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United States Patent 3,869,430
Blades March 4, 1975

High modulus, high tenacity poly(p-phenylene terephthalamide) fiber

Abstract

High strength, high modulus fibers having a density of at least 1.4o g./cm..sup.3 and consisting essentially of poly(p-phenylene terephthalamide) of inherent viscosity of at least 4.0 are provided. These fibers have a lateral birefringence of at least 0.022, a novel crystalline structure with crystalline regions having an apparent crystallite size of greater than 58 Angstrom units (A), and an orientation angle no greater than 13.degree. with the proviso that the ratio of the apparent crystallite size to the orientation angle (degrees) is at least 6A/degree.


Inventors: Blades; Herbert (Hockessin, DE)
Assignee: E. I. du Pont de Nemours and Company (Wilmington, DE)
Appl. No.: 05/268,053
Filed: June 30, 1972


Current U.S. Class: 528/348 ; 264/184; 264/203; 528/336
Current International Class: D01F 6/60 (20060101); C08g 020/20 (); C08g 020/38 ()
Field of Search: 260/78R,78A,78S

References Cited

U.S. Patent Documents
3079219 February 1963 King
3094511 June 1963 Hill et al.
3287324 November 1966 Sweeny
3354127 November 1967 Hill et al.
3414645 December 1968 Morgan
3472819 October 1969 Stephens
3542719 November 1970 Pollack
3560137 February 1971 Hahn
3574811 April 1971 Jamison
3595951 July 1971 Logullo
3627737 December 1971 Smith
3642706 February 1972 Morgan
3671542 June 1972 Kwolek
3673143 June 1972 Bair et al.
Primary Examiner: Anderson; Harold D.

Parent Case Text



This application is a continuation-in-part of my application Ser. No. 172,514, filed Aug. 17, 1971, now abandoned which is a continuation-in-part of my application Ser. No. 138,209, filed Apr. 28, 1971, now abandoned.
Claims



1. A high tenacity, high modulus fiber having a density of at least 1.40 grams per cubic centimeter and consisting essentially of poly(p-phenylene terephthalamide), the polymer of said fiber having an inherent viscosity of at least 4.0 as measured at a concentration of 0.5 gram of polymer in 100 ml. of concentrated sulfuric acid (95-98% H.sub.2 SO.sub.4) at 30.degree.C. and said fiber having 1) a lateral birefringence, .DELTA.n, of at least 0.022 as determined from measurements on oblique fiber sections of thickness, T, in microns, with a polarizing microscope equipped with a compensator, using the equation:

.DELTA.n = (K .lambda. sin 2.phi.)/T

where K is the instrumental constant of the compensator, .lambda. is the wavelength in microns of the light used and 2.phi. is the difference between the angles of the compensator to produce extinction along the minor and the major axis of the section, 2) crystalline regions with an apparent crystallite size, ACS, of greater than 58A as determined from an X-ray diffractogram scan along a line perpendicular to the fiber axis using the equation:

ACS = (K .lambda.)/(.beta. cos .theta.)

where K is taken as one; .lambda. represents the wavelength in A of the X-rays used; .theta. is the Bragg angle and .beta. is the line breadth in radians, corrected for instrumental broadening, at half intensity of the major reflection located at the smaller value of 2.theta., and 3) an orientation angle no greater than 13.degree. with the proviso that the ratio of the apparent crystallite size to the orientation angle is at

2. The fiber of claim 1 wherein the crystalline regions have an orientation

3. The fiber of claim 1 having a filament tenacity of at least 22 grams per denier.
Description



This invention relates to a greatly improved poly(p-phenylene terephthalamide) fiber particularly useful in composites with plastics.

BACKGROUND OF THE INVENTION

Resin composites reinforced with glass fibers have long been used as structural elements. Great advances in this area have been made in recent years in response to the demands for lighter, stronger and stiffer materials of construction by aircraft and aerospace designers. New high strength, high modulus inorganic fibers such as ceramics, graphite, boron, etc. have been developed but they are extremely costly and difficult to handle.

U.S. Pat. No. 3,671,542 to Kwolek teaches the use of optically anisotropic dopes of certain carbocyclic aromatic polyamides in wet spinning processes to give fibers of good strength. Heating the fibers under tension increases the tensile strength and modulus.

Fibers of even greater strength and modulus are desirable for use in preparing high grade reinforced plastic composites.

SUMMARY OF THE INVENTION

This invention provides a novel fiber having a density of at least 1.40 g./cc and consisting essentially of poly(p-phenylene terephthalamide) of inherent viscosity of at least 4.0 (preferably .gtoreq. 4.6). The fiber is characterized by a lateral birefringence of at least 0.022, crystalline regions with an apparent crystallite size greater than 58 Angstrom (A) units, and an orientation angle no greater than 13.degree. with the proviso that the ratio of the apparent crystallite size to the orientation angle (degrees) is at least 6 A/degree. The fiber has an initial modulus .gtoreq. 900 grams per denier (measured as yarn) and a filament tensile strength of at least 22 grams per denier. Preferred products have an orientation angle of less than 10.degree..

The fibers of this invention are of great value in the preparation of reinforced plastic materials of construction for very demanding applications such as for aircraft fairings, radomes, ceilings and aerospace uses. Certain composites of the fibers of this invention should resist warping caused by high humidity conditions as evidenced by results obtained in accelerated tests where the composites were exposed to boiling water. Moreover, composites of these fibers have been made which exhibit high flexural modulus, flexural offset yield strength and Charpy impact strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic views of apparatus for carrying out a process suitable for preparing the fibers of this invention.

In FIG. 1, a spin dope is pumped through transfer line 1 through a spinning block 2, through the orifices of spinneret 3 through the layer of gas 5 and into the coagulating liquid 6 in spin tube where the filments 4 are conducted. A strong as-spun multifilament yarn 15 is passed under guide 7 and is wound up on a rotating bobbin 9. The coagulating liquid 6 flows from container 11 through spin tube 10 and falls to container 12 from whence it is returned by pump 13 and tube 14 to container 11.

In FIG. 2, yarn 28 produced as described in FIG. 1 is passed over tension guide 20, around roll 21 controlled by a magnetic brake, over idler roll 22, over pulley 24 equipped with force gauge 23 and in through heated tube 27 contained in insulating box 29. The yarn exiting tube 27 is pulled by driven rolls 25 and passed to constant tension windup 26.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The products of this invention can be made by extruding a dope containing at least 30, preferably at least 40, grams of polymer per 100 ml. of solvent (volume determined at 25.degree.C.) through a thin layer of gas (or a non-coagulating liquid such as toluene, heptane, etc.) into a cold coagulating bath, washing, drying and heat treating. Where the solvent is sulfuric acid of about 98 to about 100% H.sub.2 SO.sub.4 this corresponds to dopes containing at least 14, preferably at least 18, weight % of polymer. The dope should contain less than about 2 percent water.

Suitable solvents consists essentially of sulfuric acid (containing at least 98% H.sub.2 SO.sub.4), chlorosulfuric, fluorosulfuric acid, and mixtures of these acids. The solvents can contain certain organic additives.

Additives of the type -- halogenated alkylsulfonic acids, halogenated aromatic sulfonic acids, halogenated acetic acids, halogenated lower alkyl alcohols, and halogenated ketones of aldehydes can be present in an amount up to about 30 percent of the total weight of the solvent and the additive depending upon the exact nature of the additive. The use of fluorosulfuric (rather than chlorosulfuric or sulfuric), or lower polymer concentrations permits the use of a greater amount of an additive. The greater the percent of halogen the greater the amount of additive that can be used. Trifluoromethanesulfonic acid can be present in an amount equal to the weight of the above sulfuric acids. Additionally sulfones, chlorinated phenols and nitrobenzene can be used as a solvent additive in lesser amounts than the halogenated additives described above.

Applicant has made fibers having a tenacity of less than those in the examples when conditions other than optimum were used.

The usual additives such as dyes, fillers, delusterants, UV stabilizers, antioxidants, etc., may be incorporated in the fibers of this invention.

In order to obtain the unusually high inherent viscosity (hereafter "I.V.") fibers of this invention, precautions are required to prevent degradation of the polymer throughout the process. The polymer should be dry and neutral. The exposure of the spinning dopes to temperatures above about 90.degree.C. should be minimized and the freshly spun fibers should be neutralized and washed thoroughly.

With a given spinning system -- spinning dope, jet velocity, spinneret, etc. -- the tenacity and modulus generally increase with an increase in the ratio between speed of fiber exit from the coagulating bath to the jet velocity ("spin stretch factor") until fibers are broken. The jet velocity is the average velocity of the dope in the spinneret hole or capillary as calculated from the volume of dope passing through the hole per unit time and from the cross-sectional area of the hole. The elongation of the yarn decreases as the spin stretch factor is increased.

The fibers are heated within a zone maintained at a temperature of at least 150.degree.C. under a tension of at least 0.5 gram per denier but less than that tension needed to draw the fiber (at the temperature used) more than about 1.03 times its initial length. The amount of draw is the ratio oven exit speed/oven entrance speed for a continuous treatment. Usually the draw is less than 1.02 times. For a given temperature the time of treatment and the tension are selected to give an apparent crystallite size greater than 58A (preferably greater than 70A) and an orientation angle no greater than 13.degree.. Thus the use of 150.degree.C. for a period of 60 seconds under a tension of 10 gpd has been satisfactory for a 190-denier yarn. The use of a 650.degree.C. zone for periods of from 0.6 to 1.0 second at 6 gpd tension has given excellent results with a 400-denier yarn. Extension of the time at 650.degree. C. to 2.4 seconds at 6 gpd tension have a very high modulus of 1,340 gpd but with a 20 percent loss in tensile strength and an 11 percent loss in fiber I.V. from the starting fiber. Thus zone temperatures as high as 800.degree.C. or more could be used at sufficiently short times. The use of high temperatures and long times leads to excessive degradation of the fiber resulting in losses of 30 percent or more of the starting tensile strength and/or inherent viscosity. Preferably a zone temperature of between about 250.degree.C. and 600.degree.C. (more preferably 450.degree. to 580.degree.C.) is used for from 0.5 to 5 seconds using a tension of between 1 and 8 gpd for yarn deniers of about 400 or less. Temperatures of 50.degree. to 100.degree.C. higher than the above preferred temperatures may be used with yarns of 700 to 1,500 or more denier. In general increases in temperature, tension and/or time lead to higher moduli in the heated fiber.

The heating can be done in a hot gas oven, in a liquid heating bath, by passage of the yarn over hot pins, hot plates or slots. It is preferred that the atmosphere around the yarn be inert such as nitrogen, during the heating.

The yarn is conveniently heated in the dry state, but satisfactory results can be obtained from a wet yarn direct from washing or on rewet dried yarns by slightly lengthening the time of heating. The heating may be done in stages, i.e., wet yarns may be heat treated in one step and the resulting yarn treated again with the same or different conditions. The yarns normally have no or very low twist during the heat treatment and may or may not have finish on them.

Suitable starting fibers have an inherent viscosity of at least 4.0, a lateral birefringence of at least 0.02, an orientation angle of less than about 22.degree. (preferably less than about 16.degree.) and an apparent crystallite size of less than about 52A. Such fibers will generally have a density of at least 1.40 grams/cm..sup.3 (preferably at least 1.44) and a filament tensile strength of at least 22 gpd.

In general, any of the well-known techniques of the reinforced plastics art can be used to make composites. The fibers can be filament wound or otherwise manufactured into collimated tapes, unidirectional and multidirectional fabrics and impregnated or coated with appropriate viscosity resins and resin solutions. Impregnated or coated articles can be dried to the required tack, or the resin advanced to a B-stage or cure, to produce convenient preimpregnated products commonly called "prepregs."

Furthermore, yarns can be chopped for use as reinforcement in compression molding, injection molding, etc., or sprayed on appropriate molds using processes well known in the chopped fiberglass reinforced plastics art.

Preparation of Polyamides

The polymers for use in this process are conveniently made by reacting suitable monomers in the presence of an amide type solvent by low temperatures techniques as taught in U.S. Pat. No. 3,063,966 to Kwolek et al. In order to obtain high molecular weight polymers the monomers and solvent should contain a minimum of impurities and the water content of the total reaction mixture should be less than 0.03 percent by weight.

Poly(p-phenylene terephthalamide) is conveniently made by dissolving 1,728 parts of p-phenylenediamine in a mixture of 15,200 parts of hexamethylphosphoramide and 30,400 parts of N-methylpyrrolidone, cooling to 15.degree.C. in a polymer kettle blanketed with nitrogen and then adding 3,243 parts of powdered terephthaloyl chloride with rapid stirring. The solution gels and turns into a dry crumb-like material in 3 to 4 minutes. The stirring is continued for 1.5 hours when possible with cooling to keep the product temperature at about 25.degree.C. The polymerization is essentially quantitative and the final reaction mixture contains 7.5 percent polymer of about 5.5 inherent viscosity (hereafter I.V.). The I.V. of the polymer from this system can be controlled by the ratio of monomers to solvent. Reduction of the amount of monomers from 9.83 above to 8.64 percent gives a reaction mixture containing 6.5 percent polymer of 6.0 I.V. The use of about 11.7 percent of monomers gives a reaction mixture of 9.0 percent polymer of 2.5 I.V.

The crumb-like acidic product is vigorously stirred or ground with water in a Waring Blendor or a colloid mill and the resulting polymer slurry filtered. The wet polymer is further washed by reslurrying with soft water to remove solvent and HCl and collected on a filter. This slurrying and filtering is repeated for a total of four times in sequence followed by a final wash with distilled water. To aid in neutralization, one of the soft water washes may also contain sodium carbonate or hydroxide. The polymer is then dried at 120.degree.-140.degree.C.

Polymerizations can also be carried out by the continuous mixing of the monomers.

Test Procedures

Inherent Viscosity

Inherent viscosity (I.V.) is defined by the equation:

I.V. = 1n(.eta.rel)/c

where c is the concentration (0.5 gram of polymer or fiber in 100 ml. of solvent) of the polymer solution and .eta.rel (relative viscosity) is the ratio between the flow times of the polymer solution and the solvent as measured at 30.degree.C. in a capillary viscometer. The inherent viscosity values reported and specified herein are determined using concentrated sulfuric acid (95-98% H.sub.2 SO.sub.4) unless otherwise specified.

Fiber Tensile Properties

Filament properties are measured on fibers that have been conditioned at 21.degree.C. and 65 percent relative humidity (R.H.) for at least 16 hours unless otherwise specified. Yarn properties are measured on yarn that has been conditioned at 24.degree.C. and 55% R.H. for at least 16 hours. All measurements are made in the fiber conditioning environment. Tenacity (breaking tenacity) (Ten.), elongation (breaking elongation) (E), initial modulus (Mi), and toughness (breaking toughness) (Tou.) are obtained from breaking a single filament or a multifilament yarn on an Instron tester (Instron Engineering Corp., Canton, Mass.).

Single filaments are broken with a gage length (distance between jaws) of 1.0 inch (2.54 cm.). The results on 3 filments are averaged. Yarns are twisted (under 0.1 gpd tension) to give a twist multiplier in the range of about 0.5 to 0.8 and broken with a 10 inch (25.4 cm.) gage length. All samples are elongated at a constant rate of extension (10 percent elongation/minute for fibers having an E of under 8 percent, and 60 percent elongation/minute for fibers with E of 8 to 100 percent) until the sample breaks.

The denier of a single filament (dpf) is calculated from its fundamental resonant frequency, determined by vibrating a 7 to 9 cm. length of fiber under tension with changing frequency. (A.S.T.M. D1577-66, part 25, 1968) This filament is then used for 1 break.

The denier of yarn is determined by weighing a known length (at 0.1 gpd tension); 90 cm. length is convenient.

The tenacity (grams/denier, gpd), elongation (%), initial modulus (gpd) and toughness (gram-centimeters per denier centimeter or simply gpd) as defined in A.S.T.M. D2101 part 25, 1968 are obtained from the load-elongation curve and the measured denier. In actual practice, the measured denier of the sample, test conditions and sample identification are fed to a computer before the start of a test; the computer records the load-elongation curve of the fiber as it is broken and then calculates the fiber properties.

It should be noted that different values are obtained from single filaments (filament properties) and from multifilament strands (yarn properties) of the same sample. Filament tenacities are higher than yarn tenacities -- typically about 1.2:1, filament elongations are higher than yarn elongation and filament moduli are lower than the yarn values. Unless specified otherwise all properties given herein are filament properties.

The physical properties of all yarns of the examples are measured with the yarn having 3 twists/inch (t.p.i.) (per 2.54 cm). This results in a different twist multiplier (T.M.) for yarns of different denier.

T.M. = [(twists/inch) .sqroot.denier of yarn]/73

It has been observed that the initial modulus of a yarn decreases as the T.M. is increased. For example, the modulus of a 700 denier yarn (T.M. 1.08 at 3 t.p.i.) will be lower by about 5 percent than that of an equivalent 200 denier yarn (T.M. 0.58 at 3 t.p.i.).

This effect of the twist level on the modulus is much more pronounced with larger denier yarns and rovings of two or more plys of yarn. For example, a 1,512 denier roving made up of four plys of a 378 denier yarn has moduli of 970, 920 and 810 gpd when measured at 1, 2 and 3 tpi respectively and a T.M. of 0.5, 1.1 and 1.6 respectively. The singles yarn of 378 denier has a modulus of 980 gpd at 3 tpi (0.8 T.M.). The modulus of the fibers of this invention should be measured on yarns having a twist level that will give a T.M. of between 0.5 and 0.8. In addition, the denier of the yarn or roving should be no greater than about 1,500 denier. Individual filaments should be carefully removed from oversize yarns or roving to reduce the denier of the residual bundle to 500 to 1,500 and the modulus determined (at a T.M. of between 0.5 to 0.8) on the reduced bundle. The removed filaments are not used since the handling may change their properties.

Bulk Viscosities

Bulk viscosities are measured by a Brookfield Viscometer with a No. 7 spindle at 10 rpm.

Orientation Angle

An orientation angle of the fiber similar to the one discussed in "X-Ray Diffraction Methods in Polymer Science" by Leroy E. Alexander, Wiley-Interscience (1969) Chapter 4, p. 264, is determined by the following method. A wide angle X-ray diffraction pattern (transmission pattern) of the fiber is made using a Warhus pinhole camera. The camera consists of a collimator tube 3 in. (7.6 cm.) long with two lead (Pb) pinholes 25 mils (0.0635 cm.) in diameter at each end, with a sample-to-film distance of 5 cm.; a vacuum is created in the camera during the exposure. The radiation is generated by Philips X-ray unit (Cat. No. 12045) with a copper fine-focus diffraction tube (Catalog No. 14000320) and a nickel betafilter; the unit is operated at 40 kv. and 16 ma. A fibersample holder 20 mils (0.051 cm.) thick is filled with the sample; all the filaments in the X-ray beam are kept essentially parallel. The diffraction pattern is recorded on Kodak No-Screen medical X-ray film (NS-54T) or equivalent. The film is exposed for a sufficient time to obtain a pattern which is considered acceptable by conventional standards (e.g., a pattern in which the diffraction spot to be measured has a sufficient photographic density, e.g., between 0.2 and 1.0, to be accurately readable). Generally, an exposure time of about 25 minutes is suitable; however, a lesser exposure time may be suitable, and even desirable, for highly crystalline and oriented samples to obtain a more accurately readable pattern.

The arc length in degrees at the half-maximum density (angle subtending points of 50 percent of maximum density) of one of the principal equatorial spots is measured and taken as the orientation angle of the sample. The specific arc used for orientation angle determinations on fibers described in the following examples is the one of the two principal ones which occurs at the higher value of 2.theta..

The orientation angles of fibers of this invention are determined by a densitometer method from the X-ray film. The azimuthal density distribution of the diffraction arc is obtained by use of a Leeds & Northrup Microphotometer (Catalog No. 6700-P1) whose electronic components have been replaced by a Keithley 410 Micro-Microammeter (Keithley Instruments Inc., Cleveland, Ohio). The output of this apparatus is fed to a Leeds & Northrup Speedomax Recorder, Type G.

In operation, the film is placed on the stage, the instrument is focused on the film, and the center of the diffraction pattern is made coincident with the stage center; both these centers are made coincident with the light beam of the instrument. The stage and mounted film are moved to permit the light beam to pass through the most dense area of the diffraction spot, the opposite spot is checked to insure true centering, and after any necessary fine adjustments are made, the recording of the azimuthal density trace through at least a 360.degree. rotation of the film is made on suitable coordinate paper (e.g., Graphic Controls Corp., Buffalo, N.Y., No. 578). There is obtained a curve which has two major peaks in which the density axis is considered the vertical axis and the angular displacement the horizontal axis. A base line is drawn for each peak as a straight line tangential to the minima on each side of the peak. A perpendicular line is dropped from each peak maximum to the base line. On this perpendicular at a density (the "half-density" point) equal to the average of the density at the peak maximum and the density where the base line intersects the perpendicular, is drawn a horizontal line which intersects each leg of the respective curves. The leg-to-leg length of each half-density horizontal line is converted to the degrees of arc as follows. The horizontal distance equivalent to 360.degree. of angular displacement is determined by rotation of a given point through 360.degree. followed by direct measurement of the horizontal displacement for such a rotation. For example, one of the two major peaks above may be used for this purpose. By direct proportion, the half-density leg-to-leg distance is converted to a degree value. The values for the two arcs are averaged and this is the orientation angle referred to herein. Values determined by this method have been shown to be precise to .+-. 0.7.degree. at the 95 percent probability level.

Method for Apparent Crystallite Size

Diffraction scans have been observed which are different depending on the chemical structure, crystallinity and the degree of order and orientation in the fiber. A measure of apparent crystallite size (ACS) is calculated from data obtained from an X-ray diffraction pattern using a reflection technique to record the intensity trace by means of an X-ray diffractometer.

A Philips X-ray generator, wide angle diffractometer and electronic circuit panel are used to record the diffraction pattern. Approximately 1.5 meters of yarn is wound around a modified Philips sample holder with the axis of the yarn perpendicular to the mechanical (2 .theta.) axis of the diffractometer. The modification of the sample holder consists of cutting approximately 21 notches, 0.01 inches wide, along the edge of the holder and cementing a thin sheet of lead foil across the bottom side of the rectangular opening so that only the fibers on top will be exposed to the X-ray beam. Using Nickel-filtered Copper radiation (1.5418A), a trace of the diffracted intensity is recorded from 6.degree. to 38.degree. 2.theta., at a scanning speed of 1.degree. 2.theta. per minute, a chart speed of 0.5 inches per minute, at a time constant setting of 2, with 0.5.degree. scattering and receiving slits and employing a scintillation detector with a pulse height analyzer, 2.theta. being the angle between the undiffracted and diffracted beam. The full scale deflection of the recorder is set so that the entire diffraction curve remains on scale, which is linear, but with as large a response as possible and preferably with the maximum intensity at least 50 percent of the scale.

The diffraction scans or diffractograms observed for fibers of this invention consist, when the sample is crystalline, of a pattern of multiple peaks, with two major ones located in the range of about 17.degree. to 25.degree. 2.theta., with those for most samples being in the narrower range of 19.degree. to 24.degree. 2.theta.. In a few cases, one of these two peaks will be evident only as an inflection, which, however, will be sufficient to locate its position. If the sample is not crystalline, a single very broad peak will be the only feature of the diffractogram. In this case the apparent crystallite size is taken as zero. To obtain the apparent crystallite size used herein as a structural parameter, measurements are made on that one of the two most intense peaks which is located at the smaller value of 2.theta.. The procedure is as follows (cf. Alexander, op. cit., Chapter 7).

A base line is first established on the scan by drawing a straight line between the points on the curve at 9.degree. and 36.degree. 2.theta.. Next, a vertical straight line is dropped from the top center of the selected peak to the base line, and a point midway between the top of the peak and the base line marked on this vertical line. A horizontal line is then drawn at this midpoint. This line may cut one shoulder of the peak or, if the minimum in between the two major peaks is low enough, both shoulders. The breadth of the selected peak at this point is obtained by either measuring the distance along the horizontal line from one shoulder to the vertical line and doubling it, or when possible the distance between both shoulders along the horizontal line. The distance is expressed as peak (or "line") breadth in radians, obtained by using the scale for 2.theta. (established on the chart previoulsy) to convert the observed line breadth in inches or centimeters to degrees and, eventually, radians. If B is the observed line breadth in radians, the corrected line breadth .beta. in radians is (cf. Alexander, op. cit., p. 443),

.beta. = .sqroot.B.sup.2 - b.sup.2

where b is the instrumental broadening in radians. The instrumental broadening constant b is determined by measuring the line breadth of the peak located at approximately 28.degree. 2.theta. on the diffractogram of a silicon crystal powder sample supplied by the manufacturer of the X-ray equipment (Philips Electronic Instruments, Mount Vernon, N.Y.). The constant b is this line breadth in radians. The instrument settings used are: scanning speed 0.125.degree. 2.theta. per minute, time constant setting 8 and chart speed 1 inch/min.

Finally, the apparent crystallite size associated with the selected reflection (see above) is given by

ACS = (K .lambda.)/(.beta.cos .theta.)

wherein:

K is taken as one (unity)

.lambda. is X-ray wavelength (1.5418 A here)

.beta. is corrected line breadth in radians (see above)

.theta. is the Bragg angle (1/2 of the 2.theta. value of the selected peak, as obtained from the diffractogram).

In this measurement, it is recognized that line breadth is affected by strains and imperfections in crystals (which are of unknown magnitude), as well as by crystallite size, and for this reason the value obtained by the above calculation is called the apparent crystallite size. Values determined by this method have been shown to be precise to .+-. 2 A at the 95 percent probability level.

A METHOD USING OPTICAL PROPERTIES TO MEASURE LATERAL ORDER IN FIBERS OF THIS INVENTION

Preliminary observations designed to give an idea of the range of refractive index values for n.sub..parallel. and n.sub..vertline. are made on short lengths of fibers placed in a transmission interference microscope (e.g., the two beam instrument made by E. Leitz & Co.). The fibers are mounted in a series of "Cargille" index of refraction liquid to find the point at which the index of the oil matches the index of the fiber (minimum fringe shift) first for n.sub..parallel., then for n.sub..vertline.. Fibers of this invention are characterized by a relatively uniform n.sub..parallel., and an .sub..vertline. that decreases somewhat toward the center of the fiber.

A well aligned bundle of fibers about 1 mm. in diameter and 5 cm. long is then taped to a flat Teflon TFE-fluorocarbon resin plate. A drop of epoxy embedding material, e.g., from Cargille, Inc. and prepared from 94 cc. dodecenyl succinic anhydride (hardener), 75 cc. "Araldite" 6005 resin, 8cc. dibutylphthalate (plasticizer), 3 cc. N-benzyl dimethylamine (accelerator) [the accelerator is mixed with resin and the hardener and plasticizer are added], is placed at the center of the bundle and the mounted sample placed in an oven at 60.degree.C. for about 20 hours. In that time, the embedding material flows through the bundle and polymerizes. A short segment is cut from the preparation and glued (e.g., with "DUCO" Cement) to the end of a tapered rod in such a way that when the rod is placed in the chuck of a microtome, oblique sections (about 45.degree. to the fiber axis) about 0.2 .mu. thick can be prepared. This cutting is best done with a microtome designed for preparing ultra-thin sections (e.g., the "Ultratome" made by LKB, Stockholm, Sweden) at a cutting speed of 1 mm./sec. or less. The fiber bundle should be oriented in a plane that is normal to the knife edge. Furthermore, the bundle should be inclined at about a 45.degree. angle to the cutting direction.

Sections for study in the optical microscope are picked off the microtome water trough with a small piece of microscope cover glass and transferred to a microscope slide by floating the sections on a drop of water. The water is then removed with a piece of filter paper or by evaporation. The slide is cut in half and both pieces transferred to the stages of a Leitz Interference Microscope. The piece containing the sections is placed in the measuring beam of the microscope, and the other piece of slide is placed in the reference beam. The microscope is set for interference contrast. Using green light (.lambda. = 0.546 .mu.), one records the distance (D) the wedge compensator has to be moved between black background settings, and the distance (d) between black background and black sections. It is proper to have the analyzer set with its polarization direction parallel to the short axis of the fiber sections. Then using the approximate value of n.sub..vertline. found by the method described in the first paragraph above, one can calculate the section thickness (T) from

T (in microns) = [(d/D) .lambda. (in microns)] /(n.sub..vertline. - n.sub.R)

where n.sub.R is the refractive index of the reference fluid, which in this case is air (n.sub.R = 1.00).

The sections are then mounted in an oil of refractive index near n.sub..vertline. (.about.1.64), a cover glass added and the preparation transferred to a universal stage on a polarizing microscope (e.g., a "Dialux-Pol" polarizing microscope with a five axis Fedorow Stage, both made by E. Leitz & Co.). White light is used and in the calculations below we assume a wave length, .lambda. = 0.55 .mu.. The polarizer and analyzer are crossed in the 45.degree. position, and an elliptical compensator having a maximum range of .lambda./30 (made by E. Leitz & Co.) is placed in the conventional compensator slot. Measurements are made by eye using a 32X objective lens and a 6X eyepiece.

This technique is applicable to fibers which have cross-sections that are essentially circular.

The universal stage is set to the zero tilt position and the sections rotated about the vertical axis in such a way that the long axes of the fiber sections are at 45.degree. to the polarizer, and a tilt axis set parallel to the major and minor axes of the sections. With the compensator removed, the sections are then tilted about the axis that is parallel to the minor axis of the sections to a point of minimum average intensity within the sections. The sections are then tilted about the axis parallel to the major axis of the sections to minimum intensity, or to the appearance of a maltese cross. The amount of tilt in each axis is recorded. These angles can be used to calculate the increase in path length introduced by tilting (see "Manual of the Polarizing Microscope," by A. F. Hallimond, Published by Cooke, Troughton, and Simms Ltd., York, England 1953), but this is unnecessary when considering the precision required. However, the second tilt is a useful measure of section distortion. It has been found that if the tilt about the major axis of the sections is greater than 20.degree., the section is judged to be poor and the sectioning procedure should be repeated.

The compensator is then inserted and the angle corresponding to the maximum amount of compensation required to produce extinction along the minor axis of the section is noted. The compensator is then set at the angle corresponding to the maximum amount of compensation required to produce extinction along the major axis of the section. This angle is noted and subtracted from the first compensator setting. This difference is recorded as 2.phi., maintaining the sign of the difference. As used herein, lateral birefringence (.DELTA.n) is calculated from:

.DELTA.n = (K.lambda. Sin 2.phi.)/T

where K is an instrumental constant provided by the manufacturer of the compensator, .lambda. is the wave length of light used (in microns), 2.phi. is the difference in compensator readings defined above, and T is the section thickness in microns. Positive Lateral Birefringence is defined as n.sub.r > n.sub.t, where n.sub.r is the index of refraction for light polarized such that the electric vector is along the radius of the fiber cross-section, and n.sub.t is the index of refraction for light polarized such that the electric vector is along the normal to the radius of the fiber cross-section.

Generally, compensator readings of birefringence on five or ten filaments, or on as many filaments as are necessary in order to obtain a representative sampling of the fiber bundle, are made and averaged to obtain the Lateral Birefringence. In viewing each section, there should be no cutting distortions or other artifacts which would be apparent to one skilled in the art, although a slight change in the inclination of the optic axis from its expected position is allowed if it can be compensated for by tilting. Those sections having distortions other than those allowed in the previous sentence should be disregarded. It may also be preferred, in some cases, that a new section be prepared which is free of distortion and artifacts. In most cases compensator readings to obtain the birefringence of individual filament sections are made at constant settings of the tilt axes. However, it is conceivable that in some instances the filaments will not be aligned parallel to one another within the fiber bundle prior to sectioning, causing individual filaments to be cut at different angles. This requires that the procedure for adjustment of the tilt angles be repeated for each individual filament section before compensator readings are made. As before, any section for which the tilt about the major axis of the section is greater than 20.degree. is disregarded.

The precision of the above method of determining lateral birefringence, .DELTA.n, was calculated to be .+-. 0.003 at the 90 percent confidence level, irrespective of the value of .DELTA.n.

All fibers of this invention consisting essentially of PPD-T that are essentially round in cross-section and have a d.p.f. less than about 10 will have a .DELTA.n of at least 0.022.

Fiber Densities

Fiber densities are measured using the density-gradient tube procedure for plastics specified in ASTM method D 1505-68, Part 27, 1970, modified by using heptanecarbon tetrachloride at 25.degree.C. as the liquid system for the density-gradient tube. The densities of four loosely knotted short (about 1 to 2 cm) lengths of filament or yarn are determined and the average value reported.

Following are the values (to three significant figures) of density (in grams/cubic centimeter, g/cm.sup.3) for the fibers of the examples in Table I.

______________________________________ item density a-1 1.45 b-1 1.46 c-1 1.46 d-1 1.46 d-2 1.47 ______________________________________

A minimum density of 1.40 g./cm.sup.3 is used to assure that the fibers will not have an excessive amount of voids or bubbles that would significantly reduce the expected tensile strength.

Starting fibers a, b, c and d have a density of 1.45 g./cm..sup.3.

EXAMPLE I

PPD-T polymer of 6.0 I.V. is added to sulfuric acid (99.7% H.sub.2 SO.sub.4) at 40.degree.C. in a water-jacketed commercial planetary mixer through a top entrance over about 2 minutes to give a ratio of 46 grams of polymer/100 ml. of acid. The mixer is sealed and placed under 68.5 to 76 cm. of Hg vacuum. The temperature of the water jacket is increased to 85.degree.C. and the planetary mixing blades started at a slow speed. After about 12 minutes the jacket temperature is reduced to 77.degree.C. which affords a temperature in the solution of between 79.degree.-82.degree.C. Mixing is continued for about 2 hours. The solution then has a bulk viscosity of 2,300 poises.

The dope is transferred to a glass-lined, water-jacketed (90.degree.C.) kettle. A vacuum of about 69-76 cm. of Hg is applied for about 30 minutes to remove any air or bubbles caused by the transfer. The dope is pumped from the kettle through a transfer line closely wrapped with a water line (90.degree.C.) to an electrically heated (80.degree.C.) spinning block and attached gear pump. The gear pump meters the dope through another passage in the block to a water-jacketed (80.degree.C.) spinneret pack containing a backing screen, stainless steel felt and a 0.5 inch (12.7 mm.) diameter spinneret containing 100 holes of 2 mil (0.051 mm.) diameter. The dope is extruded from the spinneret at a jet velocity of about 207 f.p.m. (63 m.p.m.) vertically through a 5 mm. layer of air into 1.degree.C. water in a spinning tube similar to that shown in FIG. 1. Items a and c are made using a freely revolving roller under the spin tube to direct the threadline to the windup while item d uses a ceramic rod. The yarn is wound up at different speeds on a bobbin under a water (50.degree.C.) spray. The bobbins of yarn are stored in a tank of water. The bobbins are then submersed in 0.1N NaHCO.sub.3 and then further extracted with water (70.degree.C.) on an advancing reel extracting device of the type shown in U.S. Pat. No. 3,659,225. The extracted yarn is wound up and dried on the bobbins at 70.degree.C. Properties of the dried yarn of 5.2 I.V. are given in Table I as items a, c, and d which are prepared at spin stretch factors of 1.5, 3.4 and 4.4 respectively.

EXAMPLE II

This example illustrates the heat treatment of fibers of PPD-T in order to increase the initial modulus.

Fibers are extruded from dopes with sulfuric acid using the general procedures of Example I. Items a, c, and d of Table I are the fibers of Example I. Yarn item e is dried under a tension of 5 gpd at 150.degree.C. Yarn item f is prepared from polymer of 6.6 I.V.

Yarn I.V. values range from 4.9 (item b) to 5.8 (items f and f-1).

The well-washed and dried yarns of about 135 to 415 denier (100 filaments) are passed through a 10 foot long (3.05 meters) stainless steel tube of about 1.5 cm. I.D. containing nitrogen at various conditions as given in Table I under "Heating Conditions" where ".degree.C." is the temperature of the wall at the midpoint of the tube, "t" is the time in seconds of the treatments and "tens" is the tension in grams/denier. The tube is electrically heated and is contained in a box of vermiculite as insulation. The nitrogen passes through a tube in the box before being fed to the yarn treatment tube. The yarns are only drawn from about 1.001 to 1.021 times their original length and do not contact the walls of the tube.

Similar results are obtained when a water-wet yarn is treated in this manner.

It is observed that with a 400 denier yarn the temperature should be about 100.degree.C. higher than that used for a 200 denier yarn when treatment times of about 1 second are used.

Composites of epoxy resin and unidirectional fibers containing about 60 volume percent of fibers b-1, e-1 and f-1 show excellent values of flexural modulus (ASTM D790-66, Procedure A with some modifications), flexural offset yield strength at 0.02 percent offset strain [ASTM D790-66 (11.5) and Appendix to ASTM D638-68], tensile strength, tensile modulus and Stress-Rupture Life test [time to break under a static axial tensile load which is a given percentage (usually 95-98 percent) of the average ultimate tensile strength of the particular sample].

The thin (about 0.25 mm thick) composites used for tensile properties show little or no warpage upon 12 hour exposure to boiling water.

Filament properties of items e-1, f and f-1 are based on four breaks; filament properites of items b and b-1 are based on seven and five breaks respectively.

Heating a yarn similar to item c at 400.degree.C. for 3 seconds at 0.7 gpd tension gives a fiber with yarn ten./E/Mi of 20.8/2.2/908, an orientation angle of 12.6.degree., an ACS of 91 A, an ACS/OA ratio of 7.2 and a density of 1.46 g./cm..sup.3.

TABLE I __________________________________________________________________________ Heating Conditions A.C.S. A.C.S. O.A Yarn Properties Filament Properties Item .degree.C-t-tens O.A. A .degree. Ten E Mi Tou Den DPF Ten E Mi Tou __________________________________________________________________________ a None 2.2 45 20.0 21.2 3.9 547 0.39 415 3.7 26 5.6 570 0.73 a-1 250-6-6 6.2 60 9.7 21.4 2.3 917 0.24 394 3.9 24 3.8 770 0.50 b None 2.6 41 15.6 22.0 3.3 649 0.34 196 2.0 25 4.4 570 .54 b-1 350-1.5-4 7.4 70 9.4 22.3 2.2 1019 0.24 179 1.8 26 3.3 890 0.145 c None 3.5 49 13.9 22.8 3.2 727 0.35 190 1.9 27 4.8 680 0.67 c-1 250-6-4 6.2 59 9.5 23.1 2.2 1080 0.27 178 1.8 29 3.7 890 0.58 d None 4.3 49 11.5 24.8 2.8 948 0.34 135 1.4 27 4.3 680 0.62 d-1 250-3-6 6.4 62 9.7 22.5 2.0 1175 0.23 136 1.4 28 3.5 830 0.50 d-2 550-6-2 12. 118 9.8 16.8 1.3 1394 0.11 136 1.3 22 2.1 1030 0.24 e Dried 150.degree. 3.0 41 13.5 23.6 2.9 862 0.35 184 1.8 e-1 400-3-4 9.1 80 8.8 20.5 1.9 1080 0.19 179 1.8 22 2.7 890 0.31 f None 4.5 50 11.2 25.1 3.2 779 0.39 191 1.9 31 4.8 710 0.76 f-1 350-1.5-6.5 10.6 84 7.9 23.9 2.1 1130 0.25 177 1.8 32 3.7 920 0.62 __________________________________________________________________________

The twist multipliers for the yarn examples in Table I range from a low of 0.477 (item d-1, 136 denier) to 0.815 (item a-1, 394 denier).

Following are values of lateral birefringence (.DELTA.n) of fibers in Table I.

______________________________________ Item .DELTA. n a 0.045 a-1 0.035 b 0.025 b-1 0.054 c 0.035 c-1 0.042 d 0.044 d-1 0.045 d-2 0.048 e-1 0.045 f 0.031 f-1 0.053 ______________________________________

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