Recently, I have helped investigate several static problems with webs having conductive layers. My "old school," conventional thinking was that structures with metalized layers should have no static problems. After all, static problems occur of polypropylene, polyester, and other insulating polymers because static charges can accumulate on the insulating materials. In fact, antistatic layers are often designed into products specifically to dissipate static. And, these antistatic layers having surface resistivities in the 10+8 to 10+11 Ohms/square range can be very effective.
So, I thought that web structures having conductive layers in the 10 to 10+3 Ohm/square range should simply have no static problems. Well, my "old school," conventional thinking is just plain wrong!
Here are three situations where we found serious static problems with web structures having metalized layers.
Tension Loss - In normal operation, web is under tension as it passes through a machine. However, web tension may be lost when the web breaks, a splice fails, or when a motor malfunctions. In any event, when web tension is lost, the web falls away from conveyance rollers and drapes over objects near the web such as static bars. In one recent case, tension was lost, and a metalized web draped across an active static bar in a solvent zone. Several pins on the static bar touched the metalized layer, sparks occurred that ignited a fire.
To prevent this, I recommend that, on lines that run metalized web, static bars should be guarded so that the web cannot touch the bar when tension is lost. Guarding might be accomplished by a plastic U-channel, for example.
Corona Treatment - Corona treatment increases the wettability or printability of polymer surfaces. And, for laminated products, corona treatment can improve adhesion between laminated layers. In one laminated structure, the bare polymer surface of a metalized web was laminated to another insulating film. To improve adhesion, the polymer surface of the metalized film was corona treated.
Corona treatment can deposit large amounts of static on the treated surface. Normally, this static may be neutralized by a static dissipator on the web span exiting the treater facing the treated surface. However, when the untreated surface is metalized, static dissipators are ineffective in removing the static from the corona treater.
The customer observable result is that the treated materials are prone to sticking and jamming after the web is cut into sheets during the converting process.
I have no recommendations to dissipate high static when corona treating the polymer surface of a metalized web. I am currently searching for good alternatives for this. Let me know if you have a good idea!
Insulating Edges - Metalized webs may have unmetalized borders. That is, the metallized coating, usually done in vacuum, does not extend to the edges of the web. These insulating borders are typically a half inch to one inch. And, as you might imagine, static accumulates on these insulating edges.
Can enough static accumulate on a half-inch insulating edge to cause a problem? I’m really not sure. I have yet to do the theoretical calculations, and I just don’t have enough experience to assess the static risks. So, just to be conservative, I advise clients to use active static bars on their machines even when running metalized webs that have insulating borders.
Now, of course, the static bars should be guarded so that the metalized web cannot touch the static bars in the event of tension loss.
I am very interested in learning more about static issues with metalized webs or laminates with foil layers. Please feel free to share your insights!