GB2255931A

GB2255931A - Particular filled composite film and method for making same abstract

Info

Publication number: GB2255931A
Application number: GB9210975A
Authority: GB
Inventors: Gwo S Swei; David J Arthur
Original assignee: Rogers Corp
Current assignee: Rogers Corp
Priority date: 1991-05-24
Filing date: 1992-05-22
Publication date: 1992-11-25
Anticipated expiration: 2012-05-22
Also published as: DE4217224A1; FR2678940A1; FR2678940B1; GB2255931B; GB9210975D0; JPH05301974A; JP3281411B2

Description

2 2.5 3,,j j i 1 PARTICULAR FILLED COMPOSITE FILM AND METHOD FOR MAKING

SAME The present invention relates to particulate filled polymer matrix composite materials and to methods of manufacturing the same and more particularly to thin films of highly filled polymer matrix composite materials.

Laminated electrical circuit substrates which include a conductive layer supported on a dielectric f luoropolymer matrix composite layer are known. Driven by the continuing trend toward increasing circuit density, very thin f ilms, e.g. less than about 1.0 mil, of highly filled fluoropolymeric matrix composite substrate materials having a substantially uniform microstructure have become desirable as allowing further reduction in the size of electronic circuits. It is technically and economically difficult to make such materials by known methods.

Fluoropolymer and particulate filled fluoropolymer matrix composite films are made by known papermaking, skiving, casting, melt extrusion and paste extrusion and calendering processes.

Films produced by paper making processes require fiber reinforcement and are limited to thicknesses greater than about 2 mil.

It is very difficult to produce thin high quality highly filled fluoropolymer matrix films by skiving due to abrasion of the skiving blade by the filler particles and tearing of the film associated with the resistance of the filler particles to the skiving blade.

The filler loading of films made by known casting processes is limited to less than about 15 volume percent.

The high melt viscosity of neat fluoropolymers complicates the production of fluoropolymer films by melt extrusion. Polyvinylidene fluoride (PVF 2) and polychlorotrifluoroethylene (PCTFE) are melt extrudable only within a narrow processing window. Polyvinylfluoride (PVF) film cannot be produced by melt extrusion due to thermal 2 instability. Polytetraf luoroethylene (PTFE) cannot be melt extruded due to its extraordinarily high melt viscosity. Fluorocopolymers are known which provide lower melting temperature and lower melt viscosity at extrusion temperatures, e.g. copolymers of tetrafluoroethylene with hexafluoropropylene (FEP) or with ethylene, copolymers of CTFE with vinylidene fluoride or hexafluoropropylene.

The introduction of fillers further complicates the melt extrusion of fluoropolymers. In the presence of certain fillers, especially at high filler loading level, the melt processability of the melt extrudable fluoropolymers is rapidly degraded due to the increase in melt viscosity associated with the presence of the filler or with filler-catalyzed thermal degradation of the polymer matrix.

A method of making highly filled PTFE composite materials wh ich exhibit excellent physical and electrical properties by paste extrusion and calendering is set forth in U.S. Patent 4,849,284 entitlea "ELECTRICAL SUBSTRATE MATERIAL". However, it is technically difficult and economically extremely difficult to produce thin, i.e. less than 2 mils, highly filled, i.e. greater.than about 40 fluoropolymer matrix composite films by the paste extrusion and calendering process. 25 The object of the present invention is to provide a method which overcomes the above noted deficiencies of known processing methods. In order to solve this problem, the present invention provides -a method for making a parti culate filled polymer matrix composite film, characterized-by mixing the polymer with a dispersion of a particulate filler in a carrier liquid to provide a casting composition wherein said casting composition includes relative amounts of polymer and filler effective to provide a composite film having greater than 15 volume percent filler particles; casting a layer of the casting composition onto a substrate; and 3 consolidating the cast layer to form the particulate filled polymer matrix composite film.

Viewed from another aspect the present invention provides a method for making a composite laminar electrical circuit material, characterized by forming a casting composition comprising an aqueous codispersion of fluoropolymer particles and glass or ceramic filler particles, said casting composition including relative amounts of the fluoropolymer particles and filler particles effective to provide a particulate filled fluoropolymer matrix composite film having greater than 15 volume percent filler particles; casting a layer of the casting composition onto a substrate, and drying the cast layer on the substrate to form the composite laminar electrical circuit material, said composite laminar electrical circuit material including a substrate layer and a particulate filled fluoropolymer matrix composite film bonded to the substrate layer.

Viewed from a further aspect the present invention provides a porous fluoropolymer film, comprising mixing the fluoropolymer with a dispersion of a particulate filler in a carrier liquid to provide a casting composition wherein said casting composition includes relative amounts of filler and fluoropolymer to provide a particulate filled composite film having greater than 15 volume percent filler, casting a layer of the casting composition onto a substrate, consolidating the layer to form a particulate filled fluoropolymer matrix composite film and removing the particulate filler from the consolidated layer to provide the porous fluoropolymer film.

A particulate filled fluoropolymer matrix composite article is disclosed. The article includes a fluoropolymer matrix and up to about 95 vol. % filler particles distributed throughout the matrix, wherein said particles have a maximum equivalent spherical diameter of less than about 10 microns.

4 0 In an alternative embodiment, a particulate filled fluoropolymer matrix composite article comprises a fluoropolymer matrix and up to about 95 vol. % filler particles distributed throughout the matrix wherein none of the particles have a single linear dimension greater than about 10 microns.

In a preferred embodiment, a particulate filled fluoropolymer matrix composite film comprises a nonfibrillated fluoropolymer matrix and greater than about 15 vol. percent filler particles distributed throughout the matrix. The film has a thickness less than about 2 mils and is free of visually evident pin holes or tears.

A porous fluoropolymer film comprising a nonfibrillated fluoropolymer matrix having a void volume of greater than about 15 vol. % and a thickness of less than about 2 mils is disclosed.

A method for making a particulate filled polymer matrix composite film is disclosed. The method comprises mixing the polymer with a dispersion of the particulate filler in a carrier liquid to provide a casting composition, wherein the casting composition includes relative amounts of polymer and filler effective to provide a film having greater than 15 volume percent filler casting a layer of the casting composition onto a substrate and consolidating the cast layer to form the particulate filled polymer matrix composite film.

A casting composition is also disclosed. The casting composition includes a mixture of liquid carrier, a polymeric matrix material, and particles of a filler material.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein:

FIGURE 1 shows a photomicrograph of a cross sectional view of a particulate filled polymer matrix composite film of the present invention.

FIGURE 2 shows a laminar circuit substrate made by the process of the present invention.

FIGURE 3 shows a cross sectional view of a conductive through-hole communicating between layers of a portion of a 5 laminated electrical circuit.

FIGURE 4 shows a cross sectional view taken along lines 4-4 of FIGURE 3.

FIGURE 5 shows a cross sectional view of a portion of a laminar electrical circuit.

Suitable fluorinated fluoropolymer matrix materials include homopolymers, e.g. polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE) and fluorinated copolymers, e.g. copolymers of tetrafluoro ethylene with a monomer selected from the group consisting of hexafluoropropylene and perfluoroalkylvinylethers, copolymers of tetrafluoroethylene with a monomer selected from the group of vinylidene fluoride, vinyl fluoride and ethylene, and copolymers of chlorotrifluoroethylene with a monomer selected from the group of hexafluoropropylene, perfluoroalkylvinylethers, vinylidene fluoride and ethylene. Blends of the above listed monomers are also suitable as the fluoropolymer matrix material -of the present invention.

Alternatively, polymer matrix material of the present invention may comprise a- thermoplastic or thermosetting polymer other than a fluoropolymer. Suitable alternative polymeric matrix materials include, e.g. polyolefins, polyimides, epoxy resins and cyanate esters. Specific examples of suitable polymer matrix materials include polyethylene, polymethylpenteneand polybutadiene.

The particulate filler material of the present invention may include any organic or inorganic particulate material. The terms "particulate" and "particles" as used herein are intended to include fibers. Suitable inorganic filler materials include, e.g. glass particles, ceramic particles, metallic particles, carbon particles and mineral 6 particles. Specific examples of suitable particles include glass beads, glass microspheres, glass fibers, silica particles, carbon black, titanium dioxide particles and barium titanate particles. Silica particles, particularly amorphous fused silica particles and silica particles made by a sol gel process, and glass particles, are preferred filler particles for applications, e.g. dielectric layers of laminar electrical circuits, requiring a low dielectric constant.

Specific examples of suitable polymeric particulate f illers include polymethylmethacrylate particles, polystyrene particles and polyimide particles. Suitable polymeric particles, e.g. LARC-TPI (Rogers, Corp.), P-84 (Lenzing).

The shape of the f iller particles, the size of the f iller particles and the size distribution of the f iller particles are important parameters with regard to characterizing the particle filled composite article of the present invention.

In a preferred embodiment of the present invention all particles of the particulate f iller exhibit an equivalent spherical diameter of less than about 10 microns (p). As used herein the "equivalent spherical diameter" of a filler particle is the diameter of a sphere which occupies the same volume as that occupied by the filler particle.

In an alternative preferred embodiment of the present invention, each of the filler particles exhibit no single linear dimension greater than about 10 ji.

For extremely thin films and in applications where a substantially uniform microstructure is an important characteristic of the film, it is preferred that all particles of the particulate filler exhibit an equivalent spherical diameter of less than about 5 L. Alternatively, it is preferred that all particles of the particulate filler exhibit no single linear dimension greater than 3 5 about 5 L.

7 In a preferred embodiment of the present invention, each of the filler particles is substantially spherical. The use of spherical filler particles provides improved processability by minimizing the filler surface area for a given particle size and filler loading. Furthermore, spherical particles provide isotropic properties to the film since the spherical particles do not become oriented during processing.

In a preferred embodiment of the present invention, the filler particles of the film are of a uniform size. The use of a monodisperse filler, i.e. wherein all the filler particles are of substantially the same size, provides a more homogeneous film having substantially uniform properties throughout.

In a particularly preferred embodiment of the present invention, the filler particles comprise spherical silica particles of a substantially uniform size, i.e. all particles within plus or minus 10 % of a nominal particle diameter. A pure silica powder known as GELSILR produced by Geltech, Inc. USA and. specified as 1 micron sphere size (+/- 10 %); density of 2.2 grams/cm2; free of hard agglomerations, has been found to be particularly suitable for use in the practice of the present invention.

The particulate filler material may be treated with a surface treatment to improve the moisture resistance and improve the mechanical properties of the composite film of the present invention.

The hydrophobic coating of the present invention may comprise any coating material that is thermally stable, exhibits a low surface energy, and improves the moisture resistance of the composite of the present invention. Suitable coating materials, include conventional silane coatings, titanate coatings and zirconate coatings. Preferred silane coatings include: phenyltrimethoxysilane, phenyltriethoxysilane, 3,3,3-trifluoropropyltrimethoxysi- lane, (tridecafluoro-1,1,2,2-tetrahydrodecyl)-l-triethoxy- 8 S) silane and mixtures thereof. Further examples of suitable fluorinated silane compounds are set forth in U.S.

Application Serial N" 279,474, filed December 2, 1988 and entitled "FLUOROPOLYMER COMPOSITE" - (now U.S. Patent Suitable titanate coatings include neopentyl(diallyl)oxytrineodecanoyl titanate, neopentyl(diallyl)oxytri(dioctyl)phosphate titanate. Suitable zirconate coatings include: neopentyl(diallyl)oxytri(dioctyl)pyrophosphate zirconate and neopentyl(diallyl)- oxytri (N- ethyl ened i amino) ethyl zirconate. Further examples of suitable titanate and zirconate coatings are set forth in European Patent Application, Serial No 91 101 884.4.

The hydrophobic coating is used in an amount effective to render the surfaces of the filler particles hydrophobic and compatible with the matrix material. The amount of coating relative to the amount of inorganic particles coated will vary with the surface area coated and density of the inorganic particles. Preferably, the coated inorganic particles of the present invention include from about 0.5 parts by weight (pbw) hydrophobic coating: 100 pbw inorganic particles to about 25 pbw hydrophobic coating: 100 pbw inorganic particles. - The polymer matrix material of the present invention is mixed with a first carrier liquid. The mixture may comprise a dispersion of polymeric particles in the first carrier liquid, a dispersion, -i.e. an emulsion, of liquid droplets of the polymer or of a monomeric or oligomeric precursor of the polymer in the first carrier liquid or a solution of the polymer in the first carrier liquid.

The choice of the first carrier liquid is based on the particular polymeric matrix material and the form in which the polymeric matrix material is to be introduced to the casting composition of the present invention. If it is desired to introduce the polymeric material as a solution, a solvent for the particular polymeric matrix material is chosen as the carrier liquid, e.g. N-methyl pyrrolidone 9 (NMP) would be a suitable carrier liquid for a solution of a polyimide. If it is desired to introduce the polymeric matrix material as a dispersion, then a suitable carrier liquid is a liquid in which the matrix material is not soluble, e.g. water would be a suitable carrier liquid for a dispersion of PTFE particles and would be a suitable carrier liquid for an emulsionf of polyamic. acid or an emulsion of butadiene monomer.

Preferably, a fluoropolymeric matrix material is introduced as an aqueous dispersion. A dispersion of PTFE in water known as TeflonR TE 30 made by DuPont has been found to be particularly suitable for use in the practice of the present invention.

A dispersion of the particulate filler of the present invention in a suitable second carrier liquid, i.e. a liquid in which the filler is not soluble. The second carrier liquid may be the same liquid or may be a liquid other than the first carrier liquid that is miscible with the first carrier liquid. For examjle, if the first carrier liquid is water, the second carrier liquid may comprise water or an alcohol. Preferably, the second carrier liquid is water.

The dispersion of filler particles may include a surfactant in an amount effective to modify the surface tension of the second carrier liquid to enable the second carrier liquid to wet the filler particles. Suitable surfactant compounds include ionic surfactants and nonionic surfactants. Triton X-100 (Rohm & Haas) has been found to be a suitable surfactant for use in aqueous filler dispersions.

Preferably-, the filler dispersion comprises from about 10 volume percent (vol %) to about 50 vol % filler particles, from about 0.1 vol % to about 10 vol % surfactant, with the remainder comprising the second carrier liquid.

The mixture of the polymeric matrix material and first carrier liquid and the dispersion of the f iller particles in the second carrier liquid are combined to form the casting composition of the present invention. Preferably, the casting composition comprises from about 10 vol. % to about 60 vol. % of the combined polymeric matrix material and f iller particles and from about 40 vol. % to about 90 vol. % combined f irst and second carrier liquids. The combined amount of polymeric matrix material and f iller particles may include from 15 vol. % to about 95 vol. % filler particles. Preferably, the combined amount of polymeric matrix material and filler particles includes from about 30 vol. % to about 70 vol. % filler particles. Most preferably, the combined amount of polymeric matrix material and filler particles includes from about 40 vol. % to about 65 vol. % filler particles.

The viscosity of the casting composition of the present invention is adjusted by the addition of suitable viscosity modifier, chosen on the basis of its compatibility in a particular carrier liquid or mixture of carrier liquids, to retard separation, i.e. sedimentation or flotation, of the filler particles from the casting composition and to provide a casting composition having a viscosity compatible with conventional casting equipment. Conventional thickening agents are suitable and are chosen on the basis of the carrier liquid selected. Conventional viscosity modifiers suitable for use in aqueous casting compositions include, e.g. polyacrylic acid compounds, vegetable gums and cellulose based compounds. Specific examples of suitable viscosity modifiers include polyacrylic acid, methyl cellulose, polyethyleneoxide, guar gum, locust bean gum, sodium carboxymethylcellulose, sodium alginate and gum tragacanth.

A minimum viscosity of the viscosityadjusted casting composition is defined according to Stoke's law, i.e.

g Dp 2 ( 0 p P L) wherein v 9 D p 119- p = L = = terminal velocity of particle = gravitational constant particle diameter particle density liquid density; and l = liquid viscosity on the basis of the size and density of the f iller particles to provide a theoretical terminal viscosity, i.e. a filler separation rate, that provides a casting composition that is stable within the time period of interest, i. e. the time period between mixing the casting composition and consolidating the film cast from the composition. For example, substitution according to the above relationship reveals that an aqueous solution having a viscosity of 10 Pa.s (10,000 cp) provides a theoretically terminal velocity of 6.5 x 1010 cm/s for silica particles (density = 2.2 g/cm3) having a diameter of 1 pm. The viscosity of the viscosity-adjusted casting composition may be further increased, i.e. beyond the minimum viscosity, on an application by application basis to adapt the casting composition to the selected casting technique. For example, a viscosity of about 0,3 Pa. s (300 cp) to about 1 Pa.s (1 000 cp) is preferred for metering rod casting.

Preferably, the viscosity adjusted casting composition exhibits a viscosity between about 0,01 Pa.s (10 cp) and about 100 Pa.s (100,000 cp). Most preferably, the viscosity adjusted casting composition exhibits a viscosity between about 0,1 Pa.s (100 cp) and 10 Pa.s (10,000 cp).

Alternatively, the viscosity modifier may be omitted if the viscosity of the carrier liquid is sufficient to provide a casting composition that does not separate during the time period of interest. Specifically, in the case of 12 extremely small particles, e.g. particles having an equivalent spherical diameter less than 0.1 11, the use of a viscosity by modifier may not be necessary.

A layer of the viscosity-adjusted casting composition is cast on a substrate by conventional methods, e.g. dip coating, reverse roll coating, knife-over-roll, knife-overplate, and metering rod coating.

Suitable substrate materials include, e.g. metallic films, polymeric films or ceramic films. Specific examples of suitable substrates include stainless steel foil, polyimide films, polyester films, fluoropolymer films. The carrier liquid and processing aids, i.e. the surfactant and viscosity modifier, are removed from the cast layer, e.g. by evaporation and/or by thermal decomposition, to consolidate a film of the polymeric matrix material and the particulate filler. Preferably, the particulate filled polymeric matrix composite film of the present invention is consoli_dated by heating to evaporate the carrier liquid. 20 The composition of the consolidated film corresponds to that of the combined amount of polymeric matrix- material and filler particles set forth above in regard to the casting composition, i.e. the film may compris e from 15 vol. % to about 95 vol. % f iller particles and from about 5 vol. % to 85 vol. % matrix material, preferably comprises from about 30 vol. % to about 70 vol. % filler particles and from 30 vol. % to about 70 vol. % matrix material and most preferably comprises from about 40 vol. % to about 65 vol. % filler particles and from about 35 vol. % to about 60 vol. % matrix material.

The consolidated film of polymeric matrix material and particulate filler may be further heated to modify the physical properties of the film, e.g. to sinter a thermoplastic matrix material or to cure and/or post cure a thermosetting matrix material.

13 The process of the present invention allows films having thickness below about 2 mils, and even below about 1 mil, to be economically produced. Film thicknesses are set forth herein in terms of "mils", wherein one mil is equal to 0.001 inch or to 0,0254 mm.

Since the process of the present invention allows thin films to be produced without deforming the film, e.g. without calendering or expanding the film, fluoropolymer matrix films can be made without the fibrillation of the matrix material characteristic of expanded films and without the associated danger of tearing or forming pinholes in the film.

If a porous film is being produced, the filler material is removed from the consolidated film. The method of removal is dependent upon the choice of filler material. If a filler material is dispersed in a matrix material wherein the matrix exhibits much higher temperature resistance then the filler material, e.g. a polymethylmethacrylate filler in a PTFE matrix the filler material can be removed thermally during the consolidation and sintering steps. Alternatively, the filler material can be dissolved in a liquid in which the filler is soluble, but within the matrix material is insoluble. Removal of filler materials from a fluorinated polymer matrices to form a porous fluoropolymer film is described in U.S. Patent N' 4,987,274, entitled "COAXIAL CABLE INSULATION AND COAXIAL CABLE MADE THEREWITH".

It should be noted that the fluoropolymer matrix of the thin porous fluoropolymer film of the present invention does not exhibit the fibrillar structure characteristic of expanded porous PTFE films.

The substrate and consolidated film may be used in combination as a laminar composite material or as a substrate for subsequent composite layers. Alternatively, the substrate may be removed from the film. The substrate may be destructively removed, e.g. by dissolution in a 14 solvent, by chemical reaction, or thermal degradation, or the substrate may be reusably removed, e.g. by overcoming interfacial adhesion between the cast film and the substrate.

The consolidated film may be used alone, e.g. as discussed below, or as the substrate for subsequent casting of further layers of the casting composition to build up a multilayer film.

The thin particulate filled fluoropolymeric matrix composite of films of the present invention have a wide range of potential applications.

A laminar circuit substrate made by the process of the present invention is shown in FIGURE 2. The substrate comprises a conductive layer 2 laminated with a particle filled fluoropolymer composite layer 4. The laminar substrate shown in FIGURE 2 can be made, e.g. by casting and consolidating layer 4 on a layer of the conductive film 2.

As discussed above, the shape, size distribution of the filler particles is very important in several applications.

Film thickness is a very important consideration in applications where a particulate filled film is used as a dielectric substrate for a high density laminar electrical circuit. As the feature dimensions of the circuit are decreased, it becomes desirable to correspondingly reduce the thickness of the dielectric layer to thereby preserve the characteristic impedance of the circuit.

In general, it is desirable that the longest characteristic dimension of the filler particles is significantly, e.g. by a factor of 10, less than the thickness of the particulate filled film to avoid bridging of particles between the surfaces of the film. Asthe desired film thickness decreases it becomes progressively more difficult to meet the criterion.

Another outgrowth of increasing circuit density is illustrated in FIGURES 3 and 4. A portion of a conventional is laminar circuit 6 includes conductive layer 8 encased within dielectric layers 10, 12. A conductive through hole, i.e. a "via", is defined by a conductive sleeve 14. The conductive sleeve 14 is separated from the conductive layer 8 by an insulating sleeve region 16 of dielectric material formed by fusing the edges of the dielectric layers 10, 12. With increasing circuit density it is desirable to decrease the dimension of the conductive through hole. As the distance between the conductive sleeve 14 and the conductive layer 8 is decreased, filler size must be correspondingly decreased to avoid bridging of filler particles between the conductive sleeve 14 and conductive layer 8.

A further aspect of increasing circuit density and decreasing via hole diameter is the need to-drill smaller via holes. As the via hole diameter decreases, filler particle size becomes an important factor in hole quality. Laser drilling of particulate filled fluoropolymer matrix films is described in coassigned U.S. Patent No 4, 915,981, entitled "METHOD OF LASER DRILLING FLUOROPOLYMER MATERIALS". Laser drilling is effective at removing precise amounts of matrix material but literally blasts whole chunks of filler particles from the hole being drilled. Consequently, the use of small filler particles allows higher quality, i.e. more precisely defined, small diameter holes to be laser drilled.

FIGURE 5 shows a portion of a laminar electrical. circuit 16 which includes a bondply layer 18 sandwiched between a pair of laminated circuit layers each comprising a conductive layer 22.supported on a dielectric layer 20. The casting process of the present invention can be used with a lower melting fluoropolymer matrix material, e.g. FEP, to produce very thin, highly filled bondplys for use in laminar electrical circuits. The ability to make a highly filled fluoropolymer matrix bondply would provide electrical properties similar to those of the substrate 16 layers being bonded, while the ability to make a very thin bondply would minimize the detrimental effect of the bondply layer on the dimensional stability of the laminar circuit.

The ability to produce very thin, highly filled fluoropolymer films with an ordered microstructure is also of advantage in regard to circuitsubstrate materials for llmm-wavell applications which require very thin films having physical and electrical properties which, given the frame of reference provided by the short wavelength radiation involved, can be considered substantially uniform for purposes'lof the particular application.

An inorganic particle filled matrix composite layer may be cast onto a polyimide f ilm to produce a composite film suitable for use in flex circuit applications. A flexible circuit including a microglass reinforced fluoropolymer matrix compqsite layer sandwiched between a polyimide film and a copper conductive pattern is described in U.S. Patent N' 4,634,631 entitled "FLEXIBLE CIRCUIT LAMINATE AND METHOD OF MAKING THE SAME".

Porous f ilms produced by the process of the present invention have a wide range of potential applications, e.g. filtration membranes, breathable fabrics.

-EXAMPLE 1

Thin films of particulate filled fluoropolymeric matrix composite material were made according to the process of the present invention.

Filler particles were pretreated with a silane coupling agent. A mixture of 6 parts by weight coated silica particles, 4 parts water and 0.05 parts surfactant (Triton X-100) was ball milled for 12 hours to form aqueous filler dispersion.

An aqueous dispersion comprising 6 parts by weight fluoropolymer particles in 4 parts by water was mixed with the aqueous filler dispersion in relative amounts appropriate to give the f iller loadings set forth below in TABLE 1.

The viscosity of the aqueous dispersion of polymer and filler was adjusted by adding a sufficient amount of a 5 polyacrylic acid, a viscosity modifying agent (Acrysol ASE 75, available from Rohm & Haas U.S. A.) to provide a coating composition having a viscosity of 1 Pa.s (1000 cp).

The viscosity-adjusted casting composition was cast on to a 2 mil thick stainless steel foil with a laboratory scale knife-over-plate coater apparatus. The cast film was dried at 2880C for 1 hour and sintered at 3710C for 10 minutes.

The matrix material, filler material, coating material, the amount of coating (expressed as weight percent of filler particles) and the film composition expressed as vol. % coated filler/vol. % matrix material, for a number of sample film compositions are given in TABLE 1.

The water absorption (24 hour immersion in 50C water) and specific gravity of sample film composition 1 and 4 were determined and are also set forth in TABLE 1.

TABLE 1

SINGLE FILM 1 2 3 4 5 6 7 9 COMPOSITION MATRIX PTFEa PTFEa PTFEa PTFEC PTFEa PTFEa PTFEa FEpb PTFEC MATERIAL FILLER SILICAd SILICAe GLASSE SILICA9 T'02 h SILICAi BARIUMi SILICAd SILICAd MATERIAL TITANATE COATING SILANEk SILANEk SILANEk SILANE1 SILANEm SILANEm SILANEM SILANEk SILANEk MATERIAL COATING 2 % 10 % 2 % 10 4 % 2 % % 2 % 2 % AMOUNT (wt % filler) VOL % FILLER/ VOL % MATRIX 50/50 45/55 45/55 40/60 40/60 65/45 40/60 40/60 40/60 MATERIAL WATER ABSORPTION 0.2861 0.7 OF FILM SPECIFIC GRAVITY 2.0651 2.14 OF FILM a) TeflonR TE-30 (DuPont) b) TeflonR FEP 120 (DuPont) c) Algoflon (Ausimont) d) GPL-10, 10 p nominal particle size (Harbison-Walker) e) amorphous fused silica, 1 p nominal particle size (Industrial Corp.) f), glass beads, 2 ji nominal particle size (Potters Corp.) g) GELSILR, 1 p monosize sol gel silica (Geltech) h) Ti Pure, 0.2 p particle size (DuPont) i) amorphous silica, 20)1 nominal particle size (Denka) j) 1 A nominal particle size (TAN Ceramic) k) phenyltrimethoxysilane 1) 3:1 mixture of phenyltrimethoxysilane/(tridecafluoro, 1, 1, 2, 2, tetrahydrodecyl)-ltriethoxysilane (Petrarch Systems) (tridecafluoro, 1, 1, 2, 2, tetrahydrodecyl)-l-triethoxysilane (Petrarch Systems) 1 v 19 The tensile properties of sample film of composition 1 were determined. The film exhibited a tensile strength of 50.105 Pa (0.731 kpsi), an elongation at break of 167.4 % and a tensile modulus of 320.105 Pa (4.62 kpsi).

A photomicrograph of a cross section of film composition 4 is shown in Figure 1.

EXAMPLE 2

A porous thin film is made by the process set forth in Example 1 by using polyme thylme thacryl ate particles having a nominal size of 0.2 p as the filler material to produce a 1.0 mil thick film having a filler loading of 60 volume percent. The polymethylmethacrylic particles are thermally decomposed during the consolidation and sintering steps to produce a porous PTFE film having a void volume of 60 percent.

EXAMPLE 3

Examplary casting compositions were continuously cast on a substrate using a # 21 Mayer Rod. The f ilms were consolidated by heating at 250 "C. The film composition, substrate, substrate speed and film thickness are set forth in TABLE 2.

TABLE 2

FILM SUBSTRATE COATING FILM COMPOSITION SPEED (fpm) THICKNESS (mils) 8 0.001" KAPTON 5 0.45 8 0.001" KAPTON 7.7 0.45 9 0.001" KAPTON.1, 11.7 0.5 1 0.001" STAIN- 10 0.45 LESS STEEL 9 0.001" STAIN- 6.0 0.8 LESS STEEL 1 fpm = 0.508 cm/s The above described films were of high quality and free of visually evident defects, e.g. pinholes, fisheyes and tears.

EXAMPLE 4

Filled fluoropolymer films were continuously cast on Mylar, Kapton and PTFE substrates and consolidated in a two zone (with zones set at 65'C and 121'C respectively) inline oven.

The casting compositions were made by mixing an aqueous fluoropolymer by mixing an aqueous fluoropolymer dispersion with an aqueous dispersion of coated filler particles to give the relative amounts of fluoropolymer and filler particles set forth in TABLE 3. The matrix material, filler material, coating, surfactant viscosity modifier, pH and viscosity for each film composition cast are set forth in TABLE 3. The amount of polymer matrix material, filler and surfactant are set forth in parts, the amount of coating is set forth as a weight percent of filler particles and the amount of viscosity modifier solids are expressed as a weight percent of total casting composition.

TABLE 3

COMPOSITION NO. 10 11 12 13 14 is 16 17 POLYMER MATRIX 60a 60b 60b 60b 60b 60b 60b 60c (parts) FILLER 40d 40d 40d 40d 40e 40e 40f 40d (parts) COATING AGENT9 2 2 2 6 8 8 10 6 (wt % filler) h SURFACTANT 3 3 3 3 3 3 3 3 (parts) VISCOSITY MODIFIER 0.20i 0.159i 0.129i 0.095k 0.129i 0.139' 0.170 0.1 503 (wt %) pH 9.2 8.0 8.8 7.5 8.8 5.2 9.5 9.6 VISCOSITY 465 460 420 425 390 380 450 380 (cp) a) b) c) d) e) f) g) h) i) j) k) FEP (FEP 120, DuPont) PTFE (D 60 Ausimont) PTFE (TE-30, DuPont) silica, Harbison-Walker 10 ji nominal particle size silica, Harbison-Walker 8 p nominal particle size silica, Harbison-Walker 1 p nominal particle size 1:3 inixture phenyltrimethoxysilane/(tridecafluoro, triethoxysilane (petrarch Systems) Triton X-100 ASE 75 (Rohm & Haas) ACRESOL GS (Rohm & Haas) Carbopol C 934 (B.F. Goodrich) (1 cp = 10-3 Pa.s) W drodecyl)-1- 1, 1, 2, 2, tetrahy 22 The composition number, substrate, coating rate (in feet per minute, 1 fpm = 0,508 cm/s), oven temperature (expressed as first zone/second zone) and the thickness of the film for films cast using the pan fed knife-overroll apparatus are given in TABLE 4 and for film cast using the nip fed three roll reverse coating apparatus are given in TABLE 5.

23 TABLE 4:

KNIFE-COATED COMPOSITION NUMBER 10 11 13 15 SUBSTRATE MYLAR MYLAR MYLAR MYLAR COATING RATE linear feet sub- 3, 5, 10 5 5 5 strate coated/ 15, 25, 35 minute (fpm) OVEN TEMPERATURE 150/250 120/150 120/150 150/250 (OF) FILM THICKNESS 0.9 - 1.0 0.6 - 0.7 0.5 1.20 (mils) COMPOSITION NUMBER 10 15 16 17 SUBSTRATE KAPTON KAPTON KAPTON KAPTON COATING RATE 5 5 5 5 (fpm) OVEN TEMPERATURE 120/150 120/150 120/150 120/250 (OF) FILM THICKNESS 0.7 - 0.9 0.8 1.0 0.8-0.9 (mils) COMPOSITION NUMBER 10 11 SUBSTRATE PTFE PTFE COATING RATE 5 5 (fpm) OVEN TEMPERATURE 150/250 120/550 (OF) FILM THICKNESS 1.1 0.5, 1.0, 2.0 (mils) TABLE 5:

3 Roll-Coated COMPOSITION NO. 14 11 15 16 17 SUBSTRATE Stainless Stainless Stainless Stainless MYLAR Steel Steel Steel Steel COATING RATE 5 5 5 5 5 (fpm) OVEN TEMPERATURE 120 / 250 120 250 120 250 120 250 120 250 (OF) FILM THICKNESS 1.0, 1.3, (mils) 1.5, 1.7 --- --- --- --- The above films cast using the knife-over-roll and reverse roll were of high quality and free. of visually evident defects, e.g. pinholes, fisheyes and tears.

26

Claims

1. A method for making a particulate filled polymer matrix composite film, characterized by mixing the polymer with a dispersion of a particulate filler in a carrier liquid to provide a casting composition wherein said casting composition includes relative amounts of polymer and filler effective to provide a composite film having greater than.15 volume percent filler particles; casting a layer of the casting composition onto a substrate and consolidating the cast layer to form the particulate filled polymer matrix composite film.

2. The method as claimed in claim 1 characterized in that the carrier liquid comprises water.

3. The method as claimed in claim 1 or 2 characterized in that the polymer comprises a fluoropolymer.

4. The method as claimed in claim 3 characterized in that the fluoropolymer comprises polytetrafluoroethylene, polychlorotrifluoroethylene, a copolymer of tetrafluoroethylene and a monomer selected from the group consisting of hexafluoropropylene and perfluoroalkyl-vinylethers, a copolymer of tetrafluoroethylene and a monomer selected from the group consisting of vinylidenefluoride, vinyl fluoride and ethylene, or a copolymer of chlorotrifluoroethylene and a monomer selected from the group consisting of hexafluoropropylene, perfluoroalkylvinylethers, vinylidenefluoride, vinyl fluoride and ethylene.

5. The method as claimed in any one of the claims 1 to 4 characterized in that the filler particles comprise inorganic particles, organic particles or a mixture thereof.

6. The method as claimed in claim 5 characterized in that the filler particles comprise inorganic particles selected from the group consisting of glass particles, ceramic particles, metallic particles and mineral particles.

j 27

7. The method as claimed in claim 5 characterized in that the filler - particles comprise organic particles selected from the group consisting of synthetic polymeric particles.

8. The method as claimed in claim 5 characterized in that the filler particles comprise coated inorganic filler particles, said filler particles including an inorganic core and a coating layer surrounding the core.

9. The method as claimed in claim 8 characterized in that the coating layer is selected from the group of silane coatings, zirconate coatings and titanate coatings.

10. The method as claimed in any one of the claims 1 to 9 characterized in that the f iller particles have a maximum equivalent spherical diameter of less than about 10 P.

11. The method as claimed in any one of the claims 1 to 9 characterized in that none of the filler particles exhibit a single linear dimension greater than 10 p.

12. The method as claimed in any one of the claims 1 to 10 characterized in that each of the filler particles is spherical.

13. The method as claimed in any one of the claims 1 to 12 characterized in that all of the filler particles are of substantially the same particle size. 25

14. The method as claimed in any one of the claims 1 to 13 characterized in that the mixing step comprises mixing a dispersion of particles of the polymer in a first carrier liquid with the dispersion of the particulate filler in a second carrier liquid and the carrier liquid of the casting composition comprises a mixture of the first and second carrier liquid.

15. The method as claimed in any one of the claims 1 to 14 characterized by adding a surfactant to the carrier liquid to modify the surface tension of the carrier liquid so that the carrier liquid wets the filler particles.

28

16. The method as claimed in claim 15 characterized in that the surfactant is selected from the group consisting of ionic surfactants and nonionic surfactants.

17. The method as claimed in any one of the claims 1 to 16 characterized by adjusting the viscosity of the casting composition to retard separation of the particulate filler from the composition.

18. The method as claimed in claim 17 characterized in that the viscosity is adjusted by adding an effective amount of viscosity modifier selected from the group consisting of polyacrylic acid, vegetable gums and cellulose-based compounds.

19. The method as claimed in claim 17 or 18 characterized in that the viscosity adjusted casting composition exhibits a viscosity between about 0,1 Pa.s (100 cp) and about 10 Pa.s (10,000 cp).

20. The method as claimed in any one of the claims 1 to 19 characterized in that the layer is cast by dispersion coating, knife-over-roll, roll-toroll coating, reverse roll coating or metering and rod coating.

21. The method as claimed in any one of the claims 1 to 20 characterized in that the substrate comprises a metallic foil, a polymeric film or a ceramic film.

22. The method as claimed in any one of the claims 1 to 21 characterized by removing the substrate from the film subsequent to the step of consolidating.

23. The method as claimed in claim 15 or 16 characterized in that the step of consolidating the layer comprises evaporating the carrier liquid and thermally decomposing the surfactant.

24. The method as claimed in claim 23 characterized in that the step of consolidating further comprises sintering the polymer matrix.

25. The method as claimed in claim 18 characterized in that the step of consolidating the layer comprises 29 evaporating the carrier liquid and thermal decomposing the viscosity modifier.

26. The method as claimed in claim 25 characterized in that the step of consolidating further comprises sintering the polymeric matrix.

27. A particulate filled polymer matrix composite film obtained by the method as claimed in any one of the claims 1 to 26.

28. The film as claimed in claim 27 characterized in that said particles have a maximum equivalent spherical diameter of less than about 10 ji.

29. The f ilm as claimed in claim 28 characterized in that the f ilm has a thickness of less than about 2 mil (0,051 mm).

30. The f ilm as claimed in claim 28 characterized in that the article comprises a f ilm having a thickness of less than about 1 mil (0,025 mm).

31. A casting composition for casting a particulate filled fluoropolymer matrix composite film according to the method as claimed in any one of the claims 3 to 26.

32. A method for making a composite laminar electrical circuit material characterized by forming a casting composition comprising an aqueous codispersion of fluoropolymer particles and glass or ceramic filler particles, said casting -composition including relative amounts of the fluoropolymer particles and filler particles effective to provide a particulate filled fluoropolymer matrix composite film having greater than 15 volume percent filler particles, casting a layer of the casting composition onto a substrate and drying the cast layer on the substrate to form the composite laminar electrical circuit material, said composite laminar electrical circuit material including a substrate layer and a particulate filled fluoropolymer matrix composite film bonded to the substrate layer.

33. The method as claimed in claim 32 characterized in that the glass or ceramic filler particles comprise coated filler particles having a glass or ceramic core and a coating layer surrounding the core.

34. The method as claimed in claim 33 characterized in that the coating layer is selected from the group of silane coatings, zirconate coatings and titanate coatings.

35. The method as claimed in claim 32, 33 or 34 characterized in that the f iller particles have a maximum equivalent spherical diameter of less than about 10 ji.

36. A laminar composite electrical material as claimed in any one of the claims 32 to 35.

37. A method for making a porous fluoropolymer film, comprising mixing the fluoropolymer with a dispersion of a particulate filler in a carrier liquid to provide a casting composition wherein said casting composition includes relative amounts of filler and fluoropolymer to provide a particulate filled composite film having greater than volume percent filler, casting a layer of the casting composition onto a substrate, consolidating the layer to form a particulate filled fluoropolymer matrix composite film and removing the particulate filler from the consolidated layer to provide the porous fluoropolymer film.

38. The method as claimed in claim 37 characterized in that the carrier liquid is water and the casting composition comprises an aqueous codispersion of fluoropolymer particles and filler particles.

39. The method as claimed in claim 37 characterized in that the filler particles comprises polystyrene particles or polymethylmethacrylate particles.

40. The method as claimed in claim 37 characterized in that the filler particles have a maximum equivalent spherical diameter of about 10 L.

31

41. The method as claimed in claim 37 characterized in that the filler particles have a maximum equivalent diameter less than 5 L.

42. The method as claimed in claim 37 characterized in that the consolidated film has a thickness less than about 2 mil.

43. The method as claimed in claim 37 characterized in that the consolidated film has a thickness of less than 1 mil.

44. The method as claimed in claim 37 characterized by adjusting the viscosity of the casting composition to prevent separation of the particulate filler from the composition.

45. The method as claimed in claim 44 characterized in that the viscosity modifier is selected from the group consisting of polyacrylic acid, vegetable gums and cellulose-based compounds.

46. The method as claimed in claim 37 characterized in that the layer is cast by dispersion coating, roll-to-roll coating, reverse roll coating or metering and rod coating.

47. The method as claimed in claim 37 characterized in that the filler particles are removed from the consolidated layer by thermal decomposition, chemical reaction or dissolution.

48. A porous fluoropolymer film made by the method as -claimed in the claims 37 to 47.

49. The method as claimed in any one of the claims 32 to 35 or the method as claimed in any one of the claims 37 to 47 or the laminar composite electrical material as claimed in claim 36 or the porous fluoropolymer film as claimed in claim 48 characterized in that the fluoropolymer comprises polytetrafluoroethylene, polychlorotrifluoroethylene, a copolymer of tetrafluoroethylene and a monomer selected from the group consisting of hexafluoropropylene and perfluoroalkylvinylethers, a copolymer of tetrafluoroethylene and a monomer selected from the group 32 consisting of vinylidenefluoride, vinyl fluoride and ethylene, or a copolymer of chlorotrif luoroethylene and a monomer selected from the group consisting of hexaf luoropropylene, perfluoroalkylvinylethers, vinylidenefluoride, 5 vinyl fluoride and ethylene.

50. A method for making a particulate filled polymer matrix composite film as claimed in claim 1 substantially as herein described.

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