Freeze Line Research Articles

CMAS deposition rate and sequence effects on cyclic life in gradient testing

Thermal barrier coating (TBC) resistance to varied CMAS deposition rates was experimentally assessed using a temperature gradient rig using a CMAS precursor spray. TBCs were composed of EB-PVD YSZ. Five different deposition rates were tested quantifying life versus deposition rate. A high deposition rate and low deposition rate were selected from those tests for sequence testing. For sequence effect testing CMAS deposition occurred at the initial rate until the expected half-life and then switched to the other rate until failure, in the order of high deposition rate first followed by low deposition rate and vice versa. The results suggest that the order of deposition rates can affect ultimate failure behavior. Specifically applying the CMAS at the low deposition rate extended the life of the sample by a nearly a factor of 2 compared to a virgin sample. At the high deposition rate, the CMAS infiltrated all the way to the bond coat past the estimated freeze line. Data showing the coating life versus CMAS deposition rate and the existence of sequence effects in CMAS dosing are fundamentally important to coating life prediction.

40 Years of Surface Warming in the Northern US Rocky Mountains: Implications for Snowpack Retreat

The northern US Rocky Mountains are experiencing rapid warming. Combined analysis of Ground Temperature (GT) measurements at two high-fidelity boreholes with Surface Air Temperature (SAT) measurements near Helena Montana spanning the past 40 years indicate the northern US Rockies have warmed on average 0.12°C - 0.32°C/decade since 1975, at least a factor of ~5 higher than the predicted 500-year-average. Warming appears to be accelerating, with warming rates since 2013 4 - 7 times higher than the 40 year average. Though uncertainty exists, the most significant GT warming appears to occur at higher elevation. Warming estimates are consistent with modelling predictions, snowpack observations, and stream temperature studies, all suggesting rapid surface temperature change in this region during the past ~40 years. The analysis indicates GT warming measured at remote borehole sites is slightly lower than regional SAT measurements collected near urban environments. We associate the discrepancy between GT/SAT measurements to both anthropogenic effects (urban development) that increase warming at the nearest SAT measurement station and a 14-year period of anomalously low snowfall that reduces surface insulation and GT warming. Using a derived average forty-year surface warming rate of 0.22°C/ decade and regional temperature-elevation trends, we calculate that the elevation of the winter freeze line during the three coldest months of the year (December, January, and February) in the northern US Rocky Mountains is retreating upward, on average, 33 m/decade. This implies a 21% reduction in freeze-line area since 1974. If this trend continues, we estimate that within the next 40 years (by 2060), the total area where ground freeze occurs during the three coldest months of the year will be ~60% of 1974 values. Since GT measurements indicate accelerated warming, this may be an underestimate. The analysis has important implications for the snowpack-water budget for Montana and the northern US Rocky Mountains.

Polymer crystallization studies under processing-relevant conditions at the SAXS/WAXS DUBBLE beamline at the ESRF

Recent developments on the experimental infrastructure and the acquisition of new detectors on the Dutch–Belgian beamline BM26B at the ESRF offer novel and promising possibilities for synchrotron X-ray experiments in the field of polymer crystallization under processing-relevant conditions. In this contribution, some of the most recent experiments mimicking conditions similar to those relevant for the plastics processing industry are discussed. Simultaneous thermal analysis and wide-angle X-ray scattering (WAXS) experiments, at the millisecond time-frame level, on β-nucleated isotactic polypropylene (i-PP) samples subjected to ballistic cooling up to 230 K s−1, show that the efficiency of the nucleating agent can be suppressed when quenched cooling rates higher than 130 K s−1are used.In situWAXS experiments using small-scale industrial equipment during a real film blowing process reveal the dependence of the onset of crystallinity (the so-called freeze line) and the crystal orientation as a function of different take-up and blow-up ratios.In situsmall-angle X-ray scattering (SAXS) experiments during high-flow fields reveal the formation of shish and kebab structures in i-PP as a function of the imposed stress. Quantitative analysis of i-PP flow-induced structures is presented. The beamline specifications required to obtain high quality and industrially relevant results are also briefly reported.

Potential Impact of Climate Change Scenarios on Whooping Crane Life History

Whooping crane (Grus americana), a rare and critically endangered species, are wetland dependent throughout their life cycle. The whooping crane’s small population size, limited distribution, and wetland habitat requirements make them vulnerable to potential climate changes. Climate change predictions suggest overall temperature increases and significant changes in precipitation regimes throughout North America. At the individual level, temperature changes should have neutral to positive effects on thermoregulation and overall energy expenditure throughout the whooping crane’s range. In the breeding grounds, earlier snow melt and increasing temperatures should improve food resources. However, increased precipitation and more extreme rainfall events could impact chick survival if rainfall occurs during hatching. Increased precipitation may also alter fire regimes leading to increased woody plant abundance thus reducing nesting habitat quality. During winter, higher temperatures will lead to a northward shifting of the freeze line, which will decrease habitat quality via invasion of black mangrove. Large portions of current winter habitat may be lost if predicted sea level changes occur. Stopover wetland availability during migration may decrease due to drier conditions in the Great Plains. Current and future conservation actions should be planned in light of not only current needs but also considering future expectations.

Prediction and experimental verification of bubble and processing characteristics in blown‐film extrusion

AbstractBlown‐film modeling is useful to the flexible packaging industry for predicting process and bubble characteristics, such as freeze line height (FLH), bubble diameter, and film thickness. The use of a suitable rheological equation to describe material properties is critical in simulating the blown‐film process. In this article, we present an improved rheological constitutive equation, which incorporates more realistic parameters of stress and deformation properties of the materials by combining the Hookean model with the Phan‐Thien Tanner (PTT) model. The proposed PTT–Hookean model is aimed at enhancing the viscoelastic behavior of the melt during biaxial stretching in the blown‐film extrusion. Predictions of the blown‐film bubble characteristics and FLH obtained with the PTT–Hookean model agreed well with the experimental data of this study and previous studies with different materials and different die geometries. The justification for combining the Hookean model with the PTT model in the blown‐film process is also reported here. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Numerical modelling of the effect of operating parameters in the plastic blown film process

The blown film process with polymer melts is modelled using non-isothermal viscoelastic rheological constitutive equations that are suitable for polyolefins. The model correlates the operating parameters such as mass flows, extruder temperature, tensile axial force and take-up force on processes such as bubble geometry and bubble temperature profile. Unlike Luo and Tanner [ Poly. Eng. Sci. , Vol. 25, 1985] who used the Maxwell constitutive equation, this study considers the viscoelastic Kelvin model that avoids any assumption regarding stress at the die exit. Like Luo and Tanner, the model uses shooting techniques to match the initial and boundary conditions from the freeze line and the die. The pressure drop and the take-up force are estimated as parameters that are optimized with the boundary conditions using the Nelder--Mead optimization method with Matlab. Effects of varying elasticity parameters and heat transfer coefficients on the bubble geometry, the pressure drop and take-up force are investigated.

Heat Transfer Analysis and Estimation of Refractory Wear in an Iron Blast Furnace Hearth Using Finite Element Method

The wear of hearth refractory by hot metal penetration and mechanical erosion is the limiting factor in the life of a blast furnace and their control and minimisation result in a direct benefit in an extended campaign. At the same time, it is difficult to directly measure the amount and location of hearth erosion during any campaign. Heat transfer mathematical model is an appropriate tool to quantify the amount of erosion based on the prediction of temperature profile particularly 1150°C freeze line isotherm in the hearth.In the present investigation, an axisymmetric conductive heat transfer model based on finite element method has been formulated and computer software is developed. Using the model and the computer code, temperature profile is predicted in the hearth zone of two different designs of industrial blast furnaces and maximum (worst) hearth wear has been estimated. The erosion pattern is calculated on the basis of worst-case location of 1150°C isotherm that can occur during the furnace campaign. Effects of hot metal temperature, cooling conditions and coke-bed states (floating and sitting) on temperature profile and refractory wear are also investigated.

Integrated experimental–modelling study of microstructural development and kinematics in a blown film extrusion process: I. Real-time Raman spectroscopy measurements of crystallinity

Integrated experimental–modelling studies on microstructural development and kinematics of blown film extrusion of low- and high-density polyethylenes (LDPE and HDPE) films are reported. First, the microstructural evolution along the machine direction of the film line is reported from Raman spectroscopy. The 1418 cm−1 peak associated with wagging of the –CH2 group in the crystalline phase was measured using mobile Raman probes. The changes in take-up ratio, blowup ratio and freeze line height displayed varied levels of influence on the rate of crystallisation. The two polyethylenes develop significantly different extents of crystallinity in the final product form, but higher strain-rates led to faster kinetics and confirmed flow-enhanced crystallisation phenomenon in both polymers. The effect of processing conditions on the development of crystallinity in LDPE and HDPE was consistent with that reported earlier for linear low-density polyethylene (LLDPE), thus confirming the use of Raman spectroscopy as an efficient and reliable analytical technqiue for monitoring microstructure in real time. These results are compared with predictions from a flow-enhanced crystallisation model in a companion paper.

Online measurements of crystallinity using Raman spectroscopy during blown film extrusion of a linear low‐density polyethylene

AbstractReal‐time, Raman spectroscopic measurements of crystallinity during the blown film extrusion of a linear low‐density polyethylene (LLDPE) are reported. The take‐up ratio (TUR), inflation air pressure, and cooling conditions were varied, and their effect on the development of crystallinity was studied. The results indicate that the crystallization process starts at the freeze line, increases along the film line, and finally plateaus. It was observed that with changing processing conditions, the differences in the profiles for crystallinity were evident when plotted as a function of time rather than distance along the machine direction. The influence of take‐up ratio and bubble inflation air pressure on the development of crystallinity clearly established the role of flow‐induced crystallization during film extrusion. Polym. Eng. Sci. 44:1484–1490, 2004. © 2004 Society of Plastics Engineers.

Computer simulation of the film blowing process incorporating crystallization and viscoelasticity

A comprehensive two-dimensional simulation of the film blowing process is developed, based on a mathematical model that incorporates the Phan-Thien and Tanner (PTT) and the neo-Hookean constitutive equations with crystallization effects. The PTT constitutive equation is employed in the liquid-like region, whereas the neo-Hookean constitutive equation is employed in the solid-like region. The orientation-induced crystallization is accounted for by incorporating the Nakamura nonisothermal equation along with the Ziabicki equation. The proposed model provides predictions of the bubble shape and dimensions, the position of the freeze line, and evolution of the temperature, crystallinity, stress and deformation. The predictions of the model show good agreement with analytical and computational solutions for some limiting cases and with experimental results reported by various workers.

Mathematical and computational modeling of heat transfer and deformation in film blowing process

AbstractThe blown‐film extrusion process was investigated both experimentally and theoretically. In experimental study, nonisothermal experiments were conducted using low‐density polyethylene. Rheological parameters were studied, considering the polymer melt as a power law fluid in nonisothermal conditions. Axial tension, bubble diameter, and film thickness at a variety of film extrusion conditions, that is, different flow rate, pressure difference across the film, and take‐up speeds were measured. In theoretical study, an analysis was employed to simulate the blown‐film extrusion process by setting up the force‐ and energy‐balance equations on the film bubble moving upward. Four nonlinear complex differential equations were integrated numerically, using an iterative backward shooting method and the fifth‐order Runge‐kutta technique. The program written, based on a mathematical model, predicts the bubble shape, temperature profile, and film thickness as a function of the distance along the machine axis. Furthermore, the model evaluates the elongational viscosity of LDPE in biaxial tension in terms of distance from die axis and take‐up speed. In this simulation, the total stress components in machine and the transverse directions were computed from the die exit up to the freeze line, the knowledge of which is necessary for evaluation of the elastic memory build up in heat‐shrinkable films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2115–2123, 2002

NUMERICAL SIMULATION OF TUBULAR BLOWN FILM PROCESSING

A numerical simulation of the film blowing process was performed using the Runge-Kutta scheme. The kinematic and force balance equations governing the process are derived, and the constitutive model proposed by Cao and Campbell is utilized. The model accounts for liquidlike behavior at the freeze line; it alters the demarcation between liquidlike behavior and solidlike behavior from the suggested kinematicaly based constraint to a rheologically based constraint, the plastic-elastic transition (PET). The paper presents a detailed discussion on how the numerical models were developed and implemented. The numerical simulation was successful in duplicating Cao and Campbell's results. Recommendations are made to gain some insight into the problem.

A Rheological Model for the Prediction of Polyethylene Blown Film Properties

In this paper, a model is presented that enables us to get a better understanding of the relationship between polymer type, machine parameters and blown film properties for PE. For the mechanical properties it appears that the key parameter is the stress at the freeze line. Validation of the model on different blown film lines has proven its reliability for LDPE. Furthermore, the model has been extended to binary LDPE/LLDPE blends, demonstrating its versatility for PE in general.

Viscoplastic‐elastic modeling of tubular blown film processing

AbstractThe blown film process has been modeled through the transition from liquidlike to solidlike behavior at the freeze line. A new constitutive equation has been developed that, when incorporated into the the kinematic and dynamic equations that describe the process, for the first time yields qualitatively correct predictions of all process variables. It is suggested that the demarcation between liquidlike behavior and solidlike behavior be altered from the conventional, kinematically based constraint, dr/dz = O, to a rheologically based constraint, the plastic‐elastic transition (PET). The results are qualitive in nature since the material is modeled as an elastic solid above the PET instead of as a viscoelastic material. The model is tested using the polystyrene data of Gupta (1981).

Development of crystalline structure during tubular film blowing of low‐density polyethylene

AbstractDevelopment of crystalline structure during the tubular film blowing of low‐density polyethylene was investigated, using wide‐angle X‐ray diffraction technique, low‐angle light scattering, and scanning electron microscopy. In the study, commercial grades of both high‐pressure low‐density polyethylene (HP‐LDPE) and low‐pressure low‐density polyethylene (LP‐LDPE) (also, commonly referred to as linear low‐density polyethylene, LLDPE) were used. The applied stresses at the freeze line were determined using theoretical expressions derived in an earlier publication [C. D. Han and T. H. Kwack, J. Appl. Polym. Sci., 28, 3399 (1983)]. The applied stresses, S11F and S33F, at and above the freeze line in the machine and transverse directions were expressed in terms of the tension at the take‐up device, take‐up ratio, blow‐up ratio, and the pressure difference across the film of the bubble. These applied stresses were used to interpret the crystalline axes' orientation in the tubular blown films. It was found that the magnitude of S11F is an important process parameter for the crystalline axes' orientation and that the biaxial stress ratio (S11F/S33F) appears to be a determining factor in the distribution of fibrillous nuclei and crystalline texture, as well as film anisotropy.

The Interaction of Crystallinity, Elastoplasticity, and a Two-Phase Model on Blown Film Bubble Shape

Historically, simulations of the blown film process have been carried out only to the freeze line; a point which is often well below the actual frost line for the process. This limitation in previous simulations was discovered to be related to the interaction of the process boundary conditions with the rheological equation of state used in previous analyses to represent the stretching of the polymer film. A new approach has been developed that allows the process to be simulated to the real frost line. This has been accomplished by using a two-phase model incorporating viscoplasticity and crystallization of the polymer melt into the physical description of the blown film process. Prediction of the bubble shape, temperature, and velocity from the die to the frost line agree fairly well with experimental data. By improving the rheological models used in the simulation, this approach has the potential of being able to pre dict physical properties of polymeric film from molecular and process parameters.

Effect of processing conditions on the orientation and properties of polyethylene blown film

AbstractThe amorphous and crystallite orientation in a number of films produced from characterised grades of high density polyethylene has been examined and some tensile properties measured. The films were produced under commercial production conditions with various blow ratios, freeze line heights and thicknesses.Orientation was assessed using optical and X‐ray methods. The relative contributions of the crystallite and amorphous components were calculated for a limited number of the films produced.Amongst the properties measured the ultimate tensile strength and elongation at break appeared to be the most variable. Birefringence was more affected by variation in blow ratio and film thickness than by freeze height changes.Attempts have been made to correlate these variations with crystallite and amorphous orientation as well as the individual behaviour of the polyethylene grades. In carrying out this correlation it became clear that the films could not generally be described by established low or high stress models of crystallite orientation. For these films produced on full‐scale commercial equipment it appears that transcrystallisation behaviour is frequently observed, with a tendency towards the low stress model.

Identifying Critical Process Variables in Poly(ethylene Terephthalate) Melt Spinning 1

A quantitative procedure for identifying critical operating conditions and material properties for the melt spinning of poly(ethylene terephthalate) (PET) is considered. The method is based on the experimental observation that for vitrified polymers like polystyrene and PET spun at speeds below 3000 m/minute, the molecular orientation of the as-spun fiber is uniquely determined by σ L, the spinline stress at the glass transition temperature. Using a constant tension model that predicts σ L, a sensitivity analysis of the fiber orientation to changes in process variables is investigated. Extrusion temperature, melt intrinsic viscosity, feed rate, and the takeup velocity are the key variables for PET melt spinning, as they strongly affect the freeze line location and the as-spun fiber orientation.

Rheology‐processing‐property relationships in tubular blown film extrusion. I. High‐pressure low‐density polyethylene

AbstractThree different grades of high‐pressure low‐density polyethylene resin were used to establish relationships between tubular film blowability and the molecular parameters, namely, the molecular weight distribution (MWD) and the degree of long‐chain branching (LCB), and also between the processing conditions and the mechanical properties of the tubular blown films produced. For the study, both the shearing and elongational flow properties of the resins were determined. During the tubular film blowing experiment we measured the freeze‐line position, the tubular bubble diameter, the takeup speed, the axial tension, the pressure inside the tubular bubble, and the mass flow rate of the resin. The thickness of the tubular blown films was measured from the samples collected. In order to determine the tubular film blowability, we measured the maximum takeup speed at which the tubular blown bubble broke, for various blowup ratios. The measurements described above permitted us to calculate the tensile stresses at the freeze line, in both the machine and transverse directions, and they were found to be correlatable to the processing conditions employed. It has been found that the tubular film blowability is increased as the resin's MWD becomes narrower and the degree of LCB is less. It has been found further that a resin having lower elongational viscosity tends to give a greater draw‐down ratio, indicating a better tubular film blowability. Finally, the tensile properties of the tubular blown films were found correlatable to the processing variables, namely, blowup and takeup ratios.

Orientation and morphology of high‐density polyethylene film produced by the tubular blowing method and its relationship to process conditions

AbstractThe orientation and crystallinity of a series of high‐density polyethylene (HDPE) tubular films is characterized using wide‐angle x‐ray scattering pole‐figure analysis and birefringence. The films ranged from uniaxial to equal biaxial. The data were used to compute biaxial orientation factors which were then plotted on an orientation‐factor triangle diagram. It was shown, within the range of conditions studied, that both the crystalline biaxial orientation factors were unique functions of the stresses exerted on the bubble at the freeze line. Both correlations are the same as those developed by Dees and Spruiell for melt‐spun HDPE fibers. SAXS measurements on the films suggest lamellar structures in both uniaxial and biaxial films.