H. X. Zhang

Studies of the Influence of Graphite and MoS2 on the Tribological Behaviors of Hybrid PTFE/Nomex Fabric Composite

Hybrid polytetrafluoroethylene (PTFE)/Nomex (DuPont, USA) composite specimens were prepared with graphite and MoS2 as fillers. The friction and wear tests were conducted under dry sliding conditions at a speed of 0.26 m/s under various loads and environmental temperatures. The results showed that the incorporation of graphite was effective in reducing the wear of the hybrid PTFE/Nomex fabric composite, whereas the MoS2 filler was unfavorable for improving the wear resistance. X-ray photoelectron spectroscopy (XPS) results revealed that MoS2 was oxidized into MoO3 during sliding. The absence of the layered structure of MoO3 resulted in a higher wear rate of fabric composite. Differential scanning calorimetry (DSC) analysis of unfilled and graphite-filled composites and their debris indicated that graphite increased the thermal stability of the unfilled composite and therefore enhanced the wear resistance.

A Study on the Sliding Wear of Hybrid PTFE/Kevlar Fabric/Phenolic Composites Filled with Nanoparticles of TiO2 and SiO2

TiO2 and SiO2 nanoparticles were introduced into hybrid polytetrafluoroethylene (PTFE)/Kevlar fabric/phenolic composites. The results showed the incorporation of TiO2 nanoparticles can reduce the wear rate of the fabric/phenolic composite at elevated temperatures, although the wear of hybrid PTFE/Kevlar fabric/phenolic composite did not change much when TiO2 or SiO2 nanoparticles were used as filler. The wear behavior was explained in terms of morphology of transfer films and worn surfaces. There was a good correlation between the morphology of transfer film and wear results.

Friction and Wear of Hybrid PTFE/Kevlar Fabric Composite Filled with ZnO Nanoparticles Sliding against Steel, Copper, and Aluminum

ZnO nanoparticles were incorporated into phenolic resin and the effect of the ZnO content on tribological properties of hybrid polytetrafluoroethylene (PTFE)/Kevlar fabric/phenolic composite was investigated. Fabric composite filled with 5 wt.% ZnO nanoparticles sliding against steel, copper, or aluminum was investigated in detail. Friction and wear tests showed that fabric composite/steel exhibited lower friction coefficient and wear rate with varied loads and speeds. It is believed that the coherent transfer film and tribochemical reactions involved in fabric composite/steel contributed to the reduced friction coefficient and wear rate of the fabric composite.

Surface Modification of CuS Nanoparticles and their Effect on the Tribological Properties of Hybrid PTFE/Kevlar Fabric/Phenolic Composite

Oleic acid-modified CuS nanoparticles were chemically synthesized. The tribological properties of surface modified and unmodified CuS nanoparticles as additives in the hybrid polytetrafluoroethylene (PTFE)/Kevlar fabric/phenolic composite were investigated in detail. The experimental results indicated that the incorporation of modified CuS nanoparticles improved the antiwear ability of the fabric/phenolic composite at varied loads and environmental temperatures. The reasons for the enhanced wear properties of the fabric/phenolic composite filled with surface-modified CuS nanoparticles were discussed based on the results of scanning electron microscopy and Fourier transform infrared spectroscopy.

In situ charge neutralization on governing particle coagulation nucleation and size distribution in macroemulsion polymerization

Fabricating monodispersed polymer latex particles with ∼300 nm size at high monomer concentrations by batch macroemulsion polymerization remains significantly challenging because of latex stability. In this study, we developed a novel approach based on in situ charge neutralization to prepare 40 wt% solid content latex containing monodispersed sub-300 nm latex particles. The cationic initiator 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AIBA) was used to induce in situ charge neutralization in the particle nucleation period and shield the negative charges of surfactant sodium dodecyl sulfate molecules to further reduce the electrostatic repulsion between primary particles, resulting in primary particle coagulation nucleation. The primary particle coagulation promoted particle number decreased to ∼1016 L−1, and the average particle size increased to ∼100 nm at a very low monomer conversion (<0.12). With increasing AIBA concentrations from 0.6 and 0.8 to 1.0 wt% (the molar ratio of AIBA/SDS is 0.64, 0.85 and 1.06, respective), the average particle size of the latex ultimately attained from 170.6 and 221.7 to 288.0 nm, respectively. Moreover, the addition of an electrolyte and copolymerization composition also governed the particle coagulation extent and affected the particle size distribution of the ultimate latex particles. To the best of our knowledge, this is the simplest, most efficient, and inexpensive approach to prepare large sized, monodispersed latex particles.

A novel approach to prepare large-scale and narrow-dispersed latex particles by emulsion polymerization based on particle coagulation mechanism

Abstract In order to understand the mechanism of narrow particle size distribution of the final latex during particle coagulation, a series of experiments were performed to investigate the effect of polymer nature on particle coagulation capability. In particular, thermodynamics and kinetics in aqueous phase were considered to illustrate the detail process of particle coagulation. The final particle size decreased with the increasing side chain length of alkyl methacrylate from 181.5 nm in MMA to 131.6 nm in EMA, 119.3 nm in PMA, and 115.1 nm in BMA, indicating that the particle coagulation capability was proportional to the hydrophilicity of polymer. With increasing polymer hydrophilicity, the affinity between surfactant molecules and particle surface decreased, thus enhancing the particle coagulation capability. Moreover, the critical length of oligomer radical also increased with increasing hydrophilicity and the efficiency of radical capture decreased, thus increasing the saturation of monomer concentration in the inner part of particle, promoting particle coagulation. Combining these results and the La Mer Diagram, a novel approach was developed to prepare large-scale, narrow-dispersed, and high solid content polymer latex based on particle coagulation mechanism. Three criteria, namely, rapid nucleation, fast coagulation, and a long growth period, should be met to produce latex with a narrow particle size distribution.