Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-03-07
2002-11-12
Jagannathan, Vasu (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S608000, C524S555000, C524S556000, C428S364000, C428S458000
Reexamination Certificate
active
06479581
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to new aqueous-based aromatic polyamide-amic acid compositions that are useful in providing formulations for coating and sizing fibers, metal surfaces, glass surfaces and other materials. The polyamide-amic acid resins comprising the aqueous-based compositions disclosed herein have a high acid number compared with prior art amide-amic acid, and amide-imide resins which results in improved solubility, providing coatings having greater adhesion to substrates because of a greater chemical reactivity. The aqueous polyamide-amic acid compositions of this invention contain only negligible amounts of organic solvents, unlike prior art coating and sizing formulations, and are therefore more desirable for use where energy costs and environmental factors are important considerations.
Aromatic amide-imide polymers and copolymers have been known for more than 30 years, and have been widely accepted commercially for use as wire enamels and electrical varnishes and in a wide variety of other coating uses. These polymers are also used in fiber sizing, as adhesives, and as impregnating resins for fabric and for composites comprising fiber or particulates. Because amide-imide polymers are intractable and substantially insoluble, coating and sizing formulations are generally applied to the work as an amide-amic acid polymer precursor. The polyamide-amic acid resin coating or matrix is then cured thermally, generally at a temperature above about 150° C., forming a polyamide-imide resin.
When used in coating applications polyamide-amic acid resins are most conveniently applied from solution. Because thermal curing generally occurs at temperatures well below the melt temperature of these resins, melt coating processes are not practical. Solvents employed for these purposes generally are polar solvents and have a high boiling point. Nitrogen-containing solvents, for example dimethylacetamide, N-methylpyrrolidone, dimethylformamide and the like are widely employed, and ethers such as tetrahydrofuran have also been found useful. These solvents are difficult to remove from the coating substrate completely, requiring extended drying periods, often at a reduced pressure. According to the art, nitrogen solvents and ether solvents such as tetrahydrofuran have a strong affinity for these polar resins, forming stoichiometric complexes on precipitation. The bound solvent may be difficult to remove from these complexes; for example, polymers precipitated from tetrahydrofuran may contain as much as 19 weight percent (wt. %) or more residual ether solvent, even after extended drying. The energy costs associated with drying and curing, together with necessity for recovery of the volatilized solvent, due to environmental considerations as well as to the cost of the solvent, substantially impacts the commercial attractiveness of polyamide imide resins as coatings.
Alternative methods disclosed in the art include forming a polyamide-amic acid in solution in a polar solvent, precipitating the resin by adding a miscible non-solvent for the polymer, for example water or an alcohol, washing the resin repeatedly to remove residual solvents and water-soluble reaction byproducts, then drying. The solid polyamide-amic acid resin may be compression molded or otherwise fabricated or formed into the desired shape, then cured in the solid state or in the melt by heating the resin article for extended periods, removing water as it forms during curing. Although useful in providing molded articles, particularly where filled resin compositions are employed, this method finds little use in coating and sizing applications because of the low melt flow and intractability of the resin.
Aromatic polyamic acid and polyamide-amic acid resins substantially free of high boiling-point, environmentally-undesirable, polar solvents produced by methods heretofore known and described in the art thus are solid resins, unsuited for direct application as coating or sizing. Aromatic polyamide-amic acid resins are made available to the trade in dry solid form. However, these compositions are neither soluble nor readily dispersible in solvents considered environmentally acceptable, particularly including water. A suitable method for forming aqueous solutions or dispersions of polyamide-amic acids with a low residual solvent level that may be applied to a substrate, dried and cured with a minimum of hydrolysis or otherwise affecting resin detrimentally would be an important advance in the coatings art. Aqueous, low residual-solvent formulations comprising polyamide-amic acid resins would find wide acceptance in the trade and means for providing such compositions would represent an important and useful advance in the coatings art.
BRIEF SUMMARY OF THE INVENTION
This invention relates to aqueous compositions comprising a polyamide-amic acid and to a method for providing aqueous polyamide-amic acid compositions. More particularly, the aqueous compositions of this invention comprise a polyamide-amic acid dispersed in water, preferably dissolved in a mixture comprising water and an amine.
The aqueous compositions of this invention are substantially free of residual polar organic solvent and are useful in providing coatings that adhere well to a variety of substrates, particularly including metals. Formulations useful as sizings and impregnates may also be prepared comprising these aqueous compositions. Coatings and films comprising the aqueous polyamide-amic acid resin component in combination with tertiary amine are readily dried and cured thermally, forming the corresponding polyamide-imide.
DETAILED DESCRIPTION OF THE INVENTION
The aqueous-based polyamide-amic acid compositions of this invention comprise an aromatic polyamide-amic acid dissolved or dispersed in water. Methods for producing polyamide-amic acid resins suitable for use in the practice of this invention are well known in the art and are generally described and disclosed therein, for example, in U.S. Pat. No. 5,230,950, which is hereby incorporated by reference in its entirety. Generally described, polyamide-amic acids are readily prepared by the polycondensation reaction of at least one suitable aromatic polycarboxylic acid or reactive derivative thereof and one or more aromatic diamines. The polymerization is conveniently carried out under substantially anhydrous conditions in a polar solvent and at a temperature below about 150° C., employing substantially stoichiometric quantities of the reactive carboxylic acid component and amine component. A slight stoichiometric excess, typically from about 0.5 to about 5 mole %, of either monomer component, preferably the carboxylic acid anhydride component, may be employed if desired in order to control molecular weight; alternatively a monofunctional reactant may be employed as an endcapping agent for this purpose, and to improve stability.
As will be described in greater detail herein below, polyamide-amic acids useful in the practice of this invention will desirably have a high level of amic acid functionality, and may also be conveniently described and characterized as having a high acid number. Polyamide-amic acids formed from reactive trimellitic acid compounds or similar tricarboxylic acid compounds in theory will comprise one amic acid grouping per tricarboxylic acid repeat unit. Thermally imidizing or curing the resin cyclizes the amic acid groups to form imide links, thereby reducing the level of amic acid functionality and thus lowering the acid number. The polyamide-amic acid resins preferred for use in forming aqueous solutions according to the invention will contain at least 50 mole %, preferably greater than 75 mole %, and more preferably will contain as great as 90 mole % to as much as 100% of the theoretical level of amic acid functionality.
It is thus essential to avoid process conditions during the polymerization and in subsequent processing to isolate and collect the solid polyamide-amic acid that may imidize the polyamide-amic acid. The polyamide-amic acid will therefore be isol
Ireland Daniel J.
Keske Robert G.
Nel Jan G.
Jagannathan Vasu
McDermott & Will & Emery
Shosho Callie
Solvay Advanced Polymers, LLC
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