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.     

Drawing in high pressure CO2—a new route to high performance fibers in memory of late Roger Porter

In this paper, we introduce a new draw technique for polymer orientation and apply it to different polymer fibers: poly(ethylene terephthalate) or PET, nylon 6,6, and ultra‐high molecular polyethylene (UHMWPE). In this technique, a polymer is drawn uniaxially in supercritical CO₂ using a custom high‐pressure apparatus. This technique can be used in replacement of a traditional drawing process or as a post‐treatment process. With PET, the technique is not effective at temperatures at or below 130°. In contrast, the process is highly effective for nylon 6,6 where CO₂ drawn fibers show significantly higher crystallinity and orientation along with improved mechanical properties. While the fibers are plasticized, the drawability of the fibers is only slightly dependent on temperature. High pressure CO₂ drawing of ultrahigh molecular weight polyethylene (UHMWPE) fibers is equally effective. Commercial high performance fibers can be drawn up to a ratio of 1.9 in asecond stage, resulting in large increases in tensile modulus and small improvements in tensile strength.     

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.     

高密度聚乙烯熔纺纤维的拉伸工艺

相对分子质量为１４万的高密度聚乙烯（ＨＤＰＥ）通过螺杆挤出机熔纺成形,然后经过高温超拉伸可制成强度为９．７ｃＮ／ｄｔｅｘ的中强聚乙烯纤维。讨论了拉伸温度、拉伸比对纤维力学性能的影响,并通过声速取向、热分析（ＤＳＣ）、广角Ｘ射线衍射（ＷＡＸＤ）和纤维力学性能的测试研究了拉伸过程中聚乙烯纤维结构与性能的变化。结果表明：拉伸温度对纤维稳定拉伸影响较大,最佳温度为１０５～１１５℃;且初生纤维的纤度越小,高倍拉伸纤维强度越大,随拉伸倍数的提高,纤维的强度和声速取向都显著提高并改善了纤维的热性能和力学性能,但断裂伸长却呈下降趋势。     

Densifying and strengthening of electrospun polyacrylonitrile‐based nanofibers by uniaxial two‐step stretching

The effects of alignment of polyacrylonitrile (PAN) nanofibers and a two‐step drawing process on the mechanical properties of the fibers were evaluated in the current study. The alignment was achieved using a high‐speed collector in electrospinning synthesis of the nanofibers. Under optimal two‐step drawing conditions (e.g., hot‐water and hot‐air stretching), the PAN nanofiber felts exhibited large improvements in both alignment and molecular chain‐orientation. Large increase in crystallinity, crystallite size, and molecular chain orientation were observed with increasing draw ratio. Optimally, stretched PAN‐based nanofibers exhibited 5.3 times higher tensile strength and 6.7 times higher tensile modulus than those of the pristine one. In addition, bulk density of the drawn PAN nanofibers increased from 0.19 to 0.33 g/cm³. Our results show that fully extended and oriented polymer chains are critical in achieving the highest mechanical properties of the electrospun PAN nanofibers. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43945.     

High modulus polypropylene fibers. II. Influence of fiber preparation upon structure and morphology

The influence of drawing on the limiting draw ratio upon formation of the morphological structure of fibers spun from binary polypropylene (PP) blends was studied. Fibers were spun from a fiber‐grade CR‐polymer and from the blends of a fiber‐grade CR‐polymer with a molding‐grade polymer in the composition range of 10–50 wt % added. As‐spun fibers were immediately moderately and additionally highly drawn at the temperature of 145°C. The structure and morphology of these fibers were investigated by small‐angle X‐ray scattering, wide‐angle X‐ray scattering, differential scanning calorimetry, scanning electron microscopy, density, birefringence, and sound velocity measurements. It was shown that continuously moderately drawn fibers are suitable precursors for the production of high tenacity PP fibers of very high modulus, because of so called oriented “smectic” structure present in these fibers. With drawing at elevated temperature, the initial metastable structure of low crystallinity was disrupted and a c‐axis orientation of monoclinic crystalline modification was developed. Hot drawing increased the size of crystallites and crystallinity degree, the orientation of crystalline domains, and average orientation of the macromolecular chains and resulted in extensive fibrillation and void formation. It was found that the blend composition has some influence on the structure of discontinuously highly drawn fibers. With increasing the content of the molding‐grade polymer in the blend, the size of crystalline and amorphous domains, density and crystallinity, as well as amorphous orientation decreased. Relationship has been established between the mechanical properties, crystallinity, and orientation of PP fibers. It was confirmed that by blending the fiber‐grade CR‐polymer by a small percentage of the molding‐grade polymer, maximization of elastic modulus is achieved, mainly because of higher orientation of amorphous domains. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1067–1082, 2006     

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     

Mechanical,thermal, and structural properties of uniaxially drawn polylactic acid/halloysite nanocomposites

The effect of solid-state drawing at different conditions including drawing ratio (DR), drawing temperature (DT), and drawing speed (DS) on mechanical, thermal, and structural properties of polylactic acid (PLA)/halloysite nanotubes (HNTs) composites were studied. PLA/HNTs composite films were prepared by melt mixing process followed by compression molding. Subsequently, drawing was performed using a tensile testing machine. Field emission scanning electron microscopy confirmed alignment and orientation of polymer chains and HNTs after stretching. Thermal and mechanical analysis of the drawn films revealed that glass transition temperature, crystallinity, ultimate tensile strength, and Young's modulus were enhanced by increasing DR, DT, and DS. However, toughness was decreased by increasing DR and DS and increased by increasing DT. In addition, the drawn nanocomposites showed superior mechanical and thermal properties compared to the drawn neat PLA films indicating the high efficiency of solid-state drawing and positive effect of HNTs. Therefore, this study could be helpful to introduce an approach to enhance the properties of biopolymers and renewable polymers by uniaxial drawing.     

Investigations of the preparation technology for polyglycolic acid fiber with perfect mechanical performance

Polyglycolic acid (PGA) fibers were prepared by melt‐spinning process in this report. The effects of spinning parameters, such as windup rates and drawn ratio, on the mechanical properties of the fibers were discussed by analyzing the internal stress of as‐spun fibers, axial sound velocity, fiber tenacity, etc. The results showed that windup rate had a slight effect on the macromolecular orientation degree of the as‐spun fibers, which was quite unusual for melt spinning, whereas, the subsequent drawing process effectively increased the macromolecular orientation degree of the PGA fibers and consequently increased the tensile strength of the fibers. Low internal stress of as‐spun fibers obtained at lower windup rate led to higher drawing ratio, and the drawn fibers possessed relatively excellent mechanical properties. As a contrast, higher windup rate resulted in the strong internal stress of the as‐spun fibers, which had a negative influence on the drawing process, and so the tensile strength of the drawn fibers was relatively poor. Therefore, PGA fiber with perfect mechanical performance could be prepared at the technical parameters of lower windup rate and higher drawing multiples as well as slow drawing rate. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Alkali treatment of jute fibers: Relationship between structure and mechanical properties

The mechanical properties of tossa jute fibers were improved by using NaOH treatment process to improve the mechanical properties of composites materials. Shrinkage of fibers during this process has significant effects to the fiber structure, as well as to the mechanical fiber properties, such as tensile strength and modulus. Isometric NaOH‐treated jute yarns (20 min at 20°C in 25% NaOH solution) lead to an increase in yarn tensile strength and modulus of ∼ 120% and 150%, respectively. These changes in mechanical properties are affected by modifying the fiber structure, basically via the crystallinity ratio, degree of polymerization, and orientation (Hermans factor). Structure–property relationships, developed for cellulosic man‐made fibers, were used with a high correlation factor to describe the behavior of the jute fiber yarns. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 623–629, 1999     

Transport and mechanical properties of iPP–sPP fibers

The drawing behavior of a blend of syndiotactic and isotactic polypropylene (iPP–sPP 50:50 w/w) was investigated at different temperatures and compared to that of pure polymers. The film of pure sPP showed that the presence of iPP allowed the blend to reach a much higher draw ratio. Fibers were obtained by drawing the blend at 110°C. The axial elastic modulus of the fibers was measured as a function of draw ratio up the highest λ = 10. The sorption and diffusion of dichloromethane vapors in the undrawn and drawn samples were studied in order to provide information about the structural organization of the amorphous phase. The elastic modulus of the fibers displayed a more‐than‐linear increase with the draw ratio, suggesting a good interconnection of the amorphous phases. The orientation of the chains with increasing λ determined a decrease of entropy and fractional free volume (FFV) and a tighter packing of the chains along the drawing direction, explaining the strong increase of the elastic modulus. The transport properties, which confirmed the mechanical properties, showed a stiffening of the amorphous phase after λ = 6, evidenced by a dual‐type sorption isotherm for the fibers and a sharp drop in the zero‐concentration diffusion coefficient. As a consequence, the permeability of the fibers was much lower than that of the unoriented sample. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 539–545, 2001     

Transparent,Lightweight, and High Strength Polyethylene Films by a Scalable Continuous Extrusion and Solid‐State Drawing Process

The continuous production of transparent high strength ultra‐drawn high‐density polyethylene films or tapes is explored using a cast film extrusion and solid‐state drawing line. Two methodologies are explored to achieve such high strength transparent polyethylene films; i) the use of suitable additives like 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐ditertpentylphenol (BZT) and ii) solid‐state drawing at an optimal temperature of 105 °C (without additives). Both methodologies result in highly oriented films of high transparency (≈91%) in the far field. Maximum attainable modulus (≈33 GPa) and tensile strength (≈900 MPa) of both types of solid‐state drawn films are similar and are an order of magnitude higher than traditional transparent plastics such as polycarbonate (PC) and poly(methyl methacrylate). Special emphasis is devoted to the effect of draw down and pre‐orientation in the as‐extruded films prior to solid‐state drawing. It is shown that pre‐orientation is beneficial in improving mechanical properties of the films at equal draw ratios. However, pre‐orientation lowers the maximum attainable draw ratio and as such the ultimate modulus and tensile strength of the films. Potential applications of these high strength transparent flexible films lie in composite laminates, automotive or aircraft glazing, high impact windows, safety glass, and displays.     

The influence of drawing,twisting, heat setting,and untwisting on the structure and mechanical properties of melt-spun high-density polyethylene fiber

The variation of crystalline morphology and mechanical properties of polyethylene fibers was studied as they were sequentially melt spun, drawn, twisted, heat set, and untwisted. Twisting of as-melt spun fibers was also investigated. The morphology was characterized using wideangle x-ray diffraction, small-angle x-ray diffraction, and scanning electron microscopy techniques. Drawing results in high crystalline orientation, fibrillation, and large increases in modulus and tensile strength. Effects due to variation of spinning conditions were noted. Twisting either as-spun or drawn fibers decreased the axial orientation, modulus, tensile strength, and usually also the elongation to break. The changes in these properties increased with twist angle. Twisting also caused transformation of a small fraction of the sample to the monoclinic form of polyethylene. Heat setting caused healing of voids generated during drawing and increased the perfection and periodicity of the stacking of lamellar crystals along the fiber axis. Heat setting also caused the monoclinic polyethylene to transform back to the orthorhombic form, and it increased the modulus and tensile strength. Untwisting returned the orientation in the fiber to essentially that which it would have if it had not been twisted, but untwisting also resulted in the formation of kink bands.     

Enhancing mechanical properties of gel‐spun polyvinyl alcohol fibers by iodine doping

High strength polyvinyl alcohol (PVA) fibers with a conventional degree of polymerization of 1500 were prepared by doping iodine with PVA spinning solution. The iodine‐doped PVA (I‐PVA) aqueous solution was extruded into cold methanol that provides dark purple PVA‐iodine complex gel fibers. Only a small amount of iodine was required to enhance drawability and molecular orientation by reducing the interaction between PVA chains. An increase of ca. 10% in the maximum draw ratio of the doped fibers compared with that of undoped PVA translated into values for the tensile strength, 2.2 Giga‐Pascal (GPa), and initial modulus (47 GPa) that were more than 30% higher than those of the neat PVA fiber. Easier chain slippage of molecules in the amorphous segments of the I‐PVA fiber during drawing leads to increased orientation in these segments, which is believed to be the source of the improvements in mechanical properties. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Investigation of a postprocessing method to tailor the mechanical properties of carbon nanotube/polyamide fibers

The incorporation of carbon nanotubes to thermoplastic fibers can potentially improve mechanical, thermal and electrical properties. In this article, a methodology to tailor the mechanical properties of carbon nanotube/nylon fibers is presented. Multiwalled nanotubes (MWNT) were combined to polyamide 12 through melt compounding and twin‐screw extrusion. Pellets containing between 0 and 5.0 wt % MWNT were extruded and subsequently melt spun with a capillary rheometer to produce filaments. To further promote the alignment of the polymer chains and MWNTs, postdrawing parameters were systematically investigated: temperature, drawing speed and elongation. The best improvements in terms of elastic modulus and yield strength were measured at 140°C and 500% elongation, whereas drawing speed was shown to have a negligible effect. It was confirmed through electron microscopy and X‐ray diffraction that these enhancements were mainly induced by the alignment of the polymer chains along the fibers' axis. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4375–4382, 2013     

拉伸对聚芳砜酰胺(PSA)纤维力学性能的影响

研究了聚芳砜酰胺(PSA)纤维制备过程中各阶段拉伸倍数对纤维最终力学性能的影响,并制备出最高断裂强度达到3.62 cN/dtex的PSA纤维。要得到综合力学性能较高的PSA纤维,合适的表观喷头拉伸倍数为-50%～30%、塑化拉伸倍数为2、热拉伸倍数为2。PSA为较难结晶的高聚物,不论是聚合物粉末还是拉伸纤维,其结晶度都较低。PSA纤维的大分子取向与纤维的断裂强度关系十分密切,大分子取向因子与纤维的断裂强度呈线性关系,提高大分子取向程度是提高PSA纤维强度的重要途径。     

Graphene oxide reinforced poly(vinyl alcohol) composite fibers via template-oriented crystallization

Here, a high breaking strength and high initial modulus fibers comprised of polyvinyl alcohol (PVA) and graphene oxide (GO) were fabricated via simple method of solution blending and wet-spinning. The structure and properties of these fibers were studied in details using two-dimensional X-ray diffractions, differential scanning calorimetry, one-dimensional X-ray diffractions, scanning electron microscopy, transmission electron microscopy, dynamic mechanical analysis and tensile test. Compared with pure PVA fiber, a 43 % improvement of breaking strength and an 81 % improvement of initial modulus were achieved by addition of 0.1 wt% of GO, and the results indicated that crystallization and orientation of GO/PVA composite fibers were both increased. GO could not only promote PVA chains ordered arrangement for increasing crystallization, but also act as a template for polymer amorphous orientation via the interactions between PVA and GO in the process of hot drawing and heat setting, which were responsible for the significant improvement in the mechanical properties of GO/PVA composite fibers.

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Graphical abstract GO could not only promote PVA chains ordered arrangement for increasing crystallization, but also act as a template for PVA amorphous orientation in the process of hot drawing. The amorphous orientation degree and the crystallization degree of PVA fibers were increased by adding GO.

     

Gel-spinning and drawing of gelatin

Gelatin fibers can be prepared by the gel-spinning method using dimethyl sulfoxide as a solvent. The use of the method and the drawing in a gel state were effective in inducing segmental orientation in gelatin fiber. The fibers showed high values for the mechanical properties of tensile strength of 180 MPa and Young's modulus of 3.4 GPa.     

Wetting of oriented and etched ultrahigh molecular weight polyethylene

Highly oriented gel‐spun ultrahigh molecular weight polyethylene (UHMWPE) fibers possess many outstanding properties desirable for composite materials but their adhesion to such matrices as epoxy is poor. This article describes the combined effects of drawing and surface modification on the bulk and surface properties of gel‐cast UHMWPE films emphasizing the effects of etching on both undrawn and drawn films. Drawing the films yields a fibrillar structural hierarchy similar to UHMWPE fibers and a significant increase in orientation, melting point, modulus, and strength. The effects of drawing on bulk properties were more significant than those of etching. The poor adhesion of epoxy to the smooth, fibrillar, and relatively nonpolar drawn film surface improves significantly with oxidization and roughening on etching. The interlaminar shear failure occurred cohesively in the UHMWPE, and thus the interlaminar shear failure strength was greater for the drawn UHMWPE with its greater tensile strength. Nitrogen plasma etching yielded the best results, both removing any low molecular weight surface layer and etching the UHMWPE beneath. Oxygen plasma etching enhanced wetting but was too harsh, causing extensive surface degradation and a significant reduction in mechanical properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 405–418, 1999     

Drawing of nylon-6 by the novel incremental drawing process

Multifilament nylon-6 fibers are drawn by the novel incremental drawing process as well as by the conventional drawing process. In this process the fibers are stretched in 36 stages along the surface of two corotating cones fitted on the incremental drawing machine. Fibers are obtained from each stage, and from their diameter measurements it is shown that they are stretched in a predicted manner. Mechanical properties, as measured by Instron and by a sonic modulus tester, show higher tenacity and modulus values for the incremental process than for the conventional one at equivalent draw ratios. Structural properties are analyzed by density measurements, wide angle X-ray diffraction and birefringence. These showed higher crystallinity and higher crystalline as well as amorphous orientation factors for the incremental process at equivalent draw ratios. The mechanical property results are explained on the basis of structural development during drawing. It has been shown that the incremental drawing process is a suitable technique for obtaining superior properties in fibers and has commercial potential.