Showing papers in "Polymer Engineering and Science in 1971"

Stress-strain behavior of styrene-acrylonitrile/glass bead composites in the glassy region

Abstract: The effects of temperature, strain rate and filler content on tensile properties of SAN/glass bead composites are studied. A point of discontinuity on the stress-strain curves for unannealed composites is investigated, annealing results in smooth curves with no discontinuities. A simple model for the filler effect on yield stress is suggested and shown to be in a good agreement with experimental data. A double shifting procedure to account for the temperature and filler effects on yield stress as a function of strain rate is proposed. A single master curve that can be represented by the equation: relates composite yield stress to strain rate, temperature and filler volume fraction.

Dynamic mechanical properties of polyurethane block polymers

Abstract: The dynamic mechanical properties of polyester and polyether urethane block polymers have been investigated at four frequencies (3.5, 11, 35 and 110 Hz) in the temperature range of — 150 to 200°C. The existence of a two phase structure was demonstrated in these systems by the observation of two major transition regions corresponding to (1) the glass transition temperature (Tg) of the ester or ether soft segments, and to (2) the softening temperature of the aromatic-urethane hard segments. Several secondary relaxations were observed in addition to the two major relaxations. It was possible to assign molecular mechanisms to each of these relaxations. All relaxation phenomena were greatly influenced by the molecular weight of the prepolymer, weight percent of hard segments, and thermal history. An increase in the molecular weight of the prepolymer above 1,000 at constant hard segment content resulted in a semi-crystalline material, which possessed a lower Tg for the macroglycol segments. Annealing to enhance crystallinity increased the Tg of the soft segments, consistent with the usual observation in semicrystalline homopolymers. These findings suggest that the relaxation mechanisms of polyurethane block polymers are not only influenced by the degree of crystallinity, but also by the nature of the domain structure.

The interaction of alkoxy silane coupling agents with silica surfaces

Abstract: The interactions of γ-aminopropyltriethoxy silane (A-1100), γ-methacryloxytrimethoxy silane (A-174), γ-glycidoxytrimethoxy silane (A-187), and ethyltriethoxy silane (A-15) with silica surfaces have been studied by means of infrared spectroscopy. The results indicate that the major force holding the silane to the silica surface after application from dilute solutions is primary chemical bonding. These bonds are formed by a condensation reaction between silanols on the hydrolyzed form of the silane and hydroxyl groups on the silica surface. In the case of the amino-silane (A-1100), hydrogen bonding was found to exist but was of minor importance in bonding the silane to the surface. In studying the effects of the addition of water, acetic acid, or n-propylamine to various silane treating solutions, it was found that n-propylamine has a unique catalytic effect on the condensation reaction. This catalytic effect explains the observation that γ-aminopropyltriethoxy silane is more reactive than the other silanes studied. It is felt that silanes not containing an amine group can be made more effective if they are applied in the presence of an amine catalyst.

Strength properties of discontinuous fiber composites

Abstract: The longitudinal strengths of unidirectional discontinuous fiber composites have been calculated based on the concept of perturbation effect and the distortional energy criterion, utilizing the finite-element method. The theoretical results thus obtained are compared with the experimental data as well as the results of shear lag analysis for tungsten-copper, boron-aluminum, glass-epoxy and boron-epoxy composites. Based on the Jackson-Cratchley equations, modified for discontinuous fibers, a formula is also proposed for calculating the strengths of randomly oriented discontinuous fiber composites. The results calculated from this formula are compared with the experimental data for aluminum oxide-aluminum-silicon and glass-epoxy composites.

Modulus of short carbon and glass fiber reinforced composites

Abstract: A theoretical model for a short fiber reinforced composite is proposed The composite is assumed to consist of an aggregate of sub-units, each sub-unit possessing the elastic properties of a reinforced composite in which the fibers are continuous and fully aligned The elastic constants of a partially oriented composite are then calculated by the Voigt and Reuss averaging procedures, giving upper and lower bounds respectively for the composite modulus Comparison is made with experimental data for such composites The measured modulus of glass and carbon fiber composites is found to be given by the Reuss or lower bound, to a good approximation compared with the difference between the bounds, for fiber orientations ranging from almost isotropic to highly aligned

A theoretical melting model for plasticating extruders

Abstract: A theoretical model for melting in plasticating extruders is described. Compared to previous models, this model introduces more accurate and less restrictive assumptions, adds a mass balance on the entire channel, and replaces certain approximate solutions by exact solutions. Flow of the solid bed is represented by a solid bed acceleration parameter, SBAP, which permits solid bed acceleration in a screw compression section. New experimental melting data for a variety of screw designs, polymers, and extruder sizes are presented and compared to the theoretical predictions. With the optimum SBAP, reasonably accurate model prediction of the melting profiles is observed for a wide variety of cases.

Cold rolling of polymers. 1 Influence of rolling on properties of amorphous polymer

Abstract: Strips of ductile, amorphous thermoplastic polymers such as polycarbonates, acrylonitrile-butadiene-styrenes, polysulfones and polyphenylene oxides have been reduced in thickness by passing them through a metalworking rolling mill at room temperature. Properties of the rolled strips were studied as a function of the thickness reduction. In addition to rolling unidirectionally, cross rolling (biaxial rolling) was also studied using equal thickness reductions in each direction. The maximum thickness reduction (to — tf)100/to which could be achieved was approximately 60 per cent regardless of the polymer studied here. Stress-strain curves, density changes, thermal stability, hardness and Izod impact strengths have been determined as a function of thickness reduction and sheet direction.

Extensional flow of non-newtonian fluids—A review

Abstract: Extensional flows have been the object of study in several laboratories in recent years. Polymeric systems have been studied in most cases because of their interesting behaviour and also because of the importance of their rheological properties to the plastics engineer. Controlled, steady elongation is more difficult to achieve in the laboratory than the more traditional viscometric flows. Moreover, it is not in general possible to predict the response of a viscoelastic material to steady extension based on knowledge of its viscometric functions. This review begins by presenting some useful expressions describing the kinematics of extensional flows. Then some results of interest from rational mechanics are presented and the behaviour predicted by a number of constitutive equations for viscoelastic fluids are discussed. After presenting the pertinent relations of linear viscoelasticity for extensional flows, experimental methods and results for steady simple extension are reviewed and some possible implications for the processing of molten polymers are discussed.

Measurements on the biaxial extension viscosity of bulk polymers: The inflation of a thin polymer sheet

Abstract: An experimental study of the inflation of a thin polymer sheet has been conducted to determine whether this technique can be used to measure the biaxial extensional viscosity of bulk polymers. Viscosities were determined at various extensional strain rates using two undiluted polyisobutylene samples having different molecular weights. Advantages, limitations, and errors associated with the method are discussed.

Uniformity of flow from sheeting dies

Abstract: A numerical method for analyzing the uniformity of flow from sheeting dies is presented. The method assumes that the flow is isothermal and that the material is a power law fluid. A uniformity index is defined and methods are presented which enable die dimensions to be established which will result in a specified uniformity at particular flow rates and pressure drops. The analysis shows that the flow index “n” of the power law is the key parameter determining the uniformity of flow from a sheeting die.

The effect of molecular weight and orientation on the sorption of n-pentane by glassy polystyrene

Abstract: The effects of polymer molecular weight, molecular weight distribution, and orientation on the rate of relaxation-controlled sorption of n-pentane by glassy polystyrene were studied. The sorption follows Case II kinetics but for films which sorb slowly the sorption rate increases at relatively long times until sorption is sharply terminated. This rate increase may be explained by the development of dispersed microvoids within the unrelaxed film core. Overshoot of the equilibrium n-pentane content occurs in sorption experiments in which accelerated sorption is pronounced. The sorption rate is independent of polymer molecular weight and molecular weight distribution per se over a broad wrange of these parameters. Essentially identical vapor sorption kinetics were observed for well annealed polystyrene films of different molecular weights and distributions. Conversely, for vapor sorption by uniaxially oriented films and for liquid sorption by partially annealed films, high molecular weight film (1,880,000) exhibits greater sorption rates than low molecular weight film (ca. 200,000). These differences in rate are not due to molecular weight differences per se, but are a consequence of the dissimilar response of free volume and strain development for films of different molecular weight prepared with a given time-temperature-strain history. Crazing of carefully annealed polystyrene films occurs during desorption of n-pentane from partially saturated films. The depth of craze penetration reflects the point of advance of the discontinuous Case II sorption boundary.

The effect of hydrostatic pressure on the tensile properties of plastics

Abstract: The effect of hydrostatic pressure, up to 112,000 lb/in.2, on the tensile properties of four polymers is reported. The pressure soaking of polystyrene in castor oil had no significant effect on the material's residual properties when tested under ambient conditions. When tested under pressure polystyrene necked, like a metal, and exhibited a brittle ductile transition at 40,000 lb/in2. Between 40,000 to 112,000 lb/in.2 the tensile strength increased by about 30%. Young's modulus and yield strength were only slightly affected by pressure. Similar results were obtained for specimens sheathed to prevent possible plasticization of the polystyrene. Polymethylmethacrylate tested at 112,000 lb/in.2 failed just short of its instability point and with only a slight increase in Young's modulus. These amorphous polymers thus behaved under pressure in a generally similar manner to metals. Pressure had a marked effect on the stress-strain curves of two crystalline polymers polyethylene and nylon. Young's modulus and tensile strength were considerably increased and elongation decreased. Pressure inhibited ‘drawing’ of the materials. Deformation was restricted to a small necked region.

Advances in instrumentation and technique of torsional pendulum and torsional braid analyses

Abstract: Instrumentation and techniques of torsional pendulum and braid analyses, studying trace moisture and cure cycle effects on thermomechanical spectra of polymeric materials

Stress‐time superposition of creep data for polypropylene and coupled glass‐reinforced polypropylene

Abstract: Glass-reinforced thermoplastics are being considered in many structural applications and fabricators require design information on these materials. Basic creep data are, in many cases, the most useful for design purposes. The work reported here concerns the development of methods of increasing our efficiency in generating creep data. The methods developed are applicable to polypropylene and coupled, glass-reinforced polypropylene. A stress-time superposition procedure has been found valid for extending creep data generated on coupled glass-reinforced polypropylene at several glass levels and at temperatures of 23 and 80°C.

Temperature profiles for polymer melts in tube flow. Part II. Conduction and shear heating corrections

Abstract: A temperature probe system to measure radial temperature profile of polymer flowing in a rod die and a method to systematically correct the conduction and the frictional shear effects were developed. Experimental data obtained on a 1-1/2-inch extruder using a blow molding compound show that both conduction and frictional shear heating effects are significant in melt temperature measurement and that the radial temperature profiles of the melt in the rod die are influenced by the RPM of the screw and the die-wall temperature. The reliability of the temperature data obtained was compared with the solution obtained from the equations of motion and energy. A good agreement between the predicted versus experimental temperature profile exists. For this polymer system, the relationship between local Nusselt number and the velocity parameter could be adequately described with the theory of Van LeeuWen.

A study of polymer melt flow instabilities in extrusion

Abstract: An experimental study is carried out to elucidate the melt fracture mechanism. For the study, the electrical outputs of the melt pressure transducers, mounted on the wall of a circular tube, are recorded on a Sanborn recording system. It has been observed from this study that the wall pressures start to fluctuate prior to the inception of visible melt fracture. The frequency and amplitude of the excursion signals are increased as the shear rate is increased up to and beyond the critical value. This behavior has been observed for three polymer samples tested: polystyrene, high density polyethylene, and low density polyethylene. The fluctuations of wall pressures are interpreted as due to an irregular flow of polymer melts at the tube wall. It is further investigated to find the effects of the die entry angle of a capillary, and capillary length-to-diameter ratio on the critical shear rate. A correlation between exit pressure and shear stress has been found for polystyrene and low density polyethylene. It is found that at and above the critical stress, the exist pressure increases abruptly. This is interpreted to be a sudden change in the elastic properties of the materials above melt fracture since Han et al have shown that the exist pressure is a measure of the elastic properties.

Melt fracture of polystyrene

Abstract: The results of an experimental study of melt fracture using polystyrene samples of narrow and broad molecular weight distribution are presented. The weight average molecular weight (Mw) ranged from 9.72 × 104 to 1.80 × 106 and the distribution breadth Mw/Mη from 1.06 to 9.21. The critical shear stress varies linearly with 1/Mw, increases slightly with temperature and is independent of the distribution of molecular weights. This type of behavior is satisfactorily explained in terms of Graessley's entanglement theory.

Crystallization studies on deformed polybutene-1 melts.

Abstract: : An apparatus was designed and built which allowed an investigation concerning the morphology and kinetics of crystallization of a deformed polybutene-1 melt. The polymer was quenched from a temperature above its melting point to one of two crystallization temperatures. The supercooled melt was then sheared and allowed to crystallize isothermally while the internal stress in the melt was continuously recorded. A polarizing microscope was employed for the simultaneous study of the resulting morphological changes. By properly accounting for thermal contractions within the apparatus caused by the quenching operation, as well as the imposed shear strain, a strain history of the polymer during crystallization was developed. From this strain history and the assumption that the volume contraction resulting from crystallization was isotropic, approximate kinetics of crystallization in a deformed polymer melt were determined. (Author)

The effect of time and temperature on the mechanical behavior of epoxy composites

Abstract: A crosslinked epoxy resin consisting of a 60/40 weight ratio of Epon 815 and Versamid 140 and composites of this material with glass beads, unidirectional glass fibers and air (foams) were tested in tension, compression and flexure to determine the effect of time and temperature on the elastic properties, yield properties and modes of failure. Unidirectional continuous fiber-filled samples were tested at different fiber orientation angles with respect to the stress axis. Strain rates ranged from 10−4 to 10 in./in.-min and the temperature from −1 to 107°C. Isotherms of tangent modulus versus strain rate were shifted to form master modulus curves. The moduli of the filled composites and the foams were predictable over the entire strain rate range. It was concluded that the time-temperature shift factors for tangent moduli and the time-temperature shift factors for stress relaxation were identical and were independent of the type and concentration of filler as well as the mode of loading. The material was found to change from a brittle-to-ductile-to-rubbery failure mode with the transition temperatures being a function of strain rate, filler content, filler type and fiber orientation angle, indicating that the transition is perhaps dependent on the state of stress. In the ductile region, an approximately linear relationship between yield stress and log strain is evident in all cases. The isotherms of yield stress versus log strain rate were shifted to form a practically linear master plot that can be used to predict the yield stress of the composites at any temperature and strain rate in the ductile region. The time-temperature shift factors for yielding were found to be independent of the type, concentration and orientation of filler and the mode of loading. Thus, the composite shift factors seem to be a property of the matrix and not dependent on the state of stress. The compressive-to-tensile yield stress ratio was practically invariant with strain rate for the unfilled matrix, while fillers and voids raised this ratio and caused it to increase with a decrease in strain rate. The yield strain of the composites is less than the unfilled matrix and is a function of fiber orientation and strain rate.

Thermally stable photoresist polymer

Abstract: A new photoresist stable to 500°C has been developed. The addition of potassium dichromate to a polyamic acid, believed to be the condensation product of pyromellitic dianhydride and 4,4′-diaminodiphenylether, results in a photosensitive polymer which can be cast or spun. The resultant film after exposure, development and post-bake is a crosslinked aromatic polyimide. An efficient developing solution consisting of a 5:1 mixture of hexamethylphosphoramide and dimethylsulfoxide has been discovered. The system has been fully characterized in terms of composition, film formation, exposure, development, hardening and removal. This material has already proven useful as an area-controlled, thermally stable dielectric and as a sputter-etch resist. It has been demonstrated that under the conditions of sputter-etching (bombardment of substrate with Ar, Ar+ species with energies from 1–300 eV) commercial photoresists, of the polyisoprene variety, char to the extent of being ineffective as a masking material. Under these same conditions, the thermally stable resist retains its film integrity and uniform sputter-etch rate. An exemplary processing sequence is included as an appendix.

Unstable flow of amorphous polymers through capillaries I. Velocity profiles of polymer having discontinuous flow curve

Abstract: Cine film was employed for recording the movement of tracer particles, thus directly determining the velocity profile in the capillary flow of model materials of polybutadiene over a range of shear stresses relating to (1.) flow conditions in which there is no distortion of the regular stream of melt emerging from a capillary, (2.) severe surface fracture, (3.) elastic turbulence, (4.) oscillatory flow of melts with discontinuous flow curves. No qualitative difference was found between the velocity profiles in conditions (1) and (2); surface fracture is not related to laminar flow disturbances and originates at the capillary exit. Surface fracture must be distinguished from elastic turbulence. The streamline velocities undergo both local and time variations within the capillary under conditions of developed elastic turbulence; the wall velocity is non-zero. A periodic pulsation of velocities occurs in the flow curve discontinuity region; it takes place throughout the capillary and is caused by stick-slipping of polymer on the capillary wall. The frequency of velocity pulsations coincides with the frequency of extrudate variations.

An experimental study of plasticating in a reciprocating‐screw injection molding machine

Abstract: Experimental studies were performed with low density polyethylene, acrylonitrile-butadiene-styrene polymers and on polyvinyl chloride to elucidate the nature of the plasticating process in a reciprocating-screw injection molding machine. Melting data, obtained by use of the “cooling experiment,” and plastic temperature data reveal that the screw recharge process is a transient plasticating extrusion process which gradually approaches the equilibrium extrusion behavior as the screw rotates. If the screw rotation time is a high percentage of the total cycle time, the plasticating behavior is very similar to steady-state extrusion behavior, but if the screw rotation time is a small percentage of the total cycle time, the plasticating behavior is significantly different. Furthermore, better plasticating is obtained by use of a low RPM and high percentage rotation time than by a high RPM and low percentage rotation time.

On the solvent stress‐cracking of polycarbonate

Abstract: The stress-cracking of polycarbonate by a gaseous or liquid agent results from the diffusion of this agent into the polymer. The low molecular weight polymer fractions and the chain ends within the bulk of the polymer become more ordered during the diffusion and swelling process by their partial solubility in the crazing agent, causing crystallization. The creation of interfaces at areas of order-disorder causes high shearing forces at this boundary and voids within the bulk of the polymer. These voids are then propagated as crazes or cracks at stresses much lower than the tensile strength of the polymer. Therefore, a stress-cracking agent need not diffuse rapidly, but must be an effective environment for swelling and/or crystallization. Data from diffusion, density, thermal and molecular analyses are presented to support this mechanism.

Influence of reservoir diameter on the melt elasticity in capillary flow

Abstract: An experimental study has been carried out to investigate the influence of the reservoir-to-capillary diameter (DR/D) ratio on the elastic properties of polymer melts flowing through capillary tubes. For the study, a capillary rheometer, described in an earlier paper by Han, was used with DR/D ratios of 3, 6, 9 and 12. Measurements were taken of wall normal stresses in high density polyethylene along the axis of the capillary, and the axial pressure profile was used to obtain the “exist pressure,” which is believed to be an elastic phenomenon. The analysis of the experimental data indicates that, for a given capillary length-to-diameter (L/D) ratio, the exit pressures first increase and then level off, as the DR/D ratio is increased. This behavior of the exit pressure is in accord with that of extrudate expansion which has also been determined in the experiment. This dependence of the exist pressure and extrudate expansion ratio on DR/D ratio is explained with the concept that the amount of recoverable elastic energy stored in the melt flowing through the capillary depends on the strain history, which the melt experiences on entering the capillary.

A theoretical melting model for a reciprocating‐screw injection molding machine

Abstract: A theoretical model for the transient melting behavior in a reciprocating-screw injection molding machine is proposed. The model is based on a steady state extrusion model, Neumann's melting problem, and a heuristic postulate for the transient behavior. The model predictions are compared to experimental melting data for low density polyethylene, acrylonitrile-butadiene-styrene polymers and polyvinyl chloride for a variety of operating conditions and two screw designs. A useful degree of correlation is demonstrated for all experimental cases.

Factors influencing the thermal oxidation of polyethylene

Abstract: The thermal stability of polyethylene containing conventional antioxidants is adversely affected by contact with copper, by certain pigments, and by the addition of a few percent of polypropylene as a processing aid. Polyethylene inhibited with 0.1% of phenolic antioxidants has approximately the same oxidative stability when in contact with a copper surface as the unprotected polymer. A mechanism is suggested to account for the loss of stability in the presence of copper. Pigments vary in the extent to which they adversely affect the oxidative stability of polyethylene. Since several factors may combine to decrease the stability of protected polyethylene compositions, as determined by accelerated tests, it is essential that their contribution under service conditions be determined. The predicted life at temperatures encountered in service is determined by extrapolating accelerated test data to that constant temperature which is calculated to cause the same degree of degradation as would occur during the daily and seasonal temperature cycles encountered in use.

Plasticating single-screw extrusion theory†

Abstract: Various solid conveying, melting, and metering theories developed for plasticating screw extrusions are reviewed. Some recommendations for improving the present melting theories are presented. It is pointed out that the overall performance of a screw extruder can be predicted only by combining the solid-conveying, melting and metering theories into a complete mathematical package because the three functions of a screw extruder depend on each other and cannot be separated. A suggestion is made on how to combine these three theories.

Holographic grating formation in photopolymers—Polymethylmethacrylate

Abstract: Polymethylmethacrylate has been sensitized to 4880 A light and used as a holographic recording medium. The polymer is sensitized with a material which may itsel be degraded by exposure to ultraviolet light. The new process is, therefore, both self-developing and fixable without chemical processing. Holographic diffraction gratings written into this material have diffracting efficiencies as high as 70 percent. Various parameters such as angular selectivity, film thickness, variation of diffracting efficiency with writing angle etc., have been studied. The spontaneously produced diffracted intensity is so high that efficient interference between first and zero ordes is observed during the writing process.

Prediction of high density polyethylene processing behavior from rheological measurements

Abstract: In order to predict the processing behavior of a high density polyethylene resin one must know the resin flow behavior over a wide range of shear rates. Low shear properties are important in applications where melt strength, sagging, etc. are critical. On the other hand, high shear flow properties are a determining factor in applications where melt instability, melt fracture and heat generation are important. The flow behavior of a resin can be established by measuring the zero shear viscosity, η0, the maximum relaxation time, τ0, and the shape of the flow curve. We have measured these basic rheological parameters on a large number of high density polyethylene resins. A shear sensitivity parameter which is independent of molecular weight was derived from a correlation between η0 and τ0. This parameter, together with η0, provide the vital information needed in order to predict the processing behavior of the resin. This method is applicable to other polymer systems provided that the rheological parameters η0 and τ0 can be experimentally obtained.

Capillary extrusion of composite materials

Abstract: Investigations on the extrusion characteristics of composite systems were performed on the Sieglaff-McKelvey capillary rheometer, with particular emphasis on the characterization of flow instability and “melt fracture” phenomena. The mechanisms of melt fracture appear to be identical for both the filled and unfilled polymers (1. Polyethylene with glass beads; 2. Ethylene-propylene copolymer with graphitized carbon black). In all cases, the flow curves exhibit a plateau at some value of the shear stress. Above this shear stress plateau, melt fracture occurs. Although slip flow is the dominant mode of transport during melt fracture, the slippage in the tube may not be a necessary condition for the subsequent severe melt fracture.