bluuebunny/crossref_metadata_2025 huggingface dataset:
ELASTIC BUCKLING OF EXTERNALLY-PRESSURIZED IMPERFECT TOROIDAL SHELLS
Authors: GALLETLY GERARD D.
Published: April 2003

This paper summarizes the results of numerical studies into the effects of initial geometric imperfections on the elastic buckling behaviour of steel circular and elliptic toroidal shells subjected to follower-type external pressure. The types of initial imperfection studied are (a) axisymmetric localized ones and (b) sinusoidal buckling modes. The principal localized imperfections studied are (i) circular increased-radius "flat spots" and (ii) smooth dimples. The buckling pressures pcr of circular toroidal shells were not very sensitive to initial imperfections. With elliptic toroids, whether the shell was sensitive to initial imperfections or not depended on the ratio k(≡ a/b) of major to minor radii of the section. The shells on the ascending part of the pcr versus k curve behaved like circular toroidal shells, i.e. they were not sensitive to initial imperfections. However, the behaviour of elliptic toroids on the descending part of the versus k curve was very different. The numerical results quoted in the paper are for limited ranges of the geometric parameters. It would be useful to extend these ranges, to explore the effects of plasticity and to conduct model tests on imperfect steel models to verify the conclusions of the numerical studies.
Tenofovir Disoproxil Fumarate
Authors: Grim Shellee A , Romanelli Frank
Published: April 2003

OBJECTIVE:

To review the pharmacology, virology, pharmacokinetics, efficacy, safety, resistance profile, and clinical use of tenofovir disoproxil fumarate.

DATA SOURCES:

A MEDLINE search was performed (1966–August 2002) using the following terms: tenofovir, tenofovir disoproxil fumarate, PMPA (9-( R)-[2-(phosphonomethoxy)propyl]adenine), and Viread. Abstracts from HIV-related meetings were reviewed.

DATA EXTRACTION AND STUDY SELECTION:

Publications and meeting abstracts regarding tenofovir were reviewed. The most recent and pertinent items were included.

DATA SYNTHESIS:

Tenofovir disoproxil fumarate is a nucleotide prodrug that is diphosphorylated to its active moiety, tenofovir diphosphate. In this form, tenofovir acts as a reverse transcriptase inhibitor to inhibit HIV-1 replication. In clinical trials, tenofovir was effective at suppressing HIV-1 RNA and boosting CD4+ cell counts. Tenofovir has a long intracellular half-life, which permits once-daily dosing. Since tenofovir does not interact with the cytochrome P450 pathway, it exhibits minimal drug interactions, with the exception of didanosine. Compared with other reverse transcriptase inhibitors, tenofovir may have advantages in terms of toxicity and medication adherence profiles. Ongoing studies are also analyzing tenofovir's activity against hepatitis B virus.

CONCLUSIONS:

Tenofovir has been shown to be active against HIV-1 in combination with other antiretrovirals. The drug's benefit as a single-agent intensifier of highly active antiretroviral therapy in treatment-experienced patients has been established, and preliminary data for treatment-naïve patients are encouraging.
Direct extrusion of polymer latex emulsions
Authors: Nichols Russell J. , Senn Richard K. , Kheradi Farokh
Published: April 2003

The synthesis of many thermoplastic and elastomeric polymers can be accomplished by dispersing the monomer (or a blend of monomers) to be polymerized in water with the aid of various surfactants. The monomer is then polymerized by the introduction of a free radical catalyst to produce a latex of the polymer. Coagulation is the first step in isolating the polymer from the latex, and typically, many subsequent process steps are required to “finish” the polymer to the pelletized or expanded crumb from suitable for end users to convert the polymer into useful products.

The process described herein combines all the process steps required to coagulate, separate and dewater, wash, devolatilize, homogenize and pump the polymer in a single‐step, short residence time system. The machine employed for this process is a counter‐rotating, tangentially opposed twin‐screw extruder. One important variation of the basic process combines an integral plasticating extrusion step with the latex conversion to make polymer blends and alloys. Critical process parameters are identified and discussed and examples of pilot plant scale experimental data for ABS (acrylonitrile‐butadiene‐styrene terpolymer) systems are reviewed.
The thermal conductivity and diffusivity of polyethylene solids and melts
Authors: Kamal Musa R. , Tan Victor , Kashani Farzad
Published: April 2003

A miniaturized version of a recently developed apparatus has been constructed to measure both the thermal conductivity and thermal diffusivity of polymer melts and solids. The apparatus was employed to measure the thermal conductivity and diffusivity of a number of polyethylene resins of varying density and molecular weight distribution. The results obtained with the miniaturized unit are in good agreement with those obtained with the original larger apparatus. Moreover, the dependence of thermal conductivity and diffusivity on temperature, resin density, and molecular weight parameters has been evaluated with the aid of the new apparatus. Empirical relationships involving the quantity (MwMn)1/2 have been found for the dependence of thermal conductivity and diffusivity on molecular weight.
The distribution of number of passes over the flights in single screw melt extruders
Authors: Manas‐Zloczower I. , Tadmor Z.
Published: April 2003

In single screw melt extruders there is a continuous flow of polymer melt over the flight clearance. The melt flowing over the clearance experiences high shear rate and may also experience very high temperature. This, in turn, may affect polymer structure and properties. The melt that flows over the flight is mixed with the bulk of the melt flowing in the channel. In the extrudate there are fluid elements that have not passed over the flights and there are fluid elements that passed over the flight once, twice, or many times. In this paper, a theoretical model is proposed for calculating the number of pass distribution function of a melt stream emerging from a melt extruder.
Permeabilities and water vapor transmission rates for commercial polymer films
Authors: Tock Richard Wm.
Published: April 2003

High molecular weight polymers have become well established as packaging or container materials. Unlike metallic and ceramic barriers, however, polymer films are known for their semipermeable nature; that is, they permit small gaseous and vapor molecules to migrate through their atomic/molecular structures. Since the intrusion of oxygen/water from the surrounding air or the loss of water/carbon dioxide from a packaged product can be detrimental to the shelf life of the product, the steady state transmission rate of a polymer film has become an important property characteristic of polymers to be used for packaging.

An attempt has been made in this study to review the transmission data of commercially available polymer films. The three molecular species O2, CO2, and H2O were of particular interest. Since these permeability data vary over five orders of magnitude, several log‐log plots were constructed in order to assist decision making in film selections. Data for 34 different polymers are given. The effects of order or crystallinity, the presence of plasticizers, and the effects of laminated layering are discernible. It is the latter modification (laminating) that possesses demonstrable potential for improving the barrier properties of polymer films used for packaging purposes.
Pressure drop for molten polymer flow through tapered dies
Authors: Vlachopoulos J. , Scott P. S.
Published: April 2003

Some approximate equations for the calculation of pressure drop for polymer melt flow through slightly tapered dies are presented. These include equations for pressure drop in a truncated cone, converging flat plates, and a converging annulus. The predicted values obtained from these equations are compared to the results of a finite element simulation for two‐dimensional flow of a power‐law fluid and some experimental data available in the literature. It is concluded that the approximate equations can be used for pressure drop calculations for dies of small taper angles and when entry, exit, and viscous dissipation effects are negligible.
Quality assurance for fiber‐reinforced composites
Authors: Frick A. , Pöllet P. , Eyerer P.
Published: April 2003

To ensure the growing use of high‐performance composites such as carbon‐fiber‐reinforced plastics (CFRP), the quality properties of these materials must be guaranteed. This goal can be reached through a quality assurance concept which still has to be developed. The individual steps of such a development are indicated. Flaws occuring in fiber‐rein‐forced composites are discussed, taking into consideration the manufacturing process, and the nondestructive and slightly destructive test methods suitable for flaw selected flaws are given as examples.
The development of phase morphology and polymer chain orientation in the processing of incompatible polymer blends
Authors: White James L. , Min Kyonsuku
Published: April 2003

A critical review is presented of experimental research on development of phase morphology and polymer chain orientation in disperse two phase flow of polymer melts. It is shown that, in stratified flow, phase morphology variations are primarily caused by viscosity differences between the two phases. For disperse two‐phase flow, the phase morphology is strongly influenced by the interfacial tension as well as the viscosity ratio. This appears to be through the capillarity or Taylor number. Large interfacial tensions and small shear stresses or matrix viscosities lead to coarse phase morphologies and phase growth under quiescent conditions. Interface modification additives (compatibilizers) may be interpreted in terms of reduction of interfacial tensions. Applications to polymer processing operations and interpretation of rheological properties are described. The development of orientation of individual polymers in blends is reviewed. It is shown that chain orientation develops differently for the individual components in both compatible and incompatible blend systems. In incompatible systems, an addition of a second polymer to a crystallizing melt would appear to reduce the levels of eventual crystalline orientation in the latter, if the former shows sharp viscosity rises prior to the crystallization.
Extruder scale‐up in a corotating twin‐screw extrusion compounding process
Authors: Agur Enno E.
Published: April 2003

The effects of processing conditions and extruder scale are examined for a compounding process involving fully intermeshing corotating twin‐screw extruders. A systematic experimental study involving a matrix of two independent variables (extruder throughput rate and screw rotational speed) was conducted on two different size twin‐screw extruders using a styrene copolymer and carbon black. Several process parameters were examined including melt temperature at the die, motor power, residence time distribution, and carbon black concentration in the extrudate. The residence time distribution data were fit to a three‐parameter plug flow/stirred tank model to obtain a time delay, a time constant, and a mean residence time for each extrusion condition. At low screw speeds, as utilized in many compounding processes, the mean residence time dropped only slightly with increasing screw diameter (for comparable screw length/screw diameter ratios). The carbon black concentrations as measured in the extrudate pellets were within ±5% of mean value, indicating that the extruder damped out the high frequency noise caused by feed rate fluctuations in the individual polymer and carbon black feeders.
Mathematical modeling of injection mold filling: A review
Authors: Mavridis H. , Hrymak A. N. , Vlachopoulos J.
Published: April 2003

The flow of molten polymers in molds is described by the conservation of mass, momentum, and energy. Several simplifications have been proposed in the last 40 years. The early models were essentially unidirectional approximations. More recently, the Hele–Shaw flow approximation has been used together with the appropriate analytical or numerical methods. Current developments include commercially available computer software based on network models for flow simulation in complex molds and modern sophisticated finite element solution of the complete conservation equations that give insight to fountain flow and small scale phenomena.
Flow patterns in the calender gap of a counterrotating twin screw extruder
Authors: Speur J. A. , Mavridis H. , Vlachopoulos J. , Janssen L. P. B. M.
Published: April 2003

The finite element method was employed to examine the flow details in the calender gap of a counterrotating twin screw extruder. For Newtonian fluids a vortex is predicted at the entry of the calender gap. Calculations show that the vortex intensity is determined uniquely by the value of relative throughput and disappears when it exceeds 24%, for the geometrical configurations examined. Shear thinning behavior of the fluid reduces the vortex intensity. Approximate equations for the pressure drop based on the lubrication approximation are compared with the finite element results and prove accurate for engineering calculations.
The glass transition in novolac phenolic molding compounds and the kinetics of its development during cure and postcure. II
Authors: Landi Vincent R.
Published: April 2003

The change in onset temperature of the glass transition in a novolac phenolic molding compound was related to the time and temperature of cure. A mathematical model consisting of a single function of time and temperature fit glass transition onset temperature ranging from 87 to 242°C. These glass transition temperatures had developed during molding at temperatures from 132 to 218°C at times of 10–1800 s. The same model predicted the glass transition and onset temperature of samples postcured to a glass transition temperature of 287°C. The mathematical model resulted from a kinetic rate law treatment. This treatment, however, did not include a secondary reaction which causes a small exotherm above 210°C in DSC testing. The resulting rate law still appears to provide a useful approximation of the development of the Tg and the cure rate in novolac phenolic molding compounds over practical molding and postcure conditions.
Chemical coatings in the eighties: How are they new?
Authors: Pilcher G. R.
Published: April 2003

The Chemical Coatings industry is examined with regard to its progress in the field of “Compliance (lower VOC) Coatings,” with emphasis on new technologies, applications systems, and ultimate goals. Various technological areas are grouped, and discussed with regard to perceived strengths, draw‐backs, and potential future developments.
Injection welding of polyamides
Authors: Maskus Peter , Gähwiler H. U.
Published: April 2003

In the following article, we will discuss welding of polyamides inside the injection mold. This process is called injection welding. The process of injection welding makes it possible to bond polyamides with quite different characteristics together without adhesive. In this way, one can achieve economic as well as constructional advantages. We will discuss two different methods in this article. In the first process, two melts are injected onto each other simultaneously; with the second technique, the melt is injected onto a thermoplastic insert. The weld line strengths of injection‐welded parts depend on material and process conditions. With an amorphous polyamide, the weld line strength can be attained up to the full tensile strength of the material, while a glass‐fiber‐reinforced polyamide falls back onto the tensile strength of the basic polymer. Injection welding achieves the greatest weld line strength when two melt streams are colliding together with the highest possible injection pressure. If a melt is injected onto an insert, the temperature of the melt should be as high as possible.
Composite models of modular intermeshing corotating and tangential counter‐rotating twin screw extruders
Authors: White James L. , Szydlowski Witold
Published: April 2003

Composite models of the flow rate and pressure profiles in modular intermeshing corotating and tangenital counter‐rotating twin screw extruders are developed. The simulation of the intermeshing corotating machine involves both screw and kneading disc elements, including left‐ and right‐handed sections. The conditions under which starvation behavior develops in sections of the screws are discussed.
Development of dispersion in the mixing of calcium carbonate into polymer blends in an internal mixer
Authors: Suetsugu Yoshiyuki , White James L. , Kyu Thein
Published: April 2003

An experimental study of agglomeration tendency and the compounding of calcium carbonates of varying particle size into polypropylene and polystyrene melts in an internal mixer and in a two roll mill is described. The tendency of particles to agglomerate was studied using the sedimentation volume method. Agglomeration increased with decreasing particle size. It was sharply reduced by coating with stearic acid. Agglomerate size distributions in thermoplastic matrices were measured using optical microscopy. The greatest problems in dispersing fillers were with the smallest particles. One unexpected result with the polystyrene compounds was that the number of large agglomerates of the smallest particles actually increased with time. This was associated with uncoated small CaCO3 particles clinging to the chamber walls of the internal mixer and being only gradually incorporated. The phenomenon was less pronounced in the polypropylene compounds. Generally large agglomerates were broken down with increased mixing. Stearic acid coating reduced the number of large agglomerates.
Orientation, crystallization, and haze development in tubular film extrusion
Authors: White James L. , Cakmak Mukerrem
Published: April 2003

A critical review of research on the development of crystallinity, orientation, and haze in tubular film is presented. We examine the basic methods of characterization of polymer films and interpretation of experimental techniques. Most studies in the literature relate to polyethylene, polystyrene, and polypropylene tubular film. Polymer chain and crystallographic axis orientation have been studied using X‐ray diffraction and birefringence. We critically discuss the representation of such data using biaxial orientation factors. The data appears correlatable in terms of expressing orientation as a function of applied stresses at solidification. It appears that film haze is predominantly caused by surface roughness. This is largely associated with the crystallization of the melt on the process line.
Inorganic superconducting materials formulated in cured polymers
Authors: VanWoert H. C. , Brumlik G. C.
Published: April 2003

Fine clusters of inorganic compounds (all lowtemperature superconductors) were mixed thoroughly with polymerizing epoxy or urethane monomers. The resulting composites were cured. These polymer composites were formulated at room temperature and pressure. The electrical resistivity of the synthesized composites were measured at room temperature and pressure. Further resistivity measurements were carried out at room temperaturehigh pressure and room pressure–liquid nitrogen temperature. Significant drops in resistivity were found for composites measured at high pressure and liquid nitrogen temperature. Composites in both epoxy and urethane polymers when subjected to pressures of up to 2000 psi had drops in resistivity of up to 6 orders magnitude. Also, composites in urethane when subjected to liquid nitrogen temperature had drops in resistivity of 6 orders in magnitude. In contrast, composites made up in two types of epoxy polymer when subjected to liquid nitrogen temperature had drops of only up to eightfold.
Compression molding of composites: Mold heating system design
Authors: Upadhyay R. K.
Published: April 2003

In compression molding of composite parts, it is important to minimize the temperature variation in the part during processing for consistent quality. A mold heating analysis and optimization scheme has been developed that allows computation of temperature variation on the mold/part surface, and optimizes the location and power of heaters. The heat transfer coefficients for a given mold can also be theoretically calculated. The validation of the analysis was carried out by two sets of experiments: the first one measured the heat loss from the mold under different boundary conditions, and the second one the temperature variation on mold/part boundary for theoretically determined power histories for a specified temperature history. The experimental data compared well with theory and provide a satisfactory validation of the analysis. The present work provides a useful tool to a design engineer and allows the mold heating system design on a scientific basis, thus eliminating the trial and error approach.
Solid bed compaction and frictional drag during melting in a simulated plasticating extruder
Authors: Hwang Chii‐Guang , McKelvey James M.
Published: April 2003

Solid bed transport has an important effect on the over all performance of a plasticating extruder. The coefficient of friction and density are the properties of the solid bed that govern its transport in the feed section of the screw. These properties have been measured, as a function of temperature and pressure, for eight common thermoplastic materials in an apparatus that simulates solid bed motion and compaction in a plasticating extruder and the results have been interpreted phenomenologically.
Properties of three types of crosslinked polyethylene
Authors: Venkatraman S. , Kleiner L.
Published: April 2003

The same base resin, a HDPE with vinyl–silane units grafted along the backbone, was subjected to three different types of crosslinking: (a) using peroxides and heat, (b) using a catalyst under high humidity, and (c) using high‐energy radiation. Macroscopic properties, such as tensile behavior‐at‐break, shear modulus, gel content, and crystallinity were measured for all three types of crosslinked material. Although the mechanical and rheological properties appear to indicate differences in distribution of crosslinks, the major differences are seen for the degree of crystallinity. These differences are rationalized on the basis of the mechanism of crosslinking and the resultant location of crosslink sites.
Reactive extrusion of polymers: A review
Authors: Tzoganakis Costas
Published: April 2003

During the last 30 years the plastics industry has experienced enormous rates of growth. Due to an increasing demand for high performance plastic materials, the use of extruders as continuous flow reactors for polymers has attracted considerable attention in polymerization and polymer modification applications. This paper is an attempt to present a survey of recent literature on the reactive extrusion of polymers. Experimental and modeling studies for various polymer systems are reviewed and some rheological, modeling, and design aspects of reactive extrusion systems are addressed.
Mechanical properties of prestressed thermoplastics
Authors: Gust H. , Hengl R. , Eyerer P.
Published: April 2003

Mechanical tests of prestressed and nonprestressed specimens show that prior stressing often significantly changes the stress–strain curve of amorphous and partly crystalline thermoplastics.1–3 Specimens subjected to a prior mechanical stress show a flatter stress–strain curve (even with a “kink”) than those not previously stressed. The original properties of prestressed specimens can be restored by tempering (at least in the range of small elongations). Specimens made of ABS react differently: After certain longterm prior stresses, followed by short‐term tensile tests, brittle fracture occurs.
Model formulation for material feeding in counter‐rotating twin‐screw extruders
Authors: Poltersdorf Bernd , Fiedler Lothar
Published: April 2003

Assuming that the solid moves in consequence of wall friction and friction of the material, transport equations have been established by means of a force and momentum balance on the volume element. These equations serve as a basis for describing the material feeding. That is, transition of the polymer from the feed hopper, or the feeding device, into the filling region of screws and the transport and compression of the solid polymer, i.e., the movement of the feed towards the plasticating zone, with simultaneous decrease of porosity. Depending on the geometry of the screws and on technological and material parameters, the model gives information on degree of filling, compression, pressure buildup, throughput of solids transport zone, and power consumption. The process model is suited for the design of the solids transport zone.
Extrusion of polymers in the plastic state through pure‐shear convergent—divergent dies
Authors: Mascia L. , Zhao J.
Published: April 2003

Extrusion experiments have been carried out by forcing the polymer at temperatures below the melting point through a variety of convergent–divergent dies in which the cross section area at entry and exit was the same, giving a state of pure‐shear deformations, with the aim of producing extrudates exhibiting biaxial orientation. PTFE has been used to model the process for the analysis of extrusion forces as it is well characterized with respect to both frictional behavior and yield characteristics. UHMWPE, on the other hand, was used to determine the extent of orientation and mechanical properties in function of the die geometry and extrusion temperature. Dies with low converging angles were found to develop extrusion forces three to four times higher than corresponding dies with larger converging angles and to produce a highly unbalanced orientation with a bias in the transverse direction. A special die design feature producing a crossply orientation pattern was also examined.
Introduction to compatibilization of polymer blends
Authors: Markham Richard L.
Published: April 2003

The key to preparation of commercially useful blends based on immiscible polymer pairs is compatibilization. Advances over the past ten years have considerably enhanced the understanding of this phenomenon. The selection of an appropriate compatibilizer and the processing technique to enhance the compatibilization is quite complex. The selection of the compatibilizer is aided by the application of the principles of miscibility and of organic interactions. It may be advantageous to prepare the compatibilizer in situ. Chemical modification of the base components to enhance interactions between the components is an important option. Processing and rheological effects must also be taken into account. The combination of the compatibilizer and the processing conditions must yield a morphology that will preserve the desired properties of each component while minimizing the undesirable properties. This morphology must be reproducible and stable in the ultimate product and through subsequent recycling processes.
Computer simulation and experiments of flow distribution in flat sheet dies
Authors: Vlcek J. , Mailvaganam G. N. , Vlachopoulos J. , Perdikoulias J.
Published: April 2003

The Control Volume Method is used in a two‐dimensional computer software package to simulate three‐dimensional nonisothermal flows of polymer melts through flat sheet dies. The results of the simulation are compared to experiments performed with different materials on an experimental die, and to a three‐dimensional simulation using the Finite Element Method. This comparison shows that for the case of negligible extensional effects, the two‐dimensional package performs reasonably well.
State of the art of injection molding of high‐performance ceramics
Authors: Sakai Tadamoto
Published: April 2003

The injection‐molding process for high‐performance ceramics is rapidly spreading into various kinds of industries, particularly the automobile industry. In this study, we provide an outline of the ceramics injection‐molding process, its characteristics, the difference between ceramics compounds and plastics, and examples of applications.
Extrusion of self‐reinforced polyethylene
Authors: Pornnimit Benjamin , Ehrenstein Gottfried W.
Published: April 2003

In recent years, efforts aimed at producing high‐strength polymers have intensified noticeably. In addition to additive‐based reinforcement, self‐reinforcement without additives represents a way to increase material strength. In this study, the effects of self‐reinforced polyethylene during extrusion with a conventional single‐screw extruder are investigated. High extrusion pressure and controlled flow conditions at a low stock temperature, in conjunction with a longitudinal flow gradient in the die, permit the production of high‐strength, extremely rigid profiles from high‐density polyethylene (HDPE). The reinforcement is due to the shish‐kebab structure, which is responsible for the improvement of thermal and mechanical properties. The growth of shish‐kebab structures with an extended chain crystal axis and the radially lamellar structure, originated by flow‐induced crystallization, depends on the orientation of the macromolecules in the melt. Compared to a conventional extruded profile, the high‐strength profile exhibits molecular chain orientation largely parallel to the direction of extrusion, homogeneous transparency, and an approximately tenfold increase in Young's modulus, as well as improved thermal properties and good dimensional stability.
A knowledge‐based design approach for injection molding of plastic balls
Authors: Tseng A. A. , Arinze B. , Kaplan J. D.
Published: April 2003

Many firms have begun to recognize the promise and potential offered by knowledge‐based systems (KBSs) to their design and manufacturing processes. KBSs incorporate the problem solving heuristics of human experts to solve difficult problems in various problem domains. A knowledge‐based approach is proposed here for the computer‐aided design of injection molded parts. The proposed KBS contains acquired knowledge of mold design alternatives and the assessment criteria for these designs. It employs this knowledge and user‐entered problem characteristics to assist in the selection of a mold design for specific requirements. The rule‐based KBS was developed using the 1st Class KBS development shell. In addition, the associated computer‐aided design system is described in some detail, along with areas for future research.
Parameters affecting process efficiency of peroxide‐initiated controlled degradation of polypropylene
Authors: Ryu S. H. , Gogos C. G. , Xanthos M.
Published: April 2003

Parameters affecting the process efficiency of the peroxide‐initiated controlled degradation of polypropylene, such as concentration of peroxide, reaction temperature, and reaction time, are investigated under in situ uniform, microsegregation, and macrosegregation conditions. Reactions were carried out under static (no mixing) and dynamic conditions on model samples and samples processed in batch mixers, respectively. As expected, MW and MWD decrease with increasing peroxide concentration. Low‐ and mid‐MW fractions increase at the expense of the high‐MW fractions. Localization of peroxide occurs under macrosegregation conditions and causes overall loss of peroxide efficiency and side reactions. MW and MWD undergo no further change after three to four times the half‐life of the peroxide. A kinetic model developed using the quasisteady‐state approximation on free radicals successfully simulates the degradation reaction kinetics for peroxide uniformly mixed. The method of moments is used to calculate the molecular weight averages. The results of this work confirm the importance of mixing uniformity as a parameter controlling the product characteristics in reactive processing.
Experimental analysis of the thermoplastic film embossing process
Authors: Haber A. , Kamal M. R.
Published: April 2003

Film embossing is a mechanical process in which a flat film is transformed into an embossed product. During the process, thermal and stress fields are applied to the polymer, causing changes in the microstructure and physical dimensions of the material. The engineering analysis of the process requires the study of various aspects relating to the characterization of the microstructure before and after embossing. A variety of experimental techniques were employed to characterize the properties and microstructure of embossed films in relation to: crystallinity, orientation, mechanical properties, and dimensions of the embossed films. Mathematical models were also developed to predict the temperature, stress, and deformation in the plastic films during embossing. The thermal treatment of the plastic film was shown to be the most significant factor in the process. The important aspects influencing the thermal treatment include the radiation heater temperature, preheat roll temperature, line velocity, and film thickness. In general, the heat transfer mathematical model predictions were in agreement with the experimental data. The stress and deformation analysis suggested that the embossing pressure and the rubber layer on the backup roll are significant factors in determining the dimensions and stress distribution in the nip region. © 1992 John Wiley & Sons, Inc.
Reactive polymers for blend compatibilization
Authors: Liu N. C. , Baker W. E.
Published: April 2003

Blending of immiscible polymers offers attractive opportunities for developing new materials with useful combinations of properties. However, simple blends often have poor mechanical properties and unstable morphologies. Compatibilization of such blends is necessary. Preformed graft or block copolymers have been traditionally added to act as compatibilizers. Another route, however, is to generate these copolymer compatibilizers in situ during melt blending using functionalized polymers. In this review, a variety of reactive polymers that have been utilized in the reactive compatibilization of polymer blends is examined. They are classified into six major categories according to the types of reactive groups they have, namely, maleic anhydride, carboxylic acids, carboxylic acid derivatives, primary and secondary amines, hydroxyl and epoxide, and reactive groups capable of ionic bonding. Their preparation methods and applications and the chemical reactions they undergo during melt blending are presented. © 1992 John Wiley & Sons, Inc.
Peroxide modified polyolefin blends—Part II: Effects on LDPE/PP blends with polymer components of dissimilar initial viscosities
Authors: Yu D. W. , Xanthos M. , Gogos C. G.
Published: April 2003

Blends of high viscosity polypropylene (PP) and low viscosity low density polyethylene (LDPE) were prepared in the Brabender batch intensive mixer. The rheological mismatch of these immiscible blend components resulted in coarse blend morphologies and poor mechanical properties. By reacting these blends with 1800 ppm Lupersol 101 peroxide at 180°C, the morphology became much finer and, at 50/50 weight ratio, assumed co‐continuous phase structure. As a result, the mechanical properties improved significantly. The above observations are attributable to the fact that the peroxide reaction at this level of concentration removes the rheological disparity of the blend components, a condition that is more favorable for efficient dispersive mixing. It has been frequently stated that the scale and uniformity of dispersive mixing affects the uniformity and controllability of reactions occuring in molten blends. This work shows, for the first time, that reactions with peroxides alter the blend morphology in a beneficial manner. The two phenomena are, therefore, mutually dependent. © 1992 John Wiley & Sons, Inc.
Advances in injection molding of fiber‐reinforced thermoplastics during 1991—Part I: Materials and processing
Authors: Kelleher Paul G.
Published: April 2003

This is the first part of a survey of developments reported in the open literature during 1991 on the subject of injection molding of fiber‐reinforced thermoplastics. Topics discussed include materials, polymers, reinforcing fibers and composite properties, advances in the injection molding process, fiber breakage, shrinkage, and warpage, as well as alternatives to the injection molding process. © 1992 John Wiley & Sons, Inc.
Synthesis of polystyrene and styrene copolymers by reactive extrusion
Authors: Michaeli W. , Grefenstein A. , Frings W.
Published: April 2003

Due to the ever growing and more complex demands on the properties of polymer materials, raw material producers and plastic processing industries are more and more interested in the continuous synthesis and modification of polymers. In this article the production of polystyrene and styrene–isoprene copolymers by anionic s‐butylithium initiated bulk polymerization in a co‐rotating close intermeshing twin screw extruder is described. In spite of high reaction temperatures of above 200°C the polystyrene stays “living,” which makes it possible to modify it in the melt by adding more monomers. The following processing methods were developed and analyzed: (1) homopolymerization of styrene; (2) copolymerization of styrene–isoprene monomer mixtures; (3) sequential polymerization of styrene and isoprene. The polymerized material resulting from the above‐mentioned processes differed considerably in structural features and final properties depending on the process parameters. As for homopolymerization of styrene, the influence of the process parameters screw rotation speed and screw geometry on the range of molecular weight distribution was investigated. Products made according to the two copolymerization methods were analyzed with regard to their structure and mechanical properties. The tests revealed that for sequential copolymerization the temperature in the polymerization zone has a decisive influence on the product quality. The influence of the screw rotation speed is less important. © 1993 John Wiley & Sons, Inc.
Biaxial orientation of poly(ethylene terephthalate). Part I: Nature of the stress—strain curves
Authors: Chandran Prashant , Jabarin Saleh
Published: April 2003

The orientation characteristics of extruded poly(ethylene terephthalate) (homopolymer) sheets are discussed with respect to: mode of extension (simultaneous versus sequential), strain rate, stretch ratio, and temperature. The nature of the stress–strain curve recorded during the process of extension, and its dependence on the parameters of extension mentioned above, are analyzed. Results show that for specimens stretched in the biaxial simultaneous mode, the stress–strain curve is concave upward, which is typical for amorphous polymeric materials. Strain‐hardening occurs at high planar extensions, and is accompanied by a steep rise in the stress measured during stretching. The magnitude of this stress increases with increasing strain rate and extension ratios, but decreases with increasing temperature of stretching. During biaxial sequential extension, the stress–strain curve is concave upward at low strain rates or at high temperatures of stretching, but becomes convex upward at higher strain rates or lower temperatures. The curve also changes from concave to convex upward at high limiting extension ratios in the first direction. The convex upward nature of the stress‐strain curve is typical of semi‐crystalline materials, indicating the occurrence of strain‐induced crystallization during stretching in the first direction. Again, the level of stress, measured during stretching, increases with increasing strain rate and limiting extension ratio in the first direction, while it decreases with increasing temperature of stretching. © 1993 John Wiley & Sons, Inc.
Biaxial orientation of poly(ethylene terephthalate). Part III: Comparative structure and property changes resulting from simultaneous and sequential orientation
Authors: Chandran Prashant , Jabarin Saleh
Published: April 2003

Commercial operations for producing Poly(ethylene Terephthalate) (PET), Blow molded oriented containers and films, usually involve stretching PET either in simultaneous or sequential modes. The specific orientation characteristics of these two modes are studied with respect to the stress‐strain curves and the strain‐hardening parameter. Degree of orientation, crystallinity, and physical properties are studied for each mode as functions of stretch ratio and temperature. The relationships among physical properties, orientation, and below molding characteristics are studies. © 1993 John Wiley & Sons, Inc.
Chemical products from cured unsaturated polyesters
Authors: Tesoro G. , Wu Y.
Published: April 2003

Recovery of value from cured unsaturated polyesters, other than the approach of grinding the resin and blending it with virgin resin for applications of minor importance, has been difficult. The long established position of the resins in the marketplace, their complex composition, and economic considerations have been major obstacles. This article reports results obtained on recovery of soluble, well‐characterized products from cured resins by neutral hydrolysis and on some experimental applications of the products. © 1993 by John Wiley & Sons, Inc.
Incremental drawing for controlling synthetic fiber structure, properties, and productivity
Authors: Sussman M. V.
Published: April 2003

The “Incremental Draw Process” (“IDP”)

“Incremental Draw Processing”* and “IDP”* are trademarks and designations for fiber orientation and elongation processes owned and patented by M. V. Sussman, Ph.D., P.E., and licensed through Arthur D. Little Enterprises, Acorn Park, Cambridge, MA 02140.

is a newly commercialized technique that controls stress and temperature profiles during high speed drawing of synthetic polymer fibers. Fiber is drawn on a single pair of shaped rolls through a large number of small draw increments within a compact space. Draw zone length and residence time are increased substantially, allowing time for morphological change. Stress relaxation between draw increments and adjustment of temperature, tension, and rate of deformation (draw) at each draw increment more effectively control stress induced crystal growth and amorphous orientation. Ultra‐orientation, improved tensile modules high tenacity, low break elongation, and low boil‐off shrinkage have been produced. Because rates of plastic deformation during drawing are drastically reduced, high fiber throughput speeds are possible, allowing direct coupling to high speed spinning lines.

Commercial applications include both textile and ultra‐high strength industrial fiber production. Limited drawing of liquid crystal fibers has been demonstrated. © 1993 John Wiley & Sons, Inc.
Computer simulations on polysilanes: A new method for modeling the energetics and dynamics of their thermochromic transitions
Authors: Welsh William J.
Published: April 2003

The present article summarizes the author's recent work on the computer‐based molecular modeling of polysilanes [‐SiR2‐]. First, conformational energies were calculated by molecular mechanics on model compounds for poly(dimethylsilane) (PDMS), poly(diethylsilane) (PDES), and poly(di‐n‐hexylsilane) (PDHS). Discrepancies in conformational preferences between calculated and experimental results are attributed to factors not accounted for in these calculations, such as electronic stabilization and packing forces. Second, molecular dynamics simulations carried out on a model compound for PDHS suggest the presence of an abrupt “gas phase” thermochromic transition, analogous to the abrupt thermochromic transitions observed for PDHS both in solution and in the solid state. Third, we develop a methodology for testing Schweizer's theory of polysilane thermochromism based on the existence of a unique polymer‐solvent conformation‐dependent stabilization energy present in conjugated chains. In his treatment of the polysilanes, Schweizer adopted reasonable estimated values for this stabilization energy term VD. Presented here is a derivation of an analytical expression for VD based on a first‐principles approach. © 1993 by John Wiley & Sons, Inc.
A review of polyethylene–polypropylene blends and their compatibilization
Authors: Teh J. W. , Rudin Alfred , Keung John C.
Published: April 2003

A major stream of post‐consumer plastics is a commingled blend of polyethylenes and polypropylenes, which usually exhibits lower performance compared with that of the homopolymers due to the incompatible nature of polyethylenes and polypropylenes. Various studies have been carried out to characterize the structure, morphology, mechanical, thermal, and rheological behavior of this blend. The effect of processing, modification, and compatibilization have also been studied by various workers. This article is a review of the work done on polyethylene–polypropylene blends to help in the understanding of this important blend. © 1994 John Wiley & Sons, Inc.
Self‐tuning cavity pressure control of injection molding filling
Authors: Gao F. , Patterson W. Ian , Kamal M. R.
Published: April 2003

Cavity pressure is an important process parameter for injection molding control. However, the nonlinear and time‐varying behavior of cavity pressure cause some difficulties in implementing a control system based on commonly used linear, time‐invariant design strategies. In this work, the nonlinearity and time‐varying characteristics of cavity pressure are examined for the mold filling phase. A self‐tuning controller has been developed, implemented, and tested for both regulating and tracking purposes over a wide range of conditions. It is found that the proposed control system can automatically adapt itself to different operating conditions, yielding quite accurate control. © 1994 John Wiley & Sons, Inc.
Oxidative effects of ozone on the aging of structural silicone elastomers
Authors: Keshavaraj Ramesh , Tock Richard W.
Published: April 2003

The changes in the physical properties of three different commercial silicone sealants used in structural glazing applications were monitored as a function of various aging factors. The variables included, simultaneous exposure to ozone and moisture in which the pH of the moisture was maintained at discrete levels of 3 and 11. The extremes of the pH range represent recorded values for acid rain (pH = 3) and basic cleaning solutions (pH = 11), respectively. A second portion of the study involved studying the effect of loading on the sealant performance. The experimental coupons were cut from cast sheets of DC‐790, DC‐995, and DC‐983. An Instron tester was used to measure the changes in the engineering properties such as Young's modulus, relaxation time, and ultimate tensile stress of the test samples as a function of time of exposure to the different variables. Unexposed samples, kept at ambient laboratory conditions, were used as the control. Results of these tests indicate that the Young's modulus of the samples decreases during an initial period as a result of their exposure. After this initial decrease the sample moduli showed a steady increase with time. Surface cracking was observed during the initial period, even though the moduli continued to increase. Moreover, the acid and ozone appear to act synergistically in the deterioration of the sealant. Samples exposed to the higher pH level did not show this kind of behavior. The effect of tensile loading on the sealant was significant only for DC‐983, the high modulus formulation. This was perhaps because of the creation of high stress centers. The medium modulus material exhibited good resistance to the synergistic effects causing deterioration. Ozone resulted in discoloration, softening, and change in structure in the low modulus formulation. Although the ozone did not react aggressively with silicone, its slow attack or degradation should still be of concern. In spite of the use of inhibitors, severe and acute cracks appeared on the surface of the rubber material at angles of ninety degrees across the line of tension. It is hoped that these accelerated test results can lead to more informed decisions concerning silicone sealant life following their exposure to ozone and acid precipitation. © 1994 John Wiley & Sons, Inc.