JP3030128B2 - Method for producing biaxially oriented polyethylene film - Google Patents

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 biaxially stretched film.
The present invention relates to a method for producing a biaxially stretched polyethylene film using a resin mainly composed of linear low-density polyethylene as a raw material.

[0002]

2. Description of the Related Art A linear low-density polyethylene resin is produced by ionic polymerization at a low temperature and a low pressure as compared with a conventional high-pressure low-density polyethylene resin. However,
The linear low-density polyethylene resin is essentially a crystalline polymer, so that biaxial stretching is difficult as in the case of conventional high-density polyethylene.

JP-A-58-90924 proposes a method for producing a biaxially stretched film using a linear low-density polyethylene resin as a raw material. The above-mentioned production method comprises: a stretching temperature: the melting point of the linear low-density polyethylene resin −20 ° C. to −5 ° C., a stretching speed: 25 to 400% / sec, a stretching ratio: at least one direction is 3 times or more and less than 8 times. In addition, 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-mentioned manufacturing method, a satisfactory stretched state cannot be obtained due to unevenness of stretching of the film or breakage of the film, and further improvement studies are required. is there. The present invention has been made in view of the above circumstances, and its object is to use a resin mainly composed of linear low-density polyethylene as a raw material, establish a good stretch state, have no stretch unevenness, and have excellent thickness accuracy. An object of the present invention is to provide a method for producing a biaxially stretched film. Still another object of the present invention is to provide a biaxially stretched film in which the unstretched remaining width of the film ear portion is small, and the film stretching ratio and the set mechanical ratio are not largely different from each other. It is to provide a manufacturing method. Furthermore,
Another object of the present invention is to provide a method for producing a biaxially stretched film having characteristics suitable as a sealant film.

[0005]

SUMMARY OF THE INVENTION The above objects of the present invention are as follows.
According to the present invention, a raw resin mainly composed of linear low-density polyethylene is cooled and solidified while being melt-extruded to form a substantially unoriented sheet, and then at least one of longitudinal and lateral directions is successively biaxially stretched. The direction is 3 times or more and less than 8 times, and the product of the stretching ratio in two directions is stretched to 9 times or more and less than 50 times, and then heat-treated, and the heat shrinkage at 100 ° C. is 30% or less in each of the longitudinal and transverse directions. In producing a polyethylene-based biaxially stretched film, the stretching temperature condition and the heat treatment temperature condition are set to a range satisfying the conditions of the formulas [Equation 1] to [Equation 3] described in claim 1. Is easily achieved by the method for producing a polyethylene-based biaxially stretched film.

Hereinafter, the present invention will be described in detail. The raw 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 an α-olefin, and is produced by a low-pressure method unlike low-density polyethylene produced by a conventional high-pressure method. Examples of the α-olefin copolymerized with ethylene include butene, pentene, hexene, octene, and 4-methylpentene. The structural difference between the high-pressure low-density polyethylene and the linear low-density polyethylene is that the former has a hyperbranched molecular structure and the latter has a linear molecular structure.

[0007] There are various methods for producing linear low-density polyethylene, and the physical properties thereof are slightly different for each production 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 smaller than 0.5, the extrudability is insufficient, and in forming a raw sheet as described later, for example, instability of sheet forming due to surging causes a thickness variation, and further, due to this. Cooling spots often cause variations in transparency or crystallinity,
It is difficult to obtain a raw material excellent in physical properties and stretchability. When the MI is larger than 3.0, the melt tension is low, and for example, there is an inconvenience in raw material molding such as occurrence of a ripple phenomenon due to unstable contact with a cooling drum in T-die molding. Furthermore, when the molecular weight is small, the stretchability and the degree of stretch orientation, which are considered to be caused by the short molecular chains and little entanglement between the molecular chains, are reduced. As a result, the physical properties of the film are also reduced, 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 resulting film is excellent in flexibility, but causes a problem in workability, and the density is 0.940 g / cc.
If it is larger, the flexibility of the film is impaired.

The linear low-density polyethylene includes a high-pressure polyethylene, an ethylene-vinyl acetate copolymer ionomer, and the like, as long as the object of the present invention is not hindered.
An ethylene-propylene copolymer or the like can be mixed. Further, according to a conventional method, heat and UV stabilizers, pigments,
An antistatic agent, a fluorescent agent, a lubricant and the like may be added.

First, in the present invention, a substantially unoriented sheet is formed from a raw resin mainly composed of linear low-density polyethylene. The formation of the unoriented sheet can be performed according to a usual sheet forming apparatus and a forming method. For example, a T-die forming method using a T-die can be used.

Next, in the present invention, the unoriented sheet is used as a raw material, and biaxially stretched in the longitudinal and transverse directions by a sequential biaxial stretching method. Stretching in the machine direction (MD) is performed by slitting the web to a predetermined width as necessary while running the raw fabric, and then passing the web through a plurality of machine-oriented vertical stretching rolls installed at right angles to the flow direction. Then, the film is stretched at a speed ratio between the rolls. Several rolls consist of a preheating roll, a stretching roll, and a cooling roll. The stretching in the transverse direction (TD) is usually performed using a tenter (transverse stretching machine) including a pre-tropical zone, a stretching zone, a heat treatment zone, a cooling zone, and the like. As a heating method, a hot air method, a radiation heating method, or the like is adopted.

The stretching ratio is biaxial stretching (easiness of stretching).
From the viewpoint of the physical properties of the obtained biaxially stretched film, at least one of the longitudinal and transverse directions is 3 times or more and less than 8 times, as in the method described in JP-A-58-90924. The product of the stretching ratios in the two directions is 9 times or more and less than 50 times. In addition, it is preferable that the height be 3 times or more and less than 8 times in both the vertical and horizontal directions.

The longitudinal stretching temperature T1 is calculated by the following equation (Equation 5).
(Same as [Equation 1]).

Tm−30 ° C. ≦ T1 ≦ Tm−10 ° C. (In the above, Tm represents the melting point of the raw material resin, and is an endothermic main peak temperature on a melting curve measured by a differential scanning calorimeter (DSC). (The same applies hereinafter.)

When the longitudinal stretching temperature T1 is lower than Tm-30 ° C., the molecular chains have poor motility, so that they are easily broken at the time of transverse stretching, and even if the stretching can be performed, the stretching ratio does not increase.
A stretched film having excellent physical properties cannot be obtained. On the other hand, when the temperature is higher than Tm-10 ° C., the orientation effect by stretching cannot be obtained, and the stretched raw material starts to adhere to the rolls, leaving sticky marks on the raw fabrics. Causes film breakage. Further, even if the film is stretched without breaking, the stretch unevenness is severe, sticky marks remain on the stretched film, the transparency and thickness accuracy are deteriorated, and the film is not of commercial value.

The transverse stretching temperature T2 is calculated by the following equation (Equation 6).
(Same as [Equation 2]).

Tm-50 ° C ≦ T2 ≦ Tm-25 ° C

The preferred transverse stretching temperature T2 is a temperature range that satisfies the following equation (Equation 7).

[Equation 7] Tm-40 ° C ≦ T2 ≦ Tm-25 ° C

When the transverse stretching temperature T2 is lower than Tm-50 ° C., it is difficult to obtain a predetermined stretching ratio, and film breakage occurs. Further, even if the film is stretched without breaking, the stretch unevenness remains, the thickness accuracy is poor, and the transparency is impaired. Conversely, if the temperature is higher than Tm-25 ° C., the film can be stretched to a desired stretching ratio, but even if it is apparently uniformly stretched, the stretching ratio differs from the set mechanical ratio, making it difficult to obtain the desired stretching ratio. In addition, magnification control becomes difficult, and stretching unevenness becomes severe. As a result, the physical properties in the film width direction are different, and the transparency is impaired, so that the film does not have commercial value. In addition, the width of the unstretched residue remaining at both ends of the film after the stretching is completed is widened, resulting in poor economic efficiency and production efficiency.

Next, in the present invention, the biaxially stretched film is subjected to a heat treatment. When a tenter is used, this heat treatment can be performed in a heat treatment zone of the tenter.

The heat treatment temperature T3 is calculated by the following equation (Equation 8).
(Same as [Equation 3]).

[Equation 8] Tm−40 ° C. ≦ T3 ≦ Tm

When the heat treatment temperature T3 is lower than Tm-40 ° C., the heat-treated film lacks dimensional stability,
When used as a sealant, the film shrinks during heat sealing, causing wrinkles on the sealing surface and impairing 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 remarkably reduced, and a whitening phenomenon accompanying the crystallization of the film occurs to impair the transparency.

The heat treatment time is preferably at least 3 seconds. If less than 3 seconds, a sufficient heat treatment effect cannot be obtained,
Since the film has a large heat shrinkage, when used as a sealant film, wrinkles may be generated during heat sealing. By the above heat treatment, the biaxially stretched film is adjusted to have a heat shrinkage at 100 ° C. of 30% or less in each of the longitudinal and transverse directions.

In the present invention, the biaxially stretched film preferably has a heat shrinkage of 8% or less, particularly 5% in the longitudinal and transverse directions, in order to more reliably prevent wrinkles during heat sealing. The heat treatment temperature T3 is preferably adjusted to the range below which satisfies the condition of the following equation (Equation 9) (same as [Equation 4]):
It is particularly preferable to set the range to satisfy the condition of the following equation (Equation 10).

[0023]

## EQU9 ## Tm−15 ° C. ≦ T3 ≦ Tm

[Expression 10] Tm−5 ° C. ≦ T3 ≦ Tm

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

[0025]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which, however, are not intended to limit the scope of the present invention. In the following examples, a sequential biaxial stretching method using a longitudinal stretching apparatus using a roll and a horizontal stretching apparatus using a hot air oven type tenter was adopted.
In addition, the measurement items shown in the text and the following examples were based on the following methods.

(1) Shrinkage A film cut into a square of 100 mm in both length and width was immersed in a silicone oil bath at a predetermined temperature for 10 minutes, taken out, and each length and length was measured and calculated by the following equation. The measurement temperatures were 90 ° C, 100 ° C, 110 ° C, 12
The test was performed at 0 ° C and 150 ° C. Shrinkage (%) = 100-A
(Or B) where A and B indicate the length (unit: mm) after immersion.

(2) Thickness accuracy Using a contact-type electronic micrometer, a maximum thickness (Tmax) and a minimum thickness (Tmin) are obtained in the film width direction.
It was calculated by the following equation (unit: μ). R = Tmax-Tmin

(3) Haze (transparency) Haze meter (SHARP PERSONAL COMPUTER) manufactured by Tokyo Denshoku
(PC-7200, COLOR ANDCOLOR DEFFRENCE MEDEL TC-1500)
Was measured at room temperature of 23 ° C. × 50% RH (unit:%).

(4) Tensile strength Measured at a tensile speed of 50 mm / min at room temperature of 23 ° C. × 50% RH according to JIS K 7113 (unit:
kg / cm ² ).

(5) Tensile elongation Measured at a tensile speed of 50 mm / min at room temperature of 23 ° C. × 50% RH in accordance with JIS K 7113 (unit:
%).

(6) Density (ρ) According to JIS K 6760, a density gradient tube is used.
C. (unit: g / cc).

(7) Melting point The endothermic main peak temperature on the melting curve measured by a differential scanning calorimeter (using DSC-II manufactured by PerkinElmer) was defined as the melting point. Measurement conditions: Measurement sample 10 to 30 mg Heating rate 10 ° C / min

(8) MI (Melt Index) Measured at 190 ° C. in accordance with JIS K 6760 (unit: g / 10 minutes).

(9) Measurement of Stretching Ratio The unstretched raw sheet was drawn by drawing a circle (diameter: A) at the center in the width direction, and the circular diameter (B) after the drawing was measured. Actual stretching ratio = B / A

(10) Unstretched Residual From the edge in the film width direction, one more than the target film thickness
Until the position where the thickness becomes 0 μ or more, the unstretched residue is used.

(11) Unstretched Residual Width Regarding the unstretched residue generated at both ends in the film width direction,
One was A and the other was B, the widths of each were summed, and the sum was divided by 2 for calculation.

(I) Examples 1-2 and Comparative Examples 1-4 (Evaluation of stretched state and heat sealability) Density at 22 ° 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, wherein an ethylene-based polymer having a main peak temperature of 125 ° C. on a melting curve by DSC at 200 to 250 ° C. The mixture was melted and kneaded, extruded from a T-die maintained at 250 ° C., and brought into close contact with a cooling roll by a known air knife method to obtain a 300 μm-thick unstretched sheet (this unstretched sheet was used as a raw material, and another example was used. Was also used.)

The above-mentioned raw material was successively guided to a biaxial stretching apparatus, and stretched under the conditions shown in Tables 1 to 3, and the stretching results shown in the same table were obtained. In the table, ○ indicates that the film has no stretch unevenness and is in a stable stretched state, Δ indicates that the film has stretch unevenness, and ×
Indicates a state in which film breakage occurs and stretching is impossible.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

As shown in Tables 1 to 3, Examples 1 and 2
In, the stretched state was good, no non-uniform stretched state was observed, and the unstretched residual width was narrow, which did not affect the film productivity. On the other hand, Comparative Examples 1 to 3
In (2), the film was broken in the transverse stretching zone, and the film obtained without breaking had a stretch unevenness, and only a film having no commercial value could be obtained. In particular, Comparative Example 2
When the film was stretched at a transverse stretching temperature of 110 ° C., the width of the unstretched residue at both ends of the film became wide, and the production efficiency was lacking. Further, in Comparative Example 4, the film adhered to the longitudinal stretching roll, the film was broken in the longitudinal stretching, and the film after the longitudinal and lateral stretching had a sticky mark, the film had a poor appearance, the transparency was lost, and the thickness was also reduced. The accuracy was also poor.

In the above example, the stretched film was immediately heat-treated at a heat treatment temperature of 120 ° C. for 5 seconds, and then cooled to room temperature and wound up. When the thickness accuracy R of the obtained film was measured, it was as shown in Table 4 below.

[0044]

Table 1-1 Example 1-1 (thickness 20μ) R = 1.2μ Example 1-2 (thickness 19μ) R = 1.0μ Example 2-1 (thickness 20μ) R = 1.0μ Example 2- 2 (thickness 20μ) R = 1.2μ

Further, according to a known laminating method, the stretched film obtained in the above example was laminated on a biaxially stretched nylon-6-film having a thickness of 15 μm (general product grade, Santo Neil SN, manufactured by Mitsubishi Kasei Polytech Co., Ltd.). Heat sealing was performed at a sealing temperature of 140 ° C., a sealing pressure of 2 kg / cm ² , and a sealing time of 1 sec, and the heat sealing property was evaluated. The stretched films obtained in Examples 1 and 2 are:
In each case, no wrinkles were observed on the sealing surface, and a good sealing portion having a sealing strength of 5 kg / 15 mm was obtained.

Example 3 In order to confirm the influence of the heat treatment temperature, the stretched film obtained in Example 2-2 (drawing ratio 5 × 5) was immediately heat-treated at 85 ° C. for 5 seconds after stretching. Cooling to room temperature, winding, measuring thickness accuracy R,
The heat sealability was evaluated in the same manner as in Example 1. As a result, the thickness accuracy R is 0.9μ (thickness 20μ),
Although slight wrinkles were observed on the sealing surface, substantially the same heat sealability as described above was obtained.

(II) Example 4 and Comparative Examples 6 and 7 (Changes in unstretched remaining width and stretch ratio) Sequential biaxial stretching apparatus for raw material (set mechanical ratio is MD 4.5)
× TD4.5), and subjected to a stretching treatment under the conditions shown in Table 5, and then immediately at a heat treatment temperature of 120 ° C.
After heat treatment for 2 seconds, it was cooled to room temperature and wound up. The unstretched residual width of each of the obtained films was measured, and the results shown in Table 5 were obtained. FIG. 1 shows a graph created based on the results shown in Table 5.

[0048]

[Table 5] Stretching temperature ° C Unstretched residual width Stretching ratio No. MD TD (mm) MD × TD ──────────────────────────────────── Example 4-1 100 100 50 4.5 × 4.5 4-2 112 80 50 4.5 × 4.4 4-3 112 95 52 4.5 × 4.5 4-4 112 100 55 4.5 × 4.5} ────────────────────────────────── Comparative example 6-1 100 110 122 4.5 × 5.7 6 -2 100 120 160 4.5 × 6.9 7-1 112 112 100 4.5 × 6.0 7-2 112 120 160 4.5 × 6.8

As is clear from the above results, Example 4
In the above, at each of the longitudinal stretching temperature and the transverse stretching temperature, the unstretched remaining width of the stretched film was almost constant, and the width did not affect the productivity. On the other hand, in Comparative Examples 6 and 7, the width of the unstretched residual portion was significantly increased, and the film productivity was extremely poor. When the stretching ratio and the set mechanical ratio at the center of the film in the film width direction were measured for each of the obtained films, in Example 4, they were almost the same as the set mechanical ratios, but in Comparative Examples 6 and 7, Was significantly different from the set mechanical magnification.

(III) Comparative Example 5 (Evaluation of physical properties of film and heat sealability) The raw material was successively guided to a biaxial stretching apparatus, and was subjected to a longitudinal stretching temperature of 112 ° C.
The film is stretched 5 times vertically and horizontally at a transverse stretching temperature of 100 ° C., and the stretched film is immediately subjected to a heat treatment temperature of 82 ° C.
And then cooled to room temperature and wound up.

In each of the above Comparative Examples, the stability of the stretched state was good, the non-uniform stretched state was not observed, the unstretched residual width was narrow, and the film productivity was not affected. The precision R was also 1.4 μ (thickness 20 μ), which was good. However, with respect to the film physical properties and the heat-sealed state of the above-mentioned film, Example 2
The stretched film of No.-2 (stretching ratio 5.0 × 5.0) is shown in Table 6 in comparison with the film heat-treated at 110 ° C. for 5 seconds, but the film of Comparative Example 5 is not satisfactory. Did not. That is, in the film of Comparative Example 5, wrinkles occurred on the sealing surface, and it was difficult to obtain a good sealing portion. In Table 6, ○ indicates that no wrinkles occur during heat sealing, and X indicates that wrinkles occur.

[0052]

[Table 6] Characteristics Example 3 Comparative Example 5 厚 Thickness (μ) 20.1 (R = 1.2) 20.4 (R = 1.4) Haze (%) 2.1 2.4 Tensile strength (MD / TD) (kg / cm ² ) 1500/1530 1393/1448 Tensile elongation (MD / TD) (%) 102/90 124/120 ──────────────────────────────── Shrinkage (%) 90 ℃ 5.2 / 5.3 25.9 / 27.3 (MD / TD) 100 ℃ 7.4 / 7.8 35.1 / 36.0 110 ℃ 15.9 / 18.4 47.6 / 49.4 120 ℃ 48.6 / 50.4 60.3 / 64.0 150 ℃ 76.3 / 72.4 77.0 / 76.5 ℃ ───────────────── Heat seal ○ × ──────────────────────────── ────

(IV) Example 5 and Comparative Example 9 (Evaluation of Heat Shrinkage Ratio) The unstretched sheet (300 μm) obtained in Example 1 was used as a raw material.
In the same manner as in Example 1, biaxial stretching and heat treatment were performed under the conditions shown in Table 7 to obtain a film having a thickness of 25 μm. The heat treatment was performed at each temperature shown in Table 7 for 5 seconds while giving a 7% relaxation, following the transverse stretching in the tenter. Table 7 shows the measurement results of the heat shrinkage of each of the obtained films.
Shown in In each of the above examples, continuous stretching was performed for 12 hours, and the stretched state was observed on the same basis as in Example 1. Table 7 also shows the results.

[0054]

[Table 7]

[0055]

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

[Brief description of the drawings]

FIG. 1 is a graph showing an example of a relationship between a transverse stretching temperature and a remaining unstretched width.

Continuation of the front page (56) References JP-A-58-90924 (JP, A) JP-A-4-93225 (JP, A) JP-A-59-215828 (JP, A) (58) Fields studied (Int .Cl. ⁷ , DB name) B29C 55/02-55/28 C08J 5/18 CES B29C 71/02

Claims (2)

(57) [Claims] 1. A raw resin mainly composed of linear low-density polyethylene is melt-extruded and solidified by cooling to form a substantially unoriented sheet. Then, at least a longitudinally and horizontally oriented sheet is formed by a sequential biaxial stretching method. After stretching in one direction is 3 times or more and less than 8 times and the product of the stretching ratios in two directions is 9 times or more and less than 50 times, heat treatment is performed, and the heat shrinkage at 100 ° C. is 30% or less in each of the longitudinal and transverse directions. In producing the polyethylene-based biaxially stretched film, the stretching temperature condition is set to a range that satisfies the following formulas [Equation 1] and [Equation 2], and the heat treatment temperature condition is the condition of the following formula [Equation 3]. A method for producing a polyethylene-based biaxially stretched film, characterized by satisfying the following conditions. Tm−30 ° C. ≦ T1 ≦ Tm−10 ° C. Tm−50 ° C. ≦ T2 ≦ Tm−25 ° C. Tm−40 ° C. ≦ T3 ≦ Tm (where Tm is a raw material (The melting point of the resin, T1 is the longitudinal stretching temperature, T2 is the transverse stretching temperature, and T3 is the heat treatment temperature.) 2. The heat shrinkage rate at 100 ° C. in each of the longitudinal and transverse directions is 8% or less, wherein the heat treatment temperature condition is within a range satisfying the following equation (Equation 4). A method for producing a biaxially stretched polyethylene film as described in the above. ## EQU4 ## Tm−15 ° C. ≦ T3 ≦ Tm