Supermolecular structure and orientation of blown cellulosic films

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Abstract
N-Methylmorpholine-N-oxide (NMMO) technology provides new possibilities for shaping fibers and films from cellulose. We discuss a blow-extrusion technique can be applied to a cellulose-amine oxide dope, yielding blown cellulosie films. Process parameters, such as the draw down ratio and the blow ratio, can be used to control the mechanical film properties in both the machine and transverse directions. In this way, a wide range of properties is covered, and a completely balanced film can be manufactured. The film thickness can be made as small as 5 μm, and the membrane properties can be varied by the conditions of precipitation. The blown cellulosic films exhibit a partially crystalline supermolecular structure and, depending on the coagulation conditions, a symmetrical or asymmetrical homogeneous morphology and pore structure. Generally, a uniplanar orientation type was found, the chains being parallel to the film surface. Around the surface normal, the chain orientation can be varied from nearly random to strongly uniaxial, in this way determining the mechanical properties in the machine and transverse directions. As compared with the conventional viscose processing route of cellophane, the blown film's NMMO processing route is less complicated and friendlier to the environment. Blown films can be made much thinner, and the mechanical properties are superior to viscose films. Possible application fields of blown cellulose films are food casings, particularly small sausage casings, packaging, and membranes.

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