U.S. patent application number 09/240342 was filed with the patent office on 2001-07-19 for angular brightness microprismatic retroreflective film or sheeting incorporating a syndiotactic vinyl aromatic polymer.
This patent application is currently assigned to 3M INNOVATIVE PROPERTIES COMPANY. Invention is credited to OJEDA, JAIME R., SMITH, KENNETH L., TABAR, RONALD J..
Application Number | 20010008679 09/240342 |
Document ID | / |
Family ID | 22906145 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008679 |
Kind Code |
A1 |
SMITH, KENNETH L. ; et
al. |
July 19, 2001 |
ANGULAR BRIGHTNESS MICROPRISMATIC RETROREFLECTIVE FILM OR SHEETING
INCORPORATING A SYNDIOTACTIC VINYL AROMATIC POLYMER
Abstract
A novel microprismatic retroreflective film or sheeting is
provided which comprises a transparent semicrystalline polymer. The
microprismatic retroreflective film or sheeting of the present
invention offers superior retroreflective brightness at large
angles of incidence and at large observational angles. Syndiotactic
vinyl aromatic polymers, especially syndiotactic polystyrene and
copolymers thereof, are preferred semicrystalline polymers, as they
impart good dimensional stability and resistance to moisture, and
can be made resistant to UV radiation. Signing materials comprising
these microprismatic retroreflective films provide improved
performance for off-angle illumination and viewing in traffic
control and other signing applications.
Inventors: |
SMITH, KENNETH L.; (ST PAUL,
MN) ; OJEDA, JAIME R.; (ST PAUL, MN) ; TABAR,
RONALD J.; (ST PAUL, MN) |
Correspondence
Address: |
JOHN A FORTKORT
3M OFFICE OF INTELLECTUAL
PROPERTY COUNSEL
PO BOX 33427
ST PAUL
MN
551333427
|
Assignee: |
3M INNOVATIVE PROPERTIES
COMPANY
|
Family ID: |
22906145 |
Appl. No.: |
09/240342 |
Filed: |
January 29, 1999 |
Current U.S.
Class: |
428/141 ;
428/156 |
Current CPC
Class: |
Y10T 428/24355 20150115;
Y10T 428/24479 20150115; G02B 5/124 20130101 |
Class at
Publication: |
428/141 ;
428/156 |
International
Class: |
B32B 003/26 |
Claims
What is claimed is:
1. A retroreflective film, comprising a polymer derived from a
syndiotactic vinyl aromatic monomer.
2. The film of claim 1, wherein said syndiotactic vinyl aromatic
polymer is a transparent semicrystalline polymer.
3. The film of claim 1, wherein said syndiotactic vinyl aromatic
polymer comprises at least 80% by weight of styrene moieties.
4. The film of claim 3, wherein said syndiotactic vinyl aromatic
polymer further comprises at least 5% by weight of
para-methylstyrene moieties.
5. The film of claim 1, wherein said syndiotactic vinyl aromatic
polymer is syndiotactic polystyrene.
6. The film of claim 1, wherein said film has at least one
microprismatic surface.
7. The film of claim 1, wherein said film has a retroreflective
surface having cube-cornered geometry.
8. The film of claim 1, wherein said syndiotactic vinyl aromatic
polymer has at least 20% by weight of syndiotactic chain
segments.
9. The film of claim 1, wherein said syndiotactic vinyl aromatic
polymer has between 30% and 98% by weight of syndiotactic chain
segments.
10. The film of claim 1, wherein said syndiotactic vinyl aromatic
polymer has between 85% and 95% by weight of syndiotactic chain
segments.
11. The film of claim 1, further comprising between about 0 5 and
2.0 parts by weight of a UV absorbing material per 100 parts by
weight of said syndiotactic vinyl aromatic polymer.
12. The film of claim 1, further comprising an antioxidant.
13. The film of claim 12, wherein said antioxidant is present in an
amount of between about 1.times.10.sup.-4 and about 2 parts by
weight per 100 parts by weight of said syndiotactic vinyl aromatic
polymer.
14. The film of claim 12, wherein said antioxidant is present in an
amount of between about 0.001 and about 1 parts by weight per 100
parts by weight of said syndiotactic vinyl aromatic polymer.
15. The film of claim 12, wherein said antioxidant is present in an
amount of between about 0.01 and about 0.5 parts by weight per 100
parts by weight of said syndiotactic vinyl aromatic polymer.
16. The film of claim 12, wherein said antioxidant is a hindered
phenolic resin.
17. A sign, comprising the film of claim 1.
Description
FIELD OF THE INVENTION
[0001] The present invention pertains to retroreflective film or
sheeting, and more specifically to retroreflective film or sheeting
comprising a microreplicated cube-corner (microprismatic)
pattern.
BACKGROUND OF THE INVENTION
[0002] Retroreflective film or sheeting utilizing microprismatic
reflecting elements is used extensively in signing applications,
including signing for traffic control. Microprismatic
retroreflectors typically comprise a sheet having a generally
planar front surface and an array of cube corner elements
protruding from the back surface. Cube corner elements comprise
interconnected, generally trihedral structures, each of which has
approximately mutually perpendicular lateral faces meeting to form
a single corner, and thus are characterized as cube-corners. In
use, the retroreflector is arranged with the front surface disposed
generally toward the anticipated location of both incident light
and intended observers. Light incident to the front surface enters
the sheet, passes through the body of the sheet and is internally
reflected by the faces of the elements so as to exit the front
surface in a direction substantially toward the light source. This
is referred to as retroreflection. The light rays are typically
reflected at the cube faces due either to total internal reflection
(TIR) from interfaces with an intentionally entrapped medium of
greatly different refractive index, such as air, or to reflective
coatings, such as vapor deposited aluminum.
[0003] In general, microprismatics reflect light back toward a
light source with high efficiency. In addition, microprismatics can
spread the retroreflected light into a zone or "cone of light"
determined by the particular cube-corner optical design. This
enables detection of reflected light at an observation angle other
than zero degrees, with zero degrees defined as the vector of
perfect retroreflection. The combination of the cube-corner optical
geometry and a material of construction having a high index of
refraction serves to maximize entrance angularity, which is to say,
to maximize the entrance angle, or angle of incidence, up to which
good retroreflective performance is observed. Angle of incidence
refers to the angle made by the vector of incident light with a
vector normal to the planar front surface of the film or sheeting.
Polymeric materials of construction are preferred because of their
physical properties; thus, within the realm of polymeric materials
typically used in these applications, an index of refraction
greater than 1.50 is considered high and is desirable. Several
cube-corner optical constructions for signing applications include
those described in U.S. Pat. No. 3,684,348 (Rowland), U.S. Pat. No.
4,588,258 (Hoopman), U.S. Pat. No. 5,138,488 (Szczech), and U.S.
Pat. No. 4,775,219 (Appledom, et al. ). U.S. Pat. No. 3,712,706
(Stamm) recognizes that a certain amount of divergence of the
retroreflected light from a microprismatic structure is always
present due to optical imperfections. In this patent, said
divergence due to optical imperfections is minimized, and the
arrangement of the optical elements is established such that the
angular divergence of the retroreflected light attributable to
diffraction is the dominant diverging factor.
[0004] U.S. Pat. No. 3,817,596 (Tanaka) seeks to diffuse the
retroreflected light by comprising the retroreflector of two types
of optical cube-corner elements, the first variety having three
reflecting planes positioned such that lines normal thereto
intersect each other at right angles, and a second variety having
three reflecting planes positioned such that lines normal thereto
intersect in a skewed manner while optical axes thereof intersect
each other at right angles.
[0005] U.S. Pat. No. 4,775,219 (Appledorn, et al.) provides
retroreflective articles which may be individually tailored so as
to distribute light retroreflected by the articles into a desired
pattern or divergence profile. This is accomplished by forming the
three lateral faces of the reflecting elements by three
intersecting sets of parallel V-shaped grooves, with at least one
of the sets including, in a repeating pattern, a groove side angle
that differs from another groove side angle of the same set.
[0006] PCT Appl. No. 96/30786 (Nilsen) seeks to redistribute light
within the retroreflected cone by texturing a surface of the
retroreflective sheeting which is in the path of the light, in
order to decrease the high degree of variation within the cone due
to the diffraction phenomena discussed in the Stamm patent.
[0007] Combining cube-corner optics with materials of construction
which further advance the performance of retroreflective articles
has become a primary focus of the industry. Polycarbonates and
acrylics are optical quality materials commonly utilized in
cube-corner retroreflectors, and polybutyrates have also been
utilized, as all three provide good optical properties and are
easily processed with conventional forming techniques. A variety of
replication techniques for manufacturing microreplicated
cube-corner materials from thermoplastics have been known to the
art. Some of these are detailed in U.S. Pat. No. 3,810,804
(Rowland), U.S. Pat. No. 4,244,683 (Rowland), U.S. Pat. No.
4,332,847 (Rowland), U.S. Pat. No. 4,486,363 (Pricone and Heenan),
U.S. Pat. No. 4,601,861 (Pricone and Roberts), U.S. Pat. No.
5,706,132 (Nestegard, et al.), Eur. Pat. Appl. 796,716 (Fujii, et
al.), and Eur. Pat. AppI. 818,301 (Fujii, et al.).
[0008] Polycarboiate (PC), which has a relatively high isotropic
index of refraction of 1.586, has been a preferred material for
microprismatics because it more effectively retroreflects to a
source which emits light at large angles of incidence to the
microprismatic sheeting. As governed by Snell's law, the higher the
index of refraction of a material is, the smaller the critical
angle (.theta..sub.c) of refraction will be, and thus, the incident
angle to which TIR can be achieved within a particular cube-corner
element will be larger (FIG. 1). Since less retroreflectivity
results as light enters a cube-corner retroreflector at
progressively larger angles of incidence, a material which enhances
high incidence retroreflectivity, while also enhancing brightness
at larger observation angles, would be of particular interest in
signing applications. This is particularly true in urban traffic
signing applications, where competition from roadway illumination
lighting, internally lit signing, automobile headlights, and other
light sources may significantly detract from the conspicuity of a
retroreflective traffic control device, sign, or the like, and a
safety premium is attached to the conspicuity of signage at
intermediate distances and wider observational angles. Relatively
less activity has been devoted to finding materials which can
enhance brightness at larger observation angles than has been the
case for finding high-index materials which can improve entrance
angularity.
[0009] Thus, microprismatic retroreflective materials of the prior
art have shortcomings in optical brightness when viewed at wider
observational angles or from intermediate distances. Also, the
three polymer types from which they are most frequently
manufactured are relatively expensive, and are subject to
dimensional instability when exposed to moisture. Many other
well-known polymeric materials which might otherwise be used in
microprismatic retroreflective materials lack sufficient resistance
to one or more of the factors involved in "weathering", such as
ultraviolet light, heat, moisture, and abrasion. Thus, there is a
need in the art for a microprismatic retroreflective material
having improved performance at wider observational angles, made
from a polymer having an index of refraction at least comparable to
that of prior art materials, conventional processability, good
weatherability, improved dimensional stability with respect to
moisture, and low cost.
[0010] These and other needs are met by the present invention, as
hereinafter described.
SUMMARY OF THE INVENTION
[0011] The present invention is a microprismatic retroreflective
film or sheeting, and a signing material comprising said
microprismatic retroreflective film or sheeting.
[0012] In one aspect of the invention, a microprismatic
retroreflective film or sheeting is provided which comprises a
transparent semicrystalline polymer. Preferably, the
semicrystalline polymer is a syndiotactic vinyl aromatic polymer;
more preferably, the semicrystalline polymer is a syndiotactic
vinyl aromatic polymer having at least 80% by weight of styrene
moieties, and most preferably, the semicrystalline polymer is a
syndiotactic polystyrene copolymer. In a particularly preferred
embodiment of the invention, the microprismatic retroreflective
film or sheeting comprises a transparent semicrystalline polymer
comprising a syndiotactic vinyl aromatic polymer comprising at
least 80% by weight of styrene moieties and further comprising at
least 5% by weight of para-methylstyrene moieties.
[0013] In another aspect of the invention, a signing material is
provided, comprising the microprismatic retroreflective film or
sheeting comprising a transparent semicrystalline polymer, which
has enhanced retroreflected brightness with respect to comparable
signing material comprising a microprismatic retroreflective film
or sheeting consisting of polycarbonate. Preferably, the signing
material, comprising the microprismatic retroreflective film or
sheeting comprising a transparent semicrystalline polymer, has
enhanced retroreflected brightness, at entrance angles greater than
about 30.degree., or at observation angles greater than about 0 20,
with respect to comparable signing material comprising a
microprismatic retroreflective film or sheeting consisting of
polycarbonate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0015] FIG. 1 is a schematic drawing illustrating the concept of
Total Internal Reflection (TIR) for a cube-corner element for an
angle of incidence smaller than the critical angle (which is a
function of the isotropic index of refraction of the material of
construction of the cube-corner element);
[0016] FIG. 2 is a graphical comparison of the Brightness in
CandlePower as a function of the Entrance Angle in Degrees for the
retroreflective sheetings of EXAMPLE 2 and COMPARATIVE EXAMPLE C1,
illustrating, for the case of an orientation angle of 0.degree. and
an observation angle of 0.33.degree., the superiority at all
entrance angles of the sheeting of EXAMPLE 2;
[0017] FIG. 3 is a graphical comparison of the Brightness in
CandlePower as a function of the Entrance Angle in Degrees for the
retroreflective sheetings of EXAMPLE 2 and COMPARATIVE EXAMPLE C1,
illustrating, for the case of an orientation angle of 0.degree. and
an observation angle of 0.5.degree., the superiority at all
entrance angles of the sheeting of EXAMPLE 2;
[0018] FIG. 4 is a graphical comparison of the Brightness in
CandlePower as a function of the Entrance Angle in Degrees for the
retroreflective sheetings of EXAMPLE 2 and COMPARATIVE EXAMPLE C1,
illustrating, for the case of an orientation angle of 0.degree. and
a very small observation angle of 0.20, the superiority at entrance
angles smaller than about 30.degree. of the sheeting of COMPARATIVE
EXAMPLE 2, and the equivalent-to-superior performance at entrance
angles larger than about 30.degree. of the sheeting of EXAMPLE 2;
and
[0019] FIG. 5 is a schematic drawing illustrating one possible
explanation for the the observation-angle dependence of the optical
behavior of the sheeting of EXAMPLE 2; to wit, that light
scattering from the semicrystalline structure of sPS causes the
redistribution, to larger observation angles, of light
retroreflected from the cube-corner element of EXAMPLE 2, resulting
in increased brightness at larger observation angles, and decreased
brightness at the smallest observation angles, for the case of
small entrance angles.
[0020] It should be understood that the invention is not limited to
the particular embodiments exemplified in the Drawings, nor those
disclosed in the following Detailed Description. On the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims. The embodiments are chosen and
described so that others skilled in the art may appreciate and
understand the principles and practices of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] Recent developments in catalysis technology have enabled the
synthesis of vinyl aromatic polymers, such as polystyrene, that
comprise chain segments having a so-called "syndiotactic"
configuration. Syndiotacticity refers to one pattern by which vinyl
monomers may be added to a growing polymer chain when one of the
carbon atoms involved in the monomer's double bond carries two
different substituents. Polymerization of such monomers in
head-to-tail fashion yields a polymer chain in which every other
carbon atom of the backbone is a site of steric isomerism. Such
carbon atoms are referred to as "pseudoasymmetric" or "chiral"
carbon atoms. Each pseudoasymmetric carbon atom can exist in one of
two distinguishable configurations. Depending upon the
configurations of such carbon atoms when the corresponding vinyl
monomers are added to a growing polymer chain, the resultant chain
can be atactic, isotactic, or syndiotactic.
[0022] For example, consider a pseudosymmetric carbon atom of a
head-to-tail backbone that carries the substituents X and Y. If the
polymer backbone is oriented so that the bonds between the main
chain carbon atoms form a planar zigzag pattern, then each X and Y
substituent will lie either above or below the plane defined by
said backbone. If all X substituents lie to one side of the
backbone while all Y substituents lie to the other side, then the
polymer chain is said to have an isotactic configuration. If the X
and Y substituents are randomly distributed above and below the
backbone, then the polymer chain is said to have an atactic
configuration. If the X and Y substituents appear alternately above
and below the backbone, the polymer is said to have a syndiotactic
configuration. In other words, the side groups of a syndiotactic
polymer chain are arranged in a symmetrical and recurring fashion
above and below the backbone when the backbone is arranged so as to
lie in a single plane. For example, in the case of syndiotactic
polystyrene, phenyl groups (the side groups), are configured
alternately above and below the plane defined by the zigzag pattern
of the fully extended carbon-carbon main chain. Syndiotacticity is
described in Rudin, "The Elements of Polymer Science and
Engineering", Academic Press, pages 128-132 (1982).
[0023] Syndiotactic vinyl aromatic polymers have been used to make
various articles that exhibit good dimensional stability, thermal
stability, and/or moisture resistance. The use of syndiotactic
polystyrene in overlay films, for example, has been described in
Assignee's copending application U.S. Ser. No. 08/761,912, filed
Dec. 9, 1996, having Attorney's Docket No. 53059USA8A. The use of
syndiotactic vinyl aromatic polymers in release liners, including
release liners having a microreplicated texture or pattern thereon,
has been described in Assignee's copending application, having
Attorney's Docket No. 53467USA3A, filed on even date with the
present application, and incorporated herein by reference.
[0024] Syndiotactic, vinyl aromatic polymers and methods of making
these polymers have been described in U.S. Pat. No. 5,496,919
(Nakano); U.S. Pat. No. 5,188,930 (Funaki et al.); U.S. Pat. No.
5,476,899 (Funaki et al.); U.S. Pat. No. 5,389,431 (Yamasaki); U.S.
Pat. No. 5,346,950 (Negi et al.); U.S. Pat. No. 5,318,839 (Arai et
al.); U.S. Pat. No. 5,273,830 (Yaguchi et al.); U.S. 5,219,940
(Nakano); U.S. Pat. No. 5,166,238 (Nakano et al.); U.S. Pat. No.
5,145,950 (Funaki et al.); U.S. Pat. No. 5,127,158 (Nakano); and
U.S. Pat. No. 5,082,717 (Yaguchi et al.). See also Japanese Patent
Application Laid-Open No. 187708/1987.
[0025] Syndiotactic polystyrene (sPS) has two significant
similarities to polycarbonate as a material of construction for a
microprismatic retroreflective film or sheet. sPS has a high
isotropic index of refraction, n, of 1.585. This is nearly
identical to the isotropic index of refraction of polycarbonate,
which is 1.586. Further, sPS can be processed into films and sheets
having very high transparencies. A significant difference, however,
is that sPS is a semicrystalline polymer. This means that sPS can
be processed in such a manner that some crystalline structures, or
crystallites, are formed. Further, it is known that the
crystallites of sPS can be organized into larger structures called
spherulites, as is the case for other well-known semicrystalline
polymers such as polyethylene, polypropylene (PP), and polyethylene
terephthalate (PET).
[0026] While it is frequently stated that semicrystalline polymers
tend to be opaque, translucent, or hazy, there are many exceptions
to this generalization. Examples of transparent semicrystalline
polymers include, but are not limited to, poly-4-methyl-1-pentene,
nucleated or biaxially oriented PP, biaxially oriented PET,
biaxially oriented polyethylene naphthalate (PEN), biaxially
oriented polyamide 6, oriented films of polyethylene, and certain
constructions (especially films) of polyvinylidene chloride,
polyvinylidene fluoride, polyvinyl fluoride,
polychlorotrifluoroethylene, poly
ethylene-alt-chlorotrifuoroethylene, polyamides (such as nylon 6,
nylon 11, nylon 12, nylon 4/6, nylon 6/6, nylon 6/9, nylon 6/10,
nylon 6/12, and nylon 6/T), polyaryl ethers (such as polyphenylene
ether and the ring-substituted polyphenylene oxides),
polyetheretherketone (PEEK), and thermoplastic polyesters (such as
PET, PEN, polybutylene terephthalate, polybutylene naphthalate, and
poly-1,4-cyclohexanedimethylene terephthalate).
[0027] There can be several reasons for a polymer, though
semicrystalline, to retain transparency. Essentially, for
crystallinity to result in opacity, light must be either reflected,
refracted, or absorbed by the polymeric material. For pure
polymers, lacking additives, most commonly it is the mechanism of
refraction which is responsible for opacity in semicrystalline
polymers. For refraction to take place at the interface between a
polymer crystallite and the polymer's amorphous phase, the
refractive indices of the crystallite and the amorphous phase must
differ. Further, the size of the crystalline entity must be at
least of the same order of magnitude as the wavelength of the
incident light. Furthermore, for refraction to result in opacity or
easily-observed deviations from transparency, such as translucency
or haziness, the amount of the incident light refracted must be
significant. The fraction of light refracted will depend on the
amount of crystallinity present, as well as on the difference in
refractive indices and the size of crystalline entities.
[0028] Thus, a semicrystalline polymer can remain transparent
because of any one of at least three factors: (1) its crystalline
refractive index very closely matches its amorphous refractive
index; (2) the size of its crystalline entities is smaller than the
wavelengths of visible light; or (3) the total amount of
crystallinity present is too small to result in a substantial
amount of refraction. One knowledgeable of these factors will
readily understand that interactions among them make it impossible
to specifically quantify the level required of any one of them for
"transparency".
[0029] However, for several of the individual cases mentioned
above, one of these factors predominates in the preservation of
transparency for that specific semicrystalline polymer composition.
Thus, poly-4-methyl- 1 -pentene remains transparent largely because
its crystalline refractive index is a nearly perfect match for the
refractive index of its amorphous phase. Nucleated polypropylene
derives its transparency from the fact that nucleating agents
simultaneously create so many centers for crystalline growth upon
cooling that no individual crystalline entity can achieve a size on
the order of the wavelengths of visible light. Biaxially oriented
polypropylene film is manufactured by stretching a precursor film
which is considerably less transparent than the finished film.
Widely-accepted theory holds that crystalline entities in the
precursor film are broken down and re-arranged during the
stretching step(s) so that they are no longer large enough to
refract visible light. Biaxially oriented polyethylene
terephthalate film, on the other hand, is typically manufactured by
stretching a transparent and nearly completely non-crystalline
precursor film at conditions which, while resulting in a large
amount of crystallinity, do not allow the crystalline entities to
grow to a size large enough to refract visible light. Many of the
other polymer films mentioned above owe transparency to the
conditions at which the molten polymer from which they are made is
"quenched", or rapidly cooled, such that both the amount and the
size of crystalline entities is kept to a minimum Techniques for
rapid quenching of film following extrusion casting are well known
in the art, and include casting onto a chilled roll, casting into a
water bath, air impingement, and other techniques.
[0030] It is significant to note, however, that any transparent
semicrystalline polymer retains some finite propensity to refract
light, unless all of its crystalline entities are significantly
smaller than optical wavelengths, or the refractive index of its
crystalline phase is a precise match to its amorphous refractive
index. Thus, almost all transparent semicrystalline polymer
constructions differ from purely amorphous polymer constructions in
their ability to refract, or "scatter", small amounts of the light
incident upon them.
[0031] The use of sPS as a material of construction for
microprismatic retroreflective film or sheeting has been found to
result in novel cube-corner articles that combine the advantages of
a high index of refraction polymer with that of a slightly diffuse
reflector, since the crystalline entities scatter some of the light
impinging the cube-corner elements. Depending on the conditions of
fabrication and the composition of the polymer resin, sPS
microprismatics can be made with a range of crystalline character,
resulting in a wide range of optical properties. In one extreme
case, sPS microprismatics can be made to have large crystalline
entities and be highly crystalline, thus having an opaque
appearance. However, sPS microprismatics having low levels of
crystallinity are very clear.
[0032] The preferred sPS retroreflective articles of the present
invention are generally characterized as having a high degree of
transparency, normally required for retroreflection, and are
further believed to have a distribution (in terms of both size and
density) of crystalline structures. The resulting combination of
good transparency, high index of refraction, and a small but
non-zero amount of light scattering makes sPS an ideal material for
increasing retroreflective brightness at wider observation angles.
Surprisingly, it has also been observed that sPS, in spite of
having an index of refraction not quite equal to that of
polycarbonate, provides superior performance at high entrance
angles. These phenomena are illustrated in FIGS. 2, 3, and 4.
[0033] In FIGS. 2 and 3, the brightness of sPS microprismatics far
exceeds that obtained for PC at observation angles of 0.33.degree.
and 0.5.degree.and this superiority is maintained over the entire
accessible range of entrance angles. Data (not shown) obtained at
many Observation angles indicates that this situation persists at
all observation angles larger than 0.33.degree.. FIG. 4 shows that,
for the smallest experimentally accessible observation angle of
0.2.degree., the sPS material underperforms PC for entrance angles
below about 30.degree., but for larger entrance angles, the
brightness of the sPS microprismatic meets or exceeds that of the
PC Clearly, the implication of these three FIGS. is that, for low
entrance angles, retroreflected light is being dispersed into a
wider "cone of light" by sPS than by PC, with brightness being
higher at most observation angles, but somewhat lower at the very
smallest observation angles; while for large entrance angles, the
sPS surprisingly widens the retroreflected cone of light without
detracting from performance at the smallest observation angles.
This implies that for the larger entrance angles, the sPS may be
providing an absolute increase in total retroreflected intensity
(integrated over all observation angles). This is a very unexpected
result for a polymer lacking any advantage in refractive index. At
minimum, the data must be interpreted as indicating an increase in
retroreflected intensity for all observers other than those quite
precisely aligned (closer than 0.2.degree.) with the light source,
which is almost equally advantageous. It is believed that process
optimization may further improve performance of sPS relative to
PC.
[0034] Without wishing to be bound by any particular theory, it
seems plausible to presume that light entering an sPS cube-corner
element is scattered slightly by the crystalline microstructure,
causing an increase in the divergence of the retroreflected light,
as shown schematically in FIG. 5. That is, it is believed that
light is dispersed toward larger observation angles by properties
of the material of construction itself, in contrast to the systems
disclosed in U.S. Pat. No. 3,712,716 (Stamm), U.S. Pat. No.
3,817,596 (Tanaka), U.S. Pat. No. 4,775,219 (Appledorn, et al.),
and PCT Appl. No. 96/30786 (Nilsen), which rely on changes in
optical construction (geometry) to spread the cone of light to
larger observation angles. This effect renders an sPS cube-corner
retroreflective construction brighter at larger observation angles
than is the case for PC, for any given cube-corner geometry.
[0035] Assuming the mechanism proposed above for spreading light is
essentially correct, any of the aforementioned semi-crystalline
polymers which can be formed into transparent articles could be
made to provide optical advantages similar to those detailed in
this invention. One skilled in the art of polymer processing will
appreciate that, through the use of nucleating agents,
stretch-orientation, melt-quenching, or other techniques for
manipulating the size and amount of crystalline entities,
additional semicrystalline polymers not listed above may also be
made to provide similar benefits. However, its combination of high
index of refraction, good processing characteristics, thermal
stability, hydrophobicity, and dimensional stability with respect
to both temperature and humidity, makes sPS a preferred cube-corner
retroreflective material.
[0036] sPS is a member of the broader class of syndiotactic vinyl
aromatic polymers, and such resins useful in the instant invention
can have a wide range of compositional characteristics, such as
molecular weight and its distribution, monomer and comonomer
identity, comonomer content, level of syndiotacticity, grafting or
long chain branching, and the like.
[0037] Syndiotactic vinyl aromatic polymers useful in the current
invention include, but are not limited to, the syndiotactic
varieties of poly(styrene), poly(alkyl styrene)s, poly (aryl
styrene)s, poly(styrene halide)s, poly(alkoxy styrene)s, poly(vinyl
ester benzoate), poly(vinyl naphthalene), poly(vinylstyrene), and
poly(acenaphthalene), as well as the hydrogenated polymers and
mixtures or copolymers containing these structural units. Examples
of poly(alkyl styrene)s include the isomers of the following:
poly(metlhylstyrene), poly(ethylstyrene), poly(propylstyrene), and
poly(butylstyrene). Examples of poly(aryl styrene)s include the
isomers of poly(phenylstyrene). As for the poly(styrene halide)s,
examples include the isomers of the following: poly(chlorostyrene),
poly(bromostyrene), and poly(fluorostyrene). Examples of
poly(alkoxy styrene)s include the isomers of the following:
poly(methoxystyrene) and poly(ethoxystyrene). Among these examples,
preferable styrene group polymers are: polystyrene, poly(p-methyl
styrene), poly(m-methyl styrene), poly(p-tertiary butyl styrene),
poly(p-chlorostyrene), poly(m-chlorostyrene),
poly(p-fluorostyrene), and copolymers of styrene and
p-methylstyrene. Of these polymers, polystyrene,
poly(p-fluorostyrene), poly(p-methylstyrene) and copolymers of
styrene and p-methylstyrene are most preferred.
[0038] Syndiotacticity can be qualitatively and quantitatively
determined by NMR analysis using the carbon isotope method
(.sup.13C-NMR) The tacticity as determined by the .sup.13C-NMR
method can be indicated in terms of either the weight percent of a
polymer which ha, a syndiotactic configuration or in terms of the
proportions of structural units (diads and pentads) continuously
connected to each other in the syndiotactic configuration. In terms
of the first approach, preferred syndiotactic polymers of the
invention include about 20 to 100, preferably 30 to 98, more
preferably 85 to 95, percent by weight of syndiotactic chain
segments. In terms of the second approach, preferred syndiotactic
polymers have a syndiotacticity such that the proportion of the
racemic diad is at least 75%, preferably at least 85%; and the
proportion of racemic pentad is at least 30%, preferably at least
50%.
[0039] In some cases, the syndiotactic vinyl aromatic polymer may
be grafted, copolymerized, or blended with various other monomeric
or polymeric species to impart desired properties to the
microprismatic retroreflective film or sheeting. For example, the
microprismatic retroreflective film or sheeting may comprise a
polymer blend of a syndiotactic vinyl aromatic polymer and
optionally, other kinds of syndiotactic and/or nonsyndiotactic
polymers. Care must be taken in formulating such blends that the
film or sheeting not be opacified via phase separation of the
blend. Within this restriction, other kinds of polymers may be
selected from among polyolefins such as polyethylene,
polypropylene, polybutene, or polypentene, polyesters such as
polyethylene terephthalate, polybutylene terephthalate, or
polyethylene naphthalate; polyamides, polythioethers, polysulfones,
polyurethanes, polyethersulfones, polyimides, halogenated vinyl
polymers such as those sold under the tradename TEFLON.TM.,
combinations of these, and the like. For polymer blends, preferably
0.01 to 50 parts by weight of other kinds of polymer(s) may be used
per 100 parts by weight of syndiotactic vinyl aromatic polymer(s).
In some embodiments, a syndiotactic polystyrene may be blended with
varying amounts of isotactic or atactic polystyrene.
[0040] While one preferred syndiotactic polystyrene polymer used in
the present invention may be derived substantially entirely from
unsubstituted styrene monomer, varying amounts of other
copolynmerizable monomers, some of which may contain alkyl, aryl,
and other substituents, are more preferably incorporated into the
polymer. Incorporation of a comonomer into a polymer serves both to
slow the rate of crystallization from the melt, and limit the size
of the crystallites. Thus, less aggressive quenching conditions are
required for processing the copolymers into transparent films or
sheeting than is the case for homopolymer For example, a preferred
syndiotactic polystyrenic copolymer may be derived from monomers
comprising about 100 parts by weight of styrene monomer and up to
about 20 parts by weight of one or more other copolymerizable
monomers, which may or may not possess pseudoasymmetry.
Representative examples of such other monomers, in addition to the
monomers for the homopolymers listed above in defining the
syndiotactic vinyl aromatic polymers group, include olefin
monomers, such as ethylene, propylene, butenes, pentenes, hexenes
octenes and decenes; diene monomers such as butadiene and isoprene;
cyclic olefin monomers; cyclic diene monomers; or polar vinyl
monomers, such as methyl methacrylate, maleic anhydride, and
acrylonitrile.
[0041] A particularly preferred syndiotactic polystyrenic copolymer
is derived from 100 parts by weight styrene and 1 to 20, preferably
5 to 15, parts by weight paramethylstyrene. Incorporating such
amounts of paramethylstyrene monomer into the polystyrene copolymer
has been found to improve the transparency of the resulting
microprismatic retroreflective film or sheeting. One example of a
particularly preferred vinyl aromatic, syndiotactic polystyrenic
polymer derived from 100 parts by weight styrene and 7 parts by
weight of paramethylstyrene, is commercially available under the
trade designation QUESTRA.TM. 406 from Dow Chemical Company.
[0042] The molecular weight of the vinyl aromatic, syndiotactic
polymer utilized in the films and sheeting of the present invention
is not critical in many applications. Polymers having molecular
weights within a wide range may be used with beneficial results.
Generally, the weight average molecular weight (M.sub.w) may be at
least 10,000, preferably 50,000 to 3,000,000, and more preferably
50,000 to about 400,000. Likewise, the molecular weight
distribution is not critical in many applications, and may be
narrow or broad. For example, the ratio of M.sub.w:M.sub.n may be
1.0 to 10, wherein M, is the number average molecular weight
[0043] The microprismatic retrorefilective film or sheeting of the
present invention may optionally comprise one or more additives to
enhance the physical properties of the film or sheeting. For
example, the film or sheeting may comprise colorants, inorganic
fillers, ultraviolet ("UV") absorbers, light stabilizers, free
radical scavengers, antioxidants, anti-static agents, processing
aids such as antiblocking agents, lubricants, cross-linking agents,
other additives and combinations thereof. Colorants typically are
added at about 0.01 to 0 5 weight percent, based upon 100 parts by
weight of the syndiotactic polymer.
[0044] Most polymeric films which are to be used in signing and
other outdoor applications are stablized against UV degradation by
compounding the base resin with UV absorbing (UVA) additives and/or
other compounds that act as excited state quenchers, hydroperoxide
decomposers, or free radical scavengers. Hidered-amine light
stabilizers (HALS) have been found to be particularly good radical
scavengers. UVA additives act by absorbing radiation in the UV
region of the spectrum. HALS, on the other hand, behave by
quenching radicals generated within the polymer matrix during
exposure to UV radiation. A review of the types of materials used
to improve UV stability may be found in R. Gachter, H. Muller, and
P. Klemchuk (Editors), "Plastics Additives Handbook", pp. 194-95
(.sub.3rd Ed., published by Hanser Publishers, New York).
[0045] UV absorbers typically are added at about 0.5 to 2.0 weight
percent based upon 100 parts by weight of the syndiotactic polymer.
Illustrative examples of suitable UV absorbers include derivatives
of benzotriazole such as TFNUVIN.TM. 327, 328, 900, and 1130, and
TINUVIN-P.TM.. available from Ciba-Geigy Corporation, Ardsley,
N.Y.; chemical derivatives of benzophenone such as UVINUL.TM. M40,
408, and D-50, available from BASF Corporation, Clifton, N.J.;
SYNTASE.TM. 230, 800, and 1200 available from Neville-Synthese
Organics, Inc., Pittsburgh, Pa.; chemical derivatives of
diphenylacrylate such as UVINUL.TM. N35, and 539, also available
from BASF Corporation of Clifton, N.J.; oxanilides such as Sanduvor
VSU, available from Sandoz Corp.; triazines such as Cyasorb UV
1164, available from Cytac Industries; and salicylate
derivatives.
[0046] Light stabilizers that may be used include hindered amines,
which are typically used at about 0.5 to 2.0 weight percent, based
upon 100 parts by weight of the syndiotactic polymer. Examples of
hindered amine light stabilizers include TINUVIN.TM. 144, 292, 622,
and 770, and CHIMASSORB.TM. 944 all available from the Ciba-Geigy
Corp., Ardsley, N.Y., and 2,2,6,6-tetraalkyl piperidine compounds.
Free radical scavengers may also be used, typically, at about 0.01
to 0.5 weight percent, based upon 100 parts by weight of the
syndiotactic polymer.
[0047] Suitable antioxidants include phosphorous antioxidants,
including monophosphites and diphosphites, and phenolic
antioxidants. Suitable monophosphites for use in the microprismatic
retroreflective film or sheeting of the present invention include,
but are not limited to, tris(2,4-tert-butyl-phenyl)phosphite) and
tris(mono- or di-nonylphenyl)phosphite Diphosphite antioxidants
suitable for use in the present invention, include, but are not
limited to, distearylpentaerythritol diphosphite, and
dioctylpentaerythritol diphosphite. Representative examples of
phenolic antioxidants include 2,6-ditertbutyl-4-methylphenol,
2,6-diphenyl-4-methoxyphenol and
2,2'-methylenbis(6-tertbutyl-4-methylphenol). Also suitable for use
as antioxidants in the present invention are hindered phenolic
resins such as IRGANOX.TM. 1010, 1076, 1035, 1425, or MD-1024, or
IRGAFOS.TM. 168, commercially available from the Ciba-Geigy Corp.,
Ardsley, N.Y.
[0048] In a preferred embodiment, the microprismatic
retroreflective film or sheeting contains an amount of the
IRGANOX.TM. 1425 antioxidant effective to enhance the transparency
of the film or sheeting. This antioxidant has a melting point of
about 260.degree. C., which is about the same as the melting point
of a syndiotactic polystyrene polymer. This material is believed to
enhance transparency by reducing the rate of crystallinity of the
syndiotactic polystyrene as the polymer solidifies from a molten
state. Specifically, it is preferred that this antioxidant be
present in an amount of from about 0.0001 to 2 parts by weight,
more preferably, from about 0.001 to 1 parts by weight, and most
preferably, from about 0.01 to 0 5 parts by weight per 100 parts by
weight of the syndiotactic vinyl aromatic polymer.
[0049] Small amounts of other processing aids, typically no more
than one part by weight per 100 parts by weight of the syndiotactic
vinyl aromatic polymer, may be added to improve the polymer's
processability. Useful processing aids include fatty acid esters,
or fatty acid amides available from Glyco Inc., Norwalk, Conn.,
metallic stearates available from Henkel Corp., Hoboken, N.J., or
WAX E.TM. available from Hoechst Celanese Corporation, Somerville,
N.J.
[0050] If desired, the syndiotactic vinyl aromatic polymer may also
contain substances such as flame retardants that optimize the
overall properties of the resultant film or sheeting.
[0051] Inorganic fillers suitable for use in the microprismatic
retroreflective films or sheeting of the present invention include,
for example, oxides, hydroxides, sulfides, nitrides, halides,
carbonates, acetates, phosphates, phosphites, organic carboxylates,
silicates, titanates or borates of the group IA, IIA, IVA, VIA,
VIlA, VIII, IB, IIB, IIIB or IVB elements, as well as hydrated
compounds thereof For example, suitable inorganic fillers
comprising a group IA element include lithium fluoride and borax
(the hydrate salt of sodium borate) Suitable inorganic fillers
comprising a group IIA element include magnesium carbonate,
magnesium phosphate, magnesium oxide and magnesium chloride. Other
suitable inorganic fillers comprising the aforementioned group
elements are disclosed in U.S. Pat. No. 5,188,930 (Funaki et al.),
incorporated herein by reference.
[0052] The use of such inorganic fillers, however, will be governed
by the effects they may have on the optical performance of the
microprismatic retroreflective film or sheeting. It will be
apparent to one skilled in the art that refractive index, particle
size, and loading level all have a potential impact on optical
performance of the present invention, which will serve to limit the
use of inorganic fillers.
[0053] However, such particulate inorganic fillers could be added
to high index of refraction amorphous polymers, such as
polycarbonate, polymethyl methacrylate, or atactic polystyrene, to
yield a similar refractive effect to that described above for
unfilled semicrystalline sPS. Thus, particulate inorganic fillers,
of appropriate particle sizes and at appropriate loading levels,
could be used as optically refracting elements in a manner similar
to that of the crystalline entities in sPS Further, such
particulate inorganic fillers could be used in a sernicrystalline
polymer such as sPS to augment or optimize the refractive effect
described herein. Additionally, such an effect may also be achieved
by combining sPS, or other transparent semicrystalline polymer,
with an appropriate amount of an appropriately dispersed
incompatible polymer, in the form of a polymer blend.
[0054] Neat (unfilled and unblended) sPS, or copolymers thereof,
however, provides several other advantageous properties that
further differentiates its utility (both optically and in ease of
processing) in the retroreflective articles detailed herein and
these are discussed below.
[0055] sPS has an inherently low surface energy (29.4 dynes/cm),
which allows for its removal, with a minimum of effort, from the
tooling used for microreplication. This property can be further
improved by the use of lubricants and additives well known in the
art for the purpose of aiding mold release. Failure of polymers to
release cleanly and easily from tooling or molds typically employed
in commercial microreplication processing has severely limited the
utility of many other amorphous and semi-crystalline polymers.
[0056] sPS has a low coefficient of hygroscopic expansion (CUE) and
good thermal stability, which render an sPS microprismatic film
dimensionally stable when exposed to extremes of environmental
temperature and humidity.
[0057] Furthermore, coating the sPS microprismatic film with a UV
blocking coating, as disclosed in U.S. patent application
08/761,912 (Ojeda) (which is incorporated herein by reference), or
laminating a transparent UV absorber loaded overlay film (see, U.S.
Pat. 4,895,428 (Nelson et al.)) to the microprismatic film or
sheeting, further enhances its utility by protecting it from the
environment and UV degradation. The former is more preferred in
commercial practice as it simplifies the manufacturing process and
reduces the cost of the final product.
[0058] Microprismatic retroreflective film and sheeting of the
present invention may be fabricated using any one of a number of
processing techniques (molding, embossing, casting, etc.), and any
of the various forms of microprismatic optical tooling disclosed in
the art may be utilized
[0059] The thickness of the microprismatic retroreflective film or
sheeting is not limited, but must be accommodated by the process
conditions. That is, sheets of different thicknesses will require
differing quenching conditions to achieve the same levels of
crystalline entity size and degree of crystallinity In some
applications, it will be desirable for the retroreflective film or
sheeting to be flexible, and in other cases, rigidity will be
required by the intended application These and processing equipment
capabilities will determine optimum thickness.
[0060] As disclosed in U. S. Pat. 4,025,159 (McGrath), a backing or
sealing film may be applied to the microprismatic sheeting by heat
sealing at discrete locations in a grid-like pattern to prevent the
entry of foreign substances (moisture, air-borne pollutants, dirt,
etc.) that can reduce the efficacy of the cube-corner film. This
film also serves to preserve the air layer adjacent to the
cube-corners, thus permitting TIR without the need for metallizing
the cube-corner elements.
[0061] The following examples, while not intended to be limiting,
illustrate various features of the present invention.
COMPARATIVE EXAMPLE C1
[0062] Polycarbonate (PC) retroreflective cube-corner film was
compression molded using microstructured nickel tooling. The
microstructured nickel tooling utilized contains microcube prism
recesses of approximately 88 micrometers (0.0035 inch). The
microcube recesses were formed as matched pairs of cube corner
elements with the optical axis canted or tilted 8.15 degrees away
from the primary groove, as generally illustrated in U.S. Pat. No.
4,588,258 (Hoopman). The nickel tooling thickness was approximately
508 micrometers (0.020 inch).
[0063] An isotropic, 500 lm thick PC (Bayer 2407) melt-cast
specimen was placed in the molding device between the
microstructured nickel tooling and a polished steel plate, to
provide an embossed (patterned) surface and an opposing smooth
surface.
[0064] The specimen was prepared using platen temperatures of 400
.degree. F. (204 .degree. C.), and a pressure of 25 kpsi for 2
minutes. The pressed specimen was then immediately quenched by
rapid immersion into ice water.
[0065] Angular brightness measurements were obtained on unsealed
specimens using the method described in U S. Pat. No 5,138,488
(Szczech), according to U.S. Federal Test Method Standard 370,
using an orientation angle of 0.degree.. The angular brightness
data is presented in Table 1, and plotted as the curves labelled
"PC" in FIGS. 2-4.
1TABLE 1 PC Brightness (cp) as a Function of Entrance Angle and
Observation Angle Observation Angle (.degree.) Entrance Angle
(.degree.) 0.2 0.33 0.5 -4 1513 483 295 15 1273 459 298 30 778 386
261 45 170 118 70 60 1.5 1.5 1
EXAMPLE 1
[0066] Syndiotactic polystyrene (sPS) cube-corner film was prepared
by compression molding a 250 .mu.m thick melt-cast specimen by the
procedures described in Comparative Example C1, with the exception
that the platen temperatures were 500 .degree. F. (260 .degree.
C.). The sPS resin used reportedly had a weight-averaged molecular
weight of 275,000, and contained 14% para-methylstyrene (pMS) as a
comonomer. The resin was obtained from the Dow Chemical Company
(Midland, Mich.). The melt-cast specimen exhibited an onset
temperature for crystallization, T.sub.c(onset), of 198.degree. C.,
a peak temperature for crystallization, T.sub.c(peak), of
193.degree. C., and a melting point peak temperature, T.sub.m, of
243.degree. C., as measured by differential scanning calorimetry
(DSC).
[0067] Angular brightness measurements were performed as in
Comparative Example C1. The data is presented in Table 2.
2TABLE 2 sPS Brightness (cp) as a Function of Entrance Angle and
Observation Angle Observation Angle (.degree.) Entrance Angle
(.degree.) 0.2 0.33 0.5 -4 1015 1124 491 15 841 887 586 30 692 630
334 45 202 160 83 60 3 2 1.3
EXAMPLE 2
[0068] sPS cube-corner film was prepared in a manner similar to
Example 1. In this example, the sPS resin used (Questra 406 - Dow
Chemical Company - Midland, Mich.) reportedly had a weight-averaged
molecular weight of 325,000, and contained 7% pMS as a comonomer.
DSC measurements on the melt-cast specimens indicated a
T.sub.c(onset) of 197.degree. C., T.sub.c(peak) of 185.degree. C.,
and a T.sub.m of 247.degree. C.
[0069] Angular brightness measurements were performed as in
Comparative Example C1. The data is presented in Table 3, and
plotted as the curves labelled "sPS" in FIGS. 2-4.
3TABLE 3 sPS Brightness (cp) as a Function of Entrance Angle and
Observation Angle Observation Angle (.degree.) Entrance Angle
(.degree.) 0.2 0.33 0.5 -4 1069 1200 586 15 945 945 620 30 819 820
413 45 232 232 184 60 3.3 2.5 2
COMPARATIVE EXAMPLES C2-C3
[0070] sPS cube-corner films were prepared in a manner similar to
Example 1. In these examples, the sPS resins used were sPS
homopolymer (Example C2) and an sPS copolymer containing 4% pMS
(Example C3). The resins were obtained from the Dow Chemical
Company (Midland, Mich.), and each reportedly had a weight-averaged
molecular weight of 275,000 DSC measurements on the melt-cast
specimens of Example C2 indicated a T.sub.c(onset) of 230.degree.
C., T.sub.c(peak) of 226.degree. C., and a T.sub.m of 263.degree.
C. DSC measurements on the melt-cast specimens of Example C3
indicated a T.sub.c(onset) of 217.degree. C., T.sub.c(peak) of
212.degree. C., and a T.sub.m of 254.degree. C.
[0071] Both films lacked sufficent transparency to be useful in
retroreflective signing materials, so angular brightness
measurements were not performed.
[0072] These two comparative examples serve to demonstrate that
quench conditions must be adapted to the polymer resin to be used
As a homopolymer and copolymer of low comonomer content,
respectively, the resins of Comp. EXAMPLES C2 and C3 are faster
crystallizers than those of EXAMPLES I and 2, and thus, require
more aggressive quenching conditions to be formed into transparent
films of the present invention.
[0073] The present invention should not be considered limited to
the particular examples described above, but rather should be
understood to cover all aspects of the invention as fairly set out
in the attached claims. Other embodiments of this invention will be
apparent to those skilled in the art, without departing from the
true scope and spirit of the invention, upon consideration of this
specification or from practices of the invention disclosed herein.
Various modifications, omissions, equivalent processes, as well as
numerous structures to which the present invention may be
applicable will be readily apparent to those of skill in the art to
which the present invention is directed upon review of the present
specification. The appended claims are intended to cover such
modifications and devices.
* * * * *