Solid PTFE Research Articles

OPTIMIZATION OF TRIBOLOGICAL PROPERTIES OF AN EPOXY HYBRID POLYMER COMPOSITE REINFORCED WITH ZrB2 AND PTFE PARTICLES USING RESPONSE SURFACE METHODOLOGY FOR HIGH-TEMPERATURE TRIBOLOGICAL APPLICATIONS

Hybrid PTFE/epoxy composites are widely used as materials for self-lubricating spherical bearing which are used in a high-temperature environment. In the present work, zirconium diboride (ZrB2) particles are incorporated to enhance high-temperature tribological properties of PTFE/epoxy composites. Pin on disc experiment is conducted with the aid of design of experiments (DOE) using central composite-response surface methodology (RSM). Under a load of 40 N and 1.25 m/s sliding speed, the optimum content 5.95 vol% of PTFE and 5.05 vol% of ZrB2, yields an ultralow coefficient of friction (COF) in conjunction with a low wear rate of the composite. The addition of ultra-high-temperature ceramic ZrB2 particles and solid lubricant PTFE is found to enhance the thermal conductivity and improve the heat transfer thereby reducing contact temperature. The use of optimum composition of the composite is capable of reducing the wear rate and high local temperature due to friction, implying its potential use as a self-lubricating spherical bearing liner material.

Microstructure and Tribological Properties of TPU/Fluoropolymer Composites

Abstract A network of poly(tetrafluoroethylene) (PTFE) microfibers in a thermoplastic polyurethane (TPU) was prepared by melt mixing the TPU with solid PTFE particles. The effect of rotor speed on ...

Microstructure and Tribological Properties of TPU/Fluoropolymer Composites

Abstract A network of poly(tetrafluoroethylene) (PTFE) microfibers in a thermoplastic polyurethane (TPU) was prepared by melt mixing the TPU with solid PTFE particles. The effect of rotor speed on the fiber dimensions was investigated. Higher shear stress was found to be the critical parameter for producing thinner PTFE fibers, rather than the shear rate imposed by the mixer. Shear stress transfer from the melt to the PTFE crystal results in solid phase plastic deformation, and the efficiency of the deformation depends on the shear stress in the matrix. All of the PTFE fiber/TPU composites show lower coefficients of friction compared with the neat TPU. The magnitude of the coefficient of friction was found to correlate with the interfacial area between PTFE and TPU generated by the microfiber network. However, for macroscale PTFE agglomerates, the reduction in the coefficient of friction is mostly affected by the uneven distribution of PTFE in the bulk and on the molded part surface.

Kinematic analysis of a lightweight periodic dielectric structure of pearls for RF coaxial power cables for space applications

The desire to reduce the mass per unit length and to increase phase stability of coaxial radio-frequency (RF) power cables for space application motivates a new design idea, namely to replace solid dielectric with a periodic chain of hollow pearls. The design of the dielectric pearls must allow for bending flexibility of the cable even if they are made from a stiff material such as silicon glass. An important requirement of RF power cables for space applications is their phase stability, which is influenced by the material-dielectric-constant tolerance over a large temperature range as well as by changes in geometry. This paper presents an original closed-form model based on rigid-body motion to predict the kinematic response of dielectric pearls to the bending of the cable. Particularly, the model maps the eccentricity of the inner and outer conductors with respect to each other and the axial strain of the bent cable along its centreline. The model results indicate that mass reductions of the periodic hollow-pearl design over conventional solid PTFE dielectrics are significant.

Enhanced performance of an expanded polytetrafluoroethylene-based triboelectric nanogenerator for energy harvesting

Triboelectric nanogenerators (TENGs) are a well-known energy supply technology. Their output performance is highly important given their intended use in real-world applications. Expanded polytetrafluoroethylene (ePTFE) membranes, which are lightweight, fibrillated, microporous membranes formed by the expansion and stretching of PTFE, possess excellent flexibility and elasticity, unlike the traditional PTFE films that they are made from. Therefore, ePTFE membranes have tremendous potential to be a triboelectric material for energy harvesting. As the most important parameter in ePTFE processing, the uniaxial expansion ratio determines the final morphology of the ePTFE and, thus, the output performance of TENGs. Hence, it requires systematic study. Here, folded-paper TENGs consisting of ePTFE membranes with different expansion ratios and nylon fabrics as two paired tribolayers (i.e., ePTFE/Nylon-TENGs) were fabricated. A systematic study concerning the effect of the expansion ratio on the output performance of the ePTFE/Nylon-TENGs was carried out. The output performance of the ePTFE/Nylon TENGs with expansion ratios ranging from 0 to 400% were measured and compared. The results showed that the output electrical signals of the ePTFE/Nylon-TENGs first increased with the expansion ratio and then decreased steadily, with an optimum performance at an expansion ratio of 100%. In addition, the output performance of the TENGs with solid PTFE was approximately the same as that of the ePTFE/Nylon-TENGs with an expansion ratio of 300%. The ePTFE/Nylon-TENGs had excellent and stable performances and yielded a maximum power density of 1.01 mW/cm2, with an effective area of 1 cm2 and a load resistance of 1 MΩ. This work suggests that ePTFE is an effective material for improving the output performance of ePTFE-based TENGs.

High load capacity with ionic liquid-lubricated tribological system

Engineering polymers with high glass transition temperature have been widely used in dynamic friction systems by oil or solid lubrication. However, in high-load systems, oil lubrication is less efficient due to the viscosity decrease at higher temperatures induced by friction heat. [Bmim][PF6] ionic liquid was used and compared with traditional L-HM46 oil and solid PTFE. Taking advantage of high [Bmim][PF6] viscosity, strong steel-[Bmim][PF6] but poor PEEK-[Bmim][PF6] interaction, the [Bmim][PF6] lubricated PEEK/steel slide falls in hydrodynamic lubrication and elastohydrodynamic lubrication region under 150–1500N. While the oil and PTFE both failed to lubricate under 800N.

Theoretical investigation of transient elastohydrodynamic lubrication with newtonian solid–liquid lubricants

AbstractThis paper reports a theoretical investigation of transient elastohydrodynamic lubrication of a line contact. A time‐dependent Reynolds equation and elasticity equations for compressible solid–liquid lubricants were solved using finite volume and multigrid techniques. The lubricants used were mineral oils mixed with very small solid particles, MoS2 and PTFE which can be treated as Newtonian fluids. The two surfaces were initially at rest and in contact. The transient oil film pressure and oil film thickness were calculated numerically. This simulation showed the significant effects of solid particles on the lubrication characteristics. Copyright © 2006 John Wiley & Sons, Ltd.

Theoretical investigation of transient elastohydrodynamic lubrication with newtonian solid-liquid lubricants

This paper reports a theoretical investigation of transient elastohydrodynamic lubrication of a line contact. A time-dependent Reynolds equation and elasticity equations for compressible solid-liquid lubricants were solved using finite volume and multigrid techniques. The lubricants used were mineral oils mixed with very small solid particles, MoS2 and PTFE, which can be treated as Newtonian fluids. The two surfaces were initially at rest and in contact. The transient oil film pressure and oil film thickness were calculated numerically. This simulation showed the significant effects of solid particles on the lubrication characteristics.

Co-adsorption of quaternary ammonium compounds—nonionic surfactants on solid–liquid interface

Adsorption of mixed surfactants on the solid substrates has received scant attention in the past. However, in recent times, there have been a few investigations, which have been overviewed. Experimental results of co-adsorption and parallel zeta potential studies using mixed quaternary ammonium compounds CTAB, TTAB and nonyl phenyl ethoxylates (NP13, NP20, NP30) have been reported in this presentation. Three solid substrates, quartz, polystyrene and PTFE were chosen for investigation. ‘S’ shaped adsorption isotherms were obtained in case of all the three substrates. The presence of nonionics had varied effect on the adsorption of CTAB/TTAB depending on concentration. The zeta potential studies, in general, support the adsorption behaviour. However, the change on the solid substrate is influenced predominantly by the positively charged CTAB. Presence of premicellar aggregates on the solid surface is indicated. The nature of solid substrate appears to be of secondary importance.

A study of the role of solid lubricant and fibrous reinforcement in modifying the wear performance of polyethersulphone

AbstractHigh‐temperature polymers are generally preferred for those tribology applications where cost is secondary and performance is the primary consideration. Since frictional heat dissipation limits the usefulness of polymers because of their poor thermal conductivity, high‐temperature polymers are preferred in applications which have harsh operating conditions. In this paper, a high‐temperature polymer, polyethersulphone (PES), was selected for some adhesive wear studies, along with two PES composites containing 18% glass‐fibre (GF) reinforcement and two solid lubricants, i.e., PTFE and MoS2 (2% each). Adhesive wear studies of these materials on two pin‐on‐disc machines indicated that neat PES was not a good tribo‐material. However, incorporation of GF and solid lubricants enhanced the wear performance by an order of two. PTFE was found to behave better than MoS2. However, after long sliding duration both the lubricants performed almost equally well. The topography of the surface of the pins and the disc was studied using SEM to investigate the wear mechanisms.

The tribological characteristics of JS material under lubrication of oil

A material composed of a steel backing, a sintered porous bronze middle layer and a layer of reinforced PTFE, which is named JS material, was prepared. The friction, wear and limiting PV values of this material under dry friction as well as the lubrication of number 20 mechanical oil were studied using a MPV-1500 friction tester. The worn surface of JS material and the transfer film formed on the counterface of carbon steel were investigated using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results show that the friction, wear and limiting PV values of JS material can be greatly improved with the lubrication of oil. The results of SEM and EPMA analyses indicate that, under dry friction conditions, the solid lubricant PTFE and Pb easily transfer to the steel counterface and results in the reduction of friction and wear; while under the lubrication of oil, little transference of PTFE and Pb to the steel surface occurs and very small friction and wear are achieved. Analyses of frictional surfaces also suggest that the Pb filler gets enrichment on the rubbing surfaces, which is beneficial in increasing the adhesion of the transfer film with the steel surface.

Formation of macromolecular iminoxyl radicals during radiolysis of fluorinated polymers in nitrogen dioxide as medium

Nitrogen-containing radicals formed in solid PTFE and in a copolymer of tetrafluoroethylene with hexafluoropropylene under the action of γ-irradiation in the presence of NO are studied by an ESR method. It is concluded from the results of analyzing the ESR spectra of polymer powders and oriented films that iminoxyl radicals are formed under these conditions. In the case of PTFE the superfine interaction constants are determined together with the nitrogen nucleus and g-tensor components. The isotropic parameters are found for the copolymer of tetrafluoroethylene with hexafluoropropylene. A mechanism is proposed for the production of iminoxyl radicals, and the effect of molecular dynamics on the method of formation of nitrogen-containing macroradicals is discussed.

Surface free energies of oral streptococci and their adhesion to solids

The adhesion of 3 strains of oral streptococci from a buffered suspension onto 3 different solid substrata was studied. Representative strains of streptococci were selected on the basis of their surface free energy (γb), namely Streptococcus mitis L1 (γb = 37 mJ·m−2), Streptococcus sanguis CH3 (95 mJ·m−2) and Streptococcus mutans NS (117 mJ·m−2). Solid substrata were also selected on basis of their surface free energy (γs), and included polytetrafluorethylene (γs = 20 mJ·m−2), polymethylmethacrylate (53 mJ·m−2) and glass (109 mJ·m−2). Bacterial adhesion was measured as the number of bacteria adhering per cm2 at equilibrium. Equilibrium was usually obtained within 20 min. S. sanguis CH3, having an intermediate surface free energy did not show a clear preference for any of the 3 solids. S. mitis L1, however, the lowest surface free energy strain, adhered in highest numbers to the low energy solid PTFE, whereas the highest γb strain, S. mutans NS, adhered in highest numbers to the highest γs solid, glass. Calculation of the interfacial free energy of adhesion (ΔFadh) for each bacterial strain showed that this parameter was predictive of bacterial adhesion to solid substrata.

Copolymerizations initiated by mechano‐radicals on particle surfaces of poly(tetrafluoroethylene)

AbstractMechanical fracture of solid poly(tetrafluoroethylene) (PTFE) produces main‐chain scissions of the polymer, and free radicals, called mechano‐radicals, are trapped on fresh surfaces generated by the fracture. These radical conversions were verified by ESR observation when several monomers—methylmethacrylate, vinylacetate, and ethylene—were brought into contact with these mechano‐radical, and the copolymerizations of PTFE with these monomers were demonstrated. It was found that a new surface property, wettability of water, was added to the solid PTFE by the copolymerization with vinyl acetate, although the bulk nature of PTFE was not changed.

Oil bath lubrication: Stringer, L. S. Industrial Lubrication, Vol 20, No 2 (February 1968) p44

Engineering polymers with high glass transition temperature have been widely used in dynamic friction systems by oil or solid lubrication. However, in high-load systems, oil lubrication is less efficient due to the viscosity decrease at higher temperatures induced by friction heat. [Bmim][PF6] ionic liquid was used and compared with traditional L-HM46 oil and solid PTFE. Taking advantage of high [Bmim][PF6] viscosity, strong steel-[Bmim][PF6] but poor PEEK-[Bmim][PF6] interaction, the [Bmim][PF6] lubricated PEEK/steel slide falls in hydrodynamic lubrication and elastohydrodynamic lubrication region under 150–1500 N. While the oil and PTFE both failed to lubricate under 800 N.

Fluid film lubrication of rubber—an interferometric study: Roberts, A. D. and Tabor, D. Wear, Vol 11, No 2 (February 1968) p163

Engineering polymers with high glass transition temperature have been widely used in dynamic friction systems by oil or solid lubrication. However, in high-load systems, oil lubrication is less efficient due to the viscosity decrease at higher temperatures induced by friction heat. [Bmim][PF6] ionic liquid was used and compared with traditional L-HM46 oil and solid PTFE. Taking advantage of high [Bmim][PF6] viscosity, strong steel-[Bmim][PF6] but poor PEEK-[Bmim][PF6] interaction, the [Bmim][PF6] lubricated PEEK/steel slide falls in hydrodynamic lubrication and elastohydrodynamic lubrication region under 150–1500 N. While the oil and PTFE both failed to lubricate under 800 N.