Polycarbonate Sheet, Sign Grade

Sign-grade polycarbonate is UV-resistant sheet made for sign faces, channel letters, backlit and illuminated signage and light boxes. It meets UL 879, the standard for electric sign components, and it carries a ten-year limited warranty - the longest in the ordinary polycarbonate range, against breakage on clear and coloured sheet and against weathering on clear. Light transmission for clear is 86% at 0.118 inch and specific gravity is 1.20.

The reason to pay for sign grade rather than general-purpose sheet is the UV package and the warranty behind it. Unprotected polycarbonate yellows, hazes and embrittles outdoors; what nobody publishes is a rate, so treat the warranty term rather than any yellowing number as the durability claim. On this grade that term is ten years. It is also available with a matte non-glare finish where reflection off the sign face is a problem.

One specification to watch: sign grade is UL 94 HB at 0.060 inch, which is 1.52 mm - the same class general-purpose sheet carries at that gauge - and no better class is published for it at any stocked thickness. General-purpose sheet does reach V-0, but only around 0.394 inch, far thicker than sign grade is made. So if the installation has a flame-rating requirement, the flame-retardant UV-stabilised grade is the one to specify. The UL file for sign-grade sheet is E146154.

Other properties are general-purpose polycarbonate's: tensile 9,500 psi ultimate on a 340,000 psi modulus, flexural 13,500 psi on 345,000, compressive 12,500 psi, Rockwell M70 and R118, heat deflection 270F at 264 psi, thermal expansion 3.75 x 10^-5 in/in/F - worth allowing for on a large sign face, because a 10-foot panel moves about a quarter of an inch over a 60 degree swing.

One thing to check before you order, and this is distributor practice rather than anything a manufacturer prints. Polycarbonate sheet sold as "1/4 inch" is commonly 0.220 inch, which is the 5.6 mm North American extruded gauge, or 0.236 inch, which is the metric 6 mm gauge - not 0.250 inch. Those are 7% apart in thickness and in weight, and the difference matters wherever the sheet has to fit a frame, a channel or a glazing pocket. The thickness on each listing here is the actual decimal, not the nominal label.

Chemical compatibility and cleaning, which is where most polycarbonate failures start. Clean it with mild diluted dish soap and a soft microfibre cloth, or an ammonia-free glass cleaner. Do NOT use ammoniated glass cleaner - the common blue kind - because ammonia attacks polycarbonate, and the resin maker's own testing shows 0.1% ammonia is enough to do damage in the presence of water. Also avoid abrasive or high-alkaline cleaners, scrapers and squeegees, dry rubbing, and solvent in direct sun. Isopropyl alcohol diluted 50/50 with water handles oil and tar. Acetone, MEK, methylene chloride, chloroform, toluene, xylene and aromatic fuels all attack it, and so do amines and the plasticisers that migrate out of flexible PVC - which is why PVC gasket, tape and cable tie contact is a real failure mode. The important subtlety is that chemical resistance is not a fixed property: stress cracking only develops where the part is in tension, so a cold-bent, over-torqued or tightly framed part will fail with chemicals a compatibility chart calls safe.