WO1990001409A1

WO1990001409A1 - Process for producing heat-shrinkable polyethylene film

Info

Publication number: WO1990001409A1
Application number: PCT/JP1989/000142
Authority: WO
Inventors: Hideo Isozaki; Tomozi Mizutani; Yoshihiro Sakamoto
Original assignee: Kohjin Co., Ltd.
Priority date: 1988-08-15
Filing date: 1989-02-13
Publication date: 1990-02-22

Abstract

A heat-shrinkable polyethylene film having a small uneveness in thickness and excellent heat-shrinkability at low temperatures and transparency, which cannot be obtained by the known tubular biaxial stretching process of a linear low-density polyethylene of 0.3 to 0.7 in g.(=[n]/[n]¿L?, wherein [n] is a limiting viscosity of a polymer and [n]L is a limiting viscosity of a straight-chain polymer of the same average molecular weight as that of the foregoing polymer) having good transparency, can be produced by selecting a resin composition containing as a main component a copolymer composed of ethylene and 1 to 10 % of at least one C4 - C12 α-olefin and having a melt index of 0.3 to 2.0 g/10 min and a density at 25°C of 0.86 to 0.92 g/cm?3¿, preferably a composition showing an endothermic area of 55 % or more based on the whole endothermic area under the melting point in a melting curve obtained by using a differential scanning calorimeter, and conducting tubular stretching under a specific stretching temperature condition.

Description

Description Method for producing polyethylene heat-shrinkable film

The present invention relates to a shrink wrapping material, and more specifically, a thickness mainly composed of ethylene and α-olefin copolymer having a parameter g ′ of 0.3 to 0.7 indicating direct molecular chain resilience. The present invention relates to a method for producing a heat-shrinkable film having small spots and excellent transparency and low shrinkage.

Background art

Conventionally, as heat shrinkable films, polyvinyl chloride, polypropylene-based biaxially stretched films, and polyamide-based biaxially stretched films have been known.

This biaxially oriented biaxially stretched film has been used practically because it has heat sealing properties and is inexpensive. In particular, in recent years, a linear low-density copolymer of ethylene and α-olefin (hereinafter simply referred to as linear Polyethylene-based heat-shrinkable films using low-density polyethylene have attracted attention because of their excellent impact resistance and heat seal strength.

However, among the linear low-density polyethylenes, those with §: · = [] / ivl with an L value of 0.3 to 0.7 have good transparency, but conventionally known tubular single-shafts using this resin. When a heat-shrinkable film is produced by a stretching method, for example, by applying the method of Patent Application Publication No. 361, 1982, No. 6142 as it is, a heat-shrinkable film is obtained, resulting in insufficient stretching stability. As the stretched film, only a film having a large thickness unevenness was obtained, and a film which was practically satisfactory could not be produced.

Censor of the invention

The present inventors have studied a method for producing a polyethylene-based heat-shrinkable film which is excellent in low-temperature shrinkage with a small thickness unevenness using the above-mentioned resin having excellent transparency. As a result, the present invention has been achieved. That is, in the present invention, the value of § = [W] / [?] L is 0.3 to 0.7 (where, is the limiting viscosity of the polymer, and is the limiting value of the linear polymer having the same weight average molecular weight as the polymer. viscosity), melt index 0. 3~2. 0 s / 1 Omin , 25 density at hand 0. 86~0. 92 g cm ³ of ethylene and at least one C j ~Ci ₂ Oa- Orefuin 1 When a heat-shrinkable film is produced by a tubular stretching method from a tubular unstretched film of a resin composition containing a 10% copolymer as a main component,

(B) The temperature at the expansion point (start point) is set to the melting point of the resin composition (the endothermic main beak in the melting curve obtained by measurement with a differential scanning calorimeter (hereinafter abbreviated as DSC)).

(Ii) The maximum temperature should be set at 1 Z4 to 1/3 of the distance of the stretching zone from the start point of expansion to the end point of expansion, with the difference between the maximum temperature and the film surface temperature at the start point of expansion. Is less than or equal to 5,

(C) Lower the temperature so that the temperature at the expansion end point is 15 to 20 lower than the maximum temperature.

(2) While traveling a distance of 0.8 times the distance of the stretching zone from the expansion end point, cool down to 60 or less.

A method for producing a polyethylene-based heat-shrinkable film having a small thickness unevenness, excellent low-temperature heat-shrinkability, and excellent transparency. At this time, the endothermic area of the resin composition, which is obtained by measuring with a differential scanning calorimeter (hereinafter abbreviated as “DSC”) a temperature lower than the melting point (endothermic main peak) by 10 or less, is preferably the total endothermic area The present invention relates to a method for producing a polyethylene-based heat-shrinkable film, which is characterized by being 55% or more.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a conceptual diagram of a tumbler stretching device used in the present invention.

FIG. 2 is an enlarged view of an extended portion of FIG. BEST MODE FOR CARRYING OUT THE INVENTION

The resin cord composition used in the present invention is a resin mainly composed of a linear low-density polyethylene or a combination of ethylene and an α-olefin having at least 4 to 12 carbon atoms. The value of _c g *, which is a composition and has a value of g ′ = ivl / ivl of 0.3 to 0.7 and a density of 0.86 to 0.92 g / cm ³ , is 0. If the ratio exceeds 7, the stretched film obtained by using this is not yet sufficiently transparent, and if it is less than 0.3, the degree of linearity is insufficient, which is characteristic of linear low-density polyethylene. All of them are unsuitable for the purpose of the present invention because of their poor mechanical strength. If the density exceeds 0.92 gZcm ^{3, the} shrinkage at low temperature is not sufficient. If the density is less than 0.86 g cm ³ , the anti-blocking agent such as inorganic fine particles is too soft or too blocky. Even if used together, it cannot be improved sufficiently.

The resin cord composition used in the present invention is mainly composed of linear low-density borylene as described above, and the melting point (endothermic main beak) in the melting curve measured using DSC is particularly important. 1) (the ratio of the endothermic area below TC lower than 塭 to the total endothermic area (hereinafter abbreviated as endothermic area ratio) is 55% or more), because it has excellent stretching stability over a relatively wide range of 塭 °. Particularly preferred.

A method of obtaining a melting curve by the above differential scanning calorimeter (in the present invention, a differential thermal analysis and concealment device (DSC-200) manufactured by Seiko Denshi Kogyo KK) is used. First, 6 to 8 mg of a sample is sealed in an aluminum van. Then, the temperature was raised to 190 ° C in a stream of nitrogen, maintained at this temperature for 1 hour, and then cooled to room temperature at a rate of about 10 / min. The melting point and the endothermic area ratio are determined from the DSC chart obtained by ascending.

Examples of the α-olefins having 4 to 12 carbon atoms copolymerized with ethylene include putene-11, pentene-11, hexene1-1, hebutin-11, octin-1,1,4-methylpentene1-1, Decene-1, Dedecene-1, Dodecene-1 Are exemplified.

The copolymerization ratio of α-olefin used in the copolymerization is preferably 0.5 to 10 mol%.

In the present invention, in addition to the above-mentioned linear low-density polyethylene, if desired, high-pressure polyethylene, an ethylene monoacetate vinyl copolymer, an ionomer, and an ethylene propylene copolymer may be used as long as the object of the present invention is not hindered. Additives such as polymers such as ethylene-based polymers, lubricants, anti-booking agents, and antistatic agents can be used in combination.

Hereinafter, a method for producing a heat-shrinkable polyethylene film by the method of the present invention will be described with reference to FIGS. 1 and 2. Fig. 1 is a conceptual diagram of the tubular extension equipment, and Fig. 2 is an enlarged view of the extension. The tubular unstretched film used in the present invention can be produced by melt-extruding the above-mentioned resin cord (hereinafter simply referred to as a resin composition) by a known method, and rapidly solidifying it.

The unstretched film obtained in this way is supplied to, for example, a stretching device as shown in Fig. 1, and when bubbles are generated by injecting air into the tube between nibroll 2 and nibroll 3, expansion and stretching are performed. Adjust the temperature so that the temperature at the expansion start point 101 is within the melting point of the resin (the main beak of the DSC chart) and the temperature range of 20 to 3 below. This can be performed by adjusting the cooling device 6. If the temperature at the expansion start point is higher than the above range, the film near the expansion start point is too soft, the stretching tension is reduced, and the film expands abnormally, resulting in uneven stretching. In addition, the orientation effect due to stretching is reduced, and the strength and heat shrinkage of the stretched film are lowered, which is not preferable. Conversely, if the temperature at the expansion start point is lower than the above temperature range, so-called neck stretching may occur. In addition, the thickness of the obtained film becomes large, the transparency becomes poor, the transparency becomes poor, and the object of the present invention cannot be achieved, and when the temperature becomes low, the internal pressure of the bubble increases, and so-called puncture frequently occurs. Is not preferred. Also, in order to make the stretching process uniform and to reduce the thickness unevenness, the expansion starting point is defined as L when the length of the stretching zone from the expansion start point 101 to the expansion end point 103 is L. An upward temperature gradient is set so that the temperature at which LZ has progressed from LZ 4 to 3/3 reaches the maximum temperature, and the difference between the maximum temperature and the expansion / starting point temperature is within 5 digits. The temperature at the expansion end point 103 should be 15 to 20 ° C lower than the maximum temperature, and during the travel of a distance of 0.8 times the length L of the stretching zone from the expansion end point. 6 (Cool to below TC.

If the above-mentioned maximum temperature exceeds a temperature 5 ° C higher than the temperature at which the expansion starts, the tensile strength of the film decreases, the thermal shrinkage of the resulting film decreases, and bubble expansion becomes unstable, resulting in unstable rocking. Conversely, if the temperature gradient is lowered during LZ 4 to 3/3 after passing through the expansion starting point 101, the stability of the stretched bubble will be improved, but the stretch will proceed in a balanced vertical and horizontal direction. In any case, the thickness unevenness of the obtained stretched film becomes large, and the object of the present invention cannot be sufficiently achieved. Further, if the temperature drop between the maximum temperature point 102 and the stretching end point 103 is 15 and the size is smaller, the bubbles become unstable and the thickness unevenness becomes larger, which is not preferable. Conversely, if the temperature drop is set so as to exceed 20, the internal pressure of the bubble will increase abnormally and the stability of the expansion starting point 101 will be lost, and the upper portion of the bubble will easily roll and the thickness unevenness will increase, which is preferable. Absent.

Furthermore, in order to stabilize the bubble, it is preferable to cool rapidly even after reaching the expansion end point 103. In other words, it is rapidly polished so as to be 60 or less while traveling a distance of 0.8 times the length L of the stretching zone from the expansion end point. If not rapidly cooled in this manner, the entire bubble is likely to sway, and the sway causes local unevenness in stretching, thereby increasing the thickness unevenness of the obtained film.

By specifying the temperature gradient in the stretching step as described above, the stability of the purple during expansion and stretching can be improved, and as a result, a film with small thickness unevenness can be obtained. (Action and effect)

Among the linear low-density polyethylenes, the resin with a relatively small g * value of 0.3 to 0.7 has excellent transparency, but in the case of the conventionally known linear low-density polyethylene, Under the same conditions as described above, stable bubble-like stretching was difficult, but by applying the method of the present invention, it was possible to obtain a film with stable stretching, small thickness unevenness, and excellent practicality. It has become.

(Example)

Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these Examples.

The measurements shown in this example were performed by the following methods.

(1) Heat shrinkage

After accurately measuring the length of each side of the film specimen cut to about 1 Ocm to the vertical and horizontal sides to the nearest 0.1 mm, immerse it in a glycerin bath adjusted to a predetermined temperature for 10 seconds and remove it After gently rinsing in water at room temperature, the length of each side was measured accurately again and calculated by the following equation.

(Α-Α ')

X 100 (%)

A

Where A and A 'are the average values of the two sides of the measured values before and after contraction, respectively.

(2) Film temperature

The temperature of the film during the stretching process is about lmm in diameter and about 15 exposed parts in length. The tip of a chrome-melon constantan type thermocouple (C-505 Thermocutable 0-100Ω manufactured by Anritsu Keiki Co., Ltd.) The film temperature was taken as the indicated value after 30 seconds.

(3) Uneven thickness of film

Using a continuous thickness gauge (Anritsu Ren 铳 thickness measuring device), the difference between the height of the highest peak and the lowest valley of the chart was measured for a sample film of 25 cm at a speed of 30 OcmZ. 2 was regarded as thickness unevenness.

(4) Transparency (haze)

The ratio of the scattered light transmittance to the parallel light transmittance was shown as a percentage using an integrating sphere light transmittance measuring device based on JIS-K6714.

(5) 8T value

g '= ivl / i l L

Where [7] is the intrinsic viscosity of the polyethylene copolymer to be measured, and decalin is used as the solvent135. Determined by dissolving in C. [^] L is the limiting viscosity of linear polyethylene having the same weight average molecular weight as the polyethylene copolymer of the above sample. This value is the weight average molecular weight <M> w measured by the light scattering method. Can be obtained from the following equation by an approximate calculation.

[? 5] and ^{= 5.29 X 10- 4 X <M} > w B - 713

Example 1,

Linear polyethylene copolymer resin of ethylene and Okuten one 1 (8: 'is 0.5 9, melting point 125.C, density at 25.C is 0. 915 g / cm ^3, Merutoinde box 1.0) It was melt-extruded from a tubular die with a diameter of 66 and rapidly cooled by internal and external water cooling to obtain a tubular unstretched film with a diameter of 65 mm and a thickness of 370. The endothermic area ratio of the unstretched film resin measured by DSC was 58.9%. The obtained unstretched film is fed to a tubular stretching machine running in the vertical direction as shown in Fig. 1, and a preheater 4, a main heater 5 in which eight annular infrared heaters are installed in four sections, and an oblique heater. The cooling air ring that can blow cold air upward is adjusted, and air is injected into the tube between the nip roll 2 and the nip roll 3 to form stretched bubbles, and the biaxially stretched film is stretched 16 m under the conditions shown in Table 1. It was possible to manufacture it stably for a long time in Z minutes.

The vertical distance L in the stretching zone was about 21 cm, the outer diameter of the baples was 240 cm, and the point of highest film temperature was about 6. Ocm below the starting point of the expansion. The biaxially stretched film was guided to another tube-shaped annealing apparatus whose temperature was adjusted to another 70, annealed for 10 seconds, cooled to room temperature, folded again, taken out and wound up.

The thickness, uneven thickness, haze, and heat shrinkage of the obtained film were measured, and the results are shown in Table 1. ·

Example 2

A linear polyethylene copolymer resin of ethylene and putin-1 (8 · 0.65: melting point: 18.7, density at 25 ° C: 0.906'grZcm ³ , melt index: 0.8) was the same as in Example 1. Thus, a tubular unstretched film with a diameter of 65 mm and a thickness of 359 i was obtained. The endothermic area ratio of the unstretched film resin measured by DSC was 53%.

A stretched bubble was formed from the obtained unstretched film in the same manner as in Example 1, and a biaxially stretched film could be stably manufactured at 13 m / min for a long time under the conditions shown in Table 1.

The vertical distance L in the stretching zone was about 20.5 cm, the outer diameter of the baples was 25 Omm, and the point of maximum film angle was about 5.8 cm below the point of expansion.

The biaxially stretched film was guided to another tubular annealing device whose temperature was adjusted to 70, and after annealing for 10 seconds, cooled to room temperature, folded again, taken out and wound.

The thickness, uneven thickness, haze, and heat shrinkage of the obtained film were measured, and the results are shown in Table 1.

Example 3

Ethylene and Puchin 1 linear Boriechiren copolymer resin of (g 'is 0.4 5, melting point 1 16 ^e C, density at 25 0. 89gZcm ^3, melt index 1.0) average particle size of 2 to 0.3% of solid fine particles of ί (trade name: Syloid # 244, manufactured by Fuji Davison) was added, and the diameter was 65 tnm and the thickness was the same as in Example 1. A 385 / z tubular unstretched film was obtained. The endothermic area ratio of the resin of this unrolled intermediate film measured by DSC was 56%.

A stretch bubble was formed from the obtained unstretched film in the same manner as in Example 1, and a biaxially stretched film could be stably manufactured at 15 mZ for a long time under the conditions shown in Table 1.

The vertical distance L in the stretching zone was about 20.2 cm, the outer diameter of the bubble was 250 strokes, and the point of maximum film angle was about 5.4 cm below the starting point of expansion.

This biaxially stretched film was guided to another tube-shaped annealing apparatus whose temperature was adjusted to another 70, and after annealing for 10 seconds, cooled to room temperature, folded again, taken out and wound up.

Comparative Example 1

A linear polyethylene copolymer resin of ethylene and butene-11 (§: · 0.89, melting point 122, 25, density 0.923 g / cm ³ , melt index 0.8) was the same as in Example 1. Thus, a tubular unstretched film with a diameter of 65 mm and a thickness of 366 was obtained. The endothermic area ratio of the unstretched film resin measured by DSC was 63.8%.

A stretched bubble was formed from the obtained unstretched film in the same manner as in Example 1, and a biaxially stretched film could be stably manufactured at 16 mZ for a long time under the conditions shown in Table 1.

The vertical distance L in the stretching zone was about 21. Ocm, the outside diameter of the tuple was 241 mm, and the point of maximum temperature of the film was about 6.2 cm below the starting point of expansion.

The film was guided to another 7 CTC tube annealing device, and after annealing for 10 seconds, it was cooled in the room, folded again, taken out and wound up. The thickness, uneven thickness, haze, and heat shrinkage of the obtained film were measured, and the results are shown in Fig.1.

As can be seen from the results of this example, there was no particular problem with the stability of the bubble, but the thickness unevenness of the obtained film was large and the haze was 8%, which was unsatisfactory in transparency. Comparative Example 2

Linear Boriechiren copolymer resin of ethylene and Okuchin one 1 (g 'to zero. 9 2, melting point 1 2 6, density at 2 5 0.9 2 g / cm ^3, a melt-in deck scan 1.0) to In the same manner as in Example 1, a tubular unstretched film having a diameter of 65 mm and a thickness of 366 was obtained. The endothermic area ratio of the unstretched film resin measured by DSC was 59.4%.

A stretched bubble was formed from the obtained unstretched film in the same manner as in Example 1, and a biaxially stretched film was produced at 15 mZ under the conditions shown in Table 1. As a result, the bubble stability was somewhat insufficient. The obtained film had a large thickness unevenness. The vertical distance L in the stretching zone was about 21.O cra, the outer diameter of the bubble was 234 mm, and the point of the highest temperature of the film was about 6.2 cm below the expansion start point.

This film was guided to another tube-shaped annealing apparatus whose temperature was adjusted to 70, annealed for 10 seconds, cooled to room temperature, folded again, taken out and wound up.

Comparative Example 3

A tubular unstretched film having a thickness of 370 was obtained in the same manner as in Example 1 except that the same linear low-density polyethylene as in Example 1 was used as the resin.

Except that the unstretched film was set to a temperature higher than the temperature range specified in the method of the present invention except that the temperature at the starting point of expansion and the temperature at the highest temperature point were higher than the temperature range specified in the method of the present invention as shown in the condition of No. A biaxially stretched film was produced at 16 m / min. During production, the bubble was abnormally expanded and unstable, and could not be manufactured continuously for a long time.

Comparative Example 4

Using the same linear low-density polyethylene as in Example 1 as a resin, a tubular unstretched film having a thickness of 370 X was obtained in the same manner as in Example 1.

As shown in Table 1, the unstretched film was subjected to the same procedure as in Example 1 except that the temperature at the expansion start point and the temperature at the highest temperature point were lower than the temperature range specified in the method of the present invention. _t produced in the bubble produced in 1 6 mZ worth a biaxially stretched film in the same manner as was only obtained film without practical results in necking. Comparative Example 5

A tubular unstretched film having a thickness of 360 // was obtained in the same manner as in Example 2 except that the same linear low-density polyethylene as in Example 2 was used as the resin.

This unstretched film was subjected to tubular stretching in the same manner as in Example 2 except that the cooling after passing through the expansion end point was insufficient as shown in Table 1. However, the nople rocked and could not maintain stable stretching.

Industrial applicability

The heat-shrinkable film produced as described above, which has a small thickness unevenness, has excellent uniformity as a film, so that it can be handled smoothly in the laminating process and packaging process with other films, and Because of its excellent transparency and heat sealability due to the physical properties of the material, it can be used as an extremely excellent packaging material.

Claims

-13- Claims

1. g '= [] / ["] L is 0.3 to 0.7 (where [] is the limiting viscosity of the polymer, [5?] _L is the limiting viscosity of the linear polymer), and the melt index There 0. 3~2. 0 g 10min, 25 density at hand is 0. 86~ 0 · 92 g / cms ethylene and at least one C4～C ₁₂ of α- old olefins 1-1 0% co of When a heat-shrinkable film is produced by a tubular stretching method from a tubular unstretched film of a resin composition containing a polymer as a component.

(B) The temperature at the starting point of the expansion is defined as the melting point of the resin composition (the endothermic main beak in the melting curve obtained by measurement with a differential scanning calorimeter (hereinafter abbreviated as DSC)).

(Mouth) The maximum temperature should be set at 1/4 to 1Ζ3 of the distance of the stretching zone from the expansion start point to the expansion end point. However, the difference between the maximum temperature and the film surface temperature at the start of expansion is 5

(C) Lower the temperature so that the temperature at the end of expansion is 15 to 20 lower than the maximum temperature.

A method for producing a bolylene-based heat-shrinkable film having a small thickness unevenness and excellent low-temperature heat-shrinkability and transparency.

2. Regarding the melting curve obtained by DSC measurement of the resin composition, the heat-absorbing surface の whose temperature is 10 or lower than the melting point (endothermic main peak) is 55% or more of the total heat-absorbing area. The method for producing a polyethylene heat-shrinkable film according to claim 1.