Morphological and mechanical properties of drawn poly(l‐lactide) electrospun twisted yarns

In this study, the continuous twisted PLLA yarns were produced using an electrospinning device consists of two oppositely charged nozzles. The electrospinning process was performed at different twist rates. The electrospun twisted yarns were drawn at different extension ratios of 50% and 100% and their morphological and mechanical properties of post‐drawn yarns were investigated. The morphological studies at all twist rates shown that uniform and smooth fibers without any bead were formed. Increasing the twist rate up to 240 rpm resulted to a decrease in the average diameter of the fibers in the yarn structure. After uniaxially drawing of the yarns, the average diameter of fibers and thus the yarn diameter decreased. The post‐drawing process enhanced the crystallinity of the fibers in the yarn structure. Furthermore, by increasing the extension ratio, the tensile strength and modulus of yarns increased, while the elongation at break (%) decreased. POLYM. ENG. SCI., 58:1091–1096, 2018. © 2017 Society of Plastics Engineers     

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     

Effect of single yarn twist and ply to single yarn twist ratio on strength and elongation of ply yarns

The amount of ply twist required to bring the surface fibers of the strand parallel to ply yarn axis is half the single yarn twist and, is experimentally verified by viewing the multifilament yarns longitudinally under Scanning Electron Microscope. The effect of single yarn twist and ply to single yarn twist ratio on strength and elongation of two‐ply cotton yarn have been studied. As the single yarn twist increases the tensile strength of the ply yarns with different levels of ply to single yarn twist ratio increases and at 130–140% of normal single yarn twist level, the ply yarns attain almost the same strength. Rate of improvement in tensile strength of cotton two‐ply yarn with respect to single yarn twist is more than that with respect to ply twist. The effect of ply to single yarn and cable to ply yarn twist ratio on strength and elongation of ply and cable multifilament yarns have been studied. Tensile strength of ply and cable multifilament yarns do not vary with the change in ratio of ply to single yarn twist and cable to ply twist respectively, particularly when the resultant yarn is finer. The cosine of average filament inclination to the ply yarn axis and that to the cable yarn axis do not vary much with different levels of ply to single yarn twist ratio and cable to ply yarn twist ratio respectively. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2245–2252, 2005     

Mechanical properties of polypropylene fibers produced from the binary polymer blends of different molecular weights

The mechanical properties of multifilament yarns, spun from the blends of a plastic‐grade polymer with a fiber‐grade CR‐polymer in the composition range of 10–50 wt % added, were investigated. The predicted modulus of a two‐phase blend, calculated from several representative equations, was compared with the elastic modulus of drawn yarns, determined from the stress vs. strain curve and dynamic modulus obtained from the sound velocity measurements. The best fit was achived with the Kleiner's simplex equation. For both the static and dynamic elastic modulus, the largest negative deviation is seen at the 80/20 and 60/40 plastic/fiber‐grade polymer blend composition, while the largest positive deviation is seen at the 90/10 plastic/fiber‐grade polymer blend composition, suggesting good compatibility of both polymers, when only a small percent of the fiber‐grade CR‐polymer is added. Improved spinnability and drawability of blended samples led to the yarns with the tensile strength over 8 cN/dtex, elastic modulus over 11 GPa and dynamic modulus over 15.5 GPa. Structural investigations have shown that the improved mechanical behavior of blended samples, compared to the yarn spun from the pure plasic‐grade polymer, is the consequence of a higher degree of crystallinity, and above all, of a much higher orientation of macromolecules. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1211–1220, 2000     

Fabrication of continuous nanofiber yarn using novel multi‐nozzle bubble electrospinning

A novel multi‐nozzle bubble electrospinning apparatus, including spinning unit, metering pump, constant flow pump, metal funnel and yarn winder, was designed for the preparation of continuous twisted polyacrylonitrile nanofiber yarns, and the principle of nanofiber yarn spinning was studied. An innovative spinning unit consisting of nozzle and air chamber was used to improve the production of nanofibers. Double conjugate electrospinning was developed using two pairs of oppositely charged spinning units to neutralize the charges. The effects of applied voltage, air flow rate, overall solution flow rate and funnel rotary speed on the fiber diameter, production rate and mechanical properties of the nanofiber yarns were analyzed. Nanofibers could be aggregated stably and bundled continuously, then twisted into nanofiber yarns uniformly at an applied voltage of 34 kV, air flow rate of 1200 mL min^?1 and overall solution flow rate of 32 mL h^?1. With an increase in the funnel rotary speed, the twist angle of the nanofiber yarns gradually increased when the take‐up speed was constant. The yarn tensile strength and elongation at break showed an increasing trend with increasing twist angle. Nanofiber yarns obtained using this novel method could be produced at a rate from 2.189 to 3.227 g h^?1 with yarn diameters ranging from 200 to 386 µm. Nanofiber yarns with a twist angle of 49.7° showed a tensile strength of 0.592 cN dtex^?1 and an elongation at break of 65.7%. © 2013 Society of Chemical Industry     

Structure changes during tensile deformation and mechanical properties of a twisted carbon nanotube yarn

The small-angle X-ray scattering measurements during tensile deformation have been performed for studying the structure and mechanical property relationships of twisted carbon nanotube (CNT) yarns. The tensile strength distribution and the diameter changes during tensile deformation have also been measured. The orientation distribution of the CNTs in the yarn has been determined and its changes during tensile deformation have been related to the variation of the tensile modulus with the twist angle. The tensile modulus and Poisson’s ratio of the yarns decreased with increasing twist angle, whereas the tensile strength of the yarn showed a maximum at the twist angle of 25°. At this twist angle, the distribution width of the tensile strength was minimum indicating the higher uniformity of the yarn structure.     

Shape memory polyurethane-based electrospun yarns for thermo-responsive actuation

Electrospun nanofibrous yarns of shape memory polyurethane (SMPU)-based nanofibers were successfully prepared. The electrospun yarns were analyzed to assess the dependence of mechanical and shape memory properties on the yarn twist angle. The yarn with a 60° twist angle has high compactness and density, leading to increased tensile strength, elastic modulus, and strain energy. In addition, this yarn shows a significant improvement in the shape memory recovery stress compared with the non-twisted SMPU nanofibers. Moreover, thermal stimuli allowed for the 60° twisted yarn to lift a load that is 103 times heavier than itself. This yarn had a shape recovery stress of 0.61 MPa and generated a 7.95 mJ recovery energy. The results suggest the electrospun yarns could be used as actuators and sensing devices in the medical and biological fields.     

Performance enhancement of nylon/kevlar fiber composites through viscoelastically generated pre‐stress

Kevlar‐29 fibers have high strength and stiffness but nylon 6,6 fibers have greater ductility. Thus by commingling these fibers prior to molding in a resin, the resulting hybrid composite may be mechanically superior to the corresponding single fiber‐type composites. The contribution made by viscoelastically generated pre‐stress, via the commingled nylon fibers, should add further performance enhancement. This paper reports on an initial study into the Charpy impact toughness and flexural stiffness of hybrid (commingled) nylon/Kevlar fiber viscoelastically pre‐stressed composites at low fiber volume fractions. The main findings show that (i) hybrid composites (with no pre‐stress) absorb more impact energy than Kevlar fiber‐only composites; (ii) pre‐stress further increases impact energy absorption in the hybrid case by up to 33%; (iii) pre‐stress increases flexural modulus by ∼40% in the hybrid composites. These findings are discussed in relation to practical composite applications. POLYM. COMPOS., 35:931–938, 2014. © 2013 Society of Plastics Engineers     

The influence of process parameters on the properties of electrospun PLLA yarns studied by the response surface methodology

Poly (l ‐lactide) (PLLA) fibrous yarns were prepared by electrospinning of polymer solutions in 2,2,2‐trifluoroethanol. Applying spinning from two oppositely charged needles the spontaneous formed triangle of fibers at a grounded substrate could be assembled into fibrous yarns using a device consisting of a take‐up roller and twister. The effect of processing parameters on the morphology, diameter and mechanical properties of PLLA yarns was investigated by the response surface methodology (RSM). This method allowed evaluating a quantitative relationship between polymer concentration, voltage, take‐up rate and distance between the needles' center and the take‐up unit on the properties of the electrospun fibers and yarns. It was found that at increasing concentrations up to 9 wt % uniform fibers were obtained with increasing mean diameters. Conversely, the fiber diameter decreased slightly when the applied voltage was increased. The take‐up rate had a significant influence on the yarn diameter, which increased as the take‐up rate decreased. The tensile strength and modulus of the yarns were correlated with these variables and it was found that the polymer concentration had the largest influence on the mechanical properties of the yarns. By applying the RSM, it was possible to obtain a relationship between processing parameters which are important in the fabrication of electrospun yarns. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41388.     

Achievement of high surface charge in poly(vinylidene fluoride) fiber yarns through dipole orientation during fabrication

Electrospun fiber materials are of scientific interest for use in multiple application areas. Charged fiber structures show enhanced properties as desired for some of these applications. One factor influencing the charge on the fiber structure that has not been explored is fiber alignment. Electrospun fiber structures, such as membranes, typically consist of randomly oriented fibers. Structural properties of the membranes such as mechanical strength are known to be affected by the random orientation of the fibers. It is suspected that fiber orientation may also affect the charge capacity of charged fiber structures. A few approaches to form electrospun yarns have been reported. Some of these approaches can also cause fibers to preferentially align along the yarn axis instead of assembling into a random structure. In this work, a rotating metal cone was used to collect Poly(vinylidene fluoride) electrospun fibers from which stretched yarns were drawn and twisted into yarns. The alignment of the fibers in the yarns was controllable to a degree that allowed exploration of the effect of alignment on charge. Long continuous oriented or random yarns of relatively uniform thickness were produced at a rate of about 10 m/h. The yarns were polarized by methods of heating, stretching, and poling. The results show that the fiber yarn formation process endows more charges to the fibers compared to the normal fiber membrane electrospinning and post polarization. This provides a facile route for the preparation of enhanced charge-functionalized fiber structures for a wide range of applications.     

Tensile fatigue behavior of polyamide 66 nanofiber yarns

Nanofiber yarns with twisted and continuous structures have potential applications in fabrication of complicated structures such as surgical suture yarns, artificial blood vessels, and tissue scaffolds. The objective of this article is to characterize the tensile fatigue behavior of continuous Polyamide 66 (PA66) nanofiber yarns produced by electrospinning with three different twist levels. Morphology and tensile properties of yarns were obtained under static tensile loading and after fatigue loading. Results showed that tensile properties and yarn diameter were dependent on the twist level. Yarns had nonlinear time‐independent stress–strain behavior under the monotonic loading rates between 10 and 50 mm/min. Applying cyclic loading also positively affected the tensile properties of nanofiber yarns and changed their stress–strain behavior. Fatigue loading increased the crystallinity and alignment of nanofibers within the yarn structure, which could be interpreted as improved tensile strength and elastic modulus. POLYM. ENG. SCI., 55:1805–1811, 2015. © 2014 Society of Plastics Engineers     

Direct drawing of gel fibers enabled by twist‐gel spinning process

A new twist‐gel spinning process for ultrahigh molecular weight polyethylene fibers is demonstrated which significantly increases the extraction rate of nonvolatile spin solvent while simultaneously reducing the consumption of extraction solvent by more than 75%. Applying twist to the gel fiber enables it to be directly hot‐drawn, allowing conventional solvent extraction to proceed significantly faster. While solvent extraction effectiveness is largely enhanced, the new process does not show reduced fiber properties. The tensile strength, Young's modulus, surface morphology, and geometry are relatively unaffected when compared to fibers produced using the conventional gel‐spinning process. The new twist‐gel spinning process is expected to improve the processing efficiency of gel‐spun high‐strength fibers, promoting broad expansion of these high performance fibers into applications that were previously prohibitive due to extremely slow production. POLYM. ENG. SCI., 55:1389–1395, 2015. © 2015 Society of Plastics Engineers     

Conjugate electrospinning of continuous nanofiber yarn of poly(L‐lactide)/nanotricalcium phosphate nanocomposite

The continuous nanofiber yarns of poly(L ‐lactide) (PLLA)/nano‐β‐tricalcium phosphate (n‐TCP) composite are prepared from oppositely charged electrospun nanofibers by conjugate electrospinning with coupled spinnerets. The morphology and mechanical properties of PLLA/n‐TCP nanofiber yarns are characterized by scanning electron microscope, transmission electron microscope, and electronic fiber strength tester. The results show that PLLA/n‐TCP nanofibers are aligned well along the longitudinal axis of the yarn, and the concentration of PLLA plays a significant role on the diameter of the nanofibers. The thicker yarn of PLLA/n‐TCP composite with the weight ratio of 10/1 has been produced by multiple conjugate electrospinning using three pairs of spinnerets, and the yarn has tensile strength of 0.31cN/dtex. A preliminary study of cell biocompatibility suggests that PLLA/n‐TCP nanofiber yarns may be useable scaffold materials. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Dynamic mechanical behavior of LLDPE composites reinforced with kevlar fibers/short glass fibers

Kevlar fibers (DuPont) and glass fibers have been used to reinforce linear lowdensity polyethylene (LLDPE) by using an elastic melt extruder and the compression molding technique. The impact behavior of hybrid composites of different compositions is compared and has been explained on the basis of volume fraction of fibers. The addition of glass fibers decreases the Izod impact strength of LLDPE. The Izod impact strength of the composite increses when glass fibers are replaced by Kevlar fibers. Dynamic mechanical α‐relaxation is studied and the effect of variation of fiber composition on the relaxation is reported in the temperature range from −50°C to 150°C at 1 Hz frequency. The α‐relaxation shifts towards the higher temperature side on addition of fibers in LLDPE. The addition of fibers increases the storage modulus, E′, of LLDPE. The hybridization of Kevlar and glass fibers helps in desiging composites with a desirable combination of impact strength and modulus. At the low temperature region, E′ increases significantly with glass fibers as compared to that noted with the addition of Kevlar fibers. The α‐transition temperature of composites increases significantly with Kevlar fibers as compared to that observed with addition of glass fibers.     

Electrical conductivity and electric modulus of stable Kevlar® fiber loaded HAF/NBR rubber composite

DC, AC conductivities and dielectric properties of high abrasion furnace carbon black (HAF)/acrylonitrile butadiene rubber (NBR) composite have been studied with varying the aramide Stable Kevlar^® fiber content, temperature, and frequency. Generally, the electrical conductivity was decreased with increasing Stable Kevlar^® fiber content, which was confirmed by the positron annihilation lifetime spectroscopy. Negative temperature coefficient of conductivity (NTCC) behavior between 353 and 413 K was detected, except for the composite containing 10 phr Kevlar which showed positive temperature coefficient of conductivity (PTCC) behavior above 383 K. These NTCC and PTCC behaviors were further manifested by differential scanning calorimetry (DSC). For the composite with 10 phr Kevlar, the interfacial polarization between the fibers and the polymeric composite can be ascribed to Maxwell‐Wagner‐Sillars mechanism. The (MWS) relaxation disappeared for higher fibers content. The analysis of the electric modulus in the frequency range from 1 kHz to 1 MHz shows that the interfacial relaxation obeys Cole–Davison distribution of relaxation times. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Dynamic viscoelasticity of hybrid kevlar and glass fiber reinforced LLDPE in the molten state

Kevlar and glass fibers were used to reinforce linear low density polyethylene (LLDPE), and composite sheets of 0.8, 1.5 and 2.5 mm thicknesses were obtained by using a compression molding technique. Dynamic viscoelastic properties of non‐hybrid and hybrid composites of various compositions at 200°C are evaluated. Storage modulus (G′) and loss modulus (G″) increase with angular frequency (ω) and reinforcement. Replacement of glass fiber by Kevlar at constant loading of fibers in LLDPE increases the value of G′, G″ and η′. The fractured surface of composite shows the gradient orientation of fibers particularly in 2.5 mm thick sheet. Top and bottom layers show relatively two‐dimensional orientation as compared to the middle layer, which shows random orientation. The orientation of fibers decreases G′ and η′ of Kevlar fiber and hybrid fiber hybrid fiber reinforced LLDPE composites. The effect of change in distance between parallel plate of rheometer (change in strain amplitude) on dynamic rheological properties is studied and reported here.     

Improved Tensile Strength and Ferroelectric Phase Content of Self‐Assembled Polyvinylidene Fluoride Fiber Yarns

An electrospinning method to obtain well‐aligned self‐assembled PVDF fibers in the form of yarn structures is presented. Post‐treatments such as stretching at 100 °C and annealing improve the tensile modulus and strength of the fibers by 17 and 41%, respectively. The results reveal that post‐treatment on fiber yarns induce crystallinity and β‐crystalline phase formation, which in turn impart a noticeable effect on the strength and stiffness of the fibers. An ≈10% improvement in the ferroelectric β‐crystalline phase fraction is estimated for the post‐treated yarns. Such yarn structures with improved strength and ferroelectric β‐phase content can be useful for nanoscale and microscale electronic devices.

     

Structure–tensile property relationship of knitted fabric composites

This paper reports some further studies on the tensile properties of plain weft knitted DuPont Kevlar fiber fabric reinforced epoxy matrix composites. One aim of this work is to investigate systematically the anisotropy of knitted fabric composites. Tensile tests were conducted at different off‐axial angles (0°, 30°, 45°, 60° and 90°) with respect to the wale direction. Fracture modes corresponding to this off‐axial variation were clearly identified. The elastic modulus and tensile strength were found to be the highest in the wale direction (0°), but decreased gradually towards the course direction (90°). This anisotropic study was carried out on both single layered and multilayered (four layered) knitted fabric composites. Another aim is to study the effect of specimen width (or number of loops per specimen width) on the tensile properties. This study indicated an optimum number of loops per sample width required to obtain meaningful tensile test results. Further, edge effect on the tensile properties of the knitted fabric composites was also investigated in the paper. This was achieved through comparative studies on cut and uncut four layered specimens. In the cut specimens the continuity of the yarns is lost at the cut edges, which show a marginal variation in the properties compared with the uncut specimens where the yarns are continuous throughout the specimen. Finally, a study was also carried out to investigate the effect of loop size (or stitch density) on the tensile strength and stiffness of the single layered composites.     

Surface modification of Kevlar by grafting carbon nanotubes

A novel and efficient method was developed for surface‐modification of Kevlar fibers by multi‐wall carbon nanotubes (MWCNTs). Kevlar fibers were immersed in a solution mixed with Hexamethylene diisocyanate, 1,4‐diazabi‐cyclo [2,2,2] octane (DABCO), and toluene to introduce pendant amine groups before the COCl‐functionalized carbon nanotubes were chemically grafted onto the surface of modified fibers under ultrasonic condition. The characterization of resulting fiber involved in SEM, infrared spectroscopy, and tensile measurement. Results indicated over 20% of the fiber surface were coated by MWCNTs even after washing, which indicated a good adhesion. Furthermore, the mean value of tensile strength of Kevlar fiber was improved by 12% compared with original one. And the interlaminar shear strength (ILSS) of the fiber‐reinforced bismaleimides composite was increased by 30%. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Application of the microbond technique. IV. Improved fiber–matrix adhesion by RF plasma treatment of organic fibers

The microbond technique is a modification of the single-fiber pullout test for measuring interfacial shear strength. Briefly, a cured microdroplet of material is debonded in shear from a single fiber. Ultra-high modulus polyethylene (Spectra) fibers and aramid fibers (Kevlar) were treated using a radio frequency plasma in order to increase the interfacial bond between the fibers and an epoxy resin. The treated fiber surface was subsequently analyzed by X-ray photoelectron spectroscopy (XPS). Plasma treatment resulted in an increased concentration of oxygen containing functionalities on the fiber surface. The interfacial shear strength as determined by the microbond test increased by 118% for the Spectra fibers and by 45% for the Kevlar fibers with the same epoxy resin. Scanning electron microscopy indicated little change of the surface topography of either fiber following plasma treatment. Effects of friction and surface composition of the plasma-treated fibers is discussed. © 1993 John Wiley & Sons, Inc.