Optical: systems and elements – Signal reflector – 3-corner retroreflective
Patent
1995-04-21
1998-09-22
Dzierzynski, Paul M.
Optical: systems and elements
Signal reflector
3-corner retroreflective
359531, 359538, 359539, 359540, G02B 5124, G02B 5128
Patent
active
058123164
DESCRIPTION:
BRIEF SUMMARY
This application is a national phase application claiming rights and benefit from International Application PCT/JP92/01407, filed Oct. 30, 1992.
FIELD OF TECHNOLOGY
This invention relates to a method for making retroreflective sheeting which is useful for road signs, number plates on vehicles such as automobiles, motorcycles, etc., construction signs and marking of sign boards and the like.
BACKGROUND TECHNOLOGY
Retroreflective sheeting which retroreflexes light towards the light source is well known in the past. Utilizing its retroreflectivity, the sheeting has been widely used in the field of road signs, number plates of vehicles, construction signs, display marking such as sign boards and the like.
In particular, such retroreflective sheetings which are normally referred to as cellular or encapsulated lens-type reflective sheeting, whose light-retroreflective performance is enhanced by utilizing low refractive index of air, are finding yearly increasing utility because of their excellent light-retroreflective ability.
Generally a cellular reflective sheeting is composed of a protective film and a support sheet which face each other across a narrow air layer, and mutually intersecting, reticular joint portions which are formed by thermally deforming a part of said support sheet to bond said two, the cellular spaces between the protective film and the support sheet, which are partitioned by said reticular joint portions, encapsulating a large number of retroreflective glass beads which are partially embedded in the support.
Retroreflective performance of a cellular reflective sheeting with such a structure almost exclusively relies on the cellular portion and the reticular joint portions make substantially, no contribution to said performance. In consequence, it is important for the sheeting to exhibit still higher retroreflective performance, that the areal ratio occupied by the joint portions per unit surface area of the retroreflective sheeting is reduced to the possible minimum.
When the areal ratio of the joint portions is reduced, i.e., when the bonded area of the joint portions between the protective film and the support sheet is reduced, however, there arises the problem that frequently deterioration in strength of the joint portions themselves or insufficient bonding strength results, causing a peeling-off trouble of the protective film from the support sheet.
Accordingly, attempts have been made to possibly reduce the area of the joint portions and to increase the bonding strength between the protective film and support sheet. For example, Japanese Patent Publication No. 1356/1986 (U.S. Pat. No. 4,025,159) has proposed "a process for making a retroreflective sheeting which comprises (a) preparing a base sheet on one surface of which a layer of a retroreflective elements is provided, and (b) forming a network of narrow intersecting bonds by thermoforming a binder material and contacting it with at least either one of a cover sheet or said base, whereby to bond the cover sheet as spaced from the layer of the retroreflective elements, said method being characterized by the steps of thermoforming a thermoformable and radiation-curable binder material and forming said network, and thereafter curing the network in situ by radioactive rays applied thereto to an insoluble and infusible state, whereby increasing the bonding strength of the network of bonds with the sheet".
According to our reconstruction of above-proposed method, however, it has been found that the interfacial bonding strength between the joint portions of the substrate sheet and the protective film is not appreciably improved by the radiation curing, while strength of the joint portions does increase. Furthermore, because the substrate sheet which is used in the above-proposed method is thermoplastic before the thermoforming (radiation curing is practiced after thermoforming), it is in the state highly susceptible to fluidization at the time of thermoforming, and the width or thickness of the joint portions are apt to fluctuate ac
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Ochi Katsura
Tanaka Osamu
Yoshizawa Masaki
Dzierzynski Paul M.
Nippon Carbide Kogyo Kabushiki Kaisha
Sikd-er Mohammad Y.
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