Amorphous Plastic Research Articles

Materialmikroskopie unter besonderer Berücksichtigung der Kunststoffe Teil II: Beobachtende Lichtmikroskopie an Kunststoffen / The Microscopy of Materials and in Particular of Plastics Part II: The Light Microscopy of Plastics

Apart from the established methods of incident light microscopy used for classical metallic surfaces, the examination of polymers requires the use of new and often unfamiliar methods of microscopy. In particular in the case of the broad pallet of filled plastics, all of the tried and trusted metallographical techniques used for the examination of opaque materials, such as bright and dark field microscopy, may also be used. In addition, for the examination of polymers, the entire spectrum of transmission microscopic techniques is also required. This is especially true in the case of colourless, amorphous plastics where the entire component e.g. in an optical stress tester can be examined non-destructively. For coloured, partially crystalline or filled and reinforced polymers, these methods can only be used on thin sections of at least adequate translucence to those with almost optically transparent properties. In this article the instrumentation required to carry out such investigations and methods of observation and evaluation which can be used are described. Further possible reasons of erroneous measurements and their prevention are explained detailed with reference to actual practical examples. In addition, the possible artefacts which might be produced are discussed, and methods by which they may be avoided suggested.

Effects of the shape of the energy director on far‐field ultrasonic welding of thermoplastics

AbstractAn energy director is widely used in ultrasonic welding to increase the welding speed and quality. In the present work, three different types of energy directors were studied—namely, a triangular, a rectangular, and an innovative semicircular energy director. Experiments were performed using far‐field test samples made of amorphous‐type (ABS) and semicrystalline‐type (PE) thermoplastics. It was found that the weld time is an important parameter of ultrasonic welding for the three types of energy directors studied. Weld pressure has different effects for the types of plastics tested. Increasing the weld pressure will decrease the welding efficiency for ABS. But for PE, increasing the weld pressure to four bars will increase the welding efficiency. The shape of the energy director was found to significantly affect the welding efficiency. In comparison, a semicircular shape was found to yield the highest welding efficiency under the same welding conditions and the triangular shape the lowest. Temperature measurements at the triangular energy director during the welding process indicate that the energy director absorbed 48.5% of the welding energy for ABS and 21.1% for PE. The different energy absorption rates are probably due to the difference in elasticity and viscosity between amorphous (ABS) and semicrystalline (PE) plastics.

Physically based dynamic model for control of cavity pressure in injection moulding of amorphous plastics

Cavity pressure plays an important role in determining the quality of injection moulded articles. However, the non-linear and time varying behaviour of the process causes difficulties in implementing an efficient control system, since these generally employ linear time invariant design strategies. Conversely, advanced control methods such as self tuning control do not provide physical information about the process. A physically based model has been developed. This model is used to control cavity pressure during the filling and packing phases. Experimental results indicate that this control yields accurate results for the injection moulding of polystyrene. Different control algorithms with different tuning criteria were implemented on a microcomputer to control the cavity pressure. The results indicate that this method is superior to the classical empirical control approach.

Amorphous Plastics Pave the Way to Widespread Holographic Applications

Noncrystalline organic photorefractive materials appear to be on the verge of widespread applications such as holographic storage and real‐time holographic processing. The performance levels of these materials, which are attracting increasing interest, are close to, and in some respects even beyond those of their long‐known inorganic counterparts.

PET/PC blends and copolymers by one-step extrusion: 1. Chemical structure and physical properties of [formula omitted] blends

Reactive blending on poly(ethylene terephthalate) (PET) and bisphenol A polycarbonate (PC) with a catalyst mechanically dispersed on polymers was performed by one-step extrusion for about 1 min at 275°C. The product appeared to be a transparent amorphous plastic with a single glass transition temperature. This approach provides a possibility to produce PET/PC block copolymers directly by extrusion of homopolymers or to create their compatible blends during one-step extrusion by varying the type and concentration of the catalyst.

Development of density distributions in injection molded amorphous engineering plastics. Part II

AbstractThe experimental results obtained on the gapwise density distributions of freely quenched and injection molded specimens of two engineering thermoplastics presented in Part I of this paper were compared with simulation results. The effect of the thermal history on the density development in the freely quenched samples was investigated by employing the fictive temperature concept in conjunction with the experimentally determined material characteristics. The density distribution in freely quenched specimens of poly(ether imide) could be readily predicted. The combined effects of the cooling rate and the pressure history on the density distributions of injection molded samples were also investigated. A phenomenological model was developed to combine the effects of thermal and pressure history on the density distribution. Overall, the predicted results for injection molded specimens agreed well with the experimental results.

Development of density distributions in injection molded amorphous engineering plastics. Part I

AbstractThe gapwise density distributions of the injection molded specimens of two engineering thermoplastics, i.e., poly(phenylene ether) and poly(ether imide), were characterized employing the density gradient column technique. The samples were molded using a 40t Van Dorn injection molding machine. The effects of the thermal history on the density distribution of unconstrained quenched specimens were also investigated. In addition, various material properties, such as pressure‐volume‐temperature, isothermal contraction, and pressure induced densification behavior were characterized, for the two resins employed in this study. The moldings of the two resins exhibited different trends in their density distributions. These findings were explained in terms of the competing effects of cooling rate and the pressure history experienced by the engineering plastic resins during the molding cycle. The data collected were also used as input to mathematical modeling of density distributions in injection molded articles, which is reported in Part II of this article.

Electrical breakdown-from liquid to amorphous solid

The electrical breakdown of dielectric liquids is considered from the viewpoint that known breakdown mechanisms in liquids should be helpful in predicting breakdown mechanisms occurring in amorphous plastics. The effect on the breakdown process of increasing liquid viscosity is charted for both positive and negative discharges. It is in the short pulse positive discharge mechanism that the greatest similarities can be expected. Similarities are also noted in negative discharge processes for some solids and liquids of very high viscosity (100000 cS). The high effective viscosity of the plastics allows for the mechanism of cumulative (multi-pulse) damage which is not generally available in the liquids. However, experiments with liquids of viscosities only one or two orders of magnitude greater than the largest (100000 cS) reported for this paper should allow a valuable comparison in this area also.

Texture and Strength Evolution inDeformed Polypropylene

Semicrystalline polypropylene samples with high concentrations of crystallinity (≳62%) are deformed in uniaxial compression at different temperatures and different strain rates. The orientation changes of the crystalline part of the material are analyzed using X‐ray diffraction methods. Pole figure evaluation is used to obtain information about the crystal structure, the geometry of slip, and its correlation with the characteristic of strength evolution in polymers. The contribution of the crystalline part of the material to plastic flow is estimated and the textures are related to the characteristic effects in plastic and visco‐elastic response of the material during deformation and subsequent stress relaxation. The observed texture effects can be explained by the rotation of crystallographic c‐axis (backbone axis) normal to the compression direction at high temperatures in a planar slip mode. The stress/strain and relaxation behavior of the material is interpreted as a combination of viscoelastic flow of amorphous volume and irreversible plastic flow of the crystalline part which contributes to an orientation dependence of deformation and stress relaxation.

Microstructure and ultimate properties of injection molded amorphous engineering plastics: Poly(ether imide) and poly(2,6‐dimethyl‐1,4‐phenylene ether)

AbstractSpecimens of two engineerig plastics i.e., poly(ether imide), PEI, and poly(2,6‐dimethyl‐ 1,4‐phenylene ether), PPE, were injection molded employing a 40t Van Dorn injection molding machine and industrial practices. The mold and melt temperatures and the injection speed were varied in a limited range which furnished acceptable samples. The density, birefringence, residual stress distributions, flexure and tensile properties, and crack development of the injection molded specimens were studied. Vacuum compression molded samples were also prepared to investigate the role played by the cooling rate in shaping microstructural distributions. The results revealed significant differences in the development of microstructure of the molded specimens of the two resins, which was related to rheology and molding conditions on one hand and to development of cracks and ultimate properties on the other hand.

Sintering of PVC

AbstractAlthough PVC is counted as one of the amorphous plastics it possesses a distinct powder grain structure. This structure is responsible for various effects; for example it is impossible to sinter thick pieces of rigid PVC dryblend possessing satisfactory mechanical properties. For this application, it is necessary to break down this powder structure into smaller parts, called primary particles, by mechanical shearing and covering these particles with processing auxiliaries enabling it to be sintered. PVC can then be melted by the influence of temperature and pressure in the same way as other thermoplastic materials. For this reason, tests were carried out to determine the best material configuration and the necessary data for the compounding and shearing process, and the sinter conditions. A continuously working roll mill was used to break down the structure. The rolls of this equipment are provided with grooves inclined at a certain angle. With this construction it is possible to achieve a high shearing rate at a low mass‐temperature. Furthermore, all additives can be added during the rolling process, so that no additional mixing process is necessary. These gelable pellets, made by the roll mill, can thus be formed in a die into plates of different thicknesses. Samples were pressed by varying compounding and sintering conditions and these were tested for mechanical properties.

Injection molding of engineering plastics

Abstract : This final report summarizes the findings of a study in injection molding of engineering plastics. Two engineering plastic resins, i.e. unmodified grades of a poly(ether imide) and a poly (2.6-dimethyl-1,4-phenylene ether) were thoroughly characterized. The characterization included rheology, thermal properties and P-V-T behavior. The data were employed to predict various microstructural distributions including density, residual stress and birefringence distributions in compression and injection molded specimens of these two engineering plastics. The detailed microstructural distributions were also studied experimentally upon processing the two engineering plastics, employing instrumented machines and industrial practices. The experimental findings were elucidated under the light of the numerical simulation results. Overall, this study should furnish a first order understanding of the microstructure development in articles injection molded from amorphous engineering plastic resins.

An Evaluation of Analytical Expressions for the Representation of Plastics Creep Data

Various analytical expressions were evaluated for their ability to represent plastics creep data resulting from isothermal, constant stress creep experi ments. The constants for each expression were determined by an iterative least squares analysis performed on published creep data for crystalline, amorphous, crosslinked and multiphased plastics. The expressions were ranked in order of effectivity for the representation of creep data using the sum of the squares of the residuals as the criterion of ranking. It was determined from the analysis that a simple 3 parameter parabolic function was the most accurate, as well as general expression for the analytical representation of the plastics creep data investigated. The best fit constants for the functions investigated were, in general, dependent upon stress. It was found that the extrapolation of 1 day creep data to periods of time as great as 1 year, generally led to large errors.

Characterization of photoviscoelastic materials by a nonlinear constitutive equation

On the basis of a phenomenological theory which is guided by the results on the macroscopic material behavior, a nonlinear constitutive equation is derived to characterize the photoviscoelastic behavior of homogeneous, isotropic, and nonaging amorphous plastics, including large deformations. It is shown how to determine four memory functions by running creep tests in pure tension and simple shear. The testing equipment and the methods of test evaluation are explained. Results are reported for some typical materials. They show how to simplify the basic equation for applications in the analysis of plane states of stress in polymeric structures under quasistatic isothermal loading conditions.

Biaxial orientation of polypropylene by hydrostatic solid state extrusion. Part III: Mechanical properties and deformation mechanisms

AbstractThe deformation of biaxially oriented polypropylene sheet produced by the BeXor process has been examined in terms of the hierarchical solid state structure described previously. The mechanical properties have been examined over a range of temperatures and strain rates. The anisotropic nature of uniaxial tensile deformation was analyzed from simultaneous measurements of longitudinal extension and lateral contraction in the width and thickness directions, by determining the shape of uniaxially deformed spherulites, and by examining fracture surfaces in the scanning electron microscope (SEM). It was determined that deformation proceeds by elastic extension of the amorphous network and plastic shear deformation of the crystalline regions in the plane of the sheet, and in the thickness dimension, by voiding with induced fibrillation. It is suggested that the property improvements, particularly at low temperatures and high strain rates, which are achieved by the BeXor process, result from changes in the hierarchical structure at the size scale of the crystalline lamellae. Specifically it appears that the small crystalline blocks formed by breakup and recrystallization of lamellae are more easily deformed than the original large coherent lamellae.

Differences between amorphous and crystalline plastics with respect to thermoforming

Advances in Polymer TechnologyVolume 5, Issue 3 p. 209-218 Article Differences between amorphous and crystalline plastics with respect to thermoforming J. J. M. Cormont, J. J. M. Cormont 6118 Ge-Nieuwstadt, The NetherlandsSearch for more papers by this author J. J. M. Cormont, J. J. M. Cormont 6118 Ge-Nieuwstadt, The NetherlandsSearch for more papers by this author First published: Autumn (Fall) 1985 https://doi.org/10.1002/adv.1985.060050305Citations: 8AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume5, Issue3Autumn (Fall) 1985Pages 209-218 RelatedInformation

A simple procedure of accounting for physical aging in the prediction of the modulus of glassy plastic materials

AbstractThe basic relationships are presented for predicting the creep and stress relaxation as a function of physical aging, loading time, and temperature of amorphous polymers processed under such conditions that their thermal treatment amounts to a quench from the melt to the glassy state. A nomogram construction, based on these relations, is proposed and developed for a particular polymer, namely an extrusion type acrylonitrile‐butadiene‐styrene (ABS), RONFALIN EST. The applicability of the procedure is restricted to the isothermal viscoelastic behavior at low stresses and strains of unplasticised amorphous plastic materials quenched from the melt to the glassy state.

Reduction of Creep Rate of Amorphous Plastics

Abstract In the present work an annealing technique for the reduction of free volume in the molded articles was demonstrated as an effective means for improving the creep resistance of high acrylonitrile polymers [1] as well as other amorphous polymers.

Thermal history and mechanical properties

An account is given of three separate experiments in which the creep modulus and the recovery after creep are shown to be strongly affected by the thermal history. The tensile creep modulus of several amorphous plastics in their glassy state increases with storage time; the recovery after creep can be very much worse than normal under some quite ordinary operating conditions; the modulus in flexure of polypropylene at 20°C is remarkably sensitive to minor thermal treatment (e.g. quenching to 0°C after a short period at 40°C). The results, including those on the polypropylene, are all consistent with the concept of slow molecular re-ordering leading to a state of lower free volume. The implications for materials evaluation programmes are discussed.

Some factors influencing cooling rates of rotationally molded parts

AbstractRecently, rotational molding engineers, concerned with warpage and uneven cooling in parts, have been “pre‐cooling” the mold in forced draft air after removal from the oven and prior to water quenching to removal temperature. In this paper, we analyze some of the factors that influence the rate of heat removal from an amorphous plastic in a metal mold. We find that mold thickness and thermal diffusivity, convection heat transfer coefficient of the cooling fluid, the thermal properties of the plastic and the initial, final and “freezing” temperatures of the plastic influence this cooling rate and the corresponding rate of volumetric shrinkage. We illustrate our analysis with several examples and discuss some guidelines in detail.