Many polyester films are coated or treated on one or both sides to enhance certain properties such as ink/coating adhesion, slip, static dissipation, etc. This coating/treatment will often have properties that are quite different than the base film which is why they work--an adhesion promoting pretreatment provides much better ink adhesion than untreated polyester for instance. The beneficial properties that these treatments provide do not come without a cost. The high performance characteristics of polyester with respect to properties such as temperature, chemical and moisture resistance do not necessarily apply to these coatings or treatments.
EXPOSURE TO EXTREME TEMPERATURE AND HUMIDITY CONDITIONS CAN REDUCE THE EFFECTIVENESS OF THE TREATMENT AND/OR CAUSE THE FILM TO BLOCK OR FUSE TO ITSELF WHEN IN A STACK OR ON A ROLL.
The symptoms of blocking are the inability to separate layers of film without tearing the film or causing the pretreatment to delaminate. Blocking occurs when moisture gets between the layers of film during processing, storage or transportation and then allowed to dry. Changes in ambient conditions can also cause this phenomenon to occur. If a roll or stack of film is stored in cool conditions and then placed in a hot, humid environment the condensation that forms will most likely wet the pretreatment and potentially cause blocking. Conversely, if the film is processed in high humidity, packaged in a moisture resistant material and then moved to a cool location the condensation within the package can also cause blocking.
To Avoid Blocking and Other Related Processing Problems:
- Store the film between 30°F and 100°F.
- Avoid storing and transporting in humid or wet conditions unless protected with “poly” wrap or other moisture resistant material.
- Process film under controlled conditions-- 40%- 60% relative humidity. (Too dry can cause other problems like excessive static or core shrinkage.
- Condition film to ambient for at least 48 hours before removing moisture resistant packaging.