Multi-layer packaging film

Details Multi-layer packaging film Multi-layer packaging film

1999-12-10

2002-03-12

Chen, Vivian (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S035700, C428S213000, C428S480000, C428S483000, C428S515000, C428S516000, C428S523000, C428S910000, C264S173110, C264S173120, C264S173150, C264S288400, C264S290200

Reexamination Certificate

active

06355336

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a multi-layer packaging film, more particularly it relates to a multi-layer packaging film suited for use as stretch film, domestic wrapping film and such designed to be used for food packaging and similar purposes.
There are known the packaging films using a polyvinylidene chloride resin or a polyvinyl chloride resin as main constituting material, but the films which are safe in use and free of the problem of environmental pollution in disposal after use are desired, and recently the films made of polyolefin resins and other halogen-free resins such as polyethylene and polymethylpentene are used depending on their properties.
These films are popularly used for food packaging when, for instance, foods are sold at retail shops in the process of distribution or stored as in a refrigerator at household. They are also used for wrapping foods when they are placed in a microwave oven for warming them up. In this case, concerning the film properties, it is preferable that the films have adhesive properties or tackiness to the vessels and won't be fused, ruptured, whitened, discolored, denatured, shrunk excessively or damaged in other forms when exposed to high temperatures.
However, the films comprising polyvinylidene chloride resins have disadvantages in that they are shrunk excessively when heated, while the films comprising polyvinyl chloride resins are at a disadvantage in that they are whitened when brought into contact with boiling water.
Also, the polyolefin resins, although comparatively inexpensive and excellent in safety to foods and permeability to water, are unsatisfactory in oxygen gas barrier properties, low in strength and heat resistance, and especially poor in oil resistance when heated, and the films made of such resins may be swollen or fused when heated with an oleaginous food packaged therein. Thus, the polyolefin resins are not suited for uses in cooking by heating.
The stretched film of polyethylene terephthalate is known as a film with excellent heat resistance. This film has high strength and rigidity, but it is unsatisfactory in flexibility, weak in adhesiveness to the vessels when used as a wrapper, and also defective in cutting properties (easiness to cut): it cannot be cut smoothly by a blunt cutter such as provided to a domestic food wrapper.
Proposals have been made for improving these defects. For example, Japanese Patent Application Laid-open (KOKAI) Nos. 5-116262 and 6-262738 disclose the multi-layer wrapping films comprising an intermediate layer composed of a polyethylene terephthalate resin and inner and outer layers composed of a polypropylene resin.
In manufacture of such multi-layer films, however, it is necessary to raise the temperature for the film molding process to the level of 280 to 300° C. or even higher, which poses the problem on stabilized work of co-extrusion with the polyolefin which is low in melting temperature. Further, a high temperature such as 90 to 130° C. is required in the film stretching step, giving rise to the problems that the polyolefin used is liable to decompose, and that the produced laminated films tend to become sticky.
SUMMARY OF THE INVENTION
The present invention aims at providing a multi-layer wrapping film having excellent transparency, adhesiveness, heat resistance, gas barrier properties, non-heat-shrinkability and easiness to cut, and also excelling in the matter of food sanitation and environmental safety in practical use.
To attain the above aim, in the first aspect of the present invention, there is provided a multi-layer packaging film comprising an intermediate layer having at least one outer layer on both surfaces of the intermediate layer, and having the thickness of not more than 40 &mgr;m,
said outer layer comprising a polyolefin resin,
said intermediate layer comprising a polytetramethylene terephthalate resin having an intrinsic viscosity of 0.7 to 2.0, a melting point of 200 to 228° C. and a crystallization temperature of not higher than 180° C., and
the thickness of said intermediate layer being 20 to 80% of the thickness of the multi-layer film.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
The polytetramethylene terephthalate resin constituting the intermediate layer in the multi-layer packaging film of the present invention has an intrinsic viscosity in the range of 0.7 to 2.0. If the intrinsic viscosity of the said resin is less than 0.7, the produced film may be unsatisfactory in elongation, and it also becomes difficult to form a film. If the intrinsic viscosity of the said resin exceeds 2.0, the melting point of the resin elevates, which may cause heat deterioration of the polyolefin resin constituting the outer layers when the polytetramethylene terephthalate resin is co-extruded with the polyolefin resin, resulting in the impaired appearance of the produced film. The preferred range of intrinsic viscosity of the polytetramethylene terephthalate resin is 0.75 to 1.8, more preferably 0.8 to 1.6. The intrinsic viscosity referred to in the present invention was determined by dissolving 0.5 g/100 ml of the resin in a 50/50 (by weight) mixed solvent of 1,1,2,2-tetrachloroethane and phenol, and measuring the solution viscosity at 30° C.
The melting point of the said polytetramethylene terephthalate resin is 200 to 228° C. if the melting point of thereof is below 200° C., the produced film may be not provided with desired heat resistance. It is usually hard to obtain a resin having a melting point exceeding 228° C. The preferred range of melting point of the polytetramethylene terephthalate resin is 205 to 228° C. In the present invention, the melting point was determined from the peak value of crystalline melting point of the resin measured by a differential scanning calorimeter (DSC) at a heating rate of 20° C./min.
The crystallization temperature of the polytetramethylene terephthalate resin is not higher than 180° C. If the crystallization temperature exceeds 180° C., the polytetramethylene terephthalate resin tends to crystallize when it is joined with the polyolefin resin in the extruder die during co-extrusion process, making it hard to obtain a transparent film. Also, in this case, the extrusion work tends to become unstable. The preferred range of crystallization temperature is 140 to 175° C. The crystallization temperature was determined from the peak value of crystallization heat absorption temperature of the resin generated when the resin heated to 300° C. by DSC at a heating rate of 20° C./min and held at this temperature for 5 minutes was cooled down to 30° C. at a temperature lowering rate of 20° C./min.
As the polytetramethylene terephthalate resin constituting the intermediate layer of the multi-layer film of the present invention, there can be preferably used the polytetramethylene terephthalate resins in which the ratio of the polytetramethylene terephthalate units to the whole structural units is 70 to 100 mol %, for example, a polytetramethylene terephthalate resin in which the ratio of the polytetramethylene terephthalate units is 100%, a polytetramethylene terephthalate copolymer resin in which the ratio of the polytetramethylene terephthalate units is 70 to 99 mol %, and the blends of these resins.
The polytetramethylene terephthalate resins can be produced from polycondensation of terephthalic acid or its ester derivatives and tetramethylene glycol. A typical example of the terephthalic acid ester derivatives is dimethyl terephthalate.
As the dicarboxylic acid unit as a components of the polytetramethylene terephthalate copolymer resin, there can be used, beside the said terephthalic acid used as main component, other aromatic dicarboxylic acids, alicyclic dicarboxylic acids, aliphatic dicarboxylic acids and their ester derivatives. In view of mechanical properties and heat resistance, the amount of the aromatic dicarboxylic acid unit is preferably in an amount of not less than 50 mol %. The amount of the terephthalic acid unit is preferably not less than 70

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