Stock material or miscellaneous articles – Structurally defined web or sheet – Including variation in thickness
Reexamination Certificate
2001-02-14
2003-07-15
Watkins, III, William P. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including variation in thickness
C428S500000, C428S523000, C428S218000, C428S163000, C428S167000, C359S529000, C359S530000, C359S519000
Reexamination Certificate
active
06592967
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to microprismatic retroreflective sheeting used to manufacture flexible retroreflective microprismatic articles such as roll-up road signs and reflective safety garments. More particularly, the invention is directed to reflective sheeting fabricated from elastic polyolefins known in the art as “plastomers.” Plastomeric retroreflective sheeting is found to exhibit a marked improvement in retention of retroreflectivity compared to prior art plasticized polyvinyl chloride (PVC) sheeting. The improvement in retroreflectivity retention results from the ability of retroreflective microprisms (e.g., corner cubes) formed in the plastomeric sheeting to rebound and/or resist permanent deformation when the sheeting is subjected to compression. Moreover, plastomeric retroreflective sheeting can be produced without plasticizers.
BACKGROUND OF THE INVENTION
Retroreflective sheeting, and methods for manufacturing it, are well known in the art. See for example Pricone, U.S. Pat. No. 4,601,861. Such sheeting typically comprises an optically clear layer, having one smooth surface for receiving light, and the opposite surface being provided with multiple microstructures, such as cube corner retroreflective elements. “Cube-corner” is an art-recognized term for structures consisting of three mutually perpendicular faces that form a corner, without regard to the size or shape of each face.
Typical cube corner retroreflective sheeting of the prior art is manufactured of a relatively rigid material such as polycarbonate or polymethylmethacrylate. Such relatively rigid materials effectively maintain the precise cube corner geometry that is necessary for good retroreflectivity. Relatively rigid sheeting of the prior art is advantageously used in flat, relatively rigid articles such as retroreflective highway signs.
For other products, a more flexible retroreflective sheeting is desirable. Retroreflective garments such as safety vests may be worn by construction workers, crossing guards, or pedestrians to make the wearer more conspicuous. The material used in retroreflective garments should be sufficiently flexible to afford comfort to the wearer, yet must retain retroreflectivity, even after being subjected to compressive forces. For example, a construction worker might store a safety vest in a trunk or truck bed with a tool box or other heavy equipment that would press against the retroreflective elements of the vest.
To date, two general types of flexible retroreflective sheeting are known. First, it is known to use plasticized polyvinyl chloride (“PVC”) as a flexible retroreflective sheeting material for use in safety garments. Unfortunately, although plasticized PVC is an ideal resin from the standpoint of flexibility, it has proven less than ideal in terms of providing resilient microprism corner cubes capable of rebounding to their original shape after a deforming load is applied to the cubes and removed. This drawback in plasticized PVC sheeting is evident from the rapid losses in retroreflectivity observed after plasticized PVC retroreflective roll-up signs have been in service for periods as short as several months. Unlike rigid forms of sheeting which typically have polymethyl methacrylate or polycarbonate microprisms, microprisms formed in plasticized PVC are subject to deformation when the sheeting material is exposed to the deforming loads typically encountered in the production, handling and ultimate roadway use of such sheetings. If microprism cube corner angles change by more than about 0.01°, or distort as little as 0.0002 mm, the retroreflectance levels will be significantly altered.
Winding of embossed plasticized PVC films prior to fabrication into an end product can also contribute to losses in retroreflectivity. After plasticized PVC sheeting is embossed with microprisms, the embossed sheeting is typically wrapped on a core until it is used to fabricate a finished product. While on the core, the microprisms are subject to a deforming load which is then released when the sheeting is unwound from the core for further manufacturing steps. This load can cause undesirable cube-corner distortion.
Retroreflectivity losses in plasticized PVC microprismatic films are also caused by the migration of plasticizer compounds typically used in the manufacture of such resins. Plasticizers such as dioctyl phthalate or diisononyl phthalate are typically used to enhance the flexibility of plasticized PVC sheeting used for roll-up signs and safety vests. Most plasticizers will eventually migrate from the plasticized PVC film. When flexible plasticized PVC is used for retroreflective applications, plasticizer migration can cause retroreflectivity losses due to slight distortion of the microprism geometry as the plasticizer exudes from the film and changes its overall material composition. This slight distortion in the microprisms can cause a significant change in the retroreflective properties of the film. Plasticizer migration increases with temperature, hence the changes in retroreflectivity due to this phenomenon are particularly noticeable in outdoor applications where the material may be exposed to elevated temperatures during the summer months.
The second general type of prior art flexible retroreflective sheeting is a multi-layer film in which a rigid or semi-rigid film containing corner-cube microprisms is adhered to a flexible film. Such films are disclosed in U.S. Pat. No. 3,684,348 and more recently in U.S. Pat. Nos. 5,450,235 and 5,491,586. Disclosures of multi-layer flexible cube corner retroreflective films are also found in U.S. Pat. Nos. 5,648,145; 5,264,063; and 5,637,173. Another example of a multi-layer film is disclosed in U.S. Pat. No. 5,175,030 which describes a composite plastic article having a tough flexible substrate, on one surface of which are microstructures formed of a cured oligomeric resin. There can be problems with the multi-layer approach of the prior art, however. First, it can be difficult to prepare a multi-layered film when the component films have different physical and chemical properties. Problems can arise from poor adhesion of the layers, and the need for costly processing techniques to overcome the physical differences in the layers. Another problem with multi-layered constructions is that they may not be truly flexible. The presence of a rigid microprism layer can impart an undesirable amount of stiffness to the product.
Attempts have been made in the prior art to overcome the problems that arise from these two approaches. U.S. Pat. No. 5,117,304 discloses a retroreflective article based upon an optically clear, aliphatic polyurethane having hard chain segments and soft chain segments. The articles are said to be characterized by flexibility and conformability over a wide range of application temperatures. Urethane polymers, however, are more costly than plasticized PVC, and present significant handling issues such as the tendency of urethane sheeting to stick to processing equipment and to itself. This problem requires the added expense of a carrier layer.
In summary, while plasticized PVC film is still viewed as a commercially satisfactory resin for some flexible retroreflective sheeting applications, the end user has had to accept a certain degree of microprism distortion, with a corresponding undesirable change in retroreflectivity and limited product longevity, as the trade-offs for good flexibility. The art has yet to meet the challenge of providing a retroreflective sheet that not only has flexibility comparable to plasticized PVC, but also provides microstructures (e.g., corner cubes) that exhibit excellent resilience in response to applied compression.
In view of the foregoing discussion, it is an object of the present invention to provide an article in which microstructures formed on a surface of the article exhibit markedly improved reboundability and retention of geometry in response to applied compression.
A further object is to provide such an article that is flexible and that ca
Avery Dennison Corporation
Day Jones
Watkins III William P.
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