Retroreflective articles return incident light back towards the light source. This unique ability has promoted widespread use of retroreflective articles on clothing.
Persons who work or exercise near motor vehicle traffic need to be conspicuously visible so that they do not get struck by passing motor vehicles. Retroreflective articles highlight a person's presence to motorists at nighttime by retroreflecting light from motor vehicle headlamps. Light from the headlamps strikes the retroreflective article on the wearer's clothing and is returned toward the motor vehicle, enabling the driver to become aware of the person's presence. The bright image displayed by the retroreflective article gives motorists more time to react.
Because retroreflective articles are frequently used on clothing, they must be able to withstand laundering conditions--otherwise, the articles cannot continue to serve their safety function after repeated washings. Investigators who work in the retroreflective art are aware of this problem, and therefore they pursue an ongoing goal of developing launderably-durable retroreflective articles so that persons who wear retroreflective clothing remain conspicuously visible after their clothing has been worn and laundered many times.
Investigators also recognize that the need to develop such articles is particularly pronounced for clothing that is frequently worn in squalid environments. Examples of such clothing includes firemen's jackets and construction workers' safety vests. Because these garments are often subjected to harsh conditions, they tend to get very dirty and therefore require cleaning under industrial laundering conditions, which typically involve wash temperatures as high as 40.degree. to 90.degree. C. (105.degree. to 190.degree. F.) and pH values of 10 to 12.5.
Some launderably-durable retroreflective articles that have been developed for use on clothing comprise an exposed layer of transparent microspheres, a polymeric binder layer, and a specularly reflective layer. The transparent microspheres are partially embedded in the polymeric binder layer, and the specularly reflective layer underlies the microsphere's embedded portions. Because the microspheres are exposed--that is, they are not covered by a light-transmissible polymeric layer--the articles are commonly referred to as "exposed lens retroreflective articles". Light that strikes the front surface of these retroreflective articles enters the transparent microspheres to have its direction altered to strike the underlying specularly reflective layer where it is reflected to reenter the microspheres where its direction is again altered but this time back in the direction of the light source.
In a successful attempt at developing an exposed lens retroreflective article that is durable under industrial laundering conditions, Wu Shyong Li in U.S. Pat. No. 5,200,262 partially embedded a monolayer of metal-coated microspheres in a binder layer that contained a flexible polymer and one or more isocyanate-functional silane coupling agents. The flexible polymers included isocyanate curable polyesters and one or two component polyurethanes.
In another successful attempt, Li used a binder layer that was formed from an electron-beam curable polymer and typically one or more crosslinkers and silane coupling agents (see U.S. Pat. No. 5,283,101). The electron-beam curable polymers included chlorosulfonated polyethylenes, ethylene copolymers comprising at least about 70 weight percent of polyethylene such as ethylene/vinyl acetate, ethylene/acrylate and ethylene/acrylic acid, and poly(ethylene-co-propylene-co-diene) polymers. The microspheres were embedded in the cured binder layer, and a specular reflective metal layer was disposed on the microsphere's embedded portions.
In another approach, Michael Crandall et al. disclosed making a retroreflective article launderably-durable by placing a compound that contains an aromatic bidentate moiety is chemical association with the reflective metal layer. In one embodiment, the compound is disposed in a polymeric binder layer that preferably includes a crosslinked or virtually crosslinked elastomer. Crosslinked polymers include polyolefins, polyesters, polyurethanes, polyepoxides, and natural or synthetic rubbers. This product is disclosed in U.S. patent application Ser. No. 08/216,404 now U.S. Pat. No. 5,474,827.