Packaging material for food to be subjected to high-temperature short-time sterilization and process for the preparation thereof

A packaging material for food to be subjected to high-temperature short-time sterilization, which comprises a flexible laminate sheet including (a) a heat-sealable inner face layer of a crystalline polypropylene, (b) a heat-resistant outer face layer of a thermoplastic resin having a melting point or decomposition point higher than the melting point of said polypropylene, (c) an oxygen-barrier layer interposed between said layers (a) and (b), and (d) a layer or layers of themoplastic resin interposed between layers (a) and (c) and/or layers (b) and (c), said heat-resistant thermoplastic resin layer (b) being selected from the group consisting of polyesters and copolyesters comprising recurring units expressed by the following formula:

• ##STR7## wherein R.sub.1 is an alkylene or cycloalkylene group having 2 to 8 carbon atoms, and R.sub.2 is an arylene group,
• said thermoplastic resin layer (d) having an impact absorbing coefficient, K.sub.I, of at least 0.5.times.10.sup.-4 cm, said value K.sub.I being defined by the formula K.sub.I =S/E in which S is the load (in kg/cm) of the resin film having a width of 1 cm under 30% elongation and E is the tensile strength (in kg/cm.sup.2) of the resin film S and E both being measured after the high-temperature short-time sterilization, said polypropylene layer being composed of a film of a polypropylene having a melt flow index of 0.5 to 3, said film being characterized in that (i) said film is one prepared by melting in an extruder a crystalline polypropylene having a melt flow index of 0.5 to 3, extruding the melt in the form of a film at a temperature of 230.degree. to 260.degree. C., through a T die, and contacting the extruded film with a chill roll having a surface temperature of 20.degree. to 70.degree. C., to thereby cool rapidly and solidify the film, (ii) when the film is extracted in salad oil at 130.degree. C. for 30 minutes, the amount extracted of the polypropylene is smaller than 1500 ppm, (iii) the polypropylene film has a pseudohexagonal crystal form or a crystal form of a combination of the pseudohexagonal system and .alpha.-monoclinic system and a degree of orientation not higher than 3.0 and (iv) the surface of the polypropylene film confronting the oxygen-barrier layer (c) has a tension in the wet state (T.sub.1) of at least 38 dyne/cm when it is allowed to stand still in an atmosphere of a temperature of 20.degree. C. and a relative humidity of 40% for 1 week.

A packaging material for food to be subjected to high-temperature short-time sterilization, which comprises a flexible laminate sheet including (a) a heat-sealable inner face layer of a crystalline polypropylene and (b) a heat-resistant outer face layer of a thermoplastic resin having a melting point or decomposition point higher than the melting point of said polypropylene, said thermoplastic resin being selected from the group consisting of

• (a) polyesters and copolyesters comprising recurring units expressed by the following formula:
• ##STR8## wherein R.sub.1 is an alkylene or cycloalkylene group having 2 to 8 carbon atoms, and R.sub.2 is an arylene group,
• (b) polyamides and copolyamides comprising recurring units expressed by the following formula:
• ##STR9## wherein n is a number of 3 to 13 and m is a number of 4 to 11, (c) polycarbonates comprising recurring units represented by the following formula:
• ##STR10## wherein R.sub.3 is a hydrocarbon group having 8 to 15 carbon atoms, (d) biaxially stretched isotactic polypropylene films, and
• (e) acetyl cellulose, said heat-sealable inner face layer of said crystalline polypropylene film being characterized in that (i) said film is one prepared by melting in an extruder a crystalline propylene-ethylene block copolymer having 2 to 10% by weight based on the total copolymer of ethylene, and extruding the melt in the form of a film at a temperature of 230.degree. to 260.degree. C., through a T die, and contacting the extruded film with a chill roll having a surface temperature of 20.degree. to 70.degree. C., to thereby cool rapidly and solidify the film, (ii) when the film is extracted in salad oil at 130.degree. C. for 30 minutes, the amount extracted of the copolymer is smaller than 1500 ppm, (iii) the film has a pseudohexagonal crystal form or a crystal form of a combination of the pseudohexagonal system and .alpha.-monoclinic system and a degree of orientation not higher than 3.0 and (iv) the surface of the film confronting the layer (b) has a tension in the wet state (T.sub.1) of at least 38 dyne/cm when it is allowed to stand still in an atmosphere of a temperature of 20.degree. C. and a relative humidity of 40% for 1 week.