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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Foaming PS With HFC-134a in Supercritical State
Because of the ban of CFC as an ozone-depleting substance and the undergoing phase-out of HCFC, the foam industry is currently looking at inert gases and HFCs as potential alternative blowing agents. However, contrarily to CFCs and HCFCs that were easily dissolved in the thermoplastic resins and could be readily expandable under relatively mild conditions with a minimum of processing difficulty, these alternative fluids unfortunately lack in solubility and are thus difficult to process. For instance extrusion foaming of polystyrene using HFC-134a is problematic above a concentration of 7wt%. Surprisingly, the pressure associated with this concentration threshold is approximately equal to the critical pressure of the HFC-134a, which may suggest that large clusters of HFC originating from its supercritical state would be responsible for the heterogeneities observed in the cell structure. This issue may be not only limited to HFC-134a as many other alternative gases (carbon dioxide, HFCs, hydrocarbons) may reach the supercritical state under the required processing conditions. Since use of mixtures of physical foaming agents in thermoplastic foam extrusion has been an industrial practice for a long time, the easiest way to circumvent this problem is to add a coagent, with the result of shifting away the critical point (pressure and temperature).
Abbreviated Terms for Plastics - Implications and Challenges
Abbreviated terms are routinely employed in the plastics industry in generic marking of molding products to assist in identification of the plastics for separation in recycling besides in trade and technical literatures. Existing ISO, ASTM and other standards provide clear guidelines for abbreviated terms for polymer families. In practice, however, a wide range of inconsistent abbreviated terms are quite common in the industry. This has significant implications for the general consumers as well as those involved with plastics recycling. The challenge is to arrive at uniform globally acceptable single set of abbreviated terms in order to addressing this issue.
CBA-Based Foaming of Thermoplastics Under Ambient Pressure
Understanding the fundamental mechanisms that govern the foaming process is the most essential universal prerequisite for developing effective processing strategies for fabricating high quality foamed plastic products using any type of foaming technology. Despite chemically blown foaming of thermoplastics under atmospheric pressure has been successfully implemented in rotational foam molding over the last decade, the related open literature does not provide substantial research addressing the fundamentals of this unique foaming mechanism. The present study focuses on clarifying the fundamental foaming mechanisms that govern the successful manufacture of thermoplastic foams using a chemical blowing agent (CBA) under low pressure (atmospheric) conditions. The presented research is mainly based on observing a series of visualization experiments conducted using a custom made visualization system including an optical microscope and a computerized CCD camera imaging system, which was utilized for investigating the behavior of foamable polyethylene (PE) samples prepared by using the compression molding method.
Influence of Carbon Nanofiber Microstructure on Electrical Proprties of LLDPE-Based Nanocomposites
Carbon nanofiber-reinforced linear low density polyethylene (LLDPE) composites were obtained from three different types of nanofibers (MJ, PR-24-PS, and PR-19-PS). A significant drop of volume resistivity was observed at 15 wt% MJ nanofiber content whereas volume resistivity started to decrease only at 25wt% of PR-24-PS or PR-19-PS nanofiber content. Further, micrographs of pure CNFs revealed that MJ nanofibers are twisted or even coiled whereas PR-24-PS and PR-19- PS are fairly straight. Scanning electron microscopy (SEM) revealed that CNFs were well dispersed in composites. We believe that the twisted or coiled structure of MJ fibers led to a better inter-fiber connectivity and reduced electrical percolation threshold.
Flash, a common injection molding defect, arises when melt flows from the cavity into thin gaps between parting surfaces. Besides rules of thumb for eliminating flash, there are few fundamental papers on flash analysis. Understanding flash as a transport phenomenon provides a systematic basis for solving flash problems. The governing equations for the gap flow are established and solved for an isothermal power law fluid, under constant pressure along the parting line where flash begins. Two shapes are investigated, rectangular and ring slits that respectively correspond to modeling flash from straight and curved parting lines. Our equation for flash length, the distance to which the melt penetrates the gap developed between the parting surfaces, is our main result. Further, adimensionalizing not only unifies the results for straight and curved parting lines, but also provides insight into how rheology, pressure and geometry govern flash. Our approach avoids tedious numerical simulation and mold structural analysis. The theory is validated by our polycarbonate flash experiments.
The Promise and Practice of Valve Gate Sequencing
Injection molders commonly use valve gates to reduce cycle times, control gate vestiges and limit gate discharge. The following are real-world examples of sequencing valve gates for balance, pressure control (particularly in family molds), reduction of knit lines, and minimization of clamp tonnage.In-cavity pressure sensing and DECOUPLED MOLDINGSM techniques are becoming more common. When combined with valve gate control these tools help create more robust processes or sometimes processes that would otherwise be impossible. This in turn has reduced scrap, material used and cycle time on a variety of tools.To put these techniques into practice molders need to pay attention to the details required for success.
Hierarchical Description of SEBS Block Copolymer Thermoplastic Elastomers
In this study we investigate the energetics and kinematics of deformation of SEBS block copolymers. The kinematics of deformation is investigated using simultaneous wide angle and small angle X-Ray diffraction on deformed samples. Results show that systematic deformation mechanisms occur in this class of thermoplastic elastomers. Moreover, these mechanisms are related to specific mechanical responses at specific levels of deformation. For the class of thermoplastic elastomers studied herein, the mechanisms include cooperative microbuckling and fragmentation of cylindrical styrenic microdomains, alignment of the fragments in the applied loading direction, followed by strain induced crystallization in selected systems.
Effect of Uniaxial Drawing of Soy Protein Isolate Film on Mechanical Properties
The effect of uniaxial drawing of soy protein isolate film on mechanical properties was investigated to accelerate efforts to develop SPI films with improved properties. The films containing 0 to 30wt% glycerol were drawn uniaxially up to a draw ratio of 2.5. The mechanical properties of the soy protein isolate film were found to be significantly improved after uniaxial drawing. The generation of crystal phase with drawing was not observed fromWAXD and DSC measurements. Therefore, the improvement in mechanical properties is ascribed to molecular orientation induced by drawing of the film.
Effects of Montmorillonite Layered Silicates on the Crystallization Properties of Polylactic Acid
Recently, polylactic Acid (PLA) has been increasingly considered for many applications due to its origin from renewable resources and its biodegradability. Separately, there has been interest in montmorillonite layered silicates (MLS), because of their remarkable ability to improve polymer properties. Strength and barrier properties are particular improvements to PLA that are considered critical. We examine the influence of MLS and processing on the crystallinity of PLA nanocomposites. Screw speed and feed rate of an extruder connected to a blown film die were systematically varied. The materials were supplied by the Naticak Army Research Laboratory and developed by Ratto and Thellen.Increasing screw speed during manufacturing decreases the residence time and is associated with the generation of smaller crystallites. Feed rate is another variable that is considered.Permeability and non isothermal Differential Scanning Calorimetry (DSC) at a single heating rate was reported recently. Here we report on the Avrami parameters of the PLA and corresponding nanocomposites.
Strategies to Balance the Flow in Profile Extrusion Dies
There are two strategies commonly adopted to balance the flow in an extrusion die for profiles: those involving and those not involving modifications of the die land cross section. In this work, a numerical code, which is being developed by the authors to perform the automatic optimization of profile extrusion dies, is used to illustrate the main issues concerning the die design strategies and to show the consequences of their application. It was concluded that the design strategies based on adjustments of the die land cross section generate dies more stable to variations of the processing conditions, but produce profiles with lower dimensional stability. On the other hand, strategies based on modifications of the die land length may be difficult to apply to profiles having significant differences in flow restriction, fact that can be overcame by the use of flow separators. However, this approach affects negatively the sensitivity of the tool and may hinder the mechanical resistance of the produced profiles.
Beam Shaping with Diffractive Optics for Laser Micro-Machining of Plastics with a Femtosecond Laser
Microfluidic devices and micro-electro-mechanical systems (MEMS) have become one of the most interesting and important applications in biotechnology, biomedical, pharmaceutical, life sciences, and agriculture. Research in manufacturing technologies used to fabricate these devices is important for improved quality as well as for time and cost savings during mass productions. A new approach for the fabrication of these MEMS devices is the laser ablation using diffractive optics elements (DOE) for beam shaping. In results described in this report, a 775 nm-wavelength high power femtosecond laser was used to ablate circular channels on polystyrene specimens. This approach to creating channels in the polystyrene was found to be promising even though the DOE used was not optimized for the femtosecond laser wavelength.
The Effect of Polyethylene Catalyst Type and Pigment Concentration on Crystal Growth during Rotomoulding
This work studied the effect of catalyst type and pigment concentration on the impact properties, crystallinity and morphology of rotationally moulded polyethylene (PE) parts. Microscopy, shrinkage and differential scanning calorimetry analysis techniques were used. It was observed that the base PE catalyst had an effect on the extent of crystallinity but that the level of pigmentation had only limited effect on the extent of crystallinity. The reduction in impact performance for turboblended pigmented samples was due to the relatively poor distributive and dispersive mixing of the pigment within the polymer matrix.
Structural and Thermal Aspects of the Performance of Hybrid Moulds with SLS
Hybrid moulds with molding blocks obtained by rapid tooling techniques are used for production of small batches. The mechanical and thermal performance of the inserts is particularly important at the design stage. This paper describes the use of Selective Laser Sintering (SLS) inserts for hybridmoulds. These inserts were experimentally study and the feasibility of this technology for the fabrication of hybrid-moulds investigated. Computer simulations using commercial software were also carried out. The obtained results show a good agreement between simulation and experimental data.
Analysis of Powder and Pellet Melting during Extrusion with a Perturbation Method
It was reported previously that the melting of a polymer during twin screw extrusion can be quantified using dynamic, on-line monitoring of a perturbation technique. Preliminary results suggested significant differences between the progression of melting of Polypropylene (PP) feed pellets and powders. This paper reports additional data on PP and Polyethylene (HDPE) feeds in the form of pellets, powder and pellet/powder mixtures. In addition to a delay of melting in the case of the powder feed, reductions in power intensity of the melting peak were also observed with the pulse perturbation technique. Unexpected behaviors in the melting of powder/pellet blends are described. The perturbation method is further examined by comparing the pulse energy input profiles with the residence time distribution and steady state power consumption over a range of throughput/screw speed (Q/N) operating conditions.
Characteristics of PP/PS/Clay Nanocomposite Produced by High-Intensity Ultrasonic Process
Polymer-clay nanocomposites of various concentrations were prepared by ultrasonically assisted melt blend process. The ultrasonic blend process using high intensity ultrasonic wave was employed to enhance nano-scale dispersion during melt mixing of polymer blend and organically modified clay. The materials studied were linear polypropylene and polystyrene reinforced with organophilic montmorillonite clay (nanoclay) at 3-5 wt% loadings. The effectiveness of the proposed ultrasonic processing technique on polypropylene matrix nanocomposites was evaluated by XRD, rheological measurements and thermal properties. We expected enhanced breakup of layered silicate bundle and further reduction in the size of dispersed phase with better homogeneity compared to the different immiscible blend pairs.Also, it was expected that generation of macroradicals in polymer mixture can lead to in-situ copolymer formation by their mutual combination, which should be an efficient path to compatibilize immiscible polymer blends and stabilize their phase morphology in the absence of other chemical agents.
A Numerical Study of Scanning Through-Transmission Laser Welding
A numerical model has been developed to simulate the heat conduction during the through-transmission laser welding (TTLW) process. The simulations were performed using the SIMPLER (Semi Implicit Method for Pressure Linked Equations Revised) software. The model was used to calculate the changes in the temperatures at points throughout the heated areas of the parts during the entire welding time. The temperature distributions and peak temperatures generated in the faying surface of welds at same line energy (LE) but different powers and speeds have been predicted and examined. Large temperature differences obtained when keeping LE constant, but doubling both power and speed were calculated and interpreted using this model.
Characterization of the Twin Screw Extrusion Process
In contrast to the well developed field of polymer characterization, the advances of extrusion process characterization lagged far behind. Engineers have to deal with problems such as process scale-up and product quality control primarily by empirical means. Practically no quantitative expressions exist between the operating conditions and the important process parameters such as degree of mixing, intensity of mixing and melting, the nature of polymer melting or plasticization, or energy consumption. We propose along with new supporting data that the parameters introduced previously, ?P/?Q and ?P/?N, that is, energy per unit of additional mass and energy per unit of screw revolution could be used to characterize the melting behavior of a polymer in an intermeshing, co-rotating twin screw extrusion process. The parameters appeared to be independent upon the extrusion rate (Q) and screw speed (N) in the ranges examined (4.5 to 36kg/hr and 100 to 500RPM). We further propose that additional parameters, such as those defining the kinetics of melting, average residence time (for melting and the over all) and residence time distribution obtainable by the pulse perturbation technique may be required in order to sufficiently define a given extrusion process.One can also predict with excellent agreement the observed specific mechanical energy input of extrusion, defined by extrusion power/throughput (P/Q), with simple linear expressions (Eqs. 1 and 2). The parameters c1 (?P/?Q), c2 (?P/?N), and a constant c3, can be determined directly by steady state extrusion experiments without employing pulse or step perturbation methods.
Ultrasonic Plunge Welding of Polypropylene Nonwovens
Polypropylene blown micro fibers with spun bonded cover web made from Polypropylene and polyester fibers were used for the experimental study of the ultrasonic plunge welding of nonwovens. A face centered central composite design of experiment (DOE) with 3 process variables and 3 levels was used to evaluate the effects of vibration amplitude, weld time and weld pressure. In addition to those settings, three different welding profiles were used resulting in forty-five different welding conditions with five specimens welded for each condition. Increasing the welding time generally improved the weld strength until it leveled off. For high welding pressures physical damage was observed around the welding area. Finally increasing the amplitude of vibration resulted in making the welding seam tougher with higher breaking forces.
Constructing Unique Micelle Structures of Charged Block Copolymers via Co-Assembly with Organic Counterions
A variety of fascinating micelle structures have been achieved by the assembly of charged triblock copolymers through the interaction with organic counterions in mixed THF/water solution. Essentially, the formation of toroidal micelles was observed in specific conditions of polymer chain constitution, the chain length of each block, the ratio of THF to water and mixing procedures. Our results showed that final assembled structures can be easily tuned by either varying the chain structure of organic counteirons or changing the ratios of THF to water.
Flash Prevention Due to Skin Solidification
The purpose of this paper is to investigate the role of skin solidification in preventing flash during injection molding of amorphous and semi-crystalline resins. In conventional injection molding, polymer melt is injected into a comparatively cold mold, resulting in the development of a solidified layer. A null hypothesis of this paper is that the development of a solidified layer reduces the exerted clamp tonnage on the machine and may even prevent the formation of flash under high pressures resulting in improved part quality. A set of Design of Experiments was implemented with control factors including barrel temperature, mold coolant temperature, pack pressure and delay time; the characterized responses included part weight, part thickness, and flash length. The results indicated that the addition of a delay time between the injection and packing stages eliminated flashing in this application.
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