JPH0352341B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat-shrinkable composite film having an opaque pearl-like luster. More specifically, a heat-shrinkable composite having an opaque pearl-like luster, which has a heat-shrinkable foamed film as a base layer and a surface layer made of an unfoamed heat-shrinkable film formed on at least one side of the base layer. Regarding film. Anisotropic materials stretched in one or two directions for use in the exterior of packaged goods, tight packaging to prevent the contents from impact, bundled packaging, label packaging that serves both to protect glass bottles and containers and to display products, etc. Shrink wrapping using plastic film (shrink film) having isotropic or isotropic heat shrinkability has been widely used for some time. Furthermore, in recent years, with advances in shrink wrapping technology, heat-shrinkable foam films with a cushioning effect have been used as a way to protect items that are easily damaged by slight impacts, such as glasses, bottles, pots, vases, and urns, from damage. It started to be done. By directly shrink-wrapping these items using this heat-shrinkable foam film, the following conventional methods are possible: a) Wrapping cushioning materials such as cardboard, tissue paper, and expanded polystyrene around the item; This eliminates the need for binding packaging with strings, bands, tape, etc., (b) preventing contact between items by partitioning the cardboard case, and (c) storing items in foam molded products with a wall thickness that matches the shape of the item. This provides great benefits in streamlining the packaging process, reducing transportation space, and reducing costs. Conventionally, polystyrene foam stretched film is well known as this type of heat-shrinkable foam film.For example, it is used as a label and protective film for glass bottles such as carbonated beverages, and is used to print patterns, trademarks, boss marks, etc. on the peripheral side of the container. It is widely practiced to wrap a polystyrene expanded stretched film around the bottle and then heat-shrink it so that it adheres tightly to the bottle (hereinafter, packaging that serves as both a label and a protective film is referred to as label packaging). However, polystyrene expanded stretched film has poor impact resistance, so if a bottle under high internal pressure, such as a bottle containing carbonated beverages, is wrapped in the film and dropped, the damaged bottle will tear the film and break the bottle into pieces. It is dangerous because it scatters widely. On the other hand, stretched films obtained from polypropylene and ethylene-propylene copolymers containing mainly propylene have superior impact resistance compared to polystyrene, and are used as unbalanced heat-shrinkable films (Japanese Patent Application Laid-Open No. 1985-1993) for label packaging. −103951),
A multi-layer heat-shrinkable film (Japanese Unexamined Patent Publication No. 57-49554) which has two or three layers of heat-sealing properties has been proposed, but the buffering effect as a protective film is not necessarily sufficient. The present inventors have conducted various studies on polystyrene-based expanded stretched films that have excellent shock resistance, shock-absorbing properties, and heat shrinkability, and have found that low-density polyethylene or/and After creating a raw rebound foam sheet from a composition containing specific amounts of ethylene-vinyl acetate copolymer, an organic decomposable blowing agent, and a metal salt of higher fatty acid, it is heated by stretching it three times or more in at least one direction. It has been found that a foamed stretched film with excellent shrinkability and cushioning effect can be obtained.However, in order to improve productivity, the foamed stretched film obtained from the above composition has a stretching temperature lower than that of crystalline propylene, which is the main component of the composition. -The closer the crystal melting point of the α-olefin copolymer is, the less opaque it becomes, which has the disadvantage that the contents appear translucent when packaged, which conversely deteriorates the product image. Furthermore, when heat sealing is applied, the foamed part melts due to the heat and pressure of the seal bar and returns to its non-foamed state, so only the sealed part becomes transparent, which can cause an unsightly appearance. had. As a result of further studies on these improvements, the present inventors added specific amounts of a polystyrene resin, an organic decomposable blowing agent, and a metal salt of a higher fatty acid to a specific crystalline propylene-α-olefin copolymer. A raw resilient foam sheet obtained by melt-extruding the composition under specific conditions is stretched in at least one direction so that the stretching ratio in one direction is at least 4 times the stretching ratio in the direction perpendicular to the stretching ratio. A heat-shrinkable composite film, which has a structure in which a specific non-foamed heat-shrinkable film is laminated on at least one side of a heat-shrinkable foamed film, has excellent heat-shrinkability, cushioning effect, and impact resistance, and has a low opacity. The present invention was achieved by discovering that it has a certain pearl-like luster and can eliminate the above-mentioned drawbacks. That is, one of the present inventions is a crystal melting point of 115 to 145
℃ crystalline propylene-α-olefin copolymer is 97-78% by weight, polystyrene resin is 3-20% by weight, organic decomposition type blowing agent is 0.1-1.0% by weight, and higher fatty acid metal salt is 0.03-1.0% by weight. The crystalline propylene-
A non-foamed heat-shrinkable thin layer separately obtained from an α-olefin copolymer that heat-shrinks more in one direction than in the perpendicular direction is laminated with the shrinking direction aligned to form the surface layer. The subject matter is a heat-shrinkable composite film characterized by the following. Another aspect of the present invention is a crystal melting point of 115 to 145.
℃ crystalline propylene-α-olefin copolymer is 97-98% by weight, polystyrene resin is 3-20% by weight, organic decomposition type blowing agent is 0.1-1.0% by weight, and higher fatty acid metal salt is 0.03-1.0% by weight. A composition consisting of A separately melt-extruded material is laminated on at least one side of the foamed unstretched base material layer to form a non-foamed unstretched surface layer, and the thus obtained unstretched composite sheet is made of the crystalline propylene-α-olefin copolymer. 10~60 than the crystal melting point of
At least one of the foamed unstretched base material layers is formed by stretching in at least one direction at least 4 times the stretching ratio in one direction at a low temperature of 4 times or more than the stretching ratio in the direction perpendicular thereto. On at least one side of the base material layer made of a heat-shrinkable foam layer that heat-shrinks in the direction of shrinkage, a surface layer made of a non-foamed heat-shrinkable thin layer that heat-shrinks in at least one direction is formed from the unstretched surface layer. The gist of the present invention is a method for producing a heat-shrinkable composite film, which is characterized in that it is formed into a composite structure in which the film is laminated in alignment with a base material layer. Each component and blending ratio of the composition used to form the base layer (hereinafter sometimes referred to as base layer composition) in the present invention will be explained. The crystalline propylene-α-olefin copolymer has a crystal melting point in the range of 115 to 145°C,
Propylene containing 70% by weight or more of propylene component and ethylene or/and α- having 4 to 8 carbon atoms
Copolymers with olefin are preferred, especially ethylene/propylene random copolymers and ethylene/propylene random copolymers.
The propylene-butene-1 terpolymer is the most preferred because it allows the foaming state of the base layer to be adjusted extremely finely and uniformly. Here, the crystal melting point (hereinafter sometimes abbreviated as Tm) is the temperature obtained by heating a sample at a rate of 10°C/min in a nitrogen atmosphere using a scanning differential calorimeter. is the peak temperature of the endothermic curve. If the Tm of the crystalline propylene-α-olefin copolymer exceeds 145° C., foaming becomes non-uniform and stretch breakage is likely to occur, and heat shrinkability is also insufficient. On the other hand, if Tm is less than 115°C, it is not preferable because it is too flexible and workability is significantly reduced, and the film tends to stick. Considering the above-mentioned physical properties and in addition to the productivity of heat-shrinkable composite films, among the crystalline propylene-α-olefin copolymers whose component composition and crystal melting point are within the above range, the ethylene component is 4 to 7% by weight. Ethylene/propylene random copolymer with a Tm of 125-140℃ in % and an ethylene component of 0.5-6
The butene-1 component is 1 to 15% by weight, and
Particularly preferred is an ethylene-propylene-butene-1 terpolymer having a Tm of 120 to 140°C. The heat-shrinkable composite film finally obtained from these copolymers has shrinkage characteristics particularly suitable for use in label packaging, and furthermore, the cost as a raw material resin is low. The melt flow rate (MFR) of the crystalline propylene-α-olefin copolymer is 0.1.
Extrusion/molding conditions (screw shape of extruder,
It is desirable to select according to the extrusion temperature, withdrawal speed, etc.). The crystalline propylene-α-olefin copolymer is the main component of the base layer and accounts for 97 to 78% by weight of the composition for the base layer, and if it is less than 78% by weight, the effects of the present invention will be insufficient. As the polystyrene resin, a styrene homopolymer or a copolymer with a styrene content of 70% or more and a softening point (ring and ball method) of 50°C or higher is used, especially polystyrene (including rubber blends),
Acrylonitrile/styrene copolymers and styrene/isobutylene copolymers are preferred. The blending ratio of polystyrene resin in the base layer composition is 3 to 3.
The content is 20% by weight, particularly preferably 5 to 15% by weight. By blending the polystyrene resin in the composition for the base layer, the base layer obtained therefrom, and hence the heat-shrinkable composite film according to the present invention, has a pearl-like luster with an opaque feel. Moreover, even if the composite film is heat-sealed, the opacity of the sealed portion can be maintained. If the above-mentioned blending ratio does not reach 3% by weight, the opacity will be insufficient, and if it exceeds 20% by weight, stretch breakage will easily occur, and furthermore, only the opacity will be emphasized and the pearl-like luster will be lost, which is not preferable. The organic decomposable blowing agent is solid at room temperature and has a decomposition temperature higher than the Tm of the crystalline propylene-α-olefin copolymer, and when heated above the decomposition temperature, it releases nitrogen, carbon dioxide gas, ammonia gas, etc. A compound that decomposes while generating is used.
Specific examples include azodicarbonamide, metal salts of azodicarbonamide, hydrazodicarbonamide, N,N'-dinitrosopentamethylenetetramine, and p-toluenesulfonylhydrazide. The proportion of these blowing agents in the base layer composition is 0.1 to 1.0% by weight, and 0.1% by weight.
If it does not reach 1.0% by weight, the amount of foaming will be small and the cushioning effect of the heat-shrinkable composite film will be inferior.
If it exceeds this, foaming will proceed too much and it will be difficult to form a foamed unstretched base material layer, and even if a foamed unstretched composite sheet is obtained by proceeding to the next step, stretching breaks will occur frequently and stretching will become difficult. This is not preferable because the resulting product will be of significantly poor quality.
As a blowing agent, use azodicarbonamide from 0.2 to
Most preferably, it is blended in an amount of 0.6% by weight. Examples of metal salts of higher fatty acids include stearic acid,
Higher fatty acids such as 12-hydroxystearic acid and periodic table a (sodium, lithium, etc.), a (calcium, magnesium, etc.), b (zinc, etc.), b
Examples include salts with metals (such as aluminum). Specific examples thereof include sodium stearate, lithium 12-hydroxystearate, calcium stearate, calcium 12-hydroxystearate, magnesium stearate, and the like. The blending ratio of the metal salt of higher fatty acid in the base layer composition is 0.03 to 1.0% by weight.
If the above-mentioned blending ratio does not reach 0.03% by weight, the dispersion of the blowing agent will be poor, resulting in non-uniform foaming, and if it exceeds 1.0% by weight, it will be extremely difficult to feed into the extruder. The composition for the base material layer contains an antioxidant, if necessary.
Ultraviolet absorbers, antistatic agents, slip agents, pigments, etc. may be added. In the present invention, the crystalline propylene-α-olefin copolymer used in the surface layer is copolymerized with the crystalline propylene-α-olefin copolymer, which is the main component of the base material layer, in order to improve the shrinkage characteristics as a heat-shrinkable composite film. By using the same polymer as the polymer, the basic unit can be increased by recycling it into the composition for the base material layer such as ears and slitter pieces. The crystalline propylene-α-olefin copolymer used for this surface layer may contain antioxidants, ultraviolet absorbers, antistatic agents, slip agents, lubricants, pigments, and the like, if necessary. The heat-shrinkable composite film (hereinafter sometimes simply referred to as composite film) according to the present invention is obtained by melt-extruding a base layer composition consisting of each of the components described above in one direction and in a direction perpendicular to this. On at least one side of the base layer made of a heat-shrinkable foam layer that heat-shrinks to a greater extent than A surface layer is formed by laminating heat-shrinkable thin layers of foam that shrink more in one direction than in the perpendicular direction to the same direction. This composite film has a surface layer formed on one or both sides of the base layer, and this surface layer may be subjected to surface treatment such as corona discharge treatment. The thickness of the base material layer is preferably 50 to 99%, particularly preferably 70 to 90%, of the total thickness of the composite film. Normally, if the total thickness of the composite film is 0.05 to 0.4 mm, the apparent specific gravity is 0.2 to 0.75, and the heat shrinkage rate in one direction at 120°C is 10% or more, it is sufficient in terms of heat shrinkability and cushioning effect. In addition, the surface layer not only sufficiently increases impact resistance due to its non-foamed and excellent strength and elongation, but also works to enhance the pearl-like luster of the base layer, as well as its good printability. This makes it possible to perform extremely beautiful printing on the surface of the composite film. Among the heat-shrinkable composite films according to the present invention, the following are particularly suitable for label packaging. That is, the total thickness is 0.10 to 0.25 mm, the apparent specific gravity is 0.3 to 0.7, and the heat shrinkage rate in one direction at 120°C is 20% or more, and the heat shrinkage rate in the direction perpendicular to this is 4 times or more. It is. If the thickness is 0.10 mm or more, the impact resistance for label packaging is sufficient, and the foam cells are uniform and the appearance is satisfactory for labels. If the thickness exceeds 0.25 mm, the time required for heat shrinkage will be too long, making it difficult to work with a large number of bottles, etc., and furthermore, the shape of the curved part after shrinkage will become uneven, which may impair the appearance. . The apparent specific gravity
If it is 0.3 or more, the foam cells will be fine and the pearl-like luster will be excellent and suitable for labels. If the apparent specific gravity exceeds 0.7, the cushioning effect will be somewhat insufficient for label packaging, and if two or more bottles are dropped in a row from a vending machine, they may collide with each other. There is a risk of the bottle breaking. Regarding heat shrinkage, due to the nature of label packaging for bottles, etc., it is necessary that the heat shrinkage in only one direction be particularly large.Therefore, if the heat shrinkage rate in one direction at 120℃ does not reach 20%, The amount of shrinkage is insufficient and the adhesion of the label to the bottle is incomplete. The heat shrinkage rate in one direction is 4
It is also important for label packaging to be more than twice as large. For example, if you create a film cylinder from a composite film, attach it to a bottle, and heat shrink it,
It is desirable to contract only in the cylindrical direction. If a large amount of shrinkage in the axial direction of the cylinder is added to the shrinkage in the circumferential direction, it will be difficult to keep the film surface clean.
The printed pattern becomes distorted, the printed characters become irregular, and the edges of the film become curved, which impairs the appearance. Therefore, a film cylinder may be created by aligning the direction in which the composite film has a thermal shrinkage rate of 20% or more with the circumferential direction, and the direction perpendicular thereto with the cylinder axis direction. In this way, for label packaging, it is necessary to have unbalanced heat shrinkability, which is a large heat shrinkage in one direction and a small heat shrinkage rate in a direction perpendicular to this direction, with virtually no dimensional change. As can be seen from the manufacturing method described below, the heat-shrinkable composite film according to the present invention generally has the shape of a long film, but the direction in which it shrinks more is the width direction of the shape of the long film. It is preferable for the composite film to be used for various purposes, and therefore, it is preferable that the base layer and the surface layer constituting the composite film also shrink more in the width direction. The heat-shrinkable composite film according to the present invention can be manufactured by the following method. That is, first, the above-described base layer composition is melted at a temperature equal to or higher than the decomposition temperature of the blowing agent contained in the composition and extruded. This melting/extrusion temperature is usually above, but close to, the decomposition temperature of the blowing agent. Since the extruded product has countless foam cells, it is rapidly cooled to below 50°C to form a foamed unstretched base material layer, and
The same copolymer as the propylene-α-olefin copolymer contained as the main component in the base layer composition (hereinafter sometimes referred to as the surface layer propylene copolymer) is separately melted and extruded. An unstretched composite sheet is obtained by laminating the foamed unstretched base material layer on at least one side of the foamed unstretched base material layer to form an unstretched surface layer. The method for laminating the foamed unstretched base material layer and the unfoamed unstretched surface layer as described above includes the following embodiments. One method is to melt and extrude the base material composition alone as described above, rapidly cool it to obtain a raw resilient foam sheet, and then extrude this raw resilient foam sheet as a foamed unstretched base material layer on at least one side of the base material. In this method, a propylene copolymer for the surface layer is melted and extruded by a lamination method, and then cooled to form an unstretched surface layer. The other method is to use a coextrusion method to melt and extrude the composition for the base layer and the propylene copolymer for the surface layer using separate extruders, while keeping them in a molten state in a T-die kept at an appropriate temperature. This is a method in which the materials are laminated into a predetermined configuration and then rapidly cooled. In this step, conditions are adjusted so that the foamed unstretched base material layer of the resulting unstretched composite sheet has an apparent specific gravity of 0.3 to 0.8 and a thickness of 0.15 mm or more. The unstretched composite sheet thus obtained is stretched at a stretching ratio in one direction at a temperature 10 to 60°C lower than the crystalline melting point Tm of the crystalline propylene-α-olefin copolymer, which is the main component of the unstretched composite sheet. Stretch at least 4 times in one direction so that the stretching ratio is 4 times or more than the stretching ratio in the direction perpendicular to the . In the case of uniaxial stretching, the stretching ratio in one direction is 4 to 10 times the stretching ratio in the vertical or horizontal direction, and in the case of biaxial stretching, the stretching ratio in one direction is at least 4 times the stretching ratio in the direction perpendicular to the stretching ratio. It is preferable to perform the stretching simultaneously or sequentially so that the area magnification becomes 6 to 30 times. In the case of uniaxial stretching, preferred stretching conditions are propylene-α-
It is stretched 4 to 8 times at a temperature 20 to 60°C lower than the Tm of the olefin copolymer. In addition, in the case of biaxial stretching, the Tm of the propylene-α-olefin copolymer is 30~
After stretching 1.05 to 2.0 times on or between two or more heating rolls kept at a temperature lower than 60°C, it is stretched in the transverse direction in a tenter at a temperature slightly higher than the stretching temperature so that the horizontal/vertical stretching ratio is 4 or more. It is preferable to use a sequential stretching method in which the film is stretched 6 to 10 times. The stretched composite film is heat-treated in a normal manner under tension or in a slightly relaxed state as required, and then cooled by air cooling, a cooling roll, a cooling belt, etc. to obtain a heat-shrinkable composite sheet according to the present invention. . After cooling in the above step, surface treatment such as corona discharge treatment may be performed in the air or in an inert gas. In the stretching process, although the main component of the unstretched composite sheet is the foamed unstretched base material layer, which is easily broken by stretching, it is the foamed unstretched base that enables stretching at such a high stretching ratio as described above. This is because an unstretched surface layer of a propylene-α-olefin copolymer having excellent strength and elongation is laminated on at least one side of the material layer, and this allows a wide range of stretching conditions for the unstretched composite sheet. , high-speed stretching can also be performed stably. Further, the heat shrinkage directions of the base layer and the surface layer are completely the same, and no non-uniform shrinkage occurs on the surface after heat shrinkage due to differences in the shrinkage direction of each layer. Incidentally, as the stretching method, a commonly practiced method can be used, but the following should be taken into consideration. With the tubular method, it is generally difficult to make the foam cells uniform and adjust the thickness when foaming polypropylene resin. In addition, in the vertical (longitudinal) uniaxial stretching of the T-die method, the heat shrinkage rate in the longitudinal direction is large, so the heat-shrinkable composite film tends to shrink at room temperature after core winding, resulting in poor rolled appearance of the heat-shrinkable composite film. In some cases, when a composite film is heat-shrinked, it may not shrink in the lateral (width) direction but instead elongate, and the breaking strength in the lateral (width) direction is small, making it easy to tear.
It is difficult to use the longitudinal direction of a composite film as it is for shrink wrapping, with the longitudinal direction of the container being the same as the longitudinal direction of the container.
Furthermore, when widening the die is carried out as a method of increasing productivity, it is difficult to control uniform foaming in the width direction of the foamed fabric due to the widening of the die. On the other hand, the T-die stretching method, which mainly involves horizontal (width direction) stretching with no or only slight stretching in the vertical (length direction), does not have the above-mentioned difficulties, and the product composite film is It is particularly suitable for use in shrink wrapping, and furthermore, the above-mentioned adjustment of the foamed material is easy, and it is an extremely preferred stretching method that can efficiently produce a wide heat-shrinkable composite film with good winding appearance. As mentioned above, the heat-shrinkable composite film according to the present invention is
(1) Due to the use of a specific composition in the base material layer, the foamed state is extremely fine and uniform, and has excellent cushioning properties; (2) It is opaque due to the combination of polystyrene resin in the base material layer. In addition to obtaining a pearl-like luster, (3) it was possible to prevent the sealed area from becoming transparent due to heat sealing, and (4) the non-foaming heat shrinkability of the crystalline propylene-α-olefin copolymer was achieved. By providing the surface layer, the extremely high gloss of the surface layer reduces the pearl-like luster of the base material layer and enables extremely clear printing; (5) the surface layer allows for high elongation; (6) It is extremely useful not only for label packaging but also for tapes, decorative packaging, buffer packaging, etc.
Furthermore, in the method for producing a heat-shrinkable composite film according to the present invention, the heat-shrinkable composite film obtained has the above-mentioned effects, and (7) a foamed unstretched base material layer and a non-foamed By laminating the unstretched surface layer and stretching the resulting unstretched composite sheet,
It has the following effects: it reduces stretch breakage and enables stable production at high draw ratios and high speeds. The present invention will be explained in more detail below using Examples and Comparative Examples, but the present invention is not limited to the following Examples. In addition, the characteristic values in the following examples and comparative examples were measured by the following method. (i) Melt fluorate (MFR): JIS K7210-1976 polypropylene and propylene-α-olerefin copolymer under conditions 14 (230°C, 2.16 kgf),
Condition 4 for polyethylene and ethylene-vinyl acetate copolymer (190°C, 2.16 kgf). (ii) Apparent specific gravity: JIS K7112-1977 Method B (vicnometer method) Distilled water is used as the immersion liquid. (iii) 120℃ shrinkage rate: A film cut into strips with a length of 10cm and a width of 1cm is immersed in a silicone oil bath at 120℃ for 30 seconds and taken out, and the length shrinkage rate (%) is determined in each direction. It was expressed as (iv) Total light transmittance: JIS K-6714 (v) Printability: Printed in a checkerboard pattern using a gravure printing machine using a commercially available printing ink for polypropylene (Polypro Color manufactured by Toyo Ink Co., Ltd.), and the printing condition Observe the image and see if it is clear with no shading or bleeding, △ if there is no shading but the shading is slightly off, or if there is no shading but there is some smearing and is unclear.
Prints with clear shading, ink bleeding, and unclear grid patterns were marked as ×. (vi) Foaming condition: Visually observe the presence and uniformity of foaming in the width and length directions of the target film. If there is no foaming unevenness and the foam cells are fine and uniform,
△: Slightly uneven foaming and rough foam cells;
A case where the foaming was uneven and the foaming cells were coarse and non-uniform was indicated as "×". Example 1 Ethylene content is 4.0% by weight, butene-1 content is
5.2% by weight, ethylene-propylene-butene-1 terpolymer with Tm of 130°C and MFR of 11.5 (contains 0.15% by weight of 2,6-di-t-butyl-p-cresol as an oxidation inhibitor) 89.4 Polystyrene (trade name Styron 679, manufactured by Asahi Dow Co., Ltd.) 10% by weight, azodicarbonamide (decomposition temperature 190℃) 0.3
% by weight, 0.3% by weight of calcium stearate, mixed with a Henschel mixer, and the resin temperature
It was melt-extruded into a strand shape at 165°C, cooled and cut to obtain an unfoamed base layer composition. This base material layer composition is passed through an extruder and a T-die to 195
It was melt-extruded at 30°C and rapidly cooled with a cooling roll kept at 30°C to obtain a raw rebound foam sheet with a thickness of 1.0 mm and an apparent specific gravity of 0.73. This raw rebound foam sheet was used as a foamed unstretched base material layer, and one side thereof was coated with ethylene propylene butene-1, which was the same as that used in the composition for the base material layer.
The terpolymer was melt-extruded at 220°C and laminated as an unstretched surface layer with a thickness of 0.15 mm on the above-mentioned resilient foam sheet between a metal roll and a silicone rubber roll kept at 30°C to form an unstretched composite sheet. After that, 110℃
After stretching 8 times in the transverse direction in a tenter and heat-treating under tension, the unfoamed surface layer was further subjected to corona discharge treatment to obtain a uniaxially stretched heat-shrinkable composite film according to the present invention. The stretching of the above-mentioned unstretched composite sheet is extremely stable with no stretching breakage.
The thickness was also uniform and better than that obtained by stretching the original resilient foam sheet alone. The resulting heat-shrinkable composite film has a thickness of 0.13 mm, an apparent specific gravity of 0.65, and a temperature of 120°C.
The heat shrinkage rate was 3% in the vertical direction and 42% in the horizontal direction, the foaming state was good, and it had an opaque pearl-like luster. In addition, when a grid pattern was printed on the surface layer of this composite film, the printing was dark and clear. This printed film was sealed with an impulse sealer into a cylindrical shape with a height of 110 mm and a diameter of 75 mm with the printed side facing outward and the horizontal direction facing the circumferential direction, and then placed over a paper cuff with a height of 120 mm and a diameter of 70 mm. ,
Heated at 220°C for 7 seconds. As a result, the cylindrical direction of the film is in perfect contact with the strip, there is no shrinkage in the height direction, there is no distortion in the printed grid, and there is an opacity that can completely shield the strip. A beautiful close-fitting package with a certain pearl-like luster was obtained. Furthermore, the portion sealed with impulse sealer also had sufficient opacity. Examples 2 to 8, Comparative Examples 1 to 9 Seven types of compositions shown in Table 1 were prepared as base layer compositions. In addition, as a propylene copolymer for the surface layer, the same ethylene content as the propylene-α-olefin copolymer contained in the base layer composition is 3.5% by weight, butene-1 content is 4.5% by weight,
An ethylene-propylene-butene-1 terpolymer with a Tm of 132°C and an MFR of 10.0 was used, and
-di-t-butyl-p-cresol 0.2% by weight,
A composition for the surface layer was prepared by blending 0.05% by weight of calcium stearate and 0.1% by weight of erucic acid amide. The above compositions for the base layer, the surface layer, and the surface layer were individually melt-extruded using two extruders, laminated in a molten state in a T-die kept at 190°C, and heated to 20°C. The unstretched three-layer sandwich-like composite consisting of a foamed unstretched surface layer in the center and unfoamed unstretched surface layers laminated on both sides of the foamed unstretched surface layer is then passed through two cooling rolls held together and quenched while being pressed. A sheet (1.5 mm thick) was obtained. This composite sheet was stretched in the vertical direction at a stretching temperature of 100℃ using a small pantograph-type stretching device (manufactured by TMLong (USA)).
After stretching 1.5 times, the film was stretched 7.0 times in the horizontal direction at the same temperature to obtain seven types of biaxially stretched heat-shrinkable composite films of the present invention having a thickness of 0.15 mm (base layer thickness 0.12 mm). Ta. In addition, for comparison, a composite film was biaxially stretched in the same manner as in the above example except that the composition shown in the column of composition for base layer in Table 1 was used as a composition corresponding to the composition for base layer. I got it. Table 1 shows the foaming state, apparent specific gravity, total light transmittance, and opacity of the sealed portion of these composite films.
The opacity of the sealed part was determined by sealing each sample with a 5 mm wide impulse sealer, then overlapping the sealed part on paper with printed letters, and checking that the letters under the sealed part were not clearly visible. , Those where the characters are unclear but can be distinguished are indicated as △, and those which are transparent and can be clearly distinguished are indicated as ×. As is clear from Table 1, the composite film according to the present invention showed a good foaming state, had an excellent shielding effect, and did not exhibit any transparent phenomenon due to heat sealing.

【table】

[Table] Examples 9 to 11, Comparative Examples 10 to 14 Crystalline propylene-α-olefin copolymer shown in Table 2 (2,6-di-t-
(contains 0.15% by weight of butyl-p-cresol) and styrene-butadiene copolymer rubber blend polystyrene (trade name "Styron 475" manufactured by Asahi Dow Co., Ltd.) 7
% by weight, 0.25% by weight of azodicarbonamide, and 0.3% by weight of calcium stearate to obtain three types of base layer compositions. This composition was melt-extruded through an extruder and a T-die at a resin temperature of 195°C, and then placed on a cooling roll kept at 25°C.
Rapidly cool while crimping using an air knife to increase the thickness.
Three types of raw rebound foam sheets of 0.85 mm were obtained. Each raw rebound foam sheet was used as a foamed unstretched base material layer, and one side of the base material layer was coated with the same crystalline propylene-α-olefin copolymer (also containing an antioxidant) as used in the composition for each base material layer. The sheets were melt-extruded and laminated on the raw rebound foam sheet between a metal roll and a silicone rubber roll kept at 40°C to form an unstretched surface layer to obtain three types of unstretched composite sheets (total thickness 0.90 mm). Ta. Each composite sheet was stretched using a small pantograph type stretching machine (TM Long) at the stretching temperature shown in Table 2.
Stretched 7.2 times in the horizontal direction using
Uniaxially stretched heat-shrinkable composite films of the present invention having a thickness of 0.13 mm were obtained (Examples 9 to 11). For comparison, the above three types of raw foam sheets (Comparative Examples 12-
14), and in Examples 9 to 11, polypropylene (Comparative Example 10) and ethylene-propylene random copolymer (Comparative Example 11) (both containing antioxidants as above) shown in Table 1 were used as propylene. - A total of five types of raw foam sheets, including a raw foam sheet with a thickness of 0.85 mm obtained by using the α-olefin copolymer in place of the α-olefin copolymer, were subjected to the same stretching device as above without laminating an unstretched surface layer. The film was stretched 7.2 times in the transverse direction at the stretching temperature shown in Table 2 to obtain a 0.12 mm thick uniaxially stretched foamed film (Comparative Examples 10 to 14). Table 2 shows the stretchability, apparent specific gravity, foaming state, printability, and heat shrinkage rate at 120°C of each sample at this time. Stretchability is expressed by the number of times of stretching breakage (unit: times) when stretching is repeated 10 times under the same stretching conditions.

[Table] As is clear from Table 2, the heat-shrinkable composite film according to the present invention has a wider stretching temperature range, has finer and more uniform foaming, and has better printing properties than a film with only a base layer without a surface layer. It was observed that the opaque pearl-like luster, which is a feature of the present invention, was clearly enhanced. Examples 12 to 15, Comparative Examples 15 to 18 Ethylene content is 4.0% by weight, butene-1 content is
Polystyrene (Asahi Dow product name “Styron 679”) 7% by weight, azodicarbonamide 0.25% by weight, and calcium stearate
0.25% by weight to obtain a base layer composition. This composition was melt-extruded at 185°C through an extruder and a T-die, and rapidly cooled on a cooling roll kept at 20°C to obtain an original rebound foam sheet with a thickness of 0.85 mm and an apparent specific gravity of 0.69. On one side of this sheet, the same ethylene-propylene-butene terpolymer used in the base layer composition was melt-extruded and laminated to a thickness of 0.10 mm to form an unstretched composite sheet. Then, using a small pantograph type stretching machine (manufactured by TMLong (USA)), stretching was carried out at a stretching temperature of 100°C in each direction, changing the stretching ratio in the vertical and horizontal directions as shown in Table 3, and stretching at 120°C. Various composite films with different vertical/horizontal heat shrinkage ratios were prepared. As in Example 1, these composite films were formed into a cylindrical shape,
It was placed on the same paper tube and heated at 220°C for 7 seconds. Table 3 shows the shrinkage state of the composite film at this time and the presence or absence of distortion in the printed grid.

【table】

Claims (1)

[Claims] 1. Crystalline propylene-α with a crystal melting point of 115 to 145°C
-Contains 97-78% by weight of olefin copolymer, 3-20% by weight of polystyrene resin, and organic decomposable blowing agent.
0.1-1.0% by weight and metal salts of higher fatty acids 0.03-1.0
% by weight, the crystalline propylene-α-olefin copolymer is applied to at least one side of a base layer consisting of a heat-shrinkable foam layer that heat-shrinks more in one direction than in the perpendicular direction. A surface layer is formed by laminating non-foamed heat-shrinkable thin layers obtained separately from the coalescence, which heat-shrink more in one direction than in the perpendicular direction, with their shrinking directions matching. Heat-shrinkable composite film. 2. The heat-shrinkable composite film according to claim 1, wherein the crystalline propylene-α-olefin copolymer is an ethylene-propylene random copolymer containing 70% by weight or more of a propylene component. 3. The heat-shrinkable composite film according to claim 1, wherein the crystalline propylene-α-olefin copolymer is an ethylene-propylene-butene-1 terpolymer containing 70% by weight or more of a propylene component. . 4 The total thickness of the heat-shrinkable composite film is 0.05 to 0.4
mm, apparent specific gravity is 0.2 to 0.75, and the heat shrinkage rate in one direction at 120°C is 10% or more and is 4 times or more the heat shrinkage rate in the direction perpendicular to this, Claims 1 to 3 The heat-shrinkable composite film described in any of the above. 5. Claim 1, wherein the thickness of the heat-shrinkable foam layer is 50 to 99% of the total thickness of the heat-shrinkable composite film.
The heat-shrinkable composite film according to any one of Items 1 to 4. 6. According to any one of claims 1 to 5, the heat-shrinkable composite film is a long film, and the base layer and the surface layer are heat-shrinkable in the width direction than in the length direction. The heat-shrinkable composite film described. 7 Crystalline propylene-α with a crystalline melting point of 115 to 145°C
-Contains 97-78% by weight of olefin copolymer, 3-20% by weight of polystyrene resin, and organic decomposable blowing agent.
0.1-1.0% by weight and metal salts of higher fatty acids 0.03-1.0
% by weight is melt-extruded at a temperature above the decomposition temperature of the blowing agent and rapidly cooled to below 50°C to form a foamed unstretched base material layer, and the crystalline propylene-α-olefin copolymer is A separately melt-extruded material is laminated on at least one side of the foamed unstretched base material layer to form a non-foamed unstretched surface layer, and the thus obtained unstretched composite sheet is made of the crystalline propylene-α-olefin copolymer. The above-mentioned foamed unstretched base material is stretched at least 4 times or more in one direction at a temperature 10 to 60°C lower than the crystal melting point of A non-foamed, heat-shrinkable thin layer formed from the unstretched surface layer, which is heat-shrinkable in at least one direction, on at least one side of the base layer, which is a heat-shrinkable foam layer that is heat-shrinkable in at least one direction. 1. A method for producing a heat-shrinkable composite film, comprising forming a composite structure in which a surface layer consisting of: 8. The method for producing a heat-shrinkable composite film according to claim 7, wherein the unstretched composite sheet is stretched in a direction perpendicular to the longitudinal stretching ratio at least 4 times the lengthwise stretching ratio at which the unstretched composite sheet was melt-extruded.