JPH0550378B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing a heat-shrinkable film for labels, and more specifically to a method for producing a polyolefin-based heat-shrinkable film having excellent heat resistance. [Prior art] Conventionally, heat-shrinkable film materials used as labels for one-way containers, especially glass bottles, include polyvinyl chloride resin and polystyrene resin, but these materials have poor heat resistance and It cannot be used as a label for contents (such as coffee drinks) that require a high temperature sterilization process exceeding ℃. On the other hand, the present inventors have disclosed that a polyolefin foam sheet is a heat-shrinkable film for labels that has superior advantages such as cushioning properties, impact strength, and heat resistance (Japanese Patent Application No. 4926/1983). However, since foam sheets are opaque, there is a limit to the height of the coating when covering glass bottles, and when compared to solid films, the thermal conductivity is low, and the process of heating the contents to a predetermined temperature is difficult. A filling line with a cooling process has the disadvantage that the line speed must be reduced. [Means for Solving the Problem] Under these circumstances, the inventors of the present invention have intensively studied a heat-shrinkable film that has excellent heat resistance and transparency, and has good mechanical suitability for mounting the film. It was discovered that a unidirectionally stretched film using a propylene copolymer is effective, and the present invention was completed. That is, the present invention provides 100 weight ethylene-propylene copolymers having an ethylene content of 2 to 10% by weight and a melt flow index (MI; according to LISK-6758) of 0.5 to 10 g/10 minutes;
Forming a sheet from a composition consisting of 0 to 50 parts by weight of a propylene homopolymer having a melt flow index of 0.5 to 10 g/10 minutes, and stretching in one direction with a surface temperature difference of 2°C or more between the front and back surfaces of the sheet. This is a method for producing an olefin-based shrink film characterized by the following. The present invention will be explained in detail below. The ethylene-propylene copolymer used in the present invention has an ethylene content of 2 to 10% by weight, preferably 3 to 7% by weight. If the ethylene content is less than 2% by weight, the stretched film will have a narrow shrinkage temperature range, and when hot air is blown onto the film, it will become molten if the hot air hits it directly, and if the hot air hits it weakly, it will not shrink enough. It is difficult to clean the entire circumference of the coated glass bottle, resulting in large losses. In addition, when the ethylene content exceeds 10% by weight, the rigidity decreases,
When attaching the film to a glass bottle, there is a lot of loss and the heat resistance decreases. Based on 100 weight parts of the ethylene-propylene copolymer, 0 to 50 parts by weight of the propylene homopolymer is selected and blended in accordance with required physical properties such as heat resistance, shrinkability upon heating, and rigidity. If the amount of propylene homopolymer blended exceeds 50 parts by weight, the shrinkage temperature range of the stretched film becomes narrow, resulting in frequent losses due to poor appearance when coating glass bottles. The melt index (MI; according to JISK-6758) of ethylene-propylene copolymer is 0.5 to 10.
g/10 minutes, preferably in the range of 1.0 to 5 g/10 minutes. The MI value of propylene homopolymer is 0.5-10g/
In 10 minutes, the MI value of the ethylene-propylene copolymer is preferably ±1 g/10 minutes. MI value is 0.5g/10
If it is less than 1 minute, uneven stretching or tearing of the film may occur. On the other hand, if it exceeds 10 g/10 minutes, the effect of stretching will be poor, especially shrinkage during heating will be insufficient and loss will increase. In the present invention, there is no problem in adding colorants, anti-static agents, antioxidants, etc. The stretching treatment for obtaining the heat-shrinkable film of the present invention involves stretching in at least one direction, either the machine direction or the transverse direction, preferably in the machine direction. When stretching in the machine direction, a roll stretching machine is usually used, preheating is performed at a temperature in the range of 100 to 140°C, and the sheet is stretched in the take-off direction at a temperature difference of 2°C or more between the front and back surfaces of the sheet. The stretching ratio is selected to be 3.0 to 7.0 times, preferably 4.0 to 6.0 times. If the preheating temperature is less than 100℃, uneven stretching will easily occur and it will be difficult to obtain a uniform shrink film.
Moreover, if the temperature exceeds 140°C, the stretching effect will be impaired. In addition, if the surface temperature difference between the front and back of the sheet is less than 2°C, when a cylindrical body is made from a stretched film by heat sealing for labels and then inserted into a heating furnace, the inner surface of the cylindrical body will contract strongly and become rolled inward. It cannot be contracted, and many folds and wrinkles occur during contraction.
A beautiful coated bottle cannot be obtained. If the stretching ratio is less than 3.0 times, the shrinkability will be insufficient, and for this reason, when the attached label is heated, it will not adhere tightly to the glass bottle and wrinkles will occur frequently on the shoulder. On the other hand, if it exceeds 7.0 times, it tends to tear in the stretching direction, causing problems in slitting and other processes. When stretching in the transverse direction, a tenter system is typically used, and the same preheating conditions and stretching ratio as in the longitudinal stretching are applied. [Example] The present invention will be described below with reference to Examples. Examples 1-2 A sheet was extruded from a 300 mm wide T-die attached to a 50 mmφ extruder using the resin shown in the table as a raw material, and a film with a thickness of 80 μm was obtained by stretching this sheet under the stretching conditions shown in the table. The shrinkage rate of this film and the results of practical tests are listed in the table. Conditions for measuring shrinkage: Immersion in an oil bath at 120°C for 1 minute. Conditions for measuring heat-resistant blocking properties: Tie the coated bottles into a set of 3 and leave in a dryer at 110°C for 20 minutes. It also had good shrinkage performance when heated, and had a good appearance after the glass bottle was covered with a film cylinder and heat-shrinked. The film does not deform even if left in a dryer at 110℃, and has excellent heat resistance. In addition, the film of Example 1 was coated (the height of the label
105 mm) bottle (inner capacity 312 c.c.) and a polyolefin foam sheet (sheet thickness 0.2 mm, foaming ratio 1.5 times,
Prepare a bottle coated with a label (label height 105 mm) and heat it at 25℃.
After filling 300c.c. of water, 105mm in a constant temperature water tank at 90℃
The time required for the water temperature in the glass bottle covered with the film to reach the same temperature as the water temperature in the bottle covered with the polyolefin foam sheet was about half the time required for immersion in the glass bottle. Comparative Example 1-3 A film was obtained using the same equipment and raw materials as in Example 1, but changing the stretching conditions as shown in the table. The film of Comparative Example 1 had large variations in thickness, resulting in large product loss. Furthermore, when the films of Comparative Examples 2 and 3 were placed on a glass bottle and heated, they did not shrink neatly and there was a lot of loss. Comparative Example 4 A styrene-butadiene copolymer was used as a raw material and stretched 5 times to obtain a film of 80 μm. This film was coated on a glass bottle, three bottles were tied into a set, and after drying at a temperature of 110°C, the bottles were left for 20 minutes. There was blocking and the film was also deformed.

〔Effect of the invention〕

The heat-shrinkable film obtained by the present invention has excellent heat resistance and can be used as a label for containers that undergo a heat sterilization process at temperatures exceeding 100°C, as well as labels for glass bottles, PET resin bottles, metal cans, etc. can be applied. Furthermore, the film obtained by the present invention has excellent transparency and impact resistance, so it can be used satisfactorily as a cover label for glass bottles for carbon beverages which are subjected to internal pressure.

Claims (1)

[Claims] 1 Ethylene content is 2 to 10% by weight, and melt flow index (MI; JIS K-6758)
A sheet is formed from a composition consisting of 100 parts by weight of an ethylene-propylene copolymer having a melt flow index of 0.5 to 10 g/10 minutes and 0 to 50 parts by weight of a propylene homopolymer having a melt flow index of 0.5 to 10 g/10 minutes. . A method for producing an olefin-based shrink film, which comprises preheating the sheet at a temperature of 100 to 140°C and stretching it in one direction with a surface temperature difference of 2°C or more between the front and back surfaces of the sheet.