Self-tacky packaging film

Abstract: A self-tacky packaging film which comprises a multi-layer film comprising: (A) two outermost, surface layers comprising a polyethylene type resin and (B) at least one core layer, between the layers (A), comprising a polypropylene type resin having a melting point of 140°C or more, at least one side of the multi-layer film being irradiated with an ionizing radiation, is excellent in tackiness, a microwave oven suitability and "saw edge"-cutting characteristics and has an appropriate flexibility.

Description: The present invention relates to a self-tacky packaging film comprising a polypropylene type resin and a polyethylene type resin, and more particularly to a self-tacky packaging film having improved tackiness, microwave oven suitability, cutting characteristics and flexibility. This kind of self-tacky packaging film is largely used in families, restaurants and hotels for packaging foodstuffs. It prevents the evaporation of water from foodstuffs during storage in a refrigerator and during heating in a microwave oven. It also prevents the diffusion of flavours and contamination with other odours during storage, as well as keeping products dust-free during business use. In particular, the popularization of microwave ovens in recent years has promoted demand for these kinds of packaging films. Thus, such films are used for tightly packaging food containers, usually made of ceramics or glass, to prevent water evaporation and loss of taste in a microwave oven. The packaging films used for such purposes are required to be tightly adhesive to the containers and simultaneously to exhibit good tackiness towards each other. Further, the packaging films must have minimal thermal shrinkage and must exhibit neither thermal fusion nor whitening in a microwave oven (hereinafter these properties are referred to as "microwave oven suitability"). As conventional self-tacky packaging films, those using polyvinylidene chloride, polyethylene or polyvinyl chloride as a main starting material are known. However, films made from polyvinylidene chloride are disadvantageous in that they show a great shrinkage when heated and are more expensive than the other two types of films. On the other hand, the films prepared from polyethylene have the shortcoming that perforations are readily formed when the films are brought to a high temperature in contact with an oily or fatty material such as meats, fried foods, etc. The films prepared from polyvinyl chloride have the disadvantage that they show a whitening phenomenon when brought into contact with boiling water. Accordingly, none of these conventional self-tacky films can be said to possess satisfactory microwave oven suitability. Many of the conventional self-tacky packaging films are furnished to users in the form of a film having a width of 20 to 45 cm and a thickness of 10 to 20 µm rolled on a core material made of paper or the like and placed in a case made of paper or the like. The film is withdrawn from the case to a desired length and cut by means of a blade called a "saw blade (edge)" fixed on the case, after which the cut-out film is used for sealing the opening of a food container or the cut section of a food, by utilizing the self-tacky property of the film itself. As the saw edge used for cutting the film, a simple edge prepared by punching out an iron plate having a thickness of about 0.2 mm into the shape of a saw is generally used. The case supporting the saw edge is usually a coated cardboard box having a base weight of about 350-700 g/m 2 , which is very low in stiffness. It needs to be possible to cut the film with such a simple cutting mechanism without difficulty. In practice, however, the cutting sometimes take place at a position remote from the case or the saw edge. Films made of polyvinylidene chloride, polyvinyl chloride or polyethylene as starting material have hitherto been used. In the case of polyvinylidene chloride film, if a tear appears somewhere in the film at the time of cutting, the tear will extend so as to cut the films not along the saw edge as desired but on a slant. Polyvinyl chloride film and polyethylene film both tend to elongate at the time of cutting, which deteriorates the sharpness of the cutting. Although the films mainly made of polyvinylidene chloride or polyvinyl chloride have appropriate flexibility (in this specification, flexibility is expressed in terms of Young's modulus) and therefore they are successfully used as domestic wrapping films, these films contain a large quantity of chloride, due to which they produce chlorine gas upon incineration. Further, the large quantity of plasticizer present in them is harmful. For these reasons, these films do not possess satisfactory environmental suitability. Thus, as a self-tacky packaging film made mainly of polypropylene, JP-B-58-46216 has proposed a film obtained by compounding polypropylene with a specified quantity of polyethylene, JP-B-63-63578 has proposed a film compounded with a specified quantity of a nucleating agent together with atactic polypropylene, rosin or the like, JP-A-4-328144 has proposed a biaxially stretched film obtained by compounding a polypropylene type copolymer with a specified quantity of a surfactant, and JP-B-2-14935 has proposed a method for producing a self-tacky packaging film by stretching a resin composition containing a tackifier. Further, JP-A-6-32952 has proposed a film composed mainly of 4-methylpentene-1. However, no self-tacky packaging film has yet been obtained which simultaneously satisfies tackiness, microwave oven suitability, satisfactory "saw edge"-cutting characteristics and has appropriate flexibility. EP-A-0049609 describes a fusion-bondable sheet which comprises an irradiated crosslinked polyolefin layer and a fusion-bondable layer laminated on a flame-retardant sheet such as aluminium. The sheet is used as an overtape to cover joint portions of a heat shrinkable cover sheet applied to a steel pipe or cable joint. It is not said to be applicable to the field of self-tacky packaging films. The present invention addresses these problems in the art. Accordingly, the present invention provides a self-tacky packaging film which comprises a multi-layer film comprising: (A) two outermost, surface layers comprising a polyethylene type resin, and (B) at least one core layer, between the layers (A), comprising a polypropylene type resin having a melting point of 140°C or more, at least one side of the multi-layer film being irradiated with an ionizing radiation. As the film or sheet which can be obtained by an irradiation of an ionizing radiation, JP-B-1-24628 has proposed a film improved in heat-sealability which can be obtained by irradiating with electron beams a film prepared by laminating ethylene/vinyl acetate copolymer on a substrate film, JP-A-5-84826 has proposed a heat-shrinkable film obtained by irradiating with ionizing radiation a sheet or a tube made of a polyethylene type resin composition, followed by a stretching treatment, JP-A-5-254074 has proposed a heat-shrinkable film obtained by irradiating with electron beams a multi-layer film made of polyethylene type resin, and JP-A-5-286087 has proposed a film or a tube including a radiation-degradational resin layer obtained by irradiating with electron beams a film containing a radiation energy-absorber in a radiation-degradational resin layer. However, a self-tacky packaging film obtained by irradiation with ionizing radiation has never been proposed so far. The present invention further provides a process for producing a self-tacky packaging film of the invention as defined above, which process comprises: (a) making a polyethylene type resin and a polypropylene type resin having a melting point of 140°C or more into a multi-layer film comprising (A) two outermost, surface layers comprising the polyethylene type resin, and (B) at least one core layer, between the layers (A), comprising the polypropylene type resin and (b) irradiating an ionizing radiation to at least one side of the surface layers (A). A further aspect of the present invention is the use of a self-tacky packaging film of the invention as defined above for tightly packaging food containers. By irradiating the multi-layer film with ionizing radiation, the polyethylene type resin constituting the two outermost, surface layers undergoes a crosslinking reaction in the hitherto known manner. As a result of this the apparent heat resistance improves and good microwave oven suitability is obtained. The surface layers show tackiness at the stage of film making. According to a study of the present inventors, this tackiness is not deteriorated by the irradiation of an ionizing radiation, but rather improved by the irradiation. On the other hand, the polypropylene type resin having a melting point of 140°C or more located between the layers (A) tends to be degraded and embrittled by irradiation with ionizing radiation. Although this property may generally be taken as an undesirable tendency deteriorating the film properties, the present inventors have found that this property is useful for imparting a "saw-edge"-cutting property to the film without deteriorating the heat resistance which is inherent to a polypropylene type resin. Further, it is possible to realize an appropriate flexibility through a selection of the thickness ratio between layers. The polyethylene type resin which can be used in the present invention includes, for example, low-density polyethylene, medium-density polyethylene and high-density polyethylene; and further includes copolymers of ethylene and α-olefins having from 3, preferably from 4 to 10 carbon atoms such as ethylene-propylene copolymer, ethylene-butene-1 copolymer, ethylene-4-methylpentene-1 copolymer, ethylene-hexene-1 copolymer, ethylene-octene-1 copolymer, ethylene-decene-1 copolymer and the like; and ethylene copolymers containing other unsaturated comonomer components such as conjugated dienes, non-conjugated dienes and the like. Acid-modified derivatives of these polymers are also included, of which examples are the polymers graft-modified with α,β-unsaturated carboxylic acids, alicyclic carboxylic acids or derivatives thereof. When a polyethylene, namely a homopolymer of ethylene, is used as said polyethylene type resin, said polyethylene type resin is preferably low-density polyethylene having a density of 0.935 g/cm 3 or less and a melt flow rate of from 0.1 to 50 g/10 minutes from the viewpoint of obtaining an appropriate tackiness. The density of the low-density polyethylene more preferably falls within the range of from 0.910 to 0.935 g/cm 3 and most preferably falls within the range of from 0.920 to 0.930 g/cm 3 . When a copolymer of ethylene and an α-olefin having from 4 to 10 carbon atoms is used as said polyethylene type resin, those having a density of from 0.88 to 0.94 g/cm 3 and a melt flow rate of from 0.1 to 50 g/10 minutes are preferable from the viewpoint of obtaining an appropriate tackiness. As for the α-olefin content, those having an α-olefin content of from 1 to 30% by weight are preferable. The kind of polypropylene type resin is not limited, provided is has a melting point of 140°C or more. Polypropylene type resins of which the melting point is lower than 140°C are not suitable for the purpose of the present invention, because such polypropylene type resins give a film having insufficient heat resistance. The polypropylene type resin which can be used in the present invention includes not only propylene homopolymer, but also a copolymer of propylene and one or more kinds of α-olefin other than propylene such as ethylene, butene-1, hexene-1 and the like. Said copolymer may be a random copolymer or a block copolymer. Further, the polypropylene type resin used in the present invention may be a single resin or a mixture thereof. In producing the film from the polypropylene type resin and polyethylene type resin in the present invention, conventionally employed additives may be incorporated into the film. For instance, an antioxidant, a stabilizer, an antistatic agent, an antifogging agent, and the like may be incorporated as desired. According to the present invention, the multi-layer film is produced by making the polyethylene type resin and the polypropylene type resin into films by the method of co-extrusion T-die method, co-extrusion inflation method or the like. The films thus obtained may be stretched, if desired. The irradiation of the ionizing radiation may be carried out either in the process of production in an "in-line" manner or in other process in an "off-line" manner. As the ionizing radiation, α-rays, β-rays, γ-rays and electron beams are suitable examples. Of these forms of radiation, electron beam radiation is preferable from the viewpoint of controllability and workability of the apparatus. Usually, the ionizing radiation is irradiated to the multi-layer film at an exposed (exposure) dose of from about 1 to about 30 Mrad and preferably from about 3 to about 20 Mrad, and at an accelerating voltage of from 100 to 300 kV and preferably from 150 to 200 kV. The radiation is usually irradiated once at a selected (an intended) exposed dose. The radiation may be irradiated two or more times to achieve a total selected exposed dose. By irradiating only one side of the multi-layer film, effects which result from irradiation are attained also in the entire film towards the opposite side of the multi-layer film in view of the property of ionizing radiation of being transmitted through an object. Accordingly, the radiation may be irradiated either to one side or to both sides of the multi-layer film so as to be irradiated at a selected exposed dose. The overall thickness of the film of the present invention is typically 50 µm or less and preferably in the range of from 8 to 15 µm. The thickness of the polypropylene type resin layer alone is preferably in the range of from 10 to 90% based on the overall thickness of the film. Layers in the film of the present invention are constituted as follows. Fundamentally, the film of the present invention is constituted from three layers, namely polyethylene type resin layer/polypropylene type resin layer/polyethylene type resin layer. If necessary, however, a layer of other composition such as a layer of a regenerated resin, a layer of a gas barrier resin or the like may be additionally provided between the two outermost, surface layers of the polyethylene type resin to form a constitution of four or more layers. The present invention will be further explained in the following examples. EXAMPLES The methods for measurements of properties mentioned in the examples were as follows. (1) Differential scanning calorimetric (DSC) measurement Melting point of polypropylene type resin was measured with DSC-7 type equipment manufactured by Perkin Elmer Co. A sheet having a thickness of about 0.5 mm was prepared from a polypropylene type resin by means of hot press, from which a test piece weighing about 10 mg was cut out. The test piece was placed in a sample pan for DSC measurement. After elevating the temperature to 230°C, the temperature was lowered to 150°C. Then, the temperature was lowered from 150°C to 40°C at a cooling rate of 5°C/minute. After maintaining the test piece at 40°C for 5 minutes, it was heated up to 180°C at a heating rate of 5°C/minute to obtain a thermogram. The main peak temperature in the thermogram at the time of the temperature elevation was taken as a melting point. (2) Density Density of polyethylene type resin was measured according to the method prescribed in JIS K6760. After annealing a sample in water at 100°C for one hour, density of the sample was measured. (3) Melt flow rate (MFR) MFR of polyethylene type resin was measured according to the method prescribed in JIS K6760. MFR of polypropylene type resin was measured according to the method prescribed in JIS K6758. (4) Comonomer content in copolymer The content of comonomer in ethylene-butene-1 copolymer was determined by measuring the absorbance at 770 cm -1 by means of an infrared spectrophotometer and referring to calibration curve. (5) A microwave oven suitability According to the Regulations Governing the Organization of Tokyo Metropolis No. 1072 "Quality indication of wrap films", a long strip film sample having a width of 3 cm and a length of 14 cm was grasped with jigs at the positions of 2.5 cm distant from the upper and lower ends, and a weight of 10 g was hung down from the lower end. The highest atmospheric temperature, at which the film kept unbroken even after a lapse for one hour under the above-mentioned condition, was measured, and expressed in ten degrees. A higher value of this temperature means a better microwave oven suitability. (6) Adhesive power A test piece film having a width of 10 cm and a length of 15 cm was tightly contacted with a 10 cm × 10 cm glass plate, and a load of 500 g was applied to the film at 23°C, 50% RH (relative humidity) for 30 minutes. Then, one longitudinal end of the film was tightly grasped with a jig and pulled up perpendicularly from the glass plate at a rate of 200 mm/minute. The force required for the pulling up was measured and expressed in terms of g/10 cm, and taken as adhesive power. (7) Young's modulus According to the method prescribed in ASTM D882, both MD (machine direction) and TD (transverse direction) were measured. A smaller value of them means a higher flexibility, provided that: shape of test piece: a long strip of 20 mm × 120 mm chuck distance: 50 mm pulling speed: 5 mm/minute. (8) Elongation at break Measured according to JIS K6781. (9) Thermal shrinkage factor After revealing MD and TD of a film, the film was cut into a size of 9 cm square. The film was dipped in a silicone oil bath kept at 140°C for 5 seconds and withdrawn, after which dimension of the film was measured. The percent shrinkage of the film was taken as thermal shrinkage factor (%). (10) Haze Measured according to ASTM D1003. A smaller value of haze means a higher transparency. (11) Saw edge-cutting property A test film was rolled on a core tube, introduced into a saw-edged case prepared by attaching a saw edge (an iron plate having a thickness of 0.2 mm punched into a form of saw) to a case made of a coated cardboard having a basis weight of 500 g/m 2 , and tried to cut with the saw edge. A sample which could be cut straightly by merely contacting the sample with the saw edge and lightly pulling it was evaluated as "good" in cutting property; while a sample which was cut either slantingly or a sample with a great elongation of film, with deformation of the edge, or with a great cutting force was evaluated as "not good" in cutting property. Example 1 As a resin (B) for constituting the polypropylene type resin layer, polypropylene (MFR = 3.5 g/10 minutes; melting point = 161°C; Noblen WF452 manufactured by Sumitomo Chemical Industry Co., Ltd.) was used. As polyethylene type resin (A) constituting the two outermost, surface layers, ethylene-butene-1 copolymer (MFR = 2.0 g/10 minutes; density = 0.905 g/cm 3 ; butene-1 content = 9% by weight; Esprene SPO N0352 manufactured by Sumitomo Chemical Industry Co., Ltd.) was used. According to the co-extrusion T die method, these resins were formed into a multi-layer film having a layer constitution of (A)/(B)/(A) and a thickness constitution of 2 µm/8 µm/2 µm. The extruder used for making the intermediate layer was a  90 mm extruder, and that used for making both sides of the surface layers was a  65 mm extruder, both manufactured by Mitsubishi Heavy Industries, Ltd. The die temperature was 260°C, the chill roll temperature was 20°C, and the working speed was 100 m/minute. Then, electron beam was irradiated, only once, to one side of the film thus obtained, by means of an electron beam irradiating apparatus (Model CB200/145/300, manufactured by Iwasaki Electric Co., Ltd.) under the conditions of an accelerating voltage of 150 kV and an exposed dose of 20 Mrad. The inner atmosphere of the irradiating apparatus was nitrogen gas. The results of evaluation are shown in Table 1. Example 2 A multi-layer film was prepared in the same manner as in Example 1, except that thickness constitution of the multi-layer film was 1.5 µm/9 µm/1.5 µm. Then, electron beam was irradiated to the film thus obtained, in the same manner as in Example 1. The results of evaluation are shown in Table 1. Comparative Example 1 A multi-layer film was prepared in the same manner as in Example 1, except that electron beam was not irradiated. The results of evaluation are shown in Table 1. Comparative Example 2 A monolayer film was prepared by extruding a polypropylene (MFR = 3.5 g/10 minutes; melting point = 161°C; Noblen WF452 manufactured by Sumitomo Chemical Industry Co., Ltd.) by T die method. Then, electron beam was irradiated to the film thus obtained, in the same manner as in Example 1. The results of evaluation are shown in Table 1. Example Comparative Example 1 2 1 2 A microwave oven suitability 150 150 150 150 Adhesive power (g/10 cm) 20 18 10 0 Young's modulus MD (kg/cm 2 ) 8,300 8,600 6,800 8,600 TD (kg/cm 2 ) 5,300 5,600 6,300 6,200 Elongation at break (MD %) 20 20 410 20 Thermal shrinkage factor (MD/TD %) 2/0 2/0 1/0 2/0 Haze (%) 1.0 0.9 1.0 2.3 Saw edge-cutting property Good Good Not good Good As has been detailed above, the self-tacky packaging film of the present invention is excellent in tackiness, a microwave oven suitability and "saw edge"-cutting characteristics, appropriate in flexibility, excellent in transparency and good in appearance. Further, the self-tacky packaging film of the present invention is much superior in environment suitability to the films made mainly of polyvinylidene chloride or polyvinyl chloride. Further, the self-tacky packaging film of the present invention is useful as packaging films such as domestic wrapping film, industrial wrapping film, etc.

Claims: A self-tacky packaging film which comprises a multi-layer film comprising: (A) two outermost, surface layers comprising a polyethylene type resin, and (B) at least one core layer, between the layers (A), comprising a polypropylene type resin having a melting point of 140°C or more, at least one side of the multi-layer film being irradiated with an ionizing radiation. A film according to claim 1, wherein the polyethylene type resin is a low-density polyethylene having a density of 0.935 g/cm 3 or less and a melt flow rate of from 0.1 to 50 g/10 minutes. A film according to claim 1, wherein the polyethylene type resin is a copolymer of ethylene and an α-olefin having from 4 to 10 carbon atoms having a density of from 0.88 to 0.94 g/cm 3 and a melt flow rate of from 0.1 to 50 g/10 minutes. A film according to claim 1, 2 or 3, wherein the irradiation is effected with an electron beam. A film according to any one of the preceding claims, wherein the polypropylene type resin is a propylene homopolymer, a copolymer of propylene and one or more α-olefins other than propylene, and mixtures thereof. A film according to any one of the preceding claims, wherein the irradiation is effected with ionizing radiation at an exposed dose of from 1 to 30 Mrad and at an accelerating voltage of from 100 to 300 kV. A film according to any one of the preceding claims, which has a thickness of 50 µm or less. A film according to any one of the preceding claims, where the layer comprising the polypropylene type resin (B) has a thickness of from 10 to 90 % of the total thickness of the self-tacky packaging film. A process for producing a self-tacky packaging film according to any one of the preceding claims which process comprises: (a) making a polyethylene type resin and a polypropylene type resin having a melting point of 140°C or more into a multi-layer film comprising (A) two outermost, surface layers comprising the polyethylene type resin, and (B) at least one core layer, between the layers (A), comprising the polypropylene type resin, and (b) irradiating at least one side of the surface layers (A) with an ionizing radiation. Use of a packaging film according to any one of claims 1 to 8 for tightly packaging food containers.

Inventors: HAYASHIDA HARUO, KASABO KAZUO, YANASE KOUICHI, HAYASHIDA, HARUO, KASABO, KAZUO, YANASE, KOUICHI
Applicants: SUMITOMO CHEMICAL CO, SUMITOMO CHEMICAL COMPANY LIMITED
Application year: 1995
Main classification: B32B27
IPC codes: C08J7, B32B27, B29D7, B65D65, B32B7