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.
Thermoforming is the process of shaping heated sheet against a cooled mold. Products range from very thin packaging disposable packaging to very thick transportation components. Thermoformed parts are recognized as having very large surface area-to-thickness ratios. They are also know for nonuniform wall thicknesses across their surfaces. As with other single-surface processes such as blow molding and to some extent, rotational molding, commercial thermoformed part wall thickness variation is typically +/- 20-30%. Three areas of computer aids have grown in popularity recently. Computer-aided multi-axis CNC trimming machines are being extensively used in heavy-gauge thermoforming to ensure accurate peripheral and mating surface dimensions. Computer programs are now available for predicting heating and cooling cycles for diverse polymers, thus allowing for more rapid material and mold changeover. And finite element analysis is used to predict local part wall thickness and plug design, among other facets. The impact of these newer developments on part-to- part dimensional reliability is discussed in this paper.
In this study, the structure development of the heat affected zone (HAZ) of PVDF based materials was investigated. For this purpose, an instrumented vibration welding machine was employed. With the precise control of built in the design of the instrumented vibration welding machine, a series of samples were welded at different welding times. This allowed for the detailed analysis of the evolution of surface features and thus fundamental mechanisms that take place during the early stages of welding. The results reveal that the mechanism of structural formations starts at highly localized regions, which coalesce in subsequent stages. Further in the process, the formation of wave-like patterns, which found to extrude fibrillar structures from the sides of the sample. This behavior was attributed to the high melt elasticity exhibited by these polymers.
The rotational molding process can be readily modified to serve as a foam processing technology so that it becomes capable of creating a foam layer or core in the interior of the rotomolded hollow articles. Although polypropylene (PP) is being traditionally considered unfeasible for foaming because of its low melt strength, its advantageous end-use properties over polyethylene (PE) render it into a preferred candidate for replacing PE in rotational foam molding. This study is primarily focused on investigating the effects of concurrently implementing the rotational foam molding technology and PP into the processing of hollow rotomolded articles with a view of reducing their inherent mechanical, insulative, and shock mitigation weaknesses. The effects of varying the processing parameters on the foaming behavior of differently formulated PP-based foamable blends were thoroughly studied through conducting extensive rotational foam molding trials. The preliminary experimental results revealed that it is feasible to successfully process PP foams in rotational foam molding. However, it is crucial to process the PP foams at the lowest processing temperature possible so that the melt strength of PP during foaming can be preserved. The PP melt strength control strategy implemented during the trials included reducing the oven temperature and/or lowering the decomposition temperature of the chemical blowing agent (CBA) by using an activator. Consequently, PP foams with acceptable cell morphologies have been produced.
This paper is intended to provide an engineering understanding of the technological potentials for processing polypropylene (PP) foams in rotational foam molding. A process proposal, based on the melt compounding material-preparation approach, capable of producing completely foamed, single-layer, single-piece PP products in rotational foam molding, is disclosed in detail. It comprises dispersing a chemical blowing agent (CBA) in the PP matrix using a twin-screw compounder, pelletizing the obtained expandable composition, and then producing foams in an uninterrupted rotational foam molding cycle by using the pre-compounded foamable pellets. Several PP grades were deliberately selected to cover a wide range of melt flow rates (MFR), starting from 5.5 up to 35 dg/min. After the raw materials participating in the study were characterized using thermal analysis instrumentation, different foamable compositions were formulated in order to prepare both 3- fold and 6-fold foamable pellets from each PP grade. The optimal foam processing strategies were identified via a systematic experimental parametric search. Foams with the best cell morphologies were obtained out of the high melt strength PP grades. In addition, the experimental results revealed that the cell morphology of the processed PP foams is not as good as that of respective PE foams. However, the cell morphologies of the PP foams processed by using the melt compounding-based approach demonstrated significant improvements in comparison with those processed by using the dry blending-based approach.
Vehicles are designed to provide as many features to the customer as possible. One of those features is the extension of the cargo area or trunk space to transport oversized items by folding the rear seats. This feature, depending on the vehicle, requires the front and/or rear seats of the vehicle to pass the luggage retention test. The performance of thermoplastic automotive rear seat back frames was evaluated using analytical tools. In this case the shoulder safety restraint for the center occupant is attached to the seat back frame. Analytical results showed that a back frame design that utilizes a thermoplastic material is possible.
Two co-continuous polyethylene/copolymer blend systems, one with a hydrogenated triblock copolymer, polyethylene/styrene-ethylene-butadiene-styrene (HDPE/SEBS) and the other with a hydrogenated diblock copolymer, polyethylene/styrene-ethylene-butadiene (HDPE/SEB) were studied in this paper. It was found that co-continuous morphologies exist over a very wide composition range in both blend systems (from 30% copolymer to 70% copolymer) and that the continuity % -composition relationship is identical for both blends. The viscosity ratio of the blend had no effect on either the composition region of co-continuity or the microstructure of the co-continuous blend. The wide region of dual phase continuity is explained using an argument based on highly stable elongated structures. The very low interfacial tension of these systems results in very effective deformation of the dispersed phase followed by long breakup times. The copolymer architecture has a substantial effect on the pore size of the blend.
Because of the increasing use of thermoplastics and thermoplastic composites in load-bearing applications, welding methods are becoming important for part cost reduction. Welding requires the melting of the surfaces to be joined, followed by a solidification of the interfacial molten layers under pressure. One widely used technique is hot-tool welding, in which the surfaces to be joined are brought to the “melting temperature” by direct contact with a heated metallic tool. In some cases, such as joining of plastic pipes, the surfaces to be joined are flat, so that the tool is a hot plate. However, in many applications, such as in automotive headlamps and rear lights, doubly curved joint interfaces require complex tools that allow the hot surfaces to match the contours of the joint interface. Applicability to complex geometries is one of the major advantages of this process. An understanding of the hot-tool welding process and how its process parameters affect weld strength has evolved over the last thirty years, but the information is scattered over many papers in the technical literature. A scheme is proposed for presenting strength data for hot-tool welds of thermoplastics to themselves and to each other. It follows an earlier scheme for a strength database for vibration welding . The main objective is to provide engineers with sufficient information to enable them to design plastic joints. The hot-tool welding method, its advantages and shortcomings, and typical applications are briefly described. Data are presented on achievable weld strengths of several thermoplastics—including welds between dissimilar materials—and their blends. A guide to the literature, including a list of references, is provided.
This paper presents an innovative dilatometer that can measure the pressure-volume-temperature (PVT) properties of polymer/CO2 solutions in a molten state. The basic rationale of the design is to determine the density (or equivalently, the specific volume) of a polymer/CO2 solution by separately measuring the mass and volume flow rates of the solution flowing in an extruder at each temperature and pressure. A positive-displacement gear pump mounted on an extruder is used to measure the volume flow rate of the solution. A single-phase polymer/CO2 solution is formed by injecting a metered amount of supercritical CO2 into a polymer melt and completely dissolving it in the melt using a foam extrusion line. The temperature of solution was precisely controlled and homogenized by using the second extruder in a tandem system and a heat exchanger with a static mixer. The pressure was controlled by the rotational speed of the screw in the second extruder. In order to reduce leakage across the gear pump, the difference between the upstream and downstream pressures was minimized using a variable resistance valve attached downstream of the gear pump. The mass flow rate was measured by directly collecting the extruded polymer melt for a fixed time after degassing CO2. A critical set of experiments was carried out to verify the functions of the system using pure polymer melts with known PVT data. Finally, the system was used to measure the specific volume of PS/CO2 solutions as a function of CO2 concentration, temperature, and pressure.
The thermal behaviors of linear and branched propylene materials with foaming additives were investigated using a high-pressure differential scanning calorimeter (DSC). In this study, the effects of material branching, dispersed additives, and dissolved blowing agent on the crystallization temperature of propylene materials were elucidated. It was observed that branching promoted the crystallization kinetics of propylene materials significantly. However the increased crystallization temperature of branched propylene materials was lowered under high pressure of gas. The effect of hydraulic pressure was determined by performing high-pressure experiments with helium which has a very low solubility in polymers. The effects of foaming additives such as talc and GMS as well as the effect of cooling rate on the crystallization temperature were also studied. The experimental results indicate that the crystallization temperature could be increased as much as 30 °C by branching the propylene resin and/or adding an additive in the propylene resin.
The melt fracture behaviors of linear and branched propylene resins with foaming additives were investigated. The effects of branching, processing temperature, additives, and blowing agent on the melt fracture of propylene materials were thoroughly studied. A CCD camera was installed at the die exit to precisely observe the onset of melt fracture of extruded foams. The critical wall shear stress was determined for various linear and branched propylene resins. It was found that the branching required to foam propylene resins also promotes melt fracture: the critical shear stress was decreased by 0.0175 MPa with an increase of 0.1 n/1000c in long-chain branching. It was also observed that the dissolved blowing agent (butane) significantly suppressed the melt fracture of both linear and branched propylene resins. On the other hand, a noticeable increase in the critical shear stress of branched propylene materials was observed with the nucleating agent (talc) and the aging modifier (glycerol mono stearate), whereas almost negligible effect of the additives on the critical shear stress was observed for linear propylene materials.
Two types of injection-molded ABS (acrylonitrile-butadiene- styrene) thermoplastics, one with MgO as the acid scavenger and one without the MgO, were used as the housings of the membrane strips for pregnancy tests and heat stressed at 37°C for two months. Membrane strips in ABS containing MgO showed premature pink color around the end of test indicator. This pink color is normally an indication of the completion of the medical test and should be colorless prior to conducting the test. No color appearance was observed for membrane strips in ABS without MgO. X-ray photoelectron spectroscopy spectra revealed that the pink color appearance was caused by the diffusion of sulfuric acid - the key compound that prevents premature color formation in membrane strips. The diffused acids went into the ABS housing and the diffusion was enhanced by the presence of MgO acting as a strong sink for the diffused sulfuric acid. This study demonstrates the importance of understanding the delicate interaction between thermoplastics and medical devices.
Isothermal physical aging of a high-Tg thermosetting epoxy/amine system was investigated at different aging temperatures (Ta) and chemical conversions (monitored by the glass transition temperature, Tg using the TBA torsion pendulum technique. The aging temperature was from 10°C (below which the effect of very small amounts of water may be involved) to 130°C (above which further reaction during isothermal aging may occur); the conversion was systematically changed from postgel (Tg > 70°C, below which microcracking occurs) to fully crosslinked (Tg=177°C). In the absence of chemical reaction during an isothermal aging process, the rate of isothermal physical aging below Ta=90°C passes through a minimum with increasing conversion. The minimum in the aging rate is related to the minimum in mechanical loss between Tg and the secondary glassy state transition (T?). If isothermal aging rates would have been measured directly from temperatures below T? to above Tg, it is concluded that two maxima in isothermal aging rate would have been observed corresponding to the two transitions. There exists a superposition in aging rate vs. Tg-Ta by shifting horizontally and vertically which implies that the aging rate is independent of the details of the changing structure due to cure. Controlling mechanisms during physical aging are segmental mobility associated with the Tg region and more localized submolecular motion associated with the glassy state relaxation, T?. Also discussed are the localized effects of isothermal physical aging which results in perturbations of the modulus and mechanical loss vs. temperature in the vicinity of Ta.
Generalizations on the cure and properties of thermosetting polymers have been formulated in terms of cure-property relationships the underlying concept for which is that transition temperatures rise with increasing chemical conversion. The relationships are summarized in four diagrams: 1) the isothermal time-temperature-transformation (TTT) cure diagram, 2) the continuous heating time-temperature-transformation (CHT) cure diagram, 3) the conversion-temperature-property (TgTP) diagram, and 4) the glass transition temperature (Tg) vs. conversion diagram. The relationships may be used to molecularly design thermosetting systems so as to optimize cure processes and glassy state properties. Glassy state properties studied vs. conversion have included modulus, density, rubber modification, and microcracking, and the dynamics of submolecular motions as represented by physical aging and transitions.
A potential material change for a pharmaceutical blister pack requires costly and time consuming re-validation. One of the major issues associated with this is moisture vapour barrier of a formed blister to guarantee an acceptable shelf life of the packaged drugs. This study considers a non-isothermal FEM analysis of the forming process, using the new model incorporated into the simulation software Polyflow, taking into account plug assist forming. This is validated on current materials before being used for the new material. A new post processor is used to calculate the moisture barrier of each blister, and evaluate the potential benefit of using a new material for medical packaging.
The drawing of nylon 6,6 and ultrahigh molecular weight polyethylene (UHMWPE) fibers has been studied at elevated temperatures in a CO2 environment in the supercritical phase regime. With nylon, the CO2 increases the crystallinity and orientation of the fiber significantly resulting in 30% higher strength values. With UHMWPE fibers, the ultimate strength is only increased slightly. However, the modulus can be improved by 50% in comparison to air drawn fibers at the same temperature. For both polymer fibers, temperature has a large effect on the final structure and the engineering properties of the fiber.
The purpose of this research is to study the pressure drop profiles of biodegradable polybutylene succinate (PBS)/CO2 solutions in a slit die and to measure the rheological properties of the solutions as a function of the blowing agent concentration. A slit die with four pressure transducers has been designed to describe the effects of shear rate, temperature, pressure, and gas content on the shear viscosity and extensional viscosity of the flowing PBS/CO2 solutions. The low shear rate viscosity of the pure polymer was measured using a cone and plate rheometer. Extensive experiments were conducted to investigate the polymer/gas solution viscosities at five different shear rates, three temperatures and five gas contents. Cross-Carreau model and generalized Arrhenius equation were used to describe the shear-viscosity behaviors of PBS/CO2 solutions. The extensional viscosity of solution was modeled based on Cogswell's equation.
Screw in-line plastic injection moulding machine has been so widely used in industry that optimization of the injector design will have great impact to industry as a whole. However, admit the extensive use of plastic injection moulding machine in industry, most studies in the injector design were based on extruders which were simpler to model. In the analysis of mass flow in the injection screw, it is founded similar to that of an extruder with reciprocating effect. This paper studies the transient model for the melting process, which is one of the most important sections in the reciprocating extruder. Based on this transient model, factors affecting the melting speed in reciprocating extruders are very clear. They include screw rotating speed, screw axial movement speed, barrel thickness, barrel heat capacity, temperature of heater and polymer, etc. The model was used to simulate conditions made by Donovan , Some phenomena observed by Donovan in his experiments were explained,. It also proves that this model is correct and applicable. Based on this model, we also concluded that to get better quality product, low rotation speed, long rotating time could be helpful. In addition, the heat capacity of the barrel affects the transient process of melting in a way that, the thinner the barrel the better. This transient model together with other models to be presented will be very helpful for controlling the whole plasticating process in reciprocating extruders in an effort to get better product quality.
To modify the properties of polymers, mineral fillers are frequently added during the compounding process. Due to adhesive forces, these pulverized fillers tend to agglomerate. Therefore, in order to achieve a good homogenisation, it is essential not only to distribute them but also to break down the solid agglomerates. A number of relating models have been published, describing observations (agglomerate rupture, erosion, clustering) made during the dispersion process in a mostly isolated manner. Here, a new model considering each observed effect have been developed in order to get a comprehensive description of the dispersion process. To verify the model, it was implemented into a program for the process simulation of co-rotating twin screw extruders (SIGMA ), and thus compared to experimental data. It showed that the model could well describe the experimentally determined data.
A limiting dilute concentration of the dispersed phase in polyamide-6 (PA6) and high-density polyethylene (HDPE) blends during extensional flow was detected. This limit concentration is explained in terms of coalescence phenomena. Blends of PA6/HDPE at different compositions and melt-drawn ratio were prepared using a twin-screw extruder with a rectangular slit at 250 °C. The rectangular sheets were cooled in water prior to being rolled with a suitable device. The extrusion velocity was maintained constant and the take up velocity was varied in order to obtain different states of deformation of the minor phase. The morphology results shown that at low take up velocity, the final state of deformation is independent of the dispersed phase composition. However, at high take up velocity the drop deformation increases with the composition. To determine a lower limit concentration, at which coalescence occurs, the average particle volume of the dispersed phase was evaluated. At the concentration range of 1 to 4 %vol. of PA6, the average volume of the particles remains constant (no coalescence) during the stretching process. However, at higher concentrations (> 5 %vol. of PA6) the coalescence process takes place and the volume increases with stretching.
The action of weathering on the mechanical and thermal properties of high density polyethylene (HDPE) without aditives in the atmosfere of Rio de Janeiro City was studied. After about 1800 hours of exposition time it was observed that degradation process was very fast and can be seen in some mechanical parameters. It was observed an increasing of 20% of elastic modulus. The stress at yield increased slightly and elongation decreased. Both, stress and elongation at break diminished. The elongation showed a decreasing about 90%. The impact resistance presented a loss of 50%. The molecular weight also decreased.
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