Converting Magazine

In Part 1, I discussed the need for excellent winding for a metallizable film. After the formulation to insure great winding, the substrate has to have several other attributes to make it a great metallizing substrate. In general, we want a bright metal appearance so the substrate surface must be very smooth. This is necessary to give a coherent reflection which is what makes a mirror give an image on reflection. The smooth surface is created by the polymer placed on the film surface. How the surface is formed determines its smoothness. Smooth surfaces are formed when the film is extruded. As the molten polymer leaves the die, the flow must be uniform and free from lines and thickness irregularities. This is controlled by the temperature of the melt and the cleanliness of the die surface. Next the liquid film surface must be set by solidification of the molten polymer.

In amorphous polymers such as polyester (PET), polycarbonate or polystyrene, it's easy to create a smooth solid surface from the smooth melt surface as the polymer forms a solid glass on cooling and will maintain the smoothness of the molten surface. However, for semicrystalline polymers, the surface will crystallize on cooling, and the size of the crystals formed on cooling will impact the surface smoothness. Generally the surface crystallinity is controlled by quenching rate as well as polymer selection. 

In PET, polystyrene or polycarbonate, amorphous polymers with high glass-transition temperatures, this can be accomplished by cooling from the opposite surface on a chill roll leaving the free surface defect-free. For polymers with a low glass-transition temperature such as polypropylene or polyethylene, rapid quenching is achieved by immersion into water or against the chill roll surface. However, if the surface is quenched against the chill roll surface, it will take the surface structure of the roll surface so mirror polished rolls (2 rms) must be used. However, in this case the polymer must be pushed (pinned) against the roll surface with enough force to exclude all air from between the film and the metal surface. This will require the use of a nip polishing roll or an electrostatic pinning system as an air knife will supply insufficient force. The lower force air knife pinning will leave small pockets of air between the roll surface and the film surface giving differential cooling to the film surface. This causes variations in surface crystallinity and therefore surface roughness on the film's chill-roll surface. 

Therefore, if an air knife pinned surface is metallized it will show variations in reflectivity which are easy for the eye to see due to the contrast difference. This is shown in the attached figure as variations in brightness (some circled in red)


Water-mark defects in metallized film

These defects are called water marks (or leopard spots) because they look like water spots which have dried on a smooth surface. They are especially prevalent in oriented films as the stretching enlarges their size. Therefore, it is best to metallize the surface opposite the chill-roll surface in air knife pinned films (and perhaps other pinned films as well).

Other means of insuring a smooth, scratch-free surface in oriented films will be discussed at a later date as well as means of controlling barrier properties by control of the surface chemistry.