Title of Invention

A PROCESS FOR THE PREPARATION OF SUB-DENIER FIBRES.

Abstract

In the present invention poly(metaphenylene isophthalamide) polymer is converted into fibres by solution wet spinning technique. A solution of the polymer is prepared in dimethyl acetamide containing 5.0 to 6% of an alkaline earth chloride such as CaCI2 or alkali chloride such as LiCI; the inorganic salt is added to improve the solubility of the polymer in the solvent. The concentration of the polymer in the spinning solution is in the range of 13.0 wt% to 17.0 wt%. These concentrations are lower compared to 20 to 25 wt% used in conventional wet spinning. The lower concentration of the spinning solution is employed to obtain low denier undrawn coagulated filaments which are then drawn to impart the desired mechanical properties to the fibre.

Full Text

This invention relates to a process for the preparation of sub-denier fibres having high yield strength. The present invention particularly relates to a process for the preparation of sub-denier fibres of poly(metaphenylene isophthalamide) with denier per filament of less than 1, where denier per filament (dpf) is defined as the weight in grams of 9000 metres in length of a single filament. Further, the sub-denier fibres of this invention have a high yield strength and a high tensile modulus. It is well known that poly(metaphenylene isophthalamide) fibres have excellent electrical insulation properties and are extensively used in the manufacture of electrical insulation paper. Poly(metaphenylene isophthalamide) high denier fibres useful for making electrical insulation papers have been described in several prior art processes, for example in US patent 3,133,138, US patent 3,079,219, European patent 0226137 and US patent 03145. The details of the prior art processes referred above are briefly summarised below. The poly(metaphenylene isophthalamide) polymer is first spun into fibres by the conventional wet spinning process, and subsequently, the as-spun fibres are hot stretched. The inherent viscosity of the polymer employed in these processes lies in the range 1.3 to 2.1 dl/g. The spin dope is either a salt-free solution or otherwise (inorganic salts such as LiCl or CaCl2 ) of the polymer in either dimethylacetamide or N-methyl pyrrolidone and the concentration of the polymer in the dope ranges from 17 to 25g per 100 ml of solvent. Aqueous solutions of CaCl2 or dimethylacetamide / CaCl2 or Ca(SCN)2 / dimethylacetamide are the different coagulating media employed in these processes, and the temperature of the coagulation ranges from 70° C to 115° C. The fibres are drawn 2 to 3 times during wet spinning and 1.6 to 4 times during hot stretching; the overall stretch of the fibres lies between 4 and 7. The hot stretching temperature is in the range 300 to 350° C and the time of exposure of the yarn to the high temperature is under 5 seconds. A comparison of the properties achieved for these fibres, as described in the above mentioned patents, indicates that the tensile strength, tensile modulus, elongation at break, and the dpf respectively lie in the range 5.8 to 8.4 gpd, 94 to 164 gpd, 10 to 37% and 1.8 to 2.2. The dpf values of 2.4, 2.8 and 3.8 have also been reported for the isophthalamide fibres in the US patent 3,079,219. However, these poly (metaphenylene isophthalamide) fibres have a high denier per filament. Dpf values in the range of 1.8 to 2.0 correspond to equivalent cylindrical diameters of 13.5 to 15 micrometres. While such high diameter fibres can be used for making useful electrical insulating papers, a significant improvement in the electrical and mechanical properties of the paper can be achieved by using low diameter fibres in the preparation of the paper. Reducing the diameters of the fibres from 15 micrometer to 8 micrometer will increase the surface area of a given volume of fibres by a factor of 2. This results in higher bonded area fraction in the paper with the attendant improvement in mechanical and electrical properties of the paper. Further, the above reduction in diameter of the fibres also means that for a given volume fraction the fibres in the paper, the number of filaments in the case of 8 micrometer fibres will be 3.5 times more than the number of filaments of 15 micrometer fibres of equal length. Further increase in the number of low diameter fibre at a given fibre volume fraction in the paper is possible, since the length of 8 micrometer fibre will be half the length of 15 micrometer fibre at constant length to diameter ratio of the filaments. Thus the number of fibres per unit volume can be increased by a factor of 7 by using low diameter or low denier fibres in place of high diameter or high denier fibres. This large increase in the number of filaments per unit volume leads to a more uniform dispersion of the fibres in the paper, and also allows the use of higher volume fraction of fibres in the paper. Since the fibres are used basically as a reinforcement in the paper, a more uniform distribution and higher volume fraction of the fibre will improve the properties of the paper particularly the mechanical properties like tensile strength and tear resistance. Reference may be made to German Patent No 2,039,254, wherein a process for the preparation of fibres with a denier per filament of ≈ 0.5 to 0.6 is provided. However the fibres described in the above patent suffer from a disadvantage that the yield strength of the fibres is only 2.6 gram per denier (gpd). Since the dimensional stability of the paper under the action of a stress ismaintained only upto the yield strength of the fibres, the paper made from the above fibres cannot be used where mechanical strength and dimensional stability are required. The main objective of the present invention is to provide a process for the preparation of sub-denier poly(metaphenylene isophthalamide) fibres with a denier of less than 1, preferably between 0.6 to 0.7, having a tensile strength greater than 5.0 gpd and particularly having a yield strength greater than 3.0 gram per denier, preferably between 3.4 to 3.7 gpd which obviates the drawbacks as detailed above. The metaphenylene isophthalamide polymer suitable for making sub-denier fibres according to this invention, should essentially be a 100% homopolymer consisting of metaphenylene isophthalamide units with a high molecular weight. The inherent viscosity of the polymer measured using a 0.5 g/100 ml concentration solution of the polymer in sulphuric acid should be greater than 2.0 dl/g and preferably be around 2.3 to 2.5 dl/g. Such high inherent viscosity polymers can be prepared by low temperature solution polycondensation of high purity monomers namely metaphenylene diamine and isophthaloyl chloride, while carefully controlling the reaction conditions such as monomer ratio and temperature. It is particularly important that monomer purities be greater than 99.8%, the purity of the solvent, dimethyl acetamide, greater than 99.9% with a water content less than 0.01% and the monomer ratio accurate to 1 ± 0.01 mole per cent. The poly(metaphenylene isophthalamide) polymer is converted into fibres by solution wet spinning technique. A solution of the polymer is prepared in dimethyl acetamide containing 5.0 to 6% of an alkaline earth chloride such as CaCl2 or alkali chloride such as LiCl; the inorganic salt is added to improve the solubility of the polymer in the solvent. The concentration of the polymer in the spinning solution is in the range of 13.0 wt.% to 17.0 wt.%. These concentrations are lower compared to 20 to 25 wt.% used in conventional wet spinning. The lower concentration of the spinning solution is employed to obtain low denier undrawn coagulated filaments which are then drawn to impart the desired mechanical properties to the fibre. It is important that a high molecular weight polymer is used while employing such low concentration spinning solutions in order to obtain the required dope viscosities and other rheological characteristics necessary for fibre spinning. The low shear rate viscosity of the spinning solutions of a 2.3 dl/g inherent viscosity metaphenylene isophthalamide polymer in dimethyl acetamide at 25°C was found to be 850 poise for a 13 wt.% solution and 2700 poise when the concentration of the solution was 16.5 wt.%. These solutions could easily be spun into sub-denier fibres according to the processes of this invention. As a comparison, the low shear rate viscosity of the 13wt% and 16wt.% solutions of a metaphenylene isophthalamide polymer of inherent viscosity 1.8 dl/g in dimethyl acetamide was found to be quite low and not suitable for spinning according to the procedures of present invention. Particularly the amount of stretch required for obtaining sub-denier fibres could not be employed in various stages of spinning without serious problems of filament breakage. Poly(metaphenylene isophthalamide) fibres were formed by extruding the solution through a spinnerette containing 720 holes of 60 micrometer diameter into a coagulation bath consisting of 40 wt.% aqueous calcium chloride solution maintained at 95°C. Spinnerettes with holes of diameter 60 micrometers are necessary to obtain sub-denier fibres. The extrusion velocity employed in the present invention is 6 metres per minute to 15 metres per minute, preferably between 9 to 10 metres per minute. In order to extrude the high viscosity dope through small diameter holes at the required velocities, the spinning solution was sheared extensively prior to extrusion by pumping the spinning solution through a filter pack made up of several layers, about 50, of stainless steel meshes varying in mesh size from 50 to 325. The filter pack served to reduce the viscosity of the spinning solution and smooth extrusion through the spinnerette was made possible. When extrusion was attempted with only a few layers of the mesh, the viscosity of the spinning solution as it entered the spinnerette hole was very high leading to bulging and distortion of the spinnerette face. The extruded filaments were coagulated in aqueous calcium chloride solution maintained at 95°C and forwarded to the first stretch bath. A stretch of 1.2 to 2.0 was applied to the filaments in the coagulation bath. In conventional wet spinning, the stretch in the coagulation bath is always less than 1 to accommodate the post-extrusion die-swell. In the present invention, the effect of die-swell was minimised by the use of a filter pack and also because of the excellent rheology of the low concentration spinning solution of high molecular weight polymer. The optimum stretch in the coagulation bath was found to be 1.5 to 1.6. This stretch reduced the denier of the coagulated filaments and thus afforded a more uniform coagulation of the filaments. When the filaments are coagulated, while being stretched, the coagulation takes place under the action of a uniaxial stress or the rheological stress. This stress minimises the relaxation of flow-induced molecular orientation and will lead to precursor structures which yield highly oriented para crystalline fibre morphology. The present investigators believe that this is one of the reasons for obtaining sub-denier poly(metaphenylene isophthalamide) fibres with a very high yield stress of 3.7 gpd, which value is almost 64% of the ultimate tensile strength of the fibres by following the procedures of this invention. The partially stretched and coagulated filaments are drawn a further 3.5 times in the stretch baths containing water maintained at 80 to 95°C. Subsequently, the filaments are washed free of dimethyl acetamide and roll dried at 85 to 90°C and wound on bobbins. The overall stretch during the wet spinning including the stretch in the coagulation bath should be between 4.5 and 6, preferably between 5.5 and 6 in order to obtain sub-denier poly(metaphenylene isophthalamide) fibres according to the procedures of this invention. The inventors have found that whenever high molecular weight poly(metaphenylene isophthalamide) polymer with an inherent viscosity of 2.3 dl/g or greater is employed, the above amount of stretches can always be given, under the conditions described above, without any filament breakage or similar associated problems in the spin-line. The dried fibres were found to be completely amorphous which could be crystallised by drawing the fibres above the glass transition temperature of the polymer, that is at temperatures between 280°C and 350°C, preferably between 310 and 325°C. In the present invention, the hot stretching of the fibres was carried out on a hot plate by forwarding the fibres to the hot plate at a given speed and the fibres leaving the hot plate were wound up on bobbins of a winding device. The surface speeds of the forwarding godet and the winding bobbin were adjusted to provide a draw ratio of 1.3 to 1.5. The forwarding and take-up speeds were chosen in such a way that the time spent by the fibres on the hot plate was between 2.0 and 10.0 secs. The hot plate was maintained at 310°C to 330°C. During the hot drawing operation, the amorphous as-spun fibres are converted to para crystalline fibres by stress-induced crystallisation of the polymer. The extent of cyrstallinity and orientation of the crystallites was found to depend on the hot drawing conditions. Particularly, the rate of hot drawing and the residence time significantly influence the final properties of the fibre. Higher rates of drawing were found to be beneficial. The residence times during drawing should be at least 2.0 sees and can be as long as 10.0 secs in some cases. The exact conditions of hot stretching also depend on the conditions under which the as-spun fibres are prepared. Thus when the fibres were spun from 13.5 wt.% solution, hot drawing had to be done at higher rates to obtain high yield strength whereas when the as-spun fibres were made using a 16.5 wt.% of the polymer in the spin dope, the high yield strength exceeding 3.5 gpd was obtained both at low rates of drawing as well as at high rates of drawing. To obtain high yield strength sub-denier poly(metaphenylene isophthalamide) fibres using the methods described in the present invention, the as-spun fibres should be drawn 1.35 to 1.4 times its length at a temperature of 320 to 330°C, providing a residence time of at least 2 secs at the above temperature, and the take - up speed after hot drawing being at least 40 metres per minute. Accordingly, the present invention provides a process for the preparation of sub-denier fibres having a dpf (denier per filament) in the range 0.53 to 0.75 and high yield strength such as have a tensile strength 5.5 to 5.9 gpd a tensile modulus 120 to 149 gpd, elongation to failure of 15.5 to 21.5% and an yield strength of 3 to 3.7 gpd , said process preparing a 10 to 17 wt% solution of poly (metaphenylene isophthalamide) polymer in organic solvent containing upto 6% of soluble alkali metal chloride or alkaline earth chloride such as herein described, to obtain a viscous solution followed by wet spinning by extruding the filtered polymer solution at velocities of 5 to 9.5 meters per minute and coagulating in CaCI2 solution at a temperature in the range of 80 to 100°C and wet drawing to 3 to 3.5 times and dry hot stretching to 1.5 to 2 times to obtain sub-denier fibres. In an embodiment of this invention the polymer used has an inherent viscosity in the range of 2.3 to 2.5 dl/g. In another embodiment of the present invention the concentration of the polymer used for the preparation of the dope is in the range 13.5 wt% to 16.7wt%. In an another embodiment of the present invention the amide type solvents used is such as Dimethylacetamide, N-methyl pyrrolidone. In yet another embodiment of the present invention the alkali metal chlorides used is LiCI or alkaline earth chloride used is CaCI2. In still another embodiment of the present invention the wet spinning operation is carried out by extruding the filtered polymer solution at velocities of 5 to 9.5 meters per minute and coagulating in CaCI2 solution at a temperature in the range of 80 to 100°C and wet drawing to 3 to 3.5 times and dry hot stretching to 1.5 to 2 times. In a preferred embodiment of this invention the fibres obtained have a dpf in the range 0.53 to 0.75, a tensile strength 5.5 to 21.5% and an yield strength of 3 to 3.7 gpd. In the present invention, high yield strength (3 to 3.7 gpd) sub-denier fibres with dpf values in the range 0.53 to 0.75 have been obtained by employing a spinning dope containing 13 to 17 wt% of the polymer of inherent viscosity 2.3 to 2.5 dl/g. The concentrations in the range of 13 to 17 wt% of the polymer in the dope makes the entanglement density of the molecular chains in the dope lower which will improve the ease of stretching of the fibres and lead to a better molecular orientation in the fibres. The novelty and inventive step of the process of the present invention resides in the fact that a low concentration of the order of 10-17 wt% solution of a high molecular weight polymer (polymetaphenylene isophthalamide) has been used as against the known art of using higher concentration of lower molecular weight polymer. This has resulted in the production of sub-denirer fibres of higher yield strength than that obtained by prior art processes. The following examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention. Example 1 This example illustrates the preparation of high molecular weight poly (metaphenylene isophthalamide) polymer. 10 kg of 99.9% pure dimethyl acetamide with water content of 0.007 wt.% was charged to a 20 litre stainless steel reactor provided with a cooling jacket, a high-speed turbine agitator and a low-speed gate agitator. 400g of finely powdered calcium chloride which had previously been fired at 350°C is added to dimethyl acetamide while stirring the contents of the reactor. The reactor was then cooled to -10°C and 813g of finely powdered freshly sublimed metaphenylene diamine was added. After the diamine had dissolved, 1529g of freshly distilled, finely powdered isophthaloyl chloride was added while the contents were being mixed using the high-speed turbine agitator. The low-speed gate agitator was switched on about 5 minutes after the addition of the acid chloride. The agitation was continued for an additional two hours. The poly(metaphenylene isophthalamide) polymer was recovered by precipitation of the reaction mixture, in water, followed by washing and drying. The inherent viscosity of the polymer measured using a 0.5 per cent solution in concentrated sulphuric acid was 2.3 dl/g. 200 g of the poly (metaphenylene isophthalamide) polymer of inherent viscosity of 2.3 dl/g was dissolved in 1.0 litre of dimethyl acetamide containing 60g of LiCl to give a 16.7 wt.% solution. This solution was filtered through a 2 µm polypropylene filter and used for fibre spinning. Poly(metaphenylene isophthalamide) fibres were spun by pumping the solution through a 50 layer stainless steel mesh filter pack and extruding the solution through a Platinum spinnerette having 720 holes, the diameter of each hole being 60 micrometers. The extruded filaments were coagulated in aqueous calcium chloride solution maintained at 95°C while being drawn by 1.6 times. The coagulated filaments were washed free of solvents and the additive by passing through two baths containing water at 90°C. The filaments were drawn a further 3.3 times during the above washing operation bringing the total wet draw ratio to 5.3. The filaments were then dried on a hot godet and wound on a bobbin. The dried fibres were hot drawn on a hot plate maintained at 330°C. The draw ratio was 1.45. The residence time on the hot plate was 10.9 secs. The hot drawn fibres had a denier of 0.64 per filament, an ultimate tensile strength of 5.8 gpd, an yield strength of 3.65 gpd, a tensile modulus of 145 gpd and an elongation to failure of 16%. Example 2 The dried as spun fibres of Example 1 were hot drawn by 1.45 times on a hot plate maintained at 330°C and the residence time was reduced to 4.2 secs. The fibres had a denier of 0.64 per filament, an ultimate tensile strength of 5.83 gpd, an yield strength of 3.37 gpd, a tensile modulus 149 gpd and an elongation to failure of 15.5%. Example 3 160 g of the poly (metaphenylene isophthalamide) polymer of inherent viscosity of 2.3 dl/g was dissolved in 1.0 litre of dimethyl acetamide containing 60g of LiCl to give a 13.8 wt.% solution. This solution was filtered through a 2 µm polypropylene filter and used for fibre spinning. Poly(metaphenylene isophthalamide) fibres were spun by pumping the solution through a 50 layer stainless steel mesh filter pack and extruding the solution through a Platinum spinnerette having 720 holes, the diameter of each hole being 60 micrometers. The extruded filaments were coagulated in aqueous calcium chloride solution maintained at 95°C while being drawn by 1.5 times. The coagulated filaments were washed free of solvents and the additive by passing through two baths containing water at 90°C. The filaments were drawn a further 3.5 times during the above washing operation bringing the total wet draw ratio to 5.3. The filaments were then dried on a hot godet and wound on a bobbin. The as-spun fibres were drawn 1.35 times on a hot plate maintained at 320°C and the residence time was 2.0 secs. The drawing was carried out at a high rate by feeding the fibres at 30 meters per minute and taking up the drawn fibres at 40 metres per minute. The fibres had denier of 0.75 per filament, a tensile strength of 5.85 gpd, an yield strength of 3.65 gpd, a tensile modulus of 130 gpd and an elongation to failure of 16.7%. Example 4 The as-spun fibres of Example 3 were drawn 1.6 times on a hot plate maintained at 320°C, the residence time being 9.5 secs. The drawing was done at a slow rate by feeding the fibres at 5.8 metres per minute and taking up the drawn fibres at 9.3 metres per minute. The fibres had a denier per filament of 0.53, a tensile strength of 5.5 gpd, an yield strength of only 3.0 gpd, a tensile modulus of 120 gpd and an elongation to failure of 21.5 %. The advantages of the present invention are: The present invention provides a process for the preparation of sub-denier (0.53 to 0.75 dpi) fibres i.e. having filament diameter 7.5 to 9 µm of high yield strength (3 to 3.7 gpd). The main advantages of the lower diameter fibres of the present invention are : 1. The use of low diameter fibres during the preparation of electrical insulating paper will increase the surface area of a given volume of fibres resulting in higher bonded area fraction in the paper and this improves the mechanical and electrical properties of the paper. 2. The use of low diameter fibres increases the number of fibres per unit volume and this leads to a more uniform dispersion of the fibres in the paper. 3. The use of low diameter fibres allows the use of higher volume fraction of fibres in the paper resulting in improved mechanical properties of the paper like tensile strength and tear resistance. 4. The higher yield strength of the fibres will be useful in obtaining high dimensional stability for the electrical insulating paper. We Claim: 1. A process for the preparation of sub-denier fibres having a dpf (denier per filament) in the range 0.53 to 0.75 and high yield strength as have a tensile strength 5.5 to 5.9 gpd ( gm per denier), a tensile modulus 120 to 149 gpd, elongation to failure of 15.5 to 21.5% and an yield strength of 3 to 3.7 gpd , said process characterized in that preparing a 10 to 17 wt% solution of poly (metaphenylene isophthalamide) polymer in organic solvent containing upto 6% of soluble alkali metal chloride or alkaline earth chloride as herein described, to obtain a viscous solution followed by wet spinning by extruding the filtered polymer solution at velocities of 5 to 9.5 meters per minute and coagulating in CaCl2 solution at a temperature in the range of 80 to 100°C and wet drawing to 3 to 3.5 times and dry hot stretching to 1.5 to 2 times to obtain sub-denier fibres. 2. A process as claimed in claim 1 wherein the polymer used is having viscosity in the range of 2.3 to 2.5 dl/g (denial /gm) 3. A process as claimed in claims 1 and 2 wherein the concentration of the polymer used is in the range 13.5 wt% to 16.7wt%. 4. A process as claimed in claims 1 to 3 wherein the organic solvent used is selected from Dimethylacetamide, N-methyl pyrrolidone. 5. A process as claimed in claims 1 to 4 wherein the alkali metal chlorides used is LiCI or alkaline earth chloride used is CaCI2. 6. A process for the preparation of sub-denier fibres substantially as herein described with reference to the examples.

Documents:

152-del-2000-abstract.pdf

152-del-2000-claims.pdf

152-del-2000-correspondence-others.pdf

152-del-2000-correspondence-po.pdf

152-del-2000-description (complete).pdf

152-del-2000-form-1.pdf

152-del-2000-form-19.pdf

152-del-2000-form-2.pdf

152-del-2000-form-3.pdf