AQUEOUS-BASED POLYAMIDE-AMIC ACID COMPOSITIONS

Abstract: An aqueous-based composition comprising an aqueous solution of the product formed of the combination of a polyamide-amic acid and a triamine, preferably wherein the polyamide-amic acid is prepared from trimellitic acid halide or its equivalent and an aromatic diamine comprising 4,4'-methylenedianiline.

Description: AQUEOUS-BASED POL AMIDE-AMIC ACID COMPOSITIONS Background of the Invention This application claims the benefit of U S Provisional Application No 60/123.961 filed March 12. 1999 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 othei materials The polvamide-amic acid resins comprising the aqueous-based compositions disclosed herein hav e a high acid number compared with prior art amide-amic acid and amide-imide resins which results in improved solubihtv 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 energv costs and env ironmental factors are important considerations Aromatic amide-imide polymers and copolymei s have been know n for more than 30 years. and have been widely accepted commercialK for use as wire enamels and electrical v arnishes and in a wide v ariety 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 generallv applied to the work as an amide-amic acid polymer precursoi The polvamide-amic acid resin coating or matrix is then cured thermal lv, generally at a temperature abov e about 150 °C. forming a polyamide-unide resin When used in coating applications polyamide-amic acid resins are most convenientlv applied tiom solution Because thermal curing generallv occui s at temperatui es w ell below the melt tempeiature of these resins, melt coating processes are not practical Solv ents employed for these purposes generally aie polar solvents and hav e a high boiling point Nitiogen-containing solvents, foi example dimethylacetamide. N-methylpyrrohdone. dimeth lformamide and the like are widelv employed, and ethers such as tetrahydrofuran have also been found useful These solvents are difficult to remove from the coating substrate completelv lequiπng extended drv mg periods, often at a l educed pressure Accoi ding to the art. nitrogen solvents and ether sol ents such as tetrahv drofuran have a stiong affinity lor these polai resins forming stoichiometnc complexes on precipitation The bound solvent be difficult to remov e from these complexes, foi example, polv mers precipitated from tetrahydrofuran may contain as much as 19 weight percent (vvt ° o) or more lesidual ether solvent, even after extended drying The energv costs associated with drv ing and curing, together with necessitv for recovei v of the volatilized solvent, due to environmental consideiations as well as to the cost of the solvent, substantial! impacts the commercial attractiveness of polyamide lmide resins as coatings Alternativ e methods disclosed in the art include forming a polyamide-amic acid in solution in a polar solv ent, piecipitating the resin by adding a miscible non-solvent for the polymer, for example water 01 an alcohol washing the resin repeatedly to remove residual solvents and water-soluble ieaction bv products, then drving The solid poiyamide-amic acid resin may be compression molded or other ise fabricated or formed into the desired shape, then cured in the solid state or the melt by heating the lesin article foi extended periods, removing water as it forms during curing Although useful in piov idmg 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 intractabilitv of the iesin Aromatic polyamic acid and polyamide-amic acid resins substantially free of high boiling- point env lionmentallv-undesirable polar solvents produced bv methods heretofore known and described in the ait thus ai e solid iesins unstated for dnect application as coating or sizing Aromatic polyamide-amic acid iesins are made available to the trade in dry solid form However these compositions aie neither soluble nor readily dispersible in solvents considered env ironmentally acceptable, particularly including water A suitable method for forming aqueous solutions or dispersions of polv amide-amic acids w ith a low residual solvent level that may be applied to a substrate dned and cured w ith a minimum of hydrolysis or otherwise affecting resin detrimentally would be an important advance in the coatings art Aqueous, low residual-solvent formulations comprising polyamtde-amic acid resins would find wide acceptance in the trade and means foi piovidmg 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 foi piovidmg aqueous polyamide-amic acid compositions Moie particularly the aqueous compositions of this inv ention comprise a polyamide-amic acid dispersed in watei. preferably dissolved in a mixtuie comprising water and an amine The aqueous compositions of this invention aie substantially free of residual polar organic solvent and aie useful in piov idmg coatings that adheie well to a v ariety of substrates, particularlv including metals Formulations usef ul 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 w ith tertiary amine are readily dried and cured thermally forming the corresponding polyamide-imide Detailed Description of the Inv ention The aqueous-based polyamide-amic acid compositions of this invention comprise an aromatic pol\ amιde-amιc acid dissolv ed or dispersed in watei Methods for producing polyamide-amic acid resins suitable foi 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 Patent 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 pol carboxylic acid or reactive derivative thereof and one or more aromatic diamines The polymerization is conveniently carried out under substantial anhvdrous conditions in a polar solvent and at a temperature below about 150°C. employ ing substantially stoichiometnc quantities of the ieactive carboxylic acid component and amine component A slight stoichiometnc excess, ty pically from about 0 5 to about 5 mole %. of either monomer component, preferably the carboxylic acid anhydride component, may be employed if desired in ordei to control molecular weight, alternativelv a monofunctional reactant may be emploved as an endcappmg agent for this purpose, and to improve stability As w ill be described in greater detail herein below , polyamide-amic acids useful in the practice of this invention ill desirably have a high lev el oi amic acid functionality, and may also be conveniently described and characterized as having a high acid number Polyamide-amic acids formed from reactive tπmellitic acid compounds or similar tπcarboxylic acid compounds in theory will comprise one amic acid grouping per tπcarboxy lie acid repeat unit Thermally imidizing or curing the resm 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 w ill contain at least 50 mole%. preferably greater than 75 mole%. and more preferably will contain as great as 90 ιnole% to as much as 100% of the theoretical level of amic acid functionality It is thus essential to avoid piocess conditions din ing the polymerization and in subsequent piocessing to isolate and collect the solid polyamide-amic acid that may lmidize the polyamide-amic acid The polyamide-amic acid will therefore be isolated in solid form under mild conditions, preferably by being coagulated or precipitated from the polar reaction solvent by adding a miscible non-solvent, for example water, a lower alkyl alcohol or the like The solid resm may then be collected and thoroughly washed with water, and centrifuaed or pressed to further reduce the water content of the solid w ithout applying heat Non-solvents othei than water and lower alkv l alcohols are known and have been used in the art for precipitating polyamic acids and poiyamide-aimc acids from solution including, for example etheis. aromatic hydrocarbons, ketones and the like Most such non-solvents are not water soluble and thus aie not leadily removed from the solid resin bv washing, and w ill therefore not be preferred Further, high boiling miscible non-solvents may be difficult to separate from the polar reaction solvent and recover for recycle To form an aqueous solution of the polyamide-amic acid, the solid resin w ill be dispersed or dissolved in a mixture comprising water and an aliphatic amine, preferably a tertiarv amine Combining the solid polyamide-amic acid resin with an amount of a suitable amine effective to substantially neutralize the amic acid functionality and form the corresponding amine salt may suffice to dissolve the polyamide-amic acid It is known that aqueous compositions comprising polyamic acids and containing greatei than 100% of the stoichiometnc or neutralizing amount of amine. more particularly from about 1 10% to about 150%. based on the amount of amic acid groups present in the polymer, are markedly more stable toward hydrolysis, hence formulations based on polyamide-amic acids comprising similarly high levels of amine will be preferred Aqueous compositions according to this invention that contain an amount of tertiary amine substantially in excess of the stoichiometnc amount, most preferably from 150% of the stoichiometnc amount to as much as a 5-fold excess w ill be more preferred, and still greater lev els of amine mav be conveniently emploved without detrimentallv affecting the coating properties of the polyamide-amic acid resin In addition to improv ing the solution stability of these compositions, large excesses of the amine may be found to improve the rate of dissolution of the solid resin Tertiary amines are known to increase the rate of thermal lmidization and curing for polyamic acids, and may be effective in promoting rapid curing of polyamide-amic acid films and coatings Though aqueous solutions of polyamide-amic acid resin comprising as little as 80% of a stoichiometnc amount of amine may be found to have a lower hydrolytic stability such solutions may find use in applications where solution stability is of less importance than rapid rates of cure Neutralizing and dissolving the solid resin may be conveniently carried out in a single operation by adding the resin, preferably in powder form, to the requisite quantity of water containing the tertiary amine The amount of water employed will generally be sufficient to provide a solution containing from 0 5 to about 20 wt %. preferably from about 1 to about 16 wt %. more preferably from about 1 to about 6 vvt % polvamide-amic acid It may be desirable in preparing the aqueous composition to first combine the solid resin directly w ith tertiary amine. then let dow n the mixture by diluting ith watei More particularly described, the polyamide-amic acid resins prefeπed foi use in forming the aqueous compositions of this invention may be prepared by the reaction of tπmellitic acid or a deriv ativ e thereof such as. for example, a lower alkyl ester of tπmellitic acid anhydride or a tπmellitic acid halide. preferably the acid chloride of tπmellitic anhydride, l e tπmellitic anhydride chloride (TMAC). with at least one aromatic diamine such as. for example, p-pheny lenediamine, m- pheny lenediamine. oxybιs(anιlιne). benzidene. 1.5-dιamιnonaphthaiene. oxvbιs(2-methylanιIιne) 2.2- -4- SUBSTΓΓUTE SHEET 26 bιs[4-(p-amιnophenoxy)pheny l] propane. bιs[4-(p-amιnophenoxy)] benzene. bιs[4-(3-amιnophenoxy)] benzene and 4.4 -methylenedianiline The preferred aromatic diamine is 4.4 " -methylenedιanιhne (MDA) Examples of other useful aromatic primary diamines are set out in U S Pat No 3.494.890 ( 1970) and U S Pat No 4,016.140 ( 1977). both incorporated herein by reference Aromatic diamines may also be polymerized with tetracarboxylic acid dianhydπdes such as benzophenone tetracarboxylic acid dianhydπde (BTDA). pyromelhtic acid dianhydπde (PMDA) or the like according to the art to provide polyamic acids On curing, these polyamic acids form poly imide resin coatings and films These and similar aromatic dianhydπdes disclosed in the art for the preparation of polyimides are also known and described in the art for use in combination with TMAC to provide polyamide-imide copolymer resins See, for example, US 4.879.345 Polyamide- amic acid iesins wherein up to 25 mole% of the TMAC monomer is replaced by one or more such additional dianhydπde monomers may also be found useful in the practice of this invention Alternatively , useful blends comprising the preferred polyamide-amic acid resins and up to 25 wt % of a prior art polyamic acid resin having a high level of acid functionality as described may also be found useful The reaction of a trimelhtic acid halide and an aromatic diamine, for example. TMAC and MDA. to form the polyamide-amic acid may be conveniently carried out in a suitable solvent such as N-methylpyrrohdone. (NMP). other polar solvents such as N,N-dιmethylformamιde (DMF). methyl ethy l ketone (MEK) and N.N-dimethylacetamide (DMAC) and hexamethylphosphoramide (HMPA) can be used The mole ratio of MDA to TMAC will preferably lie in the range of from about 0 9 I to about l l l Generally the polymerization will be carried out by first combining and dissolving MDA in the solvent in the reaction vessel and then adding TMAC monomer, w ith stirring The reaction, which is exothermic, may be conveniently controlled by regulating the rate of addition of the reactants to the reaction vessel and by means of external cooling The leaction mass will be maintained at a temperature below 150 °C to avoid curing, and preferably in a range of fro about 20°C to about 50°C. more preferably from about 27° C (80°F) to about 50° C ( 120°F) for a period of from about l to about 10 hr to complete the polymerization The reaction time is not critical, and may vary from about 1 to about 24 hr depending upon reaction temperature, w ith about 2 to about 4 hr at a temperature in the range of from 30 °C to about 50°C being preferred The polyamide-amic acid formed by the reaction may be isolated by any convenient means, for example by precipitation w ith water The precipitated polyamide-amic acid is washed, preferably with watei, and collected by filtering or centπfuging to provide a polyamide-amic acid with a low residual solvent level - The polyamide-amic acid used in this invention may be further described as a polymer material comprising a mixture of amide-amic acid units which may be represented by the following structural formula A wherein the attachment of the two amide groups to the aromatic ring as shown will be understood to represent the 1 3 and the 1 4 polyamide-amic acid configurations, together with amide-imide units which mav be represented by the following structural formula B wherein R in the structure above is the moiety derived from the aromatic diamine component R may be further described as a substituted or unsubstituted divalent arylene moiety selected from the group consisting of wherein A is a divalent radical, selected, for example, from the group consisting of -SO2- -CO-. - C(CH3)2- -0-. -S- and a dπect chemical bond Aromatic diamines hav ing other linking groups are also known in the art and used as monomers in the pioduction of polyamide-imide resins, and most will be found suitable for use according to the practice of this invention Foi the preferred polyamide-amic acid according to the invention wherein the aromatic diamine component is MDA. units A and B may be more particularly represented by the structural formulae A B I he ratio of A. the amide-amic acid units, to B. the amide-imide units in the polyamide-amic acids of this invention will be high, suitably about 20 1 to about 1 01 1 , preferably about 18 1 to about 5 1 , most preferably about 16 1 to about 8 1 The units A of the polyamide-amic acid thus will suitably comprise more than 50 mole% of the total sum of amide-amic acid units and amide-imide units in the polymer, preferably more than 75 mole% and most preferably more than 90 mole% Thus, most of the units in the polyamide-amic acid are in the amic acid form, i e , A above The acid number (milligrams of KOH/gram) of the polyamide-amic acid is suitably greater than 100. preferably in the range of from 100, more preferably 1 10 and still more preferably greater than 120. and may be up to the theoretical acid number for a resin that comprises only A structural units as show n abov e T he acid number of a fully-washed lesin is primarily a function of the amount of amic acid component, unit A. present in the resin, and may be approximated on a theoretical basis as (mole fraction of A units) • 56 . Theoretical acid no (mg KOH) 00 MW of A units For example, foi a lesin formed entirely from methy lene dianiiine and TMAC in equimolar amounts and containing 1 0 mole traction A units, the molecular weight of the repeat unit will be 372. and the theoretical acid number will be approximately 150 mg KOH/g of resm For a resin that is fully mudized. l e contains only B units, the acid number would be zero It will be understood that the acid number may be highei than the theoretical value if a stoichiometnc imbalance favoring the tπmellitic component is used in the polymerization, and also if hydrolv is of the amic acid occurs during the polymerization or washing step Acid number w ill also be increased bv any acid reaction byproducts such as HCl not removed in the washing step and. if the polymer contains acid endgroups. w ill depend to a lesser degree on polymer molecular weight These factors may. if appropriate, be taken into account in calculating a corrected theoretical acid number for a resin product Generally, however, the acid number may be taken as a rough approximation of the degree of ltnidization that has taken place, 1 e the proportion of A units and B units in the polymer As indicated, the preferred polyamide-amic acid resin products will have a low level of imidization. the mole fraction of A units will thus be greater than 0 50. and thereby the resin will have a high acid numbei The molecular weight of the polyamide-amic acid will generally be greater than about 1000 to about 10,000. more preferably from about 1,000 to about 6,000 and still more preferably from about 2000 to about 4500 g/mol The washed and pressed polyamide-amic acid wetcake. isolated from the reaction mixture by precipitation and filtration, will be a solid, wet powder comprising as much as 80 wt % water, preferably from about 40 to about 70 wt % water, based on combined weight of water and polymer It may be desirable to minimize the water content of the resin wetcake by further pressing or similar conventional means to reduce the water content However, it is essential that these processes be carried out without subjecting the resin to heat or other conditions which may lmidize or cause a reduction in molecular weight, for example by hydrolysis For most uses, including providing an aqueous solution of the polyamide-amic acid as further described herein below , the wetcake may be conveniently employed without further drying As noted, polymerization to form the polyamide-amic acid will be carried out under substantially anhydrous conditions to avoid hydrolysis of the precursors as well as hydrolysis of the polyamide-amic acid It is well understood in the art that polyamic acids are sensitive to water particulai ly when maintained at a neutral or acid pH The amide function of the amic acid grouping becomes hydrolyzed undei these conditions, breaking the polymer chain and causing a loss in molecular weight It is believed that the aromatic dicarboxvlic acid functionality that results from the hydrolysis may thermally cyclize to form an anhydride functionality that is reactive toward amine end-groups Heating and curing thus may reform the polymer chain, thereby ' • healing' " the polymer As will be apparent from an examination of structural formula A. polyamide-amic acids including those preferred for use in the practice of this invention contain an amide function in addition to the amic acid grouping The second amide functionality readily hvdroivzes under acid conditions in the presence of water, forming an aromatic carboxylic acid group that is substantially unreactive Loss in molecular weight caused by this hydrolysis step, thought to be irreversible, may result in a complete depolymeπzation of the polyamide-amic acid It w ill thus be understood that it is highly desirable to minimize contact with water under conditions that ill hydrolyze the polyamide-amic acid To form an aqueous composition in solution according to the teachings of the invention, the solid substantially organic solvent-free pol amide-amic acid described above is mixed with water in the presence of an amine. for example a tertiary amine The amine will be selected to be miscible with water and volatile, preferably a low-boiling-point tertiary amine that will be readily volatilized during a thermal treatment to cyclize and cuie the polyamide-amic acid Tπalky l amines having limited vvatei solubility such as tπ-n-buty lamine may also be found useful particularly if employed togethei w ith a suitable water-miscible co-solvent tor the amine for example isopropanol Ammonia. as well as watei -miscible primary and secondary alkyl amines, may also be effectiv e in dissolving the polyamide-amic acid Howevei. such amines are generally known to be reactive with amide functional groups and may attack the amide functionality of the polyamide-amic acid, resulting in chain scission, hence these reactive amines will be less preferred Preferably the tertiary amine w ill be a tπ-(C ] -C alkyi)amιne such as. for example, tπmethv lamine N N-dιmeth\ lethylamιne N-dimethv l propylamine tπethvlamine or the like Cyclic tertiary amines may also be found useful, as may any of the vanety of well known water- miscible, tertiary alkanol amines including N.N * -dιmethyiethanolamιne Although polyfunctional amines such as N.N dimethylpipeπdine. as well as N.N.N'N -tetraalkyl-alkaline diamines and poly- N-alkylated alkalene tπamines may also be found effectiv e, the polyfunctional amines may tend to form associative crosslinks and gel. and will be less preferred Most preferred will be tπethyl amine The amount of poiyamide-amic acid that will be dissolved in the mixture of water and amine will depend in part upon the intended use For most purposes, the solution will comprise from about 0 5 to about 15 wt %, preferably from about 1 to about 8 t %. and more preferably up to about 5 wt %. polyamide-amic acid based on combined weight of polvamide-amic acid water and tertiary amine At higher concentrations particularly at concentrations abov e about 20 wt % the solution w ill have a v erv high v iscosity and will not flow readily, and will therefore not be preferred for most coating and sizing applications At very low resin concentrations, attaining adequate coverage of the substrate at a useful coating thickness becomes difficult, the utility of solutions with a low resm concentration, generally below about 0 5 t %. will thus be limited and therefore aie not preferred The polyamide-amic acid resin w ill be combined w ith an effectiv e amount of tertiary amine sufficient to substantially neutralize the amic acid functionality of the lesin and solubi ze the polyamide-amic acid in water Generally , and depending upon the final solids content, the polyamide-amic acid solution w ill comprise from 0 5 to about 50 wt % tertiary amine. based on total combined weight of amine, polymer and water The minimum amount of tertiary amine employed will be approximately the stoichiometnc amount required to neutralize the fiee carboxylic acid groups in the polymer, and more preferably will be from about 0 8 to about 1 2 mole foi each mole of free carboxylic acid gioups in the polvamide-amic acid As indicated, an excess of the tertiary amine. as much as a ^ fold stoichiometnc excess may be desirable The ratio of amine to free carboxvlic acid groups w ill thus preferably lie in the range of from about 0 8 to about 5 The concentration of free carboxylic acid groups in the polyamide-amic acid resin may be determined bv any convenient method, for example by titration of an aliquot with a base, and may be readily calculated from the acid number for the resin Accordingly , the amount of tertiary amine that w ill be emploved in forming aqueous solutions of polyamide-amic acid resins may be determined from the acid number of the resin and the desired ratio of amine to free carboxy lic acid groups, using the follow ing toi mula (resm acid number) x (amine MW) pbw amine = resm) wherein C represents the selected ratio of amine to free carboxylic acid groups C may have a value in the range of from about 0 8 to about 5 and preferably will be greater than about 1 more preferably from about 1 1 to about 3 As noted, aqueous polyamide-amic acid compositions comprising an excess of the amme aie believed to be more stable Aqueous compositions containing from 10 wt % to as much as 50 t % tertiary amine may be found useful, and aqueous formulations comprising 4-20 wt % solids and as much as from 60 to 88 wt % tertiary amine may also find application, particularly in providing resin concentrates Any convenient method of combining the components may be employed in preparing the aqueous compositions of this invention The solid polymer may be added in increments to a stirred mixture of the amine and water, continuing the stirring until the solid resin has been dissolved Alternativ ely , tertiary amine may be added slowly to a stirred suspension of polyamide-amic acid in water, with continued stirring until the solid dissolves As with any acid-base reaction external cooling may be found necessaiv initially subsequent warming and stirring may be desirable to complete dissolution of the solid resm in a reasonable time period Aqueous-based solutions according to the invention will thus comprise a polyamide-amic acid, watei and a tertiaiy amine Generally these water-based compositions have a low level of residual organic solvent, generally less than 8 wt %. preferably less than about 5 vvt % and more preferably less than about 3 wt % and compositions comprising from about 0 5 to about 3 wt % residual organic solvent based on combined weight of residual organic solvent and polyamic acid may be found to be highly desuable for most applications Formulations containing as little as 0 l wt % and e en lowei lev els oi lesidual organic solvent may also be obtainable foi example by use of extended washings Such compositions will be highly desired for use in applications where residual solvent cannot be toleiated The aqueous-based polyamide-amic acid compositions of this invention may be found particularly desirable in formulations intended foi use in coating applications, providing an adherent - 10- SUBSTΓΓUTE SHEET RULE 26 high strength continuous coating layer having improved toughness on a coated surface Such coatings may serve as a binder layer for automotive finishes, to improv e adhesion between existing layers of automotiv e finishes oi ith other metal finishes Polvamide-amic acids are known tor hav ing good adhesion to metal surfaces, and aqueous- based compositions of this invention thus may also be found particularly useful in providing formulations for use as enamels in container coating applications, as an additive to automotive electrocoated 01 solution-based metal coating systems as a chemically corrosion-resistant coating for metal or other substrates, as a binder layer for non-stick cookware. as a coating for tie bars for usage in cement, as a pretreatment coating for polymer films such as. for example, polyester, polyamide and polyimide film when used in a metahzing operation as an adhesive to various plastic or metallic film materials such as liquid cry stal polymers and polyimides. as an additive to improve the performance of inks, in a metal powder coating system for use in magnets, and in industrial spray coating applications such as an additive for metallic or plastic thermal spray coating systems The substantially organic solvent-free aqueous compositions of this invention may be found useful for film casting where organic solvents may not be desired or tolerated Formulations commonly employed for solution casting film and for similar coating operations are viscous materials that will flow sufficiently at the coating temperature to be spread over flat surfaces, preferably using a doctoi blade or similar apparatus and level to form a smooth, uniform film surface on drying Thick films may be produced by filling shallow pans w ith even more viscous solutions, allowing the solution flow under gravity and level to form a smooth suiface. slowly evaporating the water and then drying A wide variety of apparatus and methods for dope casting are well known and widely used commercially, and may be adapted for use with the aqueous formulations of this invention The solids content of aqueous polyamide-amic acid formulations intended foi use in casting operations will be selected on the basis of the substrate and on the intended operating conditions and apparatus, and may comprise from about 0 5 up to about 25 wt % polyamide-amic acid, more preferably from about 2 to about 1 5 wt % polv amide-amic acid, and still more preferably from about 2 to about 8 wt % polyamide-amic acid Furthei uses tor these compositions include as a raw material additive for improved aramid fiber products, as a thermal ban ier layer for plastics, as an additive in rubbers for tires and othei rubbei goods as an additiv e for asphalt to improv e w eai lesistance. and as a prepreg matei ial foi manufacture of composites In these and other such compounding operations the pol amide-amic acid component may be added in aqueous solution, or it may be found desirable and useful to add directly as the solid Formulations comprising these aqueous-based polyamide-amic acid compositions may also be found useful as sizings. and particularly for fiber material that has a modulus of 8.000.000 psi oi greater and is thermally stable to at least 600 °F for at least 10 minutes Thermally-stable fiber substrates that may be used for these purposes include glass fiber, carbon and graphite fiber, alumina fiber, silicon nitride fiber, boron fiber, aramid fiber, fluorocarbon fiber and the like The term "carbon fiber" is used herein in the generic sense and includes graphite fibers as well as amorphous carbon fibers that lesult after a thermal carbonization or graphitization treatment Sizing formulations comprising the invented polyamide-amic acid compositions may be advantageously applied to other fibers as well, including fibers comprising nylon, polyester and the like, and to steel or other metal fibers Such formulations may also be useful to coat steel tire cord and as a high performance adhesive in tire and mechanical rubber goods The composition of this inv ention may be used alone or in combination with other resins known for use in sizing applications such as. for example, poly urethane- epoxy- or acrylic-based sizing compositions The sizing may be applied to individual fibers or to multiple fibers in the form of bundles, ribbons, tapes or fabrics and the like by methods well known and widely practiced in the art for these purposes including for example, by di awing the fibers through a bath containing the aqueous sizing formulation oi by spraying the sizing formulation onto the fibers and then drying the fibers Application solids for sizing formulations will usually he in the range of from about 0 05 to about 10 wt %. and preferably from about 0 5 to about 5 wt %, based on the total weight of the solution When applied as a sizing or coating, the coated or sized article will then be dried to provide an article having an adherent coating or film comprising polyamide-amic acid The composition of the dried film or coating is believed to be an ammonium salt comprising the polyamide-amic acid and at least a stoichiometnc amount of the amine as described The salt is thought to be readily dissociated on heating, reforming the polyamide-amic acid Heating for an extended period at elevated temperatuies above 150° C and generally in the range of from about 200° C to about 350° C will imidize oi cure the polymei, driving off the amine component altogether along w ith additional watei foimed in the lmidizing reaction, forming a polyamide-imide film Aqueous compositions according to this inv ention may further contain such additional solvent components and v iscosity modifiers as may be desired, with the proviso that added solvent components will be miscible with water and readily volatilized from the polyamide-amic acid on drying and curing In formulating compositions for coating and blending, conv entional polymer lubricants, colorants, fillers, stabilizers and the like may also be employed, again w ith the proviso that such additives w ill be selected to be inert and not interact detrimentally w ith the lesm Compositions compπsing high levels of polyamide-amic acid, greater than about 20 wt %. may be found useful in othei applications, and when further modified, for example by adding flow improvers and poor solv ents to reduce solution viscosity, may be found to provide additional useful compositions • 12- SUB TΓΓUTE SHEET Solid pol amide-amic acid in the form obtained by precipitation and washing as described herein above, comprising from 25-40 wt % resin and substantially free of residual organic solvent, may also be found to be directly useful without being first dissolved, for example as an additive for compounding with rubbers, inks, adhesives and the like T he inv ention w ill be more readily understood from a consideration of the following examples EXAMPLES Example 1 Preparation of Polvamide-amic Acid Bis (4-amιnophenyl)methane (4,4'-methylene dianiline or MDA), 2624 pbw ( 13 25 moles). was dissolved in 9504 pbw of N-methylpyrrolidone (NMP) The solution was cooled to 10 °C (50 °F) in a glass-lined vessel and agitated while 2800 pbw ( 13 24 moles) of 4-tπmellιtoyl anhydride chloride (TMAC) were added over a period of 4 hr and while applying external cooling to maintain the reaction mixture at or below a temperature of 39 °C ( 102 °F) After the addition was completed the reaction mass was agitated for an additional 3 hr . then mixed with water containing approximately 5% NMP over a period of 4 hr to coagulate and precipitate the polymer After filtering, the filtercake was slowly washed with deiomzed water until the NMP content of a small vacuum filtered sample was less than 3% by weight The resin was again slurπed w ith water, and then separated by centπfugation and pressed to provide a wetcake having a solids content of 37% Example 2 Preparation of Aqueous Polvamide-Amic Acid Solution To a l -liter, three-necked, round-bottom flask equipped w ith a mechanical stirrer and a thermometei were added 477 grams of deiomzed water and 22 grams of 99% pure tπethy lamine To the stu red mixture were added, in three portions at l -hi intervals. 13 1 grams of poly mide-amic acid wetcake (37% polymer. 63% water), prepared substantially according to the procedure set forth in Example 1 The mixture was then stirred overnight at room temperature to provide an aqueous solution of the polyamide-amic acid salt Example 3 Film casting Films weie cast from a polyamide-amic acid res solution prepaied. at a solids concentration of about 6 0 wt %. substantially as described in Example 2 The v iscous solution of polyamide-amic acid was coated on a clean glass plate and spread using a glass rod to doctor the v iscous solution evenly across the length of the plate The coating was then dried in a cn culating air oven at 80° C for 1 hour to provide a tack-free polyamic acid coating on the glass substrate The films weie thermally cuied by further heating at a temperature of about 200 °C for 5 hi After cooling, the cast films were stripped fiom the substrate by dipping the glass plates in water, then dried The resulting poiyamide-nmde films had a thickness of approximately 05 mm (2 mils) Filling film pans 4 x 6 x 0 5 " deep with a 6 wt % solution of the polyamide-amic acid drying by slow evaporation of the water in a circulating air oven held at 80 °C overnight, then curing the resulting film by heating at 300 °C for 4 hr . provides polyamide-imide film having a thickness of about 100 mils Example 4 Coating Aqueous polyamide-amic acid compositions of this invention are also effective in providing adherent surface coatings for articles comprising a wide variety of ferrous and nonferrous metal and metal alloys, including copper, aluminum, steel and the like A viscous. 6 wt % aqueous solution of the polyamic acid salt prepared as described, placed on a surface of a clean. 3 " x 8 " x 0 15 " copper test panel at one end. and drawn evenly across the length of the panel using a doctor blade having a 80 mil spacing, provides the panel with a uniform coating of polyamide-amic acid resin solution The coating, after being dried in a circulating air oven to 78° C for 1 hr. then cured by heating at 300 °C for 180 mm , provides the copper substrate with a tack-free, adherent polyamide-imide coating having an approximate thickness of 4 mils Fiber, in continuous strand as well as in chopped form, may be sized or coated with the aqueous polyamide-amic acid compositions of this invention Coated and sized fiber and fabric will be useful in composite materials, and may be further compounded as reinforcement for rubber goods, filled plastic materials and the like having improved substrate-fiber adhesion Examples 5 - 8 Fiber coating or size In the following examples, continuous fiber strand was coated with a solution of the polyamide-amic acid prepared as described in Example 2. generally according to the following procedure An approximately l -meter-long strand of unsized fiber was pulled by hand lengthwise through a bath containing the aqueous polyamide-amic acid solution, held at room temperature A wire fiber guide submersed in the solution ensured that the strand was fully immersed as it was being pulled through the bath The procedure was repeated three or four times to ensure complete strand coverage The coated fiber strand, hung from hooks in a circulating air oven, was then dried at 90 °C for one houi then heated at 3 °C/mιn to 150 °C and held tor 30 minutes, and then at 3 °C/mιn to 260 °C and held foi 15 mm to cuie the coating and provide a polyamide-imide coated fiber strand Four commercial fiber strand specimens were prepared and coated in this fashion, including a Kevlar® polyaramid fiber strand, a 6K PAN-based carbon fiber strand, a 12K pitch-based carbon fiber strand, and a glass fiber strand After cooling, the strands were examined and evaluated by viewing the coated fibeis using scanning electron microscopy (SEM) Coated fiber strand having from about 0 1 to about 10 wt % polyamide-amic acid resin as the coating or sizing, which may be present as the cured polvamide-imide counterpart, exhibits improved - 14- SUBSTΓΓUTE SHEET 26 adhesion when combined w ith a variety of matrix materials and thus may find application in the production of fibei reinforced composites Coated and sized fiber strand also may be used in continuous oi chopped form as reinforcement in a variety of molded and extruded rubber goods, and in formulations comprising thermoplastic and thermoset resins and the like The invention will thus be seen to be an aqueous composition comprising from about 0 5 to about 40 wt % of a polyamide-amic acid having an acid number greater than about 100. preferably from about 120 to the theoretical value for a 100 mole% polyamide-amic acid Compositions comprising water and from about 20 to about 40 wt % polyamide-amic acid resin, generally in the form of a wet powder or filter cake, combined with an effective amount of a water-miscible amine sufficient to dissolve the polyamide-amic acid, provide aqueous solutions comprising from about 0 5 to about 20 wt % resin Preferably the amine will be a tertiary aliphatic amine, soluble in water and volatile, and w ill be employed in stoichiometnc excess with respect to the amic acid functionality present in the polyamide-amic acid The aqueous solutions of the invention may also be characterized as comprising amine salts of polyamide-atnic acids On drying to remove water and at least a portion of the excess amine, films and coatings comprising amine salts of polyamide-amic acids are provided which are readily cured to form polyamide-imides The invention thus may also be described as directed to coated and sized fiber and to articles comprising metal surfaces coated or sized with an amine salt of a polyamide-amic acid, and to a method for providing films and coatings comprising amine salts of polyamide-amic acid resins U S Provisional Application No 60/123.961 filed March 12. 1999 is hereby incorporated by reference in its entirety

Claims: We claim 1 A composition containing water and a polyamide-amic acid having an acid number greater than about 100 mg KOH/g, said polyamide-amic acid comprising amide-amic acid units according to structural formula wherein R is divalent arylene radical 2 The composition of Claim 1 comprising from about 0 5 to about 40 wt % said polyamide-amic acid 3 The composition of Claim 1 wherein said acid number lies in the range of from about 100 mg KOH/g to the theoretical maximum value 4 The composition of Claim 1 wherein said polyamide-amic acid comprises amide-amic acid units according to the structural formula and further comprises the corresponding amide-imide units according to the structural formula wherein the mole ratio of said amide-amic units to said amide-imide units lies in a range of from about 18 1 to about 5 1 5 The composition of Claim 4 wherein said amide-amic acid units have the structure and said amide-imide units have the structure: 6. The composition of Claim 1 further comprising an aliphatic amine. 7. The composition of Claim 6 wherein said amine is a tertiary aliphatic amine. 8. A composition comprising water, from about 0.5 to about 20 wt.% of a polyamide- amic acid having an acid number greater than about 100 mg KOH/g, and a tertiary aliphatic amine. the amount of said tertiary aliphatic amine lying in a range determined by the relationship: (resin acid number) x (amine MW) pbw amine = — C x (pbw resin) 0.056 wherein C has a value in range of from about 0.8 to about 5. 9. The composition of Claim 8 wherein C has a value greater than about 1.0. 10. The composition of Claim 8 wherein said amine comprises from about 5 to about 50 wt.% of the combined weight of said tertiary aliphatic amine. said water and said polyamide-amic acid. 1 1 . The composition of Claim 8 wherein said tertiary aliphatic amine is triethylamine. 12. An aqueous coating composition comprising water, a polyamide-amic acid and a tertiary aliphatic amine, wherein said polyamide-amic acid comprises amic acid units A and imide units B in a ratio of A:B in the range 18: 1 - 5: 1 , and wherein said composition comprises from about 0.5 to about 15 wt.% said polyamide-amic acid, and from about 2 to about 10 wt.% said tertiary amine. based on total weight of polymer, water and amine. 13. A composition according to claim 12. said polyamide-amic acid prepared from the monomers comprising trimellitic acid halide and 4,4 " -methylenedianiline and having an acid number in the range of from about 120 to about 150 mg KOH/g. 14. A method for providing at least one surface of an article ith an adherent poiyamide- imide film comprising the steps of coating said surface with an aqueous solution comprising 0.5 to - 17- SUBSTΓΓUTE SHEET RULE 26 about 15 wt % of a poiyamide-amic acid having an acid number in the range of from about 120 to about 150 mg KOH/g, and from about 0 5 to about 50 wt % of a tertiary amine, drying said coating and thereby providing adherent film comprising a tertiary amine salt of a polyamide-amic acid, then heating said article to cure said coating 15 The method of Claim 14 wherein said polyamide-amic acid is prepared from the monomers comprising trimellitic acid halide and 4.4 -methylenedianihne 16 A film comprising a tertiary amine salt of a polyamide-amic acid 17 Coated fibers comprising the film of claim 16 as an adherent coating 18 Heat treated coated fibers of claim 17 19 Articles of manufacture comprising the film of claim 16 20 A metal article having the film of claim 16 adherent to at least one surface thereof

Inventors: IRELAND DANIEL JAMES, KESKE ROBERT G, NEL JAN GELDENHUYS, IRELAND, DANIEL, JAMES, KESKE, ROBERT, G., NEL, JAN, GELDENHUYS
Applicants: BP AMOCO CORP, BP AMOCO CORPORATION
Application year: 2000
Main classification: C08L79
IPC codes: C08G73, C08K5, C08L33, C08L79, C09D179, D06M15