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PVC-PET shrinkage and shrink labels
April 9, 2008
The attached graphs came from US Patent 6,680,097 B1 which is titled "Easily removable label for reusable containers” and uses film shrinkage to help in recycling containers with labels and makes use of film shrinkage in a very unusual manner, to my thinking. The curves show TD (transverse) and MD (longitudinal) shrinkage for both PVC and PET based films. The heat shrinkage of the film is coupled with an engineered failure in the adhesive so that when the container is placed in a hot bath for washing the adhesive force is decreased and the film shrinks and pulls itself off the container.
However, unlike the TD growth (negative shrinkage) behavior we saw in the MD stretched polyethylene film in the earlier posting on heat shrinkage, these films both show positive TD and MD shrinkage across the range of temperatures shown.
In this case the desire is to have the highest shrinkage in the circumferential direction relative to the container and this can be achieved from a wide variety of films and film manufacturing methods depending on the label application method. For cut and stack labels the printing of the label can be controlled such that the wrap direction for the container is in the MD or TD direction of the film manufacturing method as only the stack orientation needs to be changed to insure proper shrinkage characteristics.
However, In the case of higher MD oriented films, the printing would be perpendicular to the stretching direction, but in this case the label could also be applied from a roll, so the type of labeling machine will control the flexibility in the film manufacturing process. In this case we have PVC and PET films (not heat stabilized) being used and preferred due to their high shrinkage levels in the temperature range used but polystyrene would also be a candidate. Film laminations would also be possible by insuring that the heat shrinkable film would pull the other label component off the container. In this case the adhesion between the two films of the laminate should exceed the adhesive strength of the label adhesive so the shrinkage pulls the whole label off and does not separate from the lamination.
As in almost all packaging and film applications there is always a lot of substrate secret engineering hidden in the behavior.
Posted by Eldridge M. Mount on April 9, 2008 | Comments (0)