PPTA Fibers Research Articles

Structure of PPTA fibers derived directly from a liquid crystalline prepolymer dope in an organic solvent

AbstractThe structure and properties of the pulp‐like aromatic polyamide PPTA fibers prepared directly from a polymerization medium were studied by scanning electron microscopy (SEM), X‐ray diffractometry, and polarizing light microscopy. The morphology of the fiber appears significantly different from that of Kevlar and does not reveal a skin‐core structure. Regular defect bands and, thus, the regular clustering of the chain ends also could not be observed.

Microstructure and charge transport in hybrid metallophthalocyanine/rigid rod polymer fibers

Strong, environmentally stable, electrically conductive fibers can be fabricated from solutions of nickel phthalocyanine (Ni(Pc)) and the ultra-high modulus polymers poly-( p -phenyleneterephthalamide) (PPTA, Kevlar) or poly- p -phenylenebenzobisthiazole) (PBT) by dry-jet, wet-spinning techniques, followed by chemical or electrochemical doping. The fiber mechanical strength s at a particular composition is a simple linear function of the Ni(Pc) I volume fraction Φ c : s ≈ s p (1− Φ c), where s p is the of a pure PPTA or PBT fiber. The electrical conductivity σ for Φ c>0.17 obeys the empirical relationship: 1n( σ) = Φ c1n( σ c) + (1 − Φ c) 1n( σ p) where σ c represents the conductivity of polycrystalline Ni(Pc)I and σ p denotes the conductivity of pure PPTA or PBT. The temperature dependence of σ is thermally-activated and can be fit to a fluctuation-induced carrier tunnelling model. In contrast, the thermoelectric power S of the fibers is p-type and metal-like (S ∼ T), strongly resembling that of Ni(Pc) I single crystals. X-ray diffraction shows microstructural evidence for phase separation and the presence of a fine (domain size <50 nm) Ni(Pc) I dispersion embedded in the PPTA or PBT matrix.

Determination of poly( p-phenylene terephthalamide) fiber surface cleanliness by inverse gas chromatography

Inverse gas chromatography has been used to measure the surface adsorption of alkane vapors on a series of cleaned poly( p-phenylene terephthalamide) (PPTA) fibers, commercially known as Kevlar 49. From the retention volumes at infinite dilution over a range of temperatures, the differential heat of adsorption, ΔH A o, the standard free energy of adsorption, Δ G A o, and the standard entropy of adsorption, Δ S A o, have been determined for the n-alkane vapors C 6C 10 on the various substrates. The London (dispersive) component of the surface free energy, γ s d , has also been calculated. Comparison of results for the original materials with those of the cleaned samples has allowed the evaluation of surface contamination and the effectiveness of a range of solvent extraction procedures. Low values of − ΔH A ̊ , γ s d , and − ΔS A ̊ were obtained for alkane adsorption on the original materials, indicating significant levels of surface contamination. These contaminants were removed by solvent extraction with acetone or water. The subsequent changes in ΔH A ̊ , γ s d , and ΔS A ̊ revealed that higher energy adsorption sites of the highly crystalline PPTA fibers were now exposed.

Growth-Packed PPT-A Fiber for the Replacement of Native Asbestos

AbstractA now type of crystal that can be defined as a “gel crystal” was first made with oligomeric poly-p-phenyleneterephthalainide (PPT-A), associating a large amount of dimethylacetamide (DMAc) liquid molecules, in which catalytic amount of heterocyclic tertiary amine, coupled with alkali metal cations or phosphoric triamidos are incorporated. Under mechanical orientation of the growing PPT-A, before gelation of the reaction “mixture, a hark agar-like gel crystal is formed. It shows a strong optical birefrin,- gence with four clear extinction positions under crossed polarizers when the gel was sectioned either along the direction of PPT-A orientation or perpendicular to it. When the gel formed without orientation it exhibitil speckled birefringence due to a large number of tiny spherulitic crystallites. The degree of crystalline order was similar to that of lyotropic, nematic liquid crystals when evaluated from X-ray diffractograins. The same type of gel crystals with extinction positions were observed in animal and vegetable tissues. It is assumed that the gel crystal has a threedimensionally ordered molecular network structure in which oligo-PPT-A's are arranged end-to-end in parallel lines. The lines are connected laterally, to form sheets, by DMAc bridges that are linear associations of DMAc molecules, which are connected to the oligo-PPT–A at the CONH groups. PPT-A molecular growth and self-ordering occurs within this gel crystal. A similar process gives rise to wool, cotton and ol.her fibrous biological materials.The high molecular weight PPT-A fiber thus grown in the gel crystal is constructed with numerous unit fibrils which connect themselves in a three-dimensional network, that resembles native flax or ranie fibers. The molecular growth of PPT-A in the gel crystal reaches ultra high molecular weight that can rarely be attained in ordinary solvent polyinerization. The gel state of PPT-A fibrils are initially formed spontaneously by selfsubdivision of the gel crystal after the molecular growth of PPT-A. Solid fibrils and fibers are finally fortoed by removing DMAc. The PPT-A gel fibrils thus formed resemble closely “the nascent fibril” of cellulose, and the microstructure and morphology of PPT-A fibrils are consequently very similar to those of native cellulose. The thickness of PPT-A fibrils can readily be controlled by the temperature in the gel crystal before fibril formation. The direction of the fibrils depends on the initial orientation of oligo-PPT-A's immediately before the formation of the gel crystal.This growth-packed PPT-A fibril, that can be produced by a one step chemical reaction, is considered as a good candidate for the replacemerit of native asbestos, because of the lower production cost, microfibrillation property, and inherently high heat-resistance.

Vicrostructure and Electrical Properties of Kevlar/Polypyrrole Composite Fibers

AbstractElectrically conducting composite polypyrrole/poly(p-phenyleneterephthalamide) (PPTA or KEVLAR) fibers have been prepared by chemical polymerization of pyrrole within the interstices of the hydrogen-bonded gel structure of never-dried PPTA fibers. The resultant fibers contain a uniform dispersion of polypyrrole, as evidenced by scanning electron microscopy. The temperature dependence of the electrical conductivity of these hybrid fibers is presented. The conductivity is well described by the fluctuation-induced charge transport model over the entire temperature range of interest. However, the low temperature electrical conductivity also exhibits a hoppinglike temperature dependence, and an Arrhenius-like temperature dependence is observed in the high temperature limit. Measurements of the temperaturedependent tbermopower are indicative of a p-type carrier.

Studies on microbiological deterioration of textile. Part 4. Morphological deterioration of poly(p-phenylene terephthalamide)(PPTA) fiber (Kevlar 49) by Aspergillus flavus.

Poly (ρ-phenylene terephthalamide) (PPTA) fiber (Kevlar 49) was deteriorated for 8, 15 and 30 days at 30°C by Aspergillus flavus (IAM 2044) on agar slant medium containing water. The change in morphology of the fiber was observed by optical and scanning electron microscopes. A short split parallel to the direction of the fiber axis was observed for the fiber treated for 8 days. The fiber treated for 30 days was remarkably deteriorated and numerous microfibrils were observed in the fiber. The surface of the microfibrils was very smooth. We have found that the morphological change of PPTA fiber by the microbial deterioration was different from that of Nylon and PET fiber.

Microparacrystals in PPTA fibers in thermodynamic equilibrium state

It is investigated why Krenzer and Ruland, on the basis of their X-ray diffraction data, cannot justify the paracrystalline disorder in poly(p-phenylene-therephthal-amide) fibers (PPTA). In a first examination we found that the lattice factorZ of paracrystals is not considered. The correlation betweenZ and the shape factor as a consequence of theα* law is replaced by an arbitrarily chosen relation. The complete lack of literature on paracrystals published within the last decade may be responsible for the other misinterpretations which are eludicated in this paper.

Chemical modification of Kevlar fiber surfaces and of model diamides

AbstractOrganic reactions of aromatic diamides as models for PPTA fibers have been investigated to select reagents and conditions suitable for surface‐controlled heterogeneous reactions on Kevlar filaments and fabrics. Amine functional groups have been incorporated into fiber surfaces by bromination followed by ammonolysis and by nitration followed by reduction. Modification of filaments has been attained without impairment of fiber properties. Preliminary experiments have also shown that the presence of amino groups on Kevlar fabric can provide remarkably improved peel strength and apparent interlaminar shear strength in epoxy laminates, suggesting a significant role of covalent bonding in improving adhesion in aramidepoxy composites.

Lattice defects in poly(p-phenylene-terephthalamide) fibers

The lattice defects in PPTA fibers were studied by line profile analysis. The results indicate that lattice distortions are produced by local defects, probably chain ends incorporated into the crystalline lattice. The disorder is strongly anisotropic. There are indications for the existence of small domains of higher disorder which do not disappear on annealing.

Surface‐modified kevlar fiber‐reinforced ionomer: Dynamic mechanical properties and their interpretation

AbstractA quasi‐three‐dimensional model was applied to the composite of ionomer and isotropically oriented discontinuous poly(p‐phenylene terephthalamide) (PPTA) fibers having the surface modified by carboxymethyl groups in order to predict the Young's modulus from the properties of the constituent materials over a wide temperature range. The model in which the modified PPTA fibers are dispersed in ionomer matrix in a three‐dimensional network is composed of series and parallel models. A good agreement between calculated and experimental values was eventually obtained. For comparision, the complex modulus of the composite was also evaluated by application of the Halpin‐Tsai equation modified by Nielsen. The calculated values agreed with the observed values below 240 K, whereas the calculated values above 260 K were significantly lower than the observed moduli.

Morphology of poly(p‐phenylene terephthalamide) fibers

AbstractFibers of poly(p‐phenylene terephthalamide) (PPTA) have a fibrillar morphology, the individual fibrils having a high proportion of extended chains passing through periodic defect layers. A pleat structure is superimposed. The fibers are fully crystalline (within the limits of determination) with a small fraction of randomly oriented crystalline material. The major distinction between PPTA and conventional fibers lies in the high level of extended chains passing through the defect layers of the former structure. These extended chains result in crystallographic register being maintained between adjacent ordered zones. Quantitatively, a measure of this order is obtained from a comparison of the correlation length, obtained from meridional x‐ray peak widths, and the defect spacing. In conventional fibers the defect spacing, i.e., long period, is longer than the correlation length (i.e., crystal size). In PPTA, the analog of the long period, the defect spacing (about 35 nm) is smaller than the correlation length, which is over 80 nm.

The relationship between the physical structure and the microscopic deformation and failure processes of poly(p‐phenylene terephthalamide) fibers

AbstractRelations between the physical structure and the deformation and failure processes of poly(p‐phenylene terephthalamide) (PPTA) fibers are reported. The effects of the physical processes involved in fiber fabrication, including the crystallization of PPTA‐H2SO4 dopes under stress, are considered in relation to their effect on the structure of the fiber. The deformation and failure processes together with the structure of the fiber are discussed in the light of fracture‐topography studies of fiber‐epoxy composite strands, single filaments, HCl‐etched and unetched yarns, and transmission optical microscopy studies of stressed and unstressed yarns. In view of these observations, the physical structure of PPTA fibers is discussed in terms of pleated H‐bond sheets, the macromolecular chain‐end concentration and distribution, and the presence of impurities. The structural parameters that affect the failure processes of these fibers and how such parameters can be modified by service environment conditions are also addressed.

Interfacial interaction between poly(p‐phenylene terephthalamide) filament and nylon 6 matrix crystallized from the melt

AbstractNylon 6 was isothermally crystallized at 483 K for 45 in the presence of poly(p‐phenylene terephthalamide) (PPTA) filament. Two kinds of traqnscrystalline zones were observed around the surface of PPTA filament by polarized optical microscopy. The two are designated as the interfacial and the intermediate zones from the surface of PPTA filament. The aggregation state of molecular chains in these zones was characterized by means of microbeam x‐ray diffraction and birefringence. The a*‐and the a‐axes of nylon 6 oriented perpendicularly to the filament surface, aligning along the radial direction in the interfacial and intermediate zones, respectively. The b‐ and c‐axes rotate around these a*‐and a‐axes. The DSC measurement revealed that nylon 6 molecules in the interfacial zone crystallized and also melted at a higher temperature, compared with those in the intermediate zone or with the usual three‐dimensional spherulite. Viscoelastic measurement clarified the fact that the interfacial zone had a restraining effect on the molecular motion of nylon 6 in the compostes with PPTA fiber. The interfacial zone is preferable for reinforcing the composite with respect to modulus.

Deformation band studies of axially compressed poly(p‐phenylene terephthalamide) fiber

AbstractDeformation mechanism of poly(p‐phenylene terephthalamide) (PPTA) fiber during axial compression was studied. PPTA fibers were embedded in resin and axialy compressed in a tensile machine. PPTA fibers were then taken out from resin at various stages of compression. Kink band formation was examined by means of polarizing microscopy, X‐ray diffraction (WAXD), electron diffraction (ED), and electron microscopy. WAXD pattern of seriously compressed PPTA fiber shows that (200) reflection spots and arcs appear on the equator and meridian respectively. On the other hand, (110) reflection spots are confined to the equator. PPTA fiber could be splitted tangentially and radially into two kinds of thin fibrillar fragments (I and II) which reveal two distinct types of kink band and ED pattern. In fragment I obtained by tangential splitting, kink bands are formed at about 55° with respect to fibril axis, whereas in fragment II obtained by radial splitting kink bands are formed perpendicular to the fibril axis. These results were confirmed by ED studies. It was assumed that slip of (200) planes containing hydrogen bonded sheets as well as intermicrofibril slip plays an important role in the deformation of PPTA fiber during axial compression.

Surface‐modified kevlar fiber‐reinforced polyethylene and ionomer

AbstractThe surface of poly(p‐phenylene terephthalamide) (PPTA) fiber was modified by the polymer reaction via the metalation reaction in demethyl sulfoxide to provide the fiber surface with several functional groups such as the n‐octadecyl group, carboxymethyl group, and acrylonitrile. The discontinuous fibers thus modified could be isotropically dispersed in polyethylene and ionomer. The composite of ionomer and random‐in‐plane oriented discontinuous PPTA fibers with the surface modified by carboxymethyl groups was remarkably improved in its mechanical properties, and the fiber efficiency factors of about 0.4 for both strength and modulus are comparable to the values in the composite of unidirectionally oriented discontinuous PPTA fibers and ionomer. The heat resistance of ionomer was also greatly improved by the presence of this surface modified PPTA fibers, even with volume fractions as low as 2.2%.

Fine Structure of Poly (p-phenylene terephthalamide) Fibers

In order to clarify the fine structure of poly (p-phenylene terephthalamide) fibers, hereafter refered to as PPTA fibers, the absolute value of refractive index was determined and the orientations of b and c axes and chain axis of molecules in an amorphous region were evaluated quantitatively. Keviar 49R was adopted as the model sample of PPTA fibers. An interference optical microscopy and an electron microscopy were mainly adopted. By analyzing the interference fringe pattern, the refractive indexes along a, b and c axes were evaluated as follows : nα, c≤1.579, nβ, c≤1.764, nγ, c≥2.13. The refractive indexes along a molecular chain axis (nγ, α) and along the axis perpendicular to the molecular chain axis (n (α+β) /2, a) in the amorphous region were evaluated to be as nγ, a≥2.13 and n (α+β) /2, a≤1.498 respectively. The results obtained were : (1) The local orientation function of c axis changed from place to place ranging from ca. 1.0 at periphery of a fiber to 0.60.8 at the central part of a fiber. (2) b axis oriented to the radial direction of a fiber, and the local orientation function of this axis changed from 0.8 (periphery part) to 0.2∼0.4 (central part). (3) The orientation of chain axis in an amorphous region at the periphery was higher than that at the central part. (4) The volume fractions of relaxed chains, tie chains, and stretched chains in an amorphous region were evaluated to be 0.10, 0.02, and 0.17 respectively. (5) The pleated lamella structure with the period of 300500 nm, and also the micro-fibril structure of 10 nm in width were observed in the freezed fracture surface. (6) The refractive index changed periodically by the period of 1 μm along the fiber axis. By taking the above resits of (1) (6) into consideration, a new fine structure model, called “fibrious block model”, has been proposed which was constructed with the fibrious structure with many blocks of 5-4.0 nm in width and 300∼500 nm in length, for representing the fine structure of PPTA fibers.

Far-infrared spectrum of poly(p-phenylene terephthalamide)

Abstract The polarized and unpolarized far-infrared (460-30 cm−1) spectra of poly(p-phenylene terephthalamide) (PPTA) fabric and fiber arrays are reported. These spectra are discussed and tentatively analyzed on the basis of the internal and external modes of the repeat unit of PPTA. To assist in band assignments, the spectra of potassium triiodide-treated PPTA fibers and of benzanilide and its benzene solution are also reported.

RADIATION EFFECTS ON WHOLLY AROMATIC POLYAMIDES

Radiation effects on poly(m-phenylene isophthalamide) (m-PIA) and poly(p-phenylene terephthalamide) (p-PTA) fibers have been studied by use of γ-rays and electron beams.Deterioration in the mechanical properties was not observed for m-PIA or p-PTA fiber even after irradiation of 1000 Mrad in vacuo. The presence of air favored the degradation of wholly aromatic polyamides as in case of aliphatic polyamides, although a much larger irradiation dose was needed for the former than for the latter to give the same degradation in mechanical properties by irradiation. The irradiation up to 1000 Mrad with electron beams in air revealed that p-PTA is more stable against irradiation than m-PIA.Viscometric studies were carried out on the irradiated fibers. Only a small change in viscosities of p-PTA was found even after irradiation both in the presence and absence of air up to a large dose, whereas in m-PIA, gel was formed after γ-ray irradiation of about 200 Mrad in vacuo and viscosity decreased to 40% of the original value after γ-ray irradiation of about 500 Mrad in air.No or a very small change in TGA curves for m-PIA and p-PTA was observed even after the irradiation of 1000 Mrad, showing that the irradiation at room temperature had no appreciable effect on the thermal degradation of these polymers.

DYNAMIC MECHANICAL PROPERTY OF POLY(p-PHENYLENE TEREPHTHALAMIDE) FIBER

Studies on the thermal and structural properties in addition to the dynamic mechanical property were made to clarify the mechanical relaxation mechanisms of poly (p-phenylene terephthalamide) (PPTA) fiber.The modulus and density increase with increasing annealing temperature up to ca. 750K, at which PPTA fiber is thermally decomposed remarkably.Two relaxation regions are discernible in the temperature range investigated, and these are labeled β (273K) and β* (523K) relaxations. The following molecular mechanisms are associated with these relaxations on the basis of experimental results of treatment in the boiling water and annealing at various temperatures: (1) The β relaxation arises from thermal molecular motion in the amorphous region and (2) the β* relaxation is associated with that in the crystalline region.By use of X-ray measurement the thermal expansion coefficients of the (200) and (110) planes were determined. The coefficient of the b-axis (hydrogen bonded direction) reveals a distinct break at about 423K and then deviates from the linear relationship above the temperature range. The temperature range of the distinct break in the linear thermal expansion seems to be consistent with the temperature range where the raising of the tan δ curve of the β* relaxation from the base line is recognized. This fact suggests that the β* relaxation process is associated with thermal molecular motion in the crystalline region.Also, the β relaxation process was well resolved into the three relaxation ones (β1, β2, β3) by dynamic mechanical measurements at 3.5Hz.