bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of sizings on microscopic flow in resin transfer molding
Authors: Palmese G. R. , Karbhari V. M.
Published: December 2004

Sizings significantly affect the processing and final performance characteristics of resin transfer molded (RTM) parts. Manufacturers often use sizings to enhance compatability between the resin and reinforcement and to assist processing. In particular, such coatings can affect the microscopic flow characteristics of resins during the infusion stages of the RTM process. To understand the mechanisms by which sizings affect preform wet‐out and the extent of such effects, the influence of fiber sizings and fiber loadings on the flow of a vinyl‐ester resin system, through axially aligned carbon fibers, have been measured. The results and analysis of the experiments reported herein form the basis for a phenomenological model that describes the effect of sizings on micro‐flow in RTM. Furthermore, the work provides insight into the relationships among micro‐flow, macro‐flow, and preform infiltration. It is seen that sizings dramatically influence micro‐impregnation and, for the flow configuration studied, radial micro‐flow into fiber bundles was the rate limiting process for complete fiber bundle infusion.
Percolation conductivity of polymer composites filled with dispersed conductive filler
Authors: Mamunya E. P. , Davidenko V. V. , Lebedev E. V.
Published: December 2004

This paper deals with the conductivity of binary polymer composites filled with an electronically conductive material. A “dynamic cluster model” is offered to describe the conductivity of such polymer composites in the highly filled region, i.e. above the percolation threshold. The model is based on the following assumptions:

a modification of the basic statistical percolation equation, i.e. σ (φ−φc)t, where t = 1.6 to 1.9, should be applied for all systems in the highly filled region, although application is limited to the range φ = φc + Δφ, Δφ ⟹ 0 in the strict statistical percolation approach;

the most important modifications with respect to the basic equation of the statistical percolation theory are

(a) the use of a constant teff, including a constant part t1 (resembling “t” in the basic statistical percolation approach) and a variable part t2 (depending on the filler concentration φ of the specific mixture) and

(b) the definition of φc as the filler concentration where a perfect three‐dimensional network of the conductive phase has been established. This idea has been adopted from the bond‐percolation approach of Aharoni;

the resulting equation should include parameters of specific polymer composites.

The generalized equation σ = f(φ) is used to calculate the maximum possible conductivity of a certain mixture as well as the dependence of σ on the filler content.
Impact behavior of carbon fiber/ultra‐high modulus polyethylene fiber hybrid composites
Authors: Jang Jyongsik , Moon Sung‐In
Published: December 2004

Carbon fiber (CF)/ultra‐high modulus polyethylene (UHMPE) fiber hybrid composites were fabricated using vinyl ester resin as a matrix. Interfacial adhesion of carbon fiber/vinyl ester composites and UHMPE fiber/vinyl ester composites as model composites was optimized using low temperature plasma treatment. Interlaminar shear strengths of carbon fiber/vinyl ester and UHMPE fiber/vinyl ester homocomposite were greatly increased by plasma and silane coupling agent treatment. From the result of the impact test, total absorbed energy of carbon fiber/UHMPE fiber hybrid composites was correlated with laminating sequences at optimized interfacial adhesion between the reinforcing fiber and matrix resin. UHMPE fiber layers of hybrid composites played an important role in absorbing energy. Elastic and plastic deformation of UHMPE fiber layers also played a key role in improving the impact properties of carbon fiber/UHMPE fiber hybrid composites.
Influence of cellulose as a filler in vulcanized rubber composites
Authors: Nunes R. C. R. , Mano E. B.
Published: December 2004

Composites of elastomers and cellulose have been investigated. Copolymers of styrene‐butadiene rubber (SBR) and acrylonitrile‐butadiene rubber (NBR) as well as natural rubber (NR) were compounded with regenerated cellulose (Cellulose II). The technique for the incorporation of filler was based on the coprecipitation of the rubber latex‐cellulose xanthate mixture. Cellulose filler was used in the range of 0 to 30 phr. The best results were shown by the natural rubber‐regenerated cellulose systems. The different behavior of the systems is explained by experimental evidence. The reinforcement mechanism for NR compositions, which involves an induced crystallizing rubber (NR), seems to be different from the mechanism for SBR and NBR.
Effect of molecular weight distribution on fatigue crack propagation in poly(methyl methacrylate)
Authors: Kim S. L. , Janiszewski J. , Skibo M. D. , Manson J. A. , Hertzberg R. W.
Published: December 2004

It is well established that both molecular weight (M) and its distribution (MD) affect many polymer properties such as mechanical behavior. Thus studies have shown that fatigue life is enhanced by increases inM. Research here has shown that with notched specimens fatigue crack propagation (FCP) rates are dramatically decreased by increasingM, even when theMis high enough that the static fraeture energy has essentially reached its asymptotic limit. In this study, specimens of poly(methyl methacrylate) containing either high‐ or low‐Mtails were prepared and characterized. The earlier finding that FCP rates are inversely related to averageMwas confirmed, but specific effects ofMdistribution were observed. At constantMn, a low‐Mtail had little effect on FCP resistance, while a high‐Mtail improved FCP resistance of polymers whose averageMwas too low for effective entanglements. Thus with high‐Mtails, it was possible to test specimens whose averageM's were too low to permit machining. It is proposed that the effects noted are due to relative stabilization or destabilization of crazes ahead of the crack.
The temperature dependence of network forces in rubber‐like materials
Authors: Staverman A. J.
Published: December 2004

In the simplest model of rubber‐like materials only two kinds of forces are assumed: network forces with identical temperature dependence, and liquid forces which. are isotropic. For this simple model the relation between the temperature dependence of the network forces and that of the observed elastic force is calculated. When the elastic equation of the network is of the classical one term type, the well‐known equation of Flory, Ciferri and Hoeve is found without using assumptions about the physical meaning of the front factor in the elastic equation. When the elastic equation of the network is of the Mooney‐Rivlin type with two terms, the difference between the temperature dependence of the observed forces and of the network forces is found to depend on the ratio of the coefficients in the Mooney‐Rivlin equation and on the elongation. The effect of the internal pressure is to add a factor to this term. The conclusion is that detailed knowledge about the network forces is needed in order to establish the value of the correction term for thermal expansion in the equation for the temperature dependence of the elastic force in the material.
Entanglement and excluded volume effects in rubber elasticity
Authors: Gaylord Richard J.
Published: December 2004

The role of interchain excluded volume and entanglement in the elastic behavior of polymeric networks is theoretically examined. A three‐chain network model is used, with each chain confined within an infinite, rectangular cylinder. The cylinders and the network crosslinks, are assumed to deform affinely. When the cross sections of the cylinders are small, the network elastic free energy equation has the form

The λs represent the macroscopic deformation ratios. The constants A, B and C are functions of the number of each type of network chain (i.e., A contains the number of tie chains; C contains the number of loops and tie chains; B contains the number of dangling chain ends, unattached chains, loops and tie chains), their unperturbed dimensions and the sizes of the cylinders which confine them.
Fiber growth in tubular flow
Authors: McHugh A. J. , Ejike E. , Silebi C. A.
Published: December 2004

Data are presented for the seeded growth of polyethylene fibers from solutions undergoing laminar flow in tubular geometry. Radial growth rates are reported for solutions of high and low molecular weight fractions for the second stage fiber thickening process which occurs in the tube entrance region. Results are also shown for high molecular weight growth from fiberglass seeds which indicate an enhanced growth rate at elevated temperatures. A two‐stage growth pattern documented earlier for the high molecular weight fraction is shown to occur for the lower molecular weight material. The Discussion includes an analysis for the tapered fiber geometry in the second stage of growth and calculations for stress‐induced, diffusion‐limited growth in the first stage downstream from the tube entrance.
Polymerization of fibrin in shear flow fields
Authors: Clark H. G. , Puryear H. A. , Casper R. A.
Published: December 2004

The reaction of fibrinogen with thrombin to produce fibrin goes through a multistep sequence in which up to four peptides are cleaved off. When this reaction takes place at pH 6.3 in a shear flow field, the formation of fibrous product is retarded as compared with stagnant conditions. The retardation is greater at 50 s−1 than at 135 s−1. The morphology of the coagulated product appears to be different when produced at 50 s−1 than at either stagnant or 135 s−1, with the material at 50 s−1 having a sheetlike appearance in contrast to a fibrous appearance for the material at 135 s−1 and stagnant conditions: The explanation of the retardation may lie in the separation of thrombin from fibrinogen during the sheared reactions. Shear rate varies more in the vascular bed than does pH or ionic strength and probably is a more important factor in controlling physiological coagulation than these more frequently studied variables.
The classification of solvents for bis‐phenol‐a polycarbonate
Authors: Heiss Herbert L.
Published: December 2004

A procedure is described which permits the classification of solvents according to their effect on polycarbonate (PC):

Class I—dissolve amorphous or crystalline PC

Class II—dissolve only amorphous PC

Class III—convert amorphous PC to crystalline PC

Class IV—none of the above.

This classification is achieved by comparing solvents to CH2Cl2 with respect to the exotherm and rate of crystallization observed when the solvents are brought into contact with PC under controlled conditions.
Pressure dependence of diffusion coefficient for CO2 in glassy polymers
Authors: Toi Keio
Published: December 2004

The pressure dependence of the apparent diffusion and permeation coefficients were observed by using the permeation time‐lag method for CO2 in glassy poly(ethylene terephthalate) (PET), polystyrene (PS) and poly(vinyl chloride) (PVC) below 1 atm. The results show that the permeation coefficient is constant whereas the diffusion coefficient increases with pressure. According to the theoretical prediction of Paul, it can be concluded that the adsorbed CO2 in these glassy polymers is completely immobilized and does not participate directly in the diffusion. A computer was used in the numerical calculation to determine the true diffusion coefficient from the model of Paul, et al. A comparison of the curves calculated with these constants and experimental values gave excellent agreement for the three glassy polymers. But there is a large difference between the values of one of the parameters obtained by this time‐lag method and the sorption method. Relations between this difference and the magnitude of the parameters are discussed.
Selection of barrier materials from molecular structure
Authors: Lee W. M.
Published: December 2004

A prediction technique for gas permeability from polymer structure has been developed on the basis of a specific free volume diffusion theory. In this theory, the free volume available per unit mass in a polymer structure controls the rate of gas diffusion and, hence, its rate of permeation. The smaller this specific free volume is, the more difficult the gas diffusion and, thus, the better its barrier to gases becomes. Specifically, the theory predicts a linear relationship between log (permeability) and (−1/specific volume). A number of existing polymers covering six orders of magnitude in CO2 permeability and O2 permeability were found to follow this correlation. The specific free volume in a polymer was obtained from group contribution calculations. As a result, the gas permeabilities become predictable from the specific volume in a polymer which, in turn, varies with its molecular structure. The advent of this specific free volume theory for gas permeation simplifies greatly the selection of barrier materials for packaging applications. For a given barrier application, a critical specific free volume is first defined from its gas barrier requirement. The polymer structures having specific free volumes smaller than the critical value are then identified. These are the polymers that would have the necessary barrier performance. By this theory, molecular structures, with string polar‐to‐polar interactions and hydrogen‐bonding forces are found to be good barriers to CO2 and O2.
Corona‐discharge treatment of polymeric films, II: Chemical studies
Authors: Carley James F. , Kitze P. Thomas
Published: December 2004

Polymeric films, chiefly polyethylenes, were subjected to corona‐discharge treatment in a continuous treater at commercial rates in a program covering wide ranges of the main processing factors (2). Electron‐spin‐resonance measurements on freshly treated films found no free radicals. Reactions of the treated surfaces with a free‐radical compound, diphenyl picryl hydrazyl (DPPH) were studied, focusing mainly on the rate effects. The evidence indicates that corona treatment produces fairly stable peroxide structures of the forms RO2R and RO3R on polyethylene surfaces. RO3R reacts rapidly with DPPH alone, while RO2R undergoes a slower reaction after addition of the catalyst, triethylene diamine. DPPH is capable of detecting as few as 1013 peroxide groups per square centimeter. Activation energies were 12 kcal/mole for the uncatalyzed reaction and 16 kcal/mole for the amine‐catalyzed reaction. As with the physical effects reported earlier (2), the production of peroxides is most strongly dependent on the energy delivered to the film during treatment. This energy is proportional to the quotient of corona current and web speed, I/S, Regression analysis showed that air‐gap thickness, relative humidity, and number of electrodes used also were significant factors, while dielectric thickness and corona frequency were not. We found that‐γ, the polar component of surface energy of the treated film, which is nearly zero for untreated polyethylenes, is exponentially related to the concentrations of both RO2R and RO3R with a correlation coefficient for 92 specimens tested of 0.88. We believe this is the first strong evidence linking treatment factors, at commercial levels of treatment, to chemistry of the treated surface and linking both of those to changes in physical behavior of the surface.
The effects of stress cracking on the fracture toughness of polycarbonate
Authors: Petrie S. P. , Dibenedetto A. T. , Miltz J.
Published: December 2004

The growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase inkICwith time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.
Penetration behavior of the system sphere‐cylinder
Authors: Hrouz J. , Vojta V. , Ilavský M.
Published: December 2004

An equation has been derived allowing calculation of the penetration modulus A for the indentation of a hard sphere having the radius R′ into a sample in the shape of a cylinder having the radius R in a direction transverse to cylinder axis. The correction function (determined by the elliptic integrals), ℋ︁/\documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt 2 $\end{document} = ƒ(R′/R) needed in the calculation of A, has been tabulated. An approximative equation has also been suggested which adequately describes the dependence of m ℋ︁/\documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt 2 $\end{document} on R′/R; in the range 0 < R′/R ≤ 1 it describes this dependence with an accuracy of 2.5 percent. While in the case of the penetration of a cylinder the geometry is fully defined and the derived equation is generally valid, in the case of indentation of a plane sheet the equation holds only if the thickness of the sheet is infinite. Experimental results for the penetration of a steel sphere with various R′ on samples of vulcanized silicone rubber in the shape of cylinders with different R gave an A independent of R′/R, which was in agreement with the shear modulus Gt = E/3 (E is Young's modulus) with an average deviation of 2.3 percent. Further, it has been found experimentally that in the case of penetration of a cylinder the equilibrium is attained more readily than in the case of indentation of a plane.
FDT: A technique for direct study of water attack at the silane‐filler interface
Authors: Rosen M. R. , Goddard E. D.
Published: December 2004

A technique is presented for the direct study of water attack at the silane‐filler interface. This technique, Filler Desorption Test (FDT), involves observations of surface tension changes which occur when a silane‐treated filler is floated on a water surface. If all the silane has been appropriately cured to form one integral polymerized siloxane network, then the rate and degree of surface tension lowering are a sensitive measure of the adhering tendency of the polymerized silane film. Data are presented which suggest that, all other things being equal, the strength of the coupling agent (C.A.)‐filler bond under water attack can be assessed by observation of the ease with which thefirstsmall amount of polymerized silane leaves the filler and the relative degree of hydrophobicity of the resulting surface. FDT is a new tool for fundamental studies of the coupling agent‐filler interface and interphase. The method also allows rapid screening and evaluation of a wide range of chemical and physical modifications designed to improve C.A. response on various filler systems.
Evaluation for the curing characteristics of phenolic molding compounds by rockwell hardness test at an elevated temperature
Authors: Tonogai Saburo , Hasegawa Kiichi , Inada Masao
Published: December 2004

Rockwell hardness measurement at an elevated temperature has been proposed as an evaluation method for the curing characteristics of thermosetting molding compounds. This method is convenient and has a high accuracy over a wide range of curing, Using a cone indenter, the following advantages are brought about: a good correspondence with acetone extraction test, in showing the degree of cure of the internal part of a molding rather than that of the skin layer, and a higher sensitivity in measuring higher degrees of cure. On examining curing behavior of commercial phenolic molding compounds by this method, an inflection point was observed on a plot of the hardness vs log curing time. This critical point has been called “the minimum cure time”, beyond which the molding exhibits good physical properties.
The ductile‐brittle transition of low‐density polyethylene
Authors: Zewi I. G. , Rudik W. J. , Corneliussen R. D. , Lind E. V.
Published: December 2004

The impact behavior of a low‐density polyethylene was studied with an instrumented Charpy tester. A change from elastic or ductile response to brittle fracture was observed over a small temperature interval, usually within 1°C. This characteristic impact transition temperature (ITT) was highly sensitive to shallow, sharp notches. Whereas an unnotched test bar had a very low impact transition temperature of −94°C, a razor cut with a depth of only 5 percent of the total thickness raised it to −4°C. The impact transition temperature was effectively reduced by increasing the cooling rate during specimen preparation and by the addition of nonpolar liquids, On the other hand, impact properties were adversely affected by aging, annealing, and adding other thermoplastics.
Rheology of short glass fiber‐reinforced thermoplastics and its application to injection molding I. Fiber motion and viscosity measurement
Authors: Crowson R. J. , Folkes M. J. , Bright P. F.
Published: December 2004

A study has been made of the fiber orientation in short glass fiber‐filled thermoplastics resulting from convergent, divergent and shear flows. Convergent flow results in high fiber alignment along the flow direction, whereas diverging flow causes the fibers to align at 90° to the major flow direction. Shear flow produces a decrease in alignment parallel to the flow direction and the effect is pronounced at low flow rates. Non‐linear Bagley plots have been observed, under some conditions, during rheological measurements. The data are consistent with a pressure dependent viscosity.
Influence of moisture content on curing behavior of two‐step phenolic molding compounds
Authors: Tonogai Saburo , Hasegawa Kiichi , Kondo Hitoshi
Published: December 2004

Using prepared and commercial two‐step phenolic molding compounds, the influence of moisture on their curing behavior was examined by the disk cure test and the solvent extraction method for the early and middle stages of the curing process, respectively. It was determined that moisture in the compounds could enhance the curing rate and the degree of cure as well as flowability. A possible mechanism explaining the acceleration of curing was proposed, suggesting that moisture might facilitate catalytically the decomposition of hexamine or hexamine‐novolac adduct into reactive low molecular weight materials, which could then easily react with a resin even in a state of fairly advanced cure due to their facile diffusion.
Rheology of rubber‐modified polymer melts
Authors: Münstedt H.
Published: December 2004

The influence of rubber particles on rheological properties of styrene‐acrylonitrile, and poly(vinyl chloride) was investigated. The temperature dependence of the viscosity functions was found to be independent of the kind of rubber and its concentration. The extrudate swell becomes smaller with increasing particle concentration, the entrance pressure loss remaining nearly unaffected. The strong viscosity increase at small shear rates is more pronounced the higher the concentration and the smaller the size of the particles. It can be interpreted by the assumption of a yield stress caused by the fillers. This yield stress is independent of temperature within the accuracy of the measurements and comes out distinctly higher in elongation than in shear. An analytical expression of the dependence of the viscosity on particle concentration cannot be given. It is demonstrated how sensitively an agglomeration of particles is reflected in elongation and recoverable strain. Based on the morphology shown by electron micrographs of acrylonitrile‐butadiene‐styrene and acrylonitrile‐styreneacrylic ester copolymers, two different models for the occurrence of a yield stress are discussed qualitatively.
Analysis of the packing stage in injection molding
Authors: Chung T.‐S. , Ryan M. E.
Published: December 2004

An unconditionally stable explicit finite difference numerical scheme is used to determine the pressure distribution during the packing stage of a rectangular mold cavity. Different initial conditions arising from both an isothermal and nonisothermal mold filling analysis are considered in relation to subsequent packing behavior. The packing stage is of short duration, may be assumed to be isothermal, and gives rise to a more uniform pressure distribution within the mold cavity.
Comparison of eccentric rotating disk and oscillatory measurements of dynamic moduli in polymer liquids
Authors: Vrentas C. M. , Rochefort W. E. , Smith G. G. , Graessley W. W.
Published: December 2004

Measurements to obtain the dynamic moduli G′(ω) and G″(ω) for several polymer liquids were made with the Rheometrics mechanical spectrometer using the eccentric rotating disk (ERD) and the forced oscillation methods. The ERD data were corrected for instrument compliance with the Macosko‐Davis formula. Oscillatory results were confirmed in several separate studies including testing of a Newtonian liquid and an elastic solid. Agreement between the two geometries on G′(ω) and G″(ω) was generally good (±5 percent) for |G*| less than 2 × 106 dyne/cm2. If no compliance correction was applied, departures occurred near a |G*| an order of magnitude lower, indicating strongly the need for applying a compliance correction to ERD data for most polymer systems. However, as |G*| approached 107 dyne/cm2 (where compliance corrections of 50 percent or greater occurred), the two geometries yielded results which differed by 20 percent or more. No single variable was found which correlated the departures for different liquids. Nevertheless, the departures were such that values obtained in the two geometries for the plateau modulus of polyethylene, G ≈ 2 × 107 dyne/cm2, agreed within about 10 percent.
Determination of capillary entrance pressure losses and first normal stress difference in polystyrene and high impact polystyrene melts
Authors: Peña Juan J. , Guzmán Gonzalo M. , Santamaria Anton
Published: December 2004

In industrial polymer processing, polymeric melts give rise to shear stresses above 105 dynes/cm2. For the determination of rheological properties and particularly melt elasticity, only the capillary type instrument is applicable to this range. In this paper, a comparative experimental study of the first normal stress difference, obtained from end pressure losses, entrance pressure losses and die swell is reported for polystyrene and high impact polystyrene. Good agreement between results from the three methods of measurement was obtained. The results are in accordance with literature data obtained using a capillary rheometer. It is seen that the values of first normal stress difference are greater in polystyrene than in high impact polystyrene.
Design and application of an instrumented falling weight impact tester
Authors: Wnuk A. J. , Ward T. C. , McGrath J. E.
Published: December 2004

Within the past decade, instrumented impact testers have become important tools for characterizing the response of polymers to short term loads. Unlike conventional test equipment which yield only the fracture energy, the instrumented devices are capable of providing detailed force vs time plots of the impact process. An instrumented falling weight impact tester is described which is equipped with an accelerometer which permits direct observation of the weight's deceleration as it strikes and pierces the test specimen. Integration of the resulting acceleration vs time curve provides the velocity change upon impact from which the fracture energy can he calculated. The impact tester is capable of accepting a wide range of specimen shapes and sizes and is especially useful where small amounts of material are at hand. To date, it has proved its utility in characterizing thin plastic sheets as well as fiber‐reinforced laminates.
Physical aging studies of styrene‐butadiene and carbonate‐siloxane block copolymers
Authors: Tant Martin R. , Wilkes Garth L.
Published: December 2004

The physical aging process in styrene‐butadiene and carbonate‐siloxane block copolymers has been studied by monitoring the time dependent changes in mechanical and thermal properties. Specifically, stress‐strain, stress relaxation and differential scanning calorimetry experiments were utilized. For the styrene‐butadiene systems, it was found that the rate of physical aging increases with decreasing glassy content between 50 and 100 percent glass. However, the rate must decrease to zero when the percent glass reaches zero. The occurrence of physical aging was also noted in the carbonatesiloxane block copolymer investigated. The data are discussed in light of the practical ramification of utilizing these block copolymers in material applications.
Dependence of mechanical properties in polyolefin blends on shear and thermal history
Authors: Schreiber H. P. , Kapuscinski M.
Published: December 2004

Variously dispersed two‐component blends of linear polyethylene and a thermoplastic rubber were subjected to different thermal histories after compression molding and their properties were evaluated. It has been shown that ultimate properties of the blends are not defined by the composition, but vary greatly with degree of dispersion and thermal history. Stronger materials failing in a ductile manner were produced under shock cool conditions and in, finely‐dispersed blends. Brittle failures and weaker materials were favored by annealing procedures and in blends subjected to brief and/or lowshear dispersion procedures. Time and temperature dependent changes in properties of blends prepared under standard conditions have been observed and activation energies for the aging process calculated. Data interpretation was based on a hypothesis of domain morphology in the blends, and on the existence of “tie molecules” which help to determine the cohesive strength of domain boundaries.
The effect of molecular orientation on the mechanical properties of fiber‐filled amorphous polymers
Authors: Patel Prakash D. , Bogue Donald C.
Published: December 2004

The technology of using glass fibers to increase the modulus and the strength of polymeric systems is well‐developed. Much less widely exploited has been the enhancement of properties by orienting the molecules of the polymer itself. The purpose here was to look for a synergistic combination of these two strategies: using glass fiber‐filled polystyrene and introducing molecular orientation into the polystyrene matrix. For rheological reasons it is not possible to introduce large stretch ratios in the rubbery state and thus the amount of molecular orientation that can be frozen into the quenched glassy composite is small. Even so, however, the rubbery elongation (which we associate with subsequent molecular orientation) has a very significant effect on the final (room temperature) mechanical properties. Analysis of these properties was made in terms of various theoretical models (those of Brody and Ward, Smallwood‐Guth, and Russel and Acrivos for the Young's modulus; and that of Kelly and Tyson, as modified by Lees, for the tensile strength). These comparisons showed that the brittleness of unoriented polystyrene is such that the matrix does not effectively distribute the stress along the length of the fiber and thus the benefit of the length is not realized; whereas the higher ductility of oriented polystyrene allows such transfer and a consequent improvement of properties.
Curing of compression molded sheet molding compound
Authors: Lee Ly James
Published: December 2004

A recently developed kinetic model has been applied here to describe the polyester‐styrene addition copolymerization. By assuming that the termination step is negligible and the reaction rate between inhibitor and initiator free radical is much, faster than any other reactions, the kinetic mechanism can be simplified to be expressed as a single equation. The parameters, rate constant of initiator decomposition and rate constant of propagation, are estimated from the induction time and the time to the peak exotherm of isothermal differential scanning calorimetry (DSC) curves. Temperature profiles inside plate sections of SMC parts during molding are predicted by a mathematical model in which addition polymerization is coupled with heat transfer. The predicted temperature profiles compare well with the experimental results. The model is also used to predict the cure time of different part thicknesses, mold temperature and initiator concentration. Glass fibers playa role as a heat sink as well as heat conductor during curing. Adding glass fibers to SMC not only lowered the maximum exotherm but also reduced the cure time.
Rheological characterization of unsaturated polyester resin sheet molding compound
Authors: Silva‐Nieto R. J. , Fisher B. C. , Birley A. W.
Published: December 2004

This paper presents a theoretical and experimental analysis of the rheological behavior of sheet molding compound (SMC). The work analyses the squeeze flow in a parallel plate plastometer of SMC discs which contain 25 percent of fiber glass by weight. This method of flow characterization gives a good insight into the basic rheological behavior of SMC for the compression molding process when producing flat parts. The theoretical analysis applies to thickened and matured SMC at room temperature. The analysis treats SMC as a viscoelastic material having an equation of state with viscous, elastic and yield elements. The time variation of compressive force when squeezing SMC discs between two parallel plates (one fixed and one mobile) has been derived from the equation of state. The values of the viscous, elastic and yield parameters were determined by using a least squares method of curve fitting to the experimental results. There are two aspects to the reported experimental work. One aspect is concerned with showing that the three element model for the equation of state provides a realistic mathematical basis for characterizing the rheological behavior of SMC at room temperature. The other shows how the parallel plate plastometer can be used to give data which characterize SMC flow behavior under conditions similar to those of the actual compression molding process.
The toughness of copolymers of polypropylene
Authors: Fernando P. L. , Williams J. G.
Published: December 2004

A series of fracture mechanics tests of a set of random and impact copolymers of polypropylene, in which the ethylene content was varied, is described. The plane strain toughness is found to change from the homopolymer value, but thereafter to be independent of ethylene content. The yield stress of the impact copolymers decreases markedly with ethylene content, while that for random copolymers increases only slightly. This is reflected in toughness values in tension. A comparison with the behavior of other rubber‐modified materials is made and it is concluded that crazing is an important mechanism in polypropylene and is perhaps more rate and temperature sensitive than in acrylonitrile‐butadiene‐styrene (ABS).
Prediction of solubilities for volatile hydrocarbons in low‐density polyethylene using UNIFAC‐FV model
Authors: Iwai Yoshio , Anai Yoshiyuki , Arai Yasuhiko
Published: December 2004

The UNIFAC‐FV model has been applied to predict the weight‐fraction Henry's constant of n‐hexane, n‐octane, benzene, and toluene in low‐density polyethylene in the region 124 to 250°C. By adjusting the number of external degrees of freedom per hydrocarbon molecule, the weight‐fraction Henry's constant was correlated with good agreement by the model. Namely, the solubilities of hydrocarbons in the polymer can be obtained correctly from the model at low pressures, less than 2 atm. Although the model is very useful, its applicability is limited to the subcritical temperature range of a volatile hydrocarbon.
Plane strain fracture toughness testing of high density polyethylene
Authors: Chan M. K. V. , Williams J. G.
Published: December 2004

The concepts of linear elastic fracture mechanics (LEFM) are applied to three grades of high density polyethylene in an attempt to determine their fracture behavior in terms of a linear elastic fracture toughness,Kc. The effect of specimen size (thickness and width), crack length and the mode of loading onKchas been investigated in order to determine the plane strain fracture toughness,KIc, of these materials. The effect of temperature (between +23 and −180°C) on their fracture behavior has also been investigated and compared in terms of their plane strain fracture toughness values.
Kinetics of a sterically hindered amine‐cured epoxy resin system
Authors: Buckley Leonard , Roylance David
Published: December 2004

Fourier transform infrared spectroscopy (FT‐IR) and torsional braid analysis (TBA) were used to study the reaction of an epoxy resin system cured with a sterically hindered amine. Isothermal torsional braid analysis showed the apparent activation energy to be approximately 42 kJ/mol. The reaction kinetics were also considered from the unreacted freshly mixed condition and a partially reacted “B‐staged” condition using infrared spectroscopy, The B‐staged condition is unreactive and stable at room temperature due to the quenching of the primary amine reaction by the glassy structure and the steric hindrance of the secondary amine reaction. Apparent activation energies for these two conditions were found to be 49.4 kJ/mol and 49.0 kJ/mol respectively. The storage life at room temperature of the B‐staged resin system is predicted to be at least three months based on extrapolation of the experimental kinetic data.
Influence of molecular weight and molecular weight distribution on mechanical properties of polymers
Authors: Nunes Ronald W. , Martin John R. , Johnson Julian F.
Published: December 2004

This work reviews the literature published over the last ten years on polymer mechanical properties as a function of molecular weight and molecular weight distribution, Thermal properties, stress‐strain properties, impact, fracture, fatigue, creep, stress relaxation and cracking and crazing are examined for a wide variety of homopolymers and a limited number of copolymers. In general, mechanical properties increase as the molecular weight increases. However, above some limiting molecular weight the mechanical property is usually unaffected. Although much work has been done to describe the effects of molecular weight on mechanical properties, little quantitative correlation exists. The available equations to predict such properties as cracking and crazing, Tg, Tm and tensile strength from molecular characteristics are discussed in detail. However, a more quantitative description incorporating a wider range of mechanical properties would be more useful. This would facilitate use of the vast amount of information available and enable it to be applied more readily to new polymer systems.
Property‐morphology relationships of polymethylmethacrylate/polyvinylidenefluoride blends
Authors: Mijović Jovan , Luo Hong‐Lie , Han Chang Dae
Published: December 2004

An investigation was conducted to establish property‐morphology relationships in polymethylmethacrylate/polyvinylidenefluoride (PMMA/PVDF) blends. All blends were compounded in a twin‐screw extruder and then processed by injection molding Mechanical properties of blends of various compositions were studied by dynamic mechanical and impact strength measurements. The presence of crystalline regions in blends was determined by Differential Scanning Calorimetry (DSC). Morphology of fracture surfaces of blends was studied by Spinning Electron Microscopy (SEM). PMMA/PVDF blends were found to form compatible mixtures over a wide range of blend composition. Changes in dynamic mechanical properties upon annealing were found to be a direct function of blond morphology. Electron microscopic evidence showed no signs of phase separation. DSC measurements detected crystalline regions in all blends containing 40 percent or more (by weight) PVDF.
The limiting pressure‐velocity (PV) of plastics under unlubricated sliding
Authors: Yamaguchi Yukisaburo , Kashiwagi Kiyoteru
Published: December 2004

The limiting pressure‐velocity (PV) of plastics materials under normal contacting pressure P and sliding velocity V when the sliding goes on continuously unlubricated has been discussed theoretically in this study. The sliding friction tests between the rotating edge of a hollow cylinder against a stationary metal surface were carried out in room air and when the metals were cooled with circulating water.

The results obtained were as follows:

The relation between PVmax and the critical temperature τbmax of the surface above which ordinary frictional conditions are not maintained may be represented by the following formula,

Where C = Hn/μk, and H is the mean total heat conductivity between materials and environment in cal/cm2. °C · s, n the area ratio of heat radiation surface to frictional surface, μ the kinetic frictional coefficient, k the thermal equivalent of frictional work in cal/kg · cm · s, and τa the cooling or environment temperature.

The value of C ranges from 1 to 10 in cal/kg · cm s · °C, and the value of PVmax from 45 to 750 in kg/cm2 · cm/s under prevailing room temperature air cooling in this experiment, however, the value of C has been increased about 2.3 times as much as those values by circulating water of 2°C around the lower side of the test piece.
Anisotropy and dimensions of blow‐molded polyethylene bottles
Authors: Kamal Musa R. , Kalyon Dilhan , Tan Victor
Published: December 2004

A commercial blow‐molding grade, high‐density polyethylene resin was employed to produce cylindrical bottles in a commercial reciprocating screw‐extrusion blow‐molding machine. The distributions of thickness, crystallinity, birefringence, and impact strength were obtained at various positions. The thickness is greatest near the parting lines, while minimum thickness occurs at the top and bottom of the bottle. The thickness tends to be uniform in the middle section, in agreement with earlier studies of the parison during processing. Density and crystallinity distributions are closely associated with the distribution of thickness and its effect on the cooling rates prevailing during molding. Frozen stresses and birefringence are largest at the outer surface, where cooling rates are highest. The impact strength is lowest near the parting line. Photomicrographs suggest that this is associated with internal flow and crystallization phenomena.
Estimation of injection pressure during mold filling
Authors: Titomanlio G.
Published: December 2004

Dimensionless diagrams for estimating the bulk temperature of the flow front and injection pressure in the limit of small viscous generation are obtained. Also, a criterion for neglecting viscous generation is identified. The diagrams, based on the Lord and Williams model, refer to rectangular geometry and amorphous materials. A satisfactory comparison is obtained with literature data taken on polystyrene. A reasonable estimate of polyethylene injection pressure was obtained by roughly accounting for latent heat of crystallization through modified thermal diffusivity.
The compressive deformation of polymeric foams
Authors: Tsai James T.
Published: December 2004

An alternative approach has been developed to evaluate compressive stress on polymeric foams. Compressive stress depends primarily on three factors: foam density, deformation strains, and deformation rates. The density dependency derived from this approach agrees closely with the empirical models reported in the literature. Correlations between the dynamic compression and shear deformation are also derived. Experimental data are presented which show that G′ and G″ increase with increasing strain rates, while the damping factor reaches a maximum at low strain rates. Note that the proposed model for the prediction of cell‐wall rupture would not apply to foams with high open‐cell contents.
Residual stresses in polymers III: The influence of injection‐molding process conditions
Authors: Siegmann A. , Buchman A. , Kenig S.
Published: December 2004

Following the evaluation of Residual Stresses (R.S.) in quenched specimens (Part I) and the resulting mechanical‐physical properties (Part II), the, present study deals with the effect of injection‐molding process conditions on R.S. and the respective properties of amorphous polymers. Melt temperature, mold temperature, injection rate, and injection pressure were the parameters studied. Experimental results indicated that the melt temperature caused two maxima in R.S. The second one reverses from compressive to tensile. In general, most changes occur in the surface regions, while R.S. decreases with increasing melt temperature, as is the case in zones far away from the gate. Furthermore, tensile modulus increased, in general, with rising melt temperature. In the case where the effect of mold temperature was studied, it was found that R.S. are compressive in the surface layers and tend to decrease upon increase in mold temperature and distance from the entrance region. Significant changes in R.S. were also detected in the interior layers. As the mold temperature approached Tg, low values of R.S. were measured, as was the case in quenched specimens. Injection rate affects surface R.S. to a large extent. With low flow rates, tensile stresses were developed in the exterior, reversing to compressive stresses at higher speeds. The reversal in sign depends on the location relative to the gate. Once compressive stresses were formed, further increase in rate caused a reduction in R.S. In addition, variations in tensile modulus, as high as 30 percent, were measured at high injection rates. As far as injection and holding pressures are concerned, experimental results showed that a maximum in R.S. was obtained, with increasing pressure, at the surface. Close to the gate entrance, a reverse from compressive to tensile R.S. was detected at high injection pressures. As in the other cases, injection pressure influenced mostly the exterior layers. Only in zones close to the entrance and at high pressures were high levels of R.S. measured in the core regions.
Reactivity of ethylene vinyl acetate copolymers: A critical evaluation of the comprehensive patent literature on the acetoxy‐hydroxide transformation of ethylene vinyl acetate copolymers
Authors: Koopmans R. J. , Van Der Linden R. , Vansant E. F.
Published: December 2004

The comprehensive patent list concerned with the acetoxyhydroxide transformation of ethylene vinyl acetate Copolymers is examined. The possible reaction mechanisms and processing techniques for this kind of transformation are described. Finally, a survey of about 55 of the most interesting patents is given, together with some general features of the acetoxyhydroxide exchange.