Effect of areal density and fiber orientation on the deformation of thermomechanical bonds in a nonwoven fabric

This study proposes a novel method to mechanically characterize the performance of individual bonds in low‐density, thermomechanically bonded nonwoven fabrics. Commercial bicomponent, polyethylene/polypropylene (PE/PP), nonwoven fabric was laser cut into bowtie‐shaped specimens for uniaxial tensile testing so that the central region of each specimen contained an individual bond. Three groups, each composed of 20 specimens, were tested with their longitudinal axes oriented along the machine direction (MD), the cross direction (CD), and at 45° between these two directions (DD). Prior to testing, the intrinsic variation in areal density and fiber orientation in the region surrounding the individual bond were quantified via orientation and relative basis weight parameters. During testing, images of the specimens were acquired to determine the occurrence of fiber breakage, bond deformation, and bond cohesive failure. Maximum force, stiffness, and orientation parameters were found to be significantly different among the three specimen groups (p < 0.01) but the relative basis weight was not (p > 0.01). The stiffness and maximum load were linearly correlated with both the areal density and fiber orientation. Pre‐existing voids or windows within the bond lowered the maximum force for specimens with the longitudinal axes aligned with the MD. These voids had no influence on the maximum force achieved by the specimens aligned with the CD and DD. The bonds in these specimens were observed to deform less than the bonds in the specimens with the longitudinal axes aligned with the MD. The results indicate the importance of the fiber structure surrounding the bond on the tensile properties, deformation and failure mode of individual bonds within the nonwoven fabric. POLYM. ENG. SCI., 59:311–322, 2019. © 2018 Society of Plastics Engineers     

Tensile deformation of poly(p-phenylene terephthalamide) fibres,an experimental and theoretical analysis

The tensile deformation of poly(p-phenylene terephthalamide) fibres has been investigated. Functional relationships observed between stress, crystallite orientation distribution, dynamic modulus and strain are derived from an analysis of the deformational behaviour of a series model consisting of a linear arrangement of crystallites. It is shown that the deformation of these fibres is largely brought about by the elastic strain and irreversible rotation of the crystallites. A formula is derived for the stress—strain relationship of a crystalline polymeric fibre with a narrow crystallite orientation distribution.     

Plastic deformation of crystalline polymers

Under uniaxial tensile load, the plastic deformation of unoriented crystalline polymers first transforms the lamellae into a fibrous structure. Usually the drawing is inhomogeneous with a neck propagating through the sample. The higher the draw ratio, the higher the axial elastic modulus as a consequence of the larger fraction of taut tie molecules in amorphous layers connecting the crystalline blocks of each microfibril. As a consequence of the almost 1/(1 ? α) times higher strain of amorphous layers under tensile load, the taut tie molecules are much more strained than the chains in crystal blocks. Hence, their contribution to elastic modulus is substantially higher than one would guess from their fraction β. This is more so in polyethylene with higher crystallinity (α = 0.8) than in nylon 6 with low crystallinity (α = 0.5). Even for the highest modulus polyethylene E = 70 GPa ～ 0.3 × E_c, one needs less than 7.5 percent of taut tie molecules. The plastic deformation of the fibrous structure markedly enhances the number of interfibrillar tie molecules in nylon 6 and to a lesser extent in polyethylene and polypropylene. Homogeneous drawing without a neck transforms the whole sample into a fibrous structure rather uniformly so that for a long while one has the lamellar and fibrillar morphology side by side. The end effect on the structure obtained does not differ appreciably from inhomogeneous drawing with neck propagation. The drawing of polymers with a liquid crystal structure yields a highly aligned fibrous structure with very few chain folds and an exceptionally high elastic modulus and strength. But the axial connection of individual highly oriented and ordered domains is affected by a relatively small fiaction of tie molecules, and this is responsible for reduction of the elastic modulus below the value of the ideal crystal lattice.     

PA6/CaCl2复合材料络合配位的红外研究

采用傅里叶变换红外光谱分析(FTIR)研究了尼龙6/氯化钙复合材料中胺基、酰胺Ⅰ带和酰胺Ⅱ带伸缩振动峰随氯化钙用量的变化情况。通过配位化合物中氧、氮原子参与配位的优先次序,以及酰胺基团的红外形成机理,提出两种络合机理假设,证实了钙离子通过与尼龙6分子链上的氮原子发生络合作用,插入尼龙6分子链,破坏尼龙6的氢键,降低其结晶度。     

Calculation of crystallite modulus of native cellulose

Summary A crystallite modulus of native cellulose along the chain axis has been calculated based on the X-ray analysed molecular conformation and the force constants used in the vibrational analysis. The calculated values are 172.9 GPa and 70.8 GPa for the cases with and without the intramolecular hydrogen bondings taken into account, respectively. The intramolecular hydrogen bondings have been found to play an important role on the determination of crystallite modulus and the chain deformation mechanism, based on the calculation of the strain energy distribution to the internal coordinates such as bond lengths, bond angles, and so on.     

氯化钙对尼龙6/66结晶态结构的影响

本文采用示差扫描量热法(DSC)、傅立叶变换红外光谱法(FTIR)研究了CaCl2对尼龙6/66结晶态结构的影响.DSC测试结果表明,随着尼龙6/66中CaCl2含量的增加,尼龙6/66的结晶温度、熔融温度降低,结晶度逐渐减小直至变为无定型态.红外光谱研究结果证实了钙离子能够插入尼龙6/66分子链,与尼龙6/66分子链上的羰基发生络合作用,破坏尼龙6/66原有的氢键,阻碍尼龙6/66分子链的自由运动,从而使其无法结晶.     

Rheological and mechanical characteristics of poly-p-phenylene terephthalamide/nylon molecular and particulate composites

Poly-p-phenylene terephthalamide (PPTA) and nylons (nylon 6, 66, 11, and 12) molecular and particulate composites were examined with emphasis on rheological, mechanical, and morphological characteristics. These molecular composites were prepared by coagulation in water from isotropic ternary sulfuric acid solutions. Upon heating above the crystal melting temperatures of nylons, the molecular composites undergo phase separation and become two-phase particulate composites. Shear viscosity measurements indicate that the viscosity of nylon 6 and 66 systems has increased for an order of magnitude because of the presence of PPTA, but decreased in the case of nylon 11 and 12 systems. PPTA/nylon 6 and 66 systems exhibit yield behavior, particularly with high PPTA contents. Measurements show an extrudate swell of around 1 in all composition ranges and shear rates. Scanning electron microscopic investigations of low PPTA content particulate composites reveal spherical or long fibrillar morphology of PPTA phase dispersed in nylon matrices. Wide angle X-ray diffraction investigations show no preferred orientation in the extrudates, but biaxial orientation in the case of compressed films. The study on mechanical properties of extrudates of 5 to 10 wt.% PPTA compositions exhibits significant improvement in tensile modulus and tensile strength. The heat-treated extrudates show further increase in tensile modulus and tensile strength, as compared with the unannealed samples, but the elongation at break decreases.     

Epitaxial crystallization of nylon 6 cast from solution on the surface of poly(p-phenylene terephthalamide) filament

Poly(p-phenylene terephthalamide) (PPTA) fiber has the characteristics of high modulus and strength, and has been employed as the reinforcement in composite materials. The interfacial interaction between the PPTA filament and nylon 6 matrix has been investigated to make use of these characteristics of PPTA in fiber-reinforced composite systems. In the case of composites composed of the PPTA filament and nylon 6, two types of expitaxial crystallization have been observed depending on the concentration of a formic acid solution. From concentrated solution, nylon 6 forms a columnar cystal around the PPTA filament. A two-dimensional spherulite model, (i.e., a disk shape) is proposed for the aggregated structure of nylon 6 chains on the basis of wide-angles x-ray diffraction, small-angle x-ray scattering studies, and scanning electron microscopic ebservation. The a*-axis of the nylon 6 crystal is directed radially in its columnar crystal. In the inner portion, close to the interface, the be-plane is in contact with the surface of the PPTA filament, also, the directions of the b- and c-axes are paral el and perpendicular to the PPTA filament axis, respectively. On the other hand, their axes rotate about the a*-axis in the outer portion, far from the interface region. Furthermore, a different type of epitaxy was observed in the case of crystallization from a dilute solution of nylon 6 in formic acid. These two types of crystal growth were investigated from the view of crystallization condition and crystallographic lattice matching at the interface region between the PPTA filament a nylon 6. It is concluded that the hydrogen bonding between PPTA and nylon 6 molecules at the interface plays an important role for epitaxial crystallization, in addition to lattice matching.     

Proton magnetic resonance investigation of the orientation of hydrogen bonds in nylon 66, poly(vinyl alcohol), and cotton yarn

Directional dependence of the PMR-narrow-band in oriented fibers of nylon 66, poly(vinyl alcohol), and cotton has been investigated to study the orientation of hydrogen bonds in the unit cell. Filaments of oriented fibers were conditioned to 75% relative humidity, then aligned together axially and packed in a teflon tube. The teflon tube was suspended in the NMR probe in such a manner that the fiber axis was horizontal and could be rotated to a desired angle with respect to the magnetic field. Variation of the narrow-band line width as a function of the angle between the fiber axis and the magnetic field-direction shows a minimum at 0° orientation for nylon 66 and PVA, but in the case of cotton it shows a minimum at about 80° orientation. This indicates that in the case of nylon 66 and PVA, hydrogen bonds are oriented nearly prependicular to the chain axis. This would suggest that nylon 66 and PVA have interchain hydrogen bonding, but the hydrogen bonds contributing to directional dependence, in the case of cotton fiber, are intrachain. The interchain hydrogen bonds between the lateral chains, if they exist, must be random and, therefore, do not contribute to directional dependence. The interchain H bonds between central and corner chains are probably oriented in such a way that the horizontal component of p–p vectors have nearly the same orientation in the unit cell as the p–p vectors of the intrachain hydrogen bonds.     

Deformation of doubly oriented polyethylene,nylon 66 and poly(ethylene terephthalate)

Doubly oriented specimens of high density polyethylene (HDPE), nylon 66 and poly(ethylene terephthalate) (PET) were re-stretched along a direction perpendicular to the molecular chain axis at temperatures ranging between room temperature and the respective polymer melting points. Brittle failure was observed for PE samples at all the test temperatures with no significant amount of plastic deformation; whereas, for both PET and nylon 66 samples, ductile deformation was observed at elevated temperatures with plastic strain of >400%. The ductile deformation of nylon 66 and PET occurred with an anisotropic change in the cross-sectional dimensions of the specimen, the reduction taking place predominantly in only one lateral direction. The morphological change accompanying the drawing of the doubly oriented PET and nylon 66 material was examined by using X-ray and optical methods. The implications of the difference in deformation behaviour with respect to the morphological differences among oriented PE, PET and nylon 66 materials are discussed.     

Puncture deformation and fracture mechanism of oriented polymers

In this study, we investigated the effect of orientation by solid‐state cross‐rolling on the morphology, puncture deformation, and fracture mechanism of an amorphous TROGAMID material and three semicrystalline polymers: high‐density polyethylene (HDPE), polypropylene (PP), and nylon 6/6. In amorphous TROGAMID, it was found that orientation preferentially aligned polymer chains along the rolling deformation direction and reduced the plastic deformation of TROGAMID in a low‐temperature puncture test. The decrease of ductility with orientation changed the fracture mechanism of TROGAMID from ductile hole enlargement failure in the unoriented control to a more brittle delamination failure in TROGAMID cross‐rolled to a 75% thickness reduction. For semicrystalline polymers HDPE, PP, and nylon 6/6, the randomly oriented crystalline lamellae in the controls were first oriented into an oblique angle to the rolling direction (RD) before the lamellae became fragmented and preferentially oriented with the chain axis parallel to the RD. The morphological change resulted in the decrease of ductility in HDPE in the low‐temperature puncture test. In PP and nylon 6/6, the brittle fracture of unoriented controls was changed into ductile failure when they were cross‐rolled to a 50% thickness reduction. This was attributed to the tilted crystal lamellae morphology, which permitted chain slip deformation of crystals with the chain axis parallel to the maximum shear stress direction. With further orientation of PP and nylon 6/6 to a 75% thickness reduction, the failure mechanism changed back to brittle fracture as the morphology transformed into a layered discoid structure with the chain axis of the fragmented crystal blocks parallel to the RD; this prevented chain slip deformation of the crystals. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Creep enhanced adsorbtion of water or aqueous zinc chloride solution increases the creep rate of nylon 6,6

The creep behavior of nylon 6,6 at 21°C was significantly altered when the local “dry” environment was changed to water mist or an aqueous zinc chloride mist. Nylon 6,6 was found to exhibit logarithmic creep because the relation between the log of the strain rate and the creep strain was linear with a negative slope. The effect of changing the creep environment from dry to wet, with the addition of moisture from an ultrasonic humidifier was to decrease the negative slope by 50–70% within 5–10 min. This effect could be interpreted as a decrease in modulus, which allowed for easier creep deformation. Based on the stress‐free diffusivity of water in nylon and the dimensions of the test sample the time to saturate the sample was estimated to be about 100 h. Therefore, there appeared to be synergism between the creep deformation and the environment that dramatically enhanced the rate of saturation and slowed the decrease in the creep rate. The tentative explanation provided is that the aqueous solutions, by binding to the hydrogen bonds in nylon, are dragged into the sample during creep deformation, and the dragged‐in aqueous solution then plasticizes nylon. This is analogous to the conclusion in another recent study that showed that deformation, during a hardness test, in the presence of aqueous zinc chloride, transported the solution species deeper into the sample than could be reasonably explained by ordinary diffusion processes. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 494–497, 2001     

The effect of methods of drying on the fine structure,density, and some mechanical properties of jute and allied fibers

The degree of crystallinity and crystallite orientation in jute, mesta, and roselle fibers have been studied by x-ray diffraction. Removal of water, indirectly, has been carried out by alternate immersion in methanol and benzene. It has been found that an increase in crystallinity and a decrease in crystallite orientation take place in these fibers as a result of direct drying, presumably because of the formation of an increased number of hydrogen bonds associated with direct drying. Density, tenacity, and rigidity of the fiber in the two drying conditions are in agreement with the x-ray results.     

Crystallization of cyclic oligomers on the surface of nylon 6 fibers

When scoured nylon 6 filament was exposed to either water vapor or n-alcohol vapor, a portion of the cyclic [NH(CH₂)₅CO]_x, with x = 2 through 7, and the ?-caprolactam present migrated to the filament surface and crystallized. A variety of crystal forms were observed. The variations depended partly on the vapor to which the sample had been exposed. During water vapor exposures, the cyclic dimer migrated more easily to the filament surface than the cyclic monomer and the cyclic tetramer migrated more easily than the cyclic trimer. It is postulated that the secondary valence forces acting between the dimer and the nylon 6 polymer are weaker than that between the ?-caprolactam and the polymer since the dimer forms intramolecular hydrogen bonds. Consequently, the dimer migrates more easily than the monomer, which can form hydrogen bonds with the polymer. Also the cyclic trimer can form hydrogen bonds with the nylon 6 polymer and thus migrates less easily than the cyclic tetramer, which can form intramolecular hydrogen bonds. In alcohol vapor exposures, the n-alcohol used influenced the concentration of cyclic monomer and oligomer, which migrated to the surface of the nylon 6 filament. The variation of the monomer and oligomer concentration on the fiber surface with the alcohol used in the exposure is discussed.     

Orientation of poly(vinyl alcohol) nanofiber and crystallites in non-woven electrospun nanofiber mats under uniaxial stretching

The development of macroscopic nanofiber orientation and microscopic crystallite and molecular chain orientation have been investigated during uniaxial stretching of electrospun poly(vinyl alcohol) (PVA) non-woven nanofiber mats. Scanning electron microscopy and stress-strain/small-angle X-ray scattering show that the macroscopic nanofiber orientation significantly increases during the initial stage of deformation, and approaches a plateau on the way of stretching. Detailed analyses of the stress-strain/wide-angle X-ray diffraction measurement and polarized Fourier transform infrared spectroscopy indicate that the microscopic crystallite and molecular chain orientation rapidly increase at the initial stage of stretching due to macroscopic nanofiber orientation. At higher deformation, the microscopic modes of orientation continuously develop as a result of the nanofiber stretching. The complicated deformation process of non-woven nanofiber mats is discussed in terms of macroscopic nanofiber orientation and the microscopic crystallite and molecular chain orientation.     

Influence of the phase morphology on the molecular orientation at the crack tips in rubber‐modified nylon 6: A micro Fourier transform infrared study

Despite extensive efforts to understand the toughening mechanism of rubber‐modified semicrystalline polymers, the plastic deformation event at the crack tips with an extreme deformation gradient and its correlation with phase morphology is, thus far, poorly understood. In this study, micro Fourier transform infrared measurements were adopted to give direct evidence of plastic deformation at the crack tips by the molecular orientation in nylon 6/ethylene–propylene–diene terpolymer (EPDM) blends with a distinct phase morphology. Significant plastic deformation ahead of the crack tips, manifested by a high molecular orientation, was observed in the compatibilized nylon 6/EPDM blends with fine rubber particles. Moreover, the increased transverse crack‐propagation resistance due to high molecular orientation dramatically extended the plastic deformation into adjacent regions around the crack tips; this was responsible for enhanced energy dissipation during the fracture process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Elastic extension of an oriented crystalline fibre

The elastic extension of an oriented and crystalline fibre built up of rigid-rod chains is analysed. A formula for the stress-strain curve is derived. It is shown that the shape of the initial crystallite orientation distribution and the modulus for shear parallel to the chain direction are important factors determining the stress build-up during extension of the fibre. The relations predicted by this analysis agree well with the experimental data obtained from poly(p-phenylene terephthalamide) fibres.     

Numerical simulation of the mechanical properties of a carbon-fiber-reinforced hollow glass microsphere–epoxy syntactic foam

The addition of carbon fibers has a great influence on the mechanical properties of hollow glass microsphere (HGM)–epoxy syntactic foam. Thus, to elucidate the reinforcement mechanism, the numerical simulation of HGM- and carbon-fiber-filled epoxy matrixes was carried out. The effect of the orientation of carbon fibers on the elastic modulus and stress distribution was studied. The effect of the elastic modulus of the matrix on the change of force was also studied. We noted that the orientation of carbon fibers affected the elastic modulus of the matrix, and when the carbon fibers were distributed in the direction of force, the elastic modulus of the matrix reached its maximum. The maximum stress of HGMs decreased with increasing matrix elastic modulus, and the mechanical properties of the syntactic foam increased with increasing elastic modulus of the matrix. When the carbon fibers were distributed in the direction of the force, the enhancement effect was the best. Because the carbon fibers had a higher elastic modulus than the matrix, the degree of compressive deformation of the carbon fibers was smaller than that of the matrix. During compression, carbon fibers were pulled out and consumed a lot of energy. Thus, the mechanical properties of the syntactic foam were improved. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47083.     

硫氰酸钾对尼龙66聚集态结构的影响

采用差示扫描量热仪(DSC)、傅立叶红外光谱仪(FTIR)研究硫氰酸钾对尼龙66聚集态结构的影响,研究结果表明加入硫氰酸钾能够明显地改变尼龙66的聚集态结构。红外光谱结果证实钾离子能够插入尼龙66分子链,与尼龙66分子链上的羰基发生配位作用,破坏尼龙66的氢键,降低了尼龙66分子链的规整性,使尼龙66的聚集态结构发生改变。