JPH0318655B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat-shrinkable film made of linear low-density polyethylene. Conventionally, heat-shrinkable films made from polyethylene polymers are difficult to stretch using a normal tenter, which requires reheating after forming an unstretched film, so the resin is mainly cooled in a molten state during tubular extrusion. However, this method does not have enough strength or has a narrow shrinkable temperature range, so it is difficult to perform heat shrinking. There were problems such as shrinkage and insufficient adhesion to the packaged item due to the small shrinkage stress. To solve these problems, heat-shrinkable films made of cross-linked polyethylene that can be stretched normally are on the market, and manufacturers prefer to use such cross-linked polyethylene. It had some problems, such as poor heat sealability and difficulty in recycling. Therefore, the present inventors focused on linear low-density polyethylene, and as a result of repeated studies, we added a specific polymer to such linear low-density polyethylene,
It was discovered that a suitable heat-shrinkable film with excellent stretchability could be provided by forming a film, leading to the present invention.The feature is that a thin film made of linear low-density polyethylene and modified polyolefin The company discovered a heat-shrinkable film made by stretching the film, and its unique feature is that it discovered a heat-shrinkable film made by stretching a thin film made of linear low-density polyethylene, modified polyolefin, and ethylene polymer. It is at the point. The linear low-density polyethylene according to the present invention refers to a low-density polyethylene in which side chains of approximately constant length are appropriately present. Polyethylene with a large number of irregular side chains is usually called low density polyethylene, and those with short and few side chains are called medium or high density polyethylene, but the former is generally produced using a high-pressure process, and the latter is produced using a medium or high density polyethylene.
Manufactured by low pressure method. However, the polyethylene according to the present invention is usually produced by a medium-low pressure method, but by artificially introducing side chains, it becomes a bulky molecule with relatively sparse clogging, and has a linear, low-density structure. something is manufactured. At this time, in order to introduce a side chain, for example, butene-1 or the like may be added to form a copolymer, or the side chain may be introduced by any other appropriate method, and there are no particular restrictions on the manufacturing method. . Modified polyolefins are polyolefins such as polyethylene (including the above-mentioned linear low-density polyethylene), ethylene-vinyl acetate copolymers, ethylene-propylene copolymers, etc., and unsaturated carboxylates such as fumaric acid, maleic acid, and their anhydrides. It is obtained by graft copolymerizing an appropriate amount of at least one monomer selected from acids or derivatives thereof, and the preferred range of the monomer is 10 ^-4 to 10
% by weight, more preferably from 10 ^-3 to 5% by weight, but this value is not limited. Such modified polyolefin may be composed only of modified polyolefin, or it may be a mixture formed by appropriately mixing modified polyolefin with unmodified polyolefin and/or a third component to the extent that its characteristics are not lost. good. The ethylene polymer according to the present invention refers to those excluding the above-mentioned linear low density polyethylene and modified polyolefin, such as ethylene-vinyl acetate copolymer, ethylene-propylene copolymer, and ordinary low, medium, and high density polyethylene. Density polyethylene, ethylene ionomer, etc. can be mentioned, but there are no particular limitations. The reason for adding modified polyolefin is to improve these drawbacks, since linear low-density polyethylene alone had poor stretchability, insufficient thickness accuracy, and many variations in thickness. Furthermore, it was confirmed that by adding a polyethylene polymer, the stretchability was further improved and the shrinkage rate was also significantly improved, leading to the present invention. Melt extrusion, which is usually used industrially, is suitable for producing heat-shrinkable films, but other well-known methods may also be used. However, the unstretched film may then be reheated and stretched. Stretching may be carried out, for example, by longitudinal stretching, transverse stretching, biaxial stretching in longitudinal and lateral directions, etc., and can be carried out uniaxially, sequentially in two directions, etc. using a roll stretching machine, tenter method, tubular method, etc.
It may be stretched in a preferred state such as axially or simultaneously biaxially. Such stretching is different from the orientation that occurs during so-called extrusion blow-up, and is performed separately from the film forming process even if it is carried out continuously. At this time, antistatic agents, other additives, and fillers may be used in the heat-shrinkable film of the present invention as needed. Further, the present invention can be used in a wide variety of applications, such as cap stickers, shrink labels, and other general packaging, and is, of course, not subject to any particular limitations. The present invention is as described above, and the heat-shrinkable film according to the present invention has extremely excellent stretchability without causing any problems such as tearing or poor thickness accuracy even if it is subjected to normal stretching after film formation. A product with a uniform thickness was obtained, and its heat shrinkage rate also showed a high value. This also shows that the present invention has a particularly remarkable effect, and the present invention has made it possible to provide a novel heat-shrinkable film using the conventional stretching method, which had many problems in the past. Much demand is expected in the future. <Example 1> Various polymers listed in Table (1) were added to linear low-density polyethylene with a density of 0.920 and an MI of 2.1, which is linear and has side chains of approximately constant length. An unstretched film that is extruded into a flat shape by extrusion using a chill roll through a die is subjected to roll stretching in the longitudinal direction under the conditions listed in Table (1), and then tenter stretching in the transverse direction, which is the so-called sequential biaxial method. Various heat-shrinkable films were obtained by the stretching method. The physical properties of such a heat-shrinkable film are also listed in Table (1), and a heat-shrinkable film made only of linear low-density polyethylene is also listed as Comparative Example (1). In addition, among the modified polyolefins in Table (1), those in Experiment Nos. (1), (4), (5), and (6) are made by graft copolymerizing maleic anhydride onto a base of low-density polyethylene. Experiment No. (2) and Experiment No. (3) were made by graft copolymerizing maleic anhydride onto a linear low-density polyethylene base. In addition, among polyethylene polymers, Experiment No. (3),
The same (4) has a vinyl acetate content of 15% by weight, MI=
1.5 ethylene-vinyl acetate copolymer, Experiment No. (5)
The one with ethylene content is 6.0% by weight.
The propylene copolymer, Experiment No. (6), is low density polyethylene.

[Table] <Example 2> An unstretched film obtained by adding various polymers listed in Table (2) to the same linear low-density polyethylene as in Example 1 and extruding it using a water-cooled inflation method using an annular die. The film was once wound up and then subjected to simultaneous biaxial stretching using the tubular method to obtain various heat-shrinkable films. The physical properties of such a heat-shrinkable film are also listed in Table (2), and a heat-shrinkable film made only of linear low-density polyethylene is also listed as Comparative Example (2). Of the modified polyolefins in Table (1), those in experiment No. (1) are based on linear low-density polyethylene, and those in experiment Nos. (2), (3), and (4) are based on low-density polyethylene. This is the same as the base example 1. Among the polyethylene polymers, those in Experiment No. (1) and (2) are the same ethylene-vinyl acetate copolymer as in Example (1), and those in Experiment No. (3) are the same as in Example 1. The same ethylene-propylene copolymer used in Experiment No. (4) is low-density polyethylene.

【table】

[Table] As is clear from Tables (1) and (2) above, the heat-shrinkable film of the present invention has excellent stretchability and good thickness accuracy, and its heat-shrinkage rate is also extremely high. It showed a relatively high value even at low temperatures, suggesting that the temperature range in which heat shrinkage is possible is wide.

Claims (1)

[Claims] 1. A heat-shrinkable film made by stretching a thin film made of linear low-density polyethylene and modified polyolefin. 2. A heat-shrinkable film made by stretching a thin film made of linear low-density polyethylene, modified polyolefin, and ethylene polymer. 3. The heat shrinkage according to claim 2, wherein the ethylene polymer is at least one polymeric material selected from low density polyethylene, ethylene-propylene copolymer, and ethylene-vinyl acetate copolymer. sex film.