Curl in Multilayer PA/PE Film, 2001 European PLC Conference Proceedings

Dr. Walter Goetz--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 crystallization kinetics of the components of the multilayer film: Recrystallization and shrinkage of the PA layer - the first to recrystallize - does not affect the planarity of the film construction, but recrystallization 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 postcrystallization of the PA layer Humidity-induced postcrystallization 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 postcrystallization , and will experience a second shrinkage. If the shrinkage during film extrusion and the shrinkage by postcrystallization are balanced, a flat film will be obtained. Such a film exhibits further advantages as: *immediate unstability, no conditioning necessary *high thermoformability *high flexibility *high dimensional stability, no post-shrinkage of film