Retroreflective sheeting comprising thin continuous hardcoat

#The invention relates to retroreflective sheeting and articles suitable for pavement marking comprising a retroreflective layer and a thin continuous hardcoat layer comprising an inorganic oxide material or diamond-like carbon material on the outermost exposed surface. Preferably, at least one intermediate layer is provided between the retroreflective layer and hardcoat layer.

#Retroreflective pavement marking tapes and raised pavement markers are used to delineate traffic lanes on roadways. The raised markers are typically employed to improve driver visibility at night especially in wet conditions, in comparison to standard stripes of retroreflective paint or tape. Examples of various raised pavement marker designs include U.S. Pat. No. 3,332,327 (Heenan); U.S. Pat. No. 3,409,344 (Balint); U.S. Pat. No. 4,875,798 (May); U.S. Pat. No. 5,667,335 (Khieu et al.); and U.S. Pat. No. 6,127,020 (Bacon Jr. et al.). These patents all describe marker designs that include a vertically sloping face that presents a prismatic reflector toward oncoming traffic. Various pavement marking tapes are described in, for example, U.S. Pat. No. 4,117,192 (Jorgenson); U.S. Pat. No. 4,282,281 (Ethen); and U.S. Pat. No. 4,490,432 (Jordan). Pavement marking tapes and in particular, raised pavement markers are subject to abrasion and impact from vehicle tires. Such abrasion and impact cause scratches and deformation on the retroreflective surface that create optical defects that block or scatter incident light from vehicle headlamps, diminishing the retroreflected brightness of the pavement markers and tape.

#A number of approaches have been described to improve abrasion resistance and/or impact resistance. For example, U.S. Pat. No. 4,340,319 (Johnson, et al) describes a pavement marker that comprises a lens member of light-transmitting synthetic resin including a front face having a light-receiving and refracting portion adapted at an angle of at least 15° and a rear face having reflex reflective means for reflecting light transmitted through the light-receiving surface and refracting a portion back to the source. The pavement marker has an untempered glass sheet fixedly disposed on the light-receiving and refraction portion and the glass is in compression throughout the expected temperature range to which the pavement marker is exposed in use. The glass sheet may be adhesively bonded to the lens member by first applying an adhesive coating to the glass sheet or to the lens member and then placing the glass sheet in position on the lens member with the adhesive therebetween. Alternatively, the glass sheet may be bonded to the lens member during molding of the lens member. A very thin sheet of a transparent glass is provided for lamination to the lens member, the glass sheet preferably being untempered and having a thickness in the range from about 2 mils to about 15 mils. It has subsequently been found that the glass face has poor impact strength and is subject to cracking and chipping.

#A sheet of retroreflective material consisting of about a 1250 micron thick layer of polycarbonate resin having a rear surface patterned with a cube corner prismatic retroreflecting structure and having a front surface covered by about a 50 micron thick layer of acrylic resin was coated on the outer surface of the acrylic layer with a silicone hardcoat commercially available from General Electric Company, Schenectady, N.Y. under the trade designation “GE SHC 5020”. The silicone hardcoat was spray coated at a thickness of approximately 6 to 8 microns and cured according to the manufacturer recommendations as described in the GE SHC 5020 product literature. A layer of SiO2 approximately 4 to 6 microns in thickness was then deposited on the surface of the silicone hardcoat by chemical vapor deposition using plasma enhanced chemical vapor deposition.

#Several days after preparation, the sheet was tested for abrasion resistance following the method in ASTM D 4280 “Standard Specification for Extended Life Type, Nonplowable, Prismatic, Raised Retroreflective Pavement Markers”. A 25.4 millimeter diameter pad of No. 3 coarse steel wool was placed on the SiO2 coated surface. A load of 22 kg was applied to the steel wool pad and the surface was rubbed with the load 100 times. No visible signs of scratching or abrasion through the hardcoat were visible after testing.

#The sheet was also tested for Graffiti Resistance by drawing a fine line of approximately 3 centimeters on the SiO2 coated surface using a black fine point permanent marker commercially available from Sanford Corporation, Bellwood, Ill. under the trade designation “Sharpie”. The sample was stored at room temperature for about 24 hours. The sample was then wiped with both a wet and dry tissue and was found to leave a very small amount to no ink after wiping.