MXPA99003036A - Retroreflective sheeting with embedded retroreflective elements in a binder layer with shape memory and method for forming the same - Google Patents

Abstract

A method for forming retroreflective sheeting comprising a binder layer having a first surface with protrusions thereon and a plurality of retroreflective elements partially embedded in the surface, wherein the method comprises forming the binder layer in a first profile, embedding the retroreflective elements therein, and then activating that binder layer such that it is restored to the first profile via shape memory.

Description

RETRORREFLECTOR COATING WITH RETRORREFLECTOR ELEMENTS EMBEDDED IN A BINDING LAYER WITH MEMORY OF FORM AND METHOD OF FORMATION THEREOF Field of the Invention The present invention is concerned with a retroreflective coating that provides effective retroreflection at low and high angles of incidence and a method for forming such a coating.

BACKGROUND OF THE INVENTION A variety of retroreflective coatings comprising microspheres have been used for traffic signs, road signs, pavement markings, protruding ribbons, etc. Illustrative examples include those described in U.S. Patent No. 3,005,382 (Weber) and Japanese Patent A-57-27748. These coatings typically comprise a polymeric binder cap and a plurality of microspheres, normally arranged in a monolayer, partially embedded in and protruding from the binder layer. The microspheres typically have a reflective layer thereon, for example aluminum, silver or dielectric coating or in other embodiments, the binder layer contains reflecting particles, for example, pigment particles such as titanium dioxide, metal flakes, REF: 29766 pearly flakes, etc., which function in optical association with the microspheres to provide retroreflective effect. These coatings normally provide the greatest effective retroreflective performance at angles of incidence that are substantially perpendicular to the plane of the microsphere monolayer. In many applications, such as traffic signals, a signal and the coating thereon are oriented substantially perpendicular to the p-rapprochement direction and the effective retroreflective effect is obtained. To improve the retroreflective performance at high incident angles, for example such as is found in the coatings used on the sides of the vehicles, on the sides of the tunnels, protection rails, road surfaces, etc., the formation of protuberances in the coating, in such a way that at least some of the microspheres are oriented to be presented more perpendicularly to the observer. This configuration has been used extensively in pavement marking tapes, where effective retroreflection at very high angles of incidence is desirable. See, for example, U.S. Patent No. 5,316,406 (Wyckoff).

Various methods for obtaining a support layer with protuberances carrying retroreflective elements are known. In some embodiments, a foaming composition is applied to a desired spot on a flat substrate and a liquid composition, for example, a paint, which contains binder resins and reflective pigments is coated on the surface of the substrate. The microspheres are dispersed on the surface of the liquid composition, to be partially embedded therein. The composition is cured to secure the microspheres thereto and the foaming composition is activated, for example with heat, to raise the portions of the binder layer, to thereby form microspheres protrusions thereon. Examples of this process are described in the Japanese publications Nos. A-55-65524 and A-57-193352. In some embodiments, a substrate made of thermoplastic resins is etched or highlighted to form a surface with protuberances thereon. A paint containing binder resins and reflective pigments is coated on the surface in a sufficient thickness to fill the indented portions and produce a flat surface. The microspheres are partially embedded in the surface of the cured binder resins, to secure them thereto. Then the construction is heated to cause the substrate to return to a flat shape, to thereby deform the binder layer to produce protuberances corresponding to the difference in coated thickness of the binder layer. Examples of this process are described in the Japanese publications Nos. A-53-46363 and A-53-46371. In some embodiments, a paint that shrinks and wrinkles is used. The paint containing binder material and reflective pigments, is coated on a substrate, the microspheres are embedded or partially embedded in it, and then the paint is heated to dry and secure the microspheres therein and also to cause it to wrinkle, to to form, by this, protuberances with microspheres thereon. An example of this procedure is described in Japanese publication No. A-57-10102. Other methods include the formation of protuberances and microspheres embedded or partially embedded therein as described in U.S. Patent No. 4,069,281 (Eigenmann) and Japanese Publication No. A-58-237243. British Patent No. 2,251,091 describes a retroreflective coating comprising microspheres adhered to an aluminum layer and German Patent No. 3,039,037 discloses a vehicle tire with a side face having raised areas, to which microspheres have been adhered. Each of these procedures suffers from some deficiency. It is often difficult to control the depth at which the microspheres are embedded or encrusted, because the microspheres are normally applied to the binder layer before it is cured and / or dried and so is still quite soft. It is also difficult to form protuberances of desired shape and size, in which the microspheres are embedded or uniformly embedded. If the microspheres are not embedded in a sufficiently deep way, they can tend to be easily dislodged. If the microspheres are embedded too deeply the retroreflective response can be impaired. If the microspheres are not embedded or uniformly embedded, the product may not provide a desired uniformity of retroreflective effect. Finally, it is also often difficult in these processes to form protuberances in the form of pyramids and prisms that would be advantageous for retroreflection at very high angles of incidence.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a retroreflective coating that provides effective retroreflection at high angles of incidence. The invention also provides a method for the formation of such a coating. In brief summary, the method of the invention comprises: a) providing a binder layer having first and second sides, the binder layer is or can become capable of exhibiting shape memory; b) forming the binder layer to a first desired profile such that the first side has one or more protuberances thereon; c) providing a plurality of retroreflective elements; d) applying the retroreflective elements to the first side of the binder layer, in such a way that the retroreflective elements are partially embedded or embedded in the binder layer and the layer is deformed to a second profile; and e) activating the binder layer in such a manner as to substantially invert the first profile while retaining the retroreflective elements embedded or partially embedded therein. Normally, the method will furthermore comprise, after activation of the binder layer to cause inversion to the first profile, the step of: f) stabilizing the binder support layer in the first desired profile. Briefly, the retroreflective coating of the invention comprises a binder layer with retroreflective elements partially embedded in the surface thereof. The binder layer has first and second sides, the first side has one or more protuberances thereon, the protuberances have one or more retroreflective elements partially embedded therein. An important distinction with respect to previously known profiled or shaped retroreflective coatings is that the binder layer is of a so-called "shape memory" material that has been deformed and then activated after the reflective elements are embedded or embedded therein to get the desired final shape. The coatings of the invention can provide an effective retroreflective effect at very high angles of incidence over substantially all of its surface. The coating of the invention can be easily fabricated into a desired shape or profile to provide effective retroreflective performance for the proposed application.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further explained with reference to the drawings, in which: Figure 1 is a cross-sectional view of the binder layer of an illustrative coating of the invention in a first profile; Figure 2 is a cross-sectional view of an intermediate coating of the invention showing the binder layer in a second profile, with reflecting elements embedded or partially embedded in the surface thereof; Figure 3 is a cross-sectional view of the finished coating wherein the binder layer has undergone form recovery and is restored to the first profile; Figure 4 is a perspective view of a portion of a mold to form a binder layer according to the invention; and Figure 5 is a schematic illustration of the apparatus used to evaluate the retroreflective properties of the coatings in the examples. These figures, which are idealized, are not to scale and are intended to be illustrative only and not limiting.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE MODES As discussed above, the method of the invention comprises, in brief summary: a) providing a binder layer that is or can become capable of exhibiting shape memory and having first and second sides; b) forming the binder layer to the desired profile such that the first side has one or more protuberances thereon; c) providing a plurality of retroreflective elements; d) applying the retroreflective elements to the first side of the binder layer, in such a way that the retroreflective elements are partially embedded or embedded in the binder layer and the layer is deformed to a second profile; and e) activating the layer in such a way as to invert the first desired profile while retaining the retroreflective elements. Typically, the method will further include, preferably, the step of: f) stabilizing the binder support layer in the first desired profile after inversion. The binder material in the coatings of the invention comprises a polymeric material that exhibits shape recovery or shape memory, that is, a tendency, when activated, to return to a predetermined shape or profile. These are materials that can be deformed under pressure from a first predetermined profile to a second shape or profile and then will return to the first shape or profile, for example, in activation by heating without any restriction pressure. The polymer chains of such resins are in a state such that their movement is restricted by crosslinking and intermolecular physical and chemical interlacing of the molecules. Such a restriction has the effect of allowing memory and shape recovery. The resins are preferably sufficiently thermoplastic to allow the incrustation of the retroreflective elements therein. In addition, the binder material will preferably adhere sufficiently to the retroreflective elements, so that they are retained in the final product. Also, the binder material is preferably durable, that is, it will withstand use in the proposed application without the undesirable discoloration, breaking, peeling or peeling, etc., for at least a useful service life. The binder material preferably exhibits a storage modulus (referred to herein as Mo) of: (1) at least about 3 x 105 Pa (3 x 106 dynes / cm2), more preferably between about 1 x 106 Pa (1 x 107) and approximately 3 x 108 Pa (3 x 109 dynes / cm2), at 25 ° C and (2) at least approximately 9 x 105 Pa (9 x 106 dynes / cm2), more than preference between approximately 1 x 104 Pa (1 x 105) and approximately 5 x 105 Pa (5 x 106 dynes / cm2), at 120 ° C, measured by the dynamic viscoelasticity module. The Mo can be measured by using the "RSA II" device of RHEOMETRIX in a compression mode at a frequency of 1 Hz. Such materials can effectively and easily be used to form binder layers with a wide range of first desired profiles of according to the invention. If the storage module at 25 ° C is smaller than this range, the resulting binder layer will not exhibit the desired shape memory and will not effectively recover the first desired profile. In addition, the retroreflective elements may not be effectively retained by the resulting binder layer, to result in dislodging and loss of retroreflective effect. If the storage module at 120 ° C is larger than the indicated range, the binder layer will tend to be insufficiently thermoplastic to allow effective embedding of the retroreflective elements therein. As a result, the elements may not be effectively retained by the resulting binder layer.

In particular, the storage module at 25 ° C is preferably at least about 1 x 106 Pa (1 x 107 dynes / cm2) for prolonged durability in outdoor applications. The binder material preferably exhibits high flow characteristics to allow easy and effective application to a mold surface with fine characteristics. Some illustrative examples of known resins having shape memory, which may be used in the present invention, include polybenzene, styrene-isoprene copolymers, polyurethane resins, etc. They have softening points in an appropriate range to allow effective embedding of the retroreflective elements therein during production for effective retention. The softening point is usually in the range of about 80 ° to about 200 ° C, preferably between about 90 ° C and about 180 ° C. Illustrative examples of preferred monomers include at least one (meth) acrylate monomer which produces a homopolymer having a vitreous transition temperature between about -50 ° C and about 80 ° C, for example phenoxy acrylate isooctyl acrylate, tertiary butylacrylate. It has been found that such materials exhibit excellent molding properties and excellent shape memory properties in curing.

The cohesive strength and thermal resistance of the final binder layer can be improved by the incorporation of at least one second (meth) acrylate monomer which produces a homopolymer having a vitreous transition temperature of between about 80 ° and about 150 °. C. Illustrative examples include acrylic acid, isobornyl acrylate, etc. The second monomer is included in an amount that constitutes from 0 to about 85 weight percent of the total monomer components depending on the monomers. For example, if the first monomer is isooctyl acrylate and the second monomer is acrylic acid, the amount of acrylic acid is preferably in the range of about 10 weight percent and 40 weight percent. If it is less than 10 weight percent, the storage modulus at 25 ° C tends to be too low, so that the resulting binder layer does not exhibit sufficient shape memory. If the content of acrylic acid is greater than 40 weight percent, the storage module at 120 ° C is too high and the binder layer does not exhibit sufficient thermoplasticity and adhesion to the retroreflective elements is difficult. Some illustrative examples of polymers that may be used include phenoxyethyl acrylate homopolymers, copolymers comprising about 60 to about 90 weight percent isooctyl acrylate and about 10 to about 40 weight percent acrylic acid, acrylate copolymers, and isooctyl and isobornyl acrylate and tertiary butyl acrylate homopolymers. The binder layer is usually formed by pouring the liquid precursor composition onto a mold defining the first desired profile and then cured in its place. When the precursor composition has relatively high flow characteristics, for example, a viscosity at 25 ° C of between about 1 and about 100 centipoises, a geometrical shape or configuration with fine structures can be easily obtained. For example, phenoxyethyl acrylate resins have a viscosity at 25 ° C of 0.008 to 0.009 Pas (8 to 9 centipoise). A precursor composition consisting essentially of that resin can be easily poured onto a mold of fine configuration without entrapment of air bubbles at the interface with the mold surface. In some instances, it may be desirable to partially polymerize the precursor prior to its formation on the mold. Normally, the liquid precursor composition will further comprise a crosslinking or crosslinking agent. For example, when the binder layer is made with polymers of (meth) acrylics, such as phenoxyethyl acrylate, it will also further comprise polyfunctional (meth) acrylic monomers, such as 1,6-hexanediol diacrylate as a crosslinking (or bonding) agent crossed). Depending on the type of crosslinking agent, the binder resin, etc., the composition will normally comprise between about 0.01 and about 0.1 parts by weight of the crosslinking agent, more usually between about 0.02 and about 0.05 parts by weight per 100 parts. of (meth) acrylic monomers. The crosslinking agent will normally increase the thermal resistance of the resulting binder layer and contribute to effective memory properties. However, if the composition contains excessive amounts of crosslinking agent, the layer may not be sufficiently thermoplastic to allow easy embedding or imbibition of the retroreflective elements therein. The mold is designed in the shape of the desired profile or final shape of the binder layer. Typically, the shape will include a plurality of raised portions or protuberances, usually small in size, that are regularly arranged on a flat base. Illustrative examples include pyramid-like shapes, prisms, cones, partial spheres, conical trunks (or truncated cones), parallel splines or lines or spliced sinusoids, etc. The desired shapes of the protuberances are determined in part by the desired retroreflective properties and anticipated conditions of use. The pyramid-like shapes are characterized by lateral planes extending from the base to the top of the raised portions. Normally, it is preferable that the lateral planes are flat. Normally, the slope, that is, the included angle between the base and the lateral plane, is between about 30 ° and about 80 °, preferably between about 40 ° and about 75 °. If the slope is less than about 30 °, the retroreflective performance at high angles of incidence may be impaired. When the slope is more than about 80 ° C, the retroreflective performance at other angles of incidence can be undesirably reduced. The optimum slope for a specific application can be easily selected by those of ordinary skill in the art, and then easily implemented in accordance with this invention. The distance from the top of a raised portion to that of an adjacent raised portion is usually in the range of about 0.1 to about 20 millimeters, preferably between about 0.3 and about 10 millimeters. If the distance is much smaller than this range, the number of retroreflective elements that can be embedded in a protrusion may be too small, to result in reduced retroreflective brilliance. Depending on the height of the raised portions, if the distance is greater than 20 millimeters, the retroreflective performance at high angles of incidence may be impaired. Some common dimensions for the height of the raised portions is that they are approximately 2 and approximately 5 millimeters and the distance between the adjacent elevated portions is between approximately 10 and approximately 20 millimeters. The mold can be made of any of a variety of suitable materials, depending in part on the characteristics of the binder layer composition, the curing conditions, the size of the binder layer being made, etc. Illustrative examples of materials from which the molds could be manufactured include glass, plastics, ceramics and metal. The desired shape is formed in the mold by an appropriate technique, for example, molding a masterpiece, tooling, etching, needle wrap, etc. Once applied to the mold, the liquid precursor composition is polymerized, usually by application of appropriate radiation. Illustrative examples include ultraviolet radiation, electron beam and heat.

Depending on the formulation, it may be useful to incorporate one or more photopolymerization initiators, e.g., radical polymerization initiators, in the composition to shorten the cure time and obtain polymerization with minor amounts of applied radiation. Illustrative examples include benzoin, benzoin methylether, benzoin n-propyl ether, benzoin n-butyl ether, benzyl, benzophenone, p-methylbenzophenone, diacetyl, eosin, thionin, Michler's ketone, acetophenone, 2-chlorothioxanthone, anthraquinone, chloroanthraquinone, -methylanthraquinone, alpha-hydroxyisobutylphenone, p-isopropyl-hydroxyisobutylphenone, alpha, alpha-dichloro-4-phenoxyacetophenone, 1-hydroxy-1-cyclohexylacetophenone, 2,2-dimethoxy-2-phenylacetophenone, methylbenzoin formate, 2-methyl-1 - [4- (methylthio) phenyl] -2-morpholinopropene, dichlorothioxanthone, diisopropylthioxanthone, phenyldisulfide-2-nitrosofluorene, butiroin, anisoin ethyl ether, azobisisobutyronitrile, tetramethylthiuram sulfide and the like. The photopolymerization initiator is normally used at a level between about 0.1 and 5 parts by weight per 100 parts of monomer. Minor amounts will not normally provide a sufficient benefit. Quantities of more than 5 parts by weight will normally be non-economic. Also, a residual unreacted initiator can decrease the physical properties of the cured material.

Photopolymerization accelerators can be used in combination with photopolymerization initiators if desired. Illustrative examples include tertiary amines such as triethylamine, triethanolamine, 2-dimethylaminoethanol, etc .; alkyl- or arylphosphines such as triphenylphosphine, etc., and thiols such as β-thioglycol, etc. An appropriate radiation source can be easily selected by those of ordinary skill in the art. Illustrative examples of UV sources include mercury lamps, high pressure mercury lamps, ultra high pressure mercury lamps, xenon lamps, carbon arcs, metal halide lamps, such as light, etc. The construction is preferably under inert atmospheres during exposure to ultraviolet light when no base film is used. The intensity of the ultraviolet radiation is at a value between about 50 and about 250 W / cm 2 and the dose is usually between about 500 and about 5000 mJ / cm 2. Preferably, during irradiation to ultraviolet light the exposed side of the liquid precursor composition (corresponding to the second side of the binder layer) is covered by a cover sheet or base film that is transparent to ultraviolet radiation but exhibits low oxygen transmission and higher thermal resistance than the binder layer. This will allow the UV irradiation of the composition, while protecting it from the oxygen that interferes with the polymerization. Also, the base film can protect the binder layer during subsequent manufacturing steps, for example fusion to press platens during the incrustation of the retroreflective elements. Illustrative examples of suitable base films include polyester films, polyolefin films, etc. The liquid precursor composition may include other additives as desired. Illustrative examples include fine inorganic and organic particles, and fillers, stabilizers, pigments, etc. These additives preferably do not unduly interfere with the curing mechanism or impart undesirable properties to the resulting binder layer. After curing, the binder layer and the mold are separated. Depending on the nature of the binder layer and the material from which the surface of the mold is made, it may be desirable to incorporate release agents into the liquid precursor, to apply release agents to the surface of the mold prior to the application of the liquid precursor to the same or use a divider layer to obtain desired release properties.

Figure 1 shows an illustrative binder layer 3 on the base film 4. Appropriate retroreflective elements can be easily selected by those of ordinary skill in the art. The retroreflective elements may be substantially self-contained, that is, be fully capable of providing a retroreflective effect without additional components or may require additional components to provide the desired retroreflection. Normally, the retroreflective elements will be microspheres. Appropriate microspheres can be readily selected by those of ordinary skill in the art. Typically, the microspheres will be glass, ceramic or polymer, having a refractive index of between about 1.5 and about 2.2, preferably 1.9 to about 1.95 in most cases. The average diameter of the microspheres will normally be between about 40 and about 200 microns, preferably between about 50 and about 150 microns. Microspheres, outside this range can be used, however, if they are substantially smaller the retroreflective performance of the final article can be impaired due to the diffraction effects. The larger microspheres will tend to produce a thicker product. Also, the microspheres which are in the layer, can not be packed so densely and can result in poor uniformity of the retroreflective effect through the coating. Preferably, the microspheres have a reflective coating on a portion thereof, for example, a hemispherical coating of aluminum, silver or a dielectric coating. Such microspheres will be autoretrorreflective. Alternatively, the icrospheres may be free of any reflective coating and reflective material such as pigment flakes may be incorporated in the binder layer. This optical system is similar to that used in many retroreflective pavement markings. The microspheres will typically comprise a reflective layer, for example, a substantially hemispherical coating of aluminum, silver or dielectric material thereon. Alternatively, the binder layer may contain reflecting particles in at least the layer in which the microspheres are embedded. Illustrative examples include aluminum flakes, pearlescent pigment flakes and titanium dioxide. The appropriate particles can be easily selected by those skilled in the art. The retroreflective elements are normally arranged in a temporary carrier, for example, partially embedded in a thermoplastic polymer layer on a paper support. Arranged in this way, the microspheres are arranged in a monolayer of desired packing density (normally packed densely to provide maximum retroreflective performance in the resulting article) and also conveniently arranged to apply a reflective layer thereon, eg, aluminum vapor coating. This technique is well known among those skilled in the art. Also, the temporary carrier provides a convenient way to press the retroreflective elements against the first surface of the binder layer with sufficient pressure to embed the elements in the binder layer. The optimal conditions for embedding the retroreflective elements in the binder layer will be dependent in part on the equipment used (for example, passing the coatings between rubber rollers), the temperature, pressure, adhesion time of the binder layer to the elements, first profile and etc.

In general, the temperature for embedding the retroreflective elements in the binder layer is selected from the range defined by the softening point and pour point of the binder layer and is usually between about 80 ° and about 140 ° C. When the heating temperature is too low, the depth of incrustation of the retroreflective elements and the adhesion force to them developed by the binder layer may not be sufficient. When the temperature is too high, the binder layer may not effectively recover its first desired profile. As the pressure is applied to the temporary carrier, the binder layer deforms in two general ways. In a detailed localized manner, the retroreflective elements are at the point of contact with the binder layer and are forced to the binder layer under substantial pressure and force. Thus, the surface portion of the binder layer is plastically deformed. As a result, the shape memory of this surface portion of the binder layer is greatly reduced or lost. In contrast, on a more general basis, the protuberances in the binder layer are deformed under relatively low pressure. They substantially retain the shape memory properties.

Following the incrustation, the temporary carrier is separated, to leave the retroreflective elements partially embedded in the first surface of the binder layer. Preferably, the elements, the binder layer and the temporary carrier are such that the elements adhere preferably to the binder layer. It may be desirable to incorporate release agents in the temporary carrier, to reduce adhesion to the retroreflective elements and / or to incorporate coupling agents in the binder layer or to use other techniques to increase the adhesion of the binder material to the retroreflective elements. Figure 2 shows an illustrative intermediate product, after the temporary carrier is removed, the product comprises microspheres 1 with an aluminum reflector coating 2, partially embedded in the first surface of the binder layer 3 on the base film 4. In comparison with Figure 1, wherein the binder layer was in its first profile, the binder layer 3 in Figure 2 is deformed to a second profile. After the temporary carrier is removed and the pressure is removed from the first surface of the binder layer, the intermediate product is exposed to heat within the range defined by the glass transition temperature of the binder layer and the pour point of the resin .

Normally, the range will be between approximately 30 and approximately 180 ° C. If the temperature is not high enough, the recovery of form may be slow or may not occur. If the temperature is too high, the binder layer in the protuberances may undergo a plastic flow and stabilize in a deformed shape instead of in the first desired profile. Figure 3 shows a complete coating comprising microspheres 1 with an aluminum reflector coating 2 partially embedded in the first surface of the binder layer 3 in the base film 4. As can be seen in comparison with Figures 1 and 2, the layer Binder 3 has been restored to its first profile or desired shape. In some instances it may be desirable to further cross-link the binder layer after shape recovery. Sometimes this stage is referred to as a post-crosslinking or stabilization of the binder layer. Post-crosslinking is advantageous for improving environmental durability, for example, improved thermal resistance and solvent resistance. It can also return to the more stable binder layer dimensionally under conditions of use. The post-crosslinking can be induced if the binder layer contains one or more crosslinking components that substantially do not participate in the recovery mechanisms of the initial curing form. For example, it may contain components that initiate crosslinking or crosslinking at temperatures that are not greater than the temperatures at which the binder layer is subjected during initial formation, embedding of the retroreflective element and shape recovery. In another process, it may contain curing agents such as moisture curing. Polymeric radicals formed during electron beam irradiation without the use of crosslinking agents can be used. Illustrative examples of copolymerizable crosslinking agents (or crosslinkers) include thermal curing crosslinking agents such as N-alkoxyalkylacrylamide, acrylamide, N-methylolacrylamide, (meth) acrylates having a phosphoric acid group (in which are included) a phosphate group with an active hydrogen atom), glycidyl (meth) acrylates; moisture-cured crosslinking agents, such as 2-isocyanatoethyl (meth) acrylate, (meth) acrylates having a silanol group; (meth) acrylates having an amino, nitroso, or nitro group which carries out the crosslinking reaction in the presence of a redox agent; and the like. The amount of cured post-crosslinking agents is usually between about 0.1 and about 5.0 weight percent, preferably between 0.5 and 3 weight percent, based on all monomers. Catalysts can be used in combination with any of the above crosslinking agents to accelerate the crosslinking reactions. In some instances, it will be desirable to laminate a transparent cover film to the first surface of the binder layer with protruding reflective elements thereon, for example, to obtain improved retroreflective performance under humid conditions. In an illustrative process, if the binder layer exhibits thermoplastic properties, the cover film can be laminated to the first surface of the binder layer and then heat and pressure can be applied to selected portions of the second surface of the binder layer to cause portions of the binder layer flow around the retroreflective elements and in contact with the cover film. U.S. Patent Nos. 3,190,178 (McKenzie) and 4,025,159 (McGrath) describe such procedures. Further details of the invention are defined in the features of the claims.

EXAMPLES The invention will be further explained by the following illustrative examples which are only intended to be non-limiting. Unless stated otherwise, all quantities are expressed in parts by weight. The retroreflective performance of the resulting coatings was evaluated according to JIS Z 9117 standard when using an apparatus as shown in figure 5, with variable incidence and observation angles as indicated in Tables 1 and 2. The test method is schematically shown in Figure 5 where the coating 10 is shown with the retroreflective surface 12, the center 13 of the sample, the axis 14 normal to the surface 12, the light source 16, the incident or illuminating axis 17, the angle of incidence 18, receiver 20, observation axis 21, and observation angle 22.

Examples 1, 2 and comparative example A A binder layer was formed from a liquid monomer composition containing 100 parts of phenoxyethyl acrylate (PEA from Osaka Organic Chemical Company) and 1 part of photopolymerization initiator (DAROCURE ™ 1173 from Ciba-Geigy Company). The composition was poured into a mold made of silicone rubber.

The mold had a high geometric configuration of regularly formed V-shaped isocelic grooves, as shown in Figure 4. The distance a was about 1.8 millimeters and the angle b was 70 °. Each slit had a depth of approximately 2.47 mm. The composition was poured over the mold to completely fill the cavities and to overflow the elevated portions by a total thickness of the bottom of the slit to the surface of approximately 2.5 millimeters. A 50 micron thick polyester terephthalate film was laid on the exposed side of the liquid composition. The liquid was cured by exposure to ultraviolet radiation to form the binder layer with the desired profile. The ultraviolet radiation was dosed at 2500 mJ / cm2 generated by a high pressure mercury lamp (UVL-N from USHIO). The storage module, Mo at 25 ° C and 120 ° C measured by the elastic viscoelasticity method was 1.27 x 106 Pa (1.27 x 107 dynes / cm2 and 3.38 x 10"Pa (3.38 x 105 dynes / cm-) Glass microspheres having a refractive index of about 1.9 and an average diameter of about 70 microns were partially embedded in a temporary carrier comprising a polyethylene layer on paper.An aluminum reflecting layer was vapor deposited on the exposed portions of the glass microspheres to a depth of approximately 100 nanometers The microspheres were laminated to the first surface of the binder layer and a hand plate at 120 ° C pressed to the back side of the temporary carrier, to embed the aluminum coated sides from the microspheres to the binder layer The microspheres were partially embedded in the binder layer The protuberances in the binder layer were deformed The first profile was restored by carrying the construction through an oven at a temperature of about 70 ° C for about 10 minutes. The profile of the protuberances was restored and the microspheres remained firmly embedded or embedded in the binder layer as shown in Figure 3. For Example 2, a retroreflective coating was made as in Example 1, except that the mold was one different way The distance a was 0.36 millimeters and the angle b was 45 °. For Comparative Example A, a retroreflective coating was prepared as in Example 1, except that a flat plastic sheet was used in place of the mold, such that the binder layer was flat substantially without any protrusion. The retroreflective performance of the resulting coatings is tabulated in Tables 1 and 2.

Table 1: At an observation angle of 0.2 °, the retroreflective brightness of each coating (in candelas / lux / meters2) at the indicated angle of incidence (in °) was as follows: Angle Rev Rev 2 Rev A incidence 55 98.2 185 181 65 68.6 123 30.2 75 47.2 39.1 1.47 80 31.0 6.64 0 85 9.6 0 0 89.9 0 0 0 Table 2: At an observation angle of 1.0 °, the retroreflective brightness of each coating (in candelas / lux / meter2) at the indicated angle of incidence (in °) was as follows: Angule of Rev Rev 2 Rev A incidence 55 13.2 19.9 28 65 8.12 14 7.38 75 5.9 7.38 0.73 80 4.43 3.69 0 85 1.17 0 0 89.9 0 0 0 From a comparison of the results of Examples 1, 2 and A, it can be seen that the retroreflective coatings of the invention provide excellent retroreflective performance at a high incidence angle.

Examples 3 and 4 In Example 3, a retroreflective coating was prepared as in Example 1, except that a mixture of 99 parts of phenoxyethyl acrylate and 1 part of N-isobutoxymethylacrylamide (from Nitto Reiken Industries, Ltd.) was used as the curable monomer. In Example 4, a retroreflective coating was prepared as in Example 3, except that the binder layer was also post-crosslinked, by heating at 180 ° C for 30 minutes after shape recovery.

The coatings in Examples 3 and 4 each had similar retroreflective properties as those of Example 1. The coatings of Examples 1, 3 and 4 were immersed in methyl ethyl ketone. The binder layer of the coating of Example 1 was dissolved while the binder layers of the coatings of Examples 3 and 4 did not dissolve. The storage modules of the binder layers in the coatings of examples 1, 3 and 4 at 160 ° C were measured and found to be 1.05 x 104, 3.75 x 104 and 1.22 x 105 Pa (1.05 x 105, 3.75 x 105 and 1.22 x 106 dynes / cm2) respectively. These results indicate that the coating of Example 4 had the highest thermal stability among these three coatings.

Examples 5-10 In Examples 5-10, retroreflective coatings were prepared, as in Example 1, except that the following monomer compositions were used. The properties of Mo of each are indicated. Examples 5: 90% isooctyl acrylate / 10% acrylic acid Mo at 25 ° C: 4.83 x 105 Pa (4.83 x 106 dynes / cm2) Mo at 120 ° C: 1.77 x 104 Pa (1.77 x 105 dynes / cm2) Example 6: 80% isooctyl acrylate / 20% acrylic acid Mo at 25 ° C: 7.78 x 106 Pa (7.78 x 107 dynes / cm2) Mo at 120 ° C: 5.53 x 104 Pa (5.53 x 105 dynes / cm2) Example 7: 70% isooctyl acrylate / 30% acrylic acid Mo at 25 ° C: 8.75 x 107 Pa (8.75 x 108 dynes / cm2) Mo at 120 ° C: 3.60 x 105 Pa (3.60 x 106 dynes / cm2) ) Example 8: 30% isooctyl acrylate / 70% isobornyl acrylate. Mo at 25 ° C: 1.52 x 108 Pa (1.52 x 109 dynes / cm2) Mo at 120 ° C: 9.81 x 103 Pa (9.18 x 104 dynes / cm2) Example 9: 100% tertiary butyl acrylate Mo at 25 ° C: 4.82 x 107 Pa (4.82 x 108 dynes / cm2) Mo at 120 ° C: 1.14 x 104 Pa (1.14 x 105 dynes / cm2) Example 10: 97% phenoxyethyl acrylate / 3% acrylate N-methoxymethyl Mo at 25 ° C: 1.56 x 106 Pa (1.56 x 107 dynes / cm2) Mo at 120 ° C: 1.24 x 105 Pa (1.24 x 106 dynes / cm2) The coatings in examples 5-10 had each similar retroreflective properties as those of Example 1. Various modifications and alterations of this invention will become apparent to those skilled in the art, without deviating from the spirit and scope of this invention. It is noted that, in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (13)

1. Claims Having described the invention as above, the content is claimed as contained in the following: 1. A method for forming a retroreflective coating, characterized in that it comprises: a) providing a binder layer having first and second sides; the binder layer is able to exhibit shape memory; b) forming the layer to a first desired profile such that the first side has one or more protuberances thereon; c) applying retroreflective elements to the first side of the layer, in such a way that the retroreflective elements are partially embedded or embedded in the layer and the layer is deformed to a second profile; and d) activating the layer in such a way as to display shape memory and invert the second profile to the first profile while retaining the retroreflective elements. The method according to claim 1, characterized in that it further comprises the step of: e) stabilizing the binder layer in the first desired profile, after activating the layer to cause it to invert the first profile. The method according to claim 1, characterized in that the binder layer comprises a material exhibiting a storage modulus of at least 3 x 105 (3 x 106 dynes / cm2) at 250 ° C and less than 9 x 105 Pa (9 x 106 dynes / cm2) at 120 ° C after being formed to the first profile. 4. The method of compliance with the claim 1, characterized in that the binder layer comprises a material exhibiting a storage modulus preferably of between at least 1 x 10 6 Pa (1 x 107) and approximately 3 x 108 Pa (3 x 109 dynes / cm 2) at 25 ° C and between approximately 1 x 104 Pa (1 x 105) and approximately 5 x 105 Pa (5 x 106 dynes / cm2) at 120 ° C after being formed to the first profile. The method according to claim 1, characterized in that the binder layer comprises a material having a softening point of between about 80 ° C and about 200 ° C. 6. The method according to claim 1, characterized in that the retroreflective elements comprise microspheres. 7. The method of compliance with the claim 6, characterized in that the microspheres have a reflective layer thereon. The method according to claim 6, characterized in that the binder layer contains reflecting particles dispersed therein. 9. A retroreflective coating comprising a binder layer with retroreflective elements partially embedded or embedded in the surface thereof, the surface has protuberances thereon and characterized in that the binder layer has been formed via shape memory. The coating according to claim 9, characterized in that the binder layer comprises a polymerized resin, the resin has a storage modulus of at least about 3 x 105 Pa (3 x 106 dynes / cm2) at 25 ° C, and approximately 9 x 105 Pa (9 x 106 dynes / cm2) of less than 120 ° C, measured by the dynamic viscoelasticity method. The coating according to claim 9, characterized in that the retroreflective elements comprise microspheres. 12. The coating according to claim 11, characterized in that it comprises reflecting layers on the microspheres. The coating according to claim 9, characterized in that it comprises reflecting particles dispersed in the binder layer.