JPS5829208B2 - Manufacturing method of polyethylene film for twist packaging - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a polyethylene film that has excellent transparency, twist retention, and Young's modulus, has twist packaging suitability, and has improved vertical tear properties.

In recent years, in the field of twist packaging, axially stretched high-density polyethylene films, which are pollution-free and inexpensive, are rapidly gaining popularity as an alternative to the mainstay cellophane.

However, since high-density polyethylene film is uniaxially stretched, in practical use, vertical tearing may occur at the corners of the package (for example, rounded corners, candy, etc.) or at the twisted part, so improvements are desired. Ta.

The purpose of the present invention is to achieve a haze level of 3% or less and a twist retention rate of 0.7.
Above, Young's modulus is 160kg/rni and vertical tear is 3%.
The object of the present invention is to provide a method for producing a polyethylene film that has the following excellent twist packaging suitability and improved vertical tear properties.

In the present invention, the cloudiness is determined according to ASTM-D-1003-61.
The twist retention rate was measured using a photoelectric haze meter made by Toyo Seiki according to the method of Wrap the film by twisting the film at both ends 1.5 times at 20°C, 24°C.
This is the value obtained by measuring the number of twists after standing for a period of time and dividing this by 1.5.

The film at both ends is twisted 3600 times in one twist.

The vertical tear rate is the number of candies with vertical tears when 300 candies were packaged, expressed as a percentage.

Further, Young's modulus is measured by dividing stress in a straight line portion (elastic region) of a stress-strain curve based on ASTM-D638-58T by strain.

The present invention is a method for producing a uniaxially oriented polyethylene film by extruding polyethylene as a molten film from a T-die, cooling and solidifying it on a cooling roll, and then stretching the film, which is characterized by the following (1) to (5). This is a method for manufacturing polyethylene film for twist packaging.

(1) Density is 0.94.9/cfIt or more and AST
M■1o/MI216 calculated from M■2□6 measured by the method of MD1238-52T and M■1o calculated using the same method with a load of 10 kg is 1.
0 or less polyethylene (a blend of more than 1 part by weight and less than 10 parts by weight of ethylene-propylene copolymer and/or ethylene-butene copolymer) is used as the raw material resin.

(2) Manufacturing a raw film by cooling and solidifying the raw resin extruded as a molten film from a T-die to a temperature between 70 and 120°C on a cooling roll.

(3) The raw film is heated to 80 to 13
Preheat to a temperature of 0°C.

(4) Stretching the preheated raw film in the longitudinal direction at a stretching ratio of 4 times or more.

(5) Annealing the stretched film at a temperature of 80 to 130°C and then cooling it.

As mentioned above, the polyethylene used in the present invention has a density of 0.
94. @/cIf! That's all for M I 10/ M I2.
As long as 16 is 10 or less, in addition to the ethylene homopolymer, a copolymer consisting mainly of ethylene and a small amount of 1-olefin may also be included.

In the method of the present invention, the density is 0.94 g/cr! Low-density polyethylene with a density of less than 4 times has a high degree of branching and is difficult to cause crystal orientation, and stretching of 4 times or more is impossible because stretching breaks.

Further, the obtained film has a Young's modulus of 100, 9/i or less, and a twist retention rate of 0, which is unsuitable for the purpose of the present invention.

In addition, M1o/MI2.16 is an important factor, and it is necessary to form a film using polyethylene within the limits specified in the present invention and under the conditions specified in the present invention (thus, the cloudiness, which is the objective of the present invention, can be achieved for the first time) A polyethylene film having twist retention and Young's modulus is obtained.

Furthermore, it is an extremely important factor in the present invention to blend more than 1 part by weight and not more than 10 parts by weight of ethylene-propylene copolymer and/or ethylene-butene copolymer with respect to 100 parts by weight of raw polyethylene.

The amount of the ethylene/propylene copolymer or ethylene/butene copolymer added must be more than 1 part by weight and less than 10 parts by weight. If the amount is too small, and if it exceeds 10 parts by weight, the twisting properties of the polyethylene film will be significantly impaired, so that it cannot be put to practical use.

In the present invention, polynee f-L extruded from a T-die
/7 The molten film is placed on a cooling roll at a temperature of 70 to 12
It is cooled and solidified at a temperature of 0°C, preferably 90 to 100°C.

Generally, it is said that it is preferable to rapidly cool an extruded film in order to produce a transparent film, but in the method of the present invention, the film is formed by slow cooling at a relatively high temperature of 70° C. or higher, preferably 90° C. or higher.

If the above-mentioned polyethylene is used as the raw material for the present invention, the transparency will not be impaired even if it is slowly cooled, and by slow cooling using the method of the present invention, the film will adhere tightly to the roll and its flatness will be improved. A good and uniform original film for stretching can be obtained, and the stretched film obtained by stretching this film has uniform thickness and uniform transparency.

On the other hand, if the film is rapidly cooled to a temperature lower than the above temperature, the film will not adhere to the roll, and the areas that do not adhere to the roll will become white, resulting in uneven striped patterns, and the flatness will also be poor, resulting in a stretching film with mountain-like unevenness. A raw film is obtained, and even if this is stretched, only a stretched film with non-uniform transparency and thickness can be obtained.

That is, it is an unexpected discovery that by forming a film using the raw material of the present invention under the cooling conditions of the present invention, it is possible to obtain a raw material for stretching that has good roll adhesion and does not impair the transparency of the resulting film. This invention is based on this.

Note that if the cooling temperature is too high, the film will stick to the cooling roll, making film formation difficult, so the upper limit is 120°C, preferably 100°C.

In the present invention, the raw film obtained by the above-mentioned cooling and solidification is preheated to 80 to 130°C by a preheating device comprising pinch rolls having a smooth surface prior to stretching.

It is necessary to use smooth surfaces for both the heat roll and the pinch roll, and in particular, the heat roll should have a mirror finish, and the pinch roll should have a smooth surface and be appropriately crowned.

The preheating temperature of the film is 80-130℃, preferably 10
When the temperature is from 0 to 120°C and below 80°C, stretching tends to break, and when it is above 130°C, it sticks to the rolls and becomes impossible to stretch.

In the present invention, the stretching is carried out between a preheating roll and a stretching roll installed after it, similar to the method conventionally used for longitudinal stretching, and the stretching is performed between the stretching roll and the preheating roll. The stretching ratio is determined by the ratio.

In the method of the present invention, by increasing the stretching ratio in the longitudinal direction to 4 times or more, preferably 6 to 9 times, the desired twist retention rate of 0.7 or more can be obtained, and the transparency is further improved and the haze level is 3. % or less, and the Young's modulus also becomes 160 and 9/-.

In order to maintain flatness after winding, the stretched film is annealed on a hot roll at a temperature of 80 to 130°C under tension or slightly forced to shrink. The lower annealing is carried out by making the break of the next Tikuoff roll on the hot roll several percent slower than the break on the hot roll.

If the temperature is below 80℃, it will take a long time to anneal.
If the temperature is above ℃, the film will stick to the roll and become impossible to collect.

After annealing, it is necessary to cool the film after annealing in order to ensure stable winding without giving any distortion to the film before taking it off.This cooling can be done naturally, but it is better to use forced cooling. is preferred.

When the present invention is carried out in accordance with the above-mentioned conditions, it is possible to obtain a packaging film having excellent suitability for twist packaging and having high practical value.

Examples Various high-density polyethylenes and ethylene-propylene copolymers (propylene content 42%) as shown in Table 1 and Group 2
%) and ethylene-butene copolymer (butene content 30%)
Using raw materials blended with high-density polyethylene, 1
Using a 15 mmφ extruder, the extrusion temperature was 240°C.
Immediately after being discharged from the 00m1 width T-die, it is placed on a cooling roll.
The film was formed by cooling to 5° C. to ensure complete adhesion between the roll and the film to obtain an original (unstretched) film.

Immediately press this onto a preheating roll with a pinch roll for 12
The film was heated to 0° C., then stretched 2 to 8 times in the machine direction, and then annealed at 120° C. The winding speed was then reduced by several percent in accordance with the stretching ratio and wound.

The film thickness is 25 μm and the winding speed is 60 m.
/mryt.

When the stretching ratio was 2 times, it was impossible to form a film due to uneven stretching.

The haze percentage, twist retention rate (→ vertical tear rate (total), and Young's modulus (kg/vuj) of the original (unstretched) film and the 4, 6, and 8 times stretched film are shown.

For comparison, we similarly measured the physical properties of a commercially available cellophane film with a thickness of 25μ for twist packaging, and found that the haze was 1.4%, the twist retention was 0.80, the vertical tear was 1%, and the Young's modulus was 160. It was 9/-.

It is clear that the method of the present invention can produce a polyethylene film that is comparable to cellophane in haze, twist retention, warp tear, and Young's modulus.

Claims (1)

[Claims] 1. A method for producing a uniaxially oriented polyethylene film by extruding polyethylene as a molten film from a T-die, cooling and solidifying it on a cooling roll, and then stretching the film, which includes the following (1) to (5). A method for manufacturing polyethylene film for twist packaging with improved vertical tear properties. (1) Density is 0.94g/- or more and ASTMD12
1 weight per 100 parts by weight of polyethylene whose M 1 o/MI 216 calculated from MI2.16 measured by the method of 38-52T and M 1 o calculated using the same method with a load of 10 kg is IO or less. A blend of more than 10 parts by weight of ethylene-propylene copolymer and/or ethylene-butene copolymer is used as the raw material resin. (2) Manufacturing a raw film by cooling and solidifying the raw material resin extruded as a molten film from a T-die at a temperature between 70 and 120°C on a cooling roll. (3) The raw film is heated to 80 to 13
Preheat to a temperature of 0°C. (4) Stretching the preheated raw film in the longitudinal direction at a stretching ratio of 4 times or more. (5) Annealing the stretched film at a temperature of 80 to 130°C and then cooling it.