ENCAPSULATED-LENS RETROREFLECTIVE SHEETING

Abstract: An encapsulated-lens retroreflective sheeting comprising a monolayer of lenses partially embedded in a binder layer, a specularly reflective layer underlying the lenses in optical association therewith, and a cover film bonded to the binder layer, wherein said cover film comprises a polyester film or a polyvinyl chloride film. The cover film has an adhesion bonding interface on the surface in at least partial contact with the binder layer. The sheetings of the invention exhibit improved dimensional stability under heat and shrink resistance while maintaining impact resistance and other desirable properties of pavement markings.

Description: ENCAPSULATED-LENS RETROREFLECTIVE SHEETING Field of Invention The present invention relates to encapsulated- lens retroreflective sheeting that exhibits improved shrink resistance and dimensional stability under heat while maintaining strong impact resistance and other desirable properties of retroreflective sheetings. Background Retroreflective sheetings are typically used on signs, barricades, guard rails, etc. to provide improved visibility at night. Examples of enclosed-lens retroreflective sheetings include embedded-lens retroreflective sheetings (ie., as described in Japanese Unexamined Pat. Publication (Kokai) No. 5-131589) and encapsulated-lens retroreflective sheetings (described in Japanese Unexamined Pat. Publication (Kokai) No. 3-9837) . Retroreflective sheetings of the encapsulate-lens type disclosed in U.S. Pat. No. 3,190,178 (McKenzie) comprise a monolayer of retroreflective elements and a transparent cover film supported in spaced relation away from the monolayer by a thermoformed network of narrow intersecting bonds that form sealed cells within which retroreflective elements of one cell are isolated from retroreflective elements of other cells. This transparent cover film protects the retroreflective layer, allowing the sheeting to maintain retroreflective performance under wet conditions. U.S. Pat. No. 4,025,159 (McGrath) teaches the subsequent curing of the thermoformed narrow intersecting bonds to improve adhesion of the cover film to the binder layer. The sheetings disclosed in these references are sometimes referred to as encapsulated-lens retroreflective sheetings. U.S. Pat. No. 4,896,943 (Tolliver) teaches the use of polyethylene and polypropylene cover films on encapsulated-lens retroreflective sheetings to afford improved impact resistance and greater toughness to the retroreflective sheeting. In some embodiments the cover film consists of multiple layers. Japanese Examined Pat. Publication (Kokoku) No. 40-7870 and Japanese Unexamined Pat. Publication (Kokai) Nos. 52-21793, 52-110592, 60-194405, and 2-196653 disclose retroreflective sheetings with such cover films as a film composed of an acrylic polymer, a polycarbonate film, a polyester film such as polyethylene terephthalate film, a film comprising a cellulose-based polymer such as cellulose acetate, a film comprising a polyvinyl chloride-based resin, or a film comprising a polyurethane resin. Acrylic polymer cover films typically exhibit good weatherability and are considered useful when the retroreflective sheeting is used outdoors. However, acrylic polymer films are relatively rigid and brittle and therefore exhibit lower than desired impact resistance. Acrylic cover films also typically exhibit low solvent resistance and are subject to wrinkling or cracking of the cover layer or whitening of the cover layer, resulting in poor appearance of the reflective sheet. Biaxially stretched acrylic films in particular exhibit decreased dimensional stability at elevated temperatures. Polyester cover films are typically superior in impact resistance to the acrylic film. However, polyester cover films are more difficult to strongly bond to the underlying binder layer. Thus, such polyester cover films may result in inadequate durability due to this tendency for the cover film to delaminate from the underlying binder layer. Polyvinyl chloride based cover films are also typically superior in impact resistance to acrylic cover films. However they are likewise also difficult to strongly bond to the underlying binder layer. Japanese Unexamined Pat. Publication (Kokai) No. 61-255846 discloses a retroreflective sheeting wherein a film comprising an acrylic polymer containing an acrylic polyphase polymer blend is used as a cover film to provide improved impact resistance. Still, such films do not provide sufficient impact resistance for some desired applications. These films also exhibit low solvent resistance. Japanese Unexamined Pat. Publication No. 63- 370940 describes a retroreflective sheeting wherein a film comprising an ethylenic copolymer such as an ethylene-acrylic acid copolymer is used as the cover layer. Such cover films can provide enhanced adhesion to the underlying binder layer that may suffice for many applications. However, such cover films exh±bit insufficient abrasion resistance, e.g., to a pencil scratch in the scratch test, and are therefore insufficiently durable for use in many applications. Moreover, such cover films tend to shrink, causing the overall retroreflective sheeting to shrink or curl. Japanese Unexamined Pat. Publication No. 6-138312 discloses the use of a film comprising a fluoropolymer such as vinylidene fluoride as a cover film in a retroreflective sheeting. While such a film has good impact resistance and resistance to staining, such films do not strongly bond to the underlying binder layer. The resultant retroreflective sheeting is therefore- susceptible to delamination. Japanese Unexamined Pat. Publication Nos. 62- 121043 and 62-121047 disclose use of a two-layer cover film formed by laminating an outer layer comprising an acrylic copolymer and an inner layer comprising a polyurethane resin. In such a two-layer cover film, the impact resistance may be improved by making the inner polyurethane resin layer relatively thick and the acrylic copolymer layer relatively thin. However, the solvent resistance of such a two-layer cover film is low because of the acrylic copolymer used in the outer layer. In summary, there is a desire and need for new cover films in encapsulated-lens retroreflective sheetings that either maintain or improve such properties of pavement markings as: (1) impact resistance, (2) dimensional stability against heat, (3) adhesive property to the binder layer (peeling resistance of the cover layer) , (4) abrasion resistance against scratching, (5) shrinking resistance, and (6) solvent resistance. Also, "edge chipping resistance" is a particularly important property for encapsula ' ted-lens type retroreflective sheetings. Encapsulated--lens type retroreflective sheetings are particularly susceptible to edge-chipping due to the hermetically sealed cells within which the retroreflective elements are enclosed and have an air interface. Edge-chipping resistance of the above- described conventional cover films is inadequate for some applications. The overall flexibility and elongation of an encapsulated-lens retroreflective sheeting is substantially affected by the nature of the cover film. Commonly used cover films are not sufficiently flexible and occasionally not sufficiently tough for use in applications such as traffic markers where high flexibility and toughness is desirable and necessary. There is a.need for retroreflective sheetings with cover films exhibiting improved dimensional stability under heat and shrink resistance while maintaining impact resistance and other desirable properties of pavement markings. Summary of Invention The present invention provides encapsulated-lens retroreflective sheetings having an improved cover film such that the sheetings offer improved shrink resistance and dimensional stability under heat while maintaining impact resistance and other desirable properties. Furthermore, such cover films are typically less expensive than many conventional cover film materials. Encapsulated-lens type retroreflective sheetings of the invention also exhibit improved resistance to edge chipping when cut or blanked into predetermined shapes. The retroreflective sheeting is usually cut into a predetermined size and shape, and then processed and affixed to articles such as road-sign boards. Therefore, it is important that the processed edge portion of the sheeting be resistant to chipping and breakage. Briefly summarizing, encapsulated-lens, type retroreflective sheetings of the invention comprise a binder layer in which a monolayer of retroreflective elements such as glass or ceramic microspheres are partially embedded, a specularly reflective layer underlying the microspheres in optical association therewith, and a light transmissive cover film bonded to the binder layer along a network of interconnecting lines forming hermetically sealed cells within which the retroreflective elements are enclosed and have an air interface. Retroreflective sheetings of the invention differ from previously known encapsulated-lens sheetings in that the polyester or polyvinyl chloride cover film exhibits enhanced adhesion to the binder layer, as evidenced by improved shrink resistance and dimensional stability under heat while maintaining impact resistance and other desirable properties. Enhanced adhesion of the cover film to the binder layer is accomplished by an adhesion bonding interface on the surface of the cover layer in contact with the binder layer. Typically, the adhesion bonding interface is formed by coating a material onto the inner surface of the cover film, although physically treating the inner surface of the cover film with a process such as corona treatment or irradiation treatment, or mechanically roughening it, may also be used. According to the present invention, encapsulated- lens retroreflective sheetings may be made with the cover films described herein and with a variety of binder materials. Brief Description Of The Drawings The invention will be further explained with reference to the drawing, wherein: Figure 1 is a cross section of a portion of a illustrative encapsulated-lens type retroreflective sheeting of the present invention; and Figure 2 is a cross section of another illustrative encapsulated-lens type retroreflective sheeting of the present invention. These figures, which are idealized, are not to scale and are intended to be merely illustrative and non-limiting. Description of Illustrative Embodiments Encapsulated-lens type retroreflective sheetings of the invention comprise a binder layer in which a monolayer of retroreflective elements are partially embedded, a specularly reflective layer underlying the retroreflective elements in optical association therewith, and a cover film bonded to the binder layer. In the encapsulated-lens type retroreflective sheetings of the invention, the cover film is bonded to the binder layer along a network of interconnecting lines forming hermetically sealed cells within which the retroreflective elements are enclosed and have an air interface, e.g., as disclosed in U.S. Pat. No. 4,025,159. According to the present invention, encapsulated- lens type retroreflective sheetings may be made with the cover films disclosed herein and with a variety of binder materials. Enhanced adhesion of the cover film to the binder layer is accomplished by an adhesion bonding interface on the surface of the cover layer in contact with the binder layer. Typically, the adhesion bonding interface is formed by coating a material onto the inner surface of the cover film, although physically treating the inner surface of the cover film with a process such as corona treatment or irradiation treatment may also be used. In some embodiments the surface may be roughened. The components of the binder layer, the adhesion bonding interface, and the cover sheet are selected to result in enhanced adhesion of the binder layer to the cover sheet. The cover film may be colored so that the resulting encapsulated lens retroreflective sheeting appears as a solid color except for the graphics applied to the sheeting. For example, a pigmented aliphatic polyurethane resin may be coated onto the adhesion bonding interface of the cover film. The pigmented layer comprises the second layer coated onto the cover film in the case where the adhesion bonding interface is a separate layer; that is, the pigmented layer is the layer that is in at least partial contact with the binder layer. The pigmented layer also exhibits good adhesion to the binder layer. Piece 100 of encapsulated-lens type retroreflective sheeting shown in FIG. 1 has a monolayer of retroreflective elements 3, specularly reflective layer 4, binder layer 2, and cover film 11 having outer surface 1 and inner surface 12 comprising a coated layer. Inner surface 12 comprising a coated layer is the adhesion bonding interface. Narrow linking portions 21 of binder material 2 exist in a networked pattern defining individual hermetically sealed cells 7. Adhesive layer 5 may overlay optional release liner 6. Piece 200 of encapsulated-lens type retroreflective sheeting shown in FIG. 2 has a monolayer of retroreflective elements 3, specularly reflective layer 4, binder layer 2, and cover film 11 having outer surface 1 and inner surface 13. Inner surface 13 is the adhesion bonding interface. Narrow linking portions 21 of binder material 2 exist in a networked pattern defining individual hermetically sealed cells 7. Adhesive layer 5 may overlay optional release liner 6. The retroreflective elements are typically glass microspheres with a refractive index between about 1.4 and 2.7., although microspheres of other compositions and refractive indexes may also be used. A refractive index of between about 1.7 and 2.1 is preferred when the front surfaces of the microspheres are exposed as in. encapsulated-lens type sheetings of the invention. The microspheres are typically between 10 and 200 micrometers in average diameter, and preferably between about 40 and 90 micrometers in average diameter. The microspheres may also be colored by a coloring material to provide a reflected light of a color different from that of the incident light. However, the microspheres must still be light-transmissive. The microspheres may be a mixture of two or more types of microspheres, e.g., differing in refractive index from each other or differing in diameter from each other, etc. The specularly reflective layer typically comprises a layer of vapor coated metal, e.g., having a thickness usually of from about 0.01 to 10 micrometers, preferably from about 0.05 to 5 micrometers. The specularly reflective layer is typically formed from a metal such as aluminum, copper, silver, gold or zinc or a compound such as Ceθ2, Bi2θ3, ZnS, Tiθ2, CaF2, a 3 AlFg, Siθ2, or MgF2 by a thin film forming means such as vacuum evaporation. Alternatively or additionally, a resin layer comprising diffuse or specularly reflective pigment, e.g., nacreous pigment, aluminum flake, etc. may be used to provide specular reflection, where the pigment is incorporated into the resin layer instead of existing as a separate layer coated onto the microspheres. Typical nacreous pigments include BiOC- 4 and PbC0 3 . The binder layer supports a monolayer of lens elements, e.g., transparent microspheres, and bonds the cover layer or film to the other portion of the retroreflective sheeting. The binder layer typically comprises .a flexible and durable polymeric material which provides good adhesion to the retroreflective elements as well as good adhesion to the cover film in accordance with this invention. Generally, the thickness of the binder layer should be at least about equal the average diameter of the microspheres, and it is preferably up to two or three times the average diameter of the microspheres. If the binder layer is too thin, it may be difficult to secure the retroreflective elements; if the binder layer is too thick, flexibility of the retroreflective sheeting as a whole may be undesirably reduced. Such a reduction in flexibility may cause peeling of the retroreflective sheeting at the edge portion when the retroreflective sheeting is affixed to a curved surface. Further, if the binder layer is too thick it may be difficult to form the desired hermetically sealed cells. In addition to the above-described resin(s) , the binder layer may contain, if desired, one or more additives such as pigment e.g., rutile titanium dioxide, polymerization initiator, crosslinking agent, antioxidant, ultraviolet absorbent, antifungal agent, antistatic agent, and higher fatty acid. In a first illustrative embodiment of the present invention, the cover layer comprises a polyester film and has an adhesion bonding interface on the side in contact with the binder layer and the binder layer contains an acrylic resin or a polyurethane resin. Polyester is a particularly preferred cover film because of its ability to be biaxially oriented. Such biaxial orientation typically results in improved dimensional stability and scratch resistance. Binder layers comprising acrylic resins typically provide good adhesion to the adhesion bonding interface of a polyester film, and therefore can improve the delamination resistance of the cover layer from the rest of the sheeting. Such acrylic binder layers may be thermoformed to seal the cover film thereto. Further, acrylic resins exhibit good weatherability, and can prevent discoloring of the retroreflective layer by ultraviolet light which results in decreased retroreflective brightness. Illustrative examples of preferred acrylic resins suitable for use herein include polymers obtained by polymerizing monomers containing alkyl acrylate or alkyl methacrylate as a main component. The alkyl acrylate or alkyl methacrylate may be one selected from alkyl acrylates or alkyl methacrylates with the alkyl group being at least one of methyl, ethyl, isopropyl, butyl, isobutyl, isooctyl, 2-methylbutyl, 2-ethylhexyl, lauryl, stearyl, cyclohexyl, isobornyl, 2-hydroxyethyl, 2- hydroxypropyl, 3-chloro-2-hydroxypropyl, hydroxyethoxyethyl, methoxyethyl, ethoxyethyl, dimethylaminoethyl, diethylaminoethyl, or glycidyl, or may be a combination of two or more of these. The above¬ described monomer may contain as an additional component a copolymerizable monomer such as acrylic acid, methacrylic acid, θ-hydroxyethylcarboxylic acid, itaconic acid, maleic acid, fumaric acid, styrene, chlorostyrene, (x-methylstyrene, vinyltoluene, acrylamide, methacrylamide, N-methylolamide, N-methoxymethyl- acrylamide, vinyl chloride, vinyl acetate, vinyl propionate, acrylonitrile, or vinylpyridine. Specific examples of preferred acrylic polymers include copolymers obtained by polymerizing monomers containing methyl methacrylate, isooctyl acrylate, and ethyl acrylate as main components. The acrylic polymer may be thermoplastic, thermosetting or radiation-curable. Preferably, the acrylic polymer is a curable-polymer. Use of a curable polymers can improve the solvent resistance of the binder layer. The curable polymer is preferably a radiation- curable polymer. When a radiation-curable .polymer is used, a reactive diluent such as an acrylic monomer may be added. Addition of such a diluent facilitates the control of fluidity of the binder layer before hardening. Therefore, the binder layer can be formed into contact with the cover film to form an encapsulated-lens type retroreflective sheeting. As a result, a retroreflective sheeting having a uniform and high retroreflective brightness over the sheeting surface can be produced. Japanese Unexamined Pat. Publication (Kokai) No. 52- 110592 (Japanese Examined Pat. Publication (Kokoku) No. 61-13561) describes in detail retroreflective sheeting using such a radiation-curable polymer in the binder layer and the production thereof. Binder layers comprising polyurethane resin typically provide good adhesion to the adhesion bonding interface of a polyester cover film, securely bond to the microspheres, and can be used to form encapsulated-lens retroreflective sheeting of the invention. The polyurethane resin may be thermoplastic, thermosetting or radiation-curable. It is preferably a thermoplastic polyurethane resin. The thermoplastic polyurethane resin has particularly good adhesion to polyester film. The thermoplastic polyurethane resin has a softening point of preferably from 70°C to 180°C, more preferably from 80°C to 160°C. If the softening point is too low, the cohesive strength of the resin is lowered and the adhesion between the binder layer and the cover layer may be reduced. If the softening point is too high, it becomes difficult to tightly bond the binder layer and the cover layer. As a result the adhesion between the binder layer and the cover layer may be reduced, resulting in delamination. Further, if the softening point of the binder layer is less than 70°C, the hermetically sealed cells may be deformed due to external force applied to the retroreflective sheeting, resulting in decreased retroreflective brightness. If the softening point of the binder layer is higher than 180°C, the formation of hermetically sealed cells is likely to be difficult. The polyurethane resin may also be an aromatic polyurethane resin or an aliphatic polyurethane resin. An aromatic polyurethane resin is generally preferred, although the aliphatic polyurethane resins exhibit good weatherability. In a second illustrative embodiment of the present invention, the cover layer comprises a polyester film and the adhesion bonding interface comprises an ethylenic resin on the side in contact with the binder layer, and the binder layer contains an ethylenic copolymer. The ethylenic copolymer of the binder layer has good adhesion to the adhesion bonding interface comprising an ethylenic resin, can be firmly affixed to and support the retroreflective elements, facilitates the formation of hermetically sealed cells, and can prevent deformation of the cells due to external force. Illustrative examples of suitable ethylenic copolymer as the binder layer are copolymers obtained by polymerizing monomers containing ethylene and α,β- unsaturated carboxylic acid such as acrylic acid or methacrylic acid. Preferred examples thereof include (1) an ethylene/ (meth)acrylic acid copolymer or (2) an ionomer resin obtained by crosslinking.an ethylene/acrylic acid copolymer, an ethylene/methacrylic acid copolymer or an ethylene-acrylic acid/methacrylic acid copolymer by a metal ion such as Zn + or Na + . Illustrative ethylenic copolymer that can be used in the binder layer are described in detail in Japanese Unexamined Pat. Publication (Kokai) No. 62-121043. The softening point of the ethylenic copolymer is preferably from 70°C to 180°C, more preferably from 80°C to 160°C for the same reasons outlined above for the polyurethane resin. In a third illustrative embodiment of the present invention, the cover layer comprises a polyester film and the binder layer contains a polyester resin. The polyester resin in the binder layer has good adhesion to the polyester cover film, and can be firmly affixed to and support the retroreflective elements. Polyester resin binder layers also facilitate the formation of hermetically sealed cells as well as preventing deformation of the cells due to external force. Illustrative examples of suitable polyester resins include a polyester resin as a condensation product of a dicarboxylic acid (e.g., adipic acid, suberic acid, sebacic acid, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, biphenyldicarboxylic acid) with a reactant containing an alkylene diol (e.g., ethylene glycol, butane diol, or hexane diol) or a copolymer polyester resin as a condensation product of a reactant containing two or more dicarboxylic acids as described above and/or two or more alkylenediols as described above. A branched polyester resin obtained from the above-described reactant including a triol such as trimethylol, propane or glycerine and/or a tricarboxylic acid such as trimellitic acid may also be used. The polyester resin may also be an aromatic polyester resin or an aliphatic polyester resin. Aliphatic polyester resins are typically preferred for the good performance in weatherability they provide. The polyester resin may be thermoplastic, thermosetting or radiation-curable. Preferably, the polyester resin is a thermoplastic polyester resin. The thermoplastic polyester resin has particularly good adhesion to the polyester film. The softening point of such a thermoplastic polyester resin is preferably from 70°C to 180°C, more preferably from 80°C to 160°C, again for the same reasons as described above for polyurethane resin. In a fourth illustrative embodiment of the present invention, the cover layer comprises a polyester film and the adhesion bonding interface comprises a non- crystalline polyester resin. In this embodiment where the polyester film has an adhesion bonding interface comprising a non¬ crystalline polyester resin, the resin of the binder layer preferably contains one or more of a polyurethane resin,' an acrylic resin, a polyester resin, a polyvinyl chloride resin, a polyvinylidene chloride resin or a polyolefin resin. Such a resin or resins result in good adhesion to the adhesion bonding interface comprising a non-crystalline polyester resin and can firmly afffixed to and support the retroreflective elements. Among these, preferred are polyurethane resin, acrylic resin, and polyester resin. These resins are preferred for their particularly good adhesion to the adhesion bonding interface. Additionally, formation of the hermetic cells is facilitated and deformation of the cells due to external force can be prevented. Such a resin may be thermoplastic, heat-curable, or radiation-curable. It is preferably a thermoplastic resin. The thermoplastic resin has particularly good adhesion to the above-described adhesive interface. The softening point of such a thermoplastic resin is preferably from 70° to 180°C, more preferably from 80° to 160°C for the same reasons as described above for polyurethane resin. The polyester cover film can improve impact resistance, dimensional stability under heat, abrasion resistance, shrink resistance and solvent resistance of the retroreflective sheeting. The polyester film may be a film containing a polyester such as polyethylene terephthalate, polyethylene isophthalate, polypropylene terephthalate, polybutylene terephthalate, poly(ethylene- 2, 6-naphthalate), poly(1,4-cyclohexanedimethylene terephthalate), poly(1,2-propylene glycol-2,6- naphthalate) , poly[-4,4'-bis (hydroxyphenyl)2,2- propane]iso/terephthalate (the dicarboxylic acid component being composed of a mixture of an isophthalic acid and a terephthalic acid at a mixing ratio of from 90:10 to 30:70). The polyester is preferably poly¬ ethylene terephthalate or poly(ethylene-2, 6-naphthalate) . The film containing such a polyester can improve the impact resistance, dimensional stability under heat, abrasion resistance, shrinking resistance and solvent resistance of the retroreflective sheeting, and also can improve the edge-chipping resistance of an encapsulated- lens type retroreflective sheeting. Illustrative examples of cover films include a three-layer film consisting of a front surface layer comprising the above-described polyester and serving as an outer surface of the retroreflective sheeting, a back surface layer comprising the above-described polyester and being in contact with the binder layer or adhesion bonding interface or layer described below, and an intermediate layer comprising other resins and having good impact resistance. A non-crystalline polyester film formed so as to have a low crystallinity has an adhesion bonding interface comprising a non-crystalline polyester resin and improves adhesion to the resin of the above-described binder layer or the resin of the adhesion bonding interface which will be described later. In encapsulated-lens type retroreflective sheetings, the binder layer is bonded to the cover layer only at the network of interconnecting lines forming the hermetically sealed cells within which the retroreflective elements are enclosed. Since the bonded area between the binder layer (which does not substantially contribute to the retroreflective effect) , and the cover layer is reduced compared to other cover films, it is particularly preferred to use such a polyester film as a cover layer. In this, case, the crystallinity of the film can be controlled by lowering the temperature at film formation, reducing the degree of stretching, increasing the molecular weight of polyester, blending two or more kinds of polyesters or using a polyester obtained by polymerization of a reactant containing two or more dicarboxylic acids and/or diols. A preferred blend of two or ' more kinds of polyesters comprises polyethylene terephthalate and polyethylene isophthalate. Such a cover film may be a three-layer film consisting of a front surface layer comprising the above¬ described polyester and serving as an outer surface of the retroreflective sheeting, a back surface layer comprising the above-described polyester and being in contact with the binder layer or the adhesion bonding interface or layer which will be described later, and an intermediate layer comprising other resins and having good impact resistance. The above-described polyester film may contain a stabilizer such as an ultraviolet absorbent and an antioxidant so as to prevent discoloring of the film or deterioration in the strength due to heat or radiation. In a fifth illustrative embodiment of the present invention, the cover film comprises a polyvinyl chloride- based film and the adhesion bonding interface comprising an acrylic resin or a polyurethane resin on the side in contact with the binder layer. Polyvinyl chloride cover films can improve impact resistance, dimensional stability with heat durability, abrasion resistance against scratching, shrinking resistance and solvent resistance of the retroreflective sheeting. The polyvinyl chloride-based cover film preferably comprises a polyvinyl chloride homopolymer or a polyvinyl chloride copolymer. Such a polyvinyl chloride copolymer is a copolymer obtained by polymerizing vinyl chloride with a reactant containing one or more polymerizable monomers such as vinyl acetate, polyvinyl alcohol, vinyl acetal, maleic acid and styrene monomer. The polyvinyl chloride film preferably contains a plasticizer. Examples of the plasticizer include a phthalate-based plasticizer, a polyester-based plasticizer, an adipate-based plasticizer, a fatty acid based-plasticizer, trimellitate-based plasticizer and an epoxy-based plasticizer. A polyester-based plasticizer is preferred. The polyester-based plasticizer in particular can improve impact resistance and edge- chipping resistance of the encapsulated-lens type retroreflective sheetings of the invention. The plasticizer is present from about 1 to about 50 parts by weight per 100 parts by weight of the resin. Such a film may be a three-layer film consisting of a front surface layer comprising the above-described polyvinyl chloride polymer and serving as an outer surface of the retroreflective sheeting, a back surface layer comprising the above-described polyvinyl chloride copolymer and being in contact with the binder layer or the adhesion bonding interface, and an intermediate layer comprising other resins having good impact resistance. The above-described polyvinyl chloride film may contain a stabilizer such as an ultraviolet absorbent and/or an antioxidant so as to prevent discoloring of the film or deterioration in toughness due to heat or radiation. The above-described cover films comprise an adhesion bonding interface formed by subjecting the surface of the film to be brought into contact with the above-described binder layer to a treatment to enhance adhesion of the cover layer to ' the binder layer. The adhesion bonding interface treatment may be a chemical treatment or alternatively may be a physicochemical treatment (e.g., corona discharge treatment, plasma treatment, plasma polymerization treatment, plasma jet treatment, flame treatment, ultraviolet irradiation treatment, electron beams irradiation treatment, electromagnetic wave irradiation treatment) . A corona discharge treatment is preferred among physicochemical treatments. The adhesion bonding interface formed by corona discharge treatment can greatiy improve the adhesion of the cover layer to the binder layer. In the corona discharge treatment, for example, a high frequency and a high voltage (usually from 1 to 6,000 kHz and from 5 to 30 kV) are applied onto the film surface. The corona discharge treatment may be conducted in an air atmosphere but is preferably conducted in an atmosphere containing nitrogen, oxygen, hydrogen, carbon dioxide, chlorine, argon, helium, boron trifluoride, butindiol, or acrylonitrile to further improve the adhesion of the resin to the binder layer. Most preferably, the corona discharge treatment is conducted in a nitrogen atmosphere. Polyester cover film surfaces may also be subjected to electromagnetic wave irradiation treatment by use of a flash lamp. Such treatment renders the surface noncrystalline, resulting in a non-crystalline polyester resin adhesion bonding interface. A non¬ crystalline layer having a thickness of from about 0.02 micrometers to about 0.25 micrometers is formed in the vicinity of the film surface. Such an electromagnetic wave irradiation treatment is described in U-.S. Pat. Nos. 4,810,434, 4,822,451, and 4,824,899. The polyester cover film surface may also be subjected to a surface treatment. The treatment may be with (1) an aqueous solution of an acid, an alkali or an amine or (2) a solution containing a trichloroacetic acid or a phenol compound. The polyester film surface is swelled or partially dissolved by the treatment to render the surface non-crystalline, thereby forming an adhesion bonding interface comprising a non-crystalline polyester resin. The adhesion bonding interface may be a layer comprising a resin or a silicon-based composition having good adhesion to the binder layer and cover film. The resin of the adhesion bonding interface preferably comprises any one of a polyurethane resin, a polyester resin, an acrylic resin, and an ethylenic resin as a main component. Further, it may contain as an additional resin one or more of a polyvinyl alcohol resin, a carboxymethyl cellulose resin, hydroxymethyl cellulose resin, a resin composed of a carboxylated styrene butadiene copolymer, a resin composed of a carboxylated acrylonitrile-butadiene copolymer, a polyvinyl acetal resin, a resin composed of a vinylene chloride acrylonitrile copolymer, and a vinyl chloride resin. In this case, any one of the polyurethane resin, the polyester resin, the acrylic resin, and the polyethylenic resin is contained in an amount of preferably 80 weight percent or more, more preferably 90 weight percent or more based on the total resin. When the cover layer is a polyester film and the resin of the binder layer is a polyurethane resin or an acrylic resin, the resin of the adhesion bonding interface is preferably a polyurethane resin, a polyester resin, or an acrylic resin. These resins can still further improve adhesion between the cover layer and the binder layer. Preferred combinations of the resin of the adhesion bonding interface and the resin of the binder layer are: (1) adhesion bonding interface: a polyurethane resin or a polyester resin, and binder layer: a polyurethane resin; and (2) adhesion bonding interface: an acrylic resin, and binder layer: an acrylic resin. The acrylic resin of the adhesion bonding interface is preferably a resin comprising a methyl methacrylate-based copolymer. The copolymer is most preferably a methyl methacrylate-butyl methacrylate copolymer. Such a copolymer further improves adhesion between the film and- the binder layer. The polyurethane resin of the adhesion bonding interface is preferably an aliphatic polyurethane resin. The aliphatic polyurethane resin undergoes reduced deterioration with yellowing due to ultraviolet light or heat, and limits whiteness reduction of the retroreflective sheeting so as to maintain daytime brightness. When the cover layer is a polyester film and the resin of the binder layer is a polyethylenic copolymer, the resin of the adhesion bonding interface is preferably an ethylenic resin. Such a resin improves adhesion between the cover layer and the binder layer. Preferred ethylenic resin comprises a copolymer obtained by polymerizing monomers containing polyethylene or ethylene. This copolymer is obtained by copolymerizing ethylene with a monomer copolymerizable with ethylene, such as a reactant containing acrylic acid, methacrylic acid, vinyl acetate, vinyl alcohol, 0- hydroxyalkyl (meth) acrylate, mono (hydroxyalkyl (meth) acrylate) acid phosphate-based monomer, vinyl chloride, vinylidene chloride, styrene based monomer, or acrylonitrile. The copolymer is preferably an ethylene- (meth) acrylic acid copolymer, an ethylene-vinyl acetate copolymer, an ethylene-methyl (meth) acrylate copolymer, an ethylene-ethyl (meth) acrylate copolymer, or an ionomer resin. When the cover layer is a polyvinyl chloride film, the resin of the adhesion bonding interface is preferably a polyurethane resin or an acrylic resin. These resins improve adhesion between the cover layer and the binder layer. Preferred combinations of the resin of the adhesion bonding interface and the resin of the binder layer are: (1) adhesion bonding interface: a polyurethane resin, and binder layer: a polyurethane resin and (2) adhesion bonding interface: an acrylic resin, and binder layer: an acrylic resin. These combinations can further improve adhesion between the cover layer and the binder layer. When the resin of the adhesion bonding interface is a thermoplastic resin, the softening point thereof is preferably from 50°C to 150°C, more preferably from 60°C to 130°C. If the softening point is less than 50°C, the retroreflective sheeting in roll form may exhibit sticking between layers; or "blocking". Upon rewinding the film, the adhesion interface layer may lose integrity. On the contrary, if the softening point exceeds 150°C, the adhesion of the adhesion bonding interface to the binder layer may be reduced. The resin of the adhesion bonding interface has a weight average molecular weight of preferably from 10,000 to 200,000, more preferably from 20,000 to 50,000. The above-described adhesion bonding interface may contain a crosslinking agent such as an isocyanate compound, an aziridine compound, an epoxy compound, or a melamine compound as long as it does not impair transparency of the adhesion bonding interface bonding layer. The crosslinking agent can greatly improve adhesion both to the binder layer and to the film. The crosslinking agent suitable for use with an acrylic resin is a melamine compound. The adhesion bonding interface containing an acrylic resin crosslinked by a melamine compound results in particularly good adhesion between the cover layer and the binder layer. The ratio of the crosslinking agent is preferably from 0.5 to 35 parts by weight per 100 parts by weight of the resin. The adhesion bonding interface may additionally contain an additive such as an ultraviolet absorbent, an antioxidant, a surface active agent, or an adhesion aid (e.g., silane coupling agent, higher fatty acid, higher fatty acid amide) as long as it does not impair transparency of the adhesion bonding interface. The resin of the adhesion bonding interface may be a resin having the same properties as that used for the binder layer. The resin of the adhesion bonding interface may be the same as or different from the resin of the binder layer. The silicon-based composition of the adhesion bonding interface comprises an organic silicon compound such as silane coupling agent and inorganic particles such as silicon dioxide, silicon nitride, silicon carbide, magnesium fluoride, titanium oxide, zinc oxide or antimony oxide. With such a composition, the adhesion bonding interface has a continuous gelled network structure of finely dispersed inorganic particles. The inorganic particles preferably have an average particle size of less than 0.5 micrometers so that the adhesion bonding interface remains transparent. The average particle size is more preferably less than 0.05 micrometers, and most preferably less than 0.02 micrometers. The organic silicon compound is preferably a bi¬ functional silane coupling agent such as an aminoalkyltriethoxy silane coupling agent. The bi¬ functional silane coupling agent can increase adhesion to the film and the resin of the binder layer. The above-described silicon-based composition may also contain an additive such as ultraviolet absorbent, antioxidant, surface active agent, crosslinking agent, higher fatty acid or higher fatty acid amide as long as it does not impair transparency of the adhesion bonding interface. The adhesion bonding interface comprising a resin or a silicon-based composition as described above has a thickness of preferably from 0.01 to 20 micrometers, more preferably from 0.03 to 10 micrometers. The adhesion bonding interface comprising a resin or a silicon-based composition as described above can be provided on the film by any usual coating means. The adhesion bonding layer comprising a thermoplastic resin may be provided by co-extrusion at the time of film formation. Further, before providing the adhesion bonding interface, the cover film surface may be subjected to physicochemical treatment or chemical treatment as described above. The adhesion bonding interface comprising the above-described resin or silicon-based composition improves adhesion between the linking portion, i.e., seal legs, between the binder layer and the cover layer. Therefore, a cover layer having such an adhesion bonding interface is particularly preferred. In retroreflective sheeting of the present invention, a printing layer may be adhered to the surface of the cover layer. Illustrative printing layers contain a printing ink comprising a coloring agent composed of a pigment or a dyestuff and at least one resin of a thermoplastic resin, a thermosetting resin and a radiation-curable resin. Such printing layers can be provided on the above-described surface by a coating means such as gravure coating or a printing means such as screen printing. In order to increase adhesion of the printing ink, the surface of the cover layer to which printing is applied may be subjected to physicochemical treatment or chemical treatment as described above prior to providing the adhesion bonding interface. Retroreflective sheeting of the present invention may be used by affixing it to articles such as a marker board of a road sign and an indication board of an information indicator by any suitable means of adhesion. The retroreflective sheeting may be cut or blanked into a predetermined size and a predetermined shape prior to being affixed to an object. After processing steps such as cutting or blanking, fine cracks typically arise at the edges of the cover layer of conventional retroreflective sheetings. These fine, almost invisible cracks are also known as edge chipping. The retroreflective sheeting of the present invention has improved resistance to such edge chipping. The retroreflective sheeting of the invention may be affixed to an object by means typically used for retroreflective sheetings, such as by a pressure sensitive adhesive adhesive or by a heat sensitive adhesive. Preferred is a pressure sensitive adhesive or heat-sensitive adhesive comprising an acrylic polymer. The adhesive layer may be applied during the construction of the retroreflective sheeting. A plastic film may be placed between the adhesive layer and the retroreflective layer in order to improve the strength of the retroreflective sheeting as a whole. Additionally, such-a film may be used when the retroreflective sheeting is affixed to a plastic containing a large amount of plasticizer, to prevent the penetration of the plasticizer from the plastic into the retroreflective layer. A typical production process for retroreflective sheeting of the invention is described below. Typically, transparent glass microspheres are temporarily bonded in a polymeric coating on a carrier web such as kraft paper. To accomplish this temporary bonding, the carrier web, with its coated side' outward, is passed over the surface of a drum heated sufficiently to cause the polymer to become tacky. Simultaneously, the heat-tackified coating of polymer is heated sufficiently to cause the polymer to soften and partially draw into it the compact monolayer of microspheres up to about 40 to 60% of their diameter. After cooling, e.g., by blowing room-temperature air onto the structure, a bead-bond coating such as, for example, a polyurethane is squeeze roll coated over the projecting microspheres of the polymer layer. This bead- bond coating is then buffed to remove bead-bond from the outermost area of microspheres. Typically, aluminum or some other specularly reflecting metal is then vapor coated over the exposed surface of the structure. A binder layer is then applied over the specularly reflecting coating. The entire structure so formed is then stripped from the carrier web, and then placed with its exposed microsphere surface in loose contact with the selected cover film. Materials useful in the binder layer are typically room-temperature solids that will soften to a flowable state when heated to temperatures between about 25°C and 150°C. While thermosetting constituents may be employed in the binder layer, the layer as a whole must exhibit a thermoplastic or thermoadhesive phase so that it can be converted by heat into a viscous flowable or movable condition. Temperatures used for the die element and the time of pressing them against the binder material may therefore vary greatly, and are governed by the temperature at which binder material fluidizes sufficiently to move into contact with and seal to the cover film as well as the time required to reach such condition after initial die contact. With encapsulated-lens type sheetings, under pressure of a heated embossing platen the binder material flows sufficiently to wet the cover film and to flood the microspheres in the area pressed, but it does not flow significantly into areas that are not pressed, and thereby leaves the hermetically sealed cell of exposed retroreflective elements. The flooding out of binder material into areas of the structure other than the line areas of seal is controlled. This method of controlled flooding of microspheres into binder material along fine lines without flooding or swallowing adjacent microspheres in a monolayer of microspheres maintains the necessary air interface for retroreflection of encapsulated-lens type retroreflective sheetings. Permanent hermetic affixation of the transparent film over the retroreflective surface of the sheeting in a networked pattern is accomplished, while preserving maximum area of the sheeting for retroreflectivity. The assembly of the cover film and base sheet may be bonded together by inserting the two sheets between a pair of platens. One platen is heated sufficiently and pressed against the rear surface of the laminate long enough to cause heat fluidization and viscous displacement of binder material toward the cover film. In encapsulated-lens type retroreflective sheeting, one platen may be an embossing platen having a pattern of raised ridges. The ridges on the embossing platen press against the base sheet material to deform the binder layer into the desired configuration. The binder layer is heated and pressed sufficiently against the other platen so that it floods the microspheres in the areas pressed and contacts the cover film. This second platen is preferably not heated and suitably covered with rubber so as to permit yielding without loss of the moderate pressure needed for hermetic heat-sealing according to the pattern of the die elements. The pattern.of ridges on the embossing platen is such as to form a network of narrow bonds. Following the embossing operation in encapsulated-lens type retroreflective sheeting, the cover film continues to be in spaced relation with the retroreflective elements. The sheet material has the desired hermetic seals covered by a cover film, and surrounded on all borders by a polymer-based bond. If desired, a support film may be laminated to the binder layer prior to, or during, the embossing operation to separate the embossing platen from the binder layer. In addition, the sheeting may include a layer of adhesive and a release liner. Examples The invention will be further explained by the following illustrative examples which are intended to be non-limiting. Unless otherwise indicated, all amounts are expressed in parts by weight. Materials Used In Making Samples of Retroreflective Sheeting * retroreflective elements: transparent glass microspheres with an index of refraction of about 1.9, and an average diameter of from about 50 micrometers to about 80 micrometers * Adhesive: a layer of pressure sensitive adhesive comprising an isooctyl acrylate acrylic acid copolymer (monomer weight ratio 94:6) * Binder layer: various compositions (described in each example) * Cover layer: various compositions (described in each example) Process Used to Make Retroreflective Sheeting Samples Samples of the encapsulated-lens type retroreflective sheeting of the Examples were made as follows. Numerals in the text correspond to the numerals for respective constituent elements shown in Figure 1 or Figure 2. (1) Transparent microspheres 3 were partially embedded (to a depth of approximately 40% of their average diameter) into a polyethylene layer approximately 25 micrometers thick to provide a monolayer of transparent microspheres 3. The polyethylene layer was supported by a carrier web. (2) Aluminum, a specularly reflective element 4, was vapor coated at a thickness of about 0.1 micrometers onto the exposed surfaces of the microspheres 3. The focal point of the transparent microspheres 3 was substantially at the interface between the transparent microspheres 3 and the specularly reflective element 4. (3) Next, a binder layer 2 having a thickness of about 60 micrometers and a release film (not shown) were laminated onto the coated, exposed surfaces of the microspheres. The carrier web was then removed. The binder layer 2 may also be applied by coating a solution comprising components for the binder layer onto the exposed surfaces of the microspheres in addition to lamination of a film of thermoplastic binder components formed by any appropriate means. (4) Next, on the surface where the carrier web was removed, a film 11 having an adhesion bonding interface 13 (including the case of adhesion bonding layer 12) as indicated below was laminated as a cover layer 1. The binder layer 2 was embossed through the release film using a mold having a network pattern of lines (not shown) and at least one heated platen, to result in a large number of network-patterned narrow linking portions 21 forming where the binder layer 2 was partially bonded to the cover layer 1. The binder layer 2 is bonded to the cover layer 1 along these narrow linking portions 21 to form hermetically sealed cells containing transparent microspheres 3. (5) The release film was then removed from the surface of the binder layer opposite the surface in contact with the cover layer. An adhesive layer 5 with a release liner 6 was applied to produce a retroreflective sheeting 100. The adhesive layer 5 provides a means for affixing the retroreflective sheeting 100 to an article (not shown) . Test Methods Unless otherwise indicated, the following test methods were used. Impact Resistance A retroreflective sheeting of greater area than the aluminum plate adherend of 150 millimeter ("mm") by 70 mm by 1 mm was affixed to such aluminum plate using a hand squeeze roller applicator. The edge portion of the retroreflective sheeting was trimmed to make the edge portion of the retroreflective sheeting and the edge portion of the aluminum plate even to form a sample for evaluation. Dead-weights of differing amounts were dropped onto the retroreflective sheeting of the sample at a temperature of 20°C using a Gardner impact tester. The height from which the weights were dropped was varied in sequence and the damage condition of the sample was observed. The value of the impact resistance was taken as the maximum (unit: inch-pound) of the product of the weight and the height that did not result in peeling between the cover layer and the binder layer or cracks or crazings on the cover layer portion of cells. The upper limit of the product of the weight and the height in this test was set at 80 inch-pound; thus the evaluation value of a sample which resulted in no breakage at the upper limit was shown as >80. Dimensional Stability Against Heat A sample was prepared in the same manner as in the evaluation on "impact resistance" except the size of the aluminum plate was 152 mm x 152 mm x 1.6 mm. The sample was placed in a 120°C oven for 30 minutes. After removal, the sample was allowed to cool at room temperature. The area of the shrunken retroreflective sheeting was then measured. The ratio of the shrunken area to the original area (152 mm x 152 mm) was taken as the dimensional stability against heat. Edge-Chipping Resistance A sample was prepared in the same manner as in the evaluation of "impact resistance" above. However, the sample was cut and affixed so as to expose the maximum amount of cells at the edge of the sample. Using the thus-prepared sample, this same part of the edge portion was rubbed five times with a finger tip in such a manner as to peel the cover layer along the direction from the edge portion to the center portion. When the cover layer was broken or peeled to expose transparent microspheres, the evaluation was NG and when transparent microspheres were not exposed, the evaluation was OK. Shrink Resistance A retroreflective sheeting with a release paper on the adhesive layer was cut into a size of 280 mm x 215 mm to prepare a sample for evaluation. The sample was allowed to stand at a temperature of 20°C and a relative humidity of 50% for 10 days. The shrunken length (unit: mm) of the retroreflective sheeting was measured by comparing with the lengthwise direction (280 mm) of the release paper and taken as an evaluation value of the shrinking resistance. Pencil Scratch Abrasion Value A sample was prepared in the same manner as in the evaluation for "impact resistance" above except the size of an aluminum plate was 150 mm by 70 mm by 1 mm. The cover layer of the retroreflective sheeting of the resulting sample was scratched by a pencil. The least dense mark of the pencil density at the point of breakage of the cover layer was taken as the evaluation value of pencil scratching value. The evaluation in this item was conducted according to the tester method provided in Toryo-Ippan Shiken Hoho, JIS K5400. Pencils having a density mark of from 9H (most hard) to 6B (most soft) were prepared. The softer density was defined as a subordinate position. In this test method, evaluation values closer to 9H indicated a greater resistance to scratching. The evaluation value of samples which resulted in no breakage of the cover layer even with a 9H pencil were shown as ">9H". Peeling Resistance of Cover Layer A retroreflective sheeting was cut into a size of 25 mm x 150 mm, affixed to an aluminum plate cleaned with isopropyl alcohol and allowed to stand at room temperature for 24 or more hours to produce a sample for evaluation. The cleaning was conducted according to the method described in JIS Z 0237, Item 8. A pressure sensitive tape comprising a polyester base material and a pressure sensitive adhesive was fixed onto the sample to cover the surface of the retroreflective sheeting. After forming a pull portion by making a slit on the polyester base material using a cutter, the peeling strength of the cover layer was measured by using a "peeling tester" manufactured by Instron Co., Ltd. The peeling rate was 300 mm/min and the peeling angle was 90°. An average value of three measurements was taken as the evaluation value for the peeling resistance of the cover layer. Solvent Resistance A sample was prepared in the same manner as in the evaluation for "pencil scratch abrasion value" above. The sample was dipped in kerosene. After drying at room temperature, the appearance of the retroreflective sheeting sample was observed. Four separate samples were prepared in the same manner and dipped in a turpentine oil, methanol, toluene and xylene, respectively (in the former two solvents, the dipping time was 10 minutes and in the latter two solvents, the dipping time was 1 minute) , followed by the same drying as in the kerosene case. If whitening, wrinkling or cracking of the cover layer appeared in any one of solvents, the evaluation was NG. When there was no whitening, wrinkling or cracking of the cover layer in all of solvents, the evaluation was OK. Adhesion of Printing Layer A sample was prepared in the same manner as in the evaluation for "impact resistance" above. On the cover layer surface of the retroreflective sheeting of the resulting sample, a printing layer was provided using a printing ink (containing an acrylic polymer) for a road sign by a screen printing method and subjected to the following peeling test. First, a pressure sensitive adhesive cellophane tape #610, produced by 3M Co., was tightly affixed to the printing layer surface by using a squeegee and then the pressure-sensitive adhesive tape was peeled off swiftly. When the printing layer was peeled off from the cover layer surface, the evaluation was NG. When the printing layer was not peeled off, the evaluation was OK. Example 1 A biaxially stretched polyester film having a total thickness of 50 micrometers was prepared which comprised an adhesion bonding interface provided by coating an aqueous dispersion containing an aliphatic polyurethane resin on one surface of a polyethylene terephthalate film at the production step of the film. The aliphatic polyurethane resin used was NEOTAC™ R-9316 produced by ICI Resins Co., Ltd. and had a softening point of about 120°C. By using the resulting polyester film as a cover layer, a retroreflective sheeting of this example was prepared according to the above-described sample preparation process. The binder layer contained a thermoplastic aromatic polyurethane resin having a softening point of about 110°C, rutile titanium dioxide, and stearic acid. The evaluation results of the various tests for the retroreflective sheeting prepared in this example are shown in Table 1. The evaluation in subsequent examples were conducted in the same manner. Example 2 One surface of a 50 micrometers thick biaxially stretched polyethylene terephthalate film (HB Tetron Film produced by Teijin Ltd.) was subjected to a corona discharge treatment. An adhesion bonding interface was provided by coating an aqueous dispersion containing an aliphatic polyester resin (NEOREZ™ R-972 produced by ICI Resins Co., Ltd., softening point: about 120°C) to provide a polyester film having an adhesion interface layer. A retroreflective sheeting of this example was prepared in the same manner as in Example 1 except for using the resulting polyester film as a cover layer. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 3 A retroreflective sheeting of this example was prepared in the same manner as Example 2 except for using an aqueous dispersion comprising a methyl methacrylate- butyl methacrylate copolymer (NEOCRYL™ A-5045 produced by ICI Resins Co., Ltd.), a melamine compound (RESIMENE™ 750 produced by Monsanto Co., Ltd.) and an acid catalyst as an aqueous dispersion and an electron beam-curable resin comprising an ethyl acrylate-methyl methacrylate-isooctyl acrylate copolymer and a polyethylene glycol (200) diacrylate as a resin of the binder layer. The binder layer of this example was formed according to the method described in Japanese Examined Pat. Publication (Kokoku) No. 61-13561. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 4 A retroreflective sheeting of this example was prepared in the same manner as in Example 2 except for using an aqueous dispersion comprising colloidal silica, aminopropyltriethoxysilane and a surface active agent as an aqueous dispersion. The aqueous dispersion used in this example was adjusted according to the method described in Japanese Unexamined Pat. Publication (Kokai) No. 2-200476. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 5 A retroreflective sheeting of this example was prepared in the same manner as in Example 1 except that a biaxially stretched film of polyethylene terephthalate and an adhesion bonding interface composed of polyethylene was used as the polyester film. A thermoplastic film composition containing a resin comprising a polyethylene vinyl acetate copolymer and a polyethylene-methacrylic acid copolymer comprised the binder layer. The binder layer was formed in this example according to the method described in Japanese Unexamined Pat. Publication (Kokai) No. 62-121043. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 6 A retroreflective sheeting of this example was prepared in the same manner as in Example 2 except the adhesion bonding interface was formed by subjecting the polyester film surface to corona discharge treatment in a nitrogen gas. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 7 A retroreflective sheeting of this example was prepared in the same manner as in Example 1 except that a non-crystalline polyethylene terephthalate (A-PET FLL, produced by Teilin Limited) having a thickness of 50 micrometers was used as a polyester cover film. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 8 A retroreflective sheeting of this example was prepared in the same manner as in Example 2 except that a composition comprising 100 parts by weight of a polyester resin (Hot-Melt Polyester 4101, produced by Bostik Co., Ltd.) and 20 parts by weight of rutile titanium dioxide was used as the resin of the binder layer. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 9 A retroreflective sheeting of this example was prepared in the same manner as in Example 1 except that a film comprising a polyester-based plasticizer and a polyvinyl chloride homopolymer in place of polyethylene terephthalate and a film having an adhesion bonding interface having the same composition (polyurethane resin) as in Example 1 was used as the cover layer. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 10 A retroreflective sheeting of this example was prepared in the same manner as in Example 3 except that the cover film of Example 9 was used. The evaluation results of the retroreflective sheeting of this example are shown in Table 1. Example 11 A retroreflective sheeting was prepared in the same manner as Example 1 except that the adhesion bonding interface was provided by coating a solvent based solution comprising a polyester resin (Vylon series, Toyobo Co. Ltd.) onto the biaxially stretched polyester film of thickness 50 micrometers. As in Example 1, the binder layer contained an aromatic polyurethane resin having a softening point of about 110°C, rutile titanium dioxide, and stearic acid. Example 12 A retroreflective sheeting was prepared in the same manner as Example 11 except that an additional layer of pigmented aliphatic polyurethane resin was coated onto the polyester resin adhesion bonding interface. The pigmented layer also acts as an adhesion bonding surface to a binder layer. The coating solution of the pigmented layer comprised lOOg of aliphatic polyurethane solution (Dainichiseika Co., Ltd., Resamine NE-308) , 3.1g of 50% Pigment Red 168 predispersed in VAGH vinyl resin (30% solids), and 1.5g of 50% Pigment Yellow 110 predispersed in VAGH vinyl resin (26% solids) . Comparative Example A A retroreflective sheeting was prepared in the same manner as in Example 3 except that a 50 micrometers thick biaxially stretched polymethyl methacrylate film was used as the cover layer. The evaluation results of the retroreflective sheeting of this example are shown in Table 2. Comparative Example B A retroreflective sheeting was prepared in the same manner as in Example 3 except that a 50 micrometer thick biaxially stretched impact resistant polymethyl methacrylate film was used as the cover layer. The film of this example comprised a blend resin of polymethyl methacrylate and an acrylic polyphase polymer disclosed in Japanese Unexamined Pat. Publication (Kokai) No. 61- 255846. The evaluation results of the retroreflective sheeting of this example are shown in Table 2. Comparative Example C A retroreflective sheeting was prepared in the same manner as in Example 1 except that a 50 micrometer thick film comprising an ethylene-acrylic acid copolymer was used in place of the polyethylene terephthalate film. The film of this example was the film disclosed in Example 1 of Japanese Unexamined Pat. Publication (Kokai) No. 63370940. The evaluation results of the retroreflective sheeting of this example are shown in Table 2. Comparative Example D A commercially available encapsulated-lens type retroreflective sheeting was subjected to the same evaluation as the other samples. This commercially available retroreflective sheeting comprised a cover layer containing an unstretched impact resistant acrylic resin and a binder layer containing an acrylic resin. The evaluation results of the retroreflective sheeting of this example are shown in Table 2. Comparative Example E A retroreflective sheeting was prepared in the same manner as in Example 3 except that a 50 micrometer thick biaxially stretched polyethylene terephthalate film (HB Tetron Film, produced by Teijin Limited) was used for the cover film, with no additional adhesion bonding interface. The evaluation results of the retroreflective sheeting of this example are shown in Table 2. Comparative Example F A retroreflective sheeting was prepared in the same manner as in Example 1 except that a 50 micrometer thick film comprising an aliphatic polyurethane resin was used as the cover film, with no additional adhesive bonding interface. The evaluation results of the retroreflective sheeting of this example are shown in Table 2. Comparative Example G A retroreflective sheeting was prepared in the same manner as in Example 3 except that a 50 micrometer thick polyvinylidene fluoride film (DX Film produced by Denki Kagaku Kogyo K.K.) was used as the cover film, with no additional adhesion bonding interface. The evaluation results of the retroreflective sheeting of this example are shown in Table 2. Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention.

Claims: Claims : 1. An encapsulated-lens retroreflective sheeting comprising a monolayer of lenses partially embedded in a binder layer, a specularly reflective layer underlying the lenses in optical association therewith, and a cover film bonded to said binder layer, wherein said cover film comprises a polyester film or a polyvinyl chloride-based film, said cover film has an outer surface and an inner surface, said inner surface is in at least partial contact with said binder layer, said inner surface comprises an adhesion bonding interface, and said binder layer comprises an acrylic resin or a polyurethane resin. 2. The retroreflective sheeting of claim 1 wherein said adhesion bonding interface comprises a layer of polyurethane resin bonded to said inner surface of said cover film. 3. The retroreflective sheeting of claim 2 wherein said binder layer comprises a polyurethane resin. 4. The retroreflective sheeting of claim 1 wherein said adhesion bonding interface comprises a layer of polyester resin bonded to said inner surface of said cover film. 5. The retroreflective sheeting of claim 4 wherein said binder layer comprises a polyurethane resin. 6. The retroreflective sheeting of claim 1 wherein said adhesion bonding interface comprises a layer of acrylic resin bonded to said inner surface of said cover film. 7. The retroreflective sheeting of claim 6 wherein said binder layer comprises an acrylic resin. 8. The retroreflective sheeting of claim 1 wherein said adhesion bonding interface comprises a layer of a silicone based composition bonded to said inner surface of said cover film. 9. The retroreflective sheeting of claim 8 wherein said binder layer comprises a polyurethane resin. 10. An encapsulated-lens retroreflective sheeting comprising a monolayer of lenses partially embedded in a binder layer, a specularly reflective layer underlying the lenses in optical association therewith, and a cover film bonded to said binder layer, wherein said cover film comprises a polyester film, said cover film has an outer surface and an inner surface, said inner surface faces and is in partial contact with said binder layer, said inner surface comprises an adhesion bonding interface, said adhesion bonding interface comprises an ethylenic resin, and said binder layer comprises an ethylenic copolymer. 11. An encapsulated-lens retroreflective sheeting comprising a monolayer of lenses partially embedded in a binder layer, a specularly reflective layer underlying the lenses in optical association therewith, and a cover film bonded to said binder layer, wherein said cover film comprises a polyester film, said cover film has an outer surface and an inner surface, said inner surface is in at least partial contact with said binder layer, said inner surface comprises an adhesion bonding interface, and said binder layer comprises a polyester resin. 12. The retroreflective sheeting of claim 11 wherein said polyester resin in said binder layer has a softening point of from 70°C to 180°C. 13. An encapsulated-lens retroreflective sheeting comprising a monolayer of lenses partially embedded in a binder layer, a specularly reflective layer underlying the lenses in optical association therewith, and a cover film bonded to said binder layer, wherein said cover film comprises a polyester film, said cover film has an outer surface and an inner surface, said inner surface is in at least partial contact with said binder layer, said inner surface comprises an adhesion bonding interface, said adhesion bonding interface comprises a non-crystalline polyester resin. 14. The retroreflective sheeting of claim 13 wherein said polyester film is a single layer film comprising a non-crystalline polyester resin. 15. The retroreflective sheeting of claim 1 wherein said adhesion bonding interface comprises a mechanically roughened surface.

Inventors: ARAKI YOSHINORI, BAILEY TERRY R, NAGAOKA YOSHIYUKI, NGUYEN THANH-HUONG, YOKOYAMA MASAMI, ARAKI, YOSHINORI, BAILEY, TERRY, R., NAGAOKA, YOSHIYUKI, NGUYEN, THANH-HUONG, YOKOYAMA, MASAMI
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1996
Main classification: G02B5
IPC codes: E01F9, G02B5, G09F13