Overlays are most often produced from polycarbonate when the user is expected to involve limited flexing action (not a high-use keypad or membrane switch application). Polycarbonate fatigues and cracks when cycled. Therefore polyester is the preferred overlay material in these situations. When fatigue is not an issue, polycarbonate is formulated with a range of features and properties ideal for a variety of applications.
The major issues to be considered when specifying polycarbonate are texture, hard coatings, outdoor use, thickness, and fire ratings. Technical information about polycarbonate and related overlay materials can be found in this pdf file: GE Lexan, HP Lexan, FR Lexan, Valox PTX (heat stabilized polyester), Ultem (high-temperature film).
Textured polycarbonate is frequently specified if the overlay is expected to be used indoors and viewable transparent windows are not required for LCD or other types of display. Textures are available from fine matte’s to course velvets depending on the visual effect desired. General Electric’s “Lexan” brand sets the industry standard for polycarbonate film textures with identities such as 8b35, 8a35, 8a35f, etc.
Hard coated polycarbonate
Polycarbonate is a relatively soft material and therefore will easily scratch if not treated with a hard coat material. Scratches are not readily viewable when they occur on textured polycarbonate films. However, if the application requires a clear window, a hard-coated polycarbonate is nearly always appropriate. Hard coatings are available with various gloss levels depending on the application’s need for glare attenuation.
Polycarbonate for outdoor use
Polycarbonates formulated for UV resistance are essential if the overlay will be used outdoors. If the polycarbonate overlay will be used with exposure to UV light, yellowing will occur if the polycarbonate is not outdoor rated.
Polycarbonate thicknesses: Polycarbonate is available from .005” to .156” (and thicker)
Polycarbonate Limitations: In keypad applications, polycarbonate will fatigue and crack after approximately 100,000 key cycles.
Sabic (formerly GE) is the leading supplier of polycarbonate.
Polyester is the preferred overlay material in membrane switch overlay applications. Specifications for membrane switch overlay’s involving clear LCD windows (gloss and glare attenuated hard coated polyester) can be found at Marnot™ XL Hardcoat Clear Polyester. The specifications for Melrose’s textured polyester suitable for membrane switches without clear LCD windows can be found at Textured Polyester ProTek™ Hardcoat.
Polyester offers superior fatigue resistance (to 1,000,000 cycles) and is somewhat more resistant to chemical attack than polycarbonate.
Polyester is available with textured, hard-coated, and with outdoor use formulations, similar to polycarbonate.
- the range of thicknesses, fire ratings, and textures is less extensive than polycarbonate
Polyesters for use in overlay applications are limited to .008” and thinner.
Primarily due to their thin gauge, fire ratings are limited or non-existent (although usually unnecessary)
Very limited range of textures
As with polycarbonate, gloss materials will scratch easily if not hard coated
Autotype is a leading supplier of polyester.
Selecting the appropriate adhesive for an overlay application depends on:
- the 2nd surface printed ink on the backside of the overlay,
- the surface to which the overlay will be attached,
- environmental factor,
- spacer stack-up requirements (if any).
For membrane switches and overlay designs with embossed buttons, polyester is a superior choice. Polycarbonate comes in a wider range of thicknesses, so thickening an overlay can help it last longer. But since continuous flexing can produce stress fractures over time, polycarbonate is best suited for applications with low flex needs.
Melrose can assist you in making the proper adhesive selection if you advise us of these factors. Get in touch with us to receive expert consultation for your application requirements.