Antisoiling coatings for antireflective surfaces and methods...

Details Antisoiling coatings for antireflective surfaces and methods... Antisoiling coatings for antireflective surfaces and methods...

1998-01-27

2001-08-21

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C106S287130, C106S287140, C106S287160, C427S165000, C427S166000, C427S167000, C427S296000, C427S387000, C427S389700, C427S393400, C427S407200, C427S419200, C427S419500, C427S428010, C428S412000, C428S421000, C428S432000, C428S447000, C428S451000, C428S699000, C428S701000, C428S702000

Reexamination Certificate

active

06277485

ABSTRACT:

BACKGROUND OF THE INVENTION
The transparency of glass or plastic, in the form of doors, windows, lenses, filters, display devices (e.g., display panels) of electronic equipment, and the like, can be impaired by glare or reflection of light. To reduce the amount of glare, for example, on plastic or glass, the surface typically includes a single layer of a metal oxide (such as silicon dioxide), a metal fluoride, a metal nitride, a metal sulfide, or the like. Such coatings function as antireflective coatings.
Glass surfaces, for example, have about 4% surface reflection. With the aid of specialized coatings, such as metal oxides, this surface reflection can be reduced to less than about 0.5% average integrated intensity in the visible region of the spectrum at 450-650 nanometers (nm). The coatings can be multilayers of dielectric materials deposited in submicrometer thicknesses arranged to cause constructive or destructive interference of light waves of different wavelength. Antireflective materials in the visible region typically consist of three or four layers, two of which are of different materials, of alternating high and low index materials. Layers of quarter-wavelength or half-wavelength in optical thickness are typically used in the design of such materials.
Antireflective (AR) film stacks prepared by vacuum deposition (e.g., vacuum sputtering) of metal oxide thin films on substrates made of plastic, particularly flexible plastic, or glass, are particularly useful in display devices of electronic equipment. Such metal oxide films are relatively porous and consist of clusters of particles forming a relatively rough profile, which helps reduce glare and reflection. When such materials are conductive, they also help reduce static discharge and electromagnetic emissions. Thus, the primary application for these coatings is to provide contrast enhancement and antireflective properties to improve the readability of display devices, such as computer monitors.
Vacuum deposited (e.g., sputtered) metal oxide antireflective coatings are generally durable and uniform. Also, their optical properties are controllable, which makes them very desirable. They also have very high surface energies and refractive indices, however. The high surface energy of a vacuum deposited (e.g., sputtered) metal oxide surface makes it prone to contamination by organic impurities (from sources such as fingerprints). The presence of surface contaminants results in a major degradation of antireflectivity properties of the metal oxide coatings. Furthermore, because of the high refractive indices, surface contamination becomes extremely noticeable to the end-user.
Unfortunately, the high surface energy makes a vacuum deposited (e.g., sputtered) metal oxide surface difficult to clean without the use of environmentally undesirable solvent-based cleaners. Furthermore, removal of the surface contaminants can detrimentally affect the antireflective properties of the surface if the cleaning process leaves residue behind. Thus, a need exists for a protective coating on an antireflective surface that is relatively durable, and more resistant to contamination and easier to clean than the antireflective surface itself.
Numerous attempts have been made to provide antisoiling characteristics to an antireflective surface. This has been accomplished by providing antisoiling characteristics to the antireflective coating itself, or by providing an antisoiling coating over the antireflective coating. Examples of such antisoiling overcoatings are described in Applicants' Assignee's copending patent application U.S. Ser. No. 08/902,666, filed Jul. 30, 1997 (Pellerite et al.), and in JP Document 9-127307 (Sony Corp.) and U.S. Pat. No. 5,622,784 (Okaue et al.). The materials disclosed in the former document, however, are not generally appropriate for continuous coating techniques. Materials disclosed in the latter two documents, which are within the general type of compounds used in Comparative Examples B, I, O, and P herein, do not provide sufficiently durable antisoiling coatings. Although perfluoroether derivatives, such as that commercially available under the trade designation KRYTOX 157 FS(L) from E.I. DuPont de Nemours Co., Wilmington, Del., have been used as lubricants on surfaces of magnetic media articles and hard discs, they, alone, provide little antisoiling characteristics when applied to a transparent substrate as shown in Comparative Example D herein. Thus, a need still exists for materials that form durable antisoiling coatings suitable for application to substrates, particularly flexible substrates, in continuous coating techniques.
SUMMARY OF THE INVENTION
The present invention provides a protective coating on an antireflective surface that is relatively durable, and more resistant to contamination and easier to clean than the antireflective surface itself. That is, the present invention provides an antireflective article comprising a substrate having an antirefilective surface and an antisoiling coating thereon. The antisoiling coating is at least partially cured (i.e., solidified as by polymerizing and/or crosslinking) and comprises a fluorinated siloxane prepared by applying a coating composition (typically, in the form of a solution) comprising at least one fluorinated silane of the following formula (1):
ƒ
—[—R
1
—SiY
3−x
R
2
x
]
y
  (I)
wherein: R
ƒ
is a monovalent or divalent polyfluoropolyether group; R
1
is a divalent alkylene group, arylene group, or combinations thereof, optionally containing one or more heteroatoms or functional groups and optionally substituted with halides, and preferably containing about 2 to about 16 carbon atoms; R
2
is a lower alkyl group (i.e., a (C
1
-C
4
)alkyl group); Y is a halide, a lower alkoxy group (i.e., a (C
1
-C
4
)alkoxy group, preferably, a methoxy or ethoxy group), or a lower acyloxy group (i.e., —OC(O)R
3
wherein R
3
is a (C
1
-C
4
)alkyl group); x is 0 or 1; and y is 1 (R
ƒ
is monovalent) or 2 (R
ƒ
is divalent). Suitable compounds typically have a molecular weight (number average) of at least about 1000. Preferably, Y is a lower alkoxy group and R
ƒ
is a perfluoropolyether group.
The antireflective surface preferably includes a metal oxide film having one or more metal oxides, which have been preferably vacuum deposited metal (e.g., sputter coated).
The antisoiling coating is preferably at least about 15 Angstroms thick, and preferably no greater than about 150 Angstroms thick, for a desirable balance in performance with respect to antisoiling, durability, and antireflectance. Preferably, the antireflective article has a first surface antireflectivity that is different by less than about 0.5 percentage units from that of the same article without the antisoiling coating.
The present invention also provides an antisoiling coating composition comprising at least one fluorinated silane having a number average molecular weight of at least about 1000 represented by formula I above. Preferably, the coating composition (as opposed to the coating which is at least partially cured), includes a nonchlorinated solvent. The nonchlorinated solvent is preferably selected from the group of a fluorinated alkane, an alkyl perfluoroalkyl ether, and mixtures thereof. More preferably, it is an alkyl perfluoroalkyl ether. Preferably, the coating composition includes a fluorinated silane at a concentration of less than about 2.0 weight percent.
The present invention further provides a method of applying an antisoiling coating to a substrate having an antireflective surface, the method involves treating the antireflective surface with a coating composition comprising at least one fluorinated silane having a number average molecular weight of at least about 1000 represented by formula I above. Preferably, the method of applying includes a continuous process of applying which can be carried out with a continuous roll coater, such as a gravure coater, for example. Preferably, gravure coating includes feeding the coating

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