Puncture deformation and fracture mechanism of oriented polymers

In this study, we investigated the effect of orientation by solid‐state cross‐rolling on the morphology, puncture deformation, and fracture mechanism of an amorphous TROGAMID material and three semicrystalline polymers: high‐density polyethylene (HDPE), polypropylene (PP), and nylon 6/6. In amorphous TROGAMID, it was found that orientation preferentially aligned polymer chains along the rolling deformation direction and reduced the plastic deformation of TROGAMID in a low‐temperature puncture test. The decrease of ductility with orientation changed the fracture mechanism of TROGAMID from ductile hole enlargement failure in the unoriented control to a more brittle delamination failure in TROGAMID cross‐rolled to a 75% thickness reduction. For semicrystalline polymers HDPE, PP, and nylon 6/6, the randomly oriented crystalline lamellae in the controls were first oriented into an oblique angle to the rolling direction (RD) before the lamellae became fragmented and preferentially oriented with the chain axis parallel to the RD. The morphological change resulted in the decrease of ductility in HDPE in the low‐temperature puncture test. In PP and nylon 6/6, the brittle fracture of unoriented controls was changed into ductile failure when they were cross‐rolled to a 50% thickness reduction. This was attributed to the tilted crystal lamellae morphology, which permitted chain slip deformation of crystals with the chain axis parallel to the maximum shear stress direction. With further orientation of PP and nylon 6/6 to a 75% thickness reduction, the failure mechanism changed back to brittle fracture as the morphology transformed into a layered discoid structure with the chain axis of the fragmented crystal blocks parallel to the RD; this prevented chain slip deformation of the crystals. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

High modulus semi-crystalline polymers by solid state rolling

Solid state rolling of semi-crystalline polymers is shown to be an effective method of producing high strength, high modulus tape at acceptable production rates. High density polyethylene tape was produced having a tensile strength exceeding 300 MPa and a tensile modulus of 8.7 GPa at production rates exceeding 8 m/min. A significant factor in producing highly oriented tape by the rolling process is roll temperature. Increasing the roll temperature from 25°C to 125°C not only increases the maximum extent of orientation achievable, but increases the mechanical properties at a given degree of thickness reduction. Internal frictional heat development limited the maximum thickness reduction ratio of polypropylene to 6.6:1. This reduction was reached by rolling at 150°C. The resultant tape had a tensile modulus of 5.1 GPa and a tensile strength of 300 MPa.     

Very high modulus high‐density polyethylene and high‐modulus polypropylene obtained by solid‐state roll drawing

This article describes results obtained with a process developed for rolling and drawing simultaneously polymer profiles in the solid state. Solid‐state roll drawing has the advantage of being continuous, which allows relatively high production rates and the generation of high deformation ratios with some degree of biaxial orientation. The roll‐drawing process allows the extent of biaxial orientation to be controlled by the adjustment of the tension and compression loads applied to the polymers, in particular semicrystalline thermoplastics. Some experimental results obtained with a four‐station roll‐drawing apparatus are presented, particularly on high‐density polyethylene (HDPE) and polypropylene. The effect of process parameters, such as the gap between the rolls and tension, are discussed. Aspects discussed also include relaxation; structure development in terms of orientation and crystallinity as a function of draw ratio (λ); λ as a function of process parameters; and finally, mechanical and thermal properties as a function of λ. Moduli as high as 25 GPa in the longitudinal direction and about 4 GPa in the transverse direction were obtained with successively rolled, initially thick, HDPE profiles. © 2006 Government of Canada. Exclusive worldwide publication right in the article have been transferred to Wiley Periodicals, Inc. J Appl Polym Sci 102: 3391–3399, 2006     

Polymer Crystallization Induced by Biaxial Stretching During Film Manufacture

Abstract

In the manufacture of semicrystalline polymer films, orientation is commonly introduced. This orientation may be uniaxial, unbalanced biaxial, or balanced biaxial. Machine-direction and transverse stretching may be concurrent or sequential; each orienting process is characterized by a trajectory on the biaxial stress plane and the biaxial extension plane. The presence of uniaxial or biaxial tensile stress strongly affects the process of polymer crystallization, influencing the crystal-amorphous equilibrium, crystallization kinetics, and the resulting polymer morphology. Post-solidification alterations in morphology can be imposed by drawing or heat-setting under biaxial tension, below the crystalline melting point.

The machine-direction and transverse properties of a semicrystalline polymer film depend strongly on the crystalline morphology, and hence on the processing history. The dependence of film properties on processing conditions are well recognized and widely exploited; but the morphology of biaxially oriented films and the structure-property relationships involved are only partly understood.     

Orientation of polyoxymethylene by rolling with side constraints

In this paper, we describe the application of the constrained rolling process to produce highly oriented polyacetal bars with enhanced mechanical properties. In this process, the heated polymer billet is deformed in a channel formed in the circumference of the bottom roll that provides lateral constraint to the material as it deforms. It is a process that has attracted interest due to its capability to produce thick cross-sectional oriented products continuously and at moderate production speeds. Here the focus is on two commercial grades of polyoxymethylene (a) Delrin^® 100 and (b) Tarnoform^® 300. Tarnoform^®, unlike Delrin^®, is a copolymer. The compression behaviour of these grades has been investigated in a plane strain channel die to determine the optimum constrained rolling conditions. Samples were then rolled to different reduction ratios close to but below the crystalline melting temperature of the two grades.The modulus and strength increased almost linearly with reduction ratio. Rolled Delrin^® exhibited higher modulus and strength than Tarnoform^®. Under impact loading, with the initial notch perpendicular to the rolling direction, the fracture process was incomplete for both resins with the specimens exhibiting a hinge type break. Structural investigations of the rolled samples were carried out by wide and small angle X-ray diffraction. The structures produced were very similar to those produced in plane strain compression test. The pole figures from the (100) reflection suggest that the c axes of the POM crystals are oriented along the rolling direction while ab planes showed clustering of orientation of (100) normals in six directions. SAXS patterns from the rolled samples with the X-ray beam parallel to the force direction showed two-point patterns that suggest the transformation of the spherulitic morphology to a fibrillar structure in this direction. However, perpendicular to the rolling direction, four-point patterns were obtained that suggest cooperative kinking of the lamellae during deformation to produce a chevron-like structure. The enhancement in properties as a result of molecular orientation suggests that these materials can have major commercial applications.     

The effect of processing conditions on the microstructure of embossed polyethylene films

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 Lo 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 techniques were employed to characterize the properties and microstructure of the embossed film in relation to: crystallinity, orientation, mechanical properties, and dimensions of the embossed films. The thermal treatment of the polymer film was shown to be the most significant factor in the process. By controlling the thermal treatment of the film, it is possible to manipulate the properties and dimensions of the embossed film. The important aspects: influencing thermal treatment include the radiation heater temperature, preheat roll temperature, line velocity, and film thickness. The initial film orientation and embossing pressure have a minor effect on the final properties of the embossed film. The main effect of the embossing pressure is on the bulk thickness of the embossed film.     

Measurement and modelling of the film casting process: 2. Temperature distribution along draw direction

In the case film process a polymer melt is extruded through a slit die, stretched in air and cooled on a chill roll. During the path in air the melt cools and a reduction of both thickness and width takes place; obviously, temperature distribution, thickness and width reductions are function of draw ratio and stretching distance.Temperature distribution along the draw direction was measured as function of flow rate during film casting experiments performed with an iPP resin. A non-contacting method of measurement, based on a narrow-band IR pyrometer, was adopted.A good qualitative agreement is shown between experimental temperature data and predictions of a model accounting of radiation emissivity dependence upon film thickness. Differences are consistent with discrepancies of film thickness evolution along draw direction, indeed the model slightly over predicts both film thickness reduction and, parallel, temperature decrease along the draw direction.     

Experiment on and simulation of moisture transfer and rolling deformation during leaf drying

The drying characteristic and rolling deformation of eaglewood leaves are investigated experimentally, and a model is built based on Fick’s law and stress–strain relations to illustrate the leaf rolling rule. The leaves dehydrate free water and hardly roll during the initial drying period. Rolling deformation is induced by the shrink difference along leaf thickness and occurs when the moisture content reaches a critical level. The rolling index of a leaf that is dried on one side is greater than that of a leaf that is dried on both sides. In addition, the rolling index is influenced by drying temperature and leaf thickness. When a leaf is thick or when the drying temperature is high, the leaf rolls considerably.     

The effects of roll drawing on the structure and properties of oriented poly(ethylene terephthalate)

The effect of processing conditions on the structure and properties of roll drawn poly(ethylene terephthalate) (PET) was examined. It was found that, when roll drawing amorphous PET at temperatures just above the glass transition, only very low draw ratios were obtained. This is probably because there were no crystallites to lock in the applied extension. Roll drawing at high temperatures, above 130°C, where there was significant thermal crystallization, produced film of high strength. At temperatures between 130°C and 190°C, the properties were almost independent of processing temperature. Mechanical tests performed on roll drawn samples, processed in this temperature range, showed that the initial modulus and the yield stress increased linearly with draw ratio. The yield strain decreased with draw ratio up to λ = 4.0, and then became almost constant. The processing temperature that produced samples with the greatest strength was 170°C. This was because the highest draw ratios were obtained at this temperature while maintaining constant width deformation. At low draw ratios, the crystallinity increased with the processing, whereas at higher draw ratios, it was independent of temperature. This constant level of crystalline fraction may have produced the constant failure strain that was observed at high draw ratios. The orientation functions were similarly unaffected by the processing temperature, although birefringence measurements did suggest that lower processing temperatures may have produced higher levels of orientation. The orientation of the trans conformers was independent of the temperature, but the overall content was increased at higher processing temperatures.     

Structure and properties of biaxial‐oriented crystalline polymers by solid‐state crossrolling

The effect of biaxial orientation by solid‐state crossrolling on the morphology of crystalline polymers including polypropylene (PP), high density polyethylene (HDPE) and Nylon 6/6 was investigated with polarized optical microscopy, atomic force microscopy, wide‐angle X‐ray scattering, and small‐angle X‐ray scattering techniques. It was found that crossrolling gradually changed the initial spherulitic structure into a biaxially oriented crystal texture with chain axis of crystals becoming parallel to the rolling direction for all three polymers. The effect of microstructure change on the macromechanical properties was studied in tension at both ambient temperature and ?40°C. In tension at room temperature, the localized necking deformation of HDPE and PP control changed upon orientation into homogeneous deformation for the entire sample length. This was attributed to that the oriented crystal morphology eliminated the stress concentration, which existed in the original spherulitic structure from lamellae orientation in the polar and equatorial regions. At ambient conditions, the elastic moduli of HDPE and PP were found to decrease slightly with orientation whereas the modulus of Nylon 6/6 increased with increasing orientation. This was due to the fact that the amorphous chains of HDPE and PP are in a rubbery state and orientation increased the shear relaxation in the orientation direction but the amorphous chains of Nylon 6/6 are in the glassy state inhibited the shear relaxation. Both the yield stress and strain hardening exponent increased with increasing orientation for all three polymers. In tension at ?40°C, orientation changed the failure mechanism of all three polymers from brittle fracture into ductile failure, as the original spherulitic structure was changed into an oriented structure with chain axis of crystals becoming parallel to the tension direction, which allowed chain slip deformation of crystals and resulted in oriented samples showing ductile failure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

一种垫胶卷取小车衬布恒张力控制器的设计

本文主要介绍了在轮胎制造过程中,垫胶卷取小车的衬布张力是如何通过张力控制装置来实现恒张力的。该装置可以解决因衬布的挤压导致垫胶制品变形的工艺问题。     

Analytical prediction of roll coating with counter-rotating deformable rolls

Roll coating is a common technique for applying thin coating films on continuous substrates, e.g., papers and foils. Key advantages are the comparatively simple technology and the possibility of coating thin films using highly viscous fluids. Since roll coating is a self-metered process, the prediction of film thicknesses is of fundamental interest for industrial process control. In the present work, a new analytical approach for the prediction of the film thickness in roll coating with deformable rolls and negative gaps is developed. This method is based on the fluid dynamic theory of lubrication approximation. The film thickness is calculated depending on the geometry of the rolls (including the elasticity of the rubber), the fluid properties of the applied film and the roll velocities. This is gained by using boundary conditions for pressure and—in contrast to former literature—for force. The quality of the predicted results is validated with experimental data from literature. The comparison shows good agreement and thus the derived analytical model offers new possibilities for predicting film thickness and understanding of the associated influence parameters.     

Cooling and attenuation of threadline in melt spinning of poly(ethylene terephthalate)

Melt spinning of poly(ethylene terephthalate) was studied by measuring the filament tension at the take-up roll and by measuring filament diameter D(X) at various distances X below the spinnerette. A new method is developed to calculate temperature distribution both along and perpendicular to the fiber axis. Results of these calculations are compared with experimental values. The attenuation of filament diameter depends primarily on the take-up speed and the output rate. Spinning temperature and molecular weight have relatively small effects. Mass flow rate and take-up speed are the major factors controlling the cooling rate, while other spinning parameters such as polymer molecular weight and spinnerette orifice size have a small effect. The Trouton viscosity β is both temperature and molecular weight dependent. Values of β derived from these experiments can be expressed mathematically as follows:      

Cold rolling of polymers 2. Toughness enhancement in amorphous polycarbonates

The effect of cold rolling on the Izod impact strength of amorphous polycarbonates has been studied. The impact strength is a function of the roll reduction as well as the original sheet thickness. Sheets varying from 0.125 to 0.645 inches in thickness have been studied and roll reductions up to 50 percent have been utilized. It is shown that enhancement in impact strength occurs at very small percent roll reductions. The orientation release stress has been measured as a function of roll reduction and the internal stresses through the thickness of the sheets have been studied by birefringence methods. It is suggested that the residual stresses are responsible for impact enhancement rather than the molecular orientation.     

Some features of spinning hollow fibres from polycarbonatesiloxane melts

Conclusions The following parameters have been determined as a result of calculations: fibre tension, solidification path length, and the change along the spinning path of the following: stretching stresses, speed, radius, wall thickness, and fibre temperature.It has been shown that, with increase in take-up speed, the length of the section of fibre deformation is reduced, varying practically proportionally to the flow rate of the polymer system.Stability of the process of spinning hollow fibres from polycarbonatesiloxane melts is assured within a narrow range of stretching force.Translated from Khimicheskie Volokna, No. 3, pp. 41–43, May–June, 1989.     

Melt spinning and characterization of hollow fibers from poly(4-methyl-1-pentene)

In this article, the melt spinning behavior of poly(4-methyl-1-pentene) (PMP) hollow fibers (HF) is examined. The melt spinning trials are carried out on a pilot scale melt spinning plant with different settings while a 10-hole 2c-shaped spinneret is used. It is found that the winding speed mainly affects the outer fiber diameter. The influence of different melt spinning parameters is investigated, in particular temperatures, take-up velocities, and the use of quench air. For this purpose, the shape and crystalline structure of the fibers are analyzed using a light microscope, a scanning electron microscope, and wide-angle X-ray scattering. The shape of the fibers is mainly influenced by the temperature settings in the melt spinning process. As a reasonable lower limit, a melt spinning temperature of 280°C is identified. Concerning the crystallinity, a saturation going along with a slight reduction of the polymer chain orientation is observed at elevated take-up velocities.     

Evolution of phase behavior and orientation in uniaxially deformed polylactic acid films

The development of crystalline structure and orientation during uniaxial stretching of cast amorphous linear and branched lactic acid films were investigated in the rubbery temperature ranges that spans between glass transition temperature and cold crystallization temperature. This material exhibited almost ideal stress‐strain behavior in the temperature range 65–80°C. Because of its strain crystallizability, films with uniform thickness can be obtained at high deformation levels as a result of self‐leveling. Branching was found to retard this self‐leveling through its slightly detrimental effect on the strain hardening. Upon stretching the material undergoes rapid orientation in the amorphous state and beyond a critical level very sharp and highly oriented β crystalline form chains with ?3/1 helix. If the temperature is at or below Tg, with additional stretching, the films were found to revert to a highly oriented amorphous state through the destruction of the crystalline domains. At higher temperatures, further stretching results in continuation of improvement in crystalline order.     

Studies on blown film extrusion. II. Analysis of the deformation and heat transfer processes

Having investigated the elongational flow behavior of polymer melts (part I of this series), we have carried out both theoretical and experimental studies in order to better understand the deformation and heat transfer processes involved in blown film extrusion. For the experimental study, nonisothermal experiments were carried out, using high-density and low-density polyethylenes. Measurements were taken of the axial tension, bubble diameter, and film thickness at a series of extrusion conditions (i.e., flow rate, pressure difference across the film, and take-up speed). For the theoretical study, an analysis was carried out to simulate the blown-film extrusion process, by setting up the force- and energy-balance equations on the blown bubble moving upward. The approach taken in the theoretical study may be considered as an extension of the earlier work by Pearson and Petrie who considered the isothermal operation of Newtonian fluids. In the present study, However, we have considered the nonisothermal operation of power law fluids, whose rheological parameters were determined by an independent experimental study an described in part I of this series. Four highly nonlinear differential equations were solved numerically with the aid of the CDC 360 digital computer, using the fourth-order Runge-Kutta method. The mathematical model predicts the bubble shape, temperature profile, and film thickness as a function of the distance along the machine axis. Comparison is made of the experimentally observed bubble shapes with the theoretically predicted ones, showing a reasonable agreement.     

Prediction of axial and off-axis yield behavior of lsotactic polypropylene film from structural state parameters

Since only minor structural rearrangements occur prior to yielding, the axial and off-axis yield stress of a polymer sample is expected to relate to the initial structural state of the material. Samples of uniaxially oriented isotactic polypropylene films with known structural state parameters have been deformed under a uniaxial tensile stress at deformation rates of 20 percent and 100 percent per minute over an angular range from 0 to 90 degrees to the fabrication direction. The axial yield stress data was found to correlate with the initial noncrystalline orientation state of the undeformed sample. The axial and offaxis yield stress data was fit to the Hill equation derived for the application of a uniaxial tensile stress. The experimentally determined coefficients from the best fit of the yield stress data to the Hill equation were related to the measured structural state parameters. These correlations allow prediction of the yield stress of an anisotropic film at any angle to the film symmetry axis from knowledge of the measured structural state parameters alone.     

Stress optical studies of oriented poly(methyl methacrylate)

A series of samples of oriented poly(methyl methacrylate) was produced by hydrostatic extrusion at temperatures below the glass transition temperature. The development of orientation in the process was monitored by the measurement of birefringence which was shown to depend on the extrusion temperature as well as on the applied deformation. Additional information to characterize the oriented state was obtained by measuring the shrinkage force which developed when the oriented sample, constrained to constant length, was heated to a temperature just above the glass transition temperature; specimens free to contract all recovered to the isotropic state and original dimensions on annealing at this temperature. Most measurements were made on specimens ‘as extruded’ with additional studies made on specimens annealed at temperatures above the extrusion temperature but below the glass transition. The data, which have strong implications with regard to deformation mechanisms, are interpreted both at a molecular level in terms of deviations from an ideal rubber network, and at a more phenomenological level in terms of the Mooney-Rivlin equation.