Inviscid Melt Spinning of Alumina Fibers: Chemical Jet Stabilization

Alumina-calcia fibers with >50% alumina and 100% alumina fibers cannot be drawn from the melt, or conventionally melt spun, because the viscosities are too low. They can, however, be spun by inviscid melt spinning, an experimental process whereby a molten jet with a viscosity of <1 Pa.s is ejected into propane, a chemically reactive medium. The pyrolytic decomposition of propane stabilizes the molten jet. The consolidated fiber that results has a carbon-rich skin and usually, i.e., under most process conditions, but not always, a black carbon sheath. This paper identifies the chemistry and morphology of both skin and sheath by depth profile analysis, evaluates potential jet stabilization mechanisms, and concludes that the operative mechanism is rheology-dominated. Incorporation of particulate carbon in the skin of the jet increases its surface viscosity, prevents breakup into Rayleigh waves and droplets, and facilitates melt spinning of continuous filaments.     

Technique for estimating the viscosity of molten silica glass on the kinetics of the collapse of the glass capillary

The kinetics of the collapse of the melt glass capillary has been analyzed. The relation coupling the collapse time with the initial parameters—viscosity of molten glass, surface tension, and geometrical sizes of the cross section of the collapsed capillary—has been derived. A technique for estimating the viscosity of molten glass based on measuring the capillary collapse time has been elaborated. This technique has been tested on silica capillaries collapsed in a high-temperature oven of a drawing tower in the temperature interval of 1700–2050°C.     

Viscosity of glass-forming melt at the bottom of high-level waste melter-feed cold caps: Effects of temperature and incorporation of solid components

During the final stages of conversion of melter feed (glass batch) to molten glass, the glass-forming melt becomes a continuous liquid phase that encapsulates dissolving solid particles and gas bubbles that produce primary foam at the bottom of the cold cap (the reacting melter feed in an electric glass-melting furnace). The glass-forming melt viscosity plays a dominant role in primary foam formation, stability, and eventual collapse, thus affecting the rate of melting (the glass production rate per cold-cap area). We have traced the glass-forming melt viscosity during the final stages of feed-to-glass conversion as it changes in response to changing temperature and composition (resulting from dissolving solid particles). For this study, we used high-level waste melter feeds—taking advantage of the large amount of data available to us—and a variety of experimental techniques (feed expansion test, evolved gas analysis, thermogravimetric analyzer-differential scanning calorimetry, X-ray diffraction, and viscometer). Starting with a relatively low value at the moment when the melt connects, melt viscosity reached maximum within the primary foam layer and then decreased to its final melter operating temperature value. At the cold-cap bottom—the boundary between the primary foam layer and the thermal boundary layer—where physicochemical reactions of a melter feed influence the driving force of the heat transfer from the melt to the cold cap, the melt viscosity affects the rate of melting predominantly through its effect on the temperature at which primary foam is collapsing.     

Energy Budget in Atomization

An equation of mechanical energy balance in a liquid jet atomizing in an ambient gas is derived. The time rate of change of kinetic energy of the fluctuating disturbance in a given volume of the jet is shown to be equal to the sum of four types of works done per unit time on the jet and the energy dissipation rate through the agent of viscosity. The four types of works involved are the work by the surface tension, the work by the pressure fluctuation in the ambient gas, the work done by the fluctuating pressure in the jet, and the work by the viscous stress. Numerical results obtained for a wide range of relevant parameters are used to show that the surface tension work is negative in jet atomization. This is contrary to the situation in the breakup of an ink jet for which the surface tension work is positive, and thus the breakup is due to capillary pinching. It is shown that the work by fluctuating gas pressure is responsible for the atomization process, since the pressure work term is the dominant positive term in the energy budget of the jet atomization.     

Steady melting of rectangular thermoplastic bars induced by hot contacting surfaces

The steady melting of rectangular thermoplastic bars in contact with hot surfaces is analyzed by solving a simplified set of the momentum and energy balance equations, assuming a temperature and shear-rate dependent melt viscosity. A numerical model is developed for predicting the flow field and the temperature distribution in the solid and molten regions of the bar and the location of the solid/melt interface. Computer simulations show that the steady melting rate of the thermoplastic solid is mainly affected by the temperature sensitivity of the melt viscosity, by the pressure applied on the end of the bar, and by a balance between heat conduction and the convection of colder material into the molten region. For the amorphous and semicrystalline polymers considered, heat convection in the outflow direction of the molten material, viscous dissipation, and shear-thinning of the melt viscosity have a much smaller effect on the melting process. These results provide an insight into conduction-induced melting with forced melt removal caused by pressure-induced flow; they also provide a basis for developing a transient model for the hot-tool welding process.     

Models and Experiments for Sodium Evaporation From Sodium-Containing Silicate Melts

Universal simulation models based on (a) validated mass transfer relations and (b) thermodynamic modeling procedures for glass melts are developed to predict the evaporation rates of volatile species from a large range of glass melt compositions. Depending on the glass composition, temperature of the surface of the melt, local composition of the atmosphere, exposure time of a melt layer to the combustion atmosphere, and local gas velocities above the glass melt surface, the evaporation rates of volatile species can be estimated. Laboratory-scale transpiration evaporation experiments have been used to study evaporation kinetics, to derive mass transfer relations, as well as to validate the sodium evaporation modeling results for sodium-silicate melts as well as for soda-lime-silicate melts. In these investigations, the molten sodium-silicate and soda-lime-silicate melts are exposed to atmospheres of flowing gases with controlled composition and gas flow rates. In a humid atmosphere for example, sodium mainly evaporates as NaOH. From the measured NaOH evaporation rates and the mass transfer relations, the NaOH vapor pressures in equilibrium with the molten glass at prevalent temperature and furnace atmosphere composition were derived. The NaOH vapor pressures are assumed to be in equilibrium with the glass melt composition at the surface of the melt. During the evaporation test, the Na₂O surface composition will change due to depletion. This leads to equilibrium vapor pressures decreasing with time, reflecting the changing chemical activity at the glass melt surface. The results of evaporation tests for sodium-disilicate and soda-lime-silicate glass melts are shown. Chemical activities derived from these measurements are compared with the results of thermodynamic modeling, using a method based on a glass melt from ideal mixtures of associate (stoichiometric) species of structural compounds with known thermodynamic properties.     

瓶罐玻璃生产关键工艺性能测量方法与应用

综述了瓶罐玻璃生产制备过程中的关键工艺性能的测量表征方法和适用的仪器设备。涉及的关键工艺性能主要包括熔解特性、熔体特性和温黏特性等。熔解特性的分析包括原料成分分析、粒度分析、熔化均匀性分析、澄清分析等;熔体特性的测量包括表面张力测量、高温电阻率测量、羟基含量测量等;温黏特性测量主要包括高温黏度(熔融温度T_m、工作点T_w、液相线T_L)测量、中温黏度(软化点T_s)测量、低温黏度(膨胀软化点T_d、退火点T_a、转变点T_g和应变点T_st等)测量。上述工艺性能精确测量和表征,对制备高质量瓶罐玻璃制品具有重要意义。     

Surface Energy and Crystallization Phenomena of Ammonium Dinitramide

Ammonium dinitramide (ADN) was characterized during recrystallization from the melt. The surface tension of molten ADN at 97 °C was measured to be 89 mN/m. The wetting angles between molten ADN and different solid surfaces (polytetrafluoroethylene, glass, steel, and aluminum) were determined. The wettability depends on the surface tension of molten ADN, the free surface energy of the solid surfaces and the interfacial tension between the solid and liquid. Observations of the recrystallization behavior of molten ADN showed that nucleation does not occur, even at super cooling rates of 70 K. Crystallization can be initiated by the application of seed crystals.     

Two phase (air-molten carbonate salt) flow characteristics in a molten salt oxidation reactor

Molten salt oxidation process is one of the most promising alternatives to incineration that can be used to effectively destroy the organic components of mixed and hazardous wastes. To detect the flow characteristics of the molten salt oxidation process (air-molten carbonate salt two-phase flow), differential pressure fluctuation signals from a molten salt oxidation process have been analyzed by adopting the stochastic methods. Effects of the input air flow rate (0.05–0.22 m/sec) and the molten salt temperature (870–970 °C) on the phase holdup and flow characteristics are studied. The gas holdup increases with an increasing molten salt temperature due to the decrease of the viscosity and surface tension of the molten carbonate salt. It is found that a stochastic analysis of the differential pressure signals enables us to obtain the flow characteristics in the molten salt oxidation process. The experimentally obtained gas holdup data in the molten salt reactor were well described and characterized by means of the drift-flux model. This work was presented at the 7th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Melt rheological behavior of intimately mixed short sisal–glass hybrid fiber‐reinforced low‐density polyethylene composites. I. Untreated fibers

The melt rheological behavior of intimately mixed short sisal–glass hybrid fiber‐reinforced low‐density polyethylene composites was studied with an Instron capillary rheometer. The variation of melt viscosity with shear rate and shear stress at different temperatures was studied. The effect of relative composition of component fibers on the overall rheological behavior also was examined. A temperature range of 130 to 150°C and shear rate of 16.4 to 5470 s^?1 were chosen for the analysis. The melt viscosity of the hybrid composite increased with increase in the volume fraction of glass fibers and reached a maximum for the composite containing glass fiber alone. Also, experimental viscosity values of hybrid composites were in good agreement with the theoretical values calculated using the additive rule of hybrid mixtures, except at low volume fractions of glass fibers. Master curves were plotted by superpositioning shear stress and temperature results. The breakage of fibers during the extrusion process, estimated by optical microscopy, was higher for glass fiber than sisal fiber. The surface morphology of the extrudates was analyzed by optical and scanning electron microscopy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 432–442, 2003     

玻璃吹制成型熔体与模具传热耦合模拟

玻璃吹制成型过程中熔体与模具接触时间短,热交换迅速、剧烈,同时玻璃的黏度对温度极其敏感,微小的温度波动将会引起黏度的剧烈改变,并最终决定制品的厚度分布,因此熔体与模具传热的耦合求解是十分必要的。鉴于此,本文在熔体与模具接触面上引入了界面单元来处理接触面热阻区的热传递问题,建立了熔体流动与模具温度场耦合模拟的控制方程,完成了算法编制,实现了熔体流动与模具温度场的耦合模拟。算例证明,与耦合传热算法相比迭代结果不足以满足吹制成型对温度场准确性的要求;通过模拟与实验对比,在连续生产条件下模具绝大部分的温度保持稳定,但与熔体接触的型腔壁的温度却有大幅的周期性变化;模拟的最终产品壁厚较准确地反映了产品的实际壁厚分布,准确度达到88%以上。     

Effects of heating rate,quartz particle size,viscosity, and form of glass additives on high‐level waste melter feed volume expansion

Nuclear waste can be vitrified by mixing it with glass‐forming and ‐modifying additives. The resulting feed is charged into an electric glass melter. To comprehend melting behavior of a high‐alumina melter feed, we monitored the volume expansion of pellets in response to heating at different heating rates. The feeds were prepared with different particle sizes of quartz (the major additive component) and with varied silica‐to‐fluxes ratio to investigate the glass melt viscosity effects. Also, we used additional melter feeds with additives premelted into glass frit. The volume of pellets was nearly constant at temperatures <600°C. After a short period of volume shrinkage at ~600°C‐700°C, foam generation produced massive volume expansion. The low heat conductivity of foam hinders the transfer of heat from molten glass to the reacting feed. The extent of foaming increased with faster heating and higher melt viscosity, and decreased with increasing size of quartz particles and fritting of the additives. Volume expansion data are needed for the mathematical modeling of the cold cap.     

Non-contacting tension measurement for molten films

A novel air-nozzle system was developed to perform non-contacting measurement of film tension in molten films. The measurement is based on correlation between the film tension and the pressure drop across the gap from the nozzle tip to the film. Films with higher tension exhibit less deflection by the impinging air jet and, consequently, produce a higher pressure drop across a smaller gap. The correlation between film tension and pressure drop was obtained with solid films at room temperature. A procedure was developed to extend the correlation to higher temperatures. The correlation was also found to agree with film tension versus pressure drop data obtained from specially designed molten film experiments. The capability of the system was demonstrated in the film casting of a linear low-density polyethylene melt and a low-density polyethylene melt. Tensions in the molten films were measured at various values of draw ratio and extrusion speed.     

Synthesis of Glass Nanofibers Using Femtosecond Laser Radiation Under Ambient Condition

We report the unique growth of nanofibers in silica and borosilicate glass using femtosecond laser radiation at 8 MHz repetition rate and a pulse width of 214 fs in air at atmospheric pressure. The nanofibers are grown perpendicular to the substrate surface from the molten material in laser-drilled microvias where they intertwine and bundle up above the surface. The fibers are few tens of nanometers in thickness and up to several millimeters in length. Further, it is found that at some places nanoparticles are attached to the fiber surface along its length. Nanofiber growth is explained by the process of nanojets formed in the molten liquid due to pressure gradient induced from the laser pulses and subsequently drawn into fibers by the intense plasma pressure. The attachment of nanoparticles is due to the condensation of vapor in the plasma.     

ZrO2对提纯粗ZrCl4用NaCl-KCl熔盐体系物化性质影响

工业上采用NaCl-KCl熔盐体系提纯粗四氯化锆过程中,夹杂物在熔盐体系中富集,造成NaCl-KCl熔盐体系的恶化。利用旋转法、拉筒法和阿基米德原理研究了粗四氯化锆中主要夹杂物二氧化锆对NaCl-KCl熔盐体系表面张力、黏度和密度的影响。结果表明,在660~740 ℃,二氧化锆含量一定的熔盐体系的表面张力随温度的增加而减小,黏度受温度影响较大,当二氧化锆质量分数为0%、10%、15%、20%时密度与温度无关,但当二氧化锆质量分数为5%时,密度受温度的影响较大;温度一定时,熔盐表面张力随二氧化锆含量的增加先保持稳定后迅速减小,黏度随二氧化锆含量的增加逐渐增加,密度随二氧化锆含量的增加先增大后减小随后保持稳定不变。     

Nanocomposites of poly(ether ether ketone) with carbon nanofibers: Effects of dispersion and thermo-oxidative degradation on development of linear viscoelasticity and crystallinity

Poly(ether ether ketone), PEEK, is a widely used engineering plastic that is especially suitable for high temperature applications. Compounding of PEEK with carbon nanofibers, CNF, has the potential of enhancing its mechanical and thermal properties further, even at relatively low CNF concentrations. However, such enhancements can be compromised by myriad factors, some of which are elucidated in this study. Considering that the dispersion of the CNF into any high molecular weight polymer is a challenge, two different processing methods, i.e., melt and solution processing were used to prepare PEEK nanocomposites with low aspect ratio carbon nanofibers. The linear viscoelastic material functions of PEEK nanocomposites in the solid and molten states were characterized as indirect indicators of the dispersion state of the nanofibers and suggested that the dispersion of nanofibers into PEEK becomes difficult at increasing CNF concentrations for both solution and melt processing methods. Furthermore, the time-dependence of the linear viscoelastic material functions of the PEEK/CNF nanocomposites at 360-400 °C indicated that PEEK undergoes thermo-oxidative cross-linking under typical melt processing conditions, thus preventing better dispersion by progressive increases of the mixing time and specific energy input during melt processing. The crystallization behavior of PEEK is also affected by the presence of CNF and degree of cross-linking, with the rate of crystallization decreasing with increasing degree of cross-linking and upon the incorporation of CNFs both for the solution and melt processed PEEK nanocomposites.     

Aging effect of atmospheric pressure plasma jet treated polycaprolactone polymer solutions on electrospinning properties

In previous work, an atmospheric pressure plasma jet was applied to successfully improve the electrospinnability of poly-ε-caprolactone (PCL) which enabled the fabrication of beadless nanofibers. In this paper, we report the aging effect of the plasma treatment to evaluate the robustness of the developed process. For this purpose, plasma-treated PCL polymer solutions with different exposure time were stored for 1 and 8 days and the aged solutions were analyzed in terms of conductivity, viscosity, surface tension, pH, and polymer molecular weight. During storage, the surface tension and acidity of the plasma-treated solutions were maintained constant. However, the viscosity was found to be significantly lower after 8 days which was attributed to PCL degradation. Electrospinning of all stored PCL solutions resulted in the generation of beadless PCL nanofibers. The plasma treatment effects were thus found to be highly stable over time and capable of producing high-quality PCL nanofibers up to 8 days. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48914.     

Plasticization effects on bubble growth during polymer foaming

During polymer foaming with physical blowing agents, plasticization affects the melt viscosity, gas diffusivity in the melt, and the gas–melt interfacial tension. In this paper, we propose a model for plasticization during bubble growth, and estimate its effects under typical foaming conditions. The theoretical model incorporates well‐established mixture theories into a recent model for diffusion‐induced bubble growth. These include the free‐volume theories for the viscosity and diffusivity in polymer‐blowing agent mixtures and the density gradient theory for the interfacial tension. The viscoelasticity of the melt is represented by an Oldroyd‐B constitutive equation. We study the radial growth of a single bubble in an infinite expanse of melt, using parameter values based on experiments on polystyrene–CO₂ systems. Our results show that even at relatively low gas concentrations, plasticization increases the blowing‐agent diffusivity markedly and thus boosts the rate of bubble growth. In contrast, the reduction in melt viscosity and interfacial tension has little effect on bubble growth. Though not intended as quantitative guidelines for process design, these results are expected to apply qualitatively to typical foaming conditions and common polymer‐blowing agent combinations. POLYM. ENG. SCI., 46:97–107, 2006. © 2005 Society of Plastics Engineers     

Studies on structural foam processing I. The rheology of foam extrusion

An experimental study was carried out to investigate the flow behavior of gas-charged molten polymers in foam extrusion. For the study, a rectangular slit die with glass windows was constructed to permit visual observations, from the direction perpendicular to flow, of the dynamic behavior of gas bubbles when a gas-charged molten polymer flows between two parallel planes. Pictures were taken of gas bubbles in the flow channel with the aid of a camera attached to a microscope, and these were later used to determine the position at which gas bubbles start to grow. Using three melt pressure transducers mounted on the short side of the rectangular slot, pressure distributions were measured along the longitudinal centerline of the die. The polymeric materials used were high-density polyethylene and polystyrene, and the chemical blowing agents used were a proprietary hydrazide which generates nitrogen, and sodium bicarbonate which generates carbon dioxide. It was observed that the gas-charged molten polymer shows a curved pressure profile as the melt approaches the die exit, whereas the polymer without a blowing agent shows a linear pressure profile. The visual observations of the bubble growth in the flow channel, together with the pressure measurements, permitted us to determine the bubble inflation pressure, often referred to as the critical pressure for bubble inflation. It was found that the critical pressure decreases with increasing melt extrusion temperature, and increases with increasing blowing agent concentration. It was also found that the bulk viscosity of gas-charged molten polymers decreases with increasing blowing agent concentration and with increasing melt temperature. A general remark is made concerning the precaution one should take when an Instron rheometer is used for determining the bulk viscosity of gas-charged molten polymers.