Solidification structure and abrasion resistance of high chromium white irons

Superior abrasive wear resistance, combined with relatively low production costs, makes high Cr white cast irons (WCIs) particularly attractive for applications in the grinding, milling, and pumping apparatus used to process hard materials. Hypoeutectic, eutectic, and hypereutectic cast iron compositions, containing either 15 or 26 wt pct chromium, were studied with respect to the macrostructural transitions of the castings, solidification paths, and resulting microstructures when poured with varying superheats. Completely equiaxed macrostructures were produced in thick section castings with slightly hypereutectic compositions. High-stress abrasive wear tests were then performed on the various alloys to examine the influence of both macrostructure and microstructure on wear resistance. Results indicated that the alloys with a primarily austenitic matrix had a higher abrasion resistance than similar alloys with a pearlitic/bainitic matrix. Improvement in abrasion resistance was partially attributed to the ability of the austenite to transform to martensite at the wear surface during the abrasion process.     

Fluorescence study of film formation from PS/Al2O3 nanocomposites

Steady state fluorescence (SSF) and UV–vis (UVV) techniques were used to study the film formation behavior of pyrene (P) labeled polystyrene (PS) latex and Al₂O₃ (PS/Al₂O₃) composites depending on PS particles size and Al₂O₃ content. The close-packed arrays of PS spheres (SmPS: 203 nm; LgPS: 382 nm) templates on clean glass substrates were covered with various layers of Al₂O₃ by dip-coating method. Two different film series (SmPS/Al₂O₃ and LgPS/Al₂O₃) were prepared in various Al₂O₃ layer content. The film formation behavior of these composites were studied by annealing them at a temperature range of 100–250 °C and monitoring the scattered light intensity (I_sc), fluorescence intensity (I_P) from P and transmitted light intensity (I_tr) through the films after each annealing step. Optical results indicate that classical latex film formation was occurred for all Al₂O₃ content films and film formation process was unaffected by the Al₂O₃ content for both film series. Extraction of PS template produced highly ordered porous structures for high Al₂O₃ content in both film series. SEM images showed that the pore size and porosity could be easily tailored by varying the PS particle size and the Al₂O₃ content.     

Polymer/carbon nanotube composite film formation: A fluorescence study

In this study, the effect of multi‐walled Carbon nanotube (MWNT) on film formation behavior of Polystrene (PS) latex film was investigated by using steady state fluorescence technique. Films were prepared by mixing of pyrene (P)‐labeled PS latex with different amounts of MWNTs varying in the range between 0 and 20 wt%. After drying, MWNT containing films were separately annealed above glass transition temperature (T_g) of PS ranging from 100 to 270°C for 10 min. In order to monitor film formation behavior of PS/MWNT composites, Scattered light (I_s) and fluorescence intensities (I_P) from P were measured after each annealing step to monitor the stages of film formation. At 0–20 wt% range of MWNT content films, minimum film formation (T_o), void closure (T_v), and healing, (T_h) temperatures were determined. Void closure and interdiffusion stages were modeled and related activation energies were determined. It was observed that while void closure activation energies increased, backbone activation energies decreased as the percent of MWNT is increased in the composite films. POLYM. COMPOS., 35:817–826, 2014. © 2013 Society of Plastics Engineers     

Temperature dependence of oxygen diffusion into polymer/carbon nanotube composite films

This study examines the transport properties of polystyrene (PS)/multiwalled carbon nanotube (MWNT) composite films taking into consideration both MWNT composition and temperature, via fluorescence technique. Three different (3, 15, and 40 wt%) MWNT content films were prepared from PS/MWNT mixtures by annealing them at 170°C, above the glass transition temperature of PS for 10 min. The diffusivity of the PS/MWNT composite was determined by performing oxygen (O₂) diffusion measurements within a temperature range of 24 to 70°C for each film and pyrene (P) was used as the fluorescent probe. The diffusion coefficients (D) of oxygen were determined by the fluorescence quenching method assuming Fickian transport. Results indicated that D values are strongly dependent on both temperature and the MWNT content in the film and it was also observed that D coefficients obey Arrhenius behavior, from which diffusion energies were produced and increased along with increases of MWNT content. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Producing critical exponents from gelation for various photoinitiator concentrations; a photo differential scanning calorimetric study

Photoinitiated radical polymerization of an 80 wt% epoxy diacrylate (EA) and 20 wt% tripropyleneglycoldiacrylate (TPGDA) mixture with various 2-Mercaptothioxanthone (TX-SH) photoinitiator concentrations was studied by using photo-differential scanning calorimetric (Photo-DSC) technique. Photopolymerization reactions were carried out under the same conditions of temperature and light intensity. It was observed that all conversion curves during gelation at various photoinitiator concentration present nice sigmoidal behavior which suggests application of the percolation model. The critical time, where polymerization reaches the maximum rate (Rp_max) is called the glass transition point (t_g). The gel fraction exponents, β were produced from the conversion curves around t_g. The observed critical exponents were found to be around 0.55, predicting that the gel system obeys the percolation model. Rp_max and final conversion (C_s) values were found to be increased as the photoinitiator concentration was increased. On the other hand t_g values decreased as photoinitiator concentration was increased, indicating higher TX-SH concentration causes early glass transition during radical polymerization.     

Direct fluorescence technique to study evolution in transparency and crossing density at polymer–polymer interface during film formation from high-T latex particles

The Direct Fluorescence method was used to study the healing process during latex film formation above the glass transition temperature. The latex film was prepared from pyrene (P)-labeled poly(methyl methacrylate) (PMMA) particles. Heptane was used as a mixing agent. The steady-state fluorescence technique was employed to measure the density of polymer chains crossing the particle–particle interface. Various latex films with different latex content were used to study the transparency below and above the healing temperature. Crossing density was found to depend linearly on (time)^1/2, which was proposed by Prager and Tirrell. © 1996 John Wiley & Sons, Inc.     

In situ fluorescence experiments for real-time monitoring of annealed high-T latex film dissolution

A new technique, based on steady-state fluorescence measurements, is introduced for studying dissolution of polymer films. These films are formed from naphthalene and pyrene labeled poly(methyl methacrylate) (PMMA) latex particles, sterically stabilized by polyisobutylene. Diffusion of solvent (chloroform) into the annealed latex film was followed by desorption of polymer chains. Annealing was performed above T_g at various temperatures for 30-min time intervals. Desorption of pyrene labeled PMMA chains was monitored in real time by the pyrene fluorescence intensity change. Desorption coefficients were found to be between 1 and 4 × 10⁻¹⁰ cm²/s and two different dissolution mechanisms were detected. © 1996 John Wiley & Sons, Inc.     

Comparison of dissolution and mutual diffusion coefficients during dissolution of poly(methyl methacrylate) films

Steady state fluorescence measurements have been used for studying the dissolution of polymer films. These films are formed by free radical polymerization of methyl methacrylate (MMA) in which pyrene ( P_y ) was introduced as a fluorescence probe. Dissolution of poly(methyl methacrylate) (PMMA) films in chloroform–heptane mixtures were monitored in real-time by the P_y fluorescence intensity change. Dissolution coefficients D_d of P_y molecules were measured during dissolution of PMMA films, and found to be about 10⁻⁶ cm² s⁻¹. After dissolution, fluorescence quenching measurements were performed and the Stern–Volmer equation was employed to measure the mutual diffusion coefficients of heptane (D_h) and P_y (D_Py) molecules; these were found to be about 10⁻⁵ cm² s⁻¹. © 1999 Society of Chemical Industry     

Photon transmission technique for studying film formation from polystyrene latexes prepared by dispersion polymerization using various steric stabilizers

A photon-transmission method was used to monitor the evolution of transparency during film formation from various polystyrene (PS) particles which were produced using different steric stabilizers, that is, poly(acrylic acid) (PAA), poly(vinyl alcohol) (PVA), and polyvinylpyrrolidone (PVP). The latex films were prepared from PS particles at room temperature and annealed at elevated temperatures in various time intervals above the glass transition (T_g). To simulate the latex film-formation process, a Monte Carlo technique was performed for photon transmission through a rectangular lattice. The number of transmitted (N_tr) photons were calculated as a function of particle–particle interfaces that disappeared. The increase in the transmitted photon intensity (I_tr) was attributed to the increase in the number of interfaces that disappeared. The Prager–Tirrell (PT) model was employed to interpret the increase in crossing density at the junction surface. The backbone activation energy (ΔE) was measured and found to be around 120 kcal mol⁻¹ for a diffusing polymer chain across the junction surface for all PS latex films. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1257–1267, 1998     

Effect of molecular weight on latex film formation: Photon transmission study

A UV-visible (UVV) technique was used to monitor the evolution of transparency during film formation from hard latex particles. Two different latex films were prepared from particles with high and low molecular weight (HM and LM) poly(methyl methacrylate) (PMMA) separately and annealed at elevated temperatures in various time intervals above the glass transition temperature (T_g). In both films, a continuous increase in the transmitted photon intensity (I_tr) was observed above 160°C as the annealing temperature was increased. However, the reflected photon intensity (I_rf) first decreased and then increased by showing a minimum in the same temperature range as the annealing temperature was increased. The increase in the transmitted photon intensity (I_tr) is attributed to increase in the “crossing density” at the junction surface. The activation energies for back-and-forth motion (ΔE_tr) were measured and found to be around 35 and 25 kcal/mol for the reptating polymer chain across the junction surface in the LM and HM films, respectively. The decrease in I_rf was explained by the void-closure mechanism, and the increase in the I_rf above 160°C was again attributed to the increase in the crossing density at the junction surface. Back-and-forth activation energies (ΔE_rf) were measured to be around 47 and 18 kcal/mol and the void-closure constants (B) were found to be around 24 × 10³ and 12 × 10³ K for the LM and HM film samples, respectively. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 339–351, 1998