JPH04357017A - Manufacture of polyethylene-based biaxially oriented film - Google Patents

Abstract

PURPOSE:To provide the manufacture method of low shrink biaxially oriented film, which has no uneven orientation and is excellent in thickness accuracy, out of resin mainly of linear low density polyethylene. CONSTITUTION:Un-oriented linear low density polyethylene sheet is stretched under the condition that the draw ratio at least either in machine direction or in transverse direction is not less than 300% and less than 800% and the product of the draw ratios in both the directions is not less than 900% and less than 5.000%, and, after that, heat-treated so as to produce low shrink polyethylene-based biaxially oriented film. In this case, its orienting temperature conditions are set to stisfy the following numerical formula (n1) and (n2) and its heat treating temperature condition is set to satisfy the following numerical formula (n3):(n1)...Tm-30 deg.C<=T1<= Tm-10 deg.C, (n2)...Tm-50 deg.C<=T2<=Tn-25 deg.C, (n3)...Tm-40 deg.C<=T3<=Tm, in which T1 represents the lengthwise orienting temperature, T2 represents the crosswise orienting temperature and T3 represents the heat treating temperature.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] 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.

0002

BACKGROUND OF THE INVENTION Linear low-density polyethylene resins are produced by ionic polymerization at low temperatures and low pressures, compared to conventional high-pressure low-density polyethylene resins, and therefore can be produced at low cost with less equipment costs and less energy. however,
Linear low-density polyethylene resin is essentially a crystalline polymer and, like conventional high-density polyethylene, is difficult to biaxially stretch.

[0003] 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 manufacturing method includes a stretching temperature: -20°C to melting point -5°C of linear low-density polyethylene resin, a stretching speed: 25 to 400%/sec, and a stretching ratio: 3 times or more and less than 8 times in at least one direction. The film is characterized in that it is biaxially stretched under conditions where the product of stretching ratios in two directions is 9 times or more and less than 50 times.

0004

[Problems to be Solved by the Invention] However, even with the above manufacturing method, a satisfactory stretching condition cannot be obtained due to the occurrence of stretching unevenness and film breakage in the film, and further improvement studies are required. be. The present invention has been made in view of the above-mentioned circumstances, and its purpose is to establish a good stretching state using a resin mainly composed of linear low-density polyethylene, to have no stretching unevenness, and to have excellent thickness accuracy. An object of the present invention is to provide a method for manufacturing a biaxially stretched film. Furthermore, another object of the present invention is to provide a biaxially stretched film in which the width of the unstretched portion of the film edge portion is small, and the film stretching ratio and the set machine ratio do not deviate from each other. The purpose 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 properties suitable as a sealant film.

[0005]

[Means for Solving the Problems] 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 and cooled to solidify to form a substantially unoriented sheet, and then sequentially biaxially stretched in at least one direction in the longitudinal and transverse directions. After stretching in a direction of 3 times or more and less than 8 times and the product of the stretching ratios in the two directions being 9 times or more and less than 50 times, heat treatment is performed so that the heat shrinkage rate at 100 ° C. is 30% or less in each of the longitudinal and transverse directions. When manufacturing a certain polyethylene biaxially stretched film, the stretching temperature conditions and heat treatment temperature conditions are set according to the mathematical formula described in claim 1 [
This can be easily achieved by a method for producing a polyethylene biaxially stretched film, which is characterized in that the range satisfies the conditions of Equations 1 to 3.

The present invention will be explained 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 unlike low-density polyethylene, which is produced by a conventional high-pressure method, it is produced by a low-pressure method. Examples of the α-olefin copolymerized with ethylene include butene, pentene, hexene, octene, 4-methylpentene, and the like. The structural difference between high-pressure low density polyethylene and linear low density polyethylene is that the former has a multi-branched molecular structure, and the latter has a linear molecular structure.

[0007] There are various methods of manufacturing linear low density polyethylene, and the physical properties thereof differ somewhat depending on the manufacturing 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. If the MI is less than 0.5, the extrudability is insufficient, and when forming the original fabric as described below, for example, instability in sheet forming due to surging causes thickness fluctuations, and furthermore, Cooling spots often cause variations in transparency or crystallinity, and therefore,
It is difficult to obtain a raw fabric with excellent physical properties and stretchability. Furthermore, if the MI is greater than 3.0, the melt tension is low and there are disadvantages in forming the original fabric, such as the occurrence of ripples due to unstable contact with the cooling drum during T-die forming. Furthermore, due to the low molecular weight, the stretchability and degree of stretch orientation are lowered, which is thought to be due to the shorter molecular chains and less entanglement between the molecular chains. As a result, the physical properties of the film deteriorate, making it difficult to obtain a desired stretched film.

[0008] Furthermore, the linear low density polyethylene used in the present invention has a density (ρ) of 0.910 to 0.940 g/c.
c is preferred. If the density is less than 0.910 g/cc, the resulting film will have excellent flexibility, but problems will arise in processability, and if the density is less than 0.940 g/cc.
If it is larger, the flexibility of the film will be impaired.

The linear low density polyethylene may include high pressure polyethylene, ethylene-vinyl acetate copolymer ionomer, as long as it does not interfere with the purpose of the present invention.
Ethylene-propylene copolymer etc. can be mixed. Furthermore, according to conventional methods, heat and ultraviolet stabilizers, pigments,
There is no problem in adding antistatic agents, fluorescent agents, lubricants, etc.

First, in the present invention, a substantially unoriented sheet is formed from a raw material resin mainly consisting of linear low density polyethylene. The above-mentioned unoriented sheet can be molded according to a conventional sheet molding apparatus and molding method, and for example, a T-die molding method using a T-die can be used.

Next, in the present invention, the above-mentioned 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 carried out by running the original fabric, slitting it to a predetermined width as necessary, and then passing it through several longitudinal stretching rolls installed perpendicular to the machine direction. Then, it is stretched at the speed ratio between the rolls. The several rolls consist of a preheating roll, a stretching roll, and a cooling roll. Stretching in the transverse direction (TD) is usually performed using a tenter (transverse stretching machine) that includes a preheating zone, a stretching zone, a heat treatment zone, a cooling zone, and the like. As a heating method, a hot air method, a radiant heating method, etc. are adopted.

[0012] The stretching ratio is determined by biaxial stretching property (ease of stretching)
And from the viewpoint of the physical properties of the obtained biaxially stretched film, as in the method described in JP-A-58-90924, at least one of the longitudinal and transverse directions is 3 times or more and less than 8 times The product of the stretching ratios in two directions is 9 times or more and less than 50 times. And more than 3 times 8 in both vertical and horizontal directions.
It is better to make it less than double.

[0013] The longitudinal stretching temperature T1 is determined by the following formula [Equation 5] (
It is necessary to set the range to satisfy the condition (same as [Equation 1]).

[Equation 5] Tm-30℃≦T1≦Tm-10℃ (In the above, Tm represents the melting point of the raw resin, and is the endothermic main peak temperature on the melting curve measured using a differential scanning calorimeter (DSC). (hereinafter the same)

[0014]
If the longitudinal stretching temperature T1 is lower than Tm - 30°C, the molecular chains have poor mobility and are likely to break during transverse stretching, and even if stretching is possible, the stretching ratio will not increase, resulting in a stretched film with excellent physical properties. I can't do that. On the contrary, Tm-10
If the temperature is higher than ℃, the orientation effect of stretching cannot be obtained, and furthermore, the stretched original fabric starts to stick to the rolls, leaving adhesive marks on the original fabric, which causes the film to break between the rolls during stretching. . Furthermore, even if the film is stretched without breakage, the stretched film will have severe unevenness, adhesive marks will remain on the stretched film, transparency and thickness accuracy will deteriorate, and the film will not have commercial value.

[0015] The transverse stretching temperature T2 is determined by the following formula [Equation 6] (
It is necessary to set the range to satisfy the condition (same as [Equation 2]).

[Math 6] Tm-50℃≦T2≦Tm-25℃

0016
] The preferable transverse stretching temperature T2 is a temperature range that satisfies the condition of the following mathematical formula [Equation 7].

[Math. 7] Tm-40℃≦T2≦Tm-25℃

0017
] If 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 breakage, stretching unevenness remains, resulting in poor thickness accuracy and poor transparency. vice versa,
If Tm is higher than -25°C, it is possible to stretch to the desired stretching ratio, but even if the stretching is apparently uniform, the stretching ratio and the set machine ratio are different and it is difficult to obtain the desired stretching ratio. Management becomes difficult, and stretching spots become severe. As a result, the physical properties in the width direction of the film are different, and furthermore, the transparency is impaired, so that the film does not have commercial value. Moreover, the width of the unstretched parts remaining at both ends of the film after stretching is widened, which improves economic efficiency and
Production efficiency deteriorates.

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

The heat treatment temperature T3 is determined by the following formula [Equation 8] (
It is necessary to set the range to satisfy the condition (same as equation 3).

[Math. 8] Tm-40℃≦T3≦Tm

[0020] When the heat treatment temperature T3 is lower than Tm - 40°C, the heat treated film lacks dimensional stability and
It becomes shrinkable, and when used as a sealant, the film shrinks during heat sealing, causing wrinkles on the sealing surface and impairing its commercial value. On the other hand, if it is higher than Tm, the molecular orientation inside the film caused by stretching will flow and collapse, the physical properties of the film will be significantly reduced, and a whitening phenomenon will occur due to crystallization of the film, resulting in loss of transparency.

[0021] The heat treatment time is preferably 3 seconds or more. If the time is less than 3 seconds, a sufficient heat treatment effect will not be obtained and the film will have a large heat shrinkage, so when used as a sealant film, wrinkles may occur during heat sealing. By the above heat treatment, the biaxially stretched film has a heat shrinkage rate at 100° C. adjusted to 30% or less in both the longitudinal and transverse directions.

In the present invention, in order to more reliably prevent the occurrence of wrinkles during heat sealing, the heat shrinkage rate of the biaxially stretched film is set to 8% or less in each of the longitudinal and lateral directions, particularly 5%. Therefore, the heat treatment temperature T3 is preferably within a range that satisfies the following formula [Equation 9] (same as [Equation 4]):
Particularly preferably, it is within a range that satisfies the following formula [Equation 10].

[0023]

[Equation 9] Tm-15℃≦T3≦Tm

[Formula 10] Tm-5℃≦T3≦Tm

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

[0025]

EXAMPLES The present invention will be explained in more detail by way of examples below, but the present invention is not limited to the following examples unless it exceeds the gist thereof. In the following examples, a sequential biaxial stretching method using a longitudinal stretching device using rolls and a horizontal stretching device using a hot air oven type tenter was adopted. In addition, the measurement items shown in the main text and the following examples were carried out by the following method.

(1) Shrinkage rate A film cut into a square of 100 mm in length and width was immersed in a silicone oil bath at a predetermined temperature for 10 minutes and taken out.The length and width of each were measured and calculated using the following formula. Measurement temperatures are 90℃, 100℃, 110℃, 120℃
The temperature was 150°C. Shrinkage rate (%) = 100-A (
or B) However, A and B indicate the vertical and horizontal lengths (unit: mm) after immersion.

(2) Thickness Accuracy Using a contact type electronic micrometer, the maximum thickness (Tmax) and minimum thickness (Tmin) in the width direction of the film were determined and calculated using the following formula (unit: μ). R=Tmax
-Tmin

(3) Haze (transparency) Tokyo Denshoku haze meter (SHARP PERSON)
AL COMPUTER PC-7200, COL
OR AND COLOR DEFFRENCE MED
EL TC-1500) at room temperature of 23° C. and 50% RH (unit: %).

(4) Tensile strength According to JIS K 7113, 23°C x 50%R
It was measured at a tensile rate of 50 mm/min (unit: kg/cm2) at room temperature.

(5) Tensile elongation According to JIS K 7113, 23°C x 50% R
It was measured at a tensile speed of 50 mm/min in H at room temperature (unit: %).

(6) Density (ρ) Measured at 23°C using a density gradient tube in accordance with JIS K 6760 (unit: g/cc).

(7) Melting Point The endothermic main peak temperature on the melting curve measured by a differential scanning calorimeter (using PerkinElmer's DSC-II) was taken as the melting point. Measurement conditions: Measurement sample 10-30mg heating rate
10℃/min

(8) MI (melt index) JIS
Measured at 190° C. in accordance with K 6760 (unit: g/10 min).

(9) Stretching ratio measurement A circle (diameter known: A) was drawn at the center of the unstretched original fabric in the width direction and stretched, and the diameter (B) of the circle after stretching was measured and determined by the following formula. Actual stretching ratio = B/A

(10) Unstretched remaining portion 1 from the film target thickness from the edge in the width direction
The unstretched remainder is defined as the unstretched portion up to the position where the thickness is 0μ or more.

(11) Width of unstretched residue Regarding the unstretched residue generated at both ends of the film in the width direction,
One side was designated as A and the other as B, and the widths of each were summed and the total value was divided by 2.

(I) Examples 1 to 2 and Comparative Examples 1 to 4 (stretching state and heat sealability evaluation) 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 with a copolymerization amount of 10% by weight, and an ethylene polymer whose main peak temperature on the melting curve by DSC is 125°C is heated at 200 to 250°C. The mixture was melt-kneaded, extruded through a T-die kept at 250°C, and brought into close contact with a cooling roll using the known air knife method to obtain an unstretched sheet with a thickness of 300 μm (this unstretched sheet was used as a raw material, and other examples (also used in ).

[0038] The above-mentioned original fabric was sequentially introduced into a biaxial stretching apparatus, and stretched under the conditions listed in Tables 1 to 3 to obtain the stretching results shown in the same table. In the table, ○ indicates a stable stretched state with no stretching irregularities on the film, △ indicates a state where the film has stretching irregularities, ×
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 this case, the stretching condition was good, no uneven stretching condition was observed, and the width of the unstretched portion was narrow, so that the film productivity was not affected. On the other hand, Comparative Examples 1 to 3
In this case, film breakage occurred in the transversely stretched zone, and films obtained without breakage had stretching irregularities remaining and could only be obtained with no commercial value. In particular, Comparative Example 2
When the film was stretched at a transverse stretching temperature of 110° C., the width of the unstretched portions at both ends of the film became wide, resulting in a lack of production efficiency. In addition, in Comparative Example 4, the film adhered to the longitudinal stretching rolls, causing breakage during longitudinal stretching, and adhesive marks remained on the film after longitudinal and lateral stretching, resulting in poor appearance, loss of transparency, and poor thickness. The accuracy was also poor.

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

[0044]

[Table 4] 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 (general product grade/Santonil SN manufactured by Mitsubishi Kasei Polytech Co., Ltd.) with a thickness of 15 μm, and then heated. 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 sealability was evaluated. In both of the stretched films obtained in Examples 1 and 2, no wrinkles were observed on the sealing surface, and the sealing strength was 5 kg/15mm, which was a good seal.

Example 3 In order to confirm the influence of heat treatment temperature, the stretched film obtained in Example 2-2 (stretching ratio 5×5) was immediately heat treated at a heat treatment temperature of 85° C. for 5 seconds after stretching. Cool it to room temperature, wind it up, measure the thickness accuracy R,
Heat sealability evaluation was performed 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, almost the same heat sealability as above was obtained.

(II) Example 4 and Comparative Examples 6 to 7 (Changes in unstretched remaining width and stretching ratio) The original fabric was sequentially stretched using a biaxial stretching device (setting machine magnification was MD4.5)
× TD4.5), stretched under the conditions listed in Table 5, and then immediately heated at a heat treatment temperature of 120 °C for 5
After being heat treated for a second, it was cooled to room temperature and wound up. The width of the unstretched remaining portion of each film was measured, and the results shown in Table 5 were obtained. Further, a graph created based on the results shown in Table 5 is shown in FIG.

[0048]

[Table 5] Stretching temperature °C Unstretched remaining 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 1
20 160 4.5×
6.8

As is clear from the above results, in Example 4, the width of the unstretched portion of the stretched film was almost constant at each longitudinal stretching temperature and transverse stretching temperature, and the width had no effect on productivity. It wasn't something to give. On the other hand, in Comparative Examples 6 and 7, the width of the unstretched portion increased significantly and the film productivity was extremely poor. Also,
When the stretching magnification and set machine magnification at the central part in the width direction of the film were measured for each film obtained, in Example 4, it was almost the same as each set machine magnification, but in Comparative Examples 6 and 7, The machine magnification was significantly different from the set machine magnification.

(III) Comparative Example 5 (Evaluation of film physical properties and heat sealability) The original film was sequentially introduced into a biaxial stretching device, and the longitudinal stretching temperature was 112°C.
The stretched film was stretched 5 times in both length and width at a transverse stretching temperature of 100°C, and the stretched film was immediately heat-treated at a temperature of 82°C.
After being heat-treated for 5 seconds, it was cooled to room temperature and wound up.

In all of the above comparative examples, the stability of the stretching state was good, no uneven stretching was observed, and the width of the unstretched remaining portion was narrow, which did not affect film productivity, and the thickness The accuracy R was also good, being 1.4 μm (thickness 20 μm). However, regarding the film physical properties and heat sealing state of the above film, Example 2
Table 6 shows a comparison of the stretched film of Comparative Example 5 (stretching ratio 5.0 x 5.0) with a film heat-treated for 5 seconds at a heat treatment temperature of 110°C. There wasn't. That is, in the film of Comparative Example 5, wrinkles occurred on the sealing surface, making it difficult to obtain a good seal. In Table 6, ◯ indicates that wrinkles do not occur during heat sealing, and × indicates that wrinkles occur.

[0052]

[Table 6] Characteristics
Example 3 Comparative example 5
──────────────────────────
──────── Thickness (μ)
20.1 (R=1.2)
20.4 (R=1.4) Haze (%)
2.1
2.4
Tensile strength (MD/TD) (kg/cm2)
1500/1530 1393/
1448 Tensile elongation (MD/TD) (%)
102/90
124/120 ────────────
────────────────────── Shrinkage rate (%) 90℃ 5
．． 2/5.3 25.9/27.3
(MD/TD) 1
00℃ 7.4/7.8
35.1/36.0
110℃ 1
5.9/18.4 47.6/49.
4
120℃ 48.6/50.4
60.3/64.0
150℃
76.3/72.4 77.0/76
．． 5 ────────────────────
──────────── Heat sealed condition
○
× ──────────
────────────────────────

0
(IV) Example 5 and Comparative Example 9 (Evaluation of heat shrinkage rate) The unstretched sheet (300μ) obtained in Example 1 was used as a raw material,
Biaxial stretching and heat treatment were performed in the same manner as in Example 1 under the conditions shown in Table 7 to obtain a film with a thickness of 25 μm. The heat treatment was performed for 5 seconds at each temperature listed in Table 7 while giving 7% relaxation following the transverse stretching in a tenter. Table 7 shows the measurement results of the heat shrinkage rate of each film obtained.
Shown below. Further, in each of the above examples, continuous stretching was performed for 12 hours, and the stretching state was observed using the same criteria as in Example 1. The results are also shown in Table 7.

[0054]

[Table 7]

[0055]

According to the present invention described above, the following effects are achieved. ■ Enjoy the benefits of using linear low-density polyethylene as a raw material. (2) Compared to a film stretched under conventional stretching conditions, the film has no stretching irregularities, has good transparency, and also has good strength. ■
Excellent dimensional stability. ■ The width of the unstretched portion of the film edge is also small. (2) When used in a sealant film, the film has excellent performance such as no wrinkles in the sealed portion during heat sealing, and is suitable for use as a sealant film or a packaging film, for example.

[Brief explanation of the drawing]

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

Claims (1)

[Claims] Claim 1: A raw material resin mainly composed of linear low-density polyethylene is melt-extruded, cooled and solidified to form a substantially unoriented sheet, and then sequentially biaxially stretched to form a sheet with at least 100% orientation in the longitudinal and lateral directions. After stretching to a stretching ratio of 3 times or more and less than 8 times in one direction and a product of the stretching ratios in two directions of 9 times or more and less than 50 times, heat treatment is performed, and the heat shrinkage rate at 100°C is 30% or less in each of the longitudinal and transverse directions. In manufacturing a polyethylene biaxially stretched film, the stretching temperature conditions are set to a range that satisfies the following formulas [Math. 1] and [Math. 2], and the heat treatment temperature conditions are set to the conditions of the following formula [Math. 3]. A method for producing a polyethylene biaxially stretched film, characterized in that the range satisfies the following. [Math. 1] Tm-30℃≦T1≦Tm-10℃ [Math. 2] T
m-50℃≦T2≦Tm-25℃ [Math. 3] Tm-40℃
≦T3≦Tm (In the above formula, Tm is the melting point of the raw resin, T1 is the longitudinal stretching temperature, T2 is the lateral stretching temperature, and T3 is the heat treatment temperature.) [Claim 2] The heat treatment temperature condition is determined by the following formula [Equation 4] 2. The method for producing a biaxially stretched polyethylene film according to claim 1, wherein the heat shrinkage rate at 100° C. is 8% or less in each of the longitudinal and transverse directions. [Math 4] Tm-15℃≦T3≦Tm