Polyamide-imide based filaments, and a process for obtaining them
This application is a continuation-in-part of our prior copending application Ser. No. 07/408,526, filed Sep. 18, 1989, now abandoned. 
    
    
     The present invention relates to polyamide-imide based thermostable synthetic filaments with improved properties. 
     It also relates to a process for obtaining such filaments. 
     It is already known, according to FR 2,079,785 to prepare glossy polyamide-imide based filaments, containing at least 3% of alkali or alkaline earth 3,5-dicarboxybenzene sulphonate chains, by wet spinning of a polymer solution in N-methylpyrrolidone, in a coagulating aqueous bath also containing N-methylpyrrolidone, and then drawing, washing and drying. 
     But such filaments have mechanical characteristics which are too weak for certain applications, a yellow-brown colour which is too great, and show thermal degradation and a poor resistance to photodegradation. 
     The polyamide-imide filaments according to the present invention have characteristics which are clearly superior to those of the polyamide-imide filaments of the prior art. They have the following chemical structure: 
     amide-imide units (A) of formula ##STR1## 
     amide units (B) of formula 
     
         --NH--R--NH--CO--R.sub.2 --CO-- 
    
     and optionally amide units (C) of formula ##STR2## in which R represents a divalent aromatic radical, R 1  represents a trivalent aromatic radical, R 2  represents a divalent aromatic radical and M an alkali metal or alkaline earth metal, A units representing 80 to 99% of the whole of the units, B units representing 1 to 5% of the whole of the units and C units representing 0 to 20% of the whole of the units. 
     The filaments according to the invention are also characterized by a polydispersion index I≦2.2, a tenacity≧45 cN/tex, a Young&#39;s modulus≧3.8 GPa, an elongation≦25% and a colour defined by the luminance Y&gt;25%, the degree of whiteness DW&lt;30 and the yellow index YI&gt;170. 
     Preferably, the tenacity is of the order of 50 to 55 cN/tex, the modulus≧4 GPa, the luminance Y&gt;30%, the degree of whiteness&lt;28, and the yellow index&gt;190. 
     The light stability of the PAI according to the invention is also very much improved with respect to those of the PAI filaments known up to now. It is evaluated by the retention of mechanical properties, breaking tenacity≧50%, work to breaking 18% and elongation≧35%. The light stability is measured according to the method described below. 
     According to the present invention, by polyamideimide (PAI) is understood the polymers obtained by polycondensation in any appropriate solvent of: 
     at least one diisocyanate of formula: OCN-R-NCO in which R is a divalent aromatic group such as 4,4-diphenylmethane diisocyanate, and preferably 4,4&#39;-diphenylether diisocyanate or their mixtures, with: 
     an aromatic acid anhydride such as benzene-1,2,4-tricarboxylic acid anhydride, 
     an aromatic dicarboxylic acid such as terephthalic acid, 
     and optionally an alkali metal or alkaline earth metal dicarboxybenzene sulphonate, preferably sodium or potassium dicarboxybenzene sulphonate. 
     The filaments according to the invention have a chemical structure such as defined above in which R is a radical such as ##STR3## and preferably ##STR4## and R 1  is preferably a ##STR5## radical R 2  is preferably a ##STR6## radical and M is preferably Na or K. 
     According to the present application the PAI filaments are also characterized by a value of the polydispersion index I≦2.2. This index corresponds to the ratio Mw/Mn, the values of Mn and Mw being determined by gel permeation chromatography (GPC) in NMP at 80° C. and 0.1 mole/liter of lithium bromide, the masses being expressed with respect to a polystyrene standard. 
     The polydispersion index of the PAI filaments according to the invention remains low: it corresponds to a tight distribution of the molecular masses. Surprisingly, it remains low in the finished filaments in spite of the different treatments undergone by the threads during their manufacture. 
     Surprisingly, filaments according to the invention also have excellent mechanical and thermal characteristics which are much superior to those of the polyamide-imide filaments according to FR 2,079,785. In particular the tenacity is greater than or equal to 45 cN/tex, preferably greater than or equal to 50 or 55 cN/tex. Measurement is carried out with a manual or automatic dynamometer with a constant elongation gradient, on a filament sample which is subjected to longitudinal traction until it breaks. The dynamometer is linked to a calculator which provides the numerical values of the titre (dtex) of the breaking force (cN) and of the breaking tenacity= ##EQU1## and of the elongation to breaking %. The values correspond to a mean of 20 measurements. 
     Elongation to breaking is measured as indicated above. It is less than or equal to 25%. 
     The longitudinal Young&#39;s modulus E of the PAI filaments according to the invention is≧3.8 GPa, preferably≧a 4 GPa. It is the ratio: ##EQU2## obtained initially from the force/elongation curve. The specific force (tenacity) corresponds to the ratio: ##EQU3## l being the length of the sample at time t, which corresponds to the true titre, and l o  the initial length of the sample. 
     The PAI filament is, according to the invention, also characterized by a low surface colour, which is evaluated by three essential values: 
     the luminance Y %&gt;25%, preferably&gt;30% 
     the degree of whiteness DW&lt;30, preferably&lt;28 
     the yellow index YI&gt;170, preferably&gt;190, measured in the following manner: 
     the sample is ground in a coffee mill so as to obtain a flock a few millimeters thick and 4 cm 2 . The flock is placed between 2 glass plates, and the whole is placed in an &#34;Elrepho&#34; brand apparatus (Zeiss). The following results are obtained: 
     Y%: the luminance, which classifies the sample in the grey scale, and which translates the impression of light and dark: 
     Y%=100 corresponds to a perfect white 
     Y%=0 corresponds to a perfect black. 
     The degree of whiteness defines a colour point on the chromaticity map. 
     The Yellow index is an expression of the colorimetric purity in the yellow. 
     The PAI filaments according to the invention also have a good light stability, measured by exposure in severe aging conditions and quantified by: 
     a retention of breaking tenacity≧50%, preferably≧52%. 
     a retention of work to breaking≧18%, preferably≧20%. 
     a retention cf elongation≧35%, preferably 38%. 
     The measurements of light stability are carried out in an enclosure which allows the photoaging of the polymers to be studied experimentally in a dry atmosphere. The samples under test are arranged on a cylindrical tower which is driven by a circular rotatory movement and is situated at the centre of a parallelopipedal enclosure, the four corners of which are occupied by a &#34;medium pressure&#34; mercury vapour lamp type Mazda MA, 400 Watts. The envelope of the bulb only allows radiation greater than 300 nm (solar spectrum) to pass through. The temperature in the enclosure is 60° C. 
     Method: The 1.4 cm paper window on which the sample is fixed during the determination of the mechanical properties on an Instron is itself placed on one of the 24 supports of the tower in the chamber. After exposure, the paper window is recovered and the mechanical characteristics of the monofilament are determined according to the method indicated above for the determination of the mechanical properties. 
     The results are presented in the form of curves giving the percentage of the mechanical characteristics remaining in an exposed fibre, compared with a nonexposed one, as a function of the length of exposure. The values obtained depend on the starting monomers, the best results being obtained with 4,4&#39;-diphenylether diisocyanate. 
     Comparisons have also been made with PAI filaments according to FR 2,079,785. 
     The filaments according to the invention are clearly superior to the known PAI filaments. 
     The filaments according to the invention also have a much better thermal stability than the known PAI filaments: this is evaluated by the degradation speed corresponding to a percentage loss of weight as a function of the time at a temperature of 375° in air. 
     Constant temperature degradation of PAI filaments according to the invention is generally≦3% per hour, and preferably≦2% per hour. 
     The level of this degradation also depends on the starting monomers, the best results being obtained with 4,4&#39;-diphenylether diisocyanate. 
     The threads, filaments and fibres according to the present invention can be used alone, or mixed with natural or synthetic filaments or fibres with the object of modifying or improving certain properties. They are particularly useful for the manufacture of working and protective garments, by virtue of their mechanical properties and their thermal and light-resistance. In addition the filaments obtained are easy to dye to any colour with basic dyes. 
     The PAI filaments according to the present invention are obtained by wet spinning from polymer solutions in a solvent or solvent mixture. The concentration of the spinning solutions is between 4 and 35% by weight, preferably between 5 and 35%. The polymers are dissolved in a solvent or solvent mixture containing from 5 to 100% by weight of dimethylene urea of pH≦7, and 0 to 55% of an anhydrous aprotic polar solvent such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, tetramethylurea or γ-butyrolactone. 
     The solutions which are capable of being used in the process according to the invention should have a viscosity of between 100 and 200 poises, preferably between 150 and 160 poises. They can also contain various adjuvants such as pigments or roughening agents, to improve certain properties. 
     The PAI solutions ar spun in a binary or ternary aqueous coagulating bath containing a solvent or a solvent mixture in a proportion of 30 to 80% solvent and 20 to 70% water, preferably 40 to 70% solvent(s). 
     The solvent used can be dimethylformamide, dimethylethylene urea or a mixture of them. The spinning bath is maintained at between 15° and 40° C., preferably 20° to 30° C. The length of the coagulating bath is adaptable, generally as a function of the solvent concentration and the temperature. Baths having a higher solvent content generally allow filaments with better drawability properties, and therefore better final properties, to be obtained. However, when the solvent concentration is higher, a greater length of bath is necessary. The threads which come out of the coagulating bath in the state of a gel are then drawn, for example in the air, at a rate defined by the ratio V2/V1×100, V2 being the speed of the drawing rolls and V1 that of the delivery rolls. The drawing rate of the filaments in the gel state is at least 100%, preferably at least 110% or even more. 
     After drawing the threads are washed by known methods in order to remove the solvent or solvents. This washing can be carried out, for example, in successive vats in which water circulates in countercurrent, or on washing rolls, or by any other means, preferably at ambient temperature. 
     The washed threads are then dried by known methods, for example in a dryer or on rolls. The temperature of this drying can vary within wide limits, as can the speed, which is greater as the temperature is higher. It is generally advantageous to carry out drying with progressive temperature elevation, this temperature being capable of attaining, and even passing, 200° C., for example. 
     The threads then undergo hot overdrawing to improve their mechanical qualities, and in particular their tenacity, which can be advantageous for certain uses. 
     This hot overdrawing can be carried out by any known method: stove, plate, roll or roll and plate, preferably within an enclosure. It must be carried out at a temperature of at least 150° C., which can attain, and even pass, 200° to 300° C. The overdrawing rate is generally at least 150%, but it can vary within wide limits according to the qualities required for the finished filament. The total drawing rate is at least 250%, preferably at least 260%. 
     The whole drawing and overdrawing process can be carried out in one or several stages, either continuously or discontinuously with the preceding operations. In addition the secondary drawing can be combined with drying. For this it is sufficient to provide, at the end of the drying stage, a zone of higher temperature which allows overdrawing. 
    
    
     EXAMPLES 1 TO 3 
     A PAI solution is prepared from: 
     
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benzene-1,2,4-tricarboxylic acid anhydride                                
                          40 mol %                                        
(ANTM)                                                                    
terephthalic acid (AT)    8 mol %                                         
sodium salt of 5-sulphoisophthalic                                        
                          2 mol %                                         
acid (AISNa)                                                              
4,4&#39;-diphenylether diisocyanate (DIDE)                                    
                          50 mol %                                        
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     in dimethylethylene urea of pH≦7, so as to obtain a ccncentration of 21%; the polydispersion index of the polymer is 1.78. 
     The solution, with a viscosity of 598 poises, is extruded into an aqueous coagulating bath containing dimethylethylene urea. The threads come out of the coagulating bath in the gel state, and are drawn in the air at normal temperature. They are washed with water in a bath to remove the solvent, and dried on rolls. 
     The washed and dried threads undergo overdrawing in a stove maintained at a high temperature and then wound on cops. The precise process conditions are collated in Table 1 below: 
     
                       TABLE 1                                                     
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                     Example                                              
                            Example                                       
                     1      2                                             
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                 temperature °C.                                   
                                24     24                                 
                 proportion DMEU/                                         
coagulating bath water (W/W)   70/30  70/30                               
                 time(s)          4.5    4.5                              
air drawing: rate %    169      169                                       
washing (time is s)       5.5      5.5                                    
drying (°C.)    170      170                                       
                  rate %        170    170                                
overdrawing                                                               
                 temperature °C.                                   
                               270    320                                 
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     The mechanical properties of the filaments, obtained from the traction experiments, are collated in Table 2 which follows, where they are compared with filaments obtained from a solution of PAI in N-methyl-pyrrolidone spun under the conditions of French Patent FR 2,079,785 filed on Dec. 2, 1970 (Example 3C). 
     
                       TABLE 2                                                     
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                                        Polydis-                          
     Elongation                                                           
               Tenacity Modulus                                           
                               Temperature                                
                                        persion                           
Ex.  %         cN/tex   GPa    Tg °C.                              
                                        index I                           
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1      21.2    45       6.2    254      2.05                              
2    17        46       6.4    265      2.05                              
3 C  29        33       3.6    278      3                                 
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Surface colour:  Ex. 1     Ex. 2   Ex. 3C                                 
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Luminance %      31.1      33      21.8                                   
Degree of whiteness DW                                                    
                 26.4      27      37.7                                   
Yellow index YI  194       197     164                                    
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Light stability (after 40 hours of exposure in the                        
enclosure)                                                                
Retention:       Ex. 1     Ex. 2   Ex. 3C                                 
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breaking tenacity                                                         
                 52%       55%     31%                                    
work to breaking 20%       22%      5%                                    
elongation       38%       39%     17%                                    
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     After exposure in the enclosure for only 20 hours, according to the method described above, the large and spectacular fall in the mass of the PAI polymer according to Example 3C is also noted: Mw goes from 147,120 to 62,950, that is a loss of 84,170. On the other hand, a polymer according to the invention clearly loses less: 
     Mw passes from 116,400 to 99,720, that is a loss of 16,680.  In the same way, the polydispersion index of the polymer according to the invention after exposure passes from 2.05 to 2.13, while the polydispersion index of the polymer according to Example 3C passes from 3 to 4.12. 
     It is therefore noted that the fall in the mechanical properties expresses a greater degradation of the polymer chains in the filaments of Example 3C than in the filaments according to the invention. 
     Thermal Stability 
     Degradation kinetics, that is to say the loss of weight as a function of time represented by V in % hours -1   
     
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              375° C. in air                                       
              Ex. 1    Ex. 2  Ex. 3                                       
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Speed of degradation                                                      
                2          1.9    6.8                                     
% h.sup.-1                                                                
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     EXAMPLE 4 
     A polymer which is identical to that described in Examples 1, 2 and 3, but which has a polydispersion index I=1.85 is prepared. 
     The solution of this polymer in DMEU has a concentration of 21% and a viscosity of 781 poises. It is spun under the following conditions: 
     
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                   temperature °C.                                 
                                  24                                      
                   proportion DMEU/                                       
                                 60/40                                    
coagulating bath   water                                                  
                   time(s)        6                                       
air drawing: rate %      167                                              
washing (time in s)       8                                               
drying (°C.)      200                                              
                    rate %        180                                     
overdrawing                                                               
                   temperature °C.                                 
                                 260                                      
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     The mechanical properties of the filaments are the following: 
     
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                                        Polydis-                          
     Elongation                                                           
               Tenacity Modulus                                           
                               Temperature                                
                                        persion                           
Ex.  %         cN/tex   GPa    Tg °C.                              
                                        index I                           
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4    23        51       6.3    265      2.2                               
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Surface colour:                                                           
Luminance  .sub.-- Y %                                                    
                    31                                                    
Degree of whiteness DW                                                    
                    25                                                    
Yellow index YI     196                                                   
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Light stability (after 40 hours of exposure in the                        
enclosure)                                                                
                 tenacity     52%                                         
Retention:       work to breaking                                         
                              20%                                         
                 elongation   38%                                         
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 Polydispersion index: after 20 hours of exposure: 2.2                    
 Thermal stability: Degradation kinetics (% h.sup.-1): 2                  
 
    
     EXAMPLES 5 to 7 
     A solution of PAI of the same chemical structure as that described in Example 1 is prepared in a mixture of DMEU/DMF in a proportion 72:28. The polymer obtained has a polydispersion index I=1.73 and the solution has a viscosity of 405 poises at a concentration of 21%. 
     It is extruded in an aqueous coagulating bath, then the threads are drawn and treated under the conditions collated in Table 3 which follows: 
     
                                           TABLE 3                                 
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                   Example                                                
                          Example Example                                 
                   5      6       7                                       
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           temperature °C.                                         
                    20     20      20                                     
           concentration                                                  
                   DMF/water                                              
                          DMEU/DMF/                                       
                                  DMEU/DMF/                               
                          water   water                                   
coagulating bath                                                          
           proportion                                                     
                   40:60  21:9:70 13:27:60                                
           time(s)    4.5    4.5     4.5                                  
air drawing: rate %                                                       
                   167    145     167                                     
washing (time in s)                                                       
                      5.6    5.6     5.6                                  
drying (°C.)                                                       
                   160    160     160                                     
           rate %  180    180     180                                     
overdrawing                                                               
           temperature °C.                                         
                   300    300     300                                     
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     The mechanical properties of the filaments obtained are collated in Table 4 which follows: 
     
                       TABLE 4                                                     
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     Elongation                                                           
               Tenacity   Modulus                                         
                                 Polydispersion                           
Ex.  %         cN/tex     GPa    index I                                  
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5    10.2      55.3       9.87   2.2                                      
6    14.03     46         6.45   2.15                                     
7    13        56.4       9.9    2.2                                      
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         ______________________________________                                    
Surface colour:  Ex. 5     Ex. 6   Ex. 7                                  
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Luminance %      30        31      31                                     
Degree of whiteness DW                                                    
                 25        26        25.4                                 
Yellow index YI  195       194     197                                    
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Light stability: (after 40 hours of exposure)                             
Retention:    Ex. 5   Ex. 6     Ex. 7 Nomex                               
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breaking tenacity                                                         
              50%     52%       55%   47%                                 
work to breaking                                                          
              19%     21%       21%   --                                  
elongation    35%     35%       37%   16%                                 
modulus       100%    100%      100%  80%                                 
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     A polymetaphenylene isophthalamide fibre known commercially under the brand Nomex T 450 was subjected to the same photodegradation test as the fibres of the invention, for comparison. The retention of certain properties is indicated in the table below. 
     
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Thermal stability                                                         
              Ex. 5    Ex.6   Ex. 7                                       
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Degradation speed                                                         
               2.1         1.9    1.8                                     
% h.sup.-1                                                                
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