Abstract:
Disclosed is the preparation of very high molecular weight polyamide, e.g., nylon, filaments, as indicated by such filaments exhibiting a very high Relative Viscosity (RV) value. Such filaments can be used to prepare polyamide staple fibers which are especially useful for industrial applications such as in papermachine felts. The filament preparation process involves a melt phase polymerization (MPP) procedure, optionally carried out in combination with a solid phase polymerization (SPP) procedure. Both of these procedures serve to increase the molecular weight and hence the RV of the polyamide filaments produced. These procedures are conducted under selected controlled conditions which permit realization of polyamide filaments of about 2 to 100 denier and which have RV values of greater than about 190. Such filaments also exhibit excellent tenacity and tenacity resistance properties.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit of priority from Provisional Application No. 60/980,617, filed Oct. 17, 2007. This application hereby incorporates by reference Provisional Application No. 60/980,617 in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the preparation of very high molecular weight polyamide, e.g., nylon, filaments. Very high molecular weight is indicated by the filaments exhibiting a very high Relative Viscosity (RV) as defined herein. Such filaments can be used to prepare polyamide staple fibers which are especially useful for industrial applications such as in papermachine felts. 
       BACKGROUND OF THE INVENTION 
       [0003]    Industrial polyamide filaments are used in, among other things, tire cords, airbags, netting, ropes, conveyor belt cloth, felts, filters, fishing lines, and industrial cloth and tarps. When used as staple fibers for papermaking machine felts, the fibers must have generally good resistance to chemicals and generally good wear resistance (e.g., resistance to abrasion, impact and flex fatigue). Such felts are often exposed to oxidizing aqueous solutions which can seriously shorten the service life of the felt. 
         [0004]    Stabilizers are often added to polyamides for the purpose of increasing chemical resistance. The amount of stabilizer which can be introduced is limited, however, due to excess foaming that occurs during polymerization when stabilizers are added to autoclaves or continuous polymerizers (CPs). 
         [0005]    Another way of improving chemical and abrasion resistance of fibers used in papermaking machine felts is to make fibers from melt spun filaments which have relatively high molecular weight as reflected by such filaments exhibiting high Relative Viscosity (RV). However, in the past, when the polyamide supply for such filaments is polyamide flake, it was often difficult, if not impossible, to obtain filaments of the desired high RV while maintaining polymer quality, e.g., low level of cross linking and/or branching. 
         [0006]    One way to increase the RV of polyamide filaments is to increase the amount of catalyst present during polymerization in an autoclave, continuous polymerizer (CP), or elsewhere in the process. This, however, can cause process and/or product problems. Difficulties, for instance, similar to those encountered with stabilizers can occur when catalysts are added in amounts suitable to increase polymer molecular weight. Further, high quantities of catalysts in the autoclave can cause severe injection port pluggage and complications to injection timings during autoclave cycles. High quantities of catalysts injected into CPs place stringent demands on equipment capability because of high levels of water loading. 
         [0007]    In U.S. Pat. No. 5,236,652 to Kidder, a process is disclosed for making polyamide fibers for use as staple for papermaking machine felt. This process comprises (i) melt-blending polyamide flake with a polyamide additive concentrate which is made of a polyamide flake and an additive selected from the group of stabilizers, catalysts and mixtures thereof, and (ii) extruding the melt-blended mixture from a spinneret to form the higher RV fibers. The Kidder process thus requires separate preparation of a polyamide additive concentrate which is added to an extruder used in melt-blending polyamide flake. 
         [0008]    Another way to increase the RV of polyamide filaments is through solid phase polymerization (SPP) of the polymer after melt spinning. U.S. Pat. No. 5,234,644 to Schutze et al discloses a post spin SPP process for making high RV polyamide fibers for use in paper machinery webs. In this process, in contrast to prior staple fiber manufacturing processes, the post spin SPP process requires an added step after spinning the fibers with special processing equipment to increase the RV of the fibers. This special equipment adds a significant cost to the producer and the added post spinning step takes additional time to make the fibers. Furthermore, uniform fiber property control is more difficult when the post spinning SPP step is performed in a batch mode. 
         [0009]    A process and apparatus setup for preparing very high RV polyamide filaments is also disclosed in U.S. Pat. No. 6,235,390 to Schwinn and West. Such a process utilitizes both a solid phase polymerization (SPP) conditioning of polyamide flake materials followed by a melt phase polymerization (MPP) procedure to produce material suitable for spinning into filaments. The SPP phase of such a procedure utilizes a specific type of dual desiccant drying operation to condition catalyst-containing polyamide flake. Such conditioned and dried flake material is then fed to an MPP setup employing a melt-extruder and transfer lines (which optionally run to and through a booster pump and a manifold) to convey molten polyamide material to melt-spinning apparatus. The procedures and apparatus of the Schwinn/West patent permit preparation of filaments having an RV of at least about 140. Preparation of filaments having RV values as high as 169 are, in fact, disclosed in this U.S. Pat. No. 6,235,390. 
         [0010]    Prior art methods for obtaining high molecular weight polyamide fibers from high molecular weight polymers present difficulties, and have limitations. Specifically the use of high molecular weight resins, i.e., those of a molecular weight close to the desired fiber molecular weight, creates issues associated with extruding and pumping these polymers because of their high viscosity. 
         [0011]    Transporting relatively high viscosity polymers through equipment designed to produce fibers causes increased polymer temperatures due to friction. The amount of temperature increase is directly related to the viscosity (which in turn is related to the molecular weight) of the polymer. The temperature will increase at each step of the filament preparation process, e.g., in the extruder, in transfer lines, in transfer line pumps, in piping manifolds, in spinning meter pumps, and in the spin packs. This is true of conventional, relatively normal molecular weight (RV 50 to 70) polyamide fiber processes. The effect is magnified in processes involving high molecular weight polyamides due to the much higher polymer viscosities involved. The increased polymer temperatures encountered in such processes can result in degradation of the polymer, thereby actually decreasing the molecular weight of the polymer in the resulting filaments. 
         [0012]    Given all of the foregoing prior art procedures for preparing and realizing high RV polyamide filaments, and further given the issues associated with preparation of high RV polyamide filaments, it would be advantageous and desirable to identify improved procedures for efficiently producing polyamide, e.g., nylon, filaments having RV values even higher than those which have been previously reported. Such especially high molecular weight filaments would be those having tenacity and abrasion and chemical resistance properties such that they could be used to prepare polyamide staple fibers of especially desirable characteristics for industrial uses, such as, in making papermaking machine felts. 
       SUMMARY OF THE INVENTION 
       [0013]    In its process aspects, the present invention provides a process for preparing a plurality of meltspun polyamide filaments having a denier of from about 2 to about 100, a formic acid relative viscosity (RV) of greater than about 190, and tenacity and tenacity retention characteristics which render such filaments especially suitable for use in papermaking machine felts. Such a process involves melt phase polymerizing of polyamide flake material before spinning it into filaments. Preferably, the polyamide flake material to be melt phase polymerized has been prepared by a specific solid phase polymerization (SPP) procedure. 
         [0014]    In the melt phase polymerization (MPP) part of the process herein, conditioned SPP polyamide flake material having a formic acid relative viscosity (RV) of from about 90 to 120 and a moisture content of less than about 0.04 wt %, preferably prepared as hereinafter described, is used. The MPP procedure comprises the steps of A) feeding these solid phase polymerized (SPP) polyamide flakes at a temperature of from about 120° C. to 200° C. into a non-vented melt-extruder; B) melting the flakes in the melt-extruder while introducing at a flake feed end of the extruder a liquid phenolic antioxidant stabilizer which has not been premixed with polyamide material; C) extruding molten polymer resulting from the melting of said flakes from an outlet end of the melt-extruder to a transfer line wherein the temperature of the molten polymer in the transfer line within 5 feet (2.4 m) of the outlet end of the melt-extruder is from about 285° C. to 295° C.; D) conveying the molten polymer through the transfer line and via a booster pump and a manifold to at least one spinneret of at least one spinning machine; and E) spinning the molten polymer through the at least one spinneret to form a plurality of meltspun high RV polyamide filaments. 
         [0015]    In conveying the molten polymer from the melt-extruder to the spinneret, the temperature of polymer in the transfer line within 5 feet (2.4 m) of the at least one spinneret is maintained from about 295° C. to about 300° C. Further, during this transfer of molten polymer from melt-extruder to spinneret, the ratio of a) the pressure drop (ΔP in psig) between the booster pump and the manifold; to b) the molten polymer throughput (in kg/hr) is maintained in the range of from about 2.5 to 3.5. 
         [0016]    In a preferred embodiment of the process herein, SPP flake material used in the MPP process has been prepared using a certain type of conditioning procedure. In this SPP conditioning procedure, precursor polyamide flake material is used which comprises a synthetic melt spinnable polyamide polymer and a polyamidation catalyst dispersed within the flakes. Such precursor flake material has a formic acid relative viscosity (RV) of from about 40 to 60. These solid phase polymerized precursor polyamide flakes are preferably conditioned by the steps of: i) feeding the precursor polyamide flakes into a solid phase polymerization vessel; ii) contacting these precursor flakes within this vessel with a substantially oxygen free inert gas; iii) drying at least a portion of the inert gas with a serially connected dual desiccant bed regenerative drying system such that the gas entering the polymerization vessel has a dew point of no more than about 10° C.; iv) heating the inert gas to a temperature of from about 120° C. to 200° C.; v) circulating the filtered, dried, heated gas through interstices between the flakes in the polymerization vessel for 4 to 24 hours; and vi) removing from the vessel, and feeding to the melt phase polymerization part of the process, flakes which have a formic acid relative viscosity (RV) of from about 90 to 120. It is these SPP flakes, conditioned in this manner, which are preferably used as the feed to the melt-extruder in the MPP process herein. 
         [0017]    In its composition aspects, the present invention is directed to a plurality of polyamide filaments suitable for use in making fibers for papermaking machine felts. Each of the filaments comprises a synthetic melt spun polyamide polymer and has A) a formic acid relative viscosity of greater than about 190; B) a denier of from about 2 to about 100 (a decitex of about 2.2 to about 111); and C) a tenacity of from about 4.0 grams/denier to about 7.0 grams/denier (from about 3.5 cN/dtex to about 6.2 cN/dtex). Such filaments also exhibit certain retained tenacity characteristics under conditions which simulate those which occur when fibers made from such filaments are used, for example, in papermaking felts. 
         [0018]    In preferred embodiments, the polyamide polymer used to form the filaments of this invention is selected from the group consisting of poly(hexamethylene adipamide) [nylon 6,6], poly(ε-caproamide) [nylon 6] and copolymers or mixtures thereof. Also preferably the plurality of filaments will be in the form of staple fibers having a length of about 1.5 to about 5 inches (about 3.8 cm to about 12.7 cm). In other preferred embodiments, the plurality of filaments will be in the form of staple fibers having a saw tooth shaped crimp, with a crimp frequency of about 3.5 to about 18 crimps per inch (about 1.4 to about 7.1 crimps per cm). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention can be more fully understood from the following detailed description thereof in connection with accompanying drawings briefly described as follows: 
           [0020]      FIG. 1  is a schematic illustration of an apparatus for solid phase polymerizing polymer flake. 
           [0021]      FIG. 2  is a schematic illustration of a portion of a fiber manufacturing procedure wherein flake is fed to a non vented melt-extruder, melted and extruded to a transfer line, conveyed through the transfer line via a booster pump and manifold to at least one spinneret, spun into filaments, converged into tows, and placed in a storage container. 
           [0022]      FIG. 3  is a schematic illustration of a portion of a fiber manufacturing procedure wherein tows are removed from a plurality of storage containers, combined into a tow band, drawn, crimped, and cut to form crimped staple fibers. 
       
    
    
       [0023]    Throughout the following detailed description, similar reference characters refer to similar elements in all figures of the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present invention is directed to the preparation of industrial, high relative viscosity (RV) polyamide filaments, such as, for use in papermaking machine felts and other staple fiber applications. The invention is further directed to processes which preferably involve both solid phase polymerization (SPP) of polyamide flake and subsequent melt phase polymerization of molten flakes and spinning of the molten polymer into industrial high RV filaments. Accordingly, this invention represents an improvement of the processes and filaments which are disclosed in U.S. Pat. No. 6,235,390, which patent is incorporated herein by reference in its entirety. 
         [0025]    For purposes herein, the term “solid phase polymerization” or “SPP” means increasing the RV of polymer while in the solid state. Also, herein increasing polymer RV is considered synonymous with increasing polymer molecular weight. Further, for purposes herein, the term “melt phase polymerization” or “MPP” means increasing the RV (or the molecular weight) of polymer while in the liquid state. 
       Industrial High RV Filaments 
       [0026]    The invention herein is concerned with the preparation of industrial high RV filaments. For purposes herein, the term “industrial filament” means any filament having a formic acid RV of at least about 70; a denier of at least about 2 (a decitex of about 2.2); and a tenacity of about 4.0 grams/denier to about 11.0 grams/denier (about 3.5 cN/dtex to about 9.7 cN/dtex). 
         [0027]    Polymer suitable for use in the process and filaments of this invention consists of synthetic melt spinnable or melt spun polymer. Such polymers can include polyamide homopolymers, copolymers, and mixtures thereof which are predominantly aliphatic, i.e., less than 85% of the amide-linkages of the polymer are attached to two aromatic rings. Widely-used polyamide polymers such as poly(hexamethylene adipamide) which is nylon 6,6 and poly(ε-caproamide) which is nylon 6 and their copolymers and mixtures can be used in accordance with the invention. Other polyamide polymers which may be advantageously used are nylon 12, nylon 4,6, nylon 6,10, nylon 6,12, nylon 12,12, and their copolymers and mixtures. Illustrative of polyamides and copolyamides which can be employed in the process of this invention are those described in U.S. Pat. Nos. 5,077,124, 5,106,946, and 5,139,729 (each to Cofer et al.) and the polyamide polymer mixtures disclosed by Gutmann in Chemical Fibers International, pages 418-420, Volume 46, December 1996. These publications are all incorporated herein by reference. 
         [0028]    The filaments herein can include one or more polyamidation catalysts. Polyamidation catalysts suitable for use in a solid phase polymerization (SPP) process and/or a (re)melt phase polymerization (MPP) process which can be performed in making the filaments herein are oxygen-containing phosphorus compounds including those described in Curatolo et al., U.S. Pat. No. 4,568,736 such as phosphorous acid; phosphonic acid; alkyl and aryl substituted phosphonic acids; hypophosphorous acid; alkyl, aryl and alkyl/aryl substituted phosphinic acids; phosphoric acid; as well as the alkyl, aryl and alkyl/aryl esters, metal salts, ammonium salts and ammonium alkyl salts of these various phosphorus containing acids. Examples of suitable catalysts include X(CH 2 ) n PO 3 R 2 , wherein X is selected from 2-pyridyl, —NH 2 , NHR′, and N(R′) 2 , n=2 to 5, R′ and R′ independently are H or alkyl; 2-aminoethylphosphonic acid, potassium tolylphosphinate, or phenylphosphinic acid. Preferred catalysts include 2-(2′-pyridyl)ethyl phosphonic acid, and metal hypophosphite salts including sodium and manganous hypophosphite. It may be advantageous to add a base such as an alkali metal bicarbonate with the catalyst to minimize thermal degradation, as described in Buzinkai et al., U.S. Pat. No. 5,116,919. 
         [0029]    An effective amount of the catalyst(s) will generally be dispersed in the polyamide material. Generally the catalyst is added, and therefore present, in an amount from about 0.2 moles up to about 5 moles per million grams, mpmg, of polyamide (typically about 5 ppm to 155 ppm based on the polyamide). Preferably, the catalyst is added in an amount of about 0.4 moles to about 0.8 moles million grams, mpmg, of polyamide (about 10 ppm to 20 ppm based on the polyamide). This range provides commercially useful rates of solid phase polymerization and/or remelt phase polymerization under the conditions of the current invention, while minimizing deleterious effects which can occur when catalyst is used at higher levels, for example pack pressure rise during subsequent spinning. 
         [0030]    For effective solid phase polymerization, it is necessary for the amidation catalyst to be dispersed in the polyamide precursor flake. A particularly convenient method for adding the polyamidation catalyst is to provide the catalyst in a solution of polymer ingredients in which polymerization is initiated, e.g., by addition to a salt solution such as the hexamethylene-diammonium adipate solution used to make nylon 6,6. 
         [0031]    The polyamide material used to make the high RV filaments will also contain a phenolic, e.g., hindered phenolic, antioxidant stabilizer which is added in a particular manner and at a particular point during melt phase polymerization as hereinafter described. The class of useful phenolic antioxidant stabilizers employed in this invention comprises alkyl-substituted and/or aryl-substituted phenols; and mixtures thereof. 
         [0032]    Preferred phenolic antioxidant stabilizers are the alkyl-substituted, hindered phenols. Most preferably, the additive is 1,3,5-trimethyl-2,4,6-tris(3,5-tertbutyl-4-hydroxybenzyl)benzene (IRGANOX™ 1330), tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane (IRGANOX™ 1010); (N,N′hexane-1,6-diylbis(3-(3,5-di-tert-butyl-4-hydroxyphenylpropionamide) (IRGANOX™ 1098) or 3,5-bis(1,1-dimthylethyl)-4-hydroxy-2,2-bis{[3-(3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]-1-oxopropoxy}-1,3-propanediyl ester (ANOX® 20). 
         [0033]    The antioxidant stabilizer will generally be added in liquid form to the polyamide material in the extruder to form a molten polymer which contains about 0.05 wt % to about 2 wt % of the stabilizer. More preferably, the molten polymer will comprise from about 0.1 wt % to about 0.7 wt % of the antioxidant stabilizer. The filaments produced herein can also optionally contain usual minor amounts of other additives, such as plasticizers, delustrants, pigments, dyes, light stabilizers, heat stabilizers, antistatic agents for reducing static, additives for modifying dye ability, agents for modifying surface tension, etc. 
         [0034]    The polyamide filaments herein will have a formic acid RV of greater than about 190. More preferably, the filaments herein will have a formic acid RV of greater than about 200. Most preferably, the filaments herein can have a formic acid RV of from about 202 to about 230. 
         [0035]    The formic acid RV of polyamides as used herein refers to the ratio of solution and solvent viscosities measured in a capillary viscometer at 25° C. The solvent is formic acid containing 10% by weight of water. The solution is 8.4% by weight polyamide polymer dissolved in the solvent. This test is based on ASTM Standard Test Method D 789. Preferably, the formic acid RVs are determined on spun filaments, prior to drawing and can be referred to as spun fiber formic acid RVs. The RV of polyamide filaments can decrease from about 3% to about 7% upon drawing at the draw ratios described herein, but the RV of the drawn filaments will be substantially the same as the spun fiber RVs. The formic acid RV determination of a spun filament is more precise than the formic acid RV determination of a drawn filament. As such, for purposes herein, the spun fiber RVs are reported and are considered to be a reasonable estimate of the drawn fiber RVs. The RV of the filaments achievable with this invention exceeds what has been reported for prior art filament preparation processes. 
         [0036]    The filaments when drawn will generally have a denier per filament (dpf) of about 2 to about 100 (a dtex per filament of about 2.2 to 111). More preferably, the filaments herein when drawn will have a denier per filament (dpf) of about 10 to 40 (a dtex per filament of about 11.1 to about 44.4). These deniers are preferably measured deniers based on ASTM Standard Test Method D 1577. 
         [0037]    The filaments, when drawn, will generally have a tenacity of about 4.0 grams/denier to about 7.0 grams/denier (about 3.5 cN/dtex to about 6.2 cN/dtex). Preferably, the filaments will have a tenacity of about 4.5 grams/denier to about 6.5 grams/denier (about 4.0 cN/dtex to about 5.7 cN/dtex). Further, preferably the percent retained tenacity of the filaments (i) is greater than or equal to about 50% when immersed for 72 hours at 80° C. in an aqueous solution of 1000 ppm of NaOCl, or (ii) is greater than or equal to about 75% when heated at 130° C. for 72 hours. It is more preferred that the filaments have a percent retained tenacity which is greater than about 60% when immersed for 72 hours at 80° C. in an aqueous solution of 1000 ppm of NaOCl. 
         [0038]    For purposes herein, the term “filament” is defined as a relatively flexible, macroscopically homogeneous body having a high ratio of length to width across its cross-sectional area perpendicular to its length. The filament cross section can be any shape, but is typically circular. Herein, the term “fiber” is used interchangeably with the term “filament”. 
         [0039]    The filaments herein can be any length. The filaments can be cut into staple fibers having a length of about 1.5 to about 5 inches (about 3.8 cm to about 12.7 cm). Furthermore, the staple fiber can be straight (i.e., non crimped) or crimped to have a saw tooth shaped crimp along their length, with a crimp (or repeating bend) frequency of about 3.5 to about 18 crimps per inch (about 1.4 to about 7.1 crimps per cm). 
       Apparatus and Process for SPP of Precursor Polymer Flake 
       [0040]    In the initial stages of the preferred filament preparation process herein, precursor polyamide flakes are subjected to an SPP process for solid phase polymerization of such precursor flake material. This precursor flake material is made of the polyamide polymer which is ultimately suitable for use in making the filaments of the present invention. 
         [0041]    The precursor polymer flake can be prepared using batch or continuous polymerization methods known in the art, pelletized, and then fed to the SPP apparatus. As illustrated in  FIG. 1 , a typical example is to store a polyamide salt mixture/solution in a salt storage vessel  2 . The salt mixture/solution is fed from the storage vessel  2  to a polymerizer  4 , such as a continuous polymerizer or a batch autoclave. The previously mentioned polyamidation catalysts can be added simultaneously with the salt mixture/solution or separately. In the polymerizer  4 , the polyamide salt mixture/solution is heated under pressure in a substantially oxygen free inert atmosphere as is known in the art. The polyamide salt mixture/solution is polymerized into molten polymer which is extruded from the polymerizer  4 , for example, in the form of a strand. The extruded polymer strand is cooled into a solid polymer strand and fed to a pelletizer  6  which cuts, casts or granulates the polymer into flake. 
         [0042]    Other terms which can be used to refer to this “flake” material include pellets and granulates. Most conventional shapes and sizes of flake are suitable for use in the present invention. One typical shape and size comprises a pillow shape having dimensions of approximately ⅜ inch (9.5 mm) by ⅜ inch (9.5 mm) by 0.1 inch (0.25 mm). Alternatively, flake in the shape of right cylinders having dimensions of approximately 90 mils by 90 mils (2.3 mm by 2.3 mm) are convenient. Thus, it should be appreciated that the precursor polyamide material can be shaped and fed into the SPP apparatus  10  in other particulate forms than “flake”, and all such particulate forms are amenable to the initial SPP step of the filament preparation process of the instant invention. 
         [0043]    The precursor polymer flake has one or more of the polyamidation catalysts hereinbefore described dispersed within the flake. The precursor flake has a formic acid RV of about 40 to about 60. More preferably, precursor flake will have a formic acid RV of about 45 to 55. Most preferably, the precursor flake will have a formic acid RV of about 45 to 50. Further, the precursor flake can contain variable amounts of absorbed water. 
         [0044]    Suitable SPP apparatus  10  comprises a SPP assembly  12  and a serially connected dual desiccant bed regenerative drying system  14 . The SPP assembly  12  has a SPP vessel  16  and a gas system  18 . 
         [0045]    The SPP vessel  16 , otherwise known in the art as a flake conditioner, has a flake inlet  20  for receiving the precursor flake, a flake outlet  22  for removing the flake after being solid phase polymerized in the SPP vessel  16 , a gas inlet  24  for receiving circulating gas, and a gas outlet  26  for discharging the gas. The flake inlet  20  is at the top of the SPP vessel  16 . The flake outlet  22  is at the bottom of the SPP vessel  16 . The gas inlet  24  is towards the bottom of the SPP vessel  16 , whereas the gas outlet  26  is towards the top of the SPP vessel  16 . The flake can be fed one batch at a time or continuously into the flake inlet  20  of the SPP apparatus  10 . The flake can be fed into the SPP apparatus  10  at room temperature or preheated. In a preferred embodiment, the SPP vessel  16  can contain up to about 15,000 pounds (6,800 kilograms) of the flake. 
         [0046]    The gas system  18  is for circulating substantially oxygen free inert gas, such as nitrogen, argon, or helium, into the gas inlet  24 , through interstices between, thereby contacting, the flake in the SPP vessel  16 , and then out the gas outlet  26 . Thus, the gas circulates upwardly through the SPP vessel  16  counter current to the direction of flake flow when the process continually feeds flake into the flake inlet  20  and removes flake from the flake outlet  22  of the SPP vessel  16 . The preferred gas is nitrogen. Atmospheres containing other gases, for example nitrogen containing low levels of carbon dioxide, can also be used. For purposes of the present invention, the term “substantially oxygen free” gas refers to a gas containing at most about 5000 ppm oxygen when intended for use at temperatures of the order of 120° C. down to containing at most about 500 ppm oxygen for applications approaching 200° C. and containing as low as a few hundred ppm oxygen for some applications highly sensitive to oxidation. 
         [0047]    The gas system  18  has a filter  28  for separating and removing dust and/or polymer fines from the gas, a gas blower  30  for circulating the gas, a heater  32  for heating the gas, and a first conduit  34  connecting, in series and in turn, the gas outlet  26 , the filter  28 , the blower  30 , the heater  32 , and the gas inlet  24 . 
         [0048]    The filter  28  removes fine dust generally comprising volatile oligomers which have been removed from the flake and subsequently precipitated out as the gas has cooled. A suitable filter  28  is a particulate cyclone separator that impinges circulating gas on a plate causing solids to drop out, such as described on Pages 20-81 through 20-87 of the Chemical Engineers&#39; Handbook, Fifth Edition, by Robert H. Perry and Cecil H. Chilton, McGraw-Hill Book Company, NY, N.Y., published 1973. Alternatively, filters of nominally 40 microns or less are sufficient to remove the fine powder that can be created in the process. It is preferred to remove the volatile oligomers before the gas passes through desiccant beds of the drying system  14  as they can be a fire hazard during regeneration of the desiccant. 
         [0049]    Preferably, the blower  30  is adapted to force a substantially constant amount of the gas per unit time through the SSP vessel  16  while maintaining pressure of the gas in the drying system  14  at about 2 psig to about 10 psig (about 14 kilopascals to about 70 kilopascals) and to maintain gas flow and positive pressure in the SPP vessel  16 . The blower  30  can heat the circulating gas up several degrees Celsius or more depending on the make and model of the blower  30  that is used. In a preferred embodiment, the blower  30  is adapted to circulate gas through the SPP vessel  16  at a rate of about 800 to about 1800 standard cubic feet per minute (about 29 cubic meters per minute to about 51 cubic meters per minute). Gas flow is maintained low enough to preclude fluidization of the flake. 
         [0050]    The heater  32  is adapted to heat the gas in the SPP vessel  16  to a temperature of about 120° C. to about 200° C., preferably, about 150° C. to about 190° C., and most preferably to about 170° C. to about 190° C. The gas is generally heated to provide the thermal energy to heat the flake. At the gas inlet  24 , temperatures below about 150° C., require the flake residence time in the SPP vessel  16  to be too long and/or require the use of undesirably large solid phase polymerization vessels. Gas inlet temperatures greater than 200° C. can result in thermal degradation and agglomeration of the flake. The temperature of the gas existing the SPP vessel  16  through the gas outlet  26  can be at or below 100° C. requiring reheating by the heater  32  before reentry to the SPP vessel  16 . 
         [0051]    The serially connected dual desiccant bed regenerative drying system  14  is connected in parallel with the first conduit  34  between the blower  30  and the gas inlet  24 . The drying system  14  is for drying the circulating gas increasing the removal of water from the flake in the SPP vessel  16 . Water removal in turn drives the condensation reaction of the polyamide flake towards higher RV. Thus, the drying system  14  is for drying and lowering the dew point temperature of at least a portion of the circulating gas such that the dew point temperature of the gas at the gas inlet  24  is no more than about 20° C. More preferred, the dew point temperature of the gas at the gas inlet  24  is about −10° C. to 20° C. Most preferred, the dew point temperature of the gas at the gas inlet  24  is about 0° C. to about 10° C. The dew point temperature of the gas exiting the SPP vessel  16  through the gas outlet  26  can be above 30° C. and in need of drying. 
         [0052]    The portion of the gas that is passed through the drying system  14  can be up to 100% of the total gas stream circulated through the SPP vessel  16 . However, if less than 100% of the total gas stream is bypassed through the drying system  14 , then the dew point temperature at the gas inlet  24  can be controlled more accurately with a lower capacity, and therefore less expensive, drying system. Further, adjusting the portion of the gas being dried provides a fine quantity control for selecting and controlling the RV of the flake removed from the SPP vessel  16 . Such adjustments provide useful means for producing uniform RV flake. Thus, it is more preferred that the portion of the gas that is passed through the drying system  14  is about 10% to about 50% of the total gas stream circulated through the SPP vessel  16 . Most preferred, the portion of the gas that is passed through the drying system  14  is about 20% to about 40% of the total gas stream circulated through the SPP vessel  16 . 
         [0053]    Preferably, the drying system  14  is connected in parallel with the first conduit  34  and between the blower  30  and the heater  32 . There can be an adjustable valve  36  connected in the first conduit  34  between the blower  30  and the heater  32 . Then the drying system  14  can be connected in parallel with the adjustable valve  36 . 
         [0054]    The drying system  14  comprises an optional first valve  38 , an optional gas flow meter  40 , an optional second valve  42 , a serially connected dual desiccant bed regenerative dryer  50 , an optional third valve  52 , an optional fourth valve  54 , and a second conduit  56  interconnecting, in turn, the first conduit  34  (preferably between the blower  30  and the adjustable valve  36 ), the optional first valve  38 , the optional gas flow meter  40 , the optional second valve  42 , the serially connected dual desiccant bed regenerative dryer  50 , the optional third valve  52 , the optional fourth valve  54 , and the first conduit  34  (preferably between the adjustable valve  36  and the heater  32 ). The first and fourth valves  38 , 54  are useful if one wants to take the drying system  14  off line for maintenance work. As such, the first and fourth valves  38 , 54  can be, for instance, manual butterfly valves that are designed to be used in either a fully open or fully closed position. The second and third valves  42 , 52  are useful if one wants to isolate the dryer  50  from the remainder of the drying system  14  for maintenance or replacement of the dryer  50 . The second and third valves  42 , 52  can be, for instance, manual isolation valves. 
         [0055]    Referring further to  FIG. 1 , the SPP apparatus  10  can optionally include a dew point temperature measurement instrument  120  connected to the first conduit  34  for measuring the dew point temperature of the combined gas stream in the first conduit  34  downstream of the drying system  14 . The dew point temperature measurement instrument  120  can be connected to the first conduit  34  downstream of the drying system  14 , either before (as depicted in  FIG. 1 ) or after the heater  120 . In either case, the dew point temperature measurement instrument  120  should be positioned close enough to the gas inlet  24  to provide a measurement of the temperature at the gas inlet  24 . 
         [0056]    The SPP apparatus  10  is adapted such that solid state polymerization of the flake occurs in the SPP vessel  16  increasing the formic acid RV of the flake while the gas is filtered, dried, heated and circulated through the interstices between, thereby contacting, the flake in the SPP vessel  16  at a temperature of about 120° C. to about 200° C. for about 4 hours to about 24 hours, after which flake having a formic acid RV of at least about 90 can be removed from the flake outlet  22 . More preferably, the flake residence time in the SPP vessel  16  is about 5 hours to about 15 hours, most preferably about 7 hours to about 12 hours. Preferably, continuous drying of the flake in the SPP vessel  16  proceeds throughout the residence time. More preferably, the flake removed from the flake outlet  22  has a formic acid RV of about 90 to 120, most preferably, of about 100 to 120. 
         [0057]    In summary, the SPP phase of a preferred process herein can comprise the following steps. First, the precursor flake is fed into the SPP vessel  16 . Second, dust and/or polymer fines are preferably separated and removed from the gas by the filter  28 . Third, at least a portion of the gas is dried with the serially connected dual desiccant bed regenerative drying system  14  such that the gas entering the SPP vessel  16  has a dew point temperature of no more than 20° C. Fourth, the gas is heated by the heater  32  to a temperature of about 120° C. to about 200° C. Fifth, the filtered, dried, heated gas is circulated by the blower  30  through interstices between the flake in the SPP vessel  16  for about 4 to about 24 hours. Sixth, the flake having a formic acid RV of at least about 90 is removed from the flake outlet  22  of the SPP vessel  16 . 
         [0058]    The flake having a formic acid RV of at least about 90 can be withdrawn from the flake outlet  22  at the same rate that flake is fed into the flake inlet  20  to maintain the flake volume in the SPP vessel  16  substantially the same. 
       Process for MPP of Molten Polymer 
       [0059]    The filament preparation process herein includes MPP procedures for melt phase polymerizing molten polyamide polymer which is then formed into filaments. The MPP and melt-spinning phases of the process herein comprise the following steps: 
         [0060]    As shown in  FIGS. 1 and 2 , the SPP apparatus  10  can be coupled to a flake feeder  130  which, in turn, is coupled to feed the polymer flake at a temperature of about 120° C. to about 200° C. into a non-vented melt-extruder  132 . The flake feeder  130  can be, for instance, a gravimetric or volumetric feeder. In a preferred embodiment, the feeder  130  can provide a metered amount of the flake to the melt-extruder  132  in the range of about 1100 pounds per hour to about 1900 pounds per hour (500 kilograms per hour to about 862 kilograms per hour), more preferably of about 1180 pounds per hour to about 1900 pounds per hour (536 kilograms per hour to about 818 kilograms per hour). 
         [0061]    The polyamide flake that is fed into the melt-extruder  132  comprises a formic acid RV of about 90 to 120, and a polyamidation catalyst dispersed within the flake. Preferably, the flake has a formic acid RV of about 100 to 120. The flake fed to the melt-extruder will also generally have a moisture content of less than about 0.04 wt %, more preferably from about 0.01 wt % to 0.03 wt %. Flake removed from the SPP assembly  10  is quite suitable for feeding into the melt-extruder  132 . 
         [0062]    The melt-extruder  132  can be a single screw melt-extruder, but preferably a double screw melt-extruder is used. A suitable double screw melt-extruder is included in melt-extruder assembly model number ZSK120 is commercially available from Krupp, Werner &amp; Pfliederer Corporation at Ramsey, N.J. 
         [0063]    In accordance with the process of the present invention, a phenolic antioxidant stabilizer of the type described hereinbefore is introduced, e.g., injected, into the melt-extruder  132  through line  131  at or near the flake feed end of the extruder. It has been found that when such a phenolic antioxidant stabilizer material is introduced into the extruder in liquid form, without being premixed with polyamide material, the process herein is especially suitable for preparing polyamide filaments of very high RV values. 
         [0064]    The liquid antioxidant stabilizer will generally be injected into the melt-extruder  132  in amounts and at rates suitable to provide a concentration of antioxidant stabilizer in the molten polymer exiting the extruder of from about 0.2 wt % to 2.0 wt %, more preferably from about 0.5 wt % to 1.5 wt %. Water can be also be added in the melt-extruder  132  for more precise RV control in the ultimately resulting filaments. 
         [0065]    The flake is melted in the melt-extruder  132  and molten polymer is extruded from an outlet  134  of the melt-extruder  132  to a transfer line  136 . A motor assembly  138  rotates one or more screw device(s) in the melt-extruder  132  increasing the temperature of the polymer due to the mechanical work of the screw(s). As is known in the art, associated apparatus including insulation and/or heating or cooling elements maintain controlled temperature zones along the melt-extruder  132  allowing sufficient heat to melt, but not overheat, the polymer. This associated apparatus is part of the melt-extruder assembly mentioned above which is commercially available from Coperion Corporation of Ramsey, N.J. 
         [0066]    The polymer undergoes melt phase polymerization in the melt-extruder  132  and in the transfer line  136  increasing the temperature of the polymer. As such, the temperature of the molten polymer in the transfer line  136  at point P 1  within about 5 feet (2.4 m) of the outlet  134  of the melt-extruder  132  ranges from about 285° C. to about 295° C., preferably about 289° C. to about 291° C. A temperature sensor  140  can be connected to the transfer line  136  at point P 1  to measure this temperature. 
         [0067]    The extruded molten polymer is conveyed by a booster pump  142 , through the transfer line  136  to at least a spinneret  151 , 152  of at least a spinning machine. The transfer line  136  includes a conduit  144  and a manifold  146 . The conduit  136  connects the melt-extruder  132  to the manifold  146 . The manifold  146  connects to each of the spinnerets  151 , 152 . The temperature in the transfer line  136  (or, more specifically, the manifold  146  of the transfer line  136 ) at points P 2 ,P 2 ′ within 5 feet (2.4 m) of the spinnerets  151 , 152  is about 295° C. to about 300° C., preferably, of about 296° C. to about 298° C. Additional temperature sensors  148 , 150  can be connected to the manifold  146  at points P 2  and P 2 ′ to measure the temperatures at these points. An additional temperature sensor  154  can be connected to the transfer line  136  at point P 3  between the booster pump  142  and the manifold  146  to obtain an additional temperature measurement. Preferably the temperature at this point (booster pump discharge temperature) can range from about 290° C. to 300° C. The residence time of the molten polymer in the melt-extruder  132  and the transfer line  136  is about 3 to about 15 minutes, and preferably about 3 to about 10 minutes. 
         [0068]    It has been found that filaments of especially high RV can be spun if an appropriate balance is maintained between the pressure drop within the system conveying molten polymer from the extruder to the manifold and the amount of throughput of molten polymer being conveyed. In particular, in accordance with this invention, the ratio of the pressure drop (ΔP in psig) between the booster pump  142  and the manifold  146  to molten polymer throughput (in kg/hr) should be maintained within the range of from about 2.5 to 3.5, more preferably form about 2.8 to 3.2. (For purposes of this invention, pressure and throughput values are determined using transfer lines having an average of 2.83 inch (7.2 cm) inside diameter, with a total length of the distance between booster pump pressure bulb and the manifold pressure bulb being 38.3 feet (11.68 meters). 
         [0069]    Metering pumps  161 , 162  force the molten polymer from the manifold  146  through spin filter packs  164 , 166  and then the spinnerets  151 , 152 , each having a plurality of capillaries through the spinneret  151 , 152  thereby spinning the molten polymer through the capillaries into a plurality of filaments  170  having a spun fiber formic acid RV of greater than about 190, preferably of about 200 to about 250, and most preferably, of about 205 to about 230. 
         [0070]    Preferably, the molten polymer is spun through a plurality of the spinnerets  151 , 152 , each spinneret  151 , 152  forming a plurality of the filaments  170 . The filaments  170  from each spinneret  151 , 152  are quenched typically by an air flow (illustrated in  FIG. 2  by arrows) transverse to the length of the filaments  170 , converged by a convergence device  172 , coated with a lubricating spin finish, into a continuous filament tow  176 . The tows  176  are directed by feed rolls  178  and optionally one or more change of direction roll  180 . The tows  176  can be converged together forming a larger continuous filament combined tow  182  which can be fed into a storage container  184 , called a “can” by those skilled in the art. 
         [0071]    Referring to  FIG. 3  the tows  182  can be removed by a feed roll  186  from several of the cans  184 . The tows  182  can be directed by devices, such as wire loops  188  and/or a ladder guide  190  which is typically used to keep tows  182  spaced apart until desired. The tows  182  can be combined, such as at point C in  FIG. 3 , into a continuous filament tow band  192 . Then the continuous filament tow band  192  can be drawn by contact with a draw roll  194  which rotates faster than the feed roll  186 . The continuous filament tow band  192  can be drawn 2.5 to 4.0 times, according to known processes, to provide a drawn denier per filament (dpf) in a range of about 2 to about 100 (about 2.2 dtex/f to about 111.1 dtex/f). The continuous filament tow band  192  can typically have 20 to 200 thousand continuous filaments. If space requires, one or more change of direction roll(s)  196  can redirect the tow band  192 . Then the continuous filament tow band  192  can be crimped by a crimping apparatus  198 , such as by forcing the continuous filament tow band  192  into a stuffing box. Then the crimped drawn continuous filament tow band can be cut by a cutter  200  providing the staple fibers  202  of the present invention described above. 
       Test Methods 
       [0072]    The following test methods can be used in the following Examples and in connection with characterization of the present invention. 
         [0073]    Relative viscosity (RV) of nylons refers to the ratio of solution or solvent viscosities measured in a capillary viscometer at 25° C. (ASTM D 789). The solvent is formic acid containing 10% by weight water. The solution is 8.4% by weight polymer dissolved in the solvent. 
         [0074]    Denier (ASTM D 1577) is the linear density of a fiber as expressed as weight in grams of 9000 meters of fiber. The denier is measured on a Vibroscope from Textechno of Munich, Germany. Denier times (10/9) is equal to decitex (dtex). 
         [0075]    Tenacity (ASTM D 3822) is the maximum or breaking stress of a fiber as expressed as force per unit cross-sectional area. The tenacity is measured on an Instron model 1130 available from Instron of Canton, Mass. and is reported as grams per denier (grams per dtex). 
         [0076]    Denier and tenacity tests performed on samples of staple fibers are at standard temperature and relative humidity conditions prescribed by ASTM methodology. Specifically, standard conditions mean a temperature of 70+/−2° F. (21+/−1° C.) and relative humidity of 65%+/−2%. 
       EXAMPLES 
       [0077]    The invention herein can be illustrated by the following specific examples. All parts and percentages are by weight unless otherwise indicated. Examples prepared according to the process of the current invention are indicated by numerical values. Control or Comparative Examples are indicated by letters. 
         [0078]    In the examples herein, various staple fibers were produced having various spun fiber formic acid RV values. The procedures used involved an SPP phase, an MPP phase and a staple fiber production phase. 
         [0079]    In all instances, precursor polymer flake was fed to a SPP vessel  16  of a SPP apparatus like the one illustrated in  FIG. 1 . The precursor flake polymer was homopolymer nylon 6,6 (polyhexamethylene adipamide) containing a polyamidation catalyst (i.e., manganous hypophosphite obtained from Occidental Chemical Company with offices in Niagara Falls, N.Y.) in concentration by weight of 16 parts per million The precursor flake which was fed into the SPP vessel  16  had a formic acid RV of 48. 
         [0080]    A serially connected dual desiccant bed regenerative drying system  14  was connected in parallel with an adjustable solenoid activated valve  36  between the blower  30  and the dew point measurement instrument  120  of the gas system. The dryer  50  was a Sahara Dryer, model number SP-1800 commercially available from Henderson Engineering Company of Sandwich, Ill. The gas circulated through the gas system  12  was nitrogen. The regenerative dual desiccant bed circulating gas drying system  14  was used to increase the RV of the polymer flake. The pressure of the gas in the drying system  14  was about 5 psig (35 kPa). The dew point temperature of the gas exiting the dryer system  14  was measured by instrument  120 . 
         [0081]    Higher RV flake was removed from a flake outlet  22  of the SPP vessel  16  as shown in  FIG. 1  and was then fed to a melt-phase polymerization (MPP) system similar to the setup shown in  FIG. 2 . In the MPP system, a non-vented twin screw melt-extruder  132  melted and extruded the flake into molten polymer and into a transfer line  136 . A liquid hindered phenolic stabilizer (i.e., ANOX® 20, obtained from Chemtura Corporation) was injected into the front end of melt-extruder  132  through line  131 . Stabilizer was injected into the extruder so as to provide a stabilizer concentration of 0.3% by weight concentration in the molten polymer exiting the extruder. 
         [0082]    This molten polymer was pumped by booster pump  142  via transfer line  136  to a manifold  146  and metered to a plurality of spinnerets  151 , 152  and then spun into filaments  170 . The residence time of the polymer in the melt-extruder  132  and transfer line  136  was about 5 minutes. The filaments were converged into a continuous filament tows  176 . 
         [0083]    As shown in  FIG. 3 , a plurality of the continuous filament tows were converged into a continuous filament tow band  192  and then drawn. The drawn band  192  was crimped and cut into staple fibers  202 . The staple fibers  202  produced were approximately 15 denier (16.7 decitex) per filament. 
         [0084]    Process conditions and fiber RV values for the several fibers of Examples 1-5 and comparative Examples A-D are shown in Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 SPP 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 T′put 
                 SPP 
                 Gas 
                 B Pump 
                 Manif 
                 B Pump 
                 Manif 
                 Spin 
                 Delta 
               
               
                 Exple 
                 Kg/hr 
                 Gas 
                 Temp 
                 Temp 
                 Temp 
                 Press 
                 Press 
                 Press 
                 Press 
                 ΔP/T′put 
               
               
                 No. 
                 cfm 
                 Flow 
                 ° C. 
                 ° C. 
                 ° C. 
                 PSIG 
                 PSIG 
                 PSIG 
                 PSIG 
                 PSIG/Kg/hr 
                 RV 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 540 
                 1284 
                 185 
                 291 
                 296 
                 4000 
                 2246 
                 1049 
                 1754 
                 3.25 
                 229 
               
               
                 2 
                 540 
                 1220 
                 185 
                 295 
                 298 
                 4000 
                 2161 
                 991 
                 1839 
                 3.41 
                 204 
               
               
                 3 
                 540 
                 1211 
                 185 
                 295 
                 298 
                 3997 
                 2285 
                 1008 
                 1712 
                 3.17 
                 215 
               
               
                 4 
                 540 
                 1242 
                 185 
                 296 
                 298 
                 4000 
                 2272 
                 1005 
                 1728 
                 3.20 
                 202 
               
               
                 5 
                 540 
                 1170 
                 184 
                 296 
                 298 
                 3875 
                 2265 
                 967 
                 1610 
                 2.98 
                 204 
               
               
                 6 
                 455 
                 997 
                 195 
                 287 
                 296 
                 3950 
                 2241 
                 — 
                 1709 
                 3.76 
                 234 
               
               
                 7 
                 540 
                 1022 
                 193 
                 288 
                 296 
                 3925 
                 2220 
                 — 
                 1705 
                 3.16 
                 221 
               
               
                 A 
                 860 
                 1320 
                 190 
                 286 
                 298 
                 3656 
                 2476 
                 1124 
                 1180 
                 1.37 
                 173 
               
               
                 B 
                 860 
                 1318 
                 190 
                 284 
                 298 
                 3823 
                 2324 
                 1397 
                 1499 
                 1.74 
                 167 
               
               
                 C 
                 860 
                 1312 
                 190 
                 289 
                 298 
                 3902 
                 2374 
                 1374 
                 1528 
                 1.78 
                 172 
               
               
                 D 
                 860 
                 1324 
                 190 
                 288 
                 298 
                 3950 
                 2544 
                 1181 
                 1406 
                 1.63 
                 161