 |
Description  |
|
|
FIELD OF INVENTION
This invention concerns retroreflective sheetings, and specifically
concerns sheetings that incorporate a backing film, i.e., a film that
strengthens or otherwise modifies the physical properties of the
sheetings.
BACKGROUND
Microlens-based retroreflective sheetings are well-known in the art. One
form of such sheeting is embedded-lens sheeting wherein transparent
microspheres, arranged in substantially a monolayer, are at least
partially embedded in a transparent binder or spacing layer with a
specularly reflecting layer behind the microspheres underlying the spacing
layer. Examples of embedded-lens sheetings are disclosed in U.S. Pat. No.
2,407,680 (Palmqulst et al.) and in U.S. Pat. No. 4,511,210 (Tung et al.).
Typically one or more transparent cover layers are disposed in front of
the microspheres. If the microspheres are only partially embedded in the
binder layer, the front of the microspheres are embedded in the cover
layer. Such cover layers complete the optical relationships needed for
retroreflection and may further serve to provide a protective front
surface to the sheeting, such as is provided by a tough,
weather-resistant, scratch-resistant polymeric cover film. Alternatively,
instead of a cover film, a layer of substantially transparent adhesive may
be provided in front of the microspheres, such adhesive being used, for
example, to secure the sheeting to the inside of an automobile window such
that the sheeting is viewed from outside the automobile.
An advantage of embedded-lens sheetings is that, because the microspheres
are embedded within the sheeting, incident light rays are focused onto the
specularly reflecting layer irrespective of whether the front of the
sheeting is wet or dry. Thus, such sheetings are used in many
applications, for instance, providing improved visibility to traffic
signs, automobile license plates, etc. Such sheetings are often sold
having an adhesive layer covered by a removable protective liner on the
rear side of the specularly reflective layer. In use the liner is removed
and the sheeting adhered to a desired substrate with the adhesive.
In order to achieve high brightness or efficient retroreflection, it is
sometimes preferred to construct retroreflective sheeting with a very high
density of microspheres. It is also often necessary to use dimensionally
stable and what are typically very brittle and very hard materials in the
spacing layer so that the dimensional relationships between the
microspheres, spacing layer, and specularly reflecting layer remain
constant during the conditions encountered in fabrication of a finished
product with the sheeting, e.g., a license plate, and over an extended
period of time despite outdoor exposure. Also, such forms of
retroreflective sheeting may be thin and thereby flimsy. For all these
reasons, the sheetings typically require extraordinary care in handling,
and may not emboss well, suffering cracks along the outline of embossing,
e.g., alphanumeric characters or other indicia. Such cracks are
aesthetically undesirable and weaken the sheeting thereby rendering it
more susceptible to environmental degradation.
U.S. Pat. No. 3,176,584 (DeVries et al.) discloses that a reinforcing layer
may be incorporated into sheetings as taught in that patent. The
reinforcing layer may be of similar composition as the binder or space
coat material in which the microspheres are embedded and may be applied to
the back side of the specularly reflecting layer via spraying, i.e., by a
solvent-coating technique. Examples disclosed therein include methyl
methacrylate, flexible epoxy resins, chloro-sulfonated polyethylene,
polystyrene, polypropylene, polycarbonate resin, ethyl cellulose, and
cellulose acetate-butyrate. The reference does not discuss the advantages
or usefulness of such a layer, except to point out that when applied to
the contoured reflecting layer, the reinforcing layer provides a flat
surface. The specularly reflecting layers of retroreflective sheetings are
typically very thin, i.e., on the order of 0.06 micron thick, and must be
disposed in special relationship to the microspheres in order for the
sheeting to provide useful retroreflection. Because the specularly
reflecting layers are typically so thin, they are themselves very fragile
and do not provide substantial protection to the spacing layer. Thus the
sheeting may be subject to impairment of retroreflective response caused
by disturbance of the specularly reflective layer and spacing layer as the
reinforcing layer is applied. Such. disturbance may be particularly
critical where the reinforcing layer is applied with high solvent content
or at high temperature that may cause the spacing layer to deform.
U.S. Pat. No. 3,936,567 (Vesely) discloses an embedded-lens retroreflective
sheeting wherein a member, described therein as an inner supporting layer,
is adhered to the rear side of the specularly reflecting layer with an
intermediate layer of adhesive. The inner support layers are taught by the
reference to comprise a layer of plastic film such as polyvinyl chloride
film, polyethylene terephthalate film, polyethylene film, nylon film,
polypropylene film and similar such materials, and to generally be on the
order of 5 to 15 mils thick. Adhesion of the supporting layer to the
specularly reflecting layer with an intermediate layer of adhesive, as
taught by the reference, gives rise to such disadvantages as increased
costs for the adhesive and its application and imparts increased bulk to
the sheeting, rendering it thicker and therefore more difficult to roll.
U.S. Pat. No. 3,877,786 (Booras et al.) discloses a retroreflective
sheeting with a backing or reinforcing member which is intended to support
and stiffen the entire article. Examples of backing materials disclosed
therein include corrugated paper board, a plastic laminate reinforced with
glass fibers, and metal plate. Increased stiffness of the reflective film
is apparently desired to enable easier registration of the sheeting for
application of one or more colored overcoats to the reflective surface
thereof. Such increased stiffness, however, may be undesirable in many
applications, e.g., wherein the retroreflective sheeting is to be embossed
or adhered to substrate which is not flat, such as a traffic cone.
Further, although the reference does not expressly so disclose, the
reinforcing members disclosed therein would presumably have to be adhered
to the specularly reflecting layer with an intermediate layer of adhesive.
SUMMARY OF INVENTION
The present invention provides retroreflective sheetings which may have
high flexibility, improved embossability, high impact resistance, high
tear resistance, and increased tensile strength. The sheetings provided
herein may have high adhesive compatibility, enabling a wide variety of
special purpose applications, e.g., removable embodiments, and in some
embodiments may be thermally self-adhered to substrates such as metals
without use of an additional pressure-sensitive adhesive, thereby
eliminating the cost of same and of a release liner. Additionally, such
sheetings may possess sufficient body and stability such that they do not
require extraordinary care when being handled, and may typically be
printed on in roll or sheet form, e.g., flexographic printing, offset
printing or screen printing, and may also be accurately and cleanly cut,
e.g., by die cutting, into desired patterns.
In brief summary, the novel retroreflective sheeting of the invention is a
microsphere-based retroreflective sheeting comprising: (1) transparent
microspheres, typically arranged in substantially a monolayer; (2) a
transparent binder or spacing layer which the microspheres are supported
by and at least partially embedded in; (3) a specularly reflecting layer
underlying the spacing layer behind the microspheres; and (4) a backing
film. The transparent microspheres are typically glass. The spacing layer
may be a substantially thermoset material that tends to maintain its
conformed contour such that the specularly reflecting layer and
microspheres remain disposed in the desired relationship for
retroreflection. Typically the sheeting further comprises one or more
transparent cover layers disposed in front of the microspheres in which
the microspheres may be partially embedded. Such cover layers complete the
optical relationships needed for retroreflection and may further serve to
provide protection to the sheeting. Examples of suitable cover layers
include polymeric, typically weather-and/or scratch-resistant, cover
films; or a layer of substantially transparent adhesive, by which the
sheeting may be adhered, for example, to a window as a window sticker that
is visible from the opposite side of the window. According to the
invention, the backing film is self-adhered directly to the rear side of
the specularly reflecting layer. Thus, the advantages of this invention
are achieved without interference with the optical properties or
relationships of the retroreflective sheeting, i.e., the backing film is
optically nonfunctional. The backing film may be preformed, i.e., it is
formed before being adhered to the specularly, reflecting layer, or may be
formed directly thereon by extrusion.
The backing film comprises at least one layer referred to herein as the
self-adherent layer. The self-adherent layer, which is adhered directly
(i.e., without an additional adhesive agent) to the specularly reflecting
layer, comprises a polymer that is substantially thermoplastic and
nonelastomeric, is preferably extruded, and comprises: polyethylene; or
preferably, one or more copolymers of monomers comprising by weight a
major proportion of at least one of ethylene or propylene, and a minor
proportion of at least one polar comonomer; or a blend comprising (a) one
or more other polymers and (b) one or more of such copolymers or
polyethylene or combinations of such copolymers and polyethylene, the
blend comprising a major portion by weight of ethylene and/or propylene.
By "nonelastomeric", it is meant that the self-adherent layer does not
have a substantial tendency to recover or return to its initial dimensions
when stretched at room temperature and the stretching force is released.
The self-adherent layer typically has a melt index of less than 500, while
those polymers having a melt index of less than 150 are typically
preferred, and those having a melt index of less than 20 are most
preferred. Polymers having lower melt indices are typically easier to
extrude and more resistant to softening at elevated temperatures.
If desired, the backing film may further comprise at least one additional
layer, for example, a reinforcing layer, disposed on the rear side of the
self-adherent layer, i.e., the side opposite that which is adhered to the
specularly reflecting layer. Examples of materials which may be used in
such a reinforcing layer include nylon and polyester which may be used to
provide the sheeting with exceptional tear resistance and tensile
strength.
Retroreflective sheetings of the invention are typically more conveniently
handled than similar sheetings in which no backing film is incorporated.
The increased tensile strength and greater tear resistance which may be
provided herein substantially eliminate the need for extraordinary care
when handling such sheeting. Further, the backing film imparts more body
to the sheeting, which, however, may still be rolled, thereby rendering
same more convenient to work with. Sheeting of the invention typically
possesses sufficient body that it may be printed upon, in either roll or
sheet form, e.g., by flexographic printing, screen printing, or offset
printing, and may be accurately and cleanly cut, e.g., by die cutting,
into desired shapes, such as alphanumeric characters.
Utilization of a backing film according to the invention may increase the
impact resistance, particularly at low temperatures, and improve the
embossability of retroreflective sheeting in which it is incorporated.
Further, in some embodiments retroreflective sheetings of the invention
may be heat laminated directly to a substrate such as a metal surface
without another adhesive agent to produce, e.g., license plates or traffic
signs. In such embodiments, the backing film acts as adhering means to
provide a self-adhering bond to the substrate, thereby eliminating the
need for, and cost of, an adhesive and release liner. Alternatively, the
backing film may provide an adhesive-compatible surface, thereby enabling,
for instance, use of adhesives which may not provide sufficient bonding
strength to the specularly reflecting layer.
Accordingly, the backing films provided herein differ from those of the
prior art in that they provide effective self-adhesion to materials,
commonly used in the specularly reflecting layer, thereby eliminating the
need for an intermediate adhesive layer and the costs for such adhesive
and its application, while also providing desired physical properties such
as the tensile strength, high tear resistance, high impact resistance, and
cold temperature (i.e., -40.degree. C.) retention of properties.
DESCRIPTION OF THE DRAWING
The invention will be further explained in the drawing, wherein:
FIG. 1 is a cross-sectional representation of a portion of an embedded-lens
retroreflective sheeting of the present invention illustrating lamination
of a backing film directly to the rear side of the specularly reflecting
layer;
FIG. 2 is a cross-sectional representation of a portion of an embodiment of
the invention wherein a layer of adhesive and a removable liner have been
applied to the rear side of the backing film;
FIG. 3 is a cross-sectional representation of a portion of another
embodiment of the invention that has been adhered to a metal substrate
utilizing the backing film as adhering means, and
FIG. 4 is a cross-sectional representation of a portion of an embodiment of
the invention adhered to the inside of a windshield as a sticker visible
from the outside thereof, with a layer of transparent adhesive disposed in
front of the microspheres.
These figures, which are not to scale, are intended to be merely
illustrative and nonlimiting.
DETAILED DESCRIPTION OF INVENTION
The advantageous results of this invention have been achieved by
incorporation in a retroreflective sheeting, directly to the rear side of
the specularly reflecting layer thereof, of a backing film that comprises
at least a self-adherent layer and, in some embodiments, that further
comprises one or more additional layers, e.g., a reinforcing layer, on the
rear side of the self-adherent layer, i.e., the opposite side of the
self-adherent layer to that which is directly adhered to the specularly
reflecting layer.
The self-adherent layer comprises a polymer that is substantially
thermoplastic and nonelastomeric, is preferably extruded, and comprises:
polyethylene; or preferably, one or more copolymers of monomers comprising
by weight a major portion of at least one of ethylene or propylene, and a
minor portion of at least one polar comonomer. The comonomer is preferably
polar in nature, e.g., contains an 0, or a N, or combination thereof, to
improve adhesion of the resulting copolymer to the specularly reflecting
layer, which is typically a metal such as aluminum. Examples include
acrylic acid, methacrylic acid, ethyl acrylate, and vinyl acetate.
Alternatively, the self-adherent layer may comprise a blend comprising (a)
one or more other polymers and (b) one or more of the copolymers described
above or polyethylene or combinations of such copolymers and polyethylene,
with the major portion of the blend being ethylene and/or propylene. The
self-adherent layer typically has a melt index of less than 500,
preferably less than 150, and most preferably less than 20. Polymers
having lower melt indices are typically easier to extrude and more
resistant to softening at elevated temperatures.
A number of suitable polymers are commercially available at reasonable
cost, including:
______________________________________
Helt
Supplier Designation
Comonomer (Weight %)
Index
______________________________________
Dow Chemical
PRIMACOR Acrylic Acid (9)
10
Company 3440
E.I. duPont
NUCREL Methacrylic Acid (20*)
35
035
ELVAX 230 Vinyl Acetate (28)
110
SURLYN Methacrylic Acid (**)
0.7
1706
______________________________________
*Total comonomer (believed to also include a small proportion of vinyl
acetate)
**Precise composition is unknown
Each of these polymers has excellent flexibility at temperatures as low as
-40.degree. C., and retains broad adhesive compatibility, self-adhesion to
the specularly reflecting layer, and desired strength and toughness at
such temperatures. Thus, backing films made from such polymers may be
incorporated in retroreflective sheetings to be used for outdoor
applications, e.g., license plates or highway markers.
Preferably the self-adherent layer comprises a copolymer which has been
crosslinked by cations such as zinc, sodium, or potassium. Such a
crosslinked polymer is sometimes here called an "ionomeric copolymer".
Such ionomeric copolymers provide good adhesion to metals such as found in
the specularly reflecting layer and are believed to have improved tensile
strength. Ionomeric copolymers crosslinked with zinc are more preferred
because they are believed to be more weatherable and more impermeable to
moisture, and to provide better adhesion to metals than other types of
ionomeric copolymers. A preferred commercially available ionomeric
polyethylene copolymer is SURLYN Brand 1706 Resin, which is a copolymer
of, by weight, a major proportion of ethylene and a minor proportion of
methacrylic acid ionically crosslinked with zinc ions. This polymer is
particularly preferred for embossable sheetings because of the toughness
it imparts thereto.
Blends comprising (a) one or more other polymers and (b) one or more of the
copolymers described above or polyethylene or combinations of said
copolymers and polyethylene, wherein a major portion by weight of the
blend is ethylene and/or propylene, may be used to achieve lower costs and
provide a backing film having different physical properties, e.g., greater
tensile strength or higher impact resistance. In the case of a blend
comprising polyethylene, such other polymer is preferably polar in nature
to improve the self-adhesion to the specularly reflecting layer.
The self-adherent layer may further comprise such additives as colorants,
e.g., pigments; fillers, e.g., glass fibers as reinforcing agents; or
stabilizers, as long as such additives do not undesirably interfere with
the self-adhesion of the layer to the specularly reflecting layer.
The advantages of this invention, i.e., a retroreflective sheet having the
desired physical properties, may be achieved utilizing backing films
formed by many techniques. A backing film of the invention may be
preformed, such as by extrusion or solvent casting, and then adhered to
the rear side of the specularly reflecting layer, e.g., by heat
lamination, or the backing film may be formed directly upon the specularly
reflecting layer, thereby eliminating an intermediate processing step. The
backing film is preferably preformed and then adhered to the specularly
reflecting layer as such techniques are typically less strenuous upon the
fragile specularly reflecting layer. If the backing film comprises both a
self-adherent layer and a reinforcing layer, it is typically preformed and
then heat-laminated to the specularly reflecting layer, although some
combinations, e.g., a backing film comprising a self-adherent layer of
ethylene/acrylic acid copolymer and a reinforcing layer of polypropylene,
can be formed directly on the specularly reflecting layer such as by
co-extrusion.
The backing film is preferably extruded rather than cast from solution,
thus avoiding the cost of solvent and possible, pollution caused by
driving off the solvent. Furthermore, extrusion typically permits faster
production rates because the time delay incurred in drying a solvent-cast
film is eliminated. Also, if the backing film is to be formed directly
upon the specularly reflecting layer, it is preferably extruded rather
than solvent-cast to reduce the risk that solvent may leak through the
specularly reflecting layer to cause deformation of the spacing layer and
thereby impair the retroreflective response of the resultant sheeting.
In some instances, adhesion of the backing film to the specularly
reflecting layer may be improved by priming with known surface
modification treatments such as corona discharge. Polyethylene backing
films typically require such treatment to achieve sufficient adhesion to
the specularly reflecting layer and are therefore less preferred than the
aforementioned copolymers of ethylene and/or propylene which typically do
not require such a prime and tend to be stronger and tougher as well.
As described above, the backing film may further comprise at least one
layer in addition to the self-adherent layer. Such layer may be a
reinforcing layer comprising, for example, nylon, polypropylene,
acrylonitrile/butadiene/styrene, polycarbonate, or glass-filled polymers.
Biaxially oriented polyester may be used if treated such as by the method
disclosed in U.S. Pat. No. 3,188,266 (Charbonneau et al.) to provide an
effective bond between the self-adherent layer and polyester reinforcing
layer. Such backing films may be used to provide retroreflective sheetings
with desired properties such as exceptional tensile strength and tear
resistance, or sheetings which are essentially inextensible. In some
embodiments the backing film may comprise two layers in addition to the
self-adherent layer. For instance, a backing film comprising, in turn a
self-adherent layer, a nylon reinforcing layer on the rear side thereof,
and a third layer that has the same properties as the self-adherent layer
on the rear side of the reinforcing layer, possesses both the particular
strength properties imparted by the nylon reinforcing layer as well as the
particular surface properties of the third layer, e.g., self-adhesion to
metals.
Reduced thickness of the backing film and overall sheeting are typically
preferred because of cost savings provided thereby, because less bulky
sheetings may be more easily wound upon themselves into rolled form, and
because in most uses, thinner sheetings are easier to work with. The total
thickness of backing films of the invention are preferably less than 125
micrometers (5 mils) and more preferably less than 75 micrometers (3
mils).
The optimum tear resistance, tensile strength, etc., of the backing film,
and of the ultimate retroreflective sheeting, will be determined in part
by the intended use of the final construction. For instance, in some
embodiments useful backing films should provide the elongation required to
permit the composite retroreflective sheeting to stretch sufficiently to
be applied as to a traffic cone or to irregular surfaces. Good
stretchability also is required when the novel retroreflective sheeting is
to be embossed, e.g, on a license plate. To permit embossing under typical
conditions in a male/female die to the depth of 2.5 millimeters with
minimal, preferably no, cracking, the elongation of the retroreflective
sheeting, including the backing film which is part thereof, should be at
least 15 percent, is preferably at least 50 percent, and is more
preferably at least 200 percent. Conversely, the tensile strength of the
sheeting should not be so high that the sheeting, adhered to a surface
before embossing, tends to recover so as to delaminate from the precise
contours of the embossed characters, i.e., undergo "tenting". Therefore,
if the sheeting is adhered to the surface with a typical
pressure-sensitive adhesive, the retroreflective sheeting should have a
sufficient | | |
