CA1053866A - Microcellular heterocyclic polymer structures - Google Patents

Abstract

Abstract of the Disclosure Highly useful novel microcellular polymeric structures, especially films and fibers, are prepared from certain solid heterocyclic polymers by novel techniques. The polymer is first solvent cast; the structure is then precipitated by contact with a non-solvent such as water and thereafter dried completely.

Description

-1~5386~;
The present invention relates to the preparation of polymeric articles having a microcellular structure. More parti-cularly microcellular heterocyclic polymeric articles such as films and fibers are prepared by a novel solvent/non-solvent casting technique.
Solution cast opaque films have been conventionally pre-pared by adding pigments, fillers, flame retardants and solubilizers to a solution of the film-forming material which pigment acts as an opacifying agent. Without an opacifying agent such film would be colorless or transparent. Opacif~ing agents obviously increase the costs of the resultant film and also most often embrittle the film. Furthermore, such resulting films will have no greater porosity than a film which does not have pigmentation.
Various processes have been described in the art for preparing opaque films which rely for opacity upon the presence of a large number of voids in the film. Such films may be prepared by depositing a film from an emulsion i.e., either an oil-in-water or a water-in-oil emulsion.
When a water-in-oil emulsion is used -- i.e., one in which minute droplets of water are dispersed in a continuous phase of a film forming material -- the emulsion is deposited as a coat-ing and the organic solvent which comprlses the continuous phase of the emulsion is evaporated therefrom. This causes gelation of the film-forming material and entrapment of the dispersed water droplets. The water is then evaporated leaving microscopic voids throughout the film structure.
Another technique for obtaining a porous, opaque non-pigmented film is set forth in U.S. Patent No. 3,031,328, which .

-.. - . . , - . : ..................... . . .
.

-- 1~53866 contemplates preparing a solution of a thermoplastic polymer mat-erial in a mixture of a volatile organic solvent and a volatile `non-solvent liquid which has an evaporation rate substantially less than that of the solvent. The clear homogenous solution is then coated on a suitabie backing material and drled by evaporation to produce an opaque blushed film which is adapted to be rendered lo-cally transparent by heat or pressure. These films are useful as recording films.
Other techniques for ~orming opaque, porous, non-pigmented, microporous thermosetting films are set forth in U.S. 3,655,591.
Nevertheless, in spite of the above, the art has never appreciated the unique articles which result when a specific type of polymer is cast in a certain manner to produce microcellular structures which have unique and unusual properties and are in-cidentally opaque. The art has concentrated on techniques wherein the opaqueness is the sine qua non of the structure and the other properties are not of significance.
In accordance with the invention unique microcellular heterocyclic polymeric articles such as films and fibers are pre-pared by a novel solvent/non-solvent casting technique.
It is known that films, fibers and other structures can be made out of the solvent cast heterocycllc polymers such as those described in U.S. 3,661,859. Those particular polymers are referred to as 1,3-imidazolidene-2,4,5-trione-1,3-diyl. The re-peating heterocyclic ring structure of these polymers is shown as follows:
, - 3 - , :~ .

S3~
B `
~C~

n, where n is a number from 10 to l,OO0 ' ' Related but different polymers are polyhydantoins which ;
have been described in the art. See, for instance, Netherlands, 6809916, Belgium 723,772, German 1,807,742; 1,805,955; 1,812,002;
1,812,003; 1,905,367. Polyimides are well known and are described in such publications as British 1,240,665, U.S. 3,486,934, U.S.
3,536,666, French 1,488,924, French 1,549,101, Russian 218,424, German 1,301,114, Netherlands 7,001,648 and the like.
The detailed preparation of these polymers and solutions of these polymers in suitable solvents are set forth in the above-recited patents and others also in the art.
The preferred heterocyclic polymers which are used to form the microcellular structures of the invention are character-ized by high temperature thermal stability, organic petroleum sol~
vent resistance, relatively high tensile modulus,-tensile strength and ultimate elongation with low shrinkage at high temperatures.
Furthermore, they have relatively high dielectric strengths. These properties have been found by the present in-ventor and his co-workers to offer outstanding commercial advant-ages when used as films in flexible circuitry for use in air bag circuits, light monitoring circuits, and telephone circuits, because of their ability to be soldered. They also are useable for - .- . . . -, ,, ........ ~ , . ......... , .:, . . , . .... ~ . . . .
... .. , . . . . . , . : ; .............. . . ...... . . . ..

. . ' . .' . . ~ ' ' ' ' ' ' . '. ~ ' ' , '.

:10538~
magnetic tapes (where good dimensional stability at high temper-atures is required), for fibers, such as tire cord fibers, where high tenacity and modulus are required, for moldings for èlectrical connectors and bearings where high temperatures are required, mag-net wire insulation, coatings for sh:ip cables and cookware, gIass fabrics, industrial belts and the li]ce.
However, in these applications the structure has a relatively high cost per unit of weight since it is prepared from a specialty polymer. It would be desirable to have a structural article possessing essentially the outstanding properties of the above articles so that it can be used for the applications listed above, but less dense, so that it would not cost as much per unit of weight. If products of low density and still superior properties could be obtained, it would mean that a novel new structure of out-~
standing cost-performance utility would exist.
It has now been discovered and forms the fundamental substance of the invention that such relatively low density struc-tures can be prepared and are novel themselves. The technique of preparing them is also novel and forms a portion of this invention.
If the advantages delineated above for the lower density material were all that the material contributed, its existence would be welcomed and its utility would be considered outstanding in terms of cost effectiveness. Notwithstanding the outstanding utility of the lower density material, it has been discovered that the material has additional unique properties of its own which make it extremely valuable in addition to those properties enumerated above.
For purposes of convenience, the preferred heterocylic ::

1053~66 polymer species, 1,3-imidazolidene-2,4,5-trione, i.e. polypara-banic acid, herein referred to as PP~, will be featured in the following discussion. The particular conditlons, reagents and uses are especially well suited for the PPA polymers and structures resulting therefrom. Nevertheless, :;t must be emphasized that other polymers of similar structures can be handled in an analogous manner to make structures which have at least some similar pro-perties. These latter include the soluble polyimides, polyamides, and various soluble polyhydantoins.
In general, the heterocyclic polymers of the invention will comprise sufficient repeating units of a special heterocyclic ring structure to be solid at room temperature.
The heterocyclic ring will be 5-membered and will contain carbon, and nitrogen linkages wherein at least two of the carbon linkages will be carbonyl groups, i.e. ¦¦ which are separated by a nitrogen atom.
The preferred heterocyclic ring can be schematically represented as X ..
IX/ \ IX
X X
wherein X is selected from the group consisting of:
O ., f N- and -C- = C
and wherein a minimum of two carbonyl groups are present and separated by a nitrogen atom. Examples of heterocyclic rings which fall in this class are:

:. -: . ~ . . . ,, ,:;

..

1~3866 ~c~ f_~ /c~

Other suitable polymers have repeating units as ollows:

or :

R ~ R--Aromatic or sub-stitued aromat~c ::
\ / Z nucleous.

Wherein Z is a number from 50 to 2 x 107, preferably 50 to 1 x 106.
Although casting in general is a relatively welI-kDown process, for each polymer and solvent system there are unique prob-lems brought about by the particular solvents which must be used ;::
and the properties of the polymer itself. Very generally, PPA's -`
are soluble in moderate hydrogén bonding dipolar, aprotic solvents.
This presents a practical problem in casting, since solvents which are available at a reasonable cost have relatively high boiling points and are of low volatility, except at relatively high temper-atures. The effect of theseparameters is that when PPA is cast into even relatively thin structures, a film, for instance, it is iO53~6 relatively difficult to remove the last small amounts of solvent from the structure, e.g. film.
For instance, dimethylformamide (DMF) is considered to be one of the best solvents for working with PPA solution formu-lations. It boils at 156C and its excellent solvating effect results in the fast dissolution of PPA along with the formation of low viscosity solutions.
Nevertheless, this combination of low volatility and high solvation, which characterizes a good solvent, makes the removal of the last amounts of solvent from even thin structures such as films very difficult. Therefore, film casting processes must be conducted with extremely high temperatures in order to get good solvent removal at reasonable production rates.
In order to avoid these long time intervals and high temperatures, an attempt was made to find more practical techniques that could be used to effect complete solvent removal within reasonable time parameters at relatively low temperatures. As a result of these activities, the technique of making cellular PPA
film was discovered.
Very briefly, this relatively low density PPA structure, e.g. film, which has a microcellular structure, is prepared by -first solvent casting of film. Then, prior to the complete drying, which is difficult to effect anyway, one precipitates the film by contacting the film with an antisolvent, such as water. The anti-solvent should be miscible with the solvent in the polymer solution.
In general, there are four methods according to the invention that can be used to form the novel cellular articles of the invention. These are:

~353866 (a~ Method 1 - The cast article, e.g. film or solvent extruded article, e.g. fiber, is exposed to a relative:Ly high water humidi-ty, followed by a direct water washing, followed by drying. As is true of all of the techniques, the thickness and shape of the structure is controlled by its origin-al cast or extruded thickness and shape and solids content. Precipitating the structure in a high humidity environment rather than initial direct water contact is important. The reason is that too rapid precipitation and solvent removal will cause wrinkling of the structure, which is very undesirable.
(b) Method 2 - The structure, e.g. film or fiber, is solvent cast or extruded; then it is par-tially dried to a greater or lesser extent.
This serves two purposes; prevents wrinkling and increases the density of the structure.
Then it is water washed and then dried com-pletely. The density will vary according to the amount of solvent removed in the initial drying step.
(c) Method 3 - A solvent blend is prepared from a high-boiling non-solvent and a lower-boiling good solvent, then an article of film or fiber film is cast extruded from this solvent blend 1~386~
and the article is subsequently dried. This is not a precipitation as described above for Methods 1 and 2. ~owever, the article thick-ness would be set by the initial thickness of the cast or extruded article.
(d) Method 4 - The structure, e.g., film or fiber, is solvent cast or extruded onto a suitable surface, e.g. metal, glass or vellum paper, and is introduced directly into an anti-solvent without either pre-exposure to humid atmosphere as in Method 1 or pre-drying as in Method 2.
The key to this method is the solution of an anti-solvent, with either a single solvent or multi component mixture, which is a weak anti-solvent relative to water. -The precipitation thus proceeds much more slowly and prevents surface wrinkling which would occur if a strong anti-solvent such as water were used.
It is often highly desirable to extrude directly into the weak anti-solvent instead of onto a suit-able surface.
The structure is subsequently exposed to a series of progressively stronger anti-solvents to complete the precipitation and finally washed with water and dryed. The density can be controlled by the choice of anti-solvent and the percent of polymer in the original polymer solution.
Thus, Method 1 gives a density of about 0.45 g. per cubic centimeter, Method 2 gives a density varying from about ,, 10 --.. - - ~ :

~ 38~6 0.5 to 1.1 g. per c~lbic centimeter, and Methods 3 and 4 can be used to obtain a wide range of densities. When operating in the solu-tion casting mode, the following considerations will be pertinent.
Density is largely dependent on the weight fraction of polymer in the wet film at the instant precipitation occurs.
Solutions of PPA ranging above 30 weight percent cannot be con-veniently handled in conventional solvent casting equipment due to their very high viscosity associated with high molecular weight.
The Method 1 technique contemplates the use of the most viscous solution that can be handled, i.e. ~0 to 50 weight percent PPA depending upon molecular weight of the polymer.
Method 2 permits the use of a more dilute solution with its concomitant easierhandling advantages, and relies upon the evaporation of more solvent from the film after it is cast and prior to first precipitation. This allows a wider range of densities than can be obtained by Method 1.
The practical limit which sets the maximum density which can be obtained in Method 2 is the minumum amount of solvent which must remain in the polymer in order for precipitation to occur when the cast polymer solvent structure is contacted with water or other antisolvent.
The minimum density of Method 2 is limited by the max-imum amount of solvent that can be left in the wet film at the first precipitation step, which will not cause surface wrinkles on the film or fiber surface. This will vary depending on the choice of anti-solvent.
The range of densities can be further increased by (a) calendering the resulting cellular film, (b) orienting the film .~' -. .

' ' .' . ~ . ,: ' . ' .
' ' . . ..

and the fibers to elongate and reduce diameters of the cellular portions, or (c) the use of different mechanical equipment designed to handle the extremely viscous polymer solutions, for example slot extruders. That latter approach would increase the density of the microcellular material approximately proportionately to the amount of solvent reduction in the original polymer solvent solution. Thus when solution extrusion equipment is used, much higher polymer sol-vent contents can be handled as compared to the ca`sting methods described above.
In actual operation, cellular filmmade from PPA solutions in DMF were prepared on equipment which is normaIly used to make porous cellulose acetate film for electrophoresis''applications. The equipment consisted of a doctor's knife applicator, a sixty-foot continuous stainless steel belt, and four chambers equipped to con-trol humidity, temperature and the rate of air flow.
Provisions were also incorporated to spray water onto the moving continuous belt for the purposes of initiàl precipitation and for washing the solvent from the film.
The technique used was that described for Method 1. And the polymer was PPA in 20% concentration in DMF. ~The humidity was controlled at 90-95% and the film initially precipitated due to ab-sorption of water vapor. Additional precipitation and solvent re-moval was effected by direct immersion in a water bath followed by drying.
Under these conditions of high humidity'the wet film ab-sorbs water vapor xapidly due to the hygroscropic`nature of the DMF solvent but much slower than if directly immersed in water.
Films having a uniform cellular structure were obtained having - . . . - - : :

.

- - , ~ : : ~

--- 10~3866 densities of about 0.5 g. per cubic centimeterO
The belt speed varied from about .50 feet per minute to about 2.5 feet per minute. The temperature was about 100F.
The air rate was about 900 to 1,300 cu. ft. per minute. The thick~
ness of the wet film varied from about 8 mils to about 20 mils.
The total time in the oven ranged from about 6 minutes to about 15 minutes. Generally time periods above 10 minutes and less than 20 minutes appeared to be satisfactory.
Subsequent work has been done to produce cellular films which have relatively high densities, i.e. up to l.l gms./cc.
For this, the above-described equipment was modified somewhat to permit the use of a Method 2 type approach. It re-quired the addition of heaters to the first chamber in order to provide for some gradual initial solvent removal, which is a step required to control the increase in the density of the film and to prevent wrinkling. Equipment to spray water onto the moving stainless steel belt was installed in the second chamber to pre-cipitate the film. Water sprays in the third and fourth chambers were provided in order to wash out additional solvent, e.g. N,N-dimethylformamide (DMF) prior to stripping the film off the beltand subsequent drying.
Passage of the solvent loaded film into an irregular water interface produced a non-uniform surface on the precipitated film. Therefore an air knife was installed which directed an air flow downward onto the surface of the belt and provided a relatively uniform water interface for the wet film to pass into. -This type of equipment is adequate to produce a wide range of densities.

`- 1053866 Although it is predictable that mechanical properties such as modulus and tensile strength will decrease with decreasing density, it was found that these mechanical properties were not sufficiently diminished to seriously affect the utility of the cell-ular article for many applications. Moreover, in the case of the film, the propagating tear strength tended to be as good as that for the dense film.
The dielectric constant will decrease with decreasing den-sity, and therefore the dielectric constant for the cellular prod ucts are lower than that for the dense film products. This makes the cellular film more attractive for use as insulation, e.g. for microwave circuitry, especially where transmission is to be over relatively long distances. In an analogous fashion, the lower thermal conductivity make these structures desirable for thermal insulation.
As in the case of the dense film, the cellular film also withstands commercial solder bath temperatures, i.e. 500F.
One important and highly advantageous property of the cellular film, as opposed to the dense film is that copper circuits can be electroplated directly onto the cellular film, with much higher peel strengths for the electroplated copper on the cellular film than on the dense film. -For example, peel strengths for copper electro-deposited on the dense film were in the range of about 2.5 to 3.0 pounds per inch. But peel strengths on copper electrodeposited onto the micro-cellular film were in the range of about 8 pounds per inch.
This is an extremely important, aspect of the cellular film which gives it an outstanding advantage, taken in combination :, . - ~ . ~ ,.

:. 10538~i6 ` with its o-ther properties, over dense film.
Not only are the adhesion values exceedingly high for the electroplated copper circuit on dense film, and also laminates prepared with adhesives but the use of the cellular film permits the omission of a bothersome process step~ Thus, before copper laminated onto plastic films which normally contain small quant-ities of absorbed water can be soldered, the unit must be dried to remove absorbed water. If it is not, the absorbed water tends to be driven from the film during the soldering operation, because of heating of the composite unit. This rapid generation of steam causes the copper to be delaminated from the film substrate.
When the cellular film of the invention is utili~ed, the copper does not delaminate. It is theorized that this is due to the fact that there are numerous microcellular orifices into which the water can expand and through which the water can escape along the surface and through the side cross-sections of the film.
Therefore delamination is effectively prevented. This is an ex-ceedingly useful property.
These films are much more flexible than dense film of the same thickness, which is an advantage for thick multilayer structures. -Another important feature of the structures of the invention involves selective surface etching by strong bases or acids. This removes the film covering the micropores, either com-pietely or in any pattern desired. The exposed micropores, can then be electro or chemically plated with far better adhesion. In fact, grooves can be etched into the surface into which conductive metals can be deposited with excellent adhesion to the exposed -~ 15 : . : , :.

~53~36~
micropores and with excellent separation and insulation from adjacent conductor filled grooves.
All in all, cellular film because of its combination of properties and its relatively low cost is an ideal material for flexible circuits and flat conductor cables.
Numerous other highly effective uses can be made of the cellular structures, e.g. film. For instance, it can be used as is or converted to membranes Eor both liquid and gaseous separations.
The preferred structures produced by the practice of this invention are characterized by the presence therein of a large num-ber of discrete closed cells. Substantially all of these cells or voids are less than 300 microns, and preferably less than 15 microns, in size. The average cell size and cell size distribution is governed by the conditions under which the structures are made, e.g., temperature solvent, anti-solvent, polymer solids content of casting solutions, etc. The range obtainable is from about 0.1 to 300 microns.
Unless some color-forming material has been included in the composition, such as a soluble dye, the preferred films of this invention are opaque and white. Colored films may be obtained by incorporating small amounts of dye.
A film having an apparent thickness of, for example, 10 mils will have a real solid thickness which is equal to the sum of the thickness of each wall between the discrete cells lying along a path perpendicular to the outermost planar surface of the film which may be, for example, no more than 3 mils. This property ren-ders the film of this invention, particularly those having an average cell size of less than 10.0 micron, useful as vapor or liquid per-~ l6 .
: . .

: ~ t 1(~53866 meation membranes which may be utilized for a number of applications,such as for example, in desalinization processes. Thus, the film is of sufficient apparent thickness to provide the required amount of strength; yet the total thickness of solid polymer through which a molecule must pass (i.e. the cell walls) is relatively small.
Furthermore, the diffusion per unit of time of a vapor or a liquid through a unit area of some of the films of this in-vention is far greater than that in the case of non-porous film heretofore available.
The preferred films of this invention reflect light of wavelengths above 3800 angstroms which makes them useful as visible light reflectors.
The compositions of this invention are particularly use-ful when precipitated on fibers such as fiber glass, resinous yarns, vegetable or cellulosic yarns and cords. When these fibers or cords are coated with the structures of this invention, an opaque or white fabrlc is obtained without the addition of pigments as needed in the fabric heretofore employed. These coated fabrics have very desirable flexibility. The fact that pigments such as TiO2 are not needed to obtain whiteness in fibrous fabrics is quite significant since this has been a problem in the art due to the adverse effects these pigments have on the resulting fabrics. For example, it is known that pigments such as Tio2 weaken the tensile strength of the fabric.
The fibers may be coated with the compositions of this invention by any of the first three methods described above. One method found to be suitable is to dip the fibers into a solution which contains resin, solvent and non-solvent in amounts indicated - 16a -hereinabove. Upon precipitation a fabric having the desired white-ness and softness is obtained without the addition of pigments such as TiO2.
Although the above discussion has been made with re-ference to films as discrete articles, it is to be noted that films in terms of surface coatings with unique and important properties and which are bonded to a substrate can also be produced according to the technique of the invention.
The structures of this invention may be formed as surface coating films by techniques described in Methods 1-4 above. `Thus~
they may be applied by brushing/ spraying, dipping, roller coating, knife coating, electrodepositing or calendering followed by pre cipitation and drying. They can also be extrusion coated onto substrates.
The compositions of this invention are particularly use-ful when employed in spray applications using Method 3 due to the presence of the non-solvent in the composition.
The compositions of the present invention containing the non-solvent can have a lower viscosity at corresponding solids content, thereby permitting easier atomizàtion of a higher solids content of resinous materials than compositions not containing a non-solvent. Therefore, fewer coats are necessary to obtain the de-sired thickness of film by spray application using the compositions of this invention.
As already indicated compositions of this invention may be applied as films to various types of surfaces or substrates.
These surfaces may be of the type whereby the film is to be removed by a suitable method or of the type where it is adhered to the ~inal :~~

~ l[)5386~i substrate such as the metal of an automobile. Among the more suit-able surfaces which may be coated with the cellular structures of this invention are steel, treated steel, galvanized steel, concrete, glass, fabrics, fiber glass, wood, plaster board, aluminum, treated aluminum, lead, copper, and plastics. The most preferred surfaces are metals such as treated steel and treated aluminum.
Films formed from the compositions of this invention may -- -be air dried, vacuum dried or bake dried at elevated temperatures.
Although considerable emphasis has been placed on cellu-lar film formation and applications, it is an important feature ofthis invention that cellular fibers of high strength can be pro-duced utilizing the technique of the invention.
Cellular fibers made by the conventional techniques of the art are never left in cellular form, but are remelted and orient-ed in order to eliminate the cellular structure which is considered undesirable. In this invention the polymers used have such a high modulus that the porous fibers can be used with only moderate-orientation. This gives rise to a highly porous fiber which can accept dyes extremely readily. Moreover, the fiber has the capacity to absorb moisture very easily. Thus, it will be comfortable in contact with the human body. The capability of absorbing moisture is often the difference between synthetic fabrics which may feel clammy and natural fabrics such as cotton, the latter being much more comfortable because of its water absorptive capacity. Perma-nent press fabrics can be made due to high softening temperatures of these novel fibers.
The mi~rocellular films, fibers and other structures can also beelectrocoated with various metals such as copper, - - :. . .. ~ ~ :

1053~3~;6 aluminum and the like in order to form thin conductive coatings with a minimum of coating metal.
Electrocoated structures can be used in a wide variety of decorative and utilitarian applications. These involve auto-motive trim, under-the-hood uses, and radiation shields.
Electrodeposition and chemical metalizing can also be used to coat catalytic metals such as palladium, platinum, nickel, and the like, within the porous intexstices of the structure, so that it can be used to form an extremely high surface area, artificial surface for conducting catalytic reactions at relatively high temperatures. For instance, the ability of heterocyclic st-ructures to withstand temperatures up to 300C. wauld make them effective in automobile exhausts.
Also, when used for magnetic tape the cellular film is exceptionally useful because the magnetic oxide can be bonded to the surface quite readily and easily because of footholes provided by the cellular configurations.
The cellular structures of the invention are also highly useful for specialty applications where highly tenacious painted surfaces are required.
The invention is further illustrated by the following examples:
Example 1 Utilizing the technique and apparatus described above, a series of runs was carried out in the apparatus. The solutions used for each run were as follows:
(a) 19.7 weight percent PPA, 79 weight percent , - - 19 =

..... , - - ~

.. . .

DMF and 1.3 weight percent of octabromo-biphenyl twhich is an excellent flame re- . .
tardant for PPA film);
(b) a polymer solution containing 18.8 weight percent PPA, 80 weight percent DMF and 1.2 weight percent octabromobiphenyl.
All PPA utilized in these examples was prepared from di-phenyl methane diisocyanate monomer.
The properties of the film obtained are summarized below in Table I.

_ 20 _ 105;~866 X a~ o a~
a~
I
X ~ .
O H
.
~5 ~
O U~ O
~I O ~ O
a.) Q) U~ O ~ ~ O
~1 ~1 ~ ,~ ~1 U) ~1 ta o a rl ~ ~I
~-ri ~ ~ ~ CO ~ O O
O
S~ ~ ~1 P~
E~

o O O O
a) u, o o o o o o O O
-rl a~ ~r o ~ ~ ~ ~r ~ o a~ o ,, c~
E~
H
~ U~
~m ~ ~n O o E~ ,y ,1 .0 ~ U~
E~

U) ~ r~ ~ ~CO ~-a1 ~ o ~o ~ - ., rl '~ . ' ~
.,~ ~ m o,, ~ .
,~
o o o ~ ~
- a ~1 rl A
H Iq la Ul O O O
.,1 O OO
.~, C~

The resulting level of octabromobiphenyl in the dense film was about 6 weight percent. Such a level of flame retardant resulted in an oxygen index of 32 to 34, depending on thickness and density of the film, as can be seen above.
The microcellular structures of the invention can have incorporated therein a wide variety of small particle additives and/
or fillers. These can be selected so as to fit within the pores.
The resulting structures are relatively non-brittle compared to a dense structure containing a comparable amount of filler or additive.
Illustrative examples of additives are flame retardants, antioxidants, pigments and the like.
For many uses and applications, emphasis in this application has been placed on the cellular structure per se and its many uses.
It is to be emphasized that for many applications describe`d herein where the unique properties of the cellular material are not required, films, coatings, article structures, etc., composed of the dense version of the heterocyclic polymers are useable. Some of these specific uses and applications of the dense heterocyclics are novel and unobvious in their own right.
The cellular structures will also be highly useful as membranes and separators in fuel cells which do not utilize alkaline electrolytes. The high temperature resistance, strength, perme-ability and ability to be adhered to conductors such as metals and carbon as wall as the ease of electroplating a strong adherent metal film to the microporous structures make them uniquely suitable for many fuel cell and battery component applications.
Furthermore, the ability of the dense and cellular mat-erials described herein to both adhere tenaciously to metals, carbon graphite, etc., substrates as well as their high temperature solvent and corrosion resistance to virtually all chemical substances except aprotic solvents and alkalies makes these heterocyclic polymers outstanding material for tank linings, pipe coatings and other thin film protective coatings. Their low permeability properties (denser film) are also of significance in this application.
When applied in the form of a solvent solution, the aprotic solvent is so active with respect to many of the ordinary surface contaminants such as oxidized metals, salts, dirt, grease, oil, etc., that good adhesion can be obtained without rigorous surface prepar-ation.
Another unique and highly useful application of the film coatings of these heterocyclics, especially PPA, relies on the un-usual low temperature performance of these polymers. Thus, coatings for pipes and electrical cable conduit wraps of these heterocyclics can be used in extremely adverse low temperature, environments as low as about -268C;, with no adverse effect. This permits use with liquid helium and liquid nitrogen without losing flexibility and with low dissipation factors.
This permits PPA materials (either cellular or dense), to be used as insulating and protective materials in low temperature conductors. Such low temperature conductors are the clear trend of the future and PPA should play an important role in these environ-ments.

... - . , ... .. , ~ . .. - . : , -. .

Claims (35)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A shaped, microcellular solid heterocyclic polymeric article having high temperature thermal stability, organic petroleum solvent resistance, high dielectric strength, high tensile modulus, high tensile strength, and low shrinkage at high temperatures com-prising an aromatic polymer having repeating units, each unit having one aromatic nucleus and one 5-membered heterocyclic rings sche-matically represented as:

wherein X is , or and wherein a minimum of two carbonyl groups are present and separ-ated by a nitrogen atom, said article being made by precipitation from a solution of the polymer in a solvent.

2. An article according to claim 1, wherein said article is made by initial casting of a solution of said polymer in a solvent for said polymer, followed by exposing said casting to water followed by drying to produce said microcellular article.

3. An article according to either of claims 1 and 2, where-in the heterocylic ring is a parabanic acid ring.

4. An article according to either of claims 1 and 2, in which the polymer comprises polyparabanic acid prepared from di-phenylmethane diisocyanate.

5. An article according to either of claims 1 and 2, where-in the heterocyclic ring is a hydantoin ring.

6. An article according to either of claims 1 and 2, where-in the heterocyclic ring is an imide ring.

7. An article according to either of claims 1 and 2, where-in the heterocyclic ring is an imideamide ring.

8. An article according to either of claims 1 and 2, which is a film.

9. An article according to either of claims 1 and 2, which is a rod.

10. An article according to either of claims 1 and 2, where-in said article is a fibre.

11. An article according to either of claims 1 and 2, in the form of a coating attached to a substrate in order to serve as a pro-tective surface for said substrate.

12. An article according to either of claims 1 and 2, which has a density of 0.3 to 1.5 g. per cubic centimetre.

13. A flexible printed circuit article comprising:
(a) a structure according to claim 1 in the shape of a film; and (b) a conductive circuit adhered to said film in a pre-arranged configuration.

14. A flexible printed circuit according to claim 13, wherein said conductive circuit is formed of copper.

15. A printed circuit according to either of claims 13 and 14 wherein said conductive circuit has been electrodeposited on said film.

16. An article according to claim 1, whose surface has been metal coated for decorative purposes.

17. An article according to claim 16, wherein said metal is chromium.

18. An article according to claim 16, wherein said metal is aluminum.

19. An article according to claim 1, comprising a thin microcellular film, the surfaces of which are coated with a thin layer of a catalytic material so that the resulting article is a supported catalyst structure.

20. A method of producing a shaped, microcellular solid heterocyclic polymeric article having high temperature thermal stability, organic petroleum solvent resistance, high dielectric strength, high tensile modulus, high tensile strength, and low shrinkage at high temperatures comprising an aromatic polymer having repeating units, each unit having one aromatic nucleus and one 5-membered heterocyclic rings schematically represented as:

wherein X is , , or and wherein a minimum of two carbonyl groups are present and separated by a nitrogen atom, said method comprising in combination the steps of:
(a) forming a solution of said polymer in a solvent for said polymer in a concentration not to exceed a viscosity which would make the solution too difficult to handle;
(b) casting said polymer solution onto a suitable surface to form an intermediate-stage cast structure in any suitable pre-arranged con-figuration;
(c) exposing said resulting cast structure to a non-solvent either in vaporous or liquid form;
(d) precipitating solid polymer in the presence of said non-solvent;
(e) removing solvent from said precipitated solid; and (f) recovering said solid as a shaped article.

21. A method according to claim 20, wherein said non-solvent is water.

22. A method according to claim 20, wherein said solvent is di-methylformamide.

23. A method according to claim 22, wherein at least some solvent is evaporated from said intermediate stage structure prior to exposure of said structure to non-solvent.

24. A method according to claim 239 wherein said polymer is poly-parabanic acid.

25. A tape comprising a thin, narrow film of the article of claim 1, and a dispersion of magnetic metal oxide particles in a binder adhered to the surface of said narrow film.

26. A magnetic tape comprising the article of claim 1 in a narrow, thin strip configuration having magnetic oxide particles directly embedded in its outer surface in the substantial absence of conventional binders for said magnetic oxides.

27. A method according to claim 20, wherein said suitable surface is a surface to which it is intended that the resulting structure of claim 1 will become permanently bonded.

28. A method according to claim 27, wherein said suitable surface is metallic, copper, glass, or ceramic.

29. A method according to claim 27, wherein said substrate is a porous article, such as wire, wood, paper, textiles, non-wovens and other plastics.

30. An article according to claim 1, which is paper-like in appearance and is coated with a photographic emulsion.

31. An article of clothing comprising textiles prepared from fibers consisting of the article of claim 1.

32. A photographic paper according to claim 30, wherein said emulsion is a gelatin based emulsion.

33. An article according to claim 1, which is polyparabanic acid prepared from diphenylmethane diisocyanate.

34. A composite semi-conductor article comprising a film according to claim 8, which has adhered to it a pre-arranged configuration of a metal oxide semi-conductor (MOS).

35. An article comprising an elastomeric matrix reinforced with an article according to claim 1, wherein the heterocyclic ring is a parabanic acid ring.