Drawing and annealing of polypropylene fibers: Structural changes and mechanical properties

A study has been carried out on the influence of cold drawing (25°C), hot drawing (140°C), and annealing (140°C) on the structure and mechanical properties of a series of four different well-characterized melt spun polypropylene filaments. The influence of the interaction between melt spinning and drawing variables was given special attention. Cold drawing increased the orientation in the samples, disrupts the initial monoclinic crystal structure and the morphology of the filaments, and it results in extensive fibrillation. Annealing restored the monoclinic structure but eliminated only a small part of the fibrillation. Hot drawing produced changes which were qualitatively similar to the combined effects of cold drawing and annealing. The orientation and morphology of the asspun filaments were found to have major effects on drawing behavior and the mechanical properties of the drawn fibers for a given draw ratio. It was found, however, that the mechanical properties (tensile strength, tangent elastic modulus, and elongation to break) of the melt spun, hot drawn and cold drawn, and annealed fibers could all be correlated with birefringence measurements.     

Superdrawn filaments of polypropylene

Filaments of polypropylene having remarkably high strength and modulus have been prepared by the two-stage drawing process of Clark and Scott (4). In the first stage, un-oriented extruded billet is rapidly drawn to the natural draw ratio of ca 7X in hot silicone oil. The second stage of draw, a post neck draw process termed superdrawing, requires a slow rate of draw at carefully controlled temperatures to achieve maximum property values; the optimum conditions appear to be close to 4 percent/min at 130°C, The highest mechanical properties obtained were a Young's modulus of 22 GPa (240 gdtex, 3 × 10⁶ psi) and a tensile strength of 0.93 GPa (10 gdtex, 1.3 = 10⁵ psi) with an elongation to break of 7 percent. The filaments show a distinctive dead bend lack of recovery, WAXS data indicate high orientation of the molecules. SAXS data show no scattering to a resolution of 40 nm which is interpreted in terms of a continuous crystal matrix in the superdrawn filament. SEM micrographs of fractured filaments reveal a morphology of large fibrils of lath like habit with a thickness of ca. 0.1 μm. A series of experiments relating mechanical properties to the rate and temperature of superdrawing is interpreted in terms of two competing mechanisms; shearing displacement of microfibrils and unfolding of chain-fold blocks. Models are offered to explain the observed mechanical and physical property changes on superdrawing.     

Studies on production of polypropylene filaments with increased temperature stability

High Modulus high tenacity polypropylene filaments have been prepared by drawing them on a heater with a gradient of temperature. The thermal properties have been analyzed, and the effects of nature of the gradient and end temperatures on thermal properties of the filament have been investigated for such filaments. Low shrinkage values even at 150°C, high retention of storage modulus in dynamic mechanical analyses and very low change in length in thermomechanical analyses have been the characteristic of the gradient drawn filaments. The filaments have high crystallinity, crystal perfection, and crystal orientation. In addition, the reported process has the advantage of being continuous in nature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 838–842, 2006     

High modulus/tenacity filaments from blends of different molecular weights of polypropylene

Polypropylene (PP) filaments are prepared by blending two different molecular weight components of PP. A melt‐spinning process to produce filaments includes mixing of components, extrusion, and two‐stage drawing, followed by a unique Gradient Drawing? process. Blending results in highly deformable as‐spun filaments with high draw ratios. For 90:10 blends of PP samples with melt flow indexes of 35 and 3, a high level of crystallinity and crystalline and amorphous orientations are obtained. A sonic modulus of 28 GPa, dynamic modulus of 20 GPa, tensile modulus of 16 GPa, and tenacity of 667 MPa are achieved. These samples are dimensionally stable up to ～100°C. All steps in the production of the filaments are continuous. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1021–1028, 2005     

Mechanical properties and superstructure of isotactic polypropylene fibers prepared by continuous vibrating zone‐drawing

A continuous vibrating zone‐drawing (CVZD) was applied to study the effect of vibration on the mechanical properties and superstructure of isotactic polypropylene fibers. The CVZD treatment was a new drawing method by which the fiber was continuously drawn at a rate of 0.5 m/min under vibration using the specially designed apparatus. The CVZD treatment was carried out in five steps at a drawing temperature of 150°C and a frequency of 100 Hz, and applied tensions increased step by step with processing in the range of 14.8 to 207 MPa. The obtained fiber had a birefringence of 0.0373, crystallinity of 62.4%, tensile modulus of 17.6 GPa, and tensile strength of 1.11 GPa. These values are higher than those of the continuous zone‐drawn isotactic polypropylene fiber previous reported. The vibration added to the fibers during the zone‐drawing was effective in developing amorphous orientation and improving the mechanical properties. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 600–608, 2001     

The influence of drawing,twisting, heat setting and untwisting on the structure and mechanical properties of melt spun polypropylene filament

The changes in structure and mechanical properties of melt spun polypropylene filaments were determined as the filaments were successively (i) drawn, (ii) twisted, (iii) annealed, and (iv) untwisted. Filaments spun to two different melt draw down levels were studied. The effects of draw temperature and draw ratio, extent of twist, annealing temperature, and the extent of untwisting were examined. Melt spun and twisted filaments (draw ratio of 1) were also considered. Structural characterization techniques used in this study include wide angle (WAXS) and small angle (SAXS) diffraction, scanning electron microscopy (SEM), birefringence and density measurement. Drawing increased crystalline orientation and at low temperatures disrupted the monoclinic unit. cell existing in the melt spun fibers. The drawing produced considerable fibrillation under all conditions even when carried out at 120°C. The fibrillation quid void formation due to low temperature drawing was more pronounced. The variation of WAXS patterns and mechanical properties of twisted drawn filaments was interpreted by presuming that the fibrils act in a manner analogous to that of the filaments in a continuous filament twisted yarn. In the case of incompletely drawn filaments the effect of additional drawing occurring during twistin must be considered. Twisted fibers annealed at 150°C sowed a drastic reduction in tensile properties, while those annealed at 125°C did not set the twist and caused the fibers to tend to untwist. Annealing at 140°C appeared to give satisfactory heat setting. Annealing of cold drawn and cold drawn and twisted fibers increased the density, removed many defects and reformed a well defined monoclinic crystal structure and a lamellar morphology. Untwisting of heat set filaments tended to give back the properties of hot drawn fibers. In some cases, however, the opening of cracks was noted.     

Effects of mechanical drawing on the structure and properties of peek

This study examines the effects of crystallinity and temperature on the mechanical properties of PEEK. Crystallinity in PEEK Increases with annealing temperature up to a maximum of 28 percent with a melting point at 335°C. A minor melting peak also occurs about 10°C above the annealing temperature. In cold drawing the samples exhibited a yield stress and necking followed by homogeneous drawing. The yield stress increases with crystallinity, but there is no change in the modulus. The extension in the necking process also increases with crystallinity, however there is only a slight increase in extension-to-break since necking is compensated by the final amount of homogeneous drawing. The yield stress of PEEK when drawn at T_g (145°C) is significantly lower than at room temperature indicating a reduction in mechanical properties at temperatures approaching T_g. After mechanical drawing the minor melting peak disappears and on heating the material undergoes cold crystallization near the onset of T_g. There is evidence that this minor crystalline component might contribute to the yield stress changes with annealing history. Cold drawing induces crystallization of amorphous PEEK but decreases crystallinity and generates microscopic voids in crystalline PEEK, The various effects of crystallinity on mechanical properties could be important in determining the stress response of PEEK as the matrix in composites.     

Zone drawn NEW-TPI thermoplastic polyimide and its blends with Xydar liquid crystalline polymer

In this work we report the effects of single stage zone drawing on the properties of NEW-TPI thermoplastic polyimide homopolymer, and its blends with Amoco's Xydar liquid crystalline polymer. Zone drawing was performed first on homopolymer NEW-TPI films to determine the effect of load weight, heater speed, and drawing temperature on the attainable draw ratio. Degree of crystallinity and chain orientation increase as the draw ratio increases for NEW-TPI. Blends of NEW-TPI/Xydar compositions 90/10 and 70/30 were studied next. In blends, the Xydar component is not molecularly dispersed, and is initially preferentially oriented along the machine direction during the film processing stage. Xydar acts as a nucleation site and lowers the temperature for crystallization of the NEW-TPI from the rubbery amorphous state. Zone drawing was performed either parallel or perpendicular to Xydar's initial preferred orientation direction. Blends with lower Xydar fraction could be zone-drawn to higher ratios. Zone drawing perpendicular to Xydar's initial orientation direction also resulted in increased draw ratio. Dynamic mechanical properties of the zone drawn materials were studied. In homopolymer NEW-TPI, dynamic modulus increased by a factor of two to 4.0 GPa in zone drawn films, largely as a result of the formation of oriented crystallites. In the blends, the modulus parallel to Xydar's initial orientation direction was greater than that in the transverse direction. Depending upon composition and test direction, zone drawing increased the dynamic moduli of the blends from 1.5 up to 2.7 times, in the temperature range from 150°C to 300°C.     

Gel‐spinning of partially saponificated poly(vinyl alcohol)

DMSO/water (80/20 volume ratio) solutions of commercial poly(vinyl alcohol)s (a‐PVA₉₉, a‐PVA₈₈) with degrees of saponification of 99.3 and 88 mol % were gel‐spun into methanol (−20 and −70°C). The dry filaments obtained were drawn at 200°C (a‐PVA₉₉) and 150–180°C (a‐PVA₈₈). The maximum draw ratio and Young's modulus were 26 and 34 GPa for a‐PVA₉₉ and 21 and 24 GPa for a‐PVA₈₈ (drawing temperature: 160°C). So, at first, the dry filaments obtained for a‐PVA₈₈ were drawn at 150–180°C until 10 times their original length. Moreover, the predrawn a‐PVA₈₈ filaments were perfectly saponificated under fixing at the both ends and then the filaments were redrawn at 200°C. The maximum draw ratio and Young's modulus for the filaments (a‐PVA_88→99) predrawn at 150°C were 28 and 39 GPa, respectively. The a‐PVA_88→99 filaments had two melting peaks (228 and 236°C). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2872–2876, 2000     

Preparation of poly(lactic acid) fiber by dry–jet–wet spinning. II. Effect of process parameters on fiber properties

The dry–jet–wet spinning process was employed to spin poly(lactic acid)(PLA) fiber by the phase inversion technique using chloroform and methanol as solvent and nonsolvent, respectively, for PLA. The as spun fiber was subjected to two‐stage hot drawing to study the effect of various process parameters, such as take‐up speed, drawing temperature, and heat‐setting temperature on the fiber structural properties. The take‐up speed had a pronounced influence on the maximum draw ratio of the fiber. The optimum drawing temperature was observed to be 90°C to get a fiber with the tenacity of 0.6 GPa for the draw ratio of 8. The heat‐setting temperature had a pronounced effect on fiber properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3774–3780, 2006     

Effects of water on drawability,structure, and mechanical properties of poly(vinyl alcohol) melt‐spun fibers

Poly(vinyl alcohol) (PVA) melt‐spun fibers with circular cross‐section and uniform structure, which could support high stretching, were prepared by using water as plasticizer. The effects of water content on drawability, crystallization structure, and mechanical properties of the fibers were studied. The results showed that the maximum draw ratio of PVA fibers decreased with the increase of water content due to the intensive evaporation of excessive water in PVA fibers at high drawing temperature. Hot drying could remove partially the water content in PVA as‐spun fibers, thus reducing the defects caused by the rapid evaporation of water and enhancing the drawability of PVA fibers at high drawing temperature. The decreased water content also improved the orientation and crystallization structure of PVA, thus producing a corresponding enhancement in the mechanical properties of the fibers. When PVA as‐spun fibers with 5 wt % water were drawn at 180 °C, the maximum draw ratio of 11 was obtained and the corresponding tensile strength and modulus reached ～0.9 GPa and 24 GPa, respectively. Further drawing these fibers at 215 °C and thermal treating them at 220 °C for 1.5 min, drawing ratio of 16 times, tensile strength of 1.9 GPa, and modulus of 39.5 GPa were achieved. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45436.     

Combined processing of polypropylene film by coextrusion and zone-annealing

In order to improve the mechanical properties of polypropylene film, a new processing combining extrusion and zone-annealing has been applied. It was found that there are suitable conditions for each step in the combined processing. When the coextrusion draw ratio was low, the total draw ratio and modulus could be increased by the zone-annealing subsequently done. The highest modulus was obtained when the film was coextruded at extrusion draw ratio 4 and then zone-annealed at 120°C under 7 kg/mm². The value was 12 GPa in Young's modulus or 17 GPa in dynamic modulus. The peak temperature of α_c dynamic dispersion for the combinedly processed film was 109°C, which is higher by 10°C than that for the as-coextruded film. Four drawing methods were compared in dynamic viscoelasticity. These methods are the coextrusion, zone-drawing/zone-annealing, two-step coextrusion, and the combined processing by coextrusion and zone-annealing. The highest dynamic modulus for each method was arranged in the above order. The combined processing indicated the most effective improvement in mechanical properties, because it is believed that lamellae in the original film were broken by cooperating interaction of shear stress, compression, and tension on coextrusion and then the superstructure with a high crystallinity and a high molecular orientation was formed on zone-annealing.     

Two-stage drawing of poly(ethylene 2,6-naphthalate)

Initially amorphous and semicrystalline films of poly(ethylene 2,6-naphthalate) with different molecular weights were drawn by two-stage drawing, that is, coextrusion at low temperatures (25–160°C) followed by tensile drawing at high temperatures (200–245°C). Both films could be drawn up to a draw ratio of 8–10 by this method under controlled conditions. The tensile modulus and strength of drawn samples were greatly affected by the draw temperature for the first stage, predrawn morphology, and molecular weight. The remarkable effects of these variables on the tensile properties are closely related to the difference in the resultant amorphous chain orientation of the samples, reflecting the disentanglements and chain slippage during drawing, and the dissipation of chain orientation after processing.     

Mechanical properties and microstructure of poly(ethylene terephthalate) microfiber prepared by carbon dioxide laser heating

We determined that a poly(ethylene terephthalate) microfiber was easily obtained by irradiating a carbon dioxide laser to an annealed fiber. The annealed fiber was prepared by zone drawing and zone annealing. First, an original fiber was zone drawn at a drawing temperature of 90°C under an applied tension of 4.9 MPa, and the zone‐drawn fiber was subsequently zone annealed at 150°C under 50.9 MPa. The zone‐annealed fiber had a degree of crystallinity of 48%, a birefringence of 218.9 × 10^?3, tensile modulus of 18.8 GPa, and tensile strength of 0.88 GPa. The microfiber prepared by laser heating the zone‐annealed fiber had a diameter of 1.5 μm, birefringence of 172.8 × 10^?3, tensile modulus of 17.6 GPa, and tensile strength of 1.01 GPa. The draw ratio estimated from the diameter was 9165 times; such a high draw ratio has thus far not been achievable by any conventional drawing method. Microfibers may be made more easily by laser heating than by conventional technologies such as conjugate spinning. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1955–1958, 2003     

Superdrawn crystalline polymers: A new class of high-strength fiber

A new kind of acetal fiber has been discovered which has a tensile strength of 1.7 GPa (250,000 psi) and an elastic modulus of 35 GPa (5 × 10⁶ psi). This fiber is produced by a special two-stage drawing process in the solid state which requires careful control of deformation rate and temperature. Previously known drawn fibers are reported to consist of folded-chain blocks joined by a limited number of tie-molecules. It is hypothesized that the second stage of the novel drawing process eliminates the lamella (block) surfaces which act as strength-limiting stress concentrators. A new type of fiber is created in which any remaining chain-folds are distributed as defects in a continuous crystal matrix. It is the continuity of the crystal matrix which is believed responsible for the remarkable properties of the fiber.     

High modulus semi-crystalline polymers by solid state rolling

Solid state rolling of semi-crystalline polymers is shown to be an effective method of producing high strength, high modulus tape at acceptable production rates. High density polyethylene tape was produced having a tensile strength exceeding 300 MPa and a tensile modulus of 8.7 GPa at production rates exceeding 8 m/min. A significant factor in producing highly oriented tape by the rolling process is roll temperature. Increasing the roll temperature from 25°C to 125°C not only increases the maximum extent of orientation achievable, but increases the mechanical properties at a given degree of thickness reduction. Internal frictional heat development limited the maximum thickness reduction ratio of polypropylene to 6.6:1. This reduction was reached by rolling at 150°C. The resultant tape had a tensile modulus of 5.1 GPa and a tensile strength of 300 MPa.     

Viscoelastic properties and structure of the zone‐drawn polyaniline films

The zone‐drawing method was applied to chemically synthesized polyaniline cast films of emeraldine base under various applied tensions and drawing temperatures. The changes in the microstructure and viscoelastic properties of the resulting films were investigated. It was found that the microstructure was strongly affected by the drawing temperature (T_d). The crystallinity, crystallite size, and orientation factor of crystallites, respectively, attained 42%, 23 Å, and 0.975 for the film zone‐drawn at T_d = 170°C, whereas a further increase in the T_d brought about a decrease of these values. The viscoelastic measurements indicated that the dynamic storage modulus attained 12 GPa at room temperature and was 5 GPa at 280°C for the film zone‐drawn at T_d = 210°C, which was comparable to that of the typical engineering plastics. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 566–571, 2000     

Molecular orientation in drawn smectic and crystalline isotactic polypropylene

The drawing behavior of two polypropylene films of different structures was analyzed. The two films differ as a consequence of different quenching conditions. At low temperature, a biphasic smectic-amorphous system was obtained, while quenching at 100°C produced a biphasic crystalline-amorphous system. The drawing of samples was carried out at 110°C at which temperature the smectic phase is not stable and is transformed into the crystalline α-form. The initial structure affects the drawing behavior and the properties of the drawn samples. The mechanical, optical, and X-ray analyses clearly show that high molecular orientation is achieved at lower deformations in the initially smectic sample. In particular, the amorphous phase is highly oriented, inducing higher axial elastic modulus.     

Effect of hot drawing on properties of wet‐spun poly(L,D‐lactide) copolymer multifilament fibers

Polylactide stereocopolymer multifilament fibers were prepared by wet spinning and subsequent hot drawing. The stereocopolymers were poly‐(L,D ‐lactide) [P(L,D )LA], L/D ratio 96/4, and poly‐(L,DL ‐lactide) [P(L,DL )LA], L/DL ratio 70/30. They were dissolved in dichloromethane and coagulated in a spin bath containing ethanol. The hot‐drawing temperature was 65°C. The draw ratios (DR) were upto 4.5 to the P(L,D )LA 96/4 filaments and upto 3 to the P(L,DL )LA 70/30 filaments. Wet spinning decreased crystallinities of both copolymers. Hot drawing increased the crystallinity of the P(L,D )LA 96/4 filament but not to the level of the original copolymer, whereas the as‐spun and the hot‐drawn P(L,DL )LA 70/30 filaments were amorphous. The filament diameter, tenacity, Young's modulus, and elongation at break were dependent on the DR. The maximum tenacity (285 MPa) and Young's modulus (2.0 GPa) were achieved with the P(L,D )LA 96/4 filament at the DR of 4.5. Respectively, the maximum tenacity of the hot‐drawn P(L,DL )LA 70/30 filament was 175 MPa and Young's modulus 1.3 GPa at the DR of 3. Hot drawing slowed down in vitro degradation rate of both stereocopolymer filaments. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The influence of processing parameters on the structural and mechanical properties of drawn polypropylene fibres: A factorial design approach

This article reports a study of multi‐stage polypropylene fiber drawing (stretching) as a continuous, but independent stage of the overall fiber‐forming process. The fibers were drawn according to a factorial experimental design, once appropriate spinning conditions had been devised. The structures of the drawn fibers were studied using wide‐angle X‐ray diffraction and birefringence measurements. In addition, the fibers were characterized with respect to filament tenacity, elongation to break, specific secant modulus, and extent of shrinkage at 130°C. All these properties were quantitatively assessed as responses to nine specially selected process control parameters in the drawing equipment. For every property analyzed, the temperatures of the hot plates in the draw frame were found to exert no significant influence, whereas the temperatures of the initial rollers were in most cases significant. Furthermore, the speed of the final roller also played an influential role, and a number of interactions between process parameters were identified as significant. Explanations are advanced for the parts played by significant process parameters on the properties of the drawn fibers. The article also demonstrates the advantages of factorial experimental design in determining correct settings for process parameters to give drawn fibers with the properties desired. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012