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Curl of coextruded films
March 26, 2008
Today, there are many coextruded films containing a wide range of polymers combined together. In many cases the film layers are symmetrically placed and the stresses are balanced about the center line of the film. In this case the films will generally remain flat and curl free. However, there are many instances where an asymmetric structure will be better in terms of film properties.
In many instances the asymmetric structures will be prone to curl if the stresses are imbalanced. The tendency to curl will be a function of the relative thickness of the layers and which layer has the mechanical strength to control the curling tendency. For instance in an oriented four layer film of nylon/tie/PP/CoPP, if the nylon layer is less than about 5% of the structure the film will lie flat. As the nylon layer increases if thickness above 5% then the tendency to curl and the tightness of the curl will increase. This tendency could be controlled somewhat with the proper thermal treatment of the film during its manufacture.
In this example the curl was towards the nylon layer indicating that the relative shrinkage of the nylon layer was overpowering the rest of the structure. This could be offset somewhat by moisture absorption in the nylon such that the nylon layer increased in length reducing the curl in the film. However this was not enough to permit increasing the nylon layer thickness.
Any polymer pairing where one of the polymer layers increases in density faster or to a greater extent than the other polymer layer will drive the tendency of the film to curl. In cases where environmental changes such as moisture absorption are not driving the dimensional change in a polymer there is the potential to control the curling with heat treatments or annealing conditions during manufacture.
Another approach would be to modify the polymer layers modulus (stiffness) or its molecular weight to modify the crystallization rate or extent of crystallinity in the final film layer. This can be accomplished by blending polymer s to modify relative layer modulus and crystallinity, nucleation rate with quenching conditions or in some instances adding nucleating agents.
In the end, if you are facing a curling problem in a coextruded structure you will need to determine which polymer layer is controlling the curling tendency and then make the appropriate modifications to the polymer, of the structure to balance the stresses in the various film layers.
Posted by Eldridge M. Mount on March 26, 2008 | Comments (0)