JP3596621B2 - Heat shrinkable polystyrene film - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a heat-shrinkable polystyrene film suitable as a packaging material used for coating, binding, exterior, and the like. In particular, the present invention relates to a heat-shrinkable polystyrene-based film which has a small shrinkage unevenness, provides excellent shrinkage with excellent heat resistance, and stably maintains a beautiful finished appearance.
[0002]
[Prior art]
The heat-shrinkable film is used for coating, bundling, or exterior of various containers such as bottles (including glass and plastic bottles) and cans and long objects (pipes, rods, wood, various rods, etc.). Has been used as. For example, it is used to cover a part or the whole of a cap part, a shoulder part, and a body part of a bottle for the purpose of display, protection, binding, improvement of commercial value, and the like. Further, it is also used for applications such as stacking and packaging a plurality of boxes, bottles, boards, sticks, notebooks, and the like, and for packaging a skin by closely attaching a film to an object to be packaged (skin package). The above application utilizes the shrinkage and shrinkage stress of the film.
[0003]
As a material of the film, polyvinyl chloride, atactic polystyrene, polyester, polyethylene, hydrochloride rubber and the like are used. Usually, these films are formed into a tube shape, and are wrapped or bound by, for example, covering a bottle or accumulating pipes and then heat shrinking.
However, the conventional film has poor heat resistance and cannot withstand boil treatment or retort treatment at high temperatures, and thus cannot be sterilized at high temperatures. For example, when a retort process is performed, the conventional film breaks during the process.
[0004]
Further, when printing is performed on a conventional film, for example, a polyvinyl chloride film, the adhesiveness with the ink is poor. Further, since polyvinyl chloride easily forms a gelled substance of the additive, pinholes are easily generated on the printed surface. Further, since the conventional film shrinks with time after production, a change in the printing pitch due to the shrinkage occurs, and high-precision printing cannot be performed. In addition, when polyvinyl chloride is discarded or incinerated, there is a problem of corrosion by chlorine gas and acid rain by hydrogen chloride gas, and a heat-shrinkable film that does not cause pollution of waste is desired.
[0005]
In recent years, the usage of plastic bottles has been increasing rapidly. Considering the collection of this bottle, especially when coating a polyester bottle, not only when using different kinds of films such as polyvinyl chloride or polystyrene, but also when using the same type of polyester film Has a problem in that it cannot be collected and reused as it is because it is printed and used. Therefore, it is necessary to separate and remove the coating and reuse it, and a method utilizing the density difference from the polyester bottle is simple and favorable. For this reason, the polystyrene film has a low density and is easily separated, but has poor heat resistance. Further, when printing is performed, good printing cannot be performed because the polystyrene film partially dissolves or swells in a solvent contained in the ink.
[0006]
[Problems to be solved by the invention]
The present invention solves the problem of the atactic polystyrene-based heat shrinkable film among the above-mentioned conventional techniques, and the object thereof is that the heat shrinkage is sufficiently large and the film when heat shrunk is formed. An object of the present invention is to provide a heat-shrinkable polystyrene film which does not generate uneven shrinkage, has a beautiful appearance, and stably retains its appearance even under high-temperature conditions which will be experienced later.
[0007]
[Means for Solving the Problems]
In the present invention, in the stretching step, after preheating at a glass transition temperature Tg or more and Tg + 50 ° C. or less of the polymer constituting the film, and after performing uniaxial stretching 2 to 4 times, a heating zone of 30 ° C. to 150 ° C. A uniaxially stretched polystyrene film made of a resin composition containing a polystyrene-based polymer having a syndiotactic structure for 1 to 30 seconds, and a change rate ΔNz of a refractive index Nz in a thickness direction of the film. Is a polystyrene film for heat shrink packaging characterized by satisfying the following relationship and having a heat shrinkage of 48% or more when heated with hot air at 100 ° C. for 1 minute.
Polystyrene film for heat shrink packaging.
ΔNz ≦ 0.022 × (100 / Tg)
Where:
ΔNz: difference between Nz of the polystyrene film for heat shrink wrapping and Nz when not oriented,
Tg: glass transition temperature of the film.
[0008]
The polystyrene polymer having a stereoregularity of a syndiotactic structure used in the present invention has a dyad (two structural units) whose phenyl group or substituted phenyl group as a side chain is quantified by nuclear magnetic resonance. Is 75% or more . The syndiotactic structure is preferably 85% or more, more preferably 30% or more, and preferably 50% or more in a pentad (five structural units).
[0009]
Examples of the polystyrene-based polymer include polystyrene, poly (p-, m- or o-methylstyrene), poly (2,4-, 2,5-, 3,4- or 3,5-dimethylstyrene), and polystyrene. Poly (alkylstyrene) such as (p-tert-butylstyrene), poly (p-, m- or o-chlorostyrene), poly (p-, m- or o-bromostyrene), poly (p-, m -Or o-fluorostyrene), poly (halogenated styrene) such as poly (o-methyl-p-fluorostyrene), and poly (halogen-substituted alkylstyrene) such as poly (p-, m- or o-chloromethylstyrene) ), Poly (p-, m- or o-methoxystyrene), poly (alkoxystyrene) such as poly (p-, m- or o-ethoxystyrene), poly (p-, m- or o-carboxymethyl) (Polystyrene) such as poly (carboxyalkylstyrene) poly (p-vinylbenzylpropyl ether), poly (alkylsilylstyrene) such as poly (p-trimethylsilylstyrene), and further poly (vinylbenzyl). Dimethoxyphosphide) and the like.
[0010]
In the present invention, it is preferable to add a thermoplastic resin or rubber to the polystyrene-based polymer. Examples of the thermoplastic resin include polystyrene-based polymers such as polystyrene having an atactic structure, AS resin, and ABS resin; polyester-based polymers such as polyethylene terephthalate, polyethylene terephthalate, and polybutylene terephthalate; nylon 6, nylon 66, and nylon 12, Examples thereof include polyamide polymers such as nylon 4 and polyhexamethylene adipamide, and polyolefin polymers such as polyethylene, polypropylene, and polybutene. Further, as the rubber component, a rubbery copolymer containing a styrene compound as one component is preferable, for example, a partially or completely hydrogenated rubber or a styrene-butadiene copolymer of a styrene-butadiene block copolymer. Blend rubber, styrene-isoprene block copolymer, partially or completely hydrogenated rubber of butadiene part of styrene-isoprene block copolymer, methyl acrylate-butadiene-styrene copolymer rubber, acrylonitrile-butadiene-styrene copolymer Polymer rubber, acrylonitrile-alkyl acrylate-butadiene-styrene copolymer rubber, methyl methacrylate-alkyl acrylate-butadiene-styrene copolymer rubber and the like can be mentioned. Since the rubbery copolymer containing these styrene compounds as one component has a styrene unit, it has good dispersibility mainly in a styrene polymer having a syndiotactic structure, and as a result, improves the physical properties. Great effect.
[0011]
Examples of other rubbers include natural rubber, polybutadiene, polyisoprene, polyisobutylene, neoprene, ethylene-propylene copolymer rubber, urethane rubber, silicone rubber, acrylic rubber, polyether-ester rubber, polyester-ester rubber and the like. No.
[0012]
The polystyrene-based polymer used in the present invention has a weight average molecular weight of 10,000 or more, more preferably 50,000 or more. When the weight average molecular weight is less than 10,000, a film having excellent elongation characteristics and heat resistance cannot be obtained. The upper limit of the weight average molecular weight is not particularly limited. However, if the weight average molecular weight is 1500,000 or more, breakage may occur due to an increase in stretching tension, which is not preferred.
[0013]
The polystyrene-based polymer used in the present invention is used for improving electrostatic adhesion, lubricity, stretchability, workability, impact resistance, etc., and for roughening, opacity, cavitation, weight reduction, etc. For the reason, it is possible to use those obtained by blending other resins, indefinite particles, organic particles, plasticizers, compatibilizers, coloring agents, antioxidants, antistatic agents and the like in appropriate amounts.
[0014]
The polystyrene composition is formed into a film by a known method (for example, an extrusion method or a calendar method). The shape of the film is, for example, a flat shape or a tube shape, and is not particularly limited. As a stretching method, an ordinary method is employed. For example, there are a roll stretching method, a long gap stretching method, a tenter stretching method, and a tubular stretching method. In any of these methods, uniaxial stretching is performed. The stretching ratio is arbitrarily set in the range of 2.0 times to 4.0 times. In the stretching step, preheating is performed at a temperature equal to or higher than the glass transition temperature of the polymer constituting the film, for example, equal to or lower than Tg + 50 ° C. In the heat setting after stretching, after stretching, a heating zone of 30 ° C. to 150 ° C. is passed for about 1 second to 30 seconds. Further, after the stretching of the film and before or after performing the heat setting, the relaxation treatment may be performed at a predetermined degree. Further, after the above stretching, a step of cooling while applying stress to the film while maintaining the stretched or tensioned state, or a cooling step may be added after the tension is released following the treatment. The thickness of the obtained film is preferably in the range of 6 to 250 μm.
[0015]
The rate of change of the refractive index in the thickness direction of the heat-shrinkable polystyrene film of the present invention is ΔNz ≦ 0.022 × (100 / Tg)
Must be satisfied with the relationship. The syndiotactic polystyrene polymer has a higher refractive index in the thickness direction of the film as the degree of molecular orientation increases. Therefore, in order to increase the amount of shrinkage, it is necessary to increase the degree of orientation. However, as the degree of orientation is increased by stretching, crystallization proceeds, and good shrinkage characteristics are not exhibited. Further, by adding a rubber component or the like, the glass transition temperature is lowered, and crystallization accompanying this orientation does not occur to a higher orientation state. Therefore, in order to keep the relationship between the degree of orientation and the crystallization in a good range, the rate of change ΔNz of the refractive index in the thickness direction of the film needs to be 0.022 × (100 / Tg) or less. If the rate of change ΔNz of the refractive index in the thickness direction of the film is larger than 0.022 × (100 / Tg), no good shrinkage behavior is exhibited. The glass transition temperature is preferably between 60 ° C. and 80 ° C. in order to obtain the desired shrinkage characteristics.
[0016]
[Action]
The heat-shrinkable polystyrene-based film thus obtained has a smaller shrinkage speed than the conventionally proposed heat-shrinkable film, hardly causes uneven shrinkage of the film when shrunk, and has a beautiful appearance. Even under various high-temperature conditions which will be experienced later, the appearance can be stably maintained.
[0017]
【Example】
Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to only these Examples. In addition, the evaluation method of a film is shown below.
[0018]
1) Refractive index in the thickness direction (Nz)
The measurement was performed using an Abago refractometer (4-T) manufactured by Atago Co., Ltd.
[0019]
2) Glass transition temperature (Tg)
The measurement was performed at a heating rate of 20 ° C./min from room temperature using DSC210 manufactured by Seiko Electronic Industry Co., Ltd.
[0020]
3) The direction in which the heat shrinkage film shrinks is set to the long side, and the film is cut out to have a width of 15 mm. Mark lines are drawn at 200 mm intervals in the long side direction. The sample is heated by blowing hot air at 100 ° C. for 1 minute, and the amount of change in the distance between the marked lines is measured. The percentage of the change to the original length is defined as the heat shrinkage (%).
[0021]
4) The uneven shrinkage film was subjected to metallic printing for a shrinkable label, formed into a cylindrical tube, covered with a 1.5 liter square PET bottle, and passed through a shrink tunnel. The conditions of the shrink tunnel were such that the first zone had a residence time of 4.5 seconds at 100 ° C. and the second zone had a residence time of 140 seconds at 140 ° C. The light and dark of the printing due to uneven shrinkage of the obtained label was visually evaluated in four steps.
◎: Very good with no defects such as unevenness, wrinkles, looseness, etc. ○: Good with few defects, such as unevenness, wrinkles, looseness, etc. Many defects such as unevenness, wrinkles, and loosening are defective.
5) Make the boiled film a shrink label, cover the bottle with water, and shrink in hot water at 801 ° C. This was immersed again in hot water at 80 ° C. for 30 minutes, and the state of tarumi on the label was visually evaluated in four steps.
◎: Very good with no defects such as unevenness, wrinkles, looseness, etc. ○: Good with few defects, such as unevenness, wrinkles, looseness, etc. Many defects such as unevenness, wrinkles, and loosening are defective.
6) The film suitable for filling with a high-temperature substance is made into a shrink label, and shrink-fitted to the bottle by the method used for judging uneven shrinkage. This bottle is filled with hot water of 87 ° C., and water-cooled after 6 minutes. Then, the state of the tarmi of the label was visually judged.
◎: Very good with no defects such as unevenness, wrinkles, looseness, etc. ○: Good with few defects, such as unevenness, wrinkles, looseness, etc. Many defects such as unevenness, wrinkles, and loosening are defective.
Examples 1 to 3 Comparative Example 1
A mixture of syndiotactic polystyrene (weight average molecular weight: 300,000) and styrene-butadiene copolymer rubber at a weight ratio of 5: 5, 100 parts by weight and an average particle size of calcium carbonate of 1.0 μm as a lubricant. After melting and kneading 0.05 parts by weight of the material to form a polymer chip, it is dried, melted at 310 ° C., extruded from a T-die having a lip gap of 800 μm, and adhered to a cooling roll at 40 ° C. by an electrostatic imprinting method. -Cooled and solidified to obtain an amorphous sheet. The extrusion amount of the amorphous sheet was changed so that the thickness of the heat-shrinkable film became 40 μm.
[0025]
The amorphous sheet was first preheated to 110 ° C. and stretched at a stretching temperature of 100 ° C. while changing the magnification in the transverse direction. The transverse stretching ratio was 2.0 (Example 1), 3.0 (Example 2), 4.0 (Example 3), and 5.0 (Comparative Example 1). Thereafter, a heat setting treatment was performed at 60 ° C. for 15 seconds. The evaluation results of the obtained film are shown in the table.
[0026]
【The invention's effect】
The heat-shrinkable polystyrene-based film of the present invention has a practically sufficient heat-shrinkage rate, and when used for coating or bundling by heat shrinking, is uniform regardless of temperature fluctuations and unevenness in the shrinking step. Gives shrinkage results and sags and wrinkles under the high temperature conditions that the film experiences after shrinkage, such as high temperature sterilization after coating or bundling (eg, retorting, boiling) or filling the coated container with high temperature material. It was highly useful in a wide range of packaging material fields without any occurrence of cracks.
[0027]
[Table 1]

Claims (1)

In the stretching step, preheating is performed at a temperature not lower than the glass transition temperature Tg of the polymer constituting the film and not higher than Tg + 50 ° C., and after performing uniaxial stretching 2 to 4 times, the heating zone at 30 ° C. to 150 ° C. is heated for 1 to 30 seconds. A uniaxially stretched polystyrene film comprising a resin composition containing a polystyrene-based polymer having a syndiotactic structure, wherein the rate of change ΔNz of the refractive index Nz in the thickness direction of the film has the following relationship: And a heat shrinkage ratio of at least 48% when heated for 1 minute with hot air at 100 ° C. is 48% or more .
ΔNz ≦ 0.022 × (100 / Tg)
Where:
ΔNz: difference between Nz of the polystyrene film for heat shrink wrapping and Nz when not oriented,
Tg: glass transition temperature of the film.