Low-temperature impact-resistant polyamide-based stretch-oriented multilayer film

A process for producing a stretch-oriented multilayer film, comprising the steps of:

• co-extruding at least three species of melted thermoplastic resins to form a tubular product comprising at least three layers including an outer surface layer (a) comprising a thermoplastic resin other than polyamide resin, an intermediate layer (b) comprising a polyamide resin and an inner surface layer (c) comprising a sealable resin,
• cooling with water the tubular product to a temperature below a lowest one of the melting points of the thermoplastic resin, the polyamide resin and the sealable resin constituting the layers (a), (b) and (c),
• re-heating the tubular product to a temperature which is at most the lowest one of the melting points of the thermoplastic resin, the polyamide resin and the sealable resin constituting the layers (a), (b) and (c),
• vertically pulling the tubular product while introducing a fluid into the tubular product to stretch the tubular product in the vertical direction and the circumferential direction, thereby providing a biaxially stretched tubular film,
• folding the tubular film,
• again introducing a fluid into the folded tubular film to form a tubular film,
• heat-treating the tubular film from its outer surface layer (a) with steam or warm water until a relaxation ratio reaches at least 20% in at least one of the vertical direction and the circumferential direction, and
• cooling the heat-treated tubular film to provide a stretch-oriented multilayer film exhibiting an impact energy of at least 1.5 Joule at a conversion thickness of 50 μm at −10° C.