Process for forming filaments from polyamic acid
BACKGROUND OF THE INVENTION 
     Various techniques are known for producing filaments convertible into polyimide filaments from a diversity of starting materials. At present, much attention is being focused on the production of continuous monofilaments from various polymers convertible into carbon for use as a substrate material. The prior art has indicated, for example, that filaments suitable for conversion to polyimide filaments may be made by spinning an aqueous solution of an organic base salt of a polyamic acid of a diprimary diamine and a tetracarboxylic acid which is capable of forming a dianhydride into an aqueous or water miscible coagulant bath which chemically interacts with the organic base salt to regenerate the polyamide acid or otherwise convert the salt into an insoluble form. See, for example, U.S. Pat. No. 3,529,051. 
     It has now been found that by selecting the proper polyamic acid and wet spinning it into a preselected coagulating bath, a simple and extremely effective process for forming superior filaments of polyamic acid suitable for conversion to polyimide filaments may be produced. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method for producing filaments of polyamic acid suitable for conversion to polyimide monofilaments or yarn. More particularly, the invention relates to a process for producing such filaments from polyamic acid by a wet spinning technique. 
     The present invention contemplates the employment of a starting solution of polyamic acid and a solvent, wherein the polyamic acid consists essentially of essentially equimolar amounts of P,P-diaminophenyl ether and bis-(3,4-dicarboxyphenyl) sulfone dianhydride and an amount of solvent selected from the group consisting of dimethyformamide, n-methyl pyrrolidone and mixtures thereof sufficient to bring the viscosity of the starting solution to 500 to 5,000 centipoises, preferably 1,500 to 3,000 centipoises. The starting solution is extruded through an orifice directly into a coagulating bath selected from the group consisting of propanol, methyl alcohol, ethyl alcohol, butyl alcohol, a mixture of ethyl alcohol and propanol and a mixture of methyl alcohol and acetone. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the practice of the present invention it has been found that satisfactory filaments may be produced when the fiber-forming solution is a polyamic acid solution containing essentially equimolar amounts of P,P-diaminophenyl ether and bis-(3,4-dicarboxyphenyl) sulfone dianhydride, and an amount of solvent selected from the group consisting essentially of dimethyformamide, n-methyl pyrrolidone and mixtures thereof sufficient to provide a viscosity to the starting solution of 500 to 5,000 centipoises, preferably 1,500 to 3,000 centipoises. Although there may be some variation from equimolar in the ratio of the ether to the dianhydride, such variation is relatively small. The term &#34;essentially equimolar&#34; is meant to define such variation and is intended to cover a ratio range of approximately 0.95 to 1.05. The polyamic acid may be made by any suitable technique such as, for example, that described in U.S. Pat. No. 3,179,614 which disclosure is herein incorporated by reference or it may be purchased on the commercial market from duPont under the trade name 9520-162. 
     Whether the polyamic acid is prepared in the laboratory or obtained commercially, viscosity must be adjusted by solvent addition within the range of 500 to 5,000 centipoises, preferably 1,500 to 3,000 centipoises, in order to achieve continuous filaments of uniform diameter. It has been found that the very fluid solutions, i.e., those below 500 centipoises, do not fiberize while solutions with viscosities greater than 5,000 centipoises are not readily extrudable. It appears that with increasing viscosity, there is a greater tendency toward irregularity in the filament diameter. It has been further observed that fibers drawn from solutions of relatively high viscosity tend to break up at isolated points on the take-up spool and/or exhibit undesirable structural variations. 
     The basic approach which forms the basis of the present invention is a wet spinning technique which involves the extrusion of the starting solution from a small diameter spinneret hole directly into a coagulating or drying liquid bath of a particular composition. While various liquids were investigated as coagulating baths, those found suitable were propanol, methyl alcohol, ethyl alcohol, butyl alcohol, a mixture of ethyl alcohol and propanol (e.g., 30 volume percent propanol, remainder ethyl alcohol) and a mixture of methyl alcohol and acetone (e.g., 15 to 25 volume percent acetone, remainder methyl alcohol). The preferred coagulating bath is propanol. A bath of water was tried unsuccessfully. 
    
    
     In the practice of the invention, the polyamic acid starting solution was contained in a syringe which was activated by a Sage syringe pump. The syringe was connected to one end of an L- or U-shaped tube which was immersed in the coagulating bath. By means of a variable speed motor, metered amounts of the solution were extruded from a small diameter orifice (spinneret) located at the other end of the L or U tube. The extruded filament was wound up on a take-up spool located above the coagulating bath. As the primary function of the coagulating bath appears to be the removal of solvent from the fiber-forming solution, a circulating pump was utilized to prevent concentrations of solvent around the drawing fiber. A level wind mechanism was also used to spool the fibers uniformly. Various spinneret hole sizes from four to eight mils were investigated. With an eight mil diameter spinneret, it was not possible to draw fibers much less than one mil in diameter while the four mil spinneret produced fibers down to approximately 0.2 mil (five microns) in diameter. 
     It should be noted that in all cases the filament diameter is reduced by either decreasing the solution feed rate or by increasing the fiber drawing speed. Using the apparatus above described under conditions as set forth in the following table, continuous polyamic acid filaments convertible to polyimide were successfully produced in accordance with the present invention. 
     
                       Table I                                                     
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Filaments from Polyamic Acid Wet Spun in Propanol                         
                                  Spinneret                               
Example No.                                                               
          Drawing Speed                                                   
                       Feed Rate  Hole Size                               
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1         500 ft/hr     74 ml/day 6 mils                                  
2         500 ft/hr     52 ml/day 6 mils                                  
3         500 ft/hr     66 ml/day 6 mils                                  
4         200 ft/hr     74 ml/day 6 mils                                  
5         200 ft/hr     37 ml/day 8 mils                                  
6         200 ft/hr     52 ml/day 8 mils                                  
7         200 ft/hr    147 ml/day 6 mils                                  
8         200 ft/hr    147 ml/day 8 mils                                  
9-11      200-300 ft/hr                                                   
                       174 ml/day 8 mils                                  
12        300 ft/hr    174 ml/day 8 mils                                  
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     The filaments formed according to the procedures above described were dried and imidized. Drying is accomplished by heating for a time and at a temperature sufficient to remove residual solvent. In the present case, drying was accomplished by heating from room temperature to 50° in a time period of 2 hours in a vacuum of one to 10 millimeters of mercury, then raised from 50° to 90° C in the aforesaid vacuum in one hour and left overnight (i.e., approximately 16 hours). Of course the heating schedule will vary, the smaller diameters fibers (e.g., less than one mil) requiring a much lesser time at temperature. It will be appreciated that while imidization may be carried out in either a vacuum or in air, a vacuum is preferred since it is believed that this assists in removing the residual solvent from the material. Imidization satisfactory to allow subsequent carbonization occurs upon heating to temperatures in the range of approximately 250° to 300° C for at least approximately 15 minutes, preferably 270° to 290°  C for at least approximately 30 minutes. Below 250° C, the fiber tends to deform (stretch) during any subsequent carbonization while above 300° C, the imidization leads to the bonding of individual fibers together and/or to the spool. 
     After the aforesaid thermal imidization, the polyimide filament may be carbonized in order to provide a carbon filament suitable for use as a substrate. 
     Various polyimide fibers were carbonized and exhibited properties as shown in the following table. 
     
                                           Table II                                
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Carbonization of Polyimide Filaments                                      
     Temperature                                                          
            Speed                                                         
                 Percent                                                  
                      Diameter Avg.                                       
                              UTS Avg.                                    
                                    E Avg.                                
Example                                                                   
     (° C)                                                         
            (ft/hr)                                                       
                 Stretch                                                  
                      (mils)  (× 10.sup.3 psi)                      
                                    (× 10.sup.6 psi)                
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13     900  20    5   1       113   7.2                                   
14     900  30    5   .38     127   5.8                                   
15     900  40   100  .50     106   6.4                                   
16     900  60   200  .45     90    5.9                                   
17   1,150  20    5   .92     69    8.3                                   
18   1,550  20    5   .95     67    7.8                                   
19   1,150  20   100  .53     113.6 6.4                                   
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     The carbon filaments produced are well suited for meeting the requirements for usage as substrates in various coating deposition processes. In one case, for example, a 1.2 mil carbon monofilament having a resistance of 5,000 ohms/in. was utilized as a substrate in an electrically resistive vapor deposition process in which a five mil silicon carbide fiber was produced. 
     What has been set forth above is intended primarily as exemplary to enable those skilled in the art in the practice of the invention and it should therefore be understood that, within the scope of the appended claims, the invention may be practiced in other ways than as specifically described.