GB2074500A - Fusion Melt-spinning Poly(Polymethylene Terephthalamide) or Polypyrrolidone Fiber - Google Patents

Abstract

Extruding a homogeneous single phase fusion melt of a poly(polymethylene terephthalamide) or polypyrrolidone and water through a spinneret to form filaments provides fiber of desirable physical properties without significant lowering of the molecular weight of the polymer forming the fiber.

Description

SPECIFICATION Process for Spinning Poly(Polymethylene Terephthalamide) and Nylon 4 Fiber This invention relates to an improved process for preparing fiber from poly(polymethylene terephthalamides) and nylon 4. More particularly, this invention relates to such a process wherein a single phase fusion melt of poly(polymethylene terephthalamide) or nylon 4 and water is extruded through a spinneret, stretched to provide molecular orientation and further processed as desired to provide fiber of attractive physical properties.

Polypyrrolidone, nylon 4, is a highly desirable polymer for many uses. A particularly beneficial application would appear to be that of providing a textile fiber thereof which, because of the hydrophilic nature of nylon 4, would provide high moisture region and desirable esthetic properties as a result.

Poly(polymethylene terephthalamide) represents a class of nylons that have very high melting and second order transition temperatures that make it potentially useful as fiber for use as tire cords and in the apparel field such as in the fabrication of permanent-press textiles.

Poly(hexamethylene terephthalamide, otherwise referred to as nylon 6T, is particularly attractive for such uses and represents a preferred species of this general class.

Nylon 6T has a melting point of 371 CC. When this polymer is heated to temperatures approaching its melting point, decomposition of the polymer occurs before a useful melt is obtained. Previous methods for spinning this polymer were wet spinning procedures using concentrated sulfuric acid as the polymer solvent and coagulating the polymer in aqueous medium.

Formation of dilute sulfuric acid in the coagulation bath was considered to be responsible for hydrolysis of the polymer and substantial loss of molecular weight. Thus, although the concentrated sulfuric acid used to dissolve the polymer did not appear to lower the molecular weight of the polymer, the use of water in coagulating medium caused substantial reduction of the molecular weight.

However, because of the unattractive nature of concentrated sulfuric acid as the polymer solvent and other problems associated with spinning of nylon 6T and related polymers, there has been limited interest in fiber obtained from such polymers in spite of their potential benefits.

Many efforts have previously been made to provide a meltspun nylon 4 fiber. However, it was consistently found that when the polymer is heated to temperatures approaching its melting point, polymer decomposition occurs and the polymer essentially reverts to the monomer from which it was formed, pyrrolidone. This decomposition is readily evidenced by the sharp reduction in viscosity of the melt and the inability to process the resulting melt in fiber-making equipment.

Although it is potentially possible to provide nylon 4 fiber by wet and dry spinning procedures, the requirements for polymer solvent and recovery systems for the solvent to prevent environmental pollution and high solvent costs have not made such processes attractive.

Furthermore, in the coagulation steps associated with the wet and dry spinning processes, it is not certain that the polymer will not undergo degradation or hydrolysis and result in inferior or useless product.

In order to promote fiber of poly(polymethylene) terephthalate and realize its potential benefits and encourage the promotion of nylon 4 fiber for textile purposes, it is necessary to provide a process for making these fibers which avoids the deficiencies of the former fiber-forming processes while providing fiber of good physical properties. Such a provision would satisfy a longfelt need and constitute a significant advance in the art.

In accordance with the present invention, there is provided a process for preparing fiber which comprises preparing a homogeneous single phase fusion melt of a poly(polymethylene terephthalamide) or polypyrrolidone and water at a temperature above the boiling point of water at atmospheric pressure and at a temperature and pressure which maintains water in liquid state, the temperature being below the deterioration temperature of the polymer, extruding said melt through a spinneret to form filaments, and stretching the resulting filaments to provide molecular orientation.

Surprisingly, the process does not cause any significant reduction in the molecular weight of the polymers as a result of such processing. It is also surprising that adequate molecular stretching of the polymer molecules can be achieved without conducting such stretching in a steampressurized environment. In spite of the fact that the nascent filaments are solidified without the benefit of a steam-pressurized solidification zone, it is also surprising that the fibers obtained do not have a sheath-core structure and the undesirable properties associated therewith.

The process of the present invention enables poly(polymethylene terephthalamides) and nylon 4 to be spun at a temperature safely below their deterioration points and does not require undesirable polymer solvents to liquify the polymers. As a result, the process of the present invention represents an attractive route for the preparation of poly(polymethyleneterephthalamide) and polypyrrolidone fibers and encourages advantage to be taken of the desirable properties of such fibers.

The poly(polymethylene terephthalamides) useful in the process of the present invention have the structural formula in which the repeating units comprise

wherein n is an integer of about 2 to 12, preferably about 4 to 8, and more preferably 6.

Polymers containing such repeating units may be made by poiycondensation of terephthalic acid and a polymethylene diamine according to conventional procedures. Co-reactants such as caprolactam and the like may be used to provide additional repeating units in the polymer in amounts which do not detract from desirable properties of the resulting fiber. Certain of the coreactants may be used to lower the melting point of the resulting polymer or to provide other features not available from the combination of terephthalic acid and polymethylene diamine alone.

The polymers used as charge materials to the present process may be prepared in bulk or in other conventional forms. Since the poly(polymethylene terephthal imide) polymers and copolymers thereof are known, as are the polypyrrolidones, further discussion of their preparation and nature is unnecessary. Useful polymers of the type described will generally have molecular weights in the range of about 6,000 to about 100,000 or more, usually about 10,000 to 50,000, the major criteria being that they form single phase fusion melts with water In carrying out processing in accordance with the process of the present invention, it is first necessary to prepare a homogeneous single phase fusion melt of the poly(polymethylene terephthalamide) or polypyrrolidone and water.

The amount of water necessary to provide the homogeneous single phase fusion melt will generally be in the range of about 10 to 90% by weight based on the total weight of polymer and water. When the poly(poly-methlene terephthalamides) are used, from about 525% is generally employed with a preferred range being from about 7 to 15%, same basis. From about 5-1 5 weight percent is generally employed using polypyrrolidone. The water content will be influenced by the temperature of operation, the molecular weight of the polymer, and other factors. A useful method for indicating the proper polymer-water composition is to construct a phase diagram.However, there is considerable latitude in the polymer-water composition, especially at higher temperatures of operation, and, as a result, homogeneous single phase fusion melts generally can exist at a range of water contents.

Typically, solid polymer is blended with the water and the polymer adsorbs the water so that a wetted polymer solid arises. This wetted polymer solid is then processed to a melt usually by means of a screw extruder. The extruder heats and compresses the polymer-water composition and delivers the homogeneous single phase fusion melt at appropriate temperature and pressure, usually above autogeneous pressure, for extrusion. As indicated, the polymer-water composition will form the desired melt at a temperature above the boiling point of water at atmospheric pressure and at a temperature below the deterioration temperature of the polymer. In order to maintain water in liquid state at least autogenous pressure is required and higher pressures will generally arise due to compression.

It is generally preferable to prepare the melt at a temperature at least about 100C. above the minimum melting point of the polymer-water composition in order to ensure homogeneity of the melt At temperatures above the minimum melting point of the polymer-water composition there will be a range of water contents which will provide a single phase melt and greater leeway in water content is possible. The specific amount of water useful with the polymer class will vary somewhat depending upon the composition of the polymer, its molecular weight, the operating conditions, and the like. Typically, the melt is formed at a temperature in the range of about 170250 C., preferably about 180220 C.

when the poly(polymethylene terephthalamides) are used and from about 165--1750C. when polypyrrolidone is used. The melting point of polypyrrolidone is 240-2600C.

After the homogeneous ingle phase fusion melt is prepared, it is melt extruded througii a spinneret to form filaments. Usually, the pressure generated within the extruder is sufficient for extrusion but additional pumps or pressure devices may be used, if desired. The spinneret should contain fiber-forming orifices of suitable size to provide filaments of textile denier upon completion of processing. The extrusion may be directly into the atmosphere although other environments, as may be desired, may also be used. Extrusion into the atmosphere is preferably employed for the polypyrrolidone.

When nylon 4 is employed, as the filamentary extrudate emerges from the spinneret into the atmosphere, there is little or no tendency for the rapid evaporation of water vapor from the filaments. This result is believed to be due to the superior hydrophilicity of the nylon 4 polymer.

Consequently, the nylon 4 filaments obtained have a homogeneous transparent structure substantially free of sheath-core structure, voidstructure, density gradient across the filament cross-section, internal reflectance, and other deficiencies caused by uncontrolled release of water vapor from the nascent filaments.

After the filaments produced have been formed by extrusion, they are subjected to stretching in accordance with the present invention to provide molecular orientation. Such stretching results in improved physical properties of the filaments and also results in size reduction. The stretching can be conducted to a limited extent by conventional means, such as with draw down rolls and the like.

To provide additional stretch, the filaments may be drawn in conjunction with heated air impinged thereupon. Alternatively or in conjunction therewith, the filaments may be drawn over a heated surface such as a plate, pin or roll. Such drawing may be down in one or more stages as the filaments are processed during spinning.

When poly(polymethylene terephthalamides) are used, although it is possible to provide fiber of good physical properties by extruding the single phase melt into the atmosphere, it is generally preferred to extrude the melt into a steampressurized solidification zone maintained with saturated steam at a temperature which is about 1 OOC. to about 500C. below the minimum melting temperature of the polymer-water composition since such processing enables a higher degree of stretching to be achieved while the filaments remain within the solidification zone. When stretching is conducted on filaments formed in the atmosphere, it is usually obtained by drawing over a heated surface such as a platen, pin, or roll.This type of stretching may also be accomplished by spinning into heated air alone or in conjunction with drawing over a heated surface. It is also possible to effect improved stretching by extruding the filaments into hot water which can also regulate release of water from the extrudates.

Although no further processing of the filaments is necessary to carry out the process of the present invention, additional processing steps may be conducted. For example, post-stretching, drying, relaxing, crimping, etc. may be conducted.

It is usually desirable to dry the filaments under controlled conditions of temperature and humidity to improve their transparency. This is preferably conducted at dry bulb temperatures in the range of 110 to 1 80 C. and wet bulb temperatures in the range of about 60 to 1 000C.

After drying, it is also generally useful to relax the filaments in steam under pressure so as to effect shrinkage to about 5 to 40% since this processing tends to provide a favorable balance of physical properties.

The invention is more fully illustrated in the examples which follow wherein all parts and percentages are by weight unless otherwise specified.

Example 1 Poly(hexamethylene terephthalamide), nylon ST, of 1.7 relative viscosity as determined in sulfuric acid, was prepared as a fine powder of less than 100 micron particle size. Water in the amount of approximately 20% of the combined weight was physically admixed with the polymer to provide a free-flowing, dry-appearing powder.

This material was formed into dense cylindrical pellets of 9.5 millimeters diameter weighing approximately 2.5 grams and containing 19% water. These pellets were charged to an Instron capillary rheometer equipped with a single 85 micron diameter spinneret capillary and protective filter pack. The rheometer temperature was 2000C. The rheometer barrel was immediately sealed after charging of the pellets by means of the piston rod and the charge was held under approximately 500 psi pressure for 5 minutes. At the end of time, the piston drive was engaged to deliver polymer-water melt through the capillary at a linear velocity of about 9 meters per minute. A clear filament emerged which could be drawn down (stretched) in air by hand from the spinneret capillary in the manner typical of fiberforming melts.The extruded filament had a polymer content which had undergone negligible change in molecular weight from that of the starting polymer when analyzed for relative viscosity.

Example 2 The polymer of Example 1 was prepared as a coarse granulate with particle size in the range of 0.25-0.75 millimeters. This granulate was admixed with water in a double cone blender at room temperature to yield a free-flowing, dryappearing solid with approximately 8% water content. This material in the form of pellets was extruded on the Instron rehometer at 2000 C.

through a single 200 micron spinneret capillary at 5 meters per minute linear velocity to give a transparent filament showing the same excellent drawn down in air as the filament of Example 1.

The apparent melt viscosity was measured at an apparent shear rate of 10,000 reciprocal seconds and was found to be approximately 1600 poise.

Example 3 The polymer of Example 2 was supplied to a 3/4 inch single screw extruder connected by melt delivery hardware to a spinneret. The spinneret discharges into a steam chamber which contains three stages of stretch rolls and a sealing device to permit exit of drawn yarn to an external collection device.

With the extruder first being supplied with an acrylonitrile "start-up" polymer containing about 15% water and stably delivering hydrated acrylonitrile polymer melt to the spinneret, the hydrated polymer of Example 2 is added to the feed hopper. A progressive transition from 100% acrylonitrile polymer-water to 100% nylon 6Twater is made, and extruder temperatures are adjusted to achieve a melt temperature of 2000C.

The hydrated nylon 6T melt is delivered at a rate of 7.4 grams per minute to the spinneret containing 36 capillaries each of 200 micron diameter. With the steam chamber pressurized to 10 psig with saturated steam, the spun filaments are stretched at a total stretch ratio of 60 to give a tow of 1 50 total denier, or 4.1 denier per filament. On the Instron tensile testing machine, this tow was found to have a breaking tenacity of approximately 2 grams per denier and ultimate elongation of approximately 20%.

Example 4 The procedure of Example 3 is repeated in every material detail except the steam chamber is left open to the atmosphere and no steam is admitted thereto. It is found that the spun filaments show good draw down attenuation in air, giving stretch ratios of at least 1 0x and the resulting filament can be subsequently stretched further by passing it over a heated surface.

In each of the examples given above, the fiber obtained had a homogeneous transparent structure without any evidence of sheath-core structure.

In the examples which follow, extrusion of the fiber-forming nylon 4 composition was studied using an Instron capillary rheometer equipped with a single 100 micron diameter spinneret capillary and protective filterpack. Since the amount of nylon 4 available commercially is extremely limited, the amount of fiber that could be produced was insufficient to provide samples for more than cursory testing. It is known, however, from other studies involving numerous additional fiber-forming polymers that there is excellent correlation between performance on the Instron capillary rheometer and that was obtained using fullscale fiber-making equipment.In fact, when changes in polymer compositions are contemplated in full-scale commercial fibermaking processes, it is general procedure to evaluate the changed polymer composition using the Instron capillary rheometer to predict ultimate performance thereof. Accordingly, the results given in the examples, although qualitative in nature, are highly indicative of results obtained in full-scale operations and constitute practice of the present invention.

Comparative Example The polymer used in this example was a dry nylon 4 (polypyrrolidone). A sample of this polymer dissolved at 0.5% in formic acid exhibited a relative viscosity of 1.61. A portion of the dry polymer was placed in an Instron capillary rheometer equipped with a single 100 micron spinneret capillary. The polymer was heated at 24O-2600C, until a melt was formed. The melt obtained was a thin liquid consisting in the main of pyrrolidone monomer and upon extrusion of this thin melt a fiber could not be formed.

Example 5 The same polymer described in the Comparative Example was employed. A mixture of 90 parts polymer and 10 parts water after blending was introduced into the Instron capillary rheometer described in the Comparative Example.

After heating the polymer-water composition to a temperature of 1700C., a homogeneous single phase fusion melt formed. This melt was spun through the capillary into the atmosphere and formed a fiber. As the nascent filament formed, it was drawn away from the orifice by hand.

Although it was not possible to measure the extent to which stretching was effected by this stretching technique, it appeared that adequate stretching was possible to provide good fiber properties. The fiber was essentially transparent in nature and appeared to have a homogeneous structure substantially free of voids, sheath-core structure, density gradient and surface striations.

Hand tests indicated good tenacity both under A straight and loop condition. A sample of the fiber was dissolved in formic acid to provide an 0.5% solution and the relative viscosity of the polymer thus obtained was 1.56 indicating that no significant reduction in molecular weight of the polymer resulted from the fiber-making process conducted.

Claims (8)

Claims

1. A process for preparing fiber which comprises preparing a homogeneous single phase fusion melt of a polymer comprising a poly(polymethylene terephthalamide) or polypyrrolidone and water at a temperature above the boiling point of water at atmospheric pressure and at a temperature and pressure which maintain water in liquid state, the temperature being below the deterioration temperature of the polymer, extruding said melt through a spinneret to form a filament, and stretching the resulting filaments to provide molecular orientation.

2. The process of Claim 1 wherein said polymer is poly(hexamethylene terephthalamide).

3. The process of Claim 2 wherein said extruding is directly into a steam-pressurized zone wherein said stretching step is conducted.

4. The process of Claim 2 wherein the water content of the polymer-water melt is about 715%.

5. The process of Claim 4 wherein the melt temperature is about 180--2200C.

6. The process of Claim 1 wherein said polymer is polypyrrolidone.

7. The process of Claim 6 wherein said single phase fusion melt contains from about 5-12 weight percent water.

8. The process of Claim 6 wherein said temperature is in the range of about 1 65- 175"C.