A study of yarn friction in aramid fabrics

Energy dissipation by the friction forces of yarns in aramid fabrics with different weave patterns is studied. The upper theoretical limit of the ability of fibers to absorb the energy of transverse impact is determined. The maximum pull-out force for yarns of a plain-weave fabric nonlinearly depends on the number of pulled-out yarns. In twill-weave fabrics, this dependence is linear. Changing the weave pattern is an effective way to change the pull-out forces for yarns. Owing to the slippage of the yarns, the fabric behaves as a plastic material.     

Effect of nanobubbles on friction forces between hydrophobic surfaces in water

Micro- and nanoscale combined hierarchical polymer structures were fabricated by UV-assisted capillary force lithography. The method is based on the sequential application of engraved polymer molds with a UV-curable resin of polyurethane acrylate (PUA) followed by surface treatment with a trichloro(1H,1H,2H,2H-perfluorooctyl) silane in vapor phase. Two distinct wetting states were observed on these dual-roughness structures. One is “Cassie–Wenzel state” where a water droplet forms heterogeneous contact with microstructures and homogeneous contact with nanostructures. The other is “Cassie–Cassie state” where a droplet makes heterogeneous contact both with micro- and nanostructures. A simple thermodynamic model was developed to explain static contact angle, hysteresis, and wetting transition on dual-roughness structures.     

Partial carbonization of aramid fibers

Heat treatment of aramid fiber was conducted in the temperature range 300–710°C nominally for 10 and 30 s in both static air and flowing nitrogen atmosphere. Crystallinity, crystal orientation, and crystallite size were determined using x-ray diffraction. Fibers with a skin–core structure were produced at intermediate temperatures, as revealed by scanning electron microscopy of fibers after partial dissolution of the fiber in 95–98% sulfuric acid. The skin, which forms in both nitrogen and air, is amorphous and brittle. It is insoluble in sulfuric acid, suggesting it is a cross-linked polymer. Formation of the skin may be facilitated by the removal of an aggressive chemical species that forms during heat treatment. The species may diffuse out of the outer layer of the fiber, allowing it to cross-link. The molecular weight of the dissolved core, analyzed using intrinsic viscosity, decreases with increasing heat treatment temperature. The tenacity, modulus, elongation-to-break, and toughness of fibers with a skin–core structure decrease with heat treatment and the fiber loses its fibrillar character. Mechanical property reductions are greater in air than nitrogen. X-ray data are also consistent with the notion that oxygen assists attack of crystals at high temperatures. Scanning electron microscopy shows that fibers have become skin–core composites with quite different mechanical properties between the two regions. A fiber failure mechanism is proposed. © 1994 John Wiley & Sons, Inc.     

The ballistic performance of aramid based fabrics impregnated with multi-phase shear thickening fluids

Single-phase shear thickening fluids (STFs) have been extensively investigated in body protective applications. However, researchers do not have long-standing past experience of multi-phase STFs in protection. In the present work, multi-phase STFs were fabricated adding different amount of silicon carbide (SiC) additives into silica and polyethylene glycol (PEG) based suspensions. The thickening rheology of multi-phase STFs was investigated through rheological measurements. Ballistic impacts on multi-phase STF treated fabrics were carried out using lead core bullets with the impact speed of ∼330 m/s. Based on the results, multi-phase STFs improve the ballistic performance of high performance fabrics in comparison to single-phase STFs however, the mass efficiency of fabrics has a loss of performance for high velocity impact conditions.     

An anomalous dependence of the elastic modulus of aramid fibers on strain

The effect of tensile strain on the acoustic elastic modulus of Rusar, Armos, and SVM aramid fibers is studied. The elastic modulus is found to decrease during stretching to a strain on the order of 0.2%; at high strains, an increase in the modulus is observed. The effect is revealed both in a complex thread consisting of hundreds of parallel fibers and in a single fiber.     

Surface improvements of aramid fibers by physical treatments

The application of the low‐temperature plasma method, the excimer laser treatment method and the corona‐discharge method to aramid) were discussed, presenting an overview of current trends and developments in this area.     

The influence of electrostatic forces on protein adsorption

In this paper we investigate the importance of electrostatic double layer forces on the adsorption of human serum albumin by UV-ozone modified polystyrene. Electrostatic forces were measured between oxidized polystyrene surfaces and gold-coated atomic force microscope (AFM) probes in phosphate buffered saline (PBS) solutions. The variation in surface potential with surface oxygen concentration was measured. The observed force characteristics were found to agree with the theory of electrical double layer interaction under the assumption of constant potential. Chemically patterned polystyrene surfaces with adjacent 5 microm x 5 microm polar and non-polar domains have been studied by AFM before and after human serum albumin adsorption. A topographically flat surface is observed before protein adsorption indicating that the patterning process does not physically modify the surface. Friction force imaging clearly reveals the oxidation pattern with the polar domains being characterised by a higher relative friction compared to the non-polar, untreated domains. Far-field force imaging was performed on the patterned surface using the interleave AFM mode to produce two-dimensional plots of the distribution of electrostatic double-layer forces formed when the patterned polystyrene surfaces is immersed in PBS. Imaging of protein layers adsorbed onto the chemically patterned surfaces indicates that the electrostatic double-layer force was a significant driving force in the interaction of protein with the surface.     

Mechanical properties of interlayer hybrid textile composite materials based on modified aramid and UHMWPE fabrics

Aramid fibers and ultra-high molecular weight polyethylene (UHMWPE) fibers lack active surface functional groups, and the surface is smooth, limiting their practical application in textile composite materials. In this study, zinc oxide nanorods were used to grow on aramid fibers surfaces, and oxygen plasma followed by treatment with a silane coupling agent was used to modify UHMWPE fibers. The effects of surface modification on the surface morphology and composition, and mechanical properties of fibers and composites were investigated. The mechanical response of interlayer hybrid textile composite materials based on modified aramid and UHMWPE fabrics was examined. The results reveal that surface roughness, active surface functional groups, and wettability that can be controlled by treatment conditions and parameters are important for improving interface adhesion. In addition, the interlayer hybridization pattern as a result of using dissimilar layer materials and altering stacking sequence has a great impact on the mechanical behavior of hybrid textile composite materials.     

The influence of graphitization on the mechanical properties of carbon fibers

Summary Structure and preferred orientation of a series of carbon fibers with various degrees of graphitization have been studied by X-ray diffraction. The relationship between preferred orientation and Young's modulus differs from that reported for non-graphitic carbon fibers. The difference is explained by an increase of the shear compliance of the structural unit with increasing graphitization which results, for constant preferred orientation, in a decrease of the Young's modulus of the fiber with increasing graphitization.

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Zusammenfassung Struktur und Vorzugsorientierung einer Reihe von Kohlefasern mit verschiedenen Graphitierungsgraden wurden röntgenographisch untersucht. Die Beziehung zwischen Vorzugsorientierung und Elastizitätsmodul ist verschieden von der für nichtgraphitische Kohlefasern beobachteten. Der Unterschied wird durch eine Zunahme der Schernachgiebigkeit der Strukturbausteine mit zunehmender Graphitierung erklärt, die bei konstanter Vorzugsorientierung eine Verminderung des Elastizitätsmoduls der Fasern mit steigender Graphitierung zur Folge hat.

     

Investigation on influence of dimpled surfaces on heat transfer enhancement and friction factor in solar water heater

Journal of Thermal Analysis and Calorimetry - The solar water heating system efficiently converts available solar energy into useful thermal energy. The collector is the predominant unit of the...     

Study on interfacial adhesion of the aramid fibers/rubber matrix by grafting mercapto hyperbranched polysiloxane

In order to improve the interfacial adhesion between aramid fiber (AF) and rubber matrix, the mercapto hyperbranched polysiloxane (HPSi) was grafted onto the AFs via a novel in-situ growth strategy, which combines the formation of polydopamine (PDA) precursor layer and the co-dehydration condensation between (3-aminopropyl) trimethoxysilane and (3-mercaptopropyl) trimethoxysilane. Based on the results of characterizations including morphology observations and chemical structure analyses, the growth process of mercapto HPSi with the reaction time was investigated and discussed. The pull-out force tests of AFs/rubber composites, which were carried out by universal testing machine, showed that the modification strategy proposed in this study could increase the interfacial adhesion up to approximately 96.5%, and the key factor could be inferred to be the covalent interaction between mercapto groups and double bonds. It is also worth mentioning that the phenomenon of excessive surface modification will occur with the overlong growth time, which would result in the reduction of the interfacial adhesion.     

The influence of yarn parameters on thermo-physiological comfort of cotton woven fabrics

Journal of Thermal Analysis and Calorimetry - As the global temperature increases over time due to different environmental pollutions, apparel clothing with excellent thermo-physiological comfort...     

Temperature and stress effects in the creep of aramid fibers under transient moisture conditions and discussions on the mechanisms

In a previous study, a mechanosorptive phenomenon in poly(p-phenylene terephthalamide) fibers was reported. In this article, the mechanosorptive creep mechanism of aramid fibers and the temperature and stress influences on the mechanosorptive creep behavior of aramid fibers are addressed. Test results indicate that logarithmic creep rates and the mechanosorptive effects increase with temperature. The creep activation energies of the fibers tested are: 20 kJ/mole for the cyclic moisture condition, 4.4 kJ/mole for a high equilibrium moisture condition (RH = 95%), and 7.8 kJ/mole for a low equilibrium moisture condition (RH = 5%). Increase in stress may increase the logarithmic creep rates but may reduce the mechanosorptive effect. Aramid fibers contain hydrogen bonds between rodlike crystallites oriented at small angles relative to the fiber axis. Transient moisture conditions may cause slippage of hydrogen bonded elements and result in accelerated crystallite rotations due to breakage of hydrogen bonds, thus causing increases in logarithmic creep rate. The obtained activation energies and the reduction in fiber elastic compliance due to creep deformation support the proposed mechanisms. © 1992 John Wiley & Sons, Inc.     

pH dependence of friction forces between silica surfaces in solutions

The pH dependence of the friction between a silica particle and a silica wafer was investigated using lateral force microscopy. Measurements were done in the range of 3.6 < or = pH < or = 10.6 and the effect of high loading force was also examined. It is found that the friction is independent of the pH of solutions and increases linearly with the applied load, when the pH is between 3.6 and 8.6. On the other hand, once the pH is above 9.0, the friction becomes extremely small and the dependence on the applied load becomes nonlinear. It is postulated that this transition is due to the development of a gel layer composed of polymer-like segments of silicilic acid anchored on the surface; at the lower applied load, this layer acts as a boundary lubricant between the surfaces, but, at the higher applied load, the entanglements of these segments and more direct contact between two solid surfaces leads to the increase of the friction. The effects found here are expected to play an important role in elucidating the basic mechanism of the planarization process of silica wafers.     

The influence of ions on the structure of water

Journal of Structural Chemistry - The main features of the changes produced by ions in water may be summarized as follows. The influence of ions on the structure of the “free” water...     

Experimental investigation on the thermal protective performance of nonwoven fabrics made of high-performance fibers

In this study, the thermal protective performance of nonwoven fabrics made of Nomex (polyisophthaloyl metaphenylene diamine), PPS (polyphenylene sulfide), P84 (polyimide), and basalt fibers was investigated. The objective was to determine the influence of fiber type, thickness of fabric, and wet on the thermal protective performance of nonwoven fabric. The thermal resistances of different nonwoven fabrics were measured using a dry hot plate instrument, the basalt nonwoven fabrics had a highest thermal resistance in all fabric, and the thermal resistance of nonwoven fabric increased with the increase in thickness. The six nonwoven fabrics were exposed to a hot environment for a few minutes by using a self-designed apparatus. The test results showed that the nonwoven fabrics made with basalt fiber exhibited the best thermal protective performance, and the thermal protective abilities of nonwoven fabrics increased with fabric thickness. Interestingly, nonwoven fabrics with added water were found to be able to keep the fabric surface lower temperature compared to dry fabrics when exposed to a hot environment, indicating the excellent thermal protective performance of wet nonwoven fabrics.     

The influence of electrostatic forces on the structure and dynamics of molecular ionic liquids

The vast majority of molecular dynamics simulations are based on nonpolarizable force fields with fixed partial charges for all atoms. The traditional way to obtain these charges are quantum-mechanical calculations performed prior to simulation. Unfortunately, the set of the partial charges heavily relies on the method and the basis set used. Therefore, investigations of the influence of charge variation on simulation data are necessary in order to validate various charge sets. This paper elucidates the consequences of different charge sets on the structure and dynamics of the ionic liquid: 1-ethyl-3-methyl-imidazolium dicyanoamide. The structural features seem to be more or less independent of the partial charge set pointing to a dominance of shape force as modeled by Lennard-Jones parameters. This can be seen in the radial distribution and orientational correlation functions. The role of electrostatic forces comes in when studying dynamical properties. Here, significant deviations between different charge sets can be observed. Overall, dynamics seems to be governed by viscosity. In fact, all dynamical parameters presented in this work can be converted from one charge set to another by viscosity scaling.     

The effect of water immersion on the thermal degradation of cotton fibers

The decomposition behavior of cotton fibers is examined using thermogravimetric analysis. The effect of the test parameters on the thermal degradation of raw cotton fibers is determined. Focus is given to the influence of water immersion on the thermal behavior of cotton fibers. For less mature fibers a clear difference is noted between the degradation profiles of the water-immersed and untreated samples. On the contrary, only a small change is noted on the degradation profile for more mature fibers after water immersion. The maturity and variations in water-soluble content of the fiber are found to be important factors influencing the thermal behavior of raw cotton fibers. Inductively coupled plasma atomic emission spectrometry (ICP-AES) is used to underpin the effect of water immersion on cotton fibers. This improved understanding for the role of maturity and water soluble constituents in thermal degradation of cotton fibers may lead to develop routes that improve thermal stability and smoldering characteristics of cotton fibers as relevant for future applications.