CA1234518A - Thick polyimide-metal laminates with high peel strength - Google Patents

Abstract

Abstract of the Disclosure This invention relates to a laminate comprising an insoluble, intractable relatively thick layer of a polyimide bonded tightly onto a metallic substrate without the necess-ity for an intermediate adhesive bonding layer and a process for preparing such by directly extruding a polyamic acid polyimide-precursor onto said substrate and converting said polyamic acid to the polyimide in a single pass in at least two heating stages.

Description

BACKGROUND
The present invention is concerned with a flexible laminate consisting of an intractable, i.e., no longer moldable, layer o~ a fully aromatic polyimide and a substrate and a method for forming a polyimide layer directly on a substrate, e.g., copper foil or wire to Eorm a laminate in which the layers are tightly adhered without an adhesive layer intermediate the polyimide layer and the supporting substrate layer. The poly-imides of the present invention are obtained from polyamide acids by an extrusion and curing process which takes place in situ by a process not heretofore known.
The preparation of polyamidocarboxylic acids, commonly referred to as polyamide acids or polyamic acids, which are the precursors or intermediate compounds in the preparation of poly-imides, is well known and references may be made to, e.g., Meyer et al. U.S. Patent 3,981,847 and Sroog et al., J. POLY SCI., Part A, Volume III, pages 1373-1390 (1965), among others. As is well known, the polyamic acids may be cured to extremely heat-resistant, highly insulating polyamides by a cyclization reac-tion under the in~luence of heat or dehydrating agents. Because of the intractability of polyimides with high thermal stability, they must be formed into the desired shape of the end product in the form of the polyamic acid and then or subsequently subjected to cyclization reaction conditions.
The preparation of the polyamic acids is likewise well known, e.~., by the reaction of a tetracarboxylic dianhydride with a primary diamine at temperatures below about 80C in an anhydrous, polar, aprotic organic solvent such as those dis-closed in the aforementioned U.S. Patent 3,981,847. Also disclosed therein are many of the aromatic dicarboxylic acids, i ~ dicarboxylic dianhydrides and aromatic diamines useful in the `~ present invention.
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Edwards Patents 3,179,614 and 3,179,634 describe, respectively, methods of preparation of polyamic acids and con-versions thereof to the polyimides. These prior patents dis-close several methods of coating substrates with polyamic acids and subsequently curing them to polyimide films, for example, by spray gun or dipping techniques followed by air drying at room temperature for several days and then subsequently curing for 30 minutes at 300C. By these methods, films of up to about 25 ~ m thickness were prepared, but films or coatings having greater thicknesses required multiple coating and conversion cycles.
The drawback observed with multiple coatings onto planar sub-strates (as opposed to wire) is that adhesion between layers of polyimide coats is extremely low and, therefore, the layers tend to delaminate readily. Polyimide films were also made by casting polyamic acid onto glass plates, drying under vacuum at 50-80C and converting the polyamic acid films to the corre-sponding polyimides by heating to 300C for 30 minutes. The aromatic polyimides disclosed therein are all useful in said invention.
European Patent 36,330 describes a process for con-tinuously producing an aromatic polymide film that is soluble in a phenolic-type solvent, eOg., phenol itself or mono-halogenated phenol or cresol. On the other hand, the present invention relates to a continuous process for making and shaping a phenol-insoluble, intractable polyimide. While the patent sets forth the desirability of providing a continuous process for the produ~tion of polyimides and the extrusion onto a substrate, the disclosed process does not relate to the intractable polyimides of the present invention.
Published European Application No. 48,221 purports to relate to flexible foil substrates provided with a film of I l L~3~
1. 1 ¦! polyamide, polyamide-imide or polyimide adhered thereto without an intermediate adhesive layer. However, the only data on actual results relates to the preparation of aromatic polyamides which Il are soluble and coated in their final, polymerized, form from a I solution. No further curing after the film is coated is i¦ required. The adhesion of the polyamide film to the substrate is !¦ said to be excellent. However, ~his teaching has no application ¦l to polyimides of the present invention, which are insoluble; the I insoluble polyimides must be coated on the substrate in the form 1 !~ f their soluble precursor and polymerized in situ.
il As to the insoluble polyimides, no specific teaching of Il conditions required to cure polyamic acids to polyimides in situ '¦ is found in European Application 48,221, leaving one to consult 1 other references for such conditions. And, of course, the 15 1l adhesion of the films will be no greater than that obtained by i¦ the known prior art, since there is no discussion or disclosure il of any modification of conventional processes to increase the adhesion of the polyimides mentioned. One can refer, e.g. to the '614 patent, supra, and, in particular, Example 26, which ~0 ¦ only states that "good adhesion" is observed on copper, aluminum, il steel and glass. However, it has been found that peel strengths of less than 1.7 N/cm are realized.
¦ Notwithstanding the statements just mentioned in ¦¦ European Patent Application ~o. 48,221 heretofore, it has not 25 ¦I been possible to obtain a polyimide (PI) layer of sufficient thickness, i.e., greater than about 25 um, by a continuous extrusion process permitting a direct application onto a final ¦ substrate, e.g., copper foil to obtain a laminate having the 'l required peel strength and necessary electrical properties, e.g., 30 li dielectric dissipation factor, dielectric strength, volume ¦ resistivity and insulation resistance sufEicient to satisfy 1 _4.
., , !l . -5~ 1 commercial requirements or flexible printe~ circuits ukilizing such laminates. With the intractable types of polyimides of the present invention, it has been a commercial practice to form I unsupported films of the polyamic acid precursor, cure the film jl and subsequently laminate the polyimide fil~ to a copper , Il substrate with an intermediate adhesive layer, or coat the ¦
'I , Il unsupported polyimide film with a layer of copper, e.y., by vapor deposition. This processt of course, requires an extra step plus ¦l the adhesive material used in the laminating process Moreover, , the laminate suffers the technical drawback that available adhesives will not withstand the same high temperatures that the polyimide film can and therefore applications requiring such higher temperatures have been precluded, where, for example,l ll soldering or welding of connections is required. At temperatures ' ' reached in the laminate during the soldering, the adhesive~
j; softens or melts and the polyimide film "swims" or "floats" in ;
the melted adhesive on the copper surface.
A more desirable process, which would avoid the I
li aforementioned difficulties with adhesives, would be one in which ¦
20 11 a polyimide solution was extruded directly onto the ultimate¦
¦¦ supporting sheet after which the solvent thereafter would be Il removed so as to develop a strong bond between the polyimide film ¦i and the substrate. However, the polyimides of the present ~l invention are not soluble in ordinary solvents capable ofl i! carrying out the polymerization of polyamic acid. Therefore, the¦
polyimides of the present invention cannot be extruded directly lli onto the substrate in the same way as the phenol-soluble I polyimides of the European Patent 36,330, mentioned above. On~
' the other hand, the polyimides insoluble in phenolic solvents are 3~ ,I far superior to the soluble ones as regards thermal stability.

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Polyamic acid (PAC) precursors of the aromatic polyimides are soluble, but in previous attempts to coat the l polyamic acid onto the substrate and cure the polyamic acid to a I
Il polyimide, blisters and bubbles formed, due to a combination of I
I rapid volatilization of the solvent and evolution of water formed i1 by the imidization reaction in the interior of the coating and the formation of a skin of polyimide trapping solvent and water I
as voids within the film matrix. The discontinuities thereby , Il created in the polyimide layer disrupt the electrical properties , required for printed circuit applications of the polyimide laminates, lower the mechanical properties, e.g., tensile , strength, elongation to break, etc. of the film and also reduce I
~I the strength of the bond between the polyimide film and , 'isubstrate. A surface layer of polyimide or skin may be formed ,I due to premature curins at the surface layèr of PAC and can prevent the release of volatiles (solvent or moisture) by diffusion to the surface, leading to agglomeration of volatile 1 ! molecules and the formation of voids within the polyimide layer. ¦
'l We have found we can prevent this "skin effect" by programming i 20 1I the temperature of cure so that no RI skin is formed until ¦
j! substantially all of the solvent and volatile products have been 1Idiffused to the free surface of the PI layer and released from I
,' the surface of the film. Trying to cure thick films prior to !
I` substantially complete removal of the solvent leads to the I
25 1l formation of brittle, discontinuous low molecular~weight I
polyimide film.
j Patent 3,428,602 to Haller addresses the problem of ~, blowing and blistering which is encountered in casting thicker ~ sections of polyimide materials cast as films onto a `t polytetrafluorloethylene carrier film. Haller suggests that the 'I solvent must be removed from the polyamic acid solution while , .

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maintaining the temperature below the heat curing temperature of the polyamide acid because simul-taneous removal of the solvent and conversion o~ the acid to the polyimide causes the blowing and blistering. Moreover, Haller found that after the solvent is reduced to abou-t 50%, merely continuing the heat drying process is ineffectual in lowering the solvent content fur-ther. In accordance with the teaching of Haller, after a low solids polyamide acid solution of 12-15~ (by volume) is cast into a thin film and heat dried at tempera-tures below the heat curing temperature of the polyamide aciato lower the solvent content to about 50%, the concentrated polyamide acid solution is then subjected to a shearing oper-ation, for example, in a rubber mill, with further heat drying at temperatures from about 65C up to about 149C to concen-trate the polyamide acid to 75%. The concentrated polyamideacid is then shaped, e.g., by passing it through nip rolls to form a thick sheet which is heated in a curing oven at tem-peratures ranging from about 149C to about 371C. Haller found it necessary to subject the polyamide acid to the shearing action of a rubber mill to attain sufficient surface exposure to provide the additional drying of the acid solution prior to curing. Furthermore, Haller relates to the formation of free film with greater than 250 ,um thickness, but does not relate to the direct extrusion onto a substrate which will ~5 adhere thereto with the necessary strength for finished products without the need for an adhesive layer.
German Patent 1,202,981 granted October 14, 1965 discloses a method for preparing shaped polyimide articles by gradually raising the temperature during conversion of the polyamlde acid into the polyimide. For example, in Example 16 a pigmented polyamide acid is coated onto a copper substrate and converted by heating into an insoluble polyimide by introducing the film into an oven at 100C and raising the temperature to gradually over 35 minutes to 370C. The films were said to exhibit good adhesion properties, however, the peel strength in I those films has been found to be less than 1.7 N/cm. Also, a l~ bubble-free film having a thickness of greater than about 10 'l cannot be obtained unless the temperature is increased in a ¦ certain manner as discovered by Applicants herein and is ! carefully controlled in the respective temperature zones. For ll example, as seen in Example 17 of the German patent, ten separate ~ layers are required to obtain a thickness of the coating of 0.023 ,I mm or 0.0023 mm per layer on an AWG ~25 wire.
It is an object of the invention to produce a flexible, polyimide laminate without using an adhesive layer, but one which ,, will adhere to the substrate layer with peel strength equivalent 1I to adhesive-laminates and at a cost far below that of ,I conventional adhesive-joined laminates.
Another object of the invention is to produce a polyimide film laminated to a metallic substrate, such as copper, steel, aluminum, zinc, etc , without an adhesive, by directly ~0 1l extruding a polyamic acid onto a copper sheet or foil or the like and curing the laminate thus formed, in situ; the laminate is smooth and free of defects caused by blistering and bubbles llprobably due, in prior attempts, to a too rapid volatization of ¦leither solvent from the free surface of the polyamic acid coating li during the curing process or water vapor produced by the imidization reaction or both.
, Another object is to achieve, with a single, direct i! extrusion process, a PI-Cu laminate with a polyimide layer at ~ileast 10 ~m (0.4 mils) thick, having a peel strength of at least l 4 N/cm. (N = Newton), a dissipation ~actor of 1.5 x 10 3 to 5 x 1~10-3 at 1 Khz and a dielectric strength of at least about 2 IKV/mil.
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A further object is to achieve a polyimide laminate with high peel strengths able to withstand high temperatures used in processing such lamina-tes into useful praducts, such as temperatures reached in soldering connectors to printed circuit boards made ~rom said laminates, wi-thout the necessity for pre-drying to remove water from the polyimide layer or trapped in the adhesive layer.
A further object is to produce a wire coated with a single polyimide layer greater than about 64 ~m (2 ~2 mils) and a process for coating in a single pass and curing in situ.
SUMMARY OF THE INVENTION
The ob~ects o~ the invention are achieved by forming a polyamic acid (PAC) (polyimide precursor) by the reaction of an aromatic tetracarboxylic acid, e.g., pyromelli-tic acid or its dianhydride, pyromellitic dianhydride (PMDA) and an aro-matic diamine, e g. oxydianiline (4,4' diaminodiphenylether) (ODA) in a aprotic polar organic solvent, e.g. dimethyl-acetamide (DMAc), extruding a polyamic acid film onto a sub-strate, for instance, a copper foil or wire or a polymeric film or sheet, and curing the film with a thermal treatment in at least two stages to form a laminate having a polyimide layer tightly adhered to said substrate without the need for an intermediate adhesive layer to bind the polyimide film -to the substrate.
The objects of the invention are further achieved by means of a laminate, which is characterized by the factthat the polyimide layer adheres directly to the carrier material with a peeling resistance of at least 4 N/cm, the polyimide being insoluble in phenolic solvents, while the polyimide layer exhibits a tensile strength o~ 100 to 150 N per mm2, an elongation to break of 15 to 100%, and a dissipation factor of 1.5 x 10 3 to 5 x 10 3 at lKHz. PreEerably, the layer thickness _ g _ .

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of the polyimide layer is 10 ~m to 1 mm. In an additional preferred version, -the thickness of the polyimide layer is 50 to 250 ,um, and, when the carrier material consists of fibers, metal wires or cables, preference is glven to a thickness from 30 to 250 ~m.
The laminate of the invention can be planar, that is, a flexible layer of polyimide adhered to a sheet of copper or other metal such as aluminum, nickel or steel or a continuous or finite length coating on a rod, e.g., wire, or a tubular substrate. In any case, the polyimide layer is attached firmly to the substrate and, in the case of the planar lami-nate, has a high peel strength in excess of 4.0 N/cm and, preferably, greater than 5.0 N/cm.
In general, the process of the invention includes the mixing of an aromatic diamine with an aromatic tetra-carboxylic acid or the dianhydride thereof under conditions to form a polyamic acid in solution in a solvent and extruding a thick layer of the polyamic acid solution directly onto the substrate, where the solvent is partially removed from the polyamic acid layer in a first heating zone, then additional solvent is removed and the polyamic acid layer is partially cured in situ in a second heating zone a-t a higher temperature The polyamic acid layer is then completely cured by a further heat treatment in at least a third zone at a higher imidization-reaction-completing temperature. It is believed that to obtain a thick layer of polyimide, i.e. greater than about 10 ~um, which is continuous and without defects from bubbles caused by a combination of "skin effect" and too rapid evapor-ation of either the solvent or the wa-ter vapor formed in the imidization, or curing step, which will adhere strongly -to the substrate, a specific sequence of heat treatmen-ts is essential Figure 1 illustrates the preferred apparatus shown schematically for practicing the process of the invention.
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li Figure 2 is a cross-sectional view of the curing oven taken along 'I line 2-2.
¦~ DETAILED. DESCRIPTION OF THE IMVENTIOM
., il The polyamic acid (PAC) precursors of the invention, 1. formed by the reaction of an aromatic dianhydride and an aromatic diamine in a polar organic solvent, have the following structural ;'l formula:
I O o ~ HN- C C -NH -R'' _ , n ~ i, ,~ where R is an aromatic tetravalent radical, 'i R' is a divalent aromatic radical and 1S !I n is sufficient to give a polyamic acid with a nred f 0.5 or greater in DMAc containing 0.1 mole/liter lithium !, bromide.
The PAC, after extrusion onto the subst,rate, is cured by the ¦I heating process disclosed herein to form an intractable, 20 1l insoluble polyimide having the following repeating structure:
I _ _ 11, wherein R and R' are the same as above.
.~ By "no longer moldable" within the meaning of the i' invention it is understood that these polyimides: in contrast to I
!~other known polyimides, cannot be melted without decomposition ¦
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~3~ 3 and are insoluble in conventional solvents so that they cannot be given a new shape by either dissolving or melting.
The preferred startiny materials for the preparation of the polyamic acid are pyromellitic dianhydride and oxy-dianiline and the preferred solvent i5 dimethylacetamide.
Other reactants which form intractable polyimides insoluble in conventional solvents, e.g., phenol or substi-tuted phenols (halogenated phenols), can also be extruded by the process of the invention. Among the aromatic dianhydrides within the scope of this invention are pyromellitic dianhydride (PMDA); 2,3,6,7-naphthalene tetracarboxylic dianhydride;
1,2,5,6-naphthalene tetracarboxylic dianhydride; bis (3,4-dicarboxyphenyl) sulfone dianhydride; perylene 3,4,9,10-tetra-carboxylic acid dianhydride; bis (3,~-dicarboxyphenyl) ether dianhydride.
Among the aromatic diamines useful in this invention are 4,4'-diaminodiphenyl ether; 5-amino-2-(p-aminophenyl) benzothiazole; 4-amino-2-(p-aminophenyl) benzothiazole;
5-amino-2-(m-aminophenyl) benzothiazole; 5-amino-2-(p-amino-phenyl) benzoxazole; 4-amino-2-(m-aminophenyl) benzothiazole;
p- and m-phenylene diamine; 4,4'-diaminobiphenyl; bis-(4-amino-phenyl) methane; 4-amino-2-(p-aminophenyl) benzoxazole;
5-amino-2-(m-aminophenyl) benzoxazole; 4-amino-2-(m-amino-phenyl) benzoxazole; 2,5-diamino benzoxazole; 2,5-diamino benzothiazole; etc.
Although the preferred solvent is dimethylacetamide (DMAc), other polar organic solvents such as N,N-dimethyl-methoxy acetamide, dimethylformamide (DMF), diethyl formamide, N-methyl-2-pyrrolidone (NMP), and dimethylsulfoxide (DMSO) may " 30 be used. Still others may be used, e.g., N-methyl caprolactam, dimethyl sulfone, pyridine, hexamethyl phosphoramide, N-acetyl-2-pyrrolidone, tetramethyl urea and tetramethylene-sulfone.
i~ The preparation of the polyamic acid may be per~ormed ~ in accordance with prior art teachings, e.g., the above-rnentioned '614 and '634 patents. However, the preferred process will now be discussed in further detail and will be ilustrated by reference to the drawingsO In Figure 1, a dry mixture of the ~ dianhydride and diamine is prepared in a molar ratio between 0.95 and 1.05 PMDA:ODA. The mixture is loaded into a gravimetric metering feeder 3O The mixture is then fed to an extruder-reactor 4 at a precisely controlled ra~e. A polar solvent is added to the dry mixture in the extruder reactor 4 ` through a metering pump at 5. The molar ratio of dianhydride to il diamine controls the molecular weight of the polyamic acid ',l solution. The optimal range cf molecular weight of the polyamic il acid is obtained with a molar ratio between 0.98 and 1.02, and is measured as reduced viscosity (~ red) of a 0.5~ solution in ~dimethyacetamide containing 0.1 moles per liter of lithium `1I bromide. The reduced viscosity of the polyamic acid at molecular ~! ratios between 0.95 and 1.05 is 0-S to 4.0, and is between about j'l.0 and 4.0 in the optimal ratio range of 0.98 to 1.02. At a ¦Imolar weight ratio of 0.95, the average molecular weight of the l PAC formed has been ound to be about 32,000; at 1.0, about 2S I 200,000; at 1.03, about 35,000 ~determined by an ~ICA light scattering photometer, Model PGD 42000 at ~ = 436 nm).
! The temperature in the mixer-reactor 4 should ~e maintained below about 80C, but in practice, the temperature may ~Ibe increased gradually from about 20C or in zones of increasing ~ temperature to a maximum of 80C~ The solvent is added in the first zone of extruder-reactor 4. Residence time in the ~

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extruder-reactor 4 is in the order 1 to 5 minutes. The reaction to form the polyamic acid is complete at the end o~ that residence time.
The polyamic acid solution having a reduced viscosit~
of from 0.5 to 4.0, preferably more than l.O,and most preferably 1.2 to 2, is then extruded through a slit die 6 onto a substrate 7 which may be a sheet of copper or other metal or synthetic film pulled from a coil or roll supply 8 o~ the material~
The metal sheet with polyamic acid solution coated ln thereon next passes through a curing oven 10 blanke-ted with nitrogen from a supply 11 for 5-20 minutes or longer, depending on the film thickness, since longer times are required as the thickness is increased.
It has been found essential to controlthe temperatures in successive zones in the oven, but when that is controlled ithin the limits set forth herein, an intractable, bubble-free, olyimide layer is formed on the substrate 7 in a very short time, with excellent electrical and mechanical properties, which dheres to the substrate with peel strengths greater than 4.0 ~0 /cm. While not being limited to a theoritical explanation for his surprising result, it is believed that our process requires hat the solvent diffuse through the layer of polyamic acid and e released from the free film surface s~owly enough so that olvent bubbles are not formed that could grow and be trapped ithin the polymer film matrix. More particularly, it is be-ieved that about 30% of the solvent (as determined by TGA) must be released from the free side of the PAC film before there is substantial conversion of PAC to polyimide. During this time interval the temperature must be kept below 150C and preferably elow about 130C. At least about 50~ more o the solvent should hen be removed at a temperature below about 200C, preferably below about 180 C, where the pol~imidization reaction reaches at least about 80~ complete. Also, the imidization reaction must e 80-90~ completed at temperatures below about 180C, so that the major amount of water formed by the cyclization r~action i5 also diffused to the ¦
surface of the film and releasedO
In order to accomplish the foregoing, heating zones are " established in the curing oven by electrical resistance heaters ` 12, 13, 14 and 15 so that the drying (removal of solvent) and 'i curing are effected in first and second stages, respectiv~ly, as ¦
follows: in the firs~ stage, in a first zone, ~he temperature is maintained by electrical resistance heater 12 in the range from ' 100-150C; the temperature in a second zone of the first stage is ' raised to from about 130C to about 200C, preferably less than about 180DC; in a third zone, i.e., in the first zone of the second stage, after substantially all of the solvent has diffused through the surface and removed and the major amount of water of ~ reaction has been removed, the temperature is raised to from ll about 200~ to 400C; in a fourth zone, the temperature is again l raised to from about 300-600C. Each of the zones is j approximately the same length and therefore residence time in each zone is equal, but greater laminate speeds and, hence, l throughput may be achi~eved by lengthening any of the heating I zones or by adding one or more additional heating zones to either I
of the stages. In the apparatus shown in Figure 2, the oven 10 ¦
may be constructed with a lid 16, hinged for easy access to the ¦
laminate in the oven.
,` Specific examples of preferred Eorms of the invention 1i are now set forth in detail. They are intended to illustrate, ¦
but not limit the invention.
i, Example 1 I A dry mixture of pyromellitic dianhydride (PMDA) and , oxydianiline (ODA3 was prepared in a commercial powder mixer.
~ Altogeth~er approximately 5.0 kg PMDA ~ 4.54 kg ODA (PMDA : ODA =
', 1.01) were weighed into the mixer and subsequently mixed for 48 '. I
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- . , hours at the maximum mixer speed. About 1~6 kg of the mixture ;
¦ was then discharged from the mixer into a gravimetric feeder I¦ which fed the mixture to a negative-feed, double~screw extruder i~ at a rate of about 200 gm/hr. In the first zone of the extruder, I
l maintained at 201C, DMAC is added at a rate of about 430 gm/hr. I
j1 to give a solids concentration of 31.7 % by weight~ During the ¦
!~ remainder of the residence time in the extruder, the temperature ¦
is increased in succeeding zones to a maximum temperature of ¦
~I 50C. A polyamide acid having a reduced viscosity ( n ) of 1.67 i i! was formed and extruded from the extruder barrel through a thin ¦ film die. The die orifice had a rectangular cross section with Il dimensions of 200 mm x 0.35 mm. The pressure at the die head was !¦ 85 bar. The polyamic acid was extruded onto a l oz. (thickness =
1' 35 ~m) continuous sheet oE rolled annealed copper foil (Oak , '~ F-lll) and the laminate was fed into an oven having four j temperature zones of equal length at temperatures of 140C, 180C, 350C and 400C, respectively under a nitrogen atmosphere.
The total residence time of the laminate was l0 minutes during l1 which the PAC was substantially fully converted to the polyimide j Ij (PI). The PI film was strongly adhered to the copper substrate !
¦1 and was free of bubbles and discontinuities.
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¦ Example 2 j Another l.6 kg sample of the mixture was reacted in the 1¦ same manner as in Example l and the steps repeated except that I

Il now a copper foil (Oak F~ of 70 ~m was used as the substrate. , ¦l The polyimide film was strongly adhered to the copper foil and ~

,I was free of bubbles and discontinuities. The properties of the !

!1 film of Examples l and 2 are set forth in the following table: j ~, , ~ I
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Property (Polyimide Layer) Ex. 1 Ex. 2 Test Dielectric strength,4.4 4.35 ASTM D-149 KV/mil @ 60 Hz Dielectric Constant 4.0 3.9 ASTM D-150 1 KH2 @ 25C

Dielectric ~issipation .0047 .0039 ASTM D-150 Factor, 1 KHz @ 25C
Tensile Strength, N/mm2 105 110 ASTM D882 Elongation, % 4~ 31 ASTM D882 Density, g/cc1.42 1.42 ASTM D1505 Thickness, ~m 66 61 ASTM D374 Property (laminate) Peel strengt~, N/cm 8.2 4.8 IPC TM650 2.4.9 Solder Immersion no no IPC TM650 2.4.13 (unconditioned samples) blisters blisters (slightly no de- no de- modified) laminations laminations Example 3 A 3-necked flask was charged with 8~17 g PMDA to which was added 7.58 g ODA (molar ratio PMDA:ODA = .99:1.00) dissolved in 60 g DMAc while stirring continuously at full speed. An addi-tional 29.25 g DMAc, which was used to wash the ODA flask, was added to the reaction flask. The reaction was continued with stirring for 80 minutes at a temperature of 22C under a nitrogen atmosphere. A portion of the resulting PAC solution was cast onto a 23 ~m nickel-chronium foil (INCONEL~ from Somers) which had previously been etched with a ferric chloride solution contain-ing 30 g FeCl3, 60 cc 12N HCl and 180 cc water. The cast PAC was '~I'q, ~

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,I drawn down to a thi~kness ~f 356 ~lm by a ylass rvd wound with 356 1 ~m diame~er copper ~lre,. T~e alloy ~oil w~ mounted on a gl~s ,~ plate and held with tape~ The film was dried ~'c 70~C or 20 ~ minutes and then placed in a v~cuum over~ und~r a vacu~lm of 30 inl S ,, Hg at 160~ ~ under nitrogen. The temperatute o~ the o~en was ~i 'chen raised to 310 ~C ~uring a period of 4 1/~ hour~ ~ By the tilne , the tempe~atur~ of the~ film rRaches 150C, within E~bout 1-2 '~ mlnu~e~, most of the solven~ has heen drivén off ~ a~ ~etermined ~; by obse~ing the color of the filmv ~ clear, light yellowO The 10 . cu~ed dry ~ilm had a thickne6s of ~5 ,um.
I, A~ 3ampls o PA~ ~s ma~e in Ex7~inple 1~ s diluted 'co ji 22~ b5~ wei~ht PAC ~nd a red~ced vi~c~)sity ( nred ) of 1.22, ca8t and doc~ored to a ~56 llm wet film thickn~s~ onto a ~ ~m~ copper~
~ ` nickel ~lloy foil (Cupro~Nickel 30 #71S ~rc7m Somers Thin-5trip/
15 l' Brass Group, Olin Corp~ [Bomer~, Waterb~y, Connec~i~ut) the ~u~face o~ which ha~ been ~ru~hed ~n~n~chine sc~u~b~dn). The ~ast il~n wa~ dryed and cured in the same lnann~r a~ th~ above sample .
~` Both ilms had extr~nely high peel strength, while ~
i. sim~l~r sampl~ on a ~right untre~ted alloy foil peeled eAsily ~0 ~ (less than .7 N~m).. Nei~her ~he et~hed sample nor the brll3hed il sampl~ could be sep~rated wi~hout damage to the polyimide film so ., ~s to obtain a peel ~trength m~asurement. After belng ~ubj~ct to ~emperature of 2~ûC for s~ren days, th~ polylmide f ilm on the ', brushed foil exhib~ed ~xcellent adh~ion.and flexibility.
25 " Other r~ctants which will i~orm polyimides :~rom the ¦ in~e~mediate po~ya~ic a~id, in~lu~ing ~ho~ ed herei~, ~8 ,~ known tc~ those sXilled in th~ art~ are de~n~ed to fall within ~he scope of the lnven~ion~ provided ~he polylmid~ is in~oluble in ,.
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phenols or oth~?~ known polyiml~lza~ion ~olv~nt~ Thu~, ~t is possible to lamlnate ~hose poly~.mide3 dlrectly onto a metAl ~; -cubstr~te by extruding a polyamlc a~;:id (p~ecursc7~ o~ ~ polyimide) '~ ~nd ~uring, or ~onden~ing, the polyam~ c ~Cid in sltu to ~n S l! lnsoluble, ln~ able polyi~ide, Fur'che~mOre, the~e ln~olu~le polyimidec c~n be laminated tc~ other substrAtes than tho~e li~ted abov~.

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Claims (20)

?he embodiments of the invention in which an exclusive property ?r privilege is claimed are defined as follows:-

1. A planar, flexible laminate comprising a phenol-insoluble, intractable fully aromatic polyimide layer and a supporting substrate wherein said polyimide layer is directly adhered to said substrate with a peel strength greater than 4.0 N/cm, said polyimide being insoluble in phenolic solvents, the polyimide layer having a tensile strength of at least about 100 N/mm2 and a tensile elongation of 15 to 100 percent and a dissipation factor of from 1.5 X 10-3 to 5 x 10-3, at 1 Khz.

2. The laminate of Claim 1 wherein the thickness of said polyimide layer is from 10 µm to 1 mm.

3. The laminate of Claim 1 wherein the thickness of the polyimide layer is from 50 to 250 µm.

4. The laminate of Claim 1 wherein said aromatic polyimide has the following repeating structure:

wherein R = a tetravalent aromatic radical R' = a divalent aromatic radical.

5. The laminate of Claim 4 wherein R and R' are and respectively.

6. A laminate comprising a polyimide layer of at least 10 µm thickness and a supporting substrate wherein said polyimide layer is directly adhered to said substrate with a peel strength of at least about 4.0 N/cm, the polyimide layer having a tensile strength of between 100 and 150 N/mm2, a tensile elongation of 15 to 100 percent and a dissipation factor of from 1.5 x 10-3 to 5 x 10-3 at 1 Khz, said polyimide formed by heat curing a polyamic acid composition derived from the reaction of pyromellitic dianhydride (PMDA) with 4,4' oxydianiline (ODA), in the presence of a polar organic solvent, the molar ratio of PMDA
to ODA being in the range of 0.95 to 1.05, said laminate being formed by extrusion, in a continuous process, of a solution of said polyamic acid containing at least 50% by weight of said solvent, without prior removal of substantial amounts of solvent, through a die onto said substrate, and gradually removing the solvent at one or more temperatures in the range of 100°C to 200°C and curing said film at a higher temperature to obtain said polyimide.

7. The laminate of Claim 6 wherein said reaction takes place in said extruder.

8. The laminate of Claim 6 wherein said polar organic solvent is an aprotic solvent.

9. The laminate of Claim 8 wherein said aprotic, polar, organic solvent is selected from the group consisting of dimethylacetamide, dimethylformamide, N-methyl-2-pyrrolidone, and dimethylsulfoxide.

10. The flexible laminate of Claim 6 wherein said substrate is a formed fiber, metal wire or cable.

11. The laminate of Claim 10 wherein the wall thickness of the cured polyimide coating is from 30 to 250 µm.

12. The laminate of Claims 1 or 6 wherein said substrate is a metallic foil selected from the group consisting of copper, aluminum, nickel and steel.

13. The laminate of Claims 1 or 6 wherein said sub-strate is rolled annealed copper.

14. The laminate of Claim 1 wherein the peel strength of said laminate is greater than 5.0 N/cm.

15. A process for forming shaped articles comprising a laminate consisting of a substrate and a phenol-insoluble, intractable polyimide film having a thickness of at least 10 µm formed by reacting an aromatic tetracarboxylic acid or the dianhydride thereof and an aromatic diamine in the molar ratio of tetracarboxylic acid or dianhydride to diamine in the range of 0.95 to 1.05 in a polar organic solvent to form a polymeric composition consisting of a polyamic acid having the formula:

where R is an aromatic tetravalant radical, R' is a divalent aromatic radical and n is sufficient to give a polyamic acid with a ?red of 0.5 or greater extruding a film of said polyamic acid onto a substrate, removing the solvent from said film in a first stage, the temperature of said first stage being in the range from 100°
to 200°C whereby substantially all of said solvent is removed and polyamic acid partially cured to polyimide and further curing to an insoluble, intractable polyimide thereon in a second stage, wherein the temperature of said second stage is at least 200°C, whereby at least 95% of said polyamic acid is converted to a polyimide.

16. A process for forming shaped articles comprising a laminate consisting of a substrate and a phenol-insoluble, intractable polyimide film having a thickness greater than 10.gamma.m formed by reacting an aromatic tetracarboxylic acid or dianhydride thereof and an aromatic diamine in the molar ratio of tetracarboxylic acid to diamine in the range of 0.95 to 1.05 to form a polymeric composition consisting of a polyamic acid having the formula, where R is an organic tetravalent radical, R' is a divalent radical and n is sufficient to give a polyamic acid with a ?red of 0.5 or greater extruding said polyamic acid onto a substrate and curing by continuously passing said substrate having said polyamic acid coating extruded thereon through at least two stages of increasing temperature to permit removal of at least about 80% of the organic solvent in a first stage and curing said polyamic acid coating at a higher temperature and for sufficient time to form the insoluble solid polyimide in a second stage.

17. The process of Claim 14 or 15 wherein said polyamic acid solution is continuously metered from a mixing-conveying device through a shaped orifice as a film.

18. The process of Claim 14 or 15 wherein said film is extruded onto a metallic foil or wire or cable or a formed polymeric fiber, film or sheet.

19. The process of Claim 15 or 16 wherein said film is heated in said second stage to a temperature in the range of 300 to 600°C to form an insoluble, intractable solid polyimide film.

20. The process of Claim 16 with 2 stages, wherein said first stage has first and second zones, wherein the temperature of said first zone is from 100 to 150°C and said second zone is from 150 to 200°C; and said second stage has first and second zones, wherein the temperature of said first zone is from 200 to 300°C and said second zone is from 300 to 500°C.