Change in fine structure of nylon 6 gut yarn in twisting,annealing, and untwisting processes. II. Observation by the microbeam X-ray technique

The local structure in the radius direction perpendicular to the fiber axis in nylon 6 gut yarn subjected to twisting and heat setting was investigated by the microbeam x-ray technique. The observations revealed that the b-axis (or chain axis) in a crystal gradually inclines within a tangential plane parallel to the fiber axis and away from it with increase of twist, not only in the outer region but also in the center region, i.e., in the vicinity of the center axis. This local behavior of the b-axis could not be distinguished in the previous work,¹ when the usual collimator was employed in the WAXD technique.     

聚乳酸长丝与亚麻-黏胶纱的并捻工艺及性能

采用并捻联合纺纱方法,将聚乳酸长丝与亚麻-黏胶单纱直接并合加捻成复合纱线,并对影响其纱线结构和性能的因素(捻因数、张力、钢丝圈)进行了研究分析,随后通过捻因数与钢丝圈正交试验优化得出最佳的并捻纺纱工艺。研究分析得出:并捻纱的断裂强力随着捻因数的增大先增大后减小;钢丝圈号数对并捻纱断裂强力存在着较大的影响;并捻纱的纱线结构类似于包缠纱,一般张力大的组分为纱芯,另一组分在外层螺旋包覆纱芯。最佳并捻工艺参数为:捻因数220,钢丝圈选14号。     

A Study on the Relationship Between the Crystal Structure and Hard Elasticity of PVDF Fibers

Summary: Poly(vinylidene fluoride) (PVDF) fibers were prepared by melt-spinning process. The crystal structure of annealed PVDF fibers was characterized by wide- and small-angle X-ray diffraction (WAXD and SAXD) and scanning electron microscopy (SEM). Crystalline reflections of c-axis orientation of annealed PVDF fibers were illustrated by WAXD pattern. The stacked lamellar structure aligned in the direction normal to the fiber axis was found in SAXD pattern and the d-spacing of the lamellae was 13.4 nm. Such lamellar structure was supported by SEM micrographs as well. The elastic recovery of annealed PVDF fibers was above 80% from 50% extension, which was much higher than that of unannealed fibers on the first cycle. The initial elastic modulus of annealed fibers reached to a value of 3.5 GPa. The morphological and mechanical properties, all indicated that the annealed PVDF fibers had the characteristic of hard elasticity. A typical stress-strain curve at a very low strain rate indicated the deformation of crystal lamellae in the fibers and a suggested structural deformation mechanism detailed the characteristics of hard elasticity.

Cyclic loading to 50% extension of annealed PVDF fibers.     

锦纶拉伸交络丝的研究

报道了锦纶拉伸交络丝的生产过程和工艺的研究结果。通过在国产 VC442A、14S 牵伸机上采用不同类型的交络喷嘴进行试验,结果表明:①交络度随空气压力增加呈小—大—小的变化规律。并且,如果用相同的喷嘴加工较粗丝束且根数较多时,压力对交络度的影响较小;②交络区的丝束张力宜控制在0.44—1.32cN/tex;③牵伸机台的捻度控制在10—20捻/m 较适宜;④拉伸速度为426—837m/min 时对交络度的影响不大;⑤丝束总纤度在一定范围对交络度影响不大。而单丝纤度低、孔数多时,则有利于交络。拉伸交络丝具有显著的经济效益,其成本比有捻丝低340元/t。     

83dtex／72f涤纶长丝倍捻工艺探讨

为增强涤纶长丝的强力及弹性等物理性能,需对涤纶长丝进行加捻。通过对83dtex／72f涤纶全拉伸长丝的倍捻工艺参数进行调整与研究,得到了一种使长丝获得强捻的加捻工艺。采用纱线捻度的测定——直接计数法,对加捻后的长丝捻度进行了测试与分析,加捻丝线的捻度为293捻／10cm,捻度偏差率为1％,捻度不匀率为0．5％,捻缩为1．1％。结果显示,本文制定的工艺符合生产要求,达到了设计捻度,捻度均匀,能够增强产品的品质和提高织物的风格。     

Micropore formation process during stretching of polypropylene casting precursor film

Abstract

The micropore formation process during stretching of polypropylene casting precursor film was followed by DSC, SEM, AFM, SAXS and WAXD. DSC and SAXS results prove that annealing improves the lamellae periodicity and new crystals are formed. The increase of lamellae thickness is also observed by SEM and AFM. Preferentially oriented crystalline grain is along the direction perpendicular to the MD. After cold stretching, the newly formed crystals between the lamellae disappear and the voids appear in SEM and AFM. The hot stretching makes the lamellae separated step by step and many connecting bridge crystals appear between the adjacent lamellae, at the same time pores are formed. Their crystallisation behaviour is proved by the higher temperature shoulder in DSC curves and the broadening of second order peak in SAXS curves. During stretching, the long period decreases and the lamellae become twisted. More crystal grains are preferentially oriented along the direction of (110) plane.     

Measurement of the rorsional strength of chemical fibres and critical parameters of formation of snarls

The effect of the tension of chemical fibres (monofilaments, complex, reinforced) on their behavior in twisting was investigated. The use of a nylon 6 core instead of Lavsan in combination yarn decreases the stiffness of the yarn in twisting and the generated moment in additional twisting and also decreases the critical moment and increases the critical twisting value, favoring snarl formation. The practical applicability of the Greenhill and Ross criteria for estimating the critical parameters of formation of snarls in twisting fibres of different structure with low tension (fibre slackness). The best agreement of the experimental data with the calculated results was observed for nylon monofilament. St. Petersburg State University of Technology and Design; De Montfort University, Great Britain. Translated fromKhimicheskie Volokna, No. 5, pp.37–42, September–October, 1999.     

Performance enhancement and optimization for rod-baffle heat exchangers with twisted oval tubes

Twisted oval tubes were proposed to be installed in rod-baffle heat exchangers (RBHXs) to enhance heat transfer performance. A comparison between RBHXs with twisted oval tubes and those with circular tubes is performed. The results show that the heat transfer coefficient (h) per unit pressure drop (Δp) of RBHXs with twisted oval tubes is larger than 36.01%~100.24% compared with that of circular tubes when Reynolds numbers are 4577~22 889, and heat transfer enhancement is more obvious under lower inlet velocity. Then the structural parameters of RBHXs with twisted oval tubes are optimized using the response surface model and genetic algorithms. The results show that the h decreases first and then remains constant with the increase of the twisted pitch length, and decreases first and then rises again with the increase of the ratio of long axis to short axis. The Δp almost keeps constant with increasing twisted pitch length and decreases by 36.67% with the increase in the ratio of long axis to short axis. The comprehensive performance index h/Δp of the RBHXs with twisted oval tubes is enhanced by an average of 26.42% compared with that of original structure. The research results have a good guideline for heat transfer enhancement and the structural design of RBHXs.     

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     

Morphological changes in twisted nylon monofilament

The changes in structure of oriented nylon monofilaments due to an applied twisting action was investigated using wide angle x-ray scattering analysis and microscopy. The surface layer of the twisted filaments changes to a single texture after being repeatedly twisted in one direction at high temperature. Simultaneously, kink bands were produced on their surface. Characteristic features of kink bands are observed on the surface layer of the filaments first twisted in one direction and then twisted in the opposite direction at high temperature. X-ray diffraction patterns of the surface layer also changed remarkably. These changes in surface layers are related to the twist direction, the repeated number of twists, and the temperature of untwist. At lower temperatures, kink bands are not formed and a structural change to a single texture does not occur spontaneously.     

A combined process for double-twisting and heat-treating textile grade polyester yarn

Conclusions The use of a technological scheme which combines the processes of twisting and yarn heat-treatment makes it possible to intensify the process of textile grade polyester yarn manufacture as a result of shortening the series of textile-technology steps.The scheme permits one not only to set the twist well, but also to regulate the basic physicomechanical properties of the yarns over a wide range.By using the proposed technological scheme, depending on the assigned degree of deformation, one can obtained twisted yarns with a shrinkage of 1–2% at an initial elastic modulus up to 20 MPa.Translated from Khimicheskie Volokna, No. 6, pp. 32–33, November–December, 1984.     

Multiple conjugate electrospinning method for the preparation of continuous polyacrylonitrile nanofiber yarn

Continuous polyacrylonitrile nanofiber yarns were fabricated by the homemade multiple conjugate electrospinning apparatus, and the principle of yarn spinning was studied. The effects of the applied voltage, flow rate, spinning distance, and funnel rotary speed on the diameter and mechanical properties of nanofiber yarn were analyzed. The diameter of the nanofibers decreased with increasing applied voltage and the flow rate ratio of the positive and negative needles (F_P/F_N), whereas the diameter of nanofibers increased with increasing overall flow rate and needle distance between the positive and negative. Subsequently, the diameter of the yarns increased first and then decreased with increasing applied voltage, F_P/F_N, and needle distance. However, the diameters of the yarns increased dramatically and then remained stable with increasing overall flow rate. The nanofibers were stably aggregated and continuously bundled and then uniformly twisted into nanofiber yarns at an applied voltage of 20 kV, an overall flow rate of 6.4 mL/h, a needle distance of 18.5 cm, and an F_P/F_N value of 5:3. With increasing funnel rotary speed, the diameters of the nanofibers and yarns decreased, whereas the twist angle of the nanofiber yarns gradually enlarged. Meanwhile, an increase in the twist angle brought about an improvement in the yarn mechanical properties. Nanofiber yarns that prepared showed diameters between 70 and 216 μm. Nanofiber yarns with a twist angle of 65° showed a tensile strength of 50.71 MPa and an elongation of 43.56% at break, respectively. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40137.     

Focused ion beam milling of carbon nanotube yarns to study the relationship between structure and strength

The effect of twist and solvent densification on the internal structure of carbon nanotube yarns was revealed using focused ion beam milling and related to yarn strength through tensile testing. Denser carbon nanotube yarns with smaller diameters were produced either through solvent densification or with increasing twist densities from 5 to ～15 turns/mm, but led to only minor improvement in yarn tenacity. At twist densities greater than ～15 turns/mm, a core-sheath structure developed and was correlated with a decline in strength. The implications of bonding between the nanotubes in the twisted yarn are briefly considered. These results have implications for the future development of high strength carbon nanotube yarn.     

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     

Determination of twisting angle of electrospun nanofiber bundle for continuous electrospinning system

Electrospinning continuously produced twisted nanofibers with a convergence coil and a rotating ring collector. The positively charged nozzle was used in the electrospinning process to deposit electrospun fibers of polyacrylonitrile onto a rotating ring collector. By withdrawing the electrospun fibers from the rotating ring collector, it was possible to spin the electrospun fibers yarn. In this study, theoretical approaches and numerical simulations were used to determine the twisting angle of the yarn. Using the equations developed in this study, we performed numerical simulations and compared the experimental results with the numerical simulation results. Mechanical properties of the fiber bundle were analyzed for twisting angle. It was confirmed the relationship among the winding drum, the ring collector, and flux of the fibers mass per time during electrospinning in the developed system. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45528.     

Formation of lamellar structure by competition in crystallization of both components for crystalline-crystalline block copolymers

Crystallization and structure formation of poly(ethylene oxide)-poly(?-caprolactone) block copolymers (PEG-PCL) in which the melting temperatures of the components are close to each other were elucidated using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. The diblock copolymers with 33, 46 and 59 wt% of PEG composition formed ordinary single spherulites similar to those of PCL homopolymers, while concentric double-circled spherulites appeared for the PCL-PEG-PCL triblock copolymer with 66 wt% PEG composition as observed previously. For the diblock copolymers, despite of the ordinary appearance of the single spherulites, the DSC thermograms and the WAXD patterns indicated the crystallization of PEG as well as PCL. The time-resolved SAXS profiles for the diblock copolymers showed that long spacings of the crystal lamellae decreased stepwise in the crystallization process. Synthesizing these results for the single spherulites, it was concluded that PCL crystallized first followed by the crystallization of PEG with preservation of the PCL crystal lamellar structure. This means that PEG must crystallize within confined space between the formerly formed PCL crystal lamellae. Such confined crystallization of PEG caused the suppressed melting temperature, crystallinity and crystallization rate especially in the smaller PEG compositions. In the melting process of the diblock copolymers, it was observed that the PEG component first melted with a stepwise increase in the long spacing.     

Ferroelectric switching in spin-coated nylons 11 and 12

Here, we investigate the correlation between the crystal structures and the ferroelectric switching by a sinusoidal alternative electric field for spin-coated nylon 11 films as an odd nylon and for nylon 12 as an even nylon. These spin-coated nylons afforded thin films with thicknesses ranging from 101 to 125 nm. The obtained thin films were subjected to melt-quenching, melt-cooling down, annealing-quenching, or annealing-cooling down. These processes were notably related to the resulting crystal structures. In particular, the crystal structures involving twisted bonds in the molecular chains were significantly related to ferroelectricity in both nylon 11 and nylon 12. Namely, the vector component of the amide dipole moments is transverse to the direction of the molecular chains, which is induced by the presence of more twisted bonds and is significantly related to the remanent polarization P_r for both nylons. In nylon 11, the hydrogen bonding interaction between the intermolecular amide dipole moments in the α and δ crystal forms was weakened by the existence of more twisted bonds. In nylon 12, the nonpolar γ crystal form was transformed to a polarizable γ’ crystal form because of the existence of more twisted bonds. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48438.     

预网络器在锦纶66工业丝生产中的应用

研究了在锦纶66工业丝生产中预网络器对减少原丝毛丝及改善可纺性的作用.结果表明:预网络器使油剂在丝束上附着得更加均匀,可纺性显著改善,断丝率降低41.5％,捻线退绕过程中毛丝比例降低76.2％,提高了锦纶66工业丝的质量稳定性.     

A critical assessment of unbalanced surface stresses: Some complementary considerations

The wide-ranging survey of twisted growth in polymers by Lotz and Cheng cites extensive evidence consistent with the relief of surface stress being the underlying cause. This complementary note contributes to the discussion by making three main points. First, it is necessary to go further and explain the key issue of how a consistent twist is maintained when, as commonly, this habit has a lower symmetry than the crystallographic lattice. Detailed study has shown that, in polyethylene, this occurs by reorganization of the initial fold surfaces. Second, the suggested explanation by Keith and Padden that, in polyethylene, the asymmetric habit derives from molecules adding to lamellae with inclined fold surfaces is invalid being doubly inconsistent with observation. Third, twisting has now been linked to faster growth by study of row structures in polyethylene. This produces inherently rough fold surfaces in Régime II whose internal stresses drive reorganization and twisting. For slower (Régime I) growth, fold surfaces form with and maintain ordered packing so providing no basis for twisting. These new insights radically alter the context of twisted growth and provide a firm factual basis for further work.     

Study of crystallization and isothermal thickening in polyethylene using SAXD,low frequency Raman spectroscopy and electron microscopy

This study contains a combined application of three different techniques for the study of polyethy-lenes crystallized from the melt under different circumstances, small-angle X-ray diffraction (SAXD), low frequency Raman spectroscopy to examine the longitudinal acoustic (LA) mode, and electron microscopy. In particular, the combination of SAXD and Raman methods enables the separation of the situation where there is only one lamellar structure which displays several orders in the SAXD pattern, from that where there is more than one type of lamellar thickness present. The superior power of the Raman method, which does not depend on the regularity in the lamellar stacking, becomes apparent. The multiplicity of the lamellar population could be associated with lamellae formed isothermally at the preselected crystallization temperature and with lamellae which originated from material which has remained uncrystallized at this temperature and formed subsequently with smaller lamellar thickness during cooling of the sample. The existence of the corresponding double lamellar population could be made directly visible using electron microscopy on freeze-cut and stained sections. The thinner lamellae in the double population could be extracted by solvents, removing the corresponding SAXD and Raman peaks, and leaving blank image areas in place of the thin lamellae in the electron-micrographs. These extracted thinner lamellae correspond to lower molecular weights as assessed by g.p.c. Thus molecular segregation during crystallization is involved. Furthermore the segregated texture units and their arrangement within the full morphology could now be identified. Pronounced changes in lamellar thickness with crystallization time were observed throughout and were associated in the early stages of crystallization with molecular fractionation and in the later stages with thickening of lamellae already present. An unexpected interrelation between nucleation density and the final lamellar thickness through the agency of isothermal lamellar thickening has been established. Examples are quoted which are contrary to the expected trend of lamellar thickness with crystallization temperature, but which are interpretable nevertheless in the light of the effect of isothermal lamellar thickening. The potential significance of all these findings and of this kind of approach for the characterization of crystalline bulk polymers is made throughout.