Essentially colorless, transparent polyimide coatings and films

Details Essentially colorless, transparent polyimide coatings and films Essentially colorless, transparent polyimide coatings and films

2000-01-14

2001-05-15

Hampton-Hightower, P. (Department: 1711)

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

Synthetic resins

From phenol, phenol ether, or inorganic phenolate

C528S125000, C528S126000, C528S128000, C528S171000, C528S172000, C528S173000, C528S174000, C528S176000, C528S179000, C528S183000, C528S185000, C528S188000, C528S220000, C528S229000, C528S350000, C528S353000, C428S473500

Reexamination Certificate

active

06232428

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to essentially colorless, transparent polyimide coatings and films and to a process for preparing same. The present invention more particularly relates to essentially colorless, transparent polyimide coatings for optical fibers and essentially colorless, transparent polyimide films useful for thermal protective coatings and the like.
BACKGROUND OF THE INVENTION
Polyimide coatings and films have been selected for use in a number of different product applications where thermal stability and good electrical and mechanical properties are deemed necessary and/or desirable. Polyimide coatings and films having the additional property of good transparency have been used extensively as oriented films in liquid crystal display devices, fiber optic cable coatings, waveguides and protective coatings for solar cells. Although such polyimide coatings and films have good transparency, they are often colored yellow or brown as a result of a severe heat history leading to their formation. This coloration is deemed unacceptable in applications such as liquid crystal oriented films where the coloration darkens the visual field thereby impairing the function of the display device on which the film is used.
In response to this noted deficiency, various polyimide coatings and films have been developed that demonstrate a small degree of coloration and high transparency. The development of such prior art coatings and films has been guided by a series of studies that has focused on the cause of coloration in transparent polyimide films. These studies have reported that the coloration of a polyimide depends greatly upon the types of aromatic tetracarboxylic acid dianhydrides and diamino compounds selected for use as starting materials for the polyimide. In particular, these studies have reported that an aromatic diamine having the amino groups at the m-position is especially effective as the diamino compound, and the combination of it with a biphenyltetracarboxylic acid dianhydride can lead to the formation of a colorless, transparent polyimide (see Cols. 1-2, lines 64-6 of U.S. Pat. No. 4,876,330 to Higashi et al.).
The development of such prior art coatings and films has also been guided by the well-known principle that higher formation or polymerization temperatures adversely impact upon the degree of coloration of the resulting polyimide. In practice, these prior art polyimides are prepared by processes whereby the aromatic tetracarboxylic acid dianhydride and diamino compound are polymerized at temperatures of 80° C. or less to form a polyamic acid solution and then, the polyamic acid is imidized by either thermal or chemical means (see Col. 8, lines 25-39 of U.S. Pat. No. 4,876,330 to Higashi et al.).
In accordance with the above, an object of the present invention is to provide a novel, essentially colorless, transparent polyamic acid solution and polyimide coating or film that serve to negate commonly held principles and beliefs.
It is a further object of the present invention to provide a process for preparing such coatings or films that also runs contrary to established findings.
SUMMARY OF THE INVENTION
The present invention therefore relates to an essentially colorless, transparent polyamic acid solution prepared by reacting at least one aromatic tetracarboxylic acid dianhydride represented by general formula:
where X represents—O—, —S—, —SO—, —SO
2
—, —CH
2
—, —CF
2
—, —C(CH
3
)
2
—, —C(CF
3
)
2
— or a direct bond, with at least one para-substituted aromatic diamine represented by either general formula (I)
or general formula (II)
where Y represents —O—, —S—, —SO—, —SO
2
—, —CH
2
—, —CF
2
—, —C(CH
3
)
2
—, —C(CF
3
)
2
—, —CO— or a direct bond.
The present invention also relates to an essentially colorless, transparent polyimide coating or film that comprises at least one recurring structural unit of general formula (A)
or general formula (B)
where X represents —O—, —S—, —SO—, —SO
2
—, —CH
2
—, —CF
2
—, —C(CH
3
)
2
—, —C(CF
3
)
2
— or a direct bond and where Y represents —O—, —S—, —SO—, —SO
2
—, —CH
2
—, —CF
2
—, —C(CH
3
)
2
—, —C(CF
3
)
2
—CO— or a direct bond.
The present invention further relates to a process for preparing the essentially colorless, transparent polyimide coating or film described above which comprises:
preparing a polyamic acid solution by reacting the above referenced dianhydride and diamine monomer components in an organic polar solvent;
forming a coating or film of the polyamic acid from the prepared polyamic acid solution; and
imidizing the polyamic acid in the formed coating or film to a polyimide.
The foregoing and other features and advantages of the present invention will become more apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
Applicants, by way of the present invention have made the surprising discovery that aromatic diamines having amino groups at the p-position can serve as effective starting materials for essentially colorless, transparent polyimides. In particular, Applicants have found that the combination of aromatic tetracarboxylic acid dianhydrides and para-substituted aromatic diamines can also lead to the formation of essentially colorless, transparent polyimide coatings and films.
Due to the low degree of coloration achieved by way of the present invention, it is now possible to produce opaque white and blue transparent polyimide coatings and films. Such coatings and films are particularly desirable for use with optical fibers, liquid crystal display devices, solar cells and wave guides.
The aromatic tetracarboxylic acid dianhydride of the present invention is represented by the following general formula:
where X represents —O—, —S—, —SO—, —SO
2
—, —CH
2
—, —CF
2
—, —C(CH
3
)
2
—, —C(CF
3
)
2
— or a direct bond. Examples of aromatic tetracarboxylic acid dianhydrides include biphenyl dianhydride, 2,2-bis(3,4-dicarboxylphenyl) hexafluoropropane dianhydride, diphenylsulfone dianhydride, 2,2-bis(3,4′-dicarboxyphenyl) propane dianhydride, diphenylsulfide dianhydride, diphenylsulfoxide dianhydride, oxydiphthalic anhydride, biphenyltetracarboxylic acid dianhydride and benzophenone tetracarboxylic acid dianhydride. In a preferred embodiment X is a fluorine-substituted aliphatic hydrocarbon group. In a more preferred embodiment, the aromatic tetracarboxylic acid dianhydride monomer component of the present invention is 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride or 6FDA,
In another more preferred embodiment X is a direct bond and the aromatic tetracarboxylic acid dianhydride monomer component is biphenyltetracarboxylic acid dianhydride or BPDA,
The aromatic diamine of the present invention is a para-substituted aromatic diamine represented by either general formula (I)
or general formula (II)
where Y represents —O—, —S—, —SO—, —SO
2
—, —CH
2
—, —CF
2
—, —C(CH
3
)
2
—, —C(CF
3
)
2
—, —CO— or a direct bond. Examples of para-substituted aromatic diamines include bis[4-aminophenyl] sulfone, 4,4′-biphenyl diamine, oxydi aniline, 4,4′-diaminophenyl sulfone, 4,4′-diaminophenyl sulfide, 4,4′-diaminodiphenyl sulfoxide, methylene dianiline, 4,4′-diaminodiphenyl difluoromethane, 2,2′-bis[4-(4-aminophenoxy)phenyl] propane (BAPP) and bis [4-(4-aminophenoxy)phenyl] sulfone (BAPS).
In a preferred embodiment Y is a direct bond and the aromatic diamine is 4,4′-biphenyl diamine.
In another preferred embodiment Y is a sulfonyl group and the aromatic diamine is bis[4-aminophenyl]sulfone or 4,4-DDS,
When a diamine containing SO
2
functionality (e.g., 4,4-DDS) and/or Ar—O— Ar ether linkages (e.g., BAPP or BAPS) is used in preparing the polyamic acid solution of the present invention it is preferred that a second diamine monomer be employed therewith for the purpose of reducing color and/or improving toughness in the resulting polyimide. The second diamine monomer can be either a para- or meta-substituted aromatic diamine. Examples of meta-substituted aromatic diamines in

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