The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
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Extensional and Shear Rheology of Reactively Extruded Polyethylenes
In this study, the rheological characterization in extension and in shear of melts of reactively extruded low-density and linear low-density polyethylenes was carried out. The polyethylenes were extruded with peroxide, in one case, and with peroxide and diethylmaleate, in the other, using a co-rotating twin-screw extruder. A capillary rheometer with cylindrical and converging dies was employed to obtain the shear and extensional viscosities at 200 °C. The Cogswell, Binding and Mackay-Astarita models were used to estimate the extensional viscosity of the materials. An increase in both the shear and extensional viscosities was obtained due to the presence of long branches produced by the modification processes.
Extensional Rheology of Polypropylene and its Effect on Foaming of Thermoplastic Elastomers
Extensional viscosity of several polypropylene polymers and their blends was measured and compared to the relative foamability of thermoplastic elastomers (TPEs) containing these polymers. Both linear isotactic polypropylenes and branched polypropylenes were considered for this work. The extensional viscosity is measured using an RME Extensional Rheometer at various strain rates. The TPE foam is characterized based on density and cell size distribution. A small amount of branched polymer substituted for linear polymer was found to lead to improved foamability of the TPE. Results of this study will aid in the understanding of the foaming process in TPEs.
Extruder Output-Pressure Relationships for Power Law Fluids Including Leakage Flow
In order to predict output for screw extruders it is useful to have relationships of output versus pressure for various degrees of shear thinning. These relationships can be determined using numerical techniques and are usually graphically presented in dimensionless form. Various workers have presented such dimensionless graphs.It is well know that leakage flow, the flow between the flight tip and barrel, can significantly affect output and power consumption. A finite difference program was developed that accurately calculates the effect of leakage flow on output and power consumption. When the flight clearance is made dimensionless with respect to the flight height the family of output-pressure- clearance curves can be used for extruders of any diameter; in fact, creating universal output-pressure- clearance curves. This paper will present data of two different size extruders on output vs. pressure and energy efficiency vs. pressure. The utility of the information presented will be demonstrated with actual examples.
Extrusion Blow Mold Simulation Software Verification of Pinch-Off Design Parameters
The design of the pinch-off in extrusion blow mold ing has relied on past experience, company recommendation guidelines, and material supplier guidelines. This experiment was implemented to determine if simulation software could be used effectively to design a functional pinch-off design before any steel is cut. Actual bottle measurements from a Design of Experiment (DOE) (1) were contrasted to the results of the simulation software to determine the accuracy of the simulation. The actual DOE that the software was compared to is a three level DOE. The DOE was used to determine the processing parameters and flash pocket depth that produces the strongest weld line strength. The specimens collected from the DOE were tested for tensile strength and impact strength.
Failure Analysis and Redesign of a Polycarbonate Spring
An integrated approach is advocated for failure prevention of plastic products by optimising: material selection, design, processing and mould construction. The redesign of a plastic spring for a positioning device, using a buckled strip, illustrates this approach.The initial design suffered from mistakes:A low molecular PC was selected for obtaining short cycle times.The transverse deflection of the spring was too high.The mould was not tapered and caused scratches on the surface.Unlocking the positioning device caused very high bending stresses.By combining mechanical engineering, polymer engineering and practical knowledge about mould design and injection moulding, a reliable spring was redesigned.
Failure Analysis of a Large Diameter Floating Marine Hose
Floating marine hoses are often used to transport oil products from tankers to offshore moorings and, as such, their structural integrity is of major importance. Marine hoses are typically composites of various rubber compounds, textile fibers and steel reinforcement, the interaction of which is quite complex, rendering analysis of the structure difficult. In this paper, we present a detailed forensic examination of a failed marine hose, including such techniques as rubber fractography, finite element analysis, mechanical testing, and chemical testing. The various test results are assimilated to deduce the actual cause of failure, which is shown to be fatigue induced from external change.
Failure Analysis of Automotive Assemblies - Part I
Failures occurred within assemblies used in a transportation application. The cracking was observed in a relatively high number of parts immediately after cyclic thermal shock testing of the components. The cracking was found at several locations within the overmolded resin in areas immediately adjacent to underlying metal components. The focus of this investigation was a determination of the nature and cause of the failures. The results obtained during the evaluation of the failed components indicated that the cracking was the direct result of severe molecular degradation which occurred during the resin drying and molding operations, in conjunction with stresses induced through thermal shock testing of the parts. This paper will review some of the testing performed to characterize the failure mode and identify the cause of the cracking, while demonstrating the analytical procedure used in a failure investigation.
Failure Analysis of Automotive Assemblies - Part II
Failures occurred within assemblies used in a transportation application. The cracking was observed in a relatively high number of parts immediately after cyclic thermal shock testing of the components. The cracking was found at several locations within the overmolded resin in areas immediately adjacent to underlying metal components. The focus of the investigation was a determination of the nature and cause of the failures. The results obtained during the evaluation of the field samples indicated that the cracking was the direct result of molecular degradation that occurred during resin drying and molding operations, in conjunction with stresses induced through thermal shock testing of the parts. This part of the paper reviews the various methods that were used to evaluate viscosity changes in the material during drying and molding. This study illustrates the various strengths and weaknesses of the methods available to determine the molecular weight of a polymer.
Failure Behavior of Fiber-Reinforced Toughened Polymers
In fiber-reinforced toughened polymers, the influence of matrix deformation needs to be properly considered in their failure processes. There is a transition from failure dominated by massive matrix plasticity to that dominated by fiber-related mechanisms as the fiber content increases. This paper addresses some experimental observations and physical rationalization of the complex events that lead to failure of short fiber reinforced toughened nylon 6,6. The toughening behavior was studied using fracture mechanics techniques. The essential work of fracture analysis shows that the inclusions of short glass fibers not only provided a stiffening effect but also a toughening influence.
Failure of TPE Washers in Central Heating Systems
Widespread failure of TPE washers in central heating systems led to a large claim against the manufacturers and suppliers. Initial blame had been placed on plumbers who fitted the washers, either for under - or over-tightening the joints to the radiators. The material had been chosen for its ease of manufacture by injection moulding and replaced fibre washers. Inspection of failed washers showed numerous radial brittle cracks suggesting a different explanation for the failures. Trial experiments with new washers exposed to water at 85°C for three weeks stiffened, with 90% permanent set, the polyester elastomer changing dramatically in properties at short exposure times. The washers could not seal the system and extensive leakage occurred.
Failures Due to Stress Concentrations
The majority of plastic products fail due to stress concentrations. This is often due to insufficient radii of curvature, but also due to manufacturing and the condition of moulds.The stress concentration factor (SCF) can easily be determined from Peterson's book of SCF and they give about the same values as linear Finite Element Method calculations.For plastic products the values of the SCF is not sufficient to predict failure. The temperature and the strain rate are very important. The manufacturing method of the stress concentration by mechanical machining or by injection moulding gives significant differences in failure behaviour.In fatigue the stress concentration will determine the crack initiation and because of that stress concentrations are more dangerous in fatigue then in static loading.
Fatigue - Temperature Resistance of Continuous Glass Fiber/Polypropylene Composites
The flexural fatigue resistance of a continuous fiber composite (CFC) of glass and polypropylene (PP) under a fabric form has been investigated. Stress-life curves at a temperature of -40°C, 23°C and 50°C were obtained. These S-N curves show that the very high fatigue resistance of the PP/CFC composite at 23°C, in terms of normalized maximum flexural stress in fatigue (with respect to the flexural strength), remains relatively high at a testing temperature of 50°C. These curves also show that the fatigue resistance is actually improved at -40°C, i.e. at a temperature below than the glass transition temperature of the PP matrix. Comparison with a thermoset composite of similar flexural strength indicates that the intrinsic toughness and ductility of the thermoplastic PP matrix provides improved fatigue resistance, especially below the glass transition temperature.
Fatigue and Creep Crack Propagation Kinetics in PVC Pipe Material
The kinetics and mechanisms of creep and fatigue crack growth in PVC pipe material were studied in order to develop a methodology for prediction of creep fracture. In this study, fatigue crack propagation was resolved into fatigue and creep components. The creep component became more prominent with increasing temperature or decreasing loading rate. By studying the dependence of fatigue crack growth rate on loading rate and temperature, the contributions of fatigue and creep components to the overall crack growth rate were determined. The creep crack growth rate of PVC was predicted at ambient temperature by taking the ratio of the fatigue and creep contributions.
Fatigue Evaluation of Derakane* Vinylester/Glass Fiber Composites (Nabi Bus Program)
Information regarding the long-term durability of fiber reinforced structural composites is lacking in the community, but this information plays an extremely important role in the design and performance of the structure.This study is an out come of discussions that took place between TPI and Dow regarding the differentiation of the mechanical performance of composites made with DERAKANE* 8084 and XU1 MOMENTUM* 411-200. Based on clear casting data, the DERAKANE 8084 system appears to be a tougher resin however in order to obtain the same correlation on a composite, fatigue (durability) was suggested. The present document therefore details the experimental set-up, procedure and data obtained on flexure fatigue tests that were performed on the two composite systems. The data shows very clearly that whilst quasi-static flexure tests are not effective in differentiating the products that the fatigue (stress versus number of cycles) plot is able to bring out the differences. These tests were performed at 2 Hz and an R (ratio of minimum to maximum stress) = 0.2.Whilst the intent of this paper is not to generate durability data that could be used for designing the actual bus structure (as the test conditions mentioned above are far from those that exist in the field in terms of both stress level as well as frequency), it serves to present a framework for carefully obtaining and understanding data pertaining to the long-term fatigue performance of these composites. In doing so it also reinforces the importance of designing and carefully executing experiments that would provide time for damage to initiate and propagate in the composite in order to differentiate resins with varying toughness and durability.
Fatigue Resistance of Polyamide-6 and Polyamide-6/Clay Nanocomposite
Axial fatigue tests were carried out on polyamide- 6/clay nanocomposite and non-filled polyamide-6 (PA6). The results indicate that for these materials the evolution of the strain amplitude is better compared at the same maximum cyclic stress, while the accumulated strain at fracture is better compared at the same strain amplitude. The evolution of the accumulated volume variation suggests a localization of the deformation early in the fatigue process. The nanocomposite and the non-filled matrix showed similar fatigue lifetimes in the mechanically dominated fatigue regime. However, microscopic observations indicate changes in the fatigue fracture mechanisms.
Fiber Contents Effect on the Fiber Orientation in Injection Molded GF/PP Composite Plates
Short glass fiber reinforced polymer composites are common materials used to produce parts for structural applications. The attractiveness of these materials is a result of the combined advantages of the fiber reinforcing effect and the high production rates achievable by technologies such as injection molding.In fiber reinforced thermoplastic injection moldings, complex patterns of fiber orientation are obtained through-thickness. Most of the properties of the composite depend on the fiber orientation pattern. Thus, the prediction of the final properties of those composites requires either the prediction or the measurement of the fiber orientation field. The measurement of the fiber orientation field is a cumbersome and uneconomic task; thus the option has been to explore the capabilities offered by new computer modeling tools.An experimental programme was launched to systematically study the effect of fiber contents on the fiber orientation and final properties of injection molded short glass fiber reinforced polypropylene edge gated rectangular plates.
Fiber Orientation in Multilayer Tubes from a Conical Extruder
A new extruder design has been developed for the coextrusion of bi-layer annular sections. The extruder consists of a conical stator-rotor-stator assembly, which performs extrusion from each side of the rotor. Flow within this assembly is fully three-dimensional, with helicoidal streamlines in the vicinity of the rotor and the die entry region. Fiber orientation is created in the circumferential direction by these helicoidal streamlines; close to the inner and outer surfaces of the tube, the fibers are parallel to the main extrusion direction, whereas in the mid-thickness, they are oriented in the circumferential direction. The degree of orientation depends on the die design and on the polymers used.
Filler Phase Distribution in Elastomer Blends
Carbon black (CB) filler phase distributions in filled BR/BIMS blends (poly(1,4-cis-butadiene)/brominated poly (isobutylene-co-paramethylstyrene)) were quantified by image processing their tapping phase AFM blend morphologies. Preferential CB filler partition into the BIMS phase was indicated where the preference is enhanced with an increase in BIMS content suggesting a kinetic controlled filler partition process in straight mixing. 3D finite element modeling based on randomly generated blend morphologies and experimentally determined filler phase distributions could accurately predict blend tensile properties. Preferential filler partition into the BIMS phase may account for the observed synergistic improvement in abrasion resistance of these blends.
Finite Element Analysis of Impact Durablity on Extruded Plastic-Wood Fiber Composite Lineals
The impact performance of hollow profile extruded plastic-wood fiber composites depends on the profile geometry, the material properties and the damping of the system. Finite Element Analysis was used to simulate the Rosenheim and Gardner impact tests for a number of profile designs tailored for a series of plastic-wood fiber composites known as Fibrex™. In general, the analysis investigates the influence of geometry and damping properties on the impact energy and stress for fixed material properties. This investigation presents a failure criterion based on the penetration of critical impact energy. The results provide valuable information for plastic-wood fiber profile extrusion designs.
Flammability and Mechanical Properties of Vinyl Ester/Clay Nanocomposites
Flammability and mechanical properties of vinyl ester containing organically-treated montmorillonite were examined. Structure determination, done using transmission electron microscopy, revealed partial exfoliation of clay platelets. The tensile modulus was found to increase with clay content but without adversely affecting tensile strength. In small burner tests, the nanocomposites were not self-extinguishing, but vertical samples behaved quite differently from horizontal ones. Heat-release rates were also measured in a microcalorimeter at a temperature of 70°C, and these showed that clay could be effective in the enhancement of fire resistance.
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