CURL IN MULTILAYER PA/PE FILM

摘要

Non-symmetric PA/PE film offer multiple advantages as superior thermoforming, printing and sealing.However, these films will roll up or "curl", which may cause problems during slitting, winding, printing andfilling operations.Curl is generated by different crystallisation kinetics of the components of the multilayer film: Recrystallisationand shrinkage of the PA layer - the first to recrystallize - does not affect the planarity of the film construction,but recrystallisation of the PE layer - the second to recrystallize - will end up in a curl towards the PE side.Curl can be minimized by- selection of a slowly crystallizing PA- selection of a PE of low crystallinity (density)- rapid cooling, to force both layers to crystallize simultaneously- humidity-induced postcrystallisation of the PA layerHumidity-induced postcrystallisation is a method to produce low-curl or even curl-free film which is unique toPA. Water is absorbed by nylon and lowers its glass transition temperature. As soon as the glass transitiontemperature drops below the ambient temperature, a PA layer which has been only partly crystallized during thefilm-extrusion process will start postcrystallisation , and will experience a second shrinkage. If the shrinkageduring film extrusion and the shrinkage by postcrystallisation are balanced, a flat film will be obtained. Such afilm exhibits further advantages as- immediate usability, no conditioning necessary- high thermoformability- high flexibility- high dimensional stability, no post-shrinkage of film