the branched low density polyethylene were changed as shown in Table 3, and the unstretched film was formed. The same procedure as in Example 1 was carried out by an inflation molding method (molding conditions are shown in Table 3) instead of the T-die method, and the obtained unstretched film was biaxially stretched as it was (two sheets at the same time). It was The results are shown in Table 3.

Comparative Example 11 A resin composition having a molecular weight distribution index of 11.0 was obtained in the same manner as in Comparative Example 10, except that the usage rate of LLD, the type of LD, and the usage rate were changed as shown below. Then, Example 1
An unstretched film original fabric was formed in the same manner as above, and biaxially stretched using a tenter method sequential biaxial stretching device. However, in the biaxial stretching operation, when the film was stretched in the transverse direction, longitudinal tearing occurred in the film, and a good film could not be formed. The results are shown in Table 4.

[Effects of the Invention] According to the production method of the present invention, a biaxially stretched film using linear low density polyethylene, which has been difficult in the past, can be stably obtained with high efficiency, and the obtained film is excellent in transparency and uniform. It has thickness and is very excellent in strength.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Koichi Hasegawa, Koichi Hasegawa, 3-10-10, Shiodotsu, Kurashiki-shi, Okayama Mitsubishi Kasei Kogyo Co., Ltd. Mizushima Plant (56) Reference JP-A-57-181828 (JP, A)

Claims (1)

[Claims] 1. A linear low density polyethylene having a melt index of 0.3 to 4 g / 10 minutes 97 to 55 parts by weight, a branched low density polyethylene 3 having a melt index of 0.3 to 4 g / 10 minutes and a molecular weight distribution index of 3 to 15. To 45 parts by weight of a polyethylene resin composition having a molecular weight distribution index of 1 to 10 is used to form an unstretched film or sheet, and the film or sheet is then stretched 1.1 to 2 times in the machine direction and 4 to 4 times in the cross direction. A process for producing a low-density polyethylene biaxially stretched film, which comprises biaxially stretching at a stretch ratio of 7 times.