Structure–property relationships of thermally bonded polypropylene nonwovens

The effect of fiber structure and morphology on the resultant mechanical and low load deformation properties of thermally bonded nonwoven polypropylene fabrics has been studied. Commercially available staple polypropylene fibers varying in linear density and draw ratio (Herculon and Marvess staple fibers) were used in this study. The orientation of these fibers was characterized by birefrigence measurements. Differential scanning calorimetry measurements were made to determine the heat of fusion and melting point of fibers. Experiments confirm that tensile strength and stiffness of the fabrics correlate with this fiber structure. Under the same bonding conditions fabrics made from fibers with low draw ratios show higher tensile strength and stiffness than do fibers with high draw ratios. The mechanical properties of fabrics were found to be greatly affected by the thermal bonding temperature. The tenacity and flexural rigidity of fabrics made from poorly oriented fibers show higher values than those made from highly oriented fibers. The shrinkage of the fabrics was observed to increase with increasing bonding temperature in both machine and cross machine directions. The changes in fabric thickness due to the thermal bonding are considerably lower for poorly oriented fibers.     

Fibers from blends of PET and thermotropic liquid crystalline polymer

Blends of poly(ethylene terephthalate-Co-p-oxybenzoate), PET/PHB, with poly(ethylene terephthalate), PET, have been studied in the form of as-spun and drawn fibers. DSC melting and crystallization results show that the PET is compatible with LCP and the crystallization of PET decreases by the addition of LCP in the matrix. Upon heating above the crystal melting temperature of PET, the blend shows good dispersion of LCP in the PET matrix. Wide angle X-ray diffraction of drawn blended fibers show the possible formation of LCP oriented domains. The mechanical properties of drawn fiber up to 10 wt% LCP composition exhibit significant improvement in tensile modulus and tensile strength with values of 17.7 GPa and 1.0 GPa, respectively. Values of modulus are compared with prediction from composite theory, assuming the blend system as nematic domains of LCP. dispersed in PET matrix.     

The breaking strength of imperfect (real) polymer fibers

The thermodynamic fusion theory of strength of perfect polymer fibers of finite molecular weight is extended to include imperfect (i.e. real) fibers of incomplete crystallinity and orientation. Approximate equations for failure strength, strain, and work of failure are derived by extracting from the real visco-elastic fiber an equivalent reversible component suitable for thermodynamic analysis. This is facilitated by an explicit relationship between fiber breaking stress, σ_*, and breaking strain, _*, which is shown to be σ_*=0.632K_* (K=modulus) for constant strain-rate deformations. It is shown that fiber breaking time is equivalent to the fiber visco-elastic mechanical relaxation time. Experimental data shows that the activation energy of rupture of polyethylene fibers is not the activation energy of covalent bond rupture. Instead it agrees with the activation energy expected of crystal melting in accordance with the fusion theory of rupture. The activation volume of the polyethylene fibers also agrees with the value expected from this theory.     

Tensile behavior of glass fiber-reinforced acetal polymer

The tensile stress-strain behavior of glass fiber-reinforced polyacetal resin was investigated for various fiber concentrations, fiber length distributions, and finishing agents. The polyacetal fiber blends change considerably in strength and elongation at break when treated with ammonium chloride, but otherwise similar specimens still follow a common stress-strain curve to a point shortly before failure. As the mean fiber length decreases, the modulus and tensile strength fall, but the elongation at break remains almost unchanged. The observed tensile behavior is discussed in terms of a simplified model, which assigns the fibers to two categories: a fraction α parallel to the applied load, and the remainder distributed in a plane perpendicular to the load axis. By fitting this model to the stress-strain curves, two other constants of each system are derived: a length-dependent efficiency factor β for parallel fibers, whose magnitude agrees with the predictions of Rosen and his co-workers, and a factor γ which expresses the constraint of the matrix resin by the “transverse” fibers. The behavior of γ is consistent with Tsai's theory of the transverse modulus of laminates, if a reasonable amount of fiber–fiber contact is assumed. In terms of this model, possible interpretations of the behavior under repeated loading and the mechanism of tensile failure are presented.     

Effects of fiber length on the tensile strength of epoxy/glass fiber and polyester/glass fiber composites

In discontinuous fiber-reinforced composites, the shear strength at the fiber–matrix interface plays an important role in determining the reinforcing effect. In this paper, a method was devised to accurately determine this shear strength, taking the strength distribution of glass fiber into consideration. Calculated strength values based on the shear strenght obtained by the method were in better agreement with the experimental observations than those calculated by employing the shear strength obtained on the assumption that the fiber strength was uniform. The tensile strength of composites increases with increasing aspect ratio of the reinforcing fibers. This trend is almost the same regardless of the kind of matrix, the nature of interfacial treatment, and the environmental temperature. When composites are reinforced with random-planar orientation of short glass fibers of 1.5 times the mean critical fiber length, the tensile strength of composite reaches about 90% of the theoretical strength of composites reinforced with continuous glass fiber. Reinforcing with glass fibers 5 times the critical length, the tensile strength reaches about 97% of theoretical. However, from a practical point of view, it is adequate to reinforce with short fibers of 1.5–2.0 times the mean critical fiber lenght.     

Pineapple leaf fiber reinforced thermoplastic composites: Effects of fiber length and fiber content on their characteristics

Pineapple leaf fiber (PALF) was used as a reinforcement in polyolefins. Polypropylene (PP) and low‐density polyethylene (LDPE) composites with different fiber lengths (long and short fibers) and fiber contents (0–25%) were prepared and characterized. The results showed that the tensile strength of the composites increased when the PALF contents were increased. It was observed that the composites containing long fiber PALF were stronger than the short fiber composites as determined by greater tensile strength. An SEM study on the tensile fractured surface confirmed the homogeneous dispersion of the long fibers in the polymer matrixes better than dispersion of the short fibers. The unidirectional arrangement of the long fibers provided good interfacial bonding between the PALF and polymer which was a crucial factor in achieving high strength composites. Reduction in crystallinity of the composites, as evident from XRD and DSC studies suggested that the reinforcing effect of PALF played an important role in enhancing their mechanical strength. From the rule of mixtures, the stress efficiency factors of the composite strength could be calculated. The stress efficiency factors of LDPE were greater than those of PP. This would possibly explain why the high modulus fiber (PALF) had better load transfers to the ductile matrix of LDPE than the brittle matrix of PP. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Enhanced wear resistance of ultra-high molecular weight polyethylene fibers by modified-graphite oxide

A simple and feasible method to enhance the wear resistance of ultra-high molecular weight polyethylene (UHMWPE) fibers was reported. The graphite oxide (GO) prepared using improved Hummer's method was surface modified with hexadecylamine to improve its compatibility with UHMWPE. Combined with well-dispersion of modified-GO (m-GO) in dichloromethane and the fact that the viscosity of UHMWPE suspension can be decreased by dichloromethane, the well dispersed m-GO/dichloromethane was added into UHMWPE suspension to improve m-GO dispersion in UHMWPE fibers. Finally, UHMWPE fibers with different m-GO concentration were prepared using gel spinning technology. The effect of m-GO concentration on the structure and properties of modified UHMWPE fibers were investigated. The results indicated that the melting temperature and crystallinity of m-GO modified UHMWPE fibers increased with increasing of m-GO concentration, while the fiber's crystal sizes and orientation increased, thus the tensile strength of m-GO modified UHMWPE fibers remained almost undamaged. The introduction of m-GO is beneficial to the formation of smooth transfer film on fiber's surface, which enhanced the self-lubrication of UHMWPE fibers. Compared with pure UHMWPE fiber, the UHMWPE fiber containing 1.5 wt% m-GO had enhanced wear resistance by 55.4% and still maintained high tensile strength of 29.98 cN dtex⁻¹.     

聚甲醛纤维的一步法制备及其性能研究

采用自制的熔融纺丝一体化设备,使用不同熔融指数的聚甲醛的共混料为原料,通过DSC、TGA等测试以及纺丝工艺过程的分析,得出了最佳的原料配比和熔融纺丝工艺。研究了牵伸倍数对聚甲醛纤维拉伸强度的影响,结果表明:在10倍牵伸下,可以制备出拉伸强度为6.2 cN/dtex的聚甲醛纤维。     

Failure of Crossply Ceramic-Matrix Composites

The fast-fracture and stress-rupture of a crossply ceramic-matrix composite with a matrix through-crack are examined numerically to assess the importance of fiber architecture and the associated stress concentrations at the 0/90 ply interface on failure. Fiber bridging in the cracked 0 ply is modeled using a line-spring bridging model that incorporates stochastic and time-dependent fiber fracture. A finite-element model is used to determine the stresses throughout the crossply in the presence of the bridged crack. For both SiC/SiC and a typical oxide/oxide, the fast-fracture simulations show that as global failure is approached, a significant fraction of fibers near the 0/90 interface are broken, greatly reducing the stress concentration. For fibers with low Weibull moduli ( m < 10), the tensile strength is thus nearly identical to that of a unidirectional composite scaled by the appropriate fiber volume fraction, while for fibers with larger Weibull moduli ( m ≥ 10), there are modest (10−17%) reductions in tensile strength. Stress-rupture simulations show that initially high stress concentrations are relieved as fibers fail with evolving time near the 0/90 interface and shed load away from the interface. For a wide range of fiber properties, efficient load redistribution occurs such that the crossply rupture lifetime is generally within an order of magnitude of the unidirectional lifetime, when the applied stress is normalized by the relevant fast-fracture strength. Overall, stress concentrations at the 0/90 interface are largely relieved with increasing load or time due to the nonlinear bridging response and preferential fiber failure near the interface, resulting in crossplies that respond very similarly to unidirectional composites.     

The fatigue of synthetic polymeric fibers

The fatigue properties of a number of different types of fibers have been investigated and failure under cyclic loading conditions compared to that caused by simple tensile loading. Polyamide, polyester, and polyacrylonitrile fibers have been studied and all have been found to fail by fatigue mechanisms. The loading conditions have been monitored by a fiber fatigue apparatus developed for this purpose and the fracture morphologies inspected by scanning electron microscopy. In all of the cases which are considered in detail, fatigue failure of the fibers has been found to occur when cycling from zero load to a maximum load of about 60% of the tensile strength. Fatigue failure is accompanied by a distinctive fracture morphology, clearly different from the tensile fracture morphology and involving crack propagation along the fiber at a slight angle to its axis, although the mechanism which causes this in the acrylic fiber is probably different from that for the polyamide and polyester fibers.     

熔融纺丝法制备UHMWPE/MMT复合纤维的研究

以超高相对分子质量聚乙烯/蒙脱土(UHMWPE/MMT)纳米复合材料为原料,采用熔融纺丝法,在自行设计制造的实验纺丝机上制备出纤维,利用DSC、XRD、SEM等手段对其结构进行了表征,并对其性能进行了测试。拉伸条件试验表明,水浴温度85℃、拉伸倍率14倍,是纤维最佳拉伸温度与倍率;纤维在拉伸过程中存在一个最佳的甬道停留时间。微观结构分析表明,拉伸后得到的纤维熔点、取向度都得到了提高,并在一定条件下出现了正交晶形到六方晶形的转变。     

Effect of cyclic moisture absorption desorption on the mechanical properties of silanized jute-epoxy composites

The goal of this paper is to discuss the influence of water absorption-desorption cycles on the mechanical properties of natural fiber reinforced plastics. Therefore, epoxy resins with jute wovens as reinforcement with untreated and silane treated fibers were investigated. Silane treatment of fibers led to increased tensile, flexural strength, and Young's modulus of composites with up to 30%. Absorption-desorption cycles of fibers changed the fracture mechanisms of fibers without having significant effects on the tensile strength of the fibers. Light microscopic investigations showed that absorption-desorption cycles of composites led to the debonding of resin from fibers as well as to cracks in the adjacent resin. Because of these mechanisms, tensile strength and Young's modulus decrease, independent of the quality of fiber resin adhesion. For dynamic loadings, storage cycle induces damages, further bringing about a decreased dynamic modulus and an increased progress in damage with increasing load cycles during the first two environmental cycles, being constant afterwards.     

钛酸酯偶联剂对抗菌沸石/聚丙烯共混体系的影响

采用钛酸酯偶联剂对抗菌沸石进行表面修饰,然后将其与聚丙烯共混造粒并纺丝。对共混切片进行DSC、扫描电镜以及纤维拉伸强度测试。结果表明:钛酸酯偶联剂的加入能完善共混体系的结晶,提高结晶度,促进抗菌沸石的分散以及减缓共混纤维拉伸强度的下降,其影响程度与钛酸酯含量有关。同时,抗菌沸石含量也是影响共混体系结晶、分散性及纤维拉伸强度的主要因素。     

异形有机短纤维增强环氧树脂复合材料拉伸性能的研究

本文考察了纤维形态对异形有机短纤维增强环氧树脂复合材料光弹性能和拉伸性能的影响。结果表明，与平直短纤维复合材料相比，在环氧基体中加入凸端或竹节状的聚酰胺短纤维，不仅可以改变复合材料中的应力分析，有利于纤维末端的应力传递，而且能够提高短纤维复合材料的拉伸强度。     

Axial compressive strength of carbon fiber with tensile strength distribution

Measuring the fiber lengths of the broken pieces and estimating the mean tensile strength from the length just before the final fragment length in tension, efforts were made to estimate the axial compressive strengths of carbon fibers when the tensile strength varies with the length. The estimated compressive strength of carbon fibers decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compressive force owing to a decrease in the residual thermal stress and a decrease in Young's modulus of the resin matrix. There is a linear relationship between the estimated compressive strength and radial compressing force in the temperature range from room temperature to 80°C. The real compressive strength of the fibers, determined by extrapolating this straight line until the radial compressing force is zero, is about 20% higher than the compressive strength estimated by assuming that the tensile strength is uniform. It is approximately 10–50% of tensile strength. A linear relationship between the fiber axial compressive strength and compressive strength of the unidirectional composites is found. The experimental values agree with the values calculated by the rule of mixtures.     

Factors influencing the tensile properties of ramie

Many investigators have claimed that tensile breakage of native cellulose fibers occurs primarily by repture of covalent bonds in the cellulose molecules rather than by chain slippage resulting from repture of such interchain bonds as hydrogen bonds. This claim has been made partly on the basis of a comparison of tensile properties of ramie fiber and of the fully esterified counterpart. This comparison indicated that the breaking load of ramie was similar to that of the fully esterified fiber. In studying the tensile properties of ramie fiber and of fully acetylated ramie fiber, we found that the degree of polymerization of the fiber was lowered during the acetylation process. Also, it was evident that both degree of polymerization and degree of crystallinity are important factors to be considered when comparing the tensile strength of native cellulose fibers and their acetylated counterparts. Although the primary cause of tensile breakage of native cellulose fibers may be due to chain scission rather than to chain slippage, it is difficult to claim supporting evidence for this theory from studies made so for on the tensile properties of esterified ramie fibers.     

TLCP/GF混杂增强聚醚砜酮复合材料的研究

用热致性液晶聚合物(TLCP)和玻璃纤维(GF)对二氮杂萘酮联苯结构聚醚砜酮(PPESK)进行混杂增强改性,用双螺杆挤出机通过熔融共混的方式制备了改性PPESK复合材料。研究了PPESK/TLCP、PPESK/TLCP/GF复合材料的加工性能和力学性能,并对复合材料的拉伸断面形貌进行观察。结果表明,随着TLCP用量的增加,熔体的粘度逐渐降低,加工流动性得到改善;TLCP/GF的混杂增强使PPESK复合材料的拉伸强度得到一定提高,冲击强度有所降低;TLCP用量小于10份时,TLCP以球状粒子或棒状形式存在,当TLCP用量为20份时,TLCP在材料中形成直径为亚微米数量级的高长径比微纤。     

Structure and properties of thermotropic liquid crystal polymer and poly(ethylene 2,6‐naphthalate) blend fibers

BACKGROUND: The melt blending of thermotropic liquid crystal polymers (TLCPs) using conventional thermoplastics has attracted much attention due to the improved strength and tensile modulus of the resulting polymer composites. Moreover, because of their low melt viscosity, the addition of small amounts of TLCPs can reduce the melt viscosity of polymer blends, thereby enhancing the processability. RESULTS: In this study, TLCP/poly(ethylene 2,6‐naphthalate) (PEN) blend fibers were prepared by melt blending and melt spinning to improve fiber performance and processability. The relation between the structure and the mechanical properties of TLCP/PEN blend fibers and the effect of annealing on these properties were also investigated. The mechanical properties of the blend fibers were improved by increasing the spinning speed and by adding TLCP. These properties of the blend fibers were also improved by annealing. The tensile strength of TLCP5/PEN spun at a spinning speed of 2.0 km h^?1 and annealed at 235 °C for 2 h was about three times higher than that of TLCP5/PEN spun at a spinning speed of 0.5 km h^?1. The double melting behavior observed in the annealed fibers depended on the annealing temperature and time. CONCLUSION: The improvement of the mechanical properties of the blend fibers with spinning speed, by adding TLCP and by annealing was attributed to an increase in crystallite size, an increase in the degree of crystallinity and an improvement in crystal perfection. The double melting behavior was influenced by the distribution in lamella thickness that occurred because of a melt‐reorganization process during differential scanning calorimetry scans. Copyright © 2007 Society of Chemical Industry     

Effect of fiber size on structural and tensile properties of electrospun polyvinylidene fluoride fibers

We use electrospinning to obtain polyvinylidene fluoride (PVDF) fibers and demonstrate simultaneous improvements in β‐crystal microstructure and in tensile properties of fibers with reduction of their diameter. PVDF fibers with average diameters ranging from 70 to 400 nm are obtained by controlling the concentration of the polymer in the electrospinning solution. The amount of β‐crystals present is found to be greater for finer diameter fibers, yielding a maximum β‐phase fraction of 0.86 in the 70‐nm fibers. Moreover, the deformation behavior of the fibers reveals that the tensile modulus and strength improve with reductions in fiber size. Sharp increases in tensile properties are demonstrated when the size of the fibers is reduced below 175 nm. We attribute the enhanced concentration of β‐crystals and the tensile behavior of finer diameter fibers to the extensional forces experienced by the material during electrospinning. POLYM. ENG. SCI., 55:1812–1817, 2015. © 2014 Society of Plastics Engineers     

High tensile strength fiber of poly[(R)‐3‐hydroxybutyrate‐co‐(R)‐3‐hydroxyhexanoate] processed by two‐step drawing with intermediate annealing

High tensile strength fibers of poly[(R)‐3‐hydroxybutyrate‐co‐(R)‐3‐hydroxyhexanoate] [P(3HB‐co‐3HH)], a type of microbial polyesters, were processed by one‐step and two‐step cold‐drawn method with intermediate annealing. Thermal degradation behaviors were characterized by differential scanning calorimeter and gel permeation chromatography measurements. Thermal analyses were revealed that molecular weights decreased drastically within melting time at a few minute. One‐step cold‐drawn fiber with drawing ratio of 10 showed tensile strength of 281 MPa, while tensile strength of as‐spun fiber was 78 MPa. When two‐step drawing was applied for P(3HB‐co‐3HH) fibers, the tensile strength was led to 420 MPa. Furthermore, the optimization of intermediate annealing condition leads to enhance the tensile strength at 552 MPa of P(3HB‐co‐3HH) fiber. Wide‐angel X‐ray diffraction measurements of these fibers suggest that the fibers with high tensile strength include much amount of the planer‐zigzag conformation (β‐form) as molecular conformation together with 2₁ helix conformation (α‐form). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41258.