Abstract:
A method and apparatus for forming retroreflective sheeting with improved daytime whiteness performance is described in which air spheres are encapsulated into the prism structure when the prism base body is laminated to the prisms in a mold.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the formation of retroreflective microprisms in the form of small, closely packed, cube corner formations in the order of 0.025 inches, or less, in side edge dimensions. Such prisms have found many applications as retroreflective sheeting employed as safety reflective lighting devices on vehicles or clothing and in retroreflective highway signs. (See, for example, U.S. Pat. No. 3,689,356 to W. P. Rowland issued Sep. 5, 1972 and incorporated in its entirety by reference herein.) 
     SUMMARY OF THE INVENTION 
     Retroreflective sheeting is formed in accordance with the invention having air spheres incorporated into the prism structure during the process of attaching the prisms to the sheeting base body. One way of accomplishing this is to only partially fill prism mold recesses with prism material, leaving an air gap. When the base body is laminated onto the mold material and the mold material cured, the air in the recesses is entrapped in the prism body leaving air pockets in the shape of bubbles or spheres randomly distributed throughout the otherwise solid prisms. 
     As will be explained in some detail, the air bubbles redirect or scatter some of the incident light rays in a manner such that the material will have a whiter (High Cap Y) appearance when viewed from various angles. Some light rays will be redirected in a manner which scatters the light. Other rays will be retroreflected in a narrow beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary sectional view taken at location about a drum axis of a mold illustrating the deposition of liquid prism molding material thereunto from a coating head mounted thereabove. 
     FIG. 2 is a fragmentary section to an enlarged scale of the apparatus for applying a film of base material 46 by a roller 50 to the prism material 64 in the mold 16 to entrap air bubbles in the prisms 68. 
     FIG. 3 is a fragmentary view of the rear surface of reflective sheeting produced in accordance with the present invention. 
     FIG. 4 is a fragmentary sectional view to an enlarged scale taken along the lines 4--4 of FIG. 3. 
     FIG. 5 is a view as in FIG. 4 with a white backing layer 12 added to the sheeting for an air backed embodiment of the invention. 
     FIG. 6 is a view as in FIG. 4 with a metallic reflective layer 14 added for a metal reflector embodiment of the invention. 
     FIG. 7 is a schematic ray-tracing drawing to illustrate certain principles of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIGS. 1-7, the invention will now be described in detail in connection therewith. Since much of the apparatus and processes used in connection with manufacturing the embodiments herein has been previously described in connection with the above-referenced U.S. Pat. No. 3,689,356, these details will not be repeated here except where needed for a proper understanding of the present invention. 
     The apparatus of the invention includes a molding drum 100 mounted upon an axle or shaft for rotation in the direction indicated by the arrow (i.e., counterclockwise). The circumferential portion of the drum 100 consists of a multiplicity of metallic plates 16 bonded to a circumferential base portion. Each of the plates 16 is formed with a multiplicity of identical, contiguously arranged cube-corner recesses or indentations 22, and the plates 16 are provided entirely about the circumference of the drum 100 to provide a molding surface that has a substantially continuous array of cube-corner recesses 22 therein. 
     A coating head 26 is mounted about the drum 100 at one point about its circumstance for reciprocal movement thereacross. As the drum continuously rotates, a hardenable molding material 64 in fluid form is deposited thereupon from the coating head 26. Film 46 is continuously withdrawn from a feed reel (not shown) and applied against the drum 100 by the pressure roll 50, which cooperates with the drum 100 to provide a nip at which the hardenable material 64 is uniformly distributed over the surface of the mold plates 16, and at which intimate contact is effected between the material 64 and the film 46. 
     We have found that air spheres 10 can be introduced into the retroreflecting microprisms 70 (FIGS. 3-6) formed at this point in the process by the simple expedient of eliminating the customary step of pre-wetting the mold surface. The viscosity of the prism material 64 is such that, without the use of a wetting agent, the prism cavities 22 will not be completely filled. Air spheres 10 of various sizes are therefore entrapped in the prism bodies 70 when contact is made between the material 64 and the film 46. 
     The freshly applied material 64 and film 46 travel together past a bank of radiating elements (not shown) whereat hardening of the material 64 and bonding thereof to the film 46 are concurrently effected. Thereafter, a cooling medium permanently sets the material 64 which now is bonded to the film 46 so that the completed reflective sheeting 54 can be readily stripped from the drum 100 about a roll and wound upon a take-up reel. 
     The number of air spheres 10 created is dependent on the process drum speed, drum temperature and the viscosity of the prism material. It is believed that the laminating roll diameter and durometer of the roll material will also influence the number of air spheres created. 
     The location of the air spheres 10 within the individual prisms 70 and the location of the air spheres across the film appears to be random in nature. 
     The material with air spheres has a much greater whiteness (Cap Y value) 1  as can be seen from the comparative data in the following Tables I and II taken from samples of the type shown in FIG. 5 with material 12 provided as an air backing for the sheeting 54. 
    
     
                       TABLE I______________________________________Cap Y without air spheres(Polyester with acrylated epoxy prisms 330 Tooling)Orientation Angle            CAP Y______________________________________ 0               30.93 45              31.17 90              34.38135              32.08180              30.78225              31.17270              35.81315              31.41Average          32.22______________________________________ 
    
     
                       TABLE II______________________________________Cap Y with air spheres(Polyester with acrylated epoxy prisms 330 Tooling)Orientation Angle            CAP Y______________________________________ 0               46.02 45              45.58 90              47.77135              45.77180              45.94225              46.51270              48.25315              45.68Average          46.44______________________________________ Note: The data in Table II indicates an approximately 14.0 point gain in Cap Y or a 44% gain over that of Table I. The higher the Cap Y, the greater the whiteness appearance of the material. A low Cap Y material is gray in appearance. 
    
     Cap Y tests were also conducted on two metallized samples of the type shown with aluminum coating 14 on the prism faces 15 with the following results: 
     
                       TABLE III______________________________________Cap Y without air spheres(Vinyl with acrylated urethane prisms 330 Tooling)Orientation Angle            CAP Y______________________________________ 0               7.89 45              8.66 90              9.88135              7.51180              7.81225              8.93270              9.53315              7.51Average          8.47______________________________________ 
    
     
                       TABLE IV______________________________________Cap Y with air spheres(Vinyl with acrylated urethane prisms 330 Tooling)Orientation Angle            CAP Y______________________________________ 0               25.35 45              26.07 90              27.48135              25.84180              25.63225              25.86270              27.62315              26.07Average          26.24______________________________________ 
    
     In this example, Table IV Cap Y increases by approximately 17.50 points, which represents a 300% gain in Cap Y over that of Table III. The reasons, we believe, for the Cap Y increase is illustrated in the simplified one-dimensional schematic of FIG. 7, which shows how the light that is incident on a microprism retroreflector 70 reflects and refracts depending on the path of the light through the prism. 
     Ray &#34;a&#34; is incident normal to the prism window face and travels the path shown in &#34;dashes&#34; through the prism 70. If the reflecting faces F1,F2 of the prism are not metallized, ray &#34;a&#34; will exit the prism as ray a&#39; and then reflect in a diffuse manner off of the white backing 12 (shown in FIG. 5) normally used for retroreflective air backed construction. If the reflecting faces of the prism are metallized with a layer 14 of metal, such as silver or aluminum) as shown in FIG. 6, the ray will follow the longer path (shown by &#34;dashed X&#39;s.&#34;) through the prism and the air bubble 10 and exit the prism window as ray a&#34;. 
     Ray &#34;b&#34; (shown in &#34;dotted&#34; lines) which enters the prism window at an angle will follow an entirely different path through the prism. When ray &#34;b&#34; reaches the air sphere 10, its angle of incidence at the surface of the air sphere is greater than the critical angle of 41.8° (this is the critical angle assuming the index of refraction of the prism material for a cube corner prism is 1.5 and the air inside the air sphere has an index of refraction of 1.0) and the ray is totally internally reflected. When ray &#34;b&#34; reaches the reflecting face of the prism, it will exit the prism as ray b&#39; if the face is air backed. If the face is metallized, ray &#34;b&#34; will follow the longer path shown and exit the prism as b&#34;. 
     Ray &#34;c&#34; (shown in &#34;solid&#34; lines) is totally internally reflected from the prism faces and follows an undeviated path through the air sphere to emerge as rays c&#39; or c&#34;. Ray &#34;c&#34; will have the same path through the prism and air sphere independent of the coating on the prism reflecting faces. 
     It can be seen from the above described ray diagrams that the cumulative effect will be that some light will be retroreflected by a prism containing an air sphere and some light will be redirected to either: 
     a) Widen the retroreflected light pattern--This would be the case for rays that enter the face parallel to and slightly to either side of ray &#34;c&#34;. When these rays emerge from the prism, they will be diverging to either side of the direction of the rays c&#39; and c&#34;; or 
     b) Increase the Cap Y or whiteness value of the material--Rays &#34;a&#34; and &#34;b&#34; exit the prism in a direction that will increase the Cap Y value of airbacked prisms as in the case of rays a&#39; and b&#39;, or of metallized prisms, as in the case of rays a&#34; and b&#34;. Rays a&#39; and b&#39; are diffusely reflected from a White backing material to increase Cap Y. Rays a&#34; and b&#34; reflect out of the prism after only one or two reflections from the metallized faces of the prism and create a higher Cap Y or whiter appearance of the material. 
     Based upon the above examples, one can see that there are an infinite number of ray paths through corner cube microprisms containing air spheres within the prism. In practice, we find the air spheres to be random in size and location with most of the air spheres near the volumetric center of the corner cube. But, we have also seen corner cubes which contain two air spheres within the same cube and air spheres which are substantially offset from the center of the corner cubes. 
     The air spheres 10 have a beneficial effect on the retroreflected light pattern, light rays that would normally be retroreflected in a normal condensed diffraction pattern (that is, the only divergence created is due to the diffraction which occurs at the small exit aperture of the microprism) are redirected to wider observation angles and also orientation angles which are normally low in brightness. This beneficial effect is true for metallized prisms, but apparently not for all air backed prisms, as is shown in the representative data given in Tables V-VII below: 
     
                       TABLE V______________________________________Al Metallized RP1.1 Prisms, Polyester Top Film,330 Tooling (% SIA Variation from 0° to 165° of Rotation)Obs.     Entr.   With        WithoutAngle    Angle   Airspheres  Airspheres                                Δ %______________________________________0.10°     5°             4           5       -1    10       6          11       -5    20      10          30      -20    30      11          55      -44    40      34          42       -8    45      63          84      -21    50      85          104     -19    60      866         650     +216______________________________________ 
    
     The only increase in SIA variation is at the 60° entrance angle. (Bob--What is SIA variation?) 
     
                       TABLE VI______________________________________Al Metallized RP1.1 Prisms, Polyester Top Film,330 Tooling (% SIA Variation from 0° to 165° of Rotation)Obs.     Entr.   With        WithoutAngle    Angle   Airspheres  Airspheres                                Δ %______________________________________0.33°     5°             69         113     -44    10       72         159     -87    20       93         117     -24    30       65          62      +3    40       55          51      +4    45       63          50     +13    50       93          57     +36    60      583         535     +481.50°     5      288         500     -212    10      312         500     -188    20      300         475     -175    30      183         350     -167    40      183         400     -217    45      125         166     -41    50      167         250     -83    60      600         450     +150______________________________________ 
    
     The above data in Table VI shows that most of the improvement at 0.33° observation angle is at entrance angles less than 30 degrees and the improvement at a 1.50° observation angle is at all entrance angles except at a 60° entrance angle. 
     The SIA (cd/lux/m 2 ) level drops for most geometries especially at narrow observation angles. 
     
                       TABLE VII______________________________________Airbacked, Vinyl, SRV35.11 PrismsObs.     Entr.   With        WithoutAngle    Angle   Airspheres  Airspheres                                Δ %______________________________________0.1°     5°              5          5        0    10        6          5       +1    20       25          11     +14    30       337        289     +48    40       631        602     +29    45       650        491     +159    50       490        500     -19    60      6200        6700    -5000.33      5       112         42     +70    10       102         40     +62    20       78          37     +41    30       560        372     +188    40      1012        972     +40    45      1027        990     +37    50       800        525     +275    60      5450        5600    -1501.50      5       215         73     +142    10       186         76     +110    20       50          44      +6    30       533        275     +258    40       550        633     -83    45      1500        550     +950    50      2700        2100    +600    60      19900       6800    +13100______________________________________ 
    
     The above data in Table VII shows that for most geometries, the variation from 0° to 165° orientation in brightness increases. That is the material becomes more orientation sensitive when air spheres are introduced. The airbacked product construction creates the opposite effect when compared to the metallized product when air spheres are introduced. 
     The airbacked product with air spheres had, in general, lower SIA values than equivalent material without air spheres. 
     Having thus described a few particular embodiments of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.