Showing papers in "Polymer Engineering and Science in 1975"
TL;DR: In this paper, the critical strain energy release rate for impact tests may be deduced for both Charpy and Izod tests from normal energy measurements using a method of analysis, based on which a calibration factor is determined and the method is applied to a range of polymers.
Abstract: A method of analysis is given by which the critical strain energy release rate Gc for impact tests may be deduced for both Charpy and Izod tests from normal energy measurements Suitable calibration factors are determined and the method is applied to a range of polymers Very close agreement is achieved between the Charpy and Izod results except for highly ductile materials for which it was necessary to use a fully plastic analysis The method is extended to blunt notches and it is shown that the use of a strain energy per unit volume to yielding, together with a blunt notch stress analysis, gives a good description of the results
308 citations
TL;DR: In this paper, the processes used to produce high-performance carbon fibers are described from fundamentals, and the resulting fiber microstructures and the consequences of these structures on properties are presented.
Abstract: Graphite has a hexagonal close-packed crystal structure which is strong and stiff in the two directions of the basal plane and, in the third direction—perpendicular to the basal plane—is weak and compliant. High-performance carbon fibers must make use of the strong directions while suffering from the poor properties of the third. This paper describes, from fundamentals, the processes used to produce high-performance carbon fibers. The resulting fiber microstructures and the consequences of these structures on properties are presented.
186 citations
TL;DR: Experimental studies of structure development in melt spinning of polyethylene and polypropylene fibers are described in this article, where emphasis is given to the influence of applied stresses on the rates of crystallization and on the development of crystalline morphology.
Abstract: Experimental studies of structure development in melt spinning of polyethylene and polypropylene fibers are described. Emphasis is given to the influence of applied stresses on the rates of crystallization and on the development of crystalline morphology. The relationship of fiber morphology to mechanical properties, especially “hard elastic fibers” is considered. The relevance of such studies to other polymer processing operations such as film extrusion is discussed.
152 citations
TL;DR: In this paper, the technique of chemorheology is explored as a means of characterizing the viscosity-time-temperature behavior of B-staged epoxy resins.
Abstract: The work described herein was initiated in order to generate resin characterization data that could describe a B-staged epoxy material via the viscosity-time-temperature interactions that are inherently related to the nature of thermoset processing. The technique of chemorheology is explored as a means of characterizing the viscosity-time-temperature behavior of the B-staged epoxy resins. The resins studied are typical of those available as glass-impregnated prepreg bonding sheets used to manufacture multilayer printed wiring boards. The B-staged resins were characterized isothermally at various curing temperatures. Their isothermal behavior was correlated via an empirical viscosity expression in the form of a dual Arrhenius model. Experiments were then conducted in a nonisothermal temperature mode, measuring viscosity as a function of both time and temperature. The viscosity model was taken into the nonisothermal temperature mode by introducing a time-temperature integral. Good correlations between measured and predicted dynamic viscosity profiles are presented. Deviations are explained in terms of resin chemistry. The generalizations of behavior are discused. Calculations are presented which allow the comparison of the various B-staged resins in terms of their viscosity behavior under the influence of an actual lamination thermal profile.
137 citations
TL;DR: Stable poly (organophosphazenes) can be obtained from an inorganic precursor by careful control of polymerization and substitution reaction conditions as discussed by the authors, which has resulted in a new class of polymers for both specialty and large scale commercial development.
Abstract: Various aspects of polyphosphazene chemistry are reviewed. Stable poly (organophosphazenes) can be prepared from an inorganic precursor, poly(dichlorophosphazene), by careful control of polymerization and substitution reaction conditions. The bulk structure and properties of polyphosphazenes are discussed, and attention is given to those polymers which have promise as useful engineering materials. The successful preparation of stable poly(organophosphazenes) appears to have resulted in a new class of polymers for both specialty and large scale commercial development.
122 citations
TL;DR: In this article, an equation for the mechanical behavior of polyurethane foams as a function of foam density is provided, starting from a model conception and the qualitative microscopic consideration of the deformation and failure mechanism, simple relations are found for the tensile, compressive and shear strength and the elastic modulus.
Abstract: Rigid plastic foams find application in construction mainly as core materials for loaded sandwich structures—in buildings, ground vehicles, and airplanes. This work provides an equation for the mechanical behavior of polyurethane foams as a function of foam density. Starting from a model conception and the qualitative microscopic consideration of the deformation and failure mechanism, simple relations are found for the tensile, compressive and shear strength and the elastic modulus, which sufficiently express the measured results.
117 citations
TL;DR: In this article, a number of fatigue models dealing with the time and strain rate dependence of elastic modulus, yield strength, creep and localized crack tip heating are examined and confronted with available data from the literature.
Abstract: One is faced with an interesting challenge when trying to explain the effect of test frequency on polymer fatigue performance. While hysteretic heating arguments appear sufficient to explain a diminution of fatigue resistance with increasing cyclic frequency in unnotched test samples, the enhancement of fatigue resistance in many polymers with increasing cyclic frequency in notched samples is still not clearly understood. In large measure, this is due to contradictory trends in fre-quency-sensitive material properties which affect the fatigue process. In this paper, a number of proposed fatigue models dealing with the time and strain rate dependence of elastic modulus, yield strength, creep and localized crack tip heating are examined and confronted with available data from the literature. Additional fatigue crack propagation data for poly(methyl methacrylate), poly (vinyl chloride), polystyrene, poly-carbonate, nylon 66, poly(vinylidene fluoride) and poly(2,6-dimethylphenylene oxide) were obtained and are reported herein. These data were obtained over a maximum frequency range of 0.1 to 100 Hz and, for selected polymers, with various waveforms. Frequency sensitivity is shown to be greatest in those polymers that show a high tendency for crazing. Relative fatigue behavior is found to reflect a competition between strain rate and creep effects. Where creep effects dominate, the total crack growth rate may be viewed as consisting of the summation of pure fatigue and creep components, respectively. Finally, the β transition appears to have a role, with frequency sensitivity being at a maximum for polymers where the β transition at room temperature occurs in the range of the experimental test frequency.
107 citations
TL;DR: In this article, the authors have developed a method for measuring residence time distributions and characterizing axial flow behavior for 4-in. diameter twin screw extruders with a variety of rotor configurations.
Abstract: Twin screw extruders are finding increased usage in reacting and devolatilizing applications. Using self-wiping profiles, the twin screws fulfill the requirement that there be no “dead” or “unmixed” zones. Agitator design must be chosen with care so that a reasonable balance can be obtained between forwarding rate, surface-generation rate, vapor passageway, power, and axial mixing. Techniques have been developed for measuring residence time distributions and characterizing axial flow behavior. The method also permits direct determination of the holdup in starved barrel applications. Data on residence time distribution are presented for 4-in. diameter twin screw equipment with a variety of rotor configurations.
106 citations
TL;DR: A practically-oriented computer model which computes the temperature, pressure, and velocity fields in a cavity during the mold filling portion of the injection molding process is described in this paper.
Abstract: A practically-oriented computer model which computes the temperature, pressure, and velocity fields in a cavity during the mold filling portion of the injection molding process is described. The model is structured so that it can be used for cavities having non-simple shapes and for commonly used molding compounds with complicated viscosity, shear rate, temperature relationships. Predictions from the model are found to be in good agreement with results obtained from exact solutions to special cases. Model predictions in molding problems have been found to correctly describe trends such as an increase in the pressure required to fill molds as injection rate, shot temperature, and mold temperature decrease, and to be reasonably accurate when compared to data for plaque, disc, and telephone housing molds over a wide range of molding conditions. Some illustrative examples of the use of the model in solving real molding problems are provided.
96 citations
TL;DR: In this article, a finite difference analysis has been developed which predicts the temperature, pressure, and velocity distributions for the flow of thermoplastic materials in straight and tapered, hot and cold walled circular flow channels.
Abstract: A finite difference analysis has been developed which predicts the temperature, pressure, and velocity distributions for the flow of thermoplastic materials in straight and tapered, hot and cold walled circular flow channels. This analysis when combined with the cavity filling analysis described in Part II, gives the molding engineer the capability of modeling the injection molding process from the shot to the cavity during injection. The information that is obtained from these analyses is useful in equipment design and modification since it allows numerical experiments to be performed so that one may ascertain the effects on moldability of flow channel and cavity geometry, material properties, and operating conditions. In addition, the information is useful in problem diagnosis and analysis to ascertain causes of and evaluate potential solutions to molding problems.
93 citations
TL;DR: In this paper, the combination of small potassium titanate fiber reinforcement, polypropylene homopolymer and an impact modifier can produce a material with a tensile modulus of 470,000 psi, a notched Izod impact strength of 4.6 ft-lb/in.
Abstract: Reinforcement can improve the modulus, heat distortion temperature and thermal expansion coefficient of rubber modified polypropylene. These improvements canbe obtained with a minimum sacrifice in resin ductility and impact toughness. Our studies have shown that reinforcement with small discontinuous fibers can provide a material with a good balance of stiffness and impact strength. The combination of small potassium titanate fiber reinforcement, polypropylene homopolymer and an impact modifier can produce a material with a tensile modulus of 470,000 psi, a notched Izod impact strength of 4.6 ft-lb/in. and a Gardner imact strength of 320 in-lb. Glass fiber reinforcement produces similar improvements in stiffness and retains Izod impact strength but drastically reduces Gardner impact strength. Polypropylene resin viscosity has a profound effect on composite impact strength. Transmission electron microscopy showed that a correlation exists between EPDM rubber particle size and impact strength: as rubber particle size is reduced, impact strength is increased. A 5 melt flow rate polypropylene was found to have the viscosity required to adequately shear and disperse the impact modifier. The paper describes an approach to broadening the utility of polypropylene homopolymers. The properties and flow of these materials compare very favorably with medium impact ABS.
TL;DR: In this paper, the importance of molecular weight and initial morphology in determining the drawing behavior of linear polyethylenes has been demonstrated, and it has been shown that by appropriate choice of these two factors a substantial increase in draw ratio and hence stiffness can be achieved over conventionally oriented polyethylene.
Abstract: Recent work is presented on the plastic deformation of linear polyethylenes. This work demonstrates the importance of molecular weight and initial morphology in determining the drawing behavior, and shows that by appropriate choice of these two factors a substantial increase in draw ratio and hence stiffness can be achieved over conventionally-oriented polyethylenes. Under optimum conditions a modulus of approximately 700 Kbar was obtained at a draw ratio of ∼30. These very high modulus materials displayed an extensibility to break of at least 3 percent and a strength of about 4 Kbar. In some cases they also exhibited very high melting points (∼139°C) and exceptionally good thermal stability.
TL;DR: In this article, high-density polyethylene (PE) and isotactic polypropylene (PP) were melt-blended in the following percentages of PE by weight: O, 10, 33.3, 40, 50, 66.6, 90, and 100.
Abstract: High-density polyethylene (PE) and isotactic polypropylene (PP) were melt-blended in the following percentages of PE by weight: O, 10, 33.3, 40, 50, 66.6, 90, and 100. For these blends we obtained data on shear stress vs shear rate; tensile modulus and strength; density; and rates of water-vapor transmission for films. The shear-rate/shear-stress data at 190 and 210°C are well fitted by the Ellis model with a maximum relative error of 5 percent. At 190°C all the mixtures were found to exhibit flow instabilities at high shear rates except the 90 percent and pure PE compositions. These, however, were unstable at 140, 150, and 160°C.
The 10, 33.3, 40, 50, and 66.6 percent PE mixtures ruptured at elongations of less than 15 percent at the maximum tensile stress. The maximum tensile strength and modulus pass through maxima at 10 percent PE. Density is given by the equation ρ = 0.9029 + 0.0544 (wt fraction PE). Water-vapor permeability was measured using wax-sealed permeability cups, according to ASTM E96-66. Valid results were obtained for only a few compositions because of faulty seals that could be detected only during careful dismantling of the specimen dishes. Faulty seals could account for the value, about ten times ours, reported in the literature for linear PE.
TL;DR: In this article, two component interpenetrating polymer networks (IPN) composed of a polystyrene network (crosslinked with divinyl benzene) and a polyester-polyurethane network, crosslinked with trimethylolpropane, were made.
Abstract: Two component interpenetrating polymer networks (IPN) of the SIN type (simultaneous interpenetrating networks), composed of a polystyrene network (crosslinked with divinyl benzene) and a polyester-polyurethane network (crosslinked with trimethylolpropane), were made. Electron microscopy and glass-transition measurements showed that phase separation had resulted with some interpenetration, presumably occurring at the boundaries. At a composition of about 75 percent polyurethane, a phase inversion occurred, the continuous phase being polystyrene at polyurethane compositions of less than 75 percent. The stress-strain properties and hardness measurements agreed with these results. Enhanced tensile strength was observed in the IPN's in a concentration range where modulus reinforcement was not evident. A small enhancement in tear strength and thermal stability was also noted.
TL;DR: In this paper, the use of aromatic polymers for high-strength/high-modulus fibers was discussed and the factors necessary for achieving high molecular weight for the attainment of high molecular power for the heat-resistant type polymers and for the ultra-rigid types polymers was discussed.
Abstract: Current trends and developments taken from the literature are reviewed and some new work from the laboratories of Monsanto Company is presented concerning the use of aromatic polymers for high-strength/high-modulus fibers In the area of aromatic polymer synthesis, pertinent information concerning the factors necessary for the attainment of high molecular weight for the heat-resistant type polymers and for the ultra-rigid type polymers is discussed Some new polymers and new routes of synthesis for rod-like polymers are described along with the tensile properties of fibers from these novel polymers
TL;DR: In this paper, the degradation process is attributed to penetration of water into the matrix-fiber interfaces and is followed by an attack, at high temperatures, on the glass-fibers surface and coupling agent.
Abstract: Glass-fiber reinforced epoxy (GRP) and unfilled epoxy specimens were exposed to different environmental conditions consisting of hot- and cold-water absorption and subsequent drying. Effects of the environmental history on deformational and strength characteristics of the composite material were investigated.
GRP specimens exposed to hot water undergo pronounced degradation, which sets in shortly after exposure and is associated with a significant irrecoverable weight loss. Degraded specimens are characterized by higher void content and lower strength compared with their cold-water and reference counterparts. The degradation process is attributed to penetration of water into the matrix-fiber interfaces and is followed by an attack, at high temperatures, on the glass-fibers surface and coupling agent. As a result, glass constituents are leached out and then removed from the system by diffusion.
The degradation effect was also confirmed by micro-observation of the fractured surfaces and by infra-red spectroscopy. A simple and effective testing method is recommended, permitting detection of degradation onset and its progress by simultaneous measurement of weight and dimensional changes with time.
TL;DR: In this paper, the authors examined the utility of using predictive methods developed for two-component engineering composites to predict the mechanical properties of macromolecular systems including crystalline and oriented thermoplastics, block copolymers, and flexibilized thermosetting resins.
Abstract: A variety of macromolecular systems including crystalline and oriented thermoplastics, block copolymers, and flexibilized thermosetting resins can realistically be viewed as composite systems. This paper examines the utility of using predictive methods developed for two-component engineering composites to predict the mechanical properties of macromolecular systems. The concepts presently available for the prediction of stiffness and expansion coefficients of short-fiber rein-forced plastics are reviewed with respect to their engineering accuracy in structural systems design. These techniques are then applied to predict the stiffness of a hybrid polymer system lying midway between an engineering composite and a crystalline polymer. The hybrid consists of a polymer matrix (butadiene-acrylonitrile copolymer) reinforced with in-situ crystallized, low-molecular-weight filler (acetanilide). Finally, the composite approach is applied to the prediction of stiffnesses and expansion coefficients of crystalline polyethylene as a function of volume fraction crystallinity and crystallite morphology.
TL;DR: In this article, a theory for roll coating of a fluid onto a moving sheet is developed utilizing the usual lubrication approximations, and the effects of fluid and operating parameters on coating thickness and pressure distribution are determined for a Newtonian fluid, and for a purely viscous non-Newtonian fluid obeying the Power Law.
Abstract: A theory for roll coating of a fluid onto a moving sheet is developed utilizing the usual “lubrication approximations.” The effects of fluid and operating parameters on coating thickness and pressure distribution are determined for a Newtonian fluid, and for a purely viscous non-Newtonian fluid obeying the Power Law. The results for these cases are obtained analytically, and are rather straightforward.
A viscoelastic fluid is considered, of a type which shows typical non-Newtonian shear behavior observed in polymer melts and solutions and which also exhibits normal stress behavior. Analytical solutions are not possible, but a perturbation method, using a viscoelastic perturbation parameter related to a Deborah number, yields an approximate solution. Only terms to first order in the perturbation parameter are given. Subject to that degree of approximation, the following conclusions are drawn:
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Non-Newtonian shear behavior reduces the pressure distribution, and increases the coating thickness.
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Elasticity of the type usually observed in polymer solutions makes only a minor contribution to the roll-separating (load-carrying) force. The contribution is positive, but smaller than the corresponding negative contribution due to the non-Newtonian shear effects.
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An increase in load-carrying capacity would require a different viscoelastic fluid than the type considered here—one that is essentially Newtonian in shear but, independently, capable of developing significant normal stresses.
TL;DR: A second-generation apparatus was built and used to inflate heat-softened plastic sheets to shapes from hemispheres to large spheroidal bubbles as mentioned in this paper, and the results showed that the maximum areal elongations obtained were 18,600 percent for clear polystyrene at 238°F, 14,300 percent for high-impact poly styrene at 224°F and 1,010 percent for cellulose acetate butyrate at 280°F.
Abstract: A second-generation apparatus was built and used to inflate heat-softened plastic sheets to shapes from hemispheres to large spheroidal bubbles. Three sheet materials, polystyrene, high-impact polystyrene, and cellulose acetate butyrate, were successfully formed beyond the hemispherical shape at temperatures somewhat below industrial levels. Four others, rigid poly-(vinyl chloride), an acrylic-modified PVC, cast poly(methyl methacrylate), and polycarbonate, could not be deformed beyond the hemispherical shape without rupture. Measurements included bubble profiles, thickness distributions and local extension ratios for the final bubbles; two plots of profile growth measured from high-speed motion pictures; and a continuous record of bubble temperatures and blowing pressure from which the meridional temperature distributions were plotted.
Fifteen bubbles were measured and analyzed. Profiles and extension ratios were compared with those computed from the equations of isotropic, isothermal elasticity, using several different strain-energy functions with constant material parameters. Agreement over all fifteen bubbles on all properties was about ±10.8 percent (absolute) for the best models considered. Bubble growth occurred over periods of 5 to 8 seconds, with about 90 percent of the final area of the larger bubbles being generated in the final 1.5 seconds. The maximum areal elongations obtained were 18,600 percent for clear polystyrene at 238°F, 14,300 percent for high-impact polystyrene at 224°F, and 1,010 percent for cellulose acetate butyrate at 280°F.
TL;DR: In this article, two models for simulating the injection molding of thermoplastics in thin, rectangular cavities are presented, one is a much simplified bounded-radial-flow model that utilizes an existing numerical model for semi-circular cavities to adjust for the nonradial flow region bounded by the lateral walls and the other is a two-dimensional analysis which assumes that both viscosity and temperature change strongly across the narrow gap but vary weakly in the directions of flow.
Abstract: Two models are presented for simulating the injection molding of thermoplastics in thin, rectangular cavities. One is a much simplified bounded-radial-flow model that utilizes an existing numerical model for semi-circular cavities to adjust for the non-radial flow region bounded by the lateral walls. The other is a two-dimensional analysis which assumes that both viscosity and temperature change strongly across the narrow gap but vary weakly in the directions of flow. The latter analysis allows application of the potential theory and the determination of streamlines and progressing front shapes. Both models deal with a non-Newtonian viscosity with temperature variation. Comparisons between experimental and computational results are shown.
TL;DR: In this article, the hydrodynamics of the filling of a rectangular mold cavity by a molten polymer is considered in terms of lubrication theory and the relationship of the velocity field to the cavity geometry and temperature dependence of the rheological properties is predicted.
Abstract: The hydrodynamics of the filling of a rectangular mold cavity by a molten polymer is considered in terms of lubrication theory. Both isothermal and nonisothermal mold filling are analyzed. The relationship of the velocity field to the cavity geometry and temperature dependence of the rheological properties is predicted. Increasing the activation energy of viscous flow increases the tendency for channeling of melt through the center of the cavity. The results are compared to the experimental observations of our previous studies.
TL;DR: In this paper, an experimental study of the co-extrusion of polyethylene and TiO2-filled polyethylenes through capillary and rectangular cross-section dies has been carried out.
Abstract: An experimental study of the co-extrusion of polyethylene and TiO2-filled polyethylene through capillary and rectangular cross-section dies has been carried out. The influence of viscosity ratio, cross-section type, die length and duration of flow has been studied. Low viscosity melts will encapsulate high viscosity melts during flow through cylindrical and rectangular dies. Low viscosity unfilled melts can encapsulate higher viscosity filled melts and low vicosity filled melts can encapsulate higher viscosity unfilled melts. However, the rate and extent of encapsulation seems to be greater for the former case. This may be due to differences in the viscosity-shear stress behavior of the filled and unfilled melts. In rectangular dies, the extent of encapsulation for any pair is greater when the interface is initially perpendicular to the shorter cross-section dimension. The results are consistent with the idea that encapsulation primarily depends upon the ratio of die length to initial interface length.
TL;DR: In this paper, an experimental study was carried out to investigate the flow behavior and the viscosity of mica flake-filled polypropylene melts and the mechanical properties of Mica-polypropylene composites.
Abstract: An experimental study was carried out to investigate the flow behavior and the viscosity of mica flake-filled polypropylene melts and the mechanical properties of mica-polypropylene composites. The properties of the molded composites exhibit moduli which are higher than most filled polymers. Tensile strength values are noticeably improved when a silane coupling agent or a carboxyl-modified polypropylene is employed, although the extent of improvement is not great. A high processing temperature helps to minimize flake damage and to improve flow orientation of the flakes.
TL;DR: Statistical analysis has shown that the experimental accumulative curves can be best described by a single parameter exponential equation Y = 1 − e−λα where Y is the accumulative fraction of fibers, α is the orientation angle and λ is a single empirical parameter.
Abstract: Oriented short fiberglass mats were prepared by a converging flow technique of fiber/glycerine dispersions. The fiberglass contained 3 percent of identical colored fibers which were counted from enlarged photographs in regard to their deviation from the alignment axis. Representative accumulative data of fiber alignment are shown for optimal and inadequate preparation conditions. In successful experiments over 90 percent of the fibers were found in the angular range of ±15 deg and 57 percent within the range of ±5 deg. Statistical analysis has shown that the experimental accumulative curves can be best described by a single parameter exponential equation Y = 1 − e−λα where Y is the accumulative fraction of fibers, α is the orientation angle and λ is a single empirical parameter.
TL;DR: The fundamental analysis of the mechanical response of composite media involves investigations on two levels of abstructions: the micro and the macro scale as mentioned in this paper, which are known as micromechanics and lamination theory.
Abstract: The fundamental analysis of the mechanical response of composite media involves investigations on two levels of abstructions: the micro and the macro scale. These areas of study are known as micromechanics and lamination theory. This format is employed to treat a series of problems concerning (1) stiffness, creep, or viscoelastic properties; (2) strength and expansion properties for oriented continuous and short fibers; (3) randomly oriented fibers; (4) injection-molded materials; and (5) particulate reinforcements.
TL;DR: In this article, the rheological behavior of aqueous solutions of Separan AP-30, polymethylcellulose, and polyvinylpyrrolidone was studied experimentally.
Abstract: The rheological behavior of aqueous solutions of Separan AP-30®, polymethylcellulose, and polyvinylpyrrolidone was studied experimentally. These solutions exhibit non-Newtonian flow behavior in simple shear, and are characterized by one of several 2, 3, or 4 parameter rheological equations. The equations used included the power law, the Ellis model, Spriggs equation, the Herschel-Bulkley equation, and Meter's model. The power law model fits the data for each of the solutions over a limited range of shear rates, whereas the other models, which include either a lower shear rate limiting Newtonian viscosity, and/or an upper shear rate limiting Newtonian viscosity, or a yield stress, fit the data well over a wide range of shear rates from 0.00675 to 1076 sec−1. The pressure drop-flow rate data for the same aqueous solutions flowing through packed beds were correlated well by the Ergun equation using the various rheological models applied in this work to evaluate a modified fluid viscosity. In each case it was found that the rheological model which best fit the viscometric data also gave the best packed bed friction factor correlation, and that no one model, such as the powerlaw, or the Ellis model, is the best one to use in all cases for all solutions. For polyvinylpyrrolidone solutions large deviations between experimental values of friction factor and those from the Ergun equation occurred for modified Reynolds numbers greater than one. A pseudo viscoelastic parameter was used to improve the friction factor correlation empirically at high Reynolds numbers.
TL;DR: In this paper, the limiting conditions under which an oriented structure is actually possible in extensional flows are critically reviewed, and it is shown that orientation can be obtained either when entanglements are present, i.e., when the polymer molecules form some sort of network, or, if the molecules behave entirely individually, only under very special circumstances.
Abstract: A characteristic feature of extensional flows is their potential capability of determining a stable, oriented structure in liquid materials containing elongated particles. With reference to flexible linear macromolecules, either in solution or as polymer melts, the limiting conditions under which such an oriented structure is actually possible are critically reviewed. It is seen that orientation can be obtained either when entanglements are present, i.e. when the polymer molecules form some sort of network, or, if the molecules behave entirely individually, only under very special circumstances. Further, if the objective is the “freezing” of the oriented structure in a partly crystalline, high-strength material, the conditions become even more restrictive.
TL;DR: In this paper, a series of discontinuous fiber-reinforced thermoplastic composites made with random-in-plane fiber orientation are presented. But the properties of the experimental composites could not be predicted from constituent properties.
Abstract: The mechanical properties are presented for a series of discontinuous fiber-reinforced thermoplastic composites made with random-in-plane fiber orientation. The matrix and fiber materials were chosen to provide a wide range of strength, modulus, ductility and adhesive properties. In many cases strong, rigid, yet tough composites were fabricated. Strength levels of over 20,000 psi and modulus values over 1,000,000 psi were reached in several systems reinforced with short Kevlar-49 and graphite fibers. A strong dependence of composite strength and modulus on fiber strength and modulus was noted indicating good transfer of load from matrix to reinforcement. Fiber efficiency factors for modulus and strength were calculated for the experimental composite systems and averaged 0.19 and 0.11 respectively. Data were analyzed using basic composite theory. Properties of the experimental composites could not be predicted from constituent properties.
TL;DR: In this paper, a survey of environmental effects on the mechanical behavior of fiber-reinforced plastics is presented, dealing with the separate and combined effects of temperature, hygrometry and loading on mechanical characteristics.
Abstract: This survey covers the topic of environmental effects on the mechanical behavior of fiber-reinforced plastics. It deals also with the separate and combined effects of temperature, hygrometry and loading on mechanical characteristics. The response of the phases of the composite-fiber, the matrix and the interfacial region to the environmental loading history, is discussed. Conflicting hypotheses regarding the causes and mechanism of the degradation process and of the long-range durability of the composites are examined.
TL;DR: In this paper, the authors studied the effect of annealing on the sorption kinetics and equilibria of n-hexane in polystyrene and poly(2,6 dimethyl-1,4 phenylene oxide) films.
Abstract: The sorption kinetics and equilibria of n-hexane in glassy polyblends of polystyrene and poly(2,6 dimethyl-1,4 phenylene oxide) were studied as a function of annealing conditions. Cast film samples were annealed 20°C above their respective glass transition temperatures for two hours and twenty-four hours. The rate of relaxation-controlled (Case II) sorption of n-hexane in these films was reduced markedly consequent to annealing. The effect of annealing on the sorption kinetics and the independently determined film densities was more pronounced for the poly(phenylene oxide)-rich samples. Although sorption rates were reduced by as much as a factor of 100, the sorption equilibrium was insignificantly affected by annealing. Super Case II transport was observed for the slow absorbing annealed samples whereas the more rapid sorption in the unannealed samples followed ideal Case II kinetics. The more pronounced effects of annealing for the poly(phenylene oxide)-rich samples on sorption rates and film densities were explained by considering the increasing difference between the film Tg and the drying temperature used in the original film preparation for the poly(phenylene oxide)-rich samples. These results suggest that glassy polymers, cast and dried well below their glass transition temperatures, will be subject to large long-term reductions in absorption rates and specific volume. Moreover, residual, excess free volume significantly affects relaxation-controlled absorption of vapors in partially annealed glassy polymers.