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Poly(lactic acid) [PLA] is a well known biodegradable polymer which has been used in drug delivery systems, surgical repair systems such as sutures and bone fracture fixation pins and screws. PLA is biocompatible, has a high tensile strength, and has a high elastic modulus[1,2]. However, one drawback of PLA is the low elongation at break due to a brittle fracture while under tensile and bending loads. The elongation at break of PLA is typically 3 - 5 percent. The reason for this brittle behavior is due to physical aging which occurs during storage at room temperature and has been studied extensively. Plasticization is a common technique used to increase the ductility of a brittle polymer. In the case of PLA a suitable plasticizer must be miscible with PLA so as to decrease the glass transition temperature, as well as be biodegradable and nontoxic so as to provide a useful biodegradable blend. The advantages of the plasticization are low cost, ease of processing, and the ability to alter the properties of the blends by varying the amount of plasticizer. Use of a functionalized plasticizer can be more desirable such that a chemical bond is formed with the PLA polymer thereby preventing loss of the plasticizer through migration.
An experimental investigation of the inter-cavity dependencies of a modular tool using Dynamic Feed control is presented. The results showed that part weight and overall part dimension are controlled by the individual command pressure to each cavity and the material's melt temperature. The weight and dimensions of a particular cavity appear independent of the control pressures set to fill the other cavity inserts. This was demonstrated to the extent that weight and dimension showed independence from a deliberate short shot condition in the hardest to fill part. There may be a minor synergistic effect of adjacent cavity pressure and the shorting part but this would need to be confirmed with additional experimentation. Once initially stabilized both traditional machine velocity control and Dynamic Feed weight variations were in the range of +/- 0.025% or less.
The effects of antiblocking agents on the crystallization process of linear-low density polyethylene were investigated in dilute solution and in bulk. It was found that the type of antiblock has a more profound influence on crystallization than does macromolecular architecture. Silica was not found to have appreciable influence on the crystallization process either in bulk or in dilute solution. Talc, however, affected the crystallization process in both phases: in dilute solution, it decreased the apparent homopolymer fraction in TREF, and in bulk, it elevated the onset of crystallization and reduced the rate of nucleation, as monitored by DSC and DMA.
Changes in resin chemistry have a significant effect on the manufacturing parameters and mechanical properties of composite systems. A customized resin was developed for the fabrication of a water ski, which conformed to the initial manufacturing parameters of the commercial resin system. The curing properties of the resins were investigated and laminates were taken from skis to compare resistance to high temperature deformation using stress relaxation experiments. Also, real-time strain measurement systems were built and tested on skis made with both resins. Results showed that the stiffness increased under a 3-point bending load and stiffness was maintained at higher temperatures with the customized resin system.
Rapid curing of structural adhesives by ultrasonic heating has been demonstrated successfully in recent work. Therefore, it is important to examine whether ultrasound would induce non-thermal effects that accelerate the reaction rate of the adhesive. In this paper, Differential Scanning Calorimeter was used to carry out thermal analysis of the reaction kinetics of a two-part structural epoxy adhesive. A chemical model based on a four-parametric semi-empirical equation was developed to distinguish the non-thermal effects from ultrasonic vibration from the thermal effects resulting from ultrasonic heating. It was found that the non-thermal effects was more significant at the beginning of the curing process but it gradually diminished as the heating time was increased. The conversion of the epoxy adhesive produced by ultrasonic curing at 50 seconds was almost three times higher than that obtained by thermal heating.
Wetting of liquids on solid surfaces is very important for many applications including adhesive bonding. Ultrasonic forced wetting has been shown to reduce the contact angle of liquids on solid surface. The application of hydrodynamic analysis to model the process is difficult because of the well-known stress singularity that arises at the liquid/solid contact line. In this paper, dimensional analysis was utilized to establish a semi-empirical dimensionless equation for the prediction of the ultrasonic forced wetting process. Experimental data of dynamic contact angles of three liquids vibrated at different frequencies and amplitudes were produced. By correlating the dynamic contact angle with liquid properties, geometric parameters, ultrasonic vibration parameters, gravity, and thermal effects caused by viscous heating, it was shown that a dimensionless equation can be developed to predict the dynamic contact angle of liquids under ultrasonic forced wetting.
The visioplastic method is often used to model material flow for the deformation of metals. The method provides qualitative and quantitative data on material flow such as strain, strain rate and velocity throughout the flow regime. Data obtained is used to evaluate die design, troubleshoot processing problems or evaluate results of computer modeling software. The present study applies this method to the extrusion of elastomeric profiles. An advantage with elastomers is that the actual polymer being studied can be modeled directly. A general discussion of the visioplastic method is provided along with results from the evaluation of selected die geometries.
In this work, the influence of thermoplastic starch (TPS) composition, processing conditions and the hot stretch ratio (HSR) on the morphology of LDPE/TPS blends were studied. Blends were prepared in one- and two-step processes. Both series of blends were drawn at different HSR at the exit of the die. The morphology of blends was quantified using a novel methodology, which allows a more precise evaluation of the particle volume. Blends prepared in the one-step process showed increased levels of anisotropy as a consequence of a combination of coalescence and particle deformation during melt drawing. Conversely, TPS particles of reprocessed blends showed no-coalescence and a low degree of deformation.
The performance of standard mica heater bands, mineral heater bands, and heater bands containing new inorganic insulation materials were compared. The overall performance of the inorganic and mineral insulation was far better than that of standard mica insulation. Inorganic-insulated heater bands generally provided faster response, better stability, and lower power consumption than a standard mineral-filled band. Although high water retention in experimental binders led to premature heater band failure, optimized inorganic binders gave better high temperature performance than organic binders. Finally, a combination of inorganic insulating materials produced the best overall results.
A predictive melting model for polymer particulates in co-rotating twin screw extruders (Co-TSEs) is proposed. The proposed model starts with the solids conveying section where discrete mechanics is used to describe the movement and deformation of solid particulate assemblies. The interparticle forces can be estimated based on the screw geometry, processing conditions, and material properties. These forces are the sources of two important melting mechanisms: the Frictional Energy Dissipation (FED) and the Plastic Energy Dissipation (PED). The model also considers the role of barrel heating in creating a layer of preformed melt". The existence of preformed melt changes the conveying properties of solid particulates dramatically. Finally the model considers the important heat generating term the Viscous Energy Dissipation (VED) whose onset coincides with the creation of a fraction of molten polymer generated by any of the above mechanisms."
The Pyris TC Probe requires calibration with characterized standards to relate instrument response to effusivity and/or thermal conductivity. This study was under taken to determine the magnitude of any drift in calibration that occurred over time. Drift could occur due to uptake of water in the sensor's insulation or surface wear. Three calibration standards of known effusivity values were used to calibrate the instrument at 4 test times from 6 to 30 seconds. Several standards were tested during a period of 64 days with each of the calibration files. Of the 120 tests conducted, only 2 results varied more than 4%, indicating excellent stability.
The Valyi surface finishing/compression molding process (SFC) has successfully been used to produce large structural panels with Class A finishing under low pressure. The material used in the SFC process must meet certain performance requirements in order to fully exploit the capability of the process. This paper compares the mechanical properties and rheological properties of short and long glass and carbon fiber reinforced materials. The Long fiber reinforced PP resins show enhanced stiffness and impact strength. Degradation of surface appearance due to long fiber read through is an issue to be addressed in future work.
Understanding thermal behavior of molten polymers is critical to many different resin molding processes. The objective of this study was to investigate the adaptation of the Transient Plane Source (TPS) thermal analysis method for evaluating polymer feedstock raw materials in pellet form through the melt. A new sample holder was designed to accommodate the logistics of the experiment. Two sets of polymer raw materials were evaluated, one a random copolymer polypropylene with no filler and the other a polypropylene backbone modified with rubbers and mineral fillers. Thermal conductivity results for each sample were obtained at five temperatures, 250°C, 200°C, 150°C, 100°C and 50°C. Each sample was tested in triplicate to identify the precision of the TPS technique under each condition. The results of this study were correlated to thermal conductivity results obtained on the same samples by ASTM testing method D5930-97, using the transient line-source (TLS) technique.
Polymer processing equipment, batch or continuous, provides for some or all of the following mechanisms for the heating and melting of polymer particulates: Conductive Heating, Interparticle Friction Energy Dissipation (FED), Plastic Energy Dissipation from each deforming solid particulate (PED) and Viscous Energy Dissipation (VED) arising from the flow of the viscous polymer melts. Experimental evidence generated in our laboratories where PED was evaluated with individual solid polymer cylindrical samples and inside compounding equipment, such as Co-TSEs, indicates that PED arising from the irreversible deformation applied by the compounding equipment on solid particulates is often orders higher in magnitude than other heating/melting mechanisms.
Customary polycarbonate (PC) with a relatively small amount of polypropylene (PP) between 0,5 and 5 weight-% has been processed into blends and determined in extensive tests. An increase in low-temperature impact strength was shown: the impact resistance values of pure PC determined in Izod-tests could be improved by the factor 5 by adding 3 weight-% of PP. As a reason for the extremely high impact properties an special morphology of this group of blends could be stated. Because of the incompatibility of both blend partners in connection with remarkably different thermal dilatation factors concerning a common processing, fine-dispersed PP particles are created in the PC-matrix, which are surrounded by cavities. If favourable geometrical conditions of this cavity morphology (diameters, distances and so on) are present, shear mechanisms of deformation and stop processes of cracks are facilitated, which restrain or decelerate a crack propagation at a sudden load.
In this paper, we present a new continuum framework to formulate models to study flow induced crystallization in polymers. The models are developed in a general thermo-mechanical setting and are able to incorporate the main features of the crystallization process. A consistent framework is developed to model the transition from a fluid like behavior to a solid like behavior. The anisotropy of the crystalline phase is included in the model and depends on the deformation in the melt. Particular models are generated by choosing specific forms for the internal energy, entropy and the rate of dissipation. Equations governing the evolution of the natural configurations and the rate of crystallization are obtained by maximizing the rate of dissipation. The initiation criterion, marking the onset of crystallization, arises naturally in this setting in terms of the thermodynamic functions. The model is used to simulate bi-axial extension in a polymer film that is undergoing crystallization.
The torque rheometer has been an essential instrument for a wide spectrum of research and development and quality control testing laboratories throughout the years. The torque rheometer has evolved just as quickly as advances in material chemistry. Highly sophisticated software and hardware technologies have now been introduced to better serve the needs of a modern laboratory. New challenges in such areas as plastics recycling and environmentally friendly fillers for plastics are some of the needs being met by using this multifunctional instrument. This paper intends to discuss how these changes have made the instrument more relevant than ever.
Dynamic feed is the injection molding process whereby the machine's polymeric flow is attenuated by a series of independent valves that are placed in a hot runner manifold just prior to each cavities entry gate. Each valve is controlled dynamically, in real time, to follow a pre-programmed pressure profile using feedback from a pressure transducer located downstream. The advantages of having control over each cavity (or open/close sequencing along with pressure profiling) are many. Parts of widely varying fill needs can each have a tailor made pressure profile specific to the needs of that particular part geometry. Strikingly dissimilar parts can be made in a single shot that would not be possible on a conventionally equipped machine. Watkins and Hume have discussed the primary advantages of this technology previously, they focused on the particular advantages of using the technique with modular tooling1. Kazmer discussed the process in detail5. Of concern in this work is the inherent stability of the dynamic feed process and the apparent potential for increased process variation due to anticipated material variations2.
Processing flows are known to accelerate polymer crystallization kinetics, strongly altering the orientation distribution of the crystallites and producing dramatic changes in material properties. Our research probes the molecular level processes that give rise to these effects. To clarify the role of macromolecular relaxation, we investigate the effects of shear history on the crystallization of isotactic polypropylenes. A unique apparatus enables us to subject a subcooled melt to precisely controlled intervals of shear at stress levels similar to those encountered in industrial processes.(1) Brief intervals of shear enhance the rate of subsequent crystallization by orders of magnitude. Previous rheo-optical experiments have indicated that the creation of long-lived, oriented structures during flow is controlled by the dynamics of the melt.(2) We present polarimetry and synchrotron wide-angle x-ray diffraction (WAXD) data obtained during and after shear of an iPP believed to contain chains with long branches. Results suggest that shearing near the nominal melting temperature induces the formation of a slow relaxing species that templates subsequent oriented crystal growth, emphasizing the importance of rheology to shear-enhanced crystallization.
This research involves the area of rapid prototyping (RP) and a new concept called functional prototyping. The overall goal of this project was to determine if current rapid prototyping methods allow for the prediction of the mechanical performance of a molded snap-fit. The Rapid Prototyping methods that were evaluated are Fused Deposition Modeling (FDM), Selective Laser Sintering (SLS), and machining plastic from stock shape. The results show that machining is the best method to use for functional testing, followed by SLS, and FDM. An attempt to ratio the results using the modulus of elasticity and yield strength are not quite satisfactory. But it can still be used for the rough estimation. Each of the prototypes types has its own tendency to deviate from the actual value.
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Society of Plastics Engineers
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