GB2036637A - Heat-sealable film of polypropylene - Google Patents

Abstract

A film of a homopolymer of propylene, said film being non-oriented or monoaxially oriented and having a density of at least 0.904 g/cm<3> and orientation factors cos<2> X, cos<2> Y and cos<2> Z in the thickness, transverse and machine directions, respectively, of the film of from 0.20 to 0.5, said orientation factors satisfying the following relationship: <IMAGE> The film of the invention is tearable and heat-sealable and is particularly suitable for use in providing food packaging bags.

Description

SPECIFICATION Heat-sealable film of polypropylene This invention relates to a film of a homopolymer of propylene, or simply called polypropylene, and to a food packaging bag formed thereof.

A biaxially oriented polypropylene film is suitable for use as a packaging bag for various articles, especially foodstuffs, because of its excellent tensile strength and impact strength. However, it has such a poor heat-sealability that a heat-sealed portion shrinks to cause failure in sealing. On the other hand, a non-oriented polypropylene film prepared in customary manner is heat-sealable, but has an unusually high tear strength so that it is difficult to take out food from sealed bags made of such a film.

It is an object of this invention therefore to provide a tearable and heat-sealable film of a homopolymer of propylene.

Another object of this invention is to provide an easily operable packaging bag made of a heat-sealable polypropylene film.

These objects of this invention are achieved by a film of homopolymer of propylene, said film being non-oriented or monoaxially oriented and having a density of at least 0.904 and orientation factors cos2 X, cos2 Y and cos2 Z in the thickness, transverse and machine directions, respectively, of the film of from 0.20 to 0.5, said orientation factors satisfying the following relationship:

and a food packaging bag made of said film.

The orientation factors cos2 X, cos2 Y and cos2 Z can be calculated on the basis of the following equations after measuring the birefringences Ax, Ay, Az of the film in the thickness, transverse and machine directions.

Ax = n2 - ny Ay = nz- nx #z = nx - ny (n = + + ny + n2) (2) 2 nx = (n" - nl) cos X + n ny = (n - n1) cos2 Y + n n2 = (n" - nl) cos2 Z + n wherein the average refractive index n, the inherent birefringence (the birefringence in the completely monoaxially oriented state) n1, - n!, and the refractive index n in a direction perpendicular to the axis in the completely monoaxially oriented state for a homopolymer of propylene are as follows; n=1.508 n"- nl = 0.040 n = 1.495.

[See Saburo Okajima and Kazuhiko Kurihara; Journal of the Textile Society, Japan 22,206(1966).] When meeting the aforesaid conditions of film density, orientation factors and orientation, the poly-propylene film of this invention has good heat sealability and a moderate tear strength (measured by the method of JIS Z-1072) of 1.8 to 5.0 kg/cm, preferably 1.8 to 4.0 kg/cm, in the machine direction and 15.0 to 80.0 kg/cm, preferably 15.0 to 65.0 kg/cm, in the transverse direction. A food packaging sealed bag made by using this film is easily operable, has sufficient strength and impact strength, and can hold foods therein without leakage during transportation. Furthermore, the packaging bag can withstand heat sterilization treatment or retorting after enclosing foods therein.

Preferred polypropylene films of this invention have orientation factors in the following relationship.

0.50 > cos2 X, cos2 Y, cos2 Z 0.20

The homopolymer of propylene used to make the film of this invention has a melt index (MI) of 0.5 to 15, preferably 0.8 to 10.0, an isotactic index (the percent boiling n-heptane extraction residue) of 80 to 99, preferably 90 to 98, and an intrinsic viscosity, measured at 13500 in decalin, of 1.3 to 3.5. preferably 1.5 to 3.0.

The polypropylene film of this invention can be prepared, for example, by extruding the homopolymer of propylene through an annular die while maintaining the resin temperature at 200 to 2500C preferably 200 to 240"C, inflating the extrudate at a blow ratio of from 1.0 to 1.5, and cooling it from the melting temperature of the resin to 120"C at a controlled rate of 4 to 20"C, preferably 4 to 15 C, per second.

Any other film-forming methods can be employed which will afford non-oriented or monoaxially oriented films having the densities and orientation factors within the ranges specified hereinabove.

The homopolymer of propylene used as an inner layer of the packaging bag of this invention, in the form of as-produced pellets, has a density of 0.899 to 0.903. But when it is formed into a film under ordinary film-forming conditions, the film obtained has a density of 0.895 to 0.885, which is outside the range specified in this invention.

When the film of this invention is used in a retortable packaging bag, the density of the film changes upon subsequent heat sterilization treatment. The film density of at least 0.94 required in this invention is that of the film before heat sterilization. The density of the film changes only slightly upon sterilizing treatment.

Usually it increases to about 1.0001 to 1.02 times the density of the film before heat sterilization.

The polypropylene film of this invention can be used either as a single layer film or as a laminate film of two or more layers by laminating other films or sheets. Food packaging bags can be produced from these single layer or multi-layer films. When a bag is to be made of a laminate film, it can be easily heat-sealed by using the polypropylene film of this invention as the innermost layer. These single-layer or multi-layer food packaging bags are suitable as retortable bags.

There is no particular limitation on the other layers to be laminated in the laminate film or laminate bag of this invention. Any materials which have heretofore been used in laminated films or laminate bags, especially in food packaging bags, and more specifically retortable food packaging bags, may be used. When the film of this invention is used in a retortable packaging bag, it is preferred to provide a layer of a thermally stable resin.

Conveniently used as the thermally stable resin are thermoplastic resins and thermosetting resins having a higher melting temperature or decomposition temperature than the heat sealing temperature of the polypropylene film of this invention as the inner layer. Example of these resins include polyesters such as polyethylene terephthalate, polyethylene terephthalate isophthalate, poly-1 ,4-cyclohexylene dimethylene terephthalate and polyethylene hydroxybenzoate; polyamides such as polycaprolactam, polylauryllactam, polyhexamethylene adipamide, polyhexamethylene sebacamide and poly-1 ,4-cyclohexylene adipamide; polycarbonates such as polydihydroxydiphenylmethane carbonate, polydihydroxydiphenylethane carbonate, polydihydroxydiphenyl-2,2-propane carbonate and poly-p-xylene glycol bis-carbonate; cellulose esters such as cellulose acetate; and fluorocarbon resins such as polyvinyl chloride, polytetrafluoroethylene and a copolymer of tetrafluoroethylene and hexafluoropropylene. Biaxially oriented isotactic polypropylene films can also be used. Also useful are insoluble infusible resins, for example thermally stable polymers having a hetero ring in the molecular chain, such as an imide, imidazopyrrolone, imidazole, oxazole, oxadiazole or thiazole ring.Examples of the insoluble infusible resins include polyimides, polyamideimides, polyesterimides, polyamideimide esters, polyesteramideimides, polyimidoimidazopyrrolone, polyimideimidazopyrrolonimide, polyesterimideimidazopyrrolone, polybenzoxazolimide, polyimideoxadiazole, polysulfone ether imide, polyimidebenzoxazolimide, organopolysiloxanimide, polybenzimidazolimide, polyoxazinonimide, polybenzothiazolimide, polybenzimidazol, imidazopyrrolonimide, polybenzoxazinonimidazopyrrolonimide, polybenzothiazolimidazopyrrolonimide, polybenzoxazolimidazopyrrolonimide, polyimide urea, polybenzoxazole, polybenzimidazole, polybenzothiazole, and mixtures thereof.

An aluminum foil layer or a shock-absorbing thermoplastic resin layer or both may be provided between the inner layer and the outer layer described above. Or several of these additional layers may be provided.

Examples of thermoplastic resins suitable for formation of a shock-absorbing layer include polyamides such as polycaprolactam, polylauryl lactam, polyhexamethylene adipamide, poiyhexamethylene sebacamide, and poly-1 ,4-cyclohexyleneadipamide, copolyamides, polycarbonates such as polydihydroxydiphenylmethane carbonate, poly-dihydroxydiphenylethane polycarbonate, polydihydroxydiphenyl-2,2-propanecarbonate and poly-p-xylene glycol biscarbonate, polyester-polyethers such as polyoxyethylene-ethylene terephthalate, poly-1 ,4-oxybutylene-1 ,4-butylene terephthalate, polyoxyethyleneethylene terephthalate/isophthalate, poly-1 ,3-oxypropylene-1 ,4-butylene terephthalate, poly-1,3 oxypropylene-1 ,4-butylene terephthalate and poly-1 ,4-oxybutylene-oxyethylene-ethylene terephthalate, and polyester-polylactones such as polyepsilon-caprolactone polyethylene terephthalate, and polyepsiloncaprolactone-polyethylene terephthalate/isophthalate. Films of the polymers exemplified hereinabove can be used after being stretched monoaxially or biaxially or in the unstretched state. Certain stretched polypropylene films can also be used as the shock-absorbing layer.

Each layer needs not to have a specified thickness. The inner layer usually has a thickness of 30 to 100 microns, preferably 40 to 80 microns. The outer layer usually has a thickness of 1 to 30 microns, preferably 5 to 20 microns. The shock-absorbing thermoplastic resin layer usually has a thickness of 5 to 40 microns, preferably 10 to 30 microns.

In the case of a single layer, the film has a thickness of to 150 microns, preferably 5to 100 microns.

A laminate sheet composed of the individual layers described above may be formed by bonding these layers in films or foils by known means. For example, these layers can be bonded by an adhesive, for example adhesives composed of thermosetting resins such as polyester-isocyanate type adhesives, epoxy adhesives and polyester-isocyanate-epoxy adhesives, synthetic rubber adhesives, and adhesive composed of thermoplastic resins. However, the aforesaid polyester-isocyanate type adhesive, epoxy adhesives and polyester-isocyanate-epoxytype adhesives are preferred.

In bonding the films and foils using adhesives, the surface of the polypropylene film and/or thermally stable resin film which is to be bonded may be treated with ozone, corona discharge, flame and/or oxidizing agents to increase its adhesiveness.

When the thermally stable resin layer is to be formed of an insoluble infusible resin, a solution of a precursor of such a resin is coated on the surface of an aluminum foil and baked to form a thermally stable resin film layer.

When a polyamide such as nylon 6 or nylon 12 is used to form the shock-absorbing layer as an interlayer, this layer acts also as an adhesive layer and no particular adhesive needs to be used, because such polyamides are superior hot melt adhesives.

The single-layer or laminate polypropylene film of this invention or the polypropylene resin, thermally stable resin and shock-absorbing thermoplastic resin constituting the individual layers of a food packaging laminate bag produced from it may contain known additives such as other resins, natural or synthetic rubbers, plasticizers, lubricants, fillers, coloring agents, stabilizers and antistatic agents in amounts which do not impede the achievement of the objects of this invention.

The food packaging laminate bag of this invention can be made of the single-layer or laminate sheet described hereinabove by any known method, for example by superposing a pair of laminate sheets so that the polypropylene film layers face each other, and heat sealing the assembly so as to form the peripheral edge of the desired bag.

The method of heat sealing is known. There can be used, for example, a method of externally heating the desired portion by using a heated bar, heated knife, heated wire, impulse seal, etc., and a method of internally heating the desired portions by ultrasonic vibrations or induction heating.

Afood is filled in the resulting bag, and as desired, the inside of the bag is deaerated by any known metghod such as vacuum deaeration, hot filling packaging dearation, steam deaeration, steam jetting deaeration or pressure deaeration, or the food contained packed is heat sterized. Then the food packing opening is heat-sealed to pack the food in the bag.

If desired, the bag structure having packed therein the food is heat-sterized.

The heat sterilization can be formed either batchwise or continuously.

The packaging bags in accordance with this invention can enclose therein, and if desired permit sterilization of, a very wide range of cooked and semicooked foods.

Examples of foods especially suitable for packing in the bags of this invention included boiled, steamed and water-boiled products of various vegetables, fish, and/or livestock products, for example, curry, hash, various stews such as beef stews, gravies, such as meat sauce, sweet-and-sour pork, sukiyaki, Chinese liquid starch dressing, sweet and-sour boiled vegetables, water-boiled asparagus, and cream-boiled tuna; various soups such as light soup, potage, miso soup, pork soup and Japanese chowder; pastas and noodles such as spaghetti, buckwheat noodles, Japanese noodles, Chinese noodles, and other form of noodles; various forms of cooked rice such as boiled rice, boiled rice with red beans, seasoned boiled rice with fish, meat and/or vegetables, fried rice, pilaf, and porridge; additives for preparing seasoned boiled rice and Chinese noodles; desserts or sweets, such as water-boiled red beans, sweek thick bean-meal soup, and boiled peas with honey and bean jam; and meat or fish processed products such as meat dumpling, hamberg, beef steak, roast pork, corn beaf, ham, sausage, broiled fish, grilled meat, grilled chicken, roast chicken, pork chop, smoked fish, bacon, and boiled fish paste.

The following Examples and Comparative Examples illustrate the present invention more specifically.

In these examples, the tear strength and heat seal strength were measured by the methods stipulated in JIS-Z 1702, and Announcement No. 17 of the Ministry of Health and Welfare, Japan, respectively.

Example 1 A homopolymer of propylene having a melt index of 1.3 was formed into a film having a thickness of 50 microns by an air-cooling extrusion-inflation method under the condition shown below.

Resin temperature: 230or Rate of cooling: 13"C/sec. upto 120 C Inflation ratio: 1.2 The resulting film had a density of 0.905 g/cm3. The orientation factors cos2 X, cos2 Y and cos2 Z were 0.25, 0.30, and 0.45, respectively. cos2X/cos2Y = 0.83, cos2 X/cos2 Z = 0.56.

The film had a tear strength of 2.42 kg/cm in the machine direction, and 17.6 kg/cm in the transverse direction.

A 12-micron-thick aluminum foil and the polypropylene film, a 9 micron-thick aluminum foil and the polypropylene film obtained above were laminated using a urethane type adhesive. Using the polypropylene film as an inner surface, bags having a size of 100 mm x 130 mm were prepared. The resulting bag had a heat seal strength of 3.5 kg/15 mm width. Cheese was packed into these bags, and the bags were subjected to an opening test by 18 panelists.

Example 2 The same three-layer laminate sheet of polyethylene terephthalate film/aluminum foil/polypropylene film as produced in Example 1 was used as a closure material for multilayered bottle of polypropylene obtained by blow molding. The closure material withstood retorting at 120 2, and the closure could be easily opened by tearing.

Comparative Example 1 The same resin as in Example 1 was formed into a film having a thickness of 50 microns by a T-die method under usual conditions. The film had a density of 0.891. The orientation factors were as follows: cos2 X = 0.31, cos2 Y = 0.33, cos2 Z = 0.36, cos2 X/cos2 Y = 0.94, cos2 Xicos2 Z = 0.73.

The film had a tear strength of 8.75 kg/cm in the machine direction, and 25.8 kg/cm in the transverse direction.

Athree-layer laminate bag was prepared in the same way as in Example 1 using this film, and the same opening test as in Example 1 was performed. The results are shown in Table 1.

The laminated bag had a heat seal strength of 3.2 kg/15 mm width.

TABLE I Number of Number of Number of panelists who panelists who panelists who judged that judged that judged that the closure the open- the closure was very easy ability is was very dif to open ordinary ficult to open Example 1 9 7 2 Comparative 1 3 14 Comparative Example 2 A commercially available biaxially oriented polypropylene film (25 microns thick) had a density of 0.909, and the orientation factors were as follows: cos2 X = 0.10, cos2 Y = 0.63, cos2 Z = 0.27, cos2 X/cos2 Y = 0.16, cos2 X/cos2 Z = 0.37. The film had a tear strength of 3.67 kg/cm in the machine direction, and 2.4 kg/cm in the transverse direction, and had good tearability.

A th ree-layer laminate film was prepared in the same way as in Example 1 using this film. Attempt was made to produce a bag by using this laminate film and heat-sealing the polypropylene film as an inside layer.

However, the sealed portion shrank, and the appearance became poor. Furthermore, no good sealability at the sealed portion was obtained.

Claims (6)

1. A film of a homopolymer of propylene, said film being non-oriented or monoaxially oriented and having a density of at least 0.904 g/cm3 and orientation factors cos2 X, cos2 Y and cos2 Z in the thickness, transverse and machine directions, respectively, of the film of from 0.20 to 0.5, said orientation factors satisfying the following relationship:

2. Afilm according to claim 1 substantially as described in Example 1 or 2.

3. A laminate of two or more layers comprising a film of a homopolymer of propylene as claimed in claim 1 or 2 having at least one other film or sheet laminated thereto.

4. A food packaging bag composed of a film as claimed in claim 1 or 2.

5. A food packaging bag composed of a laminate as claimed in claim 3 in which the innermost layer is the film of the homopolymer of propylene.

6. A food packaging bag according to claim 5 which is a laminate bag suitable for retorting.