Identify Common Plastic Substrates

Today's screen printers are asked to print on various substrates specified by customers who may not understand the processing limitations of various plastic materials. This article will introduce some of the testing methods used for identifying common plastic substrates used for screen printing.

Testing
Initial testing for an unidentified plastic material begins with observation, and then only if necessary, chemical testing using solvents and flames. It is possible to identify many plastics by their handling characteristics, without the necessity of further testing by chemical means.
For quick and easy Solvent Testing, place a drop or two of the test solvent on the material and let it sit for 30 seconds, then rub the droplet with a gloved fingertip to check for plastic softening or swelling.
Common solvents used in the testing described here are cyclohexanone, and an aromatic solvent such as a high-flash naphtha like SC-100 (Hysol 10) or SC-150. (The numbers 100 and 150 refer to the flash points of the solvents in this instance).
Most thermoplastics are generally sensitive to some solvents, but in screen printing we commonly use several solvent-resistant materials including polyethylene, polypropylene, and polyester.
For Flame Testing or Burn Testing, use a clean flame from a Bunsen burner, propane torch, or alcohol lamp. Use a pair of tongs or tweezers to place a SMALL piece of plastic into the flame, and notice the color and appearance of the flame from the burning plastic. The flame color is an indicator of the plastic type.
Remove the plastic from the torch, and notice whether the material continues to burn on its own, or if the burning plastic flame extinguishes itself. Once the plastic is extinguished, lightly smell the plastic to assess the odor.
Do not inhale deeply! Some plastics have unpleasant, strong, or acrid smells. Do not use a match! The sulfurous smell will interfere with the smell of the plastic. The chart below shows some identifying characteristics of the more common plastics that a screen printer may encounter. Proper substrate identification will allow the screen printer to select the correct ink for the application.
For additional information on identifying plastics or for help selecting an ink for a particular plastic, call our Technical Services team at 1-800-SERICOL.

Styrene brittle, distinctive shattering or tearing; "metallic" sound when shaken; very sensitive to solvents; burns with an orange sputtery flame and dense sooty smoke.

Polypropylene (includes Coroplast?) cuts cleanly, floats on water, bends repeatedly without tearing; unaffected by solvents; burns with yellowish-blue flame with little smoke, and continues to burn when removed from the flame.

Polyethylene waxy feel; very solvent resistant, cuts cleanly, floats on water, will stretch before tearing, burns similar to polypropylene.

Vinyl softened by cyclohexanone but not by aromatics; burns with distinctive bright green-tinged flame and dirty sooty smoke.

Polycarbonate solvent testing with cyclohexanone is the easiest test as it will soften polycarbonate and Polyester within 15 to 30 seconds; polycarbonate is difficult to ignite and burns with a sputtery orange flame and smelly smoke; Polyester film has a metallic sound when it is shaken, and is difficult to tear; it burns rapidly with bluish-yellowish flame, and continues to burn when removed from the flame source.