Hygroscopic Plastics

By Jim Hingst

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. 

Polycarbonate can absorb moisture and oxygen from the air.
As the plastic outgases, bubbles can form under the applied
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.

For more information on plastics read:

Industrial Plastics: Theory and Applications

 

For  information on thermoforming read:

Thermoforming: A Practical Guide

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Tips for Screen Printing on Polycarbonate Films
Introduction to Acrylics
Prepping Acrylic Sheet Prior to Decoration
Screen Printing Acrylic Sheet
Painting Acrylic Signs

NEW INSTRUCTIONAL VIDEOS.  Five new videos have been added to RTape’s  YouTube channel. The following is a description of the new videos:

Squeegee Technique. Nothing is more basic in vinyl application than the squeegee. But some squeegees work better than others. And there are right ways and wrong ways to use this simple tool. This video clip reviews squeegee selection, squeegee care and squeegee technique. Click here to view the Squeegee Technique video.

Wet Applications. Dry applications are typically recommended for most vinyl applications. For those exceptions to the rule, this instructional video explains the right way to perform a wet application. Click here to view the Wet Applications video.

Introducing AT65. Installing multiple color overlays with a paper application tape is not much easier than driving in the dark without your headlights. For these challenging applications, RTape developed its AT65, the universal high tack film. This video clip explains the features and benefits of this remarkable new film application tape. Click here to view the Introducing AT65 video.

Application of Window Graphics. Installing vinyl graphics on glass can be challenging, because the adhesive aggressively grabs onto this high energy surface. Repositioning graphics on window is generally difficult, if not impossible. This instructional video explains how to dry apply window graphics right the first time. It also describes the necessary steps required for surface preparation. Click here to view the Application of Window Graphics video.

Transferring Frosted Window Graphics Films.  Because frosted window films are highly textured, transferring cut vinyl graphics can be problematic. This video provides direction in selection of the right application tape and how to apply these films quickly, easily and without problems. Click here to view the Transferring Frosted Window Graphics Films video.

About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 450 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.  Vinyl Sign Techniques is available at sign supply distributors and at Amazon. 

Slides of Jim's Artwork

© 2014 Jim Hingst