CA1276046C - Retroreflective sheeting and methods for making same - Google Patents
Retroreflective sheeting and methods for making sameInfo
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- CA1276046C CA1276046C CA000586270A CA586270A CA1276046C CA 1276046 C CA1276046 C CA 1276046C CA 000586270 A CA000586270 A CA 000586270A CA 586270 A CA586270 A CA 586270A CA 1276046 C CA1276046 C CA 1276046C
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Abstract
RETROREFLECTIVE SHEETING AND
METHODS FOR MAKING SAME
ABSTRACT OF THE DISCLOSURE
A retroreflective relatively flexible laminate sheet construction has a thermoplastic web with a smooth light-receiving first side and a second side coextensive with said first side. A
retroreflective pattern is formed on the second side. A slurry of granular material is deposited on the second side to cover selected portions of the formed pattern with remaining portions of the formed pattern devoid of the granular material, and said slurry is dried or cured to produce a well-defined pattern. A layer of backcoating material is deposited on the second side to overlay the granular material, the backcoating material contacting the thermoplastic web where no granular material has been deposited, thereby encapsulating the granular material between the second side and the backcoating layer. An added, outer layer provides additional weather protection for the thermoplastic web. Methods are detailed to manufacture the laminate, and compositions of backcoating mixtures also are disclosed. The backcoating mixture includes a water-borne emulsion of an acrylic/urethane copolymer, a whitening agent, a defoamer, an acrylic-based thickening agent and a pH-adjusting agent.
METHODS FOR MAKING SAME
ABSTRACT OF THE DISCLOSURE
A retroreflective relatively flexible laminate sheet construction has a thermoplastic web with a smooth light-receiving first side and a second side coextensive with said first side. A
retroreflective pattern is formed on the second side. A slurry of granular material is deposited on the second side to cover selected portions of the formed pattern with remaining portions of the formed pattern devoid of the granular material, and said slurry is dried or cured to produce a well-defined pattern. A layer of backcoating material is deposited on the second side to overlay the granular material, the backcoating material contacting the thermoplastic web where no granular material has been deposited, thereby encapsulating the granular material between the second side and the backcoating layer. An added, outer layer provides additional weather protection for the thermoplastic web. Methods are detailed to manufacture the laminate, and compositions of backcoating mixtures also are disclosed. The backcoating mixture includes a water-borne emulsion of an acrylic/urethane copolymer, a whitening agent, a defoamer, an acrylic-based thickening agent and a pH-adjusting agent.
Description
RETROREFLECTIVE SIEETING AND
METHODS FOR MAKING SAME
This application is a division of Canadian Serial No. 463,~18 filed September 19, 1984.
BACKGROUND OF THE INVENTION
Retroreflective sheeting has particular use in making highway signs, street signs and the like, and is now employed extensively. The Federal government has recognized two primary types of retroreflective sheeting: glass bead and cube-corner. Such approved sheeting materials are found in a specification entitled "FP-79", published by the U.S.
Department of Transportation, Federal Highway Administration.
Specification FP-79 presently has been adopted as a purchasing standard by many state highway departments, and it sets forth certain minimum specifications which must be met by retroreflective sheeting of the cube-corner type. Included among the specified characteristics are those for reflectivity, color, flexibility of material and resistance to cracking and weathering.
Cube-corner type reflector elements generally provide a higher specific lntensity at 0.2 observation angle and 0 entrance angle than do glass bead type reflector elements, but, to applicants' knowledge, no one successfully has furnished a sheeting material in com~ercial quantities which generally will meet the requirements for the Class IIIB sheeting set forth in the aforementioned FP-79 specification. Accordingly, the present invention seeks to provide a uni~ue sheeting product which will substantially meet such specified criteria and which can be produced in accordance with the novel methods disclosed herein in an economical fashion ar~ in com~ercial q~lantities.
Retroreflectivity is achieved by cube-corner type reflector elements primarily through the principle of total internal reflection. It is well known that any surface contact made by another material with the faces of the cube-corner elements generally has a deleterious effect on the reflectiveness of the reflector element.
However, when all of the element faces are metallized, or mirrored, then, rather than relying upon total internal reflection, retroreElection is achieved by specular reflection from the mirrored faces. Generally, metallizing will provide a grayish or black coloration under certain daylight conditions vis-a-vis unmetallized cube-corner type elements.
The present invention relates generally to methods and apparatus for producing retroreflective sheeting constructions and, more particularly, to methods and apparatus for producing a flexible laminate sheeting construction including an upper thermoplastic shee-t, the reverse of which is provided with a repeating, retroreflecting pattern of fine or precise detail, a backcoating to protect the formed pattern, and a selectively applied intermediate layer allowing bonding of the backcoating to overlay the formed pattern on the thermoplastic sheet while preserving and enhancing the retroreflective properties of both the formed pattern and the laminated sheet. More precisely, the present invention is applicable to the production of cube-corner type retroreflective sheeting laminates.
Within the art of designing reflectors and retrore~lective material, the -terms "cube-corner" or "trihedral," or "tetrahedral" are recognized in the art as describing structure or patterns consisting of three mutually perpendicular faces, not limited to any particular size or shape of the faces, or the orientation of the optical axis of the cube-corner element. Each of the cube-corner faces can assume a different size and shape relative to the others, depending upon the angular reflective response characteristics deslred, and the cube forming techniques employed.
Rxamples of prior cube-corner type reflectors may be found in U.S. Patent No. 1,906,655, issued to Stimson, and U.S. Patent No.
4,073,568, issued to Heasley. Stimson shows a reflex light reflector including an obverse face and a reverse light-reflecting face consisting of a plurality of cube-corner type reflector elements with each such element having three mutually perpendicular surfaces adapted for total internal reflection of light impinging thereon from the obverse face.
~easley describes a cube-corner type reflector in the form of a rectangular parallelpiped.
It long has been desired to obtain the benefits of cube-corner reflective properties in the form of flexible sheeting. As noted above, one advantageous aspect of such sheeting is in the manufacture of highway and street signs, markers and the like, where graphics are printed, painted, silk-screened or otherwise applied to a highly reflective substrate mounted to a flat, stifE, supportive surface. Flexible retroreflective sheeting, when used as such a substrate, can be stored and shipped while wound onto rolls, and can readily be cut or ortherwise formed into the desired shape and size required for a particular application. The reflective nature of the sheeting allows such signs, markers, and the like to reflect light from a vehicle's headlights, permitting the item to be read by the driver, without requiring a permanent light source to illuminate the sign or marker.
Production of such retroreflective sheeting has been made practicable by apparatus and methods to form precise cube-corner patterns in greatly reduced sizes on flexible thermoplastic sheeting. Desirably, such sheeting may then be assembled in the form of self-adhesive laminates.
Others have recogniæed the desireability of producing retroreflective thermoplastic material in sheet form. United States Patents Nos. 2,310,790, 2,380,447, and 2,481,757, granted to Jungersen, describe and teach the shortcomings of previously-known reflectors manufactured from glass, and the advantages inherent in providing a reflective material in a less fragile and more flexible sheet form. While so suggesting, it is not known if Jungersen in fact ever commercialized any product disclosed in such patents.
In U.S. Patents Nos. 4,244,683 and 4,332,847 issued to Rowland, the desirability of manufacturing cube-corner retroreflective sheeting in a continuous, non-stop process is presented, but the approach selected by Rowland is a "semi-continuous" process (Rowland '683, column 2, lines 18 -38), presumably so-called because the process requires frequent repositioning of the molding plates.
In United States Patent No. 3,187,068, issued to eVries, et al. continuous production of reflective sheeting is disclosed, utilizing encapsulated glass microspheres as the reflecting medium. eVries, et al.
describes the application of a pressure-activated adhesive layer to such sheeting to enable attachment of sheeting segments to selected surfaces.
In United States Patent No. 3,649,352, issued to Courneya, a beaded sheeting construction is described, portions of which become reflective when heated, and which includes a pressure-activated adhesive layer allowing attachment of the sheeting construction to other articles.
Palmquist, et al., 2,407,680 teach the utilization of glass microspheres or beads included as the reflective elements in flexible sheet forms; Tung, et al., in United States Patent No. 4,367,920, also desGribes a laminated sheet construction using glass microspheres as the reflective elements.
A co~mon problem in the construction of reflective laminate sheeting is to find means to bond the lamina firmly together in a way which preserves the required retroreflective qualities of the reflective elements selected for use. An example of prior efforts to solve this problem with respect to glass microspheres may be seen in United States Patent No. 3,190,178, issued to McKenzie, wherein a cover sheet or film is secured over exposed glass microspheres by use of die elements which force a portion oE the material in which the glass microspheres are embedded into contact with the cover sheet. The die elements thus create a grid pattern on the resulting sheeting construction, with each grid forming a separate cell. Within each cell, an air space is maintained between the microspheres and -the cover sheet, and incident light traverses the cover sheet and the air space to be retroreflected by the embedded microspheres.
~1'`
Holmen, et al., U.S. Patent No. 3,924,929, teach a cube-corner type uppex rigid sheet having upstanding walls, or septa, integrally formed as part of the cube pattern. The septa extend to form a regular geometric pattern of individual cells, with the septa extending at least as far from the upper sheet as the cube-corner elements. A particulate packing may be used to fill each of the cells, and a backing sheet is then attached to the rear of the upper sheet, with the septa serving as the attachment sites. Holmen, et al. use relatively large cube-corner elements fashioned as rigid sections bound to a flexible back, and has limited flexibility in use.
In McGrath, U.S. Patent No. 4,025,159, the cellular concept is described with respect to cube-corner type retroreflective sheeting, through use of dies to force a carrier film into contact with the reverse side of the cube-corner sheeting. The carrier film must then be cured with radiation to bind it to the cube-corner sheeting and, as in McKenzie, the resulting cells include an airspace extending between the carrier film and the reverse side of the cube-corner sheet. The air cell structure apparently was intended to provide a hermetically sealed cell, avoiding the need for metalizing the cube-corner elements, and providing an air/thermoplastic interface to enhance retroreflection.
None of the foregoing teach the assembly of molded or embossed cube-corner type retroreflective sheeting into self-adhesive laminates which protect and enhance the reflective properties of the sheeting without requiring the use of dies or of integrally-molded septa or walls included as part of the cube pattern. Further, none of the foregoing permits the material to benefit Erom encapsulated sections of cube-corner elements while enhancing and substantially meeting the requirements specified in the aforementioned DOT F'P-79 Specification.
BRIEF DESCRIPTION OF'1~E INN~rION
A thermoplastic sheet or web is provided on its reverse side with a retroreflective cube-corner type pattern. A thin layer of a liquid vehicle or solvent containing hydrophobic granular material (such as silica treated with silanes) is deposited on the reverse side of the web, as by screen printing, in a pattern leaving selected sites devold of granular materlal. The web ls then dried to drive off the solvent and, thereafter, a wa-ter-based backcoating is applled over the granular materlal pattern, with portions of the backcoating being in direc-t contact with the the~noplastic web at those sites on the web devoid of granular material. Thereafter, the backcoating is dried or cured, and a layer of adhesive such as pressure-sensitive or heat-activated adhesive is applied there-to. This procedure thus enables the assembly of patterned web materlal into lamlnates whlch include an actlvated adheslve layer while protecting the retroreflective properties of the precisely formed cube-corner pattern.
The invention to which this divisional application is directed pertains to a water-based backcoating for application and attachment to a thermoplastic web, the backcoating comprising a water-borne emulsion of an acrylic/urethane copolymer in a proportion from about 69 percent to about 80 percent, a whitening agent in a proportion from about 21 percent to about 24 percent, a defoamer in a proportion from about 0.4 percent to about 006 percent, an acrylic-based thickening agent in a proportion fron about 1.5 percent to 2.5 percent, and a pH-adjusting agent in a proportion up to about 0.3 percent.
Another aspec-t of the water-based backcoating for application and attachment to a supporting thermoplastic web, comprehends a water-borne polymeric acrylic system in a proportion from about 42 percent to about 62 percent, water in a proportion from about 2 percent to about 12 percent, an anti-skinning agent in a proportion from about 1.5 percen-t to about 2.5 percent, a whitening agent in a proportion from about 5 percent to about 36 percent, a flatting agent in a proportion from about 3 percent to about 5 percent, a pH-adjusting agent in a proportion fran about 0.3 percent to about 0.5 percent, a defoamer ln a proportion from about 0.6 percent to about 1.0 percent, a coalescent solvent in a proportion fram about 1.0 percent to 1.6 percent, and a thlckener in a proportlon from up to 3.0 percent.
In a preferred embodiment of the retroreflectlve laminate, an outer protective layer of thermoplastic material, used to provide additional weather reslstant properties, is secured to the thermoplastic web on the side opposite ~7~
from that upon which the retroreflective pattern is formed during or before -the cube fo~ming process.
The completed lamina-te is then cut, trimmed, or otherwise shaped for application to supporting surfaces, such as street or highway signs, and graphics or other indicia may thereafter be painted, printed, silk-screened, or otherwise affLxed to the uppermost surEace of the laminate, thus producing a readily and easily constructed highly retroreflective Einished product.
These and further aspects of the present invention will become more apparent upon consideration oE the accompanying drawings, wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged perspective and somewhat schematic view of one preferred aspect of the retroreflective sheeting of the present invention as a completed construction;
FIG. 2 ls a view along line 2-2 of Fig. 1, FIG. 3 is a greatly enlarged plan view illustrating a section of the formed surface of reflective sheeting comprising one aspect of the present invention;
FIG. 4 is a somewhat schematic and symbolic view of the processes and machinery utili2ed in a preferred aspect of the present invention;
FIG. 5 is a plan view of one form of screen pattern used to apply the hydrophobic granular layer of the present invention;
FIG. 6 is an enlarged view, in partial detail, of an individual cell of the sheeting of the present invention; and FIG. 7 is an enlarged perspective view illustrating a second preferred embodiment of the retroreflective sheeting of the present invention.
DETAILED DESCRIPTION OF THE DR~WINGS
Referring now to Fig. 3, the numeral 10 indicates generally a segment of cube-corner type retroreflective thermoplastic web used in fo~ming the laminate of the present invention. As seen in Fig. 3, there is depicted the rear surface of a portion of flexible retroreflective sheeting 12 fashioned from transparent thermoplastic material in web fo~m which has fo~med thereon, preferably by embossing, a re-troreflective and repeating pattern of cube-corner reflector elements characterized by cube faces 14, 16 and 18. In a preferred aspect of such sheeting, sheet 12 is formed from an impact-modified acrylic material having W inhibitors or absorbers added thereto, and which, prior to embossing, had parallel front and back surfaces and was initially on the order of about 0.006 inches thick. One such material is known as Plexiglas DR,TM
sold by the Rohm and Haas Ccmpany.
The cube-corner pattern foImed on sheeting :L2 is formed in an opticalLy precise, finely-detailed pattern. For example, as seen in Fig. 2, the depth to wh-ich the cube-corner pattern is embossed onto sheet 12 may be of the order of 0.00338 inch, (dimension X). As shown at dimension Y in Fig. 3, the cubes formed on sheet 12 may be spaced apart by a distance on the order of about 0.0072 inch, for the depth as shown at X as set forth above. While the cube pattern shown in Fig. 1 illustrates cubes formed with their optical axes normal to the face of sheet 12, it is to be understood that other versions and patterns may also be utilized as forming the retroreflective web of the laminate of the present invention.
Referring now to Fig. 1, the numeral 20 indicates generally a roll of retroreflective laminate 22 manufactured in accordance with preferred aspects of the present invention to be described hereinbelcw. As herein shown, laminate 22 is rolled onto a core 24. A thermoplastic web 26 having a front or obverse surface 28 and a rear or reverse surface 30 upon which is embossed the cube-corner type retroreflective pattern illustrated in Fig. 3. The thermoplastic web 26 may be on the order oE about 6 mils in thickness (0.006 inch).
Bonded to the reverse surface 30 of the thermoplastic web 26 is backcoating or film 32. In a preferred aspect of the present inven-tion, a hydrophobic granular silica material 34 is interposed between the backcoat film 32 and the reverse side 30 in a manner to be described hereinbelow.
In accordance with a preferred embcdiment of the present invention, a layer of adhesive 36 is bonded to a release sheet 38 in a presently well-known fashion, and is thereafter bonded to cured backcoat film 32 in order to provide a finished laminate 22 which includes a pressure-sensitive or heat-;" -8-activated adhesive layer 36 applied to sheeting 12 in a manner which preserves the retroreflective qualities and properties of the cube-corner pattern ernbossed thereon. The release sheet 38 is used to protect adhesive layer 36 until it is desired to apply laminate 22 to a given surface.
Fig. 4 shows, in schematic form, a preferred arrangement of equipment and sequence of operations to produce a retroreflective sheeting laminate of the type shown in Fig. 3.
The application of adheslve directly to the reverse side of a cube-corner ernbossed thermoplastic web 26 will cause an undesirable and unacceptable loss of retroreflective capability. This arises from the contac-t of the adhesive material wi-th the reverse side of ernbossed thermoplastic web 26, i.e., the filling of the "valleys" formed by the enbossed pat-tern and the subse~uent in-terface formed between substances tha-t are too similar in refractive indices to produce adequate retroreflection, so the transparent film can no longer utilize the phenomenon of total internal reflection to efficiently effect retroreflection of light. To solve this problen, a substantial portion of the cube-corner pattern ei-ther mus-t be hermetically sealed with an air space be-tween the back wall and the cube-corner elements, or the cube-corner elements must be backed in a way which would preserve the retroreflective properites of the forrned web while providing sites for firm at-tachment of an adhesive layer (or other adhesive material). Without such protection, and without such attaching sites, the use of, and effectiveness of a retroreflective embossed web is seriously compromised and curtailed.
Unexpec-tedly, use of hydrophobic granular materials has been found -to afford such protection of optical properties. ~nong such materials are xylenated glass particles, powdered silicone rubber, and silane-treated silica.
As part of the present invention, it has been found that a hydrophobic silica mixture consisting principally of amorphous silica treated with silanes, when used to fill the valleys formed by -the embossed pattern, preserves the retroreflective properties of the formed pattern for most practical purposes. Again, it is not known precisely why this effect obtains:
it has been theori~ed that the point contact of granules with the reverse face of the embossed thermoplastic web acts to preserve the retroreflective ~.~7~Q~
properties of the pattern, perhaps by preserving a sufficient air interface with the reverse side of the cube-corner pattern. However, the present invention obtains excellent results even where -the primary silica particles used are significantly srnaller than, for example, the particles discussed in prior art patents such as Holmen, e-t al~, U.S. Patent No. 3,924,929.
Use of such silica offers advantages such as low price, availability, and ease and precision of formulation. It further provides uni~ue color and reflective characteristics to the film which improves the appearance of the film even relative to the glass bead types heretofore com~only used.
As discussed hereinabove with respect to the Holmen, et al.
reference, others have attempted to solve the problem of loss of reflectivity by prcviding upstanding walls or septa as par-t of the rigid molded front face pattern, wi-th the septa forming individual pockets for the application of granular ccmpounds having particle sizes far in excess of the silica particles used in the present invention. The disadvantages to such an approach, particularly wi-th respect to the cube-corner type embossed pattern utilized in the present invention are manifest. Use of rigid septa limi-ts the size and shape of the cell. A separate mold must be formed for each type of retroreflective sheeting requiring a cell size other than that fonmed in the original mold. What is meant by the tenn "cell size" is the area bounded by or closed off by the walls to form a single pocket for the granular backing ma-terial.
Formation of such septa in a relatively rigid mold pattern manufactured to as fine and precise a degree of detail as that shown in the present invention also may cause problems wi-th respect -to stripping the Eonnedthermoplastic web from the fonning tool. This may particularly be a problem where the septa or walls extend inwardly into the mold to a dis-tance greater than the depth of the cube-corner pattern.
A preferred embodiment of the present invention includes -the mixing of a hydrophobic silica mixture using hydrophobic silica, organic solvents, and thickeners, and the application of this mixture, while in a liquid fo~m, to the reverse side of the fonmed thermoplastic web in a desired pattern. One b, J ` s 1 ~276~
advantage of the present process and product is that the pa-ttern can conveniently be changed to effect changes in reflective capability of the film, without changing the tools used in forming the embossed web. Thereafter, the partially coated or imprinted thermoplastic web is passed through a drying oven which drives off the solvents used to form the mixture, thereby drying the pattern on the thermoplastic sheet. The pattern in which the silica is applied to the thermoplastic web leaves selected portions or sites on the formed face of the thermoplastic web devoid of silica.
Referring now to Fig. 5, -the numeral 40 indicates generally such a selec-ted pattern. Each runner or path 42 represents an area on the reverse ~surface of thermoplastic web 26 where no silica has been deposited. Each s~uare or diamond~shaped area 44 represents an area on the surface of thermoplastic web 26 onto which the silica mixture has been deposited.
As seen in Fig. 6, the actual percentage of area covered by the silica mixture is determined by the thickness or width of each runner or path 42, and the pattern selected for deposition of the silica, with the cell 44 having an area bounded by the runners 42, and fully available for the reception and retroreflec-tion of incident light by the ~mbossed retroreflective patterns shown partially at 46.
Referring now to Fig. 4, it may be seen that thermoplastic web 26 may be drawn directly from an associated forming machine (not herein specifically shown) in a continuous process, or may be drawn from a separate supply reel onto which the embossed web 26 has been wound (not herein specifically shown). If desired, web 26 may be supported by a backing sheet (not herein specifically shown) coextensive with obverse face 28, leaving reverse surface 30 exposed.
It shollld be noted that reference to web 26 also includes reference to a laminate formed by web 26 and a backing sheet such as described hereinabove.
Web 26 is drawn by, for example, powered rollers (not herein specifically sh~wn), to silica mixture application station 48. As herein diagra~matically shown, a preferred means and method of applying the silica mixture to web 26 may be accomplished through use of a screen-printing cylinder 7~
50 which has mounted about the outer perlphery thereof, a metal screen formed to provide the shape or pattern to which it is desired to apply the silica mixture. The mixture is forced under pressure fram the interior of screen-printing drum 50 on-to the reverse side 30 of the thermoplastic web 26. As herein shown, the web 26 is directed by idler roller 52 to pass between the screen-printing drum 50 and a backing roller 54.
A preferred form of the apparatus utilized to apply the silica mixture at application station 48 consists of a drum printer manufacture~ by Stork Brabant BV of Boxmeer, Holland, of the type having a drum with electro-foImed mesh screens over which a photo-resist pattern (such as used for conventional silk screen) may be mounted, with a screæn pattern providing a diamond cell size in the range of fram about 0.096 inch to 0.300 inch, and a runner or cell wall thickness of fran about 0.010 inch to about 0.050 inch.
Variations in th~ shape of the cells, pattern repeat of the cells, and thickness of the runners may be accanplished by changing the printing screen used on screen-printing drum 50. Also, the constant width of the web may be of various sizes, and the printing screens used will be of a canpatible width.
In its preferred form, the silica mixture is made from a hydrophobic silica such as that manufactured by the Pigments Division of Degussa, of Frankfurt, West Germany, under the trade designa-tion Sipernat D10.TM. A preferred canposition of the mixture includes hydrophobic silica in a mixture containing approximately 98 percent silane-treated silicon dioxide (SiO2); 0.8 percent sodium oxide (Na20), and 0.8 percent of sulfur trioxide (SO3); a non-polar aliphatic hy~rocarbon solvent carrier; a polar solvent; and, where desired or re~uired, a thickening agent. One aliphatic non-polar hydrocarbon solvent successfully used is low odor mineral spirits, and a workable mixture has been created through use of an organic alcohol, preferably butanol, as the polar solvent material. A smectite clay-based thixotropic thickener also may be used in varying amounts to produce a well-defined screen-printed pattern of the silica slurry on the embossed thermoplastic web.
In its preferred embodiment, the primary particle size of thesilica is about 18 nananeters, and the agglomerated particle size of the hydrophobic silica in its final form is about 5 microns. However, it will be ~ 2~7~
understood that the only critical limitation on the particle size is such that the area in which it is deposited will be substantially impe~vious to the backcoating material 32, whereby the backcoating material is unable to penetrate the hydrophobic silica and interac-t with the cube-corner pattern except in those areas devoid of the silica.
The particular combination of solvents and thickeners is important to satisfactory deposition and definition of the silica in a precise and accurate pattern. Screen printing of particulate material commonly requires use of resins or other binders to hold the deposited particles in place. A
resin or binder cannot however be used in this instance because of the adverse effect on reflectivity of the web because of refractive index similarities.
Another important consideration is the rheology, or flow characteristics of the silica slurry as it is forced through the printing screen. The slurry must "relax", or thin as it is forced through the screen apertures, and thereafter regain sufficient viscosity to retain a well-defined pattern with good leveling qualities and appearance characteristics. Yet another consideration is use of a solvent vehicle which obtains the aforementioned qualities without attacking or degrading the thermoplastic web upon which the retroreflective pattern is formed.
Use of polar solvents, such as butanol, enables the slurry to maintain an increased concentration of solids (silica). Such solvents, however, react with the thermoplastic material used to form the web. Non-polar solvents, such as mineral spirits, preserve the embossed web, yet do not act to provide a satisfactory silica pattern. Therefore a blend of polar and non-polar solvents has been found to be useful in carrying enough solids without degrading reflectivity or degrading the web.
Preferably, the hydrophobic silica is present in proportions ranging from about 15 percent to about 35 percent by weight, the non-polar solvent carrier is present in amounts ranging from about 40 percent to about 70 percent, the polar solvent is present in amounts ranging from about 10 percent to about 30 percent, and the thickening agent may be present in amounts from about 2 percent to about 8 percent. One preferred formulation of the silica mixture includes 20 percent by weight Sipernat D10 hydrophobic silica, 56 percent mineral spiri-ts, 20 percent butanol, and 4 percent -thickener. It has been found that such propor-tions preserve the web while providing a useful silica pa-ttern.
After application of the silica mixture, web 26 is passed through a heating oven 56 where the resulting silica pattern is heated to drive off the organic solvents without heating web 26 to -the point where heat distortion of the cube-corner elements of the laminate will occur~
After drying, the silica is mechanically held to the cube-corner elements on the reverse face 30 of web 26 by, it is believed, electrostatic forces and physical inter-engagement of the silica particles themselves.
Thus, as web 26 exits mixture application station 48, it has -taken on the form of a firs-t modified laminate 58, l.e.' a web 26 having cube-corner elemen-ts with a precisely formed pattern of silica mixture screened thereon over a portion of the elements, with an uncovered portion of the cube-corner elements still exposed. As modified laminate 58 exits drying oven 56, it takes on a second modified lamina-te cons-truction 60 wherein the solvents present in the silica mixture have been driven off and the silica itself has remained dried into its screened-on pattern.
The second modified laminate 60 then enters a backcoating application station 62. The application of a water-based backcoating acconplishes several results. First, those areas onto which no silica has been screened or deposited will allow direct contact between the backcoating and the reverse side 30 of the embossed or otherwise formed thermoplastic web 26, thus "wetting" web 26 with the liquid backcoating mixture. Second, a layer of backcoating material will overlay the silica pattern formed on theLmoplastic web 26 and, when applied effectively, will not disturb or disrup-t the printed or screened-on silica pattern. Third, the backcoating may then be dried and/or cured to provide a firm attachment -to thermoplastic web 26 to provide a flat, smooth and integral surface upon which further layers, such as a layer of pressure-sensitive or heat-activated adhesive may be effectively and conveniently applied, and to protectively cover or encapsulate the silica pattern. A surprising and unexpected result is that the silica prevents permeation by the liguid backcoating to the cube-corner pattern. As described 3.2~
above, such permeation would adversely affect the reflectivity of the final assembled laminate.
Application of the backcoating mixture to the second modified laminate 60 may be accomplished in a number of ways, such as by spraying, roller application, squeegeeing, or the like. The manner in which the backcoat is applied will be determined by, inter alia, the precise formulation of the backcoat and the pressure, or force, which can be withstood by the silica pattern after it has been dried.
For purposes of illustration, a backcoating application station 62 may be characterized as having a supply header or tank 64 co~nunicating with an application means 66 which may be a nozzle or series of nozzles, or the like.
An implement such as a doctor blade 68 may be used to more uniEormly spread the backcoating after it has been applied without damaging the silica pattern. A
platen 70 prcvides support for the second modified laminate 60 during application of the backcoat.
After application, the third modified laminate 72 enters drying oven 74 wherein the backcoat material is heat-cured, resulting in backcoating layer 32 as sh~n in Fig. 1.
Successful use of a backcoating requires that the backcoating formulation meet several particularly important working parameters. One is that the backcoating have flow characteristics such that the relatively narrow and shallow rLmners formed by the silica pattern will be filled, while not dewetting or disturbing the dried silica pattern itself. This means that the viscosity of the backcoating must be carefully controlled to assure that the backcoating can be applied while completely encapsulating without disturbing the silica pattern. Another characteristic is that the backcoating cannot penetra-te or interact with the applied silica to reach the interface between the silica and the cube-corner pattern. Yet another requirement is that the backcoating, when dried, have the required flexibility and toughness to withstand use in a laminate. Ideally, the backcoating should also be of a color which enhances daytime visibility of articles made with such laninates.
Several preferred backcoatings have been utilized. ~ach may be characterized generally as including a water-borne or water-based polymeric - ~ -15-~ ~ ~t7 ~
mixture or system, a whitening agent, a defoamer, a thickner for use in adjusting the final viscosity, and a pH-adjusting component.
A first preferred fo~mulation of a backcoating is presented herewith as Example 1:
Example 1 *
1. DP-101, a water-borne polymeric system consisting of about 34%
acrylic/urethane copolymer, 61% water and 5% coalescent solvent, such as M-pyrol 69.7% to 79.7%
2. UCD-106C~, a pre-dispersed whitening agent (titanium dioxide) containing about 72% solids 21.5% to 23.5%
3. Balab 3017A, a defoamer 0.4% to 0.6%
4. CP-15 (50 percent in water) acrylic/based thickener to adjust viscosity1.5% to 2.5%
5. Annonia (28 percent aqueous solution) to adjust pH
to 8.5 to 10.0 None to 0.3%
The foregoing mixture is fonned by adding the defoamer to the water-borne acrylic/urethane copolymer system with gentle stirring. Thereafter, the whitening agent and the amnonia, if necessary, are added as gentle stirring is continued. The thickener is thereafter added with increasing blade speed and the entire mixture is stirred for about 30 minutes at moderate speed. A
preEerred mixer for such an operation is manufactured by Meyers Engineering of Bell, California under the trade or model designation "550".
DP-101 is a trade designation of Polyvinyl Chemical Industries, Inc. of Wilmington, Massachusetts. While the precise fo~mulation is not known, Polyvinyl Chemical Industries has assigned the trade designation DP-101 only to the particular urethane/acrylic copolymer resin utilized in the foregoing backcoat formulation. DP-101 is defined by Polyvinyl Chemical Inc. as a water * Trademarks -16-~ ~t7 ~
dispersion oE a graft copolymer of an aliphatic urethane joined to a styrene-acrylic copolymer. Its weight per gallon is 8.6 pounds, its acid value is 9.5, and its index of refraction is 1.3956. Its molecular weight, with respect to that portion of the resin soluble in tetrahydrofuran, when measured by GPC, is:
Mw 450,569; Mn 65,660; and Mz 1,204,300, and its viscosity, as measured by the Brookfield Viscosity Method at 25C is 200 cps. UCD-1060 is a trade designation of the Universal Color Dispersion Company of Lansing, Illinois, used to identify a dispersion product for water-based systems. Balab 3017-A is also identified by the trade designation 'bubble breaker' and is a product of the Organic Division of Witco Che~ical Corporation of New ~ork, N.Y. CP-15 is a trade designation of the Rohm and Haas Canpany and is an acrylic-based thickening agent. M-pyrol is a trade designation of the G.A.F. Corporation used to identify a methylpyrolictive coalescent solvent. The amount of organic coalescent in the water based systems preferably should not exceed about 10% by formula weight, otherwise the backcoa-ting might perrneate the hydrophobic granular matter into the fo~med cube-corner pattern.
A second fo~nulation for the backcoating mixture is herewith presented as exarnple 2 and adds a cross-linking agent to improve durability:
Example 2 1. DP-101, a water-borne polymeric system consisting of about 34%
acrylic/urethane copolymer, 61% water, and 5% coalescent solvent such as M-pyrol 70% to 90%
2. UCD-1060Q, a pre-dispersed whitening agent (-titanium dioxide) containing about 72% solids 10% -to 20%
*
3. BYK-W, a defoamer 12%
4. De-ionized water 5%
5. Anrnonia (28 percent a~ueous solution) to adjust pH
to 8.5 to 9.0 None to 0.3%
* Trademarks -17-~":
~ ~7~
After the foregoing ingredients have been rnixed, and irnnediatelyprior to application, a quantity of the foregoing mixture is placed in a mixing vessel, and a freshLy prepared solution of cross-linking agent is mixed therewith. A preferred cross-linking agent generally is a polyfunctional aziridine, such as CX-100, manufactured by Polyvinyl Chemical Industries, Inc.
of Wilmington, Massachussetts. A preferred preparation consists of 35 lbs. of backcoating mixture canbined with 150 grams of CX-100, dissolved in 150 grams of water.
BYK-W is a defoamer manufactured by Mallinckrodt of Melville, New York.
In this embodirnent, the addition of the cross-linking agent enhances the weatherability of the finished laminate by increasing the durability and toughness of the backcoating.
A third formulation for the backcoating material is herewith presented as exarnple 3-Example 3:
1. Emulsion E-1829, a water-borne polymeric acrylic ernulsion42.1% to 62.1%
2. Water 2.2% to 12.2%
3. Ethylene glycol, an anti-skinning flow improvernent agent 1.5% to 2.5%
4. UCD 1060Q, a pre-dispersed whitening agent (titanlum dioxide) 26.2% to 36.2%
5. Syloid 169, silicone dioxide flatting agen-t to prevent blocking 3.2% to 5.2%
6. Dimethylamino ethanol pH-adjusting solvent 0.3% to 0.5%
7. Balab 3017A defoamer 0.6% to 1.0%
8. Texanol solvent, a coalescent solvent for improved film formation 1.4% to 1.6%
* Trademarks -18-~.27~
9. CP-15 (50 percent in water) acrylic-based thickener to adjust viscosity None to 1.6%
The foregoing backcoating is prepared by adding the defoamer to the water-borne system with gentle mixing, then adding the water, the anti-skinning agent, the pre-dispersed whitening agent and the amine while continuing gentle mixing. Thereafter, the coalescent solvent is added. Blade speed is then increased and the thickener is added to adjust the viscosity to the desired level and the resulting mixture is then stirred at moderate speed for 30 minutes.
Emulsion E-1829 is a trade designation of the Rohm and Haas Company of Philadelphia, Pennsylvania, for an acrylic emulsion vehicle. Emulsion E-1829 is also sold under the trade designation 'Rhoplex AC-829' and is a 100%
acrylic emulsion polymer made by typical emulsion polymerization processes, with a molecular weight in excess of 1,000,000. Its weight per gallon is 8.85 pounds, its viscosity is 1,200 to 2,300 cP and its pH range is 8.6 to 9.1. Its glass transition temperature is 3C. Syloid is a trade designation of the Davidson Chemical Company, a division of W. R. Grace, of Baltimore, Maryland for a silicon dioxide flatting agent. Texanol is a trade designation of the Eastment Chemical Products Company of Kingsport, Tennessee, used to identify a coalescing agent.
Referring now to Fig. 2, a partial sectional view of a schematic portion of e~bossed thermoplastic web 26 after application of both silica 34 and backcoating 32 is shown. As therein seen, reverse side 30 of thermoplastic web includes a series of valleys, indicated generally at 76. Th~ valleys 76 schematically represent the cube-corner elements found in web 26 when the cube-corner pattern shown in E'ig. 1 is embossed onto thermoplastic web 26. When the silica layer 34 is applied, the valleys between adjacent cube-corner elements 76 are filled (except where the screen pattern leaves web 26 exposed) and, in a preferred embodiment of the invention, enough silica 34 is applied to extend a distance of about 0.0001 to about 0.003 inch above -the embossed surface of thermoplastic web 26, as characteriæed by dimension A of Fig. 2. In like fashion, the backcoat layer 32 is applied to a thickness B of about 0.002 to * Trademarks -19-about 0.004 inch above the silica layer 34. Where runners or paths 42 are formed, each such runner consists of the backcoat material which extends downward to wet the floor of each valley 76 to a total depth C, as shown in Fig. 2 which, preferably, is about 0.006 inch. In a preferred embo~iment of the present invention, each such runner is 0.001 inch deep and, as characterized by dimension D in Figs. 2 and 6, may be on the order of 0.015 inch wide.
In the embodiment herein illustrated, each discrete element of the applied silia pattern is s~uare in shape with the leng-th of each side of the square characterized by dimension E in Figs. 2 and 6. As hereinabove described, the percentage of surface area available for retroreflection may be adjusted by adjusting the dimensions D and E as shown in Figs. 2 and 6. Where, for example, dimension D is 0.015 inch and dimension E is 0.200 inch, the effective surface available for retroreflection is 84 percent~ Where dimension D is 0.027 inch and dimension E is 0.138 inch, approximately 70 percent of the surface of the resulting sheet preserves retroreflective characteristics. With a dimension D of 0.029 inch and a dimension E of 0.096 inch, approximately 55 percent of the total surface of the resulting sheet retains retroreflective prc~erties.
Thus, the degree to which the resulting laminar sheet returns incident light towards its source may be adjusted independent of the actual cube~corner type pattern formed on theImoplastic web 26, in a manner which is much more convenient and efficacious than changing the mold dimensions or characteristics used to produce the embossed cube-corner pattern.
Referring again to Figs. 1 and 4, after fourth modified laminate 84 exits drying oven 74, a pressure-sensitive or heat-activated adhesive layer 36 may then be applied by taking the resulting laminate 84 and drawing it past a station where a backing or release sheet 38, pre-coated with adhesive 36, may be layered directly onto backcoating 38, resulting in a completed laminate 22 as shown in Fig. 1. Finally, if one is used, the carrier sheet is stripped away, exposing obverse face 28 as the light-receiving surface of the inished laminate 22.
It should be noted that the foregoing examples and preferred embodiments have been presented with respect to a cube-corner embossed pattern having a depth characterized by dimension X in Fig. 2 of 0.00~ inch. It is contemplated -that patterns of varying depth and varying dimensions may be utilized, and that the dimensions herein discussed for the depth of silica applied, and the width and depth of the runners thereby formed, may be varied without departing fran the spirit and scope of the invention as herein discussed.
The finished sheet will have the physical characteristics enabling it to substantially meet specification FP-79 for reflective sheeting, and its reflective properties can easily be varied by utilizing a different screen pattern. Moreover, the whiteness achieved by the existing laminate backcoating substantially enhances the daylight esthetics of the finished material. The heating of the laminate during the drying and curing of the silica, backcoating or adhesive, also may have an effect on the final reflective performance of the laminate, dependent upon the characteristics of the initial tool and the material chosen for the fi~n. It has been determined that for optlm~n performance, the laminate should not be heated above l~ODF during these various processing steps for the preferred enbodiment disclosed herein.
It may also be noted that while the silica pattern herein presen-ted is a series of squares turned to present a diamond-like pattern, other cell sizes and shapes are also possible, wherever they appear efficacious for purposes of performance or appearance, and are within the spirit and scope of the invention as herein discussed and claimed.
As previously noted, Fig. 7 illustrates another preferred embodiment of the present :invention. In this embodiment, a layer 25 of a more weather resistant thermoplastic material than that forming web 26, such as unmodified or W modified, polymethyl methacrylate, is laminated to the impact modified acrylic forming web 26. In its preferred form, layer 25 will be about .0003 inch, and will not exceed .0005 inch (0.5 mil) in thickness. It has been found that the provision of this added layer provides additional weathering characteristics needed for certain environments, while, when not exceeding the noted thickness, permits the total laminate to remain sufficiently flexible.
Preferred materials in this embodiment may be that sold under the trade ,(~' designation V052 or V044 by -the Rohm & Haas Ccmpany, or a polyarylate sold under the trade designation Ardel, by Union Carbide. Various techniques may be employed to apply this outer layer to the web before the silica and backcoating is applied. For example, the additional layer of the~noplastic material may be applied by solvent casting or may be co-extruded with the initial film.
A preferred formulation for the ou-ter layer 25 includes use of Korad-D, the trade name of a modified polymethyl methacrylate manufactured by Polymeric Extruded Products, Inc. of Newark, New Jersey. Such material includes U.V. light absorbing substances, and is cross-linked -to a flexible, rubber base substance, adding flexibility. In particular, use of Tinuvin~ 34, a benzotriazol ccmpound manufactured by Geigy, is used as a W inhibitor. This substance is known chemically as 2-(2H-benzotriazol-2-yl)-4-methyl-phenol.
Korad-D thermoplastic is described in United States Patent No. 3,562,235, issued on February 9, 1971. When Korad-D thermoplastic is used, it may be applied as a 2 mil outer layer during the cube forming process, or it may be co-extruded with the web 26 before such formation, in a layer 1 mil thick, or it may be applied in solution directly to the web 26 in a layer 1/2 mil thick.
The particular thickness will depend in part on the total thickness parameters of the finished laminate.
~ hile the foregoing has presented various specific preferred e~bcdiments, it is to be understood that these embcdiments have been presente~l by way of example only. It is expected that others will perceive differences which, while varying Erom the foregoing, do not depart from the spirit and scope of the invention as herein claimed and described.
~ Trademarks -22-
METHODS FOR MAKING SAME
This application is a division of Canadian Serial No. 463,~18 filed September 19, 1984.
BACKGROUND OF THE INVENTION
Retroreflective sheeting has particular use in making highway signs, street signs and the like, and is now employed extensively. The Federal government has recognized two primary types of retroreflective sheeting: glass bead and cube-corner. Such approved sheeting materials are found in a specification entitled "FP-79", published by the U.S.
Department of Transportation, Federal Highway Administration.
Specification FP-79 presently has been adopted as a purchasing standard by many state highway departments, and it sets forth certain minimum specifications which must be met by retroreflective sheeting of the cube-corner type. Included among the specified characteristics are those for reflectivity, color, flexibility of material and resistance to cracking and weathering.
Cube-corner type reflector elements generally provide a higher specific lntensity at 0.2 observation angle and 0 entrance angle than do glass bead type reflector elements, but, to applicants' knowledge, no one successfully has furnished a sheeting material in com~ercial quantities which generally will meet the requirements for the Class IIIB sheeting set forth in the aforementioned FP-79 specification. Accordingly, the present invention seeks to provide a uni~ue sheeting product which will substantially meet such specified criteria and which can be produced in accordance with the novel methods disclosed herein in an economical fashion ar~ in com~ercial q~lantities.
Retroreflectivity is achieved by cube-corner type reflector elements primarily through the principle of total internal reflection. It is well known that any surface contact made by another material with the faces of the cube-corner elements generally has a deleterious effect on the reflectiveness of the reflector element.
However, when all of the element faces are metallized, or mirrored, then, rather than relying upon total internal reflection, retroreElection is achieved by specular reflection from the mirrored faces. Generally, metallizing will provide a grayish or black coloration under certain daylight conditions vis-a-vis unmetallized cube-corner type elements.
The present invention relates generally to methods and apparatus for producing retroreflective sheeting constructions and, more particularly, to methods and apparatus for producing a flexible laminate sheeting construction including an upper thermoplastic shee-t, the reverse of which is provided with a repeating, retroreflecting pattern of fine or precise detail, a backcoating to protect the formed pattern, and a selectively applied intermediate layer allowing bonding of the backcoating to overlay the formed pattern on the thermoplastic sheet while preserving and enhancing the retroreflective properties of both the formed pattern and the laminated sheet. More precisely, the present invention is applicable to the production of cube-corner type retroreflective sheeting laminates.
Within the art of designing reflectors and retrore~lective material, the -terms "cube-corner" or "trihedral," or "tetrahedral" are recognized in the art as describing structure or patterns consisting of three mutually perpendicular faces, not limited to any particular size or shape of the faces, or the orientation of the optical axis of the cube-corner element. Each of the cube-corner faces can assume a different size and shape relative to the others, depending upon the angular reflective response characteristics deslred, and the cube forming techniques employed.
Rxamples of prior cube-corner type reflectors may be found in U.S. Patent No. 1,906,655, issued to Stimson, and U.S. Patent No.
4,073,568, issued to Heasley. Stimson shows a reflex light reflector including an obverse face and a reverse light-reflecting face consisting of a plurality of cube-corner type reflector elements with each such element having three mutually perpendicular surfaces adapted for total internal reflection of light impinging thereon from the obverse face.
~easley describes a cube-corner type reflector in the form of a rectangular parallelpiped.
It long has been desired to obtain the benefits of cube-corner reflective properties in the form of flexible sheeting. As noted above, one advantageous aspect of such sheeting is in the manufacture of highway and street signs, markers and the like, where graphics are printed, painted, silk-screened or otherwise applied to a highly reflective substrate mounted to a flat, stifE, supportive surface. Flexible retroreflective sheeting, when used as such a substrate, can be stored and shipped while wound onto rolls, and can readily be cut or ortherwise formed into the desired shape and size required for a particular application. The reflective nature of the sheeting allows such signs, markers, and the like to reflect light from a vehicle's headlights, permitting the item to be read by the driver, without requiring a permanent light source to illuminate the sign or marker.
Production of such retroreflective sheeting has been made practicable by apparatus and methods to form precise cube-corner patterns in greatly reduced sizes on flexible thermoplastic sheeting. Desirably, such sheeting may then be assembled in the form of self-adhesive laminates.
Others have recogniæed the desireability of producing retroreflective thermoplastic material in sheet form. United States Patents Nos. 2,310,790, 2,380,447, and 2,481,757, granted to Jungersen, describe and teach the shortcomings of previously-known reflectors manufactured from glass, and the advantages inherent in providing a reflective material in a less fragile and more flexible sheet form. While so suggesting, it is not known if Jungersen in fact ever commercialized any product disclosed in such patents.
In U.S. Patents Nos. 4,244,683 and 4,332,847 issued to Rowland, the desirability of manufacturing cube-corner retroreflective sheeting in a continuous, non-stop process is presented, but the approach selected by Rowland is a "semi-continuous" process (Rowland '683, column 2, lines 18 -38), presumably so-called because the process requires frequent repositioning of the molding plates.
In United States Patent No. 3,187,068, issued to eVries, et al. continuous production of reflective sheeting is disclosed, utilizing encapsulated glass microspheres as the reflecting medium. eVries, et al.
describes the application of a pressure-activated adhesive layer to such sheeting to enable attachment of sheeting segments to selected surfaces.
In United States Patent No. 3,649,352, issued to Courneya, a beaded sheeting construction is described, portions of which become reflective when heated, and which includes a pressure-activated adhesive layer allowing attachment of the sheeting construction to other articles.
Palmquist, et al., 2,407,680 teach the utilization of glass microspheres or beads included as the reflective elements in flexible sheet forms; Tung, et al., in United States Patent No. 4,367,920, also desGribes a laminated sheet construction using glass microspheres as the reflective elements.
A co~mon problem in the construction of reflective laminate sheeting is to find means to bond the lamina firmly together in a way which preserves the required retroreflective qualities of the reflective elements selected for use. An example of prior efforts to solve this problem with respect to glass microspheres may be seen in United States Patent No. 3,190,178, issued to McKenzie, wherein a cover sheet or film is secured over exposed glass microspheres by use of die elements which force a portion oE the material in which the glass microspheres are embedded into contact with the cover sheet. The die elements thus create a grid pattern on the resulting sheeting construction, with each grid forming a separate cell. Within each cell, an air space is maintained between the microspheres and -the cover sheet, and incident light traverses the cover sheet and the air space to be retroreflected by the embedded microspheres.
~1'`
Holmen, et al., U.S. Patent No. 3,924,929, teach a cube-corner type uppex rigid sheet having upstanding walls, or septa, integrally formed as part of the cube pattern. The septa extend to form a regular geometric pattern of individual cells, with the septa extending at least as far from the upper sheet as the cube-corner elements. A particulate packing may be used to fill each of the cells, and a backing sheet is then attached to the rear of the upper sheet, with the septa serving as the attachment sites. Holmen, et al. use relatively large cube-corner elements fashioned as rigid sections bound to a flexible back, and has limited flexibility in use.
In McGrath, U.S. Patent No. 4,025,159, the cellular concept is described with respect to cube-corner type retroreflective sheeting, through use of dies to force a carrier film into contact with the reverse side of the cube-corner sheeting. The carrier film must then be cured with radiation to bind it to the cube-corner sheeting and, as in McKenzie, the resulting cells include an airspace extending between the carrier film and the reverse side of the cube-corner sheet. The air cell structure apparently was intended to provide a hermetically sealed cell, avoiding the need for metalizing the cube-corner elements, and providing an air/thermoplastic interface to enhance retroreflection.
None of the foregoing teach the assembly of molded or embossed cube-corner type retroreflective sheeting into self-adhesive laminates which protect and enhance the reflective properties of the sheeting without requiring the use of dies or of integrally-molded septa or walls included as part of the cube pattern. Further, none of the foregoing permits the material to benefit Erom encapsulated sections of cube-corner elements while enhancing and substantially meeting the requirements specified in the aforementioned DOT F'P-79 Specification.
BRIEF DESCRIPTION OF'1~E INN~rION
A thermoplastic sheet or web is provided on its reverse side with a retroreflective cube-corner type pattern. A thin layer of a liquid vehicle or solvent containing hydrophobic granular material (such as silica treated with silanes) is deposited on the reverse side of the web, as by screen printing, in a pattern leaving selected sites devold of granular materlal. The web ls then dried to drive off the solvent and, thereafter, a wa-ter-based backcoating is applled over the granular materlal pattern, with portions of the backcoating being in direc-t contact with the the~noplastic web at those sites on the web devoid of granular material. Thereafter, the backcoating is dried or cured, and a layer of adhesive such as pressure-sensitive or heat-activated adhesive is applied there-to. This procedure thus enables the assembly of patterned web materlal into lamlnates whlch include an actlvated adheslve layer while protecting the retroreflective properties of the precisely formed cube-corner pattern.
The invention to which this divisional application is directed pertains to a water-based backcoating for application and attachment to a thermoplastic web, the backcoating comprising a water-borne emulsion of an acrylic/urethane copolymer in a proportion from about 69 percent to about 80 percent, a whitening agent in a proportion from about 21 percent to about 24 percent, a defoamer in a proportion from about 0.4 percent to about 006 percent, an acrylic-based thickening agent in a proportion fron about 1.5 percent to 2.5 percent, and a pH-adjusting agent in a proportion up to about 0.3 percent.
Another aspec-t of the water-based backcoating for application and attachment to a supporting thermoplastic web, comprehends a water-borne polymeric acrylic system in a proportion from about 42 percent to about 62 percent, water in a proportion from about 2 percent to about 12 percent, an anti-skinning agent in a proportion from about 1.5 percen-t to about 2.5 percent, a whitening agent in a proportion from about 5 percent to about 36 percent, a flatting agent in a proportion from about 3 percent to about 5 percent, a pH-adjusting agent in a proportion fran about 0.3 percent to about 0.5 percent, a defoamer ln a proportion from about 0.6 percent to about 1.0 percent, a coalescent solvent in a proportion fram about 1.0 percent to 1.6 percent, and a thlckener in a proportlon from up to 3.0 percent.
In a preferred embodiment of the retroreflectlve laminate, an outer protective layer of thermoplastic material, used to provide additional weather reslstant properties, is secured to the thermoplastic web on the side opposite ~7~
from that upon which the retroreflective pattern is formed during or before -the cube fo~ming process.
The completed lamina-te is then cut, trimmed, or otherwise shaped for application to supporting surfaces, such as street or highway signs, and graphics or other indicia may thereafter be painted, printed, silk-screened, or otherwise affLxed to the uppermost surEace of the laminate, thus producing a readily and easily constructed highly retroreflective Einished product.
These and further aspects of the present invention will become more apparent upon consideration oE the accompanying drawings, wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged perspective and somewhat schematic view of one preferred aspect of the retroreflective sheeting of the present invention as a completed construction;
FIG. 2 ls a view along line 2-2 of Fig. 1, FIG. 3 is a greatly enlarged plan view illustrating a section of the formed surface of reflective sheeting comprising one aspect of the present invention;
FIG. 4 is a somewhat schematic and symbolic view of the processes and machinery utili2ed in a preferred aspect of the present invention;
FIG. 5 is a plan view of one form of screen pattern used to apply the hydrophobic granular layer of the present invention;
FIG. 6 is an enlarged view, in partial detail, of an individual cell of the sheeting of the present invention; and FIG. 7 is an enlarged perspective view illustrating a second preferred embodiment of the retroreflective sheeting of the present invention.
DETAILED DESCRIPTION OF THE DR~WINGS
Referring now to Fig. 3, the numeral 10 indicates generally a segment of cube-corner type retroreflective thermoplastic web used in fo~ming the laminate of the present invention. As seen in Fig. 3, there is depicted the rear surface of a portion of flexible retroreflective sheeting 12 fashioned from transparent thermoplastic material in web fo~m which has fo~med thereon, preferably by embossing, a re-troreflective and repeating pattern of cube-corner reflector elements characterized by cube faces 14, 16 and 18. In a preferred aspect of such sheeting, sheet 12 is formed from an impact-modified acrylic material having W inhibitors or absorbers added thereto, and which, prior to embossing, had parallel front and back surfaces and was initially on the order of about 0.006 inches thick. One such material is known as Plexiglas DR,TM
sold by the Rohm and Haas Ccmpany.
The cube-corner pattern foImed on sheeting :L2 is formed in an opticalLy precise, finely-detailed pattern. For example, as seen in Fig. 2, the depth to wh-ich the cube-corner pattern is embossed onto sheet 12 may be of the order of 0.00338 inch, (dimension X). As shown at dimension Y in Fig. 3, the cubes formed on sheet 12 may be spaced apart by a distance on the order of about 0.0072 inch, for the depth as shown at X as set forth above. While the cube pattern shown in Fig. 1 illustrates cubes formed with their optical axes normal to the face of sheet 12, it is to be understood that other versions and patterns may also be utilized as forming the retroreflective web of the laminate of the present invention.
Referring now to Fig. 1, the numeral 20 indicates generally a roll of retroreflective laminate 22 manufactured in accordance with preferred aspects of the present invention to be described hereinbelcw. As herein shown, laminate 22 is rolled onto a core 24. A thermoplastic web 26 having a front or obverse surface 28 and a rear or reverse surface 30 upon which is embossed the cube-corner type retroreflective pattern illustrated in Fig. 3. The thermoplastic web 26 may be on the order oE about 6 mils in thickness (0.006 inch).
Bonded to the reverse surface 30 of the thermoplastic web 26 is backcoating or film 32. In a preferred aspect of the present inven-tion, a hydrophobic granular silica material 34 is interposed between the backcoat film 32 and the reverse side 30 in a manner to be described hereinbelow.
In accordance with a preferred embcdiment of the present invention, a layer of adhesive 36 is bonded to a release sheet 38 in a presently well-known fashion, and is thereafter bonded to cured backcoat film 32 in order to provide a finished laminate 22 which includes a pressure-sensitive or heat-;" -8-activated adhesive layer 36 applied to sheeting 12 in a manner which preserves the retroreflective qualities and properties of the cube-corner pattern ernbossed thereon. The release sheet 38 is used to protect adhesive layer 36 until it is desired to apply laminate 22 to a given surface.
Fig. 4 shows, in schematic form, a preferred arrangement of equipment and sequence of operations to produce a retroreflective sheeting laminate of the type shown in Fig. 3.
The application of adheslve directly to the reverse side of a cube-corner ernbossed thermoplastic web 26 will cause an undesirable and unacceptable loss of retroreflective capability. This arises from the contac-t of the adhesive material wi-th the reverse side of ernbossed thermoplastic web 26, i.e., the filling of the "valleys" formed by the enbossed pat-tern and the subse~uent in-terface formed between substances tha-t are too similar in refractive indices to produce adequate retroreflection, so the transparent film can no longer utilize the phenomenon of total internal reflection to efficiently effect retroreflection of light. To solve this problen, a substantial portion of the cube-corner pattern ei-ther mus-t be hermetically sealed with an air space be-tween the back wall and the cube-corner elements, or the cube-corner elements must be backed in a way which would preserve the retroreflective properites of the forrned web while providing sites for firm at-tachment of an adhesive layer (or other adhesive material). Without such protection, and without such attaching sites, the use of, and effectiveness of a retroreflective embossed web is seriously compromised and curtailed.
Unexpec-tedly, use of hydrophobic granular materials has been found -to afford such protection of optical properties. ~nong such materials are xylenated glass particles, powdered silicone rubber, and silane-treated silica.
As part of the present invention, it has been found that a hydrophobic silica mixture consisting principally of amorphous silica treated with silanes, when used to fill the valleys formed by -the embossed pattern, preserves the retroreflective properties of the formed pattern for most practical purposes. Again, it is not known precisely why this effect obtains:
it has been theori~ed that the point contact of granules with the reverse face of the embossed thermoplastic web acts to preserve the retroreflective ~.~7~Q~
properties of the pattern, perhaps by preserving a sufficient air interface with the reverse side of the cube-corner pattern. However, the present invention obtains excellent results even where -the primary silica particles used are significantly srnaller than, for example, the particles discussed in prior art patents such as Holmen, e-t al~, U.S. Patent No. 3,924,929.
Use of such silica offers advantages such as low price, availability, and ease and precision of formulation. It further provides uni~ue color and reflective characteristics to the film which improves the appearance of the film even relative to the glass bead types heretofore com~only used.
As discussed hereinabove with respect to the Holmen, et al.
reference, others have attempted to solve the problem of loss of reflectivity by prcviding upstanding walls or septa as par-t of the rigid molded front face pattern, wi-th the septa forming individual pockets for the application of granular ccmpounds having particle sizes far in excess of the silica particles used in the present invention. The disadvantages to such an approach, particularly wi-th respect to the cube-corner type embossed pattern utilized in the present invention are manifest. Use of rigid septa limi-ts the size and shape of the cell. A separate mold must be formed for each type of retroreflective sheeting requiring a cell size other than that fonmed in the original mold. What is meant by the tenn "cell size" is the area bounded by or closed off by the walls to form a single pocket for the granular backing ma-terial.
Formation of such septa in a relatively rigid mold pattern manufactured to as fine and precise a degree of detail as that shown in the present invention also may cause problems wi-th respect -to stripping the Eonnedthermoplastic web from the fonning tool. This may particularly be a problem where the septa or walls extend inwardly into the mold to a dis-tance greater than the depth of the cube-corner pattern.
A preferred embodiment of the present invention includes -the mixing of a hydrophobic silica mixture using hydrophobic silica, organic solvents, and thickeners, and the application of this mixture, while in a liquid fo~m, to the reverse side of the fonmed thermoplastic web in a desired pattern. One b, J ` s 1 ~276~
advantage of the present process and product is that the pa-ttern can conveniently be changed to effect changes in reflective capability of the film, without changing the tools used in forming the embossed web. Thereafter, the partially coated or imprinted thermoplastic web is passed through a drying oven which drives off the solvents used to form the mixture, thereby drying the pattern on the thermoplastic sheet. The pattern in which the silica is applied to the thermoplastic web leaves selected portions or sites on the formed face of the thermoplastic web devoid of silica.
Referring now to Fig. 5, -the numeral 40 indicates generally such a selec-ted pattern. Each runner or path 42 represents an area on the reverse ~surface of thermoplastic web 26 where no silica has been deposited. Each s~uare or diamond~shaped area 44 represents an area on the surface of thermoplastic web 26 onto which the silica mixture has been deposited.
As seen in Fig. 6, the actual percentage of area covered by the silica mixture is determined by the thickness or width of each runner or path 42, and the pattern selected for deposition of the silica, with the cell 44 having an area bounded by the runners 42, and fully available for the reception and retroreflec-tion of incident light by the ~mbossed retroreflective patterns shown partially at 46.
Referring now to Fig. 4, it may be seen that thermoplastic web 26 may be drawn directly from an associated forming machine (not herein specifically shown) in a continuous process, or may be drawn from a separate supply reel onto which the embossed web 26 has been wound (not herein specifically shown). If desired, web 26 may be supported by a backing sheet (not herein specifically shown) coextensive with obverse face 28, leaving reverse surface 30 exposed.
It shollld be noted that reference to web 26 also includes reference to a laminate formed by web 26 and a backing sheet such as described hereinabove.
Web 26 is drawn by, for example, powered rollers (not herein specifically sh~wn), to silica mixture application station 48. As herein diagra~matically shown, a preferred means and method of applying the silica mixture to web 26 may be accomplished through use of a screen-printing cylinder 7~
50 which has mounted about the outer perlphery thereof, a metal screen formed to provide the shape or pattern to which it is desired to apply the silica mixture. The mixture is forced under pressure fram the interior of screen-printing drum 50 on-to the reverse side 30 of the thermoplastic web 26. As herein shown, the web 26 is directed by idler roller 52 to pass between the screen-printing drum 50 and a backing roller 54.
A preferred form of the apparatus utilized to apply the silica mixture at application station 48 consists of a drum printer manufacture~ by Stork Brabant BV of Boxmeer, Holland, of the type having a drum with electro-foImed mesh screens over which a photo-resist pattern (such as used for conventional silk screen) may be mounted, with a screæn pattern providing a diamond cell size in the range of fram about 0.096 inch to 0.300 inch, and a runner or cell wall thickness of fran about 0.010 inch to about 0.050 inch.
Variations in th~ shape of the cells, pattern repeat of the cells, and thickness of the runners may be accanplished by changing the printing screen used on screen-printing drum 50. Also, the constant width of the web may be of various sizes, and the printing screens used will be of a canpatible width.
In its preferred form, the silica mixture is made from a hydrophobic silica such as that manufactured by the Pigments Division of Degussa, of Frankfurt, West Germany, under the trade designa-tion Sipernat D10.TM. A preferred canposition of the mixture includes hydrophobic silica in a mixture containing approximately 98 percent silane-treated silicon dioxide (SiO2); 0.8 percent sodium oxide (Na20), and 0.8 percent of sulfur trioxide (SO3); a non-polar aliphatic hy~rocarbon solvent carrier; a polar solvent; and, where desired or re~uired, a thickening agent. One aliphatic non-polar hydrocarbon solvent successfully used is low odor mineral spirits, and a workable mixture has been created through use of an organic alcohol, preferably butanol, as the polar solvent material. A smectite clay-based thixotropic thickener also may be used in varying amounts to produce a well-defined screen-printed pattern of the silica slurry on the embossed thermoplastic web.
In its preferred embodiment, the primary particle size of thesilica is about 18 nananeters, and the agglomerated particle size of the hydrophobic silica in its final form is about 5 microns. However, it will be ~ 2~7~
understood that the only critical limitation on the particle size is such that the area in which it is deposited will be substantially impe~vious to the backcoating material 32, whereby the backcoating material is unable to penetrate the hydrophobic silica and interac-t with the cube-corner pattern except in those areas devoid of the silica.
The particular combination of solvents and thickeners is important to satisfactory deposition and definition of the silica in a precise and accurate pattern. Screen printing of particulate material commonly requires use of resins or other binders to hold the deposited particles in place. A
resin or binder cannot however be used in this instance because of the adverse effect on reflectivity of the web because of refractive index similarities.
Another important consideration is the rheology, or flow characteristics of the silica slurry as it is forced through the printing screen. The slurry must "relax", or thin as it is forced through the screen apertures, and thereafter regain sufficient viscosity to retain a well-defined pattern with good leveling qualities and appearance characteristics. Yet another consideration is use of a solvent vehicle which obtains the aforementioned qualities without attacking or degrading the thermoplastic web upon which the retroreflective pattern is formed.
Use of polar solvents, such as butanol, enables the slurry to maintain an increased concentration of solids (silica). Such solvents, however, react with the thermoplastic material used to form the web. Non-polar solvents, such as mineral spirits, preserve the embossed web, yet do not act to provide a satisfactory silica pattern. Therefore a blend of polar and non-polar solvents has been found to be useful in carrying enough solids without degrading reflectivity or degrading the web.
Preferably, the hydrophobic silica is present in proportions ranging from about 15 percent to about 35 percent by weight, the non-polar solvent carrier is present in amounts ranging from about 40 percent to about 70 percent, the polar solvent is present in amounts ranging from about 10 percent to about 30 percent, and the thickening agent may be present in amounts from about 2 percent to about 8 percent. One preferred formulation of the silica mixture includes 20 percent by weight Sipernat D10 hydrophobic silica, 56 percent mineral spiri-ts, 20 percent butanol, and 4 percent -thickener. It has been found that such propor-tions preserve the web while providing a useful silica pa-ttern.
After application of the silica mixture, web 26 is passed through a heating oven 56 where the resulting silica pattern is heated to drive off the organic solvents without heating web 26 to -the point where heat distortion of the cube-corner elements of the laminate will occur~
After drying, the silica is mechanically held to the cube-corner elements on the reverse face 30 of web 26 by, it is believed, electrostatic forces and physical inter-engagement of the silica particles themselves.
Thus, as web 26 exits mixture application station 48, it has -taken on the form of a firs-t modified laminate 58, l.e.' a web 26 having cube-corner elemen-ts with a precisely formed pattern of silica mixture screened thereon over a portion of the elements, with an uncovered portion of the cube-corner elements still exposed. As modified laminate 58 exits drying oven 56, it takes on a second modified lamina-te cons-truction 60 wherein the solvents present in the silica mixture have been driven off and the silica itself has remained dried into its screened-on pattern.
The second modified laminate 60 then enters a backcoating application station 62. The application of a water-based backcoating acconplishes several results. First, those areas onto which no silica has been screened or deposited will allow direct contact between the backcoating and the reverse side 30 of the embossed or otherwise formed thermoplastic web 26, thus "wetting" web 26 with the liquid backcoating mixture. Second, a layer of backcoating material will overlay the silica pattern formed on theLmoplastic web 26 and, when applied effectively, will not disturb or disrup-t the printed or screened-on silica pattern. Third, the backcoating may then be dried and/or cured to provide a firm attachment -to thermoplastic web 26 to provide a flat, smooth and integral surface upon which further layers, such as a layer of pressure-sensitive or heat-activated adhesive may be effectively and conveniently applied, and to protectively cover or encapsulate the silica pattern. A surprising and unexpected result is that the silica prevents permeation by the liguid backcoating to the cube-corner pattern. As described 3.2~
above, such permeation would adversely affect the reflectivity of the final assembled laminate.
Application of the backcoating mixture to the second modified laminate 60 may be accomplished in a number of ways, such as by spraying, roller application, squeegeeing, or the like. The manner in which the backcoat is applied will be determined by, inter alia, the precise formulation of the backcoat and the pressure, or force, which can be withstood by the silica pattern after it has been dried.
For purposes of illustration, a backcoating application station 62 may be characterized as having a supply header or tank 64 co~nunicating with an application means 66 which may be a nozzle or series of nozzles, or the like.
An implement such as a doctor blade 68 may be used to more uniEormly spread the backcoating after it has been applied without damaging the silica pattern. A
platen 70 prcvides support for the second modified laminate 60 during application of the backcoat.
After application, the third modified laminate 72 enters drying oven 74 wherein the backcoat material is heat-cured, resulting in backcoating layer 32 as sh~n in Fig. 1.
Successful use of a backcoating requires that the backcoating formulation meet several particularly important working parameters. One is that the backcoating have flow characteristics such that the relatively narrow and shallow rLmners formed by the silica pattern will be filled, while not dewetting or disturbing the dried silica pattern itself. This means that the viscosity of the backcoating must be carefully controlled to assure that the backcoating can be applied while completely encapsulating without disturbing the silica pattern. Another characteristic is that the backcoating cannot penetra-te or interact with the applied silica to reach the interface between the silica and the cube-corner pattern. Yet another requirement is that the backcoating, when dried, have the required flexibility and toughness to withstand use in a laminate. Ideally, the backcoating should also be of a color which enhances daytime visibility of articles made with such laninates.
Several preferred backcoatings have been utilized. ~ach may be characterized generally as including a water-borne or water-based polymeric - ~ -15-~ ~ ~t7 ~
mixture or system, a whitening agent, a defoamer, a thickner for use in adjusting the final viscosity, and a pH-adjusting component.
A first preferred fo~mulation of a backcoating is presented herewith as Example 1:
Example 1 *
1. DP-101, a water-borne polymeric system consisting of about 34%
acrylic/urethane copolymer, 61% water and 5% coalescent solvent, such as M-pyrol 69.7% to 79.7%
2. UCD-106C~, a pre-dispersed whitening agent (titanium dioxide) containing about 72% solids 21.5% to 23.5%
3. Balab 3017A, a defoamer 0.4% to 0.6%
4. CP-15 (50 percent in water) acrylic/based thickener to adjust viscosity1.5% to 2.5%
5. Annonia (28 percent aqueous solution) to adjust pH
to 8.5 to 10.0 None to 0.3%
The foregoing mixture is fonned by adding the defoamer to the water-borne acrylic/urethane copolymer system with gentle stirring. Thereafter, the whitening agent and the amnonia, if necessary, are added as gentle stirring is continued. The thickener is thereafter added with increasing blade speed and the entire mixture is stirred for about 30 minutes at moderate speed. A
preEerred mixer for such an operation is manufactured by Meyers Engineering of Bell, California under the trade or model designation "550".
DP-101 is a trade designation of Polyvinyl Chemical Industries, Inc. of Wilmington, Massachusetts. While the precise fo~mulation is not known, Polyvinyl Chemical Industries has assigned the trade designation DP-101 only to the particular urethane/acrylic copolymer resin utilized in the foregoing backcoat formulation. DP-101 is defined by Polyvinyl Chemical Inc. as a water * Trademarks -16-~ ~t7 ~
dispersion oE a graft copolymer of an aliphatic urethane joined to a styrene-acrylic copolymer. Its weight per gallon is 8.6 pounds, its acid value is 9.5, and its index of refraction is 1.3956. Its molecular weight, with respect to that portion of the resin soluble in tetrahydrofuran, when measured by GPC, is:
Mw 450,569; Mn 65,660; and Mz 1,204,300, and its viscosity, as measured by the Brookfield Viscosity Method at 25C is 200 cps. UCD-1060 is a trade designation of the Universal Color Dispersion Company of Lansing, Illinois, used to identify a dispersion product for water-based systems. Balab 3017-A is also identified by the trade designation 'bubble breaker' and is a product of the Organic Division of Witco Che~ical Corporation of New ~ork, N.Y. CP-15 is a trade designation of the Rohm and Haas Canpany and is an acrylic-based thickening agent. M-pyrol is a trade designation of the G.A.F. Corporation used to identify a methylpyrolictive coalescent solvent. The amount of organic coalescent in the water based systems preferably should not exceed about 10% by formula weight, otherwise the backcoa-ting might perrneate the hydrophobic granular matter into the fo~med cube-corner pattern.
A second fo~nulation for the backcoating mixture is herewith presented as exarnple 2 and adds a cross-linking agent to improve durability:
Example 2 1. DP-101, a water-borne polymeric system consisting of about 34%
acrylic/urethane copolymer, 61% water, and 5% coalescent solvent such as M-pyrol 70% to 90%
2. UCD-1060Q, a pre-dispersed whitening agent (-titanium dioxide) containing about 72% solids 10% -to 20%
*
3. BYK-W, a defoamer 12%
4. De-ionized water 5%
5. Anrnonia (28 percent a~ueous solution) to adjust pH
to 8.5 to 9.0 None to 0.3%
* Trademarks -17-~":
~ ~7~
After the foregoing ingredients have been rnixed, and irnnediatelyprior to application, a quantity of the foregoing mixture is placed in a mixing vessel, and a freshLy prepared solution of cross-linking agent is mixed therewith. A preferred cross-linking agent generally is a polyfunctional aziridine, such as CX-100, manufactured by Polyvinyl Chemical Industries, Inc.
of Wilmington, Massachussetts. A preferred preparation consists of 35 lbs. of backcoating mixture canbined with 150 grams of CX-100, dissolved in 150 grams of water.
BYK-W is a defoamer manufactured by Mallinckrodt of Melville, New York.
In this embodirnent, the addition of the cross-linking agent enhances the weatherability of the finished laminate by increasing the durability and toughness of the backcoating.
A third formulation for the backcoating material is herewith presented as exarnple 3-Example 3:
1. Emulsion E-1829, a water-borne polymeric acrylic ernulsion42.1% to 62.1%
2. Water 2.2% to 12.2%
3. Ethylene glycol, an anti-skinning flow improvernent agent 1.5% to 2.5%
4. UCD 1060Q, a pre-dispersed whitening agent (titanlum dioxide) 26.2% to 36.2%
5. Syloid 169, silicone dioxide flatting agen-t to prevent blocking 3.2% to 5.2%
6. Dimethylamino ethanol pH-adjusting solvent 0.3% to 0.5%
7. Balab 3017A defoamer 0.6% to 1.0%
8. Texanol solvent, a coalescent solvent for improved film formation 1.4% to 1.6%
* Trademarks -18-~.27~
9. CP-15 (50 percent in water) acrylic-based thickener to adjust viscosity None to 1.6%
The foregoing backcoating is prepared by adding the defoamer to the water-borne system with gentle mixing, then adding the water, the anti-skinning agent, the pre-dispersed whitening agent and the amine while continuing gentle mixing. Thereafter, the coalescent solvent is added. Blade speed is then increased and the thickener is added to adjust the viscosity to the desired level and the resulting mixture is then stirred at moderate speed for 30 minutes.
Emulsion E-1829 is a trade designation of the Rohm and Haas Company of Philadelphia, Pennsylvania, for an acrylic emulsion vehicle. Emulsion E-1829 is also sold under the trade designation 'Rhoplex AC-829' and is a 100%
acrylic emulsion polymer made by typical emulsion polymerization processes, with a molecular weight in excess of 1,000,000. Its weight per gallon is 8.85 pounds, its viscosity is 1,200 to 2,300 cP and its pH range is 8.6 to 9.1. Its glass transition temperature is 3C. Syloid is a trade designation of the Davidson Chemical Company, a division of W. R. Grace, of Baltimore, Maryland for a silicon dioxide flatting agent. Texanol is a trade designation of the Eastment Chemical Products Company of Kingsport, Tennessee, used to identify a coalescing agent.
Referring now to Fig. 2, a partial sectional view of a schematic portion of e~bossed thermoplastic web 26 after application of both silica 34 and backcoating 32 is shown. As therein seen, reverse side 30 of thermoplastic web includes a series of valleys, indicated generally at 76. Th~ valleys 76 schematically represent the cube-corner elements found in web 26 when the cube-corner pattern shown in E'ig. 1 is embossed onto thermoplastic web 26. When the silica layer 34 is applied, the valleys between adjacent cube-corner elements 76 are filled (except where the screen pattern leaves web 26 exposed) and, in a preferred embodiment of the invention, enough silica 34 is applied to extend a distance of about 0.0001 to about 0.003 inch above -the embossed surface of thermoplastic web 26, as characteriæed by dimension A of Fig. 2. In like fashion, the backcoat layer 32 is applied to a thickness B of about 0.002 to * Trademarks -19-about 0.004 inch above the silica layer 34. Where runners or paths 42 are formed, each such runner consists of the backcoat material which extends downward to wet the floor of each valley 76 to a total depth C, as shown in Fig. 2 which, preferably, is about 0.006 inch. In a preferred embo~iment of the present invention, each such runner is 0.001 inch deep and, as characterized by dimension D in Figs. 2 and 6, may be on the order of 0.015 inch wide.
In the embodiment herein illustrated, each discrete element of the applied silia pattern is s~uare in shape with the leng-th of each side of the square characterized by dimension E in Figs. 2 and 6. As hereinabove described, the percentage of surface area available for retroreflection may be adjusted by adjusting the dimensions D and E as shown in Figs. 2 and 6. Where, for example, dimension D is 0.015 inch and dimension E is 0.200 inch, the effective surface available for retroreflection is 84 percent~ Where dimension D is 0.027 inch and dimension E is 0.138 inch, approximately 70 percent of the surface of the resulting sheet preserves retroreflective characteristics. With a dimension D of 0.029 inch and a dimension E of 0.096 inch, approximately 55 percent of the total surface of the resulting sheet retains retroreflective prc~erties.
Thus, the degree to which the resulting laminar sheet returns incident light towards its source may be adjusted independent of the actual cube~corner type pattern formed on theImoplastic web 26, in a manner which is much more convenient and efficacious than changing the mold dimensions or characteristics used to produce the embossed cube-corner pattern.
Referring again to Figs. 1 and 4, after fourth modified laminate 84 exits drying oven 74, a pressure-sensitive or heat-activated adhesive layer 36 may then be applied by taking the resulting laminate 84 and drawing it past a station where a backing or release sheet 38, pre-coated with adhesive 36, may be layered directly onto backcoating 38, resulting in a completed laminate 22 as shown in Fig. 1. Finally, if one is used, the carrier sheet is stripped away, exposing obverse face 28 as the light-receiving surface of the inished laminate 22.
It should be noted that the foregoing examples and preferred embodiments have been presented with respect to a cube-corner embossed pattern having a depth characterized by dimension X in Fig. 2 of 0.00~ inch. It is contemplated -that patterns of varying depth and varying dimensions may be utilized, and that the dimensions herein discussed for the depth of silica applied, and the width and depth of the runners thereby formed, may be varied without departing fran the spirit and scope of the invention as herein discussed.
The finished sheet will have the physical characteristics enabling it to substantially meet specification FP-79 for reflective sheeting, and its reflective properties can easily be varied by utilizing a different screen pattern. Moreover, the whiteness achieved by the existing laminate backcoating substantially enhances the daylight esthetics of the finished material. The heating of the laminate during the drying and curing of the silica, backcoating or adhesive, also may have an effect on the final reflective performance of the laminate, dependent upon the characteristics of the initial tool and the material chosen for the fi~n. It has been determined that for optlm~n performance, the laminate should not be heated above l~ODF during these various processing steps for the preferred enbodiment disclosed herein.
It may also be noted that while the silica pattern herein presen-ted is a series of squares turned to present a diamond-like pattern, other cell sizes and shapes are also possible, wherever they appear efficacious for purposes of performance or appearance, and are within the spirit and scope of the invention as herein discussed and claimed.
As previously noted, Fig. 7 illustrates another preferred embodiment of the present :invention. In this embodiment, a layer 25 of a more weather resistant thermoplastic material than that forming web 26, such as unmodified or W modified, polymethyl methacrylate, is laminated to the impact modified acrylic forming web 26. In its preferred form, layer 25 will be about .0003 inch, and will not exceed .0005 inch (0.5 mil) in thickness. It has been found that the provision of this added layer provides additional weathering characteristics needed for certain environments, while, when not exceeding the noted thickness, permits the total laminate to remain sufficiently flexible.
Preferred materials in this embodiment may be that sold under the trade ,(~' designation V052 or V044 by -the Rohm & Haas Ccmpany, or a polyarylate sold under the trade designation Ardel, by Union Carbide. Various techniques may be employed to apply this outer layer to the web before the silica and backcoating is applied. For example, the additional layer of the~noplastic material may be applied by solvent casting or may be co-extruded with the initial film.
A preferred formulation for the ou-ter layer 25 includes use of Korad-D, the trade name of a modified polymethyl methacrylate manufactured by Polymeric Extruded Products, Inc. of Newark, New Jersey. Such material includes U.V. light absorbing substances, and is cross-linked -to a flexible, rubber base substance, adding flexibility. In particular, use of Tinuvin~ 34, a benzotriazol ccmpound manufactured by Geigy, is used as a W inhibitor. This substance is known chemically as 2-(2H-benzotriazol-2-yl)-4-methyl-phenol.
Korad-D thermoplastic is described in United States Patent No. 3,562,235, issued on February 9, 1971. When Korad-D thermoplastic is used, it may be applied as a 2 mil outer layer during the cube forming process, or it may be co-extruded with the web 26 before such formation, in a layer 1 mil thick, or it may be applied in solution directly to the web 26 in a layer 1/2 mil thick.
The particular thickness will depend in part on the total thickness parameters of the finished laminate.
~ hile the foregoing has presented various specific preferred e~bcdiments, it is to be understood that these embcdiments have been presente~l by way of example only. It is expected that others will perceive differences which, while varying Erom the foregoing, do not depart from the spirit and scope of the invention as herein claimed and described.
~ Trademarks -22-
Claims
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. A water-based backcoating composition for application and attachment to a thermoplastic web, said backcoating composition comprising (a) a water-borne emulsion of an acrylic/urethane copolymer in a proportion from about 69 percent to about 80 percent;
(b) a whitening agent in a proportion from about 21 percent to about 24 percent;
(c) a defoamer in a proportion from about 0.4 percent to about 0.6 percent;
(d) an acrylic-based thickening agent in a proportion from about 1.5 percent to 2.5 percent; and (e) a pH-adjusting agent in a proportion up to about 0.3 percent.
2. A water-based backcoating composition for application and attachment to a supporting thermoplastic web, said backcoating composition comprising:
(a) a water-borne polymeric acrylic system in a proportion from about 42 percent to about 62 percent;
(b) water in a proportion from about 2 percent to about 12 percent;
(c) an anti-skinning agent in a proportion from about 1.5 percent to about 2.5 percent;
(d) a whitening agent in a proportion from about 5 percent to about 36 percent;
(e) a flatting agent in a proportion from about 3 percent to about 5 percent;
(f) a pH-adjusting agent in a proportion from about 0.3 percent to about 0.5 percent;
(g) a defoamer in a proportion from about 0.6 percent to about 1.0 percent;
(h) a coalescent solvent in a proportion from about 1.0 percent to 1.6 percent; and (i) a thickener in a proportion from up to 3.0 percent.
3. The water-based backcoating composition of Claim 1 further including a water-based cross-linking agent added in a proportion from about 0.9 percent to about 1.5 percent of the compositions set forth in Claim 1.
4. The water-based backcoating composition of Claim 3 wherein said cross-linking agent is a polyfunctional aziridine.
5. The water-based backcoating composition of Claim 2 further including a water-based cross-linking agent added in a proportion from about 0.9 percent to about 1.5 percent of the compositions set forth in Claim 2.
6. The water-based backcoating composition of Claim 5 wherein said cross-linking agent is a polyfunctional aziridine.
7. The water-based backcoating composition of Claim 1 wherein the water-borne emulsion is an aliphatic urethane grafted to a styrene-acrylic copolymer.
8. The water-based backcoating composition of Claim 2 wherein the water-borne polymeric acrylic system comprises an acrylic emulsion polymer having a viscosity in the range from about 1,200 to about 2,300 cP.
9. The water-based backcoating composition of Claim 7 further including a water-based cross-linking agent added in a proportion from about 0.9 percent to about 1.5 percent of the composition set forth in Claim 7.
10. The water-based backcoating composition of Claim 9 wherein said cross-linking agent is a polyfunctional aziridine.
11. The water-based backcoating composition of Claim 8 further including a water-based cross-linking agent added in a proportion from about 0.9 percent to about 1.5 percent of the composition set forth in Claim 8.
12. The water-based backcoating composition in Claim 11 wherein said cross-linking agent is a polyfunctional aziridine.
1. A water-based backcoating composition for application and attachment to a thermoplastic web, said backcoating composition comprising (a) a water-borne emulsion of an acrylic/urethane copolymer in a proportion from about 69 percent to about 80 percent;
(b) a whitening agent in a proportion from about 21 percent to about 24 percent;
(c) a defoamer in a proportion from about 0.4 percent to about 0.6 percent;
(d) an acrylic-based thickening agent in a proportion from about 1.5 percent to 2.5 percent; and (e) a pH-adjusting agent in a proportion up to about 0.3 percent.
2. A water-based backcoating composition for application and attachment to a supporting thermoplastic web, said backcoating composition comprising:
(a) a water-borne polymeric acrylic system in a proportion from about 42 percent to about 62 percent;
(b) water in a proportion from about 2 percent to about 12 percent;
(c) an anti-skinning agent in a proportion from about 1.5 percent to about 2.5 percent;
(d) a whitening agent in a proportion from about 5 percent to about 36 percent;
(e) a flatting agent in a proportion from about 3 percent to about 5 percent;
(f) a pH-adjusting agent in a proportion from about 0.3 percent to about 0.5 percent;
(g) a defoamer in a proportion from about 0.6 percent to about 1.0 percent;
(h) a coalescent solvent in a proportion from about 1.0 percent to 1.6 percent; and (i) a thickener in a proportion from up to 3.0 percent.
3. The water-based backcoating composition of Claim 1 further including a water-based cross-linking agent added in a proportion from about 0.9 percent to about 1.5 percent of the compositions set forth in Claim 1.
4. The water-based backcoating composition of Claim 3 wherein said cross-linking agent is a polyfunctional aziridine.
5. The water-based backcoating composition of Claim 2 further including a water-based cross-linking agent added in a proportion from about 0.9 percent to about 1.5 percent of the compositions set forth in Claim 2.
6. The water-based backcoating composition of Claim 5 wherein said cross-linking agent is a polyfunctional aziridine.
7. The water-based backcoating composition of Claim 1 wherein the water-borne emulsion is an aliphatic urethane grafted to a styrene-acrylic copolymer.
8. The water-based backcoating composition of Claim 2 wherein the water-borne polymeric acrylic system comprises an acrylic emulsion polymer having a viscosity in the range from about 1,200 to about 2,300 cP.
9. The water-based backcoating composition of Claim 7 further including a water-based cross-linking agent added in a proportion from about 0.9 percent to about 1.5 percent of the composition set forth in Claim 7.
10. The water-based backcoating composition of Claim 9 wherein said cross-linking agent is a polyfunctional aziridine.
11. The water-based backcoating composition of Claim 8 further including a water-based cross-linking agent added in a proportion from about 0.9 percent to about 1.5 percent of the composition set forth in Claim 8.
12. The water-based backcoating composition in Claim 11 wherein said cross-linking agent is a polyfunctional aziridine.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US53306883A | 1983-09-19 | 1983-09-19 | |
| US533,068 | 1983-09-19 | ||
| US64000984A | 1984-08-10 | 1984-08-10 | |
| US640,009 | 1984-08-10 |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000463618A Division CA1247423A (en) | 1983-09-19 | 1984-09-19 | Retroreflective sheeting and methods for making same |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1276046C true CA1276046C (en) | 1990-11-06 |
Family
ID=27064048
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000463618A Expired CA1247423A (en) | 1983-09-19 | 1984-09-19 | Retroreflective sheeting and methods for making same |
| CA000586270A Expired - Fee Related CA1276046C (en) | 1983-09-19 | 1988-12-16 | Retroreflective sheeting and methods for making same |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000463618A Expired CA1247423A (en) | 1983-09-19 | 1984-09-19 | Retroreflective sheeting and methods for making same |
Country Status (1)
| Country | Link |
|---|---|
| CA (2) | CA1247423A (en) |
-
1984
- 1984-09-19 CA CA000463618A patent/CA1247423A/en not_active Expired
-
1988
- 1988-12-16 CA CA000586270A patent/CA1276046C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA1247423A (en) | 1988-12-28 |
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