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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Enhanced Foaming Ability through Microstructure Control of Polymer Blends
The effect of the dispersed phase morphology on the foaming ability of a blend of polyethylene (PE) and polypropylene (PP) is investigated. Two blends of PE/PP are prepared, one with the PP phase exhibiting spherical domains and the other with the PP phase exhibiting fibrillar domains. The morphological features of the PE/spherical-PP and PE/fibrillar-PP are identified using SEM. Batch foaming is conducted to show the effect of the blend morphology on the foaming ability. Measurements of the uniaxial extensional viscosity of the two types of blends are made. The crystallization kinetics is investigated using small amplitude oscillatory shear test. Changes in foaming ability are explained in terms of the differences in extensional viscosity and crystallization kinetics.
Visualization of Cell-Growth Induced Fiber Orientation in Polymer Composite Foams
In conductive polymer composites (CPCs), the alignment of fibers play a key role in determining the functional properties such as electrical conductivity, thermal conductivity, electromagnetic interference shielding, and dielectric behavior. However, in CPC foams, the evolution of fiber alignment caused by cell growth is not well understood. In this work, the interactions between carbon fibers (CF) and growing cells were in-situ visualized in a high-pressure foam injection molding process using polystyrene (PS)/CF as a model material system. The carbon fiber content, foam processing conditions, and visualization set-up were identified in a way that the interactions between individual fibers and cells can be captured. The results clearly demonstrate that the fibers in the vicinity of cell nuclei exhibit both translational and rotational movements upon the growth of the cells and the degree of rotation is a function of the cell size, initial fiber angle, and its distance from the cell nucleus. The results of this work provide a better understanding of the mechanisms by which foaming influences the functionalities of CPC foams and gives insight into the development of new orientation models that can accommodate foaming effects.
Controlled Foaming of Polystyrene/MWCNT by Carbon Dioxide
The ability to prepare various foams with a fixed relative density or a fixed cell size opens the possibility for decoupling the effects of cell size and cell density on the properties of foams, especially electrical conductivity. In this work, the PS/MWCNT composite foams were selected as a model system and various foamed samples were prepared by one step batch process with supercritical carbon dioxide (scCO2). Processing conditions were determined to produce polystyrene (PS)/multi-walled carbon nanotube (MWCNTs) composite foams that have the same relative density with significantly different cell sizes. Different cellular structures were achieved by controlling the saturation temperature, pressure, and time. The microcellular structure and relative density of PS-MWCNT nanocomposites were then characterized. Around the relative density of 0.48, the cell size varied from 4.5 -103 ?m.
From Nano-Structured iPP Formation to Nano-Cellular iPP Foam
Nanocellular foams of isotactic polypropylene (iPP) containing clarifying agent were prepared through batch foaming process with supercritical carbon dioxide (CO2). Clarifying agent, Millad NX8000, which has been introduced as a new generation of sorbitol derivatives to enhance the clarity of PP, was used to promote cell nucleation. The samples of iPP/sorbitol were prepared using twin-screw micro-compounder. Thermo-gravimetric analysis (TGA) was performed to assess the thermal degradation status of the materials during processing and characterization tests. Crystallization behavior of neat iPP and iPP/sorbitol was studied using differential scanning calorimetry and polarized optical microscopy. Cellular structure of the foams was also characterized. Depending on the foaming condition, foam structure was obtained in both micro and nano scales. Nanocellular foams with a cell density of ~1015 cells/cm3 was achieved by controlling crystallization kinetic of iPP in the presence of sorbitol. An optimum foaming temperature was found wherein the smallest cell size with highest cell density could be produced.
Uniaxial Strain Effects on the Percolatoin Threshold of Fibers in Polymer Composites: A Monte Carlo Simulation
In this research, a Monte Carlo model is built to examine the effects of compressive and tensile strains on the percolation threshold of fibers in polymer composites. Uniaxial strain was applied in the vertical direction in a system containing fibers with an aspect ratio of 10. The strain effect was modeled by introducing its corresponding alignment effect on the fibers. The critical volume fraction èc was then analyzed in both normal direction (vertical, èc?) and parallel direction (planar, èc?) to that of the cross-section plain. The results showed that the introduction of fiber alignment, caused by strain changed both the èc? and èc?, albeit with different trends. With the increase of tensile strain, èc? reached a minimum value first before starting to rise, while èc? continuously increased. On the other hand, under compression, with the increase of strain, èc? showed a minimal behavior before rise while èc? always increased. The results of this study confirm that the percolation threshold in a particular direction of interest can be decreased via a proper choice of applied strain.
Surface Quality of Parts Manufactured Using Selective Laser Sintering
Surface quality remains one of the biggest problems when manufacturing products using selective laser sintering (SLS). Several experiments were performed using different SLS input variables in order to manufacture samples with different surface characteristics. The effect of virgin or recycled powder, the laser power utilized, and the roller speed were studied and related to surface defects. An Alicona metrology system, with a 4th axis for rotating and imaging tools system was utilized to analyze samples. The three main quantities investigated were average surface roughness, root mean square roughness and ratio of areas. Bearing area curves were also examined.
A Fracture Mechancis Approach to Service Life Prediction of Hdpe Fusion Joints in Nuclear Applications
Limiting failure mode for long term performance of High Density Polyethylene (HDPE) pressure piping is slow crack growth (SCG), which is governed by the sustained stress levels (pressure and axial loads), and increases exponentially with elevated temperatures. Preliminary findings indicated the much lower resistance to SCG exhibited by HDPE butt-fusion pipe joints when compared to the parent pipe material. The integrity of HDPE pipe joints and the critical flaw size evaluation has now become a focus for the nuclear industry, regulators, and the plastic pipe industry. The ASME Boiler and Pressure Vessel Code Committee Sections III, IX, and XI address the use of HDPE piping in nuclear safety related applications. This current study is a summary of the findings till date on the crack driving force (Stress Intensity Factor), material fracture resistance (SCG tests on butt-fusion pipe joint materials), and service life prediction models for critical flaw size determination.
Pitfalls of MFI Targeting in the Specification of Golf Ball Thermoplastic Polyurethane Cover Layers
Injection moldable thermoplastic polyurethanes (TPUs) have been used in golf ball cover layers since the 1980s and can provide an attractive combination of formulation flexibility, performance, ease of processing, and overall lower cost when compared to cast thermoset polyurethane and polyurethane-urea systems. In developing raw material specifications for TPU and in their polymerization at the material supplier, melt flow index (MFI) is often used as a means to target certain processing and property requirements. However, using MFI as the only metric for quantifying a given TPU formulation can lead to problems as it does not capture polymerization and thermal history dependent structural variations inherent to these materials. In this study, final golf ball performance attributes, mechanical properties, thermal properties, dynamic mechanical properties, and dynamic rheological properties of two batches of TPU with identical composition and MFI were compared and contrasted. The results will be used to illustrate why TPU copolymer segmental structure and both temperature and time dependent morphology are important in the processing characteristics and properties of these versatile materials.
Isothermal Crystallization Behavior of Poly(Lactic Acid)/Cellulose Nanofiber Composites with Presence of CO2
The isothermal melt crystallization behavior of poly(lactic acid) (PLA)/cellulose nanofiber (CNF) at atmospheric pressure and two CO2 pressures was investigated using regular and high-pressure differential scanning calorimeters (DSC). The DSC analysis and polarized optical microscope (POM) result indicated that CNFs acted as crystal nucleating agents and accelerated the crystallization kinetics by providing more nuclei and decreasing the crystallization half-time. It was found that the crystallization kinetics of PLA materials was changed depending on the crystallization temperature, CNF content, and CO2 pressure. During isothermal process, the crystallization kinetic constant, k, increased with temperature up to a critical isothermal temperature before decreased; increasing CO2 pressure accelerated the crystallization kinetics of PLA at temperatures below 110 øC, but depressed at higher temperatures. The highest crystallinity was achieved at 15 bar.
Combinatorial Effects of Kneading Elements on Mixing in Twin-Screw Compounding
Modularity in the design of screw configurations is one of the most advantageous characteristics in twin-screw extrusion. In the compounding of polymers and solid additives, various mixing section geometries are used to distribute and disperse the minor phase within the polymer matrix. Thus, quantitative determination of the mixing efficiency of a particular screw configuration can greatly improve final product quality. Kneading blocks are the standard screw elements used to perform the mixing operation. This paper will discuss the mixing performance of various kneading blocks with regards to paddle size, mixing section length, and positioning. Mixing will be characterized through a design of experiment approach and the evaluation of the residence stress distribution for a range of operating conditions.
Injection Molding of Nano-Features ? A Study on Filling and Birefringence
The ability of the polymer melt to enter into the micro- or nano-structures is a crucial factor for successful molding operations and their final usability. When molding a rectangular plate with nano-features on top of it, it was observed that the nano-features are filled completely, away from the fan gate and only partially filled near the gate. A fundamental understanding based hypothesis driven approach helps to understand and solve the non-uniform filling behavior and leverage existing commercial modeling tools effectively. Based on the hypothesis, recommendations were made for further experimental studies, which showed a significant improvement in the filling of the nano-features completely throughout the part. Further studies focused on reducing the non-uniform birefringence by optimizing the gate design, using Moldflow? and Abaqus?.
Effect of Thin Walling and Foaming on TPO Part Performance
Two prominent strategies for decreasing injection molded part weight are thin walling of solid parts and foaming parts via supercritical fluids or chemical blowing agents. These strategies not only affect part weight but also mechanical performance. Advanced Composites has evaluated the effect of both of these strategies on part performance in a laboratory plaque study. Part density and weight were measured along with tensile, flexural, and notched impact properties. In the case of wall thickness reduction, the decrease in stiffness was more substantial than the reduction in weight on a percentage basis. Injection foaming resulted in a greater percentage of impact reduction than weight reduction but demonstrated advantages in flexural stiffness. These findings have ramifications for both part design and material selection.
Properties of Crosslinked Polyurethane-Clay Nanocomposites
Polyurethanes (PU) are versatile materials displaying desirable properties that can be modified through the addition of fillers. The introduction of organoclay into the matrix can change the mechanical and overall properties of the polymer. In this paper, the effect of clay concentration on the behavior of crosslinked polyurethane was investigated. The prepared systems were evaluated through analytical techniques in which their rheological and dynamic mechanical properties were studied. Results demonstrated that the introduction of clay improved the storage modulus by more than 30%. Furthermore, clay decreased the molecular weight and viscosity of nanocomposite solution by 40% and 90%, respectively for up to 10 wt% clay, which significantly improves processability.
Phase Morphology and Electrical Conductivity of Polypropylene/Polylactic Acid Blends Filled with Multi-Walled Carbon Nanotubphase
The selective localization of multiwalled carbon nanotubes (MWCNTs) was studied in a ternary system containing polypropylene (PP), polylactic acid (PLA), and MWCNTs. With a proper composition, the selective localization of MWCNTs in PP/PLA blends with more viscous PLA phase decreased the electrical percolation threshold of the blend to 0.15 vol. %, which was 4 times lower than that of the PP/MWCNT composites. Despite the predictions suggested by the thermodynamic measurements, the transmission electron microscopy (TEM) micrographs revealed the localization of carbon nanotubes in the PP phase instead of the PLA. Moreover, decreasing the viscosity ratio of PP/PLA did not cause the MWCNTs? to migrate to the PLA phase. The electrical conductivity measurements showed that the blend system with less viscous PLA phase had a lower conductivity due to its coarser morphology. The selective localization of MWCNTs in these blend systems was explained in terms of the relative molecular mobility of the phase during processing.
Thermal Analysis of Soy Flour Elastomer Composites
Biobased plastics are becoming viable alternatives to petroleum-based plastics because they decrease dependence on petroleum derivatives and tend to be more environmentally friendly. Raw materials such as soy flour are widely available, low cost, lightweight, and can have high strength. In this study, soy flour was utilized as a filler in thermoplastic elastomer composites. Because weak interfacial adhesion between the soy flour and the elastomer and low water resistance pose challenges, a surface pretreatment, acetylation, was investigated for composites with soy flour concentrations of 10 wt%, 15 wt% and 20 wt%. Previous studies of the mechanical properties of these composites at 10 wt% determined that acetylation resulted in ultimate strength comparable to that of the pure elastomer. In this study, the chemical pathways of the reaction were verified and the thermo-mechanical properties characterized. Interfacial adhesion was characterized through scanning electron microscopy (SEM); the study determined that the acetylation reaction increased interfacial adhesion as indicated by smaller particle sizes and less agglomeration. Thermal properties were determined though thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Acetylation reduced the intake of water by the soy flour, thus increasing the thermal stability of the composites. Increased thermal stability was indicated by a rise in decomposition temperature.
Effect of Catalyst on Compatibilization of Poly(lactic acid) / Polyamide Blends
The effects of catalysts p-toluenesulfonic acid (TsOH) on trans-reactions in poly (lactic acid) (PLA)/polyamide (PA11) blends were investigated in this study. The extent of reaction was tracked using solubility, modulated differential scanning calorimetry (MDSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR). The organic acid catalyst appeared to induce trans-reaction but also reduced overall molecular weight in PLA/PA11 blends. The interchange reactions appeared to compatibilize the blends as evidenced by calorimetry and microscopy.
Anisotropic Thermal Conduction in Polymeric Materials
The strong coupling of mechanical and thermal effects in polymer processing flows has a significant impact on both the processability and final properties of the material. Simple molecular arguments suggest that Fourier?s law must be generalized to allow for anisotropic thermal conductivity in polymers subjected to deformation. In addition, theoretical results suggest a linear relationship between the thermal conductivity and stress tensors, or a stress-thermal rule. In our laboratory we have developed a novel optical method based on Forced Rayleigh Scattering (FRS) to obtain quantitative measurements of components of the thermal diffusivity tensor in polymers subjected to deformations. We have found the stress-thermal rule to be valid for several polymer systems in both shear and elongational deformations. More recently, we have developed a novel technique based on Infrared Thermography (IRT) that complements FRS and allows for the study of a wider range of polymeric materials.
Failure Analysis of Copolyester Clamps
Cracking and subsequent failure of a clamp feature on a broken copolymer component occurred through brittle fracture as a result of environmental stress cracking. Environmental stress cracking (ESC) is a phenomenon whereby a particular plastic resin is affected by a specific chemical agent while under stress. ESC occurs commonly in polymeric components but the fractographic features of copolyester are similar in both ESC and creep. The analytical techniques employed in this article provide a guideline by which to properly determine the root cause of failure for copolyester components and describes characteristics typically observed in copolyester ESC failures.
Experimental Results of Melt Modulation Packing Parameters Control on Cold-Runner Injection Molding Final Product Quality
This paper details the experimental results performed on a clear polymer, STYRON? 685D (GPPS), to investigate the impact of varying the packing processing parameters on its final quality. Packing processing parameters, including packing pressure and packing time, have significant impact on the internal molecular orientations, mechanical properties and optical performance of injection molded polymeric products. Experimental results have been obtained from three melt modulation control methods to control the packing pressure, packing time and control valve angle during each cold-runner injection molding cycle.
Experimental Analysis of Fibers Orientation and Mechanical Properties in Injection Molding of Thermoplastic Reinforced Materials by Rapid Heat Cycle Molding
Injection molding of reinforced thermoplastic materials is increasingly used in order to obtain high specific mechanical properties and good surface finishing. In this work a rapid heat cycle molding (RHCM) technology was employed to study the influence of injection molding parameters on fibers orientation and mechanical properties of thermoplastic fiber reinforced injection molded parts. X-ray computed tomography (CT) was employed to allow a direct observation of the fibers orientation. The injection molding parameters investigated were mold temperature, injection speed and part thickness. The experiments show that the mechanical properties are maximized by high part thickness, low mold temperature and injection velocity. However, the variation of the investigated injection molding conditions had a negligible effect of fibers orientation.
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