Heat-shrinkable multi-layer film for deep-draw forming, and process for production thereof

Abstract

A polyamide resin-based heat-shrinkable multilayer film having excellent heat shrinkability and strength, as well as a high degree of deep A polyamide resin-based heat-shrinkable multilayer film having excellent heat shrinkability and strength, as well as a high degree of deep drawability as represented a shorter side-basis draw ratio (D/L) in excess of 0.6 between a maximum draw depth (D) and a shorter side length (L) of a mold opening is provided. The multilayer film is characterized by including a surface layer (a) comprising a thermoplastic resin, an intermediate layer (b) comprising a polyamide resin, and a surface layer (c) comprising a sealable resin, wherein (A) the intermediate layer (b) comprises a mixture of an aliphatic polyamide having a melting point of at least 180° C., and an amorphous aromatic polyamide comprising a polycondensation product of an aliphatic diamine with is ophthalmic acid and terephthalic acid as main acid components, including 25-45 wt. % of the amorphous aromatic polyamide, and has a thickness exceeding 25% and at most 50% of a total thickness of whole layers, and exhibiting: (B) tensile stresses at an elongation of 100% as measured in a tensile test at 100° C., including 3-22 N in at least one of a longitudinal direction (MD) and a transverse direction (TD) and an average of MD and TD values of 3-20N, and (C) a shrinkage in hot water at 90° C. of 3 to 20% in each of MD and TD. The film is produced through a process of strictly controlling biaxial stretching ratios and heat-relaxation ratios.