Optical adhesive coating having low refractive index

Details Optical adhesive coating having low refractive index Optical adhesive coating having low refractive index

2002-03-19

2004-03-09

Lipman, Bernard (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C428S421000, C428S422000

Reexamination Certificate

active

06703463

ABSTRACT:

FIELD OF THE INVENTION
This invention is directed to optical adhesives useful in light transmitting devices, and more particularly to fluorosubstituted monoacrylate based adhesives having low refractive index. The invention is further directed to transfer tapes comprised of at least one layer of a fluorosubstituted monoacrylate based adhesive.
BACKGROUND OF THE INVENTION
Optical coatings to control light distribution, i.e., anti-glare, anti-iridescence, low reflectance and interference, employ coatings of varying refractive index to obtain the desired light distribution. While fluoropolymers offer low refractive index, generally below 1.4, fluoropolymers typically have poor solvent solubility and poor adhesion to substrates.
For optical applications, fluoropolymers are usually made in situ by radiation curing. Alternatively, fluoropolymers may be extruded as melts. It is desirable, therefore, to provide a solvent-soluble fluoropolymer having low refractive index and good adhesion to substrates.
SUMMARY OF THE INVENTION
The present invention is directed to an optical adhesive having a refractive index of less than 1.40 comprising a transparent polymer comprising:
75-100% by weight, based on the total weight of the polymer of at least one fluorosubstituted monoacrylate comonomer of the formula:
wherein R
1
is hydrogen and R
2
is a linear or branched fluoroalkyl group having 2 to 20 carbon atoms; and
0-5% by weight, based on the total weight of the polymer of an ethylenically unsaturated comonomer selected from the group consisting of (a) mono- and di-carboxylic acids, (b) hydroxyalkyl monomers, (c) epoxy monomers, (d) carboxylic amides, and (e) N-vinyl lactam monomers. The optical adhesive of the present invention is soluble in organic solvents, and in particular, in non-fluorinated organic solvents. The polymer of the optical adhesive of the present invention has a low glass transition temperature and has the ability to bond well with substrates, including glass substrates and polyethylene terephthalate polyester film substrates typically used in optical devices.
The present invention is further directed to a transfer tape comprising an optical adhesive layer and a carrier layer, wherein the optical adhesive has a refractive index of less than 1.40 and comprises a transparent polymer comprising:
75-100% by weight, based on the total weight of the polymer of at least one fluorosubstituted monoacrylate comonomer of the formula:
wherein R
1
is hydrogen and R
2
is a linear or branched fluoroalkyl group having 2 to 20 carbon atoms; and
0-5% by weight, based on the total weight of the polymer of an ethylenically unsaturated comonomer selected from the group consisting of (a) mono- and di-carboxylic acids, (b) hydroxyalkyl monomers, (c) epoxy monomers, (d) carboxylic amides, and (e) N-vinyl lactam monomers.
DETAILED DESCRIPTION OF THE INVENTION
The fluorosubstituted acrylates of the present invention can be copolymerized to prepare copolymers having specifically desired physical properties, such as refractive index, glass transition temperature, light transmittance and adhesiveness.
The fluorosubstituted acrylate monomers used to form the polymer of the present invention are selected from monoacrylates. Generally, monoacrylates are preferred over monomethacrylates because of their lower refractive indices. Among the monoacrylates that may be used are, for example, those of the formula:
wherein R
1
is hydrogen and R
2
is a linear or branched fluoroalkyl group. In one embodiment, the fluoroalkyl group R
2
is a fluoroalkyl having 2 to 20 carbon atoms. For example, the fluoroalkyl group may be one of: —CH
2
CF
3
, —CH
2
C
2
F
5
, —CH
2
C
3
F
7
, —CH
2
C
4
F
9
, —CH
2
C
5
F
11
, —CH
2
C
7
F
15
, —CH
2
C
8
F
17
, —CH
2
C
9
F
19
, —CH
2
C
10
F
21
, —CH
2
CH
2
CF
3
, —CH
2
CH
2
C
2
F
5
, —CH
2
CH
2
C
3
F
7
, —CH
2
CH
2
C
4
F
9
, —CH
2
CH
2
C
5
F
11
, —CH
2
CH
2
C
7
F
15
, —CH
2
CH
2
C
8
F
17
, —CH
2
CH
2
C
9
F
18
, —CH
2
CH
2
C
10
F
21
, —CH
2
(CF
2
)
2
H, —CH
2
(CF
2
)
4
H, —CH
2
(CF
2
)
6
H, —CH
2
(CF
2
)
8
H, —CH
2
(CF
2
)
10
H, —CH(CF
3
)
2
, —CH
2
CF
2
CHFCF
3
, —CH
2
CF
2
CHF(CF
2
)
6
H, —CH
2
CF(CF
3
)CHFCF(CF
3
)
2
, —CH
2
C
6
HF
12
, —C
6
HF
12
, —CH
2
C
10
HF
20
, —CH
2
C
5
F
9
H,
Particularly useful fluorosubstituted monoacrylates include 1H,1H-heptafluorobutyl acrylate, 1H,1H-pentadecafluorooctyl acrylate, hexafluoroisopropylacrylate, 2,2,2-trifluoroethyl acrylate and 1H,1H,2H,2H-heptadecafluorodecyl acrylate. Another useful fluorosubstituted monoacrylate is a blend of 1H,1H,2H,2H-fluoroalkyl acrylates available from DuPont under the tradename ZONYL® TA-N.
The fluoropolymer used to make the optical adhesive of the present invention generally contains 75-100% by weight, based on the total weight of the polymer, of fluorosubstituted monoacrylate. In one embodiment, the polymer comprises two fluorosubstituted monoacrylates, wherein the total fluorosubstituted monoacrylate content is within the range of 75-100% by weight, based on the total weight of the polymer.
In another embodiment of the present invention, the polymer comprises 95-100% by weight, based on the total weight of the polymer, of the fluorosubstituted monoacrylate. In yet another embodiment of the present invention, the polymer comprises 99-100% by weight, based on the total weight of the polymer, of the fluorosubstituted monoacrylate.
Fluorosubstituted methacrylates may be substituted for a portion of the fluorosubstituted acrylates described above. Examples of such fluorosubstituted methacrylates include methacrylates of Formula I above, wherein R
1
is methyl, or a fluorosubstituted methyl group. Because the fluorosubstituted methacrylates generally have higher refractive index and glass transition temperature than the fluorosubstituted acrylates, only a relatively small amount of the methacrylate is included in the polymer of the optical adhesive of the present invention.
A small amount of fluorosubstituted diacrylate monomer may be added to the fluorosubstituted monoacrylate monomer. The addition of an excessive amount of fluorosubstituted diacrylate, greater than about 1% by weight, causes gelling of the optical adhesive.
In addition to the fluorosubstituted monoacrylate monomer, the adhesive polymer contains at least one ethylenically unsaturated monomer having a polar group. This ethylenically unsaturated monomer enhances the cohesive strength of the adhesive and provides a site for crosslinking. Useful ethylenically unsaturated polar monomers include ethylenically unsaturated mono-, di- and polycarboxylic acids, epoxy monomers, hydroxyalkyl monomers, carboxylic amides, and N-vinyl lactam monomers. The fluoropolymer used to make the optical adhesive of the present invention generally contains up to 5% by weight, based on the total weight of the polymer, of the ethylenically unsaturated polar monomer. In one embodiment, the fluoropolymer contains up to 2% by weight, based on the total weight of the polymer, of the ethylenically unsaturated polar monomer, and in another embodiment, the fluoropolymer contains up to 0.5% by weight, based on the total weight of the polymer, of the ethylenically unsaturated polar monomer.
Useful ethylenically unsaturated mono- and dicarboxylic acids include acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, glutaconic acid, 3-methylglutaconic acid, muconic acid, dihydromuconic acid, methylenemalonic acid, citraconic acid, mesaconic acid, and methyleneglutaric acid. Acrylic acid is particularly useful as the ethylenically unsaturated polar monomer.
Useful ethylenically unsaturated epoxy monomers include glycidyl methacrylate, methylglycidyl methacrylate and allylglycidylether. The ethylenically unsaturated carboxylic amides include N-alkylcarboxylic amides, N-methylol carboxylic amides, and alkylethers of the foregoing amides, for example, acrylamide, methacrylamide, N-methylacrylamide, &bgr;-diethylacrylamide, mono-, di- and ester-amides of maleic, fumaric, itaconic and other ethylenically unsaturated dicarboxylic acids, N-methylol acr

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