U.S. patent application number 11/143221 was filed with the patent office on 2005-10-20 for anti-reflective thin film constructions.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Ouderkirk, Andrew J., Weber, Michael F..
Application Number | 20050233123 11/143221 |
Document ID | / |
Family ID | 21720595 |
Filed Date | 2005-10-20 |
United States Patent
Application |
20050233123 |
Kind Code |
A1 |
Weber, Michael F. ; et
al. |
October 20, 2005 |
Anti-reflective thin film constructions
Abstract
A broadband anti-reflective thin film article is disclosed. The
articles includes a base substrate having a substrate surface and a
thin film polymeric multi-layer anti-reflectivity stack having a
plurality of polymer layers. Each polymer layer having a thickness
no greater than 1 micrometer. The substrate surface has a broadband
reflectivity value. The thin film polymeric multi-layer
anti-reflectivity stack is optically coupled to the substrate
surface. The thin film polymeric multi-layer anti-reflectivity
stack reduces the broadband reflectivity of the substrate surface
by a factor of at least 2 at normal angles of incidence.
Inventors: |
Weber, Michael F.;
(Shoreview, MN) ; Ouderkirk, Andrew J.; (Woodbury,
MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
21720595 |
Appl. No.: |
11/143221 |
Filed: |
June 2, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11143221 |
Jun 2, 2005 |
|
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09006379 |
Jan 13, 1998 |
|
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6926952 |
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Current U.S.
Class: |
428/215 ;
428/339; 428/421; 428/447; 428/480 |
Current CPC
Class: |
Y10T 428/2495 20150115;
G02B 1/111 20130101; Y10T 428/24942 20150115; Y10T 428/31797
20150401; Y10T 428/24967 20150115; Y10T 428/31786 20150401; Y10T
428/31663 20150401; Y10T 428/31848 20150401; Y10T 428/3154
20150401; Y10T 428/269 20150115 |
Class at
Publication: |
428/215 ;
428/339; 428/421; 428/447; 428/480 |
International
Class: |
B32B 007/02; B32B
027/36; B32B 027/06 |
Claims
What is claimed is:
1. A broadband anti-reflective article comprising; a base substrate
having a substrate surface, the substrate surface having a
broadband reflectivity value; a thin film polymeric multi-layer
anti-reflectivity stack comprising a plurality of polymer layers,
each of the polymer layers having a thickness no greater than about
1 micrometer, the thin film polymeric multi-layer anti-reflectivity
stack is optically coupled to the substrate surface; wherein, the
thin film polymeric multi-layer anti-reflectivity stack reduces the
broadband reflectivity of the substrate surface by a factor of at
least 2 at normal angles of incidence.
2. An article according to claim 1 wherein the thin film polymeric
multi-layer anti-reflectivity stack reduces visible light broadband
spectrum reflectivity of the substrate surface by a factor of at
least 2 at normal angles of incidence.
3. An article according to claim 1 wherein the thin film polymeric
multi-layer anti-reflectivity stack reduces a broadband light
spectrum from 420 nm to 750 nm reflectivity of the substrate
surface by a factor of at least 2 at normal angles of
incidence.
4. An article according to claim 1 wherein the base has an index of
refraction of about 1.5.
5. An article according to claim 1 wherein the plurality of polymer
layers comprises a low index polymer layer selected from the group
consisting of silicone polymers and fluoropolymers, and
combinations thereof.
6. An article according to claim 1 where the plurality of polymer
layers comprises a high index polymer layer selected from the group
consisting of polyethylene naphthalate, polyethylene terephthalate,
and copolymers thereof.
7. An article according to claim 1 wherein the plurality of polymer
layers comprises a low index polymer layer selected from the group
consisting of silicone polymers and fluoropolymers, and
combinations thereof and the plurality of polymer layers comprises
a high index polymer layer selected from the group consisting of
polyethylene naphthalate, polyethylene terephthalate, and
copolymers thereof.
8. An article according to claim 1 wherein each polymer layer has
an optical thickness in a range from {fraction (1/16)} to 1
wavelength.
9. An article according to claim 1 wherein each polymer layer has a
thickness in a range from 10 to 250 nm.
10. A broadband anti-reflective article comprising; a base
substrate having a substrate surface, the substrate surface having
a broadband reflectivity value measured in air of 6% or greater, at
normal angles of incidence; a thin film polymeric multi-layer
anti-reflectivity stack comprising a plurality of polymer layers,
each of said polymer layers having a thickness no greater than
about 1 micrometer, the thin film polymeric multi-layer
anti-reflectivity stack optically coupled to the substrate surface;
wherein, the thin film polymeric multi-layer anti-reflectivity
stack reduces the broadband reflectivity value measured in air to a
value of less than 6%, at normal angles of incidence.
11. An article according to claim 10 wherein the thin film
polymeric multi-layer anti-reflectivity stack reduces visible light
broadband spectrum reflectivity value measured in air to a value of
less than 3%, at normal angles of incidence.
12. An article according to claim 10 wherein the thin film
polymeric multi-layer anti-reflectivity stack reduces a broadband
light spectrum from 420 nm to 750 nm reflectivity value measured in
air to a value of less than 3%, at normal angles of incidence.
13. An article according to claim 10 wherein the thin film
polymeric multi-layer anti-reflectivity stack reduces a broadband
light spectrum from 420 nm to 750 nm reflectivity value measured in
air to a value of less than 1%, at normal angles of incidence.
14. An article according to claim 10 wherein the base has an index
of refraction of about 1.5.
15. An article according to claim 10 wherein the plurality of
polymer layers comprises a low index polymer layer selected from
the group consisting of silicone polymers and fluoropolymers, and
combinations thereof.
16. An article according to claim 10 where the plurality of polymer
layers comprises a high index polymer layer selected from the group
consisting of polyethylene naphthalate, polyethylene terephthalate,
and copolymers thereof.
17. An article according to claim 10 wherein the plurality of
polymer layers comprises a low index polymer layer selected from
the group consisting of silicone polymers and fluoropolymers, and
combinations thereof and the plurality of polymer layers comprises
a high index polymer layer selected from the group consisting of
polyethylene naphthalate, polyethylene terephthalate, and
copolymers thereof.
18. An article according to claim 10 wherein each polymer layer has
an optical thickness in a range from {fraction (1/16)} to 1
wavelength.
19. An article according to claim 10 wherein each polymer layer has
a thickness in a range from 10 to 250 nm.
Description
CROSS REFERENCE
[0001] This application is a continuation of application Ser. No.
09/006,379, filed Jan. 13, 1998, which is incorporated by reference
herein.
BACKGROUND
[0002] This invention relates to providing anti-reflective
properties.
[0003] Anti-reflective layers, e.g., in the form of thin films, are
used to prevent unwanted reflection at surfaces. Such reflection is
particularly undesirable in the case of, e.g., cathode ray tubes,
liquid crystal displays, and windows because it causes glare and
can reduce the brightness and contrast of a displayed image.
[0004] Traditional anti-reflective layers have been made from
inorganic materials, e.g., magnesium fluoride, that are coated onto
an optical element such as a glass lens. Using a quarter-wavelength
thick anti-reflective layer can reduce the reflection considerably
for the selected wavelength because of destructive
interference.
SUMMARY
[0005] In a first aspect, the invention features an article that
includes a plurality of polymer layers, each having a thickness no
greater than about 1 micrometer and collectively forming a
free-standing film. The layers are selected such that when the
article is optically coupled to a surface of a substrate having a
refractive index of about 1.50, the article reduces the
reflectivity of the substrate at the surface over a wavelength
range of interest by a factor of at least about 2 at normal angles
of incidence. The article may be optically coupled to the substrate
surface, e.g., by adhering it to the substrate surface. Materials
are said to be "optically coupled" when the air space between them
is replaced by a material with an index of refraction similar to
that of the two articles, or when the air space is reduced in
thickness to a value much less than a quarter wavelength.
[0006] In a second aspect, the invention features an article that
includes a plurality of polymer layers, each having a thickness no
greater than about 1 micrometer and collectively forming a
free-standing film, in which the polymer layers are selected such
that the reflectivity measured in air over a wavelength range of
interest at normal angles of incidence is no greater than about 6%,
preferably no greater than about 3%, and more preferably no greater
than about 1%.
[0007] In preferred embodiments of the first and second aspects of
the invention, the polymer layers are selected from the group
consisting of polymer compositions that are coextrudable with each
other. Examples of suitable polymers include relatively low index
polymers such as silicone polymers, fluoropolymers (e.g., a
vinylidene fluoridetetrafluoroethylene-- hexafluoropropylene
terpolymer), fluoro-chloropolymers, methacrylate polymers,
polyester copolymers, and combinations thereof, and relatively high
index polymers such as polyesters, polycarbonates, polysulfones,
polyethersulfones, and combinations thereof. Preferably, the
article includes two adjoining polymer layers in which the
refractive indices of the adjoining layers are different from each
other. At least one polymer layer preferably has an index of
refraction less than about 1.55.
[0008] The article may further include an inorganic layer. Examples
of suitable inorganic layers include zirconia, titania, tin oxide,
indium oxide-tin oxide, silver, aluminum, and combinations thereof.
The article may also include a layer for modifying the mechanical,
chemical, or electrical properties, or combination thereof, of the
film.
[0009] Each of the polymer layers is preferably oriented in
substantially the same direction and to substantially the same
degree as the other layers. The article preferably is provided in
the form of a flexible film.
[0010] In a third aspect, the invention features an article in the
form of a free-standing film that includes: (a) a base that
includes a polymer layer having a major surface; and (b) an
anti-reflective stack optically coupled to the major surface of the
base that reduces the reflectivity of the base at the major surface
of the base over a first wavelength range of interest. The stack
includes alternating layers of (i) high index polymers having an
index of refraction greater than about 1.55 and (ii) low index
polymers having an index of refraction less than about 1.55. The
article may be used to modify the optical properties of a substrate
by optically coupling the article to a surface of the substrate,
e.g., by adhering the article to the surface.
[0011] In preferred embodiments of the third aspect of the
invention, the polymer layers of the base and the stack are
selected from the group consisting of polymer compositions that are
co-extrudable with each other. Examples of suitable polymers for
the stack include relatively low index polymers such as silicone
polymers, fluoropolymers (e.g., a vinylidene
fluoride-tetrafluoroethylene-hexafluoropropylene terpolymer),
fluorochloropolymers, methacrylate polymers, polyester copolymers,
and combinations thereof, and relatively high index polymers such
as polyesters, polycarbonates, polysulfones, polyethersulfones, and
combinations thereof. Each of the polymer layers of the base and
stack is preferably oriented in substantially the same direction
and to substantially the same degree as the other layers.
[0012] Each of the layers of the stack preferably has a thickness
no greater than about 1 micrometer. The particular thickness values
for individual layers are chosen using computer modeling and
generally fall within the range of about {fraction (1/16)}
wavelength to about 1 wavelength.
[0013] The stack may include a polymer layer having a refractive
index that is greater than or equal to the highest refractive index
of the base. A particularly preferred layer for the stack is a
birefringent polymer layer having two orthogonal optic axes
parallel to the plane of the film.
[0014] The stack may further include an inorganic layer. Examples
of suitable inorganic layers include zirconia, titania, tin oxide,
indium oxide-tin oxide, silver, aluminum, and combinations thereof.
The stack may also include a layer for modifying the mechanical,
chemical, or electrical properties, or combination thereof, of the
film.
[0015] The base may include a pair of opposed major surfaces, each
of which is optically coupled to an anti-reflective stack. The base
may include a plurality of alternating layers of a first polymer
and a second polymer in which the first polymer has a higher index
of refraction associated with at least one in-plane axis than
adjoining layers of the second polymer. Examples of suitable
materials for the first and second polymer include polyethylene
naphthalate and a polyethylene naphthalate copolymer,
respectively.
[0016] In one preferred embodiment, the base includes a multilayer
reflective polarizer which selectively reflects light of one
polarization and transmits light of a second polarization at normal
angles of incidence over a second wavelength range of interest. The
polarizer may feature alternating layers of polyethylene
naphthalate and a polyethylene naphthalate copolymer.
[0017] In another preferred embodiment, the base includes a
multilayer mirror that reflects light of two orthogonal
polarizations at normal angles of incidence over a second
wavelength range of interest. The mirror may feature alternating
layers of polyethylene naphthalate and (a) a polyethylene
naphthalate copolymer, (b) polymethyl methacrylate, or (c) a
terephthalic acid copolymer (e.g., poly(ethylene
glycol-co-cyclohexane-1,4-dimethanol terephthalate)).
[0018] In a fourth aspect, the invention features a process for
preparing an article that includes co-extruding a plurality of
polymer compositions with each other to form a plurality of polymer
layers in the form of a free-standing film, the polymer layers
being selected such that when the article is optically coupled to a
substrate, the article reduces the reflectivity of the substrate
over a wavelength range of interest.
[0019] The polymer layers are preferably selected such that when
the article is optically coupled to a substrate having a refractive
index of about 1.50, the article reduces the reflectivity of the
substrate over a wavelength range of interest by a factor of at
least about 2. Preferably, the polymer compositions are co-extruded
such that each of the resulting polymer layers has a thickness no
greater than about 1 micrometer. The polymer compositions are
preferably extruded with one or more additional polymer
compositions to form one or more removable skin layers on a surface
of the article to protect the article.
[0020] In a fifth aspect, the invention features a process for
preparing an article that includes co-extruding a plurality of
polymer compositions with each other to form a plurality of polymer
layers in the form of a free-standing film, the polymer layers
being selected such that the reflectivity measured in air over a
wavelength range of interest at normal angles of incidence is no
greater than about 6%, preferably no greater than about 3%, and
more preferably no greater than about 1%.
[0021] Preferably, the polymer compositions are co-extruded such
that each of the resulting polymer layers has a thickness no
greater than about 1 micrometer. The polymer compositions are
preferably extruded with one or more additional polymer
compositions to form one or more removable skin layers on a surface
of the article to protect the article.
[0022] In a sixth aspect, the invention features a process for
preparing an article that includes (a) co-extruding a plurality of
polymer compositions with each other to form a free-standing film,
where the film includes (i) a base that includes a polymer layer
having a major surface, (ii) a precursor anti-reflective
construction that includes a polymer layer, and (iii) at least one
removable polymer layer; and (b) stretching the film to convert the
precursor anti-reflective construction to an anti-reflective
construction optically coupled to the major surface of the base and
selected to reduce the reflectivity of the base at the major
surface over a wavelength range of interest. In some preferred
embodiments, the base, the stack, or both, includes a plurality of
polymer layers. The process preferably includes the step of
stripping the removable polymer layer from the film prior to
stretching the film; the removable polymer layer may also be
removed subsequent to stretching the film.
[0023] Examples of preferred materials for the removable polymer
layer include polyethylene, polypropylene, atactic polystyrene, and
combinations thereof.
[0024] The invention provides lightweight, relatively inexpensive,
polymeric antireflective constructions that can be used alone or in
combination with a number of materials, including multilayer
polymeric polarizers and mirrors. The structure and properties of
the anti-reflective construction can be tailored to render it
effective over a selected portion of the electromagnetic spectrum.
In addition, unlike conventional vacuum-deposited anti-reflective
coatings, the polymeric anti-reflective constructions according to
the invention can readily be applied to non-planar (e.g., curved)
surfaces such as cathode ray tubes.
[0025] Other features and advantages of the invention will be
apparent from the following description of the preferred
embodiments thereof, and from the claims.
DESCRIPTION OF DRAWINGS
[0026] FIG. 1A is a schematic sectional view of an article
featuring a base optically coupled to a single-layer
anti-reflective ("AR") construction.
[0027] FIG. 1B is a schematic sectional view of an article
featuring a base optically coupled to a single-layer AR
construction, in which a removable skin layer is positioned over
the AR construction.
[0028] FIG. 2A is a schematic sectional view of an article
featuring a base optically coupled to a multilayer AR
construction.
[0029] FIG. 2B is a schematic sectional view of an article
featuring a pair of bases, each optically coupled to a multilayer
AR construction, in which the two structures are separated by an
internal skin layer.
[0030] FIG. 3 is a plot of reflectivity versus wavelength for a PET
base optically coupled to a 4-layer THV/PEN AR construction
calculated at 0.degree. relative to the normal (curve a) and
60.degree. relative to the normal for s polarized light (curve s)
and p polarized light (curve p).
[0031] FIG. 4 is a schematic sectional view of an article featuring
a base provided with an optical coupling agent for attachment to a
substrate, in which the base is further optically coupled to a
multilayer AR construction.
[0032] FIG. 5 is a schematic sectional view of an article featuring
a multilayer base provided with a plurality of anti-reflective
stacks separated by skin layers.
[0033] FIG. 6 is a plot of reflectivity versus wavelength for a PET
base optically coupled to a 44-layer PEN/PMMA AR construction
calculated at 0.degree. relative to the normal.
[0034] FIG. 7 is a plot of transmission versus wavelength for a
glass base optically coupled to an 81-layer THV/PMMA-PVDF/PC
construction, where the PMMA-PVDF acts as a tie layer.
DETAILED DESCRIPTION
[0035] Anti-reflective (AR) constructions are preferably provided
in the form of freestanding films, i.e., films having sufficient
mechanical integrity that they can be readily handled without the
need for additional reinforcing layers. The anti-reflective
properties can be tailored to cover a selected range of
electromagnetic frequencies, including portions of the visible,
infrared (IR), and ultraviolet (UV) regions of the electromagnetic
spectrum.
[0036] The AR constructions may be used alone (such that the film
forms an interface with air) or optically coupled to one or both
major surfaces of a base; in the latter case, the AR construction
de-reflects radiation impinging upon the surface of the base at the
base/AR construction interface. The AR construction may be adhered
to the surface of the base. Preferably, however, it is formed
simultaneously with the base by co-extrusion, as described in more
detail, below. In addition, the base/AR construction article may
itself be optically coupled, e.g., by means of an adhesive to yet
another surface, e.g., a window.
[0037] The AR construction may consist of any number of polymer
layers, typically ranging from one to several tens of layers. The
AR polymer layers may be optically thin, e.g., with a thickness of
between about 0.010 micrometer and about 0.25 micrometer, or
optically thick, e.g., with a thickness of greater than about 0.25
micrometer, or a combination of optically thin and optically thick
layers. The particular thickness values are selected depending upon
the portion of the electromagnetic spectrum over which the film is
designed to operate and, where the AR construction is optically
coupled to a base, the optical characteristics of the base. The AR
polymer layers are preferably made from polymer compositions that
are coextrudable with each other and, where the AR construction is
optically coupled to a base, the materials forming the base.
[0038] An example of a single layer AR construction is
schematically shown in FIG. 1A. As shown in the figure, article 100
features a base 102 and an anti-reflective polymer layer 104. AR
layer 104 (which may be optically thick or thin) preferably is made
from a thermoplastic polymer with a lower index of refraction than
base 102. Specifically, reflection is minimized when AR layer 104
has an index of refraction that is approximately the square root of
a multiple of the index of refraction of the base and the
surrounding medium (e.g. air), and is a quarter wavelength thick.
Preferred polymers for AR layer 104 typically have an index of
refraction less than about 1.45, more preferably less than about
1.38.
[0039] Suitable low index polymers for AR layer 104 include
silicone polymers, methacrylate polymers, fluoropolymers, polyester
copolymers, and fluoro-chloropolymers. Particularly preferred are
fluoropolymers available under the trade designation THV-500 from
Dyneon LLC, St. Paul, Minn., a vinylidene
fluoride-tetrafluoroethylenehexafluoropropylene terpolymer, which
has an index of refraction of 1.36, in the form of quarter
wavelength layers on the base. These polymers can reduce the total
surface reflectivity of a relatively high index of refraction base
by about a factor of 2 irrespective of layer thickness. As a
specific example, biaxially oriented polyethylene terephthalate
(PET) has an index of refraction of 1.66 and reflectivity for
visible light of 6.0% per surface at a normal angle of incidence.
Covering such a biaxially oriented PET base with an optically thick
layer of THV-500 fluoropolymer will reduce the reflectivity of the
combined film to a calculated value of about 3.26% per side.
Reflectivity would be reduced even further if the THV layer were a
quarter wavelength thick.
[0040] To lower the reflectivity even further, it may be preferable
to use a thin film, multilayer AR construction. Such constructions
offer the advantage of improved broad band reduction in
reflectivity relative to single-layer AR constructions while
maintaining acceptable bandwidths. A multilayer AR construction is
shown in FIG. 2A. With reference to FIG. 2A, an article 130
features a base 134 provided on two sides with a multilayer AR
stack 132; it is also possible to provide the AR stack on only one
side of the base. Each layer 136 of the AR stack 132 will generally
be optically thin, although optically thick layers, or a
combination of optically thin and optically thick layers, can be
used as well.
[0041] AR stacks 132 can consist of any number of material layers
136 depending on the optical characteristics of the base and the
desired portion of the electromagnetic spectrum over which AR
stacks 132 are designed to operate. Stacks having two or more
layers can produce lower reflectivity over a wider band than a
single layer, especially if the base has an index of refraction
below about 1.60. With multiple layers in the AR stack, reflections
from multiple interfaces can destructively interfere to reduce the
overall reflectivity.
[0042] One of the materials in a multilayer AR stack preferably has
an index equal to or higher than that of the highest index of
refraction associated with the base. Since a multilayer stack of
only two materials can be designed to function as an equivalent
single layer of almost any index, AR stacks having four or more
layers can be made using only two materials, and have a wider
bandwidth than a three material, three-layer stack. This is useful
in the case of articles prepared by co-extrusion (as described
below) because in the co-extrusion process it is easier to add
extra layers of existing materials than to add a new material.
Suitable materials for the AR layers include thermoplastic polymers
such as, for example, polyethylene terephthalate, polybutylene
terephthalate, polyethylene naphthalate, 2,6-polybutylene
naphthalate, polyamides, polycarbonates, atactic polystyrene,
syndiotactic polystyrene, and polymethyl methacrylate. Copolymers
based upon these materials may also be suitable.
[0043] Layers having different refractive indices may be separated
by "tie layers" having indices intermediate those of surrounding
layers. Such layers are particularly useful for improving the
adhesion between layers in the stack. An example of such a
construction features, in order of decreasing index of refraction,
polycarbonate/polymethyl methacrylate/polyvinylidene fluoride/THV
fluoropolymer, where the polymethyl methacrylate and polyvinylidene
fluoride act as a tie layer to improve the adhesion between the
polycarbonate and the THV fluoropolymer.
[0044] FIG. 7 illustrates calculated transmission values for an 81
layer AR construction utilizing polycarbonate ("PC") and THV
fluoropolymer as the high and low index layers, respectively, on a
glass substrate, with polymethyl methacrylate ("PMMA")
polyvinylidene fluoride ("PVDF") acting as a tie layer. The
refractive index of the tie layer was allowed to float between 1.45
and 1.55, and ended with an optimized index of 1.497.
[0045] The stack was optimized to anti-reflect a glass surface. As
shown in FIG. 7, a significant anti-reflective effect was
obtained.
[0046] Particularly preferred articles include those in which one
or more of the AR stack layers 136 are made of the same material or
materials as the base 134, or as one or more layers of the base.
For example, if a multilayer base film is to be de-reflected over a
portion of the wavelength spectrum, e.g., in the case of a base
functioning as an IR mirror provided with an AR construction
designed to de-reflect visible light, it would be desirable to
design the AR stack from the same materials as the mirror
itself.
[0047] Another example of a useful article is one featuring a
multilayer AR construction combining both polymer layers and layers
of higher index inorganic materials.
[0048] In one embodiment, the inorganic material would have an
index of refraction intermediate that of the base and the organic
polymer. For example, the inorganic material could be a sol
gel-deposited layer of alumina or a zirconia-silica mix on a PEN
base and the organic polymer could be a fluoropolymer such as
THV-500.
[0049] In another embodiment, the inorganic material would have an
index of refraction higher than that of the base. For example, the
inorganic material could be a sol gel-deposited layer of zirconia
or titania on a PEN base and the organic polymer could be a quarter
wavelength thick fluoropolymer such as THV-500.
[0050] In another embodiment, the inorganic material could be
silver, aluminum, or a quarter or half wavelength thick layer of a
transparent conductor such as indium-tin oxide (ITO) having far IR
rejection capabilities, and the polymer layer could be a quarter
wavelength thick fluoropolymer such as THV-500.
[0051] In yet another embodiment, the inorganic material could be
combined with a multilayer polymer construction.
[0052] Materials useful for the base include both organic polymers
and inorganic materials such as ceramics and glasses having
relatively high refractive indices.
[0053] Particularly preferred base materials are single and
multilayer polymer films. Examples of suitable single layer polymer
films include polyethylene terephthalate and polycarbonate films;
such films, in turn, may be uniaxially or biaxially oriented. One
example of a suitable multilayer polymer film is one in which the
thicknesses of the individual polymer layers is no greater than
about 0.5 micrometers, as described in Wheatley et al., U.S. Pat.
No. 5,278,694, the disclosure of which is herein incorporated by
reference.
[0054] A second example of a suitable multilayer polymer film is
described in commonly assigned U.S. Pat. No. 5,882,774, the
disclosure of which is herein incorporated by reference. Very
briefly, that application describes the construction of multilayer
polymer films (mirrors and polarizers) for which the Brewster angle
(the angle at which reflectance goes to zero) is very large or is
nonexistent for the polymer layer interfaces. This allows for the
construction of multilayer mirrors and polarizers whose
reflectivity for p polarized light decreases slowly with angle of
incidence, is independent of angle of incidence, or increases with
angle of incidence away from the normal. As a result, multilayer
films having high reflectivity for both s and p polarized light
over a wide bandwidth, and over a wide range of angles can be
achieved.
[0055] The relationships between the indices of refraction in each
film layer of the base to each other determine the reflectance
behavior of the base at any angle of incidence, from any azimuthal
direction. The principles and design considerations described in
U.S. Pat. No. 5,882,774 can be applied to create multilayer bases
having the desired optical effects for a wide variety of
circumstances and applications. The indices of refraction of the
layers in the multilayer base can be manipulated and tailored to
produce devices having the desired optical properties. Many useful
devices, such as mirrors and polarizers having a wide range of
performance characteristics, can be designed and fabricated using
the principles described therein.
[0056] Particularly preferred combinations of layers in the case of
polarizers include polyethylene naphthalate ("PEN")/coPEN,
polyethylene terephthalate ("PET")/coPEN, PEN/syndiotactic
polystyrene ("SPS"), PET/SPS, PEN/Estar, and PET/Estar, where
"coPEN" refers to a copolymer or blend based upon naphthalene
dicarboxylic acid, and "Estar" is a trade designation for a
copolymer of terephthalic acid, ethylene glycol, and
cyclohexane-1,4-dimethanol that is commercially available from
Eastman Chemical Co., Kingsport, Tenn.
[0057] Particularly preferred combinations of layers in the case of
mirrors include
[0058] PET/Ecdel, PEN/Ecdel, PEN/SPS, PEN/THV, PEN/polymethyl
methacrylate ("PMMA"), PEN/coPET, and PET/SPS, where "coPET" refers
to a copolymer or blend based upon terephthalic acid (as described
above), "Ecdel" is a trade designation for a copolymer of
cyclohexane dicarboxylic acid, ethylene glycol, and
cyclohexane-1,4-dimethanol that is commercially available from
Eastman Chemical Co., Kingsport, Tenn. and "THV" is a fluoropolymer
commercially available from 3M Co., St. Paul, Minn.
[0059] The number of layers in the base is selected to achieve the
desired optical properties using the minimum number of layers for
reasons of film thickness, flexibility, and economy. In the case of
both polarizers and mirrors, the number of layers is preferably
less than about 10,000, more preferably less than about 1,000.
[0060] A further criteria must be considered when designing AR
constructions for non-normal incident angles with birefringent
polymers such as oriented crystalline or partially crystalline
polymers. In these cases, it is necessary to account for the
anisotropy of the index of refraction. In other words, for
non-normal angles, the reflection will depend on the index normal
to the film plane as well the in-plane indices, for both the
substrate and the AR construction. If the two in-plane indices are
different (biaxial birefringence), these differences must be taken
into account for all angles of incidence.
[0061] FIG. 3 illustrates calculated reflectivities for a
four-layer AR construction consisting of alternating THV and PEN
layers deposited on a PET substrate. The order of indices of
refraction starting from the layer near the air interface is
1.37/1.74/1.37/1.74/1.65 (PET). The reflectivities are calculated
for normal incidence, unpolarized light (curve a), for s-polarized
light at 60.degree. relative to the normal (curve s), and
p-polarized light at 60.degree. relative to the normal (curve
p).
[0062] FIG. 6 illustrates calculated transmissivities for a 44
layer AR construction consisting of alternating PEN and PMMA layers
deposited on a PET substrate. The transmissivities are calculated
for normal incidence, unpolarized light.
[0063] Manufacture
[0064] AR constructions may be manufactured by co-extrusion using a
feedblock method, e.g., as described in co-pending U.S. Pat. No.
5,882,774.
[0065] Where the AR construction is optically coupled to a mono- or
multilayer polymer base, the article is preferably formed by
co-extruding the AR construction with the base to form the article
in a single step.
[0066] Co-extrusion of optically thin AR layers may be difficult
due to the shear forces in an extrusion system causing an unstable
polymer flow. Extrusion coating of thin layers may also be
difficult to control. Thus, to prepare optically thin AR layers, it
is generally preferred to extrude relatively thick polymer layers
and then stretch the resulting material to produce the desired
final thickness. The stretching process will also result in the
orientation of certain polymers, with the degree of orientation
related to the amount of stretching.
[0067] The effect of shear forces produced during co-extrusion can
also be reduced by co-extruding one or more relatively thick "skin
layers" when forming the article. As shown in FIG. 1B, article 112
is provided with a skin layer 110 that absorbs much of the shear
forces developed during extrusion. In one simplified construction,
skin layer 110 is placed on top of AR layer 114, and AR layer 114
is on top of base 116. By appropriate selection of the composition
of the skin layer 110, the skin layer 110 can be removed from all
or a portion of article 112. If the final article 112 is stretched,
skin layer 110 can be removed either before or after stretching.
The presence of one or more skin layers in the co-extrusion process
may also assist with the production of multilayer AR stacks.
[0068] Suitable materials for the skin layer include co-extrudable
polymers such as polyethylene, polypropylene, and atactic
polystyrene. These materials generally will not adhere strongly to
most suitable materials for the AR layer(s). The addition of the
skin layer allows the AR layer(s) to be extruded without structural
damage and, if desired, subsequently stretched to form optically
thin layer(s).
[0069] A thick "skin layer" may be useful itself as an AR layer in
reducing reflectance if it were made of a polymer having a
relatively low index of refraction such as THV-500 fluoropolymer
(n=1.36). For example, in the case of a PET substrate provided with
a THV AR layer, the air/THV interface reflects only about 2.33% of
the light. The THV/PET interface reflects 0.93%. Bare PET (n=1.65)
reflects 6.0%, so the use of a THV skin layer would reduce this
value to 3.26%. The 0.93% reflection value at the PET/THV interface
could be reduced even further by incorporating one or more
intermediate layers, or by grading the index with the use of a
third polymer which is miscible with both PET and THV-500.
[0070] Another design involving skin layers is shown in FIG. 2B. In
this design, useful in cases where the base has only one side
provided with an AR construction, two AR stacks 140 are placed
roughly in the middle of an intermediate article 146. The two AR
stacks are separated by an internal skin layer 142. The base 144
are oriented outward. From this construction, two articles 148, 150
can later be formed by separating skin layer 142 from the layers
forming each AR stack 140. In this way, the AR stacks are not
subject to high shear during the extrusion process. Additional AR
stacks and/or skin layers can be extruded along the outer surfaces,
if desired.
[0071] Following co-extrusion and removal of skin layers (if
present), additional materials may be deposited on the outermost
layer of the AR construction, in which case the stack is
pre-designed using computer modeling to factor in the presence of
these layers and their effect on the optical properties of the
stack. For example, inorganic materials such as alumina, zirconia,
silica, titania, and combinations thereof can be deposited in the
form of a sol-gel. Other inorganic materials, e.g., indium-tin
oxide and metals such as silver or aluminum can be vapor coated
onto the outermost layer of the AR construction.
[0072] If desired, the resulting construction can then be solution
coated with a fluoropolymer to form a hybrid AR construction, as
described above. The fluoropolymer could also be solution-coated
directly onto the outermost layer of the AR construction.
[0073] Applications
[0074] The AR constructions are useful in a variety of
applications. The particular design is selected based upon the
application.
[0075] FIG. 4 shows an article 200 that may be optically coupled to
a substrate to provide anti-reflective properties. It is useful in
applications for which higher optical transmission and/or reduced
glare are desired. Representative substrates include, e.g.,
goggles, eyeglasses, display windows, paper labels or sheets,
opaque but glossy surfaces, luminares and lighting fixtures, low
temperature light bulbs, computer monitors, and liquid crystal
displays (both backlit and front lit).
[0076] Article 200 is also useful as a substrate for coatings such
as photographic emulsions when it is desirable to suppress even the
minute reflection that occurs at the solid/solid interface between
the coating and substrate. Such an article may optionally be
optically coupled to another article. The AR stack serves to
de-reflect the interface between the monolayer base film and the
coating. For example, in the case of photographic emulsions
designed for laser exposure, the AR stack will prevent the typical
"woodgrain" pattern caused by interference of coherent light
reflected from both the top and bottom interfaces of the
emulsion.
[0077] Article 200 features a relatively thick, birefringent
monolayer base 202, a multilayer anti-reflective stack 204
optically coupled to the base, and an additional layer 206 for
providing desired mechanical, chemical, and/or electrical
properties. Layer 206 may be part of the final article or it may be
strippable, e.g., a strippable skin layer, that is removed to yield
the final article. An optical coupling agent 208 is used to secure
article 200 to a substrate. Stack 204, which includes a plurality
of polymer layers 210 featuring alternating layers of birefringent
polymers and low index polymers is designed to de-reflect base
202.
[0078] Article 200 is prepared by co-extruding base 202 and stack
204 in the form of a unitary article, and then stretching the
article until a pre-determined thickness is reached.
[0079] Stretching produces high in-plane indices of refraction in
the case of crystalline and semicrystalline polymers described
above.
[0080] Base 202 is selected to provide sufficient mechanical
strength and thickness for ease of handling during manufacture and
application. Preferred materials include birefringent,
strain-hardening materials having a glass transition temperature
equal to or lower than the polymers forming stack 208. A
representative polymer suitable for base 202 is polyethylene
terephthalate.
[0081] It is also possible to construct article 200 without base
202. For such constructions, stack 204 should contain a sufficient
number of layers such that it forms a free-standing film having a
thickness, e.g., in the range of about 3-5 microns. Optical
coupling agent 208 is provided directly on a surface of stack
204.
[0082] Optical coupling agent 208 may be any clear material that
will wet both the substrate and article 200. Typically, the
coupling agent is an optical adhesive, e.g., a pressure sensitive
adhesive or epoxy resin. Preferably, the index of refraction of
coupling agent 208 is intermediate that of base 202 and the
substrate.
[0083] Suitable materials for layer 206 include coatings to lower
the surface energy and/or coefficient of friction of the article to
aid in cleaning the article or preventing surface abrasions, and
antistatic or electromagnetic interference coatings. Layer 206 may
also be in the form of a protective skin layer that is either
strippable or permanent (in which case it becomes part of the final
article). For example, where article 200 is designed to reduce the
reflectance at solid/solid interfaces such as the interface between
a base film and a coating, layer 206 may take the form of a
permanent skin layer having an index of refraction matched to that
of the coating. Layer 206 may be applied after co-extrusion, but
either prior to, or after, stretching. In addition, multiple layers
may be used. In all cases where layer 206 is designed to become
part of the final article, the thickness and index of refraction of
layer 206 must be included as part of the AR stack optical
design.
[0084] Article 200 may optionally include a functional coating or
film (not shown) in between base 202 and coupling agent 208.
Because it is placed on the back side of base 202, it does not
affect the design or function of AR stack 204 located on the
opposite side of base 202. Article 146, shown in FIG. 2B, may
similarly include such a coating on the back side of base 144.
Examples of suitable coatings include transparent conducting films
for EMI shielding or IR rejection, anti-static films, UV protective
coatings, colored or neutral grey coatings that control light
transmission, and polarizing coatings.
[0085] The article shown in FIG. 2A may be used without laminating
it to a substrate. It is useful in applications where high
transparency is desired. Examples include protective face masks,
goggles, window coverings that include a thermally insulating air
gap, insulation layers inside multi-pane glass windows, overhead
projection transparencies and associated covers, and high
transparency wrapping material for packaging.
[0086] It is possible to eliminate base 134, in which case stacks
132 are provided in the form of a single, free-standing film. One
or more layers for providing desired mechanical, chemical, and/or
electrical properties may be included as well. For example, in the
case of face masks and goggles, it may be desirable to include a
hydrophilic coating to prevent fogging.
[0087] FIG. 5 shows an article 300 which may be optically coupled
to a substrate to provide anti-reflective properties, or used by
itself. It is particularly useful in applications where high color
saturation and low glare are desired. Examples of such applications
include edge filters with high transmission and sharp spectral
cut-offs (including hot and cold mirrors), reflective color filters
with high color purity in both reflection and transmission (e.g.,
for use as label stock, security/verification laminates, color
filters for projection displays, visible colored window
decorations, IR reflecting window film, colored adhesive backed
tapes, colored gift wrap, colored packaging film, color films for
advertising, and the like, ect.), multi-layer polymeric polarizers,
and multi-layer tear-resistant films having low glare.
[0088] As shown in FIG. 5, article 300 features a multilayer base
302 surrounded on each side with a multilayer anti-reflective stack
304. Each stack 304, in turn is provided with a skin layer 306,
followed by another multilayer anti-reflective stack 308. Examples
of the former include multilayer polymeric polarizers and mirrors.
To protect outer anti-reflective stacks 308 during extrusion,
Article 300 may further be provided with strippable skin layers
(not shown). Where article 300 is designed for application to a
separate substrate, one of antireflective stacks 308 may be
eliminated and replaced with an optical coupling agent (not shown)
for coupling article 300 to the substrate.
[0089] Multilayer base 304 may be an article with utility on its
own that gains functionality by addition of antireflective stacks,
or it may be a base film that is added to an antireflective stack
to give the antireflective stack additional mechanical strength or
stiffness.
[0090] Antireflective stacks 304 are particularly useful in the
case of multilayer optical film bases such as polarizers and
mirrors. Preferably, antireflective stacks 304 are designed to
suppress sideband ripples and thereby aid in providing a uniformly
low reflectivity at wavelengths outside of a high reflectivity band
stop, or on the low reflectivity side of an edge filter. Skin
layers 306 (which effectively decoupled the interference effects of
antireflective stacks 308 and antireflective stacks 304 (also help
suppress the asymptotic sideband ripple from a stopband, as they
are equivalent to immersing multilayer stack 302 in a higher index
medium. Antireflective stacks 308 further operate to eliminate the
air/skin interface which is a source of reflection from the
article.
[0091] Other embodiments are within the following claims.
* * * * *