JPH054276A - Method for producing polyethylene-based biaxially stretched film - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a method for producing a polyethylene-based biaxially stretched film which is made of a resin mainly composed of linear low density polyethylene as a raw material and has no stretch unevenness and low shrinkage characteristics. [Structure] A raw material resin mainly composed of linear low-density polyethylene is melt-extruded to be cooled and solidified to form a substantially non-oriented sheet, and then, by a simultaneous biaxial stretching method, the following formula [Numerical formula 1] After stretching 4 times or more and less than 7 times in the longitudinal and transverse directions at a stretching temperature satisfying the conditions, heat treatment is further performed at a temperature satisfying the condition of the following mathematical formula [Formula 2], and 100 ° C
To produce a polyethylene-based biaxially stretched film having a heat shrinkage ratio of 8% or less in each of the longitudinal and transverse directions. ## EQU1 ## Tm-30.degree. C..ltoreq.T1.ltoreq.Tm-10.degree. C. ## EQU2 ## Tm-15.degree. C..ltoreq.T2.ltoreq.Tm (where Tm is the melting point of the raw material resin, T1 is the stretching temperature,
T2 represents the heat treatment temperature)

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyethylene-based biaxially stretched film.
The present invention relates to a method for producing a polyethylene-based biaxially stretched film having a low shrinkage property, which is made of a resin mainly composed of linear low-density polyethylene.

[0002]

2. Description of the Related Art A linear low density polyethylene resin is produced by ionic polymerization under low temperature and low pressure as compared with a conventional high pressure method low density polyethylene resin. However,
Since linear low density polyethylene resin is essentially a crystalline polymer, it is difficult to biaxially stretch like conventional high density polyethylene.

Japanese Patent Laid-Open No. 58-90924 proposes a method for producing a biaxially stretched film using a linear low density polyethylene resin as a raw material. The above production method is as follows: Stretching temperature: melting point of linear low density polyethylene resin -20 ° C to melting point -5 ° C, stretching speed: 25 to 400% / sec, stretching ratio: at least 3 times or more and less than 8 times in one direction. It is characterized in that biaxial stretching is performed under the condition that the product of the stretching ratios in two directions is 9 times or more and less than 50 times.

[0004]

However, even with the above-described manufacturing method, a satisfactory stretched state cannot be obtained because stretch unevenness occurs in the film or film breakage occurs, and further improvement studies are required. is there. Further, the film obtained by the above manufacturing method, as a sealant film, for example, when laminated with a nylon film or a polyester film and then heat-sealed into a bag, or a general-purpose packaging film, for example, a fiber. When making a bag with a fusing seal for packaging, there is a drawback that wrinkles occur at the seal portion. The present invention has been made in view of the above circumstances, and an object thereof is to use a resin mainly composed of linear low-density polyethylene as a raw material, establish a good stretched state, have no stretch unevenness, and have excellent thickness accuracy. It is to provide a method for producing a polyethylene-based biaxially stretched film. Another object of the present invention is to provide a method for producing a polyethylene-based biaxially stretched film having suitable properties as a sealant film.

[0005]

The above objects of the present invention are as follows.
According to the present invention, a raw material resin mainly composed of linear low-density polyethylene is melt-extruded to be cooled and solidified to form a substantially non-oriented sheet, which is then subjected to a simultaneous biaxial stretching method. After stretching 4 times or more and less than 7 times in the machine and transverse directions at the stretching temperature satisfying the condition of the formula [Formula 1] described in [1], the formula [formula 2] described in claim 1 is further used. It is easily achieved by a method for producing a linear low-density polyethylene-based biaxially stretched film having a heat shrinkage ratio at 100 ° C. of 8% or less in each of the longitudinal and transverse directions, which is characterized by heat treatment at a temperature satisfying a condition. It

The present invention will be described in detail below. The raw material resin used in the present invention is a resin mainly composed of linear low density polyethylene. Linear low density polyethylene is a copolymer of ethylene and α-olefin, and is manufactured by a low pressure method unlike low density polyethylene manufactured by a conventional high pressure method. Then, as the α-olefin copolymerized with ethylene, butene, pentene, hexene, octene, 4-methylpentene and the like can be mentioned. The structural difference between the high-pressure low-density polyethylene and the linear low-density polyethylene is that the former has a multibranched molecular structure and the latter has a linear molecular structure.

There are various methods for producing linear low density polyethylene, and the physical properties thereof are somewhat different depending on the method. The linear low density polyethylene used in the present invention preferably has an MI (melt index, g / 10 min) of 0.5 to 3.0. When the MI is less than 0.5, the extrudability is insufficient, and in the case of forming a raw sheet as described later, for example, instability of sheet forming due to surging causes a thickness variation, which is further caused by this. Cooling spots often cause variations in transparency or crystallinity.
It is difficult to obtain a raw material having excellent physical properties and stretchability. Further, when MI is more than 3.0, the melt tension is low, and there is a problem in the original forming such as the occurrence of a ripple phenomenon due to the unstable contact with the cooling drum in the T-die forming. Furthermore, the low molecular weight reduces the stretchability and the degree of stretch orientation, which are considered to be due to the short molecular chains and the small entanglement of the molecular chains. As a result, the physical properties of the film are deteriorated and it is difficult to obtain a desired stretched film.

The linear low density polyethylene used in the present invention has a density (ρ) of 0.910 to 0.940 g / c.
Those of c are preferred. When the density is less than 0.910 g / cc, the flexibility of the obtained film is excellent, but there is a problem in processability, and the density is 0.940 g / cc.
If it is larger, the flexibility of the film is impaired.

The linear low-density polyethylene may be a high-pressure polyethylene, an ethylene-vinyl acetate copolymer, an ionomer, an ethylene-propylene copolymer, or an ethylene as long as it does not hinder the object of the present invention. -Saponified vinyl alcohol copolymer, carboxylic acid-modified polyolefin and the like can be mixed. Furthermore, according to the usual method,
Addition of heat and UV stabilizers, pigments, antistatic agents, fluorescent agents, lubricants and the like can be performed.

First, in the present invention, a substantially non-oriented sheet is formed from a raw material resin mainly composed of linear low density polyethylene. The above-mentioned non-oriented sheet can be formed according to a usual sheet forming apparatus and forming method, and for example, a T-die forming method using a T-die can be used.

Next, in the present invention, the above-mentioned unoriented sheet is used as a raw sheet, and biaxial stretching is carried out in the longitudinal and transverse directions by the simultaneous biaxial stretching method. The simultaneous biaxial stretching can be specifically performed by employing a known tenter system simultaneous biaxial stretching method or a tubular system simultaneous biaxial stretching method. Then, the time distribution of the stretching in the machine direction and the transverse direction is arbitrary, and for example, the stretching in the machine direction is gradually started until the stretching in the machine direction is completed, or the stretching in the machine direction and the transverse direction is performed. Starting at the same time, stretching in the transverse direction may be completed first.

The stretch ratio is biaxial stretchability (ease of stretching).
Also, from the viewpoint of the physical properties of the obtained biaxially stretched film, it is preferable to set the ratio in the machine direction and the transverse direction to 4 times or more and less than 7 times, respectively, as in the method described in JP-A-58-90924.

The stretching temperature T1 is calculated by the following formula [Equation 4].
The range must satisfy the condition (same as [Equation 1]).

## EQU00004 ## Tm-30.degree. C..ltoreq.T1.ltoreq.Tm-10.degree. C. (In the above, Tm represents the melting point of the raw material resin, and the endothermic main peak temperature on the melting curve measured by a differential scanning calorimeter (DSC). Is defined as the following.)

When the stretching temperature T1 is lower than Tm-30 ° C., the mobility of the molecular chain is poor, so that the film tends to break during transverse stretching, and even if it is stretched, the stretching ratio does not increase and the stretched film has excellent physical properties. Can't get vice versa,
If the temperature is higher than Tm-10 ° C, the orientation effect due to the stretching cannot be obtained, and further, the stretched raw fabric starts to adhere to the roll to leave an adhesive mark on the raw fabric, which causes the film rupture between the rolls during stretching. Occurs. Even if the film is stretched without breakage, the unevenness of the stretching is severe, adhesive marks are left on the stretched film, the transparency and thickness accuracy are deteriorated, and the film does not have commercial value.

Next, in the present invention, the biaxially stretched film is heat treated. The heat treatment temperature T2 is
It is necessary to set the range to satisfy the condition of the following formula [Equation 5] (same as [Equation 2]).

[Formula 5] Tm −15 ° C. ≦ T2 ≦ Tm

When the heat treatment temperature T2 is lower than Tm-40 ° C., the heat-treated film has a shrinkage ratio of more than 8%, and when it is used as a sealant material, it is heat-sealed. The film shrinks and wrinkles occur on the sealing surface, detracting from the commercial value. On the other hand, when it is higher than Tm, the molecular orientation inside the film generated by stretching flows and collapses, and the physical properties of the film are significantly deteriorated.
A whitening phenomenon occurs due to crystallization of the film, which impairs transparency.

In the case of the tenter system simultaneous biaxial stretching method, the heat treatment is continued in the tenter after stretching, and in the tubular system simultaneous biaxial stretching method, the stretched film is folded, for example, a tenter or a roll. , A heat belt or the like can be used. The heat treatment time is preferably 3 seconds or more. If it is less than 3 seconds, a sufficient heat treatment effect cannot be obtained, and the film has a large heat shrinkability, so that when used as a sealant film, wrinkles may occur during heat sealing. The heat treatment is not particularly limited, but is 5 to 15%.
It is preferable to carry out the relaxation of the above. By the above heat treatment, the biaxially stretched film is adjusted so that the heat shrinkage at 100 ° C. is 8% or less in each of the longitudinal and transverse directions.

In the present invention, in order to more reliably prevent the generation of wrinkles during heat sealing, the heat shrinkage ratio of the biaxially stretched film is adjusted to 5% or less in each of the longitudinal and transverse directions. Therefore, for this reason, the heat treatment temperature T2 is preferably set in a range satisfying the condition of the following mathematical expression [Equation 6] (same as [Equation 3]).

[0019]

[Formula 6] Tm −5 ° C. ≦ T2 ≦ Tm

The biaxially stretched film of the present invention may be subjected to a known surface treatment such as corona treatment or flame treatment, if necessary.

[0021]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. In addition, the measurement items shown in the text and the following examples were based on the following methods.

(1) Shrinkage rate A film cut into a square of 100 mm in both length and width is 10
It was immersed in a silicone oil bath at 0 ° C. for 10 minutes and taken out, and the length and width were measured and calculated by the following formula. Shrinkage rate (%) = 100-A (or B) However, A and B show the length (unit is mm) of each length and width after immersion.

(7) Melting point Using a differential scanning calorimeter (using DSC-II manufactured by Perkin Elmer Co., Ltd.), 10 mg of the raw material resin was heated to 150 ° C. and melted, followed by rapid cooling (40 ° C./min.), Then, 10 ° C./min. The endothermic main peak temperature on the melting curve when the temperature was raised at was taken as the melting point.

Examples 1 and 2 and Comparative Examples 1 to 3 (tubular system simultaneous biaxial stretching method) Density at 23 ° C. 0.922 g / cc, melt index 0.9 g / 10 min, flow ratio 21, copolymerization component 4 -Methylpentene-1, an ethylene-α-olefin copolymer having a copolymerization amount of 10% by weight, and an ethylene-based polymer having a main peak temperature of 125 ° C for a melting curve by DSC is melt-kneaded at 250 ° C, Slit diameter 75m
It was extruded from a circular die of m. The outer side is passed through a water tank while sliding the outer surface of a cylindrical mandrel (external diameter is 66 mm and cooling water of 20 ° C is circulated inside) in which the extruded molten tubular sheet is attached directly under the die. By doing so, it is cooled down to room temperature and taken out, with a diameter of 65
A tubular unoriented sheet having a thickness of 250 μm and a thickness of 250 μm was obtained.

The above-mentioned unoriented sheet was introduced into a tubular type biaxial stretching device and stretched under the conditions shown in Table 1, and then,
After holding both ends of the obtained stretched film with tenter clips, heat treatment was performed in the tenter at the temperature shown in Table 1 and cooling to room temperature to obtain a biaxially stretched film having a thickness of 16 μm. The heat treatment time was 5 seconds. Table 1 shows the observation results of the stretched state and the measurement results of the heat shrinkage ratio. In addition, in Table 1,
∘ indicates a stable stretched state in which the film has no stretching unevenness, Δ indicates a state in which the film has stretching unevenness, and x indicates a state in which the film is broken and cannot be stretched (the same applies to Table 2 below).

[0026]

[Table 1]                     Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 ─────────────────────────────────── Stretching temperature (° C) 100 110 85 85 110 125 Stretching ratio Vertical x Horizontal 4x4 4x6 4x4 4x6 5x6 Stretched state ○ ○ × ○ △ Heat set temperature (℃) 100 125-90- Heat shrinkage%     100 ℃ Vertical 7.2 2.6-32.3-             Horizontal 7.4 4.1 − 45.1 − ───────────────────────────────────

Examples 3 to 4 and Comparative Examples 4 to 6 (Tenter type simultaneous biaxial stretching method) The same ethylene polymer as in Example 1 was melt-kneaded at 250 ° C. and formed into a sheet from a 450 mm wide T-die. It was extruded and brought into close contact with a cooling roll kept at 25 ° C. by a known air knife method to obtain an unoriented sheet having a thickness of 750 μ.

The above-mentioned unoriented sheet is introduced into a tenter type simultaneous biaxial stretching device and stretched under the conditions shown in Table 2. Both ends of the obtained stretched film are held by tenter clips and heat treated at the temperature shown in Table 2. And then cool to room temperature to a thickness of 30μ
To obtain a biaxially stretched film. The heat treatment was performed while giving a relaxation of 7%, and the treatment time was 5 seconds. Table 2 shows the observation results of the stretched state and the measurement results of the heat shrinkage ratio.

[0029]

[Table 2]                     Example 3 Example 4 Comparative example 4 Comparative example 5 Comparative example 6 ─────────────────────────────────── Stretching temperature (° C) 115 95 85 115 115 Stretching ratio Vertical x Horizontal 5x5 6x4 7x4 4x6 5x6 Stretched state ○ ○ × ○ △ Heat set temperature (℃) 125 110-100- Heat shrinkage%     100 ℃ Vertical 3.3 8.0 −21.5 −             Horizontal 3.6 7.6-32.4- ───────────────────────────────────

[0030]

According to the present invention described above, the following effects are achieved. The advantages of using linear low-density polyethylene as a raw material are directly enjoyed. Compared to a film stretched under conventional stretching conditions, the film has no stretching unevenness, has good transparency, and has good strength. Excellent dimensional stability. When used for a sealant film, it has excellent performance such as no wrinkles in the seal portion during heat sealing, and for example, a sealant film,
A film suitable as a packaging film can be obtained.

Claims (2)

[Claims] 1. A raw material resin mainly composed of linear low-density polyethylene is melt-extruded to be cooled and solidified to form a substantially non-oriented sheet, which is then subjected to a simultaneous biaxial stretching method to obtain the following mathematical formula: ] The film is stretched in the machine direction and in the transverse direction at a stretching temperature of 4 times or more and less than 7 times at a stretching temperature that satisfies the following condition, and then further heat-treated at a temperature that satisfies the following mathematical formula [Equation 2]. A method for producing a polyethylene-based biaxially stretched film having a thermal shrinkage at 8 ° C of 8% or less in each of the longitudinal and transverse directions. ## EQU1 ## Tm-30.degree. C..ltoreq.T1.ltoreq.Tm-10.degree. C. ## EQU2 ## Tm-15.degree. C..ltoreq.T2.ltoreq.Tm (In the above formulas, Tm is the melting point of the raw material resin, T1 is the stretching temperature, and T2 is the heat treatment temperature. Represent) 2. The method for producing a linear low-density polyethylene-based biaxially stretched film according to claim 1, wherein the heat treatment temperature is in a range that satisfies the following mathematical expression [Formula 3]. ## EQU00003 ## Tm-5.degree. C..ltoreq.T2.ltoreq.Tm (where Tm represents the melting point of the raw material resin and T2 represents the heat treatment temperature).