JPH0523584B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a heat-shrinkable polypropylene film. More specifically, the present invention relates to a heat-shrinkable polypropylene film that has different longitudinal and horizontal heat shrinkage rates and has excellent low-temperature shrinkability and is suitable as a shrink film for packaging cylindrical objects such as bottles. [Conventional technology] Recently, pre-labeled bottles made of glass PET and made of hollow bottles with syringe labels have been rapidly increasing as containers for carbonated drinks, youth, beer, sake, etc. to prevent the bottles from scattering and to promote the products. are doing. Conventionally, polyvinyl chloride film, polystyrene film, etc. have been used as materials for pre-labels. However, although polyvinyl chloride film has excellent printability, packaging finish, and shatterproof properties, it contains chlorine in its molecules, so when it is burned in an incinerator after use, it generates toxic gas. Therefore, there are environmental problems. In addition, polystyrene film has a heat resistance of at most 80
℃ and sterilized with hot water over 85℃,
Not suitable for retort sterilization. Therefore, it has been desired to develop a pre-label material that is safe without any environmental problems, has excellent packaging finish and shatterproof properties, and can be sterilized in hot water of 85°C or higher or retort sterilized. [Problems to be Solved by the Invention] Uniaxially stretched polypropylene films are available as pre-label materials that address the above issues.Although these uniaxially stretched polypropylene films exhibit large shrinkage when heated to near the melting point of the crystals, they usually shrink. In the shrink wrapping process the film reaches 80~100
The drawback is that the shrinkage rate is low at low temperatures around ℃. To improve the constant temperature shrinkability, a method of adding petroleum resins to propylene copolymers (Japanese Patent Publication No. 1987-
No. 50064) is known. However, with a stretched film obtained simply by adding petroleum resins to a polypropylene copolymer using a normal film-forming and stretching method, the low-temperature shrinkability is improved, but this film can be used to shrink bottles, etc. Even if you package it, you will not be able to get a good finish. In other words, when a bottle, etc. is shrink-wrapped with polypropylene film from the shoulder to the bottom of the bottle, the shoulder part of the film separates from the bottle (hereinafter referred to as "shrinkage return") due to cooling after shrink-wrapping, resulting in an aesthetic disadvantage. becomes. [Means for Solving the Problems] As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that low-temperature shrinkage can be achieved by combining a specific polypropylene resin composition and specific processing conditions. The present invention was achieved by discovering that it is possible to provide a heat-shrinkable polypropylene film that has excellent properties and also has a good appearance without causing the phenomenon of "shrinkage return" during cooling after shrink-wrapping. That is, in the present invention, an unstretched film obtained by melt extruding a blend of 98 to 50% by weight of a propylene copolymer and 2 to 50% by weight of a petroleum resin is stretched at a stretching temperature of 70%.
~110℃, deformation rate of 500%/sec or more in uniaxial direction
A stretched film that is stretched 1.5 to 7 times,
Thermal shrinkage rate at 80°C is 20% or more in the stretching direction and 5% or less in the direction perpendicular to the stretching direction, and S ₂ /S ₁ ≧0.3, S ₂ ≧50g/mm ² (However, S ₁ , S ₂
is a heat-shrinkable polypropylene film characterized by a residual shrinkage stress in the stretching direction at 80°C and 25°C after 20% free shrinkage in the stretching direction at 80°C, respectively. As the propylene copolymer in the present invention, a propylene-ethylene random copolymer or a propylene-ethylene-α-olefin terpolymer is used. These propylene copolymers may be used alone or in combination of two or more. The aforementioned propylene-ethylene random copolymer having an ethylene component content of 2 to 10% by weight is used, but one having an ethylene component content of 3 to 8% by weight is particularly preferred in terms of stretchability and low-temperature shrinkability. The propylene-ethylene-α-olefin terpolymer used has an ethylene component content of 0 to 5% by weight, an α-olefin component content of 2 to 30% by weight, and a propylene component content of 98 to 65% by weight. Petroleum resins used in the present invention include petroleum resins, hydrogenated petroleum resins, terpene resins, and hydrogenated terpene resins, but commercially available products include Alcon P-90 manufactured by Arakawa Chemical Industries, Ltd.
P-115, P-125, P-140, Clearon P-85, P-105, P-115, P-125,
Examples include Escorets 5300 and 5320 manufactured by Exxon Chemical Co., Ltd., but are not limited thereto. Among them, those having a softening point of 100 to 150°C are preferably used. The blending ratio of petroleum-based resin to the propylene-based copolymer is 2 to 50% by weight, but if it is less than 2% by weight, the effect of improving low-temperature stretchability and low-temperature shrinkage is poor;
If it exceeds 50% by weight, tackiness occurs, resulting in poor processability and a decrease in the physical strength of the resulting film. It is desirable to mix the propylene copolymer and petroleum resin to a uniformly mixed state by a known method such as ordinary roll mixing, Banbury mixing, or extruder mixing. The mixed composition used in the present invention may contain antioxidants, antistatic agents, ultraviolet light inhibitors, lubricants, and other additives as required. In the extrusion step in the stretched raw fabric manufacturing method of the present invention, the material is melt-extruded from a die through a suitable extruder at a processing temperature of 300 DEG C. or lower, preferably between 160 DEG and 280 DEG C., and then cooled. If the extrusion temperature exceeds 300°C, the petroleum resin mixed in the propylene copolymer will be thermally decomposed, resulting in generation of bubbles and significant smoke generation, which is not preferable. Next, the substantially unstretched film obtained above is stretched using a longitudinal uniaxial stretching device or a horizontal uniaxial stretching device at a stretching temperature of 70 to 110°C and a deformation rate of 500%/
This is carried out by stretching 1.5 to 7 times, preferably 2 to 5 times, at sec or more. In this case, the stretching temperature is
If the temperature is lower than 70°C, problems such as uniform stretching cannot be achieved and uneven stretching occurs, or the resulting film shrinks significantly over time. The stretching temperature
If the temperature is higher than 110°C, problems may occur during the process such as the film sticking to the roll during stretching, or the resulting film may have disadvantages such as poor low-temperature shrinkability. The deformation speed must be 500%/sec or more, and if the deformation speed is less than 500%/sec, S ₂ /S ₁
becomes smaller and when cooled after heat-shrink packaging, the adhesion of the bottle shoulder becomes poor, resulting in an aesthetic defect. Moreover, if the stretching ratio exceeds 7 times, the shrinkage rate is rather reduced, and if the stretching ratio is less than 1.5 times, sufficient orientation does not occur, so the shrinkage rate is not sufficient in either case. The heat shrinkage rate of the stretched film in the direction perpendicular to the stretching direction is appropriately adjusted by selecting the blending ratio of the propylene copolymer and petroleum resin, the stretching temperature, and the stretching ratio. The stretched film thus obtained is desirably lightly heat-treated to suppress natural shrinkage. The heat treatment temperature is 40 to 100°C, and the heat treatment time may be determined as appropriate depending on the heat treatment temperature, but as the temperature increases, a shorter treatment time is used. The time is preferably 2 seconds or more, at which the treatment effect usually appears, and 180 seconds or less, which is industrially practical. The heat-shrinkable polypropylene film thus obtained has a heat shrinkage rate of 20 in the stretching direction at 80°C.
% or more and 5% or less in the direction perpendicular to the stretching direction, and S ₂ /S ₁ ≧0.3, S ₂ ≧50 g/mm ² (However,
S ₁ and S ₂ have the performance of residual shrinkage stress in the stretching direction at 80°C and 25°C after 20% free shrinkage in the stretching direction at 80°C, respectively. When shrink-wrapping a hollow bottle made of glass, PET, etc. from the bottle shoulder to the bottom with shrink-wrapping film, for example, shrink-wrapping a cylindrical object with a diameter approximately 5% larger than the maximum diameter of the bottle and the required height. (hereinafter referred to as a sleeve) is made, and the bottle is inserted into this sleeve and passed through a shrinkage tunnel. Therefore, the contraction behavior from the start of contraction to the end of contraction is different for the shoulder, body, and bottom of the bottle. In other words, the film portion located in the body part shrinks by about 5% without tension (hereinafter referred to as free contraction) and comes into close contact with the body part, and then becomes under tension and develops contraction stress. On the other hand, the bottle shoulder portion comes into close contact with the bottle shoulder after free contraction corresponding to the diameter of the sleeve and the diameter of the bottle shoulder portion, and then develops shrinkage stress. In this case, the free shrinkage is larger at the shoulder of the bottle than at the body. In most cases, the free shrinkage at the shoulder of a bottle is approximately 20% or more, and in order to completely shrink wrap such bottles, the residual shrinkage stress must be at least a certain amount after free shrinkage of 20% or more. It is necessary that there be. When a bottle or the like is shrink-wrapped from the neck to the bottom using a polypropylene film, the "return of shrinkage" at the shoulder of the bottle, which occurs when the bottle is cooled after shrink-wrapping, is not observed with the polypropylene film of the present invention. [Effects of the Invention] The heat-shrinkable polypropylene film obtained by the present invention has the following effects. (a) Since it has excellent low-temperature shrinkability, there is no need to particularly adjust the temperature of the shrink tunnel to a high and uniform temperature, and conventional shrink packaging lines can be used as is. (b) Because it has excellent low-temperature shrinkability, even containers with low heat resistance can be shrink-wrapped without deforming the container. (c) Even bottles with a large difference in diameter between the body and neck, such as glass bottles for youth use and PET bottles, and containers with various complicated shapes can be packaged in a sufficiently tight manner. (d) Low-temperature shrinkability and residual shrinkage stress change with temperature are small, so when a bottle, etc. is shrink-wrapped from the neck to the bottom, no "shrinkage return" occurs when cooling after shrink-wrapping, and a good finish can be obtained. [Example] Next, the present invention will be explained in detail with reference to Examples, but the present invention is not limited thereto. In addition, the measuring method and evaluation criteria of the characteristics in the present invention are as follows. (1) Heat shrinkage rate Cut the film to be measured into 100 mm long and 100 mm wide, and measure the vertical and horizontal dimensions of the film to 0.1.
Accurately measure to the nearest mm. (The measured value is Amm
). This film is completely immersed in a glycerin bath set at a predetermined temperature for 10 seconds, then taken out and immediately quenched in a water bath. The surface of the sample is cleared and the vertical and horizontal dimensions of the film are accurately measured to the nearest 0.1 mm (the measured value is designated as Bmm). The thermal shrinkage rate is calculated using the following formula. Thermal shrinkage rate (%) = A-B/A x 100 (2) Residual shrinkage stress Length with marked lines 5mm from both ends
Connect a film of 72.5 mm and width of 10 mm to a strain meter,
The specimen is fixed at each marked line position to a pair of gripping devices set at a gripping interval of 50 mm, and immersed in a glycerin bath whose temperature is controlled to 80°C. For S ₁ , read the shrinkage stress after 10 seconds of immersion with a strain meter, and for _S2 , after reading S ₁ , take it out of the glycerin bath and read the shrinkage stress after cooling to 25℃, and calculate each stress by unit cross-sectional area ( The values of S ₁ and S ₂ were calculated per mm ² ). (3) Label characteristics Film lengthwise (direction perpendicular to stretching direction) 117mm,
After cutting to 223mm horizontally (stretching direction), impulse seal the ends parallel to each other in the vertical direction so that the ends are 5mm apart.
I created a cylindrical sleep by overlapping mm. This sleep has a height of 140mm and a trunk diameter of 68.5mm.
mm, the diameter at the shoulder at a height of 117 mm from the bottom is approximately 55
After attaching it to a 300 ml glass bottle of 300 mm in diameter so as to protect it from the shoulder to the bottom, the stretched film was placed in an oven at 200° C. for 10 seconds to heat shrink the stretched film and obtain a labeled glass bottle. This labeled glass bottle was evaluated by the following method. [Appearance adhesion] Judgment is made based on the adhesion between the polypropylene film and the glass bottle after heat shrinkage. Those with no wrinkles, avatars, or air entrapment are marked ○, and those with partial wrinkles or avatars are marked △. Those with many marks, wrinkles, and avatars are marked with an x. [Uniformity of label edge line on bottle shoulder] Labels with almost straight edge lines are marked with a circle, and labels with wavy or wrinkled edges are marked with an x. [Shrinkage return after cooling] After shrink-wrapping and labeling, the glass bottle is left at room temperature, and after 24 hours it is determined by the adhesion between the label on the shoulder of the bottle and the glass bottle. A label that is in close contact with the bottle is marked with an ○ mark, and a label that is partially or completely separated from the bottle is marked with an x mark. [Boiling suitability] Judging the appearance when a labeled glass bottle is immersed in hot water at 90℃ for 20 minutes and then taken out. Items that have not changed at all from before treatment are marked with an ○ mark, and cases with partial loosening, cloudiness, etc. are marked with an x mark. [Retort Suitability] Judging the appearance when a labeled glass bottle is removed from steam treatment at 120℃ for 40 minutes. Those that have not changed at all from before treatment are marked with an ○, those that are partially loosened or cloudy, and those that are overall bulky and have poor adhesion are marked with an x. Example 1 90% by weight of propylene-ethylene random copolymer (ethylene content: 4% by weight, melt index 2.7g/10 min at 230°C) and petroleum resin (Alcon P-115, manufactured by Arakawa Chemical Co., Ltd., 10% by weight (softening point: 115°C) was kneaded using a twin-screw kneader to obtain a pellet-like mixture. This mixture is melt-kneaded at 170-250°C and extruded through an annular die kept at 220°C.The inside of the tube slides on the outer surface of a cylindrical mandrel that circulates cooling water, and the outside is passed through a water bath. It was cooled and taken off to obtain a tubular unstretched film with a diameter of about 186 mm and a thickness of 160 μm. This unstretched film is used as a raw material and is led to a tube stretching device.
4.0 times the length at a stretching temperature of 90℃ and a deformation rate of 600%/sec.
Stretched at 1.0 times horizontally. The stretched film was annealed with hot air at 60° C. for 8 seconds in a tube-shaped annealing device, then cooled to room temperature, folded, and rolled up. The resulting stretched film had a thickness of 42μ. The heat shrinkage rate and residual shrinkage stress of this film were measured, and the label properties were further evaluated. The results are summarized in Table 1. Example 2 90% by weight of propylene-ethylene random copolymer (ethylene content 6% by weight, melt index 5.7g/10 minutes at 230°C) and petroleum resin (Alcon P-115, manufactured by Arakawa Chemical Co., Ltd., 10% by weight (softening point: 115°C) was kneaded using a twin-screw kneader to obtain a pellet-like mixture. This mixture was melt-kneaded at 170 to 250°C and extruded into a sheet through a T-die kept at 200°C.
It was cooled to 30°C using a cooling roll. This unstretched film is heated to 90℃ and deformed at a speed of
It was stretched 3 times in the machine direction at 1500%/sec. The stretched film was then passed through a hot roll at 60° C. and annealed for 5 seconds. The thickness of the obtained stretched film is
It was 44μ. This film was subjected to the same measurement and evaluation as in Example 1. The results are summarized in Table 1. Example 3, Comparative Examples 1 to 4 A polypropylene heat-shrinkable film with a thickness of 40μ was produced in the same manner as in Example 1, except that the raw material composition, stretching temperature, deformation speed, and stretching ratio were changed as shown in Table 1. , conducted measurement evaluation. The results are summarized in Table 1. Example 4 The raw material composition was 3.0% by weight of ethylene content, 2.5% by weight of putene-1 content, 85% by weight of a propylene-ethylene-putene-1 terpolymer with a melt index of 4.0, and Alcon P-1. A polypropylene heat-shrinkable film with a thickness of 42 μm was produced in the same manner as in Example 3 under the conditions shown in Table 1, except that 15% by weight of 125 (manufactured by Arakawa Chemical Industry Co., Ltd., softening point 125°C) was used. We then conducted a measurement evaluation. The results are summarized in Table 1. As can be seen from the results in Table 1, if the stretching temperature is too high, the obtained thermal shrinkage rate will be insufficient (Comparative Example 1), and if the deformation rate is slow, the residual shrinkage stress will be insufficient (Comparative Example 2). Without the addition of resin, the heat shrinkage rate and residual shrinkage stress would be insufficient (Comparative Example 3), and even if the stretching ratio was too high, the heat shrinkage rate would actually decrease (Comparative Example 4). It can be seen that the heat-shrinkable film of the invention has excellent properties such as appearance adhesion after shrink labeling, boiling properties, and retort properties.

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Claims (1)

[Claims] 1. An unstretched film obtained by melt extruding a blend of 98 to 50% by weight of a propylene copolymer and 2 to 50% by weight of a petroleum resin is stretched at a stretching temperature of 70 to 110°C and a deformation rate. A stretched film that is stretched 1.5 to 7 times in the uniaxial direction at 500%/second or more, and whose heat shrinkage rate at 80°C is 20% or more in the stretching direction and 5% or less in the direction perpendicular to the stretching direction. Yes, and S ₂ /S ₁ ≧
0.3, S ₂ ≧50g/mm ² (However, S ₁ and S ₂ are each 80
80℃ after 20% free shrinkage in the stretching direction at ℃
A heat-shrinkable polypropylene film characterized by a residual shrinkage stress in the stretching direction at 25°C. 2 The propylene copolymer is a propylene-ethylene random copolymer with an ethylene component content of 2 to 10% by weight, or an ethylene component content of 0 to 5% by weight,
Claim No. 1, characterized in that it is made of one or more of a propylene-ethylene-α-olefin terpolymer having an α-olefin component content of 2 to 30% by weight and a propylene component content of 98 to 65% by weight. The heat-shrinkable polypropylene film according to item 1.