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Application: Selection of a slowly crystallizing polyamide and a low crystallinity polyethylene, rapid cooling to force both layers to crystallize simultaneously, and humidity-induced post-crystallization of the polyamide layer can minimize curl.
Certain applications in flexible packaging require properties beyond the realm of what commonly used polyolefins can offer. These include gas barrier, heat resistance, thermoformability, and toughness. To obtain high-performance films with superior properties, layering two or more different materials in a single structure is necessary. Polyamide has frequent use as a component in these multilayer film structures due to its unique combination of mechanical strength, puncture resistance, transparency, thermal stability, oxygen and aroma barrier, and thermoformability.
Multilayer films of three, five, seven, or more layers are common. If these multilayer film constructions are non-symmetrical, the film will roll or "curl" when placed on a flat surface. This curl will cause problems during slitting, winding, printing, and filling operations. It may also generate scrap, require a decrease of converting machine speed, or both. Five- or seven-layer film constructions allow placing the nylon layer in or near the center of the film. This produces a symmetrical or nearly symmetrical film that has virtually no tendency to curl or seal. This paper discusses the phenomena of curl in non-symmetrical film and offers suggestions to reduce or eliminate curl. Although this paper focuses on three-layer combination of polyethylene (PE)/tie resin/polyamide (PA) coextruded film, the general principles apply to more complicated multilayer constructions of PE and PA and other semi-crystalline polymers.
What Causes Curl?
Curl is the phenomenon of rolling seen in coextruded multilayer film with at least two semicrystalline polymers. It is not due to different coefficients of thermal contraction of the polymers but to their different crystallization kinetics.
When the multilayer structure of PA/tie resin/PE leaves the die of a blown or cast film extruder, all the materials are molten. The polymer chains arrange in a...