Note: Descriptions are shown in the official language in which they were submitted.
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
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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
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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
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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
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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-
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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
-11-
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
12-
~ 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
-13-
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
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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
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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-
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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-
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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
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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-
