CONVECTIVE LEACHING OF POLYBENZAZOLE FILMS This application relates to the art of leaching residual solvents from polybenzazole films For the purposes of this application, the term "film" refers to both thin film (usually 1 to 254 μm thick) and to sheet (usually a 254 μm to 1 cm thick) It is well-known to polymerize polybenzazole polymers in acid solutions, so that an acid dope is formed See, for example, Wolfe et al , U S Patent 4,533,693 (August 5, 1985) Frequently, the resulting polymers are not thermoplastic However, they can be made into films by (a) extruding the desired film out of the dope, (b) optionally stretching the dope, (c) coagulating the polymer from the dope, and (d) leaching the residual solvent (usually phosphoric acid) out of the film See, for example, Harvey et al , U S Patent 4,973,442 (November 27, 1990)

The leaching step can become a significant bottleneck in a continuous production process, because it is very time-consuming Commercially desirable films should contain no more than 2000 to 2500 ppm phosphorus, and film for very sensitive uses should contain 250 ppm phosphorus or less On the other hand, the Harvey patent shows that film washed in water for 5 minutes contains 4 weight percent phosphorus, and film washed for 48 hours still contains 0 8 weight percent phosphorus (8000 ppm) The long leaching times needed to reach acceptable levels of phosphorus are unmanageable in a commercial production

What is needed is an accelerated process to leach residual solvent from polybenzazole films

One aspect of the present invention is a process to leach residual solvent from a polybenzazole film that has two faces, wherein the wet, never-dried polybenzazole film is contacted with a leaching fluid that removes residual solvent from the film, characterized in that the pressure of leaching fluid on one face of the film is higher than the pressure on the other face of the film such that the leaching fluid passes through the film from one face to the other

A second aspect of the present invention is a film that contains polybenzazole polymer, characterized in that the film contains on average from 1 to 250 ppm phosphorus The process of this invention makes films with low phosphorus content in a relatively short time At optimum conditions, it can make films with acceptable phosphorus levels (preferably no more than 2500 ppm) in 5 minutes or less, or it can make films with very low phosphorus (100 ppm or less) in slightly longer time (30 minutes to 2 hours) The films can be used in electronic and structural applications

The present invention uses polybenzoxazole (PBO) or polybenzothiazole (PBT) polymers or copolymers that are polymerized in a mixture containing polyphosphoric acid PBO, PBT and random, sequential and block copolymers of PBO ana PBT are described in references such as Wolfe et al , Liquid Crystalline Polymer Compositions. Process and Products,

U S Patent 4,703, 103 (October 27, 1987), Wolfe e al , Liquid Crystalline Poly(2 6-Benzo- thiazole) Compositions, Process and Products, U S Patent 4,533,724 (August 6, 1985), Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U S Patent 4,533,693 (August 6, 1985), Evers, Thermo-oxidatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U S Patent 4,359,567 (November 16, 1982), Tsai et al , Method for Making

Heterocychc Block Copolymer, U S Patent 4,578,432 (March 25, 1986), 1 1 Ency Poly Sci & Eng , Polybenzothiazoles and Polybenzoxazoles, 601 (J Wiley & Sons 1988) and W W Adams et al , The Materials Science and Engineering of Rigid-Rod Polymers (Materials Research Society 1989) The polymer may contain AB-mer units, as represented in Formula 1 (a), and/or

AA/BB-mer units, as represented in Formula 1 (b)

K a ) AB

K b ) AA /BB

wherein:

Each Ar represents an aromatic group The aromatic group may be heterocyclic, such as a pyπdinylene group, but it is preferably carbocyclic The aromatic group may be a fused or unfused polycyclic system, but is preferably a single six-membered ring Size is not critical, but the aromatic group preferably contains no more than 18 carbon atoms, more preferably no more than 12 carbon atoms and most preferably no more than 6 carbon atoms Examples of suitable aromatic groups include phenylene moiet'es, tolylene moieties, biphenylene moieties and bis-pnenylene ether moieties Ar ¹ in AA/BB-mer units is preferably a 1 ,2,4,5-phenylene moiety or an analog thereof Ar m AB-mer units is preferably a 1 ,3,4-phenylene moiety or an analog thereof

Each Z is independently an oxygen or a sulfur atom Each DM is independently a bond or a divalent organic moiety that does not interfere with the synthesis, fabrication or use of the polymer The divalent organic moiety may contain an aliphatic group, which preferably has no more than 12 carbon atoms, but the divalent organic moiety is preferably an aromatic group (Ar) as previously described It is most preferably a 1 ,4-phenylene moiety or an analog thereof

The nitrogen atom and the Z moiety in each azole ring are bonded to adjacent carbon atoms in the aromatic group, such that a five-membered azole ring fused with the aromatic group is formed

The azole rings in AA/BB-mer units may be in cis- or trans-position with respect to each other, as illustrated in 1 1 Ency Poly Sci & Eng , supra, at 602 The poiymer preferably consists essentially of either AB-PBZ mer units or AA/BB-PBZ mer units, and more preferably consists essentially of AA/BB-PBZ mer units The polybenzazole polymer may be rigid rod, semi-rigid rod or flexible coil It is preferably a lyotropic liquid-crystalline polymer, which forms liquid crystalline domains in solution when its concentration exceeds a "critical concentration point " It is preferably rigid rod in the case of an AA/BB-PBZ polymer or semi-rigid in the case of an AB-PBZ polymer Azole rings within the polymer are preferably oxazole rings (Z = O) Preferred mer units are illustrated in Formulae 2(a)-(h) The polymer more preferably consists essentially of mer units selected from those illustrated in 2(a)-(h), and most preferably consists essentially of a number of identical units selected from those illustrated in 2(a)-(d)

Each polymer preferably contains on average at least 25 mer units, more preferably at least 50 mer units and most preferably at least 100 mer units The intrinsic viscosity of rigid AA/BB-PBZ polymers in methanesulfonic acid at 25°C is preferably at least 10 dLVg, more preferably at least 15 dL/g and most preferably at least 20 dL g For some purposes, an intrinsic viscosity of at least 25 dLg or 30 dL/g may be best Intrinsic viscosity of 60 dL/g or higher is possible, but tne intrinsic viscosity is preferably no more than 50 dL/g The intrinsic

viscosity of semi-rigid AB-PBZ polymers is preferably at least 5 dL/g, more preferably at least 10 dL/g and most preferably at least 15 dL/g.

The polymer is dissolved in a dope solution that contains the polymer and a solvent. The solvent is usually acidic. It preferably contains methanesulfonic acid or polyphos- phoric acid, and it is more preferably polyphosphoric acid.

The dope should contain a high enough concentration of polymer for the polymer to coagulate to form a solid film. When the polymer is lyotropic liquid-crystalline, then the concentration of polymer in the dope is preferably high enough to provide a liquid- -crystal line dope. The concentration of the polymer is preferably at least 7 weight percent, o more preferably at least 10 weight percent and most preferably at least 14 weight percent. The maximum concentration is limited primarily by practical factors, such as polymer solubility and dope viscosity. The concentration of polymer is seldom more than 30 weight percent, and usually no more than 20 weight percent.

Suitable polymers or copolymers and dopes can be synthesized by known 5 procedures, such as those described in Wolfe et al., U.S. Patent 4,533,693 (August 6, 1985); Sybert et al., U.S. Patent 4,772,678 (September 20, 1988); Harris, U.S. Patent 4,847,350 (July 1 1 , 1989); and Gregory et al., U.S. Patent 5,089,591 (February 18, 1992). In summary, suitable monomers (AA-monomers and BB-monomers or AB-monomers) are reacted in a solution of nonoxidizing and dehydrating acid under nonoxidizing atmosphere with vigorous mixing and 0 high shear at a temperature that is increased in step-wise or ramped fashion from no more than 120°C o at least 190°C. Examples of suitable AA-monomers include terephthalic acid and analogs thereof. Examples of suitable BB-monomers include 4,6-diamino-resorcinol, 2,5-diaminohydroquinone, 2,5-diamino-1 ,4-dithiobenzene and analogs thereof, typically stored as acid salts. Examples of suitable AB-monomers include 3-amino-4-hydroxybenzoic 5 acid, 3-hydroxy-4-aminobenzoic acid, 3-amino-4-thiobenzoic acid, 3-thio-4-aminobenzoic acid and analogs thereof, typically stored as acid salts.

The dope is formed into a film. For instance, films can be shaped by extruding the dope through a slit or tubular die and optionally stretching the dope film in the machine and/or transverse directions. Examples of suitable processes are shown in Harvey et al., 0 U.S. Patent 4,973,442 (November 27, 1990); Chenevey et al., U.S. Patent 4,898,924 (February 6, 1990); and Pierini et al ., U.S. Serial. No. 937,327 (filed August 27, 1992).

The film is preferably coagulated before the leaching step by contacting it with a coagulant fluid that precipitates the polybenzazole polymer from solution and removes a part of the solvent. (The term "coagulation" is commonly used in the art to describe the step in 5 which dope is initially contacted with a fluid that precipitates the polymer from solution and removes part of the solvent. The term "leaching" is commonly used in the art to describe the step of subsequently washing almost all of the residual solvent from the coagulated polymer. The term "coagulation" does not necessarily imply that the dope film is in a flowable state. The

dope may be at a temperature low enough so that the dope is essentially non-flowing before the coagulation step begins.)

Steam, water or an aqueous acid solution are all examples of suitable coagulant fluids. The coagulant fluid is preferably an aqueous acid solution. The aqueous acid solution is more preferably an aqueous phosphoric acid solution. The solution preferably contains at least 10 weight percent acid, more preferably at least 20 weight percent acid, and most preferably at least 25 weight percent acid. It preferably contains no more than 50 weight percent acid, and more preferably no more than 35 weight percent acid. The aqueous acid solution may be at any temperature at which it is liquid (usually 0°C to 100°C). Its temperature is preferably at least 5 ^c C and more preferably at least 25°C. Its temperature is preferably no more than 85°C aπd more preferably no more than 45 ^C C.

The coagulant fluid may be applied in any manner usual for washing films, such as by immersing or spraying. The residence time is preferably no more than 15 minutes, more preferably no more than 10 minutes, more highly preferably no more than 5 minutes and most preferably no more than 1 minute. It preferably reduces the residual solvent in the film to no more than 25 weight percent, more preferably no more than 15 weight percent and most preferably no more than 5 weight percent.

The film may optionally go through subsequent washing steps before it undergoes convective leaching of the present invention. Those steps may be carried out using known washing fluids, such as water, steam or acid solutions which contain less acid than the coagulant fluid.

It is important that the surface of the film should not be allowed to dry at any time after the coagulation step starts and before the leaching step is completed. It is theorized, without intending to be bound, that the wet never-dried surface of the polybenzazole film is relatively porous and provides paths to leach residual solvent from inside the film. This discovery is surprising in light of the high barrier properties reported for dried polybenzazole films. On the other hand, it is theorized that the pores close when they become dry and do not open even when they become wet again. The closed pores trap residual solvent inside the film.

After it has been solidified so that it will not tear or deform, the film undergoes convective leaching by contact with a leaching fluid that is capable of removing residual solvent from the film. Examples of suitable leaching fluids include water, liquid organic solvents and steam.

The leaching fluid may be at any temperature at which it remains fluid (such as 0°C to 100°C for water). However, the leaching fluid is preferably at an elevated temperature (above 60°C). The temperature of water is preferably at least 60°C, more preferably at least 75 ^C C and most preferably at least 85 ^C C. The temperature of steam is preferably a temperature at which the steam is "wet" or "saturated" steam. At atmospheric pressures, that temperature is preferably 100°C to 120°C and more preferably 105° to 1 10°C, but at pressures above

atmospneπc pressure the temperature is preferably higher Tne temperature ot organic solvents is preferably at least 75°C, more preferably at least 100°C, more highly preferably at least 150°C and most preferably at least 200°C The maximum temperature is not critical to the invention and depends on practical limitations It should be low enough that the polybenzazole polymer and the hot fluid do not decompose Preferably, when the hot fluid is a liquid, the temperature is low enough that the liquid does not boil

An organic solvent is preferably selected so that it remains liquid under leaching conditions Its boiling point is preferably at least 100°C, more preferably at least 150°C and most preferably at least 200°C at atmospheric pressure It preferaσly meets at least one of the following qualities solubility in water of greater than one weight percent, high polarity, and/or ability to form addition complexes with the residual polymerization solvent Examples of suitable hot organic solvents include, dimethyl sulfoxide, ethylene glycol, propylene carbonate, glycerol and hexyl alcohol

The leaching fluid is contacted with one face of the film at a pressure that is higher than the pressure on the other face of the film, so that fluid passes through the film from one face to the other When the film is in a tubular shape, this can be accomplished by putting the fluid inside the tube under pressure When the film is flat, then it is preferably placed in an apparatus that can create a high pressure zone on one side of the film (and/or pull a reduced pressure on the other side of the film) One face of the film can optionally be open to the atmosphere, but the exposed face of the film should be kept sufficiently wet to prevent the pores on the film from closing

The pressure difference across the film should be chosen to move the fluid through the film at a desired rate without damaging the film The optimal pressure varies depending upon the thickness and porosity of the film The pressure difference across the film is usually between 0 1 atm (10 kPa) and 50 atm (5 MPa) For most films, the pressure difference is preferably at least 1 atm (0 1 MPa), more preferably at least 3 atm (0 3 MPa) and most preferably at least 5 atm (0 5 MPa) It is preferably no more than 25 atm (2 5 MPa), and more preferably no more than 10 atm (1 MPa) The leaching process may optionally contain more than one convective leaching zone with more than one pressure and/or with different leaching ^{fl uιds}

The residence time in the convective leaching zone is preferably no more than 30 minutes, highly preferably no more than 20 minutes, more preferably no more than 10 minutes, more highly preferably no more than 5 minutes and most preferably no more than 2 minutes The total residence time for the entire coagulation, washing and leaching process is preferably no more than 35 minutes, highly preferably no more than 25 minutes, more preferably no more than 15 minutes, more highly preferably no more than 10 minutes and most preferably no more than 5 minutes However, longer times may be desirable to obtain very low levels of residual solvent

The leached film contains hign ouantities of the leaching fluid, frequently more than 50 weight percent After the leaching process is completed, it may optionally be dried and heat-treated according to known methods, such as those described in Harvey et al , U S Patent 4,973,442 (November 27, 1990) and Chenevey et al , U S Patent 4,898,924 (February 6, 1990) For example, it may be dried in a vacuum or circulating oven, preferably under sufficient tension in the transverse and machine directions to prevent shrinkage or curling The dried film may be a thin film or a thicker sheet, as previously described Films can be used in structural applications such as honeycomb, as a layer in a circuit board, or as a magnetic media substrate

The residual solvent content of the film varies depending upon the time and the leaching conditions When the residual solvent was a Dolyphosphoπc acid related compound, the film preferably contained no more than 2500 ppm phosphorus after a short leacnmg process, highly preferably no more than 1500 ppm phosphorus, more preferably no more than 1000 ppm and most preferably no more than 500 ppm For a somewhat longer leaching process, it preferably contains no more than 200 ppm, more preferably no more than 150 ppm, more highly preferably no more than 100 ppm and most preferably no more than 50 ppm (Residual phosphorus measurements are based upon the weight of the film after it has been substantially dried Phosphorus can be measured using an X-ray fluorescence technique described in E P Bertin, Principles and Practice of X-Ray Spectrometπc Analysis - Second Ed (Plenum Press 1984) Suitable equipment is commercially available under the tradename KEVEX 770 XRF and from Philips Electronic Instruments ) Perfect leaching is not usually feasible The film preferably contains at least 1 ppm phosphorus and more preferably at least 5 ppm residual phosphorus

The dried and heat-treated film may have biaxially balanced properties or uniaxially unbalanced properties The tensile strength of a biaxially balanced film is preferably at least 35 Ksi (240 MPa) and more preferably at least 65 Ksi (450 MPa) The tensile modulus is preferably at least 2 Msi (14 GPa) and more preferably at least 5 Msi (34 GPa) The film is preferably at least 10 μm thick and more preferably at least 25 μm It is preferably no more than 1 mm thick

The invention is further illustrated by the following illustrative examples The following examples are for illustrative purposes only, and should not be taken as limiting the scope of either the specification or the claims Unless otherwise stated, all parts and percentages are by weight (PPM = parts per million by weight )

Residual phosphorus is measured using X-ray fluorescence (XRF) on 1 25 inch (3 18 cm) diameter samples cut from the film Example 1 - Convective Steam Leaching Process

Dope films that contained 14 weight percent cis-PBO (intrinsic viscosity of 32 αL/g in methanesulfonic acid at 25 ^C C) were extruded Tne dope films were stretched to 5 times tneir original length and width The stretcned film thickness was 2 mil (51 μm) The stretched fil ms

were mounted on hoops and immersed for 5 minutes in an aqueous phosphoric acid solution having the concentration shown in Table 1 The films were washed for 5 minutes in room temperature deionized water

Samples of film were mounted in a high pressure filter assembly, making sure that the film remained wet throughout The holder was positioned with the exit up at a 45 ^c angle from horizontal Steam at 40 to 45 psig (280 to 310 kPa) pressure and a temperature of 150°C is pumped into the entrance port, and condensate was drained periodically with a valve near the entrance port Water was kept in the exit line to keep the back side of the film wet Steam can be seen exiting through the exit side After a period of time shown in Table I, the film was dried and phosphorus was measured The results are shown in Table I

₀ » • j • __ Res idual

Samp -l, e _l A _s c _fc id _βa i _t n _h u _S n _t d _e e _am r _n Phosp ,horus

(%) (min.) ⁽ PP ⁾ a 15 5 675 b 15 15 434 c 15 30 106 d 15 60 237 e 30 5 158 f 30 15 124 g 30 30 89 h 30 60 43

Example 2 - Convective Steam Leaching Process

Dope films that contained 14 weight percent cis-PBO (32 I V in methanesulfonic acid at 25°C) were extruded The dope films were stretched to 5 times their original length and width The stretched film thickness was 2 mil (51 μm) The stretched films were mounted on hoops and immersed for 3 to 4 days in room temperature water

Samples of film were mounted in a high pressure fιlte assembly and steam was passed through the film as described in Example 1 After a period of time shown in Table II, the film was dried and phosphorus was measured The results are shown in Table II

Table II

Time Residual

Sample under

Steam Phosphorus

(min. ) ( ppm)

1 15 296 j 30 197 60 153

1 120 138