Curl in Multilayer PA/PE Film, 2002 PLACE Conference Proceedings

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 and filling operations.
Curl is generated by different crystallisation kinetics of the components of the multilayer film: Recrystallisation and 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 layer

Humidity-induced postcrystallisation is a method to produce low-curl or even curl-free film which is unique to PA. Water is absorbed by nylon and lowers its glass transition temperature. As soon as the glass transition temperature drops below the ambient temperature, a PA layer which has been only partly crystallized during the film-extrusion process will start postcrystallisation , and will experience a second shrinkage. If the shrinkage during film extrusion and the shrinkage by postcrystallisation are balanced, a flat film will be obtained. Such a film exhibits further advantages as

• immediate usability, no conditioning necessary
• high thermoformability
• high flexibility
• high dimensional stability, no post-shrinkage of film