A food packaging film

摘要

Films having oxygen and moisture permeabilities both of 1,0 or less, and haze values of 1,0 or less, are made by melt-extrusion of a polyamide having the recurring structural unit -HN(CH2)6CO-, either as an inflated tubular film or as a flat film which is cooled in a liquid bath or on a chilled surface. The extrusion temperature of the melt may be 225 DEG -260 DEG C. The polyamide, which is made from 7-amino heptanoic acid, desirably has a reduced viscosity of from 0,7-3,0, preferably from 1,6-2,0. Reduced viscosity IR is defined by:- where D N is the difference between the flow-time of the polyamide solution and that of the solvent, No is the flow-time of the solvent, and C is the concentration of the polyamide in solution in grams per 100 mls., measurements being made at 30 DEG C. using a 3:2 by weight mixture of phenol and tetrachloroethane as solvent at a concentration of 0,2 gms. polyamide per 100 mls. solution. Oxygen and moisture permeabilities are expressed as gms./645 sq. cms./24 hours/0,0025 cm. thickness, and haze values are measured by the method described in the Specification. The films also have good sealability, and are capable of being successively stored at temperatures below -10 DEG C. and immersed in boiling water. (For details of their use as food packaging materials, see Group VI). The polyamide may be made by heating an ester of 7-amino heptanoic acid at atmospheric pressure in the presence of water to form a prepolymer, which is then condensed at a higher temperature and at atmospheric pressure, in the absence of a catalyst, the water and formed alcohol being first removed by distillation and the prepolymer then heated further at a desired temperature to form the high molecular weight polyamide. Preferred esters are the alkyl esters with 1-8, especially 2-6, carbon atoms in the alkyl chain, e.g. the ethyl, n-propyl, n-butyl, n-hexyl, and n-octyl, esters of 7-amino heptanoic acid and also 1 phenyl-ethyl-7-amino heptanoate, and the initial mol. ratio of ester to water may be from 1:1 to 1:30, especially 1:2,5 to 1:15, with an optimum ratio of 1:10. The prepolymer can be made by heating the ester-water mixture at up to 200 DEG C., preferably at 95 DEG -150 DEG C., while passing an inert gas, e.g. argon, nitrogen, helium or carbon dioxide, through the reaction mixture. The temperature is raised to about 200 DEG C. to remove all the water and formed alcohol, and the prepolymer then heated to above 240 DEG C., preferably 260 DEG -280 DEG C., to form a high molecular weight polyamide. The reaction mixture may contain additives such as pigments, which do not function as catalysts or impede the polymerization, and viscosity stabilizers, e.g. benzoic, stearic, p-toluic, adipic, sebacic, glutaric and pimelic acids and their esters, or ethyl 6-cyano hexanoate, ethyl 7-benzamido heptanoate, ethyl 7-acetamido heptanoate, and ethyl 7-toluamido heptanoate.ALSO:A food packaging film material which comprises a polyamide having the recurring structural unit -HN(CH2)6CO- is capable of being successively stored at temperatures below -10 DEG C. and immersed in boiling water, possesses good heat sealability, a haze value of 1.0 or less when measured by the method described in the Specification and oxygen and moisture permeabilities of 1.0 or less, expressed as gms./645 sq. cms./24 hours/0.0025 cm. thickness. (For details of the method of preparing the polyamide film from 7-amino heptanoic acid, see Group IV (a).) Foods of moisture content up to 50 or 60% may be stored for prolonged periods using these films without substantial weight loss or decomposition. Pre-cooked products may be inserted in the open end of a bag made from the film (preferably by heat sealing one end of a length of tubular film) a vacuum applied to the interior of the bag, the open end sealed, and the package stored at -10 DEG to 30 DEG C. until required for use, when the package may be heated in boiling water for 10-20 mins. and then torn open. A beef stew so packaged was stored at - 21 DEG C. for a month and then heated for 20 mins. in boiling water. The stew could be readily seen through the clear film, and the film was not damaged in handling during storage or heating.