摘要:

AbstractOn water uptake, plaster of Paris sets to gypsum, and the volume expansion associated with this particular hydration reaction has been long exploited to make casts for statuary. Immobilization of molecules of liquid water into the crystal lattice of water of crystallization gives rise to a decrease of entropy, and this is manifested by the so‐called heat of hydration. When it occurs at a calcium sulfate inclusion inside a polymer, the plaster of Paris → gypsum reaction generates a pressure, and the dependence of the equilibrium value of this pressure on temperature is expressed by Clapeyron's equation. From published data for the volume expansion ΔVand entropy decrease ΔS, it is shown that\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{dp}}{{dT}} = \frac{{\Delta V}}{{\Delta S}} = - 50{\rm bar K}^{ - 1} $$\end{document}Using as a benchmark the fact thatpis zero at 100°C, the temperature at which plaster of Paris, gypsum, and liquid solution can exist in thermodynamic equilibrium (1), we find that hydration at ambient temperatures of a typical calcium sulfate filler composition will, under conditions of thermodynamic equilibrium, generate sufficient pressure to cause internal cracking. Attention is drawn to the fact that creation of water of crystallization pressures of similar magnitude can be expected for any other filler that can exist in more than one state of hydration. As a corollary, the Clapeyron equation is used to make a similar analysis of the phenomenon of frost shattering in polymeric materials that contain free

ISSN:0032-3888    

DOI:10.1002/pen.760272202

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractThe effect of varying interaction parameters on the phase diagrams of ternary polymer blends was explored by simulating spinodals through use of the Flory‐Huggins lattice theory. Results indicate that miscibility is favored for the case of ternary mixtures of marginally miscible or marginally immiscible pairs where all pair interactions are nearly athermal. Miscibility is restricted for asymmetric ternary blends when one of the polymer pairs is either strongly miscible or strongly immiscible. For symmetric blends of partially immiscible pairs, both two‐phase and three‐phase miscibility gaps are pred

ISSN:0032-3888    

DOI:10.1002/pen.760272203

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractThe structure formation processes that occur during the flow of dilute blends of high density polyethylene (HDPE) or polypropylene (PP) In a linear low density polyethylene (LLDPE) carrier phase have been studied. Due to low surface tensions, high deformations of the dispersed minor phase can be induced under slow flow conditions leading to the formation of slender filaments. Measurements on a slit die, having a large, converging flow entrance region, demonstrate that the mechanism for filament formation is droplet bursting, yielding growing tails during shear flow, or, unsteady drop elongation during extensional flow. Tail growth can be modeled as the flow of a slightly tapering cylinder in a fluid of different viscosity, For dispersed to carrier phase viscosity ratios greater than unity, extensional flow occurs in the tail phase, which can induce oriented crystallization. For ratios less than unity, the flow is compressive, which. Inhibits crystallization. Drop deformation and crystallization in the converging flow entrance region is greatly enhanced by the extensional flow, and droplet growth can be described by a model assuming a time‐dependent, planar, extensional flow field. Data for birefringence and melting points of as‐crystallized fibers are also presented and discus

ISSN:0032-3888    

DOI:10.1002/pen.760272204

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractSheets of medium density polyethylene (MDPE) were extruded through a slit die containing an internal separator. Thus, the melt stream was momentarily split before emerging from the die. A line of separation was evident in the extruded sheets. It is attributed to incomplete welding or healing. Measurements of tear energyGcrevealed that the extruded sheets were anisotropic and that the weld line was extremely weak after extrusion start‐up, only about 1/5 of the strength elsewhere. As extrusion continued, the strength of the weld line increased to reach that of the bulk material after about 10 min. This is attributed to an increasing temperature of the melt in the die region, aiding interdiffusion. A sample containing 30% by weight of short glass fibers showed less initial weld‐line weakness but the weld line remained weak in this case, even after long extrusion ti

ISSN:0032-3888    

DOI:10.1002/pen.760272205

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractThe failure modes and failure sequences for polycarbonate specimens loaded by a clearance‐fit pin were investigated. Specimens were tested in various configurations to produce failures by different engineering failure modes. Photographs of the isochromatic fringe patterns were taken at different levels of damage development to examine the failure modes. A finite element stress analysis was used to explain the observed failure onset modes. Damage onset predictions were made based on a modified von Mises criterion. Failures involving net‐tension damage initiated in the net‐tension mode and failed ultimately in the same mode. Cleavage failures were observed for specimens with very small end‐distances. Bearing failures were preceded by damage onset in a net‐tension mode. Certain large‐hole specimens failed in a buckling f

ISSN:0032-3888    

DOI:10.1002/pen.760272206

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractPhase behavior of polymer‐polymer‐solvent systems can be determined using gel permeation chromatography. This technique enables one to generate the binodal curve and tie lines relating coexisting phases, to locate the critical point and, through the use of an appropriate solution theory, to calculate interaction parameters. Previous studies of the incompatible polystyrene‐polybutadiene‐chloroform system indicated that the classical Flory‐Huggins theory yields an anomalous behavior of negative interaction parameters. Probable causes are specific interactions due to the polar nature of chloroform plus failure to account for the concentration dependence of the polystyrene‐chloroform interaction. A modified version of the Flory‐Huggins theory incorporating concentration dependence of polymer‐solvent interaction was used to reevaluate the polystyrene‐polybutadiene‐chloroform system. This approach yields positive interaction parameters for the incompatible system polystyren

ISSN:0032-3888    

DOI:10.1002/pen.760272207

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractFibers of organic polymers (polystyrene, cellulose acetate, and polypropylene) were formed by the rapid expansion of supercritical fluid solutions through a small diameter nozzle to ambient temperature and pressure. Solutions were prepared either by dissolving the polymer directly in room temperature pentane, or in an autoclave at elevated temperatures and pressures for less soluble polymers. The fibers were collected on substrates mounted in the expansion jet. The diameters of the fibers formed (typically 1‐5 μm) were much smaller than the opening of the nozzle, although fiber diameter was observed to generally increase with nozzle diameter. The aspect ratios of the fibers, produced by this process were on the order of 103or larger. Optimum conditions for fiber formation occurred at fluid expansion temperatures near the melting point of the polymer, with particle formation mechanisms favored at higher and lower temperatur

ISSN:0032-3888    

DOI:10.1002/pen.760272208

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractDynamic mechanical measurements allow direct determination of the instant at which a network polymer gels. In such an experiment, the evolution ofG′(t,ω0) andG″ (t,ω0) is measured in small amplitude oscillatory shear as a function of cross‐linking timet. The frequency ω0is kept constant throughout. At the beginning of the experiment,G″ is orders of magnitude larger thanG′, and at completion of reaction, this order is reversed. It recently has been suggested by Tung and Dynes that the gel point (GP) might occur at the time at whichG′ andG″ cross each other. However, there is much dispute whether GP occurs exactly at the crossover or just somewhere in its vicinity. This study resolves the dispute by modeling the rheological behavior at GP: There is only one class of network polymers for which GP coincides with the crossover. This class of polymers exhibits, when reaching GP, power law relaxationG(t) ∼t−nwith a specific exponent valuen= 1/2. Examples are stoichiometrically balanced network polymers and networks with excess cross‐linker, however, only at temperatures much above the glass transition. Otherwise, the power law behavior would be masked by vitrification. Power law relaxation seems to be property of polymers at GP in general. However, some polymers have a different exponent value,n≠ 1/2, in which case the crossover occurs before GP (forn1/2); i.e. the crossover cannot be used for detecting GP. While there are no networks known to us withn1/2. A new method is suggested for measuring GP of

ISSN:0032-3888    

DOI:10.1002/pen.760272209

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractA mathematical model of a semi batch reactor was developed to investigate the oligomerization reactions in the melt transesterification of dimethyl terephthalate with ethylene glycol catalyzed by metal acetate catalyst. The detailed kinetic scheme based on the molecular species model is used to estimate the conversion of methyl ester groups and the concentrations of various oligomeric species. The numerical simulation of the model shows that the oligomerization reactions lower the overall conversion of methyl ester end groups. Effects of ethylene glycol/dimethyl terephthalate mole ratios, reaction temperature and, catalyst concentration on the conversion, oligomer concentration, oligomer molecular weight, and molecular weight distribution were also analyzed.

ISSN:0032-3888    

DOI:10.1002/pen.760272210

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractA modification of the kinetics equation of mechanical degradation of Harrington and Zimm is proposed to fit experimental data taken on a molten polystyrene. This equation is applied to each molecular weight of the discretized molecular weight distribution curve, and the limiting molecular weight is determined for each molecular weight. With this modification the theoretical curves fit bothMwandMncurves well.

ISSN:0032-3888    

DOI:10.1002/pen.760272211

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY