JPH0560413B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a foamed shrink film used for shrink labels for soft drink glass containers and the like. [Prior art and its problems] Shrink labels are made of a heat-shrinkable resin film, such as polyethylene, with a foaming ratio of about 2 to 3 times, and one side is printed to display the product name, seller, etc. This is done by winding it into a cylindrical shape, inserting it into a glass container, heating it and shrinking it, and then bringing it into close contact with the glass container for use. Since this Shrink label is made of a foamed film, it has a mechanical buffering effect and heat-insulating properties, and is particularly suitable as a packaging material for glass containers containing soft drinks such as carbonated drinks. By the way, in the case of the foamed shrinkage film used for such shrinkable labels, in order to obtain sufficient buffering effect and heat-insulating properties, it is necessary to increase the foaming ratio. However, when the expansion ratio is increased, the surface roughness of the film increases, resulting in a rough surface, resulting in a disadvantage that the printed finish, especially the sharpness, is greatly reduced and the design is lacking. On the other hand, if the foaming ratio is reduced in an attempt to improve printability, there arises the disadvantage that sufficient buffering effect and heat-insulating properties cannot be obtained. [Problems to be Solved by the Invention] Therefore, in the present invention, the roughness of the front and back surfaces of the foamed shrink film made of polyolefin resin is set to be between 10 and 20 μm, and the roughness of the front surface is set to A and the back surface is set to have a roughness of A and a back surface. The above conflicting problems were solved by satisfying the relationship A-B/A≧0.1, where B is the roughness of the surface. The surface roughness of the foamed shrink film in this invention is measured by the method specified in JIS-B-0601. Specifically, the stylus type (JIS-B-
0651), average roughness direct reading type (JIS-B-0655), light cutting type (JIS-B-0653), light wave interference type (JIS-B
-0652), NF roughness formula (JIS-B-0656), etc. can be used. The absolute value of the roughness on both sides of the foam shrink film is 10μ
If it is less than m, the foaming ratio has to be lowered, and the insulation and heat retention properties and buffering effects are insufficient. Furthermore, if the thickness exceeds 20 μm, printability deteriorates on both the front and back surfaces, making it impossible to print clearly. Furthermore, if A-B/A is less than 0.1, the surface roughness will not be sufficiently reduced;
It is unsuitable as a printing surface, the roughness of the back surface is not large enough, and it is disadvantageous in terms of cushioning properties and heat insulation properties.
Therefore, the average expansion ratio of the foam shrinkage film of the present invention is in the range of about 1.5 to 2.0 times. Further, the stretching ratio is preferably in the range of 3.5 to 5.0 times. In such a foam shrink film, the front and back surfaces have different roughness, one side is relatively smooth and suitable for printing, and the other side is relatively rough and has high cushioning properties and excellent heat and insulation properties. Become. Next, a method for producing such a foam shrink film will be explained. First, a polyolefin resin is prepared. As the polyolefin resin, low density polyethylene, high density polyethylene, polypropylene, ethylene-propylene block copolymer, ethylene-propylene random copolymer, and blend polymers thereof are used. Chemical blowing agents such as azobisisobutyronitrile, azodicarbonamide, and N,N'-dinitrosopentamethylenetetramine are added to these polyolefin resins.
Add 0.1-1.5% by weight. In addition, additives such as antioxidants, lubricants, plasticizers, and antistatic agents may be added in required amounts as necessary. This mixture is uniformly dry-blended using a mixer such as a Henschel mixer or a tumbler to obtain a resin composition. Next, this resin composition is melt-extruded to form a film and foamed by an inflation method or a T-die method using an extruder. During this forming process, a small-diameter cooling roll is brought into contact with only one side of the molten film discharged immediately after the extrusion die, or a small amount of cooling water is injected onto only one side of the film. Pre-cool. This preliminary cooling suppresses the growth of foam cells on one side of the film and the decomposition of the chemical foaming agent, resulting in a state in which foaming is suppressed. Next, the film in this state is cooled with a cooling roll or the like and then stretched to obtain a foamed shrink film. By such a manufacturing method, a film with different expansion ratios and different surface roughness can be obtained on the front and back sides, and by adjusting the degree of preliminary cooling on one side of the film, the above relational expression; A-B/A≧0.1 can be obtained. It is possible to obtain a film that satisfies the following. [Example] Hereinafter, an example will be shown to clarify the action and effect. A mixture of the four types of polyolefin resins (resin types A, B, C, and D) shown in the attached table with a blowing agent of 1 to 1.5% by weight; It is extruded using an extruder at a molding temperature of 180 to 200°C, and pre-cooled by contacting only one side of the molten film with a cooling roll immediately after the die to a thickness of 400 μm and an expansion ratio of 1.4 to 2.0.
A foam sheet of twice the size was molded. Next, this foamed sheet was stretched using a heated roll type uniaxial stretching machine at a stretching temperature of 115° C. and a stretching ratio of 4.0 to 4.5 times to obtain a foamed shrink film having a thickness of about 90 to 100 μm. For this foamed shrink film, the shrinkage rate in the machine direction is
The temperature at 40% (40% contraction temperature), surface roughness, print finish, and heat retention were measured. 40% shrinkage temperature is the JIS heat shrinkage rate in the machine direction at each temperature.
-Z-1709, and the temperature when the shrinkage rate was 40% was determined from the graph. The surface roughness was measured using an SE-3C universal surface profile measuring device manufactured by Kosaka Laboratory Co., Ltd.
It was determined by JIS-B-0601-1982 10-point average roughness method. The print finish was visually observed and evaluated as follows. ◎: Very good ○: Good △: Slightly poor ×: Poor Heat retention was determined by wrapping this film around a 300ml glass container, shrinking it, filling it with water, leaving it in a constant temperature bath at 70℃ for 1 hour, and then taking it out. The evaluation was based on the feeling of heat when holding the film part of the glass container in your hand. ○: Can be held in hand for more than 1 minute. △: Can be held in hand for up to 30 seconds. ×: Can be held in hand for less than 10 seconds. For comparison, a similar study was conducted on a foamed shrink film produced using a conventional method without pre-cooling. The results are also shown in the attached table. As is clear from the results in the attached table, the foamed shrink film of the present invention has both good printability and heat retention. [Effects of the Invention] As explained above, in the foam shrinkage film of the present invention, one side of one film is relatively smooth and the other side is relatively rough, so the smooth surface has good printability. Therefore, the rough side retains sufficient heat retention and cushioning properties. Therefore, this foamed shrink film is suitable for, for example, shrink labels for one-way containers and the like. 【table】

Claims (1)

[Scope of Claims] 1. In a foamed shrink film made of polyolefin resin, where the surface roughness of the film is A and the back surface roughness is B, A and B are 10 to 20 μm, and A-B /A≧0.1 (roughness measurement is in accordance with JIS-B-0601).