Hygroscopic Plastics

Outgassing: Problems Arising Out of Thin Air

Some plastics can absorb moisture from the air.  These materials fall into the category called “hygroscopic plastics”.  The best known product in this group is polycarbonate. It’s not, however, the only hygroscopic plastic. Nylon and polyester can also absorb water vapor.

Water vapor isn’t the only substance that polycarbonate sheet can absorb.  Researchers have discovered that polycarbonate can also become saturated with oxygen. 

While absorption of moisture and oxygen isn’t always a problem, it can be, especially when you are decorating the sheet with vinyl graphics. As the plastic outgases, bubbles can form under the applied vinyl graphics.  

Moisture in polycarbonate sheet is also an issue for sign makers, who are thermoforming signs.  Here’s why.  Sure the heating in the thermoforming process will drive out moisture, just as it will in a recirculating oven.  But the heat is higher. Much, much higher.  Exposed to this high heat, the moisture rapidly boils off, turning into a gas.  As the trapped moisture explodes from the sheet, tiny craters or pit marks can form on its surface. 

As a rule of thumb, polycarbonate sheets should be dried before decoration. At the very least, you need 180°F to drive off the moisture. At that temperature, it takes a long time to dry out a sheet – 12 or more hours.  The manufacturers of polycarbonate recommend a much higher temperature. To drive the moisture out of a polycarbonate sheet the typical recipe for successful vinyl application is to pop it in a hot air recirculating oven at for 250° F for at least four hours.

Drying times will vary depending on the thickness of the sheet. A ¼” sheet may take as long as 24 hours for drying. For specific advice on procedures for drying polycarbonate sheet, read the manufacturer’s technical bulletins covering processing and then follow their instructions to the letter of the law. As the saying goes, it’s only good advice, if you take it.

Some people feel that you can avoid many problems if you keep the plastic sheet wrapped in the plastic that it comes in, or if you use it immediately after you receive it.  While this advice sounds good, it’s no substitute for pre-drying the sheet.  It is a misconception that the plastic wrap covering a pallet load of polycarbonate provides adequate protection from moisture. All the plastic wrap does is to protect the sheet from dirt.  While bags of desiccant underneath the skid may help to a certain extent keeping the sheets drier, it may not be enough.

Others recommend removing the surface protection film laminated to the polycarbonate sheet to allow it to naturally outgas any water vapor.  Lots of luck.  Removing the masking will accelerate the moisture absorption.  It’s actually better to keep the surface protection film on to minimize the absorption of moisture.  With the masking on, only the edges of the sheet are exposed.

Unfortunately, we don’t live in a protective bubble and nobody stores their polycarbonate sheet in one either, whether they are a manufacturer, distributor or sign maker.  Polycarbonate is kept in warehouses, some of which become very humid in certain parts of the country.  The safest course of action is to pre-dry the sheets according the manufacturer’s instructions.